Minakshi Devi

Spatial and time variations in critical frequencies of the ionospheric F region above the zone of equatorial earthquake preparation

Spatial and time variations in the critical frequencies foF2 before the strong earthquake of August 15, 1963, with the magnitude M = 7.75 are analyzed. The epicenter of the earthquake was located in the vicinity of the magnetic equator in the American longitudinal sector. The data of the topside ionosonde on board Alouette-1 and of the series of ground-based ionosondes has been used for this purpose. The ground-based ionosondes made it possible to detect an insignificant anomalous decrease in foF2 within the zone of earthquake preparation a few days prior to the earthquake. This result confirms the conclusion drawn earlier on the basis of the satellite data. The modification of the ionosphere at the F-region level is more evident in the satellite than in the ground-based data. It is also noted that the character of the time variations in foF2 a day before the earthquake is similar to the so called “quiet time” Q-disturbances in the ionosphere, when the electron concentration at the F region maximum differs from the median values by more than 20% under undisturbed geophysical conditions.

Anomalous total electron content (TEC) and atmospheric refractivity prior to the very strong China earthquake of May 2008

A dual-frequency global positioning system (GPS) receiving set-up at Guwahati (26° 10′ N, 91° 45′ E), has been in operation for the last year and a half, providing total electron content (TEC) data as input for understanding pre-earthquake contributions to low-latitude atmospheric dynamics. The major China earthquake of 12 May 2008, with magnitude 8.0 and an epicentre at 31° 24′ N, 103° 58′ E is a rare event to facilitate extracting earthquake features on the TEC data, and hence low-latitude system perturbations. This paper begins with a brief discussion on the methods adopted in identifying TEC performance before an impending earthquake from ionospheric data, and presents results of analysis of the event of 12 May. TEC magnitudes recorded with latitude / longitude and elevation of satellites for every pass are linked with pre-earthquake TEC features and are used as inputs to identify epicentre position. The role of seismic-time refractive index variations is examined to explain the observed TEC characteristics.

Identification of quiet and disturbed days through IEC profile features over anomaly crest region

Abstract In this paper, the ionospheric electron content (IEC) profile features, have been utilized for extraction of a few parameters and the parameters are used for assertion of a certain day as quiet (Q) or disturbed (D). The IEC values taken for the study cover two solar activity periods (high and low) and are based on VHF RB data collected over Guwahati (26.2°N, 91.75°E). The paper describes methods for extraction of parameters like profile factor, P and anomaly factor, PEA from IEC profiles separately for three seasons (summer, winter and equinox). The definitions of Q and D days are made through profile features and the threshold values of ΣK p for each season are evaluated. The relations between ΣK p and P factor, ΣK p and PEA are established after corrections for solar activity condition. The prediction and assertion of Q/D days are then made by examining IEC profiles for the cases where IEC data were not used for the parameter extraction.

Measured and computed ionospheric electron content in the equatorial anomaly crest region

Abstract The ionospheric electron content (IEC) measured using the VHF Radio Beacon (RB) technique from a geostationary satellite at Guwahati (geographic latitude 26.2 °N, geographic longitude 91.7 °E, geomagnetic latitude 15.2 °N, dip angle 37 °N, magnetic latitude 20.64 °N), during low and high solar activity periods is analysed to examine the influence of solar activity on the IEC near the equatorial ionospheric anomaly crest region, and to assess the predictability of existing ionospheric models such as the International Reference Ionosphere (IRI) and the Semi empirical Low latitude Ionosphere Model (SLIM) on the IEC for different solar activity conditions. The paper shows that the IEC figures derived from models vary widely with solar activity and neither of the models predicts the IEC (within its maximum and minimum deviations) at this station for the entire day. The attempt here is to explain the discrepancy observed between the measured IEC and that derived through the IRI or SLIM models, with the help of the Sheffield University Plasmasphere Ionosphere Model (SUPIM) for a selected solar activity condition. Model IEC values computed from the IRI over a few other anomaly crest stations are also compared; the paper highlights the differences in the observed and model-predicted patterns between 20 ° and 25 °N magnetic latitude.

Modifications of the ionosphere prior to large earthquakes: report from the Ionosphere Precursor Study Group

The current status of ionospheric precursor studies associated with large earthquakes (EQ) is summarized in this report. It is a joint endeavor of the “Ionosphere Precursor Study Task Group,” which was formed with the support of the Mitsubishi Foundation in 2014–2015. The group promotes the study of ionosphere precursors (IP) to EQs and aims to prepare for a future EQ dedicated satellite constellation, which is essential to obtain the global morphology of IPs and hence demonstrate whether the ionosphere can be used for short-term EQ predictions. Following a review of the recent IP studies, the problems and specific research areas that emerged from the one-year project are described. Planned or launched satellite missions dedicated (or suitable) for EQ studies are also mentioned.

