Download The Equatorial Spread F/Plasma Bubble Irregularities (ESF) M. A.

1 The Equatorial Spread F/Plasma Bubble Irregularities (ESF)
M. A. Abdu
Instituto Nacional de Pesquisas Espaciais – INPE
São Jose dos Campos, SP Brazil.
(Updated on 29 October 2010)
1- Introduction:
1.1- Definition and major characteristics:
The term Equatorial Spread F (ESF) refers to plasma irregularities of the night time
equatorial ionosphere characterized by the presence of plasma structures of scale sizes
ranging from a few meters to 100s of kilometers. They are generated by plasma
instability processes initiated at the bottom side gradient region of a rapidly rising post
sunset F layer over equator. The instability initiated by density perturbations grow
nonlinearly into vertically extended flux tube aligned plasma depletions, or plasma
bubbles, with scale sizes of 10s to 100s of kms across field lines, cascading progressively
into smaller scale sizes. As the field aligned structures rise over the equator the
irregularity belt stretch to latitudes ± 20° on either side the dip equator. Once generated
the plasma bubbles irregularities can evolve for several hours till their decay past
midnight. The nighttime equatorial ionospheric conditions permit the irregularity
developments cycle to be active till sunrise.
1.2- Objectives and significance for investigation:
ESF irregularities can cause significant modifications to radio waves propagating in the
ionosphere, so that space operational systems, such as the applications based on GPS and
telecommunications satellites (among others) can be severely affected/interrupted by
their presence. Further, they constitute an important element of the Space weather effects
of the equatorial region that are highly controlled by solar variability on one side, and
vertical coupling through upward propagating atmospheric waves originating from the
lower atmosphere on the other. The objective of the ongoing investigation is to achieve a
clear understanding of the diverse causes of the ESF variability which is a fundamental
requirement in our efforts to develop tools to predict their occurrence in space and time.
2- ESF Diagnostics using different techniques
The ESF irregularity scale sizes ranging by five to six orders of magnitude necessitates the use of
diverse types of diagnostic techniques for their investigations. Broadly they are:
 Ionosondes and Radars,
 Optical imagers,
 Satellites and Rockets,
 GPS receivers etc.,
The following examples illustrate the irregularity signatures as observed by the different
techniques.
2 The ESF phenomenon was first discovered as range spreading F layer traces in the night time
ionograms over Huancayo,, Peru, by Booker and Wells (1938).
Fig.1 (a) shows some examples of spread F development in an equatorial ionogram. The range
spreading of the F layer trace at frequencies scanned from 1 to 20 MHz (by a digisonde)
indicates radio wave reflection/scattering by plasma structures of scales sizes varying from a few
kms down to a few meters.
Fig.1 (b) shows ESF ionograms, and the all-sky 630nm images of the corresponding plasma
depletions/bubbles, at two magnetic conjugate stations (Campo Grande and Boa Vista) in Brazil,.
The magnetic N-S aligned patches are the airglow/electron density depletions in the background
of high intensity background emission due to the EIA. They have east-west scale size of a few
hundreds of kms and north-south extension of around 4000 kms. The airglow depletions are
symmetric, whereas the smaller scale structures seen by the ionosonde are asymmetric, at the
conjugate sites with respect to the dip equator. More about the bubbles will be discussed later.
Fig 1(c) presents the 30 MHz radar maps of 5m irregularities over Sao Luis, Brazil, and Altair
incoherent scatter radar scans of bubble evolution from altitude modulation of the bottom side F
region.
Fig-1d: Electron density profiles measured by a rocket borne probe cutting through a plasma
bubble event over Natal, Brazil on 11 Decemebr 1985.
Fig. 1(e) shows the electron density measured (on 08 October 2009) by the C/NOFS satellite
instruments on its orbits before, during and after an intense bubble event in the South American
sector. The electron density can be depleted by two orders of magnitude inside a bubble. The
structure sizes diagnosed in situ by satellites vary from a few hundred meters to their largest
existing sizes. The plasma depletions are associated with large upward plasma drift as the plot
for 08 December 2008 shows.
The irregularities of a few hundreds of meters are responsible for causing scintillation of the
signals from GPS satellites, some examples of which are shown in Fig. 1(f).
With this brief introduction on the irregularity characteristics we will now proceed to discuss the
generation mechanism and development conditions of these irregularities.
3- Plasma Bubble irregularity generation mechanism/development
3.1- Background Ionosphere-Thermosphere conditions:
 Dynamo electric fields,
 Evening prereversal electric field enhancement (PRE),
(Thermospheric zonal wind and, E layer conductivity sunset gradient),
Over middle and high latitudes the nearly vertical geomagnetic field lines (Fig 2 a) permit
ionospheric plasma to escape to higher altitude and magnetosphere thereby resulting in low
3 ionospheric densities. Over equatorial latitudes, on the other hand, the nearly horizontal field
lines, the entire field line remaining in the dense ionosphere and connecting the equatorial F
region to its conjugate E layers, as shown in Fig. 2b causes confinement of the plasma to the low
latitude region, so that the plasma dynamics is controlled mainly by electric fields generated by
E- and F-layer dynamo. The zonal electric field causes vertical E x B drift of the plasma, the
main driver of ESF and EIA.
Fig. 3a shows the average vertical drift (zonal electric field) as measured by the Jicamarca (Peru)
incoherent scatter radar for different seasons and solar flux conditions. The day time upward drift
(eastward electric field) reverses to downward (westward electric field) after sunset by E layer
dynamo action Just before the reversal there is a large increase in the vertical drift which is
driven by the F layer dynamo that intensifies at this time. This is known as the prereversal
enhancement in the vertical drift/ zonal E-field, widely known as the PRE. The PRE is
responsible for the large rise of the F layer at sunset shown in Fig 3b for an equatorial site in
Brazil.
The PRE is produced by the dynamo action of thermospheric zonal wind (which turns eastward
in the evening as shown in Fig 4a) in the presence of the longitudinal/local time gradient in the E
layer Pederson conductivity (represented by the variation of foE shown in Fig. 4b).
3.2- Instability processes and growth rates:
 Bubble initiation, development and dynamics, drift velocities etc.,
The bottom side of the rising post sunset F layer can be unstable to density perturbations
occurring at its steepening gradient region. Instabilities grow by Rayleigh-Taylor (R-T)
mechanism, by which a light fluid situated below rise up into a denser fluid situated above, a
situation represented by positive density gradient of the bottom side ionosphere. The
electrodynamic equivalent of the R-T process is illustrated in Fig. 5a.
The current produced by the gravity which is strictly horizontal is given by: Jx = nMg/B. Jx and
depends on the electron density n, so that a perturbation in n could cause a current divergence
and charges pile up at the edge of the perturbation as shown in the figure. The perturbation
electric field E due to the charge pile up causes E  B drift of ions and electrons which is
upward in the lower density regions of the perturbation and downward in the density
enhancements. The perturbation thus increases and so does E, the process leading to instability.
The system is stable if g and n are parallel, as is the case on the topside of the F layer where
GRT instability does not develop. The gravity driven linear growth rate is given by:

