Geoeffective Solar Phenomena
[10] The solar studies of the last decade have left no doubt that
geoeffective phenomena on the Sun that completely determine the
state of near-Earth space are extremely large flare events and
coronal holes. The class of flare phenomena includes:
- solar flares with all the spectrum of dynamic manifestations
of the motion of material and radiation in all ranges of the
electromagnetic spectrum;
- solar filament eruptions with all accompanying phenomena.
[11] Solar flares take place in active regions with sunspot groups and
without them; they represent a response of the solar atmosphere
to a fast energy release, which results in a sharp local heating
of all layers of the solar atmosphere accompanied by generation of
powerful radiation in a broad band of the electromagnetic
spectrum, from
g-ray quanta to kilometer radio waves as
well as by flows of electrons, protons, and heavy nuclei. Solar
filaments are clouds of dense and cooler (than the surrounding
medium) plasma, which take in the magnetic field of the solar
atmosphere the form of structures extended along the polarity
reversal line. The mean sizes of solar filaments: length is 50
megameters (Mm), width is several megameters, and height above
the visible surface of the Sun (photosphere) is 10 Mm. Solar
filament eruptions observed with a high resolution have an
appearance of a two-ribbon flare with a slowly growing intensity
before the maximum ( > 1 hour) and a long timescale of the
intensity decline ( > 3 hours). In the great majority of cases the
phenomenon takes place outside active regions, in weak magnetic
fields ( < 50 G)
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Figure 3
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[12] Flare phenomena (Figure 3) comprise the entire set of dynamic phenomena on
the Sun; they are a consequence of the interaction of a new
emerging magnetic flux with the already present magnetic fields.
This definition includes all the spectrum of brightenings,
flares, and solar filament eruptions, whose development follows
the same scenario. The study [Ishkov, 1998] of the entire set of
flare phenomena has made possible a classification of flare
phenomena depending on the intensity of the magnetic field in
which they take place (Table 2).
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Figure 4
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[13] In the great majority of cases, solar filament eruptions take
place outside active regions, in weak magnetic fields
( < 50 G) (see Figure 4). "Spotless'' flares are phenomena that take place
in spotless active regions with a magnetic field of
< 500 G. They are
characterized by a slow growth toward the maximum and by very weak
hard X-ray radiation; the microwave burst is always less than
300 sfu, and the X-ray importance is usually
M7; in the
optical range their importance can reach the highest values, 4N.
Long-lived flares in sunspot groups differ by maximum energy
output in all ranges. Flares arising in the strongest magnetic
fields ( > 2000 G) are always impulsive ones; they are well
visible in hard X rays and
g rays up to the highest
energies; however, their X-ray importance does not reach M5, and
the optical importance does not exceed 1B.
[14] Using the observations of emerging magnetic fluxes (EMF), we can
summarize phenomena after which the flare activity increases
[Ishkov, 1998]:
- any appearance of a new EMF results in an flare activity
increase;
- for the implementation of large solar flares it is necessary
that the new EMF be rather large ( >1013 Wb) and the rate of
its emergence be not less than
109 Wb/s;
- large solar flares occur one-two days after the detection of
EMF within an active region;
- large- and medium-importance flares in active regions are
clustered in series, trains Obashev et al., 1973; Ishkov, 1998],
which take place in a restricted time interval.
[15] The time interval in which the major part of large- and
medium-importance flares is implemented in an active region is
called period of flare energy release. Depending on the degree of
development of an active region, characteristics of its magnetic
field, and power of the new EMF, this period can take from 16 to
80 hours, on the average 5530 hours, or 16% of the transit
time of an active region across the visible solar disk [Ishkov, 1998].
These tags allow us to predict the period of flare energy
release one-two days before the occurrence of large geoeffective
solar flares.
[16] The prediction of solar filament eruptions is more difficult: only
direct observations of an EMF in weak magnetic fields can give a
possibility to predict them. EMF interacts with the background
magnetic field and can emerge only to regions of the neutral line
of the axial magnetic-field component.
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Figure 5
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[17] Coronal holes are regions in the solar atmosphere with a magnetic
field open to the interplanetary space; from them, solar plasma
freely outflows to the heliosphere, forming high-speed flows of
the solar wind (see Figure 5). A large coronal hole usually exists during 4-8
solar revolutions virtually without changing its position.
However, its visible boundaries may displace by up to
20/day, changing its sizes or moving it as a whole.
Low-latitude coronal holes are considered as sources of recurrent
magnetic storms and of flows of energetic ( > 2 MeV) electrons.
Geoeffective coronal holes are those found in a heliolatitude
interval of N25-S25, having a heliolatitude extension of
10, area of
5000 msh [Joselyn, 1986], and
localized in enhanced background magnetic fields. The effect on
the near-Earth space appears when the western boundary of the
coronal hole reaches a heliolongitude of ~W40, ~3 days
after the transit of the western boundary of the coronal hole
across the central meridian of the Sun [Watari, 1990;
Solodyna et al., 1977]. We should
especially emphasize the role of a coronal hole as an enhancer of
the geoefficiency of solar flare phenomena. The presence of a
coronal hole near active regions where a solar flare event takes
place sharply increases their geoefficiency and expands the range
of their localization. An example is the event of April 14, 1994,
when an eruption of a high-latitude (S50) filament located
beneath a large coronal hole resulted in a complete modification
of magnetic structures in the southern hemisphere of the Sun
[McAllister et al., 1995] and in a major magnetic storm of 17 April 1994.
[18] The electromagnetic disturbances from flare events appear
virtually at the instant of the process development;
corpuscular and plasma disturbances from solar geoeffective
phenomena (flare events, coronal holes) propagating in the
heliosphere, through the solar wind, affect the magnetospheres of
planets, their satellites, and comets, causing considerable
deviations from the background quiet state in nearly all layers
of the considered objects. The agents causing this disturbance
are:
- transient structures, coronal mass ejections, which are a
consequence of active processes in flares and filament eruptions;
their direct observations enable us to specify the direction
of a disturbance motion in the interplanetary space and the
possibility of its incoming to near-Earth space;
- high-velocity flows of solar plasma following a shock wave from
powerful flare events or outflowing from regions with an opened
magnetic configuration (coronal holes).
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