CME-induced Outflows Observed with Hinode/EIS

1. CME-induced Outflows Observed with Hinode/EIS M. Jin, C. Fang, M. D. Ding, P. F. Chen, S. Imada Department of Astronomy, Nanjing University, Nanjing, China ( fangc@nju.edu.cn ) We investigate the outflows associated with two halo CMEs occurred on 2006 December 13 and 14 in NOAA 10930, using the Hinode/EIS observations. Each CME was accompanied by an EIT wave and coronal dimming. Dopplergrams in the dimming regions are obtained from the spectra of 7 EIS lines. The results show that strong outflows are visible in the dimming regions from the lower transition region to the corona during the CME eruption. It is found that the velocity is positively correlated with the photospheric magnetic field, as well as the magnitude of the dimming. We estimate the mass loss based on height-dependent EUV dimmings and find it to be smaller than the CME mass derived from white-light observations. The mass difference is attributed partly to the uncertain atmospheric model, and partly to the chromospheric outflows, which refill the coronal dimmings. Abstract Outflows in the Dimming Regions --- Outflow Velocity, Magnetic Field and Dimming --- The 2006 December 13 Event Left: Photospheric magnetogram (solid contours) observed by SOHO/MDI and difference intensity map (dashed contours) observed by SOHO/EIT, overlaid on the Dopplergram (gray scale) of the Fe XII 195.12 Å line observed by Hinode/EIS. The dotted boxes show the 16 regions used to obtain the velocities and magnetic fields. Right: Relationship between the velocities and magnetic fields (filled circles) and that between the velocities and relative changes in intensity (open circles) for 16 small gridded areas shown in the left panel. The 2006 December 14 Event SOHO/EIT 195 Å base difference images at 03:00:01 UT on 2006 December 13 (left) and at 01:13:42 UT on 2006 December 15 (right). The dashed boxes represent the Hinode/EIS field of view in the two events. Observations and Data Analysis Faster outflows tend to be located at the regions with stronger magnetic field, where more enhanced intensity depletion appears. Quantitatively, the outflow velocities are positively correlated with the magnetic field strengths and the magnitude of intensity depletion. Estimation of the CME Mass -- Mass Loss Estimation -- Using 5 EUV emission lines with different formation temperatures, we can estimate the net mass loss in the dimming region. The total mass loss can be expressed as: where i represents the sequence of the EUV lines, S(h) and is the area of the EIS dimming region and the decrement of the density at different heights in the corona, respectively. Note that the dimming regions are observed several hours after CME eruption, therefore the mass losses may be underestimated. The emission lines included in this study Variation of and area of dimming against height. -- Chromospheric Mass Flow -- In order to estimate the mass flow during the CME eruption, we choose He II line that has the lowest formation temperature in the EIS spectrum. The mass flow is calculated by the equation: , where is the mass density at the formation height of the line, v is the time-dependent Doppler velocity of the outflows, S is the outflow area. * The lines used for mass loss estimation To compare the different results from the 7 emission lines, we first select a region with greater than 20% intensity drop and calculate the mean velocities of that region for 7 lines. We further choose the region with velocities larger than 1.5 times the mean velocities to obtain the averaged velocity values for several phases in the two events. For the 2006 December 13 Event, it shows that, at the eruption phase, all the 7 lines have significant blue shifts, though the values depend on their formation temperatures. This result has been reported by Imada et al. (2007). For the 2006 December 14 Event, the velocity was the largest at the eruption phase. At the post-eruption phase, the velocity decreased to the pre-eruption value. In this event, however, we do not see strong temperature-dependence of the velocities in the dimming region. Outflows in the Dimming Regions --- Outflow Velocity Evolution --- Variation of mass flow after the CME eruption on 2006 December 14-15. The start time is set to be the beginning of the flare impulsive phase. The dotted and dashed lines show the linear and exponential fitting of the mass flow evolution, respectively. Estimation of CME Mass Summary • During the CME eruption, strong outflows appear in the dimming regions at different heights of the solar atmosphere, and the outflow velocity decreases gradually with time. • The outflows are concentrated at the patches with strong photospheric magnetic field. The larger the magnetic field strength, the faster the outflow velocity. The outflow velocities are positively correlated with the relative changes of the intensity during the CME eruption, which suggests that the more the coronal mass is depleted, the higher the outflow velocity is. • The net mass loss in the dimming region estimated from the EUV lines is ~1015 g that is smaller than that estimated from the LASCO observations. The discrepancy would be significantly reduced if we assume that an atmospheric model is several times denser than the quiet-Sun model, the integration of the dimming volume extends down to the chromosphere, and if the earlier observation after CME eruption is available. • There is significant mass supply from the chromosphere to refill the coronal dimmings, which finally helps the coronal dimmings to recover to normal. We propose that this is mainly due to the dynamic response of the chromosphere to the enhanced pressure gradient as the CME eruption evacuates the plasma in the low corona. Conclusions The 2006 December 13 Event The 2006 December 14 Event Doppler velocities in dependence of line formation temperatures observed by Hinode/EIS. The observation times represent different eruption phases.