RHESSI microflares - Flare Cartoons and Reality
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Number: | 101 |
1st Author: | Sigrid Berkebile-Stoiser |
2nd Author: | |
Published: | 11 May 2008 |
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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.
... 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).
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.
RHESSI Nugget Date | 11 May 2008 + |
RHESSI Nugget First Author | Sigrid Berkebile-Stoiser + |
RHESSI Nugget Index | 101 + |