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At least 199 records · Page 11
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NASA NTRS2020

The Formation of Electron Heat Flux in the Region of Diffuse Aurora

Whistler and electrostatic electron cyclotron harmonics waves are responsible for scattering and precipitating the energetic plasma sheet electrons that drive the diffuse aurora. These primary electrons with energies in the kiloelectron volt range, simultaneously precipitating in magnetically conjugate regions, produce the secondary electron population and can be reflected by the atmosphere back through the magnetosphere and precipitate into the conjugate region with additional follow‐up atmospheric backscatter. Primary, degraded, and secondary electrons can be trapped back into the magnetosphere as they travel back and forth between the two magnetically conjugate ionospheres and continuously delivering their energy to the cold plasma sheet electrons and form the electron thermal fluxes that deposit this energy at the upper ionospheric altitudes. We consider the formation of these heat fluxes focusing on the magnetosphere‐ionosphere energy interplay of the entire superthermal electron spectra from 1 eV up to 10 keV and discuss the efficiency of the different spectral energy intervals that contribute to the electron plasma heating at the magnetospheric altitudes. Our parametric studies at L = 6.8, with lower and upper band chorus whistler wave amplitudes of 10 pT and electron cyclotron harmonic wave amplitudes of 1 mVm−1, indicate the dominant role of the whistler mode in the formation of the electron heat flux coming from the magnetosphere to the ionosphere.

George V. Khazanov↗
NASA NTRS1979

Fiber Ring Optical Gyroscope (FROG)

The design, construction, and testing of a one meter diameter fiber ring optical gyro, using 1.57 kilometers of single mode fiber, are described. The various noise components: electronic, thermal, mechanical, and optical, were evaluated. Both dc and ac methods were used. An attempt was made to measure the Earth rotation rate; however, the results were questionable because of the optical and electronic noise present. It was concluded that fiber ring optical gyroscopes using all discrete components have many serious problems that can only be overcome by discarding the discrete approach and adapting an all integrated optic technique that has the laser source, modulator, detector, beamsplitters, and bias element on a single chip.

Source record↗
NASA NTRS1987

On the heating of electrons in the tail of Giacobini-Zinner

Computations for the temperature and density of electrons for Comet Giacobini-Zinner are presented which differ from those of Marconi and Mendis (1986) in the use of recently calculated cooling rates, and in the treating of photoelectrons separately from the thermalized electrons with respect to their collisions with the neutrals. The present temperatures are found to be high relative to the ICE observations, though the calculated densities are consistent with these observations.

Marconi, M. L.↗
NASA NTRS1994

Localized injection of large-amplitude Pc 1 waves and electron temperature enhancement near the plasmapause observed by DE2 in the upper ionosphere

The relation between electron temperature enhancement and large amplitude Pc 1 wave injections in the upper ionosphere is investigated using the data obtained by the Dynamics Explorer 2 spacecraft. Results can be summarized as follows: (1) The region of the temperature enhancement coincides with that of the wave injection which is latitudinally very narrow (less than 100 km) in comparison with the wavelength along the ambient magnetic field (several hundred kilometers). (2) The duration of the wave injection (or the temperature enhancement) seems to be less than a few hours even under quiet geomagnetic conditions, and/or the injection seems to be very localized, not only latitudinally, but also longitudinally. (3) The appearance and the magnitude of temperature enhancement depend on both the wave amplitude and the satellite altitude. (4) Two of the 22 events that were analyzed show a clear enhancement of low-energy electron flux (5 to 30 eV) at the wave injection, and the flux is field-aligned both downward and upward. The region of the temperature enhancement coincides with that of the downward electron flux. From these results, it is suggested that the temperature enhancement which accompanies large-amplitude waves with Pc 1 pulsation frequencies (0.2 to 5 Hz) is caused by the direct acceleration of thermal electrons at low altitudes by the parallel electric field (0.01 to 0.001 mV/m) of the ion-cyclotron waves (kinetic Alfven waves) having an oblique wave normal.

