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Lyons, L. R.

Publications and source records attributed to Lyons, L. R..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Energetic and magnetosheath energy particle signatures of the low-latitude boundary layer at low altitudes near noon

The energetic and magnetosheath energy particle signature of the low-latitude boundary layer (LBL) and its separation from the cusp are investigated using particle data from the polar-orbiting S3-3 satellite. The LBL and its boundaries, as determined from the energetic particles are compared with the LBL and boundaries determined from a modified set of low-energy plasma criteria based on Newell and Meng (NM, 1988, 1989). Excellent agreement was found in 68 percent of the 19 orbits where the LBL was identified by the energetic electron data. In 10 percent of the remaining orbits, the modified NM criteria did not provide numerical LBL identification, though a clear boundary region was visually identifiable in the data. For the remaining orbits, the modified NM criteria identified the LBL, but not its full latitudinal extent.

Roeder, J. L.↗
NASA NTRS1992

Inferences concerning the magnetospheric source region for auroral breakup

Inferences concerning the magnetospheric source region for auroral arcs obtained from particle measurements on polar orbiting satellites are presented and contrasted with other ideas. An argument that the magnetospheric source region for auroral arc breakup and substorm initiation is along Boundary Plasma Sheet (BPS) magnetic field lines is given. This source region lies beyond a distinct central plasma sheet region and sufficiently far from the Earth that energetic ion motion violates the guiding center approximation (is chaotic). The source region is not constrained to any particular range of distances from the Earth, and substorm initiation may be possible over a wide range of distances from near synchronous orbit to the distant tail. It is also argued that the layer of low energy electrons and velocity dispersed ion beams observed at low altitudes on Aureol 3 is not a different region from the region of auroral arcs. Both comprise the BPS. The two regions occasionally appear distinct at low altitudes because of the effects of arc field aligned potential drops on precipitating particles.

Lyons, L. R.↗
NASA NTRS1992

Formation of auroral arcs via magnetosphere-ionosphere coupling

The aurora results from electrons that first are energized at very high altitudes by the solar wind-earth interactions and then travel down magnetic field lines until they hit the atmosphere. The basic physical processes are described that lead to the energization of these electrons and thus to the aurora.

Lyons, L. R.↗
NASA NTRS1992

Force balance near an X line along which E x J is less than 0

Conditions for which particle motion within the current sheet in the vicinity of an X line can give a current in the direction appropriate for E x J is less than 0. The way in which the balance between gyroviscosity and the electric force along an X line is maintained for any E x J is shown. It is concluded that observational evidence for the occasional existence of E x J is less than 0 along an X line provides support for the suggestion that collisionless graviscosity, rather than resistivity, balances the electric force along an X line. It is found that there is a maximum electric field magnitude for particles to be able to carry a significant current. For parameters typical of the distant magnetotail, the critical electric field magnitude was found to be about 0.15 mV/m, which is of the order of, though somewhat less than, the potential electric field magnitudes expected in the magnetotail. This maximum allowable field magnitude is about the same for protons as it is for electrons in the magnetotail.

Lyons, L. R.↗
NASA NTRS1991

Sondrestrom radar measurements of the reconnection electric field

The possibility of using Sondrestrom incoherent radar scatter to estimate the rate of solar-wind energy transfer is examined by using plasma-velocity measurements in the separatrix reference frame. The separatrix is the boundary between open and closed field lines, and its orientation is deduced from all-sky images. The radar observations are used to determine the separatrix location and the ionospheric plasma drift. Measurements of the reconnection electric field in the midnight sector for one night are taken, revealing that the field is less than 15 mV/m during the time of local polar-cap extension. During polar-cap contraction the field range is 30-40 mV/m, and these periods correspond to substorm expansive phases. The limitations associated with measuring ionospheric plasma drift, the boundary orientation, and boundary location are enumerated. The measurements in the experimental case demonstrate the possibility of plasma transfer from closed to open field lines.

De La Beaujardiere, O.↗
NASA NTRS1990

Force balance near an X line in a collisionless plasma

The suggestion by Dungey (1988) that the gyroviscosity associated with gradients of the off-diagonal elements of the electron pressure tensor can balance a reconnection electric field along a magnetic X line in a collisionless plasma is investigated. The detailed balance of forces in the vicinity of an X line is evaluated using a two-dimensional magnetic field model and a simple model for particle motion. The results show that the gyroviscosity can indeed provide the force required to balance a reconnection electric field in that region, so that neither collisions nor wave turbulence are necessary for reconnection. The results also show that there should not be a significant increase in current from electron acceleration very near an X line. Reasonable numerical estimates are obtained for conditions expected in the vicinity of the distant X line in the geomagnetic tail.

