G. M. Le

Interaction between Two Coronal Mass Ejections in the 2013 May 22 Large Solar Energetic Particle Event

We investigate the eruption and interaction of two coronal mass ejections (CMEs) during the large 2013 Mayxa022 solar energetic particle event using multiple spacecraft observations. Two CMEs, having similar propagation directions, were found to erupt from two nearby active regions (ARs), AR11748 and AR11745, at ~08:48xa0UT and ~13:25xa0UT, respectively. The second CME was faster than the first CME. Using the graduated cylindrical shell model, we reconstructed the propagation of these two CMEs and found that the leading edge of the second CME caught up with the trailing edge of the first CME at a height of ~6 solar radii. After about two hours, the leading edges of the two CMEs merged at a height of ~20 solar radii. Type II solar radio bursts showed strong enhancement during this two hour period. Using the velocity dispersion method, we obtained the solar particle release (SPR) time and the path length for energetic electrons. Further assuming that energetic protons propagated along the same interplanetary magnetic field, we also obtained the SPR time for energetic protons, which were close to that of electrons. These release times agreed with the time when the second CME caught up with the trailing edge of the first CME, indicating that the CME-CME interaction (and shock-CME interaction) plays an important role in the process of particle acceleration in this event.

Northern Sky Galactic Cosmic Ray Anisotropy between 10 and 1000 TeV with the Tibet Air Shower Array

We report on the analysis of the 10−1000 TeV large-scale sidereal anisotropy of Galactic cosmic rays (GCRs) with the data collected by the Tibet Air Shower Array from 1995 October to 2010 February. In this analysis, we improve the energy estimate and extend the decl. range down to −30◦. We find that the anisotropy maps above 100 TeV are distinct from that at a multi-TeV band. The so-called tail-in and loss-cone features identified at low energies get less significant, and a new component appears at ∼ 100 TeV. The spatial distribution of the GCR intensity with an excess (7.2σ pre-trial, 5.2σ post-trial) and a deficit (−5.8σ pre-trial) are observed in the 300 TeV anisotropy map, in close agreement with IceCube’s results at 400 TeV. Combining the Tibet results in the northern sky with IceCube’s results in the southern sky, we establish a full-sky picture of the anisotropy in hundreds of TeV band. We further find that the amplitude of the first order anisotropy increases sharply above ∼ 100 TeV, indicating a new component of the anisotropy. All these results may shed new light on understanding the origin and propagation of GCRs.

SEED POPULATION IN LARGE SOLAR ENERGETIC PARTICLE EVENTS AND THE TWIN-CME SCENARIO

It has recently been suggested that large solar energetic particle (SEP) events are often caused by twin coronal mass ejections (CMEs). In the twin-CME scenario, the preceding CME provides both an enhanced turbulence level and enhanced seed population at the main CME-driven shock. In this work, we study the effect of the preceding CMEs on the seed population. We examine event-integrated abundance of iron to oxygen ratio (Fe/O) at energies above 25 MeV/nuc for large SEP events in solar cycle 23. We find that the Fe/O ratio (normalized to the reference coronal value of 0.134) ≤2.0 for almost all single-CME events and these events tend to have smaller peak intensities. In comparison, the Fe/O ratio of twin-CME events scatters in a larger range, reaching as high as 8, suggesting the presence of flare material from perhaps preceding flares. For extremely large SEP events with peak intensities above 1000 pfu, the Fe/O ratios drop below 2, indicating that the seed particles are dominated by coronal material rather than flare material in these extreme events. The Fe/O ratios of ground level enhancement (GLE) events, which are all twin-CME events, scatter in a broad range. For a given Fe/O ratio, GLE events tend to have larger peak intensities than non-GLE events. Using velocity dispersion analysis, we find that GLE events have lower solar particle release heights than non-GLE events, agreeing with earlier results by Reames.

Time dependence of loss-cone amplitude measured with the tibet air-shower array

The galactic cosmic-ray anisotropy at TeV energies has a large-scale deficit region distributed around 150 to 240 degrees in right ascension, which is called “Loss-Cone”. The Milagro experiment in the U.S. detected a significant increase in the Loss-Cone amplitude at 6 TeV from July 2000 to July 2007, and argued that it could be due to variations in the heliosphere in relation to solar activities. In this presentation, we report on the time dependence of the Loss-Cone amplitude from November 1999 through December 2008 measured with the Tibet air-shower array. No time dependence was found in the Loss-Cone amplitude at energies of 4.4, 6.2, and 11 TeV. If the increase in the Loss-Cone amplitude Milagro detected were genuine, the same tendency would be seen at sub-TeV energies where the anisotropy is far more sensitive to solar activities. Matsushiro underground muon observation at 0.6 TeV during the corresponding period, however, reported no significant increase of the Loss-Cone amplitude.

