Thermal structure and dynamics of the Martian upper atmosphere at solar minimum from global circulation model simulations

International audienceSimulations of the Martian upper atmosphere have been produced from a self-consistent three-dimensional numerical model of the Martian thermosphere and ionosphere, called MarTIM. It covers an altitude range of 60 km to the upper thermosphere, usually at least 250 km altitude. A radiation scheme is included that allows the main sources of energy input, EUV/UV and IR absorption by CO2 and CO, to be calculated. CO2, N2 and O are treated as the major gases in MarTIM, and are mutually diffused (though neutral chemistry is ignored). The densities of other species (the minor gases), CO, Ar, O2 and NO, are based on diffusive equilibrium above the turbopause. The ionosphere is calculated from a simple photoionisation and charge exchange routine though in this paper we will only consider the thermal and dynamic structure of the neutral atmosphere at solar minimum conditions. The semi-diurnal (2,2) migrating tide, introduced at MarTIM's lower boundary, affects the dynamics up to 130 km. The Mars Climate Database (Lewis et al., 2001) can be used as a lower boundary in MarTIM. The effect of this is to increase wind speeds in the thermosphere and to produce small-scale structures throughout the thermosphere. Temperature profiles are in good agreement with Pathfinder results. Wind velocities are slightly lower compared to analysis of MGS accelerometer data (Withers, 2003). The novel step-by-step approach of adding in new features to MarTIM has resulted in further understanding of the drivers of the Martian thermosphere

The characteristics of the lower stratospheric gravity wave field above Halley (75°S, 26°W), Antarctica, from radiosonde observations

Daily radiosonde observations between 2003 and 2013 from Halley research station, Antarctica (75°S, 26°W) are used to determine climatologies of gravity wave properties in the lower stratosphere (between 15 km and 22 km altitude). Individual waves are extracted from the radiosonde profile using wavelet analysis and separated into upward and downward propagating waves. An increase in the percentage of downward propagating waves (~30% of the waves) is seen during the winter months. For the upward and downward propagating waves their horizontal and vertical wavelength, intrinsic frequency, energy density, pseudo-momentum flux and direction of propagation are determined. The upward propagating wave field is found to be dominated by waves with short vertical wavelength (~1 km) and low intrinsic frequency (ω~f). The downward propagating wave field is composed of a wider distribution of vertical wavelength waves and has a larger proportion of higher frequency waves present. The upward propagating waves show an increase in total energy density in autumn and spring, the larger increase occurs during spring (up to 1.7 J kg-1 in September). The downward propagating waves increase in total energy density occurs during wintertime (up to 0.7 J kg-1 in June). During winter the contributions of the upward and downward propagating waves to the total energy density and pseudo-momentum flux are almost equal. This paper details the first study of individual gravity wave properties combined into upward and downward propagating wave climatologies in the lower stratosphere above Halley

Numerical and statistical evidence for long-range ducted gravity wave propagation over Halley, Antarctica

Abundant short‒period, small‒scale gravity waves have been identified in the mesosphere and lower thermosphere over Halley, Antarctica, via ground‒based airglow image data. Although many are observed as freely propagating at the heights of the airglow layers, new results under modeled conditions reveal that a significant fraction of these waves may be subject to reflections at altitudes above and below. The waves may at times be trapped within broad thermal ducts, spanning from the tropopause or stratopause to the base of the thermosphere (∼140 km), which may facilitate long‒range propagation (∼1000s of km) under favorable wind conditions

Winds and tides of the Extended Unified Model in the mesosphere and lower thermosphere validated with meteor radar observations

