UVCS/SOHO PAPERS - 2005

2005-26: Solar Wind from Coronal Funnels and Transition Region Ly-alpha

Esser, Ruth; Lie-Svendsen, Oystein; Janse, Ase Marit; Killie, Mari Anne

The Astrophysical Journal, Volume 629, Issue 1, pp. L61-L64.

Using a newly developed gyrotropic solar wind model that extends continuously from the mid-chromosphere to 1 AU and that accounts for radiative losses in the transition region, we have studied the difference between the fast solar wind emanating from a funnel geometry and a ``traditional'' rapidly expanding wind. The main aim is to determine whether or not observations of the Ly-alpha intensity in the low transition region can be reconciled with solar wind models. In a rapidly expanding geometry, we are not able to produce a Ly? intensity much higher than 1/10 of the observed values without creating a large pressure in the transition region and, as a result, a mass flux much higher than observed. In a funnel, on the other hand, we can easily obtain the observed Ly? intensity, while still having a wind solution in agreement with observations. The main reason for this is that the fast flow in the funnel causes hydrogen to be very far from ionization equilibrium, with the Ly-alpha intensity coming from temperatures of about 5?104 K. At these elevated temperatures, the radiative loss is much more efficient. The results of this Letter support the idea that the solar wind originates from small coronal funnels.

2005-25: Weak Compressible Magnetohydrodynamic Turbulence in the Solar Corona

Chandran, Benjamin D.

Physical Review Letters, vol. 95, Issue 26, id. 265004

This Letter presents a calculation of the power spectra of weakly turbulent Alfven waves and fast magnetosonic waves ('fast waves') in low-beta plasmas. It is shown that three-wave interactions transfer energy to high-frequency fast waves and, to a lesser extent, high-frequency Alfven waves. High-frequency waves produced by MHD turbulence are a promising explanation for the anisotropic heating of minor ions in the solar corona.

2005-24: Solar Wind Outflow in Polar Plumes from 1.05 to 2.4 Rsolar

Gabriel, A. H.; Abbo, L.; Bely-Dubau, F.; Llebaria, A.; Antonucci, E.

Astrophysical Journal, Volume 635, Issue 2, pp. L185-L188.

An earlier publication (Paper I), which measured the outflow velocity in solar plumes out to 1.35 Rsolar using the Doppler dimming technique, has here been extended out to 2.4 Rsolar by including observations from SOHO UVCS. It is shown that plume outflow velocities, greater than interplumes at lower heights, have lower acceleration and fall below interplume velocities at heights greater than 1.6 Rsolar. This analysis resolves what has been an apparent disagreement between previously published work. The mass flow rate in plumes is shown to decrease with height, presumably through mass transfer to the interplume regions.

2005-23: High-frequency Alfven waves in multi-ion coronal plasma: Observational implications

Ofman, L.; Davila, J. M.; Nakariakov, V. M.; Vinas, A.-F.

Journal of Geophysical Research, Volume 110, Issue A9, CiteID A09102

We investigate the effects of high-frequency (of order ion gyrofrequency) Alfven and ion-cyclotron waves on ion emission lines by studying the dispersion of these waves in a multi-ion coronal plasma. For this purpose we solve the dispersion relation of the linearized multifluid and Vlasov equations in a magnetized multi-ion plasma with coronal abundances of heavy ions. We also calculate the dispersion relation using nonlinear one-dimensional hybrid kinetic simulations of the multi-ion plasma. When heavy ions are present the dispersion relation of parallel propagating Alfven cyclotron waves exhibits the following branches (in the positive Omega-k quadrant): right-hand polarized nonresonant and left-hand polarized resonant branch for protons and each ion. We calculate the ratio of ion to proton velocities perpendicular to the direction of the magnetic field for each wave modes for typical coronal parameters and find strong enhancement of the heavy ion perpendicular fluid velocity compared with proton perpendicular fluid velocity. The linear multifluid cold plasma results agree with linear warm plasma Vlasov results and with the nonlinear hybrid simulation model results. In view of our findings we discuss how the observed nonthermal line broadening of minor ions in coronal holes may relate to the high-frequency wave motions.

