Mercury's flux transfer event (FTE) showers are dayside magnetopause crossings accompanied by large numbers (≥10) of magnetic flux ropes (FRs). These shower events are common, occurring during 52% (1,953/3,748) of the analyzed crossings. Shower events are observed with magnetic shear angles (θ) from 0°to 180°across the magnetopause and magnetosheath plasma β from 0.1 to 10 but are most prevalent for high θ and low plasma β. Individual FR duration correlates positively, while spacing correlates negatively, with θ and plasma β. FR flux content and core magnetic field intensity correlate negatively with plasma β, but they do not correlate with θ. During shower intervals, FRs carry 60% to 85% of the magnetic flux required to supply Mercury's Dungey cycle. The FTE showers and the large amount of magnetic flux carried by the FTE-type FRs appear quite different from observations at Earth and other planetary magnetospheres visited thus far. Plain Language Summary Any planet with an interior dynamo will interact with the outward streaming stellar wind and likely form a magnetosphere. The magnetopause is a boundary between the shocked solar wind and planetary magnetic field, which can prevent most of the solar wind from directly entering into the magnetosphere. The multiple X-line reconnection that frequently occurs in the magnetopause creates helical magnetic fields that are termed magnetic flux ropes (FRs) about which open and interplanetary magnetic fields drape. FTE-type FRs generally have magnetic field lines with one end embedded in the solar wind and the other end connected to the planet through the magnetospheric cusp. The investigation of FTEs in Mercury's magnetosphere is of particular interest because they often occur in large numbers with extremely small temporal spacing, i.e., FTE showers, that are not seen elsewhere. We find that the properties of the FTE-type flux ropes in these showers depend upon plasma β in the magnetosheath and the magnetic shear angle across the magnetopause. The magnetic flux carried by these flux ropes dominates magnetic flux transfer between Mercury's dayside and nightside magnetosphere. These new results may contribute significantly to our understanding of solar wind-magnetosphere-exosphere coupling at Mercury.

Jupiter, the fifth planet from the sun, has the strongest intrinsic magnetic field among planets in the solar system. The interplay between this magnetic field and the solar wind results in a magnetosphere extending from the topside of Jupiter's atmosphere/ionosphere to beyond 𝐴𝐴 𝐴𝐴∼ 100 𝐴𝐴 R𝐽𝐽 (1 𝐴𝐴 R𝐽𝐽 = 𝐴𝐴 ∼ 71,400 km, Jupiter radii; hereinafter, 𝐴𝐴 𝐴𝐴 represents the radial distance to the Jupiter) (Bagenal et al., 2007). Jupiter's magnetosphere is filled with plasma originating from various sources, including the solar wind, Jupiter's atmosphere/ionosphere, and Jupiter's moons. Among these sources, the moon Io, which supplies 𝐴𝐴 ∼ 1 ton plasma per second to the magnetosphere (e.g., Thomas et al., 2004), serves as the dominant one (e.g., Bolton et al., 2015). After entering the magnetosphere, plasma from Io (and other sources) is picked up by the magnetospheric corotating electric fields and corotates with Jupiter with a period of 9.92 hr. The corotation, in turn, induces a centrifugal force on plasma. This force tends to pull plasma radially outward against the magnetic forces, leading to the deformation of Jupiter's dipole-like magnetic fields (e.g., Hill et al., 1974). The deformation is reinforced by the plasma pressure gradient and anisotropy, which, as suggested by later observational and modeling work (e.g., Caudal, 1986;Mauk & Krimigis, 1987;Paranicas et al., 1991), even play a dominant role in balancing the magnetic forces. As a final result of the force balance, a current sheet is formed in Jupiter's middle and outer magnetosphere (Vasyliunas, 1983).Because of Jupiter's dipole tilts ( ∼10 • ), Jupiter's current sheet is generally displaced from Jupiter's rotational equator (Khurana, 1992;Khurana & Schwarzl, 2005;Connerney et al., 1981). As a result of this displacement and Jupiter rotation, a spacecraft in Jupiter's magnetosphere would periodically cross the current sheet. These periodical crossings manifest as a series of magnetic field reversals in magnetic field data (e.g., Connerney et al., 1981;Khurana & Schwarzl, 2005). According to previous observations, magnetic field reversals can be detected from 𝐴𝐴 𝐴𝐴∼ 10 𝐴𝐴 R𝐽𝐽 to almost the magnetopause (e.g., Connerney et al., 1981;Khurana & Schwarzl, 2005), suggesting the existence of the current sheet in the most of the equatorial regions of Jupiter's magnetosphere. Besides its huge size and notability in observations, the current sheet plays a Abstract Jupiter's magnetosphere contains a current sheet of huge size near its equator. The current sheet not only mediates the global mass and energy cycles of Jupiter's magnetosphere, but also provides a site for many localized dynamic processes, such as reconnection and wave-particle interaction. To correctly evaluate its role in these processes, a statistical description of the current sheet is required. To this end, here we conduct statistics on Jupiter's current sheet, by using four-year Juno data obtained in the 20-100 Jupiter radius, 0-6 local time magnetosphere. The statistics show the thickness of th...

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 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 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.

The magnetic gradient and curvature drift of energetic ions can form a longitudinal electric current around a planet known as the ring current, that has been observed in the intrinsic magnetospheres of Earth, Jupiter, and Saturn. However, there is still a lack of observational evidence of ring current in Mercury’s magnetosphere, which has a significantly weaker dipole magnetic field. Under such conditions, charged particles are thought to be efficiently lost through magnetopause shadowing and/or directly impact the planetary surface. Here, we present the observational evidence of Mercury’s ring current by analysing particle measurements from MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) spacecraft. The ring current is bifurcated because of the dayside off-equatorial magnetic minima. Test-particle simulation with Mercury’s dynamic magnetospheric magnetic field model (KT17 model) validates this morphology. The ring current energy exceeds $$5\times {10}^{10}$$ 5 × 10 10  J during active times, indicating that magnetic storms may also occur on Mercury.

Mercury possesses a global dipole magnetic field with a similar polarity to Earth's dipole field, but the magnetic field intensity near Mercury's magnetic equatorial plane (∼200 nT) is much less than the strength of Earth's field (∼30,000 nT) (see Anderson et al., 2012). Mercury's magnetic field can hold off the constantly streaming solar wind with a subsolar magnetopause distance of around one thousand kilometers above the planet's surface