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Equatorial spread-F (ESF) and vertical winds
Journal of Atmospheric and Solar-Terrestrial Physics 61 (1999) 607±617
Equatorial spread-F (ESF) and vertical winds R. Raghavarao*, R. Suhasini, H.G. Mayr, W.R. Hoegy, L.E. Wharton Goddard Space Flight Center, Mail code 910.4, Greenbelt, MD 20771, USA Received 1 October 1998; accepted 20 January 1999
Abstract The Equatorial Spread-F (ESF) phenomenon is recorded in ionograms as a hierarchy of plasma instabilities in the F-layer of the equatorial ionosphere. The ESF is characterized by irregularities in the plasma (electron and ion) density and electric ®eld distributions perpendicular to the Earth's magnetic ®eld. Large scale irregularities are generated by a primary plasma instability that develops in electric ®elds and plasma densities. Other secondary instabilities then develop and generate irregularities at several scale sizes that often produce a plasma `hole' or `bubble' that rises up with high E B velocities. The ESF/plasma bubble phenomenon has been studied extensively with experimental techniques and modeling, which revealed important features. In the bottom side F-layer, near sunset, when the vertical density gradient steepens as the layer is supported by the horizontal (North±South) Earth's magnetic ®eld lines against the omnipresent Earth's gravitational acceleration ( g ), the plasma conditions can give rise to Rayleigh±Taylor (RT) type instability. But the observed day to day variability of the ESF occurrence suggested that other agencies may also be involved in generating the instability. Sekar and Raghavarao (1987) with linear theory, and Raghavarao, Sekar and Suhasini (1992), with non-linear numerical modeling, suggested that vertical downward (upward) winds in the ambient gas have the potential to cause (inhibit) the ESF/bubble phenomenon. The presence of downward winds near the equator was reported earlier. In this paper, we show evidence for the presence of downward winds collocated with irregularities in electric ®elds and plasma densities as revealed by an unique combination of highly accurate measurements with instruments onboard the DE-2 satellite. The observations reported here are also consistent with the notion that the build-up of the equatorial ionization anomaly (EIA) prior to local sunset is important for the ESF instability. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction With ionosondes, short pulses of high frequency (HF) radio waves are radiated upward, and these are re¯ected from the ionosphere to be recorded as traces in ionograms depicting the virtual heights (h ') of re¯ection at varying frequencies ( f, in the 1±20 MHz
* Corresponding author Tel.: +1-301-286-8575; fax: +1301-286-1663. E-mail address: [email protected] (R. Raghavarao)
range). On certain nights, the spread in the width of the pulses re¯ected from the F-layer increases abnormally, showing up as a diuse trace, and this phenomenon is called Spread-F. Its occurrence characteristics at low geomagnetic latitudes are known to be distinctly dierent from those at higher latitudes and hence it is referred to as Equatorial Spread-F (ESF). The ESF has been investigated extensively with ground-based ionosondes, with scintillation measurements of radio waves from radio stars and beacon satellites (Abdu et al., 1992; Aarons, 1993, and references therein) and with VHF radars (Woodman and LaHoz, 1976; Tsunoda, 1980).
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The ESF phenomenon has been identi®ed with the plasma bubbles that are observed with rocket experiments (Szuszczewicz et al., 1980; Raghavarao et al., 1984, 1987; Kelley et al., 1986; Sridharan et al., 1997) and with satellite-borne instruments (Hoegy et al., 1982; Aggson et al., 1995; Singh et al., 1997; Kil and Heelis, 1998; and references therein). On days of ESF occurrence, the F-layer near the magnetic equator becomes unstable around local sunset time and as a consequence irregularities develop in the electric ®eld and plasma densities with a wide range of scale sizes. These irregularities are aligned with the Earth's magnetic ®eld lines forming sometimes severe density depletions called `holes' or `bubbles' (Tsunoda, 1980; Ossakow, 1981). The dynamical behavior of plasma bubbles often reveals rising plumes with dierent shapes as observed by VHF radars (Woodman and Lahoz, 1976; Patra et al., 1997). The zonal motions of bubbles and their role in causing radio scintillations have also been studied with optical imaging techniques (Mendillo et al., 1992; Weber et al., 1996; Sahai et al., 1998). The onset conditions for ESF and plasma bubbles have been investigated by measuring vertical pro®les of winds and electric ®elds with rocket released chemicals (Barium±Strontium (Ba±Sr), Sodium (Na)) and electron densities with Langmuir probes (LP) (Raghavarao et al., 1984; 1987) and by combining many of the above mentioned techniques (Sridharan et al., 1997). Among the dierent scale-sizes, the large-scale plasma irregularities are observed to develop ®rst in the bottom side F-layer and are believed to be caused by a collisional Rayleigh±Taylor (RT) type instability (Haerendel, 1974; Hudson and Kennel, 1975; Scannapieco and Ossakow, 1976). Such an instability is expected to occur when the layer is at very high altitudes where the ion-neutral collision frequencies (nin) are low as the RT instability growth rate is given as (1/L )( g/nin). Here g is the acceleration due to gravity and L is the vertical scale length of the ion density gradient. A necessary condition for the RT instability is that plasma densities form with steep positive height gradients that are supported by the horizontal Earth's magnetic ®eld against `g'. Such large density gradients can develop soon after local sunset when the ionization in the bottom-side F-layer decreases due to recombination and electric ®eld induced plasma redistribution. Several aspects of the ESF/plasma bubble phenomenon are understood. Some basic features of the phenomenon, however, are not understood such as the variations with longitude, season, and from day to day. The altitude of ESF onset where the instability is initiated can vary over an altitude range of more than 100 km either at solar maximum (Sastri and Murthy, 1978) or at solar minimum (Sastri et al., 1978). In the collisional Rayleigh±Taylor (RT) instability, changes
in the collision frequency due to variations in temperature for example can change the onset altitude; but temperature variations as large as 10% would produce only changes of about 30 km. Another factor is the scale-size of the electron density in the bottom-side Flayer, which is dicult to assess since it is aected by the transient decay of the ionization after sunset. Observationally, however, there is no evidence to suggest that the post sunset ESF is signi®cantly aected by the height dependence of the F-layer (Sastri and Murthy, 1978; Sastri et al., 1978). Under seemingly identical conditions for the plasma density and its height structure, the ESF occurs on one day but does not occur on the other day (Sekar, 1990; Ramarao et al., 1997). The post-sunset ESF occurrence (non-occurrence) has been shown to be closely linked on a day to day basis with the prior intensi®cation (non-intensi®cation) of the ionization crests in the well-known Equatorial Ionization Anomaly (EIA) that develops in the late afternoon near 208 magnetic latitude (Raghavarao et al., 1988; Alex et al., 1989; Sridharan et al., 1994; Jayachandran et al., 1997). The formation of the EIA and its intensi®cation is produced by plasma ¯owing in a fountain from the equator towards the crests, which is caused by dynamo electric ®elds. This fountain eect increases the ionization crests at the expense of the plasma density in the bottom-side F-region near the equator and thus steepens there the vertical density gradient, which is conducive to RT instability and explains in part the relationship between EIA and ESF. However, there is also another link between the EIA and the ESF that is potentially important, and this is the subject of the present paper. 2. Vertical winds and ESF/plasma bubbles With Ba±Sr releases at heights below F-layer, Anandarao et al. (1978) and Raghavarao et al. (1987) measured vertical winds at sunset time. With a ground-based Fabry±Perot spectrometer that measured the OI emission line, Biondi and Sipler (1985) had observed the temporal variations of vertical winds during one night at around 200 km. These measurements provided the impetus for theoretical investigations of the eects of winds on the RT instability. By analytically solving the plasma ¯uid equations of continuity, momentum (including zonal and vertical winds), and the current conservation for ions and electrons, Sekar and Raghavarao (1987) showed that the growth rate g ', of the generalized RT instability (GRT, since transport velocities are included) is enhanced (reduced) by downward (upward) vertical winds. Earlier, Hanson et al. (1986) showed that the growth rate can be enhanced (reduced) also by upward (down-
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ward) ion drift velocities. Based on the numerical model results, Raghavarao et al. (1992) concluded that the eects of either vertical downward winds or upward plasma drifts of equal magnitude are the same in accelerating ESF instability. Hence, the equation for the growth rate (Sekar and Raghavarao, 1987) can be written as, g 1=L g=nin Znin =o i Vi ÿ V , with 1=L 1=Ni dNi = dz
1
where Ni is the electron/ion density; oi, the ion gyrofrequency; Z, the zonal eastward winds; and V, Vi, the vertical upward velocities of the neutrals and ions, respectively, at the dip equator; nin and g as de®ned above. (In the right-handed coordinate system, x, y and z represent the magnetic east, north and upward directions, respectively.) The ®rst term in Eq. (1) represents the collisional RT instability (Haerendel, 1973), the second term represents the contribution from the zonal winds, Z (Chiu and Straus, 1979); and the last term represents the contribution from the vertical wind, V (Sekar and Raghavarao, 1987) and vertical ion drift velocities, Vi (Hanson et al., 1986). Chiu and Straus (1979) showed that the zonal winds do not contribute signi®cantly to the instability growth rate in the F-layer, since the nin/ oi is about 1/200 at 260 km and decreases exponentially with height. Sekar and Raghavarao (1987) compared the eective contributions of dierent terms in Eq. (1) to the growth rate g ' and showed that a 1 m/s downward wind (or upward ion drift velocity) at 260 km is as eective as a 200 m/s eastward wind and that a downward wind of 16 m/s at 300 km would be as eective as gravity. Furthermore, Raghavarao et al. (1992) showed from non-linear numerical simulations that downward winds of 20 m/s at the F-region altitudes (or 20 m/s upward plasma drifts) accelerate the evolutionary process of the instability signi®cantly faster than gravity ( g ) alone and that the acceleration continues beyond the height of maximum density into the topside F-region. They attributed the day to day variability in the ESF and bubble occurrence to the variability in the vertical winds. The eects of vertical winds on the bubble growth are clearly shown in the spatial distribution of the plasma velocities (Sekar et al., 1994). Hypothesizing the presence of vertical winds (of ÿ20 m/s), Raghavarao et al. (1987), Sekar and Raghavarao (1997), and Sekar et al. (1997) were able to interpret the ESF (electron) density irregularities obtained from three dierent rocket campaigns. In addition to vertical winds, they also included the contributions to the growth rate from eastward winds and westward-tilted gradients of the F-layer (Kelley et al.,
