Astrophysical Journal, Volume 643, Issue 2, pp. 1239-1244.
Observations of the extended corona obtained with the Ultraviolet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory (SOHO) during the solar minimum years 1996 and 1997 have been analyzed to derive the oxygen abundance in the outer corona. A comparison of the absolute coronal abundance, measured in the coronal regions surrounding the quiescent solar minimum streamers, to the heliospheric values confirms that these regions are the dominant sources of the slow solar wind. However, the inferred coronal abundances are consistent with the heliospheric values only in case the ion velocity distribution is anisotropic and enhanced across the coronal magnetic field. Thus this analysis also leads to the conclusion that energy is deposited in the slow coronal wind at least up to 2.7 Rsolar and that the efficiency of energy deposition is likely to be related to the local coronal magnetic topology.
Astrophysical Journal, Volume 652, Issue 1, pp. 774-792.
We present UV spectral information for 22 halo or partial halo CMEs observed
by UVCS. The CME fronts show broad line profiles,
while the line intensities are comparable to the background corona. The Doppler
shifts of the front material are generally small, showing that the motion
of gas in the fronts is mostly transverse to the line of sight. This indicates
that, at least in halo CMEs, the fronts generally correspond to coronal plasma
swept up by a shock or compression wave, rather than plasma carried outward
by magnetic loops. This favors an ice cream cone (or a spherical shell) model,
as opposed to an expanding arcade of loops. We use the line widths to discriminate
between shock heating and bulk expansion. Of 14 cases where we detected the
CME front, the line broadening in 7 cases can be attributed to shock heating,
while in 3 cases it is the line-of-sight component of the CME expansion.
For the CME cores we determine the angles between the motion and the plane
of the sky, along with the actual heliocentric distances, in order to provide
quantitative estimates of projection effects.
2006-28: On the behavior of O^(+5) in
coronal holes: Importance of sunward propagating waves
Hollweg, Joseph V.
Journal of Geophysical Research, Volume 111, Issue A12, CiteID A12106
The high thermal anisotropy of O+5 in coronal holes, as observed by the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO), suggests that these ions are being heated by the cyclotron resonance. The observations indicate that the O+5 temperature steadily increases from r ~ 1.5 Rs (where the ions become almost collisionless) out to r ~ 3.5 Rs (the outer limit of the observations). Previous models have not been able to reproduce even the qualitative result of a steady temperature rise. We suggest that the problem has been that previous models have considered O+5 resonating only with outward propagating waves. Once the ions are heated perpendicularly to the background magnetic field, they are accelerated to high outward flow speeds by the mirror force. As a result, they resonate with outgoing waves having higher (normalized) wave numbers, where there is presumably less power; they may even drop out of resonance altogether. We suggest here that resonances with inward (i.e., sunward) propagating waves may be the key to explaining the observed O+5 temperature rise. In that case, as the ions are accelerated by the mirror force, they never drop out of resonance, and they resonate with ingoing waves having lower (normalized) wave numbers where there is presumably more power. We offer a very simple strawman model to illustrate the differences between oxygen resonances with ingoing and outgoing waves and to show that the UVCS/SOHO results can be approximately reproduced if the ingoing wave power spectrum in the resonant range varies as k^(-gamma), with gamma ~ 5/3. We point out that it is really necessary to take into account the fact that O+5 (and other heavy ions) can resonate with ingoing and outgoing waves simultaneously, which can only be studied via full kinetic solutions for the ion distribution functions; however, it is possible that once the ions are accelerated by the mirror force, the resonances with sunward propagating waves will be dominant.
2006-27: Energization of Plasma Species
by Intermittent Kinetic Alfven Waves
Voitenko, Yuriy; Goossens, Marcel
Space Science Reviews, Volume 122, Issue 1-4, pp. 255-270
We propose a new phase-mixing sweep model of coronal heating and solar wind acceleration based on dissipative properties of kinetic Alfven waves (KAWs). The energy reservoir is provided by the intermittent ~1 Hz MHD Alfven waves excited at the coronal base by magnetic restructuring. These waves propagate upward along open magnetic field lines, phase-mix, and gradually develop short wavelengths across the magnetic field. Eventually, at 1.5-4 solar radii they are transformed into KAWs. We analyze several basic mechanisms for anisotropic energization of plasma species by KAWs and find them compatible with observations. In particular, UVCS (onboard SOHO) observations of intense cross-field ion energization at 1.5-4 solar radii can be naturally explained by non-adiabatic ion acceleration in the vicinity of demagnetizing KAW phases. The ion cyclotron motion is destroyed there by electric and magnetic fields of KAWs.
