Survey and Analysis of Whistler‐ and Z‐Mode Emission in the Juno Extended Mission

Whistler‐mode and Z‐mode waves may have significant impact on electron scattering and acceleration at Jupiter as recent stochastic models indicate. New observations of these waves in the Juno extended mission expand the orbital coverage from previous surveys, extending especially to smaller radial distances near the magnetic equator, and to extended ranges of dusk‐side magnetic local time. M‐shells transiting the Io torus and the orbit of Europa are recognized as a significant source region for whistler‐mode lower and upper band chorus emission with intensity peaks near 9 RJ. Ganymede is a significant source of intense whistler‐mode chorus confined to the local moon environment. New Z‐mode observations now extend from a few kHz to over 80 kHz. Inferred source regions for Z‐mode are consistent with the outer edge of the Io torus and with auroral field lines that may also support broadband kilometric or decametric emission.


Juno Waves Instrument
The Juno Waves instrument (Kurth et al., 2017) measures electric signals in the frequency range ∼10-∼150 kHz on the low frequency receiver (LFR-hi), ∼50 Hz to 20 kHz on LFR-lo, and magnetic signals in the range ∼50 Hz to 20 kHz on the LFR-B receiver as described in the previous survey.In the survey mode used in this study spectra are returned about once every second.The instrument also operates in a burst mode with greater spectral resolution.While in burst mode, the instrument can monitor frequencies close to f ce and survey at the rate of 1 spectrum/sec to frequencies up to 41 MHz.Menietti et al. (2020Menietti et al. ( , 2021) ) reported low-latitude surveys of whistler-mode and Z-mode emission observed by the Juno spacecraft during orbits 1 to 33.We now report further on the analysis of the data after Juno has progressed through 45 orbits.The methodology of the survey is the same as presented in Menietti et al. (2020Menietti et al. ( , 2021)), and we discuss this only briefly below.

Low-Latitude Whistler-Mode Survey Methodology
Each 1-min period, 43 frequency channels are sampled from the Waves low frequency magnetic receiver, LFR-B, on board Juno.The average and median intensity at each frequency is computed from the 60 measurements at each frequency.To avoid noise spikes in the data we limit the ratio (average intensity)/(median intensity) < 10 for each frequency of each 1-min data sample.We bin the wave data in M-shell, latitude, and local time, where M-shell is defined by the radial distance (R J ) a magnetic field line crosses the magnetic equator, based on the JRM09 plus current sheet model (Connerney et al., 2018).
We define relative frequency, β i = f i /f ceq , where f i is the center frequency of the frequency bin, Δβ i. and subscript "eq" refers to a point at the equator along the same M-shell as the spacecraft at the time of observation.For the lower band chorus (β i < 0.5) and upper band chorus (β i > 0.5) we define f lheq /f ceq < Δβ 1 < 0.1, 0.1 < Δβ 2 < 0.2, etc. with 0.8 < Δβ 9 < 0.9 as the highest frequency interval.We determine B 2 (β i ), proportional to the magnetic intensity, by integration of the measured magnetic spectral density over the frequency channels within Δβ i for 1-min time steps, Δτ.Both mean and median averages are calculated.

Models and Constraints
For the whistler-mode study, primarily in the Io plasma torus region, the plasma density is obtained using interpolated values of analytic density models as discussed in Imai et al. (2015) and Menietti et al. (2020Menietti et al. ( , 2021)).The magnetic field model is the JRM09+CAN model (Connerney et al., 1981(Connerney et al., , 2018)), and is used to map field lines along an M-shell from the satellite position of observation to the magnetic equator.Connerney et al. (2020) introduced a new Jovian current sheet model important for mapping auroral and ionospheric footprints with more distant magnetospheric observations of Juno.However, the size of the spatial bins of the wave intensity data used in the present survey are larger than the increase of resolution available from the new current sheet model.Hence, we continue to use the JRM09 + CAN model in the present survey.We assume a whistler-mode chorus source region near the magnetic equator (Hospodarsky et al., 2012), and trace the magnetic field line from the observation down to the magnetic equator to record the cyclotron frequency, f ceq , and lower hybrid frequency, f lheq .Whistler-mode emission for f < f ceq /2 (lower band chorus) and upper band are included.As discussed in Menietti et al. (2020Menietti et al. ( , 2021) ) chorus observations are restricted to magnetic latitudes, λ < 31° to avoid auroral hiss emission from the Jovian polar regions that have a different generation mechanism.

