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1.
In this concise review of the recent developments in relativistic shock theory in the Universe we restrict ourselves to shocks
that do not exhibit quantum effects. On the other hand, emphasis is given to the formation of shocks under both non-magnetised
and magnetised conditions. We only briefly discuss particle acceleration in relativistic shocks where much of the results
are still preliminary. Analytical theory is rather limited in predicting the real shock structure. Kinetic instability theory
is briefed including its predictions and limitations. A recent self-similar relativistic shock theory is described which predicts
the average long-term shock behaviour to be magnetised and to cause reasonable power-law distributions for energetic particles.
The main focus in this review is on numerical experiments on highly relativistic shocks in (i) pair and (ii) electron-nucleon
plasmas and their limitations. These simulations do not validate all predictions of analytic and self-similar theory and so
far they do not solve the injection problem and the self-modification by self-generated cosmic rays. The main results of the
numerical experiments discussed in this review are: (i) a confirmation of shock evolution in non-magnetised relativistic plasma
in 3D due to either the lepton-Weibel instability (in pair plasmas) or to the ion-Weibel instability; (ii) the sensitive dependence
of shock formation on upstream magnetisation which causes suppression of Weibel modes for large upstream magnetisation ratios
σ>10−3; (iii) the sensitive dependence of particle dynamics on the upstream magnetic inclination angle θ
Bn
, where particles of θ
Bn
>34° cannot escape upstream, leading to the distinction between ‘subluminal’ and ‘superluminal’ shocks; (iv) particles in
ultra-relativistic shocks can hardly overturn the shock and escape to upstream; they may oscillate around the shock ramp for
a long time, so to speak ‘surfing it’ and thereby becoming accelerated by a kind of SDA; (v) these particles form a power-law
tail on the downstream distribution; their limitations are pointed out; (vi) recently developed methods permit the calculation
of the radiation spectra emitted by the downstream high-energy particles; (vii) the Weibel-generated downstream magnetic fields
form large-amplitude vortices which could be advected by the downstream flow to large distances from the shock and possibly
contribute to an extended strong field region; (viii) if cosmic rays are included, Bell-like modes can generate upstream magnetic
turbulence at short and, by diffusive re-coupling, also long wavelengths in nearly parallel magnetic field shocks; (ix) advection
of such large-amplitude waves should cause periodic reformation of the quasi-parallel shock and eject large-amplitude magnetic
field vortices downstream where they contribute to turbulence and to maintaining an extended region of large magnetic fields. 相似文献
2.
Peter Goldreich 《Astrophysics and Space Science》2001,278(1-2):17-23
The inertial range of incompressible MHD turbulence is most conveniently described in terms of counter propagating waves.
Shear Alfvén waves control the cascade dynamics. Slow waves play a passive role and adopt the spectrum set by the shear Alfvén
waves. Cascades composed entirely of shear Alfvén waves do not generate a significant measure of slow waves. MHD turbulence
is anisotropic with energy cascading more rapidly along k
⊥ than along k
∥. Anisotropy increases with k
⊥ such that the excited modes are confined inside a cone bounded by k
∥∝ k
perp
2/3. The opening angle of the cone, θ(k
⊥)∝ k
⊥
-1/3, defines the scale dependent anisotropy. MHD turbulence is generically strong in the sense that the waves which comprise
it are critically damped. Nevertheless, deep inside the inertial range, turbulent fluctuations are small. Their energy density
is less than that of the background field by a factor θ2(k
⊥)≪. MHD cascades are best understood geometrically. Wave packets suffer distortions as they move along magnetic field lines
perturbed by counter propagating wave packets. Field lines perturbed by unidirectional waves map planes perpendicular to the
local field into each other. Shear Alfvén waves are responsible for the mapping's shear and slow waves for its dilatation.
