A study of the CDAW 9C substorm of May 3, 1986, using magnetogram inversion technique 2, and a substorm scenario with two active phases

One of the ('DAW 9(' substornts is investigated in this pa. per using the database reported by Hones et al. and supplelnented with magnetogram inversion technique (MIT) 2 data.. These latter have provided information about the dynamics of the open ta. il magnetic flux, current syst. ems in the ionosphere, and the size a. nd dyna, lnics of the current wedge. We have identified the growth, expansion, and recovery phases of this substorm, with characteristics expected from a generally accepted scenario. However, specific signatures were observed in the interval (0919- 0935) (?T, i.e., between the growth and expansion phases, indicating the concurrent development of the substorln onset and corresponding instabilities in the innermost current sheet,, and slna,ll-scale cross-tail current disruptions without the open tail reconnection. In addition to signatures of small-scale dipolariza, tion, an increase of the open tail magnetic flux, and a current system of the type close to D P 2 were observed a,t. (0919-0935) tIT, which is more likely to suggest predominance of the tail-stretching process than magnetic colla.pse. This fact was interpreted in letres of a releva, nt simple model a.s a signa.ture of the growth of the energy input h'om the solar wind which ensures the observable disturbance power. Hence the disturba,nce a,t (0919-0935) UT was more likely a driven one than a,n mfioading one. The aforementioned signa,tures make it possible t.o identify the interva.l (0919-0935) UT a,s the "pha._e of multiple onsets" or (equivalently) the "first a, ctive phase," which was previously defined by Mishin (1991, and references therein) a,s one of the four standard phases of a typical substorln (in addition to the expansion phase). Thus the ca. se st.tdy supports the substorm scenario with two active phases and, a. ccordingl.x..', with two different kinds of physics. This case study illustrates also the inforlnativity of MIT 2 data and their ability to effectively complement the database tra, ditionally used in substorm studies.