摘要:

Using a one‐dimensional hydrodynamic code we have simulated the radial evolution of heliospheric disturbances produced by slow coronal mass ejections (CMEs) embedded in much faster leading and trailing ambient solar wind. We find that pressure gradients induced by initial speed differences between slow CMEs and faster ambient wind can produce large accelerations of the CMEs, eventually nearly up to the speed of the ambient wind. The compressions, rarefactions, and shocks associated with this acceleration persist to large heliocentric distances. Comparison with observations reveals that such effects helped accelerate two of the six high‐latitude CME events observed by Ulys

ISSN:0094-8276    

DOI:10.1029/96GL02843

年代:1996

数据来源: WILEY

摘要:

Observations from the SWICS instrument on Ulysses reveal that there is little evidence for the acceleration of interstellar pick‐up ions at the forward and reverse shocks which surround Co‐rotating Interactions Regions (CIR's) in the solar wind. Rather, the pick‐up ions exhibit strong acceleration in regions within the CIR where there are increased variations in the magnitude of the magnetic field. A simple model is presented in which the pick‐up ions are statistically accelerated by transit‐time damping the magnitude variations. The model yields the observed distribution functions for the accelerated pic

ISSN:0094-8276    

DOI:10.1029/96GL02833

年代:1996

数据来源: WILEY

摘要:

A model for the generation of the Io plasma torus “ribbon” density feature is proposed. In this model a self‐regulation mechanism for plasma pressure is hypothesized, operating through ring current inhibition of flux‐tube interchange transport. Torus plasma outflow drives hot plasma from the middle magnetosphere inwards, raising its confining pressure and impeding further outflow of torus plasma. As the hot plasma cools by collisions with torus ribbon electrons its pressure diminishes, allowing more torus outflow. This negative feedback modulates torus ion flux‐tube contentNL² in such a way as to hold it relatively constant. In order to simulate this process, we have constructed numerical models which approximate plasma transport by a non‐linear diffusion process, with the feedback mechanisms built into the diffusivity. For certain reasonable parameter values, these models produce a density enhancement nearL= 5.6 to 5.8 which somewhat resembles the “ribbon” feature that is actually observed ne

ISSN:0094-8276    

DOI:10.1029/96GL02756

年代:1996

数据来源: WILEY

摘要:

The Galileo spacecraft recently measured an ≈ 40% decrease of Jupiter's magnetic field near Io from a distance of 0.5 RIo[Kivelson et al.,1996], a signature that cannot be explained in terms of the previously known Alfven wing current system alone. The Galileo observations can be explained by an additional magnetic moment of Io aligned with the ambient Jovian field. There is no Earth‐like magnetosphere at Io because its magnetic moment is not sufficient to stand off the Jovian field far above Io's atmosphere. The Alfven wing current system must close within the Io interior, implying an electrical conductivity in the outer layers much higher than that expected for solid rock alone. Since widespread sulfur‐rich aquifers are present to support Io's extensive volcanism, the thermal and electrical conductivities in Io's outer layers will be higher than would be the case for solid rock. We suggest that on average the temperature remains below the Curie point to a depth on the order of at least 300 km. Io's magnetic moment could then be explained as thermoremanent magnetization of ferromagnetic material. In this model, Io's outer layers cooled through the Curie temperature in a magnetic field about 15 times the current ambient field at Io. At that time, Io was closer to Jupiter, and/or Jupiter's dynamo magnetic moment was str

ISSN:0094-8276    

DOI:10.1029/96GL02764

年代:1996

数据来源: WILEY

摘要:

Three‐dimensional, global MHD simulations of solar wind flow onto a prescribed magnetopause obstacle are used to show that a bow shock's nose locationasand the relative subsolar magnetosheath thickness Δms/ampare strong functions of the IMF cone angle θ (betweenvswandBsw) and the Alfven Mach numberMAFor a givenMAthe shock is more distant for higher θ (restricted to the interval 0–90° by symmetries), whileas/ampand Δms/ampincrease with decreasingMAfor θ ≳ 20° but decrease with decreasingMAfor θ ∼ 0°. Large differences in Δms/ampare predicted between θ = 0° and 90° at lowMA, with smaller differences remaining even atMA∼ 10. The θ = 0° results confirm and extend the previous work ofSpreiter and Rizzi[1974]. The simulations show that successful models for the subsolar shock location cannot subsume the dependences onMAand θ into a sole dependence onMms. Instead, they confirm a recent prediction [Cairns and Grabbe, 1994] thatas/ampand Δms/ampshould depend strongly on θ andMAforMA≲ 10 (as well as on other MHD variables). Detailed comparisons between theory and data remain to be done. However, preliminary comparisons show good agreement, with distant shock locations found for lowMAand large θ ≳ 45° and closer locatio

