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. Author manuscript; available in PMC: 2020 Aug 31.
Published in final edited form as: J Geophys Res Space Phys. 2020 Feb 27;125(3):e2019JA027486. doi: 10.1029/2019ja027486

Magnetotail Reconnection at Jupiter: A Survey of Juno Magnetic Field Observations

Marissa F Vogt 1, John EP Connerney 2, Gina A DiBraccio 2, Rob J Wilson 3, Michelle F Thomsen 4, Robert W Ebert 5,6, George B Clark 7, Christopher Paranicas 7, William S Kurth 8, Frédéric Allegrini 5,6, Phil W Valek 5, Scott J Bolton 5
PMCID: PMC7458104  NIHMSID: NIHMS1615073  PMID: 32874821

Abstract

At Jupiter, tail reconnection is thought to be driven by an internal mass loading and release process called the Vasyliunas cycle. Galileo data have shown hundreds of reconnection events occurring in Jupiter’s magnetotail. Here we present a survey of reconnection events observed by Juno during its first 16 orbits of Jupiter (July 2016–October 2018). The events are identified using Juno magnetic field data, which facilitates comparison to the Vogt et al. (2010, https://doi.org/10.1029/2009JA015098) survey of reconnection events from Galileo magnetometer data, but we present data from Juno’s other particle and fields instruments for context. We searched for field dipolarizations or reversals and found 232 reconnection events in the Juno data, most of which featured an increase in |Bθ|, the magnetic field meridional component, by a factor of 3 over background values. We found that most properties of the Juno reconnection events, like their spatial distribution and duration, are comparable to Galileo, including the presence of a ~3-day quasi-periodicity in the recurrence of Juno tail reconnection events and in Juno JEDI, JADE, and Waves data. However, unlike with Galileo we were unable to clearly define a statistical x-line separating planetward and tailward Juno events. A preliminary analysis of plasma velocities during five magnetic field reconnection events showed that the events were accompanied by fast radial flows, confirming our interpretation of these magnetic signatures as reconnection events. We anticipate that a future survey covering other Juno datasets will provide additional insight into the nature of tail reconnection at Jupiter.

Plain Language Summary

Magnetic reconnection is an important physical process that allows for the release of mass and energy from a planetary magnetotail. Reconnection can be observed in the magnetic field data measured by spacecraft by looking for an increase or a reversal in the north-south direction of the magnetic field. In previous work, hundreds of reconnection events have been found in Jupiter’s magnetotail by analyzing magnetic field data from the Galileo mission at Jupiter. In this study we surveyed magnetometer data from the Juno mission to identify reconnection events, and we compared their properties to the Galileo events. Overall, we find that the Galileo and Juno data show similar results, including the ~3-day timescale for the recurrence of reconnection events and other activity in Jupiter’s magnetosphere. Though here we focused on Juno magnetometer data, we hope to extend our study to other Juno datasets in the future, and we expect those results will improve our understanding of the importance of tail reconnection in Jupiter’s magnetosphere.

1. Introduction

The primary source of plasma in Jupiter’s magnetosphere is the volcanically active moon Io, which contributes plasma to the system at an estimated rate of ~500–1,000 kg/s (e.g., Thomas et al., 2004). This plasma must ultimately be lost from the system, which could be achieved through magnetotail reconnection and plasmoid release. Reconnection at Jupiter has long been considered to be mostly rotationally driven, rather than driven by the solar wind as it is at Earth, in a process called the Vasyliunas cycle (Vasyliūnas, 1983). In the internally driven Vasyliunas cycle mass-loaded flux tubes rotate into the night side and become stretched radially by the centrifugal force until they break off and release a plasmoid. Hundreds of reconnection events have been observed by the Galileo spacecraft in Jupiter’s magnetotail (e.g., Kronberg et al., 2005; Russell et al., 1998; Vogt et al., 2010), providing initial estimates of event properties like their spatial extent, recurrence time, and location. However, the role of tail reconnection in the overall transport of mass and energy in Jupiter’s magnetosphere remains a topic of debate (e.g., Cowley et al., 2015; Vogt et al., 2014). In particular, small-scale reconnection called “drizzle” has been proposed as an internally driven mass loss mechanism (e.g., Delamere & Bagenal, 2010) and has recently been observed in the dayside magnetosphere of Saturn (e.g., Guo et al., 2018).

In this study we report on the results of a survey of tail reconnection signatures observed in magnetic field data collected by the Juno spacecraft during its first 16 orbits of Jupiter. In magnetic field data, tail reconnection events can be identified by field dipolarizations, reversals, or bipolar fluctuations in the north-south component of the magnetic field. In our analysis we also consider data from three of Juno’s particles and fields instruments to provide context for the magnetic field signatures, though we focus on the magnetometer data and compare our findings to the Vogt et al. (2010) survey of reconnection signatures observed in Galileo magnetic field data.

This paper is organized as follows. In section 2 we describe the Juno data used in this study, and in section 3 we compare the Juno and Galileo space-craft trajectories, instrumentation, and data availability. In section 4 we outline the automated reconnection event detection algorithm used to identify reconnection events in the magnetometer data. Section 5 contains our results, including an analysis of the event spatial distribution and frequency and a comparison to previous studies. Section 6 presents an over-view of data from Juno’s other particle and fields instruments from Orbit 11, which was one of the most active orbits observed to date, including Juno plasma moments from a particularly dynamic 4-day interval in January 2018. We conclude with a summary in section 7.

