Mohe is the only place in China where you can reliably see the aurora borealis, and for this reason alone it draws winter travellers from across the country. The season runs from November to March, with December through February offering the longest and darkest nights. Clear skies away from even Mohe's modest light pollution reveal curtains of pale green moving slowly above the river valley.
The standard approach is to stay two or three nights to improve your odds. The weather in Mohe is cold enough that it is dry and clear more often than not in winter, but any significant cloud cover kills the view entirely. Local guesthouses and hotels have developed a routine around aurora watching: dinner, a few hours of rest, then a 2am alarm and a walk to a dark field or riverbank. The effort is proportionate to the reward when the lights appear.