Large Electron Temperature Research Articles

Stability of H+ beams in the polar wind

In the classical picture of the polar wind, the H+ flow becomes supersonic and collisionless, and the H+ velocity distribution becomes anisotropic and asymmetric at altitudes above about 3000 km. Previously, the stability of the classical polar wind was studied and found to be stable for a wide range of electron temperatures. However, this classical picture of the polar wind results from steady state models that neglect horizontal plasma convection. The plasma convection at high latitudes can significantly affect the characteristics of the polar wind. For example, ions can be energized in the cusp region and then convected into the polar cap, which subsequently results in energetic ion beams and/or conics passing through the classical polar wind. The effect of energetic H+ beams on the stability of the polar wind was studied with regard to the excitation of electrostatic waves. The cases considered covered a wide range of electron‐to‐background ion temperature ratios (Te/Ti = 0.1, 1, 10) and beam‐to‐background ion density ratios (nb/(ni + nb) = 0.1, 0.5, 0.9). A relatively cold beam was assumed (Tb = 0.1Ti). A combination of the Nyquist technique and a direct solution of the plasma dispersion equation was used to find the minimum beam drift velocity required to destabilize the plasma. The following results were obtained: (1) The plasma can be destabilized for relative drift energy ≲1 eV, (2) The plasma is more unstable for large electron temperatures and for comparable ion and beam densities, (3) For large electron temperatures the parallel propagating ion/ion plasma‐acoustic instability is triggered first, while the obliquely propagating ion/ion cyclotron instability tends to trigger first for low Te.

Response of the Arecibo ionosphere to large HF-induced electron temperature enhancements

During recent experiments at Arecibo, Puerto Rico, large (1000 - >2500 K) enhancements in electron temperature have been observed when high-power radio waves reflect near the nighttime F region peak. The electron temperature enhancements are accompanied by significant (50 – 200 K) increases in ion temperature, large (10 – 25%) reductions in electron density, and strong HF-induced spread F. When large electron temperature enhancements develop, the ionosphere appears to become dynamically unstable resulting in the production of geomagnetic field-aligned striations having elevated electron temperatures and depleted electron densities. In the present work, a theoretical description of the F region thermal response is provided along with several new experimental results. An interesting aspect of this type of radio wave heating experiment is that it offers the possibility of directly measuring aeronomic parameters that are difficult to determine with other techniques.

The F- and E-region studies by incoherent scatter radar

The incoherent scatter radar gives the possibility to study the ionospheric parameters of E and F-regions, at the same time, from the altitude of 90 km up to 1000 km with a good time and spatial resolution. In this paper, we describe the measurements technique and data analysis strategy. We continue by discussing the main energy sources and their influence on the auroral ionosphere. We show the average behavior of the low thermosphere studied with large amount of data. On some examples we describe the answer of the auroral ionosphere to the energy input, electric field and particle precipitations. The modification of the ion composition due to the energy deposition is illustrated and discussed. We study some unusual events due to very large electric fields. The observations of very large ion temperatures rising up to 10.000 K are explained. During the same period, we observed very large electron temperatures occurring between 100 and 120 km. These phenomena are due to a non-linear wave heating. The effect of non-Maxwellian ion velocity distribution, due to large electric field on measurements and on data interpretation, is discussed. Finally, we discuss the new trends in the incoherent scatter technique.

Evaluation of integrals of the Compton scattering cross-section

Exact formulae are derived for various integrals of the Compton scattering cross-section. The interaction kernel is integrated over outgoing photon frequency and direction, and over a relativistic Maxwellian distribution for the electrons. The total Compton cross-section, the energy exchange rate, and the transport mean free path are thereby expressed in terms of single integrals of analytic functions. In addition, these integrals produce simple analytic expressions in the limiting cases of either small or large frequency or electron temperature. A numerical method based on Gaussian quadrature is used to compute the transport mean free path. A comparison with previously published results is presented.

