Search

We review the range of applications and use of multi spacecraft techniques, applicable to close formation arrays of spacecraft, focusing on spatial gradient based methods, and the curlometer in particular. The curlometer was originally applied to Cluster multi-spacecraft magnetic field data, but later was updated for different environments and measurement constraints such as the NASA MMS mission, small-scale formation of 4 spacecraft; the 3 spacecraft configurations of the NASA THEMIS mision, and derived 2-4 point measurements from the ESA Swarm mission. In general, spatial gradient based methods are adaptable to a range of multi-point and multi-scale arrays. We also review the range of other techniques based on the computation of magnetic field gradients and magnetic field topology in general, including: magnetic rotation analysis and various least squares approaches. We review Taylor expansion methodology (FOTE), in particular, which has also been applied to both Cluster and MMS constellations, as well as interpretation of simulations. Four-point estimates of magnetic gradients are limited by uncertainties in spacecraft separations and the magnetic field, as well as the presence of non-linear gradients and temporal evolution. Nevertheless, the techniques can be reliable in many magnetospheric regions where time stationarity is largely applicable, or when properties of the morphology can be assumed (for example, the expected orientation of underlying large-scale structure). Many magnetospheric regions have been investigated directly (illustrated here by the magnetopause, ring current and field-aligned currents at high and low altitudes), and options for variable numbers of spacecraft have been considered. The comparative use of plasma measurements and possible new methodology for arrays of spacecraft greater than four are also considered briefly. [ABSTRACT FROM AUTHOR]

In‐situ spacecraft missions are powerful assets to study processes that occur in space plasmas. One of their main limitations, however, is extrapolating such local measurements to the global scales of the system. To overcome this problem at least partially, multi‐point measurements can be used. There are several multi‐spacecraft missions currently operating in the Earth's magnetosphere, and the simultaneous use of the data collected by them provides new insights into the large‐scale properties and evolution of magnetospheric plasma processes. In this work, we focus on studying the Earth's magnetopause (MP) using a conjunction between the Magnetospheric Multiscale and Cluster fleets, when both missions skimmed the MP for several hours at distant locations during radial interplanetary magnetic field (IMF) conditions. The observed MP positions as a function of the evolving solar wind conditions are compared to model predictions of the MP. We observe an inflation of the magnetosphere (∼0.7 RE), consistent with magnetosheath pressure decrease during radial IMF conditions, which is less pronounced on the flank (<0.2 RE). There is observational evidence of magnetic reconnection in the subsolar region for the whole encounter, and in the dusk flank for the last portion of the encounter, suggesting that reconnection was extending more than 15 RE. However, reconnection jets were not always observed, suggesting that reconnection was patchy, intermittent or both. Shear flows reduce the reconnection rate up to ∼30% in the dusk flank according to predictions, and the plasma β enhancement in the magnetosheath during radial IMF favors reconnection suppression by the diamagnetic drift. Key Points: Simultaneous observations of the equatorial subsolar magnetopause and dusk flank during time‐extended radial interplanetary magnetic fieldThe magnetosphere enlarges ∼0.7 RE in the subsolar region but <0.2 RE in the flank due to the reduced pressure exerted by the solar windSimultaneous reconnection evidence in the subsolar and flank regions more than 15 RE apart is observed during part of the encounter [ABSTRACT FROM AUTHOR]

We report a unique conjugate observation of fast flows and associated current sheet disturbances in the near‐Earth magnetotail by MMS (Magnetospheric Multiscale) and Cluster preceding a positive bay onset of a small substorm at ∼14:10 UT, September 8, 2018. MMS and Cluster were located both at X ∼ −14 RE. A dipolarization front (DF) of a localized fast flow was detected by Cluster and MMS, separated in the dawn‐dusk direction by ∼4 RE, almost simultaneously. Adiabatic electron acceleration signatures revealed from the comparison of the energy spectra confirm that both spacecraft encounter the same DF. We analyzed the change in the current sheet structure based on multi‐scale multi‐point data analysis. The current sheet thickened during the passage of DF, yet, temporally thinned subsequently associated with another flow enhancement centered more on the dawnward side of the initial flow. MMS and Cluster observed intense perpendicular and parallel current in the off‐equatorial region mainly during this interval of the current sheet thinning. Maximum field‐aligned currents both at MMS and Cluster are directed tailward. Detailed analysis of MMS data showed that the intense field‐aligned currents consisted of multiple small‐scale intense current layers accompanied by enhanced Hall‐currents in the dawn‐dusk flow‐shear region. We suggest that the current sheet thinning is related to the flow bouncing process and/or to the expansion/activation of reconnection. Based on these mesoscale and small‐scale multipoint observations, 3D evolution of the flow and current‐sheet disturbances was inferred preceding the development of a substorm current wedge. Key Points: Evolution of localized fast flows and dipolarization front is obtained from multi‐scale multi‐point observations in near‐Earth magnetotailCurrent sheet thinning accompanied by intense field‐aligned currents is detected following the passage of the dipolarization frontFrom signatures of adiabatic electron acceleration it is confirmed that the same flow front was detected by the multi‐point measurements [ABSTRACT FROM AUTHOR]

