Six pieces of evidence against the corotation enforcement theory to explain the main aurora at Jupiter
aa r X i v : . [ phy s i c s . s p ace - ph ] M a y manuscript submitted to JGR: Space Physics
Six pieces of evidence against the corotationenforcement theory to explain the main aurora atJupiter
B. Bonfond Z. Yao , D. Grodent LPAP, STAR Institute, Universit´e de Li`ege, Belgium Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academyof Sciences, Beijing, China.
Key Points: • The corotation enforcement current system currently is the mainstream explana-tion for the main auroral emissions at Jupiter • We expose six observational pieces of evidence that this theory is not the mainexplanation for these auroral emissions • Improved theories should account for the local time variations in the magnetosphereand the importance of the plasma waves for the creation of auroral emissions.
Corresponding author: Bertrand Bonfond, [email protected] –1–anuscript submitted to
JGR: Space Physics
Abstract
The most remarkable feature of the ultraviolet auroras at Jupiter is the ever present andalmost continuous curtain of bright emissions centered on each magnetic pole and calledthe main emissions. According to the classical theory, it results from an electric currentloop transferring momentum from the Jovian ionosphere to the magnetospheric plasma.However, predictions based on these mainstream models have been recently challengedby observations from Juno and the Hubble Space Telescope. Here we review the maincontradictory observations, expose their implications for the theory and discuss promis-ing paths forward.
Plain Language Summary
The powerful auroras at Jupiter are very different from those at Earth and the mech-anisms generating them differ as well. Their most obvious features is a relatively con-tinuous auroral curtain surrounding the magnetic poles. The classical explanation forits presence involves an electric current system that allows particles in the magnetosphereto rotate with planet. While these models explain some characteristics of the auroras,recent observations from the NASA Juno spacecraft and the Hubble Space Telescope chal-lenge this theoretical framework.
The ultraviolet (UV) auroras at Jupiter can be separated into three almost equallypowerful components (Nichols, Clarke, Grard, Grodent, & Hansen, 2009; Grodent et al.,2018): 1) the main emissions (ME), which are forming an almost continuous curtain ofauroral emissions around the magnetic pole, 2) the polar emissions located poleward ofthe ME and 3) the equatorward, or outer, emissions, essentially comprised between theME and Io’s footpath (Figure 1a). The auroral footprints of the Galilean moons, are of-ten cited as a fourth component, even if their total emitted power is much smaller ( ∼ ∼
500 GW for the ME (Gro-dent et al., 2018)). The ME magnetically maps to distances typically ranging from 20to 60 Jovian radii ( R J ) (Vogt et al., 2011), though in rare instances, this distance droppedto near to the orbit of Ganymede (15 R J ) (Bonfond et al., 2012). Since it became clearthat the main emissions did correspond neither to the open-closed field line boundary(or the outer-most magnetosphere) nor to the Io torus (Dols et al., 1992), the most widelyaccepted explanation for this auroral feature involves a large scale current system cou-pling the magnetospheric plasma to the ionosphere (Hill, 1979, 2001; Cowley & Bunce,2001; Southwood & Kivelson, 2001).According to this theoretical frame, the current system transfers momentum fromthe ionosphere to the plasma sheet. It flows radially outward in the plasma sheet andthe J × B force accelerates the magnetospheric plasma towards corotation with the planet.At the other end of the circuit, equatorward Pedersen currents slow down the chargedparticles in the ionosphere, which interact with the rest of the upper atmosphere via ion-neutral collisions. Field aligned currents flow between these two sections of the loop, up-ward from the ionosphere to the magnetosphere in the middle magnetosphere and in theopposite direction in the outer magnetosphere (Figure 1b). In the plasma sheet, this cur-rent system starts at Io’s orbit, where fresh plasma is injected in the magnetosphere fromthe volcanic moon’s neighbourhood. This plasma then progressively migrates outwardto be eventually released in the Jovian magnetotail. As the radial distance increases andin the absence of additional forces, the conservation of the angular momentum dictatesthat the angular velocity of the plasma would decrease. Thus, to maintain corotation,the required momentum transfer from the ionosphere to the magnetosphere increases,as do the currents. Models predict the field aligned currents peak at a distance close tothe region where the system becomes unable to maintain full corotation with the planet, –2–anuscript submitted to JGR: Space Physics also