Polar cap boundary layer waves: An auroral zone phenomenon

Polar cap boundary layer waves are ELF/VLF electric and magnetic waves dete cted on field lines just adjacent to the polar cap. Intense waves are prese nt at this location essentially all (96%) of the time. The wave latitude-lo cal time distribution is shown to be the same as that of the Feldstein auro ral oval, a distribution centered at similar to 75 degrees at local noon an d similar to 65 degrees at local midnight. The most intense waves are detec ted coincident with the strongest magnetic field gradients (field-aligned c urrents). Statistically, the, wave intensities are greatest near local noon (10(-13) mV(2) m(-1) at 3 kHz) and midnight and are least near dawn and du sk (similar to5 x 10(-15) mV(2) m(-1) at 3 kHz). The noon and midnight wave intensities increase slightly when the interplanetary magnetic field is di rected southward. The dawn and dusk waves appear to be controlled by the so lar wind speed. Using high-resolution data, specific frequency bands of ele ctromagnetic whistler-mode waves are identified: similar to 200 Hz and 1-2 and similar to5 kHz. These may correspond to previously identified "magneti c noise bursts" and "auroral hiss", respectively. Assuming cyclotron resona nt interactions, the 1- to 5-kHz auroral hiss is shown to be resonant with similar to 50-eV to similar to1.0-keV electrons. Several mechanisms, both r esonant (nonlocal) and nonresonant (local), are suggested for the generatio n of the similar to 200-Hz electromagnetic waves. Three types of intense el ectric signals are present: solitary bipolar pulses (electron holes), waves at similar to4 x 10(2) to 6 x 10(3) Hz (lower hybrid waves), and narrowban d waves at similar to 10 kHz (electrostatic waves near the upper hybrid res onance frequency). Solitary bipolar pulse onset events have been detected f or the first time. The bipolar pulses reached 2 mV in(-1) peak-to-peak ampl itudes within 3 ms. An exponential growth rate of 0.72 ms, or 0.25f(ec), wa s determined. The previously reported "broadband nature" of the polar cap b oundary layer (and low-latitude boundary layer) waves is now postulated to be caused by a fast switching between the various electromagnetic and elect rostatic modes described above. The polar cap boundary layer waves are most likely a consequence of instabilities associated with auroral zone field-a ligned currents carried by similar to 50-eV to 1.0-keV electrons and proton s. The currents in turn have been ascribed to be driven by the solar wind-m agnetosphere global interaction. One consequence of the presence of the wav es at high altitudes is diffusion of magnetosheath plasma into the magnetos phere and magnetospheric plasma out into the magnetosheath (cross-field dif fusion, due to parasitic wave-particle interactions). It is speculated that field-aligned currents and similar wave modes will be detected at all plan etary magnetospheres.