72 research outputs found
Examining Periodic Solar Wind Density Structures Observed in the SECCHI Heliospheric Imagers
We present an analysis of small-scale, periodic, solar-wind density
enhancements (length-scales as small as \approx 1000 Mm) observed in images
from the Heliospheric Imager (HI) aboard STEREO A. We discuss their possible
relationship to periodic fluctuations of the proton density that have been
identified at 1 AU using in-situ plasma measurements. Specifically, Viall,
Kepko, and Spence (2008) examined 11 years of in-situ solar-wind density
measurements at 1 AU and demonstrated that not only turbulent structures, but
also non-turbulent periodic density structures exist in the solar wind with
scale sizes of hundreds to one thousand Mm. In a subsequent paper, Viall,
Spence, and Kasper (2009) analyzed the {\alpha} to proton solar-wind abundance
ratio measured during one such event of periodic density structures,
demonstrating that the plasma behavior was highly suggestive that either
temporally or spatially varying coronal source plasma created those density
structures. Large periodic density structures observed at 1 AU, which were
generated in the corona, can be observable in coronal and heliospheric
white-light images if they possess sufficiently high density contrast. Indeed,
we identify such periodic density structures as they enter the HI field of view
and follow them as they advect with the solar wind through the images. The
smaller periodic density structures that we identify in the images are
comparable in size to the larger structures analyzed in-situ at 1 AU, yielding
further evidence that periodic density enhancements are a consequence of
coronal activity as the solar wind is formed.Comment: 15 pages, 12 figures. The final publication is available at
http://www.springerlink.co
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
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
Speeds and arrival times of solar transients approximated by self-similar expanding circular fronts
The NASA STEREO mission opened up the possibility to forecast the arrival
times, speeds and directions of solar transients from outside the Sun-Earth
line. In particular, we are interested in predicting potentially geo-effective
Interplanetary Coronal Mass Ejections (ICMEs) from observations of density
structures at large observation angles from the Sun (with the STEREO
Heliospheric Imager instrument). We contribute to this endeavor by deriving
analytical formulas concerning a geometric correction for the ICME speed and
arrival time for the technique introduced by Davies et al. (2012, ApJ, in
press) called Self-Similar Expansion Fitting (SSEF). This model assumes that a
circle propagates outward, along a plane specified by a position angle (e.g.
the ecliptic), with constant angular half width (lambda). This is an extension
to earlier, more simple models: Fixed-Phi-Fitting (lambda = 0 degree) and
Harmonic Mean Fitting (lambda = 90 degree). This approach has the advantage
that it is possible to assess clearly, in contrast to previous models, if a
particular location in the heliosphere, such as a planet or spacecraft, might
be expected to be hit by the ICME front. Our correction formulas are especially
significant for glancing hits, where small differences in the direction greatly
influence the expected speeds (up to 100-200 km/s) and arrival times (up to two
days later than the apex). For very wide ICMEs (2 lambda > 120 degree), the
geometric correction becomes very similar to the one derived by M\"ostl et al.
(2011, ApJ, 741, id. 34) for the Harmonic Mean model. These analytic
expressions can also be used for empirical or analytical models to predict the
1 AU arrival time of an ICME by correcting for effects of hits by the flank
rather than the apex, if the width and direction of the ICME in a plane are
known and a circular geometry of the ICME front is assumed.Comment: 15 pages, 5 figures, accepted for publication in "Solar Physics
Heliospheric Observations of STEREO-Directed Coronal Mass Ejections in 2008--2010: Lessons for Future Observations of Earth-Directed CMEs
We present a study of coronal mass ejections (CMEs) which impacted one of the
STEREO spacecraft between January 2008 and early 2010. We focus our study on 20
CMEs which were observed remotely by the Heliospheric Imagers (HIs) onboard the
other STEREO spacecraft up to large heliocentric distances. We compare the
predictions of the Fixed-Phi and Harmonic Mean (HM) fitting methods, which only
differ by the assumed geometry of the CME. It is possible to use these
techniques to determine from remote-sensing observations the CME direction of
propagation, arrival time and final speed which are compared to in situ
measurements. We find evidence that for large viewing angles, the HM fitting
method predicts the CME direction better. However, this may be due to the fact
that only wide CMEs can be successfully observed when the CME propagates more
than 100 deg from the observing spacecraft. Overall eight CMEs, originating
from behind the limb as seen by one of the STEREO spacecraft can be tracked and
their arrival time at the other STEREO spacecraft can be successfully
predicted. This includes CMEs, such as the events on 4 December 2009 and 9
April 2010, which were viewed 130 deg away from their direction of propagation.
