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On 3 November 2021, an interplanetary coronal mass ejection impacted the Earth’s magnetosphere leading to a relevant geomagnetic storm (Kp = 8-), the most intense event that occurred so far during the rising phase of solar cycle 25. This work presents the state of the solar wind before and during the geomagnetic storm, as well as the response of the plasmasphere–ionosphere–thermosphere system in the European sector. To investigate the longitudinal differences, the ionosphere–thermosphere response of the American sector was also analyzed. The plasmasphere dynamics was investigated through field line resonances detected at the European quasi-Meridional Magnetometer Array, while the ionosphere was investigated through the combined use of ionospheric parameters (mainly the critical frequency of the F2 layer, foF2) from ionosondes and Total Electron Content (TEC) obtained from Global Navigation Satellite System receivers at four locations in the European sector, and at three locations in the American one. An original method was used to retrieve aeronomic parameters from observed electron concentration in the ionospheric F region. During the analyzed interval, the plasmasphere, originally in a state of saturation, was eroded up to two Earth’s radii, and only partially recovered after the main phase of the storm. The possible formation of a drainage plume is also observed. We observed variations in the ionospheric parameters with negative and positive phase and reported longitudinal and latitudinal dependence of storm features in the European sector. The relative behavior between foF2 and TEC data is also discussed in order to speculate about the possible role of the topside ionosphere and plasmasphere response at the investigated European site. The American sector analysis revealed negative storm signatures in electron concentration at the F2 region. Neutral composition and temperature changes are shown to be the main reason for the observed decrease of electron concentration in the American sector.

Lidar observations at Sodankyla (Finland) on January 9 and 11 1992, made as part of the European Arctic Stratospheric Ozone Experiment (EASOE), showed very different profiles of backscattering ratio. A prominent layer was seen between potential temperatures of 420 and 445K on January 9, which was nearly absent in the backscattering profile between these isentropic surfaces on the 11th. Examination of the trajectories of the airmasses reaching Sodankyla on these days lead us to suggest that the larger returns on January 9th could arise from the formation of a nitric acid coating on volcanic aerosols. Simulations using a microphysical model produce results consistent with this suggestion. We conclude that nitric acid condensation on January 9 can explain the measurements, but that independent information would be needed to quantify the characteristics of the aerosol substrate.

[1] A Singular Value Decomposition analysis is applied to climatological data to determine the modes of variability of monthly mean Sea Surface Temperature (SST) in the tropics coupled with the southern hemisphere stratospheric polar vortex intensity for the 1958 to 2006 period and to identify the tropical region strongly influencing the high latitude stratospheric dynamics. Two subsets of data have been considered by explicitly taking into account the phase of the Pacific Decadal Oscillation (PDO). The study evidences a shift of the tropical region affecting the polar vortex from the Western Pacific to the Indo-Pacific Ocean, driven by the modulation of the SST pattern, due to the phase of the PDO. The analysis shows in both cases (years with positive/negative PDO phases) a high degree of correlation between an SST related index, calculated from NOAA/ERSST and HadISST datasets, and an index of the polar vortex intensity based on ECMWF data.

[1] The atmospheric response to the tendency of oceanic equatorial temperature observed during recent decades has been simulated, leading to the identification of a possible link between tropical Sea Surface Temperature (SST) and Antarctic climate change. The dynamical mechanism, by which changes in tropical SSTs exert an influence on the upper latitude atmospheric dynamics, is investigated. The analysis of the simulations suggests that the perturbations of the atmospheric circulation are induced by changes in the annual cycle of the tropical convective activity, which influences the intensity of the wave forcing. In particular, the introduction of the SST perturbation generates a strong vertical wind shear at middle latitude during the Antarctic winter affecting the propagation of convectively generated waves during the following spring.

[1] The polar Antarctic atmosphere has been recently characterized by a progressive intensification of the circumpolar westerly winds, both in the stratospheric and tropospheric component. We simulate the response of the polar southern hemisphere atmosphere to a prescribed trend of oceanic equatorial temperature to demonstrate a possible link between the progressive warming of tropical sea surface temperature (SST) and Antarctic climate changes in the last decades. Model simulations produce an atmospheric response suggesting an influence of equatorial SST on polar dynamics, particularly during summer months.

The work presents measurements obtained with the multi-input spectroradiometer GASCOD/A4p, the Fourier transform far infrared interferometer SAFIRE-A and the two-channel chromatograph HAGAR, on board the M-55 Geophysica stratospheric aircraft during the APE-GAIA measurement campaign, held in September-October 1999 over the Antarctic Peninsula. The paper aims to investigate the gas mixing ratio distributions at the flight altitude, during a recovery phase of the reservoir species in the lower stratosphere in the Antarctic spring. Attention is focused on the daytime flight of 23 September, when the aircraft flew inside the vortex at two different altitudes (about 16.5 and 18.5 km). Concentration distributions were examined for NO2, N2O, O3, BrO, ClO, HCl, H2O and HNO3, during flight inside and outside the vortex. Simultaneous concentration measurements of HNO3 and H2O showed a denitrification but not dehydration inside the vortex core. The reformation of chlorine reservoirs is investigated. It was evident that the re-conversion of HCl was favoured in the Cl+CH4 branches, and its high mixing ratio up to 1 ppbv at 425 K surface confirms that low ozone air masses inside the vortex and strong sunlight greatly favour a more rapid formation of passive chlorine reservoirs.

During May 1999, data collected with two of University of L'Aquila lidar systems, Italy show an increase in the stratospheric aerosol content at altitudes between 15 and 18 km. Our initial hypothesis is that these aerosols originated from the Shishaldin (Alaska) volcanic eruption of 19 April 1999. SAGE II observations, taken between Julian day 100 and 180, at latitudes above 40°N, show several sightings that are good candidates for volcanic aerosols. A trajectory analysis shows that a few SAGE II measurements are in space and time compatible with the Shishaldin eruption, and the aerosol enhancements observed by L'Aquila lidars have been also sampled by SAGE II, later and in different locations.

A technique based on conservative properties of certain meteorological fields is used to compare ozone measurements from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) with soundings from a lidar system operated at midlatitudes by the University of L'Aquila, Italy. A few typical cases are analyzed in connection with the position of the vortex relative to the observing station, and it is shown that in general lidar observations taken within the vortex compare well with the UARS data, regardless of whether they are coincident with a satellite overpass. It is shown that such analysis may be useful for comparing measurements of the same quantity taken at different sites using different measurement techniques.