Resonant ULF absorption in storm time conditions

Abstract

_____________________________________________________________________________________________ INTRODUCTIONThe origin of ultralow-frequency (ULF) oscillations of magnetic and electric fields is often associated with the excitation of magnetohydrodynamic (MHD) resonances in near-Earth plasma frozen in the geomagnetic field. In [Chen, Hasegawa, 1974;Southwood, 1974], the authors theoretically analyze the excitation of resonant ULF oscillations of magnetic field lines by surface MHD waves in the magnetosphere. Resonant frequencies of such oscillations have been repeatedly calculated by numerical methods for both dipole [Lee, Lysak, 1989] and nondipole geomagnetic field models [Cheng, Zaharia, 2003]. The numerical calculations established typical values of minimum resonant frequencies 3-5 mHz (in the dayside magnetosphere) for quiet geomagnetic conditions. A significant limitation of such studies was the approximation of infinitely large ionospheric conductivity. This approximation restricts field line resonances only to eigenoscillations of the so-called classical half-wave mode, i.e. to standing waves for which an integer of half-waves fits between conjugated ionospheres along magnetic field lines. In this case, resonant oscillations of the magnetic field line are analogous to acoustic oscillations of a string whose ends are fixed in the E layer of the conjugated ionospheres [Nishida, 1980]. Taking the finite ionospheric conductivity into account revealed first and foremost a significant damping decrement of the fundamental harmonic of the half-wave mode for sufficiently low ionospheric conductivity [Newton et al., 1978]. In addition, the finite ionospheric conductivity supplemented field line resonances with quarter-wave oscillations, for which an odd number of quarter wavelengths fits between the conjugated ionospheres [Allan, Knox, 1979]. Such oscillations are similar to acoustic oscillations of a pipe whose one end is fixed and the The differences between the experimental and theoretical estimates can be explained by a strong effect of nondissipative energy losses on the Q factor of the resonator [Poulter, Allan, 1985]. At high latitudes, i.e. at L~10 and at low ionospheric conductivity under polar winter conditions, the theoretical Q-factor estimate taking into account only ohmic dissipation [Yumoto et al., 1995] gives Q 1~1 . At high latitudes, there appear additional non-dissipative losses produced, for example, by magnetospheric convection. Indeed, convection trajectories in the general case do not coincide with constant-period magnetic surfaces of the field line resonance. In the drift motion, magnetospheric plasma along with field-aligned currents leaves the resonance region. This causes additional losses of resonator energy and reduces the Q factor. With a low Q factor of the resonator, the eigenoscillation amplitude does not exceed the amplitude of background waves; this hinders the observation of resonant ULF oscillations at high latitudes.High oscillation amplitudes at 2.7, 3.5, and 3.9 mHz, found in...