Annual Report of New Zealand Magnetic Observatories, 2007

Transcription

1 Annual Report of New Zealand Magnetic Observatories, 2007 INTRODUCTION There were no significant changes to the equipment used in the magnetic observatories in The new fluxgate at Scott Base continued to give occasional problems, but to a smaller extent than in the preceding year as we tried different network configurations. The similar equipment at Eyrewell continued to operate very reliably. The data from the Apia Magnetic Observatory in Samoa is now handled by GNS Science, although the operation continues to be done by staff of the Samoa Meteorology Division. INSTRUMENTATION Variometers The standard configuration for our magnetic observatories is based on a DMI (Danish Meteorological Institute) model FGE 3-component fluxgate sensor, being the high-stability version in which the fluxgate unit is suspended to minimize drift. The unit at Eyrewell has a nominal resolution of 0.1 nt, while that at Scott Base has a nominal resolution of 0.2 nt. The outputs from the fluxgate units are digitised by ADAM-4017 Analog/Digital Converters, interfaced to a laptop running Linux. Each observatory also has a GEM GSM-90 Overhauser total field magnetometer, which is also interfaced to the PC. At Eyrewell, a second (desktop) PC is used for communications, with a cellphone modem to transmit hourly files to GNS Science via the internet. The Scott Base laptop is connected to the internet via the Scott Base computer network. After the new recording system at Scott Base started recording on 17 January 2006, we continued to run the old system in parallel (the new and old fluxgate sensors, and the new Proton Magnetometer bottle are in the same heated room in the Variometer Hut). The new systems at Eyrewell and Scott Base both transfer data by ftp to servers at the Lower Hutt office of GNS Science. The Apia observatory is still using the New Zealand designed 3-component fluxgate variometers and digitising systems of Tomlinson, 1993, with the data recorded by laptop PCs. The data is then ed to New Zealand for processing. Absolute measurements Absolute measurements at these observatories are made using a DIM (Declination Inclination Magnetometer) that is a single-axis fluxgate sensor (DSIR design) mounted on the telescope of a non-magnetic theodolite. (Apia uses a similar DIM donated by the United States Geological Survey.) The DIM is used in a null mode, i.e. perpendicular to the magnetic field. With the sensor level, the declination is obtained by finding the null direction, and then the telescope can be rotated in the magnetic meridian to determine the inclination. A proton magnetometer is meanwhile used to determine the total field (Eventually, these manual proton magnetometer readings will not be necessary at the observatories with continuous proton magnetometers, but the measurements will be continued for a transitional Institute of Geological and Nuclear Sciences Limited

2 period). Absolute measurements were done weekly at Eyrewell and Scott Base, and twice weekly at Apia. Plots of the absolute measurements, and the corrections made to the variometer values as a result of these measurements, are included in the report for each observatory. DATA DISTRIBUTION INTERMAGNET All three observatories are part of the INTERMAGNET network of magnetic observatories, and daily data is sent to the Edinburgh GIN (Geomagnetic Information Node) by . The 2007 CDROM is now available, although the 2008 CDROM is expected very soon. More recent data is available on the Intermagnet web-site. Currently daily data from Eyrewell and Scott Base appears on this web-site within a few hours of the end of each UT day, and that from Apia after a couple of days. Older data is available on the INTERMAGNET CDROMs, which have Eyrewell data from 1994, Scott Base data from 1996, and Apia data from The World Data Centre for Geomagnetism (Edinburgh) is currently implementing a new system which will provide access to archival data from these observatories (as well as Amberley, the site used before Eyrewell) at their website: Magnetic Indices Eyrewell is one of the two southern hemisphere mid-latitude observatories used in the derivation of the planetary Kp index of magnetic variation. The Eyrewell K-index values are calculated by the LRNS (Linear-phase, Robust, Non-Linear Smoothing) computer scaling technique. Daily Kp values are sent to the CETP (Centre d'étude des Environnements Terrestre et Planétaires) of the Institut Pierre Simon Laplace, in Saint-Maur-des-Fossés, France, and the daily values are sent bi-monthly to the Adolf-Schmidt-Observatorium für Geomagnetismus of the GeoForschungsZentrum in Potsdam, Germany. Web Availability Hourly magnetic data for 2000 to 2008 are now available on the GNS web-site for the Eyrewell (EYR) and Scott Base (SBA) observatories, together with Apia (API) data for 2006 to The programme of putting historic New Zealand magnetic data on our web-site is on hold, while we await developments at the Edinburgh World Data Centre. We have large amounts of digital data available on request for researchers, e.g. hourly values since 1965 for Amberley/Eyrewell and Apia, and since 1966 for Scott Base, whilst 20 second values are available from about 1991 onwards for these stations. Cataloguing of these resources is underway.

