共查询到20条相似文献，搜索用时 46 毫秒
1.
P. Venkatakrishnan 《Astrophysics and Space Science》1996,243(1):43-47
The mechanisms that could possibly heat the corona are briefly reviewed with emphasis on their observability. Observing enhanced wave flux at footpoints of active regions would confirm wave heating. Observation of nonthermal electrons in tiny coronal events (nanoflares) would confirm dissipation of current sheets. Presence of large scale flows in coronal arcades would underline the importance of turbulent resistivity for coronal heating. A comparison of HeI absorption in quiet and active regions demonstrates the difficulty of interpreting data that connect chromospheric dynamics with coronal heating. Finally, the implications of the search for observations of coronal heating processes are mentioned. 相似文献
2.
This article is the third in a series working towards the construction of a realistic, evolving, non-linear force-free coronal-field model for the solar magnetic carpet. Here, we present preliminary results of 3D time-dependent simulations of the small-scale coronal field of the magnetic carpet. Four simulations are considered, each with the same evolving photospheric boundary condition: a 48-hour time series of synthetic magnetograms produced from the model of Meyer et al. (Solar Phys. 272, 29, 2011). Three simulations include a uniform, overlying coronal magnetic field of differing strength, the fourth simulation includes no overlying field. The build-up, storage, and dissipation of magnetic energy within the simulations is studied. In particular, we study their dependence upon the evolution of the photospheric magnetic field and the strength of the overlying coronal field. We also consider where energy is stored and dissipated within the coronal field. The free magnetic energy built up is found to be more than sufficient to power small-scale, transient phenomena such as nanoflares and X-ray bright points, with the bulk of the free energy found to be stored low down, between 0.5?–?0.8 Mm. The energy dissipated is currently found to be too small to account for the heating of the entire quiet-Sun corona. However, the form and location of energy-dissipation regions qualitatively agree with what is observed on small scales on the Sun. Future MHD modelling using the same synthetic magnetograms may lead to a higher energy release. 相似文献
3.
Li Feng Wei-Qun Gan Purple Mountain Observatory Chinese Academy of Sciences Nanjing Max Planck Institute for Solar System Research Max-Planck-Str. Katlenburg-Lindau Germany 《中国天文和天体物理学报》2006,6(5)
It was suggested by Parker that the solar corona is heated by many small energy release events generally called microflares or nanoflares. More and more observations showed flows and intensity variations in nonflaring loops. Both theories and observations have indicated that the heating of coronal loops should actually be unsteady. Using SOLFTM (Solar Flux Tube Model), we investigate the hydrodynamics of coronal loops undergoing different manners of impulsive heating with the same total energy deposition. The half length of the loops is 110 Mm, a typical length of active region loops. We divide the loops into two categories: loops that experience catastrophic cooling and loops that do not. It is found that when the nanoflare heating sources are in the coronal part, the loops are in non-catastrophic-cooling state and their evolutions are similar. When the heating is localized below the transition region, the loops evolve in quite different ways. It is shown that with increasing number of heating pulses and inter-pulse time, the catastrophic cooling is weakened, delayed, or even disappears altogether. 相似文献
4.
High-lying, dynamic loops have been observed at transition region temperatures since Skylab observations. The nature of these loops has been debated for many years with several explanations having been put forward. These include that the loops are merely cooling from hotter coronal loops, that they are produced from siphon flows, or that they are loops heated only to transition region temperatures. In this paper we will make use of combined SOHO-MDI (Michelson-Doppler Imager), SOHO-CDS (Coronal Diagnostic Spectrometer) and Yohkoh SXT (Soft X-ray Telescope) datasets in order to determine whether the appearance of transition region loops is related to small-scale flaring in the corona, and to estimate the magnetic configuration of the loops. The latter allows us to determine the direction of plasma flows in the transition region loops. We find that the appearance of the transition region loops is often related to small-scale flaring in the corona and in this case the transition region loops appear to be cooling with material draining down from the loop top. 相似文献
5.
