**CORONAL MASS EJECTIONS: INITIATION AND DETECTION**

N. GOPALSWAMY

Center for Solar Physics and Space Weather, Department of Physics, The
Catholic University of America, Washington DC 20064, USA

**ABSTRACT**

Coronal mass ejections (CMEs) are large-scale magnetic structures
expelled from the Sun due to MHD processes involving interaction between
plasma and magnetic field in closed flux regions. I provide a summary of
the observational signatures and current models on CME initiation. CMEs
are traditionally observed using white light coronagraphs. I also
provide a summary of various signatures of CMEs detected in other
wavelengths, which have helped us obtain a complete picture of the CME
phenomenon in the inner heliosphere.

1\. INTRODUCTION

Coronal Mass ejections (CMEs) were first recognized as large-scale
structures expelled from the Sun in the early 1970\'s (Tousey, 1973
using space-based coronagraphs. Thousands of CMEs have been observed
since then, and their physical properties are well known (see
Hundhausen, 1999; St Cyr, 2000). CMEs are currently studied extensively,
owing to their role in deciding the physical conditions in the geospace;
they are also important in understanding the long-term evolution of
solar magnetism since they remove significant amounts of magnetic flux
along with material (see, e.g., Low, 2001). CMEs are responsible for
driving interplanetary (IP) shocks (Sheeley et al., 1985), and as a
consequence, the solar energetic particles (SEPs, see e.g., Tylka,
2001). How CMEs are initiated is still a mystery. In recent years,
spacecraft observations at various wavelengths (apart from the
traditional white light) have provided extensive information on the
source regions and the initial phase of CMEs. To understand CME
initiation, we need to have complete information on the structure and
dynamics of the pre-eruption, closed magnetic field regions on the Sun.
I summarize the observed signatures at various layers of the solar
atmosphere corresponding to the solar source of CMEs. I also summarize
new insights we have gained by combining the white light data with
images at other wavelengths.

2\. INITIATION

CMEs originate from large-scale closed magnetic field regions such as
active regions and filament/prominence regions. Active regions and
filament regions often form complexes. Large-scale closed filed lines
can also be found interconnecting active regions (Svestka). During solar
minima, the equatorial streamer belt constitutes a dominant closed field
structure. Thus, we need to look at the evolution of the magnetic field
for clues to the CME initiation. While we can reliably measure the
photospheric magnetic field, it is difficult to do so in the outer
layers; we can infer the topology of the field lines from coronal
images. Thus, we need to look for signatures of spatio-temporal changes
in the structure of closed field regions in order to understand CME
initiation.

2.1 PRE-ERUPTION STRUCTURES

What kind of closed field regions will result in eruption? The
helmet-streamer structure, known for a long time from eclipse pictures,
is a large-scale closed field region. The closed field part of the
streamer deforms to become the frontal structure of the CME, followed by
the coronal cavity and the prominence core (Hundhausen, 1999). Thus the
" three-part structure" of CMEs (see Fig. 1) can be readily observed in
the pre-eruption phase, and is now commonly observed by inner coronal
imagers such as the *Yohkoh* mission\'s Soft X-ray Telescope (SXT) and
the Solar and Heliospheric Observatory\'s (SOHO) Extreme-ultraviolet
Imaging Telescope (EIT). The pre-eruption configuration in active
regions is probably similar, except for the height of the filament and
the strength of the overlying magnetic field. Transequatorial and
interconnecting structures may result in CMEs without a prominence core.
However, multi-arcade eruptions (Webb et al., 1997; Gopalswamy, 1999)
that span more than one closed region may still contain a prominence
core from one of the underlying flux systems (Gopalswamy et al. 1999a;
Gopalswamy and Thompson, 2000).

