The correlation between the solar wind parameters and geomagnetic activity represented by several indices is utilized to examine the effects of the azimuthal component of the interplanetary magnetic field on solar wind-magnetosphere coupling. It is suggested that the |B_y| dependence of geomagnetic activity can be expressed by using the theoretical equation of the dayside reconnection. It is emphasized that the influence of B_y on different indices varies significantly. The seasonal variations of the B_y dependence of the AL index is discussed in the light of the viscous-like interaction hypothesis.

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When the magnetic field and the fluid velocity in the Earth's core are expressed by toroidal and poloidal modes, four types of interaction are conceivable between them: interaction of (a) poloidal velocity (ν_S) with poloidal magnetic field (B_S) [ν_S×B_S]; (b) poloidal velocity with toroidal field (B_T) [ν_S×B_T]; (c) toroidal velocity (ν_T) with poloidal field [ν_T×B_S]; and (d) toroidal velocity with toroidal field [ν_T×B_T]. The last type of interaction [ν_T×B_T] does not induce any poloidal magnetic field. We therefore examine the first three types of interaction that induce the poloidal magnetic fields, which are to be compared with the observed non-dipole field.
If we assume a dipole field and a toroidal field of the second degree and order zero, the first two types of interaction, the poloidal-poloidal interaction [ν_S×B_S] and the poloidal-toroidal interaction [ν_S×B_T], induce two modes of poloidal field, (l-1, k) and (l+1, k), from the velocity of the (l, k) mode. When a plausible radial function is assumed for the velocity, the lower mode (l-1, k) always predominates, indicating that the lower mode is more easily observable.
Among the three types of interaction, the poloidal-poloidal interaction [ν_S×B_S] is the most efficient for inducing the poloidal field, giving the largest amplitude of the induced field for the same magnitude of the velocity and the primary field. The amplitude of the field which is induced by the poloidal-toroidal interaction [ν_S×B_T] and the toroidal-poloidal interaction [ν_r×B_S] decreases with increasing velocity, if the magnitude of the velocity is constant irrespective of the degree of the velocity. The decrease is the steepest with the toroidal-poloidal interaction. If we ascribe the cause of the standing field to this type of interaction, the velocity of higher harmonics should be predominant in the core, because the observed slope of decrease for the standing field is less steep than that calculated for the constant velocity. On the other hand, if we assume other interactions, the velocity of the lower harmonics should be predominant.

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The characteristics of mid-latitude spread-F occurrence are reviewed (in the light of some recent publications) in terms of four different scale sizes. The term “macroscale” has been used to describe significant height rises and electron-density depletions which in general precede spread-F occurrence. TID wavetrains in the form of wavelike modulations of these height rises are termed “large-scale” structures. The experimental evidence suggests that off-vertical reflections from tilts associated with these wavetrains are responsible for the basic components of spread traces on ionograms. Also, it is proposed that “medium-scale” structures at the F₂-layer level are a result of stratification effects, and finally that field-aligned irregularities which sometimes produce weak spread traces, be termed “small-scale” structures. Attention is drawn to the fact that structures similar to those proposed here for mid-latitude spread-F appear to be related to other phenomena in equatorial and sub-auroral zone regions. It is suggested that the breaking of atmospheric gravity waves has a central role to play in the development of these mid-latitude spread-F structures.

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