CHAPTER 3

HEIGHT GRADIENT OF F REGION VERTICALPLASMA DRIFT IN THE EVENING EQUATORIALIONOSPHERE

3.1. Introduction

3.2. Measurement Technique And Data Analysis

3.3. Observations

3.4. Discussion

3.5. Conclusion

3. Height Gradient Of F Region Vertical Plasma Drift in the Evening Equatorial Ionosphere 54

3.1 Introduction

Extensive measurements of zonal and vertical plasma drifts of equatorial F region

have been made at Jicamarca (12° S, 76.9° W, Dip 2° N), Peru, with the incoherent

scatter radar over the past two and a half decades. The observations have helped

in establishing the day to day, seasonal, solar cycle and magnetic activity varia­

tions of the F region vertical plasma drift which is driven by east-west electric

fields (Woodman, 1970; Fejer et a/., 1979a; Gonzales et al., 1979b; Fejer, 1981;

1991). Plasma drifts are derived extensively from ground based HF techniques

like Ionosonde and Phase path sounder (Batista et aI., 1986; Balan et a/., 1992;

Sastri et al., 1992; Subbarao and Krishnamurthy, 1994) and satellite measurements

of vector electric field and ion drifts (Coley and Heelis, 1989). Several theoretical

and numerical models have been developed to explain F region drifts in terms of

electric fields generated by E and! or F region dynamos (Rishbeth, 1971; Heelis

et al., 1974; Farley et al., 1986; Crain et al., 1993a).

The early observations at Jicamarca showed the F region vertical plasma drift

to be height independent except at times when there is a rapid reversal in the drift

(Woodman, 1970; McClure and Petersen, 1972). These studies are done using drift

data averaged over the altitude range of 300-400 km. Murphy and Heelis (1986)

have derived relationship between zonal and meridional plasma drifts at low and

mid latitudes which consider the curl free electric field condition (.6.X E = 0).

This study show the presence of a vertical gradient in the drift. The assumption

of the curl free electric field implies that, if the vertical ion drift has an altitude

gradient which is a function of local time, it can be expressed in terms of the zonal

drift as10V¢ 2Vr oVr _"--- - -- + -- - uI o¢ r or

where ¢ and r are azimuthal and radial directions. Thus a local altitude gradient in

the vertical ion drift velocity may be required to .reconcile the observed east-west

and vertical drifts at any given location.

The presence of vertical gradient is confirmed by studies of Pingree and Fejer

(1987) using Jicamarca radar data. The F region zonal drift at low and dip

latitudes have shown latitudinal variation in the evening period. Simultaneous

3.1. Introduction 55

observations of rocket from Chamiacal, Argentina and Incoherent scatter radar,

Jicamarca showed that evening reversal of the F region vertical drift occurred

earlier at higher latitudes (Valenzula et al., 1980). Assuming the field lines as

equipotential, this variation can be expressed as a vertical gradient in the zonal drift

at the magnetic dip equator. This is very well explained in the F region dynamo

model by Anderson and Mendillo (1983). The improved method of observation

showed the drift to be height dependent.

McClure and Petersen (1972) have shown a negative velocity gradient value

near the sunset. The average gradient is found to be negative in the afternoon and

early evening periods and positive at all other times. Pingree and Fejer (1987)

have studied the height gradient of F region drift for a few days during March­

April, 1986. In the afternoon and early evening period, the gradient is found to

be negative with the reversal occurring earlier at higher altitudes. The results

show a negative height gradient in the 1300-2000 1ST region and weak positive

value at all other times. Subbarao and Krishnamurthy (1994), using the phase

path sounder have studied the height gradient of vertical drift in the dusk period

over the equatorial station of Trivandrum. They have reported a negative height

gradient of vertical drift on one day (September 19, 1975 from 1810-1904 hours

LT).

