Magnetic eta index and ionospheric disturbances in the E layer during solar minimum

We analysed the correlation of the changes of the magnetic vertical component with the ionospheric deviations from monthly median of the E layer characteristics. Our previous work concern the data from only one year - 2004. We extend our research to other years starting from 1996 and focusing on 2007 and 2008, years of the prolonged solar minimum. The analysis shows that under magnetically quiet circumstances the magnetic index eta indicates large magnetic disturbance, especially in vertical component when other magnetic indices inform about quiet magnetic conditions. In all analysed daily cases the high values of eta index or large positive gradients of eta correlates in time with the appearance within two hours of the sporadic E layer. An advantage of the eta index is the fact that after eliminating the effect of currents induced within the Earth, eta index bring independent and meaningful information on the system of current in the ionosphere. Hence, the eta index could be an important element of the ionosphere monitoring and can be used to predict such local phenomenon like the appearance of the sporadic E layer.