DIAL, a system for atmospheric methane measurement over Guwahati: development and results

The article discusses the development and operational details of Differential Absorption LiDAR (DIAL) for the measurement of methane concentration in the semi-urban environment of Gauhati University. The system comprises two specifically selected wavelengths in 3 μm range: one is an absorbing wavelength (λon) and the other is non-absorbed (λoff) by methane molecules. Pulses of 10 ns for the two wavelengths are transmitted alternately for interleaved sampling of differential absorption. The pulse repetition rate is variable between 1 and 20 Hz. The slope and integrated target approaches are adopted to calculate the methane concentration, and observed figures are compared with globally reported values.

Model prediction of cyclonic tracks over Bay of Bengal and resultant precipitation in the north-east region

The Varsha, a spectral hydrostatic general circulation model, is run regularly at an eight-processor Flosolver machine at Gauhati University (26°10′ N, 91°45′ E), with the aims to predicting track of cyclones generated in the Bay of Bengal and forecasting precipitation occurrence and intensity over the north-eastern part (NE) of India, induced by such events. The initial conditions of the model are prepared from FNL dataset of National Centre for Environmental Prediction (NCEP), available at 1°×1° resolutions. The predicted tracks of a few cyclonic events as SIDR (Nov, 2007), Aila (May, 2009) and Laila (May, 2010) are presented along with their contribution to precipitation in the NE. Each prediction, when assessed through observations obtained from satellite-based measurements of Tropical Rainfall Measuring Mission (TRMM) and National Oceanic and Atmospheric Administration (NOAA), and also from India Meteorological Department (IMD), shows that the model-generated zonal wind is a good precursor parameter in track determination of a cyclone. The reliability of the model-projected precipitation features over the NE, contributed by such storms, is also highlighted.

Study of Physical and Dynamical Processes in the Ionosphere at Equatorial Anomaly Crest Region during Magnetic Storm for High and Low Solar Activity Period

The ionosphere represents one of the most important error sources that affecting GPS positioning. It is strongly related to solar activity and geomagnetic field. The GPS satellites and dual frequency receivers can be used to measure the Total Electron Content (TEC) of the Earth’s ionosphere. In this paper, the total electron content (TEC) profile features, have been utilized for extraction of parameters like profile factor, P and anomaly factor, PEA separately for three seasons (summer, winter and equinox) and the parameters are used for assertion of a certain day as quiet (Q) or disturbed (D). The ionospheric Total electron content (TEC) is used to examine the influence of solar activity by taking advantage of the dispersive nature of the ionospheric medium. The definitions of Q and D days are made through profile features and the threshold values of ∑Kp for each season are evaluated. The relations between Disturbed storm time (Dst) and P factor, ∑Kp and P factor are established after corrections for solar activity condition. The prediction and assertion of Q/D days are then made by examining IEC profiles for the cases where IEC data were not used for the parameter extraction. A few case studies using these methods are also presented in these papers. Work is based on TEC/IEC data collected from VHF Radio Beacon (RB) technique from geostationary satellite and GPS receiver at Guwahati (26.20N, 91.750E), an equatorial anomaly crest station.

MEASURED AND COMPUTED TOTAL ELECTRON CONTENT OVER GUWAHATI: OBSERVED THROUGH SATELLITE AND MODEL COMPUTATION.

Bornali Chetia 1* , Santanu Kalita 2 , Minakshi Devi 1 , Kamla Prsan Ray 3 and Ananda Kumar Barbara 1 . 1. Department of Physics, Gauhati University, Guwahati 781014, Assam, India 2. Department of Computer Application, Mahapurusha Srimanta Sankaradeva Viswavidyalaya, Nagaon 782001, India 3. Society for Applied Microwave Electronics Engineering & Research (SAMEER) (DIT, MC&IT, Govt. of India),IIT Campus, Hill Side, Powai, Mumbai ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History

Study of spread-F irregularities, a comparison with beacon satellite results

Abstract Ground-based ionosonde data as well as phase and amplitude records from geostationary satellites ATS 6 and ETS II were taken at Gauhati (26°10′N, 91°45′E; geom. lat. 15°N, dip 37°). The ionospheric data were analysed for spread- F and its association with scintillations in radio beacon data has been examined through a period which covers a complete solar cycle. It is seen that this station does not show many characteristic features of low latitude stations. Postmidnight frequency spread is predominant. During geomagnetic storms dawn-time range spread is seen. The spread- F bears a positive correlation with K p . Frequency spread is associated with amplitude scintillations, suggesting thereby that irregularity structures are thick. During disturbed conditions, range spread associated with scintillations and rapid build-up of morning-time integrated electron content (IEC) have been observed. Here, an attempt has been made to offer an explanation in terms of precipitation of charged particles.