# # Program information file # PROGRAM_ID 2024B007 PROGRAM_TITLE Is the interstellar cosmic-ray flux time variable? PROGRAM_INV1 Nick Indriolo PROGRAM_INV2 Mark Wolfire PROGRAM_INV3 Arshia Jacob PROGRAM_INV4 Paola Caselli PROGRAM_INV5 Alexei Ivlev PROGRAM_SCICAT galactic/interstellar medium PROGRAM_ABSTRACT_BEG The standard picture of cosmic-ray propagation is that particles are accelerated in discrete regions such as supernova remnants, before diffusing outward and forming a uniform background sea throughout the Galaxy. In this scenario, the cosmic-ray flux and associated ionization rate of hydrogen are expected to be constant at any location far from acceleration sites. Recent observations of a diffuse molecular cloud indicate that the abundance of H3+, a molecule that traces the cosmic-ray flux, has dropped by a factor of 2 over a timescale of 20 years. This can be interpreted as due to a decrease in the cosmic-ray flux in an interstellar cloud, something never before observed, and not expected in the standard model of Galactic cosmic-ray propagation. To confirm the decreased H3+ column density with high confidence, we propose high S/N iSHELL observations of the sight line where this effect has tentatively been seen. PROGRAM_ABSTRACT_END