Lyons, L. R.↗
NASA NTRS1989

Access of energetic particles to storm time ring current through enhanced radial 'diffusion'

It is proposed that the transport of particles with energy of at least approximately 40 keV into the geomagnetic storm time ring current can result from enhanced stochastic radial transport driven by fluctuating electric fields during a storm's main phase. The effects of such electric fields are estimated by applying radial diffusion theory, assuming a preexisting trapped-particle population as the initial condition. The feasibility of explaining observed flux increases of particles of at least approximately 40 keV at no more than approximately 4 earth radii by enhanced radial 'diffusion' is then demonstrated. It is estimated that the at least approximately 40-keV portion of the storm time ring current at approximately 3 earth radii consists of about 50 percent preexisting and about 50 percent new particles. The formation of the storm time ring current is found to be perhaps explainable via a combination of direct radial transport at energies no greater than approximately 40 keV and 'diffusive' radial transport at higher energies.

Lyons, L. R.↗
NASA NTRS1989

Trapped-particle evacuation - Source of magnetotail bursts and tailward flows?

Observational and theoretical evidence that the polar cap can expand rapidly enough during the growth phase of a substorm to release geomagnetically trapped particles from previously closed drift shells, enabling the particles to escape into the tail, are examined. Observations show that the moving separatrix can overtake convecting nightside plasma during intervals of polar-cap expansion. Models of this phenomenon suggest that closed nightside field lines can be evaculated of their particle populations, and it is noted that such evacuations can account for the occurrence of energetic-particle bursts in the tail.

Lyons, L. R.↗
NASA NTRS1989

The neutral E region zonal winds during intense postmidnight diffuse aurora - Response to observed particle fluxes

Zonal winds associated with diffuse auroras were simulated using observed fluxes of electron precipitation and a high-resolution time-dependent numerical model described by Waltershcheid et al. (1985). A very strong dynamically unstable E region jet is simulated for an electric field of 50 mV/m, and the results suggest a connection with omega band. These zonal winds are not as strong as those reported by Lyons and Walterscheid (1985), but are much stronger (by a factor of 3 or greater) than the E region winds simulated by Fuller-Rowell (1985). The effects of large-scale cross-arc winds were simulated to evaluate their effect; it is shown that strong cross-arc winds are required to prevent the formation of a strong E zonal jet. The results agree qualitatively with radar observations of zonal winds by Johnson et al. (1987).

Walterscheid, R. L.↗
NASA NTRS1989

Evaluating auroral processes within a magnotospheric model

A summary of the research performed is included. Topics covered include magnetospheric model; association between discrete auroras and ion precipitation from the tail current sheet; auroral arc scale sizes and structures; polar cap size variation; low-altitude auroral boundary; auroral wave-particle interactions; thermospheric interactions; and the neutral wind 'flywheel'.

Lyons, L. R.↗
NASA NTRS1988

Description of substorms in the tail incorporating boundary layer and neutral line effects

A description of the substorm expansion phase that includes the formation of a neutral line in the relatively near-earth portion of the tail plasma sheet and phenomena observed in the plasma sheet boundary layer (PSBL) is proposed. Specifically, it is proposed that substorm onset results from the formation of a neutral line within the preexisting source region for the PSBL. The source region is presumably the tail current sheet, which is suggested to extend well earthward of 80 earth radii. Both before and after the neutral line forms, auroral field-aligned currents and large ion flows remain confined to the PSBL earthward of the source region.

Lyons, L. R.↗
NASA NTRS1988

A general association between discrete auroras and ion precipitation from the tail

Observations from the spinning polar-orbiting S3-3 satellite were used to compare the locations of discrete auroral arcs (defined to be regions containing particle distributions consistent with field-aligned potential drops of not less than 0.5 kV) with regions of isotropic ion precipitation. It was found that the regions of discrete aurora are almost exclusively confined to the region of isotropic ion precipitation at all local times studied (polar cap arcs and local times near noon were not considered). It was also found that, throughout the local time interval studied, the discrete aurora was generally associated with spatial structure and boundaries in the precipitating ions, indicating that arc generation may be associated with structure in the particle population within the tail current sheet.

