Automated detection of EIT waves and dimmings
Olena Podladchikova and David Berghmans
Royal Observatory of Belgium, Ringlaan 3, B-1180, Brussels, Belgium
January 24, 2005
Abstract. Studies of the onset of earth-directed CMEs rely on solar disk observations where CME structures are difficult to disentangle because of the diversity
and transient character of the phenomena involved. Dimmings and coronal waves
are among the best evidence of the large-scale reorganization of coronal magnetic
fields associated with the onset of CMEs. The physical mechanism behind EIT waves
is still unclear: they are considered as MHD waves or as a consequence of plasma
compression on the extending border of a dimming.
In this paper we address the problem of automatically detecting and analyzing
EIT waves and dimmings in EUV images. This paper presents a ”proof of principle”
that automated detection of EIT wave and dimmings is indeed possible. At the
current stage of work, the method can unambiguously detect dimmings and EIT
waves when applied on a typical test-case event. Moreover, we propose a way to extract these events from the data, and determine such parameters as life time, depth,
area and volume of dimmings for future catalogs. For EIT waves we unambiguously
define, in near solar minimum conditions, the eruption center, the front of EIT wave
and its propagation velocity.
In addition, we show that the presented methods yields new insights about
the geometrical shape of dimmings and the connection with the EIT wave front
properties, and the apparent angular rotation of the EIT wave under study.
1. Introduction
The large scale perturbations of the coronal magnetic field during the
onset of coronal mass ejections (CMEs) manifests itself most spectacularly by dimmings and coronal EIT waves (Tsuneta et al., 1991; Delaboudiniere et al., 1995). EIT waves were discovered by the EUV
Imaging Telescope (EIT) on board SOHO as transient wavelike structures with enhanced coronal emission followed by an expanding dimming region. Previous studies of dimmings and EIT waves have shown
their quasi-isotropic propagation in large angular sectors and quasisymmetry around eruption centers (Thompson et al., 1998; Klassen
et al., 2000; Thompson et al., 2000; Warmuth et al., 2001). However,
the mechanism for EIT waves is still unclear. In the wave-like picture, the EIT wave is considered as an MHD perturbation (Klassen
et al., 2000; Thompson et al., 2000; Warmuth et al., 2001), which is
the coronal analog of the Moreton wave observed in Hα (Moreton
et al., 1960), although the velocities of EIT waves are generally 2-3
times smaller (Smith et al., 1971; Klassen et al., 2000). However, in
c 2005 Kluwer Academic Publishers. Printed in the Netherlands.
°
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Podladchikova and Berghmans
another interpretation, EIT waves are considered as a consequence of
plasma compression on the propagating border of the dimming, i.e.
on the region of opened magnetic field lines (Delannée and Aulanier,
1999; Delannée, 2001).
Resolving this open issue of the two conflicting interpretations is currently hampered by the lack of sufficient observational data. The EIT on
SOHO is the only full-disc coronal imager in operations that routinely
detects EIT waves and dimmings. Yet, with the 12 min cadence of EIT,
the observations of EIT waves and dimmings are seriously undersampled in time. Typically only a few images are obtained of these highly
dynamic events that last up till an hour and might cover the complete
solar disc. The upcoming SECCHI/EUVI imagers on the twin STEREO
spacecraft will not only improve the time resolution significantly, but
in addition give a 3D view. However, since the SECCHI/EUVI data
rate will be much higher than the EIT data rate, it will become a
labor intensive task to scan every image for EIT waves and dimmings.
In this paper we anticipate on this problem with the development of
feature recognition software for the automated detection of EIT waves
and dimmings.
Detecting EIT waves is a particularly hard problem of feature recognition given their large variety in physical appearance. The detection
of EIT waves and dimmings, as a 2 dimensional area on the solar disc
of weak intensity variation, is hampered by the occurrence of other
phenomena that also result in local intensity variations, such as small
flares and prominence eruptions. Moreover, visual observations may not
adequately represent the location of the wave front due to projection
effects along the line of sight: observations of similar events on the limb
show strong enhancements in emission out to 1.5 solar radii (Thompson
et al., 1998).
We propose a new technique for the automatic extraction of dimmings and EIT waves. In addition the technique allows the investigation
of their structure and development and filters out the non-related phenomena. The algorithm is based on the established understanding of the
nearly circular coronal wave propagation (Thompson et al., 2000; Moses
et al., 1997): We suppose that the characteristics of EIT waves and
dimmings depend strongly on the distance from the eruption center,
and thus we work in a polar coordinate system projected on the curved
spherical solar surface centered on the eruption center (EC).
