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At least 775 records · Page 43
All sourcesNASA NTRSDOE OSTI
NASA NTRS2011

Relation Between the 3D-Geometry of the Coronal Wave and Associated CME During the 26 April 2008 Event

We study the kinematical characteristics and 3D geometry of a large-scale coronal wave that occurred in association with the 26 April 2008 flare-CME event. The wave was observed with the EUVI instruments aboard both STEREO spacecraft (STEREO-A and STEREO-B) with a mean speed of approx 240 km/s. The wave is more pronounced in the eastern propagation direction, and is thus, better observable in STEREO-B images. From STEREO-B observations we derive two separate initiation centers for the wave, and their locations fit with the coronal dimming regions. Assuming a simple geometry of the wave we reconstruct its 3D nature from combined STEREO-A and STEREO-B observations. We find that the wave structure is asymmetric with an inclination toward East. The associated CME has a deprojected speed of approx 750 +/- 50 km/s, and it shows a non-radial outward motion toward the East with respect to the underlying source region location. Applying the forward fitting model developed by Thernisien, Howard, and Vourlidas we derive the CME flux rope position on the solar surface to be close to the dimming regions. We conclude that the expanding flanks of the CME most likely drive and shape the coronal wave.

CMEs↗
NASA NTRS2018

Effect of Gravity Waves from Small Islands in the Southern Ocean on the Southern Hemisphere Atmospheric Circulation

The effect of small islands in the Southern Ocean on the atmospheric circulation in the Southern Hemisphere is considered with a series of simulations using the NASA Goddard Earth Observing System Chemistry-Climate Model in which the gravity wave stress generated by these islands is increased to resemble observed values. The enhanced gravity wave drag leads to a 2 K warming of the springtime polar stratosphere, partially ameliorating biases in this region. Resolved wave drag declines in the stratospheric region in which the added orographic gravity waves deposit their momentum, such that changes in gravity waves are partially compensated by changes in resolved waves, though resolved wave drag increases further poleward. The orographic drag from these islands has impacts for surface climate, as biases in tropospheric jet position are also partially ameliorated. These results suggest that these small islands are likely contributing to the missing drag near 60 degrees S in the upper stratosphere evident in many data assimilation products.

chemistry-climate models↗
NASA NTRS1971

Air-coupled seismic waves at long range from Apollo launchings.

Microphones and seismographs were co-located in arrays on Skidaway Island, Georgia, for the launchings of Apollo 13 and 14, 374 km to the south. Simultaneous acoustic and seismic waves were recorded for both events at times appropriate to the arrival of the acoustic waves from the source. The acoustic signal is relatively broadband compared to the nearly monochromatic seismic signal; the seismic signal is much more continuous than the more pulse-like acoustic signal; ground loading from the pressure variations of the acoustic waves is shown to be too small to account for the seismic waves; and the measured phase velocities of both acoustic and seismic waves across the local instrument arrays differ by less than 6 per cent and possibly 3 per cent if experimental error is included. It is concluded that the seismic waves are generated by resonant coupling to the acoustic waves along some 10 km of path on Skidaway Island.

Donn, W. L.↗
NASA NTRS1972

Excitation of parametric instabilities by radio waves in the ionosphere.

The excitation of parametric instabilities by radio waves in a magnetoplasma is discussed. A uniform medium is assumed and linear approximations are used. Excitation by a pump wave of ordinary polarization is hardly affected by the magnetic field. Low or zero frequency ion waves and high frequency Langmuir waves are excited simultaneously. For an extraordinary pump wave, the excited high frequency electrostatic waves are in the Bernstein mode. The threshold is slightly higher and excitation can occur only within certain 'allowed' frequency bands. A new type of parametric instability in which the excited waves are electromagnetic in nature and which is more strongly affected by the inhomogeneous nature of the medium is discussed qualitatively.

