Some statistical properties of flow bursts in the magnetotail

We investigated the properties of physical parameters during earthward flow burst (FB) events in the near‐Earth magnetic tail. For the investigation, we used the measurements made by two of the Cluster satellites, C1 and C4, from 2001 to 2003 and selected FB events based on a set of strict criteria. First, while we confirmed the well‐known fact that the FBs are characterized by generally having a lower density than that of their surrounding medium, we newly found that the maximum flow speed of FBs is statistically proportional to the density depletion amount relative to the surrounding medium density . Namely, more density‐depleted FBs relative to the surrounding medium density move faster toward the Earth. We statistically determined the flux tube entropy parameter pV 5/3 , where p is the ion pressure and V is the flux tube volume of unit magnetic flux, and found that nearly all of the FB events indicate a decrease as compared to the background values. These results clearly support the bubble model of the flow burst events examined here. Also we found that the FBs can be of high or low temperature or ion pressure: while the majority of the FB events show an increased temperature and a decreased ion pressure as compared to those of background, ∼30% of the events indicate an opposite behavior. This does not contradict the bubble idea as long as they are associated with a reduced pV 5/3 . In addition, we found a few other interesting features which can all be interpreted within the bubble model. First, the temperature is found to be overall nearly isotropic during some FB events and only weakly anisotropic during some other FB events. However, we newly found that when it is anisotropic, T ⊥ is higher than T ∥ for ∼63% of the FB events and T ⊥ /T ∥ increases during FBs as compared to that of the preceding quiet time interval. One implication of this result is that the Betatron acceleration takes place more effectively over the Fermi acceleration during the course of the earthward transportation of bubble flux tubes. Also, we newly found that for ∼23% of the FB events, even the small amount of the anisotropy exceed the classic mirror instability limit mainly due to a high plasma beta condition while the firehose instability limit is rarely violated. Last, the total pressure (plasma pressure plus magnetic pressure) decreases for ∼74% of the FB events and does not for the others. A more interesting feature is that while the total pressure change is small for most of the FBs, some events reveal a nonnegligible unbalance with the surrounding medium. This implies the existence of the total pressure gradient force acting on the bubble flux tubes of such events, which likely decelerates the earthward motion of the flux tube, in addition to the accelerating magnetic tension force.