Publications
225 results found
Lavraud B, Kieokaew R, Fargette N, et al., 2021, Magnetic reconnection as a mechanism to produce multiple protonpopulations and beams locally in the solar wind, Journal of Astrophysics and Astronomy, Vol: 656, Pages: 1-8, ISSN: 0250-6335
Context. Spacecraft observations early revealed frequent multiple protonpopulations in the solar wind. Decades of research on their origin have focusedon processes such as magnetic reconnection in the low corona and wave-particleinteractions in the corona and locally in the solar wind.Aims.This study aimsto highlight that multiple proton populations and beams are also produced bymagnetic reconnection occurring locally in the solar wind. Methods. We use highresolution Solar Orbiter proton velocity distribution function measurements,complemented by electron and magnetic field data, to analyze the association ofmultiple proton populations and beams with magnetic reconnection during aperiod of slow Alfv\'enic solar wind on 16 July 2020. Results. At least 6reconnecting current sheets with associated multiple proton populations andbeams, including a case of magnetic reconnection at a switchback boundary, arefound during this day. This represents 2% of the measured distributionfunctions. We discuss how this proportion may be underestimated, and how it maydepend on solar wind type and distance from the Sun. Conclusions. Althoughsuggesting a likely small contribution, but which remains to be quantitativelyassessed, Solar Orbiter observations show that magnetic reconnection must beconsidered as one of the mechanisms that produce multiple proton populationsand beams locally in the solar wind.
Oieroset M, Phan TD, Ergun R, et al., 2021, Spatial evolution of magnetic reconnection diffusion region structures with distance from the X-line, Physics of Plasmas, Vol: 28, ISSN: 1070-664X
We report Magnetospheric Multiscale four-spacecraft observations of a thin reconnecting current sheet with weakly asymmetric inflow conditions and a guide field of approximately twice the reconnecting magnetic field. The event was observed at the interface of interlinked magnetic field lines at the flank magnetopause when the maximum spacecraft separation was 370 km and the spacecraft covered ∼1.7 ion inertial lengths (di) in the reconnection outflow direction. The ion-scale spacecraft separation made it possible to observe the transition from electron-only super ion-Alfvénic outflow near the electron diffusion region (EDR) to the emergence of sub-Alfvénic ion outflow in the ion diffusion region (IDR). The EDR to IDR evolution over a distance less than 2 di also shows the transition from a near-linear reconnecting magnetic field reversal to a more bifurcated current sheet as well as significant decreases in the parallel electric field and dissipation. Both the ion and electron heating in this diffusion region event were similar to the previously reported heating in the far downstream exhausts. The dimensionless reconnection rate, obtained four different ways, was in the range of 0.13–0.27. This event reveals the rapid spatial evolution of the plasma and electromagnetic fields through the EDR to IDR transition region
Dunlop MW, Dong XC, Wang TY, et al., 2021, Curlometer technique and applications, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-29, ISSN: 2169-9380
We review the range of applications and use of the curlometer, initially developed to analyze Cluster multi-spacecraft magnetic field data; but more recently adapted to other arrays of spacecraft flying in formation, such as MMS small-scale, 4-spacecraft configurations; THEMIS close constellations of 3–5 spacecraft, and Swarm 2–3 spacecraft configurations. Although magnetic gradients require knowledge of spacecraft separations and the magnetic field, the structure of the electric current density (for example, its relative spatial scale), and any temporal evolution, limits measurement accuracy. Nevertheless, in many magnetospheric regions the curlometer is reliable (within certain limits), particularly under conditions of time stationarity, or with supporting information on morphology (for example, when the geometry of the large scale structure is expected). A number of large-scale regions have been covered, such as: the cross-tail current sheet, ring current, the current layer at the magnetopause and field-aligned currents. Transient and smaller scale current structures (e.g., reconnected flux tube or dipolarisation fronts) and energy transfer processes. The method is able to provide estimates of single components of the vector current density, even if there are only two or three satellites flying in formation, within the current region, as can be the case when there is a highly irregular spacecraft configuration. The computation of magnetic field gradients and topology in general includes magnetic rotation analysis and various least squares approaches, as well as the curlometer, and indeed the added inclusion of plasma measurements and the extension to larger arrays of spacecraft have recently been considered.
Mejnertsen L, Eastwood J, Chittenden J, 2021, Control of magnetopause flux rope topology by non-local reconnection, Frontiers in Astronomy and Space Sciences, Vol: 8, Pages: 1-15, ISSN: 2296-987X
Dayside magnetic reconnection between the interplanetary magnetic field and the Earth’s magnetic field is the primary mechanism enabling mass and energy entry into the magnetosphere. During favorable solar wind conditions, multiple reconnection X-lines can form on the dayside magnetopause, potentially forming flux ropes. These flux ropes move tailward, but their evolution and fate in the tail is not fully understood. Whilst flux ropes may constitute a class of flux transfer events, the extent to which they add flux to the tail depends on their topology, which can only be measured in situ by satellites providing local observations. Global simulations allow the entire magnetospheric system to be captured at an instant in time, and thus reveal the interconnection between different plasma regions and dynamics on large scales. Using the Gorgon MHD code, we analyze the formation and evolution of flux ropes on the dayside magnetopause during a simulation of a real solar wind event. With a relatively strong solar wind dynamic pressure and southward interplanetary magnetic field, the dayside region becomes very dynamic with evidence of multiple reconnection events. The resulting flux ropes transit around the flank of the magnetosphere before eventually dissipating due to non-local reconnection. This shows that non-local effects may be important in controlling the topology of flux ropes and is a complicating factor in attempts to establish the overall contribution that flux ropes make in the general circulation of magnetic flux through the magnetosphere.
