Low-energy Electron Population Research Articles

DIFFUSIVE SHOCK ACCELERATION SIMULATIONS OF RADIO RELICS

Recent radio observations have identified a class of structures, so-called radio relics, in clusters of galaxies. The radio emission from these sources is interpreted as synchrotron radiation from GeV electrons gyrating in microG-level magnetic fields. Radio relics, located mostly in the outskirts of clusters, seem to associate with shock waves, especially those developed during mergers. In fact, they seem to be good structures to identify and probe such shocks in intracluster media (ICMs), provided we understand the electron acceleration and re-acceleration at those shocks. In this paper, we describe time-dependent simulations for diffusive shock acceleration at weak shocks that are expected to be found in ICMs. Freshly injected as well as pre-existing populations of cosmic-ray (CR) electrons are considered, and energy losses via synchrotron and inverse Compton are included. We then compare the synchrotron flux and spectral distributions estimated from the simulations with those in two well-observed radio relics in CIZA J2242.8+5301 and ZwCl0008.8+5215. Considering that the CR electron injection is rather inefficient at weak shocks with Mach number M <~ a few, the existence of radio relics could indicate the pre-existing population of low-energy CR electrons in ICMs. The implication of our results on the merger shock scenario of radio relics is discussed.

Low-energy electron production by relativistic runaway electron avalanches in air

[1] This paper investigates the production of low-energy (few eV) electrons by relativistic runaway electron avalanches. This work is motivated by a growing body of literature that claims that runaway electron avalanches produce an anomalous growth of low-energy electrons and hence an anomalously large electrical conductivity, a factor of 50 larger than expected from standard calculations. Such large enhancements would have a substantial impact on properties of runaway electron avalanches and their observable effects. Indeed, these purportedly large conductivities have been used to argue that runaway electron avalanches result in a novel form of electrical breakdown called “runaway breakdown.” In this paper, we present simple analytical calculations, detailed Monte Carlo simulations, and a review of the experimental literature to show that no such anomalous growth of low-energy electron populations exists. Consequently, estimates of the conductivity generated by a runaway electron avalanche have been greatly exaggerated in many previous papers, drawing into question several of the claims about runaway breakdown.

The Radio Observatory on the Lunar Surface for Solar studies

The Radio Observatory on the Lunar Surface for Solar studies (ROLSS) is a concept for a near-side low radio frequency imaging interferometric array designed to study particle acceleration at the Sun and in the inner heliosphere. The prime science mission is to image the radio emission generated by Type II and III solar radio burst processes with the aim of determining the sites at and mechanisms by which the radiating particles are accelerated. Specific questions to be addressed include the following: (1) Isolating the sites of electron acceleration responsible for Type II and III solar radio bursts during coronal mass ejections (CMEs); and (2) Determining if and the mechanism(s) by which multiple, successive CMEs produce unusually efficient particle acceleration and intense radio emission. Secondary science goals include constraining the density of the lunar ionosphere by searching for a low radio frequency cutoff to solar radio emission and constraining the low energy electron population in astrophysical sources. Key design requirements on ROLSS include the operational frequency and angular resolution. The electron densities in the solar corona and inner heliosphere are such that the relevant emission occurs at frequencies below 10 MHz. Second, resolving the potential sites of particle acceleration requires an instrument with an angular resolution of at least 2°, equivalent to a linear array size of approximately 1000 m. Operations would consist of data acquisition during the lunar day, with regular data downlinks. No operations would occur during lunar night. ROLSS is envisioned as an interferometric array, because a single aperture would be impractically large. The major components of the ROLSS array are 3 antenna arms arranged in a Y shape, with a central electronics package (CEP) located at the center. The Y configuration for the antenna arms both allows for the formation of reasonably high dynamic range images on short time scales as well as relatively easy deployment. Each antenna arm is a linear strip of polyimide film (e.g., Kapton™) on which 16 science antennas are located by depositing a conductor (e.g., silver). The antenna arms can be rolled for transport, with deployment consisting of unrolling the rolls. Each science antenna is a single polarization dipole. The arms also contain transmission lines for carrying the radio signals from the science antennas to the CEP. The CEP itself houses the receivers for the science antennas, the command and data handling hardware, and, mounted externally, the downlink antenna. We have conducted two experiments relevant to the ROLSS concept. First, we deployed a proof-of-concept science antenna. Comparison of the impedance of the antenna feed points with simulations showed a high level of agreement, lending credence to the antenna concept. Second, we exposed a sample of space-qualified polyimide film, with a silver coating on one side, to temperature cycling and UV exposure designed to replicate a year on the lunar surface. No degradation of the polyimide film’s material or electric properties was found. Both of these tests support the notion of using polyimide-film based antennas. The prime science mission favors an equatorial site, and a site on the limb could simplify certain aspects of the instrument design. A site on the lunar near side is sufficient for meeting the science goals. While the site should be of relatively low relief topography, the entire site does not have to be flat as the fraction of the area occupied by the antenna arms is relatively small (∼0.3%). Further, the antenna arms do not have to lay flat as deviations of ±1 m are still small relative to the observational wavelengths. Deployment could be accomplished either with astronauts, completely robotically, or via a combination of crewed and robotic means. Future work for the ROLSS concept includes more exhaustive testing of the radio frequency (RF) and environmental suitability of polyimide film-based science antennas, ultra-low power electronics in order to minimize the amount of power storage needed, batteries with a larger temperature range for both survival and operation, and rovers (robotic, crewed, or both) for deployment. The ROLSS array could also serve as the precursor to a larger array on the far side of the Moon for astrophysical and cosmological studies.

