Nakata et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-018-0227-0(2018) 5:66

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open AccessObservation of equatorial plasma bubbles
by the airglow imager on ISS-IMAP
Hiroyuki Nakata1*, Akira Takahashi1 , Toshiaki Takano1 , Akinori Saito2 and Takeshi Sakanoi3

Abstract
Using 630 nm airglow data observed by an airglow imager on the International Space Station (ISS), the occurrence ofequatorial plasma bubbles (EPBs) is studied. In order to examine the physical mechanisms in the boundary region
between the Earth and the outer space, an ionosphere, mesosphere, upper atmosphere, and plasmasphere mapping(IMAP) mission had been conducted onboard the ISS since October 2012. The visible light and infrared spectrum
imager (VISI) is utilized in the ISS-IMAP mission for nadir-looking observation of the earth’s atmospheric airglow. In thisstudy, we automatically select EPBs according to the criterion for extracting the tilted dark lines from VISI data. Using
the selected events, the dependence of the occurrence rate of EPBs is examined. There is no other report of theoccurrence rate of EPBs using downward-looking visible airglow data (630 nm). In this result, the occurrence rate ishigh at all longitudes in the equinoctial seasons. In the solstice seasons, in contrast, the occurrence rate is very small
especially in the Pacific and American sectors. This result is basically consistent with previous studies, e.g., thosedetermined by plasma density data on DMSP satellites.
During the June solstice in 2013, EPBs were observed in association with geomagnetic storms that occurred due to asouthward turning of the IMF Bz. Using these events, we examined the storm-time features of the occurrence of EPBs
in the Pacific-American sectors during the June solstice. In these sectors, where the occurrence rate of EPBs is verysmall during solstice seasons, some EPBs were observed in the peak and recovery phases of the storms. This resultshows that the prompt penetration of electric fields causes the development of EPBs, in the data we analyzed, thegeomagnetic storms did not inhibit the generation of EPB in the Pacific–American sectors.
Keywords: ISS, IMAP mission, Airglow, Equatorial plasma bubble, Geomagnetic storm

Introduction
In the age of satellite-based communication, the studyof equatorial plasma bubbles (EPBs) becomes ever more
important. Radio signals from satellites scintillate and may
often become disrupted due to irregularities of various
scale sizes inside the EPBs. Therefore, since the earlystages of EPB studies, observations using radio waves havebeen utilized, including radar observation (e.g., Woodman
and LaHoz (1976); Kelley et al. (1981); Tsunoda et al.
(1981)), ionosonde (e.g., Maruyama and Matsuura (1984);
Abdu et al. (2003)), GPS scintillation (e.g., Pi et al. (1997);
Otsuka et al. (2006); Brahmanandam et al. (2012)), andtotal electron content (TEC) fluctuation (Nishioka et al.
(2008); Li et al. (2009)). Imaging of nighttime airglow has*Correspondence: nakata@faculty.chiba-u.jp
Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku,
Chiba 263-8522, Japan
Full list of author information is available at the end of the article1

also been an effective method for the study of EPBs (e.g.,
Otsuka et al. (2002); Makela and Kelley (2003); Pimenta
et al. (2003)). Since the plasma density decreases inside
EPBs, the intensity of the airglow becomes depressed.
The imaging observation from satellites are also utilized
to examine the global structure of EPBs. The IMAGE
satellite observed 135.6 nm airglow using a far-ultraviolet
imaging system (FUV). Immel et al. (2003; 2004) showedthat plasma depletions were observed by FUV data andthat the drift speed of these depletion was of the orderof 100 m/s. Similar observations were also done using
a global ultraviolet imager (GUVI) on the TIMED satellite (e.g., Kil et al. (2004); Kamalabadi et al. (2009)). In
this way, ultraviolet airglow observation is used for thesatellite-based airglow observation, although the visual
wavelengths, such as 630 nm and 777.4 nm, are used forthe ground-based observations.

© The Author(s). 2018 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 theCreative Commons license, and indicate if changes were made.

