Academic literature on the topic 'Coronal heating at small scales'

Abstract. As the dissipation mechanisms considered for the heating of the solar corona would be sufficiently efficient only in the presence of small scales, turbulence is thought to be a key player in the coronal heating processes: it allows indeed to transfer energy from the large scales to these small scales. While Direct numerical simulations which have been performed to investigate the properties of magnetohydrodynamic turbulence in the corona have provided interesting results, they are limited to small Reynolds numbers. We present here a model of coronal loop turbulence involving shell-models and Alfvén waves propagation, allowing the much faster computation of spectra and turbulence statistics at higher Reynolds numbers. We also present first results of the forward-modelling of spectroscopic observables in the UV.

Abstract Coronal heating is a longstanding issue in solar physics as well as plasma physics in general. In recent years, significant resolution improvements of satellite observations have contributed to a deeper understanding of small-scale physics, e.g., magnetic reconnection processes on fine scales inside the turbulent geo-magnetosheath. Coronal plasmas feature turbulent complexity of flows and magnetic fields with similar fine scales, and thus electron magnetic reconnection is very likely to be excited in the coronal region working as one of the ways to heat the solar corona, which offers a possible new mechanism for the nanoflare model proposed by Parker. We in this paper simulate and analyze the magnetic reconnection processes on a fine scale of the electron skin depth, with a particle-in-cell treatment, and estimate its contribution to coronal heating. The result shows that the electron magnetic reconnection can provide substantial heating efficiency for heating the corona to its observed temperature, once the reconnection events are reasonably spread.

Context. The relative importance of alternating current (AC) and direct current (DC) heating mechanisms in maintaining the temperature of the solar corona is not well constrained. Aims. We aim to investigate the effects of the characteristic time scales of photospheric driving on the injection and dissipation of magnetic and kinetic energy within a coronal arcade. Methods. We conducted three-dimensional magnetohydrodynamic simulations of complex foot point driving imposed on a potential coronal arcade. We modified the typical time scales associated with the velocity driver to understand the efficiency of heating obtained using AC and DC drivers. We considered the implications for the injected Poynting flux and the spatial and temporal nature of the energy release in dissipative regimes. Results. For the same driver amplitude and complexity, long time scale velocity motions are able to inject a much greater Poynting flux of energy into the corona. Consequently, in non-ideal regimes, slow stressing motions result in a greater increase in plasma temperature than for wave-like driving. In dissipative simulations, Ohmic heating is found to be much more significant than viscous heating. For all drivers in our parameter space, energy dissipation is greatest close to the base of the arcade, where the magnetic field strength is strongest, and at separatrix surfaces, where the field connectivity changes. Across all simulations, the background field is stressed with random foot point motions (in a manner more typical of DC heating studies), and, even for short time scale driving, the injected Poynting flux is large given the small amplitude flows considered. For long time scale driving, the rate of energy injection was comparable to the expected requirements in active regions. The heating rates were found to scale with the perturbed magnetic field strength and not the total field strength. Conclusions. Alongside recent studies that show that power within the corona is dominated by low frequency motions, our results suggest that, in the closed corona, DC heating is more significant than AC heating.

Coronal heating is at the origin of the EUV and X-ray emission and mass loss from the sun and many other stars. While different scenarios have been proposed to explain the heating of magnetically confined and open regions of the corona, they must all rely on the transfer, storage and dissipation of the abundant energy present in photospheric motions, which, coupled to magnetic fields, give rise to the complex phenomenology seen at the chromosphere and transition region (i.e. spicules, jets, ‘tornadoes’). Here we discuss models and numerical simulations which rely on magnetic fields and electric currents both for energy transfer and for storage in the corona. We will revisit the sources and frequency spectrum of kinetic and electromagnetic energies, the role of boundary conditions, and the routes to small scales required for effective dissipation. Because reconnection in current sheets has been, and still is, one of the most important processes for coronal heating, we will also discuss recent aspects concerning the triggering of reconnection instabilities and the transition to fast reconnection.

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.

