摘要:

We suggest that the polar heliospheric magnetic field, at large heliocentric distances, may deviate considerably from the generally accepted Archimedean spiral. Instead, we suggest that the large‐scale field near the poles may be dominated by randomly‐oriented transverse magnetic fields with magnitude much larger than the average spiral. The averagevectorfield is still the spiral, but the averagemagnitudemay be much larger. In addition, the field direction is transverse to the radial direction most of the time instead of being nearly radial. This magnetic‐field structure has important consequences for the transport of cosmic rays. Preliminary model calculations suggest changes in the radial gradient of galactic cosmic rays which may improve agreement with observa

ISSN:0094-8276    

DOI:10.1029/GL016i001p00001

年代:1989

数据来源: WILEY

摘要:

VEGA‐1 measurements of the plasma density and magnetic field in the coma of Comet Halley show characteristic signatures over a significant portion of the outbound pass. It is found that the assumption that there is a balance between the thermal and magnetic pressures in these features can be used to obtain estimates of the plasma temperature as a function of distance from the nucleus. These estimates indicate that the ions cool from ∼1.5 × 106°K at 105km to 2 × 105°K at 5 × 104km. The technique used here represents a novel approach whereby temperature measurements can be made in situations where only plasma density and magnetic field data are a

ISSN:0094-8276    

DOI:10.1029/GL016i001p00005

年代:1989

数据来源: WILEY

摘要:

An ion beam resonates with R‐mode waves at two frequency bands. These are a high frequency RH mode (ω » Ωp) and a low frequency RL mode (ω ∼ ΩH2O+), where Ωpand ΩH2O+are cyclotron frequencies of protons and water ions, respectively. We study the nonlinear evolution of beam‐generated RH waves by one‐dimensional hybrid computer experiments. Both wave‐particle and subsequent wave‐;wave interactions are examined. We find that the RH waves through the interaction with an ion beam formed by pick‐up heavy ions near comets, grow to a level ofBw/Bo∼ 0.23. The waves saturate by wave‐wave interactions (via the decay instability) supplying energy to backward travelling daughter R‐mode waves with slightly lowerk(and hence lower ω) as well as forward travelling longitudinal waves. The backward travelling daughter R‐mode waves decay further to granddaughter waves with still lowerk. We show that this inverse cascading process occurs faster than the growth of the RL mode instability and produces a wide band wave spectrum which extends from the wavenumber of beam‐excited RH mode to much lowerk. The nonlinearly excited longitudinal sound waves play a rol

ISSN:0094-8276    

DOI:10.1029/GL016i001p00009

年代:1989

数据来源: WILEY

摘要:

Contrary to the prevailing popular view, we find that some transient ground events with bipolar north‐south signatures are related to variations in solar wind dynamic pressure and not necessarily to magnetic merging. We present simultaneous solar wind plasma observations for two previously reported transient ground events observed at dayside auroral latitudes. During the first event, originally reported by Lanzerotti et al. [1987], conjugate ground magnetometers recorded north‐south magnetic field deflections accompanied by strong deflections in the east‐west and vertical directions. The second event was reported by Todd et al. [1986], who noted ground radar observations indicating strong northward then southward ionospheric flows. The events were associated with the postulated signatures of patchy, sporadic, merging of magnetosheath and magnetospheric magnetic field lines at the dayside magnetopause, known as flux transfer events. Conversely, we demonstrate that the event reported by Lanzerotti et al. was accompanied by a sharp increase in solar wind dynamic pressure, a magnetospheric compression, and a consequent ringing of the magnetospheric magnetic field. The event reported by Todd et al. was associated with a brief but sharp increase in the solar wind dynamic pre

ISSN:0094-8276    

DOI:10.1029/GL016i001p00013

年代:1989

数据来源: WILEY

摘要:

This paper discusses the phenomenological and formal similarities between the merging of aircraft vortex trails and the merging of magnetic field lines in a plasma. High‐resolution photographs are shown of smoke trails from the wing tips of an airplane. These photographs show that the two vortex trails merge together downstream of the aircraft in a way similar to the merging of oppositely directed magnetic field lines in a plasma. Although there are some differences, this correspondence is apparently related to the fact that the vorticity equation in a fluid has the same mathematical form as the magnetic field equation in a MHD plasma. In both cases the merging proceeds at a rate considerably faster than would be predicted from classical estimates of the viscosity and resistivity. The enhanced merging rate in the fluid case appears to result from turbulence that increases the diffusion rate in the merging regio

ISSN:0094-8276    

DOI:10.1029/GL016i001p00017

年代:1989

数据来源: WILEY

摘要:

