Detection of Hertz Frequency Multiharmonic Field Line Resonances at Low‐L (L = 1.1–1.5) During Van Allen Probe Perigee Passes.

We present new and previously unreported in situ observations of Hertz frequency multiharmonic mode field line resonances detected by the Electric Field and Waves 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, we conclude that 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. Key Points: We present previously unreported low‐L (L ∼ 1.2) Hertz frequency multiharmonic field line resonances observed in the electric field spectraThe field line resonance harmonics were regularly observed by the Van Allen probes and have frequencies consistent with odd harmonic modesThe observed harmonic frequencies are compared to models using empirical density profiles, offering a new diagnostic for density dynamics [ABSTRACT FROM AUTHOR]