On the origin and configuration of the 20 March 2003 interplanetary shock and magnetic cloud at 1 AU

Abstract

On 20 March 2003, a forward shock was observed in the near‐Earth solar wind, followed 8 hours later by an interplanetary magnetic cloud (IMC), in a configuration having several uncommon features: Both were parts of a 38‐hour interval containing transient solar outflows that occurred in an extended high‐speed stream from a Y‐shaped extension of the south polar coronal hole. (In contrast, IMCs, and ejecta in general, were rarely observed within high‐speed streams at low heliolatitudes during cycle 23.) The most likely solar source for the IMC is AR 10314, located at S15°, just above the “fork” of the Y‐shaped coronal hole. Several solar flares occurred in this active region on 17–18 March, as well as a succession of four coronal mass ejections (CMEs). Velocity considerations narrow the most likely source of the 38‐hour interval of activity to two CMEs on 17 March 2003 associated with solar flares at W33° and W39°. The IMC axis had a north‐south orientation, which is unusual for IMCs during this solar cycle. Its left‐handedness implies an association with a left‐skewed coronal arcade, which is less common in the Southern Hemisphere. Considering the shock observed ahead of the IMC, we conclude based on orientation and ram pressure arguments that this shock was not driven by the IMC, as might be presumed, but was the flank of an unrelated shock that overtook the IMC approximately halfway between Sun and Earth, heating the plasma and accelerating particles within the IMC. The CME associated with the X‐class flare, at 1208 UT on 18 March in AR 10314 appears to be the solar source for this shock.