摘要:

A multiple‐receiver radar technique for imaging the spatial distribution of ionospheric plasma irregularities is introduced and demonstrated with equatorial electrojet data obtained at the Jicamarca Radio Observatory. The images obtained with a few seconds time resolution enable the monitoring of the temporal evolution of the irregularity structures within the radar field of view. Daytime electrojet images contain signatures of localized irregularity patches which drift in the east‐west direction at about the ion‐acoustic vel

ISSN:0094-8276    

DOI:10.1029/90GL02603

年代:1991

数据来源: WILEY

摘要:

Data from the Dynamics Explorer 1 High Altitude Plasma Instrument (HAPI) and magnetometer are used to investigate the sources of field‐aligned currents in the nightside auroral zone. It is found that the formula developed by S. Knight predicts the field‐aligned current density fairly accurately in regions where a significant potential drop can be inferred from the HAPI data; there are, however, regions in which the proportionality between potential drop and field‐aligned current does not hold. In particular, we note occurrences of strong upward field‐aligned current associated not with inverted‐V events but instead with supratherrnal bursts. In addition, upward field‐aligned currents are often observed to peak near the edges of inverted‐V events, rather than in the center as would be predic

ISSN:0094-8276    

DOI:10.1029/90GL02674

年代:1991

数据来源: WILEY

摘要:

First order cyclotron or Landau resonant pitch angle scattering of electrons by oblique whistler waves propagating at large angles to the ambient field are found to be at least as large as that due to parallel propagating waves. Commonly observed precipitation of>40 keV electrons in association with ducted whistlers may thus be accompanied by substantial fluxes of lower energy (10 eV–40 keV) electrons precipitated by the nonducted component

ISSN:0094-8276    

DOI:10.1029/90GL02476

年代:1991

数据来源: WILEY

摘要:

Electric field wave observations in the central plasma sheet show the correlated occurrence of broadband electrostatic noise and electrostatic electron cyclotron harmonic emissions. A model is proposed in which the broadband emissions are electron acoustic waves generated by an observed low energy electron beam, and the cyclotron emissions are generated by the hot electron loss cone instability. The high degree of correlation between the two emissions is provided in the model by the presence of the cold electron beam population, which allows both of the plasma instabilities to grow.

ISSN:0094-8276    

DOI:10.1029/90GL02036

年代:1991

数据来源: WILEY

摘要:

Dipolarization observed in the near‐Earth plasma sheet can be initiated by the substorm expansion onset. Field‐aligned currents at the substorm expansion onset are enhanced first in the ionosphere in response to an enhanced magnetospheric convection. Closure of substorm field‐aligned currents through the plasma sheet can disrupt the cross‐tail current to cause dipolarization in the plasma sheet. The electric field induced by the current disruption can launch a fast‐mode compression wave to result in the plasma injection event. A new X line can form tailward of the dipolarization region during the substorm expansion phase. A timing sequence of substorm events is inferred from the proposed conceptual model of substorms based on the magnetosphere‐ionosphere coupling process on the Alfvén bounc

ISSN:0094-8276    

DOI:10.1029/90GL02435

年代:1991

数据来源: WILEY

摘要:

An unusual occurrence was recorded by Pioneer Venus Orbiter (PVO) in the Venusian ionosheath on November 10‐11, 1981 (orbit 1071): more than an order of magnitude higher densities of ions and electrons were observed than have ever been measured by PVO before then or since. This region of enhanced densities covered an extended region in the dayside ionosheath. The purpose of this paper is to briefly present our analysis of these unique observation

ISSN:0094-8276    

DOI:10.1029/90GL02671

年代:1991

数据来源: WILEY

摘要:

The structure of rotational discontinuities (RDs) has been studied through hybrid simulations for a range of propagation angle Θbnbetween the discontinuity normal and the upstream magnetic field and plasma β. For sufficiently narrow initial states, the simulations produce quasi‐steady “reverse rotation” magnetic field structures for 30° ≤ Θbn≤ 60° and 0<β ≤ 1 on a time scale of roughly 200 Ωi−1. This structure is characterized by a right handed field rotation upstream joined smoothly to a left handed field rotation downstream; its width decreases from 60–70 c/ωpiat Θbn= 30° to less than 25 c/ωpiat Θbn= 60°. The magnetic field hodograms of our RD results have a distinctive S‐shape which is most pronounced in simulations with small Θbnand initially right handed rotations. The reverse rotation structure is the net result of the expansion of the initial current layer via the f

