Solar Physics with Radio Observations, Proceedings of Nobeyama Symposium 1998, NRO Report 479

Thermal and Nonthermal Components in an X-Class Long Duration
FlareHiroshi Nakajima, Jun Sato
Nobeyama Radio Observatory, NAOJ, Minamimaki, Minamisaku, Nagano 384-1305, Japan
E-mail(FI): nakajima@nro.nao.ac.jp
and Ken’ichi Fujiki
Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa 442-8507, JapanAbstract
We have analyzed the 1992 November 2 GOES X9-class long-duration ﬂare that was accompanied byintense microwave and hard X-ray emission, using data from the Nobeyama Radioheliograph at 17 GHz
and the Hard X-Ray and Soft X-Ray Telescopes onboard the Yohkoh. We show that: (1) There are twoelectron components: one is a quite soft component as inferred from hard X-ray emission, and the otheris a quite hard nonthermal component, as inferred from microwave emission. (2) Hard X-ray observations
show a bar structure which is located well above the soft X-ray loop. (3) The soft electron component iscreated in the bar region. It is also suggested that the hard nonthermal electron component is also createdin or near the compact bar region and is trapped in the larger region surrounding the soft X-ray source.
(4) The bar region has a high temperature of 40 MK, a high density of 5 × 1010 cm−3 , and a high thermalenergy content of 1030 erg.
Key words: Sun: ﬂare — Sun: X-ray — Sun: radio emission — particle acceleration

1.

Introduction

Observations with the Yohkoh Soft X-ray Telescope (SXT) suggest that energy in some long-duration ﬂares isreleased by magnetic reconnection in a current sheet above the soft X-ray loop (Tsuneta, et al. 1992; Tsuneta 1996).
Masuda (1993) and Masuda et al. (1994) analyzed impulsive hard X-ray ﬂares and found that the hard X-ray source
is located above the soft X-ray loop in most of analyzed events. They considered this loop-top hard X-ray source tobe evidence that energy release and particle acceleration occur above the soft X-ray loop. Since most of Masuda’s
events belong to LDE’s from a viewpoint of GOES ﬂare classiﬁcation, the loop-top hard X-ray source is a property
of impulsive bursts which occur at the early stage of LDE’s.
There are numerous mysteries to be resolved, i.e., whether the loop-top source is nonthermal or thermal, whether
the loop-top source is located coincidently with the fast shock where the reconnection outﬂow collides with theunderlying soft X-ray loop, what the relationship between the loop-top hard X-ray source and the loop-footpoint
hard X-ray sources is. Clearly, further studies are needed.
In this paper, we report the results of an analysis of the 1992 November 2 GOES X9 ﬂare (S23, W100) which
was carried out using data from the Nobeyama Radioheliograph and the Yohkoh Hard X-ray Telescope (HXT). Thisﬂare has a duration of more than 10 hours in GOES soft X-ray emission, and has a duration of more than 30 minutes
in microwave and hard X-ray emission. A preliminary report on the microwave aspects of this ﬂare was made by
Nakajima & Metcalf (1995). Since then, the imaging software has been considerably improved by Sato (1997), andas a result, a more detailed analysis of this ﬂare has become possible. This ﬂare has been also extensively analyzedby Feldman et al. (1995) for soft X-ray aspects, and by Altyntsev et al. (1998) for microwave aspects.
2.

Observations

Figure 1 shows time proﬁles of total ﬂuxes of microwave (17 GHz) and hard X-ray (M1: 23-33 keV, M2: 33-53
keV, H: 53-93 keV) emission from the 1992 Nov. 2 ﬂare. Although the hard X-ray observations are only available
for the later half of the impulsive phase, the microwave time proﬁle at 17 GHz is similar to those at hard X- rayseven in ﬁne structures as seen in Figure 1, especially in the time proﬁles of the higher energy bands. This suggests

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Fig. 1.. Time profiles of microwave (17 GHz) and hard X-ray intensities of the 1992 November 2 flare. The ordinate is an arbitrary unit.
The microwave time profile is similar to the hard X-ray time profiles even in fine structures (at 03:01 UT and 03:06 UT), especiallyat higher energies.

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Table 1. Power-law indices and electron temperatures estimated from microwave and hard X-ray total intensity spectraγ ∗1Microwaves
Hard X-rays:M1/L
M2/M1
H/M2
*1: Photon spectral indices.

