M.E. Wiedenbeck (JPL/Caltech)

26 June 2008 SHINE, Zermatt, UT 1

High-energy Elemental, Isotopic, and Charge-State Composition in 3He-rich Solar Energetic Particle

EventsM.E. Wiedenbeck (JPL/Caltech)

R.A. Leske, C.M.S. Cohen, A.C. Cummings, R.A. Mewaldt, E.C. Stone (Caltech)

T.T. von Rosenvinge (NASA/GSFC)

26 June 2008 SHINE, Zermatt, UT 2

Breneman & Stone 1985

Leske et al. 2007

Fractionation in Gradual SEP Events

• abundance enhancements organized as power law in (Q/M)a, where the exponent a varies from event to event

• assuming that the isotopes of an element have the same distribution of charge states, correlation between different isotope ratios can be calculated with no free parameters

26 June 2008 SHINE, Zermatt, UT 3

Correlation between isotope ratios: 26Mg/24Mg versus 22Ne/20Ne

• - gradual events analyzed by Leske et al. 2007• - 3He-rich events (larger symbols for events with better statistical accuracy)• diagonal lines - expected correlation if fractionation is a power law function of Q/M• the larger, more precisely measured 3He-rich events tend to fall near the predicted correlation line

20 Aug 2002event

26 June 2008 SHINE, Zermatt, UT 4

Hypothesis: Fractionation in 3He-rich Events is also Organized as a Power Law in Q/M

Use technique introduced and applied to 6 Nov 1997 SEP Event by

Cohen et al., GRL 26, 149 (1999)

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Isotopic Fractionation in the 3He-rich Event of 20 Aug 2002

• fit with power law in the mass ratio

• fit dominated by Ne and Mg isotope ratios

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Combining Elemental and Isotopic Composition to Estimate Charge States

• fractionation power-law exponent, a, calculated from the enhancement of 22Ne/20Ne

• combining this value of a with the Fe/O ratio yields the the corresponding ionic charge state ratio, QFe/QO (given the known ratio of masses)

• same approach applied to other elemental abundance ratios yields charge states of additional elements

• fractionation exponents tend to have large negative values, -10 to -25 in many casesfractionation

exponent

Q(Fe)

20 Aug 2002

1 May 20009 Sep 1998

26 June 2008 SHINE, Zermatt, UT 7

Comparison with Direct Measurements of <QFe>

• for two of the SIS events the are direct measurements from SEPICA below ~0.6 MeV/nuc

• for the 20 Aug 2002 event Joe Mazur has obtained charge states using the geomagnetic cutoff method with LICA on SAMPEX -- may have some contamination from particles from large 3He-rich event on the preceding day

• comparison suggests that increase of QFe with E/M continues to rise above 1 MeV/nuc

• low energy values of QFe represent charge states at 1 AU, values inferred from SIS are at the site where the fractionation occured

ACE/ SEPICA

ACE/SIS

SAMPEX/LICA

9 September 1998

1 May 2000

20 August 2002

26 June 2008 SHINE, Zermatt, UT 8

Is fractionation a function of Q/M?

• Q/M is relevant for rigidity-dependent processes

• Coulomb losses depend on Q2/M

• isotope fractionation as a power law in the mass ratio would be unchanged if Q were replaced by any function of Q -- it cancels when comparing isotopes of the same element

• assume fractionation is a power law in Qk/M and use constraints that QFe26 and QFelargest value of QFe measured at lower energies -- highlighted portions of curves satisfy these conditions for the three events where lower energy data are available

• only values of k within ~20% of 1.0 are acceptable using this criterion

9 September 1998

1 May 2000

20 August 2002

26 June 2008 SHINE, Zermatt, UT 9

Inferred Charge States

• using the fractionation exponent derived from the isotopic ratios, one can derive ratios of <Q> values for any pair of elements

• assume <QC>=6.0 to obtain Q values for other elements

• plot shows Q expresses as Z-QZ (i.e., number of electrons attached)

• find sequences of elements with a given Z-QZ values -- note particularly that Ne through S have He-like structure (2 electrons attached) -- previously noted by Reames, Meyer, & von Rosenvinge (1994)

He-like ions

26 June 2008 SHINE, Zermatt, UT 10

Source Temperature?

Ca, Fe, Ni

N, O

C

4He+2

3He+2

3He+1

What about 3He/4He?

Origin of non-monotonic dependence of element enhancement on Z?

7 large 3He-rich events above 10 MeV/nuc

How well does the assumption of fractionation as a power law in Q/M organize the

composition observations?

26 June 2008 SHINE, Zermatt, UT 11

Summary

• isotope fractionation in 3He-rich SEP events appears to be organized as a power-law in the ratio of the isotope masses, at least at energies >10 MeV/nuc

• assuming that the isotopic fractionation result is due to a general fractionation that has the form of a power-law in Q/M, combining the isotope results with elemental composition measurements makes it possible to infer Q-states

• comparison with direct measurements of Q-states in a few events suggests that increase of <QFe> with increasing E/M below 1 MeV/nuc continues to higher energies

• if one assumes that the fractionation depends on Qk/M (allowing the possibility k1), find that k should be in the range ~0.8-1.2 to assure QFe26 and value measured by ACE/SEPICA below 1 MeV/nuc

• derived Q-states are not consistent with a single source temperature for all the elements in the range 6Z28 -- not surprising given that Q-states measured at lower energies have an energy dependence attributed to stripping during acceleration

• fractionation as a power-law in Q/M in 3He-rich events is similar to fractionation in gradual (shock acceleration) events -- may indicate that coronal shocks play a role in accelerating the highest energy particles in 3He-rich events

• Q-values greater than those measured below 1 MeV/nuc suggests that the seed material being fractionated had already reached a significant fraction of 1 MeV/nuc

26 June 2008 SHINE, Zermatt, UT 12

Key Questions

• Is the technique yielding correct Q-state values?

- need to compare with direct measurements in additional events -- this is possible for a number of gradual events; data are not available for many 3He-rich events unless the same fractionation pattern can be found at lower energies

• What mechanism is producing these Q-states?

- source population is not thermal

- Fe undergoes a great deal of stripping before the fractionation occurs

• What is the physical origin of the fractionation as a power-law in Q/M?

- large exponents required suggest that this may just be an approximation

- is Q/M really the important parameter in the fractionation?

• Why is the fractionation so much stronger in 3He-rich events than in gradual events?

• What exactly is the connection between the composition (Z,M,Q) below 1 MeV/nuc and that above 10 MeV/nuc?

• Can the huge enhancement of 3He be fit into this picture?