322 research outputs found
Spectroscopic Observations of Hot Lines Constraining Coronal Heating in Solar Active Regions
EUV observations of warm coronal loops suggest that they are bundles of
unresolved strands that are heated impulsively to high temperatures by
nanoflares. The plasma would then have the observed properties (e.g., excess
density compared to static equilibrium) when it cools into the 1-2 MK range. If
this interpretation is correct, then very hot emission should be present
outside of proper flares. It is predicted to be vey faint, however. A critical
element for proving or refuting this hypothesis is the existence of hot, very
faint plasmas which should be at amounts predicted by impulsive heating. We
report on the first comprehensive spectroscopic study of hot plasmas in active
regions. Data from the EIS spectrometer on Hinode were used to construct
emission measure distributions in quiescent active regions in the 1-5 MK
temperature range. The distributions are flat or slowly increasing up to
approximately 3 MK and then fall off rapidly at higher temperatures. We show
that active region models based on impulsive heating can reproduce the observed
EM distributions relatively well. Our results provide strong new evidence that
coronal heating is impulsive in nature.Comment: ApJ, 2009, in pres
Hot coronal loops associated with umbral brightenings
We analyzed AIA/SDO high-cadence images in all bands, HMI/SDO data, soft
X-ray images from SXI/GOES-15, and Halpha images from the GONG network. We
detected umbral brightenings that were visible in all AIA bands as well as in
Halpha. Moreover, we identified hot coronal loops that connected the
brightenings with nearby regions of opposite magnetic polarity. These loops
were initially visible in the 94 A band, subsequently in the 335 A band, and in
one case in the 211 A band. A differential emission measure analysis revealed
plasma with an average temperature of about 6.5x10^6 K. This behavior suggests
cooling of impulsively heated loops.Comment: A&A, 2013, in pres
Toward understanding the early stages of an impulsively accelerated coronal mass ejection
The expanding magnetic flux in coronal mass ejections (CMEs) often forms a
cavity. A spherical model is simultaneously fit to STEREO EUVI and COR1 data of
an impulsively accelerated CME on 25 March 2008, which displays a well-defined
extreme ultraviolet (EUV) and white-light cavity of nearly circular shape
already at low heights ~ 0.2 Rs. The center height h(t) and radial expansion
r(t) of the cavity are obtained in the whole height range of the main
acceleration. We interpret them as the axis height and as a quantity
proportional to the minor radius of a flux rope, respectively. The
three-dimensional expansion of the CME exhibits two phases in the course of its
main upward acceleration. From the first h and r data points, taken shortly
after the onset of the main acceleration, the erupting flux shows an
overexpansion compared to its rise, as expressed by the decrease of the aspect
ratio from k=h/r ~ 3 to k ~ (1.5-2.0). This phase is approximately coincident
with the impulsive rise of the acceleration and is followed by a phase of very
gradual change of the aspect ratio (a nearly self-similar expansion) toward k ~
1.5 at h ~ 10 Rs. The initial overexpansion of the CME cavity can be caused by
flux conservation around a rising flux rope of decreasing axial current and by
the addition of flux to a growing, or even newly forming,flux rope by magnetic
reconnection. Further analysis will be required to decide which of these
contributions is dominant. The data also suggest that the horizontal component
of the impulsive cavity expansion (parallel to the solar surface) triggers the
associated EUV wave, which subsequently detaches from the CME volume.Comment: in press, A&A, 201
Observational features of equatorial coronal hole jets
Collimated ejections of plasma called "coronal hole jets" are commonly
observed in polar coronal holes. However, such coronal jets are not only a
specific features of polar coronal holes but they can also be found in coronal
holes appearing at lower heliographic latitudes. In this paper we present some
observations of "equatorial coronal hole jets" made up with data provided by
the STEREO/SECCHI instruments during a period comprising March 2007 and
December 2007. The jet events are selected by requiring at least some
visibility in both COR1 and EUVI instruments. We report 15 jet events, and we
discuss their main features. For one event, the uplift velocity has been
determined as about 200 km/s, while the deceleration rate appears to be about
0.11 km/s2, less than solar gravity. The average jet visibility time is about
30 minutes, consistent with jet observed in polar regions. On the basis of the
present dataset, we provisionally conclude that there are not substantial
physical differences between polar and equatorial coronal hole jets.Comment: 9 pages, 8 figures, 1 table, accepted for publication in Annales
Geophysicae, Special Issue:'Three eyes on the Sun-multi-spacecraft studies of
the corona and impacts on the heliosphere
Combining particle acceleration and coronal heating via data-constrained calculations of nanoflares in coronal loops
We model nanoflare heating of extrapolated active-region coronal loops via
the acceleration of electrons and protons in Harris-type current sheets. The
kinetic energy of the accelerated particles is estimated using semi-analytical
and test-particle-tracing approaches. Vector magnetograms and photospheric
Doppler velocity maps of NOAA active region 09114, recorded by the Imaging
Vector Magnetograph (IVM), were used for this analysis. A current-free field
extrapolation of the active-region corona was first constructed. The
corresponding Poynting fluxes at the footpoints of 5000 extrapolated coronal
loops were then calculated. Assuming that reconnecting current sheets develop
along these loops, we utilized previous results to estimate the kinetic-energy
gain of the accelerated particles and we related this energy to nanoflare
heating and macroscopic loop characteristics. Kinetic energies of 0.1 to 8 keV
(for electrons) and 0.3 to 470 keV (for protons) were found to cause heating
rates ranging from to 1 . Hydrodynamic
simulations show that such heating rates can sustain plasma in coronal
conditions inside the loops and generate plasma thermal distributions which are
consistent with active region observations. We concluded the analysis by
computing the form of X-ray spectra generated by the accelerated electrons
using the thick target approach that were found to be in agreement with
observed X-ray spectra, thus supporting the plausibility of our
nanoflare-heating scenario.Comment: 11 figure
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