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The Juno Magnetic Field investigation (MAG) characterizes Jupiter’s planetary magnetic field and magnetosphere, providing the first globally distributed and proximate measurements of the magnetic field of Jupiter. The magnetic field instrumentation consists of two independent magnetometer sensor suites, each consisting of a tri-axial Fluxgate Magnetometer (FGM) sensor and a pair of co-located imaging sensors mounted on an ultra-stable optical bench. The imaging system sensors are part of a subsystem that provides accurate attitude information (to ∼20 arcsec on a spinning spacecraft) near the point of measurement of the magnetic field. The two sensor suites are accommodated at 10 and 12 m from the body of the spacecraft on a 4 m long magnetometer boom affixed to the outer end of one of ’s three solar array assemblies. The magnetometer sensors are controlled by independent and functionally identical electronics boards within the magnetometer electronics package mounted inside Juno’s massive radiation shielded vault. The imaging sensors are controlled by a fully hardware redundant electronics package also mounted within the radiation vault. Each magnetometer sensor measures the vector magnetic field with 100 ppm absolute vector accuracy over a wide dynamic range (to 16 Gauss = 1.6 × 10 6  nT per axis) with a resolution of ∼0.05 nT in the most sensitive dynamic range (±1600 nT per axis). Both magnetometers sample the magnetic field simultaneously at an intrinsic sample rate of 64 vector samples per second. The magnetic field instrumentation may be reconfigured in flight to meet unanticipated needs and is fully hardware redundant. The attitude determination system compares images with an on-board star catalog to provide attitude solutions (quaternions) at a rate of up to 4 solutions per second, and may be configured to acquire images of selected targets for science and engineering analysis. The system tracks and catalogs objects that pass through the imager field of view and also provides a continuous record of radiation exposure. A spacecraft magnetic control program was implemented to provide a magnetically clean environment for the magnetic sensors, and residual spacecraft fields and/or sensor offsets are monitored in flight taking advantage of Juno’s spin (nominally 2 rpm) to separate environmental fields from those that rotate with the spacecraft.

While Mars lacks a global intrinsic magnetic field, it does exhibit crustal magnetic anomalies (mostly in its Southern Hemisphere). These crustal magnetic anomalies directly interact with solar wind, which forms a mini‐magnetosphere and a region denoted the mini‐magnetopause. Using magnetic field and plasma measurements from the Mars Atmosphere and Volatile Evolution, we report a novel case of magnetic reconnection at the Martian mini‐magnetopause. In this process, protons and oxygen ions from the Martian atmosphere were accelerated during reconnection and likely escaped along the outflow direction. Magnetic reconnection may occur between the interplanetary magnetic field and crustal magnetic fields at the Martian mini‐magnetopause, which contributes to planetary ion escape, solar wind entering the mini‐magnetosphere and the evolution of magnetic topology in the dayside Martian mini‐magnetosphere. Plain Language Summary While Mars lacks a global intrinsic magnetic field, it does exhibit crustal magnetic anomalies. The solar wind from the sun accompanied by interplanetary magnetic field (IMF) directly interacts with this crustal magnetic field, similar to what occurs on Earth, albeit at a smaller scale. The boundary between the crustal field on Mars and the IMF is called the mini‐magnetopause. Magnetic reconnection is a fundamental process in astrophysical and space plasmas that can change the topology of magnetic field and effectively convert magnetic energy into thermal and kinetic energy. Using magnetic field and plasma measurements from the Mars Atmosphere and Volatile Evolution missions, we report direction observations of magnetic reconnection at the Martian mini‐magnetopause. Through magnetic reconnection at the mini‐magnetopause, the IMF reconnects with the magnetic field from the crustal field, forming new magnetic field lines that channel solar wind to enter the Martian atmosphere and planetary plasmas to escape. Key Points Magnetic reconnection at the Martian mini‐magnetopause is reported for the first time Proton and oxygen ions from the mini‐magnetosphere are accelerated during magnetic reconnection Magnetic reconnection at the mini‐magnetopause could cause solar wind to enter the mini‐magnetosphere

The dynamic activity of stars such as the Sun influences (exo)planetary space environments through modulation of stellar radiation, plasma wind, particle and magnetic fluxes. Energetic solar-stellar phenomena such as flares and coronal mass ejections act as transient perturbations giving rise to hazardous space weather. Magnetic fields – the primary driver of solar-stellar activity – are created via a magnetohydrodynamic dynamo mechanism within stellar convection zones. The dynamo mechanism in our host star – the Sun – is manifest in the cyclic appearance of magnetized sunspots on the solar surface. While sunspots have been directly observed for over four centuries, and theories of the origin of solar-stellar magnetism have been explored for over half a century, the inability to converge on the exact mechanism(s) governing cycle to cycle fluctuations and inconsistent predictions for the strength of future sunspot cycles have been challenging for models of the solar cycles. This review discusses observational constraints on the solar magnetic cycle with a focus on those relevant for cycle forecasting, elucidates recent physical insights which aid in understanding solar cycle variability, and presents advances in solar cycle predictions achieved via data-driven, physics-based models. The most successful prediction approaches support the Babcock-Leighton solar dynamo mechanism as the primary driver of solar cycle variability and reinforce the flux transport paradigm as a useful tool for modelling solar-stellar magnetism.

