We study the dynamics and evolution of a C2.3 two-ribbon flare, developed on 2002 August 11, during the impulsive and the long gradual phase. To this end we obtained multiwavelength observations using the CDS spectrometer aboard SOHO, facilities at the NSO/Sacramento Peak, and the TRACE and RHESSI spacecrafts. CDS spectroheliograms in the Fe XIX, Fe XVI, O V and He I lines allows us to determine the velocity field at different heights/temperatures during the flare and to compare them with the chromospheric velocity fields deduced from H alpha image differences. TRACE images in the 17.1 nm band greatly help in determining the morphology and the evolution of the flaring structures. During the impulsive phase a strong blue-shifted Fe XIX component (-200 km/s) is observed at the footpoints of the flaring loop system, together with a red-shifted emission of O V and He I lines (20 km/s). In one footpoint simultaneous H alpha data are also available and we find, at the same time and location, downflows with an inferred velocity between 4 and 10 km/s. We also verify that the instantaneous momenta of the oppositely directed flows detected in Fe XIX and H alpha are equal within one order of magnitude. These signatures are in general agreement with the scenario of explosive chromospheric evaporation. Combining RHESSI and CDS data after the coronal upflows have ceased, we prove that, independently from the filling factor, an essential contribution to the density of the post-flare loop system is supplied from evaporated chromospheric material. Finally, we consider the cooling of this loop system, that becomes successively visible in progressively colder signatures during the gradual phase. We show that the observed cooling behaviour can be obtained assuming a coronal filling factor between 0.2 and 0.5.
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