1. Rapid core field variations just before Swarm
Arnaud Chulliat, Patrick Alken and Stefan Maus
University of Colorado Boulder & NOAA/NGDC, USA
19 June 2014
3rd Swarm Science Meeting

2. Rapid core field variations at observatories
2007
jerk
nT/yr
2011
jerk
2003
jerk
Taking the second-order time derivative of the field (the secular acceleration) is a convenient way to extract the fastest variations.
19 June 2014
3rd Swarm Science Meeting

3. Rapid core field variations in models
6-yr torsional oscillations in quasi-geostrophic core flows inverted from the gufm1 model, (Gillet et al., 2010) A
Secular acceleration “pulses” in the 2000s observed in:
CHAOS models (Chulliat et al., 2010; Olsen et al., 2014)
Dedicated CHAMP data based models (Chulliat and Maus, 2014)
SA power spectrum at CMB (in (nT/yr2)2)
19 June 2014
3rd Swarm Science Meeting

4. SA at the core-mantle boundary from CHAMP data
★ MBO
★ MBO
Radial component, Hann-filtered
nT/yr2
19 June 2014
3rd Swarm Science Meeting

5. Are we seeing a wave at the core surface?
SA along the geographic equator, at CMB
19 June 2014
3rd Swarm Science Meeting

6. 3rd Swarm Science Meeting
Defense Meteorological Satellite Program (DMSP) data for main field modeling
4 satellites launched in 1999, 2003, 2006 and 2009
Tri-axis fluxgate magnetometer (“Special sensor magnetometer”, SSM) mounted on a 5-m boom on each satellite
Sun-synchroneous, near-polar orbits
835 to 850 km altitude
Alken et al., JGR, 2014:
Calibration of the SSM magnetometers (timing shifts, scale factors, offsets, non-orthogonality angles)
Euler angles
DMSP-MAG-1 model
19 June 2014
3rd Swarm Science Meeting

7. 3rd Swarm Science Meeting
SA from DMSP data
Calculate spherical harmonic SA models on 3-year time intervals of DMSP data, centered on epochs ranging from to , with a 30-day step
Extend as much as possible the analysis using shorter time intervals for models centered within and
SA modeled as a second-order Taylor expansion in time
Regularization with minimum energy norm at the core
Similar approach as for CHAMP data (Chulliat and Maus, 2014); extension of a method developed by Maus et al. (2010).
19 June 2014
3rd Swarm Science Meeting

8. 3rd Swarm Science Meeting
SA pulses
CHAMP
DMSP
(nT/yr2)2
19 June 2014
3rd Swarm Science Meeting

9. Time-longitude diagram in the equatorial region
DMSP
CHAMP
19 June 2014
3rd Swarm Science Meeting

10. Time longitude diagram in the equatorial region
CHAOS-4plus
CHAOS-4
19 June 2014
3rd Swarm Science Meeting

11. Validation of SA models at observatories
Initial SV model (sat. A, presented at the Cal/val meeting)
CHAMP models (thick magenta line)
DMSP models (thick red line)
CHAOS-4 (dashed black line)
CHAOS-4plus (black line)
19 June 2014
3rd Swarm Science Meeting

12. Validation of SA models at observatories (2)
Initial SV model (sat. A, presented at the Cal/val meeting)
CHAMP models (thick magenta line)
DMSP models (thick red line)
CHAOS-4 (dashed black line)
CHAOS-4plus (black line)
19 June 2014
3rd Swarm Science Meeting

13. 3rd Swarm Science Meeting
Conclusions
Analysis of DMSP data shows that there is a new secular acceleration (SA) pulse in 2012.
Preliminary analysis of the first six months of Swarm data seem to confirm this result.
The 2006, 2009 (from CHAMP) and 2012 (from DMSP) pulses have the time-longitude pattern of a standing wave in the equatorial region, of period 6 years.
The origin of this wave is unknown.
At the Earth’s surface, the beginnings and ends of these three pulses lead to 3 jerks: around 2004, 2007 and Next jerk in 2014?
Results to be further extended in time once enough Swarm data are available for SA calculation (about 1yr?)
19 June 2014
3rd Swarm Science Meeting

14. Is the observed wave a torsional oscillation?
A TZ flow generating a stationary wave must satisfy:
But: less than 10% of the signal can be fitted by a TZ flow
The observed wave is NOT a torsional oscillation
Other possible waves in this frequency band:
Slow MC waves (requires a large toroidal field ~ 10-1 T)
Magnetic Rossby waves in Braginsky’s stratified ocean of the core
19 June 2014
3rd Swarm Science Meeting

15. Main field and secular variation at the CMB
2002.2
2005
2009.5 nT
MF ~ 106 nT
SV ~ 104 nT/yr
nT/yr
19 June 2014
3rd Swarm Science Meeting