G. Gloeckler

The MESSENGER mission to Mercury: Scientific objectives and implementation

Abstract Mercury holds answers to several critical questions regarding the formation and evolution of the terrestrial planets. These questions include the origin of Mercurys anomalously high ratio of metal to silicate and its implications for planetary accretion processes, the nature of Mercurys geological evolution and interior cooling history, the mechanism of global magnetic field generation, the state of Mercurys core, and the processes controlling volatile species in Mercurys polar deposits, exosphere, and magnetosphere. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission has been designed to fly by and orbit Mercury to address all of these key questions. After launch by a Delta 2925H-9.5, two flybys of Venus, and two flybys of Mercury, orbit insertion is accomplished at the third Mercury encounter. The instrument payload includes a dual imaging system for wide and narrow fields-of-view, monochrome and color imaging, and stereo; X-ray and combined gamma-ray and neutron spectrometers for surface chemical mapping; a magnetometer; a laser altimeter; a combined ultraviolet–visible and visible-near-infrared spectrometer to survey both exospheric species and surface mineralogy; and an energetic particle and plasma spectrometer to sample charged species in the magnetosphere. During the flybys of Mercury, regions unexplored by Mariner 10 will be seen for the first time, and new data will be gathered on Mercurys exosphere, magnetosphere, and surface composition. During the orbital phase of the mission, one Earth year in duration, MESSENGER will complete global mapping and the detailed characterization of the exosphere, magnetosphere, surface, and interior.

Composition of quasi‐stationary solar wind flows from Ulysses/Solar Wind Ion Composition Spectrometer

Using improved, self-consistent analysis techniques, we determine the average solar wind charge state and elemental composition of nearly 40 ion species of He, C, N, O, Ne, Mg, Si, S, and Fe observed with the Solar Wind Ion Composition Spectrometer on Ulysses. We compare results obtained during selected time periods, including both slow solar wind and fast streams, concentrating on the quasi-stationary flows away from recurrent or intermittent disturbances such as corotating interaction regions or coronal mass ejections. In the fast streams the charge state distributions are consistent with a single freezing-in temperature for each element, whereas in the slow wind these distributions appear to be composed of contributions from a range of temperatures. The elemental composition shows the well-known first ionization potential (FIP) bias of the solar wind composition with respect to the photosphere. However, it appears that our average enrichment factor of low-FIP elements in the slow wind, not quite a factor of 3, is smaller than that in previous compilations. In fast streams the FIP bias is found to be yet smaller but still significantly above 1, clearly indicating that the FIP fractionation effect is also active beneath coronal holes from where the fast wind originates. This imposes basic requirements upon FIP fractionation models, which should reproduce the stronger and more variable low-FIP bias in the slow wind and a weaker (and perhaps conceptually different) low-FIP bias in fast streams. Taken together, these results firmly establish the fundamental difference between the two quasi-stationary solar wind types.

Global Observations of the Interstellar Interaction from the Interstellar Boundary Explorer (IBEX)

