In the latter half of October, huge sunspots were observed on the surface of the Sun. These sunspots appeared at the east limb of the Sun on Oct. 16, and moved to the west as the Sun rotated (Fig. 1 and Fig. 2(a)). They rotated out of view after Oct. 30. On Oct. 26, the total area of these sunspots became almost 66 times larger than the Earth’s cross section (*1). This was the largest sunspot area in this solar cycle, and the largest observed in the last 24 years (since Nov. 18, 1990). In the middle of November, these sunspots appeared again at the east limb, as the Sun’s rotation brought them back into view (Fig. 2(b)).
Fig.3 and Fig.4 show the Sunspots captured by the solar observing satellite Hinode on Oct. 24 and Nov. 15, respectively. In these figures, image (a) is a white light image, and image (b) is the magnetic field map (white = positive (N) pole, black = negative (S) pole).
Sunspots look darker because their temperature is lower than the surroundings. Strong magnetic fields in the sunspots lower the temperature, because the magnetic fields obstruct the convection that transports the heat generated in the center of the Sun to the solar surface. Strong magnetic fields sometimes cause solar flares, huge explosions which occur in the solar atmosphere. Therefore, one of the reasons why Hinode accurately measures the magnetic fields on the solar surface is to understand the mechanism of solar flares.
In both Fig. 3(b) and Fig. 4(b), the right (preceding) sunspots have N poles and the left (following) sunspots have S poles. In Fig. 4(b), the left side of the preceding sunspots appears as S poles, and the left side of the following sunspots appears as N poles. This is just an artifact caused because we observed the sunspots obliquely while they were located close to the solar limb.
In Fig. 3(b), N poles and S poles are tangled. This type of sunspot structure often causes solar flares. Actually, huge flares occurred 6 times around the end of October. Although a few middle-class flares occurred on November 15 and 16, the magnetic structure doesn’t look as complicated in November as it did in October. Will more flares occur? We will continue careful observations.
How do solar flares influence the Earth? Sometimes when flares occur, high-energy charged particles reach the Earth and magnetic storms occur. In October, the terrestrial environment was not disturbed much, although many flares occurred on the solar surface. The reason is under investigation; one possibility is that the magnetic field in the solar upper atmosphere was so strong that it suppressed the eruption of charged particles. In November, even if there are fewer large flares than in October, there could be flares which greatly influence the Earth. We should carefully watch the evolution of the sunspots.
Notice: This is a press release from the National Astronomical Observatory of Japan (NAOJ). See the original story here.