263 research outputs found
Eruptions from solar ephemeral regions as an extension of the size distribution of coronal mass ejections
Observations of the quiet solar corona in the 171A (~1MK) passband of the
Transition Region and Coronal Explorer (TRACE) often show disruptions of the
coronal part of small-scale ephemeral bipolar regions that resemble the
phenomena associated with coronal mass ejections on much larger scales:
ephemeral regions exhibit flare-like brightenings, rapidly rising filaments
carrying absorbing material at chromospheric temperatures, or the temporary
dimming of the surrounding corona. I analyze all available TRACE observing
sequences between 1998/04/01 and 2009/09/30 with full-resolution 171A image
sequences spanning a day or more within 500 arcsec of disk center, observing
essentially quiet Sun with good exposures and relatively low background. Ten
such data sets are identified between 2000 and 2008, spanning 570h of observing
with a total of 17133 exposures. Eighty small-scale coronal eruptions are
identified. Their size distribution forms a smooth extension of the
distribution of angular widths of coronal mass ejections, suggesting that the
eruption frequency for bipolar magnetic regions is essentially scale free over
at least two orders of magnitude, from eruptions near the arcsecond resolution
limit of TRACE to the largest coronal mass ejections observed in the inner
heliosphere. This scale range may be associated with the properties of the
nested set of ranges of connectivity in the magnetic field, in which
increasingly large and energetic events can reach higher and higher into the
corona until the heliosphere is reached
Detection of flux emergence, splitting, merging, and cancellation of network field. I Splitting and Merging
Frequencies of magnetic patch processes on supergranule boundary, namely flux
emergence, splitting, merging, and cancellation, are investigated through an
automatic detection. We use a set of line of sight magnetograms taken by the
Solar Optical Telescope (SOT) on board Hinode satellite. We found 1636 positive
patches and 1637 negative patches in the data set, whose time duration is 3.5
hours and field of view is 112" \times 112". Total numbers of magnetic
processes are followed: 493 positive and 482 negative splittings, 536 positive
and 535 negative mergings, 86 cancellations, and 3 emergences. Total numbers of
emergence and cancellation are significantly smaller than those of splitting
and merging. Further, frequency dependences of merging and splitting processes
on flux content are investigated. Merging has a weak dependence on flux content
only with a power- law index of 0.28. Timescale for splitting is found to be
independent of parent flux content before splitting, which corresponds to \sim
33 minutes. It is also found that patches split into any flux contents with a
same probability. This splitting has a power-law distribution of flux content
with an index of -2 as a time independent solution. These results support that
the frequency distribution of flux content in the analyzed flux range is
rapidly maintained by merging and splitting, namely surface processes. We
suggest a model for frequency distributions of cancellation and emergence based
on this idea.Comment: 32 pages, 10 figures, 1 table, accepted to Ap
Statistical Study of Emerging Flux Regions and the Upper Atmosphere Response
We statistically study the property of emerging flux regions (EFRs) and the
upper solar atmosphere response to the flux emergence by using data from the
Helioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly
(AIA) on board the Solar Dynamics Observatory (SDO). Parameters including the
total emerged flux, the flux growth rate, the maximum area, the duration of the
emergence and the separation speed of the opposite polarities are adopted to
delineate the property of the EFRs. The response of the upper atmosphere is
addressed by the response of the atmosphere at different wavelengths (and thus
at different temperatures). According to our results, the total emerged fluxes
are in the range of (0.44 -- 11.2) Mx while the maximum area
ranges from 17 to 182 arcsec. The durations of the emergence are between 1
and 12 hours, which are positively correlated to both the total emerged flux
and the maximum area. The maximum distances between the opposite polarities are
7 -- 25 arcsec and are also correlated to the duration positively. The
separation speeds are from 0.05 to 1.08 km s, negatively correlated to
the duration. The derived flux growth rates are (0.1 -- 1.3) Mx
hr, which are positively correlated to the total emerging flux. The
upper atmosphere responds to the flux emergence in the 1600\AA\ chromospheric
line first, and then tens and hundreds of seconds later, in coronal lines, such
as the 171\AA\ (T=10 K) and 211\AA\ (T=10 K) lines almost
simultaneously, suggesting the successively heating of atmosphere from the
chromosphere to the corona
Can the Solar Wind be Driven by Magnetic Reconnection in the Sun's Magnetic Carpet?