Lyons, L. R.↗
NASA NTRS1988

Unmagnetized diffusion for azimuthally symmetric wave and particle distributions

The quasi-linear diffusion of particles from resonant interactions with a spectrum of electrostatic waves is investigated theoretically, extending results obtained for no magnetic field and for strong magnetic fields to cases where the ambient magnetic field which organizes azimuthally symmetric wave and particle distributions does not have to be taken into consideration in evaluating the local interaction. The derivation of the governing equations is explained, and numerical results are presented in extensive graphs and characterized in detail. Slow-mode ion-acoustic waves are shown to be unstable under the plasma conditions studied, and the dependence of resonant-ion diffusion rates with pitch angle, speed, and the distribution of wave energy in wavenumber space is explored. The implications of the present findings for theoretical models of the earth bow shock and plasma-sheet boundary layer are indicated.

Dusenbery, P. B.↗
NASA NTRS1987

Ion precipitation from the magnetopause current sheet

Measurements from the polar-orbiting S3-3 satellite show that energetic ions frequently precipitate with isotropic pitch-angle distributions at auroral latitudes on the dayside. It is proposed that this precipitation results from nonguiding center motion of radiation-belt ions that drift into the magnetopause current sheet, and that the ion precipitation and flows of energetic ions observed in the magnetosheath originate together within the magnetopause current sheet. Ions ejected from the magnetopause toward the earth flow along open field lines that are adjacent to the separatrix between open and closed field lines. Those ejected into the magnetosheath flow along field lines that are connected to the geomagnetic field and adjacent to the separatrix between the connected field lines and purely magnetosheath fields lines. These proposals are tested by comparing the S3-3 observations of precipitating ions with previously analyzed ISEE-1 observations of energetic ions, obtained near the magnetopause and in the magnetosheath. The S3-3 observations imply that the region of ion precipitation is often continuous as a function of local time. This suggests that, at least on the dayside, there is often a continuous shell of manetospheric ions within the magnetosheath flowing from the magnetosphere. These ions are likely an important source for energetic ions in the interplanetary medium.

Lyons, L. R.↗
NASA NTRS1987

Magnetic field-aligned electric field acceleration and the characteristics of the optical aurora

The long-recognized association of brighter aurora with more deeply penetrating, and hence more energetic, electrons is examined. Using the Knight (1973) relation between the magnetic-field-aligned current density and potential drop (derived from the theory of single-particle motion in the presence of a magnetic-field-aligned electric field), an approximate expression relating the energy flux of the precipitating electrons over discrete aurora and the mean particle energy is derived. This expression is used in conjunction with an auroral optical excitation and emission model to specify the dependence of the red/blue ratio of auroral optical emissions on the brightness of the aurora. It is shown that the quantitative predictions of the discrete auroral theory are in accord with observations of the aurora.

Christensen, A. B.↗
NASA NTRS1987

Conditions for double layers in the Earth's magnetosphere and perhaps in other astrophysical objects

Double layers form along auroral field lines in the Earth's magnetosphere. They form in order to maintain current continuity in the ionosphere in the presence of a magnetospheric electric field E with nabla x E is not equal to 0. Features which govern the formation of the double layers are: (1) the divergence of E, (2) the conductivity of the ionosphere, and (3) the current-voltage characteristics of auroral magnetic field lines. Astrophysical situations where nabla x E is not equal to 0 is applied to a conducting plasma similar to the Earth's ionosphere are potential candidates for the formation of double layers. The region with nabla x E is not equal to 0 can be generated within, or along field lines connected to, the conducting plasma. In addition to nabla x E, shear neutral flow in the conducting plasma can also form double layers.

Lyons, L. R.↗
NASA NTRS1987

Conditions for double layers in the earth's magnetosphere and perhaps in other astrophysical objects

It is suggested that the features which govern the formation of the double layers are: (1) the divergence of the magnetospheric electric field, (2) the ionospheric conductivity, and (3) the current-voltage characteristics of auroral magnetic field lines. Also considered are conditions in other astrophysical objects that could lead to the formation of DLs in a manner analogous to what occurs in the earth's auroral zones. It is noted that two processes can drive divergent Pedersen currents within a collisional conducting layer: (1) sheared plasma flow applied anywhere along the magnetic field lines connected to the conducting layer and (2) a neutral flow with shear within the conducting layer.

Lyons, L. R.↗