Fejer, J. A.↗
NASA NTRS1973

Transverse low frequency wave in a two fluid solar wind

Investigation is made of the properties of low frequency transverse waves in a two-fluid solar wind having a radial magnetic field and radial streaming velocity. In order to examine what effects this streaming medium has on the waves, linearly polarized waves are decomposed into left and right circularly polarized waves. Computation is made of analytic expressions valid to first order for the radial amplitude and phase dependence of these constituent waves. It is shown that after travelling a given distance r, these waves have different amplitudes and phases. The former result causes their superposition to become elliptical rather than linear. The latter causes the axis of the ellipse of polarization to rotate through a well-defined angle. Analytic expressions are obtained for the eccentricity of the ellipse and for the angle of rotation. In analogy with regular Faraday rotation, in which the plane of polarization of a linear polarized wave rotates, the effect is denoted as generalized Faraday rotation.

Solodyna, G. V.↗
NASA NTRS1973

Whistler waves observed upstream from collisionless shocks

Waves in the frequency range 0.5 - 4 Hz were studied in the region upstream of the earth's bow shock using data from the fluxgate magnetic field experiment on IMP-6. Analysis of 150 examples of these waves during a three month interval indicates that amplitudes are generally less than 1 or 2 gammas and propagation directions generally make angles of between 20 and 40 degrees with the field direction. The waves as measured in the spacecraft frame of reference are either left or right hand polarized with respect to the average field direction. It is concluded that the observed waves are right handed waves in the plasma frame of reference with wavelengths of approximately 100 km propagating upstream in the whistler mode. Doppler shifting reduces the observed frequencies in the spacecraft frame and reverses the observed polarization for those waves propagating more directly upstream. Similar waves are seen ahead of most interplanetary shocks.

Fairfield, D. H.↗
NASA NTRS1974

Propagation of waves along an impedance boundary

A theoretical analysis of the scalar wave field due to a point source above a plane impedance boundary is presented. A surface wave is found to be an essential component of the total wave field. It is shown that, as a result of ducting of energy by the surface wave, the amplitude of the total wave near the boundary can be greater than it would be if the boundary were perfectly reflecting. Asymptotic results, valid near the boundary, are obtained both for the case of finite impedance (the soft-boundary case) and for the limiting case in which the impedance becomes infinite (the hard-boundary case). In the latter, the wave amplitude in the farfield decreases essentially inversely as the horizontal propagation distance; in the former (if the surface-wave term is neglected), it decreases inversely as the square of the horizontal propagation distance.

Wenzel, A. R.↗
NASA NTRS1975

Martian atmospheric lee waves

Mariner 9 television pictures of Mars extensive mountain lee wave phenomenon in the northern mid-latitudes during winter were evaluated. The characteristic wave length of the lee waves is readily observable, and in a few cases the boundaries of the wave patterns, as well as the wave length, are observed. The cloud patterns resulting from the waves generated by the flow across a mountain or crater are shown to be dependent upon the velocity profile of the air stream and the vertical stability of the atmosphere. Using the stability as inferred by the temperature structure obtained from the infrared spectrometer data, a two layer velocity model of the air stream is used in calculations based on the theory of mountain lee waves. Results yield magnitudes generally in agreement with various other circulation models.

Pirraglia, J. A.↗
NASA NTRS1975

Bow shock and magnetosheath waves at Mercury

Mariner 10 measurements at the Mercury bow shock provide examples where the magnetic field is approximately parallel or perpendicular to the bow shock normal. Upstream of a broad irregular parallel shock, left hand circularly polarized waves are observed which cut off very sharply at approximately 4 Hz. Upstream of a perpendicular shock, right hand circularly polarized waves are observed which persist up to the Nyquist frequency of 12 Ha. Determination of the wave propagation vector as a function of frequency helps conclusively identify the waves as whistler mode waves propagating from the shock. The magnetosheath downstream of the parallel shock is disturbed more than that downstream of the perpendicular shock particularly below 1 Hz. In the latter case regular left hand polarized waves observed slightly above the proton gyrofrequency are identified as ion cyclotron waves with wavelength approximately 300 km which are Doppler shifted up to their observed frequency.

