Dawn-dusk Asymmetrical Distribution of Saturn’s Cusp

Domain: physics
Published: 2026-04-01
Editorial outcome: accept

Abstract
Saturn’s magnetosphere, shaped by solar wind interaction with its dipole field, differs from Earth’s due to faster rotation and Enceladus’s internal plasma sources. An ongoing focus of investigation is how the internal plasma sources and rapid rotation result in a different global magnetospheric configuration. The magnetospheric cusp, a crucial interaction region between solar wind and planetary magnetic field, serves as an indicator of global magnetic configuration. Here we utilize Cassini observations from 2004 to 2010 to study dawn-dusk asymmetry in Saturn’s cusp distribution and extend the number of cusp events from about 11 reported in previous studies to 67. We find peak occurrence in the post-noon sector and signatures extending to post-dusk (near 20 local time), resembling recent observations of Jupiter’s post-dusk cusp. We further examine magnetic topology using high-resolution magnetohydrodynamics simulations to visualize the cusp asymmetry. This is consistent with previous studies, but our simulations provide a more detailed view of Saturn’s magnetic topology near the magnetopause. This asymmetry of cusp distribution demonstrates how rapid rotation and internal plasma sources fundamentally alter magnetospheric configuration, offering insights for understanding other rotating planetary systems within and beyond the solar system.

Introduction
Planetary magnetospheres are formed as a result of the continuous interaction between the high-speed solar wind and the internally generated magnetic fields present in most planets within the solar system, which display considerable variations among different planets. For example, the magnetospheres of Jupiter and Saturn are more internally driven because of the planets’ rapid rotation speeds coupled with internal plasma sources within their magnetospheres, which differs from the strongly solar wind-driven magnetospheres typically seen on Earth1,2. In the case of ice giants like Uranus and Neptune, the unique pole-on configuration of their magnetospheres – the geometry in which the upstream solar-wind flow is approximately aligned with the planet’s magnetic dipole axis – lead to distinct coupling dynamics with the solar wind3,4. Atthesametime,the highly dynamic nature of these gas giants’ plasma environments fosters complex interactions with the solar wind, which serves as an external factor in the magnetospheric dynamics5,6. Understanding these diverse magnetospheric dynamics can improve the interpretation of observations of planetary systems and offer useful perspectives for the broader field of planetary science, including star-planet interactions.

When the solar wind interacts with a planetary magnetosphere, magnetic reconnection on the magnetopause often occurs where the magnetic shear is sufficiently large. At Earth, magnetopause reconnection is strongly governed by the north–south component of the interplanetary magnetic field (IMF)—with southward IMF driving subsolar reconnection and northward IMF favoring high-latitude lobe reconnection7. In the rotationally dominated magnetospheres of giant planets, magnetopause reconnection was reported to be favored at high latitudes. In contrast, low-latitude subsolar reconnection is often suppressed by diamagnetic effects and is therefore less likely to occur there8–10. Recent simulations11–13 have suggested that reconnection at Jupiter is strongly influenced by the east–west IMF component, with reconnection occurring predominantly at high latitudes and suppressed reconnection at low-latitude magnetopause locations. During reconnection, solar wind particles are allowed to enter the magnetosphere through the newly reconnected magnetic field lines. The magnetospheric cusp is known as a vital funnel-shaped region that permit these particles to directly access the atmosphere14,15. The cusps play a crucial role in planetary magnetospheres, as they significantly contribute to various mass and energy transport processes within the magnetosphere, ionosphere, and upper atmosphere. These processes are key in the formation of geomagnetic storms16, substorms17, and auroras18. A series of in situ studies, e.g., using the Cluster multi-spacecraft mission, have revealed that Earth’s cusp is a broad, complex region characterized by various intriguing boundary layers14,19. The cusp in Earth’s quiescent magnetosphere, strongly governed by solar wind, typically exhibits a general morning-afternoon symmetry of its morphology and statistical occurrence rates, which is one of the most fundamental properties of terrestrial cusp20,21, although the distribution of its spatial location can be influenced by the IMF By component and pressure of the solar wind22,23. In contrast, due to Saturn’s rapid rotation, the Kronian magnetospheric plasma generally subcorotates relative to the planet, remaining in near-rigid corotation within approximately 20 Rs, but falling significantly below the rigid corotation rate beyond Titan’s orbit24,25. Compared to Earth, Jupiter exhibits bright and persistent polar auroras26, and Saturn’s aurora shows variable morphologies resembling both Earth-like and Jupiter-like structures27, suggesting a markedly different magnetospheric configuration and dynamics of rapidly rotating giant planets2,28,29. The rapid rotation of giant planets plays a crucial role in modifying magnetic flux transport within the magnetosphere and near the magnetopause compared to Earth12,29. This rotational influence can modify dawnside magnetopause reconnection rates associated with solar wind interaction and alter the configuration of the magnetopause and the open–closed field line boundary, where the cusps are anchored12,29.

Zhang et al.11,29 provide additional insights into the distinct magnetospheric configurations of Earth and giant planets using high-resolution simulations. In rotation-dominated Jovian-like systems, low reconnection rates on the dawn side can cause closed flux to accumulate there, resulting in an open flux fraction of approximately 20% in the polar region (above 80° magnetic latitude, MLAT20). Conversely, a terrestrial-like magnetosphere is mostly solar wind-dominated, exhibiting near-complete openness of the polar region above 80° MLAT (about 98%)29. Although Saturn’s magnetosphere is similarly rotation-dominated, its weak magnetic field (an equatorial magnetic field strength of 0.21 G, about 1/20 of Jupiter’s30) results in its reduced corotation dominance. The consequently higher solar wind merging potential-to-corotation potential ratio (0.38 at Saturn vs. 0.09 at Jupiter29) increases solar wind-driven flux opening efficiency. As a result, Kronian-like magnetosphere’s polar open flux fraction varies widely (58–93% above 80° MLAT)29. Observationally, Saturn’s high-latitude magnetosphere has been reported to be largely plasma-depleted and composed predominantly of open field lines, with the polar cap boundary located at 14.5 ± 0.6° and 12.7 ± 0.6° colatitude in the southern and northern hemispheres, respectively31,32. Compared to Jupiter’s small open flux area, Saturn’s relatively large area of open magnetic field may result in a more observable cusp, although more detailed analysis and observations are needed to confirm the prediction. Previous theoretical models by Cowley et al. (2004, 2005)33,34 proposed that rotational dynamics drive the subsolar X-line at Saturn, thereby influencing the azimuthal distribution of the polar cap and cusp in local time. This framework is based on terrestrial magnetopause reconnection pictures and the Dungey7/Vasyliūnas cycle35. However, the applicability of the Dungey cycle to giant planets remains a topic of active debate36.

Currently, the global distribution picture of Saturn’s cusp remains an active topic of ongoing research. As shown in the simple schematic Fig. 1, a comparative analysis of Saturn’s cusp distribution with Earth’s is crucial for understanding the diverse planetary magnetospheres and their interaction with the solar wind.

Some case studies have identified characteristics in Saturn’s cusp region, such as magnetosheath-like electron features and ion dispersion patterns that are similar to those observed in Earth’s cusp37–40. Additionally, variations in ion dispersion features under different reconnection conditions and magnetic depression features in Saturn’s cusp have been examined, revealing parallels with Earth’s cusp37,38,40. These similarities indicate that the microphysical processes governing the behavior of charged particles within cusps might be universal across different planetary environments. However, the study by Arridge et al.40 noted cusp encounters recurring in sync with Saturn’s rotation period, thought to be linked to the cusp position oscillating in phase with Saturn’s global magnetospheric dynamics40. Previous research utilizing global magnetohydrodynamic (MHD) simulations has demonstrated that the magnetospheric structure of rapidly rotating giant planets exhibits significant asymmetries41, markedly different from those observed at Earth11,12. For instance, simulation studies have shown that rapid rotation significantly alters the global magnetospheric configuration of Jupiter, notably causing a pronounced duskward shift in the open magnetic field regions associated with Jupiter’s polar cusp11,12. This dusk-side distribution of Jupiter’s cusp has been supported by observational results from Xu et al. (2024)13. Given Saturn’s rotation-dominated magnetosphere, a similar duskward shift in its cusp distribution is expected13,29.

