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A&A
Volume 691, November 2024
Article Number A13
Number of page(s) 8
Section Planets and planetary systems
DOI https://doi.org/10.1051/0004-6361/202450905
Published online 25 October 2024

© The Authors 2024

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1 Introduction

Magnetic reconnection is a fundamental process in laboratory and space plasma systems that involves the rearrangement of magnetic topology, leading to the conversion of magnetic energy into kinetic energy and thermal energy (i.e., particle acceleration (Parker 1979; Biskamp 1996; Priest & Forbes 2000; Yamada et al. 2010)). In planetary magnetospheres, magnetic reconnection is mostly observed at the dayside magnetopause and in the nightside magnetotail, albeit through different mechanisms at Earth and the giant planets, such as Saturn and Jupiter (Sonnerup et al. 1981; Russell et al. 1998; Øieroset et al. 2001; Jackman et al. 2007; McAndrews et al. 2008; Vogt et al. 2010, 2020; Montgomery et al. 2022). At Earth, the reconnection at both sites is primarily driven by the interplanetary magnetic field (IMF) through interaction between solar wind (SW) and the geomagnetic field, known as the Dungey cycle (Dungey 1961). At giant magnetospheres, the Dungey cycle may not play a dominant role due to a relatively lower SW merging rate and faster planetary rotation (Zhang et al. 2021, 2024). For example, at Jupiter, internal plasma sources from its volcanic moon Io and centrifugal force due to fast planetary rotation are considered the main mechanism driving nightside magnetotail reconnection near the edge of the magnetodisc, referred to as the Vasyliunas cycle (Vasyliunas 1983; Delamere & Bagenal 2013). The radial plasma pressure force may also play an important role in this process (Mauk & Krimigis 1987; Paranicas et al. 1991). According to the Vasylinuas cycle, in giant magnetospheres, magnetotail reconnection has been observed between 37.5 and 124.2 Jovian Radii (RJ) at low latitudes, with a significant dawn-dusk asymmetry caused by fast rotation (Vogt et al. 2010, 2020).

Besides the magnetopause, dayside magnetic reconnection may also occur within the magnetosphere when fast-rotating magnetodiscs form in giant magnetospheres. For example, in the Kronian magnetosphere, Cassini magnetometer datasets have revealed instances of potential dayside magnetodisc reconnection events (Bθ reversal) (Delamere et al. 2013, 2015). More recently, Guo et al. (2018) have reported conclusive observational evidence for dayside magnetodisc reconnection events at Saturn, showing other key signatures associated with the Bθ reversal in a magnetic reconnection event as predicted in the fast Hall reconnection theory (Birn et al. 2001), such as proton and electron energy enhancement, and Hall magnetic fields. The reconnection sites are patchily distributed in the magnetodisc and rotate with the magnetosphere (Guo et al. 2019). Given the similarities between the underlying driving mechanisms of the Kronian and Jovian magnetospheres, an interesting question arises as to whether dayside magnetodisc reconnection can occur in the Jovian magnetosphere. If so, we are interested in knowing what the spatial distribution and occurrence rate of such dayside magnetodisc reconnection events are.

At Jupiter, indirect evidence has indicated the possible existence of dayside magnetodisc reconnection. For example, unique auroral dawn storms and auroral injections at Jupiter might be induced by the occurrence of dayside magnetodisc reconnection (Yao et al. 2020). In the morning sector beyond 30 RJ, abrupt decreases of the magnetic field magnitude to less than 0.2–1.0 nT were first identified in Ulysses data (Southwood et al. 1993; Haynes et al. 1994) and later in reanalysis of Pioneer and Voyager data (Leamon et al. 1995). In situ Bθ reversal events in the day-side magnetodisc were first reported by Kivelson & Southwood (2005). Using Galileo magnetic field data, the observed reversal of the Bθ component occurred within the Jovian dayside magnetosphere near 30–50 RJ, indicating a possible dayside magnetodisc reconnection event. However, this dayside magnetodisc event was only characterized by a reversal (northward disturbance) of Bθ, without the observations of the particle associated with the reconnection site. Recently, Zhao et al. (2024) provided the first evidence of Jovian dayside magnetodisc reconnection at 30–60 RJ using magnetic and particle data from Galileo and Voyager. However, the identified events are still limited, and thus the general global picture for dayside magnetodisc reconnection in the Jovian magnetosphere remains uncertain.

