The Haleakala Cosmic Ray Neutron Monitor Station: 22 Aug 94 Intercalibration with the Huancayo Station Extracted from a paper presented at the 23rd Int'l Cosmic ray Conf. (Calgary) [Proc 23rd Int'l Cosmic Ray Conf., 3, 609, 1993] K. R. Pyle pyle@odysseus.uchicago.edu Enrico Fermi Institute The University of Chicago Chicago, IL 60637, USA ABSTRACT The Haleakala Neutron Monitor Station, which consists of a standard 12-tube IGY monitor and an 18-tube NM-64 supermonitor was established in February 1991 to provide a long-term replacement for the IGY neutron monitor at Huancayo, Peru, in operation since 1951. The Haleakala site was chosen to provide a close match to the geomagnetic latitude and elevation of Huancayo. Twenty-one months of simultaneous operation of the Haleakala and Huancayo stations (Feb. 1991 through Oct. 1992) has provided sufficient data to obtain a good intercalibration of the monitors. The Huancayo station was turned over to the Instituto Geofisico del Peru on 1 January 1993. 1. INTRODUCTION After the development of the neutron monitor over the years 1948-50 (Simpson, 1951) and the realization that the Earth's magnetic field could be used as a spectrometer to allow measurements of the cosmic ray spectrum down to low primary energies, the University of Chicago in the early 1950s established a network of high-altitude neutron monitor stations covering a wide range of geomagnetic latitudes. Two of these stations (at Climax, Colorado and Huancayo, Peru) have been in continuous operation since 1950/51 and are the longest-operating stations in the world-wide network of neutron monitors. These two stations, at ~3 GV and ~13 GV vertical cutoff rigidities, respectively (Figure 1), have provided continuous coverage over four solar sunspot cycles, or two solar magnetic cycles. In the mid-1980s political unrest in Peru, especially in the Huancayo area, made it increasingly difficult for University of Chicago staff to travel to the station and perform the yearly maintenance and calibration procedures essential for the long-term stability of this monitor. In 1988 we began planning for a replacement for Huancayo, knowing that the long-term outlook for its operation was questionable, and that a contin- ued lack of maintenance was going to reduce the scientific value of this station. Our search was primarily for sites in Hawaii, providing a unique combination of high altitude, low geomagnetic latitude (high geomagnetic cutoff) and political stability. A site was chosen at the summit of Mt. Haleakala on Maui, on the grounds of Haleakala Observatory, a part of the University of Hawaii's Institute for Astronomy. At this site we have installed a standard 12-tube IGY neutron monitor, as well as an 18-tube NM-64 supermonitor. The BF3 tubes and moderator for the latter, originally part of the supermonitor at Leeds University in England, were generously provided by that University. The new station was put into operation in February, 1991 and has been operating successfully since then. No data from the Huancayo station has been received at Chicago since November 1992. Table 1 summarizes the characteristics of the three University of Chicago stations. TABLE 1: Climax and Haleakala Neutron Monitor Stations Monitor | Climax, | Haleakala, | Haleakala, | Huancayo, | Colorado (IGY) | Hawaii (S/M) | Hawaii (IGY) | Peru (IGY) ---------------|----------------|---------------|----------------|------------ Configuration: | 12-IGY | 18-NM64 | 12-IGY | 12-IGY Sections: | 2 x 6-tube | 3 x 6-tube | 2 x 6-tube | 2 x 6-tube In Operation: | 1950- | 2/1991- | 2/1991- | 1951-92 1980 Rigidity | | | | Cutoff (GV): | 2.99 | 12.91 | 12.91 | 12.92 Elevation (m): | 3400 | 3030 | 3030 | 3400 Latitude: | N 39.37 | N 20.72 | N 20.72 | S 12.03 Longitude: | E 253.82 | E 203.72 | E 203.72 | E 284.67 | | | | Data | | | | Section Rates: | 2@1 minute | 3 @10 seconds | 2@1 minute | 2 @1 minute Tube Rates: | | 18 @ 1-hour | | Time Accuracy: | ~0.2 sec.(WWV)| ~0.2 sec.(WWV)| ~0.2 sec.