Predominance of ECH wave contribution to diffuse aurora in Earth's outer magnetosphere

Abstract

Due to its importance for global energy dissipation in the ionosphere, the diffuse aurora has been intensively studied in the past 40 years. Its origin (precipitation of 0.5-10 keV electrons from the plasma sheet without potential acceleration) has been generally attributed to whistler-mode chorus wave scattering in the inner magnetosphere (R <~8 R E ), while the scattering mechanism beyond that distance remains unresolved. By modeling the quasi-linear diffusion of electrons with realistic parameters for the magnetic field, loss cone size, and wave intensity (obtained from Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations as a function of magnetospheric location), we estimate the loss cone filling ratio and electron cyclotron harmonic (ECH) wave-induced electron precipitation systematically throughout the entire data set, from 6 R E out to 31 R E (the THEMIS apogee). By comparing the wave-induced precipitation directly with the equatorially mapped energy flux distribution of the diffuse aurora from ionospheric observations (OVATION Prime model) at low altitudes, we quantify the contribution of auroral energy flux precipitated due to ECH wave scattering. Although the wave amplitudes decrease, as expected, with distance from the Earth, due to the smaller loss cone size and stretched magnetic field topology, ECH waves are still capable of causing sufficient scattering of plasma sheet electrons to account for the observed diffuse auroral dissipation. Our results demonstrate that ECH waves are the dominant driver of the diffuse aurora in the outer magnetosphere, beyond~8 R E .

Oblique Whistler-Mode Waves in the Earth’s Inner Magnetosphere: Energy Distribution, Origins, and Role in Radiation Belt Dynamics

“…30, 31 andMourenas et al 2012a, Artemyev et al 2012a). In this sense, such very oblique whistler-mode lower-band chorus waves may partially take away the job from electron cyclotron harmonic (ECH) waves, which are often considered as the main wave mode responsible for keV electron scattering in the loss-cone and the subsequent formation of diffuse aurora (Horne and Thorne 2000;Horne et al 2003b;Ni et al 2011cNi et al , 2012Zhang et al 2015;Ni et al 2016), at least in the absence of significant upper-band chorus (i.e. whistler-mode waves with ω/Ω ce0 > 0.5) which may also be a good contender for electron scattering (Ni et al 2011d) and acceleration (Mozer et al 2014).…”

“…Noticeably, electron trapping and parallel acceleration up to 10-100 keV in Landau resonance could lead to the formation of high energy electron beams and produce some of the bursty precipitations which are observed in the atmosphere ). Similarly, oblique whistler-mode waves could concur with ECH waves (Zhang et al 2014(Zhang et al , 2015 in scattering (or transporting) electrons into the loss-cone and partially account for electron precipitations from the Earth's magnetotail region (Panov et al 2013). 7.…”

Understanding the Origin of Jupiter's Diffuse Aurora Using Juno's First Perijove Observations

“…In Earth's magnetosphere, diffuse aurora is originated from pitch angle scattering caused by whistler mode chorus waves in the inner magnetosphere and electrostatic electron cyclotron harmonic waves in the outer magnetosphere (Ni et al, 2016;Thorne et al, 2010;Zhang et al, 2015). At Jupiter, similarly, electron pitch angle scattering is thought to produce diffuse auroral emissions and whistler mode waves that are generated due to anisotropic electron injections are suggested to scatter energetic electrons into the loss cone (Bhattacharya et al, 2005(Bhattacharya et al, , 2001; Bolton et al, 1997;Mauk et al, 2002;Xiao et al, 2003).…”

Source of the Bursty Bulk Flow Diffuse Aurora: Electrostatic Cyclotron Harmonic and Whistler Waves in the Coupling of Bursty Bulk Flows to Auroral Precipitation

“…Recent statistical observations and theoretical predictions support the importance, and even dominance, of the whistler chorus in scattering energetic electrons out of the plasma sheet for L shells ≤8 (Li et al, 2005;Santolik et al, 2009;Thorne et al, 2010). Other observed statistics support the dominant scattering effect of electron cyclotron harmonic (ECH) waves at L shells > 8 (Liang et al, 2010;Ni, Thorne, Liang, et al, 2011;Zhang et al, 2015) and an association between intense auroral forms and ECH waves at about L = 6 (Fuzikawa et al, 2018). Other observed statistics support the dominant scattering effect of electron cyclotron harmonic (ECH) waves at L shells > 8 (Liang et al, 2010;Ni, Thorne, Liang, et al, 2011;Zhang et al, 2015) and an association between intense auroral forms and ECH waves at about L = 6 (Fuzikawa et al, 2018).…”