Author name code: wang-yi-ming
ADS astronomy entries on 2022-09-14
=author:"Wang, Y.-M."
------------------------------------------------------------------------
Title: Undetected Minority-polarity Flux as the Missing Link in
Coronal Heating
Authors: Wang, Y. -M.
Bibcode: 2022arXiv220611327W
Altcode:
During the last few decades, the most widely favored models for
coronal heating have involved the in situ dissipation of energy,
with footpoint shuffling giving rise to multiple current sheets (the
"nanoflare" model) or to Alfv{é}n waves that leak into the corona and
undergo dissipative interactions (the wave heating scenario). As has
been recognized earlier, observations suggest instead that the energy
deposition is concentrated at very low heights, with the coronal loops
being filled with hot, dense material from below, which accounts for
their overdensities and flat temperature profiles. While an obvious
mechanism for footpoint heating would be reconnection with small-scale
fields, this possibility seems to have been widely ignored because
magnetograms show almost no minority-polarity flux inside active region
(AR) plages. Here, we present further examples to support our earlier
conclusions (1) that magnetograms greatly underrepresent the amount of
minority-polarity flux inside plages and "unipolar" network, and (2)
that small loops are a major constituent of \ion{Fe}{9} 17.1 nm moss. On
the assumption that the emergence or churning rate of small-scale flux
is the same inside plages as in mixed-polarity regions of the quiet Sun,
we estimate the energy flux density associated with reconnection with
the plage fields to be on the order of 10$^7$ erg cm$^{-2}$ s$^{-1}$,
sufficient to heat the AR corona.
Title: Parameterization of Cap-Edge Dust Lifting over the Southern
Polar Region
Authors: Chow, K. C.; Xiao, J.; Wang, Y. M.
Bibcode: 2022mamo.conf.1557C
Altcode:
No abstract at ADS
Title: From Coronal Holes to Pulsars and Back Again: Learning the
Importance of Data
Authors: Wang, Y. -M.
Bibcode: 2022FrASS...9.8837W
Altcode:
Although wanting to become an astronomer from an early age, I ended up
in solar physics purely by chance, after first working in high-energy
astrophysics. I've never regretted switching from the pulsar to the
solar magnetosphere, because solar physics has a great advantage over
other areas of astrophysics—in the enormous amount of high-quality
data available, much of it underutilized. I've often wondered why
theoreticians and modelers don't spent more time looking at these data
(perhaps they feel that it is cheating, like taking a peek at the
answers to a difficult homework assignment?). Conversely, I wonder why
observers and data analysts aren't more skeptical of the theoretical
models—especially the fashionable ones.
Title: Magnetograph Saturation and the Open Flux Problem
Authors: Wang, Y. -M.; Ulrich, R. K.; Harvey, J. W.
Bibcode: 2022ApJ...926..113W
Altcode: 2021arXiv211209969W
Extrapolations of line-of-sight photospheric field measurements
predict radial interplanetary magnetic field (IMF) strengths that
are factors of ~2-4 too low. To address this open flux problem, we
reanalyze the magnetograph measurements from different observatories,
with particular focus on those made in the saturation-prone Fe I 525.0
nm line by the Mount Wilson Observatory (MWO) and the Wilcox Solar
Observatory (WSO). The total dipole strengths, which determine the
total open flux, generally show large variations among observatories,
even when their total photospheric fluxes are in agreement. However,
the MWO and WSO dipole strengths, as well as their total fluxes,
agree remarkably well with each other, suggesting that the two data
sets require the same scaling factor. As shown earlier by Ulrich et
al., the saturation correction δ -1 derived by comparing
MWO measurements in the 525.0 nm line with those in the nonsaturating
Fe I 523.3 nm line depends sensitively on where along the irregularly
shaped 523.3 nm line wings the exit slits are placed. If the slits are
positioned so that the 523.3 and 525.0 nm signals originate from the
same height, δ -1 ~ 4.5 at the disk center, falling to ~2
near the limb. When this correction is applied to either the MWO or
WSO maps, the derived open fluxes are consistent with the observed
IMF magnitude. Other investigators obtained scaling factors only
one-half as large because they sampled the 523.3 nm line farther out
in the wings, where the shift between the right- and left-circularly
polarized components is substantially smaller.
Title: First light observations of the solar wind in the outer corona
with the Metis coronagraph
Authors: Romoli, M.; Antonucci, E.; Andretta, V.; Capuano, G. E.; Da
Deppo, V.; De Leo, Y.; Downs, C.; Fineschi, S.; Heinzel, P.; Landini,
F.; Liberatore, A.; Naletto, G.; Nicolini, G.; Pancrazzi, M.; Sasso,
C.; Spadaro, D.; Susino, R.; Telloni, D.; Teriaca, L.; Uslenghi,
M.; Wang, Y. -M.; Bemporad, A.; Capobianco, G.; Casti, M.; Fabi, M.;
Frassati, F.; Frassetto, F.; Giordano, S.; Grimani, C.; Jerse, G.;
Magli, E.; Massone, G.; Messerotti, M.; Moses, D.; Pelizzo, M. -G.;
Romano, P.; Schühle, U.; Slemer, A.; Stangalini, M.; Straus, T.;
Volpicelli, C. A.; Zangrilli, L.; Zuppella, P.; Abbo, L.; Auchère,
F.; Aznar Cuadrado, R.; Berlicki, A.; Bruno, R.; Ciaravella, A.;
D'Amicis, R.; Lamy, P.; Lanzafame, A.; Malvezzi, A. M.; Nicolosi,
P.; Nisticò, G.; Peter, H.; Plainaki, C.; Poletto, L.; Reale, F.;
Solanki, S. K.; Strachan, L.; Tondello, G.; Tsinganos, K.; Velli,
M.; Ventura, R.; Vial, J. -C.; Woch, J.; Zimbardo, G.
Bibcode: 2021A&A...656A..32R
Altcode: 2021arXiv210613344R
In this work, we present an investigation of the wind in the solar
corona that has been initiated by observations of the resonantly
scattered ultraviolet emission of the coronal plasma obtained with
UVCS-SOHO, designed to measure the wind outflow speed by applying
Doppler dimming diagnostics. Metis on Solar Orbiter complements the
UVCS spectroscopic observations that were performed during solar
activity cycle 23 by simultaneously imaging the polarized visible
light and the H I Lyman-α corona in order to obtain high spatial and
temporal resolution maps of the outward velocity of the continuously
expanding solar atmosphere. The Metis observations, taken on May 15,
2020, provide the first H I Lyman-α images of the extended corona
and the first instantaneous map of the speed of the coronal plasma
outflows during the minimum of solar activity and allow us to identify
the layer where the slow wind flow is observed. The polarized visible
light (580-640 nm) and the ultraviolet H I Lyα (121.6 nm) coronal
emissions, obtained with the two Metis channels, were combined in
order to measure the dimming of the UV emission relative to a static
corona. This effect is caused by the outward motion of the coronal
plasma along the direction of incidence of the chromospheric photons
on the coronal neutral hydrogen. The plasma outflow velocity was then
derived as a function of the measured Doppler dimming. The static
corona UV emission was simulated on the basis of the plasma electron
density inferred from the polarized visible light. This study leads
to the identification, in the velocity maps of the solar corona, of
the high-density layer about ±10° wide, centered on the extension
of a quiet equatorial streamer present at the east limb - the coronal
origin of the heliospheric current sheet - where the slowest wind
flows at about 160 ± 18 km s−1 from 4 R⊙
to 6 R⊙. Beyond the boundaries of the high-density layer,
the wind velocity rapidly increases, marking the transition between
slow and fast wind in the corona.
Title: First demonstration of full ELM suppression in low input
torque plasmas to support ITER research plan using n = 4 RMP in EAST
Authors: Sun, Y.; Ma, Q.; Jia, M.; Gu, S.; Loarte, A.; Liang, Y.; Liu,
Y. Q.; Paz-Soldan, C. A.; Wu, X. M.; Xie, P. C.; Ye, C.; Wang, H. H.;
Zhao, J. Q.; Guo, W.; He, K.; Li, Y. Y.; Li, G.; Liu, H.; Qian, J.;
Sheng, H.; Shi, T.; Wang, Y. M.; Weisberg, D.; Wan, B.; Zang, Q.;
Zeng, L.; Zhang, B.; Zhang, L.; Zhang, T.; Zhou, C.; EAST Contributors
Bibcode: 2021NucFu..61j6037S
Altcode:
Full suppression of type-I edge localized modes (ELMs) using n = 4
resonant magnetic perturbations (RMPs) as planned for ITER has been
demonstrated for the first time (n is the toroidal mode number of the
applied RMP). This is achieved in EAST plasmas with low input torque
and tungsten divertor, and the target plasma for these experiments in
EAST is chosen to be relevant to the ITER Q = 10 operational scenario,
thus also addressing significant scenario issues for ITER. In these
experiments the lowest neutral beam injection (NBI) input torque is
around TNBI ~ 0.44 Nm, which extrapolates to around 14 Nm
in ITER (compared to a total torque input of 35 Nm when 33 MW of NBI
are used for heating). The q95 is around 3.6 and normalized
plasma beta βN ~ 1.5-1.8, similar to that in the ITER Q =
10 scenario. Suppression windows in both q95 and plasma
density are observed; in addition, lower plasma rotation is found to
be favourabe to access ELM suppression. ELM suppression is maintained
with line averaged density up to 60%nGW (Greenwald density
limit) by feedforward gas fuelling after suppression is achieved. It
is interesting to note that in addition to an upper density, a low
density threshold for ELM suppression of 40%nGW is also
observed. In these conditions energy confinement does not significantly
drop (<10%) during ELM suppression when compared to the ELMy H-mode
conditions, which is much better than previous results using low n
(n = 1 and 2) RMPs in higher q95 regimes. In addition,
the core plasma tungsten concentration is clearly reduced during
ELM suppression demonstrating an effective impurity exhaust. MHD
response modelling using the MARS-F code shows that edge magnetic field
stochasticity has a peak at q95 ~ 3.65 for the odd parity
configuration, which is consistent to the observed suppression window
around 3.6-3.75. These results expand the physical understanding of
ELM suppression and demonstrate the effectiveness of n = 4 RMPs for
reliable control ELMs in future ITER high Q plasma scenarios with
minimum detrimental effects on plasma confinement.
Title: A New Reconstruction of the Sun's Magnetic Field and Total
Irradiance since 1700
Authors: Wang, Y. -M.; Lean, J. L.
Bibcode: 2021ApJ...920..100W
Altcode:
We model the Sun's large-scale magnetic field and total solar irradiance
(TSI) since 1700 by combining flux transport simulations with empirical
relationships between facular brightening, sunspot darkening, and the
total photospheric flux. The photospheric field is evolved subject to
the constraints that (1) the flux emergence rate scales as the yearly
sunspot numbers, and (2) the polar field strength at solar minimum
is proportional to the amplitude of the following cycle. Simulations
are performed using both the recently revised sunspot numbers and an
average of these numbers and the Hoyt-Schatten group numbers. A decrease
(increase) in the polar field strength from one cycle to the next is
simulated either by increasing (decreasing) the poleward flow speed, or
by decreasing (increasing) the average axial tilts of active regions;
the resulting photospheric field evolution is very similar whichever
parameter is varied. Comparisons between irradiance data and both
the simulated and observed photospheric field suggest that TSI and
facular brightness increase less steeply with the field strength at
solar minimum than at other phases of the cycle, presumably because of
the dominance of small-scale ephemeral regions when activity is very
low. This relative insensitivity of the irradiance to changes in the
large-scale field during cycle minima results in a minimum-to-minimum
increase of annual TSI from 1700 to 1964 (2008) of 0.2 (0.06) W
m-2, a factor of 2-3 smaller than predicted in earlier
reconstructions where the relation between facular brightness and
field strength was assumed to be independent of cycle phase.
Title: Using Observations of Solar Vector Magnetic Field from Dual
View Points to Remove the 180° Ambiguity
Authors: Zhou, R. Y.; Wang, Y. M.; Su, Y. N.; Bi, S. L.; Liu, R.;
Ji, H. S.
Bibcode: 2021AcASn..62...41Z
Altcode:
With solar orbiter being put into successful operational observation,
solar magnetic observation has entered the era of remote sensing
from dual view points. In this paper, we carried out a simulation
of correcting the 180° ambiguity of transverse magnetic field
with magnetograms from dual view points using analytical formula as
well as observations made by the Helioseismic and Magnetic Imager
(HMI). Magnetograms of a small mature sunspot at different times were
used to simulate magnetograms from dual view points. We find that,
in order to correct the 180° ambiguity of the vector magnetograms,
it is sufficient to have a line-of-sight magnetogram from another
view point to help to judge the direction of the transverse magnetic
field. For the measuring accuracy of HMI, we estimate that a ∼30°
angle formed from two observational points is the smallest angle to
correct the 180° ambiguity around the place with the magnetic field
strength of < 50 Gs. Correction for weaker magnetic fields surely
needs larger separation angle, but considering projection effect,
we propose that separation angles of ∼30° are the optimal angles
for future space missions with the scientific aim of correcting the
180° ambiguity of vector magnetograms.
Title: Small-scale Flux Emergence, Coronal Hole Heating, and Flux-tube
Expansion: A Hybrid Solar Wind Model
Authors: Wang, Y. -M.
Bibcode: 2020ApJ...904..199W
Altcode: 2021arXiv210404016W
Extreme-ultraviolet images from the Solar Dynamics Observatory often
show loop-like fine structure to be present where no minority-polarity
flux is visible in magnetograms, suggesting that the rate of
ephemeral region (ER) emergence inside "unipolar" regions has been
underestimated. Assuming that this rate is the same inside coronal
holes as in the quiet Sun, we show that interchange reconnection
between ERs and open field lines gives rise to a solar wind energy
flux that exceeds 105 erg cm-2 s-1
and that scales as the field strength at the coronal base, consistent
with observations. In addition to providing ohmic heating in the low
corona, these reconnection events may be a source of Alfvén waves
with periods ranging from the granular timescale of ∼10 minutes
to the supergranular/plume timescale of many hours, with some of
the longer-period waves being reflected and dissipated in the outer
corona. The asymptotic wind speed depends on the radial distribution
of the heating, which is largely controlled by the rate of flux-tube
expansion. Along the rapidly diverging flux tubes associated with slow
wind, heating is concentrated well inside the sonic point (1) because
the outward conductive heat-flux density and thus the outer coronal
temperatures are reduced, and (2) because the net wave energy flux is
dissipated at a rate proportional to the local Alfvén speed. In this
"hybrid" solar wind model, reconnection heats the lower corona and
drives the mass flux, whereas waves impart energy and momentum to the
outflow at greater distances.
Title: Modeling inner boundary values at 18 solar radii during
solar quiet time for global three-dimensional time-dependent
magnetohydrodynamic numerical simulation
Authors: Wu, Chin-Chun; Liou, Kan; Wood, Brian E.; Plunkett,
Simon; Socker, Dennis; Wang, Y. M.; Wu, S. T.; Dryer, Murray; Kung,
Christopher
Bibcode: 2020JASTP.20105211W
Altcode:
We develop an empirical model of the solar wind parameters at the inner
boundary (18 solar radii, Rs) of the heliosphere that can
be used in our global, three-dimensional (3D) magnetohydrodynamic
(MHD) model (G3DMHD) or other equivalent ones. The model takes
solar magnetic field maps at 2.5 R, which is based on the
Potential Field Source Surface, PFSS model and interpolates the solar
wind plasma and field out to 18 Rs using the algorithm of
Wang and Sheeley (1990). A formula (V18Rs = V1 +
V2fsα) is used to calculate the solar
wind speed at 18 Rs, where V1 is in a range of
150-350 km s-1, V2 is in the range of 250-500
km s-1, and "fs" is the magnetic flux expansion
factor derived from the Wang and Sheeley (WS) algorithm at 2.5 R. To
estimate the solar wind density and temperature at 18 Rs, we
assume an incompressible solar wind and a constant total pressure. The
three free parameters are obtained by adjusting simulation results
to match in-situ observations (Wind) for more than 54 combinations of
V1, V2 and α during a quiet solar wind interval,
i.e., the Carrington Rotation (CR) 2082. We found that VBF =
(200 ± 50) + (400 ± 100) fs-0.4 km/s is a good
formula for the quiet solar wind period. The formula was also good
to use for the other quiet solar periods. Comparing results between
WSA (Arge et al. 2000, 2004) and our model (WSW-3DMHD), we find the
following: i) The results of using VBF with the full rotation
(FR) data as input to drive the 3DMHD model is better than the results
of WSA using FR, or daily updated.. ii) The WSA model using the modified
daily updated 4-day-advanced solar wind speed predictions is slightly
better than that for WSW-3DMHD. iii) The results of using VBF
as input to drive the 3DMHD model is much better than the using the WSA
formula with an extra parameter for the angular width (θb)
from the nearest coronal hole. The present study puts in doubt in the
usefulness of θb for these purposes.
Title: Searching for a Boundary Layer as a Source of the Slow
Solar Wind
Authors: Ko, Y. K.; Muglach, K.; Riley, P.; Wang, Y. M.
Bibcode: 2019AGUFMSH41F3330K
Altcode:
Recent investigations in the solar wind plasma and magnetic field
characteristics indicate a likely existence of a "boundary layer"
where the slow solar wind originates from. Such a boundary layer
resides at the coronal hole boundary where the open field lines
emanating from it expand super-radially into the corona. We select two
adjacent coronal holes that are the sources of two consecutive solar
wind streams measured by ACE. One is a low-latitude extension of the
north polar coronal hole that past the central meridian on August 18,
2015, and the other is an equatorial coronal hole that past the central
meridian on August 20. We use data from SDO/AIA, SDO/HMI and Hinode/EIS
in combination with PFSS and 3D MHD models to investigate the evolution
of the coronal and magnetic field properties at the boundary of these
coronal holes and search for signatures of such a boundary layer.
Title: Further Evidence for Looplike Fine Structure inside
“Unipolar” Active Region Plages
Authors: Wang, Y. -M.; Ugarte-Urra, I.; Reep, J. W.
Bibcode: 2019ApJ...885...34W
Altcode: 2021arXiv210406633W
Earlier studies using extreme-ultraviolet images and line-of-sight
magnetograms from the Solar Dynamics Observatory (SDO) have suggested
that active region (AR) plages and strong network concentrations
often have small, looplike features embedded within them, even
though no minority-polarity flux is visible in the corresponding
magnetograms. Because of the unexpected nature of these findings, we
have searched the SDO database for examples of inverted-Y structures
rooted inside “unipolar” plages, with such jetlike structures
being interpreted as evidence for magnetic reconnection between small
bipoles and the dominant-polarity field. Several illustrative cases are
presented from the period of 2013-2015, all of which are associated with
transient outflows from AR “moss.” The triangular or dome-shaped
bases have horizontal dimensions of ∼2-4 Mm, corresponding to ∼1-3
granular diameters. We also note that the spongy-textured Fe IX 17.1 nm
moss is not confined to plages, but may extend into regions where the
photospheric field is relatively weak or even has mixed polarity. We
again find a tendency for bright coronal loops seen in the 17.1,
19.3, and 21.1 nm passbands to show looplike fine structure and
compact brightenings at their footpoints. These observations provide
further confirmation that present-day magnetograms are significantly
underrepresenting the amount of minority-polarity flux inside AR plages
and again suggest that footpoint reconnection and small-scale flux
cancellation may play a major role in coronal heating, both inside
and outside ARs.
Title: Observations of Slow Solar Wind from Equatorial Coronal Holes
Authors: Wang, Y. -M.; Ko, Y. -K.
Bibcode: 2019ApJ...880..146W
Altcode: 2021arXiv210406626W
Because of its distinctive compositional properties and variability,
low-speed (≲450 km s-1) solar wind is widely believed
to originate from coronal streamers, unlike high-speed wind, which
comes from coronal holes. An alternative scenario is that the bulk
of the slow wind (excluding that in the immediate vicinity of the
heliospheric current sheet) originates from rapidly diverging flux
tubes rooted inside small coronal holes or just within the boundaries
of large holes. This viewpoint is based largely on photospheric field
extrapolations, which are subject to considerable uncertainties and
do not include dynamical effects, making it difficult to be certain
whether a source is located just inside or outside a hole boundary, or
whether a high-latitude hole will be connected to Earth. To minimize
the dependence on field-line extrapolations, we have searched for
cases where equatorial coronal holes at central meridian are followed
by low-speed streams at Earth. We describe 14 examples from the period
2014-2017, involving Fe XIV 21.1 nm coronal holes located near active
regions and having equatorial widths of ∼3°-10°. The associated in
situ wind was characterized by speeds v ∼ 300-450 km s-1
and by O7+/O6+ ratios of ∼0.05-0.15, with v
showing the usual correlation with proton temperature. In addition,
consistent with other recent studies, this slow wind had remarkably
high Alfvénicity, similar to that in high-speed streams. We conclude
that small coronal holes are a major contributor to the slow solar wind
during the maximum and early post-maximum phases of the solar cycle.
Title: Observations of Solar Wind from Earth-directed Coronal
Pseudostreamers
Authors: Wang, Y. -M.; Panasenco, O.
Bibcode: 2019ApJ...872..139W
Altcode:
Low-speed (≲450 km s-1) solar wind is widely considered
to originate from streamer loops that intermittently release their
contents into the heliosphere, in contrast to high-speed wind, which
has its source in large coronal holes. To account for the presence of
slow wind far from the heliospheric current sheet (HCS), it has been
suggested that “pseudostreamers” rooted between coronal holes
of the same polarity continually undergo interchange reconnection
with the adjacent open flux, producing a wide band of slow wind
centered on the separatrix/plasma sheet that extends outward from
the pseudostreamer cusp. Employing extreme-ultraviolet images and
potential-field source-surface extrapolations, we have identified 10
Earth-directed pseudostreamers during 2013-2016. In situ measurements
show wind speeds ranging from ∼320 to ∼600 km s-1 in the
days immediately preceding and following the predicted pseudostreamer
crossings, with the proton densities and O7+/O6+
ratios tending to be inversely correlated with the bulk speed. We also
identify examples of coronal holes that straddle the solar equator and
give rise to wind speeds of order 400 km s-1. Our results
support the idea that the bulk of the slow wind observed more than a
few degrees from the HCS originates from just inside coronal holes.
Title: Helicity Removal and Coronal Fe XII Stalks: Evidence That
the Axial Field Is Not Ejected but Resubmerged
Authors: Wang, Y. -M.; Berger, M. A.
Bibcode: 2018ApJ...868...66W
Altcode:
The magnetic/current helicity of the coronal field is closely associated
with the presence of a nonpotential axial component directed along the
photospheric polarity inversion line (PIL), which is also the source
of the axial/toroidal field in flux ropes and coronal mass ejections
(CMEs). To better understand the role of this axial component in
the evolution of coronal helicity, we use Fe XII 19.3 nm images and
longitudinal magnetograms from the Solar Dynamics Observatory to
track active regions (ARs) and their filament channels as they decay
due to flux transport processes. We find that the Fe XII loop legs
or “stalks,” initially oriented almost perpendicular to the PIL,
become closely aligned with it after ∼1-4 rotations; this alignment
is attributed to the progressive cancellation of the transverse field
component at the PIL. As the AR flux continues to decay, the PIL becomes
ever more distorted and the directions of the stalks are increasingly
randomized. These observations suggest that most of the original axial
field in ARs is not expelled in CMEs, but instead pinches off after
the eruptions and becomes concentrated at the PIL. Because the twist
of the field decreases, however, the helicity itself decreases, with
CMEs removing a significant fraction of it in the form of disconnected
flux ropes. Like most of the AR flux, the bulk of the axial field
is eventually canceled/resubmerged, brought to the equator by the
subsurface meridional flow, and annihilated (along with the remaining
helicity) by merging with its opposite-handed counterpart from the
other hemisphere.
Title: Gradual Streamer Expansions and the Relationship between
Blobs and Inflows
Authors: Wang, Y. -M.; Hess, P.
Bibcode: 2018ApJ...859..135W
Altcode:
Coronal helmet streamers show a continual tendency to expand outward and
pinch off, giving rise to flux ropes that are observed in white light
as “blobs” propagating outward along the heliospheric current/plasma
sheet. The blobs form within the r ∼ 2-6 R ⊙ heliocentric
range of the Large Angle and Spectrometric Coronagraph (LASCO) C2
instrument, but the expected inward-moving counterparts are often
not detected. Here we show that the height of blob formation varies
as a function of the underlying photospheric field, with the helmet
streamer loops expanding to greater heights when active regions
(ARs) emerge underneath them. When the pinch-offs occur at r ∼ 3-4
R ⊙, diverging inward/outward tracks sometimes appear
in height-time maps constructed from LASCO C2 running-difference
images. When the underlying photospheric field is weak, the blobs
form closer to the inner edge of the C2 field of view and only the
outward tracks are clearly visible. Conversely, when the emergence
of large ARs leads to a strengthening of the outer coronal field
and an increase in the total white-light radiance (as during late
2014), the expanding helmet-streamer loops pinch off beyond r ∼
4 R ⊙, triggering strong inflow streams whose outgoing
counterparts are usually very faint. We deduce that the visibility
of the blobs and inflows depends on the amount of material that the
diverging components sweep up within the 2-6 R ⊙ field of
view. We also note that the rate of blob production tends to increase
when a helmet streamer is “activated” by underlying flux emergence.
Title: “Twisting” Motions in Erupting Coronal Pseudostreamers
as Evidence for Interchange Reconnection
Authors: Wang, Y. -M.; Hess, P.
Bibcode: 2018ApJ...853..103W
Altcode:
Using white-light observations from the COR1 coronagraph during
2008-2013, we have identified ∼50 eruptive events in which a
narrow streamer structure appears to rotate about its radial axis
as it rises into the field of view beyond r∼ 1.4 {R}⊙
. Extreme-ultraviolet images and potential-field extrapolations
suggest that most of these eruptions involve one arcade of a
double-lobed pseudostreamer, which is surrounded by open flux of
a single polarity. The “twisting” is manifested by the cavity
of the erupting lobe, which evolves from a circular to a narrowing
oval structure as it is ejected nonradially in the direction of the
original X-point. At the same time, the loop legs on the trailing side
of the rising cavity/flux rope expand and straighten out, starting at
the outer edge of the lobe and progressing inward; this asymmetric
opening-up contributes to the impression of a three-dimensional
structure twisting away from the observer. On the leading side of
the lobe, collapsing cusps are sometimes detected, suggesting the
presence of a current sheet where the cavity loops reconnect with the
oppositely directed open flux from the adjacent coronal hole. In some
events, the inner loops of the cavity/flux rope may continue to expand
outward without undergoing interchange reconnection. The transfer of
material to open field lines, as well as the lateral confinement of
the pseudostreamer by the surrounding coronal holes, acts to produce
a relatively narrow, fan-like ejection that differs fundamentally from
the large, bubble-shaped ejections associated with helmet streamers.
Title: Surface Flux Transport and the Evolution of the Sun's Polar
Fields
Authors: Wang, Y. -M.
Bibcode: 2018smf..book..351W
Altcode:
No abstract at ADS
Title: Inflows in the Inner White-light Corona: The Closing-down of
Flux after Coronal Mass Ejections
Authors: Hess, P.; Wang, Y. -M.
Bibcode: 2017ApJ...850....6H
Altcode:
During times of high solar activity, the Solar and Heliospheric
Observatory/Large Angle and Spectrometric Coronagraph C2 coronagraph
has recorded multitudes of small features moving inward through
its 2{--}6 {R}⊙ field of view. These outer-coronal
inflows, which are concentrated around the heliospheric current sheet,
tend to be poorly correlated with individual coronal mass ejection
(CME) events. Using running-difference movies constructed from Solar
Terrestrial Relations Observatory/COR1 coronagraph images taken during
2008-2014, we have identified large numbers of inward-moving features
at heliocentric distances below 2 {R}⊙ , with the rate
increasing with sunspot and CME activity. Most of these inner-coronal
inflows are closely associated with CMEs, being observed during and in
the days immediately following the eruptions. Here, we describe several
examples of the pinching-off of tapered streamer structures in the wake
of CMEs. This type of inflow event is characterized by a separation
of the flow into incoming and outgoing components connected by a thin
spike, which is interpreted as a continually elongating current sheet
viewed edge-on; by the prior convergence of narrow rays toward the
current sheet; and by a succession of collapsing loops that form a
cusp-shaped structure at the base of the current sheet. The re-forming
streamer overlies a growing post-eruption arcade that is visible in EUV
images. These observations provide support for standard reconnection
models for the formation/evolution of flux ropes during solar eruptive
events. We suggest that inflow streams that occur over a relatively
wide range of position angles result from the pinching-off of loop
arcades whose axes are oriented parallel rather than perpendicular to
the sky plane.
Title: Surface Flux Transport and the Evolution of the Sun's Polar
Fields
Authors: Wang, Y. -M.
Bibcode: 2017SSRv..210..351W
Altcode: 2016SSRv..tmp...21W
The evolution of the polar fields occupies a central place in flux
transport (Babcock-Leighton) models of the solar cycle. We discuss
the relationship between surface flux transport and polar field
evolution, focusing on two main issues: the latitudinal profile of
the meridional flow and the axial tilts of active regions. Recent
helioseismic observations indicate that the poleward flow speed peaks
at much lower latitudes than inferred from magnetic feature tracking,
which includes the effect of supergranular diffusion and thus does not
represent the actual bulk flow. Employing idealized simulations, we
demonstrate that flow profiles that peak at mid latitudes give rise to
overly strong and concentrated polar fields. We discuss the differences
between magnetic and white-light measurements of tilt angles, noting
the large uncertainties inherent in the sunspot group measurements
and their tendency to underestimate the actual tilts. We find no
clear evidence for systematic cycle-to-cycle variations in Joy's law
during cycles 21-23. Finally, based on the observed evolution of the
Sun's axial dipole component and polar fields up to the end of 2015,
we predict that cycle 25 will be similar in amplitude to cycle 24.
Title: Small Coronal Holes Near Active Regions as Sources of Slow
Solar Wind
Authors: Wang, Y. -M.
Bibcode: 2017ApJ...841...94W
Altcode:
We discuss the nature of the small areas of rapidly diverging,
open magnetic flux that form in the strong unipolar fields at the
peripheries of active regions (ARs), according to coronal extrapolations
of photospheric field measurements. Because such regions usually have
dark counterparts in extreme-ultraviolet (EUV) images, we refer to them
as coronal holes, even when they appear as narrow lanes or contain
sunspots. Revisiting previously identified “AR sources” of slow
solar wind from 1998 and 1999, we find that they are all associated
with EUV coronal holes; the absence of well-defined He I 1083.0 nm
counterparts to some of these holes is attributed to the large flux
of photoionizing radiation from neighboring AR loops. Examining a
number of AR-associated EUV holes during the 2014 activity maximum,
we confirm that they are characterized by wind speeds of ∼300-450 km
s-1, O7+/O6+ ratios of ∼0.05-0.4,
and footpoint field strengths typically of order 30 G. The close spacing
between ARs at sunspot maximum limits the widths of unipolar regions
and their embedded holes, while the continual emergence of new flux
leads to rapid changes in the hole boundaries. Because of the highly
nonradial nature of AR fields, the smaller EUV holes are often masked
by the overlying canopy of loops, and may be more visible toward one
solar limb than at central meridian. As sunspot activity declines,
the AR remnants merge to form much larger, weaker, and longer-lived
unipolar regions, which harbor the “classical” coronal holes that
produce recurrent high-speed streams.
Title: Light-Toned Materials of Melas Chasma: Evidence for Their
Formation on Mars
Authors: Bi, X. Y.; Ling, Z. C.; Chen, J.; Zhang, J.; Cao, H. J.;
Wang, Y. M.; Song, C. Y.
Bibcode: 2017LPI....48.2794B
Altcode:
We find a distinctive terrain in Melas Chasma on Mars which has wavy
shape, determine the mineral phases, and figure out their possible
formation mechanism.
Title: The Stereo Electron Spikes and the Interplanetary Magnetic
Field
Authors: Jokipii, J. R.; Sheeley, N. R., Jr.; Wang, Y. M.; Giacalone,
J.
Bibcode: 2016AGUFMSH51G..06J
Altcode:
A recent paper (Klassen etal, 2015) discussed observations of a spike
event of 55-65 keV electrons which occurred very nearly simultaneously
at STEREO A and STEREO B, which at the time were separated in longitude
by 38 degrees. The authors associated the spikes with a flare at the
Sun near the footpoint of the nominal Archimedean spiral magnetic
field line passing through STEREO A. The spike at STEREO A was delayed
by 2.2 minutes from that at STEREOB. We discuss the observations in
terms of a model in which the electrons, accelerated at the flare,
propagate without significant scattering along magnetic field lines
which separate or diverge as a function of radial distance from the
Sun. The near simultaneity of the spikes at the two spacecraft is a
natural consequence of this model. We interpret the divergence of the
magnetic field lines as a consequence of field-line random walk and
flux-tube expansion. We show that the field-line random walk in the
absence of flux-tube expansion produces an rms spread of field lines
significantly less than that which is required to produce to observed
divergence. We find that observations of the solar wind and its source
region at the time of the event can account for the observations in
terms of propagation along interplanetary magnetic field-lines. Klassen,
A., Dresing, N., Gomez-Herrero, R, and Heber, B., A&A 580, A115
(2015) Financial support for NS and YMW was provided by NASA and CNR.
Title: Fundamental Physics of the Slow Solar Wind - What do we Know?
Authors: Ofman, L.; Abbo, L.; Antiochos, S. K.; Hansteen, V. H.;
Harra, L.; Ko, Y. K.; Lapenta, G.; Li, B.; Riley, P.; Strachan, L.;
von Steiger, R.; Wang, Y. M.
Bibcode: 2016AGUFMSH42A..01O
Altcode:
Fundamental physical properties of the slow solar wind (SSW), such
as density, temperature, outflow speed, heavy ion abundances and
charges states were obtained from in-situ measurements at 1AU in
the past from WIND, ACE, and other spacecraft. Plasma and magnetic
field measurement are available as close as 0.3 AU from Helios data,
Spektr-R, and MESSENGER spacecraft. Remote sensing spectroscopic
measurements are available in the corona and below from SOHO/UVCS,
Hinode, and other missions. One of the major objectives of the Solar
Orbiter and Solar Probe Plus missions is to study the sources of the
SSW close to the Sun. The present state of understanding of the physics
of the SSW is based on the combination of the existing observations,
theoretical and numerical 3D MHD and multi-fluid models, that connect
between the SSW sources in the corona and the heliosphere. Recently,
hybrid models that combine fluid electrons and kinetic ions of the
expanding solar wind were developed, and provide further insights of the
local SSW plasma heating processes that related to turbulent magnetic
fluctuations spectra and kinetic ion instabilities observed in the
SSW plasma. These models produce the velocity distribution functions
(VDFs) of the protons and heavier ions as well as the ion anisotropic
temperatures. I will discuss the results of the above observations
and models, and review the current status of our understanding of
the fundamental physics of the SSW. I will review the open questions,
and discuss how they could be addressed with near future observations
and models.
Title: The Oxygen Charge-state Ratio as an Indicator of Footpoint
Field Strength in the Source Regions of the Solar Wind
Authors: Wang, Y. -M.
Bibcode: 2016ApJ...833..121W
Altcode:
Because of its distinctive compositional properties and high
variability, the slow solar wind is widely believed to originate
from coronal streamers, unlike high-speed wind, which emanates from
coronal holes. Based on measurements from the Advanced Composition
Explorer, it has been proposed that an oxygen charge-state ratio
O7+/O6+ of 0.145 is the threshold that separates
streamer from coronal hole wind. During the 2007-2009 sunspot minimum,
however, the median value of O7+/O6+ fell to
only 0.06, implying that almost all of the near-Earth wind came from
coronal holes, despite the fact that the streamer belt lay much closer
to the ecliptic plane at that time than at solar maximum. Employing
extrapolations of photospheric field maps to derive the footpoint
field strengths B 0 of the near-Earth wind, we find that the
median value of B 0 decreased to only 2.6 G during 2007-2009,
from a value of 21 G during 1998-2004. The factor of ∼2 decrease in
the median value of O7+/O6+ thus reflects the
factor of ∼8 decrease in the footpoint field strength. Variations
in O7+/O6+ are strongly anticorrelated with
the wind speed on timescales of days, but not on long timescales,
which are dominated by changes in B 0. We suggest that the
charge-state ratio is determined by the amount of energy deposited near
the coronal base, which in turn depends on B 0 and the local
flux-tube expansion rate. High values of O7+/O6+
are associated with rapidly diverging flux tubes rooted just inside
the boundaries of coronal holes with strong footpoint fields.
