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At least 73 records · Page 4

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NASA NTRS1995

Particle signatures of magnetic topology at the magnetopause: AMPTE/CCE observations

Electron distributions at energies above 50 eV have been found to be a sensitive indicator of magnetic topology for magnetopause crossings of the AMPTE/CCE spacecraft. Progressing from the magnetosheath to the magnetosphere two abrupt transitions occur. First, the magnetosheath electron population directed either parallel or antiparallel to the magnetic field is replaced by a streaming, heated magnetosheath electron population. The other half of the distribution is unchanged. The region with unidirectional, heated magnetosheath electrons is identified as the magnetosheath boundary layer (MSBL). Second, the unheated magnetosheath electron population is replaced by a heated population nearly identical to the population encountered in the MSBL, resulting in a symmetric counterstreaming distribution. The region populated by the bidirectional heated magnetosheath electrons is identified as the low-latitude boundary layer (LLBL). The MSBL and LLBL identified by the electron transitions are the same as the regions identified using ion composition measurements. The magnetosheath-MSBL transition reflects a change in magnetic topology from a solar wind field line to one that threads the magnetopause, and the existence of a magnetosheath-MSBL transition implies that the magnetopause is open. When the current layer is easily identified, the MSBL-LLBL transition coincides with the magnetopause current layer, indicating that the magnetosheath electrons are heated in the current layer. Both magnetosheath-MSBL and MSBL-LLBL transitions are observed for low as well as high magnetic shears. Moreover, the transitions are particularly clear for low shear implying that magnetic topology boundaries are sharp even when abrupt changes in the field and other plasma parameters are absent. Furthermore, for low magnetic shear, solar wind ions with low parallel drift speeds make up the majority of the LLBL population indicating that the magnetosheath plasma has convected directly across the magnetosheath plasma has converted directly across the magnetopause. These observations are consistent with quasi-steady, high-latitude reconnection and indicate that the signatures of this reconnection geometry are commonly present in the subpolar region.

Fuselier, S. A.↗

NASA NTRS2011

Dayside Magnetopause Transients Correlated with Changes of the Magnetosheath Magnetic Field Orientation

The paper analyses one long-term pass (26 August 2007) of the THEMIS spacecraft across the dayside low-latitude magnetopause. THEMIS B, serving partly as a magnetosheath monitor, observed several changes of the magnetic field that were accompanied by dynamic changes of the magnetopause location and/or the structure of magnetopause layers observed by THEMIS C, D, and E, whereas THEMIS A scanned the inner magnetosphere. We discuss the plasma and the magnetic field data with motivation to identify sources of observed quasiperiodic plasma transients. Such events at the magnetopause are usually attributed to pressure pulses coming from the solar wind, foreshock fluctuations, flux transfer events or surface waves. The presented transient events differ in nature (the magnetopause surface deformation, the low-latitude boundary layer thickening, the crossing of the reconnection site), but we found that all of them are associated with changes of the magnetosheath magnetic field orientation and with enhancements or depressions of the plasma density. Since these features are not observed in the data of upstream monitors, the study emphasizes the role of magnetosheath fluctuations in the solar wind-magnetosphere coupling.

Tkachenko, O.↗

NASA NTRS2023

Finding the magnetopause location using soft X-ray observations and a statistical inverse method

Variability in the location and shape of the dayside magnetopause is attributed to magnetic reconnection, a fundamental process that enables the transfer of mass, energy, and momentum from the solar wind into the magnetosphere. The spatial and temporal properties of the magnetopause, under varying solar and magnetospheric conditions, remain largely unknown because empirical studies using in-situ observations are challenging to interpret. Global wide field-of-view (FOV) imaging is the only means to simultaneously observe the spatial distribution of the plasma properties over the vast dayside magnetospheric region and, subsequently, quantify the energy transport from the interplanetary medium into the terrestrial magnetosphere. Two upcoming missions, ESA/CAS SMILE and NASA’s LEXI will provide wide-field imagery of the dayside magnetosheath in soft X-rays, an emission generated by charge exchange interactions between high charge-state heavy ions of solar wind origin and exospheric neutral atoms. High-cadence two-dimensional observations of the magnetosheath will allow the estimation of dynamic properties of its inner boundary, the magnetopause, and enable studies of its response to changes in the solar wind dynamic pressure and interplanetary magnetic field orientation. This work introduces a statistically-based estimation approach based on inverse theory to estimate the spatial distribution of magnetosheath soft X-ray emissivities and, with this, identify the location of the magnetopause over the Sun−Earth line. To do so, we simulate the magnetosheath structure using the MHD-based OpenGGCM model and generate synthetic soft X-ray images using LEXI’s orbit and attitude information. Our results show that 3-D estimations using the described statistically-based technique are robust against Poisson-distributed shot noise inherent to soft X-ray images. Also, our proposed methodology shows that the accuracy of both three-dimensional (3-D) estimation and the magnetopause standoff distance calculation highly depends on the observational point.

