Just needs to be Snell approved. We have 'loaner' helmets that you can use too if you don't have a helmet for an event....or, as Cordell has offered (i think), you can borrow one from someone else.....
Look for this sticker on the inside of the helmet. It needs to be M2005, M2010, SA2005, or SA2010
http://www.smf.org/images/decals/m05dec.jpg
http://www.smf.org/images/decals/m2010decal.jpg
http://www.smf.org/images/decals/sa05dec.jpg
http://www.smf.org/images/decals/sa2010decal.jpg
Keep in mind that typically the 'M' rated helmets are not good at a track like Mid-Ohio, but the 'SA' helmets are going to be 2-3 times as much as the 'M' rated helmets. Just saying, if you are ever considering doing track time, this will help reduce the change of having to buy two helmets.