322 research outputs found
A decade of solar Type III radio bursts observed by the Nancay Radioheliograph 1998-2008
We present a statistical survey of almost 10 000 radio Type III bursts
observed by the Nancay Radioheliograph from 1998 to 2008, covering nearly a
full solar cycle. In particular, sources sizes, positions, and fluxes were
examined. We find an east-west asymmetry in source positions which could be
attributed to a 6(+/-)1 degree eastward tilt of the magnetic field, that source
FWHM sizes s roughly follow a solar-cycle averaged distribution dN/ds = 14
{\nu}^{-3.3} s^{-4} arcmin^{-1} day^{-1}, and that source fluxes closely follow
a solar-cycle averaged dN/dS_{\nu} = 0.34 {\nu}^{-2.9} S_{\nu}^{-1.7} sfu^{-1}
day^{-1} distribution (when {\nu} is in GHz, s in arcmin, and S_{\nu} in sfu).
Fitting a barometric density profile yields a temperature of 0.6 MK, while a
solar wind-like (\propto h^{-2}) density profile yields a density of 1.2x10^6
cm^{-3} at an altitude of 1 RS, assuming harmonic emission. Finally, we found
that the solar-cycle averaged radiated Type III energy could be similar in
magnitude to that radiated by nanoflares via non-thermal bremsstrahlung
processes, and we hint at the possibility that escaping electron beams might
carry as much energy away from the corona as is introduced into it by
accelerated nanoflare electrons.Comment: 22 pages, 18 figure
Understanding CME and associated shock in the solar corona by merging multi wavelengths observation
Using multi-wavelength imaging observations, in EUV, white light and radio,
and radio spectral data over a large frequency range, we analyzed the
triggering and development of a complex eruptive event. This one includes two
components, an eruptive jet and a CME which interact during more than 30 min,
and can be considered as physically linked. This was an unusual event. The jet
is generated above a typical complex magnetic configuration which has been
investigated in many former studies related to the build-up of eruptive jets;
this configuration includes fan-field lines originating from a corona null
point above a parasitic polarity, which is embedded in one polarity region of
large Active Region (AR). The initiation and development of the CME, observed
first in EUV, does not show usual signatures. In this case, the eruptive jet is
the main actor of this event. The CME appears first as a simple loop system
which becomes destabilized by magnetic reconnection between the outer part of
the jet and the ambient medium. The progression of the CME is closely
associated with the occurrence of two successive types II bursts from distinct
origin. An important part of this study is the first radio type II burst for
which the joint spectral and imaging observations allowed: i) to follow, step
by step, the evolution of the spectrum and of the trajectory of the radio
burst, in relationship with the CME evolution; ii) to obtain, without
introducing an electronic density model, the B-field and the Alfven speed.Comment: 17 pages, 13 figure
Further constraints on electron acceleration in solar noise storms
We reexamine the energetics of nonthermal electron acceleration in solar
noise storms. A new result is obtained for the minimum nonthermal electron
number density required to produce a Langmuir wave population of sufficient
intensity to power the noise storm emission. We combine this constraint with
the stochastic electron acceleration formalism developed by Subramanian &
Becker (2005) to derive a rigorous estimate for the efficiency of the overall
noise storm emission process, beginning with nonthermal electron acceleration
and culminating in the observed radiation. We also calculate separate
efficiencies for the electron acceleration -- Langmuir wave generation stage
and the Langmuir wave -- noise storm production stage. In addition, we obtain a
new theoretical estimate for the energy density of the Langmuir waves in noise
storm continuum sources.Comment: Accepted for publication in Solar Physic
The effects of forest degradation on soil carbon dynamics in the tropics
Plant-soil interactions and soil carbon dynamics are an essential part of soil function. Land-use change can affect the soil’s ability to accumulate and store carbon. Deforestation and conversion to croplands has decreased tree species cover and diversity in the tropics resulting in degraded and secondary forests becoming the dominant forested habitat. Understanding the effects of forest degradation on soil carbon dynamics is vital if we are to remediate these ecosystems under climate change. The overarching aim of this thesis was understanding how changes in tree and plant species composition at different levels of degradation affect soil carbon dynamics and litter decomposition in the old and neo-tropics using litter transplant experiments in the field. Malaysia is one of the two biggest producers of palm oil in the world with Indonesia. Borneo is a biodiversity hotspot, but this ecosystem is decreasing at an alarming rate. Sabah, in norther Malaysian Borneo is converting its tropical forest to oil palm plantations resulting in vast expanses of oil palm monocultures containing secondary forest fragments at various degrees of degradation. In Central America, the tree cover is also dominated by secondary forests and timber plantations; in Panama, only 21 % of the tree cover classified as intact forest and there are remediation projects in place to encourage reforestation of degraded landscapes into plantations using native timber species. The level of degradation in the habitats changed the microclimate which affected soil properties, microbial activity and litter decomposition. Litter properties also had an effect on the rate of litter decomposition and microbial activity. The loss of tree cover in the deforested habitats and monocultures resulted in lower microbial activity and decomposition rates whereas the secondary forests has similar microbial activity and decomposition rates as the old growth forests. Overall, my results suggest that mixed litter inputs are crucial for maintaining soil function and that tropical forest soils might be more resilient to change than expected
Combining visibilities from the Giant Meterwave Radio Telescope and the Nancay Radio Heliograph: High dynamic range snapshot images of the solar corona at 327 MHz
We report first results from an ongoing program of combining visibilities
from the Giant Meterwave Radio Telescope (GMRT) and the Nancay Radio Heliograph
(NRH) to produce composite snapshot images of the sun at meter wavelengths. We
describe the data processing, including a specific multi-scale CLEAN algorithm.
