Extreme two-phase change of ionospheric electron temperature overshoot during geomagnetic storms

An intense surge in the equatorial electron temperature (Te) at sunrise, known as the morning Te overshoot, has been one of the defining ionospheric features since its discovery early in the Space Age. Despite decades of study, the behavior of the morning overshoot during geomagnetic storms remains poorly understood. We report a two-stage response of the morning Te overshoot to geomagnetic activity, uncovered by a neural network model. Electron temperatures show an initial enhancement during the storm's main phase, followed by a drastic depletion exceeding 1000 K and disappearance of the overshoot in the recovery phase. This two-phase change aligns with the early influence of westward prompt penetration electric field, overtaken by the development of the eastward disturbance dynamo later in the storm. These electric field changes affect vertical plasma drifts that redistribute electron densities, modifying ionospheric cooling rates. Our findings provide new insights into the dynamics of one of the most widely studied ionospheric features and showcase the potential of new-generation digital twin models of near-Earth space environment to reveal previously unrecognized physical patterns.

Pressure-Gradient Current at High Latitude from Swarm Measurements

The pressure-gradient current is among the weaker ionospheric current systems arising from plasma pressure variations. It is also called diamagnetic current because it produces a magnetic field which is oriented oppositely to the ambient magnetic field, causing its reduction. The magnetic reduction can be revealed in measurements made by low-Earth orbiting satellites flying close to ionospheric plasma regions where rapid changes in density occur. Using geomagnetic field, plasma density and electron temperature measurements recorded on board ESA Swarm A satellite from April 2014 to March 2018, we reconstruct the flow patterns of the pressure-gradient current at high-latitude ionosphere in both hemispheres, and investigate their dependence on magnetic local time, geomagnetic activity, season and solar forcing drivers. Although being small in amplitude these currents appear to be a ubiquitous phenomenon at ionospheric high latitudes characterized by well defined flow patterns, which can cause artifacts in the main field models. Our findings can be used to correct magnetic field measurements for diamagnetic current effect, to improve modern magnetic field models, as well as to understand the impact of ionospheric irregularities on ionospheric dynamics at small-scale sizes of a few tens of kilometers.

A New Ionospheric Index to Investigate Electron Temperature Small-Scale Variations in the Topside Ionosphere

The electron temperature (Te) behavior at small scales (both spatial and temporal) in the topside ionosphere is investigated through in situ observations collected by Langmuir Probes on-board the European Space Agency Swarm satellites from the beginning of 2014 to the end of 2020. Te observations are employed to calculate the Rate Of change of electron TEmperature Index (ROTEI), which represents the standard deviation of the Te time derivative calculated over a window of fixed width. As a consequence, ROTEI provides a description of the small-scale variations of Te along the Swarm satellites orbit. The extension of the dataset and the orbital configuration of the Swarm satellites allowed us to perform a statistical analysis of ROTEI to unveil its mean spatial, diurnal, seasonal, and solar activity variations. The main ROTEI statistical trends are presented and discussed in the light of the current knowledge of the phenomena affecting the distribution and dynamics of the ionospheric plasma, which play a key role in triggering Te small-scale variations. The appearance of unexpected high values of ROTEI at mid and low latitudes for specific magnetic local time sectors is revealed and discussed in association with the presence of Te spikes recorded by Swarm satellites under very specific conditions.