May 13 2005 M8.0 Flare, CME and Geomagnetic Storm

The May 13 flare
(see more info about the flare here) occurred in an active region NOAA 0759 at N10E10 with relatively simple magnetic topology. The position, magnetic structure and polarity distribution in this active region are very similar to these of the SHINE May 12, 1997 event. The May 13 2005 AR had one large well developed sunspot and several small sunspots with partially developed penumbra. The magnetic structure could be described as bipolar with several strong magnetic flux concentrations. A prelimenary analysis of the May 13 2005 flare and CME was presented in a poster at 2005 Fall AGU meeting in San Francisco, CA.

A PDF copy of an accepted that describes this event is available here

BBSO completely covered this flare and the following 1 min cadence data have been obtained:

Please contact Vasyl Yurchyshyn for data availability

BBSO Ha 1635UT

BBSO Ha 1757UT

Ha images above show that the two ribbon flare was associated with the major neutral line (hi-res movies are available upon a request). We would like to note that a dark chromospheric filament above the neural line did not erupt and it was seen before, during and after the flare. Structure of Ha filaments as well as the orientation of the post-flare loop system shows that negative helicity (left handed twist) was dominant in this active region.

We modeled the active region's magnetic field with a linear force-free (LFFF) model with parameter alpha=-0.025 arcsec-1. Images below show magnetic force lines over plotted over Ha images and an MDI longitudinal magnetogram. The LFFF model adequately represents both post-flare loops system and the general twist in the active region, while, of course, many peculiarities in the observed field could not be modeled.

LFFF and Halpha at 1635UT

LFFF and Halpha at 1757UT


The averaged flow map shows that there was a persistent converging flow toward the major netral line associated with the flare. A DMG movie made of the line-of-sight component reveals that this converging motion was due to a strong moat outflow from the major spot. We speculate that the structure of the magnetic field above the neutral line could be represented by a sheared arcade and magnetic cancellation due to the converging flow could lead to a reconnection and the subsequent eruption. Thus, converging motion on the neutral line brought together green field lines and reconnection produced post-flare loop system (pink) and the large-scale magnetic field (yellow) that gave rise to a CME.


DMG/MAG Flow Map

May 13 Flare cartoon

Unfortunately LASCO did not cover the event and only the first image of the CME is available. CME arrived at 1AU on early may 15 the and it had strong, negative and rotating  Bz and positive By. Visual inspection of the preliminary ACE data reveals that the CME was observed at 1AU as a magnetic cloud and this magnetic cloud was left handed and its axial field pointed eastward, which corresponds to the helicity and the direction of the active region where it originated. Due to strong negative Bz, the CME on May 13 produced a strong geomagnetic storm, when the Dst index dropped to -260nT, which is 60 nT more than predicted.

(c) Last modified on Nov 13, 2006 by Vasyl Yurchyshyn