A&A 422, 337-349 (2004)
DOI: 10.1051/0004-6361:20035815
D. Tripathi - V. Bothmer - H. Cremades
Max-Planck-Institut für Aeronomie 37171 Katlenburg-Lindau, Germany
Received 5 December 2003 / Accepted 5 April 2004
Abstract
The Extreme ultraviolet Imaging Telescope (EIT) on board the Solar and
Heliospheric Observatory (SOHO) spacecraft provides unique observations of dynamic
processes in the low corona. The EIT 195 Å data taken from 1997 to the end of 2002 were
investigated to study the basic physical properties of post-eruptive arcades (PEAs) and
their relationship with coronal mass ejections (CMEs) as detected by SOHO/LASCO (Large
Angle Spectrometric Coronagraph). Over the investigated time period, 236 PEA events have
been identified in total. For each PEA, its EUV lifetime as derived from the emission time
at 195 Å, its heliographic position and length, and its corresponding photospheric
source region inferred from SOHO/MDI (Michelson Doppler Imager) data has been studied, as
well as the variation of these parameters over the investigated phase of solar cycle 23.
An almost one to one correspondence is found between EUV PEAs and white-light CMEs. Based
on this finding, PEAs can be considered as reliable tracers of CME events even without
simultaneous coronagraph observations. A detailed comparison of the white-light, soft
X-ray and EUV observation for some of the events shows, that PEAs form in the aftermath of
CMEs likely in the course of the magnetic restructurings taking place at the coronal
source sites. The average EUV emission life-time for the selected events ranged from 2 to
20 h, with an average of 7 h. The heliographic length of the PEAs was in the range
of 2 to 40 degrees, with an average of 15 degrees. The length increased by a factor of 3
to 4 in the latitude range of 20 to 40 degrees in the northern and southern hemispheres,
with longer PEAs being observed preferentially at higher latitudes. The PEAs were located
mainly in the activity belts in both hemispheres, with the southern hemispheric ones being
shifted by about 15 degree in latitude further away from the solar equator during
1997-2002. The decrease in latitude of the PEA positions was 10 to 15 degrees in the
northern and southern hemispheres over this period. The axes of the PEAs were overlying
magnetic polarity inversion lines when traced back to the MDI synoptic charts of the
photospheric field. The magnetic polarities on both sides of the inversion lines agreed
with the dominant magnetic pattern expected in cycle 23, i.e. being preferentially
positive to the West of the PEA axes in the North and negative in the South. One third
(31%) of the PEA events showed reversed polarities. The origin of PEAs is found not just
in single bipolar regions (BPRs), but also in between pairs of neighboring BPRs.
Key words: Sun: corona - Sun: coronal mass ejections (CMEs) - Sun: flares - Sun: filaments - Sun: photosphere - Sun: solar-terrestrial relations
Coronal mass ejections (CMEs) are dynamic events in which plasma with closed magnetic field structure is ejected out of the solar atmosphere. They disrupt the flow of the solar wind in the heliosphere and can cause major geomagnetic effects (e.g., Gosling et al. 1974; Zhang et al. 2003; Bothmer & Schwenn 1995). The origin of CMEs and their evolution in interplanetary space are not well understood. The LASCO (Large Angle Spectrometric Coronagraph) coronagraph on board the SOHO (Solar Heliospheric Observatory) spacecraft has provided unprecedented observations of CMEs since launch in 1995 (e.g., St. Cyr et al. 2000; Howard et al. 1997).
It is a challenge to better understand the source regions of CMEs in the low corona and photosphere. Unfortunately, the regions underlying CMEs are best observed on the visible disk, whereas CMEs are best observed at the limb. Those CMEs originating around disk center are called halos (Howard et al. 1982). They appear unstructured because they propagate mainly parallel to the line-of-sight so that their real physical parameters are difficult to determine. Front-side halo CMEs are of strong importance in terms of space weather (e.g., Webb 2000; Bothmer 1999). Detailed studies of the source regions of front-side halo CMEs will provide important information to understand the solar causes of CMEs and also to help forecast their possible arrivals at Earth's orbit.
