Sites of hot thermal plasma in solar flares

.4dv. Space Res. Vol. 30, No. 3, pp. 671-676.2002 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/02 $22.00 + 0.00

Pergamon www.elsevier.com/locatelasr

PII: SO273-1177(02)00361-7

SITES OF HOT THERMAL PLASMA FLARES

IN SOLAR

M. Tomczak’ 1Astronomical

Institute,

Wroclaw University, Kopernika 11, PL-51-622

Wroclaw, Poland

ABSTRACT The time evolution of the emission measure derived from the Bragg Crystal Spectrometer (Fe XXV channel) and from the Soft X-ray Telescope images for 27 limb flares well-observed by Yohkoh has been compared. The ratio q of these two emission measures has been used as an estimation of relative contribution of hot thermal plasma. It has been confirmed that hot plasma is situated mainly inside the bright loop-top kernels, where it co-exists with cooler plasma. The maximal relative contribution qmaz of hot plasma in kernels is larger for lower-situated events, which suggests a dependence on the amount of released energy. A method checking how well the bright loop-top kernel represents the site of hot thermal plasma has been proposed. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION There is a close relationship between hot plasma content (-20 MK) and the energy release in solar flares. Because of high energy losses by thermal conduction and radiation, such a plasma may indicate in-situ heating. Therefore an analysis of radiation of hot thermal plasma allows to investigate some details of the energy release process in solar flares. Among the instruments aboard the Japanese satellite Yohkoh, the most sensitive to the radiation of hot thermal plasma is the Bragg Crystal Spectrometer (BCS) in the channels Fe XXV and Fe XXVI. However, an important disadvantage of this instrument is that it accumulates photons from the whole Sun. Therefore, to determine sites of hot thermal plasma some X-ray images of the flare should be incorporated. The use for this aim of images derived from the Hard X-ray Telescope (HXT) is inconvenient because of a strong contribution from non-thermal radiation and some systematic errors in the image reconstruction method. The Soft X-ray Telescope (SXT) is equipped with five different. broadband filters that allow the diagnostics of the solar plasma from 3 MK up to about 30 MK. However, the values of about 20 MK usually occur in the periphery of flaring structures in the SXT temperature map, and they are at least partly due to instrumental effects: (1) small signal-@noise values, (2) differences in the Point Spread Function for individual filters of images (Siarkowski et al. 1996). (Martens et al. 1995), (3) small errors in the coalignment Doschek (1999) has revealed the existence of a hot flare component for some flares. He defined this component as a group of pixels in the SXT temperature maps appearing in sequential flare images taken at the same location, and having temperature values in the range 16-22 MK (typically derived from Fe XXV spectra). The total emission measure of the hot flare component was always significantly lower than the total Fe XXV emission measure. This means that the hotter pixels seen in the SXT maps cannot be responsible for the whole Fe XXV emission. The reported inconsistency between the SXT maps and the BCS/Fe XXV data can be explained assuming that the emitting plasma was multithermal. If this is the case, the SXT produces average temperatures along the lines of sight corresponding to different pixels. As showed by Jakimiec et al. (1998) for a twotemperature plasma (10 MK, 20 MK), the SXT temperature remains relatively low (T 5 13 MK) even for a very significant content of the hot plasma. A common feature of soft X-ray flare images is a bright loop-top kernels, or bright knots situated at the top area of the observed magnetic structure (Acton et al. 1992). Such a kernel is formed during the flare development, and is responsible for the soft X-ray light curve maximum. The following observations strongly

