White-Light Flares and HESSI Prospects H. S. Hudson (UCB and SPRC) March 8, 2002

White Light Flares White light flares are the most energetic flares observed. Optical spectra suggest two types of emission: impulsive and gradual. Impulsive WL emission correlates well with hard X-rays We presently don’t have much of a clue about the gradual component. In general the coronal plasma defines the geometry of the magnetic field; the footpoints with high-energy signatures show us the most interesting field lines.

Yohkoh white light The Yohkoh pixel size is 2.5 arc sec. The Yohkoh sensitivity is in the Fraunhofer g-band (5308A). About 30 flares observed, to below the GOES M class (catalog in preparation by Mathews, Van Driel-Gestelyi, Hudson, Nitta). Calibration ~2 x 5000A contrast.

TRACE white light The TRACE white light channel has significant contributions from 1700 Å to 1  m The TRACE pixel size is 0.5 arc sec TRACE WL images show solar granulation, sunspots and faculae similar to visible images in the blue Such broad-band observations of WLF are not customary; calibration in progress

New Yohkoh result on pressure SXT gives a clue that may point to an understanding of the gradual WLF component HESSI is nicely situated to study this

Intermediate conclusions WLF’s remain interesting, but now we are interested in the coronal geometry as well as the physics of the emission. HESSI may have a handle on the gradual component via “superhot” structures best visible at 5-15 keV. The remainder of this talk reviews the Metcalf et al. results on the flare of August 25, 2001 (TRACE white light, HXT,  - rays, submillimeter).

2001 Aug 25 White Light Flare The HXT M2 emission coincides with the TRACE WL emission. The NE and NW sources move slowly in conjunction with the TRACE kernels. In the S kernel, the HXT source moves rapidly (400 km/sec) along the TRACE ribbon.

Motion of the HXR Sources The HXR source in the South moves rapidly along the WL ribbon at a speed of 400 km/sec.

Motion of the HXR Sources - II

Potential Field Extrapolation The MDI line-of-sight magnetogram was extrapolated from the photosphere into the corona. Although the potential extrapolation is probably not accurate, we can at least get an idea of what is connected to what.

WL Flare and Separators The NE and NW kernels do not lie along separators. The S ribbon, however, lies almost perfectly along a separator. The HXR source in the S is moving directly along the S separator. The separators were calculated using a potential extrapolation of the MDI data.

Yohkoh/TRACE/MDI Alignment The TRACE pointing is stable, but not known accurately. TRACE pointing was determined using MDI white light. Yohkoh pointing is known accurately from the onboard gyros and the HXT aspect sensor. The resulting co-alignment errors are 0.5” for TRACE and 1” for Yohkoh. Hence, the co-alignment between the HXR and WL data is good to within 2-3 TRACE pixels.

Light Curves The HXR and WL light curves are similar. Given the poor time resolution of the TRACE WL images, the light curves are consistent with a common generation mechanism. Energetics implies that the WL sources are in the chromosphere, rather than the photosphere. Over- ionization via non-thermal electrons fits. This flare does not show a clear gradual WL component, need better WL time resolution.

HXR Light Curves

WL Light Curves

Conclusions The TRACE WL emission corresponds well in space and time to the HXR emission. The most likely explanation for the WL enhancement is the direct excitation by the particle beam responsible for the hard X-rays. The flare ribbons (NE and NW) do not correspond to separatrices as derived from a potential field. The NE WL ribbon moves directly across the sunspot umbra. Have umbral field lines become open?