2. Juno Data Used in This Study

The Juno spacecraft has been in a 53-day polar orbit of Jupiter since July 2016, with an apojove of 113 RJ (Jovian radii, 1 RJ = 71,492 km) and inclination up to 105.5° (Bolton et al., 2017). In our analysis we use data from Juno’s first 16 orbits, from July 2016 through late October 2018. Juno is carrying a suite of instruments that measure the magnetic field and plasma properties in Jupiter’s magnetotail. Together, data from the particles and fields instruments can provide a complete picture of the conditions in Jupiter’s magnetotail. In our work we use data from the following instruments:

  1. MAG (Magnetometer; Connerney et al., 2017):vector fluxgate magnetometer measuring the magnetic field components. Here we use data with a time resolution of 1 s per vector.

  2. JEDI (Juno Energetic particle Detector Instrument; Mauk et al., 2017): measures energy and pitch angle distributions for electrons from 25 to 500 keV and energy, pitch angle, and ion composition distributions for ions at ~20–50 keV to more than 1 MeV.

  3. JADE (Jovian Auroral Distributions Experiment; McComas et al., 2017): provides energy spectra and pitch angle distributions for electrons at 0.05–100 keV and ions ~0.01–50 keV/Q, also measures ion composition from 1 to 50 amu.

  4. Waves (Juno WAVES Investigation; Kurth et al., 2017): provides electric spectra at frequencies 50 Hz to 40 MHz and magnetic spectra at frequencies 50 Hz to 20 kHz, with a time resolution of 1–10 s per spectrum during the outer portions of Juno’s orbit.

Tail reconnection can be observed in magnetic field data by field dipolarizations or reversals in the north-south component of the magnetic field, as illustrated in Figure 1 and described in more details in section 4, and in particle data by radial plasma flow bursts, heating, and increased density. At Jupiter, tail reconnection signatures have also been associated with increases in the hectometric, or HOM, radio emissions, the portion of Jupiter’s auroral radio emissions at frequencies from a few hundred mHz to a few MHz (Louarn et al., 2007, 2014). Here we focus on Juno magnetometer observations of tail reconnection signatures, though we also analyze data from JEDI, JADE, and Waves to provide context for the MAG data. Both JEDI and JADE can provide ion plasma moments (e.g., density and velocity) as higher order data products. Increased radial plasma flows observed with JADE or JEDI can confirm that field dipolarizations and reversals observed with the magnetometer are indeed signatures of reconnection. Here we present JADE plasma moments from a dynamic interval covering 4 days in January 2018, as ion plasma moments from Juno’s first 16 orbits are not yet available for large-scale survey studies. We anticipate that a future study will incorporate a full survey of the plasma flow and magnetic field signatures.

Figure 1.

Figure 1.

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Meridian plane schematic (not to scale) showing the expected field configuration and magnetic signature of reconnection in Jupiter’s magnetotail, modified from Figures 1 and 2 of Vogt et al. (2014). (a) The initial or relaxed field configuration. (b) The field configuration and expected direction of radial flow during reconnection. (c) Illustration of how the observed magnetic field signature of tail reconnection or a plasmoid can depend on the spacecraft trajectory through the reconnection region.

3. Juno and Galileo Compared: Instrumentation, Data Availability, and Spacecraft Trajectory

In this study we draw comparisons to previous results obtained using data from the Galileo spacecraft, in particular the tail reconnection signatures observed with Galileo’s magnetometer (Kivelson et al., 1992) and Energetic Particle Detector (EPD) (Williams et al., 1992) instruments. Galileo also measured Jupiter’s auroral radio emissions with the Plasma Wave Subsystem (PWS) instrument (Gurnett et al., 1992). Galileo data rates were limited due to the failure of the high-gain antenna, so the Galileo magnetometer measurements are typically available with a time resolution of 24 s per vector, and the EPD data have a typical time resolution of 3–11 min. Separate surveys of reconnection signatures in the Galileo magnetometer and EPD data yielded many more reconnection events in the magnetic field data than in the EPD, likely due to the magnetometer data’s higher time resolution, but showed overall good agreement, especially between the sign of Bθ, the meridional component of the magnetic field in SIII coordinates, and the radial direction of plasma flow (Kronberg et al., 2005; Vogt et al., 2010; Woch et al., 2002). Jupiter’s background field is southward at the equator so that Bθ is usually positive. As illustrated in Figure 1b, we expect locations planetward (tailward) of the reconnection x-line to generally feature enhanced positive (negative) Bθ and radially inward (outward) flow. Based on the good agreement between Galileo magnetic field and EPD data we have a reasonable amount of confidence in the accuracy of identifying reconnection events using magnetometer data alone. However, it will be of interest to eventually incorporate Juno plasma moments, which can confirm the magnitude and direction of radial plasma flows, in a large survey of Juno reconnection signatures.

Figure 2a shows Galileo’s orbit and the location of reconnection events observed by the Galileo magnetometer (Vogt et al., 2010), shown here in an equatorial plane view in the JSS (Jupiter De-Spun Sun) coordinate system in which the z-axis is aligned with Jupiter’s spin axis, the y-axis is obtained from the cross product of z^ and the direction to the Sun, and the x-axis completes the set (e.g., y is dusk and x is noon local time). Galileo’s orbit was largely confined to the equatorial plane and the orbital period varied in length but was typically around 50 days. Figure 2a shows that most reconnection events were observed in the post-midnight local time sector, though more Galileo data are available at these local times, particularly at cylindrical radial distances beyond ~90 RJ. After normalizing for the duration of time Galileo spent in each region (e.g., figure 9 of Vogt et al., 2010), reconnection events were observed most frequently in the deep predawn magnetotail (roughly 80–100 RJ, 0200–0400 LT). This region includes Galileo orbit G2, which was particularly active, featuring roughly 30% of all Galileo reconnection events. Other significantly dynamic orbits were G8 (18% of all Galileo events) and C9 (11% of all Galileo events). It is not clear why these Galileo orbits were so active compared to others, and recent analysis suggests that the predicted solar wind conditions upstream of Jupiter during orbits G2, G8, and C9 were fairly typical (Vogt et al., 2019 submitted).

