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Journal articles on the topic 'Equatorial Ionization anomaly'

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Author: Grafiati

Published: 6 September 2023

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1

Bharti, Gaurav, T. Bag, and M. V. Sunil Krishna. "Effect of geomagnetic storm conditions on the equatorial ionization anomaly and equatorial temperature anomaly." Journal of Atmospheric and Solar-Terrestrial Physics 168 (March 2018): 8–20. http://dx.doi.org/10.1016/j.jastp.2017.12.014.

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2

Wang, Hai-Ning, Qing-Lin Zhu, Xiang Dong, Dong-Sheng Sheng, Yong-Feng Zhi, Chen Zhou, and Bin Xu. "A Novel Technique for High-Precision Ionospheric VTEC Estimation and Prediction at the Equatorial Ionization Anomaly Region: A Case Study over Haikou Station." Remote Sensing 15, no. 13 (July 4, 2023): 3394. http://dx.doi.org/10.3390/rs15133394.

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This paper introduces a novel technique that uses observation data from GNSS to estimate the ionospheric vertical total electron content (VTEC) using the Kriging–Kalman method. The technique provides a method to validate the accuracy of the Ionospheric VTEC analysis within the Equatorial Ionization anomaly region. The technique developed uses GNSS VTEC alongside solar parameters, such as solar radio flux (F10.7 cm), Disturbance Storm Time (Dst) and other data, and Long Short Term Memory (LSTM) Networks to predict the occurrence time of the ionospheric equatorial anomaly and ionospheric VTEC changes. The LSTM method was applied to GNSS data from Haikou Station. A comparison of this technique with the neural network (NN) model and International Reference Ionosphere model shows that the LSTM outperforms all of them at VTEC estimation and prediction. The results, which are based on the root mean square error (RMSE) between GNSS VTEC and GIM VTEC outside the equatorial anomaly region, was 1.42 TECU, and the results of GNSS VTEC and VTEC from Beidou geostationary orbit satellite, which lies inside the equatorial ionization anomaly region, was 1.92 TECU. The method developed can be used in VTEC prediction and estimation in real time space operations.

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3

Lee, I. T., J. Y. Liu, C. H. Lin, K. I. Oyama, C. Y. Chen, and C. H. Chen. "Ionospheric plasma caves under the equatorial ionization anomaly." Journal of Geophysical Research: Space Physics 117, A11 (November 2012): n/a. http://dx.doi.org/10.1029/2012ja017868.

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4

Chen, Pei-Ren. "Two-day oscillation of the equatorial ionization anomaly." Journal of Geophysical Research 97, A5 (1992): 6343. http://dx.doi.org/10.1029/91ja02445.

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5

Eastes, R. W., S. C. Solomon, R. E. Daniell, D. N. Anderson, A. G. Burns, S. L. England, C. R. Martinis, and W. E. McClintock. "Global‐Scale Observations of the Equatorial Ionization Anomaly." Geophysical Research Letters 46, no. 16 (August 19, 2019): 9318–26. http://dx.doi.org/10.1029/2019gl084199.

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6

Raghavarao, R., M. Nageswararao, J. Hanumath Sastri, G. D. Vyas, and M. Sriramarao. "Role of equatorial ionization anomaly in the initiation of equatorial spread F." Journal of Geophysical Research 93, A6 (1988): 5959. http://dx.doi.org/10.1029/ja093ia06p05959.

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7

Sharma, P., and R. Raghavarao. "Simultaneous occurrence of ionization ledge and counterelectrojet in the equatorial ionosphere: observational evidence and its implications." Canadian Journal of Physics 67, no. 2-3 (February 1, 1989): 166–72. http://dx.doi.org/10.1139/p89-028.

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In this paper we present observational evidence for the simultaneous occurrence of an ionization ledge in the topside and a counterelectrojet in the E-region altitudes of the equatorial ionosphere. The following morphological features of the ionization ledge are found to be the same as those of the counterelectrojet phenomenon: namely, occurrence on a sequence of days in succession, preferential occurrence during a solar minimum period as compared with a solar maximum period, occurrence in a limited longitudinal belt, and lunar control of the occurrence as revealed by our data.There is also a high degree of correlation on both magnetically quiet and magnetically disturbed days between the ionization ledge and the counterelectrojet. Our study brings out a close coupling in the occurrence and morphological features of the two low-latitude phenomena, even though they are widely separated in altitude. The implications of such a correlation, with special reference to the understanding of the dynamical coupling of the E and F regions in the equatorial ionosphere, are briefly discussed.We also report a hitherto unreported feature of the ionization anomaly, viz., its occurrence on certain days in a narrow (~30°) longitude belt. The observational evidence presented in this paper may lead to new insights into the understanding of the interrelationships between the three phenomena, the ionization ledge, the ionization anomaly, and the counterelectrojet, in the equatorial ionosphere.

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8

Ray, S., A. Paul, and A. DasGupta. "Equatorial scintillations in relation to the development of ionization anomaly." Annales Geophysicae 24, no. 5 (July 3, 2006): 1429–42. http://dx.doi.org/10.5194/angeo-24-1429-2006.