Te (MK)

1.5
7.8
7.1
5.8

δ ∗2
33561130*2: Electron spectral indices.

that both high-energy electrons which contribute to microwave and hard X-ray emission are accelerated or heatedby closely related acceleration mechanisms and/or in close proximity to each other.
Table 1 shows eﬀective power-law electron spectrum indices or eﬀective electron temperature which are estimatedfrom microwave and hard X-ray total ﬂuxes. The hard X-ray emission has a very soft spectrum although the indicesbecome somewhat harder at higher energies. The eﬀective electron temperature, which is derived from the M1/Lratio, is 35 MK. The power-law index of nonthermal electrons is estimated to be very hard, about 3, from theoptically-thin part of the micro-millimeter wave spectrum. Note that in the case of such a hard micro-millimeterwave spectrum, the micro-millimeter wave emission emanates from nonthermal electrons at energies of more thanseveral hundred keV. These observations show that there are mixed populations of electrons in the ﬂaring region,i.e., a very soft component derived from the hard X-ray spectrum and a very hard spectrum derived from themicro-millimeter wave spectrum.
Figure 2 shows overlays of microwave (17 GHz), hard X-ray, soft X-ray ﬂare images around 03:00 UT. We canclearly see that a hard X-ray source with a bar structure, 1.1 × 104 km in length and 4.3 × 103 km in width, is locatedabout 1.5 × 104 km above the brightest apex of the soft X-ray loop. This hard X-ray bar structure is more evident
in the M2 band image. We call this hard X-ray bar source bar-1. At lower energy bands (L- and M1-bands), alarger, diﬀuse source surrounds the compact M2-band hard X-ray source. On the other hand, the microwave source
at 17 GHz, which represents the nonthermal component, is large and covers the soft X-ray and hard X-ray sources.
The centroids of the microwave and hard X-ray sources are not located exactly above the soft X-ray source but areslightly displaced from exactly above the soft X-ray source toward the south. There is also a weak indication ofanother weaker bar source (bar-2) which is located below and parallel to bar-1, at almost the same height as the topof the soft X-ray loop.
The microwave and hard X-ray sources show temporal change during ten minutes after 03:00 UT as shown for
two typical times in Figure 3. The bar-1 source at M2-band consists of symmetrical double-sources (we designate
the southern and northern one as ds-1 and ds-2, respectively) at 03:00 UT. As time elapses, ds-1 decreases more
rapidly than ds-2 and simultaneously, bar-2 becomes visible. As a result, the weighted centroid of the double source
shifts to the north. This change of the hard X-ray source structure at M2-band can be traced in the lower-band(M1-band) image. Correspondingly, the microwave source at 17 GHz shifts toward the same direction as the hard
X-ray sources move. These observations suggest that nonthermal electrons which emit microwave emission at 17
GHz are accelerated in or near the same region as the hard X-ray bar regions and propagate along magnetic ﬁeldlines, resulting in formation of the larger region which covers the hard X-ray bar source.
Table 2 shows physical parameters at 03:00 UT for the compact bar (bar-1) region and the halo region which are
estimated from hard X-ray data. The bar-1 region has eﬀective temperature of about 40 MK. On the other hand,electron temperature of about 30 MK can be derived for the hard X-ray bar region from the soft X-ray images using
the ﬁlter ratio method. This suggests that the bar-1 region is actually ﬁlled with electrons of about 40 MK. The
halo component has slightly lower temperature of about 30 MK and is distributed, surrounding the bar-1 region,
which suggests that high-temperature thermal plasma is created in the bar-1 region and diﬀuses away from it.
3.

Summary and Discussion

The present observations are summarized as follows. (1) There are two kinds of electron components: One is aquite soft component derived from hard X-ray emission. The other is a hard nonthermal component derived frommicrowave emission. (2) Hard X-ray observations show a bar structure which is located above the soft X-ray loop.

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Fig. 2.. Overlays of hard X-ray (dotted √contours), soft X-ray (greyscale), and microwave (thick contours) flare images around 03:00
UT. All contour levels are in steps of 2 starting from 12.5% of the peak intensity in each panel. The field of view is 2.5 arcmin.
The solar north is up. The location of the west solar limb is shown in each panel.

Table 2. Physical parameters of hard X-ray compact and halo components in the case of thermal fitting.

Compact component
M1/L
M2/M1
Halo component
M1/LTe(MK)EM(cm−3)

Ne(cm−3 )Etotal(erg)3754

1 × 1049

5 × 1010

1 × 1030

335 × 1049

4 × 1010

5 × 1030

Thermal and Nonthermal Components in an X-Class Long Duration Flare247

Fig. 3.. Coaligned microwave (contours), hard X-ray (contours), and soft X-ray images (greyscale) at two typical times (03:00 UT and
03:07 UT). All contour levels are 10, 25, 50, and 75 % of peak intensities. The field of view is 2.5 arcmin.

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The other, much weaker bar structure (bar-2) is also observed at roughly the same height as the soft X-ray loop top.
(3) The soft electron component with the temperature of 40 MK is created in the bar region. It is also suggested
that the hard nonthermal electron component is also created in or near the compact bar region and is trapped in
the larger region surrounding the soft X-ray and hard X-ray sources. (4) The bar region has a high temperature of
40 MK, a high density of 5 × 1010 cm−3 , and a high thermal energy content of 1030 erg.
Masuda et al. estimated for the loop-top hard X-ray source of the 1992 January 13 GOES M2-class ﬂare that theeﬀective electron temperature is 2 × 108 K, the density is 2 × 1010 cm−3 , and the thermal energy content is 3 × 1028
erg. As compared with these properties, the 1992 November 2 event shows a lower temperature but a much higher
thermal energy content.
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