Solar Wind from Coronal Holes: A Closer Look

Craig DeForest (MDI)
Barbara Thompson (EIT)
John Cook, Gunther Brueckner (LASCO)
Andrea Modigliani (UVCS)
Klaus Willhelm, Udo Schuehle (SUMER)
Andrew Breen (EISCAT)
Jack Harvey (Kitt Peak)

First proposed: 25-July-97, to be run near 7-Sept-97
Latest revision: 4-Sep-1997

Participating Instruments and Observatories:

MDI, EIT, LASCO, UVCS, SUMER, EISCAT, Kitt Peak

Background and Scientific Justification

Studying the origin and acceleration of the solar wind is one of SoHO's primary missions. Because the solar wind has long been known to originate in the polar coronal holes, there naturally has been more than a modicum of interest in measuring solar wind flow from the coronal holes. In the first year of the SoHO mission, wind-speed-like quantities have been measured by several SoHO instruments and several ground-based observatories in collaboration with SoHO.

Unfortunately, no clear "consensus profile" of the wind speed has been thus obtained: the different measurements of wind speed have yielded results that prima facie conflict. In fact, the different techniques are somewhat incommensurate: they measure different quantities. Table 1 characterizes some of the measurements that have been made, and that were presented at the SoHO-V workshop in Oslo, in June 1997.

 ________________________________________________________________________
 | Instrument | Altitude(R₀)| "Speed"       | Principle of Measurement   |
 |------------+-------------+---------------+----------------------------|
 | SUMER      | 1.0 - 1.3   | <50 km/s      | Direct doppler shift       |
 | UVCS       | 1.5 - 4     | 50 - 200 km/s | Doppler Dimming            |
 | LASCO      | 2.0 - 6.0   | 600 km/s      | Running-difference images  |
 |            |             |               |                            |
 | EISCAT     | [NA - To be filled in]      | Running-difference ranging |
 -------------------------------------------------------------------------

Table I: Existing wind-speed measurements presented at SoHO V

The main disagreement between the different measurements appears to be between direct (Doppler) and indirect (elapsed-time) measures of wind speed. The LASCO and EISCAT measurements yield speeds up to the accepted in situ wind speed at 1AU -- as close in as 2R₀. But those measurements detect the motion of broad features rather than of the wind material itself; so that they may really be measuring the speed of propagating waves. In contrast, UVCS and SUMER both yield much lower values of the wind speed. The UVCS measurements, which involve doppler dimming, require a thermal model of the coronal hole plasma and hence their measurements are somewhat model-dependent. However, they can place an upper limit on the speed at 2R₀ of ~200km/sec almost regardless of the model used. Similarly, SUMER sees no measurable systematic doppler shift of spectral lines emitted at the base of the corona, and hence can place an upper limit of about 50 km/s on the wind speed at altitudes as high as 1.3 R₀.

Meanwhile, analysis of high cadence images from EIT has demonstrated the existence of compressive wave trains in polar plumes (the densest features in the coronal holes) at the lowest altitudes: 1 - 1.2 R₀. If these (or similar) waves can be shown to propagate all the way to (and above) 2 R₀ in the corona, they will provide a ready explanation for the difference between the LASCO and UVCS observations.

The purpose of this JOP is to obtain simultaneous measurements of wind speed from each of the main instrument types, to more clearly illustrate (and hopefully to resolve) the conflict between them. A simultaneous observation by the several instruments is needed to lend credence to any explanation that is forwarded.

Overview of Observing Campaign

Scheduling

To bring the coronal hole into the best view in the lower-coronal instruments, this JOP should be run near the times of maximum B angle. We request that it be scheduled as close to 7-Sept-97 as possible; subsequent runs may be requested (with VLA or other GBRO collaboration for radio scintillation measurements of wind speed) on or near subsequent dates of maximum B angle.

Duration

Two to three days of six to eight hours per day coverage is suggested as a baseline; however, the complete observing plan is not yet finalized (as of 25-July-97). The relatively long duration is necessary because of the need to interlace high cadence observing sequences in some of the imaging instruments.

