703 research outputs found
Quantitative Measurements of CME-driven Shocks from LASCO Observations
In this paper, we demonstrate that CME-driven shocks can be detected in white
light coronagraph images and in which properties such as the density
compression ratio and shock direction can be measured. Also, their propagation
direction can be deduced via simple modeling. We focused on CMEs during the
ascending phase of solar cycle 23 when the large-scale morphology of the corona
was simple. We selected events which were good candidates to drive a shock due
to their high speeds (V>1500 km/s). The final list includes 15 CMEs. For each
event, we calibrated the LASCO data, constructed excess mass images and
searched for indications of faint and relatively sharp fronts ahead of the
bright CME front. We found such signatures in 86% (13/15) of the events and
measured the upstream/downstream densities to estimate the shock strength. Our
values are in agreement with theoretical expectations and show good
correlations with the CME kinetic energy and momentum. Finally, we used a
simple forward modeling technique to estimate the 3D shape and orientation of
the white light shock features. We found excellent agreement with the observed
density profiles and the locations of the CME source regions. Our results
strongly suggest that the observed brightness enhancements result from density
enhancements due to a bow-shock structure driven by the CME.Comment: to be published in Astrophysical Journa
Comprehensive Analysis of Coronal Mass Ejection Mass and Energy Properties Over a Full Solar Cycle
The LASCO coronagraphs, in continuous operation since 1995, have observed the
evolution of the solar corona and coronal mass ejections (CMEs) over a full
solar cycle with high quality images and regular cadence. This is the first
time that such a dataset becomes available and constitutes a unique resource
for the study of CMEs. In this paper, we present a comprehensive investigation
of the solar cycle dependence on the CME mass and energy over a full solar
cycle (1996-2009) including the first in-depth discussion of the mass and
energy analysis methods and their associated errors. Our analysis provides
several results worthy of further studies. It demonstrates the possible
existence of two event classes; 'normal' CMEs reaching constant mass for
R_{\sun} and 'pseudo' CMEs which disappear in the C3 FOV. It shows that the
mass and energy properties of CME reach constant levels, and therefore should
be measured, only above \sim 10 R_\sun. The mass density (g/R_\sun^2) of
CMEs varies relatively little ( order of magnitude) suggesting that the
majority of the mass originates from a small range in coronal heights. We find
a sudden reduction in the CME mass in mid-2003 which may be related to a change
in the electron content of the large scale corona and we uncover the presence
of a six-month periodicity in the ejected mass from 2003 onwards.Comment: 42 pages, 16 figures, To appear in Astrophysical Journa
A Decade of Coronagraphic and Spectroscopic Studies of CME-Driven Shocks
Shocks driven by Coronal Mass Ejections (CMEs) are primary agents of space
weather. They can accelerate particles to high energies and can compress the
magnetosphere thus setting in motion geomagnetic storms. For many years, these
shocks were studied only in-situ when they crossed over spacecraft or remotely
through their radio emission spectra. Neither of these two methods provides
information on the spatial structure of the shock nor on its relationship to
its driver, the CME. In the last decade, we have been able to not only image
shocks with coronagraphs but also measure their properties remotely through the
use of spectroscopic and image analysis methods. Thanks to instrumentation on
STEREO and SOHO we can now image shocks (and waves) from the low corona,
through the inner heliosphere, to Earth. Here, we review the progress made in
imaging and analyzing CME-driven shocks and show that joint coronagraphic and
spectrscopic observations are our best means to understand shock physics close
to the Sun.Comment: 6 pages, 3 figure
First Determination of the True Mass of Coronal Mass Ejections: A Novel Approach to Using the Two STEREO Viewpoints
The twin Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI)
COR2 coronagraphs of the Solar Terrestrial Relations Observatory (STEREO)
provide images of the solar corona from two view points in the solar system.
