共查询到20条相似文献，搜索用时 15 毫秒
1.
We analyze the properties of the electric-current distribution over the cross sections of fairly dense coronal magnetic flux tubes in which the plasma pressure exceeds the magnetic pressure, so that the equilibrium is maintained by the ambient magnetic field. If the plasma is fully ionized, the distributions of the longitudinal and azimuthal currents over the cross section of the loop have the same spatial scale as the pressure distribution. However, even a small number of neutral atoms in the corona (with a mass fraction of the order of 10?5, taking into account the partial ionization of helium) substantially modifies the current distribution over the tube cross section: in this case, a considerable fraction of the full current flowing along the tube is concentrated in a thin region near the axis with a radius of the order of (10?2–10?3)r 0 (where r 0 is the characteristic scale of the plasma-pressure distribution over the tube), thus forming a sort of a jet current. This comes about because the pattern of the conductivity anisotropy is substantially modified in the presence of ion-atom collisions in the magnetoactive plasma of the tube, and the Cowling conductivity dominates over the Hall and Pedersen conductivities. The high current density near the axis of the tube can ensure heating of the plasma to coronal temperatures via Joule dissipation. 相似文献
2.
A model treating a solar coronal hole as an axially symmetrical magnetic formation that is in equilibrium with the surrounding
medium is proposed. The model is applicable in the lower corona (to heights of the order of several hundreds of Mm), where
the influence of the solar-wind outflow on the state of the system can still be neglected. The magnetic field of the coronal
hole is comprised of a relatively weak open flux that varies with height, which extends into interplanetary space, and a closed
field, whose flux closes at the chromosphere near the coronal hole. Simple analytical formulas are obtained, which demonstrate
for a given equilibrium configuration of the plasma and field the main effect of interest—the lowering of the temperature
and density of the gas in the coronal hole compared to their values in the corona at the same geometric height. In particular,
it is shown that, at heights of several tens of Mm, the temperature and density of the plasma in the coronal hole are roughly
half the corresponding values at the same height in the corona, if the cross-sectional radius of the hole exceeds the scale
height in the corona by roughly a factor of 1.5: R
h
≈ 1.5H(T
0). In the special case when R
h
≈ H(T
0), the plasma temperature in the hole is equal to the coronal temperature, and the darkening of the coronal hole is due only
to an appreciable reduction of the plasma density in the hole, compared to the coronal density. An analogy of the properties
of coronal holes and sunspots is discussed, based on the similarity of the magnetic structures of these formations. In spite
of the fundamental difference in the mechanisms for energy transport in coronal holes and sunspots, the equilibrium distributions
of the plasma parameters in these formations are determined only by the magnetic and gravitational forces, giving rise to
a number of common properties, due to their similar magnetic structures. 相似文献
3.
The excitation of the ballooning instability by the eigenoscillations of coronal loops is analyzed using the energy method. The second variation of the potential energy for the case of a plasma—plasma boundary is obtained via the linearized ideal MHD equations. It is shown that the eigenmodes of a magnetic tube and of a toroidal coronal loop coincide in a first approximation. The bending oscillations of the loops are able to excite the ballooning instability when β ? 1. The effects of the instability in solar coronal loops are discussed. 相似文献
4.
New results from electrophotometric scanning of the solar disk in the HeI λ 10830 Å and Hα lines are presented. The intensity at the center of the HeI λ 10830.30 Å line is 1–3% higher in the regions of coronal holes than in quiescent regions; this is accompanied by a decrease in the size and contrast of the chromospheric network compared to the network in quiescent regions. Our observations in the HeI line revealed chains of “dark points” surrounding coronal holes. The Hα±0.5 observations show increased velocities of ascent near the dark points compared to the velocities inside coronal holes and in quiescent regions. It is proposed that the intensification and acceleration of the flows of solar plasma from the dark points are due to reconnection of the magnetic fields of the bipolar chromospheric network and the predominantly unipolar magnetic field inside the coronal holes. Our observations suggest that the same reconnection process takes place near the temperature minimum, in the presence of certain conditions at the boundary between coronal holes and bipolar active regions. The reconnection process produces plasma flows from the chromosphere to the corona, which are sufficient to form prominences. 相似文献
5.
