2000-32: The electron velocity distribution in the high-speed solar wind: Modelling the effects of protons
O. Lie-Svendsen and E. Leer
JGR, volume 105, pp 35-46
The evolution of the electron velocity distribution function (VDF) in high-speed solar wind streams is modeled taking the expanding geometry, the polarization electric field, and Coulomb collisions into account. The VDF we find at the orbit of Mercury is composed of an isotropic, collision-dominated core, a trapped, anisotropic population called ``halo'' in this study, and a narrow, high-energy ``strahl'' that escapes along the magnetic field. The distribution function is very similar to the electron VDF observed in the low-density, high-speed solar wind by Pilipp et al. [1987] and Phillips et al. [1989]. The main features of the VDF can be obtained by considering only electron self-collisions; the effect of proton collisions is to make the distribution function more isotropic. At low energies, collisions with protons dominate the angular scattering, but electron self-collisions alone are frequent enough to keep the core of the distribution function quite isotropic. The expanding geometry produces an anisotropic halo and a narrow strahl. The angular scattering by protons reduces the anisotropy of the trapped halo particles and broadens the lower-energy part of the strahl. Along the magnetic field the resulting electron velocity distribution is composed of a relatively cold core and a halo-strahl spectrum that is ``flatter'' than the coronal spectrum. The two-temperature electron distribution function often observed in the solar wind may therefore be produced by Coulomb collisions and should not be taken as a ``proof'' of a non-Maxwellian (two-temperature) distribution function in the corona.
Gardner, L. D.; Atkins, N.; Fineschi, S.; Smith, P. L.; Kohl, J. L.; Maccari, L.; Romoli, Marco
Proc. SPIE 4139, pp. 362-369
Instrumentation for UV/EUV Astronomy and Solar Missions, eds.
Fineschi, Korendyke, Siegmund & Woodgate
We have carried out measurements
of efficiency as functions of position across the surfaces of replica
grating made from the same masters as the UVCS/SOHO flight units. Variations
in first order efficiency which significantly affect the interpretation
of UVCS data are found along the direction perpendicular to the grooves.
Variations are also found along the direction parallel to the grooves,
but these do not seriously affect UVCS data interpretation. The measurements
and their application to the radiometric calibration of UVCS/SOHO are
discussed.
2000-30: Velocity shear-induced mode conversion in solar wind and streamer plasmas
Kaghashvili, E. Kh., & Esser, R.
Astrophys. J., 539, 463, 2000
The possible scenario of Alfvén
wave transformation into other MHD waves due to inhomogeneous flow in
different regions of solar wind/corona is examined. We have chosen four examples
with different plasma parameters and velocity shear parameters as representative
cases for the plasma conditions. Two of the cases are representative of coronal
holes and streamers at a radial distance of 1.6 Rsolar. The third example
examines the effect of an increased velocity shear due to the vicinity of
a boundary. The fourth case represents solar wind conditions at 1 AU, where
the plasma parameter is about unity. To study the wave coupling in the above
described cases, we make use of ``nonmodal'' formalism, which appears to
be a useful tool for wave processes taking place in a plasma with velocity
shear. Our finding is that the possibility of coupling between Alfvén
waves and fast magnetosonic waves exists in coronal holes while for streamer
conditions the Alfvén mode couples to the slow magnetosonic mode.
In the inner corona, it seems that these two possible ways of coupling do
not overlap each other in the same structures but beyond the Alfvénic
point, where β is close to unity, both of them can operate at the same time
and the mechanism is more sensitive to the background plasma conditions.
2000-29: Nonequilibrium Ionization and First Ionization Potential Effect Diagnostics
Edgar, Richard J.; Esser, Ruth
ApJ 538 pp. L167-L170
We examine the accuracy of a common
first ionization potential effect diagnostic, the ratio of Ne VI to Mg
VI lines in the solar transition region. Since the two ions have quite similar
contribution functions near their maxima in equilibrium, the ratio of these
two ions is often taken to be the abundance ratio of Ne and Mg. First we
compute nonequilibrium ionization effects on the ratio f(Ne+5)/f(Mg+5) of
ion fractions for a variety of simple flows through the transition region.
Calculating the spectral line ratios for a few examples, we then show that
nonequilibrium effects as well as temperature and density dependence must
be evaluated for each line ratio used in the diagnostics.
2000-28: Dissipation of Slow Magnetosonic Waves in Coronal Plumes
Ofman, L.; Nakariakov, V. M.; Sehgal, N.
