94 research outputs found
Evidence for Widespread Cooling in an Active Region Observed with the SDO Atmospheric Imaging Assembly
A well known behavior of EUV light curves of discrete coronal loops is that
the peak intensities of cooler channels or spectral lines are reached at
progressively later times than hotter channels. This time lag is understood to
be the result of hot coronal loop plasma cooling through these lower respective
temperatures. However, loops typically comprise only a minority of the total
emission in active regions. Is this cooling pattern a common property of active
region coronal plasma, or does it only occur in unique circumstances,
locations, and times? The new SDO/AIA data provide a wonderful opportunity to
answer this question systematically for an entire active region. We measure the
time lag between pairs of SDO/AIA EUV channels using 24 hours of images of AR
11082 observed on 19 June 2010. We find that there is a time-lag signal
consistent with cooling plasma, just as is usually found for loops, throughout
the active region including the diffuse emission between loops for the entire
24 hour duration. The pattern persists consistently for all channel pairs and
choice of window length within the 24 hour time period, giving us confidence
that the plasma is cooling from temperatures of greater than 3 MK, and
sometimes exceeding 7 MK, down to temperatures lower than ~ 0.8 MK. This
suggests that the bulk of the emitting coronal plasma in this active region is
not steady; rather, it is dynamic and constantly evolving. These measurements
provide crucial constraints on any model which seeks to describe coronal
heating.Comment: 17 pages text, 7 figures in main body, 5 Appendix figure
Long-Term Trends in Phytoplankton Chlorophyll a and Size Structure in the Benguela Upwelling System
This is the final version. Available from American Geophysical Union (AGU) via the DOI in this record.The Benguela Upwelling System (BUS) is among the most productive ecosystems globally, supporting numerous fisheries and ecosystem services in Southern Africa. Sea-viewing Wide Field-of-view Sensor and Moderate-resolution Imaging Spectroradiometer-Aqua chlorophyll a (Chla) concentrations between September 1997 and February 2018 were used to investigate long-term trends in phytoplankton biomass and size structure (microphytoplankton [>20Ā Ī¼m], nanophytoplankton [2ā20Ā Ī¼m], and picophytoplankton [<2Ā Ī¼m]) in the Northern Benguela, Southern Benguela (SB), and Agulhas Bank (AB) shelf and open ocean regions of the BUS. Trends in upwelling and correlations with Chla and size structure were examined. Increasing Chla and microphytoplankton trends occurred in the Northern Benguela shelf and open ocean, while decreases were evident on the SB shelf in all seasons. In the SB open ocean, small increases occurred during austral winter, with a decrease in spring. On the AB shelf, increases in Chla and microphytoplankton occurred in summer with decreases during the other seasons. Patterns differed in the AB open ocean, with increases in winter and spring and decreases in summer and autumn. Although R 2 values indicated that linear trends accounted for a reasonable portion of the variance, and most trends were statistically significant, they showed only small changes on the shelf domains and little to no change in the open ocean. Strong correlations between upwelling, Chla, and the size classes were observed, but distinct seasonal differences occurred in each region. This is the first 20-year analysis of phytoplankton biomass and community structure in the BUS and provides a baseline against which future changes can be monitored.NERC National Centre for Earth ObservationSouth African National Research Foundation (NRF)South African Department of Environmental Affair
A spectral study of gamma-ray emitting AGN
In this paper we present a statistical analysis of the gamma-ray spectra of
flat-spectrum radio quasars (FSRQ) compared to those of BL Lacs. The average
spectra and possible systematic deviations from power-law behaviour are
investigated by summing up the intensity and the power-law fit statistic for
both classes of objects. We also compare the time-averaged spectrum to that at
the time of gamma-ray outbursts. The spectrum of the average AGN is softer than
that of the extragalactic gamma-ray background. It may be that BL Lacs, which
on average have a harder spectrum than FSRQs, make up the bulk of the
extragalactic background. We also find apparent cut-offs at both low and high
energies in the spectra of FSRQs at the time of gamma-ray outbursts. While the
cut-off at high energies may have something to do with opacity, the cut-off at
low energies may be taken as indication that the gamma-ray emission of FSRQs is
not a one component spectrum.Comment: 8 pages, accepted for publication in A&
Monsoons, ITCZs and the concept of the global monsoon
Earth's tropical and subtropical rainbands, such as Intertropical Convergence Zones (ITCZs) and monsoons, are complex systems, governed by both largeāscale constraints on the atmospheric general circulation and regional interactions with continents and orography, and coupled to the ocean. Monsoons have historically been considered as regional largeāscale sea breeze circulations, driven by landāsea contrast. More recently, a perspective has emerged of a Global Monsoon, a globalāscale solstitial mode that dominates the annual variation of tropical and subtropical precipitation. This results from the seasonal variation of the global tropical atmospheric overturning and migration of the associated convergence zone. Regional subsystems are embedded in this global monsoon, localized by surface boundary conditions. Parallel with this, much theoretical progress has been made on the fundamental dynamics of the seasonal Hadley cells and convergence zones via the use of hierarchical modeling approaches, including aquaplanets. Here we review the theoretical progress made, and explore the extent to which these advances can help synthesize theory with observations to better understand differing characteristics of regional monsoons and their responses to certain forcings. After summarizing the dynamical and energetic balances that distinguish an ITCZ from a monsoon, we show that this theoretical framework provides strong support for the migrating convergence zone picture and allows constraints on the circulation to be identified via the momentum and energy budgets. Limitations of current theories are discussed, including the need for a better understanding of the influence of zonal asymmetries and transients on the largeāscale tropical circulation.This is the author accepted manuscript. The final version is available from the publisher via the DOI in this recordMet Offic
Correction: Wald Sequential Probability Ratio Test for Space Object Conjunction Assessment
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