Iyemori, T.↗
NASA NTRS1994

Millimeter, microwave, hard X-ray, and soft X-ray observations of energetic electron populations in solar flares

We present comparisons of multiwavelength data for a number of solar flares observed during the major campaign of 1991 June. The different wavelengths are diagnostics of energetic electrons in different energy ranges: soft X-rays are produced by electrons with energies typically below 10 keV, hard X-rays by electrons with energies in the range 10-200 keV, microwaves by electrons in the range 100 keV-1 MeV, and millimeter-wavelength emission by electrons with energies of 0.5 MeV and above. The flares in the 1991 June active period were remarkable in two ways: all have very high turnover frequencies in their microwave spectra, and very soft hard X-ray spectra. The sensitivity of the microwave and millimeter data permit us to study the more energetic (greater than 0.3 MeV) electrons even in small flares, where their high-energy bremsstrahlung is too weak for present detectors. The millimeter data show delays in the onset of emission with respect to the emissions associated with lower energy electrons and differences in time profiles, energy spectral indices incompatible with those implied by the hard X-ray data, and a range of variability of the peak flux in the impulsive phase when compared with the peak hard X-ray flux which is two orders of magnitude larger than the corresponding variability in the peak microwave flux. All these results suggest that the hard X-ray-emitting electrons and those at higher energies which produce millimeter emission must be regarded as separate populations. This has implications for the well-known 'number problem' found previously when comparing the numbers of non thermal electrons required to produce the hard X-ray and radio emissions.

Kundu, M. R.↗
NASA NTRS1981

Tearing at the dayside magnetopause

Initial observations of the dayside magnetopause thickness using magnetometers on board ISEE 1 (International Sun-Earth Explorer) and ISEE 2 indicate that this boundary may only be a few ion gyro-radii thick during periods of observed southward interplanetary magnetic field (IMF). Such a thin current layer can destabilize the collisionless tearing mode, a probable first step in the reconnection sequence. The linear growth rate is calculated assuming typical magnetopause parameters and it is found that it is a sensitive function of IMF orientation, magnetopause thickness, and electron number density. It is also shown that the finite size of the dayside magnetopause necessitates the existence of a two-dimensional tearing mode wave vector spectrum, a consequence of the inability of a thermal electron to maintain Landau resonance with the wave for a growth period. Implications regarding reconnection are discussed.

Quest, K. B.↗
NASA NTRS1980

Recent observations of energetic electrons in solar flares

The impulsive solar X-ray burst observed by the ISEE 3 X-ray spectrometer experiment on October 5, 1978 is discussed with consideration of energetic electrons in solar flares. The X-ray spectrum is consistent with a power law electron spectrum with no apparent low energy cut-off up to energies of around 5 keV. Although an explanation of the observed X-ray spectrum in terms of the emission from a multi-thermal electron spectrum cannot be ruled out, the observations lend support to the existence of nonthermal electron spectra during the impulsive phase of solar flares.

Kane, S. R.↗
NASA NTRS1999

The Role of Instabilities in Plasmaspheric Heating, Flux Tube Refilling, and the Development of Spatial Structures

Plasma instabilities appear to play an important role in plasmasphere dynamics. Direct interactions between the plasmasphere and other plasma populations lead to energy transfer and heating, to equatorial trapping, and to changes in ionospheric outflow. Super-thermal electron and ring current populations are the dominant sources of energy for these processes. Flute (Interchange), electromagnetic lower-hybrid drift, and E x B drift instabilities can also play important roles in the distribution of thermal, plasmaspheric plasma. As our attention has returned to the remaining plasmaspheric mysteries, it has become clear that the details of plasmaspause formation and erosion and the development of localized, dense thermal plasma structures are almost completely unexplained. Radially sharp density depletions and generalized density loss are seen inside the plasmapause. Narrow and extended regions of enhanced thermal plasma density is found throughout the magnetosphere, even after extended periods of quite geomagnetic conditions. Sharp gradients are often seen on the westward edge of density enhancements, while eastward boundaries are often highly structured.