High-lying, dynamic loops have been observed at transition region temperatures since Skylab observations. The nature of these loops has been debated for many years with several explanations having been put forward. These include that the loops are merely cooling from hotter coronal loops, that they are produced from siphon flows, or that they are loops heated only to transition region temperatures. In this paper we will make use of combined SOHO-MDI (Michelson-Doppler Imager), SOHO-CDS (Coronal Diagnostic Spectrometer) and Yohkoh SXT (Soft X-ray Telescope) datasets in order to determine whether the appearance of transition region loops is related to small-scale flaring in the corona, and to estimate the magnetic configuration of the loops. The latter allows us to determine the direction of plasma flows in the transition region loops. We find that the appearance of the transition region loops is often related to small-scale flaring in the corona and in this case the transition region loops appear to be cooling with material draining down from the loop top. 相似文献
6.
Markus J. Aschwanden 《Journal of Astrophysics and Astronomy》2008,29(1-2):3-16
Celebrating the diamond jubilee of the Physics Research Laboratory (PRL) in Ahmedabad, India, we look back over the last six decades in solar physics and contemplate on the ten outstanding problems (or research foci) in solar physics:
- The solar neutrino problem
- Structure of the solar interior (helioseismology)
- The solar magnetic field (dynamo, solar cycle, corona)
- Hydrodynamics of coronal loops
- MHD oscillations and waves (coronal seismology)
- The coronal heating problem
- Self-organized criticality (from nanoflares to giant flares)
- Magnetic reconnection processes
- Particle acceleration processes
- Coronal mass ejections and coronal dimming
7.
This paper is a pedagogical introduction to avalanche models of solar flares, including a comprehensive review of recent modeling
efforts and directions. This class of flare model is built on a recent paradigm in statistical physics, known as self-organized
criticality. The basic idea is that flares are the result of an ‘avalanche’ of small-scale magnetic reconnection events cascading
through a highly stressed coronal magnetic structure, driven to a critical state by random photospheric motions of its magnetic
footpoints. Such models thus provide a natural and convenient computational framework to examine Parker's hypothesis of coronal
heating by nanoflares.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1013301521745 相似文献
8.
The evolution of hydrodynamic shocks generated by nanoflares in coronal loops is investigated in order to suggest requirements for future observations. The shocks produced by nanoflares of 1024 ergs decay within a few seconds, which sets stringent requirements for Doppler or transverse velocity measurements. With feasible improvements, the Normal Incidence X-ray Telescope could observe the time-averaged emission from a nanoflare, however. We also consider collisions between shocks and show that these could also be observed.Operated by the Association of Universities for Research in Astronomy (AURA) under agreement with the Natinal Science Foundation. 相似文献
9.
James A. Klimchuk 《Solar physics》2006,234(1):41-77
The question of what heats the solar corona remains one of the most important problems in astrophysics. Finding a definitive
solution involves a number of challenging steps, beginning with an identification of the energy source and ending with a prediction
of observable quantities that can be compared directly with actual observations. Critical intermediate steps include realistic
modeling of both the energy release process (the conversion of magnetic stress energy or wave energy into heat) and the response
of the plasma to the heating. A variety of difficult issues must be addressed: highly disparate spatial scales, physical connections
between the corona and lower atmosphere, complex microphysics, and variability and dynamics. Nearly all of the coronal heating
mechanisms that have been proposed produce heating that is impulsive from the perspective of elemental magnetic flux strands.
It is this perspective that must be adopted to understand how the plasma responds and radiates. In our opinion, the most promising
explanation offered so far is Parker's idea of nanoflares occurring in magnetic fields that become tangled by turbulent convection.
Exciting new developments include the identification of the “secondary instability” as the likely mechanism of energy release
and the demonstration that impulsive heating in sub-resolution strands can explain certain observed properties of coronal
loops that are otherwise very difficult to understand. Whatever the detailed mechanism of energy release, it is clear that
some form of magnetic reconnection must be occurring at significant altitudes in the corona (above the magnetic carpet), so
that the tangling does not increase indefinitely. This article outlines the key elements of a comprehensive strategy for solving
the coronal heating problem and warns of obstacles that must be overcome along the way. 相似文献
10.
Solar Physics - A series of plates of the solar corona were obtained during the total solar eclipse of July 10th, 1972 near Anadyr (U.S.S.R.) using a standard eclipse coronagraph of Ø = 20 cm... 相似文献
11.
