Jovian Auroral Radio Source Occultation Modeling and Application to the JUICE Science Mission Planning
AAstronomy & Astrophysics manuscript no. juice-expres © ESO 2021February 16, 2021
Auroral Radio Source Occultation Modeling and Application to theJUICE Science Mission Planning
B. Cecconi , C. K. Louis , C. Muñoz Crego , and C. Vallat LESIA, Observatoire de Paris, CNRS, PSL Research University, Meudon, Francee-mail: [email protected] School of Cosmic Physics, DIAS Dunsink Observatory, Dublin Institute for Advanced Studies, Dublin 15, Irelande-mail: [email protected] Aurora B.V., for European Space Agency, ESAC, Madrid, Spaine-mail: [email protected] Rhea Group, for European Space Agency, ESAC, Madrid, Spaine-mail: [email protected]
ABSTRACT
Aims.
Validate the use of ExPRES as an observation planning tool for the JUICE mission.
Methods.
We simulate the occultations of the Jovian auroral radio emissions during the Galileo flybys of the Galilean moons ofJupiter. The ExPRES simulations runs are configured using fixed typical parameters for the main aurora radio emissions. We comparethe simulation run results with the actual Galileo PWS observations.
Results.
The ExPRES code accurately models the temporal occurrence of the occultations in the whole spectral range observed byGalileo PWS.
Conclusions.
The method can be applied for preparing the JUICE moon flyby science operation planning.
Key words. radio continuum: planetary systems emissions – planets and satellites: individuals: Jupiter – planets and satellites:individuals: Io – planets and satellites: individuals: Europa – planets and satellites: individuals: Ganymede – planets and satellites:individuals: Callisto
1. Introduction
The magnetosphere of Jupiter produces low frequency radioemissions from its polar regions, along the active magneticfield lines connected to the Jovian auroral oval as well as tothe Galilean moon auroral magnetic footprints. The Jovian ra-dio emissions are intense and non-thermal radio frequency phe-nomena, spanning from a few kHz to about 40 MHz, and pro-duced through the Cyclotron Maser Instability (CMI), whichconverts the local plasma free-energy into electromagnetic ra-diation (Zarka 1992). They are used as a proxy for the Jovianmagnetospheric activity. They have been discovered by Burke& Franklin (1955) and have been since studied with ground ob-servatories (e.g., Nançay Decameter Array, Lamy et al. 2017))and space-borne instruments (with, e.g., the Voyager, Galileo,Cassini and Juno space missions).Observations from the Cassini Radio and Plasma Waves Sci-ence (RPWS, Gurnett et al. 1992) and Galileo Plasma WavesScience (PWS, Gurnett et al. 1992) experiments showed thatthe Jovian radio emission events are observed quasi-permanentlyalong the spacecraft orbit. Figure 1 shows 24 hours of simultane-ously observed spectral flux densities by the Cassini / RPWS andGalileo / PWS instruments, when Cassini was close to its flyby ofJupiter. Both panels of this figure display many arc-shaped ra-dio events (a few of them are highlighted with a thin plain blackline), with a few small time-frequency regions with quiet (back-ground level) conditions. Radio arcs can be classified with ori-entation of their curvature: “vertex-early” and “vertex-late” arcscorresponds to opening “(” or closing “)” parenthesis shapes. The arc shape of Jovian radio emission is well explained by theCMI mechanism at the radio source, coupled with the shape ofthe magnetic field lines, and the rotation of Jupiter with respectof the observer, as described in Fig. 1 of Louis et al. (2019).This figure also introduce two other features of the observed ra-dio spectrum around Jupiter: Type II Solar radio bursts are alsoobserved depending on the solar activity (three events are high-lighted with a thin dashed black line); and the so-called “attenu-ation lanes” (Gurnett et al. 1998) resulting from the propagationof hectometric waves through the Io plasma torus (Menietti et al.2003). The attenuation lanes are observed as a narrow-band at-tenuation feature modulated at the planetary rotation period. Theattenuation is also accompanied or replaced with an intensifica-tion of the signal, similarly to caustic optical phenomena.Although not covering the full spectral range of the Jovianradio emissions, Galileo / PWS has been routinely collecting ra-dio observations during its many orbits in the Jovian system.This data (Gurnett et al. 1997) shows quasi continuous emis-sion from a few 100 kHz up to 5.6 MHz, the upper spectrallimit of PWS. During the Galilean moon flybys, Galileo / PWSobserved full occultations of the Jovian radio emissions (Kurthet al. 1997).The ESA JUICE (JUpiter ICy moon Explorer, Witasse 2019)will explore the Jupiter system and its magnetosphere. The studyof the Jovian magnetosphere can strongly benefit from remoteobservations and modelling tools (Cecconi 2019). Two instru-ments of the JUICE scientific payload are operating in the lowfrequency radio range (below 50 MHz): the Radio and PlasmaWaves Instrument (RPWI, Wahlund 2013) has a receiver ded-
Article number, page 1 of 12 a r X i v : . [ a s t r o - ph . I M ] F e b & A proofs: manuscript no. juice-expres
Cassini/RPWS HFRGalileo PWS - LRS Electric00:002001-01-02 03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:002001-01-03102103104 F r equen cy ( k H z ) C a li b r a t ed F l u x D en s i t y ( W / m / H z ) F r equen cy ( k H z ) S pe c t r a l D en s i t y ( V / m / H z ) Fig. 1.