Due to the limited coverage of in situ satellite data in the Jovian magnetosphere, a general picture of the distribution of dayside magnetodisc reconnection events at Jupiter is unavailable, and thus extensive exploration by future Jovian missions, such as JUICE and Europa Clipper, is required. To answer the question of the existence of Jovian dayside magnetodisc reconnection and to bridge the gap between existing observational evidence and future exploration, we used a high resolving power three-dimensional global MHD code of Jupiter’s magnetosphere to quantify the possible spatial distribution of dayside Jovian magnetodisc reconnection events and explore the SW conditions that are more likely to cause dayside magnetodisc reconnection. This study not only sheds light on the mechanisms related to the generation and evolution of dayside magnetodisc reconnection, but it also provides a theoretical prediction for observing dayside magnetodisc reconnection events when future missions such as JUICE and Europa Clippers arrive at Jupiter. The paper is organized as follows: Section 2 describes the model and simulation setups, Section 3 presents the simulation results, and Section 4 discusses the formation and evolution of dayside magnetodisc reconnection in the Jovian magnetosphere.

2 Methods

We used the Grid Agnostic MHD for Extended Research Applications (GAMERA) global model (Zhang et al. 2019) to simulate the space environment of Jupiter’s magnetosphere. The GAMERA code uses finite-volume techniques to solve the ideal MHD equations on a non-orthogonal, curvilinear grid that is adapted to Jovian magnetospheric problems. The grids are oriented in solar-magnetospheric coordinates, where the X, Y, and Z axes correspond to the Sun, east (duskside), and magnetic north, respectively. The grid resolution varies with radial distance to the planetary center, with the highest radial resolution of ~0.15 RJ near the low-altitude (inner) boundary, which is located at a Jovicentric distance of 3.5 RJ. The Jovian magnetospheric simulation extends to 120 RJ in the sunward direction, −1300 RJ in the anti-sunward direction, and ±470 RJ in directions perpendicular to the Sun-Jupiter axis. To simplify the analysis, the dipole tilt angle of the Jovian magnetosphere is set to 0. Thus the periodic flapping of the Jovian magnetodisc is not considered in this study. The 10-hour rotation of Jupiter is implemented by imposing a time-stationary corotation potential onto the ionospheric potential through electrostatic magnetosphere-ionosphere coupling (Zhang et al. 2018). The simulated heavy-ion mass loading from the Io plasma torus is 1000 kg s−1, implemented in the simulation domain as a ring centered at 6.5 RJ in the equatorial plane.

The upstream SW and IMF conditions that drive the global Jovian magnetosphere model have two periods to simulate the response of dayside magnetodisc reconnection to different SW ram pressure, based on typical SW/IMF conditions measured near Jupiter’s orbit (Blanc et al. 2005; Delamere & Bagenal 2010; Jackman & Arridge 2011). During the initial 0–416 hours of simulation time (ST), the upstream SW temperature, density, velocity, and IMF east-west component (By) are set to enhanced parameters of 2.0 × 104 K, 0.5 cm−3, 650 km s−1, and 0.5 nT, respectively, corresponding to a ram pressure of 0.11 nPa and Alfvén Mach numbers of 42.1. It should be noted that only the simulation period after the start-up transit (~300 h) is suitable for analysis, within which the average radial profile of the heavy ion density was settled into a quasi-steady state in a spin-averaged sense. In this study, the data after 380 : 00 ST are analyzed. To study the response of dayside magnetodisc reconnection during SW rarefaction events, the SW density and velocity were set to 0.25 cm−3 and 350 km s−1 during 416 : 00–500 : 00 ST, corresponding to a ram pressure of 0.02 nPa and Alfvén Mach numbers of 16.0. After ~418 : 00 ST, when the reduced ram pressure starts to affect the dayside magnetopause, the compressed magnetosphere is relaxed, which causes a rapid expansion and produces highly dynamic structures (Movie A1, see Fig. A.1). After approximately 40 hours, the violently expanding magnetosphere starts to shrink but remains larger than its state prior to 380 : 00 ST. For convenience, the simulation periods 380 : 00–118 : 00, 418 : 00–458 : 00, and 458 : 00–500 : 00 ST are respectively referred to as the SW compression, rarefaction, and relaxation periods.

