How a magnetic helix can develop at solstice in a Uranus-type rotating magnetosphere

2020 ◽  
Author(s):  
Léa Griton ◽  
Filippo Pantellini
Keyword(s):  
Rotation Axis ◽  
Double Helix ◽  
Large Angle ◽  
Rotation Period ◽  
Ecliptic Plane ◽  
Characteristic Relaxation Time ◽  
Dipole Axis ◽  
Magnetospheric Tail ◽  
Planetary Magnetic Field ◽  
Fast Rotating

<p class="p1">The characteristic relaxation time of the Uranus magnetosphere is of the order  of the planet's rotation period. This is also the case for Jupiter or Saturn. However, the specificity of Uranus (and to a lesser extent of  Neptune) is that the rotation axis and the magnetic dipole axis are separated by  a large angle (~60°) the consequence of which is the development of a highly dynamic and complex magnetospheric tail. In addition, and contrary to all other planets of the solar system, the rotation axis of Uranus happens to be quasi-parallel to the ecliptic plane which also implies a strong variability of the magnetospheric structure as a function of the season. The magnetosphere of Uranus is thus a particularly challenging case for global plasma simulations, even in the frame of MHD. We present a detailed analysis of MHD simulations of a fast-rotating magnetosphere inspired from Uranus at solstice. At first, a simplified case allows us to explain in detail the formation and the internal structure of a double helix that develops in the magnetotail at solstice. Then we analyse a "real" Uranus simulation with parameters for the solar wind and planetary magnetic field defined from the measurements of Voyager II flyby in 1986.</p>

MHD simulations of an Uranus type rotating magnetosphere

2020 ◽  
Author(s):  
Filippo Pantellini ◽  
Léa Griton
Keyword(s):  
Solar Wind ◽  
Rotation Axis ◽  
Large Angle ◽  
Rotation Period ◽  
Ecliptic Plane ◽  
Characteristic Relaxation Time ◽  
Dipole Axis ◽  
Mhd Simulations ◽  
Planetary Rotation ◽  
Magnetospheric Tail

<p>The characteristic relaxation time of the Uranus magnetosphere is of the order  of the planet's rotation period. This is also the case for Jupiter or Saturn. However, the specificity of Uranus (and to a lesser extent of  Neptune) is that the rotation axis and the magnetic dipole axis are separated by  a large angle (~60°) the consequence of which is the development of a highly dynamic and complex magnetospheric tail. In addition, and contrary to all other planets of the solar system, the rotation axis of Uranus happens to be quasi-parallel to the ecliptic plane which also implies a strong variability of the magnetospheric structure as a function of the season. The magnetosphere of Uranus is thus a particularly challenging case for global plasma simulations, even in the frame of MHD. We present MHD simulations of a Uranus type magnetosphere at both equinox (solar wind is orthogonal to the planetary rotation axis) and solstice (solar wind is orthogonal to the planetary rotation axis) configurations. The main differences between the two configurations will be discussed. </p>

scholarly journals A physical model for the magnetosphere of Uranus at solstice time

2020 ◽  
Vol 643 ◽  
pp. A144
Author(s):  
Filippo Pantellini
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Magnetic Dipole ◽  
Rotation Axis ◽  
Double Helix ◽  
Mass Density ◽  
Fluid Velocity ◽  
Rotation Period ◽  
Magnetospheric Tail ◽  
Field Lines

