  We have studied the X-ray emission (both the quiescent component and the
flares) of the dM3e star AD Leo, analyzing the Einstein IPC, ROSAT PSPC and
ASCA SIS observations. Using a consistent method which explicitly considers
sustained heating we have analyzed six flares with sufficient statistics,
deriving constraints on the physical parameters of the flaring regions. In all
cases the flaring loops are likely compact (L approx 0.3 R*), and confined to a
rather narrow range of sizes, incompatible with the large (L >= R*) tenuous
loops claimed by previous analyses of flares on AD Leo and other similar stars.
The flaring loops appear to have a larger cross section (beta = r/L approx.
0.3) than customarily assumed (e.g. beta 0.1). All flares show evidence of
significant heating during the decay phase. Although the derived peak pressures
are high (up to P approx. 10^4 dyne/cm^2) with a peak temperature of approx. 50
MK, the magnetic fields required to confine such loops and to produce the
observed flare luminosity are relatively modest (B approx. 1 to 2 kG) and fully
compatible with the photospheric magnetic fields measured in several flare
stars. If the narrow range of loop sizes obtained is extrapolated to the
quiescent structures responsible for the active corona, the latter can be
naturally scaled up from the solar case through a modest (a factor of 10)
increase in pressure in otherwise solar-like active structures with a small
surface filling factor (approx 5%). The quiescent component of the corona shows
no evidence for abundance peculiarities with respect to the photosphere, and
the quiescent coronal luminosity is remarkably constant (with variations of
less than a factor of 2) across the almost 20 yr span of the observations
discussed here.
