Engineering topics

Berchem, J.

Publications and source records attributed to Berchem, J..

18 records

NASA NTRS2016

Impacts of Spontaneous Hot Flow Anomalies on the Magnetosheath and Magnetopause

Spacecraft observations and global hybrid (kinetic ions and fluid electrons) simulations have demonstrated that ion dissipation processes at the quasi-parallel bow shock are associated with the formation of structures called spontaneous hot flow anomalies (SHFAs). Previous simulations and recent spacecraft observations have also established that SHFAs result in the formation of magnetosheath filamentary structures(MFS). In this paper we demonstrate that in addition to MFS, SHFAs also result in the formation of magnetos heath cavities that are associated with decreases in density, velocity, and magnetic field and enhancements in temperature. We use the results of a global MHD run to determine the change in the magnetosheath properties associated with cavities due to ion kinetic effects. The results also show the formation of regions of high flow speed called magnetosheath jets whose properties as a function of solar wind Mach number are described in this study. Comparing the properties of the simulated magnetosheath cavities and jets to past spacecraft observations provides good agreement in both cases. We also demonstrate that pressure variations associated with cavities and SHFAs in the sheath result in a continuous sunward and anti sunward magnetopause motion. This result is consistent with previous suggestions that SHFAs may be responsible for the generation of ion cyclotron waves and precipitation of ring current protons in the outer magnetosphere.

Omidi, N.↗

NASA NTRS2012

Inner Plasma Structure of the Low-Latitude Reconnection Layer

We report a clear transition through a reconnection layer at the low-latitude magnetopause which shows a complete traversal across all reconnected field lines during northwestward interplanetary magnetic field (IMF) conditions. The associated plasma populations confirm details of the electron and ion mixing and the time history and acceleration through the current layer. This case has low magnetic shear with a strong guide field and the reconnection layer contains a single density depletion layer on the magnetosheath side which we suggest results from nearly field-aligned magnetosheath flows. Within the reconnection boundary layer, there are two plasma boundaries, close to the inferred separatrices on the magnetosphere and magnetosheath sides (Ssp and Ssh) and two boundaries associated with the Alfvén waves (or Rotational Discontinuities, RDsp and RDsh). The data are consistent with these being launched from the reconnection site and the plasma distributions are well ordered and suggestive of the time elapsed since reconnection of the field lines observed. In each sub-layer between the boundaries the plasma distribution is different and is centered around the current sheet, responsible for magnetosheath acceleration. We show evidence for a velocity dispersion effect in the electron anisotropy that is consistent with the time elapsed since reconnection. In addition, new evidence is presented for the occurrence of partial reflection of magnetosheath electrons at the magnetopause current layer.

magnetosheath↗

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 NTRS2011

Extended Magnetic Reconnection Across the Dayside Magnetopause

The extent of where magnetic reconnection (MR), the dominant process responsible for energy and plasma transport into the magnetosphere, operates across Earth's dayside magnetopause has previously been only indirectly shown by observations. We report the first direct evidence of X-line structure resulting from the operation of MR at each of two widely separated locations along the tilted, subsolar line of maximum current on Earth's magnetopause, confirming the operation of MR at two or more sites across the extended region where MR is expected to occur. The evidence results from in-situ observations of the associated ion and electron plasma distributions, present within each magnetic X-line structure, taken by two spacecraft passing through the active MR regions simultaneously.

Dunlop, M. W.↗

NASA NTRS2003

Dayside Proton Aurora: Comparisons between Global MHD Simulations and Image Observations

The IMAGE mission provides a unique opportunity to evaluate the accuracy of current global models of the solar wind interaction with the Earth's magnetosphere. In particular, images of proton auroras from the Far Ultraviolet Instrument (FUV) onboard the IMAGE spacecraft are well suited to support investigations of the response of the Earth's magnetosphere to interplanetary disturbances. Accordingly, we have modeled two events that occurred on June 8 and July 28, 2000, using plasma and magnetic field parameters measured upstream of the bow shock as input to three-dimensional magnetohydrodynamic (MHD) simulations. This paper begins with a discussion of images of proton auroras from the FUV SI-12 instrument in comparison with the simulation results. The comparison showed a very good agreement between intensifications in the auroral emissions measured by FUV SI-12 and the enhancement of plasma flows into the dayside ionosphere predicted by the global simulations. Subsequently, the IMAGE observations are analyzed in the context of the dayside magnetosphere's topological changes in magnetic field and plasma flows inferred from the simulation results. Finding include that the global dynamics of the auroral proton precipitation patterns observed by IMAGE are consistent with magnetic field reconnection occurring as a continuous process while the iMF changes in direction and the solar wind dynamic pressure varies. The global simulations also indicate that some of the transient patterns observed by IMAGE are consistent with sporadic reconnection processes. Global merging patterns found in the simulations agree with the antiparallel merging model. though locally component merging might broaden the merging region, especially in the region where shocked solar wind discontinuities first reach the magnetopause. Finally, the simulations predict the accretion of plasma near the bow shock in the regions threaded by newly open field lines on which plasma flows into the dayside ionosphere are enhanced. Overall the results of these initial comparisons between global MHD simulation results and IMAGE observations emphasize the interplay between reconnection and dynamic pressure processes at the dayside magnetopause. as well as the intricate connection between the bow shock and the auroral region.

