SEARCH · Engineering Papers

Results for “Polarity”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 163 records · Page 9

All sources NASA NTRS DOE OSTI

NASA NTRS1998

Polar Cap Area and Boundary Motion During Substorms

The area of the polar cap as a function of local time and substorm phase was measured using images from the Polar Ultraviolet Imager (UVI) for different interplanetary magnetic field (IMF) orientations during three substorms in January 1997. We measured changes in the polar cap area and motion of the poleward and equatorward boundary of the auroral oval as determined by UVI images. It was found that the polar cap boundary is strongly influenced by thinning of the oval, decrease in polar cap structures, the poleward expansion of the substorm at midnight and the fading of luminosity below the instrument sensitivity threshold. Generally these effects dominate over the latitudinal motion of the auroral oval at its equatorward edge. A new feature is that the polar cap region clears of precipitation during the substorm growth phase, which expands the size of the polar cap but may not necessarily be related to an expansion of the open flux. We present a new finding that the increase in polar cap area prior to onset and the decrease in the area following it are independent of the strength of the southward IMF component. For one case the polar cap area increased while the southward component of the IMF was no less than -0.5 nT. These observations have strong implications for models that use the polar cap area to estimate the magnitude of energy storage in the lobe magnetic field and loss during substorms.

Brittnacher, M.↗

NASA NTRS1999

Dynamics of the Auroral Luminosity Boundary of the Polar Cap During Substorms

The area of the polar cap during substorms has been measured using images from the Polar Ultraviolet Imager (UVI) for different interplanetary magnetic field (IMF) conditions. Changes in the poleward boundary of auroral luminosity have been analyzed in relation to substorm phase and IMF orientation. Reconnection models of flux transport into the polar cap during the substorm growth phase, and loss from the polar cap during the expansion phase, provide a framework by which these UVI observations can be analyzed. By comparison of the observations with the model predictions we can determine to what extent these models accurately predict the polar cap dynamics, and also where anomalous behavior calls for a new understanding of the dynamics beyond what these models provide. It was found that the polar cap boundary near noon and midnight usually shifted down in latitude by 1-2 degrees and 3-4 degrees respectively, increasing the area of the polar cap during the substorm growth phase as predicted. However, this growth phase phenomenon also unexpectedly occurs independently of the IMF Bz component, as shown for a substorm on January 9, 1997. The polar cap area also increased due to motion of the dawn and dusk aurora to lower latitudes, although the latitudinal shifts were asymmetric, not always concurrent, and continued well into the substorm expansion phase. The polar cap area decreased immediately following the expansion phase due to the poleward motion of the aurora on the nightside, consistent with the model prediction. What is not explained by the models is that the poleward auroral boundary in the nightside region sometimes reached very high latitudes (greater than 80 degrees MLat) greatly decreasing the polar cap area, independent of the magnitude of the substorm.

Brittnacher, M. J.↗

NASA NTRS2002

HST NICMOS Observations of the Polarization of NGC 1068

We have observed the polarized light at 2 microns in the center of NGC 1068 with HST (Hubble Space Telescope) NICMOS (Near Infrared Camera Multi Object Spectrometer) Camera 2. The nucleus is dominated by a bright, unresolved source, polarized at a level of 6.0 +/- 1.2% with a position angle of 122 degrees +/- 1.5 degrees. There are two polarized lobes extending tip to 8" northeast and southwest of the nucleus. The polarized flux in both lobes is quite clumpy, with the maximum polarization occurring in the southwest lobe at a level of 17% when smoothed to 0.23" resolution. The perpendiculars to the polarization vectors in these two lobes point back to the intense unresolved nuclear source to within one 0.076" Camera 2 pixel, thereby confirming that this source is the origin of the scattered light and therefore the probable AGN (Active Galactic Nuclei) central engine. Whereas the polarization of the nucleus is probably caused by dichroic absorption, the polarization in the lobes is almost certainly caused by scattering, with very little contribution from dichroic absorption. Features in the polarized lobes include a gap at a distance of about 1" from the nucleus toward the southwest lobe and a "knot" of emission about 5" northwest of the nucleus. Both features had been discussed by groundbased observers, but they are much better defined with the high spatial resolution of NICMOS. The northeast knot may be the side of a molecular cloud that is facing the nucleus, which cloud may be preventing the expansion of the northeast radio lobe at the head of the radio synchrotron-radiation-emitting jet. We also report the presence of two ghosts in the Camera 2 polarizers.

