Unusual time histories of galactic and anomalous cosmic rays at 1 AU over the deep solar minimum of cycle 23/24

Abstract

The unusually quiet Sun of the cycle 23/24 solar minimum (that ended in December, 2009) has resulted in lower values of the interplanetary magnetic field and a slower approach of the tilt angle of the heliospheric current sheet toward the solar equator than has been observed for recent solar minima. As a result of these changes, the time‐histories of galactic and anomalous cosmic rays over this period are very different from those of recent minima at the same phase of the heliomagnetic cycle. Since ∼2005.6 there has been an on‐going increase in cosmic‐ray intensity (except for one brief transient decrease) that lasted for 4.4 years. The relative rigidity dependences of these increases compared to previous cycles are complex and should provide insight into the role of various solar and interplanetary phenomena in the modulation process. The largest increase occurs in the nominal “cross‐over energy” region (where the modulation is essentially the same for each minimum of the two past 22 year heliomagnetic cycles) which extends from ∼200 MeV/n to >500 MeV/n.