Solar Physics
DOI: 10.1007/•••••-•••-•••-••••-•A New Solar Imaging System for Observing High
Speed Eruptions: Solar Dynamics Doppler Imager(SDDI)
Kiyoshi Ichimoto1,2 · Takako T. Ishii1 ·Kenichi Otsuji1 · Goichi Kimura1 ·Yoshikazu Nakatani1 · Naoki Kaneda1 ·Shin’Ichi Nagata1 · Satoru Ueno1 ·Kumi Hirose1 · Denis Cabezas1 ·Satoshi Morita2
c Springer ••••⃝Abstract A new solar imaging system was installed at Hida observatory to observe the dynamics of flares and filament eruptions. The system (Solar Dynamics
Doppler Imager; SDDI) takes full disk solar images with a field of view of 2520”x2520” at multiple wavelengths around Hα line at 6562A. Regular operation wasstarted in May 2016, in which images at 73 wavelength positions spanning from
Hα−9A to Hα+9A are obtained every 15 seconds. The large dynamic range ofmeasurement of the line-of-sight velocity (±400 km/sec) allows us to determine
the real motions of erupting filaments in 3D space. It is expected that SDDI
provides unprecedented data set to study the relation between kinematics offilament eruptions and Coronal Mass Ejections (CME), and contribute to thereal time prediction of the occurrence of CMEs that cause a significant impact
on the space environment of the earth.
Keywords: Sun, imaging system, flare, filament eruption, CME

1. Introduction
It is known that many of coronal mass ejections (CME), the primary driver ofthe solar storms that impact the space environment of the earth, are associatedwith prominence/filament eruptions, and the erupted prominence is sometimes
observed as a bright core of the CME (e.g. Gopalswamy, 2015, Howard, 2015).
Munro (1979) reported that 70% of CME are associated with a filament disappearance, while Gopalswamy et al. (2003) found that 72% of prominence

1Kwasan and Hida Observatories, Kyoto University

2

National Astronomical Observatory of Japan

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Ichimoto et.al.

eruptions are clearly accompanied by CMEs. Morimoto and Kurokawa (2003)
studied 15 events of filament disappearance and found that 8 events that wereidentified as eruptive one were all associated with CME and other 7 events thatwere identified as ’quasi-eruptive’ or non-eruptive one were not associated with
CME. Close relationship between kinematics of prominence eruption and CME
is also known (Gopalswamy. 2015), i.e., there is a good correlation between theascending speeds and directions of prominence eruption and CME. These facts
strongly suggest that observations of erupting motions of prominence/filaments
provide a valuable mean for predicting the occurrence of CME that impacts theearth.
Ascending velocities of prominence eruptions are so far mainly measured forevents observed on the solar limb, for which time variation of apparent heights oferupting prominence projected on the sky were obtained from a series of images.
Shimojo (2014) reported the ascending velocities of prominence eruptions observed by Nobeyama Radio Heliograph on the solar limb in July 1992 ∼ March
2013, and found that the yearly average of the radial velocity is about 100 km/s
except the solar minimum and some of them reached 400km/sec. Similar result
was reported by McCauley etal (2015) using 904 events observed by AtmosphericImaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO). In
these observations, we lack the information of the line-of-sight component ofvelocity of erupting prominences nor their real speed and direction in 3D space.
Line-of-sight velocities of erupting filaments, i.e., the motions towards theearth, have been rarely observed to a large traveling distance. A few exceptions are found in a spectroscopic observation from ground and a stereoscopic
observation from space, both of which observed a filament eruption that took
place near the central meridian of the sun seen from the earth. Penn (2000)
obtained the line-of-sight velocity of 200∼300km/s for an ascending filament byspectroscopic observation in HeI 10830A and determined its trajectory up tothe height of 0.5 solar radii. Filippov (2013), using Solar Terrestrial Relations
Observatory (STEREO) and the SDO, determined the ascending velocity of afilament eruption up to the height of 300Mm (0.43 solar radii) and found that,during the initial stage of the eruption, the filament moved along the magnetic
neutral surface. It is noted that such observations are rare cases that highly rely
on chance.
The lack of information of the line-of-sight velocity of filament eruptions
is largely due to limited dynamic range of Doppler measurement in synoptic
Hα observations of current ground-based facilities. Many observatories conductsynoptic solar observations in Hα with a passband width of 0.25∼0.5A and afixed wavelength at the line center. At some solar observing stations, oﬀ-band Hαimages are also taken in regular basis, but their ranges in wavelength oﬀset are
insuﬃcient to capture the Doppler shift of high speed eruptions; for examples,
they are ±0.8A for Flare Monitoring Telescope (FMT, Kurokawa et al 1995) that
was/are in operational at Hida observatory (1992-2009) and Ica University inPeru (2010 - ), ±1.2A for Solar Magnetic Activity Research Telescope (SMART;
Ueno et al. 2004, the former system of the present one reported in this paper)at Hida observatory, and ±0.5A at Fuxian Lake Observatory (ONSET; Fang etal 2013). Thus filament eruptions with a line-of-sight velocity grater than about