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1981) that is always present at sunset. Similarly, Laakso et al. (1995) invoked the presence of downward vertical winds of 15±30 m/s to explain the plasma bubbles and associated zonal electric ®elds measured by the San Marco-D satellite. Based on DE-2 satellite measurements, relatively large variations are observed in the neutral temperature and wind velocities that are closely linked to the EIA (Raghavarao et al., 1991, 1998). Near the EIA crests, the temperature is enhanced relative to the magnetic equator, while the zonal winds near the equator are signi®cantly larger than in the crest regionÐa phenomenon that is referred to as the Equatorial Temperature and Wind Anomaly (ETWA). Raghavarao et al. (1993) also reported vertical winds that tend to be upward in the crest region and downward around the magnetic equator at local times (1900±2200 LST) when the ESF is observed. The vertical winds revealed a latitudinal structure similar to that of the ETWA temperature, and Fuller-Rowell et al. (1997) suggested that both are produced by the release of chemical energy from the dissociation of the enhanced O+ 2 in the crest region. These observations are consistent with rocket measurements below the Flayer (from 100 to 300 km) that reveal at sunset near the dip equator vertical winds with signi®cant magnitudes, in addition to zonal and meridional winds and electric ®elds with large spatial gradients (Anandarao et al., 1978; Raghavarao et al., 1984, 1987; Sridharan et al., 1997). In the present paper, we show two sets of satellite data obtained (i) at 1890 LST, the expected time of ESF onset, and (ii) at 2100 LST, the expected time of fully developed ESF. These examples show evidence of downward vertical winds collocated with irregular electric ®elds and ion densities, the ESF/bubble signatures. The data thus con®rm the earlier hypothesis (Raghavarao et al., 1992) about the importance of vertical winds for the ESF. 3. DE-2 measurements The DE-2 satellite was in an elliptical orbit with 908 inclination, with perigee around 300 km, and it operated from August 1981 to February 1983, a period of maximum solar activity. The slow orbital precession of the DE-2 orbit made it possible to cover all local solar times (LSTs) in a period of about one year. Among the times of interest for ESF, 1845±2000 LST occurred during the solstice season, and 2000±0200 LST occurred during equinox. The DE-2 satellite measured a combination of several parameters (Homan et al., 1981) and provides the best data set to date for investigating a number of problems, such as the origin of ESF, which are connected with the structure, dynamics
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and electrodynamics of the Ionosphere±Thermosphere System (ITS). The pertinent parameters, measured simultaneously and some of them with unparalleled accuracy, are brie¯y described below. 3.1. Winds and temperatures The Wind and Temperature Spectrometer (WATS), described in detail by Spencer et al. (1981), measured the neutral temperature (T ) with a sampling rate of 1/ s, and the zonal (Z ) and vertical (V ) wind components each with a sampling rate of 0.5/s. Spencer et al. (1981) and Raghavarao et al. (1993) discussed the accuracies of the temperature and wind measurements. For a solar ¯ux of F10.7=175 and noon time gas densities at 300 and 400 km altitudes, respectively, the estimated errors in T are 1.5 K and 3.4 K, and in the winds 2 m/s and 4 m/s. The errors are reduced to 1/2 of these values for the available 16 s averages presented here. Since the errors increase with altitude, we used only data obtained below 400 km. There are, however, three major uncertainties in the measured values of vertical winds: 1. The zero-level of the winds is uncertain by about 15 m/s, since the satellite attitude could only be determined with 0.18 accuracy (Raghavarao et al., 1993). 2. The vertical winds measured away from perigee are in¯uenced by the ellipticity of the orbit. Since the wind component measured perpendicular to the satellite velocity vector (Vs) in the plane of the orbit is taken as the `vertical velocity', its direction deviates from the radial (height) at satellite positions away from perigee. Around perigee the eect is negligible, but away from it the measurements are aected, in particular during the solstice period when cross equatorial winds are present. During solstice, at any point away from perigee, the measured velocity V in the orbital plane can be presented as V V 0 cos q2M sin q, where V' and M are the true components of vertical and meridional winds, respectively, and q is the angle of deviation of the normal to the satellite velocity vector from the radial direction (from the Earth's center). The signs +/ÿ apply when DE-2 is in the summer and winter hemispheres respectively, when the satellite is moving from, say, the northern to southern hemisphere with its perigee near the equator. 3. Variations in the satellite attitude can produce spurious vertical winds, which are generally small compared to the horizontal winds. On occasions, such variations may be inferred from large-scale (0208± 308 in latitude) oscillations of the three components of the earth's magnetic ®eld that have been recorded by one or two of the triaxial ¯uxgate magnetometers
(Farthing et al., 1981) on board DE-2 satellite. Such magnetic ®eld oscillations in fact have been found to be correlated with the variations of vertical winds in one of the data sets discussed here. However, it cannot be ruled out that the said magnetic ®eld oscillations are generated by ionospheric currents and not by attitude variations. Such currents have been inferred from satellite (MAGSAT) borne magnetometer data (Langel et al., 1993; Olsen, 1997), in particular during evening hours when the conductivities of the F-region, dominate over those of the E-region and hence the dynamo action in the Fregion prevails (Rishbeth, 1971). The MAGSET satellite, having attitude determination and control of 6 nT accuracy, could measure magnetic ®elds in the 300±500 height region (Maeda et al., 1982) with suf®cient accuracy to infer the meridional currents driven by the F-region dynamo at dusk time around the dip equator (Takeda and Maeda, 1983). The above mentioned magnetic ®eld oscillations observed by the triaxial magnetometer on DE-2 thus may in part be generated by ionospheric currents.
3.2. AC electric ®elds The Vector Electric Field Instrument (VEFI), described in detail by Maynard et al. (1981), measured both the DC and the irregular or variable (called AC) electrostatic ®elds in the ionosphere. A double ¯oating probe technique was employed for two mutually perpendicular directions (X, Y), both in the orbital plane containing the satellite velocity vector (Vs) of the nonspinning DE-2. The double probe in the east±west direction, perpendicular to the X±Y plane, failed to get deployed. The double probes in the X and Y directions are oriented at 2458 to the spacecraft velocity, respectively. The AC ®elds, measured with the double probe technique in either one of the two directions around perigee, represent mainly the variable component of the electrostatic ®eld in the vertical direction. A small part of the variable ®eld lies in the magnetic east±west direction at longitudes where the declination angle of the Earth's magnetic ®eld is signi®cant (the DE-2 orbit being 908 inclined to the equator). The AC ®elds in each direction were processed onboard the satellite by a spectrometer with a bank of comb ®lters that provided the spectral amplitudes in 8 low frequency (LF) bands (4±1024 Hz) and 4 high frequency (HF) bands (1.02±512.0 kHz) (Maynard et al., 1981). We have chosen to present here the Y ®eld data (rms values) in the ®rst four of the eight LF bands with 1 s sampling rate along the path of the DE-2 orbit. The AC signal amplitudes in these channels represent irregularities
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with scale sizes of approximately 2, 1, 0.5, and 0.25 km (the satellite velocity being about 8 km/s), which are characteristic of ESF/plasma bubbles. 3.3. Ion densities and irregularities To describe the plasma density irregularities, the ratio of delta(Ni)/Ni was measured with a very sensitive Retarding Potential Analyzer (RPA) (Hanson et al., 1981). The values of delta(Ni)/Ni were obtained for six dierent frequency (Hz) bands, and we present the amplitude variations in the lowest frequency band (32± 86 Hz) read in 4 s intervals. The measured amplitude variations indicate the presence of ESF/bubble structure and reveal scales similar to the irregular (AC) electric ®elds observed simultaneously. The plasma density (Ni) was also measured with a Langmuir probe (LANG, Krebiehl et al., 1981), and the available 16 seconds averages are presented. In these data, however, the density irregularities related to ESF are averaged out. 4. Observations In the following, we shall describe two sets of observations from DE-2 that show the presence of ESF/ plasma bubbles along with vertical winds that are conducive to generate the phenomenon through the GRT instability. With Fig. 1 we present DE-2 measurements in orbit 1626 of 21 November 1981 (81325) near 137E longitude at 18.9 LST, which is close to the onset-time of ESF occurrence. Fig. 1(a) shows the variations in the ion density (Ni) characteristic of the EIA and the signatures of ETWA, i.e., the latitudinal variations of the zonal winds (Z ) and temperatures (T ). The values of T can be read by adding 1000 to the scale. As seen from Fig. 1(a), T forms a crest at about 208 N in the winter hemisphere collocated with the peak in Ni, which is the signature of ETWA. In the south, the crest is masked by the steep temperature increase in the summer hemisphere (Raghavarao et al., 1991). The observed variations in the vertical wind (V ), shown in Fig. 1(b), reveal upward velocities and peaks located on either side of the dip equator. The downward winds are centered around the dip equator and are collocated with the T minimum. Superimposed on the large scale variations in the vertical winds, there are also small scale variations observed with smaller amplitudes that are negatively correlated with similar structures in T. Notwithstanding the earlier mentioned uncertainties about the zero level in the vertical winds (V ), the velocities are downward around the dip equator. However, the vertical winds shown here need to be taken with caution. The vertical component of
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the magnetic ®eld measured onboard the spacecraft reveals deviations from the magnetic ®eld model to produce an oscillatory structure similar to that of the vertical winds (R. A. Homan, private communication). This indicates that the satellite attitude may change signi®cantly and may produce in this particular case spurious vertical wind velocities. We shall return to this problem later in the discussion and conclusion sections. The amplitudes of the ion density irregularities represented by delta(Ni)/Ni on a logarithmic scale are shown in Fig. 1(c). One large peak is observed north of the dip equator and three well resolved peaks south of it. These density peaks are observed above a broad background of lesser magnitude, and they are collocated with LF±AC electric ®eld peaks shown in Fig. 1(d±g). The amplitude peaks in both types of irregularities, delta(Ni)/Ni and AC ®eld, are observed to occur at the same locations and on both sides of the dip equator in a region where the vertical winds are downward [Fig. 1(b)]. (The location of the dip equator is shown with an arrow.) From the magnetic ®eld lines shown in Fig. 1(e), with height (Ht ) scale on the right side of the Figure, it is evident that the irregularity peak north of the dip equator and the one closest to it in the south are on the same ®eld line tube. The apex height of this ®eld line tube over the equator is centered at 340 km and has a width of 20 km. The other two AC ®eld and delta (Ni) peaks located further south do not have their conjugate companions in the north. At these northern conjugate locations, the observed vertical winds [Fig. 1(b)] are upward (or reduced in magnitude if downward), which could explain why irregularities do not develop there. The amplitudes of AC ®elds presented in the four LF bands [Fig. 1(d±g)] are con®ned to speci®c ®eldline tubes that do not engulf the entire bottomside Flayer around the dip equator. The AC ®elds are also observed to decrease with frequency. As discussed by Kelley et al. (1982), these features are the signatures of plasma bubbles in the formative stage associated with the GRT instability (Eq. (1)). In this context it is important to point out that the vertical ion/electron density gradients at the EIA trough may be large as required by the GRT instability, although the densities themselves are small as shown in Fig. 1(b). Large vertical gradients have been observed in ion/electron density pro®les of the bottom side F-layer measured at the post-sunset ESF onset times with rocket-borne probes (Raghavarao et al., 1987; Sridharan et al., 1997). Our observations suggest that in the ®eld tube shown the V/L (in Eq. (1)) is large enough for the GRT instability to develop to form plasma bubbles that move in the directions ((E B)/B2) perpendicular to the magnetic ®eld. The data presented here, however, show only a snapshot picture of ESF/bubbles along the sat-
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Fig. 1. DE-2 measurements at 1854 LST in orbit 1626 on 21 November 1981 of: (a) zonal winds (Z ), neutral temperatures (T ), and ion (plasma) densities (P ); (b) vertical winds (V ); (c) irregularities in plasma densities, DNi/Ni; and (d±g) the AC electric ®elds in dierent (®rst four) low frequency (LF) bands as shown. Vertical arrow in (c) shows the location of the dip equator. Dashed lines in (e) depict magnetic ®eld lines passing through the two conjugate A.C. electric ®eld signals and the height (Ht ) scale is shown on the right.
