High energy cosmic ray laboratory

Laboratory of High-Energy Cosmic Rays

The laboratory was formed in 1986 on the basis of two divisions: the laboratory of cosmic ray variation ( heads of the laboratory were A. I. Kuzmin, G. V. Skripin) and the department of cosmic ray station network in Siberia and the Far East (head of the department was N. P. Chirkov). I. A. Transky was elected as the first head of the laboratory of high-energy cosmic rays. Later, the laboratory was headed by V. G. Grigoryev (1991-1995) and P. A. Krivoshapkin (1995-2005). In 2005 S.A.Starodubtsev was elected as the head of laboratory.

Research directions:

* structure and dynamics of the heliosphere;

* modulation of cosmic rays by the solar wind

* investigation of the nature of solar cosmic rays

The laboratory conducts scientific substantiation of measurements of the cosmic rays intensity using the A. I. Kuzmin cosmic ray spectrograph at the Yakutsk and Tixie. The energy range of cosmic ray research is from 2 to 300 GeV.

Main scientific results

*Structure and dynamics of the heliosphere:

*Modulation of cosmic rays by the solar wind

*Investigation of the nature of solar cosmic rays.

Structure and dynamics of the heliosphere:

1. Observations of cosmic rays measured with neutron monitors as well as measured aboard the Ulysses spacecraft indicate that the neutral surface of the interplanetary magnetic field is shifted towards the south by about 7 ˚ (Fig. 1). The reason for the shift is a shortage of magnetic flux in the southern hemisphere, due to the fact that a part of flux exists in an unobservable form. A model of electrostatic approximation of the heliospheric modulation of cosmic rays, consistent with the shift mechanism, has been developed.

РИСУНОК

Fig. 1. North-southern asymmetry of the heliosphere according to observations aboard the Ulysses spacecraft. The black line is measurements; the red line is a theory.

2. A systematic disappearance of cosmic ray anisotropy near the minima of solar activity during the epoch of positive polarity of the magnetic field in the northern hemisphere of the Sun is found (Fig. 2). This fact is indicative of the inclination of neutral surface of the interplanetary magnetic field to the south.

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Fig. 2. Behavior of the cosmic rays anisotropy according to the data of the Yakutsk spectrograph in the epochs of solar activity minima at different polarities of the general magnetic field of the Sun. The thick vectors correspond to the projection of the Earth on the southern hemisphere, the thin ones – on the northern hemisphere of the Sun.

3. Based on the long-term continuous measurements of the muon telescopes at the Yakutsk and Nagoya stations it is shown that the tensor anisotropy of cosmic rays consists of stable annual and semi-annual waves of antisymmetric daily and semi-daily variations (Fig. 3). They are associated with loop structures of the interplanetary magnetic field which are concentrated at low helio-latitudes and shifted , on the average, to the south by 4.5 degrees.

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Fig. 3.Annual course of the antisymmetric daily (r12) and semi-daily (r22) variations of the CR anisotropy obtained by the data of the Nagoya and Yakutsk stations. The observed (a) and (c) and expected variations (b) and (d) of the CR anisotropy are shown. The months are denoted by numbers , the scale of the CR anisotropy is shown by the line segment. The ends of vectors are denoted by the dots on the clock dials.

Modulation of cosmic rays by the solar wind:

On the basis of measurements with the ASK-1 ionization chamber, it was found that the vector direction of anisotropy of cosmic rays with the energy E ~ 60 GeV undergoes quasiperiodic changes with a period of 22 years (Fig. 1). It is shown that the main physical reason for this behavior of the anisotropy is the change of direction of cosmic ray drift to the opposite direction in the solar activity cycle due to the change of polarity of the general solar magnetic field.

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ПОДПИСЬ К РИСУНКУ1. Fig.1. Angle between the vector of cosmic ray anisotropy and direction of the Sun-Earth depending on time. The results of measurements with the ASK-1 ionization chamber (Yakutsk) are presented; red dots correspond to periods of time with positive polarity of the general solar magnetic field, blue – negative. (Krymsky et al., Astron. Lett. 2009. V.35. No.5. P.333.)

