Current Sheet Magnetic Field Research Articles

Effect of magnetospheric conditions on the morphology of Jupiter’s ultraviolet main auroral emission as observed by Juno-UVS

Auroral emissions are a reflection of magnetospheric processes, and at Jupiter, it is not entirely certain how the morphology of the UV main emission (ME) varies with magnetospheric compression or the strength of the central current sheet. This work leverages the observations from Juno-UVS to link ME variability with particular magnetospheric states. We employed novel arc-detection techniques to determine new reference ovals for the ME from perijoves 1 through 54, in both hemispheres, and analysed how the size and shape of the ME vary compared to this reference oval. The morphology and brightness of the ME vary in local time: the dawn-side ME is typically expanded, while the dusk-side ME is contracted, compared to the reference oval, and the dusk-side ME is twice as bright as the dawn-side ME. Both the northern and southern ME and the day-side and night-side ME expand and contract from their reference ovals synchronously, which indicates that the variable size of the ME is caused by a process occurring throughout the Jovian magnetosphere. The poleward latitudinal shift of the auroral footprint of Ganymede correlates with the poleward motion of the ME, whereas a similar relation is not present for the footprint of Io. Additionally, the expansion of the ME correlates well with an increase in magnetodisc current. These two results suggest that a changing current-sheet magnetic field is partially responsible for the variable size of the ME. Finally, magnetospheric compression is linked to a global ME contraction and brightening, though this brightening occurs predominantly in the day-side ME. This observation, and the observation that the dusk-side ME is typically brighter than the dawn-side ME, stands in contrast to the modelled and observed behaviour of field-aligned currents and thus weakens the theoretical link between field-aligned currents and the generation of the auroral ME.

Magnetohydrodynamic simulation of a solar flare: 1. Current sheet in the corona

The results of modeling the preflare situation in the solar corona, obtained using a numerical solution for a complete set of three-dimensional MHD equations, are reviewed. Any assumptions concerning the flare development character or the active region’s behavior before a flare are not introduced. The initial and boundary conditions on the photosphere are specified from magnetic field measurements before a flare. The photospheric field sources are approximated by magnetic dipoles. The usage of the PERESVET program indicated that a current sheet is formed in the vicinity of a singular magnetic field line in the corona. The sheet is formed due to disturbances coming from the photosphere. The energy necessary for a flare is stored in the current sheet magnetic field during 2–3 days. The main construction principles of the PERESVET program, which makes it possible to use the maps of a measured photospheric field as boundary conditions, are presented.

Magnetosphere-ionosphere coupling in Jupiter's middle magnetosphere: Computations including a self-consistent current sheet magnetic field model

In this paper we consider the effect of a self-consistently computed magnetosdisc field structure on the magnetosphere-ionosphere coupling current system at Jupiter. We find that the azimuthal current intensity, and thus the stretching of the magnetic field lines, is dependent on the magnetosphere-ionosphere coupling current system parameters, i.e. the ionospheric Pedersen conductivity and iogenic plasma mass outflow rate. Overall, however, the equatorial magnetic field profiles obtained are similar in the inner region to those used previously, such that the currents are of the same order as previous solutions obtained using a fixed empirical equatorial field strength model, although the outer fringing field of the current disc acts to reverse the field-aligned current in the outer region. We also find that, while the azimuthal current in the inner region is dominated by hot plasma pressure, as is generally held to be the case at Jupiter, the use of a realistic plasma angular velocity profile actually results in the centrifugal current becoming dominant in the outer magnetosphere. In addition, despite the dependence of the intensity of the azimuthal current on the magnetosphere-ionosphere coupling current system parameters, the location of the peak field-aligned current in the equatorial plane also varies, such that the ionospheric location remains roughly constant. It is thus found that significant changes to the mass density of the iogenic plasma disc are required to explain the variation in the main oval location observed using the Hubble Space Telescope.

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.

