Paul Evenson

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.

Protons from the decay of solar flare neutrons

We have observed fluxes of energetic protons in interplanetary space which we interpret as the decay products of neutrons generated in a solar flare on 1982 June 3 at 11:42 UT. Because of the particular geometry of this event we can construct the spectrum of neutrons escaping from the Sun with great accuracy in the kinetic energy range 10--100 MeV. The resulting spectrum places stringent constraints on the free parameters used in previously published calculations of neutron production in solar flares. We also estimate the diffusion mean free path of charged particles in the interplanetary medium in a new way.

Magnetic helicity of the Parker field

The topological properties of the interplanetary magnetic field are discussed based on a calculation of the magnetic helicity of the Parker field. The most striking feature of the helicity is that it is negative north of the heliospheric current sheet and positive south of the current sheet, regardless of the sign of the solar poloidal magnetic field. Informal arguments based on MHD turbulence theory suggest that the magnetic helicity of turbulence in the interplanetary medium may be related to the large-scale Parker helicity. Because charged particle scattering in certain types of magnetic turbulence (such as slab turbulence) depends strongly upon helicity, the existence of such a relationship could have important implications to cosmic-ray transport in the heliosphere.

A quantitative test of solar modulation theory - The proton, helium, and electron spectra from 1965 through 1979

The predictions of a conventional, spherically symmetric model of solar modulation have been compared with the measured spectra of positively and negatively charged galactic cosmic-ray particles at 1 AU throughout the 1965-1976 solar cycle and through the enhanced modulation of 1979. For the proton/helium, proton/electron, and helium/electron flux ratios, there is remarkably good agreement between theory and experiment, except for small differences in 1965 and 1969. Possible systematic experimental errors are discussed, and it is concluded that: (1) the 11 year modulation process is largely independent of the sign of the particle charge; and (2) the assumption of steady state is a fairly good approximation for long term modulation.

CME geometry in relation to cosmic ray anisotropy

Strong enhancements of the cosmic ray anisotropy were observed before and during the January 1997 CME/magnetic cloud. From a multi-station analysis of neutron monitor data, we conclude that “B × ∇n” drift is a primary source of CME-related anisotropies for 5 GeV cosmic rays. Evolution of the cosmic ray density and density gradients is closely linked to magnetic properties of the ejecta, and provides information on the magnetic cloud and related features as they approach and pass Earth. Strong enhancement of the field-aligned anisotropy was observed primarily during the 9 hours prior to shock arrival. If typical, this phenomenon should prove useful for space weather forecasting.

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.

Geometry of an interplanetary CME on October 29, 2003 deduced from cosmic rays

A coronal mass ejection (CME) associated with an X17 solar flare reached Earth on October 29, 2003, causing an ∼11% decrease in the intensity of high-energy Galactic cosmic rays recorded by muon detectors. The CME also produced a strong enhancement of the cosmic ray directional anisotropy. Based upon a simple inclined cylinder model, we use the anisotropy data to derive for the first rime the three-dimensional geometry of the cosmic ray depleted region formed behind the shock in this event. We also compare the geometry derived from cosmic rays with that derived from in situ interplanetary magnetic field (IMF) observations using a Magnetic Flux Rope model. Copyright 2004 by the American Geophysical Union.

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.

Cosmic Ray Electrons

Modulation of cosmic electrons is similar to that of nuclei, but there are clear differences. At energies below 100 MeV the electron spectrum has a negative slope, which may in some way be related to electrons released from the magnetosphere of the planet Jupiter. If there is such a relationship, its nature is not established, and alternative explanations for the upturn exist. At higher energies, electrons are predominantly negatively charged, and it is probable that the difference in net charge sign from that of nuclei is responsible for many of the observed differences in the behavior of electrons and nuclei under modulation. A consistent picture of the cosmic positron abundance and its time variation may be emerging from the world dataset.