摘要:

The kinetic structure and stability of subfast intermediate shocks (IS) are investigated using a hybrid code. The shocks are formed dynamically by the interaction between a flowing plasma and a stationary piston. For θBN= 60° and plasma β = 0.46, the strong IS is found to be stable with a width in the range of 10 to 20 ion inertial lengths (λi). The rotation of the transverse component of the magnetic field is in the ion sense. The weak IS has a more complex structure and consists of both Alfvén and slow waves. The leading edge of the shock is dominated by the Alfvén mode and is associated with a S‐shape electron sense field rotation with a small decrease (increase) in the magnetic field (density) across it. Some of the ion dissipation occurs within this layer, which is relatively thin (∼ 17 – 20λi). However, the transition to the downstream density and magnetic field occurs in the much wider (∼ 150λi) trailing slow wave. The main heating associated with this trailing edge occurs in the direction parallel to the magnetic field. This slow wave has a phase velocity larger than the Alfvén speed due to kinetic corrections to linear wave properties. As a result, the slow wave stays attached to the leading edge of the shock which remains time‐stationary. The classification of IS's, based on phase velocity of MHD modes, becomes ambiguous in the kinetic limit. When the magnetic field is noncoplanar, the strong IS becomes time‐dependent and expands self‐similarly in time, whereas the weak IS di

ISSN:0094-8276    

DOI:10.1029/92GL01994

年代:1992

数据来源: WILEY

摘要:

The magnetic field often exhibits a maximum in the Earth's low‐latitude boundary layer. We show examples of this behavior, using data from the AMPTE/IRM spacecraft, and argue that two fundamentally distinct causes exist for the excess field: (i) a depression, within the layer, of the population of medium‐energy ions of magnetospheric origin; (ii) field curvature effects associated with undulations of the magnetopause its

ISSN:0094-8276    

DOI:10.1029/92GL01809

年代:1992

数据来源: WILEY

摘要:

Some implications are considered of recent theoretical work concerning the excitation of dayside ionospheric convection by magnetic reconnection at the dayside magnetopause. In particular, transient bursts of such reconnection (‘flux transfer events’) are considered as a cause of polar cap ‘patches’ of enhanced plasma density. Examples of such patches, as observed at European longitudes by the EISCAT radar, are presented and used to discuss the implications of the proposed me

ISSN:0094-8276    

DOI:10.1029/92GL01993

年代:1992

数据来源: WILEY

摘要:

On 15 October 1981 Dynamics Explorer 2 crossed the polar cusp at 1015 MLT and observed three distinct ion populations as it was moving poleward. These three populations had peak‐flux energy around 2.7 keV, 850 eV and 360 eV. At the time of observation, the IMF was southward. The first step coincided with a rotation of the flow from westward to poleward and then eastward. The second and third steps showed a flow directed principally poleward. Furthermore, the magnetic and electric perturbations in the first step are well fitted by an elongated flux tube footprint model. These results suggest that three consecutive Flux Transfer Events (FTEs) have injected solar wind plasma into the ionosphere forming the polar cusp. The individual FTE signatures can only be identified by the jumps in the precipitation pattern. The newest reconnected FTE footprint was crossed near the edge of the event while the two oldest ones were crossed around the event center. The small latitudinal size of these FTE footprints (∼ 40 km) and their short recurrence rate (3, 6 min) is consistent with an intermittent reconnection taking place at the subsolar point on a short time sc

ISSN:0094-8276    

DOI:10.1029/92GL01806

年代:1992

数据来源: WILEY

摘要:

This paper discusses important new findings obtained from global kinetic simulations of magnetotail plasma. A region of strongly non‐adiabatic ion acceleration (known as the “wall” region) exists in the near Earth tail and demarcates two very different regimes of ion motion: adiabatic and quasiadiabatic. A strong enhancement of the cross‐tail current occurs on the tailward side of the wall. Our comparison of numerical and adiabatic pressure profiles indicates that non‐adiabatic processes operating in this region may contribute significantly to a pressure balance relief in the course of quasisteady magnetospheric c

ISSN:0094-8276    

DOI:10.1029/92GL01810

年代:1992

数据来源: WILEY

摘要:

