Quantification of cold-ion beams in an earthward bursty bulk flow behind a dipolarization front

The bursty bulk flows (BBFs), probably driven by magnetic reconnection in Earth's magnetotail, carry hot plasma sheet plasmas and magnetic flux towards the inner magnetosphere, which may trigger geomagnetic storms. The magnetotail lobe contains significant amount of cold plasma of ionospheric origin. Whether and how the cold plasma impacts the generation and propagation of BBFs remains unclear. We present an observation of an earthward busty bulk flow behind a dipolarization by the Magnetospheric Multiscale (MMS) mission. Hot ions of plasma sheet origin are found throughout the entire BBF region, while cold-ion beams are widely identified in this BBF. To quantify the distribution and properties of the cold-ion beams, we classify the ion velocity distribution functions (VDFs) of the BBF as only hot ion, hot ion and cold-ion beam with a parallel velocity, hot ion and cold-ion beam with an anti-parallel velocity, and hot ion with bi-directional cold-ion beams. The distribution percentages of the four types are 43.7%, 34.0%, 11.3%, and 11.0%, respectively. The partial moment calculation shows that the cold-ion beams with total speeds of about 1100 km/s contribute 10-20% of the ion number densities. The average temperature (~300 eV) of the cold ions is one magnitude lower than that of the hot ions. We are carrying out further work to evaluate the scattering effect due to magnetic field curvature and plasma waves on the dynamics of the cold-ion beams inside this bursty bulk flow.