2006-26: Wave acceleration of the fast
solar wind
Leon Ofman
Adv. Space Research, vol 38 (issue 1), 64-74
Observations show that the solar wind exhibits two modes of outflow: the slow (400 km/s), high density, highly variable wind that emerges from equatorial solar regions, and the high latitude, fast (800 km/s), low density, steady solar wind. The bi-modal solar wind outflow is most evident near minima of solar activity. Theoretical studies of the fast solar wind in open magnetic structures motivated by SOHO, Ulysses, and Helios observations indicate that both, high frequency kinetic waves, and low frequency MHD waves play a role in its acceleration and heating. Ion-cyclotron waves have been suggested as the main energy source of the solar wind. However, there are theoretical difficulties with the ion-cyclotron wave heating of the protons, and these waves do not heat electrons. Low frequency MHD waves are still the best candidates to transport momentum and energy far from the Sun, to accelerate the solar wind on large spatial scales. I will present recently developed two-dimensional three-fluid model that includes explicit wave acceleration, and visco-resistive dissipation. The model describes electrons, protons, and minor ions as three coupled fluids that are heated by different heating processes with the parameters constrained by observations. I will present the results of 2.5D three-fluid simulations of the fast solar wind plasma that combine the effects of MHD waves self-consistently, and ion-cyclotron waves parametrically on the acceleration and heating processes. I will present the results of hybrid kinetic models of ion-cyclotron wave heating of the heavy ions in the solar wind plasma.
2006-25: Dissipation of the Perpendicular
Turbulent Cascade in the Solar Wind
Markovskii, S. A.; Vasquez, Bernard J.; Smith, Charles W.; Hollweg, Joseph
V.
Astrophysical Journal, Volume 639, Issue 2, pp. 1177-1185.
The core solar wind protons are observed to be heated perpendicularly to the magnetic field. This is taken to be a signature of the cyclotron damping of the turbulent fluctuations, which are thought to be responsible for the heating. At the same time, it is commonly accepted that the turbulent cascade produces mostly highly oblique (quasi-two-dimensional) fluctuations, which cannot be immediately cyclotron resonant with the ions because of their low frequencies and small parallel wavenumbers. To address this problem, we propose a new, indirect mechanism for damping the quasi-two-dimensional fluctuations. The mechanism involves a plasma instability, which excites ion cyclotron resonant waves. As the cascade proceeds to higher wavenumbers, it generates increasingly high velocity shear associated with the turbulent fluctuations. The shear eventually becomes unstable to waves near harmonics of the ion cyclotron frequency. Once the frequency of the waves is upshifted, they can heat ions perpendicularly, extracting the energy from the quasi-two-dimensional fluctuations. The dissipation rates of quasi-two-dimensional fluctuations are incorporated into a model of the energy transfer in the turbulent cascade. Our analysis of the observed spectra shows that the spectral break separating the inertial and dissipation ranges of the turbulence, where the dissipation sets in, corresponds to the same shear under a wide range of plasma conditions, in agreement with the prediction of the theory. The observed turbulence spectra often have power-law dissipation ranges with an average spectral index of -3. We demonstrate that this fact is simply a consequence of a marginal state of the instability in the dissipation range.
2006-24: A global 2.5-dimensional three
fluid solar wind model with alpha particles
Li, Bo; Li, Xing; Labrosse, Nicolas
Journal of Geophysical Research, Volume 111, Issue A8, CiteID A08106
A global 2.5-dimensional three fluid solar wind model is presented. Two ion species, namely protons and alpha particles, are heated by an empirical energy flux while electrons are heated by the classical heat flux and Coulomb coupling with ions. It is found that for a reasonable relative speed between alpha particles and protons at 1 AU to be achieved, the alphas need to be preferentially heated in the inner corona. No external heating is applied in the streamer base, the closed magnetic field region. A hot coronal boundary, the electron heat flux, and Coulomb coupling keep plasma species in equilibrium inside the streamer, and a nonisothermal streamer is found. The abundance of alpha particles varies within the streamer base. It is small in the streamer core compared with streamer legs, and alphas continuously drain out of the streamer core along magnetic field due to gravitational settling. The settling operates over a timescale of several days. Alpha particles in the slow wind have a smaller abundance than in the fast wind at 1 AU, in agreement with observations. This is mainly determined in the near-Sun region. For the coronal alpha abundances in the range 0.015-0.15, it is found that alpha particles play a negligible role in determining the magnetic field. In this sense, treating alphas as test particles is justified. However, alphas have an important impact on solar wind parameters. Coulomb collisions and heating drag alphas into the solar wind. The Coulomb friction with protons by itself is, however, unable to drive into the slow solar wind a flux of alphas flowing at roughly the same speed of protons as observed by in situ measurements at 1 AU.
