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Horwitz, J. L.

Publications and source records attributed to Horwitz, J. L..

106 records · Page 6
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NASA NTRS1982

The ionosphere as a source for magnetospheric ions

Ion composition measurements within the past several years have shown O(+), He(+), and other ions of terrestrial origin to compose a substantial fraction of the magnetospheric ion population. This review examines (1) observations of topside ionospheric composition, (2) mechanisms for energization and injection of ionospheric ions into the magnetosphere, and (3) observations of ions of ionospheric origin in various regions of the magnetosphere, including the plasmasphere, ring current, magnetotail plasma sheet and lobes, and boundary layer and magnetosheath.

Horwitz, J. L.↗
NASA NTRS1982

Conical pitch angle distributions of very low-energy ion fluxes observed by ISEE 1

Observations are presented of conical distributions of low-energy ion fluxes from throughout the magnetosphere. The data were provided by the plasma composition experiment (PCE) on ISEE 1. ISEE 1 was launched in October 1977 into a highly elliptical orbit with a 30 deg inclination to the equator and 22.5 earth radii apogee. Particular attention is given to data taken when the instrument was in its thermal plasma mode, sampling ions in the energy per charge range 0-100 eV/e. Attention is given to examples of conical distributions in 0- to 100-eV/e ions, the occurrence of conical distributions of 0- to 100-eV ions in local time-geocentric distance and latitude-geocentric distance coordinates, the cone angles in 0- to 100-eV ion conics, Kp distributions of 0- to 100-eV ion conics, and some compositional aspects of 0- to 100-eV ion conics.

Horwitz, J. L.↗
NASA NTRS1981

ISEE 1 observations of thermal plasma in the vicinity of the plasmasphere during periods of quieting magnetic activity

An investigation of thermal plasma behavior in the vicinity of the plasmasphere during periods of quieting magnetic activity was conducted by combining thermal ion observations made with the plasma composition experiment on ISEE 1 with plasma density profiles obtained from plasma frequency measurements made with the same satellite's plasma wave experiment. During periods in which the magnetic activity quiets, the two regions characterized by H(+):He(+):O(+) (isotropic) and H(+):O(+):He(+) (field-aligned) ion species distributions (in order of dominance) are separated by a new region in which low-energy H(+) and He(+) are found flowing along the magnetic field lines. At other times, following quieting magnetic activity, distributions having peak fluxes at 90 deg pitch angle are observed in this region.

Horwitz, J. L.↗
NASA NTRS1981

ISEE 1 observations of O/2+/ in the magnetosphere

Observations of O(2+) by ISEE 1 have revealed occasional O(2+)/O(+) density ratios of the order of unity within the plasmasphere. However, most plasmaspheric O(2+)/O(+) density ratios are generally below 0.3. These ratios agree generally with those seen by GEOS 1. Within the plasmasphere the O(2+) population appears to be primarily cold and isotropic (as is typical for major ions). Outside the plasmasphere, O(2+)/O(+) field-aligned flux ratios may be of the order of 0.1-0.2, though they are frequently much smaller, and the O(2+) distributions are typically field aligned. A rare observation of a unidirectional conic in O(2+) is also reported.

Horwitz, J. L.↗
NASA NTRS1981

Pancake pitch angle distributions in warm ions observed with ISEE 1

Observations of pancake (peak flux near 90-deg pitch angle) distributions of low-energy (not greater than 100 eV) ions are reported. Pancake distributions occur often in H(+) and He(+) simultaneously while O(+) fluxes are either undetectable or field-aligned. These H(+) and He(+) pancake distributions typically display characteristic energies of the order of 10 eV and are frequently mixed with higher density, colder isotropic, quasi-Maxwellian components. They appear often within the outer regions of the plasmasphere, and seem to occur most frequently on the dayside and near the magnetic equator.

Horwitz, J. L.↗
NASA NTRS1981

Low energy ion pitch angle distributions observed on the dayside at geosynchronous altitudes

Pitch angle distributions of low energy (20-400 eV) ions observed at geosynchronous altitudes are found to have field-aligned components almost 90% of the time on the average from dawn to midafternoon, thereafter decreasing to just less than 50%. Pancake pitch angle component occurrence frequencies vary oppositely, increasing to a peak of 40% in late afternoon. Although the occurrence frequencies for field-aligned components display no systematic variation with energy, pancake components are observed more often in the lower energy particles. Similarly, magnetic activity is found to have little effect on the occurrence frequencies of field-aligned components, whereas pancake components occur more frequently under magnetically quiet conditions.

Comfort, R. H.↗
NASA NTRS1980

New advances in thermal plasma research

Recent measurements obtained of the cold or thermal plasma of the earth's magnetosphere, which is believed to originate in the ionosphere, are reviewed. Consideration is given to the results of ATS 6 measurements which indicated unexpectedly high plasma temperatures and varied pitch-angle distributions, and the data from the low-energy plasma experiments on board GEOS 1 and 2 and ISEE 1, which were intended to clarify the ATS 6 results. These later measurements of ion composition, plasma energy and plasma distribution are noted to have confirmed earlier data and discovered new plasma components (D(+) or He(+2)), an intermixing of cold ionospheric plasma and hot magnetospheric plasma, the ordering of the plasma by the magnetic field rather than the ram direction in the outer magnetosphere, and wave phenomena. Questions remaining concerning the temperature and composition distributions of the plasmasphere and plasma trough, the relative densities of the cold and warmer components of the magnetosphere, plasma energization mechanisms, and the relative mix of the various plasma distributions are indicated.

