Simultaneous observations of ESF irregularities over Indian region using radar and GPS

Abstract

Abstract. In this paper, we present simultaneous observations of temporal and spatial variability of total electron content (TEC) and GPS amplitude scintillations on L1 frequency (1.575 GHz) during the time of equatorial spread F (ESF) while the MST radar (53 MHz) located at Gadanki (13.5 • N, 79.2 • E, Dip latitude 6.3 • N), a low latitude station, made simultaneous observations. In particular, the latitudinal and longitudinal extent of TEC and L-band scintillations was studied in the Indian region for different types of ESF structures observed using the MST radar during the low solar activity period of 2004 and 2005. Simultaneous radar and GPS observations during severe ESF events in the pre-midnight hour reveal that significant GPS L band scintillations, depletions in TEC, and the double derivative of the TEC index (DROTI), which is a measure of fluctuations in TEC, obtained at low latitudes coincide with the appearance of radar echoes at Gadanki. As expected, when the irregularities reach higher altitudes as seen in the radar map during pre-midnight periods, strong scintillations on an L-band signal are observed at higher latitudes. Conversely, when radar echoes are confined to only lower altitudes, weak scintillations are found and their latitudinal extent is small. During magnetically quiet periods, we have recorded plume type radar echoes during a post-midnight period that is devoid of L-band scintillations. Using spectral slopes and crosscorrelation index of the VHF scintillation observations, we suggest that these irregularities could be "dead" or "fossil" bubbles which are just drifting in from west. This scenario is consistent with the observations where suppression of prereversal enhancement (PRE) in the eastward electric field is indicated by ionosonde observations of the height of equatorial F layer and also occurrence of low spectral width in the radar observations relative to pre-midnight period. However, absence of L-band scintillations during post-midnight Correspondence to: S. Sripathi (ssripathi@rediffmail.com) event, when radar observed plume like structures and scintillations were recorded on VHF band, raises questions about the process of evolution of the irregularities. A possible explanation is that whereas small scale (∼3 m) irregularities are generated through secondary waves that grow on the walls of km scale size irregularities, in this case evolution of the Rayleigh-Taylor instability itself did not extend to irregularities of scale sizes of a few hundred meters that produce scintillation on a L-band signal.

A study on chaotic behaviour of equatorial/low latitude ionosphere over Indian subcontinent, using GPS-TEC time series

“…Since these plasma irregularities are highly field aligned, they are mapped also to higher latitudes as irregularities evolve non-linearly into the topside ionosphere (Whalen, 2002). Also, several observations suggest that L band scintillations are found to be strong at the equatorward edge of the EIA, where the Rayleigh-Taylor instability may give rise to a few hundred meter scale size irregularities (Rama Rao et al, 2006a;Sripathi et al, 2008).…”

The comparative study of chaoticity and dynamical complexity of the low-latitude ionosphere, over Nigeria, during quiet and disturbed days

“…These effects can also be seen more clearly in the Tsallis entropy values for the five window period for quiet days of January 2011, because the night-time value is higher and also shows a much higher series of fluctuations during this period compared with other periods. As mentioned in Unnikrishnan and Ravindran (2010), the irregular changes in the dynamical characteristics of TEC from the results of the Lyapunov exponent and Tsallis entropy may also be due to the collisional Raleigh-Taylor instability that may give rise to a few large irregularities in L band measurements (Rama Rao et al, 2006;Sripathi et al, 2008). All these can be seen as internal factors responsible for variations in the dynamical response of TEC, as recorded from the values of the Lyapunov exponents and Tsallis entropy completed for days without storms, which might be quiet or disturbed according to classification and which could also account for higher values of these qualifiers during disturbed days compared with quiet days.…”

Ionospheric scintillations at Guilin detected by GPS ground-based and radio occultation observations

“…Thus these higher-altitude effects at the magnetic equator lead to the scintillation activities of satellite signals in the equatorial anomaly region. Sripathi et al (2008) studied ESF irregularities over Indian region using the observations of MST radar (53 MHz) and GPS amplitude scintillations on L1 frequency. The MST radar is located at the low-latitude station Gadanki (13.5°N, 79.2°E, magnetic dip 6.3°N), and the used 18 GPS receivers are installed over India covering the equatorial and low-latitude regions.…”

“…It is known that ionospheric scintillations are produced when the signals travel through a turbulent medium, where changes in the refractive index produce scattering. Sripathi et al (2008) pointed out that if the irregularities are confined to a layer of thickness less than 100 km, when the radio waves emerge from the irregularity layer, only a phase perturbation is imposed due to the irregularities. Further propagation of the radio waves to the plane of the receiver produces amplitude fluctuations in addition to phase fluctuations.…”