Second Harmonic Poloidal Waves Observed by the Van Allen Probes
Tuesday, 16 December 2014: 9:30 AM
The Van Allen Probes mission provides a unique opportunity to determine the wavelength or scale size of various ULF waves in the inner magnetosphere. The spatial property is important when discussing how the waves are excited and interact with particles. In this presentation, we describe Van Allen Probes observations of a long-lasting second harmonic poloidal standing Alfvén wave event (duration ~ 5 hours) that occurred on 2 March 2014 in the noon sector. The standing wave harmonic is inferred from the relationship between the oscillations in the electric field and magnetic field. During this event, the two spacecraft were very near the magnetic equator and stayed within the plasmasphere. Most importantly, the two spacecraft had a very small separation (less than 2000 km). From the phase delay of the wave signals detected when the spacecraft were on the same L shell but were separated by 300 km (0.6 degrees) in longitude, we estimate the azimuthal wave number to be greater than 100, corresponding to an azimuthal wavelength of less than 200 km. Modulation of the flux of ions was present with amplitude and phase that depend on energy and pitch angle. We use the observed frequency, the inferred standing wave mode structure, and the estimated azimuthal wave number to predict the energy and pitch angle dependence of drift-bounce resonance of ions and compare the prediction with the observed ion flux oscillations.