SOHO Joint Observing Programme 137 TIME VARIATION AND DYNAMICS OF ACTIVE REGIONS LOOPS II Authors: O. Kjeldseth-Moe (CDS), K. Wilhelm (SUMER), W. Curdt (SUMER), K. Schrijver (TRACE), T. Fredvik (CDS), Ø. Wikstøl Progress: First Version: January 18 2001 Objective: To study the plasma dynamics and the strong temporal variability of the emission from active region loops at the solar limb, in a wide temperature range. Conditions Necessary to Run: CDS can run this alone, but high cadence images from TRACE and spectrograms from SUMER are essential whenever they can be obtained. Scientific Justification: A previous JOP, JOP75 (CDS/SUMER/EIT), has given us valuable information about active region loops at the solar limb. CDS has observed very rapid temporal variations in the emission from loops at transition region temperatures (He I, O III, O V, Ne VI). Loops change their appearance appreciably over periods of 10-20 minutes and entire loops may even turn on and off in these short time intervals. Changes are seen in coronal lines, but they are less extensive than the variation in the transition region lines. The rapid changes in morphology is coupled to a strong dynamical state. Doppler shifts corresponding to line-of-sight velocities of 50 km/s or more are commonly seen at transition region temperatures. SUMER has observed even more rapid variations in active region loops. As a part of JOP75, SUMER observed significant changes both in emission and Doppler shift on time scales as short as 30 seconds for transition region lines. EIT observations showed that there are smaller, but still pronounced, temporal variations in the intensity distribution of lower coronal loops as well, over periods of a couple of minutes. Important objectives for this JOP include: - observe with high temporal resolution in order to find the shortest time scales for the variations of the emission and Doppler shifts of loops - observe with high spatial resolution in order to investigate the possible fine structures of the loops - observe with high spectral resolution in order to detect any multiple line profiles - observe uninterrupted for long periods of time to obtain more information of all aspects of active region loop systems. Questions may be asked: Are the strong time variations of the loops related to other rapid time variations in the transition region such as the blinkers (JOP073)? How are the high velocities created? Is the observed behaviour related to extreme fine structure in loops? Whatever will be the answers to these and other questions it seems clear that the observed behaviour will require a complete change in our concept of the transition region and has profound implications for modelling and understan- ding of the solar atmosphere Pointing: This program is to be executed at the solar limb. No part of the SUMER slit must be closer to the solar limb than 10". A part of the CDS FOV must be on the disc (does not apply for O_LOOP4), since the mean wavelength on the disc will be used as a reference wavelength. The FOV of TRACE must cover the areas observed by CDS and SUMER. Operating Details: CDS: Four sequences are run: O_LOOP1, O_LOOP2, (O_LOOP4) and O_LOOP5. O_LOOP1 builds up a 160 x 240 arc second raster in 6 wavelength bands, i.e. He I 584.33 A, O III 599.66 A, O V 629.73 A, Ne VI 562 A, Mg IX 368.06 A, and Fe XVI 360.76 A, by stepping the 4 arc second slit in steps of 4 arc seconds. The exposure time at each position is 10 seconds giving a cadence of approximately 8-10 minutes. This is the ``work-horse'' for the CDS observations, and is run a number of times for a total duration of several hours. O_LOOP2 builds up a similar raster in 13 wavelength bands over a period of 45 minutes. Wavelengths include density sensitive lines and this raster is designed for a more thorough diagnostics. However, the sequence is not suitable for observing time variations since it obviously is not rapid enough. O_LOOP2 is therefore run only at the beginning or/and the end of the JOP. O_LOOP4 is similar to O_LOOP1, the difference being that the slit is not stepped across the loop system, but stays in one single position. This sequence is only an option if SUMER is not observing. O_LOOP5 builds up a 60 x 240 arc second raster in the same 6 wavelength bands as O_LOOP1 and O_LOOP4, by stepping the 4 arc second slit in steps of 8 arc seconds. The exposure time at each position is 10 seconds, giving a cadence of approximately 2 minutes. This is a fast alternative to O_LOOP1. SUMER: Two sequences covering different wavelength bands are run: The first sequence includes N V 1238 A, Mg X 624 A and O V 629 A, with an exposure time of 45 sec. The second sequence is Ne VIII 770 A, C IV 1548 A and C IV 1550 A, with an exposure time of 90 sec. In both cases, the slit is placed at a fixed position close to the solar limb, and series of spectrograms are recorded for several hours before changing to the other observing sequence. TRACE: TRACE observes mainly in Fe IX/X 171 A, but observations in C IV 1550 A channel will also be attempted. The cadence is 30 seconds and the field of view is 320 x 320 arc seconds.