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One of the longstanding questions of space science is: How does the Earth’s magnetosphere generate auroral arcs? A related question is: What form of energy is extracted from the magnetosphere to drive auroral arcs? Not knowing the answers to these questions hinders our ability to determine the impact of auroral arcs on the magnetospheric system. Magnetospheric mechanisms for driving quiescent auroral arcs are reviewed. Two types of quiescent arcs are (1) low-latitude non-Alfvénic (growth-phase) arcs magnetically connecting to the electron plasma sheet and (2) high-latitude arcs magnetically connecting near the plasma-sheet boundary layer. The reviews of the magnetospheric generator mechanisms are separated for the two types of quiescent arcs. The driving of auroral-arc currents in large-scale computer simulations is examined. Predicted observables in the magnetosphere and in the ionosphere are compiled for the various generator mechanisms.

The three Swarm satellites provide an optimum, low Earth orbit (LEO) and multi‐spacecraft platform, to explore for the first time the local correlation between field‐aligned currents (FACs), auroral electrojets, and magnetic perturbations at the Earth's surface. By combining Swarm and ground magnetic field data, one can investigate systematically the full correlation chain, whose final link controls the ground induced currents and related space weather effects. We introduce an integrated FAC product, the Sheet FAC (SFAC) index, as a convenient measure of the in‐situ FAC data, and explore the correlations SFAC‐AE, SFAC‐PEJ and SFAC‐dH, with AE the standard auroral electrojet index, PEJ the local, Swarm based, polar electrojet index, and dH the horizontal magnetic field perturbation at the Earth's surface. Given the good SFAC‐dH correlation, we also suggest an extension of SFAC to higher LEO satellites, which cannot observe any more the electrojet currents, but are fully capable to monitor SFAC. Plain Language Summary ‘Space weather’ resembles, to some extent, ordinary weather. Likewise, storms in space, termed ‘magnetic storms’, have common features with ordinary storms. Just like ordinary storms, magnetic storms can cause damage, and similar to ordinary weather, space weather needs to be monitored and, ideally, predicted. The SFAC index, introduced in the paper, is shown to be a potentially useful tool for such goals, able to capture local effects. This is essential for efficient monitoring. Moreover, the SFAC index can be extended to many satellites, which is important too. Just like for ordinary weather, space weather prediction requires measurements of key parameters that are used as input by specific models. The more and denser the measurements, the better the output, namely the prediction. Key Points The newly introduced SFAC index is shown to be robust and appropriate for space weather monitoring by low Earth orbit satellites SFAC appears to be able to capture local features related to magnetospheric dynamics, as driven, in particular, by bursty bulk flows While the introduction of SFAC takes advantage of Swarm features, the index can be extended to other low Earth orbit satellites

The fundamental processes responsible for energy exchange between large-scale electromagnetic fields and plasma are well understood theoretically, but in practice these theories have not been tested. These processes are ubiquitous in all plasmas, especially at the interface between high and low beta plasmas in planetary magnetospheres and other magnetic environments. Although such boundaries pervade the plasma Universe, the processes responsible for the release of the stored magnetic and thermal plasma energy have not been fully identified and the importance of the relative impact of each process is unknown. Despite advances in understanding energy release through the conversion of magnetic to kinetic energy in magnetic reconnection, how the extreme pressures in the regions between stretched and more relaxed field lines in the transition region are balanced and released through adiabatic convection of plasma and fields is still a mystery. Recent theoretical advances and the predictions of large-scale instabilities must be tested. In essence, the processes responsible remain poorly understood and the problem unresolved. The aim of the White Paper submitted to ESA’s Voyage 2050 call, and the contents of this paper, is to highlight three outstanding open science questions that are of clear international interest: (i) the interplay of local and global plasma physics processes: (ii) the partitioning during energy conversion between electromagnetic and plasma energy: and (iii) what processes drive the coupling between low and high beta plasmas. We present a discussion of the new measurements and technological advances required from current state-of-the-art, and several candidate mission profiles with which these international high-priority science goals could be significantly advanced.

