共查询到20条相似文献,搜索用时 31 毫秒
1.
Although the elemental composition in all parts of the solar photosphere appears to be the same this is clearly not the case
with the solar upper atmosphere (SUA). Spectroscopic studies show that in the corona elemental composition along solar equatorial
regions is usually different from polar regions; composition in quiet Sun regions is often different from coronal hole and
active region compositions and the transition region composition is frequently different from the coronal composition along
the same line of sight. In the following two issues are discussed. The first involves abundance ratios between the high-FIP
O and Ne and the low-FIP Mg and Fe that are important for meaningful comparisons between photospheric and SUA compositions
and the second involves a review of composition and time variability of SUA plasmas at heights of 1.0≤h≤1.5R
⊙. 相似文献
2.
On July 5.–6. 1983, during the EXOSAT performance verification (PV) and calibration phase, a raster scan of Cygnus X-2 was performed. In contrast to the previously observed smooth intensity variations on timescales of hours, the source revealed a behaviour unknown until now: active periods with high energy flares recurring on time scales of 300–500 s were interrupted by quiet periods of several hours. At all intensity levels the source spectra clearly require a two component continuum (blackbody + thermal bremsstrahlung). In addition, a weak iron emission line with equivalent widths between 39 an 70 eV was detected. The source has a much harder spectrum during the flares than during quiet periods, indicating drastic temperature changes within the emission region, while the absolute iron line flux does not vary. From the spectral characteristics it becomes clear that self-comptonization of the thermal bremsstrahlung spectrum plays an important role. The time variability and spectral behaviour in this peculiar state allow Cyg X-2 to be classified as a Low Mass X-ray Binary System (LMXB) very similar to the prototype of this class, Sco X-1. 相似文献
3.
S. R. Habbal 《Space Science Reviews》1994,70(1-2):37-46
The observational characteristics of the small scale magnetic structures are summarized. The temperature structure and temporal variability of the emission from coronal bright points, that pervade the source region of the solar wind in coronal holes and the quiet sun, and from active regions are shown to be remarkably similar. Particular emphasis is given to observations, potentially feasible with SOHO, that could resolve some of the outstanding issues regarding the role of the small scale magnetic structures in the energy balance and properties of the solar wind. 相似文献
4.
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
5.
The composition of the solar photosphere is believed to be uniform. Indeed a quantity that does not vary with solar surface
location or with a particular solar feature, i.e., no observational evidence is available to indicate that the photospheric
composition near the solar equator is different from the photospheric composition near the solar poles or that the photospheric
composition in quiet regions is different from the composition in active regions. In contrast, the composition of the solar
upper atmosphere is not well defined. Solar composition work in recent decades has brought the recognition that there are
systematic differences between the composition of the corona and the photosphere and revealed evidence for spatial and time
variability in the composition of various coronal features. We review the spectroscopic techniques used and the progress that
was made in recent years in deriving the plasma compositions of various solar upper atmosphere structures.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
6.
低声爆静音锥设计方法研究 总被引:1,自引:0,他引:1
声爆精确预测及低声爆设计方法已成为新一代军民用超声速飞机研制过程中必须解决的关键难题之一。基于计算流体力学(CFD)、波形参数法以及MARK-VII方法构建了高精度声爆预测方法,利用该方法对低声爆静音锥的设计展开研究。研究结果表明,静音锥的设计存在临界长度,静音锥长度小于临界长度时,静音锥产生的激波在传播过程中与机头弓形激波合并,静音锥无法起到降低声爆的作用;静音锥长度大于临界长度时,声爆水平也会略有上升。静音锥临界长度随飞行高度和飞行马赫数的变化而变化,可以根据实际飞行状态采用可伸缩设计,达到最佳的低声爆效果。多级静音锥利用多道弱激波取代机头强弓形激波,其声爆水平较单级静音锥也更低,同样,多级静音锥每一级的长度也要达到临界长度。不同静音锥头部形状产生的脱体激波形状不同,脱体距离也不同,导致阻力系数以及静音锥壁面温度有所不同,但静音锥头部形状对远场声爆信号的影响并不明显。采用静音锥的低声爆方案与原始方案比较,声爆水平得到大幅降低,阻力系数略有上升。 相似文献
7.
8.
Ya. I. Feldstein 《Space Science Reviews》1976,18(5-6):777-861
The concepts of near-pole magnetic field variations during magnetically quiet periods are explored, with special emphasis on the relationships of these variations to interplanetary magnetic field components. Methods are proposed for relating the variations which have been observed to the fields from the various sources, based on a thorough selection of reference levels. We assume that the field variations in the summer polar cap during magnetically quiet periods consist of the following components: (i) the middle-latitude S
qvariation extended to the polar region; (ii) the DPC(B
y) single-cell current system with a polar electrojet in day-side cusp latitudes; (iii) the DMC(B
z) two-cell current system of magnetospheric convection, in the form of a homogeneous current sheet in the polar cap towards the sun, with return currents through lower latitudes; (iv) the DPC(B
z) single-cell counterclockwise current system with a focus in the day-side cusp region. Quantitative relations between the near-pole variation intensities and the value and sign of the IMF azimuthal component, with a 1 hr time resolution, have been obtained and used to suggest ways of diagnosing the interplanetary magnetic field on the basis of ground observations. 相似文献
9.
