Ajith et al. Progress in Earth and Planetary Science (2018) 5:60
https://doi.org/10.1186/s40645-018-0218-1

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Unseasonal development of post-sunset Fregion irregularities over Southeast Asia on
28 July 2014: 2. Forcing from below?
K. K. Ajith1, S. Tulasi Ram1* , B. A. Carter2, S. Sathish Kumar3, M. Yamamoto4, T. Yokoyama5, S. Gurubaran1,
S. Sripathi1, K. Hozumi5, K. Groves6 and R. G. Caton7

Abstract
An unseasonal development of Equatorial Plasma Bubble (EPB)/Equatorial Spread-F (ESF) activity in a wide longitudinal
sector over India and Southeast Asia during the post-sunset hours of 28 July 2014 has been investigated in detail using
the multiple independent observations in this paper and a companion paper by (Prog Earth Planet Sci 5:10, 2018). This
post-sunset ESF/EPB event was preceded by a substantial elevation of the equatorial F-layer due to strong pre-reversal
enhancement (PRE) during the season which is climatologically unfavorable for strong PRE and post-sunset EPBs. In
this paper, it is found that a strong equatorward meridional neutral wind at off-equatorial E-region and enhanced
longitudinal gradient of equatorial electrojet appear to be the responsible candidates for the enhanced PRE and EPB
development on 28 July 2014. These changes in the local wind system and PRE are found to have an apparent
connection with the planetary wave forces of lower atmospheric origin. Both the post-sunset height rise (PSSR) andlower thermospheric meridional neutral wind consistently exhibit quasi-2-day planetary wave-like oscillations during
20–31 July 2014 with maximum amplitudes on 28 July 2014. This study emphasizes the role of planetary wave forcesfrom the lower atmosphere that can alter the local neutral wind system and E-region conductivities that may lead tothe development of strong PRE and EPB activity which were otherwise climatologically unfavorable for post-sunset
EPB/ESF development.
Keywords: Equatorial plasma bubbles, Pre-reversal enhancement, Neutral winds

Introduction
The development of Equatorial Spread-F (ESF)/Equatorial
Plasma Bubble (EPB) irregularities in the nighttime equatorial ionosphere has been studied for several decades
since it was first reported by Booker and Wells (1938).
From the detailed studies by several researchers, thegeneral morphological features of EPBs such as the occurrence variability with local time, season, latitude, longitude, solar cycle, and geomagnetic activity were more orless thoroughly understood (Abdu et al. 1981; Kelley and
McClure 1981; Tsunoda 1985; Rama Rao et al. 2005;
Tulasi Ram et al. 2008; Kumar et al. 2016). However, theanomalous day-to-day variability in the occurrence of
EPBs still remains to puzzle the researchers over the years.

* Correspondence: s.tulasiram@gmail.com
1
Indian Institute of Geomagnetism, Navi Mumbai, India
Full list of author information is available at the end of the article

Because of their potential impact on trans-ionosphericradio communications and satellite-based navigation
systems, EPBs have drawn wide attention and become oneof the important topics of space weather interest today(Kelley et al. 2011; Woodman 2009).
The ESF/EPB irregularities generally initiate at the bottom side F-layer through the Rayleigh-Taylor (RT) instability (Haerendel 1973) and grow nonlinearly to thetopside ionosphere via polarization electric fields withinthem. The pre-reversal enhancement (PRE) of the equatorial zonal electric field and steep vertical density
gradient due to bottom side recombination sets the conditions most favorable for the development of EPBs during the post-sunset hours (Farley et al. 1986; Tulasi Ram
et al. 2006). Further, the spatial large-scale wave structure (LSWS) at the base of F-layer is believed to be thepotential seed for the development of EPBs and© 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
the Creative Commons license, and indicate if changes were made.

