100 research outputs found
Understanding coronal heating and solar wind acceleration: Case for in situ near‐Sun measurements
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94903/1/rog1641.pd
Quasi-Periodic Pulsations in Solar Flares: new clues from the Fermi Gamma-Ray Burst Monitor
In the last four decades it has been observed that solar flares show
quasi-periodic pulsations (QPPs) from the lowest, i.e. radio, to the highest,
i.e. gamma-ray, part of the electromagnetic spectrum. To this day, it is still
unclear which mechanism creates such QPPs. In this paper, we analyze four
bright solar flares which show compelling signatures of quasi-periodic behavior
and were observed with the Gamma-Ray Burst Monitor (\gbm) onboard the Fermi
satellite. Because GBM covers over 3 decades in energy (8 keV to 40 MeV) it can
be a key instrument to understand the physical processes which drive solar
flares. We tested for periodicity in the time series of the solar flares
observed by GBM by applying a classical periodogram analysis. However, contrary
to previous authors, we did not detrend the raw light curve before creating the
power spectral density spectrum (PSD). To assess the significance of the
frequencies we made use of a method which is commonly applied for X-ray
binaries and Seyfert galaxies. This technique takes into account the underlying
continuum of the PSD which for all of these sources has a P(f) ~ f^{-\alpha}
dependence and is typically labeled red-noise. We checked the reliability of
this technique by applying it to a solar flare which was observed by the Reuven
Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) which contains, besides
any potential periodicity from the Sun, a 4 s rotational period due to the
rotation of the spacecraft around its axis. While we do not find an intrinsic
solar quasi-periodic pulsation we do reproduce the instrumental periodicity.
Moreover, with the method adopted here, we do not detect significant QPPs in
the four bright solar flares observed by GBM. We stress that for the purpose of
such kind of analyses it is of uttermost importance to appropriately account
for the red-noise component in the PSD of these astrophysical sources.Comment: accepted by A&
On Solving the Coronal Heating Problem
This article assesses the current state of understanding of coronal heating,
outlines the key elements of a comprehensive strategy for solving the problem,
and warns of obstacles that must be overcome along the way.Comment: Accepted by Solar Physics; Published by Solar Physic
Multiwavelength studies of MHD waves in the solar chromosphere: An overview of recent results
The chromosphere is a thin layer of the solar atmosphere that bridges the
relatively cool photosphere and the intensely heated transition region and
corona. Compressible and incompressible waves propagating through the
chromosphere can supply significant amounts of energy to the interface region
and corona. In recent years an abundance of high-resolution observations from
state-of-the-art facilities have provided new and exciting ways of
disentangling the characteristics of oscillatory phenomena propagating through
the dynamic chromosphere. Coupled with rapid advancements in
magnetohydrodynamic wave theory, we are now in an ideal position to thoroughly
investigate the role waves play in supplying energy to sustain chromospheric
and coronal heating. Here, we review the recent progress made in
characterising, categorising and interpreting oscillations manifesting in the
solar chromosphere, with an impetus placed on their intrinsic energetics.Comment: 48 pages, 25 figures, accepted into Space Science Review
Physics of Solar Prominences: II - Magnetic Structure and Dynamics
Observations and models of solar prominences are reviewed. We focus on
non-eruptive prominences, and describe recent progress in four areas of
prominence research: (1) magnetic structure deduced from observations and
models, (2) the dynamics of prominence plasmas (formation and flows), (3)
Magneto-hydrodynamic (MHD) waves in prominences and (4) the formation and
large-scale patterns of the filament channels in which prominences are located.
