CMES AND THE SOLAR CYCLE VARIATION IN THEIR GEOEFFECTIVENESS
David F. Webb
Institute for Scientific Research, Boston College
140 Commonwealth Ave., Chestnut Hill, MA 02467-3862 U.S.A.
Also at: AFRL/VSBXS, Space Vehicles Directorate, Hanscom AFB, MA U.S.A..
Tel: 1-781-377-3086 / fax: 1-781-377-3160
e-mail: [email protected]
1
ABSTRACT
Coronal mass ejections (CMEs) are an important factor in
coronal and interplanetary dynamics. They can inject
large amounts of mass and magnetic fields into the
heliosphere, causing major geomagnetic storms and
interplanetary shocks, a key source of solar energetic
particles. Recent studies using the excellent data sets from
the SOHO, Yohkoh, TRACE, Wind, ACE and other
spacecraft and ground-based instruments have improved
our knowledge of the origins and early development of
CMEs at the Sun and how they affect space weather. I
review some key coronal properties of CMEs, their source
regions, their manifestations in the solar wind, and their
geoeffectiveness. Halo CMEs are of special interest for
space weather because they suggest the launch of a
geoeffective disturbance toward Earth. However, their
correspondence to geomagnetic storms varies over the
solar cycle. Although CMEs are involved with the largest
storms at all phases of the cycle, recurrent features such
as interaction regions and high speed wind streams are
also geoeffective.
INTRODUCTION
CMEs consist of large structures containing plasma and
magnetic fields that are expelled from the Sun into the
heliosphere. Most of the ejected material comes from the
low corona, although cooler, denser material probably of
chromospheric origin can also be ejected. Much of the
plasma observed in a CME is entrained on expanding
magnetic field lines, which can have the form of helical
field lines with changing pitch angles, i.e., a flux rope.
This paper reviews the well-determined coronal
properties of CMEs and what we know about their source
regions, and some key signatures of CMEs in the solar
wind. I emphasize the recent observations of halo CMEs,
which appear as expanding, circular brightenings that
completely surround the occulting disk of the SOHO
LASCO coronagraphs, and are important for space
weather studies.
Most observations of CMEs have been made by white
light coronagraphs, operating in space. These
observations are based on the Thomson scattering
process, wherein dense plasma structures in the corona
become visible via photospheric light scattered off of the
free electrons in the plasma. The LASCO coronagraphs
(Brueckner et al., 1995) provide the most recent white
light observations on CMEs. These data have been
acquired since early 1996, providing fairly continuous
coverage of the corona and CMEs from the solar activity
minimum through the maximum of cycle 23. The LASCO
2
observations of CMEs are complemented by other SOHO
instruments operating at coronal wavelengths, especially
EIT, UVCS and CDS, and the Yohkoh and TRACE
spacecraft. Hudson and Cliver (2001) provide a recent
summary of non-white light observations of CMEs. The
geoeffective aspects of all these CME observations are
the focus of this paper.
In the next section I review our understanding of how
solar and geomagnetic activity varies over the solar cycle.
In section 3 I discuss the basic properties of CMEs and
what we know about their source regions. In Section 4 I
review the manifestations of CMEs in the heliosphere and
near Earth, their relationship to recurrent solar wind
structures, and what structures are most geoeffective. The
last section summarizes the important points.
SOLAR AND GEOMAGNETIC ACTIVITY OVER
THE SOLAR CYCLE
It well known that the level of geomagnetic activity tends
to follow the solar sunspot cycle. Sunspot counts are a
useful measure of the general level of magnetic activity
emerging through the photosphere of the Sun, and they
rise and fall relatively uniformly over the cycle. Other
major classes of solar activity also tend to track the
sunspot number during the cycle, including active
regions, flares, filaments and their eruptions, and CMEs
(e.g., Webb and Howard, 1994). This activity is
transmitted to Earth through the solar corona and its
expansion into the heliosphere as the solar wind.
The cyclical variation of the solar magnetic field can be
summarized as follows. The sunspot number indicates the
scale of the emergence through the solar surface of
regions of the strongest flux. The photospheric flux is
indicative of the strength of the toroidal flux which in a
Babcock-type dynamo model of solar activity is built up
from the basic solar dipole field over the solar cycle. The
total photospheric flux varies by about a factor of three
over the cycle. A useful technique for estimating the
interplanetary flux is to calculate the flux through the
source surface above which the field is assumed to be
radial (e.g., Luhmann et al., 1998). The source surface
flux tracks the lower order, dipole contribution of the
solar field over the cycle and is indicative of the variation
of the total open flux from the Sun. Although the total
interplanetary magnetic field (IMF) flux at 1 AU is
stronger at maximum and weaker near minimum than the
total open flux (e.g., Wang et al., 2000), it is still strong
during the declining phase when it is dominated by the
open flux in high speed wind streams from coronal holes.
Since solar activity is transmitted to Earth via the solar
wind, how closely does the cycle of geoactivity follow
that of solar activity? The overall correspondence
between the sunspot and geoactivity cycles can be seen in
long-term plots comparing various indices of the two
kinds of activity. On annual time scales the geoactivity
cycle has more structure than the solar cycle. Figure 1
shows the annual number of disturbed days with the
geomagnetic index Ap > 50 vs sunspot number over the
last 6 solar cycles. Ap is more variable than sunspot
number, but does tend to track the sunspot cycles in
amplitude.