Ajith et al. Progress in Earth and Planetary Science (2018) 5:60

determines the location of bubble onset (Tsunoda andWhite 1981; Tsunoda 2005; Tsunoda et al. 2011; Tulasi
Ram et al. 2012, 2014). Besides the PRE and initial seeding by LSWS, the symmetrical plasma distribution in the
conjugate hemispheres and meridional neutral winds
also play significant roles for determining the onset of
EPBs (Maruyama and Matuura 1984; Maruyama 1988).
Among the above necessary favorable conditions, thepost-sunset height rise (PSSR) of equatorial F-layer via
the PRE is known to be the most important factor. It isalso often shown that the EPBs would always develop
when the equatorial F-layer is elevated to sufficiently
higher altitudes (Farley et al. 1970; Rastogi 1980; Kelley
and Maruyama 1992; Fejer et al. 1999; Anderson et al.
2004; Tulasi Ram et al. 2006, 2007; Li et al. 2007; Huang
and Hairston 2015). However, the strength of the PRE(and PSSR) itself exhibits significant day-to-day variability (Scherliess and Fejer 1999), thereby leaving the early
prediction of EPBs far from the reality. The PRE is generally large during the seasons when the sunset terminator is closely aligned with the local magnetic meridian(Abdu et al. 1981; Tsunoda 1985). The close alignmentof the sunset terminator with the magnetic meridian
causes simultaneous decay of E-region conductivity atboth ends of the field line and maximizes the strength of
the PRE. Therefore, over Indian and Southeast Asianlongitudes where the magnetic declination is close tozero, the PRE strength maximizes during equinoxes andbecomes weaker during solstices (e.g., Tulasi Ram et al.
2006). As a result, the occurrence of post-sunset EPBs
maximizes during the equinoxes and becomes lowest
during the solstices, particularly during the June solstices
over Indian and Southeast Asian sector (e.g., Retterer
and Gentile 2009; Otsuka et al. 2009; Ajith et al. 2015).
There can be another class of EPBs that develop aroundpost-midnight hours of June solstices over Indian andSoutheast Asian sector which have different characteristics compared to post-sunset EPBs (Otsuka andOgawa 2009; Dao et al. 2016; Ajith et al. 2016). However, the present investigation is focused only on the
post-sunset EPBs that are associated with PRE andPSSR.
In contrast to the generally known morphology, thePRE may be occasionally suppressed during equinoxes
and, conversely, can be enhanced during solstices, thereby
instituting enigmatic day-to-day variability even underquiet geomagnetic conditions. Such unseasonal cases ofPRE suppression during equinoxes and enhancement during solstices are difficult to predict. Despite some recent
success in using coupled thermosphere-ionosphere modeling to describe day-to-day EPB variability during equinox periods (e.g., Carter et al. 2014a, 2014b, 2014c), EPB
occurrence during solstices is not well predicted by either
theoretical or climatological models, and even by the more

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sophisticated self-consistent Thermosphere-IonosphereElectrodynamics General Circulation Model (TIEGCM)(e.g. Carter et al. 2014c). Investigating such unseasonalcases in greater detail can advance our understanding on
the possible factors responsible for the day-to-day variability in the PRE and EPB occurrence. In the present paper(and in a companion paper by Carter et al. 2018), we report and investigate an interesting case of unseasonal enhancement of PRE and development of EPBs during thepost-sunset hours on July 28, 2014, over a wide sector ofIndian and Southeast Asian longitudes.

Methods/Experimental
An interesting unseasonal EPB event on July 28, 2014,
and the responsible factors have been investigated using
a suite of ground-based observations, such as Equatorial
Atmosphere Radar (EAR), magnetometers, ionosondes,
MF radar, and VHF scintillation observations spanningover Indian and Southeast Asian longitudes. Figure 1
shows the locations of various observatories consideredin the present study. The 47-MHz Equatorial Atmosphere Radar (EAR) in Kototabang, Indonesia (0.20° S,
100.32° E, 10.36° S dip latitude) is routinely operated
with a peak and average powers of 100 kW and 5 kW,
respectively, to study the F-region field-aligned irregularities (FAIs) at ~ 3 m scale size during night times (1800–
0600 LT). The EAR is regularly operated with two setsof 8 beams (total 16 beams) in F-region mode that sequentially scans the beam directions on a pulse-to-pulse
basis (Fukao et al. 2003). In the present study, the backscatter echoes and radial Doppler velocities obtained
mainly from the central (azimuth = 180°) beam of theEAR were considered. The background ionospheric conditions such as post-sunset height rise (PSSR) and theoccurrence of ESF over Indian and Southeast Asian longitudes were examined by three ground-based ionosondes at Tirunelveli, India (8.7° N, 77.7° E, 1.67° N dip);
Bac Lieu, Vietnam (9.29° N, 105.71° E, 1.5° N dip latitude); and Sanya, China (18.4° N, 109.6°E, 12.8° N dip
latitude). Further, the VHF scintillation observations
from low-latitude stations Kolkata (previously known as
Calcutta), India (22.58° N, 88.38° E, 13.19° N dip latitude) and Chung-Li, Taiwan (22.5° N, 121.4° E, 12.8° N
dip latitude) were considered to examine the VHF scintillation activity on 28 July 2014. The role of the equatorial electrojet (EEJ) current on the PRE is examinedusing the three magnetometers located at equatorial stations, Addis Ababa (8.98° N, 38.76° E, 0.5° S dip latitude), Tirunelveli (8.7° N, 77.7° E, 0.9° N dip latitude),and Phuket (7.89° N, 98.39° E, 0.15° S dip latitude). Also,
the possible role of the equatorward meridional wind on
the PRE on July 28, 2014, is examined using hourly neutral wind data obtained the medium frequency (2 MHz)partial reflection (PR) radar at Kolhapur (16.7° N, 74.3°

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Fig. 1 The geographic locations of different instruments used in the present study. The blue line indicates the geomagnetic equator

E, 11.5° N dip latitude). This radar operates in a spaced
antenna mode and determines the wind field by a fullcorrelation analysis (Gurubaran and Rajaram 1999).