However, several questions remain unresolved. A comprehensive global observational picture of cusp regions at giant planets is still lacking. At Jupiter, due to Juno’s current orbital constraints13,42,43, high-latitude observations in the middle-to-outer magnetosphere across various local times remain incomplete, leaving the properties and distribution of cusp regions in the dawn, noon, and post-noon sectors mysterious. Given Saturn’s similarly rapidly rotating magnetosphere, investigating its global cusp distribution is especially important. Such research provides valuable comparative insight into Jupiter’s reported dusk-side cusp and informs future observations of Jupiter’s cusp and high-latitude boundary layer regions. Previous studies have provided valuable analyses of several Saturn cusp events37–40. Here we extend this foundation by offering a global, dwell-time–normalized statistical view, enabling a global assessment of the cusp occurrence distribution.

In the present research, we explored the global distribution of Saturn’s cusp using data from the Cassini spacecraft, spanning from 2004 to 2010. Additionally, we employed high-resolution MHD simulations11,12,44 to visualize the magnetic configurations of Saturn and its cusp, revealing Saturn’s magnetopause-related three-dimensional magnetic topology. Both the statistical analysis and the simulation results highlight the characteristics of the morning-afternoon asymmetry in Saturn’s cusp, which diverges from the symmetric distribution observed at Earth and shows consistency with Jupiter’s recently discovered configuration.

Results
A Kronian cusp case is presented for the period between 23:00 on January 30 and 05:00 on January 31, 2009, when the Cassini spacecraft was located in the high latitude (>40°) noon region of the northern hemisphere, near 16 Rs (Fig. 2). Cassini transitioned from the magnetospheric region to the boundary layer and the cusp region, as shown in Fig. 2. In this study, we examine Cassini’s in-situ measurements from three key sources: the Cassini Magnetometer instrument (MAG)45, Cassini Plasma Spectrometer - Electron Spectrometer (CAPS-ELS)46, and Cassini Plasma Spectrometer - Ion Mass Spectrometer (CAPS-IMS)46. Before 23:51 UT, as indicated by the first dashed line in Fig. 2, Cassini recorded a magnetospheric electron spectrum with enhanced energy ranges between 1000 and 10000 eV. In this region, the ion energy spectrum displayed no distinct features, and the electron pitch angle distribution was isotropic. These characteristics are indicative of the magnetosphere at this location30,40; hence, this region is identified as part of the magnetosphere and is denoted by the letter M in Fig. 2.

After 00:36 UT marked by the second dashed line in Fig. 2, the spacecraft observed a region characterized by a notable enhancement in the electron energy spectrum from 10 to 100 eV, as depicted in Fig. 2c. This enhancement is characteristic of magnetosheath electrons47. Concurrently, the magnetic field component remained relatively stable, as shown in Fig. 2a, b, which rules out the possibility of the spacecraft being in the magnetosheath. Additionally, observations included a bidirectional distribution of low-energy electrons and a rise in high-energy electrons opposing the magnetic field direction (shown in Fig. 2e–h), along with multiple ion dispersion signals. These observations suggest that the magnetopause experienced pulsed reconnection processes37,48. These distinct features robustly indicate that the region encountered by the spacecraft is indeed a magnetospheric cusp region37,38,40. Situated between the magnetosphere and the cusp is a transitional area, characterized by plasma properties that are intermediate between those of the magnetosphere and the cusp. This region is identified as a boundary layer and is denoted by BL in Fig. 2.

To identify Kronian cusp events, the most important criteria, based on previous research on Saturn’s cusp37,38,40 and Jupiter’s cusp13, are as follows: (1) Spacecraft location, e.g., relatively high latitudes (>30 degrees) and within the magnetopause; (2) Most typical plasma characteristics within cusp regions, e.g., magnetosheath-like electron energy distributions13,37,38,40. These are the Level 1 criteria, the most critical for identification. Most Saturn cusp events can be well identified by meeting these criteria. Additionally, other important but non-essential features can further aid in identification, including ion dispersion37,40 associated with magnetopause reconnection, field-aligned electron distributions37,40, and magnetic depressions39 within cusp regions. These are referred to as the Level 2 criteria, which can be helpful but are not necessary for cusp identification. Utilizing all these criteria, we identified a total of 67 cusp events that matched the characteristics observed by the Cassini spacecraft between 2004 and 2010. These events encompass previously reported cases of Saturn’s cusp region, demonstrating the reliability of our identification and the consistency of this study with prior research. Information on Saturn’s cusp events and the corresponding criteria fulfillment is organized in the Supplementary Data 1. It is noted that the survey period of 2004–2010 corresponds to the timeframe from Cassini’s entry into Saturn’s bow shock to the final year before the CAPS instrument was placed in Operations on Hold (2011)38. Since the detection of magnetosheath-like electron spectra by CAPS provides the most direct and essential evidence for cusp identification14,20,40, data collected after 2011 were excluded to ensure the reliability of event identification. This study does not distinguish between cusp types associated with different magnetopause reconnection pictures. Recent findings suggest that magnetopause reconnection at giant planets may be significantly more complex than the subsolar/lobe-dominated patterns observed at Earth, with reconnection-related dynamics exhibiting greater variability11–13. Moreover, long-term, stable solar wind measurements are lacking at Saturn during the Cassini mission39,40, and solar wind models cannot provide real-time, accurate information on the east–west and north–south components of IMF39,40,49. As a result, identifying cusp types based on ion dispersion signatures linked to specific reconnection geometries is highly challenging and beyond the scope of this study.

The global distribution of these events is presented in Fig. 3. Fig. 3a and b demonstrate that the majority of observed cusp events are located in the northern hemisphere and predominantly on the afternoon side, with some events even detectable on the post-dusk side (after 20 local time, LT). Fig. 3c displays the distribution of cusp events at high latitudes (>30° or <-30°) across various radial distances and local times. Due to the Cassini spacecraft’s trajectory constraints, its high-latitude path in the southern hemisphere is primarily on the morning side, while in the northern hemisphere, it is largely on the afternoon side. Notably, between radial distances of 10 and 20 Rs, high-latitude tracks are present on both the morning and afternoon sides. However, there is a marked contrast in event density for 10 to 20 Rs radial range in high latitudes (Fig. 3c). Between 13 and 16 LT, a total of 17 cusp events were identified within Cassini’s accumulated dwell time of 36,626 minutes, corresponding to an event density of approximately 0.028 events per hour. In contrast, during the interval from 8 to 11 LT with a dwell time of 37,379 minutes, only 4 cusp events were detected, indicating a substantially lower event density of 0.006 events/hour. Although previous studies have reported minor differences in the period of kilometric radiation waves north and south of Saturn50,51, there is currently insufficient evidence to suggest a pronounced north-south asymmetry in Saturn’s magnetospheric configuration that can notably impact the polar cusp structure. Thus, this study posits that the observed distribution reflects the morning and afternoon asymmetry in Saturn’s cusp distribution. More importantly, three post-dusk cusp events were identified during the statistical study, with the specific identification criteria for each event detailed in Supplementary Data 1. And comprehensive plasma analyses, along with comparisons distinguishing these events from nightside ionospheric outflows, are provided in Supplementary Note 1 and Supplementary Figs. 1–3. The plasma energy spectra and spatial distributions of these events closely resemble those observed in Jupiter’s post-dusk polar cusp region as reported by Xu et al. (2024) (see Supplementary Note 1), indicating a consistent cusp microphysics and complex magnetospheric structures across different giant planets.

