Observations of Broad Longitudinal Extents of 3 He-rich SEP Events M.E. Wiedenbeck 1, G.M. Mason 2, C.M.S. Cohen 3, N.V. Nitta 4, R. Gómez-Herrero 5, D.K. Haggerty 2 1. Jet Propulsion Laboratory, California Institute of Technology 2. Johns Hopkins University / Applied Physics Laboratory 3. California Institute of Technology 4. Lockheed Martin Solar and Astrophysics Lab 5. Christian-Albrechts Universität zu Kiel 16 August nd ICRC (Beijing), Paper ID 1162 Copyright All rights reserved.

16 August nd ICRC (Beijing), Paper ID 1162 Copyright All rights reserved. Reames 1999 Shimojo & Shibata 2000 The Prevailing Model for the Origin of Impulsive ( 3 He-rich) Solar Energetic Particle Events in impulsive events, particle acceleration takes place near the reconnection region of a solar flare some particles have access to open field lines involved in the reconnection and can escape from the corona along those open field lines because the reconnection region is small, the cluster of open field lines is small and has a relatively narrow angular spread in the heliosphere in contrast, in gradual SEP events particles are accelerated along a shock front that spans a large solid angle and particles have access to a wide range of field lines

16 August nd ICRC (Beijing), Paper ID 1162 Copyright All rights reserved. Evidence for the Prevailing Model: Longitude Distribution of X-ray Flares Associated with 3 He-rich SEP Events distribution of flare locations is centered around W60, close to the nominal connection point for a Parker spiral magnetic field in a 400 km/s solar wind rms width of the distribution is ~15°-20° (neglecting eastern hemisphere events) Reames suggested that the width is primarily due to variations in the tightness of the winding of the Parker associated with to solar wind speed variations

16 August nd ICRC (Beijing), Paper ID 1162 Copyright All rights reserved. Evidence for the Prevailing Model: Longitude Distribution of X-ray Flares Associated with 3 He-rich SEP Events distribution of flare locations is centered around W60, close to the nominal connection point for a Parker spiral magnetic field in a 400 km/s solar wind rms width of the distribution is ~15°-20° (neglecting eastern hemisphere events) Reames suggested that the width is primarily due to variations in the tightness of the winding of the Parker associated with to solar wind speed variations distribution of calculated Parker spiral connection points (1998 to early 2010) agrees rather well

5 Multispacecraft 3 He-rich Events: Jan 2007 through Jan 2011 require detection in at least one of the STEREO/LETs; then examine ACE/ULEIS & SIS and the other STEREO/LET for coincident detections solar activity was low very few 3 He-rich events were available for study during the first 2 years chance coincident events from different active regions were unlikely because it is able to make measurements below an MeV/nuc, ULEIS is more sensitive to small 3 He-rich events table does not include a number of events that were detected by ACE/ULEIS but not by any of the other instruments events detectable at spacecraft separated by >60° are fairly common— contrary to expectations from the flare longitude distribution derived from associations with single-spacecraft 3 He- rich events 16 August nd ICRC (Beijing), Paper ID 1162 Copyright All rights reserved.

16 August nd ICRC (Beijing), Paper ID 1162 Copyright All rights reserved. The Best Example Observed to Date of a 3-Spacecraft Event with Wide Separation in Heliographic Longitude: 7 Feb 2010 STEREO-A leading Earth by 65° STEREO-B trailing by 71° clear detections of 3 He at all both STEREOs and at ACE 3 He clearly above background and not significantly contaminated from spillover from 4 He 3 He intensity significantly greater at STEREO-B than at STEREO-A

16 August nd ICRC (Beijing), Paper ID 1162 Copyright All rights reserved. 7 Feb He-rich SEP Event single active region located close to central meridian producing flares, type III bursts, and electron events the active region was well connected to STEREO-B 3 He-rich event onsets (arrows in upper panel) occurred between two significant near-relativistic electron events (not the source of the 3 He) the 3 He-rich event and the electron events were detected at all three spacecraft at STEREO-A the intensities were reduced and the onsets delayed relative to STEREO-B no fast CMEs were associated with the event, but a relatively slow, halo CME was launched from the same active region early on 7 Feb.

