Feasibility of using platform to observe and detect Space events

(ESA contract AO-i8735/16/NL/LF)

Executive Summary

Prepared by: Eelco Doornbos, TU Delft Lotfi Massarweh, TU Delft Alessandra Menicucci, TU Delft Elisabetta Iorfida, TU Delft Claudia Stolle, GFZ Potsdam Martin Rother, GFZ Potsdam Ingo Michaelis, GFZ Potsdam Guram Kervalishvili, GFZ Potsdam Jan Rauberg, GFZ Potsdam

August 15, 2018

EUROPEAN SPACE AGENCY CONTRACT REPORT The work described in this report was done under ESA contract. Responsibility for the contents resides in the author or organisation that prepared it. Executive summary

Introduction

Magnetometers are fundamental instruments used in planetary and space science. Mag- netometers are able to measure the magnetic fields originating in the planet’s interior, past fields encapsulated in rocks in the crust, as well as magnetic fields generated by the mo- tion of charged particles in the plasma environments of , and interplanetary space. Currents in space plasma transfer energy over very large distances. The state of the Earth’s is to a large extent dependent on the orientation and strength of the interplanetary magnetic field (IMF) just upstream. A southward orientation of the IMF en- ables magnetic reconnection between the IMF and magnetosphere, triggering geomagnetic storms and substorms. This is why IMF measurements in the Sun-Earth L1 point are essen- tial for space weather modeling and prediction. In the case of the plasma within the Earth’s magnetosphere, currents are able to deposit significant energy in the Earth’s upper atmosphere, causing ionization, heating and expan- sion of the neutral upper atmosphere, which in turn interacts with the charged particle envi- ronment in a complex, highly non-linear way. These processes induce variability in the level of atmospheric drag on , as well as perturbations of radio communications, navi- gation signals and scientific observations relying on radio links. Currents in the can also induce currents inside the Earth, affecting and possibly endangering man-made in- frastructure, such as power lines and pipes. For these reasons, measurements of ionospheric currents are essential observations in the field of space weather. Due to the geometry and high altitude of magnetospheric and ionospheric current sys- tems, measurements are needed in addition to ground magnetic observatories, in order to reveal their detailed behaviour. Due to the large spatial extent of the currents, the variety of spatial scales of variability, as well as the sometimes very rapid fluctuations, adding simultaneous observations from many satellites will be extremely useful to enhance our understanding of space weather, and to be prepared to adjust our response to space weather in more sophisticated ways than is currently possible. This is where the use of platform magnetometers comes in. On low Earth orbiting (LEO) satellites, magnetometers are often used as part of the attitude control subsystem. Although these instruments are not designed and implemented for space weather observation, they can nevertheless be used for this purpose. In this study, data from the diagnostics magnetometers on LISA Pathfinder and the AOCS magnetometers of the ESA GOCE and Swarm satellites has been used (see Figure1

