First Daytime Red‐Line Emission Measurements of the Stable Auroral Red (SAR) Arcs

Abstract

We present the first ground‐based measurements of daytime stable auroral red (SAR) arc using OI 630.0 nm emissions. SAR arc is a direct consequence of heat conduction from the inner‐magnetosphere to the ionospheric regions characterized by increased electron temperatures and low electron density in the region of mid‐latitude trough. So far, SAR arc emissions have only been reported for nighttime conditions. For the present study, daytime optical measurements were enabled using a high‐resolution imaging echelle spectrograph from Boston (42.36°N, 71.05°W, MLAT ≈ 53°). Simultaneous Millstone Hill Incoherent Scatter Radar and Defense Meteorological Satellite Program measurements of electron density and temperature confirm our findings. Forward modeling approach enabled estimation of daytime electron temperatures in 200–650 km altitude during this SAR arc event to be varying between 3,500 and 4,400 K. These observations open numerous possibilities of optical investigations of magnetospheric‐ionospheric interactions during daytime, when the upper atmosphere is dynamic with large gradients in ionospheric conductivities, temperatures, and winds.

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

“…Another important parameter that can affect O( 1 D) emissions is the electron temperature in subauroral ionosphere as discussed in Section 3. Such O( 1 D) 630.0 nm emission enhancements have been reported in the literature for both night time and daytime conditions (e.g., Baumgardner et al, 2013;Martinis et al, 2019;Mendillo et al, 2016;Upadhyay & Pallamraju, 2024). These larger emissions at F-region heights have been described by thermal emissions due to increased Te and as a plausible source for SAR arc formation (e.g., Nagy et al, 1970;Kozyra et al, 1997 and reference therein).…”

“…The night time observations suggest that sufficiently high enhancements in Te (>3,000°K) enable the excitation of atomic oxygen to its O( 1 D) state and produces OI 630.0 nm red-line emissions in the form of stable auroral red (SAR) arcs (e.g., Kozyra et al, 1997;Rees & Roble, 1975). This relation between increased Te associated with ionospheric trough and O( 1 D) emission enhancement causing SAR arc has been shown in various studies for nighttime (e.g., Baumgardner et al, 2013;Martinis et al, 2019;Mendillo et al, 2016) and recently, for the first time in daytime conditions (Upadhyay & Pallamraju, 2024). Specific causes for sudden mid-latitude electron temperature enhancements are not completely understood yet.…”

“…Temporal variations in the dayglow emissions can also occur through the photoelectron impact mechanism on occasions, such as, occurrence of solar flares (e.g., Das et al, 2010;Sumod et al, 2014). Also, during some geomagnetically disturbed periods, when additional deposition of heat from inner-magnetosphere in the mid-latitude ionosphere increases the ambient electron temperature (Te) to more than 3000 K, such enhanced Te also causes production of O( 1 D) emissions through the PEI mechanism and thereby produces SAR arcs (Rees & Roble, 1975;Upadhyay & Pallamraju, 2024).…”

“…In the study of high-latitude daytime cusps by D. Pallamraju et al (2004), the enhanced OI 630.0 nm dayglow emissions were found to be directly associated with increased Te (>4,000 K). Furthermore, the recent work by Upadhyay and Pallamraju (2024) estimated the sufficiently high Te values (reaching approximately 4,400 K) required to produce the daytime SAR arc emissions. It is worth noting that all days listed in Table 1 with observed enhancements in the OI 630.0 nm emissions were associated with increased electron temperatures.…”

“…As, increased Te peak and reduced Ne are the two important ionospheric signatures of the presence of SAR arcs (Baumgardner et al, 2013;Foster et al, 1994), several studies have used DMSP measured nighttime Ne, and Te variations to identify SAR arcs over midlatitudes (e.g., Martinis et al, 2019;Mendillo et al, 2016). In addition, a few earlier studies have reported an increased Te in both nighttime (e.g., Mendillo et al, 2013) and daytime (Upadhyay & Pallamraju, 2024) as measured by DMSP corresponding to the presence of SAR arcs. In this study, we have used daytime (6-18 magnetic local time, MLT) DMSP latitudinal measurements at 4-s resolution to examine the presence of peak Te along with its location.…”