7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Upper-hybrid waves and fluctuations in space plasmaPeter H. Yoon1 and Y. Omura2
1IPST University of Maryland College Park, USA, 2 RISH, Kyoto University, Japan
yoonp@umd.edu:
Quasi electrostatic fluctuations in the upper-hybrid
frequency range are commonly detected in the planetary
magnetospheric environment. The origin of suchphenomena may relate to the instability driven by aloss-cone feature associated with the electrons
populating the dipole-like magnetic field. The present
paper carries out a one-dimensional electrostatic
particle-in-cell simulation accompanied by a reduced
quasilinear kinetic theoretical analysis to investigate thedynamics of the upper-hybrid mode instability driven by
an initial ring electron distribution function, which is aform of loss-cone distribution. A favorable comparison isfound between the two approaches, which shows that thequasilinear theory can be an effective tool in the study ofplasma instabilities, especially if it is guided by, andvalidated against, the more rigorous simulation result.
The type of quasilinear theory employed in the present
study assumes that the particle distribution function canbe modeled by an analytical time-dependent form. Sucha modeling must be done carefully to reflect the actualphysics. In the context of solar wind research, thetime-dependent bi-Maxwellian distribution has beenquite successfully used for various temperature
anisotropy-driven instabilities – see, e.g., a recent paper[1]. However, in the present case of an initial ring, ormore generally, the loss-cone instability, the time
evolution of the electron distribution is not trivial topredict a priori. In order to guide the theoreticalmodeling, we have thus first carried out aone-dimensional electrostatic (1D ES) particle-in-cell(PIC) code simulation. Guided by the results ofsimulation, we have subsequently modeled thetime-dependent hot electron distribution by the same ring
form, except that the ring speed and thermal spread areallowed to vary in time. Upon verifying the result ofsuch a reduced quasilinear theory against the PIC
simulation, it is found that the quasilinear method quite

reasonably reproduces the simulation result.
The present paper is largely motivated by quasi
electrostatic fluctuations in the upper-hybrid frequency
range pervasively detected in the terrestrial or planetary
magnetosphere, which is presumably excited by theloss-cone electrons, ring distribution being a form ofloss-cone. However, the occurrence of ring or non
gyrotropic ring (also known as the agyrotropic crescent)electron distribution can take place in other physicalsituations. For instance, such a distribution of electrons
have been seen in numerical simulations of magnetic
reconnection exhausts [2,3]. Such electron distributions
are detected by Magnetospheric Multi-Scale (MMS)
spacecraft close to the electron diffusion region, which
are accompanied by intense high-frequency upper-hybrid
fluctuations [4]. This broadens the relevance of thepresent to situations beyond the magnetosphericapplications.
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