We present results of a) simulations for two models of the sun at 327 MHz, with
differing complexity b) observations of a complex noise storm on the sun at 327
MHz on Aug 27 2002. Our results illustrate the capacity of this method to
produce high dynamic range snapshot images when the solar corona has structures
with scales ranging from the image resolution of 49" to the size of the whole
sun.
We find that we cannot obtain reliable snapshot images for complex objects
when the visibilities are sparsely sampled.Comment: Accepted for publication in Astronomy & Astrophysics. Version with
high resolution figures available from
ftp://ftp.iucaa.ernet.in/in.coming/gmrtnr
Théâtre et Pouvoir à Brest au XVIIIe siècle
Lorsque la ville de Brest naît véritablement dans la seconde moitié du XVIIe siècle, l’activité théâtrale y est fortement déconsidérée, marginalisée, presque illicite. Les autorités du port la considèrent tout d’abord avec crainte, comme une occasion pour leurs hommes de se divertir, avant de l’envisager désormais comme un mal nécessaire, capable d’en éviter de pires. La fondation du théâtre de la marine, première salle bâtie en Bretagne, assure ainsi la légitimité et la régularité de la pratique de cet art au cœur de la cité. Mais c’est finalement l’institutionnalisation qui accentue les tensions. Dans les dernières années de l’Ancien Régime, la salle brestoise est toujours le lieu de représentation du pouvoir, mais cette fois, ce pouvoir s’est inversé. Si l’art dramatique alarme les autorités de la marine à la fin du XVIIe siècle, malgré leurs tentatives de supervision, il les tourmente tout autant cent ans plus tard.When the city of Brest was really born in the second half of the seventeenth century, all theatrical activities were strongly discredited, marginalized, almost illegal. At first, port authorities apprehensively considered them an opportunity for their employees to be entertained, then as a necessary evil in order to avoid worse ones. The founding of the Maritime Theatre, first ever built in Brittany, thus ensured the legitimacy and regularity of the practice of the theatrical arts in the heart of the city. However, the institutionalization of the system actually emphasized tensions. In the last years of the Ancien Régime, the Brest theatre was still the place where power was performed, though that power had changed sides. While at the end of the seventeenth century theatre alarmed maritime authorities despite their attempts to keep it under close supervision, it was still tormenting them a hundred years later
Space storm measurements of the July 2005 solar extreme events from the low corona to the Earth
The Athens Neutron Monitor Data Processing (ANMODAP) Center recorded an
unusual Forbush decrease with a sharp enhancement of cosmic ray intensity right
after the main phase of the Forbush decrease on 16 July 2005, followed by a
second decrease within less than 12 h. This exceptional event is neither a
ground level enhancement nor a geomagnetic effect in cosmic rays. It rather
appears as the effect of a special structure of interplanetary disturbances
originating from a group of coronal mass ejections (CMEs) in the 13-14 July
2005 period. The initiation of the CMEs was accompanied by type IV radio bursts
and intense solar flares (SFs) on the west solar limb (AR 786); this group of
energetic phenomena appears under the label of Solar Extreme Events of July
2005. We study the characteristics of these events using combined data from
Earth (the ARTEMIS IV radioheliograph, the Athens Neutron Monitor (ANMODAP)),
space (WIND/WAVES) and data archives. We propose an interpretation of the
unusual Forbush profile in terms of a magnetic structure and a succession of
interplanetary shocks interacting with the magnetosphere.Comment: Advances in Space Research, Volume 43, Issue 4, p. 600-60
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