The Cluster Magnetic Field Investigation: overview of in-flight performance and initial results
TL;DR: In this paper, the authors present an overview of the instrumentation used to measure the magnetic field on the four Cluster spacecraft and an overview the performance of the operational modes used in flight.
Abstract: . The accurate measurement of the magnetic field along the orbits of the four Cluster spacecraft is a primary objective of the mission. The magnetic field is a key constituent of the plasma in and around the magnetosphere, and it plays an active role in all physical processes that define the structure and dynamics of magnetospheric phenomena on all scales. With the four-point measurements on Cluster, it has become possible to study the three-dimensional aspects of space plasma phenomena on scales commeasurable with the size of the spacecraft constellation, and to distinguish temporal and spatial dependences of small-scale processes. We present an overview of the instrumentation used to measure the magnetic field on the four Cluster spacecraft and an overview the performance of the operational modes used in flight. We also report on the results of the preliminary in-orbit calibration of the magnetometers; these results show that all components of the magnetic field are measured with an accuracy approaching 0.1 nT. Further data analysis is expected to bring an even more accurate determination of the calibration parameters. Several examples of the capabilities of the investigation are presented from the commissioning phase of the mission, and from the different regions visited by the spacecraft to date: the tail current sheet, the dusk side magnetopause and magnetosheath, the bow shock and the cusp. We also describe the data processing flow and the implementation of data distribution to other Cluster investigations and to the scientific community in general. Key words. Interplanetary physics (instruments and techniques) – magnetospheric physics (magnetospheric configuration and dynamics) – space plasma physics (shock waves)
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TL;DR: The THEMIS Fluxgate Magnetometer (FGM) as discussed by the authors was designed to study abrupt reconfigurations of the Earth's magnetosphere during the substorm onset phase and is capable of detecting variations of the magnetic field with amplitudes of 0.01 nT.
Abstract: The THEMIS Fluxgate Magnetometer (FGM) measures the background magnetic field and its low frequency fluctuations (up to 64 Hz) in the near-Earth space. The FGM is capable of detecting variations of the magnetic field with amplitudes of 0.01 nT, and it is particularly designed to study abrupt reconfigurations of the Earth’s magnetosphere during the substorm onset phase. The FGM uses an updated technology developed in Germany that digitizes the sensor signals directly and replaces the analog hardware by software. Use of the digital fluxgate technology results in lower mass of the instrument and improved robustness. The present paper gives a description of the FGM experimental design and the data products, the extended calibration tests made before spacecraft launch, and first results of its magnetic field measurements during the first half year in space. It is also shown that the FGM on board the five THEMIS spacecraft well meets and even exceeds the required conditions of the stability and the resolution for the magnetometer.
1,198 citations
Cites methods from "The Cluster Magnetic Field Investig..."
...Experience from magnetometer experiments on more recent missions such as Freja (Zanetti et al. 1994), Equator-S (Fornacon et al. 1999), Cluster (Balogh et al. 2001), Cassini (Dougherty et al. 2004), Double Star (Carr et al. 2005), VenusExpress (Zhang et al. 2006), or Rosetta (Auster et al. 2007;…...
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...Placing just one sensor on a 2 m short boom is a novelty compared to, for example, the Cluster mission where each spacecraft has two sensors mounted on a 5 m boom....
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...Experience from magnetometer experiments on more recent missions such as Freja (Zanetti et al. 1994), Equator-S (Fornacon et al. 1999), Cluster (Balogh et al. 2001), Cassini (Dougherty et al. 2004), Double Star (Carr et al. 2005), VenusExpress (Zhang et al. 2006), or Rosetta (Auster et al. 2007; Glassmeier et al. 2007a) largely contributed to the successful design, fabrication, and operation of the Themis magnetometers....
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...This allows making use of special data analysis tools developed for the Cluster mission (e.g. Glassmeier et al. 2001)....
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...After the four-spacecraft Cluster mission it is the second mission of this kind....
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TL;DR: The first direct determination of the dissipation range of magnetofluid turbulence in the solar wind at the electron scales is reported and a remarkable agreement with theoretical predictions of a quasi-two-dimensional cascade into Kinetic Alfvén Waves (KAW).