671

M. Tomczak

612

suggest that the hot thermal plasma should be contained mainly inside the flare loop-top kernel, where it is mixed with the cooler plasma: 1. The images made by the Skylab NlU spectroheliograph (SO82A) which offered better temperature discrimination than the Yohkoh instruments (Doschek & Feldman 1996). 2. The bright loop-top kernel is distinctly hotter than the other parts of the flaring structure during a flare onset (Tomczak 1994). 3. Temperature of brighter pixels, which form the loop-top kernel, drops as fast as the BCS/Fe xxv light curve (Doschek et al. 1995). 4. Different relationship between maximum temperatures and maximum emission measures are estimated from the BCS (Fe XXV, (3x1~) and GOES data for flares whose footpoints are occulted or not, respectively (Sterling et al. 1997). 5. A very good correlation between the emission measure estimated from the BCS/Fe xxv spectra and the emission measure of the bright loop-top kernel from the SXT images is found for many flares (Jakimiec et al. 1998). SELECTION OF EVENTS AND METHOD OF ANALYSIS The subject of this paper is to compare the amounts of hotter and cooler plasma in the bright loop top kernel during the flare evolution. Following Jakimiec et al. (1998), the BCS/Fe XXV and the SXT observations have been used for the estimation of the hotter and cooler plasma content, respectively. For many flares a time variation of the ratio of the emission measures, q = EM(BCS)/EM(SXT), has been carefully investigated. Table 1 presents the list of the 27 limb flares analyzed. All events were well-observed by Yohkoh since their early onset. The selection has been limited to limb events to avoid those cases in which the characteristics of the bright loop-top kernel are altered by footpoints emission. The electron temperatures 2” and emission measures were calculated from BCS/Fe XXV spectra using the intensity ratio of the dielectronic satellite line j to the resonance line w. To determine this, synthetic spectra were fitted to observed spectra using a least-squares minimized x2 fitting routine. For a better fit, the x2 calculations were restricted to regions of the spectra including the reported temperature-sensitive lines and the nearby continuum (see Sterling et al. 1997, for details). A single-component fit was used. The bright looptop kernels were defined in the SXT Bell9 filter images at flare maximum as the areas of emission outlined by the isophote that is 50% of the brightest pixel. Sometimes, several additional pixels were included to take into account an expansion of the flaring structure. The temperature and the emission measure of the bright loop-top kernel has been calculated for the pair of SXT images made with the two thickest filters, Bell9 and All2 (filter ratio method, see Hara et al. 1992). RESULTS Figure 1 presents the time evolution of the temperature (top panel) and emission measure (middle panel) for the flare of 13 January 1992 (event No. 5, the so-called Masuda flare). The values calculated from the BCS/Fe xxv spectra and for the bright looptop kernel are marked with diamonds and crosses, respectively. Also the SXT temperatures and emission measures of the whole flaring structure are included (triangles). For an independent checking, the standard GOES diagnostic (Thomas et al. 1985) has been performed the results are marked in Fig. 1 with the solid line. If we consider the whole-flare parameters for the investigated time-interval, the following inequalities are fulfilled: T(BCS)

> T(GOES)

> T(SXT)

and

EM(BCS)

< EM(GOES)

< EM(SXT)

(1)

This proves that the flare plasma was multithermal and the results of individual instruments depend on their transmission bands. During the flare onset the bright loop-top kernel had a significant temperature excess in comparison with other parts of the flare. A small temperature deficit (- 0.5 MK) of the kernel seen after 17:32 UT is probably due to instrumental effects (Martens et al. 1995).

Sites of Hot Thermal Plasma in Solar Flares

673

Table 1. List of investigated flaresa

(1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

(2)

(3)

(4

(5)

(6)

(7)

17-NOV-91 2-DEC-91 l@DEC-91 18-DEC-91 13-JAN-92 13-JAN-92 15-JAN-92 6-FEB-92 17-FEB-92 19FEB-92 14FEB-92 26-FEB-92 l-APR-92 28-JUN-92 5-JUL-92 9-SEP-92 5OCT-92 5-NOV-92 24NOV-92 24-NOV-92 24-NOV-92 2-MAR-93 27-SEP-93 27-SEP-93 9-OCT-93 30-NOV-93 29-JAN-94

07~16 05:Ol 04:04 lo:29 17:34 19:13 19:02 21:02 15:46 03:55 14:58 01:39 lo:17 14:24 20:00 18:03 09:31 06:22 lo:05 14:29 16:04 15:lO 10:54 12:12 08:ll 06:08 11:29

Ml.1 M3.6 c9.3 M3.5 M2.0 Ml.3 M2.0 M4.1 Ml.9 M3.7 C9.2 Ml.3 M2.3 Ml.6 Ml.1 Ml.9 M2.0 M2.0 C6.9 c5.9 C5.4 c5.0 c5.7 Ml.8 Ml.1 c9.2 M2.4

Sll E86 N16 wa7 S 14 Elimb S 18 Elimb S15 Wlimb S 10 Elimb SO9E74 NO5Wlimb N16 W81 NO4 E85 NO6Elimb S16 W90 SO5Elimb N15 Elimb S13 E81 Sll W78 SO7Wlimb S18 Wlimb SO7Wlimb SO7Wlimb SO7Wlimb SO7E82 Nil E80 NlO Elimb Nil W78 S20 Elimb NO7Wlimb

6929

s

6952

C

6968 6982 6994 7012 7012 7030 7050 7067 7070 7073 7123 7216 7220 7270 7293 7323 7342 7342 7342 7440 7590 7590 7590 7627 7654

s C S S 5 C

s S S S C S C S S C

S S C

S S S C

S S

(8)

1.1 1.0 0.9 <2.2 1.6 0.9 0.8 <2.0 1.8 0.7 >1.2 1.8 0.5

(9)

07108 04:56 04:03 lo:26 17:28 19:05 18:56 20:54 15:42 03:51 14:50 01:38 10:15 13:58 19:59 17:59 09:25 06:20 10:02 14:23 15:50 15:06 lo:52 12:08 08:09 06:05 11:25