Abstract The periodic coronal rain and in-phase radiative intensity pulsations have been observed in multiple wavelengths in recent years. However, due to the lack of three-dimensional coronal magnetic fields and thermodynamic data in observations, it remains challenging to quantify the coronal heating rate that drives the mass cycles. In this work, based on the MURaM code, we conduct a three-dimensional radiative magnetohydrodynamic simulation spanning from the convective zone to the corona, where the solar atmosphere is heated self-consistently through dissipation resulting from magnetoconvection. For the first time, we model the periodic coronal rain in an active region. With a high spatial resolution, the simulation well resembles the observational features across different extreme-ultraviolet wavelengths. These include the realistic interweaving coronal loops, periodic coronal rain, and periodic intensity pulsations, with two periods of 3.0 hr and 3.7 hr identified within one loop system. Moreover, the simulation allows for a detailed three-dimensional depiction of coronal rain on small scales, revealing adjacent shower-like rain clumps ∼500 km in width and showcasing their multithermal internal structures. We further reveal that these periodic variations essentially reflect the cyclic energy evolution of the coronal loop under thermal nonequilibrium state. Importantly, as the driver of the mass circulation, the self-consistent coronal heating rate is considerably complex in time and space, with hour-level variations in 1 order of magnitude, minute-level bursts, and varying asymmetry reaching ten times between footpoints. This provides an instructive template for the ad hoc heating function and further enhances our understanding of the coronal heating process.

Abstract In this paper, we propose that flux cancellation on small granular scales (≲1000 km) ubiquitously drives reconnection at a multitude of sites in the low solar atmosphere, contributing to chromospheric/coronal heating and the generation of the solar wind. We analyze the energy conversion in these small-scale flux cancellation events using both analytical models and three-dimensional, resistive magnetohydrodynamic (MHD) simulations. The analytical models—in combination with the latest estimates of flux cancellation rates—allow us to estimate the energy release rates due to cancellation events, which are found to be on the order 106–107 erg cm−2 s−1, sufficient to heat the chromosphere and corona of the quiet Sun and active regions, and to power the solar wind. The MHD simulations confirm the conversion of energy in reconnecting current sheets, in a geometry representing a small-scale bipole being advected toward an intergranular lane. A ribbon-like jet of heated plasma that is accelerated upward could also escape the Sun as the solar wind in an open-field configuration. We conclude that through two phases of atmospheric energy release—precancellation and cancellation—the cancellation of photospheric magnetic flux fragments and the associated magnetic reconnection may provide a substantial energy and mass flux contribution to coronal heating and solar wind generation.

AbstractMagnetohydrodynamic turbulence has been proposed as a mechanism for the heating of coronal active regions, and has therefore been actively investigated in recent years. According to this scenario, a turbulent regime is driven by footpoint motions. The energy being pumped this way into active region loops, is efficiently transferred to small scales due to a direct energy cascade. The ensuing generation of fine scale structures, which is a natural outcome of turbulent regimes, helps to enhance the dissipation of either waves or DC currents.We present an updated overview of recent results on turbulent coronal heating. To illustrate this theoretical scenario, we simulate the internal dynamics of a coronal loop within the reduced MHD approximation. The application of a stationary velocity field at the photospheric boundary leads to a turbulent stationary regime after several photospheric turnover times. This regime is characterized by a broadband power spectrum and energy dissipation rate levels compatible with the heating requirements of active region loops. Also, the energy dissipation rate displays a complex superposition of impulsive events, which we associate to the so-called nanoflares. A statistical analysis yields a power law distribution as a function of their energies, which is consistent with those obtained from observations. We also study the distributions of peak dissipation rate and duration of these events.

