David Schriver

MESSENGER Observations of Magnetic Reconnection in Mercury’s Magnetosphere

MESSENGER from Mercury The spacecraft MESSENGER passed by Mercury in October 2008, in what was the second of three fly-bys before it settles into the planets orbit in 2011. Another spacecraft visited Mercury in the mid-1970s, which mapped 45% of the planets surface. Now, after MESSENGER, only 10% of Mercurys surface remains to be imaged up close. Denevi et al. (p. 613) use this near-global data to look at the mechanisms that shaped Mercurys crust, which likely formed by eruption of magmas of different compositions over a long period of time. Like the Moon, Mercurys surface is dotted with impact craters. Watters et al. (p. 618) describe a well-preserved impact basin, Rembrandt, which is second in size to the largest known basin, Caloris. Unlike Caloris, Rembrandt is not completely filled by material of volcanic origin, preserving clues to its formation and evolution. It displays unique patterns of tectonic deformation, some of which result from Mercurys contraction as its interior cooled over time. Mercurys exosphere and magnetosphere were also observed (see the Perspective by Glassmeier). Magnetic reconnection is a process whereby the interplanetary magnetic field lines join the magnetospheric field lines and transfer energy from the solar wind into the magnetosphere. Slavin et al. (p. 606) report observations of intense magnetic reconnection 10 times as intense as that of Earth. McClintock et al. (p. 610) describe simultaneous, high-resolution measurements of Mg, Ca, and Na in Mercurys exosphere, which may shed light on the processes that create and maintain the exosphere. Mercury’s magnetosphere responds more strongly to the influence of the Sun’s magnetic field than does Earth’s magnetosphere. Solar wind energy transfer to planetary magnetospheres and ionospheres is controlled by magnetic reconnection, a process that determines the degree of connectivity between the interplanetary magnetic field (IMF) and a planet’s magnetic field. During MESSENGER’s second flyby of Mercury, a steady southward IMF was observed and the magnetopause was threaded by a strong magnetic field, indicating a reconnection rate ~10 times that typical at Earth. Moreover, a large flux transfer event was observed in the magnetosheath, and a plasmoid and multiple traveling compression regions were observed in Mercury’s magnetotail, all products of reconnection. These observations indicate that Mercury’s magnetosphere is much more responsive to IMF direction and dominated by the effects of reconnection than that of Earth or the other magnetized planets.

MESSENGER Observations of Extreme Loading and Unloading of Mercury's Magnetic Tail

MESSENGERs Third Set of Messages MESSENGER, the spacecraft en route to insertion into orbit about Mercury in March 2011, completed its third flyby of the planet on 29 September 2009. Prockter et al. (p. 668, published online 15 July) present imaging data acquired during this flyby, showing that volcanism on Mercury has extended to much more recent times than previously assumed. The temporal extent of volcanic activity and, in particular, the timing of most recent activity had been missing ingredients in the understanding of Mercurys global thermal evolution. Slavin et al. (p. 665, published online 15 July) report on magnetic field measurements made during the 29 September flyby, when Mercurys magnetosphere underwent extremely strong coupling with the solar wind. The planets tail magnetic field increased and then decreased by factors of 2 to 3.5 during periods lasting 2 to 3 minutes. These observations suggest that magnetic open flux loads the magnetosphere, which is subsequently unloaded by substorms—magnetic disturbances during which energy is rapidly released in the magnetotail. At Earth, changes in tail magnetic field intensity during the loading/unloading cycle are much smaller and occur on much longer time scales. Vervack et al. (p. 672, published online 15 July) used the Mercury Atmospheric and Surface Composition Spectrometer onboard MESSENGER to make measurements of Mercurys neutral and ion exospheres. Differences in the altitude profiles of magnesium, calcium, and sodium over the north and south poles of Mercury indicate that multiple processes are at play to create and maintain the exosphere. Relative to Earth, Mercury’s magnetospheric substorms are more intense and occur on shorter time scales. During MESSENGER’s third flyby of Mercury, the magnetic field in the planet’s magnetic tail increased by factors of 2 to 3.5 over intervals of 2 to 3 minutes. Magnetospheric substorms at Earth are powered by similar tail loading, but the amplitude is lower by a factor of ~10 and typical durations are ~1 hour. The extreme tail loading observed at Mercury implies that the relative intensity of substorms must be much larger than at Earth. The correspondence between the duration of tail field enhancements and the characteristic time for the Dungey cycle, which describes plasma circulation through Mercury’s magnetosphere, suggests that such circulation determines the substorm time scale. A key aspect of tail unloading during terrestrial substorms is the acceleration of energetic charged particles, but no acceleration signatures were seen during the MESSENGER flyby.

