Engineering topics

Paschmann, G.

Publications and source records attributed to Paschmann, G..

At least 19 records

NASA NTRS1998

The Electron Drift Technique for Measuring Electric and Magnetic Fields

The electron drift technique is based on sensing the drift of a weak beam of test electrons that is caused by electric fields and/or gradients in the magnetic field. These quantities can, by use of different electron energies, in principle be determined separately. Depending on the ratio of drift speed to magnetic field strength, the drift velocity can be determined either from the two emission directions that cause the electrons to gyrate back to detectors placed some distance from the emitting guns, or from measurements of the time of flight of the electrons. As a by-product of the time-of-flight measurements, the magnetic field strength is also determined. The paper describes strengths and weaknesses of the method as well as technical constraints.

Paschmann, G.↗

NASA NTRS1997

The Electron Drift Instrument for Cluster

The Electron Drift Instrument (EDI) measures the drift of a weak beam of test electrons that, when emitted in certain directions, return to the spacecraft after one or more gyrations. This drift is related to the electric field and the gradient in the magnetic field, and these quantities can, by use of different electron energies, be determined separately. As a by-product, the magnetic field strength is also measured. The present paper describes the scientific objectives, the experimental method, and the technical realization of the various elements of the instrument.

Paschmann, G.↗

NASA NTRS1993

Double-polytropic closure in the magentosheath

The magnetosheath plasma is usually neither isotropic nor adiabatic. This paper contains an attempt to decribe its thermodynamic properties in terms of two polytropic laws, p(sub perpendicular)/rho B(exp gamma(sub perpendicular)-1) = C(sub perpendicular) and p(sub parallel)B(exp gamma(sub parallel)-1)/rho(exp gamma(sub parallel)) = C(sub parallel), such that for gamma(sub perpendicular) = 2, gamma(sub parallel) = 3 the usual Chew-Goldberger-Low double-adiabatic expressions are recovered and for gamma(sub perpendicular) = 1, gamma(sub parallel) = 1 double-isothermal conditions are obtained. Using data from the AMPTE/IRM spacecraft, we show that the subsolar magnetosheath plasma may be better described by the double-polytropic laws than by the mirror instability threshold, in particular in the low beta region near the magnetopause. The inferred polytropic exponents vary from event to event but are typically in the ranges of gamma(sub perpendicular) = 0.94 +/- 0.10 and gamma(sub parallel) = 1.14 +/- 0.13 for the 29 cases we have examined.

Hau, L.-N.↗

NASA NTRS1993

Remote sensing of two-dimensional magnetopause structures

The study presents a technique for analyzing remote measurements made by a single spacecraft of 2D disturbances in the ambient magnetosheath or magnetospheric magnetic field caused, for example, by flux transfer events or pressure pulses. The methodology is based on a recent linear theory for isentropic field-aligned MHD flow over gently sloping 2D obstacles. The technique uses only magnetic field measurements and can provide information about the orientation and actual cross-sectional shape of the event, as well as information about the spacecraft trajectory relative to the bulge. Analysis of two sample events, one recorded by the AMPTE/IRM spacecraft in the magnetosheath and the other by AMPTE/CCE in the magnetosphere, indicates that the bulges on the magnetopause surface causing the magnetic field and flow perturbations for these events did not have the semicircular cross section suggested in previous work; instead they had a more elongated shape, the dimension tangential to the magnetopause being substantially larger than that normal to it.

Walthour, D. W.↗

NASA NTRS1992

Magnetic field maxima in the low latitude boundary layer

The magnetic field often exhibits a maximum in the earth's low-latitude boundary layer. Examples of this behavior are shown using data from the AMPTE/IRM spacecraft, and it is argued that two fundamentally distinct causes exist for the excess field: (1) a depression, within the layer, of the population of medium-energy ions of magnetospheric origin and (2) field curvature effects associated with undulations of the magnetopause itself.

Sonnerup, B.↗

NASA NTRS1992

Bursty bulk flows in the inner central plasma sheet: An effective means of earthward transport in the magnetotail

High speed flows in the Earth's Inner Central Plasma Sheet (ICPS) occur during enhanced flow intervals that have been termed Bursty Bulk Flow (BBF) events. The importance of different flow magnitude samples for Earthward transport in the ICPS are statistically evaluated and several representative BBF's and their relevance to Earthward transport are discussed. The selection of BBF's is automated in a database and they are shown to be responsible for most of the Earthward transport that occurs within the ICPS. The BBF related transport is compared to the transport measured within the entire plasma sheet during the 1985 AMPTE/IRM crossings of the magnetotail. The results show that BBF's last only a small fraction of the time in the plasma sheet but can account for several tens of percent of the Earthward particle and energy transfer and possibly all of the Earthward magnetic flux transfer in the plasma sheet.

