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Apollo lunar samples reveal that the Moon generated its own global magnetosphere, lasting from ~4.25 to ~2.5 billion years (Ga) ago. At peak lunar magnetic intensity (4 Ga ago), the Moon was volcanically active, likely generating a very tenuous atmosphere, and, it is believed, was at a geocentric distance of ~18 Earth radii (RE). Solar storms strip a planet’s atmosphere over time, and only a strong magnetosphere would be able to provide maximum protection. We present simplified magnetic dipole field modeling confined within a paraboloidal-shaped magnetopause to show how the expected Earth-Moon coupled magnetospheres provide a substantial buffer from the expected intense solar wind, reducing Earth’s atmospheric loss to space.

We present the first observations of Mercury's plasma mantle, a primary region for solar wind entry into the planetary magnetosphere, located in the high-latitude magnetotail. MESSENGER observations from two orbits on 10 November 2012 have been analyzed. The main plasma mantle features are (1) a steady decrease in proton density as MESSENGER moved deeper into the magnetotail; (2) frequent flux transfer events throughout the magnetosheath and into the magnetotail, suggesting that these events are the primary source for solar wind plasma injection; (3) a diamagnetic depression, due to the presence of plasma, as pressure balance is maintained; and (4) a clear proton velocity dispersion, resulting from lower-energy protons being transported deep into the magnetosphere as higher-energy protons escape downtail. From these velocity dispersions we infer cross-magnetosphere electric potentials of 23 kV and 29 kV, consistent with estimates determined from measurements of magnetopause reconnection rate and tail loading and unloading events.

We present Venus Express observations of the impact of a slow interplanetary coronal mass ejection (ICME), which struck Venus on 23 December 2006, creating unusual quasi steady state upstream conditions for the 2 h close to periapsis: an enhanced (∼ nT) interplanetary magnetic field (IMF), radially aligned with the Sun-Venus line; and a dense (∼ cm−3) solar wind. Contrary to our current understanding and expectations, the ionosphere became partially demagnetized. We also find evidence for shocked sheathlike solar wind protons and electrons in the wake of Venus, and powerful (≈ nT2/Hz) foreshock whistler mode waves radiating from the bow shock at an unexpectedly low frequency (0.6 Hz). Given the abnormally high density of escaping heavy ions at the magnetopause boundary (295 cm−3, one of the highest of the whole mission) and the enhanced density of escaping heavy ions in the wake, we find that even weak ICMEs with no driving shocks can increase atmospheric loss rates at Venus and suggests that the Bx component of the IMF may be a factor in atmospheric escape rates.

CLJ and RMW acknowledge support from the Natural Sciences and Engineering Research Council of Canada, and CLJ acknowledges support from MESSENGER Participating Scientist grant NNX11AB84G. 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.

[1] We here document with magnetic field observations a passage of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft through Mercury's magnetosphere under conditions of a quasi-parallel bow shock, i.e., when the direction of the upstream interplanetary magnetic field was within 45° of the bow shock normal. The spacecraft's fast transition of the magnetosheath and the steady solar wind conditions during the period analyzed allow both spatial and temporal properties of the shock crossing to be investigated. The observations show that the shock reformation process can be nearly periodic under stable solar wind conditions. Throughout the 25-min-long observation period, the pulsation duration deviated by at most ~10% from the average 10 s period measured. This quasiperiodicity allows us to study all aspects of the shock reconfiguration, including ultra-low-frequency waves in the upstream region and large-amplitude magnetic structures observed in the vicinity of the magnetosheath-solar wind transition region and inside the magnetosheath. We also show that bow shock reformation can be a substantial source of wave activity in the magnetosphere, on this occasion having given rise to oscillations in the magnetic field with peak-to-peak amplitudes of 40–50 nT over large parts of the dayside magnetosphere. The clean and cyclic behavior observed throughout the magnetosphere, the magnetosheath, and the upstream region indicates that the subsolar region was primarily influenced by a cyclic reformation of the shock front, rather than by a spatial and temporal patchwork of short large-amplitude magnetic structures, as is generally the case at the terrestrial bow shock under quasi-parallel conditions.

