Detection of ionospheric perturbation due to a soft gamma ray repeater SGR J1550-5418 by very low frequency radio waves

Abstract

We wish to report our observations of ionospheric disturbances made by incident Gamma Rays from the Soft Gamma Repeater SGR J1550-5418 which took place on 22nd of January, 2009. The observations were made by a loop antenna and a Gyrator-II type receiver which was tuned to the 500KW VTX station transmitting Very Low Frequency (VLF) signal at 18.2 kHz. We looked for signatures of sudden ionospheric disturbances (SID) which commenced within seconds of the observations reported by various satellites. We used Long Wave Propagation Capability code to compute the changes in the ionospheric parameters due to the repeated passage of the high energy radiation. We detected seventy six events in our VLF detector which appear to be associated with the SGR J1550-5418. We found that 28 of them are within three seconds of the satellite observations and 38 (fifty percent) of them are within seven seconds of the satellite observations. We also compute the evolution of the electron number density of the ionosphere due to this events and found that the ionosphere was becoming increasingly charged due to repeated bombardment of the high energy radiations. The lower ionospheric height went down significantly. We found convincing evidence that the source SGR J1550-5418 repeatedly caused ionospheric disturbances on the 22nd of January, 2009. The electron number density went up significantly at all the heights. The height of the lower ionosphere went down by about 15 km due to these repeated events.