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A prolonged pattern of the ionospheric depletion in the south of the 21 August 2017 solar eclipse path
Journal Pre-proof A prolonged pattern of the ionospheric depletion in the south of the 21 August 2017 solar eclipse path Ming Guo, Na Xu, Jian Feng, Zhongxin Deng PII:
S1364-6826(20)30028-6
DOI:
https://doi.org/10.1016/j.jastp.2020.105208
Reference:
ATP 105208
To appear in:
Journal of Atmospheric and Solar-Terrestrial Physics
Received Date: 26 May 2019 Revised Date:
1 September 2019
Accepted Date: 20 January 2020
Please cite this article as: Guo, M., Xu, N., Feng, J., Deng, Z., A prolonged pattern of the ionospheric depletion in the south of the 21 August 2017 solar eclipse path, Journal of Atmospheric and SolarTerrestrial Physics (2020), doi: https://doi.org/10.1016/j.jastp.2020.105208. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
A Prolonged Pattern of the Ionospheric Depletion in the South of the 21 August 2017 Solar Eclipse Path Ming Guo1,Na Xu2, Jian Feng2, Zhongxin Deng2
1. Office of International Affairs, Wuhan University, Wuhan, Hubei, China. 2. China Research Institute of RadiowavePropagation, Qingdao, China. Corresponding author: Na Xu ([email protected])
Abstract During the American solar eclipse on 21 August 2017, the GNSS network was applied to record the ionospheric responses. Following the lunar shadow, the TEC depletion emerged over the eclipse path. After the eclipse end, the depletion in the north gradually disappeared and then the TEC enhancement emerged. The depletion in the south has continued for a long time. A sharp area between the enhancement and depletion regions appeared at 27°N latitude. Moreover, the depletion region were moving to south with larger and larger TEC decline after the eclipse maximum. The TEC over four selected places with same latitude and different eclipse obscurations is presented. Interestingly, the recovery periods of these TEC values are similar. The foF2 and hmF2 in the enhancement and depletion regions displayed very different variation tendency. The eclipse-induced thermospheric winds are considered to further reduce the TEC after the eclipse. The formation of the sharp area in TEC may be related to the fountain effect. 1
1. Introduction Formation of the earth’s ionosphere is mainly due to the solar radiation, thus the ionosphere presents great difference in different illumination conditions. Solar eclipse is a very special event that sunlight, penumbra and umbra occur successively. When the solar shadow move across the ground with the speed of sound and even supersonic speed over specific regions, not only the rapid decrease of electron density, but many other phenomena can also be observed in ionosphere. Earlier in the 1940s, the ionospheric variations during solar eclipse have been investigated by short-wave [Ledig et al., 1946; Gejer and Åkerlind, 1947]. The decrease of the electron density in ionospheric E-, F1- and F2-layes was observed and the delayed response of the F2-layer to the eclipse was also reported in the early days. As time goes on, more and more phenomena in ionosphere during a solar eclipse have been recorded and reported. The ionospheric responses to an eclipse are different with the variations of the latitude, altitude, distance to the eclipse path, obscuration, solar zenith and so on [Jakowski et al., 2008; Le et al., 2009; Bullett & Mabie, 2018; Cherniak & Zakharenkova, 2018]. The unusual cases of electron density enhancement during or after an eclipse also have been recorded [Evans, 1965;Chen et al., 2013]. As early as 1970, Chimonas and Hines [1970] have suggested that the rapid movement of the lunar shadow can produce traveling ionospheric disturbances (TIDs). The ozone layer was experimentally shown to be the major source of the gravity waves, which can propagate upward and disturb the ionosphere [Zerefos et al., 2007; Chen et al., 2015b; 2
Nayak & Yiğit, 2018]. New studies illuminated that the ionospheric bow waves in Total Electron Content (TEC) were consequences of the direct Extreme Ultraviolet (EUV) modulation [Mrak et al., 2018; Zhang et al., 2018]. On some occasion, due to the greater electron density gradient in the lunar shadow, the E-region field-aligned irregularities could be produced by gravity waves [Thampi et al., 2010; Brahmanandam et al., 2013; Chen et al, 2015a]. In a word, new phenomena were often found in the recent solar eclipses and the study of the eclipse influence on ionosphere continues deepening our understanding of the plasma dynamics over the earth. The great solar eclipse travelling across North America on 21 August 2017 provides us a very significant and rare opportunity again to investigate the ionospheric responses to eclipse. The depletion and enhancement in electron density/ TEC, as well as the TIDs during the solar eclipse have been well studied. In this paper, we mainly focus on the ionospheric variations after the eclipse. The data of Global Position System (GPS) TEC over America and two Digisondes are used to investigate the eclipse induced disturbances. We find a prolonged pattern of the ionospheric depletion in the south of the eclipse path and there is a distinct boundary on the north of the depletion region. As far as we know, this phenomenon is reported for the first time.
2. Date and Methods The solar eclipse of 21 August 2017 started from the North Pacific Ocean, traveled through the entire contiguous United States and entered the Atlantic Ocean, 3
at last ended its trip near Cape Verde off the western coast of Africa. As shown in Figure 1, the lunar shadow landed the west coast of United States at 17:16 UT on 21 August and left the east coast at 18:48 UT. Thousands of ground-based Global Navigation Satellite System (GNSS) stations and a dozen digisondes in North and South America have recorded the eclipse effects on ionosphere. The TEC data of the GNSS network covers almost all the American Continent with very high time and space resolutions. The ionogram data from the Digisondes at Idaho (Idaho National Laboratory) and Boa Vista (Instituto Nacional de Pesquisas Espaciais) are available at the Lowell GIRO Data Center, http://giro.uml.edu/ [Reinisch and Galkin, 2011]. We manually scale each ionogram to get the important ionospheric information of critical frequency and peak height of F2-layer (foF2 &hmF2) [Chen et al., 2010a; 2012]. The solar activity and magnetic conditions during 20-22 August 2017 are displayed in Figure 2. The F10.7 index (the solar radio flux at 10.7 cm) in Figure 2a was relatively quiet and there was no obvious difference between the three days. Figures 2b and 2c show the mild geomagnetic activity in the three days, with the Kp index less than 4 and the Disturbance Storm Time (DST) index larger than -25 nT [Mayaud, 1980]. Therefore, the abnormal ionospheric disturbances on 21 August should be mainly due to the solar eclipse and the ionospheric parameters on 20 and 22 August can be applied as reference. The TEC is the integration of the electron density along the ray path from a GNSS satellite to a GNSS receiver on ground. The recorded data of each receiver are converted to vertical TEC values and put on maps as colored cycles. The vertical TEC 4
maps at 17:57/ 23:27 UT on the eclipse day, control days of 20 and 22 August are shown in the first/ second row of Figure 3. The left, middle and right columns display the TEC maps recorded on 20, 21, and 22 August, respectively. Compared with the TEC on control days in Figures 3a and 3c, the TEC under the lunar shadow in Figure 3b decreased more than 5 TECU when the eclipse crossed over. As shown in Figure 3e, one and an half hour after the eclipse end, the TEC in most region between the latitudes of 0-20°N was still lower than that of the control days. To reveal the detail of the TEC variations on the eclipse day, we analyze the percentage differential TEC (dTEC) with respect to the TEC values of the control days.
