* Nikkei jumps 1.8 pct as yen plunges on intervention
* Speculators hastily cover shorts in stock futures
* Japan forex market intervention first in 6 years
* Focus on whether the yen can be kept on the back foot
By Shinichi Saoshiro
TOKYO, Sept 15 (Reuters) - Japan's Nikkei average rallied on Wednesday, reversing earlier losses as the yen plummeted from a 15-year high against the dollar as the authorities stepped in to prevent the currency from probing an all-time high against the greenback.
The intervention in the foreign exchange market by Japan, the first in six years, sent short-term speculators scurrying to cover short positions in stock futures, lifting the Nikkei to a one-month intraday high, market players said.
"The key points for the stock market going forward are if the Nikkei can remain on an uptrend once the short-covering in futures peters out," said Yutaka Miura, a senior technical analyst at Mizuho Securities.
"That in turn may depend on whether the yen remains on a weaker footing. The point here is that the yen hasn't strengthened on domestic factors alone -- the easing stance by the United States and low U.S. yields have played a key part as well."
The dollar jumped to 84.40 yen shortly after hitting a 15-year low of 82.87 yen on trading platform EBS. That earlier rise in the yen followed a Japan ruling party vote victory on Tuesday by Prime Minister Naoto Kan over heavyweight Ichiro Ozawa, who had been more strident in his calls to intervene to weaken the yen.
The dollar hit a record low of 79.95 yen in 1995.
Shares of exporters, which had been dogged by the yen's recent strength, snapped back from earlier losses and gained, with Sony Corp rising 2.4 percent to 2,553 yen after losing as much as 2.4 percent.
Electronics parts maker Kyocera Corp gained 1.8 percent to 8,000 yen. (Editing by Michael Watson)