We note that in the MHD simulation, magnetic reconnection is enabled by numerical resistivity, which occurs when opposing magnetic fluxes enter a single computational cell and are then averaged out of existence (Brambles et al. 2011). The rate of reconnection is determined by external conditions in the reconnection region, through the conservation of mass, momentum, and magnetic flux (Lyon et al. 2004; Zhang et al. 2016, 2017). In their simulation of the terrestrial magnetosphere, Ouellette et al. (2013) have demonstrated that the reconnection rate is limited to a fraction (≈0.1) of the Alfvén speed in the inflow regardless of the grid size, suggesting Petschek-like reconnection configurations in the simulation. Therefore, it is expected that the global MHD simulation can capture the large-scale configuration and distribution of Jovian dayside magnetodisc reconnection when it is driven by external conditions, with an approximate reconnection rate of around 0.08 in this study, despite a lack of micro-physics, which is mostly subgrid. Further details on why steady-state magnetic reconnection exhibits a similarly fast rate across different MHD models are discussed in Liu et al. (2017).

thumbnail Fig. 1

Basic characteristics of dayside magnetodisc reconnection. (a) Evolution of the three components of the magnetic field: Br (positive outward), Bθ (positive southward), and Bϕ (positive counterclockwise). (b) Two components of plasma flow along the equatorial plane, Vr (positive outward) and Vϕ (positive counterclockwise), in Radial–Theta–Phi coordinates (a spherical coordinate system whose Z axis is Jupiter’s rotation axis pointing north) from 435 : 00–438 : 00 ST. The results were obtained at the position with the maximum Bθ reversal (R = 80.5 RJ, MLT = 06 : 53), marked by green triangles eclosed by black lines in bottom panels. The pink-shaded area represents a magnetic reconnection event. (c–f) Evolution of equatorial Bz (= −Bθ at the equatorial plane, positive northward) in Jupiter’s dayside and near-dayside magnetosphere from 435:00–438:00 ST. Black, magenta, and red lines indicate the Bz zero lines, and green arrows represent plasma flows. Bold black lines represent the magnetopause, which is determined by the contour of Bz = 0. Cyan lines approximate the magnetodisc boundary, which is determined by the contour of the critical density (1.7 cm−3), where Bθ/Bz reversals can occur near the simulated inner magnetosphere.

3 Results

In order to compare the simulated dayside reconnection signatures with spacecraft observations, we present an evolution of magnetic field and plasma flow in Radial-Theta-Phi coordinates at a fixed position with the maximum Bθ reversal (R = 80.5 RJ, MLT = 06 : 53) in Figures 1a–1b. These results were obtained during the SW initial rarefaction from 435 : 00–438 : 00 ST, which is characterized by larger and more widespread Bθ reversals relative to other STs. Figure 1a reveals that after a slight increase, Bθ exhibits a significant reversal to the northward direction to a value of −5.4 nT near 437 : 00 ST (pink-shaded area), which is opposite to the southward planetary background field. This Bθ reversal is accompanied by rapid strength decreases in both Br and Bϕ. Similar magnetic field signatures were also recorded by Cassini during a dayside magnetodisc reconnection event at Saturn (Guo et al. 2018). Figure 1b shows that during the simulated reconnection event near 437 : 00 ST (pink-shaded area), Vr exhibits a reversal simultaneously corresponding to the Bθ reversal, while Vϕ demonstrates a rapid increase followed by a decrease, suggesting that this position may pass through both enhanced outflow areas on different sides of the reconnection. General two-dimensional interpretations only describe magnetodisc reconnection in the Radial-Theta cut plane, but the actual three-dimensional situation has to consider the azimuthal motion. This is because the equatorial current sheet near the reconnection site does not align exactly in the same azimuthal direction in the Jovian rotating frame, leading to a radial distribution of corotational angular velocity. Consequently, disturbances in Br and Bϕ, as well as in Vr and Vϕ, near the reconnection period exhibit a correlation.