The physical processes that heat the solar corona and accelerate the solar
wind remain unknown after many years of study. Some have suggested that the
wind is driven by waves and turbulence in open magnetic flux tubes, and others
have suggested that plasma is injected into the open tubes by magnetic
reconnection with closed loops. In order to test the latter idea, we developed
Monte Carlo simulations of the photospheric "magnetic carpet" and extrapolated
the time-varying coronal field. These models were constructed for a range of
different magnetic flux imbalance ratios. Completely balanced models represent
quiet regions on the Sun and source regions of slow solar wind streams. Highly
imbalanced models represent coronal holes and source regions of fast wind
streams. The models agree with observed emergence rates, surface flux
densities, and number distributions of magnetic elements. Despite having no
imposed supergranular motions, a realistic network of magnetic "funnels"
appeared spontaneously. We computed the rate at which closed field lines open
up (i.e., recycling times for open flux), and we estimated the energy flux
released in reconnection events involving the opening up of closed flux tubes.
For quiet regions and mixed-polarity coronal holes, these energy fluxes were
found to be much lower than required to accelerate the solar wind. For the most
imbalanced coronal holes, the energy fluxes may be large enough to power the
solar wind, but the recycling times are far longer than the time it takes the
solar wind to accelerate into the low corona. Thus, it is unlikely that either
the slow or fast solar wind is driven by reconnection and loop-opening
processes in the magnetic carpet.Comment: 25 pages (emulateapj style), 13 figures, ApJ, in pres
Decorrelation Times of Photospheric Fields and Flows
We use autocorrelation to investigate evolution in flow fields inferred by
applying Fourier Local Correlation Tracking (FLCT) to a sequence of
high-resolution (0.3 \arcsec), high-cadence ( min) line-of-sight
magnetograms of NOAA active region (AR) 10930 recorded by the Narrowband Filter
Imager (NFI) of the Solar Optical Telescope (SOT) aboard the {\em Hinode}
satellite over 12--13 December 2006. To baseline the timescales of flow
evolution, we also autocorrelated the magnetograms, at several spatial
binnings, to characterize the lifetimes of active region magnetic structures
versus spatial scale. Autocorrelation of flow maps can be used to optimize
tracking parameters, to understand tracking algorithms' susceptibility to
noise, and to estimate flow lifetimes. Tracking parameters varied include: time
interval between magnetogram pairs tracked, spatial binning applied
to the magnetograms, and windowing parameter used in FLCT. Flow
structures vary over a range of spatial and temporal scales (including
unresolved scales), so tracked flows represent a local average of the flow over
a particular range of space and time. We define flow lifetime to be the flow
decorrelation time, . For , tracking results represent
the average velocity over one or more flow lifetimes. We analyze lifetimes of
flow components, divergences, and curls as functions of magnetic field strength
and spatial scale. We find a significant trend of increasing lifetimes of flow
components, divergences, and curls with field strength, consistent with Lorentz
forces partially governing flows in the active photosphere, as well as strong
trends of increasing flow lifetime and decreasing magnitudes with increases in
both spatial scale and .Comment: 48 pages, 20 figures, submitted to the Astrophysical Journal;
full-resolution images in manuscript (8MB) at
http://solarmuri.ssl.berkeley.edu/~welsch/public/manuscripts/flow_lifetimes_v2.pd
Observing Evolution in the Supergranular Length Scale During Periods of Low Solar Activity
We present the initial results of an observational study into the variation
of the dominant length-scale of quiet solar emission: supergranulation. This
length-scale reflects the radiative energy in the plasma of the upper solar
chromosphere and transition region at the magnetic network boundaries forming
