Geophysical Research Letters

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1 RESEARCH LETTER Special Section: Early Results: Juno at Jupiter Key Points: There is a radiation belt within Jupiter s rings This innermost belt consists of ion masses up to sulfur but very few electrons Energetic neutral atoms may be the source of this innermost belt Supporting Information: Supporting Information S1 Correspondence to: P. Kollmann, Peter.Kollmann@jhuapl.edu Citation: Kollmann, P., et al. (2017), A heavy ion and proton radiation belt inside of Jupiter s rings, Geophys. Res. Lett., 44, , doi:. Received 7 DEC 2016 Accepted 15 MAY 2017 Accepted article online 25 MAY 2017 Published online 3 JUN 2017 A heavy ion and proton radiation belt inside of Jupiter s rings P. Kollmann 1, C. Paranicas 1, G. Clark 1, B. H. Mauk 1, D. K. Haggerty 1, A. M. Rymer 1, D. Santos-Costa 2, J. E. P. Connerney 3, F. Allegrini 2,4, P. Valek 2,W.S. Kurth 5, G. R. Gladstone 2, S. Levin 6, and S. Bolton 2 1 Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland, USA, 2 Southwest Research Institute, San Antonio, Texas, USA, 3 NASA Goddard Space Flight Center, Greenbelt, Maryland, USA, 4 Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas, USA, 5 Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA, 6 Jet Propulsion Laboratory, Pasadena, California, USA Abstract Energetic charged particle measurements by the Jupiter Energetic Particle Detector Instrument (JEDI) on board Juno have revealed a radiation belt of hundreds of kev ions up to the atomic mass of sulfur, located between Jupiter s rings and atmosphere. Proton energy spectra display an unusual intensity increase above 300 kev. We suggest that this is because charge exchange in Jupiter s neutral environment does not efficiently remove ions at such high energies. Since this innermost belt includes heavy ions, it cannot be exclusively supplied by cosmic ray albedo neutron decay, which is an important source at Earth and Saturn but only supplies protons and electrons. We find indications that the stripping of energetic neutral atoms in Jupiter s high atmosphere might be the ion source. Since the stripped off electrons are of low energy, this hypothesis is consistent with observations of the ratio of energetic electrons to ions being much less than 1. Plain Language Summary Planets with their own a magnetic field, as Earth and Jupiter, are surrounded by belts of radiation. In case of Jupiter, most of this radiation extends to relatively large distances to Jupiter (several times the size of the planet) but is so strong that it is difficult to engineer satellites that can fly through them safely. Juno is the first spacecraft that repeatedly passes close to Jupiter and its Jupiter Energetic Particle Detector Instrument (JEDI) was now able to measure and quantify a small, separate radiation belt close to Jupiter. The measured population was unknown before and is located between Jupiter s atmosphere and its tenuous rings, inward to the main radiation belt. 1. Introduction Radiation belts are regions within planetary magnetospheres that consist of very energetic charged particle populations. Prior to Juno s arrival at Jupiter, limited particle data were available near Jupiter s rings. The lowest altitude flyby mission was Pioneer 11 that reached a location magnetically connected to 1.6 R J [Van Allen, 1976]. The Galileo orbiter entered Jupiter s atmosphere, but the time resolution of its data was very low. The Galileo Probe made the first really useful measurements well within Jupiter s rings leading to the discovery of a radiation belt of >60 MeV ions and >3 MeV electrons [Fischer et al., 1996]. Juno s orbit with closest approach at 1.06 R J makes it uniquely suited to study the innermost region of Jupiter s magnetosphere. Here we report the first measurements of tens of kev to about 10 MeV ions within a region that magnetically connects near Jupiter s rings and derive previously unknown properties of Jupiter s innermost radiation belt (section 4). We also suggest an explanation for a peculiar ion population measured in this region (section 5.3) The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. 2. Data Set Data presented here were taken on 27 August/DOY 240 (day of year) of 2016, during the first Jupiter approach PJ1 where the Jupiter Energetic Particle Detector Instrument (JEDI) was taking science data. JEDI consists of three sensors referred to as JEDI-90, JEDI-180, and JEDI-270. It is described in Mauk et al. [2013] and we summarize relevant details in supporting information Text S1.1. KOLLMANN ET AL. JUPITER S INNERMOST BELT 5259