The former exceeds the latter by θ-1(k
⊥)≫ 1 which accounts for dominance of the shear Alfvén waves in controlling the cascade dynamics.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
Ramon J. Quiroga 《Astrophysics and Space Science》1983,93(1):37-54
Statistics in absorption 21-cm data show two main types of clouds at low galactic latitudes: dense small clouds, many of them
with molecular cores, with dispersions σ≈1.5 km s−1 and large clouds forming the fine features of the spiral arms (the shingle like features) with a dispersion range α≈3–4 km
s−1. Sizes and dispersions of both types of clouds are compatible with the Kolmogorov law of turbulence: σ∞d
1/3. The large clouds forming the shingle-like features can be considered as the largest clouds of a Kolmogorov spectrum (the
initial vortices), or as the hydrodynamic features with minimum sizes in the Galaxy. In order to define hydrodynamic motions
in the same sense as given by Ogrodnikov (1965) we use here the tensorial form of the Helmholtz theorem to obtain an approximation
for the hydrodynamic motions depending on distances and seen from the local standard of rest:V
r
∞r. The intermediate range of sizes between turbulent motions and hydrodynamic motions is 100<d<300 pc which is also the range of sizes of the large clouds forming the fine features of the spiral arms.
A classification on of motions in the Galaxy is postulated: (a) a basic rotation motion given by an smooth unperturbed curveΘ
b
(R) associated to the old disk population. (b) Systematic motions of the spiral arms. (c) Systematic motions in the fine structure
of the arms. For scale sizes smaller than these fine features one has turbulent motions according to the Kolmogorov law.
The densities and sizes of the turbulent clouds behave asn
H
∝d
−2 in a range of sizes 7 pc<d<300 pc. The obtained gas densities of the clouds are confirmed with the dust densities from photometric studies. The conditions
for gravitational binding of the clouds are analyzed. Factors as the geometry and the magnetic field within the clouds increases
the critic densities for gravitational binding. When we consider these factors we find that the wide component clouds have
densities below such a critical value.
The narrow component clouds have densities similar or above the critical value; but the real fraction of collapsing clouds
remains unknown as far as the factor of geometry and the inner magnetic field of each cloud are not determinated. 相似文献
4.
We develop a theory for radar signal scattering by anisotropic Langmuir turbulence in the solar corona due to a t+l ⇄ t process. Langmuir turbulence is assumed to be generated within a cone by a narrow type III burst electron beam. Using wave-kinetic
theory we obtain expressions for the frequency shift, scattering cross-section of the turbulence, coefficient of absorption
(due to scattering) and optical depth. On the basis of those expressions we give some estimates for an echo spectrum. We show
that the minimum radar echo frequency shift is determined by the minimal phase velocity of the Langmuir waves, the maximum
shift is determined by the electron beam velocity, but in any case it can not exceed −wt/2 (decay) and wt (coalescence), where
wt is the frequency of a radar signal. The angular characteristics of the scattered signal differ dramatically for the cases
of coalescence and decay. The signal is scattered into a narrow cone high above the specular reflection point (wp ≪ wt), but
in the vicinity of wp ∼ wt/2 the red-shifted echo is scattered isotropically, while the blue-shifted echo is scattered into
a even narrower cone. We show that absorption (due to scattering) increases with increasing radar frequency. The dependence
of the absorption on the local plasma frequency is strongly determined by the Langmuir turbulence spectrum. Our theory shows
that the role of the nonlinear scattering process t+l ⇄ t is essential and that such process can be used for radar studies of the spectral energy density of anisotropic Langmuir turbulence. 相似文献
5.
This paper presents the results of the optical R band and 1.5–12 keV band X-ray monitoring of the high-energy peaked BL Lacertae source 1ES 1959+650 performed during 2002–2007
with the 70 cm Meniscus Telescope of Abastumani Astrophysical Observatory (Georgia) and the All-Sky Monitor on board the Rossi
X-ray Time Explorer, respectively. The observed long- and short-term outbursts are fitted with the lightcurves obtained by
means of the modeling of synchrotron flares that are assumed to be the result of a propagation of the relativistic shock waves
through the jet of 1ES 1959+650, pointed to the observer. Different values of the input parameters (shock velocity, particles’
spectral index, sizes of emission region, minimum and maximum Lorentz factors of the particles etc.) are used in order to
fit the simulated lightcurves whose constructed by means of observational data. This investigation shows that both shock velocity
and physical conditions in the jet of 1ES 1959+650 should be variable from flare to flare. The shocks are found to be mildly
relativistic with the apparent speeds β=0.46–0.85, expressed in the units of c. Spectral index of the particle energy distribution varied from 2.10 to 2.17 for the long-term flares while it is higher
in the case of short-term outbursts: s=2.32–2.45 that is suggested to be a result of the deceleration of shock front during its passage through the shell situated
downstream the Mach disc. The average strength of a turbulent magnetic field ranged from 0.025 gauss to 0.04 gauss for different
long-term flares while the values of 0.07–0.14 gauss were adopted for the different short-term outbursts. The lengths of variable
jet area found to be of 0.13–0.47 pc with the transverse extents of (0.5–1.0)×1017 cm in the case of long-term flares. The same characteristics for short-term outbursts were (2.74–5.5)×1016 cm and (0.2–04)×1017 cm, respectively. We conclude that both shock velocity and properties of pre-shocked plasma were not the same in 1ES 1959+650
for the different flaring epochs. 相似文献
6.