2005-22: Resonant Heating of Ions by Parallel Propagating Alfven Waves in Solar Coronal Holes

Zhang, Tian-Xi; Wang, Jing-Xiu; Xiao, Chi-Jie

Chinese Journal of Astronomy and Astrophysics, Volume 5, Issue 3, pp. 285-294 (2005).

Resonant heating of H, O+5, and Mg+9 by parallel propagating ion-cyclotron Alfven waves in solar coronal holes at a heliocentric distance is studied using the heating rate derived from the quasilinear theory. It is shown that the particle-Alfven-wave interaction is a significant microscopic process. The temperatures of the ions are rapidly increased up to the observed order in only microseconds, which implies that simply inserting the quasilinear heating rate into the fluid/MHD energy equation to calculate the radial dependence of ion temperatures may cause errors as the time scales do not match. Different species ions are heated by Alfven waves with a power law spectrum in approximately a mass order. To heat O+5 over Mg+9 as measured by the Ultraviolet Coronagraph Spectrometer (UVCS) in the solar coronal hole at a region greater than 1.9 Rs, the energy density of Alfven waves with a frequency close to the O+5-cyclotron frequency must be at least double of that at the Mg+9-cyclotron frequency. With an appropriate wave-energy spectrum, the heating of H, O+5 and Mg+9 can be consistent with the UVCS measurements in solar coronal holes at a heliocentric distance.

2005-21: Hybrid simulation of ion cyclotron resonance in the solar wind: Evolution of velocity distribution functions

Li, Xing; Habbal, Shadia R.

Journal of Geophysical Research, Volume 110, Issue A10, CiteID A10109

Resonant interaction between ions (oxygen ions O+5 and protons) and ion cyclotron waves is investigated using a one dimensional hybrid code. Ion cyclotron waves are self-consistently generated by an ion cyclotron anisotropy instability. We focus on the detailed acceleration process of ions. The energization of oxygen ions due to waves is found to have two stages. During the first stage, oxygen ions are energized by ion cyclotron waves in the direction perpendicular to the background magnetic field and can develop extreme high temperature anisotropies with TOperp/TOparallel ~ 22 in an initially low beta plasma (beta value at 0.01) with very little parallel heating. During this stage, oxygen ions do not show an appreciable bulk acceleration along the background magnetic field. In the second stage, a large bulk acceleration of oxygen ions as large as 0.3 vA, where vA is the Alfven speed, is observed. Ion cyclotron waves are not able to maintain a high temperature anisotropy as inferred from observations. The nonlinear nature of wave particle interaction produces highly complex velocity distribution functions in the oxygen ions. In contrast, the heating and acceleration behavior of the major species, namely protons, is quite different. The velocity distribution functions of protons are less complex than the oxygen velocity distributions. Protons can also develop a large temperature anisotropy with preferential heating in the perpendicular direction. A bulk acceleration of protons (much smaller than the acceleration of oxygen ions) along the background magnetic field is observed to develop simultaneously with the development of a proton temperature anisotropy.

Cranmer, S. R

6th Solar-B Science Meeting, Kyoto, Japan, November 8-11, 2005

Cranmer, S. R

Solar Wind 11/SOHO-16: "Connecting Sun and Heliosphere", June 13-17, 2005, ESA SP-592, p. 159

Four decades have gone by since the discovery that the solar wind at 1 AU seems to exist in two relatively distinct states: slow and fast. There is still no universal agreement concerning the primary physical cause of this apparently bimodal distribution, even in its simplest manifestation at solar minimum. In this presentation we review and extend a series of ideas that link the different states of solar wind to the varying superradial geometry of magnetic flux tubes in the extended corona. Past researchers have emphasized different aspects of this relationship, and we attempt to disentangle some of the seemingly contradictory results. We apply the hypothesis of Wang and Sheeley (as well as Kovalenko) that Alfven wave fluxes at the base are the same for all flux tubes to a recent model of non-WKB Alfven wave reflection and turbulent heating, and we predict coronal heating rates as a function of flux tube geometry. We compare the feedback of these heating rates on the locations of Parker-type critical points, and we discuss the ranges of parameters that yield a realistic bifurcation of wind solutions into fast and slow. Finally, we discuss the need for next-generation coronagraph spectroscopy of the extended corona - especially measurements of the electron temperature above 1.5 solar radii - in order to confirm and refine these ideas.