known as the corotation breakdown distance. In the region where the upward cur-rents peak, field aligned potential are expected to form and accelerate electrons into theatmosphere, causing the main auroral emissions.Many observations gathered either by the spacecraft that have visited the Joviansystem through the years or by Earth based telescopes appear to support some elemen-tal processes in this framework. First, the magnetospheric plasma at Jupiter is eitherin full corotation with the planet, or, at least, significantly rotating with it, indicativethat momentum is indeed transferred from the ionosphere/thermosphere to the magne-tosphere. It should however be noted that this is also true for Saturn, and yet, the as-sociated current system does not give rise to significant auroras (the auroras at Saturnare mainly caused by other processes). Then, sub-corotation and velocity shears in thepolar ionosphere of Jupiter have been observed, indicative of a torque being exerted onthe inner polar regions of the ionosphere (i.e. Johnson et al., 2017). It is also notewor-thy that these models predict a location of the auroral emissions and a typical bright-ness consistent with the observations. The idea that corotation enforcement currents drivethe ME also provides an explanation for the usually dimmer main emissions on their pre-noon section, named the discontinuity (Radioti et al., 2008). As the shape of the day-side magnetopause forces the plasma in the dawn-side magnetosphere, its azimuthal ve-locity increases and the need for momentum transfer decreases. The field aligned cur-rents inferred from Galileo magnetic field measurements in the equatorial plane are alsominimum in this sector (Khurana, 2001). Furthermore, the equatorward expansion ofthe main emissions during time interval during which the mass outflow rate is expectedto have increased is also consistent with the theory (Bonfond et al., 2012). Finally, theobservation of the relationship between the precipitating energy flux in the main emis-sions and the mean electron energy were found to be consistent with the Knight-like re-lationship expected for quasi-static electric fields (Gustin et al., 2004; Grard et al., 2016),even if Clark et al. (2018) showed that Alfv´enic acceleration could lead to the same kindof relationship.However, while this model is widely accepted to be the explanation for the mainauroral emissions and despite its successes, several observations contradicting the pre-dictions of this theoretical framework concerning the main auroral emissions have recentlystarted to accumulate. Some of these observations were actually known for a long time,while others were recently revealed by the NASA Juno mission.
One of the first prediction of the corotation enforcement currents models concernedthe response to solar wind compressions and expansions. All these models predict thatthe ME aurora would dim as a response to a solar wind compression (Southwood & Kivel-son, 2001; Cowley & Bunce, 2003). The first versions only considered steady state sys-tems. A later iteration took the time variations into considerations (Cowley et al., 2007).Observations of the infrared H +3 aurora before the Ulysses Jupiter fly-by showedan increase of the total emitted power with the increase of the solar wind (Baron et al.,1996). It was however not clear at the time that this increase was due to the main emis-sions, or whether it was related to a brightening of other regions. Studies based on Hisakiobservations of the total auroral power in the ultraviolet reached the same conclusion(Kita et al., 2016). In other wavelengths (e.g. Gurnett et al. (2002) for the radio hec-tometric emissions, Dunn et al. (2016) for the X-rays owing to ion precipitation and Sin-clair et al. (2019) for the infrared hydrocarbon emissions), increase of the auroral activ-ity have also been found to correlate with compressed solar wind conditions. It should –3–anuscript submitted to JGR: Space Physics be noted that these indices possibly involve processes taking place poleward of the ME,which may or may not be correlated with the ME. Analysis of the response of the UVaurora to a solar wind compression prior to the Cassini Jupiter fly-by showed that themain emissions brightened during a solar wind compression (Nichols et al., 2007). How-ever, the exact timing of the response remained unclear, as the model of Cowley et al.