There is observational evidence for an extreme fine structure in the solar transition region, much smaller than 1 in size (Dereet al., 1987, 1988) Corresponding to this extreme fine structure there appear to be an equally complex dynamical structure. We review the evidence for such dynamical extreme fine structure as demonstrated by the frequent appearance of multiple velocities, i.e. distinctly different velocities in the transition region occurring within the angular resolution element. Multiple velocities are prominent in active regions and particularly near sunspots, where velocity components may be supersonic. However, multiple velocities are frequent also in quiet regions. The consequences of such fine structure for modeling the transition region will be outlined. Finally the appropriate CDS and SUMER observations needed to extend our knowledge of a finely structured transition region and corona, spatially or in time, are discussed. 相似文献
10.
11.
An assessment is presented of the probable magnitude of ocean signals causing aliasing in ocean bottom pressure measurements
from the GRACE satellite mission. Even after modelling as much of the high frequency signal as possible, variability between
1 mbar (in quiet ocean regions) and 10 mbar (on some shelves) is likely to remain. Interpretation of the resulting retrievals
will therefore rely on the facts that the satellite sampling will average the aliasing signal to some extent, and that the
spatial patterns of aliased signal and true signal will be different. To this end, a theoretical argument is given, and supported
by model diagnostics, suggesting that observable bottom pressure signals will be strongly constrained by the shape of the
ocean floor. The modelled magnitudes offer the prospect of significant detectable signals and, while the model accuracy can
be called into question, there are hints from Earth rotation and satellite orbit measurements that significant mass redistributions
occur in the ocean. It seems certain that we will learn something new about the oceans from GRACE.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
12.
The Hard X-ray Imaging Spectrometer aboard the SMM detected gigantic arches in the corona which are formed or, if preexisting, become excited after major two-ribbon flares. They are seen in 3.5–8 keV X-rays and extend along the H ∥ = 0 line to altitudes between 105 and 2 × 105 km. These arches are stationary and form the base of a stationary type I radio noise storm initiated by the flare. They are visible in X-rays for ten hours or more and may be revived, in temperature, density, and brightness, if another two-ribbon flare appears below them. We suggest that they are built-up through reconnection process during the flare from the upper reconnected loops in the Kopp and Pneuman model. These loops become interconnected along the H ∥ = 0 line in consequence of great shear of the reconnecting loops. Obviously, the coronal transient associated with such flares must be either accomplished prior to the formation of the arch, or it must be formed through a process different from the Anzer-Kopp-Pneuman mechanism. Striking brightness variations occur quasi-periodically in the corona below and above the arch a few hours after the flare. These variations are seen at about the same time in soft X-rays, hard X-rays, and on centimeter microwaves in the low corona, as well as at metric waves in the type I noise-storm region. In spite of their flare-like intensity, however, the variations have little response in the transition layer (O v line) and no response at all in the chromosphere (Hα). We suggest that these semi-periodic brightenings are due to repetitive acceleration processes in plasmoids that encircle the arch perpendicular to the H ∥ = 0 line from the low corona through the noise storm region, being completely detached from the lower atmospheric layers. 相似文献
13.
We present a preliminary analysis of two quiet Sun transition region areas observed with the SOHO/SUMER spectrometer, using
lines from oxygen, nitrogen and silicon. The average quiet Sun physical parameters are studied as a function of line intensity.
Systematic variations of line position and width with increasing intensity are found. A large number of small-scale active
points have been observed, and preliminary analysis of the physical properties and dynamics of these active features is presented.
A jet-like structure, found in an active point, is also investigated and its velocity along the line of sight determined.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
14.
K. P. Dere 《Space Science Reviews》1994,70(1-2):21-30
Observations of transient and steady velocities at chromospheric, transition region and coronal temperatures in the quiet Sun and coronal holes are reviewed. The relevance of fine-scale structures in governing the mass balance of the solar atmosphere is stressed. At present, a coherent picture of these mass flows does not exist. However, the current observational base of transition region and coronal velocity information is limited but should greatly improve with measurements from the SOHO satellite. 相似文献
15.
