Abstract
The Voyager 2 crossing of the termination shock indicated that most of the upstream energy from the thermal solar wind ions was transferred to pickup ions (PUIs) and other energetic ...particles downstream of the shock. We use hybrid simulations at the termination shock for the Voyager 2, flank, and tail directions to evaluate the distributions of different ion species downstream of the shock over the energy range of 0.52–55 keV. Here, we extend the work of Gkioulidou et al., which showed an energy-dependent discrepancy between modeled and energetic neutral atom (ENA) observations, and fit distributions to a hybrid model to show that a population of PUIs accelerated via diffusive shock acceleration (DSA) to become low-energy anomalous cosmic rays (ACRs) can bridge the gap between modeled and observed ENA fluxes. Our results with the inclusion of DSA via hybrid fitting give entirely new and novel evidence that DSA at the termination shock is likely to be an important physical process. These ACRs carry a significant fraction of the energy density at the termination shock (22%, 13%, and 19% in the Voyager 2, flank, and tail directions, respectively). Using these ACRs in global ENA modeling of the heliosphere from 0.52 to 55 keV, we find that scaling factors as large as 1.8–2.5 are no longer required to match ENA observations at energies of ∼1–4 keV. Large discrepancies between modeled and observed ENAs only remain over energies of 4–20 keV, indicating that there may be a further acceleration mechanism in the heliosheath at these energies.
Abstract
The shape of the heliosphere is currently under active debate. Energetic neutral atoms (ENAs) offer the best method for investigating the global structure of the heliosphere. To date, the ...Interstellar Boundary Explorer (IBEX) and the Ion and Neutral Camera (INCA) that was on board Cassini provide the only global ENA observations of the heliosphere. While extensive modeling has been done at IBEX-Hi energies (0.52–6 keV), no global ENA modeling has been conducted for INCA energies (5.2–55 keV). Here, we use an ENA model of the heliosphere based on hybrid results that capture the heating and acceleration of pickup ions (PUIs) at the termination shock to compare modeled global ENA results with IBEX-Hi and INCA observations using both a long- and short-tail model of the heliosphere. We find that the modeled ENA results for the two heliotail configurations produce similar results from the IBEX-Hi through the INCA energies. We conclude from our modeled ENAs, which only include PUI acceleration at the termination shock, that ENA observations in currently available energy ranges are insufficient for probing the shape and length of the heliotail. However, as a prediction for the future IMAP-Ultra mission (3–300 keV) we present modeled ENA maps at 80 keV, where the cooling length (∼600 au) is greater than the distance where the long- and short-heliotail models differ (∼400 au), and find that IMAP-Ultra should be able to identify the shape of the heliotail, predicting differences in the north lobe to downwind flux ratio between the models at 48%.
Abstract Determining the magnitude and direction of the interstellar magnetic field ( B ISM ) is a long-standing problem. To date, some methods to infer the direction and magnitude have utilized ...best-fit models to the positions of the termination shock and heliopause measured by Voyager 1 and 2. Other models use the circularity of the Interstellar Boundary Explorer (IBEX) ribbon assuming a secondary energetic neutral atom (ENA) mechanism. Previous studies have revealed that the B ISM organizes the orientation of the heliotail with respect to the solar meridian. Here we propose a new way to infer the direction of the B ISM based on ