Low-altitude signatures of magnetotail reconnection

Precipitating ions on the poleward edge of the nightside auroral oval sometimes exhibit sharp low-energy cutoffs in their energy spectra. These truncated spectra are interpreted as signatures of magnetic reconnection in the magnetotail. The energy cutoff is frequently smoothly dispersed in latitude, allowing an interpretation in terms of quasi-steady reconnection. These events are designated velocity-dispersed ion structures (VDIS) type 2. Roughly one third of type 2 VDIS are accompanied by a sharp transition in the polar rain near the open-closed boundary that aids in their analysis. From 886 nightside open-closed boundary crossings by DMSP spacecraft, 148 type 2 VDIS were identified. They were found most frequently within 2–3 hours of midnight and for 40% of the open-closed boundary crossings between 2200 and 0100 magnetic local time. Minimum variance fits to the cutoff energies and polar rain transition are performed on 49 of these events. For four of them the information from the minimum variance fit and observed convection velocities are used to infer distances to the reconnection site that varied from 30 to 80 RE. In three of these four cases a sharp transition in the convection velocity is observed, coincident with the arrival of ions from the reconnection site. If the reconnection region is viewed as a voltage source, lobe field lines can be insufficiently populated to carry the current necessary to impose the required voltage on the ionosphere. This explains the coincidence between the arrival of ions and a discontinuity in convection, that is, that an electric field from the reconnection site is imposed on the ionosphere but only after sufficient density populates the field lines that connect the regions.