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.