Two types of ELF hiss observed at Varanasi, India

Abstract

Abstract. The morphology of ELF hiss events observed at low-latitude ground station Varanasi (L = 1.07, geomagnetic latitude 14°55¢N) are reported, which consist of two types: (1) events which propagated in ducted mode along the geomagnetic ®eld line corresponding to observing station Varanasi and (2) events which propagated in ducted mode along higher L-values (L = 4±6), after reaching the lower edge of ionosphere excite the Earth-ionosphere wave guide and propagate towards equator to be received at Varanasi. To understand the generation mechanism of ELF hiss, incoherent Cerenkov radiated power from the low latitude and middle latitude plasmasphere are evaluated. Considering this estimated power as an input for wave ampli®cation through wave±particle interaction, the growth rate and ampli®cation factor is evaluated which is too small to explain the observed wave intensity. It is suggested that some non-linear mechanism is responsible for the generation of ELF hiss.

Observation of discrete VLF emissions at low latitudes and their generation mechanism

“…At low latitude, nights with observable VLF emissions are rather rare, and the activity is closely related to strong magnetic storms (Singh, 1993). But when it appears its occurrence rate is comparable to that of mid latitude (Singh, 1993;Singh et al, 1999). The observations of discrete VLF emissions at low latitude ground stations Varanasi, Jammu and Gulmarg are unusual in the sense that most of the reported observations are from satellites (Cornilleau-Wehrlin et al, 1978;Hattori et al, 1991;Santolik et at., 2004) or from high latitudes.…”

Generation mechanism for VLF chorus emissions observed at a low-latitude ground station

“…At low latitude, nights with observable VLF emissions are rather rare, and the activity is closely related to strong magnetic storms. However, during nights with chorus emissions the occurrence rate is comparable to that at mid latitude (Singh, 1993;Singh et al, 1999Singh et al, , 2000. Several interesting VLF events were recorded during the period January to July 1986 at the low-latitude ground station Gulmarg (geomag.…”

Hiss emissions during quiet and disturbed periods

“…The early observations of low latitude ground-based VLF hiss comes from Japanese workers [50][51][52] and later on from Indian workers [4,53]. VLF hiss at Srinagar, India was recorded in the frequency band 1-3 kHz and 5-7 kHz [53].…”

“…Global distribution of hiss is characterized by three principal zone of intense activity of which the first zone is located around invariant latitudes above 70 AE (auroral hiss), the second near 50 AE (mid-latitude) and the third below 30 AE latitude (equatorial hiss) [3]. Ground-based observations revealed that the low-latitude hiss are less intense than those observed at middle and high latitudes [4,5]. Jorgensen [6] analyzed amplitude distribution of hiss and showed that it decreased with decreasing latitude (10 dB per 1000 km) and explained it in terms of attenuation of hiss emission propagating through the earth-ionosphere wave guide from auroral zone to middle and low latitudes.…”

“…A number of generation mechanism both incoherent (Cerenkov radiation, cyclotron radiation, Doppler-shifted cyclotron radiation) and coherent (Cerenkov instability, cyclotron instability, traveling wave-tube mechanism) have been proposed from time to time to explain the observed hiss emission intensity at ground stations as well as on-board rockets and satellites [13][14][15][16][17][18][19][20][21][22]. In all the theoretical treatments of instability mechanism it was assumed that all the waves are amplified around the magnetic equator while propagating back and forth along the geomagnetic field lines, till their intensity reaches the observed intensity [4,[23][24][25][26][27][28]. In the case of incoherent generation mechanism, the wave amplitude, generated by individual charged particle distributed along the field lines, is added up to explain the observed flux density.…”