| SH 4.6.4 NEUTRON MONITOR SENSITIVITY TO SOLAR MODULATION CHANGES: ALTITUDE VS. CUTOFF RIGIDITYK. R. Pyle ABSTRACT INTRODUCTION The counting-rate response of a given monitor (IGY or supermonitor) is determined by a) the geomagnetic latitude effect, which controls the access of primaries to the top of the atmosphere, where the cascade begins, b) the design (type and number of tubes), and c) the pressure-altitude of the monitor, which determines the properties of the cascade at the monitorŐs elevation. The development of the nucleonic cascade begins in the upper atmosphere; it grows and is then attenuated with increasing atmospheric depth. At neutron monitor energies ( 500 MV) the magnitude of the effect of solar modulation on the level of the Galactic flux at Earth is inversely related to particle energy or rigidity. For instance, a monitor which has a geomagnetic vertical cutoff rigidity (VCR) of 2 GV would be expected to exhibit a larger percentage variation for a given 'modulation eventŐ than a monitor with a VCR of 3 GV all other factors being equal. It is therefore often assumed that monitors with the lowest vertical cutoff rigidity (Vcr) (i.e. those at high geomagnetic latitude) should measure the greatest variation in counting rate. However, when the simultaneous development and attenuation of the cascade (the Specific Yield Function, Simpson et al. 1953) is taken into account, the altitude of the monitor can be seen to play a very important role, especially at low to medium Vcrs (i.e. <5-6 GV). In fact, as will be demonstrated by several examples below, a mid-latitude high-altitude monitor such as ours at Climax, Colorado (Elevation 3400 M, Pcv ~3 GV) is one of the most sensitive in the world to changes in modulation. COSMIC RAY YIELD FUNCTIONS It is not the purpose of this paper to examine in detail the various measurements and calculations of the primary proton yield function, but rather to call attention to the fact that the shape of the yield function varies appreciably with altitude, and that this is often the determining factor for sensitivity. All studies of the yield function exhibit this shape change with altitude, and we have chosen just one typical example for illustration in Figure 1, adapted from DeBrunner and Flückiger (1971b). In it we show the yield functions for an IGY monitor at two altitudes: sea level and 650 g-cm-2. The latter pressure-altitude is close to that for our Climax neutron monitor (685 g-cm-2). The major point to be made here is that the higher pressure-altitude curve has a maximum at a significantly lower rigidity (~4.5 GV) than the sea level curve (~7 GV). The annotations that we have added to Figure 1 show the vertical cutoff rigidities for 4 selected neutron monitors (Climax, Haleakala, Deep River, McMurdo, and South Pole). Simply put, the counting rates registered by these monitors are the integrals to the right of the vertical lines; it is apparent from these curves that a high altitude mid-latitude monitor such as Climax responds to a significantly lower mean primary rigidity than any sea-level monitor, including polar monitors. However, equatorial high-altitude monitors (such as our IGY and supermonitor on Mt. Haleakala in Hawaii, would not exhibit significantly larger sensitivity than a sea-level equatorial monitor. In the following section, we will provide examples of the "sensitivity" of the Climax monitor to changes in the modulation level by comparisons with low-altitude monitors over several time-scales, from several days (Forbush decreases) to variations over several 11-year solar cycles. DATA COMPARISONS In Figure 2 we have plotted a typical Forbush Decrease (2/26/92) as seen at Climax and Deep River, along with their ratio. For this event, the Climax "sensitivity" is ~16% greater than Deep River; the ratio of the two counting rates drops. Figure 3 is a comparison of these same two stations over the nearly 4 decades of simultaneous operation (unfortunately, the Deep River monitor was shut down in 1995). All data has been normalized to the 1965 level, defined as 100%. In this Figure we have used Climax data which has been adjusted after 1980/81 to correct normalization problems which occurred at that time. (Our thanks to H. Ahluwahlia for calling our attention to this problem). Relative to the previously published counting rates for Climax, this renormalization results in an adopted multiplicative factor of 0.9891 from 16 Sept. 1980 through 31 Aug. 1981, and 1.0121 thereafter. These corrections will be published in the near future. The top panel displays monthly averages of the counting rates, while the bottom panel displays the ratio between the two monitors. Over the period 1958-1995 the ratio varies only ~4%, while the individual counting rates vary by more than 30%. In Figure 4 we plot the two sets of monthly averages against each other and show the best-fit straight line. Based on this, the Climax monitor is again 16% more "sensitive" to changes in solar modulation. This sensitivity appears to be independent of the phase of the solar cycle, in that the fit appears to equally good at both solar minimum and maximum. As a final illustration of the importance of atmospheric mass to the sensitivity to modulation of neutron monitors, we compare two monitors with essentially the same Pcv but very different elevations (McMurdo and South Pole), which demonstrates the altitude effect very clearly. In Figure 5 we show counting rates from these two monitors during the same Forbush decrease used in Figure 2. Again, the increased sensitivity with elevation is very apparent. SUMMARY ACKNOWLEDGMENTS REFERENCES
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