Paisan Tooprakai

ENERGETIC PARTICLE OBSERVATIONS DURING THE 2000 JULY 14 SOLAR EVENT

Data from nine high-latitude neutron monitors are used to deduce the intensity-time and anisotropytime pro—les and pitch-angle distributions of energetic protons near Earth during the major solar event on 2000 July 14 (also known as the Bastille Day event). In addition, particle and magnetic —eld measurements from W ind, the Advanced Composition Explorer, and the Solar and Heliospheric Observatory (SOHO) are used in the analysis. The observations are —tted with good agreement between two independent numerical models of interplanetary transport. The rapid decrease of anisotropy from a high initial value cannot be explained by a simple model of interplanetary transport. Hence, we invoke a barrier or magnetic bottleneck consistent with an observed magnetic disturbance from an earlier coronal mass ejec

SPACESHIP EARTH OBSERVATIONS OF THE EASTER 2001 SOLAR PARTICLE EVENT

The largest relativistic (~1 GeV) solar proton event of the current solar activity cycle occurred on Easter 2001 (April 15). This was the first such event to be observed by Spaceship Earth, an 11-station network of neutron monitors optimized for measuring the angular distribution of solar cosmic rays. We derive the particle density and anisotropy as functions of time and model these with numerical solutions of the Boltzmann equation. We conclude that transport in the interplanetary medium was diffusive in this event, with a radial mean free path of 0.17 AU. The high time resolution of the Spaceship Earth network and the fast particle speed permit accurate determination of particle injection timing at the solar source. We find that particle injection at the Sun began at 13:42 UT ±1 minute, about 14 minutes before the first arrival of particles at Earth, in close association with the onset of shock-related radio emissions and ~15 minutes after liftoff of a coronal mass ejection (CME). Our results are consistent with the hypothesis that solar particles were accelerated to GeV energies on Easter 2001 by a CME-driven shock wave.

Relativistic Solar Protons on 1989 October 22: Injection and Transport along Both Legs of a Closed Interplanetary Magnetic Loop

Worldwide neutron monitor observations of relativistic solar protons on 1989 October 22 have proven puzzling, with an initial spike at some stations followed by a second peak, which is difficult to understand in terms of transport along a standard Archimedean spiral magnetic field or a second injection near the Sun. Here we analyze data from polar monitors, which measure the directional distribution of solar energetic particles (mainly protons) at rigidities of � 1‐3 GV. This event has the unusual properties that the particle density dips after the initial spike, followed by a hump with bidirectional flows and then a very slow decay. The spectral index, determined using bare neutron counters, varies dramatically, with energy dispersion features. The density and anisotropy data are simultaneously fit by simulating the particle transport for various magnetic field configurations and determining the best-fit injection functionneartheSun.ThedataarenotwellfitforanArchimedeanspiralfield,amagneticbottleneckbeyondEarth,or particle injection along one leg of a closed magnetic loop. A model with simultaneous injection along both legs of a closed loop provides a better explanation: particles moving along the near leg make up the spike, those coming from thefarlegmakeupthehump,bothlegscontributetothebidirectional streaming,andtrappingintheloopaccountsfor the slow decay of the particle density. Refined fits indicate a very low spectral index of turbulence, q < 1, a parallel mean free path of 1.2‐2.0 AU, a loop length of 4:7 � 0:3 AU, and escape of relativistic protons from the loop on a timescale of 3 hr. The weak scattering is consistent with reports of weak fluctuations in magnetic loops, while the low q-value may indicate a smaller correlation length as well.

SQUEEZING OF PARTICLE DISTRIBUTIONS BY EXPANDING MAGNETIC TURBULENCE AND SPACE WEATHER VARIABILITY

Among the space weather effects due to gradual solar storms, greatly enhanced high-energy ion fluxes contribute to radiation damage to satellites, spacecraft, and astronauts and dominate the hazards to air travelers, which motivates examination of the transport of high-energy solar ions to Earths orbit. Ions of low kinetic energy (up to ~2 MeV nucleon–1) from impulsive solar events exhibit abrupt changes due to filamentation of the magnetic connection from the Sun, indicating that anisotropic, field-aligned magnetic flux tubelike structures persist to Earths orbit. By employing a corresponding spherical two-component model of Alfvenic (slab) and two-dimensional magnetic fluctuations to trace simulated trajectories in the solar wind, we show that the distribution of high-energy (E ≥ 1 GeV) protons from gradual solar events is squeezed toward magnetic flux structures with a specific polarity because of the conical shape of the flux structures. Conical flux structures and the squeezing of energetic particle distributions should occur in any astrophysical wind or jet with expanding, magnetized, turbulent plasma. This transport phenomenon contributes to event-to-event variability in ground level enhancements of GeV-range ions from solar storms, presenting a fundamental uncertainty in space weather prediction.

Simulations of Lateral Transport and Dropout Structure of Energetic Particles from Impulsive Solar Flares

We simulate trajectories of energetic particles from impulsive solar flares for 2D+slab models of magnetic turbulence in spherical geometry to study dropout features, i.e., sharp, repeated changes in the particle density. Among random-phase realizations of two-dimensional (2D) turbulence, a spherical harmonic expansion can generate homogeneous turbulence over a sphere, but a 2D fast Fourier transform (FFT) locally mapped onto the lateral coordinates in the region of interest is much faster computationally, and we show that the results are qualitatively similar. We then use the 2D FFT field as input to a 2D MHD simulation, which dynamically generates realistic features of turbulence such as coherent structures. The magnetic field lines and particles spread nondiffusively (ballistically) to a patchy distribution reaching up to 25 degrees from the injection longitude and latitude at

MONITORING SHORT-TERM COSMIC-RAY SPECTRAL VARIATIONS USING NEUTRON MONITOR TIME-DELAY MEASUREMENTS

r \sim 1

SIMULATIONS OF LATERAL TRANSPORT AND DROPOUT STRUCTURE OF ENERGETIC PARTICLES FROM IMPULSIVE SOLAR FLARES

AU. This dropout pattern in field line trajectories has sharper features in the case of the more realistic 2D MHD model, in better qualitative agreement with observations. The initial dropout pattern in particle trajectories is relatively insensitive to particle energy, though the energy affects the patterns evolution with time. We make predictions for future observations of solar particles near the Sun (e.g., at 0.25 AU), for which we expect a sharp pulse of outgoing particles along the dropout pattern, followed by backscattering that first remains close to the dropout pattern and later exhibits cross-field transport to a distribution that is more diffusive, yet mostly contained within the dropout pattern found at greater distances.