Rotation measures towards 500 pulsars show that there is a random and ordered magnetic field dispersed throughout the Galaxy. The ordered component has a field strength of 1-2 µG, while the random component is 5-6 µG with a cell size of about 50 pc.
The random field has a dramatic effect on the evolution of supernovae. While it can be neglected in the early stages of the explosion, as the remnant expands, the trapped magnetic field in the thin, cool shell resists the expansion, and results in a much thicker shell with much lower compression. Now, after a million years, the hot bubble has an inner edge at 60 pc and an outer edge of 90 pc. Over the next four million years, the combined thermal and magnetic pressure of the thick shell forces the hot bubble to decrease in size. After ~ 5 x 106 yr, the bubble radius is only 10 pc or so. Supernova bubbles can still overlap, but generally at a time when they are significantly weaker disturbances. This suggests that, consistent with the observations, the HIM produced by Type II supernovae should be much smaller than predicted by Heiles.