J. V. S. V. Rao

Daytime equatorial geomagnetic H field response to the growth phase and expansion phase onset of isolated substorms: Case studies and their implications

Observations are presented from the Indian magnetometer network (dipole latitude range 1.2°S to 13.5°N) of short-lived ( 40-min interval of southward Bz. A positive baylike perturbation is found to prevail during the substorm growth phase, followed by a negative baylike disturbance starting precisely at the onset of expansion phase activity. The amplitude of the positive as well as negative bay-type disturbance showed a clear-cut enhancement at locations inside the equatorial electrojet belt when compared with stations away from the electrojet influence, indicative of a significant contribution of ionospheric currents to the bays. This pattern of response, which is found in two out of the three events studied, constitutes the first-time evidence for the occurrence of equatorial H field perturbations related to the growth phase as well as the expansion phase onset for individual substorms. The H field perturbations are suggested to be signatures of prompt penetration by electric fields associated with rapid changes in magnetospheric convection brought about by the swift transitions in the IMF Bz, before shielding by the ring current becomes effective. In the third event, where the growth phase is weak and prolonged, there is no perceptible simultaneous disturbance in the equatorial H field, while the expansion phase onset is associated with a positive bay of very small amplitude (≤4 nT).

Doppler frequency fluctuations of lower thermospheric reflections in the equatorial electrojet region

Abstract A common and characteristic feature of lower F-region (~ 200 km) reflections at vertical incidence over Kodaikanal (dip 3.0°N) during the daytime is the presence of quasi-sinusoidal variations in the time rate of change of phase path, P (Doppler frequency shift, Δƒ ) with periodicities of 30–600 s. We have studied here the dependence of the lower-period segment (30–300 s) of the Doppler frequency variations on the ambient electrojet strength and associated ionospheric conditions. The level of wave activity in P in the period range 30–300 s is quantified by computing the variance of the P fluctuations synthesized through reverse FFT, in the chosen frequency band, of the FFT of original data. The variance of P (computed at ~1 h intervals) is found to bear a significant linear relationship to the ambient electrojet strength. P fluctuations in the range 30–120 s are found to be most sensitive to changes in electrojet strength compared to the other two sub-bands 120–210 s and 210–300 s, and tend to dominate, in general, the spectral content of P fluctuations in the range 30–300 s. The Doppler fluctuations in band 30–300 s are consistently seen to practically cease at times of disappearances of Esq on bottomside ionograms (i.e. during partial/complete counter-electrojet conditions). The short-period fluctuations in P are interpreted in terms of phase path changes imposed on lower F-region reflections by the refractive index variations associated with the convective motions of plasma density irregularities (types I and II) in the daytime equatorial electrojet.

Manifestations of short-period electric field fluctuations in the equatorial lower thermosphere

Measurements (90h) of phase path, P of lower F-region (~ 200km) reflections at normal incidence over Kodaikanal (dip 3.0°N) revealed the time rate of change of phase path, (Doppler frequency shift, Δf) to undergo quasi-periodic fluctuations quite regularly during the day time. The changes in which have peak-to-peak amplitudes of 6 to 30 ms−1 (0.1-0.5Hz in Δf) exhibit quasi-periods in the band 60-600s, comprising of two overlapping spectral domains: the shorter-period components (T 300s). The spectral content of the variations in is found to depend on the ambient equatorial electrojet strength and associated ionospheric conditions in that, while the longerperiod components persist almost all the time, the shorter-period components tend to cease during the times of disappearance of equatorial sporadic-E (Esq) on ionograms (partial counter-electrojet). Perturbations in the F-region vertical plasma drift, VzF are considered to be primarily responsible for the observed changes in , particularly the longer-period (T > 300s) fluctuations. Correlative and visual studies of the simultaneous data ( 52h) of phase path at Kodaikanal and of ground level geomagnetic micropulsations at Trivandrum (located similar 200km south of Kodaikanal, dip 0.6°S) showed the absence of a persistent and clear-cut association of the fluctuations in with Pc4/Pc5 micropulsations, indicating that hydromagnetic waves are not a primary source of the VzF pulsations. It is suggested that the longer-period (300 < T < 600s) fluctuations in VzF are due to gravity waves related changes in the east-west electric field at E-region altitudes outside the electrojet belt, and mapped along the field lines to F-region levels over Kodaikanal.