Figure 1 also illustrates the double-peaked nature of the
geoactivity cycle. In general, geomagnetic activity
exhibits a peak near sunspot maximum and another
during the declining phase of the cycle. These peaks vary
in amplitude and timing, and the peak around maximum
may itself consist of two peaks (Richardson et al., 2000).
The two main peaks are usually considered to represent
the maximum phases of two components of geoactivity
that have different solar and heliospheric sources. The
first peak is associated with transient solar activity, i.e.,
CMEs, that tracks the solar cycle in amplitude and phase.
The later peak is attributed to recurrent high speed
streams from coronal holes, and is often higher than the
early peak.
Richardson et al. (2000; 2001) recently studied the
relative contributions of different types of solar wind
structures to the aa index from 1972-1986. They
identified CME-related flows, corotating high-speed
streams, and slow flows near the Earth, finding that each
type contributed significantly to aa at all phases of the
cycle. For example, CMEs contribute ~50% of aa at solar
maximum and ~10% outside of maximum, and high
speed streams contribute ~70% outside of maximum and
~30% at maximum. Thus, both types of sources, CMEs
and coronal holes/high speed streams, contribute to
geoactivity all phases of the cycle.
most CMEs traveling within the heliosphere. Strong
southward fields often occur either in magnetic clouds or
in the preceding post-shock regions, or both. Slower
CMEs not driving shocks are probably associated with
many smaller storms. Zhao et al. (1993) found that 78%
of all periods with southward IMF -10 nT for durations
3 hr were associated with one or more CME signatures.
Compression and draping of fields in the preceding
ambient solar wind can also enhance southward IMF
(Gosling and McComas, 1987).
SOLAR CHARACTERISTICS OF CMES
Properties of CMEs
The measured properties of CMEs include their
occurrence rates, locations relative to the solar disk,
angular widths and speeds (e.g., Kahler, 1992; Webb,
2000; St. Cyr et al., 2000). There is a large range in the
basic properties of CMEs. Their speeds, accelerations,
masses and energies extend over 2-3 orders of magnitude,
and their widths exceed by factors of 3-10 the sizes of
flaring active regions.
CMEs, however, are responsible for the most geoeffective
solar wind disturbances and, therefore, the largest storms.
Enhanced solar wind speeds and southward magnetic
fields associated with interplanetary shocks and ejecta are
known to be important causes of storms (e.g., Tsurutani et
al., 1988; Gosling et al., 1991). The reason that any
CME/magnetic cloud encountering Earth is likely to be
geoeffective is that enhanced speeds and, particularly,
sustained southward IMF will occur within or ahead of
CMEs can exhibit a variety of forms, some having the
classical “three-part” structure and others being more
complex with interiors with bright emitting material. The
basic structure of the former kind consists of a bright
leading arc followed by a darker, low-density cavity and a
bright core of denser material (Figure 2). These may
represent pre-event structures which erupt: a prominence
and its overlying coronal cavity, and the ambient corona
(e.g., streamer) which is compressed as the system rises.
Partly because of their increased sensitivity, field of view
and dynamic range, the SOHO LASCO C2 and C3
coronagraphs have observed many different forms of
CMEs, including those with large circular regions
resembling flux ropes and halo CMEs.
Figure 1: Annual number of geomagnetic disturbed days with
Ap index > 50, (dashed line and hatched area) vs. annual
sunspot number (solid line) for solar cycles 17-23. Courtesy
NOAA National Geophysical Data Center, Boulder, CO.
Figure 2: Evolution of a “3-part” CME on 2 June 1998. The
three features are: 1) bright curved leading edge, followed by 2)
darker region, and 3) bright, interior structure, here a
prominence. Note circular structures just above the prominence,
suggesting a flux rope. From Plunkett et al. (2000).
Halo CMEs appear as expanding, circular brightenings
3
that completely surround the coronagraph occulting disk
(Howard et al., 1982). Their observation suggests that
these CMEs are moving outward either toward or away
from the Earth and are detected after expanding to a size
exceeding the diameter of the coronagraph's occulter
(Figure 3). Other observations of associated activity on
the solar disk are necessary to distinguish whether a halo
CME was launched from the frontside or backside of the
Sun. Other CMEs which have a larger apparent angular
size than typical limb CMEs, but do not appear as
complete halos, are called `partial halo' CMEs.
Halo CMEs are important to study for three reasons: 1)
They are known to be the key link between solar
eruptions and many space weather phenomena such as
major storms and solar energetic particle events; 2) The
source regions of frontside halo CMEs are usually located
within a few tens of degrees of Sun center, as viewed
from Earth (Webb et al., 2001a; Cane et al., 2000). Thus,
the source regions of halo events can be studied in greater
detail than for most CMEs which are observed near the
limb: 3) Frontside halo CMEs must travel approximately
along the Sun-Earth line, so their internal material can be
sampled in situ by spacecraft near the Earth. Three
spacecraft, SOHO, Wind and ACE, now provide solar
wind measurements upstream of Earth.
