The Dark Side of the Solar Flare Myth

Gosling [1993, 1994] reviewed the growing observational evidence that (1) traveling interplanetary shocks, (2) large solar energetic-particle (SEP) events and (3) large non-recurrent geomagnetic storms are produced by coronal mass ejections (CMEs), not by solar flares. This declaration of independence from the flare community certainly erased the attitude of benign neglect of interplanetary phenomena and observations. In one case it produced hostile dismay that "Jack Gosling and a few other revisionists" would "wage an assault on the last 30 years of solar-flare research" based on the "low-grade optical data that the CME people use" [Zirin 1994]. Calmer objections were raised by Hudson, Haisch and Strong [1995], who accept the interplanetary consequences of CMEs but suggest that "it is shortsighted to distinguish CMEs and flares."

CMEs came late to the domain of known solar eruptive phenomena, where flares stood alone for over 100 years. CMEs can be massive objects; spanning 120° in solar latitude or longitude, they can involve 10^16 g of gas that is suddenly ejected at speeds up to 2000 km/s with a kinetic energy of >10^32 ergs, all directed outward into interplanetary space. Thousands of CMEs have now been observed by the Skylab, SMM, SOLWIND and Mauna Loa coronagraphs and the Helios photometers. Their properties and their relationship with flares have been extensively reviewed [Kahler 1992; Hundhausen 1995; Webb and Howard 1994; Webb 1995]. Flares are observed with no associated CMEs, and conversely; the two phenomena are not causally related. At solar maximum the magnetic equator of the Sun, called the streamer belt or heliospheric current sheet, is highly inclined to the ecliptic so it passes near the solar poles. At this time CMEs are distributed around the streamer belt at all heliographic latitudes while classical H-alpha flares only occur in a limited latitude band.

The motivation to distinguish flares and CMEs goes far beyond semantics. The term "flare" evokes the idea of limited spatial and temporal extent. We will see that this alone causes serious errors when flares and CMEs are not distinguished. Like others, I once believed that high-energy particles were only accelerated in point-source flares. Thus, I speak as former disciple (and victim) of the flare myth. "Flares" also imply chromospheric and coronal heating and are classically observed by photons from a hot gas. The 100-year history of flare observations has produced a significant photon bias that tends to discount interplanetary observations of plasma processes that are not visible in photons.

Interplanetary phenomena near Earth can provide information on their own solar origin. In some cases this information is internal; for example, ionization states of ions carry information on the temperature of the source plasma. More often it involves extensive comparisons between local phenomena and radio, X-ray, gamma-ray and CME observations in the related solar events. All interplanetary shocks energetic enough to be seen as type II radio bursts, for example, are found to have associated CMEs [Cane, Sheeley and Howard 1987]. The interplanetary properties of CMEs have been identified and their effect on the magnetosphere has been studied directly [see Gosling 1993]. Energetic particle observations identified two populations, clearly distinguished by their abundances, ionization states, and time and longitude distributions. One population comes from impulsive flares and the other, involving the largest, most energetic events, comes from CME-driven shocks [Reames 1990, 1993, 1994, 1995]. Gosling [1993, 1994] basically combined the published and generally-accepted results from a variety of interplanetary observations into "The flare myth." Most in the interplanetary community were well aware that CMEs, not flares, cause the dominant near-Earth phenomena of relevance to solar-terrestrial studies. Except for the "sudden ionospheric disturbances" caused directly by photons, flares are not "geo-effective."

I freely admit to being a revisionist as Zirin [1994] suggests; when we are unable to revise our understanding of physical phenomena we are no longer doing productive science. However, the idea of an "assault" and the similarities between CMEs and flares deserve further attention.