Therefore, we predict that some Earth-directed CMEs will be observed by the HIs
until early 2013, when the separation between Earth and one of the STEREO
spacecraft will be similar to the separation of the two STEREO spacecraft in
2009--2010.Comment: 21 pages, accepted to Solar Physic
The large longitudinal spread of solar energetic particles during the January 17, 2010 solar event
We investigate multi-spacecraft observations of the January 17, 2010 solar
energetic particle event. Energetic electrons and protons have been observed
over a remarkable large longitudinal range at the two STEREO spacecraft and
SOHO suggesting a longitudinal spread of nearly 360 degrees at 1AU. The flaring
active region, which was on the backside of the Sun as seen from Earth, was
separated by more than 100 degrees in longitude from the magnetic footpoints of
each of the three spacecraft. The event is characterized by strongly delayed
energetic particle onsets with respect to the flare and only small or no
anisotropies in the intensity measurements at all three locations. The presence
of a coronal shock is evidenced by the observation of a type II radio burst
from the Earth and STEREO B. In order to describe the observations in terms of
particle transport in the interplanetary medium, including perpendicular
diffusion, a 1D model describing the propagation along a magnetic field line
(model 1) (Dr\"oge, 2003) and the 3D propagation model (model 2) by (Dr\"oge et
al., 2010) including perpendicular diffusion in the interplanetary medium have
been applied, respectively. While both models are capable of reproducing the
observations, model 1 requires injection functions at the Sun of several hours.
Model 2, which includes lateral transport in the solar wind, reveals high
values for the ratio of perpendicular to parallel diffusion. Because we do not
find evidence for unusual long injection functions at the Sun we favor a
scenario with strong perpendicular transport in the interplanetary medium as
explanation for the observations.Comment: The final publication is available at http://www.springerlink.co
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
On the Nature and Genesis of EUV Waves: A Synthesis of Observations from SOHO, STEREO, SDO, and Hinode
A major, albeit serendipitous, discovery of the SOlar and Heliospheric
Observatory mission was the observation by the Extreme Ultraviolet Telescope
(EIT) of large-scale Extreme Ultraviolet (EUV) intensity fronts propagating
over a significant fraction of the Sun's surface. These so-called EIT or EUV
waves are associated with eruptive phenomena and have been studied intensely.
However, their wave nature has been challenged by non-wave (or pseudo-wave)
interpretations and the subject remains under debate. A string of recent solar
missions has provided a wealth of detailed EUV observations of these waves
bringing us closer to resolving their nature. With this review, we gather the
current state-of-art knowledge in the field and synthesize it into a picture of
an EUV wave driven by the lateral expansion of the CME. This picture can
account for both wave and pseudo-wave interpretations of the observations, thus
resolving the controversy over the nature of EUV waves to a large degree but
not completely. We close with a discussion of several remaining open questions
in the field of EUV waves research.Comment: Solar Physics, Special Issue "The Sun in 360",2012, accepted for
publicatio
Origins of the Ambient Solar Wind: Implications for Space Weather
The Sun's outer atmosphere is heated to temperatures of millions of degrees,
and solar plasma flows out into interplanetary space at supersonic speeds. This
paper reviews our current understanding of these interrelated problems: coronal
heating and the acceleration of the ambient solar wind. We also discuss where
the community stands in its ability to forecast how variations in the solar
wind (i.e., fast and slow wind streams) impact the Earth. Although the last few
decades have seen significant progress in observations and modeling, we still
do not have a complete understanding of the relevant physical processes, nor do
we have a quantitatively precise census of which coronal structures contribute
to specific types of solar wind. Fast streams are known to be connected to the
central regions of large coronal holes. Slow streams, however, appear to come
from a wide range of sources, including streamers, pseudostreamers, coronal
loops, active regions, and coronal hole boundaries. Complicating our
understanding even more is the fact that processes such as turbulence,
stream-stream interactions, and Coulomb collisions can make it difficult to
unambiguously map a parcel measured at 1 AU back down to its coronal source. We
also review recent progress -- in theoretical modeling, observational data
analysis, and forecasting techniques that sit at the interface between data and
theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue
connected with a 2016 ISSI workshop on "The Scientific Foundations of Space
Weather." 44 pages, 9 figure
BepiColombo’s Cruise Phase: Unique Opportunity for Synergistic Observations
The investigation of multi-spacecraft coordinated observations during the cruise phase of BepiColombo (ESA/JAXA) are reported, with a particular emphasis on the recently launched missions, Solar Orbiter (ESA/NASA) and Parker Solar Probe (NASA). Despite some payload constraints, many instruments onboard BepiColombo are operating during its cruise phase simultaneously covering a wide range of heliocentric distances (0.28 AU–0.5 AU). Hence, the various spacecraft configurations and the combined in-situ and remote sensing measurements from the different spacecraft, offer unique opportunities for BepiColombo to be part of these unprecedented multipoint synergistic observations and for potential scientific studies in the inner heliosphere, even before its orbit insertion around Mercury in December 2025. The main goal of this report is to present the coordinated observation opportunities during the cruise phase of BepiColombo (excluding the planetary flybys). We summarize the identified science topics, the operational instruments, the method we have used to identify the windows of opportunity and discuss the planning of joint observations in the future.</p
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