Engineering topics

Maynard, N. C.

Publications and source records attributed to Maynard, N. C..

At least 55 records · Page 3

NASA NTRS1985

Comparative in situ studies of the unstable day-time equatorial E-region

Three in situ rocket observations of electrostatic fluctuations which characterize the strong, mild, and weak day-time unstable E-region are compared. The fluctuations are analyzed in relation to the ambient electron density gradients, the measured or inferred drift velocities and, where possible, to simultaneous VHF backscatter radar measurements. Based on these parameters, the linear dispersion relation is solved to find the range of altitudes and wavenumbers for which the growth rate indicates that instability will occur. These computations are compared with actual observations of the irregularities. How well the linear growth rate predicts the altitude range of the unstable layer, and to what extent the regime of the unstable wavenumbers is reflected in the spectra of the irregularities, is shown. Overall, the agreement is good.

Pfaff, R. F.↗

NASA NTRS1985

A diurnal study of the electrical structure of the equatorial middle atmosphere

Electrical parameters measured from 115 km down to below 20 km during the Project Condor campaign at the Punta Lobos Rocket Range near Lima, Peru, are presented. Ten rocket-launched payloads measured electrical conductivity. A strong diurnal influence due to solar ultraviolet radiation is shown. Nine of the payloads also measured electric fields. No large mesospheric vertical electric fields are found in the data. A calculation of the dc global conduction current density at 18 km is smaller than previously measured at low latitudes and does not show the conventional diurnal variation.

Croskey, C. L.↗

NASA NTRS1985

Auroral zone electric fields from DE 1 and 2 at magnetic conjunctions

Nearly simultaneous measurements of auroral zone electric fields are obtained by the Dynamics Explorer spacecraft at altitudes below 900 km and above 4,500 km during magnetic conjunctions. The measured electric fields are usually perpendicular to the magnetic field lines. The north-south meridional electric fields are projected to a common altitude by a mapping function which accounts for the convergence of the magnetic field lines. When plotted as a function of invariant latitude, graphs of the projected electric fields measured by both DE-1 and DE-2 show that the large-scale electric field is the same at both altitudes, as expected. Superimposed on the large-scale fields, however, are small-scale features with wavelengths less than 100 km which are larger in magnitude at the higher altitude. Fourier transforms of the electric fields show that the magnitudes depend on wavelength. Outside of the auroral zone the electric field spectrums are nearly identical. But within the auroral zone the high and low altitude electric fields have a ratio which increases with the reciprocal of the wavelength. The small-scale electric field variations are associated with field-aligned currents. These currents are measured with both a plasma instrument and magnetometer on DE-1.

Weimer, D. R.↗

NASA NTRS1985

Fine structure and dynamics of the Harang discontinuity - An event study

Two rockets were launched simultaneously from the Andoya Rocket Range in northern Norway into and through a Harang discontinuity region during a small isolated substorm. Each was equipped with an array of instruments for monitoring ionospheric electric fields and auroral particles. Different sized rockets allowed both to traverse a range of 300 km while introducing altitude separations of up to 440 km, and separations between the feet of the magnetic field lines intersecting the payloads of up to 100 km. The data sets, coupled with multi-station ground-based observations, provide information on the structure and dynamics of the Harang discontinuity region. Two boundaries were encountered. The first electric field reversal was observed simultaneously by both payloads in a region of weak precipitation, while the second reversal was associated with an intense auroral band. Since an unambiguous interpretation of these data is not possible, two alternative scenarios are presented. In one the discontinuity becomes realigned during the flight following a decay in the activity. The second involves a triple cell convection system, possibly the result of an eddy in the flow.

Egeland, A.↗

NASA NTRS1985

The westward thermospheric jet-stream of the evening auroral oval

Ground-based and satellite measurements of the thermospheric wind in jet-streams during the evening auroral oval are analyzed, in order to study the geophysical mechanisms of thermospheric wind generation. Numerical simulations using a global, three-dimensional, time-dependent model of thermospheric dynamics were compared with the satellite data, and the results are discussed in detail. The wind distribution during the storm is shown in a series of color plates.

Rees, D.↗

NASA NTRS1985

Current carriers for the field-aligned current system

The Dynamics Explorer satellites have returned a large body of data containing high resolution magnetometer measurements and distributions of charged particles of all but thermal electrons. From these data the relations of the field aligned currents to particle precipitation structures were investigated and the charge carriers were identified. Results include the following: (1) during very quiet periods, field aligned currents exist primarily as fine structure; (2) during onset of substorms, region 1 and region 2 become clearly evident but contain significant structure; (3) as magnetic activity subsides, current regions become less distinct, and structure becomes more dominant; (4) the distribution of the upward currents derived from magnetometer data and calculated from suprathermal electron data agree remarkably well in shape but not necessarily in magnitude; (5) at all local times, 5 eV electrons seldom carry most of the upward current; (6) except for the accelerated inverted-V electrons, the dominant upward current carriers which are masured are below 500 eV and are distributed in energy; (7) dusk upward currents (region 1) are associated with the boundary plasma sheet; and (8) suprathermal electron bursts are important current carrying structures.

