  Open-shutter RHESSI observations of 3-15 keV X-rays are found to exhibit
active region transient brightenings and microflares at a rate of at least 10
per hour occurring even during the periods of lowest solar activity so far in
the mission. A thermal component fitted by temperatures of 6-14 MK dominates
from 3 keV to about 9 keV, but can be traced up to 14 keV in some cases, and
has an average duration of 131(+-103) seconds at 7-8 keV. The duration
increases with decreasing photon energy. The peak count rate defined by
cross-correlation is delayed at low energies. The temperature peaks early in
the event and then decreases, whereas the emission measure increases throughout
the event. The properties are consistent with thermal conduction dominating the
evolution. In some of the bigger events, a second component was found in the
11-14 keV range extending down to 8 keV in some cases. The duration is
typically 3 times shorter and ends near the peak time of the thermal component
consistent with the Neupert effect of regular flares. Therefore the second
component is suggested to be of non-thermal origin, presumably causing the
beam-driven evaporation of the first component. The two components can be
separated and analyzed in detail for the first time. Low-keV measurements allow
a reliable estimate of the energy input by microflares necessary to assess
their relevance for coronal heating.