3. Observations and Discussion The dTEC is estimated by the following formula, dTEC% =
× 100% (1)
where, TEC is the vertical TEC value on the eclipse day and TECm is the mean TEC value of 20 and 22 August. Figure 4 presents the percentage dTEC maps from 15:27 UT on 21 August to 00:27 UT on the next day with one-hour step. The TEC variations from 40°S to 70°N are exhibited, thus the mid- and low-latitude ionospheric responses to the eclipse can be well investigated. When the lunar shadow landed on the American continent at 17:27 UT in Figure 4c, the TEC depletion appeared around the yellow circle, which indicates center of the eclipse. As the eclipse moving to the southeast, the depletion region also extended along the eclipse path. At 19:27 UT in Figure 4e, the eclipse was close to the east coast and the depletion region has covered 5
almost the whole region between the latitudes of 10-50°N. After the eclipse ended its trip at 20:02 UT, the TEC on west and north of the depletion region began to recover and the south edge of the depletion region was gradually moving to south. As shown in Figure 4g at 21:27 UT, the TEC enhancement appeared at the head of the eclipse path. The enhancement region gradually expanded and covered the north of the United States at 23:27 UT in Figure 4i. The sharp area between the enhancement and depletion regions lied on the latitude of ~27°N. At last, the TEC anomaly disappeared after 00:27 UT on 22 August. To compare the ionospheric responses with different obscuration values and latitudes, the percentage dTEC values are arranged in Figure 5 as a function of latitude and time at different longitudes. The time resolution of the TEC values is 5 min. Though this figure is similar to Figure 5 in the letter of Cherniak and Zakharenkova, [2018], the displayed latitudes are expanded to 10°S to completely exhibit the prolonged depletion region, which emerged in the south of the eclipse path and spread southward to the equator. Several or tens of minutes after the eclipse beginning at 200-km altitude, the TEC decline emerged at most of the shown latitudes. After the eclipse end, there appeared obvious boundary at the latitude of 27°N separating the TEC enhancement area in the north and the depletion area in the south in all the five plots. The TEC of the north ionosphere rapidly increased, before long exceeded the mean values and revealed apparent enhancement. The south ionosphere has maintained lower TEC values for a long time. The post-eclipse ionospheric TEC/ electron density enhancements have been recorded and studied for more than half a 6
century [Evans, 1965a, Jakowski et al., 2008]. Usually, the downward plasma flux from plasmasphere, which induced by the rapidly decreasing temperature under the lunar shadow, is considered to increase the F-layer plasma density [Evans, 1965b; Le et al., 2009; Chen et al., 2015b; Cherniak & Zakharenkova, 2018]. Not long ago, a possible mechanism was put forward applying Global Ionosphere-Thermosphere Model (GITM) that the ionospheric enhancement after the eclipse of 21 August 2017 was due to the enhanced oxygen density driven by the horizontal winds [Wu et al., 2018]. However, the eclipse-induced enhancement is not the key point for discussion in this paper, and we focus more on the prolonged pattern of the ionospheric depletion after the great solar eclipse. In the same eclipse event, the Digisonde over Idaho, the vertical and oblique-incidence ionospheric sounding, as well as the incoherent scatter radar at Millstone Hill all recorded the ionospheric plasma quickly increased and then recovered within half an hour after the eclipse end, but not report the prolonged depletion [Reinisch et al., 2018; Bullett & Mabie, 2018; Goncharenko et al., 2018]. These observations were carried out to the north of 40oN latitude, but the prolonged TEC depletion occurred in the north of 27oN latitude. Much slower ionospheric recovery has also been reported in many other eclipse cases. During the solar eclipse of 2009 and 2010, it spent more than two hours after the eclipse end for the TEC over India recovering to common value [Choudhary et al., 2011; Kumar et al., 2013]. The Korea and Japan ionosondes found that the electron density took more than one hour to recover during the solar eclipse of 2012 [Chen et al., 2015b]. More than two hours after the solar eclipse of August 1, 2008, the electron 7
density at different heights over the Ukrainian incoherent scatter radar has recovered [Domnin, et al., 2013]. Thus, the prolonged pattern of the ionospheric eclipse depletion can occur at both mid- and low-latitudes. Compared with the depletion of other solar eclipses, the prolonged ionospheric depletion after the 2017 American solar eclipse has presented many pa rticular characteristics, including the sharp area between the TEC enhancement and depletion regions at 27oN latitude. To further study the eclipse depletion, we investigate the TEC variations at the locations [15oN, 100oW], [15oN, 90oW], [15oN, 80oW], and [15oN, 70oW], firstly. As shown in Figure 6, the TEC values in the four plots all began to decrease at the start of the eclipse and then spent more than two hours after the eclipse end to recover. Compared with the TEC values at the eclipse start, the maximum TEC reductions in the four plots of Figure 6 are 1.5, 6.0, 9.3, and 10.8 TECU, respectively, which are roughly in direct proportion to the local maximum obscurations of 0.14, 0.28, 0.46 and 0.65. Although the eclipse-induced depletions in the four selected places are of great difference, the recovery periods of the depletions are similar. This implies that there was dynamic processes working after the eclipse. The latitude dependences of the dTEC at the longitudes of 100oW, 90oW, 80oW, and 70oW at different time are displayed in Figure 7. In the initial phase of the solar eclipse, the latitudinal distribution of the dTEC were just inversely proportional to that of the obscuration. The photo ionization decrease played a major role. Soon after, the depletion valley moved southward and became lower and lower, as shown in Figures 7b, 7c, and 7d, indicating the plasma diffusion or transport might gradually become dominant after 8
the eclipse maximum. The two Digisondes at Idaho (112.7°W, 43.8°N; 50.6°N Mag. latitude) and Boa Vista (60.7°W, 2.8°N; 12.1°N Mag. latitude) were located in the TEC enhancement and depletion regions with the maximum eclipse obscuration of 0.995 and 0.398, respectively. The ionospheric parameters, such as foF2 and hmF2, of the two Digisondes are displayed and compared in Figure 8. The foF2 at Idaho in Figure 8a decreased 8-min after the eclipse start and recovered quickly after the eclipse maximum. The foF2 kept rising after the eclipse end and exceeded that of the control days at ~20:00 UT to form obvious enhancement. In the recovering phase of the peak electron density, the hmF2 in Figure 8b dropped at first and then went up. Similar to previous observations during a solar eclipse and sunset, the hmF2 began to fall off while the foF2 rebounded from its minimum, and then turned to recovery [Adeniyi et al., 2007; Nayak et al., 2012; Chen, G., et al., 2013]. Except for the following foF2 enhancement, it is a typical ionospheric responses to the decrease of solar radiation. However, the observations at Boa Vista are very different. The hmF2 in Figure 8d descended at the beginning of the eclipse. The peak height reached its bottom 25-min after the eclipse maximum. When the hmF2 started to rise, the foF2 in Figure 8c decreased. The disturbance on the peak height preceded that on the electron density. Thus, compared with the photo ionization decrease, the plasma transport or drifting may play a greater role over Boa Vista. The lunar shadow can cool the earth’s atmosphere and change the atmospheric pressure to produce wind fields [Coster et al., 2017]. The GITM shows that the 9
variations of horizontal winds were related to the ionospheric electron density enhancement after the eclipse of 21 August 2017 [Wu et al., 2018]. The Thermosphere-Ionosphere Electrodynamics General-Circulation Model (TIEGCM) simulations have predicted the TEC depletion over the Central America at 21:10 UT after the same eclipse. They found that the northward winds with the speed of 30 m/s appeared over Brazil and pushed the plasma drifting downward with the electrodynamic processes. The strong downward plasma drift is suggested to induce the TEC depletion [Lei et al., 2018]. The observations of the Boa Vista Digisonde agree with the TIEGCM simulation results. The ions descent were observed at first, and then their density began to decrease. The instantaneous and localized cooling produced large temperature gradients, which might induce the convergent horizontal winds under the eclipse shadow [Chen et al., 2010b; 2010c; Harding et al., 2018]. As the numerical simulation of Dang et al., [2018], the temperature decrease of neutral atmosphere can lead to changes of winds, which have a tendency to blow more toward the center of the eclipse path. In the south of the eclipse path, the northward winds could push the ions downward along the magnetic field lines to the lower altitudes with higher recombination rate. As a result, the ionospheric electron density/ TEC started to decrease. There was no data illuminating the distribution and variation of the thermospheric wind fields during and after the solar eclipse. However, according to the TEC variations shown in Figure 7, we speculated that the wind fields on the south of the eclipse path were significantly influenced by the rapid atmospheric temperature dropping during the eclipse maximum, which agrees with the simulated 10
global meridional wind by Dang et al., [2018]. Thus, the northward thermospheric winds may be primarily responsible for the continuously decreasing TEC after the eclipse maximum as well as the southward moving depletion region. While the lunar shadow travelled southeastward from mid-latitude to low-latitude, the plasma depletion regions at different longitudes were all observed moving to south and then the TEC enhancement appeared in the north to form a sharp area at the latitude of 27°N. It is noteworthy that the sharp area was close to the north edge of the Equatorial Ionospheric Anomaly (EIA) region. The great solar eclipse occurred over America in the midday and the fountain effect was dominating the EIA region ionosphere. The lunar shadow not only had influence on the background ionosphere, but also had the ability to weaken the fountain by reducing its electron density. In addition, as mentioned above, the thermospheric winds may further reduce the TEC in the south of the eclipse path after the eclipse. The fountain effect at noon is also considered to average the plasma loss in the EIA region. In brief, the EIA region ionosphere suffered much more plasma loss than the ionosphere in the north, thus, the sharp area was found to emerge near the north edge of the EIA region in the dTEC map. A numerical model is planned to present more details of the formation of the sharp area.