Results
Figure 2a shows the altitude-time-intensity (ATI) map ofbackscatter echoes from the field-aligned irregularities(FAIs) at ~ 3 m scale size observed from the central
beam (azimuth = 180°) of the EAR on 28 July 2014 over
Kototabang, Indonesia. The corresponding Doppler velocity obtained from single-beam F-region Doppler mode
observations from the central beam of the EAR is shownin Fig. 2b. From Fig. 2a, b, one can observe the strong
plume-like structures between ~ 1245 and 1645 UT (~1945 to 2345 LT) on 28 July 2014. These FAI echoes arevery intense (> 30 dB) and have risen beyond the maximum range of EAR (~ 510 km) in this measurement
mode. Further, these FAI echoes exhibit strong upward
drift (negative Doppler velocity) with velocities greaterthan 100 m/s (Fig. 2b) during ~ 1245 to 1430 UT.
Hence, the results from Fig. 2a, b indicate that these FAI
echoes are from active and upwelling plume-like EPBs
developed over EAR longitudes. Later around 1830 UT(0130 LT), another small EPB echo is observed during
the post-midnight hours. From the fan sector backscatter maps (not shown in the figure) constructed from the
16 beam observations of EAR (Ajith et al. 2015), it is further confirmed that the observed post-sunset EPBs on
28 July 2014 are freshly developed over EAR longitudes,
and the small post-midnight EPB around 1830 UT isdrifting into the field of view (FoV) of EAR from thewestern longitudes.
The ESF/EPB activity on this day (28 July 2014) has
also been observed as range Spread-F echoes fromionosondes at Tirunelveli, Bac Lieu, and Sanya which
are spread over Indian and Southeast Asian longitudes.
Carter et al. (2018), in the companion paper, have also
shown that the scintillation activity at the GPS-L1 frequency is widespread over Indian and Southeast Asianlongitudes on 28 July 2014 using ground-based GPS and
COSMIC/Formosat-3 Radio Occultation observations.
In this study, we further examined the VHF scintillation(~ 250 MHz) observations from the Scintillation Network Decision Aid (SCINDA) network (Groves et al.
1997). For example, Fig. 3a, b shows the S4-index as afunction of universal time on the night of 28 July 2014
from Kolkata, India (22.58° N, 88.38° E, 13.19° N dip
latitude) and Chung-Li, Taiwan (22.5° N, 121.4° E,
12.8° N dip latitude), respectively. The geographic locations of these stations can be observed in Fig. 1. It
can be observed in Fig. 3a, b that the strong VHF
scintillation occurred at both Kolkata and Chung-Li
indicating that the EPBs were developed over a wide
sector over Indian and Southeast Asian longitudes(77° E to 121° E) on that night.
Figure 4 shows the stacked ATI plots of backscatter
FAI echoes observed from the EAR for all days in themonth of July 2014. The white portions in Fig. 4 indicate
lack of data (no observation). It can be seen from thisfigure that the EPB activity over EAR location is completely absent in the month of July, except on 28 July.

Ajith et al. Progress in Earth and Planetary Science (2018) 5:60

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Fig. 2 a Altitude-time-intensity (ATI) and b altitude-time-velocity (ATV) maps of F-region field-aligned irregularities (FAIs) observed by central
beam (azimuth = 180°) of Equatorial Atmosphere Radar (EAR) during 1100 to 2300 UT on 28 July 2014

Also, no significant VHF scintillations at Kolkata andChung-Li were observed during the post-sunset hours of
this month, except on this event day (not shown in the
figure). Therefore, this is a unique case of enhanced EPB
activity on 28 July 2014 over Indian and Southeast Asianlongitudes, where the PRE is generally weak/absent during July and not favorable for the development of
post-sunset EPBs.
Now, in effort to examine the background PRE conditions on this day, we analyzed the F-layer base height (h′F) observed from two equatorial ionosondes at Bac Lieu,
Vietnam, and Tirunelveli, India. Figure 5a, b shows thenighttime h′F variations from the ionosondes at two
equatorial stations, Bac Lieu and Tirunelveli, respectively. In Fig. 1, it is clear that both stations are locatedclose to the magnetic equator and separated by ~ 28° in
longitude. The red curves in Fig. 5 show the h′F variations on 28 July 2014, and the blue curves with error
bars represent the monthly mean h′F variations of July
2014 excluding 28 July. The gray dots indicate the variation of h′F values on individual days during July 2014at 10 min interval. The horizontal line in light blue color
in Fig. 5a, b indicates the occurrences and the durationsof Spread-F observed at the respective stations on 28
July. Note that the scaling of h′F from ionosondes underintense Spread-F conditions can be ambiguous, so the h′
F variations on 28 July (red curves) were not shown after
the onset of Spread-F. It can be seen in Fig. 5a that theequatorial F-layer on 28 July exhibits a significant
post-sunset height rise (PSSR) over Bac Lieu andreaches to a maximum altitude of ~ 380 km at 1200
UT (~ 1900 LT). Similarly, h′F over Tirunelveli alsorose to an altitude of ~ 440 km at 1450 UT (~ 1950
LT) during the post-sunset hours (Fig. 5b). The large
PSSR at both Bac Lieu and Tirunelveli on 28 July(red curves) compared to their monthly mean values(blue curves with error bars) indicate that the PRE issignificantly enhanced over Indian and Southeast
Asian longitudes on this day. Further, the Spread-F
started to occur just after the peak PSSR and lasted
for several hours as shown by the horizontal lines inlight blue color at both stations.

