Chen et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-019-0263-4(2019) 6:13

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Ionospheric responses to the 21 August
2017 solar eclipse by using data
assimilation approach
Chia-Hung Chen1* , Chien-Hung Charles Lin1 and Tomoko Matsuo2

Abstract
Using the physics-based thermosphere-ionosphere model (NCAR-TIEGCM) with an ensemble Kalman filter, thisstudy reports the first data assimilative analysis of the ionosphere responses to the solar eclipse on 21 August 2017.
The system, using a 2-min assimilation cycle of data from ground-based GNSS observations, show dynamic
variations of the equatorial ionization anomaly (EIA) due to the electrodynamic effects of the solar eclipse. Two
major ionospheric responses are captured: (1) an early appearance of EIA at the westward boundary of moon
shadow and (2) an enhanced EIA at lower latitudes and suppressed EIA at the higher latitudes. These eclipseinduced conjugate EIA variations are produced by an eastward electric field perturbation around the magnetic
equator and a westward electric field perturbation at the higher latitudes.
Keywords: Ionospheric data assimilation, Solar eclipse, Equatorial ionization anomaly, GNSS, TIEGCM

Introduction
The solar eclipse’s effect on variations in ionosphericelectron density have been investigated and reported bynumerous observational and modeling methodologies(e.g., Müller-Wodarg et al. 1998; Tsai and Liu 1999;
Afraimovich et al. 2002; Sridharan et al. 2002; Tomás etal. 2007, 2008, 2009; Le et al. 2008, 2009; Choudhary et
al. 2011; St.-Maurice et al. 2011; Adekoya et al. 2015;
Panda et al. 2015; Huba and Drob 2017). The above
studies showed the ionospheric electron density reductions and the F region altitude increases during
the solar eclipses. Changes in the ionospheric electrodynamic processes during solar eclipse periods is oneof the most important drivers of these ionosphericvariations. This is because eclipse-induced electron
density changes can modify ionospheric electric conductivity and electric field distribution (Rishbeth
1968). Variation of electric fields in the ionosphere
will further affect vertical plasma (E × B) drifts and inturn strongly affect the morphology of ionosphericelectron density as well as the development of the* Correspondence: koichi@mail.ncku.edu.tw
1
Department of Earth Sciences, National Cheng Kung University, No.1,
University Road, Tainan City 701, Taiwan
Full list of author information is available at the end of the article

equatorial ionization anomaly (EIA) during the solar
eclipses (Cheng et al. 1992; Tsai and Liu 1999;
Sridharan et al. 2002; Tomás et al. 2007).
The total solar eclipse event, 21 August 2017, occurredwith a path of totality over the central USA, providing agood opportunity to investigate its effects on the photochemical and electrodynamic processes of the ionosphere.
Using the ionospheric total electron content (TEC) obtained from a dense network of Global Navigation Satellite
Systems (GNSS) receivers over North America, ionospheric TEC structures, such as the large TEC depletion(Coster et al. 2017; Cherniak and Zakharenkova 2018),
large-scale traveling ionospheric disturbances (TIDs)(Coster et al. 2017), eclipse-induced ionospheric bow
waves and gravity wave-like structures of electron density(Zhang et al. 2017; Nayak and Yiğit 2018; Sun et al. 2018),
and large-scale TEC enhancement after eclipse (Cherniak
and Zakharenkova 2018) have been reported for the 2017
August eclipse. In this paper, a three-dimensional global
assimilative system is used to investigate ionospheric variation and its mechanisms during the 2017 August solar
eclipse. We primarily focus on two ionospheric responses
to the 2017 August solar eclipse: (1) the change in theionospheric electric field system and (2) the resulting perturbations of the conjugate EIAs.

© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

Chen et al. Progress in Earth and Planetary Science(2019) 6:13

Methods/Experimental
The National Center for Atmospheric Research Thermo
sphere-Ionosphere-Electrodynamics General Circulation
Model (NCAR-TIEGCM) with the high latitude forcing specified by the empirical high-latitude ion convention model(Heelis et al. 1982) is utilized as the physical model in this assimilative system. The input parameters, F10.7 and Kp
indices, in TIEGCM are specified by realistic values
that change over time. An ensemble Kalman filter(EnKF)-based community software assimilation systemcalled the Data Assimilation Research Testbed (DART)(Anderson et al. 2009) is employed to combine the
TIEGCM and observations (c.f. Chen et al. 2016a).
Note that, in this assimilative system, the half width ofthe localization radius is set equal to 1000 km in thehorizontal direction but is not employed in the verticaldirection. The ensemble number is set to 90, and theensemble simulations are initialized by perturbing solar
F10.7 and Kp indexes via centered Gaussian distributions with a standard deviation of ± 20 (1022 Wm2 Hz1)
and ± 1 (geomagnetic quiet time Kp), respectively (cf.
Chen et al. 2016a). We calculate the ionospheric TECfrom the ground-based GNSS receivers provided byInternational GNSS Service (IGS) and further assimilate
these global TEC observations into the TIEGCM every
2 min in order to catch the rapid change of
eclipse-induced ionospheric electron density. Notedthat the time step for TIEGCM is also 2 min in thisstudy. Since the TIEGCM is a self-consistent model,
the assimilation of TEC observations into the model
will simultaneously update the unobserved model statevariables, such as neutral winds, neutral temperature,
atomic and molecular oxygen missing ratios, and atomic
oxygen ion density, according to their correlative relationships to the TEC observations (Chartier et al. 2016; Chenet al. 2016a, 2016b). These updates will further adjust thecondition of the divergence-free current system in themodel and then modify the morphology of electron density to achieve a unified system (Chen et al. 2017).
Results
Figure 1 shows the comparisons of GNSS TEC observations (a), the TIEGCM assimilation results on 20
August (one day before the eclipse) (b) and 21 August (eclipse day) (c), as well as the difference between the two-days assimilation results (d) aroundthe North and South America regions at 1730 UT(top panels), 1800 UT (middle panels), and 1830 UT(bottom panels), respectively. The assimilation resulton the eclipse day (Fig. 1c) shows the TEC reduction
around the northwest side of the maximum solar
eclipse obscuration region (white circles). The difference TEC (Fig. 1d), defined by subtracting the assimilation result 1 day before the eclipse (Fig. 1b) fromPage 2 of 9

the eclipse day (Fig. 1c), further shows the clear TEC
enhancements around EIA regions, but with the TEC
reductions around the obscuration region at eachtime frame. This eclipse-induced TEC depletion isaround − 4 TECU at 1830 UT, a roughly 40% decrease
in respect to the background TEC (Fig. 1b). The
white lines in Fig. 1d display the locations of geomagnetic field line projecting to the ground surface. The
field line starts from the location of the solar eclipse(white circle) to its conjugate location in the southernhemisphere at a given time. The results show that theeclipse-induced TEC depletions were also clearly seenat their relative conjugate locations. Since there wasno TEC observation around the conjugate locationsin the southern hemisphere (Fig. 1a), the conjugate
effect of TEC depletions was generated from themodel simulation.
The altitude-latitude variation of electron density (Ne)at 1730 UT (top panels), 1830 UT (middle panels), and1930 UT (bottom panels) along the specific longitudinalplane (around geophysical longitude of 86° W, shown in
the white line in the bottom panel of Fig. 1d) are shownin Fig. 2. The top (bottom) panel of Fig. 2c is the ionospheric state at 1 h before (after) the maximum eclipse
obscuration. The maximum eclipse obscuration arrived
at the location of 36.2° N latitude on 1830 UT, denotedby the black triangles in Fig. 2c. Compared with the assimilation result on the previous day (Fig. 2a), the assimilation analysis of EIAs on the eclipse day (Fig. 2b)shows the larger EIA peak densities in both hemispheres, except for the southern EIA peak at 1930 UT(bottom panel of Fig. 2b). The shape of the EIA analysis
at 1830 UT and 1930 UT were compressed toward themagnetic equator, showing density enhancements at theequator-side and density reductions at the poleward-side(Fig. 2c). The northern edge of this electron density reduction was just over the maximum eclipse location. It
is also clearly seen that the variations in electron density(reductions and enhancements) intensified over time.
The black lines in Fig. 2c demonstrate the latitudinalvariations of eastward electric field perturbations, defined by subtracting the electric fields (positive eastward)
on the day before the eclipse from the eclipse day, at
300 km altitude calculated in the assimilative system.
Compared with the previous day, the electric field at
1830 UT on the eclipse day was perturbed westward at
the locations greater than ~ 10° N geographic latitude
and enhanced at the locations lower than ~ 10° N geographic latitude. Both reductions and enhancements ofelectric field perturbations were further intensified at1930 UT.
In order to investigate time variations as well as thelatitudinal dependences of the ionospheric responses
to the solar eclipse, we further plot the latitude-timeChen et al. Progress in Earth and Planetary Science(2019) 6:13

