532 research outputs found
Variations of high-latitude geomagnetic pulsation frequencies: a comparison of time-of-flight estimates and IMAGE magnetometer observations
No full textThe fundamental eigenfrequencies of standing Alfvén waves on closed geomagnetic field lines are estimated for the region spanning 5.9≤L < 9.5 over all MLT (Magnetic Local Time). The T96 magnetic field model and a realistic empirical plasma mass density model are employed using the time-of-flight approximation, refining previous calculations that assumed a relatively simplistic mass density model. An assessment of the implications of using different mass density models in the time-of-flight calculations is presented. The calculated frequencies exhibit dependences on field line footprint magnetic latitude and MLT, which are attributed to both magnetic field configuration and spatial variations in mass density. In order to assess the validity of the time-of-flight calculated frequencies, the estimates are compared to observations of FLR (Field Line Resonance) frequencies. Using IMAGE (International Monitor for Auroral Geomagnetic Effects) ground magnetometer observations obtained between 2001 and 2012, an automated FLR identification method is developed, based on the cross-phase technique. The average FLR frequency is determined, including variations with footprint latitude and MLT, and compared to the time-of-flight analysis. The results show agreement in the latitudinal and local time dependences. Furthermore, with the use of the realistic mass density model in the time-of-flight calculations, closer agreement with the observed FLR frequencies is obtained. The study is limited by the latitudinal coverage of the IMAGE magnetometer array, and future work will aim to extend the ground magnetometer data used to include additional magnetometer arrays.</p
Seasonal and temporal variations of field-aligned currents and ground magnetic deflections during substorms
No full textField-aligned currents (FACs), also known as Birkeland currents, are the agents by which energy and momentum are transferred to the ionosphere from the magnetosphere and solar wind. This coupling is enhanced at substorm onset through the formation of the substorm current wedge. Using FAC data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment and substorm expansion phase onsets identified using the Substorm Onsets and Phases from Indices of the Electrojet technique, we examine the Northern Hemisphere FACs in all local time sectors with respect to substorm onset and subdivided by season. Our results show that while there is a strong seasonal dependence on the underlying FACs, the increase in FACs following substorm onset only varies by 10% with season, with substorms increasing the hemispheric FACs by 420 kA on average. Over an hour prior to substorm onset, the dayside currents in the postnoon quadrant increase linearly, whereas the nightside currents show a linear increase starting 20-30 min before onset. After onset, the nightside Region 1, Region 2, and nonlocally closed currents and the SuperMAG AL (SML) index follow the Weimer (1994, https://doi.org/10.1029/93JA02721) model with the same time constants in each season. These results contrast earlier contradictory studies that indicate that substorms are either longer in the summer or decay faster in the summer. Our results imply that, on average, substorm FACs do not change with season but that their relative impact on the coupled magnetosphere-ionosphere system does due to the changes in the underlying currents.</p
Energization of the ring current by substorms
No full textThe substorm process releases large amounts of energy into the magnetospheric system, although where the energy is transferred to and how it is partitioned remains an open question. In this study, we address whether the substorm process contributes a significant amount of energy to the ring current. The ring current is a highly variable region, and understanding the energization processes provides valuable insight into how substorm-ring current coupling may contribute to the generation of storm conditions and provide a source of energy for wave driving. In order to quantify the energy input into the ring current during the substorm process, we analyze Radiation Belt Storm Probes Ion Composition Experiment and Helium Oxygen Proton Electron ion flux measurements for H+, O+, and He+. The energy content of the ring current is estimated and binned spatially for L and magnetic local time. The results are combined with an independently derived substorm event list to perform a statistical analysis of variations in the ring current energy content with substorm phase. We show that the ring current energy is significantly higher in the expansion phase compared to the growth phase, with the energy enhancement persisting into the substorm recovery phase. The characteristics of the energy enhancement suggest the injection of energized ions from the tail plasma sheet following substorm onset. The local time variations indicate a loss of energetic H+ ions in the afternoon sector, likely due to wave-particle interactions. Overall, we find that the average energy input into the ring current is ∼9% of the previously reported energy released during substorms.</p
A new technique for determining Substorm Onsets and Phases from Indices of the Electrojet (SOPHIE)
No full textWe present a new quantitative technique that determines the times and durations of substorm expansion and recovery phases and possible growth phases based on percentiles of the rate of change of auroral electrojet indices. By being able to prescribe different percentile values, we can determine the onset and duration of substorm phases for smaller or larger variations of the auroral index or indeed any auroral zone ground-based magnetometer data. We apply this technique to the SuperMAG AL (SML) index and compare our expansion phase onset times with previous lists of substorm onsets. We find that more than 50% of events in previous lists occur within 20 min of our identified onsets. We also present a comparison of superposed epoch analyses of SML based on our onsets identified by our technique and existing onset lists and find that the general characteristics of the substorm bay are comparable. By prescribing user-defined thresholds, this automated, quantitative technique represents an improvement over any visual identification of substorm onsets or indeed any fixed threshold method
Measurements of turbulence at stratocumulus top
No full textUsing 1m resolution airborne data from research flights we divide stratocumulus top region into sublayers and characterize properties of turbulence in each sublayer. Results indicate, that there are no clear differences of turbulence properties between thermodynamically different "calassical" and "non-classical" stratocumulus regimes, but there are clear signs of turbulence ainsotropy in stably stratified sublayers in the cloud top region
Application-oriented link adaptation for IEEE 802.11
No full textThe thesis addresses the ever-persistent problem of quality guarantees in wireless data networks. The quality guarantees are especially important for real-time streaming applications. While in fixed links most of the quality issues could be, and are still resolved by over-provisioning, in the world of wireless communications things get complicated. The main reason for wireless links being a bottleneck in terms of both achievable throughput and packet loss and delay guarantees, is the resource limitation that will always be there. Our work showed that while there is no magic solution that can resolve all link quality-related problems in wireless networks, a lot could be done to mitigate them. The first, and the most important, is to make the radio as adaptive to the changes of the link conditions as possible. This can be done through our advanced hybrid rate-control algorithm. The algorithm combines a stable throughput-based solution with a rapid link-quality feedback supplement into a novel controller that has both swift response and stable performance. The second solution, which is dependant on the first one, is the propagation of link status-related information from the link layer to the application. The latter can use this feedback to change its data stream properties to match those of the underlying wireless link. This behaviour proves most useful in cases when the radio cannot compensate for a deterioration of the radio connection such as a significant drop of the signal strength due to obstruction, or too many users competing to use the same medium.Electrical Engineering, Mathematics and Computer Scienc
Co-creatie in het publieke domein: Een vanzelfsprekendheid
No full textIndustrial DesignIndustrial Design Engineerin
Joint ground/space observations of transient phenomena associated with the Polar cusp
No full textSIGLEAvailable from British Library Document Supply Centre-DSC:DXN045147 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
Bubble velocity measurements using ultrasound
No full textApplied SciencesKramers Laboratorium voor Fysische Technologi
Experimental study on tranisitio characteristics of natural-concvection flow
No full textApplied Science
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