The observations reported so far on the height gradient of vertical drift veloc­

ity are extremely limited. Moreover, all these observations are made during the

equinox period. So the seasonal pattern, if any, in the height gradient of the F

region drift is not known. In addition to that, the height dependence of vertical

drift in the bottomside F region is obscure as the radar measurements at Jicamarca

typically cover the altitude region around and above the F region peak.

This chapter gives an insight into the altitude gradient of the F region vertical

drift below the F region peak during the evening hours of winter solstice in the

Indian dip equatorial region using the HF phase path (doppler) technique. The

analysis uses the HF Doppler drift velocity measured from two equatorial stations,

Trivandrum and' Kodaikanal.

3. Height Gradient Of F Region Vertical Plasma Drift in the Evening Equatorial Ionosphere 56

3.2 Measurement Technique And Data Analysis

HF pulsed phase path (doppler) sounders are operated simultaneously at Trivan­

drum (8.5° N, 77° E, Dip 0.5° N) and Kodaikanal (10.25° N, 77.5° E, Dip 4° N)

to provide continuous information on the time rate of change of phase path (or

doppler frequency shift) of reflection from discrete ionospheric regions at normal

incidence (Balan et al., 1982; Sastri et al., 1985).

The basic design of both the systems are the same. They differ only in the

mode of data acquisition. The phase path sounder at Kodaikanal is operated at a

frequency of 4 MHz and the HF Doppler radar at Trivandrum at 5.5 MHz.

In the experiment at Kodaikanal, the quadrature channel outputs (A cos <p,

A sin <p) of the receiver are used in a logic scheme (which essentially identifies

2nTT phase conditions) to provide data on the sense and magnitude of the change

in phase path to the limit equivalent of a total phase path with a time resolution

of 6 sec. Data is recorded continuously on a strip chart recorder run at a speed

of 10 cm/min, which gives a basic time resolution of 6 sec. At Trivandrum, the

analog outputs of the receiver (A cos ¢, A sin ¢), are recorded in digital mode with

the help of an ND converter and the Doppler frequency shift is computed using

FFf from the digital data acquired.

The present study incorporates simultaneous and continuous measurement of

F region vertical drift in the evening hours from 1700-2100 1ST at Trivandrum

and Kodaikanal on IS days during December, 1993 to January, 1994. The days

selected are quiet to moderately disturbed (Ap < 21 for 13 out of 15 days) at an

epoch of low to moderate solar activity (81 < 510.7 < J.22 units) under non-spread

F conditions.

The time rate of change of phase path delta 6.P / 6.t (or the doppler shift 6.f) of

F region echoes during the evening period i3 a measure of the vertical drift velocity

near the reflection level because the contribution of electron density changes is

negligible at those times. The vertical drift velocity (Vd) is then Vd = (>../2)6.P / 6.t

or (>../2)6.f. This Vd near the dip equator represent the combined effect of Ex B

drift and an apparent upward drift due to chemical loss. The drift due to chemical

loss is mostly negligible in the evening period, since the height of reflection will

be > 300 km (Bittencourt and Abdu, 1981). In the case of Trivandrum data the

3.3. Observations 57

reflection level is above 300 kIn and hence chemical loss is negligible on all the

selected days. In Kodaikanal data on some of the days Vd has to be corrected

for the layer decay due to chemical loss. The apparent vertical drift V,s due to

chemical loss is VI3 = 13L, where 13 is the loss coefficient and L is the electron

density scale length. This is calculated using the standard procedure (Sastri et al.,

1992).

The vertical drift velocity is calculated for every one minute from 1700-2100

1ST on all the fifteen days. The velocity values are smoothed by subjecting it to a

suitable smoothing window of 55 minutes. This will help in taking out the short

period fluctuations which will prevail in the evening F region Vd near the dip

equator. This yields the steady vertical drift pattern at the two stations.

The height gradient is calculated in every five minute interval as dVdjdh

where dVd = VdTri - VdJ(od and dh is the difference in reflection height be­

tween the two stations. The height is the group height which is calculated as

h = C x tj2; where c is the velocity of light and t is the time delay of the signal.

