Anisotropy Of Galactic Cosmic Rays Research Articles

North–south component of galactic cosmic ray anisotropy at 1 AU

The galactic cosmic ray (GCR) solar diurnal anisotropy (SDA) may be represented by a vector (A; Ar, Aφ, Aθ) in a spherical polar coordinate system centered on the sun. We reported (elsewhere) the results of a detailed study of time variations of yearly radial (Ar) and east–west (Aφ) components recorded by the global network of the neutron monitors (NMs) with a long track record for 1963–2013, for four sunspot number (SSN) cycles (20–23) and the rising phase of cycle 24. A powerful new technique is used to compute and study time variations of the transverse component (Aθ) due to off-ecliptic GCR contributions, with the same NM data; GCR radial particle density gradient (Gr) drives all three components. The north–south anisotropy (Aθ) is computed from yearly NM data (Gϕ=0), a flat heliospheric current sheet (HCS) model and the concept of GCR isotropic hard sphere scattering in the solar wind plasma. Relationships to SSN, rigidity and solar polarity intervals are studied. For a positive (p-) polarity the solar magnetic field in the northern hemisphere points outward and GCRs drift from polar regions toward equatorial plane and out along HCS, setting up a symmetric gradient (Gθs) pointing away from HCS (there is a local GCR density minimum on HCS); for n-polarity interval Gθs points towards HCS (there is a local GCR density maximum on HCS). Also, there exists a heliospheric asymmetric density gradient (Gθa) perpendicular to the ecliptic plane, it is the main contributor to Aθ for the period of our analysis. This is the most interesting and significant insight.

The cosmic ray anisotropy below 10<sup>15</sup> eV

Abstract. The measurement of the anisotropy in the cosmic ray (CR) arrival direction distribution provides important informations on the propagation mechanisms and on the identification of their sources. In the last decade the anisotropy came back to the attention of the scientific community, thanks to several new two-dimensional representations of the CR arrival direction distribution which clearly showed the existence of anisotropies at different angular scales in both hemispheres. The origin of the observed anisotropies is still unknown. So far, no theory of CRs in the Galaxy exists yet to explain the observations leaving the standard model of CRs and that of the local magnetic field unchanged at the same time. In this paper the observations of Galactic CR anisotropy will be briefly summarized, with particular attention to the results obtained by the ARGO-YBJ experiment in the Northern Hemisphere.

PRECURSORS TO INTERSTELLAR SHOCKS OF SOLAR ORIGIN

On or about 2012 August 25, the Voyager 1 spacecraft crossed the heliopause into the nearby interstellar plasma. In the nearly three years that the spacecraft has been in interstellar space, three notable particle and field disturbances have been observed, each apparently associated with a shock wave propagating outward from the Sun. Here, we present a detailed analysis of the third and most impressive of these disturbances, with brief comparisons to the two previous events, both of which have been previously reported. The shock responsible for the third event was first detected on 2014 February 17 by the onset of narrowband radio emissions from the approaching shock, followed on 2014 May 13 by the abrupt appearance of intense electron plasma oscillations generated by electrons streaming outward ahead of the shock. Finally, the shock arrived on 2014 August 25, as indicated by a jump in the magnetic field strength and the plasma density. Various disturbances in the intensity and anisotropy of galactic cosmic rays were also observed ahead of the shock, some of which are believed to be caused by the reflection and acceleration of cosmic rays by the magnetic field jump at the shock, and/or by interactions with upstream plasma waves. Comparisons to the two previous weaker events show somewhat similar precursor effects, although differing in certain details. Many of these effects are very similar to those observed in the region called the foreshock that occurs upstream of planetary bow shocks, only on a vastly larger spatial scale.

Effects of the Heliospheric Current Sheet on Trains of Enhanced Diurnal Variations in Galactic Cosmic Rays

We present recurrent trains of the enhanced diurnal anisotropy (DA) of Galactic cosmic rays (GCRs) during 2010 as observed by the Princess Sirindhorn Neutron Monitor at Doi Inthanon with a vertical cutoff rigidity of 16.8 GV. By investigating synoptic charts of the heliospheric current sheet (HCS) and the trains of enhanced DA, we found some consistency between them. During most time periods when recurrent trains of enhanced DA occurred, the DAs were noticeably enhanced more during the away sector than during the toward sector of the interplanetary magnetic field $\mathbf{B}$ . We interpret these temporal features in terms of a $\mathbf{B}\times\nabla n$ gradient anisotropy, which is associated with latitudinal gradients of the GCR density ( $n$ ) derived from the perpendicular component of DA in the solar wind frame. The gradual changes of DA in the trains in turn are associated with the gradual change in the latitudinal gradient of the GCR density along the HCS. We found that the north–south asymmetry (offset) of the HCS is responsible for the asymmetric recurrent modulation of the GCR intensity and suggest that particle drift along the wavy HCS and the adiabatic loss can cause the recurrent modulation, which is related to a local latitudinal gradient in the GCR density. The causes of the latitudinal gradient are discussed in light of current theories and observations.