1 TFM100G4-S BBILLINGSLEY AAEROFLIGHTSPA MAGNETOMETERCE & DDEF ENSE

TFM 100S SPACECRAFT ATTITUDE CONTROL TRIAXIAL FLUXGATE MAGNETOMETER

Figure 1 The Billingsley 3-axis fluxgateFEBRUARY magnetometers 2008 TFM100-S SPECIFICATIONS (left) of which 3 each are SPECIFICATIONSused as the ’FGM’ instruments in the AOCS subsystems of Swarm and GOCE, and the TFM100G4-S, that was used as ’MGM’ for magnetic diagnostics on LISA Pathfinder. Radiation Tolerance: > 300 kRADs (with 1.59 mm cover) > 100 kRADs (with 0.79 mm cover) Option MissionAxial Instrument Alignment: dimensions Orthogonality better than ± 1° ; ± 0.5° massspecial; calibration Axial Alignment: Orthogonalitydata provided better than to ± ±0.1 1°°, Special to ± 0.1° Swarm VFM sensor: 8.2 cm diameter, processing unit: 10×10×6 cm 280 / 750 g Input VoltageSwarm Options:Input TFM100-S Voltage: 3.66×3.58×15.44 24-34 cm VDC + 15 to + 45 VDC 200 g Input Current:GOCE TFM100-S 3.66×3.58×15.44 > 50mA cm+ 45 to + 125 VDC Option 200 g Input Current: 20 mA at 28 V Power andLPF SignalIsolation: Lines TFM100G4-S 3.51× 3.23×8.26 Galvanically cm between Isolated power and signal ground > 200 100MΩ g at 500 VDC Field MeasurementField Range Measurement Options: Range: ± 100 μ ± T 100 = ± μ10.0T Volts (Other ranges available) Table 1Accuracy: Some characteristics of selected scientific ± 0.5% and of platformfull scale magnetometers. Accuracy: Linearity: ± 0. 5% 0.015% of full scale, of full Specialscale to 0.1% Linearity: Sensitivity: ± 0.007% 100 of μ Vfull / nTscale, Typical < ±0.0025 and TableScale1), Factor to analyse Temperature the performance Shift: .0075% and limitations full scale of/ °C such instruments, for space Sensitivity: Output Ripple: 100 µV/nT 3 mV (or peak user to specified)peak @ 2nd harmonic weather use. Scale Factor TemperatureAnalog Output Shift: @ Zero Field: 0.003% ± full 0.025V scale/ ° Celsius TheZero Swarm Shift satellites with Temperature: also each carry ≤ two 1.0 scientific nT /°C magnetometers: the ASM (absolute Noise: scalar magnetometer),Susceptibility to providing Perming: a < highly 40 picoTesla ± accurate 25 nT RMS/shift absolute with Hz reference,@1± 5 HzGauss Typical and applied VFM (vector field Output Ripple:magnetometer), Output Impedance: providing very accurate 3 millivolt 332 3-axis peak Ω ± vector to5% peak measurements. @ 2nd harmonic The satellites were Frequency Response: - 3 dB @ > 150 Hz Analog Outputspecifically @E MZero I: designed Field: for making measurements± 0.020 CEO1, Volt CEO3, of the REO2, highest CS01, possible CSO2, accuracy. CSO6, RSO1, This al-RSO2, RS03 Zero Shiftlowed with Temperature: usRandom to assess Vibration: the performance < of± the1 nT/ 20G platform° Celsius RMS magnetometers(20 Typical Hz to 2 KHz) in an ideal environment, Shock: >230 G Susceptibilityand against to Perming: a highly accurate reference. ± 20 nT The shift GOCE with satellite ± 5 Gauss is used applied as a test case of a mission that wasTemperature not designed Range: with magnetic cleanliness - 40° to and +85 space °C weather measurements in mind, Output Impedance:Weight: 332 Ω ± 200 5% grams (can drive(1.59 >mm 3500’ cover) of cable) but that can nevertheless contribute to this 166 purpose. grams (0.79 mm cover) Option Frequency Response: 3 dB @ ≥ 3.5 kHz Over Load Recovery:Size: ± 5 Gauss 3.66 slew cm x < 3.58 2 milliseconds cm x 15.44 cm LISA Pathfinder E M I: Connector: Designed NON-MAGNETIC to meet CEO1, 9 CEO3, PIN "D"REO2, TYPE CS01, CSO2, CSO6, RSO1, RSO2, RS03 Random Vibration: > 20G RMS 20 Hz to 2 KHz The investigations of LISA pathfinder data starts with the downloading of magnetometer, Temperature Range: - 55° to + 85° Celsius operating orbit and attitude data from the LISA Pathfinder Legacy Archive website. The data from Acceleration:each of the three axes on the four > 60G magnetometers contain considerable biases of several

Weight: hundred nT, with respect to the IMF, 100 whichgrams normally has a strength of only a few nT. The Size: fact that these biases show similarities 3.51 in cm terms x 3.23 of magnitude cm x 8.26 andcm timing of changing magni- Connector:tude, for the four instruments, indicates Chassis that mounted they are 9 pin mainly male due "D" to type; stray mating fields originatingconnector supplied

from other equipment on the spacecraft. The timing of bias changes is furthermore closely linked to times of switching between the various experiments. In general, the biases can be considered to be piecewise constant, or at the worst, show a linear drift over time, so that the measurements are easily corrected. A simple removal of a daily mean bias already brings 20936 Theseus Terrace, Germantown, MD 20876 USA the magnetometer readings closewww.magnetometer.com together, and close to [email protected] signal that resembles the IMF. This is shown in Figure2, in which 4-minute301-540-8338 averages of the raw 301-774-7707 data sampled (LAB) at 0.25 Hz are plotted. Only for a period of about 2 hours, a clear discrepancy between the 4 magnetome- ter readings is visible in this Figure, which can be traced back to spacecraft operations. The

