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Massive galaxies like our Milky Way are orbited by satellite dwarf galaxies. Standard cosmological simulations of galaxy formation predict that these satellites should move randomly around their host. Müller et al. examined the satellites of the nearby elliptical galaxy Centaurus A (see the Perspective by Boylan-Kolchin). They found that the satellites are distributed in a planar arrangement, and the members of the plane are orbiting in a coherent direction. This is inconsistent with more than 99% of comparable galaxies in simulations. Centaurus A, the Milky Way, and Andromeda all have highly statistically unlikely satellite systems. This observational evidence suggests that something is wrong with standard cosmological simulations. Science , this issue p. 534 ; see also p. 520 A plane of satellite dwarf galaxies rotating around the Centaurus A galaxy is inconsistent with cosmological simulations. The Milky Way and Andromeda galaxies are each surrounded by a thin plane of satellite dwarf galaxies that may be corotating. Cosmological simulations predict that most satellite galaxy systems are close to isotropic with random motions, so those two well-studied systems are often interpreted as rare statistical outliers. We test this assumption using the kinematics of satellite galaxies around the Centaurus A galaxy. Our statistical analysis reveals evidence for corotation in a narrow plane: Of the 16 Centaurus A satellites with kinematic data, 14 follow a coherent velocity pattern aligned with the long axis of their spatial distribution. In standard cosmological simulations, <0.5% of Centaurus A–like systems show such behavior. Corotating satellite systems may be common in the universe, challenging small-scale structure formation in the prevailing cosmological paradigm.

We invoke the estimates of the amplitudes of the velocity perturbations and caused by the influence of a spiral density wave that have been obtained by us previously from three stellar samples. These include Galactic masers with measured VLBI trigonometric parallaxes and proper motions, OB2 stars, and Cepheids. From these data we have obtained new estimates of the spiral pattern speed in the Galaxy : , , and km s kpc from the samples of masers, OB2 stars, and Cepheids, respectively. The corotation radii for these three samples are , , and , suggesting that the corotation circle is located between the Sun and the Perseus arm segment.

Stars form by accreting material from their surrounding disks. There is a consensus that matter flowing through the disk is channelled onto the stellar surface by the stellar magnetic field. This is thought to be strong enough to truncate the disk close to the corotation radius, at which the disk rotates at the same rate as the star. Spectro-interferometric studies in young stellar objects show that hydrogen emission (a well known tracer of accretion activity) mostly comes from a region a few milliarcseconds across, usually located within the dust sublimation radius 1 – 3 . The origin of the hydrogen emission could be the stellar magnetosphere, a rotating wind or a disk. In the case of intermediate-mass Herbig AeBe stars, the fact that Brackett γ (Brγ) emission is spatially resolved rules out the possibility that most of the emission comes from the magnetosphere 4 – 6 because the weak magnetic fields (some tenths of a gauss) detected in these sources 7 , 8 result in very compact magnetospheres. In the case of T Tauri sources, their larger magnetospheres should make them easier to resolve. The small angular size of the magnetosphere (a few tenths of a milliarcsecond), however, along with the presence of winds 9 , 10 make the interpretation of the observations challenging. Here we report optical long-baseline interferometric observations that spatially resolve the inner disk of the T Tauri star TW Hydrae. We find that the near-infrared hydrogen emission comes from a region approximately 3.5 stellar radii across. This region is within the continuum dusty disk emitting region (7 stellar radii across) and also within the corotation radius, which is twice as big. This indicates that the hydrogen emission originates in the accretion columns (funnel flows of matter accreting onto the star), as expected in magnetospheric accretion models, rather than in a wind emitted at much larger distance (more than one astronomical unit). The size of the inner disk of the T Tauri star TW Hydrae is determined using optical long-baseline interferometric observations, indicating that hydrogen emission comes from a region approximately 3.5 stellar radii across.

