In this study, we present ionospheric observations of field-aligned currents from AMPERE and the ESA Swarm A satellite, in conjunction with high-resolution thermospheric density measurements from accelerometers on board Swarm C and GRACE-FO, for the 3rd and 4th February 2022 geomagnetic storms that led to the loss of 38 Starlink internet satellites. We study the global storm time response of the thermospheric density enhancements, including their growth, decay, and latitudinal distribution. We find that the thermospheric density enhances globally in response to high-latitude energy input from the magnetosphere-solar wind system, and takes at least a full day to recover to pre-storm density levels. We also find that the greatest density perturbations occur at polar latitudes consistent with the magnetosphere-ionosphere dayside cusp, and that there appeared to be a saturation of the thermospheric density during the geomagnetic storm on the 4th. Our results highlight the critical importance of high-latitude ionospheric observations when diagnosing potentially hazardous conditions for low-Earth-orbit satellites.

Super Dual Auroral Radar Network (SuperDARN) consists of more than 30 monostatic high-frequency (HF, 10-18~MHz) radars which utilise signals scattered from decameter-scale ionospheric irregularities for studying dynamic processes in the ionosphere. By combining line-of-sight velocity measurements of ionospheric scatter echoes from radars with overlapping fields of view, SuperDARN provides maps of ionospheric plasma drift velocity over mid and high latitudes. The conventional SuperDARN radars consecutively scan through sixteen beam directions with dwelling time of 3.5 s/beam, which places a lower limit of one minute to sample the entire field of view. In this work we remove this limitation by utilizing advanced capabilities of the recently developed Borealis digital SuperDARN radar system. Combining a wide transmission beam with multiple narrow reception beams allows us to sample all conventional beam directions simultaneously and to increase the sampling rate of the entire field of view by up to sixteen times without noticeable deterioration of the data quality. The wide-beam emission also enabled the implementation of multistatic operations, where ionospheric scatter signals from one radar are received by other radars with overlapping viewing areas. These novel operations required the development of a new model to determine the geographic location of the source of the multistatic radar echoes. Our preliminary studies showed that, in comparison with the conventional monostatic operations, the multistatic operations provide a significant increase in geographic coverage, in some cases nearly doubling it. The multistatic data also provide additional velocity vector components increasing the likelihood of reconstructing full plasma drift velocity vectors.

Conventional forecasting of high-frequency (HF, 3-30 MHz) radio wave propagation is based on a combination of ionospheric and propagation models. However, at very high latitudes this approach is seriously undermined by the intrinsically dynamic ionospheric conditions regularly perturbed by energetic particle precipitations and strong electric fields. From this perspective, the multi-year observations of HF propagation characteristics by SuperDARN radars across auroral and polar cap regions represent a unique opportunity for systematic validation of the conventional approach, as well as for creating an empirical propagation model directly from the radar observations. Qualitative identification and quantitative characterisation of the propagation modes requires an accurate knowledge of the vertical angle of arrival (elevation angle) across the high-latitude part of the radar network. This information has become available only in recent years, facilitated by the development of reliable data-based calibration techniques for SuperDARN interferometry. We present the solar-cycle/seasonal/diurnal climatology of HF propagation characteristics at very high latitudes derived from two-frequency observations by the Rankin Inlet SuperDARN radar.

We present three events where high-resolution electric and magnetic field measurements from the Swarm satellite constellation coincided with excellent F-region ionospheric coverage from SuperDARN. Large-scale ionospheric convection patterns from SuperDARN, together with field-aligned-current patterns from AMPERE, provide information on quasi-static ionospheric dynamics traversed by Swarm. Because the Swarm observations and orbital path coincided with favorable SuperDARN/AMPERE observing conditions, it was possible to filter the Swarm electric field observations into a quasi-static component that agreed with the SuperDARN electric field. We contend that the residual electric field from Swarm is thus indicative of small- and mesoscale dynamics not captured by the convection and FAC patterns. We compare calculations of the Poynting flux between the different instruments and show that dynamics on small- to mesoscales can be highly variable within structures like field-aligned currents. In the events shown, small- and medium-scale Poynting fluxes occasionally dominate over that from large-scale processes.

