Stochastic acceleration in the heliosheath appears to be a likely mechanism by which Anomalous Cosmic Rays (ACRs) are accelerated. However, most stochastic acceleration mechanisms are not appropriate. The energy density in the ACRs and in the interstellar pickup ions out of which the ACRs are accelerated greatly exceeds the energy density in the turbulence in the heliosheath. Thus, a traditional stochastic acceleration mechanism in which particles are accelerated by damping the turbulence will not work. A stochastic acceleration mechanism has been developed in which the total energy of the pickup ions and the ACRs is conserved. Energy is redistributed from the core pickup ions into a suprathermal tail to create the ACRs. A model for the acceleration of the ACRs in the heliosheath, based on this stochastic acceleration mechanism, is presented. The model provides reasonable fits to the spectra of suprathermal particles and ACRs observed by Voyager. 相似文献
Mounting observational evidence of the emergence of twisted magnetic flux tubes through the photosphere have now been published. Such flux tubes, formed by the solar dynamo and transported through the convection zone, eventually reach the solar atmosphere. Their accumulation in the solar corona leads to flares and coronal mass ejections. Since reconnections occur during the evolution of the flux tubes, the concepts of twist and magnetic stress become inappropriate. Magnetic helicity, as a well preserved quantity, in particular in plasma with high magnetic Reynolds number, is a more suitable physical quantity to use, even if reconnection is involved. 相似文献
In accordance with the United Nations Outer Space Treaties [United Nations, Agreement Governing the Activities of States on the Moon and Other Celestial Bodies, UN doc A/RES/34/68, resolution 38/68 of December 1979], currently maintained and promulgated by the Committee on Space Research [COSPAR Planetary Protection Panel, Planetary Protection Policy accepted by the COSPAR Council and Bureau, 20 October 2002, amended 24 March 2005, http://www.cosparhq.org/scistr/PPPolicy.htm], missions exploring the Solar system must meet planetary protection requirements. Planetary protection aims to protect celestial bodies from terrestrial contamination and to protect the Earth environment from potential biological contamination carried by returned samples or space systems that have been in contact with an extraterrestrial environment. From an exobiology perspective, Mars is one of the major targets, and several missions are currently in operation, in transit, or scheduled for its exploration. Some of them include payloads dedicated to the detection of life or traces of life. The next step, over the coming years, will be to return samples from Mars to Earth, with a view to increasing our knowledge in preparation for the first manned mission that is likely to take place within the next few decades. Robotic missions to Mars shall meet planetary protection specifications, currently well documented, and planetary protection programs are implemented in a very reliable manner given that experience in the field spans some 40 years. With regards to sample return missions, a set of stringent requirements has been approved by COSPAR [COSPAR Planetary Protection Panel, Planetary Protection Policy accepted by the COSPAR Council and Bureau, 20 October 2002, amended 24 March 2005, http://www.cosparhq.org/scistr/PPPolicy.htm], and technical challenges must now be overcome in order to preserve the Earth’s biosphere from any eventual contamination risk. In addition to the human dimension of the mission, sending astronauts to Mars will entail meeting all these constraints. Astronauts present huge sources of contamination for Mars and are also potential carriers of biohazardous material on their return to Earth. If they were to have the misfortune of being contaminated, they themselves would become a biohazard, and, as a consequence, in addition to the technical constraints, human and ethical considerations must also be taken into account. 相似文献
A postdetection design methodology for a multiple high-pulse-repetition frequency (PRF) pulse Doppler radar has been developed. The postdetection processor consists of an M out of N detector where range and target ambiguities are resolved, followed by a square-law detector which enhances the minimum signal-to-noise (S/N) power-ratio per pulse burst performance. For given probabilities of false alarm and detection, formulas are derived from which the three thresholds associated with the cascaded detector can be found. Fundamental tradeoffs between the minimum S/N required, number of ghosts, and the number of operations (NOPs) that the cascaded detector must perform are identified. It is shown that the NOPs and the number of ghosts increase and the minimum S/N required decreases as the binary M out of N detector passes more detections to the square-law detector 相似文献
