257 research outputs found
Processes and mechanisms governing the initiation and propagation of CMEs
The most important observational characteristics of coronal mass ejections (CMEs) are summarized, emphasizing those aspects which are relevant for testing physical concepts employed to explain the CME take-off and propagation. In particular, the kinematics, scalings, and the CME-flare relationship are stressed. Special attention is paid to 3-dimensional (3-D) topology of the magnetic field structures, particularly to aspects related to the concept of semi-toroidal flux-rope anchored at both ends in the dense photosphere and embedded in the coronal magnetic arcade. Observations are compared with physical principles and concepts employed in explaining the CME phenomenon, and implications are discussed. A simple flux-rope model is used to explain various stages of the eruption. The model is able to reproduce all basic observational requirements: stable equilibrium and possible oscillations around equilibrium, metastable state and possible destabilization by an external disturbance, pre-eruptive gradual-rise until loss of equilibrium, possibility of fallback events and failed eruptions, relationship between impulsiveness of the CME acceleration and the source-region size, etc. However, it is shown that the purely ideal MHD process cannot account for highest observed accelerations which can attain values up to 10 km s<sup>&minus;2</sup>. Such accelerations can be achieved if the process of reconnection beneath the erupting flux-rope is included into the model. Essentially, the role of reconnection is in changing the magnetic flux associated with the flux-rope current and supplying "fresh" poloidal magnetic flux to the rope. These effects help sustain the electric current flowing along the flux-rope, and consequently, reinforce and prolong the CME acceleration. The model straightforwardly explains the observed synchronization of the flare impulsive phase and the CME main-acceleration stage, as well as the correlations between various CME and flare parameters
Millisecond solar radio bursts in the metric wavelength range
A study and classification of super-short structures (SSSs) recorded during
metric type IV bursts is presented. The most important property of SSSs is
their duration, at half power ranging from 4-50 ms, what is up to 10 times
shorter than spikes at corresponding frequencies. The solar origin of the SSSs
is confirmed by one-to-one correspondence between spectral recordings of
Artemis-IV1 and high time resolution single frequency measurements of the
TSRS2. We have divided the SSSs in the following categories:
1. Broad-Band SSSs: They were partitioned in two subcategories, the
SSS-Pulses and Drifting SSSs;
2. Narrow-band: They appear either as Spike-Like SSSs or as Patch-Like SSSs;
3. Complex SSS: They consist of the absorption-emission segments and were
morphologically subdivided into Rain-drop Bursts (narrow-band emission head and
a broad-band absorption tail) and Blinkers.Comment: Recent Advances in Astronomy and Astrophysics: 7th International
Conference of the Hellenic Astronomical Society. AIP Conference Proceedings,
Volume 848, pp. 224-228 (2006
Characteristics of Low-Latitude Coronal Holes near the Maximum of Solar cycle 24
We investigate the statistics of 288 low-latitude coronal holes extracted
from SDO/AIA-193 filtergrams over the time range 2011/01/01 to 2013/12/31. We
analyse the distribution of characteristic coronal hole properties, such as the
areas, mean AIA-193 intensities, and mean magnetic field densities, the local
distribution of the SDO/AIA-193 intensity and the magnetic field within the
coronal holes, and the distribution of magnetic flux tubes in coronal holes. We
find that the mean magnetic field density of all coronal holes under study is
3.0 +- 1.6 G, and the percentage of unbalanced magnetic flux is 49 +- 16 %. The
mean magnetic field density, the mean unsigned magnetic field density, and the
percentage of unbalanced magnetic flux of coronal holes depend strongly
pairwise on each other, with correlation coefficients cc > 0.92. Furthermore,
we find that the unbalanced magnetic flux of the coronal holes is predominantly
concentrated in magnetic flux tubes: 38 % (81 %) of the unbalanced magnetic
flux of coronal holes arises from only 1 % (10 %) of the coronal hole area,
clustered in magnetic flux tubes with field strengths > 50 G (10 G). The
average magnetic field density and the unbalanced magnetic flux derived from
the magnetic flux tubes correlate with the mean magnetic field density and the
unbalanced magnetic flux of the overall coronal hole (cc > 0.93). These
findings give evidence that the overall magnetic characteristics of coronal
holes are governed by the characteristics of the magnetic flux tubes.Comment: 15 figure
Coronal Shock Waves, EUV waves, and their Relation to CMEs. II. Modeling MHD Shock Wave Propagation Along the Solar Surface, Using Nonlinear Geometrical Acoustics
We model the propagation of a coronal shock wave, using nonlinear geometrical
acoustics. The method is based on the Wentzel-Kramers-Brillouin (WKB) approach
and takes into account the main properties of nonlinear waves: i) dependence of
the wave front velocity on the wave amplitude, ii) nonlinear dissipation of the
wave energy, and iii) progressive increase in the duration of solitary shock
waves. We address the method in detail and present results of the modeling of
the propagation of shock-associated extreme-ultraviolet (EUV) waves as well as
Moreton waves along the solar surface in the simplest solar corona model. The
calculations reveal deceleration and lengthening of the waves. In contrast,
waves considered in the linear approximation keep their length unchanged and
slightly accelerate.Comment: 15 pages, 7 figures, accepted for publication in Solar Physic
The role of aerodynamic drag in propagation of interplanetary coronal mass ejections
Context. The propagation of interplanetary coronal mass ejections (ICMEs) and the forecast of their arrival on Earth is one of the
central issues of space weather studies.
