Evidence for Anomalous Energization of Electrons by Beam-Ionosphere Interactions in the Auroral F-region

Abstract:

We and others have previously identified anomalous Incoherent Scatter Radar (ISR) spectra confined to narrow altitude ranges near the F-region peak and correlated with dynamic auroral precipitation [Isham et al., 2012; Akbari et al., 2012, 2013]. The radar echoes are attributed to beam-generated Langmuir turbulence. In this work we used a 1-dimentional Zakharov simulation to constrain the range of physical mechanisms underlying these observational features. Our results suggest the presence of a local F-region energy source, possibly produced by non-linear wave-wave interactions.

In details we have found that:

1) Simultaneous enhancements in ion-line and plasma-line channels of the PFISR can be produced by strong Langmuir turbulence and caviton collapse generated by relatively strong soft electron beams (< 1 keV) that lack the electron population directly resonant with the detectable waves.

2) Except for cases of very high electron to ion temperature ratios (Tr > 6), caviton collapse would give rise to a dominant zero-frequency peak in the ion-line channel. In these cases simulated spectral features do not match those measured by ISRs.

3) Observations of enhanced ion-acoustic shoulders and lack of observation of such high temperature ratios in ISR measurements, therefore, suggest that the Parametric Decay Instability (PDI) operates at the observations wavenumbers. This in turns requires local energization of lower energy electrons (< 20 eV) at ~250 km.

4) The existence of other modes and processes is also supported by observations of non-linear features in the plasma-line measurements.