A quadruple magnetic source model for arcade flares and X-ray arcade formations outside active regions - I. Dark filament suspension and the magneticstructure in the pre-event regions

The high-sensitivity, wide dynamic-range observations by the Soft X-ray Tel escope (SXT) aboard Yohkoh has enabled us to look into the faint pre-event structures of arcade flares, and of even fainter X-ray arcade formation eve nts outside active regions. What we have found in the pre-event structure o f the latter, however, was not a sagged simple (bipolar) arcade, as expecte d in the classical model, but a "dual-arcades" type structure in which the inside legs of each "arcade" cross with the other's, landing at the closer part of the domain of the other. Similar features, together with some other features inexplicable in the classical arcade flare model, were also found in strong arcade flares in active regions seen axis-on at the limb. These features raised a severe problem with the classical "reclosing of the once opened simple arcade" model(s). In the present paper, we propose interpreta tions of what we discovered by Yohkoh-SXT by reviving a quadruple source mo del proposed by one of the authors (YU) years ago, pointing out a serious d ifficulty in the classical model(s). This model, based on the quadruple mag netic sources in the photosphere, has a "neutral sheet" already in the pre- event phase in the corona above the field polarity-reversal line in the pho tosphere, and turns out to explain quite nicely the structures of both fain t pre-event corona before arcade formation events, and that of arcade flare s discovered by Yohkoh-SXT. A dynamic model of arcade flares and arcade for mation events based on this dark filament model will be discussed in the fo rthcoming Paper II of this series.