Otsuka Progress in Earth and Planetary Science (2018) 5:57
https://doi.org/10.1186/s40645-018-0212-7

Progress in Earth andPlanetary Science

REVIEW

Open Access

Review of the generation mechanisms of
post-midnight irregularities in theequatorial and low-latitude ionosphere
Yuichi Otsuka

Abstract
This paper provides a brief review of ionospheric irregularities that occur in magnetically equatorial and low-latitude
regions post-midnight during low solar activity periods. Ionospheric irregularities can occur in equatorial plasma
bubbles. Plasma bubbles are well-known to frequently occur post-sunset when the solar terminator is nearly parallel tothe geomagnetic field lines (during equinoxes at the longitude where the declination of the geomagnetic field isalmost equal to zero and near the December solstice at the longitude where the declination is tilted westward),
especially during high solar activity conditions via the Rayleigh–Taylor instability. However, recent observations
during a solar minimum period show a high occurrence rate of irregularities post-midnight around the Junesolstice. The mechanisms for generating the post-midnight irregularities are still unknown, but two candidates
have been proposed. One candidate is the seeding of the Rayleigh–Taylor instability by atmospheric gravity
waves propagating from below into the ionosphere. The other candidate is the uplift of the F layer by themeridional neutral winds in the thermosphere, which may be associated with midnight temperature maximums in thethermosphere.
Keywords: Equatorial ionosphere, Ionospheric irregularity, Rayleigh–Taylor instability, Plasma bubble

Introduction
Plasma bubbles, which are localized plasma density depletions in the ionosphere at equatorial and low-latitude
regions, have been studied since their discovery byBooker and Wells (1938) with various radio and opticaltechniques (see reviews by Hysell 2000; Makela 2006;
Kelley et al. 2011; Abdu 2012). Within plasma bubbles,
various size scales of plasma density irregularities exist.
Intense backscatter echoes caused by Bragg scatteringfrom field-aligned irregularities (FAI) with spatial scalesof one half of the radar wavelength have been observedwith VHF, UHF, and L-band radars (e.g., Woodman and
LaHoz 1976; Tsunoda 1980a, b). The FAI echoes are
considered to be manifestations of plasma bubbles (e.g.,
Woodman and LaHoz 1976). Tsunoda et al. (1982)
showed that FAIs observed using the ALTAIR radar
were collocated with plasma depletions measured using
the Atmosphere Explorer-E satellite. It is well-accepted
Correspondence: otsuka@isee.nagoya-u.ac.jp
Institute for Space-Earth Environmental Research, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya 464-8601, Japanthat plasma bubbles are generated via the Rayleigh–Taylor
instability that occurs at the magnetic equator after sunset
when the eastward electric fields increase. In general,
plasma bubbles occur frequently when the solar terminator is parallel to the geomagnetic field (during the equinoxes at the longitude where the declination of thegeomagnetic field is almost equal to zero and near the
December solstice at the longitude where the declination
is tilted westward), and their occurrence rate increases
with solar activity (Abdu et al. 1983; Tsunoda 1985; Burkeet al. 2004; Nishioka et al. 2008). Such local time, seasonal,
and solar activity dependences could be attributed to themagnitude of the eastward electric field at the magnetic
equator. An enhancement of the eastward electric fieldoccurs around the evening terminator in the equatorial
ionosphere, known as prereversal enhancement, and increases the upward E × B drift. At the evening terminator,a thermospheric neutral wind blows eastward due to apressure gradient toward the dayside caused by solar
heating. Since the neutral wind traverses the geomagnetic
field through the F region dynamo mechanism, upward© 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.