Ajith et al. Progress in Earth and Planetary Science (2018) 5:60

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Fig. 3 The S4-index recorded during the night of 28 July 2014 from the ground stations located in a Kolkata and b Chung-Li

Discussion
From the results presented in Figs. 1, 2, and 3, it is evident that this unseasonal ESF/EPB event on 28 July 2014is a large-scale plasma bubble activity spanning over awide longitudinal sector from India, Indonesia, andSoutheast Asian region. Carter et al. (2018, the companion paper) have also shown the widespread L-band scintillation activity over the longitudes of Indian andSoutheast Asian region from COSMIC/Formosat-3
GPS-Radio Occultation observations. Further, Fig. 4 indicates that the EPB activity is completely absent overthe Indonesian sector during the entire month of July
2014 except this singular event day (28th). Also, theVHF scintillation observations from Kolkata (India) and
Chung-Li (Taiwan) exhibit no significant post-sunset
scintillations other than on July 28 for the entire month(not shown in the figure). Carter et al. (2018) havealready shown this unseasonal ESF event on 28 July
2014 is not associated with any disturbed geomagnetic
activity. Further, this unseasonal ESF event is precededby a substantial elevation of equatorial F-layer (h′F in
Fig. 5a, b) compared to its monthly mean variation andday-to-day variability represented by the standard deviation (error bars) at both Indian and Indonesian sectors.
This elevation of equatorial F-layer is mainly due to thelarge pre-reversal enhancement (PRE) in the zonal electric field, which is generally unexpected in the month of
July due to the misalignment between the sunset terminator and magnetic meridian (Abdu et al. 1981; Tsunoda
1985). Hence, the causes behind the unusual enhancement of PRE on this event day remain to be the most intriguing question to be addressed here. Though theexact mechanisms responsible for the enhanced PRE are
not clearly known, we put forth our views about the possible mechanisms hereunder.
There are three mechanisms generally proposed to explain the PRE in the literature: (i) the curl-free mechanism (CFM) by Rishbeth (1973) and Eccles (1998), (ii)Hall current divergence mechanism (HCDM) by Farley
et al. (1986), and (iii) the EEJ feeding to PRE by

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Fig. 4 The altitude-time-intensity (ATI) map of F-region backscatter echoes observed by the central beam of Equatorial Atmosphere Radar (EAR)
for all days in July 2014

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Fig. 5 The F-layer base (virtual) height (h′F) variations over a Bac Lieu and b Tirunelveli on 28 July 2014 (red line) along with the corresponding
monthly mean (blue lines with error bars). Cyan line indicates the duration of Equatorial Spread F (ESF)Ajith et al. Progress in Earth and Planetary Science (2018) 5:60

80

Kolhapur28 July 2014
July monthly Avg.

60
40
Velocity (m/s)

Haerendel and Eccles (1992). An excellent review on thethree competing mechanisms and their relative importance can be found in Eccles et al. (2015). The CFM proposed by Rishbeth (1971, 1973) and Eccles (1998)
indicates that the PRE magnitude is proportional to thelongitudinal gradient in the downward polarization electric field generated by the F-region neutral wind dynamo. The HCDM by Farley et al. (1986) depends on
the Hall conductivity gradient in the off-equatorial
E-region and the associated divergence of Hall current
which in turn is driven by the downward polarizationelectric fields generated by the F-region dynamo. Nevertheless, both the CFM and HCDM suggest that the
downward polarization electric fields generated by theF-region dynamo region and the simultaneous decay ofcorresponding off-equatorial E-region conductivity holdthe key for the PRE. Tsunoda (1985) has further shownthat the rate of decay of total field-line-integrated
E-region conductivity (combined E-region conductivities
of both hemispheres) controls the magnitude of thePRE. The slower decay of off-equator E-region density inthe summer hemisphere reduces the PRE magnitude
during the solstices by short-circuiting the polarizationelectric fields generated by the F-region dynamo (Tsunoda 1985). Abdu et al. (2004) have shown that the PRE
is very sensitive to local time/longitudinal gradient in
the field-line-integrated E-region Pedersen conductivity.
The declination angle of the geomagnetic field lines isnearly zero over the Indian and Indonesian sectors. Julybeing the northern summer, the off-equatorial E-region
sunset occurs later (earlier) in the northern (southern)
magnetic hemisphere with respect to apex sunset overthe equator. Therefore, the off-equatorial E-region conductivity in the northern hemisphere should generally be
responsible for short-circuiting of dynamo electric fields
during July. However, the large enhancement in PRE on
this event day (July 28) over both Indian and Indonesian
sectors indicates that the short-circuiting effect by theE-region conductivity is absent or kept minimum on thisday for some reason, perhaps, due to the forces of lower
atmospheric origin.
With a view to further investigate, the mesospheric
neutral wind measurements over the Indian sector frommedium frequency (2 MHz) partial reflection (PR) radarat an off-equatorial station, Kolhapur, were examined.
The geographic location of the Kolhapur PR radar canbe seen in Fig. 1. Figure 6 shows the meridional windvariation at 98 km altitude observed from the Kolhapur
on July 28 (red curve). The blue curve with error bars
represents the 30-day mean variation of meridional windcentered on July 28 (i.e., July 28 ± 15 days) and its standard deviation. The small vertical green arrow on the
X-axis indicates the time of equatorial E-region (100 km)
sunset over Indian (78° E) sector. It can be noticed fromPage 8 of 15

