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The precipitation of tens to hundreds of keV electrons from Earth's magnetosphere plays a crucial role in magnetosphere‐ionosphere coupling, primarily driven by chorus wave scattering. Most existing simulations of electron precipitation rely on test particle models that neglect particle feedback on waves. However, both theoretical and observational studies indicate that the feedback from energetic electrons significantly influences chorus wave excitation and evolution. In this study, we quantify electron precipitation driven by chorus waves using self‐consistent simulations at L = 6 with typical magnetospheric plasma parameters. Electrons in the ∼10–200 keV range are precipitated, exhibiting energy‐dispersive characteristics. The precipitation intensity reaches ∼108–109${10}^{8}\\!\\mathit{\\mbox{--}}\\!{10}^{9}$keV/s/sr/cm2/MeV$\\mathrm{k}\\mathrm{e}\\mathrm{V}/\\mathrm{s}/\\mathrm{s}\\mathrm{r}/{\\mathrm{c}\\mathrm{m}}^{2}/\\mathrm{M}\\mathrm{e}\\mathrm{V}$ , consistent with the typical values in observations. As a comparison, test particle simulations underestimate the precipitation intensity by nearly an order of magnitude. These results highlight the importance of self‐consistent simulations in quantifying electron precipitation and investigating wave‐particle interactions that modulate magnetospheric dynamics.

Transpolar arcs (TPAs) describe a subset of auroral emissions observed poleward of the Earth's main auroral ovals when the interplanetary magnetic field (IMF) is northward. These emissions are thought to align with a “wedge” of closed magnetic field lines extending to the high‐latitude boundary of the Earth's magnetosphere, where they are postulated to interact with the IMF through magnetic reconnection. Such an instance of “high‐latitude TPA wedge reconnection” is expected to open the Earth's magnetic field lines, but the event has never been verified by in situ observations. Here we report a detection of a high‐latitude TPA wedge reconnection site within one ion gyroradius distance on 18 March 2002, which manifests in the ionosphere as colocation of the TPA and a cusp spot. The result may explain previously reported cases of remotely detected “non‐lobe” high‐latitude magnetic reconnection, with implications for future understanding of TPAs and northward‐IMF global magnetospheric dynamics.

The density of the solar wind plasma near the Earth’s magnetosphere sometimes decreases to only several per cent of the usual value, and such density extrema result in a significant reduction of the dynamic pressure and Alfvén Mach number (MA) of the solar wind flow. While a symmetric expansion of the Earth’s magnetosphere by the low dynamic pressure was assumed in previous studies, a global magnetohydrodynamic (MHD) simulation study predicted a remarkable dawn-dusk asymmetry of the magnetospheric shape under low-density solar wind and Parker-spiral interplanetary magnetic field (IMF) configuration. Here, we present observations consistent with the asymmetric deformation of the magnetosphere under low-MA solar wind and Parker-spiral IMF conditions, focusing on the significant expansion of the dawn-flank magnetosphere detected by the Geotail spacecraft. A global MHD simulation reproduced the dawnward expansion of the near-Earth magnetosphere, which was consistent with the observation by Geotail. The solar wind flow had a non-negligible dusk-to-dawn component and partly affected the dawnward expansion of the magnetosphere. Local, roughly Alfvénic sunward acceleration of magnetosheath ions at the dawn flank magnetopause suggests magnetosheath plasma entry into the magnetosphere through open field lines generated by magnetic reconnection at the dayside magnetopause. At the same time, Cluster 1 and 3, located near the southern polar cusp, also detected continuous antisunward ion jets and occasional sunward jets, which are consistent with the occurrence of magnetic reconnection near the southern cusp. These observations suggest that enhanced plasma acceleration at the dayside magnetopause operates under the low-MA solar wind and Parker spiral IMF conditions and that plasma influx across the dawnside magnetopause is at work under such a low-MA condition. These results can be helpful in understanding interactions between low-MA solar/stellar winds and celestial objects, such as inner planets and exoplanets.Graphic Abstract

The modulation of chorus waves on several‐second timescales in Earth's magnetosphere plays a crucial role in modulating electron precipitation intensity, leading to the formation of pulsating aurora. However, the physical mechanism underlying chorus modulation remains not fully understood. In this study, we perform self‐consistent particle‐in‐cell simulations with typical magnetospheric plasma parameters to quantify chorus modulation driven by plasma density variations and compressional magnetic field fluctuations. It is demonstrated that chorus modulation is determined by nonlinear wave‐particle interactions, in which the condition for nonlinear wave growth is highly sensitive to background plasma parameters. The resulting electron precipitation in the ∼10–200 keV energy range exhibits modulation on comparable timescales, consistent with observations of pulsating aurora. This study enhances our understanding of how variations in magnetospheric plasma parameters influence chorus wave excitation and the associated particle dynamics.