Gonzalo Cucho-Padin↗

NASA NTRS2012

Spatial Distribution of Rolled up Kelvin-Helmholtz Vortices at Earth's Dayside and Flank Magnetopause

The Kelvin-Helmholtz Instability (KHI) can drive waves at the magnetopause. These waves can grow to form rolled-up vortices and facilitate transfer of plasma into the magnetosphere. To investigate the persistence and frequency of such waves at the magnetopause we have carried out a survey of all Double Star 1 magnetopause crossings, using a combination of ion and magnetic field measurements. Using criteria originally used in a Geotail study made by Hasegawa et al. (2006) (forthwith referred to as H2006), 17 candidate events were identified from the entire TC-1 mission (covering 623 orbits where the magnetopause was sampled), a majority of which were on the dayside of the terminator. The relationship between density and shear velocity was then investigated, to identify the predicted signature of a rolled up vortex from H2006 and all 17 events exhibited some level of rolled up behavior. The location of the events had a clear dawn-dusk asymmetry, with 12 (71 %) on the post noon, dusk flank suggesting preferential growth in this region.

dawn-dusk asymmetry↗

NASA NTRS1974

Observations of the internal structure of the magnetopause

Magnetic field, plasma flux, and ELF wave data have been studied for several encounters of Ogo 5 with the earth's magnetopause. In one case of a crossing in the near-earth region of the geomagnetic tail, the structure agreed closely with a simple Chapman-Ferraro type of model with nearly complete neutralization of the charge separation electric field. Departures from the simple structure were observed at other magnetopause crossings. One crossing revealed a well-defined double structure with a large change in field direction closer to the earth than the ion flux and field strength gradients. The thickness of the magnetopause often depended on whether the change in field strength, the change in field direction, or the change in ion flux was being considered. Bursts of ELF waves were occasionally observed at the magnetopause.

Neugebauer, M.↗

NASA NTRS1974

Magnetopause reconnection rate

Discussion of certain aspects of the magnetic field reconnection process at the magnetopause. It is argued that the upper limits on reconnection derived by Petschek (1964), on the one hand, and Yeh and Axford (1970) and Sonnerup (1970), on the other, may be reconciled, at least qualitatively. Further, the possibility that magnetopause reconnection proceeds at this upper limit remains a real one. It is demonstrated that under the same assumptions as those used by Petschek (1964, 1966) the theoretical upper limit on the electric field component parallel to the X type magnetic null line is proportional to sin squared (theta/2) when the magnetic fields on the two sides of the magnetopause have equal magnitude. Finally, it is shown that when the magnetic field outside the magnetopause is weaker than the field inside that surface, reconnection may cease altogether for cos theta greater than the ratio of the outside field to the inside one.

Sonnerup, B. U. O.↗

NASA NTRS1975

Waves in the vicinity of the magnetopause

Magnetic field data from the IMP 6 spacecraft are used in an attempt to clarify the magnetic field microstructure of the magnetopause. After discussing the problems in identifying the magnetopause, and the difficulties in determining normal components, the presence of waves in the vicinity of the magnetopause is discussed. The downstream magnetopause is considered and the presence of surface waves on the tail boundary is demonstrated.

Fairfield, D. H.↗

NASA NTRS1976

Accretion onto magnetized neutron stars - Normal mode analysis of the interchange instability at the magnetopause

Results are reported for a linearized hydromagnetic stability analysis of the magnetopause of an accreting neutron star. The magnetosphere is assumed to be slowly rotating, and the plasma just outside the magnetopause is assumed to be weakly magnetized. The plasma layer is assumed to be bounded above by a shock wave and to be thin compared with the radius of the magnetosphere. Under these circumstances, the growing modes are shown to be localized in the direction parallel to the zero-order magnetic field, but the structure of the modes is still similar to the flute mode. An expression for the growth rate at each magnetic latitude is obtained in terms of the magnitude of the gravitational acceleration normal to the surface, the azimuthal mode number, the radius of the magnetosphere, the height of the shock above the magnetopause, and the effective Atwood number which embodies the stabilizing effects of favorable curvature and magnetic tension. The effective Atwood number is calculated, and the stabilizing effects of viscosity and aligned flow parallel to the magnetopause are discussed.

