부산대학교 도서관

We analyze coronal loops in active region 8272, observed with TRACE on 23 July 1998 during a 70-min interval with a cadence of 1.5 min, in the temperature range of T≈0.9–1.6 MK. We focus on a compact dipolar loop system with a linear size of ≈30000 km. In this compact loop system we detect about 20 EUV brightenings at any instant of time and in each wavelength. We measure the centroid position of these EUV brightenings and determine their cospatiality in subsequent time frames. We find that EUV brightenings are not cospatial in subsequent time intervals (†t=90 s), but are almost randomly distributed in space and time.From the filter-ratio of 171 and 195 Å fluxes and the spatial diameters (w=2100±1200 km) of the EUV brightening sources we infer an average increase of the electron density by Δne≈10 cm, which corresponds to an approximate density increase by a factor of ≈1.2−2.0 with respect to the ambient EUV-emitting plasma. The temperature of the EUV brightenings is found within a similar range as the ambient plasma, i.e., Te≈0.9–1.6 MK, with no trend of heating during the brightening phase. Because these small EUV brightenings do not show temperature increases like EUV nanoflares, they cannot be associated with upflows of heated plasma by chromospheric evaporation like in flares. Consequently they also do not contribute to coronal heating. Instead, their near-isothermal density enhancements seem to be caused by compressional waves, which start near the loop footpoints and propagate along the loops with approximate sound speed (vs≈130–190 km s). These EUV brightenings are possibly related to other dynamic EUV phenomena that have been reported as ‘explosive events’, bi-directional EUV jets, ‘blinkers’, or wave-like disturbances in coronal loops and plumes.