Abstract: We report the first direct determination of the dissipation range of magnetofluid turbulence in the solar wind at the electron scales. Combining high resolution magnetic and electric field data of the Cluster spacecraft, we computed the spectrum of turbulence and found two distinct breakpoints in the magnetic spectrum at 0.4 and 35 Hz, which correspond, respectively, to the Doppler-shifted proton and electron gyroscales, ${f}_{{\ensuremath{\rho}}_{p}}$ and ${f}_{{\ensuremath{\rho}}_{e}}$. Below ${f}_{{\ensuremath{\rho}}_{p}}$, the spectrum follows a Kolmogorov scaling ${f}^{\ensuremath{-}1.62}$, typical of spectra observed at 1 AU. Above ${f}_{{\ensuremath{\rho}}_{p}}$, a second inertial range is formed with a scaling ${f}^{\ensuremath{-}2.3}$ down to ${f}_{{\ensuremath{\rho}}_{e}}$. Above ${f}_{{\ensuremath{\rho}}_{e}}$, the spectrum has a steeper power law $\ensuremath{\sim}{f}^{\ensuremath{-}4.1}$ down to the noise level of the instrument. We interpret this as the dissipation range and show a remarkable agreement with theoretical predictions of a quasi-two-dimensional cascade into Kinetic Alfv\'en Waves (KAW).
580 citations
TL;DR: It is shown that the electron Larmor radius plays the role of a dissipation scale in space plasma turbulence and the spectra form a quasiuniversal spectrum following the Kolmogorov's law at MHD scales.
Abstract: To investigate the universality of magnetic turbulence in space plasmas, we analyze seven time periods in the free solar wind under different plasma conditions. Three instruments on Cluster spacecraft operating in different frequency ranges give us the possibility to resolve spectra up to 300 Hz. We show that the spectra form a quasiuniversal spectrum following the Kolmogorov's law $\ensuremath{\sim}{k}^{\ensuremath{-}5/3}$ at MHD scales, a $\ensuremath{\sim}{k}^{\ensuremath{-}2.8}$ power law at ion scales, and an exponential $\ensuremath{\sim}\mathrm{exp} [\ensuremath{-}\sqrt{k{\ensuremath{\rho}}_{e}}]$ at scales $k{\ensuremath{\rho}}_{e}\ensuremath{\sim}[0.1,1]$, where ${\ensuremath{\rho}}_{e}$ is the electron gyroradius. This is the first observation of an exponential magnetic spectrum in space plasmas that may indicate the onset of dissipation. We distinguish for the first time between the role of different spatial kinetic plasma scales and show that the electron Larmor radius plays the role of a dissipation scale in space plasma turbulence.
437 citations
TL;DR: In this article, a sine-wave parametric model with a variable amplitude was used to analyze the lower band of chorus below one half of the electron cyclotron frequency, measured at a radial distance of 4.4 Earth's radii, within a 2000 km long source region located close to the equator.
Abstract: We discuss chorus emissions measured by the four Cluster spacecraft at close separations during a geomagnetically disturbed period on 18 April 2002. We analyze the lower band of chorus below one half of the electron cyclotron frequency, measured at a radial distance of 4.4 Earth's radii, within a 2000 km long source region located close to the equator. The characteristic wave vector directions in this region are nearly parallel to the field lines and the multipoint measurement demonstrates the dynamic character of the chorus source region, changing the Poynting flux direction at time scales shorter than a few seconds. The electric field waveforms of the chorus wave packets (forming separate chorus elements on power spectrograms) show a fine structure consisting of subpackets with a maximum amplitude above 30 mV/m. To study this fine structure we have used a sine-wave parametric model with a variable amplitude. The subpackets typically start with an exponential growth phase, and after reaching the saturation amplitude they often show an exponential decay phase. The duration of subpackets is variable from a few milliseconds to a few tens of milliseconds, and they appear in the waveform randomly, with no clear periodicity. The obtained growth rate (ratio of the imaginary part to the real part of the wave frequency) is highly variable from case to case with values obtained between a few thousandths and a few hundredths. The same chorus wave packets simultaneously observed on the different closely separated spacecraft appear to have a different internal subpacket structure. The characteristic scale of the subpackets can thus be lower than tens of kilometers in the plane perpendicular to the field line, or hundreds of kilometers parallel to the field line (corresponding to a characteristic time scale of few milliseconds during the propagation of the entire wave packet). Using delays of time-frequency curves obtained on different spacecraft, we have found the same propagation direction as obtained from the simultaneous Poynting flux calculations. The delays roughly correspond to the whistler-mode group velocity estimated from the cold plasma theory. We have also observed delays corresponding to antiparallel propagation directions for two neighboring chorus wave packets, less than 0.1 s apart.
