Study of the 27-day variations of the galactic cosmic ray intensity and anisotropy

We demonstrate that the general features of the radial and azimuthal components of the anisotropy of galactic cosmic rays can be studied by the harmonic analysis method using data from an individual neutron monitor with cut off rigidity <5 GV. In particular, we study the characteristics of the 27-day (solar rotation period) variations of the galactic cosmic ray intensity and anisotropy, solar wind velocity, interplanetary magnetic field strength and sunspot number. The amplitudes of the 27-day variations of the galactic cosmic ray anisotropy are greater, and the phases more clearly established, in A > 0 polarity periods than in A < 0 polarity periods at times of minimum solar activity. The phases of the 27-day variations of the galactic cosmic rays intensity and anisotropy are opposite with respect to the similar changes of the solar wind velocity in A > 0 polarity periods. No significant dependence of the amplitude of the 27-day variation of the galactic cosmic ray anisotropy on the tilt angle of the heliospheric neutral sheet is found. Daily epicyclegrams obtained by Chree’s method show that the 27-day variations of the galactic cosmic ray anisotropy during A > 0 polarity periods follow elliptical paths with the major axes oriented approximately along the interplanetary magnetic field. The paths are more irregular during A < 0 polarity periods.     

Observed solar cycle modulation of galactic cosmic rays over a range of rigidities

We have studied the long-term, steady-state, solar cycle modulation of galactic cosmic ray intensity for seven cycles (17–23). Our analysis is based on the data obtained with a variety of detectors on earth (neutron monitors of the global network and muon detectors) as well as telescopes flown on high altitude balloons and on-board near-earth satellites. The median rigidity of response for these detectors to galactic cosmic ray spectrum lies in the range 1–70 GV. We correlate cosmic ray data to sunspot numbers, Ap, solar wind bulk speed (V), magnetic field (B), as well as to the cycle maximum (M), minimum (m), and the epochs of the solar polar field reversals. This enables us to derive the rigidity dependence of observations, and helps us to define the characteristics of the modulation function in the heliosphere.     

Rigidity spectrum of the 27-day variation of the galactic cosmic ray intensity in different epochs of solar activity

We study the temporal evolution of the power rigidity spectrum of the first (27 days) and the second (14 days) harmonics of the 27-day variation of the galactic cosmic ray intensity measured by neutron monitors in the period of 1965–2002. The rigidity spectrum of these variations can be approximated by a power law. We show the rigidity spectra of the first and the second harmonics of the 27-day variation of the galactic cosmic ray intensity have similar time profiles. These spectra are hard (γ ≈ 0.5 ± 0.1) and soft (γ ≈ 1.1 ± 0.2) during solar maximum and minimum activity, respectively. We ascribe this to the alternation of the sizes of the modulation regions responsible for the 27-day variation of the galactic cosmic ray intensity in different epochs of solar activity. Especially, the average radial sizes of the modulation regions of the 27-day variation and the heliolatitudinal extension of the heliolongitudinal asymmetry are smaller during solar minimum than during solar maximum. We show also, that the temporal changes of the power rigidity spectra of the first and the second harmonics of the 27-day variation of the galactic cosmic ray intensity are in a negative correlation with the changes of the rigidity spectrum of the corresponding 11-year variation.     

Cosmic ray anisotropies in the outer heliosphere

The observation of the directional distribution of energetic and cosmic ray particles has been done with the Voyager spacecraft over a long period. Since 2002, when the first flux enhancements of charged particles associated with the approach of Voyager 1 to the solar wind termination shock were observed, these anisotropy measurements have become of special interest. They play an important role to understand the magnetic field and shock structure and the basics of the modulation of cosmic ray and anomalous particles at and beyond the termination shock. They also serve as motivation to study the spatial behavior of galactic and anomalous cosmic ray anisotropies with numerical modulation models in order to illustrate how the radial anisotropy, at different energies, change from upstream to downstream of the termination shock. Observations made by Voyager 1 indicate that the termination shock is a complicated region than previously thought, hence the effects of the latitude dependence of the termination shock’s compression ratio and injection efficiency on the radial anisotropies of galactic and anomalous protons will be illustrated. We find that the magnitude and direction of the radial anisotropy strongly depends on the position in the heliosphere and the energy of particles. The effect of the TS on the radial anisotropy is to abruptly increase its value in the heliosheath especially in the A > 0 cycle for galactic protons and in both polarity cycles for anomalous protons. Furthermore, the global effect of the latitude dependence of the shock’s compression ratio is to increase the radial anisotropy for galactic protons throughout the heliosphere, while when combined with the latitude dependence of the injection efficiency this increase depends on modulation factors for anomalous protons and can even alter the direction of the radial anisotropy.     