2 Data from the magnetometer [Connerney et al., 2017b] enable organization of the data with local pitch angle. Unless stated otherwise, we average over all pitch angles. 3. Regions of the Magnetosphere In Figure 1a, we show an overview of measurements around closest approach. The total particle red and orange curves use JEDI s solid-state-detector-only measurements that combine all ionizing particles above 100 kev and can be expressed as total particles valid ions + electrons (1) Either electrons or ions can dominate this sum, and the dominant species changes for different regions of the magnetosphere. The types and the geometry factors of the solid-state detectors (SSDs) that are used for the red and orange curves are identical. Since half of JEDI-180 s SSDs are identical to JEDI-270 s small ion SSDs [Mauk et al., 2013], we scaled up the rate from JEDI-180 by a factor of 2. Valid ions (shown as a blue curve) counts all ions with energies above 100 kev. It uses the same JEDI-270 SSDs as above but only includes particles with a coinciding time-of-flight (TOF) measurement. The TOF measurement distinguishes ions from electrons without ambiguity (see supporting information Text S1.1). Additional possible contributions to equation (1) are shown to be negligible in supporting information Text S1.2. We identify three regions of the magnetosphere: 1. Innermost radiation belt (12:38 to 12:54 UTC of 2016 DOY 240). This is the belt we focus on. We will argue that it is dominated by ions (section 4.1) and located within the rings (section 4.3). 2. Middle radiation belt (12:21 to 12:38 and 12:54 to 13:20). We measure that the total particle rate is much greater than the valid ion rate in this belt, which can only be true if the total particle rate is almost equal to the electron rate, meaning that there are mostly electrons in this region (different to the innermost belt). We argue in Paranicas et al. [2017] that most of the counted electrons in this belt have energies of >700 kev. 3. Outer radiation belt (12:06 to 12:21 and 13:20 to 13:40.) Similar to the middle belt, we find the total particle rate being much greater than the valid ion rate, meaning that there are mostly electrons in this belt. However, the valid ion rate is higher here than in the middle belt. The outer belt shows a cutoff in the ion population at its Jupiterward side that magnetically traces to Io s orbit (section 4.3). 4. Innermost Belt Properties 4.1. Small Electron Content JEDI uses particle TOF measurements to identify ion species (see supporting information Text S1.1). No TOF measurements are performed with JEDI-180, and the other two JEDIs did not take TOF data for 20 min near closest approach to ensure health and safety of the instrument. At the edge of the innermost radiation belt, around 12.7 h, we observe that the total particle rate is almost equal to the valid ion rate (see Figure 1a), which means that ions are dominating in this region and that there are not many energetic electrons present (see equation (1)), in marked contrast to the other belts. The small difference between the total particle and valid ion rates visible in Figure 1a at the edge of the innermost belt edge is mostly due to the nonideal ion detection efficiency: the shown valid ion/ total particle ratio in the innermost belt is 65%, which is close to the ion detection efficiency in this region of 75%. (This efficiency is calculated as in Figure 42 of Mauk et al. [2013] with event data from the innermost belt.) Since the total particle and valid ion curves in Figure 1a track each other at the edge of the innermost belt, we assume that this behavior continues throughout this belt. However, to date and in the absence of TOF data, there is no certainty about this Pitch Angle Distribution As it can be seen in Figure 1c, the pitch angle distribution in the innermost belt shows mostly equatorially mirroring particles and falls off by 3 orders of magnitude toward local pitch angles of α loc 50. This is because the loss cone near Jupiter is large, and any ion (independent on what source it had) that is too field aligned will be lost to the dense atmosphere, leaving only the observed equatorially mirroring ions. (The extent of the loss cone and its growth during approach can be seen best in JEDI s proton data [see, e.g., Mauk et al., 2017 Figure 3f].) KOLLMANN ET AL. JUPITER S INNERMOST BELT 5260

