The Influence of IMF Clock Angle on Dayside Flux Transfer Events at Mercury

Abstract

Analysis of MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) data has shown for the first time that the orientation of the interplanetary magnetic field (IMF) in the magnetosheath of Mercury plays a crucial role in the formation of flux transfer events (FTEs) at the dayside magnetopause. During the first 4 Hermean years of MESSENGER's orbit around Mercury, we have identified 805 FTEs using magnetometer data. Under conditions of near-southward IMF, at least one FTE was detected on nearly 70% of passes through the magnetopause but the observation rate during northward IMF was less than 20%. FTEs were also observed preferentially in the prenoon sector. FTEs have been observed at Earth at all locations on the magnetopause under a wide range of solar wind conditions by single spacecraft such as

The Contribution of Flux Transfer Events to Mercury's Dungey Cycle

“…Although we have regarded the solid line as a “sensible upper limit” based on the extreme value of B N , the effect of using an upper limit value for r (1600 km; based on the upper end of the duration range reported by Imber et al, ) is indicated by the dotted line. Changing this assumption increases the flux content by a factor of 3.2; a flux content of 1 MWb is obtained for an X‐line length of ∼4.5 hr of MLT, comparable to the length we might infer from the FTE statistics reported by Leyser et al (), and values of Φ total equivalent to the mean and extreme values of Φ rope reported by Imber et al () are obtained for X‐line lengths of ∼0.25 and 1 hr of MLT, respectively.…”

“…In terms of inferences from MESSENGER observations, we can draw conflicting conclusions. Leyser et al () reported that, despite fairly uniform coverage of magnetopause crossings through dayside magnetic local times, most FTEs were observed within 3 hr of MLT from noon, which suggests that a maximum typical X‐line length may be <6 hr of MLT. On the other hand, DiBraccio et al () surveyed magnetopause crossings for rotational discontinuities and found the magnetopause was open on crossings between 8 and 16 hr local time.…”

“…Conversely, Φ total could be considerably smaller if the reconnection line extent was much smaller. As discussed above, statistics of FTE locations at Mercury are perhaps suggestive of an upper limit for a coherent X‐line being below ∼6 hr of MLT (Leyser et al, ), though the presence of rotational discontinuities across a wider local time range might suggest the possibility of a longer coherent X‐line (DiBraccio et al, ).…”

“…Flux transfer events give rise to a characteristic bipolar signature in the component of the magnetic field normal to the local magnetopause surface, B N , and an enhancement is observed in the field magnitude, | B | (Russell and Elphic, ; Russell and Elphic, ; Elphic, ). The occurrence of FTEs at Mercury is found to exhibit the same interplanetary magnetic field (IMF) control as at Earth, with FTEs being preferentially observed for the southward IMF conditions that favor reconnection with the northward directed planetary field (Leyser et al, ). The high repetition rate and large relative size of FTEs at Mercury, in terms of magnetic flux content compared with the overall flux contained within the Hermean magnetosphere, mean that they are already thought to be the major driver of Mercury's magnetospheric dynamics (Slavin et al, , , , ; Imber et al, ).…”

Investigating Mercury’s Environment with the Two-Spacecraft BepiColombo Mission

“…The coupling of the interplanetary magnetic field and the solar wind with the planetary magnetosphere is much stronger than previously believed, owing to the almost- continuous dayside magnetic reconnection (e.g. , as proved by the frequent observations of flux transfer events (FTE) Leyser et al 2017) (Fig. 1 upper panels).…”