Observational evidence of ring current in the magnetosphere of Mercury

Abstract

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.

BepiColombo Mio Observations of Low‐Energy Ions During the First Mercury Flyby: Initial Results

“…During the time interval in which the MIA FOV is directed within the loss cone predicted by the KT17 model, the ion flux measured by MIA is > 10 times lower than that at the adjacent times. This is in agreement with the ion loss cone observed by MESSENGER within Mercury's magnetosphere (J. T. Zhao et al., 2020, 2022; Winslow et al., 2014), though we cannot conclusively confirm the ion loss cone from the flyby MIA data due to the limited FOV. In‐orbit observations by MIA will provide the full 4 π coverage, and in combination with magnetic field measurements, comprehensive energy–pitch‐angle distributions of ions will be available.…”

ULF Modulations on Plasma Environment and Coherent Waves of Mercury's Magnetosphere: MESSENGER's Observation

“…The magnetic field direction changes from dawnward to northward, and the strength decreases gradually as the spacecraft moves toward the equator. The increasing signature in the proton flux coincides with the previous statistics about the background proton distribution (Zhao et al., 2022). The absence of the protons with a pitch angle of ∼180° can be seen in Figure 2b, while the conjugate proton with a pitch angle of ∼0° is not measurable due to the limited field of view of FIPS.…”

Magnetic storms in Mercury’s magnetosphere

“…The total energy of the magnetic storm at Mercury for the event studied is estimated to be ~10 12 J (where MDst = −60 nT), which is three orders of magnitude smaller than for typical magnetic storms at Earth. This estimation is an order of magnitude of the previous estimation from the FIPS measurement of 50 eV-13.6 keV protons [6], implying that more comprehensive particle measurements are required in the further investigation. Additionally, the ring current morphology may differ from the Earth's ring current as demonstrated in Zhao et al [6].…”

“…This estimation is an order of magnitude of the previous estimation from the FIPS measurement of 50 eV-13.6 keV protons [6], implying that more comprehensive particle measurements are required in the further investigation. Additionally, the ring current morphology may differ from the Earth's ring current as demonstrated in Zhao et al [6]. Under such strong solar wind enforcement (R SS < 1.35 R M ), this ring current is mostly in the equatorial plane on the nightside, but splits into two parts on the dayside; one in the northern mid-latitude region and the other in the southern mid-latitude region.…”

“…Despite a faster rate of charged particle loss from Mercury's magnetosphere compared with Earth, previous literature has confirmed the presence of Mercury's ring current from analysis of particle measurements from NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft [6][7][8]. The existence of the ring current suggests that the magnetic storm is possible to occur on Mercury.…”