Page 3 of 9

Fig. 1 Comparisons of GNSS TEC observations (a), data assimilation results on the eclipse previous day (b) and the eclipse day (c), and the TEC
difference between these 2 days (d) around the North and South America regions at 1730 UT (top panels), 1800 UT (middle panels), and 1830 UT(bottom panels), respectively. Black indicates the magnetic equator. White circle is the location of maximum obscuration of the solar eclipse. The
white line in d indicates the locations of geomagnetic field line projecting to the ground surface. The field line start from the location of thesolar eclipse (white circle) to its conjugate location in the southern hemisphereChen et al. Progress in Earth and Planetary Science(2019) 6:13

Page 4 of 9

Fig. 2 The altitude-latitude variations of electron density profile from the assimilation results on the eclipse previous day (a) and the eclipse day(b) and the difference (c) between the results of a and b, along the magnetic field line (shown in the bottom panel of Fig. 1d) at 1730 UT (toppanels), 1830 UT (middle panels), and 1930 UT (bottom panels), respectively. The black line in c indicates the electric field perturbations between
the eclipse day and the previous day

variations of ionospheric TEC along the specific longitudinal plane (same with Fig. 2) at 300 km altitude.
Compared with the assimilation result of TEC on theprevious day (Fig. 3a), the TEC assimilation analysis
in Fig. 3b shows a clear TEC reduction region above
the latitude of 36° N and around the occurrence timeof 1830 UT. Figure 3c shows this TEC reduction further moved to the lower latitude regions and became
stronger over time. Furthermore, it is also seen thatthe relative TEC reduction also appeared at the conjugate southern hemisphere. Before the arrival of thesolar eclipse at this longitude, there are TEC enhancements around 1730 UT (~ 1130 LT) at the EIA regions at the northern hemispheres, which formed theEIAs earlier than the day before the eclipse. The earlier appearance of the EIAs are also shown by comparing the global ionospheric maps (GIMs) on theeclipse day with the monthly median reference (figure
not shown).
Figure 4 shows the relative latitude-time variations ofeastward electric field perturbation along the specificlongitudinal plane (same with Figs. 2 and 3) at 300 km
altitude. From the electric fields on the day before theeclipse (Fig. 4a), it shows that the eastward electricfields appeared around the morning hours and thenreversed to the westward after the evening hours. The
assimilation analysis result on the eclipse day (Fig. 4b)further shows the enhancement of eastward electricfields at the northern mid- and low-latitude regions
around 1600 ~ 2100 UT (1000~1500 LT). Theseenhancements of electric fields also occurred at the
conjugate southern hemisphere simultaneously. Furthermore, the electric fields in Fig. 4b as well as the difference between the previous day and the eclipse dayChen et al. Progress in Earth and Planetary Science(2019) 6:13

