Elsevier

Icarus

Volume 209, Issue 1, September 2010, Pages 3-10
Icarus

Modeling of the magnetosphere of Mercury at the time of the first MESSENGER flyby

https://doi.org/10.1016/j.icarus.2009.11.036Get rights and content

Abstract

The MESSENGER spacecraft flyby of Mercury on 14 January 2008 provided a new opportunity to study the intrinsic magnetic field of the innermost planet and its interaction with the solar wind. The model presented in this paper is based on the solution of the three-dimensional, bi-fluid equations for solar wind protons and electrons in the absence of mass loading. In this study we provide new estimates of Mercury’s intrinsic magnetic field and the solar wind conditions that prevailed at the time of the flyby. We show that the location of the boundary layers and the strength of the magnetic field along the spacecraft trajectory can be reproduced with a solar wind ram pressure Psw = 6.8 nPa and a planetary magnetic dipole having a magnitude of 210 RM3  nT and an offset of 0.18 RM to the north of the equator, where RM is Mercury’s radius. Analysis of the plasma flow reveals the existence of a stable drift belt around the planet; such a belt can account for the locations of diamagnetic decreases observed by the MESSENGER Magnetometer. Moreover, we determine that the ion impact rate at the northern cusp was four times higher than at the southern cusp, a result that provides a possible explanation for the observed north–south asymmetry in exospheric sodium in the neutral tail.

Introduction

On 14 January 2008 the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft completed the first of three scheduled flybys of Mercury that will, ultimately, lead it to orbital insertion about the innermost planet on 18 March 2011 (Solomon et al., 2008). During this first flyby, the MESSENGER spacecraft traversed the night side of the magnetosphere of Mercury from dusk to dawn in a quasi-equatorial trajectory and reached its closest approach altitude of 201.4 km at 19:04:39 UTC. This close encounter allowed the Magnetometer (MAG) and the Energetic Particle and Plasma Spectrometer (EPPS) to observe the planet’s magnetic field and plasma environment. Preliminary analyses of the MAG observations and those from the Fast Imaging Plasma Spectrometer (FIPS) sensor on the EPPS instrument (Anderson et al., 2008, Slavin et al., 2008, Zurbuchen et al., 2008) revealed new details of the complex dynamics of Mercury’s magnetosphere and the manner by which it interacts with the solar wind. These analyses emphasized that the structural complexity of the magnetosphere of Mercury is at least in part the result of its small spatial scale, fast temporal response, and strong coupling with the solar wind.

In recent years, our understanding of such a highly coupled magnetospheric system has been greatly advanced by increasingly sophisticated numerical models. A large number of these models use either fluid or kinetic theories with varying levels of complexity. Ideal magnetohydrodynamic (MHD) models were the first to characterize successfully the large-scale structure of the magnetosphere of Mercury (Kabin et al., 2000, Ip and Kopp, 2002). Although such models are numerically efficient, they do not take into account finite ion cyclotron effects and multi-species interactions. More accurate and rigorous treatments of the plasma dynamics and gyroradius effects were provided by several hybrid models (Kallio and Janhunen, 2004, Trávníček et al., 2007). Whereas these models were successful at capturing important finite cyclotron effects in the magnetosphere of Mercury, they had a limited spatial extent, large shot noise due to the low number of super-particles (∼5 to ∼10) in a simulation cell, and a considerable numerical burden.

Multi-fluid modeling emerged as a reasonable compromise between too simple, ideal MHD techniques and computationally complicated and costly kinetic techniques. Multi-fluid modeling was first applied to Mercury by Kidder et al. (2008) to study the interaction between the planet’s magnetosphere and the solar wind under northward and southward interplanetary magnetic field (IMF) conditions. In a related study of another body, Harnett et al. (2005) showed that this technique retains key cyclotron effects and yields results comparable to those obtained from kinetic modeling. Huba and Rudakov (2004) earlier showed that the inclusion of the Hall physics within the ideal MHD theory provides an increased reconnection rate in the modeled magnetospheric system that is consistent with the results derived from hybrid and particle simulations. All of these advantages come with a computational burden (computing time and memory) not excessively higher than those incurred with ideal MHD models.

In this paper we apply the Hall multi-fluid theory to study the interaction of the magnetosphere of Mercury with the solar wind at the time of the first MESSENGER flyby of the planet. In Section 2, we describe the model, the techniques used in this study, and how the method compares with those employed in previous studies. Section 3 details the boundary conditions imposed on the model and the techniques used to infer solar wind and intrinsic field parameters consistent with those observed during the flyby. In Section 4, we present the modeling results and how they compare to the data gathered by MESSENGER.

Section snippets

The magnetospheric model

The model presented in this paper is based on the solution of the conservation equations of a bi-fluid plasma (H+ and electrons) subjected to the intrinsic magnetic field and the gravity field of the planet in the absence of a mass-loading source. Although this paper reports the results of a bi-fluid model, we describe the plasma governing equations in their general multi-fluid form. The mass, momentum, and pressure conservation equations for ions areρst+·(ρsvs)=0t(ρsvs)+(ρsvsvs+psI)=ρsg+n

Boundary conditions

The external boundaries of our computational domain were imposed by the solar wind condition prevailing during the encounter, whereas the inner boundary, defined at Mercury’s surface, was imposed by the intrinsic magnetic field of the planet. The planet was approximated by a nonmagnetic sphere through which the magnetic flux is allowed to cross but no particle flux can penetrate. In this study, we used an iterative technique to determine the most likely solar wind condition at the time of this

Magnetospheric structure

The derived magnetospheric structure in the X–Y and X–Z planes is shown in Fig. 2. As the solar wind slows and diverts around the planetary obstacle, the ion and electron densities and temperatures increase and a bow shock forms. On the day side of the magnetosphere, the ions and electrons pile up to five times the density of the undisturbed solar wind. However, ion density decreases dramatically inside the magnetopause due to the presence of the intrinsic field of the planet, which acts as an

Comparison with MESSENGER MAG data

A comparison of the modeled magnetic field along the MESSENGER flyby trajectory and the MAG measurements is given in Fig. 5. This comparison shows that our model captures correctly the structure of the magnetic field, which suggests that no major changes in the IMF or the solar wind occurred while the spacecraft was traveling through the magnetosphere. Whereas the inbound bow shock is well located, our model spreads the interface over a larger distance than is observed. This excessive spread is

Conclusions

This paper presents results of bi-fluid modeling of the magnetosphere of Mercury at the time of the first MESSENGER flyby of Mercury. With this model, we show that the location of the boundary layers and the strength of the magnetic field along the spacecraft trajectory can be reproduced using a solar wind ram pressure Psw = 6.8 nPa and a planetary magnetic dipole with a magnitude of 210 RM3  nT and an offset of 0.18 RM to the north. This dipole is very close to the one determined by Alexeev et al.

Acknowledgments

The work presented here has been supported by the NASA MESSENGER Participating Scientist Program under grant NNX07AR61G. The MESSENGER project is supported by the NASA Discovery Program under contracts NASW-00002 to the Carnegie Institution of Washington and NAS5-97271 to the Johns Hopkins University Applied Physics Laboratory. The calculations were carried out on the high-performance computing resources of the NASA Center for Computational Sciences (NCCS). The authors thank the MESSENGER team

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