Abdu Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-019-0258-1(2019) 6:11

Progress in Earth andPlanetary Science

REVIEW

Open Access

Day-to-day and short-term variabilities inthe equatorial plasma bubble/spread F
irregularity seeding and development
Mangalathayil Ali Abdu1,2

Abstract
The background ionospheric conditions shaped by sunset electrodynamic processes are responsible for thedevelopment of equatorial plasma bubble (EPB)/equatorial spread F (ESF) irregularities of the post-sunset
ionosphere. Distinct conditions exist for the EPB/ESF development also at later hours of the night. The plasma
instability growth leading to EPB generation is dependent on the basic precursor conditions defined by thewell-known parameters: the evening prereversal enhancement in vertical plasma drift (PRE), wave structure inplasma density and polarization electric field required to initiate/seed the instability, and the F layer bottomside density gradient, as a significant factor in controlling the growth rate. Competing roles of the zonal versus
meridional thermospheric winds additionally control their development. Statistical as well as case studies haveaddressed aspects of the EPB development and occurrence under different geophysical conditions, generally
focusing attention on any one of the above specific parameters. Little is known regarding the relative importanceof concurrent presence of the precursor parameters (mentioned above) in shaping a given event. A large degreeof day-to-day variability in these parameters arise from different sources of forcing, such as upward propagating
atmospheric waves, and magnetic disturbance time electric fields in the form of prompt penetration anddisturbance dynamo electric fields that often contribute to the widely observed short-term variabilities in EPB
development and dynamics. In this paper, we will present and discuss some important aspects of the EPB/
ESF short-term variability, focusing attention on their enhanced development, or suppression and, wherever
possible, highlighting also the relative roles of the precursor parameters in such variability.
Keywords: Equatorial ionosphere, Plasma bubbles/spread F irregularities, Prereversal vertical drift, Prompt
penetration electric field, Gravity waves, Trans-equatorial wind, Planetary/Kelvin waves, ESF/EPB short-term
variability

Introduction
The nighttime equatorial ionospheric plasma structuring manifests itself in the form of geomagnetic field-aligned
plasma irregularities, also known by its generic name as
equatorial spread F (ESF) or equatorial plasma bubble(EPB) irregularities. The ESF/EPB irregularities occur in awide spectrum of scale sizes (from few centimeters to a few
hundreds of kilometers); the shortest scale measured being
11 cm (Tsunoda 1980) and the outer scale size reaching
800–1000 km. They also present a large degree of variability
with wide-ranging time scales. The long-term variability inCorrespondence: abdu.managa@gmail.com
1
Instituto Tecnológico de Aeronáutica (ITA), São Jose dos Campos, Brazil
2
Instituto Nacional de Pesquisas Espaciais (INPE), São Jose dos Campos, Brazil

their development and occurrence is dependent largely on
solar activity cycle defined by the variations in solar EUV
flux and/or sunspot number. The medium-term variability
is dependent on the season as well as solar flux, and it is by
far relatively better understood and therefore more easily
predictable. There are also short-term variabilities in theform of changes occurring on a day-to-day basis, as well asof a more transient nature in response to their driving
sources with corresponding time scales. Upward propagating atmospheric waves, such as, gravity waves, and planetary waves, produce variabilities on time scales varying fromshort-term to several days of quasi periods. It is reasonable
to consider that the shorter term and day-to-day variabilities could be largely controlled by gravity waves, whereas© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

Abdu Progress in Earth and Planetary Science(2019) 6:11

the variability on time scale of a few to several days could
be largely driven by planetary/Kelvin waves through theirmodulation of tidal winds. The variabilities arising frommagnetospheric (and high latitude) forcing can also be oftransient nature with time scales varying from minutes tohours, or of several days. Little is known about the different
drivers of these short-term and day-to-day variabilities,
which therefore are the least predictable component of the
ESF variability. Investigations on the sources and characteristics of the variabilities are important due, primarily, to theimpacts of these irregularities and the background ionospheric conditions on the different space application
systems of practical interest to us. Further, a better understanding of the possible drivers of such variabilities
is a primary requirement for advancing our knowledge
on the science of the coupling processes that govern
their development as well as to make continuing progress towards developing predictive capability on theiroccurrences.
The irregularities originate from plasma instability
process that may develop in the nighttime ionospherewhen conditions of the background ionosphere are favorable for their development. In the most typical scenario, the instability may develop through the RayleighTaylor interchange mechanism (Dungey 1956) acting
on the seed perturbations in density and polarizationelectric field produced by gravity waves at the bottomside of a rapidly rising post-sunset F layer. The rapid
post-sunset rise of layer (PSSR) occurs through the enhanced vertical plasma drift arising from the evening
prereversal enhancement in the zonal electric field (thePRE). The PRE is generated through the action of the Flayer dynamo under sunset electrodynamics processes.
The thermospheric zonal wind, which is eastward in
the evening, produces vertical polarization electric fieldin the F region that has strong longitudinal gradientacross the sunset terminator due to the decay in the Elayer conductivity towards the nightside. This situation
leads to the development of the PRE as explained by
Rishbeth (1971) (see also Heelis et al. 1974; Farley et al.
1986; Eccles et al. 2015). Based on a TIECGM simulation study, a candid explanation of the PRE development was recently presented by Heelis et al. (2012).
During low solar flux years, the PRE vertical drift velocity is relatively small, when therefore the role of an
instability seed source may become relatively more important for the ESF development. The instability seeding through perturbations in electron density andpolarization electric field is believed to be provided bygravity waves propagating upward from sources of theirlikely generation in the tropospheric convective regions(see for example, Rottger 1981; Huang et al. 1993; Mc
Clure et al. 1998; Fritts and Vadas 2008; Abdu et al.
2009a; Tsunoda 2010; Li et al. 2016). Once initiated atPage 2 of 22

the F layer bottom side gradient region under sufficiently large linear growth rate condition, the instability
may grow nonlinearly to topside ionosphere in the formof plasma-depleted magnetic field-aligned structures.
These structures may cascade into a wide spectrum of
scale sizes of irregularities (Haerendel 1973), the composite form of which is known by the generic name, the
ESF irregularities, that are observable by a wide variety
of diagnostic techniques. Being part of the coupledatmosphere-ionosphere-thermosphere system, they suffer
variabilities due to different sources of forcing related tomagnetospheric disturbances and lower atmospheric wave
activities. As a result, large degrees of day-to-day andshort-term variabilities mark the development and occurrence of the EPB/ESF irregularities, which constitute theprime focus of this paper.
Plasma bubble/ESF generation mechanism

The EPB/ESF irregularity development takes place in sequential phases, the observed details of which can varydepending upon the diagnostic techniques. The earliest
observation by ionosondes (Booker and Wells 1938)
showed that the rise of the post-sunset F layer to higher
altitudes was a precondition for the occurrence of range
spread F echoes in ionogram. The observations also revealed that the presence of satellite traces adjacent to
the main F layer trace in ionogram was often a precursor to the spread F (SF) development (Lyon et al. 1961;
Abdu et al. 1981; Tsunoda 2008). The range-timeintensity (RTI) map obtained by the Jicamarca VHF
radar showed that the generation of 3-m irregularities
was initiated at the F layer bottom side. Their upward
growth to the topside ionosphere provided the first indication that the ESF irregularities are associated withmagnetic field-aligned plasma depletions, that is, plasma
bubbles, that rise upward from the F layer bottomside following their generation through the Rayleigh-Taylor (R-T)
interchange instability mechanism (Woodman and La
Hoz 1976; Woodman 2009; Scannapieco and Ossakow
1976; Hanson and Sanatani 1971). Diagnostics by the ALTAIR fully steerable incoherent scatter radar (at UHF) has
provided important information on the characteristics ofthe two-dimensional background conditions propecious
for the irregularity growth and evolution. A notable precursor condition was identified as the post-sunset upwelling of the F layer having characteristics of a large-scalewave structure (LSWS) (e.g., Tsunoda et al. 1979; Tsunoda
and White 1981). From east-west scan measurements inthe equatorial plane, it was observed that the irregularities
first developed at the west wall of the upwelling, which
was attributed to instability growth through zonal windinteraction with the horizontal gradient region of the
upwelling or that of associated wave structure. Basedon ALTAIR results complemented by results from otherAbdu Progress in Earth and Planetary Science(2019) 6:11

instruments (such as ionosondes and optical imagers),
Tsunoda (2015) further characterized the EPB generation process as occurring in three phases: (1) amplification of the upwelling during the post-sunset rise of
the F layer (due to prereversal vertical drift, PRE), (2)launching of the EPB from the crest of the upwelling,and (3) structuring of the plasma within the upwelling.
The ALTAIR observations further revealed that EPBs
occurred often in clusters that are connected with the
ESF patches in the bottom side F layer and that their
zonal widths were comparable to those of the upwelling. Considerations on the day-to-day variability in the
background conditions and on the drivers that contribute to the phases defining the EPB/ESF development,
especially those related to the phases (1) and (2) above,
constitute the key elements of the discussion to follow
in this paper. Central to the discussion to begin with isthe basic plasma instability mechanism responsible forthe generation of the EPB/ESF irregularities, that is, theRayleigh-Taylor (R-T) interchange mechanism. A comprehensive picture of the EPB generation mechanism
and the different sources of forcing that can modify the
coupled processes leading to the EPB short-term variability is presented schematically in Fig. 1. The red colored blocks represent the parameters that play directroles in the processes leading to the EPB generation.
Their relationship contributing to the instability linear