Mercury's Magnetosphere After MESSENGER's First Flyby

Observations by MESSENGER show that Mercurys magnetosphere is immersed in a comet-like cloud of planetary ions. The most abundant, Na+, is broadly distributed but exhibits flux maxima in the magnetosheath, where the local plasma flow speed is high, and near the spacecrafts closest approach, where atmospheric density should peak. The magnetic field showed reconnection signatures in the form of flux transfer events, azimuthal rotations consistent with Kelvin-Helmholtz waves along the magnetopause, and extensive ultralow-frequency wave activity. Two outbound current sheet boundaries were observed, across which the magnetic field decreased in a manner suggestive of a double magnetopause. The separation of these current layers, comparable to the gyro-radius of a Na+ pickup ion entering the magnetosphere after being accelerated in the magnetosheath, may indicate a planetary ion boundary layer.

MESSENGER Observations of the Spatial Distribution of Planetary Ions Near Mercury

The polar regions of Mercury are important sources of material for its ionized exosphere. Global measurements by MESSENGER of the fluxes of heavy ions at Mercury, particularly sodium (Na+) and oxygen (O+), exhibit distinct maxima in the northern magnetic-cusp region, indicating that polar regions are important sources of Mercury’s ionized exosphere, presumably through solar-wind sputtering near the poles. The observed fluxes of helium (He+) are more evenly distributed, indicating a more uniform source such as that expected from evaporation from a helium-saturated surface. In some regions near Mercury, especially the nightside equatorial region, the Na+ pressure can be a substantial fraction of the proton pressure.

MESSENGER and Mariner 10 Flyby Observations of Magnetotail Structure and Dynamics at Mercury

increasing antisunward distance ∣X∣, B � ∣X∣ G , with G varying from � 5.4 for northward to � 1.6 for southward IMF. Low-latitude boundary layers (LLBLs) containing strong northward magnetic field were detected at the tail flanks during two of the flybys. The observed thickness of the LLBL was � 33% and 16% of the radius of the tail during M1 and M3, respectively, but the boundary layer was completely absent during M2. Clear signatures of tail reconnection are evident in the M2 and M3 magnetic field measurements. Plasmoids and traveling compression regions were observed during M2 and M3 with typical durations of � 1–3 s, suggesting diameters of � 500–1500 km. Overall, the response of Mercury’s magnetotail to the steady southward IMF during M2 appeared very similar to steady magnetospheric convection events at Earth, which are believed to be driven by quasi-continuous reconnection. In contrast, the M3 measurements are dominated by tail loading and unloading events that resemble the large-scale magnetic field reconfigurations observed during magnetospheric substorms at Earth.

The formation of the wall region: Consequences in the near Earth magnetotail

This paper discusses important new findings obtained from global kinetic simulations of magnetotail plasma. A region of strongly non-adiabatic ion acceleration (known as the [open quotes]wall[close quotes] region) exists in the near Earth tail and demarcates two very different regimes of ion motion: Adiabatic and quasiadiabatic. A strong enhancement of the cross-tail current occurs on the tailward side of the wall. The authors comparison of numerical and adiabatic pressure profiles indicates that non-adiabatic processes operating in this region may contribute significantly to a pressure balance relief in the course of quasisteady magnetospheric convection. 23 refs., 4 figs.

Wave-particle interactions in the equatorial source region of whistler-mode emissions

Wave-particle interactions can play a key role in the process of transfer of energy between different electron populations in the outer Van Allen radiation belt. We present a case study of wave-particle interactions in the equatorial source region of whistler-mode emissions. We select measurements of the Cluster spacecraft when these emissions are observed in the form of random hiss with only occasional discrete chorus wave packets, and where the wave propagation properties are very similar to previously analyzed cases of whistler-mode chorus. We observe a positive divergence of the Poynting flux at minima of the magnetic field modulus along the magnetic field lines, indicating the central position of the source. In this region we perform a linear stability analysis based on the locally measured electron phase space densities. We find two unstable electron populations. The first of them consists of energy-dispersed and highly anisotropic injected electrons at energies of a few hundreds eV to a few keV, with the perpendicular temperature more than 10 times higher than the parallel temperature with respect to the magnetic field line. Another unstable population is formed by trapped electrons at energies above 10 keV. We show that the injected electrons at lower energies can be responsible for a part of the waves that propagate obliquely at frequencies above one half of the electron cyclotron frequency. Our model of the trapped electrons at higher energies gives insufficient growth of the waves below one half of the electron cyclotron frequency and a nonlinear generation mechanism might be necessary to explain their presence even in this simple case.

