Using ROCSAT-1, DEMETER, and ICON Ion Velocities to Explore the Solar Activity Dependence of Daytime Ionospheric Four-peaked Structures

The plasma density and temperature and ion velocities observed by the ROCSAT-1, DEMETER, and ICON satellites are used to examine the daytime wavenumber-4 (WN4) feature in the equatorial ionosphere during various months and solar activity levels. The ROCSAT-1, DEMETER, and ICON ion density, ion temperature, and ion velocity generally yield prominent WN4 features over the center of Pacific Ocean, the west side of South America, the center of the Atlantic Ocean, and Southern India. The correlation coefficient between the ion density and upward ion drift is significant during high solar activity of 1999-2004. This confirms that the upward ion drift is essential during high solar activity, and the associated amplitude of dynamo eastward electric field is in the range of 0.10-0.14 mV/m, which is 15-19% of daily dynamo electric field. By contrast, the ion density and the northward field-aligned ion flow show a clear anti-correlation which yields a maximum coefficient in August during low solar activity. The amplitude of the ion flow is in the range of 10.44-13.91 m/s, which is 10-13% of the ambient ion flows. In addition, ICON meridional field-aligned ion flows also exhibit prominent WN4 feature in the low solar activity years of 2020-2021. The significant correlation of the DEMETER ion density versus ion flow and prominent WN4 features in the ICON ion flow demonstrate that the meridional field-aligned southward ion flow plays an important role during low solar activity.