The frequency of occurrence of CMEs observed in white
light tends to track the solar cycle in both phase and
amplitude, which varies by an order of magnitude over
the cycle (Webb and Howard, 1994). LASCO has now
Figure 3: Illustrations of 2 kinds of halo CMEs: (a)
Symmetrical, gradual CME forming a complete ring around the
C2 occulter on 12 May 1997. Associated with a C1 flare,
filament eruption and EUV wave; (b) Asymmetrical, impulsive
halo CME on 17 February 2000. The CME was fast and
associated with an M1/2N flare. These are difference images of
4
consecutive C2 images.
observed from solar minimum in early 1996 through the
rise phase and maximum of the current (23rd) solar cycle.
It has detected CMEs at a rate slightly higher than the
earlier observations, reaching >4/day at maximum (St Cyr
et al., 2000; C. St. Cyr., priv. comm., 2001). Halo CMEs
occurred during the rise phase of this cycle at a rate of 16/month, about 10% the rate of all CMEs. Full halo
CMEs were only detected at a rate of ~4% of all CMEs. If
CMEs occurred randomly at all longitudes and LASCO
detected all of them, this rate would be about 15%. This
suggests that LASCO sees as halos CMEs that are
brighter (i.e., denser) than average.
The latitude distribution of the central position angles of
CMEs tends to cluster about the equator at minimum but
broaden over all latitudes near solar maximum. This
remains true in the LASCO data. Hundhausen (1993)
noted that this CME latitude variation more closely
parallels that of streamers and prominences than of active
regions, flares or sunspots. On the contrary, the angular
size distribution of CMEs varies little over the cycle,
maintaining an average width of about 45 o (Hundhausen,
1993; Howard et al., 1985). The CME size distribution
observed by LASCO is affected by its increased detection
of very wide CMEs, especially halos (St. Cyr et al.,
2000). However, although the average width of LASCO
CMEs is 72o, the median of 50o is similar to that of the
earlier measurements.
Estimates of the apparent speeds of the leading edges of
CMEs range from about 20 to over 2000 km/s. Thus,
these speeds range from well below the sound speed in
the lower corona to well above the Alfven speed. The
annual average speeds of SOLWIND and SMM CMEs
varied over the solar cycle from about 150-475 km/s, but
their relationship to sunspot number was unclear (Howard
et al., 1986; Hundhausen et al., 1994a). The LASCO
CME speed distributions are similar in range to the prior
measurements, but do show a tendency to increase with
sunspot number in this cycle (St. Cyr et al., 2000; N.
Gopalswamy, priv. comm., 2001). The average CME
speeds have remained at their highest from 1999-2001.
The annual average speed of full halo CMEs is 1.5-2
times greater than that of all CMEs, suggesting that
LASCO sees as halos CMEs which are faster and, hence,
more energetic than the average CME.
MacQueen and Fisher (1983) first described two
kinematical classes of CMEs based on their associated
surface events: slow, gradually accelerating CMEs
associated with prominence eruptions, and fast CMEs
with constant speeds associated with flares. Above a
height of about 2RS the speeds of typical CMEs are
relatively constant. Despite its increased field of view,
only 17% of all LASCO CMEs exhibit acceleration out to
30 RS (St. Cyr et al., 2000). Clearly the acceleration for
most CMEs occurs low in the corona (St. Cyr et al.,
1999), especially the fastest ones (Zhang et al., 2001).
Using LASCO data, Sheeley et al. (1999) and Srivastava
et al. (1999) find that gradually accelerating CMEs are
balloon-like with central cores which accelerate more
slowly than the leading edge, whereas fast CMEs move at
constant speed even as far out as 30RS. However, when
viewed well out of the skyplane, gradual CMEs look like
smooth halos which accelerate to a limiting value then
fade away, while fast CMEs have ragged structure and
decelerate (Sheeley et al., 1999). Examples of these
classes of halo events are shown in Figure 3.
Finally, the masses and energies of CMEs require
difficult instrument calibrations and have large
uncertainties. The average excess masses of CMEs
derived from the older coronagraph data were a few x
1015 g (Howard et al., 1985; Hundhausen et al., 1994b).
However, more recent studies using Helios (Webb et al.,
1996) and SOHO LASCO (Howard et al., 1997;
Vourilidas et al., 2000) data indicate that CME masses
have been underestimated, probably because mass
outflow can continue well after the CME's leading edge
leaves the instrument field of view. Average CME kinetic
energies from the older data were a few x 10 31 erg,
likewise underestimated.
Solar Source Regions of CMEs
Here I briefly summarize our knowledge of the nearsurface features which appear to be involved in the
initiation of CMEs. See recent reviews for more details,
including Hundhausen (1999), Forbes (2000), Webb
(2000) and Cliver and Hudson (2002). Many CMEs
appear to arise from large-scale, closed structures,
particularly preexisting coronal streamers (e.g.,
Hundhausen, 1993). Low and Hundhausen (1995)
emphasized that the prominence/streamer involves a dual
flux system, one part of which is a flux rope with its
associated coronal cavity, which may help to drive the
eruption once the streamer is disrupted. Many energetic
CMEs actually involve the disruption (“blowout”) of a
preexisting streamer, which can increase in brightness and
size for days before erupting as a CME (Figure 1; Howard
et al., 1985; Hundhausen, 1993; Subramanian et al.,
1999). The streamer usually disappears afterwards, but
may eventually reform.
