Direct evidence of substorm-related impulsive injections of electrons at Mercury.

Mercury's magnetosphere is known to involve fundamental processes releasing particles and energy like at Earth due to the solar wind interaction. The resulting cycle is however much faster and involves acceleration, transport, loss, and recycling of plasma. Direct experimental evidence for the roles of electrons during this cycle is however missing. Here we show that in-situ plasma observations obtained during BepiColombo's first Mercury flyby reveal a compressed magnetosphere hosts of quasi-periodic fluctuations, including the original observation of dynamic phenomena in the post-midnight, southern magnetosphere. The energy-time dispersed electron enhancements support the occurrence of substorm-related, multiple, impulsive injections of electrons that ultimately precipitate onto its surface and induce X-ray fluorescence. These observations reveal that electron injections and subsequent energy-dependent drift now observed throughout Solar System is a universal mechanism that generates aurorae despite the differences in structure and dynamics of the planetary magnetospheres.

Ablation of Venusian oxygen ions by unshocked solar wind.

As an Earth-like planet Venus probably had a primordial dipole field for several million years after formation of the planet. Since this dipole field eventually vanished the ionosphere of Venus has been exposed to the solar wind. The solar wind is shocked near Venus, and then scavenges the ionospheric particles through the magnetosheath and the magnetotail. The escape rate of oxygen ions (O+) estimated from spacecraft observations over the past several decades has manifested its importance for the evolution of planetary habitability, considering the accumulated effect over the history of Venus. However, all the previous observations were made in the shocked solar wind and/or inside the wake, though some simulations showed that unshocked solar wind can also ablate O+ ions. Here we report Venus Express observations of O+ ions in the unshocked solar wind during the solar minimum. The observations suggest that these O+ ions are accelerated by the unshocked solar wind through pickup processes. The estimated O+ escape rate, 2.1 × 1024 ions/s, is comparable to those measured in the shocked solar wind and the wake. This escape rate could result in about 2 cm global water loss over 4.5 billion years. Our results suggest that the atmospheric loss at unmagnetized planets is significantly underestimated by previous observations, and thus we can emphasize the importance of an Earth-like dipole for planetary habitability.