Southwest Research Institute Origin, Acceleration, and Solar Cycle Variations of Suprathermal Particles SPP SWT Washington DC September 14-15, 2016 Mihir I. Desai Southwest Research Institute San Antonio, Texas

Outline Conflicting observations of ST tails Quiet-times selection criteria New technique for selecting quiet-times Spectra and composition above ~6xVsw Opportunities for SPP

What are Suprathermal Ions and where do they come from? Ions with speeds between ~2-20 times the solar wind speed (energy range ~2-300 keV/nucleon) Two hypotheses: Lower-energy portion of population accelerated in CME shocks, SEPs, CIRs. Continuous acceleration in IP space

Continuous Acceleration of Quiet-time Suprathermal tails Tails have universal v-5 or E-1.5 power-law spectra Fisk & Gloeckler ApJ (2006)

3He in IP space (Mason et al. 1999 ApJ, 525, L133; Wiedenbeck et al. 2003) 4He 3He As the CME itself passes Earth, a further increase in energetic particles is often observed, and in this case is clearly associated with interplanetary acceleration near the CME. This portion of the particle event is called the “Energetic Storm Particle” or ESP event because they were first discovered in 1960’s in association with geomagnetic storms. Predicting particle properties such as peak intensities, energy spectra, maximum energy extent during the CME-driven shocks has proved to be difficult. One possible reason for this is that the particles available for acceleration - the seed population - by these CME shocks includes solar wind ions and other ions that may be present in the inner heliosphere; it may even include energized particles from earlier events. The range of speeds is larger for CME shocks than for any of the other heliospheric shocks, leading to a wide range of possible energies for the accelerated particles. Image Credit: http://www.phys.ufl.edu/~meisel/sonic-boom.jpg Energetic 3He ions are present in the interplanetary medium during active solar periods

3He from flares Wiedenbeck et al., 2005 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 Solar Min Solar Max Solar Min Fe/O and C/O --SW/CIR-like during solar minimum Fe/O and C/O -- SEP-like during solar maximum 3He/4He is enhanced above SW value during all years Dayeh et al., ApJ (2009)

What criteria are suitable for selecting “quiet-times”? Reference Criteria Fisk & Gloeckler (2006; 2007) Vsw<320 km/s Gloeckler et al. (2008) Fisk & Gloeckler (2008) Low count rate in ACE/ULEIS Dayeh et al., (2009) Low count rate in Wind/STEP between 40-320 keV/n Hill et al. (2009) Low count rate in Cassini/CHEMS

Does Quiet or slow SW represent “quiet-times” for suprathermals? Gloeckler et al., (2008) Quiet time selection criteria: V(sw) < 320 km/s 2007: Low-intensity periods for CNO suprathermals and occurrence of slow SW do not show a one--to-one correspondence

Does “Quiet” or slow SW represent “quiet”-times in suprathermals? Gloeckler et al., (2008) Quiet time selection criteria: V(sw) < 320 km/s 1998: Two of the largest enhancements in the CNO intensity occur during periods when V(sw) < 320 km/s

Using V(sw) < 320 km/s includes suprathermals from CIRs and SEPs Also eliminates “quiet” periods in suprathermals.

Comparison of Quiet time Criteria Dayeh et al. “quiet-times” (in red): suprathermal CNO intensity varies within a narrow range (~factor of 5) Using Gloeckler’s criteria (blue): 1998 -- CNO includes contributions from CIRs and SEPs and varies between 3 orders of magnitude 2007 -- excludes many intervals with low count rates May 1998 SEP event Wind/STEP 80-160 keV/n

ST Composition vs solar cycle Desai et al., 2006; Dayeh et al., ApJ (2009; 2016)

New method for selecting quiet-times Calculate hourly averaged intensities of C-Fe between 0.11-1.28 MeV/n 1998-2015 Sort intensities in ascending order for each year array from 0 – max (left) Rebin sorted hourly intensities into daily bins Calculate the mean and variance for each daily bin for each year Plot variance vs mean (right) in each year 3-tiered distribution: P1 - quiet-times; P2 – CIRs, SEPs, ESPs; P3 – Largest SEPs or ESPs Dayeh et al., in review ApJ (2016)

3-tiered distribution is observed each year Dayeh et al., in review ApJ (2016)

Composition vs. Solar Cycle Desai et al., 2006; Dayeh et al., ApJ (2009; 2016)

Spectral Properties Dayeh et al., ApJ in review (2016); No systematic steepening of Fe and O spectra; O spectra are steeper in more cases

Spectral Properties Dayeh et al., ApJ in review (2016); 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

Spectral Indices vs Solar cycle No clear SC dependence; Fe spectra in SC24 somewhat steeper than SC23 Dayeh et al., ApJ in review (2016)

>0.1 MeV/n ST ion Properties Energy Range Spectral Forms Composition ~0.1-1.28 MeV/n (>10xVsw) E-1.66-E-2.5 No systematic steepening at higher energies No clear SC dependence; Fe spectra in SC24 somewhat steeper than SC23 3He enhanced in solar max. drops during solar min C/O & Fe/O: CIR/SW-like in solar minimum, SEP-like in solar maximum

Summary Suprathermal ions above ~2 keV/nucleon are important sources for CME shocks, SEPs, CIRs  Effects on acceleration mechanisms are poorly understood Theoretical models 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., Many sources contribute to the suprathermal pool, e.g., PUIs, flares, SEPs, CIRs, CME shocks etc.,  Relative contributions are not well determined Observations provide conflicting picture  Relationship between v-5 tails below ~1.3-5 Vsw and variable spectra and composition >5 Vsw is unclear

Opportunities for SPP Quiet-times will be prevalent during the early phases of the SPP mission. These periods will need to be carefully identified and selected taking account of instrument sensitivities and external conditions. This unique dataset when combined with Solar Orbiter/HIS and EPD measurements will provide an unprecedented opportunity to distinguish between the two basic theoretical concepts. SPP/EP instruments need to be on and collecting data along each orbit or for as much time as possible to measure the composition and spectra of the ST ion populations, and develop a clear understanding of the radial evolution of quiet-time ST tail intensities These new measurements will be critical for validating and refining CME shock and SEP acceleration models

Question What acceleration processes are responsible for producing the quiet-time suprathermal tails near 1 AU?

Spectral Shapes Fitted a single power-law to Wind/STEP spectra between 40-320 keV/n (~6-20 x SW speed) Fitted a power-law modulated by an exponential to ACE/ULEIS spectra between 0.1-3 MeV/n Dayeh et al., ApJ (2009)

Species-dependent roll-overs Fe spectrum rolls over at ~0.5 MeV/n (lowest among species) STEP energy range is below reported and observed roll-over energies Composition: Little effect on C/O and 3He/4He (similar M/Q) If STEP spectra are in roll-overs, then Fe/O will increase STEP Energy Spectra Fisk & Gloeckler ApJ (2008)

Two Basic Categories Continuous acceleration in the corona or 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.,

Desai & Giacalone (2016), in press in Living Reviews in Solar and Space Physics

Fractional contributions Dayeh et al. quiet-times --- nearly equal contribution by each hour (max cont. = 0.4%) -- 30% of hours contribute 50% of intensity Using Vsw<320 km/s (1998) -- only 4% of the most intense hours contribute 50% of the total yearly intensity