Homologous flares

Homologous flares are the solar equivalent of identical twins. They concern a series of solar flares taking place repetitively in the same active region with essentially the same position and with a common pattern of development, i.e. having the same main footpoints and general shape in the main phase as defined in H-alpha or EUV-imagery. Though not a requirement, homologous flares often have similar strength, and if there are more than two, they sometimes occur within similar time intervals.

A good example occurred on 24-26 November 2000, when five homologous X-class (!) flares were produced by active region NOAA 9236 southwest of its main leading spot (image above). Note in the middle image, there’s a difference of 5 hours between SOHO’s EUV image and the continuum on which it is overlaid. The study of this kind of flares is important as it might provide information on flare trigger mechanisms, as well as on the processes of energy storage and release. Hence, these studies may offer vital clues for improving space weather forecasts.

Last week, moderately sized sunspot region NOAA 2017 produced 1 X- and 3 M-class flares within less than 48 hours (see image above). On 28 March, there was an M2.0 flare (peak at 19:18UT) followed a few hours later by an M2.6 flare (peak at 23:51UT). One day later, an X1.0 flare peaked at 17:48UT, and on 30 March there was an M2.1 flare hitting its maximum at 11:55UT. The flares occurred near a delta region to the north and east of the main spot (image underneath). A delta concerns spots of opposite polarity within the same penumbra and within 2.5 degrees from each other.

A preliminary analysis of H-alpha (showing the cool inner atmosphere of the Sun) and AIA 171 (showing the Sun’s much hotter upper transition region) imagery seems to indicate that the first two flares might indeed be homologous, as they have a very similar (but not identical) outlook and also show a clear surge in H-alpha (image underneath, top). This is not the case with the X1-flare and the third M2-flare (bottom images), having an outlook that is clearly distinct from the first two, with no surge and a different post-flare loops configuration. The X1-flare was also the only event to enhance the proton flux levels, though no threshold was reached.

The first two M-class flares are similar, but not completely identical. For example, the surge from the first M2-flare is less pronounced and double. Also, the second M2-flare produced a faint Moreton wave (in H-alpha: a bright arc rapidly moving to the north), just as the much stronger X1-flare. All these differences may be due to changes in the magnetic field of the sunspot region, as can be seen in the mosaic underneath. Notice the similarity and subtle differences for the first two flares (top images), and the major changes in the delta region for the two other flares (bottom images).

Over the years, the concept of "homology" has been extended to the flare-associated EIT-waves, coronal mass ejections (CME), and even radio-bursts. This movie first shows the white light and magnetic evolution of the sunspot group NOAA 2017, followed for each of the four events by the activity in H-alpha and EUV (overlaid on white light imagery), and ends by showing the associated EIT-waves and CMEs (difference images, i.e. only changes between two subsequent images are shown).

Credits - Data and imagery for the movie clips were taken from the GONG H-alpha network, SDO, SOHO/LASCO, PROBA2, and (J)Helioviewer.

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