2006-23: Kinetic Physics of the Solar
Corona and Solar Wind
Marsch, Eckart
Living Reviews in Solar Physics, vol. 3, no. 1
Kinetic plasma physics of the solar corona and solar wind are reviewed with emphasis on the theoretical understanding of the in situ measurements of solar wind particles and waves, as well as on the remote-sensing observations of the solar corona made by means of ultraviolet spectroscopy and imaging. In order to explain coronal and interplanetary heating, the microphysics of the dissipation of various forms of mechanical, electric and magnetic energy at small scales (e.g., contained in plasma waves, turbulences or non-uniform flows) must be addressed. We therefore scrutinise the basic assumptions underlying the classical transport theory and the related collisional heating rates, and also describe alternatives associated with wave-particle interactions. We elucidate the kinetic aspects of heating the solar corona and interplanetary plasma through Landau- and cyclotron-resonant damping of plasma waves, and analyse in detail wave absorption and micro instabilities. Important aspects (virtues and limitations) of fluid models, either single- and multi-species or magnetohydrodynamic and multi-moment models, for coronal heating and solar wind acceleration are critically discussed. Also, kinetic model results which were recently obtained by numerically solving the Vlasov-Boltzmann equation in a coronal funnel and hole are presented. Promising areas and perspectives for future research are outlined finally.
2006-22: On the Lyman alpha and beta
lines in solar coronal streamers
Labrosse, N.; Li, X.; Li, B.
Astronomy and Astrophysics, Volume 455, Issue 2, August IV 2006, pp. 719-723
We investigate the formation of the H I Lyman alpha and Lyman beta lines in an equatorial coronal streamer. Particular attention is paid to frequency redistribution for the scattering of the incident radiation. The properties of the spectral lines are studied. Methods. The coronal model is given by a global 2.5D three fluid solar wind model with alpha particles. The emergent intensities and line profiles are calculated from the solution of the statistical equilibrium and radiative transfer equations for an hydrogen atom with 11 energy levels under non local thermodynamic equilibrium. The formation of the lines results from radiative excitation, collisional excitation, and takes into account the coupling with all other transitions between the hydrogen energy levels. Results. We present new estimates of the radiative and collisional contributions of the Lyman line intensities within the streamer. It is also shown that within the streamer, the full width at half-maximum (FWHM) of the Lyman beta line is a better indicator of the plasma temperature than that of Lyman alpha. These results show that care should be taken when inferring the proton temperature from the Lyman alpha line profile as observed in coronal streamers, e.g. by the Ultraviolet Coronagraph Spectrometer or the Solar Ultraviolet Measurements of Emitted Radiation experiments on board the Solar and Heliospheric Observatory.
2006-21: Effect of the latitudinal distribution
of temperature at the coronal base on the interplanetary magnetic field configuration
and the solar wind flow
Li, Bo; Habbal, Shadia Rifai; Li, Xing; Mountford, Chris
Journal of Geophysical Research, Volume 110, Issue A12, CiteID A12112
Using a two-dimensional MHD model of the corona and solar wind, we investigate the role of the temperature distribution with latitude at the coronal base on the global magnetic field configuration and solar wind properties at 1 AU. The latitudinal distribution of temperature is aimed at modeling the transition in electron temperature at the Sun from a polar coronal hole to the quiet Sun to active regions. The results of the model calculations illustrate how the variation of temperature with latitude impacts the coronal magnetic field configuration and the distribution of wave energy flux in the solar wind and consequently its thermodynamic properties. The sharp temperature changes at the coronal base lead to the formation of current sheets in the corona. They also modify the location of the streamer cusp and the neutral line originating there. Two different approaches in treating electron heat flux are also compared, one assumes a Spitzer expression throughout the computational domain and the other assumes a collisionless expression beyond some radial distance. Model results thus derived differ little in terms of proton flux and terminal speed.
2006-20: Three-dimensional Structure
of the 2002 April 21 Coronal Mass Ejection
Lee, J.-Y.; Raymond, J. C.; Ko, Y.-K.; Kim, K.-S.
The Astrophysical Journal, Volume 651, Issue 1, pp. 566-575.
A three-dimensional reconstruction of the 2002 April 21 partial halo Coronal
Mass Ejection (CME) has been made based on the O VI 1032 Å and [Fe
XVIII] 974 Å lines observed by the Ultraviolet Coronagraph Spectrometer
(UVCS) on board the Solar and Heliospheric Observatory (SOHO). We use the
Doppler velocities to derive the CME structure along the line of sight.
