Determining the Wave Vector Direction of Equatorial Fast Magnetosonic Waves. Issue 16 (18th August 2018)

• Record Type: Journal Article
• Authors: Boardsen, Scott A.; Hospodarsky, George B.; Min, Kyungguk; Averkamp, Terrance F.; Bounds, Scott R.; Kletzing, Craig A.; Pfaff, Robert F.
• Is Part Of: Geophysical research letters. Volume 45:Issue 16(2018)
• Page Start: 7951
• Page End: 7959
• Publication Date: 2018-08-18
• Subjects: equatorial fast magnetosonic -- wave vector analysis -- E field polarization analysis -- Poynting flux analysis; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2018GL078695
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: We perform polarization analysis of the equatorial fast magnetosonic waves electric field over a 20‐min interval of Van Allen Probes A waveform receiver burst mode data. The wave power peaks at harmonics of the proton cyclotron frequency indicating that the spacecraft is near or in the source region. The wave vector is inferred from the direction of the major axis of the electric field polarization ellipsoid and the sign of the phase between the longitudinal electric and compressional magnetic field components. We show that wave vector is preferentially in the azimuthal direction as opposed to the radial direction. From Poynting flux analysis one would infer that the wave vector is primarily in the radial direction. We show that the error in the Poynting flux is large ~90° . These results strongly imply that the wave growth occurs during azimuthal propagation in the source region for this event. Plain Language Summary: Near‐equatorial fast magnetosonic waves are strongly prevalent in the Earth's inner magnetosphere. They strongly interact with the proton component (<40 KeV) of the ring current and play a role in energization and pitch angle scattering of radiation belt electrons. The wave source is proton ring (shell) distributions; however, the details of wave amplification require more investigation. We analyze an event where the wave power peaks at harmonics of the proton cyclotron frequency, which suggests that this event is in or near the wave source. From polarization analysis of the electric field and the sign of the phase between the longitudinal electric and parallel magnetic field components we find that the wave vector is directed in the azimuthal direction. This suggests that wave gain mainly occurs during azimuthal propagation. The Poynting flux (energy flow) direction can also be used to estimate the wave vector direction. We show that this flux is not a reliable estimator of wave vector for this mode, because the transverse electric field component needed for computation is a factor of 100 below the noise level from other naturally occurring waves leading to large angular errors in flux direction. Key Points: For this near source event, the wave number is found to have a strong azimuthal direction, suggesting that wave gain is in this direction Fast magnetosonic wave vector is estimated from electric field polarization analysis and sign of phase between Ek and dB|| Poynting flux found to be invalid for event analyzed for harmonic numbers >8 proton cyclotron frequencies