HEATING THE HOT ATMOSPHERES OF GALAXY GROUPS AND CLUSTERS WITH CAVITIES: THE RELATIONSHIP BETWEEN JET POWER AND LOW-FREQUENCY RADIO EMISSION

We present scaling relations between jet power and radio power measured using the Giant Metrewave Radio Telescope (GMRT), Chandra, and XMM-Newton, for a sample of nine galaxy groups combined with the Bîrzan et al. sample of clusters. Cavity power is used as a proxy for mechanical jet power. Radio power is measured at 235 MHz and 1.4 GHz, and the integrated 10 MHz–10 GHz radio luminosity is estimated from the GMRT 610–235 MHz spectral index. The use of consistently analyzed, high-resolution low-frequency radio data from a single observatory makes the radio powers for the groups more reliable than those used by previous studies, and the combined sample covers 6–7 decades in radio power and 5 decades in cavity power. We find a relation of the form Pjet∝ L∼0.7radio for integrated radio luminosity, with a total scatter of σLrad = 0.63 and an intrinsic scatter of σi, Lrad = 0.59. A similar relation is found for 235 MHz power, but a slightly flatter relation with greater scatter is found for 1.4 GHz power, suggesting that low-frequency or broadband radio measurements are superior jet power indicators. We find our low-frequency relations to be in good agreement with previous observational results. Comparison with jet models shows reasonable agreement, which may be improved if radio sources have a significant low-energy electron population. We consider possible factors that could bias our results or render them more uncertain, and find that correcting for such factors in those groups we are able to study in detail leads to a flattening of the Pjet:Lradio relation.

Resonant scattering of plasma sheet electrons leading to diffuse auroral precipitation: 1. Evaluation for electrostatic electron cyclotron harmonic waves

