Markus Rapp

MAARSY: The new MST radar on Andøya—System description and first results

[1]xa0The 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.

Trends of mesospheric gravity waves at northern middle latitudes during summer

[1]xa0Recent investigations of the seasonal variation of the activity of gravity waves in the mesosphere/lower thermosphere (MLT) at middle and high latitudes suggest a semiannual variation with maxima during winter and summer and minima during the equinoxes. It is generally assumed that this annual cycle is determined by filtering processes due to the background winds in the stratosphere and lower mesosphere. On the other side, long-term observations of mesospheric winds at Juliusruh (55°N, 13°E) since 1990 indicate a stable increase of westward directed winds below 80 km (negative trends) during summer, as, e.g., clearly evident in monthly means in July. Here, we are studying how these long-term changes of winds are related to trends of the activity of gravity waves (GW) with periods between 3–6 hours. Our results show that the observed zonal wind trend at about 75 km during July goes along with an enhancement of the GW activity at altitudes above 80 km. Indeed, also the year-to-year variation of maxima of the observed westward directed winds at altitudes near 75 km and the GW activity at about 80 km are significantly correlated. Our results stimulate the further study of long-term wind changes and corresponding gravity wave trends.

Rocket-borne in situ measurements of meteor smoke: Charging properties and implications for seasonal variation

Rocket-borne observations of meteoric smoke particles (MSPs) are presented from three campaigns at polar latitudes (69 degrees N) in September 2006, and in the summers of 2007 and 2008. MSPs are detected using a novel technique based on photoelectron emission from the particles after stimulation by UV photons emitted by a xenon flashlamp. Resulting photoelectron currents are shown to be proportional to particle volume density. September results match model predictions qualitatively at altitudes from 65 to 85 km while measurements at higher altitudes are contaminated by photoelectrons from NO and O-2((1)Delta(g)). Contamination below this altitude can be excluded based on concurrent satellite observations. The observations show a large variability from flight to flight. Part of this variability can be attributed to differences in the charging of MSPs during day and night. Finally we find that MSP volume density in summer can exceed that during September. Analyzing model simulations of the global transport and microphysics of these particles, we show that our observations are in agreement with the model predictions, even though number densities of particles with radii >1 nm, which have long been thought to be suitable condensation nuclei for mesospheric ice particles, show the opposite behavior. It is shown that this discrepancy is caused by the fact that even larger particles (similar to 3 nm) dominate the volume density and that transport affects these different particle sizes in different ways. These results reinforce previous model findings according to which seasonal MSP variability is mainly driven by the global circulation and corresponding transport.

Microphysical parameters of mesospheric ice clouds derived from calibrated observations of polar mesosphere summer echoes at Bragg wavelengths of 2.8 m and 30 cm

[1]xa0The currently most widely accepted theory of polar mesosphere summer echoes (PMSE) assumes that the echoes originate from turbulence-induced scatter in combination with a large Schmidt number caused by the presence of charged ice particles. We test this theory with calibrated observations with the European Incoherent Scatter (EISCAT) Svalbard Radar (ESR) at 500 MHz (Bragg wavelength 30 cm) and the Sounding System (SOUSY) Svalbard Radar (SSR) at 53.5 MHz (Bragg wavelength 2.8 m), which are collocated near Longyearbyen on Svalbard (78°N, 16°E). Our observations in June 2006 yield volume reflectivities ranging from values of 2.5 × 10−19 m−1 to 1 × 10−17 m−1 for the case of the ESR echoes and from 5 × 10−16 m−1 to 6.3 × 10−12 m−1 for the SSR echoes. In the frame of the above-mentioned theory the expected reflectivity ratio should be equal to or larger than the ratio of the frequencies to the third power (i.e., larger than (500 MHz/53.5 MHz)3 = 816). Our experimental results show that 94% of the observations satisfy this expectation. The remaining 6%, which show too small ratios, can be tentatively attributed to calibration uncertainties and an incomplete filling of the scattering volume of the SSR, which is significantly larger than that of the ESR. Hence our observations are largely consistent with the predictions of the above-mentioned theory even though we note that it cannot prove it, which would require additional observations at different frequencies. However, this consistency is used as sufficient motivation to apply the assumed theory to the observations in order to derive Schmidt numbers and radii of the charged aerosol particles. Corresponding results are in excellent agreement with expectations from microphysical models and independent satellite and lidar observations, thereby corroborating our initial assumptions.

Rocket probing of PMSE and NLC — Results from the recent MIDAS/MaCWAVE campaign

Abstract From 29 June to 6 July 2002, the European/American MIDAS/MaCWAVE campaign took place at the Andoya Rocket Range (69°N, 16°E) in Norway. Three MMAS payloads were launched in two different salvoes to study the thermal and dynamical environment of Polar Mesosphere Summer Echoes (PMSE) and noctilucent clouds (NLC). All the payloads were equipped with instruments to measure the number density of positive ions, electrons, charged aerosol particles, and neutrals. An electric field experiment was also included aboard one of the MIDAS payloads. An overview of the preliminary results from the campaign will be presented with emphasis on the first salvo where both NLC and PMSE were present. In particular, the small-scale structures measured during passage through these layers will be compared to previous measurements performed in NLC and PMSE conditions.

