Ultra-low frequency (ULF) signals are observed using ground based magnetometers. We apply four years of data from West Greenland in a statistical analysis of ULF signal distribution as a function of season, latitude, local time, and solar wind conditions. We identify a ULF signal population associated with the magnetospheric cusp that is separate from the auroral oval during summer. Earlier studies, which were mainly performed in winter, failed to unambiguously identify these signals.

This study shows the first observations of the upflow of ions in the Earth's ionosphere generated by ultra-low frequency waves. These waves are visible as auroral arcs. Using various instruments and models, their complex dynamics and the coupling between the ionosphere and magnetosphere were highlighted. Results show significant energy dissipation and currents, even from small-scale waves, highlighting the importance of a multi-instrument approach to understanding such phenomena.

EISCAT3D is an ionospheric radar currently under construction in Northern Fenno-Scandinavia. The radar will make 3D measurements of the ionosphere at 50–1000 km altitudes. We show that the so-called multipurpose radar modulations and optimal data analysis can improve the time resolution of the measurements by more than an order of magnitude, and they enable one to measure ion-neutral collision frequencies, which are proportional to neutral particle density, in the lower ionosphere.

The magnetic mirror force bends the orbits of electrons precipitating into the atmosphere. It has been suggested that relativistic electrons make much less ionization due to the force than if it did not exist, but the actual effectivity in the atmospheric electron density has not been revealed. We used conjugated observational data from the ELFIN (Electron Losses and Fields INvestigation) satellite and the EISCAT (European Incoherent SCATter scientific radar system) Tromsø radar to find that the electron density decreased by about 40 % at 80 km altitude because of the force.

We use NO column density data from the Syowa station in Antarctica from 2012–2017. We compare these ground-based radiometer observations with results from a global atmosphere model to understand the year-to-year and day-to-day variability, shortcomings of current electron forcing, and how geomagnetic storms are driving the variability of NO. Our results demonstrate an underestimation in the magnitude of day-to-day variability in simulations, which calls for improved electron forcing in models.

We studied the first broad band emissions, called continuum, in the dayside aurora. They are similar to Strong Thermal Emission Velocity Enhancement (STEVE) with white-, pale-pink-, or mauve-coloured light. But unlike STEVE, they follow the dayside aurora forming rays and other dynamic shapes. We used ground optical and radar observations and found evidence of heating and upwelling of both plasma and neutral air. This study provides new information on conditions for continuum emission, but its understanding will require further work.

Our research explores the shock aurora, which is typically observed on the dayside due to the rapid compression of the Earth's magnetic field. We observed this rare aurora on the nightside, a region where such events are difficult to detect. Using ground-based cameras, we identified new features, including leaping and vortex-like patterns. These findings offer a fresh insight into the interactions between the solar wind and the magnetosphere, enhancing our understanding of space weather and its effects.

The complete time series of the geomagnetic disturbance index (K) from Norwegian magnetic observatories have been digitized. We compare and discuss the tree methods used to derive the index, finding that each method has strengths and weaknesses. In total, we present all K indices derived from Norwegian observatories since the 1930s until today, the used derivation methods and the long historic time series as a whole, enabling critical use for future scientific work.

We carried out a citizen science study of aurora sightings and technological disruptions experienced during the extreme geomagnetic storm of 10 May 2024. We collected reports from 696 observers from over 30 countries via an online survey, supplemented with observations logged in the Skywarden database. We found that the aurora was seen from exceptionally low latitudes and had very bright red and pink hues, suggesting that high fluxes of low-energy electrons from space entered the atmosphere.

Several tons of meteoric particles burn up in our atmosphere each day. This deposits a great deal of material that binds with other atmospheric particles and forms so-called meteoric smoke particles. These particles are assumed to influence radar measurements. Here, we have compared radar measurements with simulations of a radar spectrum with and without dust particles and found that dust influences the radar spectrum in the altitude range of 75–85 km.

We present via simulation how useful monochromatic images taken by a multi-point imager network are for auroral research in the EISCAT_3D project. We apply the generalized-aurora computed tomography (G-ACT) to modeled multiple auroral images and ionospheric electron density data. It is demonstrated that G-ACT provides better reconstruction results than the normal ACT and can interpolate ionospheric electron density at a much higher spatial resolution than observed by the EISCAT_3D radar.

Powerful radio waves transmitted into the ionosphere from the ground were used to study electron energization in the pumped ionospheric plasma turbulence, by detecting optical emissions from atomic oxygen. Our results obtained with the EISCAT (European Incoherent Scatter Scientific Association) facilities in northern Norway and optical detection with the ALIS (Auroral Large Imaging System) in northern Sweden suggest that long-wavelength upper hybrid waves are important in accelerating electrons.

