J. Plasma Fusion Res. SERIES, Vol.7 (2006) 165-168

Multi-Temperature Analysis of a Sigmoidal Active Region
HORI Kuniko, HARRA Louise K.1 , MOORE Ron L.2 and FALCONER David A.2
Solar Observatory, National Astronomical Observatory of Japan, Tokyo 181-8588, Japan
1 Mullard Space Science Laboratory, University College London, Dorking, Surrey, RH5 6NT, UK
2 NASA Marshall Space Flight Center, Huntsville, AL 35812, USA（Received: 14 November 2004 / Accepted: 26 August 2005）

Abstract
A sigmoidal structure on the solar surface, which is generally remarkable in soft X-ray images, is known
as a manifestation of highly sheared magnetic loops. It often marks the source region of a coronal massejection (CME) that can disturb Earth’s geomagnetosphere and then atmosphere. Previous observational
studies on the solar sigmoids have focused on their topology (e.g., accumulation of magnetic helicity orshear) and connection with CMEs (e.g., missing mass deduced from a dimmed area in 2D solar images) on
the basis of soft X-ray images, i.e., emission from coronal hot plasmas (> 2 MK). We investigate a sigmoidal
active region using the Coronal Diagnostic Spectrometer (CDS) instrument on the Solar and HeliosphericObservatory (SOHO) spacecraft stationed at the L1 Lagrangian point. CDS can provide both images andspectra of extreme ultraviolet (EUV) emission lines that cover formation temperatures in the range 10 4 – 106
K. By combining CDS images with images in other wavelengths, we diagnose multi-temperature structure ofa sigmoidal active region.
Keywords:
Sun, corona, transient region, magnetic field, EUV line, sigmoid
1．Introduction
Solar eruptive phenomena such as flares, filament
eruptions, and coronal mass ejections (CMEs) aredeeply involved with sheared (or twisted) magnetic
structures that store extra magnetic energy in them. Especially, sigmoidal active regions that appear in a shapeof a character “S” or “inverted-S” are known to be apossible CME source [1, 2]. Hence, sigmoids belong to
an important category for the space weather forecast.
The soft X-ray telescope (SXT) on the Japanese Yohkoh
spacecraft observed many sigmoids that partially orbodily erupted and left a cusp or an arcade-like structure. The sigmoid disappearance is often followed by anappearance of localized dark areas (so-called dimming)around the eruption site (e.g., [3]). While the cusp andarcade structures suggest the occurrence of magnetic reconnection, the dimming suggests a decrease of coronal mass due to the eruption or a temperature change in
the area (e.g., [4]). Information provided by the Yohkoh
SXT is, however, limited to hot plasmas (> 2 MK) inthe corona. The physical process involved in a sigmoidformation is thus still not well addressed. (For possible
scenarios, see, e.g., [5 - 7]).
Here we discuss a magnetic configuration of a sigmoidal active region using extreme ultraviolet (EUV)

spectroscopic observations with the Normal Incidence
Spectrometer (NIS) of the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) spacecraft [8]. The CDS can address
plasma characteristics (density, temperature, and velocity) within a single volume from the chromospherethrough to the corona. By comparing sigmoids in CDS
and SXT images, Gibson et al. [9] reported that CDS
sigmoids have the same helical orientations as their Xray counterparts. Multi-wavelength observations canprove 3D structure of sigmoids as they are sensitive totemperatures. In addition to CDS and SXT images, wealso use magnetograms from SOHO/MDI (MichelsonDoppler Imager) and NASA/MSFC (Marshall Space
Flight Center). Cool structures (filaments) are verified
with Hα images from Hiraiso Solar observatory.
2．Observations
The target sigmoidal active region was NOAA
#9254, which we selected for an international campaign
observation involving the instruments on the SOHO
spacecraft, coordinated among ground-based observatories in Japan. The observation period was December
6–9, 2000. Figure 1 left shows a soft X-ray snapshot ofC 2006 by The Japan Society of Plasma

Science and Nuclear Fusion Research

Corresponding author’s e-mail: hori@solar.mtk.nao.ac.jp165

Multi-Temperature Analysis of a Sigmoidal Active Region

Dark

(a)

(b)

(c)

(d)

BrightFig. 1 Snapshtos of AR9254 from SXT (left) and CDS (right) at 21h (UT) on Dec. 6, 2000. The CDS images show intensity(a, c) and velocity (b, d; up to ± 30 km s–1 ) distributions in Fe XVI line (a, b) and in O V line (c, d). The contours on
the SXT image and in panel (c) outline the sigmoid in panel (a). The color bar attached in the bottom left shows theintensity scales. Solar north is up and west to the right.

Fig. 2 Synthetic spectrum for a typical active region with a constant density of 1010 cm–3 . Created by CHIANTI database[10], [11].