Aims. We investigate to which degree various ICME parameters (mass, size, take-off speed) and the ambient solar-wind parameters (density
and velocity) affect the ICME Sun-Earth transit time.
Methods. We study solutions of a drag-based equation of motion by systematically varying the input parameters. The analysis is focused on ICME
transit times and 1Â AU velocities.
Results. The model results reveal that wide ICMEs of low masses adjust to the solar-wind speed already close to the sun, so the transit time is
determined primarily by the solar-wind speed. The shortest transit times and accordingly the highest 1Â AU velocities are related to
narrow and massive ICMEs (i.e. high-density eruptions) propagating in high-speed solar wind streams. We apply the model to the
Sun-Earth event associated with the CME of 25 July 2004 and compare the results with the outcome of the numerical MHD modeling
Heliospheric Evolution of Magnetic Clouds
Interplanetary evolution of eleven magnetic clouds (MCs) recorded by at least
two radially aligned spacecraft is studied. The in situ magnetic field
measurements are fitted to a cylindrically symmetric Gold-Hoyle force-free
uniform-twist flux-rope configuration. The analysis reveals that in a
statistical sense the expansion of studied MCs is compatible with self-similar
behavior. However, individual events expose a large scatter of expansion rates,
ranging from very weak to very strong expansion. Individually, only four events
show an expansion rate compatible with the isotropic self-similar expansion.
The results indicate that the expansion has to be much stronger when MCs are
still close to the Sun than in the studied 0.47 - 4.8 AU distance range. The
evolution of the magnetic field strength shows a large deviation from the
behavior expected for the case of an isotropic self-similar expansion. In the
statistical sense, as well as in most of the individual events, the inferred
magnetic field decreases much slower than expected. Only three events show a
behavior compatible with a self-similar expansion. There is also a discrepancy
between the magnetic field decrease and the increase of the MC size, indicating
that magnetic reconnection and geometrical deformations play a significant role
in the MC evolution. About half of the events show a decay of the electric
current as expected for the self-similar expansion. Statistically, the inferred
axial magnetic flux is broadly consistent with it remaining constant. However,
events characterized by large magnetic flux show a clear tendency of decreasing
flux.Comment: 64 pages, 10 figure
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
The Wave-Driver System of the Off-Disk Coronal Wave 17 January 2010
We study the 17 January 2010 flare-CME-wave event by using STEREO/SECCHI EUVI
and COR1 data. The observational study is combined with an analytic model which
simulates the evolution of the coronal-wave phenomenon associated with the
event. From EUV observations, the wave signature appears to be dome shaped
having a component propagating on the solar surface (v~280 km s-1) as well as
off-disk (v~600 km s-1) away from the Sun. The off-disk dome of the wave
consists of two enhancements in intensity, which conjointly develop and can be
followed up to white-light coronagraph images. Applying an analytic model, we
derive that these intensity variations belong to a wave-driver system with a
weakly shocked wave, initially driven by expanding loops, which are indicative
of the early evolution phase of the accompanying CME. We obtain the shock
standoff distance between wave and driver from observations as well as from
model results. The shock standoff distance close to the Sun (<0.3 Rs above the
solar surface) is found to rapidly increase with values of ~0.03-0.09 Rs which
give evidence of an initial lateral (over-)expansion of the CME. The
kinematical evolution of the on-disk wave could be modeled using input
parameters which require a more impulsive driver (t=90 s, a=1.7 km s-2)
compared to the off-disk component (t=340 s, a=1.5 km s-2).Comment: accepted for publication in Solar Physic
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