HIGHLIGHTS

Magnetospheric substorms are space weather disturbances powered by the rapid release of magnetic energy stored in the lobes of planetary magnetic tails. Despite the comprehensive observations of substorm at Earth, there are rare detail observations of substorm processes at Mercury.

Based on theories, the beam instability induced by shock-accelerated ions can generate upstream-propagating Alfven waves (UPAWs) with a power spectral bump near 0.03 Hz, while the nonlinear wave-wave interaction favors an inverse cascade to create a power-law spectrum. Here we present the first observational evidence for the upstream-propagating Alfvenic fluctuations (UPAFs) with power-law spectra. We utilize a new criterion to identify the upstream-propagating Alfvenic intervals: the propagation direction is opposite to that of solar wind strahl electron outflow.

The group of Solar and Heliospheric Physics published a paper on ApJ, reporting an observation of sunward proton beam drift in association with the sunward propagating Alfven waves. The sunward propagation is found to be in the pristine solar wind disconnected from the Earth bow shock. Although several possibilities are discussed in the end of this paper, it still remains a mystery on how to generate the sunward waves and associated proton beams.

For details please refer to: doi:10.1088/0004-637X/805/2/176, 2015