Hoffman, R. A.↗

NASA NTRS1985

Structure in the DC and AC electric fields associated with the dayside cusp region

The cusp region as seen in the AC and DC electric fields is one of intense variation. The intensity peaks within the soft particle precipitation. The only AC signal that appears to be unique to the cusp is broadband ULF-ELF magnetic noise. Other types of emissions are also found at other local times at high latitudes. The pattern of these signals, especially that of ULF-ELF broadband electrostatic noise (BEN), distinguishes the cusp region from other regions. BEN signatures are indicators of magnetosheath-like soft particle precipitation but not necessarily of open field lines. In addition, large spike-like features in the DC electric field are seen near local magnetic noon which appear to be related to the large convective electric fields that have been observed at the magnetopause. These features are not necessarily tied to convection reversals, but may appear within broader regions of zonal convective flow.

Maynard, N. C.↗

NASA NTRS1984

Current carriers for the field-aligned current system

The Dynamics Explorer satellites have returned a large body of data containing high resolution magnetometer measurements and distributions of charged particles of all but thermal electrons. From these data the relations of the field aligned currents to particle precipitation structures were investigated and the charge carriers were identified. Results include the following: (1) during very quiet periods, field aligned currents exist primarily as fine structure; (2) during onset of substorms, region 1 and region 2 become clearly evident but contain significant structure; (3) as magnetic activity subsides, current regions become less distinct, and structure becomes more dominant; (4) the distribution of the upward currents derived from magnetometer data and calculated from suprathermal electron data agree remarkably well in shape but not necessarily in magnitude; (5) at all local times, 5 eV electrons seldom carry most of the upward current; (6) except for the accelerated inverted-V electrons, the dominant upward current carriers which are measured are below 500 eV and are distributed in energy; (7) dusk upward currents (region 1) are associated with the boundary plasma sheet; and (8) suprathermal electron bursts are important current carrying structures.

Hoffman, R. A.↗

NASA NTRS1984

Electrical structure in the high-latitude middle atmosphere

Large V/m electric fields, both horizontal and vertical, have been observed within bounded regions of the upper stratosphere and lower mesosphere. They seem to occur only in regions where the electrical conductivity is a few times 10 to the -10th S/m or less and appear to be current limited. While low conductivity is necessary, it is not a sufficient condition for occurrence. The observed large horizontal electric fields were found to be anticorrelated with the local neutral wind. Large variations in the conductivity were also observed to occur with fluctuations in magnetic activity, and these were found to be consistent with measured variations in energy deposition during auroral phenomena. Theoretical concepts of mapping of electric fields downward from the thermosphere along equipotential magnetic field lines were shown to hold qualitatively in the D-region at the mV/m level. Perturbations affecting such models were determined to be small.

Maynard, N. C.↗

NASA NTRS1984

Particle acceleration parallel and perpendicular to the magnetic field observed by DE-2

Observations by the instrument payload on DE-2 have been used to study the plasma and electrodynamic properties of the ionosphere when field-aligned currents approaching 100 microA/sq m flow at about 900-km altitude. In such a situation, it is found that the thermal ions may account for a substantial fraction of the current carriers and that the bulk of the energetic electron population has undergone a net acceleration of a few hundred electron volts. Large amplitude electrostatic waves that have a maximum near 100 Hz and an energetic ion population that shows evidence for both parallel and perpendicular acceleration to about 30 eV also accompany such events. The horizontal thermal ion drift (electric field) that accompanies these plasma phenomena has extremely large gradients over small spatial scales. Such a signature is consistent with a requirement for a large field-aligned current, and a limit to the available field-aligned potential difference that may be applied to the plasma.

Heelis, R. A.↗

NASA NTRS1984

Observation of plasma deceleration at a rotational magnetopause discontinuity

This letter presents a detailed experimental analysis of the electric fields and electron flow velocities at a rotational magnetopause discontinuity. It is shown for this event that the magnetosheath plasma velocity is sharply decelerated on entry into the magnetosphere in very good agreement with a theoretical magnetohydrodynamic fit. This type of event is remarkable because the product of the electric-field vector and the Chapman-Ferraro magnetopause current is negative during the magnetopause crossing. This case represents one of two types of valid solution to the MHD boundary conditions. It differs from previously published examples of reconnective magnetopause geometries in that energy is being transferred locally from kinetic energy into electromagnetic energy at the time of these measurements.

Aggson, T. L.↗

NASA NTRS1984

Relationships between field-aligned currents, electric fields, and particle precipitation as observed by Dynamics Explorer-2

The relationships between field-aligned currents, electric fields, and particle fluxes are determined using observations from the polar orbiting low-altitude satellite Dynamics Explorer-2. It is shown that the north-south electric field and the east-west magnetic field components are usually highly correlated in the field-aligned current regions. This proportionality observationally proves that the field-aligned current equals the divergence of the height-integrated ionospheric Pedersen current in the meridional plane to a high degree of approximation. As a general rule, in the evening sector the upward field-aligned currents flow in the boundary plasma sheet region and the downward currents flow in the central plasma sheet region. The current densities determined independently from the plasma and magnetic field measurements are compared. Although the current densities deduced from the two methods are in general agreement, the degree and extent of the agreement vary in individual cases.

