SOHO/TRACE Joint Observing Programme 083 HIGH CADENCE ACTIVITY STUDIES AND THE HEATING OF CORONAL LOOPS Authors: Robert Walsh (St Andrews), Jack Ireland (ESA/GSFC) & Ineke De Moortel (St Andrews) Contacts: Ted Tarbell (TRACE), Richard Harrison (CDS), Julia Saba (MDI), Sarah Matthews (Yohkoh). E-mail Addresses: robert@mcs.st-and.ac.uk, ireland@esa.nascom.nasa.gov, ineke@mcs.st-and.ac.uk, tarbell@sag.space.lockheed.com harrison@solg2.bhnsc.rl.ac.uk, sam@msslac.mssl.ucl.ac.uk Progress: First Draft March 1998 Second Draft February 2000 Latest revision March 2000 Objective: Our aim is to investigate rapid time variation (down to the order of seconds) around and within a target coronal loop system and relate these dynamical changes to theoretical models for various dynamic coronal loop heating mechanisms. Conditions Necessary to Run: Occurrence of a suitably well defined coronal loop system (on either the disk or limb) and involvement of at minimum TRACE. Scientific Case: One of the principle objectives of the TRACE, SOHO and Yohkoh Missions is to tackle the difficult problem of how the solar corona is heated. With the wide variety of structures present in this dynamic environment, it is likely that several different mechanisms play their role in the energy deposition. However, as to which mechanism dominates in a particular solar region is under debate. Coronal loop structures are believed to outline the Sun's magnetic field as it rises through the dense photosphere and spreads out into the rarefied corona. These loops are amongst the hottest regions of the corona (typically 1-10 MK) and appear to show changes down to the cadence threshold of current instrumentation (Strong 1994). This Joint Observing Programme between TRACE, SOHO and Yohkoh seeks to investigate, through a selection of differing observing sequences over a range of timescales, the dynamic heating properties of coronal loops. NOTE: This JOP was first run sucessfully in March 1999. Results from that observing campaign are given in the following. A: Oscillations in Coronal Loops Wave heating mechanisms are thought to play an important role in the heating of coronal loops. The resonant absorption of MHD waves in loop type cavities predicts narrow regions of significant energy deposition: phase mixing of Alfven waves predicts a strongly time dependent amplitude damping. The unambiguous detection of these mechanisms would be a great leap forward in our understanding the role of MHD wave dynamics in coronal loops. Estimates of the wave periods generated by inputing typical coronal parameters to these models suggest that high cadence observations will be very important in the search for conclusive evidence. Fundamentally, we wish to determine the characteristic behaviour of oscillations in coronal loops. EUV coronal oscillations have been found before; for example, a 262s period oscillation is described by Chapman et al. 1972); Antonucci et al. (1984) see both 117s and 141s. Harrison (1987) sees a 20 minute oscillation in a loop system which could be slow standing magnetoacoustic waves as described by Roberts et al. (1984). Recently, Ireland et al. (1998b) describe a wavelet analysis of spatially restricted CDS data in which they find co-temporal 300s period oscillations at chromospheric and transition region temperatures. TRACE permits multi-temperature views of target loop systems at high cadence over fields of view bigger than those available with CDS. Using JOP 83 observations from March 23rd 1999, De Moortel, Ireland & Walsh (2000) investigate a set of TRACE 171 A observations of a bright loop footpoint of a diffuse coronal loop structure. A wavelet analysis displays outward propagating perturbations with periods 180-420 s at approx. 70 - 165 kms-1. These authors suggest that the oscillations are slow magnetoacoustic waves. B: Small scale transient EUV brightenings Recent interest has centred on the concept that the corona is the result of the cumulative effect of many small-scale, localised heating bursts. With improved spatial and temporal resolution, the observed evolution of many discrete brightenings over a wide range of temperatures has been established. Bocchialini, Vial and Einaudi (1997) perform a statistical analysis on SUMER data of an EUV bright point and deduce that self-similiar structures exist with scales much smaller that the instrument resolution. Shimizu and Tsuneta (1997) provide a survey of intensity changes in an active region viewed by SXT. They relate the small brightenings to localised regions within loops. Shimizu (1995) describes many frequent flare-like brightenings, mostly in the form of single and multiple loops observed by the Soft X-ray Telescope (SXT) on Yohkoh. The loop heating appears to occur at either the footpoints or loop intersection and he terms these events ``Active Region Transient Brightenings'' (ARTBs). EUV transient brightenings in coronal loops are described by Berghmans and Clette (1999) - their statistical analysis leads them to conclude that these ``loop events'' are EUV counterparts of ARTBs (Berghmans, McKenzie and Clette, 1999). In the previous run of JOP 83, Ireland, Wills-Davey and Walsh (1999) made a study of 27 small dynamic brightening events in 171 A line with 9s cadence and 1'' resolution. Their results showed the brightenings displayed quite complex structures consisting of small scale loops with hot, dense plasma flowing along them, often over 100 kms-1. However, co-temporal MDI magnetograms do not show any obvious bipolar structure normally indicative of a magnetic loop structure. In addition, there appears to be little connection between the variation of the photospheric magnetic field and the coronal behaviour. It may be that the relevant changes are occuring either at or below the noise limit of MDI. C: Emission and Temperature Variations along a Coronal Loop The temperature profile along a magnetic loop is very sensitive to the position in the loop where the heat is deposited, the lengthscale of the energy deposition and the lifetime of the event (Walsh, 1998). Priest (1998) applies a very simple analytical model to compare the temperature structure for several different heating profiles with SXT temperature measurements along a large (>100 Mm) loop. Kano and Tsuneta (1996) measure the temperature along X-ray active region loops and find that the temperature profile has several small fluctuations which were larger that the observational error. These variations may reflect actual spatial fluctuations of the heat input to the loop. Walsh and Georgoulis (1999) introduce a coronal loop model to examine the plasma response to energy deposition events over a range of lengthscales and timescales. They show that the spatial fluctuations in temperature can be reproduced by small-scale, localised heating events. However, the heating must be dynamic in nature. Static heating models cannot reproduce these types of spatial variation. In fact, Walsh, Bell and Hood (1996) demonstrate that even a simple sinusoidal variation in the heat input with respect to time yields an apex temperature that can differ widely from that expected with a static model. These spatial heating simulations could be used to "recreate" the observed, evolving temperature profile along a loop. Thus, a constraint would be placed on the lengthscale, location and lifetime of the heat input and consequently upon the possible heating mechanism itself. The superior cadence abilities of the TRACE instrument would be invaluable in this context with EUV observations of bright loops down to the order of tens of seconds (see Basic Method below). The diagnostic capabilities of CDS will also be utilised to determine a range of plasma parameter values and velocity measurements. D : Other Instrument Involvement Complementary to the TRACE observations, the spectral capabilities of CDS (plasma density, velocity, turbulent nature and ``average'' temperature measurements) will be essential, as will coincident MDI magnetograms when the target system is on the disc. Yohkoh will provide coverage in higher temperature lines. EIT will provide information on any significant events that occur exterior to the TRACE/CDS field of view (for example, a prominence eruption). The previous run of JOP 83 identified MDI magnetograms as a crucial component in understanding the physics of the observed phenomena. High priority should be set to ensuring that MDI magnetograms are available for the observation time. If at all possible, the high res FOV shpould be preferred. Basic Method: The JOP should be run as a target of opportunity programme - i.e. listed for a period of about 3-4 weeks. The TRACE component of this JOP is split into three related observing sequences which together form a thorough examination of the activity within coronal loops. These sequences are graded with regard to increasing cadence. They are: - TJOP1 - Variations in Loop Structure (minutes) - TJOP2 - High Cadence Temperature Measurements along Loops (orders of tens of seconds) - TJOP3 - Rapid Variations of Loop Emission (order of seconds) For each run of the JOP either one TJOP or a selection of the TJOPs will be used - dependent on the target and the choice of the JOP leaders. The instrumental support from SOHO and Yohkoh for each TJOP is given below in Operating Details. Pointing and Target Selection: Both disk and limb target loop systems will be