D. S. V. V. D. Prasad

Morphological and spectral characteristics of L-band and VHF scintillations and their impact on trans-ionospheric communications

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.

Day-to-day variability of Equatorial Electrojet and its role on the day-to-day characteristics of the Equatorial Ionization Anomaly over the Indian and Brazilian sectors†

The equatorial electrojet (EEJ) is a narrow band of current flowing eastward at the ionospheric E region altitudes along the dayside dip equator. Mutually perpendicular electric and magnetic fields over the equator results in the formation of equatorial ionization anomaly (EIA), which in turn generates large electron density variabilities. Simultaneous study on the characteristics of EEJ and EIA is necessary to understand the role of EEJ on the EIA variabilities. This is helpful for the improved estimation of total electron content (TEC) and range delays required for satellite-based communication and navigation applications. Present study reports simultaneous variations of EEJ and GPS-TEC over Indian and Brazilian sectors to understand the role of EEJ on the day-to-day characteristics of the EIA. Magnetometer measurements during the low solar activity year 2004 are used to derive the EEJ values over the two different sectors. The characteristics of EIA are studied using two different chains of GPS receivers along the common meridian of 77°E (India) and 45°W (Brazil). The diurnal, seasonal, and day-to-day variations of EEJ and TEC are described simultaneously. Variations of EIA during different seasons are presented along with the variations of the EEJ in the two hemispheres. The role of EEJ variations on the characteristic features of the EIA such as the strength and temporal extent of the EIA crest has also been reported. Further, the time delay between the occurrences of the day maximum EEJ and the well-developed EIA is studied and corresponding results are presented in this paper.

Multistation VHF scintillation studies at low latitudes

Abstract Amplitude scintillations data at 244 MHz (FLEETSAT, 73 E) recorded from a network of three stations at Waltair (177°N, 83.3°E), Nuzvid (16.8°N, 80.8°E) and Payyanur (12°N, 75.8°E) from August 1991 to April 1993 are used to study the characteristics of ionospheric irregularities at low and near-equatorial latitudes in the Indian sector. The occurrence and patch durations are found to be higher at the near-equatorial station Payyanur, when compared to the occurrence at the other two low latitude stations. Equinoctial maxima are seen at all three stations followed by secondary maxima in winter. However during summer, significant scintillations were present at Payyanur, while the occurrence is much reduced at the other two low latitude stations. Seintillations are also inhibited during magnetically disturbed days. The scintillation patches recorded at the two nearby low latitude stations (Waltair and Nuzvid) show similarities during post-sunset hours, while their characteristics are found to differ after midnight. From a comparison of similar patches simultaneously recorded at the above two nearby stations, the velocity components of the drift of ionospheric irregularities are found to show predominent eastward movements with velocities varying between 80 and 280 ms −1 . The occurretice of daytime scintillations is found to be higher in summer months.

Geomagnetic activity control on VHF scintillations over an Indian low latitude station, Waltair (17:7 - N;83:3 - E;20 - N dip)

Using the data of amplitude scintillations recorded at 244 MHz from the geostationary satellite, FLEETSAT (73‡E) at a low latitude station, Waltair (17.7‡N, 83.3‡E, 20‡N dip), during the increasing sunspot activity period of 1997–2000, the effect of the geomagnetic storms on the occurrence of ionospheric scintillations has been studied. A total of 60 SC storms studied during this period, following the Aarons’ criterion, reveals that the local time of onset of the recovery phase of the geomagnetic storms play an important role in the generation or inhibition of the ionospheric irregularities. Out of the 60 storms studied, nearly 60 to 70% satisfied the categories I, II and III of Aarons’ criteria. However, in the remaining 30 to 40% of the cases, no consistent results were observed. Thus, there is a necessity for further investigation of the effect of geomagnetic storms on ionospheric irregularities, particularly with reference to the altitude variations of the F-layer (h’F) relating to the changes in the local electric fields.