First three‐dimensional observations of polar mesosphere winter echoes: Resolving space‐time ambiguity

[1]xa0We present the first three-dimensionally resolved observations of polar mesosphere winter echoes obtained with a 25 beam-experiment covering a volume of about 50 km in diameter (horizontal distance) at altitudes between 65 and 85 km. This allows us to resolve the classical space time ambiguity of single beam observations and reveals that the echoing structure was tilted in the East–West direction but showed no considerable tilt in the North–South direction. The Doppler shifts derived from the 24 off-zenith beam directions are consistent with the mean background wind measured independently by a co-located MF-radar. The time development of the 3-D echo-pattern is consistent with scattering structures that follow the constant phase lines of a medium frequency gravity wave that is propagating against the mean flow. Wave parameters derived from these observations are independently confirmed by the analysis of co-located wind measurements with the aforementioned MF-radar. Overall, the observed echo morphology in time and space is reminiscent of gravity wave breaking which is known to lead to a maximum of turbulence activity that moves with the phase of the wave.

Microphysical Properties of Mesospheric Aerosols: An Overview of In Situ-Results from the ECOMA Project

Six sounding rockets were launched within the ECOMA (=“Existence and Charge state Of Meteoric smoke particles in the middle Atmosphere”) project to study the characteristics of meteoric smoke particles (MSPs) and mesospheric ice particles, as well as their possible microphysical relation. The launches were conducted during three campaigns from the Andoya Rocket Range (69°N, 16°E), one in September 2006, and the other two in the summers of 2007 and 2008. This chapter provides an overview of these observations and presents the corresponding geophysical results with special emphasis on our understanding of the micropyhsics of mesospheric ice particles. Most notably, we are able to confirm the existence of MSPs at all altitudes between 60 and 85 km in September, and a seasonal variation that is consistent with previous model studies in which MSP-variability is mainly driven by the global circulation. Together with these model studies as well as recent satellite observations of MSPs our results hence cast some doubt on a standard assumption of state-of-the-art microphysical models of mesospheric ice clouds, namely that ice nucleation mainly occurs heterogeneously on MSPs.

Localized mesosphere‐stratosphere‐troposphere radar echoes from the E region at 69°N: Properties and physical mechanisms

[1]xa0We present the first observations, to our knowledge, of a new class of high-latitude mesosphere-stratosphere-troposphere radar echoes from the E region as observed with the Arctic Lidar Observatory for Middle Atmosphere Research wind radar during the period 2004–2008. These echoes occur primarily during the summer months and in the altitude range from 93 to 114 km, with a pronounced peak of maximum occurrence at about 100 km. The echoes are rather short with typical durations of ∼20 min, with some examples lasting as long as 3 h. The echoes typically cover only a few hundred meters in the vertical and show both small Doppler velocities (±1–2 m/s) as well as very narrow spectral widths (just a few meters per second when converted to Doppler velocities). The echoes are highly aspect sensitive indicative of a specular-scattering mechanism and reveal a distinct diurnal variation with maxima of occurrence around noon and midnight. The latter is related to the semidiurnal tidal components of the zonal and meridional wind where times of occurrence correspond to large values of corresponding vertical wind shears. Considering possible physical mechanisms, turbulence with large Schmidt number scatter is likely ruled out as is auroral backscatter. Finally, a strong case for a close correspondence of the echoes to sporadic E layers is presented on the basis of comparisons to ionosonde data, occurrence patterns of sporadic layers, simultaneous and common volume lidar measurements of a sporadic Fe layer, as well as simultaneous measurements of sporadic E layers with the European Incoherent Scatter UHF radar at a horizontal distance of 130 km. Applying the theory of partial reflections to the observed electron density gradients, we are able to demonstrate that the observed echo strengths can likely be explained on the basis of this scattering mechanism.

Charged Aerosol Effects on the Scattering of Radar Waves from the D-Region

Charged aerosol particles are an important contributor to the D-region charge balance and affect the scattering of radar waves. Among these particles are meteoric smoke particles (MSP) which occur at all D-region altitudes and all seasons, and mesospheric ice particles whose occurrence is confined to altitudes of ∼80–90 km at polar latitudes during summer. We argue that it is the modification of electron diffusion by the heavy charged aerosol particles which is the prime effect leading to clearly detectable signatures in both incoherent and coherent radar backscatter. In the case of incoherent scatter, it is shown that the presence of charged aerosol particles modifies the incoherent scatter spectrum. Corresponding observations with the EISCAT UHF radar and the Arecibo radar have been used to detect both MSP and ice particles at D-region altitudes and characterize their radii and number densities. In the case of coherent scatter, it is argued that the modified diffusion properties of the D-region electrons lead to small scale structures at the radar Bragg wavelength due to turbulent mixing in combination with a large Schmidt number. To test this theory, calibrated echo strengths of polar mesosphere summer echoes have been measured with the EISCAT radars at Tromso (69°N) and Svalbard (78°N) and collocated 53 MHz radars, thus covering frequencies of 53 MHz, 224 MHz, 500 MHz, and 933 MHz. Importantly, the vast majority of these observations show excellent agreement with the corresponding theoretical predictions thus providing strong support for this theory. This theory was subsequently applied to the same data sets in order to derive ice particle radii. Corresponding results are in excellent agreement with independent data sets from satellite-borne and ground-based optical observations. Finally, some suggestions for future investigations are given.

Reply to comment by P. M. Bellan on “Comment on ‘Ice iron/sodium film as cause for high noctilucent cloud radar reflectivity’”

[1] There recently has been a debate about whether polar mesosphere summer echoes (PMSE) are caused by radar scattering from electrons in a metal layer on mesospheric ice particle surfaces as proposed by Bellan (2008) or from electrons in the gas phase as has been the conventional assumption for decades. In a comment to Bellan (2008) we showed strong experimental evidence against the metal film hypothesis and concluded that it should be dismissed. In his reply to our comment, Bellan (2010) does not invalidate our arguments but instead criticizes the classical PMSE theory which is strongly supported by experiments and is by now accepted by most scientists. In the current reply, we address this criticism point by point and conclude that the arguments presented by Bellan (2010) neither invalidate our criticism of the metal film hypothesis nor do they question the classical PMSE theory.