The mesosphere and lower thermosphere (MLT) is a critical region that must be accurately reproduced in general circulation models (GCMs) that aim to include the coupling between the lower and middle atmosphere and the thermosphere. An accurate representation of the MLT is thus important for improved climate modelling and the development of a whole atmosphere model. This is because the atmospheric waves at these heights are particularly large, and so the energy and momentum they carry is an important driver of climatological phenomena through the whole atmosphere, affecting terrestrial and space weather. The Extended Unified Model (ExUM) is the recently developed version of the Met Office's Unified Model which has been extended to model the MLT. The capability of the ExUM to model atmospheric winds and tides in the MLT is currently unknown. Here, we present the first study of winds and tides from the ExUM. We make a comparison against meteor radar observations of winds and tides from 2006 between 80 and 100 km over two radar stations – Rothera (68∘ S, 68∘ W) and Ascension Island (8∘ S, 14∘ W). These locations are chosen to study tides in two very different tidal regimes – the equatorial regime, where the diurnal (24 h) tide dominates, and the polar regime, where the semi-diurnal (12 h) tide dominates. The results of this study illustrate that the ExUM is capable of reproducing atmospheric winds and tides that capture many of the key characteristics seen in meteor radar observations, such as zonal and meridional wind maxima and minima, the increase in tidal amplitude with increasing height, and the decrease in tidal phase with increasing height. In particular, in the equatorial regime some essential characteristics of the background winds, tidal amplitudes and tidal phases are well captured but with significant differences in detail. In the polar regime, the difference is more pronounced. The ExUM zonal background winds in austral winter are primarily westward rather than eastward, and in austral summer they are larger than observed above 90 km. The ExUM tidal amplitudes here are in general consistent with observed values, but they are also larger than observed values above 90 km in austral summer. The tidal phases are generally well replicated in this regime. We propose that the bias in background winds in the polar regime is a consequence of the lack of in situ gravity wave generation to generate eastward fluxes in the MLT. The results of this study indicate that the ExUM has a good natural capability for modelling atmospheric winds and tides in the MLT but that there is room for improvement in the model physics in this region. This highlights the need for modifications to the physical parameterization schemes used in the model in this region – such as the non-orographic spectral gravity wave scheme – to improve aspects such as polar circulation. To this end, we make specific recommendations of changes that can be implemented to improve the accuracy of the ExUM in the MLT.</p

An 18‐year climatology of directional stratospheric gravity wave momentum flux from 3‐D satellite observations

Atmospheric gravity waves (GWs) are key drivers of the atmospheric circulation, but their representation in general circulation models (GCMs) is challenging, leading to significant biases in middle atmospheric circulations. Unresolved GW momentum transport in GCMs must be parameterised, but global directional GW observations are needed to constrain this. Here we present an 18‐year climatology of directional stratospheric GW momentum flux (GWMF) from global AIRS/Aqua 3‐D satellite observations during 2002 to 2019. Striking hemispheric asymmetries are found at high latitudes, including dramatic reductions and reversals of GWMF during sudden stratospheric warmings. During southern hemisphere winter, a lateral convergence of GWMF towards 60°S is found that has no northern hemisphere counterpart. In the tropics, we find that zonal GWMF in AIRS measurements is strongly modulated by the semi‐annual oscillation (SAO) but not the quasi‐biennial oscillation (QBO). Our results provide guidance for future GW parameterisations needed to resolve long‐standing biases in GCMs

Investigating an unusually large 28-day oscillation in mesospheric temperature over Antarctica using ground-based and satellite measurements

The Utah State University (USU) Advanced Mesospheric Temperature Mapper (AMTM) was deployed at the Amundsen‐Scott South Pole Station in 2010 to measure OH temperature at ~87 km as part of an international network to study the mesospheric dynamics over Antarctica. During the austral winter of 2014, an unusually large amplitude ~28‐day oscillation in mesospheric temperature was observed for ~100 days from the South Pole Station. This study investigates the characteristics and global structure of this exceptional planetary‐scale wave event utilizing ground‐based mesospheric OH temperature measurements from two Antarctic stations (South Pole and Rothera) together with satellite temperature measurements from the Microwave Limb Sounder (MLS) on the Aura satellite, and the Solar Occultation For Ice Experiment (SOFIE) on the Aeronomy of Ice in the Mesosphere (AIM) satellite. Our analyses have revealed that this large oscillation is a winter time, high latitude phenomenon, exhibiting a coherent zonal wave #1 structure below 80 km altitude. At higher altitudes, the wave was confined in longitude between 180‐360°E. The amplitude of this oscillation reached ~15 K at 85 km and it was observed to grow with altitude as it extended from the stratosphere into the lower thermosphere in the southern hemisphere. The satellite data further established the existence of this oscillation in the northern hemisphere during the boreal winter time. The main characteristics and global structure of this event as observed in temperature are consistent with the predicted 28‐day Rossby Wave (1,4) mode

