Lion roars and nonoscillatory drift mirror waves in the magnetosheath

摘要: A complete set of ISEE plasma wave, plasma, and field data are used to identify the instability responsible for generation extremely low frequency (ELF) electromagnetic lion roars. Lion roars detected close magnetopause generated by cyclotron anisotropic (T⊥−/T∥− ≃ 1.2) thermal electrons when local critical energy, EM = B²/8πN, falls values (EM ∼ 10–30 eV) or below electron 25 eV, as a result decreases in B. companion theoretical paper, Thorne Tsurutani (1981), demonstrates that convective growth rates under these conditions is greater than 100 dB RE−1. The terminated increases ambient magnetic magnitude consequential eV. Because there few resonant particles at high energies, rate 3 orders measurable ceases. value absolute upper limit unstable waves predicted theory, ωmax A−Ω−/(A− + 1), compared with observations. predictions observations found be general, but not exact, agreement. Several possible explanations explored. quasi-periodic, ∼20-s oscillations which cause variations hence alternately drive then stable also investigated. pressures shown out phase, while total pressure (electron ion field) remains relatively constant. Most associated particle motion. large 2∶1 strength β (8πP/B²), from 1–2 maximum 10–25 minimum. Analysis high-resolution fields two closely separated spacecraft, 1 2, rule possibility could due Cross-correlation analyses spacecraft time delays correlation consistent structures being quasi-static nature. temporal spatial convecting past magnetosheath flow speed. quasi-periodic ∼20 proton gyroradii scale rest frame. Magnetic similar lengths exist magnetosheaths Jupiter Saturn (Pioneer 11 data). results interpretation magnetohydrodynamic nonoscillatory ‘waves’ drift mirror instability. condition instability, β⊥/β∥> (1/β⊥), met cases studied this paper. instabilities intimately coupled. (>10), energy regions leads becoming unstable. whistler mode can ducted enhanced-density, low-field regions. Thus roar durations may represent propagation an wave packet travelling group velocity, correspond convection duct (drift wave) spacecraft. occurring natural relaxation processes reduce anisotropies created preferential heating solar wind it passes through bow shock further compression takes place approach near-subsolar magnetopause. One consequence onset isotropization enhanced expulsion along lines toward flanks magnetosheath. It determined if mechanism general process ‘plasma removal’ planetary magnetosheaths. Furthermore, presence have important consequences merging. alternating (T⊥ > T∥) temperature lead patchy, sporadic reconnection.