Pickup ion-induced magnetic waves observed by the Voyager spacecraft beyond Pluto Print E-mail
Tuesday, 21 August 2018 16:21

Interplanetary space is filled with magnetized plasma from the Sun (the solar wind) and interstellar gas, mainly hydrogen and helium, which continuously flows through the heliosphere. The interstellar atoms are ionized by extreme ultraviolet radiation and the solar wind. In result of ionization of the interstellar atoms, new ions in the solar wind are created.

They are called pickup ions because immediately after creation they are picked up by the local magnetic field of the solar wind and advected with the solar wind into the interplanetary space. Pickup ions gyrate in the magnetic field, producing characteristic magnetic waves. Waves of this kind can be detected in time series of the intensity and direction of the magnetic field observed by the interplanetary probes.

The magnetic waves created due to the creation of pickup ion have characteristic signatures in the magnetic field time series, different for each of the pickup ion species. However, these signatures can be detected only when the growth rate of the wave is greater than turbulence level of the solar wind. Searching for the wave-growth events is challenging, but very important, because it allows for verification of the models of the distribution of the interstellar neutral gas in the heliosphere and the pickup ion creation processes in the solar wind magnetic field. The growth rate of the magnetic waves depends, among others, on the production rate of pickup ions. The pickup ion production rate is directly proportional to the density of the interstellar neutral gas in the interplanetary space and the ionization rates, which both vary in time with the solar cycle and spatially with heliographic latitude. The spatial distributions of the interstellar neutral hydrogen and helium density and ionization rate are totally different, thus the resulting pickup ion production rates are significantly different. For solar distances smaller than 3 astronomical units, the production rate for helium ions is much greater than the production rate for hydrogen ions. In consequence, it is expected that a spacecraft finds more magnetic wave events characteristic for the creation of helium than hydrogen pickup ions. For solar distances greater than 3 au, more hydrogen pickup ions are produced.

The magnetic waves excited by the newborn interstellar pickup ions are observed almost in the whole heliosphere within the regime of the supersonic solar wind from a few tenths to several dozens of astronomical units from the Sun. They were detected by instruments on ACE (at 1 au) and Ulysses missions (from 1 to 5 au). Very important data were gathered by Voyager spacecraft between 1 and 45 au. A team of researchers from the University of New Hampshire studied magnetic field data from Voyager 1 and Voyager 2 missions collected in a period from 1997 to 1990 and identified more than 600 events associated with magnetic waves excited by newly-born interstellar pickup ions, both of helium and hydrogen. Scientists from the Laboratory for Solar System and Astrophysics from the Space Research Centre of the Polish Academy of Sciences (Justyna M. Sokół, Maciej Bzowski, and Marzena A. Kubiak) studied the pickup ion production rates for interstellar hydrogen and helium using the Warsaw Test Particle Model (WTPM) simulation code, a model of the solar wind evolution in time and heliographic latitude, and a model of solar wind extreme ultraviolet ionizing radiation. These research tools, developed by the team from CBK PAN, were used to calculate the distribution of interstellar neutral gas of hydrogen and helium inside the heliosphere and the production rates for pickup ions along Voyager 1 & 2 trajectories. Results of these studies were used by the UNH team in the research on the magnetic waves creation. The study showed that the magnetic waves due to the pickup ions can be excited as far from the Sun as the Pluto orbit and beyond, it is up to 45 au from the Sun. The team of researchers from UNH together with the researchers from CBK PAN found a good agreement between the observed rates of magnetic waves due to pickup ion creation and the model predictions.

The results of the study were published in a series of three articles by Hollick et al. in The Astrophysical Journal and The Astrophysical Journal Supplement. The articles are available at: http://iopscience.iop.org/article/10.3847/1538-4357/aac83bhttp://iopscience.iop.org/article/10.3847/1538-4357/aac839, http://iopscience.iop.org/article/10.3847/1538-4365/aac83a.

Justyna M. Sokół & Maciej Bzowski

Last Updated on Wednesday, 22 August 2018 13:39
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