Polar Mesosphere Summer Echoes Research Articles

Meteoric smoke particle properties derived using dual-beam Arecibo UHF observations of D-region spectra during different seasons

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 10 2 – 10 4 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.

Radar cross sections for mesospheric echoes at Jicamarca

Abstract. Radar cross sections (RCS) of mesospheric layers at 50 MHz observed at Jicamarca, Peru, range from 10−18 to 10−16 m−1, three orders of magnitudes smaller than cross sections reported for polar mesospheric winter echoes during solar proton events and six orders of magnitude smaller than polar mesospheric summer echoes. Large RCS are found in thick layers around 70 km that also show wide radar spectra, which is interpreted as turbulent broadening. For typical atmospheric and ionospheric conditions, volume scattering RCS for stationary, homogeneous, isotropic turbulence at 3 m are also in the range 10−18 to 10−16 m−1, in reasonable agreement with measurements. Moreover, theory predicts maximum cross sections around 70 km, also in agreement with observations. Theoretical values are still a matter of order-of-magnitude estimation, since the Bragg scale of 3 m is near or inside the viscous subrange, where the form of the turbulence spectrum is not well known. In addition, steep electron density gradients can increase cross-sections significantly. For thin layers with large RCS and narrow spectra, isotropic turbulence theory fails and scattering or reflection from anisotropic irregularities may gain relevance.

Polar mesospheric summer echoes at 78°N, 16°E, 2008: First results of the refurbished sounding system (SOUSY) Svalbard radar

The second‐generation sounding system (SOUSY) 53.5‐MHz mesosphere‐stratosphere‐troposphere (MST) radar at 78°N, 16°E on Svalbard has recently completed its inaugural summer, 2008, of polar mesospheric summer echoes (PMSE) observations. Here PMSE observations have been assembled in order to identify dates of the earliest and latest occurrences and how the frequency of PMSE occurrence correlates with dynamics and temperature, which are available from the collocated Nippon/Norway Svalbard Meteor 31‐MHz Radar (NSMR). We find strong correlations between preferred PMSE altitude and low temperature, and between equatorward flow and occurrence rate. Temperature drops cause increases in PMSE occurrence: for the height interval 82–92 km, a drop of around 7 K increases the occurrence, typically by 1–2% d−1 and similarly for a 1 m s−1 increase in equatorward wind. A temperature drop of 5 K at 90 km altitude results in a lowering of the underlying preferred PMSE altitude by 1 km. This study therefore qualifies, at least for 78°N, 16°E and 2008, the dependence of PMSE occurrence rates and preferred heights on 90 km temperature and dynamics.

Low latitude 2‐day planetary wave impact on austral polar mesopause temperatures: revealed by a January diminution in PMSE above Davis, Antarctica

A new characteristic of the austral summer polar mesopause as revealed by ground‐based radar and satellite temperature measurements is reported, that is linked to inter‐annual variability of the low‐latitude easterly wind jet. Four consecutive seasons of polar mesosphere summer echoes (PMSE) and mesosphere temperature observations above Davis, Antarctica (68.6°S) show a mid‐January diminution in PMSE occurrence rate that coincides with a minor mesopause warming of several degrees. Spectral analyses of PMSE, Aura Microwave Limb Sounder (MLS) temperatures and radar meridional winds show the presence of ∼4–5‐day planetary waves (PWs) throughout the austral summer in the polar upper mesosphere together with enhanced ∼2‐day PW activity from mid‐January to mid‐February. Analysis of MLS temperatures show that the ∼2‐day PWs have zonal wavenumbers (S) with both westward (S = −2, −3) and eastward (S = +2, +3) components. Although displaying some inter‐annual variation in the peak onset time, the mid‐January mesopause warming coincides with a weakening of the equatorward meridional wind above Davis and enhancement of low‐latitude 2‐day PW activity.

Study on the Relation Between the Reflectivity and Frequency in Dusty Plasma of the Polar Summer Mesopause

Relation between the volume reflectivity and frequency in view of the polar mesosphere summer echoes (PMSE) is studied using the small-scale structure of electron density caused by charged dust particles. A theoretical expression for the radar volume reflectivity is derived, which agrees with the statistical result. Both the theoretical and statistical results are confirmed with the data obtained from simultaneous observations at three frequencies. Hence the small-scale structure caused by charged dust particles may be a useful tool for the study on the generation mechanism of PMSE.

