2. Development of Solar Cycle 23 on the Sun and Near the Earth

Figure 1

[3]  Figure 1 shows for 1995-2004 the time history of some solar, heliospheric and cosmic ray characteristics near the Earth: the strength of the interplanetary magnetic field (IMF) B_IMF (from http://nssdcftp.gsfc.nasa.gov/spacecraft_data/omni/omni_27_av.dat, the solid line) and the sunspot area S (from http://science.nasa.gov/ssl/PAD/ SOLAR/greenwch.htm, the dotted line) in Figure 1a; the line-of-sight component of the polar photospheric magnetic field as seen from the Earth, B_is ^{N, S}, and the latitude boundary of the IMF sector structure zone, l_t ^{N, S} (Figures 1b and 1c, respectively) for the north (the dotted lines) and south (the dashed lines) solar hemispheres, (both from http://sun.stanford.edu/~wso/wso.html); and the GCR intensity (the relative count rates of the Huancayo and Climax neutron monitors (ftp://ulysses.sr.unh.edu/NeutronMonitor/DailyAverages.1951-.txt) and of the omnidirectional Geiger counter in the Pfotzer maximum in the stratosphere at Moscow and Murmansk, listing from top to bottom in Figure 1d). All the initial monthly, Carrington rotation or 27-day averaged data were smoothed with a 0.5-year period. The cosmic ray data were additionally normalized to 100% for February 1997. For the current solar cycle the maximum phase in the GCR intensity, that is, the period D_Max²³ = t_g2²³ - t_g1²³ between two main gaps ( t_g1²³ and t_g2²³ ), lasted for 3 years, from 2000.7 to 2003.7, and it is shown by the shaded band in Figure 1.

[4]  One can see from Figure 1 that the IMF strength was rather high during almost 5 years, from 1999.0 to 2004.0, while the period of maximum sunspot area is somewhat shorter, from 2000.4 to 2002.8. The long period of high B_IMF may have a bearing on the long maximum phase in the GCR intensity variation, although note that the IMF strength started decreasing a few months after the end of the maximum phase in the GCR intensity, t_g2²³. The polar magnetic fields in both hemispheres changed sign approximately simultaneously around 2000.0, but soon stopped increasing in strength and were rather small (less than a half of their maximum value) during next 3 years. This weak polar magnetic field is also reflected in rather large ( 40^o ) and constant for 3 years latitude boundaries of the IMF sector structure zone. So the long maximum phase in the GCR intensity variation in solar cycle 23 can be related in general (but not in details) both to the behavior of the IMF strength (which reflects the toroidal or sunspot branch of solar activity [see Krainev and Webber, 2004]) and to the prolonged period of the weak high-latitude poloidal solar magnetic field.

Figure 2

[5]  In order to facilitate a search of the factors responsible for the length and modulation depth of the solar cycle maximum phase in the GCR intensity we superposed in Figure 2 for solar cycles 20-23 the time histories of the solar, heliospheric and cosmic ray characteristics, already discussed for the current solar cycle, as functions of the time t' = t- tⁱ_min elapsed since the beginning tⁱ_min of the i th solar cycle. Note that we chose the stratospheric relative count rate at Murmansk, N_Mu , as a GCR intensity index (the maximum phases shown by the thicker parts of the lines in Figure 2e) and instead of the polar magnetic field and latitude boundary of the IMF sector structure zone in each hemisphere we show the average characteristics: B_ls^pol = (B_ls ^N2 + B_ls ^S2)/2 (Figure 2c) and a_t =(l_t ^N -l_t ^S)/2 (Figure 2d, the pseudotilt of the IMF current sheet) for solar cycles 21-23, when we have the systematic data on the solar magnetic fields. Besides, in Figure 2b we show (also for the cycles 21-23) one more solar factor, B2, the energy density of the solar magnetic field averaged over the photosphere [see Krainev et al., 1999, and references therein]. Naturally, as the solar cycles considered are different both in their height and duration, the modulation depth and position of the solar cycle maximum phase in the GCR intensity are also different for different solar cycles. However, one can notice some regularities.

[6]  First, there is a concentration of the GCR maximum phases in the time period t' =4-6, years since solar minima, and the corresponding concentration of the IMF strength and the solar magnetic field energy factor B2 in the ranges t' =4.5-6.5 and t'=2.5-5, respectively. Besides, the factor B2 clearly demonstrates the Gnevyshev gap effect, i.e., the pronounced double-peak structure with Gnevyshev gap between the peaks, also characteristic for the GCR intensity modulation (see [Krainev et al., 1999]). In addition, the depth of the GCR intensity modulation corresponds (at least qualitatively) to the maximum level of both B_IMF and B2. These facts make us suggest that the average energy density B2 of the photospheric magnetic fields, along with B_IMF, is one of the important factors responsible for the characteristics of the solar cycle maximum phase in the GCR intensity. Another important feature also seen in Figures 2c and 2d is the behavior of the poloidal solar magnetic field characteristics. After the deep gap in the strength of the polar magnetic field and the corresponding peak in the pseudotilt (due to the reversal of the high-latitude photospheric magnetic fields) there is a period of relatively weak polar field (and the large latitude range of the IMF sector structure zone) for solar cycle 21 and, especially, 23. It can be seen that for these cycles the length of the solar cycle maximum phase in the GCR intensity is significantly longer than for cycles 20 and 22. It strengthens our opinion that the behavior of the poloidal solar magnetic fields is another important factor for the characteristics of the solar cycle maximum phase in the GCR intensity.

Figure 3

[7]  There is a fact that casts some doubt upon the use of B2 as a factor important for the features of the maximum phase in the GCR intensity. As one can see from Figure 2 the position of this phase is different for the different cycles while in solar cycles 21-23 the reversal of the high latitude solar magnetic field (and hence the gap in B2 and the peak in the pseudotilt) occurs approximately at the same time after the beginning of the solar cycle. In order to clarify the situation we made in Figure 3 the same superposition for solar cycles 20-23 of the time histories of the solar, heliospheric and cosmic ray characteristics as in Figure 2, but plotted them as functions of the time t'' = t- tⁱ_GP elapsed since the middle of the solar cycle maximum phase in the GCR intensity (or since the Gnevyshev peak, t_GPⁱ = t_g1ⁱ+ tⁱ_g2, in the GCR intensity corresponding to the Gnevyshev gap in its modulation). We see that three solar cycles 21-23 are divided into two groups: (1) solar cycle 22 for which there is a small time advance (less than 1 year) of the Gnevyshev gap in B2 factor with respect to the Gnevyshev peak in the GCR intensity and (2) solar cycles 21 and 23 characterized by the greater time advance (  2 years) of the Gnevyshev gap in B2 and by the subsequent period of the weak poloidal solar magnetic field (and the large IMF sector structure zone). Probably, this division reflects one more aspect of the 22-year wave in the GCR intensity modulation.