Journal articles on the topic 'Solar corona problem'
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1
Gudiksen, B. V., and Â. Nordlund. "An Ab Initio Approach to the Solar Coronal Heating Problem." Symposium - International Astronomical Union 219 (2004): 488–92. http://dx.doi.org/10.1017/s0074180900182506.
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We approach the solar coronal heating problem ab initio. Starting from a potential extrapolation of a SOHO/MDI magnetogram, a FAL—C atmospheric stratification, and a realistic photospheric velocity field, Spitzer conductivity and magnetic dissipation creates a corona where more than 2 106ergs s—1 cm—2 is dissipated. The winding of the magnetic field by the horizontal velocities in the solar photosphere is sufficient to provide a major part of the heating in the solar corona. The heating is intermittent on the smallest scale, but on average follows the magnetic field strength squared, as is expected from a force free magnetic field configuration. The intermittent heating creates large temperature and density fluctuations in the corona. The total dissipated energy in the corona is at least constant if not increasing with magnetic Reynolds number, making this heating process unavoidable as a major contributor to the heating of the solar corona.
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Parhi, S., B. P. Pandey, M. Goossens, and G. S. Lakhina. "Numerical Simulation of Twisted Solar Corona." Symposium - International Astronomical Union 185 (1998): 467–68. http://dx.doi.org/10.1017/s0074180900239235.
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The solar corona supports a variety of waves generated by convective upwelling motion in the photosphere. In order to explain the observed coronal temperature profile, resonant absorption of MHD waves by coronal plasma (Goossens et al, 1995) has been proposed as a possible candidate. The physical picture is that the footpoint motion in the photosphere constantly stirs the coronal plasma leading to the MHD wave generation which is then resonantly absorbed producing the enhanced heating of the corona. Here we consider the problem of MHD wave propagation in a twisted solar corona.
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Koutchmy, S., M. M. Molodensky, and J. C. Vial. "On the 3D Solar Corona Structure." International Astronomical Union Colloquium 144 (1994): 585–88. http://dx.doi.org/10.1017/s0252921100026099.
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AbstractWe consider the 3D structure of the solar corona using eclipse observations. We use a pseudostereoscopic effect of the rigidly rotating corona to determine the true position of the main coronal structures: broad threads, rays and streamers. We find that observations collected by the “Multi-station International Coronal Experiment” are well suited for solving this problem. Formula and error estimation are given to demonstrate the feasibility of the method. An example of stereo-view deduced from a simple analysis of results coming from the 1991 eclipse is given. The observed apparent shifts allow for the first time to apprehend the true 3D structure of the corona. The structure of streamers was compared with the pecularities (pleats and cusps) of the solar heliosphere current sheet, deduced from the sourse surface. The positions of the two main streamers systems rays (near the N-E and S-limb) coincide with the pleats of the current heliosphere layer. We conclude that large helmet streamers are composed by the pleats of the heliosphere current sheet projected on the plane of the sky.
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Woods, L. C. "Heating the Solar Corona." Highlights of Astronomy 13 (2005): 124. http://dx.doi.org/10.1017/s1539299600015276.
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A typical temperature for the quiet solar corona is ~ 1.5 x 106K, whereas the photosphere – the likely source of the thermal energy – has a temperature less than 6 × 103 K. Although many theories have been advanced to explain why the corona is so much hotter than the photosphere, this old problem remains unsolved. However, there is a mechanism based on second-order transport that may provide the answer, or at least part of the answer. This process, described by the author in Thermodynamic inequalities in gases and magnetoplasmas, John Wiley & Sons Ltd, 1996, causes heat to be transported across strong magnetic fields up temperature gradients.
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Rusin, V., V. Dermendjiev, M. Rybansky, and G. Buyukliev. "Slight Disappearance of Prominence Plasma to the Solar Corona." Symposium - International Astronomical Union 142 (1990): 347–49. http://dx.doi.org/10.1017/s0074180900088240.
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The problem of prominences-corona relationship is relativelly old. Already in 1931 Lockyer [1] showed that there is a close relation between prominences distribution and the form of white-light corona. However, this problem is still debatable and poses a number of controversial questions. One of them is the question of the energy and mass exchange between prominences and the ambient corona. It is generally assumed that the mass balance exists between the corona and prominences, but unambiguous observational proofs for prominences-corona plasma exchange are very rare. There are little data [2-4], as well, that could be used to address the problem of slight plasma flows from prominences to the corona.
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Yalim, M. S., G. P. Zank, and M. Asgari-Targhi. "Coronal Loop Heating by Nearly Incompressible Magnetohydrodynamic and Reduced Magnetohydrodynamic Turbulence Models." Astrophysical Journal 944, no. 2 (February 1, 2023): 119. http://dx.doi.org/10.3847/1538-4357/acb151.
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Abstract The transport of waves and turbulence beyond the photosphere is central to the coronal heating problem. Turbulence in the quiet solar corona has been modeled on the basis of the nearly incompressible magnetohydrodynamic (NI MHD) theory to describe the transport of low-frequency turbulence in open magnetic field regions. It describes the evolution of the coupled majority quasi-2D and minority slab component, driven by the magnetic carpet and advected by a subsonic, sub-Alfvénic flow from the lower corona. In this paper, we couple the NI MHD turbulence transport model with an MHD model of the solar corona to study the heating problem in a coronal loop. In a realistic benchmark coronal loop problem, we find that a loop can be heated to ∼1.5 million K by transport and dissipation of MHD turbulence described by the NI MHD model. We also find that the majority 2D component is as important as the minority slab component in the heating of the coronal loop. We compare our coupled MHD/NI MHD model results with a reduced MHD (RMHD) model. An important distinction between these models is that RMHD solves for small-scale velocity and magnetic field fluctuations and obtains the actual viscous/resistive dissipation associated with their evolution whereas NI MHD evolves scalar moments of the fluctuating velocity and magnetic fields and approximates dissipation using an MHD turbulence phenomenology. Despite the basic differences between the models, their simulation results match remarkably well, yielding almost identical heating rates inside the corona.
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Howson, Thomas. "How Transverse Waves Drive Turbulence in the Solar Corona." Symmetry 14, no. 2 (February 15, 2022): 384. http://dx.doi.org/10.3390/sym14020384.
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Oscillatory power is pervasive throughout the solar corona, and magnetohydrodynamic (MHD) waves may carry a significant energy flux throughout the Sun’s atmosphere. As a result, over much of the past century, these waves have attracted great interest in the context of the coronal heating problem. They are a potential source of the energy required to maintain the high-temperature plasma and may accelerate the fast solar wind. Despite many observations of coronal waves, large uncertainties inhibit reliable estimates of their exact energy flux, and as such, it remains unclear whether they can contribute significantly to the coronal energy budget. A related issue concerns whether the wave energy can be dissipated over sufficiently short time scales to balance the atmospheric losses. For typical coronal parameters, energy dissipation rates are very low and, thus, any heating model must efficiently generate very small-length scales. As such, MHD turbulence is a promising plasma phenomenon for dissipating large quantities of energy quickly and over a large volume. In recent years, with advances in computational and observational power, much research has highlighted how MHD waves can drive complex turbulent behaviour in the solar corona. In this review, we present recent results that illuminate the energetics of these oscillatory processes and discuss how transverse waves may cause instability and turbulence in the Sun’s atmosphere.
