146 research outputs found
Large-scale solar wind flow around Saturn's nonaxisymmetric magnetosphere
The interaction between the solar wind and a magnetosphere is fundamental to
the dynamics of a planetary system. Here, we address fundamental questions on
the large-scale magnetosheath flow around Saturn using a 3D magnetohydrodynamic
(MHD) simulation. We find Saturn's polar-flattened magnetosphere to channel
~20% more flow over the poles than around the flanks at the terminator.
Further, we decompose the MHD forces responsible for accelerating the
magnetosheath plasma to find the plasma pressure gradient as the dominant
driver. This is by virtue of a high-beta magnetosheath, and in turn, the
high-MA bow shock. Together with long-term magnetosheath data by the Cassini
spacecraft, we present evidence of how nonaxisymmetry substantially alters the
conditions further downstream at the magnetopause, crucial for understanding
solar wind-magnetosphere interactions such as reconnection and shear
flow-driven instabilities. We anticipate our results to provide a more accurate
insight into the global conditions upstream of Saturn and the outer planets.Comment: Accepted for publication in Journal of Geophysical Journal: Space
Physic
A combined model of pressure variations in Titan's plasma environment
In order to analyze varying plasma conditions upstream of Titan, we have combined a physical model of Saturn's plasmadisk with a geometrical model of the oscillating current sheet. During modeled oscillation phases where Titan is furthest from the current sheet, the main sources of plasma pressure in the near-Titan space are the magnetic pressure and, for disturbed conditions, the hot plasma pressure. When Titan is at the center of the sheet, the main sources are the dynamic pressure associated with Saturn's cold, subcorotating plasma and the hot plasma pressure under disturbed conditions. Total pressure at Titan (dynamic plus thermal plus magnetic) typically increases by a factor of up to about three as the current sheet center is approached. The predicted incident plasma flow direction deviates from the orbital plane of Titan by âČ10°. These results suggest a correlation between the location of magnetic pressure maxima and the oscillation phase of the plasmasheet. Our model may be used to predict near-Titan conditions from âfar-fieldâ in situ measurements
Discovery of a transient radiation belt at Saturn
Radiation belts have been detected in situ at five planets. Only at Earth however has any variability in their intensity been heretofore observed, in indirect response to solar eruptions and high altitude nuclear explosions. The Cassini spacecraft's MIMI/LEMMS instrument has now detected systematic radiation belt variability elsewhere. We report three sudden increases in energetic ion intensity around Saturn, in the vicinity of the moons Dione and Tethys, each lasting for several weeks, in response to interplanetary events caused by solar eruptions. However, the intensifications, which could create temporary satellite atmospheres at the aforementioned moons, were sharply restricted outside the orbit of Tethys. Unlike Earth, Saturn has almost unchanging inner ion radiation belts: due to Saturn's near-symmetrical magnetic field, Tethys and Dione inhibit inward radial transport of energetic ions, shielding the planet's main, inner radiation belt from solar wind influences
Suprathermal electrons at Saturn's bow shock
The leading explanation for the origin of galactic cosmic rays is particle
acceleration at the shocks surrounding young supernova remnants (SNRs),
although crucial aspects of the acceleration process are unclear. The similar
collisionless plasma shocks frequently encountered by spacecraft in the solar
wind are generally far weaker (lower Mach number) than these SNR shocks.
However, the Cassini spacecraft has shown that the shock standing in the solar
wind sunward of Saturn (Saturn's bow shock) can occasionally reach this
high-Mach number astrophysical regime. In this regime Cassini has provided the
first in situ evidence for electron acceleration under quasi-parallel upstream
magnetic conditions. Here we present the full picture of suprathermal electrons
at Saturn's bow shock revealed by Cassini. The downstream thermal electron
distribution is resolved in all data taken by the low-energy electron detector
(CAPS-ELS, <28 keV) during shock crossings, but the higher energy channels were
at (or close to) background. The high-energy electron detector (MIMI-LEMMS, >18
keV) measured a suprathermal electron signature at 31 of 508 crossings, where
typically only the lowest energy channels (<100 keV) were above background. We
show that these results are consistent with theory in which the "injection" of
thermal electrons into an acceleration process involves interaction with
whistler waves at the shock front, and becomes possible for all upstream
magnetic field orientations at high Mach numbers like those of the strong
shocks around young SNRs. A future dedicated study will analyze the rare
crossings with evidence for relativistic electrons (up to ~1 MeV).Comment: 22 pages, 5 figures. Accepted for publication in Ap
A combined model of pressure variations in Titan's plasma environment
In order to analyze varying plasma conditions upstream of Titan, we have combined a physical model of Saturn?s plasma disk with a geometrical model of the oscillating current sheet. During modeled oscillation phases where Titan is farthest from the current sheet, the main sources of plasma pressure in the near-Titan space are the magnetic pressure and, for disturbed conditions, the hot plasma pressure. When Titan is at the center of the sheet, the main sources are the dynamic pressure associated with Saturn?s cold, subcorotating plasma and the hot plasma pressure under disturbed conditions. Total pressure at Titan (dynamic plus thermal plus magnetic) typically increases by a factor of up to about 3 as the current sheet center is approached. The predicted incident plasma flow direction deviates from the orbital plane of Titan by âČ 10⊠. These results suggest a correlation between the location of magnetic pressure maxima and the oscillation phase of the plasma sheet. Our model may be used to predict near-Titan conditions from ?far-field? in situ measurements.Fil: Achilleos, N.. University College London; Reino UnidoFil: Arridge, C. S.. University College London; Reino UnidoFil: Bertucci, Cesar. Consejo Nacional de InvestigaciĂłnes CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de AstronomĂa y FĂsica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de AstronomĂa y FĂsica del Espacio; ArgentinaFil: Guio, P.. University College London; Reino UnidoFil: Romanelli, Norberto Julio. Consejo Nacional de InvestigaciĂłnes CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de AstronomĂa y FĂsica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de AstronomĂa y FĂsica del Espacio; ArgentinaFil: Sergis, N.. Academy Of Athens. Office for Space Research and Technology; Greci
Local Time Variation in the Large-Scale Structure of Saturn's Magnetosphere
The large-scale structure of Saturn's magnetosphere is determined by internal and external
factors, including the rapid planetary rotation rate, significant internal hot and cold plasma sources, and
varying solar wind pressure. Under certain conditions the dayside magnetospheric magnetic field changes
from a dipolar to more disk-like structure, due to global force balance being approximately maintained
during the reconfiguration. However, it is still not fully understood which factors dominantly influence
this behavior, and in particular how it varies with local time. We explore this in detail using a 2-D
force-balance model of Saturn's magnetodisk to describe the magnetosphere at different local time sectors.