ApJ 533, pp. 1071-1083
Recently, slow magnetosonic waves were identified in polar plumes, at heights up to about 1.2 Rsolar using the Extreme Ultraviolet Imaging Telescope (EIT) observations of quasi-periodic EUV intensity fluctuations, and higher in the corona using the Ultraviolet Coronagraph Spectrometer (UVCS) white-light channel. First, we derive the linear dispersion relation for the slow waves in the viscous plasma. Next, we derive and solve an evolutionary equation of the Burgers type for the slow waves, incorporating the effects of radial stratification, quadratic nonlinearity, and viscosity. Finally, we model the propagation and dissipation of slow magnetosonic waves in polar plumes using one-dimensional and two-dimensional MHD codes in spherical geometry. The waves are launched at the base of the corona with a monochromatic source. We find that the slow waves nonlinearly steepen as they propagate away from the Sun into the solar wind. The nonlinear steepening of the waves leads to enhanced dissipation owing to compressive viscosity at the wave fronts. The efficient dissipation of the slow wave by compressive viscosity leads to damping of the waves within the first solar radii above the surface. We investigate the parametric dependence of the wave properties.
2000-27: Source Regions of the Slow Solar Wind in Coronal Streamers
Ofman, L.
Geophys. Res. Lett., 27, pp.2885-2888
Recent SOHO/UVCS observations of the O5+ ion line emission at 1032 Angstrom in coronal streamers indicate that the emission is stronger by an order of magnitude at the edges (legs) of streamers than in the central core of streamers. In contrast, the brightness of the Ly-alpha emission peaks in the core of streamers. I have developed the first 2.5D, three-fluid numerical MHD model of the slow solar wind dow in a coronal streamer. Using the model I find that the-enhancement of the oxygen line emission occurs due to the enhanced abundances of O5+ ions in the legs of streamer caused by the Coulomb friction with the outflowing protons. Thus, the enhanced O5+ emission trances the source regions of the slow solar wind in coronal streamers. The identification of these regions helps to understand the origins and the composition of the slow solar wind.
2000-26: A Kinetic Model of Coronal Heating and Acceleration by Ion-Cyclotron Waves: Preliminary Result
Isenberg, P. A.; Lee, M. A.; Hollweg, J. V.
Solar Phys., 193, pp.247-257
We present a kinetic model of the
heating and acceleration of coronal protons by outward-propagating ion-cyclotron
waves on open, radial magnetic flux tubes. In contrast to fluid models which
typically insist on bi-Maxwellian distributions and which spread the wave
energy and momentum over the entire proton population, this model follows
the kinetic evolution of the collisionless proton distribution function in
response to the combination of the resonant wave-particle interaction and
external forces. The approximation is made that pitch-angle scattering by
the waves is faster than all other processes, resulting in proton distributions
which are uniform over the resonant surfaces in velocity space. We further
assume, in this preliminary version, that the waves are dispersionless so
these resonant surfaces are portions of spheres centered on the radial sum
of the Alfvén speed and the proton bulk speed. We incorporate the
fact that only those protons with radial speeds less than the bulk speed
will be resonant with outward-propagating waves, so this rapid interaction
acts only on the sunward half of the distribution. Despite this limitation,
we find that the strong perpendicular heating of the resonant particles,
coupled with the mirror force, results in substantial outward acceleration
of the entire distribution. The proton distribution evolves towards an incomplete
shell in velocity space, and appears vastly different from the distributions
assumed in fluid models. Evidence of these distinctive distributions should
be observable by instruments on Solar Probe.
2000-25: Alfvénicity of fluctuations associated with the Kelvin-Helmholtz instability
Parhi, S., Suess, S. T.
Physics of Plasmas, 7, pp.2995-3003
The velocity shear which exists
between three layers in an ideal plasma is studied. This configuration
is modeled as a jet (or, strictly speaking, a wake) embedded in a uniform
medium using a magnetohydrodynamics (MHD) code developed for astrophysical
jet simulations. Weak and strong magnetic fields are considered both inside
and outside the jet with a shear Mach number of 6. The shear can be Kelvin-Helmholtz
(KH) unstable and evolve into a new less sheared pattern. There exists
extensive literature on the KH instability which is extended by quantitatively
describing the MHD properties of the fluctuations associated with the instability.
To do so, a time series analysis of the fluctuations at various points
inside and adjacent to the jet is performed. Specifically, points either
in the center of the jet or just outside the transition layer-the initial
location of the shear layer are considered. In the nonlinear stage, the
perturbation is found to be a sum of the fast magnetosonic mode, slow magnetosonic
mode, and the Alfven component. To quantitatively evaluate the fluctuations,
the normalized cross-helicity and Elsasser ratios are calculated, which
in turn measure the degree of Alfvenicity. Fully nonlinear fluctuations
are found to be more Alfvenic than magnetosonic in the low beta case (
beta approximately=0.833) as compared with high beta case ( beta approximately=13.3).
This is in contrast to linear modes generated by the KH instability, which
are magnetosonic modes
2000-24: SOHO Observations of a Coronal Mass Ejection
A. Akmal, J. Raymond, A. Vourlidas, B. Thompson, A. Ciaravella, Y-K. Ko, M. Uzzo and R. Wu
ApJ 553, pp.922-934.