Aims. We investigate the formation of small scales and the related dissipation of magnetohydronamic (MHD) wave energy through non-linear interactions of counter-propagating, phase-mixed Alfvénic waves in a complex magnetic field. Methods. We conducted fully three-dimensional, non-ideal MHD simulations of transverse waves in complex magnetic field configurations. Continuous wave drivers were imposed on the foot points of magnetic field lines and the system was evolved for several Alfvén travel times. Phase-mixed waves were allowed to reflect off the upper boundary and the interactions between the resultant counter-streaming wave packets were analysed. Results. The complex nature of the background magnetic field encourages the development of phase mixing throughout the numerical domain, leading to a growth in alternating currents and vorticities. Counter-propagating phase-mixed MHD wave modes induce a cascade of energy to small scales and result in more efficient wave energy dissipation. This effect is enhanced in simulations with more complex background fields. High-frequency drivers excite localised field line resonances and produce efficient wave heating. However, this relies on the formation of large amplitude oscillations on resonant field lines. Drivers with smaller frequencies than the fundamental frequencies of field lines are not able to excite resonances and thus do not inject sufficient Poynting flux to power coronal heating. Even in the case of high-frequency oscillations, the rate of dissipation is likely too slow to balance coronal energy losses, even within the quiet Sun. Conclusions. For the case of the generalised phase-mixing presented here, complex background field structures enhance the rate of wave energy dissipation. However, it remains difficult for realistic wave drivers to inject sufficient Poynting flux to heat the corona. Indeed, significant heating only occurs in cases which exhibit oscillation amplitudes that are much larger than those currently observed in the solar atmosphere.

This presentation focuses upon the coronal heating problem and reports the results of Ionson's (1984) unified theory of electrodynamic heating. This generalized theory, which is based upon Ionson's (1982) LRC approach, unveils a variety of new heating mechanisms and links together previously proposed processes. Specifically, Ionson (1984) has derived a standing wave equation for the global current, I, driven by emfs that are generated by the β≳1 convection. This global electrodynamics equation has the same form as a driven LRC equation where the equivalent inductance, L=4ℓ/πc2, scales with the coronal loop length and where the equivalent capacitance, C=c2 ℓ/4πv2A, is essentially the product of the free space capacitance, ℓ/4π, and the low frequency dielectric constant, c2/v2A. The driving emf, ∊=vBa/c, is a formal integration constant associated with the convective stressing of β≳1 magnetic fields. Since the transition from the β≳1 driver to the β<1 coronal loop is typically small compared to the “wavelength” of the associated magnetic fluctuation, this integration constant is not sensitive to details of the transition zone. The total resistance, Rtot = L(1/tdiss+1/tphase+1/tleak), represents electrodynamic energy “loss” from dissipation, magnetic stress leakage out of the loop and phase-mixing. These three processes have been parameterized by appropriate timescales. Note that Rleak=L/tleak and Rphase=L/tphase do not result in resistive heating but do participate in limiting the amplitude of the global current, I. This is fairly obvious with regard to magnetic stress leakage but not for phase-mixing. The phase-mixing resistance, Rphase, represents coupling between the global current and the local current density. Since the global current is essentially an integration of the local currents, the degree of coherency between the local currents can play an important role in determining the ultimate amplitude of I. The rate at which coherency between the local currents is lost is given by the phase-mixing time, tphase. A loss of coherency implies a corresponding reduction in the amplitude of I. In this sense, Rphase measures the phase-mixing contribution to the global current limitation process.

