RHESSI microflares - Flare Cartoons and Reality

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= RHESSI Microflares - Flare Cartoons and Reality =
= RHESSI Microflares - Flare Cartoons and Reality =

Revision as of 22:38, 7 May 2009


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

RHESSI Microflares - Flare Cartoons and Reality

Introduction

The sequence of events and the general observational signatures to be expected for solar flares have been visualized 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 consistence/disagreement with 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 microevents for which highly resolved imaging data showing the chromosphere and corona as well as magnetic field and spectroscopic data are available.


Figure 1: Top: Highly resolved (0.6’’/pixel) line of sight magnetogram of active region no. 10465, one of four active regions present on the Sun on September 26, 2006. All microflares on that day for which images could be reconstructed originated from this active region. The centroid positions of microflares are indicated by red crosses. Microflares 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))

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 2: 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). This event was observed on September 26, 2003. The images were recorded in the 17.1 nm and 1600 nm filter passbands of the TRACE instrument and have a resolution of 0.5’’/pixel. Flare loop footpoints brighten up in the impulsive phase of the microflare and a postflare loop becomes visible 15 min later. As expected from flare cartoons, the RHESSI 3-8 keV source showing hot plasma inside the flare loop (T~10 Mill. K) is situated in between the chromospheric brightenings. Magnetic field maps for the microflare site demonstrate that the RHESSI loop ends are located in areas of opposite 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 RHESSI science nugget no. 52 `A myriard 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).

Summarizing, although we found microflares to be thoroughly complex in their detailed characteristics, the general multi-wavelength and spectral observations of microflares compare soundly to flare cartoons.

Figure 3: Chromospheric fine structure 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. Data from DOT can be processed in a way that atmospheric disturbances are removed from the images (`speckle reconstruction’). This greatly improves the spatial resolution, in this case it was as good as ~0.2’’. In the chromospheric H alpha line (bottom left), three microflare brightenings are visible, the strongest of which lies in the penumbra of a sunspot. The brightenings are finely structured on the arcsec level. The thermal RHESSI X-ray emission in this microflare originates from a loop source in between two of these brightenings. The bottom right image is the same as on the left, with the position of the spectrograph slit of the Coronal Diagnostic Spectrometer (CDS) indicated. See Figure 4 for a description of the CDS observations of this (and two more homologous) microflares. The photospheric appearance of the observed region is shown in the top left panel.
Figure 4: Bottom: RHESSI X-ray light curves for three microflares which occurred on July 4, 2006. Chromospheric images of the first event are shown in Figure 3. In the top and middle panel, line spectroscopic observations of the Coronal Diagnostic Spectrometer can be seen. The two CDS images are space-time plots, i.e. one vertical row corresponds to one slit image in time in either intensity or velocity space. Time increases from left to the right. Here, we show the intensities and Doppler velocities observed for a chromospheric EUV line (He I 58.43 nm). Color scale denotes the specific intensity in the top panel (units of erg s-1 cm-2 sterad-1 A-1) and the magnitude and direction of the observed flows in this line (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. As can be seen in the intensity space-time plot, two brightenings are observed for each of the three RHESSI microflares. Especially for the first event, CDS emission correlates well in time with the RHESSI X-ray emission. Flare simulations of electron driven chromospheric evaporation predict either up- or downflows in the chromosphere during the impulsive phase of flares, depending on the amount of energy input by non-thermal electrons. Although the three microflares look similar in DOT imaging data, the chromospheric flow pattern is different. Downflows (+40 km s-1) are observed for the first event, whereas upflows (-40 km s-1) occur in the second one. In the third microflare, the flow direction changes fastly in the northern microflare brightening whereas weak downflows (+20 km s-1) are seen in the southern brightening. Interestingly, the strongest flows (+70 km s-1) during the microflares are not observed in the brightest pixels but 10’’ to the south, where the speeds attained are in agreement with plasma in free fall.


Further microflares and many more details on the work shortly outlined here can be found in


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 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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