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

We 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 × 10–2 cm–3 for He2+, 3.4 × 10–4 cm–3 for He+, 8.0 × 10–4 cm–3 for O+‐group ions, and 5.1 × 10–3 cm–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 590 km, and that for the O+ group and He+ is 1100 km. On the nightside and within 750 km 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 Mercury's magnetosphere.

[1]  A. Fitzsimmons,et al.  Sodium D2 line profiles: clues to the temperature structure of Mercury’s exosphere , 1999 .

[2]  Uwe Fink,et al.  Distribution and Abundance of Sodium in Mercury's Atmosphere, 1985–1988 , 1997 .

[3]  R. Killen,et al.  The sodium tail of Mercury , 2002 .

[4]  M. Mendillo,et al.  Imaging the sources and full extent of the sodium tail of the planet Mercury , 2008 .

[5]  A. Potter,et al.  Evidence for suprathermal sodium on Mercury , 1997 .

[6]  M. Mendillo,et al.  The Use of Small Telescopes for Spectral Imaging of Low-light-level Extended Atmospheres in the Solar System , 2009 .

[7]  S. Solomon,et al.  MESSENGER Mission Overview , 2007 .

[8]  F. Leblanc,et al.  Mercury and Moon He exospheres: Analysis and modeling , 2011 .

[9]  M. Zuber,et al.  Low‐degree structure in Mercury's planetary magnetic field , 2012 .

[10]  Daniel N. Baker,et al.  Mercury's magnetopause and bow shock from MESSENGER Magnetometer observations , 2013 .

[11]  W. Ip,et al.  Source dependency of exospheric sodium on Mercury , 2011 .

[12]  G. Thomas,et al.  Mercury's helium exosphere , 1975 .

[13]  M. Mendillo,et al.  Escape rates and variability constraints for high‐energy sodium sources at Mercury , 2011 .

[14]  S. Solomon,et al.  MESSENGER Observations of Magnetic Reconnection in Mercury’s Magnetosphere , 2009, Science.

[15]  George Gloeckler,et al.  MESSENGER Observations of the Spatial Distribution of Planetary Ions Near Mercury , 2011, Science.

[16]  Bernard V. Jackson,et al.  Evidence for space weather at Mercury , 2001 .

[17]  J. Slavin,et al.  Limits to Mercury's magnesium exosphere from MESSENGER second flyby observations , 2011 .

[18]  R. Killen.  Depletion of sulfur on the surface of asteroids and the moon , 2003 .

[19]  S. Solomon,et al.  Solar wind forcing at Mercury: WSA‐ENLIL model results , 2011 .

[20]  J. Dungey.  Interplanetary Magnetic Field and the Auroral Zones , 1961 .

[21]  M. Belton,et al.  Mercury's Atmosphere from Mariner 10: Preliminary Results , 1974, Science.

[22]  F. Leblanc,et al.  Mercury exosphere I. Global circulation model of its sodium component , 2010 .

[23]  A. Potter,et al.  Evidence for Magnetospheric Effects on the Sodium Atmosphere of Mercury , 1990, Science.

[24]  G. Gloeckler,et al.  MESSENGER Observations of the Composition of Mercury's Ionized Exosphere and Plasma Environment , 2008, Science.

[25]  K. Kabin,et al.  On the space environment of Mercury , 2004 .

[26]  G. Thomas,et al.  Neutral and ion exosphere models for lunar hydrogen and helium , 1974 .

[27]  Thomas H. Zurbuchen,et al.  Low-weight plasma instrument to be used in the inner heliosphere , 1998, Optics & Photonics.

[28]  G. Siscoe,et al.  Observations at Mercury Encounter by the Plasma Science Experiment on Mariner 10 , 1974, Science.

[29]  Thomas E. Moore,et al.  A quantitative model of the planetary Na + contribution to Mercury’s magnetosphere , 2003 .