'
L

n
n
 g


  in





(1)
L
The zonal electric field (of the PRE) also contributes to the instability growth, as also shown in
Fig.5. This leads to the generalized R-T (GRT) process for which the growth rate as modified to
include the zonal E-field is given by:

'
L

n
n
 E
g


 B
 in


 



L
(2)
4 The electric field also lifts the layer to a height of reduced in thus increasing the growth rate
further. When the electric field is westward then the growth rate will be reduced and the bottom
side will be stabilized.
The effect of a zonal wind will be present when the layer is tilted so that n has a horizontal
component as well. Under such a condition the growth rate becomes:

'
L
n
n





E
x0
B
cos  

g
cos  
E
z
 uB
B
in


sin   



(3)
L
When  , the recombination rate, is smaller than the other terms the instability grows, the
L
polarization electric field grows linearly in the initial stage turning to nonlinear phase leading to
the bubble vertical growth.
The local growth rates of Eqs 1-3 can be considered to be representative expressions when the
equatorial anomaly is symmetric with respect to the magnetic equator. In the presence of a trans
equatorial wind the EIA become asymmetric and the relative contributions of the integrated
Pederson conductivity from the E- and F- region segments of the field lines needs to be included
in the growth rate which can then be expressed in terms of field line integrated parameters using
the following expression (see, Sultan, 1996; Haerendel et al., 1992; Maruyama, 1988, see also,
Kelley, 1989; Abdu, 2006).
 FT