5
Previous statistical association studies indicated that
erupting filaments and X-ray events, especially of long
duration, were the most common near-surface activity
associated with CMEs (e.g., Webb, 1992). Most optical
flares occur independently of CMEs and even those
accompanying CMEs may be a consequence rather than a
cause of CMEs (Gosling, 1993; but see Hudson et al.,
1995). The fastest, most energetic CMEs, however, are
usually also associated with surface flares, and my studies
show that reported flares are associated with most (~85%)
frontside, full halo CMEs. This rate may be higher than
reported previously because the sources of halo CMEs
can be clearly viewed near Sun center.
Comparisons of soft X-ray data with the white light
observations have provided many insights into the source
regions of CMEs. Sheeley et al. (1983) showed that the
probability of associating a CME with a soft X-ray flare
increased linearly with the flare duration, reaching 100%
for flare events of duration >6 hours. The SMM CME
observations indicated that the estimated departure time
of flare-associated CMEs typically preceded the flare
onsets. Harrison (1986) found that such CMEs were
initiated during weaker soft X-ray bursts that preceded
any subsequent main flare by tens of minutes, and that the
main flares were often offset to one side of the CME (c.f.,
Webb, 1992). This offset is also shown by Plunkett et al.
(2002) for LASCO CMEs. The latitude distribution of the
CMEs peaks at the equator, but the distribution of EUV
activity observed by the EIT associated with these CMEs
is bimodal with peaks 30o north and south of the equator.
This offset is confirmed for the distribution of sources
associated with halo CMEs (Figure 4; Webb et al.,
2001a).
This pattern indicates that many CMEs can involve more
complex, multiple polarity systems (Webb et al., 1997)
such as modeled by Antiochos et al. (1999). This
multipolar pattern was evident in the LASCO
observations during the recent solar minimum in 19961997, in which the simple, bipolar streamer belt observed
in the outer coronagraphs overlay twin arcades flanking
the equator as observed by the inner coronagraph
(Schwenn et al., 1997). The eruption of such a streamer as
a CME would imply that it originated in a quadrupolar
magnetic region.
The most obvious X-ray signatures of CMEs in the low
corona are the arcades of bright loops which develop after
the CME material has apparently left the surface (Kahler,
1977-Skylab; Hudson and Webb, 1997-Yohkoh). Prior to
the eruption, an S-shaped structure, called a sigmoid by
Rust and Kumar (1996) often develops in association with
a filament activation. A sigmoid is indicative of a highly
sheared, non-potential coronal magnetic field. The
sigmoid structure denotes the dominant helicity in a given
Figure 4: Histogram of the great circle distances between the
surface source activity (S) associated with each full halo CME
and the sub-Earth point (C). The average of this distribution is
35o. The average is 28o when limb sources are excluded.
hemisphere, being S-shaped or right-handed in the south
and reverse-S or left-handed in the north. This type of
structure might be an important precursor of a CME (e.g.,
Canfield et al. 1999), although its geoeffectiveness has
The frequent detection of EUV waves is probably the
most exciting new aspect of the SOHO EIT observations.
The waves are most clearly seen in movies of differences
of consecutive pairs of full-disk 195 images, and appear
as expanding wave fronts moving outward from flaring
active regions. The EIT has now observed many waves in
association with CMEs, but only about half of the halo
CMEs are associated with EIT waves (Webb et al.,
2001a). Thompson et al. (1999) interpreted them as fast
mode MHD waves, similar to the Moreton waves
observed in chromospheric lines.
Distance between Sub-Earth Point and CME Source
14
12
X
Number
10
S
C
8
6
4
2
CMES IN THE HELIOSPHERE AND THEIR
GEOEFFECTIVENESS
0
0
20
40
60
80
Great Circle Distance (deg.)
100
coronal waves that propagate outward from the CME
source region, and filament eruptions. Optical flares and
filament eruptions can also be identified in H and radio
images. Specifically, Webb et al. (2001a) find that 2/3 of
the halo CMEs are associated with filament eruptions and
with dimmings. Metric radio bursts are also often
associated with the CMEs.
120
not been established. Eventually an eruptive flare can
occur, resulting in the bright, long-duration arcade of
loops. Sterling et al. (2000) call this process “sigmoid-toarcade” evolution. These arcades suggest the eruption and
subsequent reconnection of strong magnetic field lines
associated with the CME system (c.f., Svestka and Cliver,
1992).
Dimming regions observed in X-rays and in the EUV
imply that material is evacuated from the low corona (c.f.,
Hudson and Webb, 1997). The dimming regions can be
much more extensive than the flaring activity and often
map out the apparent base of the white light CME
(Thompson et al., 2000). Thus, the dimming events
appear to be one of the earliest and best-defined
signatures in soft X-ray and EUV emission of the actual
mass ejected from the low corona. Gopalswamy et al.
(1999; 2000) has reported two observations of large-scale
coronal enhancements, not dimmings, that could be the
near-surface material that appears later in the white light
CME.
Surveys of solar activity associated with frontside halo
CMEs have been made primarily with images of the low
corona from the SOHO EIT and Yohkoh SXT
instruments. Hudson et al. (1998) and Webb et al. (2000a)
examined halo CMEs observed during the first half of
1997, finding that about half of the halo events were
associated with frontside activity as expected. Studies of
CME source regions using EIT and LASCO data have
been reported by Thompson et al. (1999a,b; 2000) and
Plunkett et al. (2000). The activity associated with halo
CMEs includes the formation of dimming regions, of
long-lived loop arcades, flaring active regions, large-scale
6
Solar Wind Signatures of CMEs
CMEs carry into the heliosphere large amounts of coronal
magnetic fields and plasma, which can be detected by
remote sensing and in-situ spacecraft observations. The
passage of this material past a single spacecraft is marked
by certain distinctive signatures, but with a great degree
of variation from event to event (e.g., Gosling, 1993).