Figure 2.

Figure 2.

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(a) Equatorial plane view of Galileo’s orbit (black lines), with circles showing the locations of reconnection events identified in Galileo magnetometer data by Vogt et al. (2010). In all panels, the color of the reconnection event indicates the dominant Bθ signature in each event, with red for Bθ > 0 events, blue for Bθ < 0 events, and green for mixed or bipolar Bθ events. The location of the statistical x-line derived by Vogt et al. (2010) based on the sign of Bθ is shown in purple. Light blue lines show the locations of the Joy et al. (2002) magnetopause (both compressed and expanded). The Sun is to the left. (b) As in Figure 2a but with the Galileo orbit now shown in orange and also showing the location of Juno’s orbit projected into the equatorial plane. Black lines show intervals when Juno was within 10° of the jovigraphic equator and gray lines show the remaining portions of Juno’s orbit. (c) Meridian plane view of Juno’s orbit (black) and Galileo’s orbit (orange). Reconnection events identified in Juno data are shown by colored squares. Dashed lines show ±10° jovigraphic latitude. (d) Juno’s orbit (black and gray lines) projected onto the equatorial plane and locations of reconnection events identified in this study with Juno magnetometer data. The Sun is to the left. (e) Bins of 15 RJ and 0.5-hr local time colored by the dominant sign of Bθ for Juno reconnection events in each bin (see text), shown in the equatorial plane with the Sun to the left. Gray bins show regions with data within 10° of the jovigraphic equator but no identified events. Black bins show regions for which there are no identified events and also no data within 10° of the jovigraphic equator. (f) Bins of 15 RJ and 1-hr local time colored by the dominant sign of Bθ for Galileo reconnection events. Gray bins show regions with data but no identified events, and black bins show regions with only one event or two events that do not have the same Bθ signature. Modified from figure 11 of Vogt et al. (2010).

Figure 2b again shows the Galileo orbit and reconnection event locations but with the equatorial plane projection of Juno’s first 16 orbits overplotted as thick black and gray lines. Juno’s orbit during this interval provides excellent coverage of the deep predawn magnetotail and a considerable amount of overlap with Galileo orbit G2. Therefore, the results of our Juno data survey can help establish whether the high level of activity during Galileo orbit G2 was due to spatial or temporal effects. Juno’s orbital tilt increases with time, so that with each successive orbit Juno spends less time near the equatorial plane and the equatorial crossing point of its orbit moves radially inward, as shown in Figure 2c. This means that Juno’s orbit is not as favorable as Galileo’s for observing the plasma sheet, which is located roughly at the magnetic equator. The plasma sheet passes over the jovigraphic equator every ~5 hr due to Jupiter’s ~10° dipole tilt, particularly during the later orbits considered here. Fortunately, Juno’s trajectory places it within 10° of the equatorial plane during most of the inbound portion of the first 16 orbits (black lines in Figures 2b and 2d), in a favorable position to observe reconnection signatures.

4. Identifying Reconnection Events in Juno Magnetic Field Data

We have surveyed magnetic field data from Juno’s first 16 orbits of Jupiter, looking for signatures of tail reconnection events. For data collected roughly near the jovigraphic equator, these signatures can be easily seen as reversals of or increases in the magnitude of Bθ, the meridional component of the magnetic field. The nature of the Bθ signature depends on several factors, including the spacecraft’s trajectory with respect to the region of reconnection, as illustrated in Figure 1c.

For identifying the Juno reconnection events we applied a set of criteria that was nearly identical to what was used in the Vogt et al. (2010) event detection method, which was developed to identify reconnection events in Galileo magnetometer data. The goal of the automated detection algorithm was to identify transient increases in |Bθ|, indicating a field dipolarization or reversal suggestive of reconnection, over background levels. We adjusted the criteria several times before arriving at a robust event detection method that minimized false positive detections and successfully identified most of the reconnection signatures that we would have selected by visual inspection. We defined the background Bθ as the 10-hr running average of |Bθ|, meaning that the background varies smoothly on time scales of days, following Vogt et al. (2010). They chose this definition for the background field because it removes variations in the magnetic field that occur due to the planet’s rotation, such as plasma sheet motion with respect to the spacecraft, but allows for radial and local time variations in the background, which occur on longer time scales as the spacecraft moves through the magnetosphere. We then required that |Bθ| increase by at least a factor of 2 over background values for at least 1 min. The background Bθ is typically ~1–2 nT, and for background values less than or equal to 5 nT we required |Bθ| increase by a factor of 3 over background values. However, for exceptionally large background values (larger than 5 nT) we required only an increase in |Bθ| by a factor of 2 over the background. Furthermore, we required that Bθ > 3 nT or Bθ < −2 nT, to ensure a significant |Bθ| increase even for small values of the background Bθ; this threshold was more relaxed for negative values of Bθ (−2 nT) than for positive values (3 nT) because the background Bθ is positive, so any sustained negative excursion of Bθ is suggestive of tail activity. Finally, the algorithm stepped backward and forward in time to define the start and end points for the interval of enhanced |Bθ| as follows: we considered |Bθ| to be “enhanced” as long as Bθ remained negative or |Bθ| remained at least 1.5 times the background level (or greater than 1.5 nT if the background was less than 1 nT). Once |Bθ| failed to meet this requirement for 2 min, the algorithm identified the start or stop time of the |Bθ| enhancement. After identifying intervals with an enhanced |Bθ|, we considered intervals of enhanced |Bθ| to be part of one event if they occurred within 60 min of each other. We then discarded any events lasting less than 10 min or longer than 10 hr (one Jovian rotation). Following Vogt et al. (2010), we also restricted our analysis to data at radial distances of at least 30 RJ and to times when the spacecraft was inside the magneto-sphere, excising intervals within 10 hr of a boundary crossing (e.g., Hospodarsky et al., 2017).