The atmosphere of the Sun is characterized by a complex interplay of competing physical processes: convection, radiation, conduction, and magnetic fields. The most obvious imprint of the solar convection and its overshooting in the low atmosphere is the granulation pattern. Beside this dominating scale there is a more or less smooth distribution of spatial scales, both towards smaller and larger scales, making the Sun essentially a multi-scale object. Convection and overshooting give the photosphere its face but also act as drivers for the layers above, namely the chromosphere and corona. The magnetic field configuration effectively couples the atmospheric layers on a multitude of spatial scales, for instance in the form of loops that are anchored in the convection zone and continue through the atmosphere up into the chromosphere and corona. The magnetic field is also an important structuring agent for the small, granulation-size scales, although (hydrodynamic) shock waves also play an important role—especially in the internetwork atmosphere where mostly weak fields prevail. Based on recent results from observations and numerical simulations, we attempt to present a comprehensive picture of the atmosphere of the quiet Sun as a highly intermittent and dynamic system. 相似文献
16.
Results from a series of SOHO/Coronal Diagnostic Spectrometer (CDS) observations of coronal holes and plumes are presented,
including analysis of a low-latitude plume observed in August 1996. Spectroscopic diagnostic techniques using the CHIANTI
atomic database are applied to derive the plasma parameters: electron density, temperature, and element abundances. The results
are compared with quiet sun values. Coronal electron densities in the holes are found to be about 2 × 108 cm-3, a factor of two to three lower than in the quiet sun. The plasma thermal distribution exhibits differences between coronal
holes, the quiet sun and plumes. For example, the peak of the emission in coronal holes is at a lower temperature (T ⋍ 8 ×
105 K) than in the quiet sun (T ⋍ 1 × 106 K), while plumes are cooler (T ⋍ 7.6 × 105 K) and show a different distribution, closer to an isothermal state.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
17.
Transition region explosive events are observed throughout the quiet Sun and represent an interesting local heating phenomenon. The coronal counterparts of these events, if they exist, were not observed in a sounding rocket campaign dedicated to this objective. The coronal instrument complement on the SOHO spacecraft provides an opportunity to extend this search for the coronal counterparts of the transition region explosive events, as well as to explore the correspondence of explosive events with large scale coronal structures, such as with coronal dark lanes. 相似文献
18.
Precise measurement and spacecraft tracking are obtained by using phase-locked loops in cascade in two-way communications links. Statistics on cycle slip time are of vital importance in system planning and design. This paper presents: 1) results of a computer simulation study of the mean time to first cycle slip of cascade phase-locked loops preceded by bandpass limiters, and 2) the determination of probability distributions of cycle slip. Numerical results are obtained for a typical coherent communication system. 相似文献
19.
The AE-C spacecraft skimmed through the southern polar cusp at a 400 km altitude during a large geomagnetic storm on September 21, 1977. This period has been designated as a special IMS period, and the AE-C data were acquired close to the times that data were acquired by the DMSP satellite at nearly the same location over the southern polar cap, and by the GEOS satellite located near the noon-meridian in the northern hemisphere. Low energy electrons (1-500 eV) were measured with the photoelectron spectrometer experiment experiment onboard AE-C. This instrument was operated in the mode which measured precipitating electron fluxes and backscattered electron fluxes in alternating 4s intervals with two sensors. A region of intense precipitating electron fluxes was observed near 0924 UT on September 21, 1977 extending from 69 degree invariant latitude at 1100 MLT to 72 degree invariant latitude at 1152 MLT. From the spectra of the precipitating electrons, this region is identified as the southern polar cusp. Since the K
p equals 7- during this time, the displacement of the cusp down to these low latitudes is not unreasonable. Particle data obtained from the DMSP satellite on orbits close to AE-C, confirm that the position of the cusp was rapidly changing during this period, and was displaced to latitudes equatorward of the quiet time position. A second region of intense fluxes of precipitating electron was observed by AE-C at approximately 0933 UT from 69 degree invariant latitude near 1700 MLT to 66 degree invariant latitude near 1730 MLT. This region of low energy electron fluxes is characterized by slightly harder energy spectra and is interpreted as being the afternoon auroral zone. The remarkable and fortunate location of the AE-C, DMSP, and GEOS spacecraft during this special IMS period will allow future correlative studies aimed at the determination of the shape of the magnetosphere during very disturbed conditions. 相似文献
20.
Spiro K. Antiochos 《Space Science Reviews》1994,70(1-2):143-148
Although static loop models are often used to describe the structure of coronal loops, it is evident on both observational and theoretical grounds that mass motions play a crucial role in the physics of the corona and transition region. First we review the observations of emission-line broadening and wavelength shifts, which imply the presence of random motions and systematic downflows in coronal loops. Some discrepancies in the observations are discussed. It is argued that velocities due to gas pressure gradients are the most likely explanation for the observed flows. A number of models that have been proposed for these motions are reviewed. The implications of the various models on observations of the corona and transition region by SOHO are discussed. 相似文献