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60km/s erude observers and we are almost blind to such high speed eruptions
moving toward the earth with the current observing facilities.
To open a new eye for watching the high speed motions of filament eruptions
and to investigate the way for predicting the occurrence of geo-eﬀective CMEs,
we developed a new Hα imaging system that covers a wide range of wavelength around Hα under the Project for Solar-Telestrial Environment Prediction(PSTEP: http://www.pstep.jp/?lang=en). The new system, Solar Dynamics
Doppler Imager (SDDI), started the regular operation at Hida observatory inMay 2016. In this paper, we describe the basic features of SDDI and demonstrate
its performance with some initial data.

2. Observing system

Figure 1. Optical layout of SDDI

The imaging system was installed on an optical bench of one of four telescopesof the SMART at Hida Observatory of Kyoto University. Optical layout is shownin Figure 1. The system consists of a refracting telescope with an aperture
diameter of 20 cm, a tunable Lyot filter, a high speed CMOS camera with 2k×2k
pixels and feeding optics. At the entrance of the telescope, a bandpass filter thattransmits lights in a wavelength range of 6300∼7000A is placed to reduce theheat load into the system. The design of the imaging optics is optimized for theHα wavelength and it has nearly diﬀraction limited image quality in Hα over thefield of view covering the full solar disk. After the primary focus of the objective
lens (f =4000 mm), a lens unit collimates the beam to form a pupil image with adiameter of 33mm at the center of the tunable Lyot filter. Finally the reimaging
lens with a focal length of 170 mm forms a solar image on the camera with a platescale of 190 arcsec/mm. The spatial sampling is 1.23 arcsec/pix and the field ofview (FOV) covered by the camera is 2520 × 2520 arcsec, while there is somevignetting of lights by the tunable filter at the corners of the FOV. The camera
is mounted on a linear stage and translated remotely along the optical axis foradjusting the focus. Since the diﬀraction limited resolution of a 20cm telescopeat 6563A is 0.83 arcsec, the spatial sampling of the system is not suﬃciently
high. This is because we prioritize the field coverage and time resolution rather
than the spatial resolution by considering the scientific objective of the SDDI.

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Ichimoto et.al.
Table 1. Basic properties of SDDI
Optical system
Telescope diameterEﬀective focal length
Beam at tunable filter

200 mm1086 mm
Collimated with a ϕ33mm pupil

Tunable filter
Passband width (FWHM)
Aperture
Tuning range
Response time
Peak transmission

0.25A
40 × 40 mmHα±9A< 0.1 sec
4.8 %Camera: Hamamatsu Photonics Co. ORCA Flash 4.0Format
2048 × 2048 pixels
Pixel size
6.5 µmData depth
16 bits
Frame rate
100 frames/sec (max.)
SystemField of view
Spatial sampling
Exposure time
Time cadence
Dynamic range of LOS velocity