Finally, we classified each event according to the dominant sign of Bθ, which provides a proxy for the event location with respect to a reconnection x-line and the expected direction of radial flow. Because Jupiter’s background field is southward near the equator, we typically expect events planetward of the x-line to feature an enhanced positive Bθ and events tailward of the x-line to feature a negative Bθ signature, while events with both a positive and negative enhanced Bθ can indicate that a plasmoid or the reconnection x-line passed over the spacecraft, as illustrated in Figure 1b. Comparison between the sign of Bθ and the direction of radial plasma flow, as measured by Galileo’s EPD instrument, have shown very good agreement, but it is important to remember that the sign of Bθ is only a proxy for the flow direction, since the nature of the Bθ signature can also depend on other factors, including the spacecraft’s trajectory with respect to the region of reconnection, as illustrated in Figure 1c. Following Vogt et al. (2010), we classified an event as “Bθ > 0” if Bθ remained positive for at least 85% of the event duration, “Bθ < 0” if Bθ remained negative for at least 85% of the event duration, and “Bθ bipolar” or mixed for all other events. Our results are summarized in the next section.

5. Juno Reconnection Events, Their Properties, and Comparison to Galileo

Our automated detection algorithm found 232 events in the Juno magnetometer data. Figure 2c shows their locations in a meridian plane, and Figure 2d shows their locations projected onto the equatorial plane, with colors indicating the dominant sign of Bθ in each event. Supporting information, Table S1 contains a list of the start and stop times, locations, and Bθ sign classifications for all 232 Juno reconnection events.

Figure 3 shows the magnetic field components, field bendback and elevation angles, and field magnitude for three example events. The field bendback angle, α, indicates the sweepback (or sweep forward) of the magnetic field out of a meridian plane and is defined as α=tan−1(BφBR). The magnetic field is usually swept back with respect to the radial direction, with BR and Bφ having opposite signs, so that α is typically negative.

Figure 3.

Figure 3.

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(a) Magnetic field components, field angles, and field magnitude for a Juno reconnection event from Orbit 2 on Day 286 (12 October) of 2016, at radial distance 70 RJ and local time 05:38. The top panel shows BR and Bφ, the radial (black) and azimuthal (blue) components of the magnetic field, respectively. The second panel shows Bθ, the meridional component of the magnetic field, in black, as well as ± the background Bθ (green). The third panel shows the magnetic field elevation angle and the fourth panel shows the field bendback angle, plotted only when |BR| > 3 nT (see text). The bottom panel shows |B|, the field magnitude. The purple highlighted region in all panels shows the reconnection event interval as defined by our automated detection algorithm. (b) As in Figure 3a but for two events from Juno Orbit 8, on Day 226 (14 August) of 2017, at radial distance 106 RJ and local time 04:08.

Changes to the bendback angle can be used to infer radial flow through conservation of angular momentum; for example, inward moving plasma will experience an increase in its angular velocity and the magnetic field lines, which are frozen into the flow, will become less bent back (α will become less negative or even positive). The field elevation angle, θelevation, indicates the angle that the magnetic field makes with respect to the radial direction in the R-θ plane and is defined as θelevation=tan−1(Bθ|BR|). The elevation angle is positive for a southward field (positive Bθ) and changes smoothly as BR changes sign on either side of the current sheet. We evaluate both angles only when |BR| > 3 nT because small fluctuations in BR can lead to large fluctuations in the field angles when BR is small.

The event in Figure 3a is from Orbit 2 on Day 286 (12 October) of 2016, when Juno was located at 70 RJ in radial distance and 05:38 local time. During this event |Bθ| is enhanced above the background (green lines in Bθ panel), first with a negative value, then a positive value, then finally a negative value again. As Bθ turns positive the bendback angle briefly turns positive, suggesting inward flow, which could explain why Bθ changes sign. Overall, the sign of Bθ during the event could mean that the x-line moved over the spacecraft during the event. The field magnitude peaks before the maxima in |Bθ| and is only a few nT during part of the event, indicating that the spacecraft was located in or very close to the current sheet during the event, and consistent with previous observations that most plasmoids at Jupiter are loop-like or are “crater” flux ropes (Farrugia et al., 1988), lacking a core field, as expected in the high β (ratio of thermal to magnetic pressure) Jovian plasma environment (Kivelson & Khurana, 1995; Vogt et al., 2014). The two events in Figure 3b are from Orbit 8 on Day 226 (14 August) of 2017, when Juno was located at 106 RJ in radial distance and 04:08 local time. In the first event, from 08:53 to 11:45 UT, Bθ is initially enhanced and positive, then goes through zero, and becomes negative, with an overall morphology resembling the bipolar Bθ signature expected during a tailward-moving plasmoid. The bendback angle initially increases then decreases as Bθ turns negative, suggesting outward flow. In the second event in Figure 3b, from 13:06 to 13:58 UT, Bθ is enhanced and positive, and the bendback angle becomes positive. The Bθ behavior and positive bendback angle are both consistent with inward flow for a spacecraft location planetward of a reconnection x-line.

