29,737 research outputs found
Metal-free activation of H<sub>2</sub>O<sub>2</sub> by g-C<sub>3</sub>N<sub>4</sub> under visible light irradiation for the degradation of organic pollutants
Solar flare hard X-ray spikes observed by RHESSI: a statistical study
Context. Hard X-ray (HXR) spikes refer to fine time structures on timescales
of seconds to milliseconds in high-energy HXR emission profiles during solar
flare eruptions. Aims. We present a preliminary statistical investigation of
temporal and spectral properties of HXR spikes. Methods. Using a three-sigma
spike selection rule, we detected 184 spikes in 94 out of 322 flares with
significant counts at given photon energies, which were detected from
demodulated HXR light curves obtained by the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI). About one fifth of these spikes are also
detected at photon energies higher than 100 keV. Results. The statistical
properties of the spikes are as follows. (1) HXR spikes are produced in both
impulsive flares and long-duration flares with nearly the same occurrence
rates. Ninety percent of the spikes occur during the rise phase of the flares,
and about 70% occur around the peak times of the flares. (2) The time durations
of the spikes vary from 0.2 to 2 s, with the mean being 1.0 s, which is not
dependent on photon energies. The spikes exhibit symmetric time profiles with
no significant difference between rise and decay times. (3) Among the most
energetic spikes, nearly all of them have harder count spectra than their
underlying slow-varying components. There is also a weak indication that spikes
exhibiting time lags in high-energy emissions tend to have harder spectra than
spikes with time lags in low-energy emissions.Comment: 16 pages, 13 figure
Bipartite graph partitioning and data clustering
Many data types arising from data mining applications can be modeled as
bipartite graphs, examples include terms and documents in a text corpus,
customers and purchasing items in market basket analysis and reviewers and
movies in a movie recommender system. In this paper, we propose a new data
clustering method based on partitioning the underlying bipartite graph. The
partition is constructed by minimizing a normalized sum of edge weights between
unmatched pairs of vertices of the bipartite graph. We show that an approximate
solution to the minimization problem can be obtained by computing a partial
singular value decomposition (SVD) of the associated edge weight matrix of the
bipartite graph. We point out the connection of our clustering algorithm to
correspondence analysis used in multivariate analysis. We also briefly discuss
the issue of assigning data objects to multiple clusters. In the experimental
results, we apply our clustering algorithm to the problem of document
clustering to illustrate its effectiveness and efficiency.Comment: Proceedings of ACM CIKM 2001, the Tenth International Conference on
Information and Knowledge Management, 200
Chromospheric Evaporation in an X1.0 Flare on 2014 March 29 Observed with IRIS and EIS
Chromospheric evaporation refers to dynamic mass motions in flare loops as a
result of rapid energy deposition in the chromosphere. These have been observed
as blueshifts in X-ray and extreme-ultraviolet (EUV) spectral lines
corresponding to upward motions at a few tens to a few hundreds of km/s. Past
spectroscopic observations have also revealed a dominant stationary component,
in addition to the blueshifted component, in emission lines formed at high
temperatures (~10 MK). This is contradictory to evaporation models predicting
predominant blueshifts in hot lines. The recently launched Interface Region
Imaging Spectrograph (IRIS) provides high resolution imaging and spectroscopic
observations that focus on the chromosphere and transition region in the UV
passband. Using the new IRIS observations, combined with coordinated
observations from the EUV Imaging Spectrometer, we study the chromospheric
evaporation process from the upper chromosphere to corona during an X1.0 flare
on 2014 March 29. We find evident evaporation signatures, characterized by
Doppler shifts and line broadening, at two flare ribbons separating from each
other, suggesting that chromospheric evaporation takes place in successively
formed flaring loops throughout the flare. More importantly, we detect dominant
blueshifts in the high temperature Fe XXI line (~10 MK), in agreement with
theoretical predictions. We also find that, in this flare, gentle evaporation
occurs at some locations in the rise phase of the flare, while explosive
evaporation is detected at some other locations near the peak of the flare.
There is a conversion from gentle to explosive evaporation as the flare
evolves.Comment: ApJ in pres
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