Modulation of Jupiter's plasma flow, polar currents, and auroral precipitation by solar wind-induced compressions and expansions of the magnetosphere: a simple theoretical model

Abstract

Abstract.We construct a simple model of the plasma flow, magnetosphere-ionosphere coupling currents, and auroral precipitation in Jupiter's magnetosphere, and examine how they respond to compressions and expansions of the system induced by changes in solar wind dynamic pressure. The main simplifying assumption is axi-symmetry, the system being modelled principally to reflect dayside conditions. The model thus describes three magnetospheric regions, namely the middle and outer magnetosphere on closed magnetic field lines bounded by the magnetopause, together with a region of open field lines mapping to the tail. The calculations assume that the system is initially in a state of steady diffusive outflow of iogenic plasma with a particular equatorial magnetopause radius, and that the magnetopause then moves rapidly in or out due to a change in the solar wind dynamic pressure. If the change is sufficiently rapid (∼2-3 h or less) the plasma angular momentum is conserved during the excursion, allowing the modified plasma angular velocity to be calculated from the radial displacement of the field lines, together with the modified magnetosphere-ionosphere coupling currents and auroral precipitation. The properties of these transient states are compared with those of the steady states to which they revert over intervals of ∼1-2 days. Results are shown for rapid compressions of the system from an initially expanded state typical of a solar wind rarefaction region, illustrating the reduction in total precipitating electron power that occurs for modest compressions, followed by partial recovery in the emergent steady state. For major compressions, however, typical of the onset of a solar wind compression region, a brightened transient state occurs in which super-rotation is induced on closed field lines, resulting in a reversal in sense of the usual magnetosphereionosphere coupling current system. Current system reversal results in accelerated auroral electron precipitation occurring Correspondence to: S. W. H. Cowley (swhc1@ion.le.ac.uk) in the outer magnetosphere region rather than in the middle magnetosphere as is usual, with peak energy fluxes occurring just poleward of the boundary between the outer and middle magnetosphere. Plasma sub-corotation is then re-established as steady-state conditions re-emerge, together with the usual sense of flow of the closed field current system and renewed but weakened accelerated electron precipitation in the middle magnetosphere. Results for rapid expansions of the system from an initially compressed state typical of a solar wind compression region are also shown, illustrating the enhancement in precipitating electron power that occurs in the transient state, followed by partial reduction as steady conditions re-emerge.

Solar wind effects on Jupiter non-Io DAM emissions during Ulysses distant encounter (2003–2004)

“…But auroral brightening could follow a strong solar wind compression by the flywheel effect in the ionosphere (e.g., Cowley & Bunce 2001;Clarke et al 2009). Simulation results have shown that a transient brightening of the aurora can be expected, at slightly different latitudes, depending on the region of enhanced current, by the same principle of differential rotation rate of the plasma and field (Cowley et al 2007). This has possibly been observed in the UV aurora ).…”

“…Their results show that the magnitude of the field-aligned current is also dependent on the timescale of the response of the ionospheric neutrals relative to the ions. Cowley et al (2007) found that for a few hours following an impulsive compression, the main oval will dim because of the increased angular velocity of the equatorial plasma, and the emission at the open-closed field line boundary will brighten because of the increased flow shear across the boundary. As discussed by Nichols et al (2009), if the compression is strong enough to induce super-rotation, a condition that is achieved by more modest compressions if the neutral atmosphere is unresponsive, then the current system will reverse and instead produce emission on field lines mapping to the outer magnetosphere between the middle magnetosphere and the magnetopause.…”

“…Cowley et al (2007) constructed a model of the plasma flow, magnetosphere-ionosphere coupling currents, and auroral precipitation in Jupiter's magnetosphere, examining how they respond to compressions and expansions of the magnetosphere driven by changes in solar wind dynamic pressure. Their results show that the magnitude of the field-aligned current is also dependent on the timescale of the response of the ionospheric neutrals relative to the ions.…”

Characteristics of solar wind control on Jovian UV auroral activity deciphered by long‐term Hisaki EXCEED observations: Evidence of preconditioning of the magnetosphere?

“…The models by Southwood and Kivelson [2001] and Cowley and Bunce [2003] predict that, due to the conservation of angular momentum, the angular velocity of the magnetospheric plasma increases when the Jovian magnetosphere is compressed. On the other hand, Cowley et al [2007] expanded the modeling to the outer magnetosphere (and the polar region of the ionosphere) to find that a major compression increases the aurora brightness at the open-closed field line boundary during the rapid compression phase lasting approximately 2-3 h. Eventually, the field-aligned current decreases in the steady compressed magnetosphere, which appears 1-2 days after the start of the event. Therefore, the solar wind pressure and the aurora are expected to be anticorrelated.…”

What characterizes planetary space weather?

“…The magnetospheric dynamics as demonstrated by increases in the output of the aurora and auroral kilometric radiation (AKR) at Saturn has been found to be driven by the rotation of the planet except during periods of strong solar wind compression of the magnetosphere (Desch 1982;Clarke et al 2009;Crary et al 2005;Badman and Cowley 2007;Masters et al 2014;Badman et al 2014). The influence of the solar wind at Jupiter is thought to be less strong although solar wind compressions have been associated with changes in the aurora (Baron et al 1996;Gurnett et al 2002;Nichols et al 2007;Cowley et al 2007;Clarke et al 2009). …”