Longitudinal Magnetic Field Changes Accompanying Solar Flares

Abstract

We have used Global Oscillation Network Group (GONG) magnetograms to characterize the changes in the photospheric longitudinal magnetic field during 15 X-class solar flares. An abrupt, significant, and persistent change in the magnetic field occurred in at least one location within the flaring active region during each event. We have identified a total of 42 sites where such field changes occurred. At 75% of these sites, the magnetic field change occurred in less than 10 minutes. The absolute values of the field changes ranged between 30 and almost 300 G, the median being 90 G. Decreases in the measured field component were twice as frequent as increases. The field changes ranged between 1.4 and 20 times the rms noise of the observations. In all but one equivocal case, the field changes occurred after the start of the flare. In all cases, the field changes were permanent. At least two-thirds of the field changes occurred in the penumbrae of sunspots. During three events for which simultaneous Transition Region and Coronal Explorer (TRACE ) images are available, we have found excellent spatial and temporal correlation between the change in the magnetic field and an increase in brightness of the footpoints of flare ribbons, but not vice versa. Among many possible explanations for the observations, we favor one in which the magnetic field changes result from the penumbral field relaxing upward by reconnecting magnetic fields above the surface. One of the basic assumptions of flare theories is that the photospheric magnetic field does not change significantly during flares. These results suggest that this assumption needs to be re-examined.

Global Properties of Solar Flares

“…The continuous stream of data at a regular cadence nevertheless allows a fit to the time series pixel by pixel, and the fit also determines an epoch for each pixel even in the presence of flare interference in the magnetic signals (e.g., Qiu and Gary 2003). The epochs of the variations measured in this way by Sudol and Harvey (2005) turn out to coincide as a rule with the impulsive phase of the flare (Sect. 2.4), and the image configurations of the altered pixels resemble the flare ribbons.…”

“…The photospheric fields themselves also show striking flare effects when viewed via Zeeman splitting, rather than as an image feature such as umbra or facula (Wang 1992;Kosovichev and Zharkova 2001;Sudol and Harvey 2005) (see Fig. 23).…”

“…The inset plots show the variation of non-potential energy density with height Fig. 23 One of the examples of stepwise changes in the photospheric longitudinal magnetic field by Sudol and Harvey (2005) as "ubiquitous features of X-class flares". Left: a GONG longitudinal magnetogram (10-m average prior to the event) for the flare SOL2002-05-20T15:29 (X2.1, S21 E65); middle: a representation of the field change across the flare time; right: the field variation in a representative pixel.…”

“…In the time-series plot the vertical lines denote the GOES start, peak (15:29 UT), and end times. The total duration of the plot is four hours, and the vertical scale spans 400 G far have only been clearly recognized in the line-of-sight component of the field, partly because the sampling is much more efficient; the GONG magnetograms used by Sudol and Harvey (2005) have a regular one-minute cadence. This flare-associated variations appear as stepwise changes, usually superposed on a varying background.…”

Rapid Penumbra and Lorentz Force Changes in an X1.0 Solar Flare

“…Observationally, since the first clear evidence from Yohkoh observations of Anwar et al (1993), rapid and irreversible changes in sunspot structure associated with flares have been studied for the past two decades. In the early time, most observations focused on the photospheric magnetic-field changes in the course of major flares, such as the magnetic shear changes along the flaring polarity inversion lines (PILs; Ambastha et al 1993;Chen et al 1994;Hagyard et al 1999), the stepwise changes in the longitudinal fields (Kosovichev & Zharkova 2001;Sudol & Harvey 2005), and so on. When these observations concluded that the longitudinal field flux variations were unbalanced between the disk-and limb-ward Wang et al 2002;Yurchyshyn et al 2004;Wang 2006), the white-light sunspot structure changes during major flares were also identified, which found that the penumbrae often became darker close to the central PILs while they decayed in the peripheral region Deng et al 2005;Liu et al 2005;Chen et al 2007).…”