Constraining the IMF at Pluto Using New Horizons SWAP Data and Hybrid Simulations

Abstract

New Horizons did not carry a magnetometer, so there is no direct way to know the strength of the Interplanetary Magnetic Field (IMF) at the time of the flyby. Using a hybrid model together with a detailed model of the Solar Wind Around Pluto (SWAP) instrument (McComas et al., 2007, https://doi.org/10.1007/s11214-007-9205-3) we have identified three observables that are sensitive to IMF strength: the width of Pluto's heavy ion tail, the energy of heavy ions, and the profile of thermal pressure along the New Horizons trajectory. By comparing simulation to data we infer that the IMF strength was most likely near or below ∼0.1 nT. We also show that the shell distribution of Interstellar Pickup Ions is important for reproducing these observations due to its effect on global pressure balance.

Triton’s Variable Interaction With Neptune’s Magnetospheric Plasma

“…The hybrid approach is well‐documented and has been used to investigate the plasma interactions of many solar system objects. Examples include Earth’s Moon (e.g., Holmström et al., 2012; Liuzzo et al., 2021), the moons of Jupiter (e.g., Fatemi et al., 2016; Lipatov & Combi, 2006; Lipatov et al., 2010, 2013; Liuzzo et al., 2015, 2016; 2017, 2018; 2019a, 2019b; 2020; Lindkvist et al., 2015; Poppe et al., 2018; Sittler et al., 2013) and Saturn (e.g., Kriegel et al., 2009, 2011, 2014; Ledvina et al., 2012; Lipatov et al., 2012; Sillanpää et al., 2006; Sillanpää & Johnson, 2015), as well as Mercury (e.g., Exner et al., 2018; Kallio & Janhunen, 2003), Venus (e.g., Kallio et al., 2006; Martinecz et al., 2009), Mars (e.g., Kallio & Janhunen, 2001; Modolo et al., 2016), and Pluto (e.g., Barnes et al., 2019; Delamere, 2009; Feyerabend et al., 2017).…”

Pluto's Interaction With Energetic Heliospheric Ions

“…At Pluto, bi‐ion waves show in simulations as a result of plutogenic pickup ions (Barnes et al, 2019 ; Delamere, 2009 ). Most of the wave power is between the gyrofrequency of upstream protons and pickup heavies, which are at 0.1 and 2.9 hr, respectively, based on the parameters found at Pluto (Bagenal et al, 2015 ; McComas et al, 2016 ).…”

“…IPS ions do not create a current in the heliosphere as all species, ions, and electrons, are moving with the same bulk speed. When the solar wind encounters Pluto, most solar wind plasma is excluded from the wake (Bagenal et al, 2016 , 2019 ). IPS ions are also affected, but a fraction of them are able to enter the wake (Figure 1 ) while roughly maintaining their bulk speed (section 3.3 , Figure 3 ).…”

“…The gyroradii of 10 keV interstellar He + pickup ions, as we will study them here, is 200 R P (Pluto radius defined here as 1 R P =1,188.3 km; Nimmo et al, 2017 ), much larger than the size of the obstacle. (The exact value depends on the magnetic field that can be estimated from Voyager measurements, Bagenal et al, 2015 , or modeling, Barnes et al, 2019 .) Despite their density being lower than the solar wind, at least interstellar H + pickup ions are thought to play an important role in determining the shape of Pluto's heavy ion tail (Barnes et al, 2019 ).…”

“…(The exact value depends on the magnetic field that can be estimated from Voyager measurements, Bagenal et al, 2015 , or modeling, Barnes et al, 2019 .) Despite their density being lower than the solar wind, at least interstellar H + pickup ions are thought to play an important role in determining the shape of Pluto's heavy ion tail (Barnes et al, 2019 ).…”