Radar efficiency and the calculation of decade-long PMSE backscatter cross-section for the Resolute Bay VHF radar

Abstract. The Resolute Bay VHF radar, located in Nunavut, Canada (75.0° N, 95.0° W) and operating at 51.5 MHz, has been used to investigate Polar Mesosphere Summer Echoes (PMSE) since 1997. PMSE are a unique form of strong coherent radar echoes, and their understanding has been a challenge to the scientific community since their discovery more than three decades ago. While other high latitude radars have recorded strong levels of PMSE activities, the Resolute Bay radar has observed relatively lower levels of PMSE strengths. In order to derive absolute measurements of PMSE strength at this site, a technique is developed to determine the radar efficiency using cosmic (sky) noise variations along with the help of a calibrated noise source. VHF radars are only rarely calibrated, but determination of efficiency is even less common. Here we emphasize the importance of efficiency for determination of cross-section measurements. The significant advantage of this method is that it can be directly applied to any MST radar system anywhere in the world as long as the sky noise variations are known. The radar efficiencies for two on-site radars at Resolute Bay are determined. PMSE backscatter cross-section is estimated, and decade-long PMSE strength variations at this location are investigated. It was noticed that the median of the backscatter cross-section distribution remains relatively unchanged, but over the years a great level of variability occurs in the high power tail of the distribution.

Electric field measurements in a NLC/PMSE region during the MASS/ECOMA campaign

Abstract. We present results of electric field measurements made during the MASS rocket campaign in Andøya, Norway into noctilucent clouds (NLC) and polar mesospheric summer echoes (PMSE) on 3 August 2007. The instrument used high input-impedance preamps to measure vertical and horizontal electric fields. No large-amplitude geophysical electric fields were detected in the cloud layers, but significant levels of electric field fluctuations were measured. Within the cloud layer, the probe potentials relative to the rocket skin were driven negative by incident heavy charged aerosols. The amplitude of spikes caused by probe shadowing were also larger in the NLC/PMSE region. We describe a method for calculating positive ion conductivities using these shadowing spike amplitudes and the density of heavy charged aerosols.

Occurrence of polar mesosphere summer echoes at very high latitudes

Abstract. Observations of polar mesosphere summer echoes (PMSE) have been carried out during the summer periodes 1999–2001 and 2003–2004 at the very high latitude of 78° N using the SOUSY Svalbard Radar (53.5 MHz) at Longyearbyen. Although the measurements could not be done continuously in these seasons, PMSE have been detected over more than 6600 h of 9300 h of observation time overall. Using this data base, particular PMSE occurrence characteristics have been determined. PMSE at Svalbard appear from the middle of May to the end of August with an almost permanent total occurrence in June and July. Diurnal variations are observable in the height-depend occurrence rates and in PMSE thickness, they show a maximum around 09:00–10:00 UTC and a minimum around 21:00–22:00 UTC. PMSE occur nearly exclusively between a height of 80 km and 92 km with a maximum near 85 km. However, PMSE appear not simultaneously over the entire height range, the mean vertical PMSE extension is around 4–6 km in June and July. Furthermore, typically PMSE are separated into several layers, and only 30% of all PMSE are single layers. The probability of multiple layers is greater in June and July than at the beginning and the end of the PMSE season and shows a marked 5-day-variation. The same variation is noticeable in the seasonal dependence of the PMSE occurrence and the PMSE thickness. We finally discuss potential geophysical processes to explain our observational results.

Long-term changes of (polar) mesosphere summer echoes

Strong VHF radar echoes have been observed not only during summer months at polar latitudes (polar mesosphere summer echoes, PMSE) but also at middle latitudes (mesosphere summer echoes, MSE). These echoes are closely connected with small ice particles, thus containing information about mesospheric temperature and water vapour content. But the (P)MSE also depend on the ionisation due to solar wave radiation and precipitating high energetic particles. Observations with VHF radars at Andenes (69.3°N; 16.0°E) since 1994 and at Kühlungsborn (54.6°N; 11.8°E) since 1998 are used for investigations of the solar and geomagnetic control of the (P)MSE as well as of possible long-term changes. The (P)MSE are positively correlated with the solar Lyman α radiation and the geomagnetic activity and have slightly positive trends. Due to the limited measuring period, the significance levels of the detected (P)MSE trends are small. Positive trends in noctilucent clouds (NLC) and polar mesospheric clouds (PMC) are in general agreement with (P)MSE trends.