Berchem, J.↗

NASA NTRS1998

The Distant Tail at 200 R(sub E): Comparison Between Geotail Observations and the Results from a Global Magnetohydrodynamic Simulation

This paper reports a comparison between Geotail observations of plasmas and magnetic fields at 200 R(sub E) in the Earth's magnetotail with results from a time-dependent, global magnetohydrodynamic simulation of the interaction of the solar wind with the magnetosphere. The study focuses on observations from July 7, 1993, during which the Geotail spacecraft crossed the distant tail magnetospheric boundary several times while the interplanetary magnetic field (IMF) was predominantly northward and was marked by slow rotations of its clock angle. Simultaneous IMP 8 observations of solar wind ions and the IMF were used as driving input for the MHD simulation, and the resulting time series were compared directly with those from the Geotail spacecraft. The very good agreement found provided the basis for an investigation of the response of the distant tail associated with the clock angle of the IMF. Results from the simulation show that the stresses imposed by the draping of magnetosheath field lines and the asymmetric removal of magnetic flux tailward of the cusps altered considerably the shape of the distant tail as the solar wind discontinuities convected downstream of Earth. As a result, the cross section of the distant tail was considerably flattened along the direction perpendicular to the IMF clock angle, the direction of the neutral sheet following that of the IMF. The simulation also revealed that the combined action of magnetic reconnection and the slow rotation of the IMF clock angle led to a braiding of the distant tail's magnetic field lines along the axis of the tail, with the plane of the braid lying in the direction of the IMF.

Berchem, J.↗

NASA NTRS1998

Large-Scale Dynamics of the Magnetospheric Boundary: Comparisons between Global MHD Simulation Results and ISTP Observations

Understanding the large-scale dynamics of the magnetospheric boundary is an important step towards achieving the ISTP mission's broad objective of assessing the global transport of plasma and energy through the geospace environment. Our approach is based on three-dimensional global magnetohydrodynamic (MHD) simulations of the solar wind-magnetosphere- ionosphere system, and consists of using interplanetary magnetic field (IMF) and plasma parameters measured by solar wind monitors upstream of the bow shock as input to the simulations for predicting the large-scale dynamics of the magnetospheric boundary. The validity of these predictions is tested by comparing local data streams with time series measured by downstream spacecraft crossing the magnetospheric boundary. In this paper, we review results from several case studies which confirm that our MHD model reproduces very well the large-scale motion of the magnetospheric boundary. The first case illustrates the complexity of the magnetic field topology that can occur at the dayside magnetospheric boundary for periods of northward IMF with strong Bx and By components. The second comparison reviewed combines dynamic and topological aspects in an investigation of the evolution of the distant tail at 200 R(sub E) from the Earth.

Berchem, J.↗

NASA NTRS1997

Boundary Layer Formation in the Magnetotail: Geotail Observations and Comparisons with a Global MHD Simulation

We present Geotail plasma and field observations from the middle magnetotail near X(sub GSE) = -46 R(sub E) for the time period 1400 to 1800 UT on December 14, 1994. During that period, the Wind satellite monitored the solar wind plasma and interplanetary magnetic field (IMF) upstream of the bow shock. The IMF was northward and the plasma parameters near average. Geotail observed slow tailward flows and a northward field. The plasma and field parameters indicate that Geotail is either in the plasma sheet or in a boundary layer. We used the Wind solar wind plasma and IMF data as input for a global simulation of that time interval. Comparison of the simulation results with the observational data show very good overall agreement of the magnitudes of the plasma and field parameters. In particular, the simulation reproduces the slow tailward flows and northward field found at Geotail. Small scale temporal, variations are less well reproduced. The simulation shows the formation of a broad boundary layer (which we call tail flank boundary layer, TFBL) that consists of closed flux which is formed by magnetic magnetic reconnection of IMF and lobe field lines. The simulation results indicate that Geotail is located very close to the TFBL and may have entered the TFBL proper. We show that the TFBL plays an important role in energy transport from the solar wind into the magnetosphere during northward IMF conditions.