Page 5 of 9

Fig. 3 The latitude-time variations of TEC along the magnetic field line (shown in the bottom panel of Fig. 1d) from the assimilation results on
the eclipse previous day (a) and the eclipse day (b), and the difference (c) between the TEC of a and b. The white circle indicates the latitude
and the time of the maximum obscuration of the solar eclipse(Fig. 4c) shows the mid-latitude eastward electric fieldreversing to the westward direction right after the maximum obscuration of solar eclipse and then moving to
the lower latitude region over time. After 2230 UT, themid-latitude westward electric field turned to the eastward direction again.
The eclipsed-induced thermospheric neutral windchanges will also contribute to the electric field perturbations at the ionospheric E region through the dynamo***

process ( E0¼U  B ) and then further affect the ionospheric electron density perturbations. By subtracting
the assimilative results of the previous day from those of
the eclipse day, Fig. 5b and c show the contributions ofeastward electric field perturbations at 1830 UT by theassimilative system and the dynamo process (meridionalwind) at 110 km altitude (ionospheric E region), respectively. The eastward electric field perturbations in Fig. 5bshow the enhancement at the eastern side of the maximum eclipse obscuration as well as the reduction at thewestern side. The perturbations in the Pedersen neutral
wind dynamo effect in Fig. 5c further show theenhancement at the equatorial region and the reduction
at the mid-latitude region.

Discussion
Unlike the previous studies (Le et al. 2009; Huba andDrob 2017) to simulate the ionospheric responses to thesolar eclipses by applying a solar EUV radiation mask,
this paper only assimilates the GNSS TEC observations
into the physical model without using any mask to obscure the solar EUV effect in the model. In order tocatch the rapid change of ionospheric electron density
during the solar eclipse, the ionospheric TEC observations are assimilated into the system every 2 min. The
assimilation analysis results on the eclipse day are compared with the previous day and show the TEC reductions around the maximum obscuration of the 2017
August total solar eclipse. The results obtained from theionospheric data assimilation system are consistent withprevious papers demonstrating TEC reductions by comparison of the solar eclipse day to a reference day during
the same eclipse event (Coster et al. 2017; Zhang et al.

Chen et al. Progress in Earth and Planetary Science(2019) 6:13

Page 6 of 9

Fig. 4 Same as Fig. 3 but for the eastward electric fields

2017; Cherniak and Zakharenkova 2018; Nayak and Yiğit
2018; Sun et al. 2018). Furthermore, the related neutral
temperature in the assimilation system was also decreased around the range of solar eclipse (figure notshown), which is consistent with the expectation oftemperature reduction due to the solar eclipse. This indicates that the present ionospheric data assimilation
system has the capability to improve the thermospheric
responses to the solar eclipse.
Our results show enhanced EIAs at the low-latitude regions prior to the arrival of maximum obscuration. It isfollowed by the electron density reductions in the obscuration latitudes with the EIA enhancements at the adjacent
lower latitudes. Both features appeared in the conjugate
southern hemisphere and are consistent with the conjugated responses reported by Huba and Drob (2017). The
major difference is that our assimilation results show not
only electron density reductions at the maximum obscuration latitudes but also the lower latitude EIA enhancements. The difference may come from the fact that ourassimilation system considers the effects of the thermospheric responses (temperature and winds) to the eclipse.
The ionospheric electric field effects on the electron
density variations are shown in Fig. 2. Before the arrivalof solar eclipse at 1730 UT (top panel of Fig. 2c), theelectric field had been enhanced in the eastward direction around the geophysical latitude of − 5~20° N compared with the previous day. This eastward enhanced
electric field on the eclipse day relatively uplifted theionospheric F layer and resulted in the enhancement ofelectron density around geophysical latitude 10° N. At
1830 UT, the enhancement and reduction of eastward
electric field perturbations further appeared at low- andmid-latitude regions, respectively, in the northern hemisphere. Since the electric fields at low- and mid-latitude
regions are calculated with the assumption of equal potential along the magnetic field line (Richmond et al.
1992), the feature of enhanced and reduced electric field
perturbations overall appeared along the magnetic filelines, but had the smaller perturbations in the southernhemisphere (also seen in Fig. 3c). At 1930 UT, the electric field perturbations further intensified the enhancement and the reduction of electron density at the lowand mid-latitudes, respectively.
According to the assimilation results of the electricfield on the eclipse day (Fig. 4b), it is seen that the enhanced eastward electric field before the maximum obscuration of eclipse in the northern hemisphereChen et al. Progress in Earth and Planetary Science(2019) 6:13