Calibrated Cassini / RPWS / HFR (Zarka et al. 2004; Cecconi & Zarka 2019) (top) and GLL / PWS (see Section 2.1) (bottom) radio electricpower spectral densities during 24 hours close to the Cassini flyby of Jupiter. Significant features have been highlighted: a few radio emission“arcs” are traced in plain line; Type III Solar radio bursts are traced in dashed line; and the attenuation lanes are traced in dotted line. icated to the study of Jovian radio emissions; and the Radarfor Icy Moon Exploration (RIME) experiment (Bruzzone et al.2013) is operating with a central frequency at 9 MHz, which lieswithin the Jovian radio emission spectral range. The Jovian ra-dio emission may interfere with RIME active radar mode (Cec-coni et al. 2012), but can be also used in a passive radar exper-iment mode during icy moon flybys (Romero-Wolf et al. 2015;Schroeder et al. 2016; Kumamoto et al. 2017).The ExPRES code (Exoplanetary and Planetary Radio Emis-sions Simulator, Louis et al. 2019) simulates for a given observerthe geometrical visibility of radio emissions of a magnetisedbody. This visibility depends in particular of the emission anglebetween the magnetic field vector at the source and the emittedwave vector, which is computed self-consistently in the frame ofthe CMI theory. The anisotropic shape of the radio source andtheir geometrical observability conditions are well described inFig. 1 (panels c, d and e) of Louis et al. (2019). This computationis iterated at each time / frequency step and for each source. Theproduced time-frequency map (or dynamic spectrum) can thendirectly be compared to observations.In this study, we are modelling the Jovian radio emission oc-cultations during (past) Galileo and (planned) JUICE Galileanmoon flybys, using the ExPRES code.
2. Data Sets
Several sets of data have been used in this study and are pre-sented in this section. For the Galileo spacecraft flybys, we com-pare the actual observed radio spectra, with low frequency ra-dio occultations predicted by the ExPRES code, using the actual flyby geometry (spacecraft and moon trajectory in a Jovian refer-ence frame). The JUICE spacecraft study only includes modelledoccultation.
All Galilean moon flyby of Galileo with PWS data have beenmodelled and analysed. The Galileo PWS (hereafter referredto as GLL / PWS) data have been retrieved from Universityof Iowa das2 server interface (Piker et al. 2019), using the das2py python module. We have used the GLL / PWS LRS 152-channel calibrated electric collection from that das2 server.The data are radio-electric power spectral densities providedin units of V / m / Hz. This dataset doesn’t include the instru-ment’s antenna gain calibration, but this has no consequence onthis study. The data set has a native time resolution of 18.67seconds. These data are also available in the full resolutionGLL / PWS dataset (
GO-J-PWS-2-REDR-LPW-SA-FULL-V1.0 ,Gurnett et al. 1997), at NASA PDS (Planetary Data System) PPI(Planetary Plasma Interaction) node.
The moons and spacecraft trajectory data are computed usingSPICE kernels (Acton 1996). In this study, the ephemeris datahave been retrieved using the NASA-JPL (Jet Propulsion Labo- currently available from https://github.com/das-developers/das2py http://das2.org/browse?resolve=tag:das2.org,2012:site:/uiowa/galileo/pws/survey_electric Article number, page 2 of 12. Cecconi et al.: Auroral Radio Source Occultation Modeling ratory of the National Aeronautics and Space Administration) in-stance of WebGeocalc (Acton et al. 2018) for Galileo spacecraftand Jovian moons, and another WebGeocalc instance at ESA-ESAC (European Space Astronomy Centre of the EuropeanSpace Agency) for the JUICE spacecraft. The JUICE spacecraftSPICE kernels (ESA SPICE Service 2020) contains all the stud-ied orbital scenarii, as described in the JUICE CReMA (Consol-idated Report on Mission Analysis) documents. In this study,the selected JUICE trajectory scenario is CReMA-3.0. Theephemeris of all bodies have been retrieved in the
IAU_JUPITER reference frame, also referred to as “IAU Jupiter System III(1965)”. In the WebGeoCalc interface, we use the “planetocen-tric” representation for coordinate retrieval, in which the longi-tude is oriented Eastward.For each flyby, two ephemeris data files are retrieved, usingthe ‘State Vector’ WebGeocalc capability: (a) the location of themoon and (b) that of the spacecraft, both in the
IAU_JUPITER frame, as seen from the center of Jupiter, with a time interval ofa few hours (2 to 4 hours, depending on the spacecraft velocityrelative to the moon) centred on the closest approach epoch ofthe flyby, and a time sampling step of one minute. We do notcorrect for light time propagation. The resulting uncertainty inephemeris data timing is of the order of 1 second, which is muchbelow time resolution of the data and the simulations.
3. Jovian Radio Emissions Occultations
As shown by Kurth et al. (1997), the Jovian hectometric ra-dio emissions are occulted by Ganymede during G01 flyby(Ganymede flyby during the first orbit around Jupiter) of theGalileo spacecraft, on June 27th 1996. Figure 1 of Kurth et al.(1997) shows the GLL / PWS spectrogram during G01 flyby (alsodisplayed on the bottom-left panel of Figure 2). The full occulta-tion is observed between 05:50 and 06:20 SCET. The occultationspectral ingress and egress profiles imply that the observed radiosources at higher frequencies (located close to Jupiter) are oc-culted earlier and reappears later than the lower frequency ones,which are located further out from Jupiter. Possible occultationof the Jovian radio emissions have been also reported during thefirst Io flyby of Galileo (Louarn et al. 1997).Table 1 shows our assessment of all targeted Galilean moonflybys by the Galileo spacecraft. Appendix B provides access tothe full material used to conduct this study, with figures corre-sponding to each flyby. In this paper, we have selected one flybyof each Galilean moon, where the radio emission occultation wasclearly observed (see the grey rows in Table 1). Figure 2 showsGLL / PWS observations for each of the selected flybys, i.e., fromleft to right and top to bottom: Io (I24), Europa (E12), Ganymede(G01) and Callisto (C30) flybys.