[1] Using statistical wave power spectral profiles obtained from CRRES and the latitudinal distributions of wave propagation modeled by the HOTRAY code, a quantitative analysis has been performed on the scattering of plasma sheet electrons into the diffuse auroral zone by multiband electrostatic electron cyclotron harmonic (ECH) emissions near L = 6 within the 0000–0600 MLT sector. The results show that ECH wave scattering of plasma sheet electrons varies from near the strong diffusion rate (timescale of an hour or less) during active times with peak wave amplitudes of an order of 1 mV/m to very weak scattering (on the timescale of >1 day) during quiet conditions with typical wave amplitudes of tenths of mV/m. However, for the low-energy (∼100 eV to below 2 keV) electron population mainly associated with the diffuse auroral emission, ECH waves are only responsible for rapid pitch angle diffusion (occasionally near the limit of strong diffusion) for a small portion of the electron population with pitch angles αeq 70°. Computations of the bounce-averaged coefficients of momentum diffusion and (pitch angle, momentum) mixed diffusion indicate that both mixed diffusion and energy diffusion of plasma sheet electrons due to ECH waves are very small compared to pitch angle diffusion and that ECH waves have little effect on local electron acceleration. Consequently, the multiple harmonic ECH emissions cannot play a dominant role in the occurrence of diffuse auroral precipitation near L = 6, and other wave-particle interaction mechanisms, such as whistler mode chorus-driven resonant scattering, are required to explain the global distribution of diffuse auroral precipitation and the formation of the pancake distribution in the inner magnetosphere.

Electron capture dissociation in a linear radiofrequency‐free magnetic cell

Electron capture dissociation in a linear radiofrequency‐free magnetic cell

Effect of 13.56MHz low-frequency power on electrical characteristic of 60MHz radio-frequency capacitively coupled plasma

Dual-frequency capacitively coupled plasma (DF-CCP) driven by two frequencies of 60MHz and 13.56MHz was studied using compensated Langmuir probe technique. The change of EEPF with radial position and low-frequency input power was obtained. The experimental result shows that the input power of 13.56MHz mainly affects the population of low-energy electrons and the effect becomes greater when the gas pressure increases. The EEPF exhibits a structure of bi-energy-peak beyond the centre of discharge. The second energy peak drifts to the region of higher electron energy from the centre to the edge of electrode. The emergence of second energy peak could be attributed to the enhanced heating of middle-energy electrons. The electron density and electron temperature were calculated using EEPF. The plasma electron heating mechanism was discussed.

Formation of the delayed relativistic solar electrons

The existence of non-thermal electrons in the solar atmosphere and along the heliospheric field lines is deduced through emission of electromagnetic waves and via direct in situ measurements at 1 AU. The relation between the in situ electrons, their spectral shapes and the relative timing with respect to imaging and spectrographic observations are important in identifying potential acceleration sites and in understanding of the processes which control the formation of the delayed, (mildly) relativistic electrons which are observed in conjunction with flares or coronal mass ejections (CMEs). In contrast to previously suggested paradigms of acceleration of relativistic electrons around the CME shocks or at the flare site, it is suggested here that the delayed acceleration occurs along the stretched, closed coronal field lines, when an anisotropic seed population of low-energy electrons is injected in conjunction with the high frequency coronal radio bursts behind the large CME, as recorded by radioheliographs. The energization proceeds as a bootstrap process due to interaction with oblique whistler waves. The flare serves mainly as a time reference for the electromagnetic emissions, while the propagating CME subsequently opens an access for the relativistic electrons to the interplanetary medium.

Microdosimetric event distributions in sub-cellular volumes with a general purpose Monte Carlo code

The general purpose Monte Carlo code PENELOPE is used to calculate microdosimetric quantities including dose-weighted lineal energy spectra, which can be used to predict relative biological effect (RBE), for binary radiation therapies that utilise the photoabsorption of X-ray of high- Z materials. Spectra are calculated for Gd homogenously distributed at a concentration of 10 mg/ml in water and irradiated by 70 keV monoenergetic photons, around 20 keV above the k-edge of Gd (50.239 keV), which has been shown to give optimal dose enhancement, and for a metallic Gd surface in close proximity (within 2 μm) to a sensitive component of the nucleosome, modelled as a sphere of water of 1 μm diameter, for 60 and 70 keV monoenergetic X-rays. X-ray interactions with homogenously distributed Gd lead to a greater population of high lineal energy electrons than in liquid water, probably due to the creation of short range Auger electrons and photoelectrons, whereas interactions with Gd outside of the sensitive volume are longer ranged increasing the population of low lineal energy electrons. The data does not support increased therapeutic advantage through increased RBE in the case of Gd bearing contrast systems where little cellular absorption of Gd occurs. Homogenously distributed Gd leads to higher lineal energies than pure water, probably due to the creation of short range, high LET Auger and photoelectrons, whereas photoelectrons that originate in Gd that are outside the sensitive volume tend to have relatively higher energies and long ranges increasing the population of low LET electrons.