Title: Role of the Coronal Alfvén Speed in Modulating the Solar-wind
Helium Abundance
Authors: Wang, Y. -M.
Bibcode: 2016ApJ...833L..21W
Altcode:
The helium abundance He/H in the solar wind is relatively constant at
∼0.04 in high-speed streams, but varies in phase with the sunspot
number in slow wind, from ∼0.01 at solar minimum to ∼0.04 at
maximum. Suggested mechanisms for helium fractionation have included
frictional coupling to protons and resonant interactions with
high-frequency Alfvénic fluctuations. We compare He/H measurements
during 1995-2015 with coronal parameters derived from source-surface
extrapolations of photospheric field maps. We find that the near-Earth
helium abundance is an increasing function of the magnetic field
strength and Alfvén speed v A in the outer corona, while
being only weakly correlated with the proton flux density. Throughout
the solar cycle, fast wind is associated with short-term increases
in v A near the source surface; resonance with Alfvén
waves, with v A and the relative speed of α-particles
and protons decreasing with increasing heliocentric distance, may
then lead to enhanced He/H at 1 au. The modulation of helium in slow
wind reflects the tendency for the associated coronal Alfvén speeds
to rise steeply from sunspot minimum, when this wind is concentrated
around the source-surface neutral line, to sunspot maximum, when the
source-surface field attains its peak strengths. The helium abundance
near the source surface may represent a balance between collisional
decoupling from protons and Alfvén wave acceleration.
Title: Slow Solar Wind: Observations and Modeling
Authors: Abbo, L.; Ofman, L.; Antiochos, S. K.; Hansteen, V. H.;
Harra, L.; Ko, Y. -K.; Lapenta, G.; Li, B.; Riley, P.; Strachan, L.;
von Steiger, R.; Wang, Y. -M.
Bibcode: 2016SSRv..201...55A
Altcode: 2016SSRv..tmp...34A
While it is certain that the fast solar wind originates from coronal
holes, where and how the slow solar wind (SSW) is formed remains an
outstanding question in solar physics even in the post-SOHO era. The
quest for the SSW origin forms a major objective for the planned future
missions such as the Solar Orbiter and Solar Probe Plus. Nonetheless,
results from spacecraft data, combined with theoretical modeling, have
helped to investigate many aspects of the SSW. Fundamental physical
properties of the coronal plasma have been derived from spectroscopic
and imaging remote-sensing data and in situ data, and these results
have provided crucial insights for a deeper understanding of the origin
and acceleration of the SSW. Advanced models of the SSW in coronal
streamers and other structures have been developed using 3D MHD and
multi-fluid equations.
Title: Near-Earth heliospheric magnetic field intensity since 1750:
2. Cosmogenic radionuclide reconstructions
Authors: Owens, M. J.; Cliver, E.; McCracken, K. G.; Beer, J.; Barnard,
L.; Lockwood, M.; Rouillard, A.; Passos, D.; Riley, P.; Usoskin, I.;
Wang, Y. -M.
Bibcode: 2016JGRA..121.6064O
Altcode:
This is Part 2 of a study of the near-Earth heliospheric magnetic field
strength, B, since 1750. Part 1 produced composite estimates of B from
geomagnetic and sunspot data over the period 1750-2013. Sunspot-based
reconstructions can be extended back to 1610, but the paleocosmic ray
(PCR) record is the only data set capable of providing a record of
solar activity on millennial timescales. The process for converting
10Be concentrations measured in ice cores to B is more
complex than with geomagnetic and sunspot data, and the uncertainties
in B derived from cosmogenic nuclides (~20% for any individual year)
are much larger. Within this level of uncertainty, we find reasonable
overall agreement between PCR-based B and the geomagnetic- and sunspot
number-based series. This agreement was enhanced by excising low values
in PCR-based B attributed to high-energy solar proton events. Other
discordant intervals, with as yet unspecified causes remain included in
our analysis. Comparison of 3 year averages centered on sunspot minimum
yields reasonable agreement between the three estimates, providing a
means to investigate the long-term changes in the heliospheric magnetic
field into the past even without a means to remove solar proton events
from the records.
Title: Near-Earth heliospheric magnetic field intensity since 1750:
1. Sunspot and geomagnetic reconstructions
Authors: Owens, M. J.; Cliver, E.; McCracken, K. G.; Beer, J.; Barnard,
L.; Lockwood, M.; Rouillard, A.; Passos, D.; Riley, P.; Usoskin, I.;
Wang, Y. -M.
Bibcode: 2016JGRA..121.6048O
Altcode:
We present two separate time series of the near-Earth heliospheric
magnetic field strength (B) based on geomagnetic data and sunspot number
(SSN). The geomagnetic-based B series from 1845 to 2013 is a weighted
composite of two series that employ the interdiurnal variability index;
this series is highly correlated with in situ spacecraft measurements
of B (correlation coefficient, r = 0.94; mean square error, MSE =
0.16 nT2). The SSN-based estimate of B, from 1750 to 2013,
is a weighted composite of eight time series derived from two separate
reconstruction methods applied to four different SSN time series,
allowing determination of the uncertainty from both the underlying
sunspot records and the B reconstruction methods. The SSN-based
composite is highly correlated with direct spacecraft measurements of B
and with the composite geomagnetic B time series from 1845 to 2013 (r =
0.91; MSE = 0.24 nT2), demonstrating that B can accurately
reconstructed by both geomagnetic and sunspot-based methods. The
composite sunspot and geomagnetic B time series, with uncertainties,
are provided as supporting information.
Title: Signatures of anomalous Higgs couplings in angular
asymmetries of H → Zℓ+ℓ- and
e+e- → HZ
Authors: Beneke, M.; Boito, D.; Wang, Y. -M.
Bibcode: 2016NPPP..273..846B
Altcode:
Parametrizing beyond Standard Model physics by the SU (3) ×
SU(2)L × U(1)Y dimension-six effective
lagrangian, we study the impact of anomalous Higgs couplings in
angular asymmetries of the crossing symmetric processes H →
Zℓ+ℓ- and e+e-
→ HZ. In the light of present bounds on d = 6 couplings, we show
that some asymmetries can reveal BSM effects that would otherwise
be hidden in other observables. The d = 6 HZγ couplings as well
as (to a lesser extent) HZℓℓ contact interactions can generate
asymmetries at the several percent level, albeit having less significant
effects on the di-lepton invariant mass spectrum of the decay H →
Zℓ+ℓ-. The higher di-lepton invariant mass
probed in e+e- → HZ can lead to complementary
anomalous coupling searches at e+e- colliders.
Title: The Ubiquitous Presence of Looplike Fine Structure inside
Solar Active Regions
Authors: Wang, Y. -M.
Bibcode: 2016ApJ...820L..13W
Altcode:
Although most of the solar surface outside active regions (ARs)
is pervaded by small-scale fields of mixed polarity, this magnetic
“carpet” or “junkyard” is thought to be largely absent inside
AR plages and strong network. However, using extreme-ultraviolet images
and line-of-sight magnetograms from the Solar Dynamics Observatory,
we find that unipolar flux concentrations, both inside and outside
ARs, often have small, loop-shaped Fe ix 17.1 and Fe xii 19.3 nm
features embedded within them, even though no minority-polarity flux
is visible in the corresponding magnetograms. Such looplike structures,
characterized by horizontal sizes of ∼3-5 Mm and varying on timescales
of minutes or less, are seen inside bright 17.1 nm moss, as well as in
fainter moss-like regions associated with weaker network outside ARs. We
also note a tendency for bright coronal loops to show compact, looplike
features at their footpoints. Based on these observations, we suggest
that present-day magnetograms may be substantially underrepresenting the
amount of minority-polarity flux inside plages and strong network, and
that reconnection between small bipoles and the overlying large-scale
field could be a major source of coronal heating both in ARs and in
the quiet Sun.
Title: Converging Supergranular Flows and the Formation of Coronal
Plumes
Authors: Wang, Y. -M.; Warren, H. P.; Muglach, K.
Bibcode: 2016ApJ...818..203W
Altcode:
Earlier studies have suggested that coronal plumes are energized
by magnetic reconnection between unipolar flux concentrations and
nearby bipoles, even though magnetograms sometimes show very little
minority-polarity flux near the footpoints of plumes. Here we use
high-resolution extreme-ultraviolet (EUV) images and magnetograms
from the Solar Dynamics Observatory (SDO) to clarify the relationship
between plume emission and the underlying photospheric field. We
find that plumes form where unipolar network elements inside coronal
holes converge to form dense clumps, and fade as the clumps disperse
again. The converging flows also carry internetwork fields of both
polarities. Although the minority-polarity flux is sometimes barely
visible in the magnetograms, the corresponding EUV images almost
invariably show loop-like features in the core of the plumes, with the
fine structure changing on timescales of minutes or less. We conclude
that the SDO observations are consistent with a model in which plume
emission originates from interchange reconnection in converging flows,
with the plume lifetime being determined by the ∼1 day evolutionary
timescale of the supergranular network. Furthermore, the presence of
large EUV bright points and/or ephemeral regions is not a necessary
precondition for the formation of plumes, which can be energized
even by the weak, mixed-polarity internetwork fields swept up by
converging flows.
Title: Slow Solar Wind: Observable Characteristics for Constraining
Modelling
Authors: Ofman, L.; Abbo, L.; Antiochos, S. K.; Hansteen, V. H.;
Harra, L.; Ko, Y. K.; Lapenta, G.; Li, B.; Riley, P.; Strachan, L.;
von Steiger, R.; Wang, Y. M.
Bibcode: 2015AGUFMSH11F..03O
Altcode:
The Slow Solar Wind (SSW) origin is an open issue in the post SOHO
era and forms a major objective for planned future missions such as
the Solar Orbiter and Solar Probe Plus.Results from spacecraft data,
combined with theoretical modeling, have helped to investigate many
aspects of the SSW. Fundamental physical properties of the coronal
plasma have been derived from spectroscopic and imaging remote-sensing
data and in-situ data, and these results have provided crucial insights
for a deeper understanding of the origin and acceleration of the
SSW.Advances models of the SSW in coronal streamers and other structures
have been developed using 3D MHD and multi-fluid equations.Nevertheless,
there are still debated questions such as:What are the source regions
of SSW? What are their contributions to the SSW?Which is the role
of the magnetic topology in corona for the origin, acceleration and
energy deposition of SSW?Which are the possible acceleration and heating
mechanisms for the SSW?The aim of this study is to present the insights
on the SSW origin and formationarisen during the discussions at the
International Space Science Institute (ISSI) by the Team entitled
''Slowsolar wind sources and acceleration mechanisms in the corona''
held in Bern (Switzerland) in March2014--2015. The attached figure will
be presented to summarize the different hypotheses of the SSW formation.
Title: Capabilities of a Global 3D MHD Model for Monitoring Extremely
Fast CMEs
Authors: Wu, C. C.; Plunkett, S. P.; Liou, K.; Socker, D. G.; Wu,
S. T.; Wang, Y. M.
Bibcode: 2015AGUFMSH41F..03W
Altcode:
Since the start of the space era, spacecraft have recorded many
extremely fast coronal mass ejections (CMEs) which have resulted in
severe geomagnetic storms. Accurate and timely forecasting of the
space weather effects of these events is important for protecting
expensive space assets and astronauts and avoiding communications
interruptions. Here, we will introduce a newly developed global,
three-dimensional (3D) magnetohydrodynamic (MHD) model (G3DMHD). The
model takes the solar magnetic field maps at 2.5 solar radii (Rs)
and intepolates the solar wind plasma and field out to 18 Rs using
the algorithm of Wang and Sheeley (1990, JGR). The output is used as
the inner boundary condition for a 3D MHD model. The G3DMHD model is
capable of simulating (i) extremely fast CME events with propagation
speeds faster than 2500 km/s; and (ii) multiple CME events in sequence
or simultaneously. We will demonstrate the simulation results (and
comparison with in-situ observation) for the fastest CME in record
on 23 July 2012, the shortest transit time in March 1976, and the
well-known historic Carrington 1859 event.
Title: Coronal Mass Ejections and the Solar Cycle Variation of the
Sun's Open Flux
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 2015ApJ...809L..24W
Altcode: 2021arXiv210407238W
The strength of the radial component of the interplanetary magnetic
field (IMF), which is a measure of the Sun’s total open flux, is
observed to vary by roughly a factor of two over the 11 year solar
cycle. Several recent studies have proposed that the Sun’s open
flux consists of a constant or “floor” component that dominates
at sunspot minimum, and a time-varying component due to coronal mass
ejections (CMEs). Here, we point out that CMEs cannot account for
the large peaks in the IMF strength which occurred in 2003 and late
2014, and which coincided with peaks in the Sun’s equatorial dipole
moment. We also show that near-Earth interplanetary CMEs, as identified
in the catalog of Richardson and Cane, contribute at most ∼30% of the
average radial IMF strength even during sunspot maximum. We conclude
that the long-term variation of the radial IMF strength is determined
mainly by the Sun’s total dipole moment, with the quadrupole moment
and CMEs providing an additional boost near sunspot maximum. Most of
the open flux is rooted in coronal holes, whose solar cycle evolution
in turn reflects that of the Sun’s lowest-order multipoles.
Title: The Recent Rejuvenation of the Sun's Large-scale Magnetic
Field: A Clue for Understanding Past and Future Sunspot Cycles
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.
Bibcode: 2015ApJ...809..113S
Altcode:
The quiet nature of sunspot cycle 24 was disrupted during the second
half of 2014 when the Sun’s large-scale field underwent a sudden
rejuvenation: the solar mean field reached its highest value since
1991, the interplanetary field strength doubled, and galactic cosmic
rays showed their strongest 27-day modulation since neutron-monitor
observations began in 1957; in the outer corona, the large increase of
field strength was reflected by unprecedentedly large numbers of coronal
loops collapsing inward along the heliospheric current sheet. Here, we
show that this rejuvenation was not caused by a significant increase in
the level of solar activity as measured by the smoothed sunspot number
and CME rate, but instead was caused by the systematic emergence of flux
in active regions whose longitudinal distribution greatly increased the
Sun’s dipole moment. A similar post-maximum increase in the dipole
moment occurred during each of the previous three sunspot cycles,
and marked the start of the declining phase of each cycle. We note
that the north-south component of this peak dipole moment provides
an early indicator of the amplitude of the next cycle, and conclude
that the amplitude of cycle 25 may be comparable to that of cycle 24,
and well above the amplitudes obtained during the Maunder Minimum.
Title: Evidence of the Solar EUV Hot Channel as a Magnetic Flux Rope
from Remote-sensing and In Situ Observations
Authors: SONG, H. Q.; CHEN, Y.; ZHANG, J.; CHENG, X.; Wang, B.; HU,
Q.; LI, G.; WANG, Y. M.
Bibcode: 2015ApJ...808L..15S
Altcode: 2015arXiv150700078S
Hot channels (HCs), high-temperature erupting structures in the lower
corona of the Sun, have been proposed as a proxy of magnetic flux
ropes (MFRs) since their initial discovery. However, it is difficult
to provide definitive proof given the fact that there is no direct
measurement of the magnetic field in the corona. An alternative method
is to use the magnetic field measurement in the solar wind from in
situ instruments. On 2012 July 12, an HC was observed prior to and
during a coronal mass ejection (CME) by the Atmospheric Imaging
Assembly high-temperature images. The HC is invisible in the EUVI
low-temperature images, which only show the cooler leading front
(LF). However, both the LF and an ejecta can be observed in the
coronagraphic images. These are consistent with the high temperature
and high density of the HC and support that the ejecta is the erupted
HC. Meanwhile, the associated CME shock was identified ahead of the
ejecta and the sheath through the COR2 images, and the corresponding
ICME was detected by the Advanced Composition Explorer, showing the
shock, sheath, and magnetic cloud (MC) sequentially, which agrees
with the coronagraphic observations. Further, the MC average Fe charge
state is elevated, containing a relatively low-ionization-state center
and a high-ionization-state shell, consistent with the preexisting
HC observation and its growth through magnetic reconnection. All of
these observations support that the MC detected near the Earth is the
counterpart of the erupted HC in the corona for this event. The study
provides strong observational evidence of the HC as an MFR.
Title: Coronal Pseudo-Streamer and Bipolar Streamer Observed by
SOHO/UVCS in March 2008
Authors: Abbo, L.; Lionello, R.; Riley, P.; Wang, Y. -M.
Bibcode: 2015SoPh..290.2043A
Altcode: 2015SoPh..tmp...90A; 2015arXiv150505649A
The past solar minimum is characterized by several peculiar aspects
and by a complex magnetic topology with two different types of coronal
streamers: pseudo-streamers and bipolar streamers. Pseudo-streamers
or unipolar streamer are coronal structures that separate coronal
holes of the same polarity, without a current sheet in the outer
corona; unlike bipolar streamers, which separate coronal holes of
opposite magnetic polarity. In this study, two examples of these
structures have been identified in the period of Carrington rotation
2067 by applying a potential-field source-surface extrapolation
of the photospheric field measurements. We present a spectroscopic
analysis of a pseudo-streamer and a bipolar streamer observed in the
period 12 - 17 March 2008 at high spectral and spatial resolution by
the Ultraviolet Coronagraph Spectrometer (UVCS; Kohl et al., Solar
Phys.162, 313, 1995) onboard the Solar and Heliospheric Observatory
(SOHO). The solar wind plasma parameters, such as kinetic temperature,
electron density, and outflow velocity, were inferred in the extended
corona (from 1.7 to 2.1 R⊙) by analyzing the O VI doublet
and H I Ly α line spectra. The coronal magnetic topology was taken
into account and was extrapolated with a 3D magneto-hydrodynamic
model of the global corona. The results of the analysis show some
peculiarities of the pseudo-streamer physical parameters in comparison
with those obtained for bipolar streamers: in particular, we have
found a higher kinetic temperature and higher outflow velocities of
O VI ions and lower electron density values. In conclusion, we point
out that pseudo-streamers produce a hybrid type of outflow that is
intermediate between the slow and fast solar winds. These outflows are
a possible source of slow/fast wind in a non-dipolar solar magnetic
field configuration.
Title: First Taste of Hot Channel in Interplanetary Space
Authors: Song, H. Q.; Zhang, J.; Chen, Y.; Cheng, X.; Li, G.; Wang,
Y. M.
Bibcode: 2015ApJ...803...96S
Altcode: 2015arXiv150204408S
A hot channel (HC) is a high temperature (∼10 MK) structure in the
inner corona first revealed by the Atmospheric Imaging Assembly on board
the Solar Dynamics Observatory. Eruptions of HCs are often associated
with flares and coronal mass ejections (CMEs). Results of previous
studies have suggested that an HC is a good proxy for a magnetic
flux rope (MFR) in the inner corona as well as another well known MFR
candidate, the prominence-cavity structure, which has a normal coronal
temperature (∼1-2 MK). In this paper, we report a high temperature
structure (HTS, ∼1.5 MK) contained in an interplanetary CME induced
by an HC eruption. According to the observations of bidirectional
electrons, high temperature and density, strong magnetic field, and
its association with the shock, sheath, and plasma pile-up region,
we suggest that the HTS is the interplanetary counterpart of the
HC. The scale of the measured HTS is around 14 R ⊙ , and
it maintained a much higher temperature than the background solar wind
even at 1 AU. It is significantly different from the typical magnetic
clouds, which usually have a much lower temperature. Our study suggests
that the existence of a corotating interaction region ahead of the HC
formed a magnetic container to inhibit expansion of the HC and cool
it down to a low temperature.
Title: Pseudostreamers as the Source of a Separate Class of Solar
Coronal Mass Ejections
Authors: Wang, Y. -M.
Bibcode: 2015ApJ...803L..12W
Altcode:
Using white-light and extreme-ultraviolet imaging observations,
we confirm that pseudostreamers (streamers that separate coronal
holes of the same polarity) give rise to a different type of coronal
mass ejection (CME) from that associated with helmet streamers
(defined as separating coronal holes of opposite polarity). Whereas
helmet streamers are the source of the familiar bubble-shaped CMEs
characterized by gradual acceleration and a three-part structure,
pseudostreamers produce narrower, fanlike ejections with roughly
constant speeds. These ejections, which are typically triggered by
underlying filament eruptions or small, flaring active regions, are
confined laterally and channeled outward by the like-polarity open flux
that converges onto the pseudostreamer plasma sheet from both sides. In
contrast, helmet streamer CMEs are centered on the relatively weak field
around the heliospheric current sheet and thus undergo greater lateral
expansion. Pseudostreamer ejections have a morphological resemblance
to white-light jets from coronal holes; however, unlike the latter,
they are not primarily driven by interchange reconnection, and tend
to have larger widths (∼20°-30°), lower speeds (∼250-700 km
s-1), and more complex internal structure.
Title: Active-region Tilt Angles: Magnetic versus White-light
Determinations of Joy's Law
Authors: Wang, Y. -M.; Colaninno, R. C.; Baranyi, T.; Li, J.
Bibcode: 2015ApJ...798...50W
Altcode: 2014arXiv1412.2329W
The axes of solar active regions are inclined relative to the
east-west direction, with the tilt angle tending to increase with
latitude ("Joy's law"). Observational determinations of Joy's law
have been based either on white-light images of sunspot groups or
on magnetograms, where the latter have the advantage of measuring
directly the physically relevant quantity (the photospheric field),
but the disadvantage of having been recorded routinely only since
the mid-1960s. White-light studies employing the historical Mount
Wilson (MW) database have yielded tilt angles that are smaller and
that increase less steeply with latitude than those obtained from
magnetic data. We confirm this effect by comparing sunspot-group
tilt angles from the Debrecen Photoheliographic Database with
measurements made by Li and Ulrich using MW magnetograms taken during
cycles 21-23. Whether white-light or magnetic data are employed, the
median tilt angles significantly exceed the mean values, and provide a
better characterization of the observed distributions. The discrepancy
between the white-light and magnetic results is found to have two main
sources. First, a substantial fraction of the white-light "tilt angles"
refer to sunspots of the same polarity. Of greater physical significance
is that the magnetograph measurements include the contribution of
plage areas, which are invisible in white-light images but tend to
have greater axial inclinations than the adjacent sunspots. Given the
large uncertainties inherent in both the white-light and the magnetic
measurements, it remains unclear whether any systematic relationship
exists between tilt angle and cycle amplitude during cycles 16-23.
Title: Solar Cycle Variation of the Sun's Low-Order Magnetic
Multipoles: Heliospheric Consequences
Authors: Wang, Y. -M.
Bibcode: 2015sac..book..387W
Altcode:
No abstract at ADS
Title: Solar Cycle Variation of the Sun's Low-Order Magnetic
Multipoles: Heliospheric Consequences
Authors: Wang, Y. -M.
Bibcode: 2014SSRv..186..387W
Altcode: 2014SSRv..tmp...17W
The Sun's dipole and quadrupole components play a central role in the
solar cycle evolution of the interplanetary magnetic field (IMF). The
long-term variation of the radial IMF component approximately tracks
that of the total dipole moment, with additional contributions coming
near sunspot maximum from the quadrupole moment and from CMEs. The axial
and equatorial components of the dipole vary out of phase with each
other over the solar cycle. The equatorial dipole, whose photospheric
sources are subject to rotational shearing, decays on a timescale of
∼1 yr and must be continually regenerated by new sunspot activity; its
fluctuating strength depends not only on the activity level, but also
on the longitudinal phase relationships among the active regions. During
cycles 21-23, the equatorial dipole and IMF reached their peak strength
∼2 yrs after sunspot maximum; conversely, large dips or "Gnevyshev
gaps" occurred when active regions emerged longitudinally out of phase
with each other. The 10Be-inferred phase shift in the IMF
variation during the Maunder Minimum may be explained by a decrease in
the amplitude of the equatorial dipole relative to the axial dipole,
due either to a systematic weakening of the emerging bipoles or to
an increase in their tilt angles. In mid-2012, during the polarity
reversal of cycle 24, the nonaxisymmetric quadrupole component became
so dominant that the heliospheric current sheet (HCS) split into two
cylindrical components. Hemispheric asymmetries in sunspot activity
give rise to an axisymmetric quadrupole component, which has combined
with the axial dipole to produce a systematic southward displacement
of the HCS since cycle 20.
Title: Coronal Inflows during the Interval 1996-2014
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.
Bibcode: 2014ApJ...797...10S
Altcode:
We extend our previous counts of coronal inflows from the 5 yr interval
1996-2001 to the 18 yr interval 1996-2014. By comparing stackplots
of these counts with similar stackplots of the source-surface
magnetic field and its longitudinal gradient, we find that the
inflows occur in long-lived streams with counting rates in excess of
18 inflows per day at sector boundaries where the gradient exceeds
0.22 G rad-1. These streams are responsible for the high
(86%) correlation between the inflow rate and the longitudinal
field gradient. The overall inflow rate was several times larger in
sunspot cycle 23 than it has been so far in cycle 24, reflecting the
relatively weak source-surface fields during this cycle. By comparison,
in cycles 21-22, the source-surface field and its gradient had bursts
of great strength, as if large numbers of inflows occurred during
those cycles. We find no obvious relation between inflows and coronal
mass ejections (CMEs) on timescales of days to weeks, regardless of the
speeds of the CMEs, and only a 60% correlation on timescales of months,
provided the CMEs are fast (V > 600 km s-1). We conclude
that most of the flux carried out by CMEs is returned to the Sun via
field line reconnection well below the 2.0 R ⊙ inner limit
of the LASCO field of view, and that the remainder accumulates in the
outer corona for an eventual return at sector boundaries.
Title: An Unusual Heliospheric Plasma Sheet Crossing at 1 AU
Authors: Wu, C. C.; Liou, K.; Vourlidas, A.; Lepping, R. P.; Wang,
Y. M.; Plunkett, S. P.; Socker, D. G.; Wu, S. T.
Bibcode: 2014AGUFMSH43A4166W
Altcode:
At 11:46UT on September 9, 2011, the Wind spacecraft encountered
an interplanetary (IP) fast forward shock. The shock was followed
almost immediately (~5 minutes) by a short duration (~35 minutes),
extremely large density pulse with a density peak of ~100 cm-3. While
a sharp increase in the solar wind density is typical of an IP shock
downstream, the unusual large density increase prompts a further
investigation. After a close examination of other in situ data from
Wind, we find the density pulse was associated with (1) a spike in
the plasma beta (ratio of thermal to magnetic pressure), (2) multiple
sign changes in the azimuthal angle of magnetic field, (3) depressed
magnetic field, (4) a small radial component of magnetic field, and (5)
a large (>90 degrees) pitch-angle change in suprathermal electrons
(>200 eV) across the density pulse. We conclude that the density
pulse is the heliospheric plasma sheet and the estimated thickness is
~820,000km. The unusually large density pulse is likely to be a result
of the shock compression from behind. This view is supported by our 3D
magnetohydrodynamic simulation. The detailed result and implications
will be discussed. *This work is supported partially by ONR 6.1 program
Title: Formation of a Double-decker Magnetic Flux Rope in the
Sigmoidal Solar Active Region 11520
Authors: Cheng, X.; Ding, M. D.; Zhang, J.; Sun, X. D.; Guo, Y.;
Wang, Y. M.; Kliem, B.; Deng, Y. Y.
Bibcode: 2014ApJ...789...93C
Altcode: 2014arXiv1405.4923C
In this paper, we address the formation of a magnetic flux rope
(MFR) that erupted on 2012 July 12 and caused a strong geomagnetic
storm event on July 15. Through analyzing the long-term evolution
of the associated active region observed by the Atmospheric Imaging
Assembly and the Helioseismic and Magnetic Imager on board the Solar
Dynamics Observatory, it is found that the twisted field of an MFR,
indicated by a continuous S-shaped sigmoid, is built up from two groups
of sheared arcades near the main polarity inversion line a half day
before the eruption. The temperature within the twisted field and
sheared arcades is higher than that of the ambient volume, suggesting
that magnetic reconnection most likely works there. The driver behind
the reconnection is attributed to shearing and converging motions
at magnetic footpoints with velocities in the range of 0.1-0.6 km
s-1. The rotation of the preceding sunspot also contributes
to the MFR buildup. Extrapolated three-dimensional non-linear force-free
field structures further reveal the locations of the reconnection to
be in a bald-patch region and in a hyperbolic flux tube. About 2 hr
before the eruption, indications of a second MFR in the form of an
S-shaped hot channel are seen. It lies above the original MFR that
continuously exists and includes a filament. The whole structure
thus makes up a stable double-decker MFR system for hours prior to
the eruption. Eventually, after entering the domain of instability,
the high-lying MFR impulsively erupts to generate a fast coronal mass
ejection and X-class flare; while the low-lying MFR remains behind
and continuously maintains the sigmoidicity of the active region.
Title: Is Solar Cycle 24 Producing More Coronal Mass Ejections Than
Cycle 23?
Authors: Wang, Y. -M.; Colaninno, R.
Bibcode: 2014ApJ...784L..27W
Altcode:
Although sunspot numbers are roughly a factor of two lower in the
current cycle than in cycle 23, the rate of coronal mass ejections
(CMEs) appears to be at least as high in 2011-2013 as during the
corresponding phase of the previous cycle, according to three catalogs
that list events observed with the Large Angle and Spectrometric
Coronagraph (LASCO). However, the number of CMEs detected is sensitive
to such factors as the image cadence and the tendency (especially by
human observers) to under-/overcount small or faint ejections during
periods of high/low activity. In contrast to the total number, the
total mass of CMEs is determined mainly by larger events. Using the
mass measurements of 11,000 CMEs given in the manual CDAW catalog,
we find that the mass loss rate remains well correlated with the
sunspot number during cycle 24. In the case of the automated CACTus
and SEEDS catalogs, the large increase in the number of CMEs during
cycle 24 is almost certainly an artifact caused by the near-doubling
of the LASCO image cadence after mid-2010. We confirm that fast CMEs
undergo a much stronger solar-cycle variation than slow ones, and
that the relative frequency of slow and less massive CMEs increases
with decreasing sunspot number. We conclude that cycle 24 is not only
producing fewer CMEs than cycle 23, but that these ejections also tend
to be slower and less massive than those observed one cycle earlier.
Title: Temperature Evolution of a Magnetic Flux Rope in a Failed
Solar Eruption
Authors: Song, H. Q.; Zhang, J.; Cheng, X.; Chen, Y.; Liu, R.; Wang,
Y. M.; Li, B.
Bibcode: 2014ApJ...784...48S
Altcode: 2014arXiv1402.1602S
In this paper, we report for the first time the detailed temperature
evolution process of the magnetic flux rope in a failed solar
eruption. Occurring on 2013 January 05, the flux rope was impulsively
accelerated to a speed of ~400 km s-1 in the first minute,
then decelerated and came to a complete stop in two minutes. The
failed eruption resulted in a large-size high-lying (~100 Mm above the
surface), high-temperature "fire ball" sitting in the corona for more
than two hours. The time evolution of the thermal structure of the flux
rope was revealed through the differential emission measure analysis
technique, which produced temperature maps using observations of the
Atmospheric Imaging Assembly on board the Solar Dynamic Observatory. The
average temperature of the flux rope steadily increased from ~5 MK to
~10 MK during the first nine minutes of the evolution, which was much
longer than the rise time (about three minutes) of the associated soft
X-ray flare. We suggest that the flux rope is heated by the energy
release of the continuing magnetic reconnection, different from the
heating of the low-lying flare loops, which is mainly produced by
the chromospheric plasma evaporation. The loop arcade overlying the
flux rope was pushed up by ~10 Mm during the attempted eruption. The
pattern of the velocity variation of the loop arcade strongly suggests
that the failure of the eruption was caused by the strapping effect
of the overlying loop arcade.
Title: Evidence for Two Separate Heliospheric Current Sheets of
Cylindrical Shape During Mid-2012
Authors: Wang, Y. -M.; Young, P. R.; Muglach, K.
Bibcode: 2014ApJ...780..103W
Altcode:
During the reversal of the Sun's polar fields at sunspot maximum,
outward extrapolations of magnetograph measurements often predict the
presence of two or more current sheets extending into the interplanetary
medium, instead of the single heliospheric current sheet (HCS) that
forms the basis of the standard "ballerina skirt" picture. By comparing
potential-field source-surface models of the coronal streamer belt
with white-light coronagraph observations, we deduce that the HCS
was split into two distinct structures with circular cross sections
during mid-2012. These cylindrical current sheets were centered near
the heliographic equator and separated in longitude by roughly 180° a
corresponding four-sector polarity pattern was observed at Earth. Each
cylinder enclosed a negative-polarity coronal hole that was identifiable
in extreme ultraviolet images and gave rise to a high-speed stream. The
two current sheet systems are shown to be a result of the dominance
of the Sun's nonaxisymmetric quadrupole component, as the axial dipole
field was undergoing its reversal during solar cycle 24.
Title: On the Strength of the Hemispheric Rule and the Origin of
Active-region Helicity
Authors: Wang, Y. -M.
Bibcode: 2013ApJ...775L..46W
Altcode:
Vector magnetograph and morphological observations have shown that
the solar magnetic field tends to have negative (positive) helicity in
the northern (southern) hemisphere, although only ~60%-70% of active
regions appear to obey this "hemispheric rule." In contrast, at least
~80% of quiescent filaments and filament channels that form during the
decay of active regions follow the rule. We attribute this discrepancy
to the difficulty in determining the helicity sign of newly emerged
active regions, which are dominated by their current-free component;
as the transverse field is canceled at the polarity inversion lines,
however, the axial component becomes dominant there, allowing a more
reliable determination of the original active-region chirality. We
thus deduce that the hemispheric rule is far stronger than generally
assumed, and cannot be explained by stochastic processes. Earlier
studies have shown that the twist associated with the axial tilt of
active regions is too small to account for the observed helicity;
here, both tilt and twist are induced by the Coriolis force acting on
the diverging flow in the emerging flux tube. However, in addition to
this east-west expansion about the apex of the loop, each of its legs
must expand continually in cross section during its rise through the
convection zone, thereby acquiring a further twist through the Coriolis
force. Since this transverse pressure effect is not limited by drag
or tension forces, the final twist depends mainly on the rise time,
and may be large enough to explain the observed active-region helicity.
Title: Fe XII Stalks and the Origin of the Axial Field in Filament
Channels
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Stenborg, G.
Bibcode: 2013ApJ...770...72W
Altcode:
Employing Fe XII images and line-of-sight magnetograms, we deduce
the direction of the axial field in high-latitude filament channels
from the orientation of the adjacent stalklike structures. Throughout
the rising phase of the current solar cycle 24, filament channels
poleward of latitude 30° overwhelmingly obeyed the hemispheric
chirality rule, being dextral (sinistral) in the northern (southern)
hemisphere, corresponding to negative (positive) helicity. During
the deep minimum of 2007-2009, the orientation of the Fe XII stalks
was often difficult to determine, but no obvious violations of the
rule were found. Although the hemispheric trend was still present
during the maximum and early declining phase of cycle 23 (2000-2003),
several high-latitude exceptions were identified at that time. From
the observation that dextral (sinistral) filament channels form
through the decay of active regions whose Fe XII features show
a counterclockwise (clockwise) whorl, we conclude that the axial
field direction is determined by the intrinsic helicity of the active
regions. In contrast, generation of the axial field component by the
photospheric differential rotation is difficult to reconcile with the
observed chirality of polar crown and circular filament channels, and
with the presence of filament channels along the equator. The main role
of differential rotation in filament channel formation is to expedite
the cancellation of flux and thus the removal of the transverse field
component. We propose further that, rather than being ejected into
the heliosphere, the axial field is eventually resubmerged by flux
cancellation as the adjacent unipolar regions become increasingly mixed.
Title: Origins of Suprathermal Seed Particles in Gradual Solar
Energetic Particle Events
Authors: Tylka, A. J.; Ko, Y.; Ng, C. K.; Wang, Y. M.; Dietrich, W. F.