Sheared magnetic structures, inferred from photospheric magnetograms,
have been thought to be indicative of imminent eruption (Ambastha et
al., 1993). Combining vector magnetograms with the Yohkoh soft X-ray
images, Falconer (2001) has identified two characteristics of active
regions that may be useful in discriminating CME-productive active
regions: the length of the strong-field, strong-shear main neutral line
and the global net current in the active region. Above certain threshold
values of these parameters, production of CMEs is likely. The coronal
manifestation of the photospheric shear is the sigmoidal structure in
active regions, observed as S- and reverse S-shaped features (Rust and
Kumar, 1996) in soft X-ray images. Statistically, sigmoidal regions have
been found to be more eruptive than the non-sigmoidal regions (Canfield
et al., 1999; Glover et al., 2000). Sigmoidal structures are replaced by
arcades in the post eruption phase (Sterling and Hudson, 1997). Most of
the events studied by Canfield et al. (1999) correspond to a period with
no white light observations, so their association with CMEs is not
clear. Systematic analysis of CMEs originating from sigmoidal regions is
necessary before making firm conclusions.

![](media/image1.png){width="4.511805555555555in"
height="1.7840277777777778in"}

The formation and eruption of prominences is one of the central issues
of CME initiation. In fact some authors consider the prominence eruption
as the cause of CMEs (Filippov, 1996; Wu et al., 2000), contrary to the
arguments that prominence cores are not energetically favorable to cause
CMEs (Hundhausen, 1999). The complex relationship between the filament,
coronal cavity and the frontal structure before and during the eruption
is a subject of current debate (see e.g., Rust, 2001; Linker et al.,
2001; Low, 2001). Many CMEs in the IP medium have been found to have
flux rope structure (magnetic clouds), although it is not clear which
part of the CME becomes the cloud (see e.g., Bothmer and Schwenn, 1994).
Recently, certain concave-outward white light features in the
coronagraph field of view have been interpreted as flux ropes (Chen et
al. 1998). The question is whether the flux ropes are pre-eruption
structures or they are formed during eruption (Low and Hundhausen, 1995;
Gosling, 1990). The flux rope structure naturally provides the necessary
\"dip\" where the filament can reside (e.g. van Ballegooijen and
Martens, 1989) and a reduction in the magnetic flux could cause the flux
rope to erupt (Linker et al., 2001). Shearing and twisting of magnetic
field lines can produce the necessary dips to support the prominence
(Antiochos et al., 1994). Another idea is that the prominence is simply
a flowing material and no dip is needed for support (Martin and
McAllister, 1997).

2.2 PRE-ERUPTION EVOLUTION

Flux emergence has been considered one of the as a possible trigger of
CMEs, which initiates small-scale reconnection near the filaments.
Feynman and Martin (1995) examined the magnetic flux in the source
regions of CMEs associated with filament eruptions and found that they
occurred after the emergence of substantial amounts of new flux in the
vicinity of the filaments. One of the requirements of the eruption is a
\`\`favorable\" orientation of the newly emerging flux with respect to
the preexisting flux in such a way that small-scale reconnection can
take place. Using numerical simulation, Chen and Shibata (2000)
demonstrated the eruption of arcades overlying the filaments as CMEs due
to reconnection between emerging and existing magnetic field lines.
However, Wang and Sheeley (1999) studied a set of filament eruptions and
concluded that flux emergence serves as a catalyst, rather than a
trigger of the filament eruption. Subramanian and Dere (2001) studied a
larger sample of CME events and found flux emergence in many cases, but
there were eruptions with no substantial flux emergence. They also found
eruptions with no substantial flux emergence. The study of Subramanian
and Dere (2001) is consistent with the conclusions of Wang and Sheeley
(1999).

In another study, Lara et al. (2000) found significant changes in the
flux at the time of CMEs in small subregions within the overall region
of eruption. The flux change was also observed during flares without
CMEs, but the largest changes were found at the times of CMEs. When the
flux from the entire region of eruption was tracked, the changes were
not significant. The flux changes were also found to be substantial only
close to the photospheric neutral line. The time resolution of the
SOHO/MDI magnetograms used for this study was rather poor (90 min).
Similar study with a better time resolution may be able to shed light on
the importance of localized flux changes in the eruptive region.

