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

Yuan-Kuen Ko   Submitted: 2014-08-22 09:37

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.

Authors: Ko, Yuan-Kuen; Tylka, Allan J.; Ng, Chee K.; Wang, Yi-Ming; Dietrich, William F.
Projects: None

Publication Status: Published in the Astrophysical Journal, 2013, vol.776, p92
Last Modified: 2014-08-27 12:58
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Modeling UV and X-Ray Emission in a Post-CME Current Sheet  

Yuan-Kuen Ko   Submitted: 2010-08-09 10:15

A post-CME current sheet (CS) is a common feature developed behind an erupting flux rope in CME models. Observationally, white light observations have recorded many occurrences of a thin ray appearing behind a CME eruption that closely resembles a post-CME CS in its spatial correspondence and morphology. UV and X-ray observations further strengthen this interpretation by the observations of high temperature emission at locations consistent with model predictions. The next question then becomes whether the properties inside a post-CME current sheet predicted by a model agree with observed properties. In this work, we assume that the post-CME CS is a consequence of Petschek-like reconnection and that the observed ray-like structure is bounded by a pair of slow mode shocks developed from the reconnection site. We perform time-dependent ionization calculations and model the UV line emission. We find that such a model is consistent with SOHO/UVCS observations of the post-CME CS. The change of Fe XVIII emission in one event implies an inflow speed of ~10 km s-1 and a corresponding reconnection rate of M_A ~0.01. We calculate the expected X-ray emission for comparison with X-ray observations by Hinode/XRT, as well as the ionic charge states as would be measured in-situ at 1 AU. We find that the predicted count rate for Hinode/XRT agree with what was observed in a post-CME CS on April 9, 2008, and the predicted ionic charge states are consistent with high ionization states commonly measured in the interplanetary CMEs. The model results depend strongly on the physical parameters in the ambient corona, namely the coronal magnetic field, the electron density and temperature during the CME event. It is crucial to obtain these ambient coronal parameters and as many facets of the CS properties as possible by observational means so that the post-CME current sheet models can be scrutinized more effectively.

Authors: Yuan-Kuen Ko, John C. Raymond, Bojan Vrsnak, Eugen Vujic
Projects: None

Publication Status: Accepted for publication in the Astrophysical Journal
Last Modified: 2010-08-12 09:46
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Hot Plasma in Non-Flaring Active Regions Observed by the Extreme-ultraviolet Imaging Spectrometer on Hinode  

Yuan-Kuen Ko   Submitted: 2009-03-20 07:01

The Extreme-ultraviolet Imaging Spectrometer (EIS) on the Hinode spacecraft obtains high resolution spectra of the solar atmosphere in two wavelength ranges: 170 - 210 and 250 - 290 angstroms. These wavelength regions contain a wealth of emission lines covering temperature regions from the chromosphere/transition region (e.g., He II, Si VII) up to flare temperatures (Fe XXIII, Fe XXIV). Of particular interest for understanding coronal heating is a line of Ca XVII at 192.858 angstroms, formed near a temperature of 6 million degrees. However, this line is blended with two Fe XI and six O V lines. In this paper we discuss a specific procedure to extract the Ca XVII line from the blend. We have performed this procedure on the raster data of five active regions and a limb flare, and demonstrated that the Ca XVII line can be satisfactorily extracted from the blend if the Ca XVII flux contributes to at least ~10% of the blend. We show examples of the high-temperature corona depicted by the Ca XVII emission and find that the Ca XVII emission has three morphological features in these active regions - 1) `fat' medium-sized loops confined in a smaller space than the 1 million degree corona, 2) weaker, diffuse emission surrounding these loops that spread over the core of the active region, and 3) the locations of the strong Ca XVII loops are often weak in line emission formed from the 1 million degree plasma. We find that the emission measure ratio of the 6 million degree plasma relative to the cooler 1 million degree plasma in the core of the active regions, using the Ca XVII to Fe XI line intensity ratio as a proxy, can be as high as 10. Outside of the active region core where the 1 million degree loops are abundant, the ratio has an upper limit of about 0.5.

Authors: Yuan-Kuen Ko, George A. Doschek, Harry P. Warren, Peter R. Young
Projects: Hinode/EIS

Publication Status: ApJ (in press)
Last Modified: 2009-03-20 09:15
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Dynamical and Physical Properties of a Post-CME Current Sheet  

Yuan-Kuen Ko   Submitted: 2003-09-22 11:52

In the eruptive process of the Kopp-Pneuman type, the closed magnetic field is stretched by the eruption so much that it is usually believed to be ``open'' to infinity. Formation of the current sheet in such a configuration makes it possible for the energy in the coronal magnetic field to quickly convert into thermal and kinetic energies, and cause significant observational consequences, such as growing post-flare/CME loop system in the corona, separating bright flare ribbons in the chromosphere, and fast ejections of the plasma and the magnetic flux. An eruption on January 8, 2002 provides us a good opportunity to look into these observational signatures of and place constraints on the theories of eruptions. The event started with the expansion of a magnetic arcade over an active region, developed into a coronal mass ejection (CME), and left some thin streamer-like structures with successively growing loop systems beneath them. The plasma outflow and the highly ionized states of the plasma inside these streamer-like structures, as well as the growing loops beneath them lead us to conclude that these structures are associated with a magnetic reconnection site, namely the current sheet, of this eruptive process. We combine the data from the Ultraviolet Coronagraph Spectrometer, Large Angle and Spectrometric Coronagraph Experiment, EUV Imaging Telescope, and Coronal Diagnostic Spectrometer on board the Solar and Heliospheric Observatory, as well as from the Mauna Loa Solar Observatory Mark-IV K-coronameter, to investigate the morphological and dynamical properties of this event, as well as the physical properties of the current sheet. The velocity and acceleration of the CME reached up to 1800 km s-1 and 1 km s-2, respectively. The acceleration are found to mainly occur at the lower corona (< 2.76 Ro). The post-CME loop systems showed both behaviors of post-flare loops (upward motion with decreasing speed) and soft X-ray giant arches (upward motion with constant speed, or acceleration) according to the definition of v{S}vestka (1996). In the current sheet, the presence of highly ionized ions, such as Fe+17 and Ca+13, suggests temperature as high as 3-4 imes 106 K, and the plasma outflows have speeds ranging from 300 km s-1 to 650 km s-1. Absolute elemental abundances in the current sheet show a strong First Ionization Potential effect, and have values similar to those found in the active region streamers. The magnetic field strength in the vicinity of the current sheet is found to be of the order of 1 Gauss.

Authors: Ko, Y.-K., Raymond, J. C., Lin, J., Lawrence, G., Li, J., Fludra, A.
Projects: None

Publication Status: Published in ApJ, 2003, Vol.594, p1068.
Last Modified: 2003-09-22 11:52
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Abstracts by Author
Source Regions of the Interplanetary Magnetic Field and Variability in Heavy-ion Elemental Composition in Gradual Solar Energetic Particle Events
Modeling UV and X-Ray Emission in a Post-CME Current Sheet
Hot Plasma in Non-Flaring Active Regions Observed by the Extreme-ultraviolet Imaging Spectrometer on Hinode
Dynamical and Physical Properties of a Post-CME Current Sheet

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