JGR:Space physics

Syndicate content Journal of Geophysical Research: Space Physics
Table of Contents for Journal of Geophysical Research: Space Physics. List of articles from both the latest and EarlyView issues.
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Nine Outstanding Questions of Solar Wind Physics

Thu, 06/04/2020 - 18:25

In situ measurements of the solar wind have been available for almost 60 years, and in that time plasma‐physics simulation capabilities have commenced, and ground‐based solar observations have expanded into space‐based solar observations. These observations and simulations have yielded an increasingly improved knowledge of fundamental physics and have delivered a remarkable understanding of the solar wind and its complexity. Yet there are longstanding major unsolved questions. Synthesizing inputs from the solar wind research community, nine outstanding questions of solar‐wind physics are developed and discussed in this commentary. These involve questions about the formation of the solar wind, about the inherent properties of the solar wind (and what the properties say about its formation), and about the evolution of the solar wind. The questions focus on (1) origin locations on the Sun, (2) plasma release, (3) acceleration, (4) heavy‐ion abundances and charge states, (5) magnetic structure, (6) Alfven waves, (7) turbulence, (8) distribution‐function evolution, and (9) energetic‐particle transport. On these nine questions we offer suggestions for future progress, forward looking on what is likely to be accomplished in near future with data from Parker Solar Probe, from Solar Orbiter, from the Daniel K. Inouye Solar Telescope (DKIST), and from Polarimeter to Unify the Corona and Heliosphere (PUNCH). Calls are made for improved measurements, for higher‐resolution simulations, and for advances in plasma‐physics theory.

An Event Study of Simultaneous Earthward and Tailward Reconnection Exhaust Flows in the Earth's Midtail

Wed, 06/03/2020 - 19:00

We report an event of two‐satellite measurements of simultaneous earthward and tailward fast flows of ~500 km/s in the midtail at X  ~ −63 RE and evaluate magnetic reconnection as a responsible mechanism by comparing the observations with a particle‐in‐cell (PIC) simulation. The two satellites were near midnight separated mainly along the X direction by ~5 RE. As they moved across the current sheet from the northern to southern lobes, the one closer to the Earth crossed the x line with fast flows changing from tailward to earthward, while the other one simultaneously observed tailward flows. The observed plasma and fields showed several key reconnection signatures, including the Walén relation, the fast reconnection rate of ~0.1, the Hall magnetic and electric fields, and counterstreaming electrons in the separatrix, indicating the fast flow was the reconnection exhaust. The observed temporal variations of flow speeds and magnetic fields suggested that the x line was moving tailward to a location between the two satellites and the exhaust was moving up and down. Within the exhaust, plasma pressure was highly anisotropic, and the current sheet can be unstable to the mirror, ion cyclotron, and firehose instabilities. Current sheet flapping and enhanced compressional waves near proton's local gyro frequencies were observed around the current sheet. Comparing with the PIC simulation suggests that the waves were mainly a result of oblique firehose instability.

Comparisons Between Jupiter's X‐ray, UV and Radio Emissions and In‐Situ Solar Wind Measurements During 2007