Hardly. Most of my own recent work (apart from reviews) has involved the energetic particles that do come from impulsive flares, the 3He-rich events [e.g. Reames, Meyer & von Rosenvinge 1993] where streaming electrons generate plasma waves that are resonantly absorbed by 3He to produce ~1000-fold enhancements in 3He/4He. Enhancements of 3He and of heavy elements from Ne through Fe, always seem to occur in energetic ions from impulsive flares, and have even been observed via the broad gamma-ray lines produced by the beam interacting in the footpoints of flare loops [Murphy et al. 1991]. These processes occur in >1000 events/year, probably in all impulsive flares, yet resonant waves near the proton gyrofrequency are not visible in photons.

The physics of particle acceleration in impulsive flares is no less interesting because the events are not geo-effective. I have chosen this example from my own field, but there are many other new and exciting observations of flare phenomena. Flare research is both interesting and important; there is no assault, merely an attempt to determine the correct physics.

It is sometimes argued that we can learn about CMEs by studying flares because both are powered by magnetic energy. This is like saying that since gravity causes rain to fall and the Earth to move around the Sun, we can learn about meteorology by studying planetary dynamics. The point is that other physical mechanisms are involved. In the case of impulsive flares, detailed plasma physics produces millisecond radio spikes involving pulsed electron beams with spatial scales as small as 10 km; multiple injections combine to form both type III radio and hard X-ray bursts, and beam-generated waves are resonantly absorbed to enhance energetic 3He. In the case of CMEs, a quiescent prominence 300,000 km in length can become magnetically unstable and be launched like a giant helium balloon on time scales of tens of minutes to hours. Apart from the energy source, any relationship between the physical processes involved in these two phenomena is obscure, at best.

Where is the evidence that these diverse phenomena share any physical mechanisms in common that could justify calling both "flares"? CMEs probably have greater kinship with plasmoids emitted into the Earth's magnetotail than with classical solar flares.

Hudson et al. [1995], state "We do not know of any physical parameter showing a bimodal distribution that can distinctly divide the events ...." The title of the paper by Reames [1988] is "Bimodal abundances in the energetic particles of solar and interplanetary origin." The ionization states of Fe probably also form a bimodal distribution [Luhn et al. 1987]. We can not observe bimodal distributions in large traveling interplanetary shocks, for example, because all of them are associated with CMEs [Cane et al. 1987]. There may also be flare-induced (blast-wave) shocks from flares without CMEs, but evidently they do not survive beyond the corona. However, there are such broad distributions of energy release for both flares and CMEs that it would be truly surprising if the distribution of coronal heating, or soft X-ray intensity, were bimodal. Furthermore, time profiles of the heating curves may depend more on common electron transport in a loop than on different mechanisms of electron acceleration that might occur in flares and CMEs.

One of the difficulties in the present controversy is that flares and CMEs often occur together. However, one does not cause the other. Even when flares and CMEs do occur together, there is little relationship between their positions or timing and often there is more energy in the CME than in the associated flare [Kahler 1992]. Kahler [1982] coined the term "big flare syndrome" to point out that, despite appearances, not all processes occurring in conjunction with big flares are causally related to the flare or to each other, even when well correlated. Our greatest progress in establishing causal relationships has come from the observation of small impulsive flares without CMEs and from erupting-filament events which are CMEs without impulsive flares [Kahler 1992].

As stated previously, the term "flare" evokes the idea of limited spatial and temporal extent. Historically, the idea of flares as point sources of large SEP events led to a profound misunderstanding of the physics of particle acceleration and transport that persisted for 30 years. Flares last for hours, at most, but large SEP events last for days. Flares are also confined to a few degrees on the Sun but large SEP events (and interplanetary shocks) are seen over a span of ~180°, sometimes more. Early workers proposed that the unseen hand of "coronal diffusion" somehow transported particles uniformly across magnetic field lines to distant longitudes where slow "leakage" from the corona and intense scattering on interplanetary turbulence trapped them for days (often until just after the time of shock passage). Only recently have we understood that the shocks from CMEs cross field lines to accelerate the particles locally over vast regions of space and for long intervals of time in the largest SEP events [see Reames 1993, 1994, 1995].

