• R G Rastogi

Articles written in Journal of Earth System Science

• Total electron content at low latitudes

Radio beacon from ATS-6 at 140 MHz was used to measure the changes in the polarization angle (Faraday rotation) at Bombay, Rajkot, Ahmedabad, Udaipur and Patiala during October 1975 to July 1976. In this paper, results of diurnal, seasonal and latitudinal variations in total electron content (TEC) derived from these measurements are reported. The amplitude of diurnal peak is found to be higher at Rajkot, Ahmedabad and Udaipur as compared to that at Patiala or Bombay, indicating that the peak of Appleton anomaly in the latitudinal variation of TEC was close to the latitude of Ahmedabad. The diurnal maximum of TEC occurs around the same time during summer and winter months. The peak electron content shows a semiannual variation at all the stations with large values in equinoxes as compared to winter and summer. The TEC at Bombay shows a seasonal anamoly with high values in winter as compared to summer. The paper describes the development of latitudinal anomaly with the time of the day for different seasons. This anomaly is maximum during 1000 to 1800 LT and is located between 12° and 14° N (dip latitude) in summer and equinoxes and at about 10°N in winter.

• Theory for preliminary negative impulse in storm sudden commencement inH at equatorial stations

It is shown that the storm sudden commencement (SSC) inH field at low latitude station consists of only a positive excursion when the interplanetary shock due to the solar plasma impinging on the magnetosphere is associated with a southward excursion of the interplanetary magnetic field (IMF). When the signature of SSC at low latitude station consists of a preliminary negative excursion preceding the main positive excursion of theH field, the solar plasma causing the compression is associated with a northward excursion of the IMF. It is suggested that the signature of SSC(H) at equatorial stations is the result of combined effect of the compression of magnetosphere by the solar plasma as well as due to the electric field effects associated with the velocity of the solar plasma (v) interacting with the northward component (Bz) of the interplanetary field (i.e.,E=−vxBz).

• Geomagnetic disturbance effects on equatorial spreadF

Geomagnetic disturbance effects on the occurrence of range and frequency spread at Huancayo are studied for the period 1957–74. The occurrence of frequency spread is decreased on disturbed days forD-months andE-months while duringJ-months an increase is noted in the post-midnight period. The occurrence of range spread is decreased on disturbed days in the pre-midnight hours duringD andE-months. Post-midnight hours ofD andE-months and all hours of night duringJ-months show an increase of range spread on disturbed days, which is most prominent during low sunspot years. The mean occurrence of frequency spread (2300–0100 LT) and range spread (2000–2300 LT) decrease with increasing ΣKp. On the other hand, occurrence of range spread (0300–0500 LT) increases with increasing ΣKp, and is suggested as the consequence of the changes of the horizontal electric field in theF-region associated with the geomagnetic storms.

The post-sunset maximum in virtual height of theF region near the magnetic equator is associated with the general rise of the wholeF region from the base to the height of peak ionisation with little change in the semi-thickness of the layer. This rise ofF region is accentuated on days with large evening peak in the vertical drift velocity or the horizontal electric field in theF region. The range type of equatorial spreadF first occurs only if theF region drift velocity remains significantly upwards after sunset but the maximum intensity of spreadF occurs when the drift velocities are low or even downwards. The range spread first appears at or below the base of theF layer and later spreads into theF layer due to downward movement of the layer and/or upward movement of the irregularity. SpreadF seen on VHF backscatter records corresponds to the range type of spreadF seen on normal ionograms. The frequency type of spreadF does not produce VHF echoes. A strong peak in the electric field seems to be a necessary condition for the generation of equatorial spreadF.

• Integrated production and effective loss rates in the ionosphere

The total electron content data obtained at Ahmedabad through the Faraday fading records of the radio beacons abroad the satellites Explorer 22 and 27 are used to determine the overhead integrated production rate (Q0) and integrated loss coefficient (β′) for the epoch 1965–1968. The production rate (Q0) is shown to have two peaks during a year around the equinoctal months and for a particular monthQ0 increases linearly with the 10·7 cm solar flux. The loss coefficient β′, too, has two equinoctial peaks within a year. The semiannual variations ofQ0 and β′ are discussed in relation to similar variation in the [O]/[N2] ratio.

• Frequency dependence of equatorial ionospheric scintillations

Simultaneous observations of amplitude scintillations at 40 MHz, 140 MHz and 360 MHz radiated from ATS-6 satellite at 34° E longitude were made at Ootacamund near the magnetic equator in India. It has been found that the frequency variation of scintillation index (S4) isS4fn, withn being about 1·2 only for weak scintillations, i.e., so long as the scintillation index does not exceed 0·6 at the lower frequency. For strong scintillations (S4&gt;0·6) where multiple scattering may be present, the exponentn itself is a function of the intensity of scintillation, the scintillation indices at two frequencies are related by:S4(f1)=S4(f2) exp [1·3 log(f2/f1)(1−S4(f2)] so long asf2/f1≤3. Thus knowing scintillation index at a given frequency one can estimate the scintillation index at another frequency. This will be of significant importance for communication problems. Evidence is also shown for the reversal of the frequency law in cases of intense scintillations.