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Abstract. The irregularities in the electron density distribution of the ionosphere over the equatorial region frequently disrupt space-based communication and navigation links by causing severe amplitude and phase scintillations of signals. Development of a specification and forecast system for scintillations is needed in view of the increased reliance on space-based communication and navigation systems, which are vulnerable to ionospheric scintillations. It has been suggested in recent years that a developed equatorial anomaly in the afternoon hours, with a steep gradient of the F-region ionization or Total Electron Content (TEC) in the region between the crest and the trough, may be taken as a precursor to scintillations on transionospheric links. Latitudinal gradient of TEC measured using Faraday Rotation technique from LEO NOAA 12/14 transmissions during the afternoon hours at Calcutta shows a highly significant association with L-band scintillations recorded on the INMARSAT link, also from Calcutta, during the equinoxes, August through October 2000, and February through April 2001. The daytime equatorial electrojet is believed to control the development of the equatorial anomaly and plays a crucial role in the subsequent development of F-region irregularities in the post-sunset hours. The diurnal maximum and integrated value (integrated from the time of onset of plasma influx to off-equatorial latitudes till local sunset) of the strength of the electrojet in the Indian longitude sector shows a significant association with post-sunset L-band scintillations recorded at Calcutta during the two equinoxes mentioned earlier. Generation of equatorial irregularities over the magnetic equator in the post-sunset hours is intimately related to the variation of the height of the F-layer around sunset. Ionosonde data from Kodaikanal, a station situated close to the magnetic equator, has been utilized to calculate the vertical drift of the F-layer over the magnetic equator for the period August through October 2000. The post-sunset F-region height rise over the magnetic equator shows a remarkable correspondence with the occurrence of scintillations at Calcutta located near the northern crest of the equatorial anomaly. Existence of a flat-topped ionization distribution over the magnetic equator around sunset has been suggested as a possible indication of occurrence of post-sunset scintillations. Width of the latitudinal distribution of ionization obtained from DMSP satellite shows some correspondence with post-sunset L-band scintillations. During the period of observation of the present study (August through October 2000, and February through April 2001), it has been observed that although the probability of occurrence of scintillations is high on days with flat-topped ion density variation over the equator, there are cases when no scintillations were observed even when a pronounced flat top variation was recorded.

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9

Goncharenko, L. P., A. J. Coster, J. L. Chau, and C. E. Valladares. "Impact of sudden stratospheric warmings on equatorial ionization anomaly." Journal of Geophysical Research: Space Physics 115, A10 (October 2010): n/a. http://dx.doi.org/10.1029/2010ja015400.

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10

Rama Rao, P. V. S., P. T. Jayachandran, and P. Sri Ram. "Ionospheric irregularities: The role of the equatorial ionization anomaly." Radio Science 32, no. 4 (July 1997): 1551–57. http://dx.doi.org/10.1029/97rs00665.

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11

Liu, J. Y., P. K. Rajesh, I. T. Lee, and T. C. Chow. "Airglow observations over the equatorial ionization anomaly zone in Taiwan." Annales Geophysicae 29, no. 5 (May 5, 2011): 749–57. http://dx.doi.org/10.5194/angeo-29-749-2011.

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Abstract. Airglow imaging at mid-latitude stations often show intensity modulations associated with medium scale travelling ionospheric disturbances (MSTID), while those carried out near the equatorial regions reveal depletions caused by equatorial plasma bubbles (EPB). Two all sky cameras are used to observe plasma depletions in the 630.0 nm emission over the equatorial ionization anomaly (EIA) region, Taiwan (23° N, 121° E; 13.5° N Magnetic) during 1998–2002 and 2006–2007. The results show EPB and MSTID depletions in different solar activity conditions. Several new features of the EPB depletions such as bifurcation, secondary structure on the walls, westward tilt, etc., are discussed in this paper. Evidence of tilted depletions with secondary structures developing on the eastern wall that later evolve to appear as bifurcations, are presented for the first time. Moreover, detail investigations are carried out using International Reference Ionosphere (IRI) model as well as the electron density from Ionosonde and Global Positioning System (GPS) Occultation Experiment (GOX) onboard FORMOSAT-3/COSMIC satellite, to understand the conditions that favor the propagation of MSTID to the latitude of Taiwan.

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12

Jayachandran, P. T., P. Sri Ram, V. V. Somayajulu, and P. V. S. Rama Rao. "Effect of equatorial ionization anomaly on the occurrence of spread-F." Annales Geophysicae 15, no. 2 (February 28, 1997): 255–62. http://dx.doi.org/10.1007/s00585-997-0255-3.

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Abstract. The unique geometry of the geomagnetic field lines over the equatorial ionosphere coupled with the E-W electric field causes the equatorial ionization anomaly (EIA) and equatorial spread-F (ESF). Ionosonde data obtained at a chain of four stations covering equator to anomaly crest region (0.3 to 33 °N dip) in the Indian sector are used to study the role of EIA and the associated processes on the occurrence of ESF. The study period pertains to the equinoctial months (March, April, September and October) of 1991. The ratios of critical frequency of F-layer (ƒ0F2) and electron densities at an altitude of 270 km between Ahmedabad (33 °N dip) and Waltair (20 °N dip) are found to shoot up in the afternoon hours on spread-F days showing strengthening of the EIA in the afternoon hours. The study confirms the earlier conclusions made by Raghava Rao et al. and Alex et al. that a well-developed EIA is one of the conditions conducive for the generation of ESF. This study also shows that the location of the crest is also important in addition to the strength of the anomaly.

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13

Kulyamin, Dmitry V., and Pavel A. Ostanin. "Modelling of equatorial ionospheric anomaly in the INM RAS coupled thermosphere-ionosphere model." Russian Journal of Numerical Analysis and Mathematical Modelling 35, no. 1 (February 25, 2020): 1–9. http://dx.doi.org/10.1515/rnam-2020-0001.

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AbstractThe paper deals with the problem of the equatorial ionization anomaly (EIA) modelling and its representation in the global dynamical models of Earth’s ionosphere and thermosphere. A new version of the coupled thermosphere-ionosphere global dynamical model which reproduce the equatorial anomaly considerably well is presented. Key processes responsible for the EIA formation are outlined and their representation in the model is indicated. It was shown that the developed coupled thermosphere-ionosphere model with additional accounting of vertical electromagnetic drift at the equator realistically represents the EIA characteristics.

