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Sauvaud, J. A.

Publications and source records attributed to Sauvaud, J. A..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2016

Shift of the Magnetopause Reconnection Line to the Winter Hemisphere Under Southward IMF Conditions: Geotail and MMS Observations

At 02:13 UT on 18 November 2015 when the geomagnetic dipole was tilted by -27deg, the MMS spacecraft observed southward reconnection jets near the subsolar magnetopause under southward and dawnward interplanetary magnetic field conditions. Based on four-spacecraft estimations of the magnetic field direction near the separatrix and the motion and direction of the current sheet, the location of the reconnection line was estimated to be approx.1.8 R(sub E) or further northward of MMS. The Geotail spacecraft at GSM Z approx. 1.4 R(sub E) also observed southward reconnection jets at the dawnside magnetopause 30-40 min later. The estimated reconnection line location was northward of GSM Z approx.2 R(sub E). This crossing occurred when MMS observed purely southward magnetic fields in the magnetosheath. The simultaneous observations are thus consistent with the hypothesis that the dayside magnetopause reconnection line shifts from the subsolar point toward the northem (winter) hemisphere due to the effect of geomagnetic dipole tilt.

Kitamura, N.↗
NASA NTRS1995

The double oval UV auroral distribution. 1: Implications for the mapping of auroral arcs

During the later stages of the auroral substorm the luminosity distribution frequently resembles a double oval, one oval lying poleward of the normal or main UV auroral oval. We interpret the double oval morphology as being due to the plasma sheet boundary layer becoming active in the later stages of the substorm process. If the disturbance engulfs the nightside low-latitude boundary layers, then the double oval configuration extends into the dayside ionospheric region. The main UV oval is associated with the inner portion of the central plasma sheet and can rapidly change its auroral character from being diffuse to discrete. This transition is associated with the substorm process and is fundamental to understanding the near-Earth character of substorm onset. On the other hand, the poleward arc system in the nightside ionosphere occurs adjacent to or near the open-closed field line boundary. This system activates at the end of the optical expansion phase and is a part of the recovery phase configuration in substorms where it occurs. These two source regions for nightside discrete auroral arcs are important in resolving the controversy concerning the mapping of arcs to the magnetosphere. The dayside extension of this double oval configuration is also investigated and shows particle signatures which differ considerably from those on the nightside giving clues to the magnetospheric source regions of the aurora in the two local time sectors. Near-Earth substorm onsets are shown to be coupled to processes occurring much further tailward and indicate the importance of understanding the temporal development of features within the double oval. Using 'variance images,' a new technqiue for the investigation of these dynamics is outlined.

Elphinstone, R. D.↗
NASA NTRS1995

Centrifugal trapping in the magnetotail

Particles leving the neutral sheet in the distant magnetotail at times display adiabatic trajectory sequences characterized by an inflection toward the equator and subsequent mirroring in its vicinity. We demonstrate that this low-latitude mirroring results primarily from a centrifugal deceleration due to the fast direction-changing E x B drift. This effect which we refer to as 'centrifugal trapping' appears both in guiding centre and full particle treatments. It thus does not directly relate to nonadiabatic motion. However, pitch angle scattering due to nonadiabatic neutral sheet interaction does play a role in reducing the parallel speed of the particles. We show that centrifugal trapping is an important mechanism for the confinement of the slowest (typically below the equatorial E x B drift speed) plasma sheet populations to the midplane vicinity.

Delcourt, D. C.↗
NASA NTRS1993

Electron plasma environment at comet Grigg-Skjellerup: General observations and comparison with the environment at comet Halley

The three-dimensional electron spectrometer of the Reme plasma analyzer-complete positive ion, electron and ram negative ion measurements near comet Halley (RPA-COPERNIC) experiment aboard the Giotto spacecraft, although damaged during the comet Halley encounter in March 1986, has provided very new results during the encounter on July 10, 1992, with the weakly active comet Grigg-Skjellerup (G-S). The main characteristic features of the highly structured interaction region extending from approximately 26,500 km inbound to approximately 37,200 km outbound are presented. These results are compared to the results obtained by the same instrument during the Giotto comet Halley fly-by. Despite the large difference in the size of the interaction regions (approximately 60,000 km for G-S, approximately 2000,000 km for Halley) due to 2 orders of magnitude difference in cometary neutral gas production rate, there are striking similarities in the solar wind interactions with the two comets.

Reme, H.↗
NASA NTRS1993

Polar wind ion dynamics in the magnetotail

The circulation of polar wind ions from the high-latitude ionosphere to the plasma sheet is investigated. Single-particle trajectory simulations for the geomagnetic tail show, in addition to the breaking of adiabaticity, the possible creation of new high-altitude mirror points. This trajectory feature results from an intense parallel deceleration imparted by the magnetic field rotation during fast ExB transport. This centrifugal deceleration yields a critical parallel velocity which depends on the magnitude of the convection electric field and below which ions remain trapped inside the neutral sheet.

Delcourt, D. C.↗
NASA NTRS1992

Nonadiabatic transport features in the outer cusp region

The dayside to nightside circulation of plasma along the magnetopause inside the magnetosphere is examined by means of three-dimensional single-particle codes. It is demonstrated that particles incident upon the outer cusp region experience transient non-adiabatic motions, owing to a localized minimum in the field magnitude. Here, possibly large magnetic moment changes yield injection into the loss cone of fractions of the incoming population or, alternatively, enhanced bouncing motions at high altitudes. It is shown that particles gaining access to the magnetotail over the polar cap are progressively extracted from the weak field region by the large-scale convection electric field. In this latter case, the trajectory simulations suggest an implicit 'entry boundary' into the nightside magnetosphere, which corresponds to the sunward edge of field lines featuring monotonic decrease of the field magnitude along their length.

Delcourt, D. C.↗
NASA NTRS1992

A two satellite study of nightside flux transfer events in the plasma sheet

The short-term (about 1 min) events during two consecutive substorms on March 23 1979 have been studied using high time resolution measurements of magnetic and electric fields and of the kiloelectronvolt electron flux at X about -21 R(e). Data obtained make it possible to describe the localized magnetic structures transported or propagated within the plasma sheet and to identify these events as plasma sheet nightside magnetic flux transfer events (NFTEs). The best examples of the NFTEs found so far are all observed in a thin portion of plasma sheet where the contraction of the reconnected field lines dominates over the oppositely directed pressure gradient force. Systematic differences in the magnetic variations observed at ISEE-1 and 2 indicate current concentration at the outer plasma sheet boundary during the passage of a NFTE.

Sergeev, V. A.↗
NASA NTRS1991

Gyro-phase effects near the storm-time boundary of energetic plasma

The nonadiabatic acceleration of plasma sheet ions during the expansion phase of substorms is examined by means of single-particle codes. It is shown that, in the near-earth plasma sheet, the gyration phase at substorm onset controls the net ion energization, as the gyro-period is locally comparable to the field variation time scale. This can yield a particularly significant decrease of the particle magnetic moment. It is accordingly argued that, via adiabatic invariant violation, the dipolarization of magnetospheric field lines can induce a 'de-trapping' of inner plasma sheet populations and, hence, give rise to short-lived precipitation enhancements over the auroral zone. As this effect depends upon cyclotron frequency, it is expected to occur at distinct latitudes for different ion species. This contrasts with nonadiabatic ion behavior further out into the geotail, which is characterized by intense perpendicular heating and a likely collective trapping.

Delcourt, D. C.↗
NASA NTRS1991

Negative ions in the coma of Comet Halley

The detection of negatively charged cometary ions in the inner coma of Comet Halley is reported. These ions are observed in three broad mass peaks at 7-19, 22-65, and 85-110 AMU, with densities reaching greater than about 1/cu cm, about 0.05/cu cm, and about 0.04/cu cm, respectively, at a distance of about 2300 km from the nucleus. The ion species thought to be present include O(-), OH(-), C(-), CH(-), CN(-) and heavier complex CHO molecular ions. As negative ions are easily destroyed by solar radiation at about 1 AU, an efficient production mechanism, so far unidentified, is required to account for the observed densities. The detection of negative ions in the coma near 1 AU implies that negative ions should also be present in similar neutral gas and dust environment farther away from the sun. If the negative-ion densities are large enough, they could play an important part in physical processes such as radiative transfer or charge exchange.

Chaizy, P.↗
NASA NTRS1990

Cleft contribution to ring current formation

The storm time transport of ionospheric plasma from the 'cleft fountain' to the plasma sheet and ring current is investigated by means of three-dimensional trajectory codes. Using observations to define the source location and flow rate, test particles are traced during a 'taillike' to 'dipolelike' reconfiguration of the geomagnetic field. Emphasis is placed on the behavior of heavy ions of low charge state, O(+). As a result of their long periods of gyration, these ions are highly sensitive to rapid field variations and possibly display transient nonadiabatic motions. It is demonstrated that O(+) which have originated in the high-latitude ionosphere but which find themselves in the vicinity of the equator at substorm onset can experience considerable energization (from several keV up to several hundred keV) and pitch angle increase leading to trapping, as an effect of the induced electric field.

Delcourt, D. C.↗
NASA NTRS1989

The composition of heavy molecular ions inside the ionopause of Comet Halley

The RPA2-PICCA instrument aboard the Giotto spacecraft obtained 10-210 amu mass spectral of cold thermal molecular ions in the coma of Comet Halley. The dissociation products of the long chain formaldehyde polymer polyoxymethylene (POM) have recently been proposed as the dominant complex molecules in the coma of Comet Halley; however, POM alone cannot account for all of the features of the high resolution spectrum. An important component of the dust at Comet Halley is particles highly enriched in carbon, hydrogen, oxygen, and nitrogen relative to the composition of carbonaceous chondrites. Since this dust could be a source for the heavy molecules observed by PICCA, a search was conducted for other chemical species by determining all the molecules with mass between 20 and 120 amu which can be made from the relatively abundant C, H, O, and N, without regard to chemical structure.