Bibcode: 2013AGUSMSH51C..02T
Altcode:
Gradual solar energetic particle (SEP) events are those in which ions
are accelerated to their observed energies by interactions with a
shock driven by a fast coronal mass-ejection (CME). Previous studies
have shown that much of the observed event-to-event variability can be
understood in terms of shock speed and evolution in the shock-normal
angle. But an equally important factor, particularly for the elemental
composition, is the origin of the suprathermal seed particles upon
which the shock acts. To tackle this issue, we (1) use observed
solar-wind speed, photospheric magnetograms, and the PFSS model to map
the Sun-L1 interplanetary magnetic field (IMF) lines back to their
source region on the Sun at the time of the SEP observations; and
(2) then look for correlation between SEP composition (as measured
by Wind and ACE at ~2-30 MeV/nucleon) and characteristics of the
identified IMF-source regions. The study is based on 24 SEP events,
identified as a statistically-significant increase in ~20 MeV protons
and occurring in 1998 and 2003-2006, when the rate of CMEs was lower
than in solar-maximum years and the field-line tracing is therefore
more likely to be successful. In all cases, we are dealing with events
in which the Fe/O enhancement is well-below the highly-enhanced values
(~1) associated with "impulsive" SEP events, in which ions are believed
to have attained their observed energies through magnetic reconnection,
such as that which occurs in flares. We find that the gradual SEP
Fe/O is correlated with the magnetic field near the IMF-source,
with the largest enhancements occurring when the field is strong,
due to the nearby presence of an active region. In these cases,
other elemental ratios show a strong charge-to-mass (Q/M) ordering,
at least on average, similar to that found in impulsive events. These
results lead us to suggest that reconnection processes at footpoints
near active regions bias the heavy-ion composition of suprathermal
seed ions by processes qualitatively similar to those that produce
even larger heavy-ion enhancements in impulsive SEP events. To address
potential technical concerns about our analysis, we review efforts to
exclude impulsive SEP events from our event sample. We also discuss
the implications of our results for using coronal field models to
determine the source of the interplanetary magnetic field.
Title: Transient Brightenings Associated with Flux Cancellation
along a Filament Channel
Authors: Wang, Y. -M.; Muglach, K.
Bibcode: 2013ApJ...763...97W
Altcode:
Filament channels coincide with large-scale polarity inversion
lines of the photospheric magnetic field, where flux cancellation
continually takes place. High-cadence Solar Dynamics Observatory
(SDO) images recorded in He II 30.4 nm and Fe IX 17.1 nm during 2010
August 22 reveal numerous transient brightenings occurring along
the edge of a filament channel within a decaying active region,
where SDO line-of-sight magnetograms show strong opposite-polarity
flux in close contact. The brightenings are elongated along the
direction of the filament channel, with linear extents of several
arcseconds, and typically last a few minutes; they sometimes have
the form of multiple two-sided ejections with speeds on the order of
100 km s-1. Remarkably, some of the brightenings rapidly
develop into larger scale events, forming sheetlike structures that are
eventually torn apart by the diverging flows in the filament channel
and ejected in opposite directions. We interpret the brightenings as
resulting from reconnections among filament-channel field lines having
one footpoint located in the region of canceling flux. In some cases,
the flow patterns that develop in the channel may bring successive
horizontal loops together and cause a cascade to larger scales.
Title: The Solar Wind and Interplanetary Field during Very Low
Amplitude Sunspot Cycles
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 2013ApJ...764...90W
Altcode:
Cosmogenic isotope records indicate that a solar-cycle modulation
persists through extended periods of very low sunspot activity. One
immediate implication is that the photospheric field during such grand
minima did not consist entirely of ephemeral regions, which produce
a negligible amount of open magnetic flux, but continued to have a
large-scale component originating from active regions. Present-day
solar and heliospheric observations show that the solar wind mass
flux and proton density at the coronal base scale almost linearly
with the footpoint field strength, whereas the wind speed at Earth is
uncorrelated with the latter. Thus a factor of ~4-7 reduction in the
total open flux, as deduced from reconstructions of the interplanetary
magnetic field (IMF) during the Maunder Minimum, would lead to a
similar decrease in the solar wind densities, while leaving the
wind speeds largely unchanged. We also demonstrate that a decrease
in the strengths of the largest active regions during grand minima
will reduce the amplitude of the Sun's equatorial dipole relative to
the axial component, causing the IMF strength to peak near sunspot
minimum rather than near sunspot maximum, a result that is consistent
with the phase shift observed in the 10Be record during the
Maunder Minimum. Finally, we discuss the origin of the 5 yr periodicity
sometimes present in the cosmogenic isotope data during low and medium
amplitude cycles.
Title: Semiempirical Models of the Slow and Fast Solar Wind
Authors: Wang, Y. -M.
Bibcode: 2013mspc.book..123W
Altcode:
No abstract at ADS
Title: Semiempirical Models of the Slow and Fast Solar Wind
Authors: Wang, Y. -M.
Bibcode: 2012SSRv..172..123W
Altcode: 2011SSRv..tmp....3W; 2011SSRv..tmp..337W; 2011SSRv..tmp...93W;
2011SSRv..tmp..169W
Coronal holes can produce several types of solar wind with a variety
of compositional properties, depending on the location and strength of
the heating along their open magnetic field lines. High-speed wind is
associated with (relatively) slowly diverging flux tubes rooted in
the interiors of large holes with weak, uniform footpoint fields;
heating is spread over a large radial distance, so that most of
the energy is conducted outward and goes into accelerating the wind
rather than increasing the mass flux. In the rapidly diverging open
fields present at coronal hole boundaries and around active regions,
the heating is concentrated at low heights and the temperature maximum
is located near the coronal base, resulting in high oxygen freezing-in
temperatures and low asymptotic wind speeds. Polar plumes have a strong
additional source of heating at their bases, which generates a large
downward conductive flux, raising the densities and enhancing the
radiative losses. The relative constancy of the solar wind mass flux
at Earth reflects the tendency for the heating rate in coronal holes
to increase monotonically with the footpoint field strength, with very
high mass fluxes at the Sun offsetting the enormous flux-tube expansion
in active region holes. Although coronal holes are its main source,
slow wind is also released continually from helmet streamer loops by
reconnection processes, giving rise to plasma blobs (small flux ropes)
and the heliospheric plasma sheet.
Title: Determining the North-South Displacement of the Heliospheric
Current Sheet from Coronal Streamer Observations
Authors: Robbrecht, E.; Wang, Y. -M.
Bibcode: 2012ApJ...755..135R
Altcode:
Inferences based on interplanetary field measurements have suggested
a statistical tendency for the heliospheric current sheet (HCS)
to be displaced southward of the heliographic equator during the
past four solar cycles. Here, we use synoptic maps of white-light
streamer structures to determine more directly the longitudinally
averaged latitude of the HCS, after separating out the contribution of
streamers without magnetic polarity reversals ("pseudostreamers"). We
find a strong tendency for the HCS to be shifted southward by a few
degrees during 2007-2011, but no significant shift during the 1996-1997
sunspot minimum. Fluctuations in the magnitude and direction of the
north-south shifts often occur on timescales as short as one or two
Carrington rotations, as a result of changes in the streamer structures
due to active region emergence. The largest shifts occurred during
2010-2011 and were on the order of -6°. Such southward displacements
are consistent with the overwhelming dominance of northern-hemisphere
sunspot activity during the rising phase of the current solar cycle 24,
resulting in a strong axisymmetric quadrupole component whose sign at
the equator matched that of the north polar field; the symmetry-breaking
effect of the quadrupole was further enhanced by the weakness of the
polar fields.
Title: On the Nature of the Solar Wind from Coronal Pseudostreamers
Authors: Wang, Y. -M.; Grappin, R.; Robbrecht, E.; Sheeley, N. R., Jr.
Bibcode: 2012ApJ...749..182W
Altcode:
Coronal pseudostreamers, which separate like-polarity coronal holes, do
not have current sheet extensions, unlike the familiar helmet streamers
that separate opposite-polarity holes. Both types of streamers taper
into narrow plasma sheets that are maintained by continual interchange
reconnection with the adjacent open magnetic field lines. White-light
observations show that pseudostreamers do not emit plasma blobs; this
important difference from helmet streamers is due to the convergence
of like-polarity field lines above the X-point, which prevents the
underlying loops from expanding outward and pinching off. The main
component of the pseudostreamer wind has the form of steady outflow
along the open field lines rooted just inside the boundaries of the
adjacent coronal holes. These flux tubes are characterized by very
rapid expansion below the X-point, followed by reconvergence at greater
heights. Analysis of an idealized pseudostreamer configuration shows
that, as the separation between the underlying holes increases, the
X-point rises and the expansion factor f ss at the source
surface increases. In situ observations of pseudostreamer crossings
indicate wind speeds v ranging from ~350 to ~550 km s-1,
with O7 +/O6 + ratios that are enhanced compared
with those in high-speed streams but substantially lower than in the
slow solar wind. Hydrodynamic energy-balance models show that the
empirical v-f ss relation overestimates the wind speeds
from nonmonotonically expanding flux tubes, particularly when the
X-point is located at low heights and f ss is small. We
conclude that pseudostreamers produce a "hybrid" type of outflow that
is intermediate between classical slow and fast solar wind.
Title: Evidence for Two Separate but Interlaced Components of the
Chromospheric Magnetic Field
Authors: Reardon, K. P.; Wang, Y. -M.; Muglach, K.; Warren, H. P.
Bibcode: 2011ApJ...742..119R
Altcode:
Chromospheric fibrils are generally thought to trace out low-lying,
mainly horizontal magnetic fields that fan out from flux concentrations
in the photosphere. A high-resolution (~0farcs1 pixel-1)
image, taken in the core of the Ca II 854.2 nm line and covering
an unusually large area, shows the dark fibrils within an active
region remnant as fine, looplike features that are aligned parallel
to each other and have lengths comparable to a supergranular
diameter. Comparison with simultaneous line-of-sight magnetograms
confirms that the fibrils are centered above intranetwork areas
(supergranular cell interiors), with one end rooted just inside the
neighboring plage or strong unipolar network but the other endpoint
less clearly defined. Focusing on a particular arcade-like structure
lying entirely on one side of a filament channel (large-scale polarity
inversion), we find that the total amount of positive-polarity flux
underlying this "fibril arcade" is ~50 times greater than the total
amount of negative-polarity flux. Thus, if the fibrils represent closed
loops, they must consist of very weak fields (in terms of total magnetic
flux), which are interpenetrated by a more vertical field that contains
most of the flux. This surprising result suggests that the fibrils in
unipolar regions connect the network to the nearby intranetwork flux,
while the bulk of the network flux links to remote regions of the
opposite polarity, forming a second, higher canopy above the fibril
canopy. The chromospheric field near the edge of the network thus has
an interlaced structure resembling that in sunspot penumbrae.
Title: Asymmetric Sunspot Activity and the Southward Displacement
of the Heliospheric Current Sheet
Authors: Wang, Y. -M.; Robbrecht, E.
Bibcode: 2011ApJ...736..136W
Altcode:
Observations of the interplanetary magnetic field (IMF) have suggested
a statistical tendency for the heliospheric current sheet (HCS)
to be shifted a few degrees southward of the heliographic equator
during the period 1965-2010, particularly in the years near sunspot
minimum. Using potential-field source-surface extrapolations and
photospheric flux-transport simulations, we demonstrate that this
southward displacement follows from Joy's law and the observed
hemispheric asymmetry in the sunspot numbers, with activity being
stronger in the southern (northern) hemisphere during the declining
(rising) phase of cycles 20-23. The hemispheric asymmetry gives rise
to an axisymmetric quadrupole field, whose equatorial zone has the
sign of the leading-polarity flux in the dominant hemisphere; during
the last four cycles, the polarity of the IMF around the equator thus
tended to match that of the north polar field both before and after
polar field reversal. However, large fluctuations are introduced by
the nonaxisymmetric field components, which depend on the longitudinal
distribution of sunspot activity in either hemisphere. Consistent
with this model, the HCS showed an average northward displacement
during cycle 19, when the "usual" alternation was reversed and
the northern hemisphere became far more active than the southern
hemisphere during the declining phase of the cycle. We propose a new
method for determining the north-south displacement of the HCS from
coronal streamer observations.
Title: Morphology, dynamics and plasma parameters of plumes and
inter-plume regions in solar coronal holes
Authors: Wilhelm, K.; Abbo, L.; Auchère, F.; Barbey, N.; Feng, L.;
Gabriel, A. H.; Giordano, S.; Imada, S.; Llebaria, A.; Matthaeus,
W. H.; Poletto, G.; Raouafi, N. -E.; Suess, S. T.; Teriaca, L.; Wang,
Y. -M.
Bibcode: 2011A&ARv..19...35W
Altcode: 2011arXiv1103.4481W
Coronal plumes, which extend from solar coronal holes (CH) into the high
corona and—possibly—into the solar wind (SW), can now continuously
be studied with modern telescopes and spectrometers on spacecraft,
in addition to investigations from the ground, in particular, during
total eclipses. Despite the large amount of data available on these
prominent features and related phenomena, many questions remained
unanswered as to their generation and relative contributions to
the high-speed streams emanating from CHs. An understanding of the
processes of plume formation and evolution requires a better knowledge
of the physical conditions at the base of CHs, in plumes and in the
surrounding inter-plume regions. More specifically, information is
needed on the magnetic field configuration, the electron densities
and temperatures, effective ion temperatures, non-thermal motions,
plume cross sections relative to the size of a CH, the plasma bulk
speeds, as well as any plume signatures in the SW. In spring 2007, the
authors proposed a study on `Structure and dynamics of coronal plumes
and inter-plume regions in solar coronal holes' to the International
Space Science Institute (ISSI) in Bern to clarify some of these aspects
by considering relevant observations and the extensive literature. This
review summarizes the results and conclusions of the study. Stereoscopic
observations allowed us to include three-dimensional reconstructions
of plumes. Multi-instrument investigations carried out during several
campaigns led to progress in some areas, such as plasma densities,
temperatures, plume structure and the relation to other solar phenomena,
but not all questions could be answered concerning the details of
plume generation process(es) and interaction with the SW.
Title: The Evolution of Dark Canopies Around Active Regions
Authors: Wang, Y. -M.; Robbrecht, E.; Muglach, K.
Bibcode: 2011ApJ...733...20W
Altcode: 2011arXiv1103.4373W
As observed in spectral lines originating from the chromosphere,
transition region, and low corona, active regions are surrounded by an
extensive "circumfacular" area which is darker than the quiet Sun. We
examine the properties of these dark moat- or canopy-like areas using Fe
IX 17.1 nm images and line-of-sight magnetograms from the Solar Dynamics
Observatory. The 17.1 nm canopies consist of fibrils (horizontal fields
containing extreme-ultraviolet-absorbing chromospheric material)
clumped into featherlike structures. The dark fibrils initially
form a quasiradial or vortical pattern as the low-lying field lines
fanning out from the emerging active region connect to surrounding
network and intranetwork elements of opposite polarity. The area
occupied by the 17.1 nm fibrils expands as supergranular convection
causes the active-region flux to spread into the background medium;
the outer boundary of the dark canopy stabilizes where the diffusing
flux encounters a unipolar region of opposite sign. The dark fibrils
tend to accumulate in regions of weak longitudinal field and to become
rooted in mixed-polarity flux. To explain the latter observation,
we note that the low-lying fibrils are more likely to interact with
small loops associated with weak, opposite-polarity flux elements
in close proximity, than with high loops anchored inside strong
unipolar network flux. As a result, the 17.1 nm fibrils gradually
become concentrated around the large-scale polarity inversion lines
(PILs), where most of the mixed-polarity flux is located. Systematic
flux cancellation, assisted by rotational shearing, removes the field
component transverse to the PIL and causes the fibrils to coalesce
into long PIL-aligned filaments.
Title: The Evolution of Dark Canopies Around Active Regions
Authors: Muglach, Karin; Wang, Y. M.; Robbrecht, E.
Bibcode: 2011SPD....42.1718M
Altcode: 2011BAAS..43S.1718M
As observed in spectral lines originating from the chromosphere,
transition region, and low corona, active regions are surrounded
by an extensive 'circumfacular' area which is darker than the quiet
Sun. We examine the properties of these dark moat- or canopy-like areas
using Fe IX 17.1 nm images and line-of-sight magnetograms from the
Solar Dynamics Observatory. The 17.1 nm canopies consist of fibrils
(horizontal fields containing EUV-absorbing chromospheric material)
clumped into featherlike structures. The dark fibrils initially
form a quasiradial or vortical pattern as the low-lying field lines
fanning out from the emerging active region connect to surrounding
network and intranetwork elements of the opposite polarity. The area
occupied by the 17.1 nm fibrils expands as supergranular convection
causes the active region flux to spread into the background medium;
the outer boundary of the dark canopy stabilizes where the diffusing
flux encounters a unipolar region of the opposite sign. The dark fibrils
tend to accumulate in regions of weak longitudinal field and to become
rooted in mixed-polarity flux. To explain the latter observation,
we note that the low-lying fibrils are more likely to interact with
small loops associated with weak, opposite-polarity flux elements
in close proximity, than with high loops anchored inside strong
unipolar network flux. As a result, the 17.1 nm fibrils gradually
become concentrated around the large-scale polarity inversion lines
(PILs), where most of the mixed-polarity flux is located. Systematic
flux cancellation, assisted by rotational shearing, removes the field
component transverse to the PIL and causes the fibrils to coalesce
into long PIL-aligned filaments.
Title: Two-temperature Models for Polar Plumes: Cooling by Means of
Strong Base Heating
Authors: Grappin, R.; Wang, Y. -M.; Pantellini, F.
Bibcode: 2011ApJ...727...30G
Altcode:
In earlier one-fluid hydrodynamical calculations incorporating heat
conduction and radiative losses, it was shown that the high densities
in polar plumes could be reproduced by including a concentrated heat
source near the plume base, in addition to the global heating required
in both the plume and interplume regions of the coronal hole. The
extra heating (attributed to interchange reconnection between the open
flux and an underlying magnetic bipole) results in lower flow speeds
and temperatures relative to the interplume gas, predictions that
have since been confirmed by spectroscopic measurements. Here, the
model is extended to the two-fluid case, in which ions and electrons
are allowed to have different temperatures, coupling is via Coulomb
collisions, and heat transport is mainly by electrons. Again, we find
that depositing energy very close to the coronal base, in either the
protons or electrons (or both), raises the densities and decreases the
flow speeds everywhere along the flux tube. The higher densities in turn
act to lower the ion temperatures by coupling the protons more closely
to the energy-losing electrons. In addition, we find that energy must
be deposited globally in both the electrons and the ions; without this
direct heating, the electrons would end up cooler in the interplume
region than in the plume, contrary to observations. Increasing the
rate of flux-tube expansion has the effect of lowering the electron
and ion temperatures and reducing the asymptotic flow speed, both
in the plume and the interplume region; the observed densities and
temperatures can be matched by taking the magnetic field to fall off
with radius roughly as r -4.
Title: Spinning Motions in Coronal Cavities
Authors: Wang, Y. -M.; Stenborg, G.
Bibcode: 2010ApJ...719L.181W
Altcode:
In movies made from Fe XII 19.5 nm images, coronal cavities that graze
or are detached from the solar limb appear as continually spinning
structures, with sky-plane projected flow speeds in the range 5-10
km s-1. These whirling motions often persist in the same
sense for up to several days and provide strong evidence that the
cavities and the immediately surrounding streamer material have the
form of helical flux ropes viewed along their axes. A pronounced bias
toward spin in the equatorward direction is observed during 2008. We
attribute this bias to the poleward concentration of the photospheric
magnetic flux near sunspot minimum, which leads to asymmetric heating
along large-scale coronal loops and tends to drive a flow from higher
to lower latitudes; this flow is converted into an equatorward spinning
motion when the loops pinch off to form a flux rope. As sunspot activity
increases and the polar fields weaken, we expect the preferred direction
of the spin to reverse.
Title: Observations of the magnetic field and plasma in the
heliosheath by Voyager 2 from 2007.7 to 2009.4
Authors: Burlaga, L. F.; Ness, N. F.; Wang, Y. -M.; Sheeley, N. R.;
Richardson, J. D.
Bibcode: 2010JGRA..115.8107B
Altcode: 2010JGRA..11508107B
The density and temperature profiles of the plasma measured by Voyager
2 (V2) behind the termination shock changed abruptly near 2008.6
from relatively large average values and large fluctuations during
2007.7 to 2008.6 (interval A) to relatively low average values and
very small-amplitude fluctuations during 2008.6 to 2009.4 (interval
B). This paper shows that the change in the magnetic field strength B(t)
was less abrupt than the plasma changes, and the fluctuations of the
magnetic field strength in interval B were of moderate amplitude, with
indications of a quasiperiodic structure in part of the interval. The
magnetic field was directed away from the sun (positive polarity)
∼ 78% ± 5% of the time in both interval A and interval B, changing
in an irregular way from positive to negative polarities throughout
the interval. The polarity distribution indicates that the minimum
latitudinal extent of the heliospheric current sheet (HCS) was
near V2 throughout the interval, consistent with the extrapolated
minimum latitudes of the HCS computed from solar magnetic field
observations. Thus, V2 was observing magnetic fields from the southern
polar coronal hole most of the time. The distribution of B was lognormal
in interval A and Gaussian interval B.
Title: On the "Extended" Solar Cycle in Coronal Emission
Authors: Robbrecht, E.; Wang, Y. -M.; Sheeley, N. R., Jr.; Rich, N. B.
Bibcode: 2010ApJ...716..693R
Altcode:
Butterfly diagrams (latitude-time plots) of coronal emission show a
zone of enhanced brightness that appears near the poles just after
solar maximum and migrates toward lower latitudes; a bifurcation seems
to occur at sunspot minimum, with one branch continuing to migrate
equatorward with the sunspots of the new cycle and the other branch
heading back to the poles. The resulting patterns have been likened to
those seen in torsional oscillations and have been taken as evidence
for an extended solar cycle lasting over ~17 yr. In order to clarify
the nature of the overlapping bands of coronal emission, we construct
butterfly diagrams from green-line simulations covering the period
1967-2009 and from 19.5 nm and 30.4 nm observations taken with the
Extreme-Ultraviolet Imaging Telescope during 1996-2009. As anticipated
from earlier studies, we find that the high-latitude enhancements mark
the footpoint areas of closed loops with one end rooted outside the
evolving boundaries of the polar coronal holes. The strong underlying
fields were built up over the declining phase of the cycle through
the poleward transport of active-region flux by the surface meridional
flow. Rather than being a precursor of the new-cycle sunspot activity
zone, the high-latitude emission forms a physically distinct, U-shaped
band that curves upward again as active-region fields emerge at
midlatitudes and reconnect with the receding polar-hole boundaries. We
conclude that the so-called extended cycle in coronal emission is a
manifestation not of early new-cycle activity, but of the poleward
concentration of old-cycle trailing-polarity flux by meridional flow.
Title: On the Relative Constancy of the Solar Wind Mass Flux at 1 AU
Authors: Wang, Y. -M.
Bibcode: 2010ApJ...715L.121W
Altcode:
Employing solar wind measurements from the Advanced Composition
Explorer and Ulysses, photospheric magnetic data, and conservation
laws along open field lines, we confirm that the energy and mass flux
densities at the Sun increase roughly linearly with the footpoint
field strength, B 0. This empirical result has a number of
important physical implications. First, it supports the assumption
that the magnetic field is the source of the heating in coronal
holes. Second, because B 0 may vary by over 2 orders of
magnitude, depending on how close the footpoint is located to active
regions, the heating rate in coronal holes varies over a very wide
range, with active-region holes being characterized by much stronger
heating and much larger mass fluxes at low heights than the large,
weak-field polar holes. Third, the variation of the mass flux density
at 1 AU remains very modest because the mass flux density at the Sun
and the net flux-tube expansion both increase almost linearly with B
0, so that the two effects offset each other.
Title: The Coronal and Heliospheric 2007 May 19 Event: Coronal
Mass Ejection, Extreme Ultraviolet Imager Wave, Radio Bursts, and
Energetic Electrons
Authors: Kerdraon, A.; Pick, M.; Hoang, S.; Wang, Y. -M.; Haggerty, D.
Bibcode: 2010ApJ...715..468K
Altcode:
We study the global development of the 2007 May 19 event and investigate
the origin and the escape of the energetic electrons responsible for the
interplanetary bursts and for the solar energetic particle event. The
data analysis combines radio spectral and imaging observations with
STEREO EUV observations. We also use the direction-finding capabilities
on the Wind/Waves radio instrument. Electron acceleration and injections
into the interplanetary medium occur with some delay after the flare. It
is shown that they are related to the expansion of the coronal mass
ejection and of the extreme ultraviolet imager wave. There are two
accelerations at two different locations in the corona which correspond
to two different electron trajectories in the interplanetary medium.
Title: Formation and Evolution of Coronal Holes Following the
Emergence of Active Regions
Authors: Wang, Y. -M.; Robbrecht, E.; Rouillard, A. P.; Sheeley,
N. R., Jr.; Thernisien, A. F. R.
Bibcode: 2010ApJ...715...39W
Altcode:
The low level of solar activity over the past four years has provided
unusually favorable conditions for tracking the formation and evolution
of individual coronal holes and their wind streams. Employing
extreme-ultraviolet images recorded with the Solar Terrestrial
Relations Observatory during 2007-2009, we analyze three cases
in which small coronal holes first appear at the edges of newly
emerged active regions and then expand via flux transport processes,
eventually becoming attached to the polar holes. The holes form
gradually over timescales comparable to or greater than that for
the active regions to emerge, without any obvious association with
coronal mass ejections. The evolving hole areas coincide approximately
with the footpoints of open field lines derived from potential-field
source-surface extrapolations of the photospheric field. One of these
coronal-hole systems, centered at the equator and maintained by a
succession of old-cycle active regions emerging in the same longitude
range, persists in one form or another for up to two years. The other
two holes, located at midlatitudes and originating from new-cycle
active regions, become strongly sheared and decay away after a few
rotations. The hole boundaries and the small active-region holes, both
of which are sources of slow wind, are observed to undergo continual
short-term (lsim1 day) fluctuations on spatial scales comparable to
that of the supergranulation. From in situ measurements, we identify
a number of plasma sheets associated with pseudostreamers separating
holes of the same polarity.
Title: On the Weakening of the Polar Magnetic Fields during Solar
Cycle 23
Authors: Wang, Y. -M.; Robbrecht, E.; Sheeley, N. R., Jr.
Bibcode: 2009ApJ...707.1372W
Altcode:
The Sun's polar fields are currently ~40% weaker than they were during
the previous three sunspot minima. This weakening has been accompanied
by a corresponding decrease in the interplanetary magnetic field (IMF)
strength, by a ~20% shrinkage in the polar coronal-hole areas, and by
a reduction in the solar-wind mass flux over the poles. It has also
been reflected in coronal streamer structure and the heliospheric
current sheet, which only showed the expected flattening into the
equatorial plane after sunspot numbers fell to unusually low values
in mid-2008. From latitude-time plots of the photospheric field,
it has long been apparent that the polar fields are formed through
the transport of trailing-polarity flux from the sunspot latitudes
to the poles. To address the question of why the polar fields are
now so weak, we simulate the evolution of the photospheric field and
radial IMF strength from 1965 to the present, employing a surface
transport model that includes the effects of active region emergence,
differential rotation, supergranular convection, and a poleward bulk
flow. We find that the observed evolution can be reproduced if the
amplitude of the surface meridional flow is varied by as little as 15%
(between 14.5 and 17 m s-1), with the higher average speeds
being required during the long cycles 20 and 23.
Title: What Is the Nature of EUV Waves? First STEREO 3D Observations
and Comparison with Theoretical Models
Authors: Patsourakos, S.; Vourlidas, A.; Wang, Y. M.; Stenborg, G.;
Thernisien, A.
Bibcode: 2009SoPh..259...49P
Altcode: 2009arXiv0905.2189P
One of the major discoveries of the Extreme ultraviolet Imaging
Telescope (EIT) on SOHO was the intensity enhancements propagating
over a large fraction of the solar surface. The physical origin(s)
of the so-called EIT waves is still strongly debated with either
wave (primarily fast-mode MHD waves) or nonwave (pseudo-wave)
interpretations. The difficulty in understanding the nature of EUV waves
lies in the limitations of the EIT observations that have been used
almost exclusively for their study. They suffer from low cadence and
single temperature and viewpoint coverage. These limitations are largely
overcome by the SECCHI/EUVI observations onboard the STEREO mission. The
EUVI telescopes provide high-cadence, simultaneous multitemperature
coverage and two well-separated viewpoints. We present here the first
detailed analysis of an EUV wave observed by the EUVI disk imagers on 7
December 2007 when the STEREO spacecraft separation was ≈ 45°. Both a
small flare and a coronal mass ejection (CME) were associated with the
wave. We also offer the first comprehensive comparison of the various
wave interpretations against the observations. Our major findings are
as follows: (1) High-cadence (2.5-minute) 171 Å images showed a strong
association between expanding loops and the wave onset and significant
differences in the wave appearance between the two STEREO viewpoints
during its early stages; these differences largely disappeared later;
(2) the wave appears at the active region periphery when an abrupt
disappearance of the expanding loops occurs within an interval of 2.5
minutes; (3) almost simultaneous images at different temperatures
showed that the wave was most visible in the 1 - 2 MK range and
almost invisible in chromospheric/transition region temperatures; (4)
triangulations of the wave indicate it was rather low lying (≈ 90
Mm above the surface); (5) forward-fitting of the corresponding CME as
seen by the COR1 coronagraphs showed that the projection of the best-fit
model on the solar surface was inconsistent with the location and size
of the co-temporal EUV wave; and (6) simulations of a fast-mode wave
were found in good agreement with the overall shape and location of the
observed wave. Our findings give significant support for a fast-mode
interpretation of EUV waves and indicate that they are probably
triggered by the rapid expansion of the loops associated with the CME.
Title: Evidence for Mixed Helicity in Erupting Filaments
Authors: Muglach, K.; Wang, Y. -M.; Kliem, B.
Bibcode: 2009ApJ...703..976M
Altcode: 2009arXiv0907.4446M
Erupting filaments are sometimes observed to undergo a rotation
about the vertical direction as they rise. This rotation of the
filament axis is generally interpreted as a conversion of twist into
writhe in a kink-unstable magnetic flux rope. Consistent with this
interpretation, the rotation is usually found to be clockwise (as viewed
from above) if the post-eruption arcade has right-handed helicity, but
counterclockwise if it has left-handed helicity. Here, we describe two
non-active-region filament events recorded with the Extreme-Ultraviolet
Imaging Telescope on the Solar and Heliospheric Observatory in which
the sense of rotation appears to be opposite to that expected from
the helicity of the post-event arcade. Based on these observations,
we suggest that the rotation of the filament axis is, in general,
determined by the net helicity of the erupting system, and that the
axially aligned core of the filament can have the opposite helicity sign
to the surrounding field. In most cases, the surrounding field provides
the main contribution to the net helicity. In the events reported here,
however, the helicity associated with the filament "barbs" is opposite
in sign to and dominates that of the overlying arcade.
Title: Endpoint Brightenings in Erupting Filaments
Authors: Wang, Y. -M.; Muglach, K.; Kliem, B.
Bibcode: 2009ApJ...699..133W
Altcode:
Two well known phenomena associated with erupting filaments are
the transient coronal holes that form on each side of the filament
channel and the bright post-event arcade with its expanding double
row of footpoints. Here we focus on a frequently overlooked signature
of filament eruptions: the spike- or fan-shaped brightenings that
appear to mark the far endpoints of the filament. From a sample of
non-active-region filament events observed with the Extreme-Ultraviolet
Imaging Telescope on the Solar and Heliospheric Observatory, we find
that these brightenings usually occur near the outer edges of the
transient holes, in contrast to the post-event arcades, which define
their inner edges. The endpoints are often multiple and are rooted
in and around strong network flux well outside the filament channel,
a result that is consistent with the axial field of the filament being
much stronger than the photospheric field inside the channel. The
extreme ultraviolet brightenings, which are most intense at the
time of maximum outward acceleration of the filament, can be used to
determine unambiguously the direction of the axial field component from
longitudinal magnetograms. Their location near the outer boundary of
the transient holes suggests that we are observing the footprints of
the current sheet formed at the leading edge of the erupting filament,
as distinct from the vertical current sheet behind the filament which
is the source of the post-event arcade.
Title: Radial and solar cycle variations of the magnetic fields in
the heliosheath: Voyager 1 observations from 2005 to 2008
Authors: Burlaga, L. F.; Ness, N. F.; Acuña, M. H.; Wang, Y. -M.;
Sheeley, N. R.
Bibcode: 2009JGRA..114.6106B
Altcode: 2009JGRA..11406106B
We discuss the magnetic field strength B(t) and polarity observed by
Voyager 1 (V1) in the heliosheath at the heliographic latitude ≈34°
as it moved away from the Sun from 2005 through 2008.82 (where 2008.0
is the beginning of 1 January 2008). The pattern of the polarity of the
magnetic field changed from alternating positive and negative polarities
to predominantly negative polarities (magnetic fields pointing along
the Archimedean spiral field angle toward the Sun) at ≈2006.23). This
transition indicates that the latitudinal extent of the heliospheric
current sheet (HCS) was decreasing in the supersonic solar wind, as
expected for the declining phase of the solar cycle, and as predicted
by extrapolation of the magnetic neutral line near the photosphere to
the position of V1. However, the polarity was not uniformly negative
in during 2008, in contrast to the predicted polarity. This difference
suggests that the maximum latitudinal extent of the HCS was tending
to increase in the northern hemisphere in the heliosheath, while it
was decreasing in the supersonic solar wind. The large-scale magnetic
field strength B(t) was observed by V1 from 2005 through 2008.82. During
this interval of decreasing solar activity toward solar minimum, B(t)
at 1 AU was decreasing, and the solar wind speed V at the latitude of
V1 was increasing. Adjusting the temporal profile of B(t) observed by V1
for the solar cycle variations of B and V in the supersonic solar wind,
we find that the radial gradient of B(R) in heliosheath from the radial
distance R = 94.2 AU to 107.9 AU between 2005.0 and 2008.82 was 0.0017
nT/AU ≤ grad B ≤ 0.0055 nT/AU, or grad B = (0.0036 ± 0.0019) nT/AU.
Title: Coronal Holes and Open Magnetic Flux
Authors: Wang, Y. -M.
Bibcode: 2009SSRv..144..383W
Altcode: 2008SSRv..tmp..151W
Coronal holes are low-density regions of the corona which appear dark
in X-rays and which contain “open” magnetic flux, along which
plasma escapes into the heliosphere. Like the rest of the Sun’s
large-scale field, the open flux originates in active regions but
is subsequently redistributed over the solar surface by transport
processes, eventually forming the polar coronal holes. The total open
flux and radial interplanetary field component vary roughly as the
Sun’s total dipole strength, which tends to peak a few years after
sunspot maximum. An inverse correlation exists between the rate of
flux-tube expansion in coronal holes and the solar wind speed at 1
AU. In the rapidly diverging fields present at the polar hole boundaries
and near active regions, the bulk of the heating occurs at low heights,
leading to an increase in the mass flux density at the Sun and a
decrease in the asymptotic wind speed. The quasi-rigid rotation of
coronal holes is maintained by continual footpoint exchanges between
open and closed field lines, with the reconnection taking place at
the streamer cusps. At much lower heights within the hole interiors,
“interchange reconnection” between small bipoles and the overlying
open flux also gives rise to coronal jets and polar plumes.
Title: The Structure of Streamer Blobs
Authors: Sheeley, N. R., Jr.; Lee, D. D. -H.; Casto, K. P.; Wang,
Y. -M.; Rich, N. B.
Bibcode: 2009ApJ...694.1471S
Altcode:
We have used Sun-Earth Connection Coronal and Heliospheric Investigation
observations obtained from the STEREO A and B spacecraft to study
complementary face-on and edge-on views of coronal streamers. The
face-on views are analogous to what one might see looking down on a flat
equatorial streamer belt at sunspot minimum, and show streamer blobs
as diffuse arches gradually expanding outward from the Sun. With the
passage of time, the legs of the arches fade, and the ejections appear
as a series of azimuthal structures like ripples on a pond. The arched
topology is similar to that obtained in face-on views of streamer
disconnection events (including in/out pairs and streamer blowout
mass ejections), and suggests that streamer blobs have the helical
structure of magnetic flux ropes.
Title: Time-dependent hydrodynamical simulations of slow solar wind,
coronal inflows, and polar plumes
Authors: Pinto, R.; Grappin, R.; Wang, Y. -M.; Léorat, J.
Bibcode: 2009A&A...497..537P
Altcode:
Aims: We explore the effects of varying the areal expansion rate and
coronal heating function on the solar wind flow.
Methods: We use a
one-dimensional, time-dependent hydrodynamical code. The computational
domain extends from near the photosphere, where nonreflecting boundary
conditions are applied, to 30 R_⊙, and includes a transition region
where heat conduction and radiative losses dominate.
Results:
We confirm that the observed inverse relationship between asymptotic
wind speed and expansion factor is obtained if the coronal heating
rate is a function of the local magnetic field strength. We show
that inflows can be generated by suddenly increasing the rate of
flux-tube expansion and suggest that this process may be involved in
the closing-down of flux at coronal hole boundaries. We also simulate
the formation and decay of a polar plume, by including an additional,
time-dependent heating source near the base of the flux tube.