Table 1 gives a summary of various event properties for both the Juno reconnection events identified here and the Galileo events identified by Vogt et al. (2010). The average duration of Juno events is 83 min, compared to 59 min with Galileo. Both Juno and Galileo events were observed over nearly the full range of radial distances considered, with a median radial distance of 80 RJ for the Juno events and 84 RJ for the Galileo events. Overall, we find that most properties of the Juno reconnection events are similar to the Galileo event properties, particularly after taking into considerations differences between Galileo’s and Juno’s orbits.

Table 1.

Event Duration and Spatial Distribution.

Galileo events (all)a Juno events (all) Juno Bθ > 0 events Juno Bθ < 0 events Juno Bθ mixed events
Number of events (percent of total) 249 232 131 (56) 22 (10) 79 (34)
Average duration (min) 59 85 75 155 81
R rangeb (RJ) 33.25 to 145.6 37.5 to 113.4 38.1 to 113.4 51.7 to 112.5 37.5 to 112.7
Median R (RJ) 84 81 79 100 78
Jovigraphic latitude rangeb (degrees) −4.3 to 0.7 −17.8 to 10.3 −17.8 to 10.3 −12.4 to 5.4 −15.2 to 9.8
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a

See Vogt et al. (2010), Table 1.

b

The latitude range of the Galileo and Juno orbits varied significantly (see Figure 2c).

Additionally, the radial distance range of two datasets varies, with apoapsis ~145 RJ for Galileo and ~113 RJ for Juno (see Figure 2b).

We identified almost as many events, 232, in just under 2.5 years of Juno data as Vogt et al. (2010) did with Galileo (just 249 events in nearly 7 years of data), despite very similar event selection criteria. However, the Galileo data have significant gaps and also span all nightside local times. So far, Juno’s orbit has been confined to post-midnight local times, where the Galileo event occurrence rate is highest (more than 75% of the Galileo events were identified at post-midnight local times). Therefore, the overall event occurrence rate is roughly comparable for Juno and Galileo when considering post-midnight local times. Figure 4a shows the amount of available Juno data, Figure 4b shows the amount of Juno data within ±10° latitude of the jovi-graphic equator for a more direct comparison with the Galileo data availability shown in Figure 4c (Vogt et al., 2010). Figure 4d shows the event occurrence rate, or duration of all events in each bin divided by the amount of time Juno spent in each bin, in bins of 15 RJ and 1-hr local time projected onto the equatorial plane, and Figure 4e shows the event occurrence rate but calculated only considering Juno data within ±10° jovigraphic latitude. Figure 4f shows the Galileo event occurrence rate (Vogt et al., 2010). The Juno event occurrence rate is slightly lower than the Galileo post-midnight event occurrence rate when considering all data (Figure 4d) but slightly higher than Galileo when considering only Juno data within ±10° jovigraphic latitude (Figure 4e).

Figure 4.

Figure 4.

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Comparison of data availability and reconnection event occurrence frequency from Galileo to Juno, shown in an equatorial plane view in the JSS coordinate system. (a) Bins of 15 RJ and 0.5-hr local time colored by the number of hours that Juno spent in each bin. The location of the Joy et al. (2002) magnetopause is shown in blue. (b) As in Figure 4a but showing amount of data within ±10° latitude of the jovigraphic equator. (c) As in Figure 4a but for Galileo data, modified from Figure 1 of Vogt et al. (2010). The black square highlights the region plotted in Figure 4a. (d) As in Figure 4a but for the event occurrence rate, which is the duration of all events in each bin divided by the amount of time Juno spent in each bin. Bins with data but no observed events are colored in gray. (e) As in Figure 4d but normalizing by amount of Juno data within ±10° jovigraphic latitude. (f) As in Figure 4d but for Galileo data, modified from figure 9 of Vogt et al. (2010). Black bins indicate regions with fewer than 10 hr of data or with two or fewer identified reconnection events. The black square highlights the region plotted in Figure 4d.

As with Galileo, we find that some Juno orbits were exceptionally dynamic while others were quiet. For example, Juno Orbit 11 featured 25 events, while Orbit 12 contained just two events. On average we identified about 14 events per orbit. Since consecutive Juno orbits, like Orbits 11 and 12, have nearly identical trajectories, it is likely that the difference in magnetospheric activity from one orbit to another was due to temporal, and not spatial, effects, such as changes in Io’s plasma production rate or in the external solar wind conditions. The latter half of the Juno orbits analyzed here (roughly Orbits 8 through 16) significantly spatially overlapped with Galileo’s dynamic orbits G2 and G8, but only Juno Orbits 8 (24 events) and 11 were exceptionally active. The earlier Juno orbits were located relatively close to the expected compressed magnetopause location (see light blue line in Figure 2d) and were very active, especially Orbit 1 (30 events). Yao et al. (2019) recently examined the association between Jupiter’s auroral activity, magnetic flux loading/unloading in the magnetotail, and reconnection events. They also noted temporal variability in both the auroral activity and amount of magnetic loading/unloading, with some very quiet intervals and some more active.

While there are many similarities between the Juno and Galileo events, we found significant differences in the event Bθ sign classifications and the resulting inferred location with respect to a reconnection x-line.