Otsuka Progress in Earth and Planetary Science (2018) 5:57

dynamo current is driven in the F region. Since electric
current flows along the geomagnetic field, the E and F
regions are coupled electro-dynamically. Due to the rapid
decrease of the plasma density in the E region after sunset,
the longitudinal gradient of the conductivity in the Eregion becomes steep at the sunset terminator. To maintain curl-free of the electric fields and/or divergence-freeof the electric currents, intense eastward and westward
electric fields are generated before and after the sunset terminator, respectively. Details of these mechanisms are described by Eccles et al. (2015). Prereversal enhancement is
one of the most important parameters in relation to plasma
bubble generation (e.g., Kelley 1989; Fejer et al. 1999).
During low solar activity conditions in recent solar
cycles, however, after midnight, FAIs were frequently
observed with VHF radars at magnetically low latitudes(e.g., Patra et al. 2009; Otsuka et al. 2009; Li et al. 2012;
Yizengaw et al. 2013), and plasma density and electric fieldirregularities were observed by the C/NOFS satellite (e.g.,
Yokoyama et al. 2011; Dao et al. 2011; Yizengaw et al.
2013). The mechanisms generating these equatorial ionospheric irregularities are still unknown and extensively debated. The probability of plasma bubble occurrence (P)can be interpreted as a product of two probabilities, that
is, P = PI ∙ Ps, PI is the conditional probability for theRayleigh–Taylor instability, and Ps is the probability ofseeding an instability (e.g., McClure et al. 1998). Local
time, seasonal, longitudinal, and solar activity dependences for P are attributed to PI. PI could be represented
in terms of the growth rate of the Rayleigh–Taylor instability. Asymmetry of the equatorial ionization anomaly
between the Northern and Southern Hemispheres, which
could be caused by transequatorial winds, also contributes
to PI (Maruyama and Matuura 1984). On the other hand,the day-to-day variability of P could be controlled by Ps
because plasma bubbles occur some days but not others
even in almost identical conditions for the Rayleigh–Taylor instability. Atmospheric gravity waves andplasma velocity shear at the prereversal enhancement
are considered to be candidates for seeds for theRayleigh–Taylor instability (e.g., Hysell and Kudeki
2004; Abdu et al. 2009). Atmospheric acoustic/gravity
waves could be launched from severe convection inequatorial regions and propagate upward. Some ofthese waves could reach the ionosphere/thermosphereand seed the instability. Because the Intertropical Convergence Zone (ITCZ) is an area where tropospheric
convection is active at the equatorial region (Wallset
and Gautier 1993), the ITCZ can be a source of gravity
waves. McClure et al. (1998) have proposed that gravity
waves launched from the ITCZ could initiate theRayleigh–Taylor instability and that the gravity wave
seeding mechanisms explain the longitudinal dependence of the plasma bubble occurrence rate because thePage 2 of 13

location of the ITCZ depends on the longitude and migrates northward and southward from the geographic
equator mainly according to the subsolar point. However, Su et al. (2014) have reported that such a relationship between the ITCZ and plasma bubble occurrence
is not clearly seen for the longitudinal dependence.
During the high solar activity conditions of the equinox, PI is high and varies day to day, so identifying themost important factors for plasma bubble generation isdifficult. On the contrary, during the low solar activity
conditions of the June solstice, PI is low, so PS mayneed to work effectively to generate plasma bubbles.
Study of the post-midnight irregularities could elucidate
plasma bubble generation mechanisms. This paper reviews recent studies of the post-midnight irregularities
in equatorial and low-latitude regions and argues possible mechanisms for their generation.

Observations of post-midnight irregularities
Characteristics of occurrences

Using the Gadanki radar in India, Patra et al. (2009)
have observed F-region FAIs post-midnight during July–
August in low solar activity conditions. Figure 1 showsrange-time-intensity plots of the FAI echo observed by
the Gadanki radar during July 1–10, 2008. FAI echoes
were detected during the post-midnight periods of thosenights. Patra et al. (2009) noted that the FAI echoes wereobserved up to sunrise and beyond (the time of sunrise
in the F region (300 km) is 0430 LT) and that the echoregion extended to altitudes as high as 500 km on someoccasions, connecting to an apex altitude of 600 km overthe magnetic equator. Post-midnight FAIs were also
observed by VHF radar at Hainan, China on June of
2009–2010 (Li et al. 2012). The researchers observedthat the features of these FAI echoes are different fromthe FAIs observed post-sunset in the equinox season. In
Indonesia, Otsuka et al. (2009) have carried out continuous observations of FAIs using a 30.8 MHz radar withmultiple beams since February 2006. Figure 2 showsrange-time-intensity (RTI) plots of the F-region FAI
echoes observed in five beams on the night of August
21 and July 8, 2007. On the night of August 21, 2007
(Fig. 2 (right)), the FAI echoes on the beams with azimuths ranging from 180.0–125.8° lasted for more than
4 h during 2200–0400 LT with changing altitudes in therange from 200 to 350 km. Time delays between the FAI
echoes of the different beams are not seen, indicatingthat the FAI region did not move in the zonal direction.
On the other hand, on the night of July 8, 2007 (Fig. 2(left)), FAI echoes were observed continuously during2330–0300 LT on the easternmost beam and 0030–0500
LT on the westernmost beam. From this time delay, thetrace velocity of the echo region in the zonal direction is
120 m/s westward. Using a 30.8 MHz radar in Indonesia,