Simultaneous density and electric field fluctuation spectra associated with velocity shears in the auroral oval

Simultaneous satellite in situ measurements of density (ΔN/N) and electric field fluctuation (ΔE) spectra in the high‐latitude ionosphere are presented using two orbits of Dynamics Explorer 2 (DE 2) data traversing, respectively, the F region at 350 km altitude and the topside ionosphere at 900 km altitude. The spectral study was primarily confined to large structured velocity regions in the auroral oval. By means of the very complete set of energetic particle, dc and ac electric field, field‐aligned current, thermal plasma density, and temperature measurements available from DE 2, we were able to identify two categories of spectra associated with velocity shears irrespective of the height of the satellite. The first category was observed in very intense velocity shear regions of shear frequencies ∼10 Hz in conjunction with large field‐aligned current densities. Under these conditions the spatial spectra of ΔN/N and ΔE had identical power law indices of −1.8±0.2 between scale lengths of approximately 10 km and 300 m. At scale lengths shorter than 300 m the ΔE spectra steepened to an index of −3±0.5 while the spectral index of ΔN/N remained close to its original value of approximately −1.8±0.2, with large power spectral densities observed down to 10 m scale lengths. The second category was observed in more moderate velocity shear regions of shear frequencies ∼ 1 Hz in conjunction with weak field‐aligned currents. In this case the slopes of the density spectra were essentially unchanged, while the ΔE spectra had a much steeper slope of −3±0.5 between 10 km and a few hundred meters. Other factors identifying the two categories are as follows. The first category of spectra was characterized by the existence of upward flowing ions with conic distributions energized to 30 eV and possibly O+ ion cyclotron waves and large electron temperature enhancements. The second category of spectra was associated with wave activity in the 4‐ to 16‐kHz range, most probably O+ lower hybrid waves, and occasionally large ion temperature enhancements. The observations of ΔN/N and ΔE spectral behavior are compared to recent work on two‐dimensional plasma turbulence theory and nonlinear simulations of the collisional Kelvin‐Helmholtz (KH) instability. In particular, the spectral behavior associated with the moderate velocity shear category agrees well with some recent computations of the spatial power spectra of the KH instability (Keskinen et al., 1988).

Large F‐region electron‐temperature enhancements generated by high‐power HF radio waves

During recent experiments at Arecibo, Puerto Rico, large (1000‐2000 K) enhancements in electron temperature have been observed when high‐power radio waves reflect near the nighttime F region peak. The electron temperature enhancements are accompanied by significant (50‐300 K) increases in ion temperature, large (10‐15%) reductions in electron density, and strong HF‐induced spread F. When large electron temperature enhancements develop, the ionosphere appears to become dynamically unstable resulting in the production of geomagnetic field‐aligned striations having elevated electron temperatures and depleted electron densities.

Strong electron heating at the Earth's bow shock

The electron heating at collisionless shocks in near‐Earth space is normally found to be relatively small. We report here on two sets of bow shock crossings observed by the ISEE 1 and ISEE 2 spacecraft in which very large electron temperature increases were found. The two sets of shocks are part of a larger set of 52 bow shock crossings compiled from the ISEE data set. When the shocks with the large electron heating are compared to the rest of the shocks in the compiled set, it is found that the only upstream parameter which is out of the ordinary is the upstream solar wind flow speed Vu. Indeed, for the entire shock set the only upstream parameters which correlate well with the electron heating are the solar wind speed and quantities derived from it. The best correlation among those tested is between the temperature increase (Ted ‐ Teu) and the difference in the square of the bulk flow speed (Vu² ‐ Vd²) across the shock, which is proportional to the total amount of bulk flow energy dissipated by the shock. A subsequent search for bow shock crossings which occur during intervals of high solar wind speed confirms that the electron heating is very large under high‐speed conditions. The first‐order dependence of the electron temperature increase on the available bulk flow energy is consistent with a heating process which is dominated by the macroscopic cross‐shock electrostatic potential jump. This study suggests that the temperature difference is the appropriate measure of electron heating at shocks (rather than the ratio Ted/Teu) and that the first‐order dependence on (Vu² ‐ Vd²) should be normalized out in future studies of electron heating at space shocks.

