2,343 research outputs found
Multi-thermal dynamics and energetics of a coronal mass ejection in the low solar atmosphere
The aim of this work is to determine the multi-thermal characteristics and
plasma energetics of an eruptive plasmoid and occulted flare observed by Solar
Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA). We study an event
from 03-Nov-2010 (peaking at 12:20UT in GOES soft X-rays) of a coronal mass
ejection and occulted flare which demonstrates the morphology of a classic
erupting flux rope. The high spatial, and time resolution, and six coronal
channels, of the SDO/AIA images allows the dynamics of the multi-thermal
emission during the initial phases of eruption to be studied in detail. The
Differential Emission Measure (DEM) is calculated, using an optimised version
of a regularized inversion method (Hannah & Kontar 2012), for each pixel across
the six channels at different times, resulting in emission measure maps and
movies in a variety of temperature ranges. We find that the core of the
erupting plasmoid is hot (8-11, 11-14MK) with a similarly hot filamentary
"stem" structure connecting it to the lower atmosphere, which could be
interpreted as the current sheet in the flux rope model, though is wider than
these models suggest. The velocity of the leading edge of the eruption is
597-664 km s in the temperature range 3-4MK and between 1029-1246
km s for 2-3MK. We estimate the density (in 11-14 MK) of the
erupting core and stem during the impulsive phase to be about
cm, cm, cm in the plasmoid
core, stem and surrounding envelope of material. This gives thermal energy
estimates of erg, erg and
erg. The kinetic energy for the core and envelope is slightly smaller. The
thermal energy of the core and current sheet grows during the eruption,
suggesting continuous influx of energy presumably via reconnection.Comment: Submitted to A&A: in revisio
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
The association of the Hale Sector Boundary with RHESSI solar flares and active longitudes
The heliospheric magnetic field (HMF) is structured into large sectors of
positive and negative polarity. The parts of the boundary between these sectors
where the change in polarity matches that of the leading-to-following sunspot
polarity in that solar hemisphere, are called Hale Sector Boundaries (HSB). We
investigate the flare occurrence rate near HSBs and the association between
HSBs and active longitudes. Previous work determined the times HSBs were at
solar central meridian, using the detection of the HMF sector boundary crossing
at the Earth. In addition to this, we use a new approach which finds the HSB
locations at all times by determining them from Potential Field Source Surface
(PFSS) extrapolations of photospheric magnetograms. We use the RHESSI X-ray
flare list for comparison to the HSB as it provides accurate flare locations
over 14 years, from February 2002 to February 2016, covering both Cycles 23 and
24. For the active longitude positions we use previously published work based
on sunspot observations. We find that the two methods of determining the HSB
generally agree and that 41% (Cycle 23) and 47% (Cycle 24) of RHESSI flares
occur within of the PFSS determined-HSB. The behaviour of the HSBs
varies over the two Cycles studied, and as expected they swap in hemisphere as
the Cycles change. The HSBs and active longitudes do overlap but not
consistently. They often move at different rates relative to each other (and
the Carrington solar rotation rate) and these vary over each Cycle. The HSBs
provide a useful additional activity indicator, particularly during periods
when active longitudes are difficult to determine
Observations of Reconnection Flows in a Flare on the Solar Disk
Magnetic reconnection is a well-accepted part of the theory of solar eruptive
events, though the evidence is still circumstantial. Intrinsic to the
reconnection picture of a solar eruptive event, particularly in the standard
model for two-ribbon flares ("CSHKP" model), are an advective flow of
magnetized plasma into the reconnection region, expansion of field above the
reconnection region as a flux rope erupts, retraction of heated
post-reconnection loops, and downflows of cooling plasma along those loops. We
report on a unique set of SDO/AIA imaging and Hinode/EIS spectroscopic
observations of the disk flare SOL2016-03-23T03:54 in which all four flows are
present simultaneously. This includes spectroscopic evidence for a plasma
upflow in association with large-scale expanding closed inflow field. The
reconnection inflows are symmetric, and consistent with fast reconnection, and
the post-reconnection loops show a clear cooling and deceleration as they
retract. Observations of coronal reconnection flows are still rare, and most
events are observed at the solar limb, obscured by complex foregrounds, making
their relationship to the flare ribbons, cusp field and arcades formed in the
lower atmosphere difficult to interpret. The disk location and favorable
perspective of this event have removed these ambiguities giving a clear picture
of the reconnection dynamics.Comment: 9 pages, 5 figures, and 1 table. Accepted for publication in ApJ
First limits on the 3-200 keV X-ray spectrum of the quiet Sun using RHESSI
We present the first results using the Reuven Ramaty High-Energy Solar
Spectroscopic Imager, RHESSI, to observe solar X-ray emission not associated
with active regions, sunspots or flares (the quiet Sun). Using a newly
developed chopping technique (fan-beam modulation) during seven periods of
offpointing between June 2005 to October 2006, we obtained upper limits over
3-200 keV for the quietest times when the GOES12 1-8A flux fell below
Wm. These values are smaller than previous limits in the 17-120 keV
range and extend them to both lower and higher energies. The limit in 3-6 keV
is consistent with a coronal temperature MK. For quiet Sun periods
when the GOES12 1-8A background flux was between Wm and
Wm, the RHESSI 3-6 keV flux correlates to this as a power-law,
with an index of . The power-law correlation for microflares has
a steeper index of . We also discuss the possibility of
observing quiet Sun X-rays due to solar axions and use the RHESSI quiet Sun
limits to estimate the axion-to-photon coupling constant for two different
axion emission scenarios.Comment: 4 pages, 3 figures, Accepted by ApJ letter
The sub-arcsecond hard X-ray structure of loop footpoints in a solar flare
The newly developed X-ray visibility forward fitting technique is applied to
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data of a limb
flare to investigate the energy and height dependence on sizes, shapes, and
position of hard X-ray chromospheric footpoint sources. This provides
information about the electron transport and chromospheric density structure.