We describe a Coronal Mass Ejection (CME) observed on 23 April 1999 by the Ultraviolet Coronagraph Spectrometer (UVCS), the Extreme ultraviolet Imaging Telescope (EIT) and the Large Angle and Spectrometric COronagraphs (LASCO) aboard the Solar and Heliospheric Observatory. The event was also observed by the Nobeyama radioheliograph in microwaves. This 480 km s minus 1 CME exhibits a number of interesting and unusual features. In addition to the O VI and C III lines typical of UVCS spectra of CMEs, we also detect the forbidden and intercombination lines of O V at gamma-gamma 1213.8, 1218.4, the NIII lines at gramma-gamma 991.6, 989.8 and Ly Beta. Of particular interest are the relative intensities of the O V lines, which represent an accurate electron density diagnostic not generally available at 3.5 R. By combining the density with the column density derived from LASCO, we obtain the emission measure of ejected gas. With the help of models of the temperature and time-dependent ionization state of the expanding gas, we determine a range of heating rates required to account for the UV emission lines. The total thermal energy deposited as the gas travels to 3.5 R is comparable to the kinetic and gravitational potential energies. We note a core of colder material radiating in C III, surrounded by hotter material radiating in the O V and O VI lines. This concentration of the coolest material into small regions may be a common feature of CME's. This event thus represents a unique opportunity to describe the morphology of a CME, and to characterize its plasma parameters.
2000-23: Propagation and Dissipation of Slow Magnetosonic Waves in Coronal Plumes
Ofman, L.
ASP Conference Series, 205, p. 147
Recently, slow magnetosonic waves were identified in polar plumes at heights of up to ∼1.2Rodot using the Extreme Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) spacecraft. We model the propagation of these waves in solar polar coronal plumes, and the dissipation of the waves by compressive viscosity. First, we estimate the damping rate of the waves using linear theory. Next, we model the slow magnetosonic wave with a 2D MHD code in spherical geometry and assume azimuthal symmetry of the plumes. We find that the slow magnetosonic waves generated at the base of the plumes by periodic compressions propagate outward and dissipate in the plumes. The background atmosphere includes Parker's flow and the corresponding density stratified by gravity close to the Sun. The stratification leads to the nonlinear steepening, and to enhanced dissipation over the linear rate of the waves. The dissipation may contribute to the heating and the acceleration of the solar wind. By observing the density structure of the plumes during an eclipse it may be possible to learn more about these waves.
2000-22: NOAA AR 8210: Evolution and Flares from Multiband Diagnostics
A. Warmuth, A. Hanslmeier, M. Messerotti, A. Cacciani, P.F. Morettit, W. Otruba
Solar Physics, 194, pp.103-120
NOAA 8210 has been a region showing a remarkable level of activity well before solar maximum. Dominated by a large, rapidly rotating δ spot, it produced several intense flares during its disk passage at the end of April-beginning of May 1998. We examine the development of AR 8210 in Hα and white light (WL) and study the evolution of its complex magnetic topology. While the other principal flares are briefly reviewed, the great X1.1/3B flare of 2 May, which was observed at Kanzelhöhe Solar Observatory during a SOHO/UVCS ground support campaign, is studied in detail. This event has been documented in full-disk Hα and Na-D intensitygrams, Dopplergrams, and magnetograms, with a time cadence of one minute each. The flare was associated with a CME and produced significant geomagnetic effects. Furthermore, we point out the perspectives for our planned Flare Monitoring and Alerting System, since the two new instruments (Magneto-Optical Filter and Digital H_alpha; camera), which made their first operational run with the campaign, are crucial components for this program.
2000-21: Cyclotron Resonances in Coronal Holes 3. A Five-Beam Turbulence-Driven Model
J. V. Hollweg
J. Geophys. Res., 105, 15699-15713.
For pt.2 see ibid., vol.104, no.A11, p.24793-805 (1999). Following Hollweg and Johnson [1988], Isenberg [1990], and Li et al. [1999a], the present authors postulate that the Sun launches a flux of low-frequency Alfven waves, which dissipate via a turbulent cascade to high frequencies where the energy is absorbed by ion cyclotron resonant interactions. The plasma consists of two proton beams, which are proxies for the resonant and nonresonant halves of their distribution function, two He++ beams, which are proxies for the strongly and weakly resonant halves of their distribution, and a single beam of O+5 with vanishing density. The level of the power spectrum at the high resonant frequencies is determined by the condition that the protons and He++ resonantly absorb energy at the same rate at which the low-frequency waves are dissipating. Once the level of the high-frequency power spectrum is determined, the resonant heating and acceleration of the O+5 can be calculated. For both Kolmogorov and Kraichnan scalings of the turbulent dissipation the model yields results for the protons that are in reasonably good agreement with the UVCS/SOHO results. The He++ becomes more than mass proportionally heated and flows faster than the protons, close to the Sun. However, their model is unable to reproduce the UVCS/SOHO observation that the O+5 temperature is still increasing with heliocentric distance r out to 3.5 R_sun. Instead, the O+5 becomes very hot initially, experiences a strong mirror force, and accelerates to high speed, which in turn leads to rapid adiabatic cooling
2000-20: Characteristics of solar coronal streamers - Element abundance, temperature and density from coordinated CDS and UVCS SOHO observations
S. Parenti, B.J.I. Bromage, G. Poletto, G. Noci, J.J. Raymond & G.E. Bromage
A&A, 363, pp.800-814
This paper presents the results from coordinated observations of streamers acquired by the SOHO Coronal Diagnostic Spectrometer CDS and UltraViolet Coronograph Spectrometer UVCS experiments. Data from different altitudes within the solar corona were taken, with the purpose of determining their physical parameters-- densities, electron temperatures and element abundances -- and their changes over the altitude range between 1.02 and 1.6 R_odot.Further UVCS streamer data, taken about two months later are used for a further comparison of the behaviours seen in two different streamers. Whenever possible, alternative methods have been adopted to determine the same physical parameter, as a cross check. In particular, the DEM technique has been applied to UVCS data, in order to compare abundance values derived in this way, with those obtained using the method of Raymond et al. citeray97. We conclude that abundances in the low corona covered by CDS data do not show evidence for abundance variation, with respect to photospheric values, while, at UVCS altitudes, a depletion of allelement abundances is clearly evident. No clear evidence of a FIP effect in streamers was found we get contrasting results from the only two high FIP elements present in our spectra.
