Search

The polar summer mesosphere is the Earth’s coldest region, allowing the formation of mesospheric ice clouds. These ice clouds produce strong polar mesospheric summer echoes (PMSE) that are ...

In April 2018 the PMWE1 sounding rocket campaign was conducted at the Andøya Space Center involving coordinated measurements with rockets and ground instruments to measure parameters relevant for testing of the existing theories of polar mesospheric winter echo (PMWE) formation. The Middle Atmosphere Alomar Radar System (MAARSY) was operated to detect PMWE with multiple beam directions to detect favorable launch conditions. A dedicated experiment configuration with five different beam positions was used to point the radar beam along the planned trajectory of the payload. This special radar experiment allowed to obtain basic information about the spatial structure of the PMWE and its dynamical behavior around the flight of the two rockets. PMWE with signal strengths between 10−17 and 10−15 m−1 have been observed by MAARSY during the whole campaign period, starting with a moderate occurrence at the beginning which decreased towards the end of the campaign. Furthermore real common-volume observations by rocket instruments and radar soundings have been carried out at PMWE altitudes on up-leg and down-leg of the rocket flights. The Saura MF radar was operated during both flights probing the mesosphere with a multiple beam scan experiment to derive horizontal winds and electron density profiles. The obtained PMWE characteristics as signal strength and spectral width of the received radar signals as well as estimated horizontal winds and electron densities are presented with particular emphasis to the launch times of the sounding rockets.

The Saura radar is designed and used to measure winds and electron densities at polar latitudes (69 $$^\circ $$ N) within the D region, namely between 50 and 100 km altitude. A relatively narrow radar beam can be generated and steered into distinct pointing directions as a rather large antenna array is used. From the observed radial velocities of the individual pointing directions, the horizontal and vertical wind fields can be obtained using the Doppler beam swinging (DBS) method. With recent upgrades to the radar, the interferometric capabilities are largely improved allowing simultaneous application of different wind estimation techniques now, and also echo localization. In recent studies, Saura DBS winds assuming isotropic scattering were found to be underestimated in comparison with highly reliable winds observed with the MAARSY MST radar in the presence of polar mesospheric summer echoes (PMSE). This underestimation has been investigated by analyzing the scattering positions as well as applying the imaging Doppler interferometry technique. Besides this, Saura winds derived with the classical DBS method seem to be error prone at altitudes above 90 km and even below this altitude for periods of enhanced ionization, e.g., particle precipitations. Various methods taking into account the scattering positions have been used to correct the wind underestimation. These winds are compared to MST radar winds during PMSE, and an optimal combination of these methods for the Saura radar is presented. This combined wind data appears to be reliable; it shows reasonable amplitudes as well as tidal structures for the entire altitude region.

The exceptionally solar and geomagnetic quiet winter of 2019–2020 provides a very useful scenario to study polar mesospheric radar echoes. Such a condition limits the impact of ionisation caused by geomagnetic storms and particle precipitation and allows to investigate the role of moderate electron densities as well as neutral dynamics in formation and transport of the structures causing these echoes. For this purpose we used the continuous operation of the sensitive VHF Middle Atmosphere Alomar Radar System (MAARSY) in conjunction with the Saura partial reflection radar throughout the winter as key instruments to monitor the Mesosphere. Eight months of radar measurements are analyzed in respect to occurrence, echo power, spectral widths and their relation to practically common volume measurements of electron densities. VHF polar mesospheric echoes observed during 2019/2020 were generally more faint and occurred less frequent than in previous year. The very low occurrence rates of 6.5% at most for the months November to February as well as the complete absence of echoes below 56 km throughout the season are very remarkable. Highest occurrences of echoes with spectral widths mostly below 3 ms−1 were seen for electron densities of 3 ⋅ 108 m−3, while on average Ne ~ 5.5⋅ 108 m−3 were observed for the presence of VHF echoes, representing a necessary prerequisite to observe such VHF radar echoes. VHF radar echoes occurring at altitudes below 75 km furthermore seem to show preferences to distinct electron number density windows.

