Radiative properties of a plasma moving across a magnetic field. I: Theoretical analysis

The early‐time evolution of plasmas moving across a background magnetic field is addressed with a two‐dimensional model in which a plasma cloud is assumed to have formed instantaneously with a velocity across a uniform background magnetic field and with a Gaussian density profile in the two dimensions perpendicular to the direction of motion. This model treats both the dynamics associated with the formation of a polarization field and the generation and propagation of electromagnetic waves. In general, the results indicate that, to zeroth order, the plasma cloud behaves like a large dipole antenna oriented in the direction of the polarization field which oscillates at frequencies defined by the normal mode of the system. The magnitude of the radiation field and the amount of plasma momentum and energy carried away by and stored instantaneously in the fields are discussed only qualitatively in this paper, quantitative results for specific cloud parameters and scaling laws for the magnitude of the fields and the slowing down of the plasma cloud are presented in a companion manuscript.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70196/2/PFBPEI-5-4-1289-1.pd

Radiative properties of a plasma moving across a magnetic field. II: Numerical results

A theoretical analysis developed in a companion paper to treat the early‐time evolution of plasmas moving across a background magnetic field is applied to the modeling of low‐beta, barium chemical releases in the magnetosphere. The results indicate that radiation damping plays an important role in defining the plasma cloud evolution, causing a rapid decay of the polarization field and a loss of plasma kinetic energy and momentum on time scales comparable to several ion gyroperiods. The radiation spectrum consists of a burst of chirped, high‐frequency (in the range of the cloud plasma frequencies) waves, followed by a pulse of whistler waves, and subsequently by ion cyclotron emission. Scaling laws are derived for the plasma momentum and energy loss rates and predictions for the braking time, the amplitude and spectrum of the radiation field, and the total radiated power are presented for conditions relevant to the recent Combined Release and Radiation Effects Satellite (CRRES) experiments [Phys. Fluids B 4, 2249 (1992)].Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70589/2/PFBPEI-5-4-1306-1.pd

Impacts of a power-law non-thermal electron tail on the ionization and recombination rates

We have investigated the effects of a non-thermal electron population on the ionization and recombination rates. The considered electron distribution is defined as a Maxwellian function below a break energy E_b and as a power-law function of index alpha above this energy. We have calculated the collisional (direct and excitation autoionization) ionization coefficient rates as well as the (radiative and dielectronic) recombination rates. Practical methods are given to calculate these rates in order to be easily included in a computer code. The ionization rates are very sensitive to the non-thermal electron population and can be increased by several orders of magnitude depending on the temperature and parameters of the power-law function (E_b and alpha). The non-thermal electrons have a much weaker effect on the (radiative and dielectronic) recombination rates. We have determined the mean electric charge of elements C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe and Ni for different values of the break energy and power-law index. The ionization balance is affected significantly, whereas the effect is smaller in ionizing plasmas.Comment: 16 pages, 19 figures, accepted for publication in A&

NUV/Blue spectral observations of sprites in the 320-460 nm region: N2{\mathrm N_2} (2PG) Emissions

A near-ultraviolet (NUV) spectrograph (320-460 nm) was flown on the EXL98 aircraft sprite observation campaign during July 1998. In this wavelength range video rate (60 fields/sec) spectrographic observations found the NUV/blue emissions to be predominantly N2 (2PG). The negligible level of N2+ (1NG) present in the spectrum is confirmed by observations of a co-aligned, narrowly filtered 427.8 nm imager and is in agreement with previous ground-based filtered photometer observations. The synthetic spectral fit to the observations indicates a characteristic energy of ~1.8 eV, in agreement with our other NUV observations.Comment: 7 pages, 2 figures, 1 table, JGR Space Physics "Effects of Thunderstorms and Lightning in the Upper Atmosphere" Special Sectio

Long-duration gamma-ray emissions from 2007 and 2008 winter thunderstorms

The Gamma-Ray Observation of Winter THunderclouds (GROWTH) experiment, consisting of two radiation-detection subsystems, has been operating since 2006 on the premises of Kashiwazaki-Kariwa nuclear power plant located at the coastal area of Japan Sea. By 2010 February, GROWTH detected 7 long-duration $\gamma$-rays emissions associated with winter thunderstorms. Of them, two events, obtained on 2007 December 13 and 2008 December 25, are reported.On both occasions, all inorganic scintillators (NaI, CsI, and BGO) of the two subsystems detected significant gamma-ray signals lasting for >1 minute. Neither of these two events were associated with any lightning. In both cases, the gamma-ray energy spectra extend to 10 MeV, suggesting that the detected gamma-rays are produced by relativistic electrons via bremsstrahlung. Assuming that the initial photon spectrum at the source is expressed by a power-law function,the observed photons can be interpreted as being radiated from a source located at a distance of 290-560 m for the 2007 event and 110-690 m for the 2008 one, both at 90% confidence level.Employing these photon spectra, the number of relativistic electrons is estimated as 10^9 - 10^{11}. The estimation generally agrees with those calculated based on the relativistic runaway electron avalanche model. A GROWTH photon spectrum, summed over 3 individual events including the present two events and another reported previously, has similar features including a cut-off energy, to an averaged spectrum of terrestrial gamma-ray flashes.Comment: 46 pages, 13 figures, accepted for publication in JGR-Atmospher