Regional and temporal variability of solar activity and galactic cosmic ray effects on the lower atmosphere circulation

In this work we studied the spatial and temporal structure of long-term effects of solar activity (SA) and galactic cosmic ray (GCR) variations on the lower atmosphere circulation as well as possible reasons for the peculiarities of this structure. The study revealed a strong latitudinal and regional dependence of SA/GCR effects on pressure variations in the lower troposphere which seems to be determined by specific features of baric systems formed in different regions. The temporal structure of SA/GCR effects on the troposphere circulation at high and middle latitudes is characterized by a roughly 60-year periodicity which is apparently due to the epochs of the large-scale atmospheric circulation. It is suggested that a possible mechanism of long-term effects of solar activity and cosmic ray variations on the troposphere circulation involves changes in the evolution of the polar vortex in the stratosphere of high latitudes, as well as planetary frontal zones.     

Two intense Forbush decreases of solar activity cycle 22

We study two intense Forbush decreases that occurred during two adjacent SOLTIP (Solar connection of Transient Interplanetary Processes) intervals; namely SOLTIP 1 (22–27 March 1991) and SOLTIP 2 (1–17 June 1991); galactic cosmic ray intensity at the depth of the second Forbush decrease was the lowest ever recorded since continuous monitoring by Climax neutron monitor began in 1951 (58% below the solar minimum value of 1954), indicating extreme conditions in the heliosphere that prevented galactic cosmic rays from reaching the Earth. These decreases were seen propagating in outer heliosphere by the deep space missions Voyagers 1, 2 and Pioneer 10, 11, with suitable time delays. We analyze hourly, pressure corrected, neutron monitor data from the global sites in both hemispheres, and muon telescopes located underground; they respond to 10–300 GV range of the galactic cosmic ray spectrum. This circumstance provides us an ideal opportunity to study the rigidity dependence of the amplitudes of the two Forbush decreases. In both cases the amplitude is found to be a power law in rigidity, with negative exponents.     

Stratospheric polar vortex as a possible reason for temporal variations of solar activity and galactic cosmic ray effects on the lower atmosphere circulation

Possible reasons for the temporal instability of long-term effects of solar activity (SA) and galactic cosmic ray (GCR) variations on the lower atmosphere circulation were studied. It was shown that the detected earlier ∼60-year oscillations of the amplitude and sign of SA/GCR effects on the troposphere pressure at high and middle latitudes (Veretenenko and Ogurtsov, Adv.Space Res., 2012) are closely related to the state of a cyclonic vortex forming in the polar stratosphere. The intensity of the vortex was found to reveal a roughly 60-year periodicity affecting the evolution of the large-scale atmospheric circulation and the character of SA/GCR effects. An intensification of both Arctic anticyclones and mid-latitudinal cyclones associated with an increase of GCR fluxes at minima of the 11-year solar cycles is observed in the epochs of a strong polar vortex. In the epochs of a weak polar vortex SA/GCR effects on the development of baric systems at middle and high latitudes were found to change the sign. The results obtained provide evidence that the mechanism of solar activity and cosmic ray influences on the lower atmosphere circulation involves changes in the evolution of the stratospheric polar vortex.     

Propagation modes of energetic charged particles in the heliosphere

Several years ago, the anisotropic diffusion and convective transport accompanied by adiabatic deceleration were considered as the principal means for cosmic ray propagation. Particles of relatively small energies (～ 1 MeV) can propagate along the force lines of the magnetic field without scattering at distances of several astronomical units in the quiet heliosphere. The theory describing the 11-year variation of galactic cosmic ray intensity and the propagation of solar cosmic rays was founded on this basis. However, the anomalies of the 11-year variation of galactic cosmic ray intensity in 1969–1971 revealed the necessity to take into account the influence of the general electromagnetic field of the heliosphere giving rise to a rapid magnetic drift of particles. The particles drift either from the magnetic axis to the ecliptic plane (in the cycle of 1969–1980) or in the opposite direction depending on the sign of the general magnetic field of the sun. The neutral layers along which the drift velocity is comparable to the particle velocity is of great significance. However, in the presence of sector structure, the time of particle propagation along the neutral layer from the boundary of the modulation region to the earth orbit is substantially increased. Thus a marked adiabatic deceleration is here possible. The time delay observed in the recovery of proton intensities at various energies can be explained in terms of a transient phase of the interplanetary field following the polarity reversal.     

