Voyager and ACE Observations of Waves due to Interstellar Pickup He+

							  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, 1979
R = 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 Spectrum
Magnetic power
spectrum computed for
4.5 hr interval shown on     fpc/4
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.
Theory – extension of Lee & Ip to
     include non-resonant pitch-angle
         spread in radial IMF.
Model Parameters
Peak Power vs. Accumulation Time
 Power depends on newly-ionized density ni = n Ni Dt
                                    nl
                 Model Spectrum
Only sunward
    waves are
     unstable.
k < 0:
 RH in s/c frame
 LH in SW frame
Dominant 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?
                Conclusions
Pickup 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.