Kelvin-Helmholtz instability along the flanks of the magnetosphere exhibits a semiannual variation, instability maximums occurring at the equinoxes and instability minimums at the solstices. It is suggested that Kelvin-Helmholtz instabilities, generated at the magnetopause, initiate the modulation of geomagnetic disturbance detected as the semiannual variation of geomagnetic activity. The Kelvin-Helmholtz explanation predicts a universal time variation of geomagnetic disturbance. This prediction is confirmed by several analyses of the geomagnetic activity indices. Since the physical mechanism suggested for the semiannual variatioh of geomagnetic activity depends on the tilt of the dipole axis, the 'equinoctial' hypothesis is supported.The semiannual variation of geomagnetic activity is a well-established phenomenon, having maximums near the equinoxes and minimums near the solstices (range •15 7). There The lack of a physical mechanism to adequately explain the cause of the semiannual variation has been the major problem. A knowledge of the mechanism will identify the parameters responsible for the phenomenon and therefore make possible the understanding of the semiannual variation of geomagnetic activity. K½,nvIN-H½,nM I-IOnTZ HYPOTHESISIt is the purpose of this paper to associate the semiannual variation of geomagnetic activity with the variation of Kelvin-Helmholtz in- 6073

The previously studied variation of geomagnetic activity with lunar phase indicates a g,eneral decrease in geomagnetic activity of about 4% for several days before full moon and an increase of about 4% for several days after full moon. Substantial evidence is presented indicating that the observed variation of geomagnetic activity with lunar phase requires lunar latitudes at full moon within about 4 ø from the ecliptic plane. It is demonstrated that the influence of a variable lunar parallax must be .extremely small, if it exists at all. Some difficulties are presented with both a tidal mechanism and a geomagnetic tailneutral sheet mechanism for explaining the observed variation of geomagnetic activity.

The northern hemisphere 700-mbar contour heights from 20 ø to 70øN for the period 1947-1970 are studied in conjunction with 272 key days, where the daily increase of the Ci index equals or exceeds 1.0. The superposed epoch method is applied from 33 days before to 66 days after the key day for a variety of zonal and meridional indices. It is shown that the 700-mbar height difference between 20 ø and 55øN increases significantly in winter 4 days following geomagnetic disturbance (in summer a less prominent but statistically significant increase is found 2 days earlier). The effect is most clear in winter in the quadrant 90ø-175øW and corresponds to a 7% increase in the mean geostrophic westerly flow. The statistical significance of the results is established by applying Student's t test to the difference of each daily mean from the continuum. Synoptic analyses of the departures of the mean 700-mbar contour heights from seasonal climatology following geomagnetic disturbance reveal that the effect proceeds with the growth and development of large negative centers in the latitude belt 40ø-60øN and smaller positive departures at lower latitudes.A number of studies [Mustel et al., 1965; Mustel, 1966; Kubyshkin, 1965 Kubyshkin, , 1966 claim to have discovered surface pressure responses to solar geomagnetic disturbances. On the other hand, studies by Stolov and Spar [1968] and Stolov and Shapiro [1969] failed to substantiate the reported claims. More recent studies by Sarukhanyan and Smirnov [1970] and Mustel [1970] indicate that the zonal character of the atmospheric circulation is disrupted and that meridional processes are enhanced during disturbed periods. However, Stolov and Shapiro [1971] find no evidence for this claim.In earlier work, Shapiro [1956, 1959, 1972] working with sea level pressure data found significantly high persistence during the first week after large increases in geomagnetic activity along with an increase in the average north-south pressure gradient. A current study by Roberts and Olson [1973] confirms their earlier result connecting geomagnetic activity with a subsequent deepening of 300-rribar troughs.Encouraged by these positive results, we have been stimulated to extend our previous investigation. In this latest study the strong seasonal trend in the 700-mbar heights introduced by ordinary meteorological factors is removed as a possible obscuration in the detection of real general circulation changes following solar geomagnetic activity. Apparently, just such an approach was necessary. DATA The meteorological data consist of 1200 UT sea level pressures and 700-mbar heights from 20 ø to 70øN for the period January 1, 1947, to December 31, 1970. The data are available at 5 ø latitude intervals and 10 ø longitude intervals in a diamond-shaped grid. As measures of solar geomagnetic disturbance we selected in the preliminary investigation [Stolov and Shapiro, 1971]: (1) the 41 chromospheric flares (1956-1960) producing geomagnetic disturbance used by Mustel et al. [1965], (2) the 39 chromos...

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Explorer :t8 magnetometer and plasma probe data are used to calculate the magnitude of the Kelvin-Helmholtz instability criterion for 42 crossings of the magnetopause. The 30 crossings through the dipole field region show that for low levels of geomagnetic disturbance (Kp _• 2) large regions of the magnetopause near the nose are stable, whereas for high6r levels of geomagnetic disturbance (Kp > 2) only a small region near the nose is stable. The unstable regions are found further downstream. Similarly, the 12 crossings through the tail show that the unstable regions are downstream of the stable regions. The Kelvin-Helmholtz mechanism might be the 'viscous interaction' discussed by Axford and Hines and the relevant parameter for worldwide geomagnetic disturbance, just as the southward interplanetar• field has been firmly established as the major relevant parameter in the case of substorm activity. Boller and $tolov [1970] have explained the cause of the semiannual variation of.geomagnetic activity by examining Kelvin-Helmholtz instability at the magnetopause lAxlord1968; Southwoo:d, 1968]. This mechanism may contribute to the convection of dipole magnetic fields and may be a companion to the Dungey reconnection mechanism for the transfer of energy from the solar wind to the magnetotail [Piddington, 1960, 1962; AX]ord and Hines, 1961; Dungey, 1961]. The increase of the magnetic field in the magnetotail and the sudden release of the stored energy from the tail initiate a series of related processes known as the polar magnetic substorm [Akaso[u, 1968; Rostoker, 1972]. The magnetospheric substorm is one class of geomagnetic disturbance. The other class of geomagnetic disturbance is the 'worldwide geomagnetic activity' discussed by Hirshberg and Colburn []969]. The Kelvin-Helmholtz mechanism might be the 'viscous interaction' discussed by Ax]ord and Hines [i961] and the relevant parameter for worldwide geomagnetic disturbance, just as the southward interplanetary field has been firmly established as the major relevant parameter in the case of substorm activity. Copyright (•) 1973 by the American Geophysical Union. ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß 2330 ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß •' ß ß ß ß ß ß 0 O0 0730 0800