Page 3 of 22

growth rate (γFT) through the generalized RayleighTaylor mechanism can be expressed through field line
integrated parameters as given by Eq. 1 (Sultan 1996)
γ FT ¼ ð1=LFT Þ ΣP F =ΣP F þ ΣP E
 E=B  U P FT þ g=νin −βFTð1ÞEach of terms in Eq. 1 is represented as a corresponding box in Fig. 1. The term E/B represents the evening
prereversal vertical drift marked as PRE in Fig. 1; g is thegravitational acceleration; νin is the ion-neutral collision
frequency, with the gravity term g/νin also shown in the
figure; and LFT is the F layer flux tube integrated bottomside gradient scale length, shown in the figure box also
as such, (which has its simplified local form expressed as
Ni/(ΔNi/Δh), where Ni is the ion density). The ratio of F
region field line integrated conductivity to the sum of
the E and F region-integrated conductivities, (ΣPF/(ΣPF+ ΣP)), in Eq. 1 is shown as such in the figure box. βFT is
the recombination rate (not shown in the Figure). U PFT is
the meridional wind in the vertical plane marked as suchin the figure box, which can also be part of a transequatorial wind (TEW). We may further note that (1)the conductivity ratio (in Eq. 1) increases from a fractionof one before sunset to one as the E layer conductivity
decays into the night, (2) the linear growth rate increases
with decrease in gradient length, and (3) both the PRE

Fig. 1 A schematic representation of the coupled processes controlling the EPB/ESF development and its short-term variabilities. The boxes in
red show the connection among the specific background parameters (explained in the text) that determine the EPB growth through R-T
instability linear growth rate. The green boxes depict the parameters that modify/control the basic parameters responsible for EPB generation.
The blue boxes specify the sources of forcing that operate in the different types of the short-term variabilities as explained in the text

Abdu Progress in Earth and Planetary Science(2019) 6:11

vertical drift and the gravity terms contribute positively
to the linear growth rate while the meridional wind term
contributes mostly negatively to γFT. It is to be noted
further that the contribution to γFT arising from the PRE
vertical drift has additional aspects: (a) an increase in E/
B can result in an increase in the gravity term (g/νin) due
to the decrease in the ion-neutral collision frequency as
the F layer height rises due to the PRE, the gravity term
thereby further raising the γFT, and (b) the gradient scale
length has been found to increase with increase in F
layer height, so that an increase in E/B (that raises the Flayer height) can contribute to an increase in the gradientscale length (LFT) that, in turn, could contribute to slow
down the growth rate. We may point out that the linear
growth rate is derived from one-dimensional perturbation
analysis in which the seed perturbation is assumed not to
perturb the background conditions represented as fieldline-integrated quantities. Therefore, if the linear growthrate so obtained is found to be above a threshold value, instability growth could occur (in the form of structure
amplitude growth) along the entire field line. Considering
the equatorial plane two-dimensional situation, suchgrowth should be favored at the crest of the upwelling depending upon the degree to which the LSWS may modulate the larger scale upwelling of evening F layer producedby the PRE (to be further commented later). Even when
the linear growth rate is large enough, the instability may
develop only if a seed perturbation of sufficient amplitude
is present. The seed perturbation can be in the form of awave structure in density and polarization electric field(indicated in the fig. box as WS. Pol E-field), which may
be induced by gravity waves, as depicted in the figure. Another form of seeding process has been suggested as that
arising from instability growth in the shear flow region of
the post-sunset plasma vortex. As discussed by Kudeki
and Bhattacharyya (1999) and Hysell and Kudeki (2004),
the westward plasma flow in the presence of eastward
neutral wind in the bottomside/valley region of the F layer(below the plasma vortex focus) can be an important energy source for creating an instability that may provide theseed perturbation capable of initiating EPB developmentby the R-T mechanism. Nonlinear growth of the instability(from a background condition already distorted by the linear growth process) may lead to a more rapid rise of bubble structures to topside ionosphere. However, such
nonlinear growth can be retarded/limited by the field line
integrated conductivity that is controlled (mostly increased) by meridional/trans-equatorial winds. It should
be pointed out that the linear growth rate expression
stated above was derived on assumption of small perturbations, with higher order terms discarded (see Sultan
1996, for details) so that it represents only a tendency forthe instability growth. As a result, discrepancy may be expected between a specific ESF observation and the linear
growth rate that predicted its development.

Page 4 of 22

The parameters indicated in the top green box ofFig. 1, namely, the thermospheric zonal wind and the Elayer conductivity sunset (SS) gradient in the evening
hours, are the ones directly responsible for the development of the evening prereversal vertical drift enhancement(the PRE). The other green boxes indicate the parametersthat may, directly or indirectly, modify the intensity of the
PRE and thereby the factors (or terms) that determine thelinear and nonlinear growth rates for the EPB. These parameters, which we call as “modifying parameters” are (1)prompt penetration electric fields (PPEF), (2) disturbance
dynamo electric field (DDEF), (3) tidal winds modifying
the sunset gradient in the E layer conductivity, and (4)gravity waves. They can suffer large degrees of short-termchanges as a result of the variabilities in the sources driving them (driving sources), represented in blue boxes inFig. 1. These drivers are magnetospheric disturbances andupward propagating wave disturbances originating fromthe lower atmosphere. The latter is dominated by upward
propagating atmospheric waves in the form of gravity
waves and planetary/Kelvin waves originating from tropospheric convection processes. The planetary waves (PWs)
are very large horizontal scale atmospheric oscillations inneutral wind, density, and pressure that propagate zonallyand also vertically from their sources in the troposphericstratospheric regions. They have periodicity varying from
2 to 20 days and propagate globally, mostly westward,
with significant meridional and zonal velocity components (e.g., Forbes 1996). They have zonal wave length ofseveral thousands of kilometers, with small vertical wave
length (of a few tens of kilometers), which limits their upward propagation to middle atmosphere and lower thermosphere and perhaps up to the dynamo region. Kelvin waves
are one type of PWs (generated in the troposphere) thatare trapped in the equatorial region and propagate eastward(e.g., Salby and Garcia 1987; Liu et al. 2012). The ultra-fast
Kelvin (UFK) wave that has a period of 3–4 days and vertical wave length of 30–50 km is of particular interest to us
since it can propagate upward to even higher altitude into
the thermosphere with significant amplitude dominatedby zonal wind component. The atmospheric tides, especially the diurnal and semidiurnal waves (due to theirlarger vertical wave lengths), reach ionospheric heights
where they produce dynamo electric fields that drive
the regular ionospheric dynamics including the current
systems and the plasma drifts. Nonlinear interaction
can occur between planetary/UFK waves and tidal
waves (Pancheva et al. 2003), and the resulting modulated tidal winds interacting with the dynamo region isbelieved to control the PRE vertical drift and F layerheight variations (Abdu et al. 2015a). The variabilities

Abdu Progress in Earth and Planetary Science(2019) 6:11

in these driving sources when they are imposed on the“modifying parameters” mentioned above constitute thebasic drivers of the day-to-day and short-term variabilities in the development and occurrence of the EPB/EFS irregularities.
The main paths by which the short-term variability in the
EPB can occur are through changes in the following parameters: (1) The PRE vertical drift that mainly controls the linear growth rate. It can undergo large variations throughsuperposition of vertical drifts due to disturbance electricfields, in the form of prompt penetration electric field(PPEF) and disturbance wind dynamo electric field (DDEF),
when they occur in the evening hours. The PRE canundergo changes also through planetary/Kelvin wave
modulation of the tides (as mentioned above) that modify
the longitudinal gradient in the E layer Pedersen conductivity at sunset that controls the PRE. To understand thisprocess better, we may consider, for example, the zonal
component of a modulated tidal wind interacting with theE region plasma in the environment of large height gradientin the ratio of ion-neutral collision-to-gyro frequency. The
zonal wind can cause vertical motion of ions (both the molecular and atomic ions) that may modify the E layer density, to different degrees, on the day- and nightside of theterminator. Thus, the evening time zonal wind can causesignificant modification in the sunset longitudinal gradientin plasma density and hence in the E layer Pedersen conductivity. As indicated in Fig. 1, the PRE can be modified
also by gravity waves induced oscillations in the evening F
layer heights. (2) Variability in the seed perturbations in theform of precursor wave structure (with associated perturbations in polarization electric field), possibly induced bygravity waves, is another source of the short-term variability. When the PRE vertical drift has relatively smaller amplitudes, its modulation by gravity waves may also become
significant as will be discussed later. (3) Meridional/trans-equatorial winds (UPFT) that can modify the field line integrated conductivity that controls the instability linear
growth rate as well as the nonlinear growth of the plasma
bubble. The (UPFT) arises from the north-south asymmetry
in the thermospheric heating during quiet time (as also by
the storm time asymmetric heating of the high latitude
thermospheres). We will briefly discuss below some new
and recent results on the short-term variabilities in EPB irregularities arising from the various sources mentioned
above, highlighting their important characteristics and theelectrodynamics conditions in which they occur.

