An extensive VHF/UHF scintillation data base covering the frequency range of VHF to a few gigahertz has been utilized to determine the magnitudes of phase and intensity scintillations and their temporal/spatial structures during the sunspot maximum and minimum periods. The equatorial portion of the study has been based on geostationary satellite observations at Huancayo, a station on the magnetic equator, and at Ascension Island, which is an equatorial anomaly station having an extremely disturbed irregularity environment. The high‐latitude part of the study is based on quasistationary satellite measurements at a polar cap location (Thule) and two auroral locations (Goose Bay and Tromsø). The Tromsø observations are augmented with the Defense Nuclear Agency HiLat satellite beacon measurements during the solar minimum period. The data indicate a strong solar cycle control of scintillation activity at all locations, resulting in a drastic reduction of the magnitudes and occurrence of scintillations during the current solar minimum period. This pattern is consistent with both a reduction of F region ionization density and a reduction of irregularity generation in the solar minimum period. At the magnetic equator the magnitude of scintillations at 1.5 GHz seldom exceeds 3 dB with the percentage occurrence > 2 dB varying from 70% during high sunspot conditions to 30% during low sunspot conditions. At the crest of the equatorial anomaly, on the other hand, during the solar maximum in 1979, fades of 20 dB at 1.5 GHz are observed 30% of the time. At a decreased level of solar activity in 1982, a similar occurrence level is obtained at 1.5 GHz for fade levels of only 5 dB. During the solar minimum period, 1.5‐GHz scintillations are virtually absent. Phase scintillation measurements made at Ascension Island indicate that the median value of rms phase deviation is about 5 rad for detrend intervals of 100 s. In the auroral region, during the solar maximum period under magnetically disturbed conditions, the median values of scintillation fades and rms phase deviation (82‐s detrend) at 250 MHz are observed to be 15 dB and 3 rad, respectively. At Thule, located deep within the polar cap, the median values of scintillation fades and rms phase deviation at 250 MHz attain values as large as 20 dB and 4 rad during the sunspot maximum period. Unlike Ascension Island the scintillation activity at high‐latitude stations exhibits a threshold effect and does not decrease until 1983. However, in 1986 with sunspot numbers in the vicinity of 10, fade levels as low as 5 dB at 250 MHz are recorded in the polar cap and auroral stations only 5% of the time. It is noted that at auroral locations the most prominent feature, namely the existence of magnetic L shell‐aligned irregularity sheets, is equally evident at both sunspot maximum and minimum.

No abstract

Satellite in situ measurements made by the Dynamics Explorer 2 (DE 2) satellite were utilized to describe the nature of plasma structuring at high latitudes caused by the gradient drift instability process. Specifically, by using noon‐midnight and dawn‐dusk orbits of the DE 2 satellite it was found possible to study the simultaneous density and electric field spectra of convecting large‐scale (approximately hundreds of kilometers) plasma density enhancements in the polar cap known as “patches”) in directions parallel and perpendicular to their antisunward convection. Distinct differences were noted in the behavior of the ac and dc electric field structure and short‐scale (<125 m) density irregularities in these two mutually orthogonal directions perpendicular to the geomagnetic field. However, since these two orthogonal directions were not sampled simultaneously, the observed differences cannot be unequivocally related to the direction of convection. Structured plasma density enhancements in the auroral oval (known as “blobs”) were found to have considerable power spectral density at these short scales in the presence of significant Pedersen and Hall conductances in the 10‐ to 20‐mho range. While density irregularity amplitudes (ΔN/N)rms were found to be as large as 15–20% using 8‐s samples of the DE 2 data, the corresponding dc electric field fluctuation ΔE was found to be less than a few millivolts per meter for both patches and blobs. This (ΔN/N)RMS vis‐a‐vis ΔE behavior for the gradient drift process provided a fairly dramatic contrast with velocity shear driven processes where the ΔE magnitudes were found to be at least an order of magnitude larger for the same levels of density irregularities. The electric field spectra for the moderate shear category discussed by Basu et al. (1988a) were also found to have a significantly different spectral index as compared to such spectra associated with the gradient drift process. The results of this paper together with those of Basu et al. (1988a) provide fairly conclusive evidence for the existence of at least two generic classes of instabilities operating in the high‐latitude ionosphere: one driven by large‐scale density gradients in a homogeneous convection field with respect to the neutrals and the other driven by the structured convection field itself in an ambient ionosphere where density fluctuations are ubiquitous.

An equatorial campaign was conducted during September 25 to October 7, 1994, to investigate the neutral and plasma dynamics in the equatorial ionosphere after sunset in relation to the day‐to‐day variability of the occurrence of equatorial spread F (ESF). The campaign was organized under the auspices of National Science Foundation's Multi‐Instrumented Studies of the Equatorial Thermosphere Aeronomy program (MISETA), which included the Jicamarca radar, spaced‐antenna satellite scintillation, digisonde, all‐sky imager, and Fabry‐Perot interferometer (FPI) measurements near the magnetic equator in Peru. During a part of the period September 27 to October 3, the Geophysics Directorate of Phillips Laboratory performed measurements away from the magnetic equator at Aguaverde, Chile (magnetic latitude: 11°S) located 800 km to the east of the Jicamarca meridian using geostationary and GPS satellite scintillation, digisonde and all‐sky imager systems. The incoherent scatter radar results indicate that the postsunset enhancement of upward plasma drift, even though of the order of only 20 m s−1 during the solar minimum period, is a necessary condition for the generation of ESF. In view of the extreme difficulty of determining the neutral wind speed during the early evening hours by the FPI due to low airglow intensity, it was not possible to unequivocally associate the observed postsunset enhancements with strong eastward neutral winds. However, considering a few observations contiguous to the campaign period, it appears that such a causal relationship may exist. The scintillation drift measurements in Peru and Chile indicated that the zonal irregularity drift was smaller away from the magnetic equator, implying a variation of neutral wind with latitude. This is reproduced in the altitude variation of zonal drift observed by the Jicamarca radar. During a magnetic storm, scintillation measurements indicated that eastward drifts near the magnetic equator are accompanied by westward drifts near the anomaly peak, which is consistent with the effects of a disturbance dynamo. The campaign results indicate that in order to resolve the variability of ESF, a careful probing of neutral dynamics as a function of latitude needs to be undertaken during the postsunset period.

Abstract. In this paper we present a study of the ionospheric effects of a halo coronal mass ejection (CME) initiated on the Sun on September 20, 1999, and causing the largest magnetic storm during this month on September 22-23, 1999, with the hourly Dst index being -167 nT at -2400 UT on September 22. The recurrent CME on October 18 caused an even larger magnetic storm on October 22, 1999, with Dst of -231 nT at -0700 UT. The ionospheric effects of these two major magnetic storms are studied through their effects on a prototype of a Global Positioning System (GPS) -