Upstream propagating waves are observed to correlate with the reflected ions in the foot of a high Mach number quasi‐perpendicular collisionless shock. The respective wavefronts show time varying amplitude matching the variation of the reflected ions and the shock reformation cycles. We interpret this process in terms of the fast two‐stream instability and investigate the results of the PIC simulation as compared with the prediction of the linear analysis. Evidence for upstream propagating whistler waves supported by two‐stream instability is continuously growing, see for example, recent surveys based on MMS in‐situ observations of the Earth bow‐shock. The present study suggests that upstream propagating waves are consistent with and in particular supported by the mechanism of fast two‐stream instability developing in the upstream region of the shock foot. Plain Language Summary By using particle‐in‐cell simulations, a highly non‐stationary quasi‐perpendicular collisionless shock exhibits reformation cycles characterized by extensions and compressions of the foot, ramp, and overshoot. Waves with upstream propagation are crossing the foot, with large variation of their phase velocities with respect to the rest frame of the shock, repeated periodically. Additionally, strong downstream propagating waves driven by incoming ions ‐ incoming electrons two‐stream instability (or slow two‐stream instability) are observed close to the end of each shock cycle. In a system attached to the electron flow (regarded as proxy for the system used in the study of two‐stream instabilities), the variations of the phase velocities appear to be significantly smaller. The linear theory based on dispersion analysis supports, in our case, the use of a 1‐D code for exploring upstream whistler propagation driven by the fast two‐stream instability, associated by former studies with 2‐D configuration. At the same time, the analytical examination of the real part of the dispersion relation shows good agreement with the simulation results, while the imaginary part indicates a significant local growth rate of the fast two‐stream instability, that can presumably compensate the otherwise strong damping of the oblique whistler waves. Key Points Fast two‐stream instability excited in the foot of a collisionless shock by using 1‐D PIC simulations Cyclic emissions of whistler waves in the foot of the shocks are supported by fast and slow two‐stream instability Convergence of three independent approaches: simulations, analytical, and observations (MMS)

In the White Paper, submitted in response to the European Space Agency (ESA) Voyage 2050 Call, we present the importance of advancing our knowledge of plasma-neutral gas interactions, and of deepening our understanding of the partially ionized environments that are ubiquitous in the upper atmospheres of planets and moons, and elsewhere in space. In future space missions, the above task requires addressing the following fundamental questions: (A) How and by how much do plasma-neutral gas interactions influence the re-distribution of externally provided energy to the composing species? (B) How and by how much do plasma-neutral gas interactions contribute toward the growth of heavy complex molecules and biomolecules? Answering these questions is an absolute prerequisite for addressing the long-standing questions of atmospheric escape, the origin of biomolecules, and their role in the evolution of planets, moons, or comets, under the influence of energy sources in the form of electromagnetic and corpuscular radiation, because low-energy ion-neutral cross-sections in space cannot be reproduced quantitatively in laboratories for conditions of satisfying, particularly, (1) low-temperatures, (2) tenuous or strong gradients or layered media, and (3) in low-gravity plasma. Measurements with a minimum core instrument package (< 15 kg) can be used to perform such investigations in many different conditions and should be included in all deep-space missions. These investigations, if specific ranges of background parameters are considered, can also be pursued for Earth, Mars, and Venus.

Electric currents are fundamental to the structure and dynamics of space plasmas, including our own near-Earth space environment, or \"geospace.\"This volume takes an integrated approach to the subject of electric currents by incorporating their phenomenology and physics for many regions in one volume.

A pair of negative electric potential structures associated with inverted‐V aurora is investigated using electric and magnetic field, ion and electron data from the Cluster spacecraft, crossing the auroral acceleration region (AAR) at different altitudes above the Northern hemisphere midnight auroral oval. The spatial and temporal development of the acceleration structures is studied, given the magnetic conjunction opportunity and the one minute difference between the Cluster spacecraft crossings. The configuration allowed for estimation of characteristic times of development for the two structures and of the parallel electric field and potential drop for the more stable one. The first potential structure had a width of ∼80 km (projected to the ionosphere) and was relatively short‐lived, developing in less than 40 s and decaying in one minute. The parallel potential drop increased between altitudes of 1.13 RE and 1.3 RE, whereas the acceleration potential above 1.3 RE remained almost unchanged during that time. This intensification occurred mainly after the time when the associated upward current had reached its maximum value. The second structure had a width of ∼50 km and was subject to an increase by a factor of 3 of the parallel potential drop below 1.3 RE, during about 40 s, after which it remained rather stable for one minute or more. Similarly here, the acceleration potential above 1.3 RE remained roughly unchanged. For the more stable second structure, an average parallel electric field between 1.13 and 1.3 RE could be estimated (∼0.56 mV/m). The conductance along the flux tube was also stable for one minute or more. Key Points Multi‐spacecraft estimation of parallel electric field in AAR Estimating the growth and decay times of potential structures Measurement of altitude distribution of the electric potentials