2000-19: The Sun and the Solar Wind Close to the Sun
Suess, S. T.
Advances in Space Research 26, 761 (Proc. 1998 COSPAR Symposium, Nagoya, Japan)
One of the benefits from the Ulysses,
SOHO, and YOHKOH missions has been a strong stimulus to better understand
the magnetohydrodynamic processes involved in coronal expansion. Three topics
for which this has been especially true are described here. These are: (i)
The observed constancy of the radial interplanetary magnetic field strength
(as mapped to constant radius). (ii) The geometric spreading of coronal plumes
and coronal holes, and the fate of plumes. (iii) The plasma b in streamers
and the physics of streamer confinement.
2000-18: Identification of the Coronal Sources of the Fast Solar Wind
Giordano, S.; Antonucci, E.; Noci, G.; Romoli, M.; and Kohl, J.
ApJL, 531, pp. L79-L82.
The present spectroscopic study
of the ultraviolet coronal emission in a polar hole, detected on 1996 April
6-9 with the Ultraviolet Coronagraph Spectrometer aboard the Solar and Heliospheric
Observatory spacecraft, identifies the interplume lanes and background coronal
hole regions as the channels in which the fast solar wind is preferentially
accelerated. In interplume lanes, at heliocentric distance 1.7 R_sun, the
corona expands at a rate between 105 and 150 km s^-1, that is, much faster
than in plumes in which the outflow velocity is between 0 and 65 km s-1.
The wind velocity is inferred from the Doppler dimming of the O VI &
lambda;1032, 1037 lines, within a range of values, whose lower and upper
limit corresponds to anisotropic and isotropic velocity distribution of the
oxygen coronal ions, respectively.
2000-17: Latitudinal Dependence of Outflow Velocities from O VI Doppler Dimming Observations during the Whole Sun Month
Strachan, L., A. V. Panasyuk, D. Dobrzycka, John L. Kohl, G. Noci, S. E. Gibson, D. A. Biesecker
J. Geophys. Res., 105, 2345
Empirical determinations of outflow
velocities in the solar corona provide a much needed constraint, along with
density and temperature determinations, of the acceleration and heating mechanisms
in the extended corona. Much progress has been made on density determinations
from white light polarized brightness observations but outflow velocities
have been more difficult to determine. We present the first determinations
of outflow velocities versus height and latitude based on a three-dimensional
(3-D) reconstruction of the O VI 1032 and 1037 Angstrom emissivities. The
Doppler dimming (and pumping) of the local emissivities give true localized
outflow velocities at the selected locations in the extended corona from
~1.75 to 2.75 solar radii. The velocities are based on an empirical
model of the corona which is constrained by the reconstructed O VI emissivities
derived from the SOHO Ultraviolet Coronagraph Spectrometer (UVCS) synoptic
observations and by electron density determinations based on white light
measurements from the SOHO Large Angle Spectroscopic Coronagraph (LASCO)
and Mauna Loa Solar Observatory (MLSO) Mk III coronameter.
2000-16: Polar Plumes and Inter-plume regions as observed by SUMER on SOHO
D. Banerjee, L. Teriaca, J.G. Doyle, P. Lemaire
Solar Physics, 194, pp.43-58
We present observations of Ovi 1032 Å line profiles obtained with the SUMER instrument on SOHO extending from the solar disk to 1.5 R_solar above the limb in the north polar coronal hole. Variations of the intensity and linewidth in the polar plume and inter-plume regions are investigated. We find an anti-correlation between the intensity and the linewidth in the plume and inter-plume regions with detailed plume structures been seen out to 1.5 R_solar. Possible implications regarding the magnetic topologies of these two regions and related heating mechanisms are discussed. The Ovi linewidth measurements are combined with UVCS output to provide an overview of its variations with height extending up to 3.5 R_solar. We find a linear increase of the linewidth from 1 to 1.2 R_solar, then a plateau followed by a sharp increase around 1.5 R_solar.
