Pages 631-640

The IACG Campaign IV: Solar Sources of Heliospheric Structure Observed Out of the Ecliptic

A. B. Galvin 1 and H.S. Hudson 2

1 Department of Physics, University of Maryland, College Park, MD 20742, USA,
2 Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA


The Inter-Agency Consultative Group, representing the four space agencies with contributing missions (the European Space Agency, the Japanese Institute of Space and Astronautical Science, the Russian Space Agency, and the United States National Aeronautics and Space Administration), has initiated a data analysis campaign "Solar Sources of Heliospheric Structure Observed out of the Ecliptic" (IACG Campaign IV) based on the unique capabilities and opportunities afforded by the core campaign missions, Ulysses and Yohkoh. The campaign is strongly benefited by data sets from solar ground observatories and other spacecraft missions (SPARTAN 201, IMP, WIND, SoHO). The campaign themes emphasize the joint analysis of these remote-sensing and in-situ data sets for features such as coronal holes, coronal mass ejections, and other solar sources of heliospheric structure.


In the mid-1980's, Europe, Japan, the former USSR and Eastern European countries, and the United States of America formed the Inter-Agency Consultative Group for Space Science (IACG) to provide a global framework for enhancing scientific cooperation among their respective space agencies (the European Space Agency, ESA, the Japanese Institute of Space and Astronautical Science, ISAS, the Russian Space Agency, IKI, and the United States National Aeronautics and Space Administration, NASA). The initial task of the IACG was coordinating the international space armada to Comet Halley in 1986. The present IACG objectives include the coordination of their respective agencies' space missions to address fundamental issues in solar-terrestrial physics.

The core group of current IACG missions includes Ulysses, Geotail, Wind, Interball, Polar, FAST, Galileo, Yohkoh, Spartan 201, SoHO, Coronas, various solar and geomagnetic ground-based observatories, and incorporates other missions such as ICE, IMP-8, the Voyagers, Sampex, and Pioneer. The IACG instigates specific campaigns that focus on a well-defined scientific topic, using one or more core missions as a cornerstone. The general elements of an IACG campaign include: the determination of specific campaign objectives; the selection of key time periods that are designed to meet the objectives; the identification, coordination, and acquisition of required data sets from multi-spacecraft and ground-based observatories; followed by a collaborative data analysis and interpretation effort (including theory and modeling); and finally a wide dissemination of the scientific results. There are currently four IACG campaigns in the planning or implementation stage: Campaign I "Magnetotail Energy Flow and Non-Linear Dynamics", Campaign II "Boundaries in Collisionless Plasmas", Campaign III "Solar Events and Their Manifestation in Interplanetary Space and Geospace", and Campaign IV "Solar Sources of Heliospheric Structure Observed Out of the Ecliptic".

The Fourth IACG Campaign

A potential IACG campaign for "coordinated solar pole observations" was proposed and subsequently accepted at the 13th meeting of the IACG in October, 1993. The science goals of the campaign were decided at an IACG-sponsored solar-heliospheric workshop held in Easton, Maryland, January 27-29, 1994. Four major science topics were identified: (1) the large-scale heliosphere and its dependence on solar photospheric and coronal phenomena; (2) the topology of coronal streamers; (3) the three-dimensional shape of coronal hole boundaries; and (4) solar wind source and acceleration. Within these major topics, several subtopics emerged (Table 1). The Easton workshop identified the required spacecraft and ground observers and key observing intervals based on the Ulysses trajectory timetable (Table 2). Key intervals begin in June, 1994, and last through 1996. It is expected that the main campaign activities will end in late 1997, approximately one year after the last key observing interval.




The large-scale heliosphere and its dependence on solar photospheric and coronal phenomena.

The topology of coronal streamers.

The three-dimensional shape of coronal hole boundaries.

Solar wind source and acceleration.


The structure of the heliospheric magnetic field as a function of heliolatitude, and its relation to coronal structure

The determination of the strength and structure of the photospheric magnetic field in the polar regions of the sun

The structure, dynamics, and heliolatitude extent of transient phenomena from the sun and their manifestations in interplanetary space

The dependence of the solar wind and its properties on the topology and dynamics of polar coronal holes

Tests of solar wind acceleration models and their dependence on solar coronal field geometries

The heliolatitude dependence of the properties of the solar wind, in particular its relationship to open and closed coronal structures, and the origin of the slow solar wind

North-South asymmetries of the Sun at solar minimum, and a search for the axes of symmetry of the two hemispheres of the Sun

The extent and structure of the heliospheric current sheet and its relationship with the solar magnetic neutral line, and its relationship to the heliospheric extension of the coronal streamer belt