Simpson, Janet P.↗

NASA NTRS2003

Polar Cap Disturbances: Mesosphere and Thermosphere-Ionosphere Response to Solar-Terrestrial Interactions

The Polar Cap is the Upper-Atmosphere cum Mag-netosphere region which is enclosed by the poleward boundary of the Auroral Oval and is threaded by open geomagnetic tield lines. In this region, there is normally a steady precipition (Polar "drizzle") of low energy (w 300eV) electrons that excite optical emissions from the ionosphere. At times, enhanced ionization patches are formed near the Dayside Cusp regions that drift across the Polar Cap towards the Night Sector of the Auroral Oval. Discrete auroral arcs and auroras formed during Solar Magnetic Cloud (SMC)/Coronal Mass Ejection (CME) events are also observed in the Polar Cap. Spectrophotometric observations of all these Polar Cap phenomena provide a measure of the average energy as well a energy flux of the electrons precipitating in the Polar Cap region during these disturbances. Such measurements also point to modulations of the Polar Cap Mesosphere-Lower Thermosphere (MLT) air density and temperature by zonally symmetric tides whose Hough functions peak in the Polar region. MLT cooling during Stratospheric Warming events and their relation to Polar Vortex and associated Gravity wave activities are also observed at the Polar Cap sites.

Sivjee, G.↗

NASA NTRS2008

The Bess-Polar II Long Duration Flight Above Antarctica

The Balloon-borne Experiment with a Superconducting Spectrometer, BESS, has been developed to study elementary particle phenomena in the early universe through measurements of low energy antiprotons to investigate their origin and through a search for antihelium. The BESS collaboration carried out nine northern latitude flights between 1993 and 2002. BESS-Polar is an advanced program of the BESS collaboration to study these topics with much greater precision using long duration flights above Antarctica. The BESS-Polar spectrometer was successfully developed to accumulate much larger numbers of events during long duration flights around the South Pole. Approximately a factor of four reductions in the amount of material in the particle beam enables measurement of much lower energy antiprotons down to 100 MeV (at top of atmosphere). The first BESS-Polar flight (BESS-Polar I) of 8.5 days was carried out above Antarctica in December 2004. recording 900 million cosmic-ray events. The second BESS-Polar flight (BESS-Polar 11) was successfully carried out in the austral summer season of 2007-2008. Based on experience with BESS-Polar I, the spectrometer was improved in performance and achieved long term stability during the flight. A newly constructed magnet with a larger liquid He capacity and improved thermal insulation and an upgraded data storage system with larger capacity of hard disk drives (HDDs) enabled longer observation time. BESS-Polar II was launched on December 22, 2007 from Williams Field, McMurdo Station, in Antarctica. The spectrometer worked properly and observed cosmic rays for about 24.5 days at float altitude, recording 4.6 billion events on the HDDs until the limit of the magnet operation was reached on January 16, 2008. The flight was terminated and the spectrometer was safely landed on the West Antarctic ice sheet (1000 km from the South Pole) on January 21, 2008. Here, the BESS-Polar instrument is discussed, highlighting improvements made for BESS-Polar II, and overviews of the flight and performance are reported.

Sasaki, Makoto↗

NASA NTRS2006

Tracking the Martian CO2 Polar Ice Caps in Infrared Images

Researchers at NASA s Jet Propulsion Laboratory have developed a method for automatically tracking the polar caps on Mars as they advance and recede each year (see figure). The seasonal Mars polar caps are composed mainly of CO2 ice and are therefore cold enough to stand out clearly in infrared data collected by the Thermal Emission Imaging System (THEMIS) onboard the Mars Odyssey spacecraft. The Bimodal Image Temperature (BIT) histogram analysis algorithm analyzes raw, uncalibrated data to identify images that contain both "cold" ("polar cap") and "warm" ("not polar cap") pixels. The algorithm dynamically identifies the temperature that separates these two regions. This flexibility is critical, because in the absence of any calibration, the threshold temperature can vary significantly from image to image. Using the identified threshold, the algorithm classifies each pixel in the image as "polar cap" or "not polar cap," then identifies the image row that contains the spatial transition from "polar cap" to "not polar cap." While this method is useful for analyzing data that has already been returned by THEMIS, it has even more significance with respect to data that has not yet been collected. Instead of seeking the polar cap only in specific, targeted images, the simplicity and efficiency of this method makes it feasible for direct, onboard use. That is, THEMIS could continuously monitor its observations for any detections of the polar-cap edge, producing detections over a wide range of spatial and temporal conditions. This effort can greatly contribute to our understanding of long-term climatic change on Mars.

