3. Development of Solar Cycle 23 in the Outer Heliosphere

[8]  In general, in the part of the heliosphere where the solar wind structure does not change with distance, the GCR intensity variations at different heliocentric distances also should be similar. In this connection it is interesting to compare the development of the solar cycle variation in the GCR intensity of the same species and energy T near the Earth and in the distant heliosphere. It would be possible for the present solar cycle using the GCR intensities JH for the hydrogen, averaged in the range T = 120-240 MeV, and J_He for the helium, T_n = 180-450 MeV n^-1, measured aboard the IMP 8, Voyager 1 and Voyager 2 spacecraft, but unfortunately, the data from the near the Earth IMP 8 stopped being detected systematically in October 2001 (see http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=1973-078A).

[9]  However it is possible to estimate what the detectors aboard IMP 8 would measure after 10.2001 using the cosmic ray data from other experiments. Of course, it would be better if the GCR data in the energy ranges in question could be inferred from the direct measurements aboard some spacecraft still in operation. For the time being as an alternative we use the stratospheric data. Krainev and Webber [2003] used the count rate of the omnidirectional Geiger counter in the Pfotzer maximum at Murmansk (the cutoff rigidity R_c=0.6 GV, the medium rigidity during solar cycle maxima R_m 9 GV [Svirzhevsky, 2003]) for this purpose. However, the effective rigidities of the GCR particles contributing to N1_Mu ^max and J_H, J_He, are too different, and Krainev and Webber [2005] suggested for the purpose of estimating the IMP 8 intensities using the difference between the count rates of the Geiger counter in the Pfotzer maximum at Murmansk and Moscow, N1_MM ^max = N1_Mu ^max- N1_Mo ^max (the effective rigidity R_e < 1 GV). Krainev and Webber [2005] studied in the first approximation the test time series J_H, J_He, and N1_MM ^max, their possible trends and regression relationship and made the estimation for the time period from October 2001 to November 2004 of the IMP 8 26-day averaged intensities, smoothed with the periods 0.5 and 2 years. Below we use the results of this estimation for the 0.5-year smoothed time series.

Figure 4

[10]  In Figures 4a and 4b the 26-day averaged and smoothed with a 0.5-year period intensities measured aboard the Voyagers 1 and 2 (http://voycrs.gsfc.nasa.gov/heliopause/heliopause.html) and IMP 8 spacecraft are shown for 1995-2004. The data in Figures 4a and 4b are for the GCR protons and helium nuclei, respectively, in the energy ranges very close to those listed above for IMP 8. The first fact one can notice is that the GCR intensity modulation in the minimum epoch of the current solar cycle began near the Earth much earlier than in the outer heliosphere (the maxima in the 2-year smoothed intensity-time profiles are t_m²³ =1997.1 and 1998.9 [see Krainev and Webber, 2005]). The difference in the corresponding times of the first gap ( t_g1²³ ) in the double-gap structure of the 0.5-year smoothed intensity-time profiles is not so significant (approximately 2001.0 and 2001.7 for IMP 8 and for Voyagers 1 and 2, respectively). Krainev and Webber [2005] suggested that this fact reflects the magnetic drift effects for qA>0 phase of the solar magnetic cycle. As to the maximum phase in the GCR intensity, the main feature that one can see in Figures 4a and 4b is that although there is a double-gap structure with the Gnevyshev peak between the gaps in each intensity time profile, this structure for the intensity measured in the outer heliosphere looks rather strange, especially for the higher energy helium nuclei. Suffice it to note that the helium intensity around the Gnevyshev peak at the Voyager 1 is in excess of the maximum intensity in 1998! Krainev and Webber [2003] even suggested that the GCR intensity peaks at Voyager 1 have something in common with the very high fluxes of the low energy particles measured there in 2002-2003 and connected by some investigators [Krimigis et al., 2003; McDonald et al., 2003; Zeldovich et al., 2003] with the effects of the termination shock.

[11]  Krainev and Webber [2005] suggested that the "strangeness" of the double-gap structure of the GCR intensity-time profiles in the outer heliosphere and its difference from the corresponding double-gap structure near the Earth could be due to the strong 22 wave in the GCR intensity, observed in the outer heliosphere. In order to allow for both the changing heliocentric distance r(t) of the spacecraft and the 22 wave we suggested normalizing the absolute GCR intensity, J(r, t), using the GCR intensity radial profiles during the minimum ( J_mⁱ(r) ), and maximum ( J_Mⁱ(r) ) of the i th solar cycle, in the following way:

(1)

The radial profiles of the GCR intensity in the extreme phases for solar cycles 21-23 were determined by Krainev and Webber [2005] using the intensity time series smoothed with a 2-year period. Besides, as the radial profile for the next minimum of solar activity is still unknown, we suggested that J_m²⁴(r) = J_m²²(r) , that is, that the GCR intensity radial profile in the minimum of solar cycle depends only on the IMF polarity. Note that using (1) one should take into account the change with time of the current radial profiles of the GCR intensity in the extreme phases of solar cycle. Namely, if and are the moments when the 2-year smoothed intensity attains its maximum ( J_mⁱ ) and minimum ( J_Mⁱ ) values in the i th solar cycle, the radial profiles ( J_mⁱ(r) ) and ( J_Mⁱ(r) ) should be used in (1) for t_M^i-1< t < t_Mⁱ and t_mⁱ< t < t_mⁱ⁺¹, respectively (for the sake of simplicity, we suggested that the reversal of the high-latitude solar and heliospheric magnetic field in the i th solar cycle occurs in the moment t_Mⁱ ).

[12]  In Figures 4c and 4d the same GCR intensities are shown as in Figures 4a and 4b; however, they are normalized according to (1). Besides, we allowed for the trivial effect of the difference D r=r - 1, AU, in the radial distance of the spacecraft with respect to 1 AU, plotting J_norm(t - D r/V_sw ), with V_sw = 450 km  s^-1. One can see that the double-gap structure of the GCR intensity in the outer heliosphere took its usual form, even the positions of the first gap and Gnevyshev peak near the Earth and in the outer heliosphere being approximately the same. So probably the peak in the GCR intensity observed at Voyager 1 in 2002 does not have relation to the effects of the termination shock. The second gap in the outer heliosphere has not been completed by the 2004.7, but we expect it to be formed in the next half a year. Now we cannot state if the main cause of the significant difference in the magnitude of the Gnevyshev peak in the GCR intensity between the inner and outer heliosphere is due to smoothing of the double-gap structure with the radial distance, or just to the defects either of the method of the estimation of the IMP 8 GCR intensity since October 2001 or of the method of the GCR intensity normalization used by us. We are working on the improvement of these methods.