We investigate the role of auroral particle precipitation in small-scale (below hundreds of meters) plasma structuring in the auroral ionosphere over the Arctic. To the scope, we together analyse data recorded by an Ionospheric Scintillation Monitor Receiver (ISMR) of Global Navigation Satellite System (GNSS) signals and by an All-Sky Camera located in Longyearbyen, Svalbard (Norway). We leverage on the raw GNSS samples provided at 50 Hz by the ISMR to evaluate amplitude and phase scintillation indices at 1 s time resolution and the Ionosphere-Free Linear Combination at 20 ms time resolution. The simultaneous use of the 1 s GNSS-based scintillation indices allows identifying the scale size of the irregularities involved in plasma structuring in the range of small (up to few hundreds of meters) and medium-scale size ranges (up to few kilometers) for GNSS frequencies and observational geometry. Additionally, they allow identifying the diffractive and refractive nature of the found fluctuations on the recorded GNSS signals. Six strong auroral events and their effects on plasma structuring are studied. Plasma structuring down to scales of hundreds of meters are seen when strong gradients in auroral emissions at 557.7 nm cross the line of sight between the GNSS satellite and receiver. Local magnetic field measurements confirm small-scale structuring processes coinciding with intensification of ionospheric currents. Since 557.7 nm emissions primarily originate from the ionospheric E-region, plasma instabilities from particle precipitation at E-region altitudes are considered to be responsible for the signatures of small-scale plasma structuring highlighted in the GNSS scintillation data.

Equatorial plasma irregularities in the ionospheric F-region proliferate after sunset, causing the most apparent radio scintillation “hot-spot” in geospace. These irregularities are caused by plasma instabilities, and appear mostly in the form of under-densities that rise up from the F-region’s bottomside. After an irregularity production peak at sunset, the amplitude of the resulting turbulence decays with time. Analyzing a large database of plasma irregularity spectra observed by one of the European Space Agency’s Swarm satellites, we have applied a novel but conceptually simple statistical analysis to the data, finding in the process that post-sunset turbulence in the F-region tends to decay with a uniform, scale-independent rate at night, thereby confirming and extending the results from earlier case studies. Our results should be of utility for large-scale space weather modelling efforts that are unable to resolve turbulent effects.

Abrupt changes in the solar wind dynamic pressure can greatly affect the Earth’s magnetosphere‐ionosphere system. We present an ionospheric flow vortex in the morning side during the sudden decrease in the solar wind dynamic pressure. The flow vortex was clearly observed by both the Hankasalmi radar and the azimuthal scan mode of the European Incoherent Scatter (EISCAT) Svalbard Radar (ESR). The flow vortex was first seen in the eastern field of view (FOV) of the Hankasalmi radar, and then propagated poleward and westward into the FOV of the ESR. During the passage of the flow vortex, a gradual decrease of electron density was observed by the field-aligned ESR 42 m antenna. When the equatorward directed ionospheric flow reached the ESR site, weak and visible increases in the electron density and electron temperature were observed. This impact was likely caused by soft electron precipitation associated with the clockwise flow vortex and upward field-aligned current. The azimuthal scan mode of the ESR 32 m radar at low elevation angle (30°) allowed us to measure key ionospheric parameters over a larger area (6° in latitude and 120° in azimuthal angle). The latitudinal scan of the electron temperature was used to derive the equatorward auroral boundary, which shows that the flow vortex was located in the subauroral region. We further demonstrated that it is possible to study the weak increase of electron density by using GPS total electron content (TEC) data. A minor TEC increase was observed near the center of the flow vortex.

Using a large dataset of ground-based GNSS scintillation observations coupled with in-situ particle detector data, we perform a statistical analysis of both the input energy flux from precipitating particles, and the observed prevalence of density irregularities in the northern hemisphere cusp. By examining geomagnetic activity trends in the two databases, we conclude that the occurrence of irregularities in the cusp grows increasingly likely during storm-time, whereas the precipitating particle energy flux does not. We thus find a weak or nonexistent statistical link between geomagnetic activity and precipitating particle energy flux in the cusp. This is a result of a documented tendency for the cusp energy flux to maximize during northward IMF, when density irregularities tend not to be widespread. Their number clearly maximizes during southward IMF. At any rate, even though ionization and subsequent density gradients directly caused by soft electron precipitation in the cusp are not to be ignored for the trigger of irregularities, our results point to the need to scrutinize additional physical processes for the creation of irregularities causing scintillations in and around the cusp. While numerous phenomena known to cause density irregularities have been identified and described, there is a need for a systematic evaluation of the conditions under which the various destabilizing mechanisms become important and how they sculpt the observed ionospheric ‘irregularity landscape’. As such, we call for a quantitative assessment of the role of particle precipitation in the cusp, given that other factors contribute to the production of irregularities in a major way.

