Transpolar arcs (TPAs) describe a subset of auroral emissions observed poleward of the Earth's main auroral ovals when the interplanetary magnetic field (IMF) is northward. These emissions are thought to align with a “wedge” of closed magnetic field lines extending to the high‐latitude boundary of the Earth's magnetosphere, where they are postulated to interact with the IMF through magnetic reconnection. Such an instance of “high‐latitude TPA wedge reconnection” is expected to open the Earth's magnetic field lines, but the event has never been verified by in situ observations. Here we report a detection of a high‐latitude TPA wedge reconnection site within one ion gyroradius distance on 18 March 2002, which manifests in the ionosphere as colocation of the TPA and a cusp spot. The result may explain previously reported cases of remotely detected “non‐lobe” high‐latitude magnetic reconnection, with implications for future understanding of TPAs and northward‐IMF global magnetospheric dynamics.

Relatively little is known about the causes of fine‐scale aurora with scale sizes of km and less. We analyze images from the Auroral Structure and Kinetics (ASK) instrument, a multi‐monochromatic spectral imager located in the Norwegian High Arctic. We analyze five categories of fine‐scale aurora. The categories described in this paper are: “drift arcs”, “drift arcs (dynamic)”, “chocolate sauce”, “chocolate sauce (turbulent)”, and “psychedelic”; these are categories based on those in the citizen science project, Aurora Zoo . The analysis comprises: magnetic local time (MLT) dependence, characteristic precipitation energies, occurrence relative to the auroral oval, and adapted local intermittency measure (LIM) analysis of radially averaged image 2D power spectra. LIM analysis allows us to estimate the widths of the smallest structures typically associated with each aurora type. We find that chocolate sauce (turbulent) and psychedelic aurora are associated with nightside processes. This is based on their tendency to be higher energy, to occur at late dusk/midnight MLTs, and to occur on the poleward boundary of the auroral oval. For non‐psychedelic aurora types, we find the smallest characteristic widths ( m) and distribution of energies (typically less than 10 keV) to be consistent with the dissipative non‐linear inertial Alfvén wave acceleration mechanism described by Wu and Chao (2004a), https://doi.org/10.1029/2003ja010126 . For psychedelic aurora, the smallest characteristic widths ( m) and high energies (median 10.9 keV) make it unlikely that it is produced by the aforementioned mechanism, which predicts arc widths of km and energies less than 10 keV.

Citizen science (also referred to as participatory science or community science), in which members of the general public contribute to scientific research, is not a new concept, as early examples of such studies can be found a couple of centuries ago. With the advancement of technology in an increasingly connected world, it has never been easier to engage citizen scientists in research projects. In this paper, we review citizen science initiatives and projects in the fields of atmosphere and space physics, including both early observation campaigns prior to the twenty-first century and recent projects. Ongoing initiatives take a broad range of forms, from the collection of data by citizen scientists to their involvement in the data analysis process and to the hosting of instruments in non-scientific public structures. We also discuss some of the challenges specific to citizen science, such as training citizen scientists, maintaining their engagement, ensuring reciprocity, managing citizen science data, interfacing the academic and citizen scientist communities, and funding citizen science. To these challenges we suggest possible solutions, and we highlight the unique opportunities offered by recent software and hardware developments. These game-changing opportunities are foreshadowing the dawn of a new era for citizen science – and hence for science in general and atmosphere and space physics in particular.

We have constructed a new high-resolution auroral imager, called Embla, to simultaneously measure energy and flux of auroral precipitation, neutral temperature, and electric fields in fine scale aurora. Embla is designed primarily for studies of auroral electrodynamics, substorm onset, and neutral heating by aurora. The instrument has recently been installed at Skibotn, Norway, very close to the EISCAT_3D radar transmitter site, and we expect the combination of radar and optical observations to enable better measurements for our science than either instrument can provide alone. Embla builds on work done using the Auroral Structure and Kinetics (ASK) instrument, which has been stationed at the EISCAT Svalbard Radar since 2007. Embla has a spatial resolution in the E-region of ~30 m and a planned temporal resolution of at least 32 frames per second, allowing us to resolve the fine-scale structure and rapid dynamics of auroral features. It consists of 4 co-aligned imagers with identical 9 degree fields of view centred on magnetic zenith. Each imager is equipped with a different narrow passband interference filter, targetting emissions in N2 1P (2 imagers), OI 777.4 nm, and O+ 2P. The combination of N2 1P and OI 777.4 nm observations allows us to image the characteristic energy and flux of auroral electron precipitation. The O+ 2P emission has a long lifetime of 5 s, providing a means to observe ion drift perpendicular to the magnetic field and therefore a way to determine ionospheric electric fields at very high cadence in localised regions around the aurora. Finally, by combining observations from two of the imagers in separate regions of the N2 1P band we can image the neutral temperature at the altitude of the auroral emission. The simultaneous measurement of these properties of the aurora and ionosphere will allow us to investigate auroral electrodynamics in detail.

