H. Chandra

Solar cycle and seasonal variation of spread-F near the magnetic equator

Spread-F at equatorial stations, Ibadan, Djibouti and Kodaikanal is shown to increase with increasing solar activity. At Huancayo and other stations in the American zone there is least occurrence of spread-F in years of high solar activity. The seasonal variation of spread-F is greatest at stations in the American zone with a maximum in December and a minimum in June. At African stations a smaller seasonal variation is seen with maximum in June and minimum in December. At Asian stations the seasonal variation of spread-F is very small.

Equatorial electrojet studies from rocket and ground measurements

Combining the data of in-situ measurements of ionospheric current, Jm by rocket-borne instruments and the ground based geomagnetic H field close to the magnetic equator a linear relation has been found between the peak current density Jm and the daily range of H, (RH). This relationship has been used to convert long series of RH data into Jm. Combining Jm and the E-region peak electron density Nm, the electron velocity in the ionosphere, VE has been calculated. It is shown that after all corrections are made of the solar zenith angle variations, the ionospheric current as well as electron drift in American and Indian sectors show strong equinoctial maxima, the mean values of both the parameters are larger at American than in Indian sector. The solar cycle variation of the electrojet current is primarily due to the variations of NmE, and not due to the variations of electric field. The diurnal variation of the electric field with peak at 09–10 LT interacting with noon peak of NmE making ΔH to peak at an hour earlier than noon. It is stressed to realise the importance of electric field in diurnal, seasonal and longitudinal variations of the equatorial electrojet current.

Interplanetary magnetic field and the equatorial ionosphere

With the increase of the southward component of the interplanetary magnetic field, the magnitude of the east-west drift speed in the F-region of the ionosphere at Thumba, close to the magnetic equator, decreases for the daytime as well as for the nighttime periods. This is interpreted to be due to the decrease of the equatorial east-west electrostatic field with the increasing southward component of interplanetary magnetic field.

The disappearance of equatorial E s and the reversal of electrojet current

Based on simultaneous observations of the horizontal geomagnetic field component H, sporadic E (Es) and E-W electron drifts at stations close to the dip equator within the equatorial electrojet region, it has been found that on quiet days and sometimes on disturbed days, when there is an abnormal large decrease in H during daytime, there is a simultaneous disappearance of Es and a reversal of the direction of drift of electrons from westward to eastward. This suggests that the disappearance of equatorial Es during day-time is due to a temporary reversal of the electrojet current, which is caused by the imposition of an additional electrostatic field opposite in direction to that of normal Sa field.

Long term ionospheric trends over Ahmedabad

Ionospheric data over Ahmedabad (23°N, 73°E) during 1955–96 have been examined for the long term changes arising due to the predicted cooling of the mesosphere and thermosphere because of the increased concentration of trace gases resulting from anthropogenic causes. The long term changes in the critical frequencies of the ionospheric layers and the height of the maximum ionization as characterized by hPF2, the virtual height at 0.834 of foF2, have been obtained after removing the seasonal and solar cycle variations. While the trend of a small decrease in foE (0.15 MHz in 40 years) may not be considered significant, there is a clear indication of a decrease in foF2 (1.6 MHz for midday, 1.0 MHz for midnight). A small increase is noted in foF1 (0.3 MHz). There is a decrease in hPF2 (16 km for midday and 10 km for midnight). The lowering of F2 layer peak will affect radio propagation.

Ionospheric measurements during the total solar eclipse of 11 August 1999

A number of radio experiments were conducted at Ahmedabad (23°N, 73°E) with the aim of studying the ionospheric effects of the total solar eclipse of 11 August 1999. Rapid radio soundings from the ionosonde were made on the eclipse day and on control days. A riometer was operating at 30 MHz, and field strength measurements along the three oblique incidence paths of Colombo-Ahmedabad (11905 kHz), Bombay-Ahmedabad (558 kHz) and Rajkot-Ahmedabad (810 kHz) were also made. A reduction of about 20% was observed in the minimum frequency of reflection from the ionosonde (fmin), which indicates a reduction in D-region ionization. The critical frequency of the E-layer was not measurable beyond 1600 h IST on eclipse day due to the strong blanketing sporadic-E, but there is a 20% decrease in the critical frequency of the F1-layer. Although there was no change in the minimum virtual height of the F-layer on eclipse day, there appears to have been a decrease in the height of maximum ionization (hpF2) during the eclipse, indicating a reduction in the thickness of the F-layer. The signal strength of the Colombo-Ahmedabad path shows an initial rapid increase with the start of the eclipse (indication of a decrease in ionization in the D- and lower E-regions), but subsequently decreases until the maximum of the eclipse (excessive deviative absorption because of the wave penetrating to the E-region). The field strength measurements of the Bombay-Ahmedabad path show a large fading after sunset as the sky wave also appeared. On eclipse day the fading started about an hour earlier. Riometer recordings also show a higher signal during eclipse day, which again indicates an eclipse-associated decrease in ionization.

