An extraordinary SAR Arc Event



For at least the first 12 hours of March 6, 1981 (UT), the nightside earth was encompassed by an extraordinary example of a Stable Auroral Red (SAR) arc (see figure); one of the more intense of the last several years. Initial analysis of data received from several ground stations indicates 6300-Å (O(′D)) emission intensity of 2 to 2.5 KR, which remained rather constant throughout a major portion of the local evening and morning sectors. Interestingly, the arc dimmed significantly prior to morning twilight, perhaps as a result of reduced energy input or the diurnal variation of thermospheric composition. Simultaneous measurements from various latitudes yielded estimates of altitude of maximum emission and location which are 400–500 km and L = 2.8, respectively.

Of particular note in this event was the extremely pronounced separation from the more northerly auroral precipitation seen in the Figure, a separation in excess of 4 L-shells.