University of Maryland
Atlantic Building, Room 2400 4:30 PM Monday, October 2, 2006
Coffee, Tea & Snacks 4:15-4:30 PM

Ken McCracken
IPST, University of Maryland

Long-Term Changes in the Cosmic Radiation and the Heliomagnetic Field; 1428-200

The instrumental and cosmogenic records are combined to yield an inter-calibrated cosmic ray record for the period 1428-2005. This shows that the 11/22 year modulating effects of the heliosphere have changed greatly over the past 570 years. In particular, the cosmic ray intensity at sunspot minimum has declined in a step-wise manner from near the local interstellar value in the 15th century, to reach the low level evident since the sunspot minimum of 1954. Using the inversion methodology of Caballero-Lopez et al (2004), the inter-calibrated cosmic ray data have been inverted to provide an estimate of the annual average strength of the heliomagnetic field near Earth since 1428. The sunspot minimum value was circa 0.5nT during the Spoerer Minimum (1420-1540), rising to 1.5nT prior to the Maunder minimum (1645-1715); to circa 3.0 nT between the Maunder and Dalton (1800-1820) minima; and continuing to increase thereafter until it reached the current value of 5.2nT in 1954. It is speculated that this monotonic increase in the heliomagnetic field is the cause of the previously recognized 2300 year periodicity in the cosmic radiation at Earth. It is shown that there is a substantial inverse correlation between solar energetic particle (SEP) events, and the estimated value of the heliomagnetic field. A mechanism is advanced to explain this anti-correlation in terms of changes in Alfven velocity due to an increasing magnetic field strength in the vicinity of the Sun. The “solar weather” implications of the long- and medium term changes in the galactic and solar cosmic radiation will be outlined.