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Cremades, H., F. A. Iglesias, and L. A. Merenda. "Asymmetric expansion of coronal mass ejections in the low corona." Astronomy & Astrophysics 635 (March 2020): A100. http://dx.doi.org/10.1051/0004-6361/201936664.
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Aims. Understanding how magnetic fields are structured within coronal mass ejections (CMEs), and how they evolve from the low corona into the heliosphere, is a major challenge for space weather forecasting and for solar physics. The study of CME morphology is a particularly auspicious approach to this problem, given that it holds a close relationship with the CME magnetic field configuration. Although earlier studies have suggested an asymmetry in the width of CMEs in orthogonal directions, this has not been inspected using multi-viewpoint observations. Methods. The improved spatial, temporal, and spectral resolution, added to the multiple vantage points offered by missions of the Heliophysics System Observatory, constitute a unique opportunity to gain insight into this regard. We inspect the early evolution (below ten solar radii) of the morphology of a dozen CMEs occurring under specific conditions of observing spacecraft location and CME trajectory, favorable to reduce uncertainties typically involved in the 3D reconstruction used here. These events are carefully reconstructed by means of a forward modeling tool using simultaneous observations of the Solar-Terrestrial Relations Observatory (STEREO) Extreme Ultraviolet Imager and the Solar Dynamics Observatory Atmospheric Imaging Assembly as input when originating low in the corona, and followed up in the outer fields of view of the STEREO and the Solar and Heliospheric Observatory coronagraphs. We then examine the height evolution of the morphological parameters arising from the reconstructions. Results. The multi-viewpoint analysis of this set of CMEs revealed that their initial expansion – below three solar radii – is considerably asymmetric and non-self-similar. Both angular widths, namely along the main axes of CMEs (AWL) and in the orthogonal direction (AWD, representative of the flux rope diameter), exhibit much steeper change rates below this height, with the growth rate of AWL found to be larger than that of AWD, also below that height. Angular widths along the main axes of CMEs are on average ≈1.8 times larger than widths in the orthogonal direction AWD. The ratios of the two expansion speeds, namely in the directions of CMEs main axes and in their orthogonal, are nearly constant in time after ∼4 solar radii, with an average ratio ≈1.6. Heights at which the width change rate is defined to stabilize are greater for AWL than for AWD.
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Hansteen, V. H. "Solar Wind Acceleration." International Astronomical Union Colloquium 144 (1994): 453–60. http://dx.doi.org/10.1017/s0252921100025781.
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AbstractThe general aspects of solar wind acceleration are well described by considering the thermally driven outflow from an electron – proton corona. However, two puzzling observations remain to be explained: 1) The predicted asymptotic flow velocity is much lower than that observed in high speed streams, and 2) The proton flux observed at 1AU varies considerably less than expected when considering the sensitivity of the proton flux to the coronal temperature predicted by thermally driven models. The solution of the first problem rests upon finding a mechanism which can deposit energy and/or momentum beyond the critical point of the flow. The invariance of the proton flux requires that a mechanism for maintaining a relatively constant proton density scale height in the subsonic region of the flow is found. One such possibility lies in considering the effects of an enhanced coronal helium abundance on the force balance of the subsonic flow. This scenario is discussed in some depth.
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Minarovjech, M., and M. Rybanský. "Solar Corona Investigation by the Coronal Line Photometer at Lomnický Peak." International Astronomical Union Colloquium 144 (1994): 431–34. http://dx.doi.org/10.1017/s0252921100025744.
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AbstractThis paper deals with a possibility to use the ground-based method of observation in order to solve basic problems connected with the solar corona research. Namely:1.heating of the solar corona2.course of the global cycle in the corona3.rotation of the solar corona and development of active regions.There is stressed a possibility of high-time resolution of the coronal line photometer at Lomnický Peak coronal station, and use of the latter to obtain crucial observations.
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Virtanen, Ilpo, and Kalevi Mursula. "Photospheric and coronal magnetic fields in six magnetographs." Astronomy & Astrophysics 626 (June 2019): A67. http://dx.doi.org/10.1051/0004-6361/201935713.
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Context. Solar photospheric magnetic fields have been observed since the 1950s and calibrated digital data are available from the 1970s onwards. Synoptic maps of the photospheric magnetic field are widely used in solar research, especially in the modeling of the solar corona and solar wind, and in studies of space weather and space climate. Magnetic flux density of the solar corona is a key parameter for heliospheric physics. Aims. The observed photospheric magnetic flux depends on the instrument and data processing used, which is a major problem for long-term studies. Here we scale the different observations of the photospheric field to the same absolute level and form a uniform record of coronal magnetic flux since the 1970s. Methods. We use a recently suggested method of harmonic scaling, which scales any pair of synoptic observations of any resolution to the same level. After scaling, we use the Potential Field Source Surface (PFSS) model to calculate the scaled magnetic field at various altitudes from photosphere to coronal source surface. Results. Harmonic scaling gives effective, latitudinally dependent scaling factors, which vary over the solar cycle. When scaling low-resolution data to high-resolution data, effective scaling factors are typically largest at low latitudes in the ascending phase of solar cycle and smallest for unipolar polar fields around solar minima. The harmonic scaling method used here allows for the observations of the different data sets to be scaled to the same level and the scaled unsigned coronal flux densities agree very well with each other. We also find that scaled coronal magnetic fields show a slightly different solar cycle variation from that of the nonscaled fields.
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Parnell, Clare E., and Ineke De Moortel. "A contemporary view of coronal heating." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 370, no. 1970 (July 13, 2012): 3217–40. http://dx.doi.org/10.1098/rsta.2012.0113.
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Determining the heating mechanism (or mechanisms) that causes the outer atmosphere of the Sun, and many other stars, to reach temperatures orders of magnitude higher than their surface temperatures has long been a key problem. For decades, the problem has been known as the coronal heating problem, but it is now clear that ‘coronal heating’ cannot be treated or explained in isolation and that the heating of the whole solar atmosphere must be studied as a highly coupled system. The magnetic field of the star is known to play a key role, but, despite significant advancements in solar telescopes, computing power and much greater understanding of theoretical mechanisms, the question of which mechanism or mechanisms are the dominant supplier of energy to the chromosphere and corona is still open. Following substantial recent progress, we consider the most likely contenders and discuss the key factors that have made, and still make, determining the actual (coronal) heating mechanism (or mechanisms) so difficult.
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Vaiana, G. S., and S. Sciortino. "Observations of stellar coronae." Symposium - International Astronomical Union 122 (1987): 333–45. http://dx.doi.org/10.1017/s0074180900156670.
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We present an overview of recent stellar X-ray observations, with some discussion of the requirements for future observations. We argue that solar observations indicate that coronal X-ray emission is strongly related to surface magnetic field activity; we show that the interpretation of X-ray stellar coronal emission from late-type stars within the framework of models analogous to those developed for the solar corona is viable, and it is supported by many experimental results. The extension of this solar analogy to the early-type stars is quite questionable and remains an unsolved problem, while the working hypothesis of an X-ray phase, related to phenomena of magnetic field-related activity, as contrasted to a wind phase during the PMS evolutionary stage is suggested by the present status of observations.
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Aly, J. J. "Quasi-Static Evolution of a Force-Free Magnetic Field and Conditions for the Onset of a Stellar Flare." International Astronomical Union Colloquium 104, no. 2 (1989): 259–63. http://dx.doi.org/10.1017/s0252921100154302.