Whats Happening in the Heliosphere The influence of the Sun is felt well beyond the orbits of the planets. The solar wind is a stream of charged particles emanating from the Sun that carves a bubble in interstellar space known as the heliosphere and shrouds the entire solar system. The edge of the heliosphere, the region where the solar wind interacts with interstellar space, is largely unexplored. Voyager 1 and 2 crossed this boundary in 2004 and 2007, respectively, providing detailed but only localized information. In this issue (see the cover), McComas et al. (p. 959, published online 15 October), Fuselier et al. (p. 962, published online 15 October), Funsten et al. (p. 964, published online 15 October), and Möbius et al. (p. 969, published online 15 October) present data taken by NASAs Interstellar Boundary Explorer (IBEX). Since early 2009, IBEX has been building all-sky maps of the emissions of energetic neutral atoms produced at the boundary between the heliosphere and the interstellar medium. These maps have unexpectedly revealed a narrow band of emission that bisects the two Voyager locations at energies ranging from 0.2 to 6 kiloelectron volts. Emissions from the band are two- to threefold brighter than outside the band, in contrast to current models that predict much smaller variations across the sky. By comparing the IBEX observations with models of the heliosphere, Schwadron et al. (p. 966, published online 15 October) show that to date no model fully explains the observations. The model they have developed suggests that the interstellar magnetic field plays a stronger role than previously thought. In addition to the all-sky maps, IBEX measured the signatures of H, He, and O flowing into the heliosphere from the interstellar medium. In a related report, Krimigis et al. (p. 971, published online 15 October) present an all-sky image of energetic neutral atoms with energies ranging between 6 and 13 kiloelectron volts obtained with the Ion and Neutral Camera onboard the Cassini spacecraft orbiting Saturn. It shows that parts of the structure observed by IBEX extend to high energies. These data indicate that the shape of the heliosphere is not consistent with that of a comet aligned in the direction of the Suns travel through the galaxy as was previously thought. Observations by the Interstellar Boundary Explorer have revealed surprising features in the interaction between the heliosphere and the interstellar medium. The Sun moves through the local interstellar medium, continuously emitting ionized, supersonic solar wind plasma and carving out a cavity in interstellar space called the heliosphere. The recently launched Interstellar Boundary Explorer (IBEX) spacecraft has completed its first all-sky maps of the interstellar interaction at the edge of the heliosphere by imaging energetic neutral atoms (ENAs) emanating from this region. We found a bright ribbon of ENA emission, unpredicted by prior models or theories, that may be ordered by the local interstellar magnetic field interacting with the heliosphere. This ribbon is superposed on globally distributed flux variations ordered by both the solar wind structure and the direction of motion through the interstellar medium. Our results indicate that the external galactic environment strongly imprints the heliosphere.

Direct observation of He+ pick-up ions of interstellar origin in the solar wind

Singly-ionized helium with a velocity distribution extending up to double the solar wind velocity has been detected in interplanetary space. This distribution unambiguously determines the source: interstellar neutrals, ionized and accelerated in the solar wind. The observed significant flux increase in early December is due to the gravitational focusing of the interstellar neutral wind on the downwind side of the Sun.

A magnetic cloud containing prominence material: January 1997

This work discusses the relations among (1) an interplanetary force-free magnetic cloud containing a plug of cold high-density material with unusual composition, (2) a coronal mass ejection (CME), (3) an eruptive prominence, and (4) a model of prominence material supported by a force-free magnetic flux rope in a coronal streamer. The magnetic cloud moved past the Wind spacecraft located in the solar wind upstream of Earth on January 10 and 11, 1997. The magnetic field configuration in the magnetic cloud was approximately a constant-α, force-free flux rope. The 4He++/H+ abundance in the most of the magnetic cloud was similar to that of the streamer belt material, suggesting an association between the magnetic cloud and a helmet streamer. A very cold region of exceptionally high density was detected at the rear of the magnetic cloud. This dense region had an unusual composition, including (1) a relatively high (10%) 4He++/He+ abundance (indicating a source near the photosphere), and (2) 4He+, with an abundance relative to 4He++ of ∼1%, and the unusual charge states of O5+ and Fe5+ (indicating a freezing-in temperature of (1.6–4.0) × 105 °K, which is unusually low, but consistent with that expected for prominence material). Thus we suggest that the high-density region might be prominence material. The CME was seen in the solar corona on January 6, 1997, by the large angle and spectrometric coronagraph (LASCO) instrument on SOHO shortly after an eruptive prominence. A helmet streamer was observed near the latitude of the eruptive prominence a quarter of a solar rotation before and after the eruptive prominence. These observations are consistent with recent models, including the conceptual model of Low and Hundhausen [1995] for a quasi-static helmet streamer containing a force-free flux rope which supports prominence material and the dynamical model of Wu et al. [1997] for CMEs produced by the disruption of such a configuration.

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.

