Checking neutron monitor data

Correction for efficiency

Cosmic ray measurements with ground based observations are used for geophysical and astrophysical investigations. For this purpose the cosmic ray intensity in the near Earth space and directly above the atmosphere is computed from the neutron monitor (NM) data. Therefore the NM measurements of the individual stations have to be verified initially, and if necessary the data have to be processed. The primary data processing does not provide any physical information except on instrument performance. Incidents may arise during long term continuous NM operation that cause different kinds of 'instrumental' variations in the measured data, such as:

• exchange of counter tubes, electronics or amplifiers
• change of characteristics of counter tubes
• change in characteristics of electronics (amplifier, discriminator)
• change of detector environment (exchange of detector housing, roof, removal of snow from the monitor ..)
• relocation of detector
• one part of the detector is not in operation
• change of number of counter tubes (adding or removing counter tubes to the NM station)
• malfunction of power supplies, short power switch-off, electromagnetic interference
• malfunction of the counters or the data acquisition system

Types of count rate variations caused by artificial reasons

The above causes may produce the following types of changes in the NM count rates:

• peaks are short and considerable changes in the NM count rate with a quick recovery to the initial level,
• steplike jumps are sharp changes in the count rate for a longer time interval than only one read-out, and
• drifts are gradual and rather slow changes in the count rate.

Figure: Plot of NM time-series with typical possible malfunctions. (a) peaks (red), (b) steplike jump (blue), and (c) drift (violet).

Peaks in the count rates of a NM can occur singularly or repeatedly during a short time. Peaks are in the majority of cases the result of a malfunction, and the count rates during these times should therefore be removed.

The steplike jump is often observed when an element of the NM has been replaced or the working regime of a device has been changed dramatically. Such steplike jumps should be corrected if they can be attributed clearly to an artificial cause.

Data quality control

The data quality control of a NM station is typically made by comparing count rate ratios between different identical NM sections. The count rate ratios during a time interval that has to be examined are compared to the ratio during a period of stable NM operation. A change in the count rate ratio indicates a malfunction of one section. The resulting ratio of the count rate ratios during the interval of examination and the reference interval is used to correct the count rate of the erroneous section. This will compensate for the variations caused by changes of instrumental origin or by other external causes except for those of cosmic ray intensity origin.

For the NM data quality control the detector is usually divided into identical sections, and the count rate ratios of the sections are compared with each other (A/B, A/C, B/C). In an optimal realization of the method one section corresponds to one counter tube.

However, with this quality control method (multi-section method) it is not possible to detect malfunctions of the whole detector, i.e. when changes occur simultaneously in all channels. These variations are not always of instrumental origin, but may be associated with changes in the detector environment such as change of snow cover or building reconstruction. These influences on the count rates of an NM can be detected only by comparing the count rates with the data of nearby NM stations. To compensate for these influences, a method should be employed to compare the data of different nearby NM stations and to correct the data of the NM station with differing measurements.