XMM-Newtonobservations of the hot-gas atmospheres of 3C 66B and 3C 449

We present new XMM-Newton observations of the hot-gas environments of two lowpower twin-jet radio galaxies, 3C 66B and 3C 449, showing direct evidence for the interactions between X-ray-emitting gas and radio plasma that are thought to determine the largescale radio structure of these sources. The temperatures th at we measure for the two environments are significantly higher than those predicted by st andard luminosity-temperature relations for clusters and groups. We show that luminosity-temperature relations for radioquiet and radio-loud X-ray groups differ, in the sense that r adio-source heating may operate in most groups containing radio galaxies. If the radio lobes are expanding subsonically, we find minimum ages of 3× 10 8 years for 3C 66B, and 5× 10 8 years for 3C 449, older than the values obtained from spectral ageing, which would give the radio source sufficient time to heat the groups to the observed temperatures for plausible values of the jet power. The external pressures in the atmospheres of both radio galaxies are an order of magnitude higher than equipartition estimates of their radio-lobe pressures, co nfirming that the radio lobes are either out of equipartition or require a pressure contribution fro m non-radiating particles. Constraints from the level of X-ray emission we measure from the radio lobes allow us to conclude that a departure from equipartition must be in the direction of magnetic domination, and that the most plausible candidates for a particle contribution to lo be pressure are relativistic protons, an additional population of low-energy electrons, or entrained and heated thermal material.

Variable circular polarization associated with relativistic ejections from GRS 1915 + 105

We report the discovery of variable circularly polarized radio emission associated with relativistic ejections from GRS 1915 + 105, based on observations with the Australia Telescope Compact Array (ATCA) and the Multi-Element Radio-Linked Interferometer Network (MERLIN). Following a radio flare in 2001 January, significant and variable circular polarization, at a fractional level of 0.2–0.4 per cent, was measured with ATCA at four frequencies between 1 and 9 GHz. Following an additional outburst 65 d later in 2001 March, further ATCA observations measured a comparable sign and level of circular polarization at two frequencies. At this second epoch, contemporaneous MERLIN observations directly imaged a relativistic ejection event and allowed us confidently to associate both the circularly and the linearly polarized emission with the relativistic ejecta, allowing a detailed measurement of the full polarization properties in the optically thin phase. The fractional circular polarization spectrum appears to flatten at higher frequencies in 2001 January, when there is strong evidence for multiple components at different optical depths. While we cannot conclusively distinguish between synchrotron or propagation-induced conversion as the origin of the circularly polarized component, we do not consider that coherent or birefringent scintillation mechanisms are likely. The implication is therefore that the ejections from GRS 1915 + 105 are associated with a significant population of low-energy electrons, with associated consequences for the energetics of relativistic ejection events. We briefly compare the data for SS 433 and GRS 1915 + 105 with those for active galactic nuclei, and note that linear-to-circular polarization ratios 1 at the higher frequencies, illustrating the role of Faraday depolarization. In addition, the 2001 January ATCA observations reveal a linear polarization ‘rotator’ event, probing the (variable) orientation of the magnetic field structure in the outflow.