To quantify this asymmetry, we computed the probability distribution of the cusp with the time resolution of 1 minute, as depicted in Fig. 4b. Additionally, for comparative purposes with Saturn, we replicated Earth’s cusp probability distribution using the Cluster electron and magnetic field datasets52 as shown in Fig. 4a. It is noted that the cusp detections at different local times are normalized by the spacecraft’s high-latitude dwell time at corresponding local times for both Cluster and Cassini, minimizing potential statistical biases caused by orbital differences. For detailed information on event identification and case presentations in Earth’s cusp region, please refer to Supplementary Note 2 and Supplementary Fig. 4. The results of this distribution are consistent with previous studies20: The peak occurrence rate of Earth’s cusp is centered around noon (11-13 LT), with some disparity between the morning and afternoon sides, which can be attributed to the influence of the IMF By component and solar wind dynamic pressure, which can shift and modify the cusp’s position during spacecraft observation22,23. In contrast, the peak occurrence rate of Saturn’s cusp manifests on the afternoon side 13-15 LT. Notably, Saturn’s cusp distribution does not mirror the shift of Earth’s cusp distribution toward the afternoon under positive IMF By condition22. Instead, it shows a clear shift in overall distribution toward the dusk or even the night side. Global MHD simulations11,29,44, as illustrated in Fig. 4c and 4d, map the magnetic field configurations near the magnetopauses of Earth and Saturn, highlighting the differences between their distributions. Saturn’s magnetopause, influenced by the planet’s rapid rotational dynamics, is more expanded on the morning side compared to the afternoon side, in agreement with previous MHD simulations and observations, which found the magnetopause was typically expanded by 1-2 Rs on the dawn side compared to dusk41,53,54. The cusp must conform to this topology, leading to the observed asymmetry and enabling cusp observation even on the post-dusk side — an occurrence hardly seen at Earth20,22. Moreover, the simulation results reveal a picture of Saturn’s magnetopause reconnection under typical solar wind conditions, which closely resembles that of Jupiter12,13. Reconnection occurs predominantly at high latitudes and is suppressed in low-latitude magnetopause regions—unlike Earth, where reconnection is primarily governed by the north–south component of the interplanetary magnetic field7,14,55. For a detailed presentation and analysis of Saturn’s magnetopause reconnection results, please refer to Supplementary Note 3 and Supplementary Figs. 5–6.

Methods
Simulation information
In this study, the global MHD model of Saturn’s magnetic configuration (Fig. 4d) is built upon the GAMERA code (Grid Agnostic MHD for Extended Research Applications) to tackle equations on a specifically designed curvilinear, non-orthogonal grid suited for the study of planetary magnetospheres44. The computational model employs a 256 × 256 × 256-cell grid, implemented as a non uniform, distorted spherical grid oriented along the Solar Magnetic (SM) X-axis and possessing a symmetry axis. The grid exhibits non-uniformity in the radial-azimuthal plane, with cell sizes reducing as they approach Saturn. The simulation begins at an inner boundary of 4 Rs, with boundary conditions obtained from electrostatic potential models61,62. The outer boundary takes the form of a cylinder, stretching from SM-X = 56 Rs to SM-X = -600 Rs, with a radius around 214 Rs. The conditions at this outer boundary are controlled by the solar wind and the interplanetary magnetic field (IMF). The grid is structured so that its resolution diminishes with increasing distance from Saturn, which enhances computational efficiency by aligning with the larger scale of magnetospheric processes farther from the planet. The radial cell sizes near Saturn’s magnetopause range between approximately 0.1 and 0.2 Rs. This model is driven by simplified representations of solar wind and IMF conditions, setting IMF Bz at -0.5 nT, a particle density of 0.1 per cubic centimeter, a solar wind velocity of 400 km/s. The dipole tilt of Saturn’s magnetic field is maintained at 0 degrees to remove hemispheric asymmetries, thereby simplifying the overall analysis. Moreover, the moon Enceladus ejects plasma into Saturn’s magnetosphere63,64, creating an internal plasma source that migrates outward and influences the magnetopause. The simulation data presented in this paper are publicly available online via Xu (2026)65.

Cassini and cluster data
We analyzed magnetometer (MAG), electron spectrometer (ELS), and ion mass spectrometer (IMS) data from the Cassini spacecraft spanning 2004-2010. Data were obtained from the NASA Planetary Data System. We analyzed magnetic field observations and position data from the Cassini MAG instrument45, and thermal ion and electron measurements with energy ranges up to 28 keV (electrons) and 50 keV (ions) from Cassini-CAPS/IMS/ELS46. Cluster magnetic field and particle data were obtained from the ESA Cluster Science Archive. The datasets to analyze Earth’s cusp were obtained by the FGM66, PEACE67, CIS68, and WHISPER69 instruments onboard Cluster. Detailed identification information of Earth’s cusp is shown in Supplementary Note 2.

Data availability
All Cassini data presented here are publicly available from NASA’s Planetary Data System (https://pds-ppi.igpp.ucla.edu/). The MAG dataset of Cassini is available via https://search-pdsppi.igpp.ucla.edu/search/view/?f=yes&id=pds://PPI/cassini-mag-cal. The CAPS dataset of Cassini is available via https://search-pdsppi.igpp.ucla.edu/search/?sc=Cassini&i=CAPS. The Cluster data used in this paper can be obtained from the Cluster Science Archive via https://csa.esac.esa.int/csa-web/. The simulation data of Saturn’s magnetospheric topology presented in this paper are publicly available online via Xu (2026)65 https://doi.org/10.17605/OSF.IO/RQJYH. The datasets generated during and/or analysed during the current study are available from the corresponding author upon request.

