28, 3 /REVIEWS OF GEOPHYSICS Gorney: SOLAR CYCLE EFFECTS ß 319
Figure 3. Polar view of interplaneta•
space in the ecliptic plane, showing the
propagation of solar X rays, solar ener-
getic particles, and the solar wind from the
Sun to the vicinity of Earth. The Sun's
rotational velocity, coupled with the out-
ward velocity of the solar wind, produces
the Archimedean spiral pattern of the in-
terplanetary magnetic field.
/
CORONA
CHROMOSPH ERE
URST OF' HIGH
ENERGY PARTICLES
- •
EARTH'S
ORBIT
EARTH
The plot; on a logarithmic scale, shows that the proton
fiuence can exhibit ordtr-of-magnitude variations from
year to year, even within a single cycle. This feature is
perhaps most evident for the data from cycle 22, in which
more than a factor of 100 variation in proton fiUence was
observed between the two years preceding sunspot
maximum. The dramatic increase in fiuence in cycle 22
was due primarily to the occurrence of one or two
extremely large events dur. ing 1989. Even with the large
year-to-year variations which are evident in Figure 4, the
data still demonstrate a tendency for higher fiuences to
occur within a few years of sunspot maximum. It is also
interesting to note that the cycles with the highest sunspot
numbers (e.g., cycles 19 and 22) also appear to have higher
levels of integrated proton fiuences.
Although over 200 solar proton events (with fluxes over
10 (cm 2 s sr) q) have occurred over the past -30 years,
only about 3(} of these events have had sufficient flux at
high energies to be classified as ground level events [Smart
and Shea, 1989]. Furthermore, the integrated proton
fiuence over the same 30-year period is dominated by only
a handful of events. The sporadic occurrence of these
events, coupled with the logarithmic flux distribution of
even,ts, leads to great difficulty in formulating estimates of
expected fiuence appropriate for short time intervals [see
Feynman et al., 1988; Stassinopoulos, 1975; Chenette and
Dietrich, 1984].
Some examples of the most severe proton events ever
observed are shown in Figure 5. The plots show integral
proton flux above 10 and 30 MeV and, where available,
above 100 MeV for three -1-week-long periods. Note that
two of the events are quite recent. These two were
associated with solar cycle 22, and they occurred within 1
month of each other. For almost two decades prior to these
recent events, however, the event of August 1972 was
regarded by many as representative of a worst case, even
though the originating flare was not optimally located on
the Sun for "direct connection" to Earth. It was argued
IO
1o
SUNSPOT L"YCLE
ß ß •
•o ß
1o 8 •
7
1o
I •1 ! i i ! !
-4 -3 -8 -I 0 I • 3 4 .5 B
YEAR RELATIVE TO MAXIMUM
Figure 4. Superimposed epoch representation of the yearly
fiuence of >30-MeV solar protons as a function of time relative
to the year of sunspot maximum. Data are plotted for solar
cycles 19-21 and for the first three years of solar cycle 22.
Vertical lines through the data points indicate the complete range
of values observed prior to cycle 22. The plot is from Zwickl and
Kunches [1989] and Feynman and Gabriel [1990].
that the extreme magnitude of the August 1972 event
resulted from a unique sequence of events which was not
likely to be repeated. This sequence of events involved the
propagation of two (converging) magnetohydrodynamic
shocks in interplanetary space at the time of the flare
occurrence. Earth was situated between these two shock