Wed, 06/03/2020 - 17:18

We compare Chandra and XMM‐Newton X‐ray observations of Jupiter during 2007 with a rich multi‐instrument data set including upstream in situ solar wind measurements from the New Horizons spacecraft, radio emissions from the Nançay Decametric Array and Wind/Waves, and ultraviolet (UV) observations from the Hubble Space Telescope. New Horizons data revealed two corotating interaction regions (CIRs) impacted Jupiter during these observations. Non‐Io decametric bursts and UV emissions brightened together and varied in phase with the CIRs. We characterize three types of X‐ray aurorae: hard X‐ray bremsstrahlung main emission, pulsed/flared soft X‐ray emissions, and a newly identified dim flickering (varying on short time scales, but quasi‐continuously present) aurora. For most observations, the X‐ray aurorae were dominated by pulsed/flaring emissions, with ion spectral lines that were best fit by iogenic plasma. However, the brightest X‐ray aurora was coincident with a magnetosphere expansion. For this observation, the aurorae were produced by both flickering emission and erratic pulses/flares. Auroral spectral models for this observation required the addition of solar wind ions to attain good fits, suggesting solar wind entry into the outer magnetosphere or directly into the pole for this particularly bright observation. X‐ray bremsstrahlung from high energy electrons was only bright for one observation, which was during a forward shock. This bremsstrahlung was spatially coincident with bright UV main emission (power > 1 TW) and X‐ray ion spectral line dusk emission, suggesting closening of upward and downward current systems during the shock. Otherwise, the bremsstrahlung was dim, and UV main emission power was also lower (<700 GW), suggesting their power scaled together.

Jupiter's X‐ray Emission During the 2007 Solar Minimum

Wed, 06/03/2020 - 17:17

The 2007–2009 solar minimum was the longest of the space age. We present the first of two companion papers on Chandra and XMM‐Newton X‐ray campaigns of Jupiter through February–March 2007. We find that low solar X‐ray flux during solar minimum causes Jupiter's equatorial regions to be exceptionally X‐ray dim (0.21 GW at minimum; 0.76 GW at maximum). While the Jovian equatorial emission varies with solar cycle, the aurorae have comparably bright intervals at solar minimum and maximum. We apply atomic charge exchange models to auroral spectra and find that iogenic plasma of sulphur and oxygen ions provides excellent fits for XMM‐Newton observations. The fitted spectral S:O ratios of 0.4–1.3 are in good agreement with in situ magnetospheric S:O measurements of 0.3–1.5, suggesting that the ions that produce Jupiter's X‐ray aurora predominantly originate inside the magnetosphere. The aurorae were particularly bright on 24–25 February and 8–9 March, but these two observations exhibit very different spatial, spectral, and temporal behavior; 24–25 February was the only observation in this campaign with significant hard X‐ray bremsstrahlung from precipitating electrons, suggesting this may be rare. For 8–9 March, a bremsstrahlung component was absent, but bright oxygen O6+ lines and best‐fit models containing carbon, point to contributions from solar wind ions. This contribution is absent in the other observations. Comparing simultaneous Chandra ACIS and XMM‐Newton EPIC spectra showed that ACIS systematically underreported 0.45‐ to 0.6‐keV Jovian emission, suggesting quenching may be less important for Jupiter's atmosphere than previously thought. We therefore recommend XMM‐Newton for spectral analyses and quantifying opacity/quenching effects.

New Modes and Mechanisms of Long‐Term Ionospheric TEC Variations From Global Ionosphere Maps

Wed, 06/03/2020 - 17:09

The ionosphere is very active and complex due to photo‐ionization from the solar activity, while traditional empirical models can only give a rough description of its actual variations. Nowadays, global ionosphere maps (GIMs) derived from denser Global Navigation Satellite Systems (GNSS) world‐tracking data provide an excellent total electron content (TEC) data set for global ionospheric research and modeling. In this paper, long‐tern variations of 16‐year (2003–2018) TEC time series from GIMs are investigated by using the principal mode analysis (PCA) technique. We analyze the resulting modes in the time‐spectral domain and parameterize the main contributions in terms of solar and magnetospheric forcing, local solar time (LST), and annual variations. The results show that the TEC variability is strongly dependent on the geographical location of the Earth's magnetic field, and the Earth's diurnal rotation modulates its spatial patterns of variability. The latitudinal asymmetry in the global distribution of TEC variations is due to the effects caused by the irregular shape of the Earth's magnetic field along with its diurnal rotation. The analyses of residuals show that periodicities are correlated to the solar wind speed and magnetospheric forcing, especially those located near the southern dip pole at the night side. Furthermore, we found a TEC anomaly at about 15° from the South magnetic dip at the night side, more prominent around 52°S 155°E.