We develop a climatological model for the Northern Hemisphere based on a long-term dataset (2010-2021) of the rate of change of the total electron content (TEC) index (ROTI) maps from the International GNSS Service (IGS). The IGS ROTI maps are daily averaged in magnetic latitude and local time coordinates. To develop a climatological model, the ROTI maps are decomposed into a few base functions and coefficients using the empirical orthogonal function (EOF) method. The EOF method converges very quickly, and the first four EOFs reflect the majority (96%) of the total data variability. Furthermore, different EOF components can reflect different drivers of ionospheric irregularities. The first EOF reflects the averaged ROTI activity and the impact of the solar radiation and geomagnetic activity; the 2nd EOF reflects the impact of the interplanetary magnetic field (IMF) Bz and electric field; the 3rd and 4th EOFs reflect the dawn-dusk asymmetry around the auroral oval and polar cap, and they can be related to the IMF By. To build an empirical model, we fit the EOF coefficients using helio-geophysical indices from four different categories (solar activity; geomagnetic indices; IMF; the solar wind coupling function). The final EOF model is dependent on seven selected indices (F10.7P, Kp, Dst, Bt, By, Bz and Ekl). The statistical data-model comparisons show satisfactory results with a good correlation coefficient. However, the model cannot capture the significant expansion of the dayside ROTI activity during strong geomagnetic storms. Future effort is needed to provide corrections to the model for severe storms.

We present an investigation of polar cap plasma structure lifetimes. We analyze both simulated data from ionospheric models (International Reference Ionosphere model and Mass Spectrometer Incoherent Scatter model) and in-situ data from the Swarm satellite mission (the 16 Hz Advanced Plasma Density data set). We find that the theoretical prediction that E-region conductance is a predictor of F-region polar cap plasma structure lifetimes is indeed supported by both in-situ-based observations and by ionospheric models. In-situ plasma structure lifetimes correlate well with the ratio of F- to E-region conductance. We present explicit predictions of small scale (~1 km) structure lifetimes, which range from less than 1 hour during local summer to around 3 hours during local winter. We highlight a large discrepancy between the observational and theoretical scale-dependency of decay due to diffusion.

The Australian NWC (North West Cape) signal transmitter is known to strongly interfere with the topside ionosphere. We analyze 456 conjunctions between Swarm A, B and NWC, in addition to 58 conjunctions between NorSat-1 and NWC. The in-situ measurements provided by these satellites include the 16 Hz Swarm Advanced Plasma density dataset, and the novel 1000 Hz plasma density measurements from the m-NLP system aboard NorSat-1. We subject the data to a detailed PSD analysis and subsequent superposed epoch analysis. This allows us to present comprehensive statistics of the NWC-induced plasma fluctuations, both their scale-dependency, and their climatology. The result should be seen in the context of VLF signal transmitter-induced plasma density fluctuations, where we find counter-evidence for the existence of turbulent structuring induced by the NWC transmitter.

Ionospheric plasma irregularities can be successfully studied with the Swarm satellites. Parameters derived from the in situ plasma measurements and from the topside ionosphere total electron content provide a comprehensive dataset for characterising plasma structuring along the orbits of the Swarm satellites. The Ionospheric Plasma IRregularities (IPIR) data product summarizes these parameters and has already been used in studies related to structuring and variability of ionospheric plasma. We provide a detailed description of the algorithms behind the IPIR data product and demonstrate its use for ionospheric studies.

Auroral particle precipitation potentially plays a main role in ionospheric plasma structuring. The impact of auroral particle precipitation on plasma structuring is investigated using multi-point measurements from scintillation receivers and all sky cameras from Longyearbyen, Ny-Ålesund and Hornsund on Svalbard. This provides us with the unique possibility of studying the spatial and temporal dynamics of the aurora. Here we consider three case studies to investigate how plasma structuring is related to different auroral forms. We demonstrate that plasma structuring impacting the GNSS signals is largest at the edges of auroral forms. Here we studied two stable arcs, two dynamic auroral bands and a spiral. Specifically for arcs we find elevated phase scintillation indices at the pole-ward edge of the aurora. This is observed for auroral oxygen emissions (557.7 nm) at 150~km in the ionospheric E-region. This altitude is also used as the ionospheric piercing point for the GNSS signals as the observations remain the same regardless of different satellite elevations and azimuths. Further, there may be a time delay between the temporal evolution of aurora (f.e. commencement and fading of auroral activity) and observations of elevated phase scintillation indices. The time delay could be explained by the intense influx of particles, which increases the plasma density and causes recombination to carry on longer, which may lead to a persistence of structures - a ‘memory effect’. High values of phase scintillation indices can be observed even shortly after strong visible aurora and can then remain significant at low intensities of the aurora.

Electromagnetic energy carried by magnetohydrodynamic modes is an important mechanism in the energy transfer between the magnetosphere and the ionosphere. Through wave reflection in the ionosphere, Alfvén waves are known to carry field-aligned currents, and thus play an important role in the dynamics of the ionosphere-magnetosphere coupling. The role of Hall conductance in this interplay has been explored in magneto-hydrodynamic models of the ionosphere, but has hitherto not been observed in-situ. We use five years of observations from the Swarm mission to shed light on this interplay. We present the high-latitude climatology of both the measured Poynting flux and the measured Alfvén wave reflection coefficient. Our results indicate that high-energy deeply penetrating precipitation, which directly leads to strongly enhanced Hall conductance, is an important cause of positively interfering Alfvén wave reflection. We present such observational evidence, and with that, suggest that Hall conductance is substantially more important in the ionospheric wave reflection climatology than hitherto believed.