Exohiss is a low-frequency structureless whistler-mode emission potentially contributing to the precipitation loss of radiation belt electrons outside the plasmasphere. Exohiss is usually considered the plasmaspheric hiss leaked out of the dayside plasmapause. However, the evolution of exohiss after the leakage has not been fully understood. Here we report the prompt enhancements of exohiss waves following substorm injections observed by Van Allen Probes. Within several minutes, the energetic electron fluxes around 100 keV were enhanced by up to 5 times, accompanied by an up to 10-time increase of the exohiss wave power. These substorm-injected electrons are shown to produce a new peak of linear growth rate in the exohiss band (< 0.1fce). The corresponding path-integrated growth rate of wave power within 10° latitude of the magnetic equatorial plane can reach 13.4, approximately explaining the observed enhancement of exohiss waves. These observations and simulations suggest that the substorm-injected energetic electrons could amplify the preexisting exohiss waves.
Decametric (DAM) radio emissions are one of the main windows through which one can reveal and understand the Jovian magnetospheric dynamics and its interaction with the moons. DAMs are generated by energetic electrons through cyclotron-maser instability. For Io (the most active moon) related DAMs, the energetic electrons are sourced from Io volcanic activities, and quickly trapped by neighboring Jovian magnetic field. To properly interpret the physical processes behind DAMs, it is important to precisely locate the source field lines from which DAMs are emitted. Following the work by Hess et al. (2008, 2010), we develop a method to locate the source region as well as the associated field lines for any given DAM emission recorded in a radio dynamic spectrum by, e.g., Wind/WAVES or STEREO/WAVES. The field lines are calculated by the state-of-art analytical model, called JRM09 (Connerney et al., 2018). By using this method, we may also derive the emission cone angle and the energy of associated electrons. If multiple radio instruments at different perspectives observe the same DAM event, the evolution of its source region and associated field lines is able to be revealed. We apply the method to an Io-DAM event, and find that the method is valid and reliable. Some physical processes behind the DAM event are also discussed.
Using the test particle simulation method, we investigate the stochastic motion of electrons with energy of 300 keV in a monochromatic magnetosonic (MS) wave field. This study is motivated by the violation of the quasi-linear theory assumption, when strong MS waves (amplitude up to ~1 nT) are present in the Earth’s magnetosphere. First, electron motion can become stochastic when the wave amplitude exceeds a certain threshold. If an electron initially resonates with the MS wave via bounce resonance, as the bounce resonance order increases, the amplitude threshold of electron stochastic motion increases until it reaches the peak at about the 11th order in our study, then the amplitude threshold slowly declines. Further, we find that the coexistence of bounce and Landau resonances between electrons and MS waves will significantly reduce the amplitude threshold. In some cases, the electron motion can become stochastic in the field of an MS wave with amplitudes below 1 nT. Regardless, if neither the bounce nor Landau resonance condition is satisfied initially, then the amplitude threshold of stochastic motion shows an increasing trend for lower frequencies and a decreasing trend for higher frequencies, even though the amplitude threshold is always very large (> 5 nT). Our study suggests that electron stochastic motion should also be considered when modeling electron dynamics regulated by intense MS waves in the Earth’s magnetosphere.
Locating the source of decametric (DAM) radio emissions is a key step in the use of remote radio observations to understand the Jovian magnetospheric dynamics and their interaction with the planet’s moons. Wang YM et al. (2020) presented a method by which recorded arc-shaped DAM emissions in the radio dynamic spectra can be used to locate the source of a DAM. An Io-related DAM event on March 14, 2014 was used to demonstrate the method. A key parameter in the method is whether the DAM is emitted in the northern or the southern hemisphere; the hemisphere of origin can be determined definitively from the polarization of the emission. Unfortunately, polarization information for the emission on March 14, 2014 event was not recorded. Our analysis assumed the source to be in the northern hemisphere. Lamy et al. (2022) argue convincingly that the source was probably in the southern hemisphere. We appreciate the helpful contribution of Lamy et al. (2022) to this discussion and have updated our analysis, this time assuming that the DAM source was in the southern hemisphere. We also explore the sensitivity of our method to another parameter — the height at which the value of fce,max, which is the maximal electron cyclotron frequency reached along the active magnetic flux tube, is adopted. Finally, we introduce our recent statistical study of 68 DAM events, which lays a more solid basis for testing the reliability of our method, which we continue to suggest is a promising tool by which remote radio observations can be used to locate the emission source of Jovian DAMs.