In this paper we present a proof-of-principle by applying the proposed technique on a test-case: the clear, well known event observed
by SOHO/EIT on 12 May 1997. The EIT wave and dimmings has
been studied by a number of authors (Thompson et al., 1998; Moses
et al., 1997; Goplaswamy et al., 2000; Webb et al., 2000; Plunkett
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Automatic detection of EIT waves and dimmings
3
et al., 1998; Chertok and Grechnev, 2003; Chertok and Grechnev,
2003; Zhukov and Auchère, 2004). The eruption was subsequently detected as a halo CME by the Large Angle Spectrometric Coronagraph
(LASCO). We determine the characteristics of the coronal wave and
dimmings structure during 20 hours following the eruption including:
− the dynamics of the developing structure: EC, dimming, EIT Wave,
− the radial velocity of the coronal wave,
− the connection between the enhancement of the EIT wave front and
the angular distribution of dimmings around the eruption center,
− the apparent angular motion of the EIT coronal wave around the
eruption center, and characterize it by angular velocity or angular
moment.
Finally we also demonstrate the application to another, more complex
case (23/10/2003).
2. Preview of the event
Dynamical solar disk events (such as dimmings and EIT waves) have
been studied using two kinds of differences pictures (Chertok and Grechnev, 2003; Chertok and Grechnev, 2003). A running difference (RD)
image is obtained by substracting the previous image from the current one. A base difference or fixed difference (FD) image is created
by substracting a fixed reference image taken before the event from
each subsequent image. Running difference images (RD) show the best
contrast of the wave front but are harder to interpret. Some of the emission enhancement and dimness seen after the front of EIT wave has a
methodological origin: there will be an artificial emission enhancement
if the depth of true dimmings decreases between 2 pictures. On top
of this, the interpretation of both the RD and FD images suffers from
the difficulty posed by the solar rotation. Longitudinal displacement of
features on the difference pictures will have a bright (for dark structure)
or dark (for bright structure) artificial border at the eastward side.
Compensation of rotation only partially recovers from this problem.
FD images (Fig. 1) (left) better show the geometrical structure and
physical properties and dynamics of dimming. On the other hand, RD
images (Fig. 1) (right) are useful to study properties of EIT wave like
the radial velocity of front propagation, nonhomogeneous character of
front localization and front dynamic.
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Podladchikova and Berghmans
Figure 1. Differences images of EIT images on the 12/05/97 in 195 Å with the
solar rotation compensation Left panel shows fixed differences images relative to
04:34 UT, illustrating the development of the dimming. Right panel shows running
differences images illustrating more clearly the EIT wave propagation.
3. Detection of event occurrence
5
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Figure 2. Mean, variance, skewness, kurtosis of running difference images versus
image number N . EIT wave propagates during 16(04:50 UT - 04:34 UT), 17(05:07
UT- 04:50 UT), 18(05:24 UT - 05:07 UT), 19(05:41 UT - 05:24 UT) image of the
day 12/05/1997.
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Automatic detection of EIT waves and dimmings
5
The original images observed by SOHO/EIT in 195 Å are rebinned
from 1024 × 1024 or 512 × 512 pixels to 256 × 256 pixels. This lower resolution is sufficient for detection of large scale events and for applying
of statistical analysis methods. The choice of band pass is explained by
the best temporal cadence of observations.
Applying feature recognition techniques on EIT waves and dimming
is a non-trivial task given their large variety in size and structure. To bypass this problem, the detection of the occurrence is done without using
the spatial information in the images. Instead the detection is based on
the characteristics of the histogram-distribution of rebinned (256x256
pixels) running difference images, compensated for solar rotation.
At each moment in time the first four moments of the distributions
have been computed. The mean varies around zero, since the average
energy released by the Sun does not change widely in time (Fig. 2).
The beginning of the eruption is characterized by jumplike increase of
variance. This is easily explained by the activation of the eruption center and the propagation of the dimmings. The sign of skewness changes
during the event, showing different asymmetry, and the kurtosis during
the EIT wave and dimmings propagation increase rapidly, showing the
increasing peakness of the distribution.
Thus, from Fig. 2 one can easily concluded on the occurrence, the
start of the event and its duration.
4. Dimmings extraction
The main characteristics of dimmings such as structure, borders and
integral intensity are important for space weather applications. Dimming regions are supposed to be the regions where mass was ejected
that leads to the CME. Observing a dimming region in connection
with a halo CME thus indicates a ’front-sided’ event, which might lead
to a geomagnetic storm. In order to determine these parameters of the
dimming we assume that dimmings are simply connected areas the area
of dimmings propagation is much larger than other areas with reduced
intensity.
To extract the dimming from the background of other structures (the
noise in this problem), we start from images of fixed differences (FD).
FD in contrast to RD allows one to investigate not the variation of
the dimming but the dimming itself. Such images have been formed by
substraction of basic (before event) 15 image at 04:34 UT from the next
ones. Rotation since 04:34 UT was compensated for each subsequent
image. Furthermore we collect two groups of pixels: a maximal and a
minimal pixelmap.
