Coronal Mass Ejection from the Sun
Peter O. Taylor
The X-rays produced in many solar flares and spewed throughout interplanetary space are known to spawn short-lived effects at the Earth, and they raise legitimate concerns for anyone who ventures beyond the protection of the atmosphere. For many years, the energetic processes and effects of flares have been focal points of interest among solar and atmospheric physicists and others who study the solar-terrestrial relationship. Recently however, a group of space and solar physicists led by Dr. John T. Gosling, a scientist at Los Alamos National Laboratory, have proposed a scenario which has -- to say the least -- stirred attention among those who examine the source and effect of solar activity.
At times of high activity, great storms plague the Earth's magnetic field and dazzling light-shows (aurorae) can be viewed from much lower latitudes than normally possible. Two important events during the mid-nineteenth century significantly influenced scientific thinking concerning the source of such storms: First, the number of magnetic disturbances was linked to the newly discovered sunspot cycle, and second, the English astronomer Richard Carrington observed the first solar flare and suggested that it might have some association with a spectacular geomagnetic storm which followed.
Shortly thereafter, evidence began to accumulate which seemed to confirm flares as the source of terrestrial magnetic disturbances. Since then, the two phenomena have been reasonably well correlated, and the relationship has been generally accepted for more than half a century. In spite of such circumstantial evidence however, the flare/magnetic storm connection has never been observed on a one-for-one basis. Some powerful flares -- even those which are centered in the western solar hemisphere where the interplanetary magnetic field should make connection with the Earth's field most likely -- do not result in disturbances, and some storms do not appear to be associated with a particular flare.
The flare-origin of geomagnetic storms began to be called into question over 15 years ago, when many disturbances were tracked back to the Sun and found to originate with the solar wind and regions within the wind known as coronal holes. During the ensuing years, storms which repeat at about 27-day intervals (the apparent rotational period of the Sun) were unequivocally tied to these features. Such "recurrent" magnetic disturbances take place as matter streaming into space from a low-density portion of the Sun's atmosphere (i.e., from a coronal hole), co-rotating with the Sun, returns to a geoeffective position.
However, until recently the occurrence of large non-recurrent storms -- those which are often accompanied by the spectacular terrestrial effects we read about and hear described by the popular media -- has continued to be linked to the enormous energy bursts from especially violent solar flares.
The Gosling group has now offered evidence for an alternative theory; one which relegates the flares which seem to precede many of these storms to a secondary effect associated with gigantic ejections of matter from the Sun's atmosphere, and identifies the latter as the true source of all great geomagnetic disturbances. Moreover, Gosling's research cites the shock wave which precedes the ejected material, and the strength, speed and configuration of the magnetic fields carried within it, as the causal agent(s) of all such magnetic storms.
As with the solar wind, observations of the release of immense quantities of coronal material into space (thousands of millions of tons of solar matter in some coronal ejections) is a relative latecomer to solar physics. Such expulsions, known as coronal mass ejections or simply "CMEs," are observed with spacecraft-mounted coronagraphs which employ sensitive electronic detectors rather than photographic film.
CMEs were first observed in 1973 by instruments carried aboard the US solar satellite, OSO-7, and have subsequently been explored by a number of spacecraft including Skylab and the SOLWIND and Solar Maximum Mission (SMM) satellites. These observations are complimented by others in the inner corona from ground-based instruments at Mauna Loa Solar Observatory in Hawaii, and by the zodiacal-light photometers aboard the Helios spacecraft.
Although CMEs exhibit a number of morphological features, they often have the appearance of giant loops or bubbles of solar material tied to the photosphere. A more technical definition was provided by National Center for Atmospheric Research astronomer, A.J. Hundhausen, in 1984: ". . . an observable change in coronal structure that (1) occurs on a time scale between a few minutes and several hours, and (2) involves the appearance of a new, discrete, bright, white-light feature in the coronagraph field of view." An extension of this definition includes the requirement that the event display a predominantly outward motion rather than fall back towards the Sun.
In any event, CMEs are mammoth structures which can occupy up to a quarter of the solar limb before lifting off into space, sometimes accompanied by the remnants of an erupting prominence. They appear to originate mainly in closed magnetic field regions within the coronal streamer belt and are thought to arise from changes in the large-scale coronal magnetic field.
This image was taken in February 1997 by the NASA Extreme Ultraviolet Imaging Telescope. It shows three CMEs that appear as "hot spots" near the western limb of the Sun.