Goldman M, Newman DL, Eastwood JP, et al., 2021, Multi-beam energy moments of measured compound ion velocity distributions, Physics of Plasmas, Vol: 28, ISSN: 1070-664X
Compound ion distributions, fi(v), have been measured with high-time resolution by NASA's Magnetospheric Multi-Scale Mission (MMS) and have been found in reconnection simulations. A compound distribution, fi(v), consisting, for example, of essentially disjoint pieces will be called a multi-beam distribution and modeled as a sum of “beams,” fi(v) = f1(v) + ⋯ + fN(v). Velocity moments of fi(v) are taken beam by beam and summed. Such multi-beam moments of fi(v) have advantages over the customary standard velocity moments of fi(v), for which there is only one mean flow velocity. For example, the standard thermal energy moment of a pair of equal and opposite cold particle beams is non-zero even though each beam has zero thermal energy. We therefore call this thermal energy pseudothermal. By contrast, a multi-beam moment of two or more beams has no pseudothermal energy. We develop three different ways of approximating a compound ion velocity distribution, fi(v), as a sum of beams and finding multi-beam moments for both a compound fi(v) measured by MMS in the dayside magnetosphere during reconnection and a compound fi(v) found in a particle-in-cell simulation of magnetotail reconnection. The three methods are (i) a visual method in which the velocity centroid of each beam is estimated and the beam densities are determined self-consistently, (ii) a k-means method in which particles in a particle representation of fi(v) are sorted into a minimum energy configuration of N (= k) clusters, and (iii) a nonlinear least squares method based on a fit to a sum of N kappa functions. Multi-beam energy moments are calculated and compared with standard moments for the thermal energy density, pressure tensor, thermal energy flux (heat plus enthalpy fluxes), bulk kinetic energy density, ram pressure, and bulk kinetic energy flux. Applying this new formalism to real data demonstrates in detail how multi-beam techniques provide new insig
Desai R, Eastwood J, Horne R, et al., 2021, Drift orbit bifurcations and cross-field transport in the outer radiation belt: global MHD and integrated test-particle simulations, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-14, ISSN: 2169-9380
Energetic particle fluxes in the outer magnetosphere present a significant challenge to modellingefforts as they can vary by orders of magnitude in response to solar wind driving conditions. In thisarticle, we demonstrate the ability to propagate test particles through global MHD simulations to ahigh level of precision and use this to map the cross-field radial transport associated with relativisticelectrons undergoing drift orbit bifurcations (DOBs). The simulations predict DOBs primarily occurwithin an Earth radius of the magnetopause loss cone and appears significantly different for southwardand northward interplanetary magnetic field orientations. The changes to the second invariant areshown to manifest as a dropout in particle fluxes with pitch angles close to 90◦and indicate DOBsare a cause of butterfly pitch angle distributions within the night-time sector. The convective electricfield, not included in previous DOB studies, is found to have a significant effect on the resultant longterm transport, and losses to the magnetopause and atmosphere are identified as a potential methodfor incorporating DOBs within Fokker-Planck transport models.
LaMoury AT, Hietala H, Plaschke F, et al., 2021, Solar wind control of magnetosheath jet formation and propagation to the magnetopause, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-15, ISSN: 2169-9380
Magnetosheath jets are localized high-dynamic pressure pulses originating at Earth's bow shock and propagating earthward through the magnetosheath. Jets can influence magnetospheric dynamics upon impacting the magnetopause; however, many jets dissipate before reaching it. In this study we present a database of 13,096 jets observed by the Time History of Events and Macroscale Interactions during Substorms spacecraft from 2008 to 2018, spanning a solar cycle. Each jet is associated with upstream solar wind conditions from OMNI. We statistically examine how solar wind conditions control the likelihood of jets forming at the shock, and the conditions favorable for jets to propagate through the magnetosheath and reach the magnetopause. We see that, for each solar wind quantity, these two effects are separate, but when combined, we find that jets are over 17 times more likely to reach and potentially impact the magnetopause when the interplanetary magnetic field (IMF) orientation is at a low cone angle, and approximately 8 times more likely during high speed solar wind. Low IMF magnitude, high Alfvén Mach number, and low density approximately double the number of jets at the magnetopause, while urn:x-wiley:21699380:media:jgra56749:jgra56749-math-0001 and dynamic pressure display no net effect. Due to the strong dependence on wind speed, we infer that jet impact rates may be solar cycle dependent as well as vary during solar wind transients. This is an important step towards forecasting the magnetospheric effects of magnetosheath jets, as it allows for predictions of jet impact rates based on measurements of the upstream solar wind.
