Long-Term Trends in the Ionospheric Equivalent Slab Thickness: Is the ionosphere really shrinking?

Anthropogenic greenhouse gas (GHG) emissions are the ongoing major driver of Earth’s climate change. While the increasing concentration of GHG causes a warming of the lower atmosphere, it leads to a cooling of the upper atmosphere, which is expected to result in a thermal contraction. These changes in the neutral atmosphere have been demonstrated to also influence the ionosphere. In fact, the contraction of the whole upper atmosphere should induce a downward displacement of the ionospheric layers due to the changes in the vertical distribution of the different ion species in the ionosphere.

To study the ionosphere changes in response to climate change, we investigate the long-term trends of the ionospheric equivalent slab thickness (τ). τ represents the thickness of an ideal ionospheric slab of constant electron density equal to that of the F2-layer peak (NmF2) with a vertical total electron content (vTEC) value equivalent to that of the entire ionosphere. To achieve this, we derive τ time series from a selection of globally distributed and co-located ionosondes (providing NmF2) and ground-based GNSS receivers (providing vTEC), focusing on stations with at least two solar cycles of continuous data. For trend and coherent structures extraction and analysis, we use the Empirical Mode Decomposition (EMD), an advanced data-adaptive decomposition method. EMD is particularly suited for preserving nonlinearity in time series and for processing non-stationary data, offering a more accurate representation of long-term variations compared to traditional statistical methods. We present preliminary results showing a global long-term decrease of τ, but with magnitudes dependent on latitude, pointing out a general shrinking of the ionosphere in the last two decades.