50 research outputs found
SDO Observations of Magnetic Reconnection at Coronal Hole Boundaries
With the observations from the Atmospheric Imaging Assembly (AIA) and the
Helioseismic and Magnetic Imager (HMI) aboard the \emph{Solar Dynamics
Observatory}, we investigate the coronal hole boundaries (CHBs) of an
equatorial extension of polar coronal hole. At the CHBs, lots of
extreme-ultraviolet (EUV) jets, which appear to be the signatures of magnetic
reconnection, are observed in the 193 {\AA} images, and some jets occur
repetitively at the same sites. The evolution of the jets is associated with
the emergence and cancelation of magnetic fields. We notice that both the east
and the west CHBs shift westward, and the shift velocities are close to the
velocities of rigid rotation compared with those of the photospheric
differential rotation. This indicates that magnetic reconnection at CHBs
results in the evolution of CHBs and maintains the rigid rotation of coronal
holes.Comment: 10 pages, 5 figures, accepted for publication in ApJ
Magnetic Evolution and Temperature Variation in a Coronal Hole
We have explored the magnetic flux evolution and temperature variation in a
coronal-hole region, using Big Bear Solar Observatory (BBSO) deep magnetograms
and {\it SOHO}/EIT images observed from 2005 October 10 to 14. For comparison,
we also investigated a neighboring quiet region of the Sun. The coronal hole
evolved from its mature stage to its disappearance during the observing period.
We have obtained the following results: (1) When the coronal hole was well
developed on October 10, about 60 % of the magnetic flux was positive. The EUV
brightness was 420 counts pixel, and the coronal temperature, estimated
from the line ratio of the EIT 195 {\AA} and 171 {\AA} images, was 1.07 MK. (2)
On October 14, when the coronal hole had almost disappeared, 51 % of the
magnetic flux was positive, the EUV radiance was 530 counts pixel, and
the temperature was 1.10 MK. (3) In the neighboring quiet region, the fraction
of positive flux varied between 0.49 and 0.47. The EUV brightness displayed an
irregular variation, with a mean value of 870 counts pixel. The
temperature was almost constant at 1.11 MK during the five-day observation. Our
results demonstrate that in a coronal hole less imbalance of the magnetic flux
in opposite polarities leads to stronger EUV brightness and higher coronal
temperatures
Comparison of Magnetic Flux Distribution between a Coronal Hole and a Quiet Region
Employing Big Bear Solar Observatory (BBSO) deep magnetograms and H
images in a quiet region and a coronal hole, observed on September 14 and 16,
2004, respectively, we have explored the magnetic flux emergence, disappearance
and distribution in the two regions. The following results are obtained: (1)
The evolution of magnetic flux in the quiet region is much faster than that in
the coronal hole, as the flux appeared in the form of ephemeral regions in the
quiet region is 4.3 times as large as that in the coronal hole, and the flux
disappeared in the form of flux cancellation, 2.9 times as fast as in the
coronal hole. (2) More magnetic elements with opposite polarities in the quiet
region are connected by arch filaments, estimating from magnetograms and
H images. (3) We measured the magnetic flux of about 1000 magnetic
elements in each observing region. The flux distribution of network and
intranetwork (IN) elements is similar in both polarities in the quiet region.
For network fields in the coronal hole, the number of negative elements is much
more than that of positive elements. However for the IN fields, the number of
positive elements is much more than that of negative elements. (4) In the
coronal hole, the fraction of negative flux change obviously with different
threshold flux density. 73% of the magnetic fields with flux density larger
than 2 Gauss is negative polarity, and 95% of the magnetic fields is negative,
if we only measure the fields with their flux density larger than 20 Gauss. Our
results display that in a coronal hole, stronger fields is occupied by one
predominant polarity; however the majority of weaker fields, occupied by the
other polarity
Dipolar Evolution in a Coronal Hole Region
Using observations from the SOHO, STEREO and Hinode, we investigate magnetic
field evolution in an equatorial coronal hole region. Two dipoles emerge one by
one. The negative element of the first dipole disappears due to the interaction
with the positive element of the second dipole. During this process, a jet and
a plasma eruption are observed. The opposite polarities of the second dipole
separate at first, and then cancel with each other, which is first reported in
a coronal hole. With the reduction of unsigned magnetic flux of the second
dipole from 9.8*10^20 Mx to 3.0*10^20 Mx in two days, 171 A brightness
decreases by 75% and coronal loops shrink obviously. At the cancellation sites,
the transverse fields are strong and point directly from the positive elements
to the negative ones, meanwhile Doppler red-shifts with an average velocity of
0.9 km s-1 are observed, comparable to the horizontal velocity (1.0 km s-1)
derived from the cancelling island motion. Several days later, the northeastern
part of the coronal hole, where the dipoles are located, appears as a quiet
region. These observations support the idea that the interaction between the
two dipoles is caused by flux reconnection, while the cancellation between the
opposite polarities of the second dipole is due to the submergence of original
loops. These results will help us to understand coronal hole evolution.Comment: 23 pages, 9 figures. accepted by Ap
EUV jets, type III radio bursts and sunspot waves investigated using SDO/AIA observations
Images from the Solar Dynamics Observatory (SDO) at 211A are used to identify
the solar source of the type III radio bursts seen in WIND/WAVES dynamic
spectra. We analyse a 2.5 hour period during which six strong bursts are seen.