UVCS observed the O VI line profiles split into strongly Doppler red- and
blueshifted components, and the region of split profiles grew rapidly along
the long spectrograph slit. The more localized [Fe XVIII] bright emission
starts at the same time as the maximum Doppler redshift of O VI, indicating
that it is inside the CME. In the view from the solar west, the O VI looks
like halo CMEs seen by the Large Angle and Spectrometric Coronagraph (LASCO).
The [Fe XVIII] bright emission appears as a barlike structure seen nearly
end-on from the Earth, and side-on from the solar north and from solar west.
The reconstructed [Fe XVIII] emission allows two interpretations, as ejection
of preexisting hot plasma or as a current sheet. The evidence favors the
current sheet interpretation, although we cannot rule out the alternatives.
2006-19: Ultraviolet spectroscopy of the
extended solar corona
Kohl, John L.; Noci, Giancarlo; Cranmer, Steven R.;
Raymond, John C.
The Astronomy and Astrophysics Review, Volume 13, Issue 1-2, pp. 31-157
The first observations of ultraviolet spectral line profiles and intensities from the extended solar corona (i.e., more than 1.5 solar radii from Sun-center) were obtained on 13 April 1979 when a rocket-borne ultraviolet coronagraph spectrometer of the Harvard-Smithsonian Center for Astrophysics made direct measurements of proton kinetic temperatures, and obtained upper limits on outflow velocities in a quiet coronal region and a polar coronal hole. Following those observations, ultraviolet coronagraphic spectroscopy has expanded to include observations of over 60 spectral lines in coronal holes, streamers, coronal jets, and solar flare/coronal mass ejection (CME) events. Spectroscopic diagnostic techniques have been developed to determine proton, electron and ion kinetic temperatures and velocity distributions, proton and ion bulk flow speeds and chemical abundances. The observations have been made during three sounding rocket flights, four Shuttle deployed and retrieved Spartan 201 flights, and the Solar and Heliospheric Observatory (SOHO) mission. Ultraviolet spectroscopy of the extended solar corona has led to fundamentally new views of the acceleration regions of the solar wind and CMEs. Observations with the Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO revealed surprisingly large temperatures, outflow speeds, and velocity distribution anisotropies in coronal holes, especially for minor ions. Those measurements have guided theorists to discard some candidate physical processes of solar wind acceleration and to increase and expand investigations of ion cyclotron resonance and related processes. Analyses of UVCS observations of CME plasma properties and the evolution of CMEs have provided the following: temperatures, inflow velocities and derived values of resistivity and reconnection rates in CME current sheets, compression ratios and extremely high ion temperatures behind CME shocks, and three dimensional flow velocities and magnetic field chirality in CMEs. Ultraviolet spectroscopy has been used to determine the thermal energy content of CMEs allowing the total energy budget to be known for the first time. Such spectroscopic observations are capable of providing detailed empirical descriptions of solar energetic particle (SEP) source regions that allow theoretical models of SEP acceleration to be tailored to specific events, thereby enabling in situ measurements of freshly emitted SEPs to be used for testing and guiding the evolution of SEP acceleration theory. Here we review the history of ultraviolet coronagraph spectroscopy, summarize the physics of spectral line formation in the extended corona, describe the spectroscopic diagnostic techniques, review the advances in our understanding of solar wind source regions and flare/CME events provided by ultraviolet spectroscopy and discuss the scientific potential of next generation ultraviolet coronagraph spectrometers.
The Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO has made detailed observations of coronal streamers in the extended corona (1.5 Ro < r < 10 Ro) for almost a complete solar cycle. Measurements of ultraviolet line profiles and intensities and polarized white light intensities are used to determine plasma parameters (proton and ion temperatures, electron densities, and ion outflow speeds) for these large scale coronal structures. In this paper we compare the variations of streamer properties obtained in the extended corona with the properties found in the solar wind at 1 AU. Solar wind velocities and densities derived from in situ spacecraft data are used for comparisons. Such comparisons may be used to distinguish between solar wind variations due to changes in physical parameters of streamers at the Sun as opposed to changes in flow geometry.
M. Uzzo, J. Raymond and A. Vourlidas
SOHO-17: Ten Years of SOHO and Beyond, May 7-12, 2006, Giardini Naxos, Sicily, ESA SP-617Miralles, M. P., Cranmer, S. R., and Kohl, J. L.
SOHO-17: Ten Years of SOHO and Beyond, May 7-12, 2006, Giardini Naxos, Sicily, ESA SP-617Kohl, J. L., Panasyuk, A. V., Cranmer, S. R., Fineschi, S., Gardner,L. D., Philips, D. H., Raymond, J. C., and Uzzo, M.
SOHO-17: Ten Years of SOHO and Beyond, May 7-12, 2006, Giardini Naxos,
Sicily, ESA SP-617