A simple axisymmetric model of magnetosphere‐ionosphere coupling currents in Jupiter's polar ionosphere

Abstract

[1] We propose a simple illustrative axisymmetric model of the plasma flow and currents in Jupiter's polar ionosphere which are due both to internal magnetospheric plasma processes and the solar wind interaction. The plasma flow in the model is specified using a combination of observations, previous modeling, and theory, and the ionospheric and field-aligned currents are then calculated. With increasing latitude, the model represents conditions in the inner, middle, and outer magnetosphere on closed field lines and on open field lines mapping to the tail lobes. The model allows us to address three important topics, concerned with the closure of the upward field-aligned currents flowing in the middle magnetosphere region, the energy transfers from planetary rotation to polar upper atmosphere heating and to the magnetosphere, and the relative significance of auroral processes associated with the boundary of open field lines (and hence with the solar wind interaction) and with the middle magnetosphere. It is shown in particular that the energy transfers to the polar upper atmosphere and magnetosphere are of order hundreds of TWeach and that discrete auroral precipitation is expected both at the boundary of open field lines and in the middle magnetosphere, though being dominated by the latter. While the initial calculations assume for simplicity a constant ionospheric conductance, we also present a development of the model in which the conductance is self-consistently increased in regions of upward field-aligned current by the precipitation of accelerated electrons. It is shown that this feedback acts to spread the upward current in the region equatorward of the open field line boundary, thus reducing the energy flux and total power of precipitating auroral electrons in this region. At the same time it concentrates the upward current in the equatorward part of the middle magnetosphere, thereby increasing the energy flux and total power of precipitating electrons in this region.

Significance of Dungey-cycle flows in Jupiter's and Saturn's magnetospheres, and their identification on closed equatorial field lines

“…We also suppose that the surrounding region of mass-reduced flux tubes flowing around the boundary via dawn from the Vasyliunas-and Dungey-cycle reconnection sites in the tail together form the "outer magnetosphere" region on the dayside, characterised by relatively strong southward-pointing fields in the equatorial plane and the lack of a central current sheet, as originally suggested by Cowley et al (1996). In subsequent discussions Cowley et al (2003bCowley et al ( , 2005b emphasised the Dungey-cycle contributions to this layer, while Kivelson and Southwood (2005) emphasise the Vasyliunas-cycle contribution. Here we will discuss in more detail the relative contributions of these two processes to the formation of the layer.…”

“…It may be thought that these voltage values represent overestimates since the plasma sub-corotates throughout much of the outer magnetosphere. However, this sub-corotation region maps to within ∼20 • of the magnetic pole at Jupiter and within ∼25 • of the pole at Saturn, corresponding to only ∼10% and 20%, respectively, of the total planetary magnetic flux (Cowley et al, 2004b(Cowley et al, , 2005b, as will be discussed further below. If the plasma rotates on average at approximately half the planetary angular velocity throughout this region, then the above rotational voltage estimates will be reduced by only ∼5% and 10%, respectively, due to this effect.…”

A model of the plasma flow and current in Saturn's polar ionosphere under conditions of strong Dungey cycle driving

“…At Jupiter, however, the equivalent middle magnetosphere "corotation breakdown" currents are more intense by at least an order of magnitude (e.g. Cowley et al, 2005b), and are believed to be associated with accelerated electron precipitation that produces the bright "main oval" auroral emissions in that case Cowley and Bunce, 2001;Hill, 2001;Southwood and Kivelson, 2001;Grodent et al, 2003).…”

Magnetopause reconnection rate estimates for Jupiter's magnetosphere based on interplanetary measurements at ~5AU

“…They calculated the potential difference between 20 R J and 60 R J to be only ∼4 MV. Cowley et al (2005) recently expanded this work to include the outer magnetosphere region and computed the voltage between ∼20 R J and the magnetopause to be ∼10 MV. Thus, the magnetopause reconnection estimate of ∼1 MV is clearly not negligible in this region.…”