(a) ACE observations for event Two. From top to bottom: proton density, proton bulk speed, proton temperature, pitch angle distribution of ∼272eV electrons, magnitude, components and ϕ angle of magnetic field, and the O⁷⁺/O⁶⁺ and C⁶⁺/C⁵⁺ charge state ratios of oxygen and carbon. (b) Low density region and HCS crossing observed by ACE (vertical red dashed line). The first panel is the proton density data with 12-min resolution from SWICS, the second panel is the proton bulk speed, and the third panel is the ϕ angle of magnetic field. The vertical black dashed line and pink region in both a and b are the position of the shock and the low density region before the stream interface shown by the vertical black solid line.