(2018) 5:66

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To study the climatological characteristics of EPBs,
in situ observations also have been conducted. Defense
Meteorological Satellite Program (DMSP) satellites track
the sun-synchronous orbits at the altitude of 840 km andprovide the in situ plasma density data. Huang et al. (2001)
showed that the occurrence rate of EPBs is maximum inthe equinoctial seasons, which is in general agreementwith most ground-based observations. Burke et al. (2004),
followed by Gentile et al. (2006), showed the seasonallongitudinal distributions of the occurrence rate of EPBs.
In their results, the occurrence rate became larger where
the difference between the declination and the terminator angles at the magnetic equator was smaller. As for thesolar activity dependence, it was clear that the occurrence
rate increased with solar activity (e.g., Gentile et al. (2006);
Xiong et al. (2010)).
As compared to the clear dependence of the occurrence rate of EPBs on long-term solar activity, the effectof geomagnetic storms on the occurrence of EPBs has
been a subject of considerable discussion. Rastogi et al.
(1981) showed that the scintillations due to EPBs in thepostsunset decreased with magnetic activity. Using DE2
data, Palmroth et al. (2000) also showed that plasma density depletions were suppressed by geomagnetic activity.
These results are consistent with the calculation results by
the TIEGCM model (Carter et al. 2014a, b). In contrast,
several recent studies showed that geomagnetic storms
enhanced the EPB occurrence (Abdu et al. 2003; Kil andPaxton 2006; Li et al. 2006; Basu et al. 2007). During
geomagnetic storms, the ionospheric electric field wasaffected by the prompt penetration electric field (e.g.,
Basu et al. (2007); Kikuchi et al. 2008) and disturbance
dynamo (Blanc and Richmond 1980). During the main
phases of the storms, the direction of the prompt penetration electric field is eastward in the dayside and dusk
hours, which enhanced the growth rate of the RayleighTaylor instability. In the recovery phase, this direction
becomes opposite due to the over-shielding effect. The
polarity of the disturbance dynamo is almost the same as
the over-shielding electric field during the recovery phase(Abdu et al. 2008). However, the effect of the disturbance
dynamo is delayed as compared to the prompt penetration electric field and last longer. The EPB occurrence
is determined by the competing effects of these electricfield.
The ionosphere, mesosphere, upper atmosphere, andplasmasphere mapping (IMAP) mission is a part of theJapanese Experiment Module (JEM) second stage planon the International Space Station (ISS). In order tomake nadir-looking spectroscopic measurement of airglows, a visible imaging spectrometer instrument (VISI)
was installed on the ISS-IMAP. The target for airglow
emissions in this study is the 630 nm band. The 630 nm
airglow is emitted by the dissociative recombination of

atomic oxygen. The emission layer of the airglow is at theheight of 250 km, and the intensity of the airglow reflects
the density of the ionospheric plasma. As compared to theother satellites, the altitude of the orbit is lower (about
400 km), which is an advantage for airglow observation inthat the attenuation of the airglow becomes smaller. Since
the orbital period and inclination of the ISS are 91 min
and 51.64°, respectively, the ISS tracks the whole earth
in 1 day. Therefore, the climatology of the EPB occurrence can be determined using VISI data. In this study, amethod for automatic determination of EPBs is used togenerate the global map of the occurrence rate of EPBs.
This study provides statistical results for the EPB occurrence using nadir-looking imaging data (630 nm), which
has been never reported. It is very important to compare
the statistical results with other observational results inorder to examine the characteristics of these data.
With the results from this global map, however, theenhancement of the EPB occurrence, which is inconsistent with previous studies, appeared. The cause of thisinconsistency, namely, the effect of geomagnetic storms,
is discussed.

Nakata et al. Progress in Earth and Planetary Science

Methods/Experimental
Data

The ISS consists of a number of modules and components. The Japan Aerospace Exploration Agency (JAXA)
is one of the participating space agencies and provides
the largest single module on the ISS, the Japanese Experiment Module (JEM), called “Kibou.” The IMAP mission
was one of the onboard missions installed in the Exposed
Facility Unit on Kibou. This mission utilized the VISI
for airglow observation. The observed wavelengths of the
VISI were 730 nm (OH Meniel band), 762 nm (O2 (0-0)),
and 630 nm (OI). To examine the occurrence of EPBs,
the 630 nm airglow data, which is a good proxy for theelectron density at an altitude of 250 km, is used in thisstudy. The VISI faced the Earth’s surface and had two
slits directed forward and backward at 45° from the nadir.
The angle of the field of view (FOV) perpendicular tothe orbit is 90°, which is 300 km in width at the altitude of 250 km (Sakanoi et al. 2011). The intensity of the
airglow is extracted by the differential of the two wavelengths (630 nm and the background). This is because
the streetlights and the reflections of moonlight on clouds
near the full moon contaminate the airglow intensity. In
this study, the EPB occurrence was examined by using
630 nm forward-slit data observed from September 2012
to April 2014. Figure 1 shows a sample of the 630 nm airglow data along the track of the ISS observed at 02 UT
on 25 September 2012. The bright region around 15° to
30° in geographic latitude is due to the equatorial ionospheric anomaly (EIA). Several dark lines are recognizedin EIA. Such dark lines in the VISI image are EPBs, inNakata et al. Progress in Earth and Planetary Science(2018) 5:66

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Fig. 1 The 630 nm airglow image along the track of the ISS observed at 02 UT on 25 September 2012

which the electron density is depressed as compared tothe background.
Detection of EPBs

Because the size of the FOV is narrower as compared tothe scale of the EPBs, the EPBs are recognized as dark lines
in the VISI data. These lines are usually tilted to the trackwith various angles as shown in Fig. 2a, which is the samedata in the airglow image shown in Fig. 1. In order to manage a large amount of image data, automatic detection of
EPBs is required. To select the EPBs from the VISI data,
we developed the following criteria. A schematic figure ofthese criteria is shown in Fig. 2c.
(i) Split a part of image into 3 ×3 bins, each bin having asize of 6 ×21/22 pixels.
(ii) Calculate the average intensity of each bin.
(iii) When the average intensities of three bins
perpendicularly adjacent to the track (the bins notedby “−” in Fig. 2c) are 10% less than the leading andtrailing bins (the bins noted by “+”), select these bins
as the occurrence of an EPB.

