Abstract :

The Total Electron Content (TEC) depletion near the equatorial ionization anomaly crest region in the Indian ionospheric sector has been analyzed during the year March 2005 to February 2006, and May 2016 to August 2017, low solar activity periods of 23rd and 24th solar cycle respectively. TEC data from March 2005 to February 2006 is recorded by GPS Ionospheric Scintillation and TEC Monitor (GISTM) GSV4004A and data from May 2016 to August 2017 is obtained from a multi-frequency multi-constellation Global Navigation Satellite System (GNSS) receiver installed at Bhopal (23.2° N, 77.4°E & MLAT 14.8°N). TEC depletions have long been recognized as one of the most prominent concerns of space weather as they produce intense scintillation in trans-ionospheric satellite radio signals. All the TEC depletion in the present study occurred in pre-midnight hours and they are more frequent in equinox months during both solar cycles. Among all the events, the severe depletion in TEC was observed on magnetically disturbed day i.e. on 25 October 2016 which may be due to the occurrence of plasma bubble. From ~2 to ~43 TECU and ~5 to ~20 TECU depletions has been detected for the satellite passes along the line of sight of the signal for low solar activity period of 24th and 23rd solar cycle respectively. TEC depletions also show good correspondence with the occurrence of scintillation patches. Rate of change of TEC Index (ROTI) measurements are used to predict the presence of scintillation causing irregularities and its values show the strength of irregularity. Losses of lock were also detected during the encounter of the irregularity when the depletion in TEC is greater than 5 TECU. Results are discussed in terms of possible source mechanism responsible for the occurrence of plasma density depletion.