A basic new result from the Hard X-ray Imaging Spectrometer (HXIS) observations, is that flaring activity as seen in X-rays usually encompasses two or more interacting magnetic bipoles within an active region (Machado and Moore, 1987; Machado et al., 1987a,b). This picture contrasts with the standard Skylab classification of compact and two-ribbon flare types (Sturrock, 1980), in which single bipole flaring and even "single loop" flaring was stressed, leading to particular efforts in theoretical modelling. The fact that flaring encompasses multiple bipoles (i.e. flare loops straddle two or more neutral fines), favors in principle a different picture which is reminiscent of the earlier flare models like those of Sweet and Syrovatskii, where energy release occurs at the interface between neighboring loops, the site of X-type neutral points where current sheets can form. In an extensive study of flares from nine different active regions, which were observed by the HXIS and the Marshall Space Flight Center (MSFC) vector magnetograph, we have established the following set of properties:
- The basic structure of a flare usually consists of an "initiating" bipole and one or more adjacent bipoles impacted against it.