EPB/ESF variability operating through changes inPRE
Observationally, the dependence of EPB/ESF development on the PRE vertical drift has been investigated extensively (e.g., Abdu et al. 1983, 2009c; Fejer et al. 1999;

Page 5 of 22

Huang and Hairston 2015). The PRE can suffer variability in two ways: (1) superposition of the vertical drift
due to penetration electric field and disturbance dynamo
electric field on the PRE which will be discussed in
Section 2.1 and (2) changes in the two basic parameters
responsible for its normal development mentioned earlier, that is, (a) the thermospheric zonal wind in the evening and (b) the longitudinal gradient in the E layer
conductivity across the terminator. Item 2(a) will not be
discussed specifically. Item 2(b) on conductivity gradientthat involves the role of E layer winds (due to upward
propagating atmospheric waves) will be discussed in
Section 2.2. When the PRE is of weak intensity, it may
suffer significant modification also due to gravity waves,which will be discussed in Section 3. We may further
note that the PRE vertical drift is known to be part ofthe post-sunset plasma vortex flow, in which the E layer
zonal wind has a contribution to the vertical shear in thezonal plasma, as studied using the thermosphere ionosphere electrodynamics - general circulation model(TIE-GCM) by Rodrigues et al. (2012). We will not be
discussing here the specific role of this process in the
PRE variability. Another question concerns the identification of threshold levels in the PRE vertical drift and in
the associated post-sunset F layer height required for the
EPB/ESF development. Such threshold levels can varysignificantly depending upon the longitude, season, andsolar flux conditions of the analyzed data (for more details see, for example, Farley et al. 1970; Abdu et al.
1983; Fejer et al. 1999; Manju et al. 2007; Smith et al.
2016). It should be kept in mind that the degree of variability in the ESF occurrence, to result from a change in
the PRE or from an imposed disturbance electric fields
to be described below, should depend inevitably upon
the threshold conditions for their development prevailing at the time of the occurrence of the disturbances.
Short-term variability in PRE due to penetration electricfields and disturbance dynamo electric field

The magnetospheric electric fields penetrating to low
and equatorial latitudes under disturbed conditions cancause drastic modifications in the ionospheric currents
and plasma structuring (see, for example, Nishida 1971;
Kikuchi et al. 1996; Basu et al. 2001; Fejer 2011; Abdu
et al. 2003; Abdu 2012). Under the interplanetary magnetic field (IMF), Bz, southward conditions and the reconnection process the interplanetary electric fieldmaps to high latitudes. The energy transfer process that
follows is responsible for the rapid auroral electrojet intensification marking a substorm development, which
may often be accompanied by concurrent global storm
development manifested by rapid ring current intensification indicated by Dst decrease. The high latitude electricfield, identified also as magnetospheric convection electricAbdu Progress in Earth and Planetary Science(2019) 6:11

field, promptly penetrates to equatorial latitudes as anunder-shielding (dawn-dusk) prompt penetration electric field (PPEF) that has eastward polarity in the daysideevening sector and westward polarity in the night sector(Fejer and Scherliess 1995; Kikuchi and Hashimoto 2016).
A shielding layer due to the region-2 FAC (field aligned
current) develops in time scale of 20–30 min to balance
the convection electric field so that a subsequent northward turning of the Bz marking an auroral electrojet (AE)recovery may result in equatorward penetration of an
over-shielding electric field (OSEF). The OSEF polarity is
dusk-to-dawn, (opposite to that of the PPEF), that is, westward in the dayside-evening sector and eastward in thenight sector (Kelley et al. 1979; Fejer et al. 2008). The
OSEF is typically followed by disturbance wind dynamo(arising from auroral heating) with the associated electricfield (DDEF) dominating the equatorial latitudes. The
DDEF becomes active with a time delay of a few hours(3–4 h) from the storm development, often coinciding
with the recovery phase of the storm (Blanc and Richmond 1980; Richmond et al. 2003). The polarity of thepenetration electric field in the sunset sector, with its associated vertical drift complementing the PRE vertical drift,
is a very crucial factor for the post-sunset EPB development (Abdu et al. 2003, 2009b; Li et al. 2010). An exampleof the contrasting effects due to under-shielding andover-shielding electric fields occurring in the evening over
Fortaleza (3.9° S, 38.45° W, dip angle − 9°), Brazil, is presented in Fig. 2 (Abdu et al. 2009b, see also Abdu et al.
2012). On 25 September, the Bz turning south (with weak
amplitude) at ~ 1930 UT (LT = UT–2 h 28 min at Fortaleza) initiated an AE activity, also of weak amplitude, thatsoon intensified at ~ 20:30 (~ 1800 LT) marking a substorm. At this time, the PRE was already under development over Fortaleza longitude. The under-shielding PPEF
of eastward polarity associated with the AE intensifications and the (eventual) substorm development caused anenhancement in the evening vertical drift that was superposed on PRE vertical drift. The PRE vertical drift so enhanced attained a peak value of ~ 75 m/s at 21:30 UT(1902 LT) as compared to the 50 m/s peak drift of thequiet time PRE (shown in blue curve, bottom panel) thatusually occurs at ~ 22:00 UT (19:32 LT). The large enhancement in the PRE vertical drift due to the PPEF
caused prompt development of EPB (indicated by verticalpink bars) half hour earlier than its typical quiet time onset. In contrast to this, on 23 September, the Bz was
southward with the AE activity (and the Dst decrease notshown here) prevailing several hours prior to the evening.
The subsequent Bz turning north at 19:40 UT (17:12 LT)
caused a rapid recovery in the AE activity that approachedzero by ~ 21 UT/18:32 LT, (panel 3). The associatedover-shielding electric field of westward polarity appears
to be largely responsible for the near total suppression ofPage 6 of 22

the PRE on this evening (green curve in bottom panel). In
view of the disturbance condition prevailing several hours(at least 6 h) prior to sunset, the role of a DDEF of westward polarity may also have contributed to this PRE suppression, as a result of which the usual post-sunset EPB
did not develop on this evening.
Regarding the PRE enhancement due to under-shielding
PPEF, we note that its intensity could depend upon thestrength of the penetration electric fields as illustrated by
the example of an event sequence presented in Fig. 3(Abdu et al. 2018). On 16 September, the Bz southward
fluctuations were present starting from ~ 10 UT, which
was followed by an AE intensification (still under Bz south
condition) starting at ~ 21 UT (18 LT) that coincided with
PRE development time over Sao Luis (2.33° S, 44.2° W,
dip angle −.5°) (SL). A PPEF of eastward polarity associated with the AE intensification appears to have caused anincrease in F layer heights plotted at sequential plasma
frequencies (panel 3) as well as in the enhanced PRE vertical drift that peaked at ~ 60 m/s over Sao Luis (bottom
panel). The quiet day value of the peak PRE vertical drift
was about 27 m/s (red curve), which is normally below thethreshold value required for ESF development in September. The enhancement in the PRE vertical drift on thisevening corresponded to an eastward penetration electricfield of 0.6 mV/m. As a result, ESF developed promptly
over Sao Luis having onset at 21:30 UT (18:30 LT) (panel3) followed by its delayed onset at 22:30 UT (19:30 LT)
over Cachoeira Paulista (CP) (22.6° S, 315° E; dip angle: −28°) (panel 4). The time delay of 60 min corresponds tothe vertical growth time for the EPB observed overCachoeira Paulista. Thus, the enhanced vertical drift due
to an under-shielding PPEF of eastward polarity appears
indeed to be responsible for the EPB generation manifested in the form of the ESF development observed sequentially at SL and CP. The approximate bubble rise
velocity in this case may be estimated as 150 m/s. A more
intense disturbance activity marked the evening of 17 September when a large Bz southward turning, accompanied
by a large AE intensification, which occurred at ~ 20:00
UT. The Dst decrease starting at this time (not shownhere) dipped to − 180 nT at 23:30 UT marking an intensestorm. The beginning of this storm disturbance at ~ 20
UT also coincided with the PRE development time as was
the disturbance on the previous evening. The undershielding PPEF of eastward polarity (associated with thestorm) caused large enhancement in the vertical drift thatwas superposed on the PRE vertical drift, which peakedat ~ 100 m/s over SL near 22:00 UT/19:00 LT. The
PPEF intensity in this case was estimated as 1.7 mV/m.
This appears to have caused the prompt ESF development observed first over SL, followed by CP with a timedelay of only 30 min. This time delay is much shorter
than that was observed in the case of such sequential

Abdu Progress in Earth and Planetary ScienceFig. 2 (See legend on next page.)(2019) 6:11

