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)


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


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


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)


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


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


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


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


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


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?


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


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?

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M.E. Wiedenbeck (JPL/Caltech)