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1.
Jörn E. Kunstmann 《Earth, Moon, and Planets》1978,18(1):91-104
The frequency spectra of the interplanetary magnetic field fluctuations are the projection of their wavenumber spectra onto one dimension. Only the frequency spectra can be measured by spacecrafts. It is studied how their measured size depends on the direction of the mean fieldB
0, which structures the symmetry of the fluctuations relative to the solar wind system. It is specialized for the slab model, Alfvén waves, magneto-acoustic waves and the isotropic case. For the slab model the frequency spectra are proportional to
, whereq is the spectral index and the angle betweenB
0 and the radial direction. For the diffusion coefficientK
TT the relation
holds. 相似文献
2.
The spatial structure of the transverse oscillations in the interplanetary magnetic field at 1 AU is studied by comparing the simultaneous observations by Explorer 33 and 35 satellites at the maximum separation of about 200R
E. The anisotropy characteristics of these oscillations suggest that the oscillations sampled are Alfvén waves. It is found that the size of the region of the wave coherence is related to the solar wind velocity; the size is 80R
E when the wind velocity is lower than 500 km s–1 but becomes less than this when the wind velocity is higher. An inference is made that the solar atmospheric turbulence contributing to the faster solar wind is finer in scale than that associated with the slower wind.A postgraduate student at the Tokai University 相似文献
3.
D. B. Melrose 《Solar physics》1989,120(2):369-381
There is a characteristic maximum brightness temperature T
B 1015K for type III solar radio bursts in the solar wind. The suggestion is explored that the maximum observed values of T
Bmay be attributed to saturation of the processes involved in the plasma emission. The processes leading to fundamental and second harmonic emission saturate when T
Bis approximately equal to the effective temperature T
Lof the Langmuir waves. The expected maximum value of T
Bis estimated for this saturation model in two ways: from the growth rate for the beam instability, and from the maximum amplitude of the observed Langmuir turbulence. The agreement with the observed values is satisfactory in view of the uncertainties in the estimates (a) of the intrinsic brightness temperature from the observed brightness temperature, (b) of the actual growth rate of the beam instability, which must be driven by local, transient features (that are unobservable using available instruments) in the electron distribution, and (c) in the k-space volume filled by the Langmuir waves, and this is consistent with the observational data on two well-studied events at the orbit of the Earth and with statistical data for events over a range of radial distances from the Sun. 相似文献
4.
A quasi-steady 1-D hydrodynamic model, with mass addition, has been used to study the various modes of interaction of the solar wind with a medium-bright, H2O-dominated comet (such as P/Halley) approaching the Sun.At large heliocentric distances (d 5 AU) the solar wind penetrates unimpeded on to the surface. As the comet moves further in, mass loading of the solar wind by heavy ions from the fledgling cometary atmosphere causes it to slow down, thereby causing a significant enhancement of the interplanetary field. Still further in at d 3.14 AU, as the mass loading reaches a critical value, a collision-less standing shock is formed in the solar wind upstream of the nucleus. As d decreases further, the distance of this shock from the nucleus increases. The cometary atmosphere becomes dense enough to stand off the solar wind ahead of the nuclear surface and form a well defined tangential discontinuity surface (or ionopause) only when d reaches the value 2.65 AU. When d 2.65 AU an inner shock could, in principle, also form within the cometary ionosphere, although its existence would depend on the detailed thermodynamics of the cometary ionosphere. Resolution of this question is beyond the scope of the present analysis.The conclusions of the present study would be qualitatively valid for other comets having sizes, surface optical properties and chemical compositions, different from those adopted here. The helio-centric distances at which the various transitions take place from the one mode of solar wind interaction to another, would, of course, be different, with all these distances being smaller for less active comets. 相似文献
5.
A consistent account of plasma turbulence in magnetohydrodynamics equations describing transport processes across the magnetic field is presented. The structure of the perpendicular shock wave generated in the solar atmosphere, as a result of either local disturbance of the magnetic field or dense plasma cloud motion with a frozen-in magnetic field, has been investigated. The region of parameters in the solar atmosphere at which the electron-ion relative drift velocity u exceeds the electron thermal velocity V
eand generation of radio emission becomes possible, has been determined. The plasma turbulence inside the front has been shown, under conditions of solar corona, not to cause the oscillation structure of shock front to break down. Under chromospheric conditions, the shock profile is aperiodical. Then, the condition u > Vecan be satisfied and shock waves having an Alfvén Mach number M which exceeds the critical value M
c 3.3 for aperiodical shock waves can exist (Eselevich et al., 1971a). Arguments are given in favour of the fact that perpendicular shock waves are generated in the Sun's atmosphere when dense plasma clouds, with a frozen-in magnetic field, are expanded. 相似文献
6.
