|
Coronal MHD Waves and Oscillations: Observations and Quests (Invited Review)
|
|
Markus J. Aschwanden Submitted: 2005-03-03 16:05
Coronal seismology, a new field of solar physics that emerged over the last
five years, provides unique information on basic physical properties of the
solar corona. The inhomogeneous coronal plasma supports a variety of MHD wave
modes, which manifest themselves as standing waves (MHD oscillations) and
propagating waves. Here we briefly review the physical properties of
observed MHD oscillations and waves, including fast kink modes, fast sausage
modes, slow (acoustic) modes, torsional modes, their diagnostics of the coronal
magnetic field, and their physical damping mechanisms.
We discuss the excitation mechanisms of coronal MHD oscillations and waves:
the origin of the exciter, exciter propagation, and excitation in magnetic
reconnection outflow regions. Finally we consider the role of coronal
MHD oscillations and waves for coronal heating, the detectability of various
MHD wave types, and we estimate the energies carried in the observed MHD waves
and oscillations: Alfv'enic MHD waves could potentially provide sufficient
energy to sustain coronal heating, while acoustic MHD waves fall far short
of the required coronal heating rates.
Authors: Markus J. Aschwanden
Projects: Soho-EIT,Soho-LASCO,TRACE
|
Publication Status: Philosophical Transactions of the Royal Society A; Mathematical, Physical and Engineering Sciences, Proc. of The Royal Society S
Last Modified: 2005-03-03 16:05
|
 
 
|
|
|
Particle Acceleration in Solar Flares and Escape into Interplanetary Space
|
|
Markus J. Aschwanden Submitted: 2005-03-01 09:38
We review the physics of particle acceleration and kinematics
in solar flares under the particular aspect of their escape
and propagation into interplanetary space.
The topics include the magnetic topology in
acceleration regions, the altitude of flare acceleration regions,
evidence for bi-directional acceleration, the asymmetry of
upward versus downward acceleration, and particle access
to interplanetary space.
Authors: Markus J. Aschwanden
Projects: Yohkoh-HXT,Yohkoh-SXT,TRACE
|
Publication Status: AGU Monograph of AGU Chapman Conference ``Solar Energetic Plasmas and Particles'', Turku, Finland, 2-6 August 2004, (eds. Nat Go
Last Modified: 2005-03-01 09:40
|
 
 
|
|
|
The Sun
|
|
Markus J. Aschwanden Submitted: 2005-02-21 15:59
Chapter 1, ''The Sun'' for the ENCYCLOPEDIA OF THE SOLAR SYSTEM. Contents:
- Glossary
- I. Introduction
- II. The Solar Interior
- III. The Photosphere
- IV. The Chromosphere and Transition Region
- V. The Corona
- VI. Solar Flares and Coronal Mass Ejections
- Final Comments
- Bibliography
Authors: Markus J. Aschwanden
Projects: RHESSI,Soho-EIT,Soho-MDI,Soho-CDS,Soho-LASCO,Yohkoh-HXT,Yohkoh-SXT,TRACE
|
Publication Status: Encyclopedia of the Solar System, 2nd edition, 2005, (eds. Lucy-Ann McFadden, Paul Weissman, & Torrence Johnson), Academy Press,
Last Modified: 2005-02-21 15:59
|
 