Whistler-Mode Chorus Survey
The data for this survey were sampled from Juno orbits PJ01 through PJ45, extending the previous survey from PJ33.In Figure 1 we show survey plots of averaged lower-band whistler-mode intensity (f < f ce /2) sorted in bins of M-shell and magnetic local time (MLT) (Figure 1a) and sorted in the meridian plane in bins of radial distance and λ (Figure 1b).White bins indicate regions where the spacecraft did not sample.1-minute averaged intensity observed within each spatial bin is shown averaged over all of the observed MLTs (Figure 1a) and over all latitudes <31° (Figure 1b).In Figure 1a the bin size in MLT is 15°, and in Figure 1b the bin size is 2° over the range 0 < λ < 16°, and 5° for λ > 16°.
Compared to results reported by Menietti et al. (2021) for perijove orbits PJ01 through PJ33, this latest survey of whistler mode shows more complete coverage of the near equatorial inner magnetosphere covering the range 5 < MLT < 18.The most intense whistler-mode emission lies in the range 7< M < 11 and is within 20° of the magnetic equator in both hemispheres.The intensities range from 10 −5 nT 2 to 10 −2.2 nT 2 (Figure 1a) and 10 −2.9 nT 2 (Figure 1b).There is also a detection of higher intensity in the range −5° < λ < −2.5° and 15 < M < 16 associated with the Ganymede flyby during PJ34 (cf.Li et al., 2023;Shprits et al., 2018).This emission may be confined to the local Ganymede magnetosphere.
We display whistler-mode intensity as a function of β, M, and λ by increasing the size of the spatial bins compared to those used in Figure 1 to improve statistics.The frequency bins were defined earlier.We plot whistler-mode intensity averaged over all magnetic local times and all latitudes for each ΔM = 1.0 in the range 5.0 < M < 20.0.We note the Juno orbit is not symmetric with respect to the magnetic equator.The latitude bins range from −16° to +31°, with the width of the bins Δλ = 2° for |λ| < 16° and Δλ = 5° for |λ| > 16°.Error bars for both whistler-mode and Z-mode data (below) are one standard deviation and shown only above the data point on semi-log plots.Averages are plotted with triangles and medians with circles.After binning the data we sort relative to intensity and manually survey the strongest emissions to eliminate outliers due to spacecraft engine firings, instrumental interference, etc.
In Figure 2a the intensity versus β decreases almost linearly from ∼8 × 10 −5 nT 2 at β = 0.05 down to <3 × 10 −7 nT 2 at β = 0.75, except for the small enhancement for the upper band chorus at β = 0.55.In Figure 2b the maximum intensity occurs at M = 8.5, with a small secondary peak at M = 15.5, which is due to the strong whistler-mode emission seen during the Ganymede flyby (PJ34).We consider the dependence on magnetic latitude in 4 ranges of M-shell (Figure 3).In all cases the intensity peaks away from the magnetic equator, suggesting convective wave growth with distance from the magnetic equator.For 5 < M < 7 the intensity has a small peak of ∼3 × 10 −4 nT 2 with magnetic latitude near λ = ∼3° and another weaker peak near λ = 10°, but we note there is sparse data for λ < 2° in this range of M. For 7 < M < 10, a stronger peak of ∼2 × 10 −3 nT 2 occurs for −5° and a broader and smaller peak near λ = 7°.In the range 10 < M < 13, the intensity is smaller with peaks <2 × 10 −4 nT 2 at λ = ±3° and smaller peaks at λ = 9° and λ = 15°.The peak near 29° is likely due to increasing visibility of auroral hiss emission at higher latitudes.Finally, for 13 < M < 20 there is a peak of 2 × 10 −2 nT 2 at λ = −3° and a weak peak λ = 13°.The peak at −3° primarily results from the Ganymede flyby on PJ34, and, again, the increase at λ = 29° is likely due to increasing visibility of auroral hiss.The secondary peaks seen within each range of M-shell range may be due to temporal effects.
Whistler-mode chorus propagating from source regions near the magnetic equator is observed to increase in intensity above the equator before decreasing with latitude.The latitude of the peak chorus intensity is a function of M-shell, but is believed to be less than 30°.The whistler-mode intensity, however, begins to increase at about 30° and continues beyond 50° latitude (cf.Li et al., 2020) due to the increasing visibility of auroral hiss emission with latitude (Menietti et al., 2020(Menietti et al., , 2021)).
Upper band chorus is observed most intensely at Jupiter near the magnetic equator and when lower band chorus intensity is high.In Figure 4a we plot upper-band intensity versus M-shell which indicates the maximum intensity of the upper band chorus is ∼4 × 10 −6 nT 2 at M = 9.5, which is more than an order of magnitude lower than the intensity of the lower-band chorus.The bin at M = 7.5 contained only 24 measurements representing 2 small periods of modest intensity data that are deemed not a statistically meaningful sample compared to the many  10.1029/2023JA032037 5 of 11 hundreds of measurements (1,800 < N < 2,300) for 8 < M < 12. Upper-band chorus intensity versus magnetic latitude (Figure 4b) peaks at ∼10 −5 nT 2 and λ = −3° also much lower than lower band chorus, but at similar latitude.

