The magnetic nature of coronal mass ejections
Coronal mass ejections are eruptions of plasma and magnetic field from low in the Sun's atmosphere. They act as
a valve to release magnetic energy from the Sun's atmosphere and are the major driver of space weather events.
Theoretical and observational developments since their discovery in the early 1970's have shown that coronal mass
ejections are driven by changes in the magnetic field which occur over a wide range of time scales from
hours to weeks. Since it is not currently possible to directly observe
the magnetic field in this region of the Sun to understand these changes, other observational
signatures must be used as a proxy for the magnetic field evolution. In the past this has included the
slow rise of filaments or coronal loops which indicate an overall rise or expansion of the magnetic structure
which can in many cases be interpreted as being that of a twisted rope of magnetic field. This nugget discusses
how plasma outflows from an active region which erupts to produce a coronal mass ejection can be used to
understand when important changes in the magnetic field start to occur.
Figure 1 - XRT image of active region-coronal hole complex on 17 October 2007
The importance of outflows
Multiwavelength observations with EIS captured the evolution of an active region (Figure 1) over four days in which a magnetic flux rope formed and subsequently erupted. Six hours before the eruption, a striking increase in outflow velocities was seen in the active region which was surrounded by a coronal hole. Figures 2 and 3 show a comparison of the active region outflow velocities for 3 days and 6 hours before the coronal mass ejection. The generation of intensified flows is explained by Murray et al. (2010) who showed that compression of the nearby coronal hole field by the expanding active region leads to outflows. The active region studied here exhibits a sigmoidal shape indicating that a flux rope is present. The slow-rise and expansion of this flux rope prior to its eruption would, therefore, logically lead to stronger compression of the surrounding field and intensification of outflows, though it is possible that other, reconnection-related, mechanisms may also play a role. We believe that the intensification of outflows at the edge of this AR is a spectroscopic precursor of its CME. Further studies into the use of how plasma ouflows are affected by the evolving magnetic configutation in the time leeading up to a coronal mass ejection are planned.
Figure 2 - Top: EIS Fe XII intensity (left) and velocity (right) maps of active region-coronal hole complex on 17 October 2007. Eastern and western outflow regions are circled. Bottom: Histograms of the outflow velocities on the eastern (left) and western (right) sides of the active region from 15 to 17 October. Velocity PDFs are consistent for both sides, ranging from a few to 15 km/s. The thick black line is the average velocity PDF for 8 rasters over the 3 day period.
Figure 3 - Top; EIS Fe XII intensity and velocity maps for 18 October 2007, 6 hours before the coronal mass ejection. Bottom: Outflow velocities on the eastern side of the active region are virtually unchanged from the previous 3 days (compare the 3-day average PDF in black with 18 October PDF in red on the left). The outflow velocities show a significant enhancement to 20 km/s on the western side of the active region 6 hours before the CME is detected in LASCO's C2 coronagraph. The distinct intensification of the outflow velocities occurs where there is a filament/flux rope along the polarity inversion line.
For more details see Baker, van Driel-Gesztelyi, Green, 2011, Sol. Phys., DOI 10.1007/s11207-011-9893-4