(iv) In calculating the average intensity, vary the gap to
the leading and trailing bins (x in Fig. 2c) from 0 to
6 pixels.
(v) To pick the tilted dark lines, tilt the shapes of thebins by x as shown in Fig. 2c. In this study, x =6n (n =1, 2, · · · 10). In selecting an EPB, the sameprocedures given in (iii) and(iv) are applied.
The width of the bin in the moving direction is 6 pixels. The actual scale of this width corresponds to 70 to
80 km, depending on the path. This means that these criteria can pick larger scale EPBs. In the original plan of the
IMAP, the VISI was designed to detect a 10% variationin the total intensity for each airglow emission assumingthat the typical total intensity of the 630 nm airglow is
100 R with no background emission and that the sensitivity of the VISI is 0.028 el/R/pixel/s (Sakanoi et al. 2011).
In the real laboratory test, in contrast, the sensitivity of
the VISI is 0.032 el/R/pixel/s. The signal
 to noise ratio
2(S/N ratio) of CCD is determined as S/ S + Nbk + Nro
where S is the number of counts for the target (airglow),Nakata et al. Progress in Earth and Planetary Science(2018) 5:66

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(a)

(b)

(c)

Fig. 2 The schematic figure of the selection of the EPB events. The top panel a shows the airglow image obtained on 25 September 2012. The
middle panel b shows the model image of the tilted EPB as shown by an arrow in the top panel. To detect this EPB, the spatial variations of the
airglow shown in the bottom panel c were sought

Nbk is the number of counts for background, and Nrois read-out noise. Because the exposure time is 1 s andthe row-column binning on CCD is 1×16 pixels, S for
100 R is 0.032×100 × 1×16 = 51.2 counts. The valueof Nro is estimated as 2√ in the laboratory test. Therefore, S/N ratio is 51.2/ 51.2 + 35.8 + 22 = 5.3 when
the total intensity is 100 R and the background intensity
is 70 R (Nbk = 0.032 × 70 × 1×16 = 35.8). In addition, the average value of the airglow in 6×21/22 pixelsof the airglow image is used to evaluate the depressionof the airglow. Therefore, the S/N ratio of the VISI islarge enough to detect the depression of the airglow due
to EPBs.
Using this criterion, we selected 413 events of EPBs
from September 2012 to April 2014. Some events that are
not EPBs were also detected using this criteria because of
the scattering of moonlight by clouds and city lights. By
checking the background images, these false detections
were removed by visual inspection. Therefore, depletions
in airglow intensity other than those due to EPBs were notNakata et al. Progress in Earth and Planetary Science(2018) 5:66

selected in this study, while some weak EPBs and the edgesof EPBs were also not selected.

Results
Occurrence rate of EPBs

Selecting EPB events based on the criteria as described in
the previous section, the occurrence rate of EPBs is determined. As the EPBs are developed around the equatorial
region, only the observation region around the equatoris considered. Since the inclination of the ISS orbit is51.64°, the ISS sweeps at this angle around the equator.
Since the airglow observation was done only in the nighttime and thus was affected by the phase of the moon,
the latitude of the observation region is dependent on
day of year. Considering this situation, we have calculated the occurrence rate of EPBs in each bin using theratio of the number of EPBs to the observation duration
where the magnetic latitude is within 30° in geographicalcoordinates.
Figure 3 shows the occurrence rate of EPBs using the
VISI airglow imager data. The horizontal and vertical axes
show geographical longitude and month, respectively. In
determining the occurrence rate, the number of EPBs andthe observation duration were counted in each bin with
15° in longitude × 10 days. Then, the occurrence rate isdetermined by the number of EPBs per hour in each bin.
As for the global occurrence distribution of EPBs, Burkeet al. (2004) and Gentile et al. (2006) determined similar