[1] Mercury's bow shock is unique in our solar system as it is produced by low Mach number solar wind blowing over a small magnetized body. The availability of MESSENGER orbiter data enables us for the first time to conduct an in-depth study of upstream waves in Mercury's foreshock. This paper reports first results of an observational study of upstream ULF waves in Mercury's foreshock using high-time resolution magnetic field data from the MESSENGER spacecraft to understand the general morphology of these waves. We find that the most common wave phenomenon in Mercury's foreshock has frequencies ~ 2 Hz, with properties similar to the 1 Hz whistler waves in the Earth's foreshock. Their generation appears to be generic to the shock and not affected by the weak strength and small size of Mercury's bow shock. On the other hand, the most common wave phenomenon in the Earth's foreshock is the large-amplitude 30 second waves, identified as fast magnetosonic waves generated by backstreaming ions. Similar waves at Mercury have wave frequencies at ~ 0.3 Hz, but occur only sporadically. The general lack of strong “30 second” magnetosonic waves at Mercury can be attributed to the lack of strong backstreaming ions due to a weak bow shock and not enough time for wave growth due to the small foreshock size. Superposed on the “1 Hz” whistler waves, there are short bursts of spectral peaks at ~ 0.8 Hz that are new and have not been reported previously in Mariner 10 data.

We present results from a new three-dimensional empirical magnetopause model based on 15,089 magnetopause crossings from 23 spacecraft. To construct the model, we introduce a Support Vector Regression Machine (SVRM) technique with a systematic approach that balances model smoothness with fitting accuracy to produce a model that reveals the manner in which the size and shape of the magnetopause depend upon various control parameters without any assumptions concerning the analytical shape of the magnetopause. The new model fits the data used in the modeling very accurately, and can guarantee a similar accuracy when predicting unseen observations within the applicable range of control parameters. We introduce a new error analysis technique based upon the SVRM that enables us to obtain model errors appropriate to different locations and control parameters. We find significant east-west elongations in the magnetopause shape for many combinations of control parameters. Variations in the Earth's dipole tilt can cause significant magnetopause north/south asymmetries and deviation of the magnetopause nose from the Sun-Earth line nonlinearly by as much as 5Re. Subsolar magnetopause erosion effect under southward IMF is seen which is strongly affected by solar wind dynamic pressure. Further, we find significant shrinking of high-latitude magnetopause with decreased magnetopause flaring angle during northward IMF.

We report the simultaneous detection of a terrestrial myriametric radio burst (TMRB) by IMAGE and Geotail on 19 August 2001. The TMRB was confined in time (0830-1006 UT) and frequency (12-50kHz). Comparisons with all known nonthermal myriametric radiation components reveal that the TMRB might be a distinct radiation with a source that is unrelated to the previously known radiation. Considerations of beaming from spin-modulation analysis and observing satellite and source locations suggest that the TMRB may have a fan beamlike radiation pattern emitted by a discrete, dayside source located along the poleward edge of magnetospheric cusp field lines. TMRB responsiveness to IMF Bz and By orientations suggests that a possible source of the TMRB could be due to dayside magnetic reconnection instigated by northward interplanetary field condition.

[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 � 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 Alfven 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 reconnectionatMercuryisnotonlymoreintensethanatEarthbutalsothatitoccursfornearly allorientationsofthe interplanetarymagnetic field duetothe low-bnatureof thesolar windin the inner heliosphere.

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-f1uid 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 P(sub sw) = 6.8 nPa and a planetary magnetic dipole having a magnitude of 210 R(sub M)(exp 3)- nT and an offset of 0.18 R(sub M) to the north of the equator, where R(sub M) 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 n011hern 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.

The Kelvin-Helmholtz instability is believed to be an important means for the transfer of energy, plasma, and momentum from the solar wind into planetary magnetospheres, with in situ measurements r ...

