electrodynamics of the equatorial...
TRANSCRIPT
Chapter I
ELECTRODYNAMICS OF THE EQUATORIAL IONOSPHERE
1.1. Introduction
The Ionosphere is the ionized component of the Earth's upper
atmosphere and is a transition region between the dense, electrically neutral
atmosphere below and very thin, ionized plasmasphere and magnetosphere
above. Earth's atmosphere is divided into various regions on the basis of
temperature and composition. The temperature structure of the atmosphere is
decided by the absorption of solar radiation. On the basis of the thermal
characteristics, the neutral atmosphere is divided into troposphere, stratosphere,
mesosphere and thermosphere. The low-latitude ionosphere occupies
approximately the same altitude range as the neutral mesosphere and
thermosphere in the altitude range 60 to 800 km. The principal source of ion
production is the solar extreme ultraviolet (EUV) radiation and the soft X-rays
of solar origin. The ionizing radiation varies daily, seasonally and with location.
The ionosphere is vertically structured in to 0, E, Fl and F2 layers. These
regions differ from one another in composition, density, ionizing sources,
degree of variability, chemistry and dynamics. The 0 layer extends roughly
from 60 to 90 km and is present only during the daytime. The E layer (90-140
km) practically disappears at night. The F[ layer extends from 140 - 180 km and
the F2 layer from 180 to 600 km. F[ region disappears at night. F2 layer
is always the densest of the ionospheric regions with maximum density
105- 8 X 106 electrons/cm3
. The layer F[ coalesce with F2 at night forming a
single F- region with maximum electron density in the vicinity of 350 km. The
plasmasphere ionosphere model suggested by Jenkins et al (1997) have shown
that under certain conditions an additional layer can form in the low latitude
topside ionosphere. This layer (F3 layer) has subsequently been observed in
ionograms recorded at Fortaleza in Brazil.
2
Ionosphere interacts strongly with the geomagnetic field. The solar wind
plasma and magnetic field distorts and confines the geomagnetic field with in a
cavity called the magnetosphere. The expanding solar corona drags the solar
magnetic field outward along with the solar wind plasma, forming the
interplanetary magnetic field (lMF). The solar wind and IMF drives the
magnetospheric convection system, energizes much of the plasma on the
Earth's magnetic field lines and drives large neutral atmospheric winds.
Because of these effects, changes in the solar wind plasma parameters and IMF
are very important for ionospheric studies. Magnetospheric electric fields map
down to the ionosphere, creating plasma convection, frictional heating and
plasma instabilities. Auroral particle precipitation ionizes the high latitude
atmosphere during nighttime and heat can be conducted from the
magnetosphere down to the ionosphere. On the other hand, some of the cold
ionospheric electrons and ions evaporate into the plasmasphere, plasma sheet
and magnetotaillobes.
1.2. Ionospheric Conductivity
The free electrons and. ions in the earth's ionosphere make it
electrically conducting. The upper atmosphere has an electrical conductivity
much greater than that of the lower atmosphere. The current density J can be
expressed as a function of the electric field E by using the generalized form of
Ohm's law as,
J = O'.E (1.1)
Where 0' is the tensor conductivity, J is the current density and E, the electric
field.
The geomagnetic field inhibits the motion of charged particles in the
direction normal to the field lines, and thus the conductivity is anisotropic.
Therefore three conductivities are defined. Parallel conductivity 0'0, is for the
direction parallel to the magnetic field line. Pedersen conductivity 0'[, is for the
direction perpendicular to the magnetic field and parallel to the electric field and
3
Hall conductivity 0'2, is for the direction perpendicular to both magnetic and
electric fields. Hence the conductivity tenser 0' can be represented in terms of
0'0. 0'1 and 0'2 as
0' 0'2
o oo
0'0
(1.2)
The polarization charges at the top and bottom of the conducting layer
will modify the electric field E under equilibrium conditions, ie, when there is
no vertical current component, the vertical electric field can be eliminated. Then
the 3 x 3 tensor 0' can be replaced by a 2 x 2 tensor 0", called the layer
conductivity, whose components depend on the dip angle 1. Using co-ordinates
x, y for the magnetic southward and eastward direction, the layer conductivity is
given as
0"
where,
0' xx
0' xy
0' yy
=
=
=
=
~ xx 0' xl
lO' xy 0' YY J
0'0 Sin 2 1+ 0'1 Cos 2 I
0'0 0'2 Sin I
0'0 Sin 2 I + 0'1 Cos 2 I
0'2 2 Cos 2 I
0'0 Sin 2 I + 0'1 Cos 2 I
(1.3)
(1.4)
At the magnetic equator I = 0, then the components of 0' simplify to
0' xx = 0'0, 0' xy = 0, 0' yy = 0'1 +~0'1
where 0' 3 is called the Cowling conductivity.
(1.5)
4
During daytime, the conductivity in the E regIon IS very high and
during nighttime it decreases by a factor 1/50 (Rishbeth, 1971). It is seen that
0' , and 0' 2 maximize in the E-region where electron and ion densities behave
in a fairly regular manner and are governed by a simple balance between
production and loss (Davies, 1965; Ching and Chiu, 1973; Torr and Harper,
1977). Unlike daytime condition, the relative importance of 0', above 200 km
to that below 200 km, can be substantial at night time (Harper and Walker,
1977). The variations in the conductivities during the low and high solar
activity periods have been explained by Richmond (1995). In the mid-latitude,
during low solar activity, the parallel conductivity is much larger than the
Pedersen and Hall conductivities. At a given altitude, both Pedersen and Hall
conductivities are essentially proportional to the electron density. During high
solar activity, largest Pedersen conductivity can sometime be in the
ionospheric F- regIOn above 200 km at night. In contrast to Pedersen
conductivity, the Hall conductivity always peaks In the E- region of the
ionosphere. Changes in the Pedersen conductance IS more than Hall
conductance. This difference is more at night than that during daytime.
Similarly the changes are noticed to vary with solar activity.
1.3 Ionospheric electrodynamics
The Sun and the Moon produce tidal forces in the atmosphere, which
results in air motion across the geomagnetic field. The wind will carry ions
along with it leaving behind the electron whose collision frequency is very
much less than its gyro-frequency. The wind-induced motion will lead to a
charge separation resulting in an electric field. This electric field will produce a
current flow at the ionosphere as in a dynamo, known as ionospheric wind
dynamo. A fairly accurate picture of the current pattern and its evolution can
be deduced from the continuous observation of the surface geomagnetic field.