Characterizing Ionospheric Effect on GNSS Radio Occultation Atmospheric Bending Angle

Wed, 06/03/2020 - 16:51

Global Navigation Satellite System (GNSS) atmospheric radio occultation (RO) has been an active remote sensing method to explore the Earth's atmosphere recently. The RO products especially atmospheric bending angles have been widely used for climate study. However, the residual ionospheric error (RIE) still exists to a certain extent after the widely used linear combination in bending angle calculation. In this study, we analyzed both CHAMP and COSMIC data and did full 3‐D ray tracing to characterize RIE. It is found that the order of RIE is around ±0.1 μrad and has an obvious dependency on a variety of factors including altitude, latitude, local time, season, and solar activity level. The amplitude and complexity of RIE generally increase with altitude. It is closely related to the strength of the E layer and the asymmetry between LEO side and GNSS side. Along magnetic latitude direction, the RIE shows a pronounced feature of two peaks and three troughs. From both the simulated and observed RIE, we can identify clearly the ionospheric seasonal variation features, such as the semiannual variation, the equinox asymmetry and the annual anomaly. A spherical asymmetry factor was also defined to quantify the correlation between ionospheric spherical asymmetry degree and RIE. The study provides a comprehensive understanding of RIE, which could contribute to the correction of RIE in the future.

First Observations of Magnetosonic Waves With Nonlinear Harmonics

Wed, 06/03/2020 - 16:50

Magnetosonic waves, with the frequency between proton cyclotron frequency and lower hybrid frequency, mainly occur near the Earth's magnetic equatorial plane and play an important role in the magnetospheric dynamics. In this paper, we report unusual magnetosonic waves with nonlinear harmonics observed by Magnetospheric Multiscale mission in the magnetotail. These magnetosonic waves have multiband enhanced electromagnetic power spectral densities (B w/B 0 up to 1/45) with the frequency from below to above the lower hybrid frequency and are quasi‐perpendicular propagating and linearly polarized. The frequency of the fundamental band is much higher than the proton cyclotron frequency f ci (i.e., ~35 f ci). We identified these emissions as high‐frequency magnetosonic waves with nonlinear harmonics. These are the first observations of such high‐frequency magnetosonic waves with the frequencies of harmonics higher than the lower hybrid frequency in the Earth's magnetosphere to the best of our knowledge. Given the absence of ring distributions for the protons and the easy coupling between compressed mode and its electromagnetic term, we propose that these nonlinear harmonic structures of the observed magnetosonic waves are likely generated by the nonlinear wave‐wave coupling among electromagnetic terms of the fundamental and higher harmonic waves.

Use of the L1 Constellation as a Multi‐Spacecraft Solar Wind Monitor

Wed, 06/03/2020 - 16:16

A novel multi‐spacecraft solar wind monitor is developed which expands on the forecasting ability of OMNI by giving spatially resolved predictions. The prediction algorithm ingests all the data from the current fleet of three L1 monitors, allowing gradients in the solar wind to be resolved on scale sizes similar to the magnetosphere. Understanding structure of the solar wind is vital to determine the global magnetospheric configuration, which is important both in real‐time as a space‐weather product and also for data users wanting to know upstream conditions when interpreting observations. The model is validated by comparing the predictions with other spacecraft observing the solar wind in‐situ. We perform a statistical study with thousands of hours of Magnetospheric Multi‐Scale observations in the solar wind, comparing the prediction accuracy of the multi‐spacecraft monitor to all of the OMNIWeb single‐spacecraft monitors. The multi‐spacecraft monitor shows improvement over all three of the single‐spacecraft predictions for 44% of the cases, and also outperforms both ACE and WIND, which are the primary magnetic field contributors to the OMNI IMF prediction, 55% of the time. We propose that the realistic structure of the solar wind that is resolved with multiple solar wind monitors could be vitally important to implement as upstream conditions in global magnetospheric simulations.

