** Progress in Earth and Planetary Science is the official journal of the Japan Geoscience Union, published in collaboration with its 50 society members.

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    Progress in Earth and Planetary Science

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    A minimum in the latitude variation of spread-F at March equinox

    Balan N, Maruyama T, Patra A K, Narayanan V L

    Low-mid latitude spread-F, Minimum and maximum in spread-F

    A clear minimum in the latitude variation of Spread-F predicted by theoretical models is identified for the first time using the ionosonde data at low-mid latitudes.

    This short paper presents the statistics of spread-F occurrence at four low-mid latitude locations (17.0° N, 21.2° N, 26.5° N and 36.5° N mag. lat.) in Japan longitude sector (~ 135° E) in March to April 2002, 2003 and 2006 (high, medium and low solar activity). The location of a spread-F minimum predicted by theoretical models is identified for the first time. The spread-F minimum occurs at the poleward side of the equatorial ionisation anomaly crest and shifts equatorward from ~ 25° N mag. lat. at high solar activity to below 17° N at low solar activity. The corresponding spread-F maximum occurring on the poleward side of the minimum also shifts equatorward from ~ 35° N (or beyond) at medium solar activity to 20°–25° N at low solar activity. The spread-F occurrence increases with decreasing solar activity at all locations especially at higher latitudes (> ~ 25° N) where there is almost no occurrence at high solar activity.