Thus a global system of electric field at the ionosphere leads to a divergence
free current system. Over the equator, the electrostatic field is eastward during
daytime and westward during nighttime. Under quiet conditions, the winds and
/,
:;\, ..,,~..........
5
currents can be separated in to, Sq (solar quiet) and L (lunar quiet) variations.
During disturbed period magnetospherically produced electric fields and
currents can dominate over those produced by ionospheric dynamo (Richmond,
1995).
Horizontal winds in the thermosphere, driven by the daily pressure
variation due the solar heating, set in motion to the charged particles in the F
region (Rishbeth, 1971) and a current is induced as in a dynamo. In the low
latitude thermosphere, the a net average eastward flow is150 m/s near 350 km
and about 50 m/s at 200 km altitude which is most pronounced in the 2100-2400
LT. Since the Coriolis force vanishes at the equator, in a steady state, the winds
should blow in the pressure gradient direction from west to east across the
sunset terminator. During daytime, the F- region dynamo fields driven by F
region winds are largely short circuited by the highly conducting E- region. At
night, when the E region conductivity drops by a factor 1/50, F- region dynamo
can develop an appreciable electric field. The F- region dynamo is particularly
effective after sunset when the thermospheric winds are strong and the F- region
electron density is quite high (Rishbeth, 1977). The F- region dynamo behaves
like a constant current generator with high internal impedance.
Rishbeth (1971) discussed the role of E regIOn conductivity in
developing the F region polarization fields. Neutral air wind blowing across the
magnetic field cause a slow transverse drift of the positive ions, perpendicular
to both the wind and the magnetic field. This drift set up an electric polarization
field which can only be neutralized by currents flowing along magnetic field
lines through the E- layer. But, at night, the E - layer conductivity may be too
small to close this circuit, so that polarization fields builds up in the F- layer,
causing the plasma drift with the winds. This polarization effect may influence
the behavior of the equatorial F- layer during night-time. Electrons are highly
mobile in the direction of the magnetic field, and as a result the field lines
behave as a good conductor linking the E- and F - regions.
Besides the F region polarization field just described, there are
polarization fields of E region origin. These fields suggested by Martyn (1953)
6
are produced by the dynamo action in the E region by tidal winds, which exist
in various diurnal semidiurnal modes. These fields are of similar magnitudes
during day and night, unlike F region polarization fields which build up
quickly at sunset and decrease quickly at sunrise. Thus F region may playa
greater role than the E region in the night-time phenomena.
Heelis et ai. (1974) studied the effect of F region dynamo in modifying the
F region vertical drifts, which would otherwise be driven by E region electric
fields. Following the model developed by Heelis et ai. (1974), Farley et ai.
(1986) performed model calculations of equatorial electric fields and discussed
the physical mechanism by which F region polarization fields result in the
enhancement of zonal electric field (post sunset enhancement). Haerendel and
Eccles (1992) studied the role of equatorial electrojet (EEl) in the evening
ionosphere. They suggested that the equatorial electric field in the evening
sector results from a large current system set up by the effect of F region neutral
wind dynamo and the equatorial electrojet. This current is upward at the equator
since the upward current driven by the F region dynamo is not balanced by the
Pedersen Current (Haerendel et al., 1992), this current is upward at the equator.
They suggested that the EEl plays an important role in the evening
enhancement of upward and eastward plasma drifts.
Du and Stening (1999) found that the ionospheric process is controlled
by the E- region during daytime but by the F- region during nighttime. The F
region has a larger effect on the dynamo process during solar maximum than at
solar minimum, and during equinox than in solstice
1.4 F Region Vertical Plasma Drift
The F region vertical drift velocity is found to have a typical pattern
(Figure. 1.1) around the sunset period. The motion of the equatorial ionosphere
due to the E x B drift is generally upward in daytime and downward in
nighttime. The vertical drift reaches a maximum upward value after the sunset.
7
40
20---~E
------- 0N> t+r'
-20
15 17 19 21 23 01
lST (hours.)
Figure 1.1 Variation of vertical plasma drifts at Trivandrum
(Balan et al. 1992)
The magnitude of plasma drift slowly decreases and then reverses its direction
and in general remains downward in the midnight period. At dusk, the upward
drift velocity increases for 1 to 2 hours prior to the drift reversal. This is called
the evening enhancement, or pre-reversal enhancement, of the equatorial
ionospheric electrical field. The essential characteristics of the evening
enhancements are known to be the result of the dynamo effect by F- region
neutral winds and the effect of rapid changes in E-region electric conductivity at
sunset. The pre-reversal enhancement of the vertical drift is explained as due to
the F region dynamo effect. The thermospheric neutral wind at altitudes of the
equatorial F- region near dusk blow eastward. The eastward motion of neutral
particles causes only ions to drift upward by collision; the electric field
produced by the charge separation is projected into the E region through
magnetic field lines with high electric conductivity. However, since the F
region dynamo is a constant current source with high internal resistance, it is
readily short-circuited by the E-region with high Pedersen conductivity before
sunset. At night, conductivity is reduced by the decrease in E-region electron
density, creating a downward electrical field in the F-region. The E x B drift
induced by this electrical field has the same direction as thermospheric winds.
At the boundary of day-time and night-time conditions, the electrical field
created by the F-region dynamo effect results in a non-uniform E-W distribution
of electric conductivity in the E region, causing charge separation. Projected
8
back to the F-region, the resulting electrical field is eastward and westward to
the west and east of the boundary. In other words, the eastward electrical field
is intensified immediately before the reversal of the electrical field drift.
During daytime, the drift is found to oscillate between upward and downward
directions.
At the F region heights, the collision frequencies are so low that the
lOns and the electrons gyrate several times before they are affected by
collisions. Thus the motions perpendicular to the magnetic field are effectively
Hall drifts of ions and electrons produce by the cross product of the electric and
magnetic fields (Woodman, 1970). Since the dip angle at Trivandrum is about
0.5°, the measurement of vertical drift can be considered as the measurement of
horizontal electric field.