The Astrophysical Journal, Volume 624, Issue 1, pp. 378-391. 2005

Slow solar wind is believed to arise in the legs or near the cusp of streamers, inside the brightness boundary. In an earlier study, we used an analytic model of flow in this layer to analyze the effect of the magnetic field on the geometry of the flow. That study successfully described those conditions that can lead to a decrease of the flow speed with increasing height near the cusp of the closed magnetic helmet inside a streamer. The model was, however, restricted to a radial brightness boundary on the streamer and hence to a relatively thick outflow region near the cusp. Here this restriction is relaxed through the explicit introduction of a coronal hole-like region outside the brightness boundary. We use the model to describe flow solutions for outflow in a thin layer inside the brightness boundary. The flow geometry now can be constrictive just above the cusp, and we show solutions of this type. Many solutions then show a diverging geometry at greater heights above the cusp, out to at least 5 Rsolar. We fail to find solutions in which the geometry alone leads to slow flow but give a more general description than before of conditions favoring slow flow and, consequently, gravitational settling in the legs of streamers.

Woo, Richard; Habbal, Shadia Rifai

The Astrophysical Journal, Volume 629, Issue 2, pp. L129-L132. 2005

This Letter uses Doppler dimming measurements by SOHO UVCS to elucidate the origin and acceleration of the slow solar wind. By investigating plasma flow in the corona over an active region during 2000 May 14-16, we confirm what has been suggested by the presence of the imprint of active regions in the solar wind near Earth orbit, that active regions are a source of slow wind. The observed active region does not have an associated streamer in the outer corona. We explain how this implies that any related heliospheric current sheet must be transverse to the line of sight. It is this favorable geometry of a transverse heliospheric current sheet that allows the plasma flow over the active region to be isolated in path-integrated Doppler dimming measurements. The results also show that acceleration of the slow wind associated with active regions toward its terminal speed is faster than that along the heliospheric current sheet. These differences in acceleration explain why the signatures of the heliospheric current sheet are dissimilar in velocity, but not in density, between the corona and solar wind measured near Earth orbit.

Morgan, H.; Habbal, S. Rifai

The Astrophysical Journal, Volume 630, Issue 2, pp. L189-L192. 2005

Due to their high intensity of emission in the O VI λλ1031.9 and 1037.6 lines, even small sunspots on the solar disk can strongly influence the intensity of the radiative scattering component of O VI lines in the corona. Observations of O VI disk spectra show a λ1032/λ1038 line intensity ratio of >2.6 in a sunspot, compared to quiet-disk values of ~2. The enhancement of the λ1032 line in comparison to the λ1038 line is likely due to interaction between molecular hydrogen emission from the sunspot and the chromospheric O5+. Modeling shows that a contribution from sunspots increases the coronal O VI λ1032/λ1038 intensity ratio to values considerably higher than those achieved with a quiet-disk or coronal hole spectrum. Therefore a reexamination of flow velocities derived from SOHO UVCS streamer observations must be made. This modeling demonstrates that the inclusion of sunspots, when present, may lead to nonzero outflow velocities at lower heights in streamer cores, in contrast to some existing model results.

Frazin, R. A.; Kamalabadi, F.; Weber, M. A.

The Astrophysical Journal, Volume 628, Issue 2, pp. 1070-1080. 2005

Conventional differential emission measure (DEM) analysis allows one to determine the amount of plasma as a function of temperature along a given line of sight. A completely different technique called solar rotational tomography (SRT) exploits the view angles provided by solar rotation to determine the spatial distribution of emissivity in three dimensions. These two techniques can be combined in a procedure called differential emission measure tomography (DEMT) to determine the DEM at each point in the corona with the same spatial resolution as can normally be achieved by SRT. In this paper the theory of DEMT is presented, and numerical examples based on the Atmospheric Imaging Assembly (AIA) are given. The results demonstrate promising potential for the methods to be adapted for use with other EUV and X-ray imaging and/or spectroscopy instruments.