Angelopoulos, Vassilis↗

NASA NTRS1992

Bursty bulk flows in the inner central plasma sheet

High-speed flows in the inner central plasma sheet (first reported by Baumjohann et al. (1990) are studied, together with the concurrent behavior of the plasma and magnetic field, by using AMPTE/IRM data from about 9 to 19 R(E) in the earth magnetotail. The conclusions drawn from the detailed analysis of a representative event are reinforced by a superposed epoch analysis applied on two years of data. The high-speed flows organize themselves in 10-min time scale flow enhancements called here bursty-bulk flow (BBF) events. Both temporal and spatial effects are responsible for their bursty nature. The flow velocity exhibits peaks of very large amplitude with a characteristic time scale of the order of a minute, which are usually associated with magnetic field dipolarizations and ion temeperature increases. The BBFs represent intervals of enhanced earthward convection and energy transport per unit area in the y-z GSM direction of the order of 5 x 10 exp 19 ergs/R(E-squared).

Angelopoulos, V.↗

NASA NTRS1992

Pressure changes in the plasma sheet during substorm injections

Data from the CHEM instrument on AMPTE CCE, data from the 3D plasma instrument and the SULEICA instrument on AMPTE IRM, and magnetometer data from both spacecraft are used to determine the particle pressure and total pressure as a function of radial distance in the plasma sheet for periods before and after the onset of substorm-associated ion enhancements over the range 7-19 RE. Events were chosen that occurred during times of increasing magnetospheric activity, as determined by an increasing AE index, in which a sudden increase, or 'injection', of energetic particle flux is observed. It is shown that the simultaneous appearance of energetic particles and changes in the magnetic field results naturally from pressure balance and does not necessarily indicate that the local changing field is accelerating the particles.

Kistler, L. M.↗

NASA NTRS1990

Upstream pressure variations associated with the bow shock and their effects on the magnetosphere

The AMPTE IRM solar wind data are analyzed to determine the relationship between upstream pressure fluctuations and magnetospheric perturbations. It is argued that the upstream pressure variations are not inherent in the solar wind but rather are associated with the bow shock. This conclusion follows from the fact that the upstream field strength and density associated with perturbations are highly correlated with each other, while they tend to be anticorrelated in the undisturbed solar wind, and that the upstream perturbations occur within the foreshock or at its boundary. The results imply a mode of interaction between the solar wind upstream and the magnetosphere whereby density changes produced in the foreshock subsequently convect through the bow shock and impinge on the magnetosphere. Upstream pressure perturbations should create significant effects on the magnetopause and at the foot of nearby field lines that lead to the polar cusp ionosphere.

Fairfield, D. H.↗

NASA NTRS1989

High-speed flows of H(+) and He(2+) ions at the magnetosphere

Observations of high-speed flows of H(+) and He(2+) ions obtained by the hot plasma composition experiment on AMPTE/CCE provide direct support for the single-particle approach to acceleration in current layers. These high-speed flows are attributed to single-fluid momentum balance across a magnetopause with a normal magnetic field and a tangential electric field. The flow speeds of the two ion species are found to be approximately equal.

Paschmann, G.↗

NASA NTRS1989

On-board data analysis techniques for space plasma particle instruments

This article describes the on-board data analysis techniques used on two recent space plasma particle instruments to compute meaningful parameters of three-dimensional plasma distributions in real time. These parameters may be transmitted with much higher time resolution than the full distribution within the limited telemetry. In addition, they greatly reduce the ground processing requirements.

Curtis, D. W.↗

NASA NTRS1987

AMPTE IRM observations of waves associated with flux transfer events in the magnetosphere

The AMPTE IRM wave instrument has been applied to the study of flux transfer events (FTE's). This initial investigation concentrates on FTE's observed in the magnetosphere during the fall of 1984. The wave morphology consists of four significant features: at frequencies below the ion gyrofrequency, magnetic fluctuations occur with amplitudes of the order of 1 nT; at frequencies from a few hertz to a few hundred hertz, electric field fluctuations are observed which have a broadband amplitude of a few millivolts per meter, which are perpendicularly polarized at the lowest frequencies, and which are partly electrostatic and partly electromagnetic; in the frequency range from 100 Hz to 10 kHz, broadband spikelike waves occur with a time-averaged amplitude of about 0.1 mV/m; and near the electron plasma frequency, bursts of waves occur at the boundaries of FTE's. In none of the 25 events surveyed does the total broadband amplitude of all the waves exceed a few millivolts per meter, an amplitude far too small to provide the dissipation required by 'traditional' reconnection models in which the dissipation region has a thickness the order of an ion gyroradius. Thus, either all of the FTE's are observed at some distance from the diffusion region, or the observable waves play no significant role in the diffusion process, or 'traditional' reconnection models do not apply to reconnection in FTE's.