     Ulysses            Solar                In-situ 
     Time Period        Observatories        Spacecraft

South Polar Pass 
(June-Nov 1994)
Solar wind source and     Yohkoh            Ulysses 
 acceleration             Spartan           IMP-8 
Shape and structure in    Coronas           Voyagers (V1,V2)
 coronal hole             Ground-based      In October, 1994, Ulysses 
Large-scale structure     Solar Eclipse     crossed the longitudes of
                          (Nov 3)           Galileo and V1/V2

Perihelion Pass 
(January-June 1995)

Streamer belt/neutral     Yohkoh            Ulysses
 surface/slow wind        Coronas           IMP-8 
Solar wind source and     Ground-based      Wind
 acceleratio                                ICE-aligned period 
Shape and structure in                      Voyagers
 coronal hole 
Large-scale structure

North Polar Pass 
(July-Sept 1995)

Solar wind source and     Yohkoh            Ulysses
 acceleration             Spartan           Wind 
Shape and structure in    Coronas           IMP-8
 coronal hole             Ground-based      Voyagers 
Large-scale structure                       In August, 1995, Ulysses
                                            crossed the longitude of
                                            Pioneer 10

North Hemisphere Descent 
(Oct 1995 through 1996) 

Acceleration physics      Yohkoh           Ulysses
 imprint in solar         SoHO             Wind
 wind                     Coronas          Voyager 1 
Streamer belt/neutral     Ground-based     Pioneer 10
 surface/slow wind                         IMP-8 
Solar wind source and
Shape and structure in
 coronal hole 
Large-scale structure 

The fourth IACG campaign "Solar Sources of Heliospheric Structure Observed Out of the Ecliptic" is of special interest to the solar and heliospheric communities. This campaign has as its core campaign missions Ulysses and Yohkoh. The campaign takes advantage of the unique trajectory of the ESA/NASA Ulysses mission which allows, for the first time, the treatment of solar heliographic latitude as an independent variable for in situ measurements, while the Japanese Yohkoh satellite has remarkable soft X-ray observations with enough sampling resolution (temporal and angular) to define for the first time the structure of the entire visible corona.

The Core Missions: Ulysses and Yohkoh

Ulysses. The Ulysses spacecraft (Wenzel et al., 1992) was launched in October 1990 into an ecliptic transfer orbit to Jupiter. From the Jupiter Flyby (in February 1992), Ulysses received a gravitational assist placing the spacecraft into a high-inclination orbit directed south of the ecliptic plane. The Ulysses trajectory is shown in Figure 1. The out-of-the-ecliptic trajectory of the Ulysses spacecraft (shown against the solar disk in Figure 2) reached maximum solar latitudes of 80oS in September, 1994 (during the first south polar pass), and 80oN in July, 1995 (during the first north polar pass). A "fast latitude scan" occurred from mid-September 1994 through July 1995, when Ulysses covered the full range of latitudes (80oS to 80oN) in just 10 months, crossing the ecliptic plane with a perihelion of 1.3 AU (Smith and Marsden, 1995). Ulysses has now begun its second orbit (November 1995 - December 2001), which will include polar passes near solar maximum. The experiment complement of Ulysses includes magnetic field, solar wind plasma and composition, radio and plasma waves, suprathermal and energetic particles and composition, solar x-rays and gamma-rays, cosmic dust, and radio science investigations. (Detailed descriptions of the Ulysses investigations are available in Astron. and Astrophys. Suppl. Ser. , 92, 207-440, 1992).

ulysses orbit Fig. 1. The Ulysses trajectory, including the in-ecliptic phase (from launch in October 1990 to the Jupiter gravitational assist in February 1992) and the first southern and northern polar passes. Ulysses has now begun its second orbit.

ulysses orbit projected on solar disk Fig. 2. The Ulysses trajectory projected onto the solar disk (soft X-ray image from May 8, 1992, provided by Yohkoh SXT). The perspective is from the earth, hence the annual motion wraps the Ulysses track around the front (earth facing) hemisphere, shown as solid lines, and the back, invisible hemisphere, shown by dotted lines. Tic marks are one month apart, starting with Delta at 1/1/1993, and ending with diamond at 1/1/1997.

Yohkoh. The solar observatory Yohkoh ("Sunbeam" in Japanese) was launched in late August, 1991, into an elliptical, low-altitude (520 by 795 km) Earth orbit, with an orbital period of approximately 96 minutes. The spacecraft spends 65 to 75 minutes per orbit in daylight. The scientific payload is designed to make high-energy observations of the sun: soft x-rays (1-2 keV), hard x-rays (15-100 keV), line complexes for ions of iron, calcium and sulfur (FeXXVI, FeXXV, CaXIX, and SXV), and wideband x-ray and gamma-ray energy release measurements (Acton et al., 1992). (The Yohkoh instruments are further described in Solar Physics, Vol. 136, No. 1, 1991.)