Auchère, F.

The Astrophysical Journal, Volume 622, Issue 1, pp. 737-743. 2005

In modeling of the resonantly scattered solar coronal Lyα line of H I, the intensity of the chromospheric source is often assumed to be uniform. We investigate the validity of this assumption. After establishing a correlation between the H I 121.6 nm and He II 30.4 nm line intensities, we build Carrington maps of the Lyα chromosphere from SOHO EUV Imaging Telescope data. These maps are used to compute the Lyα irradiance throughout the corona and heliosphere. A 15% latitudinal anisotropy is found at 1 AU at solar minimum, and this value becomes larger closer to the Sun. The effect of the flux anisotropy on the total intensity of the Lyα resonantly scattered coronal radiation is quantified. We find that at solar minimum, the uniform-disk assumption leads to systematic overestimates of the total intensity of the polar regions by 15% on average. The evolution of this effect with solar activity and the case of other resonantly scattered coronal lines are discussed.

A problem of fundamental importance for future space travel to the Moon and Mars is the determination and prediction of the radiation environment generated by the Sun. The sources of solar energetic particles (SEP) and the physical processes associated with their acceleration and propagation are not well understood. Ultraviolet coronagraphic spectroscopy uniquely has the capabilities for determining the detailed plasma properties of the likely source regions of such particles. This information can be used to develop empirical models of the source regions for specific events, and it can provide the key information needed to identify and understand the physical processes that produce SEP hazards. UVCS/SOHO observations have provided the first detailed diagnostics of the plasma parameters of coronal mass ejections (CMEs) in the extended corona. These observations have provided new insights into the roles of shock waves, reconnection and magnetic helicity in CME eruptions. Next generation ultraviolet coronagraph spectrometers could provide additional diagnostic capabilities. This paper summarizes past observations, and discusses the diagnostic potential of advanced ultraviolet coronagraphic spectroscopy for characterizing two possible sites of SEP production: CME shocks and reconnection current sheets.

Cranmer, S. R.; van Ballegooijen, A. A.

The Astrophysical Journal Supplement Series, Volume 156, Issue 2, pp. 265-293, 2005

We present a comprehensive model of the global properties of Alfvén waves in the solar atmosphere and the fast solar wind. Linear non-WKB wave transport equations are solved from the photosphere to a distance past the orbit of the Earth, and for wave periods ranging from 3 s to 3 days. We derive a radially varying power spectrum of kinetic and magnetic energy fluctuations for waves propagating in both directions along a superradially expanding magnetic flux tube. This work differs from previous models in three major ways. (1) In the chromosphere and low corona, the successive merging of flux tubes on granular and supergranular scales is described using a two-dimensional magnetostatic model of a network element. Below a critical flux-tube merging height the waves are modeled as thin-tube kink modes, and we assume that all of the kink-mode wave energy is transformed into volume-filling Alfvén waves above the merging height. (2) The frequency power spectrum of horizontal motions is specified only at the photosphere, based on prior analyses of G-band bright point kinematics. Everywhere else in the model the amplitudes of outward and inward propagating waves are computed with no free parameters. We find that the wave amplitudes in the corona agree well with off-limb nonthermal line-width constraints. (3) Nonlinear turbulent damping is applied to the results of the linear model using a phenomenological energy loss term. A single choice for the normalization of the turbulent outer-scale length produces both the right amount of damping at large distances (to agree with in situ measurements) and the right amount of heating in the extended corona (to agree with empirically constrained solar wind acceleration models). In the corona, the modeled heating rate differs by more than an order of magnitude from a rate based on isotropic Kolmogorov turbulence.

