Voyager 2 Observations of Magnetic Waves due to Interstellar Pickup Ions

Colin J. JoyceCharles W. Smith, Phillip A. Isenberg,Nathan A. Schwadron, Neil Murphy

and S. Peter Gary

Two distinct forms of waves

• Low-frequency fast magnetosonic waves due to cyclotron resonance– Joyce et al., ApJ, 724, 1256, 2010.– Mean field is quasi-radial

• Bernstein waves seen at higher spacecraft frame frequencies– Joyce et al., ApJ, submitted.– Mean field is perpendicular to the flow

Newborn Ion Production

• Neutral particles from the ISM are free to stream through the heliosphere unfettered by the IMF until they become ionized.

• There are two ionization processes:– Charge exchange (interaction with solar wind

particles).

- Photo-ionization (interaction with photons).

• Most interstellar hydrogen is ionized outside of a region bounded at ~5 AU known as the H+ ionization cavity.

Ionization Rates

• Interstellar He is ionized primarily through photoionization, whereas H is ionized by a combination of both processes.

• The photoionization rate of H and He is given by:

• The charge exchange rate for H is:

Rucinski et al. (1996)

PUI Theory

Pickup ion in velocity space.

• PUIs enter velocity space with an initial distribution determined by the angle between the solar wind and IMF.

• This distribution is unstable to the generation of MHD waves, which causes the PUI to scatter to isotropy.

• The wave energy generated by this interaction should be a common and observable feature of power spectra in the heliosphere, and is believed to be cause solar wind heating via the turbulent cascade.

• The effects of newborn PUIs in a ring distribution were first modeled by Lee and Ip (1987) and later modified by Isenberg (1996) to account for the effects of a fluctuating magnetic field.

Previous Observation• The only prior observation of PUI waves was

made by Murphy et al. (1996).• They generated power spectra from Ulysses

magnometer data and looked for power and polarization signatures at the proton cyclotron frequency.

• They found events tended to occur when the IMF was highly radial.

Procedure•We follow the same basic approach as Murphy et al. using Voyager 2 magnometer data.

•We have created an archive of magnetic power and helicity spectra ranging from launch in 1977 to midway through 1979 (spanning from 1 to ~4.75 AU).

•Data intervals are typically several hours in duration and are selected to be free of solar wind transients that would affect the spectra.

•Each spectrum is then searched for power and helicity signatures at the proton cyclotron frequency. “Normal” background spectrum with

no wave enhancements present.

Low-Frequency Fast-mode waves due to PUIs

•We have found one event in this data which is indicative of waves excited by pickup H+ and He+.

• The event occurs over 4.5 hours when Voyager 2 was at 4.5 AU.

•IMF direction is quasi-radial, with field intensity remaining fairly constant throughout the interval.

•Dashed lines show direction to Jupiter.

• Solar wind speed is decreasing slightly.

• Seen as part of a prolonged rarefaction interval.

• Density is ~ constant, but low.

• Temperature is stable.

Fast-mode waves due to PUIs•Spectral analysis reveals power enhancements at the proton cyclotron frequency and the He+ cyclotron frequency.

•Location at 4.5 AU is on the edge of the H+ cavity, but He+ intensity is not diminished.

•We see a significant signature in the helicity spectrum of the event. When combined with the computed normalized cross helicity of the event, we show that the waves are sunward propagating fast mode waves.

•Strongly suggests excitation by newborn ions originating from interstellar neutrals.

Isenberg (1996) Theory

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•We apply the theoretical formalism of Isenberg (1996) to our observation.•This theory is time asymptotic and calculates the total energy generated by a population of pickup ions as they scatter completely to isotropy.•It modifies the analysis of Lee and Ip (1987) by accounting for the spread in pitch angle caused by magnetic field fluctuations, which causes a spread in the initial pitch angle of the PUIs and results in less wave energy being generated.•The equations used in this model are shown below.

Application of Isenberg Theory•Due to the limitations of the theory, the model does not capture the detailed shape of the enhancements.•It does, however, reproduce the relative power between the two enhancements which supports our interpretation.•We find that the amplitudes of these enhancements are limited by the timescale of the turbulent destruction of the waves.

Quasi-Perpendicular IMF

• When the local mean IMF is quasi-radial, the newborn pickup ion streams along it.– Magnetic instability is the beam instability– V|| > VA, wavelength matches particle orbit

• When the local mean IMF is quasi-perp, three wave modes are possible:– ion cyclotron instability,

where T > T|| (E > E|| )– whistler mode instability– Bernstein mode instability.

k B0 mode with = nci

• Instability is driven by a velocity-ring type distribution.

• Wave frequencies are harmonics of the proton cyclotron frequency.

• Wave growth is greatest at k almost perpendicular to the mean magnetic field.

• Wave fluctuations at high angles to the mean IMF.

Bernstein Mode Characteristics

Bernstein Mode Observations

• Event takes place over three hours with Voyager at 4.75 AU.

• IMF is highly azimuthal during interval.

• Event takes place eight days into a thirteen day rarefaction over which the solar wind speed decreases from 700 to 350 km/s.

• IMF geometry makes connection to Jupiter unlikely and thus Jovian electrons are not a likely source for the waves.

Bernstein Mode Observations•Spectral analysis of event reveals two power enhancements at spacecraft frame frequencies near the Nyquist frequency.

Bernstein Waves “Normal” Background

Bernstein Mode Observations•Higher resolution plotting reveals that enhancements are indeed present due to magnetic fluctuations.

•We see that the magnetic fluctuations are almost entirely perpendicular to the radial.

•This suggests that the waves propagate radially, almost perpendicular to the mean field.

Bernstein Mode Observations•The helicity spectrum of this event shows no signature, indicating that the waves are not circularly polarized.

•To get a better sense of the wave polarization, we generate hodograms of the event.

•This analysis reveals linear polarization, with fluctuations highly perpendicular to the mean IMF at the start of the event and developing more of a radial component in time.

Bernstein Mode Observations•The figure to the left shows the angle between the mean IMF and the radial, the wave frequency, and the angle between the fluctuations and the mean IMF during the course of the event.

•We see that the fluctuations begin once the IMF becomes highly azimuthal.

•We are unable to establish a correlation between frequency and any solar wind parameter.

•We also confirm that the fluctuations begin highly perpendicular to the mean IMF, but become more radial as time goes on.

Summary• We show evidence of first observation of pickup ion waves by Voyager and first observation of pickup He+ waves by any spacecraft.

• Apply theoretical model of Isenberg (1996) to observation using given solar wind parameters and find reasonable agreement given model limitations.

• Same pickup timescale produces both wave forms.

• Growth appears limited by turbulent cascade.

• Show evidence of a Bernstein wave mode not previously studied within this context and which may be quite common and contribute to solar wind heating.