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 planet's orbit in 2011. Another spacecraft visited Mercury in the mid-1970s, which mapped 45% of the planet's surface. Now, after MESSENGER, only 10% of Mercury's surface remains to be imaged up close. Denevi et al. (p. 613 ) use this near-global data to look at the mechanisms that shaped Mercury's crust, which likely formed by eruption of magmas of different compositions over a long period of time. Like the Moon, Mercury's 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 Mercury's contraction as its interior cooled over time. Mercury's 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 Mercury's exosphere, which may shed light on the processes that create and maintain the exosphere.

We have developed a method to despin data from the Hawkeye spacecraft (1974-1978) that involves analyzing the variation of solar array voltage with sun phase angle. This new method is needed because the optical aspect system on the Hawkeye spacecraft failed four months into its mission, after which the spacecraft spin index could be determined only for limited periods using two other methods. Adding our new solar array technique nearly doubles the available data. The Hawkeye mission was designed to study high-latitude, high-altitude magnetosphere (exterior cusp, surrounding magnetopause and boundary layers, the polar cap, and Auroral field lines). The scientific payload was designed to measure the in situ plasma, magnetic, and electric fields. Besides Hawkeye, only the Highly Eccentric Orbiting Satellite-2 (HEOS-2) systematically explored this region of the Earth's geospace. Determining the spin index for the rest of the data, and subsequently despinning these data and representing them in a geophysically meaningful coordinate system, is still important due to the uniqueness of Hawkeye's orbit. The solar array method presented is based on searching parameter space for the solution that results in the best self-organization in spin index of the solar array voltage measurements. The spin index computed using this method is compared against the spin index determined from other methods. The resulting despun magnetic field data are compared with magnetic field models and with in situ magnetic field measurements in the solar wind made by the IMP-8 spacecraft.

We report the observational results of upstream ULF waves in the Mercury's foreshock using high-time resolution magnetic field data, 20 samples per second, from the MESSENGER spacecraft. The Mercury's bow shock is unique in our solar system as it is produced by low Mach number solar wind blowing over a small magnetized body with a predominately radial interplanetary magnetic field. Our study has showed the existence of at least three types of upstream waves: 1) whistler waves at frequencies near 2 Hz, similar to the 1-Hz waves at the Earth; 2) waves with frequencies ∼ 0.3 Hz, similar to the large-amplitude 30-s waves at the Earth; 3) fluctuations with spectral peaks centered at ∼ 0.8 Hz. Unlike the Earth's foreshock where the most prominent upstream wave phenomenon is locally generated large-amplitude 30-s magnetosonic waves, the most common foreshock waves are whistler waves generated at the bow shock, with properties similar to the 1-Hz waves in the Earth's foreshock. Their occurrence characteristics show that the “1-Hz” wave generation is generic to the bow shock and not affected by the strength and size of the shock at Mercury. On the other hand, the “30-s” magnetosonic waves at Mercury occur only sporadically and with small amplitudes. The general lack of strong “30-s” magnetosonic waves at Mercury can be attributed to the lack of strong backstreaming ions due to a weak bow shock and not enough time for wave growth due to the small foreshock size. Superposed on the “1-Hz” whistler waves, there are short bursts of spectral peaks at ∼ 0.8 Hz that are new and have not been reported previously in Mariner 10 data. The source of the ∼ 0.8 Hz waves remains to be identified.

