RHESSI microflares - Flare Cartoons and Reality

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Latest revision as of 18:20, 10 November 2009

Introduction

RHESSI observes microflares extremely well. These are weaker versions of the powerful solar flares that have inspired so many cartoons. The sequence of events and the general observational signatures of solar flares are mostly captured in the famous 2D flare cartoons of the standard eruptive flare model. Examples can be found e.g. in Hirayama (1974), Cliver et al. (1986) and Shibata et al. (1995). Solar microflares are often analysed in a statistical sense, as the main interest in them is their occurrence rate and energy budget which are important input for coronal heating models. However, detailed multi-wavelength case studies which outline the response of the solar atmosphere to microflaring and thus reveal their consistency or disagreement with standard flare cartoons are rare. The problems for multi-wavelength observations of microflares begin with the limits of the observing instrumentswith respect to sensitivity, as well as temporal and spatial resolution. In this Nugget, we show examples of microflares observed with high-resolution images of their chromospheric and coronal components, including magnetic-field and spectroscopy data as well.

What Flare Cartoons Tell Us...

If microflares involve the same physical processes as energetic flares, we would expect them to accelerate particles to suprathermal velocities detectable in X-ray images as high energy emission from the footpoints of coronal magnetic loops. We would also expect the nonthermal hard X-ray spectrum to be powerlaw distributed as seen in large flares. A hot flare loop with temperatures greater than 10 MK will emit predominantly thermally distributed soft X-rays. Also, we should see heated flare loop footpoints in the chromosphere visible as brightenings in H-alpha or other chromospheric imaging data. Mass flows from the chromosphere into the corona should be detectable by the Doppler shifts of various spectral lines. Last but not least, we would expect the chromospheric footpoints of the flare loop to be situated in zones of opposite magnetic polarity.

Figure 1 (left): An MDI line-of-sight magnetogram of active region no. 10465. RHESSI microflare locations are marked by crosses and are seemingly not evenly distributed but accumulate in complex magnetic regions. The bottom panel shows the same region in white light (TRACE) where we see sunspots.
Figure 2 (right): The time evolution of one of these microflares (GOES A5) observed in the corona (T~1 MK) and the chromosphere (T~ 7,000 K). Features expected from the flare cartoons are obvious (e.g. chromospheric flare loop footpoints with a hot X-ray loop lying in between, as well as a postflare loop). The RHESSI loop ends are located in areas of opposite polarity.

... And the Reality:

In one whole day of RHESSI observations on September 26, 2003, we found 24 microflares for which we could reconstruct images. As shown in Figure 1, they all occurred inside a large active region in a complex magnetic field (see also the earlier RHESSI Science Nugget A myriad of microflares). As shown for one event in Figure 2, the imaging data show disturbances in the chromosphere, transition region and corona which generally resemble what we would expect from flare cartoons: a hot X-ray source (the flare loop) is flanked by chromospheric brightenings. However, we did not observe any hard X-rays from the flare loop footpoints. In many cases, postflare loops are observed by in TRACE EUV images. With a different data set, we studied microflares at still higher resolution (~0.2’’) from the Dutch Open Telescope, and found finely structured chromospheric brightenings (see Figure 3). Chromospheric and transition region lines at the location of some of the microflares were found to be Doppler shifted which indicates that chromospheric evaporation is occurring(see Figure 4).

Figure 3 (left) Chromospheric observations of a small micoflare (GOES A1) by the Dutch Open Telescope (DOT) on La Palma/Spain with an amazing spatial resolution of ~0.2’’ (~140 km)!
Figure 4 (right) In the top and middle panel, we show line spectroscopic observations of the Coronal Diagnostic Spectrometer which show plasma flows in the chromosphere. The two CDS images are space-time plots, i.e. one vertical row corresponds to one slit image showing either intensity or velocity. Time increases from left to the right. The color scale denotes the intensity along the slit in the top panel and the magnitude and direction of the observed flows (units of km s^-1). Flows away from the observer scale from red to yellow and flows towards the observer are shown in blue and black. RHESSI X-ray light curves are plotted for the same time interval as the CDS observations (bottom panel). The chromospheric brightenings show flows with different directions in each event. Interestingly, the strongest flows during the microflares are not observed in the brightest pixels.

Conclusions

Summarizing, although we found microflares to be thoroughly complex, the general multi-wavelength and spectral observations of microflares compare reasonably well with our expectations from the standard flare cartoon models - striking agreements, but with a few intriguing discrepancies. This study has been possible because of the wonderful improvements in resolution and sensitivity of the modern solar observations; such a conclusion would not have been possible without multi-wavelength observations of great power both from space- and ground-based observatories. Further microflares and many more details on the work outlined here can be found in our full scientific papers (1, 2), and in papers to be published.

References

1. Hirayama, T.: 1974, Solar Physics 34, 323

2. Cliver, E. W., Dennis, B. R., Kiplinger, A. L., Kane, S. R., Neidig, D. F., Sheeley Jr., N. R., and Koomen, M. J.: 1986, ApJ 305, 920

3. K. Shibata, K., S. Masuda, M. Shimojo, H. Hara, T. Yokoyama, S. Tsuneta, T. Kosugi, and Y. Ogawara: 1995, ApJ 451, L83

Biographical Note: Sigrid Berkebile-Stoiser recently finished her PhD at the University of Graz/Austria. The analysis shown in this Nugget was carried out in corroboration with Astrid Veronig, Peter Gömöry, Jan Rybák, Peter Sütterlin and Henry Aurass. They are affiliated with the University of Graz, the Slovak Academy of Sciences, the Royal Swedish Academy of Sciences and the Astrophysical Institute Potsdam.