No. 1

Characteristics of field-aligned currents derived from the Swarm constellation

Hermann Lühr, Jaeheung Park, Jan Rauberg, Ingo Michaelis, Guram Kervalishvili and Claudia Stolle

GFZ, German Research Centre for Geosciences

Swarm Data Quality WorkshopPotsdam, 2 – 5 Dec. 2014

Swarm Data Quality Workshop, Potsdam 2 – 5 Dec. 2014No. 2

Introduction to field-aligned currents

noon

(Iijima and Potemra, 1976)

Field-aligned currents are an important element in space plasma physics since they can carry energy and momentum lossless over large distances.

They are strongest at auroral latitudes and form a distinct local time pattern

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FAC estimates from single satellite

Current estimates from magnetic field measurements are commonly based on Ampère's law. For the vertical component one can write

From satellite data we only obtain along-track field variations. The equation has to be simplified.

where vx is the velocity component in flight direction ΔBy the

perpendicular field variations and Δt the time step.

Assumptions: (1) The FACs is stationary for the time of passage.(2) FACs are organised in sheets, oriented perpendicular to the flight direction.

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Example of single satellite FAC estimate

Large-scale FACs (>150km) show the typical morning side pattern.

Small-scale FACs (~10km) have much larger amplitudes but vary significantly in time. They are expected to be carried by kinetic Alfvén waves.

Swarm-A and –C are sampled at 7s time differences. The shift in latitude of Swarm-C indicates an equatorward motion of the FAC structure.

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Temporal variation of small-sale FACs

SW-A

SW-C

auroral oval

South Pole

We consider observations at the same location (orbital cross-over) but at different times.

Small and large-scale FACs exhibit different behaviour. Small-scale FACs (<10km) vary on time scales of the order of 10s.

Large-scale FACs (>150km) are stable for more than 60s.

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FAC longitudinal correlation length

SW-ASW-C

auroral oval

SW-B

Measurements at the same magnetic latitude but at different longitudes are considered.

For large-scale FACs we find good correlations up to ~10° in longitude separation for most LT sectors.

An exception is the dayside, cusp, a region with smaller correlation lengths.

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Résumé for FACs from single satellites

From the Swarm constellation observations we can conclude for the auroral oval:

Assumption (1, stationarity) is violated for small-scale FACs; their amplitude is varying rapidly with time. For large-scale FACs the assumption is fulfilled.

Assumption (2, sheet geometry) is fulfilled for large-scale FACs at most local times. Smaller longitudinal correlations length are found predominantly around noon, cusp region.The orientation of the current sheet can be determined by minimum variance analysis and accounted for in the analysis.

Conclusion: FAC estimates from single satellites magnetic field recordings are reliable when the data are filtered at a cutoff period of order 20s and the sheet orientation is considered.

Small-scale FACs cannot be determined reliably, but their contribution to the energy budget seems to be important for ionosphere-ionosphere coupling.

(For more information see our poster at the AGU meeting)

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Dual-satellite FACs

Much more robust FAC estimates can be derived when magnetic field measurements from several satellites are considered.

In case of Swarm we take data from the lower pair, Swarm-A/C. The reading of the leading Swarm-C are shifted in time (5-10s) such that a real side-by-side sampling is achieved.

We use the Level 1b data from which all non-ionospheric magnetic contributions are removed. Data have been corrected for the VFM-ASM disturbance (see Michaelis et al., Wednesday) and are low-pass filtered at a 20s cutoff period.

The applied algorithm for current estimate is Ampère’s integral.

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Field-aligned currents from the Swarm constellation

Using Ampère‘s integral in discrete form

11443

32210

43

212

dByBydBxBx

dByBydBxBxAj

tttt

ttttz

1

2 3

4

dℓ1

dℓ2

dℓ4

dℓ3

SwA SwC

dA

Bj0

1

4231 ddddA

Along-track variations are derived from two subsequent measurements

dt=5sec dl1,3=38km

Cross-track separation is 1.4° in longitude.

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Field-aligned currents from the Swarm constellation

Dual-FACs from Swarm-A/C are calculated for every second.

At latitudes above 86° no dual-FACs are calculated since the cross-track separation is less than 3 km.

Comparisons between single and dual-FACs are shown for different local times.

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IMAGE magnetometer

71.2°

67.0°

65.8°

64.3°

63.1°

60.5°

Swarm

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Summary

The early Swarm mission phase was useful to test underlying assumptions for FAC determination from single satellite.More details are given by a poster for AGU meeting.

Reliable FAC estimates can be achieved from the Swarm-A/C pair after the final orbit constellation has been achieved at middle of April and after correction for ASM/VFM differences.

Single and dual-satellite FAC estimates generally match quite well. Differences show up in the polar cap.

In the southern hemisphere the auroral oval overlaps with the geographic pole. In that region dual-satellite FAC estimates are not reliable.

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