 
|
|
|
2D Feature Recognition and 3D Reconstruction in Solar EUV Images
|
|
Markus J. Aschwanden Submitted: 2005-01-31 18:38
EUV images show the solar corona in a typical temperature
range of T~1 MK, which encompasses the most common coronal
structures: loops, filaments, and other magnetic structures
in active regions, the quiet Sun, and coronal holes.
Quantitative analysis increasingly demands automated 2D feature
recognition and 3D reconstruction, in order to localize, track,
and monitor the evolution of such coronal structures.
We discuss numerical tools that ``fingerprint'' curvi-linear
1D features (e.g., loops and filaments). We discuss existing
finger-printing algorithms, such as the brightness gradient method,
the oriented-connectivity method, stereoscopic methods,
time-differencing, and space-time feature recognition.
We discuss improved 2D feature recognition and 3D reconstruction
techniques that make use of additional a priori constraints,
using guidance from magnetic field extrapolations,
curvature radii constraints, and acceleration and velocity
constraints in time-dependent image sequences.
Applications of these algorithms aid
the analysis of SoHO/EIT, TRACE, and STEREO/SECCHI data,
such as disentangling, 3D reconstruction, and hydrodynamic
modeling of coronal loops, postflare loops, filaments, prominences,
and 3D reconstruction of the coronal magnetic field in general.
Authors: Markus J. Aschwanden
Projects: Soho-EIT,Soho-MDI,Soho-CDS,Soho-LASCO,TRACE
|
Publication Status: Solar Physics, (in press; accepted Feb 17, 2005)
Last Modified: 2005-02-17 12:43
|
 
 
|
|
|
The role of observed MHD oscillations and waves for coronal heating (Invited Review)
|
|
Markus J. Aschwanden Submitted: 2004-10-01 14:55
In this review we investigate the role of observed coronal wave and
oscillation phenomena with respect to the coronal heating problem.
A number of wave phenomena have been recently
identified with TRACE and SOHO observations, such as standing waves in the
fast kink mode and in the slow acoustic mode, as well as propagating acoustic
waves, along closed and open magnetic field lines. Most of these wave phenomena
have been detected with periods in the order of minutes to half an hour,
constrained by the cadence of available imaging instruments. We find that MHD
oscillations and waves with acoustic phase speed
carry insufficient energy to contribute significantly to coronal heating.
However, MHD oscillation and wave phenomena with Alfv'enic phase speed carry an energy
flux that is comparable with the coronal losses due to radiation and thermal conduction,
and thus potentially contribute to the heating of active region loops or
postflare loops. Evidence for the presence of propagating Alfv'en waves
has only been shown indirectly, e.g., by Doppler shift measurements, statistical
center-limb effects, and height-dependent scaling laws, but their wave energy flux
could acount for the heating of coronal holes.
Authors: Markus J. Aschwanden
Projects: Soho-EIT,TRACE
|
Publication Status: in Proc. Coronal Heating, SOHO-15 Workshop, held in St.Andrews, UK, ed. ESA, ESTEC Noordwijk, The Netherlands, Special Publicati
Last Modified: 2004-10-01 14:57
|
 
 
|
|
|
PHYSICS OF THE SOLAR CORONA. AN INTRODUCTION
|
|
Markus J. Aschwanden Submitted: 2004-09-15 16:45
Chapters:
1. Introduction
2. Thermal Radiation
3. Hydrostatics
4. Hydrodynamics
5. Magnetic Fields
6. Magneto-Hydrodynamics (MHD)
7. MHD Oscillations
8. Propagating MHD Waves
9. Coronal Heating
10. Magnetic Reconnection
11. Particle Acceleration
12. Particle Kinematics
13. Hard X-Rays
14. Gamma Rays
15. Radio Emission
16. Flare Plasma Emission
17. Coronal Mass Ejections (CMEs)
- Cover text:
Over the past ten years, results from a new generation of
solar space missions have become available. Based on those
missions and observations made at many wavelengths, from
hard X-rays, soft X-rays, extreme UV and radio wavelengths,
this book is a comprehensive introduction to solar physics.
It describes the solar corona, and sets it in the context
of basic plasma physics before moving on to discuss plasma
instabilities, plasma heating, magnetic reconnection and
particle acceleration processes. The latest results on
coronal heating and radiation - fundamental processes in
all stars - are presented. Spectacular phenomena, such as
solar flares and coronal mass ejections, are described in
detail.
Authors: Markus J. Aschwanden
Projects: None,RHESSI,Soho-EIT,Soho-MDI,Soho-CDS,Soho-LASCO,Yohkoh-BCS,Yohkoh-HXT,Yohkoh-SXT,Yohkoh-WBS,TRACE
|
Publication Status: ISBN: 3-540-22321-5, Praxis Publishing (Chichester, UK) and Springer (Berlin), September 2004
Last Modified: 2004-09-15 16:45
|
 