Context. Uranus is the only planet in the Solar System whose rotation axis and orbital plane are nearly parallel to each other. Uranus is also the planet with the largest angle between the rotation axis and the direction of its magnetic dipole (roughly 59°). Consequently, the shape and structure of its magnetospheric tail is very different to those of all other planets in whichever season one may consider. The only in situ measurements were obtained in January 1986 during a flyby of the Voyager II spacecraft. At that date, Uranus was near solstice time, but unfortunately the data collected by the spacecraft were much too sparse to allow for a clear view of the structure and dynamics of its extended magnetospheric tail. Later numerical simulations revealed that the magnetic tail of Uranus at solstice time is helically shaped with a characteristic pitch of the order of 1000 planetary radii. Aims. We aim to propose a magnetohydrodynamic model for the magnetic tail of Uranus at solstice time. Methods. We constructed our model based on a symmetrised version of the Uranian system by assuming an exact alignment of the solar wind and the planetary rotation axis and an angle of 90° between the planetary magnetic dipole and the rotation axis. We do also postulate that the impinging solar wind is steady and unmagnetised, which implies that the magnetosphere is quasi-steady in the rotating planetary frame and that there is no magnetic reconnection at the magnetopause. Results. One of the main conclusions is that all magnetic field lines forming the extended magnetic tail follow the same qualitative evolution from the time of their emergence through the planet’s surface and the time of their late evolution after having been stretched and twisted several times downstream of the planet. In the planetary frame, these field lines move on magnetic surfaces that wind up to form a tornado-shaped vortex with two foot points on the planet (one in each magnetic hemisphere). The centre of the vortex (the eye of the tornado) is a simple double helix with a helical pitch (along the symmetry axis z) λ = τ[vz+Bz/(μ0ρ)1/2], where τ is the rotation period of the planet, μ0 the permeability of vacuum, ρ the mass density, vz the fluid velocity, and Bz the magnetic field where all quantities have to be evaluated locally at the centre of the vortex. In summary, in the planetary frame, the motion of a typical magnetic field of the extended Uranian magnetic tail is a vortical motion, which asymptotically converges towards the single double helix, regardless of the line’s emergence point on the planetary surface.

scholarly journals Detecting activity cycles of late-type dwarfs in Kepler data

2013 ◽  
Vol 9 (S302) ◽  
pp. 224-227
Author(s):  
Krisztián Vida ◽  
Katalin Oláh
Keyword(s):  
Active Region ◽  
Frequency Analysis ◽  
Rotation Period ◽  
Light Curves ◽  
Late Type ◽  
Dwarf Stars ◽  
Time Frequency ◽  
Activity Cycles ◽  
Using Data ◽  
Fast Rotating

AbstractUsing data of fast-rotating active dwarf stars in the Kepler database, we perform time-frequency analysis of the light curves in order to search for signs of activity cycles. We use the phenomenon that the active region latitudes vary with the cycle (like the solar butterfly diagram), which causes the observed rotation period to change as a consequence of differential rotation. We find cycles in 8 cases of the 39 promising targets with periods between of 300–900 days.

scholarly journals Minimum Period of Rotation of Millisecond Pulsars and Pulsar Matter Equations of State

2018 ◽  
Vol 173 ◽  
pp. 02015
Author(s):  
Sergey Mikheev ◽  
Victor Tsvetkov
Keyword(s):  
Equations Of State ◽  
Rotation Period ◽  
Central Density ◽  
Polytropic Index ◽  
Minimum Period ◽  
Peak Period ◽  
Millisecond Pulsars ◽  
Fast Rotating

Based on the findings of our previous studies of fast-rotating Newtonian polytropes, we found the relation between the minimum pulsar rotation period, the value of pulsar central density, and the polytropic index. From this relation we come to the conclusion that the value of minimum central density of a pulsar with a peak period is 2.5088 • 1014 g/cm3.

scholarly journals The Thousand-Pulsar-Array programme on MeerKAT – VI. Pulse widths of a large and diverse sample of radio pulsars

2021 ◽  
Author(s):  
B Posselt ◽  
A Karastergiou ◽  
S Johnston ◽  
A Parthasarathy ◽  
M J Keith ◽  
...  
Keyword(s):  
Power Law ◽  
Rotation Axis ◽  
Rotation Period ◽  
Population Based ◽  
Population Synthesis ◽  
Radio Pulsars ◽  
Orthogonal Distance Regression ◽  
Cent Level ◽  
Width Measurements ◽  
Power Law Index

