By Hideyuki Sano

TOKYO, Sept 9 (Reuters) - The yen fell from a 15-year highagainst the dollar while the euro regained ground aftersuccessful bond auctions in Portugal and Poland revived someappetite for risk.

But traders remain unconvinced that this positive mood onrisk will last long enough to reduce expectations that the yenwill continue to rally, testing the willingness of Japanesepolicy makers to intervene in forex markets.

"With dollar/yen, virtually all kinds of moving averages andtrend lines are in a downtrend. So there are many hurdles toclear but the dollar needs to clear the five-day moving averageof 84.00 at the least," said Teppei Ino, an analyst at Bank ofTokyo-Mitsubishi UFJ.

Dollar/yen held firm at 83.95 yen, up from Wednesday's15-year low of 83.34 yen.

Ino said the greenback seems to have some support at thelower end of its Bollinger Band around 83.35, noting that thedollar rebounded at that level on Wednesday.

Dollar/yen needs to rise above its 21-day moving average, at84.77 on Thursday, to reverse its downtrend that started in earlyMay from around 95 yen, Ino added.

That is likely to be an uphill battle as Japanese exportersare thought to be eager to take advantage of a rise in dollar/yento sell the dollar ahead of their half-year book-closing at theend of September.

The euro stood at $1.2730, after rising about 0.3 percent onWednesday as Portugal raised 1.04 billion euros, helping soothefears about government funding in Europe. Poland's sale offive-year bonds also saw solid demand.

The Australian dollar changed hands at $0.9175, holding neara one-month high of $0.9194 marked on Wednesday, with traderslooking to Australian jobs data due at 0130 GMT.

The currency has met resistance at around $0.92 in recentsessions, though strong jobs numbers could help push the Aussieabove that level and $0.9223, a three-month high struck inAugust.

The Canadian dollar kept Wednesday's hefty gains following arate hike by the Canadian central bank. It stood at C$1.0370 perU.S. dollar.

Later on Thursday, the Bank of England will hold a monetarypolicy meeting, where it is expected to keep rates on hold at 0.5percent and to refrain from increasing its asset purchaseprogramme. The pound traded at $1.5460, off a 1 1/2-month low of$1.5296 hit on Tuesday. (Reporting by Hideyuki Sano; Editing by Joseph Radford)