Otsuka Progress in Earth and Planetary Science (2018) 5:57

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Fig. 1 Height-time variations of SNR associated with the F-region FAI observed by the Gadanki radar during July 1–10, 2008 (after Patra et al. 2009)

Otsuka et al. (2009, 2012) and Nishioka et al. (2012)
have performed statistical studies of the FAI echo characteristics. Figure 3 shows the seasonal and local time
variations of the occurrences and non-occurrences of
FAI echoes from 2006 to 2010. In their study, an FAI
echo was defined as an echo whose signal-to-noise ratio

is larger than 0 dB and that extends more than
50 km in range. FAIs appeared most frequently
post-sunset (pre-midnight) around the equinoxes andpost-midnight between May and August. The yearly
averages of solar flux F10.7 were 80, 73, 68, 70, and
80 in 2006, 2007, 2008, 2009, and 2010, respectively.

Fig. 2 Range-time-intensity plot of the FAI in the F region observed on the beam with azimuth of 234.2°, 207.0°, 180.0°, 153.0°, and 125.8° (fromtop to bottom panels) with the 30.8 MHz radar at Kototabang, Indonesia on the nights of (left) August 21, 2007, and (right) July 8, 2007. Leftvertical axis shows the range from the radar and the right axis shows the altitude at which the radar beam is perpendicular to the geomagnetic
field. (after Otsuka et al. 2009)

Otsuka Progress in Earth and Planetary Science (2018) 5:57

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Fig. 3 Seasonal and local time variations in the F-region FAI echoes observed at Kototabang, Indonesia from 2006 to 2010. A signal-to-noise ratio
larger than 0 dB was regarded to be an echo from an FAI. Red and blue sections represent FAI occurrences and non-occurrences, respectively,
against local time and day of the year for each year from 2006 to 2010. The yearly average of the solar flux F10.7 is given at the top of eachpanel (after Nishioka et al. 2012)

The occurrence rates of the pre-midnight FAIs tended toincrease with solar activity. On the other hand, post-midnight FAIs appeared more frequently in 2007, 2008, and2009 than the other 2 years, indicating a negative correlation with solar activity.
By analyzing plasma density data obtained by the C/
NOFS satellite, Dao et al. (2011) have reported seasonal
and longitudinal variations of the plasma density

irregularities occurring at magnetically equatorial andlow-altitude regions in 2008 when solar activity wasquite low (Fig. 4). Plasma density irregularities are foundto occur frequently during May–August. This seasonalvariation is consistent with the post-midnight FAIs
observed with VHF radars and different from that ofplasma bubbles, which frequently occur in high solar
activity conditions. Furthermore, Yizengaw et al. (2013)

Fig. 4 a Average nighttime ΔN/N versus longitude over time from May 2008 to March 2010. b Average nighttime ΔN/N separated by seasons
from observations dated May 2008 to March 2010 (after Dao et al. 2011)

Otsuka Progress in Earth and Planetary Science (2018) 5:57

have reported 4-year (2009–2012) statistics for the
post-midnight irregularities observed with the C/NOFS
satellite and noted that a strong occurrence peak predominantly in the African sector appears during the June solstice, indicating that the occurrence rate of post-midnight
irregularities depends on the longitude.
Otsuka et al. (2012) have investigated the zonal propagation velocity of the post-midnight FAI regions by conducting five-beam measurements with the 30.8 MHz radar at
Kototabang (0.20°S, 100.32°E; dip latitude 10.4°S), Indonesia
and reported that 46% (14%) of the post-midnight FAIs
propagated westward (eastward) and that the remaining
40% did not show discernible propagation. Figure 5 shows
the local time variation of the averaged zonal propagation
velocities of the post-midnight FAIs. The average propagation velocity is approximately 50 m/s westward and is invariant with time. Otsuka et al. (2012) have noted that thevelocities of the post-midnight FAIs are different from thatof the ambient F-region plasma observed with incoherent
scatter radar and satellite in situ measurements (e.g., Fejer
1993; Coley and Heelis 1989). However, by analyzing theion velocity data obtained by the C/NOFS satellite, Coley etal. (2014) have shown that the zonal plasma drift velocities
largely depend on solar activity, especially pre-midnight,
and that the average plasma drift velocity was westward
during the midnight and post-midnight period in 2009,
when solar activity was very low. Consequently, thepost-midnight FAIs may mostly propagate together withthe ambient plasma in the same way as the post-sunset
FAIs (plasma bubbles) propagate.
Evolution of the post-midnight FAIs

