Two types of magnetospheric ELF chorus and their substorm dependences

Abstract

Extremely low frequency (10-1500 Hz) magnetospheric chorus has been analyzed to investigate a possible dependence on substorms. Care was taken to separate spatial effects from temporal effects by analyzing an entire year of data acquired by the Ogo 5 search coil magnetometer. A major finding of the study of spatial dependences is that chorus occurs principally in two magnetic latitude regions. Equatorial chorus is detected near the equator, and high-latitude chorus is found at magnetic latitudes above 15 ø. When chorus in these two regions is analyzed separately, substorm dependences become apparent. Comparisons with A E indicate that equatorial chorus occurs primarily during substorms. High-latitude chorus is not strongly dependent on AE and often occurs during intervals of prolonged quiet with •E < 100 ,y for the previous 12 hours or more. The dependence of equatorial chorus on local time, magnetic latitude, and L is consistent with generation by a cyclotron resonance between the whistler mode chorus and 10-to 100-keV trapped substorm electrons. Equatorial chorus has an abrupt onset in the postmidnight sector and a second enhancement from dawn to noon, a pattern which is similar to that of energetic electron precipitation. The occurrence frequency of equatorial chorus peaks at the equator, I magnetic latitudel < 5 ø, a region where cyclotron resonance is most efficient. The L value of maximum chorus occurrence increases from 5-8 postmidnight to 7-11 postdawn, a dependence which is consistent with generation by electrons which have undergone drift shell splitting. Delay times between substorms and the onset of equatorial chorus are consistent with a gradient drift of •25-keV electrons. Equatorial postmidnight chorus and postdawn chorus have similar occurrence rates and wave intensities. The maximum chorus occurrence rates are 54% postmidnight and 56% postdawn. Time-averaged equatorial chorus intensities >_ l0 -8 ,V•' are detected up to 17% of the time for 6 _< L _< 7 postmidnight and up to 14% of the time for 7 _< L _< l0 from dawn to noon. Such wave intensities are sufficient to cause near-strong pitch angle diffusion of electrons for L > 6 and strong diffusion for L > 8. Instantaneous diffusion rates may be considerably higher owing to the discrete burstlike nature of the chorus. The spatial and temporal dependences of high-latitude chorus are considerably different from those of equatorial chorus. Highlatitude chorus occurs in local day and evening and at large L. The emission is detected primarily on the dayside, at 0800 _< LT _< 1600, and often within 1-2 RE of the magnetopause. The occurrence of highlatitude chorus during quiet intervals is consistent with local generation within 'minimum B pockets.' Jackerott, 1963; Egeland et al., 1965], at ionospheric altitudes [Taylor and Gurnett, 1968; Barrington et al., 1971; Thorne et al., 1977], and in the outer magnetosphere, the region where chorus is believed to be generated [Russell et al., 1969; Dunckel and Helliwell, 1969; Tsurutani and Smith, 1974; Burtis, 1...

THEMIS observations of electron cyclotron harmonic emissions, ULF waves, and pulsating auroras

“…Though the above estimations are based upon a number of simplified models and thus approximate only, they guide us to tentatively suggest that the ECH be a more promising candidate for higher L shell PsAs as in our event. However, one should be noted that the whistle-mode chorus may well have an off-equatorial source [Tsurutani and Smith, 1977;Sigsbee et al, 2010] which could not be observed by THEMIS satellites. We thus may not entirely exclude, from both theoretical and observational viewpoints, the possible existence of the chorus waves as well as their potential role in generating the observed PsAs in our event.…”

Substorm dependence of chorus amplitudes: Implications for the acceleration of electrons to relativistic energies

“…Twelve orbits, during which nontraditional configurations were deployed for testing purposes, were also excluded from the analyses. Tsurutani and Smith [1977] showed that the chorus emissions could be divided into two categories depending on magnetic latitude. The same scheme and notation are adopted here with equatorial and high-latitude chorus being defined as chorus observed within and beyond 15 ø of the magnetic equator, respectively.…”

The Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) on RBSP

“…Nightside chorus is strongest inside L = 8, and is also confined to latitudes below 15 • , due to strong Landau damping of oblique waves during their propagation towards higher latitude from the equatorial source region ). In contrast, dayside chorus is found over a broad range of latitudes, is most intense in the outer (L ≈ 8) magnetosphere, and shows less dependence on geomagnetic activity (Tsurutani and Smith 1977;. The wave normal distribution of chorus is required to accurately evaluate resonant electron energies and quantify the associated rates of scattering ).…”