3 Figure 1. (a) Overview of the JEDI measurements around closest approach: Rate of total particles above roughly 100 kev measured by solid state detectors (SSDs) of JEDI-270 (solid red curve, singles data) and JEDI-180 (dashed orange curve, sum over energy-resolved SSD-only channels). Valid ion measurements (roughly 100 kev to 10 MeV) using SSD-time-of-flight (TOF) coincidence measurements of JEDI-270 (blue curve). Background colors and dashed vertical lines indicate the regions defined in section 3. It can be seen that the particle/ion ratio is unusually low at the edge of the innermost belt, indicating that most of the particles are ions, not electrons (see section 3). (b) Orange: measurement of total particles with local pitch angles of α loc 90 and ambient energies of 830 kev (calibration assumes all particles as protons). Blue: expectation for the same particle population if their change over time is because JEDI samples different fractions of the equatorial pitch angle distribution due to Juno s change in latitude (see Appendix A). The shape of the expectation resembles the measurement. It is offset from the measurement because the magnetic equator is not exactly at the location predicted by the used field model. (c) Pitch angle distribution of total particles with 830 kev. We assume symmetry between northward and southward moving particles, which is appropriate in the innermost belt [Mauk et al. [2017], Figure 3]. Small black points indicate bins with zero intensity, white spaces show that there was no measurement. We removed the middle belt periods from Figures 1b and 1c since the used JEDI channel does not yield meaningful results there [Paranicas et al., 2017]. (d) Black: magnetic distance of Juno r meq relative to Jupiter using the Khurana magnetic field model. Blue: radial distance of Juno to Jupiter. (e) Black: Juno s magnetic latitude traced in the Khurana magnetic field model. Green: equatorial pitch angle of particles that have 90 local pitch angle at Juno s latitude. KOLLMANN ET AL. JUPITER S INNERMOST BELT 5261

4 4.3. Innermost Belt Is Located Within Rings Since charged particles are bouncing along magnetic field lines, we identify field lines by their radial distance r meq from Jupiter (similar to the commonly used L shell) at the point of minimum field strength (termed the magnetic equator). We will use the magnetic distance r meq used here to organize the JEDI data. Its value at any given time depends on the magnetic field model that is used to calculate it. We use the Khurana magnetic field model [Khurana, 1997; Figure 1d shows the calculated values of r meq. Figure 1e shows the latitude of the spacecraft relative to the magnetic equator, as well as the equatorial pitch angle that corresponds to particles that mirror locally at the spacecraft (see Appendix A). Figure 2a shows proton intensities from JEDI and previous missions as a function of magnetic distance. It can be seen that the JEDI intensities outward of the innermost belt are consistent with equivalent measurements from other missions, demonstrating that there were no extreme intensity changes since the previous missions. Figure 2b compares measurements during the inbound and outbound portion of the orbit. They are roughly symmetric, supporting the used magnetic field model. The residual asymmetry indicates that pre-juno field models still need improvement to account for the high latitudes covered by Juno. JEDI intensities at different r meq can only be directly compared if they are normalized to the same equatorial pitch angle. Data shown in Figure 2 are not normalized like this because the magnetic field near Jupiter did not behave as predicted [Bolton et al., 2017; Connerney et al., 2017a]. Since the pitch angle distribution in the innermost belt shows a large variation with pitch angle (Figure 1c) and Juno covered a large range of latitudes, this distorts the J(r meq ) intensity distribution within the innermost radiation belt (see Appendix A). That may explain why the shape of the JEDI intensity profile differs from what was seen by the Galileo Probe that measured within a relatively narrow range of equatorial pitch angles. Besides this, Galileo Probe/EPI measured at much higher energies (tens of MeV instead of hundreds of kev), where different sources and sinks can lead to different