SOHO Joint Observing Plan No. 1

Emerging/Submerging/Canceling Magnetic Flux Regions

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


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.)

Operational Details:

Phase 1. In order to get a reasonable sample, these observations should be run for periods of several hours (minimum; 8 - 16 hours optimum to see if lifetimes match those in SXR and when in lifetimes of the putative features the most distinguishable signatures occur). Note that this program cannot be run during the two-month realtime campaign periods, nor possibly during other MDI-groundbased campaigns.

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)

Compression: Rice

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


In Phase I, CDS will be looking for EUV heating signatures of its own with its FLARE program.

Spectrograph:		NI

Field of view: 4' x 4'

Slit: 4" x 240"

Lines: Fe XV 384

Fe XVI 335, 360

Cadence: 10 m

Flag generation: ?


SUMER will attempt to measure the heating in the layers of the atmosphere between the photosphere and corona, so its sequences will use a range of chromospheric and transition region lines.

Sequence reference:	Red Book

Field of view: 2' x 3'

Slit: 0.3" x 120"

Lines: Ly beta, O VI

Compression: moments

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.

After flag:

? (Philippe - au sécours!)

Particle Instruments

No participation (but any ideas?)

Ground Based Instrumentation

The target selection (on disk) rules out coronagraph involvement, but it may be useful to consider ground-based alternatives/complements to the MDI magnetograms (e.g. NSO magnetograph, HAO Stokes Polarimeter) and the SUMER chromospheric observations (e.g. SOUP). Microwave and millimeter-wave observations could also provide morphological information at high spatial resolution, though they would not contribute significantly to the total emitted energy determination.

Inter Agency Consultative Group

Not if I can help it.

Yohkoh (launch 1991):

SXT maps of the same areas as studied by MDI and EIT would extend to higher temperatures the DEM(t) determinations we propose. During solar minimum, this should not conflict frequently with Yohkoh's primary aim of studying flares.

CORONAS (launch 1994/5)

EUV and X-ray imagers and spectrometers are available on Coronas I/F which could complement the SOHO and Yohkoh operations. The launch dates and durations are not clear at this time.

TRACE (launch 1998)

This, of course, is the sort of thing TRACE is all about. With a year or so of these observations in hand, we should be able to help TRACE optimize its observations of small-scale coronal heating events.

SWATH (launch ?)

The combination of the high-resolution NIXT multilayer telescope on SWATH, TRACE, and SOHO would be pig heaven for this objective.


Since this program does not depend on ground-based collaboration as strongly as many of the other proposed JOPs or the helioseismology campaigns, it would make sense to schedule instances during periods of poor ground-based observing conditions, e.g. N hemisphere winter. If SOHO is launched in late 1995, this could even be done during the cruise phase or directly after the commissioning period in early 1996.

Latest revision: 1996 June 25 - J.B. Gurman