Flares and photospheric field configuration

2.1 Flares and photospheric field configuration

Flares can occur everywhere on the Sun, in active regions, penumbras, on the boundaries of the magnetic network of the quiet Sun, and even in the network interior (Krucker et al., 1997b

; Benz and Krucker, 1998

; Berghmans et al., 1998

). Regular (large) flares, however, have preferred locations. They occur in large active regions showing a complex geometry of the 3D magnetic field as revealed in photospheric vector magnetograms (Régnier and Canfield, 2006 , see Figure 5

). Major flares occur in active regions that exhibit a $δ$ configuration, defined as showing umbrae of opposite magnetic polarity. Flares of the largest size (X class) are associated with arcades of loops spanning a line of zero line-of-sight magnetic field on the surface. Hagyard et al. (1990

) have shown that the magnetic field in flaring locations is strongly sheared (Figure 6

). Magnetic shear alone, however, is not a sufficient condition. An additional requirement for large flares is the emergence of new magnetic flux from below (Choudhary et al., 1998 ). Vector magnetograms often show narrow channels of strong horizontal field components near the neutral line at the site of large flares (Zirin and Wang, 1993 ). Such fields may already be sheared when emerging and are still moving when they appear in the photosphere (Georgoulis and LaBonte, 2006 ).

Figure 5:

gif-Movie (4562 KB) Left: White light image of a sunspot taken with SOT on board the Hinode satellite. The resolution is 2”. Visualization by Hinode scientists. Right: Vector magnetogram showing in red the direction of the magnetic field and its strength (length of the bar). The movie shows the evolution in the photospheric fields that has led to an X class flare in the lower part of the active region. Courtesy of NAOJ/NINS.

Large-scale shear in the magnetic field can also build up through the slow motion of foot points stretching the length of a loop. This evolution can progress through stable field configurations. Large-scale sheared fields in active regions store huge amounts of free magnetic energy. The emerging flux may then provide the trigger mechanism for impulsive energy release. The energy is unleashed above the photosphere, either in the chromosphere or corona (Moore et al., 2001 ). The build-up of the unstable configuration for large flares is often well observable in photospheric magnetic fields. The flare leaves, however, few if any observable traces in photospheric magnetograms. Wang et al. (2004 ) find an increase of the photospheric magnetic field strength during the flare, consistent with the scenario of magnetic flux emerging into the corona. A frequent flare site is the separatrix between different magnetic loop systems (Démoulin et al., 1997 ) forming a current sheet, or in a cusp-shaped coronal structure (Longcope, 2005 ). There is also evidence for flares being related to helical magnetic fields (Low, 1996 ; Pevtsov et al., 1996 ).

Photospheric observations clearly support the scenario that flare energy originates from free magnetic energy in excess of the potential value (defined by the photospheric boundary) or, equivalently, from electric currents in the corona. For a rapid release, the currents must be concentrated into small regions. Such currents may be inferred observationally in regions with a tangential discontinuity of the magnetic field direction in the system of rising loops near the base of the corona (Solanki et al., 2003 ).

Figure 6:

Map of the line-of-sight magnetic field strength measured in a photospheric line by the MSFC magnetograph. Solid curves denote positive field, dashed curves the negative field, and the dotted curve the neutral line. Circles indicate where the transverse field deviates between $70 ∘$ and $80∘$ from the potential field (perpendicular to the neutral line), and filled squares indicate deviations $∘ > 80$ . A large flare (X3 class) occurred several hours later at the location of the largest shear (from Hagyard et al., 1990 ).

The flare energy resides in the magnetic field that originated in the interior and is convected to the surface. Not much can be observed of the flare processes until the energy appears in the form of accelerated particles and extremely hot plasma. In the next section we switch from the origin and build-up to the first observable signature after the primary energy release.