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Magnetic fields in the solar corona are braught into an endless evolution by the never-ceasing motions of the subphotospheric plasma in which the feet of their lines are anchored. It is generally thought that this evolution is essentially quasi-static, the field passing through a sequence of force-free equilibrium states. Sporadically, however, the equilibrium is broken in a region of limited extent, and during a relatively short interval of time a catastrophic highly dynamic evolution takes place, giving rise to such wellknown phenomena as flares or coronal transients. Understanding the factors which determine if a magnetohydrostatic coronal equilibrium is maintained or, on the contrary, destroyed, when boundary conditions change at the photospheric level, then appears as a central theoretical problem of solar physics. In this Communication, we report some recent results which shed some new light onto this old problem.
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Wiegelmann, T., B. Inhester, and L. Feng. "Solar stereoscopy – where are we and what developments do we require to progress?" Annales Geophysicae 27, no. 7 (July 23, 2009): 2925–36. http://dx.doi.org/10.5194/angeo-27-2925-2009.
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Abstract. Observations from the two STEREO-spacecraft give us for the first time the possibility to use stereoscopic methods to reconstruct the 3-D solar corona. Classical stereoscopy works best for solid objects with clear edges. Consequently an application of classical stereoscopic methods to the faint structures visible in the optically thin coronal plasma is by no means straight forward and several problems have to be treated adequately: 1) First there is the problem of identifying one-dimensional structures – e.g. active region coronal loops or polar plumes- from the two individual EUV-images observed with STEREO/EUVI. 2) As a next step one has the association problem to find corresponding structures in both images. This becomes more difficult as the angle between STEREO-A and B increases. 3) Within the reconstruction problem stereoscopic methods are used to compute the 3-D-geometry of the identified structures. Without any prior assumptions, e.g., regarding the footpoints of coronal loops, the reconstruction problem has not one unique solution. 4) One has to estimate the reconstruction error or accuracy of the reconstructed 3-D-structure, which depends on the accuracy of the identified structures in 2-D, the separation angle between the spacecraft, but also on the location, e.g., for east-west directed coronal loops the reconstruction error is highest close to the loop top. 5) Eventually we are not only interested in the 3-D-geometry of loops or plumes, but also in physical parameters like density, temperature, plasma flow, magnetic field strength etc. Helpful for treating some of these problems are coronal magnetic field models extrapolated from photospheric measurements, because observed EUV-loops outline the magnetic field. This feature has been used for a new method dubbed "magnetic stereoscopy". As examples we show recent application to active region loops.
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Schmitt, J. H. M. M. "Eclipse mapping at X-ray wavelengths." Symposium - International Astronomical Union 176 (1996): 85–94. http://dx.doi.org/10.1017/s0074180900083121.
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Stellar coronae cannot be spatially resolved, and yet stellar coronae are likely to exhibit a similar amount of structure as the solar corona. Currently, the only way to infer structure from spatially unresolved data is to look for rotational modulation of the X-ray emission or eclipses in the case of binary systems. I will discuss some of the observations obtained and critically review the methods used to infer structure from these data. Particular emphasis will be placed on the ill-conditioned nature of the inversion problem, that makes it next to impossible to infer the possibly three-dimensional structure of stellar coronae.
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Liu, XiaoJing, Xueshang Feng, Man Zhang, and Jingmin Zhao. "Modeling the Solar Corona with an Implicit High-order Reconstructed Discontinuous Galerkin Scheme." Astrophysical Journal Supplement Series 265, no. 1 (March 1, 2023): 19. http://dx.doi.org/10.3847/1538-4365/acb14f.
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Abstract The present study aims to apply an implicit high-order reconstructed discontinuous Galerkin (DG) scheme (rDG(P 1 P 2)) to simulate the steady-state solar corona. In this scheme, a piecewise quadratic polynomial solution, P 2, is obtained from the underlying piecewise linear DG solution, P 1, by least-squares reconstruction with a weighted essentially nonoscillatory limiter. The reconstructed quadratic polynomial solution is then used for the computation of the fluxes and source terms. In addition, an implicit time integration method with large time steps is considered in this work. The resulting large linear algebraic system of equations from the implicit discretization is solved by the cellwise relaxation implicit scheme which can make full use of the compactness of the DG scheme. The code of the implicit high-order rDG scheme is developed in Fortran language with message passing interface parallelization in Cartesian coordinates. To validate this code, we first test a problem with an exact solution, which confirms the expected third-order accuracy. Then we simulate the solar corona for Carrington rotations 2167, 2183, and 2210, and compare the modeled results with observations. We find that the numerical results basically reproduce the large-scale observed structures of the solar corona, such as coronal holes, helmet streamers, pseudostreamers, and high- and low-speed streams, which demonstrates the capability of the developed scheme.
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Gontikakis, Costis, Spiro K. Antiochos, and Peter R. Young. "The Transition Region of Solar Flare Loops." Astrophysical Journal 943, no. 2 (February 1, 2023): 120. http://dx.doi.org/10.3847/1538-4357/aca8a9.
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Abstract The transition region between the Sun’s corona and chromosphere is important to the mass and energy transfer from the lower atmosphere to the corona; consequently, this region has been studied intensely with ultraviolet and extreme ultraviolet (EUV) observations. A major result of these studies is that the amount of plasma at low temperatures, <105 K, is far too large to be compatible with the standard theory of thermal conductivity. However, it is not clear whether the disagreement lies with a problem in the observations or a problem in the theory. We address this issue by analyzing high–spatial and temporal resolution EUV observations from an X1.6-class flare, taken with the Interface Region Imaging Spectrograph and the Solar Dynamic Observatory/Atmospheric Imaging Assembly (AIA). These data allow us to isolate the emission of flare loops from that of surrounding structures. We compare the emission measures (EMs) derived from the C ii 1334.525 Å and Si iv 1402.770 Å transition region spectral lines, the Fe xxi 1354.066 Å flare line, and the AIA 171 Å coronal images. We find that the EM ratios are incompatible with a standard conduction-dominated transition region model. Furthermore, the large increases in the EM magnitudes due to flare heating make it highly unlikely that the disagreement between data and theory is due to observational uncertainties in the source of the emission. We conclude that the standard Spitzer–Härm thermal conductivity must be invalid for, at least, flare loops. We discuss the possibility that turbulent suppression of thermal conduction can account for our results.
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De Moortel, Ineke, and Philippa Browning. "Recent advances in coronal heating." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 373, no. 2042 (May 28, 2015): 20140269. http://dx.doi.org/10.1098/rsta.2014.0269.
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The solar corona, the tenuous outer atmosphere of the Sun, is orders of magnitude hotter than the solar surface. This ‘coronal heating problem’ requires the identification of a heat source to balance losses due to thermal conduction, radiation and (in some locations) convection. The review papers in this Theo Murphy meeting issue present an overview of recent observational findings, large- and small-scale numerical modelling of physical processes occurring in the solar atmosphere and other aspects which may affect our understanding of the proposed heating mechanisms. At the same time, they also set out the directions and challenges which must be tackled by future research. In this brief introduction, we summarize some of the issues and themes which reoccur throughout this issue.