The MF-Saura radar positioned at polar latitudes is very sensitive to enhanced ionization caused by solar and geomagnetic activity. Differing from the intensities normally detected by partial reflections from the ionospheric D region during quiet conditions, distinct layers can be seen due to intense increase of electron number density at as low as 50 km altitude caused by particle precipitation. Effects of energetic particle precipitation on observations by medium frequency (MF) radars have been rarely reported so far, generally associated with solar proton events, which are rather scarce especially during solar minimum conditions. Here, we focus on events associated with precipitation of particles with lower energy (

Emitted high frequency radio waves may be reflected, scattered or refracted by various phenomenon in the atmosphere. The intensity and propagation of the scattered radio wave depends on the properties of the individual scattering phenomenon and of the ambience. Depending on the radar frequency and the ionization intensity radio waves are partially reflected from altitudes of 50 km and above. These reflections are caused by electron density gradients and do not necessarily occur from the nominal pointing direction of the transmission radiation pattern. In several analyses the nominal beam pointing direction is assumed to be the scattering location, which is often not true. For the localization of radar echoes different techniques are known and typically called Angle of Arrival or Direction of Arrival. For this study we investigate three different methods to estimate the Angle of Arrival (AOA) of radar echoes. The first two methods base upon on interferometry, while the third is often referred as MUSIC and shows high potential for the investigated radar. We show results for synthetic, but realistic data as well as real radar data for the antenna array configuration of the radar.

Radars are used since many decades to perform measurements of the atmosphere. Several parameters like echo power, Doppler shift and with this the motion of air parcels or plasma irregularities as well as the angular spread of scatterers are investigated. Besides these examples the spectral width of radar echoes is often referred as a proxy for turbulence. The interpretation of detected echo widths, however, is typically rather challenging. The general assumption is fairly applicable for a perfect antenna radiation pattern represented as a pencil beam. Though, in reality the transmit radiation pattern will have sidelobes that also illuminate atmospheric portions off the nominal pointing direction. As soon as sufficiently strong scatterers exist at these positions, echoes will also be collected and superimposed with the echoes originating from the nominal direction. Additionally, oblique radar echoes might mistakenly be assigned to higher altitudes, from where no echoes are received and thus impair the measurements. These normally unwanted echoes and its spectral portions need to be removed prior to any spectral analysis related to turbulence.

A first sounding rocket campaign dedicated to investigate the creation mechanism of Polar Mesosphere Winter Echoes (PMWE) was conducted in April 2018 from the north Norwegian Andoya Space Center (69 ∘ N, 16 ∘ E). Two instrumented sounding rockets were launched on 13th and 18th of April under PMWE and non-PMWE conditions, respectively. In this paper we give an overview of the PMWE sounding rocket mission. We describe and discuss some results of combined in situ and ground-based measurements which allow to verify existing PMWE theories. Our measurements ultimately show that: a) polar winter mesosphere is abounded with meteor smoke particles (MSP) and intermittent turbulent layers, b) all PMWE observed during this campaign can be explained by neutral air turbulence, c) turbulence creates small-scale structures in all D-region constituents, including free electrons; d) MSP ultimately influence the radar volume reflectivity by distorting the turbulence spectrum of electrons, e) the influence of MSP and of background electron density is just to increase SNR.

