400 research outputs found
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
The Relationship of Coronal Mass Ejections to Streamers
We have examined images from the Large Angle Spectroscopic Coronagraph
(LASCO) to study the relationship of Coronal Mass Ejections (CMEs) to coronal
streamers. We wish to test the suggestion (Low 1996) that CMEs arise from flux
ropes embedded in a streamer erupting, thus disrupting the streamer. The data
span a period of two years near sunspot minimum through a period of increased
activity as sunspot numbers increased. We have used LASCO data from the C2
coronagraph which records Thomson scattered white light from coronal electrons
at heights between 1.5 and 6R_sun. Maps of the coronal streamers have been
constructed from LASCO C2 observations at a height of 2.5R_sun at the east and
west limbs. We have superposed the corresponding positions of CMEs observed
with the C2 coronagraph onto the synoptic maps. We identified the different
kinds of signatures CMEs leave on the streamer structure at this height
(2.5R_sun). We find four types of CMEs with respect to their effect on
streamers:
1. CMEs that disrupt the streamer 2. CMEs that have no effect on the
streamer, even though they are related to it. 3. CMEs that create streamer-like
structures 4. CMEs that are latitudinally displaced from the streamer.
This is the most extensive observational study of the relation between CMEs
and streamers to date. Previous studies using SMM data have made the general
statement that CMEs are mostly associated with streamers, and that they
frequently disrupt it. However, we find that approximately 35% of the observed
CMEs bear no relation to the pre-existing streamer, while 46% have no effect on
the observed streamer, even though they appear to be related to it. Our
conclusions thus differ considerably from those of previous studies.Comment: Accepted, Journal of Geophysical Research. 8 figs, better versions at
http://www.science.gmu.edu/~prasads/streamer.htm
Reconstructing CMEs with Coordinated Imaging and In Situ Observations: Global Structure, Kinematics, and Implications for Space Weather Forecasting
See the pdf for detailsComment: 45 pages, 16 figures, ApJ, in pres
Global distribution of the solar wind during solar cycle 23: ACE observations
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95439/1/grl26167.pd
Calomplification — the power of generative calorimeter models
Motivated by the high computational costs of classical simulations, machine-learned generative models can be extremely useful in particle physics and elsewhere. They become especially attractive when surrogate models can efficiently learn the underlying distribution, such that a generated sample outperforms a training sample of limited size. This kind of GANplification has been observed for simple Gaussian models. We show the same effect for a physics simulation, specifically photon showers in an electromagnetic calorimeter
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
Can Streamer Blobs prevent the Buildup of the Interplanetetary Magnetic Field?
Coronal Mass Ejections continuously drag closed magnetic field lines away
from the Sun, adding new flux to the interplanetary magnetic field (IMF). We
propose that the outward-moving blobs that have been observed in helmet
streamers are evidence of ongoing, small-scale reconnection in streamer current
sheets, which may play an important role in the prevention of an indefinite
buildup of the IMF. Reconnection between two open field lines from both sides
of a streamer current sheet creates a new closed field line, which becomes part
of the helmet, and a disconnected field line, which moves outward. The blobs
are formed by plasma from the streamer that is swept up in the trough of the
outward moving field line. We show that this mechanism is supported by
observations from SOHO/LASCO. Additionally, we propose a thorough statistical
study to quantify the contribution of blob formation to the reduction of the
IMF, and indicate how this mechanism may be verified by observations with
SOHO/UVCS and the proposed NASA STEREO and ESA Polar Orbiter missions.Comment: 7 pages, 2 figures; accepted by The Astrophysical Journal Letters;
uses AASTe
Structure and Dynamics of the Sun's Open Magnetic Field
The solar magnetic field is the primary agent that drives solar activity and
couples the Sun to the Heliosphere. Although the details of this coupling
depend on the quantitative properties of the field, many important aspects of
the corona - solar wind connection can be understood by considering only the
general topological properties of those regions on the Sun where the field
extends from the photosphere out to interplanetary space, the so-called open
field regions that are usually observed as coronal holes. From the simple
assumptions that underlie the standard quasi-steady corona-wind theoretical
models, and that are likely to hold for the Sun, as well, we derive two
conjectures on the possible structure and dynamics of coronal holes: (1)
Coronal holes are unique in that every unipolar region on the photosphere can
contain at most one coronal hole. (2) Coronal holes of nested polarity regions
must themselves be nested. Magnetic reconnection plays the central role in
enforcing these constraints on the field topology. From these conjectures we
derive additional properties for the topology of open field regions, and
propose several observational predictions for both the slowly varying and
transient corona/solar wind.Comment: 26 pages, 6 figure
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
How Many CMEs Have Flux Ropes? Deciphering the Signatures of Shocks, Flux Ropes, and Prominences in Coronagraph Observations of CMEs
We intend to provide a comprehensive answer to the question on whether all
Coronal Mass Ejections (CMEs) have flux rope structure. To achieve this, we
present a synthesis of the LASCO CME observations over the last sixteen years,
assisted by 3D MHD simulations of the breakout model, EUV and coronagraphic
observations from STEREO and SDO, and statistics from a revised LASCO CME
database. We argue that the bright loop often seen as the CME leading edge is
the result of pileup at the boundary of the erupting flux rope irrespective of
whether a cavity or, more generally, a 3-part CME can be identified. Based on
our previous work on white light shock detection and supported by the MHD
simulations, we identify a new type of morphology, the `two-front' morphology.
It consists of a faint front followed by diffuse emission and the bright
loop-like CME leading edge. We show that the faint front is caused by density
compression at a wave (or possibly shock) front driven by the CME. We also
present high-detailed multi-wavelength EUV observations that clarify the
relative positioning of the prominence at the bottom of a coronal cavity with
clear flux rope structure. Finally, we visually check the full LASCO CME
database for flux rope structures. In the process, we classify the events into
two clear flux rope classes (`3-part', `Loop'), jets and outflows (no clear
structure). We find that at least 40% of the observed CMEs have clear flux rope
structures. We propose a new definition for flux rope CMEs (FR-CMEs) as a
coherent magnetic, twist-carrying coronal structure with angular width of at
least 40 deg and able to reach beyond 10 Rsun which erupts on a time scale of a
few minutes to several hours. We conclude that flux ropes are a common
occurrence in CMEs and pose a challenge for future studies to identify CMEs
that are clearly not FR-CMEs.Comment: 26 pages, 9 figs, to be published in Solar Physics Topical Issue
"Flux Rope Structure of CMEs
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