2520” × 2520”
1.23 arcsec/pixel
2 msec (typical)
15 sec for 73 wavelengths (typical)−400 ∼ +400 km/s

Upgrading the camera with a larger format one is, however, one of the desired
improvement of the system in future. The basic parameters of the SDDI system
are summarized in Table 1.
The tunable filter is a conventional Lyot type birefringent filter consisting of7 stage calcite blocks with an aperture of 40 mm×40 mm. The bandpass width(FWHM) and the free spectral range at Hα are 0.25A and 32A, respectively. In
each block, a nematic type Liquid Crystal Variable Retarders (LCVR, productsof Meadowlark Optics) is equipped, and by applying specific voltages on eachLCVR, we can set the wavelength of the peak transmission at any wavelength
in the range of Hα±9A with a response time shorter than 0.1 sec. The peaktransmission is 4.8% and the wavefront error is 61nm (rms) after removal of tilt
and power components. Figure 2 shows the transmission profiles of the tunablefilter and blocking filter together with the solar spectrum around Hα line. The
scheme for controlling the LCVR voltages to tune the wavelength is based on theheritage from the formerly developed universal tunable filter at Hida observatory(Hagino etal 2013).
Figure 3 shows the block diagram of the controlling system. The tunable filter
and the camera are controlled through USB and Camera-link interfaces, respectively, by on-board PC (Windows10) that is mounted on the optical bench of thetelescope. The observing software is written by Visual-C and Interactive Data
Language (IDL) on the on-board PC, and is manipulated from the operation PC

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Figure 2. Transmission profiles of tunable filter (thick line) and blocking filter (dashed line)
together with the solar spectrum (solid line).

located in the observing room through the ethernet. The image data obtainedby the camera are directory stored in SSD or hard disks on the operation PC.
More details of characteristics of the instrument will be described in a separate
paper.

Figure 3. Block diagram of the observing system.

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Ichimoto et.al.

3. Operation and data archive
The system was installed on SMART at the end of April 2016 after extensive
characterization of the tunable filter with a spectrometer of the Domeless Solar
Telescope at Hida observatory. Regular operation of the system was then started
from the beginning of May. The standard sequence of data acquisition in dailyobservations is to take full field images at 73 wavelength positions spanning from−9A to +9A with a constant wavelength step of 0.25A. With a typical exposure
time of 2msec, a set of images can be obtained in 15 sec (In initial phase of
the operation until the end of July 2016, number of wavelength positions were61 or 41 in a range from −8A to +8A and time cadence were 30 or 20 sec,respectively.) . Quick look images are processed by the operation PC in realtime and are opened to the internet at the web site http://www.hida.kyotou.ac.jp/SMART/live/index2.html (see Figure 4).
10 hour observation in a day thus produces data of about 1.4TB. To reduce
the total amount of data to be stored, we discard images in continuum, i.e.,
images with the wavelength oﬀset larger than ±3A, unless high speed event isdetected in sum images in −5A ∼ −4A and +4 ∼+5A from Hα center. Quick
look and selected data are accessible on the web site at http://www.hida.kyotou.ac.jp/SMART/ .

Figure 4. Live images of SDDI for real time distribution of observing status.

In raw images, we recognized fringe patterns that vary with the wavelength.
The contrast and spatial scale of the pattern are a few percent in intensity andseveral arc minutes, respectively. This is not a serious problem for inspection ofimages to search interesting active events and morphological studies of eruptivephenomena. However correction of such artificial patterns is demanded for quantitative studies using spectral profiles. To remove the fringe pattern, we apply
gain correction using ’flat’ images constructed for each 73 wavelength by using
the Kuhn-Lin-Loranz method (Kuhn et al , 1991). Figure 5 shows an example
of flat image taken at Hα−1A and a ’Dopplergram’ made from the subtractionof flat images at Hα−0.5A and Hα+0,5A divided by the the sum of them. Itis noticed that the ’Dopplergram’ shows apparent blue shift towards the outer

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part of the field of view. This is because the gradual red shift of the transmission
peak of the Lyot filter due to the increasing incidence angle of the collimatedlight to the filter towards the outer part of the field of view. The amount of thewavelength shift is approximately 0.11, 0.19 and 0.38A at the solar limb (920arcsec from the center of FOV), edge (1260 arcsec) and corner (1782 arcsec) of
the FOV, respectively.