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Podladchikova and Berghmans
The maximal pixelmap is constructed by selecting all pixels in the
difference image below a weak (i.e. relatively high) threshold. All the
pixels corresponding to the dimming area are contained in the maximal
pixelmap, but typically much more than the dimming alone is selected.
To determine a threshold that separates the ussual variability in FD images from the unussual dimming we proceed as follows. The histogram
of FD images shows a distribution peaking close to zero and with a
certain width σ. The analysis of FD images before the event indicates
σ ≈ 150 counts. Therefore, we construct our maximal pixelmap with
pixel that have an intensities below than −σ. Fig. 3c shows the maximal
pixelmap. Borders of dimmings clearly appear here. However, also the
noise is present on these regions, which corrupt the dimming regions.
Figure 3. Steps of dimmings extraction from FD images. a) Intensive dimmings with
noise; b) Intensive dimmings after median filtering - basic for dimming building up;
c) Region of negative intensity; d) Region of negative intensity, cut on the negative
level of std.dev.
Figure 4. Left panel shows extracted from FD images final dimmings after building
up of intensive dimming up to the border of noisy weak dimming. Right panel
shows dimming structure maps. Arrows show the direction of vectorial gradients of
intensity. Interiors lines correspond to the larger depth of the dimmings. Most deep
region of dimmings directly join to the eruption center.
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Automatic detection of EIT waves and dimmings
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At the other hand, the minimal pixelmap is constructed by selecting
the 1% darkest pixels from FD image. Fig. 3a show the corresponding
regions in white. All the pixels in the minimal pixelmap (ignoring some
noise for the moment) belong, by assumption, to the dimming. However
a certain fraction of the dimming is obviously missed in the minimal
pixelmap. From Fig. 3a, one can see that besides the deep cut of the
dimmings, there are also white points without connection to dimmings,
which we interpret as noise. This noise can be removed by using the
assumption that the dimming is essentially larger than other areas on
the Sun with low intensities. We select on the obtained picture of the
structure of low intensity with an area much smaller than the area of
dimming, using median-filtering. Fig. 3b shows the results after median
filtering (5 × 5), that for sure belong only to intensive dimmings.
The final dimming region is formed by using the pixels in the minimal pixelmap as seeds for a region-growing method, keeping the condition of simple connected region. The region-growing is however restricted to pixels from the maximal pixelmap. This condition filters out
the noise. Fig. 3c show the final dimmings after the operation of regiongrowing. Fig. 4 (left) shows the final dimmings for four fixed difference
images 16(04:50 UT - 04:34 UT), 17(05:07 UT - 04:34 UT), 18(05:24
UT - 04:34 UT), 19(05:41 UT - 04:34 UT) of the day 12/05/1997.
4.1. Structure maps of dimmings
For understanding of dimmings structure we constructed structure maps
of dimmings, based on the determination of the gradient of intensity
function of selected area of dimmings propagation. The intensity function was smoothed first by median filtration. It is enough to use here
the square 2 × 2.
Fig. 4 (right) shows the structure maps of dimmings. Arrows show
the direction of vectorial gradients of intensity. Interiors lines correspond to the larger depth of the dimmings. Most deep region of dimmings directly join to the eruption center on the figure.
¿From the picture one can see that the deepest region of dimmings
are directly related to the region of EC. Weak dimming around them
are in the region between the dimmings and wave front.
Extraction of dimmings open possibilities of detailed analysis of their
development and structure: dynamics of shape changes, area of propagation, volume (volume means integral decreasing of intensity); coordinates of dimming. These characteristics are important for the study
of processes occurring on the solar disk after eruption, and therefore of
dimmings for solar weather.
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Podladchikova and Berghmans
4.2. Definition of geometrical parameters of dimmings and
their evolution in time
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Figure 5. Left panel shows the dependence of dimming area on image number from
04:50 UT 12/05/1997 to 00:43 UT 13/05/1997. Right panel shows the dependence
of dimmings integral intensity on image number during the same period.
For the study of the dynamics of dimming propagation, we estimated
the area and the integral intensity of these regions during the 20 hours
starting from the beginning of events (04:50 UT). Fig. 5 (left) shows
the dependence of the dimming area on number of image during 20 h.
Fig. 5 (right) shows the integral intensity of dimmings during this time.
During the time interval [04:50 UT, 05:24 UT] (from N = 16 to N = 18)
dimmings are propagating intensively. On the time interval [05:24 UT,
05:41 UT] (N = 18 to N = 19) small changes of these characteristics are
observed: area of dimmings increase, but the integral intensity almost
does not change. After 05:41 UT, when the EIT wave has disappeared
already from the disk, also the dimmings have the tendency to decrease.