Simulation model for lower hybrid current drive

A simulation model for steady-state, lower hybrid current drive is described which incorporates a relativistic, one-dimensional Fokker–Planck calculation and a toroidal ray tracing code. Two-dimensional (v⊥) effects are included in the Fokker–Planck analysis in the form of a large perpendicular electron temperature that results from pitch angle scattering. An increase in the parallel refractive index of the lower hybrid waves, arising from toroidal geometry effects, is proposed as a physical mechanism whereby injected rf waves at high phase velocity (ve≪v∥ ≲c) can interact via the Landau resonance with electrons at low phase velocity (v∥ ≲3ve). Numerical results relevant to the Alcator C [Phys. Rev. Lett. 53, 450 (1984)] and PLT [Phys. Rev. Lett. 49, 1255 (1982)] experiments are presented which demonstrate the dependency of the current drive efficiency on various plasma parameters.

Time evolution of ECRH power deposition in a tandem mirror plasma

A mechanism has been developed by which a three-dimensional Electron Cyclotron Resonance Heating (ECRH) ray-tracing-absorption calculation may be coupled to a tandem mirror transport code. The radial temperature and density profiles of the transport code are expanded via flux conservation to provide the three-dimensional geometry required for the ray-tracing calculation. Absorption along the ray trajectory determines an equivalent radial ECRH power deposition profile for use by the transport code. This profile must be generated by using multiple ray-tracing calculations to simulate the spatial spread of the power launched from an antenna. A technique for artificially producing these multiple ray simulations is presented and compared with results where multiple ray-tracing calculations were performed. Examples of plasma build-up simulations for a tandem mirror using ECRH in the plug are provided. A positive feedback mechanism is identified which produces locally large electron temperatures. This occurs frequently near the plasma edge, shielding the electrons near the plasma axis from the incident ECRH power, and producing a hollow temperature profile. This may lead t o the collapse of the plug plasma.

Cyclotron radio emission in plasma with steep temperature gradient

In the paper it is investigated how the non-Maxwellian electron velocity distribution function associated with steep temperature gradient affects the cyclotron radio emission at centimeter wavelengths. The results obtained were used to calculate the distribution of the brightness temperature across a sunspot. The presence of the large electron temperature gradient in the transition region may lead to a significant increase of the observed brightness temperature (up to several tens of percent) in comparison with the brightness temperature obtained by assumption of the Maxwellian distribution function.

Note on the parallel propagation effects of unstable Farley-Buneman waves at high latitudes

We have studied the effect that substantially enhanced electron temperatures produce on the higher-frequency shorter-wavelength modes of the Farley-Buneman instability at high latitudes. In all cases the value of the parallel wave vector is reduced and the results are closer to those obtained from fluid theory than previous calculations had revealed. The large electron temperatures are themselves the product of heating by unstable plasma waves so that the corrections described in the present work become increasingly important as the linear growth rate increases.

The Martian ionosphere in light of the Viking observations

A theoretical model has been constructed in which the ion density and the ion and electron temperature distributions are calculated by solving the coupled continuity‐momentum equations and the coupled energy equations. The latest experimental results from the Viking 1 and 2 landers are used to vary some of the parameters in the model in order to obtain agreement between the theoretical and experimental results. It is found that solar EUV radiation alone is not able to maintain the observed high ion temperatures. It was also established that the energy coupling between the electron and ion gas is insufficient to account for the measured ion temperatures even in the presence of very large electron temperatures. Direct heat input to the ion gas, probably due to solar wind‐ionosphere interactions, can result in ion temperature values in reasonable agreement with the observations. The ion densities calculated with the present model agree well with the Viking observations in the chemically controlled region, but at higher altitudes, dynamic transport processes need to be invoked to achieve consistency among the observed and calculated temperature and density values.

