RHESSI microflares - Flare Cartoons and Reality

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Nugget
Number: 101
1st Author: Sigrid Berkebile-Stoiser
2nd Author:
Published: 11 May 2008
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Contents

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. Problems for multi-wavelength observations of microflares arise mainly because observing instruments have to meet high demands regarding their sensitivity as well as temporal and spatial resolution. In this Nugget, we show examples of microflares 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 (characteristics) as large flares, we would expect microflares to accelerate particles to suprathermal velocities detectable in X-ray images as high energy emission from flare loop footpoints. They should also leave their signatures as a powerlaw part in the X-ray spectrum. A hot flare loop with temperatures greater than 10 Million degrees or so is expected to emit at lower X-ray energies. 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 Doppler shifts of spectral lines. The chromospheric footpoints of the flare loop should be situated in zones of opposite magnetic polarity.

Figure 1 (left): Highly resolved line of sight magnetogram of active region no. 10465. Microflare locations (crosses) are not evenly distributed but accumulate in complex, intermixed polarity regions. The bottom panel shows the same region in white light which allows to identify spots. (Data origin: Michelson Doppler Interferometer (MDI) and Transition Region and Coronal Explorer (TRACE), RHESSI)
Figure 2 (right): Time evolution of a GOES A5 microflare observed on Sebtember 26, 2003 in the corona (T~ 1 Mill. K) and the chromosphere (T~ 7 000 K). Features expected from 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. (Data Origin: TRACE, MDI, RHESSI)

... And the Reality:

In a 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 surrounding with intermixed polarities (see also the earlier RHESSI Science Nugget A myriad of microflares). As shown for one event in Figure 2, imaging data show an appearance in the chromosphere, transition region and corona which generally resemble the features we 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 become visible in TRACE coronal Extreme Ultraviolet 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 (Figure 3). Chromospheric and transition region lines recorded at the location of the microflare brightenings were found to be Doppler shifted which indicates the existence of chromospheric evaporation flows in the impulsive phase of 3 observed microevents (Figure 4).

Figure 3 Chromospheric fine structure in the H alpha and Ca II line of a very small microflare (GOES class A1) which occurred on September 4, 2006. The observations come from the Dutch Open Telescope (DOT) on La Palma/Spain and have an excellent spatial resolution of ~0.2’’ (~140 km). For this event and two more homologous microflares we have spectroscopic observations of the Coronal Diagnostic Spectrometer (see Figure 4). The photospheric appearance of the observed region is shown in the top left panel. Figure 4 In the top and middle panel, we show line spectroscopic observations of the Coronal Diagnostic Spectrometer which allow to analyze 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. 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). Two CDS brightenings are observed for each of the three microflares visible in X-rays. The chromospheric brightenings show enhanced flows with a varying direction 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 in their detailed characteristics, the general multi-wavelength and spectral observations of microflares compare reasonably with the expectations from the standard flare cartoon models. This 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.

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.

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.

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