shapes. Despite these limitations, Figure 2 allows constraining the spatial location of the innermost belt: the innermost belt at the energies measured by JEDI appears to start at r meq 2, within the two tenuous gossamer rings (inward of 3.15 R J [Ockert-Bell et al., 1999]), near Jupiter s main ring (located at 1.71 to 1.80 R J [Burns et al., 2004]). The intensity of the innermost belt extends at least until Juno s closest radial approach distance of r = 1.07 R J. For the magnetic field model used here, closest approach occurred near the magnetic equator, where r r meq. This is well inward of the inner edge of the halo associated with the main ring, which cuts off at about 1.26 R J [Showalter et al., 1987] Heavy Ions The innermost belt is also comprised of MeV sulfur ions (Figure 2b). At the energies shown in the figure, JEDI identifies sulfur without ambiguity. The sulfur ions show a profile that qualitatively follows the protons in Figure 2a with a steeper gradient within the innermost belt. Oxygen ions (not shown) are also observed to behave similarly. 5. Origin of the Innermost Belt It can be seen from Figure 2 that all ion intensities fall off planetward of Io s orbit. This raises the question as to how the ion intensity can recover in the innermost belt, especially since no energetic electrons are present there. There are several possible ion sources, and we discuss three of them in sections Other possibilities may be energy transfer to the local plasma [Szalay et al., 2017] from waves [Horne and Thorne, 2003] or from cosmic rays. These possibilities may be subject of future studies Radial Diffusion Radial transport from diffusion [Walt, 1994; Thomsen et al., 1977] or interchange [Southwood and Kivelson, 1987; Thorne et al., 1997] may supply the innermost belt with particles from larger distances. This can be tested by evaluating if phase space densities f(r meq ) at constant adiabatic invariants [Roederer, 1970] are continuously falling toward the planet. Such a calculation requires knowledge of the ion distribution at the magnetic equator. (In principle it is possible to use the locally measured pitch angle distribution for this. However, the local pitch angle required to conserve invariants changes through the orbit and moves too fast into the loss cone to allow deriving a phase KOLLMANN ET AL. JUPITER S INNERMOST BELT 5262

5 Figure 2. (a) Proton intensity from Juno/JEDI and previous missions as a function of magnetic distance r meq. Data from previous missions includes Galileo Probe Energetic Particle Investigation (EPI) [Fischer et al., 1996] (locations based on the O6 model [Connerney, 1993]), the inbound flyby of Pioneer 11/University of Iowa instrument [Van Allen, 1976] (location from O3 model [Acuna and Ness, 1975]), and mission-averaged data from Galileo Energetic Particle Detector Composition Measurement System (EPD/CMS) [Kollmann et al., 2016] (location from Khurana model [Khurana, 1997]). In this panel, inbound and outbound passes are averaged together for the Juno and Galileo missions. It can be seen that the innermost radiation belt extends from the gossamer rings to the gap between Jupiter and the ring halo. The exact shape of the JEDI intensity profile within the innermost belt is not directly comparable to previous missions since the JEDI intensities were not normalized to a constant equatorial pitch angle. We do not show Galileo orbiter/epd data inward of Io since most measurements suffered from saturation and penetrating particles. (b) Same as Figure 2a for sulfur ions. We separate in this panel the inbound and outbound passes. KOLLMANN ET AL. JUPITER S INNERMOST BELT 5263

6 space density profile over a sufficiently large r meq range.) The result will be sensitive to the assumed equatorial pitch angle distribution and field model. Since neither is currently well known, conclusions drawn from this would not be reliable. We therefore postpone this analysis for future studies Weak CRAND Saturn s ion belts [Cooper, 1983; Kollmann et al., 2013] as well as Earth s inner proton belt [Hess, 1959; Selesnick et al., 2013] derive mostly from galactic cosmic rays via cosmic ray albedo neutron decay (CRAND): Cosmic rays that hit atmospheric or ring material produce secondary neutrons. A fraction of these decay into protons within the belts. This could also happen within Jupiter s innermost belt. However, CRAND can only provide protons and electrons, not the heavier ions