1. Observations from 1 to 6 AU of Low-Frequency Magnetic Waves due to Newborn Interstellar Pickup Ions Using Ulysses, Voyager and ACE Data Charles W. Smith, Poornima Aggarwal, Matthew R. Argall, Leonard F. Burlaga, Maciej Bzowski, Bradford E. Cannon, S. Peter Gary, Meghan K. Fisher, Jason A. Gilbert, Sophia J. Hollick, Philip A. Isenberg, Colin J. Joyce, Neil Murphy, Raquel G. Nuno, Zackary B. Pine, John D. Richardson, Nathan A. Schwadron, Ruth M. Skoug, Justyna M. Sokół, David K. Taylor, Bernard J. Vasquez, and Thomas H. Zurbuchen
Abstract: Wave excitation by newborn interstellar pickup ions (PUIs) plays a significant role in theories that attempt to describe IBEX and Voyager observations in the heliosheath as well as solar wind heating. The same dynamic processes can be far-reaching and extend into the inner heliosphere to at least 1AU and likely to smaller heliocentric distances. While the high-resolution magnetic field measurements required to study these waves are not yet available in the heliosheath, we have studied a range of available observations and found evidence of waves due to interstellar PUIs using ACE ( at 1 AU), Ulysses ( at 2 to 5 AU, high and low latitudes) and Voyager ( and 2 to 6 AU) observations. Efforts to extend the Voyager observations to 35 AU are ongoing. We have examined these data sets and report on observations of low-frequency waves that result from newborn interstellar pickup H+ and He+ ions. Although not as common as theory once predicted, we presently have identified 524 independent occurrences. Our conclusion from studying these waves is that they are seen only when the ambient turbulence is sufficiently weak. The instability that leads to the generation of these waves requires a slow accumulation of wave energy over several to tens of hours to achieve the observed wave amplitudes. In regions where the turbulence is moderate to strong, the turbulence consumes the wave energy before it can reach observable levels and transports the energy to the dissipation scales where it heats the background thermal particles. Only intervals with the weakest turbulence will permit energy accumulation over this time scale. These conditions are most often, but not exclusively, achieved in solar wind rarefaction regions.
(below) We compare the background wave energy for wave events (red triangles) to control intervals (black circles) for Ulysses observations. Although control intervals were chosen to reside close to wave events, the waves are seen at times of low background spectra.
Our publications on this topic to date include:
Murphy, et al., Further studies of waves accompanying the solar wind pick-up of interstellar hydrogen, Space Science Reviews, 72(1-2), , 1995.
Joyce, et al., Excitation of low-frequency waves in the solar wind by newborn interstellar pickup ions H+ and He+ as seen by Voyager at 4.5 AU, The Astrophysical Journal, 724, , 2010.
Smith, et al., Ulysses and Voyager observations of waves due to interstellar pickup H+ and He+, Proceedings of Ninth Annual International Astrophysics Conference: Pickup Ions Throughout the Heliosphere and Beyond, AIP Conf. Proc. 1302, pp , 2010.
Joyce, et al., Observation of Bernstein waves excited by newborn interstellar pickup ions in the solar wind, The Astrophysical Journal, 745, 112, 2012.
Cannon, et al., Ulysses observations of magnetic waves due to newborn interstellar pickup ions. 1. New observations and linear analysis, The Astrophysical Journal, 784, 150, 2014.
Cannon, et al., Ulysses observations of magnetic waves due to newborn interstellar pickup ions. 2. Application of turbulence concepts to limiting wave energy and observability, The Astrophysical Journal, 787, 133, 2014.
Argall, et al., ACE observations of magnetic waves arising from newborn interstellar pickup Helium ions, Geophysical Research Letters, 42, , 2015.
Aggarwal, et al., Voyager observations of magnetic waves due to newborn interstellar pickup ions: 2 to 6 AU, The Astrophysical Journal, 822, 94, 2016.
Fisher, et al., A survey of magnetic waves excited by newborn interstellar He+ observed by the ACE spacecraft at 1 AU, The Astrophysical Journal, 830, 47, 2016.
(above left) Ellipticity of 502 wave events seen by the Ulysses spacecraft and argued to be excited by H+.
(above right) Spacecraft-frame frequency of peak wave power relative to proton cyclotron frequency. Theory predicts that wave frequencies should exceed the cyclotron frequency of the source ion.
(left) Spectra of waves due to interstellar pickup He+ observed by the ACE spacecraft at 1 AU. Note enhanced power at fsc > fic,He+, high degree of polarization and coherence, left-hand circular polarization in the spacecraft frame, and minimum variance direction along the mean magnetic field.
(middle) Spectra of waves due to interstellar pickup He+ observed by the Voyager 2 spacecraft at 2.1 AU. At this point the spacecraft is still inside the neutral hydrogen cavity.
(right) Spectra of waves due to interstellar pickup H+ observed by the Voyager 1 spacecraft at 6.2 AU.
(above) Daily spectrogram of ACE / MAG data showing enhancement in polarization parameters indicative of waves due to interstellar pickup He+. Twenty-five such events have been found and reported over the mission life. Note that right-hand polarized fluctuations arising from dissipation are also evident later in the day.
(below left) Solar wind conditions for Voyager observations showing wave events (red triangles) and control intervals (black circles). Curves give nominal values. Note that the density tends to be low, IMF tends to be radial, and the Alfven speed is unusually high. These are characteristics of rarefaction intervals where the IMF threads the expansion region.
(below) The wave excitation rate must exceed the rate that the turbulent cascade remakes the wave energy and transports it to small scales for dissipation and heating of the background plasma. We compare these two rates for waves observed by ACE, Voyager and Ulysses.
(below left) Ambient wind conditions for wave events (red triangles) and control intervals (black circles) seen by ACE. Note that conditions for control intervals are generally a good match for wave intervals.
(below right) Wave polarization parameters and spectral properties for both wave and control intervals. Note wave intervals are consistently more highly polarized and are almost always left-hand circularly polarized in the spacecraft frame.
Voyager
Ulysses
(above) ACE solar wind conditions for event shown above. Event time is marked by vertical dashed lines. Most, but not all, wave events are seen in rarefaction regions where the turbulence is weak.
Summary: The waves due to newborn interstellar pickup ions recorded by ACE, Ulysses and Voyager are seen at times when the background turbulence is low. This includes, but is not limited to, rarefaction intervals. While a weak power spectrum for the background fluctuations will make the observation of waves easier, it is also an indication that the background turbulence is weak.
Observation of the waves requires that the rate of wave energy production by newborn ions is greater than the rate of the turbulent cascade.
ACE
Comparison of the rate of wave energy growth due to the instability vs. the rate of energy cascade in the background turbulence shows that the observation of waves (red triangles) requires that the turbulence be weaker than the instability. This allows the wave energy to accumulate over a long period of time that is consistent with the relatively slow growth of the waves.
(above left) Polarization properties for waves seen by Voyager and associated with the H+ resonance.
(above right) Polarization properties for waves associated with the He+ resonance.