La couronne solaire est chauffée à plus de 1 MK. L'une des principales théories sur la formation de la couronne (Parker, 1988) suggère que l'énergie magnétique est dissipée dans la couronne par un grand nombre d'événements de chauffage impulsifs et peu énergétiques (1E24 ergs), appelés « nanoflares ». Le 30 mai 2020, lors de sa première séquence d'observation à haute résolution spatiale et temporelle, 1463 petits « événements » EUV de petite taille (400 - 4000 km) et de court temps de vie (10-200 s) ont été détectés dans le Soleil calme (QS) par l'imageur UV à haute résolution HRIEUV (174 Angström), à bord de Solar Orbiter. J'ai étudié si ces événements sont la signature du chauffage par nanoflares.Comme HRIEUV est sensible à une gamme de température continue, en particulier entre 1 MK et 0.3 MK, mon objectif était de vérifier si ces événements atteignent des températures coronales et, par conséquent, s'ils contribuent directement au chauffage coronal.Le 30 mai 2020, seules les données de SDO/AIA permettaient d'effectuer un diagnostic de température. C'est dans ce but que j'ai appliqué la méthode des « décalages temporels » aux canaux EUV de AIA. Ces décalages sont des signatures de chauffage ou de refroidissement pour des températures supérieures à 1 MK, au-delà de laquelle cinq des six canaux d'AIA ont leur pic de réponse. La comparaison des résultats entre les événements et le reste du QS a permis de conclure que les événements ont, pour la plupart, des décalages temporels inférieurs à la cadence d'AIA de 12 s. Des séquences d'observation ultérieures ont confirmé ces résultats avec une cadence d'AIA doublée. J'en ai déduit deux interprétations possibles : (1) les événements n'atteignent pas 1 MK, températures pour lesquelles les fonctions de réponse d'AIA se comportent de façon similaire ; (2) les temps de refroidissement sont trop courts pour que les décalages temporels soient résolus par AIA. Afin de mieux contraindre leur température, j'ai eu recours à la spectroscopie.J'ai donc analysé des observations coordonnées entre HRIEUV, AIA (imagerie), Solar Orbiter/SPICE et Hinode/EIS (spectroscopie) sur le QS, en 2022 et 2023. Tout d'abord, les événements sont détectés dans HRIEUV, puis identifiés dans AIA, ainsi que SPICE ou EIS. A partir des raies spectrales, j'ai construit des courbes de lumière et estimé la distribution de la densité en fonction de la température. J'ai conclu que l'émission de ces événements provient principalement de plasma froid (< 1 MK). Ainsi, la majorité d'entre eux ne contribuent pas directement au chauffage coronal.Afin de comprendre l'origine physique de ces événements, j'ai reproduit leurs signatures observationnelles avec le code d'évolution hydrodynamique 1D HYDRAD. Pour ce faire, j'ai calculé les courbes de lumière synthétiques de petites boucles soumises à un chauffage impulsif, en changeant les paramètres du modèle, tels que la longueur de la boucle ou l'amplitude du chauffage. J'ai cherché les paramètres qui reproduisent le mieux les observations, y compris le pic co-temporel des courbes de lumière. J'ai comparé les résultats pour deux types de boucles qui ont des propriétés très différentes : les boucles « chaudes » (T > 1E5 K) et les boucles « froides » (T < 1E5 K). Les résultats montrent que les boucles froides soumises à un chauffage impulsif sont de bons candidats pour expliquer l'origine des événements détectés par HRIEUV.En conclusion, ces événements ne sont probablement pas, pour la majeure partie d'entre eux, une signature du chauffage coronal, à moins que leur émission coronale ne soit inférieure aux limites instrumentales. Une des conséquences de ce travail est de réévaluer le rôle des petits événements EUV dans le chauffage coronal du QS, car ils pourraient jouer un rôle important dans le chauffage de la partie plus basse et plus froide de l'atmosphère solaire
The Solar corona temperature is maintained at more than 1 MK. One of the main theories of the coronal formation (Parker, 1988) suggests that the magnetic energy is dissipated into the corona through a high number of impulsive, low energetic (1E24 ergs) heating events, called “nanoflares.” On 30 May 2020, during its first high temporal and spatial resolutions observations, 1463 small (400 - 4000 km) and short-lived (10-200 s) EUV brightenings, referred to as “events”, were detected in the Quiet Sun (QS) by the high-resolution UV imager HRIEUV (174 Angström), on board Solar Orbiter. I tested the possibility that they might be signatures of nanoflare heating.As HRIEUV is sensitive to continuous temperature coverage, in particular between 1 MK and 0.3 MK, my goal was to verify if these events do reach coronal temperatures and, thus, if they contribute directly to the coronal heating.For the 30 May 2020 dataset, only SDO/AIA data were available to perform temperature diagnostics. To do so, I applied the “time lags” method to the coronal channels of AIA. This method provides signatures on plasma cooling or heating above 1 MK, as most AIA channels have their sensitivity peak at these temperatures. I compared the statistics between the events and the rest of the QS and concluded that the events are characterized by short time lags below the AIA cadence of 12 s. These results were confirmed by extending the study to later datasets using a higher AIA cadence of 6s. I proposed two possible interpretations: (1) the events peak below 1 MK, where the AIA response functions behave similarly; (2) the events' cooling time scale is too short to be resolved by the AIA cadence. Spectroscopic observations are thus necessary to better constrain the temperature of these events.To complete this work, I used co-temporal 2022 and 2023 QS data from HRIEUV, AIA (imagers), from Solar Orbiter/SPICE and HINODE/EIS (spectroscopy). I first detected events in HRIEUV and identified them in SPICE or EIS and in AIA. Then, I extracted the light curves from spectral lines emitted in a wide range of temperatures and applied spectroscopic diagnostics to derive the density as a function of temperature. I concluded that the emission of these events mainly originates from plasma below 1 MK. As such, most of them hardly contribute directly to the coronal heating.In order to understand the physical properties driving these events, I reproduced their observational signatures using the HYDRAD 1D hydrodynamics code. To do so, I computed the synthetic light curves from different models of short loops submitted to impulsive heating by changing parameters such as the loop length or the heating strength. I looked for the models that best reproduce the observations, including the light curves co-temporal peak. The work compares the results for two different types of loops that have very distinct properties: “hot” (T > 1E5 K) and “cool” (T < 1E5 K) loops. The results showed that cool loops submitted to impulsive heating are good candidates to explain the origin of most of the events detected by HRIEUV.To conclude, most of these events are probably not the signature of coronal heating phenomena, unless their coronal emission is below the instrumental limitations. One consequence of this work would be to reconsider their role in heating the QS corona, as they might instead provide a major contribution to the heating of the cooler lower solar atmosphere