Sometimes, a prolonged (\~ 1 hr) weak dimming in EUV and X-rays occurs
in the eruption region, followed by a deep dimming corresponding to the
CME (Gopalswamy et al., 1999a; Gopalswamy and Kaiser, 2001). Coronal
dimming represents depletion of material in the corona (see e.g., Hudson
et al., 1998). Therefore, pre-eruption dimming may correspond to
small-scale opening of magnetic field lines in the eruption region.

2.3 PRE-ERUPTION ENERGY RELEASE

Both thermal and nonthermal signatures of energy release have been
reported on various occasions. Thermal signatures are in the form of
small-scale X-ray or EUV brightening in the eruptive region. Nonthermal
signatures are in the form of radio bursts of type III, due to energetic
electrons carrying tens of keV. Fig. 2 shows a compact X-ray brightening
(pointed by arrow in Fig. 2) near the location of a filament that
subsequently erupted (see also Gopalswamy, 1999). It took about 3 hours
after the brightening for the filament to completely disappear. The
filament eruption was associated with the CME of 1998 January 21. The
compact brightening was located at the apex of a triangular dimming
region. While the dimming was observed both in X-ray and EUV images, the
compact brightening showed up only in X-ray images suggesting that the
latter is indeed a hot structure.

![](media/image2.png){width="5.749305555555556in"
height="2.8743055555555554in"}

Fig.2. (left) X-ray brightening before the eruption of the filament and
(right) magnetic neutral line and post-eruption X-ray structures
superposed on a SOHO/MDI magnetogram. The filament is plotted as white
contours.

Many years ago, Jackson et al. (1978) found that, statistically, the
number of type III radio bursts peaked about an hour before CMEs,
suggesting that small-scale energy release in the form of energetic
electrons preceded the CME eruption. Using the Clark Lake
multifrequuency radioheliograph images, Gopalswamy et al. (1987)
reported on precursor type III bursts located above the region of
eruption. Recently, Marque et al. (2000) found nonthermal radio bursts
originating near a filament that subsequently erupted.

The evidence for small-scale energy release is consistent with the flux
emergence reported by Feynman and Martin (1995) and the simulation
studies of Chen and Shibata (2000) and points to the possibility of
reconnection in the vicinity of filaments in the pre-eruption phase. It
is however, not clear whether the small-scale energy release is a
consequence of the lifting off of the cavity or overlying arcade. In the
scenario of multi-arcade eruption proposed by Antiochos et al. (1994),
reconnection takes place in the region overlying the eruptive
structures, redistributing the flux and hence enabling underlying flux
system to erupt. Observational signatures of this reconnection are
expected in the peripheral regions of the eruptive structure. Current
ideas and models that address the structure, evolution and energy
release in the pre-eruption phase are summarized in Table 1.

Table 1. Summary of some current ideas and models

+-----------------+-----------------+-----------------+-----------------+
| Model           | Pre-Eruption    | Pre-eruption    | Pre-eruption    |
|                 | Structure       | Evolution       | Energy          |
|                 |                 |                 |                 |
|                 |                 |                 | Release         |
+-----------------+-----------------+-----------------+-----------------+
| Forbes et al.,  | Flux rope in    | Flux            | None            |
| 1994, Linker,   | bipolar field:  | d               |                 |
| et al. 2001     | either emerges  | ecrease/changes |                 |
|                 | or forms in the | leading to loss |                 |
|                 | corona          | of              |                 |
|                 |                 |                 |                 |
|                 |                 | equilibrium     |                 |
+-----------------+-----------------+-----------------+-----------------+
| Chen and        | Flux rope in    | Flux emerges    | Reconnection    |
| Shibata, 2000   | multipolar      | consistent with | energization at |
|                 | field           | reconnection    | the site of     |
|                 |                 |                 | emergence       |
+-----------------+-----------------+-----------------+-----------------+
| Antiochos et    | Sheared arcade  | Continued       | Reconnection    |
| al., 1994       | in multipolar   | shearing        | energization at |
|                 | field           |                 | coronal         |
|                 |                 |                 | null/separator  |
+-----------------+-----------------+-----------------+-----------------+
| Wu et al., 2000 | Flux rope with  | Increase in the | None            |
|                 | overlying       | azimuthal flux  |                 |
|                 | streamer in the | or shear of the |                 |
|                 | solar wind      | streamer field  |                 |
+-----------------+-----------------+-----------------+-----------------+
| Chen et al.,    | Flux rope in    | Increase in the | None            |
| 1997            | equilibrium     | azimuthal flux  |                 |
+-----------------+-----------------+-----------------+-----------------+