Title: Understanding the Geomagnetic Precursor of the Solar Cycle
Authors: Wang, Y. -M.; Sheeley, N. R.
Bibcode: 2009ApJ...694L..11W
Altcode:
Geomagnetic activity late in the sunspot cycle has been used
successfully to forecast the amplitude of the following cycle. This
success is somewhat surprising, however, because the recurrent
high-speed wind streams that trigger the activity are not proxies of
the Sun's polar fields, whose strength is a critical factor in many
solar dynamo models. Instead, recurrent geomagnetic activity signals
increases in the Sun's equatorial dipole moment, which decays on the
~1-2 yr timescale of the surface meridional flow and does not survive
into the next cycle. In accordance with the original empirical method
of Ohl, we therefore argue that solar cycle predictions should be based
on the minimum level of geomagnetic activity, which is determined by
the Sun's axial dipole strength, not on the peak activity during the
declining phase of the cycle. On physical grounds, we suggest that
an even better indicator would be the total open flux (or strength of
the radial interplanetary field component) at sunspot minimum, which
in turn can be derived from the historical aa index by removing the
contribution of the solar wind speed. This predictor yields a peak
yearly sunspot number R max = 97 ± 25 for solar cycle 24.
Title: Slow Solar Wind from Open Regions with Strong Low-Coronal
Heating
Authors: Wang, Y. -M.; Ko, Y. -K.; Grappin, R.
Bibcode: 2009ApJ...691..760W
Altcode:
By comparing solar wind data taken by the Advanced Composition Explorer
during 1998-2007 with extrapolations of the observed photospheric
magnetic field, we verify that high O7+/O6+
and Fe/O ratios are associated with low wind speeds, large expansion
factors, strong footpoint fields, and high mass and energy flux
densities at the coronal base. As demonstrated by model calculations,
these correlations are consistent with the idea that the bulk of the
slow wind originates from regions of rapidly diverging open flux, where
the coronal heating is concentrated at low heights. We identify two main
components of the slow wind, one emanating from small coronal holes near
active regions and characterized by particularly strong low-coronal
heating, the other coming from just inside the polar-hole boundaries
and characterized by weaker low-coronal heating and intermediate
O7+/O6+ and Fe/O ratios.
Title: Coronal Holes and Open Magnetic Flux
Authors: Wang, Y. -M.
Bibcode: 2009odsm.book..383W
Altcode:
Coronal holes are low-density regions of the corona which appear
dark in X-rays and which contain "open" magnetic flux, along which
plasma escapes into the heliosphere. Like the rest of the Sun's
large-scale field, the open flux originates in active regions but
is subsequently redistributed over the solar surface by transport
processes, eventually forming the polar coronal holes. The total open
flux and radial interplanetary field component vary roughly as the Sun's
total dipole strength, which tends to peak a few years after sunspot
maximum. An inverse correlation exists between the rate of flux-tube
expansion in coronal holes and the solar wind speed at 1 AU. In the
rapidly diverging fields present at the polar hole boundaries and
near active regions, the bulk of the heating occurs at low heights,
leading to an increase in the mass flux density at the Sun and a
decrease in the asymptotic wind speed. The quasi-rigid rotation of
coronal holes is maintained by continual footpoint exchanges between
open and closed field lines, with the reconnection taking place at
the streamer cusps. At much lower heights within the hole interiors,
"interchange reconnection" between small bipoles and the overlying
open flux also gives rise to coronal jets and polar plumes.
Title: Hydrodynamical Simulations of Slow Coronal Wind, Coronal
Inflows and Polar Plumes
Authors: Pinto, R.; Grappin, R.; Wang, Y. -M.; Léorat, J.
Bibcode: 2008sf2a.conf..565P
Altcode:
We use a hydrodynamical time-dependent coronal flux tube model
extending from ∼ 1 R_⊙, where nonreflecting boundary conditions
are applied, to 30 R_⊙, which includes a transition region sustained
by the equilibrium between thermal conduction, radiative losses and a
prescribed mechanical heating flux. We recover the observed inverse
relationship between asymptotic wind speed and expansion factor if
the coronal heating rate is a function of the local magnetic field
strength. We show that inflows can be generated by suddenly increasing
the rate of flux-tube expansion, and suggest that this process may be
involved in the closing-down of flux at coronal hole boundaries. We
also simulate the formation and decay of a polar plume, by including
an additional, time-dependent heating source near the base of the
flux tube.
Title: Relating the Solar Wind Helium Abundance to the Coronal
Magnetic Field
Authors: Wang, Y. -M.
Bibcode: 2008ApJ...683..499W
Altcode:
We analyze the long-term variation of the solar wind helium abundance,
both in and out of the ecliptic, using stackplot displays to compare
these in situ observations with derived coronal parameters. The
coronal source regions are identified and their magnetic properties
characterized by applying a current-free extrapolation, with source
surface located at heliocentric distance r = 2.5 R⊙,
to magnetograph measurements. The density ratio AHe
of α-particles to protons is found to correlate best with the
source-surface field strength Bss, which tends to be enhanced
in high-speed flows and in the slow wind at sunspot maximum, but to be
weak in the low-speed wind that originates from the polar coronal-hole
boundaries around sunspot minimum. A much weaker correlation exists
between AHe and the proton flux density at the source
surface. These results are consistent with acceleration of the
α-particles by ion cyclotron resonance in the outer corona. However,
we are unable to explain why the helium abundance was depressed in the
recurrent high-speed streams observed in the ecliptic during 1999-2000
and 2003-2004.
Title: Observations of Low-Latitude Coronal Plumes
Authors: Wang, Y. -M.; Muglach, K.
Bibcode: 2008SoPh..249...17W
Altcode: 2008SoPh..tmp...71W
Using Fe IX/X 17.1 nm observations from the Extreme-Ultraviolet
Imaging Telescope (EIT) on the Solar and Heliospheric Observatory
(SOHO), we have identified many coronal plumes inside low-latitude
coronal holes as they transited the solar limb during the late
declining phase of cycle 23. These diffuse, linear features appear
to be completely analogous to the familiar polar plumes. By tracking
them as they rotate from the limb onto the disk (or vice versa),
we confirm that EUV plumes seen against the disk appear as faint,
diffuse blobs of emission surrounding a brighter core. When the EIT
images are compared with near-simultaneous magnetograms from the SOHO
Michelson Doppler Imager (MDI), the low-latitude, on-disk plumes are
found to overlie regions of mixed polarity, where small bipoles are in
contact with unipolar flux concentrations inside the coronal hole. The
birth and decay of the plumes are shown to be closely related to the
emergence of ephemeral regions, their dispersal in the supergranular
flow field, and the cancellation of the minority-polarity flux against
the dominant-polarity network elements. In addition to the faint polar
and nonpolar plumes associated with ephemeral regions, we note the
existence of two topologically similar coronal structures: the giant
plume-like features that occur above active regions inside coronal
holes, and the even larger scale "pseudostreamers" that separate
coronal holes of the same polarity. In all three cases, the basic
structure consists of open field lines of a given polarity overlying
a photospheric region of the opposite polarity; ongoing interchange
reconnection at the X-point separating the open field domains from
the underlying double-arcade system appears to result in the steady
evaporation of material from the closed into the open region.
Title: Heliospheric Images of the Solar Wind at Earth
Authors: Sheeley, N. R., Jr.; Herbst, A. D.; Palatchi, C. A.; Wang,
Y. -M.; Howard, R. A.; Moses, J. D.; Vourlidas, A.; Newmark, J. S.;
Socker, D. G.; Plunkett, S. P.; Korendyke, C. M.; Burlaga, L. F.;
Davila, J. M.; Thompson, W. T.; St. Cyr, O. C.; Harrison, R. A.;
Davis, C. J.; Eyles, C. J.; Halain, J. P.; Wang, D.; Rich, N. B.;
Battams, K.; Esfandiari, E.; Stenborg, G.
Bibcode: 2008ApJ...675..853S
Altcode:
During relatively quiet solar conditions throughout the spring and
summer of 2007, the SECCHI HI2 white-light telescope on the STEREO
B solar-orbiting spacecraft observed a succession of wave fronts
sweeping past Earth. We have compared these heliospheric images with
in situ plasma and magnetic field measurements obtained by near-Earth
spacecraft, and we have found a near perfect association between the
occurrence of these waves and the arrival of density enhancements
at the leading edges of high-speed solar wind streams. Virtually
all of the strong corotating interaction regions are accompanied by
large-scale waves, and the low-density regions between them lack such
waves. Because the Sun was dominated by long-lived coronal holes and
recurrent solar wind streams during this interval, there is little
doubt that we have been observing the compression regions that are
formed at low latitude as solar rotation causes the high-speed wind
from coronal holes to run into lower speed wind ahead of it.
Title: SECCHI Observations of the Sun's Garden-Hose Density Spiral
Authors: Sheeley, N. R., Jr.; Herbst, A. D.; Palatchi, C. A.; Wang,
Y. -M.; Howard, R. A.; Moses, J. D.; Vourlidas, A.; Newmark, J. S.;
Socker, D. G.; Plunkett, S. P.; Korendyke, C. M.; Burlaga, L. F.;
Davila, J. M.; Thompson, W. T.; St. Cyr, O. C.; Harrison, R. A.;
Davis, C. J.; Eyles, C. J.; Halain, J. P.; Wang, D.; Rich, N. B.;
Battams, K.; Esfandiari, E.; Stenborg, G.
Bibcode: 2008ApJ...674L.109S
Altcode:
The SECCHI HI2 white-light imagers on the STEREO A and B spacecraft
show systematically different proper motions of material moving outward
from the Sun in front of high-speed solar wind streams from coronal
holes. As a group of ejections enters the eastern (A) field of view,
the elements at the rear of the group appear to overrun the elements
at the front. (This is a projection effect and does not mean that the
different elements actually merge.) The opposite is true in the western
(B) field; the elements at the front of the group appear to run away
from the elements at the rear. Elongation/time maps show this effect
as a characteristic grouping of the tracks of motion into convergent
patterns in the east and divergent patterns in the west, consistent
with ejections from a single longitude on the rotating Sun. Evidently,
we are observing segments of the "garden-hose" spiral made visible
when fast wind from a low-latitude coronal hole compresses blobs of
streamer material being shed at the leading edge of the hole.
Title: Global structure and dynamics of large-scale fluctuations in
the solar wind: Voyager 2 observations during 2005 and 2006
Authors: Burlaga, L. F.; Ness, N. F.; Acũna, M. H.; Wang, Y. -M.;
Sheeley, N. R.; Wang, C.; Richardson, J. D.
Bibcode: 2008JGRA..113.2104B
Altcode:
The Voyager 2 (V2) observations of daily averages of the solar
wind during 2005 and 2006 from 75.3 AU to 81.6 AU between ~25.7°S
and 27.1°S show both a step-like trend in the speed V(t) and
``large-scale fluctuations'' of the magnetic field strength B, speed
V, density N and temperature T. The distribution functions of B,
N, and NV2 observed by V2 are lognormal and that of V
is approximately Gaussian. We introduce a method for specifying
the boundary conditions at all latitudes (except near the poles)
on a Sun-centered surface of radius of 1 AU, based on solar magnetic
field observations. This paper uses only the boundary conditions at
the latitude of V2 and a 1-D time-dependent MHD model to calculate
the radial evolution of the large-scale fluctuations of B(t), V(t)
and N(t) at distances between 1 and 90 AU. This model explains the
V2 observations of a lognormal distribution of B and the Gaussian
distribution of V, but not the observed lognormal distributions of
N and NV2. The lognormal distribution of B observed by V2
was produced primarily by dynamical processes beyond 1 AU.
Title: A Streamer Ejection with Reconnection Close to the Sun
Authors: Sheeley, N. R., Jr.; Warren, H. P.; Wang, Y. -M.
Bibcode: 2007ApJ...671..926S
Altcode:
We previously described coronal events that expand gradually outward
over an interval of 1-2 days and then suddenly tear apart in the
coronagraph's 2-6 Rsolar field of view to form an outgoing
flux rope and an inward system of collapsing loops. Now, we combine
LASCO white-light images of the outer corona with spectrally resolved
EIT images of the inner corona to describe a similar event for which the
separation occurs closer to the Sun. The evolution of this 2006 July
1-2 event had four phases: (1) an expansion phase in which magnetic
loops rise slowly upward and increase the amount of open flux in the
adjacent polar coronal hole and in the low-latitude hole of opposite
polarity; (2) a stretching phase in which the legs of the rising
loops pinch together to form a current sheet; (3) a transition phase
in which field line reconnection produces an outgoing flux rope and a
hot cusp of new loops; and (4) an end phase in which the reconnected
loops become visible at lower temperatures, and the outgoing flux rope
plows through the slow material ahead of it to form a traveling bow
wave. During this time, the photospheric field was relatively weak and
unchanging, as if the eruption had a nonmagnetic origin. We suppose
that coronal heating gradually overpowers magnetic tension and causes
the streamer to separate into a system of collapsing loops and a flux
rope that is carried outward in the solar wind.
Title: Instability of P-waves just below the transition region in
a global solar wind simulation
Authors: Grappin, R.; Léorat, J.; Pinto, R.; Wang, Y. -M.
Bibcode: 2007arXiv0710.0899G
Altcode:
We investigate how wave propagation is modified by the presence
of heat sources and sinks, in the simple 1D, hydrodynamical case,
including chromosphere and solar wind. We integrate the time-dependent
hydrodynamic equations of the solar wind with spherical symmetry,
including conduction, radiative cooling and a prescribed mechanical
heat flux. Once a quasi-stationary wind is established, we study the
response of the system to pressure oscillations at the photospheric
boundary. We use transparent boundary conditions. We find that
wavepackets with high enough amplitude propagating upward from the
photosphere implode just below the transition region. This implosion
is due to the radiative cooling term generating pressure holes close
to the wave crests of the wave, which make the wave collapse. In the
case where heat sources and sinks are not present in the equations,
the wave remains stable whatever the initial wave amplitude, which is
compatible with published work. Instability should be observable when
and where the TR is high enough above the optically thick regions.
Title: On the Formation of Filament Channels
Authors: Wang, Y. -M.; Muglach, K.
Bibcode: 2007ApJ...666.1284W
Altcode:
From the Hα archive of the Big Bear Solar Observatory (BBSO) we
have selected three examples showing fibril structures that change
their orientation, over 1 or 2 days, from nearly perpendicular to
nearly parallel to the polarity inversion line (PIL). In one case,
the filament channel forms within a single decaying bipole; in the
other two cases, it forms along the boundary between an active
region and its surroundings. Comparing the Hα filtergrams with
magnetograms from the Michelson Doppler Imager (MDI), we find that
the fibrils become aligned with the PIL as supergranular convection
brings opposite-polarity magnetic flux together; shearing motions
along the PIL, when present, act mainly to accelerate the rate of
diffusive annihilation. We conclude that the reorientation of the
fibrils is due to the cancellation and submergence of the transverse
field component (B⊥), leaving behind the preexisting axial
field component (B∥). The latter may have been generated
by photospheric differential rotation over longer timescales, or
else was already present when the flux emerged. The filament channel
forms slowly if B∥/B⊥ is initially small,
as along the internal neutral line of a newly emerged bipole, but
may appear within hours if this ratio is initially substantial,
as where the dipole-like loops of an active region curve around its
periphery. In all of our examples, filaments form within a day or so
after the fibrils become aligned with the PIL, while barbs appear at
a later stage, as flux elements continue to diffuse across the PIL
and cancel with the majority-polarity flux on the other side.
Title: The Solar Eclipse of 2006 and the Origin of Raylike Features
in the White-Light Corona
Authors: Wang, Y. -M.; Biersteker, J. B.; Sheeley, N. R., Jr.;
Koutchmy, S.; Mouette, J.; Druckmüller, M.
Bibcode: 2007ApJ...660..882W
Altcode:
Solar eclipse observations have long suggested that the white-light
corona is permeated by long fine rays. By comparing photographs of
the 2006 March 29 total eclipse with current-free extrapolations of
photospheric field measurements and with images from the Solar and
Heliospheric Observatory (SOHO), we deduce that the bulk of these
linear features fall into three categories: (1) polar and low-latitude
plumes that overlie small magnetic bipoles inside coronal holes,
(2) helmet streamer rays that overlie large loop arcades and separate
coronal holes of opposite polarity, and (3) ``pseudostreamer'' rays
that overlie twin loop arcades and separate coronal holes of the
same polarity. The helmet streamer rays extend outward to form the
plasma sheet component of the slow solar wind, while the plumes and
pseudostreamers contribute to the fast solar wind. In all three cases,
the rays are formed by magnetic reconnection between closed coronal
loops and adjacent open field lines. Although seemingly ubiquitous
when seen projected against the sky plane, the rays are in fact rooted
inside or along the boundaries of coronal holes.
Title: Coronal Pseudostreamers
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Rich, N. B.
Bibcode: 2007ApJ...658.1340W
Altcode:
In a recent study of the 2006 solar eclipse, we noted that there are
two kinds of coronal streamers: ``helmet streamers,'' which separate
coronal holes of opposite magnetic polarity, and ``pseudostreamers,''
which overlie twin loop arcades and separate holes of the same
polarity. It is well known that the heliospheric plasma and current
sheets represent the outward extension of helmet streamers. Using
white-light data from the Large Angle and Spectrometric Coronagraph
(LASCO), we here show that pseudostreamers likewise have plasma sheet
extensions, across which the polarity does not reverse; these multiple
sheets contribute significantly to the brightness of the K corona,
although their internal densities tend to be lower than those in
the heliospheric plasma sheet. We use current-free extrapolations of
photospheric field measurements to simulate the observed brightness
patterns in the outer corona, including the contributions of both
helmet streamer and pseudostreamer plasma sheets. Running-difference
images show that pseudostreamers are relatively quiescent, resembling
large-scale plumes; preliminary analysis suggests flow speeds as
high as 200 km s-1 at heliocentric distances of only ~3
Rsolar, supporting the prediction (based on their low
flux tube divergence rates) that pseudostreamers are sources of fast
solar wind.
Title: In/Out Pairs and the Detachment of Coronal Streamers
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.
Bibcode: 2007ApJ...655.1142S
Altcode:
We previously described coronal events that originate in the 2-6
Rsolar field of view of the LASCO white-light coronagraph
and involve the simultaneous ejection of material inward toward the
Sun and outward away from it. Now, in a study of more than 160 in/out
pairs, we have found that these features are density enhancements at
the leading and trailing edges of depletions that occur when slowly
rising coronal structures separate from the Sun. The outward component
is shaped like a large arch with both ends attached to the Sun, and the
inward component is often resolved into loops. We also found about 60
additional events in which the outward components began near the edge of
the occulting disk and inward components were not visible, as if these
events were in/out pairs that originated below the 2 Rsolar
radius of the occulting disk. We conclude that in/out pairs belong to a
broad class of streamer detachments, which include ``streamer blowout''
coronal mass ejections, and we suppose that all of these events occur
when rising magnetic loops reconnect to produce an outgoing helical
flux rope and an ingoing arcade of collapsing loops.
Title: Sources of the Solar Wind at Ulysses during 1990-2006
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 2006ApJ...653..708W
Altcode:
The Ulysses spacecraft is now well into its third polar orbit around
the Sun. Using stackplot displays, we summarize the wind speeds and
interplanetary sector polarities recorded by Ulysses since its launch
in 1990 and relate the observed patterns to the global evolution of
open magnetic regions (coronal holes) over the solar cycle. We verify
that the wind speeds are inversely correlated with the rate of flux-tube
divergence in the corona, as derived from a current-free extrapolation
of the measured photospheric field. We identify the source of each of
the long-lived, high-speed streams encountered by Ulysses and discuss
their formation, evolution, and rotational properties.
Title: Observations of Flux Rope Formation in the Outer Corona
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 2006ApJ...650.1172W
Altcode:
In previous studies employing the Large Angle and Spectrometric
Coronagraph (LASCO), we identified a class of white-light ejections
that separate into incoming and outgoing components at distances of
~3-5 Rsolar from Sun center. These events, of which up to
several per month are observed during high solar activity, are generally
preceded by a gradual outward expansion of faint loops over a period
of a day or more. The expansion terminates when the streamer material,
in the form of an elongated stalk or a sheetlike structure, suddenly
tears apart. The collapsing material is sometimes recognizable as a
collection of loops, while the ejected component is usually poorly
resolved. Here we describe a streamer detachment observed on 2005
December 11, in which the outgoing component can be clearly identified
as a cylindrical flux rope with its ends anchored in the Sun. Based
on simple three-dimensional white-light reconstructions, we conclude
that in/out pairs in general represent the pinching off of streamer
loop arcades to form flux ropes, as seen from different viewing angles.
Title: Solar Source Regions for 3He-rich Solar Energetic
Particle Events Identified Using Imaging Radio, Optical, and Energetic
Particle Observations
Authors: Pick, M.; Mason, G. M.; Wang, Y. -M.; Tan, C.; Wang, L.
Bibcode: 2006ApJ...648.1247P
Altcode:
We have identified the sources of six impulsive 3He-rich
solar energetic particle events using imaging radio, optical, and
energetic ion and electron data, together with calculated coronal fields
obtained from extrapolating photospheric magnetograms using a potential
field source surface (PFSS) model. These events were all studied in
2006 by Wang et al., who identified the particle sources as typically
small, flaring active regions lying next to a coronal hole containing
Earth-directed open field lines, located between W33° and W65°. By
introducing radio imaging data we were able in one case to conclusively
identify which of two simultaneous EUV jets was associated with the
particle source. In addition, type III radio burst and energetic
electron data introduced in this study constrain the injection times
much more accurately than possible with low-energy ion data used in
Wang et al. These new observations confirm the source identifications of
Wang et al. and remove many of the remaining uncertainties. All of these
events were associated with narrow, fast coronal mass ejections (CMEs),
which are unusual for 3He-rich solar energetic particle (SEP)
events. Although the CMEs generally were ejected in directions well off
the ecliptic plane, the PFSS calculations show the presence of magnetic
field lines that made it possible for the energetic particle to quickly
reach Earth. Some of these impulsive events were observed during periods
in which 3He was observed continuously over several days.
Title: Role of the Sun's Nonaxisymmetric Open Flux in Cosmic-Ray
Modulation
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Rouillard, A. P.
Bibcode: 2006ApJ...644..638W
Altcode:
We reexamine the empirical relationship between the Sun's open magnetic
flux and the cosmic-ray (CR) intensity over the solar cycle. The
single parameter that correlates best with the inverted CR rate is
found to be the nonaxisymmetric or longitudinally varying component
of the total open flux, rather than the sunspot number or the rate
of coronal mass ejections (CMEs). The nonaxisymmetric open flux in
turn tracks the evolution of the Sun's equatorial dipole component,
which is a function of both the strength and the longitudinal
distribution of sunspot activity. Year-long peaks in the equatorial
dipole strength coincide with steplike decreases in the CR intensity
and with the formation of global merged interaction regions (GMIRs)
in the outer heliosphere. During these periods, nonaxisymmetric open
flux (in the form of low-latitude coronal holes) is created through the
organized emergence of large active regions, resulting in the global
injection of magnetic energy into the heliosphere. At the same time,
strengthenings of the equatorial dipole are generally accompanied
by large increases in the number of fast CMEs. Rotationally induced,
compressional interactions between the nonaxisymmetric open flux, fast
CMEs, and high-speed streams then give rise to outward-propagating
diffusive barriers that extend over all longitudes and to a latitude
(>~45°) again determined by the equatorial dipole strength.
Title: Time-dependent simulations of solar wind including the
transition region
Authors: Grappin, R.; Léorat, J.; Wang, Y. -M.
Bibcode: 2006sf2a.conf..543G
Altcode:
A low resolution 1D numerical model of the solar wind including the
transition region and a part of the low, cold solar atmosphere is
proposed. It is meant as a first step toward multidimensional modeling
of wave transfer through the transition region and subsequent heating
and acceleration of the corona and wind.
Title: Solar physics: Back to the next solar cycle
Authors: Wang, Y. -M.; Sheeley, N. R.
Bibcode: 2006NatPh...2..367W
Altcode:
Many solar physicists expect the peak sunspot activity during the
next solar cycle to be at its weakest in almost a century. A recent
prediction to the contrary could turn this prevailing wisdom on
its head.
Title: The Pre-CME Sun
Authors: Gopalswamy, N.; Mikić, Z.; Maia, D.; Alexander, D.; Cremades,
H.; Kaufmann, P.; Tripathi, D.; Wang, Y. -M.
Bibcode: 2006SSRv..123..303G
Altcode: 2006SSRv..tmp...77G
The coronal mass ejection (CME) phenomenon occurs in closed magnetic
field regions on the Sun such as active regions, filament regions,
transequatorial interconnection regions, and complexes involving a
combination of these. This chapter describes the current knowledge
on these closed field structures and how they lead to CMEs. After
describing the specific magnetic structures observed in the CME source
region, we compare the substructures of CMEs to what is observed before
eruption. Evolution of the closed magnetic structures in response to
various photospheric motions over different time scales (convection,
differential rotation, meridional circulation) somehow leads to the
eruption. We describe this pre-eruption evolution and attempt to link
them to the observed features of CMEs. Small-scale energetic signatures
in the form of electron acceleration (signified by nonthermal radio
bursts at metric wavelengths) and plasma heating (observed as compact
soft X-ray brightening) may be indicative of impending CMEs. We survey
these pre-eruptive energy releases using observations taken before
and during the eruption of several CMEs. Finally, we discuss how the
observations can be converted into useful inputs to numerical models
that can describe the CME initiation.
Title: Coronal Observations of CMEs. Report of Working Group A
Authors: Schwenn, R.; Raymond, J. C.; Alexander, D.; Ciaravella, A.;
Gopalswamy, N.; Howard, R.; Hudson, H.; Kaufmann, P.; Klassen, A.;
Maia, D.; Munoz-Martinez, G.; Pick, M.; Reiner, M.; Srivastava, N.;
Tripathi, D.; Vourlidas, A.; Wang, Y. -M.; Zhang, J.
Bibcode: 2006SSRv..123..127S
Altcode: 2006SSRv..tmp...58S
CMEs have been observed for over 30 years with a wide variety of
instruments. It is now possible to derive detailed and quantitative
information on CME morphology, velocity, acceleration and mass. Flares
associated with CMEs are observed in X-rays, and several different
radio signatures are also seen. Optical and UV spectra of CMEs both on
the disk and at the limb provide velocities along the line of sight
and diagnostics for temperature, density and composition. From the
vast quantity of data we attempt to synthesize the current state of
knowledge of the properties of CMEs, along with some specific observed
characteristics that illuminate the physical processes occurring during
CME eruption. These include the common three-part structures of CMEs,
which is generally attributed to compressed material at the leading
edge, a low-density magnetic bubble and dense prominence gas. Signatures
of shock waves are seen, but the location of these shocks relative
to the other structures and the occurrence rate at the heights where
Solar Energetic Particles are produced remains controversial. The
relationships among CMEs, Moreton waves, EIT waves, and EUV dimming
are also cloudy. The close connection between CMEs and flares suggests
that magnetic reconnection plays an important role in CME eruption
and evolution. We discuss the evidence for reconnection in current
sheets from white-light, X-ray, radio and UV observations. Finally, we
summarize the requirements for future instrumentation that might answer
the outstanding questions and the opportunities that new space-based
and ground-based observatories will provide in the future.
Title: Coronal Holes, Jets, and the Origin of 3He-rich
Particle Events
Authors: Wang, Y. -M.; Pick, M.; Mason, G. M.
Bibcode: 2006ApJ...639..495W
Altcode:
Using magnetograph measurements, coronal field extrapolations,
and imaging observations, we investigate the solar origins of 25
3He-rich particle events from the period 1997-2003. In
essentially every case we find that the source of the impulsive solar
energetic particles (SEPs) lies next to a coronal hole containing
Earth-directed open field lines. Averaged over all events, the
source-hole separation is only ~4° at the photosphere. The source
itself is typically a small, flaring active region located between
longitudes ~W25 and ~W72. Around the estimated particle injection
time, EUV images often show a jetlike ejection aligned with the open
field lines. In some cases, a corresponding white-light jet is seen
at heliocentric distances >~2 Rsolar, similar to those
studied earlier by Wang & Sheeley. The jets show a tendency to
recur, a behavior that is reflected in the time variation of the
measured 3He and Fe particle intensities. We interpret the
jets as signatures of magnetic reconnection (``footpoint exchange'')
between closed and open field lines. On the basis of these findings,
we expect 3He enrichments to be observed whenever
Earth-connected open field lines undergo footpoint exchanges with
nearby active or ephemeral region fields. Because small bipoles
emerge continually inside coronal holes, moderate enhancements in
the 3He level can occur even when no significant flaring
activity is recorded.
Title: Coronal Observations of CMEs
Authors: Schwenn, R.; Raymond, J. C.; Alexander, D.; Ciaravella, A.;
Gopalswamy, N.; Howard, R.; Hudson, H.; Kaufmann, P.; Klassen, A.;
Maia, D.; Munoz-Martinez, G.; Pick, M.; Reiner, M.; Srivastava, N.;
Tripathi, D.; Vourlidas, A.; Wang, Y. -M.; Zhang, J.
Bibcode: 2006cme..book..127S
Altcode:
CMEs have been observed for over 30 years with a wide variety of
instruments. It is now possible to derive detailed and quantitative
information on CME morphology, velocity, acceleration and mass. Flares
associated with CMEs are observed in X-rays, and several different
radio signatures are also seen. Optical and UV spectra of CMEs both on
the disk and at the limb provide velocities along the line of sight
and diagnostics for temperature, density and composition. From the
vast quantity of data we attempt to synthesize the current state of
knowledge of the properties of CMEs, along with some specific observed
characteristics that illuminate the physical processes occurring during
CME eruption. These include the common three-part structures of CMEs,
which is generally attributed to compressed material at the leading
edge, a low-density magnetic bubble and dense prominence gas. Signatures
of shock waves are seen, but the location of these shocks relative
to the other structures and the occurrence rate at the heights where
Solar Energetic Particles are produced remains controversial. The
relationships among CMEs, Moreton waves, EIT waves, and EUV dimming
are also cloudy. The close connection between CMEs and flares suggests
that magnetic reconnection plays an important role in CME eruption
and evolution. We discuss the evidence for reconnection in current
sheets from white-light, X-ray, radio and UV observations. Finally, we
summarize the requirements for future instrumentation that might answer
the outstanding questions and the opportunities that new space-based
and ground-based observatories will provide in the future.
Title: Origin of impulsive 3He-rich particle events
Authors: Tan, C.; Pick, M.; Wang, Y. -M.; Mason, G.; Wang, L.
Bibcode: 2006cosp...36.1517T
Altcode: 2006cosp.meet.1517T
Using EUV white light radio observations and coronal magnetic field
extrapolations we studied the origin of solar impulsive accelerated
electrons and 3He rich events SEP These events generated type III radio
bursts in the corona and the interplanetary medium and were associated
with CMEs We showed that for these events the electron acceleration
takes place during the early development of CMEs We illustrated our
results in some cases and discuss the interpretation
Title: Consequence of a shock propagating in a preceding magnetic
cloud in aspect of SEP flux
Authors: Shen, C. L.; Wang, Y. M.; Ye, P. Z.; Wang, S.
Bibcode: 2006cosp...36.1948S
Altcode: 2006cosp.meet.1948S
Five definite cases of a shock propagating in an interplanetary magnetic
cloud MC are reported to study possible consequences signatures of
such phenomena in aspect of the SEP flux based on the magnetic field
and solar wind plasma data from the ACE spacecraft and the integral
high energy proton flux data from the GOES spacecraft Enhancement
of SEP fluxes starting at the MC front boundary and ending at the MC
rear boundary is found in two of the five cases the Nov 5-6 2001 event
and Nov 7-8 1998 event This is very different from the observations
of isolated MCs in which the energetic particle fluxes are usually
depressed The increments of the magnetic field strength and the solar
wind speed at the shocks suggest that the shocks embedded in these
two SEP-rich MCs are stronger than the shocks embedded in the other
three MCs All these results imply that a shock propagating into an
MC might make the SEP flux increase in the MC and the significance of
such a SEP enhancement depends on the strength of the embedded shock
This is consistent with the traditional view of point that MCs are a
kind of independent and relatively closed magnetic structure in the
interplanetary space the particles inside MCs are difficult to escape
and vice versa
Title: The Dependence of Characteristic Times of Gradual SEP Events
on Their Associated CME Properties
Authors: Pan, Z. H.; Wang, C. B.; Xue, X. H.; Wang, Y. M.
Bibcode: 2006cosp...36.1943P
Altcode: 2006cosp.meet.1943P
It is generally believed that coronal mass ejections CMEs are the
drivers of shocks that accelerate gradual solar energetic particles
SEPs One might expect that the characteristics of the SEP intensity
time profiles observed at 1 AU are determined by properties of
the associated CMEs such as the radial speed and the angular width
Recently Kahler statistically investigated the characteristic times
of gradual SEP events observed from 1998-2002 and their associated
coronal mass ejection properties Astrophys J 628 1014--1022 2005 Three
characteristic times of gradual SEP events are determined as functions
of solar source longitude 1 T 0 the time from associated CME launch
to SEP onset at 1 AU 2 T R the rise time from SEP onset to the time
when the SEP intensity is a factor of 2 below peak intensity and 3 T
D the duration over which the SEP intensity is within a factor of 2
of the peak intensity However in his study the CME speeds and angular
widths are directly taken from the LASCO CME catalog In this study
we analyze the radial speeds and the angular widths of CMEs by an
ice-cream cone model and re-investigate their correlationships with the
characteristic times of the corresponding SEP events We find T R and T
D are significantly correlated with radial speed for SEP events in the
best-connected longitude range and there is no correlation between T 0
and CME radial speed and angular width which is consistent with Kahler
s results On the other hand it s found that T R and T D are also have
Title: The Pre-CME Sun
Authors: Gopalswamy, N.; Mikić, Z.; Maia, D.; Alexander, D.; Cremades,
H.; Kaufmann, P.; Tripathi, D.; Wang, Y. -M.
Bibcode: 2006cme..book..303G
Altcode:
The coronal mass ejection (CME) phenomenon occurs in closed magnetic
field regions on the Sun such as active regions, filament regions,
transequatorial interconnection regions, and complexes involving a
combination of these. This chapter describes the current knowledge
on these closed field structures and how they lead to CMEs. After
describing the specific magnetic structures observed in the CME source
region, we compare the substructures of CMEs to what is observed before
eruption. Evolution of the closed magnetic structures in response to
various photospheric motions over different time scales (convection,
differential rotation, meridional circulation) somehow leads to the
eruption. We describe this pre-eruption evolution and attempt to link
them to the observed features of CMEs. Small-scale energetic signatures
in the form of electron acceleration (signified by nonthermal radio
bursts at metric wavelengths) and plasma heating (observed as compact
soft X-ray brightening) may be indicative of impending CMEs. We survey
these pre-eruptive energy releases using observations taken before
and during the eruption of several CMEs. Finally, we discuss how the
observations can be converted into useful inputs to numerical models
that can describe the CME initiation.
Title: Modeling the Sun's Magnetic Field and Irradiance since 1713
Authors: Wang, Y. -M.; Lean, J. L.; Sheeley, N. R., Jr.
Bibcode: 2005ApJ...625..522W
Altcode:
We use a flux transport model to simulate the evolution of the
Sun's total and open magnetic flux over the last 26 solar cycles
(1713-1996). Polar field reversals are maintained by varying the
meridional flow speed between 11 and 20 m s-1, with the
poleward-directed surface flow being slower during low-amplitude
cycles. If the strengths of the active regions are fixed but their
numbers are taken to be proportional to the cycle amplitude, the
open flux is found to scale approximately as the square root of the
cycle amplitude. However, the scaling becomes linear if the number of
active regions per cycle is fixed but their average strength is taken
to be proportional to the cycle amplitude. Even with the inclusion
of a secularly varying ephemeral region background, the increase in
the total photospheric flux between the Maunder minimum and the end of
solar cycle 21 is at most ~one-third of its minimum-to-maximum variation
during the latter cycle. The simulations are compared with geomagnetic
activity and cosmogenic isotope records and are used to derive a
new reconstruction of total solar irradiance (TSI). The increase in
cycle-averaged TSI since the Maunder minimum is estimated to be ~1 W
m-2. Because the diffusive decay rate accelerates as the
average spacing between active regions decreases, the photospheric
magnetic flux and facular brightness grow more slowly than the sunspot
number and TSI saturates during the highest amplitude cycles.
Title: Global structure of the out-of-ecliptic solar wind
Authors: Whang, Y. C.; Wang, Y. -M.; Sheeley, N. R.; Burlaga, L. F.
