Search

The energetic particle precipitation (EPP) indirect effect (IE) refers to the downward transport of reactive odd nitrogen (NOx = NO + NO2) produced by EPP (EPP‐NOx) from the polar winter mesosphere and lower thermosphere to the stratosphere where it can destroy ozone. Previous studies of the EPP IE examined NOx descent averaged over the polar region, but the work presented here considers longitudinal variations. We report that the January 2009 split Arctic vortex in the stratosphere left an imprint on the distribution of NO near the mesopause, and that the magnitude of EPP‐NOx descent in the upper mesosphere depends strongly on the planetary wave (PW) phase. We focus on an 11‐day case study in late January immediately following the 2009 sudden stratospheric warming during which regional‐scale Lagrangian coherent structures (LCSs) formed atop the strengthening mesospheric vortex. The LCSs emerged over the north Atlantic in the vicinity of the trough of a 10‐day westward traveling planetary wave. Over the next week, the LCSs acted to confine NO‐rich air to polar latitudes, effectively prolonging its lifetime as it descended into the top of the polar vortex. Both a whole atmosphere data assimilation model and satellite observations show that the PW trough remained coincident in space and time with the NO‐rich air as both migrated westward over the Canadian Arctic. Estimates of descent rates indicate five times stronger descent inside the PW trough compared to other longitudes. This case serves to set the stage for future climatological analysis of NO transport via LCSs. Plain Language Summary: Energetic particles from the sun and the magnetosphere impinge upon Earth's upper atmosphere and create reactive odd nitrogen (NOx) in the mesosphere and lower thermosphere. Descent in the winter polar vortex effectively transports this NOx down to the stratosphere where it can destroy ozone. State‐of‐the‐art models currently underestimate this vertical transport by a factor of 4. Previous studies have examined the NOx descent averaged over the entire polar region, but this study considers longitudinal variations. We examine a case study during late January 2009 and find a closed circulation coincident with the trough of a planetary wave over the north Atlantic at 90 km with shear zones inhibiting horizontal mixing to the north, east, and south. This circulation (1) contains elevated NOx, (2) is associated with five times stronger descent compared to other longitudes, and (3) is the natural upward continuation of the westward tilting polar vortex in the stratosphere and mesosphere. Thus, this meteorological feature near the mesopause provides a transport pathway for air to enter the top of the polar vortex. This is the first work to illustrate the zonally asymmetric nature of NOx descent in the polar winter upper mesosphere and couple it to the vortex below. Key Points: First demonstration of the impact of the split Arctic vortex on the geographic distribution of nitric oxide at the winter mesopauseFirst evidence that a Lagrangian coherent structure inhibits horizontal transport of nitric oxide at the polar winter mesopauseDescent of nitric oxide is five times stronger between 80 and 90 km in a westward traveling planetary wave trough compared to the ridge [ABSTRACT FROM AUTHOR]

The paper presents results of simulations with a high‐resolution (equivalent to ∼67‐km grid size) Martian general circulation model (MGCM) from the surface up to the mesosphere for a full Martian year. The obtained climatology of the small‐scale disturbances can serve as a proxy for gravity waves (GWs) that are largely not resolved by MGCMs with conventional grid resolution and thus have to be parameterized. GW activity varies greatly with season and geographical location, which contradicts with the constant in space and time sources in the lower atmosphere adopted by GW parameterizations employed by coarse‐grid MGCMs. In particular, lower‐atmospheric GW activity is smaller in polar regions of the troposphere throughout all seasons, and the intensity is larger in southern spring and summer and in winter hemisphere at both solstices. In the mesosphere, the peak of GW activity shifts toward middle and high latitudes, and the interhemispheric symmetry is much larger compared to the lower atmosphere. The detailed climatology created in this study can be used for prescribing sources of GWs in parameterizations utilized by MGCMs as well as for validating the parameterizations in the middle and upper atmosphere. Key Points: Gravity wave activity on Mars demonstrates significant spatial and seasonal variabilityMaxima of GW activity shifts from low latitudes in the troposphere to middle and high latitudes in the mesosphereGW activity is much larger during perihelion than aphelion [ABSTRACT FROM AUTHOR]