Wagstaff, Kiri L.↗

NASA NTRSDate not supplied

Linear Polarizer Design for Application in the Far-Ultraviolet Spectral Range

New mission concepts that are under consideration by NASA call for the design and implementation of Far Ultraviolet (FUV) polarizer technologies that have not been developed yet. A team that includes members from the NASA Goddard Space Flight Center (GSFC), Arizona State University (ASU), and Woodruff Consulting, worked on the design and development of a polarizer design that produce very high extinction ratios in the FUV spectral range (100-200 nm). This polarizer consists of reflecting light through a series of mirrors from a combination of two silicon carbide (SiC) and two lithium fluoride (LiF) crystals positioned at angles of incidence (relative to surface normal) close to the average LiF Brewster’s angle in the FUV. The output is a highly linearly polarized beam. This polarizer concept was fabricated and tested in the existing McPherson 225 Vacuum Ultraviolet (VUV) spectrometer located in the Optics Branch at NASA-GSFC. Initial testing using a MgF2 crystal at the Brewster’s angle as an analyzer has shown that this design can produce state-of-the-art extinction ratios at the Hydrogen Lyman-Alpha (Ly-α) wavelength of 121.6 nm, and that the measured extinction ratio of the two crossed polarizers, ≈114, is mostly limited by the MgF2 analyzer. A polarizer with such a performance at this wavelength has never been reported and it signifies a breakthrough in FUV polarization technology. The levels of effectiveness paired with the polarizer’s compact design allows for a new polarizer capability that would one day be implemented in a future FUV spectropolarimetry space mission.

Far Ultraviolet (FUV)↗

NASA NTRSDate not supplied

A New Linear Polarizer Design for Application in the Far-Ultraviolet Spectral Range

New mission concepts that are under consideration by NASA (such as the Polstar MIDEX mission concept) call for the design and implementation of Far Ultraviolet (FUV) polarizer technologies that have not been developed yet. A team that includes members from the NASA Goddard Space Flight Center (GSFC), Arizona State University (ASU), and Woodruff Consulting, worked on the design and development of a polarizer design that may produce extinction ratios higher than have ever been reported before in the FUV spectral range (100-200 nm). This polarizer consists of transmitting linearly polarized light through a series of reflections from a combination of two silicon carbide (SiC) and two lithium fluoride (LiF) crystals positioned at angles of incidence (relative to surface normal) close to the LiF Brewster’s angle. The performance of this polarizer concept was fabricated and tested with an existing McPherson 225 Vacuum Ultraviolet (VUV) spectrometer located in the Optics Branch at NASA-GSFC. Initial testing has shown that in the FUV spectral range, this design can produce state-of-the-art extinction ratios at the Hydrogen Lyman-Alpha (Ly-α) wavelength of 121.6 nm. A polarizer with such a performance has never been reported and it signifies a breakthrough in FUV polarization technology. The levels of effectiveness paired with the design’s compact design allows for a new polarizer capability that would one day be implemented in a future spectropolarimetry space mission. In addition, this polarizer concept could potentially be used to characterize the optical properties of new mirror coatings that will be used in a future Habitable World Observatory (HWO) mission concept as proposed by the 2020 Decadal Survey.2