Bibcode: 2005JGRA..110.3103W
Altcode: 2005JGRA..11003103W
We use the observed photospheric field maps and the wind speed observed
from Ulysses to study the out-of-ecliptic solar wind. The model
calculates the wind speed from the rate of magnetic flux tube expansion
factors using a conversion function that is determined by least squares
fit of all currently available data from Ulysses. Using the best fit
conversion function, we investigate the global solar wind covering
a 36-year period from 1968 through 2003. The results complement and
expand upon earlier studies conducted with interplanetary scintillation
and other in situ spacecraft observations. The rotationally averaged
wind speed is a function of two parameters: the heliolatitude and the
phase of the solar cycle. The out-of-ecliptic solar wind has a recurrent
stable structure, and the average wind speed varies like a sine square
of latitude profile spanning more than 5 years during the declining
phase and solar minimum in each solar cycle. Ulysses has observed
this stable structure in its first polar orbit in 1992-1997. Near
solar maximum the structure of the out-of-ecliptic solar wind is in a
transient state lasting 2-3 years when the stable structure breaks down
during the disappearance and reappearance of the polar coronal holes.
Title: The Origin of Postflare Loops
Authors: Sheeley, N. R., Jr.; Warren, H. P.; Wang, Y. -M.
Bibcode: 2004ApJ...616.1224S
Altcode:
We apply a tracking technique, previously developed to study motions
in the outer corona by Sheeley, Walters, Wang, and Howard, to 195 Å
filtergrams obtained with the Transition Region and Coronal Explorer
(TRACE) satellite and obtain height-time maps of the motions in the
hot (10-20 MK) plasma clouds above postflare loop systems. These
maps indicate the following two main characteristics. (1) Within the
plasma cloud, the motions are downward at speeds of approximately 4
km s-1. The cloud itself grows with time, its upper layers
being replenished by the arrival and deceleration of fast inflows and
its lower layers disappearing when they cool to form the tops of new
postflare loops. (2) Early in these events, the inward motions are
turbulent, showing a variety of dark elongated features resembling
``tadpoles'' and some bright features. Later, the inflows are visible
as dark collapsing loops, changing from initially cusp-shaped features
to rounder loops as they move inward. Their speeds initially lie in the
range 100-600 km s-1 but decrease to 4 km s-1
in about 3 minutes, corresponding to an average deceleration ~1500 m
s-2. Combining these observations with similar observations
obtained at reconnection sites in the outer corona by the Large Angle
Spectrometric Coronagraph (LASCO), we conclude that postflare loops are
the end result of the formation, filling, deceleration, and cooling
of magnetic loops produced by the reconnection of field lines blown
open in the flare. The formation of collapsing loops occurs in the
dark tadpoles; the filling of these initially dark loops occurs via
chromospheric evaporation, which also contributes to the deceleration
of the loops; and the radiative cooling ultimately resolves the loops
into sharply defined structures.
Title: The Sun's Large-Scale Magnetic Field and Its Long-Term
Evolution
Authors: Wang, Y. -M.
Bibcode: 2004SoPh..224...21W
Altcode: 2005SoPh..224...21W
The Sun's large-scale external field is formed through the emergence
of magnetic flux in active regions and its subsequent dispersal over
the solar surface by differential rotation, supergranular convection,
and meridional flow. The observed evolution of the polar fields and
open flux (or interplanetary field) during recent solar cycles can be
reproduced by assuming a supergranular diffusion rate of 500 - 600
km2 s−1 and a poleward flow speed of 10 -20
m s−1. The nonaxisymmetric component of the large-scale
field decays on the flow timescale of ∼1 yr and must be continually
regenerated by new sunspot activity. Stochastic fluctuations in the
longitudinal distribution of active regions can produce large peaks
in the Sun's equatorial dipole moment and in the interplanetary field
strength during the declining phase of the cycle; by the same token,
they can lead to sudden weakenings of the large-scale field near sunspot
maximum (Gnevyshev gaps). Flux transport simulations over many solar
cycles suggest that the meridional flow speed is correlated with cycle
amplitude, with the flow being slower during less active cycles.
Title: Footpoint Switching and the Evolution of Coronal Holes
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 2004ApJ...612.1196W
Altcode:
We discuss the role of footpoint exchanges between open and closed
magnetic field lines (also known as ``interchange reconnection'')
in the formation and rotational evolution of coronal holes. Such
exchanges cause open flux to jump from one location to another when
active regions emerge; they also act to untie the rotation of coronal
holes from that of the underlying plasma. We introduce a quantitative
measure of the footpoint exchange rate and apply it to a variety of
idealized configurations. During the formation of coronal holes,
footpoint switching dominates over the creation of new open flux
if the background (or polar) field is strong compared to that of
the emerging active region, so the latter acts to change mainly the
direction rather than the magnitude of the Sun's dipole vector. The
principal role of footpoint exchanges is to counteract the subsequent
rotational shearing of the holes; this result is accomplished by means
of continual sideways displacements of open and closed field lines along
the hole boundaries. Because the timescale for rotational shearing
(~3 months) is less than that for the decay of the Sun's large-scale
nonaxisymmetric field (~1 yr), interchange reconnection is expected
on average to dominate over the closing down of flux throughout the
solar cycle.
Title: Simulations of the Quiet Sun Emission at Metric and Decimetric
Radio Wavelengths
Authors: Marqué, C.; Wang, Y. M.; Thernisien, A. F.; Vourlidas, A.;
Howard, R. A.
Bibcode: 2004AAS...204.7104M
Altcode: 2004BAAS...36Q.797M
In the metric and decimetric radio range, solar emission is dominated
by non-thermal radiation from electron populations accelerated during
flares or continuous processes. When the solar activity is low, mainly
during the solar cycle minimum, the thermal emission from the corona
can be mapped, and structures such as coronal holes, active regions
or filament cavities can be observed. The radio thermal emission is
sensitive to the electron density and temperature, and radio rays
suffer refraction effects when their frequency is close to the local
plasma frequency. A model of the electron density and temperature
distribution is thus needed to compute the thermal radiation at a
given frequency. Axisymetric and homogeneous electron density models
have been successfully used for the last fourty years to described
the basic properties of this thermal emission. Nevertheless, these
density models are not suitable for describing the corona at a given
date. We present in this poster more realistic simulations using
a Potential Field Source Surface extrapolation and realistic electron
density distributions. Assuming hydrostatic equilibrium, the density
is determined by the strength of the magnetic field and the length of
the magnetic loops: n=n0(B,L)*f(r). Different n0 and f functions are
used and the corresponding results are compared to real data.
Title: The termination shock near 35° latitude
Authors: Whang, Y. C.; Burlaga, L. F.; Wang, Y. -M.; Sheeley, N. R.
Bibcode: 2004GeoRL..31.3805W
Altcode: 2004GeoRL..3103805W
The termination shock moves outwards and inwards over timescales of a
solar cycle in response to the variations in the average solar wind
speed. The amplitude is greater than 50 AU near 35° latitude; the
maximum (minimum) distance occurs during the rising (declining) phase
of the solar cycle. Shock parameters are distinctly different when
the shock moves outwards or inwards. During the period of high-speed
(low-speed) solar wind, the shock moves outward (inward) and the shock
is weaker (stronger). This study assumes that the first crossing of
Voyager 1 with the termination shock occurred at 85.5 AU on 2002.6. If
Voyager 1 did cross the shock in 2002.6, the spacecraft would likely
cross the shock at least two more times before 2010, but no second
crossing would occur close to 2003.1. If Voyager 1 did not cross the
shock in mid-2002, it might still do so before 2005.
Title: The solar cycle evolution of the large-scale photospheric field
Authors: Wang, Y. -M.
Bibcode: 2004cosp...35.1628W
Altcode: 2004cosp.meet.1628W
We discuss the evolution of the large-scale photospheric field over
the solar cycle, emphasizing the role of surface transport processes
(differential rotation, supergranular convection, and meridional
flow). The observed evolution of both the solar polar and interplanetary
fields can be reproduced by assuming a poleward flow speed of order 20 m
s-1. The combination of differential rotation and latitudinal
transport gives rise to large-scale magnetic patterns that rotate more
rigidly than the photosphere itself. Since it decays on the flow time
scale of ∼1 yr, the nonaxisymmetric component of the photospheric
field must be continually regenerated by sunspot activity. Stochastic
fluctuations in the longitudinal distribution of active regions can
produce large peaks in the Sun's equatorial dipole moment and in the
interplanetary field strength (such as occurred in 1982 and 1991),
and by the same token can lead to sudden weakenings of the large-scale
field near sunspot maximum (``Gnevyshev gaps''). Simulations over many
solar cycles suggest that the meridional flow speed is correlated with
cycle amplitude---the flow is faster during more active cycles.
Title: Coronal inflows as evidence for reconnection in the outer
corona
Authors: Wang, Y. -M.
Bibcode: 2004cosp...35.1627W
Altcode: 2004cosp.meet.1627W
We describe the inflows observed with the SOHO/LASCO white-light
coronagraph and discuss their relevance to the closing-down of magnetic
flux in the corona. The inflows are not seen above 6 R_⊙ from
Sun center, which appears to be a point of no return for the Sun's
plasmas and fields. Some inflows have characteristics (like fast,
oppositely directed ejections) that are easily interpreted in terms
of conventional models for field line reconnection. However, the vast
majority of inflows show a more subtle behavior that typically includes
the following: (1) Birth as a weak, downward-moving density enhancement
at 4--5 R_⊙, with no visible outgoing counterpart; (2) Formation of a
narrow sinking column, leaving a dark depletion tail in its wake; at the
same time, acceleration to maximum speeds of 50--100 km s-1;
(3) Deceleration and formation of an outward-pointing cusp or stretched
loop below 2.5 R_⊙. We interpret the initial downward motion as a
preparatory stage for the pinching-off of field lines, which occurs in
the depleted region behind the sinking column and is later revealed by
the appearance of the stretched loop. We contrast the coronal inflows
to the outward-moving LASCO streamer blobs, which are interpreted as
the product of interchange reconnection between closed helmet-streamer
loops and open field lines.
Title: On the Topological Evolution of the Coronal Magnetic Field
During the Solar Cycle
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 2003ApJ...599.1404W
Altcode:
Using an axisymmetric model that includes the effects of flux
emergence and surface transport processes, we calculate the evolution
of the photospheric magnetic field over the solar cycle and derive
a corresponding sequence of coronal configurations by means of a
potential-field source-surface extrapolation. By identifying magnetic
neutral points and tracking changes in the total flux within each
topological domain, we construct an overall picture of how open
and closed flux is transported as the coronal field reverses its
polarity:1. During the rising phase of the cycle, an X-point forms
above the emerging flux (represented by a bipole structure) in each
hemisphere, and the overlying, opposite-polarity field lines are
``stripped away'' (reconnected to each side); at the same time, as
the Sun's axial dipole strength decreases, open field lines from the
polar coronal holes begin to merge at the equator and close down.2. As
the rate of flux emergence peaks, the X-point rises toward the source
surface and the bipole opens up, forming a trailing-polarity hole
on its poleward side, which evolves into the new-cycle polar hole;
the leading-polarity open flux on the equatorward side of the bipole
progressively closes down by merging with its opposite-hemisphere
counterpart.3. Later in the declining phase of the cycle, the
opposite-hemisphere bipoles begin to reconnect with each other at an
equatorial X-point, producing long trailing-polarity loops that rise
toward the source surface and continue to feed flux into the new-cycle
polar holes, and short leading-polarity loops that collapse toward the
photosphere and eventually submerge.We compare the case in which the
transport of the photospheric field is by supergranular diffusion alone
with that in which both diffusion and a 20 m s-1 poleward
flow are present; the latter model is shown to reproduce more closely
the coronal topologies inferred from the observed photospheric field.
Title: Quantization on brightness about meteors in Chinese ancient
recordings
Authors: Wang, Y. M.
Bibcode: 2003AcASn..44..416W
Altcode:
The records of ``big as X'' about meteors in Chinese annals were the
values that the ancients estimated the scale of visible surface of the
stars' images. In fact, the visible surface is a special sensation
that derives from the diffraction, diffusion of light and effect of
irradiation when we observe stars with naked eyes. In substance, ``big
as X'' might be regarded as the records of brightness. These records
can be quantified to apparent scales (angular diameter) according
to the Model of Celestial Sphere used by Chinese ancients. Then we
can reduce the size to a brightness on the basis of some ``standard
points''. This paper has counted up 131 objects, which appeared in
ancient 4420 recordings in the words ``big as X'', and reduced them
to brightness.
Title: Multiple magnetic clouds: Several examples during March-April
2001
Authors: Wang, Y. M.; Ye, P. Z.; Wang, S.
Bibcode: 2003JGRA..108.1370W
Altcode:
Multiple magnetic cloud (Multi-MC), which is formed by the overtaking of
successive coronal mass ejections (CMEs), is a kind of complex structure
in interplanetary space. Multi-MC is worthy of notice due to its special
properties and potential geoeffectiveness. Using the data from the
ACE spacecraft, we identify the three cases of Multi-MC in the period
from March to April 2001. Some observational signatures of Multi-MC
are concluded: (1) Multi-MC only consists of several magnetic clouds
and interacting regions between them; (2) each subcloud in Multi-MC
is primarily satisfied with the criteria of isolated magnetic cloud,
except that the proton temperature is not as low as that in typical
magnetic cloud due to the compression between the subclouds; (3) the
speed of solar wind at the rear part of the front subcloud does not
continuously decrease, rather increases because of the overtaking of
the following subcloud; (4) inside the interacting region between the
subclouds, the magnetic field becomes less regular and its strength
decreases obviously, and (5) β value increases to a high level in
the interacting region. We find out that two of three Multi-MCs are
associated with the great geomagnetic storms (Dst ≤ -200 nT), which
indicate a close relationship between the Multi-MCs and some intense
geomagnetic storms. The observational results imply that the Multi-MC
is possibly another type of the interplanetary origin of the large
geomagnetic storm, though not all of them have geoeffectiveness. Based
on the observations from Solar and Heliospheric Observatory (SOHO) and
GOES, the solar sources (CMEs) of these Multi-MCs are identified. We
suggest that such successive halo CMEs are not required to be originated
from a single solar region. Furthermore, the relationship between
Multi-MC and complex ejecta is analyzed, and some similarities and
differences between them are discussed.
Title: Voyager 1 Studies of the HMF to 81 AU During the Ascending
Phase of Solar Cycle 23
Authors: Burlaga, L. F.; Ness, N. F.; Wang, Y. -M.; Sheeley, N. R.
Bibcode: 2003AIPC..679...39B
Altcode:
The paper analyzes the magnetic field strength B and polarity
observed in the distant heliosphere from 1996 to early 2001 and will be
discussed in relation to the variation of B from 1978 through 1996. The
observations extend the results of Burlaga et al. [1]. The polarity of
the heliospheric magnetic field (HMF) from 1997 to early 2001 is studied
in relation to the extrapolated position of the heliospheric current
sheet (HCS). These observations of polarity extend the earlier results
of Burlaga et al. [2] and Burlaga and Ness [3]. The V1 observations
of the heliospheric magnetic field strength B agree with Parker's
model of the global heliospheric magnetic field from 1 to 81 AU and
from 1978 to 2001, when one considers the solar cycle variations in
the source magnetic field strength and the latitude/time variation in
the solar wind speed. Parker's model, without adjustable parameters,
describe the general tendency for B to decrease with increasing distance
R from the Sun, and the solar cycle time variations causing the three
broad increases of B around 1980, 1990, and 2000, and the minima of B
in 1987 and 1997. The variation of magnetic polarity observed by V1
and V2 was caused by the increasing latitudinal width of the sector
zone with increasing solar activity, which in turn was related to
the increasing maximum latitudinal extent and the decreasing minimum
latitudinal extent of the footpoints of the HCS.
Title: Theoretical analysis on the geoeffectiveness of a shock
overtaking a preceding magnetic cloud
Authors: Wang, Y. M.; Ye, P. Z.; Wang, S.; Xiong, M.
Bibcode: 2003SoPh..216..295W
Altcode:
The shock compression of the preexisting southward directed magnetic
field can enhance a geomagnetic disturbance. A simple theoretical
model is proposed to study the geoeffectiveness of a shock overtaking
a preceding magnetic cloud. Our aim is to answer theoretically the
question how deep the shock enters into the cloud when the event just
reaches the maximum geoeffectiveness. The results suggest that the
minimum value of Dst* decreases initially, then increases
again while the shock propagates from the border to the center of the
cloud. There is a position where the shock compression of the preceding
cloud obtains the maximum geoeffectiveness. In different situations,
the position is different. The higher the overtaking shock speed is,
the deeper is this position, and the smaller is the corresponding
Dst*min. Some shortcomings of this theoretical
model are also discussed.
Title: An interplanetary cause of large geomagnetic storms: Fast
forward shock overtaking preceding magnetic cloud
Authors: Wang, Y. M.; Ye, P. Z.; Wang, S.; Xue, X. H.
Bibcode: 2003GeoRL..30.1700W
Altcode: 2003GeoRL..30m..33W
In the event that occurred during October 3-6, 2000, at least one
magnetosonic wave and one fast forward shock advanced into the preceding
magnetic cloud (MC). By using the field and plasma data from the ACE
and WIND spacecraft, we analyze the evolution of this event, including
the characteristics and changes of the magnetic fields and plasma. At
the rear part of the cloud, a large southward magnetic field is caused
by a shock compression. The shock intensified a preexisting southward
magnetic field. This increased the geoeffectiveness of this event
and produced an intense geomagnetic storm with Dst = -175 nT. We also
describe another event with a shock overtaking a MC on Nov. 6, 2001. A
great geomagnetic storm of intensity Dst = 292 nT resulted. These
observations are used to argue that shock compression of magnetic
cloud fields is an important interplanetary cause of large geomagnetic
storms. Our analyses suggest that the geoeffectiveness is related to the
direction of preexisting magnetic fields, the intensity of overtaking
shock, and the amount of shock penetration into the preceding MC.
Title: Modeling the Sun's Large-Scale Magnetic Field during the
Maunder Minimum
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 2003ApJ...591.1248W
Altcode:
We use a flux transport model to simulate the evolution of the Sun's
magnetic dipole moment, polar fields, and open flux under Maunder
minimum conditions. Even when the rate of active region emergence
is taken to be a factor of ~30 smaller than in recent solar cycles,
regular polarity oscillations of the axial dipole and polar fields
can be maintained if the speed of the poleward surface flow is reduced
from ~20 to ~10 m s-1 and the source flux emerges at very
low latitudes (~10°). The axial dipole is then found to have an
amplitude of the order of 0.5 G, as compared with ~4 G during solar
cycle 21. The strength of the radial interplanetary field component
at Earth is estimated to be in the range ~0.3-0.7 nT, about a factor
of 7 lower than contemporary values. We discuss the implications of
these weak fields for our understanding of geomagnetic activity and
cosmic-ray modulation during the Maunder minimum.
Title: On the Fluctuating Component of the Sun's Large-Scale
Magnetic Field
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 2003ApJ...590.1111W
Altcode:
The Sun's large-scale magnetic field and its proxies are known to
undergo substantial variations on timescales much less than a solar
cycle but longer than a rotation period. Examples of such variations
include the double activity maximum inferred by Gnevyshev, the large
peaks in the interplanetary field strength observed in 1982 and 1991,
and the 1.3-1.4 yr periodicities detected over limited time intervals
in solar wind speed and geomagnetic activity. We consider the question
of the extent to which these variations are stochastic in nature. For
this purpose, we simulate the evolution of the Sun's equatorial dipole
strength and total open flux under the assumption that the active
region sources (BMRs) are distributed randomly in longitude. The
results are then interpreted with the help of a simple random walk
model including dissipation. We find that the equatorial dipole and open
flux generally exhibit multiple peaks during each 11 yr cycle, with the
highest peak as likely to occur during the declining phase as at sunspot
maximum. The widths of the peaks are determined by the timescale τ~1
yr for the equatorial dipole to decay through the combined action of
meridional flow, differential rotation, and supergranular diffusion. The
amplitudes of the fluctuations depend on the strengths and longitudinal
phase relations of the BMRs, as well as on the relative rates of flux
emergence and decay. We conclude that stochastic processes provide a
viable explanation for the ``Gnevyshev gaps'' and for the existence
of quasi periodicities in the range ~1-3 yr.
Title: Solar Wind Speed and Temperature Outside 10 AU and the
Termination Shock
Authors: Whang, Y. C.; Burlaga, L. F.; Wang, Y. -M.; Sheeley,
N. R., Jr.
Bibcode: 2003ApJ...589..635W
Altcode:
In this paper we first present a series of pickup proton solar wind
solutions following the fluid motion in the upwind direction to show
that the wind speed V and temperature T, at a given r outside 10 AU,
are primarily functions of the 1 AU wind speed V0. This
relationship is attributed to the accumulated effects of the
pickup proton process on the heating and deceleration of the solar
wind. Because pickup protons are expected to have similar effects on
the solar wind at all latitudes in the upwind side of the heliosphere,
in the second part of the paper, the two formulae V(r, V0)
and T(r, V0) are extended to study the termination shock at
35° latitude. Wang and Sheeley have an empirical model for calculating
the 1 AU wind speed V0 from the observed photospheric
field. We use the simulated wind speed V0 to calculate V
and T outside 60 AU following the fluid motion; then we can study the
solar cycle variation of the termination shock. The shock location near
35° is unambiguously dependent on the solar cycle, with a period of
approximately 1 solar cycle; the amplitude for variation of the shock
location is greater than 50 AU. The new result supports the idea that
the first encounter of Voyager 1 with the termination shock may occur
during the declining phase of cycle 23. After the first encounter, the
spacecraft will cross the shock two more times over a period of 8 years.
Title: The Solar Wind and Its Magnetic Sources at Sunspot Maximum
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 2003ApJ...587..818W
Altcode:
We use in situ measurements from the Advanced Composition
Explorer and magnetograph data from the National Solar
Observatory to relate the properties of the solar wind during
1998-2002 to its source region magnetic fields. The great bulk
of the solar maximum wind is characterized by low proton speeds
(vp~420 km s-1) and high oxygen charge state
ratios (nO7+/nO6+~0.3). This
slow wind originates from small, sheared open-field regions located
near active regions and characterized by very large flux tube expansion
factors (fexp>>10) and high footpoint field strengths
(B0~30 G). In contrast, the occasional high-speed streams
emanate from weak-field regions (B0~5 G) with small expansion
factors (fexp~4) and show relatively low charge state
ratios (nO7+/nO6+~0.1)
their proton velocities (vp~550 km s-1) are
substantially reduced by interactions with the surrounding sea of
low-speed wind. We attribute the high freeze-in temperatures of the
slow wind to enhanced heating taking place in the low corona in the
presence of the very strong, rapidly diverging source fields, which
are found to be correlated with high mass and energy flux densities
at the coronal base.
Title: Meridional Flow and the Solar Cycle Variation of the Sun's
Open Magnetic Flux
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Lean, J.
Bibcode: 2002ApJ...580.1188W
Altcode:
We simulate the evolution of the Sun's large-scale magnetic field
during solar cycle 21, including the effect of surface transport
processes and active region emergence. As an important new constraint
on the model, we have scaled our source fluxes upward to be consistent
with the average measured strength of the interplanetary magnetic
field (IMF). By adopting a poleward bulk flow of amplitude ~20-25 m
s-1 together with a supergranular diffusion rate of ~500
km2 s-1, we are then able to match the observed
variation of the Sun's polar fields and open magnetic flux. The high
meridional flow speeds, peaking at low latitudes, prevent the buildup
of an overly strong axisymmetric dipole component at sunspot minimum,
while accounting for the giant poleward surges of flux and accompanying
polar field fluctuations observed near sunspot maximum. The present
simulations also reproduce the large peak in the equatorial dipole
and IMF strength recorded in 1982.
Title: Polarity reversal of the solar magnetic field during cycle 23
Authors: Wang, Y. -M.; Sheeley, N. R.; Andrews, M. D.
Bibcode: 2002JGRA..107.1465W
Altcode:
Using magnetograph data, coronagraph observations, and source surface
extrapolations, we examine the evolution of the photospheric magnetic
field from 1996 through the 2000-2001 polarity reversal and show how
this evolution is reflected in coronal holes, coronal streamers, the
heliospheric current sheet (HCS), and the solar wind. The photospheric
polarity reversal is completed in the more active Northern Hemisphere
in late 2000 and then in the Southern Hemisphere in 2001. The polar
coronal holes disappear in 2000 and start to re-form in 2001; during
this interval, most of the open magnetic flux resides in the active
region latitudes, where small coronal holes with strong footpoint
fields generate predominantly slow solar wind. The nondipolar nature
of the large-scale coronal field at sunspot maximum gives rise to
complex streamer/HCS topologies, in which a four-sector structure and
even a secondary, detached current sheet with cylindrical geometry are
sometimes present. Comparison of the potential field extrapolations with
coronagraph and Ulysses observations suggests that the magnetograph
measurements may have underestimated the strength of the south polar
field during late 2000.
Title: The effect of increasing solar activity on the Sun's total
and open magnetic flux during multiple cycles: Implications for
solar forcing of climate
Authors: Lean, J. L.; Wang, Y. -M.; Sheeley, N. R.
Bibcode: 2002GeoRL..29.2224L
Altcode: 2002GeoRL..29x..77L
We investigate the relationship between solar irradiance and cosmogenic
isotope variations by simulating with a flux transport model the effect
of solar activity on the Sun's total and open magnetic flux. As the
total amount of magnetic flux deposited in successive cycles increases,
the polar fields build up, producing a secular increase in the open
flux that controls the interplanetary magnetic field which modulates
the cosmic ray flux that produces cosmogenic isotopes. Non-axisymmetric
fields at lower latitudes decay on time scales of less than a year; as
a result the total magnetic flux at the solar surface, which controls
the Sun's irradiance, lacks an upward trend during cycle minima. This
suggests that secular increases in cosmogenic and geomagnetic proxies
of solar activity may not necessarily imply equivalent secular trends
in solar irradiance. Questions therefore arise about the interpretation
of Sun-climate relationships, which typically assume that the proxies
imply radiative forcing.
Title: Multiple magnetic clouds in interplanetary space
Authors: Wang, Y. M.; Wang, S.; Ye, P. Z.
Bibcode: 2002SoPh..211..333W
Altcode:
An interplanetary magnetic cloud (MC) is usually considered the
byproduct of a coronal mass ejection (CME). Due to the frequent
occurrence of CMEs, multiple magnetic clouds (multi-MCs), in which one
MC catches up with another, should be a relatively common phenomenon. A
simple flux rope model is used to get the primary magnetic field
features of multi-MCs. Results indicate that the magnetic field
configuration of multi-MCs mainly depends on the magnetic field
characteristics of each member of multi-MCs. It may be entirely
different in another situation. Moreover, we fit the data from the
Wind spacecraft by using this model. Comparing the model with the
observations, we verify the existence of multi-MCs, and propose some
suggestions for further work.
Title: A statistical study on the geoeffectiveness of Earth-directed
coronal mass ejections from March 1997 to December 2000
Authors: Wang, Y. M.; Ye, P. Z.; Wang, S.; Zhou, G. P.; Wang, J. X.
Bibcode: 2002JGRA..107.1340W
Altcode:
We have identified 132 Earth-directed coronal mass ejections
(CMEs) based on the observations of the Large Angle Spectroscopic
Coronagraph (LASCO) and Extreme Ultraviolet Imaging Telescope (EIT)
on board of Solar and Heliospheric Observatory (SOHO) from March
1997 to December 2000 and carried out a statistical study on their
geoeffectiveness. The following results are obtained: (1) Only 45%
of the total 132 Earth-directed halo CMEs caused geomagnetic storms
with Kp ≥ 5; (2) The initial sites of these geoeffective halo CMEs
are rather symmetrically distributed in the heliographic latitude
of the visible solar disc, while asymmetrical in longitude with the
majority located in the west side of the central meridian; (3) The
frontside halo CMEs accompanied with solar flares (identified from
GOES-8 satellite observations) seem to be more geoeffective; (4) Only a
weak correlation between the CME projected speed and the transit time
is revealed. However, for the severe geomagnetic storms (with Kp ≥
7), a significant correlation at the confidence level of 99% is found.
Title: Characteristics of Coronal Inflows
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.
Bibcode: 2002ApJ...579..874S
Altcode:
This paper describes coronal inflows observed with the Large Angle
Spectrometric Coronagraph (LASCO). The inflows are not seen above
5.5 Rsolar, which appears to be a ``point of no return''
for the Sun's plasmas and fields. Below this height, most inflows
seem to indicate magnetic flux that is returning to the Sun after its
reconnection at sector boundaries. Some inflows have characteristics
(like fast, oppositely directed ejections of material) that are
easily interpreted in terms of conventional models of field line
reconnection. However, the overwhelming majority of coronal inflows
have a more complex behavior that typically includes the following
characteristics:1. The birth of a very weak, localized density
enhancement about 4-5 Rsolar from Sun center and its
initially slow downward motion along a coronal ray;2. Acceleration
to a maximum speed of 50-100 km s-1, and the formation
of a sinking column;3. Deceleration and the appearance of a dark
depletion tail, visible against the bright background structures in
the lower corona;4. The formation of a stretched loop below about
2.5 Rsolar.We suppose that the initial downward motion is
a preparatory stage for reconnection, which occurs in the depleted
region in the wake of the sinking column and is later revealed by the
formation of a stretched loop in the lower corona.
Title: Heliospheric magnetic field strength and polarity from 1 to
81 AU during the ascending phase of solar cycle 23
Authors: Burlaga, L. F.; Ness, N. F.; Wang, Y. -M.; Sheeley, N. R.
Bibcode: 2002JGRA..107.1410B
Altcode:
The Voyager 1 (V1) observations of the heliospheric magnetic field
strength B agree with Parker's model of the global heliospheric magnetic
field from 1 to 81.0 AU and from 1978 to 2001.34 when one considers
the solar cycle variations in the source magnetic field strength and
the latitude/time variation in the solar wind speed. In particular,
Parker's model, without adjustable parameters, describes the general
tendency for B to decrease with increasing distance R from the Sun, the
three broad increases of B around 1980, 1990, and 2000, and the minima
of B in 1987 and 1997. During 1987 and 1997, B appears to be lower than
Parker's model predicts, but that can be attributed to the presence
of a heliospheric vortex street at these times and/or uncertainty
in the observations. There is no evidence for a significant flux
deficit increasing monotonically from 1 to 81.0 AU. By extrapolating
these results and considering the limitations of the observations,
V1 should continue to make useful measurements during the next few
years at least. The magnetic field polarity in the distant heliosphere
at V1 and Voyager 2 (V2) changed during the ascending phase of solar
cycle 23. In the Northern Hemisphere, V1 observed a decrease in the
percentage of positive polarities from ≈100% during 1997 to ≈50%
during 2000. In the Southern Hemisphere, V2 observed the opposite
behavior, an increase in the percentage of positive polarities from
≈0% during 1997 to ≈50% during 2000. The variation of magnetic
polarity observed by V1 and V2 was caused by the increasing latitudinal
width of the sector zone with increasing solar activity, which in
turn was related to the increasing maximum latitudinal extent and
the decreasing minimum latitudinal extent of the footprints of the
heliospheric current sheet (HCS). There was a tendency for the speed
and proton temperature to decrease and the density to increase at V2
from 1997 (when it observed flows from polar coronal holes) to 2001
(when it observed more complex and dynamic flows).
Title: Sunspot activity and the long-term variation of the Sun's
open magnetic flux
Authors: Wang, Y. -M.; Sheeley, N. R.
Bibcode: 2002JGRA..107.1302W
Altcode:
The interplanetary magnetic field (IMF) originates in open magnetic
regions of the Sun (coronal holes), which in turn form mainly through
the emergence and dispersal of active region fields. The radial IMF
strength is proportional to the total open flux Φopen, which
can be estimated from source surface extrapolations of the measured
photospheric field, after correction for magnetograph saturation
effects. We derive the long-term variation of Φopen during
1971-2000 and discuss its relation to sunspot activity. The average
value of Φopen was ∼20-30% higher during 1976-1996 than
during 1971-1976 and 1996-2000, with major peaks occurring in 1982
and 1991. Near sunspot minimum, most of the open flux resides in the
large polar coronal holes, whereas at sunspot maximum it is rooted
in relatively small, low-latitude holes located near active regions
and characterized by strong footpoint fields; since the decrease in
the total area occupied by holes is offset by the increase in their
average field strengths, Φopen remains roughly constant
between activity minimum and maximum, unlike the total photospheric
flux Φtot. The long-term variation of Φopen
approximately follows that of the Sun's total dipole strength, with a
contribution from the magnetic quadrupole around sunspot maximum. Global
fluctuations in sunspot activity lead to increases in the equatorial
dipole strength and hence to enhancements in Φopen and
the IMF strength lasting typically ∼1 year. We employ simulations to
clarify the role of active region emergence and photospheric transport
processes in the evolution of the open flux. Representing the initial
field configuration by one or more bipolar magnetic regions (BMRs), we
calculate its subsequent evolution under the influence of differential
rotation, supergranular convection, and a poleward bulk flow. The
initial value of Φopen is determined largely by the
equatorial dipole strength, which in turn depends on the longitudinal
phase relations between the BMRs. As the surface flow carries the BMR
flux to higher latitudes, the equatorial dipole is annihilated on a
timescale of ∼1 year by the combined effect of rotational shearing and
turbulent diffusion. The remaining flux becomes concentrated around the
poles, and Φopen approaches a limiting value that depends
on the axisymmetric dipole strengths of the original BMRs. The polar
coronal holes thus represent the long-lived, axisymmetric remnant of
the active regions that emerged earlier in the cycle.
Title: Role of a Variable Meridional Flow in the Secular Evolution
of the Sun's Polar Fields and Open Flux
Authors: Wang, Y. -M.; Lean, J.; Sheeley, N. R., Jr.
Bibcode: 2002ApJ...577L..53W
Altcode:
We use a magnetic flux transport model to simulate the evolution of
the Sun's polar fields and open flux during solar cycles 13 through 22
(1888-1997). The flux emergence rates are assumed to scale according to
the observed sunspot-number amplitudes. We find that stable polarity
oscillations can be maintained if the meridional flow rate is allowed
to vary from cycle to cycle, with higher poleward speeds occurring
during the more active cycles. Our model is able to account for a
doubling of the interplanetary field strength since 1900, as deduced
by Lockwood, Stamper, & Wild from the geomagnetic aa index. We
confirm our earlier conclusion that such a doubling of the open flux
does not imply that the base level of the total photospheric flux has
increased significantly over the last century.
Title: Coronal White-Light Jets near Sunspot Maximum
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 2002ApJ...575..542W
Altcode:
During the 1996-1997 activity minimum, the Large Angle and Spectrometric
Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO)
recorded numerous jetlike ejections above the Sun's polar regions. In
a previous study, we showed that these white-light ejections were
the outward extensions of extreme-ultraviolet (EUV) jets, which in
turn originated from flaring bright points inside the polar coronal
holes. Here we investigate a number of jetlike events observed with
LASCO during the current sunspot maximum. To identify the solar surface
counterparts of these events, we again use Fe XII λ195 images obtained
by the EUV Imaging Telescope on SOHO. The white-light jets in our sample
have angular widths of ~3°-7° and velocities typically of order 600 km
s-1 they tend to be brighter and wider than the polar jets
observed near sunspot minimum and are distributed over a much greater
range of latitudes. Many of the ejections are recurrent in nature and
originate from active regions located inside or near the boundaries
of nonpolar coronal holes. We deduce that the jet-producing regions
consist of systems of closed magnetic loops partially surrounded by
open fields; perturbations in the closed fields caused them to reconnect
with the overlying open flux, releasing the trapped energy in the form
of jetlike ejections. In some events, the core of the active region
erupts, producing fast, collimated ejections with widths of up to ~15°.
Title: Simulating the Evolution of the Large-Scale Magnetic Field
over Many Solar Cycles
Authors: Lean, J. L.; Wang, Y. -M.