4. Conclusion A solar eclipse has significant influence on the earth from surface to terrestrial space. Based on photo ionization, ionosphere is particularly sensitive to the variation 11
of the solar radiations. However, the ionospheric disturbances during a solar eclipse are more like a mixture including many kinds of linear and non-linear processes. Except for the direct effect of the decreasing sunshine, there are complex couplings between ionosphere and thermosphere, as well as the injection of matter and energy from the upper plasmasphere and the atmosphere below. Moreover, driven by magnetic and electric fields, the ionospheric plasma under lunar shadow is also in travelling. Therefore, the ionospheric responses to a solar eclipse can last for a very long time after the eclipse end and the prominent effect can emerge in the area with much lower obscuration, or even outside the lunar shadow. After analyzing the GPS TEC and ionograms of Digisondes, we find many unusual and interesting ionospheric phenomena after the 2017 great American solar eclipse, (1) The TEC in four selected places with different eclipse obscuration coefficients and different depletions spent the similar period on recovery. (2) The depletion region was found moving to south with larger and larger TEC decline after eclipse maximum. (3) After the eclipse end, the TEC enhancement occurred in the north of the eclipse depletion region to form a sharp area at the latitude of 27°N near the north edge of the EIA region. (4) The foF2 and hmF2 on the south of the eclipse path presented very different variation tendency to those on the centerline of the eclipse path. The influence of a solar eclipse on ionosphere will not finish after the eclipse end, and the ionosphere is likely to be significantly disturbed by the aftermath of the 12
eclipse, including thermospheric winds, atmospheric waves, plasma migration, chemical imbalance and so on. More observations and simulations for the ionospheric responses during and after solar eclipse can make us further understand the complex dynamic processes in both ionosphere and space environment.
Acknowledge GPS TEC data products and access through the Madrigal distributed data system are provided to the community by the Massachusetts Institute of Technology under support from US National Science Foundation grant AGS-1242204. GPS TEC data products and Millstone Hill incoherent scatter radar data, and access through the Madrigal distributed data system are provided to the community by the Massachusetts Institute of Technology under support from US National Science Foundation grant AGS-1242204. Data for the TEC processing is provided from the following organizations: UNAVCO, Scripps Orbit and Permanent Array Center, Institut Geographique National, France, International GNSS Service, The Crustal Dynamics Data Information System (CDDIS), National Geodetic Survey, Instituto Brasileiro de Geografia e Estatística, RAMSAC CORS of Instituto Geográfico Nacional de la República Argentina, Arecibo Observatory, Low-Latitude Ionospheric Sensor Network (LISN), Topcon Positioning Systems, Inc., Canadian High Arctic Ionospheric Network, Institute of Geology and Geophysics, Chinese Academy of Sciences, China Meteorology Administration, Centro di Ricerche Sismologiche, Système d'Observation du Niveau des Eaux Littorales (SONEL), RENAG : REseau 13
NAtional GPS permanent, GeoNet - the official source of geological hazard information for New Zealand, GNSS Reference Networks, Finnish Meteorological Institute, and SWEPOS - Sweden. Data of Digisondes come from Lowell Digisonde International (http://www.digisonde.com/). The F10.7, Kp and DST index data are obtained from the Goddard Space Flight Center (GSFC)/Space Physics Data Facility (SPDF) OMNIWeb interface at http://omniweb.gsfc.nasa.gov.
Reference: Adeniyi, J. O., S. M. Radicella, I. A. Adimula, A. A. Willoughby, O. A. Oladipo, and O. Olawepo (2007), Signature of the29 March 2006 eclipse on the ionosphere over
an
equatorial
station,
J.
Geophys.
Res.,
112,
A06314,
doi:10.1029/2006JA012197. Brahmanandam, P. S., Y. H. Chu, C. L. Su, T. H. Lin (2013), Daytime E-Region Irregularities During the 22 July 2009 Solar Eclipse over Chung-Li (24.9°N, 121.2°E), a Moderate Mid-Latitude Station, Terr. Atmos. Ocean. Sci., 24, 1021-1032. Bullett, T., & Mabie, J. (2018), Vertical and oblique ionosphere sounding during the 21
August
2017
solar
eclipse.
Geophysical
Research
Letters,
45.
https://doi.org/10.1002/2018GL077413. Chen, G., Z. Zhao, G. Zhu, Y. Huang, T. Li (2010a), HF Radio-Frequency Interference Mitigation, IEEE Geoscience and Remote Sensing Letters, 7(3), 479-482. 14
Chen, G.,Z. Zhao, C. Zhou, G. Yang, Y. Zhang, (2010b), Solar eclipse effects of 22 July 2009 on Sporadic-E, ANNALES GEOPHYSICAE, 28(2), 353-357. Chen, G., Z. Zhao, G. Yang, C. Zhou, M. Yao, T. Li, S. Huang, and N. Li (2010c), Enhancement and HF Doppler observations of sporadic E during the solar eclipse
of
22
July
2009,
J.
Geophys.
Res.,
115,
A09325,
doi:10.1029/2010JA015530. Chen G.,et al.(2012), Application of the oblique ionogram as vertical ionogram, Science in China - Series E: Technological Sciences, 55(5), 1240-1244. Chen, G., et al. (2013), Nighttime ionospheric enhancements induced by the occurrence of an evening solar eclipse, J. Geophys. Res. Space Physics, 118, 6588–6596, doi:10.1002/jgra.50551. Chen, G., C. Wu, Z. Zhao, D. Zhong, H. Qi, and H. Jin (2015a), Daytime E region field-aligned irregularities observed during a solar eclipse, J. Geophys. Res. Space Physics, 119, doi:10.1002/2014JA020666. Chen, G., C. Wu, X. Huang, Z. Zhao, D. Zhong, H. Qi, L. Huang, L. Qiao, and J. Wang (2015b), Plasma flux and gravity waves in the midlatitude ionosphere during the solar eclipse of 20 May 2012, J. Geophys. Res. Space Physics, 120, 3009–3020, doi:10.1002/2014JA020849. Cherniak, I., & Zakharenkova, I. (2018). Ionospheric total electron content response to the great American solar eclipse of 21 August 2017. Geophysical Research Letters, 45, 1199–1208. https://doi.org/10.1002/2017GL075989. Chimonas, G., and C. O. Hines (1970), Atmospheric gravity waves induced by a solar 15
eclipse, J. Geophys. Res., 75(4), 875–875, doi:10.1029/JA075i004p00875. Choudhary, R. K., J.
P. St.
Maurice, K. M. Ambili, S. Sunda, and B. M. Pathan
(2011), The impact of the January 15, 2010, annular solar eclipse on the equatorial and low latitude ionospheric densities, J. Geophys. Res., 116, A09309, doi:10.1029/2011JA016504 Coster, A. J., Goncharenko, L., Zhang, S.-R., Erickson, P. J., Rideout, W., & Vierinen, J. (2017). GNSS observations of ionospheric variations during the 21 August 2017 solar eclipse. Geophysical Research Letters, 44, 12,041–12,048. https://doi.org/10.1002/2017GL075774. Dang, T., Lei, J., Wang, W., Zhang, B., Burns, A., Le, H., et al. (2018). Globalresponses of the coupled thermosphereand ionosphere system to the August2017 Great American Solar Eclipse.Journal of Geophysical Research: SpacePhysics, 123, 7040–7050. https://doi.org/10.1029/2018JA025566. Domnin, I. F., L. Ya. Yemel’yanov, D. V. Kotov, M. V. Lyashenko, L. F. Chernogor (2013), Solar Eclipse of August 1, 2008, above Kharkov: 1. Results of Incoherent Scatter Observations, Geomagnetism and Aeronomy, 53(1), 113–123. Evans, J. V. (1965a), An F region eclipse, J. Geophys. Res., 70(1), 131–142, doi.org/10.1029/JZ070i001p00131. Evans, J. V. (1965b), On the behavior of foF 2 during solar eclipses, J. Geophys. Res., 70(3), 733–738, doi:10.1029/JZ070i003p00733. Gejer, Sven and, P. Åkerlind (1947), Some experimental results obtained by ionospheric investigations in Sweden during the total solar eclipse of July 9, 16
1945,
Terr.
