Science Objectives for an X-Ray Microcalorimeter Observing the Sun

We present the science case for a broadband X-ray imager with high-resolution spectroscopy, including simulations of X-ray spectral diagnostics of both active regions and solar flares. This is part of a trilogy of white papers discussing science, instrument (Bandler et al. 2010), and missions (Bookbinder et al. 2010) to exploit major advances recently made in transition-edge sensor (TES) detector technology that enable resolution better than 2 eV in an array that can handle high count rates. Combined with a modest X-ray mirror, this instrument would combine arcsecondscale imaging with high-resolution spectra over a field of view sufficiently large for the study of active regions and flares, enabling a wide range of studies such as the detection of microheating in active regions, ion-resolved velocity flows, and the presence of non-thermal electrons in hot plasmas. It would also enable more direct comparisons between solar and stellar soft X-ray spectra, a waveband in which (unusually) we currently have much better stellar data than we do of the Sun. 1. The Promise of X-ray microcalorimeTers in solar Physics Coronal heating has been the central problem of the solar outer atmosphere for over half a century (e.g. Klimchuk 2006). The advent of high quality imaging data in the last 15 years (SOHO, TRACE, STEREO, Hinode, and now SDO) has forced the realization that the solar atmosphere is significantly more dynamic and turbulent than had been previously appreciated. Since much of this behavior occurs on few seconds to minutes timescales, comparable to the integration time to record a spectrum or to scan a raster to make an image with grating instruments, what is ideally needed is an instrument capable of imaging spectroscopy, with high throughput, and high spatial and spectral resolution. In this respect, recent advances in microcalorimeter arrays for solar X-ray observations have the potential to revolutionize our understanding of coronal plasmas, specifically with regard to the non-equilibrium plasma dynamics of solar flares and active regions. The microcalorimeter is well-matched to the parameters of interest for detailed studies of the fundamental plasma physics in the Sun’s atmosphere, with excellent spectral (<2 eV), spatial (~arcsec) and temporal (millisecond) scales and broad spectral coverage from 0.2 to 10 keV. Such an instrument will be able to: 1. Detect and resolve hundreds of atomic transition lines from different ionization states of many elements in the solar atmosphere 1Naval Research Laboratory, 2Smithsonian Astrophysical Observatory, 3NASA Goddard Space Flight Center, 4Mullard Space Science Laboratory, UCL, 5Space Research Centre, Polish Academy of Sciences, 6University of Michigan, 7George Mason University, 8Rice University, 9University of Memphis, 10University of Palermo, 11Columbia University, 12Lockheed Martin, 13DAMTP, University of Cambridge,14Montana State University, 15STFC Rutherford Appleton Laboratory,16National Solar Observatory, 17Space Telescope Science Institute