As was demonstrated in earlier studies, turbulence can result in a negative contribution to the effective mean magnetic pressure, which, in turn, can cause a large‐scale instability. In this study, hydromagnetic mean‐field modelling is performed for an isothermally stratified layer in the presence of a horizontal magnetic field. The negative effective magnetic pressure instability (NEMPI) is comprehensively investigated. It is shown that, if the effect of turbulence on the mean magnetic tension force vanishes, which is consistent with results from direct numerical simulations of forced turbulence, the fastest growing eigenmodes of NEMPI are two‐dimensional. The growth rate is found to depend on a parameter β* characterizing the turbulent contribution of the effective mean magnetic pressure for moderately strong mean magnetic fields. A fit formula is proposed that gives the growth rate as a function of turbulent kinematic viscosity, turbulent magnetic diffusivity, the density scale height, and the parameter β*. The strength of the imposed magnetic field does not explicitly enter provided the location of the vertical boundaries are chosen such that the maximum of the eigenmode of NEMPI fits into the domain. The formation of sunspots and solar active regions is discussed as possible applications of NEMPI (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
7.
L.A. Pustil'nik 《Astrophysics and Space Science》1998,264(1-4):171-182
We present analysis of flare process as "phase transition" phenomena caused by frustration of current percolation in turbulent
current sheet. We show that numerous plasma instabilities in the sheet will form random resistors network with "bad resistors"-turbulent
domains and "good resistors"-normal plasma domains. We show that current percolation in random inhomogeneous turbulent current
sheet like to another percolated systems is able to produce phase transition with drastic change of global properties of system
as whole (conductivity, heat-conductivity, elasticity,) on the threshold value of critical density of "bad" elements (p= p
c
). Another property of solar flares, what may be understood on the base of percolation approach is observed universal power
dependence of frequency of flares and microflares (elementary events-spikes) on their amplitude: N
W
∝ W
−k
. It may be explained as natural sequence of universal power dependence of clusters' masses in percolated systems on their
sizes. The slope of resulted spectra is determined by the fractal dimension of clusters and depends on feedback between current
propagation and turbulence generation. We show that percolation approach allow to explain phenomena of preflare bursts-precursors
observed in radio and hard X-ray. It may be understood as results of pre-catastrophic lose of elasticity of system to small
disturbance on the percolation threshold, with formation of short life nuclear of "new phase".
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
K. Petrovay 《Solar physics》2003,215(1):17-30
The first consistent model for the turbulent tachocline is presented, with the turbulent diffusivity computed within the model instead of being specified arbitrarily. For the origin of the 3D turbulence a new mechanism is proposed. Owing to the strongly stable stratification, the mean radial shear is stable, while the horizontal shear is expected to drive predominantly horizontal, quasi-2D motions in thin slabs. Here I suggest that a major source of 3D overturning turbulent motions in the tachocline is the secondary shear instability due to the strong, random vertical shear arising between the uncorrelated horizontal flows in neighboring slabs. A formula for the vertical diffusivity due to this turbulence, Equation (9), is derived and applied in a simplified 1D model of the tachocline. It is found that Maxwell stresses due to an oscillatory poloidal magnetic field of a few hundred gauss are able to confine the tachocline to a thickness less than 5 Mm. The integral scale of the 3D overturning turbulence is the buoyancy scale, on the order of 10 km, and its velocity amplitude is a few m s–1, yielding a vertical turbulent diffusivity on the order of 108 cm2 s–1. 相似文献
9.
