New Findings Challenge Beliefs about Solar-Terrestrial Physics

Solar flares are intense, short-lived brightenings that occur near sunspots on the Sun's surface. Large geomagnetic storms, intense auroral displays, large energetic particle events in interplanetary space, and major shock wave disturbances in the solar wind often occur in close association with large solar flares. Over the years, the common association of these events in near-Earth space with solar flares led to a paradigm of cause and effect in which large solar flares came to be understood as the fundamental cause of these disturbances.

Certain aspects of this paradigm were developed in the early 1930s [e.g., Hale, 1931], and by the early 1960s it had become part of the underlying dogma central to the discipline of solar-terrestrial physics. This paradigm still dominates the popular perception of the relationship between solar activity and interplanetary and geomagnetic events and continues to provide much of the pragmatic rationale for the study of the solar flare phenomenon.

We now know, however, that the above paradigm is wrong [e.g., Kahler, 1992; Gosling, 1993] and that most major, transient disturbances in near-Earth space are produced by solar events known as coronal mass ejections (CMEs), which involve the ejection of very large quantities of solar material (10⁺¹⁵-10⁺¹⁶ g, equivalent to the mass of about 100,000 aircraft carriers) into interplanetary space. In contrast to solar flares, which are relatively easy to observe and have been studied for more than a century, CMEs are not detected easily. In fact, CMEs were not observed directly until special telescopes known as coronagraphs were first flown in space in the early 1970s. Figure 1 shows two snapshots of a CME observed with the coronagraph flown on Skylab.

Fig. 1. Two snapshots of a CME observed above the west limb of the Sun with the white light coronagraph on Skylab on August 10, 1973. The field of view of the photographs is 6 solar diameters, and the snapshots are separated in time by 24 minutes. As is common in many of these events, this CME was not associated with a solar flare.

Many CMEs, including some of the more spectacular ones, erupt from regions well away from sunspots and any apparent flaring activity. Our present understanding of CMEs is that they are not produced by flares even though CMEs and flares sometimes occur in close temporal association with one another. It is likely that both CMEs and flares arise from instabilities connected with the evolution of the magnetic field in the solar atmosphere. CMEs probably result more from changes in the large-scale magnetic field that permeates the solar corona, and flares probably result more from changes in the stronger but smaller-scale fields associated with sunspot regions lower in the solar atmosphere.

CMEs move outward from the Sun into interplanetary space with speeds as low as 50 km/s and as high as 1200 km/s or greater. The slower CMEs do not produce significant disturbances in the solar wind, nor do they seriously perturb the Earth's magnetosphere or ionosphere. On the other hand, the faster CMEs, which account for a relatively small fraction of all events, usually produce very large disturbances in the solar wind. The faster CMEs typically contain shocks on their leading edges and strong magnetic fields in extended regions following the shocks. These strong fields are primarily a result of compression that occurs as a fast CME rams into slower solar wind ahead. When the Earth's magnetosphere intercepts one of these CME-driven disturbances, large geomagnetic storms and spectacular auroral displays often result, particularly when the magnetic field carried by the solar wind is directed southward.

The strong interplanetary shocks driven by the faster CMEs are also effective in accelerating solar wind ions they intercept to energies in excess of several millions of electron volts. Only a small fraction of the solar wind ions intercepted are accelerated to these energies, but the flux of these newly accelerated ions is quite large relative to the background flux associated with galactic cosmic rays. Recent work [e.g., Reames, 1992] indicates that almost all major energetic particle events observed in the vicinity of the Earth are produced by acceleration at shocks in interplanetary space that are driven by fast CMEs rather than by acceleration at flare sites on the Sun.

It is now clear that most major transient interplanetary and geomagnetic events are produced by disturbances associated with fast CMEs. It is also clear that solar flares play no fundamental role in producing CMEs. Nevertheless, solar flares continue to be interesting events to study since a number of complex, energetic, and poorly understood processes, including particle acceleration, occur during flares.

References

Reames, D. V., Trapping and escape of the high energy particles responsible for major proton events, in Eruptive Solar Flares, Lecture Notes in Physics 399, edited by Z. Svestka, B. V. Jackson, and M. E. Machado, pp. 180-185, Springer-Verlag, New York, 1992. As is common in many of these events, this CME was not associated with a solar flare.

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