[30]  Maxim L. Khodachenko,et al.  Processes that Promote and Deplete the Exosphere of Mercury , 2007 .

[31]  S. Solomon,et al.  Solar wind alpha particles and heavy ions in the inner heliosphere observed with MESSENGER , 2012 .

[32]  T. Hill,et al.  A Bx-interconnected magnetosphere model for Mercury , 2001 .

[33]  A. Milillo,et al.  Numerical and analytical model of Mercury's exosphere: Dependence on surface and external conditions , 2007 .

[34]  Haje Korth,et al.  The Global Magnetic Field of Mercury from MESSENGER Orbital Observations , 2011, Science.

[35]  Thomas A. Bida,et al.  The calcium exosphere of Mercury , 2005 .

[36]  F. Leblanc,et al.  Mercury's sodium exosphere: Magnetospheric ion recycling , 2003 .

[37]  A. Sprague,et al.  Mercury: Sodium Atmospheric Enhancements, Radar-Bright Spots, and Visible Surface Features , 1998 .

[38]  Heliosphere,et al.  A low-weight Plasma Instrument to be used in the Inner , 2003 .

[39]  C. Russell,et al.  The magnetosphere of Mercury , 1988 .

[40]  C. Schmidt.  Mercury's sodium exosphere , 2013 .

[41]  Barry H. Mauk,et al.  The Energetic Particle and Plasma Spectrometer Instrument on the MESSENGER Spacecraft , 2007 .

[42]  D. Baker,et al.  MESSENGER observations of the plasma environment near Mercury , 2009 .

[43]  Thomas A. Bida,et al.  Discovery of calcium in Mercury's atmosphere , 2000, Nature.

[44]  R. Killen,et al.  Space weather at Mercury , 2004 .

[45]  R. Killen,et al.  Rapid changes in the sodium exosphere of Mercury , 1999 .

[46]  J. Ziegler,et al.  SRIM – The stopping and range of ions in matter (2010) , 2010 .

[47]  R. Killen,et al.  Solar radiation acceleration effects on Mercury sodium emission , 2006 .

[48]  S. Solomon,et al.  Modeling of the magnetosphere of Mercury at the time of the first MESSENGER flyby , 2010 .

[49]  J. Slavin.  Mercury's Magnetosphere , 2002 .

[50]  S. Okano,et al.  Investigation of local time dependence of Mercury's sodium exosphere based on a numerical simulation , 2008 .

[51]  F. Leblanc,et al.  Formation of a sodium ring in Mercury's magnetosphere , 2010 .

[52]  Helmut Lammer,et al.  Mapping of the cusp plasma precipitation on the surface of Mercury , 2003 .

[53]  R. Killen,et al.  Spatial distribution of sodium vapor in the atmosphere of Mercury , 1990 .

[54]  William E. McClintock,et al.  MESSENGER Observations of Mercury’s Exosphere: Detection of Magnesium and Distribution of Constituents , 2009, Science.

[55]  R. Killen,et al.  Predicting the long-term solar wind ion-sputtering source at Mercury , 2007 .

[56]  S. Krimigis,et al.  Magnetosphere, Exosphere, and Surface of Mercury , 1987 .

[57]  J. Sauvaud,et al.  Centrifugal acceleration of ions near Mercury , 2002 .

[58]  S. Barabash,et al.  Energetic neutral atom imaging of Mercury's magnetosphere 1. Distribution of neutral particles in an axially symmetrical exosphere , 2001 .

[59]  A. Summers,et al.  Hydromagnetic flow around the magnetosphere , 1966 .

[60]  Andrew F. Cheng,et al.  Mercury’s Atmosphere: A Surface-Bounded Exosphere , 2007 .

[61]  James A. Slavin,et al.  The effect of erosion on the solar wind stand-off distance at Mercury , 1979 .

[62]  M. Sarantos,et al.  Pickup ion distributions from three-dimensional neutral exospheres , 2011 .