The role of neutral atmospheric dynamics in cusp density and ionospheric patch formation

One of the most highly cited papers over the last 10 years in upper atmospheric physics literature is that describing the CHAMP satellite observation of a persistent and large density bulge over the magnetic cusp regions, which are around the north and south magnetic poles (Lühr et al., 2004). It has since been a serious challenge to atmospheric modellers to reproduce this observation. Unrealistically large quantities of heating and complicated mechanisms involving atmospheric gravity waves have been invoked. A phenomenon such as this density bulge affects satellite drag, and so has a commercial as well as scientific interest. Carlson et al (2012) proposed a simple mechanism, whereby depositing energy at a higher altitude by soft particle precipitation would produce a doubling of atmospheric density without requiring excessive energy. The experimental field trip funded by the EOARD grant number FA8655-11-1-3038 of $24,000 has produced observational data that has confirmed this mechanism, though with an important limitation detailed in the report. The collaboration between Dr Carlson and the UCL team has allowed the Carlson mechanism to be properly modelled using the UCL Coupled Middle Atmosphere (CMAT2) model in order to complete the Carlson et al (2012) paper, and the observational results to be presented in a paper in preparation (Aruliah et al, 2012). In addition to supporting the Carlson mechanism, the latter paper presents the first observations of a rapid response of the thermosphere to heating, and very large (up to 200m/s), sustained (nearly 4 hours) vertical winds using co-located radars and Fabry-Perot Interferometers. What is particularly interesting about this result is that it goes against convention. The common expectation was that energetic/hard particle precipitation (>keV) would be required to produce the large energies to lift the thermosphere and increase densities at satellite altitudes. Instead it turns out that energetic particles penetrate too deeply into the atmosphere (near altitudes of 100-120km), resulting in non-dissipative Hall currents, and, furthermore, it is highly difficult to lift the long column of air that lies above. Meanwhile soft particle precipitation (~hundreds eV) that penetrates only a short distance to altitudes of 160-200km, result in Pedersen currents, and hence Joule heating, and have only to lift a short column of rarefied gas. The soft precipitation deposits relatively little energy, yet will have a dramatic effect on satellite orbits through upwelling and the consequently density perturbation. The probability of occurrence of moderately energetic particles is higher than of energetic particles, which increases the likelihood of these density perturbations. All these considerations need to be taken into account for realistic high latitude atmospheric modelling and orbit prediction

Auroral thermosphere density study

Atmospheric drag is the 2nd largest perturbation on satellite orbits, after the gravitational force of an oblate Earth. Atmospheric drag occurs where the satellite enters the top layer of the Earth’s atmosphere, the thermosphere, which means an altitude below around 300 km at solar minimum, and around 800 km at solar maximum. The concern of our project is the “lumpiness” of thermospheric density. In particular, mesoscale density structures in the polar regions are poorly represented by empirical atmospheric models used in satellite orbit determination. They do not allow for heating from electrical currents generated by the solar wind-magnetospheric dynamo in the auroral regions. Heating causes upwelling of the denser air which can be observed by Fabry-Perot Interferometers measuring the vertical winds. On either side of a heating region there is downwelling as the atmosphere relaxes back to a normal state. Physics-based models are better able to represent the meso-scale structures, but are far too slow to be useful for orbit prediction. Two nights are presented here as case studies to indicate the size of vertical winds and the horizontal extent of up and downwelling regions for geomagnetically quiet and active conditions. We also use the UCL Coupled Middle Atmosphere Thermosphere (CMAT2) model to estimate the changes in density from a localised heat source, representative of auroral plasma flows. Finally we present initial results from the PHOENIX CubeSat which was one of the QB50 constellation of CubeSats launched in the summer of 2017. PHOENIX carries a miniaturised mass spectrometer, with the aim to provide the first in-situ mass spectrometer measurements since the Dynamics Explorer in the early 1980s. The proposed objectives were: 1) Set up the FPIs to be calibrated and ready for the experiment, and in a fit state to continue monitoring the upper thermosphere and extending the dataset of auroral measurements to provide context for the experiment. 2) Carry out a 24 hour experiment using the network of Fabry-Perot Interferometers (FPIs) with the EISCAT and SuperDARN radars to provide a case study to investigate the density variation of the upper thermosphere in the auroral and polar cap region. 3) Undertake a modelling study using the UCL CMAT2 global circulation model to investigate the contribution of frictional heating in the auroral zones to the density variation. 4) Prepare a set of CMAT2 model simulations to be used to estimate the size of orbit perturbations and to compare with conventional satellite drag models. These objectives were all achieved and highlights are listed below. • Field campaign trip to the KEOPS and Sodankylä FPI sites 9-27 January 2018 • Dr Ian McWhirter consultancy for instrumental work and maintenance for the field trip • EISCAT mainland and ESR radar experiments: 1. 16UT on 28 Jan 2017 – 04UT on 29 Jan 2017 (ESR CP2 and 2. 16UT on 30 Jan 2017 – 04UT on 31 Jan 2017 • Related activities: 3 invited talks, 3 talks, 1 poster and 2 4th year undergraduate projects (see reference list) • Extended the grant period by 4 months to 14 April 2018, in order to include the first of 2 QB50 Incoherent Scatter World Day periods (January 2018

