In this research project the properties of dipolarization fronts (DFs) and dipolarizing flux bundles (DFBs, the region led by the DF) have been statistically studied, using data from the Cluster and Magnetospheric Multiscale (MMS) spacecraft missions.
The statistical study showed that DFs are dominated by two categories: DFs with high ratios of the plasma temperature and density from before to after the DF (category A) and DFs with low temperature and density ratios (category B). Based on the following arguments relating to these DF categories, we suggest that category A DFs may evolve into category B events and that category B is at a later stage as the DF propagates earthward from the taillike region to the near-Earth dipole region:
1. Both categories, A and B, are independent of the S/C crossing location (DF dawnside or duskside) and/or the observation position in the tail (premidnight or postmidnight). This suggests that A and B are not specific to regions in the tail, e.g., premidnight or postmidnight, but characterize individual DFs at a particular stage of evolution, regardless of the observation location.
2. For category A, the plasma flow veloctiy, Vx, gradually increases over the front. That is, the magnetic flux from behind the DF moves faster and accumulates at the DF (growing DFB). For B, however, the maximum Vx is colocated with the DF onset and decreases slightly over the DF (decaying DFB). Previous studies showed that the distance between the velocity peak (maximum Vx ) and the DF decreases closer to the Earth. This suggests an evolution of the DFBs from a more growing-like type further
downtail to a more decaying-like type closer to the Earth.
3. The maximum plasma flow velocity in XY-GSM plane is on average 360±200 km/s and 230±120 km/s for Aand B, respectively. Category A has higher velocities than category B, and we interpreted this as B occurring in a later stage after reconnection onset, since flow/DF brake at near-Earth dipolefield regions where background Bz is large.
4. Category B is in a more dipolarized region: The magnetic field before the DF is on average stronger (∼ 4 nT) than for category A.
5. Category A the thermal pressure is over the entire DF structure (DF dawnside and duskside) higher before the DF crossing with a higher thermal pressure at the DF duskside. For category B, however, the thermal pressure change varies strongly between different crossing locations, indicating a disordered structure, which might be expected closer to the flow-braking region. As the flow starts to brake, this might suggest that the DF structure becomes unstable, e.g., due to the conversion of the flow energy to wave energy and thus asymmetry in the thermal pressure around the DF gets lost.
6. The results show that the plasma flow direction of category B is along the magnetic tension force
direction, while for category A the plasma flow is slightly tilted toward the DF duskside. A possible explanation for the flow tilt may be the magnetic curvature drift, which turns the jet duskward. The flow tilt and magnitude will probably depend on the width of the initial current sheet: A thinner current layer will lead to smaller curvature radius and thus to a larger curvature force. On the other hand, a thicker current layer will yield a smaller curvature force and the duskward flow tilt disappears. Accordingly, we interpret that category A is closer to the reconnection region where the current sheet is thinner, while category B is closer to the flow-braking region with a broader current layer.
With MMS at radial distances within 12 RE and Cluster at 19 RE, it is for the first time possible to compare the inner and outer magnetotail regions using multispacecraft observations of DFs. Assuming the DF to be a stable, semicircular structure, propagating along the magnetic tension force, the major results obtained in this comparative study are as follows:
1. A larger fraction of the DFs move faster closer toward Earth than farther down the tail. This fact indicates that a strong magnetic flux transport can take place even in the inner magnetosphere.
2. Larger DF velocities actually correspond to higher Bz values directly ahead of the DFs. This behavior is observed by both Cluster and MMS, although they are located in different regions in the tail (more/less dipolar magnetic field). We interpret the higher Bz to a local snow plow-like phenomenon resulting from a higher DF velocity and thus a higher magnetic flux pileup ahead of the DF.
3. There is also a significant number of tailward moving DFs observed from both, Cluster and MMS. Since it is unreasonable to assume reconnection so close to Earth, the tailward propagating events are the result of a DF rebound (bouncing) at the magnetic dipole-dominated near-Earth plasma sheet: The fast-moving DFs get first compressed at the dipole-dominated region and are then reflected tailward. Indeed the observation shows compressed DFs with smaller temporal scales and spatial thicknesses at MMS than at Cluster. As the DFs move tailward, the magnetic tension force slows them down. In agreement with this picture, there are no fast tailward moving DFs at Cluster. Only MMS observes fast tailward propagating DFs, with high elevation angles before the DFs. We interpret the high elevation angles as the remnants of previously earthward propagating DFs. Thus, we suggest that the fast tailward moving DFs are recorded directly after the rebound of the fast earthward moving DFs.