To the Formation and Heating Mechanisms of X-ray Bright Points on the Sun

We investigate the formation and heating mechanisms of X-ray bright points (XBPs) with which micro- and nanoflares in coronal holes and quiet regions, coronal heating, plasma jets and eruptions are associated. It follows from observations that XBPs consist of several compact magnetic loops that are considerably smaller than the coronal loops of active regions, but contain a plasma heated to a temperature of (1.5–4.4) MK. Using an ultrafine ( $${\sim}100$$ km) magnetic loop being formed when the magnetic field is entrained by photospheric convection as an example, we show that the plasma in such loops is heated to temperatures of (2–4) MK through the dissipation of electric currents $$I\geq 10^{9}$$ A in the chromospheric part of the loop at an increased Cowling resistivity. We have estimated the plasma density in XBP loops, $$n\approx 10^{9}{-}10^{10}$$ cm $${}^{-3}$$ , which is lower that the density of the surrounding chromosphere. Since the thermal conductivity of the loop along the magnetic field is considerably higher than that across the field, the loop rapidly heats up at the coronal level.