Three dimensional solar anisotropy of galactic cosmic rays near the recent solar minimum 23/24

Three dimensional (3D) galactic cosmic ray (GCR) anisotropy has been studied for 2006-2012. The GCR anisotropy, both in the ecliptic plane and in polar direction, were obtained based on the neutron monitors (NMs) and Nagoya muon telescopes (MT) data. We analyze two dimensional (2D) GCR anisotropy in the ecliptic plane and north-south anisotropy normal to the ecliptic plane. We reveal quasi-periodicities - the annual and 27-days waves in the GCR anisotropy in 2006-2012. We investigate the relationship of the 27-day variation of the GCR anisotropy in the ecliptic plane and in the polar direction with the parameters of solar activity and solar wind.

COROTATING SOLAR WIND STRUCTURES AND RECURRENT TRAINS OF ENHANCED DIURNAL VARIATION IN GALACTIC COSMIC RAYS

Data from the Princess Sirindhorn Neutron Monitor at Doi Inthanon, Thailand, with a vertical cutoff rigidity of 16.8 GV, were utilized to determine the diurnal anisotropy (DA) of Galactic cosmic rays (GCRs) near Earth during solar minimum conditions between 2007 November and 2010 November. We identified trains of enhanced DA over several days, which often recur after a solar rotation period (~27 days). By investigating solar coronal holes as identified from synoptic maps and solar wind parameters, we found that the intensity and anisotropy of cosmic rays are associated with the high-speed streams (HSSs) in the solar wind, which are in turn related to the structure and evolution of coronal holes. An enhanced DA was observed after the onset of some, but not all, HSSs. During time periods of recurrent trains, the DA was often enhanced or suppressed according to the sign of the interplanetary magnetic field B, which suggests a contribution from a mechanism involving a southward gradient in the GCR density, n, and a gradient anisotropy along B × ∇n. In one non-recurrent and one recurrent sequence, an HSS from an equatorial coronal hole was merged with that from a trailing mid-latitude extension of a polar coronal hole, and the slanted HSS structure in space with suppressed GCR density can account for the southward GCR gradient. We conclude that the gradient anisotropy is a source of temporary changes in the GCR DA under solar minimum conditions, and that the latitudinal GCR gradient can sometimes be explained by the coronal hole morphology.

Cosmic ray anisotropy in fractional differential models of anomalous diffusion

The problem of galactic cosmic ray anisotropy is considered in two versions of the fractional differential model for anomalous diffusion. The simplest problem of cosmic ray propagation from a point instantaneous source in an unbounded medium is used as an example to show that the transition from the standard diffusion model to the Lagutin-Uchaikin fractional differential model (with characteristic exponent α = 3/5 and a finite velocity of free particle motion), which gives rise to a knee in the energy spectrum at 106 GeV, increases the anisotropy coefficient only by 20%, while the anisotropy coefficient in the Lagutin-Tyumentsev model (with exponents α = 0.3 and β = 0.8, a long stay of particles in traps, and an infinite velocity of their jumps) is close to one. This is because the parameters of the Lagutin-Tyumentsev model have been chosen improperly.

Rigidity spectrum and cosmic ray anisotropy during sporadic events in July 2000

A spectrographic global survey is performed to study the rigidity spectrum and anisotropy of galactic cosmic rays using spacecraft data and data obtained via ground-based observations of cosmic rays (CRs) by a worldwide network of stations during the GLE of July 14, 2000, and the strong magnetic storm related to the coronal mass ejection (CME) accompanying the solar flare. The CR rigidity spectrum observed over the range of 1 to ∼20 GV during this period is shown to be described not only by the power function of particle rigidity; the distribution of CRs in the earthward direction varies over time and depends on their energy.

Extragalactic cosmic rays and their signatures

The signatures of UHE proton propagation through CMB radiation are pair-production dip and GZK cutoff. The visible manifestations of these two spectral features are ankle, which is intrinsic part of the dip, beginning of GZK cutoff in the differential spectrum and E1/2 in integral spectrum. Observed practically in all experiments since 1963, the ankle is usually interpreted as a feature caused by transition from galactic to extragalactic cosmic rays. Using the mass composition measured by HiRes, Telescope Array and Auger detectors at energy (1–3)EeV, calculated anisotropy of galactic cosmic rays at these energies, and the elongation curves we strongly argue against the interpretation of the ankle given above. The transition must occur at lower energy, most probably at the second knee as the dip model predicts. The other prediction of the dip model, the shape of the dip, is well confirmed by HiRes, Telescope Array (TA), AGASA and Yakutsk detectors, and, after recalibration of energies, by Auger detector. Predicted beginning of GZK cutoff and E1/2 agree well with HiRes and TA data. However, directly measured mass composition remains a puzzle. While HiRes and TA detectors observe the proton-dominated mass composition, as required by the dip model, the data of Auger detector strongly evidence for nuclei mass composition becoming progressively heavier at energy higher than 4EeV and reaching Iron at energy about 35EeV. The Auger-based scenario is consistent with another interpretation of the ankle at energy Ea≈4EeV as transition from extragalactic protons to extragalactic nuclei. The heavy-nuclei dominance at higher energies may be provided by low-energy of acceleration for protons Epmax∼4EeV and rigidity-dependent EAmax=ZEpmax for nuclei. The highest energy suppression may be explained as nuclei-photodisintegration cutoff.

