High latitude activity
Science Nugget: Feb 26, 1999
1. What is this about?
Ron Moore's favorite view of eruptive flares involves the appearance of
elbow-shaped magnetic loops, tied down by a stronger core magnetic field.
Reconnection ("tether cutting") allows the eruption to proceed somehow.
Yohkoh SXT has seen many examples; see the "aurora
polaris" event, discovered by Sam Freeland, for one of the first published
examples showing the behavior in soft X-rays. These events sometimes are
huge and/or powerful, launching dangerous CMEs, for example. On the other
hand, as in the case reported here, they can occur gently and innocuously.
2. Event of Feb. 23-24, 1999
At northern high latitudes, we see the following things in the Kitt Peak
magnetogram and an SXT image during the "Elbows" event in question:
Regrettably, these images aren't scaled and registered yet, but still
one can see the extreme magnetic complexity in the north. Four alternating
streams of large-scale signed field (not the "magnetic carpet"!)
vie with a complex active region a bit to the west of the central meridian.
This calls to mind Dave
Webb's hope, shared by Yutaka Uchida, that multipolar magnetic structures
can lead to eruptive instability. But detailed analysis of this type of
event - proceeding apace - has not yet come to any real conclusion about
this.
Here's a before/after comparison for the event itself:
As usual, the difference shows new things in white, and old things in
black, so that one can see the overlapping elbow loops, reminiscent of
folded arms, supplanted by overarching larger loops as the event proceeds.
In this case, no obvious cusp nor dimming exterior to the flare. Indeed,
LASCO did not report a CME, even though we think that this type of event
often appears as a low-coronal counterpart of a CME launch.
3. What have we learned?
Perhaps not too much, except to be reminded that this kind of event can
arise in the quiet Sun, not directly in association with an active region.
On the other hand, perhaps we have learned a bit: the lack of a CME in
spite of the clear signature suggests that maybe the mapping between eruptive
flares and CMEs is not one-to-one. More study needed!
February 28, 1999: Hugh Hudson (email hudson@isass0.solar.isas.ac.jp)