From September, 1991, and hence for a portion of the in-ecliptic phase and for the entire out-of-ecliptic phase of the Ulysses mission, remarkable soft X-ray images from Yohkoh are available. Yohkoh images have enough sampling (temporal and angular) to define for the first time the structure of the entire visible corona, rather than just that part above the limb. The Yohkoh SXT data (Figure 3) dramatically show the change in solar activity during the first Ulysses orbit.

Solar Soft Xrays during Ulysses first orbit Fig. 3. Two images from the Yohkoh soft X-ray telescope (SXT) during the course of the Ulysses first orbit: left, solar maximum; right, approaching solar minimum. Not only do the active regions decrease in number and brightness, but the diffuse corona has become fainter. The plot at the bottom shows the total signal summed over the entire SXT CCD as a function of time.

Other Missions

Accomplishing the full scientific objectives of the IACG Campaign IV requires data sets beyond those available through Ulysses and Yohkoh. With, for example, the in-ecliptic WIND (launched November, 1994), Interplanetary Monitoring Platform IMP-8 (launched October, 1973), and Geotail (launched July, 1992) joining Ulysses in mapping out the latitudinal aspects of heliospheric structures, heliographic latitude can now be treated as an independent variable for in situ particle and fields measurements.

As of February 1996, the Solar Heliospheric Observatory (SoHO) reached the Lagrangian L1 point and is fully operational, thereby supplementing and extending the x-ray observations available from Yohkoh to include the SoHO observations in the white light, ultraviolet, and extreme ultraviolet. Simultaneous observations from solar ground observatories and new material from the shuttle experiment SPARTAN 201 (whose flights coincided with the Ulysses highest latitudes) further contribute to the campaign objectives.

Information on these missions and access to public data bases are frequently available via the internet. Some selected world wide web sites are given in Table 3.

Table 3. Selected World Wide Web Pages

Topic                         Universal Resource Location (URL)
IACG Homepage       


Ulysses Mission Homepages
Yohkoh SXT Homepage 

SPARTAN 201         
National Solar Observatory 
 at Kitt Peak       

SoHO Homepage       




As indicated in Table 2, the key observing intervals for Campaign IV have been defined based on the unique aspects of the Ulysses trajectory. These intervals cover the South Polar Pass (June - November, 1994), the Perihelion Pass (January - June 1995), the North Polar Pass (July - September, 1995), and the North Hemisphere Descent (October 1995 through 1996).

Ulysses South and North Polar Passes

The Ulysses polar passes, defined as when the spacecraft was above 70o in solar latitude, occurred from June 26, 1994, to November 6, 1994, for the south polar pass, and from June 20, 1995, to September 30, 1995, for the north polar pass. The maximum southern and northern heliographic latitudes (80.2o) were reached on September 13, 1994, and July 31, 1995, respectively. During these time intervals, the Ulysses spacecraft was fully immersed in the high speed solar wind stream emanating from the corresponding polar coronal hole. Ulysses observations for the south polar pass have been published in special issues of Space Science Reviews (vol. 72, 1-494, 1995) and Science (vol. 268, 1005-1036, 1995).

The SPARTAN Mission. In coordination with the Ulysses maximum latitude passages, the SPARTAN 201-2 was flown in September, 1994, aboard STS-64, and the SPARTAN 201-3 was flown in September, 1995, aboard STS-69 (Kohl et al., 1995). Solar activity was decreased during the northern polar pass (Figure 4). The SPARTAN scientific payload consisted of an ultraviolet coronal spectrometer and a white light coronagraph. The measurements of ultraviolet emissions (neutral hydrogen Lyman-alpha line profiles and oxygen VI line intensities) in the corona are used to determine velocities of the plasma in the solar wind source region of the corona, and the temperature and density distribution of the coronal protons. The visible light measurements determine the coronal electron density distribution. Modeling efforts are underway, combining the SPARTAN coronal measurements with simultaneous Ulysses in-situ polar solar wind plasma measurements (temperatures, velocities, densities, and charge states), to determine various physical characteristics of the solar wind acceleration region in the corona, including proton and electron temperatures and outflow velocities (e.g., Kohl et al., 1996), and the coronal electron temperature profile (e.g., Ko et al., 1996).

