Geometry of the coronal magnetic field

3.1 Geometry of the coronal magnetic field

While there is nearly unanimous agreement that sheared or anti-parallel magnetic fields provide the flare energy released in an impulsive reconnection, the geometry of these magnetic fields in the corona at large scale is not clear. The prevalent view is depicted in Figures 13

and 16

(left), as well as illustrated by a coronagraph image in Figure 16

(right). The scenario has evolved over the past four decades and is generally credited to Carmichael (1964 ), Sturrock (1966 ), Hirayama (1974 ), and Kopp and Pneuman (1976 ). So it also named CSHKP model after these scientists. It is basically a two-dimensional geometry, involving a magnetic loop that is pinched at its legs. The loop may be extremely large or moving outward, so that its legs consist of oppositely directed (anti-parallel) fields. As a result of reconnection, the top of the loop is ejected as a plasmoid. The best evidence for this geometry are vertical cusp-shaped structures seen in soft X-rays after the flare (Tsuneta et al., 1997

; Shibata, 1999 ; Sui et al., 2006

). The cusp grows with time, and higher loops have a higher temperature (Hori et al., 1997 ), as predicted by continuous reconnection. The observational evidence also includes horizontal inflows of cold material from the side and two sources of hot plasma that move away from the X-point upward and downward. The observations of the former has been reported for a single event by Yokoyama et al. (2001 ) and Chen et al. (2004 ). Hot sources being ejected are more frequently observed in soft X-ray (Shibata et al., 1995

). They have been noticed to be associated with drifting pulsating structures in decimetric radio emission (Khan et al., 2002

) indicating the presence of non-thermal electrons. The reported association with hard X-rays and centimeter radio emission (caused by the synchrotron emission of mildly relativistic electrons) suggests that the plasmoids also include highly energetic particles (Hudson et al., 2001

). There is also occasional evidence for the downward reconnection jet in addition to the observed upward motion of the soft X-ray source (Sui et al., 2004

). Finally, the existence of two ribbons marking the footpoints of an arcade of loops in H $α$ , EUV, and X-ray emissions (Figure 8

) is long-standing evidence for the one-loop model, stretched into a third dimension.

Figure 16:

Left: A schematic drawing of the one-loop flare model. Right: Observation of an apparent X-point behind a Coronal Mass Ejection observed by LASCO/SOHO in white light.

In another also widely proposed geometry, two non-parallel loops meet and reconnect. There are several versions of the cause of interaction. Merging magnetic dipoles (Sweet, 1958 ), collision of an newly emerging loop with a pre-existing loop, proposed by Heyvaerts et al. (1977 ), or the breakthrough of the emerging loop through the corona (Antiochos, 1998 ). In this scenario the geometry is closed, i.e., no magnetic field line leads from the energy release site to interplanetary space. Ejecta leaving the Sun are still possible, but not a necessary ingredient of the model. Thus the two-loop model is often proposed for non-eruptive “compact flares”. The model predicts the existence of 4 footpoints. The number of footpoints in hard X-rays rarely exceeds two. However, radio observations have been presented that complement the number of X-ray footpoints, resulting in the frequent detection of three or more footpoints (Kundu, 1984 ; Hanaoka, 1996 , 1997 ). Nishio et al. (1997 ) find that the two loop considerably differ in size and that the smaller loop preferentially emits X-rays. There is also evidence from recent pre-flare EUV observations for multiple-loop interactions (Sui et al., 2006 ).

One-loop flare models predict the presence of plasmoids in interplanetary space, consisting of hot plasma interwoven with a closed magnetic field (i.e., magnetic field lines that close within the structure). Such plasmoids are characterized by the lack of electron heat flux because their field lines are disconnected from the Sun and have indeed been reported (Gosling et al., 1995 ). More frequently observed are counterstreaming hot electrons, indicating that the field line is closed, i.e., still connected to the Sun on both sides (Crooker et al., 2004 ). According to these authors, such features in the electron distribution are frequently observed in magnetic clouds associated with CMEs. Nevertheless, they are not associated with the great number of smaller flares.

Furthermore, there is a well-known association of nearly every large flare with type III radio bursts at meter wavelengths, produced by electron beams escaping from the Sun on open field lines connected to interplanetary space. Benz et al. (2005 ) report that 33% of all RHESSI hard X-ray flares larger than C5 in GOES class are associated with such bursts. This suggests that in a third of all flares at least one of the four ends of reconnecting field lines is open. As type III bursts represent only a small fraction of the flare energy this may not hold for the major flare energy release site. We note, furthermore, that type III bursts very often occur also in the absence of reported X-ray flares (Kane et al., 1974 ). The energy release by open and a closed field lines, termed interchange reconnection, has been proposed some time ago (Heyvaerts et al., 1977 ; Fisk et al., 1999 ) and applied more recently to in situ observations in a CME (Crooker and Webb, 2006 ).

In conclusion, there is good evidence for both the one-loop and two-loop scenarios for the geometry of the coronal magnetic field in solar flares. There is no reason to assume that they exclude each other. The combination of both in the same flare, hybrids of a closed loop reconnecting with an open loop, and other scenarios are also conceivable. Thus we do not state a standard geometry for the preflare coronal magnetic field configuration. The magnetic topologies of one loop, interchange and two loop are generally classified as dipolar, tripolar, and quadrupolar, respectively. In addition, 2D and 3D versions can be distinguished (Aschwanden, 2002 ), and various nullpoint geometries have been proposed (Priest and Forbes, 2000 ).