Distribution of ULF Wave Power in Magnetic Latitude and Local Time Using THEMIS and Arase Measurements. Issue 10 (25th October 2022)

• Record Type: Journal Article
• Authors: Sarris, Theodore E.; Li, Xinlin; Zhao, Hong; Papadakis, Kostis; Liu, Wenlong; Tu, Weichao; Angelopoulos, Vassilis; Glassmeier, Karl‐Heinz; Miyoshi, Yoshizumi; Matsuoka, Ayako; Shinohara, Iku; Imajo, Shun
• Is Part Of: Journal of geophysical research. Volume 127:Issue 10(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-10-25
• Subjects: ultra‐low‐frequency wave power -- ULF wave distribution in magnetic latitude -- pitch angle dependence of radial diffusion coefficients -- drift resonant interactions -- off‐equatorial wave power -- field line resonance amplitudes; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2022JA030469
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: Ultra‐low‐frequency (ULF) waves are known to radially diffuse hundreds‐keV to few‐MeV electrons in the magnetosphere, as the range of drift frequencies of such electrons overlaps with the frequencies of the waves, leading to resonant interactions. The theoretical framework for this process is described by analytic expressions of the resonant interactions between electrons and toroidal and poloidal ULF wave modes in a background magnetic field. However, most expressions estimate the radial diffusion rates based on estimates of the power of ULF waves that are obtained either from spacecraft close to the equatorial plane or from the ground. In this study, using multiyear measurements from the THEMIS and Arase missions, we present a statistical analysis of the distribution of ULF wave power in magnetic latitude and local time and show that the wave power of the radial and azimuthal components of the magnetic field increases away from the magnetic equator. Our result could have significant implications for the radial diffusion rates as currently estimated. Plain Language Summary: Ultra‐low‐frequency (ULF) waves are fluctuations in the electric and magnetic fields within the Earth's magnetosphere. Their frequencies are in the mHz to Hz range of frequencies, and they coexist with high‐energy (hundreds keV to few MeV) electrons in the radiation belts, which drift around the Earth at similar frequencies. This leads to a resonant interaction that causes diffusion and acceleration of electrons via their radial transport. This is well understood theoretically and is quantitatively described by radial diffusion coefficients. However, most analytic expressions for these diffusion coefficients have been derived for equatorial electrons. At the same time, ULF waves are more commonly measured either from spacecraft close to the equatorial plane or from the ground. In this study, we use multiyear measurements from the THEMIS and Arase missions and we present a statistical analysis of the distribution of ULF wave power in magnetic latitude. This is enabled by the inclinations of the spacecraft orbits, which, together with the Earth's dipole tilt, allow sampling magnetic latitudes up to ∼40°. We show that wave power of the transverse magnetic field components increases away from the magnetic equator. Our results could have significant implications for the radial diffusion rates as currently estimated. Key Points: Wave power of Pc4 and Pc5 ultra‐low‐frequency (ULF) oscillations of the azimuthal and radial magnetic field increases away from the magnetic equator Enhanced off‐equatorial ULF wave power could significantly enhance radial diffusion rates for off‐equatorially mirroring electrons Multiyear measurements of THEMIS and Arase s/c are used, whose inclinations enable measurements of magnetic latitudes up to ∼40°