Spacecraft Magnetic Field Research Articles

Evaluating the Accuracy of Magnetospheric Magnetic Field Models Using Cluster Spacecraft Magnetic Field Measurements

Magnetic fields in the inner magnetosphere can be obtained as vector sums of the Earth’s own internal magnetic field and magnetic fields stemming from currents flowing in the space plasma. While the Earth’s internal magnetic field is accurately described by the International Geomagnetic Reference Field (IGRF) model, the characterization of the external magnetic fields is significantly more complicated, as they are highly variable and dependent on the actual level of the geomagnetic activity. Tsyganenko family magnetic field models (T89, T96, T01, TA15B, TA15N), parameterized by the geomagnetic activity level and solar wind parameters, are often used by the involved community to describe these fields. In the present paper, we use a large dataset (2001–2018) of magnetospheric magnetic field measurements obtained by the four Cluster spacecraft to assess the accuracy of these models. We show that, while the newer models (T01, TA15B, TA15N) perform significantly better than the old ones (T89, T96), there remain some systematic deviations, in particular at larger latitudes. Moreover, we compare the locations of the min-B equator determined using the four-point Cluster spacecraft measurements with the locations determined using the magnetic field models. We demonstrate that, despite the newer models being comparatively slightly more accurate, an uncertainty of about one degree in the latitude of the min-B equator remains.

Analyses of residual accelerations for TianQin based on the global MHD simulation

TianQin is a proposed space-based gravitational wave observatory. It is designed to detect the gravitational wave signals in the frequency range of 0.1 mHz–1 Hz. At a geocentric distance of 105 km, the plasma in the Earth magnetosphere will contribute as the main source of environmental noises. Here, we analyze the acceleration noises that are caused by the magnetic field of space plasma for the test mass of TianQin. The real solar wind data observed by the Advanced Composition Explorer are taken as the input of the magnetohydrodynamic simulation. The Space Weather Modeling Framework is used to simulate the global magnetosphere of the Earth, from which we obtain the plasma and magnetic field parameters on the detector’s orbits at φs = 0°, 30°, 60° and 90°, where φs is the acute angle between the line that joins the Sun and the Earth and the projection of the normal of the detector’s plane on the ecliptic plane. We calculate the time series of the residual accelerations and the corresponding amplitude spectral densities on these orbit configurations. We find that the residual acceleration produced by the interaction between the TM’s magnetic moment induced by the space magnetic field and the spacecraft magnetic field (aM1) is the dominant term, which can approach 10−15 m s–2 Hz–1/2 at f ≈ 0.2 mHz for the nominal values of the magnetic susceptibility (χm = 10−5) and the magnetic shielding factor (ξm = 10) of the test mass. The ratios between the amplitude spectral density of the acceleration noise caused by the space magnetic field and the preliminary goal of the inertial sensor are 0.38 and 0.08 at 1 mHz and 10 mHz, respectively. We discuss the further reduction of this acceleration noise by decreasing χm and/or increasing ξm in the future instrumentation development for TianQin.

Drag on a spacecraft produced by the interaction of its magnetic field with the Earth's ionosphere. Physical modelling

The dynamic interaction of a “magnetized” sphere (a sphere with a magnetic field source) with a hypersonic rarefied plasma flow was studied experimentally to model the interaction of a “magnetized” spacecraft with the ionospheric plasma. The experiments were conducted at the plasma flow velocity parallel and normal to the sphere's magnetic field. The electromagnetic drag coefficient was determined as a function of the ratio of the magnetic pressure to the dynamic pressure and the ratio of the mini-magnetosphere dimension to the spacecraft dimension (sphere radius). It was shown that a spacecraft's magnetic field of about 0.8–1.5 T produces an electromagnetic drag comparable with the drag produced using ion beam technologies of space debris removal, which require dedicated spacecraft with plasma accelerators. Energy-efficient compact sources of a 0.8–1.5 Т magnetic field for space debris objects may be made using special arrangements of small-size permanent magnets – Halbach arrays.

