Modeling the evolution of chorus waves into plasmaspheric hiss

Plasmaspheric hiss (PH) is a band-limited, incoherent whistler mode emission found predominantly in the plasmasphere or high-density plasma regions in the near-Earth space environment. Since its discovery in the late 1960s, PH has been recognized as playing an important role in shaping the structure and dynamics of the Earth's electron radiation belts and creating the slot region that separates the inner and outer belts. However, the origin of PH has been a topic of intense debate for over four decades. Here we present a model for the origin of PH that involves the evolution of chorus waves into the PH spectrum. We perform extensive ray tracing using the HOTRAY code and calculate Landau damping using newly developed suprathermal flux maps from THEMIS observations, that are L and magnetic local time dependent, for both inside and outside the plasmasphere. Our results show remarkable consistency with the observed statistical characteristics of hiss, including the day/night asymmetry in wave power, frequency spectrum, geomagnetic control of PH, quasi-parallel equatorial wave normal angles, and confinement within the plasmasphere. Our model also reproduces ancillary features such as exohiss and extremely low frequency (ELF) hiss and might be related to a previously reported phenomenon called lower hybrid resonance duct trapping in the ionosphere. A detailed analysis of ray morphologies shows a separation into four distinct groups, which correspond to (1) rays that are trapped at the plasmapause, (2) PH rays, (3) ELF hiss rays, and (4) rays that represent the bulk of the chorus ray power.

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