Sunday, 20 May 2012

AR11476 - Surges at the Limb

AR11476 - when a great spot group rounds the east limb we see the chromosphere side-on, and a range of complex detail is revealed - detail often unseen when the group is on the disc and we view from above.
    Surges: here we see two large surges emerging (apparently)  from small spots half way between the preceding (p) and following (f) spots of the group. Views 1 and 2 show the outflow stage of the surge - and 3 and 4 the retraction phase. At full extent the surges reached ~120Mm, before retracting. View 4 shows some "wisps" left behind when the bulk of the material dived back into the sun.
     Surges take some explaining: they seem to be closed field structures emerging in areas of opposite magnetic polarity (see Schrijver and Zwaan "Solar and Stellar Magnetic Activity" P199). But why do they so often return along the same path? Often they do so many times over.
     Field configuration: fields emerging from spot umbrae are near-vertical, while those at the penumbra are near-horizontal - as shown in red and black (dotted) respectively.  Surges mostly emerge near the outer penumbrae and seem to follow the suggested trajectory dotted in black- the spots umbral fields  Velocity in this case was ~75km/sec - a typical surge value.

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