  The cause of quasi-periodic pulsations (QPP) in solar flares remains the
subject of debate. Recently, Nakariakov & Zimovets (2011) proposed a new model
suggesting that, in two-ribbon flares, such pulsations could be explained by
propagating slow waves. These waves may travel obliquely to the magnetic field,
reflect in the chromosphere and constructively interfere at a spatially
separate site in the corona, leading to quasi-periodic reconnection events
progressing along the flaring arcade. Such a slow wave regime would have
certain observational characteristics. We search for evidence of this
phenomenon during a selection of two-ribbon flares observed by RHESSI, SOHO and
TRACE; the flares of 2002 November 9, 2005 January 19 and 2005 August 22. We
were not able to observe a clear correlation between hard X-ray footpoint
separations and pulse timings during these events. Also, the motion of hard
X-ray footpoints is shown to be continuous within the observational error,
whereas a discontinuous motion might be anticipated in the slow wave model.
Finally, we find that for a preferential slow wave propagation angle of 25-28
degrees that is expected for the fastest waves, the velocities of the hard
X-ray footpoints lead to estimated pulse periods and ribbon lengths
significantly larger than the measured values. Hence, for the three events
studied, we conclude that the observational characteristics cannot be easily
explained via the Nakariakov & Zimovets (2011) propagating slow wave model when
only angles of 25-28 degrees are considered. We provide suggested flare
parameters to optimise future studies of this kind.
