Galactic Cosmic Ray Dose Rate at 1 A.U. During Solar Activity Cycle 24 Minimum

Measurements made by the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument aboard the Lunar Reconnaissance Orbiter (LRO) have been used to create a time history of the absorbed dose rate resulting from galactic cosmic rays (GCR) exposure at a distance from the Sun of 1 astronomical unit (A.U.). These measurements, made during the period 2009-06-26 to 2010-12-31, include the deepest point of solar minimum activity (2009 Oct), maximum GCR dose rate (2010 Jan), and the onset of the first solar energetic particle (SEP) events of solar activity cycle 24 (2010 Jun). General features of the dose rate time history include: (a) an approximately 90-day lag between an increase in the coronal mass ejection (CME) rate/IMF field strength and the modulation of the daily GCR dose rate; a 21% decrease in the average daily GCR dose rate 15 months after solar activity minimum; and the detection of a cluster of 5 particle enhancements between 2010-06-12 and 2010-08-20 at the location of the LRO orbiter. Three additional enhancements measured between 2010-09-01 and 2010-09-23 do not appear to be linked with any observed solar activity, although two of the enhancements occurred during passage of coronal high-speed solar wind streams at the location of Earth.