Abstract Medium frequency radars with multiple receivers are able to track the movement of the interference pattern on the ground from echoes from irregularities in refractive index. In particular, refractive index in the mesosphere is determined by electron density – commonly known as the ionospheric D-region. Thus using this technique it is possible to determine winds in the height regime 70-90 km, depending on the degree of ionization throughout the year. In addition, by examining the fading times of the passage of these structures, it is possible to deduce metrics pertaining to neutral air turbulence. Here, we employ a well-established method to this effect. Thereafter, comparing the turbulent intensity to the kinematic viscosity of the neutral atmosphere, we determine the turbopause altitude. Above this height, atmospheric constituents behave independently, whereas below, all components are mixed. Contrary to earlier analyses, we present evidence the turbopause altitude has been constant since approximately 2004.

Abstract Radars used to observe meteor trails in the mesosphere deliver information on winds and temperature. Use of these radars is becoming a standard method for determining mesospheric dynamics and temperatures worldwide due to relatively low costs and ease of deployment. However, recent studies have revealed that temperatures may be overestimated in conditions such as high geomagnetic activity. The effect is thought to be most prevalent at high latitude, although this is not yet proven. Here, we demonstrate how temperatures might be corrected for geomagnetic effects; the demonstration is for a particular geographic location (Svalbard, 78°N, 16°E) because it is local geomagnetic disturbances that affects local temperature measurements, therefore requiring co-located instruments. We see that summer temperatures require a correction (reduction) of a few Kelvin, but winter estimates are more accurate.

Contemporaneous multi-instrument ground-based optical and meteor radar observations of OH and O-2 airglow, temperature and neutral winds during the winter season of November 2018-February 2019 have been used to investigate the dynamics of the mesosphere/lower thermosphere (MLT) (80-100 km) region in the high Arctic at Svalbard, Norway (78 degrees N, 16 degrees E) and at Eureka, Canada (80 degrees N, 274 degrees E). Temperature observations by the MLS Aura satellite over the 20-90 km height range for the same period were also considered. The period is characterized by an unusual major sudden stratospheric warming (SSW) event that began with a displacement of the polar vortex around December 13, 2018 (DoY 347), followed by the vortex split on January 2, 2019 (DoY 367). The MLS Aura temperature observations outlined four periods of interest: 1) late November - early December (DoY 328-335) with cold temperature anomalies in the mesosphere and warm bursts at the stratopause; 2) December 13, 2018-January 2, 2019 (DoY 347-367) when the stratopause rapidly descended to 45 km and broke down, triggering the onset of a SSW; 3) January 3-20, 2019 (DoY 368-385) during the stratopause recovery phase, and 4) from January 21, 2019 (DoY 386) onward, with the formation of the elevated stratopause and gradual return to its pre-SSW height. Both airglow emissions, OH and O-2 Atm, showed simultaneously significant depletion of the integrated emission rates (IER) and temperature decrease of the order of 50-60 K, indicating upwelling, depletion of the atomic oxygen and adiabatic cooling. These cold temperature anomalies were followed by enhancements in the observed airglow IERs on December 13, 2018 (DoY 347) and January 2, 2019 (DoY 367) accompanied by a decrease in the peak altitude of the OH layer suggesting down-welling and influx of atomic oxygen from the lower thermosphere. The observations revealed oscillations with periods of 4.5-7 days, 8-10 days, and 16-21 days consistent with previously reported planetary wave activity in the winter MLT region and during major stratospheric warming events. However, the results presented here show for the first time comparisons of the multi-instrument temperature observations at 78 degrees N - 80 degrees N, providing an indispensable tool in monitoring the dynamics over the polar cap in general, and in describing the regional dynamical response of the MLT region to major large-scale phenomena like stratospheric warmings, in particular.

We present and characterize in time and three spatial dimensions a Kelvin-Helmholtz Instability (KHI) event from polar mesospheric summer echoes (PMSE) observed with the Middle Atmosphere Alomar Radar System. We use a newly developed radar imaging mode, which observed PMSE intensity and line of sight velocity with high temporal and angular resolution. The identified KHI event occurs in a narrow layer of 2.4 km thickness centered at 85 km altitude, is elongated along north-south direction, presents separation between billows of similar to 8 km in the east-west direction, and its billow width is similar to 3 km. The accompanying vertical gradients of the horizontal wind are between 35 and 45 m/s/km and vertical velocities inside the billows are +/- 12 m/s. Based on the estimated Richardson (<0.25), horizontal Froude (similar to 0.8), and buoyancy Reynolds (similar to 2.5 x 10(4)) numbers, the observed event is a KHI that occurs under weak stratification and generates strong turbulence.

