An Unexpected Whistler Wave Generation Around Dipolarization Front

Dipolarization front (DF)—a sharp boundary separating hot tenuous plasmas from cold dense plasmas—is followed by a strong B_z region, which is termed flux pileup region (FPR) or dipolarizing flux bundle (DFB). Using 9-year (2001–2009) Cluster data, we show that the FPR hosts whistler waves because of the pancake distribution of electrons, whereas the DF boundary hosts lower hybrid drift waves due to the strong density and magnetic gradients statistically. Different from statistical results, we report an unexpected case: whistler waves are generated at the DF boundary rather than inside the FPR. Using Cluster data, we observed strong whistler waves at DF boundaries but no whistlers inside FPRs, although flux tubes inside the FPRs are significantly compressed and suprathermal electrons there are perpendicularly anisotropic. We calculate wave growth rates and successfully explain the generation/damping of these whistlers. We find that 1–4 keV electrons are responsible for generation/damping of these whistlers.