Correcting Parker Solar Probe Electron Measurements for Spacecraft Magnetic and Electric Fields

AbstractThe spacecraft body of the Parker Solar Probe may interfere with electron measurements in two ways. The first is the presence of several permanent magnets near the Solar Probe Analyzers (SPAN) instruments. The second is the widely varying spacecraft potential. We estimate the effect of these interferences by performing particle tracing simulations on electrons of various energies using a simplified model of the spacecraft potential and measurements of the magnetic fields. From this we can (1) estimate the individual and combined fields of view of the SPAN‐E instruments, (2) identify regions of phase space that may be highly distorted, and (3) simulate measurements of the velocity distribution function. We compute density, temperature, and bulk velocity moments of the measured distribution functions and find that a correction table derived from the particle tracing results can be incorporated in the computation to greatly decrease the errors caused by the spacecraft potential and magnetic fields. Similar tables could be computed for a wide range of spacecraft potentials and applied during the processing of actual SPAN data.

Reducing the influence of interference of spacecraft magnetic field on magnetic measurements

The article presents constructive ways of reducing the influence of magnetic interference from spacecraft, due to its own magnetic fields, on the on-board magnetic measurements, as well as reducing the resulting magnetic moments. Well-known methods of removing magnetometer sensors from the locations of the most powerful sources of magnetic fields of a spacecraft, in particular, using extendable booms, are considered. In addition, methods for reducing the influence of spacecraft self- magnetic fields on the onboard magnetometric navigation support systems using known closed and proposed hemispherical ferromagnetic shields are considered

Control of the drag on a spacecraft in the earth’s ionosphere using the spacecraft’s magnetic field

This paper shows the possibility of active control of the drag on a spacecraft in the Earth's ionosphere using the electromagnetic force produced by the interaction of the spacecraft's magnetic field with the incident plasma flow. As a result of experimental simulation of the dynamic interaction of the magnetic field of a sphere with a hypersonic flow of the rarefied ionospheric plasma, the sphere drag coefficient is determined as a function of the ratio of the magnetic pressure to the dynamic pressure in a wide range of the angle between the incident flow velocity and the magnetic field and the angle between the incident hypersonic plasma flow and the velocity of a subsonic plasma jet injected from the sphere surface. It is shown that injecting a subsonic plasma jet into the mini-magnetosphere cavity provides a several-fold increase in the drag coefficient of a “magnetized” sphere (a sphere with its own magnetic field) in a hypersonic rarefied plasma flow in comparison with a “nonmagnetized” sphere. A 0.6 … 0.8 T magnetic field of a “magnetized” body may be an efficient means for its deorbiting through increasing the drag on the body in the Earth's ionosphere, which provides a way for removing space debris objects to lower orbits.

Can ring current stabilize magnetotail during steady magnetospheric convection?

AbstractThe present study investigates the role of the ring current in stabilizing the magnetotail during steady magnetospheric convection (SMC) events. We develop a method for estimation of the symmetric ring current intensity from the single spacecraft magnetic field observations. The method is applied to a large number of SMC events identified using three different automatic procedures adopted from the literature. It is found that the symmetric ring current can be weak or strong depending on a particular event. We find a significant fraction of events that have a rather weak symmetric ring current in spite of the strong solar wind driving during the event. These findings imply that the symmetric ring current plays no role in the magnetotail stabilization.

First results of the MAVEN magnetic field investigation

AbstractTwo Mars Atmosphere and Volatile EvolutioN magnetic field sensors sample the ambient magnetic field at the outer edge of each solar array. We characterized relatively minor spacecraft‐generated magnetic fields using in‐flight subsystem tests and spacecraft maneuvers. Dynamic spacecraft fields associated with the power subsystem (≤1 nT) are compensated for using spacecraft engineering telemetry to identify active solar array circuits and monitor their electrical current production. Static spacecraft magnetic fields are monitored using spacecraft roll maneuvers. Accuracy of measurement of the environmental magnetic field is demonstrated by comparison with field directions deduced from the symmetry properties of the electron distribution function measured by the Solar Wind Electron Analyzer. We map the bow shock, magnetic pileup boundary, the V × B convection electric field and ubiquitous proton cyclotron, and 1 Hz waves in the ion foreshock region.

