Fine structuring of whistler waves within Earth's bow shock

Details Fine structuring of whistler waves within Earth's bow shock Fine structuring of whistler waves within Earth's bow shock

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsm13d2123h&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #SM13D-2123

Physics

Plasma Physics

[2139] Interplanetary Physics / Interplanetary Shocks, [2154] Interplanetary Physics / Planetary Bow Shocks, [2159] Interplanetary Physics / Plasma Waves And Turbulence, [7851] Space Plasma Physics / Shock Waves

Scientific paper

We present observations of intense whistler waves made by Polar within Earth's perpendicular (θ Bn ≈ 90 o) bow shock under extreme solar wind conditions. The shock ramp duration was unusually extended, lasting roughly 30 s, and Polar measured all components of the electric and magnetic field waveforms at a rate of 1600 Hz. The long duration waveform measurements reveal for the first time the fine structuring of whistler waves within Earth's bow shock. The whistler waves are observed to be composed of two distinct components: a relatively broadband lower frequency component occurring near the lower hybrid frequency (a few tens of Hertz) and a higher frequency component at a few hundred Hertz which is approaching the resonance cone (f res≡ fce}\cosθ {kb). The lower frequency component is composed of right-hand polarized wave packets propagating at oblique angles with respect to the magnetic field and shock normal, with respective angles θ kb varying from 50o-70o and θ kn ˜ 50o. These waves generally have intense (dB/B ˜ 0.1-0.3) magnetic fields ranging from a few nT to 10 nT. The high-frequency component appears to be the shock analog of `` lion roars''\ often observed in magnetic field troughs of mirror mode structures in the magnetosheath. Here we find the lion roars occur within the foot and into the shock ramp in regions where there exists sufficiently intense low frequency whistlers. Similar to their magnetosheath counterpart, the lion roars reported here are packetized structures that last up to ˜10 cycles, have amplitudes reaching 1 nT, and propagate nearly parallel or antiparallel with respect to the background magnetic field (θ kb≤ 30o). Particularly noteworthy is they appear regularly with a cadence of a few tens of Hertz and are collocated with local magnetic field (and also electric field) minima of the lower frequency whistler waves. Their presence indicates that a similar generation mechanism may be operating here at Earth's bow shock. Their modulation at the lower frequency cadences favors a secondary emission scenario as an explanation. A possible candidate is that they are generated via enhancements in the perpendicular electron anisotropy due to the tens of Hertz waves occurring in the pedestal and magnetic ramp region of the shock layer.

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