Plasma evolution within an erupting coronal cavity

By David Long, Louise Harra, Sarah Matthews, Harry Warren, Kyoung-Sun Lee, George Doschek, Hirohisa Hara, Jack Jenkins


Although solar eruptions are the most energetic and spectacular events that occur in the solar system, their initiation and subsequent evolution remain areas of active research. The currently accepted model of a solar eruption (commonly called the ``standard flare model'') was originally proposed by Carmichael (1964), Sturrock (1966), Hirayama (1974) and Kopp & Pneumann (1976), and describes a solar flare as a brightening driven by magnetic reconnection of coronal loops during the eruption of a magnetic flux rope. However as magnetic structures in the solar corona, flux ropes are difficult to observe directly, and are typically inferred by either extrapolating the pre-eruption photospheric magnetic field or via direct measurement of the magnetic field of the associated coronal mass ejection (CME) in-situ following eruption. The existence of a pre-eruptive flux rope configuration can also be inferred by observing dark cavities at or near the solar limb in white light, extreme ultraviolet (EUV) and X-ray observations. However, spectroscopic observations of erupting coronal cavities, which provide valuable information on how the plasma contained within these cavities is moving in 3-dimensions, remain frustratingly rare. This is primarily due to the very small field-of-view of the available instruments and the long time-scales required to obtain the observations, both of which require precise advance knowledge of the location of the eruption.

In Long et al. (2018) we report on a unique set of spectroscopic observations of a large eruptive event with an associated coronal cavity across a broad temperature range. This is the first time that such a cavity eruption has been observed spectroscopically erupting from an active region, with the observations providing a unique physical insight into this phenomenon.

Observations of the event
The event described by Long et al. (2018) erupted from NOAA active region AR 12673 on 10 September 2017, and was associated with a GOES X8.2 class flare which began at 15:35 UT and peaked at 16:06 UT. This event was one of a series of major flares produced by the active region, with the result that the active region was the focus of a major flare watch campaign by Hinode/EIS. The primary observing plan involved a scanning raster campaign designed to study post-eruption supra-arcade plasma (Hinode Observing Plan 244), and used EIS study FlareResponse01 to raster the 2'' slit from right to left across a field of view of 239'' x 304'', observing a range of emission lines from He II (at log T=4.7) to Fe XXIV (at log T=7.2). As the active region transited the west limb, the campaign was designed to begin off-limb and raster towards the solar disk, taking ~8 minutes 52 seconds to complete each raster. As a result, the erupting cavity described here was only observed by the rasters which began at 15:42:26UT and 15:51:18UT respectively.

These narrowband spectroscopic observations from Hinode/EIS were complemented by broadband full-Sun observations from SDO/AIA. This allowed the erupting feature to be tracked at high cadence using multiple passbands at different temperatures, with the 131A, 171A, 193A and 211A passbands providing the clearest observations of the erupting cavity feature (as shown in Figure 1).

Figure 1: The eruption from 2017-Sept-10 at 15:53:06 UT as observed by SDO/AIA in the 94A, 131A, 171A, 193A, 211A and 304A passbands (panels a-f). Panel g; GOES X-ray flux showing the flare (solid black line) and Fermi nonthermal electron energy flux (dashed black line). Vertical coloured lines show the change from slow to fast rise phase in each passband (see Figure 2).

The eruption appeared to be a textbook example of the ``standard flare model'', with a thin bright feature (consistent with a current sheet) connecting the bright flare loops below to the dim cavity above (see Figure 1). Although both the current sheet and cavity were observed by Hinode/EIS, only the cavity was discussed by Long et al. (2018), with the evolution of the current sheet discussed by Warren et al. (2018).

We tracked the temporal evolution of the cavity by fitting an ellipse to the bright edge of the cavity at each time step for the 131A, 171A, 193A and 211A passbands (see Figure 2). The evolution of cavity height and width were comparable in each passband studied, with the cavity found to exhibit two distinct rise phases which were best fitted using two independent quadratic functions. This indicates that the cavity initially rose slowly then more rapidly, consistent with previous observations of solar eruptive events (cf. Regnier et al., 2011; Byrne et al., 2014).

Figure 2: Left panel; Ellipses fitted to the evolving cavity identified using the 211A passband. Right column; Temporal evolution of the height above the limb of the ellipse centroid (diamonds) and full semi-minor axis width (crosses) in each passband studied. The point at which the rise phase changed from slow to fast rise phase is indicated by the vertical dashed line and was identified using the variation in ellipse width.

The increased upward acceleration of the cavity was seen to occur during the rise phase of the X-ray flare and as the nonthermal electron energy flux measured by Fermi approached its peak (as shown in Figures 1 & 2). This indicates that the upwards acceleration of the cavity was driven by the erupting flare below (consistent with the ``standard'' flare model) and suggests that the observed cavity was an erupting flux rope.

