Magnetospheric Multiscale Observations of Ultra Low Frequency Waves in the Inner Magnetosphere
Résumé
The apogee of the Magnetospheric Multiscale (MMS) spacecraft was in the predawn local time sector early in the MMS commissioning phase. On several orbits in this phase MMS observed large amplitude (10s of nT) Pc5 pulsations, typically around 5 to 7 Earth radii, near dawn. Because MMS was in a string-of-pearls configuration we could determine the phase velocity of the waves along the spacecraft separation vector. Preliminary analysis indicates that this was of the order 30-50 km/s, much larger than the spacecraft velocity. Furthermore, the waves are propagating tailwards. Given a nominal wave period of the order 5 minutes, the wavelength of the waves is around 2 Earth radii, assuming azimuthal propagation. This corresponds to an m-number of about 20. The waves used for this initial study were observed on several successive orbits during the recovery phase of the March 17, 2015 geomagnetic storm. We will present additional analysis of the properties of these ULF waves as observed during the MMS commissioning phase, during which time the spacecraft apogee migrated from dawn to dusk through the nightside.