Ordinary-mode electromagnetic instability for counter-streaming plasmas with anisotropic temperatures

The instability of the ordinary electromagnetic mode propagating perpendicular to an external magnetic field is studied for two counter-streaming plasmas with anisotropic temperatures. A necessary and sufficient condition for this instability is derived and the growth rate is calculated. Although instability can occur in a plasma with low β‖e, provided the streaming velocity is comparable to the parallel electron thermal velocity, β‖e of the order of unity and large electron temperature anisotropy (T‖e≫ T⊥e) are required for large growth rates. These large growth rates are typically of the order of the electron cyclotron frequency. Comparison with the electrostatic two-stream instability is made.

Near-Surface Electron Temperature of Weakly Ionized Plasma According to Kinetic Theory

Properties of electrons near absorbing and emitting surfaces are studied for weakly ionized plasmas by analyzing the Boltzmann equation governing the electrons. For simplicity, it is assumed that the electric field intensity is given a priori. It is shown that there exists a “nonequilibrium absorption layer,” near the surface, wherein the kinetic distribution of electrons is completely out of equilibrium for all values of the mean free path, when the surface is highly absorbent with small or no electron emission. This layer is responsible for the large electron temperature jump at the surface, and it governs the electron temperature profile through the continuum as well as the rarefied plasmas. From the analysis it is found that the simple surface boundary condition for the continuum electron energy equation previously employed by the present author is correct when there is no surface emission. A similar simple surface boundary condition is deduced for surfaces with given finite emission rates.

Die Beeinflussung des Ionisationsgrades durch Diffusions­ und Massenströme im Plasma

Abstract A stationary, magnetically stabilized helium arc plasma is taken as an example to show that ionization degrees in such plasmas with large electron temperature and electron density gradients may not be calculated from static ionization relations valid for homogeneous plasmas (e. g. the Saha equation). Instead, ionization degrees in inhomogeneous plasmas must be determined from continuity equations of the form div (nΖυΖ)nΖ = JΖ-1 nΖ-1 -RΖnΖ nz where JΖ-1 and nΖ-1 are the ionization rate and the density of the Z -1 times charged ions, and RΖ , nΖ and υΖ are the recombination rate, the density and the centre-of-mass velocity of the Z times charged ions. Static ionization formulae may not be applied to inhomogeneous plasmas because the finite relaxation times for the attainment of static ionization equilibria result in these ionization equilibria being displaced by the motion of the ions parallel to the direction of electron temperature and density gradients. This is proved by means of two independent operations: firstly, the velocities of the helium ions, these being governed primarily by ambipolar diffusion, are calculated with the spectroscopically measured state variables from the momentum equations of the plasma; secondly, the velocities of the doubly charged ions are determined by integrating the continuity equation for these ions, use again being made of the measured state variables and of the ionization and recombination rates J1 and R2 of the helium ions that were calculated for this special plasma. The agreement between the independently obtained velocity values proves that the degree of ionization of the helium ions in this inhomogeneous plasma is described not by a static ionization formula, but by the continuity equations for these ions. Furthermore, it is shown that the ionization "equilibria" between singly charged and neutral helium particles and between three times (CIV) and four times (CY) charged carbon particles are not determined by means of static ionization formulae either. The influence of this hitherto disregarded effect on the spectroscopic determination of the electron temperature in the plasma discussed is illustrated in a diagram.

Fluctuations of electron density in the daytime F-region

Further work on the large electron density and electron temperature fluctuations observed on quiet summer and winter days is reported. The data are examined in relation to explanations based on electrodynamic effects, neutral wind variations, gravity waves, and movement of ionospherie irregularities over the site. The characteristics of the fluctuations, as measured at Arecibo, have been supplemented by ionosonde data at other sites. Vertical transport velocities have been calculated based on the assumption that the fluctuations represent a time term variation by the solution of the F-region continuity equation from 250 km to 700 km. CIRA 1965 neutral atmosphere models together with current estimates of the solar flux and loss rate coefficients have been employed. The vertical velocities calculated in this way have been compared with estimates based on magnetograms from San Juan, neutral wind velocities, E- and F-region drifts measurements by the method of similar fades in Puerto Rico, and with measurements made at Arecibo by Thome using antenna rotation.