230 research outputs found
Determining the azimuthal properties of coronal mass ejections from multi-spacecraft remote-sensing observations with stereo secchi
We discuss how simultaneous observations by multiple heliospheric imagers can
provide some important information about the azimuthal properties of Coronal
Mass Ejections (CMEs) in the heliosphere. We propose two simple models of CME
geometry that can be used to derive information about the azimuthal deflection
and the azimuthal expansion of CMEs from SECCHI/HI observations. We apply these
two models to four CMEs well-observed by both STEREO spacecraft during the year
2008. We find that in three cases, the joint STEREO-A and B observations are
consistent with CMEs moving radially outward. In some cases, we are able to
derive the azimuthal cross-section of the CME fronts, and we are able to
measure the deviation from self-similar evolution. The results from this
analysis show the importance of having multiple satellites dedicated to space
weather forecasting, for example in orbits at the Lagrangian L4 and L5 points.Comment: 7 pages, 4 figures, 1 table, accepted to Ap
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
Accuracy and Limitations of Fitting and Stereoscopic Methods to Determine the Direction of Coronal Mass Ejections from Heliospheric Imagers Observations
Using data from the Heliospheric Imagers (HIs) onboard STEREO, it is possible
to derive the direction of propagation of coronal mass ejections (CMEs) in
addition to their speed with a variety of methods. For CMEs observed by both
STEREO spacecraft, it is possible to derive their direction using simultaneous
observations from the twin spacecraft and also, using observations from only
one spacecraft with fitting methods. This makes it possible to test and compare
different analyses techniques. In this article, we propose a new fitting method
based on observations from one spacecraft, which we compare to the commonly
used fitting method of Sheeley et al. (1999). We also compare the results from
these two fitting methods with those from two stereoscopic methods, focusing on
12 CMEs observed simultaneously by the two STEREO spacecraft in 2008 and 2009.
We find evidence that the fitting method of Sheeley et al. (1999) can result in
significant errors in the determination of the CME direction when the CME
propagates outside of 60deg \pm 20 deg from the Sun-spacecraft line. We expect
our new fitting method to be better adapted to the analysis of halo or limb
CMEs with respect to the observing spacecraft. We also find some evidence that
direct triangulation in the HI fields-of-view should only be applied to CMEs
propagating approximatively towards Earth (\pm 20deg from the Sun-Earth line).
Last, we address one of the possible sources of errors of fitting methods: the
assumption of radial propagation. Using stereoscopic methods, we find that at
least seven of the 12 studied CMEs had an heliospheric deflection of less than
20deg as they propagated in the HI fields-of-view, which, we believe, validates
this approximation.Comment: 17 pages, 6 figures, 2 tables, accepted to Solar Physic
Solar-Terrestrial Simulations of CMEs with a Realistic Initiation Mechanism: Case Study for Active Region 10069
Most simulations of coronal mass ejections (CMEs) to date either focus on the
interplanetary propagation of a giant plasma "blob" without paying too much
attention to its origin and to the formation process or they focus on the
complex evolution of the coronal magnetic field due to (sub-)photospheric
motions which result in an eruption. Here, we present global simulations of
CMEs where coronal motions are used to produce a realistic evolution of the
coronal magnetic field and cause an eruption. We focus on active region 10069,
which produced a number of eruptions in late August 2002, including the August
24, 2002 CME - a fast (~2000 km/s) eruption originating from W81-, as well as a
slower eruption on August 22, 2002 (originating from W62). Using a
three-dimensional magneto-hydrodynamic (MHD) simulation of these ejections with
the Space Weather Modeling Framework (SWMF), we show how a realistic initiation
mechanism enables us to study the deflection of the CME in the corona and in
the heliosphere. Reconnection of the erupting magnetic field with that of
neighboring streamers and active regions modify the solar connectivity of the
field lines connecting to Earth and change the expected solar energetic
particle fluxes. Comparing the results at 1 AU of our simulations with in situ
observations by the ACE spacecraft, we propose an alternate solar origin for
the shock wave observed at L1 on August 26.Comment: 4 pages, 2 figures, refereed proceedings for Solar Wind 1
Effect of Solar Wind Drag on the Determination of the Properties of Coronal Mass Ejections from Heliospheric Images
The Fixed-\Phi (F\Phi) and Harmonic Mean (HM) fitting methods are two methods
to determine the average direction and velocity of coronal mass ejections
(CMEs) from time-elongation tracks produced by Heliospheric Imagers (HIs), such
as the HIs onboard the STEREO spacecraft. Both methods assume a constant
velocity in their descriptions of the time-elongation profiles of CMEs, which
are used to fit the observed time-elongation data. Here, we analyze the effect
of aerodynamic drag on CMEs propagating through interplanetary space, and how
this drag affects the result of the F\Phi and HM fitting methods. A simple drag