Finally, several outstanding issues in prominence research are discussed, along
with observations and models required to resolve them.Comment: 75 pages, 31 pictures, review pape
Can Disease Management Target Patients Most Likely to Generate High Costs? The Impact of Comorbidity
CONTEXT: Disease management programs are increasingly used to manage costs of patients with chronic disease. OBJECTIVE: We sought to examine the clinical characteristics and measure the health care expenditures of patients most likely to be targeted by disease management programs. DESIGN: Retrospective analysis of prospectively obtained data. SETTING: A general medicine practice with both faculty and residents at an urban academic medical center. PARTICIPANTS: Five thousand eight hundred sixty-one patients enrolled in the practice for at least 1 year. MAIN OUTCOMES: Annual cost of diseases targeted by disease management. MEASUREMENTS: Patients’ clinical and demographic information were collected from a computer system used to manage patients. Data included diagnostic information, medications, and resource usage over 1 year. We looked at 10 common diseases targeted by disease management programs. RESULTS: Unadjusted annual median costs for chronic diseases ranged between 1,500. Congestive heart failure (1,500), diabetes (1,400) were the most expensive. As comorbidity increased, annual adjusted costs increased exponentially. Those with comorbidity scores of 2 or more accounted for 26% of the population but 50% of the overall costs. CONCLUSIONS: Costs for individual chronic conditions vary within a relatively narrow range. However, the costs for patients with multiple coexisting medical conditions increase rapidly. Reducing health care costs will require focusing on patients with multiple comorbid diseases, not just single diseases. The overwhelming impact of comorbidity on costs raises significant concerns about the potential ability of disease management programs to limit the costs of care
Self-consistent Coronal Heating and Solar Wind Acceleration from Anisotropic Magnetohydrodynamic Turbulence
We present a series of models for the plasma properties along open magnetic
flux tubes rooted in solar coronal holes, streamers, and active regions. These
models represent the first self-consistent solutions that combine: (1)
chromospheric heating driven by an empirically guided acoustic wave spectrum,
(2) coronal heating from Alfven waves that have been partially reflected, then
damped by anisotropic turbulent cascade, and (3) solar wind acceleration from
gradients of gas pressure, acoustic wave pressure, and Alfven wave pressure.
The only input parameters are the photospheric lower boundary conditions for
the waves and the radial dependence of the background magnetic field along the
flux tube. For a single choice for the photospheric wave properties, our models
produce a realistic range of slow and fast solar wind conditions by varying
only the coronal magnetic field. Specifically, a 2D model of coronal holes and
streamers at solar minimum reproduces the latitudinal bifurcation of slow and
fast streams seen by Ulysses. The radial gradient of the Alfven speed affects
where the waves are reflected and damped, and thus whether energy is deposited
below or above the Parker critical point. As predicted by earlier studies, a
larger coronal ``expansion factor'' gives rise to a slower and denser wind,
higher temperature at the coronal base, less intense Alfven waves at 1 AU, and
correlative trends for commonly measured ratios of ion charge states and
FIP-sensitive abundances that are in general agreement with observations. These
models offer supporting evidence for the idea that coronal heating and solar
wind acceleration (in open magnetic flux tubes) can occur as a result of wave
dissipation and turbulent cascade. (abridged abstract)Comment: 32 pages (emulateapj style), 18 figures, ApJ Supplement, in press (v.
171, August 2007
Forward modeling of standing kink modes in coronal loops. I. Synthetic views
Kink magnetohydrodynamic (MHD) waves are frequently observed in various magnetic structures of the solar atmosphere. They may contribute significantly to coronal heating and could be used as a tool to diagnose the solar plasma. In this study, we synthesize the Fe ix λ171.073 emission of a coronal loop supporting a standing kink MHD mode. The kink MHD wave solution of a plasma cylinder is mapped into a semi-torus structure to simulate a curved coronal loop. We decompose the solution into a quasi-rigid kink motion and a quadrupole term, which dominate the plasma inside and outside of the flux tube, respectively. At the loop edges, the line of sight integrates relatively more ambient plasma, and the background emission becomes significant. The plasma motion associated with the quadrupole term causes spectral line broadening and emission suppression. The periodic intensity suppression will modulate the integrated intensity and the effective loop width, which both exhibit oscillatory variations at half of the kink period. The quadrupole term can be directly observed as a pendular motion at the front view
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