Day‐to‐day variability of midlatitude ionospheric currents due to magnetospheric and lower atmospheric forcing

Y Yamazaki,K Häusler,J A Wild

doi:10.1002/2016ja022817

Abstract

AbstractAs known from previous studies on the solar quiet (Sq) variation of the geomagnetic field, the strength and pattern of ionospheric dynamo currents change significantly from day to day. The present study investigates the relative importance of two sources that contribute to the day‐to‐day variability of the ionospheric currents at middle and low latitudes. One is high‐latitude electric fields that are caused by magnetospheric convection, and the other is atmospheric waves from the lower atmosphere. Global ionospheric current systems, commonly known as Sq current systems, are simulated using the National Center for Atmospheric Research thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model. Simulations are run for 1–30 April 2010 with a constant solar energy input but with various combinations of high‐latitude forcing and lower atmospheric forcing. The model well reproduces geomagnetic perturbations on the ground, when both forcings are taken into account. The contribution of high‐latitude forcing to the total Sq current intensity (Jtotal) is generally smaller than the contribution of wave forcing from below 30 km, except during active periods (Kp≥4), when Jtotal is enhanced due to the leakage of high‐latitude electric fields to lower latitudes. It is found that the penetration electric field drives ionospheric currents at middle and low latitudes not only on the dayside but also on the nightside, which has an appreciable effect on the Dst index. It is also found that quiet time day‐to‐day variability in Jtotal is dominated by symmetric‐mode migrating diurnal and semidiurnal tidal winds at 45–60° latitude at ∼110 km.

Highlights

Electrodynamics of the middle- and low-latitude ionosphere are dominated by the wind dynamo mechanism, in which electric fields and currents are generated by the action of neutral winds [e.g., Richmond, 1995a; Vasyliunas, 2012]
The present study investigates the relative importance of two sources that contribute to the day-to-day variability of the ionospheric currents at middle and low latitudes
It is found that the penetration electric field drives ionospheric currents at middle and low latitudes on the dayside and on the nightside, which has an appreciable effect on the Dst index

Summary

Introduction

Electrodynamics of the middle- and low-latitude ionosphere are dominated by the wind dynamo mechanism, in which electric fields and currents are generated by the action of neutral winds [e.g., Richmond, 1995a; Vasyliunas, 2012]. Miyahara and Ooishi [1997] examined the impact of variable neutral winds on the ionospheric wind dynamo on the basis of a thin-shell dynamo model with winds obtained from a general circulation model of the middle atmosphere They showed that wave forcing in the dynamo region from below makes a significant contribution to the day-to-day variability of the dynamo region currents and ground-level Sq variations. Joule heating associated with the polar-region electrodynamics drives equatorward disturbance winds that produce additional electric fields and currents at middle and low latitudes through the so-called disturbance dynamo mechanism [Blanc and Richmond, 1980; Fuller-Rowell et al, 2002]. We examine the relative importance of lower atmospheric forcing and high-latitude forcing in the short-term variability of the dynamo region currents at middle and low latitudes. This study addresses mechanisms by which lower atmospheric forcing and high-latitude forcing modulate the dynamo region currents at middle and low latitudes

Model and Data

Results and Discussion

Conclusions