Latitudinal variation in the occurrence of GPS L-band scintillations associated with the day-to-day changes in TEC, h′F and the E×B drift velocity and their impact on GPS satellite signals

The present study describes the day-to-day variations in the occurrence of GPS L-band scintillations from equator to the anomaly crest location associated with the changes in TEC, h′

Seasonal variation in ionospheric electron content and irregularities over Waltair — A comparison with SLIM model

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Nighttime Enhancements in IEC at Low Latitudes — A Study with Reference to F Region Dynamics

F and E ×B drift velocities. The GPS–TEC and S4 index data from an equatorial station, Trivandrum (8.47∘N, 76.91∘E), a low latitude station, Waltair (17.7∘N, 83.3∘E) and an anomaly crest location Kolkata (22.6∘N, 88.4∘E) during the low solar activity years of 2004 and 2005 are used. It is observed that the day-time ambient TEC is higher during scintillation days compared to that during the days on which there are no scintillations at the three different locations mentioned above. Further, the diurnal variation of TEC shows a rapid decay during 1700–2000 hr LT over the three different locations during scintillation days which is observed to be comparatively much less during no scintillation days. The average height of the F-layer in the post-sunset hours over Trivandrum is found to be higher, around 350 km during scintillation days while it is around 260 km during the days on which there is no scintillation activity. The average pre-reversal E ×B drift velocity observed around 19:00 hr LT is higher (20 m/s) during scintillation days, whereas during no scintillation days, it is found to be much less (7 m/s). Further, it is observed that the GPS receivers lose their locks whenever the S4 index exceeds 0.5 (>10 dB power level) and these loss of lock events are observed to be more around the anomaly crest location (Kolkata). It may be inferred from the present observations that the level of ambient ionization around noon-time, and a fast decay (collapse) of the ionization during afternoon hours followed by rapid increase in the height of the F-layer contributes significantly to the occurrence of scintillations. The present study further indicates that the S4 index at L-band frequencies increases with an increase in latitude maximizing around the crest of the equatorial ionization anomaly during the post-sunset hours resulting in more loss of lock events in the GPS receiver signals around the EIA crest region.

Satellite Beacon and Ionosonde Measurements of Ionospheric Irregularities over India During the Equinoctial Month of September 1991

Abstract Beacon satellite measurements made during 1984–1985 and 1989–1990 at a low latitude station Waltair (17.7°N, 83.3°E) are used to study the diurnal and seasonal variations in ionospheric electron content (IEC) and amplitude scintillations. In 1989–1990, under high solar activity, the daytime IEC shows winter anomaly in the peak values and local time differences in their occurrence. By night, enhancements in IEC and the occurrence of scintillations and spread-F were frequent, particularly at equinoxes. The seasonal variations of the meridional wind direction and phase computed from the SLIM model/1/ compare well with the daytime peak density anomaly and with nighttime IEC over Waltair. A probable effect of neutral winds on the occurrence of ionospheric irregularities is discussed.

Temporal and spatial variations in TEC using simultaneous measurements from the Indian GPS network of receivers during the low solar activity period of 2004–2005

The nighttime enhancement (NTE) in the ionospheric electron content (IEC) are observed over Waltair (17.7°N) and Calcutta (22°N), when the equatorial F region rises to higher altitudes during the post-sunset hours over Kodaikanal (10°N). No significant enhancement is seen at either of the stations if the altitude of the F region near the equatorial region is confined to lower altitudes. In addition to the altitude rise at the equatorial region, an earlier post-sunset reversal in the meridional neutral wind direction from poleward to equatorward also seems to be a requisite condition for the enhancements in IEC to occur at off-equatorial regions. The observed features are discussed in the light of the F region dynamics.

On the validity of the ionospheric pierce point (IPP) altitude of 350 km in the Indian equatorial and low-latitude sector

The spread-F and scintillation observations made at ten Indian stations covering 8 to 25°N geographic latitudes during the equinoctial month of September 1991 are used to study the occurrence characteristics of ionospheric irregularities and their association with the changes in the virtual height of the F layer at an equatorial station, Trivandrum (0.3° S dip). No event of scintillation occurrence is observed at off-equatorial latitudes (within the anomaly crest region) without their prior occurrence at the equatorial station (Tiruchendur, 2.3°N dip which is close to Trivandrum). However, the duration of scintillations at far equatorial stations (10 to 20°N dip) are, sometimes, longer than the durations at the near equatorial stations (0 to 10°N dip). On such occasions, the onset of scintillations and spread-F is prior to the post-sunset reversal of the height of the F layer (h′F) from upward to downward direction at the equatorial station, Trivandrum. But, if the duration of the irregularities is longer at equatorial latitudes and decreases towards higher latitudes, the onset of irregularities is observed to coincide with the time of post-sunset reversal in h′F at Trivandrum. These two types of irregularities are interpreted to be generated by the two different instability mechanisms, namely E × B drift and RT instability respectively.