Abstract We present pulse width measurements for a sample of radio pulsars observed with the MeerKAT telescope as part of the Thousand-Pulsar-Array (TPA) programme in the MeerTime project. For a centre frequency of 1284 MHz, we obtain 762 W10 measurements across the total bandwidth of 775 MHz, where W10 is the width at the 10 per cent level of the pulse peak. We also measure about 400 W10 values in each of the four or eight frequency sub-bands. Assuming, the width is a function of the rotation period P, this relationship can be described with a power law with power law index μ = −0.29 ± 0.03. However, using orthogonal distance regression, we determine a steeper power law with μ = −0.63 ± 0.06. A density plot of the period-width data reveals such a fit to align well with the contours of highest density. Building on a previous population synthesis model, we obtain population-based estimates of the obliquity of the magnetic axis with respect to the rotation axis for our pulsars. Investigating the width changes over frequency, we unambiguously identify a group of pulsars that have width broadening at higher frequencies. The measured width changes show a monotonic behaviour with frequency for the whole TPA pulsar population, whether the pulses are becoming narrower or broader with increasing frequency. We exclude a sensitivity bias, scattering and noticeable differences in the pulse component numbers as explanations for these width changes, and attempt an explanation using a qualitative model of five contributing Gaussian pulse components with flux density spectra that depend on their rotational phase.

On the genesis of quasi-steady vortices in a rotating turbulent flow

1987 ◽  
Vol 185 ◽  
pp. 121-136 ◽  
Author(s):  
Mathieu Mory ◽  
Philippe Caperan
Keyword(s):  
Kinetic Energy ◽  
Turbulent Flow ◽  
Turbulent Kinetic Energy ◽  
Thermal Convection ◽  
Rotation Axis ◽  
Rotation Period ◽  
Marginal Stability ◽  
Rotating Fluid ◽  
Mean Flow ◽  
Perturbation Equations

Turbulent flows subjected to rotation display vortices parallel to the rotation axis and exhibiting a long timescale compared to the turbulent turnover time and the rotation period. A similar flow pattern is observed arising from the thermal instability in a rotating fluid. We demonstrate the analogy between turbulence and thermal convection in a rotating fluid. A basic quasi-geostrophic turbulent flow is considered which is forced at the bottom of the layer by a stochastic component of velocity parallel to the rotation axis. The turbulent basic state has no mean flow and the gradient along the rotation axis of the turbulent kinetic energy −∂z〈ω2〉 is analogous to the mean temperature profile in thermal convection. The linear perturbation equations of this basic turbulent state are given, where the thermal diffusion equation is replaced by the turbulent kinetic energy equation. Using a simple closure of this equation the model demonstrates the occurrence of an instability when the Reynolds number exceeds a critical value. Marginal stability curves are deduced by numerical integration of the perturbation equations. The results show order-of-magnitude agreement with laboratory experiments.

scholarly journals Pulsar Polarization Limiting Radii and the Evolution of Pulsar Beams

1987 ◽  
Vol 125 ◽  
pp. 56-56
Author(s):  
John J. Barnard
Keyword(s):  
Magnetic Field ◽  
Field Strength ◽  
Radio Frequency ◽  
Magnetic Field Strength ◽  
Rotation Axis ◽  
Rotation Period ◽  
Position Angle ◽  
Polarization Angle ◽  
Beam Model ◽  
Plasma Parameters

We estimate the polarization limiting radius, rp1, as a function of rotation period P, magnetic field strength B, and radio frequency v, in radio pulsars assuming plasma parameters that are typical of polar-cap pair-creation models of pulsars. We find that rpl ⋍ 9 × 108(P/1s)0.4 cm, for a surface magnetic field strength of 1012 G, and radio frequency of 109 Hz. For short rotation periods, rpl approaches the light cylinder radius, rlc. Here the magnetic field becomes more azimuthal, and the excursion in position angle over a pulse is less, on average, than when rpl ≪ rlc. With the assumption of a vacuum magnetic field we calculate the polarization position angle as a function of pulse longitude, and the angles i (the angle between the magnetic moment and the rotation axis) and α (the angle between the line of sight and the rotation axis). We calculate the average change in polarization angle as a function of pulsar period, assuming a circular beam, and find consistency with the polarization data summarized by Narayan and Vivekand (1983). We conclude that the evidence is consistent with beams that are roughly constant in shape, providing an alternative to the evolving elliptical beam model of Narayan and Vivekand (1983). This interpretation is further supported by the frequency dependence of the polarization angle in the Crab Pulsar, the frequency of pulsars with double and multiple pulse components, the frequency of pulsars with interpulses, and the absence of pulsars in plerions. See Barnard (1986) for further details.