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14

Liu, Jing, Libo Liu, Biqiang Zhao, Jiuhou Lei, and Weixing Wan. "On the relationship between the postmidnight thermospheric equatorial mass anomaly and equatorial ionization anomaly under geomagnetic quiet conditions." Journal of Geophysical Research: Space Physics 116, A12 (December 2011): n/a. http://dx.doi.org/10.1029/2011ja016958.

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15

Pallam Raju, D., and R. Sridharan. "High resolution 2-D maps of OI 630.0 nm thermospheric dayglow from equatorial latitudes." Annales Geophysicae 16, no. 8 (August 31, 1998): 997–1006. http://dx.doi.org/10.1007/s00585-998-0997-6.

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Abstract. The first-ever high resolution 2-D maps of OI 630.0 nm dayglow obtained from equatorial latitudes clearly reveal the movement as a large-scale feature of the equatorial ionization anomaly (EIA). These also show the presence of wave-like features classified as gravity waves presumably originating at the crest of the EIA, similar to the equatorial electrojet acting as a source of these waves. These results are presented and discussed.Key words. Atmospheric composition and structure (Airglow and aurora) · Ionosphere (Equatorial ionosphere; Instruments and techniques).

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16

Seba, Ephrem Beshir, Melessew Nigussie, and Mark B. Moldwin. "The relationship between equatorial ionization anomaly and nighttime equatorial spread F in East Africa." Advances in Space Research 62, no. 7 (October 2018): 1737–52. http://dx.doi.org/10.1016/j.asr.2018.06.029.

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17

Horvath, Ildiko, and Brian C. Lovell. "Equatorial westward electrojet impacting equatorial ionization anomaly development during the 6 April 2000 superstorm." Journal of Geophysical Research: Space Physics 118, no. 11 (November 2013): 7398–409. http://dx.doi.org/10.1002/2013ja019311.

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18

Balan, Nanan, LiBo Liu, and HuiJun Le. "A brief review of equatorial ionization anomaly and ionospheric irregularities." Earth and Planetary Physics 2, no. 4 (2018): 1–19. http://dx.doi.org/10.26464/epp2018025.

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19

Mo, X. H., D. H. Zhang, J. Liu, Y. Q. Hao, J. F. Ye, J. S. Qin, W. X. Wei, and Z. Xiao. "Morphological Characteristics of Equatorial Ionization Anomaly Crest Over Nanning Region." Radio Science 53, no. 1 (January 2018): 37–47. http://dx.doi.org/10.1002/2017rs006386.

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20

Mo, X. H., D. H. Zhang, L. P. Goncharenko, Y. Q. Hao, and Z. Xiao. "Quasi-16-day periodic meridional movement of the equatorial ionization anomaly." Annales Geophysicae 32, no. 2 (February 18, 2014): 121–31. http://dx.doi.org/10.5194/angeo-32-121-2014.

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Abstract. Based on the daytime location of the equatorial ionization anomaly (EIA) crest derived from GPS observations at low latitude over China during the 2005–2006 stratospheric sudden warming (SSW), a quasi-16-day periodic meridional movement of EIA crest with the maximum amplitude of about 2 degrees relative to the average location of EIA crest has been revealed. In addition, periodic variations that are in phase with the meridional EIA movement are also revealed in the equatorial electrojet (EEJ) and F2 layer peak height (hmF2) over Chinese ionosonde stations Haikou and Chongqing. The quasi-16-day periodic component in Dst index is weak, and the 16-day periodic component does not exist in F10.7 index. Such large-scale periodic meridional movement of EIA crest is likely related to the globally enhanced stratospheric planetary waves coupled with anomalous stratospheric zonal wind connected with SSW. In addition, such large-scale periodic movement of EIA should be global, and can affect the ionospheric morphology around the low-latitude belt near the EIA region. Further case analysis, simulation and theoretical studies must proceed in order to understand the periodic movements of EIA connected with the different periodic atmospheric variations.

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21

Stolle, C., C. Manoj, H. Lühr, S. Maus, and P. Alken. "Estimating the daytime Equatorial Ionization Anomaly strength from electric field proxies." Journal of Geophysical Research: Space Physics 113, A9 (September 2008): n/a. http://dx.doi.org/10.1029/2007ja012781.

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22

Balan, N., J. Souza, and G. J. Bailey. "Recent developments in the understanding of equatorial ionization anomaly: A review." Journal of Atmospheric and Solar-Terrestrial Physics 171 (June 2018): 3–11. http://dx.doi.org/10.1016/j.jastp.2017.06.020.

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23

Balan, N., K. Shiokawa, Y. Otsuka, S. Watanabe, and G. J. Bailey. "Super plasma fountain and equatorial ionization anomaly during penetration electric field." Journal of Geophysical Research: Space Physics 114, A3 (March 2009): n/a. http://dx.doi.org/10.1029/2008ja013768.

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24

Abdu, M. A., J. H. A. Sobral, E. R. de Paula, and I. S. Batista. "Magnetospheric disturbance effects on the Equatorial Ionization Anomaly (EIA) : an overview." Journal of Atmospheric and Terrestrial Physics 53, no. 8 (August 1991): 757–71. http://dx.doi.org/10.1016/0021-9169(91)90126-r.

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25

Jose, L., S. Ravindran, C. Vineeth, T. K. Pant, and S. Alex. "Investigation of the response time of the equatorial ionosphere in context of the equatorial electrojet and equatorial ionization anomaly." Annales Geophysicae 29, no. 7 (July 19, 2011): 1267–75. http://dx.doi.org/10.5194/angeo-29-1267-2011.

