Direct detection of solar wind dynamic pressure effect on ground geomagnetic field

<jats:p>The scatter plot of the equatorial <jats:italic>D<jats:sub>st</jats:sub></jats:italic> index versus the square root of the dynamic pressure of the solar wind (<jats:italic>P</jats:italic><jats:sup>0.5</jats:sup>) showed a clear upper envelope which linearly increases with increasing <jats:italic>P</jats:italic><jats:sup>0.5</jats:sup>. This upper envelope was interpreted as showing the minimum level of the ring current intensity and thus dependence of the <jats:italic>D<jats:sub>st</jats:sub></jats:italic> index upon <jats:italic>P</jats:italic><jats:sup>0.5</jats:sup>. The similar upper envelope was also found in scatter plots of the nighttime <jats:italic>H</jats:italic>‐component versus <jats:italic>P</jats:italic><jats:sup>0.5</jats:sup> at stations in low and middle latitudes. From inclination of these upper envelopes the coefficients for the assumed linear relationship between <jats:italic>P</jats:italic><jats:sup>0.5</jats:sup> and the ground magnetic field were obtained. The coefficients derived from the envelopes for the <jats:italic>D<jats:sub>st</jats:sub></jats:italic> index and the nighttime <jats:italic>H</jats:italic>‐component at different stations agree well each other and the latter showed no latitudinal variation. The scatter plots also showed a clear linear lower envelope with negative inclination. The ground state of the magnetosphere should be specified by an intersection of the two lines showing upper and lower envelopes in the scatter plot of <jats:italic>D<jats:sub>st</jats:sub></jats:italic> which corresponds to zero <jats:italic>IMF‐ B<jats:sub>z</jats:sub></jats:italic> and the minimum value of the dynamic pressure of the solar wind.</jats:p>