(10)

38 28 9 8 36 30 28 20 21 62 31 9 9 129 12 16 12 11 15 10 18 31 27 20 11 39 77

(11)

20 10 4 3 13 10 10 10 10 12 7 8 2 33 6 4 6 4 7 10 14 7 9 6 6 8 18

(12)

occ St occ St occ, un

occ, ov un, St

occ occ occ

occ, ov

D (1) - number of event; (2) - date; (3) GOES maximum time [UT]; (4) - GOES class; (5) - location; (6) - NOAA AR; (7) - evolution of the ratio q: s - single qmaz, c - complex evolution of q (multiple . of the bright looptop kernel [in SXT qmd; (8) - qmaz; (9) - time of qmaz [UT]; (10) - Npz ( size pixels]); (11) - H (est’lmated height [in lo3 km]); (12) - remarks: occ - event with footpoints occulted by the Sun, ov - hot plasma contained in smaller volume than the bright looptop kernel, un - large amount of hot plasma is outside the bright looptop kernel, St - BCS/Fe XXV diagnostic calculated by Sterling et al. (1997).

674

M. Tomczak TEMPERATURE

TEMPERATURE

20 18 16 ;

14 12 10 8

EMISSION

MEASURE

I

I

17 28

17 32

q = EM(ECS)/EM(SXT)

,736

fcsr the

,,:‘I0

kemel

Fig. 1. The time evolution of temperature,

emission

measure, and ratio q for the flare of 13 January 1992

Fig. 2. The same as in Fig. 1 but for the flare of 5 November 1992 (event No. 18).

(event No. 5) is presented in the top, middle, and bottom panel, respectively. The values obtained from BCS/Fe xxv spectra, SXT images (kernel and whole flare), and GOES are marked with diamonds, crosses, triangles, and solid line, respectively.

I/. EM(BCS)/EM(SXT)

Fig.

3.

T(SXT)

for

the

kernel

Common evolution in the diagram log q of 22 flares (405 points) for which the hot

thermal plasma was situated inside the bright loop top kernel.

I EM(BCS)/EM(SXT)

for

the

kernel

Fig. 4. The empirical formula described by Eq. 2 ob tained as a linear fit to the points in Fig. 3. uncertainties are also plotted.

f3o

Sites of Hot Thermal Plasma in Solar Flares

615

In the bottom panel of Fig. 1 the time evolution of the ratio q = EM(BCS)/EM(SXT) for the bright loop-top kernel is presented. We see that the maximal relative content of hot thermal plasma (qmm = 1.6) occurred during the early onset of the flare with a systematic decay after that. Within the error bars the time of qmar was the same as the time of maximum for T(SXT) derived for the kernel and for T(BCS). For some events, lasting longer than 10-20 min. and having a complex light curve in soft X-rays, the time evolution of the ratio q is more complicated than in Fig. 1, namely the decay after qmaz is followed by a repeated increase (see Fig. 2). Since this is not coincided in time with a repeated emission measure increase of the kernel the conclusion is that the further production of hot thermal plasma was outside the bright loop-top kernel. Usually at that time a new kernel developed as seen in the SXT images. In the case of such complex events the analysis was limited to the bright looptop kernel responsible for the main soft X-ray maximum. If a majority of hot thermal plasma is actually situated inside the bright loop-top kernel then a strong correlation between the ratio q and T(SXT) of the kernel should be seen. This is the case for 22 flares (Fig. 3) for which the evolution in the diagram log q - T(SXT) proceeds along the line that can be described with the following empirical formula (Fig. 4): T(SXT)

= (11.4 f 0.3) + (5.63 f 0.14) log q

for

8 MK 6 T(SXT)

6 14 MK

(2)