Magn.
Atmos.
Electr.,
52
(4),
479-491,
doi.org/10.1029/TE052i004p00479. Goncharenko, L. P., Erickson, P. J., Zhang, S.-R., Galkin, I., Coster, A. J., & Jonah, O. F. (2018). Ionospheric response to the solar eclipse of 21 August 2017 in Millstone Hill (42N) observations. Geophysical Research Letters, 45, 4601–4609. https://doi.org/10.1029/2018GL077334. Harding, B. J., Drob, D. P., Buriti, R. A., & Makela, J. J. (2018). Nightside detection of a large-scale thermospheric wave generated by a solar eclipse. Geophysical Research Letters, 45. https://doi.org/10.1002/2018GL077015 Jakowski, N., S. M. Stankov, V. Wilken, C. Borries, D. Altadill, J. Chum, D. Buresova, J. Boska, P. Sauli, F. Hruska, Lj.R. Cander (2008), Ionospheric behavior over Europe during the solar eclipse of 3 October 2005, J. Atmos. Sol.-Terr. Phys., 70(6), 836-853. Kumar, S., A. K. Singh, and R. P. Singh (2013), Ionospheric response to total solar eclipse of 22 July 2009 in Discussions different Indian regions, Ann. Geophys., 31, 1549–1558. Le, H., L. Liu, X. Yue, W. Wan, and B. Ning (2009), Latitudinal dependence of the ionospheric response to solar eclipses, J. Geophys. Res., 114, A07308, doi:10.1029/2009JA014072. Ledig, P. G., M.W. Jones, A. A. Giesecke, and E. J. Chernosky (1946), Effects on the ionosphere at Huancayo, Peru, of the solar eclipse, January 25, 1944, Terr. Magn. Atmos. Electr., 51(3), 411-418, doi:10.1029/TE051i003p00411. 17
Lei, J., Dang, T., Wang, W., Burns, A., Zhang, B., & Le, H. (2018). Long-lasting response of the global thermosphere and ionosphere to the 21 August 2017 solar eclipse. Journal of Geophysical Research: Space Physics, 123, 4309–4316. https://doi.org/10.1029/2018JA025460. Mayaud, P. N. (1980), Derivation, Meaning and Use of Geomagnetic Indices, Geophys. Monogr., vol. 22, AGU, Washington, D. C. Mrak, S., Semeter, J. L., Drob, D., & Huba, J. D. (2018). Direct EUV/X-ray modulation of the ionosphere during the August 2017 total solar eclipse. Geophysical
Research
Letters,
45,
3820–3828.
https://doi.org/10.1029/2017GL076771. Nayak, C. K., Tiwari, D., Emperumal, K., Bhattacharyya, A. (2012), The equatorial ionospheric response over Tirunelveli to the 15 January 2010 annular solar eclipse: observations, Ann. Geophys., 30, 1371–1377. Nayak, C., & Yiğit, E. (2018). GPS-TEC observation of gravity waves generated in the ionosphere during 21 August 2017 total solar eclipse. Journal of Geophysical Research: Space Physics, 123, 725–738. https://doi.org/10.1002/2017JA024845. Reinisch, B.W. and I.A. Galkin (2011), Global Ionospheric Radio Observatory (GIRO), Earth Planets Space, 63(4), 377-381, doi:10.5047/eps.2011.03.001. Reinisch, B. W., Dandenault, P. B., Galkin, I. A., Hamel, R., & Richards, P. G. (2018). Investigation of the electron density variation during the 21 August 2017 solar eclipse.
Geophysical
Research
Letters,
10.1002/2017GL076572. 18
45,
1253–1261.
https://doi.org/
Thampi, S. V., M. Yamamoto, H. Liu, S. Saito, Y. Otsuka, A. K. Patra (2010), Nighttime-like quasi periodic echoes induced by a partial solar eclipse, Geophys. Res. Lett., 37, L09107. Wu, C., Ridley, A. J., Goncharenko, L., & Chen, G. (2018). GITM-data comparisons of the depletion and enhancement during the 2017 solar eclipse. Geophysical Research Letters, 45. https://doi.org/10.1002/2018GL077409. Zerefos, C. S., E. Gerasopoulos, I. Tsagouri, B. E. Psiloglou, A. Belehaki, T. Herekakis, A. Bais, S. Kazadzis, C. Eleftheratos, N. Kalivitis, and N. Mihalopoulos (2007), Evidence of gravity waves into the atmosphere during the March 2006 total solar eclipse, Atmos. Chem. Phys., 7, 4943-4951. Zhang, S.-R., Erickson, P. J., Goncharenko, L. P., Coster, A. J., Rideout, W., & Vierinen, J. (2017). Ionospheric bow waves and perturbations induced by the 21 August 2017 solar eclipse. Geophysical Research Letters, 44, 12,067–12,073. https://doi.org/10.1002/2017GL076054.
Figure 1. Map showing the eclipse path of 21 August 2017 over America as well as 19
the eclipse obscuration parameters. The locations of the two digisondes at Idaho and Boa Vista are displayed as green circles. Red lines show the eclipse path and gray dashed line shows the magnetic equator.
Figure 2. (a) F10.7, (b) Kp, and (c) DST index between 12:00 and 23:00 UT of 20-22 August 2017.
20
Figure 3. Maps showing the TEC values at 17:57 UT (Top row) and 23:27 UT (bottom row) of the eclipse and control days. Red line shows the eclipse path and gray dashed line shows the magnetic equator. Yellow circle presents the center of the solar eclipse on ground.
21
Figure 4. Maps of percentage dTEC values from 15:27 UT on 21 August to 00:27 UT on 22 August 2017 with one-hour step. Red line shows the eclipse path and gray line shows the magnetic equator. Yellow circle presents the center of the solar eclipse on ground.
22
Figure 5. Percentage dTEC values (left column) as a function of latitude and time at the longitudes of (a) 110°W, (b) 100°W, (c) 90°W, (d) 80°W, and (e) 70°W. The left vertical axis represents the geography latitude. The eclipse start, maximum, and end times at 200-km altitude are displayed as three black curves. The greatest eclipse obscuration at different latitudes is shown in the right column. The right vertical axis 23
represents the magnetic latitude.
Figure 6. The maps showing variations of the TEC values at the locations of (a) [15oN, 100oW], (b) [15oN, 90oW], (c) [15oN, 80oW], and (d) [15oN, 70oW] on the eclipse and control days. The eclipse start, maximum, and end times at 200-km altitude are displayed as three vertical dotted lines with the labels S, M, and E. The greatest eclipse obscuration is below the label M.
24
Figure 7. Maps showing the latitude dependence of the dTEC and the eclipse obscuration at the longitudes of (a) 100oW, (b) 90oW, (c) 80oW, and (d) 70oW. Blanking on the curves indicates no data recorded at that time. The maximum eclipse obscuration in the four plots is one hundred percent and labelled on the peak of the gray curves.
25
Figure 8. Variations of the foF2 and hmF2 recorded by the Digisondes at Idaho and Boa Vista on the eclipse and control days. Blanking on the curves indicates no data recorded at that time. The eclipse start, maximum, and end times at 200-km altitude are displayed as three vertical dotted lines with the labels S, M, and E. The greatest eclipse obscuration is below the label M. The Boa Vista digisonde did not operate on 22 August, thus the data of 23 August are used to replace.
26
Hightlights:
The eclipse-induced depletion was moving to south with larger and larger TEC depletion after eclipse maximum.
