Constraining Very Hot Active Region Plasma with the Multi-Order X-ray Spectral Imager

Models of impulsive heating in non-flaring active region loops have long predicted the presence of faint, “very hot” (>4 MK) plasma, the so-called “smoking gun” of nanoflare heating. Unambiguous observations of such plasma have remained elusive, due in part to the sparsity of observations of high-temperature spectral lines, particularly in the soft X-ray range. The Multi-Order X-ray Spectral Imager (MOXSI) aboard the CubeSat Imaging X-Ray Solar Spectrometer (CubIXSS), to be launched in 2025, is a slitless imaging spectrograph that will observe the entire solar disk over the spectral range 1-60 Å. Because this range contains a number of key high-temperature diagnostic lines, MOXSI is well-positioned to place unprecedented constraints on this faint, hot plasma. In this talk, we present a series of full active region simulations composed of many hundreds of impulsively heated loops with varying amounts of very hot plasma as produced by low-frequency impulsive heating. From these simulations, we forward model a series of MOXSI images, including non-equilibrium ionization as well as the effect of the dispersive grating, the point spread function, and other instrumental effects. We demonstrate that MOXSI will be able to place tighter constraints on this faint hot plasma than current observations allow, due primarily to its novel spectral range and slitless design.