Ann. Geophys., 37, 101–109, 2019https://doi.org/10.5194/angeo-37-101-2019© Author(s) 2019. This work is distributed underthe Creative Commons Attribution 4.0 License.

Magnetodisc modelling in Jupiter’s magnetosphere using Junomagnetic field data and the paraboloid magnetic field modelIvan A. Pensionerov1, Elena S. Belenkaya1, Stanley W. H. Cowley2, Igor I. Alexeev1, Vladimir V. Kalegaev1, andDavid A. Parunakian1

1Federal State Budget Educational Institution of Higher Education M.V. Lomonosov Moscow State University, SkobeltsynInstitute of Nuclear Physics (SINP MSU), 1(2), Leninskie gory, GSP-1, Moscow 119991, Russian Federation2Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK

Correspondence: Ivan A. Pensionerov (pensionerov@gmail.com)

Received: 6 July 2018 – Discussion started: 12 July 2018Revised: 18 January 2019 – Accepted: 27 January 2019 – Published: 5 February 2019

Abstract. One of the main features of Jupiter’s magneto-sphere is its equatorial magnetodisc, which significantly in-creases the field strength and size of the magnetosphere.Analysis of Juno measurements of the magnetic field duringthe first 10 orbits covering the dawn to pre-dawn sector ofthe magnetosphere (∼03:30–06:00 local time) has allowedus to determine optimal parameters of the magnetodisc usingthe paraboloid magnetospheric magnetic field model, whichemploys analytic expressions for the magnetospheric currentsystems. Specifically, within the model we determine the sizeof the Jovian magnetodisc and the magnetic field strength atits outer edge.

1 Introduction

In this paper we consider magnetic field measurements madeby the Juno spacecraft in Jupiter’s magnetosphere, payingparticular attention to the middle magnetosphere measure-ments where Jupiter’s magnetodisc field plays a major role.The structure and properties of the Jovian magnetodisc havebeen described in many papers, starting from the first space-craft flybys of Jupiter, discussed for example by Barbosaet al. (1979) and references therein. In particular, the em-pirical magnetodisc model presented by Connerney et al.(1981), derived from Voyager-1 and -2 and Pioneer-10 ob-servations, has been employed as a basis in numerous sub-sequent studies, including predictions for the Juno missionby Cowley et al. (2008, 2017). Detailed physical modelshave also been constructed by Caudal (1986), who derived

a steady-state MHD magnetodisc model in which both cen-trifugal and plasma pressure (assumed isotropic) forces wereincluded, and by Nichols (2011), who incorporated a self-consistent plasma angular velocity model. Nichols et al.(2015) have also included the effects of plasma pressureanisotropy, as observed in Voyager and Galileo particle mea-surements, which redistributes the azimuthal currents in themagnetodisc, changing its thickness.

Here we model the magnetic field observations duringJuno’s first 10 orbits for which both inbound and outboundpasses are presently available, corresponding to perijoves(PJs) 0 to 9, using the semi-empirical global paraboloid Jo-vian magnetospheric magnetic field model derived by Alex-eev and Belenkaya (2005). We focus on the middle magne-tosphere, observed on these orbits in the dawn to pre-dawnsector of the magnetosphere (∼03:30–06:00 local time, LT),for which the magnetodisc provides the main contribution tothe magnetospheric magnetic field. In the model, in whichthe field contributions are calculated using parameterised an-alytic equations, the magnetodisc is described by a simplethin plane disc lying in the planetary magnetic equatorialplane. We thus search the paraboloid model magnetodisc in-put parameters to determine the best fit to the Juno measure-ments. We note that the magnetodisc may be regarded as themost important source of magnetic field in Jupiter’s magne-tosphere, with a magnetic moment in the model derived byAlexeev and Belenkaya (2005) using Ulysses inbound data,for example, which is 2.6 times the planetary dipole moment.Consequently, the magnetodisc plays a major role in deter-mining the size of the system in its interaction with the solar

Published by Copernicus Publications on behalf of the European Geosciences Union.

