Researchers in Arcetri are involved in studies of solar activity at high temporal and spatial resolution, especially at photospheric and chromospheric levels. Our principal aim is to study the temporal correlation between the flares at coronal levels (e.g. hard X-rays emission) and the lower atmospheric signatures, mainly during the preflare and the impulsive phase.

Of particular relevance is the measure of the velocity field both in the chromosphere (e.g. in the wings of Hydrogen Balmer lines and Ca II K line) and photosphere. This signature signals the occurrence of chromospheric compression, the counterpart of chromospheric evaporation seen at soft X-ray wavelengths, and allows to constrain the boundary conditions for dynamic models. For example, some results of our work indicate that realistic flare models have to contain a velocity field that varies with height and that the primary electron beams impact the lower atmosphere in very small areas (down to few arcseconds size), that change position during the flare.

Analysis of the data acquired during a cordinated observing campaigns between the ground-based telescopes (NSO/Sac Peak) and the instruments onboard TRACE, YOHKOH and SOHO reveals interesting properties of small flares. In particular the analysis supports the concept that microflares are just {\it miniature} flares, i.e. all the relevant signatures normally detected in a major flare are visible also in the studied small event, with the same sequentiality. We find that chromospheric downward motions are a very distinctive characteristics of the flare phenomenon even in tiny events.

In the case of a small two-ribbon flare (C1 in the GOES energy scale) we compared the development of velocity fields in the whole atmosphere. We obtained for the first time simultaneous and spatially resolved observations of velocity field during the impulsive phase of the flare, both in chromosphere and upper atmosphere. In this phase, strong downflows are measured at chromospheric levels, while strong upward motions are instead measured in TR and coronal lines. For the first time, we demonstrate that such oppositely directed flows originate from the same flaring kernels in different atmospheric layers. This confirms the scenario of chromospheric evaporation predicted in theoretical models of flares during the hard X-ray burst.

In this scenario, it would be very important to obtain spectra of chromospheric lines immediately before the flare but we know that it is a difficult task to observe with a spectrograph in the right position at the right time. To this end we plan to use for ground-based observations the imaging spectrograph IBIS that allows simultaneous spectra over all the flare area and to coordinate our observing campaigns with the NASA satellite RHESSI that will provide high spectral resolution images in the range 3 KeV -- 17 MeV.

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Last Updated: 23 March, 2003