Corotation Of Plasma Research Articles

Dynamic Jovian Magnetosphere Responses to Enhanced Solar Wind Ram Pressure: Implications for Auroral Activities

AbstractThe main emission (ME) of the Jovian aurora is thought to be related to the current system associated with the breakdown of plasma corotation in the middle magnetosphere. According to the mainstream corotation breakdown model, the intensity of the Jovian ME is expected to decrease when the solar wind (SW) ram pressure increases, which is not fully consistent with auroral observations. In addition to the field‐aligned current (FAC), Alfvénic power (AP) play an important role in regulating planetary auroral emissions. We use three‐dimensional global simulations to investigate how these proxies of auroral emission respond to enhanced SW ram pressure. We found that during SW compression, both FAC and AP experience up‐down‐up trends, which is not revealed by any previous simulations, while could potentially explain many observations. The results suggest that different Jovian auroral activities including brightening or dimming can be observed during SW compression period.

Detection of coherent low-frequency radio bursts from weak-line T Tauri stars

In recent years, thanks to new facilities such as LOFAR that are capable of sensitive observations, much work has been done on the detection of stellar radio emission at low frequencies. Such emission has commonly been shown to be coherent emission, generally attributed to electron-cyclotron maser (ECM) emission, and has usually been detected from main-sequence M dwarfs. Here we report the first detection of coherent emission at low frequencies from T Tauri stars, which are known to be associated with high levels of stellar activity. Using LOFAR, we detect several bright radio bursts at 150 MHz from two weak-line T Tauri stars: KPNO-Tau 14 and LkCa 4. All of the bursts have high brightness temperatures (1013 − 1014 K) and high circular polarisation fractions (60–90%), indicating that they must be due to a coherent emission mechanism. This could be either plasma emission or ECM emission. Due to the exceptionally high brightness temperatures seen in at least one of the bursts (≥1014 K), as well as the high circular polarisation levels, it seems unlikely that plasma emission could be the source; as such, ECM is favoured as the most likely emission mechanism. Assuming this is the case, the required magnetic field in the emission regions would be 40–70 G. We determine that the most likely method of generating ECM emission is plasma co-rotation breakdown in the stellar magnetosphere. There remains the possibility, however, that it could be due to an interaction with an orbiting exoplanet.

Dynamics of nanodust in the vicinity of a stellar corona: Effect of plasma corotation

Context. In the vicinity of the Sun or other stars, the motion of the coronal and stellar wind plasma must include some amount of corotation, which could affect the dynamics of charged dust particles. In the case of the Sun, this region is now investigated in situ by the Parker Solar Probe. Charged dust particles coming from the vicinity of the Sun can also reach, and possibly be detected by, the Solar Orbiter. Aims. We use numerical simulations and theoretical models to study the effect of plasma corotation on the motion of charged nanodust particles released from the parent bodies moving in Keplerian orbits, with particular attention to the case of trapped particles. Methods. We used two methods: the motion of nanodust is described either by numerical solutions of full equations of motion, or by a two-dimensional (distance vs. radial velocity) model based on the guiding centre approximation. The models of the plasma and magnetic field in the vicinity of the star are based on analytical solutions that satisfy the freezing-in equations. Results. Including plasma corotation does not prevent trapping of nanodust in the vicinity of the Sun or other stars. This result can be understood with the help of the model based on the guiding centre approximation. For the amount of corotation expected near the Sun, the outer limit of the trapped region is almost unaffected. If the corotation persists outside the trapping region, the speed of particles ejected from the Sun is moderately increased. A strong effect of plasma corotation on charged particle dynamics occurs for the star with a high rotation rate and/or a low value of the stellar wind speed.

Global Simulation of the Jovian Magnetosphere: Transitional Structure From the Io Plasma Disk to the Plasma Sheet