Figures 1c–1f depict the evolution of the simulated Jovian dayside magnetodisc reconnection events at the equatorial plane from 435 : 00–438 : 00 ST. During this period, northward day-side Bz (= −Bθ) structures (i.e., −Bθ at the equatorial plane) ranging from 0–5.4 nT occur in the outer magnetodisc (cyan lines in Figs. 1c–1f) largely due to the interchange instabilities within the magnetodisc. Similar dayside reversal disturbances were detected by the Galileo and Voyager spacecraft at Jupiter (Kivelson & Southwood 2005; Zhao et al. 2024) and by the Cassini spacecraft at Saturn (Delamere et al. 2013, 2015; Guo et al. 2018). The extent of northward Bz disturbances range from a few RJ to tens of RJ. The northward structure emerging from magnetodisc reconnection exhibits a highly complex and dynamic structure, with distinct phases of generation, enhancement, and decay, accompanied by corotating speeds (green vectors in Figure 1). The dynamic variation of this dayside magnetodisc reconnection process is further visualized in Movie A1 (see Fig. A.1).

To better understand the generation and evolution of the simulated magnetodisc reconnection process on the dayside, we focused on two typical reversal Bz structures, denoted by the magenta and red lines in Figures 1c–1f. The magenta-enclosed Bz reversal region in the afternoon sector represents one type of dayside magnetodisc reconnection process in our simulation, which occurs on the dayside and evolves toward the dusk-side. This region underwent expansion and contraction processes from 435 : 00–438 : 00 ST, with the maximum Bz increasing up to 1.9 nT before decreasing. The Bz reversal region was transported radially from the middle magnetosphere to the near-magnetopause region, with a counterclockwise azimuthal speed of approximately 1.0–1.4 magnetic local time (MLT)/h, which is slower than Jupiter’s rotation speed of approximately 2.4 MLT/h. The red-enclosed Bz northward structures in the dawnside sector represent another type of dayside magnetodisc reconnection process identified in our simulation. This Bz northward structure is not a locally generated dayside magnetodisc reconnection event and is transported from the magnetotail reconnection on the dawnside. From 435 : 00–438 : 00 ST, the corotating dawn-side Bz northward structures first occur between 3 : 00 and 6 : 00 MLT in the pre-dawn sector, and they exhibit a significant enhancement and eventually diminish in the morning sector near 9 : 00 MLT. The maximum magnitude of northward Bz disturbance reaches 5.4 nT near 80.5 RJ and 6 : 53 MLT at 437 : 00 ST. The process of the corotating reconnection site as shown in Figure 1 and Movie A1 (see Fig. A.1) is also consistent with observations from Yao et al. (2017). Overall, our simulation suggests that the dayside Bθ reversal structure resulting from magnetodisc reconnection can either occur locally on the dayside or be transported from other earlier MLTs, including both dayside and pre-dawn regions.

The dynamic evolution of the Jovian magnetodisc reconnection process is highly complex, with periodicities ranging from minutes to hours and covering a wide radial-MLT range in the equatorial plane. Additionally, it exhibits a strong dependence on SW ram pressure, as illustrated in Movie A1 (see Fig. A.1). Prior to 418 : 00 ST, while the simulated Jovian magnetosphere is significantly compressed, the dayside extension is relatively small, with a magnetopause subsolar distance of 40–50 RJ and an almost lack of dayside reconnection structures. However, during the transition period of rarefaction (between 418 : 00 and 458 : 00 ST), the sudden decrease in SW ram pressure triggers a rapid expansion of the magnetosphere. This expansion is accompanied by the emergence of highly dynamic structures, resulting in a magnetopause subsolar distance of 75–95 RJ. It is worth noting that significant Bz reversal structures appear at most STs during this period. Following the SW rarefaction period, between 458 : 00 and 500 : 00 ST, the magnetosphere transitions into an expanded but relatively “relaxed” phase, with a magnetopause subsolar distance of 60–75 RJ. While the occurrence of Bθ reversal cases during this period has decreased compared to the rarefaction period, it is still higher than during the compressed period, where reversal structures are almost non-existent.