Echoes from meteor trails can be analyzed to derive diffusion coefficient and thereafter estimates of neutral temperatures, typically around 90 km altitude. The method entails some assumptions and, usually, use of model pressures. Even so, variations in derived temperatures have been assumed to be representative and indicators of other mesospheric processes. However, we now find that the observed variations, seen as enhancements, are very closely associated with enhancements in geomagnetic activity. Here we introduce a metric for describing geomagnetic activity (as an alternative to, e.g., the k-index) and suggest a methodology for near-real-time correction of results.

Neutral air temperatures at 90 km altitude over Svalbard (78°N, 16°E) for the period 2002 to 2019 inclusive have been estimated from observations by the Nippon/Norway Svalbard Meteor Radar (NSMR). The data are presented per se and we shall not attempt to identify contributions from extra-terrestrial or anthropogenic driving. On the other hand, comparison with the corresponding period of solar UV flux notably fails to exhibit similarities. Moreover, there is indeed evidence of systematic temporal changes, possibly with a breakpoint around 2012. Selecting winter and summer months and years before and after 2012 and fitting trend-lines, we see strong evidence for cooling during summer. An apparent winter cooling prior to 2012 lacks statistical significance. A suggestion of reversal to winter warming after 2012 is also very uncertain. The summer cooling is found to be 9.9 ± 2.9 K decade−1 between 2002 and 2012, and 4.3 ± 1.2 K decade−1 between 2002 and 2019. Importantly, there is a suggestion (although lacking statistical significance) that summer cooling alleviated after 2012, and even reversed in winter.

Common volume mesospheric meteor detections from two radar stations separated by about 130 km were used to retrieve horizontal wind fields between 82 and 96 km altitudes at high latitudes, near 69 degrees N. The horizontal wind divergence was estimated from the gradients of the wind fields. This determination is the first of its kind for the mesosphere. Twelve years of nearly continuous data sets reveal systematic summer signatures in the horizontal wind divergence field, namely, a transition from negative to positive with increasing altitude while moving across the mesopause. There are indications that the reversal altitude is also anticorrelated with the mesopause temperature in addition for a tendency for the altitude of the reversal to increase over the years. We show that the reversal in the horizontal wind divergence at the mesosphere is consistent with upward winds peaking near the mesopause. These winds indicate that adiabatic cooling was strongest at the region of the deep temperature minimum seen in the summer mesopause. Plain Language Summary The lowest atmospheric temperatures on Earth are found near 90 km altitude (the mesopause) during summer at polar latitudes. This fact is now well documented and is attributed to a predominance of large amplitude gravity waves carrying eastward momentum near 90 km altitude during summer season. Through a Coriolis deflection, the deposition of this momentum causes equatorward winds at high latitudes, an expansion of the air mass, adiabatic cooling, and an accompanying upward motion of the air. In particular, an experimental determination of the upwelling has been found lacking, owing to the very small vertical motions involved. In this paper, we present a new way to determine the mean vertical motion based on the observation of summer-long-averaged horizontal wind divergences obtained by meteor radar measurements observing the same field of view from two separate locations. For 12 consecutive years we have found that the horizontal divergence goes from negative to positive very near the altitude where the temperature reaches its minimum value. We show that this implies upward velocities of the order of 5 to 10 cm/s reaching their peak in that same region, consistent with the notion of strong adiabatic cooling as the source of the low temperatures.