20
0
-20
-40
-60
-80
-100
0

2

4

6

8

10 12 14LT (hs)

16

18

20

22

Fig. 6 The mesospheric meridional wind (positive northward) as afunction of local time on 28 July 2014 (red line) and 30-day average(July 28 ± 15 days) centered on July 28 (blue line with error bar)observed by MF radar at Kolhapur. The small vertical green arrow
indicates the time of equatorial E-region (100 km) sunset over Indian(78°E) sector

this figure that the meridional wind on July 28 is significantly different from the reference 30-day mean variation and largely southward throughout the day except
for a brief interval around midnight. Around sunset (~18.5 LT), the meridional wind turns largely southward(equatorward) and reaches approximately − 67 m/s,
which is significantly large, compared to its mean value.
Further, as will be shown later, this large equatorward
wind is a part of quasi-2-day wave that maximizes at
90 km altitude with a vertical wavelength of ~ 107 km.
Hence, the meridional wind can be assumed to be in thesame phase (direction) over lower thermospheric altitudes of ~ 90–140 km (i.e., 90+ half of the vertical wavelength), however, with a varying magnitude.
The role of thermospheric meridional winds on thestrength of PRE and ESF development has been reported
in quite a few studies (e.g., Maruyama 1988; Devasia et al.
2002; Jyoti et al. 2004; Abdu et al. 2006a and references
therein). Maruyama (1988) has shown that the summerto-winter transequatorial wind is an important factor controlling the generation of ESF. Assuming a latitudinallyuniform transequatorial wind, Maruyama (1988) has
shown that the growth rate of RT instability decreases dueto the enhanced E-region field-line-integrated conductivity. Later, Devasia et al. (2002) and Jyoti et al. (2004) havesuggested that the equatorward meridional wind could
help to trigger the instability and ESF development evenat lower F-layer heights. Further, Abdu et al. (2006a) haveshown that the larger poleward meridional wind can
reduce the PRE strength and suppress the ESF development. These studies directly or indirectly show that theAjith et al. Progress in Earth and Planetary Science (2018) 5:60

equatorward (poleward) meridional wind would enhance(suppress) the PRE and ESF. More recently, Huba andKrall (2013) have shown that the equatorward neutral
wind can cause an increase in an upward ExB drift(eastward electric field) at the equator by facilitating thepolarization electric fields to grow due to a decrease in
off-equatorial E-region conductivity. All the above studies
mainly deal with thermospheric meridional winds
around F-region altitudes. However, the meridional
wind observations around F-region altitudes are notavailable for the present case. On the other hand, theKolhapur PR radar observations indicate that the

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meridional wind is equatorward over lower thermospheric altitudes of ~ 90 to 140 km. In order toexamine the role of equatorward/poleward neutral
wind over lower thermospheric altitudes on theoff-equatorial E-region electron density and Pedersen
conductivity, we have employed the SAMI2 (Sami2 is Another Model of the Ionosphere) (Huba et al. 2000) model
simulations. Two controlled simulations were carried out
with the meridional wind (V) is set to − 60 m/s (equatorward) and + 60 m/s (poleward) over the altitudes 90 to
150 km, and the zonal wind (U) is set to zero for bothcases. Figure 7a shows the local time variations of electron

Fig. 7 The local time variations of off-equatorial (10° N dip latitude) E-region (125 km) electron density (a) derived from the controlled SAMI2
simulations with meridional wind (V) set to − 60 m/s (equatorward) and + 60 m/s (poleward); while the zonal wind (U) set to zero for both cases.
b The total field-line-integrated E-region (90 to 150 km) Pedersen conductivity

