Effect of geomagnetic storms on VLF waves at low latitudes based on the analysis of whistlers and VLF emissions observed at Indian ground stations: A Review

Abstract

. A review of the effect of geomagnetic storms on very low frequency (VLF) waves (whistlers and emissions) at low latitudes is presented, based on the spectral analysis of the storm-time VLF data collected over a period of about four decades at our low latitude ground-based Indian stations. The review begins with an introduction about the characteristics of whistlers and VLF emissions and the importance of storm-time VLF events for the developments of our theoretical knowledge in plasma physics. This is followed by four different sections (2-5) in order to understand and explain the physics of VLF events observed at low latitudes during magnetic storms. All aspects of whistler duct and geomagnetic activity are described in section 2 whereas section 3 deals with VLF wave (whistlers and emissions) activity and whistler dispersion. Section 4 presents method of analysis of whistler duct alongwith duct lifetime and VLF emission source used in the spectral analysis of storm-time VLF data reported in the present paper. Section 5 describes in brief the experimental setup used in recording of VLF data at our Indian ground stations with a presentation of some selected storm-time whistlers and emissions alongwith their detailed spectral analysis of the observed salient features. Spectral analyses of the storm-time whistlers and emissions using VLF data from our Indian stations have provided the following results (section 6): mid/high latitude emissions generated in the equatorial region of higher 𝐿 -values through the process of Doppler-shifted cyclotron resonance mechanism and propagated along the higher field lines in different ducts formed by disturbances during magnetic storm and after exiting from the ducts, they penetrated the ionosphere and are trapped in Earth-ionosphere waveguide and after propagating in the waveguide are thus recorded at low latitude ground stations. (3) The increased intensity of whistler and emission activities during magnetic storm periods are due to the formation of additional ducts by the enhanced flux of energetic electrons during magnetic storm periods. along with their dispersion analysis. The dispersion analysis of the storm-time whistlers observed at mentioned Indian stations, shows, that unusual high dispersion whistlers are registered during magnetic storm periods and their dispersions ranges from about 15 - 250 s 1/2 . From the dispersion analysis of these observed storm-time high dispersion whistlers using Dowder-Allcock method (Dowden and Allcock, 1971) and curve-fitting technique of Tarcsai (Tarcsai, 1975), we find that these whistlers have propagated along the higher geomagnetic field lines in different ducts corresponding to 𝐿 -values of 2.88 to 6.52. Hence it may be inferred that storm-time whistlers recorded at low latitudes belong to mid/high latitudes and these whistlers may have propagated in different ducts along higher 𝐿 -values and after exiting from ducts, they penetrated the ionosphere and are trapped in Earth-ionosphere waveguide and thereafter propagating in the waveguide they are received at our low latitude Indian ground stations. The wave normal angle (lying in the range of 0.2 - 2.3 0 )at the entrance into the waveguide is such that they propagated towards the equator and are received at low latitude Indian ground stations (Singh et al., 2008, 2009, 2010, 2011).