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Fig. 5 The spacial perturbations between the eclipse day and the previous day at 1830 UT and 110 km altitude. There are a Pedersen**

conductivity, b eastward electric field, and c meridional wind effect (U  B ) in the eastward direction

produced the TEC enhancements around the low- andmid-latitude regions due to the effect of upward plasma(E × B) drift. Later, the strong westward electric fields occurred right after the period of maximum obscuration ofsolar eclipse around the mid-latitude region. This westward electric field decreased the ionospheric electron
density during the eclipse window (Fig. 3c) by the downward movement of plasma vertical drift. The aforementioned features can also be seen in the longitude-latitude
perturbations of the electric field shown in Fig. 5b. On
the other hand, the occurrence time of maximum TEC
reduction was around 1850 UT at 36° N latitude shownin Fig. 3c, revealing a time delay of 20 min with the solar
eclipse obscuration (white circle in Fig. 3c at 1830 UT).
These results imply that the ionospheric electron density
morphology during the solar eclipse period is controlled
by vertical plasma drift, upward (eastward electric field)
at the low-latitude region and downward (westward electric field) at the mid-latitude region, respectively.
Figure 6 illustrates the possible mechanism of theabovementioned modification of electric field perturbations in the ionospheric data assimilation system. First ofall, the Moon’s shadow decreases the ionizing radiationfrom the Sun during the solar eclipse, which causes a reduction in electron concentration. Conductivity becomes
lower in the obscuration range of solar eclipse (Fig. 5a).
Therefore, the charge particles accumulate at the boundary of high and low conductivity. Due to the direction ofFig. 6 Eclipse-induced enhancement on the eastward and westward
electric fields at the low- and mid-latitudes, respectively. The reddotted line indicates the effect range of the solar eclipse. The
markers of “+” and “−” in the red color indicate positively andnegatively charged particles, respectively. The orange-dotted line
indicates the induced electric fieldChen et al. Progress in Earth and Planetary Science(2019) 6:13

electric fields at the low-latitude region, the positive (negative) charge particles accumulate at the southwestern(southeastern) boundary of the eclipse, which induces theeastward electric field at lower latitudes to further enhance the original background eastward electric fields. On
the other hand, at higher latitudes, the background westward electric fields are also enhanced by the same mechanism. The aforementioned mechanism is confirmed by
the assimilation results in Figs. 4b and 5c, showing the enhanced eastward (westward) electric field at the
low-latitude (mid-latitude) region.
Another interesting ionospheric feature during the 2017
August solar eclipse is shown in Fig. 3 is the early appearance of EIA (compared with the control run) in both hemispheres. The eclipse-induced early formation of EIA wasobserved and reported by Tsai and Liu (1999) during thesolar eclipse of 24 October 1995, showing the
pre-ascension of TEC observations before the solar eclipse.
They suggested that this pre-ascension of TEC is possibly
caused by the decrease of equatorial plasma fountainstrength, which results in the equatorward shift of the EIA
crest and the early forming of the EIA crest. However, thismechanism cannot fully explain the early EIA formation.
Richmond et al. (2015) and Richmond and Fang (2015)
proposed that an evening equatorial plasma vortex and the
pre-reversal enhancement (PRE) of the vertical drift are influenced by the distributions of conductivity in the E and F
regions in relation to the thermospheric neutral wind. The
temperature difference within and outside the moon
shadow area can produce a zonal pressure-gradient force todrive the thermospheric zonal wind. The electric field perturbations in Fig. 4 show the enhancement of the equatorial
eastward electric field at the eastern region of the solar
eclipse obscuration. This low-latitude eastward electric fieldmight result from the sudden darkness induced by the solar
eclipse at the termination region, like the eastward convection in the evening, and then further cause the early appearance of the EIA.