1
L 
FT
Here

E ,F
P
al., 1982);
F

E
P

E
F B
P
P 


P
g

U FT 


   FT
eff 

(5)
is the field line integrated conductivities for the E and F-regions (see also Zalesack et
U
P
is the conductivity weighted flux tube integrated vertical wind; L FT is the flux
tube integrated electron density gradient. All other parameters correspond to flux tube integrated
(or effective) quantities of the parameters defined before. The subscript FT in Eq.5 stands for
E
flux tube integrated quantity. When P approaches zero and winds are negligible the form of
FT
Eq.5 can be simplified to the local growth rate equations.
Fig.5b Animation of R-T instability growth.
The ESF bubbles are plasma depleted flux tubes, and as they rise up over the equator their
extremities extend to latitudes away from the equator, so that at the latitude of the Equatorial
anomaly ionization crest the “foot print” of a well developed bubbles (having apex height of
about 1000 km and more) can be observed. The signature of dynamics of such a depletion
structure as captured by an all-sky 630 nm imager over Cachoeira Paulista (which is located at
the EIA crest) is shown in Fig 6a (animation). The airglow depletions drift eastward (Sobral et
al., 2009; .
5 An animation of the bubble dynamics over Sao Joao de Cariri (7°S) showing drift reversal to
westward is in Fig. 6b (animation).
4- Irregularity effects on operational systems:
 Scintillation,
 GPS applications etc.
As mentioned earlier the zonal drift of the bubble irregularities produces scintillation. Fig. 7
illustrates how the scintillation is produced. The Scintillation can cause disruptions of
operational systems, such as loss of lock of the GPS signals as shown in Fig 8. Fig 9 (animation)
illustrates the GPS position error due to scintillation.
5- ESF Seasonal and longitudinal dependence
Observations by ground based and satellite born instruments shows large variability with
longitude and season in the ESF/plasma bubble irregularity spatial and temporal distribution.
Some results are shown in the following figures. Fig 10 a shows TIMED/GUVI OI 135.6 nm
images, wherein we note magnetic meridian aligned depletion streaks/bands that are nearly
symmetric with respect to the dip equator, in the background of the two bright airglow arcs that
are also nearly symmetric with the dip equator which is the EIA. We may note that EIA arcs and
the depletion intensities are more intense in the Brazilian Atlantic longitude sector. Fig. 10b
shows a global coverage of the images during equinoctial months of 2002 (Kil et al., 2006). The
bubble occurrence is intense in the Brazilian-African sector with no occurrence in the Pacific
longitude sector in this season. Fig. 11 a shows the global distribution of the magnetic signature
of the bubbles as observed by the CHAMP satellite. The season-longitude distribution of the ESF
from the CHAMP satellite is shown in Fig.11b. Spread F season-local time distribution over
Fortaleza Brazil is shown in Fig. 11c (Abdu et al., 1992). A sketch to explain the
seasonal/longitudinal control of the ESF is in Fig. 11d (Su et al., 2007, see also Abdu et al.,
1981, Tsunoda 1985).
6- Coupling processes and ESF day-to-day variability
There is a large degree of day-to-day variability in the ESF development during a season of its
regular occurrence. The most challenging problem in the present day investigation of this
phenomenon concerns improving our understanding of the causes of such variability and to
develop modeling capability for predicting the occurrence, intensity and duration of an ESF
event. For a better understanding of the possible causes of such variability we will examine the
background ionosphere-atmosphere conditions (the factors) that are known to control the ESF
development as represented in the schematic of Fig. 12. As shown here, the three key factors that
control the ESF/bubble development are:
- The evening prereversal enhancement in the F region vertical drift (the zonal electric
field), denoted as PRE,
- Seed perturbation for R-T instability initiation in the form of gravity waves (GWs), and
- Field line integrated conductivity (which is controlled by trans-equatorial/meridional
winds).
Most observational results are those that connect the PRE with the ESF. We will examine some
of these results.
6 Figures 13a shows the vertical drift over Jicamarca radar as a function of the solar flux F10.7