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Sturrock, Peter A. "The Role of Eruption in Solar Flares." International Astronomical Union Colloquium 104, no. 1 (1989): 387–97. http://dx.doi.org/10.1017/s0252921100032012.
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AbstractThis article focuses on two problems involved in the development of models of solar flares. The first concerns the mechanism responsible for eruptions, such as erupting filaments or coronal mass ejections, that are sometimes involved in the flare process. The concept of ‘loss of equilibrium’ is considered and it is argued that the concept typically arises in thought-experiments that do not represent acceptable physical behavior of the solar atmosphere. It is proposed instead that such eruptions are probably caused by an instability of a plasma configuration. The instability may be purely MHD, or it may combine both MHD and resistive processes. The second problem concerns the mechanism of energy release of the impulsive (or gradual) phase. It is proposed that this phase of flares may be due to current interruption, as was originally proposed by Alfvén and Carlqvist. However, in order for this process to be viable, it seems necessary to change one's ideas about the heating and structure of the corona in ways that are outlined briefly.
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Kuncic, Z., and P. A. Robinson. "Propagation Effects on the Cyclotron Maser Mechanism for Solar Microwave Spike Bursts." Publications of the Astronomical Society of Australia 10, no. 4 (1993): 278–82. http://dx.doi.org/10.1017/s132335800002587x.
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AbstractThe loss-cone-driven electron cyclotron maser instability is widely believed to be responsible for millisecond bursts of intense microwave emission often observed during solar flares. However, the maser radiation is strongly absorbed as it propagates outward from the corona and existing analytical models predict that this absorption should be sufficiently strong to prevent observable levels of the radiation from escaping, except under highly restrictive conditions. In order to address the problem of how microwave spike bursts can be observed at all, we present a numerical ray-tracing analysis which incorporates emission, propagation and absorption of fundamental cyclotron maser radiation in a realistic model of a coronal flux loop. It is found that the radiation can escape to a potential observer and that the physical conditions under which escape occurs are more restrictive for fundamental emission in the extraordinary (x)-mode than in the ordinary (0)-mode. Escaping radiation in the x-mode is found to be highly directional and chiefly observable toward the center of the solar disk, while escaping 0-mode radiation is found to emerge from the corona over a much wider range of directions, with some cases corresponding to observable radiation near the solar limb.
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Shivamoggi, Bhimsen K. "Stability of Parker's Steady Solar Wind Solution in the Subcritical Region." Astrophysical Journal 944, no. 1 (February 1, 2023): 96. http://dx.doi.org/10.3847/1538-4357/acb537.
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Abstract Parker’s steady solar wind solution (PSSWS) is a physically acceptable solution describing a smooth acceleration of the solar wind to supersonic speeds. Parker proposed that PSSWS possesses an intrinsic stability, like a “stable attractor” of this dynamical system. With a view to give a systematic analytical development, we restrict ourselves to the subcritical region inside the Parker critical point (PCP) where the solar wind goes through sonic flow conditions. This enables one to avoid the singularity at PCP plaguing the linear stability problem. Following Parker, we approximate the corona in the subcritical region by a static atmosphere and amend it to include an azimuthal flow and a weak radial flow. These physical simplifications enable us to pose a Sturm–Liouville problem for linearized perturbations about PSSWS. PSSWS is shown to have an intrinsic stability in the subcritical region, while leaving the solar coronal base in a state of (1) rest, (2) corotation with the Sun, and (3) slow radial motion. This result is also shown to hold when a diabatic flow in near-isothermal conditions is included in Parker’s model to explicitly account for the extended coronal heating. The isothermal gas assumption in Parker’s model is then relaxed, and a more realistic barotropic fluid representing variable extended active coronal heating conditions is considered for the gas flow. The stability of PSSWS, as the solar wind flow emerges from a state of rest at the solar surface, is shown to continue to hold.
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Browning, P. K. "Helicity injection and relaxation in a solar-coronal magnetic loop with a free surface." Journal of Plasma Physics 40, no. 2 (October 1988): 263–80. http://dx.doi.org/10.1017/s002237780001326x.
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A solar-coronal magnetic loop is rooted in the photosphere, where motions shuffle the footpoints of the field, generating currents in the corona. The dissipation of these currents provides a possible mechanism for heating the solar corona. A theory is described based on a generalization of Taylor's hypothesis, predicting that as the loop is twisted up, it relaxes towards a minimum-energy state V × B = μB. The footpoint motions inject helicity as well as energy, and the evolution is determined through a helicity-injection equation. The loop is modelled as a straight magnetic-flux tube, with twisting motions at the ends, confined by a constant external pressure at the curved surface, which is a free boundary. The problem of the loop evolution in response to given footpoint motions is solved, and an interesting example of multiple equilibria arises. The heating rate is calculated for an almost-potential loop. The model may also be regarded as representing a laboratory experiment: in particular, a simple idealization of a spheromak, with the footpoint motions replaced by an applied voltage.
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Riley, Pete, Roberto Lionello, Ronald M. Caplan, Cooper Downs, Jon A. Linker, Samuel T. Badman, and Michael L. Stevens. "Using Parker Solar Probe observations during the first four perihelia to constrain global magnetohydrodynamic models." Astronomy & Astrophysics 650 (June 2021): A19. http://dx.doi.org/10.1051/0004-6361/202039815.
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Context. Parker Solar Probe (PSP) is providing an unprecedented view of the Sun’s corona as it progressively dips closer into the solar atmosphere with each solar encounter. Each set of observations provides a unique opportunity to test and constrain global models of the solar corona and inner heliosphere and, in turn, use the model results to provide a global context for interpreting such observations. Aims. In this study, we develop a set of global magnetohydrodynamic (MHD) model solutions of varying degrees of sophistication for PSP’s first four encounters and compare the results with in situ measurements from PSP, Stereo-A, and Earth-based spacecraft, with the objective of assessing which models perform better or worse. We also seek to understand whether the so-called ‘open flux problem’, which all global models suffer from, resolves itself at closer distances to the Sun. Methods. The global structure of the corona and inner heliosphere is calculated using three different MHD models. The first model (“polytropic”), replaced the energy equation as a simple polytropic relationship to compute coronal solutions and relied on an ad hoc method for estimating the boundary conditions necessary to drive the heliospheric model. The second model (“thermodynamic”) included a more sophisticated treatment of the energy equation to derive the coronal solution, yet it also relied on a semi-empirical approach to specify the boundary conditions of the heliospheric model. The third model (“WTD”) further refines the transport of energy through the corona, by implementing the so-called wave-turbulence-driven approximation. With this model, the heliospheric model was run directly with output from the coronal solutions. All models were primarily driven by the observed photospheric magnetic field using data from Solar Dynamics Observatory’s Helioseismic and Magnetic Imager instrument. Results. Overall, we find that there are substantial differences between the model results, both in terms of the large-scale structure of the inner heliosphere during these time periods, as well as in the inferred timeseries at various spacecraft. The “thermodynamic” model, which represents the “middle ground”, in terms of model complexity, appears to reproduce the observations most closely for all four encounters. Our results also contradict an earlier study that had hinted that the open flux problem may disappear nearer the Sun. Instead, our results suggest that this “missing” solar flux is still missing even at 26.9RS, and thus it cannot be explained by interplanetary processes. Finally, the model results were also used to provide a global context for interpreting the localized in situ measurements. Conclusions. Earlier studies suggested that the more empirically-based polytropic solutions provided the best matches with observations. The results presented here, however, suggest that the thermodynamic approach is now superior. We discuss possible reasons for why this may be the case, but, ultimately, more thorough comparisons and analyses are required. Nevertheless, it is reassuring that a more sophisticated model appears to be able to reproduce observations since it provides a more fundamental glimpse into the physical processes driving the structure we observe.