 
|
|
|
Solar Magnetic Loops Observed with TRACE and EIT
|
|
Markus J. Aschwanden Submitted: 2003-11-03 15:39
We select some highlights and new results that have been obtained
from detailed ``microscopic'' observations of coronal loop structures
with the Transition Region and Coronal Explorer (TRACE) and
Extreme Ultraviolet Imager (EIT) instruments, including:
(1) the inhomogeneous substructure of EUV loops, (2) the
dynamic and non-hydrostatic nature, (3) the non-uniform heating,
(4) the magnetic topology at the loop footpoints, (5) the magnetic
energy budget for heating, and (6) oscillations and waves
in coronal loops.
Authors: Markus J. Aschwanden and Alan M. Title
Projects: Soho-EIT,TRACE
|
Publication Status: in Stars as Suns: Activity, Evolution and Planets, Symposium S219, IAU 25th Coll. and General Assembly, held in Sydney, Australi
Last Modified: 2003-11-03 15:49
|
 
 
|
|
|
On the photometric accuracy of RHESSI imaging and spectroscopy
|
|
Markus J. Aschwanden Submitted: 2003-09-16 08:25
We compare the photometric accuracy of spectra and images
in flares observed with the {sl Ramaty
High Energy Solar Spectroscopic Imager (RHESSI)} spacecraft.
We test the accuracy of the photometry by comparing the
photon fluxes obtained in different energy ranges from the
spectral-fitting software SPEX with those fluxes contained
in the images reconstructed with the {sl Clean, MEM,
MEM-Vis, Pixon,} and {sl Forward-fit} algorithms.
We quantify also the background fluxes, the fidelity
of source geometries, and spatial spectra reconstructed
with the five image reconstruction algorithms. We investigate
the effects of grid selection, pixel size, field-of-view,
and time intervals on the quality of image reconstruction.
The detailed parameters and statistics are provided in an
accompanying CD-ROM and web page.
We find that {sl Forward-fit}, {sl Pixon}, and {sl Clean}
have a robust convergence behavior and a photometric
accuracy in the order of a few percents, while {sl MEM}
does not converge optimally for large degrees
of freedom (for large field-of-views and/or small pixel sizes),
and {sl MEM-Vis} suffers in the case of time-variable
sources. This comparative study
documents the current status of the {sl RHESSI} spectral and
imaging software, one year after launch.
Authors: Aschwanden,M.J., Metcalf,T.R., Krucker,S., Sato,J., Conway,A., Hurford,G.J., and Schmahl,E.
Projects: RHESSI
|
Publication Status: Solar Physics (subm., 2003 Sept 15)
Last Modified: 2003-09-16 08:25
|
 
 
|
|
|
MHD sausage mode oscillations in coronal loops
|
|
Markus J. Aschwanden Submitted: 2003-09-16 08:21
A recent study by Nakariakov et al. pointed out that the dispersion relation
of MHD sausage mode oscillations has been incorrectly applied to coronal loops,
neglecting the highly dispersive nature of the phase speed and the long-wavelength
cutoff of the wave number. In the light of these new insights we revisit
previous observations that have been interpreted in terms of MHD sausage
mode oscillations in coronal loops and come to the following conclusions:
(1) Fast sausage MHD mode oscillations require such a high electron density
imposed by the wave number cutoff that they can only occur in flare loops;
(2) In the previously reported radio observations ($
u approx 100$ MHz to 1 GHz)
with periods of $Papprox 0.5-5$ s, the fast sausage MHD mode oscillation is likely
to be confined to a small segment (corresponding to a high harmonic node)
near the apex of the loop, rather than involving a global oscillation over
the entire loop length. The recent microwave and soft X-ray observations
of fast periods ($Papprox 6-17$ s) by Asai et al. and Melnikov et al., however,
are consistent with fast sausage MHD oscillations at the fundamental harmonic.
Authors: Aschwanden,M.J., Nakariakov,V.M., and Melnikov,V.F.
Projects: None
|
Publication Status: ApJ, in press (accepted 2003 Sept 15)
Last Modified: 2003-09-16 08:21
|
 