These signatures include transient interplanetary shocks,
depressed proton temperatures, cosmic ray depressions,
flows with enhanced helium abundances, unusual
compositions of ions and elements, and magnetic field
structures consistent with looplike topologies.
A widely used single-parameter signature of ejecta is the
occurrence of counterstreaming suprathermal electrons.
Since suprathermal electrons carry electron heat flux
away from the Sun along magnetic field lines, when
found streaming in both directions along the field they are
interpreted as signatures of closed field lines and, thus, as
a good proxy for CMEs in the solar wind (e.g., Gosling,
1993). An important multiple signature of an
interplanetary (I)CME, is a magnetic cloud (e.g., Burlaga,
1991), defined as several-hour flows with large-scale
rotations of unusually strong magnetic field accompanied
by low ion temperatures. The magnetic field data from
clouds often provide good fits to flux rope models, e,g,,
Marubashi (1986, 1997); Lepping et al. (1990); Russell
(2001).
Another class of ejecta plasma signatures are the
abundances and charge state compositions of elements
and ions which are systematically different in ICMEs
compared with other kinds of solar wind (Galvin, 1997).
As the corona expands outward, the electron density
decreases so rapidly that the plasma becomes collisionless
and the relative ionization states become constant, thus
reflecting the conditions of origin in the corona. The
charge states of minor ions (Z>2) in CME flows usually
suggest slightly hotter than normal coronal conditions
(i.e., >2 MK) at this “freezing in” location. In addition,
transient flows often exhibit element and ion abundances
that are enhanced relative to the typical solar wind.
Unusually low ionization states of He and minor ions
have also been detected in CME flows. Although rarely
observed before, enhanced He+ flows have been detected
in the flows from several recent halo CMEs with more
sensitive instruments on the SOHO, Wind and ACE
spacecraft. In each of these events an erupting filamenthalo CME could be associated with either a dense and
compact `plug' or an extended flow of cool plasma in the
trailing edge of a magnetic cloud. This is likely material
from the filament itself, consistent with near-Sun
observations showing that erupting filaments lag well
behind the leading edge of their associated CMEs. In a
recent study of Wind magnetic clouds, Lepping and
Berdichevsky (2000) find that half show a significant
increase in density toward the rear of the cloud. In a
preliminary study from 1995-2000, I have found evidence
of either dense or cool material within ~60 ICMEs, most
associated with halo CMEs.
The Geoeffectiveness of CMEs
Since the launch of SOHO, halo CMEs are being used to
study the influence of Earth-directed CMEs on
geoactivity. Analyses of the relation between halo CMEs
and geomagnetic storms have been carried out by
Brueckner et al. (1998), Webb et al. (2000), Cane et al.
(2000) and St. Cyr et al. (2000) and indicate a high degree
of correlation near solar minimum and a decreased
association near the current maximum. Webb et al. (2000)
studied the geoeffectiveness of halo CMEs in early 1997
by comparing the onset times of the frontside halo events
with storms at Earth 2-5 days later. Defining storms as
having a peak depression in Dst  -50nT, they found that
all 6 frontside halos with surface sources within 0.5 RS of
Sun center were associated with magnetic clouds, or
cloud-like structures at 1 AU, and with moderate-level
storms. Brueckner et al. (1998) also found a strong
relationship between storms and LASCO CMEs from
1996 to mid-1997, and Cane et al. (1998; 1999) found
good associations between frontside halo CMEs and
ejecta signatures at 1 AU. St. Cyr et al. (2000) found
similar, but weaker associations between halo CMEs and
storms. They studied LASCO halo CMEs from 1996-June
1998, and concluded that 83% (15 of 18) of intense
storms, were preceded by frontside halo CMEs. However,
25 of the frontside halo CMEs did not produce such large
storms and were, therefore, false alarms.
All these studies included partial halo CMEs. LASCO has
now observed a sufficient number of CMEs to permit
statistical studies of only full (360o) halo events, those
most likely to be directed along the Sun-Earth line. In a
7
study of 89 frontside full halos observed from 1996-2000,
Webb et al., (2001a) found that about 70% of the halos
were associated with shocks and/or counterstreaming
electrons or other ejecta signatures at 1AU. Magnetic
clouds or cloud-like structures were involved with ~60%
of the halos, although this rate rises to ~70% if the
peculiar year 1999 is excluded. Somewhat surprisingly,
when averaged over this entire period, only half of the
frontside halo CMEs could be well associated with
moderate-level or greater storms. The average travel time
from the onsets of the halo CMEs to the onsets of the
storms at Earth was 3.3 days.
Table 1 shows how the association degree between halo
CMEs and storms, the associated storm peaks, and the
travel times vary over the solar cycle from activity
minimum to maximum. Also shown is whether the
parameter increased or decreased and by what factor. We
see that the average CME rate and speeds increased with
the cycle and the travel time decreased, as expected for
more energetic events. However, although the storm
intensities steadily increased, the degree of association
between the halo CMEs and storms decreased
monotonically to yield the overall 50% average.