The pattern of vertical plasma drift exhibits day-to-day variability
(Woodman 1970). Studies of the F - region plasma drift concentrated mainly on
the vertical component which plays an important role in the height / latitudinal
distribution of ionization at low latitude. Different aspects of vertical plasma
drift have been extensively studied using various experimental techniques like
HF Doppler, incoherent backscatter radar, HF pulsed path sounding, Ionosonde
etc. (Woodman and Hagfors, 1969; Woodman 1970; Woodman et al., 1977;
Fejer et al., 1979a, 1979b; Gonzales et al., 1979; Abdu et al., 1981; Namboothiri
et al., 1988). The most systematic and long term study of the vertical drift was
done using Jicamara Incoherent Scatter Radar (Woodman, 1970; Abdu et al.,
1983; Batista et al., 1986; Fejer et al., 1989). The necessary criterion for the pre
reversal enhancement is a wind blowing in the F region at the time of E region
sunset (Rishbeth, 1971; Matura, 1974; Heelis et al., 1974; Farely et aI., 1986).
The observations of the vertical plasma drift using HF Doppler Radar
(Namboothiri et al., 1989 ) have shown that the average peak vertical drift is
higher in equinox compared to that during winter and summer months. Also the
9
equinoctial peak in pre-reversal enhancement is found to decrease with the
decrease in 1O.7cm solar radio flux value. The plasma drift drops by more than
a factor of 2 as the magnetic activity changes from quiet to moderate condition,
and increases well above the quiet day value for high activity. During equinox,
the pre-reversal enhancement peak is found to depend on the solar activity for
both magnetically quite and disturbed conditions. Monthly average of pre
reversal enhancement, time of occurrence of maximum value and time of its
reversal were studied by Balachandran Nair et al. (1993). They found that the
maximum plasma drift value fall off during summer and winter month and the
time of occurrence of maximum vertical velocity and reversal time do not have
much of a dependence on season. Fejer et al. (1991) determined the seasonal
averages of the equatorial F region vertical drifts from Jicamarca during 1968
1988. They found that the evening pre-reversal enhancement of vertical plasma
drifts increases linearly with solar flux during equinox but tends to saturate for
large fluxes during winter.
Hari and Krishna Murthy (1995) found that the seasonal variations of
the vertical drift is associated with the longitudinal gradients of the
thermospheric zonal wind. Similarly, the seasonal variation of the pre-reversal
enhancement of vertical drift is associated with the time difference between the
sunsets of the conjugate E region (magnetic field linked to F region) which is
indicative of the longitudinal gradient of conductivity (of E region). Ramesh
and Sastri (1995) determined the solar cycle, seasonal and magnetic activity
effects on the evening F- region vertical drifts measured with HF path sounding
at Kodaikanal. They concluded that the evening upward velocity peaks have
weaker solar flux dependence over Kodaikanal than over Jicamarca, and
suggested that this could be explained in terms of the difference in the gradients
of the thermospheric zonal wind and the E region conductivity near sunset.
Woodman (1970) has shown that the nighttime drift to be larger and
less variable during solar maximum. The pre-reversal enhancement at Jicamarca
was found to have higher amplitude during solar maximum and is almost absent
10
during winter months of solar minimum (Fejer et ai., 1979). Fejer (1981) has
shown that the pre-reversal enhancement exists only during equinoctial months
of solar minimum. Post sunset reversal time is latest during summer months of
solar maximum and earliest during winter months of solar minimum (Woodman
et ai., 1977). Fejer and Scherliess (200 1) found that the day-time average
upward drift do not vary much with solar activity, but the evening upward and
night-time downward drift increase from solar minimum to solar maximum.
The quiet-time variability of the vertical drift depends on local time, seasonal
and solar cycle.
Namboothiri et ai. (1989) have shown that the vertical velocity had
quasi-periodic fluctuations superposed on the gross pattern. Studies have shown
the presence of periodicities below 50 minute in the vertical drift (Sastri, 1988;
Subbarao and Krishna Murthy, 1983; 1994; Balachandran Nair et ai., 1992;
Sastri 1995). Earle and Kelley (1987) have studied the fluctuating components
below lOh period. Medium scale gravity waves are considered as the source of
these fluctuations. The drift velocity was found to show large fluctuations
relative to quiet time values (Gonzales et al., 1979; Fejer, 1986).
A global empirical model of the equatorial vertical plasma drift
velocity was developed from ground-based radar and satellite measurements
(Scherliess and Fejer, 1999), and the National Centre for Atmospheric Research
Thermosphere / Ionospehre / Electrodynamic General Circulation Model
(TIEGCM) successfully simulated the local time, seasonal, and solar cycle
dependence of the quite time F region plasma drifts measured at the Jicamarca
(11.90 S, 76.8° W, dip 2° S) ( Fesen et al., 2000). Equatorial F region vertical
plasma drifts were examined in detail using IDM (Ion Drift Meter) observations
on board the low inclination (19.76°) AE-E Satellite (Coley et ai., 1990; Fejer et
ai., 1995). Within a few degrees of the dip equator, the vertical plasma motions
result essentially from electrodynamic drifts driven by the zonal electric fields.
Fejer et ai. (1995) used AE-E data taken from January 1977 through December
1979 to examine the solar cycle, seasonal and longitudinal dependence of the
11
equatorial vertical plasma drift. The satellite observation of daytime upward
drift is about 20 mis, and is in good agreement with the radar data, particularly
during equinox and winter solstice. But the nighttime downward drift observed
by the satellite is usually smaller than the radar results, particularly during
summer solstice Maynard et ai. (1995) showed that the equatorial vertical drifts
obtained by vector electric field measurement on board the San Marco satellite
during the moderate solar flux this period of April- August 1988 are generally
consistent with AE-E and Jicamarca drifts.
Large longitudinal variations of vertical plasma drifts at about 14.00
LT near solar maximum during June and December solstices were inferred from
foF2 observation on board the Interkosmos-19 Satellite (Deminov et ai., 1988).
Coley et ai. (1998) used measurements from ion drift meter on the Defense
Metrological Satellite Program (DMSP) F8 and F9 satellites to examine the
vertical ion velocity at the dip equator as a function of the longitude for the year
1990, a period of high solar activity. They concluded that significant
longitudinal variation exists in the vertical plasma velocity at the dip equator
during the period of high solar activity.
