Dependence of equatorial <i>F</i> region vertical drifts on season and solar cycle

TLDR

Vertical drift measurements at Jicamarca have spanned more than half a solar cycle, revealing variations that current models cannot yet explain. The study integrates drift velocity data over time to estimate the total potential drop between reversal points near the terminators. The data show that daytime drift velocities are larger during sunspot minimum and nighttime velocities larger during maximum, with the evening reversal occurring earlier at minimum, while the period of eastward electric field is shortest during minimum and local winter; the total potential drop peaks at solar maximum and exhibits a pronounced seasonal minimum in mid‑December, indicating that F‑region dynamo polarization fields, rather than tidal winds alone, drive these variations.

Abstract

Vertical drift measurements have been made at Jicamarca for more than half a solar cycle. The data from periods of high and low activity are appreciably different. Daytime drift velocities during sunspot minimum are usually larger than during the maximum, while the opposite is true for nighttime periods. The evening reversal occurs earlier during sunspot minimum than during the maximum, but the morning reversal is not altered. The period of eastward electric field (upward drift) is thus shortest during sunspot minimum and local winter. By integrating the drift velocity data with respect to time, one can obtain a measure of the total potential drop between reversal points (near the terminators). This drop is largest at solar maximum. There is also a pronounced seasonal variation, with a minimum in mid‐December during both solar minimum and maximum. The general features of the data cannot be explained solely on the basis of tidal winds driving an E region dynamo; polarization fields related to the F region dynamo are of major importance, particularly in helping to explain the enhancement of the daytime upward drift which often occurs shortly before the drift reverses to downward in the evening. In order to account quantitatively for the observed variations, however, numerical models considerably more sophisticated than those presently available are needed.