Excluding some instrumental reasons, each persistent deviation beyond the area bordered with f3a-lines in Fig. 4 may suggest the wrong representation of the site of hot thermal plasma. The evolution above this area may suggest that the bright loop-top kernel has been choosen too large - the hot plasma is located in a smaller part of the kernel. This is the case for events Nos. 11 and 27 where sizes of kernels were probably overestimated due to the location of flaring structures along the line of sight. The evolution below this area in Fig. 4 may suggest that important amount of hot thermal plasma is situated outside the kernel. This is the case for events Nos. 4, 8 and 13 where the hot plasma were produced in several places simultaneously. The maximum relative content of the hot thermal plasma in the bright loop-top kernel can be described with the parameter qmz. For investigated flares the obtained values of this parameter were within the interval 0.5-2.8 (see Table 1). It has been found an inverse proportion between value of qand size of the bright looptop kernel described with number of SXT pixels NPc (column (10) in Table 1). The dichotomy -hotter (qmac i2 1.3) and smaller ( NPZ & 22) kernel either cooler (qmr 15 1.1) and larger (NPZ 2 27) kernel is supported by 14 events. The inverse proportion qmor - height of kernel H (column (11) in Table 1) is also evident. The dichotomy - hotter (qllM22 1.3) and lower (H < lo4 km) kernel either cooler (qmoz s 1.1) and higher (H 2 lo4 km) kernel is supported by 16 events. At lower heights the magnetic field is usually somewhat stronger. Thus, larger amount of energy can be potentially released in lower-situated flares. Therefore, the inverse proportion qmar - H for the bright loop-top kernels can be treated as an additional confirmation that the hot plasma and energy release are cospatial - both are situated in bright loop-top kernels. CONCLUSIONS The basic conclusions of this paper can be summarized as follow: 1. It has been found some additional arguments that hot thermal plasma (~20 MK) detected by the BCS/Fe XXV is situated mainly inside bright looptop kernels seen in SXT images. The hot plasma is mixed there with the cooler plasma and this is the reason why broadband instruments, e.g. the SXT, derives distinctly lower temperatures. 2. The obtained empirical formula (Eq. 2) is very useful for a checking how the bright loop-top kernel represents the site of the hot plasma. The evolution in the diagram log q - T(SXT) (Fig. 4) beyond the marked area suggests a wrong representation. Sometimes just a minor modification in the kernel definition improves the location in the diagram. On the other hand, some complex cases, like a simultaneous energy release in several places or a situation of the flaring structure along the line of sight, can happen. Nevertheless, for none of investigated flares the first item of this Section can be called in question.

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M.Tomczak

3. Eq. 2 allows to estimate emission measure of hot thermal plasma for flares stronger than about M4 for which the BCS saturates. 4. The inverse proportion qmoz - H obtained for the bright loop-top kernels can suggest that the maximal relative content of hot thermal plasma depends on the amount of energy released in a flare. This is in an agreement with Jakimiec et al. (1998) who have showed that energy of flares is released in bright looptop kernels. ACKNOWLEDGMENTS The Yohkoh satellite is a project of the Institute

of Space and Astronautical Science of Japan. I thank Prof. J. Jakimiec for many useful comments and discussions. I thank also Dr. A. Sterling for the BCS software consultation. This work was supported by the KBN grant No. 2 P03D 016 14. REFERENCES

Acton, L. W., U. Feldman, M.E. Bruner, G.A. Doschek, T. Hirayama, et al., The Morphology of 20 x lo6 K Plasma in Large Non-Impulsive Solar Flares, Publ. A&on. Sot. Japan, 44, Li’l-L75, 1992. Doschek, G.A., K.T. Strong, and S. Tsuneta, The Bright Knots at the Tops of Soft X-ray Flare Loops: Quantitative Results from Yohkoh, Astrophysical J., 440, 379-385, 1995. Doschek, G.A., and U. Feldman, The Temperature of the Bright Knots at the Tops of Solar Flare Loops, Astrophysical J., 459, 773-778, 1996. Doschek, G.A., The Electron Temperature and Fine Structure of Soft X-ray Solar Flares, Astrophysicul J., 527, 426-434, 1999. Hara, H., S. Tsuneta, J.R. Lemen, L.W. Acton, and J.M. McTiernan, High-Temperature Plasma in Active Regions Observed with the Soft X-ray Telescope Aboard Yohkoh, Publ. A&on. Sot. Japan, 44, L135L140, 1992. Jakimiec, J., M. Tomczak, R. Falewicz, K.J.H. Phillips, and A. Fludra, The Bright Loop-Top Kernels in Yohkoh X-ray Flares, Astron. Astrophys., 334, 1112-1122, 1998. Martens, P.C., L.W. Acton, and J.R. Lemen, The Point Spread Function of the Soft X-ray Telescope Aboard Yohkoh, Solar Phys., 157, 141-168, 1995. Siarkowski, M., J. Sylwester, J. Jakimiec, and M. Tomczak, Improvement of SXT Image Alignment in Order to Obtain High-Resolution Temperature Maps, Acta A&on., 46, 15-28, 1996. Sterling, C.S., H.S Hudson, J.R. Lemen, and D.A. Zarro, Temporal Variations of Solar Flare Spectral Properties: Hard X-ray Fluxes and Fe XXV, Ca XIX, and Wideband Soft X-ray Fluxes, Temperatures, and Emission Measures, Astrophysicd J. Suppl., 110, 115-156, 1997. Thomas, R.J., R. Starr, and C.J. Crannell, Expressions to Determine Temperatures and Emission Measures for Solar X-ray Events from GOES Measurements, Solar Phys., 95, 323-329, 1985. Tomczak, M., Investigation of Energy Transport in Solar Flares, Ph. D. thesis, University of Wrocfaw, Poland, 1994.