TABLE 1 Variation of Halo CMEs with Storms
1997
92
1998
54
1999
39
2000
35
Assoc. Storm Peak (-Dst)
80
96
129
149
Travel Time (days)
3.7
3.45
3.2
3.0
Halos with Storms (%)
Solar Cycle Variation (Minimum to Maximum)
Halo CME Rate
Inc.
Halo CME Speeds
Inc.
Halos & Storms
Dec.
Assoc. Storm Intensity
Inc.
Travel Time
Dec.
x10
x1.5
x3
x2
x1.2
However, confirming previous studies, I find that most
(~70%; 15 of 21) of the most intense storms (Dst 150nT) of this cycle were associated with one or more
full and/or partial halo CMEs. (The other 6 had possible
associations.) For an intense magnetic storm to occur,
there must be a sustained period of strong southward IMF
to provide an efficient transfer of energy and momentum
from the solar wind to the magnetosphere. For example,
in the relatively simple 1997 events on January 6-10 and
May 12-15, the magnetosphere's response to the magnetic
cloud in each event closely followed the southward field.
Dst rose in response to the shock compression, then
decreased in proportional response to the southward IMF
in both the compressed region before the cloud and in the
front part of the cloud. Thus, a key reason why the halo
CME studies have found that the observation of a
frontside halo CME is not a sufficient condition for a
large storm may be that not all CMEs contain or create
sufficiently strong, sustained southward field.
The January 1997 event also is a nice example of the
geoeffectiveness of a halo CME having only weak
observable solar surface manifestations (e.g., Webb et al.,
1998). Despite its occurrence near the winter solstice (see
next section), the CME caused a large magnetic storm
leading to the demise of the Telstar 4 satellite. Without
observation of the halo CME, the appearance of the
magnetic cloud and storm at Earth would not have been
predicted, and the storm would have become another
“problem” storm, one with no identifiable solar source.
+
N
Left H.
Right H. flux rope
axes
and
outer coils
S_
probably still apply.
Thus, we can conclude that halo CMEs with associated
surface features, especially if near Sun center, usually
presage at least moderate geomagnetic storms, but that at
least half as many such storms occur without halo-CME
forewarning. Therefore, the simple observation of the
occurrence of a probable frontside halolike CME has
already significantly increased our ability to forecast the
occurrence of storms of moderate or greater levels at
Earth. Moderate storms not associated with CMEs are
usually caused by Earth passage through the heliospheric
current sheet (HCS) and related corotating interaction
regions (CIRs).
CMES IN THE CONTEXT OF THE GLOBAL SOLAR
AND HELIOSPHERIC MAGNETIC FIELD
Separating the opposite-polarity, polar coronal holes in
Figure 5 is a wide band forming the base of the streamer
belt. Since CMEs can arise from and disrupt streamers,
they are intimately tied to the streamer belt. The heavy
curve is the projection of the HCS onto the solar surface
as the heliomagnetic equator. Dipolar field lines (the N–S
curved arrows) form an arcade from whose apex arises
the current sheet. The high speed wind flowing from the
polar holes constricts the oppositely-directed fields to a
narrow current sheet which appears like a ballerina's skirt.
The rotating, warped HCS appears as a sector boundary
crossing at Earth; during one solar rotation Earth will be
immersed in two large sectors of opposite polarity, each
with the relatively high speed wind of its parent hole.
We now have a new understanding of how the magnetic
topology of the Sun, CMEs, and the heliosphere are
interrelated (see Crooker, 2000, and Webb et al., 2001b,
for recent discussions). Globally the slow solar wind is
confined to the vicinity of the heliospheric extension of
the streamer belt, which is the locus of the heliospheric
current sheet at the Sun. This belt now appears to be the
source of most CMEs. Thus, CMEs tend to carry with
them the imprint of the lower harmonics of the Sun's
magnetic field and their magnetic structure tends to merge
with that of the heliosphere. This paradigm has important
implications for understanding the geoeffectiveness of
CMEs.
The short straight arrows mark the locations of two flux
ropes under the helmet streamer belt associated with
CMEs. The arrows represent the direction and orientation
of the projected axes of the flux ropes, roughly parallel to
the heliomagnetic equator. The directions of the two
arrows determine the chirality (sense of twist or helicity)
of the resulting flux ropes, which tend to be left-handed,
‘L.H.’, (right-handed, ‘R.H.’) in the northern, ‘N.’,
(southern, ‘S.’) hemisphere. These match a hemispherically-dependent magnetic field pattern associated
with filaments low in the corona (Martin et al., 1994).
Thus, Figure 5 shows how a CME and its flux rope
follows the Sun's magnetic topology.
Figure 5 is a simplified schematic view illustrating how
the solar magnetic topology fits into the larger
heliospheric context. This view is for the simplest solar
magnetic configuration at solar minimum, when the Sun's
magnetic field can be approximated as a dipole whose
axis is tilted slightly to the axis of rotation. At other
Marubashi (1986, 1997)), Rust (1994), Bothmer and Rust
(1997) and others associated sets of erupting filaments
with magnetic clouds. For the clouds that were fit by a
cylindrical flux rope model, the orientations of the rope
axes correlated with those of the filaments, and the cloud
helicities matched those predicted from the filament
chiralities. Zhao and Hoeksema (1997, 1998) found
correlations between the orientations of cloud axes fit
with flux ropes, the orientations of their filaments, and the
duration and maximum intensity of southward IMF in
those clouds.