3\. DETECTION

Material ejection from the Sun has been known for a long time from
meterwave radio observations (moving type IV bursts, Boischot, 1957) and
from H-alpha observations (Dodson and McMath, 1952). Mass ejection was
also inferred from the estimated speeds of agencies responsible for
metric type II bursts (Payne-Scott, 1947). The concept of mass ejections
(\`\`plasma clouds\") has been in use for a long time (Morrison, 1954;
Gold, 1955). How these ejections were related to CMEs as we know today
became clear only in the early seventies, when the full glory of CMEs
could be appreciated in white light coronagraph images (Tousey, 1973).
Starting from *Skylab*, a number of non-coronagraphic signatures of CMEs
in X-rays, EUV and microwaves were also recognized and are now used
extensively to identify the early phase of frontside CMEs. IP shocks are
responsible for IP type II bursts (Cane et al., 1987) and SEPs, and are
driven by CMEs. CMEs in the solar wind were found in the data acquired
by IMP, Helios and Voyager spacecraft (Burlaga et al. 1981) as magnetic
clouds or ejecta. Interplanetary scintillation (IPS) observations are
also capable of detecting CMEs over the entire Sun-Earth distance
(Manoharan et al., 1995; Tokumaru et al, 1999).

3.1 WHITE LIGHT

CMEs are detected in white light the same way the corona is observed
during eclipses: the photospheric light Thomson-scattered from the free
electrons in the corona is gathered by the optical instrument. In a
coronagrpah, an "artificial eclipse" is produced by an occulting disk to
block the direct photospheric light. The white light observations do not
depend on the temperature of the plasma electrons that scatter the
photospheric light, so that all the substructures of the CME (see
Fig. 1) are visible in coronagraphic images. Intensity of the
Thomson-scattered light is a measure of the total electron content along
the line of sight, which can be used to estimate the mass of CMEs. A
white light CME is usually described in terms of its sky plane speed,
full width and position angle measured from solar north. The CME speed
is usually measured by tracking a feature of the CME in successive
images. The measured speed is the sky plane speed, a lower limit to the
true speed except for the CMEs originating from very close to the limb.
In addition to coronagraphs, Jackson and Webb (1995) used the Helios
photometer data to reconstruct CMEs over much larger distances, although
the spatial resolution was much poorer. These authors found an increase
in CME mass when compared to what was observed close to the Sun.
Coronagraphs on future spacecraft such on the STEREO mission may be able
to obtain true speeds over the entire inner heliosphere.

[3.1.1 Halo CMEs]{.underline}

Since the coronal density decreases rapidly away from the Sun, CMEs are
best observed by coronagraphs when they are ejected close to the limb.
CMEs originating far from the limb are also observed, as enhancements
surrounding the occulting disk and hence called \`\`halo CMEs\" (Howard
et al., 1982). Although this interpretation was considered questionable
(St. Cyr and Hundhausen, 1988), halo CMEs are now routinely observed,
thanks to the high sensitivity of the SOHO/LASCO coronagraphs. The halo
CMEs also confirmed the three-dimensional nature of CMEs (Howard et al.,
1982). CMEs heading both toward and away from Earth appear as halos, so
one needs additional information on the eruption from EUV, X-ray or
H-alpha observations to determine the direction (see, Fig. 3).