102 I. A. Pensionerov et al.: Analysis of Juno magnetic field data using paraboloid magnetospheric model

Figure 1. In (a) we show a schematic of Jupiter’s magnetospherein the magnetic equatorial plane, showing various parameters of theparaboloid model. In (b) we show the definition of the planetarymagnetic dipole angle 9 in the JSM system, where XJSM pointstowards the Sun and the planetary dipole is contained in the XJSM–ZJSM plane.

wind and is thus an appropriate focus of a study using Junomagnetic field data.

2 The Jupiter paraboloid model

The paraboloid magnetospheric magnetic field model wasdeveloped for Jupiter by Alexeev and Belenkaya (2005),based on the terrestrial paraboloid model of Alexeev (1986)and Alexeev et al. (1993). It contains the internal plane-tary field, B i, calculated from the full order-4 VIP4 modelof Connerney et al. (1998); the magnetodisc field, BMD;the field of the magnetopause shielding currents, Bsi andBsMD, which screen the planetary and magnetodisc fields, re-spectively; the field of the magnetotail current system, BTS;and the penetrating part of the interplanetary magnetic field(IMF), kBIMF, where k is the IMF penetration coefficient.The magnetopause is described by a paraboloid of revolutionin Jovian solar magnetospheric (JSM) coordinates with theorigin at Jupiter’s centre:

x

Rss= 1−

y2+ z2

2R2ss, (1)

where x is directed towards the Sun, the x–z plane containsthe planet’s magnetic moment, and y completes the right-hand orthogonal set pointing towards dusk. Rss is the dis-

tance to the subsolar magnetopause, where y = 0 and z= 0.The magnetospheric magnetic field, Bm, is then the sum ofthe fields created by all these current systems:

Bm = B i(9)+BTS(9,Rss,R2,Bt)

+BMD(9,BDC,RDC1,RDC2)+Bsi(9,Rss)

+BsMD(9,Rss,BDC,RDC1,RDC2)+ kBIMF, (2)

where 9 is Jupiter’s dipole tilt angle relative to the z axis.The magnetodisc is approximated as a thin disc with outerand inner radiiRDC1 andRDC2, respectively. BDC is the mag-netodisc field at the outer boundary, while the azimuthal cur-rents in the disc are assumed to decrease as r−2. R2 is thedistance to the inner edge of the tail current sheet, and Bt isthe tail current magnetic field there. The magnetospheric cur-rent systems are thus described by nine input parameters, de-termining the physical size of the current systems, and theirmagnetic field (current) strength (9, Rss, R2, RDC1, RDC2,Bt, BDC, k, BIMF). In Fig. 1 we show sketches illustrating theparameters of the model. On the left we show a view in themagnetospheric equatorial plane, where we note that in thephysical system, the overlapping model magnetodisc and tailcurrent sheets merge together on the nightside. On the rightwe show the planetary magnetic dipole axis at angle9 in theJSM system. As shown by Alexeev and Belenkaya (2005),the magnetic moment of the model current disc is given by

MMD =BDC

2R3

DC1

(1−

RDC2

RDC1

). (3)

Alexeev and Belenkaya (2005) and Belenkaya (2004) de-termined model parameters which approximated the mag-netic field along the Ulysses inbound trajectory ratherwell. These parameters are Rss = 100RJ, R2 = 65RJ,Bt =−2.5 nT, RDC1 = 92 RJ, RDC2 = 18.4RJ, and BDC =

2.5 nT. This set of parameters is used in the present paper asa starting point for fitting parameters to the Juno data. Thedipole tilt angle 9 changes during the observations and iscalculated as a function of time in the paraboloid model.