The inner heliosphere from solar minimum to solar maximum: Short- and long-term variations in the energetic particle population

Between 1975 and 1983 HELIOS 1 scanned the interplanetary medium between 0.3 and 1 AU 31 times. The observed variations in the differential and integral flux of protons and helium nuclei in the energy range from 4 to >50 MeV/n are characterized by large temporal changes in the intensities, moderate changes in the energy spectrum and changes in the gradient below the detection level (60%). During solar minimum conditions recurrent disturbances are caused mainly by corotating interaction regions. The onset of solar activity near the end of 1977, characterized by a large number of solar events, is accompanied by a monotonous decrease of galactic cosmic radiation. The successive reduction of the cosmic ray intensity to the level of solar maximum is discussed in view of the role of large transient disturbances as compared to processes as diffusion, convection, adiabatic energy losses and drifts.     

Radiation environment due to galactic and solar cosmic rays during manned mission to Mars in the periods between maximum and minimum solar activity cycles.

A possibility of a manned mission to Mars without exceeding the current radiation standards is very doubtful during the periods of minimum solar activity since the dose equivalent due to galactic cosmic rays exceeds currently recommended standards even inside a radiation shelter with an equivalent of 30 g cm-2 aluminum. The radiation situation at the time of maximum solar activity is determined by the occurrence of major solar proton events which are exceedingly difficult to forecast. This paper discusses the radiation environment during a manned mission to Mars in the years between minimum and maximum solar activity when the galactic cosmic ray intensity is considerably reduced, but the solar flare activity has not yet maximized.     

Prediction of expected global climate change by forecasting of galactic cosmic ray intensity time variation in near future based on solar magnetic field data

A method of prediction of expected part of global climate change caused by cosmic ray (CR) by forecasting of galactic cosmic ray intensity time variation in near future based on solar activity data prediction and determined parameters of convection-diffusion and drift mechanisms is presented. This gave possibility to make prediction of expected part of global climate change, caused by long-term cosmic ray intensity variation. In this paper, we use the model of cosmic ray modulation in the Heliosphere, which considers a relation between long-term cosmic ray variations with parameters of the solar magnetic field. The later now can be predicted with good accuracy. By using this prediction, the expected cosmic ray variations in the near Earth space also can be estimated with a good accuracy. It is shown that there are two possibilities: (1) to predict cosmic ray intensity for 1–6 months by using a delay of long-term cosmic ray variations relatively to effects of the solar activity and (2) to predict cosmic ray intensity for the next solar cycle. For the second case, the prediction of the global solar magnetic field characteristics is crucial. For both cases, reliable long-term cosmic ray and solar activity data as well as solar magnetic field are necessary. For solar magnetic field, we used results of two magnetographs (from Stanford and Kitt Peak Observatories). The obtained forecasting of long-term cosmic ray intensity variation we use for estimation of the part of global climate change caused by cosmic ray intensity changing (influenced on global cloudiness covering).     

Observation of enhanced sub-iron (Sc---Cr) to iron ration in low energy cosmic rays of 50–100 MeV/N in spacelab-3

Relative abundances of sub-iron (Sc-Cr) to iron nuclei in low energy (50–100 MeV/N) galactic cosmic rays have been determined from an analysis of about 100 events of heavy ions (Z = 10−28) recorded in a detector assembly flown in the Anuradha cosmic ray experiment in the Spacelab-3 on a six day mission in April–May 1985. The measured abundance ratio of (Sc-Cr)/Fe nuclei in 50–100 MeV/N energy range is 1.1 ± 0.3, and the present result of enhanced ratio of sub-iron to iron nuclei is in agreement with other experimental results in 200–800 MeV/N range. The over-abundance of iron secondaries at these low energies cannot be explained in the conventional models for propagation of cosmic rays. Available experimental data indicate a very different time history for the low energy iron-group, as compared to those of lighter nuclei in galactic cosmic rays.     