![](media/image3.png){width="5.620833333333334in"
height="2.7444444444444445in"}

Recently, Sheeley et al. (1999) found that for a set of fast halo CMEs,
the height-time plot indicated deceleration. Since the measured heights
of halo CMEs are subject to projection effects, the true heights of halo
CMEs when they are at the edge of the coronagraph field of view is much
larger (\>\> 30 R~s~) than observed. CMEs are expected to decelerate at
large heights because they have speeds very close to the solar wind
speed at 1 AU, irrespective of their starting speed near the Sun
(Lindsay et al., 1999; Gopalswamy et al., 2000d). The subset of CMEs
that produces radio emission at long wavelengths also shows
predominantly deceleration even when they originate close to the limb
(Gopalswamy et al., 2000b). This may be due to the fact that the drag
force is significantly higher for faster events.

3.2 X-RAYS AND EUV

The early life of CMEs cannot be studied using white-light coronagraphs
because of the occulting disks employed to block the photospheric light.
Inner coronal imagers in X-rays, microwaves and EUV can detect most of
the features associated with the onset of CMEs (Gopalswamy 1999; Hudson
1999; Hudson and Cliver 2001). At these wavelengths, the radiation
emitted in the corona is imaged, unlike in white light. While most of
the CME-related near-surface phenomena were discovered during the
*Skylab* era, detailed studies became possible after the advent of SXT
and SOHO/EIT. Primary CME related signatures at these wavelengths are
coronal dimming, ejected plasmoid and arcade formation. EUV wave
transients (Thompson et al., 1999) and global enhancement (Gopalswamy et
al., 1999c) are the two newer signatures.

Coronal dimming essentially represents depletion of coronal material due
to a CME (see e.g., Hudson and Webb, 1997). Care must be taken to make
sure that the dimming is not due to temperature change in the case of
spectral line imaging. Coronal dimming is best observed in the EIT 195
angstrom images and displays considerable variation from event to event.
EIT images often show the dimming signature in the direct images, while
one has to use difference images to identify the dimming signature in
X-rays. As we pointed out before, there are weak and prolonged dimming
signatures in the pre-eruption phase and intense dimming during the
eruption phase. Combined EUV and white light observations suggest that
the coronal dimming corresponds to the depletion of the coronal
material, which later becomes the CME in the coronagraph field of view
(Gopalswamy et al., 1999b; Thompson et al., 2000).

The ejected plasmoid is most likely the heated prominence core of CMEs.
The eruptive prominence can be seen in absorption and in emission
depending upon its temperature (Gopalswamy et al., 1999c). X-ray ejecta
can also be seen on the disk in X-ray movies (Gopalswamy et al., 1997).
Recently, Nitta and Akiyama (2000) studied a handful of X-ray ejecta
associated with LASCO CMEs and found them to be slower than the CMEs,
consistent with what is known from white light observations. The CME
leading edge is not seen in X-rays because the flare loops are usually
much brighter. Under rare circumstances when there are no hot
structures, it may be possible to detect the leading edge of the CME
itself (Gopalswamy et al., 1996).

Arcade formation is a post-eruption signature (see, e.g., McAllister et
al, 1996) often used to identify the location and extent of an eruption.
Some of the eruptions with extremely weak near-surface activities have
been identified in this way (see, e.g., Webb et al., 1998). Arcade
formation is also observed in EUV, but not so complete as in X-rays.
Fig. 4 shows an arcade in X-rays and in EUV for the 1998 Jan 25 CME. An
interesting result of this observation is that different loops of the
arcade had different temperatures. Some of the darker loops in the EUV
arcade were hotter, as evidenced by the fact that they were bright in
X-rays.

![](media/image4.png){width="7.0in" height="1.875in"}

Fig. 4. Arcade formation following the 1998 January 25 CME. Contours of
X-ray intensity (from Yohkoh/SXT) are plotted on EUV intensity (from
SOHO/EIT).

