226 research outputs found
STEREO and Wind observations of a fast ICME flank triggering a prolonged geomagnetic storm on 5-7 April 2010
On 5 April 2010 an interplanetary (IP) shock was detected by the Wind
spacecraft ahead of Earth, followed by a fast (average speed 650 km/s) IP
coronal mass ejection (ICME). During the subsequent moderate geomagnetic storm
(minimum Dst = -72 nT, maximum Kp=8-), communication with the Galaxy 15
satellite was lost. We link images from STEREO/SECCHI to the near-Earth in situ
observations and show that the ICME did not decelerate much between Sun and
Earth. The ICME flank was responsible for a long storm growth phase. This type
of glancing collision was for the first time directly observed with the STEREO
Heliospheric Imagers. The magnetic cloud (MC) inside the ICME cannot be modeled
with approaches assuming an invariant direction. These observations confirm the
hypotheses that parts of ICMEs classified as (1) long-duration MCs or (2)
magnetic-cloud-like (MCL) structures can be a consequence of a spacecraft
trajectory through the ICME flank.Comment: Geophysical Research Letters (accepted); 3 Figure
The Grad-Shafranov Reconstruction of Toroidal Magnetic Flux Ropes: Method Development and Benchmark Studies
We develop an approach of Grad-Shafranov (GS) reconstruction for toroidal
structures in space plasmas, based on in-situ spacecraft measurements. The
underlying theory is the GS equation that describes two-dimensional
magnetohydrostatic equilibrium as widely applied in fusion plasmas. The
geometry is such that the arbitrary cross section of the torus has rotational
symmetry about the rotation axis , with a major radius . The magnetic
field configuration is thus determined by a scalar flux function and a
functional that is a single-variable function of . The algorithm is
implemented through a two-step approach: i) a trial-and-error process by
minimizing the residue of the functional to determine an optimal
axis orientation, and ii) for the chosen , a minimization process
resulting in the range of . Benchmark studies of known analytic solutions
to the toroidal GS equation with noise additions are presented to illustrate
the two-step procedures and to demonstrate the performance of the numerical GS
solver, separately. For the cases presented, the errors in and are
9 and 22\%, respectively, and the relative percent error in the
numerical GS solutions is less than 10\%. We also make public the computer
codes for these implementations and benchmark studies.Comment: submitted to Sol. Phys. late Dec 2016; under review; code will be
made public once review is ove
Constraining the Kinematics of Coronal Mass Ejections in the Inner Heliosphere with In-Situ Signatures
We present a new approach to combine remote observations and in situ data by
STEREO/HI and Wind, respectively, to derive the kinematics and propagation
directions of interplanetary coronal mass ejections (ICMEs). We used two
methods, Fixed-Phi and Harmonic Mean, to convert ICME elongations into
distance, and constrained the ICME direction such that the ICME distance-time
and velocity-time profiles are most consistent with in situ measurements of the
arrival time and velocity. The derived velocity-time functions from the Sun to
1 AU for the three events under study (1-6 June 2008, 13-18 February 2009, 3-5
April 2010) do not show strong differences for the two extreme geometrical
assumptions of a wide ICME with a circular front (Harmonic Mean) or an ICME of
small spatial extent in the ecliptic (Fixed-Phi). Due to the geometrical
assumptions, Harmonic Mean delivers the propagation direction further away from
the observing spacecraft with a mean difference of ~25 degree
Observations of an extreme storm in interplanetary space caused by successive coronal mass ejections
Space weather refers to dynamic conditions on the Sun and in the space
environment of the Earth, which are often driven by solar eruptions and their
subsequent interplanetary disturbances. It has been unclear how an extreme
space weather storm forms and how severe it can be. Here we report and
investigate an extreme event with multi-point remote-sensing and in-situ
observations. The formation of the extreme storm showed striking novel
features. We suggest that the in-transit interaction between two closely
launched coronal mass ejections resulted in the extreme enhancement of the
ejecta magnetic field observed near 1 AU at STEREO A. The fast transit to
STEREO A (in only 18.6 hours), or the unusually weak deceleration of the event,
was caused by the preconditioning of the upstream solar wind by an earlier
solar eruption. These results provide a new view crucial to solar physics and
space weather as to how an extreme space weather event can arise from a
combination of solar eruptions.Comment: 23 pages, 7 figure
Turbulence Properties of Interplanetary Coronal Mass Ejections in the Inner Heliosphere: Dependence on Proton Beta and Flux Rope Structure
Interplanetary coronal mass ejections (ICMEs) have low proton beta across a
broad range of heliocentric distances and a magnetic flux rope structure at
large scales, making them a unique environment for studying solar wind
fluctuations. Power spectra of magnetic field fluctuations in 28 ICMEs observed
between 0.25 and 0.95 au by Solar Orbiter and Parker Solar Probe have been
examined. At large scales, the spectra were dominated by power contained in the
flux ropes. Subtraction of the background flux rope fields reduced the mean
spectral index from to at . Rope subtraction
also revealed shorter correlation lengths in the magnetic field. The spectral
index was typically near in the inertial range at all radial distances
regardless of rope subtraction, and steepened to values consistently below
with transition to kinetic scales. The high-frequency break point terminating
