OBSERVATIONS OF LOOPS AND PROMINENCES

We review recent observations by the Yohkoh-SXT in collaboration with other spacecraft and ground-based observatories of coronal loops and p rominences. These new results point to problems that SoHO will be able to address. With a unique combination of rapid-cadence digital imagin g (greater-than-or-equal-to 32 s full-disk and greater-than-or-equal-t o 2 s partial-frame images), high spatial resolution (greater-than-or- equal-to 2.5 arcsec pixels), high sensitivity (EM less-than-or-equal-t o 10(42) cm-3), a low-scatter mirror, and large dynamic range, SXT can observe a vast range of targets on the Sun. Over the first 21 months of Yohkoh operations, SXT has taken over one million images of the cor ona and so is building up an invaluable long-term database on the larg e-scale corona and loop geometry. The most striking thing about the SX T images is the range of loop sizes and shapes. The active regions are a bright tangle of magnetic field lines, surrounded by a network of l arge-scale quiet-Sun loops stretching over distances in excess of 10(5 ) km. The cross-section of most loops seems to be constant. Loops disp laying significant increase in the ratio of the footpoint to loop-top diameter (GAMMA) are the exception, not the rule, implying the presenc e of widespread currents in the corona. All magnetic structures show c hanges. Time scales range from seconds to months. The question of how these structures are formed, become filled with with hot plasma, and a re maintained is still open. While we see the propagation of brighteni ngs along the length of active-region loops and in X-ray jets with vel ocities of several hundred km/s, much higher velocities are seen in th e quiet Sun. In XBP flares, for example, velocities of over 1000 km/s are common. Active-region loops seem to be in constant motion, moving slowly outward, carrying plasma with them. During flares, loops often produce localized brightenings at the base and later at the apex of th e loop. Quiescent filaments and prominences have been observed regular ly. Their coronal manifestation seems to be an extended arcade of loop s overlying the filament. Reliable alignment of the ground-based data with the X-ray images make it possible to make a detailed intercompari son of the hot and cold plasma structures over extended periods. Hence we are able to follow the long-term evolution of these structures and see how they become destabilized and erupt.