Such a character of dynamics of dimmings region propagation can be
considered as supporting the hypothesis of the direct connection of the
coronal wave and the propagating border of the dimming (Delannée and
Aulanier, 1999; Delannée, 2001). The extraction of dimmings allows us
determine their area and depth and to solve the problem of conservation
of space structure of dimmings.
4.3. Dimming extraction from a complex corona
The considered technique can be used for dimming extraction under
conditions of complex coronal structures, e.g. with the existence on the
disk of several active regions. Fig. 6a shows a FR image obtained by
the substraction of image at 10:16 UT from image at 15:50 UT, on
23/10/2003, with compensation of sun rotation. A big flare of class X
has been observed at 15:50 UT.
To see in the first approximation the geometrical size of dimming
and flare following regions has been preliminary extracted. Intensity
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Automatic detection of EIT waves and dimmings
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Figure 6. X flare 15:50 UT 23/10/2003. a) FD images of SUN in 195 Å (15:50 UT 10:16 UT); b) plot of flare and dimmings after preliminary filtration; c) All dimmings
appeared on the Sun to moment 15:50 UT. d) Extracted dimming connected with
the flare.
of the first region (area of low intensity) corresponds to the smallest
intensity 8% of pixels. Intensity of second region (area of flare) is larger
than 97% of intensity interval. Median filtration with the square [3] was
applied to the extracted region. Fig. 6b shows plot of these regions, low
intensity regions shown in gray, flare shown in white. Fig. 6c shows
area of low intensity radiation, area of dimmings propagating to the
moment of observation 15:50 UT. These dimmings can be caused not
only by considering flare.
For dimming extraction, one can use in principle the steps described
above. However, we have found better in this case in the first step of the
method to use RD (15:50 UT - 15:33 UT), illustrating the appearance
of low intensity regions during this interval. By this way we filter out
the dimming appearing before the flare. The next steps followed exactly
the proposed method. Fig. 6d shows the dimming created by the flare.
5. Ring analysis for the detection of EIT wave radial
velocity
In this section we define the precise location of the wave front and
its propagation velocity. The method described below of RD image
filtration for the wave front extraction uses the widely accepted assumption that EIT waves propagate nearly circularly from the eruption
site (Moses et al., 1997; Thompson et al., 1998). We suppose that the
physical law of EIT wave dynamics shall depend on the distance to the
eruption center, i.e. the length of the radius-vector ~r on solar surface
(Fig. 7), and the direction of radius-vector (angle ϕ). A polar coordinate
system centered in the EC is therefore appropriate.
The position of the eruption center is defined as the center-of-mass
of the eruption area (as determined previously by dimming extraction)
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Podladchikova and Berghmans
r
j
M
eruption
center
SUN
Figure 7. Polar coordinates ~r and ϕ of a pixel on the solar disk. The center of the
system is at the EC, and ~r is on the sphere surface
at 04h50 UT:
n
X
iEC =
Ik
k=1
n
X
n
X
· ik
; jEC =
Ik
Ik
k=1
n
X
· jk
,
Ik
k=1
k=1
with n the number of pixels in the eruption region, ik , jk , k = [1, n] are
the Cartesian coordinates of the k th pixel and Ik is the intensity of the
k th pixel.
The method is based on a set of rings (m = 1, 2, .., 40) on the solar
surface centred on the EC (see Fig. 7). The mth ring is defined by
rm−1 < |~r| < rm ,
where r is the distance from the EC. rm−1 and rm are the inner and
outer radius of the ring:
rm =
m · rmax
,
N
with rmax = 173 pixels the maximal distance.
We look for intensity variations in the pixels in these rings due
to the passage of the EIT wave front. These intensity variations can
however also be caused by artifacts, by other events not connected with
the CME, by possible observational errors, by fragmental character of
the wave front, by dimmings limited to certain angular sectors. The
total integrated intensity along a ring will be less disturbed by such
localised deviations than individual pixels. The integrated intensity of
the rings in the unperturbed region before the propagation of wave front
is expected to be close to zero (remember we work in RD images).
The global deviation due to an EIT wave passage will however be
strongly enhanced. The sequence of total integrated intensity in the
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Automatic detection of EIT waves and dimmings
concentric rings S(m) (with m = 1, 2, ..., 40) can thus be used to follow
the propagation of the front of the EIT wave and to define its radial
velocity.
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Figure 8. Dependence of the integral intensities of the ring number m. Area of
eruption: 2 ≤ m < m1 . Area of dimmings: m1 < m < m2 . Area of front wave
m2 < m < m 3 .
Fig. 8 shows the dependence of the integral intensities of the ring on
number m (or rm ) for RD 04:50 UT- 04:34 UT (16); 05:07 UT - 04:50
UT (17); 05:24 UT - 05:07 UT (18); 05:41 UT - 05:24 UT (19). Plots for
all 4 pictures have similar forms. From these plots one can define the
regions of EC, dimming and EIT wave front using the zeros of S(m).