Ajith et al. Progress in Earth and Planetary Science (2018) 5:60

density over the off-equatorial location of 10° N dip latitude at 125 km altitude. Figure 7b shows the local time
variations of total field-line-integrated Pedersen conductivity over E-region altitudes (90–150 km). From the model
simulations presented in Fig. 7a, b, it can be noticed thatboth E-region density and the field-line-integrated Pedersen conductivity exhibit a small decrease for the case ofequatorward meridional wind (blue curves) compared tothe case of poleward wind (black curves). Though thedecrease in off-equatorial E-region density and fieldline-integrated Pedersen conductivity due to the equatorward meridional wind is small, it could have a significant
influence on the PRE. For example, Fesen et al. (2000),
through TIEGCM model simulations, have shown that asmall decrease in E-region electron density from 3300 to1500 cm−3 can cause a significant enhancement in PRE

Page 10 of 15(Fig. 1 of Fesen et al. 2000) as the PRE is highly sensitive
to off-equator E-region density around sunset. Hence, one
possible explanation could be that the enhanced equatorward neutral wind in lower thermospheric altitudes as
observed from Kolhapur PR radar could cause a small
decrease in E-region plasma density at off-equatorial
latitudes via transport, particularly around sunset hours,
where the production due to photoionization is minimum
because of higher solar zenith angles. Another possible
factor that could cause a significant reduction in E-region
density is the divergent node of zonal wind shear via field
perpendicular transport of plasma. However, presently, we
do not have any observations to support the presence of
zonal wind shear. Nevertheless, this decrease in
off-equatorial E-region density (or field-line-integrated
E-region Pedersen conductivity) could reduce (or remove)

Fig. 8 The ΔH variation as a function of universal time on 28 July 2014 (red line) and July monthly average (blue line with error bar) observed bymagnetometers over a Addis Ababa, b Tirunelveli, and c Phuket and d the longitudinal variation of ΔH values for 13 UT

Ajith et al. Progress in Earth and Planetary Science (2018) 5:60

Therefore, from the results presented in Figs. 6, 7,
and 8, one can envisage that the background conditions such as large equatorward meridional wind at
northern off-equatorial E-region (Fig. 6) and enhanced
longitudinal gradient of equatorial electrojet fromdayside to nightside (Fig. 8) might have caused the favorable conditions for the enhanced PRE andlarge-scale unseasonal ESF activity on 28 July 2014.
The observed changes in the meridional wind andPRE on 28 July 2014 could have a connection withthe wave forces of lower atmospheric origin. One of
the clues that motivated our line of thinking about
the possible role of planetary wave modulation on
PRE comes from the daily variation of PSSR during
20–31 July 2014. For example, Fig. 9a, b shows thedaily variation of PSSR (peak h′F during 1900–2100
LT) at the stations Bac Lieu and Tirunelveli, respectively. One can readily observe from Fig. 9a, b that
the PSSR gradually increases from 20 July to 28 July
with a quasi-2-day wave-like modulation at both stations. Further, the PSSR reached maximum on 28 Julyat both stations.
The results presented in Fig. 9 indicate that aquasi-2-day planetary wave from the lower atmosphericorigin might have modulated the PSSR over Indian and

450(a) PSSR at Bac Lieu
400
Post Sunset height Rise (PSSR, km)

the effect of short-circuiting of the F-region dynamo electric fields in the summer hemisphere and that could leadto the enhancement in PRE.
Another mechanism proposed for PRE by Haerendel
and Eccles (1992) is the role of equatorial electrojet(EEJ) feeding to the PRE at sunset terminator. Eccles
et al. (2015) have shown that the magnitude of PRE iscontrolled by the longitudinal gradient in the EEJ fromthe dayside to nightside across the sunset terminator.
Haerendel and Eccles (1992) and Eccles et al. (2015)
have suggested that the PRE can lift the ionization out
from the E-region to further reduce the Cowling conductance and provide a positive feedback for the increase of PRE magnitude. Thus, in order to examine therole of EEJ on the enhanced PRE on July 28, the magnetometer observations from three equatorial stations,
Addis Ababa (Africa), Tirunelveli (India), and Phuket(Thailand), that are distributed from dayside to night
were examined. The geographic locations of these magnetometer stations can be seen in Fig. 1. For example,
Fig. 8a–c show the variations of ΔH (H component of
the geomagnetic field after subtracting the quiet day
mean midnight value), which is a proxy to the EEJ, from
Addis Ababa, Tirunelveli, and Phuket, respectively. The
red curves indicate the ΔH variations on July 28, and theblue curves with error bars represent the monthly meanvariations with standard deviations. It can be seen fromthese figures that the ΔH at all the three stations are significantly enhanced on July 28 compared to their respective monthly mean variations. It should be noted that theΔH variations measured from magnetometer actually represent the integrated current in the eastward direction
which is proportional to both conductivity and zonal electric field (J = σ.E). The Hall and Pedersen conductivities
maximize at approximately 110 km and 125 km altitudes,
respectively (Forbes and Lindzen 1976). Further, the zonal
current enhances significantly at equatorial latitudes,
called equatorial electrojet (EEJ) due to the Cowling effect.
The Cowling conductivity maximizes around ~ 105 km
altitude (Forbes and Lindzen 1976). Hence, the major contribution for the ΔH variations measured from the equatorial magnetometers comes from the E-region altitudes
due to the enhanced Cowling conductivity in the absenceof other solar-driven geomagnetic disturbances (quiet
periods). Figure 8d shows the ΔH values from the threestations at 13 UT during which the Addis Ababa is in dayside, Tirunelveli is close to E-region sunset terminator,and Phuket is in the nightside. The longitudinal gradientof ΔH from dayside to nightside on July 28 (red curve) appears to be high compared to its monthly mean variation(blue curve with error bars). This large longitudinal gradient in EEJ on July 28 may also contribute to the enhanced
PRE according to the mechanism proposed by Haerendel
and Eccles (1992).

