Juno In Situ Observations Above the JovianEquatorial IonosphereP. W. Valek1 , F. Bagenal2 , R. W. Ebert1,3 , F. Allegrini1,3 , D. J. McComas4 ,J. R. Szalay4 , R. J. Wilson3 , S. J. Bolton1 , and J. E. P. Connerney5,6

1Southwest Research Institute, San Antonio, TX, USA, 2Laboratory for Atmospheric and Space Physics, University ofColorado Boulder, Boulder, CO, USA, 3Department of Physics and Astronomy, University of Texas at San Antonio, SanAntonio, TX, USA, 4Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA, 5Space ResearchCorporation, Annapolis, MD, USA, 6Goddard Space Flight Center, Greenbelt, MD, USA

Abstract The arrival of Juno at Jupiter enables repeated in situ observations above the Jovianionosphere. The low altitude and high velocity of Juno at perijove permits direct sampling of ionosphericion populations. We present the first direct observations above the ionosphere made by the Jovian AuroralDistributions Experiment Ion sensor (JADE‐I). When looking into the spacecraft ram direction, JADE‐Ican measure ion energy distributions to below 1 eV/q along with ion composition. We report observationsfrom 17 Juno perijove passes. At these latitudes, the low energy ions consist of protons and heavier ions,protons being the dominant species. Heavy ions—primarily oxygen and sulfur likely originating from themagnetosphere—are seen each pass, but their intensity varies. Other trace light ions are observed duringsome of the perijoves: H3

+ (6 of 17 perijoves), He+ (2 of 17 perijoves). Ionospheric ions are observed up toaltitudes of ~7,000 km.

Plain Language Summary The high speed of the Juno spacecraft permits its lower energy ionsensor—JADE‐I—to chase down and observe low energy ions that have not been directly observed before.When looking into the direction the spacecraft is moving (i.e., the spacecraft ram direction), this sensor canobserve ionospheric ions that are at a near‐zero velocity in the rest frame of Jupiter. This permits directobservations of the ion population above Jupiter's ionosphere for the first time. Here, we present thoseobservations for 17 close fly‐bys at equatorial latitudes. We observe that the cold ion population contains arange of species. The dominant species is protons. Seen in each pass are also heavier oxygen and sulfur ions,but the intensity of these heavy ions varies from pass to pass. The presence of these heavy ions shows thatthere is some form of coupling between Jupiter and material on magnetic field lines far away from theplanet, but the mechanism is not clear from these observations. Other light ions are also seen for some of thepasses: H3

+ for six of 17 passes, He+ for two of 17 passes.

1. Introduction

Jupiter's ionosphere is expected to be a significant source of protons to Jupiter's magnetosphere (Nagyet al., 1986). Prior to the Juno mission, the Jovian ionosphere was accessible through studying ultravioletand infrared emissions from altitudes below ~1,000 km and as high as 10,000 km via radio occultations byspacecraft (Pioneers, Voyagers, and Galileo) that passed behind the planet (see review by Yelle &Miller, 2004). The radio occultations show remarkable variations between measurements. Yelle andMiller (2004) show seven Voyager and Galileo density profiles where the altitude of the density peak rangesfrom ~600 to ~1,600 km, and the peak values vary between 2 × 1010–3 × 1011 m−3 (20,000 to 300,000 cm−3).Some of the profiles show sharp variations (up to two orders of magnitude) within altitude changes of<100 km. None of the profiles are in the auroral region where the ionospheric densities are expected toincrease (Bougher et al., 2005; Egert et al., 2017). Models of the upper atmosphere and ionosphere tend tofocus on either heating or radiative processes rather than trying to match ionospheric profiles (again,reviewed by Yelle & Miller, 2004). Models of magnetosphere‐ionosphere‐thermosphere coupling tend tosimplify the ionosphere as a thin conducting layer and vary the net conductance with latitude and local time,most recently in a 3‐D circulation model by Yates et al. (2020) and coupled to auroral dynamics by Tao,Kimura, Badman, André, et al. (2016), Tao, Kimura, Badman, Murakami, et al. (2016). With the limited©2020. American Geophysical Union.

All Rights Reserved.