The Kjell Henriksen Observatory (KHO) is the world's largest optical observatory for auroral and airglow measurements, operated by the University Centre in Svalbard (UNIS). KHO is a unique site that lies underneath the dayside cusp, a funnel-shaped region where particles from the Sun can directly enter the Earth's upper atmosphere, including the ionosphere. Building on the pioneering observations of its predecessor --- the Auroral Station in Adventdalen, Svalbard --- KHO has played a pivotal role in advancing our understanding of phenomena in the polar atmosphere. The Auroral Station and KHO have amassed climatological measurements over Svalbard for an impressive 40-year period. KHO's diverse instrumentation, combined with other co-located optical and radar infrastructure and in-situ measurements from satellites and sounding rockets, has paved the way for impactful multi-instrument studies. Serving as an accessible testbed for instrument development, new types of instruments have recently been installed, both at KHO and on satellites. Beyond its scientific contributions, KHO has become an integral part of the Longyearbyen community, with students, visitors, and locals participating in tours and educational initiatives. This connection underscores KHO's multi-functional role, not only as a centre for excellent research but also as a vital hub for public outreach and engagement.

The Small Payloads for Investigation of Disturbances in Electrojet by Rockets 2 (SPIDER-2) sounding rocket was launched from Esrange, Sweden, on the 19th of February 2020 at 23:14 UT. It traversed a pulsating aurora event, deploying eight free falling units which provided in situ multi-point measurements of the electric field, magnetic field and plasma parameters. In this article, the measured plasma parameters have been analyzed and compared with each other and with optical measurements obtained by ground based instrumentation. Peaks in electron density, thermal ion flux and optical emission have been found in the E region. Electron density profiles have been derived from the data collected by the Langmuir probes in two free falling units, the electron probes in the main rocket and the wave propagation experiment. A generally good agreement has been found among the different measurements in the up-leg of the trajectory, while the effect of the rocket wake was evident in the down-leg. The observed electron density profile has been found to agree with an incoming flux of high energetic electrons with energies around 20 keV. Auroral pulsations with a periodicity of 1-2 s have been recorded by an onboard photometer, a ground-based high speed camera, and the in situ thermal ion flux. The percentages of variation between the ON and OFF phases of the pulsations have been quantified for these quantities. The brightness measured by the photometer varies up to 68%, while the thermal ion flux measurements show only a 2.5% variation.

We report the first observations of continuum emission at the poleward boundary of the dayside auroral oval. Spectral measurements of high-latitude continuum emissions resemble those of Strong Thermal Emission Velocity Enhancement (STEVE), with light characterized by colours such as white, pale pink, or mauve. The emission enhancement spans the entire visible wavelength range. However, unlike STEVE, the high-latitude dayside continuum emission events tightly follow the auroral particle precipitation, often forming field-aligned rays and other dynamic shapes. Some dayside emissions appeared as wide arcs or cloud-like structures within the red-emission-dominated dayside aurora. Our spectral measurements further suggest that the broadband continuum emission may extend into the near-infrared (NIR) regime. Similar to the STEVE emission, low-Earth-orbit measurements of plasma flow in the region of continuum emission show a strong horizontal cross-track velocity shear. Ground-based radar and optical observations provide evidence of both plasma and neutral heating, as well as upwelling, in connection to the continuum emissions. We conclude that the interplay between different heating mechanisms may be an important factor in generating high-latitude continuum emissions.

Energetic proton precipitation causes proton aurora over Svalbard, but its effects on the chemistry and heat economy of the atmosphere are not well observed. Temperature and intensity changes in the OH layer have been observed during auroral electron precipitation (Suzuki et al., 2018), but no studies have yet investigated the effect of auroral proton precipitation. This study will use observations made by the HiTIES (High-Throughput Imaging Echelle Spectrograph) instrument located at the Kjell Henriksen Observatory near Longyearbyen, Svalbard. This spectrograph observes proton precipitation in the H-alpha wavelength as well as the OH*(8-3), (9-4), (5-1) airglow vibrotational bands. These OH bands are temperature and species density dependent which can be compared to the H-alpha profile and luminosity as a measure of proton aurora energy and flux. In addition, the vibrotational states above and below v'=6 result from different production processes which can be studied. Prelimiary results will be presented.

