Low-latitude 10 eV electrons: Nighttime plasma line as a new research capability

The incoherent scatter radar (ISR) plasma line (PL) in daylight is excited by photoelectrons. Measurement of its intensity (κTp) has long been used for their study. At night, despite the absence of any other excitation mechanism, the PL intensity should have a thermal amplitude level κTe, determined by the electron gas temperature Te. To the contrary Carlson et al. (1982) found nighttime PLs over Arecibo enhanced >3 times above thermal intensities despite the absence of any known causative mechanism. Here we present discovery that nighttime PLs frequently recur, with quite variable enhancement. In the absence of direct solar EUV, these enhanced PLs must be produced by particle precipitation, manifested by the presence of variable recurring F region ~10 eV electron fluxes. We see this as offering a new tool for space environment studies, opening a new era of particle precipitation research and ISR calibration. Key Points 10 eV electron flux often weakly present at low latitude in total darkness Nighttime plasma‐line detection opens a new area of particle precipitation study Nighttime plasma lines a new diagnostic for space/geophysical environment