Ionospheric convection patterns from the Super Dual Auroral Radar Network are used to determine the trajectories, transit times and decay rates of three polar cap patches from their creation in the dayside polar cap ionosphere to their end of life on the nightside. The first two polar cap patches were created within 12 minutes of each other and travelled through the dayside convection throat, before entering the nightside auroral oval after 104 and 92 minutes, respectively. When the patches approached the nightside auroral oval, an intensification in the poleward auroral boundary occurred close to their exit point, followed by a decrease in the transit velocity. The airglow decay rates of patches 1 and 2 were found to be ≈0.6% and ≈0.9% per minute, respectively. The third patch decayed completely within the polar cap and had a lifetime of only 78 minutes. After a change in drift direction, patch 3 had a radar backscatter power half-life of 4.23 minutes, which reduced to 1.80 minutes after a stagnation, indicating a variable decay rate. 28 minutes after the change in direction, and 16 minutes after stagnation, patch 3 completely disintegrated. We relate this rapid decay to increased frictional heating, which speeds up the recombination rate. Therefore, we suggest that the stagnation of a polar cap patch is a main determinant to whether or not a polar cap patch can exit through the nightside auroral oval.

Using 65,133 hourly averages of transpolar voltage Φ(PC) from observations made over 25 years by the SuperDARN radars, with simultaneous SML and interpolated am geomagnetic indices, we study their optimum interplanetary coupling functions. We find lags of 18, 31 and 45 min. for Φ_{PC}, am and SML respectively, and fit using a general coupling function with three free fit exponents. To converge to a fit, we need to average interplanetary parameters and then apply the exponent which is a widely-used approximation: we show how and why this is valid for all interplanetary parameters, except the factor quantifying the effect of the clock angle of the interplanetary magnetic field, sin^(d)(θ/2), which must be computed at high time resolution and then averaged. We demonstrate the effect of the exponent d on the distribution, and hence weighting, of samples and show d is best determined from the requirement that the coupling function is a linear predictor, which yields d of 2.50+/-0.10, 3.00+/-0.22 and 5.23+/-0.48 for Φ_{PC}, am and SML. To check for overfitting, fits are made to half the available data and tested against the other half. Ensembles of 1000 fits are used to study the effect of the number of samples on the distribution of errors in individual fits and on systematic biases in the ensemble means. We find only a weak dependence of solar wind density for Φ_{PC} and SML but a significant one for am. The optimum coupling functions are shown to be significantly different for Φ_{PC}, am and SML.

We use 214410 hourly observations of the transpolar voltage, ΦPC, from 25 years of observations by the SuperDARN radars, to confirm the central tenet of the Expanding-Contracting Polar Cap (ECPC) model of ionospheric convection that ΦPC responds to both dayside and nightside reconnection voltages (ΦD and ΦN). We show ΦPC increases at a fixed level of nightside auroral electrojet AL index with increasingly southward IMF (identifying the well-known effect of ΦD on ΦPC) but also with increasingly negative AL at a fixed southward IMF (identifying a distinct effect of ΦN on ΦPC ). We study the variation of ΦPC with time elapsed Δt since the IMF last pointed southward and show that low/large values occur when -AL is small/large. We have to allow for the fact that at lower numbers of radar echoes, ne , the matched potential re-analysis technique used to derive is influenced by the model used: this is done by a sensitivity study of the threshold of ne required. We show that for any threshold ΦPC falls to about 15kV for & Δt greater than about 15 hours giving an upper limit to the viscous-like voltage. It is shown that both ΦPC and -AL increase with increased solar wind dynamic pressure psw , but not as much as the mid-latitude geomagnetic range index am. We conclude psw increases both ΦD and ΦN through increasing the magnetic shear across the relevant current sheet but has a bigger effect on mid-latitude geomagnetic activity indices via the additional energy stored in the tail lobes.