TeV Gamma Ray Survey on the Direction of Fermi-LAT Pulsars with the Tibet Air Shower Array

We search for steady TeV γ-rays from 18 pulsars in the Fermi-LAT pulsar catalog using the Tibet air shower array. We observe 8 sources including the Crab against expected 0.41 sources at a significance of 2σ or more among the 18 pulsars. The chance probability from Poisson statistics is estimated to be 1.4×10-8.With the Crab excluded, it is estimated to be 1.8×10-7. These low chance probabilities clearly show that the Fermi pulsars have a statistically significant correlation with TeV γ-ray excesses observed by the Tibet air shower array.

Calibration of the Yangbajing air-shower core detector (YAC) using the beam of BEPC

Aiming at the observation of cosmic-ray chemical composition at the knee energy region, a new type airshower-core detector (YAC, Yangbajing Air shower Core array) has been developed and set up at Yangbajing, 4300 m a.s.l. in Tibet, China since August, 1st, 2011. YAC will work together with the Tibet-III array and a large muon detector as a hybrid experiment. Each YAC detector unit consists of lead plates of 3.5 cm thick and a scintillation counter which detects the burst size induced by high energy electromagnetic component in the air-shower cores. The burst size is demanded to be measured from 1 MIP (Minimum Ionization Particles) to 10 MIPs. The linearity and the saturation of the plastic scintillator and PMT used in the YAC detector have been studied with the accelerator beam of the BEPCII (Beijing Electron Positron Collider, IHEP, China). The accelerator-beam experiment shows a good linearity between the incident particle flux and YAC-ADC output below 5×10 MIPs and the saturation effect of the plastic scintillator satisfies YAC detector’s requirement.

Modeling of the galactic cosmic-ray anisotropy at TeV energies

A possible origin of the large-scale anisotropy of TeV galactic cosmic rays 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 a few parsec scale surrounding the heliosphere. On the other hand, the Midscale Anisotropy would be caused by the modulation of galactic cosmic rays in the heliotail. The Midscale Anisotropy can be expressed as two intensity enhancements placed along the Hydrogen Deflection Plane, each symmetrically centered away from the heliotail direction. It is found that the separation angle between the heliotail direction and each of the two intensity enhancements monotonously decreases as energy increases from 4 TeV to 30 TeV.

Recent results on gamma-ray observation by the tibet air shower array and related topics

The Tibet-III air shower array, consisting of 789 scintillation detectors, has been operating successfully at Yangbajing in Tibet since 1999. Some recent results on gamma-ray observation by the Tibet-III air shower array are introduced and its future prospects of gamma-ray observation will be discussed.

Evaluation of the Interplanetary Magnetic Field Strength Using the Cosmic-Ray Shadow of the Sun

We analyze the Suns shadow observed with the Tibet-III air shower array and find that the shadows center deviates northward (southward) from the optical solar disk center in the away (toward) interplanetary magnetic field (IMF) sector. By comparing with numerical simulations based on the solar magnetic field model, we find that the average IMF strength in the away (toward) sector is 1.54±0.21_{stat}±0.20_{syst} (1.62±0.15_{stat}±0.22_{syst}) times larger than the model prediction. These demonstrate that the observed Suns shadow is a useful tool for the quantitative evaluation of the average solar magnetic field.

Measurement of high energy cosmic rays by the new Tibet hybrid experiment

We have started a new hybrid air shower experiment at Yangbajing (4300 m a.s.l.) in Tibet in February 2014. This new hybrid experiment consists of the YAC-II comprised of 124 core detectors placed in the form of a square grid of 1.9 m spacing covering about 500 m2, the Tibet-III air shower array with the total area of about 50,000 m2 and the underground MD array consisting of 80 cells, with the total area of about 4,200 m2. This hybrid-array system is used to observe air showers of high energy celestial gamma-ray origin and those of nuclear-component origin. In this paper, a short review of the experiment will be followed by an overview on the current results on energy spectrum and chemical composition of CRs and test of hadronic interaction models.