HOP 0283

abstract of observational proposal

The proposed Hinode observations are in support of NuSTAR investigations of hot plasma and accelerated electrons in the high corona in partly occulted flares. Hard Xray observations of electron acceleration sites in the corona can investigate the mechanisms that accelerate particles up to the extraordinarily high energies observed. Previous instruments such as the RHESSI spacecraft have rarely been able to image faint sources in the corona due to insufficient sensitivity and constraints in imaging dynamic range caused by indirect imaging. The improved sensitivity of NuSTAR will allow the systematic observation of coronal acceleration sites, but due to the instrument’s limited count rate, this investigation will best be performed in occulted flares. In addition to coronal electron acceleration sites, other observations may include pseudostreamers and the study of escaping electron beams responsible for Type III radio emission.

other participating instruments

The Nuclear Spectroscopic Array (NuSTAR) is a soft/hard X-ray instrument producing direct focused images and spectra of faint astrophysical targets from 2 to 80 keV. While not a heliophysics mission, NuSTAR occasionally observes the Sun as a target of opportunity.

remarks

Timing/duration:
NuSTAR quiet-Sun solar pointings are called as ToO’s on a 3-4 day time scale. Solar observations are 1-4 orbits (~1 to 6 hours).

Targetting:
NuSTAR has a field of view of ~12x12 arcmin. Because it was not designed/optimized for bright solar sources, NuSTAR has a limited count rate of 800 counts per second over the entire energy range. Therefore, the best opportunities to take advantage of NuSTAR’s high sensitivity and straightforward imaging come during times of low solar activity. While medium/large flares (GOES class C and higher) are probably too bright, flares that are partly occulted (i.e the base of the flare is obscured by the solar disk) offer the opportunity to observe faint sources of accelerated electrons high in the corona without the high deadtime that would be induced by the brightest part of the flare. The ideal target for this coordinated study is a region just above the solar limb at a time when a frequently-flaring active region has recently rotated off of the visible solar disk (giving a high probability of occulted flares), with its base occulted for at least one day (to avoid high deadtime due to the bright, hot thermal active-region plasma.) For studies of pseudostreamers, the target may be different.