A. Saiz

On the Estimation of Solar Energetic Particle Injection Timing from Onset Times near Earth

We examine the accuracy of a common technique for estimating the start time of solar energetic particle injection based on a linear fit to the observed onset time versus 1/(particle velocity). This is based on a concept that the first arriving particles move directly along the magnetic field with no scattering. We check this by performing numerical simulations of the transport of solar protons between 2 and 2000 MeV from the Sun to the Earth, for several assumptions regarding interplanetary scattering and the duration of particle injection, and by analyzing the results using the inverse velocity fit. We find that, in most cases, the onset times align close to a straight line as a function of inverse velocity. Despite this, the estimated injection time can be in error by several minutes. Also, the estimated path length can deviate greatly from the actual path length along the interplanetary magnetic field. The major difference between the estimated and actual path lengths implies that the first arriving particles cannot be viewed as moving directly along the interplanetary magnetic field.

Elliptical Galaxies at z = 0 from Self-consistent Hydrodynamic Simulations: Clues on Age Effects in Their Stellar Populations

We present results of a study of the stellar age distributions in the sample of elliptical-like objects (ELOs) identified at z = 0 in four hydrodynamic, self-consistent simulations operating in the context of a concordance cosmological model. The simulations show that the formation of most stars in each ELO of the sample is a consequence of violent dynamical events, either fast multiclump collapse at high zs, or mergers at lower zs. The latter can explain the age spread as well as the dynamical peculiarities observed in some ellipticals, but its relative weight is never dominant and decreases as the ELO mass at the halo scale, Mvir, increases, to such an extent that some recent mergers contributing an important fraction to the total ELO mass can possibly contribute only a small fraction of newborn stars. More massive objects have older means and narrower spreads in their stellar age distributions than less massive objects. The ELO sample shows also a tight correlation between Mvir and the central stellar line-of-sight velocity dispersion, σ. This gives a trend of the means and spreads of ELO stellar populations with σ that is consistent, even quantitatively, with the age effects observationally detected in the stellar populations of elliptical galaxies. Therefore, these effects can be explained as the observational manifestation of the intrinsic correlations found in the ELO sample between Mvir and the properties of the stellar age distribution, on the one hand, and Mvir and σ, on the other hand. These correlations hint, for the first time, at a possible way to reconcile age effects in ellipticals, and, particularly, the increase of α/Fe ratios with σ, with the hierarchical clustering paradigm. We briefly discuss the origin of the intrinsic correlations shown by ELOs in terms of the adhesion model.

Tracking cosmic-ray spectral variations with neutron monitor time-delay measurements at high cutoff rigidity during 2007-2017

We present measurements of the leader fraction of neutron monitor counts that did not follow other counts in the same counter tube from the same cosmic ray shower. We use time-delay histograms collected at the Princess Sirindhorn Neutron Monitor at Doi Inthanon, Thailand, which has the worlds highest vertical cutoff rigidity for a fixed station (16.8 GV). Changes in the leader fraction are a precise indicator of cosmic ray spectral variations above the cutoff. Our data set from 2007 to 2017 spans a full cycle of solar modulation, including the all-time cosmic ray maximum of 2009 and minimum near the end of 2014, the count rate now having returned to its initial value. The electronics to collect time-delay histograms have been upgraded twice, and we have corrected for such changes to develop a long-term leader fraction dataset. We examine the spectral variation of Galactic cosmic rays above

Cosmic Ray Modulation Observed by the Princess Sirindhorn Neutron Monitor at High Rigidity Cutoff

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Impulsive Increase of Galactic Cosmic Ray Flux Observed by IceTop

17 GV resulting from solar modulation and its solar magnetic polarity dependence.

Relationship between the neutron time delay distribution and the rigidity spectrum of primary cosmic rays up to 16.8 GV

Solar modulation refers to Galactic cosmic ray (GCR) variations with the

Observations and Monte Carlo Simulation of the Princess Sirindhorn Neutron Monitor at a Vertical Rigidity Cutoff of 16.8 GV

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Relativistic solar neutrons and protons on 28 October 2003

11-year sunspot cycle and

Largest GLE in Half a Century: Neutron Monitor Observations of the January 20, 2005 Event

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Relativistic Particle Injection and Interplanetary Transport during the January 20, 2005 Ground Level Enhancement

22-year solar magnetic cycle, and is relevant to the space radiation environment and effects on Earths atmosphere. Its complicated dependence on solar and heliospheric conditions is only roughly understood but has been empirically modeled in terms of a single modulation parameter. Most analyses of solar modulation used neutron monitor (NM) data from locations with relatively low geomagnetic cutoff rigidity, i.e., the threshold for cosmic rays to penetrate Earths magnetic field. The Princess Sirindhorn Neutron Monitor (PSNM) at Doi Inthanon, Thailand has the highest cutoff rigidity (