Statistical Analysis of the Relationships among Coronal Holes, Corotating Interaction Regions, and Geomagnetic Storms

We have examined the relationships among coronal holes (CHs), corotating interaction regions (CIRs), and geomagnetic storms in the period 1996-2003. We have identified 123 CIRs with forward and reverse shock or wave features in ACE and Wind data and have linked them to coronal holes shown in National Solar Observatory/Kitt Peak (NSO/KP) daily He I 10 830 angstrom maps considering the Sun-Earth transit time of the solar wind with the observed wind speed. A sample of 107 CH-CIR pairs is thus identified. We have examined the magnetic polarity, location, and area of the CHs as well as their association with geomagnetic storms (Dst <= -50 nT). For all pairs, the magnetic polarity of the CHs is found to be consistent with the sunward ( or earthward) direction of the interplanetary magnetic fields (IMFs), which confirms the linkage between the CHs and the CIRs in the sample. Our statistical analysis shows that ( 1) the mean longitude of the center of CHs is about 8 degrees E, (2) 74% of the CHs are located between 30 degrees S and 30 degrees N (i. e., mostly in the equatorial regions), (3) 46% of the CIRs are associated with geomagnetic storms, ( 4) the area of geo-effective coronal holes is found to be larger than 0.12% of the solar hemisphere area, and ( 5) the maximum convective electric field E-y in the solar wind is much more highly correlated with the Dst index than any other solar or interplanetary parameter. In addition, we found that there is also a semiannual variation of CIR-associated geomagnetic storms and discovered new tendencies as follows: For negative-polarity coronal holes, the percentage (59%; 16 out of 27 events) of CIRs associated with geomagnetic storms in the first half of the year is much larger than that (25%; 6 out of 24 events) in the second half of the year and the occurrence percentage (63%; 15 out of 24 events) of CIR-associated storms in the southern hemisphere is significantly larger than that (26%; 7 out of 27 events) in the northern hemisphere. Positive-polarity coronal holes exhibit an opposite tendency.