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?