(2007) predicted a possible brief ME enhancement right after the arrival of a compressedsolar wind, before a prolonged dimming of the auroral emissions. Later HST observa-tions of the aurora during either the New Horizons fly-by (Nichols, Clarke, Grard, & Gro-dent, 2009; Clarke et al., 2009) or the arrival of Juno Nichols et al. (2017) suggested thatsome auroral brightenings are consistent with intervals of solar wind compressions. Arecent study including observations from both Hisaki and the UV spectrograph on boardJuno also described brightenings correlated with the solar wind compressions, but con-cluded that the exact timing of the brightening lagged the arrival of large solar wind shocks(Kita et al., 2019). They also found that the amplitude of the brightening did not scalewith the disturbance of the dynamic pressure. In summary, these studies either concludethat the ME brightens with the arrival of a compression region, or conclude that the tim-ing of the response is unclear, but none of them report the dimming expected from thetheory.Yao et al. (2020) observed the aurora with the Hubble Space Telescope as Juno wason the dawn flank of the magnetosphere. Juno encountered several time the magnetopauseduring time intervals of compressed magnetosphere. Each time, the main emissions sig-nificantly brightened at all local times. Unlike all previous studies, this one does not relyon any propagation model of the solar wind, but directly assess the state of the magne-tosphere. It is also remarkable that even the noon sector, which is where the compres-sion effects should be the clearest, brightened compared to the quiet case. This studyalso confirms that hectometric radio emissions are systemically enhanced during solarwind compression. Finally, it should be noted that, while non-resolved enhancements ofthe auroras do not guarantee that the ME is the auroral component that caused it (seecounterexample in Kimura et al., 2015, associated with internally driven reconfigurations),the enhancements of the ME seen by HST and Juno-UVS result in an enhancement ofthe total power compatible with those Hisaki and others observed simultaneously to so-lar wind shocks.
Yao et al. (2019) directly compared the azimuthal and radial stretching of the dawn-side magnetic field as measured by Juno to the auroral output. During a time intervalfor which the magnetosphere was compressed, they noted that the auroras and the MEin particular were brighter than during quiet times. They also noted that the stretch-ing of the magnetic field, or said in other words, the loading of energy in the magneticfield, oscillated during this interval. And, contrary to classical theoretical expectation,the aurora and the radio kilometric emissions increased during the unloading phases, asif the magnetic energy was converted into particle energy, similarly to what is observedon Earth.
One of the most direct evidence of the radially outward flowing currents in the plasmasheet is the azimuthal bend back of the magnetic field. This angle is larger on the dawnflank of the magnetosphere than on the dusk flank. As a consequence, the main emis-sions are also expected to be brighter on dawn (Ray et al., 2014). However, a compar-ison of the dawn and dusk sides of the main emissions based on Hubble Space Telescopeobservations showed that the dusk side is typically 3 times brighter than the dawn side(Bonfond et al., 2015). A possible explanation is that, in addition to the corotation en-forcement currents, another current system of the same magnitude and linked to the par- –4–anuscript submitted to
JGR: Space Physics tial ring current in the magnetotail also feeds into the auroral regions. It would consis-tently strengthen the total net field aligned currents on the dusk side and weaken thecurrents on the dawn side. Analysing the equatorial magnetic field measurements of thewhole Galileo mission, Lorch et al. (2020) also concluded that azimuthal currents playa key role in determining the location of the field aligned currents. Furthermore, Vogtet al. (2019) noted that the dawn-dusk discrepancy on the bend back angle is even largerduring solar wind compressions, which should lead to a brightening of the dawn arc ofthe ME but a dimming of the dusk arc if the corotation enforcement current were driv-ing the main auroral emissions. Again, this is contrary to the observations, as the MEbrighten at all local times during compressions of the magnetopause (Yao et al., 2020).
In axisymmetric models, the velocity of the particles and the azimuthal componentof the magnetic field (i.e. the bend back) are expected to be anti-correlated. However,comparisons of the dawn and dusk flanks of the Jovian magnetosphere show that the mag-netic field bend back is larger in the dawn flank (Khurana & Schwarzl, 2005) as well asthe velocity of the charged particles (Krupp et al., 2001). When considering the threedimensional shape of the magnetosphere, this actually make much sense. As the field linesare increasingly stretched in the magnetotail, the plasma’s angular velocity decreases.Then, on the dawn side, the field lines are still considerably stretched backward (com-pared to the dusk side), but the particles angular velocity increases as the particles arenow moving radially inward. This illustrates again the limitations of axisymmetric mod-els with regard to local time effects.
