269 research outputs found
Electron acceleration sites in a large-scale coronal structure
Radio observations and interplanetary particle measurements have
shown that even in the absence of conspicuous violent processes in the low
atmosphere (such as Hα flares) electrons are accelerated in the corona, most
likely at higher altitudes than during flares (≥0.5 R above the
photosphere). The paper presents direct evidence on the acceleration sites from
a case study of radio, visible light and soft X-ray observations: electrons are
repeatedly accelerated in a large-scale coronal structure which is identified
with a streamer in coronographic observations. Energy is simultaneously
released in an active region near the base of the structure and at a height of
∼1 R , over several hours before the large-scale structure erupts. Energy
input is observed in at least two emerging active regions underneath the
streamer. The coronal configuration is three-dimensional, overlying a whole
quadrant of the Sun. It is argued that the observations trace multiple sites of
energy release presumably in current sheets embedded within the streamer, in
agreement with scenarios developed for the acceleration of electrons seen in
the corona and at 1 AU, and for the evolution of large-scale coronal structures
towards eruption
RADIO EVIDENCE OF BREAK-OUT RECONNECTION?
We reconsider the 2003 October 28 X17 flare/coronal mass ejection (CME), studying the five minutes immediately before the impulsive flare phase (not discussed in previous work). To this aim we examine complementary dynamic radio spectrograms, single frequency polarimeter records, radio images, space-based longitudinal field magnetograms, and ultraviolet images. We find widely distributed faint and narrowband meter wave radio sources located outside active regions but associated with the boundaries of magnetic flux connectivity cells, inferred from the potential extrapolation of the observed photospheric longitudinal field as a model for coronal magnetic field structures. The meter wave radio sources occur during the initial decimeter wave effects, which are well known to be associated with filament destabilization in the flaring active region (here NOAA 10486). Antiochos et al. predict in their break-out model for CME initiation that "... huge phenomena ... may be controlled by detailed plasma processes that occur in relatively tiny regions." They suggest that the expected faint energy release "... on long field lines far away from any neutral line ... may be detectable in radio/microwave emission from nonthermal particles..." In this paper, we describe meter wave sources whose properties correctly coincide with the quoted predictions of the break-out reconnection model of the CME initiation
Radio Observations of the January 20, 2005 X-Class Event
We present a multi-frequency and multi-instrument study of the 20 January
2005 event. We focus mainly on the complex radio signatures and their
association with the active phenomena taking place: flares, CMEs, particle
acceleration and magnetic restructuring. As a variety of energetic particle
accelerators and sources of radio bursts are present, in the flare-ejecta
combination, we investigate their relative importance in the progress of this
event. The dynamic spectra of {Artemis-IV-Wind/Waves-Hiras with 2000 MHz-20 kHz
frequency coverage, were used to track the evolution of the event from the low
corona to the interplanetary space; these were supplemented with SXR, HXR and
gamma-ray recordings. The observations were compared with the expected radio
signatures and energetic-particle populations envisaged by the {Standard
Flare--CME model and the reconnection outflow termination shock model. A proper
combination of these mechanisms seems to provide an adequate model for the
interpretation of the observational data.Comment: Accepted for publication in Solar Physic
Determination of 3D Trajectories of Knots in Solar Prominences Using MSDP Data
In this paper we present a new method of restoration of the true
thee-dimensional trajectories of the prominence knots based on ground-based
observations taken with a single telescope, which is equipped with a
Multi-Channel Subtractive Double Pass imaging spectrograph. Our method allows
to evaluate true three-dimensional trajectories of the prominence knots without
any assumptions concerning the shape of the trajectories or dynamics of the
motion. The reconstructed trajectories of several knots observed in three
prominences are presented.Comment: 14 pages, 9 figures, accepted for publication in Solar Physic
