Spatial and temporal relationships between UV continuum and hard x-ray emissions in solar flares Aaron J. Coyner and David Alexander Rice University June 2, th Meeting of the American Astronomical Society

Abstract Combined UV and hard x-ray observations provide a means to better understanding the spatial and temporal relationship between the energization of the solar corona in flares and the response of the solar atmosphere. In this work, we investigate the evolution of an M1 solar flare on 2004 January 19 at both UV and hard x-ray wavelengths.This is accomplished through analysis of data from the TRACE and RHESSI spacecraft respectively. Uncertainties in TRACE pointing were corrected by coaligning TRACE white light images with cotemporal MDI white light intensitygrams. RHESSI hard-xray images were created for an energy range of keV using the clean and pixon methods with a time resolution of 10 seconds per image. We compare the spatial and temporal evolution of the UV and hard x-ray emission to determine the relationship, if any, between the hard x-ray flare footpoints and the UV continuum emission.

Introduction The impulsive phase in solar flares provides us with direct evidence of energetic particle acceleration or energization in the solar atmosphere. A key factor in understanding the radiative output of a solar flare during the impulsive phase is the relationship between the high energy radiation (>10 keV) and the chromospheric and transition region emission in the UV (e.g. Cheng et al, 1988). The observed temporal and spatial correlations between these radiations provided some of the most stringent conditions for flare modeling. The combination of flare observations from RHESSI in the hard X-ray range and TRACE in the UV range provides a powerful diagnostic capability for the exploration of solar flare energetics. Bright and short-lived UV and EUV kernels before and during the impulsive phase (Fletcher et al., 2001) provide limited structural information regarding the chromospheric response Warren and Warshall (2001 ) provide evidence that UV brightenings occurring prior to the hard X-ray onset show poor correlation or in some cases anticorrelation with the hard X-ray lightcurves.

The Flare: X-rays Figure 1: GOES Soft X-ray Emission from 19 Jan : UT, showing an M1.0 flare beginning at 12:30:00 UT. Figure 2: RHESSI Hard X-ray Observing summary for the flare shown in Figure 1. The sharp jumps are due to changes in the RHESSI attenuator state.

RHESSI lightcurve for keV shows 3 distinct bursts between 12:33:00 and 12:34:20 UT. Bursts B and C have associated high cadence (~2s) TRACE UV data allows for temporal comparison of UV sources with the HXR lightcurve All 3 bursts approximately seconds in duration and show some detailed temporal structure Burst A coincides with an apparent brightening of the UV ribbon but a gap in the UV data at this time prevents a proper comparison. Hard X-Ray Time Evolution ( keV) Figure 3: Hard X-ray lightcurve for energy range keV. 3 clear bursts are marked for discussion later. Large spike in Burst A appears partially due to a change in attenuator state. A B C

UV Continuum Overview Initial brightening becomes visible at approximately 12:32:10. Consistent brightening of U- shaped ribbon structure during time span of intial HXR burst Subsequent HXR bursts cotemporal with localized UV source brightening (see figure 8) Pre-flare brightenings not shown. TRACE data from the ten minutes prior to HXR onset shows small but detectable brightenings Figure 4: TRACE 1600 A image sequence for the time frame of Bursts A and B in the hard X-ray spectrum.

Early RHESSI Burst Imaging Burst A Burst B Burst C keV keV Figure 5: Pixon images for energy range keV and sub-ranges keV and keV energy bands spanning Burst A (12:33:00-12:33:25), burst B(12:33:25-12:33:45) and burst C (12:33:45-12:34:05) keV

Initial Burst Image Findings RHESSI pixon images were taken over the complete time range of the labeled burst segments. (20-25 seconds each) Each burst images shows two bright footpoints near the center of the image. During Burst A these footpoints appear to have similar size and intensity; however, the subsequent images show an increased relative contribution from the left footpoint. This is most evident in the keV and keV images Figure 5 shows that the spatial distribution of the hard X-ray emission does not show any major dependence on the energy (above 25 keV). We increased the time resolution of the hard X-ray images to look for evidence of the burst temporal sub-structure in the spatial distribution (Figures 6 and 7).

RHESSI spatial information for 5 second time resolution images Images present are from the time range of Burst B (see Figure 3). Figure 7A shows a source which corresponds with the primary source in the 20 second image. Figure 7B shows all 3 sources which appear within Figure 6. The integration time is 10 second for this image due to an attempt to eliminate pixon artifacts due to limited counts in the initial 5 second images In Figure 7C, count rates are greatly decreased from the prior images but still shows an addition to the 2 most well- defined sources. Burst B appears dominated by a single source for its whole duration, with some lesser contribution from a second weaker source Figure 6: 20 second integration time image for Burst B (12:33:25-12:33:45). Figure 7: Temporal breakdown of the 20 second image in figure 6. Image A (12:33:25-12:33:30). Image B (12:33:30-12:33:40). Image C (12:33:40- 12:33:45) ABC

Temporal Comparisons between UV and HXR data As an initial step we compare the TRACE UV behavior over the times of the main HXR bursts shown in Figure 4. Pictured below are two series of TRACE 1600 angstrom images corresponding to the time frames of HXR bursts B and C. Each series shows the outline of 8 regions where localized sources develop. –Lightcurves for each of these regions were created and are shown in Figure 8. –Each of these lightcurves are compared in Figure 8 with the HXR lightcurve for the impulsive phase

TRACE Sequences for Bursts B and C Figure 8: TRACE 1600 image sequences burst B (above) and C (below). Lightcurves have been calculated for the labeled regions in each case.

