Field and particle data recorded on the geostationary satellite GEOS 2 are used to investigate the electric and magnetic signatures of a substorm characterized by a dispersionless injection of energetic electrons and ions. Three types of field variations are observed: (1) Long-period oscillations with period of • 300 s, interpreted as oscillations of entire field lines. These oscillations develop as second harmonic standing waves and correspond to coupled shear Alfv•n-slow magnetosonic modes. They grow after the most active period of the breakup. (2) Short-period transient osci•ations with periods of • 45-65 s, interpreted as wave modes trapped in a current layer which develops prior to the substorm breakup and is disrupted at breakup. These osci•ations also correspond to a coupled shear Alfv•n-slow magnetosonic mode (coupled via magnetic field curvature effects in a high-fi plasma). The short-period transient osci•ations are only observed during the most active period of the breakup. (3) A nonoscillatory sharp increase observed on both the parallel magnetic component and the energetic ion flux, averaged over one satellite rotation, interpreted as evidence for the fast magnetosonic mode which in view of the simultaneous large impulsive increase in the azimutha• electric field, appears to propagate radially outwards, transporting the substorm breakup downtail. Paper number 95JA00990. 0148-022 7 / 95/95J A-00990505.00 Alfv•n oscillations associated with this substorm, the electric and magnetic field measurements have been reanalyzed with new data analysis techniques. A characterization of the low-frequency oscillations at and after substorm breakup is in fact one of the prerequisites for understanding the overall mechanisms of substorm onset and recovery. We are faced with the following questions: Do the oscillations trigger the breakup, or are they a consequence of the breakup? In somewhat different terms, do they appear at onset or at a later time? Do they play a role in generating the auroral westward traveling surge (WTS) and the associated discrete auroral forms (DAF)? To answer such questions, what must be known is the frequency spectrum of the oscillations, as well as their polarizations and modal structures, the ti. me of onset, and the maximum amplitude of each frequency component, subject to the limitations of the uncertainty in the simultaneous localization of frequency and time of each transient. This is no easy task, for a number of reasons. The eigenfrequencies of the oscillation s at the satellite location are not constant in time, since the shape, length, and mass loading of the corresponding field line vary rapidly during the dipolarization process associated with the substorm; hence the determination of a time-varying eigenmode frequency spectrum is required. Furthermore, the duration of the physically significant wave activity associated with the substorm may last no more than 15 to 20 min, whereas the wave eigenperiods may be of the order of 10 min or more. Such low-frequency oscillations, if de...

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In absorbing media, where Maxwell's equations are not seif-adjoint, the adjoint field is introduced via the differential operator adjoint to the Maxwell operator. The concomitant vector can be made equal to the time averaged Poynting vector at a boundary with a non-absorbing medium. In general, the adjoint field represents an electromagnetic field in a medium other than the absorbing medium under consideration. To draw conclusions about the latter, a [conjugate field] in this medium is defined, using a conjugating transformation of the Maxwell operator and field. Relations between the conjugate and adjoint fields are established, allowing one to gather physical information about the first absorbing medium from the adjoint field.