Page 7 of 22

Abdu Progress in Earth and Planetary Science(2019) 6:11

Page 8 of 22(See figure on previous page.)
Fig. 2 Variations in the IMF Bz from ACE satellite on 23 and 25 September 2001 (green and pink curves, respectively) (top panel), auroral indices
AE/AO for the same 2 days, from WDC Kyoto site (second and third panel), and vertical drift Vz (that is, d(hF)/dt) over Fortaleza (bottom panel).
The Vz values were calculated at plasma frequencies 6 MHz and 7 MHz. They are shown for 23 and 25 September, respectively, in green and pink,
together with a quiet day reference (in blue). The ESF occurrence staring at 2140 UT on 25 September is indicated by vertical bars

occurrence on the previous day which was 60 min. For
this case of shorter time delay, the bubble rise velocity
can be estimated as ~ 300 m/s. The SF is found to be of
longer duration (and perhaps more intense as well) in
this case than it was on the previous evening. These results clearly show that an under-shielding PPEF occurring
during the evening (at the time of PRE development) may
enhance the vertical drift due to the PRE, resulting in thegeneration of EPB. The larger the intensity of the PPEF
the more intense will be the PRE vertical drift and theresulting EPB rise velocity (as judged from Digisonde observations). The increases in the intensity of the PRE andin EPB/ESF with increasing intensity of the penetrationelectric field as verified in these results might suggest an
apparent proportionality in the observed cause-effect sequence. However, such a proportionality sequence does
not seem to hold for abnormally large intensity of the disturbance electric field. For example, from analysis of radarand Digisonde data during the October 2003 magnetic
super storm, Abdu et al. (2008a, 2008b) found that a large
uplift of the F layer at a velocity approaching 1000 m/s
over Brazil that was driven by an intense eastward electricfield of ~ 30 mV/m did not produce any UHF (GPS) scintillation or enhanced spread F in ionogram. The observation was made in a region strongly influenced by theSouth Atlantic (or South American) Magnetic Anomaly(SAMA) wherein enhanced ionization by storm-induced
energetic particle precipitation is believed to produce alarge-scale gradient in the E layer conductivity that could
be responsible for the development of polarization electricfield superposed on the primary PPEF. It was intriguing,
however, that the resulting abnormally large electric field
did not support instability growth at the bottom side of
the very rapidly rising F layer. Under normal conditions,the bubble rise velocity as modeled by the R-T mechanism
is of the order of 200 m/s (e.g., Zalesak et al. 1982). ALTAIR observation showed a rise velocity of the order of
130 m/s (as inferred from Fig. 5 of the paper by Tsunoda
2015). It is not clear to what degree this rise in velocity
may increase through modification of the instability
growth parameters by the effect of an additional disturbance penetration eastward electric field occurring in the
post-sunset hours. It needs to be investigated by detailed
modeling studies. For the present discussion, we believethat the abnormally large intensity of the PPEF caused an
F layer uplift at a rate (~ 1000 m/s) that was significantly
higher than an instability vertical growth rate (rise velocity) possibly attainable under the R-T instability control

parameters (see Eq. 1) that were also modified by the samestrong PPEF. It is likely that the large uplift of the layer
might have caused a large increase in the bottom side gradient length parameter LFT that could cause a decrease in
the growth rate to a larger degree than an increase in itthat could occur by corresponding increases in the otherterms (of Eq. 1). It appears that such a situation could bea key factor in causing the breakdown of the R- T instability growth process under the extreme situation thatexisted. As a result, an enhanced development of EPB did
not occur on this night. It is not clear what should be thelimiting value of the F layer uplift rate at which the EPB
instability growth by R-T mechanism may become inoperative as we normally understand the process.
As regards the PRE vertical drift suppression by anover-shielding westward electric field, it is possible toidentify the unique role of this electric field in specificcases without the possibility of concurrent contributions to them arising from the DDEF that also has westward polarity in the evening. For this purpose, it isnecessary to consider cases of AE activity decay phase(normally indicative of northward turning of the IMF
Bz) occurring just prior to sunset hours that are preceded by weak, or insignificant, Dst decrease so thatthe westward electric field occurring at sunset could bedominantly, if not exclusively, due to over-shielding
conditions. The result of a statistical analysis of 18 casesof AE decay prior to sunset that occurred during theperiod of October to December 2003 is presented in Fig. 4.
The Sym-H/Dst index variation shown in the top panel ofthe figure represents the mean of its superposed variations
corresponding to all the cases of AE variations that presented decay in activity just preceding the evening PRE
development. The corresponding mean AE variation isshown in the middle panel. The mean of the Vz variations
for the same group of days (pink curve) is compared withthat of the 10 quietest days of the same period (bluecurve) in the bottom panel. The comparison shows a significant suppression in the PRE vertical drift due to theover-shielding westward electric field associated with therapid AE decay/recovery that just preceded the sunset(that is, the PRE vertical drift). The quiet day PRE verticaldrift peaked at 35 m/s, and the PRE decreased to 20 m/s
as a result of the AE decay that just preceded it. The weakintensity of the Dst variations that characterized the sampled days, especially during several hours prior to sunset,
may clearly rule out the possibility of any DDEF effect,
also of westward polarity, influencing the PRE suppression

Abdu Progress in Earth and Planetary Science(2019) 6:11

Page 9 of 22

Fig. 3 The panels descending from top to bottom show variations during 16, 17, and 18 September 2000 in (1) the interplanetary magnetic fieldcomponent Bz, (2) the auroral electrojet activity AE index, (3) isolines of F layer heights at fixed plasma frequencies (from 3 MHz to 12 MHz) andthe layer peak height hmF2 as observed by a Digisonde at Sao Luis, (4) the same parameters as in (3) over Cachoeira Paulista, and (5) the verticaldrift calculated as mean of dhF/dt at plasma the frequencies 4, 5, 6, and 7 MHz over Sao Luis. The spread F intensity index parameter, fop, is also
shown as vertical bar plots in orange color in panels 3 and 4 for Sao Luis and Cachoeira Paulista

effect attributed to the over-shielding westward electricfield. The weakened PRE vertical drift caused correspondingly weakened EPB development on the differentdays studied (not shown here).
In a typical sequence of magnetic disturbance phases
the disturbance dynamo electric field occurs with adelay of a few hours (usually 4–5 h) from the onset ofthe storm disturbance (Sastri 1988; Fejer and Scherliess
1995; Scherliess and Fejer 1997). In the case of a clear/isolated substorm event, the DDEF if present should be
easily observable within a few hours after the over-shielding

electric field. During a typical global storm event characterized by a significant Dst decrease, the DDEF should be observable during the Dst recovery phase. The DDEF, with its
westward polarity in the evening, may cause reduction in
the PRE vertical drift leading to suppression of the postsunset EPB development (as mentioned before). At laterhours, its eastward polarity produces upward drift(layer rise), which may result in EPB development during post-midnight hours (Carter et al. 2016; Abdu 2012).
Carter et al. (2016) analyzed scintillation data from GPS
and VHF receivers, and ionosonde data, covering a wide

Abdu Progress in Earth and Planetary Science(2019) 6:11

Fig. 4 The middle panel shows the variations in the mean of thesuperposed values of 18 cases (on 18 days) of AE decay that
occurred just prior to sunset during the period October, November,and December 2003. The top panel shows the corresponding meanvariation in the Sym-H/Dst values, and the bottom panel shows thecorresponding variation in the vertical drift velocity (pink curve) andthe mean of ten quiet days of vertical drift (blue curve),
demonstrating the PRE vertical drift suppression due to overshielding westward electric field

range of longitude, during the 17 March 2015 storm. Calculation of the Rayleigh-Taylor linear growth rate basedon coupled thermosphere –ionosphere modeling (TIEGCM) results during an event sequence showed that
post-sunset inhibition, or post-midnight development, of
scintillation could be caused by disturbance dynamo electric field contributing, respectively, to decrease or increase
the instability growth rate.
Short-term variability through PRE modifications byatmospheric waves

The PRE vertical drift and post-sunset F layer heights
can be modified significantly by upward propagating atmospheric waves originating from the sources of theirgeneration in the lower atmosphere. These waves are,as mentioned before, the gravity waves, planetary/Kelvin waves and tidal waves. The results of such modifications constitute an important component of the ESF/

Page 10 of 22

EPB variability to be considered under magneticallyquiet conditions. The planetary- and Kelvin- waves (especially the ultra-fast Kelvin waves; UFK) of 3–5 days of
periodicity in their upward propagation have been observed to produce oscillations in the PRE vertical drifts(Abdu et al. 2006a, 2015a) and in post-sunset equatorial
F layer heights (see, for example, Takahashi et al. 2006,
2007; Fagundes et al. 2009). Manifestations of thesewaves in the post-sunset F region have been establishedthrough studies comparing the oscillations characteristics of mesospheric winds with concurrent oscillationsin the evening PRE vertical drift and post-sunset F layerheights. Good correlation was found between the 3–5
day periodicity in the mesospheric zonal wind over a
low-latitude station, Cachoeira Paulista (22.6° S, 315° E),
and the PRE vertical drift over an equatorial site, Cachimbo(9.46° S, 54.83° W), as demonstrated by Abdu et al.
(2006a). Takahashi et al. (2006) showed that the planetary wave oscillations at 3–6 day period in mesosphericzonal wind (near 90 km) over the equatorial sites, Cariri(7.4° S, 36.5° W) and Ascension Island (7.9° S, 14.4° W),
were well correlated with such oscillations in the postsunset F layer heights (h’F) at 20:00 LT over Fortaleza(3.9° S, 38.4° W, Geomag. 2.1° S). Takahashi et al. (2007)
found also 3–4 day oscillations in the meteor radar zonal
wind over Cariri and in foF2 and h’F at 20:00 LT over Fortaleza correlating with the upward propagating TIMED/SABER temperature wave of the same period, which suggested the process of UFK wave modulation of the equatorial nighttime ionosphere. The above results may point
to the presence of planetary/UFK wave-induced variability
also in the ESF/EPB irregularity development that are basically controlled by the PRE vertical drift or the postsunset F layer heights that are modified by these waves.
Modulation of the PRE vertical drift, accompanied by corresponding modulation in the post-sunset ESF development, by UFK and FK waves was demonstrated by Abdu
et al. (2015a). As an example, Fig. 5 shows a wavelet
spectrum of the PRE vertical drift (bottom panel) withcorresponding variations in ESF intensity (top panel) in
response to a UFK wave episode that occurred in October2005 as corroborated from observations of upward propagating temperature wave (seen in TIMED/SABER data)
and mesospheric winds over Tirunelveli (8.7° N, 77.8° E)
and Cariri (not shown here, but see Abdu et al. 2015a).
We may note that increases in PRE vertical drift (such as
those indicated by vertical arrows in the middle panel)
due to the 3–4-day UFK wave were responsible for theearlier onset and higher intensity of the spread F on thesedays. As explained in Abdu et al. (2015a), the peak UFK
wave activity at mesospheric heights occurred some
10 days earlier to its manifestation as PRE vertical drift at
the F layer heights. From this time delay in the PRE vertical drift (with respect to the UFK activity at mesosphericAbdu Progress in Earth and Planetary Science(2019) 6:11

