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?