98 research outputs found
Widespread nanoflare variability detected with Hinode/XRT in a solar active region
It is generally agreed that small impulsive energy bursts called nanoflares
are responsible for at least some of the Sun's hot corona, but whether they are
the explanation for most of the multi-million degree plasma has been a matter
of ongoing debate. We here present evidence that nanoflares are widespread in
an active region observed by the X-Ray Telescope on-board the Hinode mission.
The distributions of intensity fluctuations have small but important
asymmetries, whether taken from individual pixels, multi-pixel subregions, or
the entire active region. Negative fluctuations (corresponding to reduced
intensity) are greater in number but weaker in amplitude, so that the median
fluctuation is negative compared to a mean of zero. Using MonteCarlo
simulations, we show that only part of this asymmetry can be explained by
Poisson photon statistics. The remainder is explainable with a tendency for
exponentially decreasing intensity, such as would be expected from a cooling
plasma produced from a nanoflare. We suggest that nanoflares are a universal
heating process within active regions.Comment: 26 pages, 7 figure
Is the Polar Region Different from the Quiet Region of the Sun?
Observations of the polar region of the Sun are critically important for
understanding the solar dynamo and the acceleration of solar wind. We carried
out precise magnetic observations on both the North polar region and the quiet
Sun at the East limb with the Spectro-Polarimeter of the Solar Optical
Telescope aboard Hinode to characterize the polar region with respect to the
quiet Sun. The average area and the total magnetic flux of the kG magnetic
concentrations in the polar region appear to be larger than those of the quiet
Sun. The magnetic field vectors classified as vertical in the quiet Sun have
symmetric histograms around zero in the strengths, showing balanced positive
and negative flux, while the histogram in the North polar region is clearly
asymmetric, showing a predominance of the negative polarity. The total magnetic
flux of the polar region is larger than that of the quiet Sun. In contrast, the
histogram of the horizontal magnetic fields is exactly the same between the
polar region and the quiet Sun. This is consistent with the idea that a local
dynamo process is responsible for the horizontal magnetic fields. A
high-resolution potential field extrapolation shows that the majority of
magnetic field lines from the kG-patches in the polar region are open with a
fanning-out structure very low in the atmosphere, while in the quiet Sun,
almost all the field lines are closed.Comment: Accepted for publication in AP
3D view of transient horizontal magnetic fields in the photosphere
We infer the 3D magnetic structure of a transient horizontal magnetic field
(THMF) during its evolution through the photosphere using SIRGAUS inversion
code. The SIRGAUS code is a modified version of SIR (Stokes Inversion based on
Response function), and allows for retrieval of information on the magnetic and
thermodynamic parameters of the flux tube embedded in the atmosphere from the
observed Stokes profiles. Spectro-polarimetric observations of the quiet Sun at
the disk center were performed with the Solar Optical Telescope (SOT) on board
Hinode with Fe I 630.2 nm lines. Using repetitive scans with a cadence of 130
s, we first detect the horizontal field that appears inside a granule, near its
edge. On the second scan, vertical fields with positive and negative polarities
appear at both ends of the horizontal field. Then, the horizontal field
disappears leaving the bipolar vertical magnetic fields. The results from the
inversion of the Stokes spectra clearly point to the existence of a flux tube
with magnetic field strength of G rising through the line forming
layer of the Fe I 630.2 nm lines. The flux tube is located at around
at =0 s and around
at =130 s. At =260 s the horizontal part is already above
the line forming region of the analyzed lines. The observed Doppler velocity is
maximally 3 km s, consistent with the upward motion of the structure as
retrieved from the SIRGAUS code. The vertical size of the tube is smaller than
the thickness of the line forming layer. The THMF has a clear
-shaped-loop structure with the apex located near the edge of a
granular cell. The magnetic flux carried by this THMF is estimated to be
Mx.Comment: 35 pages, 9 figures, Accepted for publication in Ap
Fermi acceleration at fast shock in a solar flare and impulsive loop-top hard X-ray source
We propose that non-thermal electrons are efficiently accelerated by
first-order Fermi process at the fast shock, as a natural consequence of the
new magnetohydrodynamic picture of the flaring region revealed with Yohkoh. An
oblique fast shock is naturally formed below the reconnection site, and boosts
the acceleration to significantly decrease the injection energy. The slow
shocks attached to the reconnection X-point heat the plasma up to 10--20 MK,
exceeding the injection energy. The combination of the oblique shock
configuration and the pre-heating by the slow shock allows bulk electron
acceleration from the thermal pool. The accelerated electrons are trapped
between the two slow shocks due to the magnetic mirror downstream of the fast
shock, thus explaining the impulsive loop-top hard X-ray source discovered with
Yohkoh. Acceleration time scale is ~ 0.3--0.6 s, which is consistent with the
time scale of impulsive bursts. When these electrons stream away from the
region enclosed by the fast shock and the slow shocks, they are released toward
the footpoints and may form the simultaneous double-source hard X-ray structure
at the footpoints of the reconnected field lines.Comment: 13 pages, 3 postscript figures, used AASTeX macros; accepted in
Astrophysical Journal Letter
Can High Frequency Acoustic Waves Heat the Quiet Sun Chromosphere?
We use Hinode/SOT Ca II H-line and blue continuum broadband observations to
study the presence and power of high frequency acoustic waves at high spatial
resolution. We find that there is no dominant power at small spatial scales;
the integrated power using the full resolution of Hinode (0.05'' pixels, 0.16''
resolution) is larger than the power in the data degraded to 0.5'' pixels
(TRACE pixel size) by only a factor of 1.2. At 20 mHz the ratio is 1.6.
Combining this result with the estimates of the acoustic flux based on TRACE
data of Fossum & Carlsson (2006), we conclude that the total energy flux in
acoustic waves of frequency 5-40 mHz entering the internetwork chromosphere of
the quiet Sun is less than 800 W m, inadequate to balance the radiative
losses in a static chromosphere by a factor of five.Comment: 6 pages, 8 figures, accepted for publication in PASJ (special Hinode
issue
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