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Brittnacher, M.

Publications and source records attributed to Brittnacher, M..

29 records · Page 2
All sourcesNASA NTRSDOE OSTI
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 NTRS1998

A New Synoptic Scale Feature of the Auroral Oval: The Nightside Gap

Th Polar Ultraviolet Imager (UVI) has recently identified a new morphological feature of the nightside aurora in which a well defined segment of the pre-midnight auroral oval, typically spanning 1.0- 1.5 hours in local time (710-1100 km), exhibits a significant reduction in luminosity relative to the active regions surrounding it. This feature appears in roughly 7% of the substorm period observed during our initial study spanning December 1996 through February, 1997. The energy flux (erg/sq cm/s) is shown to be lower by as much as a factor of four in this nightside gaps in comparison to the adjacent auroral activity. Substorm intensifications are often observed on either side of the nightside gap, which remains relatively inactive during these substorm periods. We demonstrate that the nightside gap is sometimes terminated by the onset of a substorm directly within the same local time boundaries. The nightside gap has strong implications for the dynamics of magnetosphere-ionosphere coupling and current closure in the midnight sector auroral oval. Specifically, we conjecture that the nightside gap occurs when transverse ionospheric currents do not feed into upward field-aligned currents in the midnight sector, perhaps due to the Harang discontinuity.

Cliua, D.↗
NASA NTRS1998

Auroral Boundaries: Comparison Between UV Images, In Situ Precipitation, and Groundbased Optical Observations

The location of poleward and equatorward boundaries are used as diagnostics of substorm phase and energy storage in the magnetosphere. Boundaries are estimated from groundbased observations (all sky imagers, meridian scanning photometers, and magnetometers), from in situ particle observations, and from spacebased global auroral observations. Each observational technique has relative advantages. However, recent studies suggest caution when comparing boundaries derived from different methods. in the study, FUV auroral images from the POLAR Ultraviolet Imager are used to estimate auroral boundaries. These boundaries are then compared with similar boundaries derived from DMSP in situ particle precipitation observations and from groundbased meridian scanning photometers. The goal is to see how well the boundaries from these three dissimilar observations correlate, to estimate dependencies on instrumental capabilities, i.e. imager resolution, and to serve as a potential first step to derive an algorithm for finding oval boundaries from groundbased optical data.

Germany, G. A.↗
NASA NTRS1997

Comparison of Dawn and Dusk Precipitating Electron Energy Populations Shortly After the Initial Shock for the January 10th, 1997 Magnetic Cloud

The observed precipitating electron energy between 0130 UT and 0400 UT of January 10 th, 1997, indicates that there is a more energetic precipitating electron population that appears in the auroral oval at 1800-2200 UT at 030) UT. This increase in energy occurs after the initial shock of the magnetic cloud reaches the Earth (0114 UT) and after faint but dynamic polar cap precipitation has been cleared out. The more energetic population is observed to remain rather constant in MLT through the onset of auroral activity (0330 UT) and to the end of the Polar spacecraft apogee pass. Data from the Ultraviolet Imager LBH long and LBH short images are used to quantify the average energy of the precipitating auroral electrons. The Wind spacecraft located about 100 RE upstream monitored the IMF and plasma parameters during the passing of the cloud. The affects of oblique angle viewing are included in the analysis. Suggestions as to the source of this hot electron population will be presented.

Spann, J.↗
NASA NTRS1997

Changes in Thermospheric O/N2 Derived from UVI Auroral Images

A rigorous test of our understanding of the coupled ionosphere-thermosphere and its response to geomagnetic storms is the ability to reproduce observed storm effects as seen in the ionosphere and neutral atmosphere. The concept of compositional change is central to studies of thermosphere response to storm conditions. In particular, information about compositional change within the highly dynamic auroral region is limited. The Ultraviolet Imager (UVI) is designed to view the full auroral region using five filters to isolate emissions from atomic oxygen (1304 and 1356) and N2 LBH. This spectral resolution allows auroral energy characteristics to be derived by two separate methods from examining ratios of observed intensities (OI 1356/LBHL or LBHS/LBHL). The LBHS:LBHL ratio is typically used as the mean energy diagnostic since the OI 1356 emission is dependent on changes in the atomic oxygen density, and these changes relative to N2 can be large. However, once the mean energy has been specified by the LBH ratio, this variability in OI 1356 emission can be exploited as a direct diagnostic of total atomic oxygen column density. This opens the potential of using UVI images to monitor the temporal and spatial response of thermospheric O to high latitude forcing within the auroral regions. Initial results of this type of analysis will be presented along with discussion of its limitations and capabilities.

Germany, G. A.↗
NASA NTRS1997

Does the Ultraviolet Imager on Polar Detect Cometesimals?