The second Korean Antarctic station, Jang Bogo Station (JBS), Terra Nova Bay (74°37.4'S, 164°13.7'E), is operational since March 2014. A Fabry-Perot Interferometer (FPI) and Vertical Incidence Pulsed Ionospheric Radar (VIPIR) were installed in 2014 and 2015 respectively, for simultaneous observations of neutral atmosphere and ionosphere in the polar region. Neutral winds observed by FPI show typical diurnal and semi-diurnal variations at around 250 km and 87 km respectively. VIPIR observations for the ionosphere also show typical electron density distributions in the polar region. Unlike conventional ionospheric sounder, it can measure ionospheric tilts to provide horizontal gradients of electron density over JBS in addition to general ionospheric parameters from sounding observation. In this article, we briefly report the preliminary results of the observations for the neutral atmosphere and ionosphere in the polar cap region. [ABSTRACT FROM AUTHOR]

Abstract: The polar vortices play a central role in vertically coupling the atmosphere from the ground to geospace by shaping the background wind field through which atmospheric waves propagate. This work extends the vertical range of previous polar vortex climatologies into the upper mesosphere. The mesospheric polar vortices are defined using the CO gradient method with Microwave Limb Sounder satellite data; the stratospheric polar vortices are defined using a stream function‐based algorithm with data from meteorological reanalyses. Strengths and weaknesses of the two vortex definitions are given, as well as recommendations for when, where, and why to use each definition. Midwinter mean vortex geometry in the mesosphere is funnel shaped in the Arctic, with a wide top and narrow bottom. The Antarctic mesospheric vortex tapers with height in early winter and broadens with height in late winter. The seasonal evolution of mesospheric vortex frequency of occurrence, size, and zonal symmetry in both hemispheres is presented. Unexpected behavior above 60 km includes late season vortex broadening in both hemispheres, especially following winters without sudden stratospheric warmings. Following extreme stratospheric disturbances the polar night jet in the mesosphere strengthens and shifts poleward, resulting in a mesospheric vortex that contracts. Overall, the mesospheric polar vortices are more similar between the two hemispheres than their stratospheric counterparts. The vortex climatology presented here serves as an observational benchmark to which the mesospheric polar vortices in high‐top climate models can be evaluated. [ABSTRACT FROM AUTHOR]

Clouds composed of CO 2 ice form throughout the Martian atmosphere. In the mesosphere, CO 2 ice clouds are thought to form via heterogeneous ice nucleation on nanoparticles of meteoric origin at temperatures often below 100 K. Lower altitude CO 2 ice clouds in the wintertime polar regions form up to around 145 K and lead to the build-up of the polar ice caps. However, the crystal structure and related fundamental properties of CO 2 ice under Martian conditions are poorly characterised. Here we present X-ray diffraction (XRD) measurements of CO 2 ice, grown via deposition from the vapour phase under temperature and pressure conditions analogous to the Martian mesosphere. A crystalline cubic structure was determined, consistent with the low-pressure polymorph (CO 2 -I, space group Pa -3 (No. 205)). CO 2 deposited at temperatures of 80-130 K and pressures of 0.01–1 mbar was consistent with dry ice and previous literature measurements, thus removing the possibility of a more complicated phase diagram for CO 2 in this region. At 80 K, a lattice parameter of 5.578 ± 0.002 Å, cell volume of 173.554 ± 0.19 Å 3 and density of 1.684 ± 0.002 g cm −3 was determined. Using these measurements, we determined the thermal expansion of CO 2 across 80–130 K that allowed for a fit of CO 2 ice density measurements across a larger temperature range (80–195 K) when combined with literature data (CO 2 density = 1.72391 - 2.53 × 10 −4 T - 2.87 × 10 − 6 T 2 ). Temperature-dependent CO 2 density values are used to estimate sedimentation velocities and heterogeneous ice nucleation rates, showing an increase in nucleation rate of up to a factor of 1000 when compared to commonly used literature values. This temperature-dependent equation of state is therefore suggested for use in future studies of Martian mesospheric CO 2 clouds. Finally, we discuss the possible shapes of crystals of CO 2 ice in the Martian atmosphere and show that a range of shapes including cubes and octahedra as well as a combination of the two in the form of cubo-octahedra are likely. [ABSTRACT FROM AUTHOR]

