Flares VI
Presiding: G Emslie, Oklahoma State University; J M McTiernan, Space Sciences Laboratory, University of California, Berkeley
SP52A-01 10:30h
Spatially Resolved Observations Confirm Transition Radiation in a Solar Radio Burst
We report the first confirmed spatially resolved observation of the decimetric Resonant Transition Radiation (RTR) in a solar radio burst, which is an unavoidable by-product of microturbulences present in dense enough astrophysical plasmas. A number of recent publications, based mainly on studies of individual events, found some indication that RTR may be produced in solar radio bursts. Most recently, we have described the observational characteristics expected for RTR in the case of solar radio bursts (Fleishman, Nita, and Gary, 2005, ApJ, 620, 506), and found that the correlations and associations predicted for total power data are indeed present in the decimetric components of a statistical sample of two-component solar continuum radio bursts. However, interpretations based on non-imaging total power data remain indirect (and, thus, ambiguous) until they can be combined with direct imaging evidence from multi-wavelength spatially resolved observations, which so far have been missing in the previous studies. The spectral components of such RTR candidate bursts (one at centimeter wavelengths due to the usual gyrosynchrotron mechanism, and one at decimeter wavelengths suspected as RTR), must be co-spatial to allow an unambiguous RTR interpretation. This study presents comprehensive (radio, optical, and soft X-ray) spatially resolved observations for one of these bursts, which, together with the already demanding spectral and polarization correlations found previously, provide direct evidence for the presence of RTR.
http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v620n1/61376/61376.html
SP52A-02 10:45h
RHESSI & Nobeyama Imaging Observations of the X 2.7 Flare of 2003 Nov 3, 00:58 UT
We describe the RHESSI HXR and Nobeyama (NoRH) microwave imaging observations of an X-class event that was observed during the great solar activity period of Oct 19--Nov 10, 2003. The flare in question of class X 2.7 was observed in AR 0488 (position N08W66) on 2003 Nov 3, 00:58:33 UT, with a large peak at 01:15:25 UT, and ending at 03:08:19 UT. The maximum fluxes at 17 and 34 GHz are respectively 678 and 536 SFU. Besides the peak at 01:15 there are two other peaks at approx 01:20 and 01:32:30 UT. In NoRP data the peak at 01:20 is the most dominant at frequencies 9.40, 3.75, 2.0 GHz. At 1.0 GHz the emission prior to this peak is fluctuating and a peak with fluctuations exists at around 01:05 UT. The third peak at 01:32:30 UT is spiky and it is nonthermal with a turnover frequency between 17 and 34 GHz. One significant property of the HXR emission (below 12 keV) during the period 01:00 - 01:08 is that the preflare emission, in projection, lies close to and above the W-limb, although the associated microwave source is on the disk. There are two preflare sources (RHESSI at 12-25 keV, and NoRH at 17 and 34 Ghz), but it is not clear how they are related. The RHESSI source seems to coincide in position with the EIT flaring source at its apex. The morphological structure seems to suggest that we are dealing with a loop-top HXR source situated at the top of the EIT flaring loop and flaring sources at 17 and 34 GHz, whose morphology suggests a complex flaring arcade structure.
SP52A-03 11:00h
The Spatial Association of OVSA and RHESSI Sources with H-alpha Ribbons
We present detailed comparisons of microwave and hard X-ray maps with H-alpha ribbons that are obtained with the Owens Valley Solar Array (OVSA), Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and H-alpha filtergrams at Big Bear Solar Observatory (BBSO), respectively, for the 2002 September 9 flare. Since microwave emission can be sensitive to nonthermal electrons residing in any part of the flaring volume depending on local magnetic field and observing frequency, our investigation is motivated to see whether the OVSA's imaging spectroscopy will reveal a link between typically confined hard X-ray sources and rather extended H-alpha kernels. For this event, we observe single hard X-ray source, three kernels at H-alpha blue wing, and more than five kernels at H-alpha line center. The microwave morphology changes from a large arcade-like structure at low frequencies to single dominant footpoint source at high frequencies, and indeed indicates the overall connection between the hard X-ray source and H-alpha kernels. However, one H-alpha kernel lying in an outer weak-field region has neither hard X-ray nor microwave counterparts, and the energetic particle transport to this kernel is yet to be explained by other means. Another non-standard feature of this event is that the H-alpha kernels remained fixed in space rather than separating away from the magnetic neutral line while the flare energy release was going on. We still argue, based on the multifrequency lightcurves, that all these separate components are energetically related, and the wavelength dependent morphology is perhaps due to energy threshold associated with each radiation.
SP52A-04 11:15h
Chromospheric Response to Non-Thermal Electrons During Flares Using RHESSI and SOHO/CDS
Many current solar flare models predict a relationship between the amount of energy deposited in the chromospheric and transition region layers of the solar atmosphere by non-thermal electrons, and the velocity at which this superheated material rises. For the first time, we present findings from simultaneous observations of loop footpoints using RHESSI and SOHO/CDS. RHESSI HXR images and spectra are used to determine the flux of non-thermal electrons using the thick-target bremsstrahlung model, while upflow velocities in the Fe XIX emission line are observed using CDS.
SP52A-05 11:30h
Chromospheric Evaporation in Solar Flares Revisited
We investigate the initial stage of chromospheric evaporation in flares using soft X-ray flare spectra obtained by the Bragg Crystal Spectrometer (BCS) experiment on Yohkoh. We determine the minimum detectable soft X-ray flare volume emission measure from BCS Ca XIX flare spectra. We find that the minimum detectable emission measure by BCS is produced by an X-ray flux that is about equal to the peak intensity of a class A5 flare. We also find that the centroid wavelength of the Ca XIX line in spectra with the first detectable emission is within about 8.5E-4 Angstroms of the rest wavelength, which is 80 km/s in terms of a Doppler shift. We interpret our results assuming sequential chromospheric evaporation into a multi-threaded flare loop envelop. Under this assumption, by comparing the BCS results with images of flares from the Soft X-ray Telescope (SXT) on Yohkoh and from TRACE, we have determined the minimum energy and electron density of multi-million degree soft X-ray plasma that can be detected using presently available spectroscopic X-ray data. In addition we consider the implications of a multi-thread loop model on TRACE and Yohkoh flare images, and the differences between the images and the multi-thread predictions. We find that the multi-million degree flare plasma in TRACE images frequently exhibits structures that do not resemble the images of loops expected from the numerical simulations of evaporation. Thus, while observational signatures of flare dynamics can be consistent with chromospheric evaporation simulations, problems still remain in understanding the loop morphology of the multi-million degree plasma.
SP52A-06 11:45h
Coronal X-ray Emission in Long-Duration Occulted Flares
To investigate coronal X-ray (≥ 6 keV) emission using RHESSI, we select long-duration (≥ 3 h) flares with completely or partially occulted footpoints. Flare footpoint sources dominate and thus obstruct the observation of coronal emission; short durations do not allow enough time for observation of various changes. We examine the location, morphology, size, height, and spectral characteristics in 20 long-duration flares seen with RHESSI whose footpoints are occulted by the solar limb to at most 30°. In 4 cases we see hard X-ray (≥ 20 keV) coronal sources; in the remaining cases hard X-ray emission comes from partially occulted footpoints only or the coronal emission is thermal.