Numerical Simulation of Ionospheric Depletions Resulting From Rocket Launches Using a General Circulation Model

Tue, 06/02/2020 - 19:00

Rocket exhaust plumes have been observed to cause large‐scale depletions of ionospheric plasmas (“ionospheric holes”). In the F region, charge exchange reactions occur between O+ ions and exhaust species such as H2O, H2, and CO2 to form ions, which then undergo rapid dissociative recombination. The Global Ionosphere‐Thermosphere Model (GITM) was extended to include these chemical reactions and appropriate source terms to represent rocket exhaust plumes. The resulting model allowed for the detailed simulation of advection, diffusion, and chemical interactions of rocket exhaust gasses in the upper atmosphere. This model was applied to ionospheric depletions resulting from the launches of Jason‐3 and FORMOSAT‐5 on SpaceX Falcon 9 rockets from Vandenberg Air Force Base. Outputs from the model were compared with GNSS, ionosonde, and satellite Langmuir probe measurements. Simulation indicated that the FORMOSAT‐5 launch resulted in a far larger and longer‐lived ionospheric depletion than the Jason‐3 launch, consistent with observations. These differences resulted primarily from the deposition of exhaust gasses at higher altitudes in the former case, which resulted in longer residence times in the upper F region where ionospheric plasma was replenished more slowly. Simulation of the FORMOSAT‐5 launch reproduced the approximate size, shape, and motion of the observed ionospheric depletion, which reflected the advection and diffusion of the rocket exhaust gasses through the thermosphere.

Solar Energetic Proton Access to the Near‐Equatorial Inner Magnetosphere

Sun, 05/31/2020 - 19:00

In this study we examine the ability of protons of solar origin to access the near‐equatorial inner magnetosphere. Here we examine four distinct solar proton events from 20–200 MeV, concurrent with both quiet time and storm time conditions using proton data from the ACE satellite in the solar wind upstream of Earth and data from the Relativistic Electron Proton Telescope (REPT) instrument aboard Van Allen Probes. We examine the direct flux correspondence between interplanetary space and the inner magnetosphere. Small substructures in interplanetary space are observable in the REPT flux profiles, which can penetrate down to L values of ≤4. Furthermore, there are orbit‐to‐orbit variations in the west‐to‐east anisotropic flux ratios. The anisotropic flux ratios are used as a proxy for cutoff energies and display cutoff variations with L shell and energy. The dependence of the anisotropic flux ratio on Dst values is shown. The results paint a picture of highly dynamic spatial and temporal proton cutoff rigidities in the near‐equatorial inner magnetosphere.

Radar Investigation of Postsunset Equatorial Ionospheric Instability Over Kwajalein During Project WINDY

Sun, 05/31/2020 - 19:00

Advanced Research Projects Agency Long‐Range Tracking and Instrumentation Radar data acquired during the NASA Waves and INstabilities from a Neutral DYnamo (WINDY) sounding rocket campaign on Kwajalein Atoll in late August and September 2017 are analyzed to study the development of postsunset F ‐region ionospheric plasma density irregularities associated with equatorial spread F (ESF) conditions. Unstable conditions existed on nine of ten observing nights during the campaign. The first indication of instability onset each night was a patchy layer of coherent scatter in the bottomside/valley region. Patchy bottom‐type layers are telltales of plasma convective instability driven by vertical current in the F region associated with bottomside shear flow. The resulting flow evolves rapidly into fully developed instability through a bootstrapping process. Sometimes, the resulting topside depletions exhibited large‐scale structuring with a spatial scale too large to be associated with the preferred scale for plasma instability. We examine incoherent scatter observations of the background ionospheric morphology prior to the appearance of bottom‐type layers and find that a small degree of large‐scale bottomside structuring is sometimes present. Numerical simulations suggest that this structuring can act as a seed and provide the initial conditions necessary to account for the clustering of the topside plumes that emerge later.