The spatial distribution of two footpoint X-ray sources is analyzed using
PIXON, Maximum Entropy Method, CLEAN and visibility forward fit algorithms at
nonthermal energies from to keV. We report, for the first
time, the vertical extents and widths of hard X-ray chromospheric sources
measured as a function of energy for a limb event. Our observations suggest
that both the vertical and horizontal sizes of footpoints are decreasing with
energy. Higher energy emission originates progressively deeper in the
chromosphere consistent with downward flare accelerated streaming electrons.
The ellipticity of the footpoints grows with energy from at keV to at keV. The positions of X-ray emission are in
agreement with an exponential density profile of scale height ~km.
The characteristic size of the hard X-ray footpoint source along the limb is
decreasing with energy suggesting a converging magnetic field in the footpoint.
The vertical sizes of X-ray sources are inconsistent with simple collisional
transport in a single density scale height but can be explained using a
multi-threaded density structure in the chromosphere.Comment: 7 pages, 7 figures, submitted to Ap
The effect of wave-particle interactions on low energy cutoffs in solar flare electron spectra
Solar flare hard X-ray spectra from RHESSI are normally interpreted in terms
of purely collisional electron beam propagation, ignoring spatial evolution and
collective effects. In this paper we present self-consistent numerical
simulations of the spatial and temporal evolution of an electron beam subject
to collisional transport and beam-driven Langmuir wave turbulence. These
wave-particle interactions represent the background plasma's response to the
electron beam propagating from the corona to chromosphere and occur on a far
faster timescale than coulomb collisions. From these simulations we derive the
mean electron flux spectrum, comparable to such spectra recovered from high
resolution hard X-rays observations of solar flares with RHESSI. We find that a
negative spectral index (i.e. a spectrum that increases with energy), or local
minima when including the expected thermal spectral component at low energies,
occurs in the standard thick-target model, when coulomb collisions are only
considered. The inclusion of wave-particle interactions does not produce a
local minimum, maintaining a positive spectral index. These simulations are a
step towards a more complete treatment of electron transport in solar flares
and suggest that a flat spectrum (spectral index of 0 to 1) down to thermal
energies maybe a better approximation instead of a sharp cut-off in the
injected electron spectrum.Comment: 6 pages, 5 figures, accepted by ApJ
The spectral difference between solar flare HXR coronal and footpoint sources due to wave-particle interactions
Investigate the spatial and spectral evolution of hard X-ray (HXR) emission
from flare accelerated electron beams subject to collisional transport and
wave-particle interactions in the solar atmosphere. We numerically follow the
propagation of a power-law of accelerated electrons in 1D space and time with
the response of the background plasma in the form of Langmuir waves using the
quasilinear approximation.}{We find that the addition of wave-particle
interactions to collisional transport for a transient initially injected
electron beam flattens the spectrum of the footpoint source. The coronal source
is unchanged and so the difference in the spectral indices between the coronal
and footpoint sources is \Delta \gamma > 2, which is larger than expected from
purely collisional transport. A steady-state beam shows little difference
between the two cases, as has been previously found, as a transiently injected
electron beam is required to produce significant wave growth, especially at
higher velocities. With this transiently injected beam the wave-particle
interactions dominate in the corona whereas the collisional losses dominate in
the chromosphere. The shape of the spectrum is different with increasing
electron beam density in the wave-particle interaction case whereas with purely
collisional transport only the normalisation is changed. We also find that the
starting height of the source electron beam above the photosphere affects the
spectral index of the footpoint when Langmuir wave growth is included. This may
account for the differing spectral indices found between double footpoints if
asymmetrical injection has occurred in the flaring loop.Comment: 10 pages, 10 FIgures, accepted for publication in A&
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