index for weak spread F (without plume) and strong spread F (with plume). It is seen that large
Vz is necessary to produce the radar plumes, that is, top side bubbles. The results from a
conjugate point experiment plotted in Fig.13b shows that larger PRE amplitude corresponds to
earliest occurrence of post sunset ESF at latitudes away from the equator. The lowest amplitude
corresponds to the SF not occurring till midnight at the conjugate sites, that is, the bubble did not
rise up to the apex height connected to the conjugate sites. These results clearly demonstrate the
control of the PRE on bubble intensity.
There is significant scatter in the relationship between the Vz/PRE and the Spread F intensity as
seen in the results of Fig. 13a. Such scatter is due to the role of other factors such as the intensity
of the seed perturbation, most likely related to the presence of precursor gravity waves as will be
shown soon. Thus while there is large day-to-day variation in the PRE intensity, its control of the
ESF can be further modified by the precursor GWs.
6.1- Vertical Coupling:
 Planetary Wave modulation of the PRE and ESF
Large variation can occurs in the amplitude of the PRE even under magnetically quiet conditions
as illustrated in Fig 14a.The data is for the stations Cachimbo and Campo Grande during the
COPEX 2002 campaign conducted in Brazil. A wavelet power spectrum of the same is
presented in Fig 14b which shows periodicities of 3-7 days centered on the day 310.
Simultaneous data of mesospheric zonal and meridional winds (at 95 and 100 km) over
Cachoeira Paulista also presents the same periodicities. The magnetic disturbance or the solar
flux index F10.7 does not contain such periodicity as shown in Fig14c. This suggests that
upward propagating planetary waves from lower atmosphere is responsible for the observed
large day-to-day variations in the PRE (Abdu et al., 2006). We can show that the ESF intensity
follows such variability.
Fig 15a presents an event of post sunset F layer height over Fortaleza (which is controlled by the
PRE and the mesospheric zonal wind over Cariri modulated by upward propagating ultra fast
Kelvin waves (as verified from the characteristics of these waves).
An explanation of how these upward propagating waves can cause modulation of the PRE and
post sunset F layer heights is presented in Fig. 15b.
 Gravity waves and instability seeding
The different ways in which the GWs can produce effects on spread F development are listed in
Fig 16. We will discuss here the direct seeding of the instability by GWs.
Examples of GW oscillations in the F layer heights at different plasma frequencies occurring at
the evening hours over Fortaleza as deduced from digisonde data are shown in Fig.17 (left
panel). The prereversal vertical drifts are shown in the right panel. The effect of such GWs in
seeding the R-T instability process leading to ESF/bubble formation is discussed through figures
18 and 19 (Abdu et al., 2009a).
Fig 18a shows that large F layer height and PRE vertical drift caused prompt development of
Bubble irregularities indicated by the plume in the RTI map on 23 Oct. The gravity wave
amplitude was very weak and marginal within the detection limit of the digisonde. On 24th
7 evening Fig 18b the GW was clearly significant. But the F layer height and PRE drift was very
weak and not sufficient to cause SF development as we note from the polarization electric field
growth rate calculated using the GW amplitude representative of the evening of 23th shown in
Fig 19. But when the GW amplitude was increased to correspond to that was observed on 24th
evening (that was 3-4 times that of the 23rd) there was sufficient exponential growth rate which is
compatible with the observed plume intensity. Thus we note that the GW did indeed contribute
to the observed irregularity development on 24 October. There are other cases studied as well but
we will not discuss them here.
6.2- Magnetosphere-Ionosphere Coupling:
 Penetrating electric field effects of ESF development
 Disturbance dynamo electric field effects on ESF development
In relation to the process of spread F development we need to consider three types of disturbance
electric fields.
They are:
1. Prompt Penetration (under-shielding) Electric Field;
2. Penetration Electric Field from over-shielding condition;
3. Disturbance Dynamo Electric Field.
With the interplanetary magnetic field component Bz turning south and sudden increase in the