Code availability
The Cassini data are processed and analyzed using the Spedas package70, which can be downloaded via the https://github.com/spedaspage. The presentation of simulated data can be done using standard plotting libraries (Python/Matlab), for example, the Python library matplotlib (which can be downloaded via the https://matplotlib.org/stable/install/page). The Matlab code used in the analysis of simulations is publicly available online via Xu (2026)65 https://doi.org/10.17605/OSF.IO/RQJYH.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_3": "I believe the authors have sufficiently addressed comments from my original review, and it would appear they have done so for the other reviewers' comments. Thus I believe the manuscript is now ready for continuation of the publication process. Many thanks to the authors for their comprehensive responses.", "reviewer_4": "The paper written by Xu, Yao et al., summarizes a new list of cusp crossings identified at Saturn’s magnetosphere, providing an updated list of this high latitude phenomenon. The papers results discuss possible dawn-dusk asymmetries in the magnetosphere, that add to the substantial body of evidence from a large number of previous papers, that Saturn is a rotationally driven magnetosphere and produces dawn-dusk asymmetries.\n\nOriginality and significance: although this paper provides a contribution to the field of Saturn’s magnetosphere, it is not however a contribution commensurate to the standards of Nature Communications which requires the results to be of high significance and “important to the field and advance understanding in a way that will move the field forward” - but instead this work is incremental. The results do not move the field forward but instead build on established work that has previously shown that a) that cusp plasma occurs after dayside magnetopause reconnection at Saturn (e.g. Jasinski et al., 2014, 2016), b) that there is dawn-dusk asymmetries in the cusp (e.g. Arridge et al., 2016) and c) that Saturn is rotationally driven and displays dawn-dusk asymmetries generally (e.g. Pilkington et al., 2015; Jia & Kivelson, 2016; Wilson et al., 2013; Sergis et al., 2015; Allen et al., 2018).\n\nIt is suggested that a more appropriate journal for this publication, would be AGU’s Journal of Geophysical Research.\n\nThere are other issues regarding originality. Below are a list of strengths and weaknesses of the paper; with the weaknesses outweighing the strengths to be published in a Nature journal.\n\nMajor Strengths:\nNone noted.\n\nMinor strengths:\n1. The paper showed a new updated list of cusp crossings, updating the field of this well-studied phenomenon at Saturn’s magnetosphere.\n2. The paper compares MHD simulations of Saturn to the Cassini dataset, and then to a simulation of the Earth.\n\nMajor weaknesses:\n1. The paper does not provide a substantial original contribution to the field (see remarks above about originality and significance).\n2. The paper did not make a compelling case that it is identifying more accurate cusp crossings instead of reproducing previous work. The authors claim that in their Figure R9 “Our identification is more accurate, excluding the periods when the spacecraft had clearly entered lobe region between the second and the third red dashed line”, citing that the cusp example shown in their figure is a more accurate identification of previously identified cusps. However, the same identification of the two cusp events is identified in the same manner as previous publications. Jasinski et al., (2016) who the authors cite, separate their cusp and show the two cusp identifications in their online supporting material (just as the authors have done here). Arridge et al., (2016) identify the two cusp crossings also, for the same interval, in their Figure 9, in exactly the same manner as Jasinski et al., (2016) as well as how the authors are identifying it. The same is true for the authors Figure R9. Therefore, the authors are not producing more accurate identification but instead reproducing previous work.\n\nMinor weaknesses:\n1. The authors claim that “The magnetopause reconnection at Saturn is more sensitive to east-west component of IMF but not north-south component” however then later contradict that this is can be true when they write a few sentences later that “due to the limited availability of simultaneous solar wind monitoring data from in-situ observations of Saturn’s and Jupiter’s magnetospheres” . Furthermore, lobe reconnection cusp identification can be completed by analyzing the ion dispersions in the dataset.\n2. Figure 1 in the paper shows a magnetospheric sketch of Saturn with the cusp labelled with a question mark (i.e. “Cusp?”), implying that this phenomenon is being identified for the first time. Considering previous publications that have reported cusp observations (e.g. Jasinski et al., 2014, 2016, 2017; Arridge et al., 2016) this labeling is misleading."}, "author_response": "Response to Referee #3’s comments\nReviewer #3 (Remarks to the Author):\nI believe the authors have sufficiently addressed comments from my original review,\nand it would appear they have done so for the other reviewers' comments. Thus I\nbelieve the manuscript is now ready for continuation of the publication process. Many\nthanks to the authors for their comprehensive responses.\nResponse\nWe are deeply grateful for the referee's positive assessment and support of our manuscript. We\nappreciate the time and effort invested in reviewing our work and the constructive feedback\nprovided throughout the review process.\nResponse to Referee #4’s comments\nWe would like to extend our sincere appreciation to the referee for the valuable suggestions\nand comments. Below are our point-by-point responses to the referee’s remarks. The original\ncomments of the referee are quoted in italics for convenience. Corresponding revisions have\nbeen made according to the referee’s suggestions. The line numbers in our response refer to\nthe highlighted draft.\nThe paper written by Xu, Yao et al., summarizes a new list of cusp crossings identified\nat Saturn’s magnetosphere, providing an updated list of this high latitude\nphenomenon. The papers results discuss possible dawn-dusk asymmetries in the\nmagnetosphere, that add to the substantial body of evidence from a large number of\nprevious papers, that Saturn is a rotationally driven magnetosphere and produces\ndawn-dusk asymmetries.\nOriginality and significance: although this paper provides a contribution to the field\nof Saturn’s magnetosphere, it is not however a contribution commensurate to the\nstandards of Nature Communications which requires the results to be of high\nsignificance and “important to the field and advance understanding in a way that\nwill move the field forward” - but instead this work is incremental. The results do\nnot move the field forward but instead build on established work that has previously\nshown that a) that cusp plasma occurs after dayside magnetopause reconnection at\nSaturn (e.g. Jasinski et al., 2014, 2016), b) that there is dawn-dusk asymmetries in the\ncusp (e.g. Arridge et al., 2016) and c) that Saturn is rotationally driven and displays\ndawn-dusk asymmetries generally (e.g. Pilkington et al., 2015; Jia & Kivelson, 2016;\nWilson et al., 2013; Sergis et al., 2015; Allen et al., 2018). It is suggested that a more\nappropriate journal for this publication, would be AGU’s Journal of Geophysical\nResearch.\nResponse #1\nWe appreciate the referee’s careful consideration of our work and their comments on originality.\nIn our previous response, we may not have articulated the novelty of the study with sufficient\nclarity. To avoid any misunderstanding and address the referee’s concern, we summarize below\nthe key aspects of the study’s novelty to more accurately convey its contributions.\n(1) Recent advances in Giant Planet Magnetosphere Understanding, and Our Novel\nContributions Within This New Framework;\n(2) Distinction from Previous Saturn Asymmetry Studies.\n Recent advances in Giant Planet Magnetosphere Understanding, and Our Novel\nContributions Within This New Framework;\nZhang et al. (2021) reported an important work who first revealed Jupiter’s complex three-\ndimensional magnetic topology through high-resolution simulations. Their study showed that\nmuch of Jupiter’s polar cap is threaded by helical magnetic flux that closes within the planetary\ninterior, extends into the outer magnetosphere, and accumulates near the dawn flank, where\nstrong differential plasma rotation drags field lines sunward, as shown in Figure R1. In addition,\nthey found that open magnetic field regions extend unusually far into the dusk and nightside\nsectors, fundamentally challenging the traditional noon-centered cusp paradigm derived from\nterrestrial observations. This work has had a significant impact within the planetary science\ncommunity, and NASA’s Jet Propulsion Laboratory has since identified further exploration of\nJupiter’s three-dimensional magnetospheric structure as a primary objective of Juno’s second\nextended mission (Bolton et al., 2025).