共查询到20条相似文献，搜索用时 265 毫秒
1.
Milan Maksimovic Viviane Pierrard Joseph Lemaire 《Astrophysics and Space Science》2001,277(1-2):181-187
We present the basics of the exospheric models of the solar wind acceleration. In these models the plasma is assumed fully
collisionless above a typical altitude in the corona. The solar wind is accelerated by the interplanetary electrostatic potential
which is needed to warrant the equality of the proton and electron fluxes. These models suggest that the fast wind emanating
from the polar regions could be due to the presence of non-thermal electron distributions in the corona.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
3.
J. G. Luhmann C. O. Lee Yan Li C. N. Arge A. B. Galvin K. Simunac C. T. Russell R. A. Howard G. Petrie 《Solar physics》2009,256(1-2):285-305
The declining phases of solar cycles are known for their high speed solar wind streams that dominate the geomagnetic responses during this period. Outstanding questions about these streams, which can provide the fastest winds of the solar cycle, concern their solar origins, persistence, and predictability. The declining phase of cycle 23 has lasted significantly longer than the corresponding phases of the previous two cycles. Solar magnetograph observations suggest that the solar polar magnetic field is also ～?2?–?3 times weaker. The launch of STEREO in late 2006 provided additional incentive to examine the origins of what is observed at 1 AU in the recent cycle, with the OMNI data base at the NSSDC available as an Earth/L1 baseline for comparisons. Here we focus on the year 2007 when the solar corona exhibited large, long-lived mid-to-low latitude coronal holes and polar hole extensions observed by both SOHO and STEREO imagers. STEREO provides in situ measurements consistent with rigidly corotating solar wind stream structure at up to ～?45° heliolongitude separation by late 2007. This stability justifies the use of magnetogram-based steady 3D solar wind models to map the observed high speed winds back to their coronal sources. We apply the WSA solar wind model currently running at the NOAA Space Weather Prediction Center with the expectation that it should perform its best at this quiet time. The model comparisons confirm the origins of the observed high speed streams expected from the solar images, but also reveal uncertainties in the solar wind source mapping associated with this cycle’s weaker solar polar fields. Overall, the results illustrate the importance of having accurate polar fields in synoptic maps used in solar wind forecast models. At the most fundamental level, they demonstrate the control of the solar polar fields over the high speed wind sources, and thus one specific connection between the solar dynamo and the solar wind character. 相似文献
4.
Liping Yang Xueshang Feng Changqing Xiang Shaohua Zhang S. T. Wu 《Solar physics》2011,271(1-2):91-110
The observations both near the Sun and in the heliosphere during the activity minimum between solar cycles 23 and 24 exhibit different phenomena from those typical of the previous solar minima. In this paper, we have chosen Carrington rotation 2070 in 2008 to investigate the properties of the background solar wind by using the three-dimensional (3D) Solar?CInterPlanetary Conservation Element/Solution Element Magnetohydrodynamic (MHD) model. We also study the effects of polar magnetic fields on the characteristics of the solar corona and the solar wind by conducting simulations with an axisymmetric polar flux added to the observed magnetic field. The numerical results are compared with the observations from multiple satellites, such as the Solar and Heliospheric Observatory (SOHO), Ulysses, Solar Terrestrial Relations Observatory (STEREO), Wind and the Advanced Composition Explorer (ACE). The comparison demonstrates that the first simulation with the observed magnetic fields reproduces some observed peculiarities near the Sun, such as relatively small polar coronal holes, the presence of mid- and low-latitude holes, a tilted and warped current sheet, and the broad multiple streamers. The numerical results also capture the inconsistency between the locus of the minimum wind speed and the location of the heliospheric current sheet, and predict slightly slower and cooler polar streams with a relatively smaller latitudinal width, broad low-latitude intermediate-speed streams, and globally weak magnetic field and low density in the heliosphere. The second simulation with strengthened polar fields indicates that the weak polar fields in the current minimum play a crucial role in determining the states of the corona and the solar wind. 相似文献
5.