We model the location of the Jovian auroral radio sources visibleat Galileo’s location using ExPRES (version 1.1.0, Louis et al.2020)). Our simulation runs are configured as follows: (a) weuse the JRM09 magnetic field model (Connerney et al. 2018) to-gether with the Connerney et al. (1981) current sheet model; (b)the sources are set every 1 ◦ in longitude along active magneticfield lines of M shell =
30 (M-shell being the measure of the mag-netic apex, i.e., the distance in Jovian radii ( R J ), of the magneticfield line at the magnetic equator), corresponding to the mainauroral oval (Grodent 2015); (c) the unstable electron tempera-ture is set to 5 keV (Louarn et al. 2017); and (d) the location ofthe visible radio sources is modeled with a temporal step of one Fig. 2.
Jovian radio emission occultations by Io (upper left), Europa(upper right), Ganymede (lower left) and Callisto (lower right), as seenby GLL / PWS. The figures are showing radio electric power spectraldensities in V / m / Hz. minute. These parameters are fixed for all simulation runs usedin this study. We also included Io-induced radio emissions inthe simulation runs, with the same unstable electron distributiontemperature (5 keV). The ExPRES configuration file is available,as described in appendix B.When the observer is located near the magnetic equator, theradio source beaming pattern implies that the visible radio sourceare split into four cluster locations, called A, B, C and D, cor-responding respectively to the North-Eastern, North-Western,South-Eastern and South-Western quadrant around Jupiter asseen from the observer (see, e.g., Fig. 2 of Marques et al. 2017,for a definition).The simulation runs have been computed using the OPUSServillat et al. (Observatoire de Paris UWS Server, 2021a)) in-stance operated by PADC for the MASER (Measurement, Anal-ysis and Simulation of Emissions in the Radio range) project(Cecconi et al. 2020). OPUS is a framework running the Univer-sal Worker Service (UWS) protocol (Harrison & Rixon 2016).The ExPRES code is available for run-on-demand from this in-terface . The occultation is computed using a simple geometric derivationof the intercept distance between the center of the Galilean moonand the straight lines passing by each visible modelled radiosource and the observer. Any source with an intercept distanceshorter than one moon radius is occulted, assuming a sphericalmoon, as sketched on Figure 3.
4. Observations
All Galileo flybys have been analysed and modelled using Ex-PRES. In this section, we present the detailed modelling resultscorresponding to the highlighted rows of Table 1. Figures 4, 5, 6and 8 present the results for these four flybys. The flybys are pre-sented in order to show the simpler to the more complex cases. Paris Astronomical Data Centre: https://padc.obspm.fr UWS MASER portal: https://voparis-uws-maser.obspm.fr/client/
Article number, page 3 of 12 & A proofs: manuscript no. juice-expres
Orbit Moon Moon Data Availability OccultationName Name Closest Approach GLL / PWS SPICE AssessmentI00 Io yes yes (?)
G01 Ganymede yes yes yes
G02 Ganymede yes yes (?)
C03 Callisto yes yes (?)
E04 Europa yes yes no E06 Europa yes yes (?)
G07 Ganymede yes yes (?)
G08 Ganymede yes yes yes
C09 Callisto yes yes (?)
C10 Callisto yes yes no E11 Europa yes yes no E12 Europa yes yes yes
E14 Europa yes yes (?)
E15 Europa yes no yes
E16 Europa yes yes (?)
E17 Europa yes yes (?)
E18 Europa no yes — E19 Europa yes yes (?)
C20 Callisto yes yes no C21 Callisto yes yes no C22 Callisto yes yes yes
C23 Callisto yes yes yes
I24 Io yes yes yes
I25 Io yes yes (?)
E26 Europa yes yes no I27 Io yes yes yes
G28 Ganymede yes yes no G29 Ganymede yes yes no C30 Callisto yes yes yes
I31 Io yes yes no I32 Io yes yes (?)
I33 Io yes yes (?)
Table 1.
List of all targeted Galilean moon flybys with PWS / Electric-Survey data. The data from all flybys (except E18) are available throughthe University of Iowa das2 server end-point. The spacecraft ephemeris data is available for all flybys except E15. The occultation assessmentindicates if the occultation is observed ( yes or no ) or unsure (?) . The grey lines correspond to the flybys shown in Figure 2 and described in detailsin this study. Table adapted from Table 1 (Orbital Facts) of the CATALOG/GO_MISSION.CAT label file available from Gurnett et al. (1997) source 1observer source 2occulting body
Fig. 3.
Simple geometric occultation scheme used in this study. Theinterception distance (dotted segment) is computed as the distance be-tween the center of the occulting body and the line of sight between theobserver and the source. In this case the source 1 is not occulted, whilesource 2 is.
The GLL / PWS data (same data as in Figure 2) are plotted to-gether with the simulations of observable auroral radio emissions(described in Section 3.1) separated into the four source types A,B, C and D (from white to black, respectively). The comparisonof observations and modelled data shows that the simulations re-produce the Jovian radio occultation during the four flybys pre-sented in Figure 2.Appendix B describes the supplementary material availablefor all flybys (Cecconi et al. 2021), which contains all the mate-rial used to conduct this study. For each Galileo flyby, we pro-vide: (a) a figure showing the GLL / PWS data and the observable radio sources modeled by ExPRES, and (b) movies showing asubset of Jovian radio sources (a selected sub-set of frequen-cies), as seen from Galileo (‘pov’ movies), or from the top of theJovian system (‘top’ movies).
The occultation occurring during the C30 flyby of Callisto isdisplayed in Figure 4. The ingress occultation time is verywell reproduced (within one minute accuracy). All sources areocculted simultaneously, with radio emissions intensity drop-ping instantly, at ∼ ∼ ∼ ∼ https://doi.org/10.25935/8ZFF-NX36 . The frames between 11:44 to 11:59 of the‘pov’ movie clearly shows the various sources reappearing oneafter the other: first, the B sources, then the A and D sourcessimultaneously and finally the C sources. The predicted reap-pearance of C sources perfectly coincides with the observed fullegress phase. This leads to two observations: (i) the main radiocontribution is that of the C sources at the time of observation, Article number, page 4 of 12. Cecconi et al.: Auroral Radio Source Occultation Modeling
Fig. 4.
