[P-EM13] Dynamics of the Inner Magnetospheric System

[PEM13-P03] Analysis of space distribution of low frequency whistler chorus waves observed by Arase

*Yugo Tamamura¹, Ayako Matsuoka¹, Mariko Teramoto², Yoshiya Kasahara³, Mitsunori Ozaki⁴, Shoya Matsuda⁵, Yoshizumi Miyoshi⁶, Tomoaki Hori⁶, Iku Shinohara⁷, Fuminori Tsuchiya⁸, Atsushi Kumamoto⁹ (1.Faculty of Science, Kyoto University, 2.Faculty of Engineering Department of Space Systems Engineering, Kyushu, 3.Emerging Media Initiative, Kanazawa University, 4.Faculty of Electrical and Computer Engineering, Institute of Science and Engineering, Kanazawa University, 5.Kanazawa University, 6.Institute for Space-Earth Environmental Research, Nagoya University, 7.Japan Aerospace Exploration Agency/Institute of Space and Astronautical Science, 8.Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University, 9.Department of Geophysics, Graduate School of Science, Tohoku University)

Keywords:magnetosphere, chorus waves

In the Earth’s inner magnetosphere, the radiation belts are torus-shaped zones at 2-6 Earth radii distance from the center of the Earth, where high energy particles are trapped by the Earth’s magnetic field. It is known that the acceleration and loss of the energetic particles in the radiation belts are deeply related with the energy transfer by electromagnetic waves. Whistler chorus waves are electromagnetic wave often observed in the low-density region outside the plasmasphere. The frequencies are generally ranging 0.1- 0.8 fce (fce is the equatorial electron cyclotron frequency). Whistler chorus waves are typically divided into lower band chorus of 0.1 - 0.5 fce and upper band chorus of 0.5 -0.8 fce, with a clear gap in wave power at 0.5 fce. The contribution of chorus waves to the acceleration and loss of the radiation belt particles has been examined by many studies. They suggested that chorus waves are generated by an anisotropic distribution of energetic electrons (a few – hundred keV) injected from the plasma sheet during substorms and energize the electrons in the radiation belt. Cattell et al. (2015) statically investigated the whistler chorus waves during moderate geomagnetic storms observed by Van Allen Probes (VAPs) and found that their frequencies often dramatically dropped and became much lower than the commonly observed frequencies of whistler chorus waves. Analysis of the Poynting flux of such chorus waves suggested that low-frequency chorus waves originated in the magnetic equator region and propagated along the magnetic field lines to high latitudes. Then, the cyclotron resonance kinetic energy may exceed the rest mass energy of the electron (~0.5 MeV) at low frequency below 0.1fce, and chorus emissions below 0.1fce can play an important role. Thus, recently, VAPs observations have led to a study of the excitation mechanism by low frequency chorus waves and their effect on relativistic electrons. However, VAPs observations are limited to magnetic low latitudes (<20°), so observations of low frequency chorus waves distant from the equator have rarely been reported, and not much is known about the propagation of low frequency chorus waves from the equator to high latitude regions. In this study, we analyzed the properties of low frequency chorus waves observed using data from the search coil magnetometer onboard the Arase satellite. We investigated the L-values, magnetic local time (MLT), and magnetic latitude (MLAT) of low-frequency chorus waves observed by the Arase satellite during the period from March 2017, when the satellite began regular observations, to October 2018. Similar to the VAPs results, low frequency chorus waves were often observed in the local times from dawn to dusk by the Arase satellite. On the other hand, we found that low frequency chorus waves are also observed in the region where the MLAT is higher than 20°, outside of the VAPs observation area. Focusing on the space distribution of low frequency chorus waves observed by the Arase satellite, we analyze to clarify their effects on the dynamics of radiation belt electrons and their propagation to high latitude regions.