Geoffrey Ye Li

Understanding large SEP events with the PATH code: Modeling of the 13 December 2006 SEP event

The Particle Acceleration and Transport in the Heliosphere (PATH) numerical code was developed to understand solar energetic particle (SEP) events in the near-Earth environment. We discuss simulation results for the 13 December 2006 SEP event. The PATH code includes modeling a background solar wind through which a CME-driven oblique shock propagates. The code incorporates a mixed population of both flare and shock-accelerated solar wind suprathermal particles. The shock parameters derived from ACE measurements at 1 AU and observational flare characteristics are used as input into the numerical model. We assume that the diffusive shock acceleration mechanism is responsible for particle energization. We model the subsequent transport of particles originated at the flare site and particles escaping from the shock and propagating in the equatorial plane through the interplanetary medium. We derive spectra for protons, oxygen, and iron ions, together with their time-intensity profiles at 1 AU. Our modeling results show reasonable agreement with in situ measurements by ACE, STEREO, GOES, and SAMPEX for this event. We numerically estimate the Fe/O abundance ratio and discuss the physics underlying a mixed SEP event. We point out that the flare population is as important as shock geometry changes during shock propagation for modeling time-intensity profiles and spectra at 1 AU. The combined effects of seed population and shock geometry will be examined in the framework of an extended PATH code in future modeling efforts.

Using the Path Code for Modeling Gradual SEP Events in the Inner Heliosphere

We model a gradual solar energetic particle (SEP) event that occurred on 2001 September 29, and was possibly caused by a coronal mass ejection related shock. A computer code PATH (particle acceleration and transport in the heliosphere) was tuned to simulate this event. The model includes local particle injection at an evolving quasi-parallel shock, first-order Fermi acceleration at the shock, and self-consistent excitation of MHD waves to enhance particle scattering, particle trapping, and escape from the shock complex, and transport in the inner heliosphere up to several AU. The shock and solar wind boundary conditions are derived from Advanced Composition Explorer (ACE) observations at 1 AU, which are then extrapolated to 0.1 AU. Modeled time-dependent spectra for energetic protons, iron, and oxygen ions are compared with ULEIS and SIS measurements onboard ACE, and with GOES-8 data. The use of the PATH code to model gradual SEP events superimposed on a pre-event background from previous SEPs is discussed.

SOLAR CYCLE ABUNDANCE VARIATIONS IN COROTATING INTERACTION REGIONS: EVIDENCE FOR A SUPRATHERMAL ION SEED POPULATION

We have surveyed the heavy ion composition of corotating interaction regions (CIRs) over the recent solar minimum and combined this with our earlier survey to cover the 1998-2011 period encompassing a full solar cycle and onset of the new cycle. We find that the solar minimum CIR intensities and spectral forms are similar to those in active periods, indicating that the basic acceleration mechanism does not vary with solar activity for energies below a few MeV nucleon{sup -1}. However, the heavy ion abundances show a clear correlation with sunspot number, where heavy ions are more enhanced during active periods. Over the mass range He-Fe, the enhancement is organized by a power law in Q/M with exponent -1.9, with Fe/O varying by a factor of {approx}6. During solar minimum CIR Fe/O was {approx}0.05, well below the corresponding solar wind ratio. Previous studies have shown that rare ions (He{sup +}, {sup 3}He) enhanced in CIRs come from the suprathermal ion pool. The observations presented here extend this evidence, indicating that in addition to rare He{sup +} and {sup 3}He the CIR major heavy ion species are accelerated out of the suprathermal ion pool, not the bulk solar wind.

Compound and perpendicular diffusion of cosmic rays and random walk of the field lines: II. Non-parallel particle transport and drifts

Compound transport of energetic charged particles across the mean magnetic field due to field line random walk is investigated by means of a Chapman–Kolmogorov equation. The probability distribution function (pdf) for the particle transport across the field P⊥ is given as a convolution of the pdf for random walk of the magnetic field, PFRW, with the pdf Pp, for particle transport relative to the random walking field. The particle propagator Pp includes the effects of advection, drift, parallel diffusion and local perpendicular diffusion of particles relative to the random walking field. At early times, the particles sub-diffuse across the field due to field line random walk. At late times, the effective cross-field diffusion coefficient has the form κ⊥e = κ⊥ + κF. The diffusion coefficient κ⊥ is the local cross-field diffusion coefficient due to particle scattering in the random magnetic field. The diffusion coefficient κF is due to coherent particle advection parallel to the mean magnetic field B0 coupled with transverse random walk of the magnetic field. Estimates of cross-field diffusion due to field line random walk, advection and drift are obtained both near to the heliospheric current sheet at Earth and at higher helio-latitudes. Cross-field diffusion due to field line random walk and advection is shown to be an important transport mechanism for low-energy particles near the current sheet, where the effects of drifts are negligible. Drift effects and field line random walk are also assessed at higher helio-latitudes off the current sheet, for a model interplanetary magnetic field, with a flat current sheet in the helio-equatorial plane.

