Analysis of Voyager 1 Magnetic Field Data From 1977 to 1990

Ab initio modulation models require as an input magnetic turbulence spectra covering the energy to the dissipation range. Most previous studies (see, e.g., Burlaga and Ness 1998, JGR, 103; Hollick et al. 2018, ApJ, 237, 849, 863) focus on either the low- or the high frequency part of the spectrum. In this study, we use 48 s resolution N-component data to calculate spectra, averaged over 13 day intervals, for the whole fourteen year period. These spectra span three decades in frequency below 0.01 Hz. Data were first filtered to eliminate spurious peaks. Hollick and co-workers provide an extensive list of interstellar pickup ion events; our 13-day averaging yields only 33. Near Earth we resolve both an energy- and a Kolmogorov inertial range. Beyond about 2.5 au we see only an inertial range, and around 4 au a 1/f noise spectrum appears at high frequencies. The first pickup ion peaks are seen in late 1979 at 6.4 au. The most such events occur during 1983 when seven were observed between about 15 and 18 au. The level of the 1/f spectrum drops by about two orders of magnitude between 4 and 25 au, but then remains fairly constant up to 40 au. Since the level of the spectrum in the inertial range drops more than ten times more over the same distance, the 1/f spectrum becomes much more prominent beyond 25 au. It is also beyond this radial distance where we see a steep increase in the magnetic variance following the well-studied decrease. If this spectrum is due to spacecraft noise, the actual magnetic variance may be decreasing significantly beyond some 20 au.

While the spectral form of the turbulence spectra to be used as input for modulation models are not significantly affected by the presence of pickup ions, a steep decrease in magnetic variance beyond 20 au would have a significant impact on cosmic-ray modulation.

The preliminary results presented here are from the Masters Dissertation of Sarah McKee. We acknowledge financial support by SANSA and the NRF.