How Does BBF/Flow Burst Interact with Ambient Field and Couples to the Ionosphere ?

How does BBF/flow burst interact with ambient field and couples to the ionosphere ?

One of the most dynamic phenomena in the magnetotail are the bursty bulk flows (BBFs), which often consists of short-time scale enhancement of flow speed, called flow burst. How these fast flows interact with the inner magnetosphere and coupled to the ionosphere and relates to the cross-tail current reconfiguration during substorm is one of essential themes to understand magnetotail reconnection, transport of energy and moment, and particle acceleration in the magnetosphere. THEMIS prime phase, which has an orbital configuration extended in a wide radial distance, is expected to cover all these key regions simultaneously for the first time and is expected to provide new information on the large- scale context of the fast flow processes. Yet, to understand the mechanism how these individual flow bursts interact with the ambient field and thereby creating field-aligned current and convert energy, local multi-point observation with optimized spatial scale at suitable location is needed.

Previous studies have shown that there exists a front layer of the fast flowing plasma jet where the plasma jets interact with ambient field and creating a strong shear both in plasma flow as well as magnetic field thereby causing a field aligned current (ex. Ref1). The spatial scale of such boundary is shown to be less than a few thousands km (Ref2), much smaller than the flow burst itself. Signature of localized field-aligned current has been observed both Cluster and conjugate ionosphere associated with the flow bursts (Ref 3, fig 1). While Cluster observations showed that local multi-point observations are a powerful tool to quantify the scale of the front layer and the field-aligned current associated with flow bursts at 15 and 19 RE region, there has never been such observation in the region where the flux transport rate decreases and energy of the flow bursts becomes diverted via field-aligned current or wave energy or particle heating. Geotail statistical study showed that such region is expected be between 10-15 Re (Ref 4). It is therefore ideal to have a local multi-point THEMIS observation concentrated at 10-12 RE region as planned in 2011-2012 to investigate the local energy conversion of the flow burst at the key region, namely where the Earthward flux transport rate strongly reduces.

Furthermore, statistical study of Cluster and TC1 showed that when Cluster observed fast flows, whether TC1 between (8-14 RE) region observed associated dipolarization or not strongly depended on the BZ at TC1 (Ref 5, Fig 2). That is, when the tail configuration was more stretched it was likely that the flow bursts penetrates to nearer Earth region and vice versa indicating that the earthward BBF penetration is controlled by the initial configuration of the inner magnetospheric magnetic field. This result suggests that the simultaneous assessment of the radial gradient of the BZ is also important to understand the flow bursts evolution together with the local shear observations, which would be studied ideally in 2010 with relatively large radial distance, 1RE, simultaneously catching the local gradient of the front layer of the flow burst. Figure 1. (a) Cluster fast flow observations with 4000 km scale separation. Flow shear direction is illustrated with pink arrows. Front layer orientation is expressed with thin dotted line. (b) Simultaneous equivalent current observations in the ionosphere. The enhanced electrojet region and expected auroral precipitation is shown. The expected shear of the fast flow in the magnetosphere was in a sense to create field-aligned current flowing from the ionosphere at the front layer and was consistent with the observation of the upward field aligned current deduced from the curl of the equivalent current in the ionosphere, near Cluster foot point. (Adapted from Nakamura et al. ,2004)

COMMENT: THIS FIGURE MIGHT BE TOO COMPLICATED BUT IS JUST MEANT TO SHOW THE USEFULNESS OF LOCAL MULTIPOINT IN MAGNETOSPHERE TOGETHER WITH IONOSPHEREIC OBSERVATIONS IN CASE YOU NEED AN EXAMPLE.

Figure 2. (a) BZ at TC1 before the dipolarization/BBFs against X position of TC1 in case of the dipolarization events (blue circles) and the non-dipolarization events (red rectangles). (b) Tail configuration for events when TC1 observed dipolarization (upper panel) and no clear dipolarization (lower panel). (Adapted from Takada et al., 2006) Reference

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