Abstract. Neutral temperatures for 90 km height above Tromsø, Norway, have been determined using ambipolar diffusion coefficients calculated from meteor echo fading times using the Nippon/Norway Tromsø Meteor Radar (NTMR). Daily temperature averages have been calculated from November 2003 to October 2014 and calibrated against temperature measurements from the Microwave Limb Sounder (MLS) on board Aura. The long-term trend of temperatures from the NTMR radar is investigated, and winter and summer seasons are looked at separately. Seasonal variation has been accounted for, as well as solar response, using the F10.7 cm flux as a proxy for solar activity. The long-term temperature trend from 2003 to 2014 is −3.6 K ± 1.1 K decade−1, with summer and winter trends −0.8 K ± 2.9 K decade−1 and −8.1 K ± 2.5 K decade−1, respectively. How well suited a meteor radar is for estimating neutral temperatures at 90 km using meteor trail echoes is discussed, and physical explanations behind a cooling trend are proposed.

A sporadic sodium layer (SSL) was detected with five‐directional lidar observation on 15 December 2012 at Tromsø, Norway. We have derived the horizontal velocity of the SSL front from the SSL onset times at the five positions and compared it with the background wind velocity from the collocated meteor radar and European Incoherent Scatter (EISCAT) radar. As a result, both velocities were fairly consistent. The increase rate in the height‐integrated sodium density around the SSL onset was 3–6 ×10 10 m −2 s −1 , which was comparable to relatively large cases in the previous studies. However, the EISCAT‐observed electric field was too small to induce such a rapid sodium atom production. In addition, the amounts of the sodium atom increases at the five positions were mostly same. Thus, there were no clear signatures for the sodium atom production. These results strongly indicate that the observed SSL was just advected by the background wind.

Four years of auroral latitude radar echo signal fading time data from the Tromsø MF radar compared with geomagnetic and solar indices yield strong evidence to suggest, although not prove, that upper mesospheric turbulence may be enhanced during periods of geomagnetic activity. We present a critique of hypotheses to explain such effects.

Abstract. Solar disturbances, depending on the orientation of the interplanetary magnetic field, typically result in perturbations of the geomagnetic field as observed by magnetometers on the ground. Here, the geomagnetic field's horizontal component, as measured by the ground-based observatory-standard magnetometer at Tromsø (70° N, 19° E) is examined for signatures of complexity. 25 year-long 10 s resolution datasets are analysed, but for fluctuations with timescales less than 1 day. Quantile-quantile (Q-Q) plots are employed first, revealing the fluctuations are better represented by Cauchy rather than Gaussian distributions. Thereafter, both spectral density and detrended fluctuation analysis methods are used to estimate values of the generalized Hurst exponent, α. The results are then compared with independent findings. Inspection and comparison of the spectral and detrended fluctuation analyses reveals that timescales between 1 h and 1 d are characterized by fractional Brownian motion with a generalized Hurst exponent of ~1.4 whereas including timescales as short as 1 min suggests fractional Brownian motion with a generalized Hurst exponent of ~1.6. This is consistent with changes in the position of the auroral electrojet that can be considered rapid during the course of an evening, whereas from minute-to-minute the electrojet moves more persistently in geomagnetic latitude.

Solar disturbances, depending on the orientation of the interplanetary magnetic field, typically result in perturbations of the geomagnetic field as observed by magnetometers on the ground. Here, the geomagnetic field's horizontal component, as measured by the ground-based observatory-standard magnetometer at Tromsø (70° N, 19° E), is examined for signatures of complexity. Twenty-five year-long 10 s resolution data sets are analysed for fluctuations with timescales of less than 1 day. Quantile–quantile plots are employed first, revealing that the fluctuations are better represented by Cauchy rather than Gaussian distributions. Thereafter, both spectral density and detrended fluctuation analysis methods are used to estimate values of the generalized Hurst exponent, α. The results are then compared with independent findings. Inspection and comparison of the spectral and detrended fluctuation analyses reveal that timescales between 1 h and 1 day are characterized by fractional Brownian motion with a generalized Hurst exponent of ~1.4, whereas including timescales as short as 1 min suggests fractional Brownian motion with a generalized Hurst exponent of ~1.6.

MF radar systems are able to determine horizontal neutral winds in the mesosphere and, to some extent in the lower thermosphere by cross-correlations of signals received at spaced antennas. Essentially, by also computing auto-correlations, signal fading may be measured which in turn is thought to be largely attributable to turbulence. Hitherto, estimates of upper limits for the turbulent energy dissipation rate have been derived from the characteristic fading times. In this paper, we propose that power spectra of the velocity components themselves may be used to yield estimates of turbulent energy dissipation rate. 2-minute resolution velocities from the Universities of Saskatchewan, Tromsø and Nagoya joint MF radar at 69°N, 19°E are used in a pilot analysis to illustrate and ratify the method.