The influence of the photosheath on the very low energy electrons during active potential control

A low and actively controlled electrostatic potential on the outer surface of a scientific spacecraft is very important for the accurate measurement of the cold plasma electrons and ions. A small potential barrier <2 eV may emerge in the photoelectron sheath during the active potential control operation using energetic (some keV) ion beams. Although the measurement conditions are improved considerably, some very low energy electrons of the ambient plasma cannot overcome the potential barrier and return to the plasma. It is designated as the screen effect of the photosheath. In this paper, the relationship between the “hidden” low energy electron population and the parameters of the photosheath potential barrier, such as the height (<2 eV) and position of the potential barrier, is analysed. The domains of the “hidden” and “detectable” ambient plasma electrons in the velocity space are presented.

Soft-electron background in x-ray telescopes using Wolter I grazing-incidence optics in near-Earth orbits

The background in x-ray telescopes in near-Earth orbits from low-energyelectrons (<100 keV) haslong been a sourceofconcern. Recent data on low-energy electron populations and improvements in the modeling of the propagation of low-energy electrons through Wolter I grazing- incidence optical systems now allow better quantitative estimates of the electron throughput of such systems. The modeling that has recently been done with respect to the U.K. soft x-ray telescope (Wide Field Camera) on ROSAT is described. The results from the modeling are then applied to several other missions using grazing-incidence optics, and the implications are discussed.

Geosynchronous environment for severe spacecraft charging

A survey of data from the first year of the P78-2 (SCATHA) satellite operations showed that severe spacecraft frame charging (fy) both in sunlight (-340 V) and in eclipse (> -8 kV) occurred on 24 April 1979. Analysis of the data indicates that if the sunlight charging environment had been present during eclipse, the vehicle would have charged in excess of 15 kV, which is above any known charging level observed to date for the SCATHA satellite. Therefore, the environment at the peak of the sunlight charging at -0650 UT 24 April 1979 was chosen for this worst case study. The environment at this time is characterized by an injection of high-energy (30-335 keV) electron fluxes whose combined current correlates with 0y with a correlation coefficient of 0.95. The fluxes were highly anisotropic, maximizing perpendicular to the magnetic field. The low-energy (<4 keV) electron population had a density <1 cm~ 3 and the low-energy ions were near background. The measured electron distribution functions, when fitted to double Maxwellians by a least-squares technique, show that throughout the sunlight charging period the high and low temperatures remained nearly the same, while the density of the high-energy component followed the charging levels. The injection occurred simultaneously with the rapid return of the magnetospheric magnetic field to a more dipolelike configuration.

Polar 5—an electron accelerator experiment within an aurora. 4. Measurements of the 391.4 nm light produced by an artificial electron beam in the upper atmosphere

The POLAR 5 sounding rocket, launched from Andøya, Norway, on February 1, 1976, was of the “mother-daughter” configuration. A rocket-borne electron accelerator, mounted on the “daughter,” produced a pulsed electron beam with a maximum current of 130 mA and electron energies up to 10 kev. Using a photometer the luminescence at 391.4nm produced by electrons colliding with ambient nitrogen molecules was studied. The observed light at 391.4 nm consisted of low background, with occasional flashes due to the natural auroral excitations, and intense sparkles when the electron beam was emitted. Below 130 km the light observed during beam injection can be explained by excitations of ambient N 2 due to high energy beam electrons. In the altitude range from 150 km to apogee at 220 km, the observed light level during beam emission is fairly constant and much larger than that produced by the high energy beam electrons. A possible source of this light is the excitation of ambient N 2 by an enhanced population of low energy electrons, created by the presence of a beam plasma discharge in the vicinity of the “daughter” payload.

POLAR 5—an electron accelerator experiment within an aurora. 2. Scattering of an artificially produced electron beam in the atmosphere

The interaction between an artificially produced narrow beam of electrons and the upper atmosphere has been studied by the POLAR 5 electron accelerator “mother”-“daughter” rocket. It is shown how the beam develops a “halo” of scattered electrons and how the low energy electron population in this “halo” is produced partly during the ionization process (at low altitudes), partly by a “wave-plasma” interaction which accelerates the ionospheric background electrons.