2000-15: The May 1997 SOHO-Ulysses Quadrature
Suess, S. T., G. Poletto, M. Romoli, M. Neugebauer, B. Goldstein, & G. Simnett
J. Geophys. Res., 105, pp.25033-51
We present results from the May
1997 SOHO-Ulysses quadrature (SOHO-Sun-Ulysses angle=90 degree), near sunspot
minimum.
Ulysses was at 5.1 AU, 10 degree north of the solar equator, and off the
east limb. It was also at the very northern edge of the streamer belt. Nevertheless,
SWOOPS detected only slow, unusually smooth wind and there was no direct
evidence of fast wind from the northern polar coronal hole or of mixing with
fast wind. LASCO images show that the streamer belt at 10 degree N was narrow
and sharp at the beginning and end of the two week observation interval,
but broadened in the middle. A related change in density, but not flow speed,
occurred at Ulysses. Under these conditions it was possible to show that
densities derived from UVCS in the lower corona are closely related to those
in the solar wind, both over quiet intervals and in transient events on the
limb. Density and velocity in one small transient observed by both LASCO
and UVCS are analyzed in detail.
2000-14: Fast Solar Wind Acceleration by Alfv\'en Waves: Observable Effects on the EUV Lines Detected by SOHO/UVCS
R. Ventura, S. Orlando, G. Peres and D. Spadaro
A&A 352, 670
SOHO/UVCS observations of the most
intense EUV spectral lines emitted by the solar corona have been providing
us a good opportunity to study in detail the acceleration regions of the
solar wind. In this work we aim at deriving useful diagnostics and
identifying possible signatures of Alfv\'en waves momentum deposition. More
specifically we investigate, with the help of a detailed wind model (Orlando
et al. 1996), the insight and the constraints that these observations give
on the presence of Alfv\'en waves, as deduced from the influence of the waves
on the solar wind structure and dynamics. The model developed by Orlando
et al. (1996) accounts for the momentum deposition by a spectrum of non-WKB
Alfv\'en waves, generated in the Sun's lower atmosphere and undergoing significant
reflection across the transition region. We compute a set of wind solutions
characterized by different physical conditions, synthesize, from them, the
emission in the Ly-alpha, Ly-beta and O VI doublet (1032~~\AA, 1038~~\AA)
lines and derive possible diagnostics. We finally compare our results
with the most recent SOHO/UVCS data.
2000-13: SOHO and radio observations of a CME shock wave
John C. Raymond, Barbara J. Thompson, O.C. St. Cyr, Nat Gopalswamy, S. Kahler, M. Kaiser, A. Lara, A. Ciaravella, M. Romoli, R. O'Neal
J. Geophys. Res. Lett., 2000, 27, 1493
A 1200 km/s Coronal Mass Ejection
was observed with the SOHO instruments EIT, LASCO and UVCS on June 11, 1998.
Simultaneously, Type II radio bursts were observed with the WAVES experiment
aboard the Wind spacecraft at 4 MHz and by ground-based instruments at metric
wavelengths. The density in the shock wave implied by the higher frequency
is close to that inferred from the SOHO/UVCS experiment. The drift
rates of the Type II radio bursts suggest shock speeds lower than the speed
derived from SOHO observations. The SOHO/UVCS spectrum shows enhanced
emission in lines of O^5+ and Si XI, consistent with modest compression
in an MHD shock.
2000-12: Reconciling spectroscopic electron temperature measurements in the solar corona with in situ charge state observations
Ruth Esser and Richard J. Edgar
ApJL, 532, pp. L71-L74.
It has been a puzzle for quite some
time that spectroscopic measurements in the inner corona indicate electron
temperatures far too low to produce the ion fractions observed in situ
in the solar wind. In the present paper we show that in order to reconcile
the two sets of measurements, a number of conditions have to exist in the
inner corona: 1) The electron distribution function has to be Maxwellian
or close to Maxwellian at the coronal base; 2) the non-Maxwellian character
of the distribution has to develop rapidlyas a function of height and has
to reach close to interplanetary properties inside of a few solar radii;
3) ions of different elements have to flow with significantly different speeds
to separate their ``freezing-in'' distances sufficiently so they can encounter
different distribution functions. We choose two examples to demonstrate that
these conditions are general requirements if both coronal electron temperatures
and in situ ion fractions are correct. However, these two examples also show
that the details of the required distribution functions are very sensitive
to the exact electron temperature, density and ion flow speed profiles in
the region of the corona where the ions predominantly form.