Z-Mode Survey
Previous surveys reported observations of Jovian Z-mode emission, the low-frequency branch of the extraordinary (X) mode (Menietti et al., 2020(Menietti et al., , 2021)).We have extended the previous Jovian Z-mode survey to include PJ01 through PJ45.The upper cutoff of Z-mode, known as Z-infinity, f zi (cf., Benson et al., 2006) is a function of wave normal angle, θ, as follows: which approaches the upper hybrid frequency, f uh = √(f ce 2 + f pe 2 ), as the wave normal angle approaches 90° (f pe = electron plasma frequency).The lower cutoff of Z-mode, also called the L = 0 cutoff (cf.Gurnett & Bhattacharjee, 2005), is given approximately by (2) In the case of low plasma density the ion cyclotron frequency becomes important and the value of f z is calculated numerically including H + ions.At Jupiter Z-mode emission is generally less intense and is usually observed at higher frequencies than whistler mode.
Z-mode emission is commonly spin-modulated with a satellite spin period of ∼30 s, producing a signal modulation of ∼15 s.Following the methodology explained in Menietti et al. (2020), we have calculated a linear least squares fit of the Z-mode intensity to a sinusoidal function, averaged over 1-min intervals.As in Menietti et al. (2020Menietti et al. ( , 2021)), the Z-mode emission frequency ranges are selected by eye and digitized in 1-min periods.At a given time we attempt to identify wave mode cutoffs that depend on f pe and f ce such as the low frequency O-mode cutoff (f pe ), upper hybrid frequency (f uh ), whistler-mode upper frequency cutoff, or Z-mode low frequency cutoff (f z ).With any of these identified cutoffs along with measured values of f ce from the Fluxgate Magnetometer (FGM) instrument on board Juno (Connerney et al., 2017), we are able to distinguish Z-mode emission within the limits of the uncertainty of mixed wave modes (cf., Elliott et al., 2021;Menietti et al., 2021;Sulaiman et al., 2021).Because polarization of the waves is not measured by the Waves instrument, the identification as Z-mode is not always certain, and the emission may sometimes be mixed with O-mode or whistler mode.Sulaiman et al. (2023) have also shown how O-mode can be identified at times by observing wave intensity spin modulation that peaks when the Juno spacecraft electric antenna lies close to the ambient magnetic field direction.This technique relies on proper orientation of the spacecraft spin axis relative to the magnetic field.A time-tagged list of the frequency range of observed Z-mode emission is available at the data archive given in the Data Availability section.
We calculate magnetic intensity for each 10 kHz synthesized frequency bin from the observed electric intensity assuming the cold plasma index of refraction, C z (f, f p , f c ) (Equation 4.4.17 of Gurnett and Bhattacharjee ( 2005)).
Observations of Z-mode emission are often made at relatively small radial distances and at mid-latitudes when Juno is near and at radial distances less than the inner edge of the Io torus (Menietti et al., 2020(Menietti et al., , 2021)).As mentioned previously, the plasma density model used in this survey of whistler mode (Imai et al., 2015) is used within the Io torus; however, in the region where Z-mode is often observed for r < 6 R J and at λ > 20° the model and observed density values may differ considerably.In these regions, where typically f pe / f ce < 1, the upper frequency cutoff for electromagnetic whistler mode is f pe .We use the observed upper cutoff of whistler mode or the lower cutoff of O-mode, to estimate f pe , consistent with the observed value of f z , which is then used in the determination of the index of refraction for Z-mode.A time-tagged list of f pe used in this study is available from data archive given in the Data Availability section.Z-mode magnetic intensities are displayed in the r-MLT and meridian planes Figures 5a and 5b, respectively.These plots be compared to Figure 8 of the previous survey (Menietti et al., 2021).The region of observed Z-mode has expanded to the previous surveys.Z-mode is observed in the mid-latitude regions of the inner Jovian magnetosphere, as in the past, but in this latest survey there are new Z-mode observations for f < 10 that are associated with regions low plasma density cavities at radial distances less than the inner edge of the Io torus.
We are also identifying some low-intensity Z-mode at higher latitudes particularly in the southern hemisphere, regions not observed by Juno in the corresponding northern hemisphere due to Juno orbit constraints.
In Figure 5 the most intense Z-mode is observed in a band extending approximately over ranges 1.5 < r < 6.5 R J , 18 hr < MLT <1 hr (counter-clockwise), and over magnetic latitude range of about 20° < λ < 60°.The intensities in the southern hemisphere are generally weaker than the northern.This is likely due to the orbit of Juno that traverses higher magnetic latitudes in southern hemisphere at larger radii.M-shells over-plotted, with outer shells M = 15, 20, and 25.Juno observes Z-mode at lower radii in this recent survey, extending down to about 1.5 R J .Wave mode discrimination is more difficult at smaller radial distances.We plot the Z-mode intensity versus frequency, M-shell, and magnetic latitude in Figure 6.The Z-mode frequency range (6a) is from ∼3 kHz to ∼80 kHz with intensity peaking at ∼2.7 × 10 −6 nT 2 below 10 kHz, and ∼1.7 × 10 −6 nT 2 between 60 and 80 kHz, but falling off sharply at 85 kHz where there are few observations.Intensity versus magnetic latitude (6b) is distinctly different in the N and S hemispheres, peaking at ∼10 −5 nT 2 between 45° and 55° in the N hemisphere and decreasing above 60°.In the more poorly sampled S-hemisphere the intensities are much weaker and the peak at 4 × 10 −7 nT 2 occurs at the highest latitudes.This is likely due to the Juno orbit as mentioned already.In Figure 6c the intensity versus M-shell does not vary significantly, displaying shallow peaks at ∼8 × 10 −6 nT 2 near M = 6 and ∼10 −5 nT 2 near M = 13.

