SAO PAULO, Oct 13 (Reuters) - Brazil"s central bank sold $1 billion worth of currency swaps in a surprise auction on Wednesday afternoon, and said later it would hold another sale on Thursday, seeking to boost the local currency from nearly six-month lows. The central bank said it sold all 20,000 foreign exchange swap contracts offered in a nonscheduled auction – double the volume offered in the bank"s previous surprise auction on Sept. 30. The sale had an immediate impact on the real, which reversed losses to trade up around 0.5% at 5.51 reais to the dollar. Later on Wednesday, the bank said it would sell up to another $1 billion in a second nonscheduled auction, due to take place on Thursday morning. The real is down by nearly a third against the U.S. dollar this year, with investors spooked by political risks, a fragile economic recovery and the prospect for rising interest rates in more wealthy economies. Last month, Brazil"s central bank said it would hold additional twice-weekly auctions of traditional currency swaps, to address demand from banks dismantling their "over-hedge" positions in derivative markets. Earlier on Wednesday, the central bank"s political economy director, Fabio Kanczuk, said there had been a smaller-than-expected market reaction to the bank"s announcement of those regular swap auctions. Reporting by Luana Maria Benedito; Additional reporting by Marcela Ayres in Brasilia; Writing by Gabriel Stargardter; Editing by Brad Haynes and Peter Cooney Our Standards: The Thomson Reuters Trust Principles.