Instrument participation:

Participation is requested from, at a minimum, LASCO, EIT, SUMER, MDI, and UVCS. Ground-based radio observatories (Nobeyama) will be contacted. If possible, a VLA-collaborative run of this JOP during March 98 B-angle maximum is suggested (after the initial September 97 run). Individual instrument plans are listed below:

LASCO (Contact: John Cook)
LASCO's purpose in this JOP is twofold: (1) to provide information n the coronal hole structure for the spectral instruments (eg UVCS); and (2) to provide one of the independent wind-speed measurements that comprise the main goal of the JOP. The main outflow measurements will be deep-field coronagraph "movies" over the hole. Each of the three days of the JOP will concentrate on one camera-field boundary: EIT/C1 on Day 1; C1/C2 on Day 2; and C2/C3 on Day 3. In addition, on Day 1, a C1 spectral scan will be made of the green line, in the hope of making a coronal dopplergram.

EIT (Contact: Barbara Thompson)
EIT will provide four-wavelength snapshots of the coronal hole at the beginning and end of each observing day to provide context and line-ratio temperatures. In addition, a high-cadence sequence will be run for three hours in either the 171A or 195A passband on Day 1, to identify the origins of wavetrains and/or discrete "blobs" that may be observed higher up.

SUMER (Contacts: Klaus Wilhelm, Don Hassler)
SUMER has the possibility of directly measuring doppler shifts of features that project into the line of sight. Just below the limb, the shifts should average toward the blue; just above the limb, they should average to a broadening as features behind the Sun project outflow into motion away from SoHO. SUMER can thus provide perhaps the only truly direct measurement of bulk flow. No systematic outflow-signature shift has been seen (prior to the Oslo SoHO-V meeting); but a carefully tailored observation may provide more sensitivity, and even a negative measurement (providing a maximum value of wind speed in the lower reaches) is useful.
SUMER will provide spectra just above and just below the limb for a differential comparison of line center; and spectra from as high up as is reasonably practical for good photon statistics, to look for Doppler-broadened lines.

UVCS (Contacts: Andrea Modigliani)
UVCS provides spectral information from slightly higher up than SUMER. This will be a key instrument to the JOP, as their measurement of wind flow speed is one of thost listed in Table I. Detailed observing plans are in progress.

MDI (Contact: Craig DeForest)
MDI will provide, at a minimum, its full-disk synoptic magnetograms to allow field extrapolation into the fields of view of the other instruments. If possible, a high cadence magnetic program will be run simultaneously with the lower coronal instruments' high cadence observations (C-1, EIT, SUMER and/or, CDS) to compare magnetic emergence/submergence with observed evolution at the base of the coronal hole.

Kitt Peak (Contacts: Jack Harvey, Frank Recely)
Kitt Peak will record high cadence Calcium ``chromospheric magnetograms'' each of the three days. Comparison with photospheric magnetograms from MDI and from BBSO will help elucidate the shape of the magnetic structures at the very base of the corona.

EISCAT (Contact: Andy Breen)
EISCAT will observe outflows with radio scintillation techniques, at distances of 28-50 R_O; this will allow ready comparison with wind speeds measured with LASCO's C-3 camera (which in turn may be compared with the instruments looking closer in).

Detailed Observing Plan:

Some of the observatories (eg MDI, Kitt Peak) will continue observing in the same mode throughout the three-day observing sequence; others will have a rotating schedule of concentration on a particular part of the corona. The rotating schedule will chiefly affect EIT, LASCO, and UVCS -- which either share resources or are photon-limited, and so must concentrate on a particular portion of the corona on each day. The days will be allocated in ascending order of altitude, as follows:

Day 1: Surface - 2.5 R_O
Day 2: 1.1 R_O - 6 R_O
Day 3: 2.0 R_O - 30 R_O

Day 1:

EIT and LASCO C-1 will generate high cadence sequences on this day. In addition, C-1 will generate a "coronal dopplergram" by scanning its narrowband filter through the visible green line band; and, at intervals through the day, C-1 will generate PB images to support UVCS's doppler dimming measurements. SUMER will observe several lines just above and below the limb of the Sun, to search for differential doppler shifting due to outflow. Kitt Peak and MDI will support the day with high cadence magnetograms. The Kitt Peak observatory will give special priority to the high-cadence chromospheric magnetogram sequence during the three hours of high cadence observation.