Summary and Conclusions
We have presented an updated analysis of Jovian low-latitude whistler-mode chorus, and Z-mode emission for orbits PJ01 through PJ45, well into the Juno extended mission.The changing orbit of Juno in the extended mission has allowed measurements of whistler-mode and Z-mode intensity in regions of MLT extending to dusk, and to lower radial distances than observed previously, and with observations of Z-mode extending to frequencies less than 10 kHz.Upper-band chorus is most commonly observed near the magnetic equator when the lower-band chorus is most intense.
Whistler-mode lower-band chorus from source regions near the magnetic equator is observed by Juno to magnetic latitudes of about 30°.At higher latitudes whistler-mode emission is believed to be a mixture of chorus and auroral hiss (Menietti et al., 2021).This new survey reports whistler-mode intensity decreases almost linearly from ∼10 −4 nT 2 at β = 0.05 down to ∼3 × 10 −7 nT 2 at β = 0.75 (β = f/f ceq ), with a small secondary peak for the upper band chorus at β = 0.55.We note that because of the low spectral resolution of the Waves instrument survey mode data, we do not often observe the intensity gap near 0.5 f ce between the lower and upper chorus bands.The lower band chorus emission at lowest frequencies is mixed with whistler-mode hiss, explaining why we see no lower-band peak near β ∼ 0.3 as is typically seen at Earth.Menietti et al. (2021) explained that the whistler-mode emission at low energy is quite bursty, which is typically a characteristic of chorus, not hiss.However, since the gyroresonance energy of whistler-mode emission decreases with frequency, the higher energies of Jovian electrons in the chorus generation region may lead to lower frequency chorus mixed with typically low frequency hiss in the equatorial region of Jupiter.The maximum whistler-mode intensity occurs near M = 8.5, with a small secondary peak at M = 15.5, which is due to strong whistler-mode emission observed near Ganymede.Intensity dependence on magnetic latitude depends on M-shell, but is a maximum near and slightly above the magnetic equator (probably due to convective wave growth) in the range 7 < M < 10.Upper-band chorus is observed by Juno near the magnetic equator especially when the lower-band chorus is most intense.The upperband chorus is generally an order of magnitude lower than lower-band chorus and seen almost exclusively in the range 8 < M < 12. Juno orbits during periods which include whistler-mode and Z-mode observations are shown in Figure 7, comparing periods of previous studies from PJ01 to PJ33 (7a,c) to periods of more recent observations from PJ34 to PJ45 (7b,d).Figures 7a and 7b are in magnetic -Z coordinates that are defined using the JRM33 magnetic field model (Connerney et al., 2022).Z is the distance above the (JRM33) magnetic equator and  is the distance from Jupiter projected onto the magnetic equator.Orbits in Figures 7c and 7d are shown in Jupiter solar equatorial coordinates, but we have rotated them to conform with Figure 1a where we have chosen + X away from the sun, and +Y toward the dawn sector.
