High-m Poloidal Waves Observed in Low Earth Orbit and Their Implications for Energetic Particles in the Magnetosphere

Abstract:

Recent studies of the magnetic field data collected by the NASA ST-5 satellites in the low Earth orbit have revealed many wave events with frequencies of 30-200 mHz (in the Pc 2-3 band). It was soon realized that these waves were in fact Doppler-shifted waves in the magnetosphere, with wave frequencies of merely a few mHz (in the Pc 5 band) and azimuthal wavenumbers (m) of the order of 100. Oscillating in the poloidal direction, high-mwaves are known to engage in drift or drift bounce resonance with energetic ring current particles, and therefore they are expected to play a role in modulating the energetic particles in the inner magnetosphere.

Using a new method that examines the differences in wave phase detected by the three ST-5 satellites in a "pearls-on-a-string" configuration, we confirm that the frequencies of the observed poloidal waves are mainly between 3 and 5 mHz in the Earth frame. In some events, the NOAA satellites were located at the equator and close to the field lines connected to the ST-5 satellites, confirming the wave frequencies estimated using only ST-5 observations. In each of the poloidal wave events observed by ST-5, the azimuthal wavenumber may change with L, but the wave frequency in the Earth frame remains the same. We have also found cases where poloidal waves were observed in both dayside and nightside of the magnetosphere when ST-5 satellite passed through the same Lshells within a single orbit, supporting the theoretical prediction of a global poloidal mode, which is also known as the transverse Alfvén resonator.

Satellite observations in low Earth orbits have shown that high-m poloidal waves can last many hours even during geomagnetically quiet conditions, suggesting that a very weak ring current may be capable of supplying enough energetic particles to excite poloidal waves. We will address the implications for energetic particles in the inner magnetosphere with theoretical and modeling considerations.