Signatures of mesospheric particles in ionospheric data

Abstract. The state of the ionosphere during the 2007 ECOMA/MASS campaign is described by in-situ observations by three sounding rockets launched from the Andøya Rocket Range and by ground based observations. The ground based measurements included the incoherent scatter radar EISCAT near Tromsø (both on UHF and VHF), as well as an MF radar, a meteor radar and an imaging riometer all located in the close vicinity of the rocket range. The pronounced electron density bite-outs seen by two of the rockets could not be detected from the ground, but the associated PMSE (Polar Mesospheric Summer Echoes) provide indirect evidence of pronounced perturbations of mesospheric electron densities.

The ECOMA 2007 campaign: rocket observations and numerical modelling of aerosol particle charging and plasma depletion in a PMSE/NLC layer

Abstract. The ECOMA series of rocket payloads use a set of aerosol particle, plasma, and optical instruments to study the properties of aerosol particles and their interaction with the ambient plasma environment in the polar mesopause region. In August 2007 the ECOMA-3 payload was launched into a region with Polar Mesosphere Summer Echoes (PMSE) and noctilucent clouds (NLC). An electron depletion was detected in a broad region between 83 and 88 km, coincident with enhanced density of negatively charged aerosol particles. We also find evidence for positive ion depletion in the same region. Charge neutrality requires that a population of positively charged particles smaller than 2 nm and with a density of at least 2×108 m−3 must also have been present in the layer, undetected by the instruments. A numerical model for the charging of aerosol particles and their interaction with the ambient plasma is used to analyse the results, showing that high aerosol particle densities are required in order to explain the observed ion density depletion. The model also shows that a very high photoionisation rate is required for the particles smaller than 2 nm to become positively charged, indicating that these may have a lower work function than pure water ice.

Observations of polar mesospheric summer echoes using PFISR during the summer of 2007

The Poker Flat Incoherent Scatter Radar (PFISR) has been running nearly continuously throughout the summer of 2007, which permits an unprecedented, long-term study of polar mesospheric summer echoes (PMSE) at UHF. When not being used for other operations, PFISR runs in its International Polar Year (IPY) mode. 176 examples of PMSE have been identified in the IPY dataset during the summer of 2007 on 42 different days. IPY data were examined from March through mid-September. The first event was observed on May 27, and the last on August 10. During the four months from May 14 through September 14 the IPY mode was run 57% of the time and PMSE was detected 3.3% of the time the mode was running. Thirty-eight percent of the days with data during these four months had at least one PMSE event. Seventy-three percent of the events occurred during the daytime, and of the 48 events at night, 31 were accompanied by auroral precipitation. The events that did not occur with aurora show a strong correlation with solar zenith angle, which suggests a dependence on background ionization. Events were observed in range gates centered at 81.2 and 85.5 km, with 141 of the 176 events in the lower gate. The events have a mean duration of 19.6 min, but events lasting as long as 106 min have been observed. The events have a mean reflectivity of 1.3 × 10 - 17 m - 1 , which is a lower bound due to the possibility that the beam is not filled. A Lorentzian power spectrum was assumed to fit for the half-power half-width (HPHW) of the measured autocorrelation functions. The HPHWs which could be measured have a mean of 14.8 m/s, with a standard deviation of 11.1 m/s.

Exploring polar mesos pheric summer echoes

Year (1932–33), Sir Edward Appleton made early radar observations of the polar atmosphere from Simavik and Tromso, Norway (Appleton et al. 1937). Radar remains an important means of studying phenomena, including aurorae, in the polar mesosphere and ionosphere. Less widely known than aurorae, PMSEs are radar features produced in regions where a fascinating combination of physical and chemical processes occur (e.g. Rapp and Lubken 2004). They involve: global atmospheric dynamics and thermodynamics; chemical and smaller scale thermodynamical processes leading to the formation of ice particles at altitudes of 80–90 km, far above ordinary rain clouds; plasma mechanisms affected by the presence of charged particles with sizes up to about 0.1 μm. Some have speculated that monitoring of PMSEs over several decades will provide diagnostics of industrial impact on the mesosphere and perhaps regions in the lower atmosphere as well. The troposphere, stratosphere and mesosphere and their boundaries are associated with structure in the plot of temperature versus altitude (e.g. Salby 1996). The stratopause and mesopause bound the mesosphere at altitudes of about 50 km and 90 km, respectively. In standard models for non-polar latitudes the temperature of the mesosphere decreases nearly linearly with altitude from around 270 K to about 180 K. The temperature increases above the mesopause to somewhat less than 200 K at 100 km. The number density of electrons increases from 10–10 cm at the mesopause by over an order of magnitude in less than 10 km (e.g. Kazil et al. 2003). Solar activity causes variations in the solar wind and, hence, in the intensity and spectrum of non-thermal particles in the atmosphere. These variations can result in significant modifications of the electron altitude profile in the mesosphere. The properties and location of the mesopause usually change more markedly with time at polar Polar mesospheric summer echoes (PMSEs) are radar features observed around 80–90 km, much higher than ordinary clouds. The ways in which nanoparticles affect radar reflectivity near the summer polar mesopause have been elucidated through the use of rocket-borne “dust” detectors and observations of scattered laser light. Artificial heating of the radar scattering regions by powerful microwave transmitters and simultaneous radar observations have also provided important data. Further efforts to understand the mechanisms producing mesospheric radar echoes and their seasonal variations will help refine our knowledge of large-scale atmospheric dynamics. AbstrAct HArtquist et al.: Polar mesosPheric summer echoes

