Stephen M. Petrinec

The Tandem Reconnection And Cusp Electrodynamics Reconnaissance Satellites (TRACERS) small explorer mission was designed to: Determine whether magnetopause reconnection is primarily spatially or temporally variable for a range of solar wind conditions . Observations from the Analyzers for Cusp Ions (ACIs) on the two TRACERS spacecrafts are used to make this determination. Tandem cusp encounters from November 2025 through February 2026 are used to distinguish cusp ion structures resulting from spatial variability of magnetopause reconnection from those resulting from temporal variability. Preliminary results indicate that magnetopause reconnection is dominantly spatially variable for all interplanetary magnetic field (IMF) orientations and all solar wind dynamic pressures. Spatial variability dominates by at least a factor of 2 for all solar wind conditions. However, almost all encounters categorized as “spatial” display some temporal variability embedded in the larger spatial variability. More tandem cusp encounters are needed to verify these preliminary results and to complete the other science objectives of the mission.

The coronal mass ejection (CME) arriving at Earth on 10 May 2024 caused the most intense geomagnetic storm in the last two decades, resulting in highly unusual cusp and magnetopause dynamics. We simulate soft X‐Ray emission due to solar wind charge exchange with exospheric neutrals to image the global dayside dynamics, focusing on the impact of a dense CME current sheet during the storm main phase. The magnetopause moves inward to , and the cusps manifest as nearly parallel emission ridges in X‐Ray. As the interplanetary magnetic field reverses, the cusp ridges move apart to higher latitudes for minutes after the reversal, addressing the open question of how the cusps evolve during and after northward turning of the IMF. Our work highlights soft X‐ray imaging enabling simultaneous observations of extreme solar‐wind impacts across multiple magnetospheric regions.

In space physics, acronyms for coordinate systems (e.g., GEI , GSM ) are commonly used; however, differences in their definitions and implementations can prevent reproducibility. In this work, we compare definitions in online resources, software packages, and frequently cited journal articles and show that implementation differences can lead to transformations between same‐named coordinate systems and position values from different data providers to differ significantly. Based on these comparisons and results, and to enable reproducibility, we recommend that (a) a standard for acronyms and definitions for coordinate systems is developed, similar to equivalents in astronomy or earth sciences; (b) a standards body develops a citable database of reference data needed for these transforms. For software that computes coordinate transforms, we also recommend that their developers provide explicit comparisons of their implementations with the results of (b) and documentation on implementation choices. Additionally, we provide recommendations for scientists and metadata developers to ensure that sufficient information is provided to enable reproducibility. Finally, we document that spacecraft positions from data providers can differ both because of differences in how they implemented transforms and because of differences in the original source of the position data, and provide recommendations to improve the documentation of spacecraft positional data sets.

Space weather predictive modeling currently utilizes real‐time data from the spacecraft ACE and DSCOVR, with SOLAR‐1 and IMAP recently online. This study assesses whether DSCOVR solar wind plasma and interplanetary magnetic field (IMF) data exhibit large‐scale discrepancies from the corresponding ACE data, and if so, whether these discrepancies can be accounted for in predictive models. We chose several month‐long intervals of ACE and DSCOVR when they were maximally separated in Y location and separately in X location. In general, DSCOVR measurements of solar wind density, velocity and magnetic field magnitude correlate quite well with ACE. However, although the measured IMF magnitude is essentially identical between the two platforms, the various IMF components show significant differences, particularly in B _ZGSE . The data frequently display a significant (10–20 min) time offset between the two sets of measurements, consistent with the plasma structures being aligned with the Parker spiral. Comparison with MMS data taken in the solar wind confirm this variation from ballistic timing. Thus, space weather predicted by models relying on L1 monitors may be significantly impacted, representing a fundamental uncertainty in the prediction accuracy. Our results suggest that for the most accurate estimate of the IMF reaching the magnetopause, modelers should choose the spacecraft which is closest to the Sun/Earth line (−20 < Y _GSE < 10 R _E ) and nearest to Earth in X _GSE . However, for predictions with the longest lead time, as for time sensitive space weather alerts, the realtime monitor used should be the one with the largest + Y _GSE and X _GSE location.

A key process in solar‐terrestrial interactions is magnetic reconnection at the magnetopause. In addition to in situ observations of plasma entry at the magnetopause, some of the important global aspects of magnetic reconnection are addressed using plasma observations in the Earth's magnetospheric cusp region. Ions injected at the magnetopause are subject to a time‐of‐flight effect, which results in distinctive ion‐energy dispersions observed in the cusp. The shape of these ion dispersions is analyzed and linked with predictions of the magnetopause reconnection location from the maximum magnetic shear model to determine the plasma entry points at the magnetopause. This study determines how those plasma entry locations affect the observed cusp ion dispersion profiles. This study uses first observations from the Analyzers for Cusp Ions (ACIs) onboard the Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites spacecraft.

