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81.
This study investigated novices' “lived experiences” of navigation within the sport of orienteering from an enactive and phenomenological approach. The objective was to qualitatively characterize elements of task-related situations that were meaningful for orienteers. The results showed that the participants continuously made judgments about the reliability of their estimations about whether they were on “the right route” on the course. When the participants judged that they were only approximately on the right route or were unable to locate themselves, elements of the situation other than map and terrain features became meaningful for them. These results demonstrate that, for novice orienteers, navigation activity must extend beyond navigation as a logical, computational way-finding problem to include embodied, social, cultural and situated dimensions. 相似文献
82.
83.
Martin Tluczykont Daniel Hampf Dieter Horns Tanja Kneiske Robert Eichler Rayk Nachtigall Gavin Rowell 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
The question of the origin of cosmic rays and other questions of astroparticle and particle physics can be addressed with indirect air-shower observations above 10 TeV primary energy. We propose to explore the cosmic ray and γ-ray sky (accelerator sky) in the energy range from 10 TeV to 1 EeV with the new ground-based large-area wide angle (ΔΩ ∼ 0.85 sterad) air-shower detector HiSCORE (Hundred∗i Square-km Cosmic ORigin Explorer). The HiSCORE detector is based on non-imaging air-shower Cherenkov light-front sampling using an array of light-collecting stations. A full detector simulation and basic reconstruction algorithms have been used to assess the performance of HiSCORE. First prototype studies for different hardware components of the detector array have been carried out. The resulting sensitivity of HiSCORE to γ-rays will be comparable to CTA at 50 TeV and will extend the sensitive energy range for γ-rays up to the PeV regime. HiSCORE will also be sensitive to charged cosmic rays between 100 TeV and 1 EeV. 相似文献
84.
Bela G. Fejer 《Space Science Reviews》2011,158(1):145-166
The low latitude ionosphere is strongly affected by several highly variable electrodynamic processes. Over the last two decades
ground-based and satellite measurements and global numerical models have been extensively used to study the longitude-dependent
climatology of low latitude electric fields and currents. These electrodynamic processes and their ionospheric effects exhibit
large ranges of temporal and spatial variations during both geomagnetic quiet and disturbed conditions. Numerous recent studies
have investigated the short term response of equatorial electric fields and currents to lower atmospheric transport processes
and solar wind-magnetosphere driving mechanisms. This includes the large electric field and current perturbations associated
with arctic sudden stratospheric warming events during geomagnetic quiet times and highly variable storm time prompt penetration
and ionospheric disturbance dynamo effects. In this review, we initially describe recent experimental and numerical modeling
results of the global climatology and short term variability of quiet time low latitude electrodynamic plasma drifts. Then,
we examine the present understanding of equatorial electric field and current perturbation fields during periods of enhanced
geomagnetic activity. 相似文献
85.
Transfers with a low thrust are considered under constraints imposed on the thrust vector direction. These constraints can be caused by peculiarities of the attitude control system and the mode of stabilization of a spacecraft, and, in the general case, they are functions of the time and state vector. The constraints specified by equalities and inequalities are investigated. It is shown that the optimal thrust is directed along the projection of the Lawden’s primer vector onto the restricting set. 相似文献
86.
Deuterium fractionations in cometary ices provide important clues to the origin and evolution of comets. Mass spectrometers
aboard spaceprobe Giotto revealed the first accurate D/H ratios in the water of Comet 1P/Halley. Ground-based observations
of HDO in Comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp), the detection of DCN in Comet Hale-Bopp, and upper limits
for several other D-bearing molecules complement our limited sample of D/H measurements. On the basis of this data set all
Oort cloud comets seem to exhibit a similar
ratio in H2O, enriched by about a factor of two relative to terrestrial water and approximately one order of magnitude relative to the
protosolar value. Oort cloud comets, and by inference also classical short-period comets derived from the Kuiper Belt cannot
be the only source for the Earth's oceans. The cometary O/C ratio and dynamical reasons make it difficult to defend an early
influx of icy planetesimals from the Jupiter zone to the early Earth. D/H measurements of OH groups in phyllosilicate rich
meteorites suggest a mixture of cometary water and water adsorbed from the nebula by the rocky grains that formed the bulk
of the Earth may be responsible for the terrestrial D/H. The D/H ratio in cometary HCN is 7 times higher than the value in
cometary H2O. Species-dependent D-fractionations occur at low temperatures and low gas densities via ion-molecule or grain-surface reactions and cannot be explained by a pure solar nebula chemistry. It is plausible that cometary
volatiles preserved the interstellar D fractionation. The observed D abundances set a lower limit to the formation temperature
of (30 ± 10) K. Similar numbers can be derived from the ortho-to-para ratio in cometary water, from the absence of neon in
cometary ices and the presence of S2. Noble gases on Earth and Mars, and the relative abundance of cometary hydrocarbons place the comet formation temperature
near 50 K. So far all cometary D/H measurements refer to bulk compositions, and it is conceivable that significant departures
from the mean value could occur at the grain-size level. Strong isotope effects as a result of coma chemistry can be excluded
for molecules H2O and HCN. A comparison of the cometary
ratio with values found in the atmospheres of the outer planets is consistent with the long-held idea that the gas planets
formed around icy cores with a high cometary D/H ratio and subsequently accumulated significant amounts of H2 from the solar nebula with a low protosolar D/H.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
87.
