资源环境科技发展态势分析平台

We report an earthward moving ion-scale flux rope embedded within the trailing edge of a hot flow anomaly (HFA) observed by the Magnetospheric Multiscale satellite constellation on 17 December 2016 upstream of Earth's quasi-parallel bow shock. The driver of the HFA, a tangential discontinuity, was observed by the Wind spacecraft without flux rope signatures around it in the solar wind. This suggests that the earthward moving flux rope was generated inside the HFA. This ion-scale flux rope is not a force free structure and expands due to a strong magnetic pressure gradient force. Solar wind ions are decelerated inside the flux rope by the static electric field likely caused by the charge separation of solar wind particles. Our observations imply that magnetic reconnection may have occurred inside the HFA. Reconnection and flux ropes may play a role in particle acceleration/heating inside foreshock transients.

Plain Language Summary Energetic particles are often observed inside the foreshock transients or in the foreshock region. The acceleration mechanisms of these energetic particles remain an open question. Possible candidates responsible for the acceleration have been put forward, such as Fermi acceleration, electron firehose, and lower hybrid drift instabilities and magnetic reconnection. However, to date, magnetic reconnection is only found in hybrid simulations during the generation of foreshock transients, but never reported by in situ observations. In this paper, we report an ion-scale flux rope observed at the trailing edge of a hot flow anomaly, which could be generated during the magnetic reconnection. Our observations indicate that reconnection could occur locally within foreshock transients and contribute to their particle acceleration.