Les grands événements de la couronne solaire (comme les flares avec une énergie de l'ordre de 10²³ J) ne suffisent pas à maintenir cette dernière aux températures de plus de un million de degrés qui y sont mesurées. La couronne doit alors être chauffée aux petites échelles, soit de façon continue, soit de façon intermittente. C'est pourquoi afin d'expliquer la température élevée de la couronne, beaucoup d'attention a été accordée aux distributions des énergies dissipées dans les plus petits événements (de l'ordre du mégamètre). En effet, si la distribution en énergie est assez pentue, les plus petits événements, qui sont inobservables, pourraient être les plus gros contributeurs à l'énergie totale dissipée dans la couronne. Des observations précédentes ont montré une large distribution en énergie mais ne permettent pas de conclure sur la valeur précise de la pente, et ces résultats s'appuient sur une estimation peu précise de l'énergie. D'autre part, des études spectroscopiques plus détaillées de structures comme les points brillants coronaux ne fournissent pas assez d'informations statistiques pour calculer leur contribution totale au chauffage. Nous voulons obtenir une meilleure estimation des distributions en énergies dissipées dans les événements de chauffage coronaux en utilisant des données de haute résolution dans plusieurs bandes de l'Extrême Ultra-Violet (EUV).Pour estimer les énergies correspondant aux événements de chauffage et déduire leur contribution, nous détectons des embrillancements dans cinq bandes EUV de l'instrument Atmospheric Imaging Assembly (AIA) à bord du satellite Solar Dynamics Observatory (SDO). Nous combinons les résultats de ces détections et nous utilisons des cartes de température et de mesure d'émission calculées à partir des mêmes observations pour calculer les énergies. Nous obtenons des distributions des surfaces, des durées de vie, des intensités et des énergies (thermique, radiative et de conduction) des événements. Ces distributions sont des lois de puissance, dont les paramètres indiquent que la population d'événements que nous avons observé n'est pas suffisante pour expliquer entièrement les températures coronales. Cependant, plusieurs processus physiques et biais observationnels peuvent être avancés pour expliquer l'énergie manquante
To explain the high temperature of the corona, much attention has been paid to the distribution of energy in dissipation events. Indeed, if the event energy distribution is steep enough, the smallest, unobservable events could be the largest contributors to the total energy dissipation in the corona. Previous observations have shown a wide distribution of energies but remain inconclusive about the precise slope. Furthermore, these results rely on a very crude estimate of the energy. On the other hand, more detailed spectroscopic studies of structures such as coronal bright points do not provide enough statistical information to derive their total contribution to heating. We aim at getting a better estimate of the distributions of the energy dissipated in coronal heating events using high-resolution, multi-channel Extreme Ultra-Violet (EUV) data. To estimate the energies corresponding to heating events and deduce their distribution, we detect brightenings in five EUV channels of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). We combine the results of these detections and we use maps of temperature and emission measure derived from the same observations to compute the energies. We obtain distributions of areas, durations, intensities, and energies (thermal, radiative, and conductive) of events. These distributions are power-laws, but their parameters indicate that a population of events like the ones we observe is not sufficient to fully explain coronal temperatures. However, several processes or observational biases can be advanced to explain the missing energy