In-situ studies of equatorial spread-F over SHAR—steep gradients in the bottomside F-region and transitional wavelength results

RH-560 rockets instrumented with Langmuir probes were launched from SHAR, India (dip 11°N) for in-situ studies of electron density irregularities associated with equatorial spread-F (ESF) when the F-region plasma was drifting down and strong range spread-F was observed with an ionosonde at SHAR. A high variability was observed in the steepness of the base of the F-region. The bases were found to be steeper during the periods when the F-region plasma was drifting down. On one of the flights irregularities were observed in the region around 280 km where the gradients in electron density were downwards, indicating that the gradient drift instability is the main mechanism for their generation. Assuming a power law of the type Pk∝ kn for irregularities of transitional scale (20–200 m), it was found that the spectral index n ranges between −1.5 and −4.6, when the mean integrated spectral power PT of the irregularities in the above scale size range varied from −45 to −12 db. A relationship between n and PT was observed and can be represented by a Gaussian function using the above expression; the altitude variation of n normalized for a PT value of −10 db showed that the nature of spectral index remains the same between 230 km and the apogee of the rocket. This is at variance with the observations of Kelley et al. [(1982), J. geophys. Res. 87, 1575] that 280 km is the threshold altitude for the steep drift wave type of spectra to a shallower spectra.

Daily variation of F-region drifts at thumba

Abstract The measurements of F-region electron drifts at Thumba during the year 1967 indicate the direction to be Westward during the day-time and Eastward during night-time, the reversals occurring between 0600–0700 hr and 1900–2000 hr. A significant increase of drift speed occurs about an hour before the morning as well as the evening reversal. The results are in excellent agreement with the electron drifts measured by Doppler shift of VHF scatter echoes at Jicamarca.

Ionization hole campaign-a coordinated rocket and ground-based study at the onset of equatorial spread-F: first results

Abstract A comprehensive multi-technique campaign involving the launch of two high altitude RH-560 rockets was carried out from Sriharikota (SHAR), India, a near-equatorial rocket launching station at the onset of equatorial spread-F, along with a host of ground-based complementary experiments at other locations spread over the country. The main objectives were to obtain the background ionospheric and thermospheric conditions at the onset of equatorial spread-F, and to be able to evaluate the relative importance of the various agencies, the neutral dynamics in particular. Multiple barium cloud releases, in situ plasma diagnostic measurements, along with complementary optical and radio probing experiments were carried out as a part of this campaign. The presence of large scale gradients in the ambient electric fields at ~185 km altitude and also of vertical winds of significant magnitudes at higher altitudes were found. The presence of large scale irregularities in the ion densities at heights above 250 km in a region of negative background density gradient is one of the significant new results. The details of the Ionization hole campaign along with the first results are presented and discussed in the context of the present knowledge of the phenomenon of equatorial spread-F.

Spectral characteristics of scintillations producing ionospheric irregularities in the Indian region

VHF amplitude scintillations were recorded at Tirunelveli (8.7°N, 77.7°E; 0.6°N dip Latitude), Pondicherry (12°N, 79.1°E; 4.4°N dip Latitude) and Mumbai, (19°N, 73°E; 13.5°N dip Latitude) for the years 1992–1996 using the 250 MHz radio beacon from the geostationary satellite FLEETSAT (73°E). The recorded digital scintillation data for few nights are analyzed to estimate scintillation index (S4), fade rates, auto-correlation functions and power spectral densities for every 2.5 minute sample during the period of the scintillation activity. The power spectral slopes are shallower for the scintillation at the generation phase and steeper towards the decay phase, which indicates the erosion of smaller scale sizes towards the decay phase.