Sugiura, M.↗

NASA NTRS1983

ULF/ELF waves and turbulence in the high-altitude dayside magnetosphere

Three groups of waves appear to be characteristic of the dayside high-lattitude magnetosphere, in view of AC electric field measurements made by polar-orbiting satellites: (1) ULF/ELF broadband noise, whose region of peak intensity is generally found to be consistent with the latitudinal extension of the cusp; (2) VLF hiss, which although present in the cusp region extends outside the established cusp projection; and (3) narrow-band ELF emissions, covering frequencies betwen 200 Hz and 1 kHz, on the equatorward side of the cusp. This classification of the waves is independent of the magnetic disturbance level. Attention is given to the manner in which the appearance of the ULF/ELF noise and the narrow-band ELF emissions can be used to localize the solar cusp position.

Holtet, J. A.↗

NASA NTRS1983

Electric field measurements at the magnetopause. I Observation of large convective velocities at rotational magnetopause discontinuities

Large convective electric fields of the order of 10 mV/m (sometimes as high as 22 mV/m) are observed at rotational magnetopause discontinuities. These observations were made with the long cylindrical (179-m base line) probes carried on the ISEE 1 satellite. These electric field observations yield convective velocity magnitudes (equal to the cross product of the vector E and the vector B, the latter divided by the square of the magnitude of B) of the order of 150 km/s. In this format for the convective velocity magnitudes, some of these observations are similar to the high speed plasma velocity observations that were made at the magnetopause with the plasma experiment carried on the ISEE 1 satellite. It is shown that, for many of these magnetopause crossings, there exists a special moving coordinate system where the observed electric fields vanish. Such a unique reference system is often used in theoretical studies of magnetic discontinuities. This special coordinate system does not move at the local plasma velocity but moves instead at a velocity intermediate between the convective velocity and the local Alfven velocity. It is used here as a diagnostic tool for the experimental investigation of rotational discontinuities at the magnetopause.

Aggson, T. L.↗

NASA NTRS1983

Relationships between field-aligned currents, electric fields and particle precipitation as observed by dynamics Explorer-2

Sugiura, M.↗

NASA NTRS1983

Observations of large magnetospheric electric fields during the onset phase of a substorm

An auroral substorm is a highly transient process which is triggered on auroral L shells and leads to an abrupt change of state in the nightside magnetosphere. Direct measurements of electric fields in the outer magnetosphere are of critical importance to understand the processes involved in sudden substorm enhancements. Shepard et al. (1980) reported direct measurements of electric fields from the GEOS 2 satellite at equatorial latitudes which showed 15 mV/m westward inductive electric fields in association with tail collapses. Similar direct electric field measurements of the westward inductive electric field at low latitudes from the ISEE 1 satellite during the onset phase of an auroral substorm are reported in the present investigation. The observations show a rapid inward magnetic field (westward electric field) motion near midnight at low or equatorial latitudes simultaneous with the sudden onset of auroral substorm enhancements near the foot of the satellite field line.

Aggson, T. L.↗

NASA NTRS1983

The plasmaspheric electric field as measured by ISEE 1

The electrodynamics of the plasmasphere has been a topic of considerable interest. Models predict a space charge buildup, or Alfven layer, at the inner edge of the ring current which opposes the dawn-dusk convection electric field in the magnetosphere and thus shields the plasmasphere from the convection electric field. The current study has the objective to present data from the ISEE 1 double cylindrical probe instrument. All measurements reported were made in the plasmasphere with electron densities of the order of 30-50 or greater per cu cm. The average electric field pattern for quiet conditions is found to be qualitatively consistent with previous average results from whistler measurements and radar backscattering measurements. The magnitudes and gross patterns are in qualitative agreement with representative ionospheric dynamo models. The basic convective flow vectors from the penetration of the magnetospheric electric field tend to follow contours which are parallel to those of the average plasmapause boundary on the nightside.

Maynard, N. C.↗

NASA NTRS1982

Characteristics of a stable auroral red arc event

The present investigation is concerned with an analysis of the measurements of the stable auroral red (SAR) arc of October 23, 1981, using data from orbit 1192 of Dynamics Explorer (DE) 2, during which a magnetic coincidence occurred with the DE-1 spacecraft near the red arc field line, and for which simultaneous ground-based intensity measurements from Richland, WA were available. The altitude of the DE-2 satellite was approximately 850 km during arc passage in the Northern Hemisphere and approximately 395 km during the conjugate hemisphere passage. The DE-1 satellite was at an altitude of approximately 6000 km during the magnetic coincidence with DE-2 in the Northern Hemisphere. The described observations and calculations reconfirm a previous understanding that the actual excitation of the O(1D) state responsible for the 6300 A emission of red arcs is caused by hot ionospheric thermal electrons.

Kozyra, J. U.↗