Simultaneous measurements of rocket-borne and ground-based instruments in a common volume were performed from Andoya, Norway (69° N, 16° E) in frame of the recent PMWE-project, devoted to clarify the formation mechanism behind polar mesosphere winter echoes (PMWE). This article focuses on measurements of April, 13th 2018. Despite low solar activity, we observe several radar echoes, giving the launch criterion. Combining precise in-situ ionization gauge and wave propagation measurements with ground-based radar measurements on 53.5 and 3.17 MHz, as well as lidar, we were able to measure key parameters of PMWE formation. Carefully analyzing the atmospheric background (i.e., temperature, viscosity, Brunt-Vaisala frequency, and scale heights of electron and neutral density), deriving turbulence parameters by means of radar and rocket, as well as estimating particle sizes of meteor smoke particles (MSP), we got a deep insight into the physical processes behind the PMWE phenomenon. Measurements clearly show that the coherent structures in refractive index variations (forming PMWE) are accompanied by neutral air turbulence, which is reflected in small-scale structures (down to some meters) of neutral and electron density. We analyze and discuss the temporal development of the radar echos by means of spectral width and wind measurements. We show that the behavior of the structures under investigation together with the atmospheric background is consistent with the interpretation, that PMWE were created by turbulence. Furthermore, it becomes clear that charged Meteor Smoke Particles (MSP) and background electron density can only enhance SNR, while turbulence is a prerequisite for their formation.

Since 1999 radar continuous observations of polar mesospheric echoes have been conducted on the Norwegian island of Andoya (69.30°N, 16.04°E), with the ALWIN radar (1999–2008) and MAARSY (since 2011). Traditionally these observations have been named after their seasonal occurrence, i.e., Polar Mesosphere Summer Echoes (PMSE) and Polar Mesosphere Winter Echoes (PMWE). PMSE are much stronger than PMWE and are known to be due to contributions of charged-dust (ice) particles and turbulence. On the other hand, most PMWE are mainly due to turbulence. Both echoes depend on electron density and its gradients. The use of MAARSY, more 17 dB sensitive than the ALWIN radar, makes it possible to observe turbulence-dominated echoes, usually below 80 km, all year round with a mean seasonal occurrence frequency of about 14%. Their minimum occurrence rate is 2% in July and August, while there are two maxima, one in March/April (22%) and the second one in October (26%). On average the dust-dominated summer echoes starts on 14 May and ends on 26 August (i.e. 105 days) with an average occurrence in June/July of 95%. These summer echoes occur mainly between 80 and 90 km, and present a maximum daily occurrence around 13:00 LT. On the other hand, the turbulence-dominated winter echoes occur mainly between 55 and 80 km with maximum daily occurrence around 11:30 LT. Besides the seasonal and daily occurrence of these echoes, we present the variability of occurrence frequency rates for the summer layers since 1994. After eliminating the effects of geomagnetic and/or solar activity the occurrence of the summer dust-dominated echoes show a positive trend of about 0.32%/yr over the last twenty seven years which might be related to the observed negative mesospheric temperature trends at polar latitudes.

The Andenes specular meteor radar shows meteor-trail diffusion rates increasing on average by ~ 20 % at times and locations where a lidar observes noctilucent clouds (NLCs). This high-latitude effect has been attributed to the presence of charged NLC but this study shows that such behaviors result predominantly from thermal tides. To make this claim, the current study evaluates data from three stations, at high-, mid-, and low-latitudes, for the years 2012 to 2016, comparing diffusion to show that thermal tides correlate strongly with the presence of NLCs. This data also shows that the connection between meteor-trail diffusion and thermal tide occurs at all altitudes in the mesosphere, while the NLC influence exists only at high-latitudes and at around peak of NLC layer. This paper discusses a number of possible explanations for changes in the regions with NLCs and leans towards the hypothesis that relative abundance of background electron density plays the leading role. A more accurate model of the meteor trail diffusion around NLC particles would help researchers determine mesospheric temperature and neutral density profiles from meteor radars.

The Middle Atmosphere Alomar Radar System (MAARSY) is a monostatic radar with an active phased array antenna designed for studies of phenomena in the mesosphere and lower thermosphere. Its design in particular the flexible beam forming and steering capability makes it to a powerful instrument to perform observations with high angular and temporal resolution. The knowledge of the actual radiation pattern is crucial to configure and analyze experiments carried out with the radar. The simulated radiation pattern is evaluated by the observation of cosmic radio emissions which are compared to a Global Sky temperature Maps model consisting of the most recent, thorough and accurate radio astronomy surveys. Additionally to these passive receive-only experiments active two-way experiments are presented, which corroborate the findings of the passive experiments.