By analyzing two-dimensional maps of the FAIs observed with the EAR in Indonesia, Ajith et al. (2016)

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have investigated the evolution of the FAI echo region
within the field-of-view of the radar. They have analyzed
116 FAI events during the equinox and 45 events during
the June solstice. Figure 6a, b shows the local time variation of FAI echo events during 2011–2012 for the equinox and the June solstice, respectively. During theequinoxes, 59% of the FAIs evolved between 1900 and2000 LT and 27% during 2000–2100 LT. On the other
hand, during the June solstices, 71% of FAIs evolved
around midnight (2200–0300 LT), as shown in Fig. 6b.
Dao et al. (2016) have investigated the vertical rise
velocities of post-midnight FAIs and found that the rise
velocities of post-midnight FAIs are between 10 and
100 m/s and are smaller than those of post-sunset FAIs
(from 35 to 260 m/s) (Fig. 7). Tulasi Ram et al. (2017)
have calculated the rise velocity of the plasma depletion
using a simulation. This velocity is consistent withthose of the observed post-midnight FAIs. Based on therise velocities, Dao et al. (2016) have estimated that the
post-midnight FAIs are generated between 21:30 LT
and 02:00 LT for 14 of the 15 events. These results suggest that post-midnight FAIs could be generated after
the prereversal enhancement of the zonal electric fields
has ceased.
Comparison with GPS scintillation

Otsuka et al. (2009) have reported seasonal and local
time variations for the GPS scintillation occurrence rate
in Indonesia during a solar minimum period fromFebruary 2006 to November 2007 and compared thisrate to the post-midnight FAI occurrence rate (Fig. 8).
While FAIs occurring after midnight were observed between May and August in 2006 and between May and
September in 2007, GPS scintillation was not observed.

Fig. 5 Local time variation of averaged zonal propagation velocity (after Otsuka et al. 2012)

Otsuka Progress in Earth and Planetary Science (2018) 5:57

Fig. 6 Histogram showing the nighttime occurrence of freshly evolved
EPBs over Kototabang during the a equinox and b June solstices from2011 to 2012. The numbers shown on top of the histogram bars indicate
percentages (after Ajith et al. 2016)

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scintillation is proportional to the amplitude of theplasma density perturbations. The difference between
the scintillation occurrences in Asian and African regions may be related to a longitudinal dependence, as
shown by satellite observations (Dao et al. 2011; Yizengaw et al. 2013). As Yizengaw et al. (2013) have shown,the post-midnight plasma irregularities in the Americanand African sectors extend to higher altitudes comparedwith those in the other longitudinal sectors. Such longitudinal differences may be responsible for why
post-midnight GPS scintillation was observed in Africa
but not in Asia.
Scintillation of a GPS signal is caused by plasma density
perturbations at scales of 300–400 m, corresponding topﬃﬃﬃﬃﬃﬃﬃ
the first Fresnel scale defined as 2λz, where λ is the radio
wavelength, and z is the altitude of the ionosphere. On theother hand, VHF radar measures meter-scale plasma
density perturbations as FAI echoes. Within plasma bubbles, plasma density irregularities of various size scales are
generated, but the smaller scale irregularities disappear
earlier than the larger scale irregularities due to plasma
diffusion (Basu et al. 1978). However, this feature is notconsistent with the FAIs observed without GPS scintillation post-midnight during a solar minimum period. Sripati(2008) have suggested that the meter-scale FAIs could be
generated at the gradient of kilometer-scale size irregularities as secondary instabilities and that the Rayleigh–Taylor instability that initiates a plasma bubble does notextend to the irregularities at size scales of a few hundred
meters that cause GPS scintillation, resulting in an absence of GPS scintillation.