Bibcode: 2002AAS...200.5711L
Altcode: 2002BAAS...34..738L
Following its emergence in the sunspot latitudes, magnetic flux
is dispersed across the Sun's surface as a result of supergranular
diffusion, meridional flow and differential rotation. Much of this flux
is annihilated when regions of opposite polarity come into contact,
but there is a net transport of flux to high solar latitudes which
eventually reverses the polarity of the polar field, and continues
the cyclic dynamo action. The accumulation of magnetic flux over many
solar cycles may provide a plausible mechanism for the generation of
a long-term secular trend in the open flux that originates in polar
regions, and possibly in the closed flux as well. Open flux extends
into interplanetary space where it modulates the levels of cosmogenic
and geomagnetic proxies of long-term solar activity, which do display
long-term secular trends in the past century. Secular trends in the
Sun's irradiance, which is associated with closed, rather than open,
flux, are speculated to track these proxies. We investigate whether
the surface transport processes applicable to the present-day Sun
permit an accumulation of flux when persisting over multiple solar
cycles that have amplitudes of increasing strength. For this purpose we
employ a database of magnetic source regions acquired for solar cycle
21 and a flux transport model developed at NRL (Wang and Sheeley,
Ap. J., 375, 761, 1991). We adjust the strengths of the sources in
each of the past 10 solar cycles such that their total flux tracks
the sunspot number. We simulate the evolution of the total, closed
and open flux and investigate the accumulation of open and closed
flux, and its sensitivity to the strengths of the various transport
processes. NASA's LWS program supported this work.
Title: Observations of Core Fallback during Coronal Mass Ejections
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 2002ApJ...567.1211W
Altcode:
White-light observations made with the Large Angle and Spectrometric
Coronagraph (LASCO) during the present solar maximum have revealed a
multitude of faint, inward-moving features at heliocentric distances of
r~2-6 Rsolar. Most of these structures appear to originate
above r~3 Rsolar and may be signatures of the closing-down
of magnetic flux at the boundaries of coronal holes or in the aftermath
of coronal mass ejections (CMEs). Here, we present observations of a
different type of inflow, in which material within the bright core of
a CME collapses back toward the Sun after rising to heights of r~2.5-6
Rsolar. We have identified roughly 20 such fallback events
during 1998-2001. The core structures, which have the form of loops or
concave-outward flux ropes, ascend into the coronagraph field of view
beyond 2 Rsolar with speeds of ~100-400 km s-1
but return with speeds of only ~50-200 km s-1. The initial
deceleration rates of ~20-100 m s-2 are comparable to the
local gravitational deceleration GMsolar/r2
but continually decrease with time. The associated CMEs tend to be
impulsive but relatively slow, with the leading front moving outward
at ~250-450 km s-1 and often showing some deceleration. It
is thus not surprising that some fraction of the core material fails
to reach escape speeds, remaining bound to the Sun by gravitational
and magnetic tension forces. We suggest that the dynamical behavior
of the core may be determined in part by momentum exchanges with the
background medium, which consists of ongoing outflows of CME material,
ambient solar wind, and inflow streams. In particular, we attribute the
asymmetry of the up-down trajectories to the action of such drag forces,
whose direction changes from inward to outward as the core decelerates.
Title: The dynamical nature of the coronal streamer belt
Authors: Wang, Y. -M.
Bibcode: 2002AdSpR..30..491W
Altcode:
SOHO/LASCO observations of the white-light corona have revealed a number
of remarkable small-scale phenomena, including plasma blobs that are
ejected continually from the cusps of streamers, fine raylike structures
that pervade the streamer belt, and ubiquitous inflows during times of
high solar activity. We discuss the implications of these observations
for the origin of the heliospheric plasma sheet, the sources of the
slow solar wind, and the variation of the interplanetary magnetic
field strength.
Title: Coronal Inflows and Sector Magnetism
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.
Bibcode: 2001ApJ...562L.107S
Altcode:
We have remeasured the rate of coronal inflows during 1996-2001,
taking into account multiple occurrences per day, and have displayed the
results in Carrington stack plots to study their long-term behavior. The
stack plots show long-lived recurrence patterns related to the sectorial
component of the Sun's large-scale field. In particular, most inflows
are parts of streams that last for several months and occur where the
sectorial field has strong gradients. The occurrence rate occasionally
approaches ~1 hr-1 near sunspot maximum when the sectorial
field is strong and the streamer belt is greatly distorted from its
flattened equatorial configuration. The link between coronal inflows
and sector magnetism suggests that the inflows are by-products of
a global recycling process in which nonaxisymmetric open flux is
provided by active regions and dissipated by differential rotation,
supergranular diffusion, and meridional flow.
Title: On the Relationship between He II λ304 Prominences and the
Photospheric Magnetic Field
Authors: Wang, Y. -M.
Bibcode: 2001ApJ...560..456W
Altcode:
As observed in He II λ304 images recorded at high cadence,
quiescent prominences resemble long-lived systems of jets, in which
chromospheric material streams continually from one footpoint area
to another. To further clarify the physical nature of the source
regions, we have compared He II λ304 images of on-disk prominences
(filaments) with line-of-sight magnetograms, employing data from the
Extreme-Ultraviolet Imaging Telescope (EIT) and the Michelson Doppler
Imager (MDI) on the Solar and Heliospheric Observatory (SOHO). The
quiescent filaments lie within channels separating the opposite-polarity
network fields on each side and containing relatively weak magnetic
flux of both polarities. The sideways extensions (``barbs'') and
endpoints of the filaments overlie smaller scale neutral lines,
where opposite-polarity flux elements are in close contact and mutual
cancellation occurs. From the chirality rules of Martin et al., we
deduce that the barbs are rooted in minority-polarity flux on the
``wrong'' side of the large-scale photospheric neutral line, and
we propose a mechanism for their formation based on the concept of
supergranular diffusion. Our results support earlier suggestions that
magnetic reconnection accompanying photospheric flux cancellation is
the dominant mechanism for injecting mass into quiescent prominences.
Title: Coronal Inflows and the Sun's Nonaxisymmetric Open Flux
Authors: Sheeley, N. R., Jr.; Knudson, T. N.; Wang, Y. -M.
Bibcode: 2001ApJ...546L.131S
Altcode:
Wang et al. recently described white-light coronagraph observations
of faint coronal features moving inward toward the Sun at heliocentric
distances of 2-6 Rsolar. In a study of these inflows during
1996-2000, we have found that they occur along bends of the coronal
streamer belt and are especially common when the magnetic field has
a four-sector structure. The measured inflow rate is dominated by
episodic bursts that are correlated with the occurrence of nonpolar
coronal holes and other indicators of the Sun's nonaxisymmetric open
flux. However, the inflow rate has only a broad long-term correlation
with conventional indicators of solar activity like the sunspot
number and coronal mass ejection rate. We conclude that most inflows
indicate collapsing field lines that occur as nonpolar coronal holes
are subjected to photospheric motions and the eruptions of new flux.
Title: The dynamical nature of coronal streamers
Authors: Wang, Y. -M.; Sheeley, N. R.; Socker, D. G.; Howard, R. A.;
Rich, N. B.
Bibcode: 2000JGR...10525133W
Altcode:
Recent high-sensitivity imaging of the Sun's white-light corona from
space has revealed a variety of unexpected small-scale phenomena,
including plasma blobs that are ejected continually from the
cusplike bases of streamers, fine raylike structures pervading the
outer streamer belt, and inflows that occur mainly during times of
high solar activity. These phenomena can be interpreted as different
manifestations of magnetic field line reconnection, in which plasma
and magnetic flux are exchanged between closed and open field regions
of the corona. The observations provide new insights into a number
of long-standing questions, including the origin of the streamer
material in the outer corona, the sources of the slow solar wind, and
the mechanisms that regulate the interplanetary magnetic field strength.
Title: EIT Waves and Fast-Mode Propagation in the Solar Corona
Authors: Wang, Y. -M.
Bibcode: 2000ApJ...543L..89W
Altcode:
During the onset of coronal mass ejections, a front of enhanced EUV
emission is sometimes seen to propagate away from a flaring active
region across the solar disk. We present model simulations to test
the hypothesis that these transients (called ``EIT waves'') represent
fast-mode MHD waves. The distribution of the magnetosonic velocity
vf in the corona is determined using a current-free
extrapolation of the measured photospheric field and a density
scaling law for coronal loops. In agreement with observations, the
waves are deflected away from active regions and coronal holes, where
vf is large; they are also refracted upward as they propagate
away from their initiation point, since vf falls off rapidly
above active regions. The average surface-projected expansion speeds are
only of order 200 km s-1, comparable to or somewhat smaller
than those of EIT waves observed during 1997-1998. The model is unable
to account for the velocities in excess of 600 km s-1
associated with Moreton waves and type II radio bursts unless it
is assumed that the initial disturbance has the form of a strong,
super-Alfvénic shock.
Title: Detection of coronal mass ejection associated shock waves in
the outer corona
Authors: Sheeley, N. R.; Hakala, W. N.; Wang, Y. -M.
Bibcode: 2000JGR...105.5081S
Altcode:
White light coronal images from the Large-Angle Spectrometric
Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO)
spacecraft show disturbances propagating away from high-speed coronal
mass ejections (CMEs). The disturbances are faintly visible ahead
of the ejected material at the noses of the CMEs but are strongly
visible along the flanks and rear ends, where they produce kinks in
the streamers and other raylike features that extend in all directions
from the Sun at this phase of the sunspot cycle. The kinks decelerate
as they move radially outward along the rays, apparently indicating the
slowing of the entire wave front as it passes by. For a fast CME seen
head on (or tail on) the deceleration occurs at virtually all position
angles around the occulting disk. However, for a CME seen obliquely
the speed varies strongly with position angle, being fast and uniform
near the nose but slower and decelerating at the sides and rear where
the deflected rays are more inclined from the sky plane and farther
from the Sun. The initial speeds (~800-1400 km/s) are faster than the
nominal MHD speed (~600 km/s) at these heights, implying that these
disturbances are shock waves, made visible like ``amber waves of grain''
[Bates, 1895] in the field of coronal rays around the Sun.
Title: Understanding the evolution of the Sun's open magnetic flux
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Lean, J.
Bibcode: 2000GeoRL..27..621W
Altcode:
The large-scale magnetic field of the Sun, including the open flux that
extends into the interplanetary medium, originates in active regions
but is redistributed over the photosphere by differential rotation,
supergranular convection, and poleward meridional flow. We use
simulations to clarify the role of the surface transport processes
in the evolution of the total open flux, Φopen,
which determines the strength of the radial interplanetary field
component. Representing the initial photospheric field configuration
by one or more bipolar magnetic regions (BMRs), we show that
Φopen varies approximately as the net dipole strength,
determined by vectorially summing the dipole moments of the individual
BMRs. As meridional flow carries the BMR flux to higher latitudes,
the equatorial dipole component is annihilated on a timescale ∼1
yr by the combined effect of rotational shearing and supergranular
diffusion. The remaining flux becomes concentrated around the poles,
and Φopen approaches a limiting value that depends on
the axisymmetric dipole strength of the original active regions. We
discuss the implications of these results for the solar cycle evolution
of Φopen.
Title: The long-term variation of the Sun's open magnetic flux
Authors: Wang, Y. -M.; Lean, J.; Sheeley, N. R., Jr.
Bibcode: 2000GeoRL..27..505W
Altcode:
The interplanetary magnetic field (IMF) has its origin in open magnetic
regions of the Sun (coronal holes). The location of these regions
and their total open flux Φopen can be inferred from
current-free extrapolations of the observed photospheric field. We
derive the long-term variation of Φopen during 1971-1998
and discuss its causes. Near sunspot minimum, the open flux originates
mainly from the large polar coronal holes, whereas at sunspot maximum
it is rooted in small, lower-latitude holes characterized by very high
field strengths; the total amount of open flux thus remains roughly
constant between sunspot minimum and maximum. Through most of the
cycle, the variation of Φopen closely follows that of the
Sun's total dipole strength, showing much less dependence on the total
photospheric flux or the sunspot number. However, episodic increases in
large-scale sunspot activity lead to strengthenings of the equatorial
dipole component, and hence to enhancements in Φopen
and the IMF strength lasting typically ∼1 yr.
Title: Evolution of coronal streamer structure during the rising
phase of solar cycle 23
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Rich, N. B.
Bibcode: 2000GeoRL..27..149W
Altcode:
White-light images of the outer corona recorded with the Large Angle
Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric
Observatory (SOHO) show a rapid widening of the streamer belt during
1998 and early 1999. The observed streamer structure and its evolution
from rotation to rotation are reproduced with a model in which the
Thomson-scattering electrons are concentrated within a narrow layer
centered around the heliospheric current sheet. The latitudinal
spreading of the streamer belt is shown to be a consequence of the
increased rate of magnetic flux emergence in the sunspot latitudes,
which led to a weakening of the Sun’s axisymmetric dipole moment, to
a rapid growth in the nonaxisymmetric components of the coronal field,
and hence to a strong tilting and warping of the plasma/current sheet.
Title: Continuous tracking of coronal outflows: Two kinds of coronal
mass ejections
Authors: Sheeley, N. R.; Walters, J. H.; Wang, Y. -M.; Howard, R. A.
Bibcode: 1999JGR...10424739S
Altcode:
We have developed a new technique for tracking white-light coronal
intensity features and have used this technique to construct continuous
height/time maps of coronal ejecta as they move outward through the
2-30Rs field of view of the Large-Angle Spectrometric
Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO)
spacecraft. Displayed as gray-scale images, these height/time maps
provide continuous histories of the motions along selected radial paths
in the corona and reveal a variety of accelerating and decelerating
features, including two principal types of coronal mass ejections
(CMEs): (1) Gradual CMEs, apparently formed when prominences and their
cavities rise up from below coronal streamers: When seen broadside,
these events acquire balloon-like shapes containing central cores,
and their leading edges accelerate gradually to speeds in the range
400-600 km/s before leaving the 2-30Rs field of view. The
cores fall behind with speeds in the range 300-400 km/s. Seen along
the line of sight, these events appear as smooth halos around the
occulting disk, consistent with head-on views of optically thin bubbles
stretched out from the Sun. At the relatively larger radial distances
seen from this ``head-on'' perspective, gradually accelerating CMEs
fade out sooner and seem to reach a constant speed more quickly than
when seen broadside. Some suitably directed gradual CMEs are associated
with interplanetary shocks and geomagnetic storms. (2) Impulsive CMEs,
often associated with flares and Moreton waves on the visible disk: When
seen broadside, these CMEs move uniformly across the 2-30Rs
field of view with speeds typically in excess of 750 km/s. At the
relatively larger radial distances seen from a head-on perspective,
impulsive events tend to have a more ragged structure than the gradual
CMEs and show clear evidence of deceleration, sometimes reducing their
speeds from 1000 to 500 km/s in 1 hour. Such decelerations are too large
to represent ballistic motions in the Sun's gravitational field but
might be caused by shock waves, sweeping up material far from the Sun.
Title: The Jetlike Nature of He II λ304 Prominences
Authors: Wang, Y. -M.
Bibcode: 1999ApJ...520L..71W
Altcode:
High-cadence He II λ304 images recorded with the Extreme-ultraviolet
Imaging Telescope on the Solar and Heliospheric Observatory show
quiescent prominences as organized systems of long-lived jets. The
prominence loop arcades consist of chromospheric material that streams
continually from one end of each loop to the other; the speeds are
typically of order 30 km s-1 and tend to increase along
the length of the loop. We suggest that the jets are triggered by
magnetic reconnection during the process of flux submergence at the
photospheric neutral line.
Title: Origin, Injection, and Acceleration of CIR Particles:
Observations Report of Working Group 6
Authors: Mason, G. M.; von Steiger, R.; Decker, R. B.; Desai, M. I.;
Dwyer, J. R.; Fisk, L. A.; Gloeckler, G.; Gosling, J. T.; Hilchenbach,
M.; Kallenbach, R.; Keppler, E.; Klecker, B.; Kunow, H.; Mann, G.;
Richardson, I. G.; Sanderson, T. R.; Simnett, G. M.; Wang, Y. -M.;
Wimmer-Schweingruber, R. F.; Fränz, M.; Mazur, J. E.
Bibcode: 1999SSRv...89..327M
Altcode:
This report emphasizes new observational aspects of CIR ions revealed
by advanced instruments launched on the Ulysses, WIND, SOHO, and
ACE spacecraft, and by the unique vantage point of Ulysses which
carried out the first survey of Corotating Interaction Region (CIR)
properties over a very wide range of heliolatitudes. With this more
complete observational picture established, this review is the basis
to consider the status of theoretical models on origin, injection, and
acceleration of CIR particles reported by Scholer, Mann et al. (1999)
in this volume.
Title: The Solar Origin of Corotating Interaction Regions and Their
Formation in the Inner Heliosphere
Authors: Balogh, A.; Bothmer, V.; Crooker, N. U.; Forsyth, R. J.;
Gloeckler, G.; Hewish, A.; Hilchenbach, M.; Kallenbach, R.; Klecker,
B.; Linker, J. A.; Lucek, E.; Mann, G.; Marsch, E.; Posner, A.;
Richardson, I. G.; Schmidt, J. M.; Scholer, M.; Wang, Y. -M.;
Wimmer-Schweingruber, R. F.; Aellig, M. R.; Bochsler, P.; Hefti, S.;
Mikić, Z.
Bibcode: 1999SSRv...89..141B
Altcode:
Corotating Interaction Regions (CIRs) form as a consequence of the
compression of the solar wind at the interface between fast speed
streams and slow streams. Dynamic interaction of solar wind streams
is a general feature of the heliospheric medium; when the sources of
the solar wind streams are relatively stable, the interaction regions
form a pattern which corotates with the Sun. The regions of origin
of the high speed solar wind streams have been clearly identified
as the coronal holes with their open magnetic field structures. The
origin of the slow speed solar wind is less clear; slow streams may
well originate from a range of coronal configurations adjacent to,
or above magnetically closed structures. This article addresses
the coronal origin of the stable pattern of solar wind streams
which leads to the formation of CIRs. In particular, coronal models
based on photospheric measurements are reviewed; we also examine
the observations of kinematic and compositional solar wind features
at 1 AU, their appearance in the stream interfaces (SIs) of CIRs,
and their relationship to the structure of the solar surface and the
inner corona; finally we summarise the Helios observations in the
inner heliosphere of CIRs and their precursors to give a link between
the optical observations on their solar origin and the in-situ plasma
observations at 1 AU after their formation. The most important question
that remains to be answered concerning the solar origin of CIRs is
related to the origin and morphology of the slow solar wind.
Title: Coronagraph observations of inflows during high solar activity
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Howard, R. A.; Cyr,
O. C. St.; Simnett, G. M.
Bibcode: 1999GeoRL..26.1203W
Altcode:
Since the start of the SOHO mission three years ago, the Large Angle
Spectrometric Coronagraph (LASCO) has recorded numerous examples of
small, faint features moving inward through the corona. The inflows
are observed at heliocentric distances of 2-4 Rs and became
increasingly common during 1998, as solar and coronal mass ejection
(CME) activity increased. The inward-moving structures, which are most
easily detected in running difference movies, often have a cusplike
appearance and tend to leave a density depletion in their wake; the
downward velocities range from less than 20 km s-1 to over
100 km s-1. The downflows are observed typically ∼1 day
after the passage of a CME, and coexist side by side with continuing
outflows of streamer material. We interpret these small-scale events
as observational signatures of the gradual closing-down of magnetic
flux dragged outward by CMEs or other transient outflows.
Title: Streamer disconnection events observed with the LASCO
coronagraph
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Howard, R. A.; Rich,
N. B.; Lamy, P. L.
Bibcode: 1999GeoRL..26.1349W
Altcode:
We present Large Angle Spectrometric Coronagraph (LASCO) observations
of two events that suggest magnetic disconnection in coronal
streamers. During the 1-2 days preceding each event, successions of
narrow looptops are seen rising slowly through the 2-6 RS
field of view, forming a bright streamer stalk which continues to
elongate with time. As the streamer becomes ever more constricted, it
eventually severs at a heliocentric distance of ∼4 RS. The
lower part of the stalk collapses back to form a cusplike structure
extending to ∼3 RS, while the disconnected segment is
observed as a kink or density enhancement that propagates outward with
a speed of order 200 km s-1. We interpret these non-CME
events as transient openings and closings of magnetic flux rooted at
the boundaries of coronal holes.
Title: Potential Field Source Surface Simulations of Soft X-ray
Corona Variability During the Solar Cycle
Authors: Lean, J. L.; Wang, Y. -M.; Mariska, J. T.; Acton, L. W.
Bibcode: 1999AAS...194.9208L
Altcode: 1999BAAS...31..987L
Magnetic fields that emerge in the solar photosphere and extend upwards
into the corona are associated with coronal heating. Some studies have
determined empirically that coronal brightness depends directly on
photospheric field strength, whereas others relate the brightness to the
length of the loops or to the sheering of opposite polarity fields. We
use the potential field source surface (PFSS) model of Wang and Sheeley
(ApJ, 392, 310, 1992) to investigate the applicability of a range of
quantitative associations between photospheric magnetic fields and the
global brightness of the non-flaring soft X-ray corona, recorded in full
disk X-rays images made by the SXT on Yohkoh. The model extrapolates all
photospheric magnetic field lines, in both active regions and smaller
scale features, into the corona. For an assigned coronal temperature of
1.5E6 K, the model determines coronal density by assuming hydrostatic
equilibrium along each closed field line and using adopted scaling
laws to relate the footpoint density to the magnetic field and/or
loop length. Integrating the brightness along the line of sight then
permits direct simulation of the independently measured SXT full disk
coronal images. With the NSO Carrington magnetic field maps as input,
the PFSS simulations can account for 85 global X-ray corona during
the six years from 1992 to 1997. This agreement is achieved using
a constant coronal temperature and a function that depends on both
the absolute strength of the photospheric magnetic field footprints,
and on the inverse loop length. Despite the overall good agreement
of the simulations and observations, significant differences occur
during some Carrington rotations. Simulations that utilize inputs
from three independent ground-based observatories (NSO, WSO and MWO)
can also at times differ significantly from each other. NASA Office
of Space Science has funded this work.
Title: Solar Polar Imager
Authors: Moses, D.; Dere, K. P.; Howard, R. A.; Korendyke, C. M.;
Socker, D. G.; Wang, Y. -M.; Goldstein, B. E.; Liewer, P. E.
Bibcode: 1999AAS...194.7613M
Altcode: 1999BAAS...31..958M
Observation of the global coronal and magnetic field structure of
the Sun requires coronal imaging and magnetograms from a perspective
out of the ecliptic. While the upcoming Solar Terrestrial Relations
Observatory (STEREO) mission will provide a great advance in the
understanding of the three-dimensional structure of the corona and
interplanetary medium, the orientation of the Sun's large scale magnetic
axis of symmetry with the STEREO spacecraft separation defines the
limits of this mission. The global structure of the streamer belts,
polar coronal holes and coronal plumes all reflect the symmetry of
the large scale solar magnetic field. Observations of Coronal Mass
Ejections (CMEs) from the LASCO and EIT instruments on the Solar and
Heliospheric Observatory (SOHO) indicate the need for synoptic coronal
imaging out of the ecliptic (Solar Polar Imager), as well as in stereo
pairs in the ecliptic (STEREO), for advancement in the understanding
of the origins and consequences of CMEs. The SOHO MDI has shown the
need for observations of the evolution of the polar magnetic fields
and convection patterns to understand the generation and transport
of the solar magnetic fields. Finally, the Ulysses mission has shown
the need for polar coronal imaging and magnetograms for understanding
the source of the solar wind. Ulysses has demonstrated the need for
on-board in situ particles and fields instruments as a link to the
remote sensing observations. Lightweight and compact instrumentation for
these observations has already been demonstrated technically. An orbital
mission involving a Jupiter assist such as Ulysses is also technically
demonstrated, although the duration of the polar observations is limited
to the point of degrading the studies of solar cycle evolution. An
orbital mission involving a circularized polar orbit is possible with
the use of solar-sail propulsion, but this involves technology that
has yet to be demonstrated.
Title: Filament Eruptions near Emerging Bipoles
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1999ApJ...510L.157W
Altcode:
It has been suggested in previous studies that quiescent prominences
and filaments erupt preferentially in the vicinity of emerging
magnetic flux. We describe three such filament eruptions observed
during 1998 with the Extreme-ultraviolet Imaging Telescope on the
Solar and Heliospheric Observatory. Comparison with magnetograms and
with potential field extrapolations suggests that the new bipole
enables the eruption by diverting the flux overlying the filament
sideways or to greater heights, allowing the filament to rise out of
its channel. Because eruptions sometimes occur in the absence of any
observable flux emergence, however, we conclude that new flux may act
as a strong catalyst but is not a necessary condition for filament
destabilization.
Title: The Solar Origin of Corotating Interaction Regions and their
Formation in the Inner Heliosphere
Authors: Balogh, A.; Bothmer, V.; Crooker, N. U.; Forsyth, R. J.;
Gloeckler, G.; Hewish, A.; Hilchenbach, M.; Kallenbach, R.; Klecker,
B.; Linker, J. A.; Lucek, E.; Mann, G.; Marsch, E.; Posner, A.;
Richardson, I. G.; Schmidt, J. M.; Scholer, M.; Wang, Y. -M.;
Wimmer-Schweingruber, R. F.; Aellig, M. R.; Bochsler, P.; Hefti, S.;
Mikić, Z.
Bibcode: 1999cir..book..141B
Altcode:
No abstract at ADS
Title: Origin, Injection, and Acceleration of CIR Particles:
Observations
Authors: Mason, G. M.; Von Steiger, R.; Decker, R. B.; Desai, M. I.;
Dwyer, J. R.; Fisk, L. A.; Gloeckler, G.; Gosling, J. T.; Hilchenbach,
M.; Kallenbach, R.; Keppler, E.; Klecker, B.; Kunow, H.; Mann, G.;
Richardson, I. G.; Sanderson, T. R.; Simnett, G. M.; Wang, Y. -M.;
Wimmer-Schweingruber, R. F.; Fränz, M.; Mazur, J. E.
Bibcode: 1999cir..book..327M
Altcode:
No abstract at ADS
Title: Observations of Correlated White-Light and Extreme-Ultraviolet
Jets from Polar Coronal Holes
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Socker, D. G.; Howard,
R. A.; Brueckner, G. E.; Michels, D. J.; Moses, D.; St. Cyr, O. C.;
Llebaria, A.; Delaboudinière, J. -P.
Bibcode: 1998ApJ...508..899W
Altcode:
Time-lapse sequences of white-light images recorded with the Large
Angle Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric
Observatory (SOHO) frequently show long, narrow structures moving
outward over the Sun's polar regions at high apparent speeds. By
comparing the LASCO observations with Fe XII λ195 spectroheliograms
made with the Extreme-ultraviolet Imaging Telescope (EIT) on SOHO
between 1997 April and 1998 February, we have identified 27 correlated
white-light and extreme-ultraviolet (EUV) jet events. In each case,
the EUV jet was observed near the limb of the polar coronal hole 20-60
minutes before the corresponding white-light jet was registered in the
coronagraph's 2-6 R⊙ field of view. The jets originate
near flaring EUV bright points and are presumably triggered by field
line reconnection between magnetic bipoles and neighboring unipolar
flux. The leading edges of the white-light jets propagate outward at
speeds of 400-1100 km s-1, whereas the bulk of their material
travels at much lower velocities averaging around 250 km s-1
at heliocentric distances of 2.9-3.7 R⊙. These lower
velocities may reflect the actual outflow speeds of the background
polar wind.
Title: Heliospheric magnetic field strength out to 66 AU: Voyager
1, 1978-1996
Authors: Burlaga, L. F.; Ness, N. F.; Wang, Y. -M.; Sheeley, N. R.
Bibcode: 1998JGR...10323727B
Altcode:
We discuss Voyager 1 (V1) observations of the heliospheric magnetic
field strength from 1978 through 1996. During this period the distance
of V1 from the Sun increased from ~3 AU to 66 AU and its heliographic
latitude increased from ~5°S to 33°N. The magnetic field strength
profile observed by V1 is consistent with Parker's spiral field model
when one considers (1) the solar cycle variation of the observed
magnetic field strength at 1 AU, B1(t) (which is a measure
of the source field strength) and (2) the latitudinal and solar cycle
variations of the solar wind speed, V(t,θ). Both B1(t) and
V(t,θ) make significant contributions to the variation of the magnetic
field strength variations observed by V1. There is no evidence for a
``magnetic flux deficit'' increasing with distance from the Sun. There
is a solar cycle variation of the magnetic field strength in the outer
heliosphere, which will affect the modulation of cosmic rays.
Title: Spatial structure of the solar wind and comparisons with
solar data and models
Authors: Neugebauer, M.; Forsyth, R. J.; Galvin, A. B.; Harvey,
K. L.; Hoeksema, J. T.; Lazarus, A. J.; Lepping, R. P.; Linker,
J. A.; Mikic, Z.; Steinberg, J. T.; von Steiger, R.; Wang, Y. -M.;
Wimmer-Schweingruber, R. F.
Bibcode: 1998JGR...10314587N
Altcode:
Data obtained by instruments on the Ulysses spacecraft during its rapid
sweep through >90° of solar latitude, crossing the solar equator
in early 1995, were combined with data obtained near Earth by the
Wind spacecraft to study the spatial structure of the solar wind and
to compare to different models of the interplanetary magnetic field
derived from solar observations. Several different source-surface
models matched the double sinusoidal structure of the heliospheric
current sheet (HCS) but with differences in latitude as great as
21°. The source-surface model that included an interplanetary
current sheet gave poorer agreement with observed current-sheet
crossings during this period than did the other source-surface models
or an MHD model. The differences between the calculated and observed
locations of the HCS were minimized when 22° of solar rotation was
added to the constant-velocity travel time from the source surface to
the spacecraft. The photospheric footpoints of the open field lines
calculated from the models generally agreed with observations in the
He 10,830 Å line of the locations of coronal holes with the exceptions
that (1) in some places, open field lines originated outside the coronal
hole boundaries and (2) the models show apparently closed-field regions
just inside some coronal hole boundaries. The patterns of mismatches
between coronal hole boundaries and the envelopes of open field lines
persisted over at least three solar rotations. The highest-speed wind
came from the polar coronal holes, with the wind originating deeper
within the hole being faster than the wind coming from near the
hole boundary. Intermediate and slow streams originated in smaller
coronal holes at low latitudes and from open field regions just
outside coronal hole boundaries. Although the HCS threaded regions
of low speed, low helium abundance, high ionization temperature,
and a high ratio of magnesium to oxygen densities (a surplus of an
element with low first-ionization potential), there was a great deal
of variation in these parameters from one place to another along
the HCS. The gradient of speed with latitude varied from 14 to 28
kms-1deg-1.
Title: Large-scale coronal heating by the small-scale magnetic field
of the Sun
Authors: Schrijver, C. J.; Title, A. M.; Harvey, K. L.; Sheeley,
N. R.; Wang, Y. -M.; van den Oord, G. H. J.; Shine, R. A.; Tarbell,
T. D.; Hurlburt, N. E.
Bibcode: 1998Natur.394..152S
Altcode:
Magnetic fields play a crucial role in heating the outer atmospheres
of the Sun and Sun-like stars, but the mechanisms by which magnetic
energy in the photosphere is converted to thermal energy in the corona
remain unclear. Observations show that magnetic fields emerge onto
the solar surface as bipolar regions with a broad range of length
scales. On large scales, the bipolar regions survive for months before
dispersing diffusively. On the smaller scales, individual bipolar
regions disappear within days but are continuously replenished by new
small flux concentrations, resulting in a sustained state of mixed
polarity. Here we determine the rate of emergence of these small
bipolar regions and we argue that the frequent magnetic reconnections
associated with these regions (an unavoidable consequence of continued
flux replacement) will heat the solar atmosphere. The model that
describes the details of these mixed-polarity regions is complementary
to the traditional diffusion model for large-scale flux dispersal and
a combination of the two should lead to a more complete understanding
of the role of magnetic fields in stellar atmospheres.
Title: Network Activity and the Evaporative Formation of Polar Plumes
Authors: Wang, Y. -M.
Bibcode: 1998ApJ...501L.145W
Altcode:
It has previously been suggested that polar plumes are generated and
maintained by strong, localized heating within coronal holes and that
this heating is somehow associated with the interaction between small
magnetic bipoles and nearby unipolar flux concentrations. These ideas
are used here as the basis of a model for the formation and evolution
of coronal plumes. The energy deposition rate in a plume is taken to
be proportional to the rate at which magnetic flux becomes reconnected
to the nearby monopole as the poles of the bipole are separated by
the supergranular flow field. The heat input is conducted down to the
base of the newly opened flux tube (located at the majority-polarity
end of the bipole), where a small fraction of the energy goes
into lifting chromospheric gas into the corona, while the rest is
radiated away. The evaporation timescale (τevap~6 hr)
represents the characteristic time for a new plume to form following
the emergence of a bipole. Once flux exchange between the bipole
and the unipolar flux concentration ceases, the plume decays on the
radiative cooling timescale τcool~4 hr. In the absence
of new bipole eruptions, the total lifetime of a plume is found to be
12-24 hr. The model accounts for the quiescent nature of polar plumes
and the tendency for the diffuse plume emission to strengthen as the
underlying bright network features disperse and decay.
Title: Origin of Streamer Material in the Outer Corona
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Walters, J. H.; Brueckner,
G. E.; Howard, R. A.; Michels, D. J.; Lamy, P. L.; Schwenn, R.;
Simnett, G. M.
Bibcode: 1998ApJ...498L.165W
Altcode:
We investigate the nature and origin of the outward-moving density
inhomogeneities (``blobs'') detected previously with the Large Angle and
Spectrometric Coronagraph on the Solar and Heliospheric Observatory. The
blobs are concentrated around the thin plasma layer that surrounds
the heliospheric current sheet and that constitutes the outer streamer
belt; they represent only a small, fluctuating component of the total
density within the plasma sheet. As noted before in Sheeley et al.,
blobs are characterized by low speeds and are continually emitted
from the elongated tips of helmet streamers at 3-4 Rsolar
from Sun center. We suggest that both the blobs and the plasma sheet
itself represent closed-field material injected into the solar wind as
a result of footpoint exchanges between the stretched helmet-streamer
loops and neighboring open field lines. The plasma sheet is thus
threaded by newly reconnected, open magnetic field lines, which lend
the white-light streamer belt its filamentary appearance. Since in
situ observations at 1 AU show that the slow wind (with speeds below
500 km s-1) spreads over an angular extent much greater
than the <~3° width of the plasma sheet, we deduce that a major
component of this wind must originate outside the helmet streamers
(i.e., from just inside coronal holes).
Title: Cyclic Magnetic Variations of the Sun
Authors: Wang, Y. -M.
Bibcode: 1998ASPC..154..131W
Altcode: 1998csss...10..131W
Magnetograph observations now provide a comprehensive picture of the
Sun's large-scale magnetic field and its evolution over the last two
sunspot cycles. The formation and reversal of the polar fields can
be understood in terms of a flux transport model that includes the
effect of a 10-20 m s^{-1} poleward surface flow. The evolution and
rotational behavior of photospheric field patterns, of large-scale
coronal structures such as coronal holes, and of solar wind streams
are discussed and given simple physical interpretations.
Title: Experimental constraints on pulsed and steady state models
of the solar wind near the Sun
Authors: Feldman, W. C.; Habbal, S. R.; Hoogeveen, G.; Wang, Y. -M.
Bibcode: 1997JGR...10226905F
Altcode:
Ulysses observations of the high-latitude solar wind were combined with
Spartan 201 observations of the corona to investigate the nature and
extent of uncertainties in our knowledge of solar wind structure near
the Sun. In addition to uncertainties stemming from the propagation of
errors in density profiles inferred from coronagraph observations [see,
e.g., Lallement et al., 1986], an assessment of the consequences of
choosing different analysis assumptions reveals very large, fundamental
uncertainties in our knowledge of even the basics of coronal structure
near the Sun. In the spirit of demonstrating the nature and extent
of these uncertainties we develop just one of a generic class of
explicitly time-dependent and filamentary models of the corona that is
consistent with the Ulysses and Spartan 201 data. This model provides a
natural explanation for the radial profiles of both the axial ratios
and apparent radial speeds of density irregularities measured at
radial distances less than 10RS using the interplanetary
scintillation technique.
Title: The high-latitude solar wind near sunspot Maximum
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1997GeoRL..24.3141W
Altcode:
We use an empirical relation between solar wind speed and coronal
flux-tube expansion to predict what Ulysses might have seen had it
flown over the solar poles during 1989-1991 instead of 1994-1996. The
wind speed patterns, derived from solar magnetograph data, show the
following characteristics: (1) high-speed streams having recurrence
rates of 28-29 days and originating from midlatitude extensions of the
polar coronal holes dominate the rising phase of the sunspot cycle
(1987-1989) (2) the persistent high-speed polar wind disappears and
low-speed wind is found at all latitudes during 1989-1990 (3) very fast,
episodic “polar jets” are generated as active region fields surge
to the poles at the time of polar field reversal (1990-1991). The wind
speed patterns that Ulysses encounters during its second polar orbit
are expected to show the same general characteristics.
Title: Solar Wind Stream Interactions and the Wind Speed-Expansion
Factor Relationship.
