UVCS/SOHO PAPERS - 2004
2004-22: The origin of the
slow solar wind in coronal streamers
Ofman, L.
Advances in Space Research, Volume 33, Issue 5, p. 681-688.
The highly variable slow solar wind has been associated with low-latitude
regions of the heliosphere most clearly by the Ulysses spacecraft. Although,
it is evident today that the slow solar wind originates in coronal helmet
streamers, the mechanism of the slow solar wind acceleration, and the origin
of the variability are still being debated. The combination of new observations
and numerical modeling are beginning to address these questions. I will
discuss how recent in-situ observations by Ulysses, white light and EUV
observations by the LASCO and UVCS instruments on SOHO advanced our understanding
of the streamer structure, dynamics, and stability. I will briefly review
the current state of numerical MHD modeling of streamers, and the possible
mechanisms that may produce the highly variable slow wind. I will present
the results of recent heat-conductive MHD modeling of multiple streamer slow
solar wind with heating function constrained by observations. I will show
how multi-fluid numerical modeling of the slow solar wind in streamers helps
to identify the regions of the slow solar wind outflow.
2004-21:
Diagnostics of the Local Interstellar Medium
using particles and UV radiation
Lallement, R.; Raymond, J. R.;
Vallerga, J.
Advances in Space Research, Volume 34, Issue 1, p. 46-52.
The chain of processes which govern trajectories and ionization of neutral
particles from the circumsolar interstellar gas to the inner heliosphere
can now be studied by combining several types of diagnostics: solar backscattered
radiation (H and He), direct detection of neutral particles (He), detection
of pickup ions of interstellar origin (H, He, C, N and O, etc.). Moreover,
continuous observations of the Sun bring the required solar parameters to
be included in the modeling and, in parallel, considerable efforts have been
made in model developments. A good example of such improvements is the synthetic
work done on interstellar helium, detailed by Möbius et al. in this
issue. This is useful because helium is a tracer of the initial state of
the interstellar gas in the Local Cloud and serves as a zero level for the
other species. In particular, the velocity distribution of interstellar neutral
hydrogen measured by SOHO/SWAN can be compared with helium characteristics,
to quantify the modifications experienced by the H flow at entrance in the
heliosphere. This provides a measurement of the interstellar circumsolar
plasma pressure. Here we focus on three aspects:
the recent EUVE data on the helium 58.4 nm glow, which provide an independent
measurement of the flow direction and velocity, and help to understand how
helium temperature has been overestimated when analyzing past glow data.
the recent SOHO/UVCS data which bring new constraints on the ionization
processes close to the Sun, namely the significant role of electron impact
and a possible explanation for the observed correlation between H+
and He+ pickup fluxes.
an update of the hydrogen-helium differentiation at the heliospheric
interface
2004-20: Shock wave driven
by an expanding system of loops
Raouafi, N.-E.; Mancuso, S.;
Solanki, S. K.; Inhester, B.; Mierla, M.; Stenborg, G.;
Delaboudinière, J. P.; Benna, C.
Astronomy and Astrophysics, v.424, p.1039-1048.
We report on a Coronal Mass Ejection (CME) observed on June 27, 1999
by the UltraViolet Coronagraph Spectrometer (UVCS) telescope operating on
board the SOHO spacecraft. The CME was also observed by the Large Angle
Spectroscopic Coronagraph (LASCO). Emission of hot material has been recorded
by UVCS propagating in front of an opening system of loops generated by
the CME. The evolution of the UVCS structure is highly correlated with the
evolution of the opening loop. The data reveal excess broadening of the
O VI doublet lines and an enhancement in the intensity of the Si XII520.66
and λ499.37 lines due to the motion of the expanding hot gas. The hot gas
emission seems to be due to a shock wave propagating in front of a very
fast gas bubble traveling along the opening loop system.
2004-19:
Solar cycle dependence of the helium focusing
cone from SOHO/UVCS observations. Electron impact rates and associated
pickup ions
Lallement, R.; Raymond, J. C.; Bertaux, J.-L.; Quémerais, E.;
Ko, Y.-K.; Uzzo, M.; McMullin, D.; Rucinski, D.
Astronomy and Astrophysics, v.426, p.867-874
The Ultraviolet Coronograph on board SOHO (UVCS) has observed the 58.4
nm glow of the interplanetary He focusing cone at regular intervals since
1996. The intensity decrease with time already observed during the first
two years (Michels et al. \cite{michels}) has dramatically amplified during
the solar activity increase. Intensities seem to reach a plateau in 2001.
Using a model of the cone emission which takes into account both photoionization
and electron impact ionization of neutral helium we show that the photoionization
increase alone cannot explain the observed intensity drop. Data can be
fitted if at minimum activity the electron impact ionization rate is the
solar cycle average rate predicted by Rucinski & Fahr (\cite{1989A&A...224..290R}),
and if this rate is increased by a factor of about 3.5 between 1996 and
2001. Assuming the Rucinski and Fahr radial dependence, such high electron
impact rates create averaged He+ pickup ion (PUI) fluxes which
may reach 50% of the fluxes of ions born after photoionization, or 35% of
PUI total fluxes, as far as 1 AU from the Sun. In slow and dense solar wind
enhancements, in particular in the presence of strong suprathermal tails,
PUIs from electron impact could be the dominant species. This could explain
a fraction of the observed correlation between He+ and H+
pickups and anticorrelation of He+ fluxes with solar wind velocity.
2004-18: Modeling the interstellar-interplanetary
helium 58.4 nm resonance glow: Towards a reconciliation with particle measurements.
Lallement, R.; Raymond, J. C.; Vallerga, J.; Lemoine, M.;
Dalaudier, F.; Bertaux, J. L
Astronomy & Astrophysics, vol. 426, pp. 875-884.
Pioneering observations of the diffuse HeI-58.4 nm background radiation
were performed with a series of satellites in the 70's. Today, their
published results on the flow of interstellar helium atoms in the heliosphere
are still in contradiction with (i) the results of the particle experiments,
i.e. in situ detection of neutrals and pickup ions; (ii) expectations
from heliospheric models and comparison with the hydrogen flow; (iii) results
of the recent helium glow observations with the Extreme Ultraviolet Explorer
(EUVE). Here we discuss these data sets and their modeling, together with
the EUVE data and the first coronographic observations of the helium glow
obtained with the Ultraviolet Coronographic Spectrometer (UVCS) on board
SOHO. We show how they can all be made compatible, and reconciled with
in situ data
We have reanalysed the Prognoz data and we derive an updated and higher
value of the background noise level. Based on this we can now fit the
data satisfactorily with the same set of helium parameters as that one
derived from recent EUVE and in situ data. We suggest that other early
data sets could be reanalyzed in the same way.
Using this updated analysis, EUVE and SOHO-UVCS measurements, we find
that all glow data are compatible with the interstellar parameters V0
≈ 25 km s-1, T0 ≈ 6500 K, λ0 ≈ 74.0
deg, β0 ≈ 6.0 deg (downwind axis, ecliptic coordinates), as
well as with the solar parameters derived from SOHO CELIAS-SEM, SUMER and
CDS observations, i.e. the helium photoionisation rate, the 58.4 nm irradiance,
and the 58.4 nm Doppler width, found to be between 60 and 90 m Å
(30 and 45 km s-1). The density is the least constrained parameter
from the glow measurements. Prognoz lateral scans, EUVE LWS and SOHO UVCS
data are compatible with an interstellar helium density n0 in
the range 0.013-0.016 cm-3. Prognoz anti-solar data and EUVE
scanner data lead to a 40% lower value, suggesting uncertainties in the
calibrations. A large part of the contradictions between particle and remote
sensing results are thus removed, since the above parameters are very similar
to those derived from in situ data.
The high electron impact rates inferred from the UVCS remote sensing
observations imply high fluxes of newly produced helium pickup ions, which
can possibly explain in part the observed correlation between H+
and He+ pickup fluxes, and the inverse correlation between He+
fluxes and solar wind velocity.
2004-17: Synopsis of the interstellar
He parameters from combined neutral gas, pickup ion and UV scattering observations
and related consequences
Möbius, E.; Bzowski, M.; Chalov, S.; Fahr, H.-J.;
Gloeckler, G.; Izmodenov, V.; Kallenbach, R.; Lallement, R.;
McMullin, D.; Noda, H.; Oka, M.; Pauluhn, A.; Raymond, J.;
Ruciński, D.; Skoug, R.; Terasawa, T.; Thompson, W.;
Vallerga, J.; von Steiger, R.; Witte, M.
