Results of DIflux Intercomparisons at the 3th Escuela Latino-Americana de Geomagnetismo in Huancayo, Perú October 22th - 29th 1997

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1 Results of DIflux Intercomparisons at the 3th Escuela Latino-Americana de Geomagnetismo in Huancayo, Perú October 22th - 29th 1997 by Oscar Veliz Castillo Instituto Geofisico del Perú Perú and Jean L. Rasson Institut Royal Météorologique de Belgique Centre de Physique du Globe B-5670 Dourbes Belgique 1. Abstract An intercomparison of geomagnetic observatory instrumentation took place during the third "Escuela Latino-Americana de Geomagnetismo" (ELAG) in the magnetic observatory of Huancayo, Peru. In total 8 DIfluxes of various makes (Zeiss 020, Zeiss 010, Ruska, EDA) where intercompared using the observatory digital fluxgate variometer baseline and proton precession magnetometer. The results for the DIfluxes are presented. We call attention to particular DIflux observational techniques in magnetic equatorial conditions. 2. Introduction For the first time since the introduction of ELAGs in 1993, the 3th escuela proposed to hold a systematic intercomparison of geomagnetic observatory instrumentation. Initially it was planned to do this in the magnetic observatory of Pilár, Argentina, but this proved to be impossible. The observatory of Huancayo, Peru, just completing its 75th anniversary, presented ideal conditions for this purpose, and the Instituto Geofisico del

2 Perú (IGP) kindly proposed to host the event there. The observatory has a large Absolute House (figure 1) with 5 usable pillars and stable variometers, both photographic and digital, for reduction to the baseline. Additionally a 1 minute proton magnetometer record is permanently available. This article focusses on intercomparisons of DIfluxes, instruments for making absolute measurements of the geomagnetic declination and inclination. Please see the "Guide for Magnetic Measurements and Observatory Practice" referenced in the bibliography for explanation of specialised jargon. 3. Special Geomagnetic Conditions in Huancayo Figure 2 gives a plot of the field components as recorded in Huancayo observatory during our ELAG. The observatory lies almost on the magnetic equator, and this shows spectacularly on the plots. Therefore the horizontal H component is quasi identical to the total field F component. Indeed the geomagnetic vector is horizontal save for a small inclination of I = 1 20'. Coincidentally the magnetic declination is still closer to zero, giving a declination oscillating around the value of D = 0 05'. This special configuration of the field vector is not without consequences on the DIflux observation procedure and reduction. For the magnetic inclination measurement the DIflux telescope will be oriented vertically, pointing towards Zenith or Nadir. As some theodolite manufacturers never foresaw an application were one would look towards the Nadir with the telescope, they did not provide the full instrument operationality in this position. For the Zeiss 020, the degree graduations markings on the vertical circle are absent over a 16 range around the 180 index, although the finer markings are present. For the Ruskas, the nadir-looking position is not possible due to a too long telescope. Also the traditional telescope position names for the inclination measurement protocol can be confusing: The labels "sensor up", "sensor down", "telescope pointing towards North", etc. are meaningless if the sensor and telescope are vertical. For the measurement of magnetic declination with a DIflux, the situation is favorable in Huancayo, as the horizontality of the sensor is not as critical as for higher latitudes. That is because the error d due to an horizontality error e during the measurement of declination is given by: d = e * tani where tani = 0.02 in Huancayo and * denotes multiplication. This also means that it is impossible to determine succesfully the Site Sensor Collimation Error ESI from the declination observations in Huancayo - but it can be obtained from the I observations. Due to the small value of D care would have to be exercised in order to avoid polarity errors in D during the data reduction. They would not reveal themselves by simple inspection. 4. Reduction to Pillar ESTE1 We give in Figure 3 and 4 pictures of the digital fluxgate variometer baselines, one for D (D0) and one for I (I0 = arctan(z0/h0) in degrees with

3 decimal fraction. Those pictures relate to the epoch of the intercomparison measurements made during the 3a ELAG. We give all the measurements reduced to pillar ESTE1, with the measurements taken by the reference DIflux HUB appearing as crosses. The baselines were reduced to pillar ESTE1 by the following method: 1.- We took as reference the complete set of measurements made with the reference DIflux HUB on the different pillars. This gives us D0 HUBpillari and I0 HUBpillari for each pillar with index i. i stands for ESTE1, ESTE2, OESTE1, OESTE2 and OESTE We computed the pillar differences δ with pillar ESTE1 using the averages of all the measurements with HUB on each pillar i: δd0 ESTE1-i = <D0 HUBpillarESTE1 > - <D0 HUBpillari > δi0 ESTE1-i = <I0 HUBpillarESTE1 > - <I0 HUBpillari > 3.- We used those δ s for reducing the complete set of observations to pillar ESTE1. 5. On Problems to Measure with the Ruska DIfluxes and a Solution We explain now how we made the reduction of the measurements by the Ruska DIfluxes, who do not allow to point the telescope towards the Nadir. This means that in Huancayo, there is only possibility to measure I in two telescope positions I a and I d : telescope pointing up with sensor North or South. This implies a defect in the measurement in the sense that it is not absolute. Indeed in taking the average of I a and I d, the measurement accuracy is now affected by the sensor magnetisation error (but not by the sensor collimation errors). Happily, we can take advantage of the declination measurement that is always done right before the I measurement: the measurement sequence for D allows to extract the sensor magnetisation error. If a, b, c and d refer to the classical four positions for the declination measurement for the DIflux, we have the Sensor Magnetisation Error ESO in nanoteslas: ESO = {D0 a -D0 b +D0 c -D0 d } * H am * (π/180) where H am is the annual mean for the horizontal component in nt for the observatory. If x = a, b, c or d, D0 x is given in degrees (with appropriate 180 adjustments) by: where D0 x = D x -δd x, δd x = asin{dd x /H am }. δd x is in degrees and dd x is the synchronous record of the digital fluxgate variometer in nt. Now we compute:

4 Z0 x = (F x +df)sin(i x ) - dz x H0 x = (F x +df)cos(i x ) - dh x. df is the pillar difference and includes also the possible proton magnetometer instrumental correction. F x is the synchronous reading of the proton magnetometer. I x is the vertical circle inclination readings or its complement with 180 correction as necessary. x = a or d. dz x, dh x : synchronous Z or H recording of the variometer. So we obtain the baseline of I: I0 = [atan(z0 a /H0 a )+atan(z0 d /H0 d )]/2 + asin(eso/f am ) and the Site Sensor Collimation Error ESI in degrees: ESI = [atan(z0 a /H0 a ) - atan(z0 d /H0 d )]/2 If one wants to calculate the baselines H0 and Z0, we must go on with the computation of the vertical circle readings corrected for the ESI and ESO Ic a and Ic b (F am current annual mean of the total field for the observatory): and we find the sought baselines: Ic x = I x - ESI + asin(eso/f am ) H0 = ½ x=a,b [(F x +df)cosic x - dh x ] Z0 = ½ x=a,b [(F x +df)sinic x - dz x ] 6. Results of the Intercomparisons Once the complete set of baseline observations was reduced to pillar ESTE1, we averaged each instrument over the complete observation interval, each for D and I. The standard deviations were also computed. We then substracted each DIflux-under-test baseline from the reference baseline, obtained with the HUB DIflux. This gave us the final E value, for D and I, wich is given in tables I and II respectively. Table III gives the particulars on each DIflux: E = Baseline HUB - Baseline DIflux Table I. Declination results in second of arc DIflux E["] stan. dev["] Number of obs. HUB PTY ZT ANC RM

5 RUA ZB RA Table II. Inclination results in second of arc DIflux E["] stan. dev.["] Number of obs. HUB PTY ZT ANC RM RUA ZB RA Table III. DIflux particulars Code DIflux HUB Zeiss010 Bartington PTY Zeiss020 Bartington ZT1 Zeiss015 Bartington ANC Zeiss010 Bartington RM1 Ruska FLM1/A RUA Ruska digital ZB2 Zeiss020 EDA RA2 Ruska FLM1/A Observatory Huancayo (pe) Patacamaya( bo) Chiripa (cr) Ancon (pe) Teoloyucan (mx) Dourbes (be) Tatuoca (br) Trelew( ar) 7. Discussion of the Results In general the measurements agree with each other on a level of 10" or better. Two instruments have apparently problems in D: PTY and ZT1. For PTY a problem was found in the horizontal circle, which would sometimes not rotate with the alidade. For ZT1, there was a faulty operation procedure leading to erroneous results. In inclination, RUA still has some magnetic cleanliness problems in the lower part of the theodolite and this shows in a 28" E value for I. ZB1 had mechanical problems in the horizontal axis clamp (this has been repaired in 1998). We appreciate the good results of the two peruvian instruments HUB and ANC from IGP along with the observers skill. Low E values were obtained along with low standard deviations. Considering only the "good" instruments, the average of the E values give 1" for D and 1" for I. Hence we may confirm as justified the use of HUB as reference for this intercomparison session. The Ruskas, participating for the first time in an intercomparison session, give low E values, establishing their good accuracy, despite their low reading resolution of 30" and their impossibility to measure with telescope pointing down in Huancayo. Finally we stress the excellent performance of the Huancayo fluxgate variometer, allowing the intercomparison to take place at a noise level of a few seconds of arc.

6 8. Conclusions We have reported on the intercomparison session of 8 DIfluxes during the 3th ELAG in the Huancayo magnetic observatory during october The baselines of a digital fluxgate variometer were used for this purpose. A total of 149 sets of D-I measurements were taken on 5 pillars in the Absolute House. Our computations show a fine agreement between most of the DIfluxes. Where a discrepancy was found, the reason could be identified and the problem solved except in the case of PTY, where a severe problem subsists in the horizontal circle. We stress the usefulness of this kind of intercomparison, as it permits raising the accuracy standards in the magnetic observatories in Latin-America. 9. Bibliography Jankowski J. and Sucksdorff C., Guide for Magnetic Measurements and Observatory Practice, published by the IAGA, Warsaw. Contact with the Secretary General of IAGA for a copy. 10. Acknowledgements We thank the Instituto Geofisico del Perú and the Huancayo magnetic observatory staff for hosting this event and providing financial support. We are indebted to the Panamerican Institute for Geography and History for providing travel support. Our thanks also go to Professor LM Barreto and Don Alberto Giesecke for the overall organisation. Last but not least, we would like to mention the skillfull archiving of the data by Mrs Elisa Orellana Cerrón (she was also one of the best observers with DIflux).