3 APIA OBSERVATORY (API) The Apia Magnetic Observatory was established by 1905 in support of the German Antarctic expeditions. Control of the observatory passed to the New Zealand Government in 1919 and was passed to the Samoan Government in Digital recording was installed at the end of 1991 after the La Cour instruments were damaged in cyclones. A theodolite with a fluxgate magnetometer attached for measuring the absolute declination and inclination, was provided by the USGS and installed in October 1998, and the observatory was accepted as an Intermagnet observatory in Until November 2004, support with data processing was given by the Institute of Geological & Nuclear Sciences Ltd, New Zealand (now known as GNS Science) by a contract with Lester Tomlinson, through his company Geoscience, Electronics and Data Services (Geoserve). For approximately the next year, Lester Tomlinson provided support on a private basis. GNS Science is now again providing support to this observatory, for electronic servicing and data processing. ACKNOWLEDGEMENT: STATION ID: LOCATION: ORGANIZATION: WEB-ADDRESS: CO-LATITUDE: LONGITUDE: ELEVATION: ABSOLUTE INSTRUMENTS: RECORDING VARIOMETER: ORIENTATION: DYNAMIC RANGE: RESOLUTION: SAMPLINGRATE: FILTERTYPE: BACKUP VARIOMETER: K-NUMBERS: K9-LIMIT: GINS: TRANSMISSION: OBSERVER: Users of the API-data should acknowledge: The Department of Agriculture, Forestry, Fisheries and Meteorology, Samoa API Apia, Samoa The Dept of Agriculture, Forestry, Fisheries and Meteorology, Samoa Deg Deg. W 4 meters DIM-fluxgate magnetometer and a proton precession magnetometer. Three-component fluxgate variometers built by NZ DSIR HDZ ±47000 nt 0.7 nt 20 sec 3 point Hamming Nil not measured 300 nt Edinburgh Faigame Sale 2007 Observations There were substantial data losses during Many of these were probably related to difficulties with the floppy disks that were used to record the data on the laptop, or to power supply problems. There were also problems with the absolute readings, as few very useful ones were made. The following figure shows the absolute readings as black dots, and a red line shows the corrections applied to the variometer values.

4 Observed and Adopted Baseline Values, API 2007 LAT: LONG: INSTITUTION: GNS INSTRUMENT: PC Magnetic North Component HN (nt) Magnetic East Component HE (nt) Vertical Component Z (nt) JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC In the period July to September 2007, as part of increasing the Samoan Government resources devoted to dealing with natural hazards, a new building was constructed in the neighbourhood of the magnetic observatory. Unfortunately, the building construction included an iron roof, and its closest corner was only 30 to 40 metres away from both the variometer and absolute measurement sites. This has produced a change of unknown size in the magnetic readings, which cannot be corrected from the absolute measurements, as they were also affected. There was no opportunity to do comparative surveys before and after the construction of the building, and the only existing magnetic survey of the area was only of the total field. We have found no indication that there were ever any auxiliary magnetic sites that we could use to re-establish the absolute values. Comparison of monthly means in 2008 with previous means suggests that the Y component has increased by about 50 nt, with only small changes in the other components. Because of this jump, no data was supplied from Apia to Intermagnet for late Minute data for all the days that were recorded is available from GNS Science. The following figures display hourly values of X, Y and Z at Apia for The main change in Y occurred during the gap in late July, although there were some further changes that are not so obvious.

5 Apia (API) - Hourly Mean Values Horizontal X Component (nt) JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Day

6 Apia (API) - Hourly Mean Values Magnetic Declination Y (nt) JAN 6840 FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Day

7 Apia (API) - Hourly Mean Values Vertical Z Component (nt) JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Day

8 EYREWELL OBSERVATORY (EYR) The Eyrewell Magnetic Observatory is located in a farmed area adjacent to Eyrewell Forest, on the north bank of the Waimakariri River about 30 km NW of Christchurch, New Zealand. The first Magnetic Observatory in this region was established in the Botanical Gardens, Christchurch in This was replaced in 1929 by a site at Amberley, about 50 kilometres to the north of Christchurch, because of interference from the electrical supply to the trams of the Christchurch Tramways Board. The Amberley site in its turn was affected by development, so Eyrewell was established in 1978 by the Department of Scientific and Industrial Research (DSIR). Its operation was taken over by the Institute of Geological and Nuclear Sciences Ltd (now known as GNS Science) in 1992 when the DSIR was disestablished. From 1993, when the Institute closed its Christchurch office, the routine operation of the observatory, data processing and maintenance was carried out under contract by Geoscience, Electronics and Data Services (Geoserve). This contract was terminated in November 2004, and GNS again took over responsibility for operating the observatory. A summary of the recording locations is as follows: Observatory Latitude Longitude Period of North East Operation Christchurch Amberley Eyrewell STATION ID: EYR LOCATION: Eyrewell, New Zealand ORGANIZATION: The Institute of Geological & Nuclear Sciences Ltd (GNS Science) WEB-ADDRESS: (refer to ) CO-LATITUDE: Deg. LONGITUDE: Deg. E ELEVATION: 120 metres ABSOLUTE INSTRUMENTS: DIM-fluxgate magnetometer and a proton precession magnetometer RECORDING VARIOMETER: DMI (Danish Meteorological Institute) model FGE Version J, suspended 3-component fluxgate variometer. ORIENTATION: HDZ DYNAMIC RANGE: ±3200 nt SAMPLINGRATE: 5 sec RESOLUTION: 0.1 nt TOTAL FIELD VARIOMETER: GEM GSM-90 Euromag Overhauser magnetometer SAMPLINGRATE: 5 sec

