LRO CRaTER Observations of the Solar Modulation Potential at 1AU

In this work, we present the solar modulation potential of galactic cosmic radiation (GCR) observed by the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument. CRaTER is one of seven science instruments flying aboard the Lunar Reconnaissance Orbiter (LRO) mission, which launched in June 2009. The telescope utilizes a stack of six silicon detectors arranged in three pairs of thin/thick detectors separated by two masses of tissue equivalent plastic. This configuration enables CRaTER to measure how the linear energy transfer (LET) spectrum of the primary GCR spectrum changes as it moves through the human body. Furthermore, the effects of varying solar modulation potential on the incident GCR spectrum are manifested as long-term trends in these data. This study is an effort to obtain solar modulation potential at 1 AU from observations made by the CRaTER instrument over the current duration of the LRO mission, which is now approaching one complete solar cycle.

Through a combination of radiation transport modeling and a prescribed relationship of how solar activity influences the primary GCR spectrum in energy, composition, and magnitude, we construct an expected response function for the CRaTER instrument. This expected response function is then used to extract the timeseries behavior of solar modulation potential from observations. The MCNP6 Monte-Carlo code is used to perform the necessary radiation transport calculations. Two independent prescriptions for GCR spectrum behavior as a function of solar activity are used: the Badhwar-O'Neill 2014 model, and the Matthia model. The results of using each method are compared.