Infer Electron Density Research Articles

Carbon Swan Spectra Measurements following Breakdown of Nitro Compound Explosive Simulants

Our measurements of micro-plasma following laser-induced optical breakdown of nitro compound explosive simulants, here 3-nitrobenzoic acid, show well-developed molecular spectra during the first several hundreds of nanoseconds. Analysis of recorded carbon spectra is accomplished using accurate line strengths for the diatomic molecular Swan system. Presence of hydrogen-beta allows us to infer electron density in the plasma evolution. Computational challenges include accounting for background variation and appropriate modeling of hydrogen embedded in molecular spectra. Recorded and computed spectra agree nicely for time delays on the order of 1.6 μs from optical breakdown when using a single temperature for local thermodynamic equilibrium plasma.

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C<sub>2</sub>, CN, and TiO Spectra Following Laser-induced Optical Breakdown

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement is used. Here we operate a Nd:YAG laser at a frequency of 10 Hz at the fundamental wavelength of 1,064 nm. The 14 nsec pulses with anenergy of 190 mJ/pulse are focused to a 50 µm spot size to generate a plasma from optical breakdown or laser ablation in air. The microplasma is imaged onto the entrance slit of a 0.6 m spectrometer, and spectra are recorded using an 1,800 grooves/mm grating an intensified linear diode array and optical multichannel analyzer (OMA) or an ICCD. Of interest are Stark-broadened atomic lines of the hydrogen Balmer series to infer electron density. We also elaborate on temperature measurements from diatomic emission spectra of aluminum monoxide (AlO), carbon (C2), cyanogen (CN), and titanium monoxide (TiO). The experimental procedures include wavelength and sensitivity calibrations. Analysis of the recorded molecular spectra is accomplished by the fitting of data with tabulated line strengths. Furthermore, Monte-Carlo type simulations are performed to estimate the error margins. Time-resolved measurements are essential for the transient plasma commonly encountered in LIBS.

NON-EQUILIBRIUM IONIZATION EFFECTS ON THE DENSITY LINE RATIO DIAGNOSTICS OF O IV

The dynamic timescales in the solar atmosphere are shorter than the ionization and recombination times of many ions used for line ratio diagnostics of the transition region and corona. The long ionization and recombination times for these ions imply that they can be found far from their equilibrium temperatures, and spectroscopic investigations require more care before being trusted in giving correct information on local quantities, such as density and temperature. By solving the full time-dependent rate equations for an oxygen model atom in the three-dimensional numerical model of the solar atmosphere generated by the Bifrost code, we are able to construct synthetic intensity maps and study the emergent emission. We investigate the method of electron density diagnostics through line ratio analysis of the O IV 140.1 nm to the 140.4 nm ratio, the assumptions made in carrying out the diagnostics, and the different interpretations of the electron density. The results show big discrepancies between emission in statistical equilibrium and emission where non-equilibrium (NEQ) ionization is treated. Deduced electron densities are up to an order of magnitude higher when NEQ effects are accounted for. The inferred electron density is found to be a weighted mean average electron density along the linemore » of sight and has no relation to the temperature of emission. This study shows that numerical modeling is essential for electron density diagnostics and is a valuable tool when the ions used for such studies are expected to be out of ionization equilibrium. Though this study has been performed on the O IV ion, similar results are also expected for other transition region ions.« less

Methods of Generation and Detailed Characterization of Millimeter-Scale Plasmas Using a Gasbag Target

Gasbag targets are useful for the research of laser-plasma interactions in inertial confinement fusion, especially in the laser overlapping regime. We report that on the Shengguang-II laser facility, millimeter-scale plasmas are successfully generated by four 0.35 μm laser beams using a gasbag target. Multiple diagnostics are applied to characterize the millimeter-scale plasmas in detail. The images from the x-ray pinhole cameras confirm that millimeter-scale plasmas are indeed created. An optical Thomson scattering system diagnoses the electron temperature of the CH filling plasmas by probing the thermal ion-acoustic fluctuations, which indicates that the electron temperature has a 600 eV flat roof in 0.7–1.3 ns. Another key parameter, i.e. the electron density of the millimeter-scale plasmas, is inferred by the spectrum of the back stimulated Raman scattering of an additional 0.53 μm laser beam. The inferred electron density keeps stable at 0.1nc in early time consistent with the controlled filling pressure and splits into a higher density in late time, which is attributed to the blast wave entering into the SRS interaction region.