Ko Yuan-Kuen Fisk Lennard A. Geiss Johannes Gloeckler George Guhathakurta Madhulika 《Solar physics》1997,171(2):345-361
The solar wind ions flowing outward through the solar corona generally have their ionic fractions freeze-in within 5 solar radii. The altitude where the freeze-in occurs depends on the competition between two time scales: the time over which the wind flows through a density scale height, and the time over which the ions achieve ionization equilibrium. Therefore, electron temperature, electron density, and the velocity of the ions are the three main physical quantities which determine the freeze-in process, and thus the solar wind ionic charge states. These physical quantities are determined by the heating and acceleration of the solar wind, as well as the geometry of the expansion. In this work, we present a parametric study of the electron temperature profile and velocities of the heavy ions in the inner solar corona. We use the ionic charge composition data observed by the SWICS experiment on Ulysses during the south polar pass to derive empirically the electron temperature profile in the south polar coronal hole. We find that the electron temperature profile in the solar inner corona is well constrained by the solar wind charge composition data. The data also indicate that the electron temperature profile must have a maximum within 2 solar radii. We also find that the velocities of heavy ions in their freeze-in regions are small (<100 km s-1) and different elements must flow at different velocities in the inner corona. 相似文献
6.
I. S. Veselovsky A. V. Dmitriev A. V. Suvorova M. V. Tarsina 《Journal of Astrophysics and Astronomy》2000,21(3-4):423-429
The cyclic evolution of the heliospheric plasma parameters is related to the time-dependent boundary conditions in the solar
corona. &#x201C;Minimal&#x201D; coronal configurations correspond to the regular appearance of the tenuous, but hot
and fast plasma streams from the large polar coronal holes. The denser, but cooler and slower solar wind is adjacent to coronal
streamers. Irregular dynamic manifestations are present in the corona and the solar wind everywhere and always. They follow
the solar activity cycle rather well. Because of this, the direct and indirect solar wind measurements demonstrate clear variations
in space and time according to the minimal, intermediate and maximal conditions of the cycles. The average solar wind density,
velocity and temperature measured at the Earth&#x2019;s orbit show specific decadal variations and trends, which are of
the order of the first tens per cent during the last three solar cycles. Statistical, spectral and correlation characteristics
of the solar wind are reviewed with the emphasis on the cycles. 相似文献
7.
Richard Woo 《Solar physics》2007,241(2):251-261
In the absence of magnetic field measurements of the solar corona, the density structure of white-light images has provided
important insight into the coronal magnetic field. Recent work sparked by highly sensitive radio occultation measurements
of path-integrated density has elucidated the density structure of unprocessed solar eclipse pictures. This paper does the
same for processed images that reveal low-contrast small-scale structures, specifically Koutchmy’s edge-enhanced white-light
image of the 11 August 1999 solar eclipse. This processed image provides visual evidence for two important results deduced
from radio occultation measurements of small-scale density variations. First, in addition to the closed loops readily seen
at the base of the corona in high-resolution EUV and soft X-ray images, open filamentary structures permeate the corona including
active regions generally thought to be magnetically closed. Observed at the image resolution, the filamentary structures are
1° wide in latitude and an order of magnitude smaller than polar plumes. Second, although inhomogeneities that are convected
along with the solar wind are also present, filamentary structures dominate the image because of their steeper density gradients.
The quantitative profile of polarized brightness (pB) at the base of the corona shows that the filamentary structures have transverse density gradients that are proportional
to their density. This explains why edge-enhanced images, limited in sensitivity to density gradients, tend to detect filamentary
structures more readily in high-density regions (e.g., active regions, streamer stalks, and prominences) than in low-density polar coronal holes, and why filamentary structures
seem more prevalent in solar eclipse pictures during solar maximum. The pB profile at the base of the corona also fills the gap in Doppler measurements there, reinforcing that open ultra-fine-scale
filamentary structures observed by the radio measurements are predominantly radial and that they are an integral part of the
radial expansion of the solar wind. 相似文献
8.
主要论述宁静日冕洞，以及日冕加热问题的研究现状。讨论了宁静日冕的理论模型、观测模型和混合模型，以及冕洞区大气模型和太阳风加热问题。最后对计划中的日冕空间探测作了简要介绍。 相似文献
9.