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Figure 1: A post-eruptive arcade imaged by TRACE at 195 Å on 04-Nov.-2003 at 22:35 UT. |
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In order to identify the source regions of CMEs, erupting prominences (disappearing
filaments) represent probably one of the best solar activity phenomena (e.g., Webb
& Hundhausen 1987), but for CMEs originating from active regions, often no
associated disappearing filament is observed (Subramanian & Dere 2001). Contrary to
the cooler prominence material seen in absorption, brightenings of S- or reverse
S-shaped hot (T
K) loop structures, called sigmoids, and
developments of X-ray arcade-like loop systems were also found to be associated
with the occurrences of CMEs (e.g., Rust & Webb 1977; Svestka et al. 1998; Hudson
et al. 1998). The post-eruptive arcades (PEAs), often also called post-flare loops,
are also visible at EUV wavelengths (e.g., Sterling et al. 2000). A recent example
for such a post-eruptive arcade imaged by TRACE (Transition Region and Coronal
Explorer, http://vestige.lmsal.com/TRACE/POD/TRACEpod.html) on November 4, 2003 at
22:35 UT, which was preceded by a very fast (V>2000 km s-1) CME and an
extremely intense X-ray flare (class X28), is shown in
Fig. 1. This PEA had formed at the solar limb an example of a
disk PEA is shown in Fig. 2. Kopp & Pneuman (1976) have
interpreted the formation of the loop systems as a consequence of magnetic
reconnection processes in the course of solar eruptions. Sometimes two ribbon
flares demark the footpoint locations and developments of these loop systems.
Whereas TRACE provides detailed views of PEAs in individual events, SOHO provides a unique data set of full disk observations for solar cycle 23. Complementary to studies of smaller sets of individual events (e.g., Hudson et al. 1998; Sterling et al. 2000), the main scope of this paper is to explore the unique set of SOHO/EIT (Extreme ultraviolet Imaging Telescope) observations in order provide the first detailed statistical analysis of the general observational characteristics of EUV PEAs and their relationship to white-light CMEs detected by LASCO.
SOHO/EIT 195 Å daily mpeg-movies taken from 1997 until the end of 2002 were
inspected to identify PEAs on the visible solar disk together with EIT movies of
higher spatial and time resolution as provided in the mvi-format (
pixels). These movies commonly have a temporal resolution of 12 min. The
instrumental characteristics of EIT have been described in detail by
Delaboudinière et al. (1995). The photon emission detected by EIT at 195 Å is
due to Fe XII ions formed at temperatures around
K (Dere et al.
2000). From 1996 until March 1997, the cadence of EIT data was limited due to
telemetry restrictions (Subramanian
Dere 2001).
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Figure 2: Top panel: running difference images taken by EIT 195 Å on 12-Sep.-2000. The first two images show the erupting prominence (EP) event and consequent post-eruptive arcade (PEA) formation in the southern hemisphere near CM. Points 1 and 2 represent the start and end points of the PEA. The last figure reveals the dimming of the arcade. Bottom panel: LASCO/C2 images showing the evolution of the associated CME in white-light. LE denotes the leading edge of the CME and EP the erupting prominence. In all figures, north is up and west is towards the right. The speed of the CME in the plane of the sky was 1550 km s-1 at PA 220. |
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EUV PEAs have been identified in the EIT movies by the appearance of transient brightenings of large-scale loop systems over periods of several hours. An event was selected for further study if it appeared as a clearly discernable feature on the solar disk visible over its full spatial extent (see e.g., Fig. 2). Note that this criterion excludes limb events. From the inspection of the EIT 195 Å data, 236 PEA events were identified during 1997-2002 (see Table 1). The number of PEAs in 1998 was strongly decreased in the second half of the year due to the SOHO recovery phase after its preliminary loss in June. For each PEA, its spatial extent was located in the EIT images at times around its maximum brightness at 195 Å. The heliographic coordinates of the arcade's extreme ends were determined in latitude and longitude listed as b1, l1, b2 and l2 in Table 1, with (b1, l1) and (b2, l2) denoting the PEA's end points to the West and East respectively. denoting the arcade's endpoint to the East (the values provided in Table 1, maybe compared with the top panel of Fig. 2 in Sect. 3). For PEAs that can be considered as fairly linear features, to first order (b1, l1), (b2, l2) represent the endpoints of the mid axis of the arcade. The calculation of the length L of each arcades based on these values is described in detail in Sect. 3. The approximated lifetime of each PEA was determined from its presence as a discernible feature in the EIT 195 Å images with a common time-cadence of 12 min.
