Substorm and convection bay compared: Auroral and magnetotail dynamics during convection bay

Abstract

Using observations from eight spacecraft and a ground network, we study two subsequent bay‐like disturbances on December 10, 1996, initiated by southward interplanetary magnetic field intervals, one being a classic substorm and another one a convection bay. Both events showed enhanced convection and Dst decreases as well as Pi2 pulsations in the auroral zone. Contrasting to the well‐defined substorm signatures of the first event (poleward auroral expansion, substorm current wedge, strong particle injection to 6.6 RE) resulting from energy loading/unloading and near‐Earth reconnection in the tail, these signatures were virtually absent during the convection bay (CB). Distinctive features of the CB event were the same as those during the Steady Magnetospheric Convection intervals: (1) wide double oval at the nightside; (2) thick plasma sheet, relaxed lobe field, and enhanced magnetic flux closure (large Bz) and multiple bursty earthward flows (BBFs) in the midtail; (3) sporadic narrow soft injections to 6.6 RE; (4) auroral streamers associated with both BBFs and narrow injections. We emphasize the development of multiple and sporadic auroral streamers which start at the poleward oval boundary, propagate equatorward (in 3–8 min) and end with a long‐duration bright spot in the equatorward oval. We conclude that the plasma sheet and auroral dynamics during the convection bay was formed by sporadic narrow (a few RE wide) plasma streams (plasma bubbles) which transported the plasma sheet material from the distant magnetotail reconnection regions to the inner magnetosphere and may significantly contribute to the magnetospheric circulation on the nightside. We modeled the nightside tail configuration using magnetotail magnetic observations and low‐altitude particle boundaries to show that at the beginning of the convection bay the increase of magnetic flux tube volume with distance was small in the midtail. Therefore the “pressure crisis” in the tail was significantly reduced during the convection bay, and the efficient earthward transport by sporadic narrow plasma streams was probably able to balance the magnetospheric circulation to avoid the large‐scale instability of the magnetotail.