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. F 2 layer is always the densest of the ionospheric regions with maximum density 10 5-8 X 10 6 electrons/cm 3. The layer F[ coalesce with F 2 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 (F 3 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 o o 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" = ~ xx 0' xl lo' xy 0' YYJ (1.3) where, 0' xx = 0'0 Sin 2 1+ 0'1 Cos 2 I 0' xy = 0'0 0'2 Sin I 0'0 Sin 2 I + 0'1 Cos 2 I 0' yy = 0' 2 2 Cos 2 I (1.4) 0'0 Sin 2 I + 0'1 Cos 2 I 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 (1.5) where 0' 3 is called the Cowling conductivity.
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 --- ~ E 20 ------- N 0 > -20 t+r' 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 ofthe 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 prereversal 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 prereversal 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 (11.9 0 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 fof 2 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... 1M 60 QC) 1.,.\..0 2;- 'g :<.0 :.> ~ (I..~ ~ -6C u lou 9 1<: u.e-l1l",t 21 100 0-100 15 17 19 21 23 LST thcursj 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 interhemispheric 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. A u-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 O 2 + in the F region. 0+ + O 2-7 O 2 ++ 0 O 2 + e- -7 0 ( 10 ) + 0 ( IS) or 0 ( ID) or 0 (3p) o (ld) -7 0 (3p) + hv (6300 A O ) O 2 + 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 A 0 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 F 2 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 h o, the level where the recombination and diffusion are balanced is calculated using model values of neutral density and temperature. The difference between h o and the observed
19 h m F 2 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 h m F 2, 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 h m F 2 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 h m F 2 can be inferred from optical measurements of 6300 A airglow emission since the volume emission rate (Photon /cm 3 / s) depends on the electron density at the altitude where dissociative recombination takes place. Therefore movements of F 2 layer will produce changes in 6300 A O emission intensity, as a function of altitude and it is possible to determine the height of the F 2 layer from the volume emission rate profile. These values of h m F 2 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 (N m ) 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 (h m ) at mid latitudes. Rishbeth et al. (1978) showed that h m 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 B z 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 B z orientation and its magnitude. Energy from the solar wind having velocity V sw is transferred to the magnetosphere in the form of electric field of the magnetospheric convection E
23 - B z V sw and precipitating particle fluxes which are also controlled by the magnitude of B z. 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--..,--...-----.--...----...---...---...-._ 5;--f-\rl---+---+--+--~-+--+-----l o-tr---t--\---t--f--t+-+---+-----iip..--+.---l -5-l---t---t---t--\4-1--i--..J---l-----...--1 Lt,h - foi-t--...l--+---'---+---'----4---.l.-.---< 6 12 18 24 60 40 20 o -20,-4'" v o a Vz ms 1 2~ Feb 1973 ~ ~ J I '\ 1/./'- / ~ ~ \.../ Lt, h 6 12 18 24 Figure 1.5. Effect of IMF Bz variations (northward changes) on the vertical drifts. (Mikhailov et al., 1996). B:z T.L~r- 20 ~ 1.9"73 5 o -5 j\ / i'-...... I--' U ~ "-./ ~V'--- 17 60 40 20 o -2-4- -6 Vz. :rn.s:-11-3 /'"'--... L7 '--- / r-...... ~ /' ""\ / ~ 'V \,j a ~ \A' L-t.h. -3 9 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 noonmidnight 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 R E (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