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Noëns, J. C., B. Pech, J. Xanthakis, H. Mavromichalaki, V. Tritakis, V. Petropoulos, and A. Paliatsos. "Observed Large-Scale East-West Asymmetries in the Solar Corona." International Astronomical Union Colloquium 144 (1994): 151–54. http://dx.doi.org/10.1017/s0252921100025227.
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AbstractSome new results are presented and discussed about the problem of the asymmetries in the observed corona between the east and west limbs. “Local effects” are analysed. Relations within one eleven-year solar activity cycle are shown.
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Nocera, L., B. Leroy, and E. R. Priest. "Phase Mixing of Propagating Alfvén Waves." Symposium - International Astronomical Union 107 (1985): 365–69. http://dx.doi.org/10.1017/s0074180900075835.
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Among MHD waves, Alfvén waves have been proved to be the best candidates to reach the solar corona and, eventually, to be responsible for the heating of this outer part of the solar atmosphere. The problem arises, however, about the mechanism able to transform the energy stored in the waves into heat.
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Parhi, S., and T. Tanaka. "Coronal Heating Mechanism in the Presence of a Flow: A Numerical Approach." Symposium - International Astronomical Union 185 (1998): 469–70. http://dx.doi.org/10.1017/s0074180900239247.
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The solar corona is a hot, tenuous plasma permeated with the structured magnetic fields. A variety of waves is generated in the corona due to the convective upwelling motion in the photosphere. The excitation of MHD fluctuations is generated by the footpoint motion of the field lines in the photosphere. Resonant absorption of the Alfvén waves in an inhomogeneous plasma has been suggested as a means of driving current and plasma heating in the corona (Sakurai et al., 1991). We study this problem in the presence of flow.
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Galzauskas, V. "VII. Active Regions: Structure and Evolution." Transactions of the International Astronomical Union 19, no. 1 (1985): 84–90. http://dx.doi.org/10.1017/s0251107x00006167.
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The corona above active regions is now recognized as an assemblage of magnetically confined loops of plasma. This advance in understanding the active upper atmosphere is documented in the monograph resulting from the Third Skylab Workshop (Solar Active Regions, 1981, ed., F.Q. Orrall). International collaborative programs during the Solar Maximum Year (SMY) have further stimulated the study of active regions with emphasis on the search for the underlying causes of solar flares. Scores of analyses of individual regions, combining space- and ground-based observations, have been published. We have as a result an improved picture of interactions between active regions: from creation of shear in the magnetic topology to inter-region connections via the corona. A revived interest in the phenomena of recurrent active regions and sunspot decay has highlighted a basic problem for solar magnetism: the removal of magnetic flux from the solar surface. The interpretation of temporary dips in the solar irradiance caused by active regions continues to generate lively debate.
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Tlatov, Andrey G., and Ivan Berezin. "Modeling the Magnetic Field of the Inner Corona in a Radially Expanding Solar Wind." Physics 5, no. 1 (January 29, 2023): 161–67. http://dx.doi.org/10.3390/physics5010012.
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The magnetic field in the interplanetary medium is formed by the action of magnetic field sources on the photosphere of the Sun and currents in the expanding atmosphere of the Sun and the solar wind. In turn, the high-speed plasma flow changes the configuration of the magnetic field lines. The problem of determining the parameters of the magnetic field near the Sun is thus a three-dimensional problem of the interaction of the magnetic field and the plasma of the solar wind. We present analytical expressions for calculating the total magnetic field vector B→(r, θ, ϕ) (in spherical coordinates) for a radially expanding solar wind flow of finite conductivity. The parameters of the solar wind are given in the form of a dimensionless magnetic Reynolds number given as an arbitrary function of the radius, r: Rm = rσμv=ξ(r), where σ, μ, and v denote, respectively, the conductivity, magnetic permeability, and velocity of the solar wind. The solution for the magnetic field components is obtained in the form of a decomposition in spherical functions and a radial part depending on the distance from the Sun. Examples of calculations of the configuration of magnetic fields and structures of the solar corona for the solar eclipse of 21 August 2017 are given.
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Feng, Xueshang, Haopeng Wang, Changqing Xiang, Xiaojing Liu, Man Zhang, Jingmin Zhao, and Fang Shen. "Magnetohydrodynamic Modeling of the Solar Corona with an Effective Implicit Strategy." Astrophysical Journal Supplement Series 257, no. 2 (November 16, 2021): 34. http://dx.doi.org/10.3847/1538-4365/ac1f8b.
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Abstract In this paper, we design an effective and robust model to solve the 3D single-fluid solar wind plasma magnetohydrodynamics (MHD) problem of low plasma β. This MHD model is formulated on a six-component composite grid system free of polar singularities. The computational domain ranges from the solar surface to the super-Alfvénic region. As common to all MHD codes, this code must handle the physical positivity-preserving property, time-step enlargement, and magnetic field divergence-free maintenance. To maintain physical positivity, we employ a positivity-preserving Harten–Lax–van Leer Riemann solver and take a self-adjusting and positivity-preserving method for variable reconstruction. To loosen the time-step limitation, we resort to the implicit lower–upper symmetric Gauss–Seidel method and keep the sparse Jacobian matrix diagonally dominant to improve the convergence rate. To deal with the constant theme of a magnetic field that is divergence-free, we adopt a globally solenoidality-preserving approach. After establishing the solar wind model, we use its explicit and implicit versions to numerically investigate the steady-state solar wind in Carrington rotations (CRs) 2172 and 2210. Both simulations achieve almost the same results for the two CRs and are basically consistent with solar coronal observations and mapped in situ interplanetary measurements. Furthermore, we use the implicit method to conduct an ad hoc simulation by multiplying the initial magnetic field of CR 2172 with a factor of 6. The simulation shows that the model can robustly and efficiently deal with the problem of a plasma β as low as about 5 × 10−7. Therefore, the established implicit solar wind MHD model is very promising for simulating complex and strong magnetic environments.
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Brooks, David H., Miho Janvier, Deborah Baker, Harry P. Warren, Frédéric Auchère, Mats Carlsson, Andrzej Fludra, et al. "Plasma Composition Measurements in an Active Region from Solar Orbiter/SPICE and Hinode/EIS." Astrophysical Journal 940, no. 1 (November 1, 2022): 66. http://dx.doi.org/10.3847/1538-4357/ac9b0b.