(WWV) | ~10 sec. Temperature: | 1-hour | 1-hour | 1-hour | 1-hour Pressure: | 1-min. | 1-min. | 1-min. | 1-min. ---------------|----------------|---------------|----------------|------------ Data are taken with two dedicated desktop computers, one for the IGY monitor and one for the supermonitor. Each computer receives the WWV time standard signal, which is used to maintain time accuracy. In case of a computer problem, only one monitor will be affected. The data are collected weekly and transferred to Chicago via the internet for subsequent reduction and analysis. The establishment of the Haleakala station was an attempt to provide not only a well intercalibrated replacement for the 40-year-old Huancayo scientific database (the longest-operating equatorial station), but also a sensitive detector for solar neutron events. Solar neutrons are the only particles that come to us directly from the site of the solar flare, unaffected by magnetic fields, and thus provide a direct measure of the neutron and proton spectrum at the flare site (Ramaty, 1986). The recent interest in the measurement of direct solar neutron production in solar flares has led to increased attention paid to neutron monitors as detectors of these solar flare neutrons, and to a substantial number of observations of these events (e.g. Chupp et al., 1987; Pyle and Simpson, 1991; Shea, Smart and Pyle, 1991). Shortly after the establishment of the Haleakala station, a small neutron event was measured in association with the 22 March 1991 solar flare event (Pyle and Simpson, 1991). An equatorial high-altitude supermonitor has several advantages for the direct detection of these neutron events: 1) the Sun is at a high elevation for a relatively large fraction of the time, thus giving a relatively low line-of-sight air mass, and hence lower attenuation of the neutron-induced cascade (DeBrunner, Fluckiger and Stein, 1990); 2) the mountain altitude gives a significantly higher count rate than the same monitor at sea-level, 3) the high geomagnetic cutoff (low magnetic latitude) greatly reduces the probability of confusion of a neutron event with a ground-level solar pro- ton event. The data collection system for the Haleakala supermonitor was designed to exploit these characteristics, and provides high time resolution (10 seconds) and high time-accuracy (continuous monitoring of the National Bureau of Standards time standard) data collection. INTERCALIBRATION OF THE HUANCAYO AND HALEAKALA MONITORS Table 1 summarized the characteristics of the four neutron monitors at the three sites. In order to provide a reliable continuation of the long Huancayo data record, I used the entire 21-month period of parallel operation to derive normalization factors between the Huancayo IGY, on the one hand, and the two Haleakala monitors, on the other hand. The vertical cutoff rigidity of the Huancayo monitor is subject to a long-term drift due to the secular variation of the Earth's geomagnetic field (Shea, 1971; Cooper and Simpson, 1979; Popielawska and Simpson, 1991). I have used the parameters derived in the latter paper to correct the Huancayo data to a constant cut-off. This correction is equivalent to subtracting 0.1105 percent/year from the counting rate, as specified in Table 2. During the period of overlap between Huancayo and Haleakala I derived a set of normalization factors which minimized the sum of the differences of daily averages. For each monitor I then express the counting rate as a percentage of the 1954 solar minimum value, 1760 counts/hour (prescale 100). These factors are summarized in Table 2. TABLE 2 Monitor: Huancayo ----------- "Solar Minimum" Haleakala Cts/Hr/100 The Huancayo data have been Climax 4316 corrected for geomagnetic drift Huancayo* 1.000 1760 with the following correction: Haleakala IGY 1.159 1519 Huancayo (corrected) = Huancayo x Haleakala S/M 0.0486 36240 (1.0-0.001105 x (Time(yrs)-1965.0)) The average deviation (after normalization) is less than 0.1 percent between both the two Haleakala monitors and the Huancayo Monitor. The twenty-one-month period from March 1991 until November 1992 covers a very large change in solar modulation level (the level on 13 June 1991 was the lowest ever observed by neutron monitors (21% below the 1954 level), and by November 1992 the equatorial neutron monitor counting rates had recovered to within 3.5% of the 1954 level. This research was supported in part by NSF Grant ATM-9215122. REFERENCES Chupp, E.L. et al.: 1987, Astrophys. J., 318, 913. Cooper, J.F. and Simpson, J.A.: 1979, Proc. 16th ICRC, Kyoto, 12, 176. DeBrunnner, H., Fluckiger, E.O., and Stein, P.: 1990, Proc. 21st ICRC, Adelaide, 5, 129. Popielawska, B. and Simpson, J.A.: 1991, Proc. 22nd ICRC, Dublin, 3, 660. Pyle, K.R., and Simpson, J.A.:1991, Proc. 22nd ICRC, Dublin, 3, 53. Ramaty, R.: 1986, in Physics of the Sun, II, ed. P.A. Sturrock, Reidel, Dordrecht. Shea, M.A.: 1971, Proc. 12th ICRC, Hobart, 3, 859. Shea, M.A., Smart, D.F., and Pyle, K.R.: 1991, Geophys. Res. Letters, 18, 1655. 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