Figure 5. An example of flat image taken at Hα−1A (right) and a ’Dopplergram’ made fromthe flat images at Hα−0.5A and Hα+0,5A. The flat image is normalized by its median value
and displayed with a range from 0.2 to 1.1.

Figure 6. An example of SDDI image taken at Hα center (left) and diﬀerence image ofHα±0.5A (right). Top is the geographical north and left is the east

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Ichimoto et.al.

4. Initial data
Figure 6 shows an example of raw image in Hα line center and accompanied
Dopplergram made from the diﬀerence of two images in Hα±0.5A. In the display, the flat field correction was not applied for Hα intensity image, while forthe Dopplaergram, flat field collection was applied to both ±0.5A images since
without the flat correction, artificial patterns are obvious in the Dopplergram.
Solar rotation and local flow patterns are clearly recognized, while global variation of Doppler signal need to be taken with a care because of gradual shift ofwavelength of the transmission peak of the Lyot filter as noted above.

Figure 7. C class flare and filament eruption on 7 July 2016 07:55:49UT occurred in NOAA
12561. Top is the geographical north and left is the east

On 7th July, we observed a compact C class flare and associated filament
eruption in NOAA 12561. Figure 7 shows images in Hα center and Hα±1.0A,and diﬀerence images of Hα±2A, Hα±4A, and Hα±6A, while the spectral profile
of ascending part of the erupting filament marked by circles is shown in Figure 8
together with a spectrum in normal region. It is noted that the profiles shown inFigure 8 is convolutions of the real solar spectra and the filter transmission curve.
Though this observation was conducted with 41 wavelength positions spanningfrom Hα-8A to Hα+8A, it clearly demonstrates the necessity of wide range ofwavelength coverage to capture dynamic eruptions, i.e., the absorption featureof the ascending blob is clearly recognized only in images with a wavelength
oﬀset larger the -2A.

5. Summary and Prospects
A new solar synoptic observation system, SDDI, was successfully installed on
SMART at Hida observatory and is conducting regular operation since the beginning of May 2016. SDDI observes the motion of filament/prominence eruptions

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Figure 8. Spectral profiles of the eruption (solid line) and disk center (dotted line) after flat
field correction.

with a line of sight velocity of up to ∼400 km/s, and enable us to capture thereal motions of large scale high speed eruptions in 3D space for the first timewith a wide field of view (2520×2520 arcsec) and a high time cadence (15 sec).
Increasing the spatial sampling with a larger format camera can be a subject for
future improvement of the system to utilize the full optical performance.
It is expected that the data obtained by SDDI will provide a new methodology of the space weather forecasting by relating the kinematic properties offilament/prominence eruptions and the occurrence of CME that reaches theearth. Identifying the activation of filament prior to its sudden destabilization
will also contribute to the prediction of its occurrence and understanding of theonset mechanism of filament eruption. Moreover the three dimensional velocity
field of erupting filaments and its time evolution informs us the spatial distribution of accelerating force acting on prominence plasma, and provide crucial
information for understanding the acceleration mechanism of the filament. The
data is available on the web site of the Hida observatory.
Acknowledgments The authors are grateful to the staﬀ of Hida observatory for supporting
the instrument development and daily observations. This work was supported by MEXT/JSPS
KAKENHI Grant Number JP15H05814, Project for Solar-Terrestrial Environment Prediction(PSTEP).

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