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Abstract A key goal of the Solar Orbiter mission is to connect elemental abundance measurements of the solar wind enveloping the spacecraft with extreme-UV (EUV) spectroscopic observations of their solar sources, but this is not an easy exercise. Observations from previous missions have revealed a highly complex picture of spatial and temporal variations of elemental abundances in the solar corona. We have used coordinated observations from Hinode and Solar Orbiter to attempt new abundance measurements with the Spectral Imaging of the Coronal Environment (SPICE) instrument, and benchmark them against standard analyses from the EUV Imaging Spectrometer (EIS). We use observations of several solar features in active region (AR) 12781 taken from an Earth-facing view by EIS on 2020 November 10, and SPICE data obtained one week later on 2020 November 17, when the AR had rotated into the Solar Orbiter field of view. We identify a range of spectral lines that are useful for determining the transition region and low-coronal-temperature structure with SPICE, and demonstrate that SPICE measurements are able to differentiate between photospheric and coronal magnesium/neon abundances. The combination of SPICE and EIS is able to establish the atmospheric composition structure of a fan loop/outflow area at the AR edge. We also discuss the problem of resolving the degree of elemental fractionation with SPICE, which is more challenging without further constraints on the temperature structure, and comment on what that can tell us about the sources of the solar wind and solar energetic particles.
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Gritsyk, P. A., and B. V. Somov. "Electron Acceleration in Collapsing Magnetic Traps during the Solar Flare on July 19, 2012: Observations and Models." Proceedings of the International Astronomical Union 13, S335 (July 2017): 90–93. http://dx.doi.org/10.1017/s1743921317008912.
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AbstractUsing the appropriate kinetic equation, we considered the problem of propagation of accelerated electrons into the solar corona and chromosphere. Its analytical solution was used for modelling the M7.7 class limb flare occurred on July 19, 2012. Coronal above-the-loop-top hard X-Ray source was interpreted in the thin-target approximation, the foot-point source - in the thick-target approximation with account of the reverse-current electric field. For the foot-point source we found a good accordance with the RHESSI observations. For the coronal source we also got very accurate estimate of the power-law spectral index, but significant differences between the modelled and observed hard X-ray intensities were noticed. The last discrepancy was solved by adding the coronal magnetic trap model to the thin target model. The former one implies that the trap collapses in two dimensions, locks and accelerates particles inside itself. In our report, we confirm an existence and high efficiency of the electron acceleration in collapsing magnetic traps during solar flares. Our new results represent (e.g. for RHESSI observations) the theoretical prediction of the double step particle acceleration in solar flares, when the first step is the acceleration in reconnection area and the second one – the acceleration in coronal trap.
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Mocanu, G., A. Marcu, I. Ballai, and B. Orza. "The problem of phase mixed shear Alfvén waves in the solar corona revisited." Astronomische Nachrichten 329, no. 8 (October 2008): 780–85. http://dx.doi.org/10.1002/asna.200811031.
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Демидов, Михаил, and Mikhail Demidov. "Possibilities and problems of solar magnetic field observations for space weather forecast." Solar-Terrestrial Physics 3, no. 1 (May 5, 2017): 26–39. http://dx.doi.org/10.12737/article_58f96ef99d4cd9.20657784.
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An essential part of the space weather problem, important in the last decades, is the forecast of near-Earth space parameters, ionospheric and geomagnetic conditions on the basis of observations of various phenomena on the Sun. Of particular importance are measurements of magnetic fields as they determine the spatial structure of outer layers of the solar atmosphere and, to a large extent, solar wind parameters. Due to lack of opportunities to observe magnetic fields directly in the corona, the almost only source of various models for quantitative calculation of heliospheric parameters are daily magnetograms measured in photospheric lines and synoptic maps derived from these magnetograms. It turns out that results of the forecast, in particular of the solar wind velocity in Earth’s orbit and the position of the heliospheric current sheet, greatly depend not only on the chosen calculation model, but also on the original material because magnetograms from different instruments (and often observations in different lines at the same), although being morphologically similar, may differ significantly in a detailed quantitative analysis. A considerable part of this paper focuses on a detailed analysis of this particular aspect of the problem of space weather forecast.
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Upendran, Vishal, Durgesh Tripathi, N. P. S. Mithun, Santosh Vadawale, and Anil Bhardwaj. "Nanoflare Heating of the Solar Corona Observed in X-Rays." Astrophysical Journal Letters 940, no. 2 (November 28, 2022): L38. http://dx.doi.org/10.3847/2041-8213/aca078.
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Abstract The existence of the million-degree corona above the cooler photosphere is an unsolved problem in astrophysics. Detailed study of the quiescent corona that exists regardless of the phase of the solar cycle may provide fruitful hints toward resolving this conundrum. However, the properties of heating mechanisms can be obtained only statistically in these regions due to their unresolved nature. Here, we develop a two-step inversion scheme based on the machine-learning scheme of Upendran & Tripathi (2021a) for the empirical impulsive heating model of Pauluhn & Solanki (2007), and apply it to disk integrated flux measurements of the quiet corona as measured by the X-ray solar monitor on board Chandrayaan-2. We use data in three energy passbands, viz, 1–1.3, 1.3–2.3, and 1–2.3 keV, and estimate the typical impulsive event frequencies, timescales, amplitudes, and the distribution of amplitudes. We find that the impulsive events occur at a frequency of ≈25 events per minute with a typical lifetime of ≈10 minutes. They are characterized by a power-law distribution with a slope α ≤ 2.0. The typical amplitudes of these events lie in an energy range of 1021–1024 erg, with a typical radiative loss of about ≈103 erg cm−2 s−1 in the energy range of 1–2.3 keV. These results provide further constraints on the properties of subpixel impulsive events in maintaining the quiet solar corona.
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Brchnelova, Michaela. "There is More Than Just One “Corona” That Can Kill Us All." Journal of the ASB Society 3, no. 1 (December 6, 2022): 7–13. http://dx.doi.org/10.51337/jasb20221206001.
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When most people hear the phrase "space exploration", they imagine preparing the colonisation of Mars, studying distant planets, searching for alien life or studying exotic black holes. Many times, however, mankind tends to leap before looking and focus on the exotic and the interesting instead of that which might not seem that special to most of us, but which which can be crucial for our survival. This way, many major and important issues in astrophysics - and sciences in general - can become overlooked and take very long to resolve. One such major issue in astrophysics is the behaviour of our closest star, the Sun. While we generally assume that we have a good idea about the most significant processes in the solar interior acting as its main energy source, our attempts at explaining the dynamics and structure of the solar atmosphere (the so-called corona) are still merely an educated guesswork at best. Due to several of the magnetohydrodynamic processes which we do not yet fully understand, the temperature of the solar atmosphere is very high compared to the layers underneath and reaches millions of Kelvins. Since this is where the major mass ejections occur, some of which might be directed towards the Earth causing geomagnetic storms, it is crucial that we start not only to fully understand these processes, but also to be able to accurately predict their outcomes. As will be shown in this text however, it is not only our limited understanding of the physics which prevents us from doing that; it is also the lack of computational resources. This paper firstly briefly discusses the basic physics behind the behaviour of the solar coronal plasma. Afterwards, it discusses the coronal heating problem in more detail and finally, it outlines the major challenges we currently face which seem to prevent us from efficient simulation and complete understanding of the behaviour of our closest star.
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Hollweg, Joseph V. "Coronal Heating: Theoretical Ideas." Highlights of Astronomy 8 (1989): 517–20. http://dx.doi.org/10.1017/s1539299600008200.
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After four decades of study, the mechanisms for heating the corona are not understood. However, the development of the field is vigorous. A variety of new ideas have been proposed, and these new ideas have generated lively debate. The goal of this short review is to present a broad overview of the ideas currently under consideration.It is now accepted that the solar magnetic field is the key ingredient. It is not known whether waves are important or not, but it is agreed that the magnetic (and possibly associated kinetic) energy must eventually appear in thin structures so that the (generally weak) dissipative processes can heat the plasma. This poses a problem of how the heat gets distributed throughout the corona.