Far Ultraviolet (FUV), polarimetry, instrumentatio↗

NASA NTRSDate not supplied

Linear Polarizer Design for Application in the Far-Ultraviolet Spectral Range

Far Ultraviolet (FUV), polarimetry, instrumentatio↗

NASA NTRSDate not supplied

X-ray and Multiwavelength Polarization of Mrk 501 Since 2022

We present multiwavelength polarization measurements for the luminous blazar Mrk 501 over a 14-month period. The 2 to 8 keV X-ray polarization was measured with the Imaging X-ray Polarimetry Explorer (IXPE) with six 100-ks observations spanning from March 2022 to June 2023. Each IXPE observation was accompanied with simultaneous X-ray data from NuSTAR, Swift/XRT, or XMM-Newton. Complementary optical polarization measurements were also available in the B,V,R,I, J bands as well as radio polarization measurements from 4.85 GHz to 225.5 GHz. Over the six IXPE observations, we find small to no variability in the X-ray polarization degree and angle with IXPE except for the most recent observation in June 2023 with slightly elevated polarization degree at ~3 sigma above the average of the other five observations. The optical and radio measurements show no apparent correlations to the X-ray polarization properties. This is consistent with the energy-stratified shock scenario established from the first two IXPE observations in which the polarized optical and radio emission is likely from different spatial regions than the polarized X-ray emission.

AGN↗

NASA NTRSDate not supplied

X-ray and Multiwavelength Polarization of Mrk 501 in 2022-2023

AGN↗

NASA NTRS1978

Search for X-ray polarization in the Crab pulsar

Results of a search for X-ray polarization in the Crab pulsar are presented which were obtained by observing the pulsar at 2.6 and 5.2 keV with identical X-ray polarimeters aboard OSO 8. The polarization in different portions of the X-ray light curve is analyzed, the polarization contribution due to the nebula is removed, and the results of the analysis are given for isolated portions of both the primary pulse and the interpulse at 2.6 and 5.2 keV. No evidence for X-ray polarization is found at the 99% confidence level; 3-sigma upper limits to the polarization at 2.6 keV are given along with the largest polarization allowed at 5.2 keV by the 99% confidence counter. It is noted that the trailing edge of the interpulse at 2.6 keV is 26% polarized at the 92% confidence level, while the 5.2-keV polarization measurement of 40% in the leading edge of the primary pulse is different from zero at 96% confidence. X-ray and optical polarization measurements are compared.

Silver, E. H.↗

NASA NTRS1980

Optical polarization of the Seyfert galaxies Mrk 3, Mrk 231, NGC 3227 and NGC 3516

The paper presents intermediate resolution observations of the emission line and continuum polarization of the Seyfert galaxies Mrk 3, Mrk 231, NGC 3227, and NGC 3516. The polarization shows a strong wavelength dependence with the polarization increasing smoothly into the blue for each galaxy. This wavelength dependence, together with the presence of polarized H(alpha) emission, indicates that the polarization of each galaxy is caused by an asymmetric dust envelope surrounding the nucleus. Observations of the polarization of the /O III/ lambda 5007 emission in Mrk 3 and NGC 3227, and the polarization through the nonstellar Na ID line absorption in Mrk 231 are used to place constraints on the extent of the polarizing clouds in these galaxies. No polarization variability was detected with time-bases ranging from a few weeks to three years.

Thompson, I.↗

NASA NTRS1982

Optical polarization of the Seyfert galaxies IC 4329A and MRK 376

Measurements of the optical polarizations of the two highly polarized Seyfert 1 galaxies IC 4329A and Mrk 376 are presented. Continuum and line polarization of the two objects were observed with the Steward Observatory 2.25-m telescope using a two-channel photoelectric Pockels cell polarimeter, a single-channel scanner, and a digicon attached to a flint prism spectrograph. Results indicate that, for both galaxies, the emission line polarization and underlying continuum polarization are identical, rising toward short wavelengths, and therefore must be explained by a common mechanism. Such a mechanism is suggested to involve polarization produced by aligned grains in the galactic disk. A model for polarization in IC 4329A by this mechanism predicts a grain size three times smaller than Galactic polarizing grains, as well as a visual extinction of about 2 magnitudes, a gas to dust mass ratio close to 100 and a polarization to extinction ratio comparable to the Galactic ratio.

