The dollar plummeted to a 15-year low against the Japanese yen Thursday, despite the Bank of Japan's reported intervention. But share prices rose on the Tokyo Stock Exchange. The dollar closed at 121.52 yen, down 2.02 yen from Wednesday's close of 123.54 yen. Opening at 122.40 yen, the currency ranged between 121.30 and 122.40 yen. Thursday's close was the lowest since the Tokyo market began floating the exchange rate in 1973.