Origin and Acceleration of Suprathermal Ions PSP-SWG Meeting October 2-6, 2017 Mihir I. Desai Southwest Research Institute San Antonio, Texas
Outline • What are suprathermal particles? • Why are they important? • Spectral Properties • Composition and Sources • Major theoretical concepts • Summary • PSP/SoLO Opportunities
What are Suprathermal Ions? • Solar wind Ions, Pickup ions, and suprathermal ion tails • SW ions: high density, Maxwellian, bulk SW speed • PUIs; ~0-2 Vsw; ring distributions • ST Ions • speeds >1.5-10 times bulk SW speed • Energy range: ~2-100 keV/nuc. Fisk & Gloeckler, PNAS 2007;104:5749-5754
CME Shocks: SEPs and ESPs • CMEs drive shocks in the corona and the interplanetary medium • Accel. Near-SunSEPs • Accel. Near-EarthESPs ESP SEP Reames.SSR, 1999
486 Seed Particles for CME shocks CMEs drive shocks in the corona and IP medium Shocks accelerate particles out of the ambient corona or the solar wind Gosling et al., 1981 Continuous distribution from solar wind to MeV energy SW or quiet SW tail as seed population
CME shocks 3He and He+ in ESP events points to multiple suprathermal sources Desai et al., 2001; Kucharek et al., 2003; Allegrini et al. 2008 ULEIS 0.25-0.8 MeV/n. SEPICA 0.25-0.8 MeV/n.
He+ & 3He in CIRs Gloeckler et al., 1996 Möbius et al. 2000 Chottoo et al., 2000 Kucharek et al., 2003 Popecki et al., 2012 Mason et al. 2012 He+ 3He PUIs and 3He-rich SEPs are important sources for CIRs
Density Variations and SEPs Large SEP events have higher fluences if the suprathermal ion densities are elevated one day before the SEP event. How does ST variability influence shock acceleration? Mewaldt et al., (2012) Mason et al., 2005
Variability in SEPs Kahler (2001) Mewaldt et al., (2006) CME speed (km/s) Peak intensities and SEP kinetic energies vary by ~3-4 orders of magnitude for a given CME speed and kinetic energy
Suprathermal Seed Particles Rare (tracer) species like flare-accelerated 3He and PU He+ found in ESPs, CIRs, large SEPs, and upstream events CME & CIR shocks accelerate suprathermal (ST) ions with energies >1.5-2 keV/n. Suprathermal origin for 3He, He+, C-Fe in CIRs, CME shocks & large SEPs Mewaldt et al., 2001
ST tails • v-5spectra ~4-40 VSW in all periodsE-1.5 in diff. intensity • Roll-over above ~40 VSW Gloeckler & Fisk ApJ (2008)
1-hr Averages Ions with speeds between ~2-8 Vsw exhibit power-laws with v-3-v-7 Fisk & Gloeckler (2012) SSR.
Spectral Indices Time-series of spectral indices Ions with speeds between ~2-8 Vsw exhibit power-laws with v-3-v-7 Fisk & Gloeckler (2012) SSR.
Histograms • Power-laws with mean v-4.5 • Index shows a broad distribution with ~1-9 Fisk & Gloeckler (2012) SSR.
Quiet-times “Quiet-times” identified using low levels in hourly averaged C-Fe intensities between 0.11-1.28 MeV/n; Dayeh et al 2017; Desai et al., 2006; Dayeh et al., ApJ (2009; 2017)
Spectral Properties Fe and O spectra have similar indices at 0.11-0.32 MeV/n Fe spectra are somewhat steeper than O spectra at 0.45-1.28 MeV/n Dayeh et al., (2017);
Spectral Indices vs Solar cycle • No clear SC dependence; • Fe spectra in SC24 somewhat steeper than SC23 Dayeh et al., (ApJ 2017)
Wiedenbeck et al., 2005 3He from flares Fraction of time 3He present varies with solar activity Constant abundance till 2004 Drops by an order of magnitude in 2005-2008 Solar Max Solar Min Desai et al. (2006) Dayeh et al. (2009)
Composition vs. Solar Cycle Desai et al., 2006; Dayeh et al., ApJ (2009; 2017)
Two Basic Categories • Continuous acceleration processes in IP space produce v-5 or E-1.5 power-law tails • Lower-energy portion of material accelerated in multiple CME shocks, SEPs, CIRs etc.,
Summary • ST ions above ~2 keV/nucleon are important sources for CME shocks, SEPs, CIRs Effects on acceleration mechanisms are poorly understood • Quiet-time ST ion composition and spectra above ~6xVsw suggest that ST tails are created from a mix of lower-energy material accelerated in CME shocks, SEPs, CIRs etc; • Relation between ~v-5 tails between ~1.3-5 Vsw and variable spectra and composition >5 Vsw is unclear • Theoretical concepts fall into two basic categories: • Continuous acceleration in the corona or IP space could result in v-5 or E-1.5 power-law tails • Superposition of lower-energy material accelerated in CME shocks, SEPs, CIRs etc SPP will determine relative contributions wrt distance
PSP/SoLO; courtesy, N. Savani • Unprecedented opportunity to distinguish between the two ST origin concepts • Cross-calibrate sensors when s/c are close • Study ST & SEP origin when s/c along same field lines • Use radial alignment to differentiate between local vs. remote sources • These new measurements are critical for validating and refining CME shock and SEP acceleration models
Specific Opportunities: PSP/SoLO • Quiet-times during the early phase (solar minimum) of PSP are ideal for studying continuous acceleration in inner heliosphere and corona • Differentiate between continuous vs. discrete sources by comparing with predictions of ST origin/acceleration models • Currently very few models predict what PSP/SoLO should observe • Need all contenders to provide specific predictions that can be tested using PSP/SoLO observations E.g., if STs are lower-energy populations from discrete events such as flares (e.g., impulsive SEP or nano/picoflares) or CME-driven shocks, then the ST density should increase as PSP/SoLO move inward – could apply Lario’s radial gradient
Large Gadual SEP Events • 3He-enrichments in many large SEPs • 3He-enrichments in many large SEPs • Large SEP events occur after a ~10 day period during which the suprathermal fluences are elevated Mason et al., 1999 Mewaldt et al.2005 AIP conf. proceedings, 525, L133