The study of the Earth’s magnetosphere through in situ observations is an important step in understanding the evolution of the Sun–Earth interaction. In this context, the long-term observation of the Earth’s magnetotail using a scientific probe in a high elliptical orbit is a challenging mission scenario due to the alignment of the magnetotail direction with the Sun–Earth line, which requires a continuous rotation of the apse line of the spacecraft’s geocentric orbit. This aspect makes the mission scenario particularly suitable for space vehicles equipped with propellantless propulsion systems, such as the classic solar sails which convert the solar radiation pressure into propulsive acceleration without propellant expenditure. However, a continuous rotation of the apse line of the osculating orbit can be achieved using a more conventional solar electric thruster, which introduces an additional constraint on the duration of the scientific mission due to the finite mass of the propellant stored on board the spacecraft. This paper analyzes the potential of a typical CubeSat equipped with a commercial miniaturized electric thruster in performing the rotation of the apse line of a geocentric orbit suitable for the in situ observation of the Earth’s magnetotail. The paper also analyzes the impact of the size of a thruster array on the flight performance for an assigned value of the payload mass and the science orbit’s characteristics. In particular, this work illustrates the optimal guidance laws that allow us to maximize the duration of the scientific mission for an assigned CubeSat’s configuration. In this sense, this paper expands the literature regarding the study of this interesting mission scenario by extending the study to conventional propulsion systems that use a propellant to provide a continuous and steerable thrust vector.

The frequency chirping of chorus waves is commonly observed in the Earth’s inner magnetosphere, but its generation remains an open question. Recently, Liu et al. (2021), https://doi.org/10.1029/2021JA029258 reported two unusual rising‐tone (upward chirping) chorus elements. Although the central frequency of constituent subpackets rises, the frequency of a single subpacket is surprisingly downward chirping. With a gcPIC‐δf$\\delta f$simulation in the dipole field, we successfully reproduce this kind of substructure, which contains alternating signs of chirping. Interestingly, both hole and hill structures are formed around the theoretical resonant velocities in the electron phase space, no matter whether the chirping is upward or downward. However, during each chirping interval, only one structure (either a hole or a hill) is associated with wave excitation: the upward chirping is related to the hole, while the hill contributes to the downward chirping. Our study provides a fresh perspective on the theory of frequency chirping in chorus waves. Plain Language Summary The frequency chirping is a typical feature of chorus waves in the Earth’s inner magnetosphere, which generally contain either rising‐tone (upward chirping) elements or falling‐tone (downward chirping) elements. Previous theory has suggested that the chirping is due to the nonlinear wave‐particle interaction, where the hole or hill structure is formed in the electron phase space. Recently, Liu et al. (2021), https://doi.org/10.1029/2021JA029258 have observed the upward chirping elements with their subpackets of downward chirping. What electron structure is associated with these elements becomes a puzzle. With a one‐dimensional (1D) general curvilinear particle‐in‐cell (gcPIC) δf simulation in the dipole magnetic field, we successfully reproduce this kind of chorus element, whose frequency contains alternating upward and downward chirping. Interestingly, both the hole and hill structures are formed during a chirping interval, but only one of the two structures is responsible for wave excitation and frequency chirping. The structure of hole‐hill combination provides an important clue into the theory of the frequency chirping in chorus waves. Key Points With a gcPIC‐δf$\\delta f$simulation in the dipole field, we reproduce the upward chirping chorus element, whose subpackets are downward chirping Both hole and hill structures can be formed in the ζ−v‖$\\zeta -{v}_{\\Vert }$phase space, no matter whether the frequency is upward or downward chirping The time evolution of the hole and hill structures in the phase space leads to the alternating frequency chirping

We report a rare regime of Earth's magnetosphere interaction with sub‐Alfvénic solar wind in which the windsock‐like magnetosphere transforms into one with Alfvén wings. In the magnetic cloud of a Coronal Mass Ejection (CME) on 24 April 2023, NASA's Magnetospheric Multiscale mission distinguishes the following features: (a) unshocked and accelerated low‐beta CME plasma coming directly against Earth's dayside magnetosphere; (b) dynamical wing filaments representing new channels of magnetic connection between the magnetosphere and foot points of the Sun's erupted flux rope; (c) cold CME ions observed with energized counter‐streaming electrons, evidence of CME plasma captured due to by reconnection between magnetic‐cloud and Alfvén‐wing field lines. The reported measurements advance our knowledge of CME interaction with planetary magnetospheres, and open new opportunities to understand how sub‐Alfvénic plasma flows impact astrophysical bodies such as Mercury, moons of Jupiter, and exoplanets close to their host stars. Plain Language Summary Like supersonically fast fighter jets creating sonic shocks in the air, planet Earth typically moves in the magnetized solar wind at super‐Alfvénic speeds and generates a bow shock. Here we report unprecedented observations of Earth's magnetosphere interacting with a sub‐Alfvénic solar wind brought by an erupted magnetic flux rope from the Sun, called a coronal mass ejection (CME). The terrestrial bow shock disappears, leaving the magnetosphere exposed directly to the cold CME plasma and the strong magnetic field from the Sun's corona. Our results show that the magnetosphere transforms from its typical windsock‐like configuration to having wings that magnetically connect our planet to the Sun. The wings are highways for Earth's plasma to be lost to the Sun, and for the plasma from the foot points of the Sun's erupted flux rope to access Earth's ionosphere. Our work indicates highly dynamic generation and interaction of the wing filaments, shedding new light on how sub‐Alfvénic plasma wind may impact astrophysical bodies in our solar and other stellar systems. Key Points MMS observed a rare regime of magnetosphere interaction with unshocked low‐beta CME plasma Wing filaments represent dynamical channels of magnetic connection between the magnetosphere and foot points of the Sun's erupted flux rope Cold CME ions observed on closed field lines, likely generated by dual‐wing reconnection