The importance of CME shocks in large SEP events is now generally recognized; yet the consequences of the old flare paradigm still linger. There are still attempts to derive diffusive interplanetary scattering parameters from particle time profiles without considering the moving, evolving shock source. 3He-rich events, with impulsive injection, tell us the ambient parallel scattering mean free path is long (~1 AU), yet in shocks it becomes quite short (~10^-3 AU) because of waves generated by the accelerated particles themselves. Neglect of this wave generation leads to the erroneous conclusion that shock acceleration is "too slow." As a CME comes out from the Sun to 1 AU, an observer's connection point to the shock swings through ~50° or more in longitude, sampling a large gradient in shock strength. Also, the speed at the nose of the shock can slow by a factor of ~2 or 3 between the Sun and the Earth. This evolution is largely ignored in the naive models we have inherited from the flare myth.

Distinguishing events as "impulsive flares" and "eruptive flares" does not end the confusion since the common perception of a "flare" is something that will easily fit in an active region. Some models of CMEs rising above "eruptive flares" begin with low-lying loops emerging from a flat solar surface. Such models do not describe real CMEs where pre-existing magnetic structures typically arch through angles of ~45° from one active region to another. These large structures are much less likely to lead to significant coronal or chromospheric heating in producing CMEs. These modelers are being misled by the flare myth just as others have been; they consider only initial conditions where flares are likely to accompany CMEs.

In changing the perceived cause of large SEP events from flares to CMEs, there is no middle ground based upon particle energy. A compromise position, that shocks accelerate MeV particles but not GeV particles, represents a reincarnation of the flare myth that has recently been disproved. Large SEP events have a 96% correlation with CMEs [Kahler et al. 1984] but a key piece of evidence for shock acceleration is the ionization state of elements such as Fe. Near 1 MeV/amu, QFe=14.1±0.2 [Luhn et al. 1985, 1987] indicates that the source plasma has a temperature ~2 MK, similar to the temperature of the corona and to the ionization states seen in the solar wind. The source cannot involve heated (>10 MK) plasma that exists in flares or reconnection sites. Only 3He-rich events show ionization states (e.g. QFe=20.5±1.2) compatible with a hot source. Recent measurements of Fe at 200-600 MeV/amu in large SEP events show ionization states [Tylka et al. 1995] in agreement with those of Luhn et al. [1985]. Ionization state measurements by 4 experiments on 3 different spacecraft now support the idea of shock acceleration of ambient, unheated coronal and solar-wind plasma.

Kahler [1994] has studied the solar injection intensities of protons up to 25 GeV as a function of the altitude of the leading edge of the CME. The intensities reach maximum only after the CME is beyond ~10 solar radii, outside the corona. In the 1989 September 29 event the leading edge of the CME was observed [Kahler 1994] to have a speed of over 1800 km/s. This was also one of the events where Fe was observed to have coronal charge states at 200-600 MeV/amu. In this event the shock came across the high corona from a source behind the west limb to accelerate ambient protons to energies as high as 25 GeV and ambient Fe as high as 600 MeV/amu at the base of the field line connected to Earth. The relativistic Fe ions must be accelerated in the tenuous high corona beyond ~2 solar radii to avoid being stripped of their orbital electrons.

As long as the flare myth persists, it continues to cause a major misunderstanding of the physics of solar, interplanetary and geomagnetic phenomena. After 30 years of such mistakes, it is time that we acknowledge the importance and the independent existence of CMEs and correctly identify them as the source of the dominant traveling interplanetary shocks, large SEP events, and major non-recurrent geomagnetic storms.

I would like to thank E. W. Cliver, H. S. Hudson, S. W. Kahler, M. A. Linzmayer, J. A. Miller, C. K. Ng, T. T. von Rosenvinge, and D. F. Webb for their comments on the manuscript.

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