3 Magnetic field calculations for the first 10 Juno orbits

As indicated above, field calculations have been made us-ing the paraboloid model for comparison with the data fromthe first 10 Juno orbits for which data are presently avail-able for study. The orbits were closely polar, with large ec-centricity, and with apoapsis initially located south of theequator in the dawn magnetosphere (e.g. Connerney et al.,2017). In Fig. 2 we show the perijove 1 trajectory versus time(in day of year (DOY) 2016) in JSM Cartesian coordinates,specifically showing the cylindrical and spherical radial dis-tances ρJSM =

√x2+ y2 and r , ZJSM, and the LT. The ver-

tical dashed line shows the time of periapsis. On later orbitsapoapsis moved towards the nightside, reaching 03:30 LT by

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I. A. Pensionerov et al.: Analysis of Juno magnetic field data using paraboloid magnetospheric model 103

Figure 2. Juno perijove 1 trajectory in JSM Cartesian coordinatesplotted versus time in DOY 2016, where the vertical dashed lineshows the time of periapsis.

perijove 9, and also rotated further into the Southern Hemi-sphere.

In this paper we confine our attention to the middle magne-tosphere, where, as we now show, the magnetic field is domi-nated by the magnetodisc and the planetary field. In the outermagnetosphere the field becomes strongly influenced by ex-ternal conditions in the solar wind, and although in some cir-cumstances these can be reasonably well predicted by MHDmodels initialised using data obtained near Earth’s orbit (e.g.Tao et al., 2005; Zieger and Hansen, 2008), they will typi-cally vary strongly on the timescale of the Juno orbit (Fig. 2),and with them too the outer magnetospheric field. In Figs. 3and 4, for example, we show the magnitudes of the modelledfield from different sources along the inbound (a) and out-bound (b) passes of perijoves 1 and 9, respectively, plottedversus radial distance. The red lines in these figures show theinternal JRM09 (“Juno reference model through perijove 9”)planetary field derived by Connerney et al. (2018), which em-ploys the well-determined degree and order 10 coefficientsfrom an overall degree 20 spherical harmonic fit to the data(plus disc model field) from the first nine Juno orbits. Theblack lines show the field of the various magnetospheric cur-rent systems in the paraboloid model as marked, where themodel parameters employed are those derived from Ulyssesinbound data by Alexeev and Belenkaya (2005), as outlinedin Sect. 2. It can be seen from Figs. 3 and 4 that for r < 60RJ

the contributions to the magnetospheric field from the mag-netopause and tail current systems (which are oppositely di-rected near the dawn–dusk meridian) are negligible com-pared with the magnetodisc field, being less than 10 % forperijove 1 and less than 16 % for perijove 9, and may thus betreated approximately inside this distance. For related rea-sons we also neglect the penetrating IMF term in Eq. (2),which is unknown when Juno is inside the magnetosphere,highly variable in direction with time, and typically of mag-nitude∼ 0.1–1 nT (Nichols et al., 2006, 2017). This field too,with penetration coefficient k < 1, is therefore similarly neg-ligible in the r < 60RJ middle magnetosphere studied here.

As a consequence of these considerations, here we employthe JRM09 model of the internal field and fit only the magne-todisc parameters to the middle magnetosphere data. For thesmall fields contributed by the magnetopause and tail currentsystems in this regime, we simply use the Ulysses parametersfrom Alexeev and Belenkaya (2005) and Belenkaya (2004)as sufficient approximations, i.e. Rss = 100RJ, R2 = 65RJ,and Bt =−2.5 nT. However, use of the Ulysses magnetodiscparameters is found to lead, for example, to a systematic un-derestimation of the field along the perijove 1 trajectory, andthus needs to be modified. Thus only three parameters,RDC1,RDC2, and BDC, need to be fitted.

To optimise the model we choose the approach of min-imising function S given by

S(BDC,RDC1,RDC2)=

√√√√√√ 1N

N∑n=1

∣∣∣B(n)mod−B

(n)obs

∣∣∣2∣∣∣B(n)obs

∣∣∣2 , (4)

where B(n)mod is the modelled field vector due to the current

systems, B(n)obs is the observed residual field following sub-

traction of the JRM09 internal field model, n is the indexnumber of the data point along the trajectory, and the to-tal number of points is N . S represents a root-mean-squarerelative deviation of the modelled magnetic field from theobserved field vectors. We used a relative deviation insteadof an absolute value to equalise the influence of all the datapoints, noting that the magnetic field varies in magnitude sig-nificantly along the part of the trajectory examined here (seeFigs. 3 and 4). Use of the absolute deviation gives good re-sults in the region closer to the planet where the field magni-tude is greater, but a poorer fit in other parts of the trajectory.