Discussion
Li et al. (2012) have reported that GPS scintillation did
not occur with the post-midnight FAIs observed by
47.5 MHz radar in Hainan (18.4°N, 109.6°E; dip latitude
12.8°N), China, although GPS scintillation coincidedwith the FAIs that occurred during the post-sunset
period. Since the intensity of amplitude scintillation is
proportional to the amplitude of plasma density perturbations, GPS scintillation could not be observed
post-midnight during solar minimum conditions when
the plasma density in the F region is low.
On the other hand, Akala et al. (2015) have studied
the climatology of GPS amplitude scintillation over Africa from 2009 to 2011 during solar minimum and ascending conditions. Post-midnight scintillation occurred
during the June solstice, although the intensity was
weak. The intensity of the scintillation observed at thestation near the magnetic equator was especially weaker
than that observed in the crest region of the equatorial
anomaly. This latitudinal variation is also interpreted
from the perspective that the intensity of the amplitude

Seeding by atmospheric gravity waves

An atmospheric gravity wave is an oscillation of neutral
air that causes ion-neutral collisions and eventually generates a polarization electric field in the F region. Huang
and Kelley (1996) have carried out numerical simulations to study the effect of gravity waves on the seedingof equatorial plasma bubbles and showed that thepolarization electric fields induced by the neutral windoscillations due to gravity waves can initiate the Rayleigh–Taylor instability and that the special resonance ofthe gravity waves and the plasma motion can speed up
plasma bubble formation. Tsunoda (2010a) have studied
the efficiency of polarization electric field generation by
gravity waves in the equatorial F region, where the magnetic field is horizontal. Ions in the F region move in the
U × B direction (where U is the thermospheric neutral
wind and B is the magnetic field) through ion-neutral
collisions and the Lorentz force, whereas electron movement is restricted to gyromotion around the magnetic
field. This difference between the ion and electron

Otsuka Progress in Earth and Planetary Science (2018) 5:57

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Fig. 7 Temporal variation of the top altitudes of the FAIs observed by the EAR during the growth phase. The left vertical axis shows the altitudeof the location where the EAR radar beam is perpendicular to the magnetic field line. The right vertical axis shows the apex altitude of the geomagnetic
field line connecting to the point measured with the EAR (after Dao et al. 2016)

Fig. 8 Seasonal and local time variations of (left) F-region FAI echoes and (right) scintillation index (S4) observed at Kototabang between
February 2006 and November 2007 (after Otsuka et al. 2009)

Otsuka Progress in Earth and Planetary Science (2018) 5:57

movements drives the polarization electric fields. The
neutral wind oscillations due to the gravity waves in thethermosphere also drive the polarization electric fields oscillating perpendicular to the wind and magnetic fields.
Tsunoda (2010a) has suggested that gravity waves with circular phase fronts effectively generate polarization electricfields through the F region dynamo mechanism rather
than gravity waves with planar phase fronts. Figure 9shows the configurations of the geomagnetic fields andphase fronts of the gravity waves with (a) planar and (b)circular phase fronts. As shown in Fig. 9a, when the phasefront is planar, a magnetic field line pierces different
phases of the gravity waves in general, except for the casewhere the gravity wave propagates precisely in the zonal
direction when the phase front of the gravity wave is parallel to the magnetic field line. Polarization electric fields
driven by the neutral wind oscillations due to gravity
waves also oscillate according to the phase of the gravity
wave. Since the polarization electric field can be transmitted along the magnetic field without attenuation, the electric field integrated along the magnetic field is smearedout. On the other hand, for the case of a circular phasefront, the polarization electric fields are not completely
smeared out because the length of the magnetic field
passes through the same phase of the gravity wave depending on the radius of the circular phase front. Gravity
waves with longer horizontal wavelength and monochromatic wavelength tend to have higher magnitudepolarization electric fields. This researcher has also suggested that such gravity waves may produce large-scale

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wave structures (LSWSs). Circular patterns of plasma
density perturbations in the F region were observed
over Japan after a large earthquake (Tsugawa et al.
2011) and over the USA after a tornado (Nishioka et
al. 2013) in two-dimensional maps of total electron
content obtained from dense GPS receiver networks,
suggesting that acoustic gravity waves can propagate
from the lower atmosphere into the thermospherewith circular patterns of phase fronts. Tsunoda(2010b) has compared tropospheric convection activity with the occurrence of equatorial spread F (ESF),
which could correspond to plasma bubbles, and foundthat the occurrence rate of the ESF is highest when
the active convection region is located very close to
the magnetic dip equator. These results indicate that
atmospheric gravity waves launched from the active
convection region in the troposphere could propagate
into the thermosphere and contribute to plasma bubble seeding.
Seeding by medium-scale traveling ionosphericdisturbances (MSTIDs)

