Observations of Low-Frequency Waves due to Newborn Interstellar Pickup Ions B. J. Vasquez, P. Aggarwal, M. R. Argall, L. F. Burlaga, M. Bzowski, B. E. Cannon, S. P. Gary, M. K. Fisher, J. A. Gilbert, P. A. Isenberg, C. J. Joyce, N. Murphy, R. G. Nuno, J. D. Richardson, N. A. Schwadron, R. M. Skoug, C. W. Smith, J. M. Sokół, D. K. Taylor, and T. H. Zurbuchen Hydrogen (right) We compute the rate of wave energy excitation by using the time derivative of the theory of Lee & Ip (1987) in combination with the observed solar wind parameters and theoretical ionization rates. The rate of wave energy excitation is given by: ACE (above left) Duration of wave events excited by pickup He+ seen by ACE spacecraft. (above right) Rate of occurrence of events as a function of the time of year. Note that ACE passes through the He+ focusing cone from Nov 15 to Jan 15 each year. Ulysses 2-5 AU (right) We compare the rate of wave energy excitation to the rate at which fluctuation energy is transported through the spectrum by the ambient turbulence. Turbulence remakes the energy and eliminates the wave signatures in favor of providing energy to the smallest scales where the ambient plasma is heated. We assume that while the waves are propagating parallel to the mean magnetic field, the turbulence is largely two-dimensional and use MHD extensions of the familiar Kolmogorov (1941) expression for the rate of energy cascade. In terms of observables, this is given by: (right top) The angle between the mean field and the radial direction for He+-excited wave events seen by ACE tend to be BR < 20. (right bottom) Control intervals have nearly identical conditions, but BR can be larger. There are more control intervals than wave events. There has been the opinion that waves due to newborn interstellar PUIs are only seen when the field is radial. Our study of Ulysses observations discredits this belief. ACE is consistent with it. (above left) Daily spectrogram of ACE/MAG data showing waves due to newborn interstellar pickup He+ from 01:00 to 03:00 UT. (above right) Detailed analysis of wave interval showing enhanced power for f > fHe,c, degree of polarization and coherence  1, ellipticity  –1, and field-aligned minimum variance direction. Taken from Argall et al., Geophys. Res. Lett., 42, 9617, 2015. ACE (right) Although the ordering is not perfect, about 90% of the wave events exhibit wave growth rates in excess of the ambient turbulence rate. This means that the instability is sufficiently strong to overcome the destructive processes of the turbulence. In about 90% of the control intervals, the turbulence is stronger than the wave growth. ACE 1AU Voyager (left) Stack showing solar wind conditions for both He+- and H+-excited wave events seen by the Voyager spacecraft. Eleven wave intervals of one or both types were seen in 1978 & 79. Red triangles represent waves. Black circles are controls. Green squares are 2 questionable intervals. Solid curves represent the nominal solar wind expansion from 1 AU. Note that densities are lower than average while Alfven speeds are high and small values of BR are favored. These intervals are often described as rarefaction intervals with nearly radial mean fields. (left) We can compute the time required for the instability to achieve the observed wave energy level. This is typically less than 40 hrs and often less than 20 hrs, which is equivalent to less than ½ AU at the nominal solar wind convection speed. This implies local generation that depends on solar wind conditions. Ulysses Red triangles represent wave intervals. Black circles represent control intervals. (above left) Conditions under which He+-excited waves are seen by ACE. (above right) Waves are characterized by high degrees of polarization and coherence, ellipticity  –1, and field-aligned minimum variance directions. (left top) He ionization rates are derived from the Warsaw Test Particle Model. Note the strong solar-cycle dependence. Ionization rates for events and associated controls are essentially identical. (left bottom) Production rates for He+, which is the product of the ionization rate and the neutral atom density, does not vary greatly from event to event or from event to controls with a few exceptions that are associated with passage through the focusing cone. Voyager 2-6AU (above left) Waves observed by Ulysses last for up to 5 hours. (above right) Waves excited by H+ tend to be seen for R > 4 AU. (above) Waves are seen over a wide range of BR from radial to nearly perpendicular to the radial direction. The ellipticity is negative in most cases, but some waves are seen with right-hand polarization. We suspect incomplete ion scattering to be the cause. Taken from Cannon et al., Astrophys. J., 784, 150, 2014 and Cannon et al., Astrophys. J., 787, 133, 2014. References: Joyce et al., Astrophys. J., 724, 1256, 2010. Joyce et al., Astrophys. J., 745, 112, 2012. Cannon et al., Astrophys. J., 784, 150, 2014. Cannon et al., Astrophys. J., 787, 133, 2014. Argall et al., Geophys. Res. Lett., 42, 9617, 2015. Aggarwall et al., Astrophys. J., 822, 94, 2016. Fisher et al., Astrophys. J., in press, 2016. Voyager 2-6AU All ACE results shown above are taken from Fisher et al., Astrophys. J., in press. This work was supported in part by NASA grants NNX07AH75G, NNX07AH75G, NNX13AF97G, NNX11AJ37G, NNX07AH75G and NNX13AH66G, by NSF grants ATM0635863, ATM0635863, AGS0962506, AGS1357893, AGS-1358103, ATM0635863 and AGS1357893, by Caltech subcontract 44A1085631, by the JPL Space Grant program, and by Polish National Space Center grant 2015/18/M/ST9/00036. The Voyager analysis is taken from Joyce et al., Astrophys. J., 724, 1256, 2010 and from Aggarwal et al., Astrophys. J., 822, 94, 2016.