Jan Laštovička

The 10 hPa level is conventionally used for determining sudden stratospheric warming in studies. Our study examines the winter of 2005–2006 from 1 to 10 hPa pressure levels, identifying three significant warming episodes in January 2006 instead of only one based on the 10 hPa level definition. The first two episodes with temperature peaks occurring around 3 and 10 January, respectively, are identified at the upper stratosphere, where they are stronger than the official warming peak on 22 January defined at 10 hPa. Increased wave activity at these upper levels, derived from the Eliassen‐Palm flux divergence, highlights the intensity of warming and circulation changes at the upper stratospheric layers. Ionospheric analysis of January 2006 revealed substantial impacts on the F2 layer ionosphere, particularly in Juliusruh. These effects are linked to planetary wave periodicities detected in wavelet spectrum analysis. These periodicities are noted in the temperature and wind, predominantly in the upper stratosphere. Further analysis employs a wavelet coherence between the upper stratosphere and the critical frequency of the F2 layer that affirms that the planetary‐scale periodicities observed in the upper stratosphere are mirrored in the F2 layer mid‐latitude ionosphere over station Juliusruh. These findings suggest a relationship between the upper stratosphere and the ionosphere, particularly at stations near the climatological vortex edge around 60°N. The novelty of this paper is that for studying the impact of sudden stratospheric warmings on the mid‐latitude ionosphere, it is better to use the major SSW definition at upper stratospheric levels rather than the traditional 10 hPa level.

The increasing concentration of greenhouse gases in the atmosphere causes net heating in the troposphere and net cooling at higher atmospheric levels, which results in long-term changes of many other quantities in the upper atmosphere and ionosphere. Here, we briefly present a scenario of global climatic changes in the Stratosphere-Mesosphere-Thermosphere-Ionosphere system. Changes in all quantities in this system together create a qualitatively consistent scenario. The main driver of these changes is carbon dioxide, but some role is played also by the stratospheric ozone/ozone depleting substances and by the secular change of the Earth magnetic field. Observation-based trends and trends derived from model simulations agree with each other qualitatively and often also quantitatively.

Previous studies have investigated the performance of reanalyses during Northern Hemisphere winters using several approaches over the period 1980-present. This study evaluates the performance of three reanalyses (ERA-5, MERRA-2, and JRA-3Q) in representing stratospheric conditions over the two most recent Northern Hemisphere winter periods (2023-2024 and 2024-2025) using temperature and zonal wind across the high-latitude stratosphere. The statistics analysis indicates that ERA-5 exhibits a warmer upper and middle stratosphere, whereas JRA-3Q shows a warmer lower stratosphere. However, for the circulation, regardless of the month and region in the stratosphere, it is observed that ERA-5 relatively shows wind values closer to easterly circulation. As stated in previous literature, the variability in both temperature and zonal wind is relatively highest in the upper stratosphere and lowest in the lower stratosphere. A monthly climatological comparison between MLS and the three reanalyses indicates that MERRA-2 is a better candidate for representing thermal conditions in the upper stratosphere (1 hPa). Additionally, zonal wind is identified as a more suitable parameter in defining SSW rather than temperature. The analysis identifies reversal and warming events originating in the upper stratosphere, highlighting the need for multi-level diagnostics that extend beyond the conventional single-level zonal wind method. The investigation further reveals that, rather than relying on the traditional 10-hPa reference in determining the SSW, these early SSW-like events occurring in the upper stratosphere initiate atmospheric-ionospheric coupling sooner. Therefore, considering the upper stratosphere helps to identify the actual onset of atmospheric-ionospheric coupling during SSW more accurately.

Sudden stratospheric warming studies usually determine SSW events at the 10 hPa level. Our study examines the winter of 2005-2006 from 1-10 hPa pressure levels, identifying three significant warming episodes in January 2006 instead of only one based on the 10 hPa level definition. The first two episodes with temperature peaks occurring around 3 and 10 January, respectively, are identified at the upper stratosphere, where they are stronger than the official warming peaking on 22 January defined at 10 hPa. Increased wave activity at these upper levels, derived from the Eliassen-Palm flux divergence, highlights the intensity of warming and circulation changes at upper stratospheric layers. Ionospheric analysis of January 2006 revealed substantial impacts on the F2 layer ionosphere, particularly in Juliusruh. These effects are linked to planetary wave periodicities detected in wavelet spectrum analysis. These periodicities are noted in the temperature and wind, predominantly in the upper stratosphere. The further analysis employs a wavelet coherence between the upper stratosphere and critical frequency of the F2 layer that affirms that the planetary-scale periodicities observed in the upper stratosphere are mirrored in the F2 layer mid-latitude ionosphere over station Juliusruh. These findings suggest a relationship between the upper stratosphere and the ionosphere, particularly at stations near the climatological vortex edge around 60oN. The main novelty of this paper is that for studying the impact of sudden stratospheric warmings on the ionosphere, it is better to use the major SSW definition at upper stratospheric levels rather than the traditional 10 hPa level.