Based on plasma observations from Juno's Joviana Auroral Distributions Experiment instrument and forward modeling, we investigate the dawn‐dusk asymmetries within Jupiter's magnetosphere between 10 and 40 RJ${\\mathrm{R}}_{J}$ . On the dawnside, the flux tubes are depleted characterized by low density and near rigid‐corotation velocity, with a low temperature and thin plasma sheet. On the duskside, the flux tubes are assimilated characterized by high density and sub‐corotation velocity, with a hotter and thicker plasma sheet. Super‐corotating hot inflows originating from reconnection events are identified in the pre‐dawn sector. These observations are consistent with the Vasyliūnas cycle, suggesting it operates in a region closer to Jupiter than previous studies suggested. Outflows are locally coupled with swept‐back magnetic fields and are frequently observed near midnight. Inflows are locally coupled with swept‐forward fields with higher temperatures. The discernible temperature difference between inflows and outflows reveals their distinct origins. Plasma beta increases with radial distance, suggesting increased instabilities at larger distances. Plain Language Summary Jupiter's magnetosphere is internally driven by plasma originating from its moons, particularly Io. As the plasma corotates around Jupiter and is transported outward, the force balance breaks with stretched magnetic field and pinched off plasma blobs, resulting in local time asymmetry. For the first time, this asymmetry is validated within 40 Jupiter radii using plasma observations from the Juno mission. Compared to the duskside, the dawnside plasma exhibits lower density, higher temperature, and larger corotational velocity (even exceeds rigid corotation sometimes). These results indicate distinct origins for inflows and outflows. This study brings us a step forward in our understanding of the dynamics of Jupiter's magnetosphere and provides a data set for comparison with simulations. Key Points The dawn‐dusk asymmetry between 10 and 40 RJ is modulated by the Vasyliūnas cycle, featuring super‐corotating flows in the pre‐dawn sector The flux tubes are depleted and accelerated on the dawnside, while they are dense and sub‐corotational on the duskside The temperature difference between inflows and outflows suggests physically distinct origins

The first experimental results on pattern transitions in the co-rotation regime (i.e. the rotation ratio $\\varOmega = \\omega _o/\\omega _i > 0$, where $\\omega _i$ and $\\omega _o$ are the angular speeds of the inner and outer cylinders, respectively) of the Taylor–Couette flow (TCF) are reported for a neutrally buoyant suspension of non-colloidal particles, up to a particle volume fraction of $\\phi = 0.3$. While the stationary Taylor vortex flow (TVF) is the primary bifurcating state in dilute suspensions ($\\phi \\leq ~0.05$), the non-axisymmetric oscillatory states, such as the spiral vortex flow (SVF) and the ribbon (RIB), appear as primary bifurcations with increasing particle loading, with an overall de-stabilization of the primary bifurcating states (TVF/SVF/RIB) being found with increasing $\\phi$ for all $\\varOmega \\geq ~0$. At small co-rotations ($\\varOmega \\sim 0$), the particles play the dual role of stabilization ($\\phi < 0.1$) and destabilization ($\\phi \\geq ~0.1$) on the secondary/tertiary oscillatory states. The distinctive features of the ‘particle-induced’ spiral vortices are identified and contrasted with those of the ‘fluid-induced’ spirals that operate in the counter-rotation regime.

We show the possible existence of a significant axial asymmetry in the reconnection separator at the Jovian magnetopause using first‐principle, physics‐based global simulations. Under eastward interplanetary magnetic field (IMF) conditions near Jupiter's orbit, reconnection occurs at the southern‐dusk and northern magnetopause with large shear angles between the magnetospheric and magnetosheath magnetic fields. When driven by the westward IMF, the reconnection position switches to the northern‐dusk and southern magnetopause. Component reconnection at the southern‐dusk/northern‐dusk magnetopause is associated with the interaction of the IMF with the nearly‐dipolar background fields. Nearly‐antiparallel reconnection near the noon‐midnight plane at the northern/southern magnetopause is related to the dawn‐dusk asymmetric, helical, closed lobe magnetic fields, which is a consequence of significant planetary corotation effects and not expected at Earth. Such configuration is testable as Juno has proceeded its orbit to the high‐altitude cusps and provides new insight into the interpretation of measurements from other rotationally‐driven systems. Plain Language Summary Magnetic reconnection is a fundamental plasma process for understanding the dynamic evolution of planetary magnetospheres. It is related to the breaking and reconnecting of magnetic fields from two different sources. Depending on the orientation of the interplanetary magnetic field (IMF), reconnection at a planetary magnetopause is a combination of antiparallel and component reconnection of magnetic fields in the outer magnetosphere and the inner magnetosheath. In the terrestrial magnetosphere, magnetopause reconnection that is dominated by solar wind‐magnetosphere interactions is generally symmetric about the Sun‐Earth axis in solar‐magnetospheric coordinates regardless of the IMF orientation. However, this axial symmetry may not exist in planetary magnetospheres where the time scale of planetary rotation is much shorter than the solar wind transition, especially at Jupiter that has a faster planetary rotation and a significantly larger magnetosphere than Earth's. This study focuses on an unusual asymmetry in magnetospheric configuration and magnetopause reconnection at Jupiter and its relationship with fast planetary rotation. Key Points Unlike at Earth, magnetopause reconnection at Jupiter may exhibit a significant axial asymmetry under east‐west interplanetary magnetic field (IMF) conditions This asymmetry is caused by the helical, closed lobe magnetic field with dawn‐dusk asymmetry due to fast Jovian rotation Reconnection may occur at northern and southern‐dusk magnetopause under eastward IMF and has a north‐south reversal for westward IMF