395 citations
TL;DR: In this paper, a flow burst was associated with a clear dipolarization ahead of the high-speed part of the predominantly Earthward directed flow, and the authors found that a ∼2000 km thick dipolarisation front moves Earthward and dawnward with a speed of ∼77 km/s.
Abstract: [1] In this paper we study a flow burst event which took place during enhanced geomagnetic activity on July 22, 2001, when Cluster was located in the postmidnight magnetotail. The flow burst was associated with a clear dipolarization ahead of the high-speed part of the predominantly Earthward directed flow. Based on the analysis of the four spacecraft data, we found that a ∼2000 km thick dipolarization front moves Earthward and dawnward with a speed of ∼77 km/s. The plasma before this front is deflected, consistent with the plasma ahead of a localized plasma bubble centered at midnight side being pushed aside by the moving obstacle. The main body of the high-speed flow is directed mainly parallel to the dipolarization front. These observations indicate that the evolution of the dipolarization front across the tail is directly coupled with the fast flow.
371 citations
References
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TL;DR: The Cluster mission as mentioned in this paper provides a new opportunity to study plasma processes and structures in the near-Earth plasma environment using four-point measurements of the magnetic field, which can enable the analysis of the three dimensional structure and dynamics of a range of phenomena which shape the macroscopic properties of the magnetosphere.
Abstract: The Cluster mission provides a new opportunity to study plasma processes and structures in the near-Earth plasma environment Four-point measurements of the magnetic field will enable the analysis of the three dimensional structure and dynamics of a range of phenomena which shape the macroscopic properties of the magnetosphere Difference measurements of the magnetic field data will be combined to derive a range of parameters, such as the current density vector, wave vectors, and discontinuity normals and curvatures, using classical time series analysis techniques iteratively with physical models and simulation of the phenomena encountered along the Cluster orbit The control and understanding of error sources which affect the four-point measurements are integral parts of the analysis techniques to be used The flight instrumentation consists of two, tri-axial fluxgate magnetometers and an on-board data-processing unit on each spacecraft, built using a highly fault-tolerant architecture High vector sample rates (up to 67 vectors s-1) at high resolution (up to 8 pT) are combined with on-board event detection software and a burst memory to capture the signature of a range of dynamic phenomena Data-processing plans are designed to ensure rapid dissemination of magnetic-field data to underpin the collaborative analysis of magnetospheric phenomena encountered by Cluster
373 citations
TL;DR: In this paper, the wave vector of low-frequency fluctuations in a space plasma is estimated based on a generalized minimum variance analysis in the terrestrial magnetosheath and the near-Earth solar wind.
Abstract: . The four Cluster spacecraft provide an excellent opportunity to study spatial structures in the magnetosphere and adjacent regions. Propagating waves are amongst the interesting structures and for the first time, Cluster will allow one to measure the wave vector of low-frequency fluctuations in a space plasma. Based on a generalized minimum variance analysis wave vector estimates will be determined in the terrestrial magnetosheath and the near-Earth solar wind. The virtue and weakness of the wave telescope technique used is discussed in detail. Key words. Electromagnetics (wave propagation) – Magnetospheric physics (MHD waves and instabilities; plasma waves and instabilities)
121 citations
TL;DR: In this paper, an outbound pass through the post-noon high-latitude magnetopause region on 14 February 2001 was studied during a CUTLASS-CUTlASS SuperDARN data collection, and the magnetic footprint of the Cluster spacecraft during the boundary passage was shown to map centrally within the fields-of-view of the CUTlass SuperDarN radars, and to pass across the field-aligned beam of the EISCAT Svalbard radar (ESR) system.
