Rare Occurrence of Off‐Equatorial Edge Initiating and Equatorward Surging Plasma Depletions Observed in OI 630‐nm Imaging

Abstract

Usual depletions in the Northern Hemisphere form a tree fork junction feature near the south (i.e., toward the equator) in all‐sky airglow images, and its branches surge toward the north (i.e., poleward). We report in this paper unusual airglow depletions in OI 630.0‐nm imaging over India that surged equatorward and formed an inverted tree fork junction feature on 3 January 2011. Airglow images showed faint signatures of medium‐scale traveling ionospheric disturbances in the beginning that were east‐west aligned and propagated toward equator. Within 06 to 18 min of its passing, turbulent structures were noted in the field of view, and two depleted patches appeared out of low airglow background over the off‐equatorial edge. The apex height of the associated geomagnetic flux tubes varied from ~ 1,400 to 1,600 km. Later, these dark regions intensified and surged equatorward while drifting slowly toward west. While remaining almost steady, one of them further intensified and continued to surge equatorward. The second dark patch got linked up with another isolated depletion to form an elongated depleted feature. Next, the southern end of this attached feature surged equatorward. When two depletions were well formed, an inverted tree fork junction was noticeable. During this time, the equatorward motion of the equatorial ionization anomaly structure is also seen. To the best of our knowledge, this is the first imaging observation of an inverted tree fork junction feature and brings out the unknown facets of ionospheric irregularities.