Figure 5: Schematic diagram illustrating the imprint of the solar
magnetic field on two CME flux ropes forming under the
helmet streamer belt, one in the northern hemisphere (N) with
left-handed (L.H.) chirality, and one in the southern hemisphere
(S.), with right-handed (R.H.) chirality. After Crooker (2000).
phases of the cycle, the dipole tilt increases and higher
harmonics distort the field, but the basic arguments here
8
In our paradigm, the direction of the leading field in a
CME should have the same orientation as the solar dipole
field. Since the dipolar field changes polarity at every
solar maximum, the direction of the leading field in
CMEs and clouds should also change at maximum. This
was confirmed by Bothmer and Rust (1997), Bothmer and
Schwenn (1998) and Mulligan et al. (1998). Mulligan et
al. also found a solar cycle variation in the orientation of
cloud axes, with low inclinations dominating during solar
minimum and the ascending phase and high inclinations
during maximum and the declining phase. Thus, in our
example in Figure 5 for solar minimum, the flux
rope/CMEs would have leading southward and trailing
northward fields, whereas at maximum a flux rope with a
northward axis parallel to a highly inclined heliomagnetic
equator would have little southward field. These
relationships provide a predictive capability for the
strength and duration of southward IMF in a magnetic
cloud based on observations of the associated filament
pattern.
edge of a high-speed stream. Since the HCS acts as a
channel for CMEs which can effectively carry the sector
boundary (Crooker et al., 1998), the probability of
encountering a CME in the ecliptic plane increases near
sector boundaries (Crooker et al., 1993).
High density can also impact the strength of a storm. For
southward IMF, the magnetosphere responds to a change
in solar wind density by an increase in plasma sheet
density. Thomsen et al. (1998) noted that the January
1997 cloud would have been much more geoeffective had
the extremely high density at its trailing edge occurred
when the IMF was southward. In the solar wind an ICME
can be associated both with high-density features of solar
origin in the slow flow itself (i.e., filament material) and
of interplanetary origin, at compression regions ahead of
the CME and sometimes trailing the CME if being
overtaken by a CIR.
The recent solar and heliospheric spacecraft observations
of CMEs help to clarify why individual CMEs seem less
well associated with storms near solar maximum. The
occurrence rate of CMEs increases over minimum by an
order of magnitude, leading to multiple CMEs per day
over the Sun or even from a single region. (These can
confuse halo CME identifications.) Gopalswamy et al.
(2001; 2002) have found since early 1998 several tens of
LASCO CMEs wherein a faster CME overtakes a slower
one within 30 RS of the Sun, producing an interaction.
Reconnection or sandwiching of each CMEs’s field lines
are likely in such cases. The combination of sequential
eruptions of CMEs and their subsequent interactions can
produce complex ejecta at 1 AU. Such ejecta often consist
of high speed flows with shocks and other ICME
signatures, but poorly defined magnetic structures with
what Burlaga et al. (2001) call ‘tangled fields’.
So the interaction of a CME with the surrounding
heliospheric patterns can affect its geoeffectiveness over
the cycle. During the declining phase of the solar cycle.
the 27-day recurrence pattern imposed by the corotation
of coronal holes and their high-speed solar wind streams
dominates the solar wind. Then, the peak strength of
storms coincides with passage of the leading edge of a
high-speed stream, not with the subsequent high-speed
flows. This is due to compression of pre-existing
southward IMF at the leading edge of CIRs and their
associated sector boundaries.(e.g., Crooker and Cliver,
1994; Tsurutani et al., 1995).
CIRs separate regions of high and low speed flows,
identified with coronal holes and streamers, respectively,
at the Sun. Thus, the solar sources of the recurrent activity
include the boundaries between coronal holes and
streamers. Crooker and Cliver (1994) showed that the
largest, usually recurrent storms associated with highspeed streams are often due to CMEs associated with the
CIRs. Since CMEs arise from closed fields in the
streamer belt and the HCS base, they are part of the slow
flows which border fast flows from coronal holes. Since
the streamer belt and HCS is tilted with respect to the
ecliptic, spacecraft there, and the Earth, will observe a
CME in the slow flow immediately preceding the leading
9
The fast flow of a high-speed stream pushes into the slow
flow ahead and compresses it, creating the CIR. ICMEs in
the slow flow also can contain southward fields which can
be compressed in the CIR. The CIR immediately follows
the ICME so that southward IMF both in the ICME and in
the CIR contribute to storm strength. Fenrich and
Luhmann (1998) noted that the geoeffectiveness of CIR
compression of the trailing part of an ICME will have a
solar cycle variation; north-leading magnetic clouds that
are favorable for this effect occur between those solar
maxima, such as cycle 21 to 22, when the Sun's dipolar
field is opposite that shown in Figure 5.
In addition, the rate at which CMEs actually encounter
Earth near maximum is modified by their broadening
latitude distribution. Thus, although the CME rate is
considerably higher at maximum, proportionally fewer
CMEs are ejected near the ecliptic because of the highly
tilted streamer belt. Because the flux ropes associated
with this tilted belt tend to be north-south oriented, the
individual CMEs that do reach the ecliptic will contain
little southward field. Finally, the “background” solar
wind into which the CMEs are injected is itself much
more complex near maximum. This creates more frequent
and complicated interactions of ejecta with the existing
structure leading to distortions and compressions which
are difficult to simulate and predict (e.g., Odstrcil and
Pizzo, 1999).
