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LOCALISATION OF CHROMOSPHERIC EVAPORATION IN SOLAR FLARES, BY THE
ANALYSIS OF X-RAY SPECTRA
Aurélie Gabriel, F. Millier, N. Lizambert
To cite this version:
Aurélie Gabriel, F. Millier, N. Lizambert. LOCALISATION OF CHROMOSPHERIC EVAPORA-
TION IN SOLAR FLARES, BY THE ANALYSIS OF X-RAY SPECTRA. Journal de Physique
Colloques, 1988, 49 (C1), pp.C1-325-C1-328. �10.1051/jphyscol:1988169�. �jpa-00227583�
LOCALISATION OF CHROMOSPHERIC EVAPORATION IN SOLAR FLARES, BY THE ANALYSIS OF X-RAY SPECTRA
A.H. GABRIEL, F. MILLIER and N. LIZAMBERT
Institut d'Astrophysique Spatiale. LPSP. BP 10, F-91371 Verri&res-le-Buisson Cedex. France
L'analyse des flares solaires collectks par le spectrombrre B cristal courM, BCS, B bord du satellite SMM rkvkle l'existence d'un plasma therrnique s'khappant verticalement avec une vitesse p m h e dc 350 km S-1.
Ce plasma en expansion, dont la temp6rature est de l'ordre de 25 106, est observb dans le spectre de l'ion Ca XIX de type hklium. La signature de ce plasma peut s'exprimer en fonction de sa vitesse, mesuree par le dkalage vers le bleu de la raie de resonance "W", et par son intensitk, relative B celle de la composante statiomaire. L'ktude de la variation de ces deux paramktres en fonction de la distance zone tmissive- limbe solaire a port6 sur 33 flares emegitrks en 1980. Les dsultats sont cornpan% B ceux pddits par deux modkles de complexitk diffkrente. Le premier, modkle A, interprkte l'absence de blue-shift lors des observations faites au limbe comrne significative d'un plasma dont l'expansion se fait perpendiculairement h la direction de v i d e . Le modble B introduit la notion de "puits" crke par l'bvaporation chrornosphbrique.Ce "puits", selon sa gkomktrie exprim& par le rapport profondeur / diambtre, peut Etre la cause d'une occultation plus ou moins importante du rayonnement kmis par la composante dynamique.
I1 est clair que ces deux modUes conduisent il des variations diffkrentes, en fonction de la position de la zone active sur le disque, de la vitesse et de I'intensitk relative du plasma dynamique. Nos rksultats qui montrent l'absence de blue-shift pour 10 flares non situCs au limbe confument l'hypothhe d'un puits et sont consistents avec le modkle B.
Abstract
Analysis of solar flares using the data from the Bent Crystal Spectrometer on the SMM solar flare satellite, shows a thermal plasma which expands vertically at a velocity of up to 350 km S-l. This plasma.
at a temperature of the order 25 106 K is observed in the l i e radiation of He-like Ca XIX. Its velocity is determined by measuring the blue shift of the resonance line "W", whereas its intensity is expressed relatively to that of the stationary component. We analyse the variations of velocity and relative intensity of the evaporating plasma as a function of its location on the solar disc for 33 flares during the year 1980.
The results are compared with the values expected from two alternative models A and B. On the basis that when observed at the limb, such flares do not usually show a blue shift, model A interprets this as due to the fact that the motion of the explosive plasma is perpendicular to the line-of-sight. Model B assumes that the process of chromospheric evaporation produces a "well" in the chromosphere, and therebye occults some of the emission of the ejected material. It is clear that the two models predict different velocity and relative intensity variations as a function of the distance of the flare from disc center. The lack of blue shift for 10 disc flares supports strongly the model B, i.e the existence of a "well" whose depth / diameter is between 0.1 and 5 as shown by our observations.
