Episodic Chromospheric Evaporation in Flare Loop Strands Observed with EIS

by Jeff Brosius (Catholic University of America/NASA Goddard Space Flight Center)


A key piece of evidence for chromospheric evaporation during the impulsive phase of flares is an upward velocity around 100 km/s or more in plasma at temperatures near 10 MK. This has been observed for about 35 years with satellite-borne, uncollimated soft X-ray spectrometers. Interestingly, with few exceptions the soft X-ray spectra at flare onset show that the whole line profile is not blueshifted, but consists of a blueshifted component accompanied by an intense stationary component. This is inconsistent with theoretical models of chromospheric evaporation in single-strand loops. The discrepancy appears to be resolved by models that incorporate multiple strands within the flare loop envelope (Hori et al. 1998, Doschek & Warren 2005, Warren & Doschek 2005, Warren 2006). In this scenario upflows of several hundred km/s occur at the onset of evaporation within each strand, but will be observed as a blueshift of the whole line profile only while the very first strand is filling with evaporated material. Upflows in subsequent strands are observed as blue wing enhancements of the intense stationary component, where the stationary component is due to emission from strands that have already been filled with evaporated plasma. (If all of the loop strands could be individually resolved, the whole line profile would appear blueshifted during evaporation in each of them.) To date the soft X-ray spectrometers seem not to be sensitive enough to observe evaporation in the very first loop strand at flare onset.

Brosius (2001) developed a rapid cadence (9.8 s) stare study to observe EUV emission lines formed over a wide range of temperatures in spatially resolved 4" X 20" CDS slit segments. Brosius (2003) and Brosius & Phillips (2004) observed M flares with this study and found that the entire single-component Fe XIX profile was blueshifted during the impulsive intensity rise, consistent with chromospheric evaporation theory. To capitalize on the higher spatial and spectral resolution of Hinode's EIS, we designed the rapid cadence (11.18 s) FLAREDOP_EIS study after its CDS counterpart. FLAREDOP_EIS includes the unblended Fe XXIII line at 263.8 A, formed at temperature around 14 MK. In this Nugget we summarize results from a run of FLAREDOP_EIS when the slit stared at the leg of a C1 flare loop. See Figure 1. For more information about this EIS and coordinated CDS observing run, see Brosius (2013).

Figure 1: AIA 131 A images of AR 11429 during the C1 flare of 2012 March 7. Times are indicated in the lower right of each frame. Some of the images appear to show multiple strands during the flare. The commanded pointings of the 2"-wide EIS and 4"-wide CDS slits are overplotted and labeled.

The entire profile of the Fe XXIII line at 263.8 A was blueshifted by an upward (negative) velocity -122+/-33 km/s when the line was first detected by EIS during the flare. The entire profile became even more blueshifted over the next two exposures, when the upward velocity reached its largest value of -208+/-14 km/s before decreasing to zero over the next 12 exposures. After that, a weak, secondary blueshifted component appeared for 5 exposures, reached a maximum upward velocity -206+/-33 km/s, and disappeared after the maximum line intensity (stationary plus blueshifted) was achieved. See Figure 2. Velocities were measured relative to the intense stationary profile observed near the flare's peak and early during its decline. The initial episode during which the entire profile was blueshifted lasted about 156 s, while the following episode during which a secondary blueshifted component was detected lasted about 56 s. See Figure 3. We attribute the first episode to chromospheric evaporation in a single loop strand, and the second to evaporation in an additional strand, as described in multi-strand flare loop models mentioned above. Line emission from progressively cooler ions (Fe XVII, XVI, and XIV) brightened at successively later times, as expected during cooling of flare-heated plasma. See Figure 4.

Figure 2: Fe XXIII line profile fits at selected times (upper left) during the flare. Frame (a) shows the profile with the largest upward relative Doppler velocity during the flare's impulsive rise; this is the 3rd EIS exposure with significant Fe XXIII emission. Frame (b) shows the profile from the 8th EIS exposure during the flare, by which time the upward velocity had decreased substantially. Note that the entire profile is blueshifted in these two frames. Frame (c) shows the profile from the 4th exposure with a significant secondary, blueshifted component; here the main component is at relative rest. Frame (d) shows a profile during the "reference" period after the flare's peak intensity; this is one of the exposures used to derive the rest wavelength and its uncertainty (1-sigma scatter, which corresponds to an uncertainty of 6.4 km/s on the reference wavelength). The extra-long vertical tic marks at the top and bottom of each frame indicate +/- 1-sigma about the reference wavelength.

Figure 3: (a) Light curve and (b) relative Doppler velocity plot for the Fe XXIII line at 263.8 A observed by EIS. Intensities displayed as vertical error bars only (with no symbols) are derived from exposures in which only one component to the emission line is observed; intensities displayed as boxes represent the sum of a main component (at rest) and a secondary blueshifted component; intensities displayed as X's correspond to exposures in which no Fe XXIII emission is detected (we fit the noise; statistical uncertainties are larger than the plotted intensities). Velocities displayed as asterisks with vertical error bars indicate exposures used to derive the reference (rest) wavelength and its 1-sigma scatter; velocities displayed as vertical error bars only (no symbols) are derived from exposures with significant Fe XXIII emission; those displayed as boxes correspond to the secondary, blueshifted component observed in five EIS exposures.

Figure 4: Light curves derived from the EIS spectra averaged over five 1-arcsec y-pixels along the slit around the spatial center of the flare. Color coded horizontal lines indicate the pre-flare intensity averaged over the first 171 exposures in the sequence (indicated with the thicker, shorter black horizontal line) plus the 1-sigma scatter over the same exposures. The flare starts in Fe XXIII emission at 18:49:48 UT (indicated with long black vertical tick marks), in Fe XVII at 18:51:40, in Fe XVI at 18:56:19, and in Fe XIV at 19:01:09 UT. S X and Si VII do not participate in the flare at the location of the EIS slit.

Brosius, J. W. 2001, ApJ, 555, 435
Brosius, J. W. 2003, ApJ, 586, 1417
Brosius, J. W. 2013, ApJ, vol. 762, in press
Brosius, J. W., & Phillips, K. J. H. 2004, ApJ, 613, 580
Doschek, G. A., & Warren, H. P. 2005, ApJ, 629, 1150
Hori, K., Yokoyama, T., Kosugi, T., & Shibata, K. 1998, ApJ, 500, 492
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Warren, H. P., & Doschek, G. A. 2005, ApJ, 618, L157

For more details, please contact: Dr. Deborah Baker.

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Last Revised: 25-Sep-2012

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