* Nikkei jumps 2.8 pct as yen plunges on intervention
* Speculators hastily cover shorts in stock futures
* Japan forex market intervention is first in 6 years
By Shinichi Saoshiro
TOKYO, Sept 15 (Reuters) - The Nikkei stock average <.N225> jumped 2.8 percent after Japan intervened to weaken the yen, boosting shares of Toyota Motor Corp <7203.T> and other exporters.
Japan intervened to sell yen for the first time in six years, bringing the currency off 15-year highs against the dollar. [ID:nTOE687026]
The action lifted the Nikkei out of negative territory and the index surged as much as 3 percent to a one-month high at one stage.
"The Japanese economy depends heavily on what it earns from exports overseas, so the intervention came at a good time and had an impact after a string of verbal threats," said Tomomi Yamashita, a fund manager at Shinkin Asset Management.
"Corporate performance hasn't been bad but the dollar hovering near 80 yen had made shares unattractive. The authorities were able to put their foot down in the markets, and the next question is can they follow through with steps to help the economy."
The benchmark Nikkei <.N225> gained 2.8 percent to 9,555.50 after touching a one-month intraday high of 9,578.53. It dropped as low as 9,199.08. The broader Topix <.TOPX> rose 1.8 percent to 850.07.
The intervention in the foreign exchange sent short-term speculators scurrying to cover short positions in stock futures, 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." (Editing by Edwina Gibbs)