Yep. Snell 95 or newer.
Either M (motorcycle) or SA (severe application) rating.
From the rule book:
Helmets meeting the following standards must be worn while on course:
All helmets meeting the current or two immediately preceding Snell Foundation standards
(SA, K, or M), or SFI standards 31.1A, 31.2A, 41.1A, or 41.2A are acceptable.
FIA approved helmets which are acceptable for use in Rally events are acceptable for Solo II events.
Next year, the requirement will be moved up to Snell 2000.