NOAA 2494's growth spurt
posted: February 09, 2016

A movie of this event can be found here.

Once again, solar flaring activity has been pretty low. The strongest flare was a C5.1 peaking on 4 February at 18:22UT. The region responsible for this event was NOAA 2494, a sunspot group that was developing near the centre of the solar disk at that time. The region already reached its maximum sunspot area about a day later. It was always smaller than two times the total surface area of the Earth, so considerably smaller than e.g. NOAA 2192. Moreover, it had a quite simple magnetic configuration, with only some polarity mixing in the middle portion and trailing main spot. The SDO white light clip below shows the evolution of NOAA 2494 from 03 till 08 February.

This movie shows this quick increase in sunspot area. It covers also the time period from 03 till 08 February. The first clip is a full disk view, where NOAA 2494 can be seen quickly developing, then steadying before a gradual decline sets in as it transits the central part of the solar disk. The second clip shows a zoom of the sunspot group in white light (see also above). The last clip shows the magnetic evolution of the group. White denotes field lines coming out of the solar surface ("positive"), whereas black represents magnetic field lines returning to the solar interior ("negative").

Note how initially (4-5 February), small patches of similar magnetic polarity are percolating ("clumping together") into mature sunspots (see Schatten 2007). This can also be seen in the images above. Low-level C-class flares occur where the newly formed sunspots of opposite polarity are close to each other. This is mostly in the middle and northern portion of the region. Starting on 6 February, positive magnetic flux can be seen emerging south of the main trailing negative spot. Correspondingly, flaring activity in NOAA 2494 shifted to the south of the trailing main spot, but only two C1 flares erupted from this location. This is illustrated by the images below showing the C5 eruption on 04 February (around 18:22UT; left), and the C1 eruption on 06 February (around 19:38UT; right), both in white light, the magnetogram, and in extreme ultraviolet around the time of maximum x-ray intensity.