Modeling the size and shape of Saturn's magnetopause with variable dynamic pressure

Abstract

[1] The location and shape of a planetary magnetopause is principally determined by the dynamic pressure, D p , of the solar wind, the orientation of the planet's magnetic dipole with respect to the solar wind flow, and by the distribution of stresses inside the magnetosphere. The magnetospheres of Saturn and Jupiter have strong internal plasma sources compared to the solar wind source and also rotate rapidly, causing an equatorial inflation of the magnetosphere and consequently the magnetopause. Empirical studies using Voyager and Pioneer data concluded that the kronian magnetopause was Earth-like in terms of its dynamics (Slavin et al., 1985) as revealed by how the position of the magnetopause varies with dynamic pressure. In this paper we present a new pressuredependent model of Saturn's magnetopause, using the functional form proposed by Shue et al. (1997). To establish the pressure-dependence, we also use a new technique for fitting a pressure-dependent model in the absence of simultaneous upstream pressure measurements. Using a Newtonian form of the pressure balance across the magnetopause boundary and using model rather than minimum variance normals, we estimate the solar wind dynamic pressure at each crossing. By iteratively fitting our model to magnetopause crossings observed by the Cassini and Voyager spacecraft, in parallel with the pressure balance, we obtain a model which is self-consistent with the dynamic pressure estimates obtained. We find a model whose size varies as $D p À1/4.3 and whose flaring decreases with increasing dynamic pressure. This is interpreted in terms of a different distribution of fields and particles stresses which has more in common with the jovian magnetosphere compared with the terrestrial situation. We compare our model with the existing models of the magnetopause and highlight the very different geometries. We find our results are consistent with recent MHD modeling of Saturn's magnetosphere (Hansen et al., 2005).Citation: Arridge, C. S., N. Achilleos, M. K. Dougherty, K. K. Khurana, and C. T. Russell (2006), Modeling the size and shape of Saturn's magnetopause with variable dynamic pressure,

Dusk-brightening Event in Saturn's H[FORMULA][F][SUP]+[/SUP][INF]3[/INF][/F][/FORMULA] Aurora

“…However, during a period shortly before our observations, a number of magnetopause boundary crossings were detected. Such signatures have been used previously by Arridge et al (2006) to develop a model relating the shape of Saturn's magnetopause to the dynamic pressure. Using this model, it is thus possible to use the location of the magnetopause crossings preceding our observations as an estimate of the solar wind pressure.…”

“…The average magnetopause standoff distance is ∼21R S during periods of compressed solar wind and ∼27R S during the more typical rarefied solar wind conditions (Arridge et al 2006).…”

“…The MAG instrument detected a total of 10 magnetopause crossings in the period between December 15 and December 17, for which the Arridge et al (2006) model calculates a range of modeled magnetopause standoff distances between 33.3R S and 31.7R S (equatorial radius of Saturn p 60,268 km), with corresponding solar wind dynamic pressures of 0.0052 nPa and 0.0065 nPa, respectively. The average magnetopause standoff distance is ∼21R S during periods of compressed solar wind and ∼27R S during the more typical rarefied solar wind conditions (Arridge et al 2006).…”

The latitudinal structure of the nightside outer magnetosphere of Saturn as revealed by velocity moments of thermal ions

“…The interplay of the centrifugal force acting on the charged particles, the magnetic stress, and the kinetic pressure of the particles leads to the formation of a complex plasma/magnetic field structure called the magnetodisk (Arridge et al, 2006;Achilleos et al, 2010). Most of the plasma is concentrated in this rotating disk, which is situated near the equatorial plane and perturbs (stretches) the planetary magnetic field.…”

Separating drivers of Saturnian magnetopause motion

“…The observed position of Saturn's magnetopause is affected by a variety of mechanisms, the most significant of which are pressure balance between the solar wind and the Kronian magnetosphere, propagation of waves on the magnetopause surface, and oscillations of the magnetopause at a constant period. Of these, pressure balance at the magnetopause has been studied extensively [e.g., Arridge et al, 2006;Kanani et al, 2010]; however, some properties of typical surface waves and magnetopause oscillations have not been well constrained.…”