6.2 Unsolved problems

Several enigmatic or outstanding problems remain regarding the origin, propagation, and heliospheric consequences of CMEs. We conclude this review by discussing several of these that we feel are the most important needing to be resolved.

The CME initiation follows storage of energy in closed magnetic field regions on the Sun over a certain period of time but we do not know what triggers the release of that energy. The stored energy that is released during eruption is the free energy available to be released in the form of CMEs, flares, and other eruptive phenomena. The magnetic source of the CME has to build up the free energy. Identification of the signatures of this energy build-up is a crucial step in deciding whether and when a CME will occur. During the build-up phase, minor energetic events occur, but it is difficult to know whether a pre-eruption energy release is a true precursor or a separate eruption. We also do not know how the free energy is apportioned among the subsequent flare energy and the CME mechanical energy. We do know, however, that flares occur without observed CMEs, just as many CMEs are not always accompanied by flares. Flares accompanying CMEs are generally of lower temperature compared to flares not accompanied by CMEs. This may indicate that the many smaller flares without CMEs may contribute to coronal heating (Yashiro et al., 2006), while CMEs carry away the mass and magnetic field into the heliosphere. Based on the highest mass (1014 kg) and speed (∼ 3500 km s–1) observed one can estimate a maximum kinetic energy of ∼ 6 × 1034 erg. Assuming that only a fraction of the stored energy is released in a single episode and that the CME derives all of its energy from a single active region, we can set a limit of ∼ 1036 erg for the maximum free energy available in a solar active region. This is consistent with the size and magnetic field strengths in solar active regions (Kahler, 2006).

CMEs are subject to propelling and retarding forces in the corona and interplanetary medium (see, e.g., Vršnak et al., 2004). The propelling force is not properly identified yet. Solar gravity and the drag force due to momentum exchange between CMEs and the ambient medium constitute the main retarding forces. The net result is that most CMEs tend to acquire the speed of the ambient solar wind at large distances from the Sun. This can be quantified as an effective interplanetary acceleration (Gopalswamy et al., 2000, 2001). However, CMEs come in all sizes and shapes and the ambient solar wind also is highly variable. The propagation of CMEs is also affected by the presence of preceding CMEs (Lyons and Simnett, 2001), especially during solar maximum years when CMEs occur in quick succession (Gopalswamy, 2004Jump To The Next Citation Point). CMEs may also be deflected by other CMEs and by nearby coronal holes (Gopalswamy et al., 2009a) and the CME itself may contain some intrinsic driving property (Howard et al., 2007). We need a proper quantification of these effects to accurately predict the arrival of a CME at a desired location in the heliosphere, once its launch has been observed and the initial speed measured. Another issue is the true speed with which CMEs propagate toward a location in the heliosphere. Coronagraphs measure speeds in the sky plane, but the travel time prediction needs space speed. For example if we consider CMEs heading towards Earth, we need to de-project the sky-plane speed and re-project it along the Sun-Earth line. There have been several attempts to convert the sky plane speed into Earth-directed speed using cone models with reasonable success, but more work is needed (Xie et al., 2006; Michalek et al., 2006; Howard et al., 2008b).

Even though the fastest CMEs produce energetic particles, we do not fully understand why some seemingly energetic events produce only low levels of SEPs. There are clear indications that particle acceleration is a complex issue with multiple sources (shocks and flares) and multiple factors deciding the acceleration efficiency (Kahler, 2001; Gopalswamy, 2004; Kahler and Vourlidas, 2005). We do not know what the flare and shock contributions are for a given SEP event. We also do not know how the ambient medium consisting of previously ejected CMEs, shocks, and SEPs determines the properties of a subsequent event.

We need to more fully understand how the remotely-sensed CMEs evolve into CMEs observed in-situ in the solar wind. Magnetic clouds observed within CMEs in the solar wind have specific magnetic properties, notably their flux rope structure. But flux rope structures near the Sun can only be inferred, although the three-point views from the STEREOs and LASCO and the continous field of view of SECCHI from the corona into the inner heliosphere have improved our understanding. Prominences are themselves thought to be flux ropes near the Sun, but observations in the interplanetary medium are not compatible with that. Magnetic clouds are observed with high charge states implying high temperature (several million K) at the source, whereas prominences are cooler structures with a temperature of only ∼ 8000 K. Such cool prominence material is rarely observed in-situ, another unsolved problem. Coronal cavities observed in eclipse pictures and inner coronal images in X-rays and EUV are thought to contain flux ropes, but an alternative explanation is that these are highly sheared magnetic structures. Recent quantitative comparison between reconnected flux at the eruption site and the azimuthal flux in flux ropes in the solar wind suggest that the two fluxes are approximately the same (Qiu et al., 2007), implying that the flux ropes are formed during the eruption process rather than present in the pre-eruption state. White-light CME observations mainly provide information on the mass content of the CME, but very little on the magnetic structure. Many related observations (magnetic and other) need to be pooled to try to obtain the magnetic structure of CMEs. Another related issue is whether all CMEs contain flux ropes, and the related question of whether all interplanetary CMEs are magnetic clouds? If they are, that implies a definite magnetic structure, from which one can infer the onset time of geomagnetic storms for space weather purposes. The magnetic cloud structure indicates a definite leading and trailing field orientation, which decides the day-side reconnection with Earth’s magnetic field that ultimately results in the magnetic storm. While flux ropes in the interplanetary medium have a well defined magnetic field strength and structure, the same cannot be said about CMEs near the Sun. At present, we have to infer the nature of interplanetary CMEs based on the magnetic properties of solar active regions at the photospheric or chromospheric levels, but the eruption itself starts in the corona.

High temporal and spatial resolution images are needed to identify and study pre-eruption signatures, which is crucial to predicting the onsets of CMEs. We still lack a quantitative understanding of how the magnetic complexity in a source region relates to CME productivity. Since vector magnetograms provide key information on the free energy available in active regions, they need to be developed and the results assimilated into various models, including MHD. Finally, the developing science of helioseismic subsurface imaging of sunspots and active regions suggests important clues to the build-up of energy in active regions that can lead to large flares and CMEs (e.g., Webb et al., 2011).

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