Wildfires propagate across vegetated canopies exhibiting complex spread patterns. Wind gusts at the fire-front extend/intensify flames and direct convective heating towards unburnt fuels resulting in rapid acceleration of spread. This behavior could be modulated by ambient atmospheric boundary layer wind turbulence. Our aim is to characterize ambient turbulence over gorse shrub experimental burns and explore how this contributes to fire behavior. Developing coupled fire-atmosphere numerical models capable of resolving most turbulent energy scales is important for understanding rapid and small-scale dynamics. However, it is equally as important to design fire-burn experiments that allow for simultaneous measurements of fire behavior and atmospheric turbulence covering a range of the turbulent spectra. We have completed six experimental burns (24-hectares) in Rakaia, New Zealand under varying wind speed and direction and atmospheric stability regimes. The ignition process ensured a fire-line propagating through dense gorse bush (1m high). Two 30m high sonic anemometer towers measured turbulent wind velocity (20Hz) at six different height levels. Visible imagery was captured for all burns by cameras mounted on Un-crewed Aerial Vehicles (UAV) at 200m AGL. Using wavelet decomposition, we identified different turbulent scales that varied relative to height above vegetation and boundary layer thermal regimes. Quadrant analysis identified statistical distribution of atmospheric sweeps (downbursts of turbulence towards vegetation) and ejections (detachment of turbulence from vegetation). Discrete analysis of sweep/ejection events revealed their temporal and spatial scales and tracked their progression as the flaming zone approached the towers. Undergoing work aims to discern these interactions with observed fire sweeps from aerial imagery by applying image velocimetry techniques and sweep structure tracking

Abstract It is paradoxical that low-energy molecular processes might be important in the hostile, energetic environments near active galactic nuclei (AGNs). It now appears that observations of the molecular component of the central gas offer crucial insight into the physics of the AGN phenomenon. As reviewed in the preceding chapter, activity in galactic centres is displayed by nuclear starbursts on scales of 102 to 103 pc as well as the true AGNs, which are the compact, luminous sources that reside in the central 10 pc of quasi-stellar objects (QSOs), radio galaxies, and Seyfert galaxies. In this chapter we are concerned with the possible ways in which molecules can be used to investigate the inner workings of the true AGN. Observations throughout the electromagnetic spectnim now suggest that many AGN are buried inside small-scale discs or tori of gas and dust. These central gas systems reprocess the non-thermal continuum into complex emission-line spectra and shield the central source from view over much of the electromagnetic spectrum. Chapter 20 contains a brief introduction to a unification hypothesis, which has been proposed in order to explain the observations of Seyfert 1 and Seyfert 2 galaxies with a single model in which the spectroscopic differences reflect different orientations of the observer’s line of sight to the central obscuring disc or torus (see Antonucci 1993 for a review). There is some hope that the descriptions of radio galaxies, BL Lacertae objects, radio-quiet QSOs, and radio-loud QSOs (the true quasars) can be unified similarly. A compact system of gas and dust is central to the unification hypothesis. Theory suggests that such a system will exist as small dense clouds (Krolik and Begelman 1986, 1988), which will be partly molecular if the pressure is sufficiently high (Krolik and Lepp 1989; Maloney, Hollenbach and Tielens 1996). Photoionization of gas by a non-thermal X-ray source allows atoms in a wide range of states of ionization to co-exist with molecules. Molecules can respond to heating and ionization by X-rays with unusual abundances (e.g. Lepp and Dalgarno 1996) and distinctive excitation patterns (Draine and Woods 1990; Gredel and Dalgarno 1995; Tine et al. 1997).

