Abstract
Abstract Amplitude scintillations recorded at 1.5 GHz frequency during the high (1998–1999) and low (2004–2005) sunspot activity periods over a low latitude station, Waltair (17.7°N, 83.3°E) revealed that the L-band scintillations mostly occur during the post-sunset to midnight hours peaking around 21:00 hr local time with maximum occurrence during equinoxes, moderate during winter and minimum during the summer months. The occurrence, as well as the intensity of scintillations, is found to be strongly dependant on both the season of the year and the sunspot number. Strong (S4-index >0.45) and fast fading scintillations (fading rates >40 fads/min) observed during the post-sunset hours of equinoxes and winter months manifest as several short duration patches at both VHF (244 MHz) and L-band (1.5 GHz) frequencies and are found to be always associated with the range or total Spread-F on ionograms and bubbles/depletions in the Total Electron Content (TEC) measured from a colocated dual frequency GPS receiver, suggesting that these scintillations are of the Plasma Bubble Induced (PBI) type. On the other hand, relatively weak and slow fading scintillations (fading rates <8 fads/min) observed around the post-midnight hours of the summer months which appear as long-duration patches (>3 hr) at 244 MHz signal (with practically no scintillation activity at the L-band frequencies) are often found to be associated with frequency Spread-F on ionograms with no depletions in TEC. Further, the presence of Fresnel oscillations observed in the spectrum of 244 MHz suggests that the long-duration scintillations observed are due to the presence of a thin layer of irregularities in the bottom side F-region which are generally known as Bottom Side Sinusoidal (BSS) irregularities. Further, the PBI-type scintillations at L-band frequencies are often found to exceed 10 dB power levels (S4 > 0.45) even during the low sunspot activity period of 2004–2005, and cause Loss of Lock in the GPS receivers resulting in a total interruption in the received signals.
Highlights
Plasma density irregularities in the night time equatorial F-region which affect the radio waves by scattering and diffraction causing scintillations on trans-ionospheric signals, Spread-F on ionograms, plume-like structures in HF/VHF radar maps, and intensity bite-outs in airglow images, are generally known as equatorial Spread-F (ESF) irregularities
Strong (S4-index >0.45) and fast fading scintillations observed during the post-sunset hours of equinoxes and winter months manifest as several short duration patches at both VHF (244 MHz) and L-band (1.5 GHz) frequencies and are found to be always associated with the range or total Spread-F on ionograms and bubbles/depletions in the Total Electron Content (TEC) measured from a colocated dual frequency Global Positioning Systems (GPS) receiver, suggesting that these scintillations are of the Plasma Bubble Induced (PBI) type
The percentage occurrence of scintillations is higher in March 2004 (55%) where the mean sunspot number Rz is 49.1, than in March 2005 (45%) where the mean sunspot number, Rz reduced to 24.8 evidencing the decrease in scintillation occurrence with the decreasing sunspot number
Summary
Plasma density irregularities in the night time equatorial F-region which affect the radio waves by scattering and diffraction causing scintillations on trans-ionospheric signals, Spread-F on ionograms, plume-like structures in HF/VHF radar maps, and intensity bite-outs in airglow images, are generally known as equatorial Spread-F (ESF) irregularities. The occurrence of scintillations is highly variable from day-to-day and is controlled by local time, season, solar cycle, latitude, longitude, and geomagnetic activity. Even after several studies on the general morphological features of scintillations (Aarons, 1982) their day-to-day variability is still one of the challenging problems for predicting iono-. With the increasing applications of Global Positioning Systems (GPS) and satellite-based communication and navigational systems, especially when the millimeter range precision approach is required, as in Satellite Based Augmentations Systems (such as WAAS in the United States and GAGAN in India), the precise occurrence characteristics and prediction of the intensity of scintillations and their effects on L-band frequencies are very much needed for a better understanding of the space-weather relating to the satellite-based communications
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