The Scale Height of Charged Particles in Jupiter’s Magnetosphere

<p>We have recently developed a new technique that uses the timings of any three consecutive current sheet crossings to determine the instantaneous motion of Jupiter’s current sheet relative to the spacecraft. Using this information on the instantaneous location of Jupiter’s current sheet, we have modeled the external field of the magnetic disc observed by Juno and Galileo spacecraft in terms of a Harris current sheet type equilibrium and obtained a map of the thickness of the Jovian current sheet over all local times and radial distances. Our modeling of Juno and Galileo magnetic field data shows that in all local times the current sheet thickness increases with radial distance. We also find that the Jovian current sheet thickness is highly asymmetric in local time, being at its thinnest in the dawn sector and the thickest in the dusk sector. The current sheet thickness on the dayside is comparable to that in the dusk sector. The nightside current sheet is intermediate in its thickness to the dawn and the dusk sectors.</p><p>In this presentation, we use the instantaneous location of the current sheet to model the electron densities measured by the plasma or plasma wave instrument. We show that overall, the scale height of electrons and the current sheet tend to be identical. However, we have encountered many cases where the electrons have a two scale-height structure where a thin plasma sheet is embedded within a thicker current sheet, the cause for which is not known. By using the magnetic field and electron density data, we have computed the plasma content of flux tubes in several local time locations in the magnetosphere. We relate the plasma content of these flux tubes to plasma rotation, plasma density and current sheet thickness. It appears that as flux tubes rotate to the dusk side, they slow down and the plasma scale height increases but the total plasma content remains constant.</p>

Wind–MRI interactions in local models of protoplanetary discs – I. Ohmic resistivity

ABSTRACT A magnetic disc wind is an important mechanism that may be responsible for driving accretion and structure formation in protoplanetary discs. Recent numerical simulations have shown that these winds can take either the traditional ‘hourglass’ symmetry about the mid-plane, or a ‘slanted’ symmetry dominated by a mid-plane toroidal field of a single sign. The formation of this slanted symmetry state has not previously been explained. We use radially local 1D vertical shearing box simulations to assess the importance of large-scale MRI channel modes in influencing the formation and morphologies of these wind solutions. We consider only Ohmic resistivity and explore the effect of different magnetizations, with the mid-plane β parameter ranging from 105 to 102. We find that our magnetic winds go through three stages of development: cyclic, transitive, and steady, with the steady wind taking a slanted symmetry profile similar to those observed in local and global simulations. We show that the cycles are driven by periodic excitation of the n = 2 or 3 MRI channel mode coupled with advective eviction, and that the transition to the steady wind is caused by a much more slowly growing n = 1 mode altering the wind structure. Saturation is achieved through a combination of advective damping from the strong wind, and suppression of the instability due to a strong toroidal field. A higher disc magnetization leads to a greater tendency towards, and more rapid settling into the slanted symmetry steady wind, which may have important implications for mass and flux transport processes in protoplanetary discs.

γ Cas stars: Normal Be stars with discs impacted by the wind of a helium-star companion?

γ Cas stars are a ∼1% minority among classical Be stars with hard (≥5−10 keV), but only moderately strong continuous thermal X-ray flux, and mostly very early-B spectral type. The X-ray flux has been suggested to originate from matter accelerated via magnetic disc-star interaction, by a rapidly rotating neutron star (NS) companion via the propeller effect, or by accretion onto a white dwarf (WD) companion. In view of the growing number of identified γ Cas stars and the only imperfect matches between these suggestions and the observations, alternative models should be pursued. Two of the three best-observed γ Cas stars, γ Cas itself and π Aqr, have a low-mass companion with low optical flux, whereas interferometry of BZ Cru is inconclusive. Binary-evolution models are examined for their ability to produce such systems. The OB+He-star stage of post-mass transfer binaries, which is otherwise observationally unaccounted, can potentially reproduce many observed properties of γ Cas stars. The interaction of the fast wind of helium stars with the circumstellar disc and/or with the wind of Be stars may give rise to the production of hard X-rays. While not modelling this process, it is shown that the energy budget is favourable, and that the wind velocities may lead to hard X-rays, as observed in γ Cas stars. Furthermore, the observed number of these objects appears to be consistent with the evolutionary models. Within the Be+He-star binary model, the Be stars in γ-Cas stars are conventional classical Be stars. They are encompassed by O-star+Wolf-Rayet systems towards higher mass, where no stable Be decretion discs exist, and by Be+sdO systems at lower mass, where the sdO winds may be too weak to cause the γ Cas phenomenon. In decreasing order of the helium-star mass, the descendants could be Be+black-hole, Be+NS, or Be+WD binaries. The interaction between the helium-star wind and the disc may provide new diagnostics of the outer disc.

