The progressive phases of three solar flares have beenobserv ed with THEMIS in July 2000, using the multiline spectropolarimetric MTR mode. A preliminary analysis of the characteristics of the polarization of the Hα and Hβ lines

observed at the beginning of the progressive phase of one of these flares is presented

Briand, C.

Chambe, G.

Hénoux, J.-C.

Vogt, E.

English

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IL NUOVO CIMENTO Vol. 25 C, N. 5-6 Settembre-Dicembre 2002THEMIS as particle detector: Spectropolarimetry of solarflares(∗)J.-C. He´noux(1), E. Vogt(2)(∗∗), G. Chambe(1) and C. Briand(3)(1)(1) Observatoire de Paris, DASOP/UMR 8645 - Place Jules Janssen, 92195 Meudon, France(2) Osservatorio Astronomico di Capodimonte - Via Moiariello 16, I-80131 Napoli, Italy(3) U.P.S. THEMIS - Via Lactea, La Laguna 38200, Tenerife, Spain(ricevuto il 10 Giugno 2002; approvato il 7 Agosto 2002)Summary. — The progressive phases of three solar flares have been observed withTHEMIS in July 2000, using the multiline spectropolarimetric MTR mode. A pre-liminary analysis of the characteristics of the polarization of the Hα and Hβ linesobserved at the beginning of the progressive phase of one of these flares is presented.PACS 96.60.-j – Solar physics.PACS 96.60.Rd – Flares, bursts, and related phenomena.PACS 01.30.Cc – Conference proceedings.1. – IntroductionThe relative contribution of ions and electrons to the flare energy budget is not wellknown yet. On the basis of the large hard–X-ray flux observed during the impulsivephase of solar flares, it has long been assumed that electrons carry practically the wholeof the energy released. For some time, only high-energy protons above 1 MeV weredetected, and only rather recently, has proton acceleration been assumed to contributesignificantly to the global flare energetics [1-3].The energy needed for protons to play a role in the solar-flare energy balance must becarried by low-energy protons (E < 1 MeV). These protons cannot be detected by γ-rayobservations and other diagnostics are needed. The shape of chromospheric line profilesand impact polarization measurements are possible diagnostics tools [4, 5].Impact polarization observations, together with hard–X-ray observations, can provideinformation on the nature and density flux of the beam particles [6,7]. Most impact line(∗) Paper presented at the International Meeting on THEMIS and the New Frontiers of SolarAtmosphere Dynamics, Rome, Italy, March 19-21, 2001.(∗∗) Present address: Department of Physics and Astronomy, University of Glasgow G12 8QQ,United Kingdom.c© Societa` Italiana di Fisica 729730 J.-C. HE´NOUX ETC.Fig. 1. – On the left: definition of impact line polarization. On the right: P (90◦, E) for theHα line and for electron, proton or ion beams as a function of the electron equivalent energyin eV, taken from [10]. Experiments: electrons: dark triangles and diamonds; protons: emptytriangles; He+: circles. Theory: electrons: dashed line; protons: dot-dashed line.polarization observations in solar flares have been limited to the Hα line [8]. The MTRmode on THEMIS offers the opportunity of multiline flare observations and we reporthere some preliminary results.2. – Impact polarization2.1. Beam of monokinetic particles. – The monochromatic radiation emitted by anatom collisionally excited by a beam of monokinetic particles of energy E may be linearlypolarized [9]. Calling respectively I‖ and I⊥ the intensities of the vibrations parallel andperpendicular to the plane defined by the beam and the line of sight and β the anglebetween these two directions (fig. 1), the degree of linear polarization P (β,E) is definedbyP (β,E) = (I‖ − I⊥)/(I‖ + I⊥) ,P (β,E) is related to the maximum of polarization observable at 90◦ from the beampropagation direction, P (90◦, E), byP (β,E) = P (90◦, E)sin2 β1− P (90◦, E) cos2 β .As shown in fig. 1, P (90◦, E) is a function of the nature of the particle, electron,proton or ion and of the particle energy. P (90◦, E) changes sign for a turnover