Acceleration of interstellar pickup ions in the disturbed solar wind observed on Ulysses

Acceleration of interstellar pickup H+ and He+ as well as of solar wind protons and alpha particles has been observed on Ulysses during the passage of a corotating interaction region (CIR) at ∼4.5 AU. Injection efficiencies for both the high thermal speed interstellar pickup ions (H+ and He+) and the low thermal speed solar wind ions (H+ and He++) are derived using velocity distribution functions of protons, pickup He+ and alpha particles from < 1 to 60 keV/e and of ions (principally protons) above ∼60 keV. The observed spatial variations of the few keV and the few hundred keV accelerated pickup protons across the forward shock of the CIR indicate a two stage acceleration mechanism. Thermal ions are first accelerated to speeds of 3 to 4 times the solar wind speed inside the CIR, presumably by some statistical mechanism, before reaching higher energies by a shock acceleration process. Our results also indicate that (1) the injection efficiencies for pickup ions are almost 100 times higher than they are for solar wind ions, (2) pickup H+ and He+ are the two most abundant suprathermal ion species and they carry a large fraction of the particle thermal pressure, (3) the injection efficiency is highest for protons, lowest for He+, and intermediate for alpha particles, (4) both H+ and He+ have identical spectral shapes above the cutoff speed for pickup ions, and (5) the solar wind frame velocity distribution function of protons has the form F(w) = F0w−4 for 1 < w < ∼5, where w is the ion speed divided by the solar wind speed. Above w ∼ 5-10 the proton spectrum becomes steeper. These results have important implications concerning acceleration of ions by shocks and CIRs, acceleration of anomalous cosmic rays, and particle dynamics in the outer heliosphere.

The Ultraviolet Coronagraph Spectrometer for the Solar and Heliospheric Observatory

The SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) is composed of three reflecting telescopes with external and internal occultation and a spectrometer assembly consisting of two toric grating spectrometers and a visible light polarimeter. The purpose of the UVCS instrument is to provide a body of data that can be used to address a broad range of scientific questions regarding the nature of the solar corona and the generation of the solar wind. The primary scientific goals are the following: to locate and characterize the coronal source regions of the solar wind, to identify and understand the dominant physical processes that accelerate the solar wind, to understand how the coronal plasma is heated in solar wind acceleration regions, and to increase the knowledge of coronal phenomena that control the physical properties of the solar wind as determined by in situ measurements. To progress toward these goals, the UVCS will perform ultraviolet spectroscopy and visible polarimetry to be combined with plasma diagnostic analysis techniques to provide detailed empirical descriptions of the extended solar corona from the coronal base to a heliocentric height of 12 solar radii.

ORIGIN OF THE SOLAR WIND FROM COMPOSITION DATA

The ESA/NASA spacecraft Ulysses is making, for the first time, direct measurements in the solar wind originating from virtually all places where the corona expands. Since the initial two polar passes of Ulysses occur during relatively quiet solar conditions, we discuss here the three main regimes of quasi-stationary solar wind flow: the high speed streams (HSSTs) coming out of the polar coronal holes, the slow solar wind surrounding the HSSTs, and the streamers which occur at B-field reversals. Comparisons between H-α maps and data taken by Ulysses demonstrate that as a result of super-radial expansion, the HSSTs occupy a much larger solid angle than that derived from radial projections of coronal holes. Data obtained with SWICS-Ulysses confirm that the strength of the FIP effect is much reduced in the HSSTs. The systematics in the variations of elemental abundances becomes particularly clear, if these are plotted against the time of ionisation (at the solar surface) rather than against the first ionisation potential (FIP). We have used a superposed-epoch method to investigate the changes in solar wind speed and composition measured during the 9-month period in 1992/93 when Ulysses regularly passed into and out of the southern HSST. We find that the patterns in the variations of the Mg/O and O7+/O6+ ratios are virtually identical and that their transition from high to low values is very steep. Since the Mg/O ratio is controlled by the FIP effect and the O7+/O6+ ratio reflects the coronal temperature, this finding points to a connection between chromospheric and coronal conditions.

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.