In coronal holes the electron (proton) density is low, and heating of the proton gas produces a rapidly increasing proton temperature in the inner corona. In models with a reasonable electron density in the upper transition region the proton gas becomes collisionless some 0.2 to 0.3 solar radii into the corona. In the collisionless region the proton heat flux is outwards, along the temperature gradient. The thermal coupling to electrons is weak in coronal holes, so the heat flux into the transition region is too small to supply the energy needed to heat the solar wind plasma to coronal temperatures. Our model studies indicate that in models with proton heating the inward heat conduction may be so inefficient that some of the energy flux must be deposited in the transition region to produce the proton fluxes that are observed in the solar wind. If we allow for coronal electron heating, the energy that is needed in the transition region to heat the solar wind to coronal temperatures, may be supplied by heat conduction from the corona. 相似文献
10.
Y. C. Whang 《Solar physics》1983,88(1-2):343-358
A one-fluid model is employed to study the global expansion of the solar wind from a two-hole corona, under the assumptions that the holes are confined to polar caps within 30° of heliographic colatitude, the flow is steady and axisymmetric, and the geometry of streamlines is prescribed. The boundary conditions are adjusted in such a way that the calculated solar wind properties at 1 AU are in a reasonable agreement with observational results. A series of numerical solutions are obtained, the series produces a maximum terminal speed of 829 km s?1 at the pole. The calculated solar wind speeds are strongly latitude dependent and are positively correlated with local divergence factor of a stream tube. The solutions imply that most plasma properties are highly inhomogeneous at the polar caps. The flow velocity, the temperature, the proton number flux and the conduction heat flux all increase towards the hole center. 相似文献
11.
V. A. Kovalenko 《Solar physics》1981,73(2):383-403
The energy balance of open-field regions of the corona and solar wind and the influence of the flow geometry in the corona upon the density and temperature, are analyzed. It is found that the energy flux arriving at the corona is constant for the corona's open regions with different flow geometries. For the waves heating the corona and solar wind, the dependence of the absorption coefficient on the corona's plasma density is found to be within the range of distances r=1.05–1.5R
. It is shown that the wave absorption is more dependent on electron density than the coronal emission. It is this difference that causes lower-density coronal holes to be colder than quiet regions. It is found that the additional energy flux necessary for providing energy balance of the corona and for producing solar wind is a flux of Alfvén waves, which can provide the energy needed for producing quasi-stationary high-speed solar wind streams. Theoretical models of coronal holes and the question of why the high-speed solar wind streams are precisely flowing out of coronal holes, are discussed. 相似文献
12.
We consider the adequacy of various solar coronal heating models. We show that the correlation between the intensity of the coronal Fe XIV 530.5 nm green line and the calculated magnetic field strength in the solar corona can be a useful tool for this purpose. We have established this correlation for coronal structures and magnetic fields of large spatial and temporal scales. The correlation found exhibits a strong dependence on both solar cycle phase and heliolatitude. The efficiency of a particular coronal heating mechanism is probably determined by the relative area occupied by low and high loops (including open structures). The direct current models based on slow field dissipation (DC) and the wave models based on Alfvén and magnetosonic wave dissipation (AC) are more efficient in the equatorial and polar zones, respectively. 相似文献
13.
The solar wind parameters were analyzed using the concept which is being developed by the authors and assumes the existence of several systems of magnetic fields of different scales on the Sun. It was demonstrated that the simplest model with one source surface and a radial expansion does not describe the characteristics of the quiet solar wind adequately. Different magnetic field subsystems on the Sun affect the characteristics of the solar wind plasma in a different way, even changing the sign of correlation. New multiparameter schemes were developed to compute the velocity and the magnetic field components of the solar wind. The radial component of the magnetic field in the solar corona and the tilt of the heliospheric current sheet, which determines the degree of divergence of field lines in the heliosphere, were taken into account when calculating the magnetic field in the solar wind. Both the divergence of field lines in the corona and the strength of the solar magnetic field are allowed for in calculating the solar wind speed. The suggested schemes provide a considerably higher computation accuracy than that given by commonly used one-parameter models. 相似文献
14.