Characterization of CHAMP magnetic data anomalies: magnetic contamination and measurement timing

The CHAMP (CHAllenging Minisatellite Payload) mission ended after more than ten years in space on 19 September 2010. For achieving a high measurement accuracy of the magnetometers on CHAMP, detailed analyses of spacecraft magnetic characteristics in orbit are required. A decade of continuous magnetometer and housekeeping data are a good basis for evaluating some of the effects of variable spacecraft magnetic fields on the ambient field determination. It was found that some perturbations of FGM (FluxGate vector Magnetometer) or OVM (OVerhauser scalar Magnetometer) measurements are caused by stray fields induced by the power system, the ASC (advanced stellar compass) instrument or magneto-torquer currents. The magnetic effect of solar currents on FGM measurements varies with the local time of the orbit and amounts to 0.2 nT. In cases when one head of the ASC instrument was blinded by the sun, sometimes transient drops in instrument current strength occur, which were accompanied by magnetic disturbance signals (∼0.3 nT) in FGM measurements. The magnetic residual contamination of OVM data by the torquer currents was of order 0.1 nT but still detectable. An improved torquer correction matrix is derived which eliminates this effect. In-flight scalar calibration parameters revealed some of the effects of timing anomalies. Time lags between FGM and OVM readings are misinterpreted by the scalar calibration as variations of the angles between some of the sensor axes. The resulting amplitudes of the anomalies presented here lie in the range of some 0.1 nT, but they are systematic in nature.

Determining the full magnetic field gradient from two spacecraft measurements under special constraints

In this exploration, it is revealed for the first time that the full magnetic field gradient and current density distribution can be derived from two spacecraft magnetic field measurements under the assumptions that the magnetic field is approximately stationary and force‐free, conditions which are particularly relevant to low altitude regions of the Earth's magnetosphere and in the regions containing the field aligned current systems. The magnetic field gradients along the spacecraft line, spacecraft velocity direction and the normal direction can be defined so that the full magnetic field gradient tensor is determined, projected into any coordinate system. One test for an ideal situation has been made, which confirms the validity of this approach with very high accuracy. This method can be applied to the determination of the magnetic field gradient tensor and current density for the forthcoming multispacecraft Swarm mission.

Automatic parameterization for magnetometer zero offset determination

Abstract. In-situ magnetic field measurements are of critical importance in understanding how the Sun creates and controls the heliosphere. To ensure the measurements are accurate, it is necessary to track the combined slowly varying spacecraft magnetic field and magnetometer zero offset – the systematic error in the sensor measurements. For a 3-axis stabilised spacecraft, in-flight correction of zero offsets primarily relies on the use of Alfvénic rotations in the magnetic field. We present a method to automatically determine a key parameter related to the ambient compressional variance of the signal (which determines the selection criteria for identifying clear Alfvénic rotations). We apply our method to different solar wind conditions, performing a statistical analysis of the data periods required to achieve a 70% chance of calculating an offset using Helios datasets. We find that 70% of 40 min data periods in regions of fast solar wind possess sufficient rotational content to calculate an offset. To achieve the same 70% calculation probability in regions of slow solar wind requires data periods of 2 h duration. We also find that 40 min data periods at perihelion compared to 1 h and 40 min data periods at aphelion are required to achieve the same 70% calculation probability. We compare our method with previous work that uses a fixed parameter approach and demonstrate an improvement in the calculation probability of up to 10% at aphelion and 5% at perihelion.