We classified roughly 56% of Juno events (131 of 232) as “Bθ > 0,” 10% (22 of 232) as “Bθ < 0,” and the remaining 34% (79 of 232) as “Bθ bipolar.” By comparison, the Galileo events were classified as 52% “Bθ > 0,” 30% “Bθ < 0,” and 18% “Bθ bipolar.” It is not clear why “Bθ < 0” events are so infrequent in the Juno event list compared to Galileo, but the difference could possibly be due to the fact that Juno data come from higher latitudes than Galileo. Because of the inclined nature of Juno’s orbit there is essentially no Juno data at cylindrical radial distance ρ > 90 RJ and local times 00:00–03:00, where most Galileo Bθ < 0 events were observed. This can be seen in the Juno data availability plot in Figure 4b and by the black bins in Figure 2e. Additionally, the spacecraft’s latitudinal position would affect its trajectory through a plasmoid or reconnection region and therefore would influence the nature of the Bθ signature recorded by the spacecraft. For example, a spacecraft like Galileo near the equator could record a bipolar Bθ signature (Trajectory A in Figure 1c) but a high-latitude spacecraft like Juno might record only a positive Bθ signature (Trajectory D in Figure 1c) for the same event. Another possible explanation for the difference in the frequency of “Bθ < 0” events from Galileo to Juno is the higher time resolution of Juno data (1 s per vector) compared to Galileo (24 s per vector). We considered this difference when testing our event detection algorithm by applying to Juno data that had been artificially down sampled to a 24-s time resolution. We found that about 90% of events were identified in both the high and low time resolution data, though the precise start and end times varied, which could affect the event Bθ sign classifications.

Because Juno observed so few negative Bθ events it is not easy to identify a statistical x-line, for example by binning events into regions of 15 RJ and 0.5 hr of local time, as shown in Figure 2e. In this figure there are not obvious regions of mostly planetward (positive Bθ, red) and mostly tailward (negative Bθ, blue) events. However, the comparable figure made using Galileo magnetometer data, reproduced here as Figure 2f, includes a clear delineation between red and blue bins near 90 RJ, with just a few green bins. The Vogt et al. (2010) x-line derived from Galileo magnetic field events, drawn in Figure 2 in purple, showed good agreement with the statistical separatrix identified using the distribution of radial flow bursts from Galileo EPD data (Woch et al., 2002). However, the Galileo x-line does not seem to match the Juno event Bθ distribution. It is also surprising and puzzling that most Juno events at ρ > 90 RJ and local times beyond 04:30 LT featured a positive Bθ signature, suggesting inward flow and a position planetward of the reconnection x-line. Future study, especially using the measured direction of radial plasma flows rather than using the sign of Bθ as a proxy, should provide insight into the differences between the Galileo and Juno results.

6. Juno Orbit 11 Multi-Instrument Overview

Thus far, our discussion has exclusively covered Juno magnetic field measurements of tail reconnection events. In this section we briefly present measurements from some of Juno’s other particles and fields instruments during Juno’s Dynamic Orbit 11 to provide context for some of the magnetic field events studied here. In a future study we intend to survey these other datasets from other orbits in more detail.

Figure 5 shows Juno JEDI, JADE, Waves and magnetic field data covering the interval from 11 January to 4 February 2018 (days of year 11–35), when the spacecraft was moving inbound from a radial distance of ~110 to ~40 RJ. The top panel shows 1-hr averages of an integrated electron quantity, a quantity similar to density, including all electrons above ~30 keV energy as measured by JEDI’s Solid State Detector 3. The next two panels show JEDI and JADE energy-time spectrograms. The fourth panel shows a Waves frequency-time spectrogram at roughly hectometric frequencies. In this figure we show frequencies from 3 to 7 MHz because of some instrumental issues like poor sensitivity and an interference band at lower frequencies. The frequencies in the figure are at the high end of the typical range given for the hectometric auroral radio emissions, a few hundred kHz to a few MHz (e.g., Gurnett et al., 1992 calculated the HOM integrated power flux over frequencies from 0.5 to 5.6 MHz). The bottom three panels show the three components of the magnetic field and the magnetic field magnitude. The JEDI, JADE, and Waves data in the top four panels all show increased fluxes or counts every 5 hr (half a Jovian rotation) as Juno passes through the plasma sheet, especially at distances inside ~100 RJ, when Juno was close to the jovigraphic equator. A similar 5-hr periodicity can be seen in all three magnetic field components and the field magnitude in the bottom three panels of Figure 5 and is most pronounced in BR, which reverses sign as Juno passes through the plasma sheet. Reconnection events we have identified here are highlighted in red in the Bθ panel (second from bottom).

Figure 5.

Figure 5.

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JEDI, JADE, Waves, and MAG data from Juno Orbit 11, spanning 11 January to 5 February 2018. From top: JEDI 1-hr averages of an integrated electron quantity (similar to density); JEDI electron energy-time spectrogram; JADE ion energy-time spectrogram; Waves frequency-time spectrogram at roughly hectometric (HOM) frequencies (3 to 7 MHz); the radial (black) and azimuthal (blue) magnetic field components; Bθ (black) and ± the 10-hr running average of |Bθ| (green), with reconnection events highlighted in red; the magnetic field magnitude. Orange vertical lines highlight quasi-periodic behavior in most datasets that occurs on a ~3-day timescale, and purple bars in the Bθ panel suggest intervals of reconnection event “clusters” that roughly occur on this ~3-day timescale.

This interval was highly dynamic, with 27 reconnection events shown in Figure 5, compared to just 12 events in the orbits immediately before and after combined.

The overview of data in Figure 5 shows that the observed reconnection events are often “clustered” into groups with a separation of ~3 days and that the other Juno datasets (JEDI, JADE, and Waves) also display temporal changes associated with the reconnection signatures and ~3-day quasi-periodicity. For example, at the times of the dashed orange lines at roughly 24, 27, and 30 January and 3 February, the JEDI spectrogram shows increased fluxes at low energies, the JADE spectrogram shows decreases near ~1 keV, and the Waves data show an increase in the hectometric radio emissions. These changes are accompanied by, or followed shortly by, a reconnection event(s) visible in the Bθ panel. Similar behavior is also seen at earlier times (see the dashed orange lines on 12, 15, and 18 January) but is less clear, possibly due to Juno’s higher latitude (and corresponding increased distance from the plasma sheet) during this interval.