We investigate how the Martian dayside ionospheric structure is modified by crustal magnetic field (CMF) strength and upstream solar wind pressure by analyzing electron density data from the Langmuir Probe and Waves instrument onboard the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft. We find that the electron density above the exobase is anticorrelated with the ratio of solar wind's normal dynamic pressure (PSW⊥${P}_{\\text{SW}\\perp }$ ) to CMF magnetic pressure (PCMF${P}_{\\text{CMF}}$ ). We also analyze the electron density behavior across different magnetic topologies as a function of PSW⊥/PCMF${P}_{\\text{SW}\\perp }/{P}_{\\text{CMF}}$ . The extremely low electron density in the draped topology relates to ionopause‐like structures. The lower electron density in the closed and open topology under higher PSW⊥/PCMF${P}_{\\text{SW}\\perp }/{P}_{\\text{CMF}}$may be attributed to a downward force, potentially the J × B force in the case of closed topology. This study highlights the complex interplay between solar wind and CMF in influencing the Martian dayside upper ionosphere. Plain Language Summary Mars is unique in the solar system because it lacks a global dipole field like Earth and instead has crustal magnetic fields (CMF, i.e., pockets of magnetic fields unevenly distributed on its surface). Such a magnetic scenario yields a very special picture of the interaction between solar wind (a stream of charged particles from the Sun) and the Martian upper atmosphere. For decades, people have found that the structure of the Martian ionosphere (an ionized layer in its upper atmosphere) can be heavily influenced by solar wind dynamic pressure (ram pressure of the stream of charged particles) and CMF strength, but the physics behind this is unclear. Our results indicate that the competition between the solar wind dynamic pressure and CMF strength can induce electromagnetic force, which affects the electron density in the Martian ionosphere. This study sheds light on the detailed physics of the interaction between solar wind and CMF and its implication for the behaviors of the Martian ionosphere. Key Points The electron density in the Martian dayside upper ionosphere is anticorrelated with pressure ratio of solar wind to crustal magnetic field The electron density in closed, open, and draped topology behaves differently as a function of this ratio The J × B force may play an important role in the effect of crustal magnetic field and solar wind conditions on the Martian upper ionosphere

About ten per cent of ‘massive’ stars (those of more than 1.5 solar masses) have strong, large-scale surface magnetic fields 1 – 3 . It has been suggested that merging of main-sequence and pre-main-sequence stars could produce such strong fields 4 , 5 , and the predicted fraction of merged massive stars is also about ten per cent 6 , 7 . The merger hypothesis is further supported by a lack of magnetic stars in close binaries 8 , 9 , which is as expected if mergers produce magnetic stars. Here we report three-dimensional magnetohydrodynamical simulations of the coalescence of two massive stars and follow the evolution of the merged product. Strong magnetic fields are produced in the simulations, and the merged star rejuvenates such that it appears younger and bluer than other coeval stars. This can explain the properties of the magnetic ‘blue straggler’ star τ Sco in the Upper Scorpius association that has an observationally inferred, apparent age of less than five million years, which is less than half the age of its birth association 10 . Such massive blue straggler stars seem likely to be progenitors of magnetars, perhaps giving rise to some of the enigmatic fast radio bursts observed 11 , and their supernovae may be affected by their strong magnetic fields 12 . Simulated mergers of two massive stars provide a solution to the long-standing puzzle of the origin of strong magnetic fields in a subset of massive stars.

We report on observations made by the Mars Atmosphere and Volatile EvolutioN spacecraft at Mars, in the region of the ion plume. We observe that in some cases, when the number density of oxygen ions is comparable to the density of the solar wind protons interaction between both plasmas leads to formation in the magnetosheath of mini induced magnetospheres possessing all typical features of induced magnetospheres typically observed at Mars or Venus: a pileup of the magnetic field at the head of the ion cloud, magnetospheric cavity, partially void of solar wind protons, draping of the interplanetary magnetic field around the mini obstacle, formation of a magnetic tail with a current sheet, in which protons are accelerated by the magnetic field tensions. These new observations may shed a light on the mechanism of formation of induced magnetospheres. Plain Language Summary There is a class of the induced planetary magnetospheres when the absence of intrinsic magnetic field allows a direct interaction of solar wind with planetary atmospheres/ionospheres. We have shown the existence of mini‐induced magnetospheres at Mars. When the density of the extracted from the ionosphere oxygen ions becomes comparable with the proton density in solar wind mini‐induced magnetospheres with all typical features of the planetary induced magnetospheres arise. Key Points Oxygen ions extracted from the Martian ionosphere interact with shocked solar wind in the magnetosheath When the ion densities of both plasmas become comparable the mini induced magnetospheres are built These Magnetospheres possess all typical features of the classical induced magnetospheres

