Active Regions

Across the solar disk, bright plages are found in a granular structure in the vicinity of active regions. These are areas of higher temperature and density than the surrounding areas, caused by the enhanced magnetic field of the active region. Fine dark fibrils are found arranged in a radial or spiral pattern around sunspots, with a clockwise rotation in the southern hemisphere, and a counter-clockwise pattern in the northern hemisphere. These are much longer (~20,000 km) than quiet-Sun mottles. The fibrils have been shown to connect areas of opposite magnetic polarity, and are aligned with the local magnetic field. On the other hand, long dark filaments mark lines of magnetic inversion. They can be found winding throughout active regions, often with one end close to a sunspot. An active region starts as a brightening in the network mottles, followed by plage filling in the network cell. Sunspots and flares follow within a few days. Newly emerging bipolar regions are connected by low-lying arch filaments. The arch filaments bring up the denser material from the lower atmosphere. At the ends of these loops the material is pulled back down, due to gravity. This is viewed as small bright Ellerman bombs which show up in the wing of Hα (with the rapid changes associated with quiet and active region chromospheric fine structure it is important to remember many of the structures will be Doppler shifted to the wings of absorption lines). Shearing and twisting of flux tubes in and around active regions also sets up currents along field lines. On a larger scale, the differential rotation of the Sun induces an extended current system. A third system of currents is due to the large conductivity of the atmosphere such that field structures cannot relax to a current free state.

Figure 1.9:
Top row¹: SoHO MDI magnetogram and continuum image of active region NOAA 10486 - the magnetic mixing around (350,-350) means this region was assigned a classification.
Middle left²: GOES  X-ray flux. The large X-class (X17.2) flare on 28 Oct 2003 came from NOAA 10486.
Middle right³: SoHO  LASCO C2 image of the CME associated with the X17.2 flare.
Bottom left⁴: Aurora in Glengormley, N.Ireland as particles from the CME slammed into the earth's atmosphere.

The magnetic field above active regions extends into the corona, where reconnection between twisted field lines can lead to solar flares (Priest & Forbes 2002), and occasionally to coronal mass ejections (CMEs). When earth-directed these can cause severe damage to satellites, astronauts and power grids, and cause the aurora in the earth's atmosphere. Figure 1.9 shows how the Michelson Doppler Imager (MDI; Scherrer et al. 1995) and Large Angle Spectroscopic Coronagraph (LASCO; Brueckner et al. 1995) on board the Solar and Heliospheric Observatory(SoHO; Fleck, Domingo, & Poland 1995) can be used to study these explosive events.

¹ Images from SoHO and courtesy of Peter Gallagher. (http://beauty.nascom.nasa.gov/arm/)
² Data from Space Environment Centre. (http://sec.noaa.gov/)
³ SoHO image. (http://sohowww.nascom.nasa.gov/)
⁴ Image courtesy of A. Fitzsimmons.