With regard to the choice of interval employed to min-imise S, we note that use of data from the innermost regionis not optimal. The JRM09 internal planetary field model dif-fers from observations at periapsis (1.06 RJ) by 0.3×105 nT(Connerney et al., 2018), which is reasonable accuracy fordescribing an observed field of magnitude ∼ 8× 105 nT, butdoes not allow us to distinguish the magnetodisc field of or-der 100 nT on this background. We thus restricted the innerborder of the interval to consider r > 5RJ only. However, onmost passes examined here, the inner radial limit is set in-

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104 I. A. Pensionerov et al.: Analysis of Juno magnetic field data using paraboloid magnetospheric model

Figure 3. Magnitude of the model magnetic fields for the Juno perijove 1 inbound (a) and outbound (b) passes, due to the internal planetaryfield (JRM09, in red), and the various model magnetospheric currents as marked (magnetopause, tail, and magnetodisc, in black).

Figure 4. As for Fig. 3, but for perijove 9.

stead at somewhat larger radii by the data that are presentlyavailable for study. A further limitation on the region of cal-culation of S in the outer magnetosphere arises from the factthat the paraboloid model does not display regions of lowfield strength during intersections with the magnetodisc, asis observed in the field at larger distances, due to the use ofthe infinitely thin disc approximation (see Sect. 4). It is thusnecessary to avoid these regions by excluding parts of thetrajectory where the spacecraft is closer than 4RJ from themagnetic equator.

We thus minimise S in the inbound and outbound radialranges between Rmin and Rmax on each pass to determine thebest-fit magnetodisc parameters. The minimisation was un-dertaken using the trust region reflective procedure (Branchet al., 1999). The best-fit values are given, together with theestimated error values and the radial ranges employed, in Ta-ble 1, where we also compare with the values derived byAlexeev and Belenkaya (2005) from Ulysses inbound data.We estimated parameter errors by choosing several differentstarting points for the algorithm in parameter space and run-ning it with a more generous termination condition in com-

parison with the normal runs. Specifically, we stopped thecalculation when dS < 0.1S, where dS is the change of func-tion S in the algorithm step. We then estimated the error as(Pmax–Pmin)/2, where Pmax and Pmin are the maximum andminimum parameter values obtained in these runs. For allthe Juno fits we found that the best-fit outer disc radius RDC1was the maximum value of 95RJ allowed in the fitting pro-cess, set by requiring that the disc radius should be less thanthe subsolar magnetopause radius (100RJ) by a few RJ. Thisindicates that the current density in the model disc, varyingas r−2, decreases somewhat too quickly with distance. Thevalues of the inner disc radius RDC2 lie between 12.5 and18.7RJ, usually smaller than the value of 18.4RJ, derivedfrom the Ulysses data, while the field strength parameterBDCvaries between 2.6 and 3.1 nT, larger than the Ulysses valueof 2.5 nT.

In Figs. 5 and 6 we provide comparisons of the observed(black) and modelled (red) residual fields for Juno perijoves 1and 6, respectively, from which the JRM09 planetary fieldhas been subtracted. Specifically we show the JSM cylindri-cal field components together with the residual field magni-

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I. A. Pensionerov et al.: Analysis of Juno magnetic field data using paraboloid magnetospheric model 105

Table 1. Magnetodisc parameters derived for the Ulysses inbound pass and the first 10 Juno orbits, together with the estimated errors and theminimum inbound and outbound radial distances available in the Juno passes.

BDC (nT) RDC2 (RJ) RDC1 (RJ) Rmin (RJ), Rmin (RJ),inbound outbound

Ulysses 2.50 18.4 92PJ-00 2.58± 0.10 18.7± 2.8 95 not available 31.5PJ-01 2.76± 0.12 12.5± 1.8 95 5.0 5.0PJ-02 2.61± 0.10 13.6± 2.3 95 13.3 not availablePJ-03 2.79± 0.10 14.5± 1.5 95 16.5 8.9PJ-04 2.65± 0.07 15.2± 1.2 95 13.7 12.3PJ-05 2.59± 0.15 14.6± 2.5 95 10.6 10.5PJ-06 2.70± 0.08 14.3± 1.2 95 8.0 17.2PJ-07 3.01± 0.09 15.5± 2.0 95 21.9 19.7PJ-08 3.07± 0.09 15.8± 1.9 95 19.5 19.5PJ-09 3.06± 0.11 13.6± 1.5 95 17.0 8.3

Figure 5. Observed (black) and modelled (red) residual fields in JSM cylindrical components, together with the residual field magnitude, forJuno perijove 1. The residual field is the observed field with the JRM09 internal field subtracted. The fields are plotted versus spherical radialdistance with inbound data shown on the left and outbound data on the right. The same model field is used for both.

tude plotted versus radial distance, where the same modelapplies to both inbound (left side) and outbound (right side)data. As can be seen, the fitted models are generally in good

accordance with the observations for the Bρ and Bz compo-nents, while the Bφ component is not adequately described,because the model does not include radial currents in the

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106 I. A. Pensionerov et al.: Analysis of Juno magnetic field data using paraboloid magnetospheric model

Figure 6. As for Fig. 5, but for perijove 6.

magnetodisc and their closure current via the ionosphere. Itis also seen in Fig. 5 that the field magnitude is underesti-mated inside of ∼ 10RJ, again probably related to the too-steep radial dependence of the azimuthal current. As the dis-tance from Jupiter decreases, a sharp increase in the residualfield is observed in the inner region to > 100 nT, while themodel field plateaus at several tens of nanoteslas (nT). At theclosest distances from the planet the increase is probably dueto inaccuracy of the JRM09 model of the internal field, not-ing that the model represents only the degree and order 10terms from an overall degree 20 fit (Connerney et al., 2018).

4 Approaches for future improvement of the Jupiterparaboloid model

We first compare the fits derived here with those obtainedusing the magnetodisc model derived by Connerney et al.(1981) from Voyager-1 and -2 and Pioneer-10 field data, butnow fitted to Juno perijove 1 data. In this model the currentflows in a planet-centred annular disc of full thickness 5RJ,with inner (R0) and outer (R1) radii at 5 and ∼ 50RJ, re-

spectively. The azimuthal current in the disc is taken to varyas I0/ρ, where ρ is the perpendicular distance from the plan-etary dipole magnetic axis. We optimised this model for Junoperijove 1 using the same method as outlined above, to findbest-fit parameters I0 = 21× 106AR−1

J (µ0I0/2≈ 185 nT),R0 = 6RJ, and R1 = 67RJ. Figure 7 shows a comparison ofthe observed residual fields (black) with the best-fit Conner-ney et al. model (blue) in a similar format to Figs. 5 and 6,where we also show the best-fit paraboloid model (red) fromFig. 5. One important difference between the model results isthe fact that the Connerney et al. (1981) model reflects wellthe observed periodic sharp drops of magnetic field strengthduring spacecraft intersections with the disc. The magne-todisc radial magnetic field component reverses sign aboveand below the disc, and at its centre becomes equal to zero.As indicated in Sect. 3, the paraboloid model with an in-finitely thin disc certainly cannot reproduce this feature andshould thus be improved by use of a disc current of finitethickness. The Connerney et al. model demonstrates reason-able coincidence with observations near Jupiter, but at greaterdistances overestimates the magnetic field strength, which in-

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I. A. Pensionerov et al.: Analysis of Juno magnetic field data using paraboloid magnetospheric model 107

Figure 7. Comparison of the observed residual field (black) and best-fit Connerney et al. (1981) magnetodisc model field (blue) in a similarformat to Fig. 5. We also show the best-fit paraboloid model (red) as in Fig. 5.

dicates that at these distances the current density variation asρ−1 is too slow.