 
|
|
|
Observational Tests of Damping by Resonant Absorption in Coronal Loop Oscillations
|
|
Markus J. Aschwanden Submitted: 2003-06-18 18:31
One of the proposed damping mechanisms of coronal (transverse) loop oscillations in the kink-mode
is resonant absorption as a result of the Alfv'en speed variation at the outer boundary
of coronal loops. Analytical expressions for the period and damping time exist for loop
models with thin non-uniform boundaries. They predict a linear dependency of the ratio
of the damping time to the period on the thickness of the non-uniform boundary layer.
Ruderman and Roberts used a sinusoidal variation of the density in the non-uniform
boundary layer and obtained the corresponding analytical expression for the damping time.
Here we measure the thickness of the non-uniform layer in oscillating loops for 11 events,
by forward-fitting of the cross-sectional density profile $n_e(r)$ and line-of-sight
integration to the cross-sectional fluxes $F(r)$ observed with TRACE 171 ang .
This way we model the internal $n_i$ and external electron density $n_e$ of the
coronal plasma in oscillating loops. This allows us to test the theoretically predicted
damping rates for thin boundaries as function of the density ratio $chi = n_e/n_i$.
Since the observations show that the loops are fully non-uniform
we also use numerical results for damping rates to determine the value of $chi$ for the loops.
We find that the density ratio predicted by the damping time,
${chi}_{LEDA}=0.53pm0.12$, is about a factor of $2.5pm2.1$ higher than the density ratio estimated from
the background fluxes, ${chi}=0.30pm 0.16$. The lower densities modeled from the background
fluxes are likely to be a consequence of the neglected hotter plasma that is not detected
with the TRACE 171 ang filter, as well as due to the neglect of the FIP-effect for relative iron
enhancement in the oscillating loops. Taking these corrections into account, resonant absorption
predicts damping times of kink-mode oscillations that are commensurable with the observed ones
and provides a new diagnostic of the density contrast of oscillating loops.
Authors: Markus J. Aschwanden, Richard W. Nightingale, esse Andries, Marcel Goossens, and Tom Van Doorsselaere
Projects:
|
Publication Status: ApJ, (in press, accepted 2003-Aug-13)
Last Modified: 2003-08-14 07:45
|
 
 
|
|
|
A new method to constrain the iron abundance from cooling delays in coronal loops
|
|
Markus J. Aschwanden Submitted: 2003-03-06 18:41
Recent observations with TRACE reveal that the time delay between
the appearance of a cooling loop in different EUV temperature filters is
proportional to the loop length, $Delta t_{12} propto L$. We model
this cooling delay in terms of radiative loss and confirm this linear
relationship theoretically. We derive an expression that can be used to
constrain the coronal iron enhancement ${alpha}_{Fe}=A_{Fe}^{cor}/A_{Fe}^{Ph}$
relative to the photospheric value as function of
the cooling delay $Delta t_{12}$, flux $F_2$, loop width $w$, and
filling factor $q_w le 1$. With this relation we find upper limits on the
iron abundance enhancement of ${alpha}_{Fe} le 4.8pm 1.7$ for 10
small-scale nanoflare loops, and ${alpha}_{Fe} le 1.4pm 0.4$ for 5
large-scale loops, in the temperature range of $Tapprox 1.0-1.4$ MK.
This result supports the previous finding that low-FIP elements, including Fe,
are enhanced in the corona. The same relation constitutes also a lower limit
for the filling factor, which is $q_w ge 0.2pm 0.1$ and
$q_w ge 0.8pm 0.2$ for the two groups of coronal loops.
Authors: Aschwanden,M.J., Schrijver,C.J., Winebarger,A.R., and Warren,H.P.
Projects:
|
Publication Status: ApJ Lett., Vol.588, p. ... (in press, 2003 May 1 issue)
Last Modified: 2003-03-31 09:19
|
 