This paper reports a transient strong diamagnetic event observed with the ISEE‐1 and ‐2 spacecraft in the near‐Earth magnetotail. Magnetic signatures associated with the event resemble those of flux rope events found in the distant magnetotail. The present event should be distinguished from those in several previous reports based on the following points: (1) The event is observed in the near‐Earth tail (X∼−17RE), not in the distant tail. (2) The tail field changes from a stressed configuration to a more dipolar one in the course of the event, and the plasma data suggest that the heating and the resultant expansion of the plasma take place simultaneously. (3) The minimum of the magnetic field strengthBTis ∼1 nT; such a strong depression is rarely observed in the near‐Earth tail. (4) An earthward plasma jet (300 to 500 km/s) continues for about 3 min, including a period of very weak magnetic field; the duration of the jet is exceptionally long compared to previously reported high‐speed flows in the plasma sheet. These magnetic field and plasma signatures are discussed in terms of two models: an earthward‐moving plasma bulge and a tai

ISSN:0094-8276    

DOI:10.1029/92GL01805

年代:1992

数据来源: WILEY

摘要:

Recent results on the comparative efficiencies of VLF versus HF heating of a typical nocturnal midlatitude ionosphere have been extended. The effects of varying the magnetic latitude and electron density profile of the ionosphere are investigated. Our results indicate that the superior heating efficiency of VLF vs HF radiation and the resulting perturbation on VLF propagation is a latitude‐dependent phenomenon, the effects being accentuated at low magnetic latitude and reversed at polar latitudes. The marked superiority of VLF heating is also found to be a feature of nocturnal ionospheres. VLF and HF radiation produce similar heating effects in ionospheres typical of summer daytime conditions. The optimum design of experiments for the study of the perturbation of VLF propagation by VLF heating transmitters is discusse

ISSN:0094-8276    

DOI:10.1029/92GL01621

年代:1992

数据来源: WILEY

摘要:

Observations of the Mount Pinatubo cloud as it moved northward and into the winter polar vortex were made with a balloon borne two‐wavelength backscattersonde. Some volcanic debris had arrived at far northerly latitudes below 20 km by October and was apparently incorporated into the initial vortex. Subsequent measurements did not show a significant increase in the central vortex aerosol until after a mid January disturbance, at which time the backscatter profiles began to increase. Above 20 km and near the center of the vortex there was no significant increase in aerosol through mid March. The column stratospheric aerosol mass loading as calculated from individual soundings at the wall of the vortex during March 1992 ranged from 18 to 24 megatonnes per unit area where the unit of area is that of the earth. This indicates that a significant amount of material was transported northward by the end of the winte

ISSN:0094-8276    

DOI:10.1029/92GL01939

年代:1992

数据来源: WILEY

摘要:

Lidar and balloonborne particle counter measurements were performed simultaneously on two days when polar stratospheric clouds were observed in late August 1991 at McMurdo, Antarctica. Both nitric acid trihydrate and ice clouds were observed in the lower stratosphere between 10 and 23 km in different formation stages and with different cooling rate; however in all cases the size distributions were bimodal. Comparison of scattering ratios measured by lidar and calculated from particle size distributions are in good agreement; however, discrepancies were observed when the lower stratosphere was highly perturbed by wave activity. Lee waves generated by air flowing over the Trans Antarctic Mountains induced ice cloud formation at altitudes as high as 20 km. No PSCs were observed after the end of August in 1991.

ISSN:0094-8276    

DOI:10.1029/92GL01941

年代:1992

数据来源: WILEY

摘要:

The potential impact of volcanic aerosols on nucleation of ice crystals in upper tropospheric cirrus clouds is examined from a microphysical perspective. The sulfuric acid aerosols which form in the stratosphere are presumably transported into the troposphere by sedimentation and tropopause folding. The tropospheric volcanic aerosol size distribution is estimated from 10 μm lidar backscatter measurements [Post, 1986] and in situ measurements [Pueschel et al., 1992b], Microphysical simulations suggest that at temperatures below about −50°C the concentration of ice crystals which nucleate may be as much as a factor of 5 larger when volcanic aerosols are present. Our simulations suggest that the presence of volcanic aerosols may increases the net radiative forcing (surface warming) of certain types of cirrus near the tropopause by as much as 8 W/m2. Further observations are required to determine whether these effects actually occur, and their global imp

ISSN:0094-8276    

DOI:10.1029/92GL01936

年代:1992

数据来源: WILEY