Onset of the magnetic explosion in solar flares and coronal mass ejections

We present observations of the magnetic field configuration and its transfo rmation in six solar eruptive events that show good agreement with the stan dard bipolar model for eruptive flares. The observations are X-ray images f rom the Yohkoh soft X-ray telescope (SXT) and magnetograms from Kitt Peak N ational Solar Observatory, interpreted together with the 1-8 Angstrom X-ray flux observed by GOES. The observations yield the following interpretation . (1) Each event is a magnetic explosion that occurs in an initially closed single bipole in which the core field is sheared and twisted in the shape of a sigmoid, having an oppositely curved elbow on each end. The arms of th e opposite elbows are sheared past each other so that they overlap and are crossed low above the neutral line in the middle of the bipole. The elbows and arms seen in the SXT images are illuminated strands of the sigmoidal co re held, which is a continuum of sheared/twisted held that fills these stra nds as well as the space between and around them. (2) Although four of the explosions are ejective (appearing to blow open the bipole) and two are con fined (appearing to be arrested within the closed bipole), all six begin th e same way. In the SXT images, the explosion begins with brightening and ex pansion of the two elbows together with the appearance of short bright shea red loops low over the neutral line under the crossed arms and, rising up f rom the crossed arms, long strands connecting the far ends of the elbows. ( 3) All six events are single-bipole events in that during the onset and ear ly development of the explosion they show no evidence for reconnection betw een the exploding bipole and any surrounding magnetic fields. We conclude t hat in each of our events the magnetic explosion was unleashed by runaway t ether-cutting via implosive/explosive reconnection in the middle of the sig moid, as in the standard model. The similarity of the onsets of the two con fined explosions to the onsets of the four ejective explosions and their ag reement with the model indicate that runaway reconnection inside a sheared core field can begin whether or not a separate system of overlying fields, or the structure of the bipole itself, allows the explosion to be ejective. Because this internal reconnection apparently begins at the very start of the sigmoid eruption and grows in step with the explosion, we infer that th is reconnection is essential for the onset and growth of the magnetic explo sion in eruptive hares and coronal mass ejections.