A fast partial-halo coronal mass ejection (CME) was observed on 2000 June 28 by instruments on the SOHO spacecraft. The CME leading edge and  filamentary cold core were detected over the northwest limb at 2.32 Rsolar by the SOHO UV Coronagraph Spectrometer (UVCS). The broad profile of the O VI lambda1032 line gives evidence of a shock front at the leading edge, supporting the identification of white-light CME sharp leading edges as fast-mode shocks. Line-of-sight speeds are as high as 1500 km s-1, comparable to the projected speed obtained from LASCO. Pumping of the O VI lambda1032 by Lybeta (v=1810 km s-1) and of O VI lambda1037 by O VI lambda1032 (v=1648 km s-1) were detected, which provide diagnostics of outflow speed and density. The angle of the ejecta with the plane of the sky is obtained, combining the projected speed from LASCO with the line-of sight-speed, and varies between 7° and 46°. In the latter case the projected height of 2.32 Rsolar was at an actual heliocentric distance of 3.6 Rsolar. An associated solar energetic particle (SEP) event was observed at the L1 point following this CME. The abundance and charge-state data are consistent with a gradual shock-accelerated SEP event. A type II radio burst was observed at the same time the shock front was detected by UVCS.

We report direct observations of the magnetic reconnection site during an eruptive process that occurred on 2003 November 18. The event started with a rapid expansion of a few magnetic arcades located over the east limb of the Sun and developed an energetic partial-halo coronal mass ejection (CME), a long current sheet, and a group of bright flare loops in the wake of the CME. It was observed by several instruments, both in space and on the ground, including the EUV Imaging Telescope, Ultraviolet Coronagraph Spectrometer, and Large Angle Spectrometric Coronagraph experiment on board the Solar and Heliospheric Observatory, the Reuven Ramaty High Energy Solar Spectroscopic Imager, and the Mauna Loa Solar Observatory Mark IV K-Coronameter. We combine the data from these instruments to investigate various properties of the eruptive process, including those around the current sheet. The maximum velocities of the CME leading edge and the core were 1939 and 1484 km s-1, respectively. The average reconnection inflow velocities near the current sheet over different time intervals ranged from 10.5 to 106 km s-1, and the average outflow velocities ranged from 460 to 1075 km s-1. This leads to a corresponding  rate of reconnection in terms of the Alfvén Mach number MA ranging from 0.01 to 0.23. The composite of images from different instruments specifies explicitly how the different objects developed by a single eruptive process are related to one another.

Ko, Y.-K.; Raymond, J. C.; Gibson, S. E.; Alexander, D.; Strachan, L.; Holzer, T.; Gilbert, H.; Cyr, O. C. St.; Thompson, B. J.; Pike, C. D.; Mason, H. E.; Burkepile, J.; Thompson, W.; Fletcher, L.

The Astrophysical Journal, Volume 623, Issue 1, pp. 519-539. 2005 On 1999 August 26, a coronal jet occurred at the northwest limb near a sigmoid active region (AR 8668) that was the target for a joint observation plan (SOHO joint observing program 106) during the third Whole Sun Month Campaign. This jet was observed by several instruments at the  limb (SOHO/CDS, SOHO/EIT, TRACE, and Mauna Loa Solar Observatory CHIP and PICS) and at 1.64 Rsolar (SOHO/UVCS). At the limb, this jet event  displayed both low- and high-temperature components. Both high- and low-temperature components were evident during the early phase (first 20 minutes) of the event. However, the low-temperature component is maintained for ~1 hr after the higher temperature component is gone. There is a second brightening (a possible second jet) seen by EIT and TRACE about 50 minutes after the onset of the first jet. The line-of-sight motion at the limb began with a 300 km s-1 redshift and evolved to a 200  km s-1 blueshift. At 1.64 Rsolar, the intensities of Lyalpha and Lybeta in the jet increased by a factor of several hundred compared with the background corona. The C III lambda977 line also brightened significantly. This indicates low-temperature [~(1-2)×105 K] emission in the jet, while the intensities of O VI lambda1032 and O VI lambda1037 increased by as  much as a factor of 8. The UVCS data show evidence of heating at the early phase of the event. The Doppler shift in the lines indicates that the line-of-sight (LOS) velocity in the jet started from ~150 km s-1 in blueshift and ended at ~100 km s-1 in redshift. This LOS motion seen at  1.64 Rsolar was apparently opposite to what was observed when the jet emerged from the limb. The Doppler dimming analysis indicates that the radial outflow speed correlates with the magnitude of the LOS speed.  Interestingly, UVCS observations at 2.33 and 2.66 Rsolar show no trace of the jet and SOHO/LASCO observations also yield no firm detection. We find  that a simple ballistic model can explain most of the dynamical properties of this jet, while the morphology and the thermal properties agree well with reconnection-driven X-ray jet models.