A sharp area between the enhancement and depletion regions appeared at the latitude of 27° N.
The fountain effect and eclipse induced thermospheric winds may play an important role for the rare phenomena.
S1364-6826(20)30028-6
DOI:
https://doi.org/10.1016/j.jastp.2020.105208
Reference:
ATP 105208
To appear in:
Journal of Atmospheric and Solar-Terrestrial Physics
Received Date: 26 May 2019 Revised Date:
1 September 2019
Accepted Date: 20 January 2020
Please cite this article as: Guo, M., Xu, N., Feng, J., Deng, Z., A prolonged pattern of the ionospheric depletion in the south of the 21 August 2017 solar eclipse path, Journal of Atmospheric and SolarTerrestrial Physics (2020), doi: https://doi.org/10.1016/j.jastp.2020.105208. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
A Prolonged Pattern of the Ionospheric Depletion in the South of the 21 August 2017 Solar Eclipse Path Ming Guo1,Na Xu2, Jian Feng2, Zhongxin Deng2
1. Office of International Affairs, Wuhan University, Wuhan, Hubei, China. 2. China Research Institute of RadiowavePropagation, Qingdao, China. Corresponding author: Na Xu ([email protected])
Abstract During the American solar eclipse on 21 August 2017, the GNSS network was applied to record the ionospheric responses. Following the lunar shadow, the TEC depletion emerged over the eclipse path. After the eclipse end, the depletion in the north gradually disappeared and then the TEC enhancement emerged. The depletion in the south has continued for a long time. A sharp area between the enhancement and depletion regions appeared at 27°N latitude. Moreover, the depletion region were moving to south with larger and larger TEC decline after the eclipse maximum. The TEC over four selected places with same latitude and different eclipse obscurations is presented. Interestingly, the recovery periods of these TEC values are similar. The foF2 and hmF2 in the enhancement and depletion regions displayed very different variation tendency. The eclipse-induced thermospheric winds are considered to further reduce the TEC after the eclipse. The formation of the sharp area in TEC may be related to the fountain effect. 1
1. Introduction Formation of the earth’s ionosphere is mainly due to the solar radiation, thus the ionosphere presents great difference in different illumination conditions. Solar eclipse is a very special event that sunlight, penumbra and umbra occur successively. When the solar shadow move across the ground with the speed of sound and even supersonic speed over specific regions, not only the rapid decrease of electron density, but many other phenomena can also be observed in ionosphere. Earlier in the 1940s, the ionospheric variations during solar eclipse have been investigated by short-wave [Ledig et al., 1946; Gejer and Åkerlind, 1947]. The decrease of the electron density in ionospheric E-, F1- and F2-layes was observed and the delayed response of the F2-layer to the eclipse was also reported in the early days. As time goes on, more and more phenomena in ionosphere during a solar eclipse have been recorded and reported. The ionospheric responses to an eclipse are different with the variations of the latitude, altitude, distance to the eclipse path, obscuration, solar zenith and so on [Jakowski et al., 2008; Le et al., 2009; Bullett & Mabie, 2018; Cherniak & Zakharenkova, 2018]. The unusual cases of electron density enhancement during or after an eclipse also have been recorded [Evans, 1965;Chen et al., 2013]. As early as 1970, Chimonas and Hines [1970] have suggested that the rapid movement of the lunar shadow can produce traveling ionospheric disturbances (TIDs). The ozone layer was experimentally shown to be the major source of the gravity waves, which can propagate upward and disturb the ionosphere [Zerefos et al., 2007; Chen et al., 2015b; 2
Nayak & Yiğit, 2018]. New studies illuminated that the ionospheric bow waves in Total Electron Content (TEC) were consequences of the direct Extreme Ultraviolet (EUV) modulation [Mrak et al., 2018; Zhang et al., 2018]. On some occasion, due to the greater electron density gradient in the lunar shadow, the E-region field-aligned irregularities could be produced by gravity waves [Thampi et al., 2010; Brahmanandam et al., 2013; Chen et al, 2015a]. In a word, new phenomena were often found in the recent solar eclipses and the study of the eclipse influence on ionosphere continues deepening our understanding of the plasma dynamics over the earth. The great solar eclipse travelling across North America on 21 August 2017 provides us a very significant and rare opportunity again to investigate the ionospheric responses to eclipse. The depletion and enhancement in electron density/ TEC, as well as the TIDs during the solar eclipse have been well studied. In this paper, we mainly focus on the ionospheric variations after the eclipse. The data of Global Position System (GPS) TEC over America and two Digisondes are used to investigate the eclipse induced disturbances. We find a prolonged pattern of the ionospheric depletion in the south of the eclipse path and there is a distinct boundary on the north of the depletion region. As far as we know, this phenomenon is reported for the first time.
2. Date and Methods The solar eclipse of 21 August 2017 started from the North Pacific Ocean, traveled through the entire contiguous United States and entered the Atlantic Ocean, 3
at last ended its trip near Cape Verde off the western coast of Africa. As shown in Figure 1, the lunar shadow landed the west coast of United States at 17:16 UT on 21 August and left the east coast at 18:48 UT. Thousands of ground-based Global Navigation Satellite System (GNSS) stations and a dozen digisondes in North and South America have recorded the eclipse effects on ionosphere. The TEC data of the GNSS network covers almost all the American Continent with very high time and space resolutions. The ionogram data from the Digisondes at Idaho (Idaho National Laboratory) and Boa Vista (Instituto Nacional de Pesquisas Espaciais) are available at the Lowell GIRO Data Center, http://giro.uml.edu/ [Reinisch and Galkin, 2011]. We manually scale each ionogram to get the important ionospheric information of critical frequency and peak height of F2-layer (foF2 &hmF2) [Chen et al., 2010a; 2012]. The solar activity and magnetic conditions during 20-22 August 2017 are displayed in Figure 2. The F10.7 index (the solar radio flux at 10.7 cm) in Figure 2a was relatively quiet and there was no obvious difference between the three days. Figures 2b and 2c show the mild geomagnetic activity in the three days, with the Kp index less than 4 and the Disturbance Storm Time (DST) index larger than -25 nT [Mayaud, 1980]. Therefore, the abnormal ionospheric disturbances on 21 August should be mainly due to the solar eclipse and the ionospheric parameters on 20 and 22 August can be applied as reference. The TEC is the integration of the electron density along the ray path from a GNSS satellite to a GNSS receiver on ground. The recorded data of each receiver are converted to vertical TEC values and put on maps as colored cycles. The vertical TEC 4
maps at 17:57/ 23:27 UT on the eclipse day, control days of 20 and 22 August are shown in the first/ second row of Figure 3. The left, middle and right columns display the TEC maps recorded on 20, 21, and 22 August, respectively. Compared with the TEC on control days in Figures 3a and 3c, the TEC under the lunar shadow in Figure 3b decreased more than 5 TECU when the eclipse crossed over. As shown in Figure 3e, one and an half hour after the eclipse end, the TEC in most region between the latitudes of 0-20°N was still lower than that of the control days. To reveal the detail of the TEC variations on the eclipse day, we analyze the percentage differential TEC (dTEC) with respect to the TEC values of the control days.
