First detection of return currents in solar flares by spectropolarimetry with THEMIS (original) (raw)
Related papers
Absence of linear polarization in H? emission of solar flares
Astronomy and Astrophysics, 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.
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.
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.
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.
Solar Physics, 2005
A multi-wavelength spatial and temporal analysis of solar high energy electrons is conducted using the August 20, 2002 flare of an unusually flat (γ1 = 1.8) hard X-ray spectrum. The flare is studied using RHESSI, Hα, radio, TRACE, and MDI observations with advanced methods and techniques never previously applied in the solar flare context. A new method to account for X-ray Compton backscattering in the photosphere (photospheric albedo) has been used to deduce the primary X-ray flare spectra. The mean electron flux distribution has been analysed using both forward fitting and model independent inversion methods of spectral analysis. We show that the contribution of the photospheric albedo to the photon spectrum modifies the calculated mean electron flux distribution, mainly at energies below ∼100 keV. The positions of the Hα emission and hard X-ray sources with respect to the current-free extrapolation of the MDI photospheric magnetic field and the characteristics of the radio emission provide evidence of the closed geometry of the magnetic field structure and the flare process in low altitude magnetic loops. In agreement with the predictions of some solar flare models, the hard X-ray sources are located on the external edges of the Hα emission and show chromospheric plasma heated by the non-thermal electrons. The fast changes of Hα intensities are located not only inside the hard X-ray sources, as expected if they are the signatures of the chromospheric response to the electron bombardment, but also away from them.
The viewing angle effect on Hα-line impact polarisation in impulsive solar events
Astronomy and Astrophysics, 2005
The effect of a viewing angle on the hydroden H α-line impact polarisation is investigated in a plane vertical atmosphere arbitrary located on the solar disk. The impact polarisation is assumed to be caused by precipitating beam electrons with pitch-angular anisotropy steadily injected into the flaring atmosphere from its top. The polarisation is calculated for a 3 level plus continuum hydrogen atom affected by Zeeman splitting in a moderate magnetic field taking into account depolarising effects of diffusive radiation and collisions with thermal electrons. The H α polarisation profiles are affected by electron beams only in the line cores whereas the wings are fully depolarized by the collisions with thermal electrons despite the extended wing emission, or "moustaches", caused by beam electrons. The full (integrated in wavelength) H α-line linear polarisation, caused by moderate electron beams, is shown to be 2−20% and either negative or positive depending on the position of a flaring loop on the solar disk and the direction of an emitted photon from the local magnetic field. The polarisation plane is projected onto a viewing angle ψ, being a superposition of the flare location on a solar disk and the magnetic field deviation from vertical on the solar surface. For viewing angles less then 50 • the H α-line impact polarisation is negative increasing up to −10% towards smaller angles, meaning that the polarisation is mostly perpendicular to the plane B × K where B is the magnetic field induction and K is the photon momentum vector. For viewing angles greater than 60 • the measured impact polarisation becomes positive, sharply increasing up to 20% towards the limb. In the range of 50−60 • the observed impact polarisation goes through a zero point despite the actual presence of beam electrons in the flaring atmosphere. The theoretical predictions of the dependence of polarisation degree on viewing angle fit remarkably well the observations of H α-line linear polarisation in small-scale flaring events such as moustaches or Ellerman bombs, located in different positions on a solar disk.
Astronomy & 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.
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.