[Show abstract][Hide abstract] ABSTRACT: Non-specular meteor echoes have been associated with field-aligned irregularities and have been observed at low and mid-latitudes sites. We present observations obtained at high latitudes with range-time features that resemble those at lower latitudes. However, these echoes cannot come from field aligned irregularities, since the radar pointing angles are almost parallel to the magnetic field. Using interferometry, we have been able to discriminate space and time features. Our echoes could be qualitatively explained by the presence of charged dust forming from the meteoric material immersed in a turbulent flow. This can lead to a high Schmidt number plasma that can sustain meter-scale turbulence just as it does for the polar mesospheric summer echoes. These rare events require relatively large meteoroids. The result emphasizes the importance of charged dust in understanding all long-duration non-specular meteor echoes. This dust will extend their diffusion times and will effect temperature estimations from specular echoes.
[Show abstract][Hide abstract] ABSTRACT: From 19 November to 19 December 2010 the fourth and final ECOMA rocket
campaign was conducted at Andøya Rocket Range (69° N, 16°
E) in northern Norway. We present and discuss measurement results
obtained during the last rocket launch labelled ECOMA09 when
simultaneous and true common volume in situ measurements of temperature
and turbulence supported by ground-based lidar observations reveal two
Mesospheric Inversion Layers (MIL) at heights between 71 and 73 km and
between 86 and 89 km. Strong turbulence was measured in the region of
the upper inversion layer, with the turbulent energy dissipation rates
maximising at 2 W kg-1. This upper MIL was observed by the
ALOMAR Weber Na lidar over the period of several hours. The spatial
extension of this MIL as observed by the MLS instrument onboard AURA
satellite was found to be more than two thousand kilometres. Our
analysis suggests that both observed MILs could possibly have been
produced by neutral air turbulence.
[Show abstract][Hide abstract] ABSTRACT: In the present paper ~ 32.5 h of EISCAT VHF PMWE observations were
analyzed with focus on spectral properties like spectral width, doppler
shift and spectral shape. Examples from two days of observations with
weak and strong polar mesosphere winter echo (PMWE) signals are
presented and discussed in detail. These examples reveal a large
variability from one case to the other. That is, some features like an
observed change of vertical wind direction and spectral broadening can
be very prominent in one case, but unnoticeable in the other case.
However, for all observations a change of spectral shape inside the
layer relative to the incoherent background is noticed.
[Show abstract][Hide abstract] ABSTRACT: This brief note describes the first measurement of the microphysical properties and variability of meteoric smoke particles (MSPs) at high latitude using the Poker Flat ISR (65.1°N, 147.5°W). We present a novel technique for determining height resolved daytime D region neutral temperatures, which takes into account the presence of charged dust. We discuss the temporal/spatial variability and the relation to meteoric input observed and MSP microphysical properties in the polar mesopause region. The derived nanometer sized MSPs are consistent with size profiles derived previously using radar/rocket techniques and we note that our results imply a lack of heavy cluster ions below 85 km during the observing period. This provides a template for potential use at many other radar sites for the determination of microphysical properties of MSPs and day-time neutral temperature in the D region that show good general agreement with model and satellite temperature data during the observing period.
Geophysical Research Letters 11/2012; · 3.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this work we present the first results of meteor smoke particles
(MSPs) detected in the D-region plasma above the 449 MHz Poker Flat
Incoherent Scatter Radar (PFISR) in Alaska (67°N, 149°W). MSPs
are believed to be the major source of condensation nuclei for the
formation of ice particles, the precursor for Polar Mesospheric Clouds
(PMCs) and Polar Mesospheric Summer Echoes (PMSE). In addition, they are
thought to contribute to D-region chemistry by providing a surface on
which heterogeneous chemistry occurs (Summers and Siskand, 1999). Our
results are obtained by utilizing a similar fitting method derived for
use at other High Power Large Aperture Radar (HPLA) sites that treats
the measured radar signal as the sum of two Lorentzian functions
[Strelnikova et al., 2007]. This method allows us to determine particle
size distributions and smoke densities (when calibrated electron density
data is available) in the range of approximately 70 to 90 km altitude
depending on background atmospheric composition. We present results from
a period of strong D-Region ionization when the detected signal-to-noise
(SNR) from the D-region is strongest (12 - 19 UT). Our results provide
insight into the presence and distribution of charged meteoric dust in
the polar mesopause region resulting from the condensation of ablated
material of meteoric origin. Furthermore, we compare our results to
other HPLA radar sites at high latitude (EISCAT) as well as low latitude
(Arecibo) to verify our results and investigate any latitudinal
variation that may exist.