RESEARCH LETTER10.1029/2020GL087623

Key Points:• First direct, in situ ion observations

above Jupiter's equatorialionosphere

• Protons and heavy, magnetosphericions are detected above theionosphere during all 17 Junoperijove passes studied here

• H3+ observed during six, and He+ on

two, of 17 Juno perijove passes

Correspondence to:P. W. Valek,pvalek@swri.edu

Citation:Valek, P. W., Bagenal, F., Ebert, R. W.,Allegrini, F., McComas, D. J., Szalay, J.R., et al. (2020). Juno in situobservations above the Jovianequatorial ionosphere. GeophysicalResearch Letters, 47, e2020GL087623.https://doi.org/10.1029/2020GL087623

Received 20 FEB 2020Accepted 8 MAY 2020Accepted article online 19 MAY 2020

VALEK ET AL. 1 of 8

information on Jupiter's ionosphere, we turn to in situ measurements by Juno (Bolton et al., 2017) aroundthe spacecraft's closest approach to the planet to address some of the open questions on ionospheric structureand variability.

The Juno spacecraft permits repeated, in situ measurements above Jupiter's ionosphere. In a study by Valeket al. (2019), observations of the high latitude ionosphere at altitudes >0.5 RJ (~36,000 km, where RJ is theJovian equatorial radius of 71,492 km) above the 1 bar level were presented. At magnetic latitudes betweenthe aurora oval and Io foot point, cold ionospheric protons were seen coincident with precipitating magneto-spheric ions. The energy distribution was used to differentiate between the ionospheric and magnetosphericions. Magnetospheric ions are warmer, generally at higher energy than the ionospheric ions, and have amagnetic latitudinal dependence as described by Szalay et al. (2017). Ionospheric ions are much colderand have energies of a few eV or less. The cold ionospheric ions consist of only protons at these high lati-tudes, but the magnetospheric ions contain protons and heavy ions, including oxygen and sulfur.Observation of a loss cone in the magnetospheric particle distribution suggests that these precipitating ionsheat the upper ionosphere and raise it to heights ~0.5 RJ (~36,000 km) above the clouds.

In this study, we present observations of the equatorial ionosphere, focusing on ion populations from twoperijoves, including their energy distribution and ion composition. In contrast to the high‐latitude,high‐altitude ionosphere which consisted only of protons, the low‐altitude equatorial ionosphere consistsof a range of species. We summarize the composition distribution above the equatorial ionosphere and showthe spatial distribution of the protons, the dominant species.

2. Data

The Juno spacecraft orbits Jupiter with a highly eccentric 53‐day trajectory that permits direct samplingabove the equatorial ionosphere (Bolton et al., 2017). At planetocentric latitudes between approximately50° and−30°, Juno is at altitudes from 3,000 to 7,500 km above the 1 bar level of Jupiter's upper atmosphere.Juno's perijoves occur at local times in the post noon sector, with the first perijove (PJ1) occurring at dusk(1,800) and move noonward at approximately a quarter hour of local time per orbit. Orbits discussed hererange from ~1630 (PJ6) to ~noon (PJ23). The spacecraft travels at speeds near 60 km/s at perijove, (i.e., itsclosest approach point). At 60 km/s, a proton and oxygen ions have energies of 19 and 300 eV, respectively.The high speed and low altitude of Juno at its perijove permits direct sampling of the upper, equatorialionosphere.

The Juno‐JADE instrument has been taking data at Jupiter's equator since Perijove 6 (2017 DOY 139). Lowenergy ions are observed with the Jovian Auroral Distributions Experiment Ion (JADE‐I) sensor (McComaset al., 2017). JADE‐I uses an electrostatic analyzer (ESA) to measure ions in an energy range of ~13 eV/q to46 keV/q in the spacecraft frame, with a ΔE/E of ~0.28 for the lowest energies. With such high spacecraftspeed, JADE‐I can observe (when looking in the ram direction) ions with sub‐eV energies in Jupiter's restframe. Because of the high ram velocity, the few eV ions are shifted up to energies on the order of 25 eVin the spacecraft frame, effectively decreasing the energy resolution so the shifted ΔE/E is ~1. The instanta-neous field of view (FOV) covers a 270° fan divided across 12 anodes (22.5° = 270°/12) by 9°. This FOV fanincludes the spin axis, so the full sky is observed every spacecraft spin (~30 s). See Valek et al. (2019) forexample observations of low energy ions of the high latitude ionosphere.

JADE‐I uses a time of flight (TOF) section to separate incident ions by their mass per charge (M/q). JADE‐Ihas sufficient mass resolution to differentiate between the different light ions (H+, H2

+/He++, H3+, and

He+) and has a sufficient mass range to measure ions to 64 amu/q. Ions pass through an ultra‐thin carbonfoil (Allegrini et al., 2016) when entering the TOF section, which liberates secondary electrons used forthe timing start signal.