G. Brunetti 《Journal of Astrophysics and Astronomy》2011,32(4):437-445
Giant radio halos in galaxy clusters probe mechanisms of particle acceleration connected with cluster merger events. Shocks
and turbulence are driven in the inter-galactic medium (IGM) during clusters mergers and may have a deep impact on the non-thermal
properties of galaxy clusters. Models of turbulent (re)acceleration of relativistic particles allow good correspondence with
present observations, from radio halos to γ-ray upper limits, although several aspects of this complex scenario still remain poorly understood. 相似文献
10.
J. F. Hansen H. F. Robey A. R. Miles R. I. Klein 《Astrophysics and Space Science》2007,307(1-3):147-152
The interaction of supernova shocks and interstellar clouds is an important astrophysical phenomenon since it can result in
stellar and planetary formation. Our experiments attempt to simulate this mass-loading as it occurs when a shock passes through
interstellar clouds. We drive a strong shock using the Omega laser (∼5kJ)into a foam-filled cylinder with an embedded Al sphere(diameterD=120 μm) simulating an interstellar cloud. The density ratio between Al and foamis∼9. We have previously reported on the interaction
between shock and cloud, the ensuing Kelvin-Helmholtz and Widnall instabilities, and the rapid stripping of all mass from
the cloud. We now present a theory that explains the rapid mass-stripping. The theory combines (1) the integral momentum equations
for a viscous boundary layer, (2) the equations for a potential flow past a sphere, (3) Spalding's law of the wall for turbulent
boundary layers, and (4) the skin friction coefficient for a turbulent boundary layer on a flat plate. The theory gives as
its final result the mass stripped from a sphere in a turbulent high Reynolds number flow, and it agrees very well with our
experimental observations. 相似文献
11.
The property of inhomogeneous turbulence in conducting fluids to expel large‐scale magnetic fields in the direction of decreasing turbulence intensity is shown as important for the magnetic field dynamics near the base of a stellar convection zone. The downward diamagnetic pumping confines a fossil internal magnetic field in the radiative core so that the field geometry is appropriate for formation of the solar tachocline. For the stars of solar age, the diamagnetic confinement is efficient only if the ratio of turbulent magnetic diffusivity ηT of the convection zone to the (microscopic or turbulent) diffusivity ηin of the radiative interior is ηT/ηin ≥ 105. Confinement in younger stars requires larger ηT/ηin. The observation of persistent magnetic structures on young solar‐type stars can thus provide evidence for the nonexistence of tachoclines in stellar interiors and on the level of turbulence in radiative cores. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
12.
Bidzina Z. Kapanadze 《Astrophysics and Space Science》2012,337(1):343-351
This paper presents a modeling of the variable synchrotron emission in the BL Lacertae sources (BLLs). Flux variability is
assumed to be a result of the interaction between a relativistic shock wave with a magnetized jet material. Long-term flares
(of months to years durations) are modeled via the propagation of a plane relativistic shock wave though the emission zone
of a cylindrical form with the radius R and length H. As for short-term bursts (lasting from days to weeks), they may result from shock passage through the jet inhomogeneities
such as a shell of enhanced density downstream to a Mach disc, originated due to pressure imbalance between the jet and its
ambient medium. Emitting particles (electrons) gain the energies, sufficient to produce synchrotron photons at optical—X-ray
frequencies, via the first-order Fermi mechanism. Observation’s frequency is the main parameter determining a rate of the
increase/ decay of the emission via the characteristic decay time of emitting electrons. The magnetic field, assumed to be
turbulent with an average field constant throughout the entire emission zone, is another key parameter determining the slope
of a lightcurve corresponding to the flare—the higher strength the magnetic field has, the steeper the lightcurve is. The
rest input parameters (shock speed, jet viewing angle, maximum/minimum energies of the electrons, particles’ density etc.),
as well the strength of average magnetic field, influence the energy output from a flare. 相似文献
13.
An intermediate shock-like event was observed by Voyager 2 on 9 January 1979. The discontinuity structure is well identified to be a 2→3 type intermediate shock by fitting the Rankine – Hugoniot
relations. The shock satisfies the following conditions: i) the plasma density increases from the upstream region to the downstream region, ii) The normal Alfvén Mach number (M
AN) is greater than unity in the preshock state and less than unity in the postshock state, iii) The fast-mode Mach numbers in the upstream and downstream regions are less than unity and both the slow-mode Mach numbers
are greater than unity, iv) The tangential component of the magnetic field changes sign across the shock front. 相似文献
14.
