MIST
Magnetosphere, Ionosphere and Solar-Terrestrial
Acceleration of Electrons by Whistler-Mode Hiss Waves at Saturn
By Emma Woodfield (British Antarctic Survey)
Whistler-mode hiss waves are well known for causing losses of energetic electrons from the radiation belts at the Earth through wave-particle interactions. The result of the interactions of charged particle with plasma waves, whether energy is transferred from wave to particle or vice-versa, is dependent on many factors including the background plasma conditions. In Saturn’s magnetosphere there is a torus of charged particles, the primary source of this plasma torus is neutral water particles emitted from the moon Enceladus which are then ionised. The combination of pressure, ambipolar electric field, centrifugal and gravitational forces on this moon sourced plasma creates a regime where density is highest near the magnetic equator and notably lower at higher latitudes. Consequently, the ratio of plasma frequency to electron gyrofrequency frequently falls below one at higher latitudes. This also coincides with the region where hiss mode waves are observed and our simulations show that this very low ratio leads hiss waves at Saturn to accelerate electrons rather than scattering them out of the radiation belt. This new finding has important implications for the radiation belt dynamics at Saturn since hiss waves are strong and frequently observed.
Another result of the high latitude occurrence of hiss (> 25 degrees) is that only electrons which bounce a good distance along the magnetic field lines will encounter these particular wave-particle interactions. Therefore, the energy increase in the electrons due to the hiss waves is only seen in these particles. We can describe how far along the magnetic field a particle will reach using the equatorial pitch angle, which is the angle between the particle velocity and the magnetic field at the magnetic equator. An electron with an equatorial pitch angle of 90 degrees is confined to the equator whereas one of 0 or 180 degrees will reach all the way down to the planet in different hemispheres. The result of the hiss wave interactions is to drive the pitch angle distributions of the electrons towards a “butterfly shape” with peaks at low (and very high) equatorial pitch angle reflecting the hiss interactions at high latitudes in both hemispheres. The strength and speed of the interaction also varies with electron energy, the figure shows how our simulations of the electron pitch angle distributions at different L-shells (radial distance along the magnetic equator of a magnetic field line) progress after one Earth day for three typical radiation belt energies. These simulations consider only the effect of the hiss waves to isolate their effect from radial diffusion and transport and any other wave-particle interactions or collisional losses. Highly anisotropic pitch angle distributions (with the peak at lowest and highest pitch angle) are apparent in all three energies in even this relatively short timescale simulation.
![Equatorial pitch angle distributions from 2D model runs at a given L-shell after 24 hours with a resolution of 0.1L. Each run considers the energy and pitch angle diffusion, no radial diffusion or radial transport is included. Each pitch angle distribution is normalised to the flux value at 90 degrees. (a) initial condition for all energies, (b,c,d) flux at 0.4, 1.0 and 3.0 MeV respectively. From: Emma Woodfield - BAS Sent: 27 January 2022 13:50 To: Walach, Maria <m.walach@lancaster.ac.uk> Subject: RE: [External] MIST Nugget Hi Maria, Will do 😊 There’s a BAS news twitter – I’ll double check the twitter handle. Thanks Emma From: Walach, Maria <m.walach@lancaster.ac.uk> Sent: 27 January 2022 12:36 To: Emma Woodfield - BAS <emmwoo@bas.ac.uk> Subject: Re: [External] MIST Nugget Hi Emma, Great! Could you prepare one within the next 1-2 weeks? I also advertise new nuggets via the MIST twitter page. Please let me know if you have any twitter account(s) that you would like me to tag in the post or any suggested wordings. Thanks, Maria On 27 Jan 2022, at 12:29, Emma Woodfield - BAS <emmwoo@bas.ac.uk> wrote: This email originated outside the University. Check before clicking links or attachments. Hi Maria, That was a quick spot – I haven’t even checked the online version myself yet! :-) Yes happy to do a nugget, when would you like it by? Kind regards Emma From: Walach, Maria <m.walach@lancaster.ac.uk> Sent: 27 January 2022 12:28 To: Emma Woodfield - BAS <emmwoo@bas.ac.uk> Subject: MIST Nugget Hi Emma, I hope all is well with you! I am contacting you on MIST Council business, due to your recent article (https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2021GL096213?campaign=wolacceptedarticle). MIST nuggets are short, 1-2 paragraphs (100 - 300 words) accompanied by a figure/animation that describes the importance and some key result/aspect of your work, as well as a caption for the figure and any references.. For some examples please see http://www.mist.ac.uk/nuggets. They are aimed to be understandable for the general MIST audience - at the level of a 1st year PhD student, with a focus on clear language and the wider impact of the results. We would like to invite you to contribute to the series of MIST nuggets and provide a short nugget on your recently published work. Hopefully this should not take much time to write, and it is a great way to advertise your work to the wider MIST community! Please would you be able to let me know whether or not you will be able to contribute at your earliest convenience. Many thanks, Maria On behalf of MIST council -------------------------------------------------- Maria-Theresia Walach Senior Research Associate Space and Planetary Physics Group Physics Department Lancaster University Lancaster LA1 4YB UK This email and any attachments are intended solely for the use of the named recipients. If you are not the intended recipient you must not use, disclose, copy or distribute this email or any of its attachments and should notify the sender immediately and delete this email from your system. UK Research and Innovation (UKRI) has taken every reasonable precaution to minimise risk of this email or any attachments containing viruses or malware but the recipient should carry out its own virus and malware checks before opening the attachments. UKRI does not accept any liability for any losses or damages which the recipient may sustain due to presence of any viruses.](/images/Emma_Woodfield_20220204.png)
Figure Caption: Equatorial pitch angle distributions from 2D model runs at a given L-shell after 24 hours with a resolution of 0.1L. Each run considers the energy and pitch angle diffusion, no radial diffusion or radial transport is included. Each pitch angle distribution is normalised to the flux value at 90 degrees. (a) initial condition for all energies, (b,c,d) flux at 0.4, 1.0 and 3.0 MeV respectively.
See full paper for details:
Woodfield, E. E., Glauert, S. A., Menietti, J. D., Horne, R. B., Kavanagh, A. J., & Shprits, Y. Y. (2022). Acceleration of electrons by whistler-mode hiss waves at Saturn. Geophysical Research Letters, 49, e2021GL096213. https://doi.org/10.1029/2021GL096213
Publication URL: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GL096213