Global budget for an eruptive active region

Abstract

Abstract. We present results on the magnetic structure of NOAA Active Region #7912 which was involved in a long duration flare on 14 October 1995, and was the source region for a magnetic cloud observed by the WIND spacecraft from October 18-20. Using vector magnetograms from the Imaging Vector Magnetograph ("IVM"), we reconstruct the magnetic field above this active region, assuming it is in a non-linear force-free state. This reconstruction is used to determine global properties of the active region magnetic field including topology, magnetic energy, and relative magnetic helicity. A comparison of some global quantities before and after the eruptive event is discussed. We show that the magnetic energy and relative helicity of the active region decreased after the eruption, consistent with the ejection of a large amount of helicity (in the magnetic cloud). We also show that the relaxed post-flare state still contains nonlinearities and is not consistent with a linear force-free state as predicted by Taylor's theory of relaxation. These results agree with those of recent numerical simulations concerning plasmoid ejection and helicity redistribution in the disruption of magnetic configurations. We propose as an explanation that the anchoring of field lines in the photosphere prevents a full cascade to the Taylor state, and that a variational formulation in which the action functional would describe this constraint should be derived.

Magnetic Helicity in Sigmoids, Coronal Mass Ejections and Magnetic Clouds

“…The helicity loss rate depends on two important assumptions: that most CMEs include a flux rope-like structure that evolves into the well-documented flux rope structure of a MC, and that magnetic helicity is conserved as the event propagates from sun to Earth. No one has ever directly measured the magnetic helicity of a CME, but recently, Bleybel et al (2002) used non-linear force-free field reconstructions based on a series of vector magnetograms to show that the magnetic helicity in an AR decreased by 0.7 x 10 42 Mx 2 after a CME: This quantity is consistent with CME helicity estimates. Longcope, Fisher & Pevtsov (1998) proposed that solar AR helicity is produced by the combined effects of the Coriolis force and convective turbulence on rising flux tubes.…”

“…New techniques (e.g. Chae 2001; Kusano et al 2002;Bleybel et al 2002) should enable better comparisons of helicity flow with CME rates. A second implication is that since helicity (mostly right-handed in the southern hemisphere and lefthanded in the northern hemisphere) is carried off by CMEs, there must be a compensating flow of helicity through the solar equatorial plane, as pointed out by Berger and Ruzmaikin (2000).…”

3D Coronal magnetic field from vector magnetograms: non-constant-αforce-free configuration of the active region NOAA 8151

“…According to the Taylor's hypothesis (Taylor 1974), the minimum magnetic energy for a given magnetic helicity is obtained for a constant-α force-free field. In a more general case, the magnetic energy released during an eruptive event is less than expected in the Taylor's hypothesis (see Bleybel et al 2002). Therefore, ∆E m overestimates the free magnetic energy available to trigger an eruptive event.…”

The magnetic field in the solar atmosphere

“…One generally finds a higher free energy content prior to larger eruptive events and a small amount of free energy ( 10 23 J) prior to the weakest flaring activity (Gilchrist et al 2012), where "weakest" activity refers to smaller than C-class events. A free energy of ≈10 24 J to ≈10 25 J has been reported prior to weak flaring activity (Bleybel et al 2002;Régnier and Priest 2007;Sun et al 2012a), where "weak" means C-class flares. Moderate to strong (M-to X-class) events tend to occur only when free energies of some 10 25 J to some 10 26 J are present (Régnier et al 2005;Metcalf et al 2005;Thalmann and Wiegelmann 2008;Jing et al 2009;Sun et al 2012a;Feng et al 2013).…”