Plasma and magnetic field parameter variations across fast forward interplanetary shocks are analyzed during the last solar
cycle minimum (1995–1996, 15 shocks), and maximum year 2000 (50 shocks). It was observed that the solar wind velocity and
magnetic field strength variation across the shocks were the parameters better correlated with Dst. Superposed epoch analysis centered on the shock showed that, during solar minimum, B
z
profiles had a southward, long-duration variation superposed with fluctuations, whereas in solar maximum the B
z
profile presented 2 peaks. The first peak occurred 4 hr after the shock, and seems to be associated with the magnetic field
disturbed by the shock in the sheath region. The second peak occurred 19 hr after the shock, and seems to be associated with
the ejecta fields. The difference in shape and peak in solar maximum (Dst peak =−50 nT, moderate activity) and minimum (Dst peak =−30 nT, weak activity) in average Dst profiles after shocks are, probably, a consequence of the energy injection in the magnetosphere being driven by different
interplanetary southward magnetic structures. A statistical distribution of geomagnetic activity levels following interplanetary
shocks was also obtained. It was observed that during solar maximum, 36% of interplanetary shocks were followed by intense
(Dst≤−100 nT) and 28% by moderate (−50≤Dst <−100 nT) geomagnetic activity. During solar minimum, 13% and 33% of the shocks were followed by intense and moderate geomagnetic
activity, respectively. Thus, during solar maximum a higher relative number of interplanetary shocks might be followed by
intense geomagnetic activity than during solar minimum. One can extrapolate, for forecasting goals, that during a whole solar
cycle a shock has a probability of around 50–60% to be followed by intense/moderate geomagnetic activity. 相似文献
15.
Helioseismic measurements indicate that the solar tachocline is very thin, its full thickness not exceeding 4% of the solar
radius. The mechanism that inhibits differential rotation to propagate from the convective zone to deeper into the radiative
zone is not known, though several propositions have been made. In this paper we demonstrate by numerical models and analytic
estimates that the tachocline can be confined to its observed thickness by a poloidal magnetic field B
p of about one kilogauss, penetrating below the convective zone and oscillating with a period of 22 years, if the tachocline
region is turbulent with a diffusivity of η∼1010 cm2 s−1 (for a turbulent magnetic Prandtl number of unity). We also show that a similar confinement may be produced for other pairs
of the parameter values (B
p, η). The assumption of the dynamo field penetrating into the tachocline is consistent whenever η≳109 cm2 s−1.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1013389631585 相似文献
16.
Wim J. Weber 《Solar physics》1981,69(1):119-130
If a solar flare originates from the dissipation of magnetic energy, available in abundance in a larger region, this dissipation must take place very rapidly. A local topological change in the magnetic field structure may be sufficient to start the dissipation process. Such a change in topology might be obtained by fast reconnection in a smaller region, such as e.g. in the Sweet-Parker model, as a result of current-driven microinstabilities.Among the candidates satisfying the requirements to obtain large enough currents, such as magnetically neutral or current sheets and MHD shocks, the latter are shown to be most probable. In a fast MHD shock the (thermal) results of turbulence do in fact destroy the conditions for turbulence. However, in this work we show numerically that the nonlinear steepening mechanism of such a shock is able to restore the driving current for a large range of parameters and over a long time. This is still true if the most difficult threshold for turbulence, being that for Langmuir turbulence, is to be achieved. The critical parameter, not only for the occurrence of turbulence but also for the restoration of the driving current, is the shock thickness. 相似文献
17.
The Bastille Day (14 July) 2000 CME is a fast, halo coronal mass ejection event headed earthward. The ejection reached Earth
on 15 July 2000 and produced a very significant magnetic storm and widespread aurora. At 1 AU the Wind spacecraft recorded a strong forward shock with a speed jump from ∼ 600 to over 1000 km s−1. About 6 months later, this CME-driven shock arrived at Voyager 2 (∼ 63 AU) on 12 January 2001 with a speed jump of ∼ 60 km s−1. This provides a good opportunity to study the shock propagation in the outer heliosphere. In this study, we employ a 2.5-D
MHD numerical model, which takes the interaction of solar wind protons and interstellar neutrals into account, to investigate
the shock propagation in detail and compare the model predictions with the Voyager 2 observations. The Bastille Day CME shock undergoes a dramatic change in character from the inner to outer heliosphere. Its
strength and propagation speed decay significantly with distance. The model results at the location of Voyager 2 are in good agreement with in-situ observations.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1014293527951 相似文献
18.