Superimposed GLL / PWS data and ExPRES simulations duringJovian radio emission occultations by Callisto (flyby C30). The fourtypes of emission (A, B, C, D) are separated (from white to dark grey,resp.) and (ii) the radio sources are occulted by the moon’s surface (orvery close to it).The low-frequency cut-o ff is not perfectly simulated (espe-cially on the ingress side), with a di ff erence of about 200 kHz.This lower frequency limit is mostly due to the ratio of theplasma and cyclotron electronic frequencies in the radio source,as configured in the ExPRES simulations. The error of a few10s-100s kHz is thus probably due to an incorrect estimationof the electron density around Jupiter. An attenuation feature isobserved starting at ∼ ∼ ∼
800 kHz and ∼ Fig. 5.
Superimposed GLL / PWS data and ExPRES simulations dur-ing Jovian radio emission occultations by Europa (flyby E12). The fourtypes of emission (A, B, C, D) are separated (from white to dark grey,resp.)
The E12 Europa’s flyby (Fig. 5) is similar to C30 Callisto’sflyby case, where all sources are occulted simultaneously atingress. Faint and sporadic radio signatures are observed dur-ing the full occultation phase (as in the case of C30). At egress,the simulation well reproduces the end of the occultation, exceptin the (cid:39) [800-1000] kHz frequency range where we observedemission during the modelled occultation.The attenuation lane feature is observed at and below ∼ ∼ ∼ ∼
250 kHz (at the begin-ning of the interval) and ∼
100 kHz (at the end) corresponds tothe local plasma upper-hybrid frequency line ( f UH = (cid:113) f pe + f ce ,where f pe and f ce are the local plasma, and electron cyclotronfrequencies, respectively). Fig. 6.
Superimposed Galileo PWS data and ExPRES simulations dur-ing Jovian radio emission occultations by Ganymede (flyby G01). Thefour types of emission (A, B, C, D) are separated (from white to darkgrey, resp.)
Figure 6 displays the occultation modeling of the Jovian ra-dio emissions during the G01 flyby of Ganymede. At ingress,unlike the previous two cases, all sources are not occulted si-multaneously. First the A sources are occulted, then the B, Cand D sources, with the beginning of the full occultation. Be-fore observed egress, a broadband noise burst at ∼ ∼
800 kHz. This has been interpreted as the signature ofGanymede’s magnetopause crossing by Gurnett et al. (1996). Ategress, the modelled reappearance of the A sources (white line)well reproduce the end of the observed occultation at frequencieshigher than a few MHz. At lower frequencies, the egress occurslater than predicted. At ∼
700 kHz, it is observed at ∼ ∼ Article number, page 5 of 12 & A proofs: manuscript no. juice-expres
Fig. 7.
G01 Flyby visualised in the Cosmographia tool. The scene is set with an observer on the Galileo spacecraft, pointing to Jupiter. Ganymedeis in the field of view. The ExPRES-modelled visible radio sources are also shown, at 700 kHz, 1 MHz, 2 MHz, 5 MHz and 10 MHz. The radiosources are naturally grouped in four sets (named A, B, C and D). A the time of the snapshot ( ), Ganymede isocculting the radio source at 2 MHz in the A-group. in supplementary material at: https://doi.org/10.25935/8zff-nx36 . Fig. 8.
Superimposed GLL / PWS data and ExPRES simulations duringJovian radio emission occultations by Io (flyby I24). The four types ofemission (A, B, C, D) are separated (from white to dark grey, resp.)
Figure 8 displays the occultation during the I24 Io’s flyby. Atingress, the occultation of the higher intensity is well modelled.We observe radio signals during the modelled full occultation,and the observed egress seems to occur earlier than the predic-tion. Intense radio arcs are visible above 2 MHz, showing thelower frequency part of vertex-late arcs. The radio signal is at-tenuated below 1 to 2 MHz, especially after the flyby. The f UH line is also observed ( ∼
300 kHz to ∼
500 kHz).This Io flyby also shows a noticeable feature. The modelledNorthern radio sources (A and B, respectively in white and light-grey) are not observed at the beginning of the studied interval.This is due to the “Equatorial Shadow Zone” (ESZ) e ff ect re- ported at Saturn (Lamy et al. 2008), see Appendix A for moredetails. In the case of flybys with partial occultations (e.g., G02, G07,E11, E26 or G28), the Jovian radio signals are observed dur-ing the flyby, with no full occultation interval. During the G07flyby, only D sources are occulted. The GLL / PWS data (see https://doi.org/10.25935/8zff-nx36 ) shows an at-tenuation of the Jovian radio signals that fits the predicted occul-tation (see Figure 9), hence suggesting that several radio sourceswere active, including the D sources.
Fig. 9.
Superimposed Galileo PWS data and ExPRES simulations dur-ing Jovian radio emission occultations by Ganymede (flyby G07). Thefour types of emission (A, B, C, D) are separated (from white to darkgrey, resp.)
Most of the other flybys (e.g., C03, E04, E06, G08, C10,E19) are occurring on the Jovian-facing side of the moon, whereno occultation can occur.
