The EUV Snapshot Imaging Spectrograph II (ESIS)

Magnetic reconnection is expected to play a major role in the mass and energy balance in the solar corona. One byproduct of magnetic reconnection is the acceleration of high energy electrons, frequently called electron beams, that flow down magnetic field lines and impact the transition region and chromosphere, heating and ablating plasma to form hot coronal plasma (Emslie, 1978). Recently, significant evidence of electron beams has been discovered in transition region and chromospheric images and spectra at the footpoints of high temperature loops in solar active regions(Testa et al., 2013, 2014, 2023). Beam heating predicts the transition region will significantly brighten (by up to two orders of magnitude) and exhibit blue shifts on short (< 20 s) time scales. These observations have been used to limit the time scale, location, and magnitude of coronal reconnection. The importance of heating by magnetic reconnection can be determined by measuring both short term evolution of the intensity and velocity in a transition region spectral line over a large field of view (FOV) at a rapid cadence. A major limitation of instruments designed to measure velocity in transition region spectral lines is the limited FOV and cadence afforded by typical slit spectrographs, such as the Interface Region Imaging Spectrograph (IRIS). An alternative to slit spectrographs are computed tomography imaging spectrographs (CTIS), where velocity information can be determined over a wide FOV in a single exposure. A CTIS is a combination of multiple imaging spectrographs, or channels, where each channel captures the same FOV and wavelength range, but with different dispersion angles. An example of a CTIS is the Extreme ultraviolet Snapshot Imaging Spectrometer (ESIS) instrument built and launched on a sounding rocket on September 30th, 2019. (Hereafter, the first instrument and flight of ESIS will be referred to as ESIS-1.) ESIS-1 flew with four channels with dispersion directions separated by 45 degrees, all observing quiet Sun in the O v 630 Å spectral line (Parker et al., 2022). Data processing and/or inversion techniques can be required to recover the velocity information from the CTIS data. Analysis of the ESIS-1 data set have successfully recovered velocity information of small scale explosive events (Parker et al., 2022). Explosive events provide multiple methods of validating the recovered velocity information. ESIS-1 also observed an extended erupting source. Though the ESIS-1 data can be analyzed to find the velocity profiles of the extended source validation of the results are difficult. The scientific goal of this proposed mission is to determine the role of magnetic reconnection in heating the solar corona. Specifically, we will 1) determine the frequency and spatial location of heating by electron beams and 2) determine the heating parameters for all beam heating events. The technology goal of this mission is to validate inversion techniques. Specifically, we will validate the inversion of two-dimensional extended structures observed with CTIS, including steady structures such as coronal loops. We propose to address the science and technology objectives by modifying and reflying the ESIS instrument, hereafter referred to as ESIS-2. Unlike ESIS-1 which observed the quiet Sun, ESIS-2 will target an active region observed with the Ne vii and Si xii spectral lines, which are formed at approximately 0.5 MK and 3 MK respectively. The upper transition region emission in the Ne vii observations will be used to address the science objectives, while the extended structures observed in the Si xii emission will be used to address the technology objective. ESIS-2 will be built around the same optical design as ESIS-1, but refitted with new optical components to change the observing passband. We will add two additional channels to perform validation of the inversion of extended structures to achieve the technology objective.

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