Solar geoeffective phenomena: Impact on the near-Earth space...

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)

Figure 3

[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).

Figure 4

[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 leq

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.

Figure 5

[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 geq

10, area of geq

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).