A quantitative, static, empirical model of the high-latitude magnetopause is developed for GSM coordinates and parameterized by dipole tilt angle (ψ), solar wind pressure, and interplanetary magnetic field (IMF) Bz. We fit 691 high-latitude magnetopause crossings by the Hawkeye 1 spacecraft to a generalized second-order surface using only crossings for which both solar wind pressure and IMF data are available. These Northern Hemisphere crossings are shown to lie within the spatial coverage of Hawkeye for different bins of ψ spanning the range of −35° to 35°, demonstrating that the independence of the crossings is not due to a bias in coverage. At high latitudes, solar wind pressure and ψ are found to be of major and equal importance in modeling magnetopause position. In the Northern Hemisphere the high-latitude magnetopause is displaced outward for positive ψ and inward for negative ψ. Additional inward displacement of the magnetopause surface is reduced for extreme negative ψ values. IMF Bz dependence is separable only after the effects of ψ and pressure are removed. The radial dependence on IMF Bz weakens near the cusp and becomes stronger antisunward of the cusp, where the magnetopause is displaced outward for negative IMF Bz, and inward for positive IMF Bz. This is consistent with findings along the low-latitude flanks. Both AE and Dst dependencies are found in the high-latitude magnetopause crossings after removing ψ and pressure dependencies from the crossings. This model is only valid at high latitudes, antisunward of the cusp, out to a xGSM value of about −5 Re. The ψ dependence of the nose is also modeled using a subset of magnetopause crossings from Roelof and Sibeck [1993] along with Hawkeye crossings below the cusp region. For positive ψ the most Sunward point of the nose is displaced below the xGSM-yGSM plane. Both the nose model and the high-latitude model are in reasonable agreement with the theoretical model of Sotirelis and Meng [1999].

Hawkeye plasma, magnetic field, and plasma wave instruments directly sampled the throat of the northern polar cusp as the orientation of the Interplanetary Magnetic Field (IMF) changed from southward to northward on July 3, 1974. Two distinct regions in the polar cusp were identified based on magnetic field, plasma flow and magnetic and electric noise: the interior and exterior cusps. The observations show highly variable flows in the exterior portion of the cusp and constantly strong dawn-dusk flows in the interior portion during periods of strong IMF By component. Results of a minimum variance analysis of the magnetic field at each cusp interface crossing provides evidence that the magnetopause surface normal deviated highly from empirical models. During intervals of relatively steady solar wind dynamic pressure, the motion of the cusp relative to the slow moving spacecraft was modulated by the varying IMF clock angle as observed by IMP 8 in the upstream solar wind. The motion did not show a correlation with internal processes monitored by the A E index. We propose that observed plasma flow patterns and cusp motion are results of reconnection between the IMF and the magnetospheric magnetic field. Flow velocity observed in the interior cusp is consistent with stress balance for a reconnection process. This unique interval provides an opportunity for detailed studies of the plasma, magnetic field, and plasma wave properties in both the exterior and interior cusp.

Reconnection is accepted as an important process for driving the solar wind/magnetospheric interaction although it is not fully understood. In particular, reconnection for northward interplanetary magnetic field (IMF) at high-latitudes tailward of the cusp, has received little attention in comparison with equatorial reconnection for southward IMF. Using Hawkeye data we present the first direct observations of reconnection at the high-latitude magnetopause (75°) during northward IMF in the form of sunward flowing protons. This flow is nearly field aligned, approximately Alfvenic, and roughly obeys tangential momentum balance. The magnetic field shear is large at the magnetopause and there is a non-zero BN component suggesting the existence of a rotational discontinuity and reconnection. The Hawkeye observations support several recent simulations at least qualitatively in terms of flow directions expected for high-latitude reconnection during northward IMF.

We present a multi-instrument study of a hot flow anomaly (HFA) observed by the Venus Express spacecraft in the Venusian foreshock, on 22 March 2008, incorporating both Venus Express Magnetometer and Analyzer of Space Plasmas and Energetic Atoms (ASPERA) plasma observations. Centered on an interplanetary magnetic field discontinuity with inward convective motional electric fields on both sides, with a decreased core field strength, ion observations consistent with a flow deflection, and bounded by compressive heated edges, the properties of this event are consistent with those of HFAs observed at other planets within the solar system.

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.