Electromagnetic Radial Diffusion in the Earth's Radiation Belts as Determined by the Solar Wind Immediate Time History and a Toy Model for the Electromagnetic Fields

Sun, 05/31/2020 - 19:00

Diffusion‐driven radiation belt models require multiple physics‐based inputs to specify the radiation environment through which spacecraft travel, including diffusion coefficients. Even though event‐specific coefficients are necessary for model accuracy, their routine integration in operational models has not yet been achieved. In fact, one of the key inputs, the radial diffusion coefficient, is still commonly determined by a Kp ‐driven parameterization. This work presents a method to determine continuous time series of time‐varying radial diffusion coefficients. A theoretical model is developed in which electromagnetic radial diffusion is controlled by the magnetopause immediate time history. Specifically, radial diffusion is described as a function of the average, variance, and autocorrelation time of the geocentric standoff distance to the subsolar point on the magnetopause. Because the magnitudes of these three magnetopause parameters vary with time and magnetic activity, so does radial diffusion. To a lesser extent, radial diffusion is also controlled by the drift frequency of the radiation belt population. Moreover, radial diffusion is quantified using a standard model in which the magnetopause is controlled by the solar wind. Although the resulting diffusion coefficients span several orders of magnitude per Kp index, the median magnitudes are remarkably similar to the ones provided by the standard Kp ‐driven statistical parameterization.

Polar Electron Content From GPS Data‐Based Global Ionospheric Maps: Assessment, Case Studies, and Climatology

Sun, 05/31/2020 - 19:00

The electron content distribution of the north and south polar ionosphere from 2001 to the beginning of 2019 is analyzed by using the UQRG global ionospheric map (GIM) of vertical total electron content (VTEC), computed every 15 min by UPC‐IonSAT with a tomographic‐kriging combined technique. We first show that the accuracy of UQRG GIM is slightly better than that of the GIMs of other analysis centers on the whole and also over both poles. Second, we show examples of polar VTEC features in UQRG GIM, previously reported by different authors and with higher‐resolution techniques. Third, by means of an unsupervised clustering algorithm, learning vector quantization, we characterize the main features of the ionospheric electron content climatology, separately for the north and south polar regions.

Propagating and Dynamic Properties of Magnetic Dips in the Dayside Magnetosheath: MMS Observations

Sun, 05/31/2020 - 19:00

The magnetosheath is inherently complex and rich, exhibiting various kinds of structures and perturbations. It is important to understand how these structures propagate and evolve and how they relate to the perturbations. Here we investigate a kind of magnetosheath structure known as a magnetic dip (MD). As far as we are aware, there have been no previous studies concerning the evolution (contracting or expanding) of these types of structures, and their propagation properties cannot be unambiguously determined. In this study, using Magnetospheric MultiScale (MMS) high‐temporal resolution data and multispacecraft analysis methods, we obtain the propagation and dynamic features of a set of MDs. Four different types of MDs are identified: “frozen‐in,” “expanding,” “contracting,” and “stable‐propagating.” Significantly, a stable‐propagation event is observed with a sunward propagation component. This indicates that the source of the structure in this case is closely associated with the magnetopause, which provides strong support to the contention in earlier research. We further reveal the mechanism leading to the MD contraction or expansion. The motion of the MDs boundary is found closely related with the dynamic pressure. The scale of the contracting and expanding events are typically ~5–20 ρi (ion gyroradius), significantly smaller than that of frozen‐in events (~40 ρi). The observations could relate large‐scale (more than several tens of ρi) and kinetic‐scale (less than ρi) MDs, by revealing an evolution that spans these different scales, and help us better understand the variation and dynamics of magnetosheath structures and plasmas.