Introduction
Tne most complex part of a solar flare, usually lasting only a few minutes, is its initial, impulsive phase, as indicated by Grineva et al. [l], Doschek et al. [2] and Feldman et al [3] from their observations of blue-shifted and broadened FeXXV resonance line.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988169
Cl-326 JOURNAL DE PHYSIQUE
Soft X-ray spectra, collected with the Bent Crystal Spectrometer (BCS) , on board the Solar Maximum Mission spacecraft (SMM), have a high temporal resolution which permits the observation of featms of the onset phase of a flare. As the resolution is high in the spectral region covering the emission of He-like ions Ca XVIII-XIX and Fe XXIV-XXV
.
spectroscopic diagnostic properties permit the derivation of the physical conditions of the flaring plasma. Thereby BCS observations lead to a better understanding of the dynamics and energetics involve in the explosive phase of flares (Antonucci and Dennis [4], Antonucci, Gabriel and Dennis [5]). From the analysis of the impulsive phase of the largest soft X-ray flares ( class M or X ) observed with BCS during 1980, an observational model has emerged: the soft X-ray emitting plasma initially appears with large upward velocity and non thermal broadening, both of which lasting for only a short time (Antonucci et al.[6]). Blue-shifted emission is interpreted as chromospheric evaporation ([4[,[5] and [6]). The primary flare energy release occurs in coronal flux tubes. Heating of the chromosphere to multimillion degree temperature follows.The heated gas moves upward into the flaring tubes, and the Doppler effect causes the X-ray emission from this ascending plasma to be blue-shifted relative to the non moving plasma already present in the flux t bes. This upwelling plasma is well1
observed in the helium-like resonance line " w " of Ca XIX (3.176 ) and can be characterised by the two following computable parameters: its velocity, determined by measuring the blue shift of the resonance Line, and its intensity relative to that of the stationary component.
When observed at the limb, most of the flares do not show a blue shift. Is this behaviour due to the fact that the expanding plasma is moving upward, perpendicular to the line of sight ? In other words is such a systematic and complete cancellation of the velocity vector likely and does it account for the observations on the limb as well as everywhere on the solar disk ?
An other explanation for the observations is that the process of chromospheric evaporation can produce a depression, or "well" in the chromosphere @e Jager [7]), which will give rise to progressive occultation effects when the flaring region is viewed obliquely. Then, how does this model fit the observations ? We analyse the BCS observations in the Ca XVIII-XIX channel for 33 flares during the year 1980 and focus our study on the variation of the velocity and relative intensity of the evaporating plasma as a function of its location on the solar disc. Our object is to investigate wether the observations are best fitted to the effect of a vertical velocity vector, or to the occultation of the region of emission.
Chromospheric evaporation model-Spectra selection-data handling We consider the two limiting models:
-Model A: the model, usually assumed for interpreting the BCS blue-shift observations, supposes a velocity vector normal to the surface for the ejected material as the explanation for the lack of blue- shift for limb flares. For this model we ignore the possibility of a well in the chromosphere, which would lead to an obscuration of the emission when at the limb.
-Model B: we adopt a model similar to that proposed by De Jager [7]. In this, the deposition occurs at the bottom of a cylindrical "well", in the chromosphere, created by the evaporation. The absorption of the side of this well obscures the high velocity component when one views limb flares. We no longer need to assume a high directivity, and take instead a hemispherical velocity distribution for the evaporated material. 'This latter assumption is not critical in the interpretation of the results.
The following procedure was used to select the spectra and extract the data:
-broadening and blue- shifted components of the soft X-ray lines are typically greatest at the peak of the hard X-ray burst emission, as shown by the SMM correlated observations made with the BCS and the Hard X-ray Burst Spectrometer (HXRBS), which detects events in the energy range from 25 to 510 keV (Antonucci et al. [6]). So, the temporal indication of the HXRBS light curve peak intensity gives us the time selection of our data. As the counting rate is low, our data are typically averaged over 20 to 30 seconds to improve the statistical significance of the spectra
-among the 36 largest flares detected during the year 1980, we have considered 33 collected with the best spectral resolution, that is with a spectrometer bin spacing of 0.3 mA.
An example of an impulsive-phase flare spectrum is shown in Figure 1: The most prominent feature is the w resonance line with its blue-shifted component referred to here as the w' line. Fom a spectral analysis discribed in [6] the thwretical spectrum, which best fits the data, is shown on Figure 1 as a continuous line superposed on the observed counting rates. It consists of the sum of two thwretical spectra (the second one blue shifted with respect to the first), each defined by a set of adjustable and independant parameters such as electron temperature and ionization conditions.
and Volontt [9]. During the gradual phase following the impulsive phase, blue-shift are absent as well as non thennal excess in line profiles.