A Study of Fast Flareless Coronal Mass Ejections

Two major processes have been proposed to convert coronal magnetic energy into the kinetic energy of a coronal mass ejection (CME): resistive magnetic reconnection and the ideal macroscopic magnetohydrodynamic instability of a magnetic flux rope. However, it remains elusive whether both processes play a comparable role or one of them prevails during a particular eruption. To shed light on this issue, we carefully studied energetic but flareless CMEs, i.e., fast CMEs not accompanied by any flares. Through searching the Coordinated Data Analysis Workshops database of CMEs observed in Solar Cycle 23, we found 13 such events with speeds larger than 1000 km s–1. Other common observational features of these events are: (1) none of them originated in active regions, they were associated with eruptions of well-developed long filaments in quiet-Sun regions; (2) no apparent enhancement of flare emissions was present in soft X-ray, EUV, and microwave data. Further studies of two events reveal that (1) the reconnection electric fields, as inferred from the product of the separation speed of post-eruption ribbons and the photospheric magnetic field measurement, were generally weak; (2) the period with a measurable reconnection electric field is considerably shorter than the total filament-CME acceleration time. These observations indicate that for these fast CMEs, the magnetic energy was released mainly via the ideal flux-rope instability through the work done by the large-scale Lorentz force acting on the rope currents rather than via magnetic reconnections. We also suggest that reconnections play a less important role in accelerating CMEs in quiet-Sun regions of weak magnetic field than those in active regions of strong magnetic field.

OBSERVATIONS OF ENERGETIC PARTICLES BETWEEN A PAIR OF COROTATING INTERACTION REGIONS

We report observations of the acceleration and trapping of energetic ions and electrons between a pair of corotating interaction regions (CIRs). The event occurred in Carrington Rotation 2060. Observed by the STEREO-B spacecraft, the two CIRs were separated by less than 5 days. In contrast to other CIR events, the fluxes of the energetic ions and electrons in this event reached their maxima between the trailing edge of the first CIR and the leading edge of the second CIR. The radial magnetic field (Br ) reversed its sense and the anisotropy of the flux also changed from Sunward to anti-Sunward between the two CIRs. Furthermore, there was an extended period of counterstreaming suprathermal electrons between the two CIRs. Similar observations for this event were also obtained with the Advanced Composition Explorer and STEREO-A. We conjecture that these observations were due to a U-shaped, large-scale magnetic field topology connecting the reverse shock of the first CIR and the forward shock of the second CIR. Such a disconnected U-shaped magnetic field topology may have formed due to magnetic reconnection in the upper corona.

OBSERVATION OF FLUX-TUBE CROSSINGS IN THE SOLAR WIND

Current sheets are ubiquitous in the solar wind. They are a major source of the solar wind MHD turbulence intermittency. They may result from nonlinear interactions of the solar wind MHD turbulence or are the boundaries of flux tubes that originate from the solar surface. Some current sheets appear in pairs and are the boundaries of transient structures such as magnetic holes and reconnection exhausts or the edges of pulsed Alfven waves. For an individual current sheet, discerning whether it is a flux-tube boundary or due to nonlinear interactions or the boundary of a transient structure is difficult. In this work, using data from the Wind spacecraft, we identify two three-current-sheet events. Detailed examination of these two events suggests that they are best explained by the flux-tube-crossing scenario. Our study provides convincing evidence supporting the scenario that the solar wind consists of flux tubes where distinct plasmas reside.

Dynamical small-scale magnetic islands as a source of local acceleration of particles in the solar wind

We present observations of energetic particle flux increases up to 1 MeV at 1 AU, which cannot be associated with ordinary mechanisms of particle acceleration, such as acceleration at shocks or at the Sun. Such unusual energetic particle events very likely have a local origin. Multi-spacecraft observations show that numerous cases of energetic particle flux enhancements and spikes correspond to passages of spacecraft through areas filled with magnetic islands with a typical width similar to 0.01-0.001AU that experience dynamical merging or/and contraction. The presence of magnetic islands inside magnetically confined cavities in the solar wind may lead to local particle energization, especially in the case when the particles have already been pre-accelerated to keV energies, for example, at shocks or due to magnetic reconnection at the heliospheric current sheet. We consider different magnetic configurations that provide favourable conditions for both the appearance of small-scale magnetic islands and their confinement.

Spectral properties of large gradual solar energetic particle events - II -Systematic Q/M-dependence of heavy ion spectral breaks

We fit the

ANGULAR DISTRIBUTION OF SOLAR WIND MAGNETIC FIELD VECTOR AT 1 AU

\sim