The intensity is maximal for the rings between 1 ≤ m < m1 , close to
EC. Region of intensity decreasing (m1 < m < m2 ), corresponds to the
region of dimming. It is worth noting that this ring include also false
dimming of methodological origin, appearing as the result of wave front
expansion. Borders of the wave front expanding before dimmings are
defined by zeros (m2 , m3 ) of the S(m) function. At the point mmax , the
intensity of the wave front achieves its maximum value. Ring of wave
front is limited by radius rm2 , rm3 . From the picture one can see that the
values m1 , m2 , m3 increase monotonously from image to image, which
is explained by the expansion of the active area by the propagating
wave front.
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20
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25
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40
Figure 9. Dependence of integral intensity S(m,k) from m for each sector k = 1...8.
Figure 10. Wave front propagation
5.1. Automatic definition of radial EIT wave velocity
To study the direction of propagation, we divide the range 0 ≤ ϕ < 2π
into 8 equal intervals (sectors). The integrated intensity was defined
S(m,
X k) of all pixels belonging to ring m and sectors k, such that
S(m, k) = S(m). The Fig. 9 shows the dependence of the integrated
k
intensities on the ring of number m (or rm ) for RD 05:07 UT - 04:50
UT in each of the 8 sectors, and it appears that the intensity of the
coronal wave changes depending on the direction. However, the phase
propagates with the same velocity. Fig. 9 confirms the assumption
about quasi-isotropy around EC. However the intensity of pixels on
a ring strongly varies, and therefore cannot be used for the definition
of dynamics of the EIT wave. But the change of intensity of each pixel
in the ring m by the integral intensity of the ring opens the possibility
to investigate the law of change of the wave front depending on the
distance to EC.
The Fig. 10 presents high intensities larger than 98 % of the intensity interval of image, illustrating the expansion of coronal wave front.
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Automatic detection of EIT waves and dimmings
13
Expanding bright area correspond then to the expanding peaks on the
Fig. 8. The numerical dependence of the distance to the wave front as
a function of time can be used to estimate the wave radial velocity.
For automatic definition of the wave front, we use the point mmax ,
i.e. the maximum of the wave front for each image on the interval from
rm2 , rm3 .
The velocity is then defined as the difference between two rmmax
positions in subsequent images, divided by the elapsed time. Table I
shows an overview of the velocities corresponding to different image
pairs. The average velocity 247 km/s is very close to average velocity 245 km/s obtained in (Thompson et al., 1998) using bright spots
position relatively to EC.
Table I. EIT wave radial velocity. The
average velocity is 247 km/s.
VEIT km/s
time
258
225
258
05:07 UT - 04:50 UT
05:24 UT - 05:07 UT
05:41 UT - 05:24 UT
6. Angular structure of the coronal wave
One can see from RD images (Fig. 1, right) that the coronal wave,
although it propagate quasi isotropically around the eruption center,
has a fragmented structure: the brightness is localized in specific sectors
limited by angular coordinates (Fig. 9). We will discuss this sector
localization in relation to the position of the dimmings. The angular coordinate 0 ≤ ϕ < 2π counted from south-west direction from
the center against clockwise direction will be used. The choice of the
direction is caused by the requirement of a good presentation of the
dimmings, placed symmetrically relative to the line of changing polarities of magnetic field before and after the eruption arcade (Thompson
et al., 1998; Zarro et al., 1999).
6.1. Dependence of wave front and dimming regions on ϕ.
In all four RD images (16,17,18,19), the rings have been extracted that
correspond to the area of the wave front and of the dimmings. In Fig. 11
we show the angular dependence of the rings corresponding to the wave
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Podladchikova and Berghmans
front (ring limited by rm2 , rm3 ) and of the rings corresponding to the
dimming (ring limited by rm1 , rm2 ) for RD images (05:24 UT- 05.07
UT). The figure suggests that pixels with decreasing intensity in the
area of dimming are localized around two areas of dimming changes.
But this plot practically does not give information about the regions
of wave front localization.
S
4000
E
W
N
dimming
wave front
2000
Intensity
0
−2000
−4000
−6000
05:24 UT − 05:07 UT
−8000
0
π/2
π
3π/2
ϕ
2π
Figure 11. Intensity dependence of wave front and dimming regions on ϕ. The
information about the wave front localization is noisy.