AbstractJupiter has a strong magnetic field, and a huge magnetosphere is formed through the solar wind‐Jupiter interaction. The generated magnetosphere–ionosphere system is reproduced based on the 9‐component Magnetohydrodynamics (MHD) and the current conservation in the ionosphere. Assuming Io plasma emission rate 1.4 t/sec, this paper reproduces self‐consistently global magnetic configuration, generations of the field‐aligned current (FAC) and aurora, formation of the Io plasma disk at 8–20 RJ, plasma corotation, instability in the plasma disk, transition from the Io plasma disk to the plasma sheet at 20–150 RJ, and the plasmoid ejection. The rotating Io plasma in the disk forms instabilities that promotes radial diffusion. H+ is supplied from the ionosphere along high‐latitude magnetic field lines and mixed with heavy ions around 15–20 RJ. Beyond 20 RJ, mixed plasma diffuses further outward by the centrifugal force that can exceed magnetic tension. In the ionosphere, the main oval occurs at 13.7°–15.5° colatitude. The Io disk is inner side of magnetic field lines traced from the low‐latitude edge of the main oval. Along magnetic field lines, the main oval is mapped from the outer edge of the Io disk to the entire plasma sheet accompanying rotation delay. Due to the corotation limit, convection is accompanied by plasmoid ejection. Back reaction of plasmoid ejection affects even transport process in the Io disk. The downward FAC occurs in the polar cap showing variability. The region of externally driven Dungey convection seems quite narrow.

Dust trajectory simulations around the Sun, Vega, and Fomalhaut

Context. Vega and Fomalhaut display a thermal emission brightness that could possibly arise from hot dust near the stars, an inner extension of their planetary debris disks. An idea has been suggested that nanometer-sized dust particles are kept in the vicinity of the stars by electromagnetic forces. This resembles the trapping that model calculations show in the corotating magnetic field in the inner heliosphere within approximately 0.2 AU from the Sun. Aims. The aim of this work is to study whether the trapping of dust due to electromagnetic forces acting on charged dust near the Sun can occur around Vega and Fomalhaut and what are the conditions for trapping. Methods. We studied the dust trajectories with numerical calculations of the full equation of motion, as well as by using the guiding center approximation. We assumed a constant dust charge and a Parker-type magnetic field, which we estimated for the two stars. Results. We find no bound trajectories of charged particles around Vega or Fomalhaut as long as the radiation pressure force exceeds the gravitational force, that is, for particles smaller than 1 μm. A trapping zone could exist inside of 0.02 AU for Vega and 0.025 AU for Fomalhaut, but only for those particles with radiation pressure force smaller than gravitational force. In comparison to the Sun, the trapping conditions would occur closer to the stars because their faster rotation leads to a more closely wound-up magnetic field spiral. We also show that plasma corotation can be consistent with trapping. Our model calculations show that the charged particles are accelerated to stellar wind velocity very quickly, pass 1 AU after approximately three days, and are further ejected outward where they pass the debris disks at high velocity. We find this for particles with a surface charge-to-mass ratio larger than 10−6 elementary charges per proton mass for both negatively and positively charged dust and independent of the strength of the radiation pressure force. Based on charging assumptions, this would correspond to dust of sizes 100 nm and smaller.

Model of Solar Wind in the Heliosphere at Low and High Latitudes

The spatial distributions of the magnetic field, plasma density, and current at distances of (20–400)RS from the Sun (where RS is the solar radius) are investigated within a stationary axisymmetric MHD model of the solar wind (SW) at all latitudes in the inertial frame of reference with the origin at the center of the Sun. The model takes into account differential (with respect to the heliolatitude) rotation of the Sun and full corotation of plasma inside a boundary sphere of radius 20RS, which breaks down beyond this sphere. Self-consistent distributions of the plasma density, current, and magnetic field in the SW are obtained by numerically solving a set of time-independent MHD equations in spherical coordinates. It is demonstrated that the calculated results do not contradict observational data and describe a gradual transition from the fast SW at high heliolatitudes to the slow SW at low heliolatitudes, as well as the steepening of the profiles of the main SW characteristics with increasing distance from the Sun. The obtained dependences extend understanding of the SW structure at low and high latitudes and agree with the well-known Parker model in the limit of a small Ampere force.

Kelvin Helmholtz Instability in Planetary Magnetospheres

Kelvin–Helmholtz instability plays a particularly important role in plasma transport at magnetospheric boundaries because it can control the development of a turbulent boundary layer, which governs the transport of mass, momentum, and energy across the boundary. Waves generated at the interface can also couple into body modes in the plasma sheet and inner magnetosphere where they can play an important role in plasma sheet transport and particle energization in the inner magnetosphere. Kinetic and electron-scale effects are important for the development of K–H instability, leading to secondary instabilities and plasma mixing. The development of vortices that entwine magnetosheath field lines with magnetospheric field lines also allows reconnection and the interchange of plasma blobs from open to closed field lines. Dawn-dusk asymmetries in Kelvin–Helmholtz development at planetary boundary layers may result from several effects including plasma corotation, kinetic effects, magnetic geometry, or asymmetric distribution of plasma. Examples are provided throughout the solar system illustrating the pervasive effects of the Kelvin–Helmholtz instability on plasma transport.