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Figure 3: LASCO C2/C3 h-t diagram for the CME detected on 12-Sep.-2000, around 12:30 UT. The solid triangle marks the estimated CME onset time at 1 solar radii and the solid circle represents the identified onset time of the PEA based on EIT 195 Å images. The h-t diagram was taken from the CME catalogue at http://cdaw.gsfc.nasa.gov/CME_list/. |
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In order to study the underlying photospheric field signature of the PEAs, magnetograms from the SOHO/MDI (Michelson Doppler Imager, see Scherrer et al. 1995) instrument were used. MDI measures the line-of-sight, i.e. the longitudinal component of the magnetic field with a resolution of 2 arcsec via the Ni I (6768 Å) line. In order to compare the EIT PEA events with other solar activity features, such as disappearing filament/prominence eruptions, X-ray flares or soft X-ray brightenings, data from the Paris/Meudon (http://bass2000.obspm.fr/home.php), and Big Bear (ftp://ftp.bbso.njit.edu/pub/archive/) observatories, the GOES satellite (ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SOLAR_FLARES/XRAY_FLARES/) and the Soft X-ray telescope (SXT) on board Yohkoh (http://cdaw.gsfc.nasa.gov/CME_list/daily_mpg/; see Tsuneta et al. 1991) were consulted.
Table 2: The columns from left to right are: Date of observation; start-, end of rising- and peak-time of GOES X-ray flares; flare locations on the solar disk in heliographic coordinates; times of coronal brightenings observed by Yohkoh/SXT, onset times of PEA observed by SOHO/EIT at 195 Å; estimated onset times of the associated CMEs based on the investigation of h-t diagrams.
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Figure 4: Distribution of PEAs in heliographic longitude in bins of 10 degrees as identified in EIT 195 Å images from 1997 to 2002. The portions of the bars represented as spotted areas represent those PEAs for which no white-light CME had been detected by LASCO/C2. |
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The association with a white-light CME was investigated for every identified PEA
listed in Table 1 based on LASCO/C2 observations as
described in Sect. 2. Figure 2 shows an example of a PEA located
near central meridian (CM) in the southern solar hemisphere and its associated
white-light CME first detected in the FOV of C2 on 12 September 2000, at 12:30 UT.
The detection of the CME in C2 was preceded by a filament eruption observed by EIT
at around 11:48 UT, becoming visible in C2 at 12:54 UT. The estimated onset time
of the CME was 11:45 UT according to the h-t diagram (see
Fig. 3), which in this case is in good agreement with the
onset time of the prominence eruption imaged by EIT. However, the validity of the
estimated CME onset time based on the extrapolation of the h-t diagram can be
misleading, e.g., for CMEs that were slowly accelerated in its initiation phase or
in case of halo CMEs. The identified onset time of the corresponding PEA at 195 Å in the EIT images was 12:24 UT, showing its maximum development around 13:48 UT and disappearing at about 17:48 UT. The EUV PEA formed about 39 min after
the eruption of the prominence and CME lift-off, and was still lasting when the
CME had already reached a height of more than 40
.
The timing of the
individual phenomena is summarized in Fig. 3.
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Figure 5:
Top panel:
the first image shows a running difference image taken by EIT at 195 Å on 17-Feb.-2000 displaying the PEA that
had formed in the southern hemisphere. The second and third images show that a filament (F) disappeared from
this solar region based on H![]() |
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Figure 6:
Multi-wavelengths observations for the source region of the 17-Feb.-2000 PEA event around 21:30 UT.