Ground based optical observations of aurora reveal fine scale structures with brightness width less than 10 kms in the direction perpendicular to B. These fine scale structures exhibit a phenomenon called arc splitting, also known as bifurcating elemental arcs or packets. One can witness the arc splitting in the auroral image when elemental arcs peel away from a central bright arc which is then followed by the generation of new arcs. Dispersive Alfvén waves have been suggested as a possible generation mechanism for this phenomenon. Semeter et al., JGR 2008 interpreted the observations of splitting elemental arcs with respect to inertial Alfvén waves. They suggested that the energy of the precipitating electrons should decrease as the arc packets move away from the central bright arc. We tested this theory by using the data from the multi spectral imager called ASK (Auroral Structure and Kinetics) stationed on Svalbard. The energy and flux of the precipitating electrons are calculated from the ASK data and Southampton ionospheric model. We used empirical number density and IGRF (International Geomagnetic Reference Field) model to calculate the properties of the inertial Alfvén waves. A comparison between the splitting elemental arcs and Alfvén waves indicates that the wave particle interaction between Alfvén waves and the precipitating electrons is a possible generation mechanism for the production of these splitting elemental arcs. From our data and calculations, we infer an acceleration height of precipitating electrons just under 3000 km. References:Semeter, J., M. Zettergren, M. Diaz, and S. Mende (2008), Wave dispersion and the discrete aurora:New constraints derived from high-speed imagery, J. Geophys. Res., 113, A12208.

Auroral Kilometric Radiation (AKR), the dominant radio emission from Earth, has been extensively studied, though previous analyses were constrained by limited spacecraft coverage. This study utilizes long-term observations from Polar, Wind, and Arase spacecraft to generate comprehensive global AKR occurrence rate maps, revealing a high-latitude and nightside preference. A detailed investigation of the equatorial shadow region confirms that the dense plasmasphere blocks AKR emissions across all wave frequencies. Low-frequency emissions (<100 kHz) are presents outside the shadow region at larger radial distance, which is attributed to magnetosheath reflection, while higher-frequency emissions (>100 kHz) propagate via plasmaspheric ducting and leakage, filling the equatorial region immediately outside the plasmasphere. Ray-tracing simulations identify low-density ducts within the plasmasphere as crucial channels that enable AKR to penetrate the dense plasmasphere, particularly at higher frequencies. These results align with meridional AKR observations, offering new insights into AKR propagation patterns.

Abstract. We investigate the origin of the continuum emissions observed in the poleward boundary dayside aurora discovered in Partamies et al. 2025, known as GHOST, and propose that they arise from highly excited, hot N2. Using spectral modelling and fits to ground-based measurements of high-resolution GHOST spectra, we demonstrate that vibrationally and rotationally excited N2 and N2+ can reproduce the observed structured continuum without requiring emission from NO. Spectral fitting indicates that GHOST events coincide with extreme ion heating and high neutral temperatures. Background conditions from additional events indicate that strong ionospheric flows are typically present, which can help to provide the necessary energy input for producing hot neutral and ionised N2. Proton aurora observations and EISCAT incoherent scatter radar measurements of ionospheric plasma parameters indicate that two of our three events are located in the cusp. These results suggest that the combination of strong flow, heating, particle precipitation, and cusp conditions produce thermally excited N2 populations which can account for the continuum spectrum of GHOST.

ABSTRACT This white paper is highly topical as it relates to the upcoming solar wind magnetosphere ionosphere link explorer (SMILE) mission: SMILE is a joint mission between the European Space Agency and the Chinese Academy of Sciences and it aims to build a more complete understanding of the Sun–Earth connection by measuring the solar wind and its dynamic interaction with the magnetosphere. It is a fully funded mission with a projected launch in 2025. This paper outlines a plan for action for SMILE’s first Northern hemisphere winter campaign using ground-based instruments. We outline open questions and which data and techniques can be employed to answer them. The science themes we discuss are: (i) Earth’s magnetosheath, magnetopause, and magnetic cusp impact on the ionospheric cusp region; (ii) defining the relationship between auroral processes, solar wind, and magnetospheric drivers; (iii) understanding the interhemispheric properties of the Earth’s magnetosphere–ionosphere system. We discuss open questions (different to the mission goals) which may be answered using existing ground-based instrumentation together with SMILE data to leverage the maximum scientific return of the mission during the first winter after launch. This paper acts as a resource for planning, and a call to collaborative action for the scientific community.

