Estimation of magnetospheric plasma ion composition for 1956–1975 by using high time resolution geomagnetic field data created from analog magnetograms

Abstract

AbstractThis study addresses the ion composition in the magnetosphere before the satellite era. We estimate the plasma ion mass for 1956–1975 from the period of low‐latitude Pi2 pulsations found in digital geomagnetic field data that are created from analog magnetograms at Kakioka. The period of investigation covers most of solar cycle 19 and the whole solar cycle 20. To consider long‐term variation, the moving average of the estimated plasma ion mass is calculated with a 1 year time window. We find that 1 year moving average of the plasma ion mass changed by a factor of ∼2 during one solar cycle (i.e., between ∼1.1 amu and ∼2.4 amu for solar cycle 19 and between ∼1.1 amu and ∼2.0 amu for solar cycle 20). The correlation coefficient between the 1 year moving average of the plasma ion mass and that of the F10.7 index is 0.86. This result supports the idea that in long‐term variation, solar radiation increases the density and the temperature of O+ ions in the ionosphere, leads to the outflow of O+ ions, and contributes to the enhancement of the plasma ion mass in the nightside magnetosphere. The digital data created from analog magnetograms provide an important clue to know the space environment in old days and are advantageous for studies of the space weather and space climate.