Shivalika Sarkar

Ionospheric irregularities at Antarctic using GPS measurements

The purpose of this work is to study the behaviour of the ionospheric scintillation at high latitude during geomagnetically quiet and disturbed conditions which is one of the most relevant themes in the space weather studies. Scintillation is a major problem in navigation application using GPS and in satellite communication at high latitudes. Severe amplitude fading and strong scintillation affect the reliability of GPS navigational system and satellite communication. To study the effects of the ionospheric scintillations, GPS receiver installed at Antarctic station Maitri (Geog. 70.76°S; 11.74°E) was used. The data is collected by using GISTM 4004A, NOVATEL’S GPS receiver during March 2008. Studies show that percentage occurrence of phase scintillation is well correlated with geomagnetic activity during the observation period. The result also shows that very intense scintillations can degrade GPS based location determination due to loss of lock of satellites. These findings indicate that the dependence of scintillations and irregularity occurrence on geomagnetic activity is associated with the magnetic local time (MLT). Large number of patches are reported and their activity depends on the magnetic activity index.

Analysis of space-and ground-based parameters prior to an earthquake on 12 December 2009

The present study includes observations of anomalies at ionospheric and ground levels using data from global positioning system (GPS)-based total electron content (TEC), ionospheric electron density by Detection of Electromagnetic Emissions Transmitted from Earthquake Regions (DEMETER), and ground-based ultra-low-frequency (ULF) emission prior to an earthquake that occurred on 12 December 2009. The analysis of the data from these three different techniques show relations with preseismic activities. The GPS-based TEC showed enhancement three days prior to the day of the earthquake. On converting the satellite azimuth into latitude and longitude, a large number of passes giving anomalous TEC values were observed covering a zone 17–23° N and 70–74° E (i.e. around the epicentre). Electron density and temperature as measured by DEMETER also show enhancement of the tracks around the epicentre one day prior to the earthquake. For ground-based ULF emission, we used different analysis techniques for the detection of abnormal changes in ULF emission before the earthquake. We observed changes in different parameters such as polarization ratio and fractal dimension. Clear enhancement is observed for polarization parameters (Z/X, Z/Y) and fractal analysis (for the Z component). The results provide useful information in acquiring data prior to earthquakes.

Analysis of GPS-based TEC and electron density by the DEMETER satellite before the Sumatra earthquake on 30 September 2009

An analysis of ionospheric total electron content (TEC) variations obtained using Global Positioning System (GPS) measurements and electron density observations by the Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER)/ISL before the Sumatra earthquake (magnitude 7.6) on 30 September 2009 is presented. The data used are global TEC maps created in the IONEX format to estimate the spatial scale of ionospheric irregularities associated with this earthquake and DEMETER/ISL observations to analyse plasma parameter changes during earthquakes. This study for the first time integrates ground-based GPS receiver and space-based DEMETER/ISL observations to probe seismo-ionospheric anomalies before a large earthquake. The pre-seismic behaviour of the TEC was detected within a few days before the main event. The anomaly appeared as local TEC enhancement in the vicinity of the forthcoming earthquake epicentre. These structures were generated in the ionosphere at a similar local time at 1, 3 and 5 days prior to the main shock. The equatorial anomaly trough moved northwards (5°) on the earthquake day and 7.5° southwards the next day. The results demonstrate that both enhanced and reduced anomalies in the ionospheric electron density appeared before the Sumatra earthquake. The results from International GNSS Service (IGS) GPS data demonstrate that TEC variations can be used to detect seismo-ionospheric signatures.

Plasma instabilities in the ionosphere at the crest of anomaly region

Comparison of the in situ density fluctuations measured by the DEMETER satellite with ground based GPS receiver measurements at the equatorial anomaly station Bhopal [geographic coordinates (23.2°N, 77.6°E); geomagnetic coordinates (14.29° N, 151.12°E)] for the low solar activity year, 2005, are presented in this paper. The Langmuir Probe experiment and Plasma Analyzer onboard DEMETER measure the electron and ion densities respectively. It is interesting to note that in situ density fluctuations observed on magnetic flux tubes that pass over Bhopal can be used as indicator of ionospheric scintillations at that site. Many cases of density fluctuations and associated scintillations have been observed during descending low solar activity period.

Study of GPS ionospheric scintillations over equatorial anomaly station Bhopal during low solar activity period

Summary form only given. The paper deals with the statistical analysis of GPS L-band scintillations at the equatorial anomaly station Bhopal (geographic coordinates: 23.2°N, 77.6°E; geomagnetic coordinates 14.29°N, 151.12°E) for the low solar activity period. During the period of observation (January 2005 to December 2006), the variation of scintillations with local time, season and magnetic activity are reported. The occurrence of scintillation is found to be maximum in equinox months and minimum in summer months. We got different levels of scintillation in some months, but found that the weak scintillation ((0.2<S 4 ≤0.4) is dominating throughout the period. The suppression or enhancement of pre-midnight scintillations during magnetic disturbed and quiet periods is found to be a seasonal and local time dependent factor. Pre-midnight and post midnight occurrence of scintillation is also reported in the paper. Pre-midnight scintillation was found to be maximum in equinox whereas it is minimum in winter months. Importance of the work lies in the evaluation of ionospheric irregularities at equatorial latitudes during the low solar activity period.