The Middle Atmosphere Alomar Radar System (MAARSY) on the North‐Norwegian island Andøya 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.

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 m 2 . 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 800kW. 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.Correspondence to: R. Latteck (latteck@iap-kborn.de) Fig. 1. Sketch of the antenna array of MAARSY. Each cross represents crossed three-element Yagi antennas mounted on a concrete block (small boxes). The six rectangles outside the array, indicated as A-F, represent containers accommodating the transmit-receive modules.

Very low frequency (VLF: 3-30 kHz) radio waves propagate inside the Earth-ionosphere waveguide monitoring the electrical conductivity of its boundaries. The upper boundary properties of the waveguide can be represented by Wait parameters (Wait & Spies, 1964), namely, the reference height and conductivity gradient of the D-region. The quiescent ionospheric condition can be disturbed by different types of physical phenomena, originating in space (Clilverd et al., 2010;Macotela et al., 2017) or on Earth (Macotela, Clilverd, Manninen, Thomson, et al., 2019). These disturbances, interpreted as perturbations of the D-region ionization levels, produce changes in the Wait parameters, which show up as phase and/or amplitude variations in the VLF signals.It is well known that the long-term variation of the daytime lower ionosphere exhibits distinct seasonal characteristics with high variability in winter, and lower variability in summer (