Prediction of Magnetic Orientation in Driver Gas Associated <i>B_z</i> Events

Can the magnetic orientation of driver gas in the interplanetary medium observed at Earth be predicted when its solar source is identified? Based on the exploratory assumption that the free magnetic energy present when a coronal mass ejection (CME) is initiated is transferred almost totally to other forms of energy by the time the CME stops acceleration, the potential field model can appropriately be used to calculate the magnetic orientation in the ejected plasma. As a first test of this hypothesis we have investigated the source regions of five strong -B z events detected at I AU for which solar sources have been identified (Tang et al., 1989; Tsurutani et al., 1992). Three were traced to flare associated CMEs, one to an eruptive prominence associated CME, and one to three possible solar sources. The computed magnetic orientations at the candidate "release height" (the height where the front of a CME ceases to accelerate) above the flare sites associated with CMEs show the existence of the expected southward field component. On the other hand, the predicted field orientation at the candidate release height above the erupting prominence associated with a CME does not have a dominant southward field component. The result shows that the exploratory assumption may be proper only for the case of flare CMEs. With such a severely limited number of cases the results are only suggestive, but the magnetic orientation in flare associated CME generated driver gas may be predictable.