 
|
|
|
Review of Coronal Oscillations - An Observer's View (INVITED REVIEW)
|
|
Markus J. Aschwanden Submitted: 2002-10-22 14:13
Recent observations show a variety of oscillation modes in the corona.
Early non-imaging observations in radio wavelengths showed
a number of fast-period oscillations in the order of seconds,
which have been interpreted as fast sausage mode oscillations.
TRACE observations from 1998 have for the first time revealed
the lateral displacements of fast kink mode oscillations, with
periods of $approx$3-5 minutes, apparently triggered by nearby flares
and destabilizing filaments. Recently, SUMER discovered with
Doppler shift measurements loop oscillations with longer periods
(10-30 minutes) and relatively short damping times in hot flare loops,
which seem to correspond to longitudinal slow magnetoacoustic
waves. In addition, propagating longitudinal waves have also
been detected with EIT and TRACE in the lowest density scale
height of loops near sunspots. All these new observations seem to
confirm the theoretically predicted oscillation modes and
can now be used as a powerful tool for ``coronal seismology''
diagnostic.
Authors: Markus J. Aschwanden
Projects:
|
Publication Status: in {em Turbulence, Waves, and Instabilities in the Solar Plasma},
NATO Advanced Research Workshops, 16-20 Sept 2
Last Modified: 2002-10-22 14:13
|
 
 
|
|
|
Tomographic 3D-Modeling of the Solar Corona with FASR
|
|
Markus J. Aschwanden Submitted: 2002-10-14 10:17
The {sl Frequency-Agile Solar Radiotelescope (FASR)} litteraly opens up a new
dimension in addition to the 3D Euclidian geometry: the frequency dimension.
The 3D geometry is degenerated to 2D in all images from astronomical telescopes,
but the additional frequency dimension allows us to retrieve the missing third
dimension by means of physical modeling. We call this type of 3D reconstruction
{sl Frequency Tomography}. In this study we simulate a realistic 3D model
of an active region, composed of 500 coronal loops with the 3D geometry
$[x(s),y(s),z(s)]$ constrained by magnetic field extrapolations and the
physical parameters of the density $n_e(s)$ and temperature $T_e(s)$ given
by hydrostatic solutions. We simulate a series of 20 radio images in a
frequency range of $
u=0.1-10$ GHz, anticipating the capabilities
of {sl FASR}, and investigate what physical information can be retrieved
from such a dataset. We discuss also forward-modeling of the chromospheric and
Quiet Sun density and temperature structure, another primary goal of future
{sl FASR} science.
Authors: Aschwanden,M.J., Alexander,D., and DeRosa,M.
Projects:
|
Publication Status: Kluwer ASSL Volume - Solar and Space Weather Radiophysics - eds. D.E.Gary and C.O.Keller, subm (Oct 14, 2002)
Last Modified: 2002-10-14 10:17
|
 
 
|
|
|
Overview of the US System for Post-Docs, Contractors, and Careers in Solar Physics (Invited Review)
|
|
Markus J. Aschwanden Submitted: 2002-09-27 13:28
This article is intended to offer young physicists
specific information on job opportunities and careers in
solar physics in the United States (US), a traditional
post-Doc country for many young European physicists.
About 500 solar physicists life in the United States,
which are employed at universities and colleges (43\%), in
government laboratories (35\%), in private companies (8\%),
or have no official affiliation (14\%).
We provide a brief overview of the affiliations of the US solar
physicists, where the academic institutions are located, what
the government laboratories consist of, and what private
companies have the largest contracts in the solar physics
business. We compile also some demographic and sociological
statistics from the larger groups of US astronomers and
physicists, that may be of interest to prospective post-Docs.
Authors: Markus J. Aschwanden
Projects:
|
Publication Status: Proc. 10th European Solar Physics Meeting on Solar Variability: From Core to Outer Frontiers, Special Session Careers of Young E
Last Modified: 2002-09-27 13:28
|
 