Since their launch in late 2006, the STEREO Ahead (A) and Behind (B)
spacecrafts have been slowly separating from Earth at a rate of 22.5 degrees
per year. By the end of 2007, the two spacecraft were separated by more than 40
degrees from each other. At this time, we began to see large-scale differences
in the morphology and total intensity between coronal mass ejections (CMEs)
observed with SECCHI-COR2 on STEREO-A and B. Due to the effects of the Thomson
scattering geometry, the intensity of an observed CME is dependent on the angle
it makes with the observed plane of the sky. From the intensity images, we can
calculate the integrated line of sight electron density and mass. We
demonstrate that is is possible to simultaneously derive the direction and true
total mass of the CME if we make the simple assumption that the same mass
should be observed in COR2-A and B
Toward understanding the early stages of an impulsively accelerated coronal mass ejection
The expanding magnetic flux in coronal mass ejections (CMEs) often forms a
cavity. A spherical model is simultaneously fit to STEREO EUVI and COR1 data of
an impulsively accelerated CME on 25 March 2008, which displays a well-defined
extreme ultraviolet (EUV) and white-light cavity of nearly circular shape
already at low heights ~ 0.2 Rs. The center height h(t) and radial expansion
r(t) of the cavity are obtained in the whole height range of the main
acceleration. We interpret them as the axis height and as a quantity
proportional to the minor radius of a flux rope, respectively. The
three-dimensional expansion of the CME exhibits two phases in the course of its
main upward acceleration. From the first h and r data points, taken shortly
after the onset of the main acceleration, the erupting flux shows an
overexpansion compared to its rise, as expressed by the decrease of the aspect
ratio from k=h/r ~ 3 to k ~ (1.5-2.0). This phase is approximately coincident
with the impulsive rise of the acceleration and is followed by a phase of very
gradual change of the aspect ratio (a nearly self-similar expansion) toward k ~
1.5 at h ~ 10 Rs. The initial overexpansion of the CME cavity can be caused by
flux conservation around a rising flux rope of decreasing axial current and by
the addition of flux to a growing, or even newly forming,flux rope by magnetic
reconnection. Further analysis will be required to decide which of these
contributions is dominant. The data also suggest that the horizontal component
of the impulsive cavity expansion (parallel to the solar surface) triggers the
associated EUV wave, which subsequently detaches from the CME volume.Comment: in press, A&A, 201
Deriving the radial distances of wide coronal mass ejections from elongation measurements in the heliosphere - Application to CME-CME interaction
We present general considerations regarding the derivation of the radial
distances of coronal mass ejections (CMEs) from elongation angle measurements
such as those provided by SECCHI and SMEI, focusing on measurements in the
Heliospheric Imager 2 (HI-2) field of view (i.e. past 0.3 AU). This study is
based on a three-dimensional (3-D) magneto-hydrodynamics (MHD) simulation of
two CMEs observed by SECCHI on January 24-27, 2007. Having a 3-D simulation
with synthetic HI images, we are able to compare the two basic methods used to
derive CME positions from elongation angles, the so-called "Point-P" and
"Fixed-Phi" approximations.
We confirm, following similar works, that both methods, while valid in the
most inner heliosphere, yield increasingly large errors in HI-2 field of view
for fast and wide CMEs. Using a simple model of a CME as an expanding
self-similar sphere, we derive an analytical relationship between elongation
angles and radial distances for wide CMEs. This relationship is simply the
harmonic mean of the "Point-P" and "Fixed-Phi'' approximations and it is aimed
at complementing 3-D fitting of CMEs by cone models or flux rope shapes. It
proves better at getting the kinematics of the simulated CME right when we
compare the results of our line-of-sights to the MHD simulation. Based on this
approximation, we re-analyze the J-maps (time-elongation maps) in January
26-27, 2007 and present the first observational evidence that the merging of
CMEs is associated with a momentum exchange from the faster ejection to the
slower one due to the propagation of the shock wave associated with the fast
eruption through the slow eruption.Comment: 10 pages, 4 figures, accepted in Annales Geophysicae (Special Issue:
Three eyes on the Sun - multi-spacecraft studies of the corona and impacts on
the heliosphere
- …