Detrital remanent magnetization in the solar nebula

We introduce the theoretical basis of a new form of remanent magnetization that likely formed on primitive bodies in the solar system. Accretional detrital remanent magnetization (ADRM) operates via “compass needle”‐type alignment of ferromagnetic solids with locally uniform background fields in the solar nebula. Accretion of coherently aligned magnetic particles should have formed aggregates up to centimeters in size with significant net magnetic moment. We quantify several processes that constrain the likelihood of ADRM formation, finding that rotational gas damping and background field intensities expected for the solar nebula are sufficient to mutually align magnetic particles with diameters between ∼30 μm and several cm. The lower bound is dictated by Brownian motion or radiative torque while the upper bound is set by aerodynamic torque on non‐spherical particles. Processes important for interstellar dust dynamics such as Larmor‐type precession and Purcell torque are less significant in the solar nebula. ADRM can be potentially observed as zones of coherent magnetization in primitive chondrites and may be detected by spacecraft magnetic field observations on the surfaces of small bodies. Observational identification and characterization of ADRM would constrain the strength and geometry of magnetic fields in the early solar system, the accretion process of sub‐meter sized objects, the formation regions of chondrite parent bodies, and the alteration history of chondritic components.

Reconstruction of propagating Kelvin–Helmholtz vortices at Mercury's magnetopause

A series of quasi-periodic magnetopause crossings were recorded by the MESSENGER spacecraft during its third flyby of Mercury on 29 September 2009, likely caused by a train of propagating Kelvin–Helmholtz (KH) vortices. We here revisit the observations to study the internal structure of the waves. Exploiting MESSENGER's rapid traversal of the magnetopause, we show that the observations permit a reconstruction of the structure of a rolled-up KH vortex directly from the spacecraft's magnetic field measurements. The derived geometry is consistent with all large-scale fluctuations in the magnetic field data, establishes the non-linear nature of the waves, and shows their vortex-like structure. In several of the wave passages, a reduction in magnetic field strength is observed in the middle of the wave, which is characteristic of rolled-up vortices and is related to the increase in magnetic pressure required to balance the centrifugal force on the plasma in the outer regions of a vortex, previously reported in computer simulations. As the KH wave starts to roll up, the reconstructed geometry suggests that the vortices develop two gradual transition regions in the magnetic field, possibly related to the mixing of magnetosheath and magnetospheric plasma, situated at the leading edges from the perspectives of both the magnetosphere and the magnetosheath.

Recalibration of the CHAMP satellite magnetic field measurements

The eight years of continuous CHAMP operation allows us to investigate the long-term behaviour of the vector magnetometer on board the satellite. For the processing of the FGM vector magnetometer data primarily nine instrument parameters are needed (three offsets, three scale factors and three misalignment angles). These are determined in pre-flight calibration and routinely in the in-flight scalar calibration. A review of the temporal evolution shows that the scale factors exhibit a clear change with time. This variation of the sensitivity can be well described by a logarithmic function. The offsets and the misalignment angles show no long-term trend. The Z component offset performs medium-term variations within the range of 1 nT. These have been corrected on a daily basis. When processing the FGM data over the whole period with the derived parameters, the difference between the FGM and OVM readings stays below 0.5 nT (corresponding to about 0.2 nT standard deviation). For a further improvement the error model has to be extended beyond the scalar calibration. Dynamic spacecraft magnetic fields varying over an orbit are falsely interpreted by the scalar calibration and cause apparent variations of the instrument parameters. In the case of CHAMP, the magnetic field generated by the power system is responsible for most of the remaining errors. We have applied an extended error model that can account for the time-varying disturbance fields. When considering them properly the standard deviation of scalar errors less than 0.1 nT can be achieved.