2000-11: A four-fluid turbulence-driven solar wind model for preferential acceleration and heating of heavy ions
You Qiu Hu, Ruth Esser, and Shadia R. Habbal
J. Geophys. Res. 105, 5093
We present for the first time a
one-dimensional, four-fluid turbulence-driven solar wind model in order to
investigate the preferential acceleration and heating of heavy ions by the
resonant cyclotron interaction with parallel-propagating left-hand-polarized
ion cyclotron waves. The model contains four species: electrons, protons,
alpha particles, and one species of minor ions. A Kolmogorov type of cascade
effect is introduced to transfer energy from the low-frequency Alfv\'en waves
to the high-frequency ion cyclotron waves, which are assumed to be entirely
dissipated by the wave-particle interaction. The quasi-linear theory of the
wave-particle interaction is invoked to distribute the dissipated wave energy
among the three ion species based on a given power-law spectrum of the ion
cyclotron waves and the cold plasma dispersion relation. It is found that
in terms of the cold plasma dispersion relation, the dispersiongenerated
by all ion species has an appreciable influence on both the behavior of the
major species and the preferential acceleration and heating of the minor
ions. The larger the number of species included in the dispersion relation
is, the stronger preferential acceleration and heating produced by the waves
for the heavy ions close to the Sun will be. A detailed comparison is carried
out between two cases, one with and the other without including the dispersive
effect of the minor ions. Although the solutions for the two cases are somewhat
different, they predict a more or less similar behavior of the minor ions,
which essentially agrees with recent observations from SOHO. This indicates
that the resonant cyclotron interaction may be responsible for the preferential
acceleration and heating of minor ions in the fast solar wind. Furthermore,
the influence of minor ions on the proton-alpha solar wind is found to be
dominated by the dispersive effect of the minor ions. Even though such an
influence is exaggerated by the cold plasma dispersion relation, it is still
small and remains within the present observational uncertainties. Therefore,
minor ions may be treated approximately as test particles in the solar wind.
2000-10: Ultraviolet Spectroscopy of Polar Coronal Jets
D. Dobrzycka, J. C. Raymond, & S. R. Cranmer,
ApJ, 538, 922
We have observed a total of five
UVCS/{\it SOHO} polar jets that correlate with the Extreme-Ultraviolet Imaging
Telescope (EIT) and
Large Angle Spectrometric Coronagraph (LASCO) jet events. We analyzed spectroscopic
observations of these jets and found that they typically undergo two phases:
at the first phase the \ion{O}{6} lines show a brief intensity enhancement
(by a factor of 1.4) and narrowing (by a factor of 0.8), while the HI Ly-alpha
line is not enhanced, and the second phase, about 25 minutes later, when
the HI Ly-alpha line shows an intensity enhancement (by a factor of 1.3)
and narrowing (by a factor of 0.8), while the \ion{O}{6} line is relatively
unchanged. We modeled the observable properties of the jets from 1997 August
5, detected at 1.71 R_sun. We interpret the first phase as the fast,
dense centroid of the jet passing by the UVCS slit. The empirical jet
model was able to reproduce the observed line properties with electron density
enhancement by a factor of 3.2 (with a resulting density of 4.5 x 10^6 cm^-3,
an electron temperature decrease (change by a factor of 0.50 to 750,000 K),
and the centroid outflow velocity larger than 280 km s^-1. During the second
phase, the model required a further decrease in the electron temperature
(change by a factor of 0.10, with a jet temperature of only 150,000 K), along
with a weaker electron density (1.7 x 10^6 cm^-3) and an outflow velocity
of 205 km s^-1. Possible scenarios of the electron temperature variations
needed to account for observed conditions on 1997 August 5 indicate that
some heating is required. We computed models of the temperature and nonequilibrium
ionization state of an expanding plasma using various forms for the heating
rates. We found that the jet had to leave the Sun at an electron temperature
below 2.5 x 10^6 K and that a heating rate of the same order as the average
coronal hole heating is required. Such low initial temperatures are consistent
with the idea that the jets observed by LASCO, EIT, and UVCS are different
than previously observed coronal X-ray jets.
2000-9: Ion Cyclotron Wave Dissipation in the Solar Corona: The Summed Effect of More than 2000 Ion Species
S. R. Cranmer
Ap. J. (2000), 532, pp.1197-1208.
Using empirical velocity distributions
derived from UVCS and SUMER ultraviolet spectroscopy, we construct theoretical
models of anisotropic ion temperatures in the polar solar corona. The primary
energy deposition mechanism we investigate is the dissipation of high frequency
(10--10000 Hz) ion-cyclotron resonant Alfven waves which can heat and accelerate
ions differently depending on their charge and mass. We find that it is possible
to explain the observed high perpendicular temperatures and strong anisotropies
with relatively small amplitudes for the resonant waves. There is suggestive
evidence for steepening of the Alfven wave spectrum between the coronal base
and the largest heights observed spectroscopically. Because the ion-cyclotron
wave dissipation is rapid, even for minor ions like O^{5+}, the observed
extended heating seems to demand a constantly replenished population of waves
over several solar radii. This indicates that the waves are generated gradually
throughout the wind rather than propagated up from the base of the corona.