1.5 Zonal Plasma Drift
The equatorial F region zonal plasma drifts are driven by the vertical
electric field which is coupled along the magnetic field lines to the E region
away from the magnetic equator. The nighttime F region electric fields are
mainly governed by the F region dynamo action due to thermospheric zonal
wind. With no shorting of the electric fields by the E region, the F region
plasma drifts along with the thermospheric zonal wind and depending upon the
extent of shorting, the plasma lags the zonal wind.
The characteristics of low latitude F region zonal plasma drift was
studied by Fejer (1991) using incoherent scatter radar data. The day-time
westward drift having an amplitude of about 40 mls is independent of solar
12
activity. While the night time eastward drift is largest in the pre-midnight
sector with amplitude increasing from about 90 to 160 rn/s from solar
minimum to maximum. Balan et ai. (1992) studied the zonal drift at
Trivandrum using HF Doppler radar in a spaced receiver configuration during
moderate solar flux condition. They observed that, the average zonal drift
pattern has nearly constant westward drifts of about 30 rn/s between 1500 and
1800 LT, an evening reversal around 1840 LT, and nighttime eastward drifts
with a maximum value of 110 rn/s between 2100 and 2300 LT. The zonal
plasma drift from westward to eastward at Trivandrum is about 2 h later than
that over Jicamarca. Figure 1.2 shows the average pattern of zonal drifts at
Jicamarca and Trivandurm.
211<:u.e-l1l",t
19 21 23
LST thcursJ
9
17
1M
60
QC)
1., .\..02;-
'g :<.0..:.>
~ (I.
.~
~
-6Cu
lOU
100
0
-10015
Figure1.2: Comparison of the variation of zonal plasma drift at Jicamarca(top: Fejer, 1997) and Trivandrum. (Bottom: Balan et ai., 1992)
Extensive measurements of night-time F region zonal drifts using the
spaced receiver scintillation technique were made at equatorial and low latitude
13
stations. Kumar et ai. (1995) obtained F region zonal drifts at night-time from
the time differences in the onset of VHF scintillations at two low latitude
stations near the peak of the Appleton anomaly crest in India. The eastward
irregularity drift decreased from about 180 rn/s to 55 rn/s during the course of
night. F region zonal plasma drift velocities can conceivably have different
magnitudes and / or altitudinal variations depending on whether or not plasma
irregularities are present.
Coley et ai. (1994) studied the relationship between low latitude F
region zonal ion drifts and neutral winds measured simultaneously by the Ion
Drift Meter (IDM) and by the wind and temperature spectrometer (WATS) on
the Dynamic Explorer-2 (DE-2) spacecraft. The equatorial zonal plasma drifts
from the IDM on DE-2 is in good agreement with the Jicamarca data (Coley and
Heelis, 1989; Fejer, 1991). Maynard et ai. (1995) showed that the diurnal
pattern of the equatorial F region zonal drifts derived from vector electric field
observations on board the San Marco satellite are consistent with earlier results.
1.5.1 Theory of Zonal Drift
The efficiency of the F region dynamo is controlled by F region zonal
neutral winds, field-line integrated E and F region Pedersen conductivities, and
local time (longitudinal) gradients on the F region zonal winds and in the E
region conductivity. Haerendel and Eccles (1992) have suggested that the
evening equatorial electric fields result from the effects of the F region zonal
neutral winds and the upward divergence of the equatorial electrojet. Crain et ai.
(1993 b) obtained as self-consistent solution for the global potential and
ionospheric plasma distributions to study equatorial electrodynamic plasma
drifts. This model accounts for the ionospheric / plasmaspheric and inter
hemispheric plasma transport. They also propose that the pre-dawn and post
sunset enhancements of the vertical plasma drift (zonal electric field) are well
correlated with the reversals of the zonal drift (and zonal wind) in the region
where the dominant dynamo driver exists. This may be illustrated with the aid
14
of a diagram of vertical equatorial plane and a horizontal plane described in
Figure 1.3. The vertical plane consists of E and F regions with a magnetic field
B entering the plane from the south. The horizontal plane represents the E
region of the southern hemisphere with a magnetic field passing through it from
below and connecting to the vertical plane.
Au-o
8
Figure 1.3 Simplified representation of the electric fields and current produced
by the reversal of the F- region zonal wind. A wind reversing from eastward to
westward (plane A) tends to produce a negative charge at the reversal boundary.
A wind reversing from westward to eastward tends to produce a positive charge
at the reversal boundary (plane B) (Crain et ai, 1993b).
They consider a much simplified, and somewhat unrealistic, ionosphere
in which only Pederson currents flow in the F region (aH =0), only Hall currents
flow in the E region (ap = 0), the only neutral wind U is a zonal wind in the F
region, and there are no local time gradients in the conductivity. In Figure 1.3, at
plane A, where the zonal wind reverses from eastward to westward, the F region
15
polarizes such that a downward, then upward, electric field is produced, E = - U
x B. These electric fields map down to the E region where they would produce
Hall currents JH that are divergent at plane A. The plasma polarizes to prevent
buildup of negative charge at the reversal point, A. These polarization fields Ep,
then produces an upward, then downward, drift on each side of A. This is
consistent with the pre-dawn enhancement of the vertical drift. Similarly, for the
case when the zonal wind reverses from westward to eastward (plane B), the
plasma polarizes to produce an electric field that is downward, then upward, on
each side of B. These electric fields, when mapped to the E region, produce Hall
currents convergent at the reversal boundary, B. The plasma polarizes,
preventing a buildup of positive charge at B, and these polarization fields, Ep,
produce downward, then upward drifts on each side of B.
1.6 Thennospheric Neutral Wind
Neutral wind is a very important thermospheric parameter which
significantly influences the distribution of F region ionization and its peak
density through transport of ionization and various other inter related processes.