Miller et al. (2009) have conducted simultaneous observations using an all-sky airglow imager and a VHF
coherent radar and showed that after southwestwardpropagating medium-scale traveling ionospheric disturbances (MSTIDs) passed through a magnetically lowlatitude (close to the magnetic equator), post-midnightplasma bubbles appeared. MSTIDs, which appear frequently at mid-latitudes and propagate southeastward

Fig. 9 Diagram showing the polarization response to the presence of gravity waves with a planar and b circular phase fronts (after
Tsunoda 2010a, b)

Otsuka Progress in Earth and Planetary Science (2018) 5:57

(northwestward) in the Northern (Southern) Hemisphere, could be caused by the Perkins instability andare accompanied by polarization electric fields generated to maintain a divergence-free ionospheric current(e.g., Kelley and Makela 2000; Shiokawa et al. 2003;
Otsuka et al. 2004). Miller et al. (2009) have suggested
that the perturbations of the electric fields can seed
the Rayleigh–Taylor instability at the magnetic equator. The seeding of plasma bubbles by MSTIDs could
be a candidate for the generation of a post-midnightplasma bubble because MSTIDs accompanying electric fields occur after sunset at mid-latitudes. Candido
et al. (2011) have presented seasonal and local time
variations of spread F occurrences over Cachoeira
Paulista (22.7°S, 45.0°W; MLAT 16°S), Brazil andfound that the spread F occurrence rate is high around
the June solstice (winter in the southern hemisphere)
during solar minimum conditions. By carrying out
simultaneous observations with an all-sky airglow
imager at the same site, they have suggested that
MSTIDs are one of the main sources of post-midnight
irregularities. MSTIDs also can be seeded by atmospheric gravity waves (Miller et al. 1997; Kelley andMiller 1997), suggesting that atmospheric gravity
waves may directly and indirectly play an importantrole in post-midnight irregularities.
However, such electrified MSTIDs do not always
reach the magnetic equator. An equatorward boundary
for the electrified MSTIDs could exist at the crest region of the equatorial anomalies (Shiokawa et al. 2002;
Narayanan et al. 2014). On the other hand, at
Kototabang, Indonesia, near the geographic equator,
MSTIDs propagating magnetically poleward were frequently observed (Shiokawa et al. 2006; Fukushima etal. 2012). The researchers have ascertained that thesepoleward-propagating MSTIDs could not be accompanied by polarization electric fields. The polarizationelectric fields generated to maintain current continuity
should be in the meridional direction because thephase front of the plasma density perturbations iselongated along the zonal direction. The electric fields
in the meridional direction move ionospheric plasma
only horizontally by E × B drift and thus do not form
plasma density perturbations (MSTIDs). The polewardpropagating MSTIDs are assumed to be generated byatmospheric gravity waves. Further investigation isneeded regarding the mechanisms of plasma bubble
seeding by electrified MSTIDs.
Preferable conditions for Rayleigh–Taylor instability
post-midnight

The linear growth rate (γ) of the Rayleigh–Taylor instability, which could cause an equatorial plasma bubble,
is written as

