Within the last 10 years , strong , narrow Na i absorption has appeared at v _ { \sun } \sim - 38 km s ^ { -1 } toward the halo star HD 219188 ; that absorption has continued to strengthen , by a factor 2–3 , over the past three years .
The line of sight appears to be moving into/through a relatively cold , quiescent intermediate velocity ( IV ) cloud , due to the 13 mas/yr proper motion of HD 219188 ; the variations in Na i probe length scales of 2–38 AU/yr .
UV spectra obtained with the HST GHRS in 1994–1995 suggest N ( H _ { tot } ) \sim 4.8 \times 10 ^ { 17 } cm ^ { -2 } , “ halo cloud ” depletions , n _ { H } \sim 25 cm ^ { -3 } , and n _ { e } \sim 0.85–6.2 cm ^ { -3 } ( if T \sim 100 K ) for the portion of the IV cloud sampled at that time .
The relatively high fractional ionization , n _ { e } / n _ { H } \gtrsim 0.034 , implies that hydrogen must be partially ionized .
The N ( Na i ) / N ( H _ { tot } ) ratio is very high ; in this case , the variations in Na i do not imply large local pressures or densities .