Daniel J. Gershman

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 observations of Mercury's dayside magnetosphere under extreme solar wind conditions

CLJ and nRMW acknowledge support from the nNatural Sciences and Engineering nResearch Council of Canada, and CLJ nacknowledges support from MESSENGER nParticipating Scientist grant nNNX11AB84G. The MESSENGER project nis supported by the NASA Discovery nProgram under contracts NASW- n00002 to the Carnegie Institution of nWashington and NAS5-97271 to The nJohns Hopkins University Applied nPhysics Laboratory.

Magnetic flux pileup and plasma depletion in Mercury’s subsolar magnetosheath

[1]xa0Measurements from the Fast Imaging Plasma Spectrometer (FIPS) and Magnetometer (MAG) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft during 40 orbits about Mercury are used to characterize the plasma depletion layer just exterior to the planets dayside magnetopause. A plasma depletion layer forms at Mercury as a result of piled-up magnetic flux that is draped around the magnetosphere. The low average upstream Alfvenic Mach number (MA ~3–5) in the solar wind at Mercury often results in large-scale plasma depletion in the magnetosheath between the subsolar magnetopause and the bow shock. Flux pileup is observed to occur downstream under both quasi-perpendicular and quasi-parallel shock geometries for all orientations of the interplanetary magnetic field (IMF). Furthermore, little to no plasma depletion is seen during some periods with stable northward IMF. The consistently low value of plasma β, the ratio of plasma pressure to magnetic pressure, at the magnetopause associated with the low average upstream MA is believed to be the cause for the high average reconnection rate at Mercury, reported to be nearly 3 times that observed at Earth. Finally, a characteristic depletion length outward from the subsolar magnetopause of ~300u2009km is found for Mercury. This value scales among planetary bodies as the average standoff distance of the magnetopause.

Distribution and compositional variations of plasma ions in Mercury's space environment: The first three Mercury years of MESSENGER observations

[1]xa0We have analyzed measurements of planetary ions near Mercury made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Fast Imaging Plasma Spectrometer (FIPS) over the first three Mercury years of orbital observations (25 March 2011 through 31 December 2011). We determined the composition and spatial distributions of the most abundant species in the regions sampled by the MESSENGER spacecraft during that period. In particular, we here focus on altitude dependence and relative abundances of species in a variety of spatial domains. We used observed density as a proxy for ambient plasma density, because of limitations to the FIPS field of view. We find that the average observed density is 3.9 × 10–2u2009cm–3 for He2+, 3.4 × 10–4u2009cm–3 for He+, 8.0 × 10–4u2009cm–3 for O+-group ions, and 5.1 × 10–3u2009cm–3 for Na+-group ions. Na+-group ions are particularly enhanced over other planetary ions (He+ and O+ group) in the northern magnetospheric cusp (by a factor of ~2.0) and in the premidnight sector on the nightside (by a factor of ~1.6). Within 30° of the equator, the average densities of all planetary ions are depressed at the subsolar point relative to the dawn and dusk terminators. The effect is largest for Na+-group ions, which are 49% lower in density at the subsolar point than at the terminators. This depression could be an effect of the FIPS energy threshold. The three planetary ion species considered show distinct dependences on altitude and local time. The Na+ group has the smallest e-folding height at all dayside local times, whereas He+ has the largest. At the subsolar point, the e-folding height for Na+-group ions is 590u2009km, and that for the O+ group and He+ is 1100u2009km. On the nightside and within 750u2009km of the geographic equator, Na+-group ions are enhanced in the premidnight sector. This enhancement is consistent with nonadiabatic motion and may be observational evidence that nonadiabatic effects are important in Mercurys magnetosphere.