Cosmic ray rigidity spectrum and anisotropy during GLE on 14 July 2000

Using the method of spectrographic global survey, rigidity spectrum and anisotropy of galactic cosmic rays during GLE on 14 July 2000 have been studied with the data from ground-based observations of cosmic rays (CR) at the world-wide network of stations. The CR rigidity spectrum observed during this period over the range 1 to ~ 20 GV is shown to be described not only by power function of particle rigidity: distribution of CRs in the earthward direction varies with time and depends on their energy.

July 2000 Forbush-effect from world network of cosmic rays stations

We studied variations in the galactic cosmic ray (CR) rigidity spectrum and anisotropy during the July 2000 strong magnetic storm from the ground-based observations of cosmic rays within the world network of neutron monitor stations through spectrographic global survey. We show the time history for amplitude variations in the rigidity spectrum of cosmic rays, their anisotropy and planetary system of the geomagnetic cutoff rigidity during the Forbush-effect evolution over that period.

Observation of anisotropy in the arrival direction distribution of cosmic rays above TeV energies with IceCube

The IceCube neutrino telescope, completed in December 2010, is a cubic-kilometer scale detector buried under the South Pole ice. Between May 2009 and May 2010, IceCube recorded 32 billion of atmospheric muons generated in air showers produced by cosmic rays in the TeV energy range. This high statistics data sample can be used to look for anisotropy in the arrival directions of the cosmic ray particles at the per-mille level. IceCube observes, for the first time in the southern hemisphere, an energy dependence in the Galactic cosmic ray anisotropy up to a few hundred TeV. This study shows that the same large-scale anisotropy observed at median energies around 20TeV is not present at 400TeV; the anisotropy observed at 400TeV shows substantial differences with respect to that at lower energy. In addition to the large-scale features observed at 20TeV in the form of strong dipole and quadrupole moments, the data include several localized regions of excess and deficit on scales between 10° and 30°. The features observed at both large and small scales are statistically significant, but their origin is currently unknown.

OBSERVATION OF ANISOTROPY IN THE GALACTIC COSMIC-RAY ARRIVAL DIRECTIONS AT 400 TeV WITH ICECUBE

In this paper we report the first observation in the Southern hemisphere of an energy dependence in the Galactic cosmic ray anisotropy up to a few hundred TeV. This measurement was performed using cosmic ray induced muons recorded by the partially deployed IceCube observatory between May 2009 and May 2010. The data include a total of 33$\times 10^{9}$ muon events with a median angular resolution of $\sim3^{\circ}$ degrees. A sky map of the relative intensity in arrival direction over the Southern celestial sky is presented for cosmic ray median energies of 20 and 400 TeV. The same large-scale anisotropy observed at median energies around 20 TeV is not present at 400 TeV. Instead, the high energy skymap shows a different anisotropy structure including a deficit with a post-trial significance of -6.3$\sigma$. This anisotropy reveals a new feature of the Galactic cosmic ray distribution, which must be incorporated into theories of the origin and propagation of cosmic rays.

Effects in cosmic rays in March 1991

Using the spectrographic global survey method, variations in the rigidity spectrum and anisotropy of galactic cosmic rays (March 1991) have been studied using data from ground-based observations of cosmic rays (CR) at the worldwide network of stations. Variations in geomagnetic cutoff rigidity (GCR) have been calculated. The paper also presents latitudinal GCR variations at certain moments of the considered period for different geomagnetic field disturbance levels. Calculation results of GCR variations have been compared with those of effect of the westward current flowing with a strength proportional to the latitude cosine along parallels on the sphere, for different radii of the current ring in the dipole field.