A model of Jupiter’s magnetodisk

An axially symmetric equilibrium model of Jupiter’s magnetodisk is developed in the MHD approximation that takes the plasma corotation and the centrifugal force into account. The model is constructed for two cases: (1) the magnetodisk plasma is assumed to have a uniform temperature; (2) the plasma pressure is assumed to be an adiabatic function of density. Analytical expressions for the magnetic field, current density, and magnetodisk temperature and thickness distributions are obtained as functions of the system parameters, viz., the radial distribution of plasma pressure in the equatorial plane, the transverse magnetic field in the center of the layer, and the angular velocity of the plasma rotation.

Auroral evidence of Io's control over the magnetosphere of Jupiter

Contrary to the case of the Earth, the main auroral oval on Jupiter is related to the breakdown of plasma corotation in the middle magnetosphere. Even if the root causes for the main auroral emissions are Io's volcanism and Jupiter's fast rotation, changes in the aurora could be attributed either to these internal factors or to fluctuations of the solar wind. Here we show multiple lines of evidence from the aurora for a major internally‐controlled magnetospheric reconfiguration that took place in Spring 2007. Hubble Space Telescope far‐UV images show that the main oval continuously expanded over a few months, engulfing the Ganymede footprint on its way. Simultaneously, there was an increased occurrence rate of large equatorward isolated auroral features attributed to injection of depleted flux tubes. Furthermore, the unique disappearance of the Io footprint on 6 June appears to be related to the exceptional equatorward migration of such a feature. The contemporary observation of the spectacular Tvashtar volcanic plume by the New‐Horizons probe as well as direct measurement of increased Io plasma torus emissions suggest that these dramatic changes were triggered by Io's volcanic activity.

Improved mapping of Jupiter's auroral features to magnetospheric sources

[1] The magnetospheric mapping of Jupiter's polar auroral emissions is highly uncertain because global Jovian field models are known to be inaccurate beyond ∼30 RJ. Furthermore, the boundary between open and closed flux in the ionosphere is not well defined because, unlike the Earth, the main auroral oval emissions at Jupiter are likely associated with the breakdown of plasma corotation and not the open/closed flux boundary in the polar cap. We have mapped contours of constant radial distance from the magnetic equator to the ionosphere in order to understand how auroral features relate to magnetospheric sources. Instead of following model field lines, we map equatorial regions to the ionosphere by requiring that the magnetic flux in some specified region at the equator equals the magnetic flux in the area to which it maps in the ionosphere. Equating the fluxes in this way allows us to link a given position in the magnetosphere to a position in the ionosphere. We find that the polar auroral active region maps to field lines beyond the dayside magnetopause that can be interpreted as Jupiter's polar cusp; the swirl region maps to lobe field lines on the night side and can be interpreted as Jupiter's polar cap; the dark region spans both open and closed field lines and must be explained by multiple processes. Additionally, we conclude that the flux through most of the area inside the main oval matches the magnetic flux contained in the magnetotail lobes and is probably open to the solar wind.

Plasma mass loading from the extended neutral gas torus of Enceladus as inferred from the observed plasma corotation lag

At Saturn, significant plasma mass loading and radial transport occur together throughout the extended neutral torus, both producing corotation lags. We present a theory that depends upon the ratio of two classes of process: those injecting new mass (simple ionization) and those effectively removing momentum without changing plasma mass density (elastic ion‐neutral collisions, like‐species charge exchange). Both induce ionospheric torques, but only the former increases total outward mass flux. From an observed angular velocity, our model calculates the distribution of mass loading with radial distance. We present solutions based on Cassini Plasma Spectrometer measurements and theoretical chemistry models for Saturn's magnetosphere. We find a pronounced interaction peak near Enceladus' orbit and a broad ionization region between 5 and 8 Saturn radii (Rs). The total mass outflux at 8Rs is 100 kg/s × (Σ/0.1 S), where Σ is Saturn's ionospheric Pedersen conductance. The reported decrease in lag beyond 8Rs requires that Σ ≠ const or reducing the total mass outflux.

