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Arcetri Solar Physics Group: High Resolution Studies

Arcetri Solar Physics Group

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Small Scale Activity


High resolution observations at different heights in the solar atmosphere have emphasized that there is no such thing as a "quiet" Sun (e.g. Schrijver et al., 1999). The study of the countless small scale "nano"-and "micro"-flares, visible even in absence of large scale magnetic structures, is of great relevance for at least two reasons: a) being ubiquitous on the Sun, they could provide at least part of the energy input necessary to coronal heating (e.g., Krucker & Benz, 1998); b) if small flares are governed by the same physical processes as large ones, they could more clearly manifest the elementary physical mechanisms that occur in larger, inherently more complex, flares (Cauzzi et al., 1995; Aschwanden et al., 2000).

Our group has a long experience in observational studies of small scale activity, combining ground-based and space-based data. We feel that the simultaneous sampling at different heights in the solar atmosphere is a necessary tool for an unambiguous identification of a flare, and especially so for small scale events. As an example, in figure we show a tiny flare occurring in region NOAA 8456 on Feb. 9, 1999.

Spectral Profiles

The brightening is visible in the chromospheric Halpha line (red wing, panel a)) and in the concomitant TRACE image acquired at 171 Angstrom (panel b)). The maximum emission at both wavelength is around 16:52 UT. The black straight line in panel a) indicates the position of the spectrograph's slit, and panel d) illustrates the CaII K profiles acquired within the patch of maximum brightening. The vertical direction represents time. It is obvious that the line goes into emission at the time of the flare, and that it shows strong shifts towards the red, i.e. strong downward motions betraying the impact of accelerated particles or conduction fronts onto the chromosphere (panel d)). The motions are of the order of 20 km/s, quite comparable with those registered in large flares. Even more interesting are the CaII K profiles depicted in panel c), that correspond to a point where only little brightening is observed. The chromospheric motions here measured at 16:48, before the "main" flare, are even stronger than those shown in panel d), even though the event itself is scarcely recognizable in intensity images. The fact that the amplitude of the flows in such tiny events is quite comparable with that measured in large flares (max 100 km/s ) suggests that the same fundamental physical processes are at work. This, and the temporal coincidence of other signatures, supports the idea that microflares are indeed just miniature flares, but that one needs a very high resolution in order to observe them (Cauzzi et al., 2003, in preparation)

In order to improve this kind of diagnostics for small flares, we plan to utilize IBIS, to obtain 2-D maps of velocity and intensity in both chromospheric (CaII 854.2 nm) and photospheric lines. This should overcome an observational difficulty peculiar to flare studies, namely the fact that flares are extremely localized both in time and space, hence difficult to pin-point in classic spectrographic modes.


  • Aschwanden, M. J., Nightingale, R. W., Tarbell, T. D., & Wolfson, C.J., 2000, ApJ, 535, 1027
  • Cauzzi, G., Falchi, A., Falciani, R., Smaldone, L. A.,Schwartz, R. A., Hagyard, M., 1995, A&A, 299, 611
  • Krucker, S., & Benz, A.O., 1998, ApJ, 501
  • Schrijver, C. J., et al., 1999, Sol. Phys., 187, 261

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    Last Updated: 03 June, 2003