\nFigure R1. Jupiter’s complex three-dimensional magnetic field structure was first revealed by\nZhang et al. (2021) using high-precision, low-dissipation simulations. Unlike traditional\nmagnetic field models, this framework successfully explains the persistently bright polar\nauroras observed at Jupiter, which differ fundamentally from Earth’s auroral morphology.\nBuilding on this foundation, Chen et al. (2023) demonstrated that magnetopause\nreconnection at Jupiter is primarily controlled by the east–west component of the interplanetary\nmagnetic field, rather than the north–south component dominant at Earth, with reconnection\noccurring predominantly at high latitudes, as shown in Figure R2. Xu et al. (2024) subsequently\nprovided the first observational evidence of Jupiter’s dusk-side and nightside cusps.\nNevertheless, because of Juno’s orbital constraints, the global distribution of Jupiter’s cusps\nremains incomplete, particularly in the dawn, noon, and post-noon sectors.\nFigure R2. Simulation results and schematic diagrams of Jupiter’s global magnetic field\nstructure associated with magnetopause reconnection, adapted from the supplementary\nmaterials of Xu et al. (2024) and Chen et al. (2023). The simulations include open field lines\nthat illustrate the interaction between the magnetopause and the solar wind. Jupiter’s magnetic\nreconnection topology is highly complex and markedly different from that of Earth.\n Our Novel Contributions in This Study Within This New Framework\n(Correspondingly addressing the comments on points a, b, and c raised by the referee):\n(a) First Comprehensive Global Cusp Survey at Giant Planets: While previous Saturn\nstudies provided valuable case analyses (Jasinski et al., 2016 examined ~10 cases limited to 9-\ncomprehensive statistical analysis of 67 cusp events spanning local times 7-21 with proper\nnormalization by spacecraft dwell time. This normalization is crucial for obtaining unbiased\nprobability distributions.\n(b) Discovery of Post-Dusk Cusp Extension—A Different Picture Compared to the\nEarth: We provide the first observational evidence of Saturn's cusp extending to the post-dusk\nsector (near 20 LT), which is fundamentally different from the previously understood cusp\nconfigurations at Saturn and Earth (local time far away from dayside/normal afternoon side).\nInstead, it closely resembles the recently reported location and plasma characteristics of\nJupiter’s dusk-side cusp. These post-dusk cusps exhibit magnetosheath-like electron\npopulations similar to those observed at Jupiter, providing important cross-planetary\ncomparison of a new reconnection paradigm at rapidly rotating giant planets.\n(c) Saturn's Magnetopause Topology and Reconnection Pattern: Our high-resolution\nGAMERA simulations reveal, for the first time, Saturn's magnetopause magnetic topology\nunder typical solar wind conditions. The results show Saturn exhibits a reconnection pattern\nsimilar to Jupiter's—occurring predominantly at high latitudes with suppressed low-latitude\nreconnection (please see Supplementary Note 3, Figures S4-S5 in the Supplementary\nInformation). This discovery complements the traditional Dungey cycle framework used in\nprevious studies of Saturn’s asymmetry (Cowley et al., 2003, 2005) and provides new insight\ninto the complexity of Saturn’s asymmetric magnetospheric system.\n(d) Unified Understanding Across Giant Planets: By demonstrating that Saturn's cusp\ndistribution and magnetopause topology closely parallel Jupiter's recently discovered\nconfiguration (Zhang et al., 2021; Chen et al., 2023; Xu et al., 2024), our work establishes that\nrapidly rotating giant planets share fundamental magnetospheric characteristics distinct from\nEarth. This provides the support for Zhang et al. (2024)’s unified framework of planetary\nauroral morphologies.\nFigure R3. Comparison between Simulated and Schematic Diagrams of Jupiter’s and\nSaturn’s Magnetopause Reconnection Patterns and Global Magnetic Topologies, Shown from\na Sun view. Under westward solar wind conditions, both simulations exhibit similar\nreconnection configurations at Jupiter and Saturn.\nFigure R4. Sketch of the reconnection plasma flow and related processes in the equatorial\nplane of Saturn’s magnetosphere out to the magnetopause (Allen et al., 2018; Cowley et al.,\n2005).\n Distinction from Previous Saturn Asymmetry Studies:\nAs the referee mentioned, we fully acknowledge that previous Cassini observations have\nrevealed pervasive asymmetries throughout Saturn's magnetosphere (e.g., Pilkington et al.,\n2015; Jia & Kivelson, 2016; Wilson et al., 2013; Sergis et al., 2017; Allen et al., 2018), which\ncan be divided into two main categories:\n(1) Within the Magnetodisc Region: Numerous studies documented asymmetric plasma\nflow patterns (Wilson et al., 2017; Thomsen et al., 2014; Kane et al., 2014), with dawn-side\noutward and dusk-side inward radial velocities. Particle distributions show pronounced\nasymmetries, with energetic particle pressure exhibiting a midnight maximum to noon\nminimum ratio of ~5 (Arridge et al., 2015; Sergis et al., 2017). Allen et al. (2018) revealed that\nwater group ions prefer the dusk-to-premidnight sector, while H⁺ and H₂⁺ show dual-flank\nenhancement. Geometrical asymmetries include plasma sheet thinning from noon to midnight\n(Krimigis et al., 2007) and stronger ring currents in the dusk-to-midnight sector (Kellett et al.,\n2011);\n(2) At High-Latitude Boundaries: Pilkington et al. (2015) found the dawn-side\nmagnetopause extends 7±1% farther than other regions. Jia & Kivelson (2016) simulations\nshowed dawn-side expansion arising from differential plasma expansion/contraction rates\nalong rotating flux tubes.\nAs comprehensively reviewed by Arridge et al. (2015), these asymmetries have been\ninterpreted within two main theoretical frameworks:\n1. Solar wind–driven models: The framework of Cowley et al. (2003, 2005), which\ncombines the Vasyliūnas and Dungey cycles, predicts dusk-side Vasyliūnas X-lines\nand dawn-side Dungey X-lines, shown by Figure R4. Although this framework is\nconsistent with some observations, direct evidence for such X-lines remains limited.\nAn alternative is the viscous interaction model (Delamere & Bagenal, 2010, 2013),\nwhich does not require large-scale convection.\n2. Rotational dynamics models: The model proposed by Kivelson and Southwood (2005)\nattributes the asymmetry to firehose instability in the dusk-side plasma sheet, an\ninterpretation supported by simulations demonstrating the development of pressure\nanisotropy (Vogt et al., 2014).\nDespite these frameworks, fundamental questions remained unanswered:\n The detailed magnetic field topology of giant planet magnetospheres remains\nincompletely understood, and previous simulation studies have not adequately\nillustrated how magnetic connectivity between the magnetosphere and the solar wind\nis established (Jia & Kivelson, 2016).\n The suppression of low-latitude magnetopause reconnection at Saturn (Lai et al., 2012;\nFuselier et al., 2014, 2020; Jasinski et al., 2021) contradicted expectations from Dungey\ncycle models (Cowley et al.; 2003, 2005).\nHow Our Work Advences This Understanding:\nOur study takes a further step toward understanding the giant planet picture linked to these\ncritical gaps by revealing, for the first time:\n1. The Detailed Magnetic Topology Picture: Our high-resolution GAMERA\nsimulations (Zhang et al., 2019) demonstrate Saturn's global magnetic field\nconfiguration, showing how open field lines extend to the post-dusk sector—a\nconfiguration not very likely to to predict from previous frameworks based on Earth-\nlike reconnection. The post-dusk cusp extension we observe is closely associated with\nthis high-latitude reconnection geometry. The dawn-side magnetopause expansion\nnoted by Pilkington et al. (2015) and Jia & Kivelson (2016) now can be potentially\nunderstood as part of the global magnetic topology we revealt.\n2. The Magnetopause Reconnection Geometry: We show that Saturn's magnetopause\nreconnection occurs predominantly at high latitudes, governed by the east-west IMF\ncomponent rather than north-south as in Dungey models. This explains why low-\nlatitude reconnection is suppressed—it's not an anomaly but a fundamental\ncharacteristic of the system.\n3. Cross-Planetary Comparison: By demonstrating that Saturn exhibits the same\nreconnection pattern recently discovered at Jupiter (Chen et al., 2023; Xu et al., 2024),\nas well as exhibiting location distributions and particle characteristics closely\nresembling those of Jupiter’s post-dusk cusp events we establish this as a fundamental\ncharacteristic of rapidly rotating magnetospheres, not a planet-specific anomaly. This\nstrong correspondence indicates similar cusp microphysics across giant planets and\nhighlights a distinct pattern of magnetosphere–solar wind interactions in giant planets,\nfundamentally different from those at Earth.\nIn essence, although previous studies have thoroughly documented asymmetries in\nSaturn’s magnetosphere, our work advances understanding by proposing a framework\nthat can account for the suppression of low-latitude reconnection at Saturn—a feature\nnot adequately explained by Dungey cycle–dominated models. Through combined\nobservational and simulation-based evidence of Saturn’s magnetic topology (e.g., cusp\nregions), we provide a helpful framework for interpreting these asymmetries.