Incorporating Inner Magnetosphere Current-driven Electron Acceleration in Numerical Simulations of Exoplanet Radio Emission

Abstract

We present calculations of auroral radio emission for an Earth-like planet produced by field-aligned current (FAC) driven electron acceleration using a coupled global magnetohydrodynamic (MHD) and inner magnetosphere model, extending the capabilities of previous works which focus solely on the direct transmission of magnetic energy between the stellar wind and ionosphere. Magnetized exoplanets are expected to produce radio emission via interaction between the host star’s stellar wind and planetary magnetosphere-ionosphere system. The empirically derived Radiometric Bode’s Law (RBL) is a linear relation between the magnetic solar wind power and total emitted radio power from magnetized Solar System planets, and is often extrapolated to extreme exoplanet systems. It has been shown that the magnitudes of the FACs coupling the stellar wind to planetary ionospheres are likely to be significantly limited (often referred to as ionospheric saturation), resulting in an estimated radio power up to several orders of magnitude less than that predicted by RBL. In this paper, we demonstrate the significance of intense, sporadic FACs, driven by nightside magnetic reconnection and inner magnetosphere plasma flow, to the total radio power produced by wind–ionosphere interaction in terrestrial planets. During periods of strong stellar wind variability, the contribution from these secondary currents can be over an order of magnitude greater than the primary current systems that previous models describe. The results highlight the role of the variability of the stellar wind on the magnitude and location of the resulting emission, subsequently affecting the conditions for detectability.

Detecting Magnetospheric Radio Emission from Giant Exoplanets

“…Nichols & Milan (2016) applied the concept of ionospheric saturation, well observed and parameterized at Earth (Reiff et al 1981;Weimer et al 1990;Hairston et al 2005;Ridley 2005;Siscoe 2011), to the typical environment of hot Jupiters, and found that the energy available for CMI emission from the Region 1 current system may be up to several orders of magnitude lower than what the RBL predicts. Sciola et al (2021) performed numerical MHD simulations of an Earth-like planet under extreme saturation conditions (separate from the simulation shown here) and found that the emission power produced by these primary FACs calculated by the analytic model and numerical simulation are in good agreement. However, Sciola et al (2021) also found that FACs generated via secondary wind-magnetosphere coupling processes, which map to lower magnetic latitudes, can drive significantly stronger CMI emission under the right stellar wind conditions (i.e., strong variability).…”

“…Sciola et al (2021) performed numerical MHD simulations of an Earth-like planet under extreme saturation conditions (separate from the simulation shown here) and found that the emission power produced by these primary FACs calculated by the analytic model and numerical simulation are in good agreement. However, Sciola et al (2021) also found that FACs generated via secondary wind-magnetosphere coupling processes, which map to lower magnetic latitudes, can drive significantly stronger CMI emission under the right stellar wind conditions (i.e., strong variability). These secondary processes are highly coupled and nonlinear and must be resolved with a numerical MHD model, which is impossible to perform for the entire set of exoplanets considered in this study.…”

“…A thorough overview of the subject can be found in the reviews by Zarka et al (2015a) and Grießmeier (2015. Starting with the foundational theoretical studies (Winglee et al 1986;Zarka et al 1997;Farrell et al 1999;Zarka et al 2001;Zarka 2007) a large body of theoretical work has been published (e.g., de Pater 2000; Farrell et al 2004;Lazio et al 2004;Grießmeier et al , 2007aGrießmeier et al , 2007bStevens 2005;Jardine & Cameron 2008;Vidotto et al 2010Vidotto et al , 2012Vidotto et al , 2015Vidotto et al , 2019Hess & Zarka 2011;Nichols 2011Nichols , 2012See et al 2015;Fujii et al 2016;Nichols & Milan 2016;Vidotto & Donati 2017;Weber et al 2017aWeber et al , 2017bWeber et al , 2018Lynch et al 2018;Zarka 2018;Kavanagh et al 2019;Wang & Loeb 2019;de Gasperin et al 2020;Shiohira et al 2020;Hori 2021;Narang et al 2021;Sciola et al 2021;Cendes et al 2022). The goals of these studies are to predict the frequency of emission and the radio flux observed from Earth.…”

Star-exoplanet interactions: A growing interdisciplinary field in heliophysics

“…However, the auroral radio emission mentioned above is influenced by the ionosphere, resulting in potentially observable ionospheric effects. The modulating effects of the ionosphere must be taken into account for predicting or analyzing observations of auroral radio emission (Sciola et al, 2021). Radio emission may only be observable under certain planetary and astrospheric conditions, but testing models on observable planets will provide key constraints and validation of our models.…”

“…Even in the case of supersonic stellar winds, the conditions can be so extreme that the magnetopause boundary is close to the planetary surface (Dong et al, 2017b;Slavin et al, 2019). This also presents significant numerical challenges because the larger intrinsic wave speeds in the region of the stronger magnetic field near the planet require a much smaller timestep for numerical stability, making the computations quite expensive (e.g., Sciola et al, 2021). The adaptation of sophisticated heliophysics models for exoplanet applications requires acknowledgment of areas where we are missing basic physics in our models and so may help us to better understand the limitations of our knowledge, to better define our computational needs, and to incorporate new or different physics in the models.…”