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Phillips, J. L.; Goldstein,
B. E.
Bibcode: 1997ApJ...488L..51W
Altcode:
Previous studies have shown that the solar wind speed observed near
Earth is inversely correlated with the divergence rate of magnetic flux
tubes near the Sun. We test the global validity of this relationship
by employing Ulysses wind speed measurements during 1990-1997 as well
as inecliptic data for 1976-1997. When the correspondence between wind
speeds and expansion factors is adjusted to yield optimal agreement with
the high-latitude Ulysses measurements, the model matches the overall
patterns of fast and slow wind near the ecliptic but predicts too much
very fast wind there. We show how this discrepancy can be resolved
by taking account of wind stream interactions, where we apply a crude
algorithm based on the propagation times of neighboring wind parcels;
the interactions reduce the amount of very fast wind at latitudes
where slow wind is present. We also test and reject an alternative
model in which the wind speed is assumed to be a function only of
angular distance from the heliospheric neutral sheet.
Title: On ``Torqueless'' Accretion from a Magnetically Truncated Disk
Authors: Wang, Y. -M.
Bibcode: 1997ApJ...487L..85W
Altcode:
In a number of recent studies, it has been suggested that a star may
accrete matter from a magnetically truncated Keplerian disk without
experiencing any significant spin-up torque. Thus, it is asserted
that the toroidal field component at the stellar surface is too small
to transmit angular momentum from the disk to the star; instead, the
material angular momentum at the inner edge of the disk is transported
back outward by magnetic stresses acting on the disk. We assess the
arguments for ``torqueless'' accretion and conclude that they are
physically unfounded.
Title: Origin and Evolution of Coronal Streamer Structure During
the 1996 Minimum Activity Phase
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Howard, R. A.; Kraemer,
J. R.; Rich, N. B.; Andrews, M. D.; Brueckner, G. E.; Dere, K. P.;
Koomen, M. J.; Korendyke, C. M.; Michels, D. J.; Moses, J. D.;
Paswaters, S. E.; Socker, D. G.; Wang, D.; Lamy, P. L.; Llebaria,
A.; Vibert, D.; Schwenn, R.; Simnett, G. M.
Bibcode: 1997ApJ...485..875W
Altcode:
We employ coronal extrapolations of solar magnetograph data to interpret
observations of the white-light streamer structure made with the LASCO
coronagraph in 1996. The topological appearance of the streamer belt
during the present minimum activity phase is well described by a model
in which the Thomson-scattering electrons are concentrated around a
single, warped current sheet encircling the Sun. Projection effects
give rise to bright, jet-like structures or spikes whenever the current
sheet is viewed edge-on multiple spikes are seen if the current sheet is
sufficiently wavy. The extreme narrowness of these features in polarized
images indicates that the scattering layer is at most a few degrees
wide. We model the evolution of the streamer belt from 1996 April to
1996 September and show that the effect of photospheric activity on
the streamer belt topology depends not just on the strength of the
erupted magnetic flux, but also on its longitudinal phase relative
to the background field. Using flux transport simulations, we also
demonstrate how the streamer belt would evolve during a prolonged
absence of activity.
Title: The Green Line Corona and Its Relation to the Photospheric
Magnetic Field
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Hawley, S. H.; Kraemer,
J. R.; Brueckner, G. E.; Howard, R. A.; Korendyke, C. M.; Michels,
D. J.; Moulton, N. E.; Socker, D. G.; Schwenn, R.
Bibcode: 1997ApJ...485..419W
Altcode:
Images of the green line corona made with the LASCO C1 coronagraph
on SOHO are analyzed by applying current-free extrapolations to the
observed photospheric field. The Fe XIV λ5303 emission is shown to
be closely related to the underlying photospheric field strength. By
modeling the observed intensity patterns as a function of latitude and
height above the solar limb, we derive an approximate scaling law of the
form nfoot ~ <Bfoot>0.9, where
nfoot is the density of the green line-emitting plasma and
<Bfoot> is the average field strength at the footprints
of the coronal loop. The observed high-latitude enhancements in the
green line corona are attributed to the poleward concentration of the
large-scale photospheric field. The strongest such enhancements occur
where the high-latitude unipolar fields become reconnected to active
region flux at lower latitudes; the global emission pattern rotates
quasi-rigidly at the rate of the dominant active region complex. The
validity of the current-free approximation is assessed by comparing
the topology of the observed and simulated green line structures.
Title: Measurements of Flow Speeds in the Corona Between 2 and 30
R⊙
Authors: Sheeley, N. R.; Wang, Y. -M.; Hawley, S. H.; Brueckner,
G. E.; Dere, K. P.; Howard, R. A.; Koomen, M. J.; Korendyke, C. M.;
Michels, D. J.; Paswaters, S. E.; Socker, D. G.; St. Cyr, O. C.;
Wang, D.; Lamy, P. L.; Llebaria, A.; Schwenn, R.; Simnett, G. M.;
Plunkett, S.; Biesecker, D. A.
Bibcode: 1997ApJ...484..472S
Altcode:
Time-lapse sequences of white-light images, obtained during sunspot
minimum conditions in 1996 by the Large Angle Spectrometric Coronagraph
on the Solar and Heliospheric Observatory, give the impression of
a continuous outflow of material in the streamer belt, as if we
were observing Thomson scattering from inhomogeneities in the solar
wind. Pursuing this idea, we have tracked the birth and outflow of
50-100 of the most prominent moving coronal features and find that:
1. They originate about 3-4 R⊙ from Sun center as
radially elongated structures above the cusps of helmet streamers. Their
initial sizes are about 1 R⊙ in the radial direction and
0.1 R⊙ in the transverse direction. 2. They move
radially outward, maintaining constant angular spans and increasing
their lengths in rough accord with their speeds, which typically
double from 150 km s-1 near 5 R⊙ to 300 km
s-1 near 25 R⊙. 3. Their individual speed
profiles v(r) cluster around a nearly parabolic path characterized
by a constant acceleration of about 4 m s-2 through most
of the 30 R⊙ field of view. This profile is consistent
with an isothermal solar wind expansion at a temperature of about
1.1 MK and a sonic point near 5 R⊙. Based on their
relatively small initial sizes, low intensities, radial motions, slow
but increasing speeds, and location in the streamer belt, we conclude
that these moving features are passively tracing the outflow of the
slow solar wind.
Title: Association of Extreme-Ultraviolet Imaging Telescope (EIT)
Polar Plumes with Mixed-Polarity Magnetic Network
Authors: Wang, Y. -M.; Sheeley, N. R.; Dere, K. P.; Duffin, R. T.;
Howard, R. A.; Michels, D. J.; Moses, J. D.; Harvey, J. W.; Branston,
D. D.; Delaboudinière, J. -P.; Artzner, G. E.; Hochedez, J. F.;
Defise, J. M.; Catura, R. C.; Lemen, J. R.; Gurman, J. B.; Neupert,
W. M.; Newmark, J.; Thompson, B.; Maucherat, A.; Clette, F.
Bibcode: 1997ApJ...484L..75W
Altcode:
SOHO EIT spectroheliograms showing the polar coronal holes during the
present sunspot minimum are compared with National Solar Observatory
(Kitt Peak) magnetograms taken in Fe I λ8688 and Ca II λ8542. The
chromospheric λ8542 magnetograms, obtained on a routine, near-daily
basis since 1996 June, reveal the Sun's strong polar fields with
remarkable clarity. We find that the Fe IX λ171 polar plumes occur
where minority-polarity flux is in contact with flux of the dominant
polarity inside each polar hole. Moreover, the locations of ``plume
haze'' coincide approximately with the patterns of brightened He
II λ304 network within the coronal hole. The observations appear
to be consistent with mechanisms of plume formation involving
magnetic reconnection between unipolar flux concentrations and nearby
bipoles. The fact that minority-polarity fields constitute only a small
fraction of the total magnetic flux within the polar holes suggests
that plumes are not the main source of the high-speed polar wind.
Title: Torque Exerted on an Oblique Rotator by a Magnetically Threaded
Accretion Disk
Authors: Wang, Y. -M.
Bibcode: 1997ApJ...475L.135W
Altcode:
Simple analytical expressions are derived for the torque exerted by
an accretion disk on a star whose magnetic dipole axis is inclined to
the rotation axis. Spin-down stresses are transmitted to the star by
the field lines that penetrate the disk beyond the corotation radius
Rc. As the dipole inclination angle χ increases, the
vertical magnetic flux through the disk decreases, and the spin-down
contribution to the torque weakens. For inclinations exceeding some
limiting value χc in the range of approximately 54°-67°,
the braking component is unable to offset the spin-up resulting from the
accretion of matter, even when the inner radius of the Keplerian disk,
R0, is located very close to Rc, so that the
fastness parameter ω ≡ (R0/Rc)3/2
approaches unity. Thus, for large tilt angles, there can be no steady
equilibrium state in which the net torque on the accreting star
vanishes, unless some of the material is simultaneously expelled by
centrifugal forces or accumulates within the disk.
Title: Near-Sun Magnetic Fields and the Solar Wind
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.; Phillips, J. L.
Bibcode: 1997cwh..conf..459S
Altcode: 2006mslp.conf..459S
No abstract at ADS
Title: ULYSSES plasma parameters: latitudinal, radial, and temporal
variations.
Authors: Goldstein, B. E.; Neugebauer, M.; Phillips, J. L.; Bame, S.;
Gosling, J. T.; McComas, D.; Wang, Y. -M.; Sheeley, N. R.; Suess, S. T.
Bibcode: 1996A&A...316..296G
Altcode:
Observations by the Ulysses SWOOPS plasma experiment are used
to investigate spatial and temporal gradients during the mission,
with emphasis on more recent high latitude observations including the
recent South Pole to North Pole passage during solar minimum. Compared
to lower latitudes, the high latitude solar wind had higher average
speed, proton temperature, and momentum flux, and lower number flux
density. As the average momentum flux observed in the high speed wind
was 21% greater than at the equator, during solar minimum the distance
to the heliopause will be comparatively less in the solar equatorial
plane than over the poles. The long term temporal gradients of momentum
flux over the life of the mission are considerably larger than the
latitudinal gradient observed by Ulysses during solar minimum. A
modest North-South high latitude asymmetry is observed in the plasma
parameters; the velocity is on the average 13km/s to 24km/s greater at
Northern latitudes than at Southern, and temperature is also higher. The
North-South temperature asymmetry is greater than can be explained by
the North-South velocity difference and the dependence of solar wind
temperature upon speed. The power law dependence of temperature on
heliocentric distance, r, at high latitudes is in range r^-0.81^ to
r^-1.03^, where r^-0.81^ is the Southern latitude result and r^-1.03^
the Northern. The parameter T/n^1/2^, where T is temperature and n
is proton number density, can be better predicted from speed than can
temperature alone. Comparison with calculations based on source models
and magnetograph data indicate that the expansion of open coronal
field lines close to the Sun was greater in the Southern hemisphere
than in the Northern; this anticorrelation with the expansion factor
is consistent with previous observational and theoretical work.
Title: Constraints on high-speed solar wind structure near its
coronal base: a ULYSSES perspective.
Authors: Feldman, W. C.; Barraclough, B. L.; Phillips, J. L.; Wang,
Y. -M.
Bibcode: 1996A&A...316..355F
Altcode:
Ulysses plasma data at high heliographic latitudes were studied to
develop constraints on the structure of the corona at the base of the
high-speed solar wind. Salient features of the flow poleward of +/-60°
revealed: 1) low variances of all bulk flow parameters, 2) parameter
values that agree with those measured during high-speed conditions
in the ecliptic plane when all are scaled to 1AU, 3) the continuous
presence of two interpenetrating proton streams that are not resolved
in velocity space, 4) a single alpha-particle beam that travels at a
speed that is close to the local Alfven speed faster than the primary
proton beam, 5) a proton temperature that is a factor of 2.4 times that
of the electrons, and 6) a constant helium abundance that averages 4.4%,
about half that inferred from helioseismic data in the solar convection
zone. These data are combined with a host of other remote-sensing
solar data and solar wind data to develop support for a model of a
well-mixed solar atmosphere that is driven by reconnection-generated
plasma-jet transients. In this model, acceleration of the solar wind
to its terminal speed is complete within a heliocentric distance of
about 5R_s_.
Title: Location of the Inner Radius of a Magnetically Threaded
Accretion Disk
Authors: Wang, Y. -M.
Bibcode: 1996ApJ...465L.111W
Altcode:
In models for disk accretion onto magnetized objects, the inner radius
R0 of the Keplerian disk is conventionally expressed in the form R0 =
xi rA, where rA is the Alfven radius for spherical accretion and the
parameter xi is usually taken to be 0.5. We point out that the value of
xi in fact depends on the fraction of the star's magnetic flux threading
the disk, which is poorly known from theory: in general, xi ~= 1.35 eta
4/7, where eta <= 1 denotes the threading coefficient. Application of
the beat frequency model to binary X-ray pulsars showing quasi-periodic
oscillations suggests strongly that eta ~ 1 for these objects: the
stellar dipole field essentially fully threads the disk and xi ~=
1. When combined with improved accretion torque models characterized
by critical fastness parameters near unity, the corrected values of xi
≡ R0/rA should allow more reliable determinations of the accreting
star's dipole moment and mass-radius relationship.
Title: Element Separation by Upward Proton Drag in the Chromosphere
Authors: Wang, Y. -M.
Bibcode: 1996ApJ...464L..91W
Altcode:
The extremely close collisional coupling between protons and heavy
ions in the upper chromosphere suggests that proton drag is the main
agent for ion-neutral separation there. We argue that a small upward
drift of protons and electrons relative to the stationary neutral
hydrogen component can explain the observed enrichment of elements
with low first ionization potential (FIP) in the corona. The resulting
abundances are determined by the ionization fractions of the different
elements relative to that of hydrogen. We suggest that the required
ambipolar flow may be induced by transient coronal heating leading
to chromospheric evaporation. The model predicts that the FIP effect
should be weak inside coronal holes, where most of the energy released
in the corona is carried outward by the solar wind rather than being
conducted downward as in closed magnetic regions and coronal plumes.
Title: Nonradial Coronal Streamers
Authors: Wang, Y. -M.
Bibcode: 1996ApJ...456L.119W
Altcode:
The appearance of white light streamers in the outer solar corona is
simulated by applying simple extrapolation models to the photospheric
magnetic field and taking line-of-sight electron scattering into
account. Highly nonradial streamers are obtained when the electrical
currents are confined to thin sheets and the photospheric field contains
a strong quadrupole (or higher order multipole) component, in addition
to the normally dominant dipole. The finite angular resolution of
the simulations gives rise to faint, multiple raylike structures or
striations, which may mimic the effect of density inhomogeneities in
the heliospheric current sheet.
Title: Coronal Plumes and Their Relationship to Network Activity
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1995ApJ...452..457W
Altcode:
Using Skylab extreme-ultraviolet spectroheliograms, we address the
question of what lies under a coronal plume. Plumes and their base
areas, both inside polar coronal holes and within lower latitude holes
near central meridian, are identified in the Mg IX λ368 emission
line. While some (usually spike-shaped) plumes show a strongly enhanced
Mg IX core, others (sheetlike in appearance) are characterized by a
much more diffuse base which may extend over several supergranules. The
base areas are found to contain collections of compact (although not
always intense) Ne VII λ465 features, whose locations in turn generally
coincide with enhancements in the He II λ304 network inside the coronal
hole. Bright plumes always show intense network features within their
base areas, but the converse does not hold: not every Ne VII or He II
bright point has an associated Mg IX plume. By comparing the locations
of plumelike Mg IX "haze" in a lower latitude hole with a simultaneous
high-resolution magnetogram, we infer that coronal plumes occur near
regions of mixed magnetic polarity. We suggest a mechanism for plume
formation, whereby small bipoles within a coronal hole reconnect with
unipolar flux concentrations located at network junctions.
Title: Empirical Relationship between the Magnetic Field and the
Mass and Energy Flux in the Source Regions of the Solar Wind
Authors: Wang, Y. -M.
Bibcode: 1995ApJ...449L.157W
Altcode:
Using daily averages of the solar wind speed and mass density measured
at Earth together with an improved method for extrapolating the observed
photospheric field to 1 AU, we construct scatter plots relating the
coronal field strength B0 to the mass flux density rho 0v0 and the total
energy flux density Fw0 at the coronal base. On average, both rho 0v0
and Fw0 increase roughly linearly with B0; they also show a monotonic
increase with the coronal flux-tube expansion factor. However, the wind
speed at Earth, determined by the ratio Fw0/( rho 0v0), is essentially
independent of B0, while tending to decrease with increasing expansion
factor (as found in earlier studies).
Title: On the Torque Exerted by a Magnetically Threaded Accretion Disk
Authors: Wang, Y. -M.
Bibcode: 1995ApJ...449L.153W
Altcode:
Simple expressions are derived for the torque exerted by an accretion
disk on a rotating, magnetized object (which may, for example, be a
neutron star, a white dwarf, or a T Tauri star). In the equilibrium
state (for which there is no net torque on the star), the inner edge
of the Keplerian disk R0 is located very close to the corotation
radius Rc, for physically plausible assumptions about the dissipation
time of the wound-up field component; the equilibrium value omega
crit of the "fastness parameter" omega ≡ (R0/Rc)3/2 lies in the
range 0.875--0.95. Empirical constraints requiring omega crit to
be significantly less than unity are thus incompatible with the
magnetically threaded disk model.
Title: Coronal flux-tube expansion and the polar wind
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Phillips, J. L.
Bibcode: 1995AdSpR..16i.365W
Altcode: 1995AdSpR..16..365W
Empirical and theoretical studies indicate an inverse correlation
between the areal expansion rate of magnetic flux tubes near the
Sun and the solar wind speed far from the sun. This relationship is
combined with solar magnetograph measurements to predict the wind
speed structures at high latitudes, and the results are compared with
observations now being carried out by Ulysses. Based on the evolution
of the polar fields during previous sunspot cycles, we also discuss
how the high-latitude wind is likely to evolve between 1994 and the
next solar maximum. Our main predictions are as follows: (1) As the
cycle declines, the fastest wind streams are expected to be centered
at mid-latitudes (above the polar-hole extensions), not at the poles
themselves. (2) The fastest wind at the poles is predicted to occur
not at sunspot minimum, when the polar fields are strongest and large
axisymmetric polar coronal holes are present, but just after sunspot
maximum, when the polar fields undergo their polarity reversal.
Title: Solar Implications of ULYSSES Interplanetary Field Measurements
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1995ApJ...447L.143W
Altcode:
Recent observations by the Ulysses magnetometer team have shown that the
strength of the radial interplanetary field component, |Br| ,
is essentially independent of latitude, a result which implies that the
heliospheric currents are confined entirely to thin sheets. Using such
a current sheet model, we extrapolate the observed photospheric field to
1 AU and compare the predicted magnitude and sign of Br with spacecraft
measurements during 1970--1993. Approximate agreement can be obtained
if the solar magnetograph measurements in the Fe I lambda 5250 line are
scaled upward by a latitude-dependent factor, similar to that derived
by Ulrich from a study of magnetic saturation effects. The correction
factor implies sharply peaked polar fields near sunspot minimum,
with each polar coronal hole having a mean field strength of 10 G.
Title: Comparing ULYSSES wind speed with coronal flux-tube expansion
factor
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.; Phillips, J. L.; Bame,
S. J.; Goldstein, B. E.
Bibcode: 1995sowi.confR..63S
Altcode:
We have been comparing measurements of solar wind speed at the
Ulysses spacecraft with coronal flux-tube expansion rates, derived from
photospheric field measurements using a current-free coronal model. The
large-scale patterns of derived speed have continued to reproduce the
observed patterns from launch through south polar passage to the present
40S latitude of the spacecraft. The fastest non-transient wind speeds
of approx. 860 km/s were encountered at midlatitudes en route to the
south pole, rather than during polar passage when the peak speeds were
approx. 820 km/s. Although this result is in qualitative agreement with
the idea that the wind speed is controlled by the coronal flux-tube
expansion rate, the 40 km/s difference is significantly smaller than
the 100-150 km/s difference based on our in-ecliptic calibration. This
paper will summarize our attempts to resolve this discrepancy and will
show the observational status of our coronal/interplanetary comparison
at the time of the meeting.
Title: Identification of Low-Latitude Coronal Plumes in
Extreme-Ultraviolet Spectroheliograms
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1995ApJ...446L..51W
Altcode:
Using Skylab extreme ultraviolet images of the solar disk, we have
identified plumelike features inside low-latitude coronal holes
undergoing limb passage. Like their polar counterparts, these diffuse
Mg IX structures are located above enhancements in the weak neon and
helium background emission within the coronal hole. We conclude that
coronal plumes are not unique to the polar regions but may occur in
open magnetic regions at any latitude.
Title: Source regions of the solar wind
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1995sowi.conf...31W
Altcode:
Using Skylab XUV data, we examine some properties of the source regions
of the solar wind. In particular, we discuss the physical nature of
polar plumes and their relationship to the polar wind, the nature of
the source regions of the slow solar wind, and the relationship between
abundance anomalies (the FIP effect) determined from the Skylab data
and the sources of fast and slow wind.
Title: Latitude and Solar-Cycle Dependence of Radial IMF Intensity
Authors: Wang, Y. -M.
Bibcode: 1995SSRv...72..193W
Altcode:
I describe a simple procedure for extrapolating the observed solar
magnetic field into the heliosphere, which averages the asymptotic
fields computed using the standard “source surface” and “current
sheet” models. The resultant field is characterized by strong
latitudinal gradients (maintained by volume currents outside the
source surface) and by abrupt reversals in direction at the current
sheets. The model yields good agreement with the observed long-term
variation of the radial IMF component in the ecliptic, and is used
to predict the variation of |B r | along the latitudinal
trajectory of Ulysses during 1990 1994. As found in earlier studies,
the magnitude ofB r at any latitude is determined largely
by the strength and relative orientation of the Sun's dipole moment.
Title: Latitude and Solar-Cycle Dependence of Radial IMF Intensity
Authors: Wang, Y. -M.
Bibcode: 1995hlh..conf..193W
Altcode:
No abstract at ADS
Title: Two Types of Slow Solar Wind
Authors: Wang, Y. -M.
Bibcode: 1994ApJ...437L..67W
Altcode:
Slow solar wind is associated with rapidly diverging magnetic field
occurring (1) at the boundaries of the large polar holes and (2) above
small coronal holes. Coronal energy balance models are developed for
these two types of sources. We find that the 'reconvergence' of flux
tubes at the polar hole boundaries can explain the high mass flux
density of the slow wind near the heliospheric current sheet. However,
to account for the high-density wind originating from the small holes
prevalent at sunspot maximum, substantially enhanced rates of coronal
heating are required.
Title: Polar Plumes and the Solar Wind
Authors: Wang, Y. -M.
Bibcode: 1994ApJ...435L.153W
Altcode:
The mass flow within a polar plume is modeled including the effect
of coronal heating and radiative losses. In addition to the 'global'
heating (on a scale Hm approximately solar radius) required
to drive high-speed wind from the plume and interplume regions of
the polar coronal hole, we find that a large amount of energy must
be dissipated very near the coronal base to produce the high plasma
densities observed in plumes. This concentrated heating, over a
scale Hb much less than solar radius, results in a steep
temperature gradient with a local temperature maximum just above the
plume base, where the gas is essentially stagnant; at greater heights,
the plume is cooler than the interplume region. Although the mass
flux densities are somewhat higher within the plumes, the interplume
regions occupy most of the polar hole area and are therefore the main
source of the high-speed polar wind.
Title: Effect of areal expansion and coronal heating on the solar wind
Authors: Wang, Y. -M.
Bibcode: 1994SSRv...70..387W
Altcode:
Empirical studies have shown that the solar wind speed at Earth
is inversely correlated with the areal expansion rate of magnetic
flux tubes near the Sun. Recent model calculations that include a
self-consistent determination of the coronal temperature allow one to
understand the physical basis of this relationship; they also suggest
why the solar wind mass flux is relatively constant.
Title: The Rotation of Photospheric Magnetic Fields: A Random Walk
Transport Model
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1994ApJ...430..399W
Altcode:
In an earlier study of solar differential rotation, we showed that
the transport of magnetic flux across latitudes acts to establish
quasi-stationary patterns, therby accounting for the observed rigid
rotation of the large-scale photospheric field. In that paper, the
effect of supergranular convection was represented by a continuum
diffusion, limiting the applicability of the calculations to large
spatial scales. Here we extend the model to scales comparable to that
of the supergranulation itself by replacing the diffusive transport
with a discrete random walk process. Rotation curves are derived
by cross-correlating the simulated photospheric field maps for a
variety of time lags and spatial resolutions. When the lag between
maps is relatively short less than or approximately = 15 days), the
midlatitude correlation functions show two distinct components: a broad
feature associated with the large-scale unipolar patterns and a narrow
feature originating from small magnetic structures encompossing from
one to several supergranular cells. By fitting the broad component
we obtain the rigid rotation profile of the patterns, whereas by
fitting the narrow component, we recover the differential rate of
the photospheric plasma itself. For time lags of 1 month or greater,
only the broad feature associated with the long-lived patterns remains
clearly identifiable in the simulations.
Title: Ulysses at 50° south: constant immersion in the high-speed
solar wind
Authors: Phillips, J. L.; Balogh, A.; Bame, S. J.; Goldstein, B. E.;
Gosling, J. T.; Hoeksema, J. T.; McComas, D. J.; Neugebauer, M.;
Sheeley, N. R., Jr.; Wang, Y. -M.
Bibcode: 1994GeoRL..21.1105P
Altcode:
We present speed observations from the Ulysses solar wind plasma
experiment through 50° south latitude. The pronounced speed modulation
arising from solar rotation and the tilt of the heliomagnetic current
sheet has nearly disappeared. Ulysses is now observing wind speeds
in the 700 to 800 km s-1 range, with a magnetic polarity
indicating an origin in the large south polar coronal hole. The
strong compressions, rarefactions, and shock waves previously seen have
weakened or disappeared. Occasional coronal mass ejections characterized
by low plasma density caused by radial expansion have been observed. The
coronal configuration was simple and stable in 1993, indicating that
the observed solar wind changes were caused by increasing spacecraft
latitude. Trends in prevailing speed with increasing latitude support
previous findings. A decrease in peak speed southward of 40° latitude
may indicate that the fastest solar wind comes from the equatorial
extensions of the polar coronal holes.
Title: Global evolution of interplanetary sector structure, coronal
holes, and solar wind streams during 1976-1993: Stackplot displays
based on solar magnetic observations
Authors: Wang, Y. M.; Sheeley, N. R., Jr.
Bibcode: 1994JGR....99.6597W
Altcode:
We use potential field calculations and solar magnetic observations
during 1976-1993 to infer the evolution of interplanetary sector
structure, coronal holes, and solar wind streams at heliographic
latitudes ranging from 80°S to 80°N. The results are presented in
the form of stackplots, which show long-lived patterns that rotate
quasi-rigidly at rates determined by the photospheric distribution
of nonaxisymmetric magnetic flux. The fastest wind streams and their
coronal hole sources form slowly rotating patterns near the poles just
after sunspot maximum but migrate to lower latitudes and tend to rotate
at near-equatorial rates as sunspot activity declines.
Title: Returning to the random walk
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.
Bibcode: 1994ASIC..433..379S
Altcode:
No abstract at ADS
Title: Understanding the Rotation of Coronal Holes
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1993ApJ...414..916W
Altcode:
In an earlier study we found that the rotation of coronal holes
could be understood on the basis of a nearly current-free coronal
field, with the holes representing open magnetic regions. In this
paper we illustrate the model by focusing on the case of CH1, the
rigidly rotating boot-shaped hole observed by Skylab. We show that the
interaction between the polar fields and the flux associated with active
regions produces distortions in the coronal field configuration and
thus in the polar-hole boundaries; these distortions corotate with the
perturbing nonaxisymmetric flux. In the case of CH1, positive-polarity
field lines in the northern hemisphere 'collided' with like-polarity
field lines fanning out from a decaying active region complex located
just below the equator, producing a midlatitude corridor of open field
lines rotating at the rate of the active region complex. Sheared coronal
holes result when nonaxisymmetric flux is present at high latitudes,
or equivalently, when the photospheric neutral line extends to high
latitudes. We demonstrate how a small active region, rotating at the
local photospheric rate, can drift through a rigidly rotating hole
like CH1. Finally, we discuss the role of field-line reconnection in
maintaining a quasi-potential coronal configuration.
Title: Flux-Tube Divergence, Coronal Heating, and the Solar Wind
Authors: Wang, Y. -M.
Bibcode: 1993ApJ...410L.123W
Altcode:
Using model calculations based on a self-consistent treatment of
the coronal energy balance, we show how the magnetic flux-tube
divergence rate controls the coronal temperature and the properties
of the solar wind. For a fixed input of mechanical and Alfven-wave
energy at the coronal base, we find that as the divergence rate
increases, the maximum coronal temperature decreases but the mass
flux leaving the sun gradually increases. As a result, the asymptotic
wind speed decreases with increasing expansion factor near the sun,
in agreement with empirical studies. As noted earlier by Withbroe, the
calculated mass flux at the sun is remarkably insensitive to parameter
variations; when combined with magnetohydrodynamic considerations,
this self-regulatory property of the model explains the observed
constancy of the mass flux at earth.
Title: Coronal Flux-Tube Expansion and the Solar Wind Speed at Ulysses
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.; Bame, S. J.; Phillips,
J. L.; Goldstein, B. E.
Bibcode: 1993BAAS...25.1203S
Altcode:
No abstract at ADS
Title: Flux-Tube Expansion, Coronal Heating and the Solar Wind
Authors: Wang, Y. -M.
Bibcode: 1993BAAS...25.1204W
Altcode:
No abstract at ADS
Title: On the latitude and solar cycle dependence of the
interplanetary magnetic field strength
Authors: Wang, Y. -M.
Bibcode: 1993JGR....98.3529W
Altcode:
Using 20 years of solar magnetograph and in-ecliptic interplanetary
magnetic field (IMF) measurements, we verify that the radial IMF
component (Br) can be approximated by a superposition of
the Sun's inclined magnetic dipole moment and a current sheet normal
to the dipole axis. The net field is found empirically to be ~3 times
stronger along the dipole axis than near the current sheet, whose
effect is to redistribute flux toward the dipole equator. The radial
IMF intensity at a given latitude and phase of the sunspot cycle is
determined by the changing strength and inclination of the dipole, which
attains its maximum amplitude when its axis is aligned with the solar
poles near sunspot minimum. The model predicts that over the sunspot
cycle, ||Br|| should undergo the least variation near the
heliographic equator and the greatest variation above the Sun's poles,
where it decreases by a factor of 10 between sunspot minimum and sunspot
maximum. The latitudinal gradients in Br are expected to
be steepest near sunspot minimum and flattest near maximum. The model
suggests that Ulysses will encounter very strong fields when it flies
over the solar poles during the declining phase of sunspot cycle 22.
Title: Flux Emergence and the Evolution of Large-Scale Photosphenc
Field Patterns (Abstract)
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1993ASPC...46..487W
Altcode: 1993mvfs.conf..487W; 1993IAUCo.141..487W
No abstract at ADS
Title: On the optimal combination of potential coefficient model
with terrestrial gravity data for FFT geoid computations.
Authors: Wang, Y. M.
Bibcode: 1993MGeo...18..406W
Altcode:
The local geoid undulation is usually computed by combining a
potential coefficient model with terrestrial gravity data. In order
to get the best estimate of the geoid, a procedure is proposed that
has the following advantages: (1) the error of the geoid caused by
the terrestrial gravity data error, the coefficient error and the
truncation error is minimized in a least squares sense; (2) the fast
Fourier techniques may be used. The procedure is simple, but it is
accurate and efficient for practical geoid computations.
Title: A New Determination of the Solar Rotation Rate
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.; Nash, A. G.
Bibcode: 1992ApJ...401..378S
Altcode:
We use 'stackplot' displays to compare observations of the photospheric
magnetic field during sunspot cycle 21 with simulations based on the
flux-transport model. Adopting nominal rates of diffusion, differential
rotation, and meridional flow, we obtain slanted patterns similar
to those of the observed field, even when the sources of flux are
assigned random longitudes in the model. At low latitudes, the slopes
of the nearly vertical patterns of simulated field are sensitive to
the rotation rate used in the calculation, and insensitive to the
rates of diffusion and flow during much of the sunspot cycle. Good
agreement between the observed and simulated patterns requires a
synodic equatorial rotation period of 26.75 +/- 0.05 days.
Title: On Potential Field Models of the Solar Corona
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1992ApJ...392..310W
Altcode:
It is shown that the line-of-sight matching procedure involved in
potential field models of the solar corona do not make good use of the
available data because there is strong evidence that the magnetic field
is nearly radial, and therefore nonpotential, at the photosphere. It is
argued that the observed photospheric field should first be corrected
for line-of-sight projection and then matched to the radial component
of the potential field. It is shown that this procedure yields much
stronger polar fields than the standard method and produces better
agreement with high-latitude coronal holes and with white-light
structures in the outer corona. The relationship of both methods to
the observed inclination angles of polar plumes is also discussed.
Title: The relationship between solar wind speed and the areal
expansion factor
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1992sws..coll..125W
Altcode:
Empirical studies indicate that the solar wind speed at Earth is
inversely correlated with the divergence rate of the coronal magnetic
field. This result suggests that the mechanical energy flux at the
coronal base (in the form of Alfven waves, for example) is roughly
constant within open field regions.
Title: A New Solar Cycle Model Including Meridional Circulation
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Nash, A. G.
Bibcode: 1991ApJ...383..431W
Altcode:
A kinematic model is presented for the solar cycle which includes
not only the transport of magnetic flux by supergranular diffusion
and a poleward bulk flow at the sun's surface, but also the effects
of turbulent diffusion and an equatorward 'return flow' beneath
the surface. As in the earlier models of Babcock and Leighton, the
rotational shearing of a subsurface poloidal field generates toroidal
flux that erupts at the surface in the form of bipolar magnetic
regions. However, such eruptions do not result in any net loss of
toroidal flux from the sun (as assumed by Babcock and Leighton);
instead, the large-scale toroidal field is destroyed both by 'unwinding'
as the local poloidal field reverses its polarity, and by diffusion as
the toroidal flux is transported equatorward by the subsurface flow
and merged with its opposite hemisphere counterpart. The inclusion
of meridional circulation allows stable oscillations of the magnetic
field, accompanied by the equatorward progression of flux eruptions,
to be achieved even in the absence of a radial gradient in the angular
velocity. An illustrative case in which a subsurface flow speed of
order 1 m/s and subsurface diffusion rate of order 10 sq km/s yield
22-yr oscillations in qualitative agreement with observations.
Title: Out-of-ecliptic tests of the inverse correlation between
solar wind speed and coronal expansion factor
Authors: Sheeley, N. R., Jr.; Swanson, E. T.; Wang, Y. -M.
Bibcode: 1991JGR....9613861S
Altcode:
In this paper we address the question of whether out-of-ecliptic
measurements satisfy the inverse correlation between wind speed at
1 AU and flux tube divergence in the corona, already found from
measurements in the ecliptic. Using the in-ecliptic calibration,
we derive out-of-ecliptic speeds from coronal expansion factors
determined from global observations of photospheric field and their
current-free coronal extension. These derived speeds are compared with
speeds inferred from interplanetary scintillation measurements during
1972-1988 and with in situ speeds measured by the Pioneer 11 spacecraft
at 16°N latitude during 1984-1988. These three sets of wind speed
show the same overall variation with latitude and time during the
sunspot cycle, with higher latitudes having more years of fast wind
than lower latitudes and all latitudes having slow wind at sunspot
maximum. Although some detailed discrepancies are also present, the
overall agreement is comparable to that achieved in the ecliptic plane.
Title: Magnetic Flux Transport and the Sun's Dipole Moment: New
Twists to the Babcock-Leighton Model
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1991ApJ...375..761W
Altcode:
The mechanisms that give rise to the sun's large-scale poloidal
magnetic field are explored in the framework of the Babcock-Leighton
(BL) model. It is shown that there are in general two quite distinct
contributions to the generation of the 'alpha effect': the first is
associated with the axial tilts of the bipolar magnetic regions as they
erupt at the surface, while the second arises through the interaction
between diffusion and flow as the magnetic flux is dispersed over the
surface. The general relationship between flux transport and the BL
dynamo is discussed.
Title: Why Fast Solar Wind Originates from Slowly Expanding Coronal
Flux Tubes
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1991ApJ...372L..45W
Altcode:
Empirical studies indicate that the solar wind speed at earth is
inversely correlated with the divergence rate of the coronal magnetic
field. It is shown that this result is consistent with simple wind
acceleration models involving Alfven waves, provided that the wave
energy flux at the coronal base is taken to be roughly constant within
open field regions.