\nBased on the above points and in response to the referee’s comments, we have made\ncorresponding improvements to the manuscript. The main text has been updated to better\nhighlight the novelty of this study and to better place our results in the context of existing\nliterature. Please see lines 15-26, 90-94, 123-126, 170-172, 254-258 in the revised manuscript.\nReference\nAllen, R. C., Mitchell, D. G., Paranicas, C. P., Hamilton, D. C., Clark, G., Rymer, A. M., ... &\nVandegriff, J. (2018). Internal versus external sources of plasma at Saturn: Overview\nfrom Magnetospheric imaging investigation/charge‐energy‐mass spectrometer\ndata. Journal of Geophysical Research: Space Physics, 123(6), 4712-4727.\nArridge, C. S., Kane, M., Sergis, N., Khurana, K. K., & Jackman, C. M. (2015). Sources of\nlocal time asymmetries in magnetodiscs. In The magnetodiscs and aurorae of giant\nplanets (pp. 301-333). New York, NY: Springer New York.\nArridge, C. S., Jasinski, J. M., Achilleos, N., Bogdanova, Y. V., Bunce, E. J., Cowley, S.\nW., ... & Krupp, N. (2016). Cassini observations of Saturn's southern polar cusp. Journal\nof Geophysical Research: Space Physics, 121(4), 3006-3030.\nBolton, S., & Juno Science Team. (2025). Juno’s 2nd extended mission (EGU25-5184). EGU\nGeneral Assembly 2025. EGUsphere. doi:10.5194/egusphere-egu25-5184\nChen, J., Zhang, B., Lin, D., Delamere, P. A., Yao, Z., Brambles, O., ... & Lyon, J. G. (2023).\nPrediction of axial asymmetry in jovian magnetopause reconnection. Geophysical\nResearch Letters, 50(9), e2022GL102577.\nCowley, S. W. H., Bunce, E. J., Stallard, T. S., & Miller, S. (2003). Jupiter's polar ionospheric\nflows: Theoretical interpretation. Geophysical research letters, 30(5).\nCowley, S. W. H., Badman, S. V., Bunce, E. J., Clarke, J. T., Gérard, J. C., Grodent, D., ... &\nYeoman, T. K. (2005). Reconnection in a rotation‐dominated magnetosphere and its\nrelation to Saturn's auroral dynamics. Journal of Geophysical Research: Space\nPhysics, 110(A2).\nDelamere, P. A., & Bagenal, F. (2010). Solar wind interaction with Jupiter's\nmagnetosphere. Journal of Geophysical Research: Space Physics, 115(A10).\nDelamere, P. A., & Bagenal, F. (2013). Magnetotail structure of the giant magnetospheres:\nImplications of the viscous interaction with the solar wind. Journal of Geophysical\nResearch: Space Physics, 118(11), 7045-7053.\nFuselier, S. A., Frahm, R., Lewis, W. S., Masters, A., Mukherjee, J., Petrinec, S. M., &\nSillanpaa, I. J. (2014). The location of magnetic reconnection at Saturn's magnetopause:\nA comparison with Earth. Journal of Geophysical Research: Space Physics, 119(4),\n2563-2578.\nFuselier, S. A., Petrinec, S. M., Sawyer, R. P., Mukherjee, J., & Masters, A. (2020).\nSuppression of magnetic reconnection at Saturn's low‐latitude magnetopause. Journal of\ngeophysical research: Space physics, 125(5), e2020JA027895.\nJasinski, J. M., Arridge, C. S., Coates, A. J., Jones, G. H., Sergis, N., Thomsen, M. F., ... &\nWaite Jr, J. H. (2016). Cassini plasma observations of Saturn's magnetospheric\ncusp. Journal of Geophysical Research: Space Physics, 121(12), 12-047.\nJasinski, J. M., Akhavan‐Tafti, M., Sun, W., Slavin, J. A., Coates, A. J., Fuselier, S. A., ... &\nMurphy, N. (2021). Flux transfer events at a reconnection‐suppressed magnetopause:\nCassini observations at Saturn. Journal of Geophysical Research: Space Physics, 126(2),\ne2020JA028786.\nJia, X., & Kivelson, M. G. (2016). Dawn‐dusk asymmetries in rotating magnetospheres:\nLessons from modeling Saturn. Journal of Geophysical Research: Space Physics, 121(2),\n1413-1424.\nKane, M., Mitchell, D. G., Carbary, J. F., & Krimigis, S. M. (2014). Plasma convection in the\nnightside magnetosphere of Saturn determined from energetic ion anisotropies. Planetary\nand Space Science, 91, 1-13.\nKellett, S., Arridge, C. S., Bunce, E. J., Coates, A. J., Cowley, S. W. H., Dougherty, M. K., ...\n& Wilson, R. J. (2011). Saturn's ring current: Local time dependence and temporal\nvariability. Journal of Geophysical Research: Space Physics, 116(A5).\nKivelson, M. G., & Southwood, D. J. (2005). Dynamical consequences of two modes of\ncentrifugal instability in Jupiter's outer magnetosphere. Journal of Geophysical Research:\nSpace Physics, 110(A12).\nKrimigis, S. M., Sergis, N., Mitchell, D. G., Hamilton, D. C., & Krupp, N. (2007). A\ndynamic, rotating ring current around Saturn. Nature, 450(7172), 1050-1053.\nLai, H. R., Wei, H. Y., Russell, C. T., Arridge, C. S., & Dougherty, M. K. (2012).\nReconnection at the magnetopause of Saturn: Perspective from FTE occurrence and\nmagnetosphere size. Journal of Geophysical Research: Space Physics, 117(A5).\nPilkington, N. M., Achilleos, N., Arridge, C. S., Guio, P., Masters, A., Ray, L. C., ... &\nDougherty, M. K. (2015). Asymmetries observed in Saturn's magnetopause geometry.\nGeophysical Research Letters, 42(17), 6890-6898.\nSergis, N., Jackman, C. M., Thomsen, M. F., Krimigis, S. M., Mitchell, D. G., Hamilton, D.\nC., ... & Wilson, R. J. (2017). Radial and local time structure of the Saturnian ring\ncurrent, revealed by Cassini. Journal of Geophysical Research: Space Physics, 122(2),\n1803-1815.\nThomsen, M. F., Jackman, C. M., Tokar, R. L., & Wilson, R. J. (2014). Plasma flows in\nSaturn's nightside magnetosphere. Journal of Geophysical Research: Space\nPhysics, 119(6), 4521-4535.\nVogt, M. F., Kivelson, M. G., Khurana, K. K., Walker, R. J., Ashour‐Abdalla, M., & Bunce,\nE. J. (2014). Simulating the effect of centrifugal forces in Jupiter's\nmagnetosphere. Journal of Geophysical Research: Space Physics, 119(3), 1925-1950.\nWilson, R. J., Bagenal, F., Delamere, P. A., Desroche, M., Fleshman, B. L., & Dols, V.\n(2013). Evidence from radial velocity measurements of a global electric field in Saturn's\ninner magnetosphere. Journal of Geophysical Research: Space Physics, 118(5), 2122-\n2132.\nWilson, R. J., Bagenal, F., & Persoon, A. M. (2017). Survey of thermal plasma ions in\nSaturn's magnetosphere utilizing a forward model. Journal of Geophysical Research:\nSpace Physics, 122(7), 7256-7278.\nXu, Y., Arridge, C. S., Yao, Z. H., Zhang, B., Ray, L. C., Badman, S. V., ... & Wei, Y.\n(2024). In situ evidence of the magnetospheric cusp of Jupiter from Juno spacecraft\nmeasurements. Nature communications, 15(1), 6062.\nZhang, B., Sorathia, K. A., Lyon, J. G., Merkin, V. G., Garretson, J. S., & Wiltberger, M.\n(2019). GAMERA: A three-dimensional finite-volume MHD solver for non-orthogonal\ncurvilinear geometries. The Astrophysical Journal Supplement Series, 244(1), 20.\nZhang, B., Delamere, P. A., Yao, Z., Bonfond, B., Lin, D., Sorathia, K. A., ... & Lyon, J. G.\n(2021). How Jupiter’s unusual magnetospheric topology structures its aurora. Science\nAdvances, 7(15), eabd1204.\nZhang, B., Yao, Z., Brambles, O. J., Delamere, P. A., Lotko, W., Grodent, D., ... & Lyon, J.\nG. (2024). A unified framework for global auroral morphologies of different\nplanets. Nature Astronomy, 8(8), 964-972.\nThere are other issues regarding originality. Below are a list of strengths and\nweaknesses of the paper; with the weaknesses outweighing the strengths to be published\nin a Nature journal.\nMajor Strengths:\nNone noted.\nMinor strengths:\n1. The paper showed a new updated list of cusp crossings, updating the field of this\nwell-studied phenomenon at Saturn’s magnetosphere.\n2. The paper compares MHD simulations of Saturn to the Cassini dataset, and then to\na simulation of the Earth.\nResponse #2\nWe thank the referee for evaluating the manuscript and for recognizing the two contributions\nlisted under the minor strengths. We also appreciate the referee’s broader concern regarding\noriginality. To avoid any misunderstanding, we would like to clarify that the elements\nhighlighted by the referee are in fact central to the manuscript’s novelty.\n(1) Updated list of Kronian cusp crossings.\nWe appreciate the referee’s recognition of the updated cusp-crossing list. Here we would like\nto clarify that the value of the catalog lies not simply in “updating” previous lists, but in several\nsubstantive advances:\n Statistical robustness: The 67 identified events represent a larger sample than those\nreported in earlier Saturn cusp studies (e.g., ~10 events in Jasinski et al., 2016), enabling\na statistically meaningful characterization of cusp occurrence and variability.\n More complete local-time coverage: Previous surveys were only limited to restricted local-\ntime sectors (approximately 9–15 LT, e.g., Jasinski et al., 2014, 2016, 2017; Arridge et al.,\n2016), whereas our systematic search spans more complete local times (approximately\n7-21 LT). This comprehensive coverage reveals a pronounced post-dusk extension of cusp\noccurrence that is not apparent in earlier study lists and is crucial for interpreting Saturn’s\nglobal magnetic geometry and comparison with Jupiter's post-dusk cusp.\n Dwell-time normalization: We introduce a spacecraft dwell-time–normalized occurrence\nprobability, which is essential for obtaining an unbiased global distribution，allowing for\na more quantitative comparison with Earth using a consistent framework.