A. Dasgupta

Bio: A. Dasgupta is an academic researcher from University of Calcutta. The author has contributed to research in topics: F region & Scintillation. The author has an hindex of 4, co-authored 5 publications receiving 178 citations.

Occurrence of nighttime VHF scintillations near the equatorial anomaly crest in the Indian sector

Abstract: The behavior of nighttime F region irregularities near the northern crest of the equatorial anomaly in the Indian sector has been investigated by using VHF amplitude scintillation measurements made at Calcutta (27°N dip subionospheric) during the period April 1977 through February 1980. With the increase in solar activity the occurrence of scintillations increases remarkably during the equinoxes and to a lesser extent during the December solstice, while the local summer occurrence shows little change. The observed patterns are assessed in terms of the variation of the F layer height with solar activity and upwelling motion of the depleted flux tubes associated with small scale irregularities. It is shown that the propagation path is more likely to intercept these equatorial irregularities during periods of high solar activity, while during periods of low solar activity the equatorial irregularities lie south of the propagation path.

Scintillations associated with bottomside sinusoidal irregularities in the equatorial F region

Abstract: Multisatellite scintillation observations and spaced receiver drift measurements are presented for a category of equatorial F region plasma irregularities characterized by nearly sinusoidal waveforms in the ion number density. The observations were made at Huancayo, Peru, and the measurements at Ancon, Peru, associated with irregularities observed by the Atmospheric-Explorer-E satellite on a few nights in December 1979. Utilizing ray paths to various geostationary satellites, it was found that the irregularities grow and decay almost simultaneously in long-lived patches extending at least 1000 km in the east-west direction.

Observations of fast Faraday polarization fluctuations at VHF and amplitude scintillations at VHF and UHF near the equatorial anomaly crest in the Indian sector

Abstract: Simultaneous measurements of fast Faraday polarization fluctuations (FPF's) and amplitude scintillations at 136 MHz during April 1977 to February 1980 and of amplitude scintillations at 136 MHz and 244 MHz during August 1981 to December 1982 made at Calcutta (27° dip subionospheric) have been utilized to investigate the nature of ionospheric irregularities during different seasons near the equatorial anomaly crest in the Indian sector. The irregularities occurring in the equinoxes under high solar activity conditions are found to be responsible for intense VHF/UHF scintillations and FPF's. The June solstice is characterized by irregularities producing slow and moderate to weak VHF scintillations but no significant FPF's and UHF scintillations. The occurrence of FPF's is mostly confined to premidnight hours and reduces significantly with respect to the amplitude scintillation occurrence in postmidnight hours. The results are assessed in terms of the strength of irregularities and the longitudinal behavior of global equatorial scintillation activities.

Global morphology of ionospheric scintillations

Abstract: Starting with post World War II studies of fading of radio star sources and continuing with fading of satellite signals of Sputnik, vast quantities of data have built up on the effect of ionospheric irregularities on signals from beyond the F layer. The review attempts to organize the available amplitude and phase scintillation data into equatorial, middle-, and high-latitude morphologies. The effect of magnetic activity, solar sunspot cycle, and time of day is shown for each of these three latitudinal sectors. The effect of the very high levels of solar flux during the past sunspot maximum of 1979-1981 is stressed. During these years unusually high levels of scintillation were noted near the peak of the Appleton equatorial anomaly (∼ ±15° away from the magnetic equator) as well as over polar latitudes. New data on phase fluctuations are summarized for the auroral zone with its sheet-like irregularity structure. One model is now available which will yield amplitude and phase predictions for varying sites and solar conditions. Other models, more limited in their output and use, are also available. The models are outlined with their limitations and data bases noted. New advances in morphology and in understanding the physics of irregularity development in the equatorial and auroral regions have taken place. Questions and unknowns in morphology and in the physics of irregularity development remain. These include the origin of the seeding sources of equatorial irregularities, the physics of development of auroral irregularity patches, and the morphology of F-layer irregularities at middle latitudes.

