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The last solar minimum activity period, and the consequent minimum modulationconditions for cosmic rays, was unusual. The highest levels of galactic protonswere recorded at Earth in late 2009 in contrast to expectations. Proton spectraobserved for 2006 to 2009 from the PAMELA cosmic ray detector on-board theResurs-DK1 satellite are presented together with the solutions of acomprehensive numerical model for the solar modulation of cosmic rays. Themodel is used to determine what mechanisms were mainly responsible for themodulation of protons during this period, and why the observed spectrum for2009 was the highest ever recorded. From mid-2006 until December 2009 we findthat the spectra became significantly softer because increasingly more lowenergy protons had reached Earth. To simulate this effect, the rigiditydependence of the diffusion coefficients had to decrease significantly below ~3GeV. The modulation minimum period of 2009 can thus be described as relativelymore "diffusion dominated" than previous solar minima. However, we illustratethat drifts still had played a significant role but that the observablemodulation effects were not as well correlated with the waviness of theheliospheric current sheet as before. Protons still experienced global gradientand curvature drifts as the heliospheric magnetic field had decreasedsignificantly until the end of 2009, in contrast to the moderate decreasesobserved during previous minimum periods. We conclude that all modulationprocesses contributed to the observed increases in the proton spectra for thisperiod, exhibiting an intriguing interplay of these major mechanisms.

Modulation of galactic protons in the heliosphere during the unusual solar minimum of 2006 to 2009