# Presentation: 600378

## 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)

## “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

## “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.

## “New” applications II

- “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!

- Detailed European SV model by E. Thébault andthe MagNetE (Magnetic Network of Europe) group

- Differences between European observatory data and the CM4 model (Sabaka et al., 2002) show insufficient SV description at several locations (Verbanac et al., 2007)

## 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 station“annual mean”of component C

- Observatoryannual meanof component C

- Repeat stationmeasurementvalue at time ti

- Observatoryrecording attime ti

**This difference determined most robustly****from quiet night time values (on-site variometer)**

## 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

## 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

## 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!

## Test comparison: stable temperature

- E

- S

- Stable temperature

## Test comparison: sensor temperature change

- E

- S

## Test comparison: electronics temperature change

- E

- S

## Improvement to results by variometer

-  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

- Experience values from repeat station surveys in Germany and southern Africa

## 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 (6:00 pm to 6:00 am) and quiet night time  annual means (0:00 to 4:00 am, *Kp* < 2) in general agree within 1 nT- However, the night time annual means differ from the standard  annual means by up to 9 nT

## 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 (6:00 pm to 6:00 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

## 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