A physical model for the magnetosphere of Uranus at solstice time

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.

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

<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>

(Non)-radial propagation of the solar wind flow

<p>The solar wind aberration due to non-radial velocity components and the Earth orbital motion is important for the overall magnetosphere geometry because the magnetospheric tail is aligned with the solar wind flow. This paper investigates an evolution of non-radial components of the solar wind flow along the path from the Sun to 6 AU. A comparison of observations at 1 AU and closer to or further from the Sun based on measurements of many spacecraft at different locations in the heliosphere (Wind, ACE, Spektr-R, THEMIS B and C, Helios 1 and 2, Mars-Express, Voyager 1 and 2) shows that (i) the average values of non-radial components vary with the distance from the Sun and (ii) they differ according to solar wind streams.</p>

MHD simulations of an Uranus type rotating magnetosphere

<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>

Using PC indices to predict violent GIC events threatening power grids

The aim of the present contribution is to investigate the timing relations between enhancements in the Polar Cap (PC) indices and power grid disturbances related to geomagnetically induced currents (GIC). The polar cap indices, PCN (North) and PCS (South), are based on measurements of geomagnetic variations in the central polar caps. These variations are strongly related to the transpolar convection of plasma and magnetic fields driven by the solar wind. During cases of enhanced merging processes at the front of the magnetosphere and subsequent tailward convection of plasma and embedded magnetic fields, the magnetospheric tail configuration may accumulate excess energy, which upon release may cause violent substorm activity. Earlier reports have disclosed remarkably lengthy intervals, ranging up to several hours, of elevated PC index values preceding GIC-related power grid disruptions. The present investigation has shown that the delays of typically 3–4 h between increases in the PC indices and GIC-related power grid disturbances are related to displacements of the substorm processes responsible for strong GIC events to subauroral latitudes where vulnerable power grids reside. The results have shown that PC index values remaining above an “alert level” of 10 mV/m through more than 1 h indicate a high risk for violent GIC events that may threaten power grids and other vulnerable technical systems. These results support the application of real-time PC indices in space weather monitoring and forecast services.