Structure and dynamics of Mercury's magnetospheric cusp: MESSENGER measurements of protons and planetary ions

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft has observed the northern magnetospheric cusp of Mercury regularly since the probe was inserted into orbit about the innermost planet in March 2011. Observations from the Fast Imaging Plasma Spectrometer (FIPS) made at altitudesu2009 10u2009cm−3) that are exceeded only by those observed in the magnetosheath. These high plasma densities are also associated with strong diamagnetic depressions observed by MESSENGERs Magnetometer. Plasma in the cusp may originate from several sources: (1) Direct inflow from the magnetosheath, (2) locally produced planetary photoions and ions sputtered off the surface from solar wind impact and then accelerated upward, and (3) flow of magnetosheath and magnetospheric plasma accelerated from dayside reconnection X-lines. We surveyed 518 cusp passes by MESSENGER, focusing on the spatial distribution, energy spectra, and pitch-angle distributions of protons and Na+-group ions. Of those, we selected 77 cusp passes during which substantial Na+-group ion populations were present for a more detailed analysis. We find that Mercurys cusp is a highly dynamic region, both in spatial extent and plasma composition and energies. From the three-dimensional plasma distributions observed by FIPS, protons with mean energies of 1u2009keV were found flowing down into the cusp (i.e., source (1) above). The distribution of pitch angles of these protons showed a depletion in the direction away from the surface, indicating that ions within 40° of the magnetic field direction are in the loss cone, lost to the surface rather than being reflected by the magnetic field. In contrast, Na+-group ions show two distinct behaviors depending on their energy. Low-energy (100–300u2009eV) ions appear to be streaming out of the cusp, showing pitch-angle distributions with a strong component antiparallel to the magnetic field (away from the surface). These ions appear to have been generated in the cusp and accelerated locally (i.e., source (2) above). Higher-energy (≥1u2009keV) Na+-group ions in the cusp exhibit much larger perpendicular components in their energy distributions. During active times, as judged by frequent, large-amplitude magnetic field fluctuations, many more Na+-group ions are measured at latitudes south of the cusp. In several cases, these Na+-group ions in the dayside magnetosphere are flowing northward toward the cusp. The high mean energy, pitch-angle distributions, and large number of Na+-group ions on dayside magnetospheric field lines are inconsistent with direct transport into the cusp of sputtered ions from the surface or newly photoionized particles. Furthermore, the highest densities and mean energies often occur together with high-amplitude magnetic fluctuations, attributed to flux transfer events along the magnetopause. These results indicate that high-energy Na+-group ions in the cusp are likely formed by ionization of escaping neutral Na in the outer dayside magnetosphere and the magnetosheath followed by acceleration and transport into the cusp by reconnection at the subsolar magnetopause (i.e., source 3 above).

Solar Wind Alpha Particles and Heavy Ions in the Inner Heliosphere Observed with MESSENGER

obstructed by the spacecraft sunshade, a data analysis technique has been developed that recovers both bulk and thermal speeds to 10% accuracy and provides the first measurements of solar wind heavy ions (mass per charge >2 amu/e) at heliocentric distances within 0.5 AU. Solar wind alpha particles and heavy ions appear to have similar mean flow speeds at values greater than that of the protons by approximately 70% of the Alfven speed. From an examination of the thermal properties of alpha particles and heavier solar wind ions, we find a ratio of the temperature of alpha particles to that of protons nearly twice that of previously reported Helios observations, though still within the limits of excessive heating of heavy ions observed spectroscopically close to the Sun. Furthermore, examination of typical magnetic power spectra at the orbits of MESSENGER and at 1 AU reveals the lack of a strong signature of local resonant ion heating, implying that a majority of heavy ion heating could occur close to the Sun. These results demonstrate that the solar wind at � 0.3 AU is a blend of the effects of wave–particle interactions occurring in both the solar corona and the heliosphere.

