R. S. Dabas

C and L band transionospheric scintillation experiment: Some results for applications to satellite radio systems

Satellite radio systems suffer loss of information in a wide band of frequencies during periods of intense ionospheric scintillation activity when the received signal undergoes rapid and deep fading. In order to assess the problem and to determine a proper fade margin for an Earth-space link, system engineers require information on signal statistics as well as on the morphological aspects of scintillations. Our observations near the northern boundary of the equatorial scintillation belt at (18.9° N geomagnetic) within the Indian zone show that the signal at 4/1.5 Ghz has faded often beyond 10 dB pp, and at times beyond 24 dB pp at 4 Ghz during equinoctial months of high solar activity during the years of 1989–1990. In addition to the morphology at 4 Ghz, information on signal statistics, such as cumulative amplitude distribution function, fade rate distribution, and signal reliability for different message lengths for some events of scintillations, both at C and L band, has been presented. The theoretical Nakagami m distribution has been found to be the best for describing various levels of fade. Autocorrelation and power-spectrum analysis have been used to estimate average fade rates and ground correlation distances. Performance evaluation of satellite Earth terminals using small antennas has been carried out to show the vulnerability of the system in the hostile ionospheric environment notwithstanding the advanced modulation systems being employed.

Day‐to‐day variability in the occurrence of equatorial and low‐latitude scintillations in the Indian zone

Nighttime ionospheric scintillation data at 4 GHz, recorded simultaneously (during September and October of 1989) from two satellites at Sikandarabad (dip 42.0°N) and Chenglepet (dip 10.5°N), along a common magnetic meridian (149.0°E), are analyzed. Scintillation occurrences on a daily basis at these two locations are examined in relation to presunset hour variations in h′F and their rate of rise, as well as with solar and magnetic activity conditions. The main objective of the study is to identify the ionospheric conditions over the magnetic equator necessary for observing intense gigahertz scintillation at 21°N magnetic latitude. The characteristics and occurrence pattern of scintillations at these two stations suggest that they are equatorial plasma bubble induced events. The scintillations and spread F at the equatorial location occur whenever h′F rises to levels of more than 400 km during evening hours. However, at 21°N magnetic latitude, scintillations were observed only on those nights when h′F rises to more than 500 km with dh′F/dt of 30 m/s or more. Also, whenever h′F was less than 400 km, no scintillations were observed at any of the locations. Scintillation activity was found to be inhibited during magnetic disturbances at both the locations except during postmidnight hours, where it is found to increase. With increasing magnetic activity, h′F values during evening hours decrease. The day-to-day variability in the occurrence of scintillations seems to be controlled mainly by the electric field, neutral winds, and magnetic activity. Scintillation intensity at low latitudes is found to be positively correlated with ionospheric electron content values observed during evening hours (2000 LT), as well as with their diurnal maximum values. In addition, scintillation intensity is also found to be dependent on the geometries of the ray paths to the respective locations and their intersections with the magnetic field lines.

On the possible use of recent EUV data for ionospheric predictions

Abstract The various ionospheric layers produced and maintained by solar ionising radiations support a variety of long distance HF communications around the world. Traditionally, long-term ionospheric predictions are based on a predicted qualitative sunspot index and correlations established between observed sunspot indices and ionospheric parameters. As the techniques are essentially statistical the long series of sunspot observations stand in good stead and have given satisfactory long-term predictions of the monthly median ionospheric parameters. However, with the recent availability of EUV data from satellites, it is considered more appropriate to use these radiations, which are directly responsible for the production of the ionosphere, for prediction purposes rather than the surrogate sunspot index. The present study has shown that the use of EUV flux for ionospheric predictions has certain advantages over the usual solar indices such as R12 and F10.7. Saturation effects, that are usually seen in the foF2 variation with sunspot numbers at low latitudes, are not apparent with EUV flux. EUV flux values also require much less smoothing when compared to sunspot numbers and are more suitable for the prediction of ionospheric parameters one or two months in advance.

Ionospheric perturbations over Delhi caused by the 26 December 2004 Sumatra earthquake

The death and destruction caused by the Sumatra earthquake on 26 December 2004 have once again jolted seismologists to find a reliable early warning system of an impending major earthquake. The F‐region ionospheric parameters probed remotely by a digital ionosonde over Delhi (28.6° N, 77.2° E) have shown precursory signatures of 26 December 2004 Sumatra earthquake. Results indicate severe perturbations in foF2 and hmF2 several hours before the deadly earthquake. The wavelike perturbations in foF2 continued for several days after the event. It is important to note that this earthquake struck at a time when there were no solar or geomagnetic disturbances to cause any anomalous ionospheric changes.

A multi-station satellite radio beacon study of ionospheric variations during total solar eclipses

Faraday rotation data obtained at Delhi, Kurukshetra, Hyderabad, Bangalore, Waltair, Nagpur and Calcutta during the total solar eclipse of 16 February 1980 and at Delhi during the total solar eclipse of 31 July 1981 have been analysed to detect the gravity waves generated by a total solar eclipse as hypothesized by Chimonas and Hines (1970, J. geophys. Res. 75, 875). It has been found that gravity waves can be generated by a total solar eclipse but their detection at ionospheric heights is critically dependent on the location of the observing station in relation to the eclipse path geometry. The distance of the observing station from the eclipse path should be more than 500 km in order to detect such gravity waves.