Breaking the centrifugal barrier to giant planet contraction by magnetic disc braking

ABSTRACT During the runaway phase of their formation, gas giants fill their gravitational spheres of influence out to Bondi or Hill radii. When runaway ends, planets shrink several orders of magnitude in radius until they are comparable in size to present-day Jupiter; in 1D models, the contraction occurs on the Kelvin–Helmholtz time-scale tKH, which is initially a few thousand years. However, if angular momentum is conserved, contraction cannot complete, as planets are inevitably spun up to their breakup periods Pbreak. We consider how a circumplanetary disc (CPD) can de-spin a primordially magnetized gas giant and remove the centrifugal barrier, provided the disc is hot enough to couple to the magnetic field, a condition that is easier to satisfy at later times. By inferring the planet’s magnetic field from its convective cooling luminosity, we show that magnetic spin-down times are shorter than contraction times throughout post-runaway contraction: tmag/tKH ∼ (Pbreak/tKH)1/21 ≲ 1. Planets can spin-down until they corotate with the CPD’s magnetospheric truncation radius, at a period Pmax/Pbreak ∼ (tKH/Pbreak)1/7. By the time the disc disperses, Pmax/Pbreak ∼ 20–30; further contraction at fixed angular momentum can spin planets back up to ∼10Pbreak, potentially explaining observed rotation periods of giant planets and brown dwarfs.

Device-technological simulation of the magnetosensitive sensor with integrated magnetic concentrator

The paper presents results on research and optimization on the basis of device-technological modeling of the structural and operational characteristics of the magnetosensitive sensor with a disk-shaped integrated magnetic concentrator (IMC). The high magnetic permeability of the IMC material provides a high value of the induction of the magnetic field along its edges, which leads to a significant enhancement of the applied external field. The IMC plays the role of a magnetic amplifier, and also affects the signal-to-noise and signal-to-bias ratios; the magnetic gain depends not only on the size of the IMC, but also on its shape. This research is devoted to the development of a disc-shaped magnetic concentrator integrated into the Hall sensor. The concentrator has a high magnetic flux gain and can be used in 3D magnetic field recording systems. Analysis of the geometric dimensions, deflection angle and the material of the integrated magnetic concentrator influence on the characteristics of a three-dimensional magnetic field sensor showed that the inclusion of a ferromagnetic concentrator in the Hall sensor design provides a significant (up to 10 times) increase in the magnetic flux gain. This makes it possible to use the investigated sensor designs to detect weak magnetic fields (from 0.01 µT to 2 mT). It is shown that a supermindure integrated magnetic disc-shaped concentrator with a diameter of D = 200 µm, a thickness of l = 10 µm and an angle of deflection of  = 60° provides a magnetic flux gain G = 10.81 with a maximum external magnetic field of B0 = 120 mT. The obtained results indicate the prospects of using the proposed constructive solution for the practical manufacture of three-dimensional sensors of weak magnetic fields with a magnetic sensitivity up to 3026 V/(A∙T) along the sensor surface. The type of sensor devices studied extends the scope of Hall sensors as an elemental base of medical equipment, equipment for magneto-resonant imaging (MRI), and also in instruments for geological and geodetic research.