The role of neutral atmospheric dynamics in cusp density and ionospheric patch formation – 2nd campaign

This report will present observations from three field trips on Svalbard which were undertaken to test a mechanism that explains unexpected density enhancements seen by the CHAMP satellite. The CHAMP satellite observed up to double the surrounding atmospheric density in the region of the magnetic cusp at altitudes of 400km (Lühr et al., 2004). This is a significant enough perturbation to be included in satellite drag models, and consequently inspired several modelling studies. The proposed mechanism by Carlson et al. (2012) requires that soft particle precipitation increases the conductivity at 150‐200km altitude and simultaneously there should be bursts of fast plasma convection to provide strong frictional heating. Heating at this high altitude means that it requires little energy to lift the rarefied gas above, and thereby bring denser air from below into the region passed through by CHAMP (~400km). The atmospheric drag increases at altitudes where satellites orbit as a consequence of upwelling, The EOARD grant FA8655-13-1-3012 funded a field trip in January 2013 to Svalbard for a joint optical and radar experiment. This provided two case studies that test and augment the first case study from January 2012. The optical observations were provided by two University College London (UCL) Fabry-Perot Interferometers (FPIs) measuring the neutral (non-ionised) component of the upper atmosphere. Independent measurements of the ionosphere were made using the European Incoherent SCATter (EISCAT) Svalbard Radar (ESR). The radar time was won by competitive peer review from radar time awarded to the UK as part of its membership of the EISCAT consortium. Svalbard is currently the only site that passes under the magnetic cusp that is equipped with radar, optical and other suitable observational instrumentation. Further data have been sought out from the University of Oslo Meridian Scanning Photometer (MSP) and the SuperDARN coherent scatter radars and all are currently being analysed and interpreted, to be written up in a paper for submittal to the Journal of Geophysical Research (JGR). Our aim is to determine what are the conditions that caused upwelling on the nights of 22nd Jan 2012 (the first cusp upwelling experiment, reported in Aruliah et al., 2014) and on the 14th January 2013; but not on the 12th January 2013. The first experiment appeared to confirm the Carlson et al. (2012) mechanism. The second and third experiments will be a test that we have correctly identified the mechanism. The second and third experiments also have the advantage of the radar beam scanning, which was not possible for the first owing to a broken driver motor. During the period of this award, a paper by Aruliah et al. (submitted in 2013) reported the first cusp upwelling experiment in January 2012, also funded by an earlier EOARD grant. The paper is currently undergoing the refereeing process. The results of the 2012 and 2013 cusp upwelling experiments were used as part of a 3 year grant proposal to the UK Natural Environment Research Council (NERC) for a modelling and experimental study to improve the modelling of the lower thermosphere. This proposal went through to the final selection, but we heard recently that it was not funded. Over the last few years it has been realized by the atmospheric community that the middle atmosphere plays a valuable role in coupling the lower atmosphere, where weather and climate are, with the upper atmosphere, which is strongly influenced by solar variability and the solar wind, i.e. Space Weather. Investigating the cusp upwelling mechanism was one of the coupling mechanisms used in our NERC proposal and is a motivation in addition to improving satellite drag modelling

Constitutive Models for Tumour Classification

The aim of this paper is to formulate new mathematical models that will be able to differentiate not only between normal and abnormal tissues, but, more importantly, between benign and malignant tumours. We present preliminary results of a tri-phasic model and numerical simulations of the effect of cellular adhesion forces on the mechanical properties of biological tissues. We pursued the following three approaches: (i) the simulation of the time-harmonic linear elastic models to examine coarse scale effects and adhesion properties, (ii) the investigation of a tri-phasic model, with the intent of upscaling this model to determine effects of electro-mechanical coupling between cells, and (iii) the upscaling of a simple cell model as a framework for studying interface conditions at malignant cells. Each of these approaches has opened exciting new directions of research that we plan to study in the future