as a result of the relentless interaction of magnetic fields and convective
motions of the Sun's interior. We demonstrate that a net difference of ~0.5Mm
in the supergranular emission length-scale occurs when comparing observations
cycle 22/23 and cycle 23/24 minima. This variation in scale is reproduced in
the datasets of multiple space- and ground-based instruments and using
different diagnostic measures. By means of extension, we consider the variation
of the supergranular length-scale over multiple solar minima by analyzing a
subset of the Mt Wilson Solar Observatory (MWO) Ca II K image record. The
observations and analysis presented provide a tantalizing look at solar
activity in the absence of large-scale flux emergence, offering insight into
times of "extreme" solar minimum and general behavior such as the phasing and
cross-dependence of different components of the spectral irradiance. Given that
the modulation of the supergranular scale imprints itself in variations of the
Sun's spectral irradiance, as well as in the mass and energy transport into the
entire outer atmosphere, this preliminary investigation is an important step in
understanding the impact of the quiet sun on the heliospheric system.Comment: 6 pages, 5 figures - ApJL. We thank Frank Eparvier, Tom Woods, Stan
Solomon, Anna Malanushenko, and Rachel Hauser for useful discussions and help
with the text, SOHO, STEREO, and the Canadian Space Agency for making their
data publicly availabl
Two-step Emergence of the Magnetic Flux Sheet from the Solar Convection Zone
We perform two-dimensional MHD simulations on the solar flux emergence. We
set the initial magnetic flux sheet at z=-20,000 km in the convection zone. The
flux sheet rises through the convective layer due to the Parker instability,
however, decelerates beneath the photosphere because the plasma on the flux
sheet piles up owing to the convectively stable photosphere above. Meanwhile,
the flux sheet becomes locally unstable to the Parker instability within the
photosphere, and the further evolution to the corona occurs (two-step emergence
model). We carry out a parameter survey to investigate the condition for this
two-step model. We find that magnetic fluxes which form active regions are
likely to have undergone the two-step emergence. The condition for the two-step
emergence is 10^21 - 10^22 Mx with 10^4 G at z=-20,000 km in the convection
zone.Comment: 41 pages, 15 figures, 1 table, Accepted for publication in Ap
Solar Magnetic Carpet I: Simulation of Synthetic Magnetograms
This paper describes a new 2D model for the photospheric evolution of the
magnetic carpet. It is the first in a series of papers working towards
constructing a realistic 3D non-potential model for the interaction of
small-scale solar magnetic fields. In the model, the basic evolution of the
magnetic elements is governed by a supergranular flow profile. In addition,
magnetic elements may evolve through the processes of emergence, cancellation,
coalescence and fragmentation. Model parameters for the emergence of bipoles
are based upon the results of observational studies. Using this model, several
simulations are considered, where the range of flux with which bipoles may
emerge is varied. In all cases the model quickly reaches a steady state where
the rates of emergence and cancellation balance. Analysis of the resulting
magnetic field shows that we reproduce observed quantities such as the flux
distribution, mean field, cancellation rates, photospheric recycle time and a
magnetic network. As expected, the simulation matches observations more closely
when a larger, and consequently more realistic, range of emerging flux values
is allowed (4e16 - 1e19 Mx). The model best reproduces the current observed
properties of the magnetic carpet when we take the minimum absolute flux for
emerging bipoles to be 4e16 Mx. In future, this 2D model will be used as an
evolving photospheric boundary condition for 3D non-potential modeling.Comment: 33 pages, 16 figures, 5 gif movies included: movies may be viewed at
http://www-solar.mcs.st-and.ac.uk/~karen/movies_paper1
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