Page 11 of 22

Fig. 5 (Top panel) the variation, during the period 01 August to 31 October 2005, in the spread F intensity represented by fop (the top frequencyof the spread F trace in ionogram representative of its intensity); (middle panel) the PRE vertical drift peak on each of the days, and (bottom) the
wavelet spectrum of the PRE vertical drift

heights), the vertical propagation velocity of the UFK
waves was estimated as ~ 5 km/day, which is in agreement with previous results (e.g., Pancheva et al. 2003).
In this way, it was possible to conclude that the waves

propagating up to the dynamo region (~ 140 km) was
responsible for the effects observed in the F region in
the form of the PRE vertical drift and ESF development.
Such PRE vertical drift modulation is possible throughAbdu Progress in Earth and Planetary Science(2019) 6:11

a corresponding modulation of the longitudinal/local
time gradient in the E layer conductivity around sunset
that may be shaped by the E layer zonal wind (Abduet al. 2006a). As explained by Abdu et al. (2006a), thetidal modes of the E layer winds could be modified by
nonlinear interaction with upward propagating UFK
waves (similar to the cases of planetary wave modulation of the tides through nonlinear interaction as explained by Pancheva et al. 2003). The E layer winds so
modified could in turn cause changes in the longitudinal gradient in E layer conductivity (as was explainedin Section 1.1), which could lead to the PRE modulation by the UFK waves (Abdu et al. 2006a; Abdu andBrum 2009). This cause-effect sequence highlights theimportant role of E layer winds in causing the variabilities in PRE and ESF/EPB developments, a point we will
be discussing again. It is relevant to point out in thiscontext that the ability by the TIE-GCM to predict realistic PRE vertical drift depended on the adjustments inthe night E region density and in the amplitude andphase of atmospheric tides (Fesen et al. 2000).
We may note here that the upward propagating waves
in the form of tidal modes and planetary/Kelvin waves
are of global scale and therefore their modulation of the
PRE vertical drift and ESF may not present any perceivable zonal (longitude) variation as such. In contrast tothis, the typical gravity wave scales are of the order of afew hundred kilometers. For example, the large-scalewave structure (LSWS) in which the EPB seeding may
take place, and which can also be associated with (oridentified as) the seeding gravity waves, have local scalesof the order of a few hundreds of kilometers, typically
400–600 km, (e.g. Tsunoda and White 1981), which may
suffer significant zonal variation depending upon the locations of the wave generation source. As a result, we
may expect significant zonal variation in the EPB seeding process due to gravity waves, whereas such zonal
variation due to the modulation effects by tidal andplanetary/Kelvin waves appears to be absent.

Gravity waves and EFS/EPB variability
An instability seed perturbation is an important requirement for initiating the EPB development although its
diagnostics based on observational data has been a challenging task. Gravity waves originating from sources inlower atmosphere were invoked to explain the spatial
structures observed in HF radar maps of the ESF irregularities (Rottger 1981). On a statistical basis, the role ofgravity waves in the longitudinal and seasonal distribution of spread F has been in part attributed to gravity
wave generation at the inter-tropical convergence zone
(ITCZ) and to its geographical and seasonal migration(McClure et al. 1998; Tsunoda 2010). From analyses ofthe seasonal/global distribution of plasma irregularities

Page 12 of 22

observed by the ROCSAT-1 and from the OLR/ITCZ
data, Su et al. (2014) found that the role of gravity waves
was recognizable mainly in South American and African
longitude sectors, and not over all longitudes. More recently, from comparison of bubble irregularity occurrences at close-by locations in Asian sector, Li et al.
(2016) found that enhanced plasma bubble generation
was associated with more active ITCZ sector. On a casestudy basis, the role of gravity waves in seeding the ESF
irregularities has been investigated for different longitude sectors (see, for example, Abdu et al. 2009a, Sreeja
et al. 2009). Based on ionosonde and radar data, it was
found that for smaller (larger) PRE vertical drift velocities a larger (smaller) amplitude of the gravity wave perturbations (in density and polarization electric field) was
required to produce a given instability growth rate for
the ESF development (Abdu et al. 2009a). In other words,
an evaluation of the role of gravity waves (in the form of aprecursor seed perturbation) in a given EPB irregularity
development would also require a simultaneous assessment primarily of the prevailing PRE vertical drift amplitude and possibly other related parameters as well.
The precursor condition for ESF development has been
identified also in the form of large-scale wave structures(LSWS) in the background electron density as observed
by east-west scan incoherent scatter radar (Tsunoda andWhite 1981). It has been observed also in the form of longitudinal wave structures in the sub-ionospheric total electron content (TEC) as measured from C/NOFS satellite
passes (for example, Thampi et al. 2009; Tsunoda et al.
2011) and Tulasi Ram et al. 2014). It may also appear as
an additional F layer trace (satellite trace) in ionograms(e.g., Abdu et al. 2014; Tsunoda 2008; Li et al. 2012).
These wave structures have scale sizes of a few hundredsof kilometers (400–800 km), and they are present in the Fregion well before sunset. They may resemble the wave
structures represented by gravity wave-induced F layerheight oscillations analyzed by Abdu et al. 2009a. Statistically, their amplitude may grow towards sunset and canbe maintained by polarization electric field whose sustenance under daytime/afternoon conditions and amplification towards sunset have been explained by Abdu et al.
(2015b). These features are illustrated in Fig. 6, whichshows two sets of examples presented for two solar activity maximum epochs: (a) October–December 2002 (F10.7:
170) and (b) October 2014 (F10.7: 155). Figure 6a showsin the upper two panels the F layer height oscillations in
the period range 0.5–1.5 h, as superposed plots, for twogroups of days of the epoch (a) that include a total of 66
days of observations. In one group, we may note oscillations that appear nearly in-phase (coherent) on all days,
with the amplitude increasing towards sunset, and transitioning into larger amplitudes corresponding to postsunset spread F indicated by the gray segment of theAbdu Progress in Earth and Planetary Science

(a)

(b)

Fig. 6 (See legend on next page.)(2019) 6:11

Page 13 of 22

Abdu Progress in Earth and Planetary Science(2019) 6:11

Page 14 of 22(See figure on previous page.)
Fig. 6 a Top panel: F layer height oscillations at 8-MHz plasma frequency obtained by band-pass filtering (0.5–1.5 h period) of the true heightmeasured at 5 and 7.5-min cadence by a Digisonde at Cachimbo, Brazil. The superposed oscillations for a selected set of days (from a 66-day
period) are nearly in-phase (coherent) on all days. Middle panel: Similar plots for the group of remaining days of the 66-day period that show theoscillations mostly in random phase. The gray segment of each curve in both panels shows transition to spread F activity after sunset, with nonspread F hours continuing colored. Bottom panel: The mean of vertical drifts calculated as Vz = dhF/dt separately for the above two groups ofdays. (These data are from the COPEX Campaign, Abdu et al. 2009c at Cachimbo, LT = UT–3 h 40 min.) b Similar results as in Fig. 6a, but for theperiod of 1–31 October 2008 over Fortaleza, where LT = UT–2 h 32 min

