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.
[ 1 ] We investigate the dependence of polar mesosphere summer echoes (PMSE) and mesosphere summer echoes (MSE) on the background electron number density. Both a lower and upper limit are quantified below and above which PMSE cannot exist. The result is that PMS E occur for a very wide range of electron number densities between $300 -500/ cm 3 and $10 5 /cm 3 . A comparison of the diurnal variation of MSE observed at Kü hlungsborn (54 °N) with current model estimates of the electron number density at midlatitudes shows that at least $300 -500 electrons /cm 3 are necessary for PMSE to exist. This lower limit is consistent with all available electron number density measurements obtained from sounding rockets in the vicinity of PMSE. It is shown that the existence of a lower electron number density limit can be understood in terms of the standard theory of the scattering of VHF waves in the D region. We have then analyzed PMSE observations during the major solar proton event on 14 July 2000. We have estimated the electron number density at PMSE altitudes based on proton and electron flux measurements obtained with detectors on board the GOES-8 and ACE spacecrafts in combination with an ion-chemical model. Comparing the electron number densities at 87 km altitude with the average PMSE signal to noise observations (SNR) we find a significant negative correlation between SNR and the electron number densities for densities on the order of $10 5 /cm 3 . We propose that this negative correlation is due to a limited amount of aerosol particles: current PMSE theories assume that electron irregularities in the VHF band can only exist if more than $ 50% of the free electrons are bound to aerosol particles which thus reduce the electron diffusivity due to ambipolar forces. If, however, the electron number density increases significantly above the aerosol number density, this condition can no longer be fulfilled. On the other hand, the fact that PMSE is present up to electron number densities of $ 10 5 /cm 3 raises several important questions on our current understanding of aerosol particles around the polar summer mesopause and their role in the creation of PMSE: either many more aerosol particles exist than has been anticipated so far (with corresponding implications for the nucleation of these particles) or our current understanding of the role of aerosol particles in the creation of PMSE is not complete. INDEX T ERMS: 0305 Atmospheric
Abstract. We report the results from simultaneous radar and rocket measurements of a PMSE event where for the first time the rocket measured dust and plasma within the radar beam. We find very clear correspondence between the measured dust charge density profile and the radar backscatter profile as a function of height. We find that even very small amounts of charged dust is associated with an appreciable PMSE radar backscatter. Although we find it likely that the dust layer corresponds fully with the PMSE layer there is a possibility that the upper part of the PMSE layer may be influenced by ion clusters which are too small to be detected by the rocket dust probe.
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