Latitude Survey Investigation of Galactic Cosmic Ray Solar Modulation during 1994-2007 Paul Evenson, Waraporn Nuntiyakul, David Ruffolo, Alejandro Saiz, John Bieber, John Clem, Roger Pyle, Marcus Duldig, and John Humble

Overview As Professor Ruffolo has just explained, neutron monitors are large ground-based detectors that provide accurate measurements of variations in the cosmic ray flux at the top of the atmosphere above the detector. At any given location the magnetic field of the Earth excludes particles below a well-defined rigidity (momentum per unit charge) known as the cutoff rigidity, which can be accurately calculated using detailed models of the geomagnetic field. By carrying a neutron monitor to different locations, e.g., on a ship, the Earth itself serves as a magnet spectrometer. By repeating such latitude surveys with identical equipment a sensitive measurement of changes in the spectrum can be made.

Magnetic Cycle Effect

Relation to Particle Charge Sign Electrons and Helium at 1.2 GV show the same “peaked” and “flat” behavior but not the same pattern of high and low peaks. Particle spectrum is also important. Neutron monitors respond to a broad range of particle energies. We must use a special technique to understand what about the energy spectrum causes the difference. Magnetic Polarity Time profile of helium and electron observations at a rigidity of 1.2 GV

Latitude Survey We can use the shielding effect of the magnetic field of the earth to find out exactly how much each rigidity interval of the spectrum contributes to the count rate of a neutron monitor carried on a ship.

Differential Response Function By smoothing the data using a fit to an arbitrary function we can take the derivative, which is called the “Differential Response Function” This is not exactly the spectrum, but it is the product of the spectrum and a fixed function called the “Yield Function” As long as the same neutron monitor is used year after year you do not need to know the exact yield function to explore spectral differences

Spectral Crossover Moraal et al. (1989 JGR 94, 1459-1464) reported a curious “crossover” in differential response functions at solar minimum from opposite polarity states. Thus, our first question for our series of surveys: Is this something that happens only at solar minimum, or is there some systematic behavior of the spectrum that can be directly related to the polarity reversal? Stay awake for the answer!

First the Sine Qua Non: We Clearly See the Crossover Comparing response functions for the two solar minima the crossover is apparent. Trying to trace the evolution of the response function from one to the other does not offer a clear answer to our question. However a surprisingly simple, but really novel, approach to the data shows that a significant change in the spectrum happened at the time of the polarity reversal.

Our New Method Plotting the (corrected) counting rate of the mobile monitor for different cutoff intervals against the counting rate of the McMurdo neutron monitor yields remarkably straight lines, with different slopes before and after the polarity reversal.

Summary of Analysis The slopes of the lines are systematically different before 2000 (red squares) and after (blue triangles). They are also in line with what we would expect from a simulation based on the “force field” model of solar modulation (black dots). The force field model does not have any direct provision for magnetic polarity dependence

Conclusions We have demonstrated that a basic change in the spectrum of cosmic rays occurs at the solar magnetic polarity reversal, which results in the observed “crossover” at solar minimum. Our analysis says nothing about the alternating "peaked" and "flat" nature of the count rate maxima, the observation that the pointed maxima are higher than the flat maxima. This makes it easier to understand why there is not a nice progression of response functions, as our analysis has in fact distilled an aspect of the spectral evolution that is not contained in simple comparisons. Clarifying what this all really means is of course the subject of future work!