curves (top panel). The other group (middle panel) showsnearly random phase (non “coherent”) oscillations that
also present amplification towards sunset (to a smaller degree than in the top panel) and evolve into spread F on
most of the days. These results clearly demonstrate thepresence of precursor seed perturbations that evolve into
post-sunset ESF development. The precise nature of thecompeting roles of these precursor oscillations vis-à-vis
the amplitude of the PRE vertical drift required for any
specific ESF development has not been evaluated in thesegroups (as was evaluated in the case studies presented in
Abdu et al. 2009a). The vertical drift on individual days
was calculated as dhF/dt using the procedure described by
Abdu et al. (1983). The variations in the mean verticaldrift for the two groups are plotted in the bottom panel,
which show that the peak of the PRE vertical drift for the
first group of days (red curve) is significantly larger (byabout 18 m/s) than for the second group of days (bluecurve). In other words, the days of “coherence” in thegravity wave-induced F layer height oscillation during theafternoon pre-sunset hours corresponds to the same set ofdays during which the PRE vertical drift is larger than on
the days of smaller height oscillations that are in “random
phase” (lack of coherence). Similar relationship appears tohold also during the epoch (b) of solar maximum (plottedin Fig. 6b). In this case, however, the data sample issmaller (31 days only) and the PRE vertical drift velocities
are also generally smaller than during the epoch (a). We
may note that the day groups with “coherence” and “noncoherence” in oscillations are apparent in this sample aswell (but to a smaller degree). The difference between the
PRE vertical drift velocity peaks in the two groups of days
during this epoch is about 8 m/s only. It should be pointed
out here that the grouping was based on whether the PRE
vertical drift peak values were < 40 m/s or > 40 m/s, but
possible displacements in the vertical drift peak times on
different days must have caused some reduction/smearing in the mean values of the drift peak calculated atfixed local times. The amplification of the oscillation
amplitude towards sunset evolving into spread F development is clearly brought out even though the “coherence” characteristics of the oscillations is less conspicuousin this case (of smaller group of days) than it was during
the previous solar maximum epoch. What appears to bean apparent connection between the PRE vertical drift andthe oscillation wave structure might indicate a possible
interaction between the upward propagating gravity waves
and the tidal modes of winds (as was suggested by Abdu
et al. 2015b) since it is known that the tidal winds in the Elayer could modify the E layer conductivity longitudinal
gradient that controls the PRE development (Abdu et al.
2006a).
When the PRE vertical drift is relatively weaker, its
peak value may suffer modification (even becoming further suppressed) by a gravity waves of strong enoughamplitude depending upon its propagation phase. As aresult, the post-sunset ESF may become weaker, delayedin development, or even suffer total suppression. Figure 7
shows (in top panel) the F layer height variations at sequential plasma frequencies, ranging from 3 MHz up to
12 MHz, (recorded by a Digisonde at Fortaleza) during a
3-day period in October 2008, an epoch of the extended
solar minimum of the cycle 23. On 10 October, a very
quiet day, the evening PRE vertical drift (second panel)peaked around 12 m/s (typical for this epoch), which isnormally below the threshold value for ESF development, but spread F developed by 20:15 LT (as indicatedin panel 3). We may point out here that the ESF development under such low PRE vertical drift could be occurring due to a relatively large amplitude of the gravity
wave seed perturbation and/or very likely also due to an
enhanced bottom side gradient (this aspect will be thesubject of a separate study). Considering the evening of
12 October, which followed a moderate degree of magnetic disturbances as indicated by the AE variations (accompanied by a weak storm with Dst peaking at − 65 nT
at 11:30 UT on 11 October, not shown here), the PRE
vertical drift was ~ 15 m/s and range spread F developed
at 19:15 LT. In this case, a slight enhancement in the
PRE vertical drift (compared to that of 10 October) may
be noticed. The enhancement appears to be caused by apenetration electric field of eastward polarity and of relatively weak intensity associated with a rapid AE intensification (see panel 4, Fig. 7) that occurred right at thetime of the PRE (18 LT) (see panel 2) also indicated by adashed vertical line. Spread F onset in this case was at
19:45 LT, earlier in LT than on 10 October (a quiet day)when the onset was at 20:15 LT. In contrast to these twocases (of 10 and 12 October), the PRE vertical drift
reached negative values on the evening of 11 October

Abdu Progress in Earth and Planetary Science(2019) 6:11

Page 15 of 22

Fig. 7 The local time variations during 10, 11, and 12 October 2008 in: Isolines of F layer height (hF) at sequential plasma frequencies (from 3
MHz to 12 MHz, and hmF2) (top panel); the vertical drift obtained as the mean of the drifts calculated as dhF/dt at the plasma frequencies, 4MHz, 5 MHz, 6 MHz, and 7 MHz (second panel); the spread F intensity represented by the range of spreading in kilometers (third panel). Auroral activity
index AE (bottom panel)

when the observed peak drift was approximately − 3 m/s.
But the real drift must be more negative than this if acorrection is applied for recombination effect, which is
not done, since we are comparing here only the relative
behavior during these days. The post-sunset SF did not
develop as a result, but SF did develop near midnight(around 23 LT), which may be related to the F layerheight increase due likely to the AE activity that intensified around this time. An important point to note about
this evening is that the suppression/reversal of the PRE
vertical drift did not result from the magnetic disturbance that was present at this time as can be verified
from the fact that the phase of the Vz variation did notconform to that of AE variation. (For example, the AE
activity decreased from 15 LT to 17 LT when the F layerheights and the Vz increased contrary to their expected
response, which is predicted to be a decrease in theirvalues due to the over-shielding westward electric field
expected to be associated with the AE decrease at thistime.) The only clear case of an F layer height responseto the AE activity on this day can be noticed near 11 LT(14 UT) when the height reached a peak value in association with the AE intensification that occurred at thistime. The small decrease in the heights of the plasma
frequency isolines around 18 LT (indicated by a dashedvertical line on 11 October) cannot be in response to the
AE activity that was on a minor rising trend during thisperiod (17–19 LT). A careful examination of the F layerheight variation around 18 LT reveals the presence of adownward phase propagation in its oscillations, which isa clear signature of an upward propagating gravity wave.
We are therefore led to conclude that the PRE suppression/reversal on this evening was the result of a gravity
wave activity, which thereby leads to the stability of the
post-sunset F layer. This is perhaps the first observation ofa case in which a gravity wave activity apparently acted tosuppress the ESF development which was otherwise favored by the existing background conditions (detailedAbdu Progress in Earth and Planetary Science(2019) 6:11

quantitative evaluation of this effect will be the topic ofseparate paper).

Variability in ESF due to disturbance trans-equatorial
winds
The role of trans-equatorial (or meridional) winds in ESF/
EPB development operates through the effect of thesewinds in modifying the field line integrated conductivity
and the F layer bottom side gradient, as was shown
from model study by Maruyama (1988). The ratio of the Fregion—to the total—field line-integrated conductivity
should control the instability linear growth rate as represented by the Eq. 1. Further, a trans-equatorial wind
(TEW) lifts up the F layer on the upwind side of themagnetic equator while bringing it down on the downwind side thereby producing correspondingly opposing(but unequal) effects on the bottomside density gradients. The field line perpendicular (meridional) components of the TEW on either sides of the equator are
also in opposite directions. The net result is that the ratio
U PFT =LFT always contributes negatively to Eq. 1 thereby
causing a decrease in the net growth rate. The consequence of such reduction in the growth rate has been
invoked by Su et al. (2017) to explain the longitudedependent solstice minimum in post-midnight irregularity occurrence observed in ROCSAT-1 data. Additionally, while a TEW may cause an increase in thefield line-integrated conductivity (∑P) on the upwindside (where the layer is raised), it may cause decrease in∑P on the downwind side where the layer is lowered.
But the latter effect is stronger than the former due tothe height-dependent molecular ion composition, sothat a net increase can occur in the total (field line integrated) value of ∑P (Maruyama 1988). As a result, thenonlinear growth of the bubble to topside ionospherecan be retarded or even suppressed. Observational results corroborating the effect of TEW in suppressing
the ESF/EPB growth during quiet time have been presented by, e.g., Abdu et al. (2006b), Maruyama et al.
(2009), and Su et al. (2017). While the TEW can be
more intense during the solstice seasons under quiet
conditions, it can become intensified any time of theyear during magnetic storm disturbances occurring
under conditions of hemispheric asymmetry in auroral
heating. We present below an interesting case of ESF
suppression due to storm time TEW as inferred fromconjugate point observations of hmF2 by Digisondes in
Brazil.
Figure 8 shows in the upper panel the variations in the
F layer vertical drift velocity (Vz) at three plasma frequencies (6 MHz, 7 MHz, and 8 MHz) during the severemagnetic storm event of 17–18 March 2017 together
with its variation on 16 March as a quiet time reference

Page 16 of 22

curve (shown in the second panel). The vertical drift wascalculated as dhF/dt from Digisonde ionograms. The
spread F occurrence during the period is also shown(plotted using the intensity parameter “fop”). The variations in the AE and Dst indices are shown in the bottom
two panels. Following the storm onset at 06 UT (as per
the AE index) on 17 March, a series of AE intensifications occurred when it was mostly daytime over Fortaleza. This was accompanied by the Dst decrease thatsuggested a major category storm development. A few
episodes of vertical drift enhancements (and reversals)
due to the storm-associated prompt penetration electricfield of eastward (and westward) polarity occurred during this period. Two of the major episodes in Vz peakedat ~ 13 UT and ~ 18 UT. Further, we may note in thesecases that the drift enhancements varied with the plasma
frequency, being larger for higher plasma frequency, thatis, higher reflection height. This feature appears to be
due to the height-dependent photo-chemistry (the ion
production and recombination rates decreasing to higher
altitudes) dominating the daytime ionosphere, whereby
the vertical gradient in electron density becomes shallower as the layer rises under the penetration electricfield. The next Vz enhancement occurred near 21:00 UT(18:00 LT) that coincided with the PRE vertical drift (indicated by the vertical line “1”). The quiet time PRE vertical drift (that of 16 March in panel 2) had a peak valueof ~ 18 m/s, and spread F occurred on the evening of 16
March and continued till the morning hours of 17
March (as can be noted in panel 3). A PPEF of eastward
polarity associated with an AE intensification occurred
around 21:00 UT (of 17 March) that caused a large increase in the PRE vertical drift, which peaked at 40 m/s(twice higher than that of the quiet day), but unexpectedly, spread F did not develop on this evening. Further,
all the three plasma frequencies presented the same vertical drift at this time, thereby indicating that the F layer
bottom side density gradient remained unaltered during
the layer rise. This is an important feature to be notedbecause it shows that the density gradient (1/LFT) did
not contribute to any alteration in the R-T instability
growth rate (as per in Eq. 1) that could occur during thelayer rise. The large increase in Vz resulted in a large increase in the F layer height (not shown here) whereby
the reduced νin could result in an enhanced contribution
to the linear growth rate arising from the gravity term(g/νin) in Eq. 1. Thus, we note from Eq. 1 that increases
in both the vertical drift (E/B) term and the gravity term,
under the condition of an unchanging LFT, should havecontributed to increase the linear growth rate. However,spread F did not occur on this evening, which mightsuggest that other factors must be responsible for suppressing the development of the ESF. A possible decrease in the growth rate γFT arising from a decrease inAbdu Progress in Earth and Planetary Science(2019) 6:11

