European Solar Physics Meeting 'The Dynamic Sun: Challenges for Theory and Observations' edition:11 location:Leuven, Belgie date:11-16 September 2005
Observations of coronal loops at high temporal resolution reveal their highly dynamic nature. Bright features (”blobs”) suddenly appear in the upper corona and sub-
sequently rain down along arched paths towards the solar surface. The acceleration of these blobs is signiﬁcantly smaller than free-fall, and some blobs brighten up before
reaching the surface. In order to explain these phenomena, we compare Hα time series of high-resolution images of coronal rain taken with the Swedish Vacuum Solar Telescope on La Palma with detailed numerical simulations. Our simulations show that any heating mecha-
nism which dissipates energy predominantly at the footpoints of coronal loops is able to trigger a highly dynamic evolution due to a loss of thermal equilibrium at the loop apex. The resulting process of ”catastrophic cooling” gives rise to fast downﬂows of cool, dense plasma blobs which emit strongly in emission lines formed at transition region and chromospheric temperatures. We study the
evolution of the observed blobs by tracking the intensities along their paths, and then deduce their projected velocities. From this analysis, we ﬁnd a good qualitative agreement with the catastrophic cooling model. This ﬁnding is supported by data from several other recent observing programs, e.g. an EIT shutterless campaign and a joint observing program (JOP 174) of SOHO and TRACE.