Astronomy and Astrophysics, v.426, p.897-907.
A coordinated effort to combine all three methods that are used to
determine the physical parameters of interstellar gas in the heliosphere
has been undertaken. In order to arrive at a consistent parameter set that
agrees with the observations of neutral gas, pickup ions and UV backscattering
we have combined data sets from coordinated observation campaigns over three
years from 1998 through 2000. The key observations include pickup ions
with ACE and Ulysses SWICS, neutral atoms with Ulysses GAS, as well as
UV backscattering at the He focusing cone close to the Sun with SOHO UVCS
and at 1 AU with EUVE. For the first time also the solar EUV irradiance that
is responsible for photo ionization was monitored with SOHO CELIAS SEM,
and the He I 58.4 nm line that illuminates He was observed simultaneously
with SOHO SUMER. The solar wind conditions were monitored with SOHO, ACE,
and WIND. Based on these data the modeling of the interstellar gas and its
secondary products in the heliosphere has resulted in a consistent set of
interstellar He parameters with much reduced uncertainties, which satisfy
all observations, even extended to earlier data sets. It was also established
that a substantial ionization in addition to photo ionization, most likely
electron impact, is required, with increasing relative importance closer
to the Sun. Furthermore, the total combined ionization rate varies significantly
with solar latitude, requiring a fully three dimensional and time dependent
treatment of the problem.
2004-16:
Effect of the electron density stratification
on off-limb O VI line profiles: How large is the velocity distribution
anisotropy in the solar corona?
Raouafi, N.-E.; Solanki, S. K.
Astronomy and Astrophysics, v.427, p.725-733
Coronagraphic spectral observations carried out with UVCS on SoHO
have shown that the velocity distribution in the solar corona is highly
anisotropic. Here we examine the influence of the density stratification
on the interpretation of such observations. In particular, we investigate
the profiles of O VI lines emitted in the corona by employing an analytical
2-D model of the large scale coronal magnetic field and solar wind. We concentrate
on the polar coronal holes and take into account the integration along the
line of sight. We find that at distances greater than 1 R⊙ from
the solar surface the widths of the emitted lines are significantly affected
by the details of the adopted electron density profiles. In particular, the
densities deduced by Doyle et al. (\cite{Doyle1999a},b) from SoHO data result
in O VI profiles whose widths and intensity ratio are relatively close
to the values observed by UVCS/SOHO although only isotropic velocity distributions
are employed. Hence we expect that the magnitude of anisotropy of the velocity
distribution deduced from UVCS data depends strongly on the adopted density
profile.
2004-15: On Filaments, Flux
Ropes, & Three-Part CME Structure
R. M. Suleiman, N. U. Crooker, J. C.
Raymond and A. van Ballegooijen
in preparation
The coronal mass ejection (CME) observed on 12 September 2000 by
the instruments aboard the Solar and Heliospheric Observatory (SOHO) provides
new insights into the origin of the classical three-part structure and
its relationship to magnetic flux rope formation. A complex helix was measured
by the Ultraviolet Coronagraph Spectrometer (UVCS) at heliocentric distances
of 3.5 and 6 R_sun. A difference of 300 km/sec in line-of-sight velocities
for two segments of the helix obtained from Doppler measurements implies
expansion and allows one to distinguish which segment was closest to the
observer. Knowledge of the different tilts of the leading and trailing
segments provides one of two components needed to determine magnetic chirality.
Obtaining the second component from a magnetogram, we show agreement with
the chirality predicted from filament properties. The leading field of the
helix matches the pre-maximum orientations of polar fields, consistent with
eruption from a switch-back in the neutral line. Observed Ly-alpha and C
III line emissions indicate that the helix was threaded with filament plasma
of varying density. While the erupting filament was organized in the form
of helix, the pre-erupting filament was likely not threaded by a helical
field. One possibility is that the filament plasma lost its pre-existing
magnetic coherence. EIT data show a concurrent arcade event, consistent with
helix formation by reconnection as part of the CME lift off process and
with possible entrainment of filament plasma onto that helix.