FORTE satellite constraints on ultra-high energy cosmic particle fluxes

The FORTE (Fast On-orbit Recording of Transient Events) satellite records bursts of electromagnetic waves arising from near the Earth's surface in the radio frequency (RF) range of 30 to 300 MHz with a dual polarization antenna. We investigate the possible RF signature of ultra-high energy cosmic-ray particles in the form of coherent Cherenkov radiation from cascades in ice. We calculate the sensitivity of the FORTE satellite to ultra-high energy (UHE) neutrino fluxes at different energies beyond the Greisen-Zatsepin-Kuzmin (GZK) cutoff. Some constraints on supersymmetry model parameters are also estimated due to the limits that FORTE sets on the UHE neutralino flux. The FORTE database consists of over 4 million recorded events to date, including in principle some events associated with UHE neutrinos. We search for candidate FORTE events in the period from September 1997 to December 1999. The candidate production mechanism is via coherent VHF radiation from a UHE neutrino shower in the Greenland ice sheet. We demonstrate a high efficiency for selection against lightning and anthropogenic backgrounds. A single candidate out of several thousand raw triggers survives all cuts, and we set limits on the corresponding particle fluxes assuming this event represents our background level.Comment: added a table, updated references and Figure 8, this version is submitted to Phys. Rev.

HST/STIS High Resolution Echelle Spectra of alpha Centauri A (G2 V)

We describe and analyze HST/STIS observations of the G2 V star alpha Centauri A (alpha Cen A, HD 128620), a star similar to the Sun. The high resolution echelle spectra obtained with the E140H and E230H gratings cover the complete spectral range 1133-3150 Angstrom with a resolution of 2.6 km/s, an absolute flux calibration accurate to +/-5%, and an absolute wavelength accuracy of 0.6-1.3 km/s. We present here a study of the E140H spectrum covering the 1140-1670 Angstrom spectral range, which includes 671 emission lines representing 37 different ions and the molecules CO and H_2. For alpha Cen A and the quiet and active Sun, we intercompare the redshifts, nonthermal line widths, and parameters of two Gaussian representations of transition region lines (e.g., Si IV, C IV), infer the electron density from the O IV intersystem lines, and compare their differential emission measure distributions. One purpose of this study is to compare the alpha Cen A and solar UV spectra to determine how the atmosphere and heating processes in alpha Cen A differ from the Sun as a result of the small differences in gravity, age, and chemical composition of the two stars. A second purpose is to provide an excellent high resolution UV spectrum of a solar-like star that can serve as a proxy for the Sun observed as a point source when comparing other stars to the Sun.Comment: A&A in pres

A study of the possibility of sprites in the atmospheres of other planets

Sprites are a spectacular type of transient luminous events (TLE) which occur above thunderstorms immediately after lightning. They have shapes of giant jellyfish, carrots or columns and last tens of milliseconds. In Earth's atmosphere, sprites mostly emit in red and blue wavelengths from excited N2 and N2+ and span a vertical range between 50 and 90 km above the surface. The emission spectra, morphology and occurrence heights of sprites reflect the properties of the planetary atmosphere they inhabit and are related to the intensity of the initiating parent lightning.. This paper presents results of theoretical calculations of the expected occurrence heights of sprites above lightning discharges in the CO2 atmosphere of Venus, the N2 atmosphere of Titan and the H2-He atmosphere of Jupiter. The expected emission features are presented and the potential of detecting sprites in planetary atmospheres by orbiting spacecraft is discussed.Comment: 26 pages, 4 figures, to appear in J. Geophys. Res. - Planet

Optically perceptible characteristics of sprites observed in Central Europe in 2007–2009

Sprites are luminous optical emissions accompanying electric discharges in the mesosphere. 489 sprite events have been observed with a TV frame rate video system in Central Europe from Sopron (47.68 °N, 16.58 °E, ~230m MSL), Hungary between 2007 and 2009. Characteristic sprite forms, i.e. column, wishbone, tree, angel, and carrot have been identified in the set of records. Characteristic morphological properties corresponding to each type are given; earlier definitions and observations as well as the related theoretical considerations are reviewed. Based on the knowledge and experience from high-speed imaging in sprite observations, probable time sequences of streamer propagation directions were associated with the characteristic sprite types. It is suggested that different streamer propagation sequences corresponding to different dynamic processes may result in similar sprite forms. Several occasionally detectable sprite features are noted and described: tendrils, glows, puffs, beads, and spots. Spots are distinguished from the similar beads by their characteristic brightness, size, and location relative to the bright body of the sprite. The events observed in Central Europe have been classified by the number of individual sprites and by the variety of types appearing in them. More than 90% of the recorded sprites were found to occur in clusters rather than alone, and more than half of the sprite clusters contained more than one sprite types. Jellyfish and dancing sprite events are described as being special subsets of sprite clusters. Statistical analysis of the occurrences of morphological types, various sprite features, and event durations indicated that jellyfish sprites and clusters of column sprites with glows and tendrils do not tend to have long optical lifetimes. Sprite events with more morphological types, on the other hand, more likely have extended durations. The maximum of the encountered event duration was lower for events with many sprite elements. Observed rates of glows and puffs may refer to the occupied height range of sprites. The importance of understanding the driving factors behind the development of various sprite types and sprite features is emphasized and some topics are suggested for further investigation