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WHAT DO WE REALLY KNOW ABOUT SOLAR WIND COUPLING?

J.G. Luhmann

Space Sciences Laboratory University of California Berkeley, CA 94720 USA, E-mail:jgluhman@sunspot.ssl.berkeley.edu

ABSTRACT

During the ISTP era we will have unprecedented opportunities to study the cause-and-effect relationships in the solar wind/magnetosphere/upper atmosphere system. Only by recognizing these relationships in both our classification schemes and research can we hope to unravel the complex subject of solar wind coupling.

INTRODUCTION

Three decades of space measurements and complementary ground-based solar and magnetometer observations have provided insights on solar wind coupling that are actually quite impressive in scope and depth. Now, with the advent of the ISTP era in its (almost) full glory, it is important to take stock of the picture that has emerged if we are to move forward. Indeed, the literature is full of so many highly specific and detailed studies that often the paradigm-level implications are lost. It is the paradigm-level understanding, on the other hand, that should be the primary target of the ISTP.

WHAT PRE-ISTP OBSERVATIONS HAVE REVEALED ABOUT THE SOLAR WIND CONTROL OF THE MAGNETOSPHERE

Although many interpretations of solar-terrestrial data have been published, only a handful of key results have altered our most basic ideas about how the coupled system works. These include:

1) The realization that the interplanetary magnetic field (IMF) orientation with respect to Earth's dipole modulates geomagnetic activity due to the interconnection of the two fields (Dungey, 1961), and that the reconnection geometry and location influences everything from polar cap convection patterns (e.g. Crooker, 1979) to seasonal preferences (Russell and McPherron, 1973a).

2) The recognition of two basic types of magnetospheric disturbance- one a global phenomenon with clear connections to solar and interplanetary disturbances, called "storms" (e.g. Gonzales et al., 1994 and references therein), and the more localized auroral zone disturbances called "substorms" (Akasofu, 1968; Arnoldy, 1971; Aubry and McPherron, 1971; Russell and McPherron, 1973b).

3) The establishment of fast coronal mass ejections (CMEs) as the cause of major geomagnetic storms (Tsurutani et al., 1988; Gosling et al., 1991), and the role of coronal holes and solar wind stream structure in producing recurrence (e.g. Sheeley et al., 1976; Crooker et al., 1993).

In addition, we have come to regard certain "patterns" of magnetospheric behavior as highly predictable responses to particular types of interplanetary conditions. For example, we know that interplanetary shocks cause Sudden Impulses and sometimes Storm Sudden Commencements, transient radiation belts, and plasmoid-like compression structures that travel down the tail. We know that the passage of CMEs brings on the low-latitude magnetic perturbation associated with storm-time ring current injection and in many cases a (possibly associated) plasmoid ejection down the tail. We know that "substorms" seem to have a variety of triggers including Southward or Northward turnings of the IMF and solar wind pressure pulses, but that they also occur under steady interplanetary conditions with sustained Southward interplanetary fields. We know that like storm-time events, substorm events such as auroral surges and electrojet current intensifications sometimes follow a sequence. On the other hand, the sequence of substorm events exhibits more variations than the sequence of major storm events. Finally, we know that substorms can be embedded within storms. These patterns clearly give us clues regarding solar wind coupling that we have yet to fully decipher.

WHAT MODELS HAVE REVEALED ABOUT THE SOLAR WIND CONTROL OF THE MAGNETOSPHERE

Probably the most valuable contemporary tools for studying the paradigms of solar wind coupling are the global MHD models of the solar wind-magnetosphere interaction (e.g. Usadi et al., 1993; Ogino et al., 1994; Fedder et al., 1995; Raeder et al., 1995). While arguments can be made about the shortcomings of the approximations used for the ionosphere description, the role of numerical diffusion that is inherent in numerical solutions of the MHD equations, and even the applicability of the latter to the physics of the coupling, no other technique is capable of giving us such realistic geometries and boundary conditions for the fully coupled, time-dependent system. Indeed, rather than being critical of all of the faults of these models, we should be testing what they are telling us against the observations, and trying to understand whether there is significant "missing" physics that can be added in some way. For without these models, we will never be able to unravel the complexity of what we observe.