AR9254 taken at 21:03 UT on Dec 6 from the SXT. The
region consists of several low-lying bright loops (shownin black, see the color bar attached in the bottom) andfaint tall loops lying outside or above the bright loops.
The low-lying loops as a whole form an S shape. Eachloop has footpoints in positive (right-hand or west side)and negative (left-hand or east) magnetic polarity fields.
Thus the S-arrangement almost traces the magnetic neutral line that separates the opposite magnetic polarities. The boxed area shows the CDS field-of-view,122”×121” (∼89000 km), which was rastered with theCDS long slit. The CDS observed AR9254 from 0h to4h (UT) on each day with a time resolution of 30 minutes in 10 spectral lines that are listed in Table 1. Theselines were selected to cover a wide range of temperatures from the transient region (> 104 K) to the corona(> 1 MK). Figure 2 shows spectrum for two spectral
ranges that were calculated by CHIANTI database fora typical active region with a constant density of 10 10
cm−3 [10, 11].
166

Multi-Temperature Analysis of a Sigmoidal Active Region

Table 1 Observed Spectral Lines.
( ∗ Corresponding to a 2nd-order emission line.)

IonWL(Å) Transition
23．Discussion
By comparing the CDS and SXT images, it is clearthat our sigmoid was not an S-shaped single loopthat was bodily formed and emerged from beneath thephotosphere as demonstrated by numerical simulations(e.g., [6]). Instead, it consisted of sheared loops lying
below the tall loops that have more or less potentialfield like shapes. The SXT observed a cusp structureon Dec. 8, which presumably emerged from one of
the tall loops. Figure 3 illustrates a possible magneticconfiguration of AR9254. The oblique-lined and grayareas correspond to negative and positive magnetic polarities, respectively. The three characteristic features
observed with CDS correspond to vertical slices of thesigmoidal active region that consists of a pair of piled
arcade loops: The inner (low-lying) arcade loops (indicated by the dashed parallelogram) apparently form thehot (> 2 MK) sigmoid. In the upper transition region,
the footpoints of both inner and outer (tall) arcade loops
might appear as the double ribbon due to heat conduction (e.g., [12]). In the lower transition region, the heat
conduction is less eﬀective and thus footpoints of only
active loops might appear as the bright patches.
Del Zanna et al. [13] diagnosed an evolution of asigmoidal active region with CDS. They found postflare
upflows (∼30 km s−1 ) in coronal lines (Mg X, Si XII)at the two ends of the sigmoid. They argued that these
upflows might be chromospheric evaporation following
the flare energy release. Because of the limited data set,T (MK)2Fe XVISi XII
Mg X361521
610

3s S1/2 - 3p P1/2
1s2 2s 2 S1/2 - 1s2 2p 2 P1/2
1s2 2s 2 S1/2 - 1s2 2p 2 P3/22.5
2.0
1.3

Mg IXCa X
Ne VI
OVO IIIHe II∗
He I

368558563
630
600
6085842s2 1 S0 - 2s 2p 1 P1
3s 2 S1/2 - 3p 2 P3/2
2s2 2p 2 P3/2 - 2s 2p2 2 D5/2
2s2 1 S0 - 2s 2p 1 P1
2s2 2p2 1 D2 - 2s 2p3 1 D2

1.0
0.6
0.4
0.3
0.1

1s2 1 S0 - 1s 2p 1 P1

0.03

In SXT full-disk images, AR9254 became active on
Dec 6, but no major flare activity was reported. The enhanced activity was also observed in other wavelengths.
The MSFC vector magnetograph showed a development of a magnetic shear along the magnetic neutralline, which almost coincides with the central part of thesigmoid. In this region, Hα brightenings were intermittently observed during the four observation days. On
Dec 7, an Hα filament rooted in a positive preceding
spot apparently extended its length northward along themagnetic neutral line. The filament formed a kinked
structure on Dec. 8, while in SXT images a tall cusp
appeared to stride across the sigmoid.
The CDS high temperature line (Fe XVI; 2.5 MK)
images show both formation and decay processes of thesigmoid. In lower temperature lines, on the other hand,the active region appeared as a double ribbon along thesigmoid (Si XII, Mg X; 1–2 MK) or patches around thecenter of the sigmoid (Ne VI and O V; 0.2–0.4 MK).
Figure 1 right panels show CDS images in Fe XVI line
(a and b) and in O V line (c and d) taken at 21:32 UT
on Dec 6 when the sigmoid was most clearly seen. Thepanels (a) and (c) show intensity distributions (see thecolor bar), and the panels (b) and (d) show velocity distributions. In the velocity maps, black and white colors
correspond to blue- and red-shifts up to ±30 km s−1 ,respectively (low emission region was masked). Clear
line-of-sight flows were detected only in Fe XVI and O
V emission lines. The Fe XVI map (b) shows upwardand downward flows along the left- and right-edge ofthe sigmoid, respectively. In the O V map (d), there are
upward motions outside of the sigmoid. After Dec 7,
the brightness of AR9254 gradually faded in all wavelengths.

+Fig. 3 Schematic magnetic conﬁguration of AR9254.
(For details, see text.)

167

Multi-Temperature Analysis of a Sigmoidal Active Region

we could not specify the start and end times of the minor flare activity in AR9254 on Dec. 6–9. However,
we conjecture that the flows detected in AR9254 wereassociated with some energy release during the activity. As the next step, we will calculate the accumulation of currents along the magnetic neutral line from theMSFC vector magnetograph [14], which is expected tobe largest just before the flare on Dec. 6. Considering
the direction of plasma flows, we will also try to specify
the energy release site within AR9254.

[6]
[7]
[8]
[9]

[10]
[1]
[2]
[3]
[4]
[5]

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