energyE0. For the Hα line, E0 is of about 200 eV for electrons and 200 keV for protons.2.2. Monoenergetic particles with an axially symmetrical velocity distribution functionf(v, α). – Presumably the angular velocity distribution function of energetic particles willnot be a delta-function. However, assuming an axially symmetrical velocity distributionfunction per unit solid angle f(v, α), where α is the angle between the particle velocityvector and the symmetry axis, it was found [11] that the preceding equation for P (β,E)THEMIS AS PARTICLE DETECTOR: SPECTROPOLARIMETRY OF SOLAR FLARES 731Fig. 2. – Possible origin of radial and tangential linear polarization: on the left electrons ofenergy E > E0 and on the right protons of energy E < E0. The double arrow and the smallellipse near the flare location F on the solar surface represent particle beams or particles movinghorizontally. Fp is the projection of F on the solar disk.is unchanged provided that β now represents the line of sight to symmetry axis angleand that P (90◦, E) is replaced byP (90◦, E) (3J2 − J0)2J0 + P (90◦, E)(J2 − J0)  P (90◦, E)(3J22J0− 12),whereJn =∫ π0f(v, α) cosn α sinα dα.The correction factor 3J2/2J0−1/2 is positive (negative) for propagation mainly parallel(perpendicular) to the symmetry axis.2.3. Deriving the nature of the particles from the direction of polarization. – At-mospheric bombardment by energetic particles with non-isotropic velocity distributionfunctions (v.d.f.) is expected to lead to the emission of polarized light with two possibledirections of polarization differing by 90◦. For a vertical axis of symmetry of the v.d.f.,these privileged directions are respectively the radial flare to disk center direction andthe direction tangential to the solar limb for a flare on the limb.Conversely, a priori, depending on particle energy, two main particle travelling di-rections can explain a given observed direction of linear polarization, and that for bothelectrons and protons. Fortunately, beam electrons reaching the chromosphere with anenergy below E0 (≈ 200 eV) will have lost their directivity and would not generate po-larized radiation, and protons with energy below 200 keV at chromospheric level will732 J.-C. HE´NOUX ETC.Fig. 3. – Spectral profiles of the intensity, degree of polarization and θ, the angle between thedirection of linear polarization and the flare to disk center direction, for the Hα line. Theseprofiles are spatially integrated over 2 arc second along the spectrograph slit in five regions.The same representation is used for all profiles and for the positioning of the spatial cuts on thespectrogram at bottom left. A Hα picture of the field of view is at the bottom right.presumably dominate over the high-energy part of the proton energy distribution spec-trum. Then, as illustrated in fig. 2, only two possibilities are left to explain a givenobserved direction of polarization:– a radial direction of polarization could be due either to low-energy protons movingpredominantly in the vertical direction or to high-energy electrons moving horizontally.– A tangential direction of polarization could be due either to low-energy protonsmoving horizontally or to high-energy electrons moving predominantly in the verticaldirection.Other complementary data, especially hard–X-ray observations, which detect high-energy electrons, would help to discriminate between these possibilities.3. – THEMIS observationsTHEMIS, as a polarization-free telescope, is well suited to linear polarization mea-surements. Moreover, the MTR mode allows multiline spectroscopy. Four lines were si-multaneously observed, the Hα, Hβ, Na D1 and Na D2 lines. A grid at the polarimeter’sentrance in F1 allows easy separation, without additional optics, of the extraordinaryand ordinary beams that carry the I + S and I − S signals, where S is a given Stokesparameter (U , Q or V ) [12]. The field of view is divided into three bands of 16 secondof arc separated by 16 second of arc. The progressive phases of three solar flares