9 RESOLUTION: 0.01 nt FILTERTYPE: Minute values produced using 19 point INTERMAGNET filter BACKUP VARIOMETER: DSIR variometer, as used until February K-NUMBERS: LRNS-method K9-LIMIT: 500 nt OBSERVER: Tim O Neill Interference from DC power system A high voltage DC power transmission line passes within 20 km of the observatory and the currents flowing in this line together with the earth return can produce a vertical field at the observatory of over 20 nanoteslas. A fluxgate sensor has been located under the line to measure the effect and the information is recorded at the observatory and a correction applied to the recorded data Observations There were no significant data losses at Eyrewell during The Benmore line correction was out of action from 1 to 7 February 2007, due to Telecom erroneously disconnecting the telephone circuit. This has been corrected from power line current data obtained from Transpower, the operator of the DC line. Absolute readings were performed weekly during this period, with no significant problems. The following figure shows the absolute readings as black dots, and a red line shows the corrections applied to the variometer values. Observed and Adopted Baseline Values, eyr Magnetic North Component HN (nt) Magnetic East Component HE (nt) Vertical Component Z (nt) JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC The following figures display hourly values of X, Y, Z and F at Eyrewell for 2007:

10 Eyrewell (EYR) - Hourly Mean Values Horizontal X Component (nt) JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Day

11 Eyrewell (EYR) - Hourly Mean Values Magnetic Declination Y (nt) JAN 8310 FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Day

12 Eyrewell (EYR) - Hourly Mean Values Vertical Z Component (nt) JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Day

13 SCOTT BASE OBSERVATORY (SBA) The Scott Base Magnetic observatory was established in 1958 by the Department of Scientific and Industrial Research (DSIR). It is located in the immediate vicinity of the New Zealand research station of Scott Base, on Ross Island, Antarctica. Its operation was taken over by the Institute of Geological and Nuclear Sciences Ltd (GNS) in 1992 when the DSIR was disestablished. After 1993, when the Institute closed its Christchurch office, responsibility for the routine operation of the observatory, including training new staff each year, data processing and equipment maintenance was carried out under contract by Geoscience, Electronics and Data Services (Geoserve). From November 2004, GNS again took over responsibility for operating the observatory. The observations at Scott Base are made by staff of Antarctica New Zealand. The observatory consists of an Absolute Hut and a Variometer Hut for observations, while the recording systems are located in the Hatherton Laboratory of Scott Base. STATION ID: LOCATION: ORGANIZATION: WEB-ADDRESS: CO-LATITUDE: LONGITUDE: ELEVATION: ABSOLUTE INSTRUMENTS: RECORDING VARIOMETER: ORIENTATION: DYNAMIC RANGE: RESOLUTION: SAMPLINGRATE: FILTERTYPE: BACKUP VARIOMETER: K-NUMBERS: K9-LIMIT: OBSERVERS: SBA Ross Island, Antarctica The Institute of Geological & Nuclear Sciences Limited (GNS Science) Deg Deg. E 10 meters DIM-fluxgate magnetometer and proton precession magnetometer Suspended three-component fluxgate variometers, model FGE of the Danish meteorological Institute. Gem Systems GSM-90 Overhauser magnetometer. Three-component fluxgate variometer built by NZ DSIR (Backup) HDZ ±6400 nt 1 nt 5 sec standard INTERMAGNET 19 point Gaussian Nil Sr variation assumed negligibly small 2000 nt Antarctica New Zealand Science Technicians