Fig. 3 The occurrence rate of EPBs observed by the VISI data

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occurrence maps using the DMSP satellites. In their studies, the EPB occurrence was determined by the fluctuations in the plasma density profiles (Huang et al. 2001).
The basic characteristics of the occurrence rate in thisstudy are almost the same as in their results: (i) the
occurrence rate is high in the equinox seasons, (ii) the
occurrence rate is highest during the December solstice in
the Atlantic sector (300–360° longitude) and African sector (0–30°), (iii) around the June solstice, the occurrence
rate is low in the Atlantic sector and Indian sector (60–150°). It is worth noting that the observation altitude of the
DMSP satellite (840 km) is higher than that of the VISI. In
contrast, the observation of EPBs in the VISI data requiresthat the background 630 nm airglow is rather bright. As
shown in Fig. 1, this condition is usually met around theEIA, whose geomagnetic latitude is about 10° to 25°. EPBs
are generated around the magnetic equator and develop
to higher altitudes. Assuming that EPBs elongate along themagnetic field lines, the highest altitude of EPBs observedby the VISI seems to be larger than 500 km in the equatorial plane. This means that the VISI observes the EPBs thathave developed to some extent. The altitude of the DMSP
satellites is 840 km, independent of latitude. These DMSP
observations also correspond to well developed EPBs.
From these facts, the seasonal and longitudinal dependence of the occurrence of EPBs determined by the VISI
is almost the same as that of DMSP satellites as shownin Burke et al. and Gentile et al. However, the difference(2018) 5:66

Nakata et al. Progress in Earth and Planetary Sciencein the observation altitudes between the DMSP and the
IMAP is reflected in the absolute value of the occurrence
rate. This will be discussed later. It is also noticed thatthe occurrence rate in the Pacific–American sector (200–300° in longitude) around the June solstice is enhancedin this study, while it is very low in the DMSP results.
As described later, it is considered that the main reason
for this difference is consecutive EPB events during geomagnetic storms. In addition, the under developed EPBs
indicated by Makela et al. (2004) may contribute to thisdifference. In the next section, we look at the relationshipbetween these consecutive events and the geomagnetic
storms. The effect of under developed EPBs will be also
discussed later.

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Table 2 A list of EPBs detected during June–July 2013
#

Date

UT

LT

GeograLat

Geomag
Lon

Lat

Lon

1

6/1

07:49:04

20:49:04

24

200

24

− 92

2

6/22

13:21:04

23:21:04

−6

155

− 13

− 131

3

6/25

21:46:42

21:46:42

−3

6

−1

78

4

6/25

21:46:51

21:46:51

−3

7

−1

79

5

6/29

03:13:08

22:13:08

− 20

280

− 10

−7

6

6/29

03:15:02

22:15:02

− 25

285

− 15

−2

7

6/29

04:45:46

21:45:46

− 19

256

− 10

− 30

8

6/29

06:19:28

22:19:28

− 22

235

− 16

− 50

9

6/29

06:21:03

22:21:03

− 26

239

− 19

− 46

10

6/29

07:49:11

21:49:11

− 13

203

− 12

− 82

EPBs during geomagnetic storms

11

6/29

10:51:58

20:51:58

−4

150

− 11

− 137

Although Solar cycle 24 is a period of weaker solar activity, many geomagnetic storms occurred in 2013 since it
was the local minimum between the peaks in 2012 and2014. In June and July 2013, six storms occurred as shownin Table 1. In this table, the geomagnetic storms whose
minimum Dst is less than − 50 nT are listed. In association with these storms, VISI observations were carriedout except for storm S2. As described previously, the dataof background emission intensity are necessary to detect
EPBs. Unfortunately, the background emissions were not
identified in storms S4 and S5. In the remaining threestorms (S1, S3, and S6), the detection of EPBs during geomagnetic storms was examined. Table 2 shows a list of
the EPBs automatically detected in June and July 2013.
Figure 4 shows the locations of these EPBs and corresponding VISI images mapped along the ISS orbits. In
Table 2, the storm-associated events are EPBs #1 and #5–
14. These events correspond to storms S1 and S3. Regarding the event associated with storm S6, EPB #15 might
be a prospective candidate. However, storm S6 started at
20 UT on July 13 just after the occurrence of EPB #15.
Therefore, there is no event associated with storm S6.
Here, we checked the events left over from the automatic

12

6/29

10:53:43

20:53:43

−9

154

− 16

− 132

13

6/30

03:58:57

21:58:57

− 22

265

− 12

− 21

14

6/30

04:00:23

22:00:23

− 26

269

− 16

− 17

15

7/13

00:27:38

03:27:38

−3

39

−7

111

16

7/21

12:51:05

00:51:05

18

181

15

109

17

7/22

07:12:50

22:12:50

− 18

230

− 12

− 55

18

7/27

11:05:50

21:05:50

−1

155

−8

− 132

19

7/29

22:02:54

22:02:54

26

0

28

77.3

Table 1 The magnetic storms that occurred during June–July
2013
#

Date

SYM-H (min.)VISI obs.

