Engineering topics

Wilber, M.

Publications and source records attributed to Wilber, M..

8 records

NASA NTRS2012

Shocklets, SLAMS, and Field-Aligned Ion Beams in the Terrestrial Foreshock

We present Wind spacecraft observations of ion distributions showing field- aligned beams (FABs) and large-amplitude magnetic fluctuations composed of a series of shocklets and short large-amplitude magnetic structures (SLAMS). The FABs are found to have T(sub k) approx 80-850 eV, V(sub b)/V(sub sw) approx 1.3-2.4, T(sub perpendicular,b)/T(sub paralell,b) approx 1-8, and n(sub b)/n(sub o) approx 0.2-11%. Saturation amplitudes for ion/ion resonant and non-resonant instabilities are too small to explain the observed SLAMS amplitudes. We show two examples where groups of SLAMS can act like a local quasi-perpendicular shock reflecting ions to produce the FABs, a scenario distinct from the more-common production at the quasi-perpendicular bow shock. The SLAMS exhibit a foot-like magnetic enhancement with a leading magnetosonic whistler train, consistent with previous observations. Strong ion and electron heating are observed within the series of shocklets and SLAMS with temperatures increasing by factors approx > 5 and approx >3, respectively. Both the core and halo electron components show strong perpendicular heating inside the feature.

Wilson, L. B.↗

NASA NTRS2010

Large-Amplitude Electrostatic Waves Observed at a Supercritical Interplanetary Shock

We present the first observations at an interplanetary shock of large-amplitude (> 100 mV/m pk-pk) solitary waves and large-amplitude (approx.30 mV/m pk-pk) waves exhibiting characteristics consistent with electron Bernstein waves. The Bernstein-like waves show enhanced power at integer and half-integer harmonics of the cyclotron frequency with a broadened power spectrum at higher frequencies, consistent with the electron cyclotron drift instability. The Bernstein-like waves are obliquely polarized with respect to the magnetic field but parallel to the shock normal direction. Strong particle heating is observed in both the electrons and ions. The observed heating and waveforms are likely due to instabilities driven by the free energy provided by reflected ions at this supercritical interplanetary shock. These results offer new insights into collisionless shock dissipation and wave-particle interactions in the solar wind.

Wilson, L. B., III↗

NASA NTRS2007

Flux Transfer Events Simultaneously Observed by Polar and Cluster: Flux Rope in the Subsolar Region and Flux Tube Addition to the Polar Cusp

The phenomenon called flux transfer events (FTEs) is widely accepted as the manifestation of time-dependent reconnection. In this paper, we present observational evidence of a flux transfer event observed simultaneously at low-latitude by Polar and at high-latitude by Cluster. This event occurs on March 21, 2002, when both Cluster and Polar are located near local noon but with a large latitudinal separation. During the event, Cluster is moving outbound from the polar cusp to the magnetosheath, and Polar is in the magnetosheath near the equatorial magnetopause. The observations show that a flux transfer event occurs between the equator and the northern cusp. Polar and Cluster observe the FTE s two open flux tubes: Polar encounters the southward moving flux tube near the equator; and Cluster the northward moving flux tube at high latitude. The low latitude FTE appears to be a flux rope with helical magnetic field lines as it has a strong core field and the magnetic field component in the boundary normal direction exhibits a strong bi-polar variation. Unlike the low-latitude FTE, the high-latitude FTE observed by Cluster does not exhibit the characteristic bi-polar perturbation in the magnetic field. But the plasma data clearly reveal its open flux tube configuration. It shows that the magnetic field lines have straightened inside the FTE and become more aligned to the neighboring flux tubes as it moves to the cusp. Enhanced electrostatic fluctuations have been observed within the FTE core, both at low- and high-latitudes. This event provides a unique opportunity to understand high-latitude FTE signatures and the nature of time-varying reconnection.

Le, G.↗

NASA NTRS1999

Global Auroral Response to a Solar Wind Pressure Pulse

A global intensification of the aurora was observed by the Ultraviolet Imager on the NASA Polar spacecraft in conjunction with the arrival of the sheath from a solar coronal mass ejection. The aurora was first observed to brighten on the dayside and then the intensification progressed rapidly toward the nightside. During this time the IMP-8 spacecraft in the solar wind recorded a 35-minute period of increased solar wind dynamic pressure. A small substorm (or, possibly pseudobreakup) occurred within a minute of the arrival of the auroral intensification on the nightside in conjunction with a second peak in the dynamic pressure. We propose that the intensification of the aurora can be explained on the basis of the compression of the magnetopause and the generation of hydrodynamic waves by the rapid increase in the solar wind dynamic pressure. It is also evident that the substorm was triggered by waves, generated by a second rise in the dynamic pressure, that propagated to flux tubes connected to the premidnight aurora region.