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Fig. 8 Variations during 17–18 March 2015 (intense storm) plotted in panels from top to bottom in: (1) F region vertical drift over Fortaleza
obtained as dhF/dt at plasma frequencies 6 MHz, 7 MHz, and 8 MHz (top panel); (2) the vertical drift obtained as mean of the drift at the same
three plasma frequencies on 16 March, a quiet day, as reference (panel 2); (3) spread F intensity represented by the fop parameter (the top
frequency of the range spreading F trace (panel 3); (4) the difference between the hmF2 values at the conjugate Digisonde stations, CampoGrande, and Boa Vista, denoted as hmF2(CG)–hmF2(BV), or dhmF2(CG − BV), marked only as dhmF2 on the y-axis as dhmF2 (panel 4); the auroral
Activity index AE (panel 5); and the Sym-H representing 1-min Dst values (bottom panel). Note that the Vz reference curve of 16 March isrepeated on two days in the second panel, whereas all other plots cover two running days, 17–18 March

the ratio of F region—to total—field line-integrated conductivity of the Eq. 1 is unlikely because there was no indication of any enhancement in the E region conductivity as
may occur under disturbed conditions due to stormassociated enhancement in energetic particle precipitation,
which can be assessed from Es layer data. No Es layer waspresent. Therefore, it looks possible to conclude that the
only factor that could have caused a decrease in thegrowth rate on this evening must be that arising from theterm in trans-equatorial/meridional wind in Eq. 1.
We examined the possible role of a TEW during thisevent by analyzing the hmF2 variations measured by
the Digisondes operated at the conjugate sites, CampoGrande (Lat. − 20:26:34; Long. 54:38:47, dip − 22° 19′)and Boa Vista (Lat. 02:49:11; Long. 60:40:24, dip 22°),(see also Abdu et al. 2009c). We have used the variationin the parameter, dhmF2(CG − BV), obtained by subtracting the hmF2 over Boa Vista (BV) from that of CampoGrande (CG), to represent the variation in the TEW. It
was shown by Abdu et al. (2009c), that a change of 1
km in this parameter corresponded to a change in
TEW of ~ 1.5 m/s in the evening hours of our interest
here. Panel 4 in Fig. 8 shows the variation in dhmF2(CG− BV) during 17 and 18 March 2015 (see also, Batista
et al. 2017). Positive values in dhmF2 correspond to a
TEW directed northward across the equator. We may
note that on 17 March the dhmF2 that was mildly positive (for most of the time) turned negative starting at
18:00 UT (~ 14:30 LT) and remained so for the next
few hours. This indicated that the TEW that was mostly
northward turned southward to attain a value of around
90 m/s (corresponding to the dhmF2(CG − BV): − 60 km)

Abdu Progress in Earth and Planetary Science(2019) 6:11

at 21:00 UT (~ 17:30 UT). We believe that this southward increase in the TEW could be responsible for decreasing the instability growth rate that caused the
suppression of the ESF development, which otherwise
was strongly favored by a PRE vertical drift that wasenhanced by the disturbance eastward electric field.
Numerical simulation of plasma bubble development
was realized by Maruyama et al. (2009) in which transequatorial wind inferred from conjugate point ionosonde observations in Asian longitude (around 100° E)
was incorporated. It was found that the growth time of
the bubble increased by a factor two (meaning a slower
growth) when the TEW velocity increased from 10 to
40 m/s. The result of Maruyama et al. (2009) corresponded to the conditions of quiet time equinoctial
asymmetry in irregularity development. But it appears
to be relevant to the present case of irregularity suppression under disturbed conditions as well, since thecentral question concerns the role of a TEW in effecting such suppression. In the present case, however, theTEW appears to require a relatively larger intensity due
to the fact that the evening PRE vertical drift (accompanied by an additional vertical drift due to the PPEF)
was larger (being twice that of its quiet time value)
when the irregularity development was suppressed. Here,
we have evoked the role of a TEW based on considerations of the terms in Eq. 1 that apply to linear growthphase of the R-T process. The role of a TEW to slow down
the nonlinear growth towards suppressing the bubble vertical growth is not clear from this analysis, because there
was no evolution to topside bubble to be monitored starting from an SF onset at the bottom side which did notoccur to begin with.

Discussion
We have presented results above illustrating the different paths through which the short-term and day-to-day
variabilities may occur in the development of the EPB/
ESF irregularities. The range of time scales defining suchvariabilities was discussed in the introduction section. It
is possible to consider the sources driving these variabilities as belonging to two broad categories (types): category 1: upward propagating atmospheric wave activity
associated with tropospheric and/or stratospheric weather disturbances and category 2: magnetic activity thatis typically associated with space weather disturbances. In
both of the cases, different degrees of modifications are
imposed on the specific parameters that are directly responsible for the instability growth leading to EPB/ESF
development, as summarized below: (a) The PRE verticaldrift (which perhaps is the most widely discussed parameter in the literature) that may suffer modification through(i) the penetrating magnetospheric electric field and the
disturbance dynamo electric field as was described inPage 18 of 22

Section 2.1 (category 2), (ii) the changes in E layer conductivity longitudinal gradient at sunset due to changes intidal winds originating from PW/UFK wave modulation of
the tides as discussed in Section 2.2, (category 1), and (iii)
gravity wave modulation of the PRE vertical drift (when
it is weak) as was described in the last paragraph of
Section 3 (category 1); (b) the instability seed in theform of wave structure in density and polarization electric field, which is shaped by upward propagating gravity
waves believed to be originating mostly from tropospheric
convective activity associated with the ITCZ, as describedin Section 3 (category 1); and (c) the total field line integrated conductivity of the E and F region as well as the ratio of the F region conductivity to the total field line
integrated conductivity that can be modified by the TEW/
meridional wind, during storm time (category 2) as well as
under “quiet” conditions.
Regarding the item (a) above, we have shown that enhanced development, or suppression, of post-sunset EPB
can arise from vertical drift velocity modifications that
may occur under magnetic disturbances (in the form of
sub-storm or storm) occurring right at the evening local
time of the PRE development. When disturbances occur
in the form of AE intensification under Bz south conditions, often accompanied by a Dst decrease indicating astorm development phase, the PRE vertical drift may become enhanced leading to EPB generation. The disturbance intensity may vary from that of a weak substorm to
one of an intense storm, or may even be representativeof an unidentifiable storm condition. It was shown that
the PRE vertical drift as well as the associated plasma
bubble rise velocity could increase with increase in theintensity of the under-shielding eastward PPEF. However, as was pointed out based on previous studies, anincrease in the bubble rise velocity in a way proportional
to the intensity of the PPEF will not be operative for abnormally large vertical drifts such as that may occur during some super storms. It was also discussed that while a
DDEF of westward polarity in the evening hours may
suppress the PRE vertical drift and, thereby, also thepost-sunset bubble growth, such effects may also be produced by an over-shielding electric field of westward polarity associated with a substorm/storm recovery phaseoccurring just prior to sunset. Results of a statistical
study based on a few cases of AE recovery phase occurring just prior to sunset hours clearly showed that theover-shielding westward electric field associated withsuch recovery phases could result in significant suppression of the PRE vertical drift, which may lead to a drastic
decrease in EPB development, a detailed evaluation ofwhich needs to be realized in future investigations.
Previous studies (see, e.g., Abdu 2012) have pointed outthat disturbance electric fields could be present over equatorial region for space weather disturbances represented