A secondary result of passing through the carbon foil is that the charge state of the incident ions is rando-mized, though the distribution is well characterized (e.g., Funsten et al., 1993). Since the JADE‐I TOF sectionis not a field‐free region, the different exit charge states produce multiple peaks in the TOF spectra for someions. This results in an overlap between the incident H+ and H2

+ signature. The relative amplitudes of theprimary and secondary peaks are used to separate H+ from H2

+, but in this paper, we do not differentiatethese ions, but rather group both as “protons.” However, significant H2

+ is not expected at these altitudes

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when we compare to similar observations at Saturn. Waite et al. (2018) found that H+ was more than 1,000×more abundant than H2

+ during the Cassini Grand Finale. We group all ions with M/q ≥ 7 as heavy ions. Adetailed description of the TOF section and how secondary foil effects are used to extend the mass resolutionis given in Kim et al. (2019, 2020).

3. Observations

Figure 1 shows two example passes above Jupiter's equatorial ionosphere. Trajectories of Juno Perijoves 7(2017 DOY 192) and 8 (2017 DOY 244) are shown on the left of Figure 1 in red and blue, respectively. Thetrajectories—as viewed from the sunward direction—are nearly identical in latitude and altitude, differingprimarily in longitude. The thick lines indicate the portions of the orbits where we observe the equatorialionosphere. They are offset slightly to make viewing easier. For these passes, the minimum altitude is~3,500 km, and the peak spacecraft velocity is 58.1 km/s.

In the red (blue) border around the plots to the right hand are the observations from Perijove 7 (8). Energy—time spectrograms include the JADE‐I data from the period indicated by the thick lines on the left. All ionsspecies are plotted as a function of the energy in the spacecraft frame. The lowest energy ions (tens of eV inthe spacecraft frame) are ionospheric protons. At higher energies (~300 eV/q in the spacecraft frame) areheavy ions (M ≥ 16 amu/q), primarily oxygen and sulfur. Accounting for the spacecraft ram velocity, bothof these populations have energies well below 1 eV. The highest energy ions (> 104 eV/q) are the low energytail of the energetic ion population discussed by Kollmann et al. (2017).

The periodic nature of the ion signal results from the spacecraft 30‐s spin. Ions are observed when the FOV ofJADE‐I is looking into the spacecraft ram direction. At perijove, the spacecraft velocity vector is nominallyorthogonal to the spacecraft spin axis. The spin effect is most obvious in the heavy ions. The heavier ions areonly seen during one energy sweep and angular bin of the instrument. The cold ionosphere cannot beresolved by the higher energy channels (i.e., unable to resolve the spatial extent of the distribution). Thelighter ions are seen in a large fraction of the spin near perijove. We believe that this is in part due to space-craft charging to a few −10's of volts, attracting the lighter species.

In each energy‐time spectrogram, there is a white box that indicates the data included in the energy‐TOFplots (Figure 2). The energy‐TOF plots show ion counts as a function of energy in the spacecraft frameand their TOF in the TOF section. The thick white lines indicate where mass per charge (M/q) of 1, 16,and 32 amu/q (H+, O+ or S++, and S+) will be observed. The dashed line indicates where ions traveling atthe same velocity as the peak of the protons distributions are expected. These data are summed over a fullspin and all look directions. Because the spacecraft velocity corrections are not included, there isspreading/smearing in energy per charge.

A strong proton signature is seen at all times above the ionosphere. In addition to the peak, there is a tail thatextends to longer times of flight. This long tail can obscure the signal from heavier ions observed at the sameenergy per charge. For Perijove 7 (inside the red box), there is a clear signature of protons at tens of eV/q andheavy ions at ~500 eV/q. The TOF signature indicates the heavy ions include O and S ions. For Perijove 8(blue box), there is again a strong proton signature. Heavy ions with energies of ~500 eV/q are again present,but at a much lower intensity than the previous orbit. There also appear to be heavy ions at M/q of 16 amu/qat lower energies, but their signal is partially obscured by the long tail of the protons. Close examination ofthe short times of flight (insert) shows a peak at M/q of 3 amu/q (i.e., H3

+).

Including the two perijoves shown in Figure 1, we have examined 17 orbits to understand how the composi-tion varies globally. This includes Perijoves 6 through 23. JADE was not operated in the equatorial regionprior to Perijove 6 to limit potentially excessive counts to the detectors from large particle fluxes. JADEwas not operated during Perijove 19 due to spacecraft maneuvers. Each pass shows a variety of masses, simi-lar to those shown in Figure 1.