Galactic cosmic ray modulation for sunspot cycle 23

We present a study of the galactic cosmic ray modulation for sunspot cycle 23. We use the monthly and the annual mean hourly, pressure corrected, data from neutron monitors of the global network (monthly rate is calculated as the average of the hourly pressure corrected values). We draw attention to an asymmetry in the galactic cosmic ray (GCR) recovery during odd and even cycles for the monthly mean hourly rate data. For over half a century of observations, we find that the recovery for the odd cycles is to a higher level than for the even cycles. Qualitatively the effect is ascribed to charged particle drifts in inhomogeneous interplanetary magnetic field. Even so it has not been possible to arrive at a quantitative, self-consistent, explanation in terms of drifts at higher and lower GCR rigidities. We also study the rigidity dependence of the amplitude of 11-year modulation over a wide range (1–200 GV) of GCR spectrum; it is a power law in rigidity with an exponent −1.22. We discuss the implication of these findings on quasi-linear diffusion theories of modulation. We reflect on GCR recovery pattern for 2006–2009.     

The onset of sunspot cycle 24 and galactic cosmic ray modulation

The annual mean sunspot number (SSN) has a minimum value in 2008, while the monthly mean SSN has a value of zero in August 2009. The galactic cosmic ray modulation for cycle 24 began at earth orbit in January 2010. We study the onset characteristics of the new modulation cycle using data from the global network of neutron monitors. They respond to time variations in different segments of the galactic cosmic ray rigidity spectrum. The corresponding temporal variations in the interplanetary magnetic field intensity (B) and solar wind velocity (V) as well as the tilt angle of the heliospheric current sheet are also studied. There is a lag of 3 months between a large, sharp increase of the tilt angle of the heliospheric current sheet and the onset of modulation. Some neutron monitors are undergoing long-term drifts of unknown origin.     

Radial and latitudinal gradients of galactic cosmic rays in the heliosphere at solar maximum

Our understanding of galactic cosmic ray (GCR) modulation has advanced greatly in the last three decades. However, we still need an appropriate knowledge of the GCR intensity gradient. Numerical simulations of the transport particle equation allow interpretation of cosmic ray intensities in the heliosphere. We use the numerical solution of the GCR transport equation during solar maximum epoch to compute the radial and latitudinal gradients. Our analysis indicates that adiabatic energy loss plays an important role in the radial distribution of GCR in the inner heliosphere, while in the outer region the diffusion and convection are the relevant processes. The latitudinal gradient is small.     

Research by the Tasmanian cosmic ray group during the International Geophysical Year

Systematic recording of the cosmic radiation commenced in Hobart in 1946 and at Mawson in Antarctica in 1955, making these two of the longest running cosmic ray observatories in the world. For the IGY, observations were also made at a sub-Antarctic island and near the equator, and an airborne survey of the nucleonic component was made from Geomagnetic Latitude −60°, south of Australia, to Japan and back. At Hobart there were neutron monitors, vertical and inclined muon telescopes, an ionization chamber, and two muon telescopes at ∼40 m of water equivalent underground. The research based on these and other observations determined the energy dependence of the Forbush and 11-year variations and concentrated, in particular, on understanding the anisotropic nature of galactic cosmic rays up to 150 GeV; the anisotropies in the onset phase of Forbush decreases; and the anisotropies in solar cosmic ray events. An investigation was initiated to calculate the trajectories and cutoff rigidities of cosmic rays in a high order simulation of the geomagnetic field. This was completed in 1959–60.     

Estimation of long-term cosmic ray intensity variation in near future and prediction of their contribution in expected global climate change

On the basis of results obtained in our paper [Dorman, L.I. Long-term cosmic ray intensity variation and part of global climate change, controlled by solar activity through cosmic rays, Paper D2.1/C2.2/E3.1-0097-04. Adv. Space Res., 2004 (accepted)], we determine: the dimension of the Heliosphere (modulation region), radial diffusion coefficient and other parameters of convection–diffusion; drift mechanisms of long-term variations of cosmic ray (CR) dependence on particle energy; level of solar activity (SA); and generally, the solar magnetic field. We obtain this important information on the basis of CR and SA data in the past, taking into account the theory of convection–diffusion and global drift modulation of galactic CR in the Heliosphere. By using these results and other regularly published predictions of expected SA variation in the near future, as well as predictions of the next SA cycle, we may make predictions of long-term cosmic ray intensity variation expected in the near future (up to 10–12 years). In [Dorman, L.I. Long-term cosmic ray intensity variation and part of global climate change, controlled by solar activity through cosmic rays, Paper D2.1/C2.2/E3.1-0097-04. Adv. Space Res., 2004 (accepted)], properties of connections between long-term variation in CR intensity and some part of a global climate change were estimated, controlled by solar activity through CR. We show that in this way we may make predictions of some part of a global climate change expected in the near future (up to 10–12 years and maybe more, depending upon the period during which definite predictions of SA can be made), controlled by solar activity through CR. In this case, estimations of expected long-term changes in the planetary distribution of cutoff rigidities, which also influence CR intensity, as well as CR-influenced effects on global climate variation, become important.     