Raeder, J.↗

NASA NTRS1995

Interactive visualization of numerical simulation results: A tool for mission planning and data analysis

We report on the development of an interactive system for visualizing and analyzing numerical simulation results. This system is based on visualization modules which use the Application Visualization System (AVS) and the NCAR graphics packages. Examples from recent simulations are presented to illustrate how these modules can be used for displaying and manipulating simulation results to facilitate their comparison with phenomenological model results and observations.

Berchem, J.↗

NASA NTRS1987

Cusp displacement at the magnetopause for large IMF Y component

The magnetic field orientation just earthward of the dayside magnetopause from 95 ISEE and 11 HEOS crossings with /By/ greater than /Bz/ in the adjacent magnetosheath indicate a statistically significant-1-earth-radius shift of the cusps toward dawn (dusk) in the Northern Hemisphere and dusk (dawn) in the Southern Hemisphere for positive (negative) By. This small shift near the magnetopause and the contrasting large shifts of some reported ionospheric cusp signatures are explained in terms of a finite thickness magnetopause model through which the cusps shift gradually from the inner to the outer edge.

Crooker, N. U.↗

NASA NTRS1985

Electric fields and convection velocities associated with flux transfer events

A survey of the electric field and (E x B)/B-squared convection velocities associated with 23 flux transfer events (FTEs) in the magnetosheath and magnetosphere is presented. Twelve-second averaged electric field data show perturbations in the component of the electric field perpendicular to the magnetopause, with an average magnitude of 3.5 mV/m associated with the FTEs. Direct flux transfer events have a northward, and reverse flux transfer events a southward, E x B velocity component, in agreement with the current FTE model. The flux tubes moved across the magnetopause plane at an average E x B speed of 125 km/s with respect to the ambient E x B flow, with events located on the dawn (dusk) side tending to move downward (duskward). No large spiky electric fields of the type observed elsewhere in the magnetosphere were detected.

Dailey, R.↗

NASA NTRS1985

Nonresonant interaction of heavy ions with electromagnetic ion cyclotron waves

The motion of a heavy ion in the presence of an intense ultralow-frequency electromagnetic wave propagating along the dc magnetic field is analyzed. Starting from the basic equations of motion and from their associated two invariants, the heavy ion velocity-space trajectories are drawn. It is shown that after a certain time, particles whose initial phase angles are randomly distributed tend to bunch together, provided that the wave intensity b-sub-1 is sufficiently large. The importance of these results for the interpretation of the recently observed acceleration of singly charged He ions in conjunction with the occurrence of large-amplitude ion cyclotron waves in the equatorial magnetosphere is discussed.

Berchem, J.↗

NASA NTRS1985

On the source region of flux transfer events

An examination has been conducted of the region of occurrence of flux transfer events for three distinct orientations of the interplanetary magnetic field: nearly horizontal in the solar magnetospheric equator, diagonally southward at 45 deg to the magnetospheric equator and nearly due south. For horizontal IMF conditions the FTE's occur in a horizontal band about + or - 6 earth radii wide. For diagonally southward IMF conditions, the FTE's occur in a diagonal swath about + or - 6 earth radii wide passing through the subsolar point. For duskward but nearly due southward IMF conditions, the observations reveal FTE's throughout the northern morning quadrant. These observations are consistent with a near equatorial source for flux transfer events and hence with component merging and not anti-parallel merging. These observations also help understand the energetic ion anisotropies seen in these events.

Russell, C. T.↗

NASA NTRS1984

Magnetic field observations at the dayside magnetopause: ISEE-1 and -2 contributions during the IMS

Dayside magnetopause macroscopic characteristics such as velocities and thicknesses of the boundary and internal magnetic field structure of the current layer were studied by ISEE-1 and 2. The ISEE results suggest that the magnetopause is much thicker than previously thought. A current sheet thicker than several proton gyroradii rules out classical kinetic theories of a magnetopause current layer of the order of the ion gyroradius. Such a thick boundary may explain the failure of theories which predict a unique polarization for the field rotation inside the current sheet.

Berchem, J.↗

NASA NTRS1984

Flux transfer events on the magnetopause - Spatial distribution and controlling factors

The spatial distribution of flux transfer events (FTE) of magnetic flux tubes pulled from the earth's magnetopause is analyzed using ISEE 1 and 2 data from 1977-82. Attention is given to interplanetary conditions influencing different observed FTE polarities. FTEs were observed on nearly 25 percent of the passes near the dayside magnetopause. Direct FTEs were located mainly in the northern dawn sector and reverse FTEs appeared in the southern dusk sector. The distribution indicated an origin in the equatorial sector, and data correlate the appearance of FTEs only when the interplanetary magnetic field (IMF) had southward or nearly horizontal orientation. The presence of a southward component in the IMF was coincident with the appearance of an FTE 45 percent of the events. E-W components in the IMF exhibited no connections with the occurrence of an FTE.

Berchem, J.↗

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 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 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.↗