Solar Soft Xrays during Spartan 201 Fig. 4. Yohkoh images of the solar corona near the time of the shuttle deployment of the SPARTAN 201 subsatellite: left: Yohkoh SXT soft X-ray image at 12:17 UT 23 September, 1994; right: SXT image at 13:44 UT 2 September, 1995. Note the decreased brightness of the later image due to the decrease of solar activity. The SPARTAN 201 made advanced coronagraphic observations for correlative studies with Ulysses.

Ulysses Perihelion Pass

The Ulysses Perihelion Pass, part of the "Fast Latitude Scan", covers January, 1995, through June, 1995. Ulysses traveled from approximately 40oS to 70oN, crossing perihelion (1.34 AU) on March 13, 1995. (The ecliptic plane was crossed March 12, 1995.) Ulysses results have been reported in a special issue of Geophysical Research Letters (vol. 22, 3297-3432, 1995). During this passage, Ulysses exited the south polar wind at about 22oS, encountered one interplanetary coronal mass ejection, entered the mid-latitude region in which several crossings of the heliospheric current sheet were observed at various heliographic longitudes and latitudes, and then at 21oN entered the fast solar wind emanating from the north polar hole (Phillips et al., 1995).

The Ulysses Perihelion Pass was selected for a collaborative study by the International Space Science Institute (ISSI) / IACG Campaign IV Working Group "Open and Closed Fields and the Solar Wind", held in Bern, Switzerland, July 8-12, 1996. Of particular interest are Carrington rotations 1892, 1893, and 1894. Ulysses and near-earth, in-ecliptic satellites such as WIND and IMP-8 simultaneously observe some of the same structures (particularly during CR 1893, which encompasses the Ulysses ecliptic plane crossing). At this time, Ulysses was in the opposing solar hemisphere from the earth (see Figure 2). This provides an opportunity for combining Ulysses observations with those of earth-orbiting satellites to track the same heliospheric structures approximately 2 weeks apart (i.e., half a Carrington rotation). This observing interval is especially interesting for testing heliospheric current sheet footprint models by tracing multispacecraft solar wind plasma observations to the source region on the coronal disk. Ground-based 10830 Å HeI images, solar magnetograms, and Yohkoh spacecraft observations are available for solar disk coronal hole estimates.

Ulysses North Hemisphere Descent

The Ulysses North Hemisphere Descent covers the end of the North Polar Pass (October 1, 1995) until the end of 1996, as the spacecraft descends from 70oN to 20oN. Although its primary mission phase is over, the Yohkoh data continue to be excellent. Ulysses is currently into its second orbit and is also in excellent health. As of September 1996, Ulysses was still observing high speed solar wind from the north polar hole.

The North Hemisphere Descent interval includes data from the SoHO spacecraft (launched December, 1995). The SoHO spacecraft, located at L1 since February, 1996, has nearly 100% data coverage. Part of the SoHO science planning is implemented through the use of Joint Observing Programs (JOPs), in which various SoHO experiments coordinate their individual observation plans for the study of specific solar phenomena. Other missions and ground-based observatories are frequently invited to participate. The SoHO JOP-44, "Large Scale Structure of the Solar Minimum Corona", is designed to test solar magnetostatic and solar wind acceleration models for the solar minimum corona (specifically for long-lived structures such as helmet streamers and coronal holes) by incorporating boundary conditions imposed from transition region and coronal observations (e.g., SoHO, Yohkoh, Ground-based) and in-situ solar wind and magnetic field measurements (e.g., WIND, Ulysses, IMP-8). The first run of this JOP covered August 8 - September 10, 1996, and was colloquially known as "Whole Sun Month".

This time interval is exceptionally interesting in that it includes the appearance of a trans-equatorial coronal hole, extending nearly up to the north polar hole (Figure 5). The active region, NOAA 7986, which lies adjacent to the coronal hole, was on its second rotation at the time of these observations. Earlier it had made the first X-class flare of the year - this type of flare is rare during solar minimum. In the left of the soft x-ray image in Figure 5, one can see large cusped loops. This region was characterized by large coronal structures and in fact was accompanied by a general brightening of its entire hemisphere of the corona, as observed by Yohkoh.

Whole Sun Month 

Fig. 5. Yohkoh soft X-ray image and Kitt Peak NSO coronal hole map for August 27, 1996. Central meridian passage of a trans-equatorial coronal hole is observed as a high speed stream in interplanetary space at L1 a few days later (August 29) by the CELIAS/MTOF solar wind sensor on SoHO. This time interval coincided with the SoHO "Whole Sun Month" observing campaign.