In-flight spacecraft magnetic field monitoring using scalar/vector gradiometry

Earth magnetic field mapping from planetary orbiting satellites requires a spacecraft magnetic field environment control program combined with the deployment of the magnetic sensors on a boom in order to reduce the measurement error caused by the local spacecraft field. Magnetic mapping missions (Magsat, Oersted, CHAMP, SAC-C MMP and the planned ESA Swarm project) carry a vector magnetometer and an absolute scalar magnetometer for in-flight calibration of the vector magnetometer scale values and for monitoring of the inter-axes angles and offsets over time intervals from months to years. This is done by comparing the two magnetometer outputs for several days and for as many different external field directions and amplitudes in the satellite frame as available. The vector and the scalar sensor may be placed of the order of 2 m apart and at the end of an about 10 m long boom counted from the spacecraft centre-of-gravity. In line with the classical dual vector sensors technique for monitoring the spacecraft magnetic field, this paper proposes and demonstrates that a similar combined scalar/vector gradiometry technique is feasible by using the measurements from the boom-mounted scalar and vector sensors onboard the Oersted satellite. For Oersted, a large difference between the pre-flight determined spacecraft magnetic field and the in-flight estimate exists causing some concern about the general applicability of the dual sensors technique.

Cluster electric current density measurements within a magnetic flux rope in the plasma sheet

On August 22, 2001 all 4 Cluster spacecraft nearly simultaneously penetrated a magnetic flux rope in the tail. The flux rope encounter took place in the central plasma sheet, βi ∼ 1–2, near the leading edge of a bursty bulk flow. The “time‐of‐flight” of the flux rope across the 4 spacecraft yielded Vx ∼ 700 km/s and a diameter of ∼1 Re. The speed at which the flux rope moved over the spacecraft is in close agreement with the Cluster plasma measurements. The magnetic field profiles measured at each spacecraft were first modeled separately using the Lepping‐Burlaga force‐free flux rope model. The results indicated that the center of the flux rope passed northward (above) s/c 3, but southward (below) of s/c 1, 2 and 4. The peak electric currents along the central axis of the flux rope predicted by these single‐s/c models were ∼15–19 nA/m2. The 4‐spacecraft Cluster magnetic field measurements provide a second means to determine the electric current density without any assumption regarding flux rope structure. The current profile determined using the curlometer technique was qualitatively similar to those determined by modeling the individual spacecraft magnetic field observations and yielded a peak current density of 17 nA/m2 near the central axis of the rope. However, the curlometer results also showed that the flux rope was not force‐free with the component of the current density perpendicular to the magnetic field exceeding the parallel component over the forward half of the rope, perhaps due to the pressure gradients generated by the collision of the BBF with the inner magnetosphere. Hence, while the single‐spacecraft models are very successful in fitting flux rope magnetic field and current variations, they do not provide a stringent test of the force‐free condition.

Fast flow during current sheet thinning

In this paper we report Cluster observation of a fast flow event in the plasma sheet associated with a small auroral substorm intensification at 1838 UT on August 12, 2001. Cluster, located in the plasma sheet, experienced significant thinning of the current sheet associated with a high‐speed Earthward flow of 900 km/s. By using the four spacecraft magnetic field data and a Harris‐type current sheet model, it was estimated that the thickness of the current sheet changes from about 1 RE before the flow observation down to 400 km, i.e., close to the ion inertia length. In the vicinity of this thin current sheet there were also signatures of enhanced current density off the center of the neutral sheet, consistent with recent Geotail results.

In-flight calibration of the NEAR magnetometer

The science objectives for the Near Earth Asteroid Rendezvous (NEAR) Magnetometer Experiment (MAG) are to measure a possible magnetic field of 433 Eros to 5 nT accuracy and secondarily to detect asteroid-solar wind interaction signatures. Because the MAG sensor is body mounted, achieving this accuracy required detailed analysis of spacecraft magnetic fields during cruise. Sources of magnetic contamination identified prior to launch and during cruise are: propulsion latch valves, fixed 190 nT residual held, solar arrays and power harness, variable 15 to 60 nT field, power distribution terminal board, /spl sim/30 nT field with 5 nT variations, power shunting circuitry, 1-5 nT variations, the MAG sensor survival heater, 6 nT steps and attitude control momentum wheels, and 1 nT amplitude at 0.5 to 10 Hz from each of four wheels. Analysis of cruise data was used to create accurate /spl plusmn/1 nT models for the fixed and variable fields with signals below 0.5 Hz to provide correction of the raw data. The spacecraft field corrections and MAG calibration were validated with data from the Earth swing-by of January 1997. Comparison of solar wind magnetic held measurements from the WIND spacecraft and NEAR from January 22-24, 1997, before and after the Earth swing-by maneuver, confirm that the resulting NEAR magnetic field measurements are accurate to 1-2 nT.