WIRA is a ground-based microwave Doppler spectro radiometer specifically designed for the measurement of profiles of horizontal wind in the upper stratosphere and lower mesosphere region where no other continuously running measurement technique exists. A proof of principle has been delivered in a previous publication. Since a technical upgrade which improved the signal to noise ratio by a factor of 2.4 the full horizontal wind field comprising zonal and meridional wind profiles is continuously measured. A completely new retrieval based on optimal estimation has been set up. Its characteristics are detailed in the present paper. Since the start of the routine operation of the first prototype in September 2010, WIRA has been measuring at four different locations at polar, mid and tropical latitudes for time periods between 5.5 and 11 months. A comparison between the data series from WIRA and ECMWF model data revealed agreement within 10% in the stratospheric zonal wind. The meridional wind profiles agree within their error bars over the entire sensitive altitude range of WIRA. However, significant differences in the mesospheric zonal wind speed of up to 40% have been found. [ABSTRACT FROM AUTHOR]

Using spectral, cross-spectral, and regression methods, we analyzed the effect of the 11-year cycle of solar activity on the ozone content in the stratosphere and lower mesosphere via satellite measurement data obtained with the help of SBUV/SBUV2 instruments in 1978-2003. We revealed a high coherence between the ozone content and solar activity level on the solar cycle scale. In much of this area, the ozone content varies approximately in phase with the solar cycle; however, in areas of significant gradients of ozone mixing ratio in the middle stratosphere, the phase shift between ozone and solar oscillations can be considerable, up to π/2. This can be caused by dynamical processes. The altitude maxima of ozone sensitivity to the 11-year solar cycle were found in the upper vicinity of the stratopause (50-55 km), in the middle stratosphere (35-40 km), and the lower stratosphere (below 25 km). Maximal changes in ozone content in the solar cycle (up to 10% and more) were found in winter and spring in polar regions. [ABSTRACT FROM AUTHOR]

Diurnal variations of ClO, HO2, and HOCl were simultaneously observed by the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) between 12 October 2009 and 21 April 2010. These were the first global observations of the diurnal variation of HOCl in the upper atmosphere. A major reaction for the production of HOCl is ClO + HO2 →HOCl + O2 (Reaction (R1)) in extra-polar regions. A model study suggested that in the mesosphere, this is the only reaction influencing the amount of HOCl during the night. The evaluation of the pure reaction period, when only Reaction (R1) occurred in the Cly chemical system, was performed by checκing the consistency of the HOCl production rate with the ClO loss rate from SMILES observation data. It turned out that the SMILES data at the pressure level of 0.28 hPa (about 58 κm) in the autumn mid-latitude region (20-40° S, February-April 2010) during night (between modified local time 18:30 and 04:00) were suitable for the estimation of the rate constant, κ1. The rate constant obtained from SMILES observations was κ1(245 K)=(7.75±0.25) ×10-12 cm3 molecule-1 s-1. This result is consistent with results from a laboratory experiment and ab initio calculations for similar low-pressure conditions. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: Observations of noctilucent clouds have revealed a surprising coupling between the winter stratosphere and the summer polar mesopause region. In spite of the great distance involved, this inter-hemispheric link has been suggested to be the principal reason for both the year-to-year variability and the hemispheric differences in the frequency of occurrence of these high-altitude clouds. In this study, we investigate the dynamical influence of the winter stratosphere on the summer mesosphere using simulations from the vertically extended version of the Canadian Middle Atmosphere Model (CMAM). We find that for both Northern and Southern Hemispheres, variability in the summer polar mesopause region from one year to another can be traced back to the planetary-wave flux entering the winter stratosphere. The teleconnection pattern is the same for both positive and negative wave-flux anomalies. Using a composite analysis to isolate the events, it is argued that the mechanism for inter-hemispheric coupling is a feedback between summer mesosphere gravity-wave drag (GWD) and zonal wind, which is induced by an anomaly in mesospheric cross-equatorial flow, the latter arising from the anomaly in winter hemisphere GWD induced by the anomaly in stratospheric conditions. [Copyright &y& Elsevier]

