Polar cap boundary layer waves: An auroral zone phenomenon

Abstract

Polar cap boundary layer waves are ELF/VLF electric and magnetic waves detected on field lines just adjacent to the polar cap. Intense waves are present at this location essentially all (96%) of the time. The wave latitude-local time distribution is shown to be the same as that of the Feldstein auroral oval, a distribution centered at ∼75° at local noon and ∼65° at local midnight. The most intense waves are detected coincident with the strongest magnetic field gradients (field-aligned currents). Statistically, the wave intensities are greatest near local noon (10−13 mV2 m−1 at 3 kHz) and midnight and are least near dawn and dusk (∼5 × 10−15 mV2 m−1 at 3 kHz). The noon and midnight wave intensities increase slightly when the interplanetary magnetic field is directed southward. The dawn and dusk waves appear to be controlled by the solar wind speed. Using high-resolution data, specific frequency bands of electromagnetic whistler-mode waves are identified: ∼200 Hz and 1-2 and ∼5 kHz. These may correspond to previously identified “magnetic noise bursts” and “auroral hiss”, respectively. Assuming cyclotron resonant interactions, the 1- to 5-kHz auroral hiss is shown to be resonant with ∼50-eV to ∼1.0-keV electrons. Several mechanisms, both resonant (nonlocal) and nonresonant (local), are suggested for the generation of the ∼200-Hz electromagnetic waves. Three types of intense electric signals are present: solitary bipolar pulses (electron holes), waves at ∼4 × 102 to 6 × 103 Hz (lower hybrid waves), and narrowband waves at ∼10 kHz (electrostatic waves near the upper hybrid resonance frequency). Solitary bipolar pulse onset events have been detected for the first time. The bipolar pulses reached 2 mV m−1 peak-to-peak amplitudes within 3 ms. An exponential growth rate of 0.72 ms, or 0.25 fce, was determined. The previously reported “broadband nature” of the polar cap boundary layer (and low-latitude boundary layer) waves is now postulated to be caused by a fast switching between the various electromagnetic and electrostatic modes described above. The polar cap boundary layer waves are most likely a consequence of instabilities associated with auroral zone field-aligned currents carried by 50-eV to 1.0-keV electrons and protons. The currents in turn have been ascribed to be driven by the solar wind-magnetosphere global interaction. One consequence of the presence of the waves at high altitudes is diffusion of magnetosheath plasma into the magnetosphere and magnetospheric plasma out into the magnetosheath (cross-field diffusion, due to parasitic wave-particle interactions). It is speculated that field-aligned currents and similar wave modes will be detected at all planetary magnetospheres.