Magnetic shear in flaring regions

Abstract

We have evaluated the shear angle of the neutral line of the non-potential magnetic field for one or two days prior to and after the flare event for 10 cases. We have used the He filament positions to evaluate the shear in the neutral line. We find from the samples we have studied that it is the change in the shear that occurs a day prior to the flare that can lead to the event. This change can be in either direction, i.e., it can be a large increase from a small value or a decrease from a large initial value. Thus it is the change in the shear angle that seems to be a deciding criterion for a flare to occur and not a large value for the shear angle itself. We have one instance where there was no significant change in the shear angle over a period of a few days and this region, although similar to other active regions studied, did not produce any flare activity.

The magnetic field in the solar atmosphere

“…Consequently, θ = 0 • implies a field with no shear and θ = 90 • would account for a transverse field parallel to the PIL. Sivaraman et al (1992) followed a different approach: they used Hα observations to determine the orientation of the major axis of filaments (as an indication for the direction of the PIL) with respect to an approximation to the orientation of the potential field azimuth. The latter has been defined by the direction perpendicular to a straight connection between the two main spots of the associated bipolar sunspot group.…”

The flares associated with the abnormal rotation rates of the bipolar sunspots: Reconnection probably below the surface

“…Sivaraman et al (1992) have given the details of the telescope and observations of daily white light pictures. Using similar criterion (Hiremath 2002) in selecting the sunspot groups, we compute rotation rate ω i of the bipolar sunspots as follows:…”

“…Many studies (Hagyard et al 1982;Hagyard et al 1984;Venkatakrishnan et al 1989;Ambastha et al 1993), from the vector magnetograms, show the relevance of magnetic shear with the eventual triggering of the solar flares. By considering the H-α filament as a proxy for the magnetic neutral line, Sivaraman et al (1992) quantitatively estimated change in the shear that corresponds with the occurrence of the flare. Schmieder et al (1994) showed that in order to have the flare occurrences, the following two conditions are necessary: (i) the break up motions of different polarity regions maintained a high shear level for the continuous build up of the magnetic flux and, (ii) the rapid motions and the changes in the magnetic sources.…”

Productivity of Solar Flares and Magnetic Helicity Injection in Active Regions

“…For this reason, many studies of relationship between the solar flare and photospheric magnetic field properties have been carried out since the flare was first observed and recorded by Carrington (1859) and Hodgson (1859). Some examples include the unbalanced changes in the photospheric line-of-sight magnetic field (Cameron & Sammis 1999;Spirock et al 2002;Wang et al 2002); rapid changes of the sunspot structure associated with a substantial fraction of flares (Liu et al 2005;Deng et al 2005;Wang et al 2004aWang et al , 2005Chen et al 2007); the magnetic shear angle evolution (Hagyard et al 1984;Hagyard & Rabin 1986;Sivaraman et al 1992;Schmieder et al 1994;Wang et al 1994Wang et al , 2004a; the horizontal gradient of longitudinal magnetic fields (Zirin & Wang 1993;Zhang et al 1994;Tian et al 2002); electric current (Canfield et al 1993;Lin et al 1993); magnetic helicity injection (Moon et al 2002a(Moon et al , 2000bSakurai & Hagino 2003;Yokoyama et al 2003;Park et al 2008). Based on the above mentioned studies, current flare forecasting models are moving toward multiple-magnetic parameter-based approaches (Leka & Barnes 2003aLi et al 2008) from sunspot-morphological evolution-based approaches (McIntosh 1990;Gallagher et al 2002).…”