Magnetosphere-ionosphere coupling currents in Jupiter's middle magnetosphere: effect of magnetosphere-ionosphere decoupling by field-aligned auroral voltages

Abstract. We consider the effect of field-aligned voltages on the magnetosphere-ionosphere coupling current system associated with the breakdown of rigid corotation of equatorial plasma in Jupiter's middle magnetosphere. Previous analyses have assumed perfect mapping of the electric field and flow along equipotential field lines between the equatorial plane and the ionosphere, whereas it has been shown that substantial field-aligned voltages must exist to drive the field-aligned currents associated with the main auroral oval. The effect of these field-aligned voltages is to decouple the flow of the equatorial and ionospheric plasma, such that their angular velocities are in general different from each other. In this paper we self-consistently include the field-aligned voltages in computing the plasma flows and currents in the system. A third order differential equation is derived for the ionospheric plasma angular velocity, and a power series solution obtained which reduces to previous solutions in the limit that the field-aligned voltage is small. Results are obtained to second order in the power series, and are compared to the original zeroth order results with no parallel voltage. We find that for system parameters appropriate to Jupiter the effect of the field-aligned voltages on the solutions is small, thus validating the results of previously-published analyses.

Emission Altitude from Relativistic Phase Shift Due to Plasma Corotation in Pulsars

The radio emission from a relativistic plasma in a pulsar magnetosphere experiences a relativistic phase shift due to spin and difference in the emission altitude. Using the single pulse data of PSR B0329 + 54 at 325 MHz and 606 MHz we studied the structure of the pulsar emission beam. The distribution of the pulse components around the central core component indicates that the emission beam consists of four nested cones. The asymmetry in the location of the conal components in the leading versus trailing parts of the profile is interpreted as being due to aberration and retardation in the pulsar magnetosphere. These measurements allow us to determine the precise location of the 4 conal rings of emission. We find that the successive outer cones are emitted at higher altitudes in the magnetosphere. Further, for any given cone, the emission height at the lower frequency is found to be larger than that at the higher frequency. We discuss the implications of our new findings on our understanding of the pulsar emission geometry and its impact on the emission mechanisms. The relativistic plasma moving along the dipolar magnetic field lines emit coherent curvature radiation. The beamed emission occurs in the direction of tangents to the field lines, and to receive it, the sight line must align with the tangent within the beaming angle 1/γ, where γ is the particle Lorentz factor. By solving the viewing geometry in an inclined and rotating dipole magnetic field, we show that, at any given pulse phase, the observer tends to receive radiation only from the specific heights allowed by the geometry.

Toroidal Magnetic Field Generation in the Magnetosphere of Crab Pulsar

Plasma mechanism for the generation of toroidal magnetic field in the magnetosphere of Crab pulsar is presented. The mechanism is based on the development of parametric type instability in the relativistic electron-positron plasma of the pulsar magnetosphere. As a result of plasma corotation with pulsar and its magnetic field, the effect of plasma radial braking takes place and the time dependence of plasma particle radial velocity is harmonic. This triggers the development of parametric type instability in the relativistic plasma of the pulsar magnetosphere. The energy for this process is drawn from the slowing down of pulsar rotation.

Transport-driven Scrape-Off-Layer flows and the boundary conditions imposed at the magnetic separatrix in a tokamak plasma

Plasma profiles and flows in the low- and high-field side scrape-off-layer (SOL) regions in Alcator C-Mod are found to be remarkably sensitive to magnetic separatrix topologies (upper-, lower- and double-null) and to impose topology-dependent flow boundary conditions on the confined plasma. Near-sonic plasma flows along magnetic field lines are observed in the high-field SOL, with magnitude and direction clearly dependent on X-point location. The principal drive mechanism for the flows is a strong ballooning-like poloidal transport asymmetry: parallel flows arise so as to re-symmetrize the resulting poloidal pressure variation in the SOL. Secondary flows involving a combination of toroidal rotation and Pfirsch–Schlüter ion currents are also evident. As a result of the transport-driven parallel flows, the SOL exhibits a net co-current (counter-current) volume-averaged toroidal momentum when B × ∇B is towards (away from) the X-point. Depending on the discharge conditions, flow momentum can couple across the separatrix and affect the toroidal rotation of the confined plasma. This mechanism accounts for a positive (negative) increment in central plasma co-rotation seen in L-mode discharges when B × ∇B is towards (away from) the X-point. Experiments in ion-cyclotron range-of-frequency-heated discharges suggest that topology-dependent flow boundary conditions may also play a role in the sensitivity of the L–H power threshold to X-point location: in a set of otherwise similar discharges, the L–H transition is seen to be coincident with central rotation achieving roughly the same value, independent of magnetic topology. For discharges with B × ∇B pointing away from the X-point (i.e. with the SOL flow boundary condition impeding co-current rotation), the same characteristic rotation can only be achieved with higher input power.