The EIT transients are newly discovered wave-like phenomenon
representing the early phase of CMEs (Thompson et al., 1999). One of the
interpretations is that the EIT waves are fast mode MHD waves associated
with solar eruptions and that they eventually become MHD shocks to
produce metric type II bursts (Klassen et al., 2000). Under some
circumstances, the EIT waves may be interpreted as shocks, especially
when they have the \"brow\" structure (Gopalswamy, 2000) and are closely
associated with H-alpha Moreton waves (Thompson et al., 2000), and
metric type II bursts (Gopalswamy et al., 2000c). Khan and Hudson (2000)
observed some transient features in X-rays during flares associated with
transequatorial eruptions. They believe that the transients they
observed are counterparts of EIT waves and may be the manifestations of
coronal shocks responsible for metric type II bursts.

A relatively new X-ray signature, the \"global enhancement,\" was
identified by Gopalswamy et al. (1999d) during the filament eruption
event of 1994 April 30. A weak, large-scale and diffuse enhancement was
imaged by

Yohkoh/SXT. The area of enhancement was much larger than the region of
arcade formation. The global enhancement was interpreted as the early
form of the CME itself. Later on, a number of such events were also
observed in EIT 195 A images during the early phase of CMEs. A detailed
investigation of this phenomenon is needed to determine whether it can
be used to detect disk CMEs.

3.3 RADIO

In radio, both thermal and nonthermal signatures of CMEs can be
detected. Thermal emission depends on the temperature, density and
magnetic field of the region as well as on the observing frequency.
Nonthermal emission depends on quantities such as the density and energy
of the nonthermal electrons. A number of mechanisms operate in producing
thermal and nonthermal radio emission, so a careful identification of
the mechanism is crucial in using the radio technique for CME studies.

[3.3.1 Microwaves]{.underline}

Filament/prominence eruptions (Hanaoka et al., 1994; Gopalswamy et al.,
1996, Hanaoka and Shinkawa, 1998), arcade formations (Hanaoka, et al.,
1994) and even coronal dimming can be studied using microwave images.
Filament eruptions are most spectacular in microwaves and provide
quantitative information on the eruptive process (Gopalswamy and
Hanaoka, 1998). Comparison between eruptive prominences in H-alpha and
in microwaves in Fig. 5 shows that microwave images provide a more
complete picture of the eruptive prominence. Coronal dimming in
microwaves is somewhat difficult to observe because the coronal
structures are optically very thin. Nevertheless, the observed radio
dimming is consistent with X-ray dimming, confirming the depletion of
material from the corona (see Fig.6).

![](media/image5.png){width="5.061111111111111in"
height="2.5305555555555554in"}

.

![](media/image6.png){width="4.0in" height="2.0in"}

[3.3.2 Meterwaves]{.underline}

In meter waves, various types of radio bursts are observed which are
produced by nonthermal electrons escaping from the region of eruption
along open field lines (type III bursts), electrons confined to shock
front (type II bursts) or trapped in moving coronal structures (moving
type IV bursts). Type II and type IV bursts are closely associated with
CMEs, although it is not clear whether the shock is driven by the CME or
the CME presents favorable conditions for shock propagation (Cane, 1984;
Gopalswamy et al., 1998; Cliver et al., 1999).

The moving type IV bursts come in three varieties: advancing fronts,
expanding arches and isolated plasmoids (see the review by Stewart,
1985). The isolated sources originate from heated prominence material,
also detected in X-rays and EUV. The advancing fronts and expanding
arches must be structures associated with the CME itself. Gopalswamy and
Kundu (1989) tracked a moving type IV burst for several solar radii at
three frequencies and found that the radio source moved roughly with the
speed of the associated white light CME. They were also able to infer
the structure of the loops by connecting the centroids of the radio
sources. These substructures are \"visible\" only because of the
nonthermal particles trapped in them. Recent images made by the Nancay
radioheliograph show radio structures that correspond to the frontal
structure of CMEs (Maia et al., 2000).