Control of the seasonal and longitudinal occurrence of equatorial scintillations by the longitudinal gradient in integrated E region Pedersen conductivity

Abstract: An enigma of equatorial research has been the observed seasonal and longitudinal occurrence patterns of equatorial scintillations (and range-type spread F). We resolve this problem by showing that the seasonal maxima in scintillation activity coincide with the times of year when the solar terminator is most nearly aligned with the geomagnetic flux tubes. That is, occurrence of plasma density irregularities responsible for scintillations is most likely when the integrated E-region Pedersen conductivity is changing most rapidly. Hence the hitherto puzzling seasonal pattern of scintillation activity, at a given longitude, becomes a simple deterministic function of the magnetic declination and geographic latitude of the magnetic dip equator. This demonstrated relationship is consistent with equatorial irregularity generation by the collisional Rayleigh-Taylor instability and irregularity growth enhancement by the current convective and (wind-driven) gradient drift instabilities. Some discrepancies in this relationship, however, have been found in scintillation data obtained at lower radio frequencies (below, say, 300 MHz) that suggest the presence of other irregularity-influencing processes. The role of field-aligned currents, associated with the longitudinal gradient in integrated E-region Pedersen conductivity produced at the solar terminator, in equatorial irregularity generation via the current convective instability has not been discussed previously.

The longitudinal morphology of equatorial F-layer irregularities relevant to their occurrence

Abstract: Determining the morphology of F-layer irregularities as a function of longitude in the equatorial region is vital for understanding the physics of the development of these irregularities. We aim to lay the observational basis which then can be used to test theoretical models. Theoretical models have been developed, notably in the papers by Tsunoda (1985) and by T. Maruyama and N. Matuura (1984). The question is whether the models are consistent with the morphology as we see it. According to our criteria, the data used should be confined to observations taken near the magnetic equator during quiet magnetic periods and at times within a few hours after sunset. Anomaly region scintillation data have to be used in a limited manner for studying the generation mechanism. The questions to be answered by proposed mechanisms are (1) why do the equinox months have high levels of occurrence over all longitudes and (2) why are there relatively high levels of occurrence in the Central Pacific Sector in the July–August period and in the 0–75° West Sector in the November-December period and (3) why are there very low levels of occurrence in November and December in the Central Pacific Sector and in July and August in the 0–75° West Sector. In the paper by Maruyama and Matuura, the authors have taken observations of topside soundings of spread-F. With this data set in hand, they conclude: “During the northern winter periods, there is maximum enhancement at the Atlantic longitudes of large westward geomagnetic declination and during the northern summer at the Pacific longitude of large eastward declination”. Tsunoda's conclusions from his use of scintillation data is that “scintillation activity appears to maximize at times of the year when the suset nodes occur”. The emphasis of one paper is on the maximum enhancement during the solstices and in the other paper on variations from the equinox as determined by latitude and declination. Each stresses certain characteristics of the morphology. While the two papers explain relatively different morphologies, each makes contributions. However there remain problems to be resolved before certifying a solution as to the physics explaining the longitudinal pattern of F-region irregularities. Satellitein-situ data, scintillation and spread-F observations will be reviewed. The limitation of each data set will be outlined particularly as relevant to the bias produced by the existence of thin versus extended layers of irregularities. A cartoon as to the occurrence pattern, as we see it, as a function of longitude will be shown.

DMSP observations of equatorial plasma bubbles in the topside ionosphere near solar maximum

Abstract: The Defense Meteorological Satellite Program (DMSP) flights F9 and F10 crossed postsunset local time sectors approximately 14 times per day in Sun-synchronous orbits at an altitude of ∼840 km. We have examined a large database of postsunset plasma density measurements acquired during ∼ 15,000 equatorial crossings made by DMSP F9 in 1989 and 1991 and DMSP F10 in 1991. On 2086 of these crossings equatorial plasma bubbles (EPBs) were observed as intervals of depleted and irregular plasma densities. We have analyzed these EPB events to determine their distributions with season, longitude (S/L), and levels of geomagnetic activity. The global S/L distributions of EPBs observed by the DMSP satellites are shown to be in general agreement with results from discrete ground-based measurements. That is, the seasonal variations detected at 840 km in longitude bins hosting radar/scintillation observatories appear similar to those reported from the ground. Over the Atlantic sector where EPBs occur frequently, we found good agreement with predictions of a simple model proposed by Tsunoda [1985]. In the Pacific sector the frequency of EPB occurrence is considerably lower, and poor counting statistics preclude confident predictions regarding the absolute value of seasonal variations. We suggest that relatively large equatorial magnetic fields at Flayer altitudes in the Pacific (∼0.34 G) sector more strongly inhibit the growth of the Rayleigh-Taylor instability than at Atlantic (∼0.25 G) longitudes. Contrary to general belief, we found that EPBs occurred regularly during geomagnetic storms, especially in the initial and main phases. EPB activity appears to have been suppressed from many hours to clays during storm recovery phases.