Sensitivity of solar off-limb line profiles to electron density
 stratification and the velocity distribution anisotropy

Raouafi, N. E.; Solanki, S. K.

doi:10.1051/0004-6361:20042568

Cited by 24 publications

(15 citation statements)

References 54 publications

(85 reference statements)

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“…For example, Raouafi & Solanki (2004, 2006) simulated the properties of the O VI λλ1032, 1037 lines after first fixing the radial variation of the outflow speed and ion temperature. Figure 5b shows an illustrative "cut" through the data cube at fixed values of u i = 600 km s −1 and w i⊥ = 215 km s −1 at r = 3.09 R ⊙ (similar to the values used by Raouafi & Solanki 2006). In this case, the most probable value of the anisotropy ratio is surprisingly close to 1, as was also assumed by Raouafi & Solanki.…”

Section: Deriving Ion Properties From the Data Cubessupporting

confidence: 64%

Improved Constraints on the Preferential Heating and Acceleration of Oxygen Ions in the Extended Solar Corona

Cranmer

Panasyuk

Kohl

2008

ApJ

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We present a detailed analysis of oxygen ion velocity distributions in the extended solar corona, based on observations made with the Ultraviolet Coronagraph Spectrometer (UVCS) on the SOHO spacecraft. Polar coronal holes at solar minimum are known to exhibit broad line widths and unusual intensity ratios of the O VI λλ1032, 1037 emission line doublet. The traditional interpretation of these features has been that oxygen ions have a strong temperature anisotropy, with the temperature perpendicular to the magnetic field being much larger than the temperature parallel to the field. However, recent work by Raouafi and Solanki suggested that it may be possible to model the observations using an isotropic velocity distribution. In this paper we analyze an expanded data set to show that the original interpretation of an anisotropic distribution is the only one that is fully consistent with the observations. It is necessary to search the full range of ion plasma parameters to determine the values with the highest probability of agreement with the UVCS data. The derived ion outflow speeds and perpendicular kinetic temperatures are consistent with earlier results, and there continues to be strong evidence for preferential ion heating and acceleration with respect to hydrogen. At heliocentric heights above 2.1 solar radii, every UVCS data point is more consistent with an anisotropic distribution than with an isotropic distribution. At heights above 3 solar radii, the exact probability of isotropy depends on the electron density chosen to simulate the line-of-sight distribution of O VI emissivity. The most realistic electron densities (which decrease steeply from 3 to 6 solar radii) produce the lowest probabilities of isotropy and most-probable temperature anisotropy ratios that exceed 10. We also use UVCS O VI absolute intensities to compute the frozen-in O 5+ ion concentration in the extended corona; the resulting range of values is roughly consistent with recent downward revisions in the oxygen abundance.

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Section: Deriving Ion Properties From the Data Cubessupporting

confidence: 64%

Improved Constraints on the Preferential Heating and Acceleration of Oxygen Ions in the Extended Solar Corona

Cranmer

Panasyuk

Kohl

2008

ApJ

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“…In contrast to many prior analyses of UVCS data, which concluded that there must be both intense preferential heating of the O +5 ions and a strong field-aligned anisotropy, Raouafi and Solanki (2004), Raouafi and Solanki (2006), and Raouafi et al (2007) reported that there may not be a compelling need for O +5 anisotropy depending on the assumptions made about the other plasma properties of the coronal hole (e.g., electron density). However, Cranmer et al (2008) performed a detailed re-analysis of these observations and concluded that there remains strong evidence in favor of both preferential O +5 heating and acceleration and significant O +5 ion anisotropy (in the sense T ⊥i > T i ) above r ≈ 2.1 R ⊙ in coronal holes.…”