We examine plasma heating due to the dissipation of acoustic waves excited in coronal magnetic loops by parametric resonance
with the five-minute oscillations in the velocity of the photospheric convection. The energy of acoustic waves excited in
the coronal magnetic loop, rate of dissipation of acoustic waves, and rate of heating of the coronal plasma are determined.
The maximum temperature predicted for the apex of the loop is calculated as a function of the velocity of photospheric oscillations,
length of the loop, and electric current in the loop. It is shown that the mechanism proposed can explain the origin of quasi-stationary
X-ray loops with temperatures of 3–6 MK. The lengths of these loops are resonant for acoustic waves excited by the five-minute
photospheric oscillations. The use of the proposed mechanism to explain heating of the X-ray loops expected to be on stars
of late spectral types is discussed. 相似文献
6.
Some mechanisms for Joule dissipation of electric currents flowing in partially ionized hydrogen plasmas are analyzed in a
three-fluid approximation. It is shown that ambipolar diffusion can be responsible for the annihilation of magnetic flux during
collisions of ions with neutral atoms. The relative influences of the electron conductivity and the Cowling conductivity on
the magnetic-field annihilation rate are examined in the frameworks of the Sweet-Parker reconnection model. The escape of
plasma is an efficient mechanism for cooling current sheetswith thicknesses of hundreds of kilometers in the solar chromosphere.
The origin of the solar chromospheric jets observed by the Hinode spacecraft is discussed. 相似文献
7.
A. P. Kropotkin 《Astronomy Reports》2011,55(12):1132-1143
The basis is laid out for a theory relating various phenomena in the solar atmosphere, including localized concentrations
of magnetic field at the bases of coronal magnetic arches, chromospheric spicules, twisted coronal magnetic flux tubes, and
flows of energy carried by Alfvén waves propagating upward into the corona. The structure of photospheric currents localized
in the vicinity of supergranule boundaries and excited by convective motions is studied. These currents exist primarily in
a “dynamo layer” of sharply enhanced transverse conductivity, which forms in the weakly ionized thermal photospheric plasma
located in the solar gravitational field. The motions of the electrons and ions in this layer have appreciably different characters:
the ions are collisionly driven by the flows of neutral atoms, while the electrons drift in the crossed electric and magnetic
fields. The electric field supporting the current arises due to the polarization of the electrons and ions. This field also
gives rise to Alfvén perturbations that propagate upward into the corona, together with their associated longitudinal currents.
The character of this “loading” makes the system of fields and currents uniquely defined. Moreover, the momentum flux carried
by these Alfvén waves should be transferred to the cool chromospheric gas, facilitating the vertical ejection of this gas
in the form of spicules, as was first proposed in 1992 by Haerendel. 相似文献
8.
The coronal and chromospheric emission of several hundred late-type stars whose activity was recently detected are analyzed. This confirms the previous conclusion for stars of HK project that there exist three groups of objects: active red M dwarfs, G-K stars with cyclic activity, and stars exhibiting high but irregular activity. The X-ray fluxes, EUV-spectra, and X-ray cycles can be used to study the main property of stellar coronas—the gradual increase in the number of high-temperature (T ≥ 10 MK) regions in the transition from the Sun to cyclically active K dwarfs and more rapidly rotating F and G stars with irregular activity. The level of X-ray emission is closely related to the spottedness of the stellar surface. The correlation between the chromospheric and coronal emission is weak when the cycles are well-defined, but becomes strong when the activity is less regular. Unexpectedly, stars whose chromospheric activity is even lower than that of the Sun are fairly numerous. Common and particular features of solar activity among the activity of other cyclically active stars are discussed. Our analysis suggests a new view of the problem of heating stellar coronas: the coronas of stars with pronounced cycles are probably heated by quasistationary processes in loops, while prolonged nonstationary coronal events are responsible for heating the coronas of F and G stars with high but irregular activity. 相似文献
9.
