55 research outputs found
Observation of galactic cosmic ray spallation events from the SoHO mission 20-Year operation of LASCO
A shower of secondary Cosmic Ray (CR) particles is produced at high altitudes
in the Earth's atmosphere, so the primordial Galactic Cosmic Rays (GCRs) are
never directly measured outside the Earth magnetosphere and atmosphere. They
approach the Earth and other planets in the complex pattern of rigidity's
dependence, generally excluded by the magnetosphere. GCRs revealed by images of
single nuclear reactions also called spallation events are described here. Such
an event was seen on Nov. 29, 2015 using a unique LASCO C3 space coronagraph
routine image taken during the Solar and Heliospheric Observatory (SoHO)
mission observing uninterruptedly at the Lagrangian L1 point. The spallation
signature of a GCR identified well outside the Earth's magnetosphere is
obtained for the 1st time. The resulting image includes different diverging
linear "tracks" of varying intensity, leading to a single pixel, this frame
identifies the site on the silicon CCD chip of the coronagraph camera. There
was no solar flare reported at that time, nor Coronal Mass Ejection (CME) and
no evidence of optical debris around the spacecraft. More examples of smaller
CR events have been discovered through the 20 years of continuous observations
from SoHO. This is the first spallation event from a CR, recorded outside the
Earth's magnetosphere. We evaluate the probable energy of these events
suggesting a plausible galactic source.
Key words: Sun: Galactic Cosmic Rays, Solar Energetic Particles, Heliosphere.Comment: 8 Pages, 5 Figs., accepted in MNRA
Improved SOT (Hinode mission) high resolution solar imaging observations
We consider the best today available observations of the Sun free of
turbulent Earth atmospheric effects, taken with the Solar Optical Telescope
(SOT) onboard the Hinode spacecraft. Both the instrumental smearing and the
observed stray light are analyzed in order to improve the resolution. The Point
Spread Function (PSF) corresponding to the blue continuum Broadband Filter
Imager (BFI) near 450 nm is deduced by analyzing i/ the limb of the Sun and ii/
images taken during the transit of the planet Venus in 2012. A combination of
Gaussian and Lorentzian functions is selected to construct a PSF in order to
remove both smearing due to the instrumental diffraction effects (PSF core) and
the large-angle stray light due to the spiders and central obscuration (wings
of the PSF) that are responsible for the parasitic stray light. A
Max-likelihood deconvolution procedure based on an optimum number of iterations
is discussed. It is applied to several solar field images, including the
granulation near the limb. The normal non-magnetic granulation is compared to
the abnormal granulation which we call magnetic. A new feature appearing for
the first time at the extreme- limb of the disk (the last 100 km) is discussed
in the context of the definition of the solar edge and of the solar diameter. A
single sunspot is considered in order to illustrate how effectively the
restoration works on the sunspot core. A set of 125 consecutive deconvolved
images is assembled in a 45 min long movie illustrating the complexity of the
dynamical behavior inside and around the sunspot.Comment: 15 pages, 22 figures, 1 movi
Increasing the Fine Structure Visibility of the Hinode SOT Ca II H Filtergrams
We present the improved so-called Madmax (OMC) operator selecting maxima of
convexities computed in multiple directions around each pixel rewritten in
MatLab and shown to be very efficient for pattern recognition.
The aim of the algorithm is to trace the bright hair-like features (for ex.
chromospheric thin jets or spicules) of solar ultimate observations polluted by
a noise of different origins. This popular spatial operator uses the second
derivative in the optimally selected direction for which its absolute value has
a maximum value. Accordingly, it uses the positivity of the resulting intensity
signal affected by a superposed noise. The results are illustrated using a test
artificially generated image and real SOT (Hinode) images are also used, to
make your own choice of the sensitive parameters to use in improving the
visibility of images.Comment: 12 pages, 3 figurs, submitted in Solar Physic
Energy Distribution of Micro-events in the Quiet Solar Corona
Recent imaging observations of EUV line emissions have shown evidence for
frequent flare-like events in a majority of the pixels in quiet regions of the
solar corona. The changes in coronal emission measure indicate impulsive
heating of new material to coronal temperatures. These heating or evaporation
events are candidate signatures of "nanoflares" or "microflares" proposed to
interpret the high temperature and the very existence of the corona. The energy
distribution of these micro-events reported in the literature differ widely,
and so do the estimates of their total energy input into the corona. Here we
analyze the assumptions of the different methods, compare them by using the
same data set and discuss their results.