Gigahertz scintillation observations at 22.0° N magnetic latitude in the Indian zone

Postsunset ionospheric scintillation measurements at 4 GHz from the INSAT 1B (74°E) satellite taken during the increasing half of the current solar cycle 22 at Sikandarabad (22.0°N magnetic latitude sub ionospheric) along 149°E geomagnetic meridian in the Indian zone have been analyzed and presented here. Results show that during the low solar activity period, only weak scintillations (peak-to-peak fluctuations < 2 dB) were observed mainly during the summer months. As the solar activity increased, occurrence of summer scintillations more or less remained the same, but the intensity and occurrence probability increased substantially in the equinoxes and to some extent in the December solstice also. During October 1989, severe scintillations, with peak-to-peak fluctuations exceeding 20 dB were observed at this latitude, which is normally beyond the daytime crest of the equatorial anomaly. The characteristics of scintillations during equinoctial periods of high sunspot year were such that their onsets were mostly abrupt and they developed fully within a few seconds. Simultaneous measurements taken from an equatorial station Chenglepet (5.2°N magnetic latitude) along the same geomagnetic meridian plane during September–October 1989 show that scintillations at Sikandarabad were conditional to their prior occurrence at Chenglepet. As compared to the equatorial location, the onset of scintillations at 22.0°N magnetic latitude was always delayed and also died out first at the higher latitude. Simultaneous observations showed that the intensity of scintillations at the equatorial location never exceeded 5 dB peak-to-peak, whereas the intensity of scintillations at 22.0°N magnetic latitude frequently exceeded 10 dB peak-to-peak. This is explained on the basis of background ionization and the geometry of the ray paths relevent for these locations. Scintillation activity is in general found to be suppressed during geomagnetic disturbances, but it is observed to be enhanced in the post midnight hours of the same night for those magnetic storms whose recovery phase starts between the midnight and dawn local time sector. Results are also compared with the observations reported from other regions of the world.

Results of foF2 and Ne-h profiles at low latitude using recent digital ionosonde observations and their comparison with IRI-2000

Abstract Using digital ionosonde observations at low-latitude station, Delhi (28.6 N, 77.2 E, mag. dip 42.4 N), the diurnal and seasonal variations of the critical frequency of F2 layer (foF2) are analyzed from August 2000 to July 2001 during a high solar activity period. Also, noontime bottomside electron density (Ne-h) profiles, below the F2-peak, are derived from ionogram, using the POLAN (Report UAG-93, WDC-A, for Solar Terrestrial Physics, Boulder, Co.) program during the same period, and these profiles are then normalized to the peak height and density (hmF2, NmF2) of the F2-region. These observations are used to assess the predictability of the International Reference Ionosphere, IRI-2000 model (Radio Sc. 36(2) (2001) 261). Results show in general, a large variability, (1σ, σ is standard deviation), in foF2 during nighttime than daytime during winter and equinox, the variability of foF2 about the mean is about ±25% by night and ±15% by day. The IRI model shows a fairly good agreement with foF2 observations during daytime, however during nighttime, the discrepancies between the two exist. Comparative studies of the normalized observed profiles with those obtained with the IRI model (Bilitza, 2001) using both the options namely: Gulyaevas (Adv. Space Res. 7 (1987) 39) model and B0-Table (Adv. Space Res. 25(1) (2000) 89), show that during all the seasons, in general, the B0-Tab option, reveals a better agreement with the observations, while the IRI model using Gulyaevas option, overestimates the electron density distribution during summer and equinox, however, during winter, the model is close to the observations. The comparisons of average profile shape parameters (B0,B1) derived from noontime observed profiles, with those obtained, using B0-Tab option, in the IRI model, show a good agreement during all the seasons. However, B0, B1 obtained, using Gulyaevas option in the IRI model, show a disagreement with the derived B0, B1 values during all the seasons, except during winter, for B0 parameter.

Study of equatorial plasma bubble dynamics using GHz scintillation observations in the Indian sector

Abstract To study equatorial plasma bubble dynamics, telemetry signals (4 GHz) were recorded simultaneously from two geostationary satellites. INSAT-1B (74°E) and INSAT-1C (94°E) at Sikandarabad satellite Earth station (dip 42.0°) from January to December 1989 and at the Chenglepet satellite Earth station (dip 10.5°) during September–October 1989 along the same geomagnetic meridian. The characteristics and occurrence pattern of the scintillations suggest that these are equatorial plasma bubble induced events. Observations from the two satellites recorded simultaneously at each of these locations were utilized to estimate the east-west plasma bubble irregularity motion. Plasma bubble rise velocities over the magnetic equator were calculated from the systematic onset time differences observed between an equatorial and a low latitude station. The east-west plasma bubble velocity estimated at Sikandarabad, corresponding to 1200 km altitude in the equatorial plane, shows a night time variation pattern with a peak at around 2100 LT. The mean values over Chenglepet, which correspond to 400 km altitude, start decreasing right from 1900 LT and seem to be influenced by the plasma bubble rise velocities. The differences in magnitude observed between the present results and those reported elsewhere by other techniques are interpreted in terms of vertical shears in the plasma zonal flow over the equator. The near alignment of the two observing stations along a common geomagnetic meridian and the simultaneous use of two satellites located twenty degrees apart in longitude provided an excellent data base to study plasma bubble dynamics.

A study of low-latitude VHF scintillations in relation to electric fields during magnetic storms

Nighttime VHF scintillations observed at Lunping (magnetic latitude 14.7°N) during four specific seasonal storms showed four different types of behaviour: Scintillation activity was found to be either inhibited or triggered during magnetic storms depending on the phase of the storm and its local time. A qualitative explanation for this behaviour is attempted in terms of storm time electric fields. The triggering of postmidnight scintillation activity during the recovery phase seems to be a result of penetration of the dusk-dawn electric fields due to shielding charges developed in the ring current region,while inhibition of usual premidnight scintillation activity seems to be related to the ionospheric disturbance dynamo electric fields acting opposite to normal fields. One case where postsunset scintillation activity was triggered seems to be a result of penetration of dawn-dusk convection electric field which developed following the main phase onset.