Comparison of Field-Aligned Currents calculated by single spacecraft and dual spacecraft methods. Yulia V. Bogdanova, Malcolm W. Dunlop RAL Space, STFC,

Comparison of Field-Aligned Currents calculated by single spacecraft and dual

spacecraft methods.

Yulia V. Bogdanova, Malcolm W. DunlopRAL Space, STFC, UK

Field-Aligned Currents Large-scale statistical R1/R2 current density distribution. In addition, smaller-scale and more variable current systems: - cusp and FTE-associated currents, - NBz currents, - R0 current system, - substorm current wedge, - BBFs-associated current systems, - aurora.

1. Single-SC method

Slavin et al., 2008

2. Dual-SC method

Ritter and Luhr, 2006:

~

Luhr et al., 1996:

Assumptions: - Stable current sheet. - Indefinite current sheet. - SC crosses the current sheet at 90º.

Assumptions: - Static system. - Constant current within the area. - 1Hz data are filtered.

Aim and outline

The aim was to find: - How reliable the currents estimates are.

- How well currents’ estimated by two methods agree with each other.

- Have a statistical picture of currents’ agreement/disagreement.

Agreement depends on: - Calibration of the magnetic field data.

- Cross-calibration between two SC, A and C.

- Quality of methods of current estimates and validity of the assumptions made.

Outline:

- Discussion of few events.

- Results of statistical study.

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Plot of FAC currents from 2-SC method, SC-A, and SC-C for the date 2014 - 4 - 18

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FAC data from one orbit

~14 LT ~14 LT~2 LT

Single SC currents show small-scale variations and higher amplitudes than currents estimated by dual-SC method.

• Small-scale current systems estimated by single SC method are not visible in 2-SC method – as that data set was averaged over ~ 20 sec. Can compare only large scale behaviour.

• For comparison, averaged single SC data over 20 sec using 2 methods:

(i) moving box average with window of 20 sec;

(ii) low-pass filter to remove high frequency fluctuations above 0.05 Hz

(corresponds to t = 20 sec).

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SC-ASC-C2-SC method

Field-aligned currents , dusk-side (~14 LT), south

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Current comparison from 2-SC method and SC-A, averaged and filtered for the date 2014 - 4 - 18

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2-SC methodSC-ASC-A av.SC-A filt.

SC-AC vs SC-A

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Current comparison from 2-SC method and SC-C, averaged and filtered for the date 2014 - 4 - 18

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2-SC methodSC-CSC-C av.SC-C filt.

SC-AC vs SC-C

Both averaging methods agree quite well, remove high frequency fluctuations, and give averaged, lower amplitude, current density.

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2-SC methodSC-A av.SC-A filt.

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2-SC methodSC-C av.SC-C filt.

SC-AC, SC-A, and SC-C comparison Only averaged & filtered values are shown from SC-A and SC-C.

Good agreement between two methods of filtering high-frequency signal from single SC data, although there are some differences.

Relatively good agreement with dual-SC data – fluctuations of the current density.

Dual-SC method gives larger peaks.

Close to pole: large disagreement between the data – seen in many events.

SC-A vs SC-AC

SC-C vs SC-AC

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SC-ASC-C2-SC method

9050 9100 9150 9200 9250 9300 9350 9400 9450 9500-2.5

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2Current comparison from 2-SC method and SC-C, averaged and filtered for the date 2014 - 9 - 14

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SC-AC, SC-A, and SC-C comparison, dayside (11/12 LT), southern hemisphere

Can see the similar fluctuations, but absolute values do not agree at the centre of the interval.

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SC-ASC-C2-SC method

1.955 1.96 1.965 1.97 1.975 1.98 1.985 1.99 1.995 2 2.005

x 104

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2-SC methodSC-A av.SC-A filt.

1.955 1.96 1.965 1.97 1.975 1.98 1.985 1.99 1.995 2 2.005

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• Active interval, high Kp index.

• Very good agreement for the strong current system

• Less so for smaller currents.

• Off-set in the data?

SC-AC,SC-A, and SC-C comparison, nightside (1-2 LT), northern hemisphere

• Used all available data: dates 18/04/2014 – 05/11/2014• Version: V01_01.• Only high-latitude regions have been used, Abs(LAT): 55-85º.• Estimated averages of single-SC current density by two methods as discussed

above.• Resampled dual-SC data to the same timeline.• Split the data set into 4 different sectors: dayside, 10-14 LT; dusk, 14-20 LT;

dawn, 4-10 LT; nightside, 20-4 LT.• Estimated:

(i) ratio of the current density strengths estimated by dual- SC (SC-AC) and single spacecraft methods .

(ii) difference between the current densities.• Looked at ratios/differences as function of local time, LAT and strength of the

current density estimated by dual-SC method.

Method for statistical study

Dayside, 10-14 LT ~965,000 data points

Field-aligned current density distribution from dual-SC method vs LT:• Majority of measurements in ± 2 µA/m^2

range.• There are some erroneous data points.