ESTIMATES OF DENSITIES AND FILLING FACTORS FROM A COOLING TIME ANALYSIS OF SOLAR MICROFLARES OBSERVED WITHRHESSI

We use more than 4,500 microflares from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) microflare data set (Christe et al., 2008, Ap. J., 677, 1385) to estimate electron densities and volumetric filling factors of microflare loops using a cooling time analysis. We show that if the filling factor is assumed to be unity, the calculated conductive cooling times are much shorter than the observed flare decay times, which in turn are much shorter than the calculated radiative cooling times. This is likely unphysical, but the contradic- tion can be resolved by assuming the radiative and conductive cooling times are comparable, which is valid when the flare loop temperature is a maximum and when external heating can be ignored. We find that resultant radiative and con- ductive cooling times are comparable to observed decay times, which has been used as an assumption in some previous studies. The inferred electron densities have a mean value of 10^11.6 cm^-3 and filling factors have a mean of 10^-3.7. The filling factors are lower and densities are higher than previous estimates for large flares, but are similar to those found for two microflares by Moore et al. (Ap. J., 526, 505, 1999).

X-rays from T Tauri: a test case for accreting T Tauri stars

Context. The generation of X-rays in accreting T Tauri stars (TTS) is thought to be predominantly due to energy dissipation in magnetic elds, but alternative X-ray generation mechanisms have been proposed, such as heating in accretion shocks near the stellar surface, or in shocks forming in jets. Aims. We test models and trends discussed in the literature using X-ray data from the classical TTS T Tau. Methods. High-resolution spectroscopy from the Reection Grating Spectrometers on XMM-Newton is used to infer electron densities, element abundances and the thermal structure of the X-ray source. We also discuss the ultraviolet light curve obtained by the Optical Monitor, and complementary ground-based photometry. A high-resolution image from Chandra constrains contributions from the two companions of T Tau N. Results. The X-ray grating spectrum is rich in emission lines, but shows an unusual mixture of features from very hot ( 30 MK) and very cool (1n3 MK) plasma, both emitted by similar amounts of emission measure. The cool plasma conrms the picture of a soft excess in the form of an enhanced O vii/O viii Ly ux ratio, similar to that previously reported for other accreting TTS. Diagnostics from lines formed by this plasma indicate low electron densities (< 10 10 cm 3 ). The Ne/Fe abundance ratio is consistent with a trend in pre-main sequence stars in which this ratio depends on spectral type, but not on accretion. Conclusions. On the basis of line density diagnostics, we conclude that the density of the cool isoft-excessi plasma is orders of magnitude below that predicted for an accretion shock, assuming previously determined accretion rates of (3 6) 10 8 M yr 1 . We argue that loading of magnetic eld lines with infalling material suppresses the heating process in a part of the corona. We thus suggest that the X-ray production of T Tau is inuenced by the accretion process although the X-rays may not form in the bulk of the accretion footpoints.

First whistler observed in the magnetosphere of Saturn

[1] This paper describes and analyzes the first whistler observed in the magnetosphere of Saturn. The whistler was detected by the Radio and Plasma Wave Science (RPWS) instrument on the Cassini spacecraft during the inbound pass on October 28, 2004, at a radial distance of 6.18 RS (Saturn radii). Based on the measured dispersion, a travel time computation shows that the whistler originated from lightning in the northern hemisphere of Saturn. Using a simple centrifugal potential model for the plasma distribution along the magnetic field line, we have determined the scale height and fractional concentrations of water group ions and protons that give the best fit to the observed dispersion. In addition, a more complex diffusive equilibrium model was also analyzed. The inferred electron density profiles for the two models are very similar. Representative scale heights and equatorial ion fractional concentrations that provided good fits to the dispersion of the whistler were found to be = 0.89 RS and = 0.7 for water group ions, and = 2.67 RS and = 0.3 for protons.