the inertial range evolved approximately linearly with radial distance and was
closer in scale to the proton inertial length than the proton gyroscale, as
expected for plasma at low proton beta. Magnetic compressibility at inertial
scales did not show any significant correlation with radial distance, in
contrast to the solar wind generally. In ICMEs, the distinctive spectral
properties at injection scales appear mostly determined by the global flux rope
structure while transition-kinetic properties are more influenced by the low
proton beta; the intervening inertial range appears independent of both ICME
features, indicative of a system-independent scaling of the turbulence.Comment: 12 pages, 5 figures; accepted for publication in the Astrophysical
Journal Letters 2023 September 2
Linking remote imagery of a coronal mass ejection to its in situ signatures at 1 AU
In a case study (June 6-7, 2008) we report on how the internal structure of a
coronal mass ejection (CME) at 1 AU can be anticipated from remote observations
of white-light images of the heliosphere. Favorable circumstances are the
absence of fast equatorial solar wind streams and a low CME velocity which
allow us to relate the imaging and in-situ data in a straightforward way. The
STEREO-B spacecraft encountered typical signatures of a magnetic flux rope
inside an interplanetary CME (ICME) whose axis was inclined at 45 degree to the
solar equatorial plane. Various CME direction-finding techniques yield
consistent results to within 15 degree. Further, remote images from STEREO-A
show that (1) the CME is unambiguously connected to the ICME and can be tracked
all the way to 1 AU, (2) the particular arc-like morphology of the CME points
to an inclined axis, and (3) the three-part structure of the CME may be
plausibly related to the in situ data. This is a first step in predicting both
the direction of travel and the internal structure of CMEs from complete remote
observations between the Sun and 1 AU, which is one of the main requirements
for forecasting the geo-effectiveness of CMEs.Comment: The Astropyhsical Journal Letters (accepted); 4 figure
The Magnetic Field Geometry of Small Solar Wind Flux Ropes Inferred from their Twist Distribution
This work extends recent efforts on the force-free modeling of large flux
rope-type structures (magnetic clouds, MCs) to much smaller spatial scales. We
first select small flux ropes (SFRs) by eye whose duration is unambiguous and
which were observed by the Solar Terrestrial Relations Observatory (STEREO) or
Wind spacecraft during solar maximum years. We inquire into which analytical
technique is physically most appropriate. We consider three models: (i) linear
force-free field ( B = B) with a
specific, prescribed constant (Lundquist solution), and (ii) with
as a free constant parameter (Lundquist-alpha solution), (iii) uniform
twist field (Gold-Hoyle solution). We retain only those cases where the impact
parameter is less than one-half the FR radius, , so the results should be
robust (29 cases). The SFR radii lie in the range [ 0.003, 0.059] AU.
Comparing results, we find that the Lundquist-alpha and uniform twist solutions
yielded comparable and small normalized values in most cases. We then
use Grad-Shafranov (GS) reconstruction to analyze these events further. We then
considered the twist per unit length, , both its profile through the FR
and its absolute value. We find to lie in the range [5.6, 34] turns/AU.
The GH model-derived values are comparable to those obtained from GS
reconstruction. We find that twist unit length () is inversely proportional
to , as . We combine MC and SFR results on and
give a relation which is approximately valid for both sets. The axial and
azimuthal fluxes, and , vary as Mx and Mx/AU. The
relative helicity per unit length,
Mx/AU.Comment: abstract shortened for arxiv, 31 pages, 15 Figures, in press at Solar
Physic
Comparing plasma and faecal measures of steroid hormones in Adelie penguins Pygoscelis adeliae
Physiological measurements of both stress and sex hormones are often used to estimate the consequences of natural or human-induced change in ecological studies of various animals. Different methods of hormone measurement exist, potentially explaining variation in results across studies; methods should be cross-validated to ensure that they correlate. We directly compared faecal and plasma hormone measurements for the first time in a wild free-living species, the Adelie penguin (Pygoscelis adeliae). Blood and faecal samples were simultaneously collected from individual penguins for comparison and assayed for testosterone and corticosterone (or their metabolites). Sex differences and variability within each measure, and correlation of values across measures were compared. For both hormones, plasma samples showed greater variation than faecal samples. Males had higher mean corticosterone concentrations than females, but the difference was only statistically significant in faecal samples. Plasma testosterone, but not faecal testosterone, was significantly higher in males than females. Correlation between sample types was poor overall, and weaker in females than in males, perhaps because measures from plasma represent hormones that are both free and bound to globulins, whereas measures from faeces represent only the free portion. Faecal samples also represent a cumulative measure of hormones over time, as opposed to a plasma ‘snapshot’ concentration. Our data indicate that faecal sampling appears more suitable for assessing baseline hormone concentrations, whilst plasma sampling may best define immediate responses to environmental events. Consequently, future studies should ensure that they select the most appropriate matrix and method of hormone measurement to answer their research questions
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