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Astron. Astrophys. 322, 311-319 (1997)

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4. Looking for intense dynamical phenomena on the Sun

Past researches have indicated (without emphasizing the fact) that the double-peaked maxima are easier to find considering the growing importance of the examined solar events. Among them we recall:

1. Gnevyshev (1977), in which Fig. 5 shows the progressive emergence of a double peak in the yearly number of sunspots, with areas increasing from 200 to more than 500 area units;

2. Roy (1977), taking into account all the major flares meeting Dodson's and Hedeman's criteria, found a double peak in the flare occurrence for cycles 19 and 20.

Hence, we re-examined the annual counts of grouped solar flares according to their [FORMULA] importance, i.e. subflares (Sb), = 1, [FORMULA], as reported by Mouradian & Soru-Escaut (1995). They were plotted in Fig. 4 together with Rz, the 10.7- [FORMULA] radio flux and the total number of flares.

[FIGURE] Fig. 4. [FORMULA] flare events arranged by importance for activity cycles 20 to 22. Panels 1-3, from the top: sunspot number Rz, 10.7- [FORMULA] radio flux, total number of flares. Panels 4-6: number of subflares (Sb), of flares with [FORMULA] importance = 1 and [FORMULA] (data obtained from Mouradian & Soru-Escaut 1995).

A double-peaked structure emerges by moving from subflares (low energy events; fourth panel from the top of Fig. 4) to flares with [FORMULA] importance [FORMULA] (high energy events; sixth panel). On the contrary, activity indices accumulating events whatever their importance, as for example the total number of flares (third panel), Rz or the 10.7- [FORMULA] radio flux (first and second panels, respectively), do not always display structured maxima. Remarkable similarities between their profiles and those obtained for indices of little importance (such as the subflares) suggest that the former activity parameters are dominated by non energetic events.

Nevertheless, we observe that, only for cycle 22, the annual averages of Rz and 10.7- [FORMULA] flux present a double-structured maximum, suggesting that this cycle contains a major number of events of large importance with respect to the previous ones. In this case double peaks clearly arise without the use of "filtering criteria" (such as the classification of events on the ground of the event's importance). We recall that Gnevyshev's maxima are not easy to find in Rz (Fig. 1) because it counts together sunspots and sunspot groups irrespectively of their importance (i.e. the sunspot area).

To investigate better the link between double-peak shape and the event's importance, we analyse several activity parameters available for cycle 21 (Fig. 5) and their hemispherical distribution on the Sun (Fig. 6):

1. the annual number of long-duration events (LDE-type flares) in the soft X-ray flux, according to their importance: X ([FORMULA]), M2 ([FORMULA]), M1 ([FORMULA]) and C ([FORMULA]) as computed from Antalová (1990); the flare life-time ranges from 120 to 270 minutes;

2. the annual number of long duration ([FORMULA] hours) 10.7- [FORMULA] bursts with flux [FORMULA] and [FORMULA] (Kahler & Cliver 1988);

3. the monthly radio flux at 410, 2695, 4995 and 15400 [FORMULA] (73.17, 11.13, 6.01, and 1.95 [FORMULA] ; Sagamore Hill Observatory - Massachusetts);

4. the monthly sunspot areas separately for northern and southern hemispheres (Makarov & Makarova 1996);

5. the semi-annual hemispherical flare index, derived from Ataç (1987), which roughly gives a measure of the total energy emitted by a chromospheric flare;

6. the semi-annual hemispherical distribution of solar LDE-type flares for M-X classes (computed from Antalová 1990).

The double peak structure is clearly seen in the radio flux trends with increasing energy from 410 to 15400 [FORMULA] (73.17 to 1.95 [FORMULA] ; Fig. 5, upper panels). Alternatively to the double peak, it is possible to speak in terms of the "Gnevyshev gap" (as called by Storini & Pase, 1995; it is defined as a relative decrease in the strength of solar activity indices connected with the maximum phase) whose depth increases with the energy of the events (see Sects. 5.1 and 5.2).

[FIGURE] Fig. 5. Several parameters for cycle 21. Upper panels: monthly averages (thin lines) of radio flux and the corresponding 13-running averages (denoted by 13s, thick lines). First and second lower panels: total annual number of LDE-type flares of C, M1, M2 and X class (Antalová 1990). Third lower panel: annual number of LDE-type flares with life-time ranging from 120 to 270 [FORMULA] (Antalová 1990). Fourth lower panel: annual number of 10.7- [FORMULA] bursts of long duration, with flux [FORMULA] and [FORMULA] (Kahler & Cliver 1988). Arrows indicate the double peak emergence.
[FIGURE] Fig. 6. Indices of northern (left) and southern (right) activity for cycle 21. Upper panels: monthly spot area (thin line) and 13-point running average (thick line; data derived from Makarov & Makarova 1996). Middle panels: Ataç semi-annual flare index (Ataç 1987). Lower panels: number of solar LDE-type flares of M-X classes on half-yearly basis (data from Antalová 1990).

The lower panels of Fig. 5 show the number of LDE-type flares separated into energy (classes X, M1, M2 and C), the number of LDE-type flares of increasing life-time (from 120 to 270 minutes) and the radio burst number. Data of LDE-flares lasting 270 minutes are multiplied by a factor of 5 to compare them with trends of LDE-flares of minor importance. The same occurred for 10.7- [FORMULA] burst numbers of energy [FORMULA]. The dependence of the double-peak occurrence with the increasing energy of single events is confirmed, as for previous data (Fig. 5, upper panels).

Fig. 6 shows another aspect of the problem: the double peak structure emergence on sunspot areas (upper panels), the total energy emitted by chromospheric flares (middle panels) and the M-X LDE-flares (lower panels) related to their hemispherical distribution. The figure shows that the bimodal data distribution is not a north-south anisotropic effect (see also the time sequence of the sunspot-area variability from 1874 to 1971 reported by White & Trotter 1977).

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998