5/09119)0 (521-W2)
TE - I 4X.07 I ' E - I + Y e 0 7 TO - J O E.07 l'D - 3 0 C.01
-220 -200 -180 -160 -140 -120 -100 -80 .-60 BIN MsER
Figure 1. The Ca XIX BCS spectrum covuing 3.165A-3.217A is observed at 07:12:14 UT during the impulsive phase of the May 9. 1980 flare. The best fit is found by superposing two theoretical spectta, the second is blue shifted (M) by 2.93 m A and with intensity Iw' a factor 0.16 of the fyst Iw.
The distance of the flaring region from the Sun's centre, p, is related to the view angle 0 defined as the angle between the line of sight to the flare and the radius vector from Sun's centre to the flare site b=sine). In this study we are concerned with the variation as a function of of two measurable parameters describing the morphology of the chromospheric evaporating material:
-the velocity of the dynamic soft X-ray source is derived from the relative wavelength shift, M, of the two spectral components required to fit the data. This blue-shift can be determined by comparing the wavelength at the peak of the resonance line "w" with the comsponding shifted peak wavelength "w"' (see Figure 1.). When the separation between these two lines is less than a shift corresponding to about 120 km s-1, we are unable to obtain a reliable measure of the velocity. The absolute error in measuring the velocity is close to 50 km s-1.
-the intensity Iw4 of the dynamic soft X-ray source relative to the one, Iw, of the stationary component.
These intensities are the peak intensities of the fitted resonance lines. Accuracy in the ratio Iw8/Iw depends on the counting rates and thereby is variable from flare to flare. Nevertheless, we assume a constant error of 25% for every flare.
Results
Figure 2 shows the results obtained in comparison with the values expected from the models designated A and B (see above). For each model, the solid lines show the values predicted for the two measured quantities: the degree of blue shift M, and the ratio of intensities Iw8/Iw = R.
For Model A, M is expected to decrease towards the limb following the cosine effect whereas R would expect to remain constant.
In contrast for Model B we expect M to m a i n reasonably constant, whereas R will decrease by reason of occultation at a rate dependent on the aspect ratio of the "well " (depth /diameter = 2).
Evidently we have a different curve for each " z ", and no reason to assume that all flares have the same value of " z
".
A special question arises when we observe no shifted component, since we cannot say wether M or R or both are zero. These points have been plotted on curve M for Model A and on curve R for Model B, since they are formally disallowed on the other two plots.Cl-328 JOURNAL DE PHYSIQUE
MODELA
-
as*-
1-I I
--
9..
0.2 0.4 0.6 e.8 I.. RP
Figure 2. The obswved values of M (uppu panel ) and R ( lower panel ) are plotted along with their
M[V bars as a function of & Figure 2a shows the expected cwes M f a model A. and Figure 2b R as a function of "2" f a model B.The solid line M ( p ) in Figure 2b is the avaage experimental values.
Solid lines R ( p ) in Figure 2 . are the mean and standard deviation expuimental data.
Inspection of the figures shows a degree of fit which is always worse than the error estimates. This is to be expected since the errors include only statistical effects and not the flare to flare variability. If we look at the points in the body of the graphs we can conclude that Model B shows a somewhat better fit that Model A. However the critical set of points on the horizontal axis can only be explained on the basis of Model B. We therefore claim that the observations support strongly the Model B (existence of a "well") and find a range of values of z (depth/diarneter) between 0.1 and 5. It is most likely that this variation in z
is due primarily to a variation in the diameter of the structure, bearing in mind that we are talking of the elementary flux tube in rhose cases where the region has a low filling factor and filanientary form.
Referena
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[4lAntcmucci. E., and Dennis. B. E.. Solar Phys. 86 (1983) 67.
151 Anmnucci, E., Gabriel, A. H., and Dennis, B. R., Astrophys 1.287 (1984) 917.
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