236 research outputs found
An alternative to the plasma emission model: Particle-In-Cell, self-consistent electromagnetic wave emission simulations of solar type III radio bursts
1.5D PIC, relativistic, fully electromagnetic (EM) simulations are used to
model EM wave emission generation in the context of solar type III radio
bursts. The model studies generation of EM waves by a super-thermal, hot beam
of electrons injected into a plasma thread that contains uniform longitudinal
magnetic field and a parabolic density gradient. In effect, a single magnetic
line connecting Sun to earth is considered, for which several cases are
studied. (i) We find that the physical system without a beam is stable and only
low amplitude level EM drift waves (noise) are excited. (ii) The beam injection
direction is controlled by setting either longitudinal or oblique electron
initial drift speed, i.e. by setting the beam pitch angle. In the case of zero
pitch angle, the beam excites only electrostatic, standing waves, oscillating
at plasma frequency, in the beam injection spatial location, and only low level
EM drift wave noise is also generated. (iii) In the case of oblique beam pitch
angles, again electrostatic waves with same properties are excited. However,
now the beam also generates EM waves with the properties commensurate to type
III radio bursts. The latter is evidenced by the wavelet analysis of transverse
electric field component, which shows that as the beam moves to the regions of
lower density, frequency of the EM waves drops accordingly. (iv) When the
density gradient is removed, electron beam with an oblique pitch angle still
generates the EM radiation. However, in the latter case no frequency decrease
is seen. Within the limitations of the model, the study presents the first
attempt to produce simulated dynamical spectrum of type III radio bursts in
fully kinetic plasma model. The latter is based on 1.5D non-zero pitch angle
(non-gyrotropic) electron beam, that is an alternative to the plasma emission
classical mechanism.Comment: Physics of Plasmas, in press, May 2011 issue (final accepted version
Statistical Survey of Type III Radio Bursts at Long Wavelengths Observed by the Solar TErrestrial RElations Observatory (STEREO)/Waves Instruments: Radio Flux Density Variations with Frequency
We have performed a statistical study of Type III radio bursts observed
by Solar TErrestrial RElations Observatory (STEREO)/Waves between May 2007 and
February 2013. We have investigated the flux density between kHz and
MHz. Both high- and low-frequency cutoffs have been observed in of
events suggesting an important role of propagation. As already reported by
previous authors, we observed that the maximum flux density occurs at MHz on
both spacecraft. We have developed a simplified analytical model of the flux
density as a function of radial distance and compared it to the STEREO/Waves
data.Comment: published in Solar Physic
First-order thermal correction to the quadratic response tensor and rate for second harmonic plasma emission
Three-wave interactions in plasmas are described, in the framework of kinetic
theory, by the quadratic response tensor (QRT). The cold-plasma QRT is a common
approximation for interactions between three fast waves. Here, the first-order
thermal correction (FOTC) to the cold-plasma QRT is derived for interactions
between three fast waves in a warm unmagnetized collisionless plasma, whose
particles have an arbitrary isotropic distribution function. The FOTC to the
cold-plasma QRT is shown to depend on the second moment of the distribution
function, the phase speeds of the waves, and the interaction geometry. Previous
calculations of the rate for second harmonic plasma emission (via Langmuir-wave
coalescence) assume the cold-plasma QRT. The FOTC to the cold-plasma QRT is
used here to calculate the FOTC to the second harmonic emission rate, and its
importance is assessed in various physical situations. The FOTC significantly
increases the rate when the ratio of the Langmuir phase speed to the electron
thermal speed is less than about 3.Comment: 11 pages, 2 figures, submitted to Physics of Plasma
Energetic particles in solar flares. Chapter 4 in the proceedings of the 2nd Skylab Workshop on Solar Flares
The recent direct observational evidence for the acceleration of particles in solar flares, i.e. radio emission, bremsstrahlung X-ray emission, gamma-ray line and continuum emission, as well as direct observations of energetic electrons and ions, are discussed and intercorrelated. At least two distinct phases of acceleration of solar particles exist that can be distinguished in terms of temporal behavior, type and energy of particles accelerated and the acceleration mechanism. Bulk energization seems the likely acceleration mechanism for the first phase while Fermi mechanism is a viable candidate for the second one
Working time flexibility components and working time regimes in Europe: using company-level data across 21 countries
Working time ?exibility comprises a wide variety of arrangements, from part-time,
overtime, to long-term leaves. Theoretical approaches to grouping these arrangements
have been developed, but empirical underpinnings are rare. This article investigates
the bundles that can be found for various ?exible working time arrangements, using the
Establishment Survey on Working Time and Work–Life Balance, 2004/2005, covering 21
EU member states and 13 industries. The results from the factor analyses con?rmed that
working time arrangements can be grouped into two bundles, one for the employee-centred
arrangements and second for the employer-centred arrangements, and that these two
bundles are separate dimensions.Wealso tested the stability of the factor analysisoutcome,
showing that although we ?nd some deviations from the pan-Europe and pan-industry
outcome, the naming of the components as ?exibility for employees and ?exibility for
employers can be considered rather stable. Lastly, we ?nd three country clusters for the 21
European countries using the bundle approach. The ?rst group includes the Northern
European countries along side Poland and Czech Republic, the second group the
continental European countries with UK and Ireland, and lastly, the southern European
countries with Hungary and Slovenia
Does shade improve light interception efficiency? A comparison among seedlings from shade-tolerant and -intolerant temperate deciduous tree species
• Here, we tested two hypotheses: shading increases light interception efficiency (LIE) of broadleaved tree seedlings, and shade-tolerant species exhibit larger LIEs than do shade-intolerant ones. The impact of seedling size was taken into account to detect potential size-independent effects on LIE. LIE was defined as the ratio of mean light intercepted by leaves to light intercepted by a horizontal surface of equal area. • Seedlings from five species differing in shade tolerance (Acer saccharum, Betula alleghaniensis, A. pseudoplatanus, B. pendula, Fagus sylvatica) were grown under neutral shading nets providing 36, 16 and 4% of external irradiance. Seedlings (1- and 2-year-old) were three-dimensionally digitized, allowing calculation of LIE. • Shading induced dramatic reduction in total leaf area, which was lowest in shade-tolerant species in all irradiance regimes. Irradiance reduced LIE through increasing leaf overlap with increasing leaf area. There was very little evidence of significant size-independent plasticity of LIE. • No relationship was found between the known shade tolerance of species and LIE at equivalent size and irradiance
Spectrum of Solar Type I Continuum Noise Storm in the 50 - 80 MHz band, and Plasma characteristics in the associated source region
Continuum observations of a solar noise storm in the frequency range of 50 -
80 MHz observed with the Gauribidanur radio spectrograph during 2000 September,
26 & 27, are presented here. The radio spectral index of the noise storm
continuum in the band 50 - 80 MHz is found to be ~3.65 during the above period.