Abstract: We present the first results of gravity wave signatures on polar mesospheric clouds (PMCs) during the summer of 2007, in the northern hemisphere polar region. The Cloud Imaging and Particle Size (CIPS) experiment has one of the three instruments on board the NASA Aeronomy of Ice in the Mesosphere (AIM) spacecraft, which was launched into a sun-synchronous orbit on April 25, 2007. CIPS is a four-camera, wide-field (120°×80°) imager designed to measure PMC morphology and particle properties. One of the objectives of AIM is to investigate gravity wave effects on PMC formation and evolution. CIPS images show distinct wave patterns and structures in PMCs that are similar to ground-based photographs of noctilucent clouds (NLCs). The observed horizontal wavelengths of the waves were found to vary between 15 and 320km, with smaller-wavelength structures of less than 50km being the most common. In this paper we present examples of individual quasi-monochromatic wave events observed by CIPS and statistics on the wave patterns observed in the northern hemisphere during the summer months of 2007, together with a map showing the geographic locations of gravity wave events observed from CIPS. [Copyright &y& Elsevier]

We present the validation of a water vapour dataset obtained by the Airborne Microwave Stratospheric Observing System AMSOS, a passive microwave radiometer operating at 183 GHz. Vertical profiles are retrieved from spectra by an optimal estimation method. The useful vertical range lies in the upper troposphere up to the mesosphere with an altitude resolution of 8 to 16 km and a horizontal resolution of about 57 km. Flight campaigns were performed once a year from 1998 to 2006 measuring the latitudinal distribution of water vapour from the tropics to the polar regions. The obtained profiles show clearly the main features of stratospheric water vapour in all latitudinal regions. Data are validated against a set of instruments comprising satellite, ground-based, airborne remote sensing and in-situ instruments. It appears that AMSOS profiles have a dry bias of 0 to -20%, when compared to satellite experiments. Also a comparison between AMSOS and in-situ hygrosondes FISH and FLASH have been performed. A matching in the short overlap region in the upper troposphere of the lidar measurements from the DIAL instrument and the AMSOS dataset allowed water vapour profiling from the middle troposphere up to the mesosphere. [ABSTRACT FROM AUTHOR]

Polar mesospheric summer echoes (PMSE) often show different layers. Artificial electron heating experiments are an important diagnostic tool to investigate the parameters in the PMSE regions. The response of PMSE to high frequency heating in the lower (80-85 km) and upper (85-90 km) PMSE layers is studied by analyzing PMSE observations carried out by the EISCAT VHF radar in July 2007. Different characteristics of modulated PMSE (e.g., PMSE intensity reduction, recovery, overshoot, ratios during the heating-on and heating-off times) are analyzed and compared at different situations such as only in one layer (lower or upper) and in both layers simultaneously without high-energy particle precipitation. Based on statistical results, it is found that in the lower layer all characteristics of modulated PMSE except PMSE recovery are greater than that in the upper layer. Electron temperature enhancement and PMSE modulation index due to ionospheric heating were calculated to show the enhanced electron temperature effect on PMSE. It is found that in both layers usually higher electron temperature enhancement corresponds to higher PMSE modulation index. However, the comparison of statistical results shows that in the lower layer electron temperature enhancement and PMSE modulation index are greater than that in the upper layer. Based on the relation of electron temperature enhancement to the PMSE modulation index, it is concluded that the variability of electron temperature enhancement may be responsible for the variations in different modulated PMSE characteristics between the lower and upper layers. [ABSTRACT FROM AUTHOR]