Abdu Progress in Earth and Planetary Science(2019) 6:11

by AE activity as low as ~ 100 nT and higher, and theprobability of occurrence of such disturbances in the sunset sector over Brazil could vary on an average from 20%
during solar minimum to 60% during solar maximum. For
this reason, the short-term variability arising from space
weather-related disturbance electric fields should be considered to be an important component of the overall variability in the ESF/EPB occurrence.
Another important component of the PRE/ESF variability may operate through the E region. In this case,the longitudinal gradient in the E layer conductivity
across the sunset terminator is modified by E layer
winds (as explained in Section 1.1) that may be modified
by nonlinear interaction between the planetary/UFK
waves and tides. The E layer conductivity longitudinal
gradient so modified has been shown to be a critical parameter in controlling the amplitude and phase of the
PRE vertical drift (Abdu and Brum 2009). The resultspresented in Section 2.2 (supported by results from previous studies) have shown that the PRE vertical drift,and hence the ESF/EPB development, can be significantly modified through such E layer processes. Another
path of a potential E layer role is through the association
observed between the PRE vertical drift and the wave
structure represented by the F layer height oscillations(as in Fig. 6). These oscillations present a certain degreeof phase coherence on a day-to-day basis, and getting
amplified towards sunset as was consistently observed indifferent solar activity epochs (as discussed in Section 3).
The possibility of interaction between upward propagating tides and gravity waves may be thought of as a possible link in the connection observed between the Flayer wave structures and the PRE vertical drift, which
possibly has important consequences for the ESF/EPB
variability. On these bases, the variability in the evening
sector E layer, arising from nonlinear interaction involving tidal wind modes on one side and the planetary/UFK
waves and gravity waves on the other side, should beconsidered to be an important source of ESF/EPB
short-term variability, and merits added attention for future research.
As regards the item (b) above concerning the ESF/EPB
variability arising from the seed perturbations, important
progress has been made as discussed (though in a limited way) in Section 3. There is increasing evidence on
statistical as well as case study bases that the role of upward propagating gravity waves from sources in troposphere is an important factor in the ESF/EPB variability.
In case studies for evaluating the gravity wave effect on
EPB generation, an important consideration should be
the phasing of the gravity wave oscillations relative tothe peak in the PRE vertical drift that is used to determine the maximum linear growth rate in a given situation for the R-T instability to evolve. This question is a

Page 19 of 22

parallel one to the two-dimensional narrative in which
EPB may develop from the crest region of the upwelling
due to LSWS, as pointed out in Section 1.1. The phasingof the gravity wave/LSWS with respect to the peak of
PRE vertical drift appears to gain importance for EPB
generation especially when the PRE vertical drift is relatively small. This situation is illustrated in the resultspresented in Section 3 (and mentioned in item a-iii)
showing an example of ESF suppression due to the positioning in anti-phase of the PRE vertical drift peak andgravity wave oscillations that occurred on 11 October(Fig. 7). Such a situation of ESF suppression was the result of a weak PRE vertical drift whose modulation by arelatively “stronger” gravity wave amplitude was possible.
On this basis, we may expect the contribution fromgravity wave dynamics to the overall ESF short-term
variability to depend also upon the degree of the variability in the PRE vertical drift that is known to depend
also upon season, longitude, and solar activity. The
ITCZ/OLR parameters (that is, the Outgoing Long Wave
Radiation from Inter-Tropical Convergence Zone) havebeen used to represent the tropospheric convective activity as a source of gravity wave seed perturbation. It
has been noted that the degree of statistical correlation
between the ITCZ/OLR parameters and the irregularity
distribution on global or regional scale is not always uniformly conclusive for all longitudes/seasons (see, for example, Tsunoda 2010, Su et al. 2014). It appears possiblethat simultaneous considerations on the associated vertical drift information (wherever available) could help
understand better the observed degrees of correlations
in such statistical comparisons.
The nature of the ESF short-term variability due to theTEW/meridional winds (item c) appears to have beenpoorly investigated. Under quiet conditions, a few studies have established their role in suppressing the ESF as
mentioned in Section 4. We presented what appears to
be a clear case of ESF suppression by TEW during amagnetic storm event (Fig. 8). In this respect, we needto distinguish among the cases of ESF suppression occurring at different storm phases. For example, the ESF
suppression at the recovery phase of a substorm occurs
due to the westward disturbance electric field associatedwith the over-shielding effect. In this case, the PRE vertical drift will be weakened. A westward electric field due
to disturbance dynamo effect occurring in the evening
hours may also suppress the ESF, which is also accompanied by reduced PRE vertical drift and F layer height.
In contrast to this, during the ESF/EPB suppression by a
TEW/meridional wind, the PRE vertical drift may notnecessarily suffer any change or sometimes may become
even enhanced if the storm (AE) intensification occurs
during the PRE as was found in the case presented here.
Important questions remain to be answered as to what

Abdu Progress in Earth and Planetary Science(2019) 6:11

should be the magnitude of the PRE vertical drift to produce an instability growth rate that can dominate/prevailover the retarding/suppressing effect due to a TEW/meridional wind. This question can be answered from modeling studies supported by more observations.

Conclusions
The main conclusions of this study may be highlighted
as follows:
1- The PRE vertical drift, which is a basic requirement
for the post-sunset ESF/EPB development, cansuffer significant short-term variability due tomagnetospheric forcing in the form of penetrationelectric fields and disturbance dynamos electric field,
as well as due to atmospheric wave disturbances in
the form of planetary/Kelvin waves and gravity waves,originating from lower atmosphere.
2- An under-shielding electric field of eastward polarity,
associated with auroral activity or Dst development,
occurring near sunset may cause an upward verticaldrift that can add to the vertical drift due to the
PRE thereby favoring/enhancing the ESF/EPB
development. The vertical drift velocity during
the PRE may increases in proportion to anincreasing intensity of the penetration electricfield (under-shielding electric field of eastward
polarity) occurring in the evening hours. An
upper limit for this proportional relationship is
uncertain, however.
3- An over-shielding electric field of westward polarity,
associated with the recovery phase of an AE activity,
occurring just prior to the sunset, causes a decrease(or suppression) of the PRE vertical drift, which
thereby contributes to stabilize the post-sunset F
layer.
4- Upward propagating planetary/Kelvin waves(especially the UFK wave) may cause large degree ofmodulation in the PRE vertical drift (and thereby in
the post-sunset F layer heights) that may contributeto intensify, or advance the onset time of, the spreadF development. The underlying mechanism isbelieved to be nonlinear interaction of thesewaves with tidal mode winds, which in turnmodify the longitudinal gradient in the E layer
conductivity that controls the PRE development.
5- Gravity wave-induced F layer height oscillationsin the form of large-scale wave structure getting
amplified towards sunset appear to be a regularfeature as a precursor to post-sunset ESF/EPB
that may develop when the PRE vertical drift isfavorable. Additionally, there appears to be present a
connection between the intensity of the PRE verticaldrift and the amplitude and phase of the precursor

Page 20 of 22

oscillations, with implications on a possible tidalgravity wave interaction modifying the winds inevening E layer, and thereby, modifying also the
PRE development.
6- The PRE vertical drift, when weakly developed, canbe suppressed or even reversed to downward by
upward propagating gravity waves of large enoughintensity. In this way, the gravity wave activity may
even lead to suppression of the post-sunset ESF/
EPB development.
7- The ESF/EPB development can be suppressed bya trans-equatorial wind during magnetic storm
disturbances even when the PRE vertical drift,enhanced by a prompt penetration electric field,
has larger than the normal amplitude required forsuch development.
There are additional aspects not addressed here, including modeling results that are essential for a moredetailed understanding of the short-term variability inthe EPB/ESF development, which is a highly challenging task and needs to be pursued further with the support of more diverse observational results.
Abbreviations
AE: Auroral electrojet; DDEF: Disturbance dynamo electric field;
EPB: Equatorial plasma bubble; ESF: Equatorial spread F; FAC: Field aligned
current; IMF: Interplanetary magnetic field; ITCZ: Inter-tropical convergencezone; OLR: Outgoing long wave radiation; PPEF: Prompt penetration electricfield; PRE: Prereversal enhancement in the zonal electric field; PW: Planetary
waves; SYM-H: Symmetric Ring Current; TEW: Trans-equatorial wind; TIEGCM: Thermosphere ionosphere electrodynamics - general circulation model;
UFK: Ultra-fast Kelvin waves
Acknowledgements
The author acknowledges the support received from the Sao Paulo State
Foundation for Promotion of Research (FAPESP) through the Process 2016/
24970-7. The author acknowledges also the support received form the
Ministry of Science and Technology and Conselho Nacional de Desenvolvimento
Científico e Tecnológico (CNPq) through the process: CNPq 300883/2008-0. The
author wishes to thank the Coordenação de Aperfeiçoamento de Pessoal deNível Superior (Capes) for a senior visiting professorship at ITA/DCTA. The Dst/
Sym-H data were downloaded from WDC/Kyoto site. The IMF data wereobtained from the website http://omniweb.gsfc.nasa.gov/from/omni_min.html.
Digisonde data over Sao Luis and Fortaleza were obtained from ionosphericobservatory operated by INPE.
Funding
The sources of funding are: The Ministry of Science and Technology of Brazil
and CNPQ Process number: 302014/2016–0, and the Foundation for Promotionof Research in the State of Sao Paulo (FAPESP) through the Process: 2016/
24970–7.
Availability of data and materials
Data sharing not applicable to this article as no datasets were generated oranalyzed during the current study.
Authors’ contributions
The author prepared the article, read and approved the final manuscript.
Author’s information
The author holds Ph.D. degree in Ionospheric Physics. Currently he is an
emeritus scientist at the National Institute for Space Research (Instituto
Nacional de Pesquisas Espaciais –INPE) and a visiting professor at the Aeronautic

Abdu Progress in Earth and Planetary Science(2019) 6:11

Technology Institute (Instituto Tecnologico de Aeronautica – ITA) at Sao
Jose dos Campos, Brazil. His research interest covers: Solar-Terrestrial Physics,
Space Weather, Aeronomy, Ionosphere physics and vertical coupling in theatmosphere-ionosphere system.
Competing interests
The author declares that he has no competing interest.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Received: 17 October 2017 Accepted: 7 January 2019

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