H. Kananen

Correlative study of solar activity and cosmic ray intensity

We perform a correlative study of solar activity (sunspot numbers) and cosmic ray intensity (neutron monitor count rates) for the last four solar cycles. Analysis of the running cross correlation between the two series shows that the behavior of cosmic ray modulation is similar, in general, for particles with different energy. However, a strong rigidity dependence as well as an unusual behavior of the cross correlation function is found for the descending phase of cycle 20. We study the evolution of cosmic ray and solar activity cycles in a three-dimensional phase space by means of the delayed component method. While all solar activity cycles and most cosmic ray cycles are planar, cosmic ray cycle 20 is significantly three-dimensional. A concept of the momentary phase of a cycle is introduced, and the phase evolution of cosmic ray and solar activity cycles is studied. We also discuss the heliospheric conditions responsible for the unusual behavior of cosmic ray modulation in the descending phase of cycle 20.

Dependence of cosmic rays on solar activity for odd and even solar cycles

Abstract We study the relationship between solar activity and cosmic ray intensity for the last four full cycles, using the time delayed component method (Usoskin et al., 1998) in a 2 D phase space. We present a new method to define the cosmic ray cycle which is free from ambiguousness related to the exact timing of cosmic ray maxima and minima. Using this definition, we confirm that the evolution of cosmic ray intensity is different for odd and even cycles and we show that odd cosmic ray cycles are longer and have longer autocorrelation interval lengths than even cycles. The momentary time lag between cosmic ray intensity and sunspot activity is about one year for odd cycles and small or negative for even cycles. This reflects the difference in the cosmic ray modulation conditions for odd and even cycles and is probably associated with the influence of drift effects.

Connections between neutron monitor count rate and solar modulation strength

We suggest a new approach to the normalisation of neutron monitor response to galactic cosmic rays. The reference normalisation count rate is the neutron monitor response to the model unmodulated flux of galactic cosmic rays. A comparison of the actually recorded neutron monitor count rate with the calculated normalisation count rate can provide one with an observationally obtained true-of-date integral measure of the current level of solar modulation of galactic cosmic rays.

PHASE EVOLUTION OF SOLAR ACTIVITY AND COSMIC-RAY VARIATION CYCLES

Cycles of phase evolution of solar activity and cosmic-ray variations are reconstructed by means of the delay component method, which allows us to study the temporal behaviour of time lag between solar activity and cosmic-ray cycle phases. It is shown that the period of the late 20th cycle was very unusual. We have found a delay in the phase of the solar activity cycle with respect to that of cosmic rays and discuss the heliospheric conditions responsible for this delay.

Sensitivity of a neutron monitor to galactic cosmic rays

Abstract We studied the sensitivity of a neutron monitor to Galactic cosmic rays depending on energy of cosmic ray particles in the interstellar medium. This is important for studying of the interstellar spectrum of cosmic rays and details of solar modulation by means of the world neutron monitor network. We calculated the expected response of a standard sea-level neutron monitor vs. energy of cosmic ray particles. First, we studied modulation of a monoenergetic flux of cosmic rays. Then the specific atmospheric yield function of a neutron monitor was applied to the calculated flux of cosmic rays at the Earths orbit. The obtained response function, being convoluted with the model interstellar spectrum of cosmic rays, gives a maximum of neutron monitor sensitivity at around several GV of rigidity of cosmic ray particles. We performed calculations for weak (φ = 350 MV) and medium (φ = 750 MV) modulation strength. A normalisation of neutron monitor count rate is suggested which can give an experimental measure of the overall solar modulation of cosmic rays.

ESTIMATION OF HIGH ENERGY SOLAR PARTICLE TRANSPORT PARAMETERS DURING THE GLE's IN 1989

ABSTRACT Analysis of five ground level enhancements in 1989 is carried out for the observations of the Lomnicky Stit and Oulu neutron monitors. The objective of the analysis is the estimation of the particle transport parameters in the solar corona and in the interplanetary space at relativistic proton energies. For the September 29 and October 24 flares the observations at both stations reveal fine structures which can be interpreted as a double injection process into IP space.

Neutron monitor data on the 15 June 1991 flare: Neutrons as a test for proton acceleration scenario

Response of Alma-Ata neutron monitor for solar neutrons from the 15 June 1991 was studied. We considered this response as a test for various scenarios of proton acceleration during the flare. The analysis of neutron monitor is an evidence in favour of the assumption of two acts of proton acceleration at impulsive and post-impulsive phases of the flare.

Search of periodic scintillations in cosmic radiation in the range 2–50 mHz

Abstract Neutron monitor measurements using 10-second recording intervals were started at Oulu in September 1987. A special analysis method based on the power spectra was adopted to compensate for the relative low statistics associated with short recording intervals. The preliminary result based on the analysis of 6274 one-hour length data records strongly indicates the existence of periodic scintillations in the whole frequency range 2–50 mHz.