A statistical study has been made of dynamic small-scale auroral events in order to understand the drivers of the large variability in the electrodynamics of auroral arcs at fine scales. We used the Auroral Structure and Kinetics (ASK) instrument, located on Svalbard, in order to measure various electrodynamic properties of fine scale auroral arcs. We performed Spearman and Kendall statistical tests and found two significant correlations. The first is between the mean precipitation flux and the variability of flux, which we assume is because of the dynamic and bursty nature of the acceleration mechanism and its dependence on Alfv & eacute;n waves. The second correlation is between the variability of the precipitating electron flux and the variability of the tangential component of the electric field close to the arc and perpendicular to the magnetic field. We propose that both variabilities occur because of the variability of the upward (field-aligned) current sheet in and around the arc, which is dynamic and non-uniform. The correlation between the two variabilities can therefore be explained by their common source. We report a statistical analysis of small scale electrodynamics, to better understand the drivers of small scale variability There is a significant correlation between average flux and variability of the flux in our events There is a significant correlation between variability of tangential component of the electric field and variability in flux We propose that variability of the dynamic non-uniform field aligned current sheet would be a source of variability in both electric fields and in flux, causing them to be correlated

Auroral Kilometric Radiation (AKR) is radio emission that originates in particle acceleration regions along magnetic field lines that coincide with discrete auroral arcs. Radio astronomy instruments aboard various spacecraft have been used to derive the flux density, source direction and other parameters of emissions of various origin. The Wind spacecraft has been in operation for 25 years and the WAVES radio instrument has previously been considered for a technique to also derive the Stokes parameters of a partially polarised radio source. While previous applications of the technique have seen it modified to study solar radio emissions, further examination is needed for its application to AKR. After correcting appropriately for the characteristics of the AKR emissions, this technique can be used to produce a utile dataset of AKR observations. Statistical properties of AKR can be examined, with the extent of local time sampling of Wind bolstering previous studies. The previously observed correlation between morphological changes in the source region and magnetospheric substorm onset can be studied further, and lists of substorm phase timings can be used to examine the general variability during these events.

We present three pieces of observational evidence to conclude that EMIC waves are not the mechanism responsible for the acceleration of auroral protons in the polar cusp. It is widely believed that ElectroMagnetic Ion Cyclotron (EMIC) plasma waves are the mechanism responsible for the acceleration of auroral protons - however, measurements of auroral proton precipitation and Pc1 pulsations from Svalbard under the cusp region indicate that there is no significant link between the two phenomena. Spectrograph measurements of proton aurora over Svalbard are studied alongside co-located magnetometer measurements of Pc1 pulsations. No evidence of a link between proton aurora and Pc1 waves was found by three different methods. Firstly, accelerated protons and Pc1 pulsations have no coincident occurrence. Secondly, the proton energy spectrum does not change between Pc1 activity and quiet times. Finally, no imprint of the EMIC wave is found in periodicity of the intensity and blue-shift of the proton H-$\alpha$ line, unlike in flickering electron aurora where intensity fluctuations are caused by EMIC waves. It may be possible that EMIC waves are causing acceleration but not propagating down to cause Pc1 pulsations, however we deem this unlikely. Therefore we conclude that EMIC waves are not the mechanism responsible for accelerating auroral protons in the cusp.

We have performed a large statistical study of the peak emission altitude of green O(1D2–1S0) (557.7 nm) and blue N2+ 1 N (427.8 nm) aurora using observations from a network of all-sky cameras stationed across northern Finland and Sweden recorded during seven winter seasons from 2000 to 2007. Both emissions were found to typically peak at about 114 km. The distribution of blue peak altitudes is more skewed than that for the green, and the mean peak emission altitudes were 114.84 ± 0.06 and 116.55 ± 0.07 km for green and blue emissions, respectively. We compare simultaneous measurements of the two emissions in combination with auroral modelling to investigate the emission production mechanisms. During low-energy electron precipitation (<∼ 4 keV), when the two emissions peak above about 110 km, it is more likely for the green emission to peak below the blue emission than vice versa, with the difference between the two heights increasing with their average. Modelling has shown that under these conditions the dominant source of O(1S), the upper state of the green line, is energy transfer from excited N2 (A3Σu+), with a rate that depends on the product of the N2 and O number densities. Since both number densities decrease with higher altitude, the production of O(1S) by energy transfer from N2 peaks at lower altitude than the N2 ionisation rate, which depends on the N2 number density only. Consequently, the green aurora peaks below the blue aurora. When the two emissions peak below about 110 km, they typically peak at very similar altitude. The dominant source of O(1S) at low altitudes must not be energy transfer from N2, since the rate of that process peaks above the N2 ionisation rate and blue emission due to quenching of the long-lived excited N2 at low altitudes. Dissociative recombination of O2+ seems most likely to be a major source at these low altitudes, but our model is unable to reproduce observations fully, suggesting there may be additional sources of O(1S) unaccounted for.