Weak, Quiet Magnetic Fields Seen in the Venus Atmosphere.

The existence of a strong internal magnetic field allows probing of the interior through both long term changes of and short period fluctuations in that magnetic field. Venus, while Earth's twin in many ways, lacks such a strong intrinsic magnetic field, but perhaps short period fluctuations can still be used to probe the electrical conductivity of the interior. Toward the end of the Venus Express mission, an aerobraking campaign took the spacecraft below the ionosphere into the very weakly electrically conducting atmosphere. As the spacecraft descended from 150 to 140 km altitude, the magnetic field became weaker on average and less noisy. Below 140 km, the median field strength became steady but the short period fluctuations continued to weaken. The weakness of the fluctuations indicates they might not be useful for electromagnetic sounding of the atmosphere from a high altitude platform such as a plane or balloon, but possibly could be attempted on a lander

Variability in Saturn's bow shock and magnetopause from pioneer and voyager: Probabilistic predictions and initial observations by Cassini

Probability distributions for the location of the Saturnian bow shock and magnetopause have been derived by extrapolating observations of dynamic solar wind pressures to the position of Saturn's orbit. These observations are those made by the Pioneer 11, Voyager 1 and 2 spacecraft near Saturn's orbit and by the Ulysses spacecraft near its aphelion. The magnetopause subsolar distance (measured from Saturn's center) is obtained using pressure equilibrium. The bow shock standoff distance is determined using empirical relations between bow shock size and solar wind dynamic pressure. Simple 2-D geometric models of the magnetopause and bow shock surfaces have been used to determine their morphologies over a large range in local time. Three cases have been studied: (1) An Earth-type magnetosphere with low internal plasma pressure; (2) An intermediate case calibrated with Voyager 1 observations; and (3) A Jupiter-like inflated magnetosphere. The comparison of these models with initial observations from the initial sunward orbits of the Cassini spacecraft indicates a more inflated magnetosphere than postulated by the previous modelling of the Pioneer-Voyager encounters

Transport of magnetic flux and mass in Saturn's inner magnetosphere

It is well accepted that cold plasma sourced by Enceladus is ultimately lost to the solar wind, while the magnetic flux convecting outward with the plasma must return to the inner magnetosphere. However, whether the interchange or reconnection, or a combination of the two processes is the dominant mechanism in returning the magnetic flux is still under debate. Initial Cassini observations have shown that the magnetic flux returns in the form of flux tubes in the inner magnetosphere. Here we investigate those events with 10 year Cassini magnetometer data and confirm that their magnetic signatures are determined by the background plasma environments: inside (outside) the plasma disk, the returning magnetic field is enhanced (depressed) in strength. The distribution, temporal variation, shape, and transportation rate of the flux tubes are also characterized. The flux tubes break into smaller ones as they convect in. The shape of their cross section is closer to circular than fingerlike as produced in the simulations based on the interchange mechanism. In addition, no sudden changes in any flux tube properties can be found at the “boundary” which has been claimed to separate the reconnection and interchange-dominant regions. On the other hand, reasonable cold plasma loss rate and outflow velocity can be obtained if the transport rate of the magnetic flux matches the reconnection rate, which supports reconnection alone as the dominant mechanism in unloading the cold plasma from the inner magnetosphere and returning the magnetic flux from the tail

mmSIM: an open toolbox for accessible structured illumination microscopy

Since the first practical super-resolution structured illumination fluorescence microscopes (SIM) were demonstrated more than two decades ago, the method has become increasingly popular for a wide range of bioimaging applications. The high cost and relative inflexibility of commercial systems, coupled with the conceptual simplicity of the approach and the desire to exploit and customize existing hardware, have led to the development of a large number of home-built systems. Several detailed hardware designs are available in the scientific literature, complemented by open-source software tools for SIM image validation and reconstruction. However, there remains a lack of simple open-source software to control these systems and manage the synchronization between hardware components, which is critical for effective SIM imaging. This article describes a new suite of software tools based on the popular Micro-Manager package, which enable the keen microscopist to develop and run a SIM system. We use the software to control two custom-built, high-speed, spatial light modulator-based SIM systems, evaluating their performance by imaging a range of fluorescent samples. By simplifying the process of SIM hardware development, we aim to support wider adoption of the technique. This article is part of the Theo Murphy meeting issue 'Super-resolution structured illumination microscopy (part 1)'