Every perijove pass revealed a strong proton signature and cold, heavy ions, though the intensity of theheavy ions can vary dramatically from pass to pass. For six of the passes, direct observations of H3

+ are seen.Two of the passes show an indication of M/q of 4 amu/q (He+). Two other passes include observations ofheavy ions with M/q _x0003C; 16 amu/q, but further analysis is required. This rich ion composition envir-onment is different from the high latitude, higher altitude observations ionosphere reported by (Valek

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et al., 2019), where only protons were observed. A summary of the observations in the equatorial ionosphereis given in Table 1.

The 17 perijoves included in this study provide excellent spatial coverage above the equatorial ionosphere.As the spacecraft travels north to south, it maps the ionosphere with a longitudinal resolution of ~20°. Asummary of the spatial distribution of the equatorial ionospheric protons as measured by JADE‐I is shownin Figure 3. The proton distribution is shown as functions of radial distance, latitude, and altitude. Since theionospheric ions are such a cold population, JADE‐I does not always resolve the energy distribution. Thevalues of the flux are a lower bound of the actual flux. Figures 3a to 3c show proton flux vs. latitude and long-itude, vs. radial distance and latitude, and vs. altitude and longitude, respectively.

Figure 3a shows the distribution of the proton flux in latitude and longitude. Only times when the Juno isbelow 1.25 RJ (~89,360 km) Jovicentric distance are plotted. It is important to note that all closest approachpoints occur in the northern hemisphere, resulting in a sampling bias for northern latitudes. Juno has lim-ited in situ observations of the equatorial ionosphere at southern latitudes.

The distribution as a function of altitude and latitude is shown in Figure 3b. The ionospheric protons areobserved beginning at altitudes of ~0.1 RJ. The proton flux increases with decreasing altitude. This is

Figure 1. Summary of equatorial observations for Perijove 7 (2017DOY192; red) and Perijove 8 (2017DOY244; blue). The Juno trajectories are shown as viewedfrom the sunward direction in the left‐hand panel. The thick lines indicate the periods shown in the accompanied energy‐time spectrograms. All ions are includedin the energy‐time spectrograms, and the energy is in the spacecraft frame. Ions observed in the white boxes are shown in the E/q‐time of flight panels (Figure 2).The spacecraft position is given in planetocentric latitude and radial distance in RJ (equatorial 1 bar radius = 71,492 km).

Figure 2. Energy TOF spectrograms. Each TOF spectrogram includes data from the period highlighted in Figure 1. Perijove 7 (8) is shown in panel a (b). Theenergy is in the spacecraft frame and includes all look directions. The thick white lines indicate the locations of the principal peak for M/q of 1, 16, and32 amu/q. The dashed line is the energy for ions moving at the ram velocity.

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consistent with occultation observations that indicate the ionospheric proton density peaks below 2,500 km(Yelle & Miller, 2004). Above an altitude of ~7,000 km (1.1 RJ Jovicentric), the proton flux drops to near thedetection levels. Above an altitude of ~15,000 km (1.2 RJ Jovicentric), the enhanced count rates are due pri-marily to penetrating radiation.

In Figure 3c, the proton flux rates are shown as a function of radial distance and longitude. The fluxes do notdepend strongly on longitude. In addition to the proton fluxes, the perijoves where H3

+ and He+ are seen are

Table 1Summary of Perijove Observations

PJ#Date/time of perijove yyyy‐ddd/hh:

MmLata

[Deg.]Lon[Deg.]

Localtime

Radial distance[RJ]

Altb

[km] H+cHeavyionsc

Other lightions

PJ6 2017‐139/06:00 8.5 140 1633 1.047 3,581 Yes Yes H3+

PJ7 2017‐192/01:54 9.5 49 1617 1.047 3,578 Yes Yes He+

PJ8 2017‐244/21:49 10.4 318 1601 1.047 3,488 Yes Yes H3+

PJ9 2017‐297/17:43 11.3 228 1546 1.054 4,005 Yes YesPJ10 2017‐350/17:57 12.1 295 1530 1.057 4,317 Yes Yes H3

+

PJ11 2018‐038/13:51 13.1 205 1515 1.045 3,560 Yes YesPJ12 2018‐091/09:46 13.9 114 1459 1.045 3,470 Yes Yes H3

