EARTH-DIRECTED CME STUDY
Author(s): R.A. Harrison (CDS)
Progress:
Draft Scheme June 7 1994
Discussion at SPWG June 14 1994
Detailed PlanMinor Modifications June 22
1994November 4
1994
CDS ID: EDCME
Objective: To study the onset phase of coronal mass ejections on the disc, i.e. Earth-directed, and their signatures at L1.
Scientific Case: A Coronal Mass Ejection (CME) represents a significant restructuring of the solar corona. Some 10^12 - 10^13 kg of matter can be expelled from the corona as a magnetic loop system, some tens of degrees across, expands into interplanetary space at speeds of several hundred km/s. Such events often involve prominence eruption and even flares, though the relationships are unclear. Also, soft X-ray brightenings within huge coronal loops have been associated with such events. Numerous models have been presented to explain the CME onset but we have very little information on the properties of the source magnetic structures, on the plasma within them and on the precise sequence of events leading to eruption. Studies of such structures as they erupt will not only allow us to predict the onsets of CMEs and understand their relationship to flare/prominence activity, they will pave the way to predicting geomagnetic activity since such activity is generated by CME-magnetospheric interaction.
Since mass ejection involves the eruption of hot (10^6 K) coronal plasma and cool (10^4 K) prominence material, with activity in the high corona, low corona and chromosphere, observations of this kind require a multidisciplinary approach, with large fields of view and a large temperature range.
This study is designed as a "first look" exercise, to be refined as we learn to view and study CMEs. It homes in on events near disc centre, i.e. CMEs which are unlikely to be Earth-directed and, therefore, detected by the SOHO particle instruments and the Wind and Cluster particle and field detectors. Complementray to this study if the study for CME onsets using limb observations. The disadvantage of disc observations is that we are forced to use proxi data, such as a prominence disappearance, to identify the onset of a CME. Coronagraph observations will, in general, be unabel to detect the CMEs in question.
Method: The basic method is for CDS,SUMER and EIT to monitor specific structures as they cross central meridian. In particular we suggest that large, active prominences are suitable targets. Meanwhile, LASCO can provide full 360 degree coverage in the hope that any ejection is seen as a halo event. The following points are taken into account:
CMEs are huge - on average 45 degree (heliographic) across at the solar limb. Thus, we need large fields of view.
There are clear associations between CMEs and prominences and active regions. Such regions should be among our initial targets.
Past studies suggest that temporal resolutions of order 5 arc minutes are appropriate.
Since this is a sit-and-wait programme, the observing schemes are chosen to provide useful data in the event of no eruption.
Pointing and Target Selection: A target should be chosen, such as a prominence or active region, which is at about E20 degree. The programme should be run on that target for as much time as possible, i.e. about 6 or more hours per day, for the following 5-6 days until it is well beyond about W60 degrees. This range covers the events most likely connected to the Earth. Prime targets should be (i) large prominences (which can be identified in ground based H alpha data), (ii) active regions which have recent history of activity (again, identified using ground based H alpha data), (iii) prominences associated with regions of emerging magnetic flux (identified using H alpha and magnetogram data), (iv) large active region interconnecting loops (identified using X-ray images - e.g. Yohkoh).
Operating Details:
The following raster and image details give the basic operations which should be repeated throughout the CME programme.
CDS
This scheme involves the largest field of view with a selection of lines appropriate
for a wide temperature range. This should provide temperature, density, abundance,
flow and morphological information over a large area in the CME source region.
Note: Since we are summing across lines, lines are selected which are appropriately
separated from surrounding lines. Mg and Fe ranges are used to cover the
temperature range, with some density coverage.
Normal incidence operation
4x240 arcsec slit
4x4 arcmin field of view - i.e. 60 location raster
3 Sec exposure at each location
Total duration = 60x3 = 3 min per raster.
Line Selection - Synoptic Line Selection [NIS]
Ion Wavelength(Å) LogTe Comment
He I 584.33 4.3 cool, granulation, depleted in coronal holes
Mg VI 349.13 5.6
Mg VIII313.73 5.9
Mg IX 368.06 6.0 good for c.hole boundary/structure
Mg X 624.94 6.1
Fe XI 369.16 6.1
Fe XII 364.47 6.2
Fe XII 338.26 6.2 density sensitive ratio w. 364.47
Fe XIII 320.80 6.2
Fe XIII 318.14 6.2 density sensitive ratio w. 320.80
Fe XIV 334.17 6.3
Fe XV 327.02 6.3
Data Compression: Sum 11 pixels across each line
Telemetry = 12 lines x 120 pixels x 16 bits / 10000 = 2.3 Sec. No bottleneck.
SUMER
The SUMER operation includes one wavelength selection with a wide range of
lines which complements the CDS selection. Particularly useful for flow
and morphological study.
1x300 arcsec slit
300x300 arcsec field of view - centred on CDS field
0.76 arcsec steps - 4 steps over 2 Sec - accumulate simultaneously.
Total duration = 395 steps x 0.5 Sec = 197.5 Sec per raster (3.3 min)
Line Selection - 1204-1244Å (Covers: Si III 1206Å, H I Ly 1216Å, N V 1238Å,
Fe XII 1242Å.)
EIT
The EIT images will provide an overview of the CME source area and
surrounding structures with images in each of four bands covering the
target area.
Extract 8x8 arcmin field - centred on CDS field
10:1 compression (2.5 arcsec) with all four bands
Total extraction time - 4 Min.
LASCO
The LASCO coronagraphs should concentrate on monitoring the 360 degree
corona, looking for halo events, i.e. Earth-directed CMEs. The
specific operational details are listed below:
C1 - Fe 5303Å green line. Images every 24(?) minutes.
C2 - White light images every 6(?) minutes (including compression)
C3 - White light images every 60-120(?) minutes
MDI
MDI will provide magnetogram support during the operation.
High resolution and standard resolution magnetograms of the target
area should be produced - about ????? per day.
SWAN
SWAN has the potential for detecting cooler material crossing the inner
heliosphere, possibly the reminder of prominence eruptions.
Monitor near Sun region.
COSTEP/CELIAS
This programme is geared toward Earth directed CMEs,. Some
events will interact with the Earth. Particle data should be monitored for such
events, though this does not involve changes in the mode of operation.
Monitor solar wind for correlations with CME events.
Ground Based Instrumentation
Ground based instrumentation is essential for identifying prominences, magnetic
structure and, possibly the ejecta themselves.
The following data are required:
- H alpha images of the full Sun and target region (e.g. Big Bear, the SOON network etc)
- H alpha limb observations (e.g. Mauna Loa Solar Observatory, Wroclow Observatory)
- Coronal white light images (from Mauna Loa Solar Observatory, Hawaii)
- Magnetogram images (e.g. Marshall SFC, Beijing, the SOON network etc)
Notes:
It is recognised that instruments on SOHO will be operating synoptic programmes which should not be interrupted and other pressures from target of opportunity operations will exist. Thus, although a single operation will last for 5-6 days, we may expect between 25 and 75% of the time dedicated to the CME Onset effort in that time.
This programme should be run on a number of occasions to ensure reasonable observations of CME onsets.