Article number, page 6 of 12. Cecconi et al.: Auroral Radio Source Occultation Modeling
5. Results and Discussion
The first result of this study is the fact that Jovian auroral radioemissions are present almost continuously, although this may notbe obvious in some observational data sets, due to the limitedsensitivity of instruments. Ground based radio telescopes (suchas the Nançay Decameter Array, in France) provide the longestobservation collections (Marques et al. 2017), but the distance toJupiter limits the detection of low intensity events despite theirintrinsic sensitivity. Space borne radio instruments (such as Voy-ager / PRA, Cassini / RPWS, GLL / PWS or Juno / Waves), provideobservations from a closer range to Jupiter. Cassini and Galileodata show quasi-continuous radio signals (as shown on Figure1). The ExPRES modeling of auroral radio sources, configuredwith radio sources every 1 ◦ in longitude, is consistent with thisobservation (with the noticeable exception of the ESZ, for an ob-server located around Io’s orbital distance to Jupiter). The occul-tation modeling analysis shows that all sources must be occultedin order to remove the natural radio signature of Jupiter’s mag-netospheric activity. It is also noticeable that faint emissions arestill visible during some occultations (e.g., during the G01, E12and C30 Galileo flybys).The JUICE / RPWI and RIME instruments will observe simi-lar radio signal levels. The JUICE / RIME and Cassini / RPWS in-struments have similar antenna characteristics, leading to an an-tenna resonance at about ∼ / RIME will observesimilar signals as those shown on Figure 1 (with a spectral rangerestricted around 9 MHz).The low frequency limit is usually well predicted by Ex-PRES, as shown on Figures 4, 5, 6, 8 and 9. The ExPRES lowfrequency emission limit is determined by the CMI theory: theemission can occur only when f pe / f ce < .
01. This finding im-plies that our modeling of the magnetic field and plasma densityis consistent with the radio observations. Discrepancies (e.g.,during flyby C30) may be related to the modeling of the twoaforementioned characteristic frequencies: f pe depends on themagnetospheric current sheet model; and f ce is determined bythe magnetic field model. The JRM09 magnetic field model isderived from the Juno magnetic measurements in the polar re-gions of Jupiter. This model is thus perfectly adapted for ourapplication. Conversely, the current sheet model could be im-proved. The results of this study will be check and updated ifneeded when a new model is published. The e ff ect of the SolarWind conditions may also play a role (as studied by Hess et al.2012), but the influence at the lowest frequencies remains to bestudied.Figure 10 displays the occultation of the Jovian radio sourcesduring the flyby I24, for two sets of simulation runs. In the topand bottom panels, the sources are located on the magnetic fieldlines with an M-shell of 50 and 30 R J , respectively, with thelarger M-shell, corresponding to radio sources located at highermagnetic latitudes. We observe no di ff erence for the occultationprediction. The di ff erence in the low cut-o ff frequency of thesimulated emissions is very small (with a slightly lower low cut-o ff frequency for sources with M shells of 50 R J ). The maindi ff erence is observed for the A and B sources (white and light-grey curve), with the ESZ feature occurring at a di ff erent time.The ExPRES modeling results show that the occultationmodeling is predicting accurately the observed occultation (e.g.,C30 flyby, or E12 and I24 flyby ingress). The ExPRES sim-ulation runs have been configured with a 1 minute samplingstep, which sets the general temporal accuracy of this study.The discrepancies between the predictions and observation have M-Shell = 50M-Shell = 30
Fig. 10.
Comparison between the jovian radio occultation during the I24flyby for sources on magnetic field lines at M shell M =
50 (top panel)and M =
30 (bottom panel). to be further studied. The mismatch mostly occurs at frequen-cies lower than ∼ ff ects (suchas refraction e ff ects) are known to occur, with, e.g., attenuationlanes. We observe them in all studied flybys, e.g., on E12, wherethe Jovian auroral radio waves are attenuated below ∼ ∼ f pe along this line. From G01,the radio source at 600 kHz reappears at 06:21 SCET, whichcorresponds to an altitude of ∼
300 km above the Ganymede’slimb. The observation frequency would translate into a plasmadensity of about 4500 cm − . The same analysis for the radiosource at 2 MHz leads to an estimation of ∼ cm − close tothe moon’s surface. This value is inconsistent with previouslypublished plasma density models for Ganymede environment(Gurnett et al. 1996; Kliore 1998; Eviatar et al. 2001). Figure 11shows how the same observations can be interpreted consideringrefraction e ff ects on the ionosphere of the moon.In Figures 4, 5 and 6, we observe faint and sometimes spo-radic radio signals, which are visible during the occultation in-terval, despite the predicted full occultation. Since the moonis geometrically occulting all the Jovian radio sources, refrac-tion e ff ects must be taken into account to interpret the observa-tion. These e ff ects can occur either in the moon’s atmosphereand ionosphere, in the Io plasma torus or in the magnetosphericplasma sheet. ExPRES assumes a straight line propagation be-tween the radio source and the observer. Article number, page 7 of 12 & A proofs: manuscript no. juice-expres
Modelling (no refraction) Observationradio source @ 700 kHz (at infinity) Galileo spacecraft trajectory 06:2106:16
Fig. 11.
Sketch of G01 configuration, with a modelled radio source at 700 kHz, assumed to be at infinity on the left-hand side. The plain lineis a sketched refracted ray path. The plain grey line represents the ionosphere of the moon. The dashed lines are the straight line propagation(no refraction) for the same source. Two locations of the Galileo spacecraft trajectory are illustrated on the right-hand side of the Figure. The“Modelling (no refraction)” column shows frames extracted from the G01 ‘pov’ movie at 06:16 and 06:21. The right-most column (“Observation”)shows the radio source visibility, including refraction e ff ects at low frequencies. The two latter results (occultation ingress and egress predic-tion mismatch and faint signals during full occultation) indicatethat propagation e ff ects play an important role in the fine un-derstanding of the Galilean radio occultations. Further analysisrequires the coupling to a ray-tracing code, such as ARTEMIS-P ( Anisotropic Ray Tracer for Electromagnetism in Magneto-sphere, Ionosphere and Solar wind including Polarization , Gau-tier et al. 2013).
