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Observational data sets for the high latitude middle atmosphere are key to understand the dynamics over those latitudes and the coupling between the lower and middle atmosphere. Utilizing long‐term data sets from an all‐sky imager at Tromsø, Norway (69.6°N, 19.2°E), the characteristics of 18 mesospheric frontal events in the Arctic winter mesosphere from 2011 to 2015 were studied. These frontal events exhibit horizontal extensions exceeding 500 km and were characterized by a sharp leading front, sometimes followed by a quasi‐monochromatic wave train or a turbulent region. A subset of these frontal gravity wave events has been identified in the past as "bores." While there have been numerous previous reports from low‐ and mid‐latitude sites, and also from southern high latitudes, there have been a few from northern high latitudes. This study focuses on the frontal events in the northern high latitudes and provides new insights into the characteristics of these events. Their horizontal wavelengths primarily ranged from 20 to 40 km, and they exhibited phase speeds in the range 30–80 m/s. Most events were observed before local midnight. No clear link between these events and auroral activity was found. The majority of fronts were found propagating in the north‐west direction, which might be due to the wind filtering effects. Plain Language Summary: To better understand the dynamics of the high latitude middle atmosphere, we used long‐term data from various instruments in Tromsø, Norway. Our focus was on examining mesospheric frontal events that occurred in the Arctic winter mesosphere between 2011 and 2015. These events were notable for their large horizontal extent, often exceeding 500 km, and their distinctive characteristics, such as a sharp leading front, followed at times by a wave train or a region of turbulence. Some of these events were previously referred to as "bores." While similar events have been reported for lower and mid‐latitudes, there has been limited research focusing on northern high latitudes. Our study fills this gap, providing new insights into this phenomenon. We found that such events are less frequent in the northern high latitudes compared to the southern high latitudes. These frontal structures mostly traveled in directions ranging from northwest to north, and also to the southeast. While there was no clear link between these events and auroral activity, the majority of fronts were found to be propagating in the northwest direction, which might be attributed to wind filtering effects. Key Points: The study investigated the characteristics of mesospheric frontal structures observed in airglow emissions during Arctic winters using long‐term data setsThe occurrence of frontal structure at northern high latitude site Tromsø is less frequent than South PoleThe majority of the observed fronts were likely associated with tropospheric meteorological processes, which serve as potential sources [ABSTRACT FROM AUTHOR]

This study presents a first simultaneous observation of N 2 + Meinel (0,0) band (hereafter, N 2 + (M)) aurora by cutting-edge short-wavelength infrared imaging spectrograph (NIRAS-2) and monochromatic camera (NIRAC) installed at the Kjell Henriksen Observatory (78 ∘ N, 16 ∘ E). On January 21 2023, N 2 + (M) intensification that is associated with a band-shape aurora structure was observed by the NIRAS-2 and the NIRAC having temporal resolutions of 30 s and 20 s, respectively. In addition, the European incoherent scatter Svalbard Radar also observed electron density variations at the same time. Electron density measured at altitude range from 100 km 120 km shows similar variations as of N 2 + (M) intensity, which implies that a primary source of N 2 + (M) emissions is direct collisions of N 2 by precipitating electrons penetrating down to around 100 km altitude (up to 10 keV). However, the observation also demonstrated moderate correlations between N 2 + (M) intensity and electron density above 140 km, which implies that different N 2 + (M) generation process, N 2 charge exchange with O + , may work up to near 160 km and make a non-negligible contribution to N 2 + (M) emissions. This hypothesis would be verified with further radar observations or stereo imaging observations useful to estimate the vertical distribution of the emission layers. The N 2 + (M) is a very promising target wavelength for aurora observation because the quality of sensors is highly expected to improve further and further. Continuous observations with our new instruments will undoubtedly provide an important information of N 2 + (M) characteristics, for future missions of both balloon-borne and satellite-borne imaging. [ABSTRACT FROM AUTHOR]