The Seasonal Variation
Although not directly related to solar activity, another
periodic variation in geoactivity has important
consequences for geoeffectiveness. This is the semiannual
or seasonal variation characterized by a higher average
level of geoactivity at the equinoxes than at the solstices.
This effect is quite apparent in statistical analyses using
common indices of geoactivity, including Dst (Cliver and
Crooker, 1993; Cliver et al., 2000). Surprisingly a
pronounced seasonal effect is also seen in the occurrences
of intense storms (Crooker et al., 1992).
Historically, three hypotheses have been put forth to
explain the seasonal variation. These are the RussellMcPherron (1973) effect, the equinoctial hypothesis, and
the axial hypothesis (see Cliver et al., 2000). The RussellMcPherron (RM) mechanism is a projection effect which
depends upon season. The IMF, ordered in the Sun's
heliographic coordinate system, projects a southward
component in Earth's dipole-ordered coordinate system
when it points toward the Sun during spring and away
from the Sun during fall. In the equinoctial hypothesis
geoactivity is maximum when the angle between the SunEarth line (i.e., the solar wind flow direction) and Earth's
dipole axis is 90o, and less at other times. This effect
works by reducing the coupling efficiency of the
magnetosphere at the solstices. Finally, in the axial
hypothesis, geoactivity peaks when Earth is at its highest
heliographic latitudes, or best aligned with any radially
propagating transient ICME flows and recurrent highspeed streams from mid-latitude coronal holes.
Although all these mechanisms may play some role in the
seasonal variation, the RM effect has generally been the
accepted mechanism because of its apparent success in
explaining geoactivity data sets (e.g., Crooker and Cliver,
1994). However, Cliver et al. (2000) find that the
equinoctial hypothesis better accounts for most of the
semi-annual modulation, ~65%,whereas the other two
mechanisms provide only 15-20% each.
CONCLUSIONS
The distribution of geomagnetic disturbances over the
solar activity cycle has traditionally been considered to
have two peaks representing two major components of
geoactivity with different solar and heliospheric sources:
one associated with transient solar activity that peaks with
the sunspot cycle, and the other associated with recurrent
CIRs, CMEs and high speed streams from coronal holes
during the declining phase. In terms of space weather,
CMEs are the most important kind of transient activity
because they link the activity at the Sun and its
propagation through the heliosphere to the Earth.
However, we now know that both CMEs and CIRs-high
speed stream ensembles can be geoeffective at all phases
of the cycle. As we have shown, the coronal streamer belt
and its extension as the heliospheric current sheet appears
to play a major role in both kinds of geoactivity.
What makes solar and heliospheric disturbances
geoeffective, in the sense that they cause storms, can be
summarized as southward IMF and compression.
10
Southward IMF is important because it allows merging of
the IMF and Earth's magnetic field, which transfers solar
wind energy and mass into the magnetosphere.
Compression is important because it strengthens existing
southward IMF and, to a lesser extent, increases density.
CMEs, the most geoeffective structures, usually contain
long duration flows of southward IMF and fast CMEs
compress any southward IMF at their leading edges and
behind shocks created by the speed difference. In
addition, CMEs themselves can carry high-density
structures, such as solar filaments. High-speed streams are
geoeffective if they compress any southward IMF in
CIRs. This compression can be enhanced when CIRs
interact with CMEs erupting through the HCS.
The recent observations of CMEs provide new insight
into the problem of why halo CMEs seem less well
associated with storms around solar maximum. Although
the CME rate is much higher at maximum, because of the
highly inclined HCS there are fewer injected into the
ecliptic and their inclined flux ropes tend to contain little
southward field. CMEs near maximum do tend to be more
energetic and faster with shorter travel times in the
heliosphere, and to be associated with more intense
storms. However, much of this increased geoactivity is
due not just to single (halo) CMEs, but also to complex
ejecta caused by sequential and/or interacting CMEs, and
their interaction with more complicated “background”
solar wind structures. Future analyses of the rich data sets
now in hand will improve our understanding of the role
CMEs play in space weather throughout the solar cycle.
Finally, recent studies have demonstrated that CMEs
carry the imprint of the solar magnetic field out into the
heliosphere. These findings imply that many characteristics of ejecta from CMEs can be predicted, at least
statistically, from characteristics of the solar field and of
associated solar source features, such as filament
orientation and chirality and HCS orientation.
ACKNOWLEDGMENT
I thank the Organizing Committee of the SOHO 11
Symposium for inviting my presentation at the meeting. I
benefitted from data from the SOHO mission, which is an
international collaboration between NASA and ESA, and
also from the SOHO/ LASCO CME catalog, generated
and maintained by the Center for Solar Physics and Space
Weather, The Catholic University of America in
cooperation with the Naval Research Laboratory and
NASA. I am grateful to C. St. Cyr and N. Gopalswamy
for use of their preliminary results, and to E. Cliver for
helpful comments. This work was supported at Boston
College by Air Force contract AF19628-00-K-0073,
AFOSR grant AF49620-98-1-0062, and NASA grant
NAG5-10833.
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