Stefan Eriksson

Reconnection observations involving large density gradients and multiple heavyion species are scarce, motivating the analysis of satellite observations at Mars. The analyzed event exhibits anti-parallel magnetic fields with a guide field, accompanied by a substantial density gradient of approximately one to two orders of magnitude involving multiple species: e−, H+, O+, O+2 , and CO+2. The event is examined in terms of the temporal and spatial scales of the heavy ions of an assumed reconnection event, with the expectation that the structure is maintained by electron currents. The distance between the shocked solar wind and the collision-dominated ionosphere–thermosphere is small compared with the characteristic lengths of the X-line and outflow region. The ionospheric ions are accelerated within the reconnection region but transition to predominantly outflow-directed motion outside the exhaust. Away from the X-line, a boundary layer forms populated by accelerated ionospheric ions. The observations suggest that multiple X-lines form within the same areographic region and collectively sustain the outflow region over an extended period. For each newly formed Xline, reconnection is proposed to advance to lower altitudes, locally eroding the Martian atmosphere. This event demonstrates that magnetic reconnection can efficiently exchange plasma between the solar wind and the Martian ionosphere.

In October 2024, the Juno spacecraft observed an extreme compression of Jupiter's magnetosphere. This event pushed the magnetosheath into an unprecedented distance of only 30 Jupiter radii from Jupiter. This suggests an enhanced Solar Wind (SW) dynamic pressure (DynP), that was likely produced from a coronal mass ejection after an X1.8 class solar flare. The compression brought the magnetosheath, observed at a high southern latitude, to the closest distance ever observed from Jupiter. Using data from multiple instruments on Juno to identify magnetosheath signatures, it is shown that Juno had many encounters with the magnetosheath as it moved inward from apojove to perijove. This included many short‐lived encounters that were not readily identifiable from a single data set. From model estimates, a SW DynP of around 1.2 is suggested for this event, consistent with previous upper estimates observed in the vicinity of Jupiter's orbital path. Remote imaging suggests the magnetosphere remained compressed for several days after this event. The event raises the possibility that Jupiter's magnetosphere on the dayside could potentially shrink inside of Callisto's orbital path, and that if Callisto happened to be on the dayside of Jupiter at the time (although it was on the nightside during this event), it could find itself outside of the jovian magnetosphere and exposed directly to the SW.

We present a first record of significantly enhanced occurrences of magnetic reconnection exhausts as measured by the Wind satellite across a stream interaction region (SIR) at 1 AU from 10:11:20 UT on 4 Jan 2019 to 09:58:00 UT on 5 Jan 2019. The activity is clustered in a slow wind compression regime ahead of the SIR interface with a deflected, compressed fast wind. The 43 exhausts of this 1-day SIR dominate a distribution of 71 exhausts as obtained by a multi-window sliding technique application to the 8-day period on 1-9 Jan 2019. Active current sheets inside the SIR are associated with normal directions mostly near the ecliptic plane and a more azimuthal-than-Parker magnetic field direction at 1 AU. We find that exhausts wider than 500 ion inertial lengths are typically present just upstream and inside this SIR rather than within unperturbed slow and fast winds beyond a shocked solar wind. The observations suggest that plasma and field compressions may be crucial elements in driving a break-up of large-scale current sheets embedded in SIRs into smaller, multi-layered current sheet segments through magnetic reconnection.

Motivated by Parker Solar Probe observations of magnetic reconnection exhausts in current sheets near the Sun, we present particle-in-cell simulations of reconnection in the presence of shear flow perpendicular to the plane of reconnection. Increasing the strength of the flow shear decreases the reconnection rate, in agreement with observations and theory, and aligns with similar decreases that have been observed in other geometries. In addition, the shear produces asymmetries in the out-of-plane current bounding the reconnection exhaust, similar to those seen in observations.

A key open question in astrophysics is how plasma is transported within strongly magnetized, rapidly rotating systems. Magnetic reconnection and flux tube interchange are possible mechanisms, with Jupiter serving as the best local analog for distant systems. However, magnetic reconnection at Jupiter remains poorly understood. A key indicator of active magnetic reconnection is the ion diffusion region, but its detection at Jupiter had not been confirmed previously. Here, we report a magnetic reconnection event in Jupiter's inner magnetosphere that presents the detection of an ion diffusion region. We provide evidence that this event involves localized flux tube interchange motion driven by centrifugal forces, which occurs inside a thin current sheet formed by the collision and twisting of two distinct flux tubes. This study provides insights into Io-genic plasma transport at Jupiter and the unique role of magnetic reconnection in rapidly rotating systems, two key unresolved questions.

Understanding the spatial variability and correlation lengths throughout Stream Interaction Regions (SIRs) has importance both for fundamentally understanding the structuring of the solar wind and SIRs and for space weather operations. SIR observations from Advanced Composition Explorer and Wind are utilized to characterize the spatial variability of these structures and to compute correlation lengths of the solar wind speed and magnetic field magnitude for different regions of the SIR. In general, the probability of agreement between time‐shifted magnetic field magnitudes and solar wind speeds decreases with increasing spacecraft separation, with this degradation being more pronounced in the magnetic field magnitude. While the slow compressed region of the SIR has the steepest decline in agreement for the magnetic field magnitude, it is the shallowest decline for the percent agreement of solar wind speed. Despite these trends, the correlation length is longest in the slow compressed region for both magnetic field magnitude and solar wind speed, with the magnetic field magnitude generally having a longer correlation length than speed. These findings are consistent with current sheet layering within the compressed regions of the SIR and impacts of large‐amplitude Alfvénic fluctuations within the compressed and pristine fast solar wind. These results also further demonstrate the limitations of treating structures in the solar wind as laminar slabs, rather than being highly structured and variable, which may have impacts on uncertainties in space weather forecasting.

