Diamagnetic depression observations at Saturn’s magnetospheric cusp by the Cassini spacecraft

The magnetospheric cusp is a region where shocked solar wind plasma can enter a planetary magnetosphere, after magnetic reconnection has occurred at the dayside magnetopause or in the lobes. The dense plasma that enters the high‐latitude magnetosphere creates diamagnetic effects whereby a depression is observed in the magnetic field. We present observations of the cusp events at Saturn’s magnetosphere where these diamagnetic depressions are found. The data are subtracted from a magnetic field model, and the calculated magnetic pressure deficits are compared to the particle pressures. A high plasma pressure layer in the magnetosphere adjacent to the cusp is discovered to also depress the magnetic field, outside of the cusp. This layer is observed to contain energetic He++ (up to ∼100 keV) from the solar wind as well as heavy water group ions (W+) originating from the moon Enceladus. We also find a modest correlation of diamagnetic depression strength to solar wind dynamic pressure and velocity; however, unlike at Earth, there is no correlation found with He++ counts.Key PointsDiamagnetic depressions are found in the cusp and are observed to continue into the adjacent magnetosphereA heated plasma layer of mixed composition is found to depress the adjacent magnetospheric fieldDiamagnetic depression strength is correlated to solar wind dynamic pressure and velocity but not to the observed He++ counts, like at EarthPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137687/1/jgra53517_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137687/2/jgra53517-sup-0001-supinfo.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137687/3/jgra53517.pd

Testing the necessity of transient spikes in the storm time ring current drivers

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95070/1/jgra20908.pd

Long‐lived plasmaspheric drainage plumes: Where does the plasma come from?

Long‐lived (weeks) plasmaspheric drainage plumes are explored. The long‐lived plumes occur during long‐lived high‐speed‐stream‐driven storms. Spacecraft in geosynchronous orbit see the plumes as dense plasmaspheric plasma advecting sunward toward the dayside magnetopause. The older plumes have the same densities and local time widths as younger plumes, and like younger plumes they are lumpy in density and they reside in a spatial gap in the electron plasma sheet (in sort of a drainage corridor). Magnetospheric‐convection simulations indicate that drainage from a filled outer plasmasphere can only supply a plume for 1.5–2 days. The question arises for long‐lived plumes (and for any plume older than about 2 days): Where is the plasma coming from? Three candidate sources appear promising: (1) substorm disruption of the nightside plasmasphere which may transport plasmaspheric plasma outward onto open drift orbits, (2) radial transport of plasmaspheric plasma in velocity‐shear‐driven instabilities near the duskside plasmapause, and (3) an anomalously high upflux of cold ionospheric protons from the tongue of ionization in the dayside ionosphere, which may directly supply ionospheric plasma into the plume. In the first two cases the plume is drainage of plasma from the magnetosphere; in the third case it is not. Where the plasma in long‐lived plumes is coming from is a quandary: to fix this dilemma, further work and probably full‐scale simulations are needed. Key Points Plasmaspheric drainage plumes can persist for weeks The source of the plasma supplying the long‐lived plumes is unknown Candidate sources include outflow from the tongue of ionizationPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108632/1/jgra51234.pd

Internally driven large-scale changes in the size of Saturn’s magnetosphere

Saturn’s magnetic field acts as an obstacle to solar wind flow, deflecting plasma around the planet and forming a cavity known as the magnetosphere. The magnetopause defines the boundary between the planetary and solar dominated regimes, and so is strongly influenced by the variable nature of pressure sources both outside and within. Following from Pilkington et al. (2014), crossings of the magnetopause are identified using 7 years of magnetic field and particle data from the Cassini spacecraft and providing unprecedented spatial coverage of the magnetopause boundary. These observations reveal a dynamical interaction where, in addition to the external influence of the solar wind dynamic pressure, internal drivers, and hot plasma dynamics in particular can take almost complete control of the system’s dayside shape and size, essentially defying the solar wind conditions. The magnetopause can move by up to 10–15 planetary radii at constant solar wind dynamic pressure, corresponding to relatively “plasma-loaded” or “plasma-depleted” states, defined in terms of the internal suprathermal plasma pressure

Influence of epoch time selection on the results of superposed epoch analysis using ACE and MPA data