Observations of mesospheric gravity waves generated by geomagnetic activity

Gravity waves (GWs) play an important role in the dynamics and energetics of the mesosphere. Geomagnetic activity is a known source of GWs in the upper atmosphere. However, how deep the effects of geomagnetic activity induced GWs penetrate into the mesosphere remains an open question. We use temperature measurements from the SABER/TIMED instrument between 2002 and 2018 to study the variations of mesospheric GW activity following intense geomagnetic disturbances identified by AE and Dst indices. By considering several case studies, we show for the first time that the GWs forced by geomagnetic activity can propagate down to about 80 km in the high latitude mesosphere. Only regions above 55° latitudes show a clear response. The fraction of cases in which there is an unambiguous enhancement in GW activity following the onset of geomagnetic disturbance is smaller during summer than other seasons. Only about half of the events show an unambiguous increase in GW activity during non-summer periods and about one quarter of the events in summer show an enhancement in GWs. In addition, we also find that the high latitude mesopause is often seen to descend in altitude following onset of geomagnetic activity in the non-summer high latitude region

First ground-based conjugate observations of Stable Auroral Red (SAR) Arcs

During the geomagnetic storm of 1 June 2013 all‐sky imagers located at geomagnetically conjugate locations at Millstone Hill, USA (42.6 o N, 71.4 o W, 50.9o mag lat) and at Rothera, Antarctica (67.5o S, 68.1o W, ‐ 53.2 o mag lat) allowed us to measure a stable auroral red (SAR) arc simultaneously in both hemispheres for the first time. The arc measured in one hemisphere was observed very close to its conjugate location in the opposite hemisphere. While spatial characteristics, such as equatorward motion and latitudinal extent, were similar at both sites, morphological properties, e.g., arc brightness and shape of the poleward edges, differed. The overall brightness of the northern hemisphere arc was considerably weaker, by a factor of ~2‐3, throughout the night. Reduced magnetospheric forcing, in a short time interval between ~0345 UT and 0445 UT, led to decreased SAR arc brightness and reduced equatorward motion at both sites. A substorm occurring near 0500UT provided additional energization that increased the SAR arc brightness as well as the speed of the equatorward motion. These results provide evidence of a complex coupling between energy sources in the inner magnetosphere and the ionospheric receptor conditions within the sub‐auroral domain at opposite ends of the same geomagnetic field line

Ductal-lobar organisation of human breast tissue, its relevance in disease and a research objective: vector mapping of parenchyma in complete breasts (the Astley Cooper project)

A human breast has many lobes, which are highly variable in size and shape, each with one central duct, its peripheral branches and their associated glandular tissues. Realising the potential of new endoductal approaches to breast diagnosis and improving our understanding of breast cancer precursors will require greatly improved knowledge of this ductal-lobar anatomy and the distribution of cancer precursors within it. This architecture is very challenging to study in its entirety: whole-breast lobe mapping has only been achieved for two human breasts. Clearly, much more efficient techniques are required. Streamlined data capture and visualisation of breast parenchymal anatomy from thin and thick sections in a vector format would allow integrated mapping of whole-breast structure with conventional histology and molecular data. The 'Astley Cooper digital breast mapping project' is proposed as a name for this achievable research objective. Success would offer new insights into the development of breast cancer precursor lesions, allow testing of the important 'sick lobe' hypothesis, improve correlation with imaging studies and provide 'ground truth' for mathematical modelling of breast growth