Desai RT, Freeman M, Eastwood J, et al., 2021, Interplanetary shock-induced magnetopause motion: Comparison between theory and global magnetohydrodynamic simulations, Geophysical Research Letters, Vol: 48, Pages: 1-11, ISSN: 0094-8276
The magnetopause marks the outer edge of the Earth’s magnetosphere and a distinct boundary between solar wind and magnetospheric plasma populations. In this letter, we use global magneto-hydrodynamic simulations to examine the response of the terrestrial magnetopause to fast-forward interplanetary shocks of various strengths and compare to theoretical predictions. The theory and simulations indicate the magnetopause response can be characterised by three distinct phases; an initial acceleration as inertial forces are overcome, a rapid compressive phase comprising the majority of the distance travelled, and large-scale damped oscillations with amplitudes of the order of an Earth radius. The two approaches agree in predicting subsolar magnetopause oscillations with frequencies2–13 mHz but the simulations notably predict larger amplitudes and weaker damping rates. This phenomenon is of high relevance to space weather forecasting and provides a possible explanation for magnetopause oscillations observed following the large interplanetary shocks of August 1972 and March 1991.
Laker R, Horbury TS, Bale SD, et al., 2021, Multi-spacecraft study of the solar wind at solar minimum: Dependence on latitude and transient outflows, Astronomy and Astrophysics: a European journal, Vol: 652, Pages: 1-10, ISSN: 0004-6361
Context. The recent launches of Parker Solar Probe, Solar Orbiter (SO), and BepiColombo, along with several older spacecraft, have provided the opportunity to study the solar wind at multiple latitudes and distances from the Sun simultaneously.Aims. We take advantage of this unique spacecraft constellation, along with low solar activity across two solar rotations between May and July 2020, to investigate how the solar wind structure, including the heliospheric current sheet (HCS), varies with latitude.Methods. We visualise the sector structure of the inner heliosphere by ballistically mapping the polarity and solar wind speed from several spacecraft onto the Sun’s source surface. We then assess the HCS morphology and orientation with the in situ data and compare this with a predicted HCS shape.Results. We resolve ripples in the HCS on scales of a few degrees in longitude and latitude, finding that the local orientations of sector boundaries were broadly consistent with the shape of the HCS but were steepened with respect to a modelled HCS at the Sun. We investigate how several CIRs varied with latitude, finding evidence for the compression region affecting slow solar wind outside the latitude extent of the faster stream. We also identified several transient structures associated with HCS crossings and speculate that one such transient may have disrupted the local HCS orientation up to five days after its passage.Conclusions. We have shown that the solar wind structure varies significantly with latitude, with this constellation providing context for solar wind measurements that would not be possible with a single spacecraft. These measurements provide an accurate representation of the solar wind within ±10° latitude, which could be used as a more rigorous constraint on solar wind models and space weather predictions. In the future, this range of latitudes will increase as SO’s orbit becomes more inclined.
LaMoury AT, Hietala H, Plaschke F, et al., 2021, Solar Wind Control of Magnetosheath Jet Formation and Propagation to the Magnetopause
Fargette N, Lavraud B, Rouillard A, et al., 2021, Magnetic increases with central current sheets: Observations with Parker Solar Probe, Astronomy & Astrophysics, Vol: 650, Pages: 1-12, ISSN: 0004-6361
Aims. We report the observation by Parker Solar Probe (PSP) of magnetic structures in the solar wind that present a strong peak intheir magnetic field magnitude with an embedded central current sheet. Similar structures have been observed, either at the Earth’smagnetopause and called interlinked flux tubes, or in the solar wind and called interplanetary field enhancements.Methods. In this work, we first investigate two striking events in detail; one occurred in the regular slow solar wind on November 2,2018 and the other was observed during a heliospheric current sheet crossing on November 13, 2018. They both show the presenceof a central current sheet with a visible ion jet and general characteristics consistent with the occurrence of magnetic reconnection.We then performed a survey of PSP data from encounters 1 to 4 and find 18 additional events presenting an increase in the magneticfield magnitude of over 30% and a central current sheet. We performed a statistical study on the 20 "magnetic increases with centralcurrent sheet" (MICCS), with 13 observed in the regular slow solar wind with a constant polarity (i.e., identical strahl direction), and7 which were specifically observed near a heliospheric current sheet (HCS) crossing.Results. We analyze and discuss the general properties of the structures, including the duration, location, amplitude, and magnetictopology, as well as the characteristics of their central current sheet. We find that the latter has a preferential orientation in the TNplane of the RTN frame. We also find no significant change in the dust impact rate in the vicinity of the MICCS under study, leadingus to conclude that dust probably plays no role in the MICCS formation and evolution. Our findings are overall consistent with adouble flux tube-configuration that would result from initially distinct flux tubes which interact during solar wind propagation.
