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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.

It is commonly believed that because of the direct solar wind interaction with the Martian atmosphere/ionosphere, the planet could have lost a significant part of its atmosphere. Closed field lines of the crustal magnetic field can weaken a transport of the ionospheric ions to the tail. Reconnection of the interplanetary magnetic field lines draping around Mars and the crustal magnetic field can also lead to a presense of sunward fluxes of planetary ions that might affect the total ion loss. The LatHyS (LATMOS Hybrid Simulation) three‐dimensional multispecies hybrid model is used here to characterize sunward fluxes of O+ ions and the magnetic field topology at Mars. It is shown that although reconnection between the interplanetary magnetic field (IMF) and the crustal magnetic fields strongly modifies the field topology, then sunward ion fluxes are rather small and do not significantly change the total ion loss. Plain Language Summary Although Mars has no a global intrinsic magnetic field and solar wind interacts directly with the planetary atmosphere/ionosphere, the existence of strong but localized crustal magnetic field modifies the field topology around Mars. As a result, the Martian magnetosphere contains elements of the intrinsic and the induced magnetospheres. Reconnection between the interplanetary magnetic field and the crustal magnetic field can generate the plasma flows toward the planet and decrease the ionospheric losses, which is very important for the evolution of the Mars atmosphere/ionosphere. We have performed the numerical simulations of these potential effects and shown that the sunward ion fluxes are significantly less than the losses induced by the solar wind impact on the Martian ionosphere. Key Points Hybrid simulations show a drastic change of the field topology at altitudes less than ∼1,000 km due to crustal field sources Although the magnetic field topology is modified, the sunward fluxes do not essentially affect the total ion loss Sunward fluxes of oxygen ions in the tail vary between ∼5% and ∼20% compared to the anti‐sunward fluxes

Observations of Uranus in the near‐infrared by ground‐based telescopes from 1992 to 2018 have shown that the planet's upper atmosphere (thermosphere) steadily cooled from ∼700 to ∼450 K. We explain this cooling as due to the concurrent decline in the power of the solar wind incident on Uranus' magnetic field, which has dropped by ∼50% over the same period due to solar activity trends longer than the 11‐year solar cycle. Uranus' thermosphere appears to be more strongly governed by the solar wind than any other planet where we have assessed this coupling so far. Uranus' total auroral power may also have declined, in contrast with the power of the radio aurora that we expect has been predominantly modulated by the solar cycle. In the absence of strong local driving, planets with sufficiently large magnetospheres may also have thermospheres predominantly governed by the stellar wind, rather than stellar radiation. Plain Language Summary So far, we have only explored the Uranus planetary system with the Voyager 2 spacecraft, which flew past in 1986. This encounter led to many discoveries, and as many mysteries. One of these mysteries has only become clear since the flyby, as ground‐based telescopes have been monitoring the temperature of Uranus' tenuous upper atmosphere and have found that this layer has been getting colder and colder since the Voyager era, unlike the deeper atmosphere that has stayed about the same temperature. By 2018 the temperature of this upper layer had almost halved, and neither the 11‐year cycle of solar activity nor Uranus' changing seasons appear to have been in control. We finally provide a solution to this long‐standing problem by identifying that the energy input to Uranus' magnetic field by the tenuous, high‐speed flow of charged particles from the Sun has been similarly declining over decades. This interaction is what drives energy flow through space around the planet, and this energy ultimately does most of the heating of the upper atmosphere, where auroras are generated. We highlight that the situation may be similar at exoplanets with similarly large magnetospheres. Key Points Ground‐based telescopes have shown that Uranus' thermosphere steadily and dramatically cooled from ∼1992 to ∼2018 We explain this cooling as due to declining solar wind kinetic power incident on Uranus' magnetosphere controlling thermosphere temperature Uranus' thermosphere appears to be governed by the solar wind, total auroral power may have also declined over the same period

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.

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.

Mars is typically regarded as a non‐magnetic planet. Currents in the Martian ionosphere generate a Venus‐like induced magnetosphere which deflects the solar wind flows and piles up the interplanetary magnetic fields. However, crustal magnetic fields in the southern hemisphere influence local plasma properties. Using observations from the MAVEN mission, we characterize the distinguishing plasma characteristics of a mini‐magnetosphere that forms on the Martian dayside. We establish three criteria to differentiate this mini‐magnetosphere from the induced magnetosphere. Notably, the mini‐magnetosphere exhibits higher plasma beta (values near 1), with a balance between planetary ions, crustal magnetic fields, and the solar wind at the magnetopause. Observations show that the crustal magnetosphere reaches an altitude of 1,300 km, larger than one‐third of the Martian radius, indicating a dichotomy between the induced northern and the crustal southern magnetospheres. These findings offer novel insights into the distinctive properties of hybrid magnetospheres in the near‐Mars space. Plain Language Summary Mars lacks a global intrinsic magnetic field. Currents in the Martian ionosphere generate a Venus‐like induced magnetosphere which deflects the solar wind flow and piles up the interplanetary magnetic field. However, local crustal magnetic fields in Mars' southern hemisphere significantly influence the nearby plasma. With the support of the MAVEN mission, this work analyses observations from passes of the spacecraft through the mini‐magnetosphere during suitable orbits and investigates plasma pressures in both single orbit data and by a 4‐year statistical analysis. We present an observation of a mini‐magnetosphere filled by trapped heavy ions above the crustal magnetic fields on the Martian dayside. Furthermore, we establish three criteria to distinguish this mini‐magnetosphere from the induced magnetosphere. Observations show that the mini‐magnetosphere reaches an altitude of 1,300 km, larger than one‐third of the Martian radius. The observed mini‐magnetosphere and the dichotomy between the crustal southern and induced northern Martian magnetosphere forms a distinct environment that may help us to test the interactions between stellar winds and magnetic or nonmagnetic bodies. Key Points The crustal magnetic fields trap ionospheric plasma to form a mini‐magnetosphere in the near‐Mars plasma environment The mini‐magnetosphere reaches 1,300 km on the Martian dayside The mini‐magnetosphere balances the solar wind through contributions from plasma thermal pressure and the crustal magnetic pressure

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