No abstract

Determining the state of the corona prior to CMEs is crucial to understanding and ultimately predicting solar eruptions. A common and compelling feature of CMEs is their three-part morphology, as seen in white-light observations of a bright expanding loop, followed by a relatively dark cavity, and finally a bright core associated with an erupting prominence/filament. This morphology is an important constraint on CME models. It is also quite common for a three-part structure of loop, cavity, and prominence core to exist quiescently in the corona, and this is equivalently an important constraint on models of CME-precursor magnetic structure. These quiescent structures exist in the low corona, primarily below approximately 1.6 R , and so are currently observable in white light during solar eclipses, or else by the Mauna Loa Solar Observatory Mk4 coronameter. We present the first comprehensive, quantitative analysis of white-light quiescent cavities as observed by the Mk4 coronameter. We find that such cavities are ubiquitous, as they are the coronal limb counterparts to filament channels observed on the solar disk. We consider examples that range from extremely long-lived, longitudinally extended polar-crown-filament-related cavities to smaller cavities associated with filaments near or within active regions. The former are often visible for days and even weeks at a time and can be identified as long-lived cavities that survive for months. We quantify cavity morphology and intensity contrast properties and consider correlations between these properties. We find multiple cases in which quiescent cavities directly erupt into CMEs and consider how morphological and intensity contrast properties of these cases differ from the general population of cavities. Finally, we discuss the implications that these observations may have for the state of the corona just prior to a CME, and more generally for the nature of coronal MHD equilibria.

Solar coronal sigmoidal active regions have been shown to be precursors to some coronal mass ejections. Sigmoids, or S-shaped structures, may be indicators of twisted or helical magnetic structures, having an increased likelihood of eruption. We present here an analysis of a sigmoidal region's three-dimensional structure and how it evolves in relation to its eruptive dynamics. We use data taken during a recent study of a sigmoidal active region passing across the solar disk (an element of the third Whole Sun Month campaign). While S-shaped structures are generally observed in soft X-ray (SXR) emission, the observations that we present demonstrate their visibility at a range of wavelengths including those showing an associated sigmoidal filament. We examine the relationship between the S-shaped structures seen in SXR and those seen in cooler lines in order to probe the sigmoidal region's three-dimensional density and temperature structure. We also consider magnetic field observations and extrapolations in relation to these coronal structures. We present an interpretation of the disk passage of the sigmoidal region, in terms of a twisted magnetic flux rope that emerges into and equilibrates with overlying coronal magnetic field structures, which explains many of the key observed aspects of the region's structure and evolution. In particular, the evolving flux rope interpretation provides insight into why and how the region moves between active and quiescent phases, how the region's sigmoidicity is maintained during its evolution, and under what circumstances sigmoidal structures are apparent at a range of wavelengths.

Many properties of Coronal Mass Ejections (CMEs), such as size, location and brightness, have been determined from measurements of white light coronal observations. We expect the average properties derived from these measurements contain systematic inaccuracies due to projection effects and suggest that CME properties are most accurately determined for those events occurring near the plane‐of‐the‐sky (i.e., over the solar limb as observed from Earth), where projection effects are minimized. A set of 111 such “limb” events have been identified in Solar Maximum Mission (SMM) white light observations through associations with Erupting Prominences at the Limb (EPLs), and X‐ray and limb optical flares. These “limb” CMEs have greater average speeds (519 ± 46 km/sec) and masses (4.5 ± .5 × 1015 grams) than the average values obtained from all SMM CMEs, consistent with the expected behavior of projection effects. Only a very small percentage of “limb” CMEs are centered at high latitudes, suggesting there are many fewer “true” high latitude CMEs than has previously been reported. No “limb” CMEs have widths greater than 110°, consistent with the interpretation that very wide CMEs (i.e., halos) are actually events of more typical widths originating away from the solar limb and viewed in projection. Only a small percentage of “limb” CMEs have measured speeds below 200 km/sec, indicating there may be fewer “true” subsonic SMM CMEs than previously reported. The correlation detected between the kinetic energy of the “limb” CMEs and the peak intensity of the associated GOES X‐ray flares, is stronger than was previously found using a set of CMEs of undetermined limb distances. All these results provide strong evidence that projection effects systematically influence the deduced properties of CME events.