Geostationary L-band signal scintillation observations near the crest of equatorial anomaly in the Indian zone

Abstract

Microwave L-band signal at 1.5 GHz from geostationary satellite INMARSAT (64°E) has been recorded on a continuous basis since 1990 at Calcutta (22.58°N, 88.38°E geographic; 32°N magnetic dip). The location of Calcutta, virtually under the northern crest of the equatorial anomaly in the Indian longitude sector, provides an excellent platform for studying equatorial scintillations. This paper presents a long-term study of scintillations at 1.5 GHz observed from Calcutta extending from minimum to high sunspot number (1996–2000). The variations of scintillations with local time, season, solar and magnetic activity are presented. L-band scintillations have been found to be essentially a pre-midnight phenomenon during the equinoctial months of high sunspot number years. Both the occurrence and intensity increase dramatically with solar activity level. During the solar maximum phase of 1998–2000, scintillations in excess of 20 dB occurred for 20% of total time in the pre-midnight hours of March 2000. Considering individual hours, a maximum percentage occurrence of 37 was recorded in the interval 19–20 LT. Although the percentage occurrence appears to be low, scintillations with Scintillation Index (SI)>20 dB were recorded on 23 out of 31 nights during the same month, indicating that scintillations do not occur continuously over a long interval of time, but in small patches with clear gaps in between. The distribution of patch duration during years 1998–2000 has a maximum in the time interval of 1–3 min. From the cumulative distributions of patch duration with SI⩾10 and 20 dB, the 99th percentile values of patch duration have been found to be 49.3 and 39.6 min, respectively. This information may be used to obtain an estimate of longitudinal separation of geostationary satellites to be used in Satellite Based Augmentation System (SBAS). During the equinoctial months of August–October and February–April, occurrence of scintillations is less during magnetically active (Kp=3+–9) conditions than during quiet (Kp=0−3). In the local summer months of May–July, the probability of occurrence of scintillations is more under magnetically active condition than in quiet condition. Equatorial scintillation is essentially controlled by the pre-reversal enhancement of the eastward electric field near sunset, which in turn has season and solar activity dependences. During magnetic activity, the generation of equatorial irregularities resulting in scintillations may be ascribed to the prompt penetration of magnetospheric electric field and disturbance dynamo effects.