For model inputs, we use recent observational results that suggest a significant local time asymmetry in
the pressure of the hot (>3 keV) plasma population, and magnetopause location. We make calculations
under different solar wind conditions, in order to investigate how these local time asymmetries influence
magnetospheric structure for different system sizes. We find significant day/night asymmetries in
the model magnetic field, consistent with recent empirical studies based on Cassini magnetometer
observations. We also find dawn-dusk asymmetries in equatorial current sheet thickness, with the varying
hot plasma content and magnetodisk radius having comparable influence on overall structure, depending
on external conditions. We also find significant variations in magnetic mapping between the ionosphere
and equatorial disk, and ring current intensity, with substantial enhancements in the night and dusk
sectors. These results have consequences for interpreting many magnetospheric phenomena that vary with
local time, such as reconnection events and auroral observations
Spinning, breathing, and flapping: Periodicities in Saturnâs middle magnetosphere
In Saturnâs magnetosphere, ubiquitous fluctuations with a period of ~10.7âh have been observed in Saturn kilometric radiation (SKR), auroral emissions, the magnetic field, the electron density, and energetic particle fluxes. Here we characterize previously unstudied periodicities in plasma properties inside of 15âRS near the equatorial plane. Although periodically varying magnetic perturbations rotate relatively smoothly (spinning), plasma properties do not. The phase of the peak value of plasma density or pressure perturbations can change substantially across a few hours of local time or RS. As a means of interpreting observations, we use a magnetohydrodynamic simulation that generates fieldâaligned currents centered at 70° invariant latitude in Saturnâs southern ionosphere and rotating at the SKR period. The simulation reproduces many periodic features of the data including not only spinning perturbations but also globalâscale compression and expansion (breathing). Simulated plasma properties are also modulated by periodic largeâscale northâsouth motion (flapping) in regions beyond ~15 Saturn radii (RS), which we do not analyze here. Inside of 15âRS, plasma responds to a superposition of spinning and breathing at the spin period, developing perturbations that peak at different phases depending on what is measured and where. Strong compressional effects act impulsively over a limited range of rotation phase. Superposition of local and globalâscale variations produces phase jumps across short distances and can introduce multiple peaks in the variation of plasma properties within one rotation period, accounting for anomalies in the phase dependence of periodic fluctuations identified in the sparse data available.Key PointsEquatorial plasma and field moments in the core region are modulated at the SKR periodThe peak phase of observed plasma properties depends on the location of measurementPeriodic changes in the magnetosphere can be described as spinning, breathing, and flappingPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136320/1/jgra53132-sup-0001-Text_SI-S01.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136320/2/jgra53132_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136320/3/jgra53132-sup-0003-Figure_SI-S01.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136320/4/jgra53132.pd
A new form of Saturn's magnetopause using a dynamic pressure balance model, based on in situ, multiâinstrument Cassini measurements
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95429/1/jgra19967.pd
Internally driven large-scale changes in the size of Saturn's magnetosphere
Saturnâs 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 systemâs
dayside shape and size, essentially defying the solar wind conditions. The magnetopause can move by up to
10â15 planetary radii at constant solar wind dynamic pressure, corresponding to relatively âplasma-loadedâ
or âplasma-depletedâ states, defined in terms of the internal suprathermal plasma pressure
Cassini in situ observations of long duration magnetic reconnection in Saturnâs magnetotail
Magnetic reconnection is a fundamental process in solar system and astrophysical plasmas, through which stored magnetic energy associated with current sheets is converted into thermal, kinetic and wave energy1, 2, 3, 4. Magnetic reconnection is also thought to be a key process involved in shedding internally produced plasma from the giant magnetospheres at Jupiter and Saturn through topological reconfiguration of the magnetic field5, 6. The region where magnetic fields reconnect is known as the diffusion region and in this letter we report on the first encounter of the Cassini spacecraft with a diffusion region in Saturnâs magnetotail. The data also show evidence of magnetic reconnection over a period of 19?h revealing that reconnection can, in fact, act for prolonged intervals in a rapidly rotating magnetosphere. We show that reconnection can be a significant pathway for internal plasma loss at Saturn6. This counters the view of reconnection as a transient method of internal plasma loss at Saturn5, 7. These results, although directly relating to the magnetosphere of Saturn, have applications in the understanding of other rapidly rotating magnetospheres, including that of Jupiter and other astrophysical bodies
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