To obtain a more comprehensive understanding of the spatial distribution and SW dependence of dayside magnetodisc reconnection, we performed a statistical analysis to predict the occurrence rate of Bθ-reversal structures during periods characterized by SW ram pressure. The statistics are focused on the radial distance and MLT of the simulated magnetodisc reconnection events during periods of compression (380 : 00–418 : 00 ST), rarefaction (418 : 00–458 : 00 ST), and relaxation (458 : 00–500 : 00 ST). In order to assess the probability that a spacecraft detects magnetic field reversals in comparable locations and SW conditions, the occurrence rate was determined by dividing the number of times Bθ reversal cases occurred within each grid by the total number of times recorded during the respective period under consideration. We note that Bθ reversals occurring near the magnetopause (with a distance <3 RJ) were excluded from the analysis to avoid any magnetic field disturbances caused by the Kelvin-Helmholtz instability.

Figure 2 illustrates the simulated distribution of occurrence rates for dayside magnetodisc reconnection during SW compression, rarefaction, and quiet-time periods. The results indicate that the highest occurrence rate and radial coverage were observed during SW rarefaction (Figure 2b), followed by SW relaxation (Figure 2c), and that the lowest rates were recorded during SW compression (Figure 2a). This SW dependence is likely attributable to the volume of the dayside magnetosphere, which is approximated by the magnetopause location (the magenta lines in Figure 2). A larger magnetospheric volume generates greater centrifugal forces, which can sufficiently stretch magnetic field lines to enable magnetodisc reconnection. Regarding the MLT dependence, simulated magnetodisc reconnection occurs at nearly all dayside MLTs, consistent with recent limited observations detecting it at 6–7, 11–13, and 15–17 MLT (Zhao et al. 2024). Specifically, simulated reconnection occurs more frequently on the dawnside (6 : 00–8 : 00 MLT) during all periods, compared to other MLTs (8 : 00–18 : 00). This dawn-dusk asymmetry may be a consequence of the corotating reconnection processes originating from the pre-dawn regions, where magnetodisc reconnection occurs more easily than on the day-side, primarily due to the presence of a larger magnetospheric volume. The slight preference for dayside reconnection on the dawnside may also occur at Saturn, which could potentially explain the distribution of the occurrence frequency of injections into the inner Kronian magnetosphere (Azari et al. 2018).

thumbnail Fig. 2

Responses of dayside magnetodisc reconnection to upstream solar wind (SW) ram pressure conditions. Occurrence rate distribution of equatorial dayside magnetodisc reconnection during SW compression, rarefaction, and relaxation periods, from 380 : 00–418 : 00, 418 : 00458 : 00, and 458 : 00–500 : 00 ST, respectively. The magenta lines represent an approximate outer limit of the dynamic magnetopause, calculated as the sum of the temporal average and corresponding the standard deviation of the magnetopause’s radial position in each period.

4 Discussion

Dayside Jovian magnetodisc reconnection is a highly intricate three-dimensional process that involves topological alterations to significantly stretched field lines rotating with the magnetodisc. Figure 3 depicts a snapshot of the simulated magnetic field lines associated with the Bθ reversal event in the afternoon sector at 437 : 00 ST, as indicated by magenta-enclosed areas in Figure 1e. Due to the centrifugal force caused by rotation, unlike the field lines near the inner magnetodisc (red lines), the magnetic field lines in the middle magnetodisc (blue lines) are no longer dipolar. Instead, they are stretched radially outward with azimuthal orientations caused by the radially decreasing corotation angular speed. When these magnetic field lines with sheared arcade structures are further stretched (green lines), the associated current sheet within the magnetodisc becomes thinner and unstable, resulting in magnetodisc reconnection (marked by the magenta X-line) and the formation of magnetic flux rope structures surrounding the plasmoid (black lines). The dynamic evolution of these rope field lines is illustrated in Movie A2 (see Fig. A.2), where the generation, enhancement, and decay of equatorial Bθ reversal structures are accompanied by similar processes of corresponding flux ropes near these regions. The enhancement of toroidal flux in the early phase is due to magnetodisc reconnection, which transforms the sheared arcade into twisted field lines. Its decay in the later phase results from reconnection between field lines in the interior of the magnetic flux rope and the possibility that the stronger external magnetic field of the flux rope may alter the internal topology through MHD waves.

