1. Discussion on Daily Variation of Baseline Value Recorded by Fluxgate Magnetometer in China Suqin Zhang Dongmei Yang Institute of geophysics, China earthquake administration Abstract In order to investigate quantitatively the accuracy of geomagnetic daily variation recorded by fluxgate magnetometer (FHDZ-M15 and GM4), we analyzed the stability and precision of some groups baseline values continuously recorded one day at some observatories in 2009. The standard deviation and variable amplitude of the baseline value in D, H and Z components was given. Figure 3. Baseline values determined by the difference variometers (FHDZ- M15 and GM4) and the same fluxgate theodolite （ MINGEO DIM ） at TAY and THJ observatories Sponsor Logo Conclusion and Discussion We draw the following conclusions from the above figures and table: 1.The absolute measurements of different time periods have little infection on the stability of the baseline value, except for CDP, KSH and HHH. 2.The baseline values have better observation accuracy. The standard deviations are below 1nT in all instruments. 3.The baseline values are steady in whole observation time in the most instruments and the variable amplitudes are below 1nT. But the amplitudes are beyond 1nT in D at DED for FHDZ-M15, in D, H at DED for GM4, and in D at MCH for GM4. 4.The problem need to be discussed and studied further is why the baseline values of FHDZ-M15 at CDP, KSH, and of GM4 at HHH changed with geomagnetic daily variation; Why the variable amplitudes are larger at DED and MCH. Data In this section, the baseline values of 10 sets of FHDZ-M15 and 7 sets of GM4 were analyzed in contrast to the diurnal variation curves. The baseline values were measured twice at intervals of one hour from 8:30 local time, and lasted nine hours at least. We get one average value from every group baseline values (two). Because of the limitation of the space, the baseline values curves, only 8 sets of FHDZ-M15 (Fig.1 and Fig.3) and 5 sets of GM4 (Fig.2 and Fig. 3) were given. The baseline values determined by the difference variometers and the same fluxgate theodolite were analyzed also (Fig.3). Figure 1. Baseline values measured hourly and the diurnal variation curve of D, H and Z components of FHDZ-M15 at six observatories Data Analyze Fig.1 shows, the baseline values of FHDZ-M15 is steady in whole observation time. Variable amplitudes of baseline are almost below 1nT, except for D component of DED. The baseline of CDP and KSH may be affected by geomagnetic daily variation. Fig.2 shows, the baseline values of GM4 is steady. The variable amplitude of baseline in D is larger than in H and Z. The baseline of HHH may be affected by geomagnetic daily variation. Fig.3 shows, the baseline values curve determined by the difference two variometer and the same fluxgate theodolite are not coincident well. Table 1 gives the standard deviation and variable amplitude of baseline values measured hourly of FHDZ-M15 and GM4. Figure 2. Baseline values measured hourly and the diurnal variation curve of D, H and Z components of GM4 at MCH and HHH observatories StationInstrument Standard DeviationVariable Amplitude DHZDHZ WMQFHDZ-M150.050.30.20.111.00.5 TAYFHDZ-M150.020.60.40.061.10.7 LZHFHDZ-M150.020.2 0.060.60.5 WHNFHDZ-M150.020.2 0.060.70.6 CNHFHDZ-M150.010.20.10.030.50.3 CDPFHDZ-M150.020.1 0.060.3 DEDFHDZ-M150.050.20.10.180.70.4 KSHFHDZ-M150.020.20.10.050.50.1 JIHFHDZ-M150.040.40.30.121.00.7 JYGFHDZ-M150.030.2 0.090.80.5 TAYGM40.020.30.20.060.70.5 LYHGM4(1)0.040.30.20.110.80.6 LYHGM4(2)0.030.20.30.080.71.0 DEDGM40.060.50.20.171.50.4 HHHGM40.010.10.20.040.20.6 MCHGM4(1)0.050.2 0.120.6 MCHGM4(2)0.060.30.10.151.00.4 Figure 4. Standard deviation of baseline values measured hourly of FHDZ-M15 and GM4 Table 1. Standard deviation and variable amplitude of baseline values measured hourly of FHDZ-M15 and GM4