Low-frequency chorus emissions have recently attracted much attention due to the suggestion that they may play important roles in the dynamics of the Van Allen Belts. However, the mechanism (s) generating these low-frequency chorus emissions have not been well understood. . In this letter, we report an interesting case in which background plasma density lowered the lower cutoff frequency of chorus emissions from above 0.1 fce (typical ordinary chorus) to 0.02 fce (extremely low-frequency chorus). Those extremely low-frequency chorus waves were observed in a rather dense plasma, where the number density Ne was found to be several times larger than has been associated with observations of ordinary chorus waves. For suprathermal electrons whose free energy is supplied by anisotropic temperatures, linear growth rates (calculated using in-situ plasma parameters measured by the Van Allen Probes) show that whistler mode instability can occur at frequencies below 0.1 fce when the background plasma density Ne increases. Especially when Ne reaches 90 cm–3 or more, the lowest unstable frequency can extend to 0.02 fce or even less, which is consistent with satellite observations. Therefore, our results demonstrate that a dense background plasma could play an essential role in the excitation of extremely low-frequency chorus waves by controlling the wave growth rates.
Previous studies suggest that dipolarization fronts (DFs) are 1 to 3RE (RE is the earth radius) wide in the dawn-dusk direction. Recent kinetic simulations have found that DFs may break up into small-scale structures after they are produced by reconnection. Motivated by this simulation, we revisited the scale size of DFs in the dawn-dusk direction by using Cluster observations during the years when the inter-distance among Cluster spacecraft was between 1000 and 2000 km. We selected the DFs that were detected by more than one spacecraft and estimated the radii of these DFs by a simple geometrical analysis, which is based on comparison of DF normals observed by different spacecraft. We found a few DFs that were only a few ion inertial lengths in the dawn-dusk direction. These results point out the importance of multi-scale coupling during the evolution of DFs.