We present observations from two consecutive TRACERS‐2 orbits through the northern low‐altitude cusp. During the first crossing, TRACERS‐2 observed reversed cusp ion dispersion and sunward convection, consistent with magnetopause reconnection tailward of the cusp during this northward IMF interval. Simultaneous THEMIS‐D observations at the equatorial magnetopause show heated magnetosheath plasma captured on closed field lines, with similar particle spectra as in the low‐altitude cusp, indicating that reconnection indeed occurred tailward of the cusp and in both hemispheres. When TRACERS‐2 traversed the northern cusp again, 95 min later, the IMF was dominated by a negative B _X component. Despite the different IMF conditions, TRACERS‐2 recorded nearly the same cusp signatures as before, that is, reversed ion dispersion and sunward convection. The observations indicate that tailward‐of‐cusp reconnection can occur for both northward and B _X ‐dominated IMF and that these distinct IMF geometries can produce remarkably similar plasma and field signatures in the low‐altitude cusp.

Magnetic reconnection is a process that occurs at the boundary between the Earth's magnetic field and the interplanetary magnetic field (IMF) called the magnetopause. Magnetic reconnection allows solar wind plasma to enter Geospace and precipitate along the newly opened geomagnetic field lines into the cusp regions. To fully understand the interaction due to magnetic reconnection between these two disparate plasma regimes, it is important to understand the location along the magnetopause where reconnection occurs. The location of the dayside X‐line at Earth's magnetopause has been well described with the maximum magnetic shear model, which correctly predicts the location within 2 R _E (Petrinec et al., 2014, https://doi.org/10.1002/2014ja020209) for 82% (e.g., Trattner, Burch, et al., 2017, https://doi.org/10.1002/2017ja024488). Events where the model fails are not randomly distributed throughout the parameter space, rather they cluster in clearly defined parameter spaces. One of these event groups are so called “Knee” events, wherein one of the two anti‐parallel reconnection regions along the dayside magnetopause includes a sharp bend (kink or “knee”) such that this region maps through both the northern and southern hemispheres. This scenario causes a deflection of the entire dayside X‐line, which is controlled by the motion of the anchor point (the transition between the component and the anti‐parallel sections along the X‐lines) to where the last draped IMF line crosses the anti‐parallel reconnection region.

The Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS) mission observes electron energy‐latitude dispersion at the equatorward edge of the magnetospheric cusp, and high‐cadence Analyzer for Cusp Electrons (ACE) measurements resolve the dispersed edge. The inverse velocity dispersion (low energy before high energy) encountered by TRACERS as it travels southward through the northern cusp rules out pure energy‐time dispersion from nearby injections or Alfvén wave‐driven acceleration. TRACERS observes electron dispersion at the equatorward edge of the northern cusp roughly half of the time for southward interplanetary magnetic field (IMF), and almost never for northward IMF. The TRACERS measurements therefore provide strong observational support for the hypothesis that the observed electron dispersion results from dayside magnetic reconnection and plasma convection, much like the ion dispersion that commonly extends across the cusp. Observations of multiple electron dispersions and electron steps suggest fine‐scale spatial and/or temporal variability in magnetic reconnection.

Predicting the location of magnetopause reconnection remains a major challenge. Existing models often fail to predict the location of the reconnection line seen in global MHD simulations, particularly under northward IMF. This work presents a new X-line model that identifies a dominant reconnection line by maximizing the reconnection rate on both local and global scales. First, it determines the orientations at the magnetopause that locally maximize the rate and then finds the global path with the highest integrated rate. Across four global MHD simulations with diverse dipole tilts and IMF orientations, the new model was consistently more accurate than both the maximum magnetic shear and the global rate maximization models in predicting the location of the magnetic separator in between the magnetospheric cusps. Crucially, it succeeds for a challenging northward IMF case where previous models have failed. This model suggests the X-line's location is determined by a fundamental principle of maximizing the conversion of magnetic to plasma energy.

For cold and heavy magnetospheric ion populations that reach the dayside magnetopause, how those populations evolve across magnetopause separatrices into the reconnection exhaust, and how the populations may affect or be affected by reconnection, are still not well understood. Observations from the Magnetospheric Multiscale (MMS) mission from January 2019 are analyzed for a series of magnetopause crossings during a time period with a “string-of-pearls” configuration of the MMS constellation. With inter-spacecraft separations of ~100-300 km, this configuration allows for simultaneous measurements of the cold ion populations in different regions of the magnetopause boundary and current layers. For several magnetopause crossings on 2019-01-25, while magnetospheric heavy ions (He+ and O+) are not observable, a significant amount of cold (temperatures of ~1’s-10 eV) magnetospheric H+ is present in the outer magnetosphere. This cold H+ population is accelerated by the E×B drift near the magnetopause, but remains as a cold beam (temperatures of 10’s eV) well into the boundary layers and reconnection exhaust. While wave modes are present that could potentially contribute to ion heating, temperature changes are small and occur primarily at the edge of the boundary layer, and so more likely related to the initial acceleration by the normal electric field than wave-particle interactions. The lack of heating for the magnetopause crossings on 2019-01-25 differs from that observed in previous work where MMS was farther away from the X-line, pointing to the highly spatially structured nature of reconnection sites along the separatrices and the importance of the relative density of the cold ion population reaching the magnetopause.