Arons, J.↗

NASA NTRS1978

Initial ISEE magnetometer results - Magnetopause observations

Magnetic-field profiles across the magnetopause are examined which were obtained by the ISEE 1 and 2 spacecraft on four passes when the spacecraft separation was only a few hundred kilometers. The velocity of the magnetopause is found to be highly irregular, ranging from 4 to over 40 km/s and varying on a shorter time scale than required for a spacecraft to cross the boundary. The thickness of the magnetopause is estimated to range from 500 to over 1000 km. Clear evidence for field-line reconnection is discerned in data obtained when the magnetosheath field was southward. This evidence consists of flux-transfer events in which reconnection begins and ends on a time scale of minutes or less, leading to oscillations in the position of the magnetopause.

Russell, C. T.↗

NASA NTRS1979

Structure of the magnetopause - Observations and implications for reconnection

The earth's magnetopause is the boundary between a hot tenuous plasma in the magnetosphere and a cooler denser plasma in the magnetosheath. Both of these plasmas contain magnetic fields whose directions are usually different but whose magnitudes are often comparable. Efforts to understand the structure of the magnetosphere have been hampered by the variability and complexity of this boundary. Waves on the magnetopause surface propagate toward the magnetotail and produce the multiple boundary crossings frequently seen by spacecraft. Boundary velocities are poorly known and range anywhere within an order of magnitude of 10 km/s. Typical thicknesses are probably of the order of a few hundred km, which is a few times the gyroradius of a thermal proton. Although conclusive direct evidence for a field component, Bn, across the magnetopause has not been found; this lack of evidence may reflect the difficulty in determining Bn in the presence of magnetopause waves rather than the real absence of this component. Considerable indirect evidence exists for an open magnetosphere, but the importance of the reconnection process thought to produce open field lines has recently been questioned.

Fairfield, D. H.↗

NASA NTRS1979

Electromagnetic structure of the magnetopause and boundary layer

After a review of the properties and predictions of the closed and open models of the magnetopause, OGO-5 magnetometer data are used to illustrate various observed signatures of the magnetopause current layer and the adjacent plasma boundary layer. Among the topics touched upon are: fluctuations, diamagnetic effects, and field aligned currents in the boundary layer; one dimensionality of the magnetopause; presence and absence of a magnetic field component perpendicular to the magnetopause; finite ion gyroradius effects. A brief summary is given of existing Vlasov theory for the description of tangential, rotational, and contact discontinuities. Special attention is paid to the tangential momentum balance and the jump conditions at a rotational discontinuity. Low frequency fluctuations are discussed with emphasis on the signatures of the tearing mode.

Sonnerup, B. U. O.↗

NASA NTRS1979

An energetic particle perspective of the magnetopause

The present analysis deals with energetic (above 24 keV) particle data from the Isee satellites during a series of magnetopause crossings. The primary energetic particle data employed in the analysis are the three-dimensional distributions from the Isee A satellite. Correlative magnetic field measurements are used to relate the particle behavior to magnetic field characteristics at and earthward of the magnetopause. It is shown that, to first order, the magnetopause may be regarded as a perfectly absorbing boundary for the trapped energetic particles, that it is nearly always in motion, and that boundary waves are often present. The observed dayside magnetopause motion is consistent with a large-scale radial motion having an approximately 10-min period plus superimposed boundary waves with a 90- to 150-sec period.

Williams, D. J.↗

NASA NTRS1981

Tearing at the dayside magnetopause

Initial observations of the dayside magnetopause thickness using magnetometers on board ISEE 1 (International Sun-Earth Explorer) and ISEE 2 indicate that this boundary may only be a few ion gyro-radii thick during periods of observed southward interplanetary magnetic field (IMF). Such a thin current layer can destabilize the collisionless tearing mode, a probable first step in the reconnection sequence. The linear growth rate is calculated assuming typical magnetopause parameters and it is found that it is a sensitive function of IMF orientation, magnetopause thickness, and electron number density. It is also shown that the finite size of the dayside magnetopause necessitates the existence of a two-dimensional tearing mode wave vector spectrum, a consequence of the inability of a thermal electron to maintain Landau resonance with the wave for a growth period. Implications regarding reconnection are discussed.

