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

Abstract

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.

Using the WSA Model to Test the Parker Spiral Approximation for SEP Event Magnetic Connections

“…Klein et al (2008) used PS+PFSS to connect L1 footpoints to open fields in active regions for seven simple SEP events. Ko et al (2013) similarly correlated SEP Fe/O abundances with the footpoint field strengths for 24 gradual SEP events. In various STA/STB SEP event comparisons with EUV waves, Nitta (2012), Park et al (2013Park et al ( , 2015, and Gómez-Herrero et al (2015) also followed this approach.…”

The prolonged southward IMF‐Bz event of 2–4 May 1998: Solar, interplanetary causes and geomagnetic consequences

“…The arrival of the second shock just before the end of the first ICME raised the question whether the eruption of the second CME had destroyed the magnetic configuration of the field lines during the trace back period, and whether information of the background IMF conditions on the solar surface was destroyed. From our analysis, however, we showed that the eruption of CME took place right after our trace back period, indicating that the source regions of the IMF determined by the velocity trace back technique would be valid [ Ko et al , ]. It is also to be noted that the active region AR 8210 remained south of the magnetic neutral line (MNL) during the whole trace back period, indicating no change in the IMF configuration.…”

“…The open field lines, which remained constantly to the north of the heliospheric current sheet both on 29 April and 1 May, clearly suggest no change in their polarity during the trace back period, from 29 April to 1 May. It is also known from observations that, depending on the hemisphere, the open magnetic field lines spiral either inward or outward, and the configuration changes, generally, either during periodic solar polar reversals occurring at the maximum of each solar cycle [ Janardhan et al , , , ] or due to the wake of CME eruption [ Ko et al , ]. However, the polarity reversal of solar cycle 23 occurred in CR1949, while our event was during CR1935.…”

“…In an attempt to link outflows from an equatorial coronal hole and solar wind observed at 1 AU, McIntosh et al [] used space‐based data from the Advanced Composition Explorer ( ACE ) to trace the solar wind back to the source surface using potential field source surface models. To identify the source regions of gradual solar energetic particle (SEP) events at 1 AU, Ko et al [] first used solar wind speed measurements to map the Sun‐L1 interplanetary magnetic field lines back to its source regions on the Sun at the time of SEP observations and compared the characteristics of the SEP events to the identified sources of IMF.…”

Analysis of multi-eruption solar energetic particle event on March 17-18, 2003

“…Analysis of MESEP events on March (17)(18) CME1, and accelerated by the CME driven shock [14]. Another study [21] considered this event gradual based on their investigation of the source region of the IP magnetic field and variability in heavy ion elements composition in gradual SEP events . The CME2 associated with the X1.5 flare, on 18 March appears to be the solar source of shock wave observed on 20 March.…”