Thermospheric winds transport en~rgy and momentum between various regions
especially during geomagnetic storms. The thermal expansion of the
atmosphere during daytime forms the so called diurnal bulge, which is centered
on the equator at about 14.00 LT. This bulging of the atmosphere gives rise to
horizontal gradients of the air pressure driving horizontal winds from the
hottest part of the thermosphere, which is in the afternoon sector, and towards
the coldest part in the early morning sector. The neutral wind therefore blow
across the polar regions and zonaly around the earth in low latitudes. The
frictional force or ion drag is generally the major factor limiting the wind speed
in the thermosphere. The winds can freely move the F region ions and
electrons in the direction of the magnetic field. If the field lines are inclined,
the ion motion has a vertical component which can affect the ion and electron
concentration, mainly because of loss coefficient has a significant height
dependence. The effect of the wind depends on its orientation with respect to
16
the geomagnetic field. poleward wind causes downward drift and tends to
reduce the ion concentration, while equatorward wind causes upward drift and
tends to increase the ion concentration. These effects being dependent on the
geometry of the magnetic field vary with latitude and with magnetic
declination. At the magnetic equator, since the field lines are nearly horizontal,
the plasma is transported with the same velocity as the neutral wind. Hence the
plasma drift in the north-south direction at the magnetic equator can be taken to
represent the meridional neutral wind velocity.
1.7 Techniques for Measurement of Neutral Wind
Measurement of thermospheric winds at middle and low latitudes is
important for an understanding of the mean circulation as well as the
propagation characteristics of waves and perturbations originating at high
latitudes. Various experimental techniques are available to measure / deduce
thermospheric neutral winds.
1.7.1 Fabry-Perot Interferometer Method
In Fabry-Perot Interferometer (FPI) method, the Doppler shift caused
by neutral wind on the air glow spectrum is determined (Biondi and Feibelman,
1968; Armstrong 1969; Meriwether et aJ.., 1986; Biondi et aJ.., 1988). Airglow
originates as a result of various photochemical reactions of neutral and ionized
constituents of the atmosphere. A major source of nightglow 6300 A° emission,
which is used for the wind determination, is associated with the dissociative
recombination of O2+ in the F region.
0+ + O2 -7 O2++ 0
O2 + e- -7 0 ( 10 ) + 0 ( IS) or 0 ( ID) or 0 (3p)
o (lD) -7 0 (3p) + hv (6300 AO)
O2+ ions, which are responsible for the 6300 A° emissions, move along
the magnetic field line prior to the recombination process. Once the
recombination takes place, the resulting excited oxygen atoms move with the
17
velocity of the neutral wind. A part of the oxygen atoms thus generated will be
in the 10 state and will emit after a period of ~ 110 s, this interval being the life
time of 0 ('D) state. This is a sufficiently long period for a particle to be in
the thermal equilibrium in F region altitudes. If there exists a gross movement
of the excited atoms with respect to the observer the airglow emission will show
a frequency shift proportional to the component of wind velocity in the line of
sight direction. For altitudes above about 400 km, thermalisation ceases to be
complete before the emission. So, if the source of 6300 A0 emission is above
this is altitude, the Doppler shift due to neutral wind on this region would not be
detectable though the optical method provides direct measurements of total
wind vector. These measurements are also limited by factors such as clouds,
daylight and phases of the moon. An important advantage of FPI method is that
it can be used independent of geographical location.
1.7.2 mcoherent Scatter Method
It is known from theoretical considerations that due to the close
coupling of the ionized and neutral constituents in the F region, the steady state
field aligned ion velocity is equal to the component of the neutral wind along
the magnetic field line, in the absence of diffusion. Vasseur (1969), Amayenc
and Vasseur (1972) and Salah and Holt (1974) made use of this fact in
determining the magnetic meridional component of neutral wind using the
Incoherent Scatter Radar (ISR) facility. In an incoherent scatter radar, the
signal transmitted from the high power radar is scattered by plasma density
fluctuations produced by thermal motions. Ionospheric parameters are
determined from the strength and spectral characteristics of the returned signal.
Any net motion of the bulk plasma gives rise to an overall Doppler shift on the
frequency spectrum of the received signal. For monostatic (back scatter) radar
this shift corresponds to the component of the transport velocity along the line
of sight of the radar. ISR can be used to measure electron density, electron and
ion temperatures, and the component of ion motion in the line of sight direction
(Rishheth and Lanchester, 1992). Thus it is possible to evaluate the partial
18
pressure gradient of the ionization and hence the diffusion speed of ions which
makes a significant contribution to the ionization drift at mid latitudes. In the
case where the ISR line of sight is not parallel to the magnetic field lines, the
electromagnetic drift will also have a contribution to the ion velocity. This
necessitates the knowledge of the time variations of the electric field for an
accurate deduction of meridional wind.
1.7.3 Meridional Winds from Ilm F2 Measurements
Due to the interaction between thermosphere and ionosphere, the F
region ionization is pushed up/down by an equatorward/poleward wind at a
place away from the dip equator. Hanson and Patterson (1964) and Rishbeth
(1967) showed that there exists a linear relation between changes in hmF2 and
changes in meridional wind in a steady state condition for small magnitudes of
meridional winds U i.e, ~hmF2 = a ~ u. Hanson and Patterson (1964) and
Richards and Torr (1986) suggested using this parameter deduced neutral wind.
Miller et al. (1986) demonstrated that this method could be employed to
estimate the meridional component of the neutral wind, from ground based
measurements of the peak of F layer, with accuracy comparable to that of
Fabry-Perot interferometer and incoherent scatter radar methods. A refined
version of the method of Miller et al. (1986) was presented by Richards (1991)
reducing the amount of computation time and taking into account the errors
caused by the assumption of steady state.
Buonsanto (1986) described a method to deduce meridional wind from
observed hmF2 data by using the servo model of Rishbeth (1967). According to
Rishbeth (1967) in the absence of neutral wind and electric field, the hmF2 lies at
a height where the effects of recombination and diffusion are balanced. A
vertical drift due to a neutral wind and/or electric field pushes the layer to a new
height. In Buonsanto's method, the balance height ho, the level where the
recombination and diffusion are balanced is calculated using model values of
neutral density and temperature. The difference between ho and the observed
19
hmF2 is attributed to meridional wind and is calculated in accordance with servo
model of Rishbeth (1967). A comparison of incoherent scatter radar, methods
of Miller et ai. (1986) and Buonsanto (1986) are given in Buonsanto et ai.
(1989, 1990). Equatorial thermospheric meridional wind during night-time has
been derived using h'F data from two equatorial stations nearly on the same
magnetic meridian by Krishna Murthy et ai. (1990).