The solar event SOL2012–10–23T03:13, which was associated with a X1.8 flare without an accompanying coronal mass ejection (CME) and with a Type II radio burst, is analyzed. A method for constructing the spatial and temporal profiles of the difference brightness detected in the AIA/SDOUVand EUV channels is used together with the analysis of the Type II radio burst. The formation and propagation of a region of compression preceded by a collisional shock detected at distances R < 1.3R ⊙ from the center of the Sun is observed in this event (R ⊙ is the solar radius). Comparison with a similar event studied earlier, SOL2011–02–28T07:34 [1], suggests that the region of compression and shock could be due to a transient (impulsive) action exerted on the surrounding plasma by an eruptive, high-temperature magnetic rope. The initial instability and eruption of this rope could be initiated by emerging magnetic flux, and its heating from magnetic reconnection. The cessation of the eruption of the rope could result from its interaction with surrounding magnetic structures (coronal loops). 相似文献
10.
V. V. Zaitsev A. G. Kislyakov S. Urpo A. V. Stepanov E. I. Shkelev 《Astronomy Reports》2003,47(10):873-882
Low-frequency pulsations of 22 and 37 GHz microwave radiation detected during solar flares are analyzed. Several microwave bursts observed at the Metsähovi Radio Observatory are studied with time resolutions of 100 and 50 ms. A fast Fourier transformation with a sliding window and the Wigner-Ville method are used to obtain frequency-time diagrams for the low-frequency pulsations, which are interpreted as natural oscillations of coronal magnetic loops; the dynamical spectra of the pulsations are synthesized for the first time. Three types of low-frequency fluctuations modulating the flare microwave radiation can be distinguished in the observations. First, there are fast and slow magneto-acoustic oscillations with periods of 0.5–0.8 s and 200–280 s, respectively. The fast magneto-acoustic oscillations appear as trains of narrow-band signals with durations of 100–200 s, a positive frequency drift dν/dt=0.25 MHz/min, and frequency splitting δν=0.01–0.05 Hz. Second, there are natural oscillations of the coronal magnetic loops as equivalent electrical circuits. These oscillations have periods of 0.5–10 s and positive or negative frequency drift rates dν/dt=8×10?3 Hz/min or dν/dt=?1.3×10?2 Hz/min, depending on the phase of the radio outburst. Third, there are modulations of the microwave radiation by short periodic pulses with a period of 20 s. The dynamical spectra of the low-frequency pulsations supply important information about the parameters of the magnetic loops: the ratio of the loop radius to its length r/L≈0.1, the plasma parameter β≈10?3, the ratio of the plasma densities outside and inside the loop ρe/ρi≈10?2, and the electrical current flowing along the loop I≈1012 A. 相似文献
11.
We have begun an investigation of the possible origins of considerable of powerful solar flares. This effect is manifest, first and foremost, in the existence of high-temperature plasma in flare loops over many hours. Analysis of the soft X-ray emission in two energy bands detected by the GOES satellites for about 20 powerful solar flares reveals long time intervals during the decay phase when the source temperature decreases, in general, exponentially. The characteristic time t i for a decrease in the temperature by a factor of ten is 3–10 hours for most powerful events. In addition, another interval of very slow decrease with a characteristic time t i of tens of hours can be identified in some cases. We found a gradual change in the dependence of the temperature on the square root of the emission measure for the source as a whole, which characterizes the transition from purely coronal processes to powerful flares with a prolonged inflow of plasma from the chromosphere. Modeling the energy balance in a loop can yield the requirements for the source of plasma heating in a long-lived arch system. A necessary condition for the development of prolonged flares seems to be a powerful coronal mass ejection, which initiates the formation of a source of plasma heating at coronal heights. Our analysis shows that a considerable fraction of the energy is often released in the region of the cusp, and that systems of giant coronal arches rising to heights of about 100 000 km above the limb are formed in most prolonged events (called dynamical flares in the terminology of Svestka). 相似文献
12.
