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The main scientific objective of WG3 is to study the origins and transport of solar emissions through the heliosphere. The emphasis of the group is on observational and theoretical studies of dynamic activity in the mid to outer corona and its influence on the solar wind. Thus, our studies differ from those of Working Group 2 which emphasize observations and modeling of dynamic magnetic field activity in the low solar atmosphere and inner corona. WG3 has three subareas of study: 1) Dynamic solar emissions in the increasing and maximum portion of solar cycle 23; 2) Propagation in the interplanetary medium (coronal mass ejections, interplanetary shocks, and energetic particles); and 3) Interaction of coronal mass ejections and solar wind streams with Earth's magnetosphere.
Solar activity provides the driving force for disturbances in the interplanetary medium and geomagnetic storms. The Sun has been well observed during the past four decades with data obtained by many spacecraft, including Skylab, SMM, Yohkoh and SOHO, and a host of ground-based observatories. In addition the interplanetary medium has been probed with in-situ measurements by many spacecraft: inside of 1 AU by HELIOS and in the outer and high-latitude heliosphere by Ulysses and the Pioneer and Voyager spacecraft. The most recent near-Earth missions collecting solar wind data are SOHO, WIND and ACE, and later in solar cycle 23 several new missions, SMEI, HESSI, Solar-B and STEREO, are planned. Presently SOHO (since early 1996), Yohkoh (since 1991) and TRACE (1998) are providing high resolution images of the Sun at a variety of wavelengths. Fortunately, the SOHO observations have been recovered after a 5-month hiatus and should continue to be available during our study period. We propose to use these observations of solar and interplanetary phenomena, and geomagnetic activity where appropriate, to address specific questions of importance to studies of solar and interplanetary disturbances and their coupling to the magnetosphere, including recent new initiatives on "Space Weather".
Past observations such as from ISEE-3, HELIOS, IPS (Interplanetary Scintillations) and at long radio wavelengths, have shown that CMEs can inject considerable amounts of solar plasma and magnetic field into the heliosphere in correspondence with the sunspot cycle. Although the range of CME speeds is large, even the slowest CMEs (slower than the in-ecliptic solar wind) are never observed to fall back to the Sun. This suggests that their internal magnetic fields control the dynamical evolution of CMEs near the Sun. The magnetic and plasma structures associated with CMEs significantly perturb the solar wind. Faster CMEs drive transient interplanetary shocks ahead of them, and these shocks can accelerate particles to cause at least lower energy SEP events. The CME-related disturbances are the most likely cause of sporadic geomagnetic storms and, therefore, a key component of space weather. Observations of Sun-centered coronal activity and "halo" CMEs as observed by SOHO provide an important new way of evaluating the origin and propagation of material toward Earth.
CMEs are linked to other solar activity including eruptive filaments, large flares, long-duration X-ray events, transient coronal holes and EUV waves. There is a broad spectrum of solar mass ejecta, ranging from small transient outflows primarily at low latitudes to large, extended events with widths of over 90o. Most large CMEs arise from closed coronal magnetic fields. But we know little about how the global magnetic field evolves and leads to the opening of these structures and the loss of magnetized material to space. The coronal observations from TRACE and SOHO (LASCO, EIT, CDS, UVCS) are providing new data on CME source regions and streamers and how this material is accelerated along with the solar wind.
An important class of mass ejecta are erupting filaments, called prominences when seen above the limb (e.g., Webb et al., 1998). Since filament plasma is embedded in helical, horizontal magnetic fields, the close association of CMEs with filament eruptions and shearing fields near the surface has led to the modeling of CMEs as flux ropes. Although CMEs clearly involve the ejection of magnetic fields from the Sun, our understanding of the strength and topology of these structures is poor (Burlaga, 1991). Partly this is because we view CME structures in white light as 2-D projections against the sky of 3-D structures such as arcades, flux ropes or shells. A prominence and its associated helmet streamer can be modeled as a dual flux system, one part of which is a flux rope and its surrounding coronal cavity which may help to drive the eruption once the streamer is disrupted. It appears that the prominence itself is only a small part of this system, lagging the bulk of the CME/flux rope in the solar wind. This seems to be supported by recent WIND observations of dense plugs of material at the trailing edges of magnetic clouds (e.g., Burlaga et al., 1998). Recently, the twist (helicity) of filaments has been found to have both hemispherical and solar cycle dependencies and to be correlated with the helicity of magnetic clouds at 1 AU (e.g., Bothmer and Rust, 1997). Our working group will study the relationship of filament eruptions and CMEs to flux ropes and magnetic clouds, and to the shedding of the Sun's flux and helicity built up over the cycle. In addition, we need to study patterns in the long-term evolution of magnetic polarity boundaries and of filament eruptions, and their roles as sites of CMEs and other eruptive activity.