 
|
|
|
Observations and Models of Coronal Loops: From Yohkoh to TRACE (Invited Review)
|
|
Markus J. Aschwanden Submitted: 2002-07-25 14:58
We review highlights of recent observations of coronal loops
in EUV and soft X-rays from {sl Yohkoh} and {sl TRACE}. We review
(1) hydrostatic loops,
(2) non-hydrostatic loops,
(3) oscillating loops,
(4) nanoflare loops, and
(5) coronal heating,
in the light of new observations, with a critical discussion of
previous interpretations and theoretical models.
Authors: Markus J. Aschwanden
Projects:
|
Publication Status: in Proc. Euroconference and IAU Colloquium 188 on Magnetic Coupling of the Solar Atmosphere, 11-15 June 2002, Santorini, Greece,
Last Modified: 2002-07-25 14:58
|
 
 
|
|
|
Chromospheric height and density measurements in a solar flare observed with RHESSI: II. Data analysis
|
|
Markus J. Aschwanden Submitted: 2002-07-25 14:48
We present an analysis of hard X-ray imaging observations from one
of the first solar flares observed with the {sl Ramaty High
Energy Solar Spectroscopic Imager (RHESSI)} spacecraft, launched
on 2002-Feb-5. The data were obtained from the 2002-Feb-22, 11:06
UT flare, which occurred close to the northwest limb. Thanks to
the high energy resolution of the germanium-cooled hard X-ray
detectors on {sl RHESSI} we can measure the flare source
positions with a high accuracy as a function of energy. Using a
forward-fitting algorithm for image reconstruction, we find a
systematic decrease in the altitudes of the source centroids
$z(varepsilon)$ as a function of increasing hard X-ray energy $varepsilon$,
as expected in the thick-target bremsstrahlung model of Brown. The
altitude of hard X-ray emission as a function of photon energy
$varepsilon$ can be characterized by a powerlaw function in the
$varepsilon=15-50$ keV energy range, viz. $z(varepsilon )
approx 2.3 (varepsilon / 20$ keV$)^{-1.3}$ Mm. Based on a purely
collisional 1-D thick-target model, this height dependence can be
inverted into a chromospheric density model $n(z)$, as derived
in Paper I, which follows
the powerlaw function $n_e(z)=1.25 imes 10^{13} (z/1$
Mm$)^{-2.5}$ cm$^{-3}$. This density is comparable with models
based on optical/UV spectrometry in the chromospheric height range
of $hlapprox 1000$ km, suggesting that the collisional
thick-target model is a reasonable first approximation to hard
X-ray footpoint sources. At $happrox 1000-2500$ km, the hard X-ray
based density model, however, is more consistent with the {sl
``spicular extended-chromosphere model''} inferred from radio
sub-mm observations, than with standard models based on hydrostatic
equilibrium. At coronal heights, $happrox 2.5-12.4$ Mm, the average
flare loop density inferred from {sl RHESSI} is comparable with
values from hydrodynamic simulations of flare chromospheric
evaporation, soft X-ray, and radio-based measurements, but below
the upper limits set by filling-factor insensitive iron line pairs.
Authors: Aschwanden,M.J., Brown,J.C., and Kontar,E.P.
Projects:
|
Publication Status: Solar Physics, (accepted Aug 13), in press.
Last Modified: 2002-08-16 17:25
|
 
 
|
|
[Older Entries]
|
|
Key
|
 | Go to main E-Print page. |
 | Download Preprint. |
 | Submitters Homepage. |
 | Edit Entry. |
 | Delete abstract. |
|
|
|