Electron acceleration by magnetosheath jet‐driven bow waves

Thu, 05/28/2020 - 14:50

Magnetosheath jets are localized fast flows with enhanced dynamic pressure. When they supermagnetosonically compress the ambient magnetosheath plasma, a bow wave or shock can form ahead of them. Such a bow wave was recently observed to accelerate ions and possibly electrons. The ion acceleration process was previously analyzed, but the electron acceleration process remains largely unexplored. Here we use multi‐point observations by Time History of Events and Macroscale during Substorms from three events to determine whether and how magnetosheath jet‐driven bow waves can accelerate electrons. We show that when suprathermal electrons in the ambient magnetosheath convect towards a bow wave, some electrons are shock‐drift accelerated and reflected towards the ambient magnetosheath and others continue moving downstream of the bow wave resulting in bi‐directional motion. Our study indicates that magnetosheath jet‐driven bow waves can result in additional energization of suprathermal electrons in the magnetosheath. It implies that magnetosheath jets can increase the efficiency of electron acceleration at planetary bow shocks or other similar astrophysical environments.

Thank You to Our 2019 Reviewers

Tue, 05/26/2020 - 19:00
Journal of Geophysical Research: Space Physics, Volume 125, Issue 5, May 2020.

Preliminary Evidence of Madden‐Julian Oscillation Effects on Ultrafast Tropical Waves in the Thermosphere

Mon, 05/25/2020 - 19:00

Over the past two decades mounting evidence demonstrated that terrestrial weather significantly influences the dynamics and mean state of the thermosphere. While important progress has been made in understanding how this coupling occurs on hourly to daily time scales, large uncertainty still exists on this effect around intraseasonal (∼30–90 days) time scales. In this work, analyses of Thermosphere Ionosphere Mesosphere Energetics Dynamics‐Sounding of the Atmosphere using Broadband Emission Radiometry temperatures near 110 km and Gravity field and steady‐state Ocean Circulation Explorer cross‐track winds near 260 km reveal prominent intraseasonal oscillations in the equatorial (±15°) zonal mean lower and middle thermosphere. Similar intraseasonal oscillations are found in the amplitudes of the diurnal eastward propagating tide with Zonal Wavenumber 3 (DE3) and the quasi‐3‐day ultrafast Kelvin wave, two prominent ultrafast tropical waves (UFTWs) excited by deep tropical tropospheric convection. Numerical simulations from the Specified‐Dynamics Whole Atmosphere Community Climate Model eXtended demonstrate a significant connection between these UFTW and the Madden‐Julian Oscillation (MJO). Compared to the boreal winter mean state, thermospheric UFTW amplitudes are larger (+5 to +12%) during MJO Phases 2–3 and smaller (−3% to −12%) during MJO Phases 6–8. Significant variations are also found with respect to the phase of the mesospheric semiannual oscillation (MSAO) and stratospheric quasi‐biannual oscillation (SQBO), with larger (±12–16%) thermospheric amplitudes during westward MSAO/SQBO phase and smaller (±3–6%) amplitudes during eastward MSAO/SQBO phase, in accordance with theoretical interpretations. This study suggests that UFTW may play a large role in coupling tropospheric intraseasonal variability to the thermosphere, raising important questions including implications for the whole atmosphere system.

Simultaneous Observations of ELF/VLF Rising‐Tone Quasiperiodic Waves and Energetic Electron Precipitations in the High‐Latitude Upper Ionosphere