Martin, P. G.↗

NASA NTRS1989

Evolution of the sun's polar fields during sunspot cycle 21 - Poleward surges and long-term behavior

Longitudinally averaged observations of the photospheric field during 1976-1986 are analyzed using a flux transport model. The way in which source eruptions, supergranular diffusion, and meridional flow collaborate to produce strong, highly concentrated polar fields near sunspot minimum is clarified as follows: (1) widespread eruptions of individual bipolar magnetic regions, with their leading polarity flux equatorward of their trailing polarity flux, collectively establish a large-scale separation of polarities in latitude; (2) the low-latitude, leading polarity flux diffuses across the equator and merges with its opposite hemisphere counterpart; and (3) meridional flow carries the resulting surplus of trailing polarity flux to the poles, and concentrates it there against the spreading effect of diffusion. Episodic 'surges' of flux to the poles are induced by fluctuations in the source eruption rate. Simulations indicate that relatively weak, trailing polarity surges may occur even in a steady flow field. However, in order to account for the giant surges of alternating polarity and the resulting oscillations in the polar field strength observed during 1980-1982, both accelerated flow and enhanced eruption rates are required.

Wang, Y.-M.↗

NASA NTRS1989

The wavelength dependence of polarization in NGC 2023

NGC 2023 is a bright reflection nebula illuminated by the central star HD37903. At 2 microns the nebula is seen solely by reflected light from the central star but in the NIR there is excess radiation that is supposed to arise from thermal emission from a population of small grains (Sellgren, 1984). The unexpectedly high surface brightness at R and I wavelengths has led to the suggestion that even at these wavelengths there is a significant contribution from this thermal emission process (Witt, Schild, and Kraiman, 1984). If the nebula is seen by reflected starlight then this radiation will be linearly polarized. The level of polarization depends on the scattering geometry, grain size distribution, etc., and is typically 20 to 40 percent for nebulae such as NGC 1999 which is morphologically similar to NGC 2023. If, in any waveband, there is a contribution of radiation from emission processes this radiation will be unpolarized and will serve to dilute the scattered radiation to give a lower level of observed polarization. A study of the wavelength dependence of polarization in nebulae in which there may be thermal emission from grains will indicate the contribution from this process to the total luminosity. Polarization maps were produced in BVRI wavebands for the NGC 2023 nebulosity which confirm that at all wavelengths it is a reflection nebula illuminated by a central star. The wavelength dependence of polarization at representative points in the nebula and in a scatter plot of polarization in V and I wavebands at all points at which measurements are given. Results indicate that throughout the nebula there is a general trend for the level of polarization to increase with wavelength and that maximum levels of polarization occur at the longest wavelengths. No evidence is seen in the data for any significant contribution from the thermal emission from grains in the BVRI luminosity of NGC 2023.

Rolph, C. D.↗

NASA NTRS1990

Polarization considerations for optical systems II; Proceedings of the Meeting, San Diego, CA, Aug. 9-11, 1989

Various papers on polarization considerations for optical systems are presented. Individual topics addressed include: use of polarization methods in earth resources investigation, measurements of polarization scattering in the vacuum UV, the Space Shuttle as a polarization observation platform, analysis of spatial pseudodepolarizers in imaging systems, analysis of polarization effects in optical systems, and a low-polarization solar vector magnetograph. Consideration is given to polarization effects on astronomical spatial interferometry, measurement of the instrumental polarization of a high-resolution UV spectrometer, and the use of KD(asterisk)P modulators for polarization measurements of the sun.

Chipman, Russell A.↗

NASA NTRS1990

Polarization Aberrations

The analysis of the polarization characteristics displayed by optical systems can be divided into two categories: geometrical and physical. Geometrical analysis calculates the change in polarization of a wavefront between pupils in an optical instrument. Physical analysis propagates the polarized fields wherever the geometrical analysis is not valid, i.e., near the edges of stops, near images, in anisotropic media, etc. Polarization aberration theory provides a starting point for geometrical design and facilitates subsequent optimization. The polarization aberrations described arise from differences in the transmitted (or reflected) amplitudes and phases at interfaces. The polarization aberration matrix (PAM) is calculated for isotropic rotationally symmetric systems through fourth order and includes the interface phase, amplitude, linear diattenuation, and linear retardance aberrations. The exponential form of Jones matrices used are discussed. The PAM in Jones matrix is introduced. The exact calculation of polarization aberrations through polarization ray tracing is described. The report is divided into three sections: I. Rotationally Symmetric Optical Systems; II. Tilted and Decentered Optical Systems; and Polarization Analysis of LIDARs.

Mcguire, James P., Jr.↗