Nighttime D region electron density profiles and variabilities inferred from broadband measurements using VLF radio emissions from lightning

Lightning discharges radiate most of their electromagnetic energy in the very low frequency (VLF, 3–30 kHz) and extremely low frequency (ELF, 3–3000 Hz) bands and are, consequently, an effective tool for remotely sensing the nighttime ionospheric D region electron density profile. Using broadband lightning‐generated VLF signals, we derived the night‐to‐night variations of the midlatitude ionospheric D region electron density profiles. For 16 nights, between 1 July and 4 August 2004, we examined measured VLF data from lightning occurring near the United States east coast (∼530–860 km away from Duke University) and received at Duke University from 0400 to 0600 UT. From these observed VLF radio atmospherics (sferics), we extracted the nighttime D region electron density profiles covering the range of electron densities from 100 to 103 cm−3, in the altitude range of approximately 70–95 km, using a two‐dimensional, laterally homogenous model of VLF propagation in the Earth‐ionosphere waveguide. The inferred electron density profile variabilities were in good agreement with those from past nighttime rocket experiments at similar latitudes. Using the rocket‐measured profiles in our propagation simulations, we determined that the two‐parameter exponential D region electron density profiles we inferred were the best exponential fit, in the electron density range of ∼3 to ∼500 cm−3, to the rocket‐measured D region electron density profiles. In an initial effort to determine the sources of the observed variabilities, we compared the SAMPEX precipitating electron measurements to the electron density profiles inferred during July 2000. The results indicate that high‐energy electron precipitation might account for at least part of the night‐to‐night variations of the D region electron densities at the midlatitudes.

Atomic Data and Spectral Line Intensities for Mgviii

EUV spectral lines of Mg VIII have been observed from the solar corona by Skylab, SERTS, and more recently by SOHO. Atomic data for this astrophysically important ion are presented in this paper. The atomic data include energy levels, oscillator strengths, transition rates, and electron-impact excitation collision strengths. Twenty levels are included by using the configurations 2s22p, 2s2p2, 2p3, 2s23s, 2s23p, and 2s23d. Collision strengths are calculated at five incident energies (15.0, 22.5, 30.0, 37.5, and 45.0 Ry) in the distorted wave approximation. The temperature of maximum abundance for this ion is log Te = 5.9 K. The rate coefficients are calculated at this temperature by averaging the collision strengths over a Maxwellian distribution. Proton rates and solar background radiation have been included in calculating level populations, and intensity ratios are calculated by including proton rates only. The relative intensity ratios are compared with observed ratios obtained from SERTS and SOHO/CDS to infer electron density in different solar features such as an active region, quiet-Sun, and off-limb. With a few exceptions, the predicted intensity ratios match observed values very well over a variety of instrumentation and solar conditions. Since Mg VIII has a number of strong EUV lines, we expect that the spectra of this ion should prove useful for abundance studies and other plasma diagnostics.

Electron Trapping Times and Trap Densities in Solar Flare Loops Measured withCOMPTONandYOHKOH

We measure energy-dependent time delays of ≈ 20-200 keV hard X-ray (HXR) emission from 78 flares observed simultaneously with the Compton Gamma Ray Observatory and Yohkoh. Fast time structures ( 1 s) are filtered out, because their time delays have been identified in terms of electron time-of-flight (TOF) differences from directly precipitating electrons (Aschwanden et al.). For the smooth HXR flux, we find systematic time delays in the range of τS = t50 keV-t200 keV ≈ -(1 ... 10) s, with a sign opposite to TOF delays, i.e., the high-energy HXRs lag the low-energy HXRs. We interpret these time delays of the smooth HXR flux in terms of electron trapping, and we fitted a model of the collisional deflection time tDefl(E) E -->3/2n -->−1e to the observed HXR delays in order to infer electron densities n${r Trap}e$ --> in the trap. Independently, we determine the electron density n${r SXR}e$ --> in flare loops from soft X-ray (SXR) peak emission measures EM= -->∫ n -->2e dh, using loop width (w) measurements to estimate the column depth dh ≈ w. Comparing the two independent density measurements in HXR and SXR, we find a mean ratio of q -->e=n${r Trap}e$ -->/n${r SXR}e$ --> ≈ 1, with a relatively small scatter by a factor of ≈ 2. Generally, it is likely that the SXR-bright flare loops have a higher density than the trapping regions (when qe 1). Our measurements provide comprehensive evidence that electron trapping in solar flares is governed in the weak-diffusion limit, i.e., that the trapping time corresponds to the collisional deflection time, while pitch-angle scattering by resonant waves seems not to be dominant in the 20-200 keV energy range. The measurements do not support a second-step acceleration scenario for energies ≤200 keV.

