20 research outputs found
Origins of the Ambient Solar Wind: Implications for Space Weather
The Sun's outer atmosphere is heated to temperatures of millions of degrees,
and solar plasma flows out into interplanetary space at supersonic speeds. This
paper reviews our current understanding of these interrelated problems: coronal
heating and the acceleration of the ambient solar wind. We also discuss where
the community stands in its ability to forecast how variations in the solar
wind (i.e., fast and slow wind streams) impact the Earth. Although the last few
decades have seen significant progress in observations and modeling, we still
do not have a complete understanding of the relevant physical processes, nor do
we have a quantitatively precise census of which coronal structures contribute
to specific types of solar wind. Fast streams are known to be connected to the
central regions of large coronal holes. Slow streams, however, appear to come
from a wide range of sources, including streamers, pseudostreamers, coronal
loops, active regions, and coronal hole boundaries. Complicating our
understanding even more is the fact that processes such as turbulence,
stream-stream interactions, and Coulomb collisions can make it difficult to
unambiguously map a parcel measured at 1 AU back down to its coronal source. We
also review recent progress -- in theoretical modeling, observational data
analysis, and forecasting techniques that sit at the interface between data and
theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue
connected with a 2016 ISSI workshop on "The Scientific Foundations of Space
Weather." 44 pages, 9 figure
The free energy of NOAA active region AR 11029
The NOAA active region AR 11029 was a small but highly active sunspot region
which produced 73 GOES soft X-ray flares. The flares appear to show a departure
from the well known power-law frequency-size distribution. Specifically, too
few GOES C-class and no M-class flares were observed by comparison with a
power-law distribution (Wheatland in Astrophys. J. 710, 1324, 2010). This was
conjectured to be due to the region having insufficient magnetic energy to
power large events. We construct nonlinear force-free extrapolations of the
coronal magnetic field of active region AR 11029 using data taken on 24 October
by the SOLIS Vector-SpectroMagnetograph (SOLIS/VSM), and data taken on 27
October by the Hinode Solar Optical Telescope SpectroPolarimeter (Hinode/SP).
Force-free modeling with photospheric magnetogram data encounters problems
because the magnetogram data are inconsistent with a force-free model, and we
employ a recently developed `self-consistency' procedure which addresses this
and accommodates uncertainties in the boundary data (Wheatland and Regnier in
Astrophys. J. 700, L88, 2009). We calculate the total energy and free energy of
the self-consistent solution and find that the free energy was 4x10^29 erg on
24 October, and 7x10^31 erg on 27 October. An order of magnitude scaling
between RHESSI non-thermal energy and GOES peak X-ray flux is established from
a sample of flares from the literature and is used to estimate flare energies
from observed GOES peak X-ray flux. Based on the scaling, we conclude that the
estimated free energy of AR 11029 on 27 October when the flaring rate peaked is
sufficient to power M-class or X-class flares, and hence the modeling does not
appear to support the hypothesis that the absence of large flares is due to the
region having limited energy.Comment: Accepted for publication in Solar Physic
Particle acceleration mechanisms
We review the possible mechanisms for production of non-thermal electrons
which are responsible for non-thermal radiation in clusters of galaxies. Our
primary focus is on non-thermal Bremsstrahlung and inverse Compton scattering,
that produce hard X-ray emission. We briefly review acceleration mechanisms and
point out that in most astrophysical situations, and in particular for the
intracluster medium, shocks, turbulence and plasma waves play a crucial role.
We consider two scenarios for production of non-thermal radiation. The first is
hard X-ray emission due to non-thermal Bremsstrahlung by nonrelativistic
particles. Non-thermal tails are produced by accelerating electrons from the
background plasma with an initial Maxwellian distribution. However, these tails
are accompanied by significant heating and they are present for a short time of
<10^6 yr, which is also the time that the tail will be thermalised. Such
non-thermal tails, even if possible, can only explain the hard X-ray but not
the radio emission which needs GeV or higher energy electrons. For these and
for production of hard X-rays by the inverse Compton model, we need the second
scenario where there is injection and subsequent acceleration of relativistic
electrons. It is shown that a steady state situation, for example arising from
secondary electrons produced from cosmic ray proton scattering by background
protons, will most likely lead to flatter than required electron spectra or it
requires a short escape time of the electrons from the cluster. An episodic
injection of relativistic electrons, presumably from galaxies or AGN, and/or
episodic generation of turbulence and shocks by mergers can result in an
electron spectrum consistent with observations but for only a short period of
less than one billion years.Comment: 22 pages, 5 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 11; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
Stochastic Acceleration by Turbulence
The subject of this paper is stochastic acceleration by plasma turbulence, a
process akin to the original model proposed by Fermi. We review the relative
merits of different acceleration models, in particular the so called first
order Fermi acceleration by shocks and second order Fermi by stochastic
processes, and point out that plasma waves or turbulence play an important role
in all mechanisms of acceleration. Thus, stochastic acceleration by turbulence
is active in most situations. We also show that it is the most efficient
mechanism of acceleration of relatively cool non relativistic thermal
background plasma particles. In addition, it can preferentially accelerate
electrons relative to protons as is needed in many astrophysical radiating
sources, where usually there are no indications of presence of shocks. We also
point out that a hybrid acceleration mechanism consisting of initial
acceleration by turbulence of background particles followed by a second stage
acceleration by a shock has many attractive features. It is demonstrated that
the above scenarios can account for many signatures of the accelerated
electrons, protons and other ions, in particular He and He, seen
directly as Solar Energetic Particles and through the radiation they produce in
solar flares.Comment: 29 pages 7 figures for proceedings of ISSI-Bern workshop on Particle
Acceleration 201