Abstract: . Cluster magnetic field data are studied during an outbound pass through the post-noon high-latitude magnetopause region on 14 February 2001. The onset of several minute perturbations in the magnetospheric field was observed in conjunction with a southward turn of the interplanetary magnetic field observed upstream by the ACE spacecraft and lagged to the subsolar magnetopause. These perturbations culminated in the observation of four clear magnetospheric flux transfer events (FTEs) adjacent to the magnetopause, together with a highly-structured magnetopause boundary layer containing related field features. Furthermore, clear FTEs were observed later in the magnetosheath. The magnetospheric FTEs were of essentially the same form as the original "flux erosion events" observed in HEOS-2 data at a similar location and under similar interplanetary conditions by Haerendel et al. (1978). We show that the nature of the magnetic perturbations in these events is consistent with the formation of open flux tubes connected to the northern polar ionosphere via pulsed reconnection in the dusk sector magnetopause. The magnetic footprint of the Cluster spacecraft during the boundary passage is shown to map centrally within the fields-of-view of the CUTLASS SuperDARN radars, and to pass across the field-aligned beam of the EISCAT Svalbard radar (ESR) system. It is shown that both the ionospheric flow and the backscatter power in the CUTLASS data pulse are in synchrony with the magnetospheric FTEs and boundary layer structures at the latitude of the Cluster footprint. These flow and power features are subsequently found to propagate poleward, forming classic "pulsed ionospheric flow" and "poleward-moving radar auroral form" structures at higher latitudes. The combined Cluster-CUTLASS observations thus represent a direct demonstration of the coupling of momentum and energy into the magnetosphere-ionosphere system via pulsed magnetopause reconnection. The ESR observations also reveal the nature of the structured and variable polar ionosphere produced by the structured and time-varying precipitation and flow. Key words. Ionosphere (auroral ionosphere) Magentospheric physics (magnetopause, cusp and boundary layers; magnetosphere-ionosphere interactions)
102 citations
TL;DR: In this article, it has been shown that when upstream waves and reflected ions are present, the waves inevitably steepen up and lead to a cyclic behavior, which can be explained by the interaction of reflected ions with low-frequency upstream waves.
Abstract: It has been demonstrated by hybrid simulations of collisionless shocks that the shock itself is not stationary but exhibits a cyclic behavior. We have performed a number of one-dimensional hybrid simulations of collisionless shocks and of the interaction of two plasma streams in order to assess the role of upstream waves in the re-formation process. We found that when upstream waves and reflected ions are present, the waves inevitably steepen up and lead to shock re-formation. To investigate the interaction of the reflected ions with upstream waves, the hybrid code is then used in two stages. In the first stage a shock together with upstream diffuse particles and waves is generated. In the second stage, upstream wave trains are isolated, and their subsequent interaction with a finite length ion beam as well as with ions emitted from a moving point source is investigated. The results show that the original wave amplitude grows and the wave steepens. This is due to a deceleration and deflection of the beam ions once they encounter a wave crest with a large local value of the angle ΘBn between the magnetic field and the direction of beam propagation: the deflection leads to a local density increase and due to the compressibility to a corresponding magnetic field increase. This sets up a positive feedback loop during which the background ions are also decelerated and can even get reflected. It is concluded that shock re-formation can be caused by the interaction of reflected ions with low-frequency upstream waves.
78 citations
"The Cluster Magnetic Field Investig..." refers background in this paper
...A particularly interesting objective of Cluster on this topic is the exploration of processes occuring at quasi-parallel bow shock geometries, where the generally accepted shock reformation occurs (Scholer and Burgess, 1992)....
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TL;DR: In this article, the authors present an analysis of data recorded on 10 November 2000, when the four spacecrafts observed an interval of strong mirror-like activity, and use correlation analysis between spacecraft pairs to examine the scale size of the mirror structures in three dimensions.
Abstract: . The Cluster spacecraft have returned the first simultaneous four-point measurements of the magnetosheath. We present an analysis of data recorded on 10 November 2000, when the four spacecrafts observed an interval of strong mirrorlike activity. Correlation analysis between spacecraft pairs is used to examine the scale size of the mirror structures in three dimensions. Two examples are presented which suggest that the scale size of mirror structures is ~ 1500–3000 km along the flow direction, and shortest along the magnetopause normal ( Key words. Magnetosphenic physics (magnetosheath; plasma waves and instabilities)
75 citations
"The Cluster Magnetic Field Investig..." refers background in this paper
...Interplanetary physics (instruments and techniques) – magnetospheric physics (magnetospheric configuration and dynamics) – space plasma physics (shock waves)...
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...On the flanks and on the dayside of the magnetosphere, there is a wealth of plasma phenomena on many scales at the bow shock and in the magnetosheath (for a first four-point study of mirror structures in the magnetosheath, see Lucek et al., 2001)....
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