1,156 research outputs found
An ecological study of Panax quinquefolius in central Appalachia: Seedling growth, harvest impacts and geographic variation in demography
American ginseng (Panax quinquefolius L.) is a long-lived, slow-maturing understory perennial herb found in eastern North American forests. The economic value of P. quinquefolius on the world market has remained strong for nearly 300 years. It has high economic and social value in the central Appalachians as well. Persistent harvest of ginseng combined with habitat loss has reduced populations in the wild, threatening the future of the species and its continued harvest. American ginseng was placed on the CITES Appendix II list in 1973 due to population declines. These concerns led to a series of studies investigating the population dynamics of ginseng in the center of its range. In the first study, I found that populations in the range center were projected to decline at a rate of 7 percent per year while those at the northern margin were projected to increase at a rate of 3 percent per year. The causes for the difference included significantly lower fertilities in the range center, and lower stasis and growth in all classes but growth from seedlings to 1-leaf plants in the populations in West Virginia. In the second study, the size structure of a wild population of ginseng was slow to recover following a fully destructive harvest, but the presence of seeds in the soil conveyed some resilience to the removal of all juvenile and adult plants from a site. In the third study, a seed bank viable beyond 20 months was documented for the first time for P. quinquefolius, indicating the need to restructure future demographic models to incorporate seed dormancy. In a series of harvester simulation studies, I found that harvester behavior dramatically impacts projected population growth rates of ginseng. By planting seeds at a depth of 2 cm, harvesters can reverse declining population growth rates. Current regulations for legal harvest in nearly three quarters (i.e., 71%) of the states in the range center are not adequate to protect P. quinquefolius in the long-term. This research led to the improvement of demographic models and documented the critical role that harvesters can play in maintaining healthy populations of wild ginseng
Dynamics of the solar magnetic bright points derived from their horizontal motions
The sub-arcsec bright points (BP) associated with the small scale magnetic
fields in the lower solar atmosphere are advected by the evolution of the
photospheric granules. We measure various quantities related to the horizontal
motions of the BPs observed in two wavelengths, including the velocity
auto-correlation function. A 1 hr time sequence of wideband H
observations conducted at the \textit{Swedish 1-m Solar Telescope}
(\textit{SST}), and a 4 hr \textit{Hinode} \textit{G}-band time sequence
observed with the Solar Optical telescope are used in this work. We follow 97
\textit{SST} and 212 \textit{Hinode} BPs with 3800 and 1950 individual velocity
measurements respectively. For its high cadence of 5 s as compared to 30 s for
\textit{Hinode} data, we emphasize more on the results from \textit{SST} data.
The BP positional uncertainty achieved by \textit{SST} is as low as 3 km. The
position errors contribute 0.75 km s to the variance of the observed
velocities. The \textit{raw} and \textit{corrected} velocity measurements in
both directions, i.e., , have Gaussian distributions with standard
deviations of and km s respectively. The BP
motions have correlation times of about s. We construct the power
spectrum of the horizontal motions as a function of frequency, a quantity that
is useful and relevant to the studies of generation of Alfv\'en waves.
Photospheric turbulent diffusion at time scales less than 200 s is found to
satisfy a power law with an index of 1.59.Comment: Accepted for publication in The Astrophysical Journal. 24 pages, 9
figures, and 1 movie (not included
The multi-thermal and multi-stranded nature of coronal rain
In this work, we analyse coordinated observations spanning chromospheric, TR
and coronal temperatures at very high resolution which reveal essential
characteristics of thermally unstable plasmas. Coronal rain is found to be a
highly multi-thermal phenomenon with a high degree of co-spatiality in the
multi-wavelength emission. EUV darkening and quasi-periodic intensity
variations are found to be strongly correlated to coronal rain showers.
Progressive cooling of coronal rain is observed, leading to a height dependence
of the emission. A fast-slow two-step catastrophic cooling progression is
found, which may reflect the transition to optically thick plasma states. The
intermittent and clumpy appearance of coronal rain at coronal heights becomes
more continuous and persistent at chromospheric heights just before impact,
mainly due to a funnel effect from the observed expansion of the magnetic
field. Strong density inhomogeneities on spatial scales of 0.2"-0.5" are found,
in which TR to chromospheric temperature transition occurs at the lowest
detectable scales. The shape of the distribution of coronal rain widths is
found to be independent of temperature with peaks close to the resolution limit
of each telescope, ranging from 0.2" to 0.8". However we find a sharp increase
of clump numbers at the coolest wavelengths and especially at higher
resolution, suggesting that the bulk of the rain distribution remains
undetected. Rain clumps appear organised in strands in both chromospheric and
TR temperatures, suggesting an important role of thermal instability in the
shaping of fundamental loop substructure. We further find structure reminiscent
of the MHD thermal mode. Rain core densities are estimated to vary between
2x10^{10} cm^{-3} and 2.5x10^{11} cm^{-3} leading to significant downward mass
fluxes per loop of 1-5x10^{9} g s^{-1}, suggesting a major role in the
chromosphere-corona mass cycle.Comment: Abstract is only short version. See paper for full. Countless pages,
figures (and movies, but not included here). Accepted for publication in the
Astrophysical Journa
Unresolved fine-scale structure in solar coronal loop-tops
New and advanced space-based observing facilities continue to lower the resolution limit and detect solar coronal loops in greater detail. We continue to discover even finer substructures within coronal loop cross-sections, in order to understand the nature of the solar corona. Here, we push this lower limit further to search for the finest coronal loop substructures, through taking advantage of the resolving power of the Swedish 1 m Solar Telescope/CRisp Imaging Spectro-Polarimeter (CRISP), together with co-observations from the Solar Dynamics Observatory/Atmospheric Image Assembly (AIA). High-resolution imaging of the chromospheric Hα 656.28 nm spectral line core and wings can, under certain circumstances, allow one to deduce the topology of the local magnetic environment of the solar atmosphere where its observed. Here, we study post-flare coronal loops, which become filled with evaporated chromosphere that rapidly condenses into chromospheric clumps of plasma (detectable in Hα) known as a coronal rain, to investigate their fine-scale structure. We identify, through analysis of three data sets, large-scale catastrophic cooling in coronal loop-tops and the existence of multi-thermal, multi-stranded substructures. Many cool strands even extend fully intact from loop-top to footpoint. We discover that coronal loop fine-scale strands can appear bunched with as many as eight parallel strands within an AIA coronal loop cross-section. The strand number density versus cross-sectional width distribution, as detected by CRISP within AIA-defined coronal loops, most likely peaks at well below 100 km, and currently, 69% of the substructure strands are statistically unresolved in AIA coronal loops.Publisher PDFPeer reviewe
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