Quest, K. B.↗

NASA NTRS1982

The thickness of the magnetopause current layer - ISEE 1 and 2 observations

ISEE 1 and 2 magnetometer data over the dayside magnetosphere are analyzed to present a global survey of the magnetopause thickness. It is shown that earlier magnetopause velocities have been somewhat underestimated, with results indicating constant rapid and irregular motion and velocities ranging from 10-80 km/s in 80% of the cases. The magnetopause thickness is found to be between 400-1000 km, with larger thicknesses also observed, and the thickness does not seem to depend on the orientation of the magnetosheath magnetic field, but does appear to be ordered by dipole magnetic latitude so that the magnetopause current sheet is thinnest near the magnetic equator, which suggests that reconnection is initiated in the equatorial regions rather than in the polar cusps.

Berchem, J.↗

NASA NTRS1982

Evidence for quasi-stationary reconnection at the dayside magnetopause

The paper investigates several highly unusual encounters with the earth's magnetopause, that occurred during an approximately 5-hour period on November 22-23, 1979, when the ISEE 1 and 2 were near orbit apogee. A large decrease in the dynamic pressure exerted by the solar wind resulted in an expansion of the magnetosphere to and beyond the apogee of the ISEE 1 and 2 orbit, and the subsolar magnetopause of about 20.4 earth radii is farther than normal in geocentric distance by a factor of about 2. Field rotations varying from about 80 to 120 deg were involved in the transition from the magnetosheath to the magnetosphere, and hodograms of the tangential component of the magnetic field vector suggest that the magnetopause was a rotational discontinuity. These observations indicate that on occasion reconnection at the dayside magnetopause can be a quasi-stationary process.

Gosling, J. T.↗

NASA NTRS1982

Inductive electric field at the magnetopause

The electric field data for two crossings of the magnetopause by ISEE-1 on November 20, 1977, have been analyzed with high time resolution. In both cases the electric field has a negative dawn-dusk component in the boundary layer, so it must reverse somewhere within the current layer to the positive value outside. If there is a component parallel to the moving magnetopause current it is small, and by no means obvious. In the case of the exit crossing from the boundary layer to the magnetosheath the data show that the electric field vector is turning for about two seconds at roughly the satellite spin rate; this changing direction suggests that the electric field has a curl. Such a curl could be caused by a travelling localized perturbation of the magnetopause surface current associated with impulsive plasma transport through the magnetopause.

Heikkila, W. J.↗

NASA NTRS1982

Magnetic field rotation through the magnetopause - ISEE 1 and 2 observations

ISEE 1 and 2 magnetic field data obtained during dayside magnetopause crossings are analyzed using a minimum variance technique in order to determine the characteristics of the rotation of the magnetic field across the magnetopause. Only a small fraction of the magnetopause crossings examined present a sufficiently coherent structure that they can be compared with theoretical predictions and only a few of the hodograms of the magnetic field at the crossings display the ideal rotation expected for a rotational discontinuity structure. It is found that the observed senses of rotation are not consistent with the predictions of the electron whistler polarization theory. Rotations exceeding 180 deg are very seldom observed; they usually occur across multiple crossings, either as 360 deg rotation about the minimum variance direction or, occasionally, in more complicated patterns associated with S-shaped hodograms. These observations are seen as indicating that the sense of the magnetic field rotation through the magnetopause is controlled by the relative orientation of the magnetosheath and the magnetospheric field and that the angular rotation is minimized when the magnetic field changes from one orientation to the other.

Berchem, J.↗

NASA NTRS1983

On the plasma conditions at the dayside magnetopause of Saturn

The energy densities epsilon(P) of the low energy ions measured by the Low Energy Charged Particle experiment on Voyager 1 and 2 are compared to the magnetic field energy densities derived from the magnetometer instrument during the crossing of the dayside Saturn magnetopause. The ratios of the proton to magnetic field energy densities are greater than about 0.5 during the Voyager 2 crossing when the magnetopause was at about 18 R(s). During the Voyager 1 crossing of the magnetopause at about 23 R(s), the ratios were about 0.1, although they approached one as the spacecraft entered the region near 18 R(s). The observations show that the dayside Saturnian magnetopause can at times have high beta plasma conditions, similar to the situation found at Jupiter.

Lanzerotti, L. J.↗