The line of sight to βCMa has been probed by Copernicus observations. This particular line of sight is remarkable for the low mean densities. We find n̄H ∼ .002 cm-3. However we can distinguish two separate regions:
1) A local nearby HI region extends over a few parsecs from the sun with a density of the order of 0.1 cm-3 and a temperature of 11000 to 12500 K.
2) An HII region lies somewhere beyond the HI region and is spread over about 60pc. Its total hydrogen mean density is of the same order as the HI region, i.e. of ∼0.1 cm-3 and it contains only elements in low ionization state. All the data are coherent with the picture of a cloud in ionization equilibrium at T ∼ 23 000° K.