1. Observed 27-Day Variations in Cosmic Ray Intensities During the Cycle 23/24 Solar Minimum
Rick Leske, A. C. Cummings,
R. A. Mewaldt, and E. C. Stone
California Institute of Technology, Pasadena, CA USA
T. T. von Rosenvinge
NASA/Goddard Space Flight Center, Greenbelt, MD USA
M. E. Wiedenbeck
Jet Propulsion Laboratory, Caltech, Pasadena, CA USA
32nd International Cosmic Ray Conference
Beijing, China
17 August 2011

2. From mid-2007 to mid-2008, ~27-day variations were clearly present in GCRs (at neutron monitor energies and at ACE/CRIS) and in ACRs (at ACE/SIS), associated with a series of high-speed solar wind streams. Although the heliospheric current sheet (HCS) was crossed twice per rotation, cosmic ray peaks occurred only at the south-to-north (outward-to-inward polarity) crossings.

3. We looked for correlations between the neutron monitor rate and: solar wind speed, solar wind density, solar wind dynamic pressure, magnetic field strength, and rms variation of the magnetic field, and find the highest (inverse) correlation with the solar wind speed:

4. Both ACRs and GCRs are clearly well anti-correlated with the solar wind speed.

5. Binning the ACR and GCR data in 50 km/s-wide solar wind speed bins, we find a good power-law relationship, with a larger variability for ACRs than for GCRs.

6. The amplitude of the GCR variations (or their index with respect to Vsw) does not show much dependence on species or rigidity within the CRIS energy interval. The average index of / corresponds to a peak-to-valley intensity variation of 5.3 +/- 0.4% for a factor of 2 change in the solar wind speed.

7. Energy spectra at low solar wind speeds (<v>=316 km/s) and high speeds (<v>=613 km/s) illustrate the differences in the spectra at the intensity peaks and valleys. GCR intensities were enhanced at low speeds by ~5%, while the lowest energy ACRs were enhanced ~50-60%, with a strong energy dependence.

8. von Rosenvinge & Paizis 17th ICRC 10, 69 (1981) and Paizis et al
von Rosenvinge & Paizis 17th ICRC 10, 69 (1981) and Paizis et al. JGR ,241 (1999) describe the amplitude variation using force-field modulation: Δc/c=-3CΔφ/βK(P), where c is the counting rate, φ the modulation parameter, β the particle speed, K the diffusion coefficient as a function of rigidity P (taken as P0.5), and C the Compton-Getting factor, which for power law spectra in E with index γ is: C=(2-αγ)/3, where α=(E+2E0)/(E+E0), with E0 the rest mass. Using this formula results in the dashed curve below. Simply assuming an energy-independent amplitude of 55% for ACR O and 5% for GCR O and mixing by their relative abundances gives the solid curve, which is virtually identical.

9. Reames & Ng found cosmic ray intensity peaks at north-to-south (outward-to-inward polarity) HCS crossings during A>0 periods. We also find peaks at outward-to-inward polarity crossings in the A<0 minimum, but this now corresponds to south-to-north crossings.
GSE !
Reames & Ng, ApJL 563, L179 (2001)

10. from Wilcox Solar Observatory
Although there may be 2 HCS crossings per rotation, they are often not equivalent, with one being a rapid crossing and the other nearly skimming along the sheet. We find peaks during the flatter, more gradual crossings, which makes sense since intensities should peak at the HCS during A<0. However, Reames & Ng found peaks at the rapid crossings in A>0, for reasons that are not clear.
1996 A>0
2008 A<0
from Wilcox Solar Observatory

11. Richardson, Cane, & Wibberenz, JGR 104, 12,549 (1999)
Richardson et al. found that 27-day amplitudes are larger in A>0 cycles than in A<0. We find amplitudes in the A<0 period similar to those at the A>0 minimum. But was not solar minimum; by the true minimum the 27-day variations had vanished.
Richardson, Cane, & Wibberenz, JGR 104, 12,549 (1999)

12. Summary
27-day variations in ACRs and GCRs in mid-2007 to mid-2008 were:
anti-correlated with solar wind speed (as a power law),
with larger amplitudes for ACRs than GCRs, and
little to no energy dependence in CRIS GCRs but large energy dependence for ACRs.
Relative amplitudes of the ACRs and GCRs agree well with expectations from simple force-field modulation.
Peaks are seen only at every second sector crossing:
At south-to-north (outward-to-inward polarity) crossings; north-to-south (also outward-to-inward polarity) crossings had peaks at each of the last two A>0 minima.
But the sector crossings are asymmetric at this time; “flat” crossings skimming the HCS are favored during this A<0 period.
Although amplitudes should (empirically, based on past cycles) be smaller at an A<0 minimum, in mid-2007 to mid-2008 they were comparable to those seen at the last A>0 minimum
But ~2008 was not solar minimum; by the true minimum 27-day variations were gone
Did the unusual minimum conditions drive the amplitudes high, or did the extreme quiet simply make them easier to see far from minimum?