CDS OBSERVING SEQUENCE ====================== TITLE: Transient brightenings in active region loop systems ----- ID: trans -- CONTRIBUTORS: P.R.Young, V.S.Airapetian, R.N.Smartt, H. Hudson, H.E.Mason ------------ SCIENTIFIC JUSTIFICATION ------------------------ A primary objective of SOHO is to look at different scales of flaring activity in the Sun's atmosphere, as it is thought that the summation of all such flares may account for the heating of the corona. Here we present an observing sequence that will allow the study of small flaring regions in active region loop systems. We base the observing sequence on two types of transient brightenings that have been seen in recent observations of the corona, and which are outlined below. Smartt et al. (1993) reported optical brightenings seen in images obtained in the red and green coronal lines (Fe X 5303 AA and Fe XIV 6374 AA, respectively) by a ground-based coronagraph at Sacramento Peak Observatory. The spatial resolution of these images varies with the seeing conditions at the time of the observations but can be 1 arcsec or less. The red and green lines offer two small "windows" on the corona, sampling material at temperatures of 6.0+/-0.1 and 6.25+/-0.1 in the log. The brightenings are characterized by an initial rise in the green line intensity for around 15 minutes (as the temperature enters the `green window') followed 10 minutes or so later by the red line maximum, showing the material passing through the "red window". These timescales for the brightenings imply radiative as opposed to conductive cooling, which is strengthened by the compactness of many of the brightenings. By the nature of observations of the corona in the visible, only off-the-limb images can be obtained, and the brightenings are commonly observed to coincide with the projected point of intersection of two coronal loops. Thus they have been attributed to loops interacting with each other as they come into close proximity. Airapetian & Smartt (1995) suggest that the different rise rates of loops seen in post-flare loop systems could provide one mechanism for this scenario, but also attracting currents in the loops and photospheric motions are also possibilities. (It is noted that the loop configurations generally remain the same after the interactions suggesting only partial reconnection is taking place (Airapetian, private communication).) (Smartt et al, 1993), such systems would be considered to be primary targets; otherwise normal active-region loop systems would be observed. It is postulated that relatively weak events occur commonly in the corona, and extended time resolution that is still consistent with the raster rate might be needed. But an angular resolution of 2 x 2 arcsec^2 might reveal a relatively high count rate in some lines if such "weak" events are also compact, dependent to some extent presumably on the conduction rates. Hence, some experience will be needed in the search for these events as a guide in selecting optimum exposure times. Shimizu et al. (1994) studied X-ray brightenings in active regions observed by the Soft X-ray Telescope (SXT) onboard Yohkoh. These observations, in contrast to those with the coronagraph, are of the central part of the solar disk and the images are obtained through broad-band filters that sample a number of X-ray emission lines simultaneously, giving a wide temperature coverage. In general, the filters are geared towards covering the hot corona (logT >= 6.5), but are still sensitive to cooler (6.0 < log T <6.5) material. The spatial resolution of SXT is, at best, 2.65 arcsec. The morphologies of the X-ray brightenings have been classified by Shimizu et al., who show that 61% of brightenings have a Y-type magnetic configuration whereby two or more loops have a common footpoint (where the brightening takes place). 25% of brightenings were observed to have an X-type configuration, the contact point being at the tops of the loops. Shimizu et al. suggest similar mechanisms to Airapetian & Smartt as a cause of the X-ray brightenings: loop-loop attraction by electric currents; flux tubes emerging through the photosphere; and loop buffeting by photospheric motions. It is thus tempting to suggest a link between the two, in particular, the brightenings with an X-type configuration may be the same phenomenon as the coronal loop interactions. We stress however that the two instruments involved are looking at different temperature plasmas with different resolutions and different positions on the Sun, and even if they both look at the same active region at the limb a correspondence may be difficult to make. For this reason co-ordinated observations with SOHO (in particular CDS) are essential. CDS can take images in both Fe X and Fe XIV lines for direct comparion with the coronagraph images, and also take simultaneous images in higher temperature lines (such as Ca XVIII 344.77) for comparison with