2004-14: An Ion-Cyclotron
Resonance-driven Three-Fluid Model of the Slow Wind near the Sun
Chen, Y.; Li, X.
ApJ, volume 609, Issue 1, pp. L41-L44.
A three-fluid slow wind consisting of electrons, protons, and
O+5 ions is constructed to account for a recent UVCS/SOHO observation. In this model, ion-cyclotron
waves, which are assumed to be generated by a Kolmogorov turbulent cascade,
are assumed to play a key role in driving the slow solar wind. By reproducing
recent measurements on the O+5 parameters along a streamer
axis and the well-known average proton flux in the slow wind near the
Earth, it is demonstrated that the ion-cyclotron resonance mechanism proposed
to explain the observations in the coronal hole and fast wind may also
be important to the ions in the slow solar wind.
2004-13: Spectroscopic measurement
of the plasma electron density and outflow velocity in a polar coronal
hole
Antonucci, E.; Dodero, M. A.; Giordano, S.;
Krishnakumar, V.; Noci, G.
Astronomy and Astrophysics, v.416, p.749-758.
A new spectroscopic method, aimed to derive the plasma electron
density and outflow velocity in expanding solar coronal regions, is discussed
in this paper. The method is based on the analysis of a pair of coronal
lines emitted via collisional and radiative excitation by the same ion,
such as the O VI 1032, 1037 Å, doublet. The merit of this technique
consists in allowing us to derive at the same time electron density and
outflow velocity of the coronal plasma from nearby lines detectable with
the same instrument, provided the constraint on mass flux conservation
along the flow tube connecting solar corona and heliosphere is taken into
account. The results obtained from the analysis of the OVI emission imply
that the physical conditions of a polar coronal hole plasma, observed during
minimum activity, are the following. The electron density decreases from
4× 105 cm-3 at 1.7 Ro to 2-4×
104 cm-3 at 3.1 Ro, whereas the outflow
velocity of the oxygen ions is monotonically increasing to reach 350-500
km s-1 at 3.1 Ro, depending on the assumptions on
the degree of anisotropy of the velocity distribution of the ions. These
results of the velocity of expansion of the fast wind confirm those obtained
with Doppler dimming techniques when assuming the lowest observed density
values for the coronal hole plasma. This implies that, for a rarified corona,
the outflow velocity of the fast solar wind in polar holes can be traced by
the motion of the O VI ions at least up to 2.4 Ro. The analysis
also shows that the degree of anisotropy of the oxygen ions, due to the acceleration
of the ions across the magnetic field in a coronal hole, exhibits a steep
increase and that the geometry of the flow tube diverges very rapidly low
down in the inner corona/transition region. The observations of the extended
corona analysed in this paper are obtained with the Ultraviolet Coronagraph
Spectrometer of the SOHO space mission.
2004-12: A slow streamer
blowout at the Sun and Ulysses
Suess, S. T.; Bemporad, A.;
Poletto, G.
Geophysical Research Letters, Volume 31, Issue 5, CiteID L05801
On 10 June 2000 a streamer on the southeast limb slowly disappeared
from LASCO/C2 over ~10 hours. A small CME was reported in C2. A substantial
interplanetary CME (ICME) was later detected at Ulysses, which was at
quadrature with the Sun and SOHO at the time. This detection illustrates
the properties of an ICME for a known solar source and demonstrates that
the identification can be done even beyond 3 AU. Slow streamer blowouts
such as this have long been known but are little studied.
2004-11: Solar wind:
The solar wind and the Sun-Earth link
Habbal, Shadia Rifia and Woo, Richard
Astronomy & Geophysics, Volume 45, Issue 4, pp. 4.38-4.43.
The solar wind fills the space between the Sun and its planets,
shapes the planetary environments and the heliosphere, and comes to a
screeching halt at the heliopause, the boundary with the interstellar
medium. This tenuous medium is a fertile environment for exotic plasma
processes, most of which are not fully understood. It also holds the intimate
secrets of the mechanisms heating the corona that continue to elude us.
As the only accessible space plasma laboratory, we must continue its exploration
in search of the processes that impact the Earth's environment and govern
the evolution of stars and their planetary systems.
2004-10: Hydrogen Lyα Intensity
Oscillations Observed by the Solar and Heliospheric Observatory Ultraviolet
Coronagraph Spectrometer
Morgan, H.; Habbal, S. Rifai; Li, X.