\n(2) MHD simulation comparisons\nWe also thank the referee for highlighting the simulation–observation comparison. We agree\nthat model comparison alone does not establish novelty. Accordingly, we clarify that the\noriginality of this work lies in the following key advances:\n First global view of Saturn’s magnetic topology near magnetopause/reconnection picture:\nOur high-resolution GAMERA simulations provide the first comprehensive global picture\nof Saturn's magnetic topology near the magnetopause, revealing how its magnetic\ngeometry significantly differs from that of Earth.\n Cross-planetary validation: By showing that Saturn’s magnetopause reconnection\nconfiguration closely resembles the recently identified structure at Jupiter (Zhang et al.,\n2021; Chen et al., 2023; Xu et al., 2024), we demonstrate that this behavior is a common\nfeature of rapidly rotating magnetospheres rather than a jupiter-specific phenomenon.\n Direct simulation–observation linkage: The close correspondence between the observed\ncusp distributions and the simulated magnetic topology simultaneously validates our\nobservational results, providing confidence in our interpretation.\n Comparative planetology framework: The Earth–Saturn comparison quantitatively\nhighlights fundamental differences in magnetospheric physics between solar wind–\ncontrolled and rapidly rotating planets, with important implications for interpreting\nexoplanetary magnetospheres.\nOverall, we hope these clarifications address the reviewer’s concern about originality. We\nbelieve the manuscript provides a statistically grounded cusp occurrence map with appropriate\nnormalization, together with a physically interpretable simulation-based framework for\nunderstanding Saturn’s magnetopause reconnection geometry in a comparative planetary\ncontext, which we consider well aligned with the scope of Nature Communications.\nTo further clarify the strengths and contributions of this study, we have revised the text to\narticulate their significance more clearly and in a more balanced manner. Please see line 123 to\n126, 242 to 243 in the revised manuscript.\nReference\nChen, J., Zhang, B., Lin, D., Delamere, P. A., Yao, Z., Brambles, O., ... & Lyon, J. G. (2023).\nPrediction of axial asymmetry in jovian magnetopause reconnection. Geophysical\nResearch Letters, 50(9), e2022GL102577.\nJasinski, J. M., Arridge, C. S., Lamy, L., Leisner, J. S., Thomsen, M. F., Mitchell, D. G., ... &\nWaite, J. H. (2014). Cusp observation at Saturn's high‐latitude magnetosphere by the\nCassini spacecraft. Geophysical Research Letters, 41(5), 1382-1388.\nJasinski, J. M., Arridge, C. S., Coates, A. J., Jones, G. H., Sergis, N., Thomsen, M. F., ... &\nWaite Jr, J. H. (2016). Cassini plasma observations of Saturn's magnetospheric\ncusp. Journal of Geophysical Research: Space Physics, 121(12), 12-047.\nJasinski, J. M., Arridge, C. S., Coates, A. J., Jones, G. H., Sergis, N., Thomsen, M. F., &\nKrupp, N. (2017). Diamagnetic depression observations at Saturn's magnetospheric cusp\nby the Cassini spacecraft. Journal of Geophysical Research: Space Physics, 122(6),\n6283-6303.\nXu, Y., Arridge, C. S., Yao, Z. H., Zhang, B., Ray, L. C., Badman, S. V., ... & Wei, Y.\n(2024). In situ evidence of the magnetospheric cusp of Jupiter from Juno spacecraft\nmeasurements. Nature communications, 15(1), 6062.\nZhang, B., Delamere, P. A., Yao, Z., Bonfond, B., Lin, D., Sorathia, K. A., ... & Lyon, J. G.\n(2021). How Jupiter’s unusual magnetospheric topology structures its aurora. Science\nAdvances, 7(15), eabd1204.\nMajor weaknesses:\n1. The paper does not provide a substantial original contribution to the field (see\nremarks above about originality and significance).\nResponse #3\nWe thank the referee for emphasizing the importance of clearly articulating originality. We\nhave responded comprehensively in Response #1 and #2, including a clearer statement of the\nmanuscript’s main contributions and several targeted wording adjustments to ensure the\nnovelty is readily apparent to readers. We therefore refer the referee to Response #1 and #2\nfor the full discussion and avoid repeating it here.\n2. The paper did not make a compelling case that it is identifying more accurate cusp\ncrossings instead of reproducing previous work. The authors claim that in their Figure\nR9 “Our identification is more accurate, excluding the periods when the spacecraft\nhad clearly entered lobe region between the second and the third red dashed line”,\nciting that the cusp example shown in their figure is a more accurate identification of\npreviously identified cusps. However, the same identification of the two cusp events is\nidentified in the same manner as previous publications. Jasinski et al., (2016) who\nthe authors cite, separate their cusp and show the two cusp identifications in their online\nsupporting material (just as the authors have done here). Arridge et al., (2016) identify\nthe two cusp crossings also, for the same interval, in their Figure 9, in exactly the same\nmanner as Jasinski et al., (2016) as well as how the authors are identifying it. The same\nis true for the authors Figure R9. Therefore, the authors are not producing more\naccurate identification but instead reproducing previous work.\nResponse #4\nWe appreciate the referee for pointing out that our previous response and wording may have\ncaused confusion. In this study, we do not intend to claim superior accuracy in identifying cusp\nevents relative to the excellent work of Jasinski et al. (2016) and Arridge et al. (2016). Our\nidentification of the two cusp intervals (1–2 February 2007, as the referee pointed out in the\ncomment) in the example provided in the previous response is meant to demonstrate\nconsistency with established methods, not superiority. Our original intention was to show\nthat our event list encompasses and extends the cusp lists reported in earlier studies (e.g.,\nJasinski et al., 2014, 2016, 2017; Arridge et al., 2016), thereby demonstrating the robustness\nand completeness of our event identification in the statistical analysis.\nFollowing the referee’s comment, we have carefully checked the manuscript and accordingly\nrevised the relevant wording in the main text to avoid any unnecessary misunderstanding\nregarding the accuracy of cusp event identification. Please see line 164 , 170 to 172 in the\nrevised manuscript.\nReference\nArridge, C. S., Jasinski, J. M., Achilleos, N., Bogdanova, Y. V., Bunce, E. J., Cowley, S.\nW., ... & Krupp, N. (2016). Cassini observations of Saturn's southern polar cusp. Journal\nof Geophysical Research: Space Physics, 121(4), 3006-3030.\nJasinski, J. M., Arridge, C. S., Lamy, L., Leisner, J. S., Thomsen, M. F., Mitchell, D. G., ... &\nWaite, J. H. (2014). Cusp observation at Saturn's high‐latitude magnetosphere by the\nCassini spacecraft. Geophysical Research Letters, 41(5), 1382-1388.\nJasinski, J. M., Arridge, C. S., Coates, A. J., Jones, G. H., Sergis, N., Thomsen, M. F., ... &\nWaite Jr, J. H. (2016). Cassini plasma observations of Saturn's magnetospheric\ncusp. Journal of Geophysical Research: Space Physics, 121(12), 12-047.\nJasinski, J. M., Arridge, C. S., Coates, A. J., Jones, G. H., Sergis, N., Thomsen, M. F., &\nKrupp, N. (2017). Diamagnetic depression observations at Saturn's magnetospheric cusp\nby the Cassini spacecraft. Journal of Geophysical Research: Space Physics, 122(6),\n6283-6303.\nMinor weaknesses:\n1. The authors claim that “ The magnetopause reconnection at Saturn is more sensitive\nto east-west component of IMF but not north-south component” however then later\ncontradict that this is can be true when they write a few sentences later that “due to\nthe limited availability of simultaneous solar wind monitoring data from in-situ\nobservations of Saturn’s and Jupiter’s magnetospheres” . Furthermore, lobe\nreconnection cusp identification can be completed by analyzing the ion dispersions in\nthe dataset.\nResponse #5\nWe thank the referee for highlighting ion dispersion signatures in planetary cusps. We agree\nthat ion dispersion can be an important diagnostic for analyzing magnetopause reconnection\ngeometry and upstream solar wind conditions in cusp regions. At rapidly rotating giant planets,\nhowever, the situation may differ when ion dispersion signatures are used to identify\nmagnetopause reconnection–associated cusps at Saturn. To address the referee's concerns, we\nhave expanded the discussion in this response to explain why applying this approach at giant\nplanets is considerably more challenging.\n Ion Dispersion at Earth\nAt Earth, ion dispersion in the cusp has been extensively studied and provides clear\ndiagnostics of magnetopause reconnection modes (e.g., Cargill et al., 2005; Pitout et al.,\n2006; Lockwood & Smith, 1994). These dispersion patterns arise from the combined\neffects of (1) convection of newly reconnected field lines and (2) the energy-dependent\ntravel times of ions along magnetic field lines, often referred to as velocity filtering\n(Lockwood & Smith, 1994).\nConsider a situation that a spacecraft traversing the cusp from lower to higher latitudes.\nUnder southward interplanetary magnetic field (IMF) conditions, when subsolar\nreconnection dominates through the classical Dungey cycle, a spacecraft traversing the\ncusp from lower to higher latitudes encounters progressively older/later open field lines.