We use a nonstationary generalization of the higher-order structure function technique to investigate statistical properties of the magnetic field fluctuations recorded by MESSENGER spacecraft during its first flyby (01/14/2008) through the near-Mercury space environment, with the emphasis on key boundary regions participating in the solar wind - magnetosphere interaction. Our analysis shows, for the first time, that kinetic-scale fluctuations play a significant role in the Mercury's magnetosphere up to the largest resolvable timescale (approx.20 s) imposed by the signal nonstationariry, suggesting that turbulence at this plane I is largely controlled by finite Larmor radius effects. In particular, we report the presence of a highly turbulent and extended foreshock system filled with packets of ULF oscillations, broad-band intermittent fluctuations in the magnetosheath, ion-kinetic turbulence in the central plasma sheet of Mercury's magnetotail, and kinetic-scale fluctuations in the inner current sheet encountered at the outbound (dawn-side) magnetopause. Overall, our measurements indicate that the Hermean magnetosphere, as well as the surrounding region, are strongly affected by non-MHD effects introduced by finite sizes of cyclotron orbits of the constituting ion species. Physical mechanisms of these effects and their potentially critical impact on the structure and dynamics of Mercury's magnetic field remain to be understood.

[1] Using 2 Hz magnetic field data from the MESSENGER mission, we have investigated more than 300 strong narrowband ion cyclotron wave (ICW) events at a heliocentric distance of about 0.3 AU during 31 May to 9 June 2008. These nearly circularly polarized transverse waves are observed extensively and discretely in the solar wind, with a median duration of 21 s. They are preferentially observed when the magnetic field is more radial than the ambient solar wind. The waves appear both left-handed and right-handed in the spacecraft frame. Their wave frequencies in the spacecraft frame are generally larger than the local proton cyclotron frequency (fpc), with a median of 1.44 fpc. The wave power spectra do not cutoff at the local fpc. On the basis of their wave characteristics, we conclude that they are intrinsically left-handed in the solar wind frame and they are generated closer to the Sun and carried out to the spacecraft by the super Alfvenic solar wind. After removing the Doppler shift, the wave frequencies in the solar wind frame are all below the local fpc, with a median of 0.35 fpc. The ICWs propagate nearly parallel to the magnetic field, and the median wave amplitude is about 0.73 nT, 3% of the background magnetic field. We compare these observations with earlier Helios observations at 0.3 AU in 1976 and contemporary 1 AU observations.

During the third MESSENGER flyby of Mercury on 29 September 2009, 15 crossings of the dusk-side magnetopause were observed in the magnetic field data over a 2-min period, during which the spacecraft traveled a distance of 0.2 R(sub M) (where R(sub M) is Mercury's radius). The quasi-periodic nature of the magnetic field variations during the crossings, the characteristic time separations of approx.16 s between pairs of crossings, and the variations of the magnetopause normal directions indicate that the signals are likely the signature of surface waves highly steepened at their leading edge that arose from the Kelvin-Helmholtz instability. At Earth, the Kelvin- Helmholtz instability is believed to lead to the turbulent transport of solar wind plasma into Earth's plasma sheet. This solar wind entry mechanism could also be important at Mercury. Citation: Boardsen, S. A., T. Sundberg, J. A.Slavin, B. J. Anderson, H. Korth, S. C. Solomon, and L. G. Blomberg (2010), Observations of Kelvin-Helmholtz waves along the dusk-side boundary of Mercury s magnetosphere during MESSENGER's third flyby

Six flux transfer events (FTEs) were encountered during MESSENGER's 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.

At 23:08 UTC on 5 June 2007 the MESSENGER spacecraft reached its closest approach altitude of 338 kin during its final flyby of Venus en route to its 2011 orbit insertion at Mercury. The availability of the simultaneous Venus Express solar wind and interplanetary magnetic field measurements provides a rare opportunity to examine the influence of upstream conditions on this planet's solar wind interaction. We present MESSENGER observations of new features of the Venus - solar wind interaction including hot flow anomalies upstream of the bow shock, a flux rope in the near-tail and a two-point determination of the timescale for magnetic flux transport through this induced magnetosphere. Citation: Stavin, J. A., et al. (2009), MESSENGER and Venus Express observations of the solar wind interaction with Venus

