Author: Vanessa Polito at DAMTP, University of Cambridge.
Chromospheric evaporation is thought to occur as the result of the rapid heating of chromospheric plasma to coronal temperatures during flares, resulting in an overpressure and consequent expansion of the plasma along the flare loops. The details of the heating are still widely debated and we lack a definitive model which can reproduce the wide range of observations over the past decades. 1D hydrodynamics models are used to simulate the plasma response to different energy deposition mechanisms . One of predictions of these models is the presence of entirely blueshifted spectral lines during the chromospheric evaporation process, which have recently been observed with IRIS [2,3,4,5] , in contrast to some previous EIS observations reported by e.g., [6,7].
In this nugget we present the study of the X2.0 class flare which occurred on the 27 October 2014 from 14:00 UT to 16:30 UT in the AR 12192, as observed by SDO, IRIS, Hinode/EIS and RHESSI. Our aim is to compare simultaneous observations with EIS and IRIS of the Fe XXIII and
Fe XXI high-temperature emission ( >10 MK) at the flare ribbon during the chromospheric evaporation process. In addition, we compare the observations with the results of 1D hydrodynamics simulations with the HYDRAD code [8,9,10].
IRIS/EIS/AIA observations of the 27 October 2014 X2 class flare
The fields of view of the IRIS and EIS spectrometers are indicated in Fig. 1, which shows the evolution of the flare over time as observed by AIA (131 Å, Fe XXI at~10 MK), the IRIS SJI (1330 Å, showing the ribbons in CII at ~0.05 MK) and EIS (Fe XXIII at ~10 MK). The ribbons present a complex morphology and evolution, but we focus on the study of the flare loops which are clearly visible in the Fe XXIII emission (middle panel).
Blue shifts with IRIS and EIS
We measured the IRIS Fe XXI Doppler shift over time, at one slit position with a cadence of 26 s (Fig2a). The Fe XXI line profile is symmetric and completely blueshifted (up to ~200 km s−1 ) during all the observation. Almost simultaneously (within 20 s), EIS observed the same footpoint region in two consecutive rasters during the peak of the chromospheric evaporation (Fig.2b). In contrast to the Fe XXI observation, the Fe XXIII line shows asymmetric profiles with enhanced blue wings indicative of plasma upflows of up to ∼ 226 km s−1. We suggest that the EIS slit is observing a superposition of high-temperature plasma originating from distinct locations along the line of sight. For instance , Fig 2c shows an image of the IRIS detector at 14:19 UT, where we can observe distinct Fe XXI emissions at different velocities along the slit, as indicated by the horizontal arrows. These locations are separated by few arc seconds, and would not be resolved by the EIS spectrometer, whose spatial resolution is around 3”.
Plasma diagnostics with IRIS/EIS/AIA
The ratio of the IRIS O IV 1399.77 Å and 1401.16 Å lines is density sensitive and thus provides an important density diagnostic of the transition region plasma during flares, although we have identified some blendings potentially affecting the line ratio. We observe that during all the impulsive phase, the ratio is close or above the high density limit calculated in CHIANTI v7.1 [11,12] (Fig.3), indicating a ribbon density higher than 1012 cm-3. In addition, we produced emission measure (EM) curves for the EIS Fe XXIII 263.76 Å and Fe XXIV 255.11 Å spectral lines and AIA 131 Å emission observed at the loop top of the flare loops at two times ( 14:27 and 14:40 UT). The loci of these curves shows an isothermal temperature cooling from about 18.5 MK to 15.4 MK and plasma densities of ~ 1011 cm-3 (lower limit).
Comparison with the results of the HYDRAD simulationsIn order to compare the observational results against theoretical predictions, we have run 1D hydrodynamics simulations of a flare loop undergoing heating with the HYDRAD code. We adopted an electron beam heating model, whereby high energy electrons stream from the loop top to the chromosphere, depositing their energy through Coulomb collisions with the ambient plasma [13,14]. The time profile of the heating is taken as the time derivative of the GOES soft X ray light-curve, starting at 14:00 UT and lasting until the derivative becomes negative.
We compare the evolution of densities and temperatures at the apex of the loop obtained with HYDRAD with the results of the EM loci method finding good agreement. In addition, the synthesized Fe XXI and Fe XXIII Doppler shifts are compared to the observed values in Figure 4. We emphasize that the duration of the evaporation and magnitude of the flows are consistent with the observations. This is very pleasing considering that we have approximated a complex flare geometry with a simple model (a single flare loop where the energy is deposited at the same footpoint over time).
The site of high-temperature upflows during the 27 October X2 class flare seems to be resolved by IRIS, which observes totally blueshifted Fe XXI line profiles during the rising phase of the flare. The Fe XXIII line is in contrast asymmetric, which we interpret as due to the lower spatial resolution of EIS. The plasma parameters at the flare footpoints and loops obtained with AIA, EIS, and IRIS are consistent with the HYDRAD results of 1D hydrodynamics simulations of a flare loop heated by an electron beam.
This work is taken from: Polito V., Reep, J., Reeves, K., Simões, P. , Dudík, J., Del Zanna, G., Golub L., Mason, H., “Simultaneous IRIS and Hinode/EIS observation and modelling of an X-class flare“, Astrophysical Journal, in prep. (2015).
-  Emslie, G. A. & Alexander, D. 1987, Sol. Phys., 110, 295
-  Young, P. R., Tian, H., & Jaeggli, S. 2015, ApJ, 799, 218
-  Polito, V., Reeves, K. K., Del Zanna, G., Golub, L., & Mason, H. E. 2015, ApJ, 803, 84
-  Graham, D. R. & Cauzzi, G. 2015, ApJ, 807, L22
-  Tian, H., Young, P. R., Reeves, K. K., et al. 2015, ArXiv e-prints
-  Watanabe, T., Hara, H., Sterling, A. C., & Harra, L. K. 2010, ApJ, 719, 213
-  Young, P. R., Doschek, G. A., Warren, H. P., & Hara, H. 2013, ApJ, 766, 127
-  Bradshaw, S. J. & Mason, H. E. 2003, A&A, 401, 699
-  Bradshaw, S. J. & Cargill, P. J. 2013, ApJ, 770, 12
-  Reep, J. W., Bradshaw, S. J., & McAteer, R. T. J. 2013, ApJ, 778, 76
-  Dere, K. P., Landi, E., Mason, H. E., Monsignori Fossi, B. C., & Young, P. R. 1997, A&AS, 125,149
-  Del Zanna, G. 2013, A&A, 558, A73
-  Brown, J. C. 1971, Sol. Phys., 18, 489
-  Emslie, A. G. 1978, ApJ, 224, 241