6. Usage for the JUICE mission planning tools
The science planning activity, coordinated by the JUICE Sci-ence Operations Center, relies on the identification at each pointin time of the science observations opportunities, using sciencemodels or / and geometry. For JUICE, some of those opportunityperiods depend on the Jupiter Radio emission simulation; iono-sphere characterization, active and / or passive radar sounding ac-tivities opportunities can benefit from an accurate simulation ofJupiter radio emission.Since the ExPRES Tool can provide as by-product the po-sitions of the radio sources as seen from the spacecraft for arange of frequencies (i.e., 1-40 MHz), the Juice Science Oper-ation Centre (JUICE SOC) has implemented a standalone tool,which wraps the ExPRES tool and identify science opportunitywindows based on the radio source position.This can be used to identify opportunity windows for oneof the high priority science objectives of the Juice RPWI instru-ment, the icy moon ionosphere characterization through iono-sphere refraction / distortion measurement.The opportunity periods currently generated to drive the cor-responding RPWI measurements are linked to the ingress andegress occultation events of the Jupiter Radio Sources, where atleast one Jupiter radio sources reach Juice spacecraft with a fre-quency between 0.1-5 MHz crossing the moon ionosphere (withthickness 0-100km).In the figure 12, the Jupiter auroral radio source as seen fromJUICE during 21C13 (the last Callisto flyby of the tour) as afunction of frequency (MHz) and UTC time is displayed: the reddots correspond to opportunity for ionosphere characterization,i.e., whenever one of the radio source types (A, B, C or D) is seenby JUICE with a line of sight passing through the ionosphere within 0-100 km. This section is using the CReMA 3.0 trajectoryscenario.Figure 13 shows the resulting ionosphere characterizationopportunities as a function of the Juice spacecraft altitude in km(green background). There are a few gaps within the ingress andegress ionosphere characterization opportunities. Those gaps of1-2 minutes are ignored to compute the final iono_ingress and iono_egress envelops used for the observations planning. Ta-ble 2 lists the RPWI in-situ and radio measurement sequence forthe 21C13 scenario based on the Closest Approach (CA), andthe ingress and egress windows envelope for ionosphere charac-terization.Finally figure 14 is a screenshot of this measurement se-quences as shown by the JUICE SOC Mapping and PlanningPayload Science software (MAPPS), used to simulate the space-craft and payload resources status (i.e., power, data rate, on-board solid-stat mass memory (SSMM)). The RPWI opera-tions are planned around the CA and around ingress and egressionosphere characterization opportunities as described in Table2. The high-resolution in-situ measurement mode is scheduled +/ - 10 minutes around the closest approach (CA), while thehigh resolution radio measurement mode is scheduled duringthe ionosphere characterization opportunities. High-resolutionmodes are the more demanding in term of resources (power, datagenerated, stored in SSMM and to be downloaded (data rate))and there are reserved for priority scientific objectives. So it iscrucial to be able to calculate the corresponding opportunitieswindows.The JUICE mission is in development phase, with a plannedlaunch in June 2022. At this stage the JUICE mission scienceplanning process is being exercised by analyzing representativescience scenarios similar to 21C13. In this study, the scenarioanalysis covers ±
12 hours around the Callisto closest approach.The JUICE SOC is identifying the same type of opportunitywindows whenever a new candidate trajectory is available forJUICE.The ExPRES tool simulation results can also be useful forother measurement types, and will be made available for byJUICE SOC instrument teams. This includes the measurementlinked to the icy shell characterization of the icy moons: – Passive radar measurement by the radar (RIME instrument)or by RPWI: Opportunities can be identified whenever any
Article number, page 8 of 12. Cecconi et al.: Auroral Radio Source Occultation Modeling
Fig. 12. dots represent the Jupiter auroral radio source state is a function of frequency (MHz) and UTC time for the last Callisto flyby (21C13).Red dots means that at least one of the source type (A, B, C and D) is visible from JUICE and that the line of sight between the source andthe spacecraft goes through the ionosphere within 0-100 km altitude above Callisto surface. The blue, orange and green dots mean that no radiosource is visible by JUICE at those frequency ranges. Here "No Radio Source" means that there is no emission from the corresponding source;Jupiter ionosphere does not generate emissions at high frequencies while emissions at low frequencies are too weak, and therefore do not produceemissions.
Fig. 13.
Juice spacecraft distance to the Callisto moon surface in km for the 21C13 Flyby (blue line); the Callisto surface (resp. the 1000km altitudeabove surface) is represented by the red dashed line (resp. green dashed line), and the ionosphere characterization opportunities windows are filledin greenish background. radio source is visible from the spacecraft for any source type(ABCD), and per source type (to di ff erentiate source fromnorth and south hemisphere) between 1 and 40 MHz (al-though lower frequencies are better since they have a deeperpenetration into the ice); – Active Radar measurement by the radar (RIME instrument):when the spacecraft is protected from Jupiter radio emission due to moon occultation for sources with frequency between9 and 11 MHz (i.e., flybys and Ganymede phase) and whenthe spacecraft is within the RIME instrument operating range(altitude < Article number, page 9 of 12 & A proofs: manuscript no. juice-expres
Data time Event Relative Time in-situ radio − slow full − normal full − burst full + normal full + normal burst + normal full + normal burst + normal full + slow full − slow full Table 2.
RPWI in-situ and radio observations mode sequence during the 21C13 Callisto flyby scenario. Some mode changes are scheduled w.r.t.Closest Approach event (CA), while ionosphere characterization observations are scheduled w.r.t ingress and egress events as identified usingExpress.
Fig. 14.
This picture is a screenshot of this measurement sequences as shown by the JUICE SOC MAPPS tool, which allows to simulate thespacecraft and science instrument resources status (i.e., power, data rate, on-board memory). The top line is colour coded to reflect the di ff erentmodes used by RPWI during the sequence. Instrument resources are displayed in the bottom plots.