SXT. The spatial resolution, at 2 arcsecs, lies between the coronagraph and SXT. REFERENCES: Airapetian & Smartt, 1995, ApJ 445, 489 Ichimoto, Hara, Takeda, Kumagai, Sakurai, Shimizu & Hudson, 1995, ApJ 445, 978 Shimizu, Tsuneta, Acton, Lemen, Ogawara & Uchida, 1994, ApJ 422, 906 Smartt, Zhang & Smutko, 1993, Sol.Phys. 148, 139 Scheme ------ Only X-ray brightenings on the disk have been analysed, while the optical brightenings are only seen on the limb, thus to relate the two it will be necessary to observe active regions both on the disk and at the limb. We note that SXT brightenings occur less regularly (around 1 per hour) in the quieter active regions expected to be seen at solar minimum. Thus we suggest at least two hours of observations at a time with SOHO to capture the brightening(s). For the disk observations of the active region, SXT images will be required to relate the expected EUV brightenings with the X-ray brightenings. As Yohkoh is more restricted in its observing time, it may be necessary to switch off this observing sequence during Yohkoh 'night-time'. At the limb, SXT images will again be useful, while ground-based observations of the red, green and yellow lines will be essential to relate the optical brightenings to the EUV brightenings. Again, there will be problems with telescope 'night-time' which may be alleviated by using observations from two different observatories (Sac Peak and Japan?). SOHO Observing Sequence ----------------------- CDS should be pointed at a pre-selected active region. If the region is too large for the telescope's 4' x 4' field-of-view, then a region of complex loop topology is preferable. We divide the CDS observations into 2 phases, both using the NIS. The first is a patrol phase, whereby the active region is quickly scanned to search for regions of enhanced intensity. Once a brightening is found, phase 2 is initiated, with a smaller region being scanned more slowly so that weaker line intensities can be obtained. It may be advantageous to use GIS to examine the brightenings in Phase 2 as it has access to a greater number of lines, at the expense of being a lot slower in observing the same region. This can be counteracted by reducing the observing region to 16" x 16", say, however this will require the brightening to be compact and to be accurately found during Phase 1. The switch between NIS and GIS should not be a problem if the look-up tables for GIS are loaded prior to the start of the observing sequence. Phase 1 ------- This is a 'patrol' phase, whererby the active region is rapidly scanned by NIS in strong lines of differing temperatures of maximum abundance. If the intensity of one of these strong lines exceeds a threshold (to be determined from preliminary CDS data), then the position of the brightening is determined and phase 2 started. For comparison with the ground-based observations, we use Fe XIV 334.17 and Mg X 368.07 (has a similar Tmax to Fe X, but line is far stronger than any of the Fe X lines). For higher temperature brightenings, Fe XVI 335.40, Ni XVIII 320.56 and Ca XVIII 344.77 are used. The rise time of the X-ray brightenings is <= 2mins and so, ideally, the active region should be scanned on timescales of less than a minute. The X-ray brightenings are most commonly seen in loops of length 20-40 arcseconds and so we recommend scanning the active region over a 4' x 4' region with a step size of 4" x 240". SPECTROMETER: NIS SLIT: 4" x 240" RASTER AREA: 240" x 240" STEP: 4", 0" RASTER LOCATIONS: 60 x 1 EXPOSURE TIME: 1s DURATION OF RASTER: 60s NUMBER OF RASTERS: until brightening is found POINTING: post-flare loop system on the limb LINES STUDIED: Ca XVIII (6.3 10^6) 344.77 Ni XVIII (3.2 10^6) 320.56 Fe XVI (2.5 10^6) 335.40 Fe XIV (2.0 10^6) 334.17 Mg IX (1.0 10^6) 368.06 Phase 2 ------- The brightenings seen by SXT can be over 1' in size (i.e., the entire length of an active region loop) and so while reducing the field of view over phase 1, we still keep it large enough to observe such brightenings. It is hoped that means exist to reduce the number of raster locations once a brightening has been found - this will significantly reduce the total raster exposure time, allowing better time resolution. SPECTROMETER: NIS SLIT: 2" x 240" RASTER AREA: 120" x 240" (or less) STEP: 2", 0" RASTER LOCATIONS: 60 x 1 EXPOSURE TIME: 5s DURATION OF RASTER: 300s NUMBER OF RASTERS: 6 (or until brightening has faded LINES STUDIED: Ca XVIII (6.3 10^6) 344.77 Ni XVIII (3.2 10^6) 320.56 Fe XVI (2.5 10^6) 360.75 Fe XV (2.0 10^6) 312.55*, 321.78, 327.03 Fe XIV (2.0 10^6) 334.17, 353.83 Fe XIII (1.6 10^6) 320.80, 348.18* 318.17, 348.18 Fe XII (1.5 10^6) 364.47*, 338.27 Fe XI (1.3 10^6) 352.67* Fe X (1.0 10^6) 345.74* Mg IX (1.0 10^6) 368.06 Mg VIII (7.9 10^5) 315.04 Mg VII (6.3 10^5) 367.67 Mg VI (4.0 10^5) 349.16 Si X (1.3 10^6) 356.05, 347.40 Si IX (1.0 10^6) 349.87, 341.87 345.13, 341.87 Si VIII (7.9 10^5) 319.83