Mitchell, David L.↗
NASA NTRS1989

Analysis of suprathermal electron properties at the magnetic pile-up boundary of Comet P/Halley

Among the plasma discontinuities detected by the Giotto spacecraft around Comet P/Halley, the magnetic pile-up boundary, located at about 135,000 km from the nucleus, has a sharpness which was not foreseen by theoretical models. At this boundary, which marks the beginning of the region where the field lines draped around the nucleus have been piled up, the magnetic field jumps sharply. Electron measurements provided by the RPA experiment show that a clear plasma discontinuity coincides with this magnetic feature. Significant changes occur here in the suprathermal electron distribution function. A magneto-plasma sheet is clearly defined after the boundary. Inside this sheet, close correlations exist between the parameters describing the magnetic field and the electron population. The polytropic equation of state governing the suprathermal electrons in the sheet has been deduced from RPA measurements. Some implications of this law are discussed.

Mazelle, C.↗
NASA NTRS1989

On the global nature of the solar wind interaction with Comet Halley

Data obtained by two instruments of the RPA-Copernic experiment aboard Giotto during its encounter with Comet Halley are used to determine the positions of several sharp boundaries delineating transitions from one flow state to another. Production rates of the neutrals are obtained, along with ion-neutral drag coefficients. It is suggested that the cometopause observed between the shock and the ionopause coincides with the expected position of a previously proposed collisionopause.

Mendis, D. A.↗
NASA NTRS1987

A numerical study of suprathermal ionospheric ion trajectories in three-dimensional electric and magnetic field models

A numerical study of the magnetospheric trajectories of ions upflowing from the ionosphere has been performed using three-dimensional magnetic and electric field models. Ion trajectories have been divided into three main types corresponding to (1) gravitational ions trapped between the magnetic and gravitational mirror points and drifting nearly along the local electric equipotential, (2) ions just escaping from the gravitational field and then convected to the magnetopause due to the loss of the main part of their kinetic energy, and (3) trapped and pseudo-trapped ions whose trajectories are mainly determined by the magnetic field topology (beams and conics) because of their large kinetic energy. Particles from conic distributions injected at inadequate local times and/or altitudes to gain enough energy to precipitate reach the dayside magnetopause after several bounce periods.

Sauvaud, J. A.↗
NASA NTRS1987

The Giotto electron plasma experiment

The RPA-Copernic experiment aboard Giotto is described. The experiment is designed to measure the three-dimensional distributions of electrons between 10 eV and 30 keV (by the RPA-1 EESA spectrometer) and the composition and distribution, close to the comet, of thermal positive ions in the mass range 10-213 amu (by the RPA-2 PICCA electrostatic mass analyzer). Three microprocessors interface RPA-1 EESA with RPA-2 PICCA and with the spacecraft and perform extensive onboard data processing. The experiment was operated successfully aboard the spacecraft in September 1985 during the encounter of Giotto with the comet Halley. The results provided by the EESA-1 indicate that the solar wind interaction with the comet Halley forms a well-defined bow shock with features quite different from the features of the comet Giacobini-Zinner bow shock; the data also showed a presence of accelerated keV electrons at the cometary bow shock, upstream and in the transition region.

Reme, H.↗
NASA NTRS1987

Large scale response of the magnetosphere to a southward turning of the interplanetary magnetic field

A set of coordinated measurements obtained on March 4, 1979 in the solar wind, in the magnetotail, at geostationary orbit, in the auroral ionosphere, and on the ground has been used to conduct a detailed study of the overall magnetospheric response to a southward turning of the IMF. The change of the IMF direction is found to be associated with a delay of less than about 5 min for increasing eastward and westward electrojets, and precedes by about 10 min the beginning of a taillike deformation of the B field geometry at geostationary orbit near midnight. After the IMF reversal and before the main substorm onset, the general motion of the westward electrojet is southward. At 6.6 R(E) in the nightside, the equatorially trapped high-energy particles adiabatically respond to the local change of the B field by moving earthward. Short-lived substorms occur soon after the IMF reversal.

Sauvaud, J. A.↗
NASA NTRS1986

Derivation of heavy (10-210 AMU) ion composition and flow parameters for the Giotto PICCA instrument

The Giotto heavy ion analyzer RPA2-PICCA was designed to identify the composition and distribution of heavy thermal positive ions in the coma of comet Halley. The instrument is an electrostatic analyzer which takes advantage of the large relative flyby velocity and the fact that the ions in the inner coma should be cold and predominantly singly charged. A method of deconvolving E/Q measurements to yield ion mass composition, temperature, and flow velocity is presented. The measurements are used to investigate the dynamic behavior of the ions and variations in mass composition as a function of cometary distance.

Mitchell, D. L.↗