2 Outliers analysis for each magnetometers [S/C frame] | 39

Figure 2 Measurements by the 4 individual magnetometers on LISA Pathfinder. For each of the 4 Figure 27: Examplemagnetometer’s as in Figure 326 axes,, where a dailya time mean-variable has local been perturbation removed is fromacting the between measurements. hour 3 and 4. To be observed how the effects along each S/C direction depend mostly upon the location of the instrument responsible for this perturbation. In addition, the magnitude is strongly influenced by its relative distance from each MAG. day of data plotted in this Figure was specifically selected to show this. On most days of our analysis,At this point such, we discrepancies can proceed were with not an present.examination of what have been defined “outliers”, as observationFigure3 shows point a comparison that is distant of the from readings other observations from one of the. (Grubbs LPF magnetometers F.E., 1969). with science data from the NASA ACE mission, which is dedicated to measuring the strength and orientation of the IMF. It is clear that both magnetometers measure the same variations in IMF. An analysis with the help of ACE wind speed data has shown that the time shift in5.5 the IMFOutliers measurements analysis between for the each missions magnetometers can be completely attributed[S/C frame] to the different �� positionsIn a very ofsimple LPF andway ACE it is possible in their respective to plot the orbits difference around between the Sun-Earth the Δ���� L1 point.measurement During the LPF mission a total of four magnetometer-carrying���� spacecraft were available of each magnetometer and the mean value computed Δ���� . Assuming once again that in orbit around L1: WIND, ACE, DSCOVR and LPF. The three other missions were dedi- the value of the IMF (Δ����) is the same at each location within LPF’s science module, it cated to measuring the IMF, and were therefore better calibrated and provided higher data is possible to express this difference � as rates. The LPF magnetometer measurements�� therefore do not offer unique measurements of how the IMF� affects Earth, but the data is nevertheless very valuable� in order to study � = Δ� � − Δ����� = (Δ� + Δ� )�� − (Δ� + Δ� )���� = Δ� � − Δ����� spatial� scales� of��� IMF variability��� at� L1. ��� � ��� �/� �/� Although the study report focuses only on the first three months of LPF data, a publica- It follows, considering the M2 as example, that tion on the analysis of the full mission duration is being planned.

�2 �1 �2 �3 �4 Δ��/� Δ��/� + Δ��/� + Δ��/� + Δ��/� �� � �� � �� � � = 4 ∙ − = 2 1 + 2 3 + 2 4 Swarm�2 and GOCE4 4 4 4 4

�2 �1 Forwith Swarm ��2�1 and= Δ� GOCE,�/� − Δ platform��/�, as difference magnetometer between and the torquer perturbation data was measured obtained fromby M2 house- and keepingthe one datarelative archives, to M1. while In a precisemore simple orbit andway, attitude this parameter information provides was availablean estimation from theof mission’show large science is the data error products. of one magnetometer For GOCE, also in thruster comparison activation with datathe other, was used, in this that way had alreadyexploiting been the available magnetometers for thermosphere configurations data product based o processing.n such differential analysis. In the followingFigure4 Figure provides 28 the an outliers overview for of the time DOY98 series case of various (see Figure data 26 during) is given, this calibration while in Figure pro- cess. 29 a same plot is shown for the “alternative case” with very large temporary perturbations (see Figure 27). The 3� standard deviation over the 24h-period is shown in dashed line. 3 Summary of the “data processing” algorithm | 47

FigureFigure 36 3: Example Comparison of LPF MAG of the-data mean-removed after the conversion measuresements to the GSE frame, of one considering of the LPFthe mean magnetometers measurements among all MAGswith for those the selected of the well-calibrated 24h-period. Data ACE still refer satellite to the magnetometer. LPF DAU-time (mean between DAU1 and DAU2). The reference baseline is the ACE one, always 8:00 am, so only few seconds different from the LPF initial time ��. This last step is part of the LPF data processing, where at this point we have obtained an expression (every 4.8s) of the local IMF, along with local effects due to a time-varying magnetic perturbation of the local magnetic environment. We provide now a summary of this “data processing” algorithm, while any further discussions on these last magnetic measurements is left to the next Chapter 6, based on the “data validation” algorithm.