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Abstract. Equatorial Electrojet (EEJ) and Equatorial Ionization Anomaly (EIA) are two large-scale processes in the equatorial/low latitude ionosphere, driven primarily by the eastward electric field during daytime. In the present paper we investigate the correlation between the Integrated EEJ strength (IEEJ) and the EIA parameters like the total electron content at the northern crest, location of crest in Magnetic latitude and strength of the EIA for the Indian sector. A good correlation has been observed between the IEEJ and EIA when a time delay is introduced between IEEJ and EIA parameters. This time delay is regarded as the response time of equatorial ionosphere in context of the evolution of EIA vis-à-vis EEJ. Further, a seasonal variation in the time delay has been observed, which is believed to be due to changes in thermospheric wind. Using the response time and the linear relationship obtained, the possibility of near-real time prediction of EIA parameters has been attempted and found that the prediction holds well during the geomagnetically quiet periods. The paper discusses these aspects in detail.

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26

Kulyamin, Dmitry V., Pavel A. Ostanin, and Valentin P. Dymnikov. "INM-IM: INM RAS Earth ionosphere F region dynamical model." Russian Journal of Numerical Analysis and Mathematical Modelling 37, no. 6 (December 1, 2022): 349–62. http://dx.doi.org/10.1515/rnam-2022-0028.

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Abstract A new INM RAS global dynamical model of Earth’s ionosphere F region (100–500 km), which takes into account plasma-chemical processes, ambipolar diffusion, and advective ion transport due to electromagnetic drifts and neutral wind is presented. The model includes parameterizations of polar electric fields induced by magnetospheric convection and simplified equatorial drifts considerations. The focus of the paper is directed on the description of specific methods developed and utilized in the ionospheric model. Key processes responsible for the formation of global ionospheric features are outlined and their representation in the model is evaluated. The main global ionospheric characteristic features, such as seasonal and diurnal cycles, the equatorial ionization anomaly (EIA), polar ionization caps and the main trough have been adequately reproduced based on this model.

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27

Li, G., B. Ning, W. Wan, and B. Zhao. "Observations of GPS ionospheric scintillations over Wuhan during geomagnetic storms." Annales Geophysicae 24, no. 6 (July 3, 2006): 1581–90. http://dx.doi.org/10.5194/angeo-24-1581-2006.

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Abstract. During the two geomagnetic storms which occurred on 1 October 2002 and 22 January 2004, the strong ionospheric scintillations of the GPS L1 band were observed at Wuhan station (30.6° N, 114.4° E, 45.8° Dip), which is situated near the northern crest of the equatorial ionosphere anomaly. We found that the intense scintillations were associated with the main phases of the storms and were co-located with the enhancement of the equatorial ionization anomaly (EIA); the co-existence of large- and small-scale irregularities at post-midnight was also found. The results may be relevant regarding the influence of the equatorial ionospheric eastward electric field during geomagnetic storms. On the other hand, GPS L1 band scintillations were not observed during the other two similar storms on 16 July 2003 and 20 November 2003. One of the reasons is probably that the sporadic E layer observed at the storms inhibited the generation of spread F by changing the Pedersen conductivity and suppressing the upward plasma drift.

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28

Li, Guozhu, Baiqi Ning, Libo Liu, Biqiang Zhao, Xinan Yue, S. Y. Su, and Sarita Venkatraman. "Correlative study of plasma bubbles, evening equatorial ionization anomaly, and equatorial prereversalE×Bdrifts at solar maximum." Radio Science 43, no. 4 (August 2008): n/a. http://dx.doi.org/10.1029/2007rs003760.

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29

Sales, G. S., B. W. Reinisch, J. L. Scali, C. Dozois, T. W. Bullett, E. J. Weber, and P. Ning. "SpreadFand the structure of equatorial ionization depletions in the southern anomaly region." Journal of Geophysical Research: Space Physics 101, A12 (December 1, 1996): 26819–27. http://dx.doi.org/10.1029/96ja01946.

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30

Yue, Xinan, William S. Schreiner, Ying-Hwa Kuo, and Jiuhou Lei. "Ionosphere equatorial ionization anomaly observed by GPS radio occultations during 2006–2014." Journal of Atmospheric and Solar-Terrestrial Physics 129 (July 2015): 30–40. http://dx.doi.org/10.1016/j.jastp.2015.04.004.

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31

Manju, G., V. Sreeja, Sudha Ravindran, and Smitha V. Thampi. "Toward prediction of L band scintillations in the equatorial ionization anomaly region." Journal of Geophysical Research: Space Physics 116, A2 (February 2011): n/a. http://dx.doi.org/10.1029/2010ja015893.

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32

Bolaji, Olawale, Oluwafisayo Owolabi, Elijah Falayi, Emmanuel Jimoh, Afolabi Kotoye, Olumide Odeyemi, Babatunde Rabiu, et al. "Observations of equatorial ionization anomaly over Africa and Middle East during a year of deep minimum." Annales Geophysicae 35, no. 1 (January 20, 2017): 123–32. http://dx.doi.org/10.5194/angeo-35-123-2017.

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Abstract. In this work, we investigated the veracity of an ion continuity equation in controlling equatorial ionization anomaly (EIA) morphology using total electron content (TEC) of 22 GPS receivers and three ground-based magnetometers (Magnetic Data Acquisition System, MAGDAS) over Africa and the Middle East (Africa–Middle East) during the quietest periods. Apart from further confirmation of the roles of equatorial electrojet (EEJ) and integrated equatorial electrojet (IEEJ) in determining hemispheric extent of EIA crest over higher latitudes, we found some additional roles played by thermospheric meridional neutral wind. Interestingly, the simultaneous observations of EIA crests in both hemispheres of Africa–Middle East showed different morphology compared to that reported over Asia. We also observed interesting latitudinal twin EIA crests domiciled at the low latitudes of the Northern Hemisphere. Our results further showed that weak EEJ strength associated with counter electrojet (CEJ) during sunrise hours could also trigger twin EIA crests over higher latitudes.