7. Conclusions and Perspectives
The Galileo radio occultations observed on the PWS data setare well modeled by ExPRES simulation, with a accuracy ofthe order of one minute. Discrepancies between predicted andobserved ingress or egress times can be attributed to refractiona ff ects, which are not included in the current modeling scheme.The validation on GLL / PWS data allows us to apply the samemodeling to the JUICE mission planning, in order to support thescientific segmentation of the Galilean moon flybys.On a technical point of view, the JUICE modeling have beendone running ExPRES through the PADC operated UWS inter-face based on OPUS. This framework is fully adapted to us-age presented in this study. Future developments of the ExPREScode and its implementation at PADC will include better man-agement of Provenance metadata (Servillat et al. 2020, 2021b),to enhance the scientific traceability and reproducibility of theresults.Several means of refining the occultation modeling havebeen identified. The main one is involving ray-tracing in the Jo-vian system (Io Torus) and the moon’s environment. This extramodeling step requires models of the magnetic field and plasmadensity environments in the vicinity of the studied moon, includ-ing the magnetospheric and moon contributions. A second orderimprovement may also be provided by the use of a more accu-rate magnetospheric current sheet model, which will refine thelocation of the radio sources on the low frequency end.
Acknowledgments
The authors acknowledge support from Observatoire de Paris–PSL, CNRS, CNES and ESA for funding the research. The au-thors also acknowledge support from the Europlanet 2024 Re-search Infrastructure (EPN2024RI) project, which has receivedfunding from the European Union’s Horizon 2020 research andinnovation programme under grant agreement No 871149). Theywant to emphasize the use of community developed tools andstandards, which greatly facilitated this study (such as Autoplot,Das2, OPUS, UWS and Cosmographia and WebGeoCalc). Theythank PADC for providing computing and storage resources.They also thank Marc Costa (from the NAIF SPICE team atNASA / JPL) for his very helpful feedback on configuring andusing Cosomographia.
Article number, page 10 of 12. Cecconi et al.: Auroral Radio Source Occultation Modeling
Appendix A: Equatorial Shadow Zone
In the equatorial region, in the innermost magnetosphere, the au-roral radio sources are not visible, due to combination of theshape of the radio emission beaming patterns and the topologyof the magnetic field lines bearing the radio source. This e ff ectis named “Equatorial Shadow Zone” (ESZ). This e ff ect has beenidentified at Saturn (Lamy et al. 2008), using a preliminary ver-sion of the ExPRES code. It has also been observed at Earth (see,e.g., Morioka et al. 2011, Figure 1), but not explicitly described.Our simulation runs show that some of the Jovian auroralradio sources are not always visible at the distance of Io’s or-bit. Figure A.1 shows the observability for each auroral radiosource for an observer located on Io’s orbit, during one rotationof Jupiter. The simulation shows that the Southern radio sources(namely, the C and D source) are not visible in the CML range180 to 210 degrees. The Northern radio sources (namely, the Aand B source) are observable from all CML, with a drasticallyreduced spectral range at a CML of about 25 degrees. Since theNorthern and Southern ESZs do not occur at the same time, anobserver will not experience a full dropout of radio signals, con-trarily to what is observed at Saturn. Fig. A.1.
Jovian auroral radio source observability from Io’s orbit.
Appendix B: Supplementary Material Description
The material used to conduct the Galileo flybys’ study are pro-vided as a separate data collection (Cecconi et al. 2021, avail-able at https://doi.org/10.25935/8zff-nx36 ) hosted byPADC (Paris Astronomical Data Centre). For each flyby, twosets of material is available: (a) the ExPRES products; (b) theCosmographia context products. The content of each subsectionis described below.
Appendix B.1: ExPRES products
The ExPRES data set is composed of four files: (a) an Ex-PRES configuration file (JSON format), (b) a Galileo spacecraft ephemeris data exported from WebGeoCalc (CSV format), (c)the output ExpRES simulation run data (CDF format), and (d)the observed GLL / PWS data with the superimposed occultationcontours (PNG format). Files (a) and (b) contains the ExPRESparameters and the SPICE Kernels used for the simulation, en-suring the results are reproducible.
Appendix B.2: Cosmographia context products
The Cosmographia data set is composed of a series of subdirec-tories, organised according to Cosmographia’s documentation. Itcontains all the required configuration catalogue files: the SPICEcatalogue file listing the kernels in use for a scene; the space-craft catalogue file defining the time interval of the scene and thespacecraft reference frame; the modeled radio source cataloguefile, derived from an ExPRES simulation run; the ExPRES con-figuration file and simulation run; and the scripts used to producethe movie output. Two output movies are provided, showing theflyby scenes, as seen from the spacecraft (‘pov’ labelled movie)and from the top of the Jovian system (‘top’ labelled movie).
