Richard E. Denton

We review a theoretically predicted, hyperbolic MHD feature, exhibited in the field and plasma flow at the magnetopause, namely that of upstream‐traveling, slow‐mode waves, first predicted in the 1950s, but, so far, not identified in spacecraft data. Today's high‐quality observations at Earth's magnetopause now invites a serious attempt to check the prediction. The required theoretical background and analysis methodology is summarized and further developed. The study is limited to elongated disturbance structures, such as bulges, on the magnetopause that are approximately two‐dimensional and, in their proper frame, time independent. Most of the analysis tools are applicable to data from a single spacecraft. Subsequent papers could employ data from multi‐spacecraft missions, such as the four‐spacecraft Magnetospheric Multi‐Scale mission. Such data will permit the study of many additional details. Because they are mathematically intertwined, both elliptic and hyperbolic signatures will be examined. The three key points illustrate the important role played by magnetic‐variance analysis in the study of field perturbations in the vicinity of the magnetopause. Its most common use to date has been for the analysis of the magnetic fields from spacecraft crossings of the magnetopause, where the minimum variance direction can provide an estimate, not always of high quality, of the direction normal to the magnetopause layer. In the present paper, magnetopause traversals play only a secondary role; our primary objective is to analyze field perturbations near the magnetopause, for which we show that the ideal minimum‐variance direction is tangential, rather than normal, to the unperturbed magnetopause surface.

Measurements of singly ionized oxygen () ions in the inner magnetosphere during geomagnetic storms are important because the ions affect various magnetospheric processes. We apply a magnetoseismic technique to Van Allen Probes data to statistically determine the spatial and temporal development of the region of elevated number density , referred to as the oxygen torus, during geomagnetic storms. This study is motivated by previous studies that reported magnetic local time (MLT) localization of the torus to the morning side. In our study, we first determine the frequencies of the fundamental through third harmonics of toroidal standing Alfvén waves to estimate the mass density and then use the electron density derived from plasma wave spectra to define the average ion mass (=). We obtain from by making an assumption about the number density relative to the number density. All quantities are evaluated in a 15 min moving data window that moves in 5 min steps. By generating ‐MLT maps of for different phases of 102 storms with SYMH minimum lower than nT, we find that a region of high appears on the morning side during the storm main phase and lasts for a few days. is also high on the morning side during the main phase, but this MLT skewing disappears during the recovery phase. Therefore, the MLT asymmetry of the oxygen torus depends on what parameter is considered.

The magnetic cloud (MC) of the Coronal Mass Ejection on 24 April 2023, contains sub‐Alfvénic solar wind, transforming Earth's magnetosphere from conventional bow‐shock magnetotail configuration to Alfvén wings. Utilizing measurements from the Magnetosphere Multiscale (MMS) mission, we present for the first time electron distribution signatures as the spacecraft traverses through various magnetic topologies during this transformation. Specifically, we characterize electrons inside the sub‐Alfvénic MC, on the dawn‐dusk wing field lines and on the closed field lines. The signatures include strahl electrons in MC regions and energetic keV electrons streaming along the dawn and dusk wing field lines. We demonstrate the distribution signatures of dual wing reconnection, defined as reconnection between dawn‐dusk Alfvén wing field lines and the interplanetary magnetic field (IMF). These signatures include four electron populations comprised of partially depleted MC electrons and bi‐directional energetic electrons with variations in energy and pitch‐angle. The distributions reveal evidence of bursty magnetic reconnection under northward IMF.

We present observations on 24 April 2023 by the Magnetospheric Multiscale spacecraft at the dayside, mid‐latitude magnetopause, when an interplanetary magnetic cloud (MC) with sub‐Alfvénic flows and northward and dawnward interplanetary magnetic field components impacted Earth's magnetosphere. The aim is to reveal the processes of solar wind‐magnetosphere interaction under sub‐Alfvénic solar wind with northward magnetic field. Our analysis of electron and ion data suggests that magnetopause reconnection occurred near both polar cusps, forming boundary layers on closed magnetic field lines on both the solar wind (i.e., MC) and magnetospheric sides of the magnetopause. Grad‐Shafranov, electron‐magnetohydrodynamics, and polynomial reconstructions of magnetopause current layers show that local (equator‐of‐the‐cusp) reconnection occurred in a sub‐ion‐scale magnetopause current sheet with a low magnetic shear angle (30°). Interestingly, the local reconnection was observed between the two (MC‐side and magnetosphere‐side) layers of closed field lines. It indicates that reconnected field lines from double cusp reconnection were interacting to induce another reconnection at the mid‐latitude magnetopause. Our results suggest that magnetopause reconnection was more efficient or frequent under sub‐Alfvénic solar wind with much lower beta plasma conditions than typical conditions. We discuss the role of such efficient reconnection in the formation of low‐latitude boundary layers.