4
x10
−6
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E
W
N
4
dimming
wave front
x10
4
S
E
N
W
dimming
wave front
3
2
−8
1
Intensity
Intensity
NW dimming
−10
0
−1
−2
−12
NW dimming
−3
SE dimming
SE dimming
05:07 UT − 04:50 UT
04:50 UT − 04:34 UT
−14
0
1
x10
π/2
5
S
π
3π/2
ϕ
E
−4
0
2π
W
N
0.5
π/2
x10
π
3π/2
ϕ
2π
5
S
E
W
N
0.5
0
0
−0.5
−1
−1.5
Intensity
Intensity
−0.5
NW dimming
−1
NW dimming
−1.5
−2
−2.5
−3
−3.5
0
−2
SE dimming
SE dimming
05:24 UT − 05:07 UT
π/2
π
ϕ
3π/2
05:41 UT − 05:24 UT
2π
−2.5
0
π/2
π
ϕ
3π/2
2π
Figure 12. Dependence on ϕ of weighted values of intensities of dimming zone and
wave zone.
archi8DBLP.tex; 31/01/2005; 21:21; p.14
Automatic detection of EIT waves and dimmings
15
Figure 13. Angular rotation of EIT wave around EC (S → E → N → W ) together
with the radial expansion of front forming spiral-like motion.
6.2. Extraction of high intensity areas.
To each pixel with angular coordinates belonging to the zone of the
front wave or to a dimming, a weight was attributed equal to the
sum of all pixels on the angular interval of length π/8 centered on the
considered point. In this way, the area of localization of the front wave
will correspond to the pixels surrounded by the ensemble of pixels with
high intensities. Then, bright points not belonging to such areas were
filtered out as noise. In such a way we extract only large scale intensity increasing and we excluding point-like perturbations. We proceed
similarly with low-intensity points for dimmings.
Fig. 12 shows the dependence on ϕ of the intensity of weighted
values of intensities of dimming zone and wave zone. This figure shows
that the proposed filtration algorithm gives a possibility to extract
explicitly the intervals angular changes which correspond to the zone
perturbed by dimmings. The first dimming has south-east direction
while the second one has a north-west direction. The wave front localization is also found to be on the almost same radial direction from
EC. The figure shows that during the time interval 04:50- 04:34, takes
place a perturbation of south-east dimming and a small perturbation of
north-west dimming. Perturbation of south-east dimming corresponds
localization of the wave front in the angular interval 0 < ϕ < 0.6π.
Intensity increasing in the north-west direction in the beginning of the
event is not necessarily explained by front wave propagation. During
the time interval 05:07- 04:50 we see activity of north-west dimming.
Area of propagation of south-east dimming expands also. Perturbation
of the south-east dimming corresponds to the localization of wave front
in the interval 0 < ϕ < 0.9π. Perturbation of north-west dimming
corresponds localization of wave front in the interval 1.3π < ϕ < 2π.
archi8DBLP.tex; 31/01/2005; 21:21; p.15
16
Podladchikova and Berghmans
On time interval 05:24- 05:07 UT, the area of propagation of both
dimmings (in particular the south-east dimming) expands. Wave front
intensity localizes also in south-east direction for 0.3π < ϕ < 0.7π
and in north-west direction for 1.5π < ϕ < 2π. The figure shows
that during the time interval 05:41- 05:24 UT, both dimmings continue
to expand. Practically, there is no possibility to extract the intensity
increasing area of the wave front in the south-east direction. The second area of localization of intensity which correspond to north-west
dimming moves to the west. The angle of the propagation of this area
is 1.7π < ϕ < 2π. To compare these pictures, we can see that both
regions of wave front intensity have systematic angular displacement
in time counterclockwise. This allows to state the hypothesis not only
about radial propagation of coronal wave but also about the existence
of an angular momentum associated to rotation.
Fig. 13 shows modified RD images in which intensity of each pixels
of the ring m and of the sector k changed on the value S(m, k). The
localization of the brightness of wave front is observed in 2 regions. The
highest increasing of intensity is in south-east direction from EC. The
second highest brightness increasing concentration is observed in the
opposite direction. From the comparison shown on the picture, one can
see that during the time between 2 observations bright regions, beside
of radial expanding, turn out in the sense S → E → N → W .
Fig. 12 shows that the coronal wave front localizes in two regions.
Such as dimmings-twins, these areas are located symmetrically with
respect to the line of magnetic field polarities changing, in the angular
sectors on the borders of dimmings. Moreover, an area with brighter
wave front is situated on the border of larger volume and area SE
dimming. In the NE direction, where intensive dimmings are absent,
wave front brightening is also very weak.
6.3. Determination of angular wave velocity
The plot of the intensity as a function of the angle ϕ in both investigations has a complex form. That is why for definition of angular velocity
as angular coordinate of each region we choose the coordinate of the
intensity center of this area analogically to a center of mass.