Forbes et al. (1988) described another method for obtaining meridional
wind from hmF2, by modifying the ionospheric simulations carried out for
Arecibo by Crary and Forbes (1986) and Miller et ai. (1986), with the
appropriate geometrical factors to extend its applicability to other mid latitude
stations. The method of Forbes et ai. (1988) deals with storms related
perturbation winds rather than the total meridional wind.
Meridional wind deductions from in-situ measurements of hmF2 have
also been carried out using satellite data. Burrage et al. (1990) obtained
thermospheric wind information from the brightness measurements of 6300 A°
emission line obtained using AE-E satellite on the low latitude region.
Essentially he has followed the method of Miller et al. (1986). Vertical
excursion of hmF2 can be inferred from optical measurements of 6300 A°
airglow emission since the volume emission rate (Photon /cm3/ s) depends on
the electron density at the altitude where dissociative recombination takes place.
Therefore movements of F2 layer will produce changes in 6300 AO emission
intensity, as a function of altitude and it is possible to determine the height of
the F2 layer from the volume emission rate profile. These values of hmF2 were
then used in conjunction with a thermosphere model and accounting for the
effect of the zonal electric field to determined meridional wind. Jicamarca
incoherent scatter radar measurements of electric field were used for the
appropriate geophysical conditions assuming that over the latitude range of
interest, zonal electric field values to be independent of both latitude and
longitude.
20
In addition to the above mentioned methods, vapour release method
(Haerendel et al., 1967; Rosenberg, 1963) and observations from satellites
(Miller et al., 1986) are also used to evaluate the meridional wind.
1.7.4 Role of Neutral Wind in the F Region phenomena
Apart from the electromagnetic drift the neutral wind in the
thermosphere also contribute to the ionization distribution pattern in the
equatorial ionosphere (Bramley and Young, 1968). The effect of wind on the
peak of the F layer is discussed quantitatively by Rishbeth (1967). Neutral
winds could explain the decrease in peak density (Nm) on summer days and (in
part) the persistence of ionization over night. It could also be responsible for
observed day to night changes in the peak height (hm) at mid latitudes. Rishbeth
et al. (1978) showed that hm followed changes in the meridional wind with a
time constant around one hour. Sethia et al. (1983; 1984) showed that wind has
a marked effect on the electron content at the F region. Reasonable variations in
the magnitude and phase of the wind could explain the different types of daily
variations that observed during summer. Winds also play a major part in
producing the initial positive phase of ionospheric storms, at mid and low
latitudes (e.g, McDonald et al., 1985; Mazaudier and Bernard, 1985; Yagi and
Dyson 1985b; Titheridge and Buonsanto 1988). Equatorial therrnospheric
meridional wind during night time has been derived using h'F data from two
equatorial stations nearly on the same magnetic meridian by Krishna Murthy et
al. (1990). Figure 1.4 shows the nocturnal variation of the meridional wind in
September 1988 (Krishna Murthy et al., 1990). Their results show that the
meridional wind becomes equatorward around 1915 LT and reaches a peak at
about 2000 LT. The equatorward wind abates after midnight for a few hours,
and it even became northward. It again reverses to southward during the post
mid night period. It returns to a northward direction in the morning hours.
The direction of the meridional wind is one of the important driving
parameter for the occurrence of equatorial spread F. The work of Maruyama
21
(1988; 1996) show that the effect of strong meridional wind is only to inhibit
the development of range-type spread F. Jyothi and Devasia (2000) studied
some of the observed characteristics of thermospheric merdional wind
associated with the occurrence of equatorial spread F (ESF) during equinoctial
period. Their results showed that the ESF occurrence with h'F > 300 km show
on an average the presence of a poleward (northward) wind of smaller
amplitude before the onset of ESF while the ESF
Figure 1.4 Nocturnal variation of meridional wind using h' F data
from two equatorial stations nearly on the same magnetic meridian
( Krishna Murthy et ai., 1990).
occurrence of h'F < 300 km shows the presence of an equatorward (southward)
wind of comparatively larger magnitude. Sastri et ai. (1994) noticed that the
equatorial midnight temperature maximum (MTM) is responsible for. the
midnight poleward reversal of meridional wind there which, in tum, leads to the
post-mid- night collapse of the F layer at low latitude locations on the same
meridian. The effect of meridional winds and neutral temperatures on the F
layer heights over low latitudes were studied by Gurubaran and Sridharan
(1993) and concluded that the effect of the neutral temperature and its
variability should be properly accounted for in the determination of meridional
wind from the existing ground based ionosonde data. Devasia et ai. (2002)
noticed some of the characteristic features of the thermospheric meridional wind
during equinoctial period, associated with equatorial spread F and their possible
22
role in the triggering of ESF. Their study reveals that the polarity and magnitude
of the meridional wind become significant with the equatorward wind being
present when the h'F is below a critical height for the instability to get
triggered.
Seasonal variations of equatorial night time thermospheric meridional
wind using h' F data have been deduced by Hari and Krishna Murthy (1995).
They found that the wind is poleward (Northward) in all the seasons. The peak
value of the poleward wind (at the beginning of the night 1830-1900 LT) is
greatest in winter and least is summer. In winter, the pole-ward wind at the
beginning of the night decreases with time but remains pole-ward till early
morning hours. The equinoxes are marked by a late night reversal of the
equatorward wind to pole-ward. This reversal occurs before midnight. Later, in
the early morning hours the wind again turns equatorward.
1.8 Ionospheric Changes in Response to IMF Variations.
Ionospheric response to interplanetary magnetic field (IMF) deals with
the problem of the transfer of solar wind energy in to the magnetosphere and
then to the ionosphere. The solar wind energy may be transferred in to the
middle and low latitude ionosphere, either directly from the magnetosphere in
the form of electric fields and currents or indirectly through the high latitude in
the form of wave disturbances and winds. The IMF is most commonly
represented by three components Bx, By and Bz in the Geocentric Solar
Equatorial (GSE) co-ordinate system. Where x, y and z represents sunward,
eastward and northward respectively. The z direction is taken to be the normal
to the ecliptic plane.
The IMF Bz component plays the key role, SInce the degree of
reconnection between geomagnetic field and IMF, and consequently the energy
input into the magnetosphere, depends on Bz orientation and its magnitude.