Coronal density, temperature, and heat-flux distributions for the equatorial and polar corona have been deduced from Saito’s model of averaged coronal white-light (WL) brightness and polarization observations. These distributions are compared with those determined from a kinetic collisionless/exospheric model of the solar corona. This comparison indicates similar distributions at large radial distances (>?7 R⊙) in the collisionless region. However, rather important differences are found close to the Sun in the acceleration region of the solar wind. The exospheric heat flux is directed away from the Sun, while that inferred from all WL coronal observations is in the opposite direction, i.e. conducting heat from the inner corona toward the chromosphere. This could indicate that the source of coronal heating extends up into the inner corona, where it maximizes at r>1.5 R⊙, well above the transition region. 相似文献
15.
16.
《Physics and Chemistry of the Earth, Part C: Solar, Terrestrial & Planetary Science》1999,24(4):415-419
Using the high-resolution mass spectrometer MTOF on board SOHO we have measured the solar wind isotopic abundance ratios of Ne, Mg, and Si in different solar wind regimes with bulk velocities ranging from 350 km/s to 650 km/s. Data indicate a systematic depletion of the heavier isotopes in the slow solar wind compared to their abundances in the fast solar wind from coronal holes. These variations in the solar wind isotopic composition represent a pure mass-dependent effect because the different isotopes of an element pass the inner corona with the same charge state distribution. The influence of particle mass on the acceleration of minor solar wind ions is discussed in the context of theoretical models and recent optical observations with other SOHO instruments. 相似文献
17.
In this paper, we consider the implications of the observed inverse correlation between solar wind speed at Earth and the expansion rate of the Sun-Earth flux tube as it passes through the corona. We find that the coronal expansion rate depends critically on the large-scale photospheric field distribution around the footpoint of the flux tube, with the smallest expansions occurring in tubes that are rooted near a local minimum in the field. This suggests that the fastest wind streams originate from regions where large coronal holes are about to break apart and from the facing edges of adjacent like-polarity holes, whose field lines converge as they transit the corona. These ideas lead to the following predictions:
- Weak holes and fragmentary holes can be sources of very fast wind.
- Fast wind with steep latitudinal gradients may be generated where the field lines from the polar hole and a lower-latitude hole of like polarity converge to form a mid-latitude ‘apex’.
- The fastest polar wind should occur shortly after sunspot maximum, when trailing-polarity flux converges onto the poles and begins to establish the new polar fields.
18.
V.A. Kovalenko 《Planetary and Space Science》1988,36(12):1343-1358
An analysis has been made of the origin of long-term variations in flux density of solar wind particles (nv) for different velocity regimes. The study revealed a relationship of these variations to the area of the polar coronal holes (CH). It is shown that within the framework of the model under development, the main longterm variations of nv are a result of the latitude redistribution of the solar wind mass flux in the heliosphere and are due to changes in the large-scale geometry of the solar plasma flow in the corona.
A study has been made of the variations of nv for high speed solar wind streams. It is found that nv in high speed streams which are formed in CH, decreases from minimum to maximum solar activity. The analysis indicates that this decrease is attributable to the magnetic field strength increase in coronal holes.
It has been found that periods of rapid global changes of background magnetic fields on the Sun are accompanied by a reconfiguration of coronal magnetic fields, rapid changes in the length of quiescent filaments, and by an increase in the density of the particle flux of a high speed solar wind. It has been established that these periods precede the formation of CH, corresponding to the increase in solar wind velocity near the Earth and to enhancement of the level of geomagnetic disturbance. 相似文献
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20.
The electron distribution functions from the solar corona to the solar wind are determined in this paper by considering the
effects of the external forces, of Coulomb collisions and of the wave – particle resonant interactions in the plasma wave
turbulence. The electrons are assumed to be interacting with right-handed polarized waves in the whistler regime. The acceleration
of electrons in the solar wind seems to be mainly due to the electrostatic potential. Wave turbulence determines the electron
pitch-angle diffusion and some characteristics of the velocity distribution function (VDF) such as suprathermal tails. The
role of parallel whistlers can also be extended to small altitudes in the solar wind (the acceleration region of the outer
corona), where they may explain the energization and the presence of suprathermal electrons. 相似文献