Executive Summary Chapter 1 Introduction History, heritage and operation

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Neutron Monitors in the IGY


The International Geophysical Year (IGY) from July 1957 to December 1958 provided a unique opportunity for scientists to conduct multi-discipline geophysical studies using data acquired from a large variety of sensors located around the world. One of the objectives of the IGY was the worldwide exchange of scientific data. To meet this objective the World Data Centers were established. These were established at places where there was scientific research in specific disciplines. Four World Data Centers for Cosmic Rays were established with Center A at the University of Minnesota.


Standard Cosmic Radiation Data Format Submissions to World Data Centers during the IGY

The initial plans for the exchange of cosmic radiation data during the IGY called for bi-hourly atmospheric pressure values and bi-hourly neutron monitor data corrected for atmospheric pressure. Special data forms were prepared for uniform reporting; these forms were to be sent to each cosmic ray data center every three months with a letter of data transmittal sent to the IGY Headquarters in Belgium. An example of a month of data is shown in Figure 1.

The initial publication of the Carnegie ionization chamber data (Lange and Forbush, 1948) contained only bi-hourly data, and this may have influenced the IGY organizers in their initial planning. At the time of the IGY planning meetings, only four high energy solar proton events had been identified: two in 1942, 1946 and 1949, and the publication of those results contained graphs of bi-hourly ionization chamber data (Forbush 1946; Forbush et al., 1950). Relativistic solar proton events were considered so rare that no provision was made for the exchange of data in smaller time increments than the bi-hourly values.

There were other factors to be considered. The scientific data prior to and during the IGY were primarily recorded on paper either handwritten or typed. Digital counters were installed at most sites with a camera recording the counters at set time intervals. After the film was developed, the digital values had to be manually read, and since the counters did not reset themselves at specified intervals, each reading had to be subtracted from the following value to obtain the counting rate within the specified period. The barometric pressure was usually recorded on analog charts with hourly values recorded by hand. Next the cosmic ray counting rate for the specified interval had to be corrected for the atmospheric pressure for that time period using a pre-determined barometric pressure coefficient appropriate for the station. Once the values were obtained, they had to be typed or hand written on the official IGY data forms. The entire process of recording the data was time consuming and laborious.

As preparations were underway for reporting and archiving cosmic radiation data, a major solar proton event occurred on 23 February 1956. The neutron monitor at Leeds, UK recorded an increase of 4581% over a 15-minute period. The event was world-wide in scope with significant increases in the equatorial region indicating the presence of particles in excess of 15 GV at the top of the atmosphere. Sarabhai et al. (1956) estimated the maximum energy of solar protons to be >50 GeV. Plans were quickly revised to recommend recording intervals of 15 minutes (or less) so that unusual "events" would permit detailed post event analyses (Nicolet, 1959). The routine bi-hourly reporting intervals to the World Data Centers remained unchanged.

The ground-level enhancement (GLE) of 23 February 1956 remains as the highest increase in 15 minute data recorded by neutron monitors. While there are higher increases for the 20 January 2005 GLE, these increases are for much shorter time intervals and were recorded at polar stations (Bieber et al., 2005). When averaged over a 15-minute interval, the percentage increase for the 20 January 2005 event is less than the increase at the Leeds neutron monitor on 23 February 1956. The GLE of 23 February 1956 is still actively studied today (Rishbeth et al., 2009).

Many groups planning their neutron monitor data recordings for the IGY had already implemented plans for smaller time interval data with some groups installing "flare alarms" which increased the recording interval if there was a rapid increase in the counting rate. While the IGY was during one of the most active solar cycles, there were no ground-level solar cosmic ray events during this time interval. The shorter time interval data was, however, extremely useful in the study of Forbush decreases of which there were many.

At the start of the IGY in July 1957 there were 43 IGY neutron monitors operating world wide. Nine of these monitors were in the USA; two others at Mexico City and Huancayo were under the auspices of the University of Chicago. (The Thule neutron monitor did not become operational until August 1957.) In December 1959, at the end of the International Geophysical Cooperation (the one year extension of the IGY) there were 58 IGY neutron monitors in operation around the world including the same nine monitors in the USA and those in Mexico City, Huancayo and Thule.

During the IGY, in addition to submitting bi-hourly data to the World Data Centers, the scientists initiated an exchange of data amongst themselves. It was relatively common for one group involved in a specific study, to request detailed data from another group, and this cooperation was readily acknowledged in subsequent publications. In addition to the exchange of data within the cosmic ray community, other scientists conducting research on various solar and geomagnetic phenomena were utilizing cosmic radiation data as a baseline describing the cosmic ray intensity during periods of interest. The use of neutron monitor data, particularly from the Climax neutron monitor, greatly increased with the advent of the space era.

Neutron Monitor Operations
In most stations each proportional counter has a dedicated electronics interface that records the total number of pulses that occur above a pre-set threshold over different integration periods. A new generation of electronics recently developed by the Bartol Research Institute, also records the time between pulses. For further details of this instrumentation and its use for determining cosmic ray spectra, see Bieber et al. 2002. These systems are currently in operation at the Newark and Thailand stations. The average number of evaporated neutrons produced in an inelastic interaction is energy dependent and can be roughly described as a power law, consequently spectral information can be in principle extracted this timing data. The capabilities of this new system is describe in Chapter 2 of this paper.