Over the last decade, the solar wind has exhibited low densities and magnetic field strengths, representing anomalous states that have never been observed during the space age. As discussed by Schwadron, Blake, et al. (2014, https://doi.org/10.1002/2014SW001084), the cycle 23–24 solar activity led to the longest solar minimum in more than 80 years and continued into the “mini” solar maximum of cycle 24. During this weak activity, we observed galactic cosmic ray fluxes that exceeded theERobserved small solar energetic particle events. Here we provide an update to the Schwadron, Blake, et al. (2014, https://doi.org/10.1002/2014SW001084) observations from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter. The Schwadron, Blake, et al. (2014, https://doi.org/10.1002/2014SW001084) study examined the evolution of the interplanetary magnetic field and utilized a previously published study by Goelzer et al. (2013, https://doi.org/10.1002/2013JA019404) projecting out the interplanetary magnetic field strength based on the evolution of sunspots as a proxy for the rate that the Sun releases coronal mass ejections. This led to a projection of dose rates from galactic cosmic rays on the lunar surface, which suggested a ∼20% increase of dose rates from one solar minimum to the next and indicated that the radiation environment in space may be a worsening factor important for consideration in future planning of human space exploration. We compare the predictions of Schwadron, Blake, et al. (2014, https://doi.org/10.1002/2014SW001084) with the actual dose rates observed by CRaTER in the last 4 years. The observed dose rates exceed the predictions by ∼10%, showing that the radiation environment is worsening more rapidly than previously estimated. Much of this increase is attributable to relatively low‐energy ions, which can be effectively shielded. Despite the continued paucity of solar activity, one of the hardest solar events in almost a decade occurred in September 2017 after more than a year of all‐clear periods. These particle radiation conditions present important issues that must be carefully studied and accounted for in the planning and design of future missions (to the Moon, Mars, asteroids, and beyond).

Recent analyses of ice core samples indicate that the Carrington flare of 1859 was the largest event observed in the past 500 years. These ice core data yield estimates of the proton fluence for energies greater than 30 MeV, but provide no other spectrum information. Assuming that the proton energy distribution for such an event is similar to that measured for other recent, large events, total ionizing doses in deep space are estimated for these hypothetical worst-case spectra. These estimated doses, as large as 50 krad (Si), could be catastrophic for sensitive electronic devices unless substantial shielding is provided.

1] Galactic cosmic rays (GCRs) are extremely difficult to shield against and pose one of the most severe long-term hazards for human exploration of space. The recent solar minimum between solar cycles 23 and 24 shows a prolonged period of reduced solar activity and low interplanetary magnetic field strengths. As a result, the modulation of GCRs is very weak, and the fluxes of GCRs are near their highest levels in the last 25 years in the fall of 2009. Here we explore the dose rates of GCRs in the current prolonged solar minimum and make predictions for the Lunar Reconnaissance Orbiter (LRO) Cosmic Ray Telescope for the Effects of Radiation (CRaTER), which is now measuring GCRs in the lunar environment. Our results confirm the weak modulation of GCRs leading to the largest dose rates seen in the last 25 years over a prolonged period of little solar activity. Citation: Schwadron, N. A., A. J. Boyd, K. Kozarev, M. Golightly, H. Spence, L. W. Townsend, and M. Owens (2010), Galactic cosmic ray radiation hazard in the unusual extended solar minimum between solar cycles 23 and 24, Space Weather, 8, S00E04,

PREDICCS (Predictions of Radiation from Release, EMMREM, and Data Incorporating the CRaTER, COSTEP and other SEP measurements, http://prediccs.sr.unh.edu) is an online system designed to provide a near real‐time characterization of the radiation environment of the inner heliosphere. PREDICCS utilizes data from various satellites in conjunction with numerical models such as the Earth‐Moon‐Mars Radiation Environment Module (EMMREM) to produce dose rate and particle flux data at the Earth, Moon and Mars. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument launched aboard the Lunar Reconnaissance Orbiter (LRO) spacecraft in 2009 and designed to measure energetic particle radiation, offers an opportunity to test the capability of PREDICCS to accurately describe the lunar radiation environment. We provide comparisons between dose rates produced by PREDICCS with those measured by CRaTER during three major solar energetic particle (SEP) events that occurred in 2012. In addition, using EMMREM data products together with our archive of measured CRaTER dose rates, we compute the modulation potential at the Moon throughout the LRO mission and, using this, compute the background GCR dose rate during each event. We demonstrate reasonable agreement between PREDICCS and CRaTER dose rates and come to the conclusion that PREDICCS provides credible characterization of the lunar radiation environment. This study represents the first multi‐event validation, via in situ measurement, of radiation models such as EMMREM, which should prove to be valuable in future efforts in risk assessment and in the study of radiation in the inner heliosphere.

1] The Earth-Moon-Mars Radiation Environment Module (EMMREM) is a comprehensive numerical framework for characterizing and predicting the radiation environment of the inner heliosphere. We present a study of the October/November 2003 Halloween solar energetic particle events with an energetic particle acceleration and propagation model that is part of EMMREM, highlighting the current ability of the framework to make predictions at various locations of the inner heliosphere. We compare model predictions with Ulysses observations of protons at energies above 10 MeV in order to obtain realistic proton fluxes and calculate radial gradients for peak fluxes, event fluences, and radiation dosimetric quantities. From our study, we find that a power law with an index of −3.55 at energy of 200 MeV describes the time-integrated energetic proton fluence dependence on radial distances beyond 1 AU for the 2003 Halloween events, and an index of −4.18 is appropriate for peak proton fluxes at that energy. Calculations of radiation doses based on these simulations show average power law indices of −4.32 and −3.64 for peak dose rates and accumulated doses, respectively. In an effort to improve the predictions, we have coupled our kinetic code to results from a 3-D heliospheric magnetohydrodynamic model, WSA/Enlil. While predictions with the coupled model overall show worse agreement than simulations with steady state solar wind conditions for these large events, the capability to couple energetic particle propagation and numerical models of the solar wind is an important step in the future development of space weather modeling.