At the ionospheric level, the substorm onset (expansion phase) is marked by the initial brightening and subsequent breakup of a pre-existing auroral arc. According to the field line resonance (FLR) wave model, the substorm-related auroral arc is caused by the field-aligned current carried by FLRs. The FLRs are standing shear Alfvén wave structures that are excited along the dipole/quasi-dipole lines of the geomagnetic field. The FLRs (that can cause auroral arc) thread from the Earthward edge of the plasma sheet and link the auroral arc to the plasma sheet region of 6–15 R
E. The region is associated with magnetic fluctuations that result from the nonlinear wave-wave interactions of the cross-field current-instability. The instability (excited at the substorm onset) disrupts the cross-tail current which is built up during the growth phase of the substorms and results in magnetic fluctuations. The diversion of the current to polar regions can lead to auroral arc intensification. The current FLR model is based on the amplitude equations that describe the nonlinear space-time evolution of FLRs in the presence of ponderomotive forces exerted by large amplitude FLRs (excited during substorms). The present work will modify the FLR wave model to include the effects arising from magnetic fluctuations that result from current disruption near the plasma sheet (6–15 R
E). The nonlinear evolution of FLRs is coupled with the dynamics of plasma sheet through a momentum exchange term (resulting from magnetic fluctuations due to current disruption) in the generalized Ohm's law. The resulting amplitude equations including the effects arising from magnetic fluctuations can be used to study the structure of the auroral arcs formed during substorms. We have also studied the role of feedback mechanism (in a dipole geometry of the geomagnetic field) in the formation of the discrete auroral arc observed on the nightside magnetosphere. The present nonlinear dispersive model (NDM) is extended to include effects arising from the low energy electrons originating from the plasma sheet boundary layer. These electrons increase the ionospheric conductivity in a localized patch and enhance the field-aligned current through a feedback mechanism. The feedback effects were studied numerically in a dipole geometry using the the NDM. The numerical studies yield the magnitude of the field-aligned current that is large enough to form a discrete auroral arc. Our studies provide theoretical support to the observational work of Newell et al. that the feedback instability plays a major role in the formation of the discrete auroral arcs observed on the nightside magnetosphere. 相似文献
88.
P. M. E. Décréau P. Fergeau V. Krannosels'kikh M. Lévêque Ph. Martin O. Randriamboarison F. X. Sené J. G. Trotignon P. Canu P. B. Mögensen 《Space Science Reviews》1997,79(1-2):157-193
The WHISPER sounder on the Cluster spacecraft is primarily designed to provide an absolute measurement of the total plasma density within the range 0.2–80 cm-3. This is achieved by means of a resonance sounding technique which has already proved successful in the regions to be explored. The wave analysis function of the instrument is provided by FFT calculation. Compared with the swept frequency wave analysis of previous sounders, this technique has several new capabilities. In particular, when used for natural wave measurements (which cover here the 2–80 kHz range), it offers a flexible trade-off between time and frequency resolutions. In the basic nominal operational mode, the density is measured every 28 s, the frequency and time resolution for the wave measurements are about 600 Hz and 2.2 s, respectively. Better resolutions can be obtained, especially when the spacecraft telemetry is in burst mode. Special attention has been paid to the coordination of WHISPER operations with the wave instruments, as well as with the low-energy particle counters. When operated from the multi-spacecraft Cluster, the WHISPER instrument is expected to contribute in particular to the study of plasma waves in the electron foreshock and solar wind, to investigations about small-scale structures via density and high-frequency emission signatures, and to the analysis of the non-thermal continuum in the magnetosphere. 相似文献
89.
The dynamic response of a 1-dimensional plasma diode to an applied step voltage is studied during a few electron transit times by numerical simulations when the initial state has an ion density minimum (an ion density cavity). Depending on the cavity depth and the applied voltage the potential drop distributes over the cavity or concentrates in a cathode sheath. The transistion between the two states as well as the cavity potential profiles are predicted by an analytical model. Simulations with periodic cavities as initial state show that the applied voltage can be shared between the cavities. A double layer, steady on the ion time scale, is created by introducing a steady cavity by ion losses. 相似文献
90.
Matthew A. Siegler Suzanne E. Smrekar Matthias Grott Sylvain Piqueux Nils Mueller Jean-Pierre Williams Ana-Catalina Plesa Tilman Spohn 《Space Science Reviews》2017,211(1-4):259-275
The 2018 InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) Mission has the mission goal of providing insitu data for the first measurement of the geothermal heat flow of Mars. The Heat Flow and Physical Properties Package (HP3) will take thermal conductivity and thermal gradient measurements to approximately 5 m depth. By necessity, this measurement will be made within a few meters of the lander. This means that thermal perturbations from the lander will modify local surface and subsurface temperature measurements. For HP3’s sensitive thermal gradient measurements, this spacecraft influence will be important to model and parameterize. Here we present a basic 3D model of thermal effects of the lander on its surroundings. Though lander perturbations significantly alter subsurface temperatures, a successful thermal gradient measurement will be possible in all thermal conditions by proper (\(>3~\mbox{m}\) depth) placement of the heat flow probe. 相似文献