+

PJ13 2018‐144/05:40 14.8 24 1444 1.044 3,481 Yes Yes H3+

PJ14 2018‐197/05:17 15.7 68 1429 1.044 3,549 Yes Yes He+

PJ15 2018‐250/01:40 16.6 338 1414 1.043 3,523 Yes Yes H3+

PJ16 2018‐302/21:06 17.4 247 1359 1.043 3,498 Yes YesPJ17 2018‐355/17:00 18.2 156 1344 1.064 4,984 Yes YesPJ18 2019‐043/17:35 18.9 234 1329 1.041 3,438 Yes YesPJ20 2019‐149/08:08 20.4 9 1259 1.093 7,287 Yes YesPJ21 2019‐202/04:03 21.0 278 1243 1.103 7,936 Yes YesPJ22 2019‐255/03:41 21.6 322 1228 1.102 7,935 Yes YesPJ23 2019‐307/22:18 22.4 186 1211 1.039 3,541 Yes Yes

Note. JADE began acquiring equatorial data after Perijove 6. No data was taken during Perijove 19 while Juno was in an orientation to support other instrumentscience. Summary of Perijove Observations when the spacecraft is at its minimum altitude.a

Planetocentric latitude.b

Altitude calculated above the 1‐bar level and includes the planet's oblateness.c

Heavy ions have M/q ≥ 7 amu/q.d

Proton signalmay include H2

+.

Figure 3. Summary of the spatial distribution of the proton flux in the near equatorial region of Jupiter's ionopshere. Theprotons are shown as a function of System III latitude and longitude in (a). The distribution in altitude and latitude isshown in (b). Panel (c) shows the distribution in altitude and longitude. In panel (d), the distribution is as a function ofaltitude and local time. The apparent northward shift in the proton fluxes in (a) is from Juno's perijove being northwardof the equator. Because the ions are cold, the flux estimates are lower bounds of the actual values.

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indicated. It is curious to note that we do not see a clear signature of these light ions between the System IIIwest longitudes ~150° and 280°. Heavy ions are observed at every pass, but their relative intensity varies.

4. Discussion

We present the first in situ observations above Jupiter's equatorial ionosphere where low‐energy, cold ionsare observed. Protons and heavy ions are observed every pass, with the protons being the dominant species.The ratio of heavy to light ions varies with orbit. On a subset of orbits, we directly observe light ions otherthan protons. Specifically, of the 17 passes considered here, H3

+ is seen six times and He+ twice. The H3+

and He+ ions are not observed simultaneously in the orbits included here. It is noted that the strong protonsignal can swamp the signature of ions at the same energy, so further study of the composition will beconducted.

In a previous study, we presented observations of the high latitude ionosphere (Valek et al., 2019). The mag-netospheric ions precipitate into the high‐latitude, upper atmosphere, raising the scale height of the iono-sphere. Ionospheric protons were seen at Jovicentric distances of up to 1.8 RJ (~128,000 km altitude). Thehigh latitude ionosphere is only seen at the locations where the precipitating magnetospheric ions areobserved, an unambiguous example of magnetosphere‐ionosphere (MI) coupling.

For the equatorial ionosphere, cold, heavy magnetospheric ions (O and S) are observed mixed with thelighter ions originating from Jupiter's upper atmosphere. This is again evidence of MI coupling, but themechanism is not as clear. We do not observe the magnetospheric ions precipitating, so it is not knownhow the heavy ions get into the equatorial ionosphere. Possible mechanisms include the following:

1. Energetic heavy neutrals from the magnetosphere bombard the upper atmosphere where they undergocharge exchange in a similar process as described by Krupp et al. (2018) at Saturn. The newly createdheavy ions are then picked up and become part of the ionosphere.

2. Magnetospheric ions that precipitated into the upper atmosphere at high latitudes migrate equatorially.

The first mechanism involves energetic neutral atoms (ENAs) that are produced in the magnetosphere viacharge exchange of charged particles with neutrals near Io or Europa. When these ENAs hit Jupiter's atmo-sphere, they are reionized and become part of the ionosphere. Such populations produced by this “doublecharge exchange” are observed at Mars (Halekas et al., 2015; Ramstad et al., 2015). At Jupiter, we know thatsodium ENAs are produced at Io (Mendillo et al., 1990, 2007), and higher fluxes of S and O ENAs are likelyalso produced near Io but hard to detect (see review by Bagenal & Dols 2020). Corotating torus ions that areneutralized via charge exchange with Io's neutral clouds will come off as an outward stream (forming theMendillo‐disk that extends to ~500 RJ). Ions that charge exchange close to Io (e.g., in theplasma‐atmosphere interaction) could come off as a jet or spray directed towards Jupiter's equatorial regionwhere reionization on impacting the planet's atmosphere would be a source of cold, ionospheric heavy ionsnear the planet. Alternatively, inward‐moving energetic particles charge exchange with Europa's neutralhydrogen cloud which produces more uniform, spherical cloud (Lagg et al., 2003; Mauk et al., 2003) thatwould hit Jupiter's atmosphere more uniformly.