Solar flares on the other hand, result more from the reorganization of small, but intense fields generally located near active sunspot regions. It is interesting that CMEs can occur in both normal sunspot/flare zones and also at much higher solar latitudes. As with most of the Sun's active phenomena, the number of CMEs rises and falls with the solar cycle and their occurrence seems to follow the traditional butterfly-pattern: appearance at high heliographic latitudes around solar maximum and then at progressively lower latitudes as the cycle declines to minimum.
It is unlikely that CMEs are caused by solar flare eruptions. Flares do ccompany a fair portion of these mass ejections, but interestingly, those CMEs usually begin their departure before the flare occurs; a factor which has contributed to some astronomers' belief that CMEs -- and not flares -- are the crucial link between the solar disturbance, its propagation through the heliosphere, and effects at the Earth. Further evidence of their disassociation lies in the observation that flares which occur with CMEs typically erupt far to one side of the mass ejection. Of course solar flares occur in much larger numbers than do CMEs, so only a portion of flares could be initiated by these events.
The way that such flares are caused is a matter of conjecture. Some suggest that when CMEs lift off, segments of the massive bubble containing oppositely-directed magnetic fields, become elongated and temporarily remain attached to the Sun. As the fields are distorted and brought together during the ejection's exodus, they could reconnect and cause the flare. However, an exact mechanism is unclear at this juncture.
The leading edges of fast-moving CMEs -- about one-third of all these events -- drive giant shock-waves through the solar wind at speeds up to 1200 km/sec or more. (Since they move at or below the speed of the solar wind, low-speed CMEs do not drive shock disturbances.) Although flares certainly impart energy to solar particles, CME-initiated shock-waves are thought to be capable of energizing particles on a much more impressive scale.
As the trailing mass of plasma moves out into interplanetary space, it can grow to an enormous size. Eventually, some expand to encompass a greater amount of space than the Sun itself; far larger than the narrow cone covered by outward-moving flare particles. When the shock-wave and cloud of material reaches the Earth and conditions are right, the geomagnetic field can undergo a major disturbance. Gosling reports that, on average, the Earth intercepts about six CMEs per month at cycle maximum, and less than one for any month around minimum. He and his colleagues have determined that the origins of all but one of the 37 major geomagnetic storms which occurred during the maximum of cycle 21 are associated with the passage of a shock, a CME, or both.
The direction of the looping interplanetary magnetic fields driven by fast CMEs plays an important role in the new scheme. If the fields happen to point north as the shock and plasma cloud pass the Earth, nothing much happens; the geomagnetic field simply deflects the flow around the magnetosphere. However, when the fields are directed southward (opposite the Earth's field), they can reconnect with the terrestrial field, and the probability of a magnetic storm rises in accordance with the speed and strength of the rapidly moving flow.
Some very complicated magnetic and electrical processes are then set into motion within and around the magnetosphere. The energy spawned by this mechanism drives the ensuing magnetic storm and accelerates both solar and local particles down through the magnetosphere and into the Earth's upper-atmosphere, where they strike atoms and molecules. As we know, when struck the atmospheric components become "excited" (ionized - raised to a higher energy state) and begin to glow, resulting in the aurorae.
Unfortunately, geomagnetic storm forecasters do not receive regular reports of CMEs. As a result, they must look for more visible signs of magnetic instabilities on the Sun (such as disappearing filaments) and base their predictions on the observation of events which may actually be secondary to the true storm source. Adding to the difficulty, those CMEs which would affect the terrestrial environment most dramatically -- events which erupt while facing towards the terrestrial neighborhood -- are difficult to measure from locations near the Earth. In fact, Gosling suggests that at some future time CMEs be monitored by spacecraft stationed at the L4 and L5 Langrangian points, where the view will be unobstructed by the Sun's glare.
Some scientists, however, are still hesitant to climb on the CME bandwagon. They question the CME connection because it is based on about 50% of the available SMM spacecraft observations, while the other half are ignored. The two white-light coronagraph databases of CME reports are the SMM (1980 and 1984-1989) and SOLWIND (1979 to 1985) satellite data. Unfortunately, there are discrepancies between these two databases that have not been explained. Moreover, there is strong evidence that big flares accompanied by shock waves are also very geoeffective.
We now know definitely that fast CMEs (derived from satellite data) are associated with severe geomagnetic storms. However -- as things currently stand -- when deciding between CMEs and solar flares as the "culprit" that is affecting our environment, most of us are like nudists crossing a barbed-wire fence. Until we have routine monitoring of CMEs, we cannot completely answer the important question regarding the actual source of random geomagnetic storms.
Peter O. Taylor
AAVSO Solar Division Chairman