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Fig. 2. DE-2 measurements at 2100 LST in orbit 3838 on 17 April 1982. The description is the same as in Fig. 1. Panels (a) and (b) are reproduced from Raghavarao et al. (1993).
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ellite path, which does not reveal when and at what height precisely the bubbles were ®rst formed prior to the observation. In addition to the above described features, Fig. 1(d) also shows three bursts of AC ®elds between ÿ24 and ÿ32 dip latitudes and weakened conjugate signals in the north that reveal alignment along ®eld-lines crossing the equator at altitudes from 2000 to 2800 km. These signals will not be discussed further in the paper. Another example of ESF/bubbles and vertical winds is presented with Fig. 2 for the orbit 3838 on 17 April 1982 (day 82,107) that describes conditions at 2100 LST, well beyond the post sunset time at which these features begin to develop. In Fig. 2(a), we show the observed variations in temperature, electron density, zonal winds, and in Fig. 2(b) the vertical winds that have been discussed earlier by Raghavarao et al. (1993). Characteristic of ETWA, symmetric and prominent T crests form on both sides of the dip equator. The vertical winds are upward collocated with the T crests and are downward around the dip equator. For the equinox condition discussed here, Raghavarao et al. (1993) estimated the error in the vertical winds due to the uncertainty in the satellite attitude determination and suggested that the zero level should be approximately at ÿ11 m/s. Hence the downward winds around the equator may be closer to ÿ11 m/s instead of ÿ22 m/s and the upward winds collocated with the T crests may be about 15 m/s instead of 4 m/s. It is important to note that during this orbit, unlike the previous one, the onboard magnetometer data for any of the three magnetic ®eld components did not show oscillatory structures of dimension similar to those of the vertical winds around the dip equator (R. A. Homan, private communication). Variations in the spacecraft attitude are not a problem in this case, and the vertical winds presented here are considered reliable. In Fig. 2(c) we present the observed irregularities in ion densities from the RPA and in Fig. 2(d±g) the AC electric ®eld signals in the ®rst four LF bands from the VEFI instrument. The AC ®eld amplitudes in the LF bands are prominent around three spatial locations (ÿ5.98, 5.18, 10.38 dip latitudes), all collocated with the peaks in the ion density irregularities. The ®rst one in the south and the other two in the north of the dip equator are in regions where the vertical winds are downward with relatively large velocities. The ®ve pulses preceding the ®rst AC signal (at 81,920 s) that are prominent in channel one (d) and weaker in channel two (e) are calibration marks; these were applied by command to the preampli®er while disconnected from the probe (Maynard et al., 1981). The ®rst AC signal has its conjugate counterpart in the north within the ®eld line tube shown in Fig. 2(e). As in the pre-
vious example, the other signal recorded further north does not have its conjugate counterpart in the south where the vertical downward winds are small or even upward considering the zero-level correction of ÿ11 m/s discussed above. As in Fig. 1, the AC ®eld amplitudes are decreasing with frequency, but unlike the previous example weak signals occur in the region across the dip equator. 5. Discussion As pointed out earlier for the (post sunset) case (Fig. 1), the magnetometer data (R. A. Homan, private communication) show oscillatory structures with latitudinal extent similar to those observed in the vertical winds. This suggests that the velocities are spurious, produced by variations in satellite attitude. No such contamination is found in the (pre midnight) case (Fig. 2), and the following discussion will therefore apply primarily to that. In Fig. 2, we show that the irregularities in both plasma densities (delta(Ni)/Ni) and AC electric ®elds (LF), which are the signatures of the ESF/bubble phenomenon, are collocated with downward vertical winds. The examples presented here were recorded near the dip equator in the bottom side F-layer (300± 320 km) during pre-midnight periods. Although the magnitudes of the vertical winds are uncertain they are downward where the ESF irregularities are generated. And the winds are shown to be upward or small where the ESF irregularities do not develop. At a height of 300 km, downward winds of about ÿ16 m/s contribute as much as g/nin to the growth rate of the RT instability (Sekar and Raghavarao, 1987), while upward winds contribute to suppress the instability. While the observations show the presence of ESF under conditions that are favorable for generating the phenomenon, it is not possible to determine the exact heights and times at which the underlying instability develops. The ion density measurements on DE-2 do not provide information on vertical gradients (1/L ) in Eq. (1), and from a polar orbit at a single LST the initiation time for the instability cannot be ascertained. The observations presented in this paper, however, provide evidence that the vertical winds (ÿV ) are important in addition to gravity ( g/nin), ion drift velocities, Vi, and zonal winds, Z sin d. In the height region of minimum L value, downward winds contribute to initiate the instability. The examples presented here also indicate that dierences in the vertical winds between the two hemispheres may be responsible for initiating the instability on one end of the ®eld line and suppressing it on the other. As shown in Fig. 2, the AC ®eld signals and delta (Ni)/Ni further away
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from the equator and recorded on one side of the ®eld-line tube are shown to be associated with downward winds; but no conjugate signals are recorded in the other hemisphere where the vertical winds are small in magnitude or upward directed. Thus there is clear evidence that the polarity of vertical winds and their magnitudes are important in generating the ESF/ bubble phenomenon. The timing of the vertical winds, directed downwards in the evening hours, may play an important role in generating the observed ESF/plasma bubbles and may contribute to the duration and day to day variability of this phenomenon. Non-linear modeling studies of the development of ESF/bubbles that account for vertical winds reveal that their polarity, strength and vertical structure play an important role in accelerating the bubble/ESF growth (Raghavarao et al., 1992). Raghavarao et al. (1993) proposed that the vertical winds are associated with ETWA, that the winds are upward near the EIA crests where temperature crests are observed and are downward around the equator. In the context of ESF it is relevant that during solar maximum the ETWA phenomenon is characterized by temperatures that increase after sunset and reach peak values (100±120 K) at around 2000 LST (Fig. 6 in Raghavarao et al., 1998). During these times, vertical winds are observed, on a number of occasions, with a latitudinal structure similar to that in the temperature and they are downward around the dip equator where the temperature has a minimum (Raghavarao et al., 1993). The observed downward winds are in the 10±40 m/s range as is shown in the present paper. Fuller-Rowell et al. (1997) recently reported that release of chemical energy, through dissociative recombination of O+ 2 , contributes to heat the neutral atmosphere at the EIA crests. Their model produces downward winds at the equator, in qualitative agreement with the DE-2 measurements. During maximum solar activity, the temperature dierences between crests and trough reach maximum values from 1900 to about 2100 LST, and this behavior is similar to that of the EIA (Raghavarao et al., 1998). The large scale circulation cells generated by the temperature/pressure ridges in the above time zone are also modulated by small scale variations of vertical winds (V ), which are inversely related to variations in T with similar scales (Raghavarao et al., 1993). Near the magnetic dip equator, these small scale variations may be either in phase or in opposite phase to the large scale variations of V and thus can give rise to dynamical conditions that either cause or inhibit ESF.
6. Conclusions 1. We present evidence for downward vertical winds
615
around the dip equator that are collocated with ESF associated irregularities in both the ion density and the electric ®eld at 1900 LST, the ESF start-time, and at 2100 LST, the time of maximum ESF. The observations provide support to the suggestion, made by Sekar and Raghavarao (1987) based on linear theory and Raghavarao et al. (1992) based on non-linear numerical modeling, that vertical winds may play an important role in generating the ESF/bubble phenomenon. 2. Since the EIA and ETWA appear to be the source for the downward winds around the dip equator (Raghavarao et al., 1993), the above conclusion is consistent with the ®nding that the EIA intensi®cation is a prerequisite, on a day to day basis, for the post-sunset ESF occurrence. 3. The occurrence characteristics of the EIA and related vertical winds may contribute signi®cantly to the observed longitudinal, seasonal and solar cycle variations of the ESF/bubble phenomenon. The vertical wind measurements on the DE-2 satellite that was launched in 1981 have provided a unique opportunity to carry out the investigation reported here. Unfortunately, the attitude control and knowledge for the satellite were rather limited. As discussed in the main body of the paper, the vertical winds in the ®rst data sample (Fig. 1) thus need to be taken with much caution. We conclude with a plea for future satellite missions, which can provide the improved attitude stability, control and knowledge needed to carry out the unique kind of measurements reported here.
Acknowledgements This work was performed at the NASA Goddard Space Flight Center, while R.R. held a National Research Council (NRC) Award of a Senior Research Associateship. R.R. thanks the NRC for the Award. We thank the National Space Science Data Center (NSSDC) at NASA/GSFC for supplying the electric ®eld data. We are grateful to Dr N.C. Maynard, Mission Research Corporation, Nashua, NH, for con®rming the identi®cation of calibration signals and the signals of A.C. electric ®elds. We are also grateful to Dr R.A. Homan, NASA/GSFC for providing an analysis of the onboard (DE-2) magnetometer data for a few examples and for helpful discussions. We thank one Referee for raising critical questions regarding the attitude of the DE-2 satellite, which helped to improve the quality of the paper.