Antonucci, E.; Abbo, L.; Dodero, M. A.

This study examines the physical conditions of the outer solar corona in order to identify the regions where the slow solar wind is accelerated and to investigate the latitudinal transition from slow to fast wind during the minimum of the solar cycle. The analysis is based on observations of six streamers obtained during the years of solar minimum, 1996 and 1997, with the Ultraviolet Coronagraph Spectrometer (UVCS) onboard the Solar and Heliospheric Observatory (SOHO). The outflow velocity of the oxygen ions and the electron density of the coronal plasma are determined in altitude ranging from 1.5 to 3.5 solar radii (R⊙). The adopted diagnostic method, based on spectroscopic analysis of the O VI 1032 and 1038 Å lines, fully accounts for the large expansion factor of the magnetic field lines expected in the regions surrounding the streamers. The analysis leads to the conclusion that the slow coronal wind is observed (i) in the region external to and running along the streamer boundary; and (ii) in the region above the streamer core beyond 2.7 R_⊙, where the transition between closed and open magnetic field lines takes place and the heliospheric current sheet forms. Regions in the immediate vicinity of the streamer boundary can be identified with the edges of the large polar coronal holes that characterize solar minimum. Results point to gradual variations of the properties of a coronal hole from the streamer boundary to its polar core, most likely related to the topology of the coronal magnetic field.

Spadaro, D.; Ventura, R.; Cimino, G.; Romoli, M.

Astronomy and Astrophysics, v.429, p.353-360 (2005) 

Ventura, R.; Spadaro, D.; Cimino, G.; Romoli, M.

Astronomy and Astrophysics, v.430, p.701-712 (2005)

UVCS/SOHO observations of the O VI resonance doublet and H I Lyα line intensities and profiles, together with measurements of the visible linearly polarized radiance, have been performed during two MEDOC campaigns in 1997 and 2000, i.e. near solar minimum and approaching the solar maximum phase, respectively. During both observational runs mid-latitude coronal regions in the West limb of the Sun have been scanned over a range of heliocentric distance from 1.39 to 4.1 R⊙, to study the plasma properties of streamers and adjacent regions, such as ion kinetic temperature, electron density and outflow velocity, paying particular attention to comparing plasma conditions deduced for different ions in coronal structures observed on different days and during different phases of solar activity. Besides confirming some previous findings on significant differences between open and closed field-line structures at solar minimum, our results provide some evidence for differences in kinetic temperature among mid-latitude solar minimum streamers observed on different days from about 2 R⊙ outwards, as well as in their dynamical conditions at heliocentric distances greater than 3.6 R⊙. For observations carried out in 2000, conversely, the mid-latitude coronal streamers and their surroundings are about 3 times, and more than one order of magnitude brighter, respectively, than their solar minimum counterparts and exhibit very similar kinetic and dynamical conditions. The kinetic temperatures in adjacent regions are higher than in streamers (by about a factor of 2) only within 2 R⊙, while at greater heights such differences vanish, making it difficult to discriminate between open and closed structures. This is opposite to the behaviour detected at solar minimum, when adjacent regions appear to be characterized by kinetic temperatures progressively higher and higher than in streamers with increasing height, from 2 R⊙ outwards. Therefore, a clear characterization of open and closed configurations near the solar maximum might be quite difficult, probably due both to the intrisically more complex magnetic configuration of the corona in this phase of the solar activity and the line-of-sight contamination effects that in a highly structured solar corona may strongly mix background and foreground plasma with different properties. The transition from the solar minimum to maximum also seems characterized by a global increase in the electron density inside streamers of about a factor of 4 at 1.7 R⊙ and then it progressively decreases with height.