Phan TD, Lavraud B S J, Halekas, et al., 2021, Prevalence of magnetic reconnection in the near-Sun heliospheric current sheet, Astronomy & Astrophysics, Vol: 650, Pages: 1-14, ISSN: 0004-6361
During three of its first five orbits around the Sun, Parker Solar Probe (PSP) crossed the large-scale Heliospheric Current Sheet (HCS)multiple times and provided unprecedented detailed plasma and field observations of the near-Sun HCS. We report the commondetections by PSP of reconnection exhaust signatures in the HCS at heliocentric distances of 29.5-107 solar radii during Encounters1, 4 and 5. Both sunward and antisunward-directed reconnection exhausts were observed. In the sunward reconnection exhausts,PSP detected counterstreaming strahl electrons, indicating that HCS reconnection resulted in the formation of closed magnetic fieldlines with both ends connected to the Sun. In the antisunward exhausts, PSP observed dropouts of strahl electrons, consistent withthe reconnected HCS field lines being disconnected from the Sun. The common detection of reconnection in the HCS suggests thatreconnection is almost always active in the HCS near the Sun. Furthermore, the occurrence of multiple long-duration partial crossingsof the HCS suggests that HCS reconnection could produce chains of large bulges with spatial dimensions of up to several solarradii. The finding of the prevalence of reconnection in the HCS is somewhat surprising since PSP has revealed that the HCS is muchthicker than the kinetic scales required for reconnection onset. The observations are also in stark contrast with the apparent absenceof reconnection in most of the small-scale and much more intense current sheets encountered near perihelia, many of which areassociated with ‘switchbacks’. Thus, the PSP findings suggest that large-scale dynamics either locally in the solar wind or within thecoronal source of the HCS (at the tip of helmet streamers) plays a critical role in triggering reconnection onset.
Robertson SL, Eastwood JP, Stawarz JE, et al., 2021, Electron trapping in magnetic mirror structures at the edge of magnetopause flux ropes, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-17, ISSN: 2169-9380
Flux ropes are a proposed site for particle energization during magnetic reconnection, with several mechanisms proposed. Here, Magnetospheric Multiscale mission observations of magnetic mirror structures on the edge of two ion‐scale magnetopause flux ropes are presented. Donut‐shaped features in the electron pitch angle distributions provide evidence for electron trapping in the structures. Furthermore, both events show trapping with extended 3D structure along the body of the flux rope. Potential formation mechanisms, such as the magnetic mirror instability, are examined and the evolutionary states of the structures are compared. Pressure and force analysis suggest that such structures could provide an important electron acceleration mechanism for magnetopause flux ropes, and for magnetic reconnection more generally.
Hapgood M, Angling MJ, Attrill G, et al., 2021, Development of space weather reasonable worst‐case scenarios for the UK national risk assessment, Space Weather, Vol: 19, Pages: 1-32, ISSN: 1542-7390
Severe space weather was identified as a risk to the UK in 2010 as part of a wider review of natural hazards triggered by the societal disruption caused by the eruption of the Eyjafjallajökull volcano in April of that year. To support further risk assessment by government officials, and at their request, we developed a set of reasonable worst‐case scenarios and first published them as a technical report in 2012 (current version published in 2020). Each scenario focused on a space weather environment that could disrupt a particular national infrastructure such as electric power or satellites, thus, enabling officials to explore the resilience of that infrastructure against severe space weather through discussions with relevant experts from other parts of government and with the operators of that infrastructure. This approach also encouraged us to focus on the environmental features that are key to generating adverse impacts. In this paper, we outline the scientific evidence that we have used to develop these scenarios, and the refinements made to them as new evidence emerged. We show how these scenarios are also considered as an ensemble so that government officials can prepare for a severe space weather event, during which many or all of the different scenarios will materialize. Finally, we note that this ensemble also needs to include insights into how public behavior will play out during a severe space weather event and hence the importance of providing robust, evidence‐based information on space weather and its adverse impacts.
Stawarz J, Matteini L, Parashar T, et al., 2021, Comparative Analysis of the Various Generalized Ohm's Law Terms in Magnetosheath Turbulence as Observed by Magnetospheric Multiscale
<jats:p>&lt;p&gt;&lt;span&gt;Electric fields (&lt;strong&gt;E&lt;/strong&gt;) play a fundamental role in facilitating the exchange of energy between the electromagnetic fields and the changed particles within a plasma. &lt;/span&gt;Decomposing &lt;strong&gt;E&lt;/strong&gt; into the contributions from the different terms in generalized Ohm's law, therefore, provides key insight into both the nonlinear and dissipative dynamics across the full range of scales within a plasma. Using the unique, high&amp;#8208;resolution, multi&amp;#8208;spacecraft measurements of three intervals in Earth's magnetosheath from the Magnetospheric Multiscale mission, the influence of the magnetohydrodynamic, Hall, electron pressure, and electron inertia terms from Ohm's law, as well as the impact of a finite electron mass, on the turbulent electric field&lt;strong&gt; &lt;/strong&gt;spectrum are examined observationally for the first time. The magnetohydrodynamic, Hall, and electron pressure terms are the dominant contributions to &lt;strong&gt;E&lt;/strong&gt; over the accessible length scales, which extend to scales smaller than the electron gyroradius at the greatest extent, with the Hall and electron pressure terms dominating at sub&amp;#8208;ion scales. The strength of the non&amp;#8208;ideal electron pressure contribution is stronger than expected from linear kinetic Alfv&amp;#233;n waves and a partial anti&amp;#8208;alignment with the Hall electric field is present, linked to the relative importance of electron diamagnetic currents within the turbulence. The relative contributions of linear and nonlinear electric fields scale with the turbulent fluctuation amplitude, with nonlinear contributions playing the dominant role in shaping &lt;strong&am
Dunlop MW, Wang TY, Dong XC, et al., 2021, Multispacecraft Measurements in the Magnetosphere, Magnetospheres in the Solar System, Pages: 637-656, ISBN: 9781119507529
This chapter covers a selection of the range of multispacecraft techniques that have been initially developed to analyze Cluster data. We begin the chapter with a short introduction, following this with an account of the methods and their application. The topics are separated into those dealing with magnetic field gradients and topology (which include the curlometer, magnetic rotation analysis, and least squares approach); magnetic field reconstruction and the analysis of magnetic field nulls (which are significant for magnetic reconnection and other geometries); time series analysis, adapted for multispacecraft data (including boundary identification, dimensional, and motion analysis); and wave vector analysis methods in the Fourier domain.