The factor 1/2 comes because the time delay t is the time the signal takes to reach

the reflecting height and then return back to the ground station.

3.3 Observations

The F region vertical drift due to zonal electric field is the same at Trivandrum

and Kodaikanal due to the proximity to each other and to the dip equator. The ratio

of 1I,t at Kodaikanal to that at Trivandrum is found to be 0.9981. Thermospheric

meridional neutral winds could affect the F layer vertical drift preferentially at

Kodaikanal due to the large dip angle at that location. Hence the vertical drift

at Kodaikanal is to be corrected for meridional wind effects to derive those due

to zonal electric fields alone. Direct measurement of meridional neutral winds

(using optical Fabry-Perot interferometer technique) :.n the Indian region is not

available for the days of our observation. Hence, the empirical HWM87 model of

Hedin et al., (1988) is used for correcting the drift n:.easurements at Kodaikanal

for meridional wind effects. The poleward neutral wind predicted by HWM87

steadily decreases in amplitude with time in the evenin,5 period for the geophysical

3. Height Gradient Of F Region Vertical Plasma Drift in the Evening Equatorial Ionosphere 58

conditions relevant to the present mea'iurements. The magnitude of neutral wind

is in the range 25-56 mis, and its effect on the Vd at Kodaikanal is in the range

1.5-3.5 mls. Thus the measurement of vertical drift at these two dip equatorial

stations on two different frequencies of 5.5 MHz and 4 MHz after correcting for

the meridional wind effects and chemical loss (when the reflection height is < 300

km) is equivalent to multifrequency probing of the bottomside F region at a single

station.

The gross pattern of F region drift at Trivandrum and Kodaikanal are found

to be the same with an enhancement and decay of upward drift in the dusk period,

followed by the reversal to downward direction around 2000 1ST. The magnitude

of the drift showed significant difference on all the days. This confirms the

presence of a height gradient in the vertical drift. The height gradient is calculated

from the difference in group height reflection at the two operating frequencies.

Since the difference in group height over the two frequencies are taken, the use

of group height instead of true height do not introduce any significant error in

the calculation of gradient. Also the probing frequency is well below the critical

frequency of the layer at the individual stations during the observation.

The height gradient of the vertical drift is found to be positive. Its value

varies with local time and attain high value at the dusktime enhancement in the

upward drift. A plot of the vertical height gradient and reflection height for the

7th December 1993 (Fig 3.1) shows that the upward drift at Trivandrum is higher

than that at Kodaikanal throughout the dusk period. The height gradient is found

to be positive from 1730 1ST. It attained a maximum value of 0.20 mlsec/km at

about 1800 1ST and then decayed more or less steadily thereafter reaching very

low values around the time of downward reversal of drift.

The growth and decay of the positive height gradient in vertical drift is a con­

sistent feature of all the winter data samples taken. Mass plot representing the

gradient for all the fifteen days are shown in Fig. 3.2. The height gradient is

positive during the evening enhancement of upward drift (1815-1925 1ST, max­

imum value 0.68 mlsec/km) and is small and sometimes negative in the period

prior to and after the evening enhancement. The magnitude of the gradient shows

day-to-day variability at a given local time. In the present 15 days samples the

3.3. Observations 59

200019301800

(a)

T - TrivandrumK - Kodaikanal

280

- 480EoX........r:.t- 440I~

~ 400

~.... 360

frl...JLL 320lLJ0::

---..L-~-'-~-l.----'L.--...L--l-! I1830 1900

1ST

Fig. 3.1. Time variation of (a) F re9ion ver'tical drift (Vd ),(b) Reflection heioht (h) ot Trivandl"um and Kodai­kanol and (c) gradient of vertical drift on 7-12-93

J. Height Grauient Of F Region Vertical Plasma Drift in the Evening Equatorial Ionosphere 60

0.24

0.20

,..."

E 0.12~

"-() 0.08Q)

III.......E 0.04

.........J::

<J 0.00.......