The Leibniz-Institute of Atmospheric Physics in Kühlungsborn, Germany (IAP) installed a new powerful VHF radar on the North-Norwegian island Andøya (69.30° N, 16.04° E) from 2009 to 2011. The new Middle Atmosphere Alomar Radar System (MAARSY) replaces the existing ALWIN radar which has been in continuous operation on Andøya for more than 10 yr. MAARSY is a monostatic radar operated at 53.5 MHz with an active phased array antenna consisting of 433 Yagi antennas each connected to its own transceiver with independent control of frequency, phase and power of the transmitted signal. This arrangement provides a very high flexibility of beam forming and beam steering. It allows classical beam swinging operation as well as experiments with simultaneous multiple beams and the use of modern interferometric applications for improved studies of the Arctic atmosphere from the troposphere up to the lower thermosphere with high spatial-temporal resolution. The installation of the antenna was completed in August 2009. An initial expansion stage of 196 transceiver modules was installed in spring 2010, upgraded to 343 transceiver modules in December 2010 and the installation of the radar was completed in spring 2011. Beside standard observations of tropospheric winds and Polar Mesosphere Summer Echoes, multi-beam experiments using up to 91 beams quasi-simultaneously in the mesosphere have been carried out using the different expansion stages of the system during campaigns in 2010 and 2011. These results provided a first insight into the horizontal variability of Polar Mesosphere Summer and Winter Echoes in an area of about 80 km by 80 km with time resolutions between 3 and 9 min.

In 2009/2010 the Leibniz-Institute of Atmospheric Physics (IAP) installed a new powerful VHF radar on the island Andøya in Northern Norway (69.30° N, 16.04° E). The Middle Atmosphere Alomar Radar System (MAARSY) allows studies with high spatial and temporal resolution in the troposphere/lower stratosphere and in the mesosphere/lower thermosphere of the Arctic atmosphere. The monostatic radar is operated at 53.5 MHz with an active phased array antenna consisting of 433 Yagi antennas. Each individual antenna is connected to its own transceiver with independent phase control and a scalable power output of up to 2 kW, which implies high flexibility of beam forming and beam steering. During the design phase of MAARSY several model studies have been carried out in order to estimate the radiation pattern for various combinations of beam forming and steering. However, parameters like mutual coupling, active impedance and ground parameters have an impact on the radiation pattern, but can hardly be measured. Hence, experiments need to be designed to verify the model results. For this purpose, the radar has occasionally been used in passive mode, monitoring the noise power received from both distinct cosmic noise sources like e.g. Cassiopeia A and Cygnus A, and the diffuse cosmic background noise. The analysis of the collected dataset enables us to verify beam forming and steering attempts. These results document the current status of the radar during its development and provide valuable information for further improvement.

The Leibniz-Institute of Atmospheric Physics (IAP) in Kühlungsborn started to install a new MST radar on the North-Norwegian island Andøya (69.30° N, 16.04° E) in 2009. The new Middle Atmosphere Alomar Radar System (MAARSY) replaces the previous ALWIN radar which has been successfully operated for more than 10 years. The MAARSY radar provides increased temporal and spatial resolution combined with a flexible sequential point-to-point steering of the radar beam. To increase the spatiotemporal resolution of the observations a 16-port Butler matrix has been built and implemented to the radar. In conjunction with 64 Yagi antennas of the former ALWIN antenna array the Butler matrix simultaneously provides 16 individual beams. The beam forming capability of the Butler matrix arrangement has been verified observing the galactic cosmic radio noise of the supernova remnant Cassiopeia A. Furthermore, this multi beam configuration has been used in passive experiments to estimate the cosmic noise absorption at 53.5 MHz during events of enhanced solar and geomagnetic activity as indicators for enhanced ionization at altitudes below 90 km. These observations are well correlated with simultaneous observations of corresponding beams of the co-located imaging riometer AIRIS (69.14° N, 16.02° E) at 38.2 MHz. In addition, enhanced cosmic noise absorption goes along with enhanced electron densities at altitudes below about 90 km as observed with the co-located Saura MF radar using differential absorption and differential phase measurements.