Since CMEs are denser than the ambient corona, they must be optically
thick at low metric frequencies. For this reason, coronal streamers are
clearly visible in meter-decameter images. Sheridan et al. (1978)
detected changes in the radio streamer associated with a slow CME
observed by *Skylab*. Gopalswamy and Kundu (1992) imaged a CME using the
Clark Lake multifrequency radioheliograph at three frequencies and were
able to estimate the mass of the CME from radio observations alone. The
main difficulty in imaging CMEs at these frequencies is the simultaneous
occurrence of nonthermal bursts that easily mask the thermal emission.

![](media/image7.png){width="6.058333333333334in"
height="3.029166666666667in"}

Fig. 7. Decameter-Hectometric type II burst and type III bursts due to
shock-accelerated electrons (right) associated with the 1998 may 19 CME
observed by SOHO/EIT and LASCO (left).

[3.3.3 Decameter-Hectometric Waves]{.underline}

One of the most recent additions to instruments which detect CMEs in
radio is the RAD2 receiver of the Radio and Plasma Wave (WAVES)
experiment (Bougeret et al., 1995) on board the Wind spacecraft. The
Decameter-Hectometric (DH) domain corresponds to 21 m to 300 m in
wavelength (14 to 1 MHz in frequency) and fills the gap that existed for
a long time between the ground based observations (\> 15 MHz) and space
based observations (\< 2 MHz). Radio bursts associated with shocks (type
II), electron beams escaping from the shock front (type III bursts) and
moving coronal structures (type IV bursts) are observed in this domain.
Fig. 7 shows a CME from the northwest quadrant of the sun that produced
a type II burst in the metric domain and continued into the DH domain.
Note that there were no radio bursts when the CME was at a height of 1.5
R~s~ as seen in the EIT image at 09:28 UT. The type II burst starts only
at 09:50 UT, with simultaneous type III bursts due to shock-accelerated
electrons (Cane et al., 1981). Gopalswamy et al. (2000b) showed that
most DH type II bursts are associated with faster and wider CMEs. DH
type II bursts provide an essential tool to detect faster and wider
CMEs, which are potentially more geoeffective when Earth-directed. While
the relation between metric and DH type II bursts is controversial
(Reiner et al., 2001), there is no doubt that the DH type II bursts are
indicative of CMEs leaving the outer corona (Reiner and Kaiser, 1999;
Gopalswamy et al., 2001).

3.4 IN-SITU OBSERVATIONS

Spacecraft instruments can directly sample the CMEs. Since CMEs
propagate long distances before reaching the spacecraft, they are
considerably evolved and hence it is difficult to relate the *in situ*
observations to the CME substructures observed near the Sun. The generic
term used for CMEs in the solar wind is \'ejecta\'. Interplanetary CME
(ICME) is another general term used to describe the CME-like disturbance
in the solar wind. Most often one encounters the following sequence of
structures detected by a spacecraft: IP shock, sheath, and ejecta. The
ejecta can contain ordered magnetic field, in which case it is termed as
a magnetic cloud (Burlaga et al., 1981). Disordered magnetic fields are
also observed in the ejecta (e.g., Hundhausen, 1972) and hence magnetic
clouds constitute only a subset of the IP ejecta. On rare occasions, one
observes cool dense material towards the end of the ejecta that resemble
the prominence resting at the bottom of the coronal cavity in the
pre-eruption phase of CMEs (Burlaga et al., 1999; Gopalswamy et al.,
1998). Based on this correspondence, one can make the associations
between CMEs observed near the Sun and in the solar wind listed in Table
2.

  ----------------------------------- -----------------------------------
  **CME near the Sun**                **CME in the Solar Wind**

  Coronal Shock                       IP Shock

  Frontal Structure                   Sheath

  Cavity                              Magnetic Cloud/Ejecta

  Prominence Core                     Pressure Plug
  ----------------------------------- -----------------------------------

The fact that the prominence material is rarely observed in the solar
wind is consistent with the fact that the prominence core of CMEs heat
up early on as they are visible in X-rays and hence may be
indistinguishable from other coronal structures.