N - 0.04><J

- 0.08

- 0·12

- 0.16

- 0.20

-0.24

-MEANN= 15

'-""'-"'---'--.L..-L.......L-..L..-..L.-J"--J.---L-L! I /

1730 1800 1830 1900 192,0 2000 2030 2100

1ST

Fig.3.2. Mass 'plot of the time varia~'ion of height gradientof F-region vertical drift (Vd) in the eveninghours. The average pattern is shown by thethick line.

3.4. Discussion 61

gradient varies from -0.24m1sec/km to 0.22m1sec/km in the interval 1730-2030

1ST. The sunset at conjugate E regions corresponding to F region (300 kIn) over

Trivandrum occurs during the period 1745-1820 1ST in local winter months.

The fixed frequency measurements of F region vertical drift do not correspond

to any particular height, but to the reflection level which varies with local time and

from day-to-day at a given local time. The reflection height on a given frequency

increases significantly during the evening hours because of the enhancement of

upward vertical drift and reaches maximum values around the time of the down­

ward reversal in vertical drift (Fig.3.1). The evening pattern of the height gradient

of vertical drift (Fig.3.2) suggest an altitude dependence of the height gradient.

To assess this physical situation, the height gradient of vertical drift is plotted as

a function of reflection height at Kodaikanal. The gradient values are taken at

5 minute intervals. The mean pattern of the altitude dependence is derived by

grouping the data in height bins of 20 kIn and computing the mean values of base

height at Kodaikanal and the height gradient. The resultant plot is as shown in

Fig.3.3. This shows that the gradient is essentially positive and large (0.05 - 0.1

mlsec/km on the average) between 290-340 km but also assumes negative values at

times. But above 355 kIn, the gradient is very small and this significant decrease

in the gradient is the important feature of the altitude dependence of the height

gradient of vertical drift in the bottomside F region.

3.4 Discussion

The important result of this study is that the dusktime vertical plasma drift in the

lower F region is characterised by a positive height gradient during the winter

solstice in the Indian dip equatorial region. The gradient itself is height dependent

in that it is very small at heights above 360 km. Though the positive height

gradient in the winter solstice seems to be at variance with the earlier observations

during equinox, using the incoherent scatter radar at Jicamarca, a closer analysis

shows that it is not totally inconsistent. The height profile of dusktime (1912-1924

hours) vertical drift at Jicamarca showed a slight increase of vertical drift with

height below 400 km and a steeper decrease above 400 kIn (McClure and Petersen,

3. Height Gradient Of F Region Vertical Plasma Drift in the Evening Equatorial Ionosphere 62

•• ••• ••

•••••_.

o

• •

• tIDOX' 0o 0

•

380I• 1730 - 2020 1ST•

360•I • ••• • •....:. •

'.. • • • •340 ..

: .",. , •• •••• ••• •." •• .. , .. •

320 • • • •0 • •• •c1J ,..• .I.•• , • •·1 •

260

280

t­I(!)

w 300:r:

E~........

240 I-0.10 -0.05 0.0 0.05 0.10 0.15 0.20 0.25 0.30 0.35

~VZ/6h(m/sec /km)

Fig. 3. 3 ~ Variation In height gradient with height atKodaikanal

3.4. Discussion 63

(972). The improved Jicamarca radar measurements showed by Pingree and Fejer

(1987) represented occasional presence of positive gradient in the dusk sector.

The positive height gradient in F region vertical drift observed in the present

study provides a logical explanation for the recent observations that pre-reversal

peak in F region vertical drift at Kodaikanal (4 MHz) is lower than that at Trivan­

drum (5.5 MHz) for the same season and epoch of the solar cycle (Sastri et aL.,

1994).

The vertical and zonal drifts of equatorial F region plasma are due to electric

fields generated by the dynamo action of tidal winds in E region and thermospheric

winds in F region. During daytime the electric fields are determined primarily

by the tidal E region dynamo because the fields due to F region dynamo are

largely shorted out by the highly conducting E region. At the beginning of sunset,

the control of the electrodynamics passes from the E region to the F region.

