Markus Fränz , Harald Krüger , Jim M. Raines , Austin N. Glass , Daniel J. Gershman , Fabio Prencipe , Norbert Krupp , Lina Z. Hadid , Dominique Delcourt , Sae Aizawa , Shoichiro Yokota , Yuki Harada , Yoshifumi Saito
{"title":"He+ ions in the vicinity of mercury observed by the MESSENGER and BepiColombo spacecraft","authors":"Markus Fränz , Harald Krüger , Jim M. Raines , Austin N. Glass , Daniel J. Gershman , Fabio Prencipe , Norbert Krupp , Lina Z. Hadid , Dominique Delcourt , Sae Aizawa , Shoichiro Yokota , Yuki Harada , Yoshifumi Saito","doi":"10.1016/j.pss.2025.106152","DOIUrl":null,"url":null,"abstract":"<div><div>The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft was launched in 2004, and between March 2011 and April 2015 it was the first spacecraft in orbit around Mercury. The Fast Imaging Plasma Spectrometer (FIPS) instrument on board MESSENGER measured the ion composition in the vicinity of Mercury and in the inner solar system.</div><div>We aim to determine the origin of He<span><math><msup><mrow></mrow><mrow><mo>+</mo></mrow></msup></math></span> ions in the inner solar system and in the environment of Mercury, continuing earlier work by Gershman et al., (2013).</div><div>We have analyzed measurements of He<span><math><msup><mrow></mrow><mrow><mo>+</mo></mrow></msup></math></span> and He<span><math><msup><mrow></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msup></math></span> ions made by the FIPS instrument during the interplanetary cruise phase of MESSENGER and its entire orbital mission at Mercury. We determined the spatial distributions of He<span><math><msup><mrow></mrow><mrow><mo>+</mo></mrow></msup></math></span> ions in the regions sampled by MESSENGER during that period and compare the spectra to a similar observation by the Mass Spectrum Analyzer instrument which is part of the Mercury Plasma Particle Experiment (MPPE-MSA) onboard BepiColombo. We consider two possible sources of He<span><math><msup><mrow></mrow><mrow><mo>+</mo></mrow></msup></math></span>: (1) interstellar neutral helium ionized close to Mercury and (2) solar He<span><math><msup><mrow></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msup></math></span> ions converted close to or at the surface of Mercury. We also compare the observed densities with a simple model of the ionization of the interstellar helium flow.</div><div>The FIPS data show a continuous evolution of the He<span><math><msup><mrow></mrow><mrow><mo>+</mo></mrow></msup></math></span> energy spectra from solar wind towards Mercury - changing from a shape typical for pick-up ions to a thermalized spectrum. This could mean that interstellar He<span><math><msup><mrow></mrow><mrow><mo>+</mo></mrow></msup></math></span> is concentrated at Mercury by increased electron impact ionization close to the planet. We find a remarkably similar high mean ratio of He<span><math><msup><mrow></mrow><mrow><mo>+</mo></mrow></msup></math></span>/He<span><math><msup><mrow></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msup></math></span> ions in the upstream solar wind and in the inner magnetosphere, while the ratio is reduced in the magnetosheath.</div><div>Outside Mercury’s magnetosphere the source of He<span><math><msup><mrow></mrow><mrow><mo>+</mo></mrow></msup></math></span> ions is interstellar helium, while inside the planet’s magnetosheath and the magnetosphere both interstellar helium and solar wind helium may be of similar magnitude. The observed median upstream He<span><math><msup><mrow></mrow><mrow><mo>+</mo></mrow></msup></math></span> densities are in good quantitative agreement with a simple model — though the existence of an upstream density crescent cannot yet be confirmed.</div></div>","PeriodicalId":20054,"journal":{"name":"Planetary and Space Science","volume":"265 ","pages":"Article 106152"},"PeriodicalIF":1.8000,"publicationDate":"2025-06-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Planetary and Space Science","FirstCategoryId":"101","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0032063325001199","RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q3","JCRName":"ASTRONOMY & ASTROPHYSICS","Score":null,"Total":0}
引用次数: 0
Abstract
The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft was launched in 2004, and between March 2011 and April 2015 it was the first spacecraft in orbit around Mercury. The Fast Imaging Plasma Spectrometer (FIPS) instrument on board MESSENGER measured the ion composition in the vicinity of Mercury and in the inner solar system.
We aim to determine the origin of He ions in the inner solar system and in the environment of Mercury, continuing earlier work by Gershman et al., (2013).
We have analyzed measurements of He and He ions made by the FIPS instrument during the interplanetary cruise phase of MESSENGER and its entire orbital mission at Mercury. We determined the spatial distributions of He ions in the regions sampled by MESSENGER during that period and compare the spectra to a similar observation by the Mass Spectrum Analyzer instrument which is part of the Mercury Plasma Particle Experiment (MPPE-MSA) onboard BepiColombo. We consider two possible sources of He: (1) interstellar neutral helium ionized close to Mercury and (2) solar He ions converted close to or at the surface of Mercury. We also compare the observed densities with a simple model of the ionization of the interstellar helium flow.
The FIPS data show a continuous evolution of the He energy spectra from solar wind towards Mercury - changing from a shape typical for pick-up ions to a thermalized spectrum. This could mean that interstellar He is concentrated at Mercury by increased electron impact ionization close to the planet. We find a remarkably similar high mean ratio of He/He ions in the upstream solar wind and in the inner magnetosphere, while the ratio is reduced in the magnetosheath.
Outside Mercury’s magnetosphere the source of He ions is interstellar helium, while inside the planet’s magnetosheath and the magnetosphere both interstellar helium and solar wind helium may be of similar magnitude. The observed median upstream He densities are in good quantitative agreement with a simple model — though the existence of an upstream density crescent cannot yet be confirmed.
期刊介绍:
Planetary and Space Science publishes original articles as well as short communications (letters). Ground-based and space-borne instrumentation and laboratory simulation of solar system processes are included. The following fields of planetary and solar system research are covered:
• Celestial mechanics, including dynamical evolution of the solar system, gravitational captures and resonances, relativistic effects, tracking and dynamics
• Cosmochemistry and origin, including all aspects of the formation and initial physical and chemical evolution of the solar system
• Terrestrial planets and satellites, including the physics of the interiors, geology and morphology of the surfaces, tectonics, mineralogy and dating
• Outer planets and satellites, including formation and evolution, remote sensing at all wavelengths and in situ measurements
• Planetary atmospheres, including formation and evolution, circulation and meteorology, boundary layers, remote sensing and laboratory simulation
• Planetary magnetospheres and ionospheres, including origin of magnetic fields, magnetospheric plasma and radiation belts, and their interaction with the sun, the solar wind and satellites
• Small bodies, dust and rings, including asteroids, comets and zodiacal light and their interaction with the solar radiation and the solar wind
• Exobiology, including origin of life, detection of planetary ecosystems and pre-biological phenomena in the solar system and laboratory simulations
• Extrasolar systems, including the detection and/or the detectability of exoplanets and planetary systems, their formation and evolution, the physical and chemical properties of the exoplanets
• History of planetary and space research