16 August nd ICRC (Beijing), Paper ID 1162 Copyright All rights reserved. Dependence on 3 He Fluence on Heliographic Longitude spacecraft and flare geometry 1 AU compare 3 He fluences in 2.3 to 3.8 MeV/nuc energy interval direct measurement by STEREO-A & -B LET; interpolated between ULEIS and SIS measurements at ACE fluence is a strong function of heliographic longitude, fluence at STEREO- A more than a factor of 30 less than at STEREO-B well fit with a Gaussian centered on the well-connected longitude (dotted line in right panel), rms width is ~47°

16 August nd ICRC (Beijing), Paper ID 1162 Copyright All rights reserved. simple diffusion from a point source produces Gaussian spatial distributions “drop-outs” in 3 He-rich SEP events (Mazur et al. 2000) indicates very little transverse diffusion best examples of drop-outs are in events showing velocity dispersion in their onsets, indicating long parallel mean free paths in the 7 Feb 2010 event velocity dispersion was not observed and anisotropies were weak, so scattering environment may be significantly different than in events where drop-outs are clearly present Can the Longitudinal Spread be Due to Perpendicular Diffusion? Mazur et al. 2000

16 August nd ICRC (Beijing), Paper ID 1162 Copyright All rights reserved. Field Expansion between Photosphere and Source Surface— Can the PFSS Model Explain the Broad Longitudinal Extent? several PFSS realizations checked predicted sources of fields reaching the three spacecraft do not all agree none of the models gave a longitudinal spread as large as 90° look at PFSS longitudinal spreads statistically examine 12 years of daily PFSS maps make distribution of largest longitudinal extent at heliographic equator of fields from localized (<5° radius) photospheric source no instances found with a spread as large as the 136° between the two STEREOs at the time of the 7 Feb 2010 event typically, the largest spread found in a map is ~60°—80°

16 August nd ICRC (Beijing), Paper ID 1162 Copyright All rights reserved. STEREO-A COR2 STEREO-B COR2 Could the Earth-directed CME that preceded the 3 He-rich event have been responsible for the wide longitudinal spread of the SEPs? a slow CME such as this one (~420 km/s) should not be significant in accelerating particles, but could its effects on the magnetic fields (distortion or reconnection) have significantly modified the transport of SEPs from the flare? electron access to all three spacecraft in events preceding and following the 3 He-rich event suggests that transient modification of the field was not essential to the transport in heliographic longitude

16 August nd ICRC (Beijing), Paper ID 1162 Copyright All rights reserved. How can the broad width derived from the multi-spacecraft observations be reconciled with the earlier single-spacecraft results on the distribution of flare sites associated with 3 He-rich SEP events? 3 He-rich events tend to have low intensities finite instrument sensitivity requires that event sizes exceed some threshold for detection because of the strong fall-off of event fluence (or intensity) with longitudinal separation from the well-connected longitude, events near the instrument threshold will only be detected when the observer is close to the well-connected longitude while this effect surely plays a role, it is not clear that it is sufficient to narrow the source longitude distribution enough to reconcile the two results the ACE/ULEIS instrument, which measures 3 He down to a few hundred keV/nuc, is significantly more sensitive to 3 He-rich events than instruments operating above an MeV/nuc some of the multi-spacecraft events we have identified would have not been recognized as such without the higher sensitivity of ULEIS

16 August nd ICRC (Beijing), Paper ID 1162 Copyright All rights reserved. Relationship to other 3 He-rich SEP Observations broad longitudinal spread of the 3 He may account for: occasional association of eastern hemisphere flares with observed 3 He-rich events some of the instances where there is no good candidate flare to associate with a 3 He-rich SEP event detection—flare may have occurred beyond the west limb inference that at solar maximum 3 He-rich SEP events occur at a rate ~1000/yr (Reames 1999) depends on both the assumed longitudinal spread of the 3 He and on instrument sensitivity—should be reevaluated low-level background of 3 He even when no distinguishable impulsive SEP injections are identified could contain contributions from the tails of events occurring over much of the solar surface—may help explain low-energy turn-ups observed in the quiet time 3 He energy spectra (Wiedenbeck 2007) the energetic 3 He observed >60% of the time at 1 AU at solar maximum likely contains contributions from impulsive events occurring over a large range of longitudes on the Sun points—individual solar rotations solid line—7 rotation averages dashed line—13 year shift

16 August nd ICRC (Beijing), Paper ID 1162 Copyright All rights reserved. 14 Summary 3 He from impulsive SEP events is frequently distributed over a wide range of heliographic longitudes, contrary to the prevailing model for the origin of these events the fluence of 3 He has a strong dependence on separation from the well connected longitude the physical mechanism for the longitudinal spreading is not yet established