ISSN:0094-8276    

DOI:10.1029/90GL02436

年代:1991

数据来源: WILEY

摘要:

This paper analyzes the large‐scale fluctuations of the interplanetary magnetic field strength observed by Voyager 2 from 23.3 AU to 27.8 AU during the period from day 190, 1987 to day 345, 1988. The q‐th moments of Bτshow scaling behavior in the range of scales from 16 hours to 21 days for −10 ≤ q ≤ 10, i.e., 〈Bτq〉 ∼ τs(q)in this range. s(q) is approximately a quadratic function of q for −4 ≤ q ≤ 4, as one expects for a distribution that is approximately lognormal, but the higher moments diverge from those of a lognormal distribution. The function Dq(q) = 1 + s(q)/(q ‐ 1) has the form that is characteristic of multifractals. For a multifractal magnetic field, the moments of the field should scale as ταon a set with fractal dimension f (α), where α has a continuum of values over some limited range. For the large‐scale magnetic field fluctuations, the function f(α) computed from Dq(q) is approximately a fourth order polynomial for −10 ≤ q ≤ 10, and positive values of f(α) occur for 0.8 ≤ α ≤ 1.2. The multifractal character of the magnetic field strength fluctuations generalizes the concept that the magnetic field is organized into interaction regions (regions in which the magnetic field strength and pressure are higher than average for several hours) and rarefaction regions (regions in which the magnetic field strength and pressure are lower than average). Near solar maximum the interaction regions in the distant heliosphere might be viewed as clusters of strong disturbed fields with considerable fine structure on various scales. The multifractal description of the magnetic fie

ISSN:0094-8276    

DOI:10.1029/90GL02596

年代:1991

数据来源: WILEY

摘要:

We have surveyed the Solar Maximum Mission coronagraph/polarimeter observations for evidence of magnetic disconnection of previously open magnetic structures and have identified several sequences of images consistent with this interpretation. Such disconnection occurs when open field lines above helmet streamers reconnect, in contrast to previously suggested disconnections of CMEs into closed plasmoids. In this paper a clear example of open field disconnection is shown in detail. The event, on June 27, 1988, is preceded by compression of a preexisting helmet streamer and the open coronal field around it. The compressed helmet streamer and surrounding open field region detach in a large U‐shaped structure which subsequently accelerates outward from the Sun. The observed sequence of events is consistent with reconnection across the heliospheric current sheet and the creation of a detached U‐shaped magnetic structure. Unlike CMEs, which may open new magnetic flux into interplanetary space, this process could serve to close off previously open flux, perhaps helping to maintain the roughly constant amount of open magnetic flux observed in interplanetary sp

ISSN:0094-8276    

DOI:10.1029/90GL02480

年代:1991

数据来源: WILEY

摘要:

Resonant absorption of shear Alfvén waves is thought to be a likely candidate to explain heating of the solar corona and acceleration of the solar wind. A difficulty with the theory is that the absorption process is slow. It takes a long time ∼S⅓τA, whereS≫ 1 is the magnetic Reynolds number, and τAa typical Alfvén wave propagation time, for wave fronts to steepen sufficiently for absorption to occur. Moreover, heating occurs in a very thin layer whose width scales asS−⅓. A faster absorption mechanism is nonresonant absorption by compressional viscosity, in a curved magnetic field. Heating is nonresonant and is not localized to a narrow layer. The effect could be quite important where the solar coronal magnetic field is strongly curved, in the chromosphere. It could also be important on open field lines in the upper corona, where the compressional viscosity is large. It might imply that a significant part of outgoing Alfvén waves are absorbed

ISSN:0094-8276    

DOI:10.1029/90GL02047

年代:1991

数据来源: WILEY