Magnetic Waves Excited by Newborn Interstellar Pickup Ions Measured by the Voyager Spacecraft from 1 to 45 au. III. Observation Times

In the companion papers Hollick et al. (2018a, 2018b) we analyzed 637 intervals of Voyager 1 and 2 magnetic field data from its launch in mid-1977 through 1990 that show evidence of wave excitation by newborn interstellar pickup ions (PUI). A characteristic signature of waves due to PUIs is that the wave enhancement is seen only at spacecraft-frame frequencies larger than the ion cyclotron frequency for the source ion, ${f}_{\mathrm{sc}}\gt {f}_{i,c}$, where ${f}_{i,c}\equiv {e}_{i}B/(2\pi {m}_{i}c)$, e_i is the ion charge, B is the heliospheric magnetic field (HMF) intensity, m_i is the ion mass, and c is the speed of light (Lee & Ip 1987). The waves are predicted to be left-hand, circular polarization in the spacecraft frame with magnetic fluctuations transverse to the mean magnetic field. Theory predicts that the waves are sunward-propagating and Doppler-shifted into the spacecraft frame, so they are right-hand-polarized in the plasma frame. About 10% of the waves found in the Advanced Composition Explorer (ACE) and Ulysses magnetic field data have the opposite polarization in the spacecraft frame for reasons that are not yet understood (Cannon et al. 2014a; Fisher et al. 2016).

Past observations using ACE, Ulysses, and Voyager magnetic field data include waves excited by interstellar H⁺ (Murphy et al. 1995; Joyce et al. 2010; Cannon et al. 2013,2014a, 2014b; Aggarwal et al. 2016; Smith et al. 2017) and He⁺ (Joyce et al. 2010; Argall et al. 2015, 2017; Aggarwal et al. 2016; Fisher et al. 2016; Smith et al. 2017). The published Voyager observations were limited to just 12 low-frequency events that occurred between mid-1977 and 1979 (Joyce et al. 2010; Aggarwal et al. 2016), 1 event having apparent Bernstein waves (Joyce et al. 2012), and 2 observations recorded at ∼30 au (Argall et al. 2017). This paper lists the events used by Hollick et al. (2018a, 2018b) to extend the Voyager event catalog through 1990.

In order to find these relatively rare observations, we have adopted a method used by Cannon et al. (2014a, 2014b), Argall et al. (2015, 2017), and Fisher et al. (2016) where daily spectrograms that include polarization analyses are routinely computed over an extensive volume of data. The two distant events seen by Voyager 2 at ∼30 au, and previously reported by Argall et al. (2017), were found using this technique. Since the HMF spectrum is largely unpolarized at ion cyclotron scales, the strongly polarized signals associated with PUI-excited waves are readily seen. This has permitted us to build a database of 639 wave observations. This database has been subsequently analyzed by Argall et al. (2017) and Hollick et al. (2018a, 2018b). So that other researchers may conduct their own investigation of these intervals, we have constructed tables of their times and present those tables here.

To gain a more complete perspective on the Voyager data, we offer plots showing the trajectories of the spacecraft, along with data coverage. Hollick et al. (2018a, 2018b) used both 1.92 s and 9.6 s magnetic field data from both the Voyager 1 and 2 spacecraft. Figure 1 shows the trajectories and data coverage for the magnetic field instrument for the two spacecraft over the years used in that study. The Voyagers do not provide complete data coverage, owing in large part to competition for resources at the Deep Space Network. Planetary encounters are marked in the second panel of each figure. Table 1 lists the times of closest approach, as well as the spans of time that were omitted from the lists of waves due to newborn interstellar PUIs because planetary dynamics such as shock acceleration and atmospheric leakage of neutral atoms may be involved in the measured wave properties (Ness et al. 1979a, 1979b, 1981, 1982, 1986, 1989; Goldstein et al. 1983, 1984, 1985, 1986; Smith et al. 1983, 1984, 1989, 1991; Krimigis et al. 1985). Observations in close proximity to interplanetary shocks were also omitted, as the acceleration processes at shocks can alter the wave characteristics, leading to incorrect conclusions regarding the ion source (Lee 1983; Kennel et al. 1986; Smith 1989; Bamert et al. 2004; Gloeckler et al. 2005). After the Saturn encounters and starting in 1981, data coverage is reduced for both spacecraft and there is significantly less time covered by the 1.92 s data (∼10%) than by the 9.6 s data (∼50%). Both cadences see increases in data coverage after 1986, but the coverage for 1.92 data remains only ∼15%. Since B ∼ 1/R in the outer heliosphere, the ion cyclotron frequencies are significantly reduced when at the same time the data coverage by 1.92 s data is reduced. It becomes both convenient and feasible to use the 9.6 s data at this time to obtain more and longer-duration wave intervals.