We also estimate the different forms of energy input and output, keeping in
mind that the observed brightenings are most likely secondary phenomena. A
rough estimate of the energy input observed by EIT on the SoHO satellite is of
the order of 10% of the total radiative output in the same region. It is
considerably smaller for the two reported TRACE observations. The discrepancy
can be explained partially by different thresholds for flare detection. There
is agreement on the slope and the absolute value of the distribution if the
same method were used and a numerical error corrected. The extrapolation of the
power law to unobserved energies that are many orders of magnitude smaller
remains questionable. Nevertheless, these micro-events and unresolved smaller
events are currently the best source of information on the heating process of
the corona
Prominence-cavity regions observed using SWAP 174A filtergrams and simultaneous eclipse flash spectra
Images from the SWAP (Proba 2 mission) taken at 174A in the Fe IX/X lines are
compared to simultaneous slitless flash spectra taken during the last solar
total eclipse of July, 11th 2010. Many faint low excitation emission lines
together with the HeI and HeII Paschen Alpha chromospheric lines are recorded
on eclipse spectra where regions of limb prominences are obtained with
space-borne imagers. We consider a deep flash spectrum obtained by summing 80
individual spectra to show the intensity modulations of the continuum.
Intensity depressions are observed around the prominences in both eclipse and
SWAP images. The prominence cavities are interpreted as a relative depression
of plasma density, produced inside the corona surrounding the prominences.
Photometric measurements are shown at different scales and different,
spectrally narrow, intervals for both the prominences and the coronal
background.Comment: 22 pages, 14 figures, accepted to publish in Sol. Phy
G-band Spectral Synthesis in Solar Magnetic Concentrations
Narrow band imaging in the G-band is commonly used to trace the small
magnetic field concentrations of the Sun, although the mechanism that makes
them bright has remained unclear. We carry out LTE syntheses of the G-band in
an assorted set of semi-empirical model magnetic concentrations. The syntheses
include all CH lines as well as the main atomic lines within the band-pass. The
model atmospheres produce bright G-band spectra having many properties in
common with the observed G-band bright points. In particular, the contrast
referred to the quiet Sun is about twice the contrast in continuum wavelengths.
The agreement with observations does not depend on the specificities of the
model atmosphere, rather it holds from single fluxtubes to MIcro-Structured
Magnetic Atmospheres. However, the agreement requires that the real G-band
bright points are not spatially resolved, even in the best observations. Since
the predicted G-band intensities exceed by far the observed values, we foresee
a notable increase of contrast of the G-band images upon improvement of the
angular resolution. According to the LTE modeling, the G-band spectrum emerges
from the deep photosphere that produces the continuum. Our syntheses also
predict solar magnetic concentrations showing up in continuum images but not in
the G-band . Finally, we have examined the importance of the CH
photo-dissociation in setting the amount of G-band absorption. It turns out to
play a minor role.Comment: To appear in ApJ, 554 n2 Jun 20, 33 pages and 9 figure
Quiet Sun coronal heating: statistical model
Recent observations of Krucker & Benz (1998) give strong support to Parker's
hypothesis (Parker 1988) that small scale dissipative events make the main
contribution to quiet Sun coronal heating. They also showed that these small
scale events are associated not only with the magnetic network, but also with
the cell interiors (Benz & Krucker, 1998). Taking into account in addition the
results of the analysis performed by Priest with co-authors (Priest et al.
