Estimation of Downward Heat Flux Into the F‐Region From the Inner‐Magnetosphere During Stable Auroral Red (SAR) Arc Events in the Daytime Obtained Using OI 630.0 nm Red‐Line Emissions.

Stable Auroral Red (SAR) arcs are enhanced OI 630.0 nm emissions formed due to an increased electron temperature (Te) near the equatorward wall of mid‐latitude trough during geomagnetic disturbances. The Te enhancement associated with SAR arcs is driven by electron heating through heat flux precipitation from near plasmapause region to ionospheric F‐region via heat conduction. Although Te enhancements have been reported by radar/satellite measurements along with increased 630.0 nm brightness during SAR arc events, measurements of corresponding heat flux are sparse, and almost none in the daytime. This work presents the results on the estimation of electron heat flux incident during SAR arcs formed during daytime obtained by a comprehensive suite of measurements, and forward modeling. We present observations of several SAR arc events when the ground‐based OI 630.0 nm emissions were larger than the model values during disturbed periods and were found to be existing in conjunction with increased Te at the altitude of DMSP (∼840 km). Forward modeling was carried out to determine the values of Te that would cause an enhancement in these emissions during daytime at much lower altitudes (∼400–500 km). These values of Te were used to estimate the required electron heat flux varying in the range of ∼1.0–4.6 × 1010 eV‐cm−2‐s−1. These results present the first estimates of F‐region heat flux enabled using ground‐based OI 630.0 nm emissions and open a new approach in the investigations of energy released into the ionosphere through heat conduction for daytime conditions during geomagnetically disturbed periods. Plain Language Summary: Stable Auroral Red (SAR) arcs are atomic oxygen redline (630.0 nm) emissions observed in the mid‐latitude ionosphere. These are optical manifestations of direct energy transfer from high‐altitude regions of inner‐magnetosphere to the ionospheric altitudes. The process of energy transfer occurs in the form of electron heat flux or soft particle precipitation (<10 eV) along closed geomagnetic field lines. Downward heat flux increases the ambient electron temperature in the topside ionosphere, which contributes in imparting energy to the Oxygen atoms and exciting them to higher energy states. The return of excited atoms to ground‐state results in the formation of SAR arcs. Although the electron temperature can be measured at high‐altitudes by satellites during SAR arc events, the corresponding electron heat flux cannot be measured directly using existing experimental space plasma techniques. Therefore, model calculations are preferred. Important to this study is the estimation of required heat flux that caused enhancements in the measured daytime redline emissions from the ground, which were beyond the model estimates. This study finds the daytime energy flux required for producing SAR arcs is larger than that for the nighttime SAR arcs. Such findings are being reported for the first time. Key Points: Enhancements were observed in daytime O(1D) red‐line emissions during geomagnetically disturbed days over Boston, a mid‐latitude locationDirect measurement of electron temperature (Te) by DMSP showed increased brightness accompanied by increased Te at mid‐latitudesThe downward heat flux associated with larger Te in the daytime using O(1D) emissions has been obtained for the first time [ABSTRACT FROM AUTHOR]