Absence of linear polarization in H? emission of solar flares (original) (raw)
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Absence of linear polarization in Halpha emission of solar flares
Arxiv preprint astro-ph/ …, 2005
High sensitivity observations of Hα polarization of 30 flares of different sizes and disk positions are reported. Both filter and spectrographic techniques have been used. The ZIMPOL system eliminates spurious polarizations due to seeing and flat-field effects. We didn't find any clear linear polarization signature above our sensitivity level which was usually better than 0.1%. The observations include an X17.1 flare with gamma-ray lines reported by the RHESSI satellite. These results cast serious doubts on previous claims of linear polarization at the one percent level and more, attributed to impact polarization. The absence of linear polarization limits the anisotropy of energetic protons in the Hα emitting region. The likely causes are isotropization by collisions with neutrals in the chromosphere and defocusing by the converging magnetic field.
Polarization of the hydrogen Hα line in solar flares
Astronomy and Astrophysics, 2001
Linear polarization of the hydrogen Hα line was observed during solar flares. The polarization vector is directed towards disk center and its degree is of the order of 5%. The best explanation for this polarization is anisotropic collisional excitation of the n = 3 level of hydrogen by vertical beams of protons with an energy greater than a few keV. However, previous calculations gave an expected polarization degree of 2.5% or less, a factor of two below the observations. In this paper, the theoretical model for the formation of the line polarization has been refined, including the effect of polarization in the local radiation field that is created by hydrogen proton anisotropic excitation. We have also increased the spectral index of the proton energy distribution from 4 to 5, giving more weight to the low energy protons which are the most efficient for impact polarization, without ionizing the atmosphere too much. It is found that the inclusion of the polarization of the local radiation field does not increase the Hα polarization very significantly; however, going from a spectral index of 4 to 5 results in an expected polarization degree of 4.5%, compatible with the observations.
X-Ray Polarization of Solar Flares Measured with Rhessi
Solar Physics, 2006
The degree of linear polarization in solar flares has not yet been precisely determined despite multiple attempts to measure it with different missions. The high energy range, in particular, has very rarely been explored, due to its greater instrumental difficulties. We approached the subject using the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) satellite to study six X-class and 1 M-class flares in the energy range between 100 and 350 keV. Using RHESSI as a polarimeter requires the application of strict cuts to the event list in order to extract those photons that are Compton scattered between two detectors. Our measurements show polarization values between 2 and 54%, with errors ranging from 10 to 26% in 1σ level. In view of the large uncertainties in both the magnitude and direction of the polarization vector, the results can only reject source models with extreme properties.
The Astrophysical Journal, 2000
Observations of polarization of chromospheric lines in solar Ñares can constrain the energy Ñux in accelerated protons. In this paper we analyze recently reported observations of Ha linear polarization obtained during a rather well observed Ñare on 1989 June 20. Modeling of the magnitude and orientation of the Ha polarization provides a constraint on the Ñux of low energy keV) protons, while (Z200 simultaneous gamma-ray and hard X-ray observations provide constraints on the Ñuxes of MeV Z10 protons and keV electrons, respectively. These, plus information on the energetics of the low-Z50 temperature and high-temperature thermal emissions, permit evaluation of both the absolute and relative roles of electrons and protons in the Ñare energy budget. We Ðnd that accelerated protons with energies keV can contain a signiÐcant portion of the total energy released during the Ñare, consistent with Z200 a steep extrapolation of the proton spectrum to such relatively low energies. We discuss these results in light of a uniÐed electron/proton stochastic particle acceleration model and show that the energetics are indeed consistent with this large proton energy content.
Gamma‐Ray Polarimetry of Two X‐Class Solar Flares
The Astrophysical Journal, 2006
We have performed the first polarimetry of solar flare emission at γ-ray energies (0.2-1 MeV). These observations were performed with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) for two large flares: the GOES X4.8-class solar flare of 2002 July 23, and the X17-class flare of 2003 October 28. We have marginal polarization detections in both flares, at levels of 21 ± 9% and-11 ± 5% respectively. These measurements significantly constrain the levels and directions of solar flare γ-ray polarization, and begin to probe the underlying electron distributions.
Multiwavelength Analysis of the Impact Polarization of 2001 June 15 Solar Flare
The Astrophysical Journal, 2005
We report here the impact polarization of the Hα, Hβ and MgI (552.8 nm) lines during an M6.3 solar flare observed on 2001 June 15th with the THEMIS telescope in the multi-wavelength spectropolarimetric mode. Typical spectral intensity and polarization profiles are presented. All of these lines are linearly polarized and polarization degree vary 3%-6% at line center. The directions of polarization are either parallel or perpendicular to the local transverse magnetic field, which are investigated by simultaneous observation of FeI (630.2 nm). The polarization islands are located at the edges of flare kernels. After eliminating scattering, Zeeman effect and intensity gradient, as possible origin of the observed polarization, this polarization is interpreted as due either to low energy proton beam or to the return current associated with electron beams.