The secondmechanism could result frommeridional flows in the upper atmosphere carrying heavy ions thatprecipitate from the magnetosphere at high latitudes (as suggested from Juno data by Valek et al., 2019).Global circulation models show that substantial heating of the auroral region upper atmosphere can drivesuch flows from high to low latitudes (Tao et al., 2009; Yates et al., 2020). These models are at altitudes belowthe JADEmeasurements and do not include the heavy ions. This second potential mechanismwould requirethe heavy ions to remain at high altitudes during their meridional flow.

Trace light ions such as H3+ and He+ are observed on a fraction of the passes. Light ions are known to ori-

ginate in the Jovian upper atmosphere (Stallard et al., 2018; von Zahn et al., 1998). We are uncertain as towhy we do not see them during every perijove pass. For Perijoves 20, 21, and 22, the spacecraft stays above6,500 km, so nondetection may indicate that Juno is above and altitude where the density of these trace lightion species is too low to be measured by JADE‐I. The other passes where the light ions are not seen occur atperijoves with longitudes between ~150° and 250° (Figure 2c). For these longitudes, the magnetic equator isin the southern hemisphere (<−15°, Connerney et al., 2018). Since the Juno perijoves occur in the northern

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hemisphere, perhaps, a bias in the coverage prevents us from observing light ions at these longitudes. Lightions are observed for local times after 1,400 (Figure 2d), indicating a potential local time bias. However, thereare passes where the light ions are absent at low altitudes, notably Perijoves 9 and 11.

With further Juno measurements, it will be interesting to compare the JADE in situ measurements of H3+

with the map of IR emissions reported by Stallard et al. (2018). They identified a dark ribbon of low H3+

intensity located near the magnetic equator. The perijoves where we do not observe H3+ or He+ are at long-

itudes where Stallard et al. (2018) report the ribbon to be significantly southward of 0° latitude. We wish tofurther investigate these intriguing differences between the low altitude remote observations and the in situobservations as Juno makes further observations, including covering more of local times.

5. Conclusions

Data from the Juno‐JADE‐I sensor shows fluxes of cold ions, both light (M/q ≤ 4) and heavy (M/q > 7) spe-cies, in the upper ionosphere near Jupiter's equator. Our main conclusions are as follows:

1. Juno‐JADE‐I has made the first direct, in situ ion observations of Jupiter's equatorial ionosphere. Theseionospheric ions were detected on 17 perijove passes to date at altitudes down to ≈3,000 km.

2. Heavy ions of O and S, presumably of magnetospheric origin, are detected in the ionosphere, with theirrelative concentration varying over the different perijoves.

3. Out of 17 perijove passes, H3+ ions were observed during six, and He+ on two. Further passes through the

ionosphere will map out the distribution of these minor ions.4. The equatorial ionosphere is dramatically different from the high latitude ionosphere. At the equator, the

ions are confined to lower altitudes indicating a lower temperatures and a steeper density profile.

Juno has executed 25 out of 34 orbits in its prime mission. Future studies will further explore how the iono-spheric ions reported here are distributed in latitude, longitude, and altitude. It will be interesting to com-pare these in situ measurements with ground‐based observations of H3

+ such as reported by Stallardet al. (2018). The Juno perijove will continue to move northward and to lower local times. At the end ofthe prime mission (PJ33) Juno's perijove will be at planetocentric latitude ~30° and local time ~0900.During an extended mission, Juno's perijove will continue to move northward and to lower local times,crossing the dawn terminator into the nightside at PJ50. An extended Juno mission would provide severalradio occultations as well as in situ measurements to higher latitudes.

Data Availability Statement

The JNO‐J/SW‐JAD‐3‐CALIBRATED‐V1.0 data set, version 02 files, were obtained from the Planetary DataSystem (PDS; https://pds.nasa.gov/).

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AcknowledgmentsThis work was supported as a part of thework on NASA's Juno mission. Wethank the many individuals whocontinue to make the Juno missionsuch a success.

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