Evidence of meso-scale structure in the high-latitude thermosphere

International audienceThere is a widely held assumption that the thermospheric neutral gas is slow to respond to magnetospheric forcing owing to its large inertia and therefore, may be treated as a steady state background medium for the more dynamic ionosphere. This is shown to be over simplistic. The data presented here compare direct measurements of the thermospheric neutral winds made in Northern Scandinavia by Fabry-Perot Interferometers (FPIs) with direct measurements of the ionosphere made by the EISCAT radar and with model simulations. These comparisons will show that the neutral atmosphere is capable of responding to ionospheric changes on mesoscale levels, i.e., spatial and temporal scale sizes of less than a few hundred kilometres and tens of minutes, respectively

Statistical analysis of thermospheric gravity waves from Fabry-Perot Interferometer measurements of atomic oxygen

Data from the Fabry-Perot Interferometers at KEOPS (Sweden), Sodankylä (Finland), and Svalbard (Norway), have been analysed for gravity wave activity on all the clear nights from 2000 to 2006. A total of 249 nights were available from KEOPS, 133 from Sodankylä and 185 from the Svalbard FPI. A Lomb-Scargle analysis was performed on each of these nights to identify the periods of any wave activity during the night. Comparisons between many nights of data allow the general characteristics of the waves that are present in the high latitude upper thermosphere to be determined. Comparisons were made between the different parameters: the atomic oxygen intensities, the thermospheric winds and temperatures, and for each parameter the distribution of frequencies of the waves was determined. No dependence on the number of waves on geomagnetic activity levels, or position in the solar cycle, was found. All the FPIs have had different detectors at various times, producing different time resolutions of the data, so comparisons between the different years, and between data from different sites, showed how the time resolution determines which waves are observed. In addition to the cutoff due to the Nyquist frequency, poor resolution observations significantly reduce the number of short-period waves (5 h) detected. Comparisons between the number of gravity waves detected at KEOPS and Sodankylä over all the seasons showed a similar proportion of waves to the number of nights used for both sites, as expected since the two sites are at similar latitudes and therefore locations with respect to the auroral oval, confirming this as a likely source region. Svalbard showed fewer waves with short periods than KEOPS data for a season when both had the same time resolution data. This gives a clear indication of the direction of flow of the gravity waves, and corroborates that the source is the auroral oval. This is because the energy is dissipated through heating in each cycle of a wave, therefore, over a given distance, short period waves lose more energy than long and dissipate before they reach their target

High time resolution measurements of the thermosphere from Fabry-Perot Interferometer measurements of atomic oxygen

Recent advances in the performance of CCD detectors have enabled a high time resolution study of the high latitude upper thermosphere with Fabry-Perot Interferometers(FPIs) to be performed. 10-s integration times were used during a campaign in April 2004 on an FPI located in northern Sweden in the auroral oval. The FPI is used to study the thermosphere by measuring the oxygen red line emission at 630.0 nm, which emits at an altitude of approximately 240 km. Previous time resolutions have been 4 min at best, due to the cycle of look directions normally observed. By using 10 s rather than 40 s integration times, and by limiting the number of full cycles in a night, high resolution measurements down to 15 s were achievable. This has allowed the maximum variability of the thermospheric winds and temperatures, and 630.0 nm emission intensities, at approximately 240 km, to be determined as a few minutes. This is a significantly greater variability than the often assumed value of 1 h or more. A Lomb-Scargle analysis of this data has shown evidence of gravity wave activity with waves with short periods. Gravity waves are an important feature of mesospherelower thermosphere (MLT) dynamics, observed using many techniques and providing an important mechanism for energy transfer between atmospheric regions. At high latitudes gravity waves may be generated in-situ by localised auroral activity. Short period waves were detected in all four clear nights when this experiment was performed, in 630.0 nm intensities and thermospheric winds and temperatures. Waves with many periodicities were observed, from periods of several hours, down to 14 min. These waves were seen in all parameters over several nights, implying that this variability is a typical property of the thermosphere

Statistical Modeling of the Coupled F‐Region Ionosphere‐Thermosphere at High Latitude During Polar Darkness

Statistical models have been developed for predicting the behavior of the coupled high‐latitude ionosphere‐thermosphere system. The modeled parameters were the F‐layer peak electron density, plasma structuring, ion temperature, neutral temperature, and the difference between these temperatures, which is a key term in the Joule heating equation. Ionospheric measurements from the European Incoherent Scatter Svalbard Radar and neutral atmosphere measurements from the colocated University College London Fabry‐Perot Interferometers have been made across a solar cycle. These data were all acquired during nighttime conditions as the observations with the Fabry‐Perot Interferometers are restricted to such times. Various geophysical proxies were tested to represent the processes that influence the modeled parameters. The dominant geophysical proxy for each modeled parameter was then determined. Multivariate models were also developed showing the combinations of parameters that best explained the observed variability. A comparison with climatology showed that the models give an improvement in every case with skill scores based on the mean square error of up to 0.88