MESSENGER observations of planetary ion enhancements at Mercury’s northern magnetospheric cusp during Flux Transfer Event Showers

At Mercury, several processes can release ions and neutrals out of the planet’s surface. Here we present enhancements of dayside planetary ions in the solar wind entry layer during flux transfer event (FTE) “showers” near Mercury’s northern magnetospheric cusp. In this entry layer, solar wind ions are accelerated and move downward (i.e. planetward) toward the cusps, which sputter upward-moving planetary ions within 1 minute. The precipitation rate is enhanced by an order of magnitude during FTE showers and the neutral density of the exosphere can vary by >10% due to this FTE-driven sputtering. These in situ observations of enhanced planetary ions in the entry layer likely correspond to an escape channel of Mercury’s planetary ions, and the large-scale variations of the exosphere observed on minute-timescales by ground-based telescopes. Comprehensive, future multi-point measurements made by BepiColombo will greatly enhance our understanding of the processes contributing to Mercury’s dynamic exosphere and magnetosphere.

Dayside magnetopause reconnection and flux transfer events: BepiColombo earth-Flyby observations

This study analyzes the flux transfer event (FTE)-type flux ropes and magnetic reconnection around the dayside magnetopause during BepiColombo’s Earth flyby. The magnetosheath corresponds to a high plasma β (~ 8) and the IMF has a significant radial component. Six flux ropes are identified. The motion of flux rope together with the maximum magnetic shear model suggests that the reconnection X-line swipes BepiColombo near the magnetic equator due to an increase of the radial IMF. The flux rope with the highest flux content contains a clear coalescence signature, i.e., two smaller flux ropes merging, supporting theoretical predictions the flux content of flux ropes can grow through coalescence. The secondary reconnection associated with coalescence exhibits a large normalized guide field and a reconnection rate comparable to the reconnection rate measured at the magnetopause (~ 0.1).

Dayside Magnetopause Reconnection and Flux Transfer Events: BepiColombo Earth-Flyby

<p>BepiColombo is a joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) to the planet Mercury. The BepiColombo mission consists of two spacecraft, which are the Mercury Planetary Orbiter (MPO) and Mercury Magnetospheric Orbiter (Mio). The mission made its first planetary flyby, which is the only Earth flyby, on 10 April 2020, during which several instruments collected measurements. In this study, we analyze MPO magnetometer (MAG) observations of Flux Transfer Events (FTEs) in the magnetosheath and the structure of the subsolar magnetopause near the  flow stagnation point. The magnetosheath plasma beta was high with a value of ~ 8 and the interplanetary magnetic field (IMF) was southward with a clock angle that decreased from ~ 100 degrees to ~ 150 degrees.  As the draped IMF became increasingly southward several of the flux transfer event (FTE)-type flux ropes were observed. These FTEs traveled southward indicating that the magnetopause X-line was located northward of the spacecraft, which is consistent with a dawnward tilt of the IMF. Most of the FTE-type flux ropes were in ion-scale, <10 s duration, suggesting that they were newly formed. Only one large-scale FTE-type flux rope, ~ 20 s, was observed. It was made up of two successive bipolar signatures in the normal magnetic field component, which is evidence of coalescence at a secondary reconnection site. Further analysis demonstrated that the dimensionless reconnection rate of the re-reconnection associated with the coalescence site was ~ 0.14. While this investigation was limited to the MPO MAG observations, it strongly supports a key feature of dayside reconnection discovered in the Magnetospheric Multiscale mission, the growth of FTE-type flux ropes through coalescence at secondary reconnection sites.</p>