For example if the wave front is localized in the interval 0 < ϕ <
0.6π, then angular coordinate of this area is determined as follows:
Pn
ϕC =
j=1 Ij
· ϕj
;
j=1 Ij
Pn
Here, n is a number of pixels in the interval 0 < ϕ < 0.6π, with
ϕj , j = [1, n] are the angular coordinate of j pixel, Ij is the weight of the
archi8DBLP.tex; 31/01/2005; 21:21; p.16
Automatic detection of EIT waves and dimmings
17
Table II. Intensity centers of the regions of wave front localization
N o image
ϕC [rad] SE direction
ϕC [rad] NW direction
16
17
18
19
0.91
1.12
1.68
-
5.39
5.66
5.89
Table III. Angular velocities of EIT wave front
∆t [s]
ω [rad/s] SE direction
ω [rad/s] NW direction
1020 (17 - 16)
1022 (18 - 17)
1018 (19 - 18)
2.06 × 10−4
5.48 × 10−4
-
2.64 × 10−4
2.26 × 10−4
j-th pixel. In such a way, the localization center corresponding to southeast dimming has been determined: Thus the centers of intensities have
been determined for each area of intensity localization of wave front,
lying on the same radial direction as the intensive dimmings, see table.
II.
From the obtained data, the angular velocity ω has been defined
(table III).
6.4.
Determination of final vector velocity of EIT wave
M2
r
2
M1
r
1
j=0
SUN
Figure 14. Propagation of EIT wave on the sphere
As far as EIT wave propagates as a spiral and has radial and angular
velocity, one can find the vector velocity of EIT wave.
archi8DBLP.tex; 31/01/2005; 21:21; p.17
18
Podladchikova and Berghmans
Table IV. Absolute velocities of EIT wave front
∆t [s]
Velocity [km/s] (SE direction)
Velocity [km/s] (NW direction)
1020 (17 - 16)
1022 (18 - 17)
1018 (19 - 18)
263
365
-
265
313
Fig. 14 shows position of the centers M1 , M2 of localization of coronal
wave intensity in the 2 moments of time. Coordinates of points M1 , M2
are defined by the length and direction on sphere of radius r~1 , r~2 .
Position M of the wave front defined by radius-vector with length
from EC to maximum of expanding wave front rmmax . The direction
of the radius vector is defined by the angular coordinate ϕC . Thus,
velocity vector of EIT wave can be determined as:
~ = r~2 − r~1 ,
V
∆t
where ∆t is the interval between the observations of points m1 and m2 .
The absolute value of velocity vector is
~|=
|V
|r~2 − r~1 |
,
∆t
¯ ¯
¯~ ¯
From this formula one can get the resulting values of ¯V
¯, for intervals
between images (table IV).
Thus, one can define position of EIT wave front and its localization
on sphere. It has been shown that the velocity of EIT wave is a vector
characterized not only by radial component, characterized the velocity of the wave propagation, but explained by the presence angular
moment. The problem of angular displacement of EIT wave, moreover
the definition of it quantitative characteristics demand the statistical
technique of data processing. One of possible approach is presented in
this section.
6.5. Possible reasons for EIT wave rotation
Here we present the schema of possible scenario of EIT wave rotation
mechanism and dimmings creations, which shall be later expanded into
a more complete model. Fig. 15a shows the initial curved magnetic
configuration before eruption, with magnetic flux at one footpoint much
larger than the other one. Reconnection site is located in the dense regions. The transfer of such a curved magnetic configuration to the state
archi8DBLP.tex; 31/01/2005; 21:21; p.18
Automatic detection of EIT waves and dimmings
19
xp[-
h]
Height
Pho
tosp
ere
r(h
)
~e
Pk >> PB
Figure 15. Possible scenario of EIT wave rotation and dimmings origin: a)
Pre-eruption 3D magnetic field line configuration. The transition from this state
to another one with smaller potential energy after reconnection explains the spiral
motion of the EIT wave. b) Pre-EIT wave 2D magnetic field line configuration. c)
Magnetic configuration during the eruption. The magnetic lines tend to be open.
Reconnection occurs in a dense region where the magnetic pressure is much smaller
than the kinetic one, Pk >> PB d) Magnetic configuration after the EIT wave.
Dimmings correspond to the region with open magnetic field lines
with the minimum potential energy after reconnection will dimmish the
curvature of field lines. Fig. 15b shows 2D preflaring configuration. The
next step Fig. 15c suggests that during reconnection magnetic lines will
tend to be open, as far as kinetic pressure is much larger than magnetic
one in such dense regions. Fig. 15d shows after eruption configuration,
where large footpoints will be seen like dimmings, as the dark region
with the open magnetic lines, that lost plasma.
7. Conclusions
We have developed filtering algorithms that can be applied to EUV
difference images of the solar corona for the study of EIT waves and
dimmings. The algorithms proposed can not only be used for the automatic detection of these events, but also to extract some of their
physical characteristics and to study effectively the dynamic and the
structure of dimmings and coronal waves.
− Under conditions of a relatively simple structure of the solar corona,
areas of dimming propagation have been extracted, their structure
maps have been constructed, and their areas and volumes have
been determined and followed in time.