Backgrd. airglow

S1

6/1–3

− 140 nT





S2

6/7–8

− 85 nT

None

–

S3

6/28–30− 100 nT





S4

7/6–7

− 80 nT



None

S5

7/9–11− 55 nT



None

S6

7/13–15− 75 nTIn the events with checkmark in “VISI obs.”, VISI observations were carried out. The
background emissions were strong enough to detect EPBs in the events thecheckmark in “Backgrd. airglow”

selection of the EPBs. Figure 5 shows the VISI image on
15 July 2013. We recognize an event in the African sector as shown by a circle. In this event, the tip of an EPB
appeared. In the next orbit, the slight depletion of the airglow is also recognized. Including these additional events,
EPBs were excited in association with all storms in which
the observation of EPBs is feasible. Therefore, we can saythat the storms do not inhibit the occurrence of EPBs.
The events that occurred in the Pacific–American sector in association with storm S3 are shown by blue crosses
in Fig. 4. These occurrences cause the difference in theoccurrence rate between the present result and DMSP
results. Note that EPBs #5–10 (listed in Table 2) that
occurred in the Pacific-American sector were detected infour consecutive paths of the ISS. The path of the ISS
shifts westward while the EPBs usually drift eastward.
During storms, the EPBs may drift westward (e.g., Abdu
et al. (2003); Sobral et al. (2011); Santos et al. (2016)).
The longitudinal distances between the EPBs observed on
29 June are about 20° or 2200 km. If it is assumed thatthese EPBs are the same events observed in the different
paths, the drift speed of the EPBs is about 400 m/s, whichis too fast for drift speed. Therefore, it is confirmed that
EPBs #5–10 are not the same events but individual events.
Since the EPB occurrence is very low during the June solstice in the Pacific–American sector, these events are very
noticeable. In Fig. 6, the solar wind parameters (solar windNakata et al. Progress in Earth and Planetary Science(2018) 5:66

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7/29 2209
7/27 1120

6/1 0754
7/22 0729

Latitude (deg.)

7/21 1254

6/22 1310
6/25 2140

6/29 0304
6/29 0743 6/29 0610 6/29 0437
6/30 0351

6/25 2140

7/13 0029

6/1 0739

7/29 2147
7/27 1055
7/13 0000
6/29 0639
6/29 0812

6/25 2208
7/22 0701

6/30 0420
6/29 0334
7/21 1226

6/22 1338

6/25 2208

Longitude (deg.)
Fig. 4 The EPB events occurring between June and July 2013, and the associated airglow images along the ISS-tracks. The locations of EPBs areshown by crosses. Blue crosses show the events on 29 June 2013. The red and blue numbers show the start and end times of the observation ofeach track, respectively

south at 07 UT on 28 June and maintained a southward
direction until 11 UT on 29 June. During this period,
the magnetic flux on the dayside moved tailward dueto the magnetic reconnection. Then, the accumulationof the magnetic flux in the magnetotail triggered theLatitude (deg.)

speed and IMF-Bz) and the geomagnetic index (AU/AL
and SYM-H) from 28 to 30 June are shown. The determination of the SYM-H index is almost the same as thatof Dst index, although the SYM-H index is a 1 min value(Iyemori 1990). In this storm, the IMF Bz turned to the

-135

-90

-45

0

45

90

135

Longitude (deg.)
Fig. 5 The airglow images along the ISS-tracks obtained on 15 July 2013. The blue numbers indicate the end time of the observations of each track

Nakata et al. Progress in Earth and Planetary Science(2018) 5:66

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Fig. 6 The temporal variations of solar wind parameters and the geomagnetic index during 28–30 June 2013. From top to bottom, solar windspeed, IMF Bz, AU/AL index, and SYM-H index are shown. In the AU/AL index panel, the positive and negative values show the AU and AL index,
respectively. In the bottom panel, the occurrence of EPBs listed in Table 2 are shown

reconnection in the tail. The storm started after the southward turning of the IMF Bz and reached its peak around
06 UT on 29 June. Just after the peak, the storm turned
into the recovery phase. By the end of 30 June, the storm
reached almost steady state. In association with the variation of the SYM-H index, the polar regions were also
in a magnetically disturbed condition. The occurrence of

EPBs #5–10 are shown by the vertical arrow in association with the variation of the SYM-H index. These EPBs
were observed around the peak of this magnetic storm(the arrows for EPBs #5 and #6 and those for #9 and #10
overlap, respectively). In addition, EPBs were observedin the recovery phase of the storm on the next day(EPBs #13 and #14).