3. Observations and Discussion The dTEC is estimated by the following formula, dTEC% =
× 100% (1)
where, TEC is the vertical TEC value on the eclipse day and TECm is the mean TEC value of 20 and 22 August. Figure 4 presents the percentage dTEC maps from 15:27 UT on 21 August to 00:27 UT on the next day with one-hour step. The TEC variations from 40°S to 70°N are exhibited, thus the mid- and low-latitude ionospheric responses to the eclipse can be well investigated. When the lunar shadow landed on the American continent at 17:27 UT in Figure 4c, the TEC depletion appeared around the yellow circle, which indicates center of the eclipse. As the eclipse moving to the southeast, the depletion region also extended along the eclipse path. At 19:27 UT in Figure 4e, the eclipse was close to the east coast and the depletion region has covered 5
almost the whole region between the latitudes of 10-50°N. After the eclipse ended its trip at 20:02 UT, the TEC on west and north of the depletion region began to recover and the south edge of the depletion region was gradually moving to south. As shown in Figure 4g at 21:27 UT, the TEC enhancement appeared at the head of the eclipse path. The enhancement region gradually expanded and covered the north of the United States at 23:27 UT in Figure 4i. The sharp area between the enhancement and depletion regions lied on the latitude of ~27°N. At last, the TEC anomaly disappeared after 00:27 UT on 22 August. To compare the ionospheric responses with different obscuration values and latitudes, the percentage dTEC values are arranged in Figure 5 as a function of latitude and time at different longitudes. The time resolution of the TEC values is 5 min. Though this figure is similar to Figure 5 in the letter of Cherniak and Zakharenkova, [2018], the displayed latitudes are expanded to 10°S to completely exhibit the prolonged depletion region, which emerged in the south of the eclipse path and spread southward to the equator. Several or tens of minutes after the eclipse beginning at 200-km altitude, the TEC decline emerged at most of the shown latitudes. After the eclipse end, there appeared obvious boundary at the latitude of 27°N separating the TEC enhancement area in the north and the depletion area in the south in all the five plots. The TEC of the north ionosphere rapidly increased, before long exceeded the mean values and revealed apparent enhancement. The south ionosphere has maintained lower TEC values for a long time. The post-eclipse ionospheric TEC/ electron density enhancements have been recorded and studied for more than half a 6
century [Evans, 1965a, Jakowski et al., 2008]. Usually, the downward plasma flux from plasmasphere, which induced by the rapidly decreasing temperature under the lunar shadow, is considered to increase the F-layer plasma density [Evans, 1965b; Le et al., 2009; Chen et al., 2015b; Cherniak & Zakharenkova, 2018]. Not long ago, a possible mechanism was put forward applying Global Ionosphere-Thermosphere Model (GITM) that the ionospheric enhancement after the eclipse of 21 August 2017 was due to the enhanced oxygen density driven by the horizontal winds [Wu et al., 2018]. However, the eclipse-induced enhancement is not the key point for discussion in this paper, and we focus more on the prolonged pattern of the ionospheric depletion after the great solar eclipse. In the same eclipse event, the Digisonde over Idaho, the vertical and oblique-incidence ionospheric sounding, as well as the incoherent scatter radar at Millstone Hill all recorded the ionospheric plasma quickly increased and then recovered within half an hour after the eclipse end, but not report the prolonged depletion [Reinisch et al., 2018; Bullett & Mabie, 2018; Goncharenko et al., 2018]. These observations were carried out to the north of 40oN latitude, but the prolonged TEC depletion occurred in the north of 27oN latitude. Much slower ionospheric recovery has also been reported in many other eclipse cases. During the solar eclipse of 2009 and 2010, it spent more than two hours after the eclipse end for the TEC over India recovering to common value [Choudhary et al., 2011; Kumar et al., 2013]. The Korea and Japan ionosondes found that the electron density took more than one hour to recover during the solar eclipse of 2012 [Chen et al., 2015b]. More than two hours after the solar eclipse of August 1, 2008, the electron 7
density at different heights over the Ukrainian incoherent scatter radar has recovered [Domnin, et al., 2013]. Thus, the prolonged pattern of the ionospheric eclipse depletion can occur at both mid- and low-latitudes. Compared with the depletion of other solar eclipses, the prolonged ionospheric depletion after the 2017 American solar eclipse has presented many pa rticular characteristics, including the sharp area between the TEC enhancement and depletion regions at 27oN latitude. To further study the eclipse depletion, we investigate the TEC variations at the locations [15oN, 100oW], [15oN, 90oW], [15oN, 80oW], and [15oN, 70oW], firstly. As shown in Figure 6, the TEC values in the four plots all began to decrease at the start of the eclipse and then spent more than two hours after the eclipse end to recover. Compared with the TEC values at the eclipse start, the maximum TEC reductions in the four plots of Figure 6 are 1.5, 6.0, 9.3, and 10.8 TECU, respectively, which are roughly in direct proportion to the local maximum obscurations of 0.14, 0.28, 0.46 and 0.65. Although the eclipse-induced depletions in the four selected places are of great difference, the recovery periods of the depletions are similar. This implies that there was dynamic processes working after the eclipse. The latitude dependences of the dTEC at the longitudes of 100oW, 90oW, 80oW, and 70oW at different time are displayed in Figure 7. In the initial phase of the solar eclipse, the latitudinal distribution of the dTEC were just inversely proportional to that of the obscuration. The photo ionization decrease played a major role. Soon after, the depletion valley moved southward and became lower and lower, as shown in Figures 7b, 7c, and 7d, indicating the plasma diffusion or transport might gradually become dominant after 8
the eclipse maximum. The two Digisondes at Idaho (112.7°W, 43.8°N; 50.6°N Mag. latitude) and Boa Vista (60.7°W, 2.8°N; 12.1°N Mag. latitude) were located in the TEC enhancement and depletion regions with the maximum eclipse obscuration of 0.995 and 0.398, respectively. The ionospheric parameters, such as foF2 and hmF2, of the two Digisondes are displayed and compared in Figure 8. The foF2 at Idaho in Figure 8a decreased 8-min after the eclipse start and recovered quickly after the eclipse maximum. The foF2 kept rising after the eclipse end and exceeded that of the control days at ~20:00 UT to form obvious enhancement. In the recovering phase of the peak electron density, the hmF2 in Figure 8b dropped at first and then went up. Similar to previous observations during a solar eclipse and sunset, the hmF2 began to fall off while the foF2 rebounded from its minimum, and then turned to recovery [Adeniyi et al., 2007; Nayak et al., 2012; Chen, G., et al., 2013]. Except for the following foF2 enhancement, it is a typical ionospheric responses to the decrease of solar radiation. However, the observations at Boa Vista are very different. The hmF2 in Figure 8d descended at the beginning of the eclipse. The peak height reached its bottom 25-min after the eclipse maximum. When the hmF2 started to rise, the foF2 in Figure 8c decreased. The disturbance on the peak height preceded that on the electron density. Thus, compared with the photo ionization decrease, the plasma transport or drifting may play a greater role over Boa Vista. The lunar shadow can cool the earth’s atmosphere and change the atmospheric pressure to produce wind fields [Coster et al., 2017]. The GITM shows that the 9
variations of horizontal winds were related to the ionospheric electron density enhancement after the eclipse of 21 August 2017 [Wu et al., 2018]. The Thermosphere-Ionosphere Electrodynamics General-Circulation Model (TIEGCM) simulations have predicted the TEC depletion over the Central America at 21:10 UT after the same eclipse. They found that the northward winds with the speed of 30 m/s appeared over Brazil and pushed the plasma drifting downward with the electrodynamic processes. The strong downward plasma drift is suggested to induce the TEC depletion [Lei et al., 2018]. The observations of the Boa Vista Digisonde agree with the TIEGCM simulation results. The ions descent were observed at first, and then their density began to decrease. The instantaneous and localized cooling produced large temperature gradients, which might induce the convergent horizontal winds under the eclipse shadow [Chen et al., 2010b; 2010c; Harding et al., 2018]. As the numerical simulation of Dang et al., [2018], the temperature decrease of neutral atmosphere can lead to changes of winds, which have a tendency to blow more toward the center of the eclipse path. In the south of the eclipse path, the northward winds could push the ions downward along the magnetic field lines to the lower altitudes with higher recombination rate. As a result, the ionospheric electron density/ TEC started to decrease. There was no data illuminating the distribution and variation of the thermospheric wind fields during and after the solar eclipse. However, according to the TEC variations shown in Figure 7, we speculated that the wind fields on the south of the eclipse path were significantly influenced by the rapid atmospheric temperature dropping during the eclipse maximum, which agrees with the simulated 10
global meridional wind by Dang et al., [2018]. Thus, the northward thermospheric winds may be primarily responsible for the continuously decreasing TEC after the eclipse maximum as well as the southward moving depletion region. While the lunar shadow travelled southeastward from mid-latitude to low-latitude, the plasma depletion regions at different longitudes were all observed moving to south and then the TEC enhancement appeared in the north to form a sharp area at the latitude of 27°N. It is noteworthy that the sharp area was close to the north edge of the Equatorial Ionospheric Anomaly (EIA) region. The great solar eclipse occurred over America in the midday and the fountain effect was dominating the EIA region ionosphere. The lunar shadow not only had influence on the background ionosphere, but also had the ability to weaken the fountain by reducing its electron density. In addition, as mentioned above, the thermospheric winds may further reduce the TEC in the south of the eclipse path after the eclipse. The fountain effect at noon is also considered to average the plasma loss in the EIA region. In brief, the EIA region ionosphere suffered much more plasma loss than the ionosphere in the north, thus, the sharp area was found to emerge near the north edge of the EIA region in the dTEC map. A numerical model is planned to present more details of the formation of the sharp area.