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350
300
250
200
450

(b) PSSR at Tirunelveli
400
350
300
250
200
20 21 22 23 24 25 26 27 28 29 30 31
Day in July 2014

Fig. 9 The daily variation of PSSR at a Bac Lieu and b Tirunelveli
indicating quasi-2-day planetary wave-like oscillation during 20–31
July 2014. The maximum PSSR occurred on 28 July 2014 at
both stations

Ajith et al. Progress in Earth and Planetary Science (2018) 5:60

Southeast Asian region, possibly, through the modulationin the neutral wind. Abdu et al. (2006b) have shown theplanetary wave (~ 2 to 7 day) oscillations in the

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mesospheric winds can cause similar oscillations in theequatorial evening pre-reversal electric field through Eand F-region electro-dynamical coupling. With a view toFig. 10 a The 2-day planetary wave packet (1.5–2.5-day bandpass filtered) in the meridional wind from Indian low-latitude station, Kolhapur, during 01
July 2014–31 August 2014. b Amplitude spectra of meridional wind at 80–98 km altitudes showing the quasi-2-day wave periodicity of ~ 48–52 h.
c Altitudinal variation of amplitude (red) and phase (black) of quasi-2-day wave taken at 48 h periodicity

Ajith et al. Progress in Earth and Planetary Science (2018) 5:60

further examine this, the meridional neutral wind observations at 98 km altitude from the Kolhapur PR
radar were analyzed using a digital bandpass filter. A
Butterworth digital bandpass filter centered at 48 h
(2 days) with a 3-dB bandwidth of 36–50 h (1.5–
2.5 day) is applied on the time series data of hourly
meridional wind observations during the months ofJuly to August 2014. Figure 10a shows the filteredquasi-2-day wave oscillations in the meridional wind
at 98 km altitude over Indian low-latitude station
Kolhapur during July to August 2014. It can be seenfrom Fig. 10a that the quasi-2-day wave amplitude exhibits significant enhancement during 20–30 July
2014. Further, the amplitude of the quasi-2-day wave
increases from 20 July and reaches a maximum
around 27–28 July.
Figure 10b shows the amplitude spectra of meridional
wind at altitudes from 80 to 98 km at 2 km interval during 20–30 July 2014 computed using least squares
method (Malin and Schlapp 1980; Gu et al. 2013). The
dominant quasi-2-day wave with a periodicity of ~ 48–
52 h can be observed at all altitudes. Figure 10c shows
the altitude variations of amplitude and phase ofquasi-2-day wave (at 48 h period). The quasi-2-day wave
in the meridional wind maximizes at ~ 90 km, and thephase decreases gradually with the height. From theslope of the phase variation, the vertical wavelengthof the quasi-2-day wave can be estimated as ~107 km. It is known that the vertical wavelengths ofquasi-2-day wave are generally large of the order of~ 70–200 km (e.g., Gu et al. 2013). The large vertical
wavelengths of quasi-2-day wave indicate that the meridional wind will be in the same phase (direction)over an altitude range of approximately half of thevertical wavelength. Therefore, it could be possiblethat the meridional wind would be in the same direction (equatorward) over lower thermospheric altitudesof ~ 90–143 km in the present case. Abdu et al.
(2006b) have shown that the neutral wind oscillations
associated with the planetary waves can significantly
alter the evening PRE via modulating the integrated
E-region Pedersen conductivity. Therefore, a quasi2-day planetary wave in the meridional neutral windmight have caused a similar modulation in PSSR
(Fig. 9) through the modulation of the E-region
conductivity at off-equatorial latitudes (such as at
Kolhapur) and PRE as discussed above (Fig. 6). This
study indicates the possible role of planetary wave forcesof lower atmospheric origin that can significantly modify
the neutral wind at mesosphere and lower thermospheric(MLT) region and PRE system leading to the developmentof large PSSR and unseasonal large scale EPB/ESF activity.
Nevertheless, modeling studies are further needed to substantiate this hypothesis.