Multifarious temporal variations of low-energy relativistic cosmic-ray electrons

Detailed examination of the intensity variations of 3- to 12-MeV interplanetary electrons. The data are from the Goddard cosmic-ray experiment on the Imp satellites and cover the period from just before the last solar minimum through the onset of the present solar maximum (i.e., from December 1963 through August 1969). A morphology for the intensity changes is tentatively proposed that includes solar-flare-associated events, solar co-rotating increases, Forbush decreases, quiet-time increases, and the long-term 11-year variation. It is contended that the electron components observed both during quiescent times and during quiet-time increases are galactic in origin. The quiet-time increases represent a completely new phenomenon that appears to be unique to the low-energy electron population. During a quiet-time increase the electron intensity is enhanced by a factor of 3 to 5 over a period of days, and, in general, these periods anticorrelate with low-energy solar particle events. Qualitatively, their amplitude diminishes with increasing solar activity.

A survey of low-energy electrons in the evening sector of the magnetosphere with OGO 1 and OGO 3

Observations of electrons of energy 125 ev to ∼2 kev with the OGO 1 satellite and of electrons of energy 40 ev to ∼2 kev with OGO 3 (by means of modulated Faraday cup detectors) are used to investigate the low-energy electron population in the magnetosphere within the local-time range ∼17 to ∼22 hours. Intense fluxes of these electrons are confined to a spatial region, termed the plasma sheet, which is an extension of the magnetotail plasma sheet discovered by the Vela satellites and is identified with the soft electron band first detected by Gringauz. The plasma sheet extends over the entire local-time range studied in this investigation, from the magnetospheric tail past the dusk meridian toward the dayside magnetosphere. In latitude it is confined to within 4–6 RE of the geomagnetic and/or solar magnetospheric equatorial plane, in agreement with observations already reported from the Vela satellites; no electron fluxes are detected high above the equator, not even very near the magnetopause. In radial distance the plasma sheet is terminated by the magnetopause on the outside and by a well-defined sharp inner boundary on the inside. The inner boundary has been traced from the equatorial region to the highest latitudes investigated, ∼40°; during geomagnetically quiet periods, it is observed at an equatorial distance of 11 ± 1 RE and appears to extend to higher latitudes along magnetic field lines. Weak or no electron fluxes are found between the inner boundary of the plasma sheet and the outer boundary of the plasmasphere. Detection (by an indirect process) of the very high ion densities within the plasmasphere gives positions for its boundary in good agreement with other determinations. During periods of magnetic bay activity, the plasma sheet extends closer to the earth; the inner boundary of the plasma sheet is then found at equatorial distances of 6–8 RE. This is most simply interpreted as the result of an actual inward motion of the plasma during a bay. In one case, it was possible to associate the beginning of this motion with the onset of the bay and to estimate an average radial speed of ∼12 km/sec, from which an electric field corresponding to ∼48 kilovolts across the magnetospheric tail was inferred. Within the plasma sheet, the electron population is characterized by number densities from 0.3 to 30 cm−3 and mean energies from 50 to 1600 ev and higher, with a strong anticorrelation between density and mean energy, so that the electron energy density (∼1 kev cm−3) and energy flux (∼3 ergs cm−2 sec−1) show relatively little variation. The lower energies and higher densities tend to occur during periods of geomagnetic disturbance. The nonobservation of electrons in regions above the plasma sheet implies an upper limit on the electron number density of 5 × 10−2 cm−3 if their mean energy is assumed to be ∼50 ev (typical of the magnetosheath) and 10−2 cm−3 if the energy is ∼1 kev (typical of the plasma sheet). At the inner boundary of the plasma sheet there is a sharp softening of the electron spectrum with decreasing radial distance but apparently little change in the electron number density. The electron energy density decreases across the inner boundary roughly as ∼exp (distance/0.4 RE) during quiet periods; during times of magnetic bay activity the energy density decreases as ∼exp (distance/0.6 RE), and there is a more complicated spatial structure of density and mean energy.