The Noise Storm continuum radiation is explained as a consequence of the
non-thermal, plasma emission mechanism. The beam-density of suprathermal
electrons is estimated for the coronal plasma near the source region of storm
radiation. Supplementary evidence for the density-estimate is provided by way
of analysing the imaging data from the SXT on-board the Yohkoh spacecraft, and
the LASCO, MDI, and EIT on board the SoHO spacecraft.Comment: 43 pages; 5 tables; 15 figures (9 color). ApJ (Part I : accepted
A powerful bursting radio source towards the Galactic Centre
Transient astronomical sources are typically powered by compact objects and
usually signify highly explosive or dynamic events. While radio astronomy has
an impressive record of obtaining high time resolution observations, usually it
is achieved in quite narrow fields-of-view. Consequently, the dynamic radio sky
is poorly sampled, in contrast to the situation in the X- and gamma-ray bands
in which wide-field instruments routinely detect transient sources. Here we
report a new transient source, GCRT J1745-3009, detected in 2002 during a
moderately wide-field radio transient monitoring program of the Galactic center
(GC) region at 0.33 GHz. The characteristics of its bursts are unlike those
known for any other class of radio transient. If located in or near the GC, its
brightness temperature (~10^16 K) and the implied energy density within GCRT
J1745-3009 vastly exceeds that observed in most other classes of radio
astronomical sources, and is consistent with coherent emission processes rarely
observed. We conclude that GCRT J1745-3009 is the first member of a new class
of radio transient sources, the first of possibly many new classes to be
identified through current and upcoming radio surveys.Comment: 16 pages including 3 figures. Appears in Nature, 3 March 200
Strength of Coronal Mass Ejection-Driven Shocks Near the Sun, and Its Importance in Predicting Solar Energetic Particle Events
Coronal shocks are important structures, but there are no direct observations
of them in solar and space physics. The strength of shocks plays a key role in
shock-related phenomena, such as radio bursts and solar energetic particle
(SEP) generation. This paper presents an improved method of calculating Alfven
speed and shock strength near the Sun. This method is based on using as many
observations as possible, rather than one-dimensional global models. Two
events, a relatively slow CME on 2001 September 15 and a very fast CME on 2000
June 15, are selected to illustrate the calculation process. The calculation
results suggest that the slow CME drove a strong shock, with Mach number of
3.43 - 4.18, while the fast CME drove a relatively weak shock, with Mach number
of 1.90 - 3.21. This is consistent with the radio observations, which find a
stronger and longer decameter-hectometric (DH) type II radio burst during the
first event, and a short DH type II radio burst during the second event. In
particular, the alculation results explain the observational fact that the slow
CME produced a major solar energetic particle (SEP) event, while the fast CME
did not. Through a comparison of the two events, the importance of shock
strength in predicting SEP events is addressed
Morphology and density of post-CME current sheets
Eruption of a coronal mass ejection (CME) drags and "opens" the coronal
magnetic field, presumably leading to the formation of a large-scale current
sheet and the field relaxation by magnetic reconnection. We analyze physical
characteristics of ray-like coronal features formed in the aftermath of CMEs,
to check if the interpretation of this phenomenon in terms of reconnecting
current sheet is consistent with the observations. The study is focused on
measurements of the ray width, density excess, and coronal velocity field as a
function of the radial distance. The morphology of rays indicates that they
occur as a consequence of Petschek-like reconnection in the large scale current
sheet formed in the wake of CME. The hypothesis is supported by the flow
pattern, often showing outflows along the ray, and sometimes also inflows into
the ray. The inferred inflow velocities range from 3 to 30 km s,
consistent with the narrow opening-angle of rays, adding up to a few degrees.
The density of rays is an order of magnitude larger than in the ambient corona.
The density-excess measurements are compared with the results of the analytical
model in which the Petschek-like reconnection geometry is applied to the
vertical current sheet, taking into account the decrease of the external
coronal density and magnetic field with height. The model results are
consistent with the observations, revealing that the main cause of the density
excess in rays is a transport of the dense plasma from lower to larger heights
by the reconnection outflow
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