Energy from the solar wind having velocity Vsw is transferred to the
magnetosphere in the form of electric field of the magnetospheric convection E
23
- Bz Vsw and precipitating particle fluxes which are also controlled by the
magnitude of Bz. The solar wind energy, which is mainly put in to the high
latitude region, is then dissipated by several mechanisms (electric field
variation, Joule heating, wave disturbance etc.) through the ionosphere. The
dynamics of the ionosphere as a whole is controlled directly or indirectly by the
Bz component of IMF. Variations in the other two components of IMF Bx and
By control the changes of the magnetosphere configuration even at quite
periods.
1.8.1 Response of Equatorial Ionosphere to the Variation in IMF Bz
The equatorial ionospheric response to the IMF influence has been
studied through the connection between equatorial and high latitude ionospheres
in various experiments in which electric fields have been measured at both these
latitudes (Gonzales et aI., 1979). Averaged values of the plasma drift (electric
field) at the equator was found to exhibit stronger coupling with IMF Bz.
Vertical drifts calculated from the ground based ionosonde on the
equatorial station Huancayo, showed a strong dependence on IMF Bz changes
(Mikhailov et aI., 1996). It is confirmed that the Bz turning to a northward
direction result in a decrease (up to reversal) of normal Sq (eastward during day
time and westward at night time) in the zonal component of the electric field.
The effect of IMF Bz variation on vertical plasma drift is shown in
Figures 1.5 & 1.6 (Mikhailov et aI .1996). During the interval 00-03 LT (night)
the northward turning of Bz decreased the westward vertical drift 14-18 LT
(day) the northward turning decreases the eastward vertical drift. Rastogi and
Patel (1975) and Patel (1978) suggest that strong IMF Bz reversals from south
to northward direction impose an electric field on the ionosphere opposite to the
normal Sq field. Several examples of east-west electric field (vertical drift)
reversal shown that they are well correlated with sudden northward turning of
IMF Bz (Fejel et aI., 1979b; Gonzales et aI., 1979).
24
10-y--..,--....-----.--....----...---...---...-._
Lt,h
18 24126-·f OI-t--...L--+---'---+---'----4---.L.-.---<
o
5;--f-\rl---+---+--+--~-+--+-----l
-5-l---t---t---t--\4-1--i--..J---l-----.........--1
o-tr---t--\---t--f--t+-+---+-----IIp..--+.---l
2418126a
Vz ms 1 2~ Feb 1973
~~
./ '- / ~~
I '\ 1/0
J \......./
'"Lt, h
v,-4
60
-2
o
20
40
Figure 1.5. Effect of IMF Bz variations (northward changes)
on the vertical drifts. (Mikhailov et al., 1996).
5
o
-5
B:z T.L~r- 20~ 1.9"73
j\/ i'-... ............
I--' U ~ "-./~V'---17
93
Vz. :rn.s:-11- /'"'--...L7 '---
/ r-.......
~ /' ""\ / ......~ 'V \ ,ja
~ \A' L-t • .h.
-3
60
40
20
o-2
-4-
-6
Figure 1.6 Effect of IMF Bz variations(southward changes)
on the vertical drifts. (Mikhailov et aI, 1996).
25
Large southward changes in the IMF increase the dawn to dusk magnetospheric
dynamo electric field, corresponding to an eastward electric field on the day
side and westward at night side, ie, with the same polarity as the quite time
equatorial electric field (Fejer, 1986). Mikhailov et ai. (1996) noticed that the
southward Bz excursion enhance normal zonal electric field both in the daytime
and night hours. It can be seen from Figure 1.6. that the southward Bz
excursion results in an increase in westward zonal electric field (downward
vertical drift) in the 0000-0600 LT . The daytime southward Bz excursion
leading to an increase of the eastward zonal electric field can be seen in Figure
1.5.
1.8.2 IMF By Component
Under quiet conditions IMF By causes a displacement of the Sq system
foci (Mastushita, 1977). Zakharov et ai. (1989) have carried out theoretical
analysis of the By effect during disturbed conditions. The zonal component of
the electric field Ep at the equator is directed opposite to the dynamo electric
field for most local times under By > O. Under By < 0 the Ep direction
coincides in phase with the dynamo field on the day and evening local time
interval and is anti-phase in the other LT intervals. Experimental evidence of
magnetospheric electric field penetration into the equatorial ionosphere under
By turning has been obtained from cosmos-184 satellite data (Galperin et aI.,
1978). It was shown that turning of By from negative to positive causes an ion
density (Ni) increase in the night time equatorial ionosphere associated with
additional upward plasma drift.
IMF By component effect on the East-west drift velocities of the
ionization irregularities in the ionospheric E and F region were studied by
Vyas and Chandra (1981). E and F region drift exhibit a linear relation with
By. Signature of IMF By component on the low latitude geomagnetic field was
studied by Nayar (1978). It can seen that the By component of IMF has its
signature on low latitude geomagnetic field and this signature vary with time of
the day and with season. Nayar and Revathy (1979) discussed the effect of
26
diurnal and seasonal variations of By component of TMF on the low latitude
horizontal intensity in detail. The relation between By and H is found to vary
with time of the day, season and polarity of the IMF component.
1.8.3 Bx Component
The IMF Bx component influence the magnetosphere and ionosphere
less strongly than By and Bz components, but still quite noticeable. Cowley et
al. (1991) have shown that By and Bx action may be described by a simple
model, dipole plus uniform field. According to this model, magnetic tension
caused by By results in the asymmetry of the magnetospheric convection
system in relation to the noon-midnight meridian. At ionospheric heights this
manifests itself as the displacement of the auroral oval as a whole in the
direction of By on the southern hemisphere and the opposite direction on the
northern hemisphere. The Bx component influence is similar but the asymmetry
is observed in relation to the dawn-dusk meridian; the auroral oval is displaced
along the noon-midnight meridian. Due to the sector and spiral structure of the
IMF, By positive usually corresponds to negative Bx, and vice versa. Thus we
can separate By and Bx assuming that By displaces the auroral oval only in the
direction of the dawn-dusk-meridian and Bx displaces it only in the noon
midnight direction.