An algorithm for estimating field-aligned currents from single spacecraft magnetic field measurements: a diagnostic tool applied to Freja satellite data

Introduces an algorithm which allows under certain assumptions to estimate the current density distribution along the track from single spacecraft magnetic field measurements. The assumptions are chosen that they meet at least partly the conditions encountered by the Freja satellite on its northernmost orbital segment, namely that all currents are field-aligned, ionospheric currents do not contribute significantly to the magnetic field measurements, and the velocity component perpendicular to the field direction is large (>5 km/s). Problems arise in the case of moving or temporally varying current systems. In those cases additional data like ground-based observations are needed to resolve the spatio-temporal ambiguity. With the help of simulated data the authors can show that for most of the encountered stationary current geometries the estimates fall into an uncertainty band of /spl plusmn/20%. Current density estimates are a local quantity with a spatial resolution of order 1 km. They are thus very suitable for use in studying the fine structure of auroral plasma processes.

The WIND magnetic field investigation

The magnetic field experiment on WIND will provide data for studies of a broad range of scales of structures and fluctuation characteristics of the interplanetary magnetic field throughout the mission, and, where appropriate, relate them to the statics and dynamics of the magnetosphere. The basic instrument of the Magnetic Field Investigation (MFI) is a boom-mounted dual triaxial fluxgate magnetometer and associated electronics. The dual configuration provides redundancy and also permits accurate removal of the dipolar portion of the spacecraft magnetic field. The instrument provides (1) near real-time data at nominally one vector per 92 s as key parameter data for broad dissemination, (2) rapid data at 10.9 vectors s−1 for standard analysis, and (3) occasionally, snapshot (SS) memory data and Fast Fourier Transform data (FFT), both based on 44 vectors s−1. These measurements will be precise (0.025%), accurate, ultra-sensitive (0.008 nT/step quantization), and where the sensor noise level is <0.006 nT r.m.s. for 0–10 Hz. The digital processing unit utilizes a 12-bit microprocessor controlled analogue-to-digital converter. The instrument features a very wide dynamic range of measurement capability, from ±4 nT up to ±65 536 nT per axis in eight discrete ranges. (The upper range permits complete testing in the Earth's field.) In the FTT mode power spectral density elements are transmitted to the ground as fast as once every 23 s (high rate), and 2.7 min of SS memory time series data, triggered automatically by pre-set command, requires typically about 5.1 hours for transmission. Standard data products are expected to be the following vector field averages: 0.0227-s (detail data from SS), 0.092 s (‘detail’ in standard mode), 3 s, 1 min, and 1 hour, in both GSE and GSM coordinates, as well as the FFT spectral elements. As has been our team's tradition, high instrument reliability is obtained by the use of fully redundant systems and extremely conservative designs. We plan studies of the solar wind: (1) as a collisionless plasma laboratory, at all time scales, macro, meso and micro, but concentrating on the kinetic scale, the highest time resolution of the instrument (=0.022 s), (2) as a consequence of solar energy and mass output, (3) as an external source of plasma that can couple mass, momentum, and energy to the Earth's magnetosphere, and (4) as it is modified as a consequence of its imbedded field interacting with the moon. Since the GEOTAIL Inboard Magnetometer (GIM), which is similar to the MFI instrument, was developed by members of our team, we provide a brief discussion of GIM related science objectives, along with MFI related science goals.