Modulations of the microwave emission of the Sun at 11.7 GHz have been studied using more than 40 events observed in 2001
at the Mets?hovi Radio Observatory. In nearly all the observed events, low-frequency modulations with periods of 3–90 min
were detected. As a rule, simultaneous modulation of the emission at several frequencies was observed. One possible origin
of such modulations with periods 5–10 min is parametric resonance arising in coronal magnetic loops as a result of interactions
with the 5-min photospheric oscillations, while the long-period modulations could be a manifestation of sunspot oscillations.
Torsional (ϑ-mode) and radial (r-mode) oscillations have such periods. The frequency of occurrence of oscillations with the determined periods is considered,
and a lower limit for the brightness temperature of the oscillations is estimated. 相似文献
13.
Results of studies of motions in a filament during its slow ascent and eruption based on spectral observations obtained at the Sayan Solar Observatory are presented. SDO/HMI data on the longitudinal magnetic field and SDO/AIA images in the EUV are also considered. Short-period (∼5 min) vertical oscillations of the filament as a whole were detected during its ascent. An acceleration of the rise of the filament was accompanied by the rupture of an orthogonal loop above the filament, which was observed in 193 A EUV images obtained with SDO/AIA over a long time preceding the event. Two hours before the partial eruption of the filament, SDO/HMI data indicate an increase in the magnetic flux by 2 × 1019 Mx at the footpoints of the loop. The emission from the loop rupture piont propagated toward the east and west along a neutral line, and brightenings were observed at the boundaries of the filament channel. Emission loops were visible in all SDO/AIA channels, testifying to strong heating of the filament plasma. During the rapid phase of the eruption, the filament moved with an acceleration ∼21 m/s2. Hα images show the filament splitting into fragments parallel to its axis during the eruption. The results of these studies of the eruption of the filament are in agreement with other results in the literature, and are supplemented by new observational facts. Vertical oscillations (∼5 min) of the filament as a whole are observed before the ascent phase. During the ascent phase, an interaction of the filament with a higher-lying coronal loop is observed.
相似文献14.
The process of post-flare loop formation, including the heating of flux tubes by hot chromospheric sources and their filling with plasma, is demonstrated by simulations in an MHD approximation. The loop is additionally heated at its apex by the interaction of oppositely directed plasma streams. Local coronal heating over the loop is also possible due to magnetic-field-line reconnection. A new version of the PERESVET code that can take into account anisotropy of the thermal conductivity of a plasma in a magnetic field was used for the computations. 相似文献
15.
The drift loss cone instability, propagating nearly transverse to the ambient magnetic field, is studied in the ring current
plasma taking into account the relative driftU between electrons and protons due to density gradients. The growth rates attain maxima and then decrease as the wave number
parallel to the magnetic fieldk
II increases. The peak values of the growth rates, maximised with respect tok
II, are enhanced by the increase in number density, electron temperature and loss cone index, and by the decrease in βt, the ratio of the proton thermal pressure to magnetic field pressure. The unstable frequencies fall in the range of 5 to
30Ωp with the growth rate γ ≥Ωp. In the ring current region betweenL=4 and 5, the instability will generate a strong turbulence in the frequency range between 5–500 Hz which can produce fluctuating
electric fields 0. 5–5 mV/m and magnetic field 0.8–80mγ. This instability can also occur on the auroral field lines, which
connect to the region of intense earthward plasma flow in the distant magnetotail and produce a broad band electrostatic noise. 相似文献
16.
