27 research outputs found
Can coronal hole spicules reach coronal temperatures?
We aim with the present study to provide observational evidences on whether
coronal hole spicules reach coronal temperatures. We combine multi-instrument
co-observations obtained with the SUMER/SoHO and with the EIS/SOT/XRT/Hinode.
The analysed three large spicules were found to be comprised of numerous thin
spicules which rise, rotate and descend simultaneously forming a bush-like
feature. Their rotation resembles the untwisting of a large flux rope. They
show velocities ranging from 50 to 250 km/s. We clearly associated the red- and
blue-shifted emissions in transition region lines with rotating but also with
rising and descending plasmas, respectively. Our main result is that these
spicules although very large and dynamic, show no presence in spectral lines
formed at temperatures above 300 000 K. The present paper brings out the
analysis of three Ca II H large spicules which are composed of numerous dynamic
thin spicules but appear as macrospicules in EUV lower resolution images. We
found no coronal counterpart of these and smaller spicules. We believe that the
identification of phenomena which have very different origins as macrospicules
is due to the interpretation of the transition region emission, and especially
the He II emission, wherein both chromospheric large spicules and coronal X-ray
jets are present. We suggest that the recent observation of spicules in the
coronal AIA/SDO 171 A and 211 A channels is probably due to the existence of
transition region emission there.Comment: 4 pages, 4 figures, accepted for publication in A&
Coronal response to an EUV wave from DEM analysis
EUV (Extreme-Ultraviolet) waves are globally propagating disturbances that
have been observed since the era of the SoHO/EIT instrument. Although the
kinematics of the wave front and secondary wave components have been widely
studied, there is not much known about the generation and plasma properties of
the wave. In this paper we discuss the effect of an EUV wave on the local
plasma as it passes through the corona. We studied the EUV wave, generated
during the 2011 February 15 X-class flare/CME event, using Differential
Emission Measure diagnostics. We analyzed regions on the path of the EUV wave
and investigated the local density and temperature changes. From our study we
have quantitatively confirmed previous results that during wave passage the
plasma visible in the Atmospheric Imaging Assembly (AIA) 171A channel is
getting heated to higher temperatures corresponding to AIA 193A and 211A
channels. We have calculated an increase of 6 - 9% in density and 5 - 6% in
temperature during the passage of the EUV wave. We have compared the variation
in temperature with the adiabatic relationship and have quantitatively
demonstrated the phenomenon of heating due to adiabatic compression at the wave
front. However, the cooling phase does not follow adiabatic relaxation but
shows slow decay indicating slow energy release being triggered by the wave
passage. We have also identified that heating is taking place at the front of
the wave pulse rather than at the rear. Our results provide support for the
case that the event under study here is a compressive fast-mode wave or a
shock.Comment: Accepted for publication in Ap
Magnetic Reconnection resulting from Flux Emergence: Implications for Jet Formation in the lower solar atmosphere?
We aim at investigating the formation of jet-like features in the lower solar
atmosphere, e.g. chromosphere and transition region, as a result of magnetic
reconnection. Magnetic reconnection as occurring at chromospheric and
transition regions densities and triggered by magnetic flux emergence is
studied using a 2.5D MHD code. The initial atmosphere is static and isothermal,
with a temperature of 20,000 K. The initial magnetic field is uniform and
vertical. Two physical environments with different magnetic field strength (25
G and 50 G) are presented. In each case, two sub-cases are discussed, where the
environments have different initial mass density. In the case where we have a
weaker magnetic field (25 G) and higher plasma density (
cm), valid for the typical quiet Sun chromosphere, a plasma jet would be
observed with a temperature of 2--3 K and a velocity as high as
40 km/s. The opposite case of a medium with a lower electron density
( cm), i.e. more typical for the transition region,
and a stronger magnetic field of 50 G, up-flows with line-of-sight velocities
as high as 90 km/s and temperatures of 6 10 K, i.e. upper
transition region -- low coronal temperatures, are produced. Only in the latter
case, the low corona Fe IX 171 \AA\ shows a response in the jet which is
comparable to the O V increase. The results show that magnetic reconnection can
be an efficient mechanism to drive plasma outflows in the chromosphere and
transition region. The model can reproduce characteristics, such as temperature
and velocity for a range of jet features like a fibril, a spicule, an hot X-ray
jet or a transition region jet by changing either the magnetic field strength
or the electron density, i.e. where in the atmosphere the reconnection occurs.Comment: 11 pages, 13 figures, 2 table
Off-limb (spicule) DEM distribution from SoHO/SUMER observations
In the present work we derive a Differential Emission Measure (DEM) dis-
tribution from a region dominated by spicules. We use spectral data from the
Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer
on-board the Solar Heliospheric Observatory (SoHO) covering the entire SUMER
wavelength range taken off-limb in the Northern polar coronal hole to construct
this DEM distribution using the CHIANTI atomic database. This distribution is
then used to study the thermal properties of the emission contributing to the
171 {\AA} channel in the Atmospheric Imaging Assembly (AIA) on-board the Solar
Dynamics Observatory (SDO). From our off-limb DEM we found that the radiance in
the AIA 171 {\AA} channel is dominated by emission from the Fe ix 171.07 {\AA}
line and has sparingly little contribution from other lines. The product of the
Fe ix 171.07 {\AA} line contribution function with the off-limb DEM was found
to have a maximum at logTmax (K) = 5.8 indicating that during spicule
observations the emission in this line comes from plasma at transition region
temperatures rather than coronal. For comparison, the same product with a quiet
Sun and prominence DEM were found to have a maximum at logT max (K) = 5.9 and
logTmax (K) = 5.7, respectively. We point out that the interpretation of data
obtained from the AIA 171 {\AA} filter should be done with foreknowledge of the
thermal nature of the observed phenomenon. For example, with an off-limb DEM we
find that only 3.6% of the plasma is above a million degrees, whereas using a
quiet Sun DEM, this contribution rises to 15%.Comment: 12 pages, 6 figures accepted by Solar Physic
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
Homogenized dynamics of stochastic partial differential equations with dynamical boundary conditions
A microscopic heterogeneous system under random influence is considered. The
randomness enters the system at physical boundary of small scale obstacles as
well as at the interior of the physical medium. This system is modeled by a
stochastic partial differential equation defined on a domain perforated with
small holes (obstacles or heterogeneities), together with random dynamical
boundary conditions on the boundaries of these small holes.
A homogenized macroscopic model for this microscopic heterogeneous stochastic
system is derived. This homogenized effective model is a new stochastic partial
differential equation defined on a unified domain without small holes, with
static boundary condition only. In fact, the random dynamical boundary
conditions are homogenized out, but the impact of random forces on the small
holes' boundaries is quantified as an extra stochastic term in the homogenized
stochastic partial differential equation. Moreover, the validity of the
homogenized model is justified by showing that the solutions of the microscopic
model converge to those of the effective macroscopic model in probability
distribution, as the size of small holes diminishes to zero.Comment: Communications in Mathematical Physics, to appear, 200
Geometrical optics approximation for general scalar conservation laws
This article does not have an abstract
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