Near-earth spacecraft detected high-speed solar wind streams (about 600 kilometers per second) a few days after the central meridian passage of the coronal hole. Ulysses during this period is mainly immersed in the north polar hole solar wind, but there was an "excursion" at the end of August to early September, when the solar wind speeds dropped to about 500 kps, before returning to the normal polar hole values of about 700 kps.

Data sets for the Whole Sun Month campaign are currently undergoing evaluation. Information on this and other SoHO JOPs can be obtained on the internet:

Other Coordinated Data Studies

Coordinated observations of transient-related phenomena is another Campaign IV objective, in particular in establishing their structure, dynamics, and heliolatitude extent, both on the sun and in their manifestation in interplanetary space. In this regard, there is an overlap in objectives with the IACG Campaign III, which is concerned with the origin of Coronal Mass Ejections (CMEs) on the sun, and the CME propagation through interplanetary space and effects in geospace. Therefore, joint projects are anticipated in the future, as SoHO (the cornerstone mission for Campaign III) acquires its CME event database.

Other collaborative efforts involve the SOLTIP community (Solar connection to Transient Interplanetary Phenomena). One of the SOLTIP event intervals include the polar crown arcade event of April 14, 1994, which was observed by Yohkoh SXT, and was associated with a subsequent restructuring of the polar coronal hole boundary. This event was seen as a high latitude Interplanetary CME by Ulysses.

The latitudinal extent of energetic solar particle events are being studied with multi-spacecraft observations by Ulysses, IMP-8, Voyagers, and/or WIND (e.g., Roelof et al., 1995). Their solar origins are determined with data from the Nan¨ay radioheliograph (e.g., Pick et al., 1995).


In order to broaden the participatory base in this time of limited resources, one of the major tools for disseminating information about the IACG Campaign IV is through the World Wide Web (WWW). The homepage address is given in Table 3.

Initial workshops have taken place, providing some structure for defining the campaign goals, the selection of key event periods, and initialization of collaborative efforts. These include the original IACG solar-heliospheric workshop (Easton, MD, January, 1994), the Yohkoh Coordinated Data Analysis Workshop (Honolulu, Hawaii, October, 1994), and the ISSI/IACG-C4 Working Group "Open and Closed Fields and the Solar Wind" (Bern, Switzerland, July, 1996). Future workshops and symposia are being organized.

Some Campaign IV results have already been presented at professional meetings (e.g., special session American Geophysical Union Fall Meeting, San Francisco, CA, December, 1995; Chapman Conference on Coronal Mass Ejections: Causes and Consequences, Bozeman, MT, August, 1996). A joint IACG Campaign III and Campaign IV symposium is being planned for the fall of 1997.


The campaign heavily relies on the good efforts of the solar and space science communities. We would especially like to thank the project scientists of the various IACG-related missions, including (but not limited to) Ulysses, Yohkoh, WIND, IMP-8, and SoHO. We appreciate the efforts of R. von Steiger and K. Harvey, who co-chaired with us the ISSI/IACG-C4 Working Group, and we thank the ISSI for hosting the workshop. We thank S. Gibson and D. Biesecker as SoHO JOP-44 Leaders. We would like to thank the IACG WG-1 for their support.


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Ko, Y-K., L.A. Fisk, J. Geiss, G. Gloeckler, and M. Guhathakurta, An Empirical Study of the Electron Temperature and Heavy Ion Velocities in the South Polar Coronal Hole, submitted to Solar Physics (1996).

Kohl, J.L., L.D. Gardner, L. Strachan, R. Fisher, and M. Guhathakurta, Spartan 201 Coronal Spectroscopy During the Polar Passes of Ulysses, Space Science Reviews, 72, 29 (1995).

Kohl, J.L., L.D. Gardner, L. Strachan, C.M.S. Cohen, A.B. Galvin, G. Gloeckler, M. Guhathakurta, R.R. Fisher, Y.-K. Ko, J. Geiss, and R. von Steiger, Proton Temperatures, Electron Temperatures and Outflows in the Extended Solar Corona, in Ninth Cambridge Workshop on Cool Stars, Stellar Systems and the Sun, eds. R. Pallavicini and A.K. Dupree, ASP Conference Series, in press (1996).

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Pick, M., L.J. Lanzerotti, A. Buttighoffer, S. Hoang, and R.J. Forsyth, Detection of a Solar Particle Event at an Heliolatitude of 73.8oS, Geophys. Res. Lett., 22, 3377 (1995).

Roelof, E.C., G.M. Simnett, and T.P. Armstrong, IMF Connection of Solar Energetic Protons Observed at Ulysses Via Recurrent Mid-Latitude Solar Wind Rarefaction Regions, Space Sci. Rev., 72, 309 (1995).

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