14 2007 Observations Although the new Scott Base system was very similar to that running at Eyrewell (except that a lower sensitivity was used to handle the large magnetic variations at Scott Base) there were still problems with this system, which were more prevalent during the colder part of the year. This is a little surprising, as both the sensors and their electronics are in a temperaturecontrolled room. The problems at Scott Base have been with the link between the A/D converter unit, and a laptop computer in the Scott Base laboratory, which fails erratically for no apparent reason. An improved automatic restart meant that very little data was lost, as most gaps were only a few minutes. The few longer gaps were able to be filled with data from the old system, so there were no gaps longer than about an hour in the X, Y and Z components. The Proton Magnetometer also had some problems, and there were significant periods of F data loss on 9-10 April, 1 May and 13 October Absolute readings were performed approximately weekly during this period, with requested extra observations when the corrections were changing rapidly. Strong magnetic disturbances sometimes mean that accurate measurements cannot be made for some days, so there are some gaps. The following figure shows the absolute readings as black dots, and a red line shows the corrections applied to the variometer values. The very large annual variations are discussed below. Observed and Adopted Baseline Values, SBA 2007 LAT: LONG: INSTITUTION: GNS INSTRUMENT: LC Magnetic North Component HN (nt) Magnetic East Component HE (nt) Vertical Component Z (nt) JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC The following figures display hourly values of X, Y, Z and F at Scott Base for 2007.

15 Scott Base (SBA) - Hourly Mean Values Horizontal X Component (nt) JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Day

16 Scott Base (SBA) - Hourly Mean Values Magnetic Declination Y (nt) JAN 4940 FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Day

17 Scott Base (SBA) - Hourly Mean Values Vertical Z Component (nt) JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Day

18 Site effects There have always been problems with Scott Base as an observatory. It is not in an ideal situation as it is on Ross Island, a volcanic island off the coast of continental Antarctica. The surface rocks around Scott Base are highly magnetised and their magnetic properties have significant temperature coefficients. Like all sites in Antarctica, there are very large annual temperature variations. The surface temperatures at Scott Base can vary from just over freezing in summer to less than -50 C. The seasonal temperature variations are rapidly attenuated with depth so that rocks in the top few metres undergo different temperature regimes during the year. The net effect of these large seasonal temperature variations and large temperature coefficients of some of the surface rocks is to produce seasonally varying localised gradients in the magnetic field. Measurements of total force have been made in the vicinity of the magnetic installation periodically during one year. The survey was made at a height of about 1.2 metres on a 10- metre grid so it was not able to give an accurate picture of the spatial variations but it did show that large magnetic gradients exist and that there are large seasonal variations in these magnetic gradients. Also, since there are differences in the form of these seasonal variations from one place to another, they must be caused by multiple sources. These sources will be at different depths and hence undergoing different temperature regimes throughout the year. The varying magnetic gradients produce an annual variation in the baselines as shown in the baseline figures. It is important to realise that the annual variation in baselines is not an instrumental effect but a result of the varying local magnetic gradients. Both the variometers and absolute instruments are housed in thermostatically controlled heated huts. The figure below shows the variation in D through 2007 (with extra months at each end). These variations, which correlate with the seasonal temperature variations in the Scott Base weather records, are showing that the magnetic field difference between the recording site and the absolute hut is strongly affected by the temperature changes in the surface rocks.

19 Scott Base Seasonal Changes Declination Correction Arc Min /10/06 29/11/06 18/01/07 9/03/07 28/04/07 17/06/07 6/08/07 25/09/07 14/11/07 3/01/08 22/02/ Temperature C D D fit External Temp Sensor Temp The annual variation in the baseline is less than the annual variation in the daily mean values and both are small compared to the daily variations as shown up in plots of hourly mean values. One way of looking at the situation is to divide the magnetic field at any place into three parts: 1. The main field from sources deep in the earth (core), 2. A part from external sources (ionospheric currents), and 3. A part from local geological sources. Normally, the last part is assumed to be virtually constant with time, or at least any changes with time are uniform across any observatory site. Unfortunately this is not the case at Scott Base, resulting in the annual variation in baseline values. The geomagnetic data from Scott Base are a record of the temporal variations at the position of the absolute observations. The seasonal magnetic variations include variations from both external (ionospheric and/or magnetospheric) and local geological sources. Longer term (secular variation) changes will be free from the effects of local geological sources which will average out to a constant effect from year to year. Neither will shorter term magnetic variations be affected by local geological sources as short term temperature variations are very quickly attenuated as they penetrate into the ground, affecting only a fraction of a metre at the surface. It is not practicable to move the observatory to any less disturbed site. The nearest undisturbed site would be 60 km away on the Antarctic continent and there is no logistic support available for such a site.

20 The Antarctic is a very dry site and electrostatic charges can cause electrical interference, especially during windy conditions. There is also a lot of other electrical equipment, including high power pulse transmitters, close to the geomagnetic recording equipment. Any noise on the records from these sources of interference is small compared to natural variations in the magnetic field. REFERENCE Tomlinson, L.A., New Zealand s Magnetic Observatory Instrumentation. Exploration Geophysics. 24 p83-86.