39 research outputs found
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 1 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 1 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
Large amplitude oscillatory motion along a solar filament
Large amplitude oscillations of solar filaments is a phenomenon known for
more than half a century. Recently, a new mode of oscillations, characterized
by periodical plasma motions along the filament axis, was discovered. We
analyze such an event, recorded on 23 January 2002 in Big Bear Solar
Observatory H filtergrams, in order to infer the triggering mechanism
and the nature of the restoring force. Motion along the filament axis of a
distinct buldge-like feature was traced, to quantify the kinematics of the
oscillatory motion. The data were fitted by a damped sine function, to estimate
the basic parameters of the oscillations. In order to identify the triggering
mechanism, morphological changes in the vicinity of the filament were analyzed.
The observed oscillations of the plasma along the filament was characterized by
an initial displacement of 24 Mm, initial velocity amplitude of 51 km/s, period
of 50 min, and damping time of 115 min. We interpret the trigger in terms of
poloidal magnetic flux injection by magnetic reconnection at one of the
filament legs. The restoring force is caused by the magnetic pressure gradient
along the filament axis. The period of oscillations, derived from the
linearized equation of motion (harmonic oscillator) can be expressed as
, where represents the Alfv\'en speed based on the
equilibrium poloidal field . Combination of our measurements with
some previous observations of the same kind of oscillations shows a good
agreement with the proposed interpretation.Comment: Astron. Astrophys., 2007, in pres
Understanding the physical nature of coronal "EIT waves"
For almost 20 years the physical nature of globally propagating waves in the solar corona (commonly called "EIT waves") has been controversial and subject to debate. Additional theories have been proposed over the years to explain observations that did not fit with the originally proposed fast-mode wave interpretation. However, the incompatibility of observations made using the Extreme-ultraviolet Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory with the fast-mode wave interpretation was challenged by differing viewpoints from the twin Solar Terrestrial Relations Observatory spacecraft and higher spatial/temporal resolution data from the Solar Dynamics Observatory.
In this article, we reexamine the theories proposed to explain "EIT waves" to identify measurable properties and behaviours that can be compared to current and future observations. Most of us conclude that "EIT waves" are best described as fast-mode large-amplitude waves/shocks that are initially driven by the impulsive expansion of an erupting coronal mass ejection in the low corona
Understanding the physical nature of coronal "EIT waves"
For almost 20 years the physical nature of globally propagating waves in the solar corona (commonly called "EIT waves") has been controversial and subject to debate. Additional theories have been proposed over the years to explain observations that did not fit with the originally proposed fast-mode wave interpretation. However, the incompatibility of observations made using the Extreme-ultraviolet Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory with the fast-mode wave interpretation was challenged by differing viewpoints from the twin Solar Terrestrial Relations Observatory spacecraft and higher spatial/temporal resolution data from the Solar Dynamics Observatory.
In this article, we reexamine the theories proposed to explain "EIT waves" to identify measurable properties and behaviours that can be compared to current and future observations. Most of us conclude that "EIT waves" are best described as fast-mode large-amplitude waves/shocks that are initially driven by the impulsive expansion of an erupting coronal mass ejection in the low corona
Morphology and density of post-CME current sheets
Eruption of a coronal mass ejection (CME) drags and "opens" the coronal
magnetic field, presumably leading to the formation of a large-scale current
sheet and the field relaxation by magnetic reconnection. We analyze physical
characteristics of ray-like coronal features formed in the aftermath of CMEs,
to check if the interpretation of this phenomenon in terms of reconnecting
current sheet is consistent with the observations. The study is focused on
measurements of the ray width, density excess, and coronal velocity field as a
function of the radial distance. The morphology of rays indicates that they
occur as a consequence of Petschek-like reconnection in the large scale current
sheet formed in the wake of CME. The hypothesis is supported by the flow
pattern, often showing outflows along the ray, and sometimes also inflows into
the ray. The inferred inflow velocities range from 3 to 30 km s,
consistent with the narrow opening-angle of rays, adding up to a few degrees.
The density of rays is an order of magnitude larger than in the ambient corona.
The density-excess measurements are compared with the results of the analytical
model in which the Petschek-like reconnection geometry is applied to the
vertical current sheet, taking into account the decrease of the external
coronal density and magnetic field with height. The model results are
consistent with the observations, revealing that the main cause of the density
excess in rays is a transport of the dense plasma from lower to larger heights
by the reconnection outflow
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
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
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