Manifestations of Magnetotail Flow Channels in Energetic Particle Signatures at Low‐Altitude Orbit

• Published in: Geophysical research letters Vol. 48; no. 15
• Main Authors: Sergeev, V. A., Sun, Weiqin, Yang, Jian, Panov, E. V.
• Format: Journal Article
• Language: English
• Published: 16.08.2021
• Subjects: energetic particles; flow burst; magnetotail; plasma sheet
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL093543

Although magnetotail bursty bulk flows (BBFs)/plasma bubbles are widely recognized as major plasma transport structures that provide particle acceleration and injection, their ionospheric signatures in energetic particles have not been established. In this study, we combine the ionospheric energetic proton flux (as a proxy to the equatorial pressure) and energetic electron loss‐cone filling rate (as a marker of the equatorial magnetic field) to detect a rapid traversal from stretched to dipolarized field lines across the BBF channel. During a nonsubstorm episode when multiple flow bursts were observed in the plasma sheet, we show remarkable step‐like increases of energetic proton flux (approaching 103 in some events) observed in the middle of nightside auroral zone. We also conduct a high‐resolution Rice Convection Model (RCM) simulation of a plasma‐sheet bubble intrusion into the inner magnetosphere. Besides confirmation of the observed signatures, the simulation illustrates variability of the ionospheric signatures of bubbles and provides guidance in future studies of flow burst interactions. Plain Language Summary Flow bursts (also known as fast flow channels, or BBFs/bubbles) are sporadic transient narrow structures in the magnetotail which transport energy, and accelerate and inject energetic particles into the outer radiation belt and the ring current in the inner magnetosphere. Sparse spacecraft coverage severely limits experimental studies of such complicated structures and their flow braking process. For this purpose, an image of related precipitation on the ionospheric screen can be of great help if we could specify its magnetospheric context. In this study, we demonstrate a combination of two specific energetic particle signatures in the ionosphere to identify the border of BBFs. Sharp transition from isotropic to depleted loss‐cone distributions of energetic electrons suggests the passage from stretched magnetic field lines outside the BBF to dipolarized lines inside the BBF. Sharp increase of energetic proton flux indicates the change of the equatorial pressure between stretched and dipolarized field lines. Observed together, these two signatures robustly indicate a sharp change in the magnetotail connectivity, a characteristic of BBFs. We also use the self‐consistent numerical simulation of a plasma‐sheet bubble injection to demonstrate these signatures on a physics‐based model as well as to discuss the inherent variability of the signatures. Key Points Strong step‐like increases of energetic proton flux in the nightside auroral zone accompanied by depleted loss‐cone of energetic electrons Such low‐altitude spacecraft signatures were observed 12–14 min after flow burst passage over MMS in the near‐conjugate plasma sheet RCM bubble simulation confirms such signature of azimuthal crossing of plasma bubble ionospheric projection and predicts its variability