Nakata et al. Progress in Earth and Planetary Science(2018) 5:66

Discussion
In the previous section, EPBs were observed by the VISI
data during the geomagnetic storms with high probability.
Though there has been controversy over the relationshipbetween geomagnetic storms and the occurrence of EPBs,
many recent studies have shown that EPBs occur during geomagnetic storms. Using GPS observation, Basuet al. 2001 showed that scintillations were observed overwestern South America around the peaks of geomagnetic
storms during the equinox seasons. In the Brazilian sector,
an EPB event was initiated in the dusk sector by prompt
penetration of the magnetospheric electric field eventhough a disturbance dynamo was expected to be active(Abdu et al. 2003). Li et al. (2010) also showed that thevariations in TEC occurred over a wide longitudinal range
associated with geomagnetic storms. Combining DMSP
plasma density observations with ROCSAT-1 satellite
observations, Kil and Paxton (2006) showed that plasma
density depletions and irregularities were observed during
a geomagnetic storm. These studies show that geomagnetic storms cause the development of EPBs.
Usually, the equatorial electric field, which contributes
to the development of EPBs, may be perturbed in association with many geomagnetic indices. When the IMF
Bz is southward, the magnetic reconnection occurs in thedayside of the magnetosphere. As the magnetic fluxes aretransported to the magnetic tail, the magnetic reconnection also occurs in the magnetic tail. In this process, theelectric field entered at the polar region penetrates toward
the low latitudes. In the dusk ionosphere, this penetrating
electric field (PE) is eastward and causes the plasma bubbles to grow (e.g., Abdu et al. 2009). This enhancementof the PE is shown by both of calculation and observation. For example, the numerical simulation by Hubaet al. 2005 showed the increase in the E×B drift due toenhancement of the eastward electric field. Rice Convection Model simulations suggest the penetration of theelectric field during the growth phase of a magnetic storm(Spiro et al. 1988; Fejer et al. 1990). Jicamarca radar observations also show the enhancement of vertical drift of
the ionospheric plasma (Huang 2011). During the recovery phases of storms, in contrast, the region 2 shielding
current develops and the over-shielding effect dominates(e.g., Kelley et al. (1979); Kikuchi et al. 2008). The opposite polarity of the electric field increases in this situation.
Therefore, this implies that the electric field in the recovery phase inhibits the EPB occurrence (e.g., Huang et al.
2001; Abdu et al. 2009).
In addition to the PE, the disturbance dynamo electric field (DD) also affects the ionospheric electric fieldin the equatorial region during storms. As the polarity of
the DD electric field is almost the same as that of the PE
in the recovery phase (e.g., Richmond et al. (2003)), theeffect of the DD may suppress the EPBs in the evening orPage 9 of 13

post sunset hours. As for the appearances of both PE andDD electric fields, Maruyama et al. (2005) also calculated
the vertical E×B drift speed of the ionospheric plasma.
In their results, at the later stage of a storm, the highlatitude magnetospheric sources of ion convection andauroral precipitation affect the global neutral wind field
and the ionospheric conductivities. Then, the PE effect isreduced and the development of the DD is also affected.
This means that the electric field affected by both the PE
and DD is not the simple summation of the PE and DD
effects, and the effect of the DD cannot be estimated independently. However, it is interesting that the enhancementof the vertical drift in the pre-reversal enhancement (PRE)reaches almost the same value whether the effect of the
DD is considered or not. Surely, the appearance of theeffect of the DD must be carefully examined; however, it
is possible that the effect of DD is not large enough toinhibit the occurrence of EPBs. In addition to the effectof the electric field, the specific feature of this storm is ahighly fluctuating polar electric field as shown in the AE
index from 12UT on 28 June to 12UT on 29 June. Even
during the event on 26–27 August 1998 as shown in Abdu
et al. (2003), the AE index was quite disturbed, and EPBs
were observed in the Brazilian sector. As noted in theirstudy, sustained AE activity contributes to the generationof EPBs, which may contribute to the consecutive events
during the storm.
During storm S3, EPBs #13 and #14 shown in Table 2
occurred in the recovery phase of the storm. In the previous studies concerning EPBs in the recovery phase,the enhancement of the S4 index was observed during
postmidnight period in the African and South American region in the 2015 St. Patrick’s Day storm Carter
et al. 2016. In association with an intense storm, the density variations in the early morning were observed by the
C/NOFS satellite (Zhou et al. 2016). Comparing these previous studies, EPBs #13 and #14 were observed before
midnight, which is the typical time zone for an EPB occurrence. As previously described, EPBs observed around
premidnight during the recovery phase are rare. Therefore, the occurrence of EPBs #13 and #14 seems to beaffected by some unusual circumstances. One possibility
is the fluctuation of the IMF Bz. As shown in Fig. 6, at theoccurrence of EPBs #13 and #14, the IMF Bz was highly
fluctuating. Generally, the over-shielding effect appears
during the recovery phase of the storm due to a northward
IMF following a southward IMF. In this case where the
IMF Bz is fluctuating, it is possible that the over-shielding
effect does not appear clearly. Therefore, the enhancement of the eastward electric field still continues at somelevel, and the occurrence of the EPBs is not inhibited, evenin the recovery phase.
In the previous section, we have shown that the occurrence rate of EPBs determined by the VISI is similar to(2018) 5:66