Two competing fundamental hypotheses are usually postulated in the solar coronal heating problem: heating by nanoflares and heating by waves. In the latter it is assumed that acoustic and magnetohydrodynamic disturbances whose amplitude grows as they propagate in a medium with a decreasing density come from the convection zone. The shock waves forming in the process heat up the corona. In this paper we draw attention to yet another very efficient shock wave generation process that can be realized under certain conditions typical for quiet regions on the Sun. In the approximation of stationary dissipative hydrodynamics we show that a shock wave can be generated in the quiet solar chromosphere–corona transition region by the fall of plasma from the corona into the chromosphere. This shock wave is directed upward, and its dissipation in the corona returns part of the kinetic energy of the falling plasma to the thermal energy of the corona. We discuss the prospects for developing a quantitative nonstationary model of the phenomenon. 相似文献
12.
This paper is the second in a series of studies working towards constructing a realistic, evolving, non-potential coronal
model for the solar magnetic carpet. In the present study, the interaction of two magnetic elements is considered. Our objectives
are to study magnetic energy build-up, storage and dissipation as a result of emergence, cancellation, and flyby of these
magnetic elements. In the future these interactions will be the basic building blocks of more complicated simulations involving
hundreds of elements. Each interaction is simulated in the presence of an overlying uniform magnetic field, which lies at
various orientations with respect to the evolving magnetic elements. For these three small-scale interactions, the free energy
stored in the field at the end of the simulation ranges from 0.2 – 2.1×1026 ergs, whilst the total energy dissipated ranges from 1.3 – 6.3×1026 ergs. For all cases, a stronger overlying field results in higher energy storage and dissipation. For the cancellation and
emergence simulations, motion perpendicular to the overlying field results in the highest values. For the flyby simulations,
motion parallel to the overlying field gives the highest values. In all cases, the free energy built up is sufficient to explain
small-scale phenomena such as X-ray bright points or nanoflares. In addition, if scaled for the correct number of magnetic
elements for the volume considered, the energy continually dissipated provides a significant fraction of the quiet Sun coronal
heating budget. 相似文献
13.
Interaction of weak shock waves with a current sheet is investigated by a two-dimensional numerical magnetohydrodynamic model. In accordance with solar coronal conditions, a ratio of thermal to magnetic pressures of 0.1 and a shock Alfvén Mach number slightly above 1 are considered. It is found that even weak shock waves trigger magnetic field reconnection in current sheets. Based on this result, it is suggested that drifting chains of type I radio bursts are radio manifestations of the interactions of weakly super-Alfvénic shock waves with pre-existing current sheets distributed in an active region. This model of type I noise storms is then discussed in connection with the concept of nanoflares (localized reconnections) and the heating of the solar corona. 相似文献
14.
A. I. Poland R. A. Howard M. J. Koomen D. J. Michels N. R. Sheeley Jr. 《Solar physics》1981,69(1):169-175
The Naval Research Laboratory's most recent Earth-orbiting coronagraph, called Solwind, has been observing the Sun's outer corona (2.6–10.0 R
) at 10-min intervals since March 28, 1979. These observations provide the first comprehensive view of coronal transients near the peak of a sunspot cycle. Six, well-defined transients in our quick-look data have masses ranging from 7 × 1014 g to 2 × 1016 g and outward speeds ranging from 150 km s–1 to 900 km s–1. These values are comparable to the ones that were obtained with the OSO-7 and Skylab observations during the declining phase of the last sunspot cycle. Although the amount of quick-look data is not sufficient to provide meaningful statistics, the coronal transients near sunspot maximum seem to occur with a greater frequency and a wider latitude range than the transients during the declining phase of the cycle. In both eras, there is a good, but imperfect, association between the occurrence of coronal transients and surface phenomena such as eruptive prominences and flares.On leave from the High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colo. 80307, U.S.A. Now at Goddard Space Flight Center; Greenbelt, Md. 20771, U.S.A. 相似文献
15.
The cooling of plasmas in closed coronal loops by thermal conduction is important when considering their detectability at
X-ray and EUV wavelengths. A non-local formalism of thermal conduction originating in laboratory plasmas is used and it is
shown that while the effect is unlikely to be important for loops that are in a steady state, it does play a significant role
in loops that are impulsively heated (e.g., by nanoflares). Such loops are “under-dense”, and so hot electrons have a relatively long mean free path. Analytic and numerical
models are presented, and it is shown that conduction cooling times are lengthened quite considerably. A comparison of various
cooling times with ionisation times is also presented, and it is noted that this conductive physics may enhance the chances
of observing hot nanoflare-heated plasma. 相似文献
16.
