K. B. Ramesh

Penetration of polar electric fields to the nightside dip equator at times of geomagnetic sudden commencements

Measurements of Doppler Velocity, VD of F - region reflections at normal incidence over Kodaikanal (dip 3°N, 77° 28′E) are used to study the nature of perturbations in F - region vertical plasma drift, Vz associated with the geomagnetic sudden commencements (sc) on July 8, 1991, and January 1, 1992. Both the events which occurred in a narrow time window, 1630-1700 UT (2200-2230 IST) were of sc* type at middle- and high- latitude stations in the afternoon sector. At Kodaikanal, which is on the nightside, the sc of January 1, 1992, is characterized by a double-step main impulse (MI) in H - component (the structure of the sc on July 8 could not be ascertained from normal run magnetograms because of the large amplitude and very small rise time of the sc). It is found that the usual downward motion of F - region plasma during the premidnight hours at Kodaikanal suddenly ceased (and even reversed to upward in one event) for ∼1 min coincident with the preliminary impulse (PI) and was immediately enhanced in association with the MI of the sc*. This pattern which is consistently seen in the two events implies that an eastward electric field prevails near the nightside dip equator at the time of the first impulse of double-step MI there and the PI of sc* at high latitudes. Our Doppler velocity observations constitute the first and direct experimental evidence of vertical plasma motions due to the sc - associated electric fields in the nighttime dip equatorial ionosphere. They substantiate the view based on theory (Kikuchi and Araki, 1979) and ground-based magnetic observations (Araki et al, 1985) that the dusk-to-dawn electric field imposed on the polar ionosphere with the onset of PI of sc* is instantaneously transmitted to the dip equator on the nightside as on the dayside.

Transient composite electric field disturbances near dip equator associated with auroral substorms

Ionosonde data of Kodaikanal (Geog.Long. 77° 29′E, dip 3.0°N) and Huancayo (Geog.Long. 75°18′W, dip 2.0°N) are used to show the simultaneous occurrence of a transient disturbance in F region height of composite polarity in day and night sectors near the dip equator during the auroral substorm activity on 20 August 1979. At Kodaikanal which is on the nightside at the time of the substorm activity, h′F first underwent an abrupt and rapid decrease (80km in 1 hr) followed by a much larger increase (120km in 1 hr). Perturbation in hpF2 of exactly opposite polarity was simultaneously seen at Huancayo which is on the dayside. The decrease in h′F at Kodaikanal (increase in hpF2 at Huancayo) occurred in association with an increase in polar cap potential drop, o (estimated from IMF parameters), and the subsequent increase (decrease at Huancayo) with a decrease in polar cap potential. The F-region height disturbance is interpreted as the manifestation of a global transient composite disturbance in equatorial zonal electric field caused by the prompt penetration of substorm-related high latitude electric fields into the equatorial ionosphere. The polarity pattern of the electric field disturbance is consistent with the global convection models which predict westward (eastward) electric fields at night (by day) near the geomagnetic equator in response to an increase in polar cap potential drop, and fields of opposite signs for a decrease in polar cap potential.

On the nature of substorm-related transient electric field disturbances in the equatorial ionosphere

Abstract Data derived from 5-min interval ionograms of Kodaikanal (10°14′N, 77°29′E, geomag. lat. 0.6°N) on the night of 29–30 August 1957 showed the presence of a marked perturbation in F-region height (h′F) in the midnight-to-morning period. The perturbation is characterized by a sudden decrease in h′F (65 km in 1 hr, with h′F reaching a low value of 200 km) followed by a prominent increase (200 km in 75 min) over a 3-h period. Changes in h′F of essentially the same nature were also noticed at Calcutta (geomag. lat. 12.25°N) and Ahmedabad (geomag. lat. 14°N) in the same longitude sector simultaneous with those at Kodaikanal. Examination of the auroral electrojet (AE) index and magnetograms of high latitude stations widely separated in longitude revealed the prevalence of an isolated substorm of moderate strength at the time of the perturbations in equatorial F-region height. The decrease in h′F is found to occur around the onset of the substorm and the subsequent increase during the substorm recovery phase. The observed F-region height disturbance is interpreted as the signature of a transient composite disturbance in the equatorial east-west (E-W) electric field caused by the prompt penetration of substormrelated perturbations in high latitude electric fields. The nature of the electric field disturbance is in good agreement with the recent modelling results which predict a westward peturbation in E-W field at equatorial latitudes in response to an increase in polar cap potential (around the onset of a substorm) and an eastward one with a decrease in polar cap potential (during the recovery phase) in the midnight-dawn sector.

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.

Dependence of SSNM on SSNm – a Reconsideration for Predicting the Amplitude of a Sunspot Cycle

An improved correlation between maximum sunspot number (SSNM) and the preceding minimum (SSNm) is reported when the monthly mean sunspot numbers are smoothed with a 13-month running window. This relation allows prediction of the amplitude of a sunspot cycle by making use of the sunspot data alone. The estimated smoothed maximum sunspot number (126±26) and time of maximum epoch (second half of 2000) of cycle 23 are in good agreement with the predictions made by some of the precursor methods.

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.