Regulation of ionospheric plasma velocities by thermospheric winds

Abstract

Earth's equatorial ionosphere exhibits substantial and unpredictable day-to-day variations in density and morphology. This presents challenges in preparing for adverse impacts on geopositioning systems and radio communications even 24 hours in advance. The variability is now theoretically understood as a manifestation of thermospheric weather, where winds in the upper atmosphere respond strongly to a spectrum of atmospheric waves that propagate into space from the lower and middle atmosphere. First-principles simulations predict related, large changes in the ionosphere, primarily through modification of wind-driven electromotive forces: the wind-driven dynamo. Here we show the first direct evidence of the action of a wind dynamo in space, using the coordinated, space-based observations of winds and plasma motion made by the National Aeronautics and Space Administration Ionospheric Connection Explorer. A clear relationship is found between vertical plasma velocities measured at the magnetic equator near 600 km and the thermospheric winds much farther below. Significant correlations are found between the plasma and wind velocities during several successive precession cycles of the Ionospheric Connection Explorer's orbit. Prediction of thermospheric winds in the 100-150 km altitude range emerges as the key to improved prediction of Earth's plasma environment.

Fig. 1 |. ICoN’s unique orbit and observational geometry support simultaneous observation of lower thermospheric winds and ionospheric plasma velocities.

Diagram showing the location of the observatory at ~590 km altitude and a succession of magnetic field lines that uniquely connect the observatory to two remote locations at lower altitudes, north and south of the orbit track. Near the equator, in situ ion drift observations are made on field lines whose footpoints fall in the vicinity of the horizontal neutral wind vectors measured at the remote tangent point.

Fig. 2 |. Predicted and observed meridional drifts in three successive measurement periods in early 2020.

a–f, Measured meridional drift v₂ versus that predicted from wind driving parameters using equation (2) around the planet for 10-day periods starting on 3 February (a,d), 29 February (b,e) and 22 March 2020 (c,f), shown as averages of observations between 12:00 and 14:00 solar local time for individual equator crossings (a–c) with Pearson correlation coefficient and linear fit, and longitudinally binned data (d–f). Error bars represent 1 s.d. confidence in the mean value in each bin.

Fig. 3 |. Each of the four terms in the v_pred drift calculation.

The individual terms informed by MIGHTI wind measurements are shown (coloured lines) for the 23–31 March 2020, comparing the resultant v_pred drift values (black) with the observed drifts (grey) and the standard error of the mean value calculated for each term over the observing period (error bars).

Fig. 4 |. Key properties of the equatorial ionosphere and apex coordinate system.

This coordinate system is defined such that the zonal (x₁) and meridional (x₂) directions are both perpendicular to the local magnetic field (defined as the x₃ direction). The meridional direction (x₂) is defined to be positive downward at the magnetic apex, and the zonal direction (x₁) completes the coordinate system, generally being horizontal and eastward. For illustration, a zonal wind vector u is shown as sampled continuously along the blue dashed track from the moving orbital position of ICON (blue dotted track).