Title: Deriving Solar Wind Speed from Solar Magnetic Field
Measurements
Authors: Nash, A. G.; Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1991BAAS...23..821N
Altcode:
No abstract at ADS
Title: Magnetic Field Configurations Associated with Fast Solar Wind
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.
Bibcode: 1991SoPh..131..165S
Altcode:
In this paper, we consider the implications of the observed inverse
correlation between solar wind speed at Earth and the expansion rate
of the Sun-Earth flux tube as it passes through the corona. We find
that the coronal expansion rate depends critically on the large-scale
photospheric field distribution around the footpoint of the flux tube,
with the smallest expansions occurring in tubes that are rooted near
a local minimum in the field. This suggests that the fastest wind
streams originate from regions where large coronal holes are about to
break apart and from the facing edges of adjacent like-polarity holes,
whose field lines converge as they transit the corona. These ideas
lead to the following predictions: Weak holes and fragmentary holes
can be sources of very fast wind.
Title: Magnetic Flux Transport and the Sunspot-Cycle Evolution of
Coronal Holes and Their Wind Streams
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1990ApJ...365..372W
Altcode:
The relationships between magnetic flux transport from active regions
and the formation and evolution of coronal holes are examined through
numerical simulations. The model utilized is based on the assumption
that coronal holes represent open field regions, and that the solar-wind
speed at 1 AU is universely correlated with the divergence rate of the
coronal field. The evolution of coronal holes and wind streams during
1980 - 1990 is discussed, along with flux transport and the evolution
of open field regions, and focus is placed on declining, rising, and
maximum phases. It is concluded that supergranular diffusion spreads
active region flux over the solar surface and wipes out pockets of
mixed polarity, thus creating unipolar areas containing open field
lines; differential rotation spreads flux in longitude and it combines
with diffusion to create axisymmetric polar holes from the original
active-region fields; and meridional flow accelerates the decay of
low-latitude holes by carrying flux to midlatitudes.
Title: Iterative solutions for the limited angle inverse-scattering
problems
Authors: Wang, Y. M.; Chew, W. C.
Bibcode: 1990SPIE.1351..189W
Altcode:
One major character of the limited angle inverse scattering problem is
the sparsity of the information contained in the measurement data. The
sparsity of information content in the limited angle inverse scattering
problems makes it difficult to reconstruct the object functions by
using the conventional inversion techniques. To overcome the above
difficulty, a boosting procedure is employed to obtain the maximum
amount of information for an arbitrary predefined experiment set-up. The
numerical simulations are performed in order to ascertain the role
of the boosting procedure in the limited angle inverse scattering
problems. The results of the computer simulations for two-side view
tomography demonstrate that by applying the boosting procedure, the
quality of the reconstruction and the speed of the convergence are
improved significantly. Furthermore, for subsurface detection where
both the sources and sensors are confined on one side of the object,
the reconstruction has become possible after applying the boosting
procedure.
Title: Latitudinal distribution of solar-wind speed from magnetic
observations of the Sun
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Nash, A. G.
Bibcode: 1990Natur.347..439W
Altcode:
Empirical studies suggest a close relationship between the solar-wind
speed near the Earth and the magnetic structure of the solar corona. The
correlation can be used to infer the latitudinal distribution of wind
speed at different phases of the sunspot cycle, and to identify the
sources of fast, high-latitude wind streams such as those that might
be encountered by the Ulysses spacecraft on its journey toward the
solar poles during 1992-1995.
Title: Solar Wind Speed and Coronal Flux-Tube Expansion
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1990ApJ...355..726W
Altcode:
The hypothesis that the solar wind speed at 1 AU and the rate of
magnetic flux-tube expansion in the corona are inversely correlated
is shown to be consistent with observations extending over the last
22 years. This empirical relationship allows the daily wind speeds
at earth to be predicted from a current-free extrapolation of the
observed photospheric field into the corona. The narrow boundaries
of high-speed wind streams are attributed to steep gradients in
the flux-tube expansion rates at the edges of coronal holes. When a
heliospheric current sheet is included in the model, it is found that
the flux tubes near the hole axis, although diverging more slowly
than those near the hole boundary in the corona, have undergone the
greatest net expansion at 1 AU, an effect consistent with the low
densities within high-speed streams.
Title: Preliminary Results from Modeling the White-Light Corona
at 3.5R
Authors: Nash, A. G.; Wang, Y. -M.
Bibcode: 1990BAAS...22Q.869N
Altcode:
No abstract at ADS
Title: Evolution of the Sun's Polar Fields during Sunspot Cycle 21:
Poleward Surges and Long-Term Behavior
Authors: Wang, Y. -M.; Nash, A. G.; Sheeley, N. R., Jr.
Bibcode: 1989ApJ...347..529W
Altcode:
Longitudinally averaged observations of the photospheric field during
1976-1986 are analyzed using a flux transport model. The way in
which source eruptions, supergranular diffusion, and meridional flow
collaborate to produce strong, highly concentrated polar fields near
sunspot minimum is clarified as follows: (1) widespread eruptions
of individual bipolar magnetic regions, with their leading polarity
flux equatorward of their trailing polarity flux, collectively
establish a large-scale separation of polarities in latitude; (2) the
low-latitude, leading polarity flux diffuses across the equator and
merges with its opposite hemisphere counterpart; and (3) meridional
flow carries the resulting surplus of trailing polarity flux to
the poles, and concentrates it there against the spreading effect
of diffusion. Episodic 'surges' of flux to the poles are induced
by fluctuations in the source eruption rate. Simulations indicate
that relatively weak, trailing polarity surges may occur even in a
steady flow field. However, in order to account for the giant surges
of alternating polarity and the resulting oscillations in the polar
field strength observed during 1980-1982, both accelerated flow and
enhanced eruption rates are required.
Title: The Effect of Newly Erupting Flux on the Polar Coronal Holes
Authors: Sheeley, N. R.; Wang, Y. -M.; Harvey, J. W.
Bibcode: 1989SoPh..119..323S
Altcode:
He I 10830 Å images show that early in sunspot cycles 21 and 22,
large bipolar magnetic regions strongly affected the boundaries of the
nearby polar coronal holes. East of each eruption, the hole boundary
immediately contracted poleward, leaving a band of enhanced helium
network. West of the eruption, the boundary remained diffuse and
gradually expanded equatorward into the leading, like-polarity part of
the bipolar magnetic region. Comparisons between these observations
and simulations based on a current-free coronal model suggest that:
The Sun's polar magnetic fields are confined to relatively small caps of
high average field strength, apparently by a poleward meridional flow.
Title: Magnetic Flux Transport on the Sun
Authors: Wang, Y. -M.; Nash, A. G.; Sheeley, N. R., Jr.
Bibcode: 1989Sci...245..712W
Altcode:
Although most of the magnetic flux observed on the sun originates in
the low-latitude sunspot belts, this flux is gradually dispersed over
a much wider range of latitudes by supergranular convective motions
and meridional circulation. Numerical simulations show how these
transport processes interact over the 11-year sunspot cycle to produce
a strong ``topknot'' polar field, whose existence near sunspot minimum
is suggested by the observed strength of the interplanetary magnetic
field and by the observed areal extent of polar coronal holes. The
required rates of diffusion and flow are consistent with the decay
rates of active regions and with the rotational properties of the
large-scale solar magnetic field.
Title: Average Properties of Bipolar Magnetic Regions during Sunspot
CYCLE-21
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1989SoPh..124...81W
Altcode:
We examine the statistical properties of some 2700 bipolar magnetic
regions (BMRs) with magnetic fluxes ≥3 × 1020 Mx which
erupted during 1976-1986. Empirical rules were used to estimate
the fluxes visually from daily magnetograms obtained at the National
Solar Observatory/Kitt Peak. Our analysis shows the following: (i) the
average flux per BMR declined between 1977 and 1985; (ii) the average
tilts of BMRs relative to the east-west line increase toward higher
latitudes; (iii) weaker BMRs had larger root-mean-square tilt angles
than stronger BMRs at all latitudes; (iv) over the interval 1976-1986,
BMRs with their leading poles equatorward of their trailing poles
contributed a total of 4 times as much flux as BMRs with `inverted'
tilts, but the relative amount of flux contributed by BMRs with
inverted or zero tilts increased as the sunspot cycle progressed;
(v) only 4% of BMRs had `reversed' east-west polarity orientations;
(vi) although the northern hemisphere produced far more flux during
the rising phase of the sunspot cycle, the southern hemisphere largely
compensated for this imbalance during the declining phase; (vii)
southern-hemisphere BMRs erupted at systematically higher latitudes
than northern-hemisphere ones through most of sunspot cycle 21.
Title: Implications of a Strongly Peaked Polar Magnetic Field
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.; DeVore, C. R.
Bibcode: 1989SoPh..124....1S
Altcode:
Using the flux-transport equation in the absence of sources, we study
the relation between a highly peaked polar magnetic field and the
poleward meridional flow that concentrates it. If the maximum flow
speed νm greatly exceeds the effective diffusion speed
κ/R, then the field has a quasi-equilibrium configuration in which the
poleward convection of flux via meridional flow approximately balances
the equatorward spreading via supergranular diffusion. In this case,
the flow speed ν(θ) and the magnetic field B(θ) are related by
the steady-state approximation ν(θ) ≃ (κ/R)B'(θ)/B(θ) over
a wide range of colatitudes θ from the poles to midlatitudes. In
particular, a general flow profile of the form sinpθ
cosqθ which peaks near the equator (q ≪ p) will correspond
to a cosnθ magnetic field at high latitudes only if p =
1 and νm = n κ/R. Recent measurements of n ∼ 8 and κ
∼ 600 km2 s−1 would then give νm
∼ 7 m s−1.
Title: The Evolution of the Sun's Polar Magnetic Field
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.; Nash, A. G.
Bibcode: 1989BAAS...21..827S
Altcode:
No abstract at ADS
Title: Average Magnetic Properties of Active Regions during Sunspot
Cycle 21
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1989BAAS...21Q.827W
Altcode:
No abstract at ADS
Title: The solar origin of long-term variations of the interplanetary
magnetic field strength
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1988JGR....9311227W
Altcode:
Spacecraft measurements over the past two sunspot cycles have shown
that the average strength of the interplanetary magnetic field (IMF)
undergoes surprisingly modest long-term variation, unlike the total
magnetic flux observed on the Sun. Our attempt to model the IMF during
sunspot cycle 21, based on a current-free extrapolation of the observed
photospheric field out to a fixed source surface where the field lines
become radial, yields calculated IMF intensities which vary by an order
of magnitude and which are far too low near sunspot minimum. We obtain
much better agreement with a model containing both heliospheric sheet
currents, which deflect polar flux toward the ecliptic, and volume
currents, which maintain a residual latitudinal gradient in the IMF
intensity. In order to match the observed IMF intensity levels, however,
the measured photospheric fields had to be scaled up by approximately a
factor of 2. Our composite model has the following main consequences:
(1) The source of the radial component of the IMF may be represented
to a first approximation by the dipole component of the photospheric
field. (2) The radial IMF intensity is strongest in the direction of
the dipole axis, which is aligned with the Sun's rotation axis near
sunspot minimum but tilts toward the ecliptic near sunspot maximum. (3)
The average strength of the photospheric field above latitude 55° is
of order 10 G around sunspot minimum.
Title: Mechanisms for the Rigid Rotation of Coronal Holes
Authors: Nash, A. G.; Sheeley, N. R., Jr.; Wang, Y. -M.
Bibcode: 1988SoPh..117..359N
Altcode:
We show that the rotation of coronal holes can be understood in
terms of a current-free model of the coronal magnetic field, in which
holes are the footpoint locations of open field lines. The coronal
field is determined as a function of time by matching its radial
component to the photospheric flux distribution, whose evolution is
simulated including differential rotation, supergranular diffusion,
and meridional flow. We find that ongoing field-line reconnection allows
the holes to rotate quasi-rigidly with their outer-coronal extensions,
until their boundaries become constrained by the neutral line of
the photospheric field as it winds up to form stripes of alternating
magnetic polarity. This wind-up may be significantly retarded by a
strong axisymmetric field component which forces the neutral line to
low latitudes; it is also gradually halted by the cross-latitudinal
transport of flux via supergranular diffusion and a poleward bulk
flow. We conclude that a strong axisymmetric field component is
responsible for the prolonged rigid rotation of large meridional holes
during the declining phase of the sunspot cycle, but that diffusion and
flow determine the less rigid rotation observed near sunspot maximum,
when the holes corotate with their confining polarity stripes.
Title: The Quasi-rigid Rotation of Coronal Magnetic Fields
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Nash, A. G.; Shampine,
L. R.
Bibcode: 1988ApJ...327..427W
Altcode:
Assuming that the coronal magnetic field can be approximated by
a current-free extension of the photospheric field, the authors
use spherical harmonic analysis and numerical simulations to
study its rotational properties. In the outer corona, they find
that the rotation rate is determined by three principal factors:
1. "Coronal filtering". 2. Global averages of the photospheric
rotation rate. 3. Ongoing source eruptions. These principles allow
to understand the observationally inferred rotational properties of
the outer coronal field. The overall rigidity of the rotation profile
reflects the tendency for the photosphere's non-axisymmetric flux to
be concentrated toward lower latitudes, where the rotational shear
is small; increased curvature and asymmetry occur during the rising
phase of the sunspot cycle because of the presence of higher latitude
flux. The coronal rotation rate shows a progressive acceleration due
to the equatorward migration of sunspots.
Title: A Model for Long-Term Variation of Interplanetary Magnetic
Field-Strenght
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.
Bibcode: 1988BAAS...20..705W
Altcode:
No abstract at ADS
Title: Disc accretion by magnetized neutron stars : a reassessment
of the torque.
Authors: Wang, Y. -M.
Bibcode: 1987A&A...183..257W
Altcode:
The torque exerted on a magnetized neutron star undergoing steady,
axisymmetric disc accretion is re-evaluated. As in the widely adopted
model of Ghosh and Lamb, the magnetospheric field is assumed to
thread the disc both inside and outside the radius of corotation,
yielding opposing contributions to the net torque from the forward- and
backward-swept field lines. It is shown, however, that the particular
pitch distribution postulated by Ghosh and Lamb is not consistent,
since the resulting magnetic pressure would disrupt their disc beyond
the corotation point. By taking the winding rate proportional to the
poloidal rather than the toroidal component of the field, the author
obtains a more realistic behaviour for the magnetic stresses generated
by the rotational shear. He then discusses the results in the light
of period-change measurements for several binary X-ray pulsars.
Title: Sunspot - Cycle Variations of the Interplanetary Field
Strength: Implications for Coronal Models
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; DeVore, C. R.
Bibcode: 1987BAAS...19.1133W
Altcode:
No abstract at ADS
Title: A Mechanism for the Rigid Rotation of Coronal Holes
Authors: Nash, A. G.; Sheeley, N. R., Jr.; Wang, Y. -M.
Bibcode: 1987BAAS...19.1133N
Altcode:
No abstract at ADS
Title: The Origin of Rigidly Rotating Magnetic Field Patterns on
the Sun
Authors: Sheeley, N. R., Jr.; Nash, A. G.; Wang, Y. -M.
Bibcode: 1987ApJ...319..481S
Altcode:
Using analytical calculations and numerical simulations, it is shown
that a meridional component of magnetic-flux transport will offset
the shearing effect of differential rotation and give rise to rigidly
rotating patterns of large-scale magnetic field. The nonaxisymmetric
field attains a striped polarity pattern which rotates rigidly like a
barber pole while its individual small-scale flux elements rotate at
the differential rate of the latitudes they are crossing. On the sun,
the meridional transport is provided by supergranular diffusion possibly
assisted by a small poleward flow. New sources of flux retard this
process and exclude the rigid rotation from the sunspot belts until
well into the declining phase of the sunspot cycle. This mechanism
accounts for a number of heretofore unexplained phenomena including
the tendency for coronal holes to rotate rigidly during the declining
phase of the sunspot cycle.
Title: The Origin of Rigidly Rotating Solar Magnetic Field Patterns
Authors: Nash, A. G.; Sheeley, N. R., Jr.; Wang, Y. -M.
Bibcode: 1987BAAS...19..938N
Altcode:
No abstract at ADS
Title: The Quasi-Rigid Rotation of Coronal Magnetic Fields
Authors: Wang, Y. -M.; Sheeley, N. R., Jr.; Nash, A. G.; Shampine,
L. R.
Bibcode: 1987BAAS...19..939W
Altcode:
No abstract at ADS
Title: Smearing of a Beaming Pattern by an Isotropic Cloud: an
Analysis with Applications to Nonpulsing X-Ray Sources and Cosmic Rays
Authors: Wang, Y. -M.; Schlickeiser, R.
Bibcode: 1987ApJ...313..200W
Altcode:
There is increasing observational and theoretical evidence to suggest
that many low-mass X-ray binaries contain neutron stars shrouded
by optically thick plasma. The diffusion approximation was used to
examine the effect of an isotropic, spherical scattering cloud on
anisotropic or periodic emission from an embedded source. Compton
scattering is treated by extending the 'scattering time' method of
Sunyaev and Titarchuk (1980) to include the contribution of higher
angular harmonics. Applications to cosmic-ray diffusion are also
discussed. It is shown that: the smearing-out of a beaming pattern
is largely determined by the ratio of the cloud radius to that of the
source; that periodic variations of the source are damped exponentially,
and a phase lag is introduced, if the oscillation period is short
compared with the time for a photon to diffuse through the cloud;
that Comptonization is accompanied by isotropization, so that any
underlying modulation of the source is most likely to survive in the
spectral range corresponding to its intrinsic radiation temperature.
Title: Interpreting Coronal Evolution in Terms of the Eruption and
Transport of Photospheric Magnetic Fields
Authors: Sheeley, N. R., Jr.; Wang, Y. -M.; Nash, A. G.; Shampine,
L. R.
Bibcode: 1987sowi.conf..322S
Altcode:
No abstract at ADS
Title: Production of ultra-high-energy γ-rays by accreting neutron
stars
Authors: Wang, Y. -M.
Bibcode: 1986Ap&SS.121..193W
Altcode:
Several well-known binary X-ray sources have been reported to emit
copious γ-radiation at energies up to and exceeding 1015
eV. It is proposed here that the observed events occur during episodes
of non-steady accretion onto neutron stars, when MHD instabilities
give rise to vortex motions onvery large scales deep inside the
magnetosphere. The magnetic lines of force are strongly distorted and
reconnect in neutral sheets, along which extremely high voltage drops
are maintained and a small fraction of the particles are accelerated
to ultra-relativistic energies. The γ-rays are produced in nuclear
collisions undergone by runaway ions traversing regions of high-density,
diamagnetic plasma in the accretion flow.
Title: Late stages of the Rayleigh-Taylor instability - A numerical
study in the context of accreting neutron stars
Authors: Wang, Y. -M.; Robertson, J. A.
Bibcode: 1985ApJ...299...85W
Altcode:
The development of the Rayleigh-Taylor instability in the fully
nonlinear regime is explored using a two-dimensional, compressible MHD
code. The motivation for this work was provided by models of spherical
accretion onto slowly rotating, magnetized neutron stars, where the
diamagnetic external plasma may be regarded as the superposed 'heavy
fluid' and the closed magnetosphere constitutes the underlying 'light
fluid'. The numerical procedure, including an improved algorithm for
tracing the boundary between the two fluids, is briefly described. Some
important aspects of single-mode behavior are considered, including
the 'cascade' to small scales via Kelvin-Helmholtz instability and the
dependence of the penetration and mixing rates on the initial wavelength
and density contrast. The evolution of noise-induced instabilities is
examined, describing the nonlinear interactions between the mushroom
structures. The effect of magnetic fields on the interchange process
is discussed.
Title: The axisymmetric pulsar magnetosphere.
Authors: Mestel, L.; Robertson, J. A.; Wang, Y. -M.; Westfold, K. C.
Bibcode: 1985MNRAS.217..443M
Altcode:
A tentative model is discussed for the steady spin-down of an aligned
rapidly rotating magnetic neutron star within the framework of classical
physics. The electron currents which exert the braking torque on the
star flow from the polar caps out to and beyond the light-cylinder,
picking up energy from the electric force and angular momentum from the
magnetic torque. At high enough energies the electrons emit incoherent
gamma radiation; the associated loss of angular momentum enables the
currents to cross magnetic field lines and so ultimately to return
to the star at lower latitudes. Several examples are constructed
that illustrate the required flow, but no claim is made that an
acceptable approximation to a fully self-consistent model has yet been
achieved. Reasons are given why such a classical model may spontaneously
convert itself through pair production into a quantum model, in which
circulation of primary electrons would still play an essential part.
Title: 'Propeller' action by rotating neutron stars
Authors: Wang, Y. -M.; Robertson, J. A.
Bibcode: 1985A&A...151..361W
Altcode:
The interaction between a fast-rotating, magnetized neutron star,
and matter that it captures gravitationally from an external
source, is examined with the help of a 2-dimensional MHD code. The
plasma-magnetospheric boundary is shown to be subject to strong
Kelvin-Helmholtz and gravity-driven interchange instabilities,
which result in efficient mixing as well as the rapid 'shredding'
of the component of the magnetic field along the shear flow. In a
quasi-steady 'propeller', a comparatively dense envelope builds up
around and compresses the magnetosphere (with matter being sucked
in preferentially near the direction of the rotation axis and being
expelled perpendicular to it). As it attempts to enforce corotation,
the magnetospheric field is twisted by the boundary-layer vortex motions
into loops, which float outward, transferring angular momentum through
the atmosphere. The magnitude of the spindown torque exerted on the
neutron star through this dynamo-like process is sufficient to account
for the long periods of many of the observed binary X-ray pulsars.
Title: A numerical investigation of the Kelvin-Helmholtz instability
in the context of accreting neutron stars
Authors: Wang, Y. -M.; Robertson, J. A.
Bibcode: 1984A&A...139...93W
Altcode:
The development of the Kelvin-Helmoltz instability is investigated
numerically using a two-dimensional magnetohydrodynamic code. First,
some idealized field-free configurations involving two initially uniform
fluids in relative motion are considered. Attention is given to the
formation and evolution of vortices, the width and structure of the
developing shear layer, the nonlinear growth rate and saturation of
the Kelvin-Helmholtz modes, and their dominant scale. The effect of
a magnetic field frozen into one of the fluids and aligned parallel
or perpendicular to the direction of streaming is then examined. A
vertical gravitational field is introduced, and the transition from
Rayleigh-Taylor to Kelvin-Helmholtz instability as the Mach number of
the flow increases is discussed. Finally, the astrophysical implications
are briefly examined.
Title: Further numerical studies of the Rayleigh-Taylor instability
in the context of accreting X-ray sources.
Authors: Wang, Y. -M.; Nepveu, M.; Robertson, J. A.
Bibcode: 1984A&A...135...66W
Altcode:
An earlier investigation of the nonlinear Rayleigh-Taylor instability
for accreting X-ray sources is extended to allow for more realistic
initial conditions. The two-dimensional computations show the heavy
and light fluids undergoing complementary circulatory motions which
result in the formation of alternating inverted and upright 'mushroom'
structures along the interface. The structures develop independently
of the shape of the initial perturbation. Short wavelength modes have
a strong tendency to dominate long ones, with the lower bound being
set by viscous damping. A relatively modest vertical magnetic field
will act to suppress the vortex motions and produce a 'bubble and
spike' structure. A crude simulation of the instability occurring in a
radiation-supported accretion column is presented; after a slow start,
the magnetically constrained plasma drips down into the photon medium
in the form of long narrow fingers, the dominant scale-length being
determined by radiative viscosity.
Title: The role of dissipation in the formation of spiral and
elliptical galaxies
Authors: Wang, Y. -M.; Scheuerle, H.
Bibcode: 1984A&A...130..397W
Altcode:
The effect of dissipation through cloud-cloud collisions on the
formation of spiral and elliptical galaxies is discussed. After
reviewing related galaxy formation models based on systems of
interacting gas clouds, it is shown theoretically that collisional
dissipation may control both the angular momentum gain and the rate of
star formation in a protogalaxy, thus largely determining its dynamical
and morphological properties. A similarity model is developed which
inversely correlates the initial spin of the protogalaxy with its
total mass and rate of star formation, in relation to cloud collision
dissipation. The model's calculations are found to agree with recent
observational data, but it is recommended that future models incorporate
a numerical scheme which accounts for the effects of density gradients
and shocks on the dynamical relaxation of protoelliptical galaxies.
Title: Fluid Instabilities around Accreting X-Ray Sources
Authors: Wang, Y. M.
Bibcode: 1984heac.conf..327W
Altcode:
No abstract at ADS
Title: Fluid instabilities around accreting X-ray sources
Authors: Wang, Y. -M.
Bibcode: 1984AdSpR...3j.327W
Altcode: 1984AdSpR...3..327W
Some aspects of fluid instabilities occurring in the magnetospheres
of accreting neutron stars are discussed. It is pointed out that (i)
in the absence of strong differential rotation, the accreting plasma
should be drawn out into spiralling, sheet-like structures, resulting
in efficient mixing between the two media; (ii) the Rayleigh-Taylor
instability also acts to limit the X-ray luminosity in super-critical
sources; and (iii) magnetic shear has a strong stabilizing influence
on Kelvin-Helmholtz modes, and its presence may allow substantial
amounts of material to be supported around the magnetosphere.
Title: A numerical study of the nonlinear Rayleigh-Taylor instability,
with application to accreting X-ray sources
Authors: Wang, Y. -M.; Nepveu, M.
Bibcode: 1983A&A...118..267W
Altcode:
With a view toward applications to accreting X-ray sources, the
Rayleigh-Taylor instability is followed numerically, using a 2-D
magnetohydrodynamic code. The presence of a uniform magnetic field
in the underlying medium is allowed for. The infalling plasma is
found to develop elongated, trailing loops; at least when the initial
perturbation is highly symmetric, a narrow neck also forms through
the action of the surrounding ram pressure. It is suggested that
the swirling motion present in the nonlinear phase could produce
some effective large-scale mixing between accreting plasma and the
magnetospheric field of a neutron star. Another potentially significant
tendency is for the curvature of the infalling plasma pocket to sharpen
as the instability develops: magnetic tension may therefore become
increasingly effective as a stabilizing influence.
Title: Plasma-magnetospheric interaction in X-ray sources - an
analysis of the linear Kelvin-Helmholtz instability
Authors: Wang, Y. M.; Welter, G. L.
Bibcode: 1982A&A...113..113W
Altcode:
The Kelvin-Helmholtz instability is analyzed for the case of a
magnetized plasma streaming over a vacuum magnetic field with arbitrary
orientation in the plane parallel to the interface. It is shown that
the presence of even a very weak magnetic field within the plasma may
suppress the instability, suggesting that turbulent mixing of plasma
into the magnetosphere around an accreting X-ray source is probably
not initiated solely through the growth of Kelvin-Helmholtz modes.
Title: Super-critical X-ray luminosities - The structure and stability
of a radiation-supported plasma layer
Authors: Wang, Y. -M.
Bibcode: 1982A&A...112...24W
Altcode:
The structure and stability of a plane-parallel Thomson-opaque plasma
layer, supported against gravity by radiation and thermal pressures,
are examined. The effects of Compton heating by the X-rays incident from
below and of thermal bremsstrahlung by the layer itself are taken into
account, and an approximate model for the radiative energy transfer is
developed. The layer is shown to be dynamically unstable, suggesting
that additional stabilizing factors must be present in X-ray sources
with steady luminosities substantially exceeding the Eddington limit.
Title: Changing orientation of dipole and spin axes in binary
X-ray pulsars
Authors: Wang, Y. -M.; Robnik, M.
Bibcode: 1982A&A...107..222W
Altcode:
It is shown that the inclination angle between the dipole and spin
axes of a neutron star in an X-ray binary system should increase during
spinup episodes and decrease during spindown, on a timescale comparable
with the change in spin period. This will lead to secular variations
in the pulse shapes, and, under the assumption that a typical X-ray
pulsar has spun down on net during its history of interaction with its
mass-losing companion, may account for an apparent preponderance of
'single-pulse' profiles.
Title: The non-aligned pulsar magnetosphere - an illustrative model
for small obliquity
Authors: Mestel, L.; Wang, Y. -M.
Bibcode: 1982MNRAS.198..405M
Altcode:
The electromagnetic field outside a pulsar of small obliquity
is approximated by Goldreich-Julian conditions out to the light
cylinder and by an outgoing vacuum wave beyond the cylinder, matched
by the appropriate surface charge-current distribution. The energy
supply for the wave requires current flow between the pulsar and
the light cylinder. The cold electrons carrying the current achieve
relativistic energies near the light cylinder; the consequent inertial
and radiation damping forces enable the electrons to drift across the
field lines and so complete their circuits back to the pulsar. It is
suggested that low-obliquity pulsars are essentially emitters of a
plasma-modified low-frequency wave and of gamma radiation near the
light cylinder. Illustrative models are constructed as perturbations
about an analogous approximate model for the aligned case.
Title: An analysis of the pulse profiles of the binary X-ray pulsars.
Authors: Wang, Y. -M.; Welter, G. L.
Bibcode: 1981A&A...102...97W
Altcode:
Pulse profiles of the known binary X-ray pulsars are analyzed to derive
information on beaming geometries and the structure of the plasma
magnetosphere. The X-ray light curves are interpreted, and the pulsars
are considered individually. The frequency-to-frequency variation of the
pulse profiles is also discussed with emphasis on the origin of phase
differences. Evidence for a bias of the magnetic and rotation axes of
the underlying neutron stars toward alignment is found, and the light
curves of some of the less luminous pulsars are modeled on the basis of
broad isotropically-emitting hot spots. Fan-beam geometries are required
for most of the faster, more luminous sources, and geometrical effects
are found to produce very sharp features in the X-ray light curves.
Title: Spin-reversed accretion as the cause of intermittent spindown
in slowX-ray pulsars.
Authors: Wang, Y. -M.
Bibcode: 1981A&A...102...36W
Altcode:
The magnitude and sign of the angular momentum captured by an X-ray
pulsar from its mass-losing binary companion are very sensitive to
the distribution of the relative flow velocity across the capture
cross-section. The observed period increases in some slow X-ray pulsars
are attributed to temporary reversals in the spin of the material being
accreted by the neutron star. It is shown that, for the majority of
the slow rotators, the incoming plasma will interact with the neutron
star's magnetic field at a radius where the effect of the magnetospheric
rotation can be neglected. Any spindown will be very slow unless the
star and plasma rotate in opposite directions, or the magnetic field
strength is very high. The angular momentum transmitted in a moderately
fast stellar wind is estimated, taking into account gradients in both
the wind velocity and density. The conditions under which the captured
matter would spin in a retrograde sense are evaluated. Accretion
from a slow wind, in which a Keplerian disk may be formed, is also
evaluated under similar conditions. The simultaneous measurement of
pulse periods and X-ray fluxes during spindown episodes would help
discriminate between spin-reversal and propeller type torques.
Title: Magnetic Alignment and Counter-Alignment in Binary X-Ray
Pulsars
Authors: Wang, Y. -M.
Bibcode: 1981SSRv...30..341W
Altcode:
The magnetic torque exerted on a binary X-ray pulsar acts to decrease
the angle between the dipole and spin axes during braking episodes,
but to increase it whenever spinup occurs. The likely effect on the
X-ray pulse profiles is considered.
Title: Plasma infall and X-ray production in the magnetic funnel of
an accretning neutron star.
Authors: Wang, Y. -M.; Frank, J.
Bibcode: 1981A&A....93..255W
Altcode:
The plasma and radiation structure of the accretion funnel
of a binary X-ray pulsar is investigated from a hydrodynamic
viewpoint. Two-dimensional numerical computations are presented for
the radiation-pressure and gravity-dominated region above the neutron
star's surface, taking approximate account of the effect of a strong,
nonuniform magnetic field on the radiative transfer. The velocity and
radiation fields depend sensitively on the size of the polar cap and
the strength and structure of the magnetic field, as well as on the
accretion rate. In most cases the X-rays diffuse sideways and escape
roughly perpendicular to, and close to the bottom of, the accretion
channel (forming a fan beam). The structure of a collision-dominated
regime near the star surface, which is the source of a relatively cool,
blackbody radiation field (subsequently upgraded by inverse Compton
scattering in the upper zone), is also discussed. It is suggested
that strong cyclotron line emission could originate in a ring at the
bottom of the accretion funnel, where the infalling plasma is stopped
primarily by collisions.
Title: The Test of Light Energy Concentration for 1-METER Reflector
at Yunnan-Observatory
Authors: Wang, Y. M.; Qin, S. N.
Bibcode: 1981AcASn..22..310W
Altcode:
No abstract at ADS
Title: The axisymmetric pulsar magnetosphere - II.
Authors: Mestel, L.; Wang, Y. -M.
Bibcode: 1979MNRAS.188..799M
Altcode:
A Fourier transform method is presented for the construction of
model pulsar magnetic fields. The method can be employed as part
of an iterative scheme for the construction of a self-consistent
magnetosphere. Three illustrative models are computed, for which
respectively: (1) the entire region as far as the light-cylinder
consists of massless corotating charges; (2) some of the corotating
ions are removed to produce a thin gap near the light-cylinder; (3)
a large wedge of ions is removed.
Title: The axisymmetric pulsar magnetosphere - I.
Authors: Mestel, L.; Phillips, P.; Wang, Y. -M.
Bibcode: 1979MNRAS.188..385M
Altcode:
The paper proposes a model for the magnetosphere of a pulsar with
the magnetic and rotation axes aligned. There is a basic similarity to
Jackson's model, in that there is no energy loss through a steady pulsar
wind. Electrons pulled out of the polar regions are driven across the
light-cylinder by centrifugal force, moderated by the electric force
component along the magnetic field. Just beyond the light-cylinder
the electrons become highly relativistic and emit high-frequency
radiation, which carries away energy and angular momentum, so braking
the star. This radiative dissipation forces the electrons to migrate
across the magnetic field-lines towards the equator, to be driven
back to the star by the electric field. A program is outlined for
the construction of mutually self-consistent, magnetic, electric and
particle fields.
Title: Propeller Spindown of Rotating Magnets
Authors: Holloway, N.; Kundt, W.; Wang, Y. -M.
Bibcode: 1978A&A....70L..23H
Altcode:
Summary: We reassess the torque felt by a rapidly spinning magnetized
star or planet inside plasma, and argue that earlier investigations
tended to underestimate its strength. Keywords: propeller spindown -
rotating magnetospheres - binary neutron stars
Title: Rotational history of a binary X-ray pulsar.
Authors: Wang, Y. -M.
Bibcode: 1978Obs....98..209W
Altcode:
No abstract at ADS
Title: On the role of finite inertia and resistivity in axisymmetric
pulsar magnetospheres.
Authors: Wang, Y. -M.
Bibcode: 1978MNRAS.182..157W
Altcode:
Charge-separated and quasineutral-plasma models of axisymmetric pulsar
magnetospheres are examined in a study of inertial and resistivity
effects as a function of the plasma density. In the quasineutral-plasma
model, the plasma inertia remains small throughout the open field
region; to prevent separation of the two charge species in the stellar
gravity field, an anomalous resistivity implying nonvanishing poloidal
flow must be assumed. In the steady charge-separated magnetosphere,
charge conservation is satisfied by inclusion of particle inertia
and/or radiation reaction effects in the momentum equation and/or its
divergence. Motions of the polar and equatorial charge species are
also assessed.
Title: Observations of Hercules X-1 with SAS-3 during 1975 July.
Authors: Joss, P. C.; Li, F. K.; Wang, Y. -M.; Hearn, D. R.
Bibcode: 1977ApJ...214..874J
Altcode:
X-ray pulsations from Her X-1 with energies between 0.1 and 30 keV
were observed for four days with the SAS-3 satellite, during the 1975
July-August ON state of the source. The existence of a strong flux
between 0.1 and 0.4 keV, with pulsations that are out of phase with
those above 1 keV, is confirmed. A pulsed flux in the 19-30 keV band
was discovered. The average fractional rate of change in pulse period
between 1972 and 1975 was about 3 x 10 to the -6th/yr, and the absolute
value of the average fractional rate of change in orbital period during
the same interval was not greater than 5 x 10 to the -7th/yr
Title: Nature of Her X-1
Authors: Wang, Y. -M.
Bibcode: 1975Natur.253..249W
Altcode:
IYENGAR et al.1 have reported observations of hard X rays
from Her X-1, which seem to set an upper limit of 10% on the pulsating
component in the energy range 20 to 45 keV. I offer a simple explanation
that does not require the additional source of X rays postulated by
Iyengar et al.