Thermally excited 630.0nm O(1D) emission in the cusp: A frequent high-altitude transient signature

We highlight why 630.0nm auroral emissions excited by thermal electrons are expected to be significant in the cusp and are occurring more often than generally recognized. We note conditions when they are likely to occur and provide a simple formula to calculate the altitude discriminated (R/km) and line-of-sight integrated 630.0nm intensity (kR). The formula is applied to incoherent scatter radar data near the cusp to produce 2-D maps of thermal red line aurora. We estimate when the thermally excited red aurora should be negligible or not, for given electron density (Ne), electron temperature (Te), and exospheric temperature (Tn) conditions. Sensitivity to Te dominates that to Tn and Ne. We test these formulas against radar and all-sky imaging photometer data for 2days. The time/space agreement, as transient strong red arcs pass over the cusp, confirms detection of a thermal 630.0nm aurora in ~400-450km altitude, twice the height (4 times the area) conventionally assumed for 630.0nm emissions. Among many potential uses for this technique is application to the long-standing question of the degree to which magnetic reconnection events contribute to the net magnetic flux entering the polar cap. We conclude that it is more often than now recognized and provide a tool and guidelines to facilitate improvement over present underestimates. Key Points Cusp ISR maps thermally excited aurora Cusp thermal aurora unexpectedly common Transient reconnection unexpectedly important