Stawarz JE, Matteini L, Parashar TN, et al., 2021, Comparative analysis of the various generalized Ohm's law terms in magnetosheath turbulence as observed by magnetospheric multiscale, Journal of Geophysical Research: Space Physics, Vol: 126, Pages: 1-14, ISSN: 2169-9380
Decomposing the electric field (E) into the contributions from generalized Ohm's law provides key insight into both nonlinear and dissipative dynamics across the full range of scales within a plasma. Using high‐resolution, multi‐spacecraft measurements of three intervals in Earth's magnetosheath from the Magnetospheric Multiscale mission, the influence of the magnetohydrodynamic, Hall, electron pressure, and electron inertia terms from Ohm's law, as well as the impact of a finite electron mass, on the turbulent E spectrum are examined observationally for the first time. The magnetohydrodynamic, Hall, and electron pressure terms are the dominant contributions to E over the accessible length scales, which extend to scales smaller than the electron inertial length at the greatest extent, with the Hall and electron pressure terms dominating at sub‐ion scales. The strength of the non‐ideal electron pressure contribution is stronger than expected from linear kinetic Alfvén waves and a partial anti‐alignment with the Hall electric field is present, linked to the relative importance of electron diamagnetic currents in the turbulence. The relative contribution of linear and nonlinear electric fields scale with the turbulent fluctuation amplitude, with nonlinear contributions playing the dominant role in shaping E for the intervals examined in this study. Overall, the sum of the Ohm's law terms and measured E agree to within ∼ 20% across the observable scales. These results both confirm general expectations about the behavior of E in turbulent plasmas and highlight features that should be explored further theoretically.
Eastwood JP, Goldman M, Phan TD, et al., 2020, Energy flux densities near the electron dissipation region in asymmetric magnetopause reconnection, Physical Review Letters, Vol: 125, Pages: 1-6, ISSN: 0031-9007
Magnetic reconnection is of fundamental importance to plasmas because of its role in releasing and repartitioning stored magnetic energy. Previous results suggest that this energy is predominantly released as ion enthalpy flux along the reconnection outflow. Using Magnetospheric Multiscale data we find the existence of very significant electron energy flux densities in the vicinity of the magnetopause electron dissipation region, orthogonal to the ion energy outflow. These may significantly impact models of electron transport, wave generation, and particle acceleration.
Goldman MV, Newman DL, Eastwood JP, et al., 2020, Multibeam energy moments of multibeam particle velocity distributions, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-19, ISSN: 2169-9380
High‐resolution electron and ion velocity distributions, f(v), which consist of N effectively disjoint beams, have been measured by NASA's Magnetospheric Multiscale Mission and in reconnection simulations. Commonly used standard velocity moments assume a single mean‐flow velocity for the entire distribution. This can lead to counterintuitive results for a multibeam f(v). An example is the standard thermal energy density moment (at a given space‐time point) of a pair of equal and opposite cold particle beams. This standard moment is nonzero even though each beam has zero thermal energy density. By contrast, a multibeam moment of two or more cold beams at a given position and time has no thermal energy. A multibeam moment is obtained by taking a standard moment of each beam and then summing over beams. In this paper we will generalize these notions, explore their consequences, and apply them to an f(v) which is a sum of tri‐Maxwellians. Both standard and multibeam energy moments have coherent and incoherent pieces. Examples of incoherent moments are the thermal energy density, the pressure, and the thermal energy flux (enthalpy flux plus heat flux). Corresponding coherent moments are the bulk kinetic energy density, the ram pressure, and the bulk kinetic energy flux. The difference between a standard incoherent moment and its multibeam counterpart will be defined as the “pseudothermal part” of the standard moment. The sum of a pair of corresponding coherent and incoherent moments is the undecomposed moment. Undecomposed standard moments are always equal to the corresponding undecomposed multibeam moments.
