Chee K. Ng

On the Remote Detection of Suprathermal Ions in the Solar Corona and their Role as Seeds for Solar Energetic Particle Production

Forecasting large solar energetic particle (SEP) events associated with shocks driven by fast coronal mass ejections (CMEs) poses a major difficulty in the field of space weather. Besides issues associated with CME initiation, the SEP intensities are difficult to predict, spanning three orders of magnitude at any given CME speed. Many lines of indirect evidence point to the pre-existence of suprathermal seed particles for injection into the acceleration process as a key ingredient limiting the SEP intensity of a given event. This paper outlines the observational and theoretical basis for the inference that a suprathermal particle population is present prior to large SEP events, explores various scenarios for generating seed particles and their observational signatures, and explains how such suprathermals could be detected through measuring the wings of the H I Ly{alpha} line.

USE OF INCIDENT AND REFLECTED SOLAR PARTICLE BEAMS TO TRACE THE TOPOLOGY OF MAGNETIC CLOUDS

Occasionally, large solar energetic particle (SEP) events occur inside magnetic clouds (MCs). In this work, the onset time analysis, the peak intensity analysis, and the decay phase analysis of SEPs are used to investigate two large SEP events inside MCs: the 1998 May 2 and 2002 April 21 events. The onset time analysis of non-relativistic electrons and ∼MeV nucleon −1 heavy ions shows the stability of the magnetic loop structure during a period of a few hours in the events examined. The joint analysis of pitch-angle distributions and peak intensities of electrons exhibits that, depending on the particle pitch angle observed at 1 AU, in the April event the reflection point of particles may be distributed along a wide spatial range, implying that the magnetic loop is a magnetic bottle connected to the Sun with both legs. In contrast, in the May event particle reflection occurs abruptly at the magnetic mirror formed by a compressed field enhancement behind the interplanetary shock, consistent with its open field line topology.

What Causes Scatter-free Transport of Non-relativistic Solar Electrons?

We have examined the cause of the scatter-free transport of non-relativistic solar electrons. Electron scatter-free transport events are compared with the diffusive transport event. The emphasis of our examination is on the energy dependence of electron angular distributions and the steepening of interplanetary magnetic field (IMF) power spectral densities (PSDs). Near and above the proton gyrofrequency, the effects of both R-mode (whistler) and L-mode (electromagnetic ion cyclotron, EMIC) waves need to be taken into account separately. The PSD spectral steepening due to the EMIC wave damping by solar-wind thermal ions becomes essential. In a fast-rise-fast-decay impulsive electron event we have observed such steepening, which significantly reduces PSD levels at frequencies above the proton gyrofrequency. The spectral steepening thus produced favors the occurrence of scatter-free transport of low-energy electrons. Consequently, within the Wind/3D Plasma and Energetic Particle Instrument/Silicon Semiconductor Telescope measured energy range (~25-500 keV), there appears to be an electron energy window, across which the scatter-free transport of lower energy electrons would change to the diffusive transport of higher energy electrons. We have observed such a change and found it is correlated with the occurrence of broken power-law spectra of electrons. Thus the connection between the transition from diffusive to scatter-free electron transport and the concurrent transition from high to low IMF PSD levels with corresponding breaks in the electron power-law energy spectrum and PSD spectrum has been recognized.

Source Regions of the Interplanetary Magnetic Field and Variability in Heavy-ion Elemental Composition in Gradual Solar Energetic Particle Events

