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\begin{document}

\title{Detection of Hertz Frequency Multi-Harmonic Field Lines Resonances at
Low-L (L~1.2) during Van Allen Probe Perigee Passes}



\author[1]{Francesco Rossi Lena}%
\author[1]{Louis Godwin Ozeke}%
\author[2]{John Wygant}%
\author[2]{Sheng Tian}%
\author[2]{Aaron Breneman}%
\author[1]{Ian Mann}%
\affil[1]{University of Alberta}%
\affil[2]{University of Minnesota}%


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  \date{May 25, 2024}


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\begin{abstract}
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 \textasciitilde{}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 Alfv\selectlanguage{ngerman}én 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.%
\end{abstract}\selectlanguage{ngerman}%



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