high latitude convection the sudden increase in the Auroral Electrojet (AE) Activity marks a
substorm/storm onset when high latitude electric fields promptly penetrate to equatorial latitudes.
Fig. 20 (from Fejer et al GRL 2008) show the longitudinally averaged Prompt Penetration (PP)
drifts from ROCSAT -1 following a step function increase in the AE index by 300 nT.
The PPEF is eastward during day time extending to 23 LT, with corresponding upward drift.
Thereafter the drift is downward till morning ~ 06- 08 LT.
We may also note that there is similarity between the PPEF LT variations with the quiet time Efield/vertical drift variation by comparing the Figs 20 a and c.
As a next phase a shielding layer is established with the development of the ring current (This is
to balance the convection E field). Therefore with the weakening of the convection and/or by the
Bz turning northward, an over-shielding electric field penetrates to equatorial latitude. Its
polarity is opposite to that of the PPEF as shown here (Fig. 20b).(For examples see, Fejer, 2002;
Abdu et al., 2003; Martinis et al., 2005)
Fig 21(a,b) shows a comparison between the over-shielding penetration electric field and the
disturbance dynamo electric field DDEF. The DDEF is produced by thermospheric disturbances
circulation originating from Joule heating in auroral region during the storm. It is delayed by the
time it takes for the disturbance winds to reach midlatitudes, usually by a few hours.
We will now examine some cases of ESF bubble development or suppression by prompt
penetration electric fields.
Fig. 22a shows the start of sub-storm on 26 Aug 1998 with Bz turning south (and with AE
intensification) when it is near the PRE over Brazil. As a result, the Vz over Fz is enhanced (Fig
8 22b), which caused bubble development (Fig. 22c). The bubble soon drifted westward (Fig.22d)
in contrast with the usual eastward drift (Abdu et al., 2003).
Another example of bubble development and dynamics due disturbance electric field is shown in
Fig. 23a and b. Fig-23a shows the auroral indices, AE/AO (AU/AL), variations on 08 November
2001, and Fig.23b is an animation of bubble velocity reversal to westward coinciding with the
auroral activity intensification. The velocity reversal is believed to be produced by a HALL
vertical electric field induced by a primary zonal eastward prompt penetration electric field (in
the presence of enhanced E layer conductivity) associated with the storm.
Fig. 24 a shows some cases of PRE suppression by overshielding PP electric field during a
storm that occurred on 31 March 2001, and Fig 24b shows a similar case that occurred during the
9-10 November 2004 storm (Abdu, et al. 2009b).
Fig. 25a Shows more examples of PRE enhancement and suppression by undershielding and
overshielding electric fields respectively on 25 and 23 September 2001.3during two days.
Fig.25b shows results of simulation of bubble polarization electric field growth rates for the ESF
development and non development cases.
Some cases of the effects of DDEF on ESF during 9-11 November 2004 storm events are
presented in Fig 26.
7- Summary:
Detailed understanding of the ESF responses to atmosphere-ionosphere coupling processes,
involving upward propagating atmospheric waves, as well as to space weather effects arising
from magnetosphere-ionosphere coupling involving penetration electric fields and disturbance
thermosphere circulation, needs to be achieved for advancing our capability to predict the ESF
development in space and time.
List of References
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Advances in Space Research 35 (2005) 771–787.
M. A. Abdu, J. A. Bittencourt, and I. S. Batista, Magnetic declination control of the equatorial F
region dynamo electric field development and spread F, J. Geophys. Res., 86, NO. A13,11,44311,446, 1981
M. A. Abdu, I. S. BATISTA, AND J. H. A. Sobral, A New Aspect of Magnetic Declination
Control of Equatorial Spread F and F Region Dynamo, JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 97, NO. A10, PAGES 14,897-14,904, OCTOBER 1, 1992.
Abdu, M. A., I. S. Batista, H. Takahashi, J. MacDougall, J. H. Sobral, A. F. Medeiros, and N. B.
Trivedi, Magnetospheric disturbance induced equatorial plasma bubble development and
9 dynamics: A case study in Brazilian sector, J. Geophys. Res., 108(A12), 1449,
doi:10.1029/2002JA009721, 2003