Cold plasma (up to few tens of electron volts) of ionospheric origin is present most of the time, in most of the regions of the Earth's magnetosphere. However, characterizing it using in situ measurements is difficult, owing to spacecraft electrostatic charging, as often this charging is at levels comparable to or even higher than the equivalent energy of the cold plasma. To overcome this difficulty, active potential control devices are usually placed on spacecraft that artificially reduce spacecraft charging. The electrostatic potential structure around the spacecraft is often assumed to be spherically symmetric, and corrections are applied to the measured particle distribution functions. In this work, we show that large deviations from the spherical model are present, owing to the presence of long electric field booms. We show examples using Magnetospheric MultiScale spacecraft measurements of the electrostatic potential structure and its effect on the measurement of cold ion beams. Overall, we find that particle detectors underestimate the cold ion density under certain conditions, even when their bulk kinetic energy exceeds the equivalent spacecraft potential energy and the ion beam reaches the spacecraft. Active potential control helps in reducing this unwanted effect, but we show one event with large cold ion density (∼10 cm−3) where particle detectors provide density estimates a factor of 3–5 below the density estimated from the plasma frequency. Understanding these wake effects indirectly constrains some properties of the magnetospheric cold ion component, such as their drift energy, direction, and temperature. Plain Language Summary: The near‐Earth space environment is filled with plasma, that is, ionized gas that interacts with electromagnetic fields. Owing to its relative accessibility, it constitutes an invaluable laboratory for understanding how plasmas behave in nature. Many spacecraft missions have been launched with the purpose of studying space plasmas since the 1960s, when the space era began. They carry in situ instrumentation capable of measuring the properties of electric and magnetic fields, as well as the properties of ions and electrons. One problem these missions encounter is that the spacecraft produce their own electromagnetic fields that locally interact with the plasma and modify their properties. In this work, we quantify the effects of electric field booms mounted on spacecraft, which have length scales much larger than the spacecraft itself. The electromagnetic properties of these spacecraft booms strongly affect the detection and characterization of cold plasma, that is, low‐temperature plasma. Cold plasma in the near‐Earth space environment originates in the ionosphere, populates the whole magnetosphere, and constitutes the most abundant magnetospheric population. Key Points: The modeled MMS spacecraft electrostatic potential distribution deviates from spherical because of the electric field boomsMeasurements by particle detectors in the magnetosphere are strongly affected and biased by the complex potential structureCold ion beam properties are constrained by characterizing the ion wake and using a combination of particle and electric field measurements [ABSTRACT FROM AUTHOR]