Overall, the activity during this interval is similar to the ~2–3 day quasi-periodic modulations observed by Galileo in multiple datasets, including the ion spectral index, hectometric radio emissions, magnetic field, energetic particle anisotropies, and occurrence of flow bursts and tail reconnection events (e.g., Kronberg et al., 2007, 2008, 2009; Louarn et al., 1998, 2000; Vasyliūnas et al., 1997; Vogt et al., 2010; Woch et al., 1998). This quasi-periodic behavior is generally thought to be associated with internally driven mass loading and release of the Vasyliunas cycle (e.g., Kronberg et al., 2007), though the characteristic time scale has been observed to vary from 1 to 7 days (Kronberg et al., 2009). At least some quasi-periodic behavior has been reported in almost all Galileo post-midnight orbits (Kronberg et al., 2009) but was most pronounced, and seen in multiple datasets, during select Galileo orbits like G2, G8, and C9. Therefore, it is interesting that at least a cursory analysis of Juno data shows quasi-periodic changes in some orbits, like Orbit 11 in Figure 5, but not others like Orbits 10 or 12. As with the occurrence of reconnection events, it is likely that the difference in magnetospheric activity from one orbit to another is due to temporal, and not spatial, effects.

Finally, Figure 6 presents detailed JADE plasma measurements for the interval 17–21 January 2018. The proton and heavy ion moments are three-dimensional, where the protons are obtained by summing the JADE-I Species 3 and 4 products to capture all the protons (mass per charge ratio m/q = 1/1), and the heavies are from JADE-I Species 5 with a m/q of 24/1.5. The moments use the 2019 calibrations and do not account for any spacecraft potential or background in the data. We plot the density, temperature, and velocity only for times when the ratio of the measured quantity to its uncertainty is large. Note that these criteria are not satisfied for many times in the early part of this interval, due in part to the very low counts during that time (see spectrograms in the top two panels of Figure 6). Finally, we note also that the heavy ion velocities should be treated with caution because in some cases the energy of the heavy ions is approaching or exceeding the energy range of the instrument (see top panel of Figure 6).

Figure 6.

Figure 6.

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JADE and MAG data for the interval 17–21 January 2018, during Juno Orbit 11. From top: JADE energy-time spectrograms for heavy ions and protons; JADE plasma densities for heavy ions (blue) and protons (red); plasma temperatures; plasma radial velocity VR; plasma meridional velocity Vθ; plasma azimuthal velocity Vφ; the radial (black) and azimuthal (blue) magnetic field components; Bθ (black) and ± the 10-hr running average of |Bθ| (green), with reconnection events highlighted in red; the magnetic field magnitude.

During the interval plotted in Figure 6 the magnetic field data recorded a chain of reconnection events, including several plasmoid signatures with a classic bipolar Bθ signature. The first, second, and fifth events in this interval were classified as “Bθ < 0” or “Bθ mixed,” suggesting that the spacecraft was located either close to or tailward of the reconnection x-line so that we would expect to observe outward radial plasma flow. We note that the fourth event, at 9:56 UT on 19 January, was classified as a “Bθ > 0” event, because the event detection algorithm only identified an enhanced positive Bθ signature. However, the event was shortly followed by an interval of negative Bθ reaching almost −2 nT, though our automated event detection algorithm did not include the negative Bθ interval in the event definition because the background |Bθ| was relatively large (see green line in Bθ panel). Therefore, the Bθ signature during and following the fourth event may be more accurately classified as a classic bipolar Bθ plasmoid signature. In all but the third event from this interval, at 22:10 UT on 18 January, the JADE plasma data show a large positive or outward radial velocity, as we would expect for these “Bθ < 0” and “Bθ mixed” events. During the third event Bθ remains positive and the plasma radial velocity briefly turns weakly negative or inward. It is therefore possible that this event may have been misidentified as tail reconnection and that the increase in |Bθ| is produced by some other process, like magnetotail flapping (e.g., Volwerk et al., 2013). However, overall, the JADE plasma data from this interval confirm our interpretation of these magnetic signatures as reconnection events. The direction of radial flow (outward) is consistent with the event negative or bipolar Bθ signature, confirming that the sign of Bθ can be used as a good proxy for the radial flow direction and spacecraft position with respect to a reconnection x-line.

The detailed JADE plasma measurements are also useful for calculating the mass of each plasmoid event and the overall mass loss rate. These calculations have been attempted in the past using Galileo data, but previous studies used average values for the plasmoid radial velocity and density because of the low time resolution of the Galileo EPD data (e.g., Kronberg et al., 2005, 2008; Vogt et al., 2014). These previous studies calculated plasmoid mass loss rates of ~1–120 kg/s, significantly lower than the estimated ~500–1,000 kg/s plasma input rate from Io (e.g., Thomas et al., 2004). The plasmoid mass is calculated by multiplying the density by plasmoid volume. The plasmoid volume depends on the plasmoid length, which in turn is calculated by multiplying the plasmoid duration by the radial velocity. It is convenient to define the plasmoid duration as the time from the maximum to minimum Bθ, or from the peak southward to northward field, though this definition may underestimate the plasmoid size by a factor of ~4–8 (e.g., Slavin et al., 1993; Kivelson & Khurana, 1995; see discussion in Vogt et al., 2014).