SEQUENCE OF OBSERVATION:
Day 1 - 6-Sep-1997

Start time Observing phase Purpose
INST. Observation
15:00 Doppler Dimming (till 00:00) Speed measurements at two altitudes UVCS Spectra at two slit positions through the day: 1.5 and 1.7 R₀.
15:00 Pre-scan (est. 30 min) context & ionization temp. EIT 171Å, 195Å,and 284Å images
C-3 deep WL exposure
~15:30 PB images (est. 40 min) UVCS support C-1 PB sequence over North coronal hole
16:00 Coronal Dopplergram (est. 1 hr) Doppler coronagraph C-1 10 x 8-sample scan over the spectral line
SUMER differential-Doppler scan (10 positions; 3 hours total)
17:00 High Cadence (3 hours)
(LASCO and EIT measurements repeat on 4-minute cadence.) Wave / phase speed measurement and Doppler measurement EIT 171Å exposure over pole
C-1 on- and off- line exposures (N 1/2 of field)
SUMER differential-Doppler scan (10 positions; 3 hours total)
KPNO High-cadence chromospheric magnetograms
20:00 C-1 Dopplergram (est. 1 hr) Doppler coronagraph C-1 10 x 8-sample scan over the spectral line
21:00 PB images (est. 40 min) UVCS support C-1 PB sequence over North coronal hole
~21:30 Post-scan (est. 30 min) context & ionization temp. EIT 171Å, 195Å,and 284Å images
C-3 deep WL exposure

Start time	Observing phase	Purpose
INST.	Observation
15:00	Doppler Dimming (till 00:00)	Speed measurements at two altitudes	UVCS	Spectra at two slit positions through the day: 1.5 and 1.7 R₀.
15:00	Pre-scan (est. 30 min)	context & ionization temp.	EIT	171Å, 195Å,and 284Å images
C-3	deep WL exposure
~15:30	PB images (est. 40 min)	UVCS support	C-1	PB sequence over North coronal hole
16:00	Coronal Dopplergram (est. 1 hr)	Doppler coronagraph	C-1	10 x 8-sample scan over the spectral line
SUMER	differential-Doppler scan (10 positions; 3 hours total)
17:00	High Cadence (3 hours) (LASCO and EIT measurements repeat on 4-minute cadence.)	Wave / phase speed measurement and Doppler measurement	EIT	171Å exposure over pole
C-1	on- and off- line exposures (N 1/2 of field)
SUMER	differential-Doppler scan (10 positions; 3 hours total)
KPNO	High-cadence chromospheric magnetograms
20:00	C-1 Dopplergram (est. 1 hr)	Doppler coronagraph	C-1	10 x 8-sample scan over the spectral line
21:00	PB images (est. 40 min)	UVCS support	C-1	PB sequence over North coronal hole
~21:30	Post-scan (est. 30 min)	context & ionization temp.	EIT	171Å, 195Å,and 284Å images
C-3	deep WL exposure

Day 2:

LASCO C-1 and C-2 will generate high cadence sequences on this day. In addition, C-1 will generate a "coronal dopplergram" by scanning its narrowband filter through the visible green line band; and, at intervals through the day, C-2 will generate PB images to support UVCS's doppler dimming measurements. SUMER will observe several lines just above and below the limb of the Sun, to search for differential doppler shifting due to outflow. Kitt Peak and MDI will support the day with high cadence magnetograms. While chromospheric magnetograms have been requested from Kitt Peak, no special priority is given to high cadence during the three hours from 20:00 - 23:00.

Because of the late start of the DSN pass on this day, the sequence start time is 18:00 rather than 15:00 for the main observation block and for the magnetograms. Also because of this time delay, the PB images and C-1 dopplergram after the high cadence period are reversed in order, to bring the PB images into UVCS's observing window.