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Note that the data listed as available above includes data that are not usable in this analysis. The data include unresolved maneuvers such as "mag rolls" and occasional noise problems at the frequencies of interest here. There are also files where the time tags jump backward by a significant degree. Given the ∼50% data coverage, it is possible that with a complete data set the number of wave events beyond the start of 1981 could be twice that what we found in our analysis.

Figure 1 also shows both the times of wave events used in the related studies (Hollick et al. 2018a, 2018b) as well as the duration of the events. The red (green) symbols represent events when PLS thermal proton data are (are not) available. Triangles (circles) represent 1.92 s (9.6 s) magnetic field data. Wave intervals due to He⁺ and H⁺ PUIs are shown separately. Years without any symbols were searched and did not reveal reliable wave events. Typical event durations are under 10 hr and normally closer to 5 hr. This bears no relationship to the duration of the waves in the plasma frame, as the plasma evolves and reflects only the scale size of the regions of space that contain the waves.

The detailed event times analyzed by Hollick et al. (2018a, 2018b), along with spacecraft location and wave ellipticity E_lip averaged over ${f}_{i,c}\leqslant {f}_{\mathrm{sc}}\leqslant 2{f}_{i,c}$, are listed in Tables 2–5. E_lip is listed specifically because theory predicts that the waves should be left-hand polarized in the spacecraft frame (Lee & Ip 1987) and there presently exists no clear explanation for the observation of right-hand polarized waves. For each interval listed we denote the presence or absence of waves due to pickup He⁺ or H⁺, with "Y" for yes and "N" for no. Some listings contain "??" to represent uncertainty on our part as to the presence or absence of waves. This usually means either a disturbed spectrum for indeterminate reasons or very weak, but mildly suggestive, polarization signatures. Sometimes the event duration is simply too short to obtain statistically reliable spectra at the necessary frequencies (most often the He⁺ cyclotron frequency). Uncertain observations are not used in the associated analyses (Hollick et al. 2018a, 2018b) and are treated by us as a "No" interval. They are included here only if there is evidence of waves in association with the other ion species. We likewise list the availability of PLS thermal proton data.

We have provided the times when waves due to newborn interstellar pickup He⁺ and H⁺ were seen in the Voyager 1 and 2 magnetic field data. These are the same events that were studied by Hollick et al. (2018a, 2018b). The plots and listings omit the two events studied by Argall et al. (2017), as these were not included in the Hollick et al. analyses. We have not listed the control intervals of typical solar wind turbulence conditions that were used in these same papers for comparison against the wave spectra. The 1675 control intervals are too numerous to list here and were chosen solely on the basis of not having observable wave activity in their magnetic spectra. In principle, any such intervals should serve equally well for this purpose.

C.W.S., P.A.I., S.H., and Z.B.P. are supported by NASA grant NNX17AB86G. B.J.V. is supported by NSF grant AGS1357893. C.J.J. and N.A.S. are supported by the Interstellar Boundary Explorer mission as part of NASA's Explorer Program, and partially by NASA SR&T Grant NNG06GD55G, and the Sun-2-Ice (NSF grant number AGS1135432) project. C.W.S., B.J.V., P.A.I., and N.A.S. were partially supported by NASA grant 80NSSC17K0009. J.M.S., M.B., and M.A.K. acknowledge the support by the Polish National Science Center grant 2015/19/B/ST9/01328. N.F.N. and L.F.B. are supported by the NASA Voyager Project under a cooperative agreement with the University of Maryland, Baltimore County. The data used in this analysis are available from the NSSDC.