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References Aarons, J., 1993. The longitudinal morphology of equatorial spread-F-layer irregularities relevant to their occurrence. Space Sci. Rev. 63, 209±243. Abdu, M.A., Batista, I.S., Sobral, J.H.A., 1992. A new aspect of magnetic declination control of equatorial spread-F and F-region dynamo. J. Geophys. Res. 97, 14,897±14,904. Aggson, T.L., Laakso, H., Maynard, N.C., Pfa, R.F., 1995. In-situ observations of bifurcation of equatorial ionospheric plasma depletions. J. Geophys. Res. 101, 5125± 5132. Alex, S., Koparker, P.V., Rastogi, R.G., 1989. Spread-F and ionization anomaly belt. J. Atmos. Terr. Phys. 51, 371± 379. Anandarao, B.G., Raghavarao, R., Desai, J.N., Haerendel, G., 1978. Vertical winds and turbulence at 93 km over Thumba. J. Atmos. Terr. Phys. 40, 157±163. Biondi, M.A., Sipler, D.P., 1985. Horizontal and vertical winds and temperatures in the equatorial thermosphere: measurements from Natal, Brazil during August± September 1982. Planet. Space Sci. 33, 817±823. Chiu, Y.T., Straus, J.M., 1979. Rayleigh±Taylor and winddriven instabilities of the nighttime equatorial ionosphere. J. Geophys. Res. 84, 3283±3290. Farthing, W.H., Sugiura, M., Ledley, B.G., 1981. Magnetic ®eld observations on DE-A and -B. Space Sci. Instrum. 5, 551±560. Fuller-Rowell, T.J., Codrescu, M.V., Fejer, B.G., Borer, W., Marcos, F., Anderson, D.N., 1997. Dynamics of the lowlatitude thermosphere: quiet and disturbed conditions. J. Atmos. and Sol-Terr. Phys. 59, 1533±1540. Haerendel, G. 1974 Theory of equatorial spread-F, (unpublished report). Max Plank Inst. fuÈr Phys. und Astrophys., Garching, Germany. Hanson, W.B., Heelis, R.A., Power, R.A., Lippincott, C.R., Zuccaro, D.R., Holt, B.J., Harmon, L.H., Santani, S., 1981. The retarding potential analyzer for Dynamics Explorer-B. Space Sci. Instrum. 5, 503±511. Hanson, W.B., Crain, B.L., Dennis, A., 1986. The eect of vertical drift on the equatorial F-region stability. J. Atmos. Terr. Phys. 48, 205±212. Hoegy, W.R., Curtis, S.A., Brace, L.H., Maynard, N.C., Heelis, R.A., 1982. Dynamics Explorer observations of equatorial spread-F: evidence for drift waves. Geophys. Res. Lett. 9, 993±996. Homan, R.A., Hogan, G.D., Maehl, R.C., 1981. Dynamics Explorer space craft and ground operations system. Space Sci. Instrum. 5, 349±369. Hudson, M.K., Kennel, C.F., 1975. Linear theory of equatorial spread. F. J. Geophysics Res. 80, 4581±4590. Jayachandran, T., Sri, Ram P., Somayajulu, V.V., Ramarao, P.V.S., 1997. Eect of equatorial ionization anomaly on the occurrence of spread-F. Ann. Geophys. 15, 255±262. Kelley, M.C., Larsen, M., Lahoz, C., 1981. Gravity wave interaction of equatorial spread-F: a case study. J. Geophys. Res. 86, 9087±9100. Kelley, M.C., Labelle, J., Kudeki, E., Fejer, B.G., Basu, Sa, Basu, Su, Baker, K.D., Hanuise, C., Argo, P., Woodman, R.F., Swartz, W.E., Farley, D.T., Meriwether Jr, J.W., 1986. The Condor equatorial spread-F campaign: overview
and results of the large scale measurements. J. Geophys. Res. 91, 5487±5503. Kelley, M.C., Livingston, R.C., Rino, C.L., Tsunoda, R.T., 1982. The critical wave number spectrum of topside equatorial spread-F: estimates of backscatter levels and implications for a uni®ed theory. J. Geophys. Res. 87, 5217± 5221. Kil, H., Heelis, R.A., 1998. Global distribution of density irregularities in the equatorial ionosphere. J. Geophys. Res. 103, 407±417. Krebiehl, J.P., Brace, L.H., Theis, R.F., Pinkus, W.H., Kaplan, R.B., 1981. The Dynamics Explorer Langmuir probe instrument. Space Sci. Intrum. 5, 493±503. Laakso, H., Aggson, T.L., Herrero, F.A., Pfa, R.F., Hanson, W.B., 1995. Vertical neutral wind in the equatorial F-region deduced from electric ®eld and ion density measurements. J. Atmos. and Terr. Physics 57, 645±651. Langel, R.A., Purucker, M., Rajaram, M., 1993. The equatorial electrojet and the associated currents as seen in Magsat data. J. Atmosph. Terr. Phys. 55, 1233±1269. Lloyd, K.H., Haerendel, G., 1973. Numerical modelling of the drift and deformation of ionospheric plasma clouds and their interaction with other layers of the ionosphere. J. Geophys. Res. 78, 7389±7415. Maynard, N.C., Bielecki, E.A., Burdick, H.F., 1981. Instrumentation for vector electric ®eld measurements from DE-B. Space Sci. Instrum. 5, 523±535. Maeda, H., Iyemori, T., Araki, T., Kamei, T., 1982. New evidence of a meridional current system in the equatorial ionosphere. Geophys. Res. Lett. 9, 337±340. Mendillo, M., Baumgardner, B., Pi, Xi-Q, Sultan, P., Tsunoda, R., 1992. Onset conditions for equatorial spreadF. J. Geophys. Res. 97, 13,865±13,876. Olsen, N., 1997. Ionospheric F-region currents at middle and low latitudes estimated from Magsat data. J. Geophys. Res. 102, 4563±4576. Ossakow, S.L., 1981. Spread-F theoriesÐa review. J. Atmos. Terr. Phys. 43, 437±452. Patra, A.K., Rao, P.B., Anandan, V.K., Jain, A.R., 1997. Radar observations of 2.8 m equatorial spread-F irregularities. J. Atmos. Solar-Terr. Phys. 59, 1633±1642. Raghavarao, R., Desai, J.N., Anandarao, B.G., Narayanan, R., Sekar, R., Gupta, R., Babu, V.V., Sudhakar, V., 1984. Evidence for the large scale electric ®eld gradient at the onset of equatorial spread-F. J. Atmos. Terr. Phys. 46, 355±362. Raghavarao, R., Gupta, S.P., Sekar, R., Narayanan, R., Desai, J.N., Sridharan, R., Babu, V.V., Sudhakar, V., 1987. In-situ measurements of winds, electric ®elds and electron densities at the onset of equatorial spread-F. J. Atmos. Terr. Phys. 49, 485±492. Raghavarao, R., Hoegy, W.R., Spencer, N.W., Wharton, L.E., 1993. Neutral temperature anomaly in the equatorial thermosphere-A source of vertical winds. Geophys. Res. Lett. 20, 1023±1026. Raghavarao, R., Nageswararao, M., Sastri, J.H., Vyas, G.D., Rao, S.M., 1988. Role of equatorial ionization anomaly in the initiation of equatorial spread-F. J. Geophys. Res. 93, 5959±5964. Raghavarao, R., Sekar, R., Suhasini, R., 1992. Non-linear nu-
R. Raghavarao et al. / Journal of Atmospheric and Solar-Terrestrial Physics 61 (1999) 607±617 merical simulation of equatorial spread-FÐeects of winds and electric ®elds. Adv. Space Res. 12 (6), 227±230. Raghavarao, R., Suhasini, R., Hoegy, W.R., Mayr, H.G., Wharton, L.E., 1998. Local time variation of equatorial temperature and zonal wind anomaly (ETWA). J. Atmos. and Solar-Terr. Phys. 60, 631±642. Raghavarao, R., Wharton, L.E., Spencer, N.W., Mayr, H.G., Brace, L.H., 1991. An equatorial temperature and wind anomaly (ETWA). Geophys. Res. Lett. 18, 1193±1196. Ramarao, P.V.S., Jayachandran, P.T., Sri, Ram P., 1997. Ionospheric irregularities: the role of equatorial ionization anomaly. Radio Sci. 32, 1551±1557. Rishbeth, H., 1971. The F-layer dynamo. Planet. Space Sci. 19, 263±267. Sahai, Y., Fagundes, P.R., Bittencourt, J.A., Abdu, M.A., 1998. Occurrence of large scale equatorial F-region plasma depletions during geomagnetic disturbances. J. Atmos. and Solar Terr. Phys. 60, 1593±1604. Sastri, J.H., Murthy, B.S., 1978. On the onset of equatorial spread-F. Ann. Geophys. 34, 47±49. Sastri, J.H., Subrahmanyam, V., Sasidharan, K., Rao, M.S., 1978. On the height control of the onset of equatorial spread-F. Curr. Sci. 47, 451±452. Scannapieco, A.J., Ossakow, S.L., 1976. Non-linear equatorial spread-F. Geophys. Res. Lett. 3, 451±454. Sekar, R., 1990. Plasma instabilities and the dynamics of the equatorial F-region. Ph. D. Thesis., Gujarat University, Ahmedabad-9, India. Sekar, R., Raghavarao, R., 1987. Role of vertical winds on the Rayleigh±Taylor mode instabilities of the night time equatorial ionosphere. J. Atmos. Terr. Phys. 49, 981±985. Sekar, R., Raghavarao, R., 1997. A case study on the evolution of equatorial spread-F by a non-linear numerical model using the results from a set of coordinated measurements. J. Atmos. Terr. Phys. 59, 343±350. Sekar, R., Suhasini, R., Raghavarao, R., 1994. Eects of ver-
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tical winds and electric ®elds in the non-linear evolution of equatorial spread-F. J. Geophys. Res. 99, 2205±2213. Singh, S., Johnson, F.S., Power, R.A., 1997. Gravity wave seeding of equatorial plasma bubbles. J. Geophys. Res. 102, 7399±7410. Spencer, N.W., Wharton, L.E., Niemann, H.B., Hedin, A.E., Carignan, G.R., Maurer, J.C., 1981. The Dynamics Explorer wind and temperature spectrometer. Space Sci. Instrum. 5, 417±429. Sridharan, R., Chandra, H., Das, S.R., Sekar, R., Sinha, H.S.S., Raju, D.P., Narayanan, R., Raizada, S., Mishra, R.N., Raghavarao, R., Vyas, G.D., Rao, P.D., Ramarao, P.V.S., Somayajulu, V.V., Babu, V.V., Danilov, D.A., 1997. Ionization hole campaignÐa coordinated rocket and ground-based study at the onset of equatorial spread-F: ®rst results. J. Atmos. and Solar-Terr. Phys. 59, 2051±2067. Sridharan, R., Raju, D.P., Raghavarao, R., Ramarao, P.V.S., 1994. Precursor to equatorial spread-F in OI 630.0 nm dayglow. Geophys. Res. Lett. 21, 2797±2800. Szuszczewicz, E.P., Tsunoda, R.T., Narcisi, R., Holmes, J.C., 1980. Coincident radar and rocket observations of equatorial spread-F. Geophys. Res. Lett. 7, 537±540. Takeda, M., Maeda, H., 1983. F-region dynamo in the evening-interpretation equatorial delta D anomaly found by MAGSAT. J. Atmos. Terr. Phys. 45, 401±408. Tsunoda, R.T., 1980. Magnetic ®eld-aligned characteristics of plasma bubbles in the nighttime equatorial ionosphere. J. Atmos. Terr. Phys. 42, 743±752. Weber, E.J., Basu, S., Bullett, T.W., Valladares, C., Bishop, G., Groves, K., Kuenzler, H., Ning, P., Sultan, P.J., Sheehan, R.E., Araya, J., 1996. Equatorial plasma depletion precursor signature and onset observed at 11 south of the magnetic equator. J. Geophys. Res. 101, 26,829± 26,838. Woodman, R.F., LaHoz, C., 1976. Radar observations of Fregion equatorial irregularities. J. Geophys. Res. 81, 5447± 5466.