Brittnacher, M.↗

NASA NTRS1998

What the Polar Cap Tells Us about the Substorm Growth Phase

The polar cap region in the 30 to 60 minute period prior to the onset of the auroral substorm has been examined using global images from the Polar Ultraviolet Imager (UVI) to look for observational evidence of processes related to the substorm growth phase. In particular, the area of the polar cap has been measured to determine changes in its size in relation to the orientation of the interplanetary magnetic field (IMF). It was found that the size of the polar cap region increases during the growth phase even if the IMF has no southward component. Three phenomena have been observed to produce the increase in the size of the polar cap: (1) motion of the auroral oval to lower latitude, (2) thinning of the auroral oval, and (3) reduction of intense auroral precipitation in the polar cap region. The first phenomenon has been considered to be a result of the growth of the tail lobe magnetic field and the second is related to the thinning of the plasma sheet. Both of these have been supported by in situ observational evidence and are consistent with current models of substorm development. However, the third phenomenon appears to be unrelated to the first two and does not appear to be the result of opening of the polar cap flux tubes to the solar wind IMF. This reduction of auroral precipitation provides evidence of a growth phase process, or change in auroral precipitation processes, that is not explained by current substorm models.

Brittnacher, M. J.↗

NASA NTRS1998

Understanding Substorms from the Auroral Ionosphere to the Distant Plasma Sheet

The global polar UVI images have been correlated with observations from the ground, ionosphere, geomagnetic tail between 10-20 earth radii and the interplanetary space. One of the objectives of our study is to better understand the connection among many complex phenomena occurring close to Earth and those in the near--earth plasma sheet. We have examined the details of how the auroral and polar cap boundaries at different local times behave in relation to variations occurring in the solar wind, ionosphere and plasma sheet during substorms. We have also compared locations of boundaries deduced from images to electron flux "boundaries" observed by polar orbiting spacecraft. Our results indicate that the ionospheric dynamics is important and polar cap and auroral oval boundaries expand and contract in a complicated but systematic way. These variations are correlated to solar wind parameters and growth and recovery phenomena in the plasma sheet. These results can be interpreted in terms of directly driven and/or unloading substorm processes.

Parks, G. K.↗

NASA NTRS1998

Global Observation of Substorm Growth Phase Processes in the Polar Caps

Global images of the polar cap region during the substorm growth phase by the Polar Ultraviolet Imager reveals evidence of the processes which are not completely explained by current models. In particular, it was found that size of the polar cap region increases during the growth phase even if the interplanetary magnetic field has no southward component. Three phenomena were observed to produce an increase in the size of the polar cap: (1) motion of the auroral oval to lower latitude, (2) thinning of the auroral oval, and (3) reduction of intense aurora[ precipitation in the polar region. Correlation of image intensities with in situ particle measurements from the FAST satellite are being conducted to study the three growth phase phenomena; and to help identify the source regions of the particles, the mechanisms involved in producing the auroral structures and what may be reducing the polar cap precipitation during the substorm growth phase.

Brittnacher, M.↗

NASA NTRS1995

Dawn-dusk asymmetries in the low-latitude boundary layer arising from the Kelvin-Helmholtz instability: A particle simulation

Along the low-latitude boundary layer (LLBL), the Kelvin-Helmholtz (K-H) instability can provide a means for injection of solar wind plasma across closed field lines of the magnetosphere. A fully electromagnetic, two-dimensional (three-velocity) particle code is used to investigate dawn-dusk asymmetries that can arise from velocity differences due to gradient drifts and electric field gradients, and ion acceleration across the narrow field transition layers in the dawn and dusk flanks. The model includes the dawn and dusk sides of the LLBL simultaneously in a slab geometry, incorporating seperate populations of ions and electrons for the magnetosphere and the magnetosheath plasmas. We report several effects: (1) asymmetries in the observed morphology of the turbulent structures, with familiar fluidlike vortex formation on the duskside, but tongues of magnetosheath plasma penetrating into the magnetosphere on the dawnside; (2) the formation of discrete current layers, characterized by strong currents and sharp gradients in the magnetic field, and discrete charge layers, having net charges and constant, weaker currents; (3) increasing asymmetry in dawn/dusk behavior with a decrease of initial currents; (3) increasing asymmetry in dawn/dusk behavior with a decrease of initial boundary layer width; and (4) enhancement of the dawn-to-dusk electric field as magnetosheath particles and momenta are transported across the magnetopause.

Wilber, M.↗