Radiative heating calculations in the atmosphere involve four distinguishable scales of frequency. First, there is the comparatively slow variation with frequency of the Planck function and its derivative with respect to temperature. About one-half of the radiation from a black body at terrestrial temperatures lies in a wave number range of 500 cm-1. The second scale is that of the unresolved contour of a band. For atmospheric molecules other than water vapor, the Planck function is effectively constant over a single band; water vapor bands must be divided into sections of the order of 50 cm-1 wide before this is so. For a rotating molecule, the next relevant scale of frequency is that of the spacing between rotation lines, approximately 1-5 cm-1. Finally, there is the monochromatic scale on which the absorption coefficient may be treated as a constant, and for which Lambert’s absorption law is obeyed: of the order of one-fifth of a line width ≃ 2 x 10-2 cm-1 for a gas at atmospheric pressure, down to 2 x 10-4cm-1 for a Doppler line in the middle atmosphere. This step takes us to a division of the frequency scale that, when taken together with other features of the calculation, presents a formidable computation task. Calculations can, of course, be made and are made at this limiting spectral resolution (line-by-line calculations) but, despite the fact that they are technically feasible with modern computers, such calculations are rare and are usually performed to provide a few reference cases. The great majority of investigations make use of averages over many lines, embracing spectral ranges that are small compared to a band contour (narrow-band models), or over complete bands (wide-band models), or over the entire thermal spectrum (emissivity models.) There are a number of reasons for working with spectral averages. Practical considerations are that important classes of laboratory measurements, and most atmospheric observations (e.g., satellite radiometry) are made with some spectral averaging, often comparable to that of narrow-band models.

Magnetic nanoparticles are currently under intense investigation as a heating strategy for hyperthermia cancer treatment because they are promising as a means to target the heating specifically to the tumor.[1–4] Currently, our ability to create practical and useful numerical models in dimensional spaces similar to ordinary small tumors is severely hampered by the multiple orders of magnitude of the relative scales: nm to mm. Consequently, the preponderance of literature on the topic describes experimental studies only. Detailed individual nanoparticle model spaces with moderate dimensions up to mm would be nearly intractably computationally intensive, requiring peta-scale computing resources. It would advance the state of the art to be able to apply practical computing machinery to analyze realistic medium scale in vivo systems. Such a tool could be reasonably applied in experimental analysis, and potentially in treatment planning and assessment.

Experiments have been performed to study the combustion criteria of aluminum particles at atmospheric pressure. The primary goal is to quantify the outcome for a particle into which thermal energy has been deposited. Experiments utilized instantaneous joule heating of an aluminum wire. Once the particle was generated, it fell under gravity and the flight was recorded by video; in some cases, the ignited particle quenched or fragmented, and the residue was collected for SEM and EDS imaging. This provided information related to the aluminum oxide shell which was formed when combustion occurred. These experiments produced particles of approximately 150450 microns in the arc heating tests. In a second set of experiments, particles were produced under more observable time scales. This provided observation of the oxide skin, which is known to influence the ignition process. This experiment utilized a pressure pulse to eject a small droplet of molten aluminum through a small orifice. From this experiment, particle sizes ranging 2–3 mm were produced.

The ability to eliminate freezing damage using “vitrification” (or the formation of glass) has long been an area of intense interest in cryobiology. Typically vitrification is achieved when biological systems are cooled at rates ranging from ∼8,000 °C/min to ∼10,000 °C/min [1–5]. Using traditional cooling methods (immersion in liquid nitrogen), such high cooling rates are currently not achievable, in large tissue sections (∼cm’s). In the present study we investigate a novel method to achieve high cooling rates in large tissue sections by pulsed laser heating in conjunction with cryogenic temperatures, i.e. high cooling rates are achieved by the localized difference in temperature between the laser heated tissue (∼1000’s of °C) and the surrounding liquid nitrogen (∼−160 °C). Additionally, the use of pulsed lasers allows localized heating of the tissue coupled with small time scales of energy deposition (0.1 to 1 pico seconds) such that the heating/thermal damage in tissues is minimized. To amplify this idea further, we developed a numerical model to predict the temperature transients in tissues exposed to laser heating and cryogenic temperatures. Analysis of our numerical simulations suggest that a perturbation of ∼3500 °C in a 5mm thick tissue leads to cooling rates in excess of ∼8000 °C/min throughout the tissue slice. These results indicate the possibility of vitrifying large tissue sections of cryobiological relevance using a combination of laser heating and liquid nitrogen cooling.