XYZ 5.9 Summary of the “data processing” algorithm TheUncalibrated final algorithm FGMa for the data processing 104.94 can be summarized 442.07 as follow 274.54 Added calibration 57.20 15.01 40.91 ALGORITHM:Added MTQ correction LPF magnetometer 52.13 data processing 13.93 32.82 Added orbit correction 23.36 9.97 17.78 4. Computation of the Simple Moving Average over 4 minutes [MAG frame] VFM 5.89 10.43 9.24 5. Computation of the mean over a 24h-period [MAG frame] 6. Transformation of the 4min data from MAG frame to S/C frame Table 2 Median Absolute Deviation (MAD) of calibration residuals (in nT) with respect to CHAOS6 7. Removalon Swarm of Alpha, mean for value uncalibrated over a 24h and-period several applied calibration and characterization [S/C frame] 8. Assessmentsteps. Comparison of the “outliers” was done infor FGM’s each referencemagnetometer frame. [S/C frame] a. Check on the coil’s perturbation due to power variation 9. Assessment of LPF position in GSE frame 10. Assessment of LPF attitude in GSE frame b. Check on the LPF attitude matrix daily variations 11. Transformation of the 4min data from S/C frame to GSE frame 4 END ALGORITHM

Figure 4 Evolution of residuals for Swarm Alpha (FGMa - CHAOS6) by applying calibration steps. 1st panel (FGMa), 2nd panel (FGMa, calibrated), 3rd panel (FGMa, calibrated, MTQ cor- rected), 4th panel (FGMa, calibrated, MTQ Corrected, orbit corrected). FGMa tempera- ture, effect of Magneto-Torquers (MTQ) and Geocentric Latitude have been added in the last panels.

5 Figure 5 Field-aligned currents determined from GOCE and Swarm calibrated magnetometer data, binned by magnetic latitude and local solar time.

Table2 provides a comparison of CHAOS-6 residuals for uncalibrated and calibrated Swarm FGMa data, as well as equivalent data from the VFM science instruments. It can be seen that the final calibrated FGMa residuals are of the same order of magnitude as the VFM residuals, although somewhat higher for the X- and Z-axes, as expected. Note that these numbers include the natural variability of the magnetic field, for example due to mag- netospheric and ionospheric currents during quiet and moderate days. The signal of interest due to field-aligned currents during space weather events is many times the level of these residuals. Calibration of the GOCE platform magnetometers was performed in a similar way. Al- though of course a scientific magnetometer was not available on GOCE, the data was com- pared with that of the CHAMP satellite for verification. The calibrated platform magne- tometer data of both Swarm and GOCE was subsequently processed, using the Swarm Field-Aligned Current processor. Figure5 shows maps, in geomagnetic local time and latitude coordinates, of the average pattern of field-aligned currents from the AOCS instruments, as well as from the Swarm VFM instrument. The Figure shows that all instruments were able to nicely capture the expected pattern of region 1 and region 2 field-aligned currents. As an example of the multi-instrument space weather case studies that were performed, Figure6 shows GOCE space weather data, IMF data and ground-magnetometer based ge- omagnetic activity indices. The GOCE data includes the new field-aligned currents, for 2.5 days surrounding the onset of the April 5, 2010 geomagnetic storm, as well as thermosphere density and wind data derived from the satellite’s accelerometer measurements. We can see clearly in this Figure that the field-aligned currents and auroral electrojets are driven by the interplanetary magnetic field. It is well known that the energy exchange

