21 September 2010 IAGA Workshop 2010 1

A. Chulliat, E. Thébault

Institut de Physique du Globe de ParisSorbonne Paris Cité

Application of quasi-definitive observatory data to the detection of

geomagnetic jerks and the validation of IGRF-11 candidate models

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Quasi-definitive data

The idea of fast baseline corrected data emerged within the geomagnetic field modeling community around 2008. It was later endorsed by:

• the Swarm « Mission Advisory Group » (June 2008)• INTERMAGNET (June 2008, September 2009)• an official IAGA resolution (Sopron, August 2009)

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As of today, two methods have been proposed:

• BGS : the baseline is extrapolated over one month; quasi-definitive data are produced on a next day basis (Baillie et al., IAGA Sopron, 2009).

• IPGP : a new baseline is calculated every month; quasi-definitive data are produced at the beginning of the following month, with a latency of a few days (Peltier & Chulliat, EPS, 2010; see poster by Peltier et al.)

Methods

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The difference with definitive data is less than 1nT, even for observatories having irregular baselines (for ex. BOX), provided there is enough absolute measurements.

At IPGP, we have been doing it routinely since July 2009 for 9 observatories: AAE, BOX, CLF, KOU, LZH, MBO, PHU, PPT, TAM

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Is it feasible?

Yes, and it is even relatively easy!

Plus:• It helps detecting problems with the data and absolute measurements in a timely manner.• Producing definitive data at the end of the year is greatly simplified.

Is it useful?

Let’s have a look at two examples of recent use.

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The 2007 geomagnetic jerk

o CHAOS-2 model• observatory monthly means (annual differences)

20 nT/yr2

(4 times the 1979 jerk at NGK!)

Quasi-definitive data Jan 2009 – Mar 2010

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Signature at the Earth’s surface

Using the CHAOS-2 model, we calculate the acceleration change between t1 = 2005.5 and t2 = 2008.5

=> The 2007 jerk is not worldwide in occurrence.

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Signature at the CMB

2007 2003

2005

The maximum jerk (at the Earth’s surface) occurs under St Helena Island

The maximum jerk (at the Earth’s surface) occurs under Cocos Islands

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The 2006 secular acceleration pulse

Degrees n = 5 or 6 are attenuated at the Earth’s surface

The 2003 and 2007 jerks are Earth’s surface signatures of a single phenomenon at the core surface : a global acceleration pulse peaking between around 2006.

21 September 2010 IAGA Workshop 2010 11

IGRF model, the 11th generation

Under the auspices of IAGA Working Group V-MOD (chair: S. Maus; vice-chair: Chris Finlay)

Used by scientists, commercial organizations and private individuals (mostly for orientation purpose).

Three types of models:• main field 2005 (definitive) 7 candidates• main field 2010 (preliminary) 7 candidates• secular variation 2010-2015 8 candidates

Evaluation process took place in November-December 2009. Several participants contributed evaluations with various methods.

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As part of the IGRF-11 evaluation process, candidate secular variation models were tested against observatory data, i.e., an independent dataset.

Black = observatories providing definitive data until 12/2008.Red = observatories providing definitive data until 12/2008

and quasi-definitive data from 01 to 10/2009

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Some models are much closer to the SV measured by observatory quasi-definitive data than others.

Tests against quasi-definitive data

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Quasi-definitive data (9 obs) until Oct. 2009

Definitive data (87 obs) until Dec. 2008

Means an standard deviations of model-data differences

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Conclusions

1. Producing quasi-definitive data on a regular basis proved easier than expected, even at “difficult” observatories, and paid off when preparing the 2009 definitive data.

2. The quasi-definitive data produced by IPGP since July 2009 were used for detecting the 2007 jerk and for testing IGRF-11 candidate SV models.

1. Extensive use of quasi-definitive data is expected during the Swarm mission, starting 2012.

Chulliat, A., E. Thébault and G. Hulot, Core field acceleration pulse as a common cause of the 2003 and 2007 geomagnetic jerks, Geophys. Res. Lett., 37, L07301, doi:10.1029/2009GL042019, 2010.

Chulliat, A. and E. Thébault, Testing IGRF-11 candidate models against CHAMP data and quasi-definitive observatory data, Earth Planets Space, in press.