In this paper we give an explanation for a control mechanism for velocityV of solar wind (SW) streams for coronal holes (CHs) based on the idea suggested by Rudenko and Fainshtein (1993). In accordance with that idea, the difference of values ofV in high-speed SW streams from different CHs is due to the spread in magnitude of magnetic fieldB
a in the region of acceleration of such streams near the Sun. In this case, with increasing magnitude ofB
a, there is an increase in velocity of the high-speed stream.Through calculations of the coronal magnetic field (potential-field approximation) it is shown that on the source surface the magnetic field B
s, averaged over the cross-section of the magnetic tube from a CH, can vary for different tubes over a wide range and correlates quite well with the area of this tube's base as well as depending on the radial component of the magnetic field at the base of the tube on the source surface B
or.It is found that the value of superradial divergence of the magnetic tube from a CH depends not only on the area of its base (as shown in prior work) but also on B
or. A positive correlation at the Earth's orbit between velocityV of the high-speed SW and the radial component of the magnetic field in the region of this stream is detected, which agrees indirectly with theV-control mechanism under discussion. 相似文献
7.
Since most Alfvén-waves in the solar wind are observed to come from the Sun, nonlinear wave-particle interactions can be expected to constitute their dominant dissipation process. The growth or damping of two circularly-polarized Alfvén-waves with wave vectors parallel to the ambient magnetic field is calculated using kinetic theory. If the waves are oppositely polarized they both damp proportional to their frequency. If the waves are of the same polarization, both the lower frequency wave and the plasma particles gain energy at the expense of the higher frequency wave. Thus, with increasing distance from the Sun, a steepening of the power spectrum is expected. For waves propagating in the same direction, the interaction is negligible for small , while it becomes appreciable for 10–1. For conditions typical of the solar wind near 1 AU an observed half-hour linearly-polarized wave, for example, with B=0(B
0) has a damping time of about 10 h. 相似文献
8.
We investigate the physical processes occurring in the supergranule boundary cylinder layer (SBCL). Taking into account the Coriolis force, we obtain an expression for the component of the magnetic field and velocity in the SBCL. Within the framework of linear MHD, we consider the formation and coalescence of magnetic tubes, i.e. spicules, in the course of the reconnection of the SBCL magnetic field. The estimated number of spicules appearing on each supergranule cell is in agreement with observations. This number depends on the solar latitude : (1) if the normal component of the magnetic fieldB
z
is assumed to be independent of , then the maximum number of spicules should be at = 71°; (2) ifB
z
is assumed to be the component of the dipolar fieldB
z
sin , then the maximum number should be at the pole: = 90°. The timescale of the formation and the coalescence of the magnetic tubes is 10–20 min, which is of the order of the observed lifetime of the spicules. 相似文献
9.
V. G. Fainshtein 《Solar physics》1991,136(1):169-189
By analyzing observational data, it has been possible to determine quantitative relationships that represent the role of the interaction of fast and slow solar wind (SW) streams in the formation of characteristic SW properties at the Earth's orbit.It is shown that maximum values of magnetic field B
M and density n
M peaks in the neighbourhood of the sector boundary (SB) at the base of the high-speed stream front are associated with solar wind characteristics such as the SW minimum velocity near the SB, V
m, the maximum velocity in the central part of the fast stream, V
M, and the slope of the magnetic field neutral line to the solar equatorial plane at R = 2.5 R
(R
is the solar radius).It is concluded that enhancements of absolute values of the z-component of the magnetic field, ¦B
z¦, recorded at the Earth's orbit, are largely attributable, at sufficiently large values of , to the interaction of different-velocity SW streams. 相似文献
10.
S. Chatterjee 《Astrophysics and Space Science》1989,162(1):167-170
It is shown that, in the non-interacting limit, a one-dimensional metallic system has a transition to an insulating phase, in presence of external magnetic fieldH, if B
H>E
B where B is the Bohr magneton andE
Bis the energy band width of the electronic states. Possible realization of this effect in the atmosphere of neutron stars is pointed out. 相似文献
11.
A 3-D particle simulation of excitation of whistler waves driven by an electron temperature anisotropy (T
> T
) is presented. Results show that whistler waves can have appreciable growth driven by the anisotropy. The maximum intensity of the excited whistler waves increases as a quadratic function of the anisotropy. Due to the presence of a threshold, one needs a relatively large electron temperature anisotropy above threshold to generate large-amplitude whistler waves. The average amplitude of turbulence in the context of whistler waves is up to as large as about 1% of the ambient magnetic field when T
/T
. The total energy density of the whistler turbulence is adequate for production of relativistic electrons in solar flares through stochastic acceleration. 相似文献
12.