− We have shown that the interval of dimming expansion coincides
with the time of EIT wave front propagation. When the coronal
wave is not present on the disk already dimmings start to fade.
− Under conditions of quite complex solar corona, where several active centers, coronal holes and other event are observed on the
archi8DBLP.tex; 31/01/2005; 21:21; p.19
20
Podladchikova and Berghmans
Sun, an algorithm of dimming extraction connected directly with
investigated structure has been proposed.
− Ring analysis technique of EIT wave dynamics have been proposed.
This method is based on the construction of integral intensity of
sequence of concentric circles surrounding the eruption center.
− Sector analysis of the wave front quantitatively confirms the quasiisotropic character of the EIT wave front propagation, in the case
where there is one active region on the Sun
− Radial velocities of coronal wave during time interval between images and average value of radial velocity during wave observation
on the disk were automatically determined.
− The fragmented structure of the EIT wave front was investigated
in details for the event of 12/05/1997. It was shown that coronal
wave front localizes in two regions, such as dimmings-twins these
areas are located symmetrically with respect to the line of magnetic
field polarities changing, in the angular sectors on the borders of
dimmings. Moreover, the area with brighter wave front is located
on the border of the larger volume and area SE dimming. In the
angular sectors where intensive dimmings are absent, the wave
front brightenings is also very weak.
This fact together with the character of dimmings regions can
be considered as a strong argument in the favor of the hypothesis about the development of coronal wave as the consequence of
plasma compression on the propagating border of the dimming.
− Angular rotation of both extracted regions of brightest intensity of
front combined with the radial expansion of front forming results
in spiral like motion.
− Angular velocities of EIT wave during the time between images,
vector velocities, their absolute values and average linear velocity
of coronal wave during it observation on the disk were determined.
The proposed algorithms are supposed to be used for dimmings and
EIT wave detection for data processing of SECCHI EUV telescopes on
board the STEREO mission, as well as SWAP on board of the PROBA
II mission.
archi8DBLP.tex; 31/01/2005; 21:21; p.20
Automatic detection of EIT waves and dimmings
21
Acknowledgements
The authors are grateful to the EIT consortium for providing the data,
to F. Clette, S. Koutchmy, V. Krasnoselskikh and B. Lefebvre for fruitful scientific discussions, and to B. Thompson for a detailed catalog of
EIT waves and recommendations. SOHO is a project of international
collaboration ESA/NASA. The work in this paper was completed under
the BELSPO/PRODEX project ”STEREO/SECCHI preparation to
exploitation”.
References
Chertok I. M., V. V. Grechnev, Astronomicheskiy Jurnal, v 80, 2 pp 162 - 174,
(2003).
Chertok I. M., V. V. Grechnev, Astronomicheskiy Jurnal, v 80, 2 pp 1013 - 1025,
(2003).
Delaboudiniere J. P., G. E. Artzner, J Brunaud, et al. , Solar Phys. 162, 291 (1995).
Delannée C. and G. Aulanier, Solar Phys. 190 ,107 (1999).
Delannée C., Astrophys. J. 545 ,512 (2001).
Goplaswamy N., B. J, Thompson, J. Atmos. and Solar-Terr. Phys,62, 1458 (2000).
Klassen A., H. Aurass, G. Mann, et al. , Astron. and Astrophys. 141, 357,(2000).
Klassen, A., Aurass, H. , Mann, G. ,& Thompson, B. J. 2000, A&AS,141,357
Moreton G. E. and H. E. Ramsey, PASP 72 ,357 (1960).
Moses, D. , et al. 1997, Sol. Phys. , 175, 571
Plunkett S. P. , B. J. Thompson, R. A. Howard et al, Geophys. Res. Let, 25, 2477,
1998
Smith, S. F. ,& Harvey, K. L. 1971, in Physics of the Solar Corona,ed. C. J. Macris
(Dordrecht: Reidel), 156
Thompson B. J., S. P. Plunkett, J. B. Gurman, et al. , Geophys. Res. Lett. 25, 2465
(1998).
Thompson B. J., B. Reynolds, H. Aurass, et al. , Solar Phys. 193, 161, (2000).
Tsuneta, S. , Acton, L. , Bruner, M. et al. , Solar Phys. 136, 37 (1991).
Warmuth A., B. Vrsnak, H. Aurass,et al., Astrophys. J. 560 , L105 (2001).
Webb D. F. , Lepping, R. P. Burlaga, et al. , J. Geophys. Res. 105, 2725 (2000)
Zarro D. M., A. C. Sterling, B. J. Thompson, et al. , Astrophys. J. 520, L139 (1999).
Zhukov A. , Auchere F. , A&A, 427, 705 (2004)
archi8DBLP.tex; 31/01/2005; 21:21; p.21
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