Abdu, M. A., P. P. Batista, I. S. Batista, C. G. M. Brum, A. J. Carrasco, and B. W. Reinisch
(2006), Planetary wave oscillations in mesospheric winds, equatorial evening prereversal
electric field and spread F, Geophys. Res. Lett., 33, L07107, doi:10.1029/2005GL024837.
M. A. Abdu, E. Alam Kherani, I. S. Batista, E. R. de Paula, D. C. Fritts, and J. H. A. Sobral,
Gravity wave initiation of equatorial spread F/plasma bubble irregularities based on
observational data from the SpreadFEx campaign, Ann. Geophys., 27, 2607–2622, 2009a.
Abdu, M. A., E. A. Kherani, I. S.Batista, and J. H. A. Sobral (2009b), Equatorial evening
prereversal vertical drift and spread F suppression by disturbance penetration electric fields,
Geophys. Res. Lett., 36, L19103, doi:10.1029/2009GL039919.
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Terr. Magnetism. Atmos. Electr., 43, 249, 1938.
Fejer, B. (2002), Low latitude storm time ionospheric electrodynamics, J. Atmos. Sol. Terr.
Phys., 64, 1401.
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equatorial disturbance vertical plasma drifts, Geophys. Res. Lett., 35, L20106,
doi:10.1029/2008GL035584.
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occurrence of equatorial spread F. J.Geophys. Res. 101, 26875–26891, 1996
Haerendel, G., Eccles, J.V., Cakir, S. Theory of modeling the equatorial evening ionosphere and
the origin of the shear in the horizontal plasma flow. J. Geophys. Res. 97, 1209–1223, 1992.
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Inc., New York, pp 484.
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equatorial spread F: A case study, J. Geophys. Res., 86, 9087.
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using the TIMED/GUVI limb retrievals, Ann. Geophys., 24, 1311–1316, 2006.
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11 Figures:
Fig. 1a: Ionograms taken over an equatorial site, Cachimbo, in Brazil, showing the F layer traces
before and after the range spread F development at 22:25 UT.
12 Fig. 1b: Ionograms with spread F traces and simultaneous airglow all-sky images with elongated
depletions, bubbles, at conjugate points , Boa Vista and Campo Grande, in Brazil.
13 Fig. 1c: Sao Luis 30 MHz radar RTI map and Altair incoherent scatter radar scans showing
bubble evolution from F layer bottom-side upwelling/undulations.
14 Fig 1: (d)- Examples of bubbles irregularities encountered by rocket borne instruments over
Natal, Brazil and (e)- by C/NOFS satellite in the South American-Pacific sector.
15 Fig.1 f: Scintillation records at L‐band signals received from GPS satellites 16 (a) (b) Fig 2 ab: Ionospheric plasma distribution is controlled by dipole magnetic field line: (a) nearly vertical over mid‐to0high latitude and (b) horizontal/low nclination over equatorial/low latitude. 17 (a) (b) Fig.3 a: Average F region vertical plasma drift over Jicamarca, Peru, for three seasons and three levels of F10.7, (b) F region peak height hmF2 over an equatorial site Cachimbo, Brazil, during October‐December 2002. The evening prereversal enhancement in the vertical drift (PRE) over Jicamarca and the large uplift of the hmF2 caused by the PRE over Cachimbo may be noted 18 (a) (b) Fig 4ab: (a) Thermospheric zonal and meridional winds distribution as a function of latitude and local time and (b) E layer peak density diurnal variation. 19 Fig. 5‐ Rayleigh‐Taylor interchange instability mechanism illustrated (Kelley 1989) 20 (a) (b) Fig. 6 (a) Airglow 630nm all‐sky image and the zonal drift of plasma depletion (animation)over Cachoeira Paulista, (b) animation over Cariri showing drift reversal to westward after midnight. 21 Fig. 7: An illustration of the Scintillation of radio waves from satellites received at ground.
22 Fig. 8: Example of GPS loss of lock due to severe scintillation effect.
23 Fig.9 (animation) illustrates position error due to GPS signal scintillation.
(a) 24 (b) Fig. 10 a b: TIMED/GUVI coverage of global distribution of UV airglow emission images with
meridionally stretched depletions streaks in the background of the two bright emission arcs
representing the equatorial ionization anomaly, nearly symmetric with respect the magnetic
equator.
25 Fig.11: (a) Global distribution of ESF magnetic signature as obserevd by CHAMP satellite, (b)
ESF statistics plotted in longitude versus season format, (c) ESF season-local time distributuion
over Fortaleza Brazil, (d) representation of thermospheric wind vectors in relation to magnetic