Free-standing electrically conductive nanotube and nano-bridge structures offer a simple, small-scale, low-power option for pressure and temperature sensing. To sense pressure, a constant voltage is applied across the bridge. At small scales, the heat transfer coefficient is pressure-dependent. The change in the heat transfer coefficients result in the circuit operating at higher temperatures, with different resistances, at low pressures. This in turn will lead to a change in the electrical resistivity of the system. If the system is held at constant voltage, this can be measured as a change in the current in such systems, representing a simple alternative to existing Pirani gauges. The current work simulates the Joule heating, conduction and convection heat transfer of a 5 micron long suspended single-wall carbon nanotube, incorporating temperature-sensitive material properties. The simulation allows prediction of the thermo-electrical response of the systems. The results agree with the trends observed in existing devices. Additional results look at the effects of system length, temperature, and contact resistances between the substrate and the device.

Hydronic solar systems with copper tube collectors or vacuum glass tube collectors are commonly used thermal-solar systems. These systems run well in residential buildings or in small scales for domestic water heating. These systems can be scaled up and applied to commercial buildings. The solar energy collected by these systems can be used to drive absorption chillers in summer and heat the feed water for boiler in winter. Properly designed and installed systems can save energy by utilizing free solar energy. However, operation and maintenance do have an important impact on whether these systems truly save energy. This paper presents a technical review of a large scale hydronic solar system installed in Carbondale, Illinois and compares this system with a photovoltaic system which was proposed to retrofit the hydronic solar system in energy conversion efficiency, system components, initial cost, operation and maintenance etc.

AlGaN/GaN based high electron mobility transistors (HEMTs) have been intensively used due to their high-efficiency power switching and large current handling capabilities. However, the high power densities and localized heating in these devices form small, high temperature regions called hotspots. Analysis of heat removal from hotspots and temperature control of the entire device is necessary for the reliable design of HEMT devices. For accurate analysis of heat transfer using thermal simulations in such devices with heat transfer occurring at different length scales, a roadmap is needed. For this purpose relative importance of different heat transfer modes in removing heat from devices with different substrate materials, operating at different power densities while different boundary conditions are analyzed using two and three-dimensional COMSOL Multiphysics simulations. Results give the relative importance of different parameters on the heat removal mechanism from devices and provide a roadmap for building simpler yet still accurate thermal models for AlGaN/GaN HEMTs and similar devices.

Slug flow is a commonly encountered flow regime in microchannels due to the influence of surface tension and vapor confinement at small length scales. Few experimental studies have considered diabatic vapor-liquid slug flow, owing to difficulties in generating a well-controlled and repeatable slug flow regime; generation of vapor by wall heating typically leads to large, stochastic variations in the vapor bubble characteristics. To facilitate the study of flow behavior and vapor-liquid interfaces under precisely controlled conditions, a diabatic, one-component, two-phase microchannel flow was generated by separately injecting HFE-7100 vapor and liquid into a T-junction. Injection at independently controllable liquid and vapor flow rates allows the creation of vapor-liquid slug flow patterns in a downstream borosilicate microchannel of circular cross-section with a 500 μm inside diameter. The outside surface of the microchannel was coated with a 100 nm-thick layer of indium tin oxide (ITO) to generate a uniform wall heat flux via Joule heating while allowing full optical access for flow visualization. The growth of individual vapor bubbles was quantitatively visualized at different imposed heat fluxes, in terms of the percentage change in vapor bubble length along the heated microchannel. The results demonstrate the ability of the T-junction to generate diabatic, one-component, two-phase microchannel slug flow that is suitable for generating results for the validation of flow boiling models.