Search NASA⌕ Search

SEARCH · Search NASA

Results for “shock polar”

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.

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

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Whistler wave bursts upstream of the Uranian bow shock

Observations of magnetic field wave bursts upstream of the Uranian bow shock are reported which were recorded prior to the inbound shock crossing. Three wave types are identified. One exhibits a broad spectral enhancement from a few millihertz to about 50 mHz and is seen from 17 to 10 hr prior to the inbound shock crossing. It is argued that these waves are whistler waves that have propagated upstream from the shock. A second wave type has a spacecraft frame frequency between 20 and 40 mHz, is seen only within or immediately upstream of the shock pedestal, is right-hand polarized in the spacecraft frame, and has a typical burst duration of 90 s. The third wave type has a spacecraft frame frequency of about 0.15 Hz, is seen exclusively within the shock pedestal, is left-hand polarized in the spacecraft frame, and has a burst duration lasting up to 4 min. It is argued that the low-frequency bursts are whistler waves with phase speed comparable to, but in excess of, the solar wind speed.

Smith, Charles W.↗
NASA NTRS1984

The structure of oblique subcritical bow shocks - ISEE 1 and 2 observations

ISEE 1 and 2 spacecraft magnetic field data are used to determine the scale lengths of various elements of shock structure, as well as wavelengths and wave polarizations, in a study of structural elements which include shock ramps and precursor wave trains over a series of oblique, low Mach number terrestrial bow shocks. Dissipative processes are reflected in the damping of the precursors, and dissipative scale lengths are of 200-800 km, or several times greater than shock thicknesses. The source of dissipation in the shocks does not appear to be the wave-wave decay of the whistlers, for which no evidence is found. The interaction of the whistler itself with upstream electrons is suggested as a simple and self-consistent explanation for the observed wave train damping.

Mellott, M. M.↗
NASA NTRS1993

Magnetospheric response to solar wind dynamic pressure variations: Interaction of interplanetary tangential discontinuities with the bow shock

Some magnetic impulse events observed in the polar region are related to vortices associated with plasma convection in the ionosphere. Recent analyses of satellite and ground data suggest that the interaction of solar wind dynamic pressure pulses and the magnetosphere may lead to the formation of velocity vortices in the magnetopause boundary layer region. This can in turn lead to the presence of vortices in the polar ionosphere. However, before reaching the Earth's magnetopause, these interplanetary pressure pulses must interact with and pass through the bow shock. A variation of the solar wind dynamic pressure may be associated with shocks, magnetic holes, or tangential discontinuities (TDs) in the interplanetary medium. We study the interaction of interplanetary TDs with the Earth's bow shock (BS) using both theoretical analysis and MHD computer simulations. It is found that as a result of the collision between a TD and the BS, the jump in the solar wind dynamic pressure associated with the TD is significantly modified, the bow shock moves, and a new fast shock or fast rarefaction wave, which propagates in the downstream direction, is excited. Our theoretical analysis shows that the change in the plasma density across the interplanetary TD plays the most important role in the collision process. In the case with an enhanced dynamic pressure behind the interplanetary TD, the bow shock is intensified in strength and moves in the earthward direction. The dynamic pressure jump associated with the transmitted TD is generally reduced from the value before the interaction. A fast compressional shock is excited ahead of the transmitted TD and propagates toward the Earth's magnetosphere. For the case in which the dynamic pressure is reduced behind the interplanetary TD, the pressure jump across the transmitted TD is substantially weakened, the bow shock moves in the sunward direction, and a rarefaction wave which propagates downstream is excited. We also simulate and discuss the interaction of a pair of tangential discontinuities, which may correspond to a magnetic hole, with the BS.

Wu, Bor-Han↗
NASA NTRS2000

Can Pseudobreakups be Predicted?

We statistically analyze many interplanetary shock events where there are both WIND interplanetary data and POLAR UV imaging data. We find that the shock compression events trigger pseudobreakups (PBs), substorm onsets, or enhance (pre-existing) substorm intensities. Using WIND interplanetary data and ground-based data, it is now believed that we can predict when PBs will occur and when substorm expansion phases will occur.

Zhou, X.-Y.↗
NASA NTRS1972

Study of waves in the earth's bow shock.