Coordinated optical and radar image measurements of noctilucent clouds and polar mesospheric summer echoes

Novel coincident 3-D radar, lidar and optical image measurements of dynamical structures in polar mesosphere summer echoes (PMSE) and noctilucent clouds (NLC) are presented. Common volume mesospheric measurements were made over central Alaska using the new Poker Flat Incoherent Scatter Radar (PFISR), a co-located Rayleigh lidar and remote, two-station digital image observations, enabling the first detailed investigation of the horizontal and vertical structures of NLC and PMSE. Coincident measurements were made of an unusual NLC display recorded on 10–11 August 2007, characterized by a broad luminous band that contained several prominent wave forms. Concurrent lidar and image measurements established the presence of NLC within the radar volume from ∼09:00 UT (01:00 LT), when the solar depression angle was 10.4°, until dawn. Strong but intermittent PMSE were detected by PFISR, with distinct patchy structures that exhibited a similar southward motion as the NLC. Detailed comparison of the 3-D PMSE structures and the NLC lidar and image data have revealed striking similarities when account was taken of the NLC layer altitude, suggesting a direct link between their small-scale spatial signatures (within the current resolution of the radar measurements). At the same time, the lidar detected a sustained increase in the backscatter signal, while the imagers revealed the development of copious short horizontal wavelength (4.9 km) billow waves. We conclude that strong wind shears associated with the Kelvin–Helmholtz billow instabilities played a key role in the development of a neutral turbulence layer in close proximity to the NLC layer resulting in the strong but intermittent PMSE detected at 450 MHz on this occasion.

One Analysis on the Rocket Detection of Polar Mesosphere Summer Echoes

利用仅发生极区中层夏季回波 (Polar Mesosphere Summer Echoes, PMSE) 现象时的ECT-02 火箭探空数据, 反演出电子数密度和尘埃电荷数密度. 这些典型的极区中层顶的基本参量为研究PMSE提供了良好的数据基础. 通过分析得出了极区中 层顶区域属于弱电离、弱耦合尘埃等离子体的结论. 根据电子数密度数扰动分析PMSE现象发现, 电子数密度的扰动与PMSE有密切的联系, 扰动强烈的地方回波就强, 扰动弱的地方回波就弱, 或者不发生PMSE 现象.

First Southern Hemisphere common-volume measurements of PMC and PMSE

[1] Using Rayleigh lidar and Mesosphere-Stratosphere-Troposphere radar at Davis Antarctica (68.6°S) during the 2005–2006 austral summer, we obtained the first Southern Hemisphere common-volume measurements of Polar Mesospheric Clouds (PMC) and Polar Mesosphere Summer Echoes (PMSE). PMC were observed with 6% probability during 349 hours of lidar observations. PMSE were simultaneously observed during 70% of the PMC events. The mean centroid altitudes of simultaneous PMC and PMSE were 83.97 ± 0.10 km and 85.31 ± 0.14 km, respectively. The altitude difference (∼1.3 km) is comparable to earlier boreal common-volume measurements. Both phenomena show clear diurnal variations in altitude and occurrence. PMC had a unimodal local time occurrence distribution which peaked 4–6 hours after solar midnight. PMSE were most prevalent at this time, but had a secondary occurrence peak ∼6 hours after solar noon when PMC were absent. We explain the altitude-time distributions of PMC and PMSE in terms of the thermal structure of the mesopause region determined using measurements from the Aura satellite.