SdhA blocks disruption of the Legionella-containing vacuole by hijacking the OCRL phosphatase
Legionella pneumophila grows intracellularly within a replication vacuole via action of Icm/Dot-secreted proteins. One such protein, SdhA, maintains the integrity of the vacuolar membrane, thereby preventing cytoplasmic degradation of bacteria. We show here that SdhA binds and blocks the action of OCRL (OculoCerebroRenal syndrome of Lowe), an inositol 5-phosphatase pivotal for controlling endosomal dynamics. OCRL depletion results in enhanced vacuole integrity and intracellular growth of a sdhA mutant, consistent with OCRL participating in vacuole disruption. Overexpressed SdhA alters OCRL function, enlarging endosomes, driving endosomal accumulation of phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2), and interfering with endosomal trafficking. SdhA interrupts Rab guanosine triphosphatase (GTPase)-OCRL interactions by binding to the OCRL ASPM-SPD2-Hydin (ASH) domain, without directly altering OCRL 5-phosphatase activity. The Legionella vacuole encompassing the sdhA mutant accumulates OCRL and endosomal antigen EEA1 (Early Endosome Antigen 1), consistent with SdhA blocking accumulation of OCRL-containing endosomal vesicles. Therefore, SdhA hijacking of OCRL is associated with blocking trafficking events that disrupt the pathogen vacuole.Peer Reviewe
Magnetic Reconnection with Asymmetry in the Outflow Direction
Magnetic reconnection with asymmetry in the outflow direction occurs in the
Earth's magnetotail, coronal mass ejections, flux cancellation events,
astrophysical disks, spheromak merging experiments, and elsewhere in nature and
the laboratory. A control volume analysis is performed for the case of steady
antiparallel magnetic reconnection with asymmetric downstream pressure, which
is used to derive scaling relations for the outflow velocity from each side of
the current sheet and the reconnection rate. Simple relationships for outflow
velocity are presented for the incompressible case and the case of symmetric
downstream pressure but asymmetric downstream density. Asymmetry alone is not
found to greatly affect the reconnection rate. The flow stagnation point and
magnetic field null do not coincide in a steady state unless the pressure
gradient is negligible at the flow stagnation point.Comment: 12 pages, 8 figures. Submitted to JGR. Any comments will be
appreciate
Vlasov-Maxwell, self-consistent electromagnetic wave emission simulations in the solar corona
1.5D Vlasov-Maxwell simulations are employed to model electromagnetic
emission generation in a fully self-consistent plasma kinetic model for the
first time in the solar physics context. The simulations mimic the plasma
emission mechanism and Larmor drift instability in a plasma thread that
connects the Sun to Earth with the spatial scales compressed appropriately. The
effects of spatial density gradients on the generation of electromagnetic
radiation are investigated. It is shown that 1.5D inhomogeneous plasma with a
uniform background magnetic field directed transverse to the density gradient
is aperiodically unstable to Larmor-drift instability. The latter results in a
novel effect of generation of electromagnetic emission at plasma frequency.
When density gradient is removed (i.e. when plasma becomes stable to
Larmor-drift instability) and a density, super-thermal, hot beam is
injected along the domain, in the direction perpendicular to the magnetic
field, plasma emission mechanism generates non-escaping Langmuir type
oscillations which in turn generate escaping electromagnetic radiation. It is
found that in the spatial location where the beam is injected, the standing
waves, oscillating at the plasma frequency, are excited. These can be used to
interpret the horizontal strips observed in some dynamical spectra. Quasilinear
theory predictions: (i) the electron free streaming and (ii) the beam long
relaxation time, in accord with the analytic expressions, are corroborated via
direct, fully-kinetic simulation. Finally, the interplay of Larmor-drift
instability and plasma emission mechanism is studied by considering
electron beam in the Larmor-drift unstable (inhomogeneous) plasma.
http://www.maths.qmul.ac.uk/~tsiklauri/movie1.mpg *
http://www.maths.qmul.ac.uk/~tsiklauri/movie2.mpg *
http://www.maths.qmul.ac.uk/~tsiklauri/movie3.mpgComment: Solar Physics (in press, the final, accepted version
On the relationship of shock waves to flares and coronal mass ejections
Context: Metric type II bursts are the most direct diagnostic of shock waves
in the solar corona.