Flux transfer events: Scale size and interior structure

We report the first direct investigation of the spatial properties of flux transfer events (FTEs) at the Earth's dayside magnetopause. Simultaneous magnetometer and plasma data from the ISEE 1 and 2 satellites are combined to show that magnetosheath FTEs can have a scale size of order an Earth radius in the magnetopause normal direction. We confirm that the magnetic field within the events appears to be twisted, this twisting corresponding to a core field‐aligned current of magnitude a few × 105 A. We also show evidence for plasma vorticity in FTEs. The transverse flow and field perturbations accompanying the three events studied obey approximately the Walén relation for a propagating Alfvén wave

Electrostatic Turbulence and Debye-scale Structures in Collisionless Shocks

We present analysis of more than 100 large-amplitude bipolar electrostatic structures in a quasi-perpendicular supercritical Earth's bow shock crossing, measured by the Magnetospheric Multiscale spacecraft. The occurrence of the bipolar structures is shown to be tightly correlated with magnetic field gradients in the shock transition region. The bipolar structures have negative electrostatic potentials and spatial scales of a few Debye lengths. The bipolar structures propagate highly oblique to the shock normal with velocities (in the plasma rest frame) of the order of the ion-acoustic velocity. We argue that the bipolar structures are ion phase space holes produced by the two-stream instability between incoming and reflected ions. This is the first identification of the ion two-stream instability in collisionless shocks

How Accurately Can We Measure the Reconnection Rate E M for the MMS Diffusion Region Event of 11 July 2017?

We investigate the accuracy with which the reconnection electric field E M can be determined from in situ plasma data. We study the magnetotail electron diffusion region observed by National Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) on 11 July 2017 at 22:34 UT and focus on the very large errors in E M that result from errors in an L M N boundary normal coordinate system. We determine several L M N coordinates for this MMS event using several different methods. We use these M axes to estimate E M. We find some consensus that the reconnection rate was roughly E M = 3.2 ± 0.6 mV/m, which corresponds to a normalized reconnection rate of 0.18 ± 0.035. Minimum variance analysis of the electron velocity (MVA-v e), MVA of E, minimization of Faraday residue, and an adjusted version of the maximum directional derivative of the magnetic field (MDD-B) technique all produce reasonably similar coordinate axes. We use virtual MMS data from a particle-in-cell simulation of this event to estimate the errors in the coordinate axes and reconnection rate associated with MVA-v e and MDD-B. The L and M directions are most reliably determined by MVA-v e when the spacecraft observes a clear electron jet reversal. When the magnetic field data have errors as small as 0.5% of the background field strength, the M direction obtained by MDD-B technique may be off by as much as 35°. The normal direction is most accurately obtained by MDD-B. Overall, we find that these techniques were able to identify E M from the virtual data within error bars ≥20%

Landslides on Ceres: Diversity and Geologic Context.

Landslides are among the most widespread geologic features on Ceres. Using data from Dawn's Framing Camera, landslides were previously classified based upon geomorphologic characteristics into one of three archetypal categories, Type 1(T1), Type 2 (T2), and Type 3 (T3). Due to their geologic context, variation in age, and physical characteristics, most landslides on Ceres are, however, intermediate in their morphology and physical properties between the archetypes of each landslide class. Here we describe the varied morphology of individual intermediate landslides, identify geologic controls that contribute to this variation, and provide first-order quantification of the physical properties of the continuum of Ceres's surface flows. These intermediate flows appear in varied settings and show a range of characteristics, including those found at contacts between craters, those having multiple trunks or lobes; showing characteristics of both T2 and T3 landslides; material slumping on crater rims; very small, ejecta-like flows; and those appearing inside of catenae. We suggest that while their morphologies can vary, the distribution and mechanical properties of intermediate landslides do not differ significantly from that of archetypal landslides, confirming a link between landslides and subsurface ice. We also find that most intermediate landslides are similar to Type 2 landslides and formed by shallow failure. Clusters of these features suggest ice enhancement near Juling, Kupalo and Urvara craters. Since the majority of Ceres's landslides fall in the intermediate landslide category, placing their attributes in context contributes to a better understanding of Ceres's shallow subsurface and the nature of ground ice