This is due to the significant decrease in E region conductivity which allows the

development of polarization electric fields of the F region dynamo. The numerical

modelling stl:ldies have highlighted the importance of the F region dynamo to the

F region plasma motions at night, in particular the post-sunset enhancement of the

upward vertical drift near the dip equator (Heelis et aL., 1974; Farley et aL., 1986;

Crain et aL., 1993a; 1993b). The effectiveness of E or F region to generate electric

field depends on several factors such as, the flux-tube integrated conductivity of

the region and the latitude variation of the wind system. In the dusk sector, it

was shown that the ratio of F region to E region flux tube integrated Pedersen

conductivity undergoes a large enhancement (dependent on the apex altitude over

the equator) due to the difference in the decay properties of the conductivities of

the two regions at sunset. This increase is an important parameter in producing

the sunset enhancement of the upward vertical drift, the other being the local time

gradient of the zonal neutral wind (Farley et aL., 1986; Crain et aL., 1993a; 1993b).

The relative role of E and F region dynamos in generating electric fields

depends on altitude. Around the F layer peak and above (about 400 km and

higher) most of the conductivity is located in the F region and the polarization

fields (vertical and zonal) due to F region dynamo contribute most to the zonal

and vertical plasma drifts. As the altitude decreases, the E region dynamo begins

3. Height Gradient Of F Region Vertical Plasma Drift in the Evening Equatorial Ionosphere 64

to gain importance and may become dominant at lower altitudes (around 300 km).

The net result of this altitude dependence of E and F region dynamo fields is

the dusk time development of vertical shear in zonal plasma drift with eastward

drift at and above F peak and westward drift well below the peak which is

widely observed (Kudeki et al., 1981; Fejer et al., 1985; Aggson et al., 1987) and

adequately modelled (Anderson and Mendillo, 1983; Crain et al., 1993b). The

development of the positive height gradient in vertical drift in the lower F region

during this winter solstice is believed to be a signature of this physical situation.

The relative importance of F region dynamo to the E region dynamo increases

with altitude in the lower F region. Since the polarisation electric fields of E

region dynamo are weaker than those of F region dynamo, a positive gradient in

vertical plasma drift has to develop in the lower F region. The large variability of

the height gradient of vertical drift is not surprising since the electrical coupling

of E and F region in the evening quadrant depends on several factors like altitude

profiles of plasma density and wind systems in the two regions.

Simultaneous measurement of zonal plasma drift and vertical drift is not avail­

able for both the stations to substantiate the simple and qualitative explanation

given. The earlier observations of Balan et al., (1992) clearly show that ilt Trivan­

drum the evening reversal of zonal plasma drift from westward to eastward in the

lower F region occ~rs 2-3 hours later than at Jicamarca at F region peak and

above. This is expected in view of the increase in importance of E region dynamo

over F region dynamo with decrease in altitude below the F peak. Hence it can be

concluded that large positive height gradient of equatorial F region vertical drift

at altitudes below 360 km reported is an additional facet integral to the complex

electrodynamics of the dusktime equatorial ionosphere.

3.5 Conclusion

Simultaneous observations at Kodaikanal and Trivandrum after applying for the

necessary corrections for meridional wind effects and chemical loss accounts for

multi-height probing of the bottomside F region at a single location for determin­

ing the height gradient of F region vertical plasma drift. The important result is

3.5. Conclusion 65

the existence of a positive height gradient in the interval 1815-1925 1ST preceded

by negative values. The growth and decay of the positive height gradient occurs

regularly in the dusk period and has slight day-to-day variations. This positive

height gradient of vertical plasma drift is interpreted in terms of altitude depen­

dence of the relative contributions of E and F region dynamos to the electric

fields responsible for plasma drifts (vertical and zonal) of the dusk time equatorial

F region.