As indicated above, neither of the magnetodisc modelsconsidered here describe the azimuthal field well at mediumand large distances, which shows short-term modulationsof the field between positive and negative values related tocrossings of the current sheet near the planetary rotation pe-riod (see for example the inbound data in Fig. 6). This pointsto the well-known existence of radial currents in the magne-todisc associated with sweepback of the field into a “lagging”configuration (e.g. Hill, 1979). Neither of models consideredhere, the Connerney et al. (1981) model and the paraboloidmodel of Alexeev and Belenkaya (2005), include these cur-rents, but only the azimuthal current in the magnetodisc.Such radial currents have been included in the models byKhurana (1997) and Cowley et al. (2008, 2017), and couldbe a useful addition to the paraboloid model, together withtheir field-aligned and ionospheric closure currents.

5 Discussion and conclusions

As shown in Figs. 3 and 4, in the middle part of the Jovianmagnetosphere selected for study here, the main contribu-tion to the field due to the magnetospheric current systems isthe equatorial magnetodisc. Here we have refined the magne-todisc parameters within the Jovian paraboloid model to bestfit the Juno data from the first 10 orbits in this region, forwhich both inbound and outbound data are presently avail-able. Analysis of the field at very close radial distances re-quires better knowledge of the internal planetary field, whilethe field at large distances is strongly influenced by the solarwind, whose simultaneous parameters remain unknown andare generally varying rapidly with time on the scale of theJuno passes.

As the simplest approximation we took magnetopause andtail current parameters derived using the Ulysses missiondata (Alexeev and Belenkaya, 2005; Belenkaya, 2004) andchanged only the radial and field strength parameters of themagnetodisc. We found that the best-fit model consistentlyhad a large outer radius comparable with the subsolar mag-

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108 I. A. Pensionerov et al.: Analysis of Juno magnetic field data using paraboloid magnetospheric model

netopause distance (taken to be 100RJ from the Ulyssesmodel), an inner radius usually between ∼ 12 and 14RJsmaller than the Ulysses model (∼ 18RJ), and a compara-ble field strength parameter (at the outer edge of the disc) of∼ 2.5 nT.

To further refine the Jovian paraboloid magnetosphericmodel, it will be necessary to take into account the finitethickness of the magnetodisc current, and also to accuratelydetermine its dependence on the radial distance from theplanet. The existence of radial currents in the disc, as well astheir closure via field-aligned currents in the planetary iono-sphere, should also be incorporated.

Code availability. Those who would like to work with theparaboloid model may contact Igor I. Alexeev at alex-eev@dec1.sinp.msu.ru.

Author contributions. IAP – Formal analysis, Visualization, Writ-ing – original draft; ESB – Supervision, Writing – original draft,Writing – review and editing; SWHC – Supervision, Writing – re-view and editing; IIA – Methodology, Software; VVK – Methodol-ogy, Software; DAP – Software, Data curation.

Competing interests. The authors declare that they have no conflictof interest.

Acknowledgements. Work at the Federal State Budget EducationalInstitution of Higher Education M.V. Lomonosov Moscow StateUniversity, Skobeltsyn Institute of Nuclear Physics (SINP MSU),was partially supported by the Ministry of Education and Scienceof the Russian Federation (grant RFMEFI61617X0084). Workat the University of Leicester was supported by STFC grantST/N000749/1. The Juno magnetometer data were obtained fromthe Planetary Data System (PDS). We are grateful to the Juno teamfor making the magnetic field data available (FGM instrumentscientist John E. P. Connerney; principal investigator of Junomission Scott J. Bolton).

Edited by: Elias RoussosReviewed by: two anonymous referees

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