Abstract

Hard X-ray, soft X-ray, and microwave images of about 20 solar flares that occurred in NOAA 7270 are compared for investigating where electrons are energized and emit hard X-rays. We find the following: (i) Usually hard X-ray sources appear inside a soft X-ray flaring loop or loops. The hard X-ray emission tends to originate from the two ends of the soft X-ray loop in the impulsive phase (double footpoint sources), and later the double sources evolve into a single source located near the loop top (loop-top gradual source). (ii) In a few cases, another impulsive hard X-ray source with comparable intensity with the double footpoint sources appears at a site remote from the footpoint sources. This source shows only weak brightening in soft X-rays but is bright in microwaves.