The Leibniz-Institute of Atmospheric Physics in Kühlungsborn, Germany (IAP) is installing a new powerful VHF radar on the North-Norwegian island Andøya (69.30° N, 16.04° E) in 2009/2010. The new Middle Atmosphere Alomar Radar System (MAARSY) replaces the existing ALWIN radar which has been operated continuously on Andøya for more than 10 years. The new system is a monostatic radar operated at 53.5 MHz with an active phased array antenna consisting of 433 Yagi antennas. The 3-element Yagi antennas are arranged in an equilateral triangle grid forming a circular aperture of approximately 6300 m2. Each individual antenna is connected to its own transceiver with independent phase control and a scalable output up to 2 kW. This arrangement allows very high flexibility of beam forming and beam steering with a symmetric radar beam of a minimum half power beam width of 3.6°, a maximum directive gain of 33.5 dB and a total transmitted peak power of approximately 800 kW. The IF signals of each 7 transceivers connected to each 7 antennas arranged in a hexagon are combined to 61 receiving channels. Selected channels or combinations of IF signals are sent to a 16-channel data acquisition system with 25 m sampling resolution and 16-bit digitization specified which will be upgraded to 64 channels in the final stage. The high flexibility of the new system allows classical Doppler beam swinging as well as experiments with simultaneously formed multiple beams and the use of modern interferometric applications for improved studies of the Arctic atmosphere from the troposphere up to the lower thermosphere with high spatiotemporal resolution.

The Middle Atmosphere Alomar Radar System (MAARSY) is a radar with a large aperture active phased array antenna designed for studies of atmospheric phenomena in the mesosphere and lower thermosphere. Its design in particular the flexible beam forming and steering capability makes it to a powerful instrument to perform observations with high angular and temporal resolution. The knowledge of the actual radiation pattern and radar parameters is crucial to configure and analyze experiments carried out with the radar. For this purpose the simulated radiation pattern is evaluated by the observation of cosmic radio emissions which are compared to a global sky model. Additionally to these passive receive-only experiments active two-way experiments are presented, which corroborate the findings of the passive experiments.

[1] The Middle Atmosphere Alomar Radar System (MAARSY) on the North-Norwegian island Andoya is a 53.5 MHz monostatic radar with an active phased array antenna consisting of 433 Yagi antennas. The 3-element Yagi antennas are arranged in an equilateral triangle grid forming a circular aperture of approximately 6300 m2. Each individual antenna is connected to its own transceiver with independent phase control and a scalable power output up to 2 kW. This arrangement provides a very high flexibility of beam forming and beam steering with a symmetric radar beam of a minimum beam width of 3.6° allowing classical beam swinging operation as well as experiments with simultaneous multiple beams and the use of interferometric applications for improved studies of the Arctic atmosphere from the troposphere up to the lower thermosphere with high spatio-temporal resolution. The installation of the antenna array was completed in August 2009. The radar control and data acquisition hardware as well as an initial expansion stage of 196 transceiver modules was installed in spring 2010 and upgraded to 343 transceiver modules in November 2010. The final extension to 433 transceiver modules has recently been completed in May 2011. Beside standard observations of tropospheric winds and Polar Mesosphere Summer Echoes, the first multi-beam experiments using up to 97 quasi-simultaneous beams in the mesosphere have been carried out in 2010 and 2011. These results provide a first insight into the horizontal variability of polar mesosphere summer and winter echoes with time resolutions between 3 and 9 minutes. In addition, first meteor head echo observations were conducted during the Geminid meteor shower in December 2010.