Section: Ultraviolet Spectroscopycontrasting

confidence: 64%

Coronal Holes

Cranmer¹

The Encyclopedia of Astronomy and Astrophysics

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Coronal holes are the darkest and least active regions of the Sun, as observed both on the solar disk and above the solar limb. Coronal holes are associated with rapidly expanding open magnetic fields and the acceleration of the high-speed solar wind. This paper reviews measurements of the plasma properties in coronal holes and how these measurements are used to reveal details about the physical processes that heat the solar corona and accelerate the solar wind. It is still unknown to what extent the solar wind is fed by flux tubes that remain open (and are energized by footpoint-driven wave-like fluctuations), and to what extent much of the mass and energy is input intermittently from closed loops into the open-field regions. Evidence for both paradigms is summarized in this paper. Special emphasis is also given to spectroscopic and coronagraphic measurements that allow the highly dynamic non-equilibrium evolution of the plasma to be followed as the asymptotic conditions in interplanetary space are established in the extended corona. For example, the importance of kinetic plasma physics and turbulence in coronal holes has been affirmed by surprising measurements from the UVCS instrument on SOHO that heavy ions are heated to hundreds of times the temperatures of protons and electrons. These observations point to specific kinds of collisionless Alfvén wave damping (i.e., ion cyclotron resonance), but complete theoretical models do not yet exist. Despite our incomplete knowledge of the complex multi-scale plasma physics, however, much progress has been made toward the goal of understanding the mechanisms ultimately responsible for producing the observed properties of coronal holes. more active periods of the solar cycle, coronal holes can exist at all solar latitudes, but they may only persist for several solar rotations before evolving into different magnetic configurations.Despite not being as visually spectacular as active regions, solar flares, or coronal mass ejections (CMEs), coronal holes are of abiding interest for (at least) three main reasons.1. The extended corona and solar wind connected with coronal holes tends to exist in an ambient timesteady state, at least in comparison with other regions. This makes coronal holes a natural starting point for theoretical modeling, since it often makes sense to begin with the simplest regions before attempting to understand more complex and variable structures.2. Coronal hole plasma has the lowest density, which makes it an optimal testbed for studies of collisionless kinetic processes that are the ultimate dissipation mechanisms in many theories of coronal heating. Other regions tend to have higher densities and more rapid Coulomb collisions, and thus the unique signatures of the kinetic processes (in their particle velocity distributions) are not as straightforward to measure as in coronal holes.3. Coronal holes and their associated high-speed wind streams are also responsible for a fraction of major geomagnetic storms at 1 AU. Corotating interaction regions (CIRs...

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“…In addition, we notice that the integration along the line of sight has a significant effect on the spectral line profiles emitted in the solar corona (Raouafi & Solanki 2004, 2006, which could be very important for the resulting polarization of the scattered line. This has to be taken into account for a realistic interpretation of coronal observations.…”

Section: Resultsmentioning

confidence: 97%

“…The calculations are restricted to an emitting elementary volume around the plane of the sky. Integration along the line of sight is outside the scope of the present works, but see Raouafi & Solanki (2004, 2006. In fact, polarization measurements of scattered lines which are optically thin most often concern observations made at the limb.…”

Section: Introductionmentioning

confidence: 95%

Doppler redistribution of anisotropic radiation and resonance polarization in moving scattering media

Sahal‐Bréchot¹,

Raouafi

2005

A&A

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The present paper puts the Doppler redistribution theory of anisotropic radiation and resonance polarization in moving scattering media into a predictive application following Sahal-Bréchot et al. (1998, A&A, 340, 579; P II ), using the example of the O vi 1032 Å line of the corona.Numerical results are presented for different heights over the limb compatible with observations of different instruments on SOHO (SUMER, CDS, UVCS) but far from coronal holes in order to avoid inhomogeneities of the transition zone incident radiation. The orders of magnitude of our numerical results confirm the theoretical predictions of P II and show that interpretation of polarimetric data of the O vi 1032 Å line of the corona, associated to the shift and the Doppler-dimming effect, offers a powerful diagnostic method of the complete velocity field vector, provided that the partial anisotropy of the incident radiation field be taken into account. These results also explain the precision of the diagnostic of interpretation polarimetric observations achieved by the ultraviolet spectrometer SUMER in the south polar coronal hole at 270 above the solar limb; e.g. the solar wind acceleration region.

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Sensitivity of solar off-limb line profiles to electron density stratification and the velocity distribution anisotropy

Cited by 24 publications

References 54 publications

Improved Constraints on the Preferential Heating and Acceleration of Oxygen Ions in the Extended Solar Corona

Improved Constraints on the Preferential Heating and Acceleration of Oxygen Ions in the Extended Solar Corona

Coronal Holes

Doppler redistribution of anisotropic radiation and resonance polarization in moving scattering media

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