Low‐energy ions of ionospheric origin with energies below 10s of electron volt dominate most of the volume and mass of the terrestrial magnetosphere. However, sunlit spacecraft often become positively charged to several 10s of volts, which prevents low‐energy ions from reaching the particle detectors on the spacecraft. Magnetospheric Multiscale spacecraft (MMS) observations show that ultralow‐frequency (ULF) waves drive low‐energy ions to drift in the E × B direction with a drift velocity equal to VE × B, and low‐energy ions were accelerated to sufficient total energy to be measured by the MMS/Fast Plasma Investigation Dual Ion Spectrometers. The maximum low‐energy ion energy flux peak seen in MMS1's dual ion spectrometer measurements agreed well with the theoretical calculation of H+ ion E × B drift energy. The density of ions in the energy range below minimum energy threshold was between 1 and 3 cm−3 in the magnetosphere subsolar region in this event. Plain Language Summary: Low‐energy ions of ionospheric origin dominate most of the volume and mass of the terrestrial magnetosphere. The amount of low‐energy ions that exist and the way they are distributed are open areas of study important to understanding the magnetized plasma environment surrounding Earth. Magnetospheric Multiscale spacecraft (MMS) is a four‐spacecraft mission carrying numerous instruments for characterizing the particles and fields making up this environment and which can be applied to investigating low‐energy ions. In this study, we found an event based on the MMS data showing that ultralow‐frequency waves drive low‐energy ions to drift in the direction normal to the plane defined by the electric and magnetic fields. This charged particle motion is called the E × B drift. During this event, the observations taken by the MMS from a distance of 64,000 km in space were consistent with the theoretical calculation of the charged particle E × B drift. The results were applied to show density of low‐energy ions in the subsolar magnetosphere region during this event was in the range of 1 to 3 cm−3. This event provides evidence for the E × B drift action based on magnetospheric particle and field observations that can be further applied to detect low‐energy ions in other magnetospheric locations. Key Points: An ultralow‐frequency wave and correlated low‐energy ion monoenergetic acceleration was observed by MMS1The maximum ion flux energy level detected by MMS1 DIS agreed well with the theoretical calculation of H+ ion E × B drift energyThe density of low‐energy ions with energy below minimum energy threshold was between 1 and 3 cm−3 in the magnetosphere subsolar region [ABSTRACT FROM AUTHOR]

Abstract: Turbulence in the Earth's magnetosheath at ion kinetic scales is investigated with the magnetospheric multiscale spacecraft. Several possibilities in the wave paradigm have been invoked to explain plasma turbulence at ion kinetic scales such as kinetic Alfvén, slow, or magnetosonic waves. To differentiate between these different plasma waves is a challenging task, especially since some waves, in particular, kinetic slow waves and kinetic Alfvén waves, share some properties making the possibility to distinguishing between them very difficult. Using the excellent time resolution data set provided from both the fluxgate magnetometer and the Fast Plasma Instrument, the ratio of trace velocity fluctuations to the magnetic fluctuations (in Alfvén units), which is termed the Alfvén ratio, can be calculated down to ion kinetic scales. Comparison of the measured Alfvén ratio is performed with respect to the expectation from two‐fluid magnetohydrodynamic theory for the kinetic slow wave and kinetic Alfvén wave. Moreover, the plasma data also allow normalized fluctuation amplitudes of density and magnetic field to be compared differentiating between magnetosonic‐like and kinetic Alfvén‐like turbulence. Using these two different ratios, we can rule out that the fluctuations at ion scales are dominated by magnetosonic‐like fluctuations or kinetic slow‐like fluctuations and show that they are consistent with kinetic Alfvén‐like fluctuations. This suggests that in the wave paradigm, heating in the direction of the parallel magnetic field is predominantly by the Landau damping of the kinetic Alfvén wave. [ABSTRACT FROM AUTHOR]

NASA’s magnetospheric multiscale (MMS) mission was successfully launched in March 2015. The scientific objectives of MMS are to explore and understand fundamental plasma physics processes in the earth’s magnetosphere: magnetic reconnection, particle acceleration, and turbulence. The region of scientific interest of MMS is in a tenuous plasma environment where the positive spacecraft potential may reach an equilibrium as high as several tens of volts. The active spacecraft potential control (ASPOC) instrument neutralizes the spacecraft potential by releasing the positive charge produced by indium ion emitters. While the method has successfully been applied to other spacecraft such as Cluster and Double Star, new developments in the design of the emitters and the electronics are enabling lower spacecraft potentials and higher reliability compared to previous missions. In this paper, we report the initial results from the tests of the ASPOC performance during the commissioning phase and discuss the different effects on the particle and field instruments observed at different plasma environments in the magnetosphere. [ABSTRACT FROM PUBLISHER]

An analytical model of magnetosheath plasma flow is described and compared with a large dataset of magnetosheath ion flow velocity measurements from Cluster and THEMIS spacecraft. The model is based on previous works by Kobel and Flückiger (1994) and Génot et al. (2011) and has been modified to overcome the restrictions of these models on the shape of model magnetopause and bow shock. Our model is compatible with any parabolic bow shock model and arbitrary magnetopause model. The model is relatively simple to implement and computationally inexpensive, and its only inputs are upstream solar wind parameters. Comparison with observed data yields a good correspondence: median error in the direction of flow velocity is comparable with the instrumental error, and flow magnitude is predicted with a reasonable accuracy (relative error in flow speed was less than 25% for 86.5% of observations). [ABSTRACT FROM AUTHOR]