Atmospheric parameters from the troposphere above Adventdalen, Svalbard, 78 degrees N, 16 degrees E, are examined for signatures of complexity in their respective stochastic components over time scales from similar to 1h to 1 year. Several approaches are used, all of which can estimate values of the generalized Hurst exponent, , which can in turn be compared with each other and with similar independent characterizations, usually via the classic Hurst exponent, H, obtained from location-specific and globally averaged time series. For tropopause altitude, the stochastic component exhibits the signature of a persistent fractional Gaussian noise (fGn) with approximate to 0.75. For surface air temperature, the indications are for fractional Brownian motion (fBm) with approximate to 1.4. Using recent high time-resolution data from a single high-latitude location, this identification of fBm is relevant for short-term memory as opposed to findings from many other studies addressing possible long-term memory, which demonstrate fGn with =H approximate to 0.7. Furthermore, the lack of similarity between the results for surface air temperature and tropopause altitude suggests that different underlying processes are responsible for stochastic variability.Key PointsComplexity signatures found in the arctic tropopause and air temperature Tropopause and temperature variabilities exhibit different complex signatures Very specific time-series results supplement earlier more general studies

Mesospheric wind data from meteor wind radars situated on Svalbard (78 degrees N, 16 degrees E) and the Norwegian mainland (70 degrees N, 19 degrees E) are examined for evidence of systematic change during the interval 2001-2012. For both locations, we find changes that suggest a strengthening of the summer westward jet, a weakening of the local winter eastward flow and, yet at the same time, weak evidence for any significant corresponding trend in the winter poleward flow. There is also a suggestion of an increase in the altitude of the summer polar jet, but more data will be required to confirm this. The main finding of a strengthening zonal flow is consistent with earlier studies and also with a contemporary scenario of progressive strengthening of the Brewer-Dobson circulation. We show that inclusion of sudden stratospheric warmings strongly influences trends. There is no obvious causality between the changes detected over the 2001-2012 time interval and the solar cycle parameterized by total solar irradiance.

A continuous 5 year time series of tropopause altitude made by the SOUSY VHF radar on Svalbard (78°N, 16°E) is examined for diurnal variation. The data have a nominal time resolution of 1 h, allowing for investigation of intraday timescales not normally possible using radiosonde observations. Periodicities identified in tropopause altitude are found to vary with season. Surface air temperature measurements naturally exhibit diurnal variations which depend on season: however, it is demonstrated that, using a simple scenario of constant adiabatic lapse rate in the troposphere, the tropopause altitude variation that might be anticipated from that of the surface air temperature is not in agreement with the radar observation. Of several mechanisms that could be postulated for this discrepancy, it is demonstrated that a likely candidate is intraday top‐down forcing from the stratosphere.

A new solid‐state sodium lidar installed at Ramfjordmoen, Tromsø (69.6°N, 19.2°E), started observations of neutral temperature together with sodium density in the mesosphere‐lower thermosphere (MLT) region on 1 October 2010. The new lidar provided temperature data with a time resolution of 10 min and with good quality between ∼80 and ∼105 km from October 2010 to March 2011. This paper aims at introducing the new lidar with its observational results obtained over the first 6 months of observations. We succeeded in obtaining neutral temperature and sodium density data of ∼255.5 h in total. In order to evaluate our observations, we compared (1) the sodium density with that published in the literature, (2) average temperature and column sodium density data with those obtained with Arctic Lidar Observatory for Middle Atmosphere Research Weber sodium lidar, and (3) the neutral temperature data with those obtained by Sounding of the Atmosphere with Broadband Emission Radiometry/Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. For the night of 5 October 2010, we succeeded in conducting simultaneous observations of the new lidar and the European Incoherent Scatter UHF radar with the tristatic Common Program 1 (CP‐1) mode. Comparisons of neutral and ion temperatures showed a good agreement at 104 km between 0050 and 0230 UT on 6 October 2010 when the electric field strength was smaller, while significant deviations (up to ∼25 K) are found at 107 km. We evaluated contributions of Joule heating and electron‐ion heat exchange, but derived values seem to be underestimated.