Top to bottom: First panel: EIT 195 Å images taken at 20:12 UT, 20:24 UT and 20:36 UT on February 17, 2000 showing
the pre-eruption configuration of the halo CME detected later by LASCO and the initial phase of the arcade
formation. Second panel: Yohkoh/SXT images taken at 20:03 UT, 20:23 UT and 21:23 UT on the same day showing
the brightening of an S-shaped sigmoid in the S-hemisphere in the CME source region. Third panel: H![]() |
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Figure 7: Left to right: zoomed view of the post-eruptive arcade (PEA) observed by EIT at 195 Å on 15-Apr.-2001 at 05:12 UT followed by two H- alpha images taken on 14 and 15 April showing the position of the corresponding disappearing filament (F) on the solar disk. |
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A remarkable feature of Fig. 2 is the apparent similarity between the shape of the CME core and structure of erupting prominence and PEA system. As can be inferred from the bottom panel of Fig. 2, the CME resembles a large-scale curved cylindrical flux tube that has originated from a source site bounded by the PEA's start and end points which are likely to be considered as the two legs of the CME.
The systematic investigation of the association of the entire set of PEA events listed in Table 1, with white-light CMEs detected by LASCO in close space and time relationship, showed that 210 (92%) out of 236 events identified from 1997 until the end of 2002 had clear CME associations. In seven events there were no LASCO data taken, so that only 19 (8%) PEAs were found to be lacking an associated CME.
Figure 4 shows the distribution of all identified PEAs in
heliographic longitude presented in bins of ten degrees. As expected, the
distribution peaks near CM, where PEA events can best be observed over its full
extent. The few number of PEAs without associated LASCO/C2 CMEs were all observed
around disk center in the range 40
East to 30
West.
Taking into account, that front-side halo CMEs originating from near disk center are often hard to detect because of the dependence of the efficiency of Thomson scattered light on the viewing angle of the CME with respect to the line of sight (e.g., Plunkett et al. 1998; Brueckner et al. 1998), it seems reasonable to assume that the small number of PEAs without CME association may have been caused by sensitivity limitations of LASCO. It thus seems plausible to assume that PEAs are definite indicators of CMEs that originated from the corresponding regions of the visible disk.
Disappearing filaments (prominence eruptions) are considered as one of the most
unique signatures of CMEs (e.g., Bothmer
Schwenn 1994). As an example for this
connection, Fig. 5 shows that the front-side halo CME on February 17, 2000, that originated from the SE part of the Sun near CM, was accompanied by a
filament disappearance in H
(Fig. 5, middle and right
images in top panel). Figure 6 shows a multi-wavelength view of
the CME's source region based on SOHO/EIT and MDI, Yohkoh/SXT, and ground-based
observations from Paris/Meudon. The filament closely followed the orientation of
the neutral line separating regions of opposite magnetic polarity as inferred from
the SOHO/MDI data. Bright loops in form of an S-shaped sigmoid, prominent in the
Sun's southern hemisphere (e.g., Canfield et al. 1999), are often visible shortly
before the eruption of a CME, visible here at around 20:00 UT on February 17
(Fig. 6, second panel from top, first image). A strong
brightening of the sigmoidal system occurred near the CME's lift-off time at around
20:23 UT (see Yohkoh/SXT images in Fig. 6), followed by the
onset of the EUV arcade around 21:36 UT. The X-ray flare observed by GOES 8
(available at http://www.lmsal.com/SXT/plot_goes.html) started between 20:10 and
20:15 UT (see Table 2), increasing in intensity and reaching its peak value near 20:32 UT. The onset of the PEA seen by EIT follows in time the peak of the X-ray flare.
The CME onset time corresponds to the rising phase of the X-ray flare, coinciding with
its acceleration phase, in agreement with the findings of Zhang et al. (2001). Assuming that the peak intensity of the flare indicates the CME's
lift-off from the Sun, the PEA may be interpreted as a consequence of magnetic
reconnection processes that were initiated underneath the rising CME as suggested by
Kopp & Pneuman (1976).
