On the production of singly ionized He in the solar wind from the first observations of Solar Orbiter’s Heavy Ion Sensor

Abstract:

The He+ ion is expected to be rare in the solar wind, and it is usually associated with material from the interstellar medium, interplanetary dust, comets, or planetary atmospheres that originate as neutral, undergo ionization, and are subsequently picked up by the interplanetary magnetic field. The newly picked up ions can be differentiated from typical solar wind ions through their characteristic non-thermal velocity distribution functions that contrasts the narrowly peaked Maxwellian profile of the solar wind. However, through inspection of He+ measurements from ACE/SWICS throughout solar cycle 23 (1998-2011), we find a significant population of He+ of solar origin that are unaccounted for with current ionization modeling. The population of He+ observed is independent of wind speed, composition, and source region suggesting the He+ is formed after leaving the Sun, making the interaction with dust a likely source. Through nonequilibrium ionization modeling, we find charge exchange with low energy neutrals outgassed from dust to be effective in creating solar He+ from He2+. We compare the present He+ of solar origin to first light measurements from the Heavy Ion Sensor on Solar Orbiter in the inner heliosphere to further constrain the solar wind-dust interaction.