Magnetic reconnection is a fundamental and omnipresent energy conversion process in plasma physics. Novel observations of fields and particles from Parker Solar Probe (PSP) have shown the absence of reconnection in a large number of current sheets in the near-Sun solar wind. Using near-Sun observations from PSP encounters 4–11 (2020 January–2022 March), we investigate whether reconnection onset might be suppressed by velocity shear. We compare estimates of the tearing mode growth rate in the presence of shear flow for time periods identified as containing reconnecting current sheets versus nonreconnecting times, finding systematically larger growth rates for reconnection periods. Upon examination of the parameters associated with reconnection onset, we find that 85% of the reconnection events are embedded in slow, non-Alfvénic wind streams. We compare with fast, slow non-Alfvénic, and slow Alfvénic streams, finding that the growth rate is suppressed in highly Alfvénic fast and slow wind, and reconnection is not seen in these wind types, as would be expected from our theoretical expressions. These wind streams have strong Alfvénic flow shear, consistent with the idea of reconnection suppression by such flows. This could help explain the frequent absence of reconnection events in the highly Alfvénic, near-Sun solar wind observed by PSP. Finally, we find a steepening of both the trace and magnitude magnetic field spectra within reconnection periods in comparison to ambient wind. We tie this to the dynamics of relatively balanced turbulence within these reconnection periods and the potential generation of compressible fluctuations.

Context. Magnetic reconnection events are frequently observed in the solar wind. Understanding the patterns and structures within the solar wind is crucial to put observed magnetic reconnection events into context, since their occurrence rate and properties are likely influenced by solar wind conditions. Aims. We employed unsupervised learning techniques such as self-organizing maps (SOM) and K-Means to cluster and interpret solar wind data at 1 AU for an improved understanding of the conditions that lead to magnetic reconnection in the solar wind. Methods. We collected magnetic field data and proton density, proton temperature, and solar wind speed measurements taken by the Wind spacecraft. After preprocessing the data, we trained a SOM to visualize the high-dimensional data in a lower-dimensional space and applied K-Means clustering to identify distinct clusters within the solar wind data. We then compare the results with the Xu, F., & Borovsky, J. E. (2015, J. Geophys. Res. Space Phys., 120, 70) classification of the solar wind. Results. Our analysis revealed that the reconnection events are distributed across five different clusters: (a) slow solar wind, (b) compressed slow wind, (c) highly Alfvénic wind, (d) compressed fast wind, and (e) ejecta. Compressed slow and fast wind and ejecta are clusters associated with solar wind transients such as stream interaction regions and interplanetary coronal mass ejections. The majority of the reconnection events are associated with the slow solar wind, followed by the highly Alfvénic wind, compressed slow wind, and compressed fast wind, and a small fraction of the reconnection events are associated with ejecta. Conclusions. Unsupervised learning approaches with SOM and K-Means lead to physically interpretable solar wind clusters based on their transients and allow for the contextualization of magnetic reconnection exhausts’ occurrence in the solar wind.

Understanding the spatial variability and correlation lengths throughout Stream Interaction Regions (SIRs) has importance both for fundamentally understanding structuring of the solar wind and SIRs and for space weather operations. SIR observations from ACE and Wind are utilized to characterize the spatial variability of these structures and to compute correlation lengths of the solar wind speed and magnetic field magnitude for different regions of the SIR. In general, the probability of agreement between time-shifted magnetic field magnitudes and solar wind speeds decrease for increasing spacecraft separation, with this degradation being more pronounced in the magnetic field magnitude. While the slow compressed region of the SIR has the steepest decline in agreement for the magnetic field magnitude, it is the shallowest decline for the percent agreement of solar wind speed. Despite these trends, the correlation length is longest in the slow compressed region for both magnetic field magnitude and solar wind speed, with the magnetic field magnitude having generally a longer correlation length than speed. These findings are consistent with current sheet layering within the compressed regions of the SIR and impacts of large-amplitude Alfvénic fluctuations within the compressed and pristine fast solar wind. These results also further demonstrate the limitations of treating structures in the solar wind as laminar slabs, rather than being highly structured and variable, which may have impacts on uncertainties in space weather forecasting.

The solar wind plasma is strongly accelerated near the Sun—this acceleration and heating remain beyond 1 au, which contradicts cooling by adiabatic expansion. Wave–particle interaction has been recognized as one of the key mechanisms for particle acceleration and heating in the solar wind. In this context, it is relevant to analyze the origin of the ion acoustic waves (IAWs) observed by the Parker Solar Probe (PSP) and their possible role in solar wind acceleration and heating. PSP has recorded various phenomena during its close encounters with the Sun, including IAWs, current sheets, ion velocity streams, and reconnection exhausts. The origin of the observed IAWs remains unknown. This study investigates the temporal correlation between IAWs, current sheets, ion speed streams, and reconnection exhausts as possible sources of ion beams. Our analysis reveals that current sheets and ion speed streams exhibit a statistically significant temporal proximity with IAWs and also between them, while reconnection exhausts did not show a statistical association with IAWs. These results suggest that current sheets may be associated with proton beams, which then excite IAWs.