The adiabatic theory of interaction between high and low frequency waves has been studied for the case of electron plasma oscillations and ion acoustic waves and the results are applied to the solar wind. The modified dispersion relation for ion acoustic waves has been derived, taking a Gaussian distribution for plasmons. Two limiting cases of the spectrum are studied. For a broad spectrum, the plasma turbulence has a destabilising effect by introducing a growth rate denoted by turbulence, which is positive for k
0 > (m
e/
m
i
)1/2
De
–1
, k
0 being the central wave numger of the spectrum,
De the electron Debye length. Also, even for v
d(drift velocity between electrons and ions) < c
s, we arrive at unstable ion acoustic modes. For narrow spectrum, the plasma turbulence has a stabilising effect. 相似文献
13.
G. S. Lakhina 《Astrophysics and Space Science》1985,111(2):325-334
Low frequency electromagnetic lower hybrid waves (so-called hybrid whistlers) propagating nearly transverse to the magnetic field can be driven unstable by a resonant interaction with halo electron distributions carrying solar wind heat flux. The electromagnetic lower hybrid instability is excited when the halo electron drift exceeds the parallel phase velocity of the wave. The growth rate attains a maxima at a certain value of the wavenumber. The maximum growth rate decrease by an increase in e (the ratio of electron pressure to magnetic field pressure) and halo electron temperature anisotropy. At 0.3 AU the growth time of the electromagnetic lower hybrid instability is of the order of 25 ms or shorter, whereas the most unstable wavelengths associated with the instability fall typically in a range of 27 to 90 km. The instability would give rise to a local heating of solar wind ions and electrons in the perpendicular and parallel directions relative to the magnetic field, B0. The observations of low frequency whistlers having high values ofB/E ratios (B andE being the magnitude of the wave magnetic and electric field, respectively) and propagating at large oblique angles to B0 behind interplanetary shocks, can be satisfactorily explained in terms of electromagnetic lower hybrid instability. The instability is also relevant to the generation mechanism of correlated whistler and electron plasma oscillation bursts detected on ISEE-3. 相似文献
14.
Steven R. Spangler 《Astrophysics and Space Science》1996,243(1):65-75
I discuss the use of Very Long Baseline Interferometer (VLBI) phase scintillations to probe the conditions of plasma turbulence in the solar wind. Specific results from 5.0 and 8.4 GHz observations with the Very Long Baseline Array (VLBA) are shown. There are several advantages of phase scintillation measurements. They are sensitive to fluctuations on scales of hundreds to thousands of kilometers, much larger than those probed by IPS intensity scintillations. In addition, with the frequency versatility of the VLBA one can measure turbulence from the outer corona 5–10R
to well past the perihelion approach of the Helios spacecraft. This permits tests of the consistency of radio propagation and direct in-situ measurements of turbulence. Such a comparison is made in the present paper. Special attention is dedicated to measuring the dependence of the normalization coefficient of the density power spectrum,C
N
2
on distance from the sun. Our results are consistent with the contention published several years ago by Aaron Roberts, that there is insufficient turbulence close to the sun to account for the heating and acceleration of the solar wind. In addition, an accurate determination of theC
N
2
(R) relationship could aid the detection of transients in the solar wind. 相似文献
15.
Peter J. Catto 《Astrophysics and Space Science》1974,26(1):47-94
The steady state interaction of the solar wind with the Moon is modeled as a uniform, magnetized, quasi-neutral, collisionless, hypersonic, and hyper-Alfvénic flow of an electronproton plasma past a perfectly ion absorbing, non-magnetized sphere. For the temperature of the electronsT much less than that of the ionsT i , steady state equations are derived self-consistently from the Vlasov and Maxwell equations by taking advantage of the fact that the ion gyration ratio is small compared to the radius of the Moon, by employing an ordering which requires different scale lengths along the magnetic fieldB and center of mass velocity, and by expanding in a small parameter ? that measures the smallness of ?B terms compared to a dominant term retained. A partial numerical solution is presented and discussed for the limit in which ? is much less than β=(ion pressure/magnetic pressure). In addition, a simple technique is presented whereby the steady state equations can be approximately extended to cases in whichT?T i for arbitrary ?/β. 相似文献
16.
Roman M. Häberli Michael R. Combi Tamas I. Gombosi Darren L. De Zeeuw Kenneth G. Powell 《Icarus》1997,130(2):373-386
The observation of ions created by ionization of cometary gas, either by ground-based observations or byin situmeasurements can give us useful information about the gas production and composition of comets. However, due to the interaction of ions with the magnetized solar wind and their high chemical reactivity, it is not possible to relate measured ion densities (or column densities) directly to the parent gas densities. In order to quantitatively analyze measured ion abundances in cometary comae it is necessary to understand their dynamics and chemistry. We have developed a detailed ion–chemical network of cometary atmospheres. We include production of ions by photo- and electron impact-ionization of a background neutral atmosphere, charge exchange of solar wind ions with cometary atoms/molecules, reactions between ions and molecules, and dissociative recombination of molecular ions with thermal electrons. By combining the ion–chemical network with the three-dimensional plasma flow as computed by a new fully three-dimensional MHD model of cometary plasma environments (Gombosiet al.1996) we are able to compute the density of the major cometary ions everywhere in the coma. The input parameters for our model are the solar wind conditions (density, speed, temperature, magnetic field) and the composition and production rate of the gas. We applied our model to Comet P/Halley in early March 1986, for which the input parameters are reasonably well known. We compare the resulting column density of H2O+with ground-based observations of H2O+from DiSantiet al.(1990). The results of our model are in good agreement with both the spatial distribution and the absolute abundance of H2O+and with their variations with the changing overall water production rate between two days. The results are encouraging that it will be possible to obtain production rates of neutral cometary constituents from observations of their ion products. 相似文献
17.