The above analyses demonstrate that dayside Jovian magnetodisc reconnection is largely driven by the stretching of magnetic field lines caused by planetary rotation. This process shares similar fundamental characteristics with the nightside reconnection identified in observations (Nishida 1983; Russell et al. 1998; Woch et al. 2002; Kronberg et al. 2005; Vogt et al. 2010, 2020) and proposed in theoretical configurations (Vasyliunas 1983; Delamere & Bagenal 2013). However, when considering the role of the dayside magnetopause, two major differences arise. First, the presence of the magnetopause restricts the radial plasma motion and the stretching of field line near the outer dayside magnetosphere. As a consequence, a more pronounced azimuthal motion occurs in the equatorial Bθ reversal structure and plasmoids. In contrast, the Bθ reversal structure and plasmoids associated with magnetotail reconnection are more likely to be lost radially in the distant magnetotail. Second, the smaller volume of the dayside magnetosphere makes it insufficient to facilitate significant stretching of magnetic field lines in order to generate dayside magnetodisc reconnection under most conditions. Therefore, the occurrence rate of magnetodisc reconnection on the dayside in our simulations is much lower than that on the nightside (Chen et al. 2024). This also explains why the occurrence rate of magnetodisc reconnection increases after the sudden expansion of the magnetosphere due to a decrease in SW ram pressure in Figure 2.

It should be noted that although this MHD simulation assumes ideal upstream SW conditions, the responses of the Jovian magnetosphere are inherently dynamic. Therefore, it is expected that the complexity of Jupiter’s magnetosphere is even greater under real SW conditions. Moreover, the variations in the rate of Io mass loading may significantly influence the position of Jupiter’s magnetopause (Feng et al. 2023), resulting in different spatial distributions and occurrence rates of dayside Jovian magnetodisc reconnection. Additionally, the presence of hot plasma populations may also have an impact on the overall volume of the Jovian magnetosphere, which is not implemented in current global simulations. These factors, which are not considered in the MHD simulations, may affect the details of the dayside magnetodisc reconnection. Therefore, future investigations should include a quantitative analysis of such potential effects.

thumbnail Fig. 3

Jovian magnetospheric magnetic field lines near the dayside magnetodisc reconnection in the simulation. The blue, black, and green lines represent the stretched inner, flux rope, and outer magnetic fields passing through areas near the considered x-line (magenta line), respectively. The red lines denote the quasi-dipole fields in the inner magnetosphere. The Bθ zero lines at the equatorial plane are marked in cyan, and the considered x-line is marked in magenta. The equatorial bold black lines represent the magnetopause. The simulation time (ST) is 437 : 00.

5 Conclusions

In summary, the global magnetospheric simulations under ideal SW and IMF conditions indicate that Jupiter’s magnetodisc reconnection can occur on the dayside, particularly during SW rarefaction (when the dayside magnetosphere has a larger volume). In addition to the generation, enhancement, and delay phases, the magnetodisc reconnection structure can also corotate with the planetary spin. The simulated magnetodisc reconnection signatures at Jupiter are similar to those of rare direct observation for dayside magnetodisc reconnection at Jupiter (Zhao et al. 2024) and Saturn (Guo et al. 2018). Notably, Juno has accumulated a certain amount of dayside magnetospheric data that make it possible to further examine the new picture proposed in this study in the near future.

Data availability

Movies associated to Figs. A.1 and A.2 are available at https://www.aanda.org

Acknowledgements

This work is supported by the General Program of National Natural Science Foundation of China (grant No. 42374216) and Research Grants Council (RGC) General Research Fund (grant Nos. 17308221, 17308520, 17315222, and 17308723). The model outputs used to generate the figures for analysis presented in this paper are being preserved online (https://doi.org/10.17605/OSF.IO/5YUTH).