4. Conclusion A solar eclipse has significant influence on the earth from surface to terrestrial space. Based on photo ionization, ionosphere is particularly sensitive to the variation 11
of the solar radiations. However, the ionospheric disturbances during a solar eclipse are more like a mixture including many kinds of linear and non-linear processes. Except for the direct effect of the decreasing sunshine, there are complex couplings between ionosphere and thermosphere, as well as the injection of matter and energy from the upper plasmasphere and the atmosphere below. Moreover, driven by magnetic and electric fields, the ionospheric plasma under lunar shadow is also in travelling. Therefore, the ionospheric responses to a solar eclipse can last for a very long time after the eclipse end and the prominent effect can emerge in the area with much lower obscuration, or even outside the lunar shadow. After analyzing the GPS TEC and ionograms of Digisondes, we find many unusual and interesting ionospheric phenomena after the 2017 great American solar eclipse, (1) The TEC in four selected places with different eclipse obscuration coefficients and different depletions spent the similar period on recovery. (2) The depletion region was found moving to south with larger and larger TEC decline after eclipse maximum. (3) After the eclipse end, the TEC enhancement occurred in the north of the eclipse depletion region to form a sharp area at the latitude of 27°N near the north edge of the EIA region. (4) The foF2 and hmF2 on the south of the eclipse path presented very different variation tendency to those on the centerline of the eclipse path. The influence of a solar eclipse on ionosphere will not finish after the eclipse end, and the ionosphere is likely to be significantly disturbed by the aftermath of the 12
eclipse, including thermospheric winds, atmospheric waves, plasma migration, chemical imbalance and so on. More observations and simulations for the ionospheric responses during and after solar eclipse can make us further understand the complex dynamic processes in both ionosphere and space environment.
Acknowledge GPS TEC data products and access through the Madrigal distributed data system are provided to the community by the Massachusetts Institute of Technology under support from US National Science Foundation grant AGS-1242204. GPS TEC data products and Millstone Hill incoherent scatter radar data, and access through the Madrigal distributed data system are provided to the community by the Massachusetts Institute of Technology under support from US National Science Foundation grant AGS-1242204. Data for the TEC processing is provided from the following organizations: UNAVCO, Scripps Orbit and Permanent Array Center, Institut Geographique National, France, International GNSS Service, The Crustal Dynamics Data Information System (CDDIS), National Geodetic Survey, Instituto Brasileiro de Geografia e Estatística, RAMSAC CORS of Instituto Geográfico Nacional de la República Argentina, Arecibo Observatory, Low-Latitude Ionospheric Sensor Network (LISN), Topcon Positioning Systems, Inc., Canadian High Arctic Ionospheric Network, Institute of Geology and Geophysics, Chinese Academy of Sciences, China Meteorology Administration, Centro di Ricerche Sismologiche, Système d'Observation du Niveau des Eaux Littorales (SONEL), RENAG : REseau 13
NAtional GPS permanent, GeoNet - the official source of geological hazard information for New Zealand, GNSS Reference Networks, Finnish Meteorological Institute, and SWEPOS - Sweden. Data of Digisondes come from Lowell Digisonde International (http://www.digisonde.com/). The F10.7, Kp and DST index data are obtained from the Goddard Space Flight Center (GSFC)/Space Physics Data Facility (SPDF) OMNIWeb interface at http://omniweb.gsfc.nasa.gov.
Reference: Adeniyi, J. O., S. M. Radicella, I. A. Adimula, A. A. Willoughby, O. A. Oladipo, and O. Olawepo (2007), Signature of the29 March 2006 eclipse on the ionosphere over
an
equatorial
station,
J.
Geophys.
Res.,
112,
A06314,
doi:10.1029/2006JA012197. Brahmanandam, P. S., Y. H. Chu, C. L. Su, T. H. Lin (2013), Daytime E-Region Irregularities During the 22 July 2009 Solar Eclipse over Chung-Li (24.9°N, 121.2°E), a Moderate Mid-Latitude Station, Terr. Atmos. Ocean. Sci., 24, 1021-1032. Bullett, T., & Mabie, J. (2018), Vertical and oblique ionosphere sounding during the 21
August
2017
solar
eclipse.
Geophysical
Research
Letters,
45.
https://doi.org/10.1002/2018GL077413. Chen, G., Z. Zhao, G. Zhu, Y. Huang, T. Li (2010a), HF Radio-Frequency Interference Mitigation, IEEE Geoscience and Remote Sensing Letters, 7(3), 479-482. 14
Chen, G.,Z. Zhao, C. Zhou, G. Yang, Y. Zhang, (2010b), Solar eclipse effects of 22 July 2009 on Sporadic-E, ANNALES GEOPHYSICAE, 28(2), 353-357. Chen, G., Z. Zhao, G. Yang, C. Zhou, M. Yao, T. Li, S. Huang, and N. Li (2010c), Enhancement and HF Doppler observations of sporadic E during the solar eclipse
of
22
July
2009,
J.
Geophys.
Res.,
115,
A09325,
doi:10.1029/2010JA015530. Chen G.,et al.(2012), Application of the oblique ionogram as vertical ionogram, Science in China - Series E: Technological Sciences, 55(5), 1240-1244. Chen, G., et al. (2013), Nighttime ionospheric enhancements induced by the occurrence of an evening solar eclipse, J. Geophys. Res. Space Physics, 118, 6588–6596, doi:10.1002/jgra.50551. Chen, G., C. Wu, Z. Zhao, D. Zhong, H. Qi, and H. Jin (2015a), Daytime E region field-aligned irregularities observed during a solar eclipse, J. Geophys. Res. Space Physics, 119, doi:10.1002/2014JA020666. Chen, G., C. Wu, X. Huang, Z. Zhao, D. Zhong, H. Qi, L. Huang, L. Qiao, and J. Wang (2015b), Plasma flux and gravity waves in the midlatitude ionosphere during the solar eclipse of 20 May 2012, J. Geophys. Res. Space Physics, 120, 3009–3020, doi:10.1002/2014JA020849. Cherniak, I., & Zakharenkova, I. (2018). Ionospheric total electron content response to the great American solar eclipse of 21 August 2017. Geophysical Research Letters, 45, 1199–1208. https://doi.org/10.1002/2017GL075989. Chimonas, G., and C. O. Hines (1970), Atmospheric gravity waves induced by a solar 15
eclipse, J. Geophys. Res., 75(4), 875–875, doi:10.1029/JA075i004p00875. Choudhary, R. K., J.
P. St.
Maurice, K. M. Ambili, S. Sunda, and B. M. Pathan
(2011), The impact of the January 15, 2010, annular solar eclipse on the equatorial and low latitude ionospheric densities, J. Geophys. Res., 116, A09309, doi:10.1029/2011JA016504 Coster, A. J., Goncharenko, L., Zhang, S.-R., Erickson, P. J., Rideout, W., & Vierinen, J. (2017). GNSS observations of ionospheric variations during the 21 August 2017 solar eclipse. Geophysical Research Letters, 44, 12,041–12,048. https://doi.org/10.1002/2017GL075774. Dang, T., Lei, J., Wang, W., Zhang, B., Burns, A., Le, H., et al. (2018). Globalresponses of the coupled thermosphereand ionosphere system to the August2017 Great American Solar Eclipse.Journal of Geophysical Research: SpacePhysics, 123, 7040–7050. https://doi.org/10.1029/2018JA025566. Domnin, I. F., L. Ya. Yemel’yanov, D. V. Kotov, M. V. Lyashenko, L. F. Chernogor (2013), Solar Eclipse of August 1, 2008, above Kharkov: 1. Results of Incoherent Scatter Observations, Geomagnetism and Aeronomy, 53(1), 113–123. Evans, J. V. (1965a), An F region eclipse, J. Geophys. Res., 70(1), 131–142, doi.org/10.1029/JZ070i001p00131. Evans, J. V. (1965b), On the behavior of foF 2 during solar eclipses, J. Geophys. Res., 70(3), 733–738, doi:10.1029/JZ070i003p00733. Gejer, Sven and, P. Åkerlind (1947), Some experimental results obtained by ionospheric investigations in Sweden during the total solar eclipse of July 9, 16
1945,
Terr.
