Utility and accuracy of geomagnetic repeat station surveys

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Utility and accuracy of geomagnetic repeat
station surveys 

• M. Korte(1), M. Mandea(1) and P. Kotze(2)
• (1) GeoForschungsZentrum Potsdam, Germany
• (2) Hermanus Magnetic Observatory, South Africa
• Acknowledgements for participation in field work
  and data processing M. Fredow(1) , A.
  Hemshorn(1), E. Julies(2), E. Nahayo(2), B.
  Pretorius(2) , M. Schüler(1)  

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Traditional applications

• Mapping of regional reference main and secular
  variation field- more accurate than IGRF in
  regions with sparse observatory coverage- more
  frequent updates compared to IGRF

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New applications I

• Models of lithospheric vector anomalies by
  combination of satellite (large scale, vector),
  aeromagnetic (small scale, F only) and repeat
  station ground (localized intermediate scale,
  vector) data.

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New applications II

• There is increasing evidence of small-scale
  secular variation, which might be due to
  mantle/lithospheric conductivity (induction)
  e.g.- models from satellite data do not describe
  SV completely e.g. in the southern African
  region with strong gradients- decadal models
  fail to describe long-term SV at some European
  locations
• Repeat station time series are useful to
  investigate this
• Very high data accuracy necessary!

Differences between European observatory data and
the CM4 model (Sabaka et al., 2002) show
insufficient SV description at several locations
(Verbanac et al., 2007)
Detailed European SV model by E. Thébault andthe
MagNetE (Magnetic Network of Europe) group
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The problem of accuracy

• Measurement errors- likely to be higher than at
  an observatory- can be kept small by careful
  set-up and measurement procedure
• Main problem elimination of external field
  influences- reduction to quiet night time or
  annual mean- variation recordings from nearest
  observatory or on-site variometer

Repeat stationannual meanof component C
Observatoryannual meanof component C
Repeat stationmeasurementvalue at time ti
Observatoryrecording attime ti
This difference determined most robustlyfrom
quiet night time values (on-site variometer)
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Example for using a local variometer
Differences between recordings of the LEMI
variometer at a German repeat station and NGK
observatory recordings for 3 days
Quiet hours used for final datareduction to
annual means
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Local variometers I

• Short term variations well known
• Data reduction by means of quiet night time
  values, which are assumed to represent
  undisturbed core (lithospheric) field
• Optimum on-site variometer for several days to
  include truly quiet night timeCompromises -
  on-site variometer for one full night with
  measurements in the evening and morning-
  regional variometers no further than 100 km away
  for several nights

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Local variometers II

• Short term variations well known
• Data reduction by means of quiet night time
  values, which are assumed to represent
  undisturbed core (lithospheric) field
• Optimum on-site variometer for several days to
  include truly quiet night timeCompromises -
  on-site variometer for one full night with
  measurements in the evening and morning-
  regional variometers no further than 100 km away
  for several nights
• Stability of variometer should be checked by
  several measurements (baseline)
• Temperature control for variometer is important!

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Test comparison stable temperature
Stable temperature
S
E
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Test comparison sensor temperature change
S
E
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Test comparison electronics temperature change
E
S
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Improvement to results by variometer
Experience values from repeat station surveys in
Germany and southern Africa

• Baseline difference between evening and morning
  or over up to 10 days in general in the order
  of 1 nT compared to a couple of nT if using only
  observatory recordings
• Differences to the nearest observatory often
  differ by several nT for individual
  measurements without variometer, but are robust
  for quiet night time means
• Differences to the nearest observatory can
  differ systematically from quiet night time
  differences
• Maximum deviation from the mean of 8
  measurements mostly no larger than 1.5 nT
• However, the time for the variometer temperature
  to stabilize can vary significantly depending
  on climatic conditions Germany often up to 6
  hours Southern Africa mostly only 2 to 3 hours

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Quiet night times

• How well do quiet night time values reflect
  undisturbed internal field?
• Is an average over one arbitrary night comparable
  to a quiet night time average?
• Studies on data from southern African
  observatories HER, HBK and TSU- all night
  annual means (600 pm to 600 am) and quiet night
  time annual means (000 to 400 am, Kp lt 2) in
  general agree within 1 nT- However, the night
  time annual means differ from the standard
  annual means by up to 9 nT

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Quiet night times

• Quiet night time differences between 2
  observatories over a year (2001), averages
  subtracted

- Individual minute differences lie in the order
of up to 5 nT- Individual one night averages
(600 pm to 600 am) mostly lie within /- 2 nT
of the monthly average of quiet night time values
for quiet and moderately disturbed nights (Kp
up to about 4) - Secular variation has to be
taken into account in reduction to annual mean
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Conclusions

• Repeat station surveys, particularly long time
  series, can be useful for several purposes
• New applications require high accuracy of repeat
  station data. A suitable processing to eliminate
  the external field influences is extremely
  important
• The use of on-site variometers is optimal, but
  their stability has to be ensured
• Variometer recordings (averages) over only one
  full night can be a good compromise under quiet
  to moderately disturbed conditions