we observe. In principle, these heavy ions could derive from trapping of anomalous cosmic rays [Selesnick et al., 1995] or from cosmic ray spallation of ring material. This is unlikely as the resulting composition would not be rich in sulfur, as observed. The fact that spatial profiles and energy spectra of protons and heavy ions in the innermost belt are similar suggests that all species are supplied by the same processes, making CRAND alone unlikely to explain the observations Energetic Neutral Atom Source Another potential source process is stripping of energetic neutral atoms (ENAs). ENAs are produced in the Europa and Io gas tori by charge exchange of ions with neutral gas [Mauk et al., 2004]. Most ENAs escape into space or hit the planet and heat its atmosphere. Other ENAs may pass through neutral material between Jupiter s exosphere and main rings (as the hydrogen corona or ring material, see below) where an electron can be stripped off, yielding an energetic ion with almost the same energy [Jasperse and Basu, 1982] as it had in the magnetosphere. Some of the stripped ions are subsequently lost again via charge exchange in the neutral material surrounding Jupiter and may yield the ENA emission from Jupiter reported in Mauk et al. [2004]. The surviving ions form the innermost ion belt, where the balance of stripping source and charge exchange loss determines its steady state intensity. Similar processes occur at Earth [Moritz, 1972; Gusev et al., 2003], Mars [Halekas et al., 2015], and possibly Saturn [Krimigis et al., 2005]. A cartoon of the described series of charge exchange and stripping is shown in Figure 3a. The neutral material around Jupiter responsible for the stripping of the ENAs is likely the hydrogen corona, which has a scale height of about 0.01 R J [Gladstone et al., 2004] and reaches a density comparable to the Io and Europa gas tori initially producing the ENAs (10 cm 3 [Smyth and Marconi, 2006]) at 1.2 R J. Since we also measure ions outward of this distance, there may also be a contribution from stripping in gas associated with the ring halo that reaches to 1.3 R J. An alternative explanation is that the stripped ions are not staying at a fixed r meq but distribute due to radial diffusion or nonaxisymmetric drift shells (due to the asymmetries in the magnetic or electric fields). We test in the following if the observed spectra are consistent with this scenario. Figure 3 shows proton and heavy ion spectra of the outer (b and d) and innermost belt (c and e). The spectra in the outer belt have a power law shape, as is normal in magnetospheres. The local increase in the outer belt proton spectrum around 200 kev is an artifact from penetrating electrons [Mauk et al., 2017]. The spectra of the innermost belt are unusual: The innermost proton spectrum has a minimum at 300 kev, the heavy ion spectrum at about 600 kev. The innermost belt spectra of both species are rising toward higher energies. We now demonstrate that such a spectral shape can arise if protons are supplied as a steep power law and this source is balanced by charge exchange loss, as described above. We also show that the rise toward high energies can exist because charge exchange loss is inefficient at high energies and cannot keep up well with the supply of fresh energetic ions. To show this quantitatively, we assume that some process (discussed below) delivers protons to the region of the innermost belt and that the source rate (particles per volume, momentum space volume, and time) is described by a function S(E). The innermost belt protons pass through the outskirts of Jupiter s exosphere, hydrogen corona, and ring material and some precipitate into the dense atmosphere. We assume that all material is H [Gladstone et al., 2004] and optically thin enough that ENAs can escape without being stripped again. The protons will charge exchange with a rate L = n J σ ce,j vf J (with gas density n J near Jupiter, KOLLMANN ET AL. JUPITER S INNERMOST BELT 5264