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

 
E
g 1 dn0þγ¼B νin n0 dz

ð1Þ

where E, B, g, νin, n0, and z are the eastward electric fiemagnetic field, gravity acceleration, ion-neutral collision
frequency, plasma density in the F region, and altitude,
respectively. The growth rate increases with eastward
effective electric field ðEB þ νgin Þ. Post-sunset, an enhancement of the eastward electric field (E), the so-called prereversal enhancement, plays an important role ingenerating a plasma bubble. During nighttime, however,
the electric field is on average westward (e.g., Fejer
1991), making the growth rate negative or low. On theother hand, g/νin is larger during nighttime than daytime
and increases with decreasing solar activity because νin is
proportional to the neutral density, which is smaller atnight than during the day and decreases with decreasing
solar activity (NRLMSISE; Picone et al. 2002). The valueof g/νin also increases with altitude. Therefore, the altitude of the F layer is a key parameter for plasma bubble
generation. Nishioka et al. (2012) have investigated theseasonal variations of post-midnight FAI occurrence andF-layer altitude using an ionosonde near the magnetic
equator and showed that both post-midnight FAIs anduplift of the F layer frequently occur around midnight
between May and August. Figure 10 shows the seasonal
and local time variations in the peak altitude of the F2
layer, hmF2, at Chumphon, Thailand, near the magnetic
equator. The value for hmF2 is calculated from foF2 andM(3000)F2. This figure shows that hmF2 enhances postsunset (pre-midnight) around the equinoxes and postmidnight between May and August. The post-sunset enhancements are caused by prereversal enhancement of
the zonal electric fields. The post-midnight enhancements coincide with the occurrences of post-midnight
FAIs. Based on the observed altitude of the F layer, theresearchers have evaluated the linear growth rate of theRayleigh–Taylor instability and showed that the uplift of
the F layer can enhance the growth rate due to the increase in g/νin. Figure 11a shows the seasonal variationof the F-layer altitude. Each cross represents hmF2 at
Chumphon at midnight, and the red curve shows the 3month running average of hmF2. Since plasma bubbles
are initiated at the bottom of the F layer, an altitude of
40 km below hmF2 is assumed to be where a plasma
bubble could be initiated. Figure 11b shows the seasonalvariation of g/νin at a fixed altitude of 220 km (black)and at an altitude of the bottom of the F layer defined as(hmF2–40 km) (blue). An enhancement in g/νin at thebottom of the F layer (blue) is seen between May andAugust, whereas g/νin at an altitude of 220 km does notshow distinct enhancement. This result indicates thatthe uplift of the F layer plays an important role in thegrowth rate. During the night, the E × B drift velocity isOtsuka Progress in Earth and Planetary Science (2018) 5:57

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Fig. 10 Seasonal and local time variations of hmF2 at Chumphon in (a) 2006, (b) 2008, and (c) 2009 (after Nishioka et al. 2012)

Fig. 11 (a) Seasonal variation of ionospheric altitude; crosses represent hmF2 derived from foF2 and M(3000)F2 at Chumphon, Thailand. The
running average of the daily hmF2 with a window of 3 months is shown by the red curve. The blue curve shows the altitude of the bottom-side
ionosphere, which is defined as the altitude 40 km lower than the averagedhmF2 (red curve). (b) Estimated g × B drift velocity; the blue curve shows
the g × B drift at the bottom side of the ionosphere as defined in the top panel. The black curve shows the g × B drift at a fixed altitude of 220 km(after Nishioka et al. 2012)

Otsuka Progress in Earth and Planetary Science (2018) 5:57

downward, but the seasonal average of E × B drift velocity(E/B) is approximately 10 m/s for solar minimum conditions (e.g., Fejer et al. 1991). g/νin compensates for the negative value of E × B drift velocity, and thus the growth rateor the Rayleigh–Taylor instability becomes positive. Therefore, the Rayleigh–Taylor instability may occur even near
midnight through the enhanced gravity-driven eastward
electric current, and plasma bubbles could grow.
Nicolls et al. (2006) have also shown observational evidence of F-layer uplift around midnight using an ionosondein Brazil. Performing model calculations of the F layer, they
have reported that the uplifts of the F layer around midnight are in general not caused by eastward electric fields,
but a decreasing westward electric field in conjunction with
sufficient recombination and plasma flux can be responsible
for an uplift of the F layer. The weakening of the westward
electric fields can be caused by an enhancement of theequatorward wind or an abatement of the poleward wind
through the F-region dynamo mechanism. These meridional wind variations may be associated with the midnighttemperature maximum (MTM). Using the SAMI2 modelwith E × B drift observed with the C/NOFS satellite as the
input for the background electric fields, Ajith et al. (2016)
have evaluated each term of the field-line-integrated growthrate and showed that a steep altitude gradient in thefield-line-integrated electron content is the most importantparameter for the increase of the growth rate at midnightaround the June solstice. Using the SAMI3/ESF model,
Huba and Krall (2013) have investigated the effect of themeridional neutral winds on the growth rate of the Rayleigh–Taylor instability. These researchers have shown thatthe equatorward winds at low latitudes (i.e., at a nonzero
inclination of the magnetic field) could destabilize the bottom of the F layer by decreasing the Pedersen conductivity
by the same mechanisms proposed by Maruyama et al.
(2009). The parallel component of the neutral wind along
the geomagnetic field carries the plasma to higher altitudes,
thus reducing the Pedersen conductivity such that the bottom of the F layer is destabilized by the Rayleigh–Taylor instability. These researchers also observed that the eastward
Pedersen current induced by the equatorward wind helps
to increase the RT growth rate. Dao et al. (2017) haveshown that post-midnight plasma bubbles develop towardhigher altitudes/latitudes when the poleward neutral windabated and the neutral wind became equatorward, suggesting that the equatorward neutral winds increased thegrowth rate of the Rayleigh–Taylor instability.
Relation to midnight temperature maximum

As described above, the uplift of the F layer may be apossible cause for conditions preferable for theRayleigh–Taylor instability around midnight. However,
we have a missing link; what causes the F-layer upliftaround midnight? Nicolls et al. (2006), Yokoyama et al.