Gradual solar energetic particle (SEP) events are those in which ions are accelerated to their observed energies by interactions with a shock driven by a fast coronal mass ejection (CME). Previous studies have shown that much of the observed event-to-event variability can be understood in terms of shock speed and evolution in the shock-normal angle. However, an equally important factor, particularly for the elemental composition, is the origin of the suprathermal seed particles upon which the shock acts. To tackle this issue, we (1) use observed solar-wind speed, magnetograms, and the potential-field source-surface model to map the Sun-L1 interplanetary magnetic field (IMF) line back to its source region on the Sun at the time of the SEP observations and (2) then look for a correlation between SEP composition (as measured by Wind and Advanced Composition Explorer at ~2-30 MeV nucleon–1) and characteristics of the identified IMF source regions. The study is based on 24 SEP events, identified as a statistically significant increase in ~20 MeV protons and occurring in 1998 and 2003-2006, when the rate of newly emergent solar magnetic flux and CMEs was lower than in solar-maximum years, and the field-line tracing is therefore more likely to be successful. We find that the gradual SEP Fe/O is correlated with the field strength at the IMF source, with the largest enhancements occurring when the footpoint field is strong due to the nearby presence of an active region (AR). In these cases, other elemental ratios show a strong charge-to-mass (q/M) ordering (at least on average), similar to that found in impulsive events. Such results lead us to suggest that magnetic reconnection in footpoint regions near ARs bias the heavy-ion composition of suprathermal seed ions by processes qualitatively similar to those that produce larger heavy-ion enhancements in impulsive SEP events. To address potential technical concerns about our analysis, we also discuss efforts to exclude impulsive SEP events from our event sample.

Solar energetic particles: Shock acceleration and transport through self-amplified waves

This article reviews our work on the powerful influence of self-amplified Alfven waves on the interplanetary (IP) transport and shock acceleration of solar energetic particles (SEPs). In large gradual events, a huge number of shock-accelerated protons stream through the IPmediumand amplify ambient Alfven waves by orders of magnitude. Nonlinear models that take account of selfamplified waves semi-quantitatively explain many intriguing SEP observations at 1 AU: (a) upper limits to early SEP intensities, (b) flat intensity energy spectra up to ~ 30 MeV/amu before shock arrival, and (c) complex temporal, energy, and event-to-event variations of elemental abundances. Streaming limit complicates estimation of the number and energy of SEPs accelerated in a solar event but provides a safety window for astronauts to seek shelter before a potential hazardous intensity rise at shock passage. Self-amplified waves help bootstrap shock acceleration and the high near-shock SEP intensity predicted at ≤ 20r⊙. is relevant ...

CORRELATION OF ELECTRON PATH LENGTHS OBSERVED IN THE HIGHLY WOUND OUTER REGION OF MAGNETIC CLOUDS WITH THE SLAB FRACTION OF MAGNETIC TURBULENCE IN THE DISSIPATION RANGE

Three magnetic cloud events, in which solar impulsive electron events occurred in their outer region, are employed to investigate the difference of path lengths L 0eIII traveled by non-relativistic electrons from their release site near the Sun to the observer at 1 AU, where L 0eIII = v l × (t l – t III), v l and t l being the velocity and arrival time of electrons in the lowest energy channel (~27 keV) of the Wind/3DP/SST sensor, respectively, and t III being the onset time of type III radio bursts. The deduced L 0eIII value ranges from 1.3 to 3.3 AU. Since a negligible interplanetary scattering level can be seen in both L 0eIII > 3 AU and ~1.2 AU events, the difference in L 0eIII could be linked to the turbulence geometry (slab or two-dimensional) in the solar wind. By using the Wind/MFI magnetic field data with a time resolution of 92 ms, we examine the turbulence geometry in the dissipation range. In our examination, ~6 minutes of sampled subintervals are used in order to improve time resolution. We have found that, in the transverse turbulence, the observed slab fraction is increased with an increasing L 0eIII value, reaching ~100% in the L 0eIII > 3 AU event. Our observation implies that when only the slab spectral component exists, magnetic flux tubes (magnetic surfaces) are closed and regular for a very long distance along the transport route of particles.

On the relationship between heavy-ion composition variability in gradual SEP events and the associated IMF source regions

We investigated the relationship between heavy-ion composition variability in gradual solar energetic particle (SEP) events and the source regions of the associated interplanetary magnetic field (IMF) along which these SEPs are transported. In particular, we found that the SEP elemental composition has a positive correlation with the unsigned photospheric magnetic field strength around the IMF footpoints. Furthermore, the IMF source region can be generally categorized into two classes, depending on whether the associated solar wind, and correspondingly the footpoints of the open magnetic field along which the SEPs are transported, are from a coronal hole (CH) near an active region (an ‘AR field-source’) or from a CH without an AR in its close proximity (a ‘CH field-source’). This study is based on 24 events observed in years 1998 and 2003-2006. We present in this paper the results for Fe/O as an example, and show that the observed SEP Fe/O ratio at 3-30 MeV/nucleon is preferentially enhanced in the case o...