Heliospheric Energetic Neutral Atoms: Numerical Modelling and Comparison with IBEX-Hi data
Authors: Baliukin Igor , Izmodenov Vladislav , Alexashov Dmitry 
Published: March 2020

&lt;p align=&quot;justify&quot;&gt;&lt;span&gt;The &lt;/span&gt;&lt;span&gt;&lt;em&gt;Interstellar Boundary Explorer&lt;/em&gt;&lt;/span&gt;&lt;span&gt; (IBEX) spacecraft has been measuring fluxes of &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;Energetic Neutral Atoms (ENA&lt;/span&gt;&lt;span&gt;s)&lt;/span&gt;&lt;span&gt; using the IBEX-Hi (0.3 &amp;#8211; 6 keV) instrument since 2008. We have developed &lt;/span&gt;&lt;span&gt;the&lt;/span&gt;&lt;span&gt; numerical model to calculate ENA hydrogen fluxes &lt;/span&gt;&lt;span&gt;employing&lt;/span&gt;&lt;span&gt; reconstruction of &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;trajectories &lt;/span&gt;&lt;span&gt;of the atoms &lt;/span&gt;&lt;span&gt;(backward in time) from 1 au, where they are observed by IBEX, to the point of their origin in the inner heliosheath, &lt;/span&gt;&lt;span&gt;i.e. &lt;/span&gt;&lt;span&gt;the region of the perturbed solar wind between the termination shock and the heliopause, where the plasma is strongly heated (T ~ 10&lt;/span&gt;&lt;sup&gt;&lt;span&gt;6&lt;/span&gt;&lt;/sup&gt;&lt;span&gt; K). Along the trajectory of &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;atom, differential fluxes of &lt;/span&gt;&lt;span&gt;the newly originated &lt;/span&gt;&lt;span&gt;ENA&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; with a given speed are integrated, and losses due to ionization processes (charge exchange &lt;/span&gt;&lt;span&gt;with&lt;/span&gt;&lt;span&gt; protons and photoionization) are also taken into account.&lt;/span&gt;&lt;/p&gt;&lt;p align=&quot;justify&quot;&gt;&lt;span&gt; The &lt;/span&gt;&lt;span&gt;key &lt;/span&gt;&lt;span&gt;factor in the simulation is &lt;/span&gt;&lt;span&gt;a &lt;/span&gt;&lt;span&gt;detailed consideration of the supra-thermal component of &lt;/span&gt;&lt;span&gt;pickup ions (PUI&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt;) &lt;/span&gt;&lt;span&gt;that &lt;/span&gt;&lt;span&gt;originate &lt;/span&gt;&lt;span&gt;in the region of the supersonic solar wind and &lt;/span&gt;&lt;span&gt;picked&lt;/span&gt; &lt;span&gt;by the heliospheric magnetic field since this component is &amp;#171;parent&lt;/span&gt;&lt;span&gt;al&amp;#187;&lt;/span&gt;&lt;span&gt; to the ENA. We t&lt;/span&gt;&lt;span&gt;ake&lt;/span&gt;&lt;span&gt; into account &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;supra-thermal component&lt;/span&gt;&lt;span&gt; by solving the kinetic equation under the assumption that &lt;/span&gt;&lt;span&gt;PUI &lt;/span&gt;&lt;span&gt;distribution function is isotropic everywhere in the heliosphere. This method compares favorably with other existing approaches &lt;/span&gt;&lt;span&gt;since it is based on fundamental physical laws.&lt;/span&gt;&lt;/p&gt;&lt;p align=&quot;justify&quot;&gt;&lt;span&gt;We have calculated m&lt;/span&gt;&lt;span&gt;odel &lt;/span&gt;&lt;span&gt;maps of the ENA fluxes &lt;/span&gt;&lt;span&gt;based on &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;previously developed kinetic-MHD models &lt;/span&gt;&lt;span&gt;of &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;SW/LISM interaction &lt;/span&gt;&lt;span&gt;(Izmodenov &amp; Alexashov, 2015, 2020), and &lt;/span&gt;&lt;span&gt;performed &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;comparison&lt;/span&gt;&lt;span&gt; with IBEX-Hi data&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;The &lt;/span&gt;&lt;span&gt;IBEX-Hi data is one of the few sources of knowledge about the structure of the heliospher&lt;/span&gt;&lt;span&gt;ic&lt;/span&gt;&lt;span&gt; boundary, imposing significant limitations on the parameters of &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;model &lt;/span&gt;&lt;span&gt;of the heliosphere&lt;/span&gt;&lt;span&gt;. As a result of the comparison, we concluded that 1) ENA &lt;/span&gt;&lt;span&gt;fluxes &lt;/span&gt;&lt;span&gt;from the region of the inner &lt;/span&gt;&lt;span&gt;heliosheath&lt;/span&gt;&lt;span&gt; are extremely sensitive to the form of &lt;/span&gt;&lt;span&gt;PUI &lt;/span&gt;&lt;span&gt;distribution function; 2) the model &lt;/span&gt;&lt;span&gt;of the heliosphere &lt;/span&gt;&lt;span&gt;Izmodenov &amp; Alexashov &lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;2020) &lt;/span&gt;&lt;span&gt;that &lt;/span&gt;&lt;span&gt;differs from the model Izmodenov &amp; Alexashov &lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;2015) in configuration of the interstellar magnetic field reproduces the IBEX-Hi data &lt;/span&gt;&lt;span&gt;better&lt;/span&gt;&lt;span&gt;; 3) despite a relatively good agreement, &lt;/span&gt;&lt;span&gt;there are&lt;/span&gt;&lt;span&gt; some qualitative differences between the &lt;/span&gt;&lt;span&gt;model calculations &lt;/span&gt;&lt;span&gt;and IBEX-Hi data in &lt;/span&gt;&lt;span&gt;some energy channels &lt;/span&gt;&lt;span&gt;of IBEX-Hi&lt;/span&gt;&lt;span&gt;. The reasons for these differences are discussed.&lt;/span&gt;&lt;/p&gt;