27-day variations of the galactic cosmic ray intensity and anisotropy in the minimum of the 23rd solar activity cycle

We study the 27-day variations of the solar wind velocity, galactic cosmic ray (GCR) intensity and anisotropy in the last minimum epoch of solar activity (2007–2009, A<0). The average amplitude of the 27-day variation of the galactic cosmic ray anisotropy (A27A) in the current minimum epoch of solar activity (2007–2009, A<0) is lesser than in previous positive polarity period as it is expected from the drift theory. So, polarity dependence rule for the 27-day variation of the GCR anisotropy is fully kept. It is a universal principle for the amplitudes of the 27-day variation of the GCR anisotropy. At the same time, the average amplitude of the 27-day variation of the GCR intensity (A27I) remains at the same level as for previous minimum epoch 1995–1997 (A>0) showing by the same token an violation of its polarity dependence rule established earlier. We assume that this phenomenon could be generally related with the well established 27-day variation of the solar wind velocity being in anti-correlation with the similar changes of the 27-day variation of the GCR intensity. Generally, a character of the heliolongitudinal asymmetry of spatial large-scale structure of the solar wind velocity (SWV) established in the recent minimum epoch, preferentially pronounces in the behavior of the 27-day variation of the GCR intensity than anisotropy. The formation of the 27-day variation of the GCR anisotropy preferentially takes place in a restricted disk like local vicinity in the helioequatorial region, whilst the 27-day variation of the GCR intensity is formed in the global three dimensional vicinity of the heliosphere.

Analysis of the Forbush effect in May 2005 using the spectrographic global survey method

Based on the data from ground observations of cosmic rays (CRs) obtained by the spectrographic global survey method on the worldwide network of stations, the variations in the rigidity spectrum and anisotropy of galactic cosmic rays were investigated, along with changes in the geomagnetic cutoff rigidity in May 2005. A high degree of anisotropy (∼40–60% for the first spherical harmonics and ∼5–6% for the second spherical harmonics) was observed for particles with a rigidity of 4 GV at the moments of the maximum modulation of cosmic rays.

Definition of anisotropy of galactic cosmic rays in real time

Definition of anisotropy of galactic cosmic rays in real time

Modeling of the high-energy galactic cosmic-ray anisotropy

A possible origin of the large-scale anisotropy of galactic cosmic rays at TeV energies is discussed. It can be well modeled by a superposition of the Global Anisotropy and the Midscale Anisotropy. The Global Anisotropy would be generated by galactic cosmic rays interacting with the magnetic field in the local interstellar space of scale 2 pc surrounding the heliosphere. On the other hand, the Midscale Anisotropy, possibly caused by the modulation of galactic cosmic rays in the heliotail, is expressed as two intensity enhancements placed along the Hydrogen Deflection Plane, each symmetrically centered away from the heliotail direc- tion. We find that the separation angle between the helio- tail direction and each enhancement monotonously decreases with increasing energy in an energy range 4-30 TeV.

Applications and usage of the real-time Neutron Monitor Database

A high-time resolution Neutron Monitor Database (NMDB) has started to be realized in the frame of the Seventh Framework Programme of the European Commission. This database will include cosmic ray data from at least 18 neutron monitors distributed around the world and operated in real-time. The implementation of the NMDB will provide the opportunity for several research applications most of which will be realized in real-time mode. An important one will be the establishment of an Alert signal when dangerous solar cosmic ray particles are heading to the Earth, resulting into ground level enhancements effects registered by neutron monitors. Furthermore, on the basis of these events analysis, the mapping of all ground level enhancement features in near real-time mode will provide an overall picture of these phenomena and will be used as an input for the calculation of the ionization of the atmosphere. The latter will be useful together with other contributions to radiation dose calculations within the atmosphere at several altitudes and will reveal the absorbed doses during flights. Moreover, special algorithms for anisotropy and pitch angle distribution of solar cosmic rays, which have been developed over the years, will also be set online offering the advantage to give information about the conditions of the interplanetary space. All of the applications will serve the needs of the modern world which relies at space environment and will use the extensive network of neutron monitors as a multi-directional spectrographic detector. On top of which, the decreases of the cosmic ray intensity – known as Forbush decreases – will also be analyzed and a number of important parameters such as galactic cosmic ray anisotropy will be made available to the users of NMDB. A part of the NMDB project is also dedicated to the creation of a public outreach website with the scope to inform about cosmic rays and their possible effects on humans, technological systems and space-terrestrial environment. Therefore, NMDB will also stand as an informative gate on space research through neutron monitor’s data usage.

ON TEMPORAL VARIATIONS OF THE MULTI-TeV COSMIC RAY ANISOTROPY USING THE TIBET III AIR SHOWER ARRAY

We analyze the large-scale two-dimensional sidereal anisotropy of multi-TeV cosmic rays by Tibet Air Shower Array, with the data taken from 1999 November to 2008 December. To explore temporal variations of the anisotropy, the data set is divided into nine intervals, each in a time span of about one year. The sidereal anisotropy of magnitude about 0.1% appears fairly stable from year to year over the entire observation period of nine years. This indicates that the anisotropy of TeV Galactic cosmic rays remains insensitive to solar activities since the observation period covers more than a half of the 23rd solar cycle.