Spatially-Resolved Nonthermal Line Broadening During The Impulsive Phase of a Solar Flare

This paper presents a detailed study of excess line broadening in EUV emission lines during the impulsive phase of a C-class solar flare. In this work, which utilizes data from the EUV Imaging Spectrometer (EIS) onboard Hinode, the broadened line profiles were observed to be co-spatial with the two HXR footpoints as observed by RHESSI. By plotting the derived nonthermal velocity for each pixel within the Fe XV and Fe XVI rasters against its corresponding Doppler velocity a strong correlation (|r| > 0.59) was found between the two parameters for one of the footpoints. This suggested that the excess broadening at these temperatures is due to a superposition of flows (turbulence), presumably as a result of chromospheric evaporation due to nonthermal electrons. Also presented are diagnostics of electron densities using five pairs of density-sensitive line ratios. Density maps derived using the Mg XII and Si X line pairs showed no appreciable increase in electron density at the footpoints, while the Fe XII, Fe XIII, and Fe XIV line pairs revealed densities approaching 10^(11.5) cm^(-3). Using this information, the nonthermal velocities derived from the widths of the two Fe XIV lines were plotted against their corresponding density values derived from their ratio. This showed that pixels with large nonthermal velocities were associated with pixels of moderately higher densities. This suggests that nonthermal broadening at these temperatures may have been due to enhanced densities at the footpoints, although estimates of the amount of opacity broadening and pressure broadening appeared to be negligible.Comment: 11 pages, 10 figures. Accepted to Ap

Establishing a Connection Between Active Region Outflows and the Solar Wind: Abundance Measurements with EIS/Hinode

One of the most interesting discoveries of the X-ray Telescope and EUV Imaging Spectrometer (EIS) on board the Hinode solar observatory is the presence of persistent high temperature high speed outflows from the edges of active regions. Measurements by EIS indicate that the outflows reach velocities of 50 km/s with spectral line asymmetries approaching 200 km/s. It has been suggested that these outflows may lie on open field lines that connect to the heliosphere, and that they could potentially be a significant source of the slow speed solar wind. A direct link has been difficult to establish, however. In this letter, we use EIS measurements of spectral line intensities that are sensitive to changes in the relative abundance of Si and S as a result of the first ionization potential (FIP) effect, to measure the chemical composition in the outflow regions of AR 10978 over a period of 5 days in December 2007. We find that Si is always enhanced over S by a factor of 3--4. This is generally consistent with the enhancement factor of low FIP elements measured in-situ in the slow solar wind by non-spectroscopic methods. Plasma with a slow wind-like composition was therefore flowing from the edge of the active region for at least 5 days. Furthermore, on December 10--11, when the outflow from the western side was favorably oriented in the Earth direction, the Si/S ratio was found to match the value measured a few days later by ACE/SWICS. These results provide strong observational evidence for a direct connection between the solar wind, and the coronal plasma in the outflow regions.Comment: Version to be published in ApJ

A Hot Microflare Observed With RHESSI and Hinode

RHESSI and Hinode observations of a GOES B-class flare are combined to investigate the origin of 15 MK plasma. The absence of any detectable hard X-ray emission coupled with weak blueshifted emission lines (indicating upward velocities averaging only 14 km/s) suggests that this was a result of direct heating in the corona, as opposed to nonthermal electron precipitation causing chromospheric evaporation. These findings are in agreement with a recent hydrodynamical simulation of microflare plasmas which found that higher temperatures can be attained when less energy is used to accelerate electrons out of the thermal distribution. In addition, unusual redshifts in the 2 MK Fe XV line (indicating downward velocities of 14 km/s) were observed cospatial with one of the flare ribbons during the event. Downflows of such high temperature plasma are not predicted by any common flare model.Comment: 6 pages, 4 figures, ApJL (Accepted

Evidence of Explosive Evaporation in a Microflare Observed by Hinode/EIS

We present a detailed study of explosive chromospheric evaporation during a microflare which occurred on 2007 December 7 as observed with the EUV Imaging Spectrometer (EIS) onboard Hinode. We find temperature-dependent upflows for lines formed from 1.0 to 2.5 MK and downflows for lines formed from 0.05 to 0.63 MK in the impulsive phase of the flare. Both the line intensity and the nonthermal line width appear enhanced in most of the lines and are temporally correlated with the time when significant evaporation was observed. Our results are consistent with the numerical simulations of flare models, which take into account a strong nonthermal electron beam in producing the explosive chromospheric evaporation. The explosive evaporation observed in this microflare implies that the same dynamic processes may exist in events with very different magnitudes.Comment: 14 pages, 8 figures. Accepted for publication in the Astrophysical Journa

The Temperature Dependence of Solar Active Region Outflows

Spectroscopic observations with the EUV Imaging Spectrometer (EIS) on Hinode have revealed large areas of high speed outflows at the periphery of many solar active regions. These outflows are of interest because they may connect to the heliosphere and contribute to the solar wind. In this Letter we use slit rasters from EIS in combination with narrow band slot imaging to study the temperature dependence of an active region outflow and show that it is more complicated than previously thought. Outflows are observed primarily in emission lines from Fe XI - Fe XV. Observations at lower temperatures (Si VII), in contrast, show bright fan-like structures that are dominated by downflows. The morphology of the outflows is also different than that of the fans. This suggests that the fan loops, which often show apparent outflows in imaging data, are contained on closed field lines and are not directly related to the active region outflows.Comment: Movies are available online at: http://tcrb.nrl.navy.mil/~hwarren/temp/papers/flow_temperatures/ To be submitted to ApJ