2000-08: Oxygen Outflow Velocities in a Polar Coronal Hole
S. Giordano, E. Antonucci & M. A. Dodero
Advances in Space Research, 25, No. 9, pp.1927-1930
The expansion velocity of the solar corona in a polar coronal hole during
the minimum of solar activity is inferred from an analysis of the intensities
and profiles of the O VI lambda1032, 1037 lines observed with the Ultraviolet
Coronagraph Spectrometer (UVCS), operating onboard the Solar Heliospheric
Observatory (SOHO). The outflow velocity of the oxygen ions carried by the
solar wind reaches values greater than or equal to 300 km s(-1) above 2.1
R.. This is evidence for ion acceleration primarily near 1.6-2.1 R.. This
analysis also shows that beyond 1.8 R. the velocity distribution of the oxygen
ions in the frame of reference of the solar wind is highly anisotropic, that
is, this distribution is much broader along the line of sight than along
the radial. The velocity anisotropy found in the region of acceleration of
the fast solar wind is evidence for a heating process operating preferentially
in the direction perpendicular to the magnetic field.
2000-07: Hydrogen and Oxygen Temperature in Coronal Holes
E. Antonucci, S. Giordano, M.A. Dodero
Advances in Space Research, 25, No. 9, pp.1923-1926
The analysis of the O VI lambda 1032 and 1037 and I-I I Ly alpha lambda
1216 line profiles, observed with the Ultraviolet Coronagraph Spectrometer
(UVCS) onboard the Solar Heliospheric Observatory (SOHO) in a polarcoronal
hole within 1.5 R. and 3.5 R. during the solar activityminimum, indicates
the existence of a large anisotropy in the velocity distribution of the oxygen
ions across the magnetic field lines. This is evidence for temperatures across
the field lines that at 3.1 R. exceed by greater than or equal to 1.6-1.7
x10(8) K the oxygen temperature inferred along the radial direction, which
is similar to 10(7) K. The upper limit for the neutral hydrogen/proton temperature
is 2.4-3.0 x10(6) K. The bulk motions across the magnetic field lines, due
to wave motions and nonradial coronal expansion, cannot exceed similar to
170 km s(-1). The results of the present analysis imply that oxygen ions
are heated much more effectively than protons in the first solar radius of
the solar atmosphere and that protons can be heated more efficiently than
electrons. Furthermore, in addition to the process which heats the oxygen
ions to 10(7) K, a very strong acceleration operating only perpendicularly
to the magnetic field has to be invoked to justify the temperature excess,
of the order of 10(8) K, in this direction.
2000-06: Fast Solar Wind Velocity in a Polar Coronal Hole during Solar Minimum
E. Antonucci, M.A. Dodero, and S. Giordano
Solar Physics, 197, pp.115-134
We present a study of the outflow
velocity of the fast wind in the northern polar coronal hole observed on
May 21, 1996, during the minimum of solar activity, in the frame of a joint
observing program of the SOHO (Solar Heliospheric Observatory) mission.
The outflow velocity is inferred from an analysis of the Doppler dimming
of the intensities of the O VI lambda 1032, 1037 and H I Ly alpha lambda
1216 lines observed between 1.5 R{sun}, and 3.5 R{sun}, with the Ultraviolet
Coronagraph Spectrometer (UVCS), operating onboard SOHO. The analysis shows
that, for a coronal plasma characterized by low density, as derived for
a polar hole at solar minimum by Guhathakurta et al. (1999), and low temperature,
as directly measured at the base of this coronal hole by David et al. (1998),
the oxygen outflow speed derived spectroscopically is consistent with that
implied by the proton flux conservation. The hydrogen outflow is also consistent
with flux conservation if the deviation from isotropy of the velocity distribution
of the hydrogen atoms is negligible. Hence, for this cool and tenuous corona,
the oxygen ions and neutral hydrogen atoms flow outward roughly at the same
speed, that increases from 40 km/s, at 1.5 R{sun}, to 360 km/s, at 3.1 R{sun},
with average acceleration of the order of ~4.5 times 10^3 cm/(s^{-2}). The
highly anisotropic velocity distributions of the O~VI ions found in the analysis
confirm that the process which is heating the oxygen ions acts preferentially
across the magnetic field.
2000-05: Structure of a Large low-Latitude Coronal Hole
B. J. I. Bromage, D. Alexander, A. Breen, J.R. Clegg, G. Del Zanna, C. DeForest, D. Dobrzycka, G. Gopalswami, B. Thompson, Browning, P. K.
Solar Physics, 193, pp.181-193
A series of multi-wavelength, multi-instrument observations obtained during the "Whole Sun Month" campaign made it possible to make a more detailed study than ever before of a large coronal hole which appeared on the Sun in August 1996. It extended from the north pole of the Sun, across the equator to a large active region in the southern hemisphere. The near-continuous observations by instruments on SOHO (the Solar and Heliospheric Observatory satellite) allowed studies of both the earlier development and also the later evolution of this striking feature. For the first time, the "birth" of such a large coronal hole was seen to develop from the interaction between the north polar magnetic arcade structure and large magnetic loops extending northward from an active region to the south of the equator.