wereTHEMIS AS PARTICLE DETECTOR: SPECTROPOLARIMETRY OF SOLAR FLARES 733Fig. 4. – Same as fig. 3 for Hβ.observed in July 2000. We report here some characteristics of the Hα and Hβ lines ina bright flare kernel at 07:19:35 UT, near the maximum of soft–X-ray emission, of theJuly 19th 3N flare in AR9087 (S21E12).As shown in figs. 3 and 4, the lines observed are linearly polarized with a polarizationdegree that reaches 5 to 7 per cent. The orientation of the polarization at five positionsalong the slit is not distributed randomly but shows clearly two preferred directions.For Hα these directions are at 35◦ (±7◦) and 125◦ (±7◦) from the flare to disk-centerdirection. These angles differ by 90◦. For Hβ the polarization directions are at 45◦and 120◦ from the flare to disk-center direction, differing by 75◦. The about 30◦ to 40◦deviation from the radial and tangential directions can both be interpreted as due to theinclination along the local vertical of the lines of forces followed by the particles.For three positions, the polarization is close to radial for Hα and to tangential for Hβ,that is formed deeper, and vice versa for the two others. Consequently, there is a nearly90◦ change in the direction of polarization. It is worth noticing that the directions ofpolarization of the Na D2 line, formed still deeper, i.e. into the high photosphere below,are found to be close to the Hβ polarization directions.Unfortunately no hard–X-ray observation is available for the flare under study. Sowe cannot be certain of the presence or absence of accelerated electrons. A model wherebeams penetrate into the atmosphere and where the inclination on the solar vertical ofparticle velocities is expected to increase with depth due to mirroring is the most naturalone in order to explain the observed changes in polarization direction. With such model,proton beams would be required for the three cases where the Hα polarization is almostradial and electron beams for the two other cases.Another possibility is that mirroring takes place rather at the Hα formation level and734 J.-C. HE´NOUX ETC.that escaping particles penetrate mostly vertically into the Hβ formation region. Thismodel leads to the similar conclusion that both types of particles, electrons and protons,are needed. But, this time, a radial (tangential) direction of polarization would requireelectrons (protons).4. – ConclusionsJust preliminary results have been presented above, based on a very limited sampleand in the absence of hard–X-ray measurements. They confirm the presence in thelower atmosphere of non-thermal particles during the flare progressive phase. Indeed theanalysis of the full data set must be pursued. In future, in order to check the impactpolarization signature of high-energy electrons, the impulsive phase of a few solar flareshas to be catched.∗ ∗ ∗We thank Dr. J. Rayrole andM. Semel, S. Sahal-Bre´chot and N. Feautrierfor valuable discussions.REFERENCES[1] Simnett G. H., EoS, 77 (1996) 353.[2] Ramaty R., Mandzhavidze N., Kolovsky B. and Murphy R., Astrophys. J., L193(1995) 455.[3] Emslie A. G., Miller J. A., He´noux J.-C., Vogt E. and Sahal-Bre´chot S.,Astrophys. J., 542 (2000) 513[4] He´noux J.-C., in High Energy solar Physics - Anticipating HESSI, edited by R. Ramatyand N. Mandzhavidze, ASP Conf. Ser., Vol. 206 (2000) 27-36.[5] Falchi A., this issue, p. 639.[6] He´noux J.-C.and Vogt E., Physica Scripta, T78 (1998) 60.[7] Vogt E., Sahal-Bre´chot S. and He´noux J.-C., Astron. Astrophys., 324 (1997) 1211.[8] Vogt E. and He´noux J.-C., Astron. Astrophys., 349 (1999) 283.[9] Percival I. C. and Seaton M. J., Philos. Trans. R. Soc. London, Ser. A, 251 (1959)113.[10] Werner A. and Schartner K. H., J. Phys. B, 29 (1996) 125.[11] He´noux J.-C., Chambe G., Semel M., Sahal-Bre´chot S., Woodgate B., Shine R.,Beckers J. and Machado M., Astrophys. J., 265 (1983) 1066.[12] Semel M., Astron. Astrophys., 91 (1980) 369.

THEMIS as particle detector: Spectropolarimetry of solar flares

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