The Earth's dayglow images obtained by the Ultraviolet Imager (UVI) on Polar should show darkened pixels if cometesimals bombard Earth. This darkening occurs because the UV emissions are absorbed by water between the altitude where the dayglow emissions are produced and the Polar spacecraft. We have asked: (1) Does the UVI detect darkened pixels? (2) If so, what are their characteristics? (3) What can the UVI say about the presence/absence of cometesimals? The UVI does see dark pixels, which tend to be clustered. But the calibration data obtained from the same instrument just before the instrument was launched show similarly darkened pixel clusters. After a though statistical analysis of these images, we conclude that the darkened pixels are instrumental artifacts. Therefore, the UVI does not provide evidence for the existence of the cometesimals.

Parks, G. K.↗
NASA NTRS1997

Compensation for Spherical Geometric and Absorption Effects on Lower Thermospheric Emission Intensities Derived from High Earth Orbit Images

Remote sensing of the atmosphere from high earth orbit is very attractive due to the large field of view obtained and a true global perspective. This viewpoint is complicated by earth curvature effects so that slant path enhancement and absorption effects, small from low earth orbit, become dominant even at small nadir view angles. The effect is further complicated by the large range of local times and solar zenith angles in a single image leading to a modulation of the image intensity by a significant portion of the diurnal height variation of the absorbing layer. The latter effect is significant in particular for mesospheric, stratospheric and auroral emissions due to their depth in the atmosphere. As a particular case, the emissions from atomic oxygen (130.4 and 135.6 nm) and molecular nitrogen (two LBH bands, LBHS from 140 to 160 nm and LBHL from 160 to 180 nm) as viewed from the Ultraviolet Imager (UVI) are examined. The LBH emissions are of particular interest since LBHS has significant 02 absorption while LBHL does not, In the case of auroral emissions this differential absorption, well examined in the nadir, gives information about the height of the emission and therefore the energy of the precipitating particles. Using simulations of the viewing geometry and images from the UVI we examine these effects and obtain correction factors to adjust to the nadir case with a significant improvement of the derived characteristic energy. There is a surprisingly large effect on the images from the 02 diurnal layer height changes. An empirical compensation to the nadir case is explored based on the local nadir and local zenith angles for each portion of the image. These compensations are demonstrated as applied to the above emissions in both auroral and dayglow images and compared to models. The extension of these findings to other instruments, emissions and spectral regions is examined.

Swift, W.↗
NASA NTRS1995

A new approach to the linear theory of single-species tearing in two-dimensional quasi-neutral sheets

We have developed the linear theory of collisionless ion tearing in a two-dimensional magnetotail equilibrium for a single resonant species. We have solved the normal mode problem for tearing instability by an algorithm that employs particle-in-cell simulation to calculate the orbit integrals in the Maxwell-Vlasov eigenmode equation. The results of our single-species tearing analysis can be applied to ion tearing where electron effects are not included. We have calculated the tearing growth rate as a function of the magnetic field component B(sub n) normal to the current sheet for thick and thin current sheets, and we show that marginal stability occurs when the normal gyrofrequency Omega(sub n) is comparable to the Harris neutral sheet growth rate. A cross-tail B(sub y) component has little effect on the growth rate for B(sub y) approximately = B(sub n). Even in the limit B(sub y) much greater than B(sub n), the mode is strongly stabilized by B(sub n). We report than random pitch angle scattering can overcome the stabilizing effect of B(sub n) and drive the growth rate up toward the Harris neutral sheet (B(sub n) = 0) value when the pitch angle diffusion rate is comparable to Omega(sub n).

Brittnacher, M.↗
NASA NTRS1994

On the energy principle and ion tearing in the magnetotail

We re-examine the use of the energy principle as applied to the tearing instability in the magnetotail. We demonstrate that when a magnetic field component normal to the current sheet is present, electron pitch-angle diffusion (PAD) either by micro-turbulence or by chaotic orbits cannot remove the strong stabilization of the tearing mode caused by electron compressibility. We find that our conclusions are in agreement with those of Pellat et al. (1991), who argued on the basis of canonical P(sub y) conservation that the stabilization of the ion tearing mode cannot be removed by the introduction of PAD. Our results are at variance with those of Kuznetsova and Zelenyi (1991), who argued that the application of the energy principle used by Pellat et al. (1991) is incorrect, and that tearing is in fact unstable in the limit of strong PAD. We show that the disagreement between these two studies can be traced to an incorrect orbit evaluation first introduced by Coroniti (1980) and subnsequently used by Kuznetsova and Zelenyi (1991).

Brittnacher, M.↗
NASA NTRS1990

Magnetotail acceleration using generalized drift theory - A kinetic merging scenario

It is possible to describe particle behavior in the magnetotail, including particle energization, by means of generalized drift theory. Generalized drift velocities are obtained by using the generalized first invariant which has been shown to be useful in such current sheet configurations. Particles whose generalized invariant is preserved gain energy entirely in the field-aligned direction. The form of the accelerated particle velocity distribution is obtained and self-consistency conditions are derived.

Whipple, E. C.↗