Observations of High-m Ultra-Low Frequency Waves at Low Altitudes
Thursday, 4 September 2014
Regency Ballroom (Hyatt Regency)
Guan Le, NASA Goddard Space Flight Center, Greenbelt, MD, United States, Peter J Chi, University of California Los Angeles, Los Angeles, CA, United States, Robert J Strangeway, UCLA IGPP/ESS, Los Angeles, CA, United States and James A Slavin, University of Michigan Ann Arbor, Ann Arbor, MI, United States
Abstract:
With short azimuthal scale lengths and high azimuthal wavenumbers (m), the high-m ultra-low-frequency (ULF) waves in the magnetosphere occur due to drift and drift bounce resonances of energetic particles. Measuring high-m ULF waves can be challenging in observations because it requires multiple satellites that meet stringent separation requirements to confirm the short azimuthal wavelengths. The vast number of ground-based magnetometers cannot detect high-m waves because the variations in short horizontal scales are screened by the ionosphere. Recently, we discovered many events of high-m ULF waves in the magnetic field data from the Space Technology 5 (ST-5) mission at altitudes ranging from several hundred km to over 1000 km. ST-5 is a three micro-satellite constellation in a 300 x 4500 km, dawn-dusk, and sun synchronous polar orbit with 105.6 degree inclination angle. Due to the Earthâs rotation and the dipole tilt effect, the spacecraftâs dawn-dusk orbit track can reach as low as subauroral latitudes during the course of a day. Whenever the spacecraft traverse across the dayside closed field line region at subauroral latitudes, they frequently observe strong transverse oscillations at 30-200 mHz, or in the Pc 2-3 frequency range. As the maximum separations of the ST-5 spacecraft are in the order of 10 minutes, the three ST-5 satellites often observe very similar wave packets, implying these wave oscillations occur in a localized region. The coordinated ground-based magnetic observations at the spacecraft footprints, however, do not see waves in the Pc 2-3 band; instead, the waves appear to be the common Pc 4-5 waves associated with field line resonances. We concluded that these unique Pc 2-3 waves seen by ST-5 are in fact the Doppler-shifted Pc 4-5 waves as a result of rapid traverse of the spacecraft across the resonant field lines azimuthally at low altitudes. The observations with the unique spacecraft dawn-disk orbits at proper altitudes and magnetic latitudes reveal the azimuthal characteristics of field-aligned resonances. These observations suggest a new opportunity for using low-altitude satellites to monitor the occurrence of the high-m waves and infer the state of energetic ions in the magnetosphere.