ApJ, volume 605, Issue 1, pp. 521-527.
We report on a search for significant oscillations in different
coronal structures by applying a wavelet analysis to Solar and Heliospheric
Observatory UVCS observations of the hydrogen Lyα 1216 Å line intensity
taken between 1.5 and 2.2 Rsolar. Significant periodic oscillations,
unlikely to be a result of instrumental effects, are shown to exist in
a coronal hole, the quiet Sun, and a streamer. Observations made sequentially
at different heights but at the same latitude often share similar power
spectra. Neighboring pixels at the same radial distance also share similar
power spectra. These results indicate both a localized structure to the
periodicity and a long-range preservation of oscillation patterns in the
radial expansion of the solar wind. We show that a preference for significant
oscillations with periods of 7-8 minutes exists in three out of the four
observations presented here. Other bands of preferred periodicity are observed
at different heights.
2004-9: Stagnated Outflow
of O+5 Ions in the Source Region of the Slow Solar Wind at
Solar Minimum
Chen, Y.; Esser, R.; Strachan, L. and Hu, Y.
ApJ, volume 602, Issue 1, pp. 415-421.
Using recent coordinated UVCS and LASCO measurements by Strachan
and coworkers to constrain the heating parameters of a one-dimensional
single-fluid minor ion model, we calculate the outflow velocity profile
of O+5 ions in the flow tube overlying the helmet streamer,
which has been supposed to be the source region of the slow solar wind
at least during solar minimum. The background solar wind parameters and
the flow tube geometry are taken from a recent two-dimensional magnetohydrodynamic
solar wind model. We show that in this slow-wind source region the O+5
outflow speed varies nonmonotonically with increasing heliocentric distance.
There is a local minimum of the outflow speed near the streamer cusp point
(about 3 Rsolar), which is below the current observational sensitivity.
This type of ion outflow in the slow solar wind is termed ``stagnated outflow''
in this paper. We also show that the observed effective temperature in the
perpendicular direction (to the magnetic field) and the outflow speed of
the O+5 ions can be used to put limits on their parallel thermal
temperature.
2004-8: CME-Flare Association
Deduced from Catastrophic Model of CMEs
Lin, J.
Solar Physics, v. 219, Issue 1, p. 169-196.
Based on our previous works regarding solar eruptions, we focus
on the relationships among different eruptive phenomena, such as solar
flares, eruptive prominences and coronal mass ejections (CMEs). The three
processes show clear correlations under certain circumstances. The correlation
between a CME and solar flare depends the energy that stored in the relevant
magnetic structure, which is available to drive the eruption: the more
energy that is stored, the better the correlation is; otherwise, the
correlation is poor. The correlation between a CME and eruptive prominence,
on the other hand, depends on the plasma mass concentration in the configuration
prior to the eruption: if the mass concentration is significant, a CME starts
with an eruptive prominence, otherwise, a CME develops an without an apparent
associated eruptive prominence. These results confirm that solar flares,
eruptive prominences and CMEs are different significances of a single physical
process that is related to the energy release in a disrupted coronal magnetic
field. The impact of gravity on CME propagation and the above correlations
is also investigated. Our calculations indicate that the effect of gravity
is not significant unless the strength of the background field in the disrupted
magnetic configuration becomes weak, say weaker than 30 G.
2004-7: The Role of
Magnetic Reconnection in the Observable Features of Solar Eruptions
Lin, J.; Raymond, J. C. and van Ballegooijen, A. A.
ApJ, volume 602, Issue 1, pp. 422-435.
There are two competing classes of models for coronal mass ejections
(CMEs): those that assume a preexisting magnetic flux rope and those that
can make a flux rope during the eruption by magnetic reconnection. The
present work is based on the model with a preexisting flux rope. We investigate
the evolution of morphological features of the magnetic configuration in
a CME according to a catastrophe model of flux rope CMEs developed previously.
For the parameters chosen for the present work, roughly half of the total
mass and magnetic flux are contained in the initial flux rope, while the
remaining plasma and poloidal magnetic flux are brought by magnetic reconnection
from the corona into the current sheet and from there into the CME bubble.