Conclusions
In this study, the ionospheric response during the total
solar eclipse of 21 August 2017 was investigated using thecoupled thermosphere-ionosphere data assimilation
model for the first time. By assimilation of ground-based
GNSS TEC observations on the eclipse day and the previous day, compared with their differences, the electron
density decreased up to 4 TECU (~ 40% of the background TEC on the eclipse previous day) around the solar
eclipse region at 1830 UT. The analysis of latitudinal variations of vertical electron density profiles further shows
significant enhancements and reductions in electron density at the equator-side and the polar-side of EIA crests, respectively. This enhancement of equator-side of northern(southern) EIA was driven by the upward plasma drift

Page 8 of 9from the eastward (westward) electric fields. Conversely,
the reduction of EIA crests at the two poleward-side regions were driven by the reverse direction of verticalplasma drifts (or electric fields). These two electrodynamic
processes cause the two EIA crests to be compressed toward the magnetic equator, revealing a TEC increase at
the low-latitude region and a TEC decrease at the
mid-latitude region. The morphology of low-latitude eastward and mid-latitude westward electric fields might becaused by the eclipse-induced ionospheric conductivity
changings. Moreover, we further found that the EIA crests
in both hemispheres on the eclipse day were formed earlier than that on the previous day, due to the electrodynamic processes in the wake of the eclipse. The
assimilation analysis results show stronger eastward electric fields around the magnetic equator region at the beginning of the solar eclipse (around the eastern boundaryof moon shadow), providing upward plasma (E × B) drift
to uplift the ionospheric layer and lead the earlier formingof EIA crests.
Abbreviations
DART: Data Assimilation Research Testbed; EIA: Equatorial ionization anomaly;
GNSS: Global Navigation Satellite System; TEC: Total electron content;
TIEGCM: Thermosphere-Ionosphere-Electrodynamics General Circulation
Model
Acknowledgements
The source code for the assimilation system and simulation model used inthis study, the DART and TIEGCM, are available at https://www.image.ucar.edu/DAReS/DART/ and https://www.hao.ucar.edu/
modeling/tgcm/, respectively. The authors are grateful for the NCAR High
Altitude Observatory and Data Assimilation Research Section for theirsupport of TIEGCM and DART software.
Funding
This paper is supported by Ministry of Science and Technology (MOST) andNational Space Organization (NSPO) of Taiwan to National Cheng Kung
University under MOST-107-2119-M-006-023 and NSPO-S-108002.
Availability of data and materials
The observations data from ground-based GPS receivers are available at IGS(https://garner.ucsd.edu/).
Authors’ contributions
C-HC proposed the topic, performed the data assimilation system, andcontributed the manuscript. C-HL designed the study and contributed themanuscript. TM collaborated with the corresponding author in the construction of the manuscript. All authors read and approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Author details
1
Department of Earth Sciences, National Cheng Kung University, No.1,
University Road, Tainan City 701, Taiwan. 2Ann and H.J. Smead Department
of Aerospace Engineering Sciences, University of Colorado, Boulder, CO
80309-0429, USA.

Chen et al. Progress in Earth and Planetary Science(2019) 6:13

Received: 11 July 2018 Accepted: 21 January 2019

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