Equatorial spread-F (ESF) and vertical winds R. Raghavarao*, R. Suhasini, H.G. Mayr, W.R. Hoegy, L.E. Wharton Goddard Space Flight Center, Mail code 910.4, Greenbelt, MD 20771, USA Received 1 October 1998; accepted 20 January 1999
Abstract The Equatorial Spread-F (ESF) phenomenon is recorded in ionograms as a hierarchy of plasma instabilities in the F-layer of the equatorial ionosphere. The ESF is characterized by irregularities in the plasma (electron and ion) density and electric ®eld distributions perpendicular to the Earth's magnetic ®eld. Large scale irregularities are generated by a primary plasma instability that develops in electric ®elds and plasma densities. Other secondary instabilities then develop and generate irregularities at several scale sizes that often produce a plasma `hole' or `bubble' that rises up with high E B velocities. The ESF/plasma bubble phenomenon has been studied extensively with experimental techniques and modeling, which revealed important features. In the bottom side F-layer, near sunset, when the vertical density gradient steepens as the layer is supported by the horizontal (North±South) Earth's magnetic ®eld lines against the omnipresent Earth's gravitational acceleration ( g ), the plasma conditions can give rise to Rayleigh±Taylor (RT) type instability. But the observed day to day variability of the ESF occurrence suggested that other agencies may also be involved in generating the instability. Sekar and Raghavarao (1987) with linear theory, and Raghavarao, Sekar and Suhasini (1992), with non-linear numerical modeling, suggested that vertical downward (upward) winds in the ambient gas have the potential to cause (inhibit) the ESF/bubble phenomenon. The presence of downward winds near the equator was reported earlier. In this paper, we show evidence for the presence of downward winds collocated with irregularities in electric ®elds and plasma densities as revealed by an unique combination of highly accurate measurements with instruments onboard the DE-2 satellite. The observations reported here are also consistent with the notion that the build-up of the equatorial ionization anomaly (EIA) prior to local sunset is important for the ESF instability. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction With ionosondes, short pulses of high frequency (HF) radio waves are radiated upward, and these are re¯ected from the ionosphere to be recorded as traces in ionograms depicting the virtual heights (h ') of re¯ection at varying frequencies ( f, in the 1±20 MHz
* Corresponding author Tel.: +1-301-286-8575; fax: +1301-286-1663. E-mail address: [email protected] (R. Raghavarao)
range). On certain nights, the spread in the width of the pulses re¯ected from the F-layer increases abnormally, showing up as a diuse trace, and this phenomenon is called Spread-F. Its occurrence characteristics at low geomagnetic latitudes are known to be distinctly dierent from those at higher latitudes and hence it is referred to as Equatorial Spread-F (ESF). The ESF has been investigated extensively with ground-based ionosondes, with scintillation measurements of radio waves from radio stars and beacon satellites (Abdu et al., 1992; Aarons, 1993, and references therein) and with VHF radars (Woodman and LaHoz, 1976; Tsunoda, 1980).
1364-6826/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 4 - 6 8 2 6 ( 9 9 ) 0 0 0 1 7 - 6
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The ESF phenomenon has been identi®ed with the plasma bubbles that are observed with rocket experiments (Szuszczewicz et al., 1980; Raghavarao et al., 1984, 1987; Kelley et al., 1986; Sridharan et al., 1997) and with satellite-borne instruments (Hoegy et al., 1982; Aggson et al., 1995; Singh et al., 1997; Kil and Heelis, 1998; and references therein). On days of ESF occurrence, the F-layer near the magnetic equator becomes unstable around local sunset time and as a consequence irregularities develop in the electric ®eld and plasma densities with a wide range of scale sizes. These irregularities are aligned with the Earth's magnetic ®eld lines forming sometimes severe density depletions called `holes' or `bubbles' (Tsunoda, 1980; Ossakow, 1981). The dynamical behavior of plasma bubbles often reveals rising plumes with dierent shapes as observed by VHF radars (Woodman and Lahoz, 1976; Patra et al., 1997). The zonal motions of bubbles and their role in causing radio scintillations have also been studied with optical imaging techniques (Mendillo et al., 1992; Weber et al., 1996; Sahai et al., 1998). The onset conditions for ESF and plasma bubbles have been investigated by measuring vertical pro®les of winds and electric ®elds with rocket released chemicals (Barium±Strontium (Ba±Sr), Sodium (Na)) and electron densities with Langmuir probes (LP) (Raghavarao et al., 1984; 1987) and by combining many of the above mentioned techniques (Sridharan et al., 1997). Among the dierent scale-sizes, the large-scale plasma irregularities are observed to develop ®rst in the bottom side F-layer and are believed to be caused by a collisional Rayleigh±Taylor (RT) type instability (Haerendel, 1974; Hudson and Kennel, 1975; Scannapieco and Ossakow, 1976). Such an instability is expected to occur when the layer is at very high altitudes where the ion-neutral collision frequencies (nin) are low as the RT instability growth rate is given as (1/L )( g/nin). Here g is the acceleration due to gravity and L is the vertical scale length of the ion density gradient. A necessary condition for the RT instability is that plasma densities form with steep positive height gradients that are supported by the horizontal Earth's magnetic ®eld against `g'. Such large density gradients can develop soon after local sunset when the ionization in the bottom-side F-layer decreases due to recombination and electric ®eld induced plasma redistribution. Several aspects of the ESF/plasma bubble phenomenon are understood. Some basic features of the phenomenon, however, are not understood such as the variations with longitude, season, and from day to day. The altitude of ESF onset where the instability is initiated can vary over an altitude range of more than 100 km either at solar maximum (Sastri and Murthy, 1978) or at solar minimum (Sastri et al., 1978). In the collisional Rayleigh±Taylor (RT) instability, changes
in the collision frequency due to variations in temperature for example can change the onset altitude; but temperature variations as large as 10% would produce only changes of about 30 km. Another factor is the scale-size of the electron density in the bottom-side Flayer, which is dicult to assess since it is aected by the transient decay of the ionization after sunset. Observationally, however, there is no evidence to suggest that the post sunset ESF is signi®cantly aected by the height dependence of the F-layer (Sastri and Murthy, 1978; Sastri et al., 1978). Under seemingly identical conditions for the plasma density and its height structure, the ESF occurs on one day but does not occur on the other day (Sekar, 1990; Ramarao et al., 1997). The post-sunset ESF occurrence (non-occurrence) has been shown to be closely linked on a day to day basis with the prior intensi®cation (non-intensi®cation) of the ionization crests in the well-known Equatorial Ionization Anomaly (EIA) that develops in the late afternoon near 208 magnetic latitude (Raghavarao et al., 1988; Alex et al., 1989; Sridharan et al., 1994; Jayachandran et al., 1997). The formation of the EIA and its intensi®cation is produced by plasma ¯owing in a fountain from the equator towards the crests, which is caused by dynamo electric ®elds. This fountain eect increases the ionization crests at the expense of the plasma density in the bottom-side F-region near the equator and thus steepens there the vertical density gradient, which is conducive to RT instability and explains in part the relationship between EIA and ESF. However, there is also another link between the EIA and the ESF that is potentially important, and this is the subject of the present paper. 2. Vertical winds and ESF/plasma bubbles With Ba±Sr releases at heights below F-layer, Anandarao et al. (1978) and Raghavarao et al. (1987) measured vertical winds at sunset time. With a ground-based Fabry±Perot spectrometer that measured the OI emission line, Biondi and Sipler (1985) had observed the temporal variations of vertical winds during one night at around 200 km. These measurements provided the impetus for theoretical investigations of the eects of winds on the RT instability. By analytically solving the plasma ¯uid equations of continuity, momentum (including zonal and vertical winds), and the current conservation for ions and electrons, Sekar and Raghavarao (1987) showed that the growth rate g ', of the generalized RT instability (GRT, since transport velocities are included) is enhanced (reduced) by downward (upward) vertical winds. Earlier, Hanson et al. (1986) showed that the growth rate can be enhanced (reduced) also by upward (down-
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ward) ion drift velocities. Based on the numerical model results, Raghavarao et al. (1992) concluded that the eects of either vertical downward winds or upward plasma drifts of equal magnitude are the same in accelerating ESF instability. Hence, the equation for the growth rate (Sekar and Raghavarao, 1987) can be written as, g 1=L g=nin Znin =o i Vi ÿ V , with 1=L 1=Ni dNi = dz
1
where Ni is the electron/ion density; oi, the ion gyrofrequency; Z, the zonal eastward winds; and V, Vi, the vertical upward velocities of the neutrals and ions, respectively, at the dip equator; nin and g as de®ned above. (In the right-handed coordinate system, x, y and z represent the magnetic east, north and upward directions, respectively.) The ®rst term in Eq. (1) represents the collisional RT instability (Haerendel, 1973), the second term represents the contribution from the zonal winds, Z (Chiu and Straus, 1979); and the last term represents the contribution from the vertical wind, V (Sekar and Raghavarao, 1987) and vertical ion drift velocities, Vi (Hanson et al., 1986). Chiu and Straus (1979) showed that the zonal winds do not contribute signi®cantly to the instability growth rate in the F-layer, since the nin/ oi is about 1/200 at 260 km and decreases exponentially with height. Sekar and Raghavarao (1987) compared the eective contributions of dierent terms in Eq. (1) to the growth rate g ' and showed that a 1 m/s downward wind (or upward ion drift velocity) at 260 km is as eective as a 200 m/s eastward wind and that a downward wind of 16 m/s at 300 km would be as eective as gravity. Furthermore, Raghavarao et al. (1992) showed from non-linear numerical simulations that downward winds of 20 m/s at the F-region altitudes (or 20 m/s upward plasma drifts) accelerate the evolutionary process of the instability signi®cantly faster than gravity ( g ) alone and that the acceleration continues beyond the height of maximum density into the topside F-region. They attributed the day to day variability in the ESF and bubble occurrence to the variability in the vertical winds. The eects of vertical winds on the bubble growth are clearly shown in the spatial distribution of the plasma velocities (Sekar et al., 1994). Hypothesizing the presence of vertical winds (of ÿ20 m/s), Raghavarao et al. (1987), Sekar and Raghavarao (1997), and Sekar et al. (1997) were able to interpret the ESF (electron) density irregularities obtained from three dierent rocket campaigns. In addition to vertical winds, they also included the contributions to the growth rate from eastward winds and westward-tilted gradients of the F-layer (Kelley et al.,
