Atmospheric Mass Flux as a Function of Ionospheric Emission on Unmagnetized Earth

TL;DR

This study uses kinetic simulations to analyze how ionospheric emission influences atmospheric mass loss and solar wind deposition on an unmagnetized Earth-like planet, suggesting magnetic fields may not be essential for atmospheric retention.

Contribution

It introduces a simulation-based analysis of ion escape and solar ion deposition as a function of ionospheric emission rates on unmagnetized Earth-like planets, highlighting the potential for atmospheric retention without a magnetic field.

Findings

Solar ion deposition can be comparable to ion escape rates.

Atmospheric mass loss over a billion years is less than 3%.

Ionospheric emission may evolve to a critical rate with zero net mass flux.

Abstract

We explore ion escape from, and solar ion deposition to, \hll{an unmagnetized Earth-like planet}. We use RHybrid, an ion-kinetic electron-fluid code to simulate the global plasma interaction of unmagnetized Earth with the solar wind. We vary the global ionospheric emission rate, and quantify the resultant planetary ion escape rates ($O^+$ and $H^+$) and the solar wind deposition rate ($H^+$). We use these results to compute the net mass flux to the atmosphere and find that the solar ion deposition rate could be comparable to planetary ion escape rates. For the emission rates simulated, our results show that under typical solar wind conditions ($v_{sw} = 400 \ km \ s^{-1}$, $n_{sw} = 5 \ cm^{-3}$), the mass of the atmosphere would decrease by less than 3\% over a billion years, indicating that Earth's intrinsic magnetic field may be unnecessary for retention of its atmosphere. Lastly, we present a hypothesis suggesting that ionospheric emission may evolve through time towards a critical emission rate that occurs at a net mass flux of zero.