Recent VIKING satellite observations in the auroral zone have shown the association of elevated ion conics (conics with a low energy cutoff above zero) with upward streaming electrons in the presence of low frequency electric field fluctuations. A self‐consistent particle simulation was developed which assumed the presence of a steady state electric field on auroral zone field lines capable of accelerating ions up the magnetic field lines. Results from this study show that a low frequency ion‐ion two stream instability can be excited. This low frequency instability creates a fluctuating electric field which heats the ions oblique to the magnetic field forming distributions similar to the elevated ion conies. The ion‐ion waves also interact resonantly with electrons and accelerates them in the direction of the ion

ISSN:0094-8276    

DOI:10.1029/GL016i001p00021

年代:1989

数据来源: WILEY

摘要:

In order to study nonlinear interactions of water group ions with large amplitude whistler wave packets detected at the leading edge of steepened magnetosonic waves observed near comet Giacobini‐Zinner, we carried out test particle simulations of water ion interactions with a model wave based on the G‐Z data. As the model wave, we adopted a linearly polarized magnetosonic (MS) wave as the trailing portion of the wave, and circularly polarized whistler waves in the leading edge of the wave. Both the MS and whistler waves area prioriassumed to have large amplitudes. We found that some of the water ions are decelerated in the steepened portion of the magnetosonic wave to the resonance velocity with the whistler wave packets. Through resonance and related nonlinear interaction with the large amplitude whistler waves, the water ions become trapped by the whistler wave packet. An energy balance calculation demonstrates that the trapped ions do lose their kinetic energy during the trapped motion in the whistler wave packet. It is thus demonstrated that the nonlinear trapping motion in the wave structure leads to the effective energy transfer from the water group ions to the whistler wave packets in the leading edge of the steepened MHD wa

ISSN:0094-8276    

DOI:10.1029/GL016i001p00025

年代:1989

数据来源: WILEY

摘要:

Using a hydrodynamic model for the plasma, it is demonstrated that the auroral plasma cavity is capable of drawing an appreciably large flux of oxygen ions, which are normally gravitationally bound. This escape mechanism ofO+does not involve any additional heating or acceleration of the plasma in the ionosphere. The temporal evolution of the outflow shows that it starts near the cavity and penetrates into the ionosphere with a flux front moving down with the ion‐acoustic speed. A steady outflow is reached in a few hours for a cavity at a height of one Reif the cavity is maintained. This time reduces to<1 hour for cavity heights<3000 km. During the transient state (107. An analysis for the steady state shows that outflow is controlled by the density scale height in

ISSN:0094-8276    

DOI:10.1029/GL016i001p00029

年代:1989

数据来源: WILEY

摘要:

Combined observations by meridian‐scanning photometers and the EISCAT radar show that the "midday‐auroral breakup" phenomenon is associated with major increases in ionospheric flow. A sequence of nine events is observed in the early afternoon MLT sector during a period when the IMF is strongly southward with a large positive Bycomponent. Each auroral structure is seen at both 630 and 557.7nm and initially moves westward, accompanied by an increase in potential of 30‐60kV across the north‐south dimension of the EISCAT field‐of‐view. After a few minutes the arc (or arc fragment) moves into the polar cap and fades, and the velocities observed by the radar swing from westward toward northward. We conclude that dayside auroral breakup is closely associated with momentum transfer across the magnetopause which occurs in a series of events 5‐15 minutes apart. The largest of the observed events has dimensions of about 300km (in the direction of westward motion) by 700km, is bounded on its poleward edge by a 5kR arc and is associated with a potential of

ISSN:0094-8276    

DOI:10.1029/GL016i001p00033

年代:1989

数据来源: WILEY

摘要:

Reported here are DE 1 auroral imager observations of an auroral configuration which is given the name “horse‐collar aurora.” The horse‐collar pattern comprises the total area of auroral emissions from a single hemisphere and derives its name from the shape of the emitting area. The pattern is found in images recorded during quiet geomagnetic conditions and is possibly related to the theta aurora, another quiet time configuration of the auroras. This initial report of the DE 1 observations illustrates the horse‐collar aurora with a 2‐hour image sequence that displays its basic features and shows an example of its evolution into a theta‐like auroral pattern. The interplanetary magnetic field was northward during this image sequence and there is some evidence for IMF Byinfluence on the temporal development of the horse‐collar pattern. A preliminary statistical analysis found the horse‐collar pattern appearing in one‐third or more of image sequences recorded during quiet conditions; it did not appear during disturbed conditions. Further study is required to establish more fully the characteristics of the horse‐collar aurora and to determine its implications concerning solar wind‐magnetosphere coupling when

ISSN:0094-8276    

DOI:10.1029/GL016i001p00037

年代:1989

数据来源: WILEY