I've heard that, on Thursday, Thinkorswim temporarily increased intraday margin requirements on futures to 50% of the contract value. I realize it was for Thursday only, but that sounds like a drastic jump that would have caused people to get instant margin calls at the worst possible moment. The margin on ES is normaly roughly 5620 per contract, and if they suddenly increase it to 50% of the contract value, then that would be a roughly 5 fold increase in equity needed in one's account per contract. Seems to me that, if true, this would cause forced selling at low prices. Is this really true, or did they merely increase the requirements by 50% rather than implement a huge jump in requirements to 50% of contract value? Was this for new positions only, or for any open positions (which would cause margin calls for many people)?