Listing of 502 Times When the Ulysses Magnetic Fields Instrument Observed Waves Due to Newborn Interstellar Pickup Protons

In previous papers (Murphy et al. 1995; Smith et al. 2010; Cannon et al. 2013, 2014a, 2014b) we analyzed 502 intervals of waves due to newborn interstellar pickup protons that were observed by the Ulysses magnetic field instrument (Balogh et al. 1992) from 1996 through 2005 exclusive of the years 2000–2002. Figure 1 shows the trajectory of the Ulysses spacecraft from launch until the end of 2009. Note that the years 2000–2002 constitute the fast latitudinal scan when the spacecraft was inside ∼4 au. In this region there is a severe shortage of neutral hydrogen due to aggressive ionization in the range 4–6 au (Zank 1999). This region is called "the hydrogen cavity." Without a sufficient supply of neutral hydrogen, there is insufficient pickup H⁺ to excite waves and we demonstrated the general absence of waves inside 4 au (Cannon et al. 2014a, 2014b). For this same reason, we omitted the initial trajectory to Jupiter (1991–1995) and the fast latitude scan from 2006 to 2008.

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Lee & Ip (1987) computed predicted spectra for magnetic waves due to newborn interstellar pickup ions (PUIs). The waves are expected to exist at spacecraft-frame frequencies ${f}_{{sc}}\geqslant {f}_{p,c}$, where ${f}_{p,c}={eB}/({m}_{p}c2\pi )$ is the proton cyclotron frequency and e is the proton charge, B is the magnetic field intensity, m_p is the proton mass, and c is the speed of light. They should be right-hand polarized in the plasma frame and sunward propagating. The Doppler shifted polarization in the spacecraft frame should be left handed. The magnetic fluctuations should be transverse to the mean magnetic field.

We used these general characteristics to build the data set of 502 wave events that can be argued to arise from newborn interstellar pickup H⁺. The fact that spectra at frequencies ${f}_{{sc}}\lt {f}_{p,c}$ should show typical interplanetary forms (unpolarized power law spectrum) is the result of the PUIs not undergoing significant acceleration in the solar wind except when in the vicinity of shocks. This helps to set the waves due to PUIs apart from other kinetic processes and is a significant aid in building the event set.

The wave observations are not easily found without the use of daily polarization spectrograms that can scan the years of data to reveal the waves when they occur. This is the method we used to find the wave intervals as was later done for ACE observations (Argall et al. 2015; Fisher et al. 2016). We attempted to match the general wave characteristics described above when searching the spectrograms, but we admitted some candidate intervals with wave signatures at ${f}_{{sc}}\lt {f}_{p,c}$ if there were no shocks or sources of acceleration in the vicinity. Likewise, we admitted intervals when the wave signatures were limited to ${f}_{{sc}}\geqslant {f}_{p,c}$ as expected, but the wave polarization was right handed in the spacecraft frame. We did this with the expectation that there may still be more to learn. About 10% of the wave intervals were right-hand polarized in the spacecraft frame. A similar number holds for ACE (Argall et al. 2015; Fisher et al. 2016) and Voyager (Aggarwal et al. 2016). There was no rigorous test for admitting a wave interval, but Figure 11 of Cannon et al. (2014a) clearly shows the wave intervals have a degree of polarization $\gt 0.4$ and generally higher. Sometimes the wave energy did not rise above the background spectrum, but the polarization showed clear, strong signs of wave generation by PUIs and the interval was admitted. We found that the polarization spectrum was often a better indicator of waves excited by PUIs than the power spectrum was. Figure 8 of Cannon et al. (2014a) shows that fewer than 50 events have peak wave energy at ${f}_{{sc}}\lt {f}_{p,c}$. The magnetic fluctuations are consistently transverse to the mean magnetic field.

Due to the considerable effort required in identifying these events, which are rare and often subtle in nature, we provide a list of the times for the 502 wave event intervals previously identified so that they may be independently examined by other researchers. Table 1 lists the wave event times analyzed to produce Cannon et al. (2014a, 2014b). We used a comparable number of control intervals in Cannon et al. (2014b). These were times near the wave events when the bulk plasma parameters and flow conditions were similar to the wave events, but no waves were seen. We are not listing these intervals because any reasonable selection of non-wave events should do as well.

We argued that these waves may only be observable when the wave growth time is shorter than that of the characteristic nonlinear time of the background turbulence to transport the wave energy through the inertial range cascade. Since the wave growth time is on the order of 20 hr, this requires that the background turbulence be weak and thus, this threshold explains the fact that these waves are seldom seen.

The fate of wave energy excited by newborn interstellar PUIs is that it will be absorbed by the background turbulence and used to heat the thermal ions (Zhou & Matthaeus 1990; Zank et al. 1996, 2012; Matthaeus et al. 1999; Smith et al. 2001, 2006; Isenberg et al. 2003, 2010; Breech et al. 2005, 2008; Isenberg 2005; Cranmer et al. 2009; Ng et al. 2010; Oughton et al. 2011; Usmanov et al. 2011, 2012, 2014, 2016; Adhikari et al. 2015a, 2015b). The absence of observable waves does not mean that wave excitation is not present in the plasma. The source of neutral atoms and the ionization processes that produce PUIs are approximately constant, suggesting that wave energy is continually being injected into the plasma by the PUIs. The relative scarcity of the observations is an indication that only rarely is the turbulence weak enough to allow the waves to reach observable levels. This also explains why there are marginal wave observations without significant enhancements in the power spectra. For this same reason, we do not claim to have found an exhaustive list of wave events. We made a good faith effort to be as complete as possible while spanning the broadest range of solar wind conditions subject to the reality that marginal events exist and are difficult and unreliable to identify.