The Super Dual Auroral Radar Network (SuperDARN) currently consists of more than thirty high-frequency (HF, 3–30 MHz) radars covering mid-latitude to polar regions in both hemispheres. Their major task is to map ionospheric plasma circulation which provides information about the interactions between the solar wind and the near-Earth’s space plasma environment. One of the major factors defining radar data quality is the signal-to-noise ratio (SNR), which requires an accurate characterisation of the HF noise. The standard SuperDARN data analysis software uses the SNR as part of a set of empirical procedures designed to remove low-quality data from further analysis. In this study we found that the currently used empirical algorithm systematically underestimates the noise level by up to 40%. Based on comparison of theoretical and observational noise statistics, we resolve this issue by designing and validating a procedure for accurate background noise level estimation. We then propose a simple SNR threshold to replace the existing criteria for excluding low-quality data. In addition, we show that several aspects of the radar operational regime design, as well as short-lived anthropogenic radio interference, can adversely affect the quality of the noise estimates at selected radar sites, and we propose ways to mitigate these problems.

An interplanetary shock can abruptly compress the magnetosphere, excite magnetospheric waves and field-aligned currents, and cause a ground magnetic response known as a sudden commencement (SC). However, the transient (<~1 min) response of the ionosphere-thermosphere system during an SC has been little studied due to limited temporal resolution in previous investigations. Here, we report observations of a global reversal of ionospheric vertical plasma motion during an SC on 24 October 2011 using ~6 s resolution SuperDARN ground scatter data. The dayside ionosphere suddenly moved downward during the magnetospheric compression due to the SC, lasting for only ~1 min before moving upward. By contrast, the post-midnight ionosphere briefly moved upward then moved downward during the SC. Simulations with a coupled geospace model suggest that the reversed E X B vertical drift is caused by a global reversal of ionospheric zonal electric field induced by magnetospheric compression during the SC.

For over a decade, the Super Dual Auroral Radar Network (SuperDARN) and the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) have been measuring ionospheric convection and field-aligned currents in the high-latitude regions, respectively. Using both, whole hemisphere maps of the magnetosphere-ionosphere energy transfer rate (the Poynting flux) have been generated with a time resolution of two minutes between 2010 and 2017. These uniquely data driven Poynting flux patterns are used in this study to perform a superposed epoch analysis of the northern hemisphere ionospheric response to transitions of the IMF B_z component. We discuss the difference in the distribution of Poynting flux between the magnetosphere-ionosphere Dungey cycle “switching on” and “switching off” to solar wind driving, revealing that they are not symmetric temporally or spatially.

Large thermospheric neutral density enhancements in the cusp region have been examined for many years. The CHAMP satellite for example has enabled many observations of the perturbation, showing that it is mesoscale in size and exists on statistical timescales. Further studies examining the relationship with magnetospheric energy input have shown that fine-scale Poynting fluxes are associated with the density perturbations on a case-by-case basis, whilst others have found that mesoscale downward fluxes also exist in the cusp region statistically. In this study, we use nearly 8 years of the overlapping SuperDARN and AMPERE datasets to generate global-scale patterns of the high-latitude and height-integrated Poynting flux into the ionosphere, with a time resolution of two minutes. From these, average patterns are generated based on the IMF orientation. We show the cusp is indeed an important feature in the Poynting flux maps, but the magnitude does not correlate well with statistical neutral mass density perturbations observed by the CHAMP satellite on similar spatial scales. Mesoscale height-integrated Poynting fluxes thus cannot fully account for the cusp neutral mass density enhancement, meaning energy deposition in the F-region or on fine-scales, which is not captured by our analysis, could be the primary driver.

This White Paper outlines the importance of addressing the fundamental science theme “How are charged particles energized in space plasmas” through a future ESA mission. The White Paper presents five compelling science questions related to particle energization by shocks, reconnection, waves and turbulence, jets and their combinations. Answering these questions requires resolving scale coupling, nonlinearity, and nonstationarity, which cannot be done with existing multi-point observations. In situ measurements from a multi-point, multi-scale L-class Plasma Observatory consisting of at least seven spacecraft covering fluid, ion, and electron scales are needed. The Plasma Observatory will enable a paradigm shift in our comprehension of particle energization and space plasma physics in general, with a very important impact on solar and astrophysical plasmas. It will be the next logical step following Cluster, THEMIS, and MMS for the very large and active European space plasmas community. Being one of the cornerstone missions of the future ESA Voyage 2050 science programme, it would further strengthen the European scientific and technical leadership in this important field.