ISSN:0094-8276    

DOI:10.1029/96GL02755

年代:1996

数据来源: WILEY

摘要:

Scattering by enhanced fluctuations from the electromagnetic proton cyclotron instability imposes an upper bound on the proton temperature anisotropy of the formT⟂p/T∥p−1 =Sp/β∥pαp. Here β∥pis the proton β based onT∥pand the directional subscripts indicate directions perpendicular and parallel to the background magnetic field. This paper describes the analysis of the proton temperature anisotropy in two‐dimensional hybrid simulations of quasi‐perpendicular shocks and their associated magnetosheaths. Three simulations are considered: at low, intermediate, and high upstream Mach numbers. Considering results from well downstream of all three shocks as one data set, an upper bound on the proton temperature anisotropy fits the above equation with αp=0.5. This compares favorably to αpvalues from other simulations and fro

ISSN:0094-8276    

DOI:10.1029/96GL02882

年代:1996

数据来源: WILEY

摘要:

We use a two dimensional model of the magnetosheath, not including the bow shock, to examine the effects of pressure anisotropy on magnetosheath structure. The simulation plane includes the Earth‐Sun direction and that of the incoming interplanetary magnetic field. Thus the crucial physical effects of compression and draping are both included. Flow of magnetic flux and plasma around the magnetopause in the third direction is modeled by a finite electric field at the magnetopause which allows plasma and flux to cross the magnetopause. The magnetpause is a flux surface, which is a reasonable approximation for low shear conditions. We use three models for the plasma pressure: isotropic adiabatic, double adiabatic, and the bounded anisotropy model [Denton et al.,1994; 1995]. In an isotropic plasma, density depletion results from draping which produces parallel (toB) flow around the magnetopause. This flow is greatly enhanced by the parallel pressure gradient. In an anisotropic plasma, enhancement of density depletion comes about not by the ordinary parallel pressure gradient, but by action of the mirror force. Because this mirror force is large for an anisotropic plasma, use of double adiabatic or bounded anisotropy pressure equations results in greater density depletion than that resulting from isotropic adiabatic theor

ISSN:0094-8276    

DOI:10.1029/96GL01590

年代:1996

数据来源: WILEY

摘要:

We analyze of ion populations observed by the NOAA‐12 satellite within dayside auroral transients. The data are matched with an open magnetopause model which allows for the transmission of magnetosheath ions across one or both of the two Alfvén waves which emanate from the magnetopause reconnection site. It also allows for reflection and acceleration of ions of magnetospheric origin by these waves. From the good agreement found between the model and the observations, we propose that the events and the low‐latitude boundary precipitation are both on open field l

ISSN:0094-8276    

DOI:10.1029/96GL02761

年代:1996

数据来源: WILEY

摘要:

The strong correlation between magnetic storms and southward interplanetary magnetic field (IMF) is well known from linear prediction filter studies using the Dst and IMF data. However, the linear filters change significantly from one storm to another and thus are limited in their predicting ability. Previous studies have indicated nonlinearity in the magnetospheric response as the ring current decay rate varies with the Dst value during storms. We present in this letter nonlinear models for the evolution of the Dst based on the OMNI database for 1964–1990. When solar wind data are available in advance, the evolution of storms can be predicted from the Dst and IMF data. Solar wind data, however, are not available most of the time or are available typically an hour or less in advance. Therefore, we have developed nonlinear predictive models based on the Dst data alone. In the absence of solar wind data, these models cannot predict the storm onset, but can predict the storm evolution, and may identify intense storms from moderate ones. The input‐output model based on IMF and Dst data, the autonomous model based on Dst alone, and a combination of the two can be used as forecasting tools for space weat

ISSN:0094-8276    

DOI:10.1029/96GL02828

年代:1996

数据来源: WILEY

摘要:

Magnetospheric convection can be seen as a form of interchange motions, but unlike in an ideal MHD system, is subject to the ionospheric boundary condition. For general time‐dependent interchanges, the Hall current in the ionosphere generates a field‐aligned current independent of any background gradients. The effect of the Hall conductivity on time‐dependent convection is isolated through a simple box model where pressure gradient and field line curvature are suppressed. Mathematical analysis suggests that the Hall conductivity can destabilize interchange modes with long longitudinal wavelength. The finding comes at the expense of a high degree of geometrical idealization. More realistic computations are needed to test the t

ISSN:0094-8276    

DOI:10.1029/96GL02844

年代:1996

数据来源: WILEY