1. 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

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

3. 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

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

5. 3He in IP space
(Mason et al 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:
Energetic 3He ions are present in the interplanetary medium during active solar periods

6. 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
Solar Max
Solar Min
Desai et al. (2006)
Dayeh et al. (2009)

7. 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)

8. 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 keV/n
Hill et al. (2009)
Low count rate in Cassini/CHEMS

9. 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

10. 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

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

12. 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):
CNO includes contributions from CIRs and SEPs and varies between 3 orders of magnitude
excludes many intervals with low count rates
May 1998 SEP event
Wind/STEP keV/n

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

14. New method for selecting quiet-times
Calculate hourly averaged intensities of C-Fe between MeV/n
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)

15. 3-tiered distribution is observed each year
Dayeh et al., in review ApJ (2016)

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

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

18. Spectral Properties Dayeh et al., ApJ in review (2016);
Fe and O spectra have similar indices at MeV/n
Fe spectra are somewhat steeper than O spectra at MeV/n

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

20. >0.1 MeV/n ST ion Properties
Energy Range
Spectral Forms
Composition
~ 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

21. 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

22. 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

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

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

25. 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)

26. 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.,

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

28. 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