1.9 Effect of Magnetic Stonns and substonns
A geomagnetic storm results in the decrease of horizontal component
of the geomagnetic field and subsequent recovery. At low and middle latitudes
a westward flowing ring current at the magnetosphere heights depresses
geomagnetic field. The world-wide magnetic disturbance produced during
magnetic storm is generally understood in terms of the amount of solar wind
energy transferred to the inner magnetosphere due to the solar wind
magnetosphere coupling (Gonzales et aI., 1994).
27
The principal defining property of a magnetic storm is the creation of
an enhanced ring current formed due to the enhancement of the trapped
radiation belt particle population. The ring current consists of ions and electron
transferred to the earth's environment by interaction of the solar wind with the
geomagnetic field. These interactions occur kinetically via the energy of the
solar wind particles and electrodynamically via the interplanetary magnetic and
electric fields. Electro- dynamic interactions cause the interplanetary electric
field to extent into the geomagnetic field. This electric field is transmitted along
the geomagnetic field lines to the ionosphere, which is highly conducting at
altitude between 100-150 km. The combination of electric field and high
conductivity causes significant oxygen ions and electrons in the 10-300 keY
range, located usually between 2 to 7 RE (where R E is the earth's radius) and
producing a magnetic field disturbance, which at equator, is opposite in
direction to the earth's dipole field.
Substorms are viewed as the fundamental energy release element
during solar wind-magnetosphere interactions. The response of the equatorial
ionosphere to the magnetosphere-polar-auroral processes can manifest itself into
two ways. One is due to the direct penetration of magnetospheric convective
electric field to the low latitudes and the other due to disturbance dynamo
effects (Blanc and Richmond, 1980; Blanc, 1983; Fejer, 1986). The direct
penetration of electric field can be expected during rapid changes of electric
fields during substorms (Nishida, 1968; Somayajulu et aI., 1987; Abdu et aI.,
1988).The substorm related electric fields are important not only in auroral
latitudes but also in the middle and low latitudes. The Whistler observation
show that substorm electric field of 0.5 mV/m penetrate deep within the
plasmasphere (Carpenter, 1970; Park and Carpenter, 1970). In the ionosphere,
the height of the F2 layer has been known to change considerably during
geomagnetic disturbance and this effect is attributed to electrodynamic drift
(Martyn, 1953; Maeda and Sato, 1959; Kohl, 1960). The response of the
equatorial night-time F region to the magnetic storm time disturbance has been
examined by Somayajulu et aI. (1991) using Ionogram recorded at Trivandrum
28
and magnetogram recorded at high, middle and low latitudes. During early
morning hours, there is an unusual F region height rise and a sudden onset of
the range type of spread F during the storm. Sastri et al. (1992) noticed that
there is decrease in h'F is found to occur around the onset of the substorm and
the subsequent increase during the substorm recovery phase. The observed F
region height disturbance is interpreted as the signature of a transient composite
disturbance in the equatorial east-west electric field caused by the prompt
penetration of substorm- related perturbations in high latitude electric fields.
Studies of the extensive plasma drift measurements from Jicamarca have
revealed the local and storm-time- dependent disturbance drift patterns
associated with magnetic activity (Fejer and Scherliess, 1997; Scherliess and
Fejer, 1997).
Figure 1.7 shows the temporal evolution of the Jicamarca vertical
disturbance drifts following the change in the AE index shown in the top panel.
Local time variations of the equatorial vertical average disturbance drift pattern
at the storm time bottom panel (Fejer. 2002). Following a sudden increase in the
AE index, the high latitude electric fields penetrate nearly instantaneously in to
the low latitude ionosphere since the region-2 field aligned current lags behind
the change in the polar cap potential drop. Under these "under-shielding"
conditions the equatorial perturbation vertical drifts are upward during the
day and downward at night, which correspond to eastward and westward
electric fields, respectively. Observation using incoherent / coherent scatter
radars and ground based ionosonde and magnetometers have established the
appearance of perturbations in electric fields and currents on geomagnetic
substorms/storms (Fejer, 1986; Reddy, 1989; Ganguly et ai., 1987; Fejer et ai.,
1990; Forbes et ai., 1995). Abdu et ai. (1998) viewed that the magnetospehric
electric field responsible for DP2 penetrates to the equatorial ionosphere on the
dust side as on the day-side and leads to electric field perturbations of the same
polarity (eastward) as on the day side. Sastri et ai. (2000) showed conspicuous
quasi-periodic fluctuations in F region vertical plasma drift and are found to be
29
~ ~it: ;:;;~ '[ :'I ;: :::TiT ;Tt: ~-2 -1 00 01 02 03 04 05 06
Storm-TIme (Hours)
JICAMAACA 1968-872010
0-10
100
en -10:s10
;S 0ca -10~
-~10:>
0-10
100
-10-20
Figure 1.7 Ideal change of AE index during storm time (top panel).
Local time variations of the equatorial vertical average disturbance drift
pattern at the storm times indicated in the bottom panel (Fejer, 2002).
coherent with variations in Bz (North-South) component of interplanetary
magnetic field.
Efforts have focused on the morphology of equatorial current and
plasma drift disturbances associated with several high latitude processes and the
development of empirical and theoretical electric field models (e.g., Sakharov et
aI., 1989; Denisenko and Zamay, 1992; Fejer and Seherliess, 1995). Sastri
(1989) suggested that reduction in the equatorial electrojet strength near noon,
associated with IMF polarity effects, is in fact largely due to westward
disturbance dynamo electric fields. Evidence for zonal electric field
disturbances associated with storm sudden commencements in the equatorial
nighttime ionosphere was presented by Sastri et al. (1993). Fejer and Seherliess
(1997) showed that the equatorial storm time zonal electric field pattern is in
excellent agreement with that from the disturbance dynamo model developed by
30
Blanc and Richmond (1980). Fejer (2002) has been reviewed the low latitude
storm time ionospheric electrodynamics.
1.10 Scope of the present study
This thesis mainly deals with the study of equatorial ionospheric F
regIOn electric field and its response to magnetospheric dawn-dusk electric
field. The spectra of Vz and Bz are investigated to find out similarities between
them and to find their characteristics. The pre-reversal enhancement of the
vertical plasma drift and its relation to the zonal drifts, reversal of plasma drifts
near the sunset period etc. have been studied. The HF Doppler system was
operated in the spaced receiver configuration, and these observations are used to
calculate the zonal and meridional component of the plasma drift at F region
altitudes.