We use computed tomography to reconstruct the three-dimensional distributions of the Hall and Pedersen conductivities of pulsating auroras, a key research target for understanding the magnetosphere–ionosphere coupling process. It is suggested that the high-energy electron precipitation associated with pulsating auroras may have a greater impact on the closure of field-aligned currents in the ionosphere than has been previously reported.

At times when auroras erupt on the sky, the magnetic field surrounding the Earth undergoes rapid changes. On the ground, these changes can induce harmful electric currents in technological conductor networks, such as powerlines. We have used magnetic field observations from northern Europe during 28 such events and found consistent behavior that can help to understand, and thus predict, the processes that drive auroras and geomagnetically induced currents.

Potential users of all-sky aurora images even include power companies, tourists, and aurora enthusiasts. However, these potential users are normally not familiar with interpreting these images. To make them comprehensive for more users, we developed an automatic evaluation system of auroral activity level. The method involves two steps: first making a simple set of numbers that describes the auroral activity and then further simplifying them into several levels (Level 6 is an auroral explosion).

Poleward moving auroral forms (PMAFs) are specific types of aurora believed to be the signature of the connection of Earth's magnetic field to that of the sun. In this paper, we discuss the evolution of PMAFs with regard to their auroral morphology as observed in all-sky camera images. We interpret different aspects of this evolution in terms of the connection dynamics between the magnetic fields of Earth and the sun. This sheds more light on the magnetic interaction between the sun and Earth.

We measured the height of green and blue aurorae using thousands of camera images recorded over a 7-year period. Both colours are typically brightest at about 114 km altitude. When they peak at higher altitudes the blue aurora is usually higher than the green aurora. This information will help other studies which need an estimate of the auroral height. We used a computer model to explain our observations and to investigate how the green aurora is produced.

The Arecibo radar was used to probe free electrons of the ionized atmosphere between 70 and 100 km altitude. This is also the altitude region were meteors evaporate and form secondary particulate matter, the so-called meteor smoke particles (MSPs). Free electrons attach to these MSPs when the sun is below the horizon and cause a drop in the number of free electrons, which are the subject of these measurements. We also identified a different number of free electrons during sunset and sunrise.

The pulsating auroral generation mechanism has been investigated by observing precipitating electrons using rockets or satellites. However, it is difficult for such observations to distinguish temporal changes from spatial ones. In this study, we reconstructed the horizontal 2-D distribution of precipitating electrons using only auroral images. The 3-D aurora structure was also reconstructed. We found that there were both spatial and temporal changes in the precipitating electron energy.

When radio waves from our galaxy enter the Earth's atmosphere, they are absorbed by electrons in the upper atmosphere. It was thought that by measuring the amount of absorption, it would allow the height of these electrons in the atmosphere to be determined. If so, this would have significance for future instrument design. However, this paper demonstrates that it is not possible to do this, but it does explain how multiple-frequency measurements can nevertheless be useful.

In this study, we present the comparison between an auroral model and EISCAT radar electron densities during pulsating aurorae. We test whether an overpassing satellite measurement of the average energy spectrum is a reasonable estimate for pulsating aurora electron precipitation. When patchy pulsating aurora is dominant in the morning sector, the overpass-averaged spectrum is found to be a reasonable estimate – but not when there is a mix of pulsating aurora types in the post-midnight sector.

This paper presents an analysis of high-resolution optical and radar observations of a phenomenon called fragmented aurora-like emissions (FAEs) observed close to aurora in the high Arctic. The observations suggest that FAEs are not caused by high-energy electrons or protons entering the atmosphere along Earth's magnetic field and are, therefore, not aurora. The speeds of the FAEs and their internal dynamics were measured and used to evaluate theories for how the FAEs are produced.

This paper is the first to simulate and analyse the pulsating aurorae impact on middle atmosphere on monthly/seasonal timescales. We find that pulsating aurorae have the potential to make a considerable contribution to the total energetic particle forcing and increase the impact on upper stratospheric odd nitrogen and ozone in the polar regions. Thus, it should be considered in atmospheric and climate simulations.

Energetic particle precipitation has the potential to change the neutral atmospheric temperature at the bottom of the ionosphere. We have searched for events and investigated a possible correlation between lower-ionosphere electron density enhancements and simultaneous neutral temperature changes. Six of the 10 analysed events are associated with a temperature decrease of 10–20K. The events change the chemical composition in the mesosphere, and the temperatures are probed at lower altitudes.

This is a review paper that summarises the current understanding of the lower thermosphere–ionosphere (LTI) in terms of measurements and modelling. The LTI is the transition region between space and the atmosphere and as such of tremendous importance to both the domains of space and atmosphere. The paper also serves as the background for European Space Agency Earth Explorer 10 candidate mission Daedalus.