Proton Properties in Mercury's Magnetotail: A Statistical Study

• Published in: Geophysical research letters 2020-10, Vol.47 (19), p.n/a
• Main Authors: Zhao, J.‐T., Zong, Q.‐G., Slavin, J. A., Sun, W.‐J., Zhou, X.‐Z., Yue, C., Raines, J. M., Ip, W.‐H.
• Format: Article
• Language: English
• Subjects: Adiabatic; Adiabatic flow; Analogs; Density profiles; Distribution; Earth magnetosphere; Energy spectra; Exosphere; Fluxes; Ganymede; Ions; Jupiter; Jupiter atmosphere; Magnetic properties; Magnetospheric ions; Magnetospheric-solar wind relationships; Magnetotail plasma; Magnetotails; Mercury; Mercury (planet); Mercury surface; MESSENGER Spacecraft; Normal distribution; Pitch (inclination); Planetary magnetospheres; Planetary surfaces; Plasma; Properties; Proton density (concentration); Protons; Skewed distributions; Solar system; Solar wind; Spacecraft; Statistics; Stress concentration
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL088075

This study investigates the properties of protons in the magnetotail plasma sheet of Mercury. By superposing 5‐year measurements from the MESSENGER spacecraft, we obtain the average energy spectrum of protons in the plasma sheet, which can be fitted nicely by the Gaussian‐Kappa model. The proton density, pressure, and energy spectral index κ are found to be higher on the dawnside than on the duskside. The proton temperature shows a clearly outward radial gradient. The field‐aligned density profile indicates that the protons in the outer plasma sheet move adiabatically. The pitch angle distribution reveals the reflected fluxes to be always less than the incident fluxes and indicates the loss of protons due to their impact on the planetary surface. Plain Language Summary Mercury has a miniature magnetosphere subject to intense solar wind forcing. This magnetosphere, among the smallest in the solar system, resembles the Earth's in many key respects. It is also an analog for other small and outside‐driven magnetospheres, such as Ganymede's inside Jupiter's magnetosphere. Mercury does not have a significant atmosphere but a tenuous exosphere. Therefore, Mercury's magnetospheric ions are thought to come predominately from the solar wind, and only about 10% of the ions are of planetary origin. This study presents a statistical picture of the protons in Mercury's magnetotail plasma sheet measured by the Fast Imaging Plasma Spectrometer (FIPS) onboard MESSENGER spacecraft. Many parameters are obtained through a best fit procedure with a Gaussian‐Kappa distribution. The proton number density, proton pressure, and spectral index κ show clear dawn‐dusk asymmetric features. The results also suggest that the motion of the protons is adiabatic in the outer plasma sheet and non‐adiabatic in the central plasma sheet. Furthermore, the loss feature of the protons is also revealed by their asymmetric pitch angle distributions. Key Points The asymmetric pitch angle distribution is caused by the proton loss and the loss population is dominated by the thermal protons (>830 eV) The distributions (both density, thermal pressure, and spectral index κ) exhibit a clear dawn‐dusk asymmetry systematically The proton density profile in the meridian plane suggests that the protons are adiabatic in the outer plasma sheet