The first Juno observations of the magnetic field above the Jovian auroras did notreveal the strong field aligned currents expected from the theory (Connerney et al., 2017),but a later analysis covering the first 11 Juno perijoves did reveal significant currents,with a combined mean value of 82 MA for the two hemispheres, which is in line with theestimates of the radial currents in the magnetosphere (Kotsiaros et al., 2019). However,they found that the current did not take the form of thin and regular current shells, butwere fragmented and confined on longitudinal extent. Another unexpected feature wasthe strong asymmetry between the two hemispheres, with southern currents being ap-proxymately twice as large as in the north (58 MA compared to 24 MA). They attributedthis difference to the magnetic field asymmetries leading to differences in the Pedersenconductivity between the two polar ionospheres.
One of the main finding of the Juno mission so far is the ubiquity of the stochas-tic acceleration processes for the charged particles in the polar regions. At Earth, themost steady and brightest auroral emissions are related to quasi-static potentials abovethe ionosphere which accelerate the charged particles (mostly electrons) into the upperatmosphere. Because the ME at Jupiter are even brighter and permanent, it was thoughtthat such quasi-static potentials would also dominate the energization of the charged par-ticles. Such quasi-static potentials have indeed seldom been discovered by Juno, but evenin these specific locations, the precipitating energy flux remains dominated by stochas-tic processes and most electron distributions are bi-directional along the field lines (Mauket al., 2018). This finding is a surprise since corotation enforcement models rely on theformation of such electric potentials through the Knight relationship (or a variation thereof)between the precipitating energy flux and the electron energy (e.g. Cowley & Bunce, 2001;Ray et al., 2010; Tao et al., 2016). Since bi-directional electron acceleration appear tobe the norm, the UV auroral brightness, which is almost solely related to the precipi- –5–anuscript submitted to
JGR: Space Physics tating electron energy flux, is not a reliable proxy for the intensity of the net up-goingfield aligned currents. An even more unexpected and important finding is the discoveryof bi-directional electron beams and proton inverted-V structures on the same field line(Mauk et al., 2018, 2020), meaning that a downward current is compatible with down-ward moving electrons producing UV aurora. This indicates that several processes canco-exist at different altitudes on the same field line. Thus the presence of UV aurora isnot even an indication of up-going currents.
It is not expected for 1-D (quasi-)stationary models to explains all the details ofthe auroras at Jupiter, as they are simplifications built to better understand the mostimportant processes at play. Nevertheless, specific predictions can be made out of thesemodels and a number of them were challenged by the measurements. It is noteworthythat most of the observations mentioned here above concern the generation of the au-roras, which are associated, but not strictly equivalent, to the magnetosphere-ionospherecoupling processes and their related currents. After all, the magnetospheric plasma atJupiter is rotating and field aligned current have indeed been observed. Thus, it is notclear yet whether the question of the origin of the main emissions at Jupiter requires someadjustments of the mainstream theory or a complete paradigm shift. However, recentworks have suggested possible paths forward. First, the idea that the auroral emissionsare a direct image of the up-going field aligned currents is invalidated by Juno’s mea-surements (Mauk et al., 2018). If the particle acceleration process is stochastic, even re-gions of down-going currents would have a significant flux of down-going electrons cre-ating auroral emissions. Then, it appears that the explanatory power of axisymmetricmodels is limited at Jupiter, as local time effects, fragmentation phenomena and non-axisymmetric current systems are critically important. Finally, the findings reported hereabove also suggest that wave processes and wave-particle interactions should be assessedmore carefully rather than assuming steady state continuous currents. A closer exam-ination of the energy transferred by Alfv´en waves already showed some promising results,both theoretically (Saur et al., 2018) and observationally (Gershman et al., 2019). Fi-nally, it could also be of critical importance for magneto-hydrodynamic simulations ofthe Jovian magnetosphere to focus on the Poynting flux and the contribution of Alfv´enwave power rather than on the field aligned currents when comparing their outputs toauroral images in order to provide crucial insight in understanding the origin of the mainemissions.
Acknowledgments
The authors are grateful to Pr. Jean-Claude G´erard for helpful discussions and advise.B.B. is a Research Associate of the Fonds de la Recherche Scientifique - FNRS. B.B. andD.G. acknowledge financial support from the Belgian Federal Science Policy Office (BEL-SPO) via the PRODEX Programme of ESA. The data included herein are archived inNASA’s Planetary Data System ( http://pds-atmospheres.nmsu.edu/data and services/atmospheres data/JUNO/juno.html ). References
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