Ratio of currents’ strength vs LAT

JAC/JA-averaged

JAC/JA-filtered

JAC/JC-averaged

JAC/JC-filtered

1000

-1000

Large spread in the data points; negative points indicate opposite polarity if the currents, asymmetry between southern and northern hemisphere, both methods give similar picture, both SC also produce similar results.

Ratio of currents’ strength vs LAT JAC/JA-averaged

south north

Most data points are in ± 20 range.

Colour: log10(# of points in the bin)

Large number of events with opposite polarity.

The most populated bins are with ratio between 0 and 1, and 1 and 2.

Reasonably good agreement on a statistical level.

Ratio of currents’ strength vs LAT JAC/JA-filtered

south north

Low-pass filter method gives very similar statistical results, but with less spread in the distribution.

Ratio of currents’ strength vs LT JAC/JA-averaged

JAC/JA-filtered

JAC/JC-averaged

JAC/JC-filtered

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Some spread in the data, two methods give similar results, two SC, A and C, give similar results.

Ratio of currents’ strength vs LT

JAC/JA-averaged JAC/JA-filtered

More broad distribution around at 11.5-12.5 LT: expected from the cusp small-scale currents variability

Ratio of currents’ strength vs SC-AC FAC strengthJAC/JA-averaged

JAC/JA-filtered

JAC/JC-averaged

JAC/JC-filtered

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Majority of the FAC SC-AC current density measurements are in ±1 µA/m2

Ratio of currents’ strength vs SC-AC FAC strength


Most of the data points are with the current values close to zero and with ratios in ± 1 bin.

Large spread in ratios at low current values – not surprising.

Better agreement at higher current values.

Magnitude of difference in currents’ strength vs LAT

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Abs(JAC-JC-averaged)

Abs(JAC-JA-filtered) Abs(JAC-JC-filtered)10

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• Majority of events are in range < 2 µA/m2. • Spread increases towards the poles – more noticeable in the northern hemisphere.


Abs(JAC-JA-averaged)

Abs(JAC-JA-filtered)

Majority of events have small differences between current estimates by two methods.

Spread of events increases towards the poles.

Systematic difference in the southern hemisphere.

Magnitude of difference in currents’ strength vs LTAbs(JAC-JA-averaged) Abs(JAC-JC-averaged)

Abs(JAC-JA-filtered) Abs(JAC-JC-filtered)

Similar results are produced by different averaging methods and on SC-C and SC-A

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Magnitude of difference of currents’ strength vs LT

• Most events have difference in the current estimates less than 0.1 µA/m2.• Possibly slighter larger spread at around mid-day.

Difference in averaged valuesMagnitude of difference in currents’ strength vs SC-AC strength

Abs(JAC-JA-averaged) Abs(JAC-JC-averaged)

Abs(JAC-JA-filtered) Abs(JAC-JC-filtered)

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• Most of events are in the centre, with low current and low difference.• Large current estimates have larger differences. Low-pass filter gives better results.

Magnitude of difference in currents’ strength vs SC-AC strength


• Zero-centred distribution, almost symmetrical for positive and negative currents.• Disagreement between the current densities increases with the current’s value.• Low-pass filter produces slightly better results.

Dusk, 14-20 LT, ~775,000 data points

Other sectors showed very similar results: - Quick look at dusk sector.- Spread in the data points is smaller at

dusk/dawn than at other sectors – as expected.

Ratio of currents’ strength vs LAT


Similar to the dayside sector: differences between hemispheres.

Most events are in the ratio range of 0-2.

Ratio of currents’ strength vs LT


Better agreement between the currents’ estimates at ~ 17 LT: less disturbed and larger scale currents are expected.

Ratio of current strength vs LT


More points with the same polarity than in the dayside sector.

SC-A



Abs(JAC-JA-filtered)

Similar to the dayside sector.

Magnitude of difference of current strength vs LT

Abs(JAC-JA-averaged) Abs(JAC-JA-filtered)

Better agreement between the currents’ estimates at ~ 17 LT: less disturbed and larger scale currents are expected.

Magnitude of difference of current strength vs SC-AC FAC strength

Abs(JAC-JA-averaged) Abs(JAC-JA-filtered)

Similar to the dayside sector.

Conclusions

Statistical comparison of the L2 data product with single (filtered) and dual-SC

estimates of the FAC current densities shows:

• There is a large spread in the data points, including the currents of opposite

polarity as calculated by two methods.• However, statistically the currents’ estimates agree very well => confidence in the

data products.• Both SC-A and SC-C give very similar results statistically.• Both averaging methods give similar results, possibly low-pass filter is slightly

better.• Disagreement between the current estimates increases towards the polar regions.• Near southern pole: systematic disagreement between the estimates (off-set).• Dusk/dawn estimates agree better than dayside/nightside – in agreement with

more stable and large-scale current systems expected at dusk/dawn. • Such statistics can be used for production of a ‘quality flag’ for both data sets.• Possible improvement: different averaging time, removing data point with current

estimates very close to zero.

Documents

Comparison of Field-Aligned Currents calculated by single spacecraft and dual spacecraft methods. Yulia V. Bogdanova, Malcolm W. Dunlop RAL Space, STFC,