Page 11 of 13(2011), and Nishioka et al. (2012) have suggested that
meridional neutral winds associated with midnighttemperature maximum (MTMs) can be drivers to increase the growth rate of the Rayleigh–Taylor instability
by uplifting the F layer. MTM is a phenomenon where
the neutral temperature in the thermosphere reaches alocal maximum around local midnight. Niranjan et al.
(2006) have suggested that MTMs more frequently
occur during low solar activity conditions. This feature
is consistent with the solar activity dependence of the
post-midnight irregularities. Faivre et al. (2006) haveshown that the amplitude of the observed neutral
temperature is as high as 50–200 K. MTM could be a
result of tidal phenomena and accompanied by the midnight density maximum (MDM) (Akmaev et al. 2010).
The convergence of thermospheric tidal winds aroundthe equator is considered to be responsible for generating MDMs and MTMs through adiabatic heating(Herrero 1982). Using the Thermosphere IonosphereElectrodynamics General Circulation Model (TIEGCM),
Fesen (1996) have shown that the MTM is related to atmospheric tides and that interaction of semidiurnal tides
with (2, 2) and (2, 3) modes could be responsible forgenerating MTMs and MDMs. This researcher has further shown that these two modes reinforce each other insummer, causing frequent occurrences of MTMs in thesummer hemisphere. By carrying out long-term simulations with the Whole Atmosphere Model, Akmaev et al.
(2010) have revealed that the terdiurnal tide propagating
upward from the lower atmosphere also plays an important role for the enhancement of the neutral temperaturearound midnight.

Conclusion
During solar minimum conditions, post-midnight irregularities may occur mostly in association with plasma
bubbles initiated around midnight. The statistical characteristics of these post-midnight irregularities can besummarized as follows: (1) The post-midnight irregularities occur frequently around the June solstice in low
solar activity conditions. Their occurrence rate increases
with decreasing solar activity. (2) The regions withirregularities move mostly westward. (3) The post-midnight irregularities do not accompany GPS scintillation
or weak GPS scintillation.
The mechanisms for generating these post-midnight
irregularities are argued from the perspective of two aspects. The first aspect includes a seeding of theRayleigh–Taylor instability by atmospheric gravity waves
and MSTID. Gravity waves with circular phase patterns
could effectively initiate the instability through the electric field perturbations caused by dynamo mechanisms.
Gravity waves launched from the ITCZ may contributeto seeding the instability. MSTID associated with theOtsuka Progress in Earth and Planetary Science (2018) 5:57

polarization electric fields may also initiate the instability. The second aspect includes F region conditions
favorable for the Rayleigh–Taylor instability, especiallythe altitude of the F layer. The uplift of the F layer
frequently occurs at midnight around the June solsticeseason and coincided with the post-midnight irregularities. The decrease in the ion-neutral collision frequency
at higher altitudes increases the growth rate of theinstability. The F layer uplift could be caused by themagnetically equatorward winds associated with MTMs.
Abbreviations
FAI: Field-aligned irregularity; MSTID: Medium-scale traveling ionosphericdisturbance; MTM: Midnight temperature maximum
Acknowledgements
This work was done as a part of “Workshop on Ionospheric Plasma
Bubble Seeding and Development” organized by the Institute forSpace-Earth Environmental Research, Nagoya University.
Funding
This work was supported by JSPS KAKENHI Grant Number 15H05815 and
16H05736.
Availability of data and materials
Data sharing not applicable to this article as no datasets were generated oranalyzed during the current study.
Author’s contributions
YO wrote all the parts of the manuscript.
Author’s information
YO studies the Earth’s ionosphere mainly by using GNSS receivers, radar, andoptical instruments.
Competing interests
The author declares that he has no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in publishedmaps and institutional affiliations.
Received: 15 November 2017 Accepted: 29 August 2018

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