Results of an examination of the perturbation vectors of waves upstream and downstream from the region of maximum compression in the bow shock on Ogo 5 under particularly steady solar-wind conditions. The polarization of the upstream waves was right-hand circular, and that of the downstream waves left-hand elliptical in the spacecraft frame. By observing that the polarization of the waves remained unchanged as the shock motion swept the wave structure back and forth across the satellite three times in eight minutes, it was found that the waves were not stationary in the shock frame. A study of the methods of determining the shock normal indicates that the normal estimated from a shock model should be superior to the normal based on magnetic coplanarity. The propagation vectors of the waves examined did not coincide with the shock-model normal, the average magnetic field, or the plasma-flow velocity. However, the major axis of the polarization ellipse of the downstream wave was nearly parallel to the upstream propagation vector.

Holzer, R. E.↗
NASA NTRS1971

A study of waves in the earth's bow shock

The perturbation vectors of waves up and downstream from the region of maximum compression in the bow shock were examined on OGO-5 under particularly steady solar wind conditions. The polarization of the upstream waves was RH, circular and of the downstream waves LH, elliptical in the spacecraft frame. By observing that the polarization of the waves remained unchanged as the shock motion swept the wave structure back and forth across the satellite three times in eight minutes, it was found that the waves were not stationary in the shock frame. A study of the methods of determining the shock normal indicates that the normal estimated from a shock model should be superior to one based upon magnetic coplanarity. The propagation vectors of the waves examined did not coincide with the shock model normal, the average magnetic field, or the plasma flow velocity. However, the major axis of the polarization ellipse of the downstream wave was nearly parallel to the upstream propagation vector.

Holzer, R. E.↗
NASA NTRS1986

On a difficulty with accelerating particles at slow-mode shocks

The available information on slow-mode shocks is briefly reviewed in order to determine whether these shocks are capable of accelerating particles to high energies, thus providing a possible mechanism for energetic particle production at reconnection sites. It is noted that in contrast to fast shocks, the evolutionary conditions at a steady slow shock require the upstream plasma to flow into the shock with a normal component slower than the intermediate speed. Consequently, the right-polarized waves will escape from a slow shock, and backstreaming particles scattered by these waves will not be convected back to the shock. There will be no effective compression, and further acceleration will not occur.

Isenberg, P. A.↗
NASA NTRS1987

The electromagnetic ion cyclotron beam anisotropy instability

Electromagnetic instabilities driven by an anisotropic, relatively cool ion beam are studied for the case in which both the beam and the instabilities propagate parallel or antiparallel to a uniform magnetic field. At modest beam-core relative drift speeds, sufficiently large perpendicular-to-parallel beam temperature ratios and sufficiently large plasma beta, the mode of fastest growth rate is the ion cyclotron beam anisotropy instability. Because the right-hand polarized waves observed upstream of slow shocks in the earth's magnetotail can lead to the appropriate beam anisotropy, the ion cyclotron instability may be present and account for the left-hand polarized magnetic waves observed there. Also, because of its relatively low phase speed, the ion cyclotron beam anisotropy instability may provide the scattering necessary for ion Fermi acceleration at slow shocks of sufficiently high plasma beta.

Peter Gary, S.↗
NASA NTRS2000

Interplanetary Shock Triggering of Substorms

We use WIND solar wind data a POLAR UV imaging data to study magnetospheric responses and substorm triggering mechanisms during and after interplanetary (IP) shock events.

wind solar data Polar UV substorms↗
NASA NTRS1991

Evidence for particle acceleration in AE Aqr from radio and X-ray observations

The novalike cataclysmic variable AE Aquarii shows evidence for radio synchrotron emission from expanding magnetized clouds. The radio and UV flare timescales are similar indicating that the central engine is located close to the white dwarf. A collisionless shock may develop above the polar cap, which may explain the observed temperature. Protons and ions accelerated up to energies of more than 1 TeV can diffuse away from the central engine, enter magnetized clouds, and generate waves which in turn heat and accelerate electrons. An embarrassing situation exists where reprocessed optical and X-ray pulsations (33 s) are observed, but the expected direct X-ray pulse from the white dwarf is absent. The standard theory for CVs cannot explain this, and one possible explanation is that a beam of relativistic protons and ions from the polar cap may give the observed reprocessed optical to X-rays in a thick target fixed in the system.

