资源环境科技发展态势分析平台

We present new and previously unreported in‐situ observations of Hertz frequency multi‐harmonic mode field line resonances detected by the Electric Field and Waves (EFW) instrument on‐board the NASA Van Allen Probes during low‐L perigee passes. Spectral analysis of the spin‐plane electric field data reveals the waves in numerous perigee passes, in sequential passes of Probes A and B, and with harmonic frequency structures from ∼0.5 to 3.5 Hz which vary with L‐shell, altitude, and from day to day. Comparing the observations to wave models using plasma mass density values along the field line given by empirical power laws and from the International Reference Ionosphere model (IRI), we conclude the waves are standing Alfvén field line resonances, and that only odd‐mode harmonics are excited. The model eigenfrequencies are strongly controlled by the density close to the apex of the field line, suggesting a new diagnostic for equatorial ionospheric density dynamics.

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