Chappell, C. R.↗
NASA NTRS1980

Initial thermal plasma observations from ISEE-1

The initial measurements of magnetospheric thermal ions by the Plasma Composition Experiment on ISEE-1 are presented to demonstrate the surprising variety in this plasma population. The data provide evidence that the adiabatic mapping of the high latitude ionosphere to the equatorial plasma trough provides an insufficient description of the origin, transport, and accumulation processes which supply low energy ions to the outer plasmasphere and plasma trough.

Baugher, C. R.↗
NASA NTRS1980

Conical distributions of low-energy ion fluxes at synchronous orbit

Low-energy ion fluxes observed at synchronous orbit often display conical pitch angle distributions, that is, peak fluxes between 0 and 90 deg to the magnetic field direction. They are observed at all local times and are an especially common occurrence near dusk. The conic peaks normally occur in the 15-35 deg pitch angle range. Many of the low-energy ion conics in the late afternoon-dusk sector appear to be associated with low-energy plasma from the vicinity of the dusk plasmasphere bulge region. Several hypotheses for the production of these conics are advanced, and it is judged likely that cyclotron acceleration of ions at geocentric distances of 3.1 to 5.1 earth radii is operative in many cases. It is suggested that cyclotron acceleration in this altitude range may be an effective mechanism in trapping ionospheric ions within the magnetosphere.

Horwitz, J. L.↗
NASA NTRS1980

Magnetospheric research

Research concerning the magnetosphere is discussed in terms of satellite data analysis, instrument studies and developments, solar terrestrial studies, meetings, and consultants.

Comfort, R. H.↗
NASA NTRS1979

Observations of warm plasma in the dayside plasma trough at geosynchronous orbit

Positive ions in the energy range E = 3-50 eV have been measured on the dayside at geosynchronous orbit with the University of California at San Diego auroral particles experiment on ATS 6. It is found that the near-equatorial plasma trough contains a warm plasma component with a relatively stable temperature 11 eV and a low 'isotropic component' of density typically in the range 0.1-0.4 ion/cc. The pitch angle distributions of these warm ions are typically field-aligned, indicating an ionospheric origin, and have pitch angle half widths in the range 20-40 deg. Pancake and conical distributions are also observed. Abrupt enhancements of the plasma density to 1-10 ions/cc, or more, are seen in the late afternoon and last from several minutes to a few hours.

Horwitz, J. L.↗
NASA NTRS1979

On the relationship of the polar cap current system to the north-south component of the interplanetary magnetic field

The daily magnetic variation at the Resolute Bay polar cap station is examined for its dependence on the N-S component Bz of the IMF. The magnetic variations are found to be approximately in the same direction in the nightside polar cap for both northward and southward IMF, the amplitudes of the perturbations being larger for southward IMF. In the dayside, significant differences between the northward and southward IMF cases are observed. It is suggested that the overall pattern consists of two standard two-cell current patterns placed in juxtaposition such that the interface between them is the site of the antisunward current (sunward convection). Within the centers of each of these two patterns the currents are sunward (convection is antisunward).

Horwitz, J. L.↗
NASA NTRS1978

The latitudinal distributions of auroral zone electric fields and ground magnetic perturbations and their response to variations in the interplanetary magnetic field

Measurements of the latitudinal distributions of electric fields obtained with the Chatanika, Alaska, incoherent radar have been employed in determining the influence of the north-south component of the interplanetary magnetic field (IMF) on the electric field pattern. Poleward (or equatorward) shifts produced by the northward (or southward) transitions of the IMF north-south component are given particular attention. The behavior of the electric field patterns and magnetic perturbations in the midnight sector during substorms near the Harang discontinuity is analyzed.

Horwitz, J. L.↗
NASA NTRS1978

Chatanika radar observations of ionospheric and field-aligned currents

A method for estimating field-aligned current densities from ground-based measurements of the horizontal ionospheric currents at two or more latitudes using the Chatanika incoherent scatter radar is presented. It is assumed that the one-dimensional divergence of the horizontal currents approximates the current density of a field-aligned current sheet. The field-aligned current densities found via radar data agree with those given by simultaneous magnetic observations by the Triad satellite. It is noted that downward or upward field-aligned currents having densities of 10 to the -6th to 10 to the -5th A/sq m are achieved in the evening or morning sector. The densities are further increased during periods of increased geomagnetic activity and when current directions are reversed. This reversal generally occurs during auroral oval expansion.

Kamide, Y.↗
NASA NTRS1977

The response of the dayside aurora to sharp northward and southward transitions of the interplanetary magnetic field and to magnetospheric substorms

Latitudinal shifts of the discrete dayside aurora are examined with respect to variations in the interplanetary magnetic field (IMF) and to magnetospheric substorms. Within 10-15 min after steplike southward (northward) transitions in the IMF the dayside auroral oval moves equatorward (poleward). In at least some cases the auroral shift is similar to an exponential relaxation in latitude from an initial to a final steady state value; for these cases the average exponential time constant is estimated to be 17 min. Substorm features in the dayside aurora include (1) an equatorward shift by 1-3 deg of the equatorward discrete auroral boundary, (2) a brightening of the discrete aurora near substorm onset, (3) the formation of multiple auroral bands, and (4) poleward motion of short-lived individual auroral arcs occurring nearly coincident in time with the equatorward boundary shifts, the average poleward velocity of these arcs being approximately 800 m/s.

Horwitz, J. L.↗