SEQUENCE OF OBSERVATION:
Day 2 - 7-Sep-1997

Start time Observing phase Purpose
INST. Observation
15:00 Doppler Dimming
(till 00:00) Speed measurements UVCS Spectra at 2.5R₀.
PB images (est. 40 min) C-2 PB sequence over North coronal hole
18:00 Pre-scan (est. 30 min) context & ionization temp. EIT 171Å, 195Å,and 284Å images
C-3 deep WL exposure
~18:30 PB images (est. 40 min) UVCS support C-2 PB sequence over North coronal hole
19:00 Coronal Dopplergram (est. 1 hr) Doppler coronagraph C-1 10 x 8-sample scan over the spectral line
SUMER differential-Doppler scan (10 positions; 3 hours total)
20:00 High Cadence (3 hours)
(LASCO measurements repeat on 4-minute cadence.) Wave / phase speed measurement and Doppler measurement C-1 on- and off- line exposures (N 1/2 of field)
C-2 orange-filter deep exposures (just under 1/2 of field over N pole)
SUMER differential-Doppler scan (10 positions; 3 hours total)
KPNO Chromospheric magnetograms
23:00 PB images (est. 40 min) UVCS support C-2 PB sequence over North coronal hole
~23:30 C-1 Dopplergram (est. 1 hr) Doppler coronagraph C-1 10 x 8-sample scan over the spectral line
~00:30 Post-scan (est. 30 min) context & ionization temp. EIT 171Å, 195Å,and 284Å images
C-3 deep WL exposure

Start time	Observing phase	Purpose
INST.	Observation
15:00	Doppler Dimming (till 00:00)	Speed measurements	UVCS	Spectra at 2.5R₀.
PB images (est. 40 min)	C-2	PB sequence over North coronal hole
18:00	Pre-scan (est. 30 min)	context & ionization temp.	EIT	171Å, 195Å,and 284Å images
C-3	deep WL exposure
~18:30	PB images (est. 40 min)	UVCS support	C-2	PB sequence over North coronal hole
19:00	Coronal Dopplergram (est. 1 hr)	Doppler coronagraph	C-1	10 x 8-sample scan over the spectral line
SUMER	differential-Doppler scan (10 positions; 3 hours total)
20:00	High Cadence (3 hours) (LASCO measurements repeat on 4-minute cadence.)	Wave / phase speed measurement and Doppler measurement	C-1	on- and off- line exposures (N 1/2 of field)
C-2	orange-filter deep exposures (just under 1/2 of field over N pole)
SUMER	differential-Doppler scan (10 positions; 3 hours total)
KPNO	Chromospheric magnetograms
23:00	PB images (est. 40 min)	UVCS support	C-2	PB sequence over North coronal hole
~23:30	C-1 Dopplergram (est. 1 hr)	Doppler coronagraph	C-1	10 x 8-sample scan over the spectral line
~00:30	Post-scan (est. 30 min)	context & ionization temp.	EIT	171Å, 195Å,and 284Å images
C-3	deep WL exposure

Day 3:

LASCO C-2 and C-3 will generate relatively high cadence sequences on this day. Because the time scales for outer coronal change are longer than for the inner corona, and because the likelihood of detecting the 12-minute waves in C-3 seems slight, the LASCO observation will be for 5 hours on a c. 20-minute cadence. UVCS will continue integrating its 2.5-R₀ spectrum. To continue supporting the doppler-dimming measurement in progress, C-2 will take occasional PB images. During the principal observation block, EIT will (for other reasons) be taking 171/195 doublets on a 20-minute cadence; these may also prove useful for this JOP.

SEQUENCE OF OBSERVATION:
Day 3 - 8-Sep-1997

Start time Observing phase Purpose
INST. Observation
15:00 Doppler Dimming (till 00:00) Speed measurements UVCS Spectra at 2.5 R₀.
15:00 Pre-scan (est. 30 min) context & ionization temp. EIT 171Å, 195Å,and 284Å images
C-3 deep WL exposure
~15:30 PB images (est. 40 min) UVCS support C-1 PB sequence over North coronal hole
16:00 Moderate Cadence (5 hours)
(LASCO & EIT measurements repeat on ~20 min. cadence) Wave / phase speed measurement in outer corona C-2 Orange-filter exposures
C-3 Clear esposures
EIT 171Å and 195Å images
21:00 PB images (est. 40 min) UVCS support C-2 PB sequence over North coronal hole
~21:30 Post-scan (est. 30 min) context & ionization temp. EIT 171Å, 195Å,and 284Å images
C-3 deep WL exposure