Apparent Temporal Links of TRACE Region with HXR Bursts Images show specific brightening of the UV sources within the time range of notable X-ray peaks First row corresponds to peaks within Burst B. Sequent rows correspond to peaks within Burst C Region corresponding to box 1 in the previous images clearly is not present in the images from the burst C time range Regions corresponding to 2 and 3 are active to varying degrees throughout these two burst times. Majority of the brightening during Burst C appears to originates from sources 5-8 from previous images. (See Figure 8) Figure 9: Series of running difference images over time ranges corresponding to HXR peaks. Images clearly show the UV sources which brighten over the intervals listed.

Comparison of TRACE Region Lightcurves with HXR lightcurve during the impulsive phase Figure 10: Overlay of the normalized TRACE lightcurves(black) and the normalized RHESSI keV lightcurves of 1 sec resolution lightcurve(gray).

TRACE Temporal Results The TRACE sequences in figure 8 show that each of the eight sources which appear to react to various parts of the HXR burst sequence The difference images in figure 9. show specifically the brightening areas for each of the times in which the hard x-rays recorded a notable peaks within bursts B and C The early difference images from burst B show clear brightenings from region 1 and 4; however, these regions are not visible in the difference images for burst C. Comparing the light curves for these two regions, there appears to be strong evidence for these two sources to be conjugate brightenings. Region 2 and 3 in all of the images show a degree of activity although the level of activity appears greatest in both cases during Burst C (see lightcurves in figure 10.

Pre-Flare Activity from TRACE Pre-flare information viewed from 12:20:09 until the onset of the flare. Three small regions shown in figure 11 show brightening prior to the onset of the hard X-rays The count rates for these pre-flare range from DN/s which is considerably lower than the count rates (~18000 DN/s) occurring during the flare In contrast to Warren and Warshall (2001), these pre-flare brightenings are also active during the HXR impulsive phase. Figure 11: Pre-flare sequence of UV continuum brightenings (12:20:09-12:30:45). Three regions in the images appear to be consistently brighter than the rest of the frame.

TRACE/RHESSI spatial comparisons TRACE images shown were coaligned using MDI through TRACE white light images* Darker trace regions actually reflect extreme brightening and saturation Region 1 from the TRACE is clearly spatially separate from any of the hard X-ray emission. The images to the right show a tendency for the UV sources to be spatially separate from the X-ray footpoints. Warren and Warshall (2001) reported similar results for a number of events analyzed with TRACE and HXT. Count rates for RHESSI in this flare provide significant resolution limitations Figure 12 TRACE images from 12:33:31 and 12:33:44 shown with contour overlay of kev RHESSI Images corresponding to integrations between 12:33: :40 and 12:33:40-12:33:45 respectively * Alignment performed using trace_mdi_align procedure written by T. R. Metcalf - LMSAL

Summary and Conclusions (Temporal) For our M1.0 flare the hard X-ray lightcurves show three clear bursts of seconds in duration. Within each of these bursts we see specific sources within the UV continuum which appear to brighten with specific bursts –Regions 1 and 4 show evidence for coincident brightening during burst B of the hard X-rays –Some sources (Regions 2 and 3) show activity throughout the hard-X-ray impulsive phase though the activity varies in intensity with different bursts –Regions 5-8 shows a sharp increase in intensity during the majority of burst C The RHESSI images for each burst show in general two sources of varied intensity –These sources vary in time as shown in figure 7. UV sources of pre-flare brightenings also showed significant brightening during the hard X-ray impulsive phase.

Summary and Conclusions (Spatial) UV sources active during the HXR bursts appear to occur in different spatial locations. The brightening in TRACE Region 1 does not appear to have an X-ray source in its proximity –Possible investigation for future work would be to look at magnetic field connectivities to see if there is a magnetic field connection creating the brightening in this region Within the RHESSI bursts, the spatial location of the dominant sources does not show a strong dependence on energy level above 25 keV

Future Work (Temporal) We will further investigate the temporal relationships in this and other flares with specific intent in performing correlation analysis on both pre-flare and impulsive phase brightenings –Pre-flare brightenings during our flare appear to also show brightening during the hard X-ray bursts. –Correlation analysis on those brightenings that appear cotemporal with the X-rays will provide a mechanism to isolate the UV sources which have strong responses to the individual burst Lightcurves for individual RHESSI sources will be calculated to try and correlate the individual RHESSI sources to specific peaks within a hard X- ray burst RHESSI imaging will be performed over shorter integration times to increase the time resolution with the imaged lightcurves We will investigate magnetic connectivities between regions of UV brightenings to verify whether conjugate brightening does occur.

Future Work (Spatial) Spatial analysis for this and other flares will be performed with finer spatial resolution with RHESSI. We will make further attempts to determine structure of hard x-ray sources responsible for specific bursts. Additional analysis will be performed on the pre-flare brightenings to determine a relationship (if any) between the spatial location of the pre-flare brightenings and those that coincide with the HXR impulsive phase.

Acknowledgements We gratefully acknowledge the support of NASA under contract NAS Aaron would also like to thank the Solar Physics Division for the studentship to attend this meeting.