Magn.
Atmos.
Electr.,
52
(4),
479-491,
doi.org/10.1029/TE052i004p00479. Goncharenko, L. P., Erickson, P. J., Zhang, S.-R., Galkin, I., Coster, A. J., & Jonah, O. F. (2018). Ionospheric response to the solar eclipse of 21 August 2017 in Millstone Hill (42N) observations. Geophysical Research Letters, 45, 4601–4609. https://doi.org/10.1029/2018GL077334. Harding, B. J., Drob, D. P., Buriti, R. A., & Makela, J. J. (2018). Nightside detection of a large-scale thermospheric wave generated by a solar eclipse. Geophysical Research Letters, 45. https://doi.org/10.1002/2018GL077015 Jakowski, N., S. M. Stankov, V. Wilken, C. Borries, D. Altadill, J. Chum, D. Buresova, J. Boska, P. Sauli, F. Hruska, Lj.R. Cander (2008), Ionospheric behavior over Europe during the solar eclipse of 3 October 2005, J. Atmos. Sol.-Terr. Phys., 70(6), 836-853. Kumar, S., A. K. Singh, and R. P. Singh (2013), Ionospheric response to total solar eclipse of 22 July 2009 in Discussions different Indian regions, Ann. Geophys., 31, 1549–1558. Le, H., L. Liu, X. Yue, W. Wan, and B. Ning (2009), Latitudinal dependence of the ionospheric response to solar eclipses, J. Geophys. Res., 114, A07308, doi:10.1029/2009JA014072. Ledig, P. G., M.W. Jones, A. A. Giesecke, and E. J. Chernosky (1946), Effects on the ionosphere at Huancayo, Peru, of the solar eclipse, January 25, 1944, Terr. Magn. Atmos. Electr., 51(3), 411-418, doi:10.1029/TE051i003p00411. 17
Lei, J., Dang, T., Wang, W., Burns, A., Zhang, B., & Le, H. (2018). Long-lasting response of the global thermosphere and ionosphere to the 21 August 2017 solar eclipse. Journal of Geophysical Research: Space Physics, 123, 4309–4316. https://doi.org/10.1029/2018JA025460. Mayaud, P. N. (1980), Derivation, Meaning and Use of Geomagnetic Indices, Geophys. Monogr., vol. 22, AGU, Washington, D. C. Mrak, S., Semeter, J. L., Drob, D., & Huba, J. D. (2018). Direct EUV/X-ray modulation of the ionosphere during the August 2017 total solar eclipse. Geophysical
Research
Letters,
45,
3820–3828.
https://doi.org/10.1029/2017GL076771. Nayak, C. K., Tiwari, D., Emperumal, K., Bhattacharyya, A. (2012), The equatorial ionospheric response over Tirunelveli to the 15 January 2010 annular solar eclipse: observations, Ann. Geophys., 30, 1371–1377. Nayak, C., & Yiğit, E. (2018). GPS-TEC observation of gravity waves generated in the ionosphere during 21 August 2017 total solar eclipse. Journal of Geophysical Research: Space Physics, 123, 725–738. https://doi.org/10.1002/2017JA024845. Reinisch, B.W. and I.A. Galkin (2011), Global Ionospheric Radio Observatory (GIRO), Earth Planets Space, 63(4), 377-381, doi:10.5047/eps.2011.03.001. Reinisch, B. W., Dandenault, P. B., Galkin, I. A., Hamel, R., & Richards, P. G. (2018). Investigation of the electron density variation during the 21 August 2017 solar eclipse.
Geophysical
Research
Letters,
10.1002/2017GL076572. 18
45,
1253–1261.
https://doi.org/
Thampi, S. V., M. Yamamoto, H. Liu, S. Saito, Y. Otsuka, A. K. Patra (2010), Nighttime-like quasi periodic echoes induced by a partial solar eclipse, Geophys. Res. Lett., 37, L09107. Wu, C., Ridley, A. J., Goncharenko, L., & Chen, G. (2018). GITM-data comparisons of the depletion and enhancement during the 2017 solar eclipse. Geophysical Research Letters, 45. https://doi.org/10.1002/2018GL077409. Zerefos, C. S., E. Gerasopoulos, I. Tsagouri, B. E. Psiloglou, A. Belehaki, T. Herekakis, A. Bais, S. Kazadzis, C. Eleftheratos, N. Kalivitis, and N. Mihalopoulos (2007), Evidence of gravity waves into the atmosphere during the March 2006 total solar eclipse, Atmos. Chem. Phys., 7, 4943-4951. Zhang, S.-R., Erickson, P. J., Goncharenko, L. P., Coster, A. J., Rideout, W., & Vierinen, J. (2017). Ionospheric bow waves and perturbations induced by the 21 August 2017 solar eclipse. Geophysical Research Letters, 44, 12,067–12,073. https://doi.org/10.1002/2017GL076054.
Figure 1. Map showing the eclipse path of 21 August 2017 over America as well as 19
the eclipse obscuration parameters. The locations of the two digisondes at Idaho and Boa Vista are displayed as green circles. Red lines show the eclipse path and gray dashed line shows the magnetic equator.
Figure 2. (a) F10.7, (b) Kp, and (c) DST index between 12:00 and 23:00 UT of 20-22 August 2017.
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Figure 3. Maps showing the TEC values at 17:57 UT (Top row) and 23:27 UT (bottom row) of the eclipse and control days. Red line shows the eclipse path and gray dashed line shows the magnetic equator. Yellow circle presents the center of the solar eclipse on ground.
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Figure 4. Maps of percentage dTEC values from 15:27 UT on 21 August to 00:27 UT on 22 August 2017 with one-hour step. Red line shows the eclipse path and gray line shows the magnetic equator. Yellow circle presents the center of the solar eclipse on ground.
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Figure 5. Percentage dTEC values (left column) as a function of latitude and time at the longitudes of (a) 110°W, (b) 100°W, (c) 90°W, (d) 80°W, and (e) 70°W. The left vertical axis represents the geography latitude. The eclipse start, maximum, and end times at 200-km altitude are displayed as three black curves. The greatest eclipse obscuration at different latitudes is shown in the right column. The right vertical axis 23
represents the magnetic latitude.
Figure 6. The maps showing variations of the TEC values at the locations of (a) [15oN, 100oW], (b) [15oN, 90oW], (c) [15oN, 80oW], and (d) [15oN, 70oW] on the eclipse and control days. The eclipse start, maximum, and end times at 200-km altitude are displayed as three vertical dotted lines with the labels S, M, and E. The greatest eclipse obscuration is below the label M.
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Figure 7. Maps showing the latitude dependence of the dTEC and the eclipse obscuration at the longitudes of (a) 100oW, (b) 90oW, (c) 80oW, and (d) 70oW. Blanking on the curves indicates no data recorded at that time. The maximum eclipse obscuration in the four plots is one hundred percent and labelled on the peak of the gray curves.
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Figure 8. Variations of the foF2 and hmF2 recorded by the Digisondes at Idaho and Boa Vista on the eclipse and control days. Blanking on the curves indicates no data recorded at that time. The eclipse start, maximum, and end times at 200-km altitude are displayed as three vertical dotted lines with the labels S, M, and E. The greatest eclipse obscuration is below the label M. The Boa Vista digisonde did not operate on 22 August, thus the data of 23 August are used to replace.
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Hightlights:
The eclipse-induced depletion was moving to south with larger and larger TEC depletion after eclipse maximum.
A sharp area between the enhancement and depletion regions appeared at the latitude of 27° N.
The fountain effect and eclipse induced thermospheric winds may play an important role for the rare phenomena.