[1] The second MESSENGER flyby of Venus on 5 June 2007 provided a new opportunity to study the response of the induced magnetosphere of the planet to changes in the direction of the interplanetary magnetic field (IMF). At the time of the MESSENGER flyby, the European Space Agency's Venus Express spacecraft was located outside the magnetosphere and provided a monitor of solar wind conditions. Measurements by the Venus Express magnetometer show that the IMF underwent four major changes in direction and magnitude while MESSENGER was traveling through the inner magnetosphere of Venus. The response of the magnetosphere to each of these IMF changes was determined with a semi-time-dependant global magnetohydrodynamic (MHD) model, and the results were compared with magnetic and compositional measurements by the Magnetometer (MAG) and the Energetic Particle and Plasma Spectrometer (EPPS) on MESSENGER. Our modeling results show that this semi-time-dependant MHD technique produces magnetic field profiles that can account for both of the field reversals seen by MAG and the peak in the pick-up ion density measured by EPPS. Moreover, these results reveal that the plasma sheet that confines most of the pick-up ions has a barred disk shape and continuously rotates along the Sun-planet axis to align its smallest dimension with the transverse direction of the IMF.

[1] MESSENGER's January 14, 2008, flyby of Mercury has provided new observations of the planet's magnetosphere for northward interplanetary magnetic field (IMF). The dusk magnetopause was located inward from the mean magnetopause surface, possibly due to reduced tail magnetic flux content for IMF Bz > 0 and/or the pressure of planetary pickup ions as they respond to the dawnward – v × B electric field in the magnetosheath. Within the plasma sheet rotations of the magnetic field are observed consistent with, Kelvin–Helmholtz vortices ∼1 RM in diameter (RM is Mercury's radius). MESSENGER exited through a 1,000 km-wide boundary layer bordered by inner and outer current sheets that resemble rotational and tangential discontinuities, respectively. The total magnetic field change across this layer is consistent with the predicted solar wind ram pressure at Mercury during the MESSENGER flyby.

[1] The magnetospheric electron number density and the magnetic field strength near 8 R E over the polar cap increased dramatically after the arrival of an interplanetary magnetic cloud on 31 March 2001. These parameters were determined with high accuracy from the plasma resonances stimulated by the Radio Plasma Imager (RPI) on Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) near apogee during both quiet (30 March 2001) and disturbed (31 March 2001) days. The quiet day and disturbed day values were each compared with magnetospheric magnetic field and electron density models; good agreement was found with the former but not the latter. The magnetospheric response was also expressed in terms of the ratio of the electron plasma frequency f pe to the electron cyclotron frequency f ce , which is proportional to the ratio of the electron gyroradius to the Debye radius. Simultaneous Wind measurements of the solar wind magnetic field strength, speed, and plasma density were used to calculate the solar wind quasi-invariant QI. This index is equivalent to the ratio of the solar wind magnetic pressure to the solar wind ram pressure or to the inverse of the magnetic Mach number squared. These nondimensional quantities, QI and f pe /f ce , have fundamental meanings in the solar wind MHD regime and in the relation between electric and magnetic forces on electrons in the magnetosphere, respectively. During the large 31 March 2001 storm, IMAGE was at the right place at the right time so as to enable comparisons between RPI f pe /f ce and Wind QI determinations. Both QI and f pe /f ce formed maxima during 6-hour observing intervals during this storm that were found to be highly correlated (87%) with a magnetospheric time lag of about 3 hours for f pe /f ce . These results, based on a detailed case study of this important event, suggest that the plasma parameter f pe /f ce may serve as a useful magnetospheric index.

[1] We report an event observed by the Low-Energy Neutral Atom (LENA) imager on 18 April 2001, in which enhanced neutral atom emission was observed coming from the direction of the Sun and from the general direction of the subsolar magnetopause. The enhanced neutral atom emission is shown to be primarily a result of increased solar wind charge exchange with the Earth's hydrogen exosphere, that is, enhanced neutral solar wind formation, occurring in conjunction with a southward turning of the interplanetary magnetic field (IMF) which moves the magnetopause closer to the Earth. It is shown that the neutral atom flux under compressed magnetopause conditions is extremely sensitive to changes in the IMF north-south component.