Weekly Notes from the Yohkoh Soft X-Ray Telescope

(Week 11, 2002)

Science Nugget: March 15, 2002


Small-scale flux emergence and flare-CME events

Introduction

More than a year ago, we reported on the recurrence of six flare-CME events from an active region, and indicated that the vigorous flux emergence may be a key to this phenomenon. The region showed very dynamic changes around the leading sunspot, with lots of small magnetic features coming and going. In the meantime, the story made a press release at NASA Goddard Space Flight Center, on the basis of a published paper. Because of the obvious flux increase, we thought that the small magnetic features were not so-called moving magnetic features (MMFs). According to Harvey and Harvey (1973), who invented the term MMF, MMFs are associated only with decaying suspot.

Recently, Zhang and Wang studied the same region and argued that MMFs are an important ingredient for the homologous CMEs, together with "active region loops" which are comparable to the extent of coronal dimming. They proposed that emerging flux in the form of MMFs carry twist, which is transferred to the active region loops. While the basic idea may contain some truth, we enconter some difficulties which are given in the following.
 

Relation of MMFs with UV brightenings and flares

The Zhang and Wang article concentrated on the first flare-CME event in the series. One of their statements is that UV brightenings as observed in TRACE 1600 Å data are correlated with the disappearances of MMFs due to cancellation. Cancellation represents magnetic reconnection of unconnected opposite polarities at the photospheric level. The images below (click to enlarge) compare TRACE and MDI data prior to the second event, in an attempt to associate UV bright points with pairs of opposite polarities (white: positive, black: negative).

Indeed, one may say that all the encircled UV bright points (the leftmost image) are close to mixed polarities in the middle figure (magnetogram). But we should note that such mixed polarity areas are everywhere. In order to know which pairs are MMFs and which pairs are unconnected, it is necessary to trace the evolution of individual pairs. We will show a magnetogram movie from MDI. But before that, we point out that the UV bright points do not appear to be "the early appearances of the flare" as claimed, as far as the second flare is concerned -- see the next figure.

This figure shows where the flare foot-points are located (see the arrows) on the basis of images in three wavelengths. The bottom panels show SXT and HXT images, respectively. We remember in certain cases SXT soft X-ray images taken in the early phase of a flare show emission from areas close to foot-points. An HXT 23-33 keV image is shown (bottom right) in contours on top of an MDI continuum image. We note that the main flare loop is not really associated with the UV brightenings, and that one foot-point is located close to the sunspot umbra. Interestingly, MDI magnetograms recorded spurious signals due to the flare (top left), and they probably represent the flare foot-points, consistent with the locations in the HXT image. Note that all three X-class flares on 24 November showed this artifact in MDI magnetograms, at nearly the same locations. Furthermore, the flares also left signals in MDI continuum images, i.e., they are white-light flares (cf. last week's nugget.) As usual, TRACE 1600 Å data also show features other than the main flare loop (the image at top right is saturated.)
 

Relation between the flare and the dimming

The relation of the flare and the associated CME is always a tricky business, because we still do not know for sure what the low coronal signatures are for CME initiation. But if we assume that coronal dimming represents the foot prints of the CME loop or the "flux rope," then the following figure may indicate the relation. Although, empirically, dimming seen in EIT images is well correlated with CMEs, it is always better to look for dimming at different wavelengths to ensure that dimming is not a temperature effect. An increase (or decrease) of temperature off the instrument sensitivity range will lead to the disappearance of features. Here we compare SXT and EIT data, and find that the dimming areas match. The box indicates the field of view of the figures given above.  It is not clear what the toplogical relation is between what appear to be MMFs and the hypothetical loops connecting the two dimming regions.


Unusual MMFs

A movie of MDI magnetograms, taken from full disk data, was given in the previous nugget. In the following we show a movie of MDI high-resolution magnetograms taken during November 23/24.  In order to make it lighter on computer memory and more importantly to increase the signal to noise ratio, we did 15-minute averaging; the original data were taken every minute.  We the confirm very dynamic changes in the area (the "moat") surrounding the sunspot, and find different patterns from those described by Harvey and Harvey and others in such a way that (1) the motion is not always radial, and that (2) the magnetic features do not necessarily cancel and disappear at the moat outer boundary, (3) where the flux (notably the negative polarity, opposite to the sunspot polarity) builds up. More importantly, (4) there is a persistent azimuthal motion, clockwise in a area west of the spot, the area at which a flare foot-point is located. Therefore, we conclude that the major flare/CME activity started only on November 24, by which time enough magnetic flux (and shear) accumulated to the west of the sunspot, even though the MMF activity started on the 23rd.


Conclusions

There have been several published analyses on MMFs since the 1970s, and different authors emphasized different results as to how the MMF bipolar patterns are seen with respect to the radial direction, what evolution MMFs take, and how MMFs are correlated with coronal activity. But there is a possibility that apples and bananas were compared, that is "MMFs" may mean a range of things as long as there are outwardly moving magnetic features. In our region, it seems to be the flux emergence and the accumulated shear west of the sunspot that were a direct cause of the repeated CME/flare events. We acknowledge an interesting work by V. Yurchyshyn et al., who suggested the possible correlation between the behavior of MMFs and the twist of the sunspot.

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March 15, 2002

N. Nitta (NVN) (nitta@lmsal.com)