Abstract
The Equator-S magnetometer is very sensitive and has a sampling rate of normally 128 Hz. The high sampling rate allows for the first time fluxgate magnetometer measurements of ELF waves between the ion cyclotron and the lower hybrid frequencies in the equatorial dayside magnetosheath. The so-called lion roars, typically seen by the Equator-S magnetometer at the bottom of the magnetic troughs of magnetosheath mirror waves, are near-monochromatic packets of electron whistler waves lasting for a few wave cycles only, typically 0.25 s. They are right-hand circularly polarized waves with typical amplitudes of 0.5â1 nT at around one tenth of the electron gyrofrequency. The cone angle between wave vector and ambient field is usually smaller than 1.5°.Key words. Interplanetary physics (MHD waves and turbulence; plasma waves and turbulence)
Highlights
More than 20 years ago, Smith and Tsurutani (1976) published the ®rst search-coil magnetometer observations of what they called lion roars inside magnetic troughs in the dayside magnetosheath
They found that the lion roars are narrow-banded right-hand polarized waves, basically electron cyclotron waves, that are relatively short-lived, about 2 s, and that they have typical frequencies of about 120 Hz and typical amplitudes of 0.1 nT
Zhang et al (1998) used the waveform capture instrument onboard Geotail to study 20±300 Hz lion roars with typical amplitudes of 0.1 nT within mirror waves and similar waves without ambient magnetic ®eld depletion closer to the bow shock
Summary
Due to the instrumentation employed in the earlier studies, the actual waveform of the lion roars was unknown until very recently. Zhang et al (1998) used the waveform capture instrument onboard Geotail to study 20±300 Hz lion roars with typical amplitudes of 0.1 nT within mirror waves (their type A, similar to the lion roars analyzed in the earlier studies and the present paper) and similar waves without ambient magnetic ®eld depletion closer to the bow shock (their type B). Zhang et al (1998) did not discuss the wave form itself very much, but rather the angle between the wave vector, k, and the ambient ®eld. Zhang et al (1998) did not discuss the wave form itself very much, but rather the angle between the wave vector, k, and the ambient ®eld They did this by applying a minimum variance analysis to the magnetic waveform and removed the Æ180-ambiguity of the k-vector using electric ®eld data. In the present paper we will study the waveform of mirror trough lion roars, but using theuxgate magnetometer onboard Equator-S This will limit our study to waves below 64 Hz, but extend the frequency range down to 8 Hz. Using a dierent approach we will show for single cases as well as in statistics covering 356 cases that (1) lion roars can be found down to 8 Hz (the lower limit set in our analysis), (2) the lion roar wave forms have a clear packet structure, and (3) the average cone angle found in our frequency range, hk % 0X3, is much smaller than previously thought and that lion roars travel essentially parallel to the background ®eld
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