MESSENGER observations of large flux transfer events at Mercury

Six flux transfer events (FTEs) were encountered during MESSENGERs first two flybys of Mercury (M1 and M2). For M1 the interplanetary magnetic field (IMF) was predominantly northward and four FTEs with durations of 1 to 6 s were observed in the magnetosheath following southward IMF turnings. The IMF was steadily southward during M2, and an FTE 4 s in duration was observed just inside the dawn magnetopause followed approx. 32 s later by a 7 s FTE in the magnetosheath. Flux rope models were fit to the magnetic field data to determine FTE dimensions and flux content. The largest FTE observed by MESSENGER had a diameter of approx. 1 R(sub M) (where R(sub M) is Mercury s radius), and its open magnetic field increased the fraction of the surface exposed to the solar wind by 10 - 20 percent and contributed up to approx. 30 kV to the cross-magnetospheric electric potential.

Excitation of electron acoustic waves near the electron plasma frequency and at twice the plasma frequency

In this paper we investigate the nonlinear development of the electron acoustic instability that can lead to the transfer of wave energy to frequencies just above the electron plasma frequency (ωpe) and to waves with approximately twice the electron plasma frequency (2ωpe). Using plasma conditions in the upstream electron foreshock region based on data from the AMPTE-UKS spacecraft, an electron beam is considered in plasma containing a background of hot and cold electrons. This leads to the linear excitation of large-amplitude electron acoustic waves at frequencies between about 0.8 and 1.0 ωpe. A modified decay instability then excites waves in the spectrum just above ωpe. This is followed by a second nonlinear coalescence process that causes the excitation of waves at frequencies just below 2ωpe. The linear and nonlinear properties of the electron acoustic instability are examined for observed conditions using analytical theory, particle-in-cell simulations, and Vlasov simulations. These results have application to observations made inside the electron foreshock region, as well as the polar cap and auroral zone, where plasma oscillations and waves at 2ωpe are observed.

On the origin of the ion-electron temperature difference in the plasma sheet

The results of a study of proton and electron acceleration in the Earths magnetotail are presented. By following the trajectories of thousands of charged particles launched from mantle and tail lobe source regions, distribution functions are calculated at different locations in a model magnetotail. The magnetic field is based on the Tsyganenko [1989] model combined with a constant cross-tail convection electric field. Despite the simplicity of the model and the lack of self-consistent fields, a qualitative picture of the proton/electron plasma sheet emerges including an ion to electron temperature ratio Ti/Te ranging from about 4 to 6 in the magnetotail, in approximate agreement with plasma sheet observations. To explain this result, an analytic expression for Ti/Te is derived based on the particle motion of ions and electrons in a current sheet configuration with a varying normal magnetic field component. The derived expression depends on the ion to electron mass ratio to the one-third power (mi/me)1/3 and a factor that takes the local field gradient into account. Using numerical values from the Tsyganenko [1989] field model in the derived equation gives Ti/Te ∼ 5. Another result is that the heated electron distribution functions formed in the plasma sheet are not Maxwellian but instead have power law high-energy tails much like the so-called “kappa” distributions reported by Christon et al. [1989]. At the edge of the plasma sheet, the calculations show the electron plasma sheet boundary layer extends further towards the lobe than the ion plasma sheet boundary layer, also in agreement with observations [Takahashi and Hones, 1988]. Nonisotropic distribution functions form at different locations in the plasma sheet, including electron beams streaming along field lines just inside the separatrix in the deep magnetotail. Electron distributions that are highly skewed in velocity space are found very near the magnetic null point. The nonisotropic distributions suggest that plasma instabilities and wave-particle interactions could occur in those regions. That such a simple model should reproduce many of the features of the observed plasma sheet indicates that adiabatic and nonadiabatic single-particle motion play important roles in the quiet time magnetotail and suggests that ion and electron plasma sheet formation is a natural consequence of single-particle motion in an X line type magnetotail geometry.