A STATISTICAL STUDY OF THE RELATIONSHIP BETWEEN THE TRANSPORT RATE OF MAGNETIC HELICITY AND SOLAR FLARES

We present a statistical study which is aimed at understanding the fact that some flares (type I flare) are associated with sharp variations of the transport rate of magnetic helicity (dH/dt) while others are not (type II flare). The sample consists of 49 M-class and X-class flares which were produced by nine isolated active regions. Using high temporal magnetograms obtained by the Michelson Doppler Imager instrument on the Solar and Heliospheric Observatory, we calculate the temporal variation of dH/dt during the flaring time, and compare its profile with the soft X-ray flux. We find that type I flares have longer duration and higher peak flux in soft X-ray than type II flares. Furthermore, the ratio of the total unsigned magnetic flux of the host active region to that of the visible solar disk is also higher for type I flares, while the total flux itself is independent of the flare type. Our results show that whether the flare is associated with sharp variations of dH/dt depends on the properties of the flare and of its host active region. The relationship between dH/dt and microwave bursts is also discussed.