To explain the nature of origin of Forbush decreases with a hard energy spectrum observed mainly in the solar activity minimum, a model of a piston shock wave appearing as a result of sudden jump of the solar wind speed in the surrounding spiral magnetic field is proposed. Within the framework of the model, the modulation depth of cosmic rays with the energies of 9.4 and 40 GeV, measured with the neutron monitor and muon telescope located at the Earth’s surface, has been calculated. The comparison of theoretical calculations with experimental data shows a good agreement. (Fig. 2).

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Подпись к рисунку 2. Fig. 2. Dependence of the modulation depth of cosmic rays of various energies on the velocity of shock wave front for the solar activity minimum. Points are the observed average amplitude of Forbush decreases , a thick vertical line is the observed average velocity of shock front in the solar activity minimum.

On the basis of measurements of cosmic ray intensity carried out on the world neutron monitor network as well as measurements using the Yakutsk spectrograph and spacecraft, it is shown that the 23rd solar activity cycle is characterized by a significantly lower level of turbulence of the interplanetary magnetic field in comparison with three previous ones that is manifested in a harder energy spectrum of Forbush effects in the 23rd cycle (Fig. 3).

РИСУНОК 3

Подпись к рисунку3 Fig. 3. Average annual numbers of sunspots (a), an indicator of energy spectrum of Forbush decreases by data of neutron monitors (b) and muon telescopes (c) in the 20-23 solar activity cycles.

Using the base theory of heliospheric cosmic ray modulation developed earlier (Krymsky G.F. et al. // JETP. 2007. V. 104. Issue 2. P.189.), the cosmic ray intensity in the energy range E <1 GeV expected in the 20-23 solar activity cycles, has been calculated .The calculations are in a satisfactory agreement with the results of stratospheric measurements of the cosmic ray intensity (Fig. 4). It is shown that the 23rd solar cycle (1996-2009) is distinct in its following anomalous characteristics: a lower level of magnetic turbulence in the solar wind, a harder spectrum of Forbush decreases, and anomalously low values of cosmic ray anisotropy .

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Подпись к рисунку4. Fig. 4. Dependence of the cosmic ray intensity on time by data of stratospheric measurements over Murmansk (solid circles, the geomagnetic cutoff threshold Rc = 0.6 GV) and Moscow (open circles, Rc = 2.4 GV) in the 21st (a) and 23rd (b) solar cycles. Lines are the expected cosmic ray intensity at k = 5 (a) and at k = 15 (b), where k is the ratio of regular component intensity to the turbulent component intensity of the interplanetary magnetic field.

Investigation of the nature of solar cosmic rays:

A new method has been developed for the determination of solar cosmic ray spectrum by neutron monitor data (Krymsky G.F, Grigor’ev V.G, Starodubtsev S.A. // JETP Lett. 2008. V.88. No. 7. P.483 -485.). The essence of method consists in the determination of effective momentum or energy for which the particle flux derived from the neutron monitor count rate is weakly sensitive to small variations of the index of power-law spectrum. The comparison of calculations with the direct measurements aboard spacecraft and results of other authors shows a satisfactory agreement (Fig. 1). Differential energy spectra of solar cosmic rays during the isotropic phase of flares were determined for 15 ground-level increases from 1977 to 2012. It is found that in most events a softening of index of the power-law energy spectrum is observed. In this case the interval of its changes ranges from 0.3 to 1.7, but in some cases the spectrum index remains practically unchanged within 3 hours.

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ПОДПИСЬ К РИСУНКУ1 Fig. 1. Spectrum of solar cosmic rays for ground-level enhancement on December 13, 2013.

Symbols are results of experiments;

the solid line is approximation: J ~ E-1.6 exp (-E / 1200 MeV)

Acting Head of laboratory

Pyotr Yurievich Gololobov

Junior research worker