Appendix A Movies A1 and A2

thumbnail Fig. A.1

Evolution of Jupiter’s magnetosphere under different solar wind conditions. (top) North-south component of the magnetic field (Bz, positive northward), plasma number density, radial flow (Vr, positive outflow), and azimuthal flow (Vϕ, positive anticlockwise) at the equatorial plane of Jupiter’s magnetosphere during at 437 : 00 simulation time (ST) from the GAMERA. (bottom) Solar wind ram pressure conditions driving the GAMERA. Movie A1 shows the evolution during 380 : 00–500 : 00 ST. The movie is available online.
thumbnail Fig. A.2

Simulated magnetic field lines (red lines) through the equatorial Bθ reversal structures (red ranges at the equatorial plane) on the dayside at 437 : 00 ST. The green lines denote the quasi-dipole fields in the inner magnetosphere. Movie A2 shows the evolution during 435 : 00–438 : 00 ST. The movie is available online.

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All Figures

thumbnail Fig. 1

Basic characteristics of dayside magnetodisc reconnection. (a) Evolution of the three components of the magnetic field: Br (positive outward), Bθ (positive southward), and Bϕ (positive counterclockwise). (b) Two components of plasma flow along the equatorial plane, Vr (positive outward) and Vϕ (positive counterclockwise), in Radial–Theta–Phi coordinates (a spherical coordinate system whose Z axis is Jupiter’s rotation axis pointing north) from 435 : 00–438 : 00 ST. The results were obtained at the position with the maximum Bθ reversal (R = 80.5 RJ, MLT = 06 : 53), marked by green triangles eclosed by black lines in bottom panels. The pink-shaded area represents a magnetic reconnection event. (c–f) Evolution of equatorial Bz (= −Bθ at the equatorial plane, positive northward) in Jupiter’s dayside and near-dayside magnetosphere from 435:00–438:00 ST. Black, magenta, and red lines indicate the Bz zero lines, and green arrows represent plasma flows. Bold black lines represent the magnetopause, which is determined by the contour of Bz = 0. Cyan lines approximate the magnetodisc boundary, which is determined by the contour of the critical density (1.7 cm−3), where Bθ/Bz reversals can occur near the simulated inner magnetosphere.
In the text
thumbnail Fig. 2

Responses of dayside magnetodisc reconnection to upstream solar wind (SW) ram pressure conditions. Occurrence rate distribution of equatorial dayside magnetodisc reconnection during SW compression, rarefaction, and relaxation periods, from 380 : 00–418 : 00, 418 : 00458 : 00, and 458 : 00–500 : 00 ST, respectively. The magenta lines represent an approximate outer limit of the dynamic magnetopause, calculated as the sum of the temporal average and corresponding the standard deviation of the magnetopause’s radial position in each period.
In the text
thumbnail Fig. 3

Jovian magnetospheric magnetic field lines near the dayside magnetodisc reconnection in the simulation. The blue, black, and green lines represent the stretched inner, flux rope, and outer magnetic fields passing through areas near the considered x-line (magenta line), respectively. The red lines denote the quasi-dipole fields in the inner magnetosphere. The Bθ zero lines at the equatorial plane are marked in cyan, and the considered x-line is marked in magenta. The equatorial bold black lines represent the magnetopause. The simulation time (ST) is 437 : 00.
In the text
thumbnail Fig. A.1

Evolution of Jupiter’s magnetosphere under different solar wind conditions. (top) North-south component of the magnetic field (Bz, positive northward), plasma number density, radial flow (Vr, positive outflow), and azimuthal flow (Vϕ, positive anticlockwise) at the equatorial plane of Jupiter’s magnetosphere during at 437 : 00 simulation time (ST) from the GAMERA. (bottom) Solar wind ram pressure conditions driving the GAMERA. Movie A1 shows the evolution during 380 : 00–500 : 00 ST. The movie is available online.
In the text
thumbnail Fig. A.2

Simulated magnetic field lines (red lines) through the equatorial Bθ reversal structures (red ranges at the equatorial plane) on the dayside at 437 : 00 ST. The green lines denote the quasi-dipole fields in the inner magnetosphere. Movie A2 shows the evolution during 435 : 00–438 : 00 ST. The movie is available online.
In the text

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