The Relationship of Exohiss Waves With Plasmaspheric Hiss Distribution and Solar Wind Parameters

Exohiss waves below 0.1 electron cyclotron frequency (fce) are structureless whistler‐mode emissions typically observed in the plasmatrough. Plasmaspheric hiss may possibly propagate from the plasmasphere into the plasmatrough and evolve into exohiss waves. We investigated the relationship of exohiss occurrence and characteristics with plasmaspheric hiss occurrence and solar wind parameters, analyzing Van Allen Probe observations from 1 October 2012 to 28 February 2018. Exohiss waves observed in the plasmatrough occurred more frequently on the dayside than the nightside, which was consistent with the plasmaspheric hiss distribution in the plasmasphere. Exohiss occurrence gradually increased up to ∼4 hr after hiss measurements and showed a magnetic local time dependence on the plasmaspheric hiss amplitude. We also determined the relative contribution of each solar wind parameter to exohiss distribution as based on exohiss measurements made 0–4 hr after plasmaspheric hiss measurements. A stronger southward interplanetary magnetic field (IMF) BZ limited the region of exohiss occurrence to the prenoon sector, again consistent with the distribution of plasmaspheric hiss. Prenoon exohiss was also observed for stronger dynamic pressure (PSW), but the plasmaspheric hiss appeared in the postnoon sector. This discrepancy indicated that prenoon exohiss is locally excited rather than a product of plasmaspheric hiss leakage. In addition, during enhanced solar wind conditions with southward IMF BZ or higher PSW, the intensity of the lower‐band chorus was enhanced even below 0.1fce, corresponding to the frequency range of the exohiss, implying that the nightside exohiss may be related to the evolution of low‐frequency chorus waves. Key Points Exohiss waves occur mainly on the dayside. They coincide with and increase up to 4 hr after intense plasmaspheric hiss waves Exohiss shows magnetic local time dependence, and dayside exohiss is also dependent on solar wind conditions A low‐frequency chorus most likely evolves into nightside exohiss