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Pallam Raju, D., R. Sridharan, S. Gurubaran, and R. Raghavarao. "First results from ground-based daytime optical investigation of the development of the equatorial ionization anomaly." Annales Geophysicae 14, no. 2 (February 29, 1996): 238–45. http://dx.doi.org/10.1007/s00585-996-0238-9.

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Abstract. A meridional scanning OI 630.0-nm dayglow photometer was operated from Ahmedabad (17.2°N dip lat.) scanning a region towards the south in the upper atmosphere extending over ~5° in latitude from 10.2°N to 15.2°N dip latitude. From the spatial and temporal variabilities of the dayglow intensity in the scanning region we show for the first time, evidence for the passage of the crest of the equatorial ionization anomaly (EIA) in the daytime by means of a ground-based optical technique. The relationship between the daytime eastward electric field over the dip equator in the same longitude zone as inferred from the equatorial electrojet strength and the evolutionary pattern of EIA is clearly demonstrated. The latter as inferred from the dayglow measurements is shown to be consistent with our present understanding of the electrodynamical processes in the equatorial region. The present results reveal the potential of this ground-based optical technique for the investigation of ionospheric/thermospheric phenomena with unprecedented spatial and temporal resolution.

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Chen, C. H., J. Y. Liu, K. Yumoto, C. H. Lin, and T. W. Fang. "Equatorial ionization anomaly of the total electron content and equatorial electrojet of ground-based geomagnetic field strength." Journal of Atmospheric and Solar-Terrestrial Physics 70, no. 17 (December 2008): 2172–83. http://dx.doi.org/10.1016/j.jastp.2008.09.021.

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35

Chakraborty, S. K., and R. Hajra. "Electrojet control of ambient ionization near the crest of the equatorial anomaly in the Indian zone." Annales Geophysicae 27, no. 1 (January 6, 2009): 93–105. http://dx.doi.org/10.5194/angeo-27-93-2009.

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Abstract. A long-term (1978–1990) database of total electron content (TEC) from a location (Calcutta: 22.58° N, 88.38° E geographic, dip: 32° N) near the northern crest of the equatorial ionization anomaly has extensively been studied to characterize the contribution of fountain effect in the maintenance of ambient ionization. The equatorial electrojet (EEJ) data obtained from ground magnetometer recording are used to assess the contribution of equatorial fountain. Analysis made with instantaneous values, day's maximum values and time-integrated values of EEJ strength exhibit more or less similar features. When instantaneous values of EEJ are considered TEC variations exhibit two maxima in correlation, one around 10:00–12:00 IST and the other around 18:00–20:00 IST. The later maximum in correlation coefficient is conspicuously absent when integrated values of EEJ are considered. An impulse-like feature is reflected in the diurnal TEC variation during the time intervals (09:00–10:00 IST) and (18:00–19:00 IST). The statistical analysis reveals greater correspondence with high level of significance between diurnal TEC and EEJ in the descending epoch of solar cycle than in the ascending one. On the seasonal basis, TEC in the summer solstitial months are observed to be more sensitive to the changes in EEJ strength than in the equinoctial and winter solstitial months. Combining the effects of solar flux, season, local time and EEJ an empirical formula for monthly mean diurnal TEC has been developed and validated using observed TEC data. An estimation of the relative contributions of the several terms appearing in the formula reveals much more solar flux contribution (~50–70%) in the maintenance of ambient ionization around the present location than the EEJ effects (maximum~20%).

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36

Krall, J., J. D. Huba, G. Joyce, and T. Yokoyama. "Density enhancements associated with equatorial spread <I>F</I>." Annales Geophysicae 28, no. 2 (February 1, 2010): 327–37. http://dx.doi.org/10.5194/angeo-28-327-2010.

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Abstract. Forces governing the three-dimensional structure of equatorial spread-F (ESF) plumes are examined using the NRL SAMI3/ESF three-dimensional simulation code. As is the case with the equatorial ionization anomaly (IA), density crests within the plume occur where gravitational and diffusive forces are in balance. Large E×B drifts within the ESF plume place these crests on field lines with apex heights higher than those of the background IA crests. Large poleward field-aligned ion velocities within the plume result in large ion-neutral diffusive forces that support these ionization crests at altitudes higher than background IA crest altitudes. We show examples in which density enhancements associated with ESF, also called "plasma blobs," can occur within an ESF plume on density-crest field lines, at or above the density crests. Simulated ESF density enhancements reproduce all key features of those that have been observed in situ.

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37

Tulasi Ram, S., P. V. S. Rama Rao, D. S. V. V. D. Prasad, K. Niranjan, A. Raja Babu, R. Sridharan, and C. V. Devasia. "The combined effects of electrojet strength and the geomagnetic activity (<I>K<sub>p</sub></I>-index) on the post sunset height rise of the F-layer and its role in the generation of ESF during high and low solar activity periods." Annales Geophysicae 25, no. 9 (October 2, 2007): 2007–17. http://dx.doi.org/10.5194/angeo-25-2007-2007.

