| Component | Temperature [K] | Average hydrogen number density $n_H$ | Fractional ionization $n_e/n_H$ | Comment |
|---|---|---|---|---|
| Cold neutral medium (CNM) | $80$ | $4\times 10^7$ | $10^{-3}$ | HI clouds, 3-30 ly, 1-1000 solar mass |
| Warm neutral medium (WNM) | $6000$ | $4\times 10^5$ | $0.1$ | Intercloud medium |
| Warm ionized medium (WIM) | $8000$ | $3\times 10^5$ | $0.7$ | 30% of ISM mass |
| Hot ionized medium (HIM) | $10^6$ | $2\times 10^3$ | $1.0$ | Residence of the Sun |