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Figure 8: GOES 8 X-ray plots for the CME and PEA events on February 9, 2000 ( top) and 6 July, 2000 ( bottom). The approximate onset times of the CMEs and PEAs are labeled. |
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Figure 9: YOHKOH/SXT image taken on 25-Jan.-1998 at 15:54 UT showing the coronal brightening (CB) that was detected on the NE part of the solar disk and the corresponding post-eruptive arcade (PEA) observed by EIT at 195 Å at 17:53 UT. |
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Figure 10: Frequency distribution of the lifetimes of the identified PEAs during 1997-2002 based on SOHO/EIT 195 Å observations in bins of 2 h. The average value for the lifetime was 7 h. |
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For three more PEA events, the flare position, as provided by the GOES catalogue
(ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SOLAR_FLARES/XRAY_FLARES), was located
not further away from the PEA location than 5
in heliographic
latitude and longitude and which were observed within a time interval of three
hours prior to the onset of the PEA, the chronological evolution of the coronal
features seen by EIT 195 Å and SXT were compared with the GOES 8 X-ray flare
timings (Table 2). The time evolution in the events is similar: A rising phase in soft
X-ray intensity during the acceleration phase of the CME, its consequent
lift-off and propagation phase, followed some minutes later by the peak in X-ray
intensity, finally followed by the formation of the PEA underneath it.
Figure 8 shows the time evolution of the GOES 8 X-ray flare for the
PEAs and CMEs observed on February 9 and July 6, 2000. Whereas on July 6, 2000, the
PEA causes a second increase of the X-ray intensities, it can not be distinguished
as a separate feature in the GOES 8 X-rays on February 17, 2000. The coronal
brightenings observed by SXT preceded the onset of the EUV PEAs by about 5 to 30 min (see Table 2), but due to the irregular cadence of the SXT observations, no
detailed statistics could be performed. Figure 9 shows an example of
a transient coronal brightening detected by SXT on 25-Jan.-1998 and post-eruptive arcade seen by EIT.
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Figure 11: Carrington synoptic map representing the positions of all identified PEAs during 1997-2002 based on the analysis of SOHO/EIT 195 Å observations. |
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Figure 12:
Frequency distribution showing the heliographic lengths of PEAs identified from SOHO/EIT 195 Å images during 1997-2002 in bins of 5![]() ![]() |
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Figure 13:
Variation of PEA lengths with heliographic latitude during 1997-2002. The data points represent the
latitude for the midpoint of the PEA axes. Note that no events were identified above 60![]() ![]() ![]() |
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A PEA event associated with a disappearing filament, but without corresponding LASCO white-light
CME is shown in Fig. 7. For better visibility, only a portion of the full disk
EIT image, comprising the full spatial extent of the PEA is displayed. The H
and EIT
features are basically
the same as observed for the event on 17 February 2000, visualized in Figs. 5 and 6, except that a CME was not identified in the LASCO data.
If one considers H
filament eruptions as valid CME proxies, 14 more events out of the 19
cases where no white-light CME was detected, can be clearly considered as probably CME associated.
Unfortunately, in 3 cases no H
data were available and in 2 cases the events were very complex
so that the interpretation of the data was difficult (see Table 1). If one takes into account
these informations, the total number of PEAs associated with CME increases
to 98% (224 out of 229 events) i.e. almost a one to one correspondence is found between EUV PEAs and CMEs.
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Figure 14: SOHO/MDI Carrington synoptic charts for rotations 1928 a) in 1997 and 1963 b) in 2000 and identified source regions of nine post-eruptive arcades (PEAs), marked by solid and dashed lines. In the synoptic charts, white colors represent areas of positive magnetic polarity, black colors those of negative polarity. Note that in the North the leading areas reveal positive magnetic polarities in this cycle and vice versa in the South. The solid and dashed lines represent the individual PEA axes. Case i) and j) indicates a linear shaped PEA. A PEA located in between two bipolar regions is labeled as k) and a case untypical for the dominant polarity in the northern hemisphere in cycle 23 as l) polarity configuration. |
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For each PEA, the start and end points of the loop system's long axis were calculated as described in
Sect. 2 (see Table 1). Figure 11 displays the
heliographic positions of all PEAs identified during the years 1997 to 2002 in a single Carrington map.