Fairfield, D. H.↗
NASA NTRS1975

Neutral composition gravity waves in the thermosphere observed by ESRO 4

Neutral composition waves with a wave length of about 5000 km and a wave period of about 2.5 hours were observed by ESRO 4 in the altitude region of 250 km. The amplitudes are of the order of 25 percent for Ar, 15 percent for N2, and roughly 10 percent for He and O at 25 percent geographic latitude. The wave amplitudes are seen to decrease towards lower latitudes suggesting that the composition waves are launched by auroral sources. Simultaneous ground based ionosonde measurements of the F2 layer critical frequency at mid and low latitudes show a wave period consistent with the satellite observations. The theoretical investigation with a multi-component model shows that diffusion plays a major role in explaining these wave phenomena. The phase and amplitude relation between atmospheric constituents are sensitive to the altitude region in which energy is deposited thus suggesting that Joule heating or soft particle precipitation are the predominant energy sources.

Trinks, H.↗
NASA NTRS1975

Alteration of the ionosphere by man-made waves

In recent years powerful radio wave transmissions beamed at the ionosphere at frequencies somewhat below the penetration frequency of the F2 layer produced the following, mostly unexpected, spectacular effects: (1) artificial spread F seen on ionograms and implying the presence of large scale field-aligned irregularities in the ionospheric plasma density; (2) very strong additional absorption of probing waves reflected by the F2 layer; (3) field-aligned 'on frequency' scattering of UHF waves; (4) scattering of UHF waves by Langmuir waves which are believed to be parametrically excited; (5) the 630 nm airglow is artificially enhanced by a modifying wave of ordinary polarization but it is reduced in intensity by a modifying wave of extraordinary polarization; (6) the shape of the F2 layer is modified. The results of the observations are described and their tentative interpretation in terms of different parametric instabilities is outlined.

Fejer, J. A.↗
NASA NTRS1976

Tropical waves and the quasi-biennial oscillation in the lower stratosphere

By means of spectrum analysis of 11 years of lower stratospheric daily winds and temperatures at Balboa, Ascension and Canton-Singapore, evidence is presented supporting the existence of two principal wave modes with periods of about 11-17 days (Kelvin waves) and about 4-5 days (mixed Rossby-gravity waves). The structure of the two wave modes, as well as the vertical eddy momentum flux by the waves, is shown to be related to the quasi-biennial cycle, although for the mixed Rossby-gravity waves this is obvious only at Ascension. In addition, the Coriolis term, suggested as a source of vertical easterly momentum flux for the mixed Rossby-gravity waves, is investigated and found to be of the same magnitude as the vertical eddy flux term. Finally, we have examined the mean meridional motion and the meridional eddy momentum flux for its possible association with the quasi- biennial variation.

Miller, A. J.↗
NASA NTRS1976

Martian atmospheric lee waves

Areas of extensive mountain lee wave phenomenon in the northern midlatitudes during late winter are observed in Mariner 9 television pictures of Mars. The lee wave pattern is due to the discrete spectrum of waves generated by a crater ridge in an airstream. The cloud patterns resulting from the waves generated by the flow across a mountain or crater are dependent on the velocity profile of the airstream and the vertical stability of the atmosphere. A two-layer velocity model of the airstream is used in calculations based on the theory of mountain lee waves. Regardless of the simple model used for the airstream, the calculations appear to be capable of reproducing the observed wave patterns. The many observations of lee waves suggest that the near surface wind speeds as implied by the lower stream velocities do not greatly exceed, and most of the time are less than, 40 m/sec in the northern midlatitudes during late winter except possibly on a very localized scale. Results are in agreement with circulation models, especially the Leovy and Mintz (1969) two-layer numerical model.

