Electron Velocity Moments in the Solar Wind. II. Statistical Properties

Between 2005 January 1 and 2017 November 25, the Wind spacecraft measured 3,996,051 electron velocity distribution functions (VDFs) in the near-Earth solar wind. The instrument was calibrated, and velocity moments were calculated up to the third moment (i.e., electron heat flux). The magnetic field and ion velocity moments have been interpolated to the midpoint time of the VDFs, where we have 3,947,607 finite electron velocity moments (most prior studies used <300,000 moments). The median values for the velocity moments are as follows: n_e,int ∼ 5.43 cm⁻³, ∣V_oe∣ ∼ 421 km s⁻¹, T_e,tot ∼ 12.7 eV, and ∣q_e,∥∣ ∼4.92 μW m⁻². The median values for the electron-to-proton temperature ratio is (Formula presented) Te,tot/Tp,tot ∼2.55, the electron temperature anisotropy A_e = (Formula presented) Te,⊥/Te,∥ ∼ 0.89, total electron beta β_e,tot ∼ 1.27, electron–electron collision frequency ν_ee​​​​ ∼ 0.18 day⁻¹, and electron–electron Knudsen number K_n,ee ∼ 2.57. The results are mostly consistent with previous studies except that K_n,ee is significantly larger than some other works, which implies that the near-Earth solar wind is less collisional than may have been inferred.