References
Acton, C., Bachman, N., Semenov, B., & Wright, E. 2018, Planetary and SpaceScience, 150, 9Acton, C. H. 1996, Planet. Space Sci., 44, 65Bruzzone, L., Plaut, J. J., Alberti, G., et al. 2013, 2013 IEEE International Geo-science and Remote Sensing Symposium - IGARSS, 3907Burke, B. & Franklin, K. 1955, J. Geophys. Res., 60, 213Cecconi, B. 2019, Workflow studies: Magnetospheres science and support toESA / JUICE mission planningCecconi, B., Hess, S., Hérique, A., et al. 2012, Planet. Space Sci., 61, 32Cecconi, B., Loh, A., Le Sidaner, P., et al. 2020, Data Science Journal, 19, 1062Cecconi, B., Louis, C., Muñoz Crego, C., & Vallat, C. 2021, Auroral RadioSource Occultation Modeling and Application to the JUICE Science MissionPlanning. Supplementary Material: Galileo FlybysCecconi, B. & Zarka, P. 2019, Cassini RPWS Jupiter Encounter CalibratedDatasetConnerney, J. E. P., Acuna, M. H., & Ness, N. F. 1981, Journal of GeophysicsResearch, 86, 8370Connerney, J. E. P., Kotsiaros, S., Oliversen, R. J., et al. 2018, Geophys. Res.Lett., 45, 2590ESA SPICE Service. 2020, JUICE SPICE Kernel DatasetEviatar, A., Vasyliunas, V. M., & Gurnett, D. 2001, Planet. Space Sci., 49, 327Gautier, A.-L., Cecconi, B., & Zarka, P. 2013, Proceedings of the 2013 Interna-tional Symposium on Electromagnetic Theory, 1Grodent, D. 2015, Space Sci. Rev., 187, 23Gurnett, D., Kurth, W. S., Menietti, J., & Persoon, A. 1998, Geophys. Res. Lett.,25, 1841Gurnett, D., Kurth, W. S., Shaw, R., et al. 1992, Space Sci. Rev., 60, 341Gurnett, D. A., Kurth, W. S., & Granroth, L. J. 1997, NASA PlanetaryData System, GO-J-PWS-2-REDR-LPW-SA-FULL-V1.0 [ https://doi.org/10.17189/1519681 ]Gurnett, D. A., Kurth, W. S., Roux, A., Bolton, S. J., & Kennel, C. F. 1996,Nature, 384, 535Harrison, P. A. & Rixon, G. 2016, Universal Worker Service Pattern Version 1.1,IVOA Recommendation 24 October 2016Hess, S. L. G., Echer, E., & Zarka, P. 2012, Planetary and Space Science, 70, 114Kliore, A. J. 1998, Highlights of Astronomy, 11B, 1065Kumamoto, A., Kasaba, Y., Tsuchiya, F., et al. 2017, in Planetary Radio Emis-sions VIII, ed. G. Fischer, G. Mann, M. Panchenko, & P. Zarka, 127–136Kurth, W. S., Bolton, S. J., Gurnett, D. A., & Levin, S. 1997, Geophys. Res. Lett.,24, 1171Lamy, L., Zarka, P., Cecconi, B., Hess, S., & Prangé, R. 2008, J. Geophys. Res.,113Lamy, L., Zarka, P., Cecconi, B., et al. 2017, in Planetary Radio Emissions VIII,ed. G. Fischer, G. Mann, M. Panchenko, & P. Zarka, 455–466Louarn, P., Allegrini, F., McComas, D. J., et al. 2017, Geophys. Res. Lett., 44,4439Louarn, P., Perraut, S., Roux, A., et al. 1997, Geophys. Res. Lett., 24, 2115Louis, C. K., Hess, S. L. G., Cecconi, B., et al. 2019, Astronomy and Astro-physics, 627, A30
Article number, page 11 of 12 & A proofs: manuscript no. juice-expres
Louis, C. K., Hess, S. L. G., Cecconi, B., et al. 2020, maserlib / ExPRES: Version1.1.0, This work has also been supported by the EPN-2024-RI (Europlanet2024 Research Infrastructure) project, under contract number 871149 withthe EC.Marques, M. S., Zarka, P., Echer, E., et al. 2017, Astronomy & Astrophysics,604, A17Menietti, J., Gurnett, D., Hospodarsky, G., et al. 2003, Planet. Space Sci., 51,533Morioka, A., Miyoshi, Y., Tsuchiya, F., et al. 2011, Journal of Geophysical Re-search Space Physics, 116, A04211 (15 pages)Piker, C., Granroth, L., Mukherjee, J., et al. 2019, Earth and Space Science OpenArchive [ https://doi.org/10.1002/essoar.10500359.1 ]Romero-Wolf, A., Vance, S., Maiwald, F., et al. 2015, Icarus, 248, 463Schroeder, D. M., Romero-Wolf, A., Carrer, L., et al. 2016, Planetary and SpaceScience, 134, 52Servillat, M., Aicardi, S., Cecconi, B., & Mancini, M. 2021a, arXiv e-prints,arXiv:2101.08683Servillat, M., Bonnarel, F., Louys, M., & Sanguillon, M. 2021b, arXiv e-prints,arXiv:2101.08691Servillat, M., Riebe, K., Boisson, C., et al. 2020, IVOA Provenance Data ModelVersion 1.0, IVOA Recommendation 11 April 2020Wahlund, J. E. 2013, in European Planetary Science Congress, EPSC2013–637Witasse, O. 2019, in EPSC-DPS Joint Meeting 2019, Vol. 2019, EPSC–DPS2019–400Zarka, P. 1992, Adv. Space. Res., 12, 99Zarka, P., Cecconi, B., & Kurth, W. S. 2004, J. Geophys. Res., 109, A09S15]Romero-Wolf, A., Vance, S., Maiwald, F., et al. 2015, Icarus, 248, 463Schroeder, D. M., Romero-Wolf, A., Carrer, L., et al. 2016, Planetary and SpaceScience, 134, 52Servillat, M., Aicardi, S., Cecconi, B., & Mancini, M. 2021a, arXiv e-prints,arXiv:2101.08683Servillat, M., Bonnarel, F., Louys, M., & Sanguillon, M. 2021b, arXiv e-prints,arXiv:2101.08691Servillat, M., Riebe, K., Boisson, C., et al. 2020, IVOA Provenance Data ModelVersion 1.0, IVOA Recommendation 11 April 2020Wahlund, J. E. 2013, in European Planetary Science Congress, EPSC2013–637Witasse, O. 2019, in EPSC-DPS Joint Meeting 2019, Vol. 2019, EPSC–DPS2019–400Zarka, P. 1992, Adv. Space. Res., 12, 99Zarka, P., Cecconi, B., & Kurth, W. S. 2004, J. Geophys. Res., 109, A09S15