The enhancement of cosmic radio noise absorption due to hiss-driven energetic electron precipitation during substorms

The Van Allen probes, low-altitude NOAA satellite, MetOp satellite, and riometer are used to analyze variations of precipitating energetic electron fluxes and cosmic radio noise absorption (CNA) driven by plasmaspheric hiss with respect to geomagnetic activities. The hiss-driven energetic electron precipitations (at L ∼ 4.7-5.3, magnetic local time (MLT) ∼ 8-9) are observed during geomagnetic quiet condition and substorms, respectively. We find that the CNA detected by riometers increased very little in the hiss-driven event during quiet condition on 6 September 2012. The hiss-driven enhancement of riometer was still little during the first substorm on 30 September 2012. However, the absorption detected by the riometer largely increased, while the energies of the injected electrons became higher during the second substorm on 30 September 2012. The enhancement of CNA (ΔCNA) observed by the riometer and calculated with precipitating energetic electrons is in agreement during the second substorm, implying that the precipitating energetic electrons increase CNA to an obviously detectable level of the riometer during the second substorm on 30 September 2012. The conclusion is consistent with Rodger et al. (2012), which suggest that the higher level of ΔCNA prefers to occur in the substorms, because substorms may produce more intense energetic electron precipitation associated with electron injection. Furthermore, the combination of the observations and theory calculations also suggests that higher-energy electron (>55 keV) precipitation contributes more to the ΔCNA than the lower energy electron precipitation. In this paper, the higher-energy electron precipitation is related to lower frequency hiss.