Magnes W, Hillenmaier O, Auster H-U, et al., 2020, Space weather magnetometer aboard GEO-KOMPSAT-2A, Space Science Reviews, Vol: 216, Pages: 1-36, ISSN: 0038-6308
The South Korean meteorological and environmental satellite GEO-KOMPSAT-2A (GK-2A) was launched into geostationary orbit at 128.2∘ East on 4 December 2018. The space weather observation aboard GK-2A is performed by the Korea Space Environment Monitor. It consists of three particle detectors, a charging monitor and a four-sensor Service Oriented Spacecraft Magnetometer (SOSMAG).The magnetometer design aims for avoiding strict magnetic cleanliness requirements for the hosting spacecraft and an automated on-board correction of the dynamic stray fields which are generated by the spacecraft. This is achieved through the use of two science grade fluxgate sensors on an approximately one meter long boom and two additional magnetoresistance sensors mounted within the spacecraft body.This paper describes the instrument design, discusses the ground calibration methods and results, presents the post-launch correction and calibration achievements based on the data which were acquired during the first year in orbit and demonstrates the in-flight performance of SOSMAG with two science cases.The dynamic stray fields from the GK-2A spacecraft, which was built without specific magnetic cleanliness considerations, are reduced up to a maximum factor of 35. The magnitude of the largest remnant field from an active spacecraft disturber is 2.0 nT. Due to a daily shadowing of the SOSMAG boom, sensor intrinsic offset oscillations with a periodicity up to 60 minutes and peak-to-peak values up to 5 nT remain in the corrected data product.The comparison of the cleaned SOSMAG data with the Tsyganenko 2004 magnetic field model and the magnetic field data from the Magnetospheric Multiscale mission demonstrates that the offset error is less than the required 5 nT for all three components and that the drift of the offsets over 10 months is less than 7 nT.Future work will include a further reduction of the remaining artefacts in the final data product with the focus on lessening the temperature driv
Barnes D, Davies JA, Harrison RA, et al., 2020, CMEs in the heliosphere: III. a statistical analysis of the kinematic properties derived from stereoscopic geometrical modelling techniques applied to CMEs detected in the heliosphere from 2008 to 2014 by STEREO/HI-1, Solar Physics: a journal for solar and solar-stellar research and the study of solar terrestrial physics, Vol: 295, Pages: 1-25, ISSN: 0038-0938
We present an analysis of coronal mass ejections (CMEs) observed by the Heliospheric Imagers (HIs) onboard NASA’s Solar Terrestrial Relations Observatory (STEREO) spacecraft. Between August 2008 and April 2014 we identify 273 CMEs that are observed simultaneously, by the HIs on both spacecraft. For each CME, we track the observed leading edge, as a function of time, from both vantage points, and apply the Stereoscopic Self-Similar Expansion (SSSE) technique to infer their propagation throughout the inner heliosphere. The technique is unable to accurately locate CMEs when their observed leading edge passes between the spacecraft; however, we are able to successfully apply the technique to 151, most of which occur once the spacecraft-separation angle exceeds 180∘, during solar maximum. We find that using a small half-width to fit the CME can result in inferred acceleration to unphysically high velocities and that using a larger half-width can fail to accurately locate the CMEs close to the Sun because the method does not account for CME over-expansion in this region. Observed velocities from SSSE are found to agree well with single-spacecraft (SSEF) analysis techniques applied to the same events. CME propagation directions derived from SSSE and SSEF analysis agree poorly because of known limitations present in the latter.
Horbury TS, OBrien H, Carrasco Blazquez I, et al., 2020, The Solar Orbiter magnetometer, Astronomy & Astrophysics, Vol: 642, Pages: A9-A9, ISSN: 0004-6361
The magnetometer instrument on the Solar Orbiter mission is designed to measure the magnetic field local to the spacecraft continuously for the entire mission duration. The need to characterise not only the background magnetic field but also its variations on scales from far above to well below the proton gyroscale result in challenging requirements on stability, precision, and noise, as well as magnetic and operational limitations on both the spacecraft and other instruments. The challenging vibration and thermal environment has led to significant development of the mechanical sensor design. The overall instrument design, performance, data products, and operational strategy are described.