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Perrone, Denise, Francesco Valentini, and Pierluigi Veltri. "Hybrid Vlasov simulations for alpha particles heating in the solar wind." Proceedings of the International Astronomical Union 6, S274 (September 2010): 168–71. http://dx.doi.org/10.1017/s1743921311006843.
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AbstractHeating and acceleration of heavy ions in the solar wind and corona represent a long-standing theoretical problem in space physics and are distinct experimental signatures of kinetic processes occurring in collisionless plasmas. To address this problem, we propose the use of a low-noise hybrid-Vlasov code in four dimensional phase space (1D in physical space and 3D in velocity space) configuration. We trigger a turbulent cascade injecting the energy at large wavelengths and analyze the role of kinetic effects along the development of the energy spectra. Following the evolution of both proton and α distribution functions shows that both the ion species significantly depart from the maxwellian equilibrium, with the appearance of beams of accelerated particles in the direction parallel to the background magnetic field.
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Hundhausen, J. R., and B. C. Low. "Magnetostatic structures of the solar corona. 1: A model based on the Cauchy boundary value problem." Astrophysical Journal 429 (July 1994): 876. http://dx.doi.org/10.1086/174372.
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Obergaulinger, Martin, and Manuel García-Muñoz. "Energetic particle acceleration and transport by Alfvén/acoustic waves in tokamak-like Solar flares." Proceedings of the International Astronomical Union 6, S274 (September 2010): 162–64. http://dx.doi.org/10.1017/s174392131100682x.
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AbstractAlfven/acoustic waves are ubiquitous in astrophysical as well as in laboratory plasmas. Their interplay with energetic ions is of crucial importance to understanding the energy and particle exchange in astrophysical plasmas as well as to obtaining a viable energy source in magnetically confined fusion devices. In magnetically confined fusion plasmas, an experimental phase-space characterisation of convective and diffusive energetic particle losses induced by Alfven/acoustic waves allows for a better understanding of the underlying physics. The relevance of these results in the problem of the anomalous heating of the solar corona is checked by MHD simulations of Tokamak-like Solar flare tubes.
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Podladchikova, O., B. Lefebvre, V. Krasnoselskikh, and V. Podladchikov. "Classification of probability densities on the basis of Pearson’s curves with application to coronal heating simulations." Nonlinear Processes in Geophysics 10, no. 4/5 (October 31, 2003): 323–33. http://dx.doi.org/10.5194/npg-10-323-2003.
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Abstract. An important task for the problem of coronal heating is to produce reliable evaluation of the statistical properties of energy release and eruptive events such as micro-and nanoflares in the solar corona. Different types of distributions for the peak flux, peak count rate measurements, pixel intensities, total energy flux or emission measures increases or waiting times have appeared in the literature. This raises the question of a precise evaluation and classification of such distributions. For this purpose, we use the method proposed by K. Pearson at the beginning of the last century, based on the relationship between the first 4 moments of the distribution. Pearson's technique encompasses and classifies a broad range of distributions, including some of those which have appeared in the literature about coronal heating. This technique is successfully applied to simulated data from the model of Krasnoselskikh et al. (2002). It allows to provide successful fits to the empirical distributions of the dissipated energy, and to classify them as a function of model parameters such as dissipation mechanisms and threshold.
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Демидов, Михаил, and Mikhail Demidov. "Possibilities and problems of Solar magnetic field observations for space weather forecast." Solnechno-Zemnaya Fizika 3, no. 1 (April 17, 2017): 22–33. http://dx.doi.org/10.12737/23279.
Full textAbstract:
An essential part of the space weather problem, important in the last decades, is the forecast of near-Earth space parameters, ionospheric and geomagnetic conditions on the basis of observations of various phenomena on the Sun. Of particular importance are measurements of magnetic fields as they determine the spatial structure of outer layers of the solar atmosphere and, to a large extent, solar wind parameters. Due to lack of opportunities to observe magnetic fields directly in the corona, the almost only source of various models for quantitative calculation of heliospheric parameters are daily magnetograms measured in photospheric lines and synoptic maps derived from these magnetograms. It turns out that results of the forecast, in particular of the solar wind velocity in Earth’s orbit and the position of the heliospheric current sheet, greatly depend not only on the chosen calculation model, but also on the original material because magnetograms from different instruments (and often observations in different lines at the same), although being morphologically similar, may differ significantly in a detailed quantitative analysis. A considerable part of this paper focuses on a detailed analysis of this particular aspect of the problem of space weather forecast.
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Terradas, Jaume. "The Interplay between Coronal Holes and Solar Active Regions from Magnetohydrostatic Models." Physics 5, no. 1 (February 28, 2023): 276–97. http://dx.doi.org/10.3390/physics5010021.
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Coronal holes (CHs) and active regions (ARs) are typical magnetic structures found in the solar corona. The interaction of these two structures was investigated mainly from the observational point of view, but a basic theoretical understanding of how they are connected is missing. To address this problem, in this paper, magnetohydrostatic models are constructed by numerically solving a Grad–Shafranov equation in two dimensions. A common functional form for the pressure and temperature in the CH and in the AR are assumed throught the study. Keeping the parameters of the CH constant and modifying the parameters of the nearby bipolar AR, one finds essentially three types of solutions depending on the magnitude and sign of the magnetic field at the closest foot of the AR to the CH. Two of the three solutions match well with the observation, but the third solution predicts the existence of closed magnetic field lines with quite low density and temperature with opposite characteristics to those in typical ARs. Simple analytical expressions are obtained for the pressure, temperature and density at the core of the AR and their dependence upon several major physical parameters are studied. The results obtained in this paper need to be contrasted with observations.
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Suzuki, Takeru K. "Evolution of stellar winds from the Sun to red giants." Proceedings of the International Astronomical Union 4, S257 (September 2008): 589–99. http://dx.doi.org/10.1017/s1743921309029901.
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AbstractBy performing global 1D MHD simulations, we investigate the heating and acceleration of solar and stellar winds in open magnetic field regions. Our simulation covers from photosphere to 20-60 stellar radii, and takes into account radiative cooling and thermal conduction. We do not adopt ad hoc heating function; heating is automatically calculated from the solutions of Riemann problem at the cell boundaries. In the solar wind case we impose transverse photospheric motions with velocity ~1 km/s and period between 20 seconds and 30 minutes, which generate outgoing Alfvén waves. We have found that the dissipation of Alfvén waves through compressive wave generation by decay instability is quite effective owing to the density stratification, which leads to the sufficient heating and acceleration of the coronal plasma. Next, we study the evolution of stellar winds from main sequence to red giant phases. When the stellar radius becomes ~10 times of the Sun, the steady hot corona with temperature 106K, suddenly disappears. Instead, many hot and warm (105– 106K) bubbles are formed in cool (T< 2 × 104K) chromospheric winds because of the thermal instability of the radiative cooling function; the red giant wind is not a steady stream but structured outflow.
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Zhang, Jun, Yijun Hou, Yue Fang, Feng Chen, Ting Li, Xiaoli Yan, Tao Ding, Zhiping Song, Yongyuan Xiang, and Zhong Liu. "Propagating and Stationary Bright Knots in the Quiet Sun." Astrophysical Journal Letters 942, no. 1 (December 27, 2022): L2. http://dx.doi.org/10.3847/2041-8213/aca97b.
