Polar X-Ray Arcade Formation and Giant Cusp
The pre-event structure and its change in the polar arcade formation event of February 24–25, 1993 are examined. The pre-event coronal structure was a double bipolar array (or a quadruple array) type, with the middle region having a mixed photospheric magnetic polarity. A bundle of longitudinal threads (“spine”) in soft X-rays rose with a tooth-brush type foot connections, and the arcade formation progressed westwards in time. The westmost part of the “spine” ballooned up, and the shape finally took a form like the giant cusp of Jan 25, 1992. Alerted by this, we examined the latter event in detail. We found that the giant cusp, against the expectation from the classical model, showed its southern foot remaining on the visible disk, whereas the main body of the cusp took place near or beyond the limb. These suggest that, contrary to the conventional interpretation, it may just have formed from a swollen “spine” structure similar to that of the Feb 25, 1993 event.