Main Objective: We seek to obtain EIS and IRIS flare stare spectra within the same active region as that from which SPICE flare stare spectra will be obtained.
Scientific Justification:
Accelerated protons may make up half the energy budget of solar flares, but evidence for their presence is very difficult to observe. The SPICE observations for the upcoming Solar Flare SOOP are focused on detecting accelerated flare protons via the Orrall-Zirker effect (Orrall & Zirker1976). This mechanism posits that flare accelerated protons stream through the solar atmosphere and, through charge exchange, collisional excitation, and radiative de-excitation, produce very red-shifted photons that may be detected in the wings of H Ly α and H Ly β. Our simulations indicate this highly red-shifted radiation may be detectable with SolO/SPICE observations of solar flare ribbons (Kerr et al. 2023, in preparation). The only previous detection of such emission was reported by Woodgate et al. (1992) based on GHRS observations of a flare on the M dwarf star AU Mic. Our SPICE program is focused on sit-and-stare observations to detect red wing enhancements of Ly β without corresponding blue wing enhancements, plus other lines of interest for flare modeling.
We seek to obtain EIS and IRIS flare stare spectra within the same active region as that from which SPICE flare stare spectra will be obtained. Given the uncertainties on the pointings of spectrometer slits (e.g., ~15 arcsec for EIS), the probability of precisely coaligning the three slits of SPICE, EIS, and IRIS in stare mode is negligible. Although raster scans with EIS and IRIS would be useful, we prefer to capitalize on the rapid cadence stare capabilities of these instruments, and rely upon coordinated observations with imagers to provide context images. With this in mind, the slit pointings of SPICE, EIS, and IRIS would be deliberately offset from each other. In this way we can investigate the observed feature’s (ideally a flare ribbon) behavior simultaneously at three different locations. Although the EIS and IRIS stare spectra will not provide direct evidence of nonthermal proton beams, they will each provide the time evolution of the flaring solar atmosphere over nearly three orders of magnitude in temperature at their respective pointing positions. This will tell us, at both pointing positions, the onset time of a flare, whether chromospheric evaporation proceeds explosively or gently based on line-of-sight Doppler velocities as functions of temperature, and whether nonthermal electrons are accelerated continuously or quasi-periodically (in bursts). Density sensitive line intensity ratios will also be used to determine the time evolution of flare plasma density at the formation temperatures of density sensitive line pairs, at their separate locations. Coordinated, deliberately offset stare spectra have been obtained with EIS and IRIS in the past, and provided valuable diagnostics of the flaring solar atmosphere at the two different locations in a flare ribbon (e.g., Brosius, Daw, & Inglis 2016). |
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