Zouganelis I, 2020, The Solar Orbiter Science Activity Plan: translating solar and heliospheric physics questions into action, Astronomy & Astrophysics, Vol: 642, Pages: 1-19, ISSN: 0004-6361
Solar Orbiter is the first space mission observing the solar plasma both in situ and remotely, from a close distance, in and out of the ecliptic. The ultimate goal is to understand how the Sun produces and controls the heliosphere, filling the Solar System and driving the planetary environments. With six remote-sensing and four in-situ instrument suites, the coordination and planning of the operations are essential to address the following four top-level science questions: (1) What drives the solar wind and where does the coronal magnetic field originate?; (2) How do solar transients drive heliospheric variability?; (3) How do solar eruptions produce energetic particle radiation that fills the heliosphere?; (4) How does the solar dynamo work and drive connections between the Sun and the heliosphere? Maximising the mission’s science return requires considering the characteristics of each orbit, including the relative position of the spacecraft to Earth (affecting downlink rates), trajectory events (such as gravitational assist manoeuvres), and the phase of the solar activity cycle. Furthermore, since each orbit’s science telemetry will be downloaded over the course of the following orbit, science operations must be planned at mission level, rather than at the level of individual orbits. It is important to explore the way in which those science questions are translated into an actual plan of observations that fits into the mission, thus ensuring that no opportunities are missed. First, the overarching goals are broken down into specific, answerable questions along with the required observations and the so-called Science Activity Plan (SAP) is developed to achieve this. The SAP groups objectives that require similar observations into Solar Orbiter Observing Plans, resulting in a strategic, top-level view of the optimal opportunities for science observations during the mission lifetime. This allows for all four mission goals to be addressed. In this paper, w
Mihailescu AT, Desai R, Shebanits O, et al., 2020, Spatial variations of low mass negative ions in Titan's upper atmosphere, The Planetary Science Journal, Vol: 1, Pages: 1-8, ISSN: 2632-3338
Observations with Cassini’s Electron Spectrometer discovered negative ions in Titan’s ionosphere,at altitudes between 1400 and 950 km. Within the broad mass distribution extending up to severalt housand amu, two distinct peaks were identified at 25.8-26.0 and 49.0-50.1 amu/q, corresponding to the carbon chain anions CN−and/orC2H−for the first peak and C3N−and/orC4H−for the second peak. In this study we present the spatial distribution of these low mass negative ions from 28 Titanflybys with favourable observations between 26 October 2004 and 22 May 2012. We report a trend of lower densities on the night side and increased densities up to twice as high on the day side at small solar zenith angles. To further understand this trend, we compare the negative ion densities to the total electron density measured by Cassini’s Langmuir Probe. We find the low mass negative ion density and the electron density to be proportional to each other on the dayside, but independent of each other on the night side. This indicates photochemical processes and is in agreement with the primary production route for the low mass negative ions being initiated by dissociative reactions with suprathermal electron populations produced by photoionisation. We also find the ratio ofCN−/C2H−toC3N−/C4H−highly constrained on the day-side, in agreement with this production channel, but notably displays large variations on the nightside.
AkhavanTafti M, Palmroth M, Slavin JA, et al., 2020, Comparative analysis of the vlasiator simulations and MMS observations of multiple X‐line reconnection and flux transfer events, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-22, ISSN: 2169-9380
The Vlasiator hybrid‐Vlasov code was developed to investigate global magnetospheric dynamics at ion‐kinetic scales. Here, we focus on the role of magnetic reconnection in the formation and evolution of the magnetic islands at the low‐latitude magnetopause, under southward interplanetary magnetic field (IMF) conditions. The simulation results indicate that: 1) the magnetic reconnection ion kinetics, including the Earthward‐pointing Larmor electric field on the magnetospheric‐side of an X‐point and anisotropic ion distributions, are well‐captured by Vlasiator, thus enabling the study of reconnection‐driven magnetic island evolution processes, 2) magnetic islands evolve due to continuous reconnection at adjacent X‐points, ‘coalescence’ which refers to the merging of neighboring islands to create a larger island, ‘erosion’ during which an island loses magnetic flux due to reconnection, and ‘division’ which involves the splitting of an island into smaller islands, and 3) continuous reconnection at adjacent X‐points is the dominant source of magnetic flux and plasma to the outer layers of magnetic islands resulting in cross‐sectional growth rates up to +0.3 RE2/min. The simulation results are compared to the Magnetospheric Multiscale (MMS) measurements of a chain of ion‐scale flux transfer events (FTEs) sandwiched between two dominant X‐lines. The MMS measurements similarly reveal: 1) anisotropic ion populations, and 2) normalized reconnection rate ~0.18, in agreement with theory and the Vlasiator predictions. Based on the simulation results and the MMS measurements, it is estimated that the observed ion‐scale FTEs may grow Earth‐sized within ~10 minutes, which is comparable to the average transport time for FTEs formed in the subsolar region to the high‐latitude magnetopause. Future simulations shall revisit reconnection‐driven island evolution processes with improved spatial resolutions.
Eggington JWB, Eastwood JP, Mejnertsen L, et al., 2020, Dipole tilt effect on magnetopause reconnection and the steady‐state magnetosphere‐ionosphere system: global MHD simulation, Journal of Geophysical Research: Space Physics, Vol: 125, Pages: 1-17, ISSN: 2169-9380
The Earth’s dipole tilt angle changes both diurnally and seasonally and introduces numerous variabilities in the coupled magnetosphere‐ionosphere system. By altering the location and intensity of magnetic reconnection, the dipole tilt influences convection on a global scale. However, due to the nonlinear nature of the system, various other effects like dipole rotation, varying IMF orientation and non‐uniform ionospheric conductance can smear tilt effects arising purely from changes in coupling with the solar wind. To elucidate the underlying tilt angle‐dependence, we perform MHD simulations of the steady‐state magnetosphere‐ionosphere system under purely southward IMF conditions for tilt angles from 0°‐90°. We identify the location of the magnetic separator in each case, and find that an increasing tilt angle shifts the 3‐D X‐line southward on the magnetopause due to changes in magnetic shear angle. The separator is highly unsteady above 50° tilt angle, characteristic of regular FTE generation on the magnetopause. The reconnection rate drops as the tilt angle becomes large, but remains continuous across the dayside such that the magnetosphere is open even for 90°. These trends map down to the ionosphere, with the polar cap contracting as the tilt angle increases, and region‐I field‐aligned current (FAC) migrating to higher latitudes with changing morphology. The tilt introduces a north‐south asymmetry in magnetospheric convection, thus driving more FAC in the northern (sunward‐facing) hemisphere for large tilt angles than in the south independent of conductance. These results highlight the strong sensitivity to onset time in the potential impact of a severe space weather event.