7 Figure 3. (a) Cartoon illustrating the source for the innermost belt that we suggest in section 5.3. (Jupiter image credit NASA GSFC, STScI.) (b) Red: measured proton spectrum in the outer belt, between Io and Europa. Cyan: same spectrum after converting the protons to ENAs in the Io gas torus and converting them back by stripping in Jupiter s hydrogen corona. This is a possible source spectrum for the innermost belt. The dimension is protons per volume, momentum volume, and time, expressed in arbitrary units. (c) Red: measured proton spectrum in the innermost belt. Blue: model spectrum based on a source from stripped ENAs (blue curve above) and a sink from charge exchange (see equation (2)). Amplitude is arbitrary. (d) Measured heavy ion spectrum (for masses from oxygen to sulfur inclusive) in the outer belt. Since JEDI does not distinguish ion species well for 300 kev, the calibration used here assumes for simplicity that all ions in this mass range can be treated as sulfur. (e) Measured heavy ion spectrum in the innermost belt. KOLLMANN ET AL. JUPITER S INNERMOST BELT 5265

8 charge exchange cross section σ ce,j, proton speed v, and proton phase space density f J (E) in the innermost belt, which is particles per volume and momentum volume). For a steady state, the sum of the charge exchange rate L and source rate S needs to be zero. This can be solved for the proton spectrum and yields S f J = (2) n J σ ce,j v Using H + -on-h charge exchange cross sections from Lindsay and Stebbings [2005] and a source following S E γ, we find that f J with a minimum at hundreds of kev is retrieved if the exponent is 6 γ 8. This result is independent of the mechanism that provides S. Stripping of ENAs that were produced in the magnetosphere provides to zeroth order a source spectrum with the required shape. How to calculate the source rate S for this mechanism was given in Kollmann et al. [2013] and is summarized in supporting information Text S1.3. The result is overplotted in Figure 3b in cyan. A power law fit to S yields γ = 7. As discussed above, this exponent means that the steady state spectrum f J will have a minimum about where it is observed. We overplot the modeled spectrum f J (using the power law fit to the source rate) in Figure 3c and find that it indeed qualitatively matches the measurements. This is a proof of principle that stripped ENAs may supply the innermost belt. Energetic heavy ions can exist in high charge states [Clark et al., 2016] that do not allow them to easily convert to ENAs. However, there is evidence that also singly charged ions exist [Laggetal., 1998] for which the proposed mechanism can work. Because of the lack of charge exchange and stripping cross sections for heavy ions at the required energies, we cannot test this quantitatively. A drawback of our model is that it is sensitive to the shape of S. It does not show a clear minimum if we do not use a power law fit to S (blue curve in Figure 3b) but S itself (cyan). One explanation is that the model is not valid and some unknown process is supplying the innermost radiation belt. The other explanation is that a process faster than charge exchange modifies the spectrum in a way that it becomes more power law like. Candidate processes are, for example, energy dependent scattering into the loss cone, cascades of stripping and charge exchange, or energy loss Electron Loss The stripping process discussed in section 5.3 is consistent with the low energetic electron abundance found in section 4.1: Stripped electrons have energies at least 3 orders of magnitude below the ENA energy [Edgar et al., 1973; McNeal and Birely, 1973], which is outside JEDI s energy range. The Jovian Auroral Distributions Experiment (JADE) covers these energies, but its response in the innermost belt is still being analyzed. Electrons scatter more easily than ions when interacting with atmospheric gas or ring material. The scatter will bring them into the loss cones and the deep atmosphere where they are lost. This means that electrons are not a good indication of the source process: Stripped low-energy electrons may not be detectable because they are lost too fast. Same is true for energetic electrons if they are provided by another source than stripped ENAs. (CRAND, for example, produces also electrons with a wide range of energies in a Jupiter-centered frame.) In such case, the electrons would need to be removed more efficiently than ions to explain observations. 6. Summary We use energetic charged particle data in the hundreds of kev to MeV range and discuss the properties and origin of Jupiter s innermost radiation belt: 1. The innermost radiation belt was measured by JEDI from the gossamer rings to inward of the ring halo (Figure 2). It is the first time that this population is measured at high latitudes in this energy range. 