From Figures 7a and 7c we note that during orbits PJ01 to PJ33 the inbound orbits are almost all in the northern magnetic latitudes in the region <15 R J where Z-mode and most chorus emissions are observed.However, during orbits PJ34 to PJ45 the line of apsides of the Juno orbits has shifted southward.Figures 7c and 7d indicate that during PJ01 to PJ33 the orbital plane was primarily in the dawn sector (+Y), while the orbital plane for orbits PJ34 to PJ45 was always in the dusk sector.We have calculated the average chorus intensity independently for the dawn and dusk sectors in Figure 1a.For the dawnside the average chorus intensity is 7.96 × 10 −5 nT 2 , while for the duskside it is 1.4 × 10 −4 nT 2 , which is a dawn/dusk ratio of 0.57.This difference may have several causes.The observations of the dawn and dusk sectors occurred at separate times, during a period of over 6 years and may reflect increasing magnetic activity levels of the entire Jovian magnetosphere over the Juno orbital period.However, we cannot discount a dawn-dusk electric field as reported at Jupiter by Barbosa and Kivelson (1983) and Ip and Goertz (1983).Schneider and Trauger (1995) reported enhanced Io torus brightness on the Jovian duskside, which they attributed to a Jovian dawndusk electric field.Bonfond et al. (2015) and Murakami et al. (2016) present more recent observations of dawn-dusk asymmetries.The chorus source region is located within the Io torus near the magnetic equator and is dependent on instabilities in the energetic electron population.Chorus waves are correlated with enhanced energetic electron populations at Earth and Jupiter (cf., Liu et al., 2015;Menietti et al., 2020).More careful investigation of the electron population in the chorus growth region is necessary to better understand the nature and source of the dawn-dusk anisotropy of the chorus intensity.
The extension of the orbits to higher latitudes in the southern hemisphere during PJ34 to PJ45 has allowed new observations of Z-mode.The new survey of Jovian Z-mode is improved from previous surveys.Because of large temporal and spatial effects, the plasma density in the region of Z-mode observation is calculated using the observed cutoffs of whistler mode, O-mode, and Z-mode.The density is used to obtain the ratio of magnetic to electric intensity, B/E, from the Z-mode dispersion relation.Z-mode is observed at highest intensity at middle latitudes in the approximate range 2 < r < 7 R J , with highest intensities at smaller radial distances.These waves are typically one to two orders of magnitude less intense than whistler-mode chorus, but are about two orders of magnitude larger than typical intensities of Z-mode observed at Saturn (Menietti, Averkamp, Ye, Persoon, et al., 2018;Menietti, Averkamp, Ye, Sulaiman, et al., 2018).Menietti et al. (2023) have identified both Z-mode and O-mode propagating in the same region with different intensity, pointing out the difficulties in always distinguishing Z-mode without polarization measurements.in the Jovian inner magnetosphere can provide quantitative evidence of the role of plasma wave interactions and electron energy gain or loss in the radiation zone environment.

Figure 1 .
Figure 1.Survey plots of whistler-mode intensity (f < f ce /2) sorted in bins of M-shell and the meridian plane (a), and in bins of M-shell and magnetic local time (b).

Figure 5 .
Figure 5. Z-mode magnetic intensities in the r-MLT plane (a) and the meridian plane (b).

Figure 6 .
Figure 6.Z-mode intensity versus (a) frequency, (b) M-shell, and (c) magnetic latitude.In (c) we plot the intensity in both hemispheres.In the more poorly sampled S-hemisphere the intensities are weaker because the Juno orbit samples the S. hemisphere at larger radial distances than the N. hemisphere.