We present observations on 24 April 2023 by the Magnetospheric Multiscale spacecraft at the dayside, mid-latitude magnetopause, when an interplanetary magnetic cloud (MC) with sub-Alfvénic flows and northward and dawnward interplanetary magnetic field components impacted Earth’s magnetosphere. The aim is to reveal the processes of solar wind-magnetosphere interaction under sub-Alfvénic solar wind with northward magnetic field. Our analysis of electron and ion data suggests that magnetopause reconnection occurred near both polar cusps, forming boundary layers on closed magnetic field lines on both the solar wind (i.e., MC) and magnetospheric sides of the magnetopause. Grad-Shafranov, electron-magnetohydrodynamics, and polynomial reconstructions of magnetopause current layers show that local (equator-of-the-cusp) reconnection occurred in a sub-ion-scale magnetopause current sheet with a low magnetic shear angle (30°). Interestingly, the local reconnection was observed between the two (MC-side and magnetosphere-side) layers of closed field lines. It indicates that reconnected field lines from double cusp reconnection were interacting to induce another reconnection at the mid-latitude magnetopause. Our results suggest that magnetopause reconnection was more efficient or frequent under sub-Alfvénic solar wind with much lower beta plasma conditions than typical conditions. We discuss the role of such efficient reconnection in the formation of low-latitude boundary layers.

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.

The high density plasmasphere in the magnetosphere is often separated from a lower density region outside of the plasmasphere, called the plasmatrough, by a sharp gradient in electron density called the plasmapause. Here we use plasmapause events identified from electron density data from the ISEE, CRRES, Polar, and IMAGE missions, and the nonlinear genetic algorithm TuringBot, to find models for the electron density at the midpoint of the plasmapause, n_e,pp. A good model for n_e,pp should include dependence on L, which is the strongest dependence. But models can be improved by including weaker dependencies on the magnetic local time, MLT, the solar EUV index F10.7, and geomagnetic activity as indicated by averages of Kp and AE. The most complicated model that we present predicts n_e,pp within a factor of 1.64, and is within the range of observed plasmapause densities for about 96% of our events. These models can be useful for separating plasma populations into plasmasphere-like and plasmatrough-like populations. We also make available our database of electron density measurements categorized into various populations.

An LMN coordinate system for magnetic reconnection events is sometimes determined by defining N as the direction of the gradient across the current sheet and L as the direction of maximum variance of the magnetic field. The third direction, M, is often assumed to be the direction of zero gradient, and thus the orientation of the X line. But when there is a guide field, the X line direction may have a significant component in the L direction defined in this way. For a 2D description, a coordinate system describing such an event would preferably be defined using a different coordinate direction M′ oriented along the X line. Here we use a 3D particle‐in‐cell simulation to show that the X line is oriented approximately along the direction bisecting the asymptotic magnetic field directions on the two sides of the current sheet. We describe two possible ways to determine the orientation of the X line from spacecraft data, one using the minimum gradient direction from Minimum Directional Derivative analysis at distances of the order of the current sheet thickness from the X line, and another using the bisection direction based on the asymptotic magnetic fields outside the current sheet. We discuss conditions for validity of these estimates, and we illustrate these conditions using several Magnetospheric Multiscale (MMS) events. We also show that intersection of a flux rope due to secondary reconnection with the primary X line can destroy invariance along the X line and negate the validity of a two‐dimensional description.

We examine a Dipolarization Front (DF) event with an embedded electron diffusion region (EDR), observed by the Magnetospheric Multiscale (MMS) spacecraft on 08 September 2018 at 14:51:30 UT in the Earth's magnetotail by applying multi‐scale multipoint analysis methods. In order to study the large‐scale context of this DF, we use conjunction observations of the Cluster spacecraft together with MMS. A polynomial magnetic field reconstruction technique is applied to MMS data to characterize the embedded electron current sheet including its velocity and the X‐line exhaust opening angle. Our results show that the MMS and Cluster spacecraft were located in two counter‐rotating vortex flows, and such flows may distort a flux tube in a way that the local magnetic shear angle is increased and localized magnetic reconnection may be triggered. Using multi‐point data from MMS we further show that the local normalized reconnection rate is in the range of R ∼ 0.16 to 0.18. We find a highly asymmetric electron in‐ and outflow structure, consistent with previous simulations on strong guide‐field reconnection events. This study shows that magnetic reconnection may not only take place at large‐scale stable magnetopause or magnetotail current sheets but also in transient localized current sheets, produced as a consequence of the interaction between the fast Earthward flows and the Earth's dipole field.