Time variability of EUV brightenings in coronal loops observed with TRACE

We analyze coronal loops in active region 8272, observed with TRACE on 23 J uly 1998 during a 70-min interval with a cadence of 1.5 min, in the tempera ture range of T approximate to 0.9-1.6 MK. We focus on a compact dipolar lo op system with a linear size of approximate to 30 000 km. In this compact l oop system we detect about 20 EUV brightenings at any instant of time and i n each wavelength. We measure the centroid position of these EUV brightenin gs and determine their cospatiality in subsequent time frames. We find that EUV brightenings are not cospatial in subsequent time intervals (Delta t = 90 s), but are almost randomly distributed in space and time. From the filter-ratio of 171 and 195 Angstrom fluxes and the spatial diamet ers (w = 2100 +/- 1200 km) of the EUV brightening sources we infer an avera ge increase of the electron density by Delta n(e) approximate to 10(9.0 +/- 0.3) cm(-3), which corresponds to an approximate density increase by a fac tor of approximate to 1.2-2.0 with respect to the ambient EUV-emitting plas ma. The temperature of the EUV brightenings is found within a similar range as the ambient plasma, i.e., T-e approximate to 0.9-1.6 MK, with no trend of heating during the brightening phase. Because these small EUV brightenin gs do not show temperature increases like EUV nanoflares, they cannot be as sociated with upflows of heated plasma by chromospheric evaporation like in flares. Consequently they also do not contribute to coronal heating. Inste ad, their near-isothermal density enhancements seem to be caused by compres sional waves, which start near the loop footpoints and propagate along the loops with approximate sound speed (v(s) approximate to 130-190 km s(-1)). These EUV brightenings are possibly related to other dynamic EUV phenomena that have been reported as 'explosive events', bi-directional EUV jets, 'bl inkers', or wave-like disturbances in coronal loops and plumes.