In the Earth’s inner magnetosphere, there exist regions like plasmasphere, ring current, and radiation belts, where the population of charged particles trapped along the magnetic field lines is more. These particles keep performing gyration, bounce and drift motions until they enter the loss cone and get precipitated to the neutral atmosphere. Theoretically, the mirror point latitude of a particle performing bounce motion is decided only by its equatorial pitch angle. This theoretical manifestation is based on the conservation of the first adiabatic invariant, which assumes that the magnetic field varies slowly relative to the gyro-period and gyro-radius. However, the effects of gyro-motion cannot be neglected when gyro-period and gyro-radius are large. In such a scenario, the theoretically estimated mirror point latitudes of electrons are likely to be in agreement with the actual trajectories due to their small gyro-radius. Nevertheless, for protons and other heavier charged particles like oxygen, the gyro-radius is relatively large, and the actual latitude of the mirror point may not be the same as estimated from the theory. In this context, we have carried out test particle simulations and found that the L-shell, energy, and gyro-phase of the particles do affect their mirror points. Our simulations demonstrate that the existing theoretical expression sometimes overestimates or underestimates the magnetic mirror point latitude depending on the value of L-shell, energy and gyro-phase due to underlying guiding centre approximation. For heavier particles like proton and oxygen, the location of the mirror point obtained from the simulation deviates considerably (∼ 10°–16°) from their theoretical values when energy and L-shell of the particle are higher. Furthermore, the simulations show that the particles with lower equatorial pitch angles have their mirror points inside the high or mid-latitude ionosphere.

The magnetosheath flow may take the form of large amplitude, yet spatially localized, transient increases in dynamic pressure, known as “magnetosheath jets” or “plasmoids” among other denominations. Here, we describe the present state of knowledge with respect to such jets, which are a very common phenomenon downstream of the quasi-parallel bow shock. We discuss their properties as determined by satellite observations (based on both case and statistical studies), their occurrence, their relation to solar wind and foreshock conditions, and their interaction with and impact on the magnetosphere. As carriers of plasma and corresponding momentum, energy, and magnetic flux, jets bear some similarities to bursty bulk flows, which they are compared to. Based on our knowledge of jets in the near Earth environment, we discuss the expectations for jets occurring in other planetary and astrophysical environments. We conclude with an outlook, in which a number of open questions are posed and future challenges in jet research are discussed.

Magnetic fields in plasmas can rapidly rearrange themselves in a process known as magnetic reconnection, which releases energy and accelerates particles. Torbert et al. used the Magnetospheric Multiscale (MMS) mission to probe a reconnection event in Earth's magnetotail—the region of plasma downstream from the planet as it moves through the solar wind. MMS has previously studied reconnection in the upstream magnetopause, but a different orbit was used to study the magnetotail, where the symmetry of the process is different. The authors measured plasma properties on scales of the electron dynamics, leading to insights that will apply in other regions where magnetic reconnection occurs. Science , this issue p. 1391 Magnetic reconnection in Earth’s magnetotail is observed on electron dynamics scales. Magnetic reconnection is an energy conversion process that occurs in many astrophysical contexts including Earth’s magnetosphere, where the process can be investigated in situ by spacecraft. On 11 July 2017, the four Magnetospheric Multiscale spacecraft encountered a reconnection site in Earth’s magnetotail, where reconnection involves symmetric inflow conditions. The electron-scale plasma measurements revealed (i) super-Alfvénic electron jets reaching 15,000 kilometers per second; (ii) electron meandering motion and acceleration by the electric field, producing multiple crescent-shaped structures in the velocity distributions; and (iii) the spatial dimensions of the electron diffusion region with an aspect ratio of 0.1 to 0.2, consistent with fast reconnection. The well-structured multiple layers of electron populations indicate that the dominant electron dynamics are mostly laminar, despite the presence of turbulence near the reconnection site.