We present the results of radio sounding observations probing the inner solar wind near the minimum of the solar-activity cycle, using polarized pulses from PSR B0525+21 and PSR B0531+21 received when the lines of sight toward these pulsars were close to the Sun. The observations were obtained in June 2005 and June 2007 on the Large Phased Array of the Lebedev Physical Institute at 111 MHz. An upper limit for the scattering of giant pulses from PSR B0531+21 due to their passage through the turbulent solar-wind plasma is determined. The arrival-time delays for pulses from PSR B0531+21 are used to derive the radial dependence of the mean density of the circumsolar plasma. The resulting density distribution indicates that the acceleration of fast, high-latitude solar-wind outflows continues to heliocentric distances of 5–10R ⊙, where R ⊙ is the solar radius. The mean plasma density at heliocentric distances of about 5R ⊙ is 1.4 × 104 cm?3, substantially lower than at the solar-activity maximum. This is associated with the presence of polar coronal holes. The Faraday rotation measure at heliocentric distances of 6–7R ⊙ is estimated. Deviations of the spatial distribution of the magnetic field from spherical symmetry are comparatively modest in the studied range of heliocentric distances. 相似文献
17.
Five-second observations of the solar corona carried out in the FeIX 171 Å line by the KORONAS-FOTON instrument TESIS are used to study the dynamics of small-scale coronal structures emitting in and around coronal bright points. The small-scale structures of the lower corona display complex dynamics similar to those of magnetic loops located at higher levels of the solar corona. Numerous detected oscillating structures with sizes below 10 000 km display oscillation periods from 50 to 350 s. The period distributions of these structures are different for P < 150 s and P > 150 s, which implies that different oscillation modes are excited at different periods. The small-scale structures generate numerous flare-like events with energies 1024–1026 erg (nanoflares) and with a spatial density of one event per arcsecond or more observed over an area of 4 × 1011 km2. Nanoflares are not associated with coronal bright points, and almost uniformly cover the solar disk in the observation region. The ejections of solar material from the coronal bright points demonstrate velocities of 80–110 km/s. 相似文献
18.
本文报道了天然Ⅱ型CaCO3 矿物。该矿物发现于海洋表层沉积物,成分与方解石、文石及六方球方解石相同,但结构完全不同,它们共同组成了天然CaCO3 的同质多象变体。天然Ⅱ型CaCO3 矿物的空间群为P21/c,单位轴长为a0= 0.6290±0.0002 nm,b0= 0.4934±0.0002 nm,c0= 0.7979±0.0003nm,β= 107.571°±0.002°,Z= 4,单胞体积为0.23605±0.1749nm3;理论密度为2.82 g/cm3,实测值为2.76 g/cm3;实测硬度H= 4.天然Ⅱ型CaCO3 矿物是在深水环境中较高静水压力下形成的珊瑚体生物矿物。 相似文献
19.
The structures called Whitney gathers in the catastrophe theory are not unusual among the various types of chromospheric Hα filaments and coronal helmets. They result from a projection of smooth surfaces onto a plane of the sky. The apices of the helmets can be described as gathers of the surface B r = 0 in the corona (where B r is the magnetic field). Near the limb, Hα filaments are frequently observed as similar types of structures. A model of the B r = 0 surface with sources located inside the Sun (∫ Bds = 0) is constructed here in this context. Gathers and folds of this surface are shown to be possible in relatively simple fields. Our model computations are supported by the available observational data. 相似文献
20.
RATAN-600 observations of a microwave source located above the active region NOAA 10105 obtained on September 7–20, 2002 with a frequency resolution of ～10% have revealed a spectral feature near 3.04 cm that can be interpreted as a neutral hydrogen line. This feature was observed September 11, 2002, in both absorption and emission, and was detected in the spectra of various portions of the source (sunspot, flocculus, and background). The maximum line depth of (35 ± 5)% of the source brightness was observed at the start of the observations (9.2h UT) in the flocculus in absorption. The line intensity decreased rapidly with time, becoming less than the measurement errors by 9.7h UT. It is most likely that the 3.04 cm emission is related to a 2B chromospheric flare (M2.2 X-ray burst) observed at ～7.5h UT in the floccular field, near the main sunspot of NOAA 10105. In this case, the total duration of the event was about two hours. These observations are consistent with earlier statistical studies, and refines these based on data with higher spatial resolution. Recommendations for further observational studies of the solar 3.04-cm hydrogen line are presented; requirements for theories of the 3.04 cm line taking into account nonequilibrium states of the active-region plasma are indicated. 相似文献