We present the behaviour of the coronal index (CI) of solar activity over the period 1986–(May) 1996. These data are not only a good tool for studying the activity of the Sun as a star, but they also represent an invaluable source of information in our quest to understand the properties of the heliosphere as a whole. Having compared the variations of CI and of the solar 0.05–0.8 nm soft X-ray flux over the period 1986–1995 we did not find any significant correlation between the two quantities. This favours a scenario in which the sources of the soft X-ray flux are small-scale regions of the corona, and processes of both ionization and recombination do not occur in the same volume of the solar corona as for the green corona. 相似文献
17.
David McConnell 《Solar physics》1983,84(1-2):361-369
Some interpretations of solar S burst spectra are presented. It is shown that the spectra provide evidence for small (～ 500 km) radio sources in the corona which radiate at the fundamental plasma frequency. The possibility of S burst fringes corresponding to coronal MHD waves of wavelength λ ～- 103 km is discussed. 相似文献
18.
The concept of the nanoflare, used in interpreting the solar X-ray corona, is extended to RS CVn stars which, unlike the Sun, exhibit non-thermal quiescent radio spectra. The theoretical synchrotron-radiation radio spectrum emitted by a regular series of nanoflare-electron pulses, injected into the coronal magnetic field, is derived: for an electron energy spectrum N ( γ )∝ γ − s , the spectral power density is given by P ( ν )∝ ν − s /2 . This result is valid for the observation of a series of nanoflares with total time duration ≳ the characteristic electron radiation lifetime, which is the case for electrons trapped in extensive coronal regions such as exist in RS CVn stars on the magnetic-dipole magnetospheric model. The tenuous coronal plasma allows the electrons to give a radio spectrum unaffected at high frequencies (≳5 GHz) by electron collision loss, while the emission of bremsstrahlung X-rays by the electrons also occurs with a spectrum that is related to their radio emission. The observation of individual X-ray bursts, which would provide direct evidence for microflares, is not, however, attainable with current instrumentation. 相似文献
19.
Coronal heating by nanoflares is presented by using observational, analytical, numerical simulation and statistical results.
Numerical simulations show the formation of numerous current sheets if the magnetic field is sheared and bipoles have unequal
pole strengths. This fact supports the generation of nanoflares and heating by them. The occurrence frequency of transients
such as flares, nano/microflares, on the Sun exhibits a power-law distribution with exponent α varying between 1.4 and 3.3.
For nanoflares heating α must be greater than 2. It is likely that the nanoflare heating can be reproduced by dissipating
Alfven waves. Only observations from future space missions such as Solar-B, to be launched in 2006, can shed further light
on whether Alfven waves or nanoflares, heat the solar corona. 相似文献
20.
L. K. Harra T. Magara H. Hara S. Tsuneta T. J. Okamoto A. J. Wallace 《Solar physics》2010,263(1-2):105-119
Active region magnetic flux that emerges to the photosphere from below will show complexity in the structure, with many small-scale fragmented features appearing in between the main bipole and then disappearing. Some fragments seen will be absorbed into the main polarities and others seem to cancel with opposite magnetic field. In this paper we investigate the response of the corona to the behaviour of these small fragments and whether energy through reconnection will be transported into the corona. In order to investigate this we analyse data from the Hinode space mission during flux emergence on 1?–?2 December 2006. At the initial stages of flux emergence several small-scale enhancements (of only a few pixels size) are seen in the coronal line widths and diffuse coronal emission exists. The magnetic flux emerges as a fragmented structure, and coronal loops appear above these structures or close to them. These loops are large-scale structures – most small-scale features predominantly stay within the chromosphere or at the edges of the flux emergence. The most distinctive feature in the Doppler velocity is a strong ring of coronal outflows around the edge of the emerging flux region on the eastern side which is either due to reconnection or compression of the structure. This feature lasts for many hours and is seen in many wavelengths. We discuss the implications of this feature in terms of the onset of persistent outflows from an active region that could contribute to the slow solar wind. 相似文献