The global superrotation index S compares the integrated axial angular momentum of the atmosphere to that of a state of solid-body corotation with the underlying planet. The index S is similar to a zonal Rossby number, which suggests it may be a useful indicator of the circulation regime occupied by a planetary atmosphere. We investigate the utility of S for characterizing regimes of atmospheric circulation by running idealized Earthlike general circulation model experiments over a wide range of rotation rates Ω, 8Ω E to Ω E /512, where Ω E is Earth’s rotation rate, in both an axisymmetric and three-dimensional configuration. We compute S for each simulated circulation, and study the dependence of S on Ω. For all rotation rates considered, S is on the same order of magnitude in the 3D and axisymmetric experiments. For high rotation rates, S ≪ 1 and S ∝ Ω −2 , while at low rotation rates S ≈ 1/2 = constant. By considering the limiting behavior of theoretical models for S , we show how the value of S and its local dependence on Ω can be related to the circulation regime occupied by a planetary atmosphere. Indices of S ≪ 1 and S ∝ Ω −2 define a regime dominated by geostrophic thermal wind balance, and S ≈ 1/2 = constant defines a regime where the dynamics are characterized by conservation of angular momentum within a planetary-scale Hadley circulation. Indices of S ≫ 1 and S ∝ Ω −2 define an additional regime dominated by cyclostrophic balance and strong equatorial superrotation that is not realized in our simulations.

Over the hours of 05:00–09:00 UT on 8 June 2001, the extreme ultraviolet (EUV) instrument on board the IMAGE satellite observed a shoulder-like formation in the morning sector and a post-noon plume-like structure. The plasmapause formation is simulated using the test particle model (TPM), based on a drift motion theory, which reproduces various plasmapause structures and evolution of the shoulder feature. The analysis indicates that the shoulder is created by sharp reduction and spatial non-uniformity in the dawn–dusk convection electric field intensity. The TPM-modeled event is found to develop an initial pre-dawn asymmetric bulge that becomes a shoulder as a result of increased “corotation” rate with an increasing L-shell that is preceded by localized outward convection. The shoulder structure rotates sunward and develops into a single- or double-plume structure during an active time period in simulation.

Studying solar-wind conditions is central to forecasting the impact of space weather on Earth. Under the assumption that the structure of this wind is constant in time and co-rotates with the Sun, solar-wind and thereby space-weather forecasts have been made quite effectively. Such co-rotation forecasts are well studied with decades of observations from STEREO and near-Earth spacecraft. Forecast accuracy is primarily determined by three factors: i) the longitudinal separation of spacecraft from Earth determines the corotation time (and hence forecast lead time) [ δ t] over which the solar wind must be assumed to be constant, ii) the latitudinal separation (or offset) between Earth and spacecraft [ δ θ ]] determines the degree to which the same solar wind is being encountered at both locations, and iii) the solar cycle, via the sunspot number (SSN), acts as a proxy for both how fast the solar-wind structure is evolving and how much it varies in latitude. However, the precise dependencies factoring in uncertainties are a mixture of influences from each of these factors. Furthermore, for high-precision forecasts, it is important to understand what drives the forecast accuracy and its uncertainty. Here we present a causal inference approach based on information-theoretic measures to do this. Our framework can compute not only the direct (linear and nonlinear) dependencies of the forecast mean absolute error (MAE) on SSN, Δ θ , and Δ t , but also how these individual variables combine to enhance or diminish the MAE. We provide an initial assessment of this with the potential of aiding data assimilation in the future.

We investigate the properties of small-amplitude inertial waves propagating in a differentially rotating incompressible fluid contained in a spherical shell. For cylindrical and shellular rotation profiles and in the inviscid limit, inertial waves obey a second-order partial differential equation of mixed type. Two kinds of inertial modes therefore exist, depending on whether the hyperbolic domain where characteristics propagate covers the whole shell or not. The occurrence of these two kinds of inertial modes is examined, and we show that the range of frequencies at which inertial waves may propagate is broader than with solid-body rotation. Using high-resolution calculations based on a spectral method, we show that, as with solid-body rotation, singular modes with thin shear layers following short-period attractors still exist with differential rotation. They exist even in the case of a full sphere. In the limit of vanishing viscosities, the width of the shear layers seems to weakly depend on the global background shear, showing a scaling in ${E}^{1/ 3} $ with the Ekman number $E$ , as in the solid-body rotation case. There also exist modes with thin detached layers of width scaling with ${E}^{1/ 2} $ as Ekman boundary layers. The behaviour of inertial waves with a corotation resonance within the shell is also considered. For cylindrical rotation, waves get dramatically absorbed at corotation. In contrast, for shellular rotation, waves may cross a critical layer without visible absorption, and such modes can be unstable for small enough Ekman numbers.