2000-04: SOHO Observations of a Helical Coronal Mass Ejection
A. Ciaravella, J. C. Raymond, B. J. Thompson, A. van Ballegooijen , L. Strachan, J. Li, L. Gardner, R. O'Neal, E. Antonucci, J. Kohl, G. Noci
ApJ 529, 575 (2000)
The EIT, LASCO and UVCS intruments
aboard the SOHO satellite observed a prominence eruption CME on 12 December
1997. Ejected plasma moved at about 130 $\rm km~s^{-1}$ in the plane of the
sky and showed Doppler shifts between -350 and +30 $\rm km~s^{-1}$. The eruption
appeared as a strongly curved arch in EIT images low in the corona. Emission
in ions ranging from Si III to O VI in the UVCS spectra indicates a temperature
range between $10^{4.5}$ and $10^{5.5}$ K. The morphology of the bright emission
regions seen by all three instruments suggests several strands of a helical
structure of moderate pitch angle. A reasonable fit to the spatial structure
and the velocity evolution measured by UVCS is provided by a left-handed
helix untwisting at a rate of about 9 times 10^{-4} radians per second.
2000-03 Compressibility of ion-cyclotron and whistler waves: can radio measurements detect high-frequency waves of solar origin in the corona?
Hollweg, J. V.
J. Geophys. Res. 105, pp.7573-7581
The Ultraviolet Coronagraph Spectrometer
(UVCS) on SOHO has provided several lines of evidence strongly suggesting
that coronal holes and the high-speed solar wind are heated by resonant interactions
with ion-cyclotron waves. Related evidence has also been provided by the
SUMER (Solar Ultraviolet Measurements of Emitted Radiation) instrument on
SOHO. However, the source of the waves is still unclear. Hollweg (1986),
Hollweg and Johnson (1988), and Isenberg (1990) developed models in which
the high frequency waves are the result of a turbulent cascade from lower
frequency waves which are launched by the Sun. Axford and McKenzie (1992)
suggested that solar reconnection events launch the high frequency waves
directly; the frequencies of these waves must be in the kHz range if they
are to resonate with the coronal protons. In this paper we point out that
the waves suggested by Axford and McKenzie can in principle be detected using
interplanetary scintillation (IPS) techniques. If the ion-cyclotron waves
are obliquely propagating, they will be compressive, and the corresponding
density fluctuations will induce phase, intensity, and Faraday rotation fluctuations
on radio signals passing through the corona. Tu and Marsch (1997) and Marsch
and Tu (1997) provided some detailed models based on Axford and McKenzie's
suggestion, including the wave magnetic power spectrum. From the latter,
we calculate the associated density power spectrum at 5 RS, which at high
wavenumbers turns out to be above the actual density power spectrum at 5
RS inferred from IPS by Coles and Harmon (1989). It is tempting to conclude
that the density fluctuations implied by the models of Tu and Marsch are
not present, and thus that the postulated ion-cyclotron waves of solar origin
are not present. However, we offer several reasons why such a conclusion
would be premature. We do suggest, though, that IPS has the potential to
verify or refute whether the Sun launches very high frequency waves into
the coronal holes.
2000-02: Tomography of the Solar Corona: I. A Robust, Regularized, Positive Estimation Method
Frazin, R.A.
ApJ, 530, pp.1026-1035 (2000)
A general method for the reconstruction of emissivity structures in the solar corona from projection data is presented. The method creates a robust, regularized, positive estimate (RRPE) of the coronal emissivity distribution. The problem of reconstructing the three-dimensional emissivity from a time series of coronagraph images is considered in detail. The discussion is set in the framework of the robust solution of under-determined, linear systems of equations. In two-dimensional numerical experiments, the RRPE method is compared to the multiplicative algebraic reconstruction technique (MART) and the superiority of RRPE is demonstrated. The most important and restrictive assumptions upon which the methodology is based are 1) that the structure of the corona does not vary with time and 2) that there are enough coronagraph images to resolve the longitudinal structures of the corona. The consequences of these assumptions are not addressed here but will be studied in future work. The three dimensional reconstruction of the coronal electron density from a time series of actual coronagraph images is deferred to a subsequent paper in this series.
2000-01: UVCS WLC Observations of Compressional Waves in the South Polar Coronal Hole
Ofman, L., Romoli, M., Poletto, G., Noci G., Kohl, J.L.
ApJ 529, 592 (2000)
In recent UVCS/SOHO White Light Channel (WLC) observations of the south polar coronal hole plumes and interplume regions we found quasi-periodic variations in the polarized brightness (pB) at a heliocentric distances of 1.9 solar radii. Fourier power spectrum of the polarized brightness (pB) time series shows significant peaks at about 1.5-1.8 mHz and possible fluctuations on longer time scales. The new observations corroborates our earlier findings of quasi-periodic density fluctuations - likely signatures of compressional waves propagating in the coronal hole. The detection of compressional waves may have important implications to the solar wind acceleration and heating mechanism in coronal holes.