considered where apparent loop structure is well-defined in EIT and SXT images. With increased solar activity at time of writing (February 2000) the JOP leaders will on this occasion concentrate upon loop systems which are displaying some clear signs of activity (microflaring for example) as compared to the ``static, quiescent'' loops examined in the March 1999 JOP 83 campaign. However, see the note under TJOP3 for certain constraints on this. The JOP leaders are not looking for a region that is flaring and/or erupting. Note that the CDS LOOPS_3 sequence has a specfic need for precise pointing. For example, within one run, the LOOPS_3 slit could be placed on the apex of the loops and then at the footpoint areas. For on disc targets, MDI magnetograms will be of immense help in target positioning assuming that for a simple on-disk bipolar region, the loop apex lies between the areas of opposite polarity while the footpoints are essentially the areas of strong magnetic field. (see Operating Details below). On disk targets would involve all the instruments as indicted below. It would be nice to select region within the MDI high resolution field. Limb targets would be studied using TRACE, CDS, EIT and SXT only. Operating Details: (i) TRACE - studies to be run for each TJOP: TJOP1 : High cadence EUV (195, 171,284) and Ly alpha exposures with intermittent CIV for transition region monitoring. Area : 1024 x 1024 at 0.5'' Cadence : approx. 30s-60s TJOP2 : Higher cadence EUV Observations in two wavelengths (195, 171) alternatively to produce "temperature along loop" measurements. Area : 512 x 512 at 1.0'' Cadence : approx 10-15 s with 3-5 compression. TJOP3 : Rapid imaging of part of loop structure with one line (brightest of 195 or 171) producing fastest cadence possible with TRACE for oscillation analysis and brightening study. Area : 512 x 512 at 1.0'' Cadence : 9 s for approx. 25 minute period NOTE: The clearest way of understanding how this cadence was achieved would be to look back to the last run on March 23rd 1999. In brief, Ted Tarbell put together some experimental runs using 2x2 binning of the CCD which would only run during the radiation-free portions of the TRACE orbit ie for about 25 minutes at a time. In that way, the radiation testing was skipped and AEC was trusted to keep us at good exposures and avoids using too much mass memory. Ted calculated the .utim files for the 4 wavelength combinations for the specific day of observations, computed the HLZ's and checked the mass memory with the timeline program. However, this means that you are relying on AEC (not safeframe) for lumogen protection and thus probably should not be pointed at a very hot active region. (ii) CDS - Studies to be run in conjunction with the observing sequences introduced in JOP59. This study is split into three sub-JOPS as indicated below (see actual JOP for more details). Sub-JOP1: LARGEBP2 (CDS Study 10, Variations 25 or 26) NIS, 2x240 slit, 240x240 area, 120 locations Exposure 45s (Var 25) or 20s (Var 26 - if very bright!) Duration 6949s (Var 25) or 6579s (Var 26). Lines: He I 584, O III 599, O V 629, Ca X 557, Mg IX 368, Mg X 625, Si X 346, 356, Fe XII 364, Fe XIII 348, Fe XIV 333, Fe XVI 360, Si XII 520, backgrnd 355, 335. Sub-JOP2: Run EJECT_V3 once followed by a series of LOOPS_2 and a final EJECT_V3. Total duration should be several hours - the longer the better. EJECT_V3 (CDS Study 11, Variation 18) NIS, 4x240 slit, 240x240 area, 60 locations, Exposure 10s, Duration 991s. Lines: He I 584, O V 629, Si X 347, 356, Mg IX 368, Fe XVI 360. LOOPS_2 (CDS Study 12, Variation 7) NIS, 2x240 slit, 10x240 area, 5 locations Exposure 20s. Duration 134 per raster. Lines: He I 584, O V 629, Mg IX 368, Fe XVI 360. Sub-JOP3: Run EJECT_V3 once followed by a series of LOOPS_3 and a final EJECT_V3. Total duration should be several hours - the longer the better. EJECT_V3 - see above. LOOPS_3 (CDS Study 123, Variation 1) NIS, 4x240 slit, 4x120 area, 1 location Exposure 10s - 50 rasters. Duration 707s. Lines: He I 584, O V 639, Mg IX 368, Fe XVI 360. SPECIAL CASE : When TJOP2 is being run, the CDS PROFILE study should be given priority for producing the temperature along the loop for comparison with TRACE runs. PROFILE (CDS Study 283) NIS, 2x240 slit, 240x240 area, 120 location Exposure 30s - 120 rasters. Duration 1hr14m22s. Lines: He I 584, O V 639, Mg IX 368, Fe XVI 360, Fe XVI 335, Fe XIV 334, Fe XIV 353, Fe XII 352, Fe XII 364. (iii) EIT - Ensure that there are full-Sun images in Fe IX/X, Fe XII within the TRACE/CDS Study timeframe. Ensure in both cases that there is a He II image near in time for alignment with CDS He I. 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