X-ray and H-alpha Flare Impulses

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Introduction

Impulsive brightenings in hard X-ray flares have been known about for many years, and have been commonly but not universally interpreted as due to nonthermal electrons accelerated in the corona and then beamed down to locations in the chromosphere. There they emit X-rays by the bremsstrahlung process. They may also heat up the chromosphere, with resulting localized brightening visible in the well-known H-alpha line in the red part of the spectrum. There should be a very close correlation of time between the hard X-ray impulses and chromospheric brightenings. Searches have been made for these correlations and measurements made of the time differences between the two in a program started in 2003 at the Białkow Observatory, part of the Astronomical Institute of the University of Wrocław, in south-west Poland. Many dozens of flares have been observed on the declining phase of Cycle 23 with our instrument. With RHESSI providing the hard X-ray data, the time differences seem to be falling into a definite pattern that is now emerging, and seen for the first time, which we are now in a position to interpret.

The H-alpha Observations and correlation with RHESSI

The Białkow Observatory has two solar telescopes - a Large Coronagraph (LC - 53 cm diameter of the main objective and 14500 cm effective focal length) and a Horizontal Telescope (HT), with the LC being the most used. In a coronagraph mode, there is an occulting bar which blocks out the Sun's photosphere to allow the faint white-light corona to be observed, but the observatory's altitude is too low to see the corona clearly. Instead, the occulting bar is removed in our observations and an instrument called the Multi-channel Subtractive Double Pass (MSDP) spectrograph is placed at the telescope's focal plane. This French-made instrument is an imaging spectrograph that splits the H-alpha line profiles of all points inside its 2D field of view into nine positions, separated by 0.4 Angstroms. Using this data we were able to obtain 2D H-alpha quasi-monochromatic images of 41x325 arcsec sq. on the Sun in several wavelengths across the H-alpha line profile (plus/minus 1.2 Angstroms from the line centre with a band-width of 0.06 Angstroms for each channel) or complete H-alpha line profiles (in range plus/minus 1.6 Angstroms from line centre) for all pixels in the field of view. It allows us to see moving plasma through the Doppler shifts. The spectra-images are recorded with a fast-frame CCD camera, previously used as part of the SECIS system at solar eclipses to look for wave motions in the corona, at frame rates of up to 25 images per second. During an observing run, up to 10,000 images can be captured on the hard drive of a specially adapted computer. Our procedure is to wait for a time when the Sun is likely to produce many flares, and observe a particular active region. An observing sequence is started, the data stored, then if a flare did not occur, the sequence re-started, overwriting the old data, until a flare occurs. The data are accurately timed using DCF77 atomic time signal transmitted from Germany. If all goes well, the data can then be compared with RHESSI data. As RHESSI is sometimes in the Earth's shadow or in the South Atlantic Anomaly correlation with hard X-ray data is not always guaranteed. When we do have a good sequence of H-alpha data and simultaneous RHESSI data, then we can begin to analyze the data. The time differences between the H-alpha and hard X-ray impulses are as little as 1 second, but RHESSI spacecraft spin rate is one per 4 seconds. Thus a demodulation procedure, written by Gordon Hurford, must be used on the RHESSI data to get the required time resolution so as to compare with our H-alpha data which has a time resolution of down to 0.05 seconds.

Results

Among the many flares we observed over the 2003-2005 period, we show the light curves of two: see Figure 1, where the demodulated RHESSI (20-50 keV X-ray) light curve is shown (as the histogram) on the H-alpha curve (black jagged curve) at the start of a modest (C1.2) flare on 2003 July 16 in NOAA active region 10410. A sharp impulse in hard X-rays is apparent just before 16:04 UT, and the H-alpha light curve in a small bright area of kernel shows a sharp impulse too, but very slightly delayed. The three light curves are for the emission in the H-alpha kernel at the H-alpha line center and at 0.6 Angstroms either side of the line center. The hard X-rays, which are not demodulated in this case (and so have 4-second time resolution) are plotted logarithmically, while the H-alpha data are on a linear scale, with 0.05 second resolution.

Figure 1: Light curve for 2003 July 16 flare.

The second case is a C8.3 flare on 2005 July 12, with two bright kernels (K19 and K20). The RHESSI 20-50 keV X-ray light curve has been demodulated, but because the signal was weak we smoothed the data with a 1-second box car filter. H-alpha light curves are shown for each of two kernels, labeled K19 and K20. The time resolution of these data is again 0.05 second. There is a high correlation between the hard X-ray and H-alpha emission in each case, with cross-correlation coefficients (r) greater than 0.94 and 0.96.

Figure 2: Two flare kernels (K19 and K20) and their light curves, 2005 July 12 flare.

For 16 H-alpha flaring kernels observed in 12 flares in the 2003-2005 period (mostly in our summer months as cloudy weather during the winter prevents much observation), we found similarly strong correlations in the total hard X-ray emission and the H-alpha emission from particular flare kernels. We have been able to make 72 measurements of the amount that the H-alpha emission was delayed compared with hard X-ray emission, finding that most delays are only 1 to 2 seconds. However, there was a small but significant number of longer delays, up to about 10-18 seconds.

Interpretation and Conclusions

The short delays, around 1 seconds, are fully consistent with the time response of a heated chromosphere to a beam of nonthermal electrons, assuming that is the model for the flare impulsive phase. (Lyndsay Fletcher and Hugh Hudson have recently discussed an alternative model for this phase, but it would probably imply similar correlations.) Our colleague Petr Heinzel (with co-workers) at the Ondřejov Observatory in the Czech Republic and others have made detailed calculations suggesting time delays of about a second, with sometimes the additional feature of a dip in the H-alpha light curve before the hard X-ray impulse - which we have not seen. The larger time delays, of 10-18 seconds, are less obvious, though they are about equal to the ion-sound travel time from the top of a flaring loop to the chromosphere, perhaps suggesting a wave motion resulting from the presence of an ultrahot plasma formed at the flare's impulsive stage.

Figure 3: Distribution of time delays of the H-alpha emission compared with RHESSI hard X-ray emission for 72 measurements of 16 flaring kernels in 12 flares, observed in 2003-2005. The distributions are segregated into observations made in the H-alpha line center and in the red and blue wings of the line.

We (Kenneth Phillips, Paweł Rudawy and Krzysztof Radziszewski) are continuing our observations but with an improved CCD camera that was recently purchased by the Astronomical Institute at Wrocław. Solar activity has been disappointingly low until just recently; with possibly increasing activity levels we hope to fill in more details of the time delay histogram in Figure 3.

Biographical note

Krzysztof Radziszewski and Paweł Rudawy are working in Heliophysics and Space Physics Division of the Astronomical Institute of the University of Wrocław (Poland); Kenneth Phillips, a Visiting Prof at the Mullard Space Science Laboratory, University College London, is a frequent visitor to Wrocław through a Royal Society/Polish Academy of Sciences joint project with the Space Research Center, Wrocław. All three co-authors of Nugget (based on the recent publication - ref. [1]) are engaged in research of fast changes of solar flares emission since 2003.

Reference

[1] High time resolution observations of solar H-alpha flares - II. Search for signatures of electron beam heating