As incident solar wind encounters the martian upper atmosphere, it undergoes deflection particularly in the magnetosheath. However, the plasma flow exhibits asymmetrical distribution features within this transition region, which is investigated by employing a three‐dimensional Hall magnetohydrodynamic (MHD) model from an energy transfer perspective in this study. Simulation results reveal that solar wind protons transfer momentum to ionospheric heavy ions through motional electric field in the hemisphere where the motional electric field points outward from the planet. In the opposite hemisphere, solar wind flow tends to be effectively accelerated by ambipolar and Hall electric fields. The distinct dynamics of solar wind protons in both hemispheres result in the asymmetrical deflection. Furthermore, the extent of asymmetry grows as the cross‐flow component of the upstream interplanetary magnetic field increases, but diminishes as the density of the solar wind proton increases, contingent upon the energy effectively acquired from ambipolar and Hall electric fields. Plain Language Summary Due to the lack of a global intrinsic magnetic field at Mars, the solar wind has a direct interaction with the upper atmosphere of the planet. During this interaction, heavy ions from the martian ionosphere can be accelerated by the motional electric field of the solar wind, resulting in an excess of momentum in the martian system that necessitates the deflection of solar wind protons in the opposite direction to maintain balance. In this study, we utilize a Hall‐MHD model to study the asymmetrical deflection of the solar wind in the martian magnetosheath from an energy transfer perspective. Simulation results indicate that solar wind protons tend to effectively acquire energy from the ambipolar and Hall electric fields in the hemisphere opposite to the direction of the motional electric field and transfer its energy to heavy ions through the motional electric field in the opposite hemisphere, leading to an asymmetrical deflection of the solar wind. Furthermore, the degree of asymmetry is impacted by external solar wind conditions, including the strength of interplanetary magnetic field cross‐flow component and the density of solar wind protons. These findings provide valuable insights into the flow asymmetries that arise during the interaction between Mars and solar wind. Key Points The multi‐fluid MHD model effectively reproduces the asymmetrical deflection of solar wind flow within the magnetosheath The asymmetrical deflection of solar wind is a consequence of the discrepancy in energy transfer patterns between the two hemispheres The impact of the strength of interplanetary magnetic field By and solar wind density on asymmetrical deflection is individually examined

The fast solar wind that fills the heliosphere originates from deep within regions of open magnetic field on the Sun called ‘coronal holes’. The energy source responsible for accelerating the plasma is widely debated; however, there is evidence that it is ultimately magnetic in nature, with candidate mechanisms including wave heating 1 , 2 and interchange reconnection 3 – 5 . The coronal magnetic field near the solar surface is structured on scales associated with ‘supergranulation’ convection cells, whereby descending flows create intense fields. The energy density in these ‘network’ magnetic field bundles is a candidate energy source for the wind. Here we report measurements of fast solar wind streams from the Parker Solar Probe (PSP) spacecraft 6 that provide strong evidence for the interchange reconnection mechanism. We show that the supergranulation structure at the coronal base remains imprinted in the near-Sun solar wind, resulting in asymmetric patches of magnetic ‘switchbacks’ 7 , 8 and bursty wind streams with power-law-like energetic ion spectra to beyond 100 keV. Computer simulations of interchange reconnection support key features of the observations, including the ion spectra. Important characteristics of interchange reconnection in the low corona are inferred from the data, including that the reconnection is collisionless and that the energy release rate is sufficient to power the fast wind. In this scenario, magnetic reconnection is continuous and the wind is driven by both the resulting plasma pressure and the radial Alfvénic flow bursts. Measurements of fast solar wind streams from the Parker Solar Probe spacecraft provide strong evidence for the interchange reconnection mechanism being responsible for accelerating the fast solar wind.

The Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) provides photospheric vector magnetograms with a high spatial and temporal resolution. Our intention is to model the coronal magnetic field above active regions with the help of a nonlinear force-free extrapolation code. Our code is based on an optimization principle and has been tested extensively with semianalytic and numeric equilibria and applied to vector magnetograms from Hinode and ground-based observations. Recently we implemented a new version which takes into account measurement errors in photospheric vector magnetograms. Photospheric field measurements are often affected by measurement errors and finite nonmagnetic forces inconsistent for use as a boundary for a force-free field in the corona. To deal with these uncertainties, we developed two improvements: i) preprocessing of the surface measurements to make them compatible with a force-free field, and ii) new code which keeps a balance between the force-free constraint and deviation from the photospheric field measurements. Both methods contain free parameters, which must be optimized for use with data from SDO/HMI. In this work we describe the corresponding analysis method and evaluate the force-free equilibria by how well force-freeness and solenoidal conditions are fulfilled, by the angle between magnetic field and electric current, and by comparing projections of magnetic field lines with coronal images from the Atmospheric Imaging Assembly (SDO/AIA). We also compute the available free magnetic energy and discuss the potential influence of control parameters.