Origin of short‐period (30–300 s) Doppler frequency fluctuations of lower F region reflections in the equatorial electrojet region

Measurements of phase path P of lower F region reflections at normal incidence at Kodaikanal (10°14′N, 77° 28′E, dip 3.0°N) revealed the ubiquitous presence of 30–300 s quasi-sinusoidal variations in the time rate of change of phase path, P (Doppler frequency shift) during day time. A study is made of the influence of the irregularities in the equatorial electroject on the P fluctuations using simultaneous observations of F region phase path at Kodaikanal and of equatorial electrojet with the VHF backscatter radar at Thumba (08° 29′N, 76° 56′E, dip 0.9°S). It is shown that the spectral content of the Doppler fluctuations (quantified in terms of variance, σ2 computed from P time series synthesized through FFT−1 (fast Fourier transfrom) in the chosen period bands, 30–300 s/30–120 s of the FFT of original P time series) bears a significant positive linear relationship to the horizontal phase velocity (Vp) of electrojet irregularities (3-m scale size)on a hourly basis. This result is in consonance with our earlier findings (Sastri et al., 1990) of a significant linear relationship of σ2 to the electrojet strength (estimated from H field data) and a practical cessation of the P fluctuations at times of disappearance of Esq on ionograms (partial/complete counterelectrojet). The present work substantiates the interpretation that the short-period Doppler frequency fluctuations are due to phase path changes imposed on lower F region reflections by the refractive index variations associated with the convective motions of plasma density irregularities (type I and II) in the daytime equatorial electrojet.

Geomagnetic disturbances associated with disappearing solar filaments

To gain an insight into the origin of enhanced geomagnetic activity that is recently reported to follow the ‘disparition brusque’ (DB) of quiescent solar filaments, a study is made of the interplanetary plasma and magnetic field data at 1 AU, in relation to DBs over the period January 1967 through March 1978. The investigation revealed that the ‘minor’ (Ap ≥ 30) and ‘major’ (Ap ≥ 50) geomagnetic disturbances that manifested within 8 days of DBs, almost invariably occurred (in 28 out of the 30 events studied) in association with the passage at Earth of high-speed streams in the solar wind. A majority of the streams (19 out of the 28 streams) exhibited a 27-day recurrence pattern and, thus, the associated enhancement in geomagnetic activity (and apparently followed DBs). The date of transit of the high-speed stream at Earth seems to control the delay time of the geomagnetic disturbance, rather than the size of the filament. A systematic spatial relationship between DBs and the potential solar sources of the high-speed streams (coronal holes and flares) does not appear to be present. The results point out the relevance and a prominent role of recurrent and transient high-speed structures in the solar wind in the enhancement of geomagnetic activity that accompagnies DBs.

HF Doppler Observations of Small Scale Plasma Motions in the Equatorial Electrojet

Recordings of the variations in phase path, P of reflections, at vertical incidence, from blanketing sporadic-E layers over Kodaikanal (dip 3.5?N) revealed small amplitude, quasi-sinusoidal oscillations of 45-180 s periods in the time rate of change of phase path, (Doppler frequency). Geomagnetic micropulsation activity at ground level in the Pc4 range (45-150 s) was not evident at Trivandrum (dip 0.6?S, located ~ 200 km south of Kodaikanal) on all the 26 occasions (out of the total 29 events studied) when geomagnetic data simultaneous with the phase path observations were available. Besides, the short period quasi-periodic fluctuations in were also consistently seen in the data of equatorial sporadic E reflections. We interpret the observed pulsations in as representative of those in the vertical electron drift associated with the longitudinal electrostatic drift waves generated in the equatorial electrojet by the gradient drift plasma instability mechanism.