Mon, 05/25/2020 - 19:00

The quasiperiodic (QP) waves accompanied by simultaneous energetic electron precipitations in the high‐latitude ionosphere were recorded by the sun‐synchronous circular orbit China Seismo‐Electromagnetic Satellite (CSES). The new features of QP waves observed by CSES are the well‐pronounced rising‐tone structures and very short repetition periods, which are not ofen reported by previous studies. The repetition period of QP waves varies from ~1 to over 20 s, with wave spectral properties displaying dynamic structures and clear cutoff frequencies. The majority of QP waves appear at geomagnetic latitudes from ~50° to 65°, and L shell from ~3.5 to 4, mostly inside the plasmapause. The QP waves obliquely propagate towards decreasing L shell directions with right‐handed polarization, with wave normal angles varying from ~30° to 50°. Out of the 68 events examined, 19 of them show synchronous variations of ultralow frequency (ULF) magnetic field pulsations in certain portions of the wave event. The energetic electrons predominately precipitate in a double‐peak pattern (~400 to 550 keV and ~700 to 800 keV). No clear periodic precipitating fluxes were found. The estimated time interval of energetic electrons driving free energy to modulate the whistler‐mode waves varies from 0.04 to 30°s, which is roughly consistent with the observed repetition periods (10 to over 20 s). For the fast repetition period (~1 to 2s) QP wave, it is likely caused by the bouncing of the extremely/very low frequency (ELF/VLF) whistler‐mode wave packets from the conjugate ionosphere.

Statistical study of magnetosheath jet‐driven bow waves

Sat, 05/23/2020 - 19:19

When a magnetosheath jet (localized dynamic pressure enhancements) compresses ambient magnetosheath at a (relative) speed faster than the local magnetosonic speed, a bow wave or shock can form ahead of the jet. Such bow waves or shocks were recently observed to accelerate particles, thus contributing to magnetosheath heating and particle acceleration in the extended environment of Earth’s bow shock. To further understand the characteristics of jet‐driven bow waves, we perform a statistical study to examine which solar wind conditions favor their formation and whether it is common for them to accelerate particles. We identified 364 out of 2859 (~13%) magnetosheath jets to have a bow wave or shock ahead of them with Mach number typically larger than 1.1. We show that large solar wind plasma beta, weak interplanetary magnetic field (IMF) strength, large solar wind Alfvén Mach number, and strong solar wind dynamic pressure present favorable conditions for their formation. We also show that magnetosheath jets with bow waves or shocks are more frequently associated with higher maximum ion and electron energies than those without them, confirming that it is common for these structures to accelerate particles. In particular, magnetosheath jets with bow waves have electron energy flux enhanced on average by a factor of 2 compared to both those without bow waves and the ambient magnetosheath. Our study implies that magnetosheath jets can contribute to shock acceleration of particles especially for high Mach number shocks. Therefore, shock models should be generalized to include magnetosheath jets and concomitant particle acceleration.

Magnetospheric interactions of Saturn's moon Dione (2005‐2015)

Sat, 05/23/2020 - 19:00

The moon Dione orbits Saturn at 6.2 Saturn radii $R_S$ deep in the Kronian magnetosphere. In‐situ studies of the moon‐magnetosphere interaction processes near Dione were possible with the Cassini/Huygens mission which flew by close to Dione five times at distances between 99 km and 516 km. In addition, Cassini crossed Dione’s L‐shell more than 400 times between 2004 and 2017 and documented the variability of Saturn’s magnetosphere. Different flyby geometries allowed to study the interaction processes upstream, in the low‐energy wake, and above the north pole of Dione. We describe here the energetic particle measurements from the Low Energy Magnetospheric Measurement System LEMMS, part of the Magnetosphere Imaging Instrument MIMI onboard Cassini. We also use hybrid simulation results from "A.I.K.E.F." to interpret the signatures in the particle fluxes. This paper is a continuation of Krupp et al. (2013) and Kotova et al. (2015). The key results are:

(1) Saturn’s magnetosphere at Dione's orbit is highly variable with changes in energetic charged particle fluxes by 1‐2 orders of magnitude. (2) The dropout signatures near Dione are basically consistent with a fully absorbing obstacle but some features point to more complex interaction processes than plasma and energetic particle absorption; (3) Absorption signatures are found to be asymmetric with respect to the orientation of the moon, indicative of the presence of radial drift components for electrons; (4) The deepest absorption signatures were observed at the edge of the low‐energy wake pointing to gradient‐B drifts strongest in that part of the interaction region.

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