Pluto's Interaction With Energetic Heliospheric Ions

Abstract

Pluto energies of a few kiloelectron volts and suprathermal ions with tens of kiloelectron volts and above. We measure this population using the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument on board the New Horizons spacecraft that flew by Pluto in 2015. Even though the measured ions have gyroradii larger than the size of Pluto and the cross section of its magnetosphere, we find that the boundary of the magnetosphere is depleting the energetic ion intensities by about an order of magnitude close to Pluto. The intensity is increasing exponentially with distance to Pluto and reaches nominal levels of the interplanetary medium at about 190R _P distance. Inside the wake of Pluto, we observe oscillations of the ion intensities with a periodicity of about 0.2 hr. We show that these can be quantitatively explained by the electric field of an ultralow-frequency wave and discuss possible physical drivers for such a field. We find no evidence for the presence of plutogenic ions in the considered energy range.

Keywords: New Horizons; Pluto; pickup ions; wave.

Figure 1

Overview of interstellar pickup and suprathermal ion measurements around Pluto. We only show data from a single sector of the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) for clarity. This sector points into different look directions as a function of time, depending on the changing spacecraft attitude. (first panel) The x symbols and colored solid lines show intensities dominated by 18‐keV He⁺ ions. The color coding marks measurements at different cone angles relative to the Sun. Dashed lines indicate the Poisson error envelope of the intensities. Black lines show the average IPS intensity in the solar wind and an extrapolation of the Pluto wake intensity. The crossing point is an estimate when New Horizons passed the magnetic boundary of the Pluto wake. Blue and red shaded areas call out two regions discussed in section 3.1 and 3.2. Gray and green shading provide context, showing regions observed at lower energies than discussed in this paper (McComas et al., 2016). (second panel) Cone angle of the measurement shown in the first panel. (third panel) x _PSH coordinate (orange) of New Horizons and the radial distance r to Pluto (black). (fourth panel) Cylindrical distance ρ of New Horizons to the wake center axis of Pluto (black) and of Charon (orange). PSH = Pluto Solar Heliographic coordinate system; DOY = day of year.

Figure 2

Zoom of Figure 1 into the energetic particle wake of Pluto. It can be seen that the interstellar pickup and suprathermal ion intensities increase exponentially and have oscillations superimposed. (first panel) Sum of all ions at all energies and directions that are detected by the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) as a function of time. (second and third panels) He⁺ ion intensities for two different energies (middle and lower panel) and different look directions (different colors). Error bars are Poisson errors. vertical dashed lines = locations of the major peaks of the oscillation to guide the eye; vertical shaded areas = averaging ranges for a typical peak (green shading) and valley (red shading); red points = average respective intensities. (fourth panel) Cone angle of the two sectors shown. It can be seen that there is no correlation between cone angle and intensities, meaning that the ion distribution is symmetric around the Sun direction. The clock angle (not shown) is constant during this time period. DOY = day of year.

Figure 3

He⁺ energy spectra upstream of Pluto (blue) and within Pluto's wake (green). The spectra are shown in the spacecraft frame. It can be seen that the Pluto interaction mostly affects the overall amplitude but only barely the spectral shape. Measurements are taken with a cone angle of 34° to the Sun. The vertical dashed line shows the expected interstellar pickup ion cutoff energy based on the upstream solar wind measurements, the observation angle, and assuming the ions are He⁺. Error bars show the Poisson error of the event data counts. DOY = day of year.

Figure 4

Energy spectra measured at a peak within Pluto's energetic particle wake (red x symbols) and in a valley (orange x symbols) for two different ion velocity directions (two panels). The ion velocity direction is opposite to the instrument look direction, meaning that an ion direction of 146° and a look direction of 34° are labeling the same spacecraft attitude. Either number labels measurements by sector S0 (lower panel), which is measuring further from the Sun than the other used sector, S1 (upper panel). We only connect data points with lines that are reliable in the sense that the Poisson error bars are small and where the bars of peak and valley do not overlap. Blue and black dotted curves: Model spectra assuming a time‐varying electric field (section 4.2). Blue and black diamonds = averages of the model spectra over the energy range that is also used for the measurements.

Figure 5

Sketch of the geometry and wave interaction in Pluto's wake. Locations are shown as projections on the x y plane of the Pluto Solar Heliographic (PSH) coordinate system, essentially looking down on Pluto's magnetosphere. The Sun direction is on the right. We show the location of New Horizons when observing the selected intensity peak and valley. Green = outline of the magnetic boundary of the wake; dotted lines = trajectories of the detected ions for the two considered look directions. Red and black curves are for two wave phases where the direction of the electric field is opposite. It can be seen that the electric field changes the trajectories of the ions. The electric field vector in the x y plane is shown as a blue line. Each arrowhead is for one of the wave phases shown here. Note that particularly the ions shown in red enter the wake from the side without ever being close to Pluto.

Figure 6

Energy change along Pluto's wake for ions with different starting energies (blue horizontal lines). All ions shown here are observed coming from a cone angle of 166°. It can be seen that the ions either lose (red) or gain (black) energy, depending on the wave phase. The x axis shows the x _PSH coordinate of the ions. All ions start at x _PSH=−100R _P , because this is the location we are studying here, and change their energy over about 10R _P until they leave the wake. PSH = Pluto Solar Heliographic coordinate system.

Figure 7

Summary sketch of the interaction between Pluto and the interplanetary medium. Pluto is embedded in a medium of interstellar pickup ions with kiloelectron volt energies and suprathermal ions with energies of tens of kiloelectron volts and above. These IPS (interstellar pickup and suprathermal) ions are shown in green to emphasize their extrasolar origin. Pluto is illustrated as a bright dot. The Sun is to the right of the figure. The primary effect of Pluto on IPS ions is that it depletes their intensities and forms a wake in the antisunward direction that is illustrated in black. The wake gradually refills with distance to Pluto until the intensities become indistinguishable from the unperturbed interplanetary medium (left and right areas). We find no evidence for energetic ions of plutogenic origin. Pluto is “singing” in the sense that there is an electric field wave acting within the wake that affects the observed IPS ion intensities. We symbolize this wave through the stripe pattern in the wake even though in reality the wavelength is larger than the length of the wake. IPS ions have the highest intensities when coming from the sunward direction (when measured in the Pluto or spacecraft frame) but can generally come from any direction. The ions are deflected (green arrows) from their original direction near the magnetic boundary that encompasses the wake. Ions that penetrate into the wake (red arrow) are further deflected by the electric field wave. The boundary of the energetic particle wake might be less sharp than plasma boundaries such as a bow shock or magnetic barrier. Sketch illustrated by Mike Yakovlev (JHU/APL).