2. The edge of the innermost belt includes not only protons (Figure 2a) but also ions up to the mass of sulfur (Figure 2b), which we assume is also the case for the center of the belt. The composition rules out CRAND as the only source process (section 5.2). 3. The innermost ion spectra show a deep minimum at hundreds of kev, not a power law as in the rest of the magnetosphere (Figure 3). Such a spectrum can be produced if ions are supplied with a steep spectrum by an arbitrary process and lost via charge exchange in Jupiter s high atmosphere or ring material (section 5.3). 4. Energetic neutral atoms (ENAs) that are stripped in Jupiter s high atmosphere may be that source process for the innermost belt. This is supported by modeling of the spectral shape (section 5.3) and seems to KOLLMANN ET AL. JUPITER S INNERMOST BELT 5266

9 be a common mechanism since it is also thought to be the source of low-altitude radiation at Earth and Saturn [Moritz, 1972; Krimigis et al., 2005]. Quantitative estimates if ENAs can support the observed intensity magnitude are still pending. 5. The pitch angle distribution in the innermost belt is strongly peaked for equatorially mirroring particles (Figure 1c). This is because field-aligned particles at this small distance to Jupiter mirror in the dense atmosphere, where they are lost. 6. The innermost radiation belt has an unusually low content of energetic electrons (section 4.1). This is consistent with ENA stripping, which only produces low-energy electrons (section 5.4). Appendix A: Importance of the Pitch Angle Distribution The intensity peak J(t) for times t within the innermost belt can in principle be due to JEDI s change in magnetic distance (where the peak occurs at the time where minimum r meq is reached) or in magnetic latitude (the peak occurs at the time when λ = 0 for an equatorially mirroring distribution). The timing of the peak cannot be used as a distinction since both times are close to each other in all considered field models. In the following, we test the hypothesis that J(t) is due to Juno s change in latitude λ. We use the orange curve in Figure 1b as a representation of J(t) that only includes particles with local pitch angles of α loc 90. Under conservation of the first adiabatic invariant [Roederer, 1970], the local pitch angle α loc = 90 maps to an equatorial pitch angle α meq depending on the magnetic field B(λ) as B(0) α meq (t,α loc )=arcsin B(λ(t)) sin2 (α loc ) The Khurana model is used to calculate B. We trace the magnetic field lines and refer to the location of minimum B as the magnetic equator. The angle relative to this equator is the magnetic latitude λ. λ(t) is shown in Figure 1e and used to calculate α meq (t, 90), also shown in the figure. We assume that the peak in J(t) at h is because Juno passed the magnetic equator and use this as the equatorial pitch angle distribution j(α meq ). To calculate our expectation j(t) how the measured intensity J(t) should evolve, we determine the equatorial pitch angles α meq (t, 90) (calculated via equation (A1) and shown in Figure 1e) corresponding to locally mirroring intensities at each measurement point and interpolate j(α meq ) to these values, which immediately yields j(α meq (t, 90)) = j(t) and is shown in Figure 1b as a blue curve. It can be seen that there is a slight time offset between expectation and measurement because the actual magnetic equator is not exactly at the location predicted by the Khurana model. This is expected given that the magnetic field measurements around closest approach significantly deviate from pre-juno models [Bolton et al., 2017; Connerney et al., 2017a]. Besides this, our expectation resembles the measurement since both intensity profiles change over about 3 orders of magnitude within a similar time period. This similarity supports our assumption. This suggests that to a large extent the time profile of the innermost belt reflects the pitch angle distribution. (A1) Acknowledgments Juno/JEDI data will be available through NASA s planetary data system (PDS) in May Earlier access can be provided by the instrument team. This work was funded by the NASA New Frontiers Program for Juno via subcontract with the Southwest Research Institute. The authors like tothankl.brown,j.m.peachey,and J. Vandegriff (all APL) for software development and data processing. We thank K. K. Khurana (UCLA) for his magnetic field model and A. Shinn (LASP) for its IDL adaptation. 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