Field-aligned currents (FACs) in the magnetosphere–ionosphere (M–I) system exhibit a range of spatial and temporal scales that are linked to key dynamic coupling processes. To disentangle the scale dependence in magnetic field signatures of auroral FACs and to characterize their geometry and orientation, Bunescu et al. (2015) introduced the multiscale FAC analyzer framework based on minimum variance analysis (MVA) of magnetic time series segments. In the present report this approach is carried further to include in the analysis framework a FAC density scalogram, i.e., a multiscale representation of the FAC density time series. The new technique is validated and illustrated using synthetic data consisting of overlapping sheets of FACs at different scales. The method is applied to Swarm data showing both large-scale and quiet aurora as well as mesoscale FAC structures observed during more disturbed conditions. We show both planar and non-planar FAC structures as well as uniform and non-uniform FAC density structures. For both synthetic and Swarm data, the multiscale analysis is applied by two scale sampling schemes, namely the linear and logarithmic scanning of the FAC scale domain. The local FAC density is compared with the input FAC density for the synthetic data, whereas for the Swarm data we cross-check the results with well-established single- and dual-spacecraft techniques. All the multiscale information provides a new visualization tool for the complex FAC signatures that complements other FAC analysis tools.

The lower-thermosphere–ionosphere (LTI) system consists of the upper atmosphere and the lower part of the ionosphere and as such comprises a complex system coupled to both the atmosphere below and space above. The atmospheric part of the LTI is dominated by laws of continuum fluid dynamics and chemistry, while the ionosphere is a plasma system controlled by electromagnetic forces driven by the magnetosphere, the solar wind, as well as the wind dynamo. The LTI is hence a domain controlled by many different physical processes. However, systematic in situ measurements within this region are severely lacking, although the LTI is located only 80 to 200 km above the surface of our planet. This paper reviews the current state of the art in measuring the LTI, either in situ or by several different remote-sensing methods. We begin by outlining the open questions within the LTI requiring high-quality in situ measurements, before reviewing directly observable parameters and their most important derivatives. The motivation for this review has arisen from the recent retention of the Daedalus mission as one among three competing mission candidates within the European Space Agency (ESA) Earth Explorer 10 Programme. However, this paper intends to cover the LTI parameters such that it can be used as a background scientific reference for any mission targeting in situ observations of the LTI.

In situ satellite exploration of the lower thermosphere–ionosphere system (LTI) as anticipated in the recent Daedalus mission proposal to ESA will be essential to advance the understanding of the interface between the Earth's atmosphere and its space environment. To address physical processes also below perigee, in situ measurements are to be extrapolated using models of the LTI. Motivated by the need for assessing how cost-critical mission elements such as perigee and apogee distances as well as the number of spacecraft affect the accuracy of scientific inference in the LTI, the Daedalus Ionospheric Profile Continuation (DIPCont) project is concerned with the attainable quality of in situ measurement extrapolation for different mission parameters and configurations. This report introduces the methodological framework of the DIPCont approach. Once an LTI model is chosen, ensembles of model parameters are created by means of Monte Carlo simulations using synthetic measurements based on model predictions and relative uncertainties as specified in the Daedalus Report for Assessment. The parameter ensembles give rise to ensembles of model altitude profiles for LTI variables of interest. Extrapolation quality is quantified by statistics derived from the altitude profile ensembles. The vertical extent of meaningful profile continuation is captured by the concept of extrapolation horizons defined as the boundaries of regions where the deviations remain below a prescribed error threshold. To demonstrate the methodology, the initial version of the DIPCont package presented in this paper contains a simplified LTI model with a small number of parameters. As a major source of variability, the pronounced change in temperature across the LTI is captured by self-consistent non-isothermal neutral-density and electron density profiles, constructed from scale height profiles that increase linearly with altitude. The resulting extrapolation horizons are presented for dual-satellite measurements at different inter-spacecraft distances but also for the single-satellite case to compare the two basic mission scenarios under consideration. DIPCont models and procedures are implemented in a collection of Python modules and Jupyter notebooks supplementing this report.