Solar Wind Forcing at Mercury: WSA-ENLIL Model Results

[1]xa0Analysis and interpretation of observations from the MESSENGER spacecraft in orbit about Mercury require knowledge of solar wind “forcing” parameters. We have utilized the Wang-Sheeley-Arge (WSA)-ENLIL solar wind modeling tool in order to calculate the values of interplanetary magnetic field (IMF) strength (B), solar wind velocity (V) and density (n), ram pressure (~nV2), cross-magnetosphere electric field (Vu2009×u2009B), Alfven Mach number (MA), and other derived quantities of relevance for solar wind-magnetosphere interactions. We have compared upstream MESSENGER IMF and solar wind measurements to see how well the ENLIL model results compare. Such parameters as solar wind dynamic pressure are key for determining the Mercury magnetopause standoff distance, for example. We also use the relatively high-time-resolution B-field data from MESSENGER to estimate the strength of the product of the solar wind speed and southward IMF strength (Bs) at Mercury. This product VBs is the electric field that drives many magnetospheric dynamical processes and can be compared with the occurrence of energetic particle bursts within the Mercury magnetosphere. This quantity also serves as input to the global magnetohydrodynamic and kinetic magnetosphere models that are being used to explore magnetospheric and exospheric processes at Mercury. Moreover, this modeling can help assess near-real-time magnetospheric behavior for MESSENGER or other mission analysis and/or ground-based observational campaigns. We demonstrate that this solar wind forcing tool is a crucial step toward bringing heliospheric science expertise to bear on planetary exploration programs.

Ion kinetic properties in Mercury's pre‐midnight plasma sheet

With data from the Fast Imaging Plasma Spectrometer sensor on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft, we demonstrate that the average distributions for both solar wind and planetary ions in Mercurys pre-midnight plasma sheet are well-described by hot Maxwell-Boltzmann distributions. Temperatures and densities of the H+-dominated plasma sheet, in the ranges ~1–10u2009cm−3 and ~5–30u2009MK, respectively, maintain thermal pressures of ~1u2009nPa. The dominant planetary ion, Na+, has number densities about 10% that of H+. Solar wind ions retain near-solar-wind abundances with respect to H+ and exhibit mass-proportional ion temperatures, indicative of a reconnection-dominated heating in the magnetosphere. Conversely, planetary ion species are accelerated to similar average energies greater by a factor of ~1.5 than that of H+. This energization is suggestive of acceleration in an electric potential, consistent with the presence of a strong centrifugal acceleration process in Mercurys magnetosphere.

MESSENGER observations of magnetospheric substorm activity in Mercury's near magnetotail

MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) magnetic field and plasma measurements taken during crossings of Mercurys magnetotail from 2011 to 2014 have been examined for evidence of substorms. A total of 26 events were found during which an Earth-like growth phase was followed by clear near-tail expansion phase signatures. During the growth phase, just as at Earth, the thinning of the plasma sheet and the increase of the magnetic field intensity in the lobe are observed, but the fractional increase in field intensity could be ∼3 to 5 times that at Earth. The average timescale of the growth phase is ∼1u2009min. The dipolarization that marks the initiation of the substorm expansion phase is only a few seconds in duration. During the expansion phase, lasting ∼1u2009min, the plasma sheet is observed to thicken and engulf the spacecraft. The duration of the substorm observed in this paper is consistent with previous observations of Mercurys Dungey cycle. The reconfiguration of the magnetotail during Mercurys substorm is very similar to that at Earth despite its very compressed timescale.

Plasma Distribution in Mercury's Magnetosphere Derived from MESSENGER Magnetometer and Fast Imaging Plasma Spectrometer Observations

We assess the statistical spatial distribution of plasma in Mercurys magnetosphere from observations of magnetic pressure deficits and plasma characteristics by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. The statistical distributions of proton flux and pressure were derived from 10u2009months of Fast Imaging Plasma Spectrometer (FIPS) observations obtained during the orbital phase of the MESSENGER mission. The Magnetometer-derived pressure distributions compare favorably with those deduced from the FIPS observations at locations where depressions in the magnetic field associated with the presence of enhanced plasma pressures are discernible in the Magnetometer data. The magnitudes of the magnetic pressure deficit and the plasma pressure agree on average, although the two measures of plasma pressure may deviate for individual events by as much as a factor of ~3. The FIPS distributions provide better statistics in regions where the plasma is more tenuous and reveal an enhanced plasma population near the magnetopause flanks resulting from direct entry of magnetosheath plasma into the low-latitude boundary layer of the magnetosphere. The plasma observations also exhibit a pronounced north-south asymmetry on the nightside, with markedly lower fluxes at low altitudes in the northern hemisphere than at higher altitudes in the south on the same field line. This asymmetry is consistent with particle loss to the southern hemisphere surface during bounce motion in Mercurys offset dipole magnetic field.