Magnetic Field Topology and Observed Energy Release Locations

MANDRINI, C.H.

Praga

Congreso; 26th IAU General Assembly - Joint Discussion 3: Solar Active Regions and 3D Magnetic Structure; 2006

IAU

Conferencia invitada     The magnetic field is thought to be the source of the energyreleased in diverse observed coronal phenomena, from the less energeticcoronal heating to the most violent flares and prominence eruptions.These phenomena involve not only very different scales from theenergetic, but also from the temporal, point of view. Magnetic fieldreconnection, which is efficient only at very small spatial scales, has beenthe energy release mechanism so far proposed.From a theoretical point of view, magnetic configurations with acomplex topology, i.e. having separatrices, are the ones where current sheetscan form in 2D. When going to 3D, and if the photospheric magnetic fieldis described by a series of isolated polarities (surrounded by fieldfree regions), a complete topological description is given by the skeletonformed by null points, spines, fans and separators, and associatedseparatrices (see the review by [Longcope, 2006] andthe examples therein discussed).However, if the photosphere is fully magnetized, most of the abovetopologicalstructures disappear: only separatrices associated tocoronal magnetic nulls remain.  Separatrices of a different origin are linkedto the field lines curved up at the photosphere (defining the bald-patchlocations) ([Titov etal, 1993]).For some observed magnetic configurations, those topological structures areenough to understand where flare brighteningsappear as a result of magnetic field reconnection. Few examples havebeen found where coronal null points, computed using eithersubphotospheric sources or magnetic field extrapolations to representthe coronal field, were associated with coronal activity(see e.g. [Mandrini etal, 1991];[Gaizauskas etal, 1998];[Aulanier etal, 2000];[Fletcher etal, 2001a];[Mandrini etal, 2006]).Concerning bald patches, they have been only found in connection withlow energyrelease events: small flares ([Aulanier etal, 1998]),EUV brightenings ([Fletcher etal, 2001b], andchromospheric events ([Mandrini etal, 2002],[Pariat etal, 2004]).The analysis of the topological structure of numerous active regionshas shown that flares occur in a larger variety of configurationsthan those just discussed (see e.g. [D´emoulin etal,1994]). Quasi-separatrix layers (QSLs) ([Priest andD´emoulin, 1995]; [D´emoulin etal,1996]; [Titov etal, 2002]), which areregions where there is a drastic change in field-line linkage,generalize the concept of separatrices to  magnetic configurationswithout magnetic null points and/or bald patches. Using coronalmagnetic field models, QSLs have been computed in the largestvariety of observed magnetic configurations ([D´emoulin etal,1997]; cite[Mandrini etal, 1997]; [Fletcher etal, 2001b]; [Bagal´a etal,2000], and references therein). QSLs have been foundlocated in coincidence with chromospheric and coronal loopbrightenings of varied intensity, these brightenings could be alsoconnected by field lines in the way expected by magneticreconnection theory. Moreover, in cases where vector magnetogramswere available, photospheric current concentrations were alsolocated at the photospheric trace of QSLs. Recently, to relate QSLswith the formation of strong current concentrations and study thecharacteristic of the reconnection process occurring at QSLs, 3D MHDsimulations of characteristic observed field distributions andphotospheric motions have been developed by [Aulanieretal~(2005)] and [Aulanieretal~(2006)]. The results from these simulations implythat electric currents at QSLs are amplified in time only if theQSLs are broader than the dissipative scale length, what wassuggested in the previous observational analysis. Magneticreconnection at QSLs occurs when the self-pinching of the currentlayers is strong enough to efficiently enhance the dissipation termin the induction equation. A property of this reconnection processis the continuous slippage of field lines alongeach other as they pass through the current layers.Our reviewed examples of observed regions showing variedlevels of activity and their topologies, teachus that magnetic reconnection can occur in magnetic configurationswith much wider topological characteristics than traditionally thought.