The curious case of two X-class flares - where one sets the stage for the other

Frequently, sunspots and solar flares are strongly related. The European Solar Telescope will offer a unique opportunity to compare small-scale magnetic and flow fields in the vicinity of flaring sunspots at various stages of evolution. A post written by Dr. Meetu Verma, from the Leibniz- Institute für Astrophysik Postdam, in Germany.  


The cosmos is filled with many fascinating heavenly bodies. However, most crucial and vitally important to the human race is our nearest star - the Sun. It provides us with all the energy needed to sustain life on the Earth. Sometimes this energy is released in an eruptive way, through so-called solar flares.

Active region 12673 contained the type of sunspots producing major flares. Initially, its main spot was classified as an alpha-spot (a simple, round spot). It soon attained a more complex configuration after new magnetic flux emerged as a bipolar region following the alpha-spot. Later, a delta-configuration appeared when the negative polarity of the bipolar region collided head-on with the main spot and umbrae of different polarities were surrounded by a common penumbra.

Horst Künzel discovered at the Einstein Tower in Potsdam in 1960 that these delta-spots are the most flare-prolific class of sunspots. On 6 September 2017, a major X2.2 flare occurred in the active region, followed by another powerful X9.3 flare three hours later. They produced extremely high X-ray emission and were even visible in white light. The X9.3 event was the strongest flare in solar cycle 24. However, the two flares differed significantly. While the first was confined and limited to small patches, the second was more extended and formed a two-ribbon configuration. Both flares altered the magnetic field topology in the surroundings, creating regions with penumbral decay and umbral strengthening, as can be seen in the movie.

The changes in the flow field during the pre- and post-flare phases traced the energy buildup and energy release in the active region. The observed photospheric shear motions created a highly non-potential field configuration, which provided the energy that powered the flares. Shear flows are like cars driving in opposite directions on both sides of the center divider on a highway, and can drag magnetic field lines with them.

All this supports the scenario where the X2.2 flare set the stage for more extended emission in the X9.3 flare. This conclusion is based on data from the Solar Dynamics Observatory spacecraft. The European Solar Telescope, with its 4 metre aperture and proposed instruments, will provide higher spatial and temporal resolution, offering a unique opportunity to compare small-scale magnetic and flow fields in the vicinity of flaring sunspots in different environments and at various stages of evolution.

The movie shows maps of active region NOAA 12673 on 6 September 2017, just before the X9.3 flare. From left to right and top to bottom: continuum intensity, masks of the penumbra (gray) and the umbra (black), horizontal magnetic flux density, line-of-sight velocity, vertical magnetic field density, and total flux density.

Data courtesy of NASA/SDO and the AIA and HMI science teams.

Publication: Verma, 2018, A&A, 612, 101.

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