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1981) that is always present at sunset. Similarly, Laakso et al. (1995) invoked the presence of downward vertical winds of 15±30 m/s to explain the plasma bubbles and associated zonal electric ®elds measured by the San Marco-D satellite. Based on DE-2 satellite measurements, relatively large variations are observed in the neutral temperature and wind velocities that are closely linked to the EIA (Raghavarao et al., 1991, 1998). Near the EIA crests, the temperature is enhanced relative to the magnetic equator, while the zonal winds near the equator are signi®cantly larger than in the crest regionÐa phenomenon that is referred to as the Equatorial Temperature and Wind Anomaly (ETWA). Raghavarao et al. (1993) also reported vertical winds that tend to be upward in the crest region and downward around the magnetic equator at local times (1900±2200 LST) when the ESF is observed. The vertical winds revealed a latitudinal structure similar to that of the ETWA temperature, and Fuller-Rowell et al. (1997) suggested that both are produced by the release of chemical energy from the dissociation of the enhanced O+ 2 in the crest region. These observations are consistent with rocket measurements below the Flayer (from 100 to 300 km) that reveal at sunset near the dip equator vertical winds with signi®cant magnitudes, in addition to zonal and meridional winds and electric ®elds with large spatial gradients (Anandarao et al., 1978; Raghavarao et al., 1984, 1987; Sridharan et al., 1997). In the present paper, we show two sets of satellite data obtained (i) at 1890 LST, the expected time of ESF onset, and (ii) at 2100 LST, the expected time of fully developed ESF. These examples show evidence of downward vertical winds collocated with irregular electric ®elds and ion densities, the ESF/bubble signatures. The data thus con®rm the earlier hypothesis (Raghavarao et al., 1992) about the importance of vertical winds for the ESF. 3. DE-2 measurements The DE-2 satellite was in an elliptical orbit with 908 inclination, with perigee around 300 km, and it operated from August 1981 to February 1983, a period of maximum solar activity. The slow orbital precession of the DE-2 orbit made it possible to cover all local solar times (LSTs) in a period of about one year. Among the times of interest for ESF, 1845±2000 LST occurred during the solstice season, and 2000±0200 LST occurred during equinox. The DE-2 satellite measured a combination of several parameters (Homan et al., 1981) and provides the best data set to date for investigating a number of problems, such as the origin of ESF, which are connected with the structure, dynamics
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and electrodynamics of the Ionosphere±Thermosphere System (ITS). The pertinent parameters, measured simultaneously and some of them with unparalleled accuracy, are brie¯y described below. 3.1. Winds and temperatures The Wind and Temperature Spectrometer (WATS), described in detail by Spencer et al. (1981), measured the neutral temperature (T ) with a sampling rate of 1/ s, and the zonal (Z ) and vertical (V ) wind components each with a sampling rate of 0.5/s. Spencer et al. (1981) and Raghavarao et al. (1993) discussed the accuracies of the temperature and wind measurements. For a solar ¯ux of F10.7=175 and noon time gas densities at 300 and 400 km altitudes, respectively, the estimated errors in T are 1.5 K and 3.4 K, and in the winds 2 m/s and 4 m/s. The errors are reduced to 1/2 of these values for the available 16 s averages presented here. Since the errors increase with altitude, we used only data obtained below 400 km. There are, however, three major uncertainties in the measured values of vertical winds: 1. The zero-level of the winds is uncertain by about 15 m/s, since the satellite attitude could only be determined with 0.18 accuracy (Raghavarao et al., 1993). 2. The vertical winds measured away from perigee are in¯uenced by the ellipticity of the orbit. Since the wind component measured perpendicular to the satellite velocity vector (Vs) in the plane of the orbit is taken as the `vertical velocity', its direction deviates from the radial (height) at satellite positions away from perigee. Around perigee the eect is negligible, but away from it the measurements are aected, in particular during the solstice period when cross equatorial winds are present. During solstice, at any point away from perigee, the measured velocity V in the orbital plane can be presented as V V 0 cos q2M sin q, where V' and M are the true components of vertical and meridional winds, respectively, and q is the angle of deviation of the normal to the satellite velocity vector from the radial direction (from the Earth's center). The signs +/ÿ apply when DE-2 is in the summer and winter hemispheres respectively, when the satellite is moving from, say, the northern to southern hemisphere with its perigee near the equator. 3. Variations in the satellite attitude can produce spurious vertical winds, which are generally small compared to the horizontal winds. On occasions, such variations may be inferred from large-scale (0208± 308 in latitude) oscillations of the three components of the earth's magnetic ®eld that have been recorded by one or two of the triaxial ¯uxgate magnetometers
(Farthing et al., 1981) on board DE-2 satellite. Such magnetic ®eld oscillations in fact have been found to be correlated with the variations of vertical winds in one of the data sets discussed here. However, it cannot be ruled out that the said magnetic ®eld oscillations are generated by ionospheric currents and not by attitude variations. Such currents have been inferred from satellite (MAGSAT) borne magnetometer data (Langel et al., 1993; Olsen, 1997), in particular during evening hours when the conductivities of the F-region, dominate over those of the E-region and hence the dynamo action in the Fregion prevails (Rishbeth, 1971). The MAGSET satellite, having attitude determination and control of 6 nT accuracy, could measure magnetic ®elds in the 300±500 height region (Maeda et al., 1982) with suf®cient accuracy to infer the meridional currents driven by the F-region dynamo at dusk time around the dip equator (Takeda and Maeda, 1983). The above mentioned magnetic ®eld oscillations observed by the triaxial magnetometer on DE-2 thus may in part be generated by ionospheric currents.
3.2. AC electric ®elds The Vector Electric Field Instrument (VEFI), described in detail by Maynard et al. (1981), measured both the DC and the irregular or variable (called AC) electrostatic ®elds in the ionosphere. A double ¯oating probe technique was employed for two mutually perpendicular directions (X, Y), both in the orbital plane containing the satellite velocity vector (Vs) of the nonspinning DE-2. The double probe in the east±west direction, perpendicular to the X±Y plane, failed to get deployed. The double probes in the X and Y directions are oriented at 2458 to the spacecraft velocity, respectively. The AC ®elds, measured with the double probe technique in either one of the two directions around perigee, represent mainly the variable component of the electrostatic ®eld in the vertical direction. A small part of the variable ®eld lies in the magnetic east±west direction at longitudes where the declination angle of the Earth's magnetic ®eld is signi®cant (the DE-2 orbit being 908 inclined to the equator). The AC ®elds in each direction were processed onboard the satellite by a spectrometer with a bank of comb ®lters that provided the spectral amplitudes in 8 low frequency (LF) bands (4±1024 Hz) and 4 high frequency (HF) bands (1.02±512.0 kHz) (Maynard et al., 1981). We have chosen to present here the Y ®eld data (rms values) in the ®rst four of the eight LF bands with 1 s sampling rate along the path of the DE-2 orbit. The AC signal amplitudes in these channels represent irregularities
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with scale sizes of approximately 2, 1, 0.5, and 0.25 km (the satellite velocity being about 8 km/s), which are characteristic of ESF/plasma bubbles. 3.3. Ion densities and irregularities To describe the plasma density irregularities, the ratio of delta(Ni)/Ni was measured with a very sensitive Retarding Potential Analyzer (RPA) (Hanson et al., 1981). The values of delta(Ni)/Ni were obtained for six dierent frequency (Hz) bands, and we present the amplitude variations in the lowest frequency band (32± 86 Hz) read in 4 s intervals. The measured amplitude variations indicate the presence of ESF/bubble structure and reveal scales similar to the irregular (AC) electric ®elds observed simultaneously. The plasma density (Ni) was also measured with a Langmuir probe (LANG, Krebiehl et al., 1981), and the available 16 seconds averages are presented. In these data, however, the density irregularities related to ESF are averaged out. 4. Observations In the following, we shall describe two sets of observations from DE-2 that show the presence of ESF/ plasma bubbles along with vertical winds that are conducive to generate the phenomenon through the GRT instability. With Fig. 1 we present DE-2 measurements in orbit 1626 of 21 November 1981 (81325) near 137E longitude at 18.9 LST, which is close to the onset-time of ESF occurrence. Fig. 1(a) shows the variations in the ion density (Ni) characteristic of the EIA and the signatures of ETWA, i.e., the latitudinal variations of the zonal winds (Z ) and temperatures (T ). The values of T can be read by adding 1000 to the scale. As seen from Fig. 1(a), T forms a crest at about 208 N in the winter hemisphere collocated with the peak in Ni, which is the signature of ETWA. In the south, the crest is masked by the steep temperature increase in the summer hemisphere (Raghavarao et al., 1991). The observed variations in the vertical wind (V ), shown in Fig. 1(b), reveal upward velocities and peaks located on either side of the dip equator. The downward winds are centered around the dip equator and are collocated with the T minimum. Superimposed on the large scale variations in the vertical winds, there are also small scale variations observed with smaller amplitudes that are negatively correlated with similar structures in T. Notwithstanding the earlier mentioned uncertainties about the zero level in the vertical winds (V ), the velocities are downward around the dip equator. However, the vertical winds shown here need to be taken with caution. The vertical component of
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the magnetic ®eld measured onboard the spacecraft reveals deviations from the magnetic ®eld model to produce an oscillatory structure similar to that of the vertical winds (R. A. Homan, private communication). This indicates that the satellite attitude may change signi®cantly and may produce in this particular case spurious vertical wind velocities. We shall return to this problem later in the discussion and conclusion sections. The amplitudes of the ion density irregularities represented by delta(Ni)/Ni on a logarithmic scale are shown in Fig. 1(c). One large peak is observed north of the dip equator and three well resolved peaks south of it. These density peaks are observed above a broad background of lesser magnitude, and they are collocated with LF±AC electric ®eld peaks shown in Fig. 1(d±g). The amplitude peaks in both types of irregularities, delta(Ni)/Ni and AC ®eld, are observed to occur at the same locations and on both sides of the dip equator in a region where the vertical winds are downward [Fig. 1(b)]. (The location of the dip equator is shown with an arrow.) From the magnetic ®eld lines shown in Fig. 1(e), with height (Ht ) scale on the right side of the Figure, it is evident that the irregularity peak north of the dip equator and the one closest to it in the south are on the same ®eld line tube. The apex height of this ®eld line tube over the equator is centered at 340 km and has a width of 20 km. The other two AC ®eld and delta (Ni) peaks located further south do not have their conjugate companions in the north. At these northern conjugate locations, the observed vertical winds [Fig. 1(b)] are upward (or reduced in magnitude if downward), which could explain why irregularities do not develop there. The amplitudes of AC ®elds presented in the four LF bands [Fig. 1(d±g)] are con®ned to speci®c ®eldline tubes that do not engulf the entire bottomside Flayer around the dip equator. The AC ®elds are also observed to decrease with frequency. As discussed by Kelley et al. (1982), these features are the signatures of plasma bubbles in the formative stage associated with the GRT instability (Eq. (1)). In this context it is important to point out that the vertical ion/electron density gradients at the EIA trough may be large as required by the GRT instability, although the densities themselves are small as shown in Fig. 1(b). Large vertical gradients have been observed in ion/electron density pro®les of the bottom side F-layer measured at the post-sunset ESF onset times with rocket-borne probes (Raghavarao et al., 1987; Sridharan et al., 1997). Our observations suggest that in the ®eld tube shown the V/L (in Eq. (1)) is large enough for the GRT instability to develop to form plasma bubbles that move in the directions ((E B)/B2) perpendicular to the magnetic ®eld. The data presented here, however, show only a snapshot picture of ESF/bubbles along the sat-
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Fig. 1. DE-2 measurements at 1854 LST in orbit 1626 on 21 November 1981 of: (a) zonal winds (Z ), neutral temperatures (T ), and ion (plasma) densities (P ); (b) vertical winds (V ); (c) irregularities in plasma densities, DNi/Ni; and (d±g) the AC electric ®elds in dierent (®rst four) low frequency (LF) bands as shown. Vertical arrow in (c) shows the location of the dip equator. Dashed lines in (e) depict magnetic ®eld lines passing through the two conjugate A.C. electric ®eld signals and the height (Ht ) scale is shown on the right.
