Bernhard Kliem, Mullard Space Science Laboratory, University College London and Institute of Physics and Astronomy, University of Potsdam;
Terry G. Forbes, EOS Institute, University of New Hampshire;
Spiros Patsourakos, Department of Physics, University of Ioannina;
Angelos Vourlidas, Space Science Division, Naval Research Laboratory
Coronal mass ejections (CMEs) observed by white-light coronagraphs in the outer corona and inner solar wind generally have a three-part structure – core, cavity and front. The cavity is interpreted as the cross section of the CME flux rope in the plane of sky. Although a flux-rope cavity is now thought to always be present in CMEs, it is not known whether this is true also for the pre-eruption configuration.
Observations of the CME cavity in the inner and middle corona, where the fast ejections are accelerated, had been severely limited prior to the launch of the STEREO and SDO missions. Their EUV images revealed cases of very rapid cavity expansion early in the event for a number of impulsive and fast CMEs which had no clear signature of a preexisting cavity [1-3]. This EUV cavity then evolved into the white-light cavity at greater heights. However, the pronounced initial expansion is not always correlated with the rise profile of the soft X-ray emission, usually taken as a proxy for reconnection. Therefore, these observations are difficult to reconcile with the standard interpretation that the expansion of the flux rope is driven by reconnection of ambient flux into the rope. In this nugget we present SDO observations of a flux rope and its cavity, and describe a new ideal MHD model for its observed evolution.
SDO Observations of the CME Cavity and Flux Rope in the Inner Corona
The wide temperature range of the SDO/AIA channels allows us to image both cavity and flux rope in the inner corona. The flux rope is very hot (T > 7 MK) as a result of flare reconnection, while the cavity and rim are seen at standard coronal temperatures. Although the first such observation showed the cavity rim located just outside the hot plasma  (in agreement with the observations of white-light CMEs at greater heights), a different situation is found in other, faster eruptions (as well as in the eruption studied in  at later times). Figure 1 displays an overlay of AIA observations of an eruption in the 171 Å and 131 Å channels, sensing 0.7 MK and 11 MK plasma, respectively. These images clearly show a quickly rising loop-shaped hot structure – the flux rope. The cavity forms under a set of preexisting loops, eventually sweeping them up. The cavity is considerably larger than the rope at any point in the sequence, with the ratio of their projected heights decreasing gradually from 1.9 to 1.5 as they traverse the AIA field of view.
Mechanism of Cavity Formation and Expansion
The rapid formation and expansion of a cavity in the ambient volume can be readily understood within the framework of flux-rope CMEs as an inverse pinch effect, which is an ideal MHD effect not relying on reconnection. The pinch effect is the contraction and confinement of a current-carrying plasma from the magnetic field generated by the current, and so the inverse pinch effect operates here as follows. If a current-carrying flux rope becomes unstable and rises, the current through the rope must decrease, since both the number of field line turns in the rope and the flux passing through the area between the rope and the photosphere are preserved in ideal MHD. The magnetic energy released from the current can then power the CME and flare. Both effects are equivalent to decreasing the azimuthal field strength which is directly proportional to the current. The conservation of the azimuthal flux then causes the flux surfaces in the ambient medium to move away from the rope, forming a cavity with swept-up material at its edge.
As the current is roughly inversely proportional to the length of the rope, it changes most rapidly in the early stage of the eruption. The inverse pinch effect is therefore strongest in the phase of main CME acceleration, with the temporal rate of change increasing with the velocity of the CME. These properties correspond perfectly to the observations of EUV cavities which expand very rapidly early in fast CMEs and have sizes clearly larger than the flux rope. Subsequently, the different expansion mechanisms acting in the flux rope and the cavity let the flux rope grow slightly faster. The resulting gradual approach to the size of the cavity reconciles the observations of CME cavities in the inner and outer corona. Figure 2 demonstrates the inverse pinch effect in a simulated flux rope eruption and indicates also the subsequent gradual decrease of the size ratio. Further detail will be given in a forthcoming publication.
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