Magnetic reconnection can take place between two plasma regions with antiparallel magnetic field components. In a time-dependent reconnection event, the plasma outflow region consists of a leading bulge region and a trailing reconnection layer. Magnetohydrodynamic (MHD) discontinuities, including rotational discontinuities, can be formed in both the bulge region and the trailing layer. In this paper, we suggest that the rotational discontinuities observed in the solar wind may be generated by magnetic reconnection associated with microflares in coronal holes. The structure of the reconnection layer is studied by solving the one-dimensional Riemann problem for the evolution of an initial current sheet after the onset of magnetic reconnection as well as carrying out two-dimensional MHD simulations. As the emerging magnetic flux reconnects with ambient open magnetic fields in the coronal hole, rotational discontinuities are generated in the region with open field lines. It is also found that in the solar corona with a low plasma beta ( 0.01), the magnetic energy is converted through magnetic reconnection mostly into the plasma bulk-flow energy. Since more microflares will generate more rotational discontinuities and also supply more energy to the solar wind, it is expected that the number of rotational discontinuities observed in the solar wind would be an increasing function of solar wind speed. The observation rate of rotational discontinuities generated by microflares is estimated to be dN
RD/dt - f/63 000 s (f > 1) at 1 AU. The present mechanism favors the generation of rotational discontinuities with a large shock normal angle. 相似文献
18.
Frank Verheest 《Astrophysics and Space Science》1987,138(1):209-215
The large differences in drift velocities between the solar wind protons and the picked-up ions of cometary origin cause the Alfvén waves (among others) to become unstable and generate turbulence. A self-consistent treatment of such instabilities has to take into account that these cometary ions affect the solar wind plasma in a decisive way. With the help of a previously developed formalism one finds the correct Alfvén instability criterion, which is here nondispersive, in contrast to recent calculations where the cometary ions are treated as a low-density, high-speed, and non-neutral beam through an otherwise undisturbed solar wind. The true bulk speed of the combined solar wind plus cometary ion plasma clearly shows the mass-loading and deceleration of the solar wind near the cometary nucleus, indicating a bow shock. The instability criterion is also used to determine the region upstream where the Alfvén waves can be unstable, based upon recent observations near comet Halley. 相似文献
19.
The processes of ion acceleration and Alfvén wave generation by accelerated particles at the Earth’s bow shock are studied within a quasi-linear approach. Steady-state ion and wave spectra are shown to be established in a time of 0.3–4 h, depending on the background level of Alfvénic turbulence in the solar wind. The Alfvén waves produced by accelerated ions are confined within the frequency range 10?2–1 Hz and their spectral peak with a wave amplitude βB ～ B comparable to the interplanetary magnetic field strength B corresponds to the frequency v = (2–3) × 10?2 Hz. The high-frequency part of the wave spectrum (v > 0.2 Hz) undergoes damping by thermal ions. The calculated spectra of the accelerated ions and the Alfvén waves generated by them reproduce the main features observed in experiments. 相似文献
20.
Magnetic and thermal pressures in the solar wind 总被引：1，自引：0，他引：1
Explorer 34 solar wind data for the period June to December, 1967 show that(a) The magnetic pressure, P
BB
2/8, and thermal pressure,P
kn
p
kTp+n
kT+n
e
kTe,are variable and positively correlated on a scale of 2 days, but (b) changes in P
b and P
k are anticorrelated on a scale 1 hr (0.01 AU). Thus, dynamical hydromagnetic processes (dv/dto) must occur on the mesoscale, but the solar wind tends to be in equilibrium(P
B+P
Kconstant) on a smaller scale, the microscale. The 3-hr averages show that the most probable value of P
k/P
B is =1.0±0.1, which implies that the most probable state of the solar wind at 1 AU is not one of equipartition between the thermal energy and magnetic energy. The average total pressure for a given bulk speed(P(V)=P
k+P
k+P
B) is essentially independent of V, implying that P is not determined by the heating or acceleration mechanisms of the solar wind; the average pressure is P=(2.9±1.5)×10-10dyne/cm2. 相似文献