Thiranee Khumlumlert

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.

Measurement and simulation of neutron monitor count rate dependence on surrounding structure

Neutron monitors are the premier instruments for precise measurements of time variations (e.g., of solar origin) in the galactic cosmic ray (GCR) flux in the range of ∼1–100 GeV. However, it has proven challenging to accurately determine the yield function (effective area) versus rigidity in order to relate a neutron monitors count rate with those of other monitors worldwide and the underlying GCR spectrum. Monte Carlo simulations of the yield function have been developed, but there have been few opportunities to validate these observationally, especially regarding the particular environment surrounding each monitor. Here we have precisely measured the count rate of a calibration neutron monitor near the Princess Sirindhorn Neutron Monitor (PSNM) at Doi Inthanon, Thailand (18.59∘N, 98.49∘E, 2560 m altitude), which provides a basis for comparison with count rates of other neutron monitors worldwide that are similarly calibrated. We directly measured the effect of surrounding structure by operating the calibrator outside and inside the building. Using Monte Carlo simulations, we clarify differences in response of the calibrator and PSNM, as well as the calibrator outside and inside the building. The dependence of the calibrator count rate on surrounding structure can be attributed to its sensitivity to neutrons of 0.5–10 MeV and a shift of sensitivity to nucleons of higher energy when placed inside the building. Simulated calibrator to PSNM count rate ratios inside and outside agree with observations within a few percent, providing useful validation and improving confidence in our ability to model the yield function for a neutron monitor station.

Deconvolution of interplanetary transport of solar energetic particles

We address the problem of deconvolving the effects of interplanetary transport on observed intensity and anisotropy profiles of solar energetic particles with the goal of determining the time profile and spectrum of particle injection near the Sun as well as the interplanetary scattering mean free path. Semi-automated techniques have been developed to quantitatively determine the best fit injection profile, assuming (1) a general piecewise linear profile or (2) a Reid profile of the form [C/(t − t0)] exp[−A/(t − t0) − (t − t0)/B]. The two assumptions for the form of the injection profile yielded similar results when we tested the techniques using ISEE 3 proton data from the solar flare events of July 20, 1981 (gradual flare), and January 2, 1982 (impulsive flare). For the former event, the duration of injection was much shorter for protons of higher energy (75–147 MeV), which may be interpreted as indicating that the coronal mass ejection-driven shock lost the ability to accelerate protons to ∼100 MeV after traveling beyond a certain distance from the Sun.