[Show abstract][Hide abstract] ABSTRACT: Motivated by a recent controversy in the literature about the agreement or disagreement of spectral widths (after conversion to Doppler-velocities) of PMSE observed simultaneously with the EISCAT VHF and UHF radars in Tromsø, Norway, this issue was reconsidered taking into account a much larger data set than in any of the previous studies. While these previous studies only considered case studies of a few minutes in length, we considered a total of 380min of simultaneous and common volume observations with these two radars at a time resolution of 30s, i.e., a total of 760 samples. This study is important to discern whether PMSE at these two frequencies originate from the same coherent scattering mechanism or if different physical mechanisms are at work. We considered both shape and width of the considered spectra and found that – within the available spectral resolution – the VHF spectra are on average well described by a Gauss shape (with spectral parameter n=2—which is derived from a generalized formulation of the autocorrelation function), whereas the UHF-spectra show a small deviation from this shape with an average n of 1.6. Spectral widths do largely agree but show a small systematic difference, i.e., the UHF spectra are on average 0.1m/s narrower than the VHF spectra at an average spectral width of 3.5m/s. This small systematic effect is largely explained considering the overall effect of beam-, shear-, and wave-broadening. This means that the slightly different beam widths of the VHF and UHF-radars and the hence slightly different observing volumes account for the small systematic difference in mean spectral width. Finally, we demonstrated that the small deviation of UHF-spectra from a perfect Gauss shape can be either due to relatively small signal-to-noise ratios of the UHF-data and/or a superposition of incoherent and coherent scatter in cases where both contributions add equally to the total observed power. Since VHF PMSE are much more stronger than the incoherent scatter background this effect cannot be recognized in VHF observations. This superposition effect may occasionally also lead to larger deviations between VHF and UHF spectra in that it can potentially narrow the UHF-spectra significantly. However, excluding these rather rare and exotic cases, we conclude that the majority of our observations are compatible with a single coherent scattering at both VHF and UHF.
Journal of Atmospheric and Solar-Terrestrial Physics 01/2011; 73(14):2142-2152. · 1.42 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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 Andøya 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 85km 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.
[Show abstract][Hide abstract] ABSTRACT: A total of six sounding rockets were launched during three field campaigns in the years 2006, 2007, and 2008 from the North-Norwegian Andøya Rocket Range to study the Existence and Charge state Of Meteoric smoke in the middle Atmosphere (ECOMA) and its relation to mesospheric ice particles. A new particle detector was successfully developed which combines the conventional technique of a Faraday-Cup with the active photo ionization of particles and subsequent detection of corresponding photo electrons. In this paper we will give an overview of results from these rocket campaigns. Some noteworthy findings are the experimental verification of meteor smoke existence throughout the entire mesosphere, the first direct in situ measurement of mesospheric ice volume, and new insights into the charging properties of meteoric smoke under the conditions of polar summer. Finally, we will outline future plans for a concluding ECOMA campaign that is scheduled for December 2010 to study the effect of the Geminid meteor shower on the properties of meteor smoke particles in the middle atmosphere.
[Show abstract][Hide abstract] ABSTRACT: Rocket-borne observations of meteoric smoke particles (MSPs) are presented from three campaigns at polar latitudes (69°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 O2(1Deltag). 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 (˜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.