6 GOCE thermosphere neutral density 50 06:22 LST DN 06:22 LST DN 60 70 80 90 North Magnetic Pole 80 70 60 18:22 LST AN 18:22 LST AN Magnetic latitude (deg) 50 GOCE thermosphere crosswind speed 50 06:22 LST DN 06:22 LST DN 60 70 80 90 North Magnetic Pole 80 70 60 18:22 LST AN 18:22 LST AN Magnetic latitude (deg) 50 GOCE large−scale field−aligned currents 50 06:22 LST DN 06:22 LST DN 60 70 80 90 North Magnetic Pole 80 70 60 18:22 LST AN 18:22 LST AN Magnetic latitude (deg) 50 1000 Eastward current 0

−1000 AE (nT) −2000 Westward current

100 Eastward current 0

−100

−200 Westward current SYM−H (nT)

40 North / Dusk 30 (nT) 20 +/−Bt

, 10

Bz 0 ,

By −10 −20 −30 South / Dawn ACE IMF −40 12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00 April 4 April 5 April 6 April 7

Figure 6 Detailed view of GOCE data surrounding the April 5, 2010 geomagnetic storm. The hor- izontal space between orbits corresponds to a field-aligned current density of 2 µA/m2, a wind speed of 800 m/s and density of 50 g/km3.

7 between the solar wind and magnetosphere, and the magnetosphere and ionosphere is es- pecially effective during a southward pointing IMF. The field-aligned currents measured by GOCE feed into the horizontal currents, as indi- cated by the increase in the AE index, measured by ground magnetometers at high latitudes, during times of high field-aligned currents. These currents cause extensive Joule heating of the upper atmosphere as well as heating due to particle precipitation. This heating is evi- dent from the increase of the thermosphere neutral density, in the top panel of both Figures. The heating causes a redistribution of the mass in the thermosphere, which induces wave activity, which is also clearly visible in the density plot in the form of density enhancements spanning up to a few tens of degrees of latitude at most. Because the waves are traversing the globe, these enhancements occur at different latitudes as the storm progresses. The mass redistribution due to the heating by the currents, as well as the motion of the ions, also clearly induces changes in the thermospheric wind, as measured by GOCE. The general pattern, seen in Figure6 is that during times of high activity, the wind speed in- creases in the polar cap region (magnetic latitudes above 80 degrees). A return flow pattern at magnetic latitudes in the 60-70 degrees magnetic latitude range on the dusk side is also enhanced, and moves equatorward, even reaching 50 degrees North.

Conclusions and recommendations

The project has clearly shown, through the analysis of data from the Swarm, GOCE and LISA Pathfinder missions, that platform magnetometer data are extremely useful to com- plement science class magnetometers in the detection and characterization of field-aligned currents and the strength and direction of the interplanetary magnetic field. The somewhat lower data product quality from these instruments, when compared to reference instru- ments and missions, are due to the lower instrument specifications, differences in platform handling (lower sampling rate in telemetry) and higher level of stray field perturbations. But in all cases the resulting data are entirely acceptable for the selected purposes, when augmenting high accuracy measurements made by reference missions. The usefulnesss of the data has been demonstrated in several space weather case studies. It should be noted that the reference missions, such as CHAMP and Swarm, and the overall field models de- rived from their measurements, are essential for calibration of the platform instruments. The increased temporal/spatial sampling that is obtained by having additional satellites is of major benefit. Within the scope of this project, we have only been able to study some first cases. The results of the study make it clear that for those cases, the data processing is mature enough for the data to be used in space weather and space science analysis, and that expansion to space missions with similar design characteristics is warranted. Further study is needed, however, on the obtainable performance when using platform magnetometer data on mechanically and electrically more complex satellites, such as those with reaction wheels and rotating solar arrays. This needs to be further investigated. In addition, as follow-on activity it would be particulary interesting to partner with one or more large European space contractors, to assess the cost aspects of integration of the use of platform magnetometers into a dedicated space weather data stream, during mission preparation phases. It is expected that this cost will be significantly lower than making adjustments to data streams for already operational missions, and ceretainly lower than having dedicated magnetometer space weather satellites. The results of the project, and the recommendations put forward here, and much more extensively in the final report, should provide a baseline for such further studies.

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