The effects and signatures of individual ejections on the interplanetary medium remain uncertain. Major CMEs, especially if accompanied by large flares, can produce large interplanetary disturbances. However, since the typical CME is often not associated with a flare or interplanetary shock, its interplanetary characteristics are likely more subtle than those associated with large flares. At present we know that these signatures consist of abnormal flows relative to the average solar wind, with about half of all transient interplanetary disturbances accompanied by shocks, magnetic clouds or clear "piston"-like plasma enhancements. We need to study and model the magnetic topology of these structures using both the interplanetary magnetic field and energetic particles as "tracers". During solar cycle 23 in-situ measurements from Ulysses, WIND, SOHO and other deep space missions should greatly improve our understanding of these transient flows and how they vary over the cycle. Perspective views from one or more STEREO-type missions would provide valuable complementary data on the 3-D structure of interplanetary CMEs and shocks.
We also need to understand the role of reconnection in coronal and CME evolution. This is important in the heliospheric context since the injection of magnetic flux should increase the interplanetary flux indefinitely, which is clearly not observed. Before SOHO, evidence of the disconnection of solar fields had been observed in only a small fraction of all CMEs, but in 1997 LASCO detected concave-outward structures in nearly half of all CMEs. Intermingling of both open and closed fields may also be common in interplanetary ejecta. The SOHO observations of CMEs within 30 Rs combined with in-situ interplanetary magnetic field and plasma measurements will be ideal for studies of magnetic reconnection and topologies during solar cycle 23.
WG3 will also study the relationships among interplanetary shocks, CMEs, solar flares and solar energetic particles utilizing these data sets. Coronal shocks must propagate through coronal streamers and neutral sheets which extend into the interplanetary medium. The streamers extend from the corona into the interplanetary medium and become the HCS. Since the trajectories of CMEs and the production and propagation of low-velocity SEPs may depend significantly on the presence of a neutral sheet in space, our group will also investigate the effects of neutral sheets on CME, shock and SEP propagation during solar cycle 23. We also need to understand how and where solar energetic particles are accelerated, e.g., the fraction produced in the corona vs the interplanetary medium, and how they are transported in the low corona. In the outer heliosphere corotating interaction regions play an important role in producing charged particles.
Although our studies will involve basic scientific research, there is no doubt that increased knowledge of the solar-interplanetary environment, particularly during the rising and maximum portions of the solar cycle, will enhance our ability to forecast the occurrence, magnitude and consequences of various perturbations at the Earth. Because of its involvement with CMEs and the solar wind, the studies of WG3 are a key part of the ISCS program that will contribute to the national and international Space Weather programs.
We know that geomagnetic storms have two main causes: transient solar eruptions (mainly CMEs) from streamers that form the base of the heliospheric current sheet, and recurrent interaction regions and high- speed wind streams which arise from solar coronal streamers and holes, respectively. However, we do not understand the interplanetary conditions necessary to produce geomagnetic storms and other geoactivity. Good associations have been found between geomagnetic storms and other interplanetary signatures of ejecta, especially magnetic clouds. Although magnetic clouds which have strong southward fields are clearly related to the coupling of these disturbances to the magnetosphere, we do not have a detailed physical understanding of this process. WG3 will utilize the many space observations to be obtained during solar cycle 23 to investigate the characteristics of geoeffective interplanetary phenomena and the coupling process and how they vary over the cycle.
These various scientific issues can be summarized in a series of important questions that will be addressed by Working Group 3. These questions are not meant to be exhaustive, but only to provide a flavor of the kinds of studies that are being undertaken.