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Fig. 2. DE-2 measurements at 2100 LST in orbit 3838 on 17 April 1982. The description is the same as in Fig. 1. Panels (a) and (b) are reproduced from Raghavarao et al. (1993).
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ellite path, which does not reveal when and at what height precisely the bubbles were ®rst formed prior to the observation. In addition to the above described features, Fig. 1(d) also shows three bursts of AC ®elds between ÿ24 and ÿ32 dip latitudes and weakened conjugate signals in the north that reveal alignment along ®eld-lines crossing the equator at altitudes from 2000 to 2800 km. These signals will not be discussed further in the paper. Another example of ESF/bubbles and vertical winds is presented with Fig. 2 for the orbit 3838 on 17 April 1982 (day 82,107) that describes conditions at 2100 LST, well beyond the post sunset time at which these features begin to develop. In Fig. 2(a), we show the observed variations in temperature, electron density, zonal winds, and in Fig. 2(b) the vertical winds that have been discussed earlier by Raghavarao et al. (1993). Characteristic of ETWA, symmetric and prominent T crests form on both sides of the dip equator. The vertical winds are upward collocated with the T crests and are downward around the dip equator. For the equinox condition discussed here, Raghavarao et al. (1993) estimated the error in the vertical winds due to the uncertainty in the satellite attitude determination and suggested that the zero level should be approximately at ÿ11 m/s. Hence the downward winds around the equator may be closer to ÿ11 m/s instead of ÿ22 m/s and the upward winds collocated with the T crests may be about 15 m/s instead of 4 m/s. It is important to note that during this orbit, unlike the previous one, the onboard magnetometer data for any of the three magnetic ®eld components did not show oscillatory structures of dimension similar to those of the vertical winds around the dip equator (R. A. Homan, private communication). Variations in the spacecraft attitude are not a problem in this case, and the vertical winds presented here are considered reliable. In Fig. 2(c) we present the observed irregularities in ion densities from the RPA and in Fig. 2(d±g) the AC electric ®eld signals in the ®rst four LF bands from the VEFI instrument. The AC ®eld amplitudes in the LF bands are prominent around three spatial locations (ÿ5.98, 5.18, 10.38 dip latitudes), all collocated with the peaks in the ion density irregularities. The ®rst one in the south and the other two in the north of the dip equator are in regions where the vertical winds are downward with relatively large velocities. The ®ve pulses preceding the ®rst AC signal (at 81,920 s) that are prominent in channel one (d) and weaker in channel two (e) are calibration marks; these were applied by command to the preampli®er while disconnected from the probe (Maynard et al., 1981). The ®rst AC signal has its conjugate counterpart in the north within the ®eld line tube shown in Fig. 2(e). As in the pre-
vious example, the other signal recorded further north does not have its conjugate counterpart in the south where the vertical downward winds are small or even upward considering the zero-level correction of ÿ11 m/s discussed above. As in Fig. 1, the AC ®eld amplitudes are decreasing with frequency, but unlike the previous example weak signals occur in the region across the dip equator. 5. Discussion As pointed out earlier for the (post sunset) case (Fig. 1), the magnetometer data (R. A. Homan, private communication) show oscillatory structures with latitudinal extent similar to those observed in the vertical winds. This suggests that the velocities are spurious, produced by variations in satellite attitude. No such contamination is found in the (pre midnight) case (Fig. 2), and the following discussion will therefore apply primarily to that. In Fig. 2, we show that the irregularities in both plasma densities (delta(Ni)/Ni) and AC electric ®elds (LF), which are the signatures of the ESF/bubble phenomenon, are collocated with downward vertical winds. The examples presented here were recorded near the dip equator in the bottom side F-layer (300± 320 km) during pre-midnight periods. Although the magnitudes of the vertical winds are uncertain they are downward where the ESF irregularities are generated. And the winds are shown to be upward or small where the ESF irregularities do not develop. At a height of 300 km, downward winds of about ÿ16 m/s contribute as much as g/nin to the growth rate of the RT instability (Sekar and Raghavarao, 1987), while upward winds contribute to suppress the instability. While the observations show the presence of ESF under conditions that are favorable for generating the phenomenon, it is not possible to determine the exact heights and times at which the underlying instability develops. The ion density measurements on DE-2 do not provide information on vertical gradients (1/L ) in Eq. (1), and from a polar orbit at a single LST the initiation time for the instability cannot be ascertained. The observations presented in this paper, however, provide evidence that the vertical winds (ÿV ) are important in addition to gravity ( g/nin), ion drift velocities, Vi, and zonal winds, Z sin d. In the height region of minimum L value, downward winds contribute to initiate the instability. The examples presented here also indicate that dierences in the vertical winds between the two hemispheres may be responsible for initiating the instability on one end of the ®eld line and suppressing it on the other. As shown in Fig. 2, the AC ®eld signals and delta (Ni)/Ni further away
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from the equator and recorded on one side of the ®eld-line tube are shown to be associated with downward winds; but no conjugate signals are recorded in the other hemisphere where the vertical winds are small in magnitude or upward directed. Thus there is clear evidence that the polarity of vertical winds and their magnitudes are important in generating the ESF/ bubble phenomenon. The timing of the vertical winds, directed downwards in the evening hours, may play an important role in generating the observed ESF/plasma bubbles and may contribute to the duration and day to day variability of this phenomenon. Non-linear modeling studies of the development of ESF/bubbles that account for vertical winds reveal that their polarity, strength and vertical structure play an important role in accelerating the bubble/ESF growth (Raghavarao et al., 1992). Raghavarao et al. (1993) proposed that the vertical winds are associated with ETWA, that the winds are upward near the EIA crests where temperature crests are observed and are downward around the equator. In the context of ESF it is relevant that during solar maximum the ETWA phenomenon is characterized by temperatures that increase after sunset and reach peak values (100±120 K) at around 2000 LST (Fig. 6 in Raghavarao et al., 1998). During these times, vertical winds are observed, on a number of occasions, with a latitudinal structure similar to that in the temperature and they are downward around the dip equator where the temperature has a minimum (Raghavarao et al., 1993). The observed downward winds are in the 10±40 m/s range as is shown in the present paper. Fuller-Rowell et al. (1997) recently reported that release of chemical energy, through dissociative recombination of O+ 2 , contributes to heat the neutral atmosphere at the EIA crests. Their model produces downward winds at the equator, in qualitative agreement with the DE-2 measurements. During maximum solar activity, the temperature dierences between crests and trough reach maximum values from 1900 to about 2100 LST, and this behavior is similar to that of the EIA (Raghavarao et al., 1998). The large scale circulation cells generated by the temperature/pressure ridges in the above time zone are also modulated by small scale variations of vertical winds (V ), which are inversely related to variations in T with similar scales (Raghavarao et al., 1993). Near the magnetic dip equator, these small scale variations may be either in phase or in opposite phase to the large scale variations of V and thus can give rise to dynamical conditions that either cause or inhibit ESF.
6. Conclusions 1. We present evidence for downward vertical winds
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around the dip equator that are collocated with ESF associated irregularities in both the ion density and the electric ®eld at 1900 LST, the ESF start-time, and at 2100 LST, the time of maximum ESF. The observations provide support to the suggestion, made by Sekar and Raghavarao (1987) based on linear theory and Raghavarao et al. (1992) based on non-linear numerical modeling, that vertical winds may play an important role in generating the ESF/bubble phenomenon. 2. Since the EIA and ETWA appear to be the source for the downward winds around the dip equator (Raghavarao et al., 1993), the above conclusion is consistent with the ®nding that the EIA intensi®cation is a prerequisite, on a day to day basis, for the post-sunset ESF occurrence. 3. The occurrence characteristics of the EIA and related vertical winds may contribute signi®cantly to the observed longitudinal, seasonal and solar cycle variations of the ESF/bubble phenomenon. The vertical wind measurements on the DE-2 satellite that was launched in 1981 have provided a unique opportunity to carry out the investigation reported here. Unfortunately, the attitude control and knowledge for the satellite were rather limited. As discussed in the main body of the paper, the vertical winds in the ®rst data sample (Fig. 1) thus need to be taken with much caution. We conclude with a plea for future satellite missions, which can provide the improved attitude stability, control and knowledge needed to carry out the unique kind of measurements reported here.
Acknowledgements This work was performed at the NASA Goddard Space Flight Center, while R.R. held a National Research Council (NRC) Award of a Senior Research Associateship. R.R. thanks the NRC for the Award. We thank the National Space Science Data Center (NSSDC) at NASA/GSFC for supplying the electric ®eld data. We are grateful to Dr N.C. Maynard, Mission Research Corporation, Nashua, NH, for con®rming the identi®cation of calibration signals and the signals of A.C. electric ®elds. We are also grateful to Dr R.A. Homan, NASA/GSFC for providing an analysis of the onboard (DE-2) magnetometer data for a few examples and for helpful discussions. We thank one Referee for raising critical questions regarding the attitude of the DE-2 satellite, which helped to improve the quality of the paper.