Page 10 of 13

those determined by the DMSP satellites, although theobservation altitudes are different. However, it is possiblethat the difference in the observation altitudes between
the DMSP and the IMAP is reflected in the absolutevalue of the occurrence rates. Gentile et al. (2006), in
which the occurrence rates of EPBs were determined by
DMSP data, examined individual plasma density profiles
acquired within ± 20° with respect to the magnetic equator during all DMSP satellite passes at local times greaterthan 19:30. They examined EPB encounters in a particular pass. Subsequently, the occurrence rate of EPBs
was calculated by the ratio of the number of orbits with
EPBs divided by the total number of orbits in each bin.
As shown in Figs. 1 and 2 in Gentile et al. (2006), themaximum rates were approximately 70% and 50% aroundthe Atlantic–African sectors in 1989–1992 and 1999–
2002, respectively. On the other hand, the occurrence
rate in this study was determined by the number of EPBs
per hour. To compare the present results with those of
Gentile et al. (2006), we converted the number of EPBs perhour to EPBs per orbit. In this study, the number of EPBs
and the observation duration were counted within a stripof 60° latitude, which extended from 30°S to 30°N in geographic latitude, in a bin with 15° in longitude and 10 days
duration. The total duration of a bin corresponds to thatof the orbital element of the 60° latitude range, in hours,
multiplied by the total number of orbits during 10 days
(calculated for each of 15° longitude intervals). The occurrence rate estimated in the present result as 1.8 per hour,in fact corresponds to the total number of EPBs observedin all the orbital elements (each of 60° range in each orbit)of the bin. Therefore, the occurrence rate as shown in
Fig. 3 corresponds to the total number of EPBs observedin all the orbital elements of the bin. Each such orbital element has a duration of a fraction of the orbital period of
101 min. This fraction is 60°/360° ×101 min = 16.8 min.
1 h therefore contains 60/16.8 = 3.6 orbital elements. As
a result, the number of EPB per orbit is 1.8/3.6 = 0.5,
i.e., the occurrence rate is approximately 50%, which corresponds to the maximum rate obtained by DMSP in
1999–2002 as shown in Fig. 2 in Gentile et al. (2006). As
discussed by Gentile et al. (2006), the occurrence rate of
EPB is dependent on the sunspot number. In the periods of 1999–2002 and 2012–2014, the monthly averagesof the sunspot number were about 250 and 150, respectively. Therefore, the maximum of the occurrence rate in
the present study, which examined the data obtained in
2012–2014, would be smaller than that determined by
the DMSP. Since the observation altitude by the IMAP
(250 km) is lower than that by the DMSP (840 km), on
the other hand, it is expected that the occurrence rateof the present result would be larger than that of the
DMSP. In view of the effects of both the sunspot number and the observation altitudes, the occurrence rates ofthe present study and that determined by the DMSP arecomparable.
As for the difference of the distribution of the occurrence rate, the occurrence rate in the Pacific–American
sector (200–300° longitude) around the June solstice isenhanced in this study, while it is very low in the DMSP
results. Consecutive EPB events occur in this sector inassociation with magnetic storms during June–July in
2013. Considering the geomagnetic condition as shownin the previous section, it is believed that these consecutive events occur due to the geomagnetic storms. In
contrast, the observational results in the lower equatorial
region as shown in Makela et al. (2004) may have a relationship with the difference in the occurrence map in the
Pacific–American sector. In Makela et al. (2004), dual airglow imaging systems, Cornell All-Sky Imager (CASI) andthe Cornell Narrow-Field Imager (CNFI), are used. Thesesystems are located in Maui Island, Hawaii. The FOV of
CASI and CNFI cover the higher and lower latitudinal
regions, respectively. The CASI observes over a circularregion whose geographic latitude spreads over 13 to 28°
in geographic latitudes. The FOV of CNFI is directed tothe south with a look-angle of 17.9° elevation and covers the equatorial region with geographic latitudes of 3
to 18°. In mapping their FOVs to the magnetic equatorial plane, the altitudes of the FOV of CNFI and CASI
in the equator are 400 km to 1000 km and 700 km to2500 km, respectively. A schematic figure of the relationship of these FOV altitudes is shown in Fig. 7. In thisfigure, the emission layer is assumed to be at the altitudeof 300 km, as shown in Makela et al. (2004). Using imager
data obtained by CNFI and CASI, the occurrence rates of
EPBs in both regions are obtained. In the higher latitudinal region, the occurrence is high in the equinox seasons,which is almost the same as in the previous studies. In thelower latitudinal region, in contrast, the occurrence rateis also high in the June solstice season in addition to theequinox seasons. This result indicates that EPBs whose
altitudes in the equatorial plane are smaller than 700 km
are often generated around the June solstice. Because the
IMAP can observe the EPBs that occur around the equatorial anomaly, these smaller EPBs are observable fromthe IMAP. As shown in Table 2, the geomagnetic latitudes of several events that are considered to be excitedby the geomagnetic storms are less than 16°. Based on
the results of Makela et al. (2004), these events may occur
without geomagnetic storms. However, the latitudes forsome events, e.g., EPB #9 and #14, are larger than 16°,
implying that the geomagnetic storm surely affected thedevelopment of these EPBs as discussed previously. In thepresent study, it is possible that the enhancement of theoccurrence rate of EPBs in the Pacific–American sector
during the June solstice is due to the geomagnetic storms.
However, the number of EPBs observed by the VISI (413

