Low-Frequency Waves Excited by Newborn

Interstellar Pickup Ions H+ and He+ at 4.5 AU

Charles W. Smith, Colin J. Joyce,

Philip A. Isenberg, Neil Murphy,

and Nathan A. Schwadron

Past Observations of Pickup Ion Waves [Murphy et al.,SSR, 95]

Ulysses spacecraft, quasi-radial IMF.Proton scattering from newly ionized beam generates primarily fast-mode waves above s/c proton gyrofrequency.Waves are only rarely, sporadically seen.

Solar Wind Heating

Voyager 2: Jan 7, 1979R = 4.5 AU

Note northward deflection of the IMF.

Dashed lines represent direction to Jupiter 0.5 AU away.

Spacecraft is within extended rarefaction interval starting ~4 days before with strong north/south variations spanning the region.

One event only, but high-time-res B-field data does not exist past 5 AU!

Power SpectrumMagnetic power spectrum computed for 4.5 hr interval shown on previous slide.

Note broad spectral enhancement at the proton cyclotron frequency.

Note, too, the added enhancement, lower in frequency consistent with He+ resonance.

Both waves are left-hand polarized in the s/c frame.

fpc/4

Theory – extension of Lee & Ip to include non-resonant pitch-angle spread in radial IMF.

Model Parameters

Peak Power vs. Accumulation TimePower depends on newly-ionized density ni = Ni t

nl

Model SpectrumOnly sunward waves are unstable.

k < 0: RH in s/c frame LH in SW frameDominant for radial IMF.

k > 0: LH in s/c frame RH in SW frame

Questions:

• Why are waves generally NOT seen?– Theory suggest should be seen ALWAYS.– Turbulent cascade seems NOT the answer.

• Why waves seen only during ~ radial IMF?– He+ waves NOT preferred in focusing cone.

• If STEREO waves are monochromatic…– Why are ours not? Or, why are theirs?

ConclusionsPickup ion waves are seen, but only sporadically and only in quasi-radial IMF.

Not sure why, but could involve growth rates vs. turbulent transport rate as functions of initial pickup angle.

Events observed at Ulysses, now at Voyager, maybe at STEREO, Messenger.

Need re-instatement of high-res Voyager data past Jupiter to study proton waves.

Recognition of He+ waves could allow more detailed wave-particle studies, since particle distributions are available at STEREO.

Questions:

1.Why are they seen so seldom and only during radial IMF?

2.Are they seen only on Ulysses?

3.Are pickup protons the only source ions?

Why are They so Seldom Seen?Lee and Ip [JGR, 87] obtained large amplitude wave growth under assumption that ALL wave energy accumulates without dissipation or distortion as interstellar particles are picked up within a fluid element.

Isenberg et al. [ApJ, 03] point out that the turbulent transport of wave energy removes any features on the nonlinear time scale.

Also, large fluctuations can both mask peaks and spread initial beam in pitch angle to reduce available free energy. [Isenberg, SW8, 95]

Observing Favors Radial IMF

Williams & Zank [JGR, 94] suggest that quasi-parallel waves in quasi-perp field will not be Doppler-shifted: expected frequencies and features will not be seen.

Could be part of the explanation, but would need

Ionization in radial IMF produces ions with the highest free energy and largest wave growth rates.

Jian et al. (ApJL, 701, L105, 2009)

Report observations of magnetic waves in STEREO data seen at fpc/4.

Observed most often when IMF is ~ radial.

Could these be due to He+?

Jian et al. (ApJL, 701, L105, 2009)

Are the waves due to pickup He+?

The acid test would be if assumption of Sunward propagation leads to dominant fast mode polarization in the plasma frame.