Detection of the inner plasma pileup region at comet Halley during the Vega 1 flyby by the radio sounding experiment

The two Vega spacecraft sounded the ionosphere of Comet P/Halley during their flybys at two coherent one‐way downlink frequencies. The frequency recordings were processed with software adapted from a software code used for processing Ulysses radio science data. An electron density model was developed in order to model the electron content profile observed during the Vega 1 flyby ±1500 s (±120,000 km) about the closest approach (8900 km). The electron density for distances larger than 11,000 km followed a modified r−2 law, while the electron density in the “pileup” region between 8900 km and 11,000 km increased with distance according to a single power law r4. The inferred electron density is a factor of 2 lower than that measured by the Vega BD 3 in situ experiment during inbound flight and agrees with the outbound observations. The electron density profile inferred by the radio science experiment is an average over all inhomogeneities along the ray path and is probably a better representation of the large‐scale density profile originating from the comet nucleus than the in situ observations, which are affected by inhomogeneities along the trajectory.

Dynamics of laser-produced carbon plasma

Carbon plasmas produced by radiation from a ruby laser (wavelength 694.3 nm) focussed onto a carbon target in vacuum are studied spectroscopically with a time resolution of 40 ns. Measured line profiles of several ionic species (CI-CIV) were used to infer electron density and temperature at several positions above the target surface as function of time elapsed after the beginning of the laser pulse. The particle density at several positions above the target surface as function of time was judged from corrected line intensities. Experimental data are compared with theoretical predictions made with the effusion model of plasma expansion (Kelly R and Braren B 1991 Appl. Phys. B 53 160). The effusion model provided the relative particle density in the expanding plasma cloud as a function of initial target temperature. By comparing predicted and measured time evolution of particle density, an initial target temperature of about 125eV was inferred. The coupling of the laser beam energy to the plasma itself was inferred from the failure of the model of the direct target surface heating (Andreić Z, Henč-Bartolić V and Kunze H-J 1993 Physica Scripta 48 331) to produce the required target temperature.

Spectral and Temporal Characteristics of X-Ray--bright Stars in the Pleiades

We follow up our deep ROSAT imaging survey of the Pleiades (Stauffer et al. 1994) with an analysis of the spectral and temporal characteristics of the X-ray-bright stars in the Pleiades. Raymond &amp; Smith (1977) one-and two-temperature models have been used to fit the position-sensitive proportional counter (PS PC) pulse-height spectra of the dozen or so brightest sources associated with late-type Pleiades members. The best-fit temperatures suggest hot coronal temperatures for K, M, and rapidly rotating G stars, and cooler temperatures for F and slowly rotating G stars. In order to probe the many less X-ray-luminous stars, we have generated composite spectra by combining net counts from all Pleiades members according to spectral type and rotational velocity. Model fits to the composite spectra confirm the trend seen in the individual spectral fits. Particularly interesting is the apparent dependence of coronal temperature on L<SUB>X</SUB>/L<SUB>bol</SUB>. A hardness-ratio analysis also confirms some of these trends. The PSPC data have also revealed a dozen or so strong X-ray flares with peak X-ray luminosities in excess of ∼10<SUP>30</SUP> ergs s<SUP>-1</SUP>. We have modeled the brightest of these flares with a simple quasi-static cooling loop model. The peak temperature and emission measure and the inferred electron density and plasma volume suggest a very large scale flaring event. The PSPC data were collected over a period of ∼18 months, allowing us to search for source variability on timescales ranging from less than a day (in the case of flares) to more than a year between individual exposures. On approximately year-long timescales, roughly 25% of the late-type stars are variable. Since the Pleiades was also intensively monitored by the imaging instruments on the Einstein Observatory, we have examined X-ray luminosity variations on the 10 yr timescale between Einstein and ROSAT and find that up to 40% of the late-type stars are X-ray variable. Since there is only marginal evidence for increased variability on decade-long timescales, the variability observed on long and short timescales may have a common physical origin.