Figure 3: Panel a; SDO/AIA 193A image at 15:49:28UT showing the full Hinode/EIS field-of-view (large solid box) and the field of view used in panels b & c (small solid box). Panels b & c; Fe XV and He II intensity images from Hinode/EIS. Blue contours in both panels b & c indicate where the plasma is blue-shifted by 214 km/s. The cavity is most apparent in the Fe XV image (panel b), with the filamentary material is most apparent in the He II image (panel c). Panels d & e show sample Doppler-shifted spectra from the Fe XV and He II emission lines respectively, illustrating their complex nature. These Doppler-shifted spectra were constructed by converting the spectrum in the pixel defined in the legend from wavelength to velocity space using a rest wavelength estimated by averaging the spectra in a nearby portion of quiet activity.

Although the cavity was best observed using the images from SDO/AIA, it was first observed by Hinode/EIS between 15:48:00 and 15:49:20 UT (i.e. during the raster which began at 15:42 UT) as it was first developing, and again at 15:53:20-15:54:40 UT as it erupted (during the raster which began at 15:51 UT). The cavity is most clearly seen in the Fe XV emission line shown in Figure 3 for the raster beginning at 15:42 UT and the Fe XXIV emission line for the raster beginning at 15:51 UT. While no clear filamentary material can be observed in the Fe XV image at 15:42 UT shown in Figure 3b, strongly blue-shifted emission was observed in the region enclosed by the blue contours, corresponding to the bright filament emission observed in He II shown in Figure 3c. Although the line spectra observed were incredibly complex (e.g. Figure 3d), a comparable blue-shift to that seen in Fe XV emission was also observed in Fe XVI emission, indicating that the blue-shift is due to plasma motion rather than a contribution from nearby Al IX and Fe XVII emission lines.

The filamentary material observed in each SDO/AIA passband associated with the erupting cavity was also clearly observed in the EIS He II data as a bright, very strongly blue-shifted blob (see Figure 3c & 3e). In fact, Figure 3d shows that some of the spectra were nearly blue-shifted out of the spectral window, suggesting Doppler velocities greater than 200 km/s (consistent with the velocity of 214 km/s shown by the blue contours in Figure 3c & 3d). This indicates that as the cavity expanded and erupted, the plasma contained within it flowed rapidly towards the observer, most likely draining down the legs of the flux rope defined by the cavity. There is also some diffuse Fe XXIV emission around and across the cavity, consistent with hot, low density plasma in the cavity core.

The subsequent EIS raster began at 15:51:18UT, with the cavity observable in all AIA passbands at this time, indicating a significant drop in density and/or temperature compared to the observations at the time of the earlier EIS raster shown in Figure 3. Although the intensity of the Fe XV emission line had dropped too much by this time to provide any usable observations, a bright edge around the cavity was observed in the Fe XXIV emission line (at Log T=7.1). However the corresponding spectra show that the strong plasma flow had mainly stopped by this time, with the plasma velocity peaking at 0 km/s, and some slight broadening of the profile. This indicates that while the significant downward plasma flow associated with the eruption of the flux rope had mostly ceased by this time, there continued to be some draining of hot material from the cavity.

The temporal evolution of the coronal cavity indicates that it initially began to rise slowly (with a velocity v~74-181 km/s) before rising much more rapidly (at a velocity of v~439-513 km/s), with the Fermi non-thermal electron energy flux indicating that this was driven by continuous energy input from the flare below. The strongly blue-shifted plasma observed by Hinode/EIS suggests that the rapid increase in height of the flux rope driven by the flare forced the plasma from the apex of the flux rope towards the legs, decreasing the density at the apex of the flux rope. As the flux rope initially rose gradually, this drop in plasma density would have been matched by a rise in plasma temperature, giving the observed combination of hot and cool plasma in the core of the flux rope. However, the subsequent rapid rise of the flux rope and drop in its density also forced the expansion of the flux rope volume, producing the sudden appearance of the cavity in all wavelengths. As the cavity erupted and increased in height, the drop in pressure would have stopped the flow of plasma towards the legs of the flux rope, consistent with the lack of any clear Doppler motion in the Fe XXV emission line.

While the sudden off-loading of material from an erupting filament or flux rope has been previously observed (e.g. Jenkins et al., 2017), this is the first time it has been observed spectroscopically for an eruption associated with an active region. These observations indicate that the eruption of the cavity was initially driven by the impulsive phase of the solar flare, with the increases in non-thermal electron energy matching the changing rise phase of the cavity. This rapid injection of a significant amount of energy forced a dramatic downflow of plasma from the apex of the erupting flux rope and meant that the erupting flux rope was much less dense and exhibited a different structure to other flux rope observations (cf. Hannah & Kontar, 2013). These observations are consistent with the ``standard flare model'', and highlight the vital insight provided by spectrometers such as Hinode/EIS.

The link to the paper is here:
Plasma Evolution within an Erupting Coronal Cavity

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