MESSENGER observations of flux transfer event showers at Mercury: Contributions to Dungey cycle and influences on magnetospheric sodium ions

Abstract.

Flux transfer events (FTEs) are magnetic flux ropes formed by multiple reconnection X-lines on the magnetopause between the planetary magnetic field and the interplanetary magnetic field (IMF). During FTE shower intervals, FTEs appear extremely frequently with typical rates of ~ 10 FTEs per minute. This study shows that FTE showers are observed during half of the dayside magnetopause crossings over the entire MESSENGER mission. The occurrence and physical properties of FTE showers depend on the magnetic shear angle across the magnetopause and the magnetosheath plasma  (ratio of the plasma thermal pressure to the magnetic pressure). Modeling of these flux ropes in FTE shower intervals are determined to carry most (i.e., 60% - 85%) of the magnetic flux supplying Mercury’s Dungey cycle. Furthermore, sodium ions (Na⁺) are substantially enhanced during FTE shower intervals as compared with non-shower intervals. FTE showers lead to much larger precipitation rates on the cusp’s surface and would sputter large numbers of planetary atoms into the exosphere, which then be photon-ionized resulting in the enhancement of Na⁺.

1. Introduction.

Interactions between Mercury’s global intrinsic magnetic field and supersonic solar wind form a miniature magnetosphere. Without a significant neutral atmosphere, Mercury’s magnetosphere is able to prevent most of the planet’s surface from being directly hit by the streaming solar wind most of the times [1]. Magnetic reconnection on the dayside magnetopause generates FTEs, in which the magnetic field lines have one end connected to the solar wind and the other end connected to the planet. Magnetic flux ropes concentrate magnetic flux in the FTEs (FTE-type FRs), which transport magnetic flux from the dayside magnetosphere into the nightside following the convection of FTEs. Solar wind plasma can be transported along the magnetic field lines and precipitate to the surface at the planet’s cusps causing sputtering [2]. MESSENGER observations have shown that FTE showers correspond to a very high occurrence of FTEs (~ 10 FTEs per minute) [1, 3].

In this work, we investigate how the low Alfvénic Mach number (M_A = V_SW/V_A) solar wind near Mercury’s orbit influences the occurrence and physical properties of FTE showers, and how FTE showers contribute to the Dungey cycle and influence sodium ions dynamics in the dayside magnetosphere.

2. FTE Shower at Mercury

Figure 1. Overview of MESSENGER observations of a dayside magnetosphere crossing on 10 April 2011. (a) and (b) Differential and integrated particle flux for protons. (c) Densities of alpha particle (He⁺⁺) and sodium group (Na⁺, m/q = 21 to 30). (d) to (g) Magnetic field measurements. The black arrows in (a) annotate the injected solar wind particles. FTEs and plasma filaments are marked in (g).

 

Figure 1 shows a FTE shower event observed by MESSENGER on 10 April 2011. Thirty-four FTE-type FRs appeared in two and half minutes surrounding the dayside magnetopause (~ 14 FTEs/minute). The occurrence of FTEs at Mercury is much higher than those at Earth, Jupiter and Saturn, which are commonly separated by several minutes [4]. The magnetic field intensity in the magnetosheath is even slightly larger than the magnetic field intensity in the magnetosphere, implying a thick plasma depletion layer with a low plasma  ( 0.1) adjacent to the magnetopause. Magnetic shear angle across the dayside magnetopause is around 120.

The injected solar wind particles are marked in Figure 1a. In the low altitude region, especially at the cusp, the injected particles diamagnetically reduce the field strength and form the plasma filaments (Figure 1g, see also ref. [2]). The injected solar wind particles could precipitate to the surface and sputter planetary atoms refilling the exosphere. Planetary Na⁺ appear in a density of ~ 1 cm^-3, which is around an order of magnitude higher than the average Na⁺ densities in the subsolar region [5].

3. Statistical Results

This study conducts a survey over the entire MESSENGER mission. In a total number of 3748 magnetopause crossings, around half (52%) of them were found to be accompanied by FTE showers. Further detailed investigations show that FTE showers occur more frequently when the magnetic shear angle is large and the magnetosheath plasma  is small. The repetition time, i.e., the separation between neighboring FRs, and core magnetic field of FRs are negatively correlated with the shear angle and the plasma . The duration of FRs is positively correlated with the shear angle and the plasma . FTE showers could carry 60% to 85% of the magnetic flux needed to supply Mercury’s Dungey cycle.

The planetary Na⁺ is substantially enhanced during the FTE shower intervals compared with the Na⁺ during other intervals. The total number of Na⁺ on the dayside magnetosphere in the FTE shower intervals is more than twice the number of Na⁺ during the intervals without FTE showers. Since FTE shower events depend on solar wind variations, which last tens of minutes to an hour, the corresponding enhancements of exospheric Na⁺ should be on the scale of same order, i.e., short-term variations.

4. Conclusions

We have shown that the occurrence and physical properties of FTE showers clearly depend on the magnetic shear angle across the magnetopause current sheet and the magnetosheath plasma β. FTE showers play a dominant role in the flux circulation of Mercury’s Dungey cycle. Furthermore, FTE showers are closely associated with substantial enhancements in the Na⁺ density in the dayside magnetosphere.