Author(s): J.B. Gurman, J.-P. Delaboudinière (EIT), R.A. Harrison (CDS), P. Lemaire, D. Hassler (SUMER), T. Tarbell (MDI)
Draft Scheme January 1994 Discussion at SPWG January 1994 Detailed Plan December 1994 Distributed to PI Teams TBD Substantial Revision August 1995 Minor revision June 1996
CDS ID: ?
Objective: (Phase 1)To determine whether there is (are) clearly distinguishable coronal signature(s) of small-scale, emerging, submerging, or canceling magnetic flux in any of the four EUV bandpasses of EIT, and if so, (Phase 2) to measure the UV and EUV emission measure of those signatures to determine quantitatively the amount of coronal heating that is contributed by small magnetic regions emerging from the photosphere.
Scientific Case: Either solar physicists are colossally stupid, or there is a good physical reason why "the mechanism that heats the corona" has gone undiscovered for 50 years after the high coronal temperature was demonstrated. It may be that the mechanism has simply been very difficult to observe, as might be the case for resonant Alfvén wave dissipation, or it may be that the corona is in fact heated by a wide variety of mechanisms that each only account for 10% or less of the required heating rate.
While the latter possibility is unattractive for those of us accustomed to shaving with Occam's razor, the sheer complexity of the solar atmosphere, and the continued EUV emission of the corona even at solar minimum suggest that it is not impossible that there are more than one mechanisms at work. In any case, the relative contributions of the various phenomena have not been determined quantitatively. With the ultraviolet instruments and MDI on SOHO, however, it should be possible to make such measurements for clear-cut coronal manifestations of emerging -- or submerging or canceling -- magnetic flux.
Method: This approach is divided into two phases. The first presupposes that EUV signatures of small-scale, emerging/submerging/canceling ("ESC") magnetic flux can be observed by EIT in one or more of its multilayer bandpasses. The second phase depends on:
* being able to detect these signals onboard, with the processing capabilities of the LEB,
* sending a flag with positional information to the other participating instruments, and then
* monitoring the coronal manifestation through enough of its life cycle, with sufficient temperature and DEM resolution, to yield measurements of the radiative energy as both a function of time, and integrated over the lifetime of the event.
To use such information to produce an estimated coronal heating rate, it will be essential to perform enough Phase 1 observations to determine the frequency of such events in the UV and EUV, and enough Phase 2 observations to achieve some statistical idea of mean properties.
Pointing and Target Selection: Pointing will be driven by both the need to observe features clearly on the disk, to allow estimating projected areas, and by the need to use the magnetography from the MDI 5' x 5' high-resolution field of view, which is centered on the central meridian, but offset N of the solar equator in nominal spacecraft pointing. To obtain a reasonable diversity of small-scale flux changes, we should perform the observations in both closed and open (coronal hole) regions. (Indeed, if conditions are similar to the soft X-ray observations, and Skylab and OSO experience indicates that they will be, it may only be in coronal holes that "EUV bright points" are clearly distinguishable.)
The following raster and image details give the basic operations which should be carried out during the search phase of the ESC region program:
The EIT images will determine whether there are easily identifiable signatures of ESC regions in the underlying target area. (i.e., the MDI high-resolution FOV).
Field of view: 5 x 5 blocks (416" x 416")
Bandpasses: all four (170, 195, 284 304 Å)
Cadence: 10 m or better
The MDI high-resolution magnetograms will provide the basic information for Phase 1: where are small-scale magnetic regions emerging from, submerging back into, or canceling at the solar photosphere? The MDI high-rate observations will also give continuum intensity and Doppler velocity maps of the same region.
A few minutes of VC2 telemetry near the start of the observing sequence would yield a few realtime images at the EOF.
Field of view: 7' x 7', not following solar rotation, but coordinates adjustable each operational day
Data products: 1 Dopplergram, 1 magnetogram, 1 intensity image
Cadence: 1 m
Field of view: 4' x 4'
Slit: 4" x 240"
Lines: Fe XV 384
Fe XVI 335, 360
Cadence: 10 m
Flag generation: ?
Sequence reference: Red Book 188.8.131.52
Field of view: 2' x 3'
Slit: 0.3" x 120"
Lines: Ly beta, O VI
Cadence: 20 m
Phase 2. Once a valid, repeatable, coronal signature of ESC regions has been identified in the EIT or CDS data from Phase 1, we can proceed to amassing some statistics on DEM and Te as a function of time in such features. (Note that all the above assumes we know enough about abundance variations as to be able to ignore them or fudge them in our DEM and Te determinations.)
Phase 2 observations consist of the same observations in Phase 1, but with EIT or CDS generating a flag, and CDS and SUMER then switching to different modes.
As in Phase 1, above.
As in Phase 1, above.
Before flag: FLARE program
Spectrometer: NI Slit: 2 x 240 arcsec Raster Area: 10 x 60 arcsec Step Size: 2 arcsec Number of Steps: 5 Dwell/Exposure: 0.5 sec Duration: 5x0.5 + overheads = 16 sec Number of Rasters: Open Total Duration: Open (16 x n seconds) Bins across Line: 11 Compression: Select only 30 pixels (60 arcsec) along slit 16 to 12 bit compression Line Selection: 4 very bright, well separated lines representing different temperature regimes. Ion Wavelength (A) Log Te He I 537.03 4.3 O IV 554.52 5.3 Mg IX 368.06 6.0 Fe XIV 334.17 6.3
After flag: much smaller area
Spectrometer: GI Slit: 4 x 4 arcsec Raster Area: 20 x 20 arcsec Step Size: 4 arcsec, 4 arcsec Number of Steps: 5 x 5 = 25 Dwell/Exposure: 13 sec Duration: 375 sec Number of Rasters: Open Total Duration: Open (375 x n seconds) Bins across Line: n/a Compression: None Line Selection: All GIS data.
Before flag: As in Phase 1, above.
? (Philippe - au sécours!)
No participation (but any ideas?)
Not if I can help it.