Figure 11 shows that most of the PEAs formed in the heliographic latitude range
40
North and South and that transequatorial cases were extremely rare. No PEA was
observed at latitudes above 60
North or South. The orientation of the PEA axes followed
Joy's law for the tilt of sunspots (Hale et al. 1919), i.e. the tilt angle of sunspots is half of the
latitude, i.e. a sunspot at 60
North would be expected to have a tilt of about 30
,
as measured from East towards North. The tilt angles in sunspots are measured from the equator towards
North in the northern hemisphere and vice versa in the southern hemisphere.
Based on the estimated heliographic coordinates, the length (L) of each PEAs was calculated as:
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(1) |
Figure 12 shows the frequency distribution of the calculated lengths of the
PEAs. The length of the PEAs varied from 2 to 40 degrees, with an average value of about
15.
As indicated by Fig. 11, PEAs exceeding the average length
seem to be observed preferentially at higher heliographic latitudes. The length variation with
heliographic latitude in both solar hemispheres is shown in Fig. 13. From
a linear polynomial fit it is found, that the length (L) of PEAs increases with latitude in
the North as
and in the South as
,
with b being the heliographic latitude. For latitudes of 20, 30 and 40
degrees North and South this corresponds to PEA lengths of 8, 37 and 66 degrees in the North,
and 11, 27 and 43 degrees in the South. Since PEAs can be considered as tracers of the source
regions of CMEs, this finding may imply that the longitudinal extension of CMEs increases with
heliographic latitude. The increase of the PEA length with latitude appears to be associated
with the larger sizes of disappearing filaments at higher latitudes.
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Figure 15: Sketch showing the possible pre-eruption field configuration of PEAs forming along neutral lines/filament sites in single bipolar regions (A) and in between pairs of them (B). The pre-eruption configuration is adapted from Tandberg-Hanssen (1974). |
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The estimated heliographic position for each PEA was used to locate its photospheric source region in the SOHO/MDI magnetogram synoptic charts taken during the corresponding CR. For simplicity, no consideration of the time differences between the arcade occurrences and the observation dates of the single magnetograms that contributed to the individual synoptic maps were taken into account here. The synoptic charts are generated from the definitive MDI magnetograms (available at http://soi.stanford.edu/magnetic/index5.html). For this study, the charts produced from near central meridian observations were used. This implies that arcade events identified in the eastern solar hemisphere had appeared in time after the magnetograms used to construct the synoptic maps were taken, whereas western hemispheric events had preceded the magnetogram observations.
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Figure 16: Carrington synoptic charts of the heliographic positions of post-eruptive arcades identified during 1997-2002 for which the magnetic polarities of the source regions were identified from MDI synoptic charts. The colors at the equatorward ends of the PEAs denote the magnetic polarity to the West of the individual polarity inversion line, with the red color being assigned to the positive magnetic polarity and the blue color assigned to the negative one. |
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Figure 17: a) Total number of PEAs identified in the Sun's northern and southern hemispheres (solid lines) during 1997-2002 based on SOHO/EIT 195 Å observations displayed together with the number of events with positive and negative magnetic polarities (dashed lines) to the West of the neutral line in northern and southern hemisphere respectively. The dotted lines represent the number of events with reversed polarities in both hemispheres. b) Same notation as in a), but with PEAs exhibiting quadrupolar-like (PEAs that formed in between pairs of BPRs) photospheric field source regions removed. Note that no events were recorded by EIT during the SOHO recovery phase in the second half of 1998. |
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Figures 14a,b show the positions of the ten EUV arcades identified in CRs 1928 and 1963. White and black colors in the magnetograms represent positive (field lines pointing away from the photosphere) and negative (field lines pointing towards the photosphere) magnetic polarities. The solid and dashed lines in Fig. 14 mark the calculated PEA long axes positions of the three selected PEAs of CR 1928 and the seven of CR 1963. The axes positions always matched the positions and orientations of polarity inversion lines (PILs, neutral lines) separating regions of opposite magnetic polarities in both solar hemispheres similar to the conclusion of Subramanian & Dere (2001).