For example, it is not always appreciated that the global MHD models have already provided a much improved picture of the "average" magnetosphere. While the Dungey (1961) conceptual pictures for perfectly Northward and Southward IMF were instrumental in our thinking about the cause of the apparent "half wave rectifier" magnetospheric response to the North-South IMF component, the coupled system actually has the average appearance illustrated in Figure 1 shows a projection of magnetic field lines as viewed from the Sun from the Fedder et al. (1995) model in which the interplanetary field has a typical orientation close to the magnetic equatorial plane. These results tell us that the tail lobe fields are normally sheared and twisted. The reconnected fields generally map asymmetrically into the polar caps with attendant asymmetries at the ionospheric boundary. Field merging for the typical IMF orientations occurs both in the subsolar region when there is an antiparallel field component in the magnetosheath, and adjacent to the cusps where the external and internal fields are more nearly antiparallel. While this configuration changes with the addition of Northward or Southward IMF components in ways suggestive of Dungey's original sketches of the interconnection between IMF and magnetosphere fields, the asymmetries produced by the ecliptic or "By" components prevail except on rare occasions. Evidence of the configuration in Figure 1 abounds in the literature and in the observations (e.g. Cowley, 1981; Kaymaz et al., 1992, 1994 and references therein). A recent observational study of magnetopause merging signatures (Scurry et al., 1994) further supports the gross realism of the model description, even without the detailed specification of the microphysical processes responsible for dayside reconnection.

Figure 1. View from the Sun of some field lines from the 3-D MHD model of Fedder et al. (1995). The interplanetary field is in the plane of the magnetic equator. Shown are some lines that have reconnected near the cusps, some closed dipolar field lines, and some open, draped magnetosheath field lines. The tick marks on the axes are spaced at 10 Earth radius intervals. (Figure courtesy J. Fedder).

The 3-D models have already been used to study magnetospheric transients. In particular, rotations of the North-South IMF, thought to initiate the substorm process, were tried (e.g. Usadi et al., 1993; Ogino et al., 1994; Slinker et al., 1995), and indeed produced the tail plasmoid ejection thought to occur in substorms. In the past year, simulated interplanetary shocks were propagated through one model both to study the effects on the magnetosphere, and to provide a field description for the kinetic simulation of radiation belt enhancements seen following sudden commencements. The latter produced an impressive comparison with spacecraft observations of energetic particles (Li et al., 1995). In another exercise, (Chen et al., 1995), a simulated interplanetary CME flux rope or "cloud" interaction was examined in order to compare with storm-time magnetic perturbations measured at low latitude and in the auroral zone (near the inner boundary of the model). In this case, the enhanced, rotating IMF associated with the flux rope produced "ground" signatures resembling storm-time AE and Dst (the latter minus the ring current contribution, of course). Thus both "quiet" and "disturbed" magnetospheres are revealing their secrets due to the synergistic use of the coupled models and the observations. It seems that while understanding the microscopic aspects of ionosphere coupling and field diffusion in the dayside and tail are ultimately necessary, important progress can still be made on the "macroscopic" front by using the 3-D MHD models with their numerical and parameterized versions of these processes.   

WHAT WE REALLY DQNT KNOW: TASKS FOR THE ISTP ERA

In spite of the accomplishments described above, key elements of the coupling picture remain sources of constant debate and confusion. Particular hurdles that the ISTP, with its complementary spacecraft, ground-based observational, and modeling components can move us beyond are:

1) First and foremost, the redefinition and reclassification of substorms. This involves separation in observational and modeling analyses of responses to interplanetary changes (e.g. IMF rotations or pressure "pulses") from "spontaneous" activity in the presence of steady interplanetary conditions. At a minimum, this will tell us how often and under what circumstances a substorm is merely the magnetosphere's transient response to changing external boundary conditions (necessitating reconfiguration of the coupled system), as opposed to some internal "instability" in the coupled system.

2) The clarification of the conditions under which tail plasmoid ejection occurs, and its connections to auroral features and both auroral zone current and ring current injection. This includes understanding plasmoid properties for typical IMFs (large By), as well as the difference between storm-time and substorm plasmoids (if any).

3) The clarification of the role of the ionosphere in the response of the magnetosphere to particular interplanetary conditions. This includes the role of conductivity (e.g. in balancing dayside flux erosion and tail return-flux for Southward IMF), as well as the role of the injection of ionospheric particles and of parallel electric fields (both absent in global MHD models). The importance of feedback effects of disturbance-related conductivity enhancements on the subsequent development of a storm or substorm should also be resolved.

4) The clarification of the role of the ring current (not in the existing global models) in storm and substorm development (e.g. by modification of the dayside erosion or by its contribution to energy dissipation).

While "CDAW-style" case studies and pre-arranged "campaigns" have their merit, they are not generally the best way to approach such specific questions of a more general nature. Instead, retrospective selections of data sets based mainly on interplanetary conditions are called for. To complement these, some basic model runs that experiment with specific interplanetary features can provide a "catalogue" of idealized global responses (e.g., to an East-West IMF rotation characteristic of a sector boundary, to a pressure pulse, to steady southward IMF with different ionospheric conductivities, to an interplanetary shock, to a CME flux rope with and without a preceding shock) for reference.

The ISTP has the potential to provide us with answers that improve our most basic knowledge of solar wind coupling. However, it will also provide us with a large amount of new data that while invaluable, can be distracting. If we want to influence our paradigm-level thinking and our textbook pictures, it is up to us to pose the most central questions first (whether or not the list above) and to focus on answering them.

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