Mapping Jupiter's outer radiation belt

Flux maps of energetic ions in Jupiter's outer radiation belt, calculated with the aid of a model of the current sheet magnetic field, are consistent with a spatial distribution that is uniform along field lines and monotonically decreasing with radial distance. This result is supported by numerical ion trajectory calculations that predict uniform filling of drift shells at high L values. Adiabatic compression by radial transport then provides predictions of radial energy dependence and flux gradients. At lower L values, conservation of the second adiabatic invariant predicts some anisotropy with higher equatorial fluxes.

Multidomain analytical-numerical solution for a rotating magnetic field with a finite-length conducting cylinder

This paper treats the rotating magnetic field produced by a cylindrical sheet of temporally and azimuthally periodic axial electric current with a finite-length, electrically conducting cylinder surrounded by an electrical insulator inside the current sheet. As the frequency is increased, the magnetic field produced by the induced current in the cylinder cancels progressively more of the current-sheet magnetic field in the interior of the cylinder. This paper presents results for several different frequencies and for several different length-to-diameter ratios for the cylinder. It compares the results at the midplane of a long cylinder to the analytical solution for an infinitely long cylinder. It also compares results for a high frequency to the predictions from the first two terms in an asymptotic expansion for high frequency.

Magnetic reconnection in the neutral current sheet in the presence of a small fraction of hot anisotropic ions

The effect of a small fraction of hot ions with anisotropic temperature on spontaneous reconnection of the neutral current sheet magnetic field is studied using a hydrodynamical approach. The linear dispersion equations are obtained for symmetric and antisymmetric current sheet perturbations. Solutions of these equations near the instability boundaries are found in analytical form. The numerical solutions of full dispersion equations are also represented. In the model under discussion, the magnetic reconnection is fast. The typical reconnection time for the conditions in the solar outer chromosphere and corona is of the order of several Alfvén times, which is considerably less than the values given by the current fast reconnection models.

Pioneer 11 observations of effects of Ganymede and Callisto on Jupiter's trapped radiation

Beyond about 10 RJ from Jupiter the configuration of magnetic field lines in Jupiter's magnetosphere is strongly affected by the presence of an equatorial current sheet. Using a Voyager 1 magnetic field model that explicitly includes the effects of this current sheet to describe the magnetic field line geometry, we have reexamined charged particle observations from Pioneer 11's high‐latitude flyby of Jupiter in 1974. We find that if the trajectory of Pioneer 11 is mapped to the equatorial plane along the model magnetic field lines, significant features in the time‐intensity profiles of trapped protons and electrons, including one microsignaturelike feature, are found to correspond to shells of closed field lines crossed by the orbits of the satellites Ganymede and Callisto. Although many of the characteristics of the features, including their magnitude, persistence, and similarity of appearance for protons and electrons, are difficult to explain, we suggest that these features are signatures of interaction of the trapped particles with Ganymede and Callisto. If this interpretation is correct, it offers explanations for several large features in the time‐intensity profile of trapped particles observed during Pioneer 11's flyby that could not be explained using earlier magnetic field models which did not include effects of the current sheet, and it leads to reinterpretation of some features previously associated with Ganymede as signatures of Callisto. It also implies that the configuration of the current sheet during the Pioneer 11 flyby in 1974 was very similar to that observed by Voyager 1 in 1979 and provides confirmation of the essential accuracy of current sheet magnetic field models for describing the magnetic field configuration out to radii of about 30 RJ from Jupiter. Finally, the characteristics of the signatures described suggest interesting problems for future research on the dynamics and interactions of trapped particles with satellites in the outer regions of the Jovian trapped radiation zone.

Relationship of dusk‐sector electric field to energy dispersion at the inner edge of the electron plasma sheet for nonequatorially mirroring electrons

The steady state relationships between the convection electric field and electron boundary energy dispersion in the dusk sector are obtained for off‐equatorially mirroring electrons in dipole and dipole plus current sheet magnetic field configurations. Comparison of these predictions with dusk‐sector ionospheric satellite measurements of electron boundaries and electric fields shows reasonable agreement in magnitude and general increasing trend with latitude, particularly when the dipole‐plus‐current sheet magnetic field configuration is used.