Gallagher, D. L.↗
NASA NTRS1999

A Plasma Instability Theory of Gamma-Ray Burst Emission

A plasma instability theory is presented for the prompt radiation from gamma-ray bursts. In the theory, a highly relativistic shell interacts with the interstellar medium through the filamentation and the two-stream instabilities to convert bulk kinetic energy into electron thermal energy and magnetic field energy. The processes are not efficient enough to satisfy the Rankine-Hugoniot conditions, so a shock cannot form through this mechanism. Instead, the interstellar medium passes through the shell, with the electrons radiating during this passage. Gamma-rays are produced by synchrotron self-Compton emission. Prompt optical emission is also produced through this mechanism, while prompt radio emission is produced through synchrotron emission. The model timescales are consistent with the shortest burst timescales. To emit gamma-rays, the shell's bulk Lorentz factor must be $\simg 10(exp 3)$. For the radiative processes to be efficient, the interstellar medium density must satisfy a lower limit that is a function of the bulk Lorentz factor. Because the limits operate as selection effects, bursts that violate them constitute new classes. In particular, a class of optical and ultraviolet bursts with no gamma-ray emission should exist. The lower limit on the density of the interstellar medium is consistent with the requirements of the Compton attenuation theory, providing an explanation for why all burst spectra appear to be attenuated. Several tests of the theory are discussed, as are the next theoretical investigations that should be conducted.

Brainerd, Jerome J.↗
NASA NTRS1977

HF excited instabilities in space plasmas

It has now become technologically feasible to construct ground-based HF radar systems which can deliver RF energy to the ionospheric plasma, with power densities sufficient to alter the ionospheric electron thermal budget and plasma characteristics. Effects produced by a ground-based transmitter of a power aperture of the order of 10,000 Mw sq m in the frequency range from 4 to 12 MHz are shown in a graph. Another graph presents a profile of electron gas temperature enhancement due to energy deposition by a high power HF transmitter. The conduction of high-power HF ionospheric plasma experiments is discussed. Attention is given to plasma instabilities, wave-particle acceleration effects, spacial and time scales, parametric instabilities, spread-F, and the possibility of placing high power-aperture radars on an orbiting space platform.

Carlson, H. C., Jr.↗
NASA NTRS1984

Radio-frequency heating of the coronal plasma during flares

A model is developed for the radio-frequency (RF) heating of soft X-ray emitting plasma in solar flares due to absorption of amplified cyclotron radiation. The radiation, carrying approximately 10 to the 27th to approximately 10 to the 30th erg/s, is generated through maser emission following partial precipitation of electrons in one or more flaring loops. The maser operates in a large number of small regions, each producing an 'elementary burst' (EB) of short duration. This radiation propagates either directly or after reflection to the second-harmonic absorption layer, where it is absorbed by thermal electrons. The properties of EBs and the heating of the electrons in the absorption layer are discussed in detail. RF heating and evaporation models for the production of soft X-ray emitting plasma are compared. Properties of the RF heating model that explain observed features are energy transport across field lines, rapid heating (in approximately 1 s) of coronal plasma to approximately 3 x 10 to the 7th K, and instigation of turbulent velocities up to the ion sound speed.

Melrose, D. B.↗
NASA NTRSDate not supplied

How Is the Solar Wind Shocked? A Distribution-Function Perspective From the MMS Mission

In this talk, we will discuss how plasma distribution functions from the Magnetospheric Multiscale (MMS) mission transformed the picture on how the solar wind is shocked. For quasi-parallel shocks with typical Alfven Mach numbers, we will examine how ion deceleration, reflection, and thermalization take place, and illustrate how the new picture expands on the framework envisioned by Schwartz and Burgess in 1991. For quasi-perpendicular shocks with the range of Alfven Mach numbers (~1-60) captured by MMS, we will examine examples showing the processes of ion and electron thermalization go beyond the existing paradigm and call for future investigation. The knowledge advanced by MMS measurements provides a ground to understand planetary shocks in our solar system as well as astrophysical shocks arising from stellar wind interaction with exoplanets and supernova remnants.

Li-Jen Chen↗
NASA NTRS1983

The interpretation of hard X-ray polarization measurements in solar flares

Observations of polarization of moderately hard X-rays in solar flares are reviewed and compared with the predictions of recent detailed modeling of hard X-ray bremsstrahlung production by non-thermal electrons. The recent advances in the complexity of the modeling lead to substantially lower predicted polarizations than in earlier models and more fully highlight how various parameters play a role in determining the polarization of the radiation field. The new predicted polarizations are comparable to those predicted by thermal modeling of solar flare hard X-ray production, and both are in agreement with the observations. In the light of these results, new polarization observations with current generation instruments are proposed which could be used to discriminate between non-thermal and thermal models of hard X-ray production in solar flares.