The radio bursts correlate very well with the EUV jets coming from the western
side of a sunspot in AR11092. The EUV jet emission also correlates well with
brightening at what looks like their footpoint at the edge of the umbra. For
10-15 min after strong EUV jets are ejected, the footpoint brightens at roughly
3 min intervals. In both the EUV images and the extracted light curves, it
looks as though the brightening is related to the 3-min sunspot oscillations,
although the correlation coefficient is rather low. The only open field near
the jets is rooted in the sunspot. We conclude that active region EUV/X-ray
jets and interplanetary electron streams originate on the edge of the sunspot
umbra. They form along a current sheet between the sunspot open field and
closed field connecting to underlying satellite flux. Sunspot running penumbral
waves cause roughly 3-min jet footpoint brightening. The relationship between
the waves and jets is less clear.Comment: 4 pages, 7 figures, Accepted by A&A Letters. For associated gif
movie, see http://www.mps.mpg.de/data/outgoing/innes/jets/losb_304_211_rd.gi
VLA multifrequency observations of RS CVn binaries
We present multiepoch Very Large Array (VLA) observations at 1.4 GHz, 4.9
GHz, 8.5 GHz and 14.9 GHz for a sample of eight RS CVn binary systems. Circular
polarization measurements of these systems are also reported. Most of the
fluxes observed are consistent with incoherent emission from mildly
relativistic electrons. Several systems show an increase of the degree of
circular polarization with increasing frequency in the optically thin regime,
in conflict with predictions by gyrosynchrotron models. We observed a reversal
in the sense of circular polarization with increasing frequency in three
non-eclipsing systems: EI Eri, DM Uma and HD 8358. We find clear evidence for
coherent plasma emission at 1.4 GHz in the quiescent spectrum of HD 8358 during
the helicity reversal. The degrees of polarization of the other two systems
could also be accounted for by a coherent emission process. The observations of
ER Vul revealed two U-shaped flux spectra at the highest frequencies. The
U-shape of the spectra may be accounted for by an optically thin
gyrosynchrotron source for the low frequency part whereas the high frequency
part is dominated by a thermal emission component.Comment: 12 pages, 8 figures, LaTeX, uses aa.cls. Accepted for publication in
A&
Physics of Solar Prominences: I - Spectral Diagnostics and Non-LTE Modelling
This review paper outlines background information and covers recent advances
made via the analysis of spectra and images of prominence plasma and the
increased sophistication of non-LTE (ie when there is a departure from Local
Thermodynamic Equilibrium) radiative transfer models. We first describe the
spectral inversion techniques that have been used to infer the plasma
parameters important for the general properties of the prominence plasma in
both its cool core and the hotter prominence-corona transition region. We also
review studies devoted to the observation of bulk motions of the prominence
plasma and to the determination of prominence mass. However, a simple inversion
of spectroscopic data usually fails when the lines become optically thick at
certain wavelengths. Therefore, complex non-LTE models become necessary. We
thus present the basics of non-LTE radiative transfer theory and the associated
multi-level radiative transfer problems. The main results of one- and
two-dimensional models of the prominences and their fine-structures are
presented. We then discuss the energy balance in various prominence models.
Finally, we outline the outstanding observational and theoretical questions,
and the directions for future progress in our understanding of solar
prominences.Comment: 96 pages, 37 figures, Space Science Reviews. Some figures may have a
better resolution in the published version. New version reflects minor
changes brought after proof editin
Effect of suprathermal particles on the quiet Sun radio emission
The bremsstrahlung emissivity and absorption coefficient in the
radiofrequency range are derived under
the assumption that the electron population is not purely thermal, but
presents a tail of high energy particles.
This population is approximated by a two-component Maxwellian
distribution and by the kappa-functions of different (integer) index.
It is shown that, if the temperature ratio
of the two Maxwellians is larger than 10, the absorption coefficient
and the effective temperature (the quantities entering the radio
transfer equation) depend
only on the fraction R of particles in the highest temperature Maxwellian.
In the case of kappa-functions the above quantities depend on the index n of
the functions.
The microwave radio spectrum is computed for different values of R and
for , finding, in all cases,
brightness temperatures lower than those computed with a pure thermal
distribution. This could explain some inconsistencies found between
radio and EUV observations