Figure 14a shows data from October 1997, i.e. in the rising phase of solar cycle 23, and Fig. 14b shows data for June 2000, i.e. around solar activity maximum. The number of active regions and the photospheric magnetic flux is considerably higher near solar maximum. This difference is also reflected by the minor number of PEAs identified in CR 1928. The comparison of the EIT and MDI observations yielded, that PEAs can either comprise an entire PIL of a bipolar region (Fig. 14a, case i) or they just a fraction of it (Fig. 14a, case j). Besides the cases in which PEAs had formed associated with specific single BPRs, some PEAs were found to be located in between two neighboring bipolar region (see Fig. 14a(k)). The magnetic configurations shown in Fig. 15, in analogy to the ones suggested by Tandberg-Hanssen (1974, p. 118), naturally represents the pre-eruption magnetic structure for both types of events. In Fig. 15, the presence of a filament is assumed in both configurations, although it is not necessarily required. Although the magnetic polarities on both sides of the PILs were most often the expected ones for cycle 23, with leading positive polarities in the sense of solar rotation in the North and leading negative polarities in the South, some of the PEAs formed in regions of reverse polarity configuration as compared to the one dominating the given cycle (Fig. 14b(l)).
Out of the 236 PEA events listed in Table 1,
in 216 cases the magnetic polarities on both sides of the arcade's axis could be uniquely identified.
The remaining events occurred at latitudes above 40
or in regions with very diffuse as
well as highly complex photospheric fields that made the field associations doubtful. In some
cases MDI data was not available. The spatial distribution of the 216 PEAs in heliographic
coordinates and their magnetic polarities between 1997 and 2002 are displayed as yearly Carrington maps in
Fig. 16. The color of each point at the lower latitude end of each
PEA axis represents the magnetic polarity to the West of the polarity inversion line in the
sense of solar rotation, with the red color labeling positive (N) magnetic polarity and the
blue color labeling negative (S) magnetic polarity.
Figure 16 shows that the number of PEA events increases until solar maximum in 2000 and then remains at high level in 2001 and 2002, resembling the frequency of the yearly number of CMEs detected by LASCO (Gopalswamy et al. 2003). However, it should be noted that high latitude PEAs are probably lacking because of the selection criteria which require the PEA to be observable over its full spatial extent. The clustering of PEAs in some years at specific CR longitudes (e.g., near 250 degrees in 2000) may be compared with the clustering of active regions in the rising to maximum phase of solar cycle 23 pointed out by Pojoga & Cudnik (2002) and the appearance of longitudinal bands of active regions reported by Benevolenskaya et al. (1999).
The yearly variation of the respective magnetic polarities found West of the neutral lines (PILs) in each solar hemisphere during 1997-2002 is presented in Fig. 17. In analogy to Fig. 16, the magnetic polarities to the West of the PILs are indicated with red and blue colors. As noted earlier, the expected sign of the leading magnetic polarities of BPRs is expected to be positive (North) in the northern hemisphere and vice versa in the southern hemisphere (compare with Figs. 14a,b). Out of the 216 events in Fig. 17a, 111 events were seen in the North (solid black line) and 105 in the South (dashed black line), i.e. the distribution was about equal in both hemispheres. The maximum number of PEAs peaks in both hemispheres at times of solar maximum in 2000. In the northern hemisphere 81 (73%) out of 111 PEAs and in the southern hemispheres 63 (60%) out of 105 matched the expected magnetic polarity (dashed lines in Fig. 17a). However, a fraction of the events showed reversed polarities (dotted lines). The PEAs that had formed in between pairs of BPRs, yielding reversed polarities in the given cycle, have not been taken into account. These cases do not contradict Hale's law, and have been subtracted in the numbers provided in Fig. 17b. The refined consideration reveals, that the expected polarity dominance for cycle 23 is getting more close to the total number of events, but still with a minority of reversed cases in both hemispheres. A hemispheric asymmetry in the distribution of evolving photospheric magnetic flux, as supported by the results of Li et al. (2002) who found more sunspot groups appearing in the northern hemisphere of the Sun in the rising to maximum phase of cycle 23, from 1996 until 2000, is not apparent in the PEA frequency distribution.