scholarly journals WASP-166b: a bloated super-Neptune transiting a V  = 9 star

2019 ◽  
Vol 488 (3) ◽  
pp. 3067-3075 ◽  
Author(s):  
Coel Hellier ◽  
D R Anderson ◽  
A H M J Triaud ◽  
F Bouchy ◽  
A Burdanov ◽  
...  
Keyword(s):  
Radial Velocity ◽  
Rotation Axis ◽  
Rotation Period ◽  
Magnetic Activity ◽  
Stellar Rotation ◽  
Velocity Measurements

Abstract We report the discovery of WASP-166b, a super-Neptune planet with a mass of 0.1 MJup (1.9 MNep) and a bloated radius of 0.63 RJup. It transits a V = 9.36, F9V star in a 5.44-d orbit that is aligned with the stellar rotation axis (sky-projected obliquity angle λ = 3 ± 5 deg). Variations in the radial-velocity measurements are likely the result of magnetic activity over a 12-d stellar rotation period. WASP-166b appears to be a rare object within the ‘Neptune desert’.

A Rapid Maneuver Method with High Accuracy for Spacecraft Based on CMG and RW

2012 ◽  
Vol 591-593 ◽  
pp. 2395-2400
Author(s):  
Yun Gang Yang ◽  
Feng Wang ◽  
Zhao Wei Sun
Keyword(s):  
Mathematical Simulation ◽  
Trajectory Tracking ◽  
Rotation Axis ◽  
Large Angle ◽  
High Accuracy ◽  
Tracking Algorithm ◽  
Constant Speed ◽  
New Method ◽  
Attitude Stability ◽  
Minimum Path

The mission of ground tracking satellite requires not only the ability of large angle rapid maneuver, but also the ability of attitude stability with high accuracy during and after maneuver. A rapid maneuver method with high accuracy for spacecraft based on CMG and RW is proposed to solve this problem. By use of initial quaternions and target quaternions, a minimum path trajectory tracking algorithm is designed along the instantaneous Euler rotation axis. This algorithm requires that during the accelerating stage and the decelerating stage, the torque command is assigned to CMG system; however, during the constant-speed stage and after maneuver, the torque is assigned to RW system. Mathematical simulation indicates that, compared with the situation of just using CMG, the new method endows the spacecraft with the ability of rapid maneuver with high accuracy.

Librational forcing of a rapidly rotating fluid-filled cube

2018 ◽  
Vol 842 ◽  
pp. 469-494 ◽  
Author(s):  
Ke Wu ◽  
Bruno D. Welfert ◽  
Juan M. Lopez
Keyword(s):  
Stokes Equations ◽  
Rotation Axis ◽  
Rotation Period ◽  
Rotation Frequency ◽  
Shear Layers ◽  
Navier Stokes ◽  
Rotating Fluid ◽  
Ekman Number ◽  
Slip Boundary ◽  
The Mean

The flow response of a rapidly rotating fluid-filled cube to low-amplitude librational forcing is investigated numerically. Librational forcing is the harmonic modulation of the mean rotation rate. The rotating cube supports inertial waves which may be excited by libration frequencies less than twice the rotation frequency. The response is comprised of two main components: resonant excitation of the inviscid inertial eigenmodes of the cube, and internal shear layers whose orientation is governed by the inviscid dispersion relation. The internal shear layers are driven by the fluxes in the forced boundary layers on walls orthogonal to the rotation axis and originate at the edges where these walls meet the walls parallel to the rotation axis, and are hence called edge beams. The relative contributions to the response from these components is obscured if the mean rotation period is not small enough compared to the viscous dissipation time, i.e. if the Ekman number is too large. We conduct simulations of the Navier–Stokes equations with no-slip boundary conditions using parameter values corresponding to a recent set of laboratory experiments, and reproduce the experimental observations and measurements. Then, we reduce the Ekman number by one and a half orders of magnitude, allowing for a better identification and quantification of the contributions to the response from the eigenmodes and the edge beams.

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