model is used to analytically construct time-elongation profiles which are then
fitted with the two methods. It is found that higher angles and velocities give
rise to greater error in both methods, reaching errors in the direction of
propagation of up to 15 deg and 30 deg for the F\Phi and HM fitting methods,
respectively. This is due to the physical accelerations of the CMEs being
interpreted as geometrical accelerations by the fitting methods. Because of the
geometrical definition of the HM fitting method, it is affected by the
acceleration more greatly than the F\Phi fitting method. Overall, we find that
both techniques overestimate the initial (and final) velocity and direction for
fast CMEs propagating beyond 90 deg from the Sun-spacecraft line, meaning that
arrival times at 1 AU would be predicted early (by up to 12 hours). We also
find that the direction and arrival time of a wide and decelerating CME can be
better reproduced by the F\Phi due to the cancellation of two errors:
neglecting the CME width and neglecting the CME deceleration. Overall, the
inaccuracies of the two fitting methods are expected to play an important role
in the prediction of CME hit and arrival times as we head towards solar maximum
and the STEREO spacecraft further move behind the Sun.Comment: Solar Physics, Online First, 17 page
Numerical Investigation of a Coronal Mass Ejection from an Anemone Active Region: Reconnection and Deflection of the 2005 August 22 Eruption
We present a numerical investigation of the coronal evolution of a coronal
mass ejection (CME) on 2005 August 22 using a 3-D thermodynamics
magnetohydrodynamic model, the SWMF. The source region of the eruption was
anemone active region (AR) 10798, which emerged inside a coronal hole. We
validate our modeled corona by producing synthetic extreme ultraviolet (EUV)
images, which we compare to EIT images. We initiate the CME with an
out-of-equilibrium flux rope with an orientation and chirality chosen in
agreement with observations of a H-alpha filament. During the eruption, one
footpoint of the flux rope reconnects with streamer magnetic field lines and
with open field lines from the adjacent coronal hole. It yields an eruption
which has a mix of closed and open twisted field lines due to interchange
reconnection and only one footpoint line-tied to the source region. Even with
the large-scale reconnection, we find no evidence of strong rotation of the CME
as it propagates. We study the CME deflection and find that the effect of the
Lorentz force is a deflection of the CME by about 3 deg/Rsun towards the East
during the first 30 minutes of the propagation. We also produce coronagraphic
and EUV images of the CME, which we compare with real images, identifying a
dimming region associated with the reconnection process. We discuss the
implication of our results for the arrival at Earth of CMEs originating from
the limb and for models to explain the presence of open field lines in magnetic
clouds.Comment: 14 pages, 8 Figures, accepted to Astrophysical Journa
Using an Ellipsoid Model to Track and Predict the Evolution and Propagation of Coronal Mass Ejections
We present a method for tracking and predicting the propagation and evolution
of coronal mass ejections (CMEs) using the imagers on the STEREO and SOHO
satellites. By empirically modeling the material between the inner core and
leading edge of a CME as an expanding, outward propagating ellipsoid, we track
its evolution in three-dimensional space. Though more complex empirical CME
models have been developed, we examine the accuracy of this relatively simple
geometric model, which incorporates relatively few physical assumptions,
including i) a constant propagation angle and ii) an azimuthally symmetric
structure. Testing our ellipsoid model developed herein on three separate CMEs,
we find that it is an effective tool for predicting the arrival of density
enhancements and the duration of each event near 1 AU. For each CME studied,
the trends in the trajectory, as well as the radial and transverse expansion
are studied from 0 to ~.3 AU to create predictions at 1 AU with an average
accuracy of 2.9 hours.Comment: 18 pages, 11 figure
The Brightness of Density Structures at Large Solar Elongation Angles: What is Being Observed by STEREO/SECCHI?
We discuss features of coronal mass ejections (CMEs) that are specific to
heliospheric observations at large elongation angles. Our analysis is focused
on a series of two eruptions that occurred on 2007 January 24-25, which were
tracked by the Heliospheric Imagers (HIs) onboard STEREO. Using a
three-dimensional (3-D) magneto-hydrodynamic simulation of these ejections with
the Space Weather Modeling Framework (SWMF), we illustrate how the combination
of the 3-D nature of CMEs, solar rotation, and geometry associated with the
Thomson sphere results in complex effects in the brightness observed by the
HIs. Our results demonstrate that these effects make any in-depth analysis of
CME observations without 3-D simulations challenging. In particular, the
association of bright features seen by the HIs with fronts of CME-driven shocks
is far from trivial. In this Letter, we argue that, on 2007 January 26, the HIs
observed not only two CMEs, but also a dense corotating stream compressed by
the CME-driven shocks.Comment: 5 pages, 2 figures, accepted for ApJ Lette
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