Dipolarizations in Mercury's Magnetotail: Characteristics and Consequences in a Miniature Magnetosphere

Abstract

Mercury’s global magnetic field forms a terrestrial-like magnetosphere with its interaction with the upstream solar wind. While Mercury and Earth’s magnetospheres share similar structure and many similar dynamics, the weaker planetary field, stronger solar wind forcing, and lack of ionosphere at Mercury result in smaller spatiotemporal scales and stronger effects from magnetic reconnection. These magnetospheric differences influence substorm dynamics at the two planets, including magnetotail dipolarizations. Dipolarizations result from intense magnetic reconnection in the magnetotail, and at Earth, are important for magnetic flux transport, particle energization, and substorm current wedge formation. We use in situ observations of Mercury’s space environment from the MESSENGER spacecraft to identify the characteristics and consequences of dipolarizations to Mercury’s magnetosphere. In the pursuit to improve our understanding of dipolarizations at Mercury, we develop new techniques to determine plasma flow from limited observations, identify energetic electron bursts from indirect measurements, and select dipolarizations from a magnetic field time series.

Employing statistical analysis on Mercury’s dipolarizations, we find that they share many similar features to those at Earth. Dipolarizations at Mercury are characterized by a rapid (~2 s) increase in the northward field (ΔBz ~ 30 nT) that persists for ~10 s, accompanied by a depletion (Δn/n ~ –0.3) and heating (ΔT/T ~ 0.2) of thermal plasma, rapid sunward flow (vx ~ 200 km/s), strong cross-tail electric field (Ey ~ 11 mV/m), and enhancement of energetic electron flux. We find that dipolarizations typically transport ~0.06 MWb of magnetic flux. Although a single dipolarizations transports substantially less flux than a typical substorm loads into the magnetotail (~0.7 MWb), we find that dipolarizations are typically observed in series with others, allowing dipolarizations to transport the majority of magnetic flux during a substorm. As they transport magnetic flux from the reconnection site to Mercury’s inner magnetosphere, dipolarizations can energize electrons to ~120 keV via betatron and Fermi acceleration mechanisms. The frequency of dipolarizations in Mercury’s magnetotail (~1 min-1) indicates that dipolarizations may be the dominant source of Mercury’s energetic electron environment. Finite gyroradius effects prevent ions from experiencing the same degree of acceleration. Finally, we find that despite Mercury’s relatively weak planetary magnetic field and the small spatial distance from the nightside reconnection site to the planetary surface, Mercury’s dipole field is strong enough to cause dipolarizations to brake before reaching the planet’s nightside. Braking typically occurs within a region ~500 km in thickness, located ~900 km in altitude above Mercury’s nightside surface and is evidenced by strong decreases in dipolarization frequency and in sunward flow speed. As dipolarizations brake, their transported magnetic flux accumulates and allow for the possibility of a current wedge to develop. Dipolarizations, therefore, share similar characteristics and consequences in Mercury’s magnetosphere as Earth’s, informing our understanding of the substorm process at terrestrial-like planets.