ON THE MEASUREMENT SESSION DURING THE XIVth IAGA WORKSHOP AT CHANGCHUN MAGNETIC OBSERVATORY

The Absolute Measurement Session during the XIVth IAGA Workshop on Geomagnetic Observatory Instruments, Data Acquisition and Processing was held at the Changchun Magnetic Observatory (CNH) September 14-17, 2010. Approximately 50 participants joined in this session. During the session, 27 DI-Flux magnetometers were used to make and compare measurements, and several total field comparison measurements were conducted to look for errors within 4 total field instruments. This session also included absolute measurement training with lectures and practical training, an introduction to INTERMAGNET, and the demonstration of Autodif MKII and DI3.


INTRODUCTION
The Absolute Measurement Session during the XIVth IAGA Workshop on Geomagnetic Observatory Instruments, Data Acquisition and Processing was held at the Changchun Magnetic Observatory (CNH) September 14-17, 2010.Approximately 50 participants joined in this session.During the session, 27 DI-Flux magnetometers were used to make and compare measurements, and several total field comparison measurements were conducted to look for errors within 4 total field instruments.This session also included absolute measurement training with lectures and practical training, an introduction to INTERMAGNET, and the demonstration of Autodif MKII and DI3.

DI FLUX COMPARISON
Absolute measurements of declination and inclination were made at pillars 1, 3, 4, 5, and 6 (the Autodif MKII was demonstrated on pillar 2) fixed in the absolute house of the observatory.The pillars are 3 meters away from each other.The plan of the absolute house is displayed in Figure 1.The observatory baselines were determined on pillar 4 in the absolute house.Reduction of the measurements was made using the suspended FGE fluxgate magnetometer and GSM-90 magnetometer at the observatory.The stability of these magnetometers is approximately 3-5nT/year.See Figure 2 below.

TOTAL FIELD MAGNETOMETER COMPARISON
In this session, the total field magnetometers were tested in two ways.One method compared their samples with the simultaneous samples of the continuous recording Overhauser magnetometer at the Changchun Observatory.The other method calibrated these sensors by use of a frequency generator.
Several observers participated in the session.They were Dr. Hans-Joachim Linthe from Germany, Haizhi Liu, Guihua Qi, Fuxi Yang, and Suqin Zhang from China.Dr. Linthe provided the frequency generator for the test.The instruments and the corresponding information are listed in Table 3.The data collected by test systems were directly compared to the corresponding samples collected by the Changchun Observatory system.The pier difference (ΔF) for each system is plotted in Figure 6, and the mean values and standard deviations were also calculated as presented in Table 4.The frequency test was also applied to total field magnetometer comparison.The proton precession magnetometer or simply proton magnetometer is based on free precession of protons in a liquid.The angular precession ω of protons depends linearly on the magnetic field (Jankowski & Sucksdorff, 1996), ω=2πf=γpF (1) where γp is the gyromagnetic ratio of the proton, which is a natural constant known with high accuracy.The value adopted by IAGA is γp=2.6751525*10 8T -1 s -1 .The gyrofrequency f of protons is measurable in magnetic fields that are of the same order of magnitude as the field of the Earth.F and f are connected by Equation (1).From this equation it follows that 1 Hz corresponds to 23.48720 nT.Therefore, the frequency generator can be used to calibrate the sensors.The test data for each system are listed in Tables 5 to 8, and the Summary of Proton Magnetometer Frequency Test is shown in Table 9.

CONCLUSIONS
After intense systematic comparison measurements made by all the participating observers, rather good and encouraging results were achieved in the measurement session.In addition, information and experience exchanged amongst participants will be beneficial to future studies of the Earth's magnetic field at geomagnetic observatories.

Figure 1 .
Figure 1.The plan of the absolute house

Figure 4 .
Figure 4. Individual observer's difference from workshop average (I B ) with measurements outside of two standard deviations being removed

Figure 5 .
Figure 5. Second data curve of GSM-90 at Changchun Observatory in the observation period

Table 1 .
Data Science Journal, Volume 10, 30 August 2011 List of DI-fluxes and observers

Table 2 .
Pillar differences for D, I, and F referred to pillar 4

Table 3 .
Instruments and testing specificationsAll tests were performed on Sep 16 th , 2010.These test systems were compared against the Changchun Observatory GSM-90 Overhauser magnetometer with 1Hz sampling rate.The data collected by the GSM-90 Overhauser magnetometer at Changchun Observatory were displayed in Figure5.

Table 5 .
Proton Magnetometer Frequency Test of GSM19

Table 6 .
Proton Magnetometer Frequency Test of G856AX

Table 9 .
Summary of Proton Magnetometer Frequency Test