15 research outputs found
Magnetic Energy Spectra in Active Regions
Line-of-sight magnetograms for 217 active regions (ARs) of different flare
rate observed at the solar disk center from January 1997 until December 2006
are utilized to study the turbulence regime and its relationship to the flare
productivity. Data from {\it SOHO}/MDI instrument recorded in the high
resolution mode and data from the BBSO magnetograph were used. The turbulence
regime was probed via magnetic energy spectra and magnetic dissipation spectra.
We found steeper energy spectra for ARs of higher flare productivity. We also
report that both the power index, , of the energy spectrum, , and the total spectral energy are comparably
correlated with the flare index, , of an active region. The correlations are
found to be stronger than that found between the flare index and total unsigned
flux. The flare index for an AR can be estimated based on measurements of
and as , with and . We found that the regime of the fully-developed turbulence occurs in
decaying ARs and in emerging ARs (at the very early stage of emergence).
Well-developed ARs display under-developed turbulence with strong magnetic
dissipation at all scales.Comment: 14 pages, 4 figure
25 Years of Self-Organized Criticality: Numerical Detection Methods
The detection and characterization of self-organized criticality (SOC), in
both real and simulated data, has undergone many significant revisions over the
past 25 years. The explosive advances in the many numerical methods available
for detecting, discriminating, and ultimately testing, SOC have played a
critical role in developing our understanding of how systems experience and
exhibit SOC. In this article, methods of detecting SOC are reviewed; from
correlations to complexity to critical quantities. A description of the basic
autocorrelation method leads into a detailed analysis of application-oriented
methods developed in the last 25 years. In the second half of this manuscript
space-based, time-based and spatial-temporal methods are reviewed and the
prevalence of power laws in nature is described, with an emphasis on event
detection and characterization. The search for numerical methods to clearly and
unambiguously detect SOC in data often leads us outside the comfort zone of our
own disciplines - the answers to these questions are often obtained by studying
the advances made in other fields of study. In addition, numerical detection
methods often provide the optimum link between simulations and experiments in
scientific research. We seek to explore this boundary where the rubber meets
the road, to review this expanding field of research of numerical detection of
SOC systems over the past 25 years, and to iterate forwards so as to provide
some foresight and guidance into developing breakthroughs in this subject over
the next quarter of a century.Comment: Space Science Review series on SO
Photospheric Signatures of Granular-scale Flux Emergence and Cancellation at the Penumbral Boundary
We studied flux emergence events of sub-granular scale in a solar active
region. New Solar Telescope (NST) of Big Bear Solar Observatory made it
possible to clearly observe the photospheric signature of flux emergence with
very high spatial (0".11 at 7057{\AA}) and temporal (15 s) resolution. From TiO
observations with the pixel scale of 0".0375, we found several elongated
granule-like features (GLFs) stretching from the penumbral filaments of a
sunspot at a relatively high speed of over 4 km s-1. After a slender arched
darkening appeared at a tip of a penumbral filament, a bright point (BP)
developed and quickly moved away from the filament forming and stretching a
GLF. The size of a GLF was approximately 0.5" wide and 3" long. The moving BP
encountered nearby structures after several minutes of stretching, and a
well-defined elongated shape of a GLF faded away. Magnetograms from SDO/HMI and
NST/IRIM revealed that those GLFs are photospheric indicators of small-scale
flux emergence, and their disappearance is related to magnetic cancellation.
From two well-observed events, we describe detailed development of the
sub-structures of GLFs, and different cancellation processes that each of the
two GLFs underwent.Comment: Accepted for publication in The Astrophysical Journa
Intermittency in the photosphere and corona above an active region
Recent studies undoubtedly demonstrate that the magnetic field in the
photosphere and corona is an intermittent structure, which offers new views on
the underlying physics. In particular, such problems as the existence in the
corona of localized areas with extremely strong resistivity (required to
explain magnetic reconnection of all scales) and the interchange between small
and large scales (required in study of the photosphere/corona coupling), to
name a few, can be easily captured by the concept of intermittency. This study
is focused on simultaneous time variations of intermittency properties derived
in the photosphere, chromosphere and corona. We analyzed data for NOAA AR 10930
acquired between Dec 08, 2006 12:00 UT and Dec 13, 2006 18:45 UT. Photospheric
intermittency was inferred from Hinode magnetic field measurements, while
intermittency in the transition region and corona was derived from Nobeyama 9
GHz radio polarization measurements, high cadence Hinode/XRT/Be-thin data as
well as GOES 1-8\AA flux. Photospheric dynamics and its possible relationship
with the intermittency variations were also analyzed by calculating the kinetic
vorticity. For this case study we found the following chain of events.
Intermittency of the photospheric magnetic field peaked after the specific
kinetic vorticity of plasma flows in the AR reached its maximum level (4 hour
time delay). In turn, gradual increase of coronal intermittency occurred after
the peak of the photospheric intermittency. The time delay between the peak of
photospheric intermittency and the occurrence of the first strong (X3.4) flare
was approximately 1.3 days. Our analysis seems to suggest that the enhancement
of intermittency/complexity first occurs in the photosphere and is later
transported toward the corona
Parameters of the Magnetic Flux inside Coronal Holes
Parameters of magnetic flux distribution inside low-latitude coronal holes
(CHs) were analyzed. A statistical study of 44 CHs based on Solar and
Heliospheric Observatory (SOHO)/MDI full disk magnetograms and SOHO/EIT 284\AA
images showed that the density of the net magnetic flux, , does
not correlate with the associated solar wind speeds, . Both the area and
net flux of CHs correlate with the solar wind speed and the corresponding
spatial Pearson correlation coefficients are 0.75 and 0.71, respectively. A
possible explanation for the low correlation between and
is proposed. The observed non-correlation might be rooted in the structural
complexity of the magnetic field. As a measure of complexity of the magnetic
field, the filling factor, , was calculated as a function of spatial
scales. In CHs, was found to be nearly constant at scales above 2 Mm,
which indicates a monofractal structural organization and smooth temporal
evolution. The magnitude of the filling factor is 0.04 from the Hinode SOT/SP
data and 0.07 from the MDI/HR data. The Hinode data show that at scales smaller
than 2 Mm, the filling factor decreases rapidly, which means a mutlifractal
structure and highly intermittent, burst-like energy release regime. The
absence of necessary complexity in CH magnetic fields at scales above 2 Mm
seems to be the most plausible reason why the net magnetic flux density does
not seem to be related to the solar wind speed: the energy release dynamics,
needed for solar wind acceleration, appears to occur at small scales below 1
Mm.Comment: 6 figures, approximately 23 pages. Accepted in Solar Physic