Page 13 of 15

Conclusions
An unseasonal enhancement of PRE and development ofintense EPBs were observed over Indian and South East
Asian longitudes during the post-sunset hours of 28 July
2014. Using multiple independent ground-based observations, the presence of intense EPBs over a wide longitudinal sector ranging from 77° E to 125° E is confirmed.
The ATI maps of an EAR for the month of July 2014indicate that the EPB development on July 28 is an unusual and unique event over this sector. The observations from two equatorial ionosondes at Tirunelveli and
Bac Lieu show that F-layer is elevated to higher altitudes
prior to the development of these post-sunset EPBs dueto the pre-reversal enhancement of zonal electric field.
The unusual enhancement of the PRE on this day might
be due to the reduction of the E-region conductivity inthe northern (summer) low latitudes probably due to enhanced equatorward meridional wind or divergent nodeof zonal wind shear. Further, it is also speculated thatthe large longitudinal gradient in equatorial electrojet
may also conducive for the enhanced PRE according to
the Haerendel and Eccles (1992) mechanism.
A quasi-2-day wave-like oscillation in the PSSR is consistently observed at both equatorial stations, Bac Lieu
and Tirunelveli, during 20–31 July 2014. The maximum
PSSR occurred on 28 July at both stations. A similar
quasi-2-day wave-like oscillation in the meridional neutral wind with a large vertical wavelength of ~ 107 km isobserved at the off-equatorial station, Kolhapur, during
20–30 July 2014 with maximum amplitude on 28 July.
This indicates the possible coupling between the 2-day
planetary wave of lower atmospheric origin and PSSR
over Indian and southeast Asian sector. This study emphasizes the coupling from lower atmosphere and therole of planetary wave forcing that can potentially alter
the local wind system and E-region conductivities thatcan lead to the unusual enhancement of PRE and development of EPBs over the wide longitudinal sector,
during the season which is climatologically unfavorable for EPBs.
Abbreviations
ATI: Altitude-time-intensity; EEJ: Equatorial electrojet; EPB: Equatorial plasma
bubble;; ESF: Equatorial Spread-F; FAI: Field-aligned irregularities; FoV: Field ofview; HCDM: Hall current divergence mechanism; LSWS: Large-scale wave
structure; MLT: Mesosphere and lower thermosphere; PR: Partial reflection;
PRE: Pre-reversal enhancement; PSSR: Post-sunset height rise; RT: RayleighTaylor; SCINDA: Scintillation Network Decision Aid; TIEGCM: ThermosphereIonosphere Electrodynamics General Circulation Model
Funding
This work was supported by SERB-DST Sanction Number ECR/2017/002271
and JSPS KAKENHI Grant Number 15H02135
Availability of data and materials
Please contact M. Yamamoto (yamamoto@rish.kyoto-u.ac.jp) for EAR data,
S. Gurubaran (gurubara@iigs.iigm.res.in) for Kolhapur PR data, T. Yokoyama(tyoko@nict.go.jp) for Bac Lieu ionosonde data, S. Sripathi

Ajith et al. Progress in Earth and Planetary Science (2018) 5:60

(sripathi@iigs.iigm.res.in) for Tirunelveli ionosonde data, K. Hozumi(kukkai@nict.go.jp) for Phuket magnetometer data, and Keith Groves(keith.groves@bc.edu) for SCINDA VHF scintillation data. The magnetometerdata of Tirunelveli is available from the World Data Center for Geomagnetism,
Mumbai (http://wdciig.res.in/WebUI/Home.aspx) and the magnetometer dataof Addis Ababa is available at Intermagnet (http://www.intermagnet.org/).
Authors’ contributions
KK, ST, and BC proposed the topic and conceived and designed the study.
ST and KK drafted the manuscript and carried out the data analysis. SSK and
KK analyzed the meridional wind data measured from Kolhapur PR radar. MY
contributed the EAR data. TY contributed the Bac Lieu ionosonde data. SG
contributed the Kolhapur PR radar data. SS contributed Tirunelveli ionosondedata. KH contributed the Phuket magnetometer data. KG and RC provided
the SCINDA VHF scintillation data of Kolkata and Chung-Li. All authors readand approved the final manuscript.
Competing interests
The authors declare that they have no competing interest.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in publishedmaps and institutional affiliations.
Author details
1
Indian Institute of Geomagnetism, Navi Mumbai, India. 2RMIT University,
Melbourne, Australia. 3Equatorial Geophysical Research Laboratory, Indian
Institute of Geomagnetism, Tirunelveli, India. 4Research Institute for
Sustainable Humanosphere (RISH), Kyoto University, Uji, Japan. 5National
institute of Information and Communications Technology, Tokyo, Japan.
6
Boston College, Boston, USA. 7Space Vehicles Directorate, Air Force ResearchLaboratory, Kirtland AFB, Albuquerque, NM, USA.
Received: 31 August 2017 Accepted: 12 September 2018

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