Simultaneous flares are unrelated solar flares in different active regions that occur at nearly the same time. If the flares coincide in time not by chance, but by the triggering of some physical connection, they are called sympathetic flares. More loosely, a sympathetic flare can also be defined as the initiation of a solar flare resulting from a transient phenomenon occurring elsewhere on the Sun. Hence, sympathetic flares are a subset of simultaneous flares.
The distinction between the two, i.e. is the pair of flares somehow physically connected or not, is a difficult problem. Though the existence of sympathetic flares was suggested already in the mid 1930's, their existence is still being debated. Important support for the existence of a physical connection between two far-away regions was given on 1 August 2010, when pretty much the entire visible northern hemisphere erupted. In a matter of hours, various flares and filament eruptions occurred though they were several 100.000 km separated from each other (Note 1).
There may be several causes for sympathetic flares. In EUV (SDO/AIA), coronal loops can sometimes be seen connecting active regions, occasionally even crossing the solar equator (image underneath). Any disturbance in one region can then quickly be "transported" to the other region, where it may induce another flare or filament eruption. Also EIT waves (see this STCE Newsitem) may disturb the magnetic conditions of a distant region or filament enough to trigger an eruption. It is generally believed that further examination of these sympathetic events may lead to a better understanding of flaring mechanisms.
Last week, scientists got a few additional simultaneous flares requiring further investigation. No less than 8 flare events had coinciding brightenings in two or even three well separated sunspot regions. Half of these occurred between NOAA 2035 and 2038. This movie shows two examples on 22 and 23 April. It concerns a C2 and C4 flare peaking resp. at 18:41UT and 01:04UT (images underneath). In both cases, the brightening peaked almost at the same moment in the sunspot groups. Note in the movie also the small flare south of NOAA 2042 around 23:50UT, which may or may not have initiated the 01:04UT events hardly an hour later. Further research is required to determine if there was a true physical connection or not.
Note 1 - Further reading and movies on this event in this STCE Newsitem and in NASA's Science News.
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