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Abstract The question of what heats the solar chromosphere and corona remains one of the most important puzzles in solar physics and astrophysics. Up to now, two mechanisms are considered to work in heating the chromosphere and corona: magnetic reconnection and wave (turbulent flow) dissipation. But it is still not understood which mechanism is dominant. To solve the heating problem, one important topic at this stage is that we should understand how much energy is contributing from the two mechanisms respectively to the heating. In the quiet Sun, the thermal energy signal is observed as brightenings. Here we report two kinds of bright knots with a total of 3605 in the chromosphere of the quiet Sun, using the data from the New Vacuum Solar Telescope at Yunnan Observatories. The first kind of 1537 bright knots, which is first detected in chromospheric fibrils where waves and their dissipation are ubiquitous, propagates along these fibrils with velocities from 5 to 69 km s−1. The second kind of 2068 knots keeps stationary, and always appears at the footpoints of these fibrils where network magnetic fields exist, suggesting that magnetic reconnection locally produces these stationary knots. Based on the observations of thousands of bright knots, we display the different distribution patterns of the two kinds of bright knots in the quiet Sun, and deduce that half of the energy for heating the chromosphere is supplied by wave dissipation, and the other half by magnetic reconnection.
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Vilangot Nhalil, Nived, Chris J. Nelson, Mihalis Mathioudakis, J. Gerry Doyle, and Gavin Ramsay. "Power-law energy distributions of small-scale impulsive events on the active Sun: results from IRIS." Monthly Notices of the Royal Astronomical Society 499, no. 1 (September 22, 2020): 1385–94. http://dx.doi.org/10.1093/mnras/staa2897.
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ABSTRACT Numerous studies have analysed inferred power-law distributions between frequency and energy of impulsive events in the outer solar atmosphere in an attempt to understand the predominant energy supply mechanism in the corona. Here, we apply a burst detection algorithm to high-resolution imaging data obtained by the Interface Region Imaging Spectrograph to further investigate the derived power-law index, γ, of bright impulsive events in the transition region. Applying the algorithm with a constant minimum event lifetime (of either 60 s or 110 s) indicated that the target under investigation, such as Plage and Sunspot, has an influence on the observed power-law index. For regions dominated by sunspots, we always find γ < 2; however, for data sets where the target is a plage region, we often find that γ > 2 in the energy range (∼1023, ∼1026) erg. Applying the algorithm with a minimum event lifetime of three time-steps indicated that cadence was another important factor, with the highest cadence data sets returning γ > 2 values. The estimated total radiative power obtained for the observed energy distributions is typically 10–25 per cent of what would be required to sustain the corona indicating that impulsive events in this energy range are not sufficient to solve coronal heating. If we were to extend the power-law distribution down to an energy of 1021 erg, and assume parity between radiative energy release and the deposition of thermal energy, then such bursts could provide 25–50 per cent of the required energy to account for the coronal heating problem.
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Deliu, Ionica. "COVID 19 - THE CHALLENGE OF CORONAVIRUSES." Current Trends in Natural Sciences 10, no. 19 (July 31, 2021): 416–21. http://dx.doi.org/10.47068/ctns.2021.v10i19.055.
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About corona viruses many discusses were occur in the last two years because of Covid 19, the pandemic disease of our days. The Family of coronaviruses includes the positive sense single-stranded RNA viruses with helical symmetry of enveloped nucleocapsid, which determine respiratory or intestinal infection in humans and animals and causing disorders of different organs. The name of viral genus derives from their surface with club-shaped spikes like solar corona. SARS-CoV-2 causes the COVID-19 disease and spread all over the world. The scientific communities analyse this virus and investigate the effects about the human organisms. The threat of the coronavirus become seriously enough in 11 March 2020, when the World Health Organization (WHO) declared a pandemic. No doubt the world was not prepared for this important problem of public health. Till May 2021, 1055265 cases of infection were registered in Romania. In Arges County the number of infection was 26476 in the same period.
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Afanasyev, An N., A. M. Uralov, and V. V. Grechnev. "Using the nonlinear geometrical acoustics method in the problem of moreton and EUV wave propagation in the solar corona." Geomagnetism and Aeronomy 51, no. 8 (December 2011): 1015–23. http://dx.doi.org/10.1134/s0016793211080159.
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Němeček, Zdeněk, Jana Šafránková, František Němec, Tereza Ďurovcová, Alexander Pitňa, Benjamin L. Alterman, Yuriy M. Voitenko, Jiří Pavlů, and Michael L. Stevens. "Spectra of Temperature Fluctuations in the Solar Wind." Atmosphere 12, no. 10 (September 30, 2021): 1277. http://dx.doi.org/10.3390/atmos12101277.
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Turbulent cascade transferring the free energy contained within the large scale fluctuations of the magnetic field, velocity and density into the smaller ones is probably one of the most important mechanisms responsible for heating of the solar corona and solar wind, thus the turbulent behavior of these quantities is intensively studied. The temperature is also highly fluctuating quantity but its variations are studied only rarely. There are probably two reasons, first the temperature is tensor and, second, an experimental determination of temperature variations requires knowledge of the full velocity distribution with an appropriate time resolution but such measurements are scarce. To overcome this problem, the Bright Monitor of the Solar Wind (BMSW) on board Spektr-R used the Maxwellian approximation and provided the thermal velocity with a 32 ms resolution, investigating factors influencing the temperature power spectral density shape. We discuss the question whether the temperature spectra determined from Faraday cups are real or apparent and analyze mutual relations of power spectral densities of parameters like the density, parallel and perpendicular components of the velocity and magnetic field fluctuations. Finally, we compare their spectral slopes with the slopes of the thermal velocity in both inertial and kinetic ranges and their evolution in course of solar wind expansion.
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Niedziela, R., K. Murawski, and S. Poedts. "Chromospheric heating and generation of plasma outflows by impulsively generated two-fluid magnetoacoustic waves." Astronomy & Astrophysics 652 (August 2021): A124. http://dx.doi.org/10.1051/0004-6361/202141027.
Full textAbstract:
Context. The origin of the heating of the solar atmosphere is still an unsolved problem. As the photosphere and chromosphere radiate more energy than the solar corona, it is challenging but important to reveal all the mechanisms that contribute to plasma heating there. Ion–neutral collisions could play an important role. Aims. We aim to investigate the impulsively generated two-fluid magnetoacoustic waves in the partially ionized solar chromosphere and to study the associated heating and plasma outflows, which higher up may result in nascent solar wind. Methods. To describe the plasma dynamics, we applied a two-fluid model in which ions+electrons and neutrals are treated as separate fluids. We solved the two-fluid equations numerically using the JOANNA code. Results. We show that magnetoacoustic waves triggered in the photosphere by localised velocity pulses can steepen into shocks which heat the chromosphere through ion–neutral collisions. Pulses of greater amplitude heat plasma more effectively and generate larger plasma outflows. Rising the altitude at which the pulse is launched results in opposite effects, mainly in local cooling of the chromosphere and slower plasma outflows. Conclusions. Even a solitary pulse results in a train of waves. These waves can transform into shock waves and release thermal energy, heating the chromosphere significantly. A pulse can drive vertical flows which higher up can result in the origin of the solar wind.