Relativistic solar protons in the ground level event of 23 February 1956: New study

In this study we applied again to the outstanding solar particle event of 23 February 1956, the largest one in the entire history of observations of solar cosmic rays. Due to significant improvement of the analysis/modeling techniques and new understanding of physical processes in the solar atmosphere and interplanetary space, a possibility arises to interpret the old data in the light of modern concept of multiple particle acceleration at/near the Sun. In our new analysis the data of available then neutron monitors and muon telescopes are used. The technique of the analysis includes: (a) calculation of asymptotic cones of ground-based detectors; (b) modeling of cosmic ray detector responses at variable parameters of the flux of solar relativistic protons; (c) determination of primary solar proton parameters outside magnetosphere by comparison of computed responses with observations. Certain evidence was obtained that the flux of relativistic solar protons consisted of two distinct components: prompt and delayed ones. The prompt component with exponential energy spectrum caused a giant impulse-like increase at a number of European cosmic ray stations. The delayed component had a power-law spectrum and was a cause of gradual increase at cosmic ray stations in the North American region. A numerical simulation of the proton acceleration in the vicinity of the magnetic reconnection region brings to the proton spectrum with exponential dependence on energy. This agrees with observational data for the prompt component. It is also shown that the huge increase in ∼5000% on neutron monitors was due to the prompt component only with the exponential proton spectrum. The power-law spectrum of comparable intensity gave considerably smaller effect.     

Peculiarities of galactic cosmic ray Forbush effects during October–November 2003

Features of two successive Forbush effects of the galactic cosmic ray intensity in October–November 2003 have been studied based on the neutron monitors data. The rigidity spectrum of the galactic cosmic ray intensity in the course of the first Forbush effect (22–27 October) is gradually hardening, while the rigidity spectrum of the second Forbush effect (28 October–10 November) from the starting moment is very hard. As far, the energy range of the turbulence of the interplanetary magnetic field is in general responsible for the diffusion of galactic cosmic ray particles of the energy 5–50 GeV (to which neutron monitors are sensitive), we postulate that the gradually hardening (from day to day) of the rigidity spectrum of the first Forbush effect is associated with the enhancement of the power spectral density in the energy range of the interplanetary magnetic field turbulence caused by the large scale irregularities generated due to the interaction of the extending high speed disturbances with the background solar wind. The very hard rigidity spectrum (from the starting moment) of the second Forbush effect is generally associated with the well established new structure of the energy range of the interplanetary magnetic field turbulence enriched by the already created large scale irregularities. The gradually softening of the rigidity spectrum during the recovery phase of the second Forbush effect confirms that the disturbed interplanetary magnetic field turbulence step by step returns to the initial state.     

Solar proton events recorded in the stratosphere during cosmic ray balloon observations in 1957–2008

Long-term balloon observations have been performed by the Lebedev Physical Institute since 1957 up to the present time. The observations are taken several times a week at the polar and mid latitudes and allow us to study dynamics of galactic and solar cosmic ray as well as secondary particle fluxes in the atmosphere and in the near-Earth space. Solar energetic particles (120) – mostly protons – (SEP) events with >100 MeV proton intensity above 1 cm⁻² s⁻¹ s⁻¹ were recorded during 1958–2006. Before the advent of the SEP monitoring on spacecraft these results constituted the only homogeneous series of >100 MeV SEP events. The SEP intensities and energy spectra inferred from the Lebedev Physical Institute observations are consistent with the results taken in the adjacent energy intervals by the spacecraft and neutron monitors. Joint consideration of the SEP events series recorded by balloons and by neutron monitors during solar cycles 20–23 makes it possible to restore the probable number of events in solar cycle 19, which was not properly covered by observations. Some correlation was found between duration of SEP event production in a solar cycle and sunspot cycle characteristics.