MESSENGER observations of magnetopause structure and dynamics at Mercury

Abstract

[1] On 18 March 2011, MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) became the first spacecraft to orbit Mercury, providing a new opportunity to study the outer boundary of the planet's magnetosphere-the magnetopause. Here we characterize Mercury's magnetopause using measurements collected by MESSENGER's Magnetometer and Fast Imaging Plasma Spectrometer. Analysis of measurements from two of MESSENGER's "hot seasons," when the orbital periapsis is on Mercury's dayside and the magnetopause crossing takes place in the subsolar region, resulted in 43 events with well-determined boundary normals. The typical duration of a magnetopause traversal was~5 s. The average normal magnetic field component was~20 nT, and the dimensionless reconnection rate, i.e., the ratio of the normal magnetic field component to the total field magnitude just inside the magnetopause, was 0.15 AE 0.02. This rate is a factor of~3 larger than values found during the most extensive surveys at Earth. The ratio of the reconnection rate at Mercury to that of the Earth is comparable to the ratio of the solar wind Alfvén speeds at their respective orbits. We also find that the magnetopause reconnection rate at Mercury is independent of magnetic field shear angle, but it varies inversely with plasma b, the ratio of total thermal pressure to magnetic pressure, in the magnetosheath. These results suggest that reconnection at Mercury is not only more intense than at Earth but also that it occurs for nearly all orientations of the interplanetary magnetic field due to the low-b nature of the solar wind in the inner heliosphere.

A modelling approach to infer the solar wind dynamic pressure from magnetic field observations inside Mercury’s magnetosphere

“…Slavin et al 2009Slavin et al , 2010DiBraccio et al 2013;Raines et al 2015). For example, the magnetic reconnection rate is estimated to be ∼0.15 at Mercury, which is nearly an order of magnitude higher than that observed at Earth .…”

“…The solar wind dynamic pressure P dyn = m i n sw u 2 sw , where m i is the solar wind proton mass, and the Alfvén Mach number M A = µ 0 P dyn /|B| are the main parameters that control the shape and structure of the magnetospheric boundaries (e.g. Slavin et al 2008Slavin et al , 2009DiBraccio et al 2013;Winslow et al 2013 and they are a function of variables including B, n sw , and u sw . Since the magnetic field B is already known from observations and listed in Table 1, we only consider n sw and u sw as the variables in our simulations.…”

What Controls the Structure and Dynamics of Earth’s Magnetosphere?

“…Whilst Mercury has a dipole magnetic field (Anderson et al 2011), its magnetosphere is very small (Johnson et al 2012), and is strongly driven by the solar wind (DiBraccio et al 2013) (as mentioned in Sect. 1).…”

“…At Mercury, close to the Sun, the magnetosheath plasma beta is low and in theory reconnection can occur for a wide variety of IMF orientations; this has been investigated using MESSENGER spacecraft data (DiBraccio et al 2013). In contrast, at the outer planets, e.g.…”

Open and partially closed models of the solar wind interaction with outer planet magnetospheres: the case of Saturn

“…Various spacecraft have succeeded in detecting PDL signatures at various different planets, as we now briefly discuss. At Mercury's magnetopause, the observed reconnection rate is substantially higher than at Earth (Slavin et al, 2009;DiBraccio et al, 2013), yet it is still insufficient to transfer and distribute the entire magnetic flux pileup around the magnetosphere quickly enough. Consequently, the pileup still generates a persistent large-scale PDL, which in turn leads to further growth of the average magnetopause reconnection rate (Gershman et al, 2013).…”