M. Mehdipoor 《Astrophysics and Space Science》2012,338(1):73-79
Korteweg-de-Vries-Burger (K-dVB) equation is derived for ion acoustic shock waves in electron-positron-ion plasmas. Electrons
and positrons are considered superthermal and are effectively modeled by a kappa distribution in which ions are as cold fluid.
The analytical traveling wave solutions of the K-dVB equation investigated, through the (G′/G)-expansion method. These traveling wave solutions are expressed by hyperbolic function, trigonometric functions are rational
functions. When the parameters are taken special values, the shock waves are derived from the traveling waves. It is observed
that the amplitude ion acoustic shock waves increase as spectral index κ and kinematic viscosity η
i,0 increases in which with increasing positron density β and electron temperature σ the shock amplitude decreases. Also, numerically the effect different parameters on the nonlinearity A and dispersive B terms and wave velocity V investigated. 相似文献
19.
Observations indicate that in plage areas (i.e. in active regions outside sunspots) acoustic waves travel faster than in the quiet Sun, leading to shortened travel times
and higher p-mode frequencies. Coupled with the 11-year variation of solar activity, this may also explain the solar cycle variation of
oscillation frequencies. While it is clear that the ultimate cause of any difference between the quiet Sun and plage is the
presence of magnetic fields of order 100 G in the latter, the mechanism by which the magnetic field exerts its influence has
not yet been conclusively identified. One possible such mechanism is suggested by the observation that granular motions in
plage areas tend to be slightly “abnormal”, dampened compared to the quiet Sun.
In this paper we consider the effect that abnormal granulation observed in active regions should have on the propagation of
acoustic waves. Any such effect is found to be limited to a shallow surface layer where sound waves propagate nearly vertically.
The magnetically suppressed turbulence implies higher sound speeds, leading to shorter travel times. This time shift Δ
τ is independent of the travel distance, while it shows a characteristic dependence on the assumed plage field strength. As
a consequence of the variation of the acoustic cutoff with height, Δ
τ is expected to be significantly higher for higher frequency waves within the observed regime of 3 – 5 mHz. The lower group
velocity near the upper reflection point further leads to an increased envelope time shift, as compared to the phase shift.
p-mode frequencies in plage areas are increased by a corresponding amount, Δ
ν/ν=ν
Δ
τ. These characteristics of the time and frequency shifts are in accordance with observations. The calculated overall amplitudes
of the time and frequency shifts are comparable to, but still significantly less than (by a factor of 2 to 5), those suggested
by measurements. 相似文献
20.
A double discontinuity is a rarely observed compound structure composed of a slow shock layer and an adjoining rotational
discontinuity layer in the downstream region. In this paper, we report the observations of a double discontinuity detected
by Wind on May 15, 1997. This double discontinuity is found to be the front boundary of a magnetic cloud boundary layer. We
strictly identify the shock layer and the rotational discontinuity layer by using the high-resolution plasma and magnetic
field data from Wind. The observed jump conditions of the upstream and downstream region of the slow shock layer are in good
agreement with the Rankine – Hugoniot relations. The flow speeds in the shock frame U
n
<V
Acos θ
Bn
on both sides of the slow shock layer. In the upstream region, the slow Mach number M
s1=U
n1/V
s1 is 1.95 (above unity), and in the downstream region, the slow Mach number M
s2=U
n2/V
s2 is 0.31 (below unity). Here V
A and V
s represent the Alfvén speed and the local slow magnetosonic speed, respectively, and θ
Bn
is the angle between the direction of the magnetic field and the shock normal. The magnetic cloud boundary layer observed
by Wind was also detected by Geotail 48 min later when the spacecraft was located outside the bow shock of the magnetosphere.
However, Geotail observations showed that its front boundary was no longer a double discontinuity and the rotational discontinuity
layer disappeared, indicating that this double discontinuity was unstable when propagating from Wind to Geotail. 相似文献