Journal of Geophysical Research Atmospheres 01/2010; 115. · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The nature of PMSE in the VHF and UHF frequency range is considered taking into account the shape of corresponding Doppler spectra. Assuming a turbulence-based model of PMSE it is argued that for cases where a VHF radar detects strong PMSE, the UHF radar could either detect enhanced coherent scattering caused by the same physical process as in the VHF (i.e., turbulence with large charged ice particles), there could be incoherent scattering modified by the charged ice particles, or there could be a mixture of both. In order to distinguish these cases a simple but robust method is introduced to characterize the shape of the Doppler spectra derived from observations at both frequencies. Spectral shapes are quantified with one simple fitting parameter of a generalized fit to the autocorrelation function (=Fourier transform of the Doppler spectrum). This parameter takes a value of 1 for a Lorentzian spectrum indicative of pure incoherent scatter from the D-region, a value of 2 for coherent scatter owing to turbulence, and a value of less than 1 for incoherent scatter modified by the presence of charged aerosol particles. This method is applicable to observations at altitudes between ∼70 and ∼90 km. Simultaneous observations with the EISCAT VHF and UHF radar are presented in which all three cases mentioned above are identified. For the case of incoherent scatter modified by the presence of charged aerosol particles we quantify the radius of the involved ice particles to exceed ∼5 nm. Most importantly, however, for the case where the UHF-signal exceeded the incoherent scatter signal significantly, the spectrum revealed a clear Gaussian shape indicative of a coherent scattering process with identical spectral width as for the VHF-observations. This finding gives strong support that both echoes are created by the same turbulence-based mechanism and not by different mechanisms as speculated by several previous authors.
Advances in Space Research 01/2010; · 1.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Polar mesosphere winter echoes (PMWE) are strong coherent radar echoes observed with VHF-Radars and primarily occur during the winter months at altitudes between 60-80 km. In recent years, the morphology of these echoes has been well documented, however, the physical mechanism leading to these radar echoes is yet to be established. In the current study we consider PMWE-observations with the EISCAT VHF-radar (224 MHz, Bragg wavelength = 0.67 m) in Tromsø, Northern Norway. Doppler spectra obtained inside and outside PMWE-regions are analyzed in terms of spectral shape, spectral width, and Doppler shift. For the analysis of spectral shape we employ a simple generalized fit to the autocorrelation function (ACF; note that the ACF is the Fourier transform of the Doppler power spectrum) which yields a spectral parameter that attains a value of 1 in case the spectra reveal a Lorentzian shape and a value of 2 if the spectra are Gaussian. Spectral widths and Doppler shifts are analyzed with respect to gravity wave signatures where we will also scrutinize to which extent the possible effect of high frequency gravity waves affects the spectral width and the spectral shape. Finally, these results will be discussed in the framework of current PMWE-theories.
[Show abstract][Hide abstract] ABSTRACT: There is today substantial interest in aerosols in the mesosphere and their interaction with their neutral and charged environment. These aerosols comprise both ice particles in the polar summer mesopause region and smoke particles of meteoric origin that are expected to occur in the entire middle atmosphere and during all seasons. The presence of ice particles in the mesosphere has been known for many decades and is most prominently revealed in the form of noctilucent clouds, also known as polar mesospheric clouds. Smoke particles, on the other hand, have sizes of few nanometers only such that their detection by remote sensing techniques has long been deemed impossible. In consequence, sporadic rocket borne in-situ measurements have long been the only source of experimental evidence regarding the existence and properties of these particles. However, it has recently been realized that charged mesospheric aerosol particles modify the plasma properties of the D-region and thereby influence the characteristics of radar backscatter from these altitudes (i.e., radar reflectivity and/or spectral properties). Hence, it is possible to infer properties of these charged aerosol particles in the D-Region using radar observations. In this paper we present two independent methods yielding particles properties based on such measurements and give an overview of recent results.
[Show abstract][Hide abstract] ABSTRACT: More than 10 years ago, simultaneous and common volume radar and lidar observations of polar mesosphere summer echoes (PMSE) and noctilucent clouds (NLC) provided the first compelling evidence of their joint origin. Today, it is well established that both phenomena are different observational evidence of mesospheric ice clouds. However, while the light scattering mechanism leading to NLC is essentially understood, a similarly robust quantitative understanding of PMSE has long been elusive. This paper starts with reporting the results of correlative studies of PMSE and NLC based on a very large data set (several ten thousands of simultaneous and common volume observations) collected over the years with the ALOMAR wind radar and the ALOMAR RMR lidar in Northern Norway complemented by results obtained with the Solar Occultation for Ice Experiment (SOFIE) on the AIM satellite. We then continue describing recent progress made in the quantitative understanding of PMSE based on observations of these echoes using well separated frequencies in the VHF and UHF. Today, such observations even allow the derivation of microphysical parameters like ice particle radii which are in close agreement with independent estimates from SOFIE. We will further discuss cases where NLC were observed with and without the presence of PMSE and demonstrate that all these cases can now be understood in a quantitative manner. Finally, this understanding will be applied to observed long term variations of PMSE which are recognized of being substantially different from similar NLC observations.