Characteristics of Saturn's FUV aurora observed with the Space Telescope Imaging Spectrograph

We analyze a set of 15 FUV images obtained between October 1997 and January 2001 with the Hubble Space Telescope Imaging Spectrograph (STIS), providing a good view of Saturn's south auroral oval. It is found that the morphology and brightness distribution of the aurora are dynamical with variations occurring on time scales of hours or less. The dayside main oval lies between 70° and 80° and is generally brighter and thinner in the morning than in the afternoon sector. The afternoon sector is characterized by more diffuse emission at higher latitudes. Weak emission is also observed poleward of the main oval up to the pole. A spot of enhanced auroral precipitation, tentatively identified as the optical signature of the dayside cusp, is sometimes observed poleward of the main oval in the noon sector, especially during periods when the morning arc is not fully developed. A spiral structure of the main oval with arcs at two latitudes in the same sector is occasionally observed. The brightness of the main oval ranges from below the STIS threshold of 1 kR of H2 emission up to ∼75 kR. The total electron precipitated power varies between 20 and 140 GW, that is, comparable to the Earth's active aurora but about two orders of magnitude less than on Jupiter. An increasing trend of the precipitated power between the 1997 and the 2000–2001 observations may be related to the rising solar activity. Six spectra of the aurora in the noon sector covering the 1200–1700 Å range are dominated by emissions of the Lyman‐α line and H2 Werner and Lyman bands. Their comparison with a synthetic model of electron excited H2 emissions indicates the presence of a weak absorption below 1400 Å by a column of methane ranging between 7 × 1015 and 2 × 1016 cm−2. The corresponding energy of the primary auroral electrons is estimated 12 ± 3 keV, using a low‐latitude model atmosphere based on Voyager occultation measurements. The main oval brightness and the characteristic electron energy are generally consistent with recent models of Saturn's aurora, which colocate the main oval with the narrow upward field‐aligned current system associated with departure from plasma corotation near the open‐closed field line boundary. The latitude of the bright morning arc is somewhat lower than model predictions based on the plasma flow velocity measured by Voyager in the middle magnetosphere.

Magnetosphere-ionosphere coupling currents in Jupiter's middle magnetosphere: effect of precipitation-induced enhancement of the ionospheric Pedersen conductivity

Abstract. We consider the effect of precipitation-induced enhancement of the Jovian ionospheric Pedersen conductivity on the magnetosphere-ionosphere coupling current system which is associated with the breakdown of the corotation of iogenic plasma in Jupiter's middle magnetosphere. In previous studies the Pedersen conductivity has been taken to be simply a constant, while it is expected to be significantly enhanced in the regions of upward-directed auroral field-aligned current, implying downward precipitating electrons. We develop an empirical model of the modulation of the Pedersen conductivity with field-aligned current density based on the modelling results of Millward et al. and compute the currents flowing in the system with the conductivity self-consistently dependent on the auroral precipitation. In addition, we consider two simplified models of the conductivity which provide an insight into the behaviour of the solutions. We compare the results to those obtained when the conductivity is taken to be constant, and find that the empirical conductivity model helps resolve some outstanding discrepancies between theory and observation of the plasma angular velocity and current system. Specifically, we find that the field-aligned current is concentrated in a peak of magnitude ~0.25µAm-2 in the inner region of the middle magnetosphere at ~20 RJ, rather than being more uniformly distributed as found with constant conductivity models. This peak maps to ~17° in the ionosphere, and is consistent with the position of the main oval auroras. The energy flux associated with the field-aligned current is ~10mWm-2 (corresponding to a UV luminosity of ~100kR), in a region ~0.6° in width, and the Pedersen conductivity is elevated from a background of ~0.05mho to ~0.7mho. Correspondingly, the total equatorial radial current increases greatly in the region of peak field-aligned current, and plateaus with increasing distance thereafter. This form is consistent with the observed profile of the current derived from Galileo magnetic field data. In addition, we find that the solutions using the empirical conductivity model produce an angular velocity profile which maintains the plasma near to rigid corotation out to much further distances than the constant conductivity model would suggest. Again, this is consistent with observations. Our results therefore suggest that, while the constant conductivity solutions provide an important indication that the main oval is indeed a result of the breakdown of the corotation of iogenic plasma, they do not explain the details of the observations. In order to resolve some of these discrepancies, one must take into account the elevation of the Pedersen conductivity as a result of auroral electron precipitation.Key words. Magnetospheric physics (current systems, magnetosphere-ionosphere interactions, planetary magnetospheres)70d