Identification of CME signatures in the solar wind is complicated and is
often not unique (see e.g., Neugebauer, 1997 and references therein).
Commonly used signatures are: counterstreaming superthermal electrons
and protons, helium abundance enhancements, anomalously low electron and
proton temperatures, strong magnetic fields, smooth rotation of magnetic
fields, low plasma low magnetic field variance, flux reduction of low
energy cosmic rays, and unusual ionization states and elemental
abundances. It is important to point out that not all signatures are
seen in all events and, different signatures do not coincide temporally
and spatially.

3.5 INTERPLANETARY SCINTILLATIONS (IPS)

By observing a large number of natural radio sources distributed all
around the Sun, it is possible to produce all-sky maps to detect IP
disturbances (see, e.g., Manoharan et al., 1995; Tokumaru et al., 2000).
Tokumaru et al. (2000) studied the three-dimensional propagation of four
IP disturbances between 0.2 and 1 AU, using IPS data. They were able to
show that the propagation speed depends on both the latitude and
longitude, and that the direction of maximum speed is not along the
flare normal. The disturbances seem to be distorted by the slow and fast
solar winds. Deceleration of the IP disturbances has also been detected,
sometimes in steps (Manoharan et al., 2001). One of the major advantages
of the IPS techniques is that it can track CMEs throughout the inner
heliosphere while other techniques are limited to near-Sun or near-Earth
domains. It has been shown that the scintillation index, which is a
measure of the density fluctuations in the medium, is proportional to
the density in the medium (Moore and Harrison, 1994). In the coronagraph
field of view, a CME represents a density enhancement with respect to
the ambient. The frontal structure and core are denser, while the cavity
has a density less than the ambient. For fast events, there is also a
shock ahead of the CME with a density jump. Thus it is not clear which
part of the CME is detected by the IPS technique. A careful comparison
between coronagraphic and higher resolution IPS observations of more
CMEs is needed to settle this issue. Future observations of CMEs over
the entire heliosphere by the STEREO mission will provide a good
opportunity to cross-calibrate between IPS and white light observations
of CMEs.

4\. CONCLUDING REMARKS

We have discussed most of the observational signatures related to the
initiation and detection of CMEs, mainly using imaging data at various
wavelengths. It is clear that we need to put together multiwavelength,
multi-instrument data to obtain a complete picture of the CME
phenomenon. There are also other ways of detecting CMEs based on their
ability to produce SEPs via IP shocks (see e.g., Tylka, 2001), complex
compositional signatures (see e.g., Galvin, 1997), and ejecta signatures
at 1 AU (Richardson, 1997). The CME phenomenon is intimately connected
to solar magnetism as we can infer from signatures at the solar surface.
However, we do not fully understand how the evolution of closed field
regions result in CMEs. It is not clear if all the CMEs are produced the
same way. If the complex interaction between plasma and magnetic field
is the basic process in all CMEs, differences in CMEs may be understood
in terms of the magnitude and duration of the propelling force. This
approach may also clarify whether the difference between fast, constant
speed, flare-related CMEs and slow, accelerating, prominence-related
CMEs is just quantitative or qualitative. In order to understand the
relation between CMEs near the Sun and ICMEs, one has to consider the
interaction between CMEs and the background solar wind as well as their
internal changes during propagation. A proper synthesis of all the
observations from the solar surface to the inner heliosphere might
eventually answer this question. The front-side halos (Earth-directed
CMEs) are of importance to space weather because the severest of
geomagnetic storms are caused by such CMEs (see e.g., Gosling et al.,
1991). Considerable progress has been made in recent times, and it is
now possible to study the correspondence between Earth-directed CMEs and
solar wind disturbances as well as their geoeffects (Webb et al, 2000;
Gopalswamy et al., 2000d). Ironically, it is most difficult to detect
and measure the speeds of these CMEs. We need to develop methods to
correct for the projection effects so that the space speed of CMEs could
be obtained for space weather applications.

ACKNOWLEDGMENTS

This work was supported by NASA, Air Force Office of Scientific
Research, and NSF.

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