Asymmetries in the Io plasma torus

Using Ulysses radio wave data taken during the 1992 Jupiter encounter, we conclude that there are significant large and small spatial scale azimuthal asymmetries at high latitudes in the Io plasma torus. During a period of time near perijove when the spacecraft motion was predominantly in the azimuthal direction and was relatively fixed in both latitude and radial distance, inferred electron densities depart significantly from the common assumption of longitudinal symmetry. Specifically, electron plasma concentrations near 0° system III longitude (and 0400 LT) are greater than those near 180° (and 0000 LT). Superposed on this large‐scale variation are regularly spaced density depletions, 30‐50% in magnitude, and having a spatial periodicity of about 17°. Some of these depletions may drive various known radio and plasma wave sources by means of large B parallel electric potentials. The observations are compared with recent models and with the in situ Voyager observations.

Plasma conditions generated by interaction of a high brightness, prepulse free Raman amplified KrF laser pulse with solid targets.

A high brightness, Raman amplified KrF laser has been used to irradiate solid targets with 12 ps laser pulses at intensities above ${10}^{15}$ W/${\mathrm{cm}}^{2}$ without the presence of a preformed plasma caused by low level amplified spontaneous emission prepulse. Time-resolved x-ray spectroscopy of the K-shell emission from aluminum was used to infer electron densities in excess of ${10}^{23}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ at temperatures of several hundred electronvolts.

Forbidden line ratios from Si viii and S x coronal ions

The off-limb spectra, recorded by the NRL Skylab spectrograph, have been used to infer electron densities from theoretical curves of Si viii and S x density-sensitive line ratios. The derived densities are found to be in agreement with those reported by Feldman et al. (1978). Computed line intensities using the Kopp and Orrall (1976) model have also been used to estimate densities in the quite Sun and coronal holes from these curves.

Density Diagnostics of Solar Emission Lines from the Nitrogen-like Mg VI Ion

The variation of theoretical line intensity ratios with electron density for Mg VI ion are presented. This study indicates that the line intensity ratios for Mg VI can be used to infer electron density for solar plasma.

Variability of d-region electron densities at tromsø

The 2.75 MHz partial-reflection radar at Ramfjordmoen near Tromsø has been used for a study of D-region electron densities by the differential-absorption method on a number of days during 1978–79. Received signals are generally stratified in several layers, typically over 60–80 km. Strong stable echoes are seen down to 55 km during periods of enhanced riometer absorption. Inferred electron densities vary between ~ 100–1000 cm −3 at ~ 60–80 km and show well-defined features which persist for ~ 10–20 min. During periods of high absorption, enhanced electron densities (~ 600 cm −3) are observed below 65 km. During a Polar Cap Absorption event, the inferred electron densities at 60–70 km show a very stable profile. Possible sources of D-region ionization at high latitudes are briefly discussed

? 5303 Fe XIV density models of the inner solar corona

The Sacramento Peak Observatory's 40 cm coronagraph was used with an emission line photometer to observe the distribution of λ5303 Fe XIV brightness as a function of position angle, height above the limb, and time. These data were used to construct models of the volume emissivity as a function of solar latitude and longitude. These models in turn yield estimates of the distribution of electron density in the lower solar corona as a function of latitude and longitude for several specific periods in 1973 and 1975. Three observational results are obtained. An upper limit for the inferred electron density in coronal hole regions is set at log Ne = 7.4 for an altitude of 1.15R⊙. Density models from late 1973 demonstrate an evolutionary trend toward a rather regular four-lobed appearance of coronal material; models from 1975 suggest that this characterization persisted for at least 27 solar rotations. A decrease in the total integrated λ5303 intensity of a factor of 2.9 is inferred to have taken place between 1973 and 1975.

Observation of photoelectrons and secondary electrons reflected from a potential barrier in the vicinity of ATS 6

The electron detectors on ATS 6 frequently have peaks in the counting rate at low energies from 4 to 10 eV in addition to other peaks at much higher energies. It is shown from the shape of the corresponding velocity distribution functions, the inferred electron densities, and the behavior of the counting rate versus angle that the low-energy electrons must at times be photoelectrons or secondary electrons emitted from the spacecraft. The data indicate the presence of a potential barrier in the spacecraft environment which is as much as 50 V negative with respect to the spacecraft. The barrier turns back the low-energy spacecraft-emitted electrons and prevents the low-energy ambient electrons from reaching the detector. It is argued that the magnitude of the observed barrier is too large to be explained in terms of a simple photoelectron or secondary electron sheath around a uniformly charged spacecraft. The most likely explanation is the presence of differential charging of the spacecraft surfaces.