\nThis produces normal ion dispersion, in which high-energy ions are observed first on\nyounger/new-generated field lines near the equatorward boundary, followed by\nprogressively lower-energy ions on older field lines toward the polar cap. In contrast,\nunder northward IMF conditions, high-latitude lobe reconnection can occur, leading to\nreversed or anomalous dispersion as the spacecraft encounters the newest reconnected\nfield lines at higher latitudes.\nThis direct relationship between dispersion signatures and reconnection modes is effective\nat Earth because (1) the magnetosphere is strongly controlled by the solar wind, (2) the\nreconnection picture is relatively simple, typically involving subsolar versus lobe\nreconnection, and (3) the associated field-line convection patterns are well constrained\n(Lockwood & Smith, 1994; Pitout & Bogdanova, 2021; Smith & Lockwood, 1996).\n More Complex Picture at Giant Planets\nAt rapidly rotating giant planets, the situation is very different from that at Earth. Given\nthe similarity between the magnetopause reconnection configuration of Saturn’s\nmagnetosphere revealed in this study and that of Jupiter (Chen et al., 2023), as shown in\nFigure R3, below we use ion dispersion in Jupiter’s cusp as an illustrative example\n(discussed by Xu et al., 2024 in supplementary information). We here demonstrate in detail\nthe complex ion dispersion patterns observed by spacecraft under different combinations\nof east–west solar wind conditions, spacecraft velocity, and magnetic field line convection\nvelocity.\n(1) When the spacecraft and field lines move in the same direction:\n(a) If the spacecraft speed is greater than the convection speed |V | > |V | (shown in Figure\nsc conv\nR5, note that different solar wind conditions correspond to different magnetic field\nreconnection pictures and magnetic field convection velocities), the spacecraft overtakes\nsuccessive field lines. It first encounters the most recently reconnected field line, which\ncarries high-energy ions, followed by progressively older field lines whose high-energy\nions have been lost through bounce motion. This produces a normal dispersion pattern.\n(b) If the spacecraft speed is comparable to the convection speed |V | ~ |V |,\nthe spacecraft\nsc conv\nremains on a single reconnected field line. It observes the full energy spectrum from that\nreconnection event, with high-energy ions arriving first and lower-energy ions arriving later,\nagain resulting in normal dispersion.\n(c) If the spacecraft speed is less than the convection speed |V | < |V |,\nnewly reconnected\nsc conv\nfield lines sweep past the spacecraft. The spacecraft first detects fresh reconnection\nproducts with high-energy ions, followed by older field lines with depleted high-energy\npopulations, also yielding normal dispersion.\n(d)\nFigure R5. The schematic of velocity-filter effect of ions observed in cusp for different\nsituations at Jupiter, as a example to understand the reconnection in Saturn’s picture (a, b, c)\nSchematic when Juno moves in the same direction of magnetic field line convection motion;\n(d) Schematic when Juno moves against the convection direction of magnetic field line; (e) The\nexpected ion energy distribution detected by the spacecraft in the case corresponding to (a) and\nwhen the spacecraft velocity is greater than the convection velocity; (f) The expected ion energy\ndistribution corresponding to (b) and when the spacecraft velocity is approximately equal to the\nconvection velocity; (g) The expected ion energy distribution corresponding to (c) and when\nthe spacecraft velocity is less than the convection velocity. (h) The expected reversed ion\ndispersion distribution corresponding to (d). The 'X' flag marks the reconnection point. Red\narrow and black dashed arrows indicate the convection direction of the magnetic field lines\nand the direction of the spacecraft's motion, respectively. The red, orange, and blue colors of\nthe magnetic field lines indicate the magnetic lines from near to far from the reconnection point,\nrespectively. The red indicates that a fresh magnetic reconnection is occurring and the blue\nindicates the opposite (Xu et al., 2024).\n(2) When the spacecraft and field lines move in opposite directions:\n(a) In this case, regardless of their relative speeds, the spacecraft encounters field lines in\nreverse chronological order. It first crosses older reconnected field lines dominated by low-\nenergy ions and then progressively younger field lines containing higher-energy\npopulations. This produces reversed dispersion patterns.\nGiven that our study reveals Saturn's magnetopause reconnection geometry closely resembles\nJupiter's—with high-latitude reconnection controlled by east-west IMF components rather than\nnorth-south—we expect similar complexity in ion dispersion patterns at Saturn.\nOverall, the following factors make ion dispersion analysis challenging for Saturn dataset:\n(1) At rapidly rotating giant planets, rapid planetary rotation generates complex convection\npatterns that vary strongly with local time and radial distance. Convection speeds can\nchange substantially across the cusp region, and the observed ion dispersion depends\nsensitively on the spacecraft trajectory relative to these moving structures. Unlike Earth,\nwhere solar wind conditions largely control convection, giant planets are dominated by\nintrinsic rotational dynamics that govern field line motion. Moreover, Saturn's 10.6-hour\nrotation period drives strong azimuthal flows that vary with radial distance and local time,\ncreating spatially variable convection velocities that change the relative motion between\nspacecraft and field lines;\n(2) Lack of upstream solar wind monitoring: Without simultaneous solar wind measurements,\nwe cannot determine the IMF orientation during each cusp crossing, making it impossible\nto predict expected dispersion patterns.\nIn conclusion, while ion dispersion analysis is a helpful tool, the combination of rapid rotation,\ncomplex reconnection geometry, and lack of solar wind monitoring makes this approach\ndifficult for determining reconnection modes at Saturn. Although detailed ion dispersion\nanalysis is beyond the scope of this work, it represents an important future research direction\nworth attention. Thanks again to the referee for the comment.\nReference\nCargill, P. J., Lavraud, B., Owen, C. J., Grison, B., Dunlop, M. W., Cornilleau-Wehrlin, N., ...\n& Nykyri, K. (2005). Cluster at the magnetospheric cusps. Space Science\nReviews, 118(1), 321-366.\nLockwood, M., & Smith, M. F. (1994). Low and middle altitude cusp particle signatures for\ngeneral magnetopause reconnection rate variations: 1. Theory. Journal of Geophysical\nResearch: Space Physics, 99(A5), 8531-8553.\nPitout, F., Escoubet, C. P., Klecker, B., & Rème, H. (2006). Cluster survey of the mid-altitude\ncusp: 1. size, location, and dynamics. In Annales Geophysicae (Vol. 24, No. 11, pp.\n3011-3026). Göttingen, Germany: Copernicus Publications.\nPitout, F., & Bogdanova, Y. V. (2021). The polar cusp seen by Cluster. Journal of\nGeophysical Research: Space Physics, 126(9), e2021JA029582.\nSmith, M. F., & Lockwood, M. (1996). Earth's magnetospheric cusps. Reviews of\nGeophysics, 34(2), 233-260.\nXu, Y., Arridge, C. S., Yao, Z. H., Zhang, B., Ray, L. C., Badman, S. V., ... & Wei, Y.\n(2024). In situ evidence of the magnetospheric cusp of Jupiter from Juno spacecraft\nmeasurements. Nature communications, 15(1), 6062.\n2. Figure 1 in the paper shows a magnetospheric sketch of Saturn with the cusp\nlabelled with a question mark (i.e. “Cusp?”), implying that this phenomenon is\nbeing identified for the first time. Considering previous publications that have\nreported cusp observations (e.g. Jasinski et al., 2014, 2016, 2017; Arridge et al.,\n2016) this labeling is misleading.\nResponse #6\nMany thanks to the referee for the comment. We fully agree with the referee that the question\nmark in Figure 1 is misleading which would make readers confusing. The question mark was\nintended to indicate uncertainty about the global distribution pattern of Saturn's cusp\n(particularly its potential extension to post-dusk sectors corresponding to the Jupiter’s cusp as\nreported by Xu et al., 2024), not to question the existence of cusps at Saturn, which have been\nwell-documented in previous studies as the referee pointed out.\nFollowing the referee’s suggestion, we have revised Figure 1 by removing the question mark\nfrom Saturn’s schematic and updated the caption from “The global distribution of Saturn’s cusp\nremains poorly understood” to “The global distribution of Saturn’s cusp remains incompletely\nunderstood.” This revision provides a more precise expression and more accurately reflects the\ncurrent state of knowledge regarding Saturn’s cusp.\nReference\nXu, Y., Arridge, C. S., Yao, Z. H., Zhang, B., Ray, L. C., Badman, S. V., ... & Wei, Y.\n(2024). In situ evidence of the magnetospheric cusp of Jupiter from Juno spacecraft\nmeasurements. Nature communications, 15(1), 6062."}]

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