Influence of tides and gravity waves on layering processes in the polar summer mesopause region

Abstract. Polar Mesosphere Summer Echoes (PMSE) have been studied at Andenes (69° N, 16° E), Norway, using VHF radar observations since 1994. One remarkable feature of these observations is the fact that {during 50% of the time,} the radar echoes occur in the form of two or more distinct layers. In the case of multiple PMSE layers, statistical analysis shows that the lower layer occurs at a mean height of ~83.4 km, which is almost identical to the mean height of noctilucent clouds (NLC) derived from observation with the ALOMAR Rayleigh/Mie/Raman lidar at the same site. To investigate the layering processes microphysical model simulations under the influence of tidal and gravity waves were performed. In the presence of long period gravity waves, these model investigations predict an enhanced formation of multiple PMSE layer structures, where the lower layer is a consequence of the occurrence of the largest particles at the bottom of the ice cloud. This explains the coincidence of the lowermost PMSE layers and NLC. During periods with enhanced amplitudes of the semidiurnal tide, the observed NLC and PMSE show pronounced tidal structures comparable to the results of corresponding microphysical simulations. At periods with short period gravity waves there is a tendency for a decreasing occurrence of NLC and for variable weak PMSE structures.

A new height for the summer mesopause: Antarctica, December 2007

Two VHF atmospheric radars operating in Antarctica during austral summer 2007/2008 found the Polar Mesosphere Summer Echo (PMSE) layer at 3–5 km higher altitude during the early season, compared to the late season, and to earlier seasons. Temperatures from the microwave limb sounder on the Aura satellite show that the height of the cold summer mesopause was ∼3 km higher than usual at the same time. The winter polar vortex over Antarctica did not break up until late December, so that eastward winds in the lower stratosphere were as strong as westward winds in the upper stratosphere during the early part of the austral summer. We find that a combination of limited gravity wave forcing from below in the same hemisphere and interhemisphere coupling between the winter stratosphere/mesosphere and the summer mesopause may explain the observations, and suggest a need for reappraisal of the formation mechanisms for the summer mesopause.

Simultaneous observations of Polar Mesosphere Summer Echoes at two different latitudes in Antarctica

Abstract. Simultaneous observations of Polar Mesosphere Summer Echoes (PMSE) at Wasa and Davis in Antarctica have been compared. Data with simultaneous observations were obtained for 16 days between 18 January and 5 February 2007. Wasa is at a higher geographic latitude than Davis, but at lower geomagnetic latitude. PMSE strength and occurrence frequency were significantly higher at Wasa. The variation of daily PMSE occurrence over the measurement period was in agreement with temperature and frost-point estimates from the Microwave Limb Sounder on the Aura spacecraft for both Wasa and Davis. The diurnal variation of PMSE strength and occurrence frequency as well as the shape of the altitude profiles of average PMSE strength and occurrence frequency were similar for the two sites. The deepest part of the evening minimum in PMSE occurrence frequency occurred for the same magnetic local time at the two sites rather than for the same local solar time. The study indicates that PMSE strength and occurrence increase between 68.6° and 73° geographic latitude, consistent with observed differences in mesospheric temperatures and water vapor content. The average altitude distribution of PMSE varies relatively little with latitude in the same hemisphere.

Five-day planetary waves in the middle atmosphere from Odin satellite data and ground-based instruments in Northern Hemisphere summer 2003, 2004, 2005 and 2007

Abstract. A number of studies have shown that 5-day planetary waves modulate noctilucent clouds and the closely related Polar Mesosphere Summer Echoes (PMSE) at the summer mesopause. Summer stratospheric winds should inhibit wave propagation through the stratosphere and, although some numerical models (Geisler and Dickinson, 1976) do show a possibility for upward wave propagation, it has also been suggested that the upward propagation may in practice be confined to the winter hemisphere with horizontal propagation of the wave from the winter to the summer hemisphere at mesosphere heights causing the effects observed at the summer mesopause. It has further been proposed (Garcia et al., 2005) that 5-day planetary waves observed in the summer mesosphere could be excited in-situ by baroclinic instability in the upper mesosphere. In this study, we first extract and analyze 5-day planetary wave characteristics on a global scale in the middle atmosphere (up to 54 km in temperature, and up to 68 km in ozone concentration) using measurements by the Odin satellite for selected days during northern hemisphere summer from 2003, 2004, 2005 and 2007. Second, we show that 5-day temperature fluctuations consistent with westward-traveling 5-day waves are present at the summer mesopause, using local ground-based meteor-radar observations. Finally we examine whether any of three possible sources of the detected temperature fluctuations at the summer mesopause can be excluded: upward propagation from the stratosphere in the summer-hemisphere, horizontal propagation from the winter-hemisphere or in-situ excitation as a result of the baroclinic instability. We find that in one case, far from solstice, the baroclinic instability is unlikely to be involved. In one further case, close to solstice, upward propagation in the same hemisphere seems to be ruled out. In all other cases, all or any of the three proposed mechanisms are consistent with the observations.