Meteoric ablation in the Earth's atmosphere produces particles of nanometer size and larger. These particles can become charged and influence the charge balance in the D region (60–90 km) and the incoherent scatter observed with radar from there. Radar studies have shown that, if enough dust particles are charged, they can influence the received radar spectrum below 100 km, provided the electron density is sufficiently high (>109 m 3). Here, we study an observation made with the EISCAT VHF radar on 9 January 2014 during strong particle precipitation so that incoherent scatter was observed down to almost 60 km altitude. We found that the measured spectra were too narrow in comparison to the calculated spectra. Adjusting the collision frequency provided a better fit in the frequency range of ± 10–30 Hz. However, this did not lead to the best fit in all cases, especially not for the central part of the spectra in the narrow frequency range of ±10 Hz. By including a negatively charged dust component, we obtained a better fit for spectra observed at altitudes of 75–85 km, indicating that dust influences the incoherent-scatter spectrum at D-region altitudes. The observations at lower altitudes were limited by the small number of free electrons, and observations at higher altitudes were limited by the height resolution of the observations. Inferred dust number densities range from a few particles up to 104 cm -3 , and average sizes range from approximately 0.6 to 1 nm. We find an acceptable agreement with the dust profiles calculated with the WACCM-CARMA (Whole Atmosphere Community Climate Model-Community Aerosol Radiation Model for Atmospheres) model. However, these do not include charging, which is also based on models. [ABSTRACT FROM AUTHOR]

Atmospheric Gravity Waves and Traveling Ionospheric Disturbances can be observed in the neutral atmosphere and the ionosphere at a wide range of spatial and temporal scales. Especially at medium scales, these oscillations are often not resolved in general circulation models and are parameterized. We show that ionospheric disturbances forced by upward propagating atmospheric gravity waves can be simultaneously observed with the EISCAT Very High Frequency incoherent scatter radar and the Nordic Meteor Radar Cluster. From combined multi-static measurements, both vertical and horizontal wave parameters can be determined by applying a specially developed Fourier filter analysis method. This method is demonstrated using the example of a strongly pronounced wave mode that occurred during the EISCAT experiment on 7 July 2020. Leveraging the developed technique, we show that the wave characteristics of Traveling Ionospheric Disturbances are notably impacted by the fall transition of the mesosphere/lower thermosphere. We also demonstrate the application of using the determined wave parameters to infer the thermospheric neutral wind velocities. Applying the dissipative anelastic gravity wave dispersion relation, we obtain vertical wind profiles in the lower thermosphere. [ABSTRACT FROM AUTHOR]

Atmospheric gravity waves and traveling ionospheric disturbances can be observed in the neutral atmosphere and the ionosphere at a wide range of spatial and temporal scales. Especially at medium scales, these oscillations are often not resolved in general circulation models and are parameterized. We show that ionospheric disturbances forced by upward-propagating atmospheric gravity waves can be simultaneously observed with the EISCAT very high frequency incoherent scatter radar and the Nordic Meteor Radar Cluster. From combined multi-static measurements, both vertical and horizontal wave parameters can be determined by applying a specially developed Fourier filter analysis method. This method is demonstrated using the example of a strongly pronounced wave mode that occurred during the EISCAT experiment on 7 July 2020. Leveraging the developed technique, we show that the wave characteristics of traveling ionospheric disturbances are notably impacted by the fall transition of the mesosphere and lower thermosphere. We also demonstrate the application of using the determined wave parameters to infer the thermospheric neutral wind velocities. Applying the dissipative anelastic gravity wave dispersion relation, we obtain vertical wind profiles in the lower thermosphere. [ABSTRACT FROM AUTHOR]

In Sect. 1, the original planning of Japanese Svalbard IS (incoherent scatter) radar with phased-array antennas is described. In 1988, this plan was proposed as one of the major projects for the forthcoming Solar–Terrestrial Environment Laboratory, Nagoya University, Japan, to be reorganized by the Research Institute of Atmospherics at Nagoya University. On the other hand, in 1989, UK scientists proposed a plan of polar cap radar with parabolic dish antennas in Longyearbyen to the EISCAT (European incoherent scatter) Council. In Sect. 2, the circumstances leading to Japan's participation in the EISCAT Scientific Association, with details of its processes with strong collaborations with Norwegian scientists and the EISCAT Scientific Association are described. In 1995, Japan participated EISCAT Scientific Association as the seventh member country with funds contributing to the second dish antenna of the EISCAT Svalbard Radar. In Sect. 3, a summary of the EISCAT-related achievement by Japanese scientists is described, where major interests are the lower thermosphere wind dynamics, the magnetosphere–ionosphere–thermosphere coupling, characteristics, and driving mechanisms of ion upflow, electrodynamics of current, electric field and particles, characteristics and production mechanisms of auroras, such as pulsating aurora, and aurora tomography. In Sect. 4, a summary of the scientific collaborations between Japan and Europe, particularly those between Japan and Norway, and hopes for the forthcoming EISCAT_3D and further collaboration with EISCAT community are described. [ABSTRACT FROM AUTHOR]