• Daily variations ofE andF region drifts over Tiruchirapalli

Daily variations of the E-W and N-S components of ionospheric drifts, measured using closely spaced receiver method over Tiruchirapalli near magnetic equator, are described for the periods 1973–75 forE region and 1974–75 forF region. N-S component is not observed except for a few occasions during summer months for theE region. E-W component is generally eastward during night hours and westward during day hours. There is a large day-to-day variability in the magnitude as well as in the evening reversal time. Daily variations primarily consist of the diurnal component with average amplitude of about 90 m/sec forF region and of about 80 m/sec forE region. The drift speeds are found to decrease with magnetic activity. It is concluded that the abnormal electrojet region extends upto north of Tiruchirapalli in the Indian zone and drifts here can be used as a monitor of electric field.

Almost saturated scintillations of radio beacons from geostationary satellites received at an equatorial station during night-time have been shown to occur even during complete absence of spreadF on the vertical incidence ionograms at the same location. These scintillation events were observed when the ionograms showed blanketing type of sporadicE layers simultaneously at different heights. It is suggested that strong equatorial radio wave scintillations during night-time are caused by multiple scattering between different levels of large plasma density gradients in theF or sometimes in theE regions of the ionosphere.

• Scintillations ofvhf radio beacons from two satellites

Analysis of scintillations ofvhf beacons from two very closely spaced geostationary satellites shows that the drift of irregularities is generally westward at the initial stages and changes to eastward during the later part of the night when the irregularities are fully developed or decaying.

• Equatorial electrojet in the Indian region during the geomagnetic storm of 13–14 November 1998

The geomagnetic storm of November 1998 is a unique event where IMF-Bz remained southward withvalues exceeding –15 nT for more than a day. The SYM/H index decreased from about 07 hr on 13November 1998 reaching a minimum of about –120 nT around midnight of 13–14 November 1998.Features of the equatorial electrojet in the Indian region are studied during the geomagnetic storm eventof 13–14 November 1998, based on the geomagnetic data from the chain of observatories in India. Suddennorthward turning of IMF-Bz for a very short duration around 08 hr on 13 November 1998 resultedin a small and very short duration counter electrojet. A strong (–50 nT) and a long duration counterelectrojet, right from 08 to 13 hr on 14 November 1998 was observed resulting in the absence of equatorialEs at Thumba. Absence of the equatorial ionization anomaly was also observed as seen from theionograms over Thumba and ionspheric data from Ahmedabad. The delayed effect on 14 November 1998is due to the disturbance dynamo effect.

• Effect of solar flare on the equatorial electrojet in eastern Brazil region

The effect of solar flare, sudden commencement of magnetic storm and of the disturbances ring current on the equatorial electrojet in the Eastern Brazil region, where the ground magnetic declination is as large as 20W is studied based on geomagnetic data with one minute resolution from Bacabal during November–December 1990. It is shown that the mean diurnal vector of the horizontal field was aligned along 2E of north at Huancayo and 30W of north at Bacabal during the month of December 1990. Number of solar flares that occurred on 30 December 1990 indicated the direction of solar flare related $\Delta H$ vector to be aligned along 5E of north at Huancayo and 28W of north at Bacabal. This is expected as the solar flare effects are due to the enhanced conductivity in the ionosphere. The SC at 2230 UT on 26 November 1990 produced a positive impulse in $\Delta X$ and negative impulse in $\Delta Y$ at Bacabal with $\Delta H$ vector aligned along 27W of north. At Huancayo the $\Delta H$ vector associated with SC is aligned along 8E of north, few degrees east to the alignment of the diurnal vector of H. The magnetic storm that followed the SC had a minimum Dst index of –150 nT. The corresponding storm time disturbance in $\Delta X$ at Huancayo as well as at Bacabal were about –250 nT but $\Delta Y$ at Bacabal was about +70 nT and very small at Huancayo, that give the alignment of the H vector due to ring current about 16∘W of north at Bacabal and almost along N–S at Huancayo. Thus alignment of the $\Delta H$ vector due to ring current at Bacabal is 14E of the mean direction of $\Delta H$ vector during December 1990. This is consistent with the direction of ring current dependent on the dipole declination at the ring current altitude which is about 5W of north over Bacabal and the deviation of declination due to the ring current during disturbed period given by the angle $(\psi-D)$.

• # Journal of Earth System Science

Current Issue
Volume 128 | Issue 4
June 2019