A detailed plasmoid mass loss rate calculation is beyond the scope of this initial study. However, we note that the observations from this interval—JADE radial velocities of 200–600 km/s, densities ~0.02–0.03/cm3, and plasmoid duration ~15–40 min—are comparable to or slightly larger than the values observed or assumed by previous authors (velocity 450 km/s, density 0.01–0.02/cm3, and duration 7–20 min; Kronberg et al., 2005, 2008; Vogt et al., 2014). To provide one example, in the fifth event in Figure 6, around 23:00 UT on 19 January, the plasmoid duration is about 40 min, and the average proton radial velocity is ~520 km/s. These values lead to a plasmoid length of ~17.5 RJ, nearly seven times the average plasmoid length, 2.6 RJ, calculated by Vogt et al. (2014) using Galileo data. We expect that future, more detailed calculations using Juno data will further constrain the plasmoid mass loss rate and will therefore help establish the role of tail reconnection in the overall transport of mass and energy in Jupiter’s magnetosphere.

7. Summary and Conclusions

We have surveyed magnetic field data from Juno’s first 16 orbits of Jupiter, covering the interval from July 2016 through late October 2018, to identify signatures of magnetotail reconnection events. The focus of our study was the Juno magnetometer data, in part to facilitate comparison to a previous survey of reconnection events in Galileo magnetometer data at Jupiter (Vogt et al., 2010). However, we also presented data from Juno’s JEDI, JADE, and Waves instruments from an especially dynamic interval during Orbit 11 in January 2018 to provide context for the magnetic field measurements, and we expect that a future study will incorporate a full survey of the Juno plasma flow and magnetic field signatures.

Reconnection can be identified in magnetometer data as a field dipolarization or reversal compared to background values. To select events in the Juno magnetometer data, we developed an automated event detection algorithm based on the Vogt et al. (2010) event detection method, which was developed to identify reconnection events in Galileo magnetometer data. We typically required that |Bθ| increase by a factor of 3 over the background, which we defined as the 10-hr running average of |Bθ|. Our algorithm identified 232 reconnection events in the Juno data.

Overall, we found that most properties of the Juno reconnection events like their spatial distribution, occurrence frequency, and duration are comparable to their Galileo counterparts, particularly after accounting for differences between the Galileo and Juno orbital coverage. Galileo’s orbit was largely confined to the equatorial plane, while Juno is in a polar orbit around Jupiter (see Figure 2c). Juno events have an average duration of 85 min and a median radial distance of 81 RJ, while the Galileo events have an average duration of 59 min and a median radial distance of 84 RJ. As with Galileo, we found that some Juno orbits were exceptionally dynamic, like Orbit 11 in January 2018, while others were quiet. However, the Juno data have a key advantage in that consecutive orbits have nearly identical trajectories, so we could conclude that differences in magnetospheric activity from one orbit to another are most likely due to temporal, and not spatial, effects, such as changes in Io’s plasma production rate or in the external solar wind conditions.

One significant difference between the Galileo and Juno results is in how events with different radial flow direction, as inferred from the sign of Bθ, are spatially distributed. We found that negative Bθ events, suggesting outward radial flow, were significantly less frequent in the Juno data than in Galileo. We were therefore unable to clearly define a statistical x-line in the Juno events, though the distribution of Galileo reconnection events showed a clear delineation between mostly planetward (positive Bθ, red) and mostly tailward (negative Bθ, blue) events at cylindrical radial distance ~90 RJ. Similar analysis at Saturn using Cassini magnetometer data has also failed to identify a clear statistical reconnection x-line (Smith et al., 2016). We expect that future Juno data surveys, particularly using measured plasma flows from Juno’s other instruments, will help provide insight into the differences between the Galileo and Juno results and possibly define a Juno x-line.

Finally, we presented an overview of Juno JEDI, JADE, Waves and magnetic field data during the inbound portion of Juno Orbit 11, which was particularly active. These data showed that the observed reconnection events are often “clustered” into groups with a separation of ~3 days and that the other Juno datasets (JEDI, JADE, and Waves) also displayed temporal changes associated with the reconnection signatures and ~3-day quasi-periodicity. This time scale is similar to the ~2- to 3-day quasi-periodicity that has been reported in multiple Galileo datasets and thought to be associated with internally driven mass loading and release of the Vasyliunas cycle. We also showed JADE plasma moments for 4 days in January 2018, covering five reconnection events identified in the magnetometer data. Most of our events were accompanied by large radial outward flow, confirming our interpretation that the magnetic signature indicates tail reconnection and that the sign of the Bθ during each event can be used as a good proxy for the flow direction. In the future we intend to expand our study to a full analysis of other Juno datasets, which we expect will provide additional insight into the nature of tail reconnection at Jupiter and its role in the overall transport of mass and energy in Jupiter’s magnetosphere.

Supplementary Material

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Key Points:

  • Juno has observed magnetic signatures of reconnection in Jupiter’s magnetotail

  • The overall distribution and frequency of Juno events is similar to previous Galileo observations

  • Other Juno instruments also show a response during magnetic field reconnection events and show a ~3-day periodic activity in Jupiter’s magnetosphere

Acknowledgments

MFV was supported by the Juno Participating Scientist program (Grant 80NSSC19K1263) and by NASA Grant 80NSSC17K0733. MFV gratefully acknowledges past discussions with Margaret Kivelson, Krishan Khurana, Steve Joy, and Ray Walker, who were all coauthors on previous work that motivated this study. We thank George Hospodarsky for providing the list of Juno magnetopause boundary crossings through the end of 2018. The research at the University of Iowa is supported by NASA through Contract 699041X with the Southwest Research Institute. Juno MAG data used in this study were the 1-s PC files from dataset “JNO-J-3-FGM-CAL-V1.0” that can be found on the Planetary Data System at https://pds-ppi.igpp.ucla.edu/. The Juno JADE dataset “JNO-J_SW-JAD-3-CALIBRATED V1.0” version 02 files, the JEDI dataset “JNO-J-JED-3-CDR-V1.0,” and the Juno Waves dataset

Footnotes

Supporting Information:

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