Labelle, J.↗

NASA NTRS1986

Waves and electric fields associated with the first AMPTE artificial comet

The solar-wind Ba release experiment with the Active Magnetospheric Particle Tracer Explorers is discussed. The explosion of the two Ba-containing canisters produced a rapidly expanding cloud of neutral Ba, which was rapidly ionized by the UV radiation from the sun. A static electric field of about 1-2 mV/m was detected inside the diamagnetic cavity created by the ion cloud. This field is in the same direction as the solar-wind electric field, suggesting that this field may be able to penetrate into the cloud. Electrostatic emissions were observed inside the diamagnetic cavity at the Ba ion plasma frequency.

Gurnett, D. A.↗

NASA NTRS1985

ISEE observations of the magnetopause - Reconnection and the energy balance

The total energy balance for two events with the objective of obtaining check on the interpretation in terms of reconnection is examined. To within experimental uncertainties, the plasma and magnetic field data are consistent with reconnection. An enthalpy increase comparable to the kinetic energy increase occurs in the magnetopause. Thus substantial dissipation is present in the rotational discontinuity. An ion heat flow associated with a beam of reflected magnetosheath particles carried away some 20 percent of the total converted electromagnetic energy.

Paschmann, G.↗

NASA NTRS1984

Suprathermal ions upstream from interplanetary shocks

Low energy (10 eV-30 keV) observations of suprathermal ions ahead of outward propagating interplanetary shock waves (ISQ) are reported. The data were taken with the fast plasma experiment on ISEE 1 and 2 during 17 events. Structure was more evident in the suprathermal ion distribution in the earth bow shock region than in the upstream region. Isotropic distributions were only observed ahead of ISW, although field alignment, kidney-bean distributions, ion shells in velocity space and bunches of gyrating ions were not. The data suggest that the solar wind ions are accelerated to suprathermal energies in the vicinity of the shocks, which feature low and subcritical Mach numbers at 1 AU.

Gosling, J. T.↗

NASA NTRS1984

The magnetopause as a tangential discontinuity for large field rotation angles

Three passes of the ISEE 1 and 2 satellites through the dayside terrestrial magnetopause are discussed where the magnetopause is identified as a tangential discontinuity. This identification is based primarily on the failure of the plasma and magnetic field data to satisfy the conditions for a rotational discontinuity. In all these cases the interplanetary magnetic field was directed strongly southward and the angles between the fields on the two sides of the magnetopause ranged between 136 deg and 170 deg. As this is precisely the field geometry thought to be most conducive for reconnection, the magnetopause would be expected to be a rotational discontinuity. The simplest explanation of this result would appear to be that the magnetic field orientation is not the only factor controlling the onset of reconnection. However, as the identification of the discontinuity applies only locally, it cannot be excluded that for the magnetic field conditions investigated here, different portions of the magnetopause can be described as tangential and rotational discontinuities simultaneously.

Papamastorakis, I.↗

NASA NTRS1983

Field-aligned ion beams upstream of the earth's bow shock Evidence for a magnetosheath source

High time resolution ISEE-1 and -2 observations of upstream field-aligned ion beams at several crossings of the earth's bow shock indicate that some beams are due to high energy magnetosheath particles leaking through the shock into the upstream region. The distribution immediately downstream of these oblique shocks consists of a 'core' of directly transmitted, slightly heated ions, plus a crescent-shaped, high-velocity distribution, centered roughly on the magnetic field in the direction toward the upstream region, with a fairly well defined low velocity cutoff.

Thomsen, M. F.↗

NASA NTRS1983

Energetic particles in the vicinity of a possible neutral line in the plasma sheet

Combined plasma, magnetic field, and energetic particle data obtained from ISEE-1 in the geomagnetic tail during two successive energetic particle burst events are presented. The behavior of protons with energies of more than about 100 keV is very different from that of the 30-100 keV protons which represent the suprathermal tail of the plasma sheet distribution. The more energetic ions appear on a time scale of several minutes following a northward turning of the tail magnetic field. At about the same time the plasma measurements show a velocity of about 200 km/s in the tailward direction. From these results, it is argued that two successive magnetic neutral lines are created well within the plasma sheet and move close to the satellite position in the earthward direction. The extent of the neutral line is then limited to the dusk side of the tail.

Moebius, E.↗