[Show abstract][Hide abstract] ABSTRACT: We present a new design of an in situ detector for the study of meteor smoke particles (MSPs) in the middle atmosphere. This detector combines a classical Faraday cup with a xenon-flashlamp for the active photoionization/photodetachment of MSPs and the subsequent detection of corresponding photoelectrons. This instrument was successfully launched in September 2006 from the Andøya Rocket Range in Northern Norway. A comparison of photocurrents measured during this rocket flight and measurements performed in the laboratory proves that observed signatures are truly due to photoelectrons. In addition, the observed altitude cut-off at 60 km (i.e., no signals were observed below this altitude) is fully understood in terms of the mean free path of the photoelectrons in the ambient atmosphere. This interpretation is also proven by a corresponding laboratory experiment. Consideration of all conceivable species which can be ionized by the photons of the xenon-flashlamp demonstrates that only MSPs can quantitatively explain the measured currents below an altitude of 90 km. Above this altitude, measured photocurrents are most likely due to photoionization of nitric oxide. In conclusion, our results demonstrate that the active photoionization and subsequent detection of photoelectrons provides a promising new tool for the study of MSPs in the middle atmosphere. Importantly, this new technique does not rely on the a priori charge of the particles, neither is the accessible particle size range severely limited by aerodynamical effects. Based on the analysis described in this study, the geophysical interpretation of our measurements is presented in the companion paper by Strelnikova, I., et al. [2008. Measurements of meteor smoke particles during the ECOMA-2006 campaign: 2. results. Journal of Atmospheric and Solar-Terrestrial Physics, this issue, doi:10.1016/j.jastp.2008.07.011].
Journal of Atmospheric and Solar-Terrestrial Physics 03/2009; · 1.42 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a seasonal study of the presence and characteristics of meteoric smoke particles (MSPs) in the D-region plasma derived from observations using the Gregorian and line feeds of the 430 MHz dual-beam Arecibo Observatory (AO) incoherent scatter radar (ISR) in Puerto Rico (18∘N,67∘W). MSPs are the product of re-condensation of ablated meteoric material and are believed to be the condensation nuclei for the formation of ice particles in the polar mesopause region. These CNs can then be responsible for the formation of polar mesospheric clouds (PMCs), noctilucent clouds (NLCs) and polar mesospheric summer echoes (PMSEs). For this work, we simultaneously employed both AO antenna feeds to define two radar beams inclined 15∘ east and west of zenith (Janches et al., 2006). Because of the non-vertical pointing, the sampled spectra are Doppler shifted due to the rapidly changing neutral dynamics of the MLT region. We correct this effect by removing the Doppler shift using the radial velocities estimated every ∼2 min and then integrate the corrected spectra for longer periods to enhance signal-to-noise ratio (SNR) and better investigate the variability of MSP properties. We determine MSP radii and number densities utilizing a method similar to the one developed by Strelnikova et al. (2007) in which the autocorrelation function (ACF) is approximated as the sum of two exponential decays, i.e., the power spectrum is approximated as the sum of two Lorentzians. This method, which assumes mono-disperse particles, allows us to determine mean particle properties in the 80–95 km altitude range during the hours of 10–14 AO LT when the detected SNR from the D-region is highest. Results from this work show MSP radii approximately 0.6–1.5 nm is size depending on altitude and season. Also, MSP densities as a function of altitude and season are determined with the aid of the IRI model resulting in values on the order of 102–104 per cubic centimeter. Our error analysis shows that spectral broadening from atmospheric sources such as neutral turbulence induced by gravity wave vertical variance, and non-zero vertical winds results in <10% change in derived MSP radii. Also, since our dataset covers different seasons, we investigate a potential correlation between the seasonal variations of the derived MSPs properties with that of the meteoric input function (MIF) in the MLT above Arecibo.
Journal of Atmospheric and Solar-Terrestrial Physics 01/2009; · 1.42 Impact Factor