Aims: There are two main competing views about the origin of coronal shocks:
that they originate in either blast waves ignited by the pressure pulse of a
flare or piston-driven shocks due to coronal mass ejections (CMEs). We studied
three well-observed type II bursts in an attempt to place tighter constraints
on their origins.
Methods: The type II bursts were observed by the ARTEMIS radio spectrograph
and imaged by the Nan\c{c}ay Radioheliograph (NRH) at least at two frequencies.
To take advantage of projection effects, we selected events that occurred away
from disk center.
Results: In all events, both flares and CMEs were observed. In the first
event, the speed of the shock was about 4200 km/s, while the speed of the CME
was about 850 km/s. This discrepancy ruled out the CME as the primary shock
driver. The CME may have played a role in the ignition of another shock that
occurred just after the high speed one. A CME driver was excluded from the
second event as well because the CMEs that appeared in the coronagraph data
were not synchronized with the type II burst. In the third event, the
kinematics of the CME which was determined by combining EUV and white light
data was broadly consistent with the kinematics of the type II burst, and,
therefore, the shock was probably CME-driven.
Conclusions: Our study demonstrates the diversity of conditions that may lead
to the generation of coronal shocks.Comment: 13 pages, 14 figures. "Astronomy and Astrophysics", in pres
Formation of current sheets and sigmoidal structure by the kink instability of a magnetic loop
We study dynamical consequences of the kink instability of a twisted coronal
flux rope, using the force-free coronal loop model by Titov & D\'emoulin (1999)
as the initial condition in ideal-MHD simulations. When a critical value of the
twist is exceeded, the long-wavelength () kink mode develops. Analogous to
the well-known cylindrical approximation, a helical current sheet is then
formed at the interface with the surrounding medium. In contrast to the
cylindrical case, upward-kinking loops form a second, vertical current sheet
below the loop apex at the position of the hyperbolic flux tube (generalized X
line) in the model. The current density is steepened in both sheets and
eventually exceeds the current density in the loop (although the kink
perturbation starts to saturate in our simulations without leading to a global
eruption). The projection of the field lines that pass through the vertical
current sheet shows an S shape whose sense agrees with the typical sense of
transient sigmoidal (forward or reverse S-shaped) structures that brighten in
soft X rays prior to coronal eruptions. The upward-kinked loop has the opposite
S shape, leading to the conclusion that such sigmoids do not generally show the
erupting loops themselves but indicate the formation of the vertical current
sheet below them that is the central element of the standard flare model.Comment: Astron. Astrophys. Lett., accepte
High-sensitivity observations of solar flare decimeter radiation
A new acousto-optic radio spectrometer has observed the 1 - 2 GHz radio
emission of solar flares with unprecedented sensitivity. The number of detected
decimeter type III bursts is greatly enhanced compared to observations by
conventional spectrometers observing only one frequency at the time. The
observations indicate a large number of electron beams propagating in dense
plasmas. For the first time, we report weak, reversed drifting type III bursts
at frequencies above simultaneous narrowband decimeter spikes. The type III
bursts are reliable signatures of electron beams propagating downward in the
corona, apparently away from the source of the spikes. The observations
contradict the most popular spike model that places the spike sources at the
footpoints of loops. Conspicuous also was an apparent bidirectional type U
burst forming a fish-like pattern. It occurs simultaneously with an intense
U-burst at 600-370 MHz observed in Tremsdorf. We suggest that it intermodulated
with strong terrestrial interference (cellular phones) causing a spurious
symmetric pattern in the spectrogram at 1.4 GHz. Symmetric features in the 1 -
2 GHz range, some already reported in the literature, therefore must be
considered with utmost caution.Comment: Astronomy and Astrophysics, in pres
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