Flux Transfer Event Showers at Mercury: Dependence on Plasma β and Magnetic Shear and Their Contribution to the Dungey Cycle

Abstract

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.

Review of Mercury’s dynamic magnetosphere: Post-MESSENGER era and comparative magnetospheres

“…This open magnetic flux is much larger than the maximum open lobe magnetic flux observed during Mercury's Dungey cycle (~42%). Second, Sun et al (2020a) obtained a cross-magnetosphere potential drop of ~45 kV, which is triple the average value in Mercury's magnetosphere. The cross-magnetosphere potential drop was derived from the energy dispersion of the plasma mantle in Figure 18a.…”

“…The dimensionless reconnection rate was estimated to be ~0.093, and the dawn-dusk extent of the X-line took ~20% of the tail width. Magnetic reconnection in the plasma sheet produced many flux ropes at a high frequency, which appeared as quasi-periodic flux rope groups (Figure 18), and see Figures 4 and 5 in Sun et al (2020a). The flux rope group has a period of ~70 s, and, in each flux rope group, larger-scale flux ropes at the leading edge of the group were followed by the smaller-scale flux ropes.…”

Flux Transfer Events at a Reconnection‐Suppressed Magnetopause: Cassini Observations at Saturn

“…FTEs are commonly observed at the inner planet magnetospheres of Earth and Mercury. At Mercury, FTEs are so common and are generated at such high rates at the dayside magnetopause that their observation during a typical magnetopause crossing by a spacecraft have earned the name of “FTE showers” (Slavin et al., 2012; Sun, Slavin, Smith et al., 2020). FTEs are also commonly observed and well‐studied at Earth's magnetopause by a variety of spacecraft as well as in numerical simulations (e.g.…”

Electron Bulk Heating at Saturn's Magnetopause

“…In addition, all the events to the left of the L = d i solid line contain the strongest heating observed at ΔT e ≥ 20 eV including Case studies 1 and 2 (see Sections 3.1 and 3.2). Sun et al (2020) found that FTE showers (MP crossings with ≥10 flux ropes) at Mercury are most prevalent for high magnetic shear and low plasma β conditions. Both studies were consistent with the theoretical modeling of reconnection by Swisdak et al (2010).…”

“…Sun et al. (2020) found that FTE showers (MP crossings with ≥10 flux ropes) at Mercury are most prevalent for high magnetic shear and low plasma β conditions. Both studies were consistent with the theoretical modeling of reconnection by Swisdak et al.…”