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Abstract. Several investigations have been carried out to identify the factors that are responsible for the day-to-day variability in the occurrence of equatorial spread-F (ESF). But the precise forecasting of ESF on a day-to-day basis is still far from reality. The nonlinear development and the sustenance of ESF/plasma bubbles is decided by the background ionospheric conditions, such as the base height of the F-layer (h'F), the electron density gradient (dN/dz), maximum ionization density (Nmax), geomagnetic activity and the neutral dynamics. There is increasing evidence in the literature during the recent past that shows a well developed Equatorial Ionization Anomaly (EIA) during the afternoon hours contributes significantly to the initiation of ESF during the post-sunset hours. Also, there exists a good correlation between the Equatorial Ionization Anomaly (EIA) and the Integrated Equatorial ElectroJet (IEEJ) strength, as the driving force for both is the same, namely, the zonal electric field at the equator. In this paper, we present a linear relationship that exists between the daytime integrated equatorial electrojet (IEEJ) strength and the maximum elevated height of the F-layer during post-sunset hours (denoted as peak h'F). An inverse relationship that exists between the 6-h average Kp-index prior to the local sunset and the peak h'F of the F-layer is also presented. A systematic study on the combined effects of the IEEJ and the average Kp-index on the post-sunset, peak height of the F-layer (peak h'F), which controls the development of ESF/plasma bubbles, is carried out using the ionosonde data from an equatorial station, Trivandrum (8.47° N, 76.91° E, dip.lat. 0.5° N), an off-equatorial station, SHAR (13.6° N, 79.8° E, dip.lat. 10.8° N) and VHF scintillations (244 MHz) observed over a nearby low-latitude station, Waltair (17.7° N, 83.3° E, dip.lat. 20° N). From this study, it has been found that the threshold base height of the F-layer at the equator for the development of plasma bubbles is reduced from 405 km to 317 km as the solar activity decreases from March 2001 (mean Rz=113.5) to March 2005 (mean Rz=24.5). This decrease in threshold height with the decreasing solar activity is explained on the basis of changes in the local linear growth rate of the collisional Rayleigh-Taylor instability, due to the variability of various terms such as inverse density gradient scale length (L−1), ion-neutral collision frequency (νin) and recombination rate (R) with the changes in the solar activity.

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Walker, G. O., J. H. K. Ma, and E. Golton. "The equatorial ionospheric anomaly in electron content from solar minimum to solar maximum for South East Asia." Annales Geophysicae 12, no. 2/3 (January 31, 1994): 195–209. http://dx.doi.org/10.1007/s00585-994-0195-0.

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Abstract. Median hourly, electron content-latitude profiles obtained in South East Asia under solar minimum and maximum conditions have been used to establish seasonal and solar differences in the diurnal variations of the ionospheric equatorial anomaly (EIA). The seasonal changes have been mainly accounted for from a consideration of the daytime meridional wind, affecting the EIA diffusion of ionization from the magnetic equator down the magnetic field lines towards the crests. Depending upon the seasonal location of the subsolar point in relation to the magnetic equator diffusion rates were increased or decreased. This led to crest asymmetries at the solstices with (1) the winter crest enhanced in the morning (increased diffusion rate) and (2) the same crest decaying most rapidly in the late afternoon (faster recombination rate at lower ionospheric levels). Such asymmetries were also observed, to a lesser extent, at the equinoxes since the magnetic equator (located at about 9°N lat) does not coincide with the geographic equator. Another factor affecting the magnitude of a particular electron content crest was the proximity of the subsolar point, since this increased the local ionization production rate. Enhancements of the EIA took place around sunset, mainly during the equinoxes and more frequently at solar maximum, and also there was evidence of apparent EIA crest resurgences around 0300 LST for all seasons at solar maximum. The latter are thought to be associated with the commonly observed, post-midnight, ionization enhancements at midlatitudes, ionization being transported to low latitudes by an equatorward wind. The ratio increases in crest peak electron contents from solar minimum to maximum of 2.7 at the equinoxes, 2.0 at the northern summer solstice and 1.7 at northern winter solstice can be explained, only partly, by increases in the magnitude of the eastward electric field E overhead the magnetic equator affecting the [E×B] vertical drifts. The most important factor is the corresponding increase in ionization production rate due to the increase in solar radiation flux. The EIA crest asymmetries observed at solar maximum were less significant, and this is probably due to the corresponding increase in ionization densities leading to an increase of the retarding effect of ion-drag on the daytime meridional winds.

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Bhuyan, P. K., P. K. Kakoty, and S. B. Singh. "Theoretical simulation of O<sup>+</sup> and H<sup>+</sup> densities in the Indian low latitude F-region and comparison with observations." Annales Geophysicae 20, no. 12 (December 31, 2002): 1959–66. http://dx.doi.org/10.5194/angeo-20-1959-2002.

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Abstract. The O+ and H+ ion density distributions in the Indian low latitude F-region, within ± 15° magnetic latitudes, are simulated using a time dependent model developed on the basis of solution of the plasma continuity equation. The simulated ion densities for solar minimum June and December solstices are then compared with ionosonde observations from the period 1959–1979 and measurements made by the Indian SROSS C2 satellite during 1995–1996 at an altitude of ~ 500 km. The simulated O+ density has a minimum around pre-sunrise hours and a maximum during noontime. H+ density is higher at nighttime and lower during the day. The simulations reproduced the well-known equatorial ionization anomaly (EIA) observed in electron density at the peak of the F2-region in the Indian low latitude sector during solar minimum. In situ measurement of O+ density by the SROSS C2, however, showed a single peak of ionization around the equator.Key words. Ionosphere (equatorial ionosphere; modeling and forecasting)

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Amaechi, P. O., E. O. Oyeyemi, and A. O. Akala. "Variability of the African equatorial ionization anomaly (EIA) crests during the year 2013." Canadian Journal of Physics 97, no. 2 (February 2019): 155–65. http://dx.doi.org/10.1139/cjp-2017-0985.