Since November 1978 a set of total solar irradiance (TSI) measurements from space is available, yielding a time series of
more than 25 years. Presently, there are three TSI composites available, called PMOD, ACRIM and IRMB, which are all constructed
from the same original data, but use different procedures to correct for sensitivity changes. The PMOD composite is the only
one which also corrects the early HF data for degradation. The results from the detailed analysis of the VIRGO radiometry
allow a good understanding of the effects influencing the long-term behaviour of classical radiometers in space. Thus, a re-analysis
of the behaviour of HF/NIMBUS-7 and ACRIM-I/SMM was indicated. For the former the situation is complicated by the fact that
there are no in-flight means to determine changes due to exposure to solar radiation by comparison with a less exposed radiometer
on the same spacecraft. The geometry and optical property of the cavity of HF is, however, very similar to the PMO6-type radiometers,
so the behaviour of the PMO6V radiometers on VIRGO can be used as a model. ACRIM-I had to be revised mainly due to a henceforth
undetected early increase and a more detailed analysis of its degradation. The results are not only important for solar radiometry
from space, but they also provide a more reliable TSI during cycle 21. The differences between the revised PMOD composite
and the ACRIM and IRMB are discussed by comparison with a TSI reconstruction from Kitt-Peak magnetograms. As the PMOD composite
is the only one which has reliable data for cycle 21, the behaviour of the three solar cycles can now be compared and the
similarities and differences discussed. 相似文献
Auroral emission caused by electron precipitation (Hardy et al., 1987, J. Geophys. Res. 92, 12275–12294) is powered by magnetospheric driving processes. It is not yet fully understood how the energy transfer mechanisms
are responsible for the electron precipitation. It has been proposed (Hasegawa, 1976, J. Geophys. Res. 81, 5083–5090) that Alfvén waves coming from the magnetosphere play some role in powering the aurora (Wygant et al., 2000, J. Geophys. Res. 105, 18675–18692, Keiling et al., 2003, Science299, 383–386). Alfvén-wave-induced electron acceleration is shown to be confined in a rather narrow radial distance range of
4–5 RE (Earth radii) and its importance, relative to other electron acceleration mechanisms, depends strongly on the magnetic disturbance
level so that it represents 10% of all electron precipitation power during quiet conditions and increased to 40% during disturbed
conditions. Our observations suggest that an electron Landau resonance mechanism operating in the “Alfvén resonosphere” is
responsible for the energy transfer. 相似文献
It is widely accepted that diffusive shock acceleration is an important process in the heliosphere, in particular in producing
the energetic particles associated with interplanetary shocks driven by coronal mass ejections. In its simplest formulation
shock acceleration is expected to accelerate ions with higher mass to charge ratios less efficiently than those with lower
mass to charge. Thus it is anticipated that the Fe/O ratio in shock-accelerated ion populations will decrease with increasing
energy above some energy. We examine the circumstances of five interplanetary shocks that have been reported to have associated
populations in which Fe/O increases with increasing energy. In each event, the situation is complex, with particle contributions
from other sources in addition to the shock. Furthermore, we show that the Fe/O ratio in shock-accelerated ions can decrease
even when the shock is traveling through an Fe-rich ambient ion population. Thus, although shock acceleration of an Fe-rich
suprathermal population has been proposed to explain large Fe-rich solar particle events, we find no support for this proposal
in these observations. 相似文献
A Time-Delay Integration (TDI) image acquisition and processing system has been developed to capture ICON’s Far Ultraviolet (FUV) Spectrographic Imager data. The TDI system is designed to provide variable-range motion-compensated imaging of Earth’s nightside ionospheric limb and sub-limb scenes viewed from Low Earth Orbit in the 135.6 nm emission of oxygen with an integration time of 12 seconds. As a pre-requisite of the motion compensation the TDI system is also designed to provide corrections for optical distortions generated by the FUV Imager’s optical assembly. On the dayside the TDI system is used to process 135.6 nm and 157.0 nm wavelength altitude profiles simultaneously. We present the TDI system’s design methodology and implementation as an FPGA module with an emphasis on minimization of on-board data throughput and telemetry. We also present the methods and results of testing the TDI system in simulation and with Engineering Ground Support Equipment (EGSE) to validate its performance.