Nakata et al. Progress in Earth and Planetary Science

Nakata et al. Progress in Earth and Planetary Science(2018) 5:66

CASI FOV

Page 11 of 131000 km
CNFI FOV

700 kmDMSP orbit(840 km)

507 km
400 kmemission layer(300 km)Equatorial
Anomaly

Equatorial
AnomalyFig. 7 The altitudes of EIA and FOVs of CASI and CNFI in the equatorial plane and their latitudinal locations mapping along the field lines

events) is much less than that by the DMSP (14 412 events)(Gentile et al. 2006). Because the number of EPB in eachbin is also small, the occurrence map determined in thepresent study is easily affected by additional events generated for any reasons. In order to determine the differencesin the occurrence map in detail, further longer observations of airglow are required.

Conclusions
In this study, the EPB occurrence is examined using nadirlooking 630 nm airglow data observed by the VISI imager
on the ISS-IMAP during 2013–2014. Based on the criteria
for selecting EPBs, EPBs were automatically detected. The
occurrence rate of EPBs determined in this study is consistent with previous studies. Even though several EPBs
were missed due to the automatic detection, the distribution of the occurrence rate is almost the same as in theprevious solar maximum periods. In contrast, the absolute value of the occurrence rate of EPBs in this study islarger than those determined by the DMSP satellites. Thisis explained by the difference in the observation altitudes.
Moreover, the occurrence rate determined in this study
is somewhat higher in the Pacific and American sectors
around the June solstice, which is different compared toprevious studies. The cause of this difference is the consecutive EPB events observed in association with geomagnetic storms. It is possible that the prompt penetrating
electric field, which enhances during storms, favors theoccurrence of EPBs in this sector. However, the imager
observation near the magnetic equator from Maui Island

as shown in Makela et al. (2004) showed the enhancement of the occurrence rate around the June solstice. This
result may have a relationship with the difference. Further,imager observation from space is necessary in order toexamine the difference in the occurrence rate.
Even though the criteria for EPB detection proposed inthis study select a number of events from the VISI data,some complicated events are missed. To detect EPBs moreaccurately, it is necessary to capture the intensity profile of
the airglow after removing contamination and other noise
sources, such as city lights and the reflection of the moonlight from the clouds. Further investigation of the occurrence of EPB will help the progress of projects imaging theboundary regions between the Earth and outer space.
Abbreviations
DD: Disturbance dynamo electric field; DMSP: Defense Meteorological Satellite
Program; EPB: Equatorial plasma bubble; EFU: Exposed Facility Unit; EIA:
Equatorial ionospheric anomaly; EUVI: Extra ultraviolet imager; FUV: Far
ultraviolet imager; FOV: Field of view; GUVI: Global ultraviolet imager; ISS:
International Space Station; IMAP: Ionosphere, mesosphere, upper
atmosphere, and plasmasphere mapping; JAXA: Japan Aerospace eXploration
Agency; JEM: Japan Experiment Module; MCE: Multi-mission Consolidated
Equipment; PE: Penetrating electric field; PRE: Pre-Reversal enhancement; TEC:
Total electron content; TIMED: Thermosphere ionosphere mesosphereenergetics and dynamics; VISI: Visible light and infrared spectrum imager
Acknowledgements
We thank the IMAP team and ISS staff for their extensive help in the success ofthe mission. Solar wind data from the ACE satellite and sunspot number wereobtained from NASA/GSFC OMNIWeb Plus. The AE and SYM-H values wereacquired from WDC for Geomagnetism, Kyoto, Japan.
Funding
This work was carried out by the joint research program of the Solar-Terrestrial
Environment Laboratory (reorganized to Institute for Space-EarthNakata et al. Progress in Earth and Planetary Science(2018) 5:66

Environmental Research (ISEE)), Nagoya University, and the joint research
program of CEReS, Chiba University (2014).
Availability of data and materials
ISS-IMAP data is available from AS. Solar wind data and sunspot number areavailable from NASA/GSFC OMNIWeb Plus; https://omniweb.gsfc.nasa.gov.
The AE and SYM-H values are available from WDC for Geomagnetism, Kyoto,
Japan; http://wdc.kugi.kyoto-u.ac.jp
Authors’ contributions
HN proposed the topic, conceived, and designed the study. AT carried out themain analysis in this study. TT analyzed the data and helped in theirinterpretation. AS is the principal investigator of the ISS-IMAP project andprovided the VISI imager data. ST calibrated and provided the VISI imager data.
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 Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku,
Chiba 263-8522, Japan. 2 Graduate School of Science, Kyoto University,
Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan. 3 Planetary Plasma andAtmospheric Research Center, Graduate School of Science, Tohoku University,
Aramaki-aza-aoba, Aoba-ku, Sendai 980-8578, Japan.
Received: 30 August 2017 Accepted: 8 October 2018

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