2000) who demonstrated that the heating is quasi-homogeneous along the arcs we
come to the conclusion that the sources driving these dissipative events are
also small scale sources. Typically they are of the order of or smaller than
the linear scale of the events observed, that is smaller than 2000 km. To
describe statistical properties of quiet Sun corona heating by microflares,
nanoflares, and even smaller events, we consider a cellular automata model
subject to uniform small scale driving and dissipation. The model consists of
two elements, the magnetic field source supposed to be associated with the
small scale hydrodynamic turbulence convected from the photosphere and local
dissipation of small scale currents. The dissipation is assumed to be provided
by either anomalous resistivity, when the current density exceeds a certain
threshold value, or by the magnetic reconnection. The main problem considered
is how the statistical characteristics of dissipated energy flow depend upon
characteristics of the magnetic field source and on physical mechanism
responsible for the magnetic field dissipation. As the threshold value of
current is increased, we observe the transition from Gaussian statistics to
power-law type. In addition, we find that the dissipation provided by
reconnection results in stronger deviations from Gaussian distribution.Comment: 14 pages, 12 figures, submitted to A&
Physics of Solar Prominences: I - Spectral Diagnostics and Non-LTE Modelling
This review paper outlines background information and covers recent advances
made via the analysis of spectra and images of prominence plasma and the
increased sophistication of non-LTE (ie when there is a departure from Local
Thermodynamic Equilibrium) radiative transfer models. We first describe the
spectral inversion techniques that have been used to infer the plasma
parameters important for the general properties of the prominence plasma in
both its cool core and the hotter prominence-corona transition region. We also
review studies devoted to the observation of bulk motions of the prominence
plasma and to the determination of prominence mass. However, a simple inversion
of spectroscopic data usually fails when the lines become optically thick at
certain wavelengths. Therefore, complex non-LTE models become necessary. We
thus present the basics of non-LTE radiative transfer theory and the associated
multi-level radiative transfer problems. The main results of one- and
two-dimensional models of the prominences and their fine-structures are
presented. We then discuss the energy balance in various prominence models.
Finally, we outline the outstanding observational and theoretical questions,
and the directions for future progress in our understanding of solar
prominences.Comment: 96 pages, 37 figures, Space Science Reviews. Some figures may have a
better resolution in the published version. New version reflects minor
changes brought after proof editin
Limb Spicules from the Ground and from Space
We amassed statistics for quiet-sun chromosphere spicules at the limb using
ground-based observations from the Swedish 1-m Solar Telescope on La Palma and
simultaneously from NASA's Transition Region and Coronal Explorer (TRACE)
spacecraft. The observations were obtained in July 2006. With the 0.2 arcsecond
resolution obtained after maximizing the ground-based resolution with the
Multi-Object Multi-Frame Blind Deconvolution (MOMFBD) program, we obtained
specific statistics for sizes and motions of over two dozen individual
spicules, based on movies compiled at 50-second cadence for the series of five
wavelengths observed in a very narrow band at H-alpha, on-band and in the red
and blue wings at 0.035 nm and 0.070 nm (10 s at each wavelength) using the
SOUP filter, and had simultaneous observations in the 160 nm EUV continuum from
TRACE. The MOMFBD restoration also automatically aligned the images,
facilitating the making of Dopplergrams at each off-band pair. We studied 40
H-alpha spicules, and 14 EUV spicules that overlapped H-alpha spicules; we
found that their dynamical and morphological properties fit into the framework
of several previous studies. From a preliminary comparison with spicule
theories, our observations are consistent with a reconnection mechanism for
spicule generation, and with UV spicules being a sheath region surrounding the
H-alpha spicules
Review of Coronal Oscillations - An Observer's View
Recent observations show a variety of oscillation modes in the corona. Early
non-imaging observations in radio wavelengths showed a number of fast-period
oscillations in the order of seconds, which have been interpreted as fast
sausage mode oscillations. TRACE observations from 1998 have for the first time
revealed the lateral displacements of fast kink mode oscillations, with periods
of ~3-5 minutes, apparently triggered by nearby flares and destabilizing
filaments. Recently, SUMER discovered with Doppler shift measurements loop
oscillations with longer periods (10-30 minutes) and relatively short damping
times in hot (7 MK) loops, which seem to correspond to longitudinal slow
magnetoacoustic waves. In addition, propagating longitudinal waves have also
been detected with EIT and TRACE in the lowest density scale height of loops
near sunspots. All these new observations seem to confirm the theoretically
predicted oscillation modes and can now be used as a powerful tool for
``coronal seismology'' diagnostic.Comment: 5 Figure
- …