Pirraglia, J. A.↗
NASA NTRS1976

Cloud patterns, waves and convection in the Venus atmosphere

Detailed descriptions and interpretations are provided for phenomena seen in the UV markings on Venus during the Mariner 10 encounter with that planet. The phenomena include the dark horizontal Y, circumequatorial belts, bowlike waves, the subsolar disturbance, midlatitude spiral streaks, the polar ring, and the polar region. Interpreting some of these phenomena in terms of physical processes that are familiar in earth's atmosphere, it is proposed that the large-scale brightness distribution can be most simply described in terms of a pattern with zonal wavenumber of unity which extends between about + and - 50 deg latitude and which progresses around the planet in about 4.2 days. The large-scale UV markings are interpreted as a wave phenomenon, and it is shown how the observed Y pattern could be produced by the superposition of a Rossby-Haurwitz wave dominant at middle latitudes and a Kelvin wave dominant near the equator. It is suggested that the bowlike waves may be true bow waves formed by the interaction of the rapid supercritical zonal flow with internal gravity waves of lower horizontal phase speeds generated by the subsolar disturbance.

Belton, M. J. S.↗
NASA NTRS1976

Bow shock and magnetosheath waves at Mercury

Mariner 10 measurements at the Mercury bow shock provide examples where the magnetic field is approximately parallel or perpendicular to the bow shock normal. Upstream of a broad irregular parallel shock, left-hand circularly polarized waves are observed which cut off very sharply at about 4 Hz. Upstream of a perpendicular shock, righ-hand circularly polarized waves are observed which persist up to the Nyquist frequency of 12 Hz. Determination of the wave propagation vector as a function of frequency helps to identify conclusively the waves as whistler mode waves propagating from the shock. The magnetosheath downstream of the parallel shock is disturbed more than that downstream of the perpendicular shock, particularly below 1 Hz. In the latter case, regular left-hand-polarized waves observed slightly above the proton gyrofrequency are identified as ion cyclotron waves with wavelengths of about 300 km which have been Doppler shifted up to their observed frequency.

Fairfield, D. H.↗
NASA NTRS1977

On the existence of finite amplitude, transverse Alfven waves in the interplanetary magnetic field

Interplanetary magnetic field data from the Mariner 10 spacecraft were examined for evidence of small and finite amplitude transverse Alfven waves, general finite amplitude Alfven waves, and magnetosonic waves. No evidence for transverse Alfven waves was found. Instead, the field fluctuations were found to be dominated by the general finite amplitude Alfven wave. Such wave modes correspond to non-plane-wave solutions of the nonlinear magnetohydrodynamic equations.

Sari, J. W.↗
NASA NTRS1977

On two-dimensional packets of capillary-gravity waves

The motion of a two-dimensional packet of capillary-gravity waves on water of finite depth is studied. The evolution of a packet is described by two partial differential equations: the nonlinear Schroedinger equation with a forcing term and a linear equation, which is of either elliptic or hyperbolic type depending on whether the group velocity of the capillary-gravity wave is less than or greater than the velocity of long gravity waves. These equations are used to examine the stability of the Stokes capillary-gravity wave train. The analysis reveals the existence of a resonant interaction between a capillary-gravity wave and a long gravity wave. The interaction requires that the liquid depth be small in comparison with the wavelength of the (long) gravity waves and the evolution equations describing the dynamics of this interaction are derived.

Djordjevic, V. D.↗
NASA NTRS1977

Kelvin-Helmholtz waves in the ocean

Large amplitude short waves confined near the crests of a swell have been observed when a stiff breeze was blowing against the swell. This would seem to imply the existence of both a wavelength-selective generating mechanism and a trapping mechanism, neither of which is to be expected of surface gravity waves of the observed length. It is suggested that there are significant changes in the dynamics of such waves if allowance is made for the dynamic coupling between wind and waves. For a Kelvin-Helmholtz model it is shown that energy transfer rates from the turbulent pressure fluctuations are greatly enhanced for subcritical conditions by the inclusion of the dynamic coupling. The group velocity of subcritical waves is profoundly affected, becoming infinite at the stability boundary. Thus subcritical waves could be trapped on a swell. An examination of the effects of wind shear suggest that Kelvin-Helmholtz type instability could still be present, although for stronger winds, particularly for rather longer waves.

Mahony, J. J.↗