Leach, J.↗
NASA NTRS1985

The interpretation of hard X-ray polarization measurements in solar flares

Observations of polarization of moderately hard X-rays in solar flares are reviewed and compared with the predictions of recent detailed modeling of hard X-ray bremsstrahlung production by non-thermal electrons. The recent advances in the complexity of the modeling lead to substantially lower predicted polarizations than in earlier models and more fully highlight how various parameters play a role in determining the polarization of the radiation field. The new predicted polarizations are comparable to those predicted by thermal modeling of solar flare hard X-ray production, and both are in agreement with the observations. In the light of these results, new polarization observations with current generation instruments are proposed which could be used to discriminate between non-thermal and thermal models of hard X-ray production in solar flares.

Leach, J.↗
NASA NTRS2012

Ambipolar Electric Field, Photoelectrons, and Their Role in Atmospheric Escape From Hot Jupiters

Atmospheric mass loss from Hot Jupiters can be large due to the close proximity of these planets to their host star and the strong radiation the planetary atmosphere receives. On Earth, a major contribution to the acceleration of atmospheric ions comes from the vertical separation of ions and electrons, and the generation of the ambipolar electric field. This process, known as the "polar wind," is responsible for the transport of ionospheric constituents to Earth's magnetosphere, where they are well observed. The polar wind can also be enhanced by a relatively small fraction of super-thermal electrons (photoelectrons) generated by photoionization.We formulate a simplified calculation of the effect of the ambipolar electric field and the photoelectrons on the ion scale height in a generalized manner. We find that the ion scale height can be increased by a factor of 2-15 due to the polar wind effects. We also estimate a lower limit of an order of magnitude increase of the ion density and the atmospheric mass-loss rate when polar wind effects are included.

atmospheres↗
NASA NTRS2013

New Evidence for Efficient Collisionless Heating of Electrons at the Reverse Shock of a Young Supernova Remnant

Although collisionless shocks are ubiquitous in astrophysics, certain key aspects of them are not well understood. In particular, the process known as collisionless electron heating, whereby electrons are rapidly energized at the shock front, is one of the main open issues in shock physics. Here, we present the first clear evidence for efficient collisionless electron heating at the reverse shock of Tycho's supernova remnant (SNR), revealed by Fe K diagnostics using high-quality X-ray data obtained by the Suzaku satellite. We detect K beta (3p yields 1s) fluorescence emission from low-ionization Fe ejecta excited by energetic thermal electrons at the reverse shock front, which peaks at a smaller radius than Fe K alpha (2p yields 1s) emission dominated by a relatively highly ionized component. Comparisons with our hydrodynamical simulations imply instantaneous electron heating to a temperature 1000 times higher than expected from Coulomb collisions alone. The unique environment of the reverse shock, which is propagating with a high Mach number into rarefied ejecta with a low magnetic field strength, puts strong constraints on the physical mechanism responsible for this heating and favors a cross-shock potential created by charge deflection at the shock front. Our sensitive observation also reveals that the reverse shock radius of this SNR is about 10% smaller than the previous measurement using the Fe K alpha morphology from the Chandra observations. Since strong Fe K beta fluorescence is expected only from low-ionization plasma where Fe ions still have many 3p electrons, this feature is key to diagnosing the plasma state and distribution of the immediate postshock ejecta in a young SNR.

Fe K diagnostics↗
NASA NTRS1986

A double layer model for solar X-ray and microwave pulsations

The wide range of wavelengths over which quasi-periodic pulsations have been observed suggests that the mechanism causing them acts upon the supply of high energy electrons driving the emission processes. A model is described which is based upon the radial shrinkage of a magnetic flux tube. The concentration of the current, along with the reduction in the number of available charge carriers, can rise to a condition where the current demand exceeds the capacity of the thermal electrons. Driven by the large inductance of the external current circuit, an instability takes place in the tube throat, resulting in the formation of a potential double layer, which then accelerates electrons and ions to MeV energies. The double layer can be unstable, collapsing and reforming repeatedly. The resulting pulsed particle beams give rise to pulsating emission which are observed at radio and X-ray wavelengths.

Tapping, K. F.↗