In Situ Observations of Preferential Pickup Ion Heating at an Interplanetary Shock

Nonthermal pickup ions (PUIs) are created in the solar wind (SW) by charge-exchange between SW ions (SWIs) and slow interstellar neutral atoms. It has long been theorized, but not directly observed that PUIs should be preferentially heated at quasiperpendicular shocks compared to thermal SWIs. We present in situ observations of interstellar hydrogen (${\mathrm{H}}^{+}$) PUIs at an interplanetary shock by the New Horizons’ Solar Wind Around Pluto (SWAP) instrument at $\sim 34\mathrm{au}$ from the Sun. At this shock, ${\mathrm{H}}^{+}$ PUIs are only a few percent of the total proton density but contain most of the internal particle pressure. A gradual reduction in SW flow speed and simultaneous heating of ${\mathrm{H}}^{+}$ SWIs is observed ahead of the shock, suggesting an upstream energetic particle pressure gradient. ${\mathrm{H}}^{+}$ SWIs lose $\sim 85\%$ of their energy flux across the shock and ${\mathrm{H}}^{+}$ PUIs are preferentially heated. Moreover, a PUI tail is observed downstream of the shock, such that the energy flux of all ${\mathrm{H}}^{+}$ PUIs is approximately six times that of ${\mathrm{H}}^{+}$ SWIs. We find that ${\mathrm{H}}^{+}$ PUIs, including their suprathermal tail, contain almost half of the total downstream energy flux in the shock frame.

Figure 1

SWAP observations at an interplanetary shock (IPS) in October 2015. The top $x$ axis labels show the distance from the shock derived in the shock frame. Since the PUI data cadence is $\sim 1\mathrm{day}$, we connect the data points with lines and plot horizontal lines from the two PUI data points nearest to the shock. Note that there is no PUI data $\sim 2\mathrm{day}$ after the shock due to culling [7].

Figure 2

(left) SWAP observations at the interplanetary shock. The middle of the SWAP PUI measurement times are outside the $x$-axis range; thus we show horizontal lines at their levels before and after the shock. (right) Voyager 2 observations at the TS. We show daily averaged particle moments and hourly averaged magnetic field. Red lines indicate the average before and after the shocks for SWAP and Voyager 2 data shown in the Supplemental Material [18], except for the magnetic field. SW speeds are transformed to the shock frame ($V-u$), then normalized to the average downstream value indicated by the red line.

Figure 3

(a) Energy flux for ${\mathrm{H}}^{+}$ SWIs (blue), ${\mathrm{H}}^{+}$ PUIs (red), and their total (black) in the shock frame. We perform 1 h boxcar smoothing over SW density and speed. PUI data are interpolated to the SWI measurement resolution. (b) Energy flux close to the shock. We also show the estimated energy flux contribution from the magnetic field, alphas, ${\mathrm{He}}^{+}$ PUIs, ${\mathrm{H}}^{+}$ PUI tail, electrons, and energetic particles (gray open circles), and the estimated total (black open circles). Note that the PUI density and temperature are assumed constant in panel (b) using the two PUI data points closest to the shock (horizontal lines in Fig. 1 right before and after the shock).

Figure 4

(a) SWAP day-averaged count rates in the spacecraft frame before (black) and after (blue) the shock (see Fig. 1). Fits to the ${\mathrm{H}}^{+}$ PUIs before and after the shock are shown in gray and cyan, respectively. A fit to the ${\mathrm{H}}^{+}$ PUI tail after the shock is shown in red. Models of the ${\mathrm{He}}^{+}$ PUIs are shown as dashed lines. (b) Data are normalized to the SW frame.