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Figure 18:
Variation of the heliographic latitude of post-eruptive arcades in solar cycle 23 from 1997 to 2002.
The straight line represents fits for the Sun's northern (positive latitudes) and southern (negative
latitudes) hemisphere. Note that no events were
detected above 60![]() |
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In order to investigate the solar cycle variation of the PEAs heliographic locations, the
variation of the midpoints of the PEA axes during 1997-2002 were investigated.
Figure 18 shows, that the PEAs locations follow, as expected, the butterfly
pattern of sunspots. Similar to active regions, the PEAs move in latitude from shortly before
solar minimum until sometime shortly after the following minimum from the higher latitudes
towards the solar equator. Some PEA events at higher latitudes seem to coincide with periods
when strong pulses of new magnetic flux caused a migration of the following polarity fields in
BPRs towards the Sun's poles in agreement with the findings of Benevolenskaya et al. (2002). A
linear polynomial fit yielded for the latitude variation of the PEA locations in the two
hemispheres:
for the North and
for the South, with the time t given in years from 1997-2002. The source
regions of the PEAs on average occurred at higher latitudes in the Sun's southern hemisphere
compared to those in the northern hemisphere. The difference between 1997-2000 is about
15
in latitude. This effect may have caused the more frequent appearance of reversed
polarity PEAs in the southern hemisphere.
From a careful inspection of the full set of SOHO/EIT 195 Å observations taken during the period 1997-2002, a set comprising 236 post-eruptive arcade (PEA) events has been selected, with the requirement that each PEA needed to be observable over its full spatial extent on the visible solar disk. The selected PEA events were correlated with SOHO/LASCO observations to investigate the association of PEAs with white-light CMEs and their photospheric source regions were located in the SOHO/MDI synoptic charts.
The derived basic characteristics of EUV PEAs over the years 1997-2002 can be summarized as follows:
Beyond the potential of PEAs as disk signatures for improved space weather predictions, they also serve as important source region tracers of CMEs. The PEAs selected in this study provide a unique data set to study the underlying photospheric field configuration and possible onset mechanisms of CMEs. These aspects are of special interest for the interpretation of data provided by the NASA Solar-B and STEREO missions, with planned launches in 2005 and 2006.
Acknowledgements
The usefull comments and suggestions provided by Dr. K. P. Dere have greatly improved the quality of the manuscript. We are grateful to Prof. Dr. Rainer Schwenn and Prof. Dr. Sami K. Solanki for valuable discussions and comments. This study is part of the scientific investigations of the project Stereo/Corona supported by the German "Bundesministerium für Bildung und Forschung'' through the "Deutsche Zentrum für Luft- und Raumfahrt e.V.'' (DLR, German Space Agency) under project number 50 OC 0005. Stereo/Corona is a science and hardware contribution to the optical imaging package SECCHI, currently being developed for the NASA STEREO mission to be launched in 2005. We like to thank the SOHO/LASCO/EIT/MDI consortium for providing the data and the software libraries and we acknowledge use of the CME catalog generated and maintained by NASA and The Catholic University of America in cooperation with the Naval Research Laboratory. SOHO is a project of international cooperation between ESA and NASA. We acknowledge the use of data from the American-Japanese satellite Yohkoh provided by the MSSL SURF group (http://surfwww.mssl.ucl.ac.uk/surf) and of H
data from the Observatory at Paris/Meudon distributed via the French BASS2000 (http://mesola.obspm.fr/present_en.html). The TRACE image is courtesy of http://vestige.lmsal.com/TRACE. TRACE is a mission of the Stanford-Lockheed Institute for Space Research (a joint program of the Lockheed-Martin Advanced Technology Center's Solar and Astrophysics Laboratory and Stanford's Solar Observatories Group), and part of the NASA Small Explorer program.