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References Aarons, J., 1993. The longitudinal morphology of equatorial spread-F-layer irregularities relevant to their occurrence. Space Sci. Rev. 63, 209±243. Abdu, M.A., Batista, I.S., Sobral, J.H.A., 1992. A new aspect of magnetic declination control of equatorial spread-F and F-region dynamo. J. Geophys. Res. 97, 14,897±14,904. Aggson, T.L., Laakso, H., Maynard, N.C., Pfa, R.F., 1995. In-situ observations of bifurcation of equatorial ionospheric plasma depletions. J. Geophys. Res. 101, 5125± 5132. Alex, S., Koparker, P.V., Rastogi, R.G., 1989. Spread-F and ionization anomaly belt. J. Atmos. Terr. Phys. 51, 371± 379. Anandarao, B.G., Raghavarao, R., Desai, J.N., Haerendel, G., 1978. Vertical winds and turbulence at 93 km over Thumba. J. Atmos. Terr. Phys. 40, 157±163. Biondi, M.A., Sipler, D.P., 1985. Horizontal and vertical winds and temperatures in the equatorial thermosphere: measurements from Natal, Brazil during August± September 1982. Planet. Space Sci. 33, 817±823. Chiu, Y.T., Straus, J.M., 1979. Rayleigh±Taylor and winddriven instabilities of the nighttime equatorial ionosphere. J. Geophys. Res. 84, 3283±3290. Farthing, W.H., Sugiura, M., Ledley, B.G., 1981. Magnetic ®eld observations on DE-A and -B. Space Sci. Instrum. 5, 551±560. Fuller-Rowell, T.J., Codrescu, M.V., Fejer, B.G., Borer, W., Marcos, F., Anderson, D.N., 1997. Dynamics of the lowlatitude thermosphere: quiet and disturbed conditions. J. Atmos. and Sol-Terr. Phys. 59, 1533±1540. Haerendel, G. 1974 Theory of equatorial spread-F, (unpublished report). Max Plank Inst. fuÈr Phys. und Astrophys., Garching, Germany. Hanson, W.B., Heelis, R.A., Power, R.A., Lippincott, C.R., Zuccaro, D.R., Holt, B.J., Harmon, L.H., Santani, S., 1981. The retarding potential analyzer for Dynamics Explorer-B. Space Sci. Instrum. 5, 503±511. Hanson, W.B., Crain, B.L., Dennis, A., 1986. The eect of vertical drift on the equatorial F-region stability. J. Atmos. Terr. Phys. 48, 205±212. Hoegy, W.R., Curtis, S.A., Brace, L.H., Maynard, N.C., Heelis, R.A., 1982. Dynamics Explorer observations of equatorial spread-F: evidence for drift waves. Geophys. Res. Lett. 9, 993±996. Homan, R.A., Hogan, G.D., Maehl, R.C., 1981. Dynamics Explorer space craft and ground operations system. Space Sci. Instrum. 5, 349±369. Hudson, M.K., Kennel, C.F., 1975. Linear theory of equatorial spread. F. J. Geophysics Res. 80, 4581±4590. Jayachandran, T., Sri, Ram P., Somayajulu, V.V., Ramarao, P.V.S., 1997. Eect of equatorial ionization anomaly on the occurrence of spread-F. Ann. Geophys. 15, 255±262. Kelley, M.C., Larsen, M., Lahoz, C., 1981. Gravity wave interaction of equatorial spread-F: a case study. J. Geophys. Res. 86, 9087±9100. Kelley, M.C., Labelle, J., Kudeki, E., Fejer, B.G., Basu, Sa, Basu, Su, Baker, K.D., Hanuise, C., Argo, P., Woodman, R.F., Swartz, W.E., Farley, D.T., Meriwether Jr, J.W., 1986. The Condor equatorial spread-F campaign: overview
and results of the large scale measurements. J. Geophys. Res. 91, 5487±5503. Kelley, M.C., Livingston, R.C., Rino, C.L., Tsunoda, R.T., 1982. The critical wave number spectrum of topside equatorial spread-F: estimates of backscatter levels and implications for a uni®ed theory. J. Geophys. Res. 87, 5217± 5221. Kil, H., Heelis, R.A., 1998. Global distribution of density irregularities in the equatorial ionosphere. J. Geophys. Res. 103, 407±417. Krebiehl, J.P., Brace, L.H., Theis, R.F., Pinkus, W.H., Kaplan, R.B., 1981. The Dynamics Explorer Langmuir probe instrument. Space Sci. Intrum. 5, 493±503. Laakso, H., Aggson, T.L., Herrero, F.A., Pfa, R.F., Hanson, W.B., 1995. Vertical neutral wind in the equatorial F-region deduced from electric ®eld and ion density measurements. J. Atmos. and Terr. Physics 57, 645±651. Langel, R.A., Purucker, M., Rajaram, M., 1993. The equatorial electrojet and the associated currents as seen in Magsat data. J. Atmosph. Terr. Phys. 55, 1233±1269. Lloyd, K.H., Haerendel, G., 1973. Numerical modelling of the drift and deformation of ionospheric plasma clouds and their interaction with other layers of the ionosphere. J. Geophys. Res. 78, 7389±7415. Maynard, N.C., Bielecki, E.A., Burdick, H.F., 1981. Instrumentation for vector electric ®eld measurements from DE-B. Space Sci. Instrum. 5, 523±535. Maeda, H., Iyemori, T., Araki, T., Kamei, T., 1982. New evidence of a meridional current system in the equatorial ionosphere. Geophys. Res. Lett. 9, 337±340. Mendillo, M., Baumgardner, B., Pi, Xi-Q, Sultan, P., Tsunoda, R., 1992. Onset conditions for equatorial spreadF. J. Geophys. Res. 97, 13,865±13,876. Olsen, N., 1997. Ionospheric F-region currents at middle and low latitudes estimated from Magsat data. J. Geophys. Res. 102, 4563±4576. Ossakow, S.L., 1981. Spread-F theoriesÐa review. J. Atmos. Terr. Phys. 43, 437±452. Patra, A.K., Rao, P.B., Anandan, V.K., Jain, A.R., 1997. Radar observations of 2.8 m equatorial spread-F irregularities. J. Atmos. Solar-Terr. Phys. 59, 1633±1642. Raghavarao, R., Desai, J.N., Anandarao, B.G., Narayanan, R., Sekar, R., Gupta, R., Babu, V.V., Sudhakar, V., 1984. Evidence for the large scale electric ®eld gradient at the onset of equatorial spread-F. J. Atmos. Terr. Phys. 46, 355±362. Raghavarao, R., Gupta, S.P., Sekar, R., Narayanan, R., Desai, J.N., Sridharan, R., Babu, V.V., Sudhakar, V., 1987. In-situ measurements of winds, electric ®elds and electron densities at the onset of equatorial spread-F. J. Atmos. Terr. Phys. 49, 485±492. Raghavarao, R., Hoegy, W.R., Spencer, N.W., Wharton, L.E., 1993. Neutral temperature anomaly in the equatorial thermosphere-A source of vertical winds. Geophys. Res. Lett. 20, 1023±1026. Raghavarao, R., Nageswararao, M., Sastri, J.H., Vyas, G.D., Rao, S.M., 1988. Role of equatorial ionization anomaly in the initiation of equatorial spread-F. J. Geophys. Res. 93, 5959±5964. Raghavarao, R., Sekar, R., Suhasini, R., 1992. Non-linear nu-
R. Raghavarao et al. / Journal of Atmospheric and Solar-Terrestrial Physics 61 (1999) 607±617 merical simulation of equatorial spread-FÐeects of winds and electric ®elds. Adv. Space Res. 12 (6), 227±230. Raghavarao, R., Suhasini, R., Hoegy, W.R., Mayr, H.G., Wharton, L.E., 1998. Local time variation of equatorial temperature and zonal wind anomaly (ETWA). J. Atmos. and Solar-Terr. Phys. 60, 631±642. Raghavarao, R., Wharton, L.E., Spencer, N.W., Mayr, H.G., Brace, L.H., 1991. An equatorial temperature and wind anomaly (ETWA). Geophys. Res. Lett. 18, 1193±1196. Ramarao, P.V.S., Jayachandran, P.T., Sri, Ram P., 1997. Ionospheric irregularities: the role of equatorial ionization anomaly. Radio Sci. 32, 1551±1557. Rishbeth, H., 1971. The F-layer dynamo. Planet. Space Sci. 19, 263±267. Sahai, Y., Fagundes, P.R., Bittencourt, J.A., Abdu, M.A., 1998. Occurrence of large scale equatorial F-region plasma depletions during geomagnetic disturbances. J. Atmos. and Solar Terr. Phys. 60, 1593±1604. Sastri, J.H., Murthy, B.S., 1978. On the onset of equatorial spread-F. Ann. Geophys. 34, 47±49. Sastri, J.H., Subrahmanyam, V., Sasidharan, K., Rao, M.S., 1978. On the height control of the onset of equatorial spread-F. Curr. Sci. 47, 451±452. Scannapieco, A.J., Ossakow, S.L., 1976. Non-linear equatorial spread-F. Geophys. Res. Lett. 3, 451±454. Sekar, R., 1990. Plasma instabilities and the dynamics of the equatorial F-region. Ph. D. Thesis., Gujarat University, Ahmedabad-9, India. Sekar, R., Raghavarao, R., 1987. Role of vertical winds on the Rayleigh±Taylor mode instabilities of the night time equatorial ionosphere. J. Atmos. Terr. Phys. 49, 981±985. Sekar, R., Raghavarao, R., 1997. A case study on the evolution of equatorial spread-F by a non-linear numerical model using the results from a set of coordinated measurements. J. Atmos. Terr. Phys. 59, 343±350. Sekar, R., Suhasini, R., Raghavarao, R., 1994. Eects of ver-
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tical winds and electric ®elds in the non-linear evolution of equatorial spread-F. J. Geophys. Res. 99, 2205±2213. Singh, S., Johnson, F.S., Power, R.A., 1997. Gravity wave seeding of equatorial plasma bubbles. J. Geophys. Res. 102, 7399±7410. Spencer, N.W., Wharton, L.E., Niemann, H.B., Hedin, A.E., Carignan, G.R., Maurer, J.C., 1981. The Dynamics Explorer wind and temperature spectrometer. Space Sci. Instrum. 5, 417±429. Sridharan, R., Chandra, H., Das, S.R., Sekar, R., Sinha, H.S.S., Raju, D.P., Narayanan, R., Raizada, S., Mishra, R.N., Raghavarao, R., Vyas, G.D., Rao, P.D., Ramarao, P.V.S., Somayajulu, V.V., Babu, V.V., Danilov, D.A., 1997. Ionization hole campaignÐa coordinated rocket and ground-based study at the onset of equatorial spread-F: ®rst results. J. Atmos. and Solar-Terr. Phys. 59, 2051±2067. Sridharan, R., Raju, D.P., Raghavarao, R., Ramarao, P.V.S., 1994. Precursor to equatorial spread-F in OI 630.0 nm dayglow. Geophys. Res. Lett. 21, 2797±2800. Szuszczewicz, E.P., Tsunoda, R.T., Narcisi, R., Holmes, J.C., 1980. Coincident radar and rocket observations of equatorial spread-F. Geophys. Res. Lett. 7, 537±540. Takeda, M., Maeda, H., 1983. F-region dynamo in the evening-interpretation equatorial delta D anomaly found by MAGSAT. J. Atmos. Terr. Phys. 45, 401±408. Tsunoda, R.T., 1980. Magnetic ®eld-aligned characteristics of plasma bubbles in the nighttime equatorial ionosphere. J. Atmos. Terr. Phys. 42, 743±752. Weber, E.J., Basu, S., Bullett, T.W., Valladares, C., Bishop, G., Groves, K., Kuenzler, H., Ning, P., Sultan, P.J., Sheehan, R.E., Araya, J., 1996. Equatorial plasma depletion precursor signature and onset observed at 11 south of the magnetic equator. J. Geophys. Res. 101, 26,829± 26,838. Woodman, R.F., LaHoz, C., 1976. Radar observations of Fregion equatorial irregularities. J. Geophys. Res. 81, 5447± 5466.