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This paper discusses the variability of the position and magnitude of the crests of African Equatorial Ionization Anomaly during noon and post sunset periods. Total electron content data covered the year 2013, and were obtained from a chain of global positioning system receivers in both hemispheres around 37°E longitude. Local magnetometer data were used to infer the direction and magnitude of the E × B drift, while the solar extreme ultraviolet proxy index was used as a measure of solar activity. It was found that the time of formation of both crests varied from 1400 to 1700 local time. Additionally, the position of the crests was found to be asymmetric with respect to the magnetic equator. During the noon period, the position of the northern and southern crests varied from 4.91° to 7.36° and −9.17° to −12.62°, respectively. During the post-sunset period, it varied from 8° to 11.7° and −9° to −16°, respectively. Seasonally, with reference to the magnetic equator, both crests moved poleward during equinoxes and collapsed towards the equator during winter and summer. Equinoxes recorded the greatest crest magnitude followed by winter then summer over both hemispheres during the noon period. However, this trend persisted over the northern crest only during the post-sunset period. Overall, during the noon period, we recorded correlation coefficients of 0.67 and 0.68 between crest magnitudes and ΔH, a proxy for equatorial electrojet current, and 0.88 and 0.81 between crest positions and ΔH, for the northern and southern crests, respectively. During the Halloween day storm of 30 October 2013, a westward electric field inhibited the development of the post-sunset crests.

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Kassa, T., B. Damtie, A. Bires, E. Yizengaw, and P. Cilliers. "Storm-time characteristics of the equatorial ionization anomaly in the East African sector." Advances in Space Research 56, no. 1 (July 2015): 57–70. http://dx.doi.org/10.1016/j.asr.2015.04.002.

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42

Talha, Madeeha, Nabeel Ahmed, Muneeza M. Ali, and Ghulam Murtaza. "Variability of NmF2 during solar minima at the Equatorial Ionization Anomaly crest region." Advances in Space Research 64, no. 11 (December 2019): 2321–30. http://dx.doi.org/10.1016/j.asr.2019.09.014.

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43

Kwak, Young-Sil, Hyosub Kil, Woo Kyoung Lee, and Tae-Yong Yang. "Variation of the Hemispheric Asymmetry of the Equatorial Ionization Anomaly with Solar Cycle." Journal of Astronomy and Space Sciences 36, no. 3 (September 2019): 159–68. http://dx.doi.org/10.5140/jass.2019.36.3.159.

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In solstices during the solar minimum, the hemispheric difference of the equatorial ionization anomaly (EIA) intensity (hereafter hemispheric asymmetry) is understood as being opposite in the morning and afternoon. This phenomenon is explained by the temporal variation of the combined effects of the fountain process and interhemispheric wind. However, the mechanism applied to the observations during the solar minimum has not yet been validated with observations made during other periods of the solar cycle. We investigate the variability of the hemispheric asymmetry with local time (LT), altitude, season, and solar cycle using the electron density taken by the CHAllenging Minisatellite Payload satellite and the global total electron content (TEC) maps acquired during 2001–2008. The electron density profiles provided by the Constellation Observing System for Meteorology, Ionosphere, and Climate satellites during 2007–2008 are also used to investigate the variation of the hemispheric asymmetry with altitude during the solar minimum. During the solar minimum, the location of a stronger EIA moves from the winter hemisphere to the summer hemisphere around 1200–1400 LT. The reversal of the hemispheric asymmetry is more clearly visible in the F-peak density than in TEC or in topside plasma density. During the solar maximum, the EIA in the winter hemisphere is stronger than that in the summer hemisphere in both the morning and afternoon. When the location of a stronger EIA in the afternoon is viewed as a function of the year, the transition from the winter hemisphere to the summer hemisphere occurs near 2004 (yearly average F10.7 index = 106). We discuss the mechanisms that cause the variation of the hemispheric asymmetry with LT and solar cycle.

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44

Luo, Xiaowen, Di Wang, Jinling Wang, Ziyin Wu, Jinyao Gao, Tao Zhang, Chunguo Yang, Xiaoming Qin, and Xiaolun Chen. "Study of the Spatiotemporal Characteristics of the Equatorial Ionization Anomaly Using Shipborne Multi-GNSS Data: A Case Analysis (120° E–150° E, Western Pacific Ocean, 2014–2015)." Remote Sensing 13, no. 15 (August 3, 2021): 3051. http://dx.doi.org/10.3390/rs13153051.

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Ground-based GNSS (Global Navigation Satellite System) reference stations lack the capacity to provide data for ocean regions with sufficient spatial-temporal resolution, limiting the detailed study of the equatorial ionization anomaly (EIA). Thus, this study collected kinematic multi-GNSS data on the ionospheric Total Electron Content (TEC) during two research cruises across the equator in the Western Pacific Ocean in 2014 (31 October–8 November) and 2015 (29 March–6 April). The purpose of the study was to use sufficient spatial–temporal resolution data to conduct a detailed analysis of the diurnal variation of the equatorial ionization anomaly in different seasons. The two-year data collected were used to draw the following conclusions. During the test in 2014, the EIA phenomenon in the Northern and Southern Hemispheres was relatively obvious. The maximum values occurred at local time (LT) 15:00 (~136TECu) and LT22:00 (~107TECu) in the Northern Hemisphere and at LT14:00 (100TECu) and LT22:00 (80TECu) in the Southern Hemisphere. During the test in 2015, the EIA in the Southern Hemisphere reached its maximum level at LT14:00 (~115TECu) and LT20:00 (~85TECu). However, the EIA phenomenon in the Northern Hemisphere was weakened, and a maximum value occurred only at LT 15:00 (~85TECu). The intensity contrast was reversed. The EIA phenomenon manifests a strong hemisphere asymmetry in this region.

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45

Dabas, R. S., R. M. Das, V. K. Vohra, and C. V. Devasia. "Space weather impact on the equatorial and low latitude F-region ionosphere over India." Annales Geophysicae 24, no. 1 (March 7, 2006): 97–105. http://dx.doi.org/10.5194/angeo-24-97-2006.

