5.1. General aspects of double-peaked activity maxima
The above results indicate that dynamical activity phenomena (revealed from S.D. datasets) are superimposed on the quasi-stationary 11-year trend (long-term average datasets) and can be identified on intermediate time-scales (about 5 to 12 months). The dual pattern is also evident in the standard deviation plots of Fig. 3. It follows that the formation of the activity maximum as dual- or multi-peaked pattern concerns all the solar atmospheric layers. In this paper we have not attempted to make a fine analysis of the exact time occurrence of peaks. Consequently, even though underlining their existence from lower to upper atmospheric layers, we are not able to examine their synchronism which, of course, plays a key-role in the analysis of the dual pattern propagation through the solar atmosphere up to interplanetary space and the terrestrial magnetosphere. In particular, from Figs. 2 (10.7- flux averages), 4 and 5 (flares and radio bursts) we observe that, for activity indices related to layers higher than the photosphere, double-peaked structures clearly appear in the average profiles (i.e., not only in the S.D. plots).
We believe that only intense events have the right energy to emerge in the upper solar layers. These considerations open the way to the discussion of the energy role in the double peak appearance. First, we observe that, using solar activity parameters of low energy events (e.g., subflares) or cumulative indices of activity (e.g., Rz), the evidence of a dual activity shape abruptly degrades towards a single-peaked cycle. Analysing the percentage of energetic flare events (those with importance ) with respect to the global ones for the three years centered on sunspot maxima, we found an increasing factor of about 1.4 from cycle 20 to 22. In other words, as only energetic phenomena (probably associated to the interaction between global and local magnetic fields; see Sect. 5.2) are involved in the double-peak structure, their relative increase leads to the shape in question. On the contrary, enhanced low-energy phenomena (relating mostly to local magnetic field variability and with their typical single-peaked cycle) mask the contribution of intense events to the sunspot maximum shape. A relevant dip in the outstanding events should occur during the inversion of the general heliomagnetic field (see also Nagashima et al. 1991). We underline the gap's role: in the bimodal behaviour of solar activity maxima the gap gives a net separation between the two peaks. As seen in Figs. 1, 2 and 3, peaks can appear well separated, or partially or completely merged; the use of the gap to individualize the double-peak appearance can be a less ambiguous method than looking for single peaks. As shown in Fig. 5, the gap's depth increases with the growing energy (and the importance) of the activity events. It should be noticed that the gap's depth increases abruptly at (with respect to that of 410, 2695 and 4995 radio flux trends); we suspect the existence of an energy threshold in the gap's generation but we underline the need for further studies on this subject.
5.2. Double-peak cycle structure and solar magnetic field
As pointed out above, we believe that the origin and evolution of dual-peak behaviour in solar activity cycles are related to the space-time variability of the heliomagnetic field. Looking for its role, we concentrated our attention on cycle 21, for which Obridko & Shelting (1992) introduced a magnetic energy index: = , where the average of the radial field component is computed over two solar surfaces of radius r = (photosphere: i()) and r = 2.5 (source surface: i()) for each Carrington rotation.
Fig. 7 (obtained from a point-reading of parts of Figs. 1 and 2 reported by those authors) illustrates in the upper panel the i() trend. Connecting relative minima of i() we notice a "background cycle" which peaks at Rotation 1691 (1979.9), the maximum of sunspot cycle 21. If we hypothesize that the "field background" is made up of integrated small-scale (local) fields we can explain why the occurrence of low-energy phenomena tends to follow a single-peaked activity cycle, particularly those associated with deeper atmospheric layers. We observe six peaks (denoted by A, B, C, D, E and F) emerging over the background trend, with two absolute maxima occurring on Rotation 1680 (about 1979.3) and Rotation 1712 (1981.8). Moreover, an extended relative minimum is present around the sunspot cycle maximum (Rotation 1691).
Looking at the background of i() (Fig. 7, bottom panel) we learn that its lower envelop at the source surface practically disappeared. This reinforces the idea that, at this atmospheric height, effects of small-scale fields and low-energy phenomena are irrelevant. Only peak features dominate the temporal trend. We notice the strong stability of peak A (entity, average life-time and atmospheric depth involved). On the contrary, peaks B and C seem to combine together and lose in magnitude. Probably there is a damping of the magnetic energy transmission from the lower to the upper atmosphere. Peak D appears as the outstanding impulse, i.e. the absolute maximum (1982.5). Peaks E and F appear as for the B + C couple.
Because of the high stability of peak A (indicated by dashed arrows in the upper and lower panels of Fig. 7) we expect its strong connection with the high energy phenomena on the Sun. In fact, there is a very good relationship between this peak and the first one remarked in our figures. Good synchronic activation is presumed in all the atmospheric layers. This explains why it was easier for Kopecký (1973) to find the first Gnevyshev maximum on sunspot groups rather than the second one.
As far as the B + C couple is concerned, we explain the damping effect with the contemporary inversion of the general magnetic field. In this cycle the earliest time for switched polarity occurs above heliographic latitude on Rotations 1692-93 (March 1980) at the North Pole and on Rotation 1699 (September 1980) at the southern one, but the reversal at the northern pole was not stable enough until Rotation 1719 (February-March 1982), as reported by Webb et al. (1984). The time interval between Rotation 1699 and 1719 corresponds very well with the B + C epoch. Hence, during this period the emergence of high energy phenomena in the outer solar atmosphere is unlikely. We believe that this is not the case for peak D, which occurs at the source surface level when the general magnetic field has reinforced its strength. This is the epoch for Gnevyshev's second maximum in the coronal layer. Its evidence should be clearly found in the upper solar atmosphere together with relevant effects in the interplanetary medium. On this ground even the role of Gnevyshev's gap is clarified: according to the inversion of the general magnetic field, a decrease (or a gap) in the number of high energy events occurs; consequently, two main peaks emerge on both sides of the gap. Previous considerations also throw light on what we call "large-scale dynamical phenomena": they are clusters of solar events originating in strong magnetic fields with the necessary energy to affect the heliospheric environment. We suggest that intense local fields strongly interacting with the global heliomagnetic field cause the large-scale restructuration of the solar corona; hence, they are deeply involved in the maximum shape of activity cycles.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998