In a recent study of a search for enhancements from the Galactic center with muons at sea level using the TUPI muon telescope, we have found several ground-level enhancements (GLEs) as very sharp peaks above the count rate background. This paper reports a consistent analysis of two GLEs observed in 2003 December and detected after an upgrade of the data acquisition system, which includes a noise filter and allows us to verify that the GLEs are not mere background fluctuations. The main target of this study is a search for the origin of the GLEs. The results show that one of them has a strong correlation with a solar flare, while the other has an unknown origin, because there is no satellite report of a solar flare, no prompt X-ray emission, and no excess of nuclei during the raster scan in which the GLE was observed. Even so, two possibilities are analyzed: the solar flare hypothesis and the gamma-ray burst (GRB) hypothesis. We show, by using the FLUKA Monte Carlo results for photoproduction, that under certain conditions there is the possibility of an enhancement of muons at sea level from GeV GRBs.

We present a systematic study of the large asymmetries in neutral pion fraction distribution in high energy cosmic ray families (100 TeVϽE vis Ͻ700 TeV) detected at high mountain altitudes at Pamir ͑4300 m, 595 g/cm 2 ). With this in mind we have constructed robust observables, ratios of factorial moments, in experimental and simulated families in a similar way. We have found that our experimental data do not exclude the possibility of a DCC formation mechanism in high energy interactions. ͓S0556-2821͑99͒06901-5͔ PACS number͑s͒: 96.40.De, 13.85.Tp PHYSICAL REVIEW D, VOLUME 59, 054001 0556-2821/99/59͑5͒/054001͑6͒/$15.00

On 2017 September 10 Neutron Monitors (NMs) apparatus located at ground level and high latitudes detected an increase in the counting rate associated to solar energetic particles (SEPs) emission from X8.2-class solar flare and its associated CME. This was the second-highest flare of the current solar cycle. The origin was the active region AR 12673 when it was located at the edge of the west solar disk, magnetically poorly connected with Earth. However, there was a peculiar condition: the solar protons accelerated by the CME shocks were injected within a heliospheric current sheet (HCS) region when Earth was crossing this region. We show that often HCS and SEPs propagation are closely related. If the source locations of SEPs are within or close to HCS, the HCS play the role of a Sun−Earth magnetic connection. SEPs drift around HCS paths, and SEPs are also drift in a wide range of longitudes by the HCSs. In some cases, and especially when Earth crosses the HCS sector, a fraction of these particles can reach Earth with a harder energetic particle flux, triggering a ground-level enhancement (GLE). The blast on 2017 September 10, which triggered the GLE #72, was the second in the current solar cycle. We show that the two GLEs, including all sub-GLEs observed in the current solar cycle, comes from solar explosions that happened within an HCS structure; this behavior is also observed in the GLEs of the previous solar cycle. In general, solar explosions from active regions poorly connected with Earth can trigger GLEs, through the mechanism described above. In all cases, the SEPs drift processes by HCS structures provides an efficient particle transport, allowing the observation of these solar transient events.

Rotational ideal divergence-free magnetohydrodynamic (MHD) equations are expressed in terms of transformed variables w→*=(μρ)1/2v→ and μp* = (μp + w*2/2), where v→, p, and ρ are plasma velocity, pressure, and mass density, respectively. With divergence-free flows, ∇·v→=0, the plasma density ρ does not appear in the MHD equations written in terms of w→* and μp*. The non field-aligned rotational Grad-Shafranov equation is represented in spherical coordinates. Tokamak-like axisymmetric equilibria with v→ ⊥∇ρ are obtained analytically by solving for torus solutions under only three source functions.

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