Origins of Jupiter's main oval auroral emissions

We review recent theoretical progress in understanding the origins of the jovian “main auroral oval” emissions, based on the hypothesis that they are connected with the magnetosphere‐ionosphere coupling current circuit which maintains plasma corotation in the middle magnetosphere. After the development of the basic theory, we compare results obtained with differing models of the background magnetic field, showing that an appropriate choice is crucial in determining the strength, width, and location of the precipitating auroral electron energy flux associated with the region of upward field‐aligned current. Specific comparison is made between results for a dipole field and an empirically based current sheet model, in which the angular velocity profile of the plasma is calculated self‐consistently using Hill‐Pontius theory. We also develop simple approximation regimes appropriate to small and large equatorial distances, showing that the currents depend on the mass outflow rate of iogenic plasma and not on the effective ionospheric Pedersen conductivity in the small‐distance regime, and vice versa at large distances. We also discuss the expected anticorrelation of the auroral emission intensity with solar wind dynamic pressure and present steady state results representing differing degrees of magnetospheric compression. Significant modulation of the emission is anticipated on this basis.

Distributions of current and auroral precipitation in Jupiter's middle magnetosphere computed from steady-state Hill–Pontius angular velocity profiles: solutions for current sheet and dipole magnetic field models

We present an analysis of the magnetosphere–ionosphere coupling current system which is associated with the maintenance of plasma corotation in Jupiter's middle magnetosphere. The formulation follows Cowley and Bunce (Planet. Space Sci. 49 (2001) 1067), but here the angular velocity of the equatorial plasma is computed using the steady-state theory of Hill (J. Geophys. Res. 84 (1979) 6554) and Pontius (J. Geophys. Res. 102 (1997) 7137) rather than being determined by a simple empirical model. Results are compared both for a realistic magnetospheric current sheet magnetic field model and for a simple dipole field. The results for the current sheet model confirm previous conclusions concerning current and auroral precipitation distributions based on the empirical angular velocity model. Specifically, we find that upward field-aligned currents flow from the ionosphere to the equatorial current sheet over the whole radial range from ∼20 R J to the outer boundary of the region considered at 100 R J, thus returning from the current sheet to the ionosphere at larger radial distances outside this region. The upward currents are of sufficient intensity to require the existence of field-aligned voltages which accelerate magnetospheric electrons down into the atmosphere, thus creating intense auroras. At ionospheric heights, the upward field-aligned current densities are several tenths of a μA m −2 , flowing in a narrow latitudinal band less than ∼1° wide, located at ∼16° dipole co-latitude. The associated field-aligned voltages are several tens of kV, resulting in precipitated electron energy fluxes capable of exciting UV auroras of hundreds of kR intensity. The model thus forms an appropriate basis on which to understand the origin of Jupiter's ‘main auroral oval’ emissions. For the dipole field, the steady-state equatorial angular velocity profile is found to be very similar to that for the current sheet model. Nevertheless, the different mapping of the field lines to the ionosphere results in a magnetosphere–ionosphere coupling current system which has significantly different properties. In particular, the height-integrated ionospheric Pedersen current intensity is found to be factors of ∼2–3 less than that for the current sheet model, and since the field-aligned current is also found to be spread over a much broader latitudinal band, ∼5° wide centred at ∼10° dipole co-latitude, the field-aligned current density is reduced by more than an order of magnitude. Only small (few kV) field-aligned voltages are thus required to carry the upward current in this case, resulting in much weaker electron precipitation, and much weaker, few kR, UV auroras. Our results thus emphasise that while the magnetic field model may not greatly effect the steady-state angular velocity profile, it nevertheless is of crucial significance in determining the consequent magnetosphere–ionosphere coupling current system, and the resulting auroral precipitation.

The Jovian auroral oval

Jupiter's aurora shows, among other features, a persistent, continuous oval of luminosity that encircles each magnetic pole and maps along magnetic field lines to the middle magnetosphere (r ≈ 30 Rj). This auroral oval is interpreted here as the ionospheric footprint of the upward Birkeland (magnetic field aligned) current that enforces partial corotation of magnetospheric plasma moving outward from its source (the Io plasma torus) to its sink (the outer magnetosphere and ultimately the solar wind). A simplified model of this current system, based on the assumption of a constant, uniform plasma outflow in a spin‐aligned dipole magnetic field, has a maximum upward Birkeland current density at an ionospheric latitude that maps to the middle magnetosphere. Strong upward Birkeland currents are known, from terrestrial studies, to produce bright aurora.