Swarm ASM-VFM meeting 9-10 Apr 2015ESTEC (NL) Ideas for improving the disturbance model or Welcome to the Null-Space! Nils Olsen, Lars Tøffner-Clausen,

Swarm ASM-VFM meeting 9-10 Apr 2015 ESTEC (NL)

Ideas for improving the disturbance model or

Welcome to the Null-Space!

Nils Olsen, Lars Tøffner-Clausen, Chris C. Finlay, Jonas NielsenDTU Space


Vector Disturbance Maps

These results (LTC model) are obtained by describing the dependence of the disturbance field on sun position using spherical harmonics, and using TSVD to regularize the model

Use of localized basis functions instead ?

Other regularisation method?


Disturbance field modeled by point-sources (monopoles)

• 1280 point sources distributed

on icosahedron grid at depth 0.9

• Mean horizontal separation: 0.056


Model Parametrisation

• Data set: 0403 data set without disturbance field correction(fully calibrated, but uncorrected data)

• Preliminary data screening: removal of 0.5% outliers (> 5s wrt to preliminary disturbance field model)

• Data subsampling: 10 minute values (615 000 vector triplets)


Disturbance field modeled by point-sources (monopoles)

Model regularisation: minimizing ||m|| (quadratic regularisation)

l = 5 x 105

larger l(smoother model)

smaller l


Vector Disturbance MapsLTC model (spherical harmonics) point sources


Why modeling using point sources instead of spherical harmonics?

• Disturbance model is overparameterized• d/o 25 in case of LTC model, corresponding to 3 x 25*27 = 3 x 675 = 2025

coefficients• 3 x 1280 point sources

• Model regularisation is needed• Quadratic regularisation (Tikhonov, TSVD, …) minimizes the

mean energy• ”smoothed peaks”, possible spurious features in ”weak field” regions

• Maximim Entropy Regularisation• Sharp(er) boundaries


Experience with crustal field modeling

Kother et al (GJI, in review)


Experience with crustal field modeling

Kother et al (GJI, in review)


First resultsQuadratic regularisation Maximum Entropy

regularisation

rms misfit: 169 pT


Preliminary Conclusion

• Disturbance field modeling using localized basis function (point sources) results in almost identical ruesults compared to use of spherical harmonics

• Maximum Entropy instead of quadratic regularisation leads to minimal changes• … difference to crustal field situation because disturbance field is basically

large-scale?


Assumption: disturbance depends on Sun position (a,b) wrt S/C

First challenge: VFM disturbance

Swarm Alpha

nT


Second Challenge: Calibration of VFM on Charlie

• ”Mapping” of FASM(A) F(C)

• Use of F(C) (instead of missing FASM(C)) to calibrate BVFM(C)

Assessment of these two approaches: Difference DFASM(A) –D|BVFM(C)|

no ASM available

230 pT rms 410 pT rms

This value includes contributions from

remaining VFM disturbance field

Ionospheric contributions (dawn-dusk orbit)

Data from night side regions


Assessment of VFM disturbance correction, VFM calibration and alignment

Difference of vector components DB(A) – DB(C)Difference in vector components between Alpha and Charlie provides independent check of

• VFM calibration• VFM disturbance correction• VFM – STR alignment

(Euler angle determination)

for both satellites …

• … and of the geomagnetic field model that has been used for the mapping A C

• 1 nT noise spec on field measurements appears to be met with margins - as far as one can tell

440 pT rms

380 pT rms

360 pT rms


SIFM+: Model using Vector Gradient Data

SIFMno gradient:no gradient data

SIFM:with scalar gradient datano vector gradient data

SIFM+:with scalar gradient datawith vector gradient data

Model from 2 years of CHAMP satellite data at 320 km altitude (10 x more crustal field power at n=70)

Degree correlation rn



Difference to MF7Br at surface, n = 16 - 65

SIFM:no vector gradient dataSIFM+:with vector gradient data

Inclusion of vector gradient data alleviates Backus effect



Br at surface, n = 16 - 75

Documents

Swarm ASM-VFM meeting 9-10 Apr 2015ESTEC (NL) Ideas for improving the disturbance model or Welcome to the Null-Space! Nils Olsen, Lars Tøffner-Clausen,