De Jager, O. C.↗
NASA NTRS1995

Ultra low frequency waves at the Earth's bow shock

The Earth's bow shock is a bountiful generator of waves. Some of these waves have group velocities that exceed the solar wind velocity directed into the shock and can propagate upstream against the flow. Upstream whistlers observed close to one Hertz in the spacecraft frame have been seen many Earth radii upstream. A second whistler mode wave, called the precursor, propagates upstream along the shock normal but is phase standing in the solar wind flow. The damping of both whistler mode waves is consistent with Landau damping. At low Mach numbers the precursor is connected to the non-coplanarity component in the shock ramp. At higher Mach numbers the upstream waves cannot propagate upstream and ion reflection becomes more important in providing free energy for wave particle interactions. The non-coplanarity component is still present but it now initiates a downstream wave train. Generally the waves just downstream from the bow shock are left hand circularly polarized ion cyclotron waves propagating along the magnetic field at the Alfven velocity. When the upstream Mach number is high and the helium content of the plasma is high, mirror mode waves are observed.

Russell, C. T.↗
NASA NTRS1995

Nonlinear Waves in Magnetized Accretion Disks

Time-dependent accretion for a magnetized disk can exhibit short timescale outbursts of high-power jets associated with bursts of disk radiation. The burst of energy in jets can produce outward propagating disturbances in preexisting (+/- z) jet flows. the disturbances are strong and can be expected to steepen and form strong shock waves. The strong linear polarization (up to approximately 30%) and large polarization position angle changes (90 deg and more) seen in the optical outbursts of some quasars clearly indicate synchrotron radiation, and this component of the radiation is probably associated with the burst in jet energy in the present model.

Lovelace, R. V. E.↗
NASA NTRS2022

The X-Ray Polarization View of Mrk 421 in an Average Flux State as Observed By the Imaging X-Ray Polarimetry Explorer

Particle acceleration mechanisms in supermassive black hole jets, such as shock acceleration, magnetic reconnection, and turbulence, are expected to have observable signatures in the multiwavelength polarization properties of blazars. The recent launch of the Imaging X-Ray Polarimetry Explorer (IXPE) enables us, for the first time, to use polarization in the X-ray band (2–8 keV) to probe the properties of the jet synchrotron emission in high-synchrotron-peaked BL Lac objects (HSPs). We report the discovery of X-ray linear polarization (degree Π x = 15% ± 2% and electric vector position angle ψ x = 35° ± 4°) from the jet of the HSP Mrk 421 in an average X-ray flux state. At the same time, the degree of polarization at optical, infrared, and millimeter wavelengths was found to be lower by at least a factor of 3. During the IXPE pointing, the X-ray flux of the source increased by a factor of 2.2, while the polarization behavior was consistent with no variability. The higher level of Π x compared to longer wavelengths, and the absence of significant polarization variability, suggest a shock is the most likely X-ray emission site in the jet of Mrk 421 during the observation. The multiwavelength polarization properties are consistent with an energy-stratified electron population, where the particles emitting at longer wavelengths are located farther from the acceleration site, where they experience a more disordered magnetic field.

acceleration of particles↗
NASA NTRS2011

The Strength and Radial Profile of the Coronal Magnetic Field from the Standoff Distance of a Coronal Mass Ejection-Driven Shock

We determine the coronal magnetic field strength in the heliocentric distance range 6-23 solar radii (Rs) by measuring the shock standoff distance and the radius of curvature of the flux rope during the 2008 March 25 coronal mass ejection imaged by white-light coronagraphs. Assuming the adiabatic index, we determine the Alfven Mach number, and hence the Alfven speed in the ambient medium using the measured shock speed. By measuring the upstream plasma density using polarization brightness images, we finally get the magnetic field strength upstream of the shock. The estimated magnetic field decreases from approximately 48 mG around 6 Rs to 8 mG at 23 Rs. The radial profile of the magnetic field can be described by a power law in agreement with other estimates at similar heliocentric distances.

heliosphere↗
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

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↗