I use yearly average noontime foF2 from six ionospheric stations from four continents, Juliusruh, Pruhonice, Roma, Boulder, Kokubunji and Canberra over periods 1976-1995 and 1996-2014, and six solar activity indices F10.7, F30, Mg II, solar H Lyman-α flux, sunspot numbers and He II. The results reveal somewhat and sometimes even substantially different trends of foF2 when the effect of solar cycle is removed/reduced from foF2 data with different solar activity proxies. Only F30 provides for all stations and both periods the trends of the same sign (negative), other indices reveal both positive and negative trends for different stations and periods. Therefore together with results of other criteria F30 is considered to be the most reliable solar activity index for long-term studies of midlatitude foF2.

Climatologically, the wintertime-evolved stratospheric polar vortex comprises zonal wind, which is westerly. During extreme events like Sudden Stratospheric Warming (SSW), the polar vortex disrupts, showing easterly wind at stratospheric altitudes. Defining extreme events in the stratosphere, like SSW at a definite pressure scale, depends on the region explaining its effects. For instance, the standard definition of SSW is 10 hPa pressure suites primarily for the lower atmosphere. SSW's current definition is unsuitable for the upper atmosphere, especially the ionosphere. So, with this viewpoint, we study the fundamental behaviour of the zonal mean of temperature during the reversal of the zonal mean of zonal wind by using superposed epoch analysis. We use the MERRA2 dataset for this analysis. From MERRA-2, we analyse the zonal mean of temperature and zonal wind from 1980 – 2022 northern hemispheric winters. The analyses are done at 10 hPa, where the standard definition of SSW is defined, and at 1 hPa, 0.5 hPa and 0.1 hPa. Temperature behaviours at different reversal periods are studied at various latitudinal values, starting at 60oN and ending at 90oN. With this analysis, a more general picture of the temperature–wind relation can be understood, which will help to understand and define SSW in a much better way for upper atmospheric studies.

Total electron content (TEC) is an important parameter for ionospheric dynamics, global navigation satellite system/Global Positioning System (GNSS/GPS) signal propagation and related applications of GNSS/GPS signals. Despite this fact, the long-term trends in TEC have been studied only a little. Here, we analyze the JPL-35 (Jet Propulsion Laboratory-35) homogeneous series of global TEC data for 1994–2014 for selection of the optimum solar activity proxy for TEC analyses and the UPC (Universitat Politècnica de Catalunya) TEC data over 2003–2023 for estimating long-term trends in TEC. TEC trends are predominantly negative. TEC trends reveal a clear wavenumber 2 longitudinal structure in low/equatorial latitudes with strong negative trends in belts 0–60 and 180–240° E and weak trends in 90–150 and 270–330° E. For more detailed information on TEC trends, a longer series of reliable TEC data is required.

The total electron content (TEC) is an important parameter for the ionospheric dynamics, GNSS/GPS signal propagation and related applications of GNSS/GPS signals. Despite this fact the long-term trends in TEC have been studied a little only. Here we analyze the homogeneous series JPL-35 of global TEC data for 1994–2014 for selection of the optimum solar activity proxy for TEC analyses, and the UPC TEC data over 2003–2023 for estimating long-term trends in TEC. TEC trends are very predominantly negative. TEC trends reveal a clear wavenumber 2 longitudinal structure in low/equatorial latitudes with strong negative trends in belts 0–60° E and 180–240° E and weak trends in 90–150° E and 270–330° E. For more detailed information on TEC trends a longer series of reliable TEC data is required.

The total electron content (TEC) is an important parameter for the ionospheric dynamics, GNSS/GPS signal propagation and related applications of GNSS/GPS signals. Despite this fact the long-term trends in TEC have been studied a little only. Here we analyze the homogeneous series JPL-35 of global TEC data for 1994–2014 for selection of the optimum solar activity proxy for TEC analyses, and the UPC TEC data over 2003–2023 for estimating long-term trends in TEC. TEC trends are very predominantly negative. TEC trends reveal a clear wavenumber 2 longitudinal structure in low/equatorial latitudes with strong negative trends in belts 0–60° E and 180–240° E and weak trends in 90–150° E and 270–330° E. For more detailed information on TEC trends a longer series of reliable TEC data is required.

The total electron content (TEC) is an important parameter for the ionospheric dynamics, GNSS/GPS signal propagation and related applications of GNSS/GPS signals. Despite this fact the long-term trends in TEC have been studied a little only. Here we analyze the homogeneous series JPL-35 of global TEC data for 1994–2014 for selection of the optimum solar activity proxy for TEC analyses, and the UPC TEC data over 2003–2023 for estimating long-term trends in TEC. TEC trends are very predominantly negative. TEC trends reveal a clear wavenumber 2 longitudinal structure in low/equatorial latitudes with strong negative trends in belts 0–60° E and 180–240° E and weak trends in 90–150° E and 270–330° E. For more detailed information on TEC trends a longer series of reliable TEC data is required.