meridian to explain the season/longitude dependence of ESF/ bubble irregularities.
26 Fig 12: A schematic of the interconnected processes leading to the developmen tof ESF/pasma
bubble irregularities.
27 Fig 13: (a ) The control of the Spread F by the evening vertical drift/PRE and F10.7 solar flux
index , over Jicamnarca, Peru, (b) F layer vertical drift over equator, Cachimbo, in Brazil,
showing the intensity of the PRE for different spread F onset times at off-equatorial sites, so that
the earlier ESF onset times which correspond to faster bubble rise velocity, are associated with
larger PRE amplitude.
28 Fig.14: (a)- Day-to-day variations in the PRE (prereversal evening vertical drift) over Caximbo
and Campo Grande and the corresponding variations in the indices of F10.7, Kp and Dst, (b)The same PRE values and their wavelet spectra (top three panels) and the wavelet power spectra
of mesospheric zonal and meridional winds at 100 km and 95 km obtained from meteor radar at
Cachoeira Paulista (Abdu et al-2006); (c)- similar analysis results for the indices F10.7, and Dst.
29 Fig.15ab: The captions are under the figures. 30 Fig 16 (text)
31 Fig 17: Left panel: examples of gravity wave oscillations in F layer heights at plasma
frequencies 5, 6,7, and 8 MHz over Fortaleza, Brazil. Right panel: F layer vertical drift over
Fortaleza as determined by dhF/dt at the same plasma frequencies and days as thoses of the left
panel plot.
32 Fig 18: (a)- top panel: RTI map of 5.m irregularity plumes of night time F region as observed by
the 30MHz radar; the lower panels: F layer true heights at plasma frequencies 5, 6, 7, and 8
MHz, vertical drift Vz, band-pass filtered F layer height oscillations to check for GWs, and the
R-T instability growth rate factor Vz/L , all for the night of 23-24 September 2005, and (b)
similar plots as those of (a), but for the night of 24-25 Sept 2005.
33 Fig 19: R-T instability polarization electric field growth rate simulated using the equation above
for the nigts of 23 and 24 October 2005 corresponding to the data of Fig. 18.
34 Fig 20: (a)- Seasonal dependence of longitudinally averaged equatorial prompt penetration
vertical plasma drifts (positive upward) from ROCSAT-1 following a step function increase in
the AE index by 300 nT. This drift corresponds to under-shielding electric field. (b) the inverted
form of the same plots to illustarte the drift variation patern if produced by overshielding electric
field occurring during the recovery phase of a substorm. (c) quiet time vertical drift variation.
(Fejer et al., 2008)
35 Fig 21: A comparison between the vertical perturbation drift due to overshielding electric field
(lower panel) and the disturbance dynamo electric field (upper panel) from ROCSAT -1 data.
(Fejer et al 2008).
36 Fig 22: (a) The variations of the interplanetary magnetic field componenet Bz and the auroral
indices AU/AL during the storm of 26-27 August 1998. Note the Bz south turning near
21UT/18LT on 26 Aug. (b)-The variations in the F layer heights at a number of plasma
frequencies and the vertical drift over Sao Luiz during and before the eventes in (a); (c)- All-sky
630nm airglow emission images with plasma depletions streatched in mag meridian. The bubble
drifted westward with the velocity shown in (d). (Abdu et al., 2003)
37 Fig 23: A case of bubble drifting westward associated with an AE increase that caused prompt
penetration electric field (PPEF).
38 . Fig 24: (a) and (b) cases of PRE suppression or inversion due to superimposed westward electric
field (over-shielding electric field) associated with storm recovery in the evening hours, over Sao
Luis Brazil where LT = UT – 3 hrs.
39 Fig 25: Left panel- Cases of PRE enhancement and ESF development due to PPEF of eastward
polarity on 25 September, and PRE suppression due to westward electric field from
overshielding effect on 23 September 2001. Right panel- Model simulation of the instability
polarization electric field growth rates for the conditions of bubble developmemnt on the left
panel.
40 Fig 26: Illustration of disturbance dynamo electric field effects on ESF development. In general
the ESF can develop during late night hours while they are inhibited during post sunset hours.
The horizontal rectangular bars (in the right panel) indicates the ESF occurrence duration over
Sao Luis, Fortaleza and Jicamarca, during the storm interval of 09-11 November 2004.