While X-ray and EUV imagers take images at a cadence close to the characteristic evolutionary timescales of solar phenomena, narrow slit spectroscopy is better suited for many plasma diagnostics. This improvement in our ability to measure the properties of solar plasmas, however, is often achieved at the expense of longer exposure times and a slower cadence. Wide slit spectroscopy ("overlapograms") is a compromise between both worlds as it produces images of over a relatively large field-of-view in bands as narrow as a single spectral line.

The EUV Imaging Spectrometer (EIS) has a 40" wide slit that can take 40"x512" images at a 0.9 Å spectral resolution. Solar spectral lines are about 0.3 Å wide so that a wide slit (slot) image of an isolated emission line is monochromatic. Completely isolated lines in the EUV spectrum are rare, but there are a few lines that are sufficiently dominant contributors to their spectral range as to consider them isolated in practical terms. Some of these lines are: Si VII 275.37 Å (T=0.6 MK), Fe XI 180.40 Å, Fe XII 195.12 Å (T=1.3 MK), Fe XIII 202.04 Å (T=1.6 MK), Fe XV 184.16 Å (T=2 MK), Fe XVI 262.98 Å (T=2.5 MK).



By executing scans at a number of positions on the Sun, EIS is able to build up images of the entire solar disk. The first dataset of this kind was taken June 27, 2009. Clean monochromatic images of the full disk were taken in nine spectral lines, spanning a range of temperatures from 0.05 to 2.5 MK. These full disk mosaics were developed in anticipation of NASA's Solar Dynamic Observatory (SDO) mission. Since then these full disk mosaics have been performed about every three weeks as part of the Hinode Operational Plan: HOP130. The main objectives of the HOP are two-fold: monitoring the EIS calibration in direct comparison with EVE on SDO and monitoring the solar cycle dependent EUV radiance at multiple temperatures. EVE, the Extreme-ultraviolet Variability Experiment, measures the EUV irradiance in the 1-1050 Å range at ~1 Å resolution. The spectral overlap makes it an ideal benchmark to monitor the EIS calibration over time. So far, only a few datapoints are available, but the preliminary comparison suggests that the calibration of both instruments agrees within 20% for several strong and isolated lines: Fe XI 180.40 Å, Fe XII 195.12 Å, Fe XVI 262.98 Å.



Imagers like AIA/SDO, EUVI/STEREO, EIT/SOHO already take full Sun images in a fairly narrow spectral window at a much faster cadence. Do then these EIS observations have anything unique that makes them a good complement to that suite of state-of-the-art imaging instruments? Yes, the spectral purity, which gives EIS a greater range of plasma diagnostics. In addition to the intensities, which can be used to derive emission measures and temperatures, the ratio of Fe XIII 203.8 Å to Fe XIII 202.04 Å can be used to derive electron densities from the wide slit data. The first line is blended with a Fe XII line, but for typical densities (log N ~ 9.0 - 10.0) the contribution of the blend is less than 10% in active region and 15% in the quiet Sun (Ugarte-Urra & Warren 2011).



From the ratio map and the density dependence of the ratio, calculated with CHIANTI, we can produce a map of the density in the upper solar atmosphere. To the best of our knowledge these are the first full Sun density maps produced. The map below shows at once several well known features: high densities at the core of active regions, low densities at the poles and small density concentrations spread rather uniformly across the disc, coronal bright points.



This analysis is not restricted to full Sun images. In a recent paper, Ugarte-Urra & Warren (2011) show examples of the emission measure and density analysis on loops and jets as function of time. The Doppler information is mostly lost in the wide slit data due to the overlap of spatial and spectral information on the detector, however, it is possible to retrieve it under certain conditions (Innes et al. 2008) or assumptions (Sun et al., in preparation).

For more details, please contact: Dr. Lucie Green.