Information on particle acceleration and transport derived from solar flare spectropolarimetry
Advances in Space Research, 2005
The hydrogen Ha line has been found to be linearly polarized at some locations and times during a June 15th 2001 flare observed with THEMIS. This flare was accompanied by radio pulses and hard X-ray emission. Linear polarization is below the noise level in the flare kernels. However, it is present at the edges of these kernels, in the line center and near wings where the polarization degree exceeds 4%. The directions of polarization are not random but close within ±15°to the tangential and radial directions. This polarization can be due either to electron beams and their associated return currents or to electron and proton beams.
Astronomy and Astrophysics, 2011
Aims. We study the polarization of hard X-ray (HXR) sources in the solar atmosphere, including Compton backscattering of photons in the photosphere (the albedo effect) and the spatial distribution of polarization across the source. Methods. HXR photon polarization and spectra produced via electron-ion bremsstrahlung emission are calculated from various electron distributions typical for solar flares. Compton scattering and photoelectric absorption are then modelled using Monte Carlo simulations of photon transport in the photosphere to study the observed (primary and albedo) sources. Polarization maps across HXR sources (primary and albedo components) for each of the modelled electron distributions are calculated at various source locations from the solar centre to the limb. Results. We show that Compton scattering produces a distinct polarization variation across the albedo patch at peak albedo energies of 20-50 keV for all anisotropies modelled. The results show that there are distinct spatial polarization changes in both the radial and perpendicular to radial directions across the extent of the HXR source at a given disk location. In the radial direction, the polarization magnitude and direction at specific positions along the HXR source will either increase or decrease with increased photon distribution directivity towards the photosphere. We also show how high electron cutoff energies influence the direction of polarization at above ∼100 keV. Conclusions. Spatially resolved HXR polarization measurements can provide important information about the directivity and en-ergetics of the electron distribution. Our results indicate the preferred angular resolution of polarization measurements required to distinguish between the scattered and primary components. We also show how spatially resolved polarization measurements could be used to probe the emission pattern of an HXR source, using both the magnitude and the direction of the polarization.
First detection of return currents in solar flares by spectropolarimetry with THEMIS
Astronomy & Astrophysics, 2003
Using THEMIS French-Italien telescope with the MTR mode, the Hydrogen Hα and Hβ lines have been observed to be linearly polarized up to a few percent by impact during the impulsive phase of two solar flares associated with high-frequency radio pulses. Two privileged directions of linear polarization are present, respectively radial (in the disk center to flare direction) and tangential (perpendicular to the radial direction). This 90 degree change in the linear polarization direction is interpreted as due to the chromospheric return current generated by the penetration of a non-thermal electron beam into the chromosphere.
Diagnostics of energetic electrons with anisotropic distributions in solar flares
Astronomy and Astrophysics, 2010
Aims. The paper aims are to simulate steady-state distributions of electrons beams precipitating in collisional and Ohmic losses with pitch angle anisotropy into a flaring atmosphere with converging magnetic field and to apply these to the interpretation of HXR photon spectra, directivity and polarization observed for different photon energies and flare positions on the solar disk. Methods. Summary approximation method is applied to a time-dependent Fokker-Planck equation by splitting the temporal derivative equally between the derivatives in depth, energy and pitch angles and finding the solutions in forward and backward directions for each variable. Results. For softer beams, there is a noticeable flattening of the photon spectra at lower energies caused by the self-induced electric field that increases for larger viewing angles. For the models with an electric field, the HXR emission with lower energies (30 keV) becomes directed mainly upwards at upper atmospheric levels owing to the increased number of particles moving upwards, while in deeper layers it again becomes directed downwards. The polarization maximum shifts to higher energies with every precipitation depth approaching 25 keV for the models with pure collisions and 100 keV for the models with return currents. At deeper layers, the polarization decreases because of the isotropization of electrons by collisions. The maximum polarization is observed at the viewing angle of 90 • , becoming shifted to lower angles for softer beams. The integrated polarization and directivity shows a dependence on a magnetic field convergence for harder beams, while for softer beams the directivity is strongly affected by the self-induced electric field changing from a downward motion to an upward one at upper atmospheric depths. Conclusions. The proposed precipitation model for an electron beam with wider pitch angle dispersion of 0.2 taking into account collisional and Ohmic losses allowed us to fit the double power law HXR photon spectra with a spectrum flattening at lower energies observed in the flares of 20 and 23 July 2002. The observed directivity of HXR photons of 20 keV derived for a large number of flares located from the disk center to limb is also reproduced well by the theoretical directivity calculated for an electron beam with a very narrow pitch angle dispersion of 0.02. The simulated polarization of this narrowly-directed electron beam fits up to 90% of all the available polarimetric observations carried out at various locations across the solar disk.