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94710/1/jgra19440.pd

CIR ver sus CME drivers of the ring current during intense mag netic storms

Ninety intense magnetic storms (minimum Dst value of less than −100 nT) from solar cycle 23 (1996-2005) were simulated using the hot electron and ion drift integrator (HEIDI) model. All 90 storm intervals were run with several electric fields and nightside plasma boundary conditions (five run sets). Storms were classified according to their solar wind driver, including corotating interaction regions (CIRs) and interplanetary coronal mass ejections (ICMEs). Data-model comparisons were made against the observed Dst index (specifically, Dst*) and dayside hot-ion measurements from geosynchronous orbiting spacecraft. It is found that the data-model goodness-of-fit values are different for CIR-driven storms relative to ICME-driven storms. The results are also different for the same storm category for different boundary conditions. None of the CIR-driven events was overpredicted by HEIDI, while the dayside comparisons were comparable for the different drivers. The results imply that the outer magnetosphere is responding differently to the two kinds of solar wind drivers, even though the resulting storm size might be similar. That is, for ICME-driven events, magnetospheric currents inside of geosynchronous orbit dominate the Dst perturbation, while for CIR-driven events, currents outside of this boundary have a systematically larger contribution

Inflow Speed Analysis of Interchange Injections in Saturn's Magnetosphere

During its more than 13 years in orbit, the Cassini spacecraft detected a large number of plasma and energetic charged particle injections in Saturn's inner magnetosphere. In the corotating frame of the planet, the plasma contained within an injection moves radially inward with the component particles gaining energy. The highest energy particles in the injection experience stronger gradient‐curvature drifts in the longitudinal direction and can drift out of the main body of the injection. We have used these drift‐out effects to estimate the inflow speed of 19 injections by surveying cases from the available plasma data. We find that the average inflow speed from our sample is 22 km/s, and the values are well distributed between 0 and 50 km/s, with a few higher estimates. We have also computed the radial travel distance of interchange events and found that these are typically one to two Saturn radii. We discuss the implications of these quantifications on our understanding of transport

Understanding storm‐time ring current development through data‐model comparisons of a moderate storm

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94635/1/jgra18489.pd

Interchange Injections at Saturn: Statistical Survey of Energetic H+ Sudden Flux Intensifications

We present a statistical study of interchange injections in Saturn’s inner and middle magnetosphere focusing on the dependence of occurrence rate and properties on radial distance, partial pressure, and local time distribution. Events are evaluated from over the entirety of the Cassini mission’s equatorial orbits between 2005 and 2016. We identified interchange events from CHarge Energy Mass Spectrometer (CHEMS) H+ data using a trained and tested automated algorithm, which has been compared with manual event identification for optimization. We provide estimates of interchange based on intensity, which we use to investigate current inconsistencies in local time occurrence rates. This represents the first automated detection method of interchange, estimation of injection event intensity, and comparison between interchange injection survey results. We find that the peak rates of interchange occur between 7 and 9 Saturn radii and that this range coincides with the most intense events as defined by H+ partial particle pressure. We determine that nightside occurrence dominates as compared to the dayside injection rate, supporting the hypothesis of an inversely dependent instability growth rate on local Pedersen ionospheric conductivity. Additionally, we observe a slight preference for intense events on the dawnside, supporting a triggering mechanism related to large‐scale injections from downtail reconnection. Our observed local time dependence paints a dynamic picture of interchange triggering due to both the large‐scale injection‐driven process and ionospheric conductivity.Plain Language SummaryStudying high‐energy particles around magnetized planets is essential to understanding processes behind mass transport in planetary systems. Saturn’s magnetic environment, or magnetosphere, is sourced from a large amount of low‐energy water particles from Enceladus, a moon of Saturn. Saturn’s magnetosphere also undergoes large rotational forces from Saturn’s short day and massive size. The rotational forces and dense internal mass source drive interchange injections, or the injection of high‐energy particles closer to the planet as low‐energy water particles from the inner magnetosphere are transported outward. There have been many strides toward understanding the occurrence rates of interchange injections, but it is still unknown how interchange events are triggered. We present a computational method to identify and rank interchange injections using high‐energy particle fluxes from the Cassini mission to Saturn. These events have never been identified computationally, and the resulting database is now publically available. We find that the peak rates of interchange occur between 7 and 9 Saturn radii and that this range coincides with the highest intensity events. We also find that interchange occurrence rates peak on the nightside of Saturn. Through this study, we identify the potential mechanisms behind interchange events and advance our understanding of mass transport around planets.Key PointsWe developed a novel classification and identification algorithm for interchange injection based on Cassini CHEMS 3–220 keV H+ energetic ionsRadial occurrence rates and maximum partial H+ pressure in interchange peaked between 7 and 9 Saturn radii for all intensity categoriesOccurrence rates peak on the nightside (1800–0600 LT) as compared to the dayside (0600–1800 LT)Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/145315/1/jgra54283.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145315/2/jgra54283_am.pd