  The nuclear reaction network within the interior of the Sun is an efficient
MeV physics factory, and can produce long-lived particles generic to dark
sector models. In this work we consider the sensitivity of satellite
instruments, primarily the RHESSI Spectrometer, that observe the Quiet Sun in
the MeV regime where backgrounds are low. We find that Quiet Sun observations
offer a powerful and complementary probe in regions of parameter space where
the long-lived particle decay length is longer than the radius of the Sun, and
shorter than the distance between the Sun and Earth. We comment on connections
to recent model-building work on heavy neutral leptons coupled to neutrinos and
high-quality axions from mirror symmetries.