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Abstract. For a detailed study of the space weather impact on the equatorial and low latitude F-region, the ionospheric response features are analysed during the periods of three recent and most severe magnetic storm events of the present solar cycle which occurred in October and November 2003, and November 2004. The F-layer base height (h'F), peak height (hmF2) and critical frequency (foF2) data, from Trivandrum, an equatorial station and Delhi, a low latitude location, are examined during the three magnetic storm periods. The results of the analysis clearly shows that the height of the F-region (both h'F and hmF2), at the equator and low latitude, simultaneously increases by 200 to 300 km, in association with maximum negative excursion of Dst values around the midnight hours with a large depletion of ionization over the equator, which is followed by an ionization enhancement at low latitude during the recovery phase of the storm. At Delhi, fast variations up to 200 m/s are also observed in the F-layer vertical upward/downward velocity, calculated using Doppler shifts, associated with the maximum negative excursion of Dst. This shows that during magnetic disturbances, the equatorial ionization anomaly (EIA) expands to a much wider latitude than the normal fountain driven by the E/F-layer dynamo electric fields. It is also observed that during the main phase of the storm, at low latitude there is generally an enhancement of F-region ionization with an increase in h'F/hmF2 but in the equatorial region, the ionization collapses with a decrease in h'F/hmF2, especially after sunset hours. In addition, at the equator the normal pre-sunset hours' enhancement in h'F is considerably suppressed during storm periods. This might be due to changes in magnitude and direction of the zonal electric field affecting the upward E×B drift and hence the plasma distribution in the form of a decrease in electron density in the equatorial region and an increase in the low latitude region. In association with disturbance electric fields, the enhanced storm-induced equatorward meridional winds in the thermosphere can also further amplify the F-layer height rise at low latitudes during the post-midnight hours, as observed in two of the storm periods.

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Liu, Jing, Donghe Zhang, Yongqiang Hao, and Zuo Xiao. "The time delay between the equatorial ionization anomaly and the equatorial electrojet in the eastern Asian and American sectors." Advances in Space Research 69, no. 1 (January 2022): 187–96. http://dx.doi.org/10.1016/j.asr.2021.10.004.

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47

Aswathy, R. P., G. Manju, and Surendra Sunda. "The Response Time of Equatorial Ionization Anomaly Crest: A Unique Precursor to the Time of Equatorial Spread F Initiation." Journal of Geophysical Research: Space Physics 123, no. 7 (July 2018): 5949–59. http://dx.doi.org/10.1029/2018ja025469.

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48

Zhao, B., W. Wan, and L. Liu. "Responses of equatorial anomaly to the October-November 2003 superstorms." Annales Geophysicae 23, no. 3 (March 30, 2005): 693–706. http://dx.doi.org/10.5194/angeo-23-693-2005.

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Abstract. The responses of Equatorial Ionization Anomaly (EIA) to the superstorms of October-November 2003 were investigated using the total electron content (TEC) measured with global positioning system (GPS) receivers in China, Southeast Asia, Australian (CSAA), and the American regions. Enhanced EIA was seen to be correlated with the southward turning of the interplanetary magnetic field Bz. In both the CSAA and American regions, EIA was intensified, corresponding to a large increase in the F-layer peak height (hmF2) measured by ionosonde and digisonde at middle and equatorial latitudes. However, the enhanced EIA was shown to be more significant during the daytime in the American region, which was associated with a series of large substorms when Bz was stable southward. The prompt penetration electric field and the wind disturbances dynamo electric field are suggested to be responsible for this observation according to current theory, although some features cannot be totally decipherable. Both the ionogram and magnetometer data show the existence of a weak shielding effect whose effect still needs further study. A clear asymmetric ionospheric response was shown in our TEC observations, even though it was only one month after autumnal equinox. The southern EIA crest was totally obliterated on 29 and 30 October in the CSAA region and on 31 October in the American region. Ion temperatures from the Defense Meteorological Satellite Program (DMSP) spacecraft revealed that the unequal energy injection at the polar region might be the reason for this effect. It is concluded that different physical processes have varying degrees of importance on the evolution of EIA in the CSAA and American regions.

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Manju, G., T. K. Pant, C. V. Devasia, S. Ravindran, and R. Sridharan. "Electrodynamical response of the Indian low-mid latitude ionosphere to the very large solar flare of 28 October 2003 – a case study." Annales Geophysicae 27, no. 10 (October 9, 2009): 3853–60. http://dx.doi.org/10.5194/angeo-27-3853-2009.

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Abstract. The electrodynamic effects on the low-mid latitude ionospheric region have been investigated using GPS (global positioning system) data, ionosonde data and ΔH values, during the very large solar flare (X17.2/4B) of 28 October 2003. The results bring out the flare induced unusual behaviour of the equatorial ionosphere on this day just prior to sunset. The important observations are i) Large and prolonged Ne enhancements observed from ionosonde data just after the flare-related peak enhancement in EUV flux. The observed enhancement in Ne is due to the increase in ionization production due to the enhanced EUV flux and the persistence of the enhancement is probably due to the prompt penetration related upliftment of the F layer (just prior to the flare peak phase) to higher altitudes, where recombination rates are lower. ii) A significant enhancement in total electron content (TEC) (~10 TEC units) at regions around the Equatorial Ionization Anomaly (EIA) crest region (Ahmedabad) during the flare in association with the flare related EUV flux enhancement. iii) Similar enhancements seen at stations of Jodhpur and Delhi in the mid latitude sector. iv)The flare related flux enhancements in different longitude sectors in the equatorial electrojet region have been shown to produce positive and negative variations in electrojet strength indicating the presence of current systems having positive and negative polarities in different longitude sectors. Thus the flare effect reveals the longitudinal variation of the counter electrojet events in the Equatorial Electrojet (EEJ) region.

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Yamazaki, Yosuke. "Quasi‐6‐Day Wave Effects on the Equatorial Ionization Anomaly Over a Solar Cycle." Journal of Geophysical Research: Space Physics 123, no. 11 (November 2018): 9881–92. http://dx.doi.org/10.1029/2018ja026014.

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