Stawarz JE, Matteini L, Parashar TN, et al., 2020, Generalized Ohm's Law Decomposition of the Electric Field in Magnetosheath Turbulence: Magnetospheric Multiscale Observations
Tilquin H, Eastwood JP, Phan TD, 2020, Solar wind reconnection exhausts in the inner heliosphere observed by helios and detected via machine learning, The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 895, Pages: 1-10, ISSN: 0004-637X
Reconnecting current sheets in the solar wind play an important role in the dynamics of the heliosphere and offer an opportunity to study magnetic reconnection exhausts under a wide variety of inflow and magnetic shear conditions. However, progress in understanding reconnection can be frustrated by the difficulty of finding events in long time-series data. Here we describe a new method to detect magnetic reconnection events in the solar wind based on machine learning, and apply it to Helios data in the inner heliosphere. The method searches for known solar wind reconnection exhaust features, and parameters in the algorithm are optimized to maximize the Matthews Correlation Coefficient using a training set of events and non-events. Applied to the whole Helios data set, the trained algorithm generated a candidate set of events that were subsequently verified by hand, resulting in a database of 88 events. This approach offers a significant reduction in construction time for event databases compared to purely manual approaches. The database contains events covering a range of heliospheric distances from ~0.3 to ~1 au, and a wide variety of magnetic shear angles, but is limited by the relatively coarse time resolution of the Helios data. Analysis of these events suggests that proton heating by reconnection in the inner heliosphere depends on the available magnetic energy in a manner consistent with observations in other regimes such as at the Earth's magnetopause, suggesting this may be a universal feature of reconnection.
Lavraud B, Fargette N, Réville V, et al., 2020, The heliospheric current sheet and plasma sheet during Parker Solar Probe’s first orbit, Letters of the Astrophysical Journal, Vol: 894, Pages: 1-8, ISSN: 2041-8205
We present heliospheric current sheet (HCS) and plasma sheet (HPS) observations during Parker Solar Probe's (PSP) first orbit around the Sun. We focus on the eight intervals that display a true sector boundary (TSB; based on suprathermal electron pitch angle distributions) with one or several associated current sheets. The analysis shows that (1) the main density enhancements in the vicinity of the TSB and HCS are typically associated with electron strahl dropouts, implying magnetic disconnection from the Sun, (2) the density enhancements are just about twice that in the surrounding regions, suggesting mixing of plasmas from each side of the HCS, (3) the velocity changes at the main boundaries are either correlated or anticorrelated with magnetic field changes, consistent with magnetic reconnection, (4) there often exists a layer of disconnected magnetic field just outside the high-density regions, in agreement with a reconnected topology, (5) while a few cases consist of short-lived density and velocity changes, compatible with short-duration reconnection exhausts, most events are much longer and show the presence of flux ropes interleaved with higher-β regions. These findings are consistent with the transient release of density blobs and flux ropes through sequential magnetic reconnection at the tip of the helmet streamer. The data also demonstrate that, at least during PSP's first orbit, the only structure that may be defined as the HPS is the density structure that results from magnetic reconnection, and its byproducts, likely released near the tip of the helmet streamer.
Adhikari S, Shay MA, Parashar TN, et al., 2020, Reconnection from a turbulence perspective, Physics of Plasmas, Vol: 27, Pages: 1-10, ISSN: 1070-664X
The spectral properties associated with laminar, anti-parallel reconnection are examined using a 2.5D kinetic particle in cell simulation. Both the reconnection rate and the energy spectrum exhibit three distinct phases: an initiation phase where the reconnection rate grows, a quasi-steady phase, and a declining phase where both the reconnection rate and the energy spectrum decrease. During the steady phase, the energy spectrum exhibits approximately a double power law behavior, with a slope near −5/3 at wave numbers smaller than the inverse ion inertial length and a slope steeper than −8/3 for larger wave numbers up to the inverse electron inertial length. This behavior is consistent with a Kolmogorov energy cascade and implies that laminar reconnection may fundamentally be an energy cascade process. Consistent with this idea is the fact that the reconnection rate exhibits a rough correlation with the energy spectrum at wave numbers near the inverse ion inertial length. The 2D spectrum is strongly anisotropic with most energy associated with the wave vector direction normal to the current sheet. Reconnection acts to isotropize the energy spectrum, reducing the Shebalin angle from an initial value of 70° to about 48° (nearly isotropic) by the end of the simulation. The distribution of energy over length scales is further analyzed by dividing the domain into spatial subregions and employing structure functions.
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