We calculated the collision frequencies based on the C2 model, and found that collisions with neutral H dominate the transport of heavy neutral atoms such as O below 3.5Rp. At altitudes higher than this, collisions with H+ are more frequent. In Paper II we demonstrate that a mass loss rate of 6 × 106 kg s−1 is required to prevent diffusive separation of O (the heaviest neutral species detected so far) in the thermosphere. The mass loss rate in our models is Ṁ>107kgs-1 and thus O is dragged along to high altitudes by H. On the other hand, collisions with H+ dominate the transport of heavy ions such as Si+ as long as the ratio [H+]/[H] ≳ 10−4 (Paper II). This explains why Coulomb collisions in our models are more frequent than heavy ion–H collisions at almost all altitudes apart from the immediate vicinity of the lower boundary. These collisions are much more efficient in preventing diffusive separation than collisions with neutral H.
collisions with neutral H dominate the transport of heavy neutral atoms
below 3.5Rp
Rp
mass
oss rate
6 × 106 kg s−1
kg s−1
models
Ṁ
>107kgs-1
kgs-1
[H+]/[H]
ratio
≳ 10−4