These features and the corresponding physical processes are identical to
those described by the non-flux rope models. Thus, the flux rope and non-flux
rope models are less distinct than is generally assumed. The reconnected
magnetic flux can account for the rapid expansion of the ejecta, and the
plasma flowing out of the current sheet fills the outer shell of the ejecta.
We tentatively identify the outer shell, the expanded bubble, and the flux
rope with the leading edge, void, and core of the three-component CME structure,
respectively. Thus, the final mass, speed, and magnetic energy-the quantities
that determine the geoeffectiveness of the CME-are determined not in the
initial eruption but during the CME expansion, at heights of a few solar
radii. The aspects of this explanation that need improvement are also discussed.
2004-6: Motions of Flare
Ribbons and Loops in Various Magnetic Configurations
Lin, Jun
Solar Physics, v. 222, Issue 1, p. 115-136.
Kopp-Pneuman-type magnetic configurations, which include a vertical
current sheet, with various background fields are investigated. Dissipation
of the current sheet as a result of magnetic reconnection produces bright
flare ribbons on the solar disk and a growing flare loop system in the
corona. In principle, the growth of flare loop system is governed by a
reconnection process only, and the behavior of flare ribbons is also controlled
by the background field. The flare ribbons may appear either separate
or attached to one another at the onset of the flare depending on the
background field distribution on the boundary surface. We calculate the
decrease in height that magnetic field lines undergo after they have reconnected
to form closed loops. Following previous practice, we refer to this decrease
as field line shrinkage. Unlike the motions of flare ribbons, the shrinkage
of flare loops depends weakly on the background field. Individual loops
always shrink fastest at the moment it is produced by reconnection and just
starts to leave the current sheet. The earlier the loop forms, the more
and faster it shrinks. The relevant observations are explained on the basis
of our calculations, and the aspects of the explanation that need improvement
are also discussed.
2004-5:
Coronal transients and metric type II radio
bursts. I. Effects of geometry
Mancuso, S.; and Raymond, J. C.
Astronomy and Astrophysics, v.413, p.363-371.
In this paper we investigate the relationship between type II
and coronal transient activity in terms of emission originating from
the top or the flanks of a bow/piston shock surface, extending just above
the coronal mass ejection (CME) leading edge surface. For this purpose,
we used ground-based metric type II radio burst observations of twenty-nine
events in conjunction with Large Angle and Spectrometric Coronagraph
(LASCO/SOHO) and UltraViolet Coronagraph Spectrometer (UVCS/SOHO) observations.
With the refined density diagnostic offered by the UVCS instrument, we
analyzed the type II dynamics in conjunction with the associated CME
dynamics. Although we found some correlation, in all but a few cases the
coronal transients appeared to lead the type II emission locations by
several minutes, in apparent disagreement with a CME-driven origin interpretation.
By applying a simple model, we found however that a piston-driven origin
is certainly viable for all the events under study on the hypothesis that
the radio emission originates in discrete locations above the top or the
flanks of bow/piston shock surfaces extending just above the transient
leading edges.
2004-4:
The Role of Magnetic Reconnection in the
Observable Features of Solar Eruptions
Lin, J.; Raymond, J. C.; and van Ballegooijen, A. A.
ApJ, volume 602, Issue 1, pp. 422-435.
There are two competing classes of models for coronal mass ejections
(CMEs): those that assume a preexisting magnetic flux rope and those
that can make a flux rope during the eruption by magnetic reconnection.
The present work is based on the model with a preexisting flux rope. We
investigate the evolution of morphological features of the magnetic configuration
in a CME according to a catastrophe model of flux rope CMEs developed
previously. For the parameters chosen for the present work, roughly half
of the total mass and magnetic flux are contained in the initial flux rope,
while the remaining plasma and poloidal magnetic flux are brought by magnetic
reconnection from the corona into the current sheet and from there into
the CME bubble. These features and the corresponding physical processes
are identical to those described by the non-flux rope models. Thus, the
flux rope and non-flux rope models are less distinct than is generally
assumed. The reconnected magnetic flux can account for the rapid expansion
of the ejecta, and the plasma flowing out of the current sheet fills the
outer shell of the ejecta. We tentatively identify the outer shell, the
expanded bubble, and the flux rope with the leading edge, void, and core
of the three-component CME structure, respectively. Thus, the final mass,
speed, and magnetic energy-the quantities that determine the geoeffectiveness
of the CME-are determined not in the initial eruption but during the CME
expansion, at heights of a few solar radii. The aspects of this explanation
that need improvement are also discussed.
2004-3:
Active Region Streamer Diagnostics 2001
September 14-16
Uzzo, M., Ko, Y.-K. and Raymond, J. C.
ApJ, volume 603, Issue 2, pp. 760-775.
From 2001 September 14 to 16 the Ultraviolet Coronal Spectrometer
aboard the Solar and Heliospheric Observatory conducted a series of observations
designed to study the absolute elemental abundances of an active region
streamer. The absolute elemental abundances for O, Si, Fe, Mg, N, S, and
Ar were calculated utilizing a constant electron temperature technique
and an alternative approach to the emission measure technique, both producing
complementary results. The manifestation of the first ionization potential
(FIP) effect was observed here as in previous streamers. Two aspects of
this streamer make it unusual and worth special attention. The active region
streamer observed possessed an abundance-depleted core typically, although
not exclusively, seen in quiescent streamers. A coronal mass ejection (CME)
occurred at the active region during the observations. This event resulted
in the second unexpected aspect of this streamer, its quick return to the
same general abundance characteristics that existed before the CME occurred.
The re-formation of a depleted core and the presence of the FIP effect immediately
after a highly disruptive event are surprising given the 1 day timescale
predicted by streamer theories.
2004-2:
Densities and Velocities in Fast Coronal
Mass Ejections: Radiative Pumping of the O VI Doublet
Raymond, J. C., and Ciaravella, A.
ApJ, volume 606, Issue 2, pp. L159-L162.
In very fast coronal mass ejections (CMEs), it is possible for
O VI ions to scatter Lyβ photons that originate in the solar chromosphere
and for the λ1037 transition of O VI to scatter λ1032 photons from the
solar transition region. This scattering process can be identified by departures
of the O VI I1032/I1037 intensity ratio from its collisional
value of 2. We report the first detection of this effect in a CME that
occurred on 2000 June 28, and we show that this pumping provides a density
diagnostic for CMEs faster than 1600 km s-1. At heliocentric
distances near 3 Rsolar, this diagnostic is useful for densities
in the 5×105-107 cm-3 range.
2004-1: Low-latitude Coronal Holes
during Solar Maximum
M. P. Miralles, S.
R. Cranmer, J. L. Kohl
Advances in Space
Research, Volume 33, Issue 5, p. 696-700 (2004).
Analyses of it in
situ observations have shown that some small coronal holes are sources of
slow solar wind near solar maximum when polar coronal holes become smaller
and disappear.However, not all coronal holes at solar maximum produce slow
wind.The Ultraviolet Coronagraph Spectrometer (UVCS) aboard it SOHO has been
used to observe large low-latitude coronal holes during solar maximum that
produced fast solar wind. UVCS observations show that large equatorial holes
at solar maximumhave plasma properties that seem to bridge the gap between
solar minimum polar coronal holes and streamers. The ion kinetic temperatures
in equatorial holes are about 2 times larger than those in a solar minimum
equatorial streamer, and about a factorof 2 smaller than those in polar coronal
holes above 2 R_sun. The outflow speeds for the large equatorial holes observed
by UVCSare only about 100km s-1, a factor of 4 smaller than those in polarholes,
at 3 R_sun. However, it in situ dat! a corresponding to these equatorial
coronal holes showed asymptotic wind speeds of 600--700km s-1. These wind
speeds are similar to those observed over polar coronal holes at solar minimum.
In contrast to the polar coronal holes, the bulk of the solar wind acceleration
in large equatorial coronal holes at solar maximum must occur above
3 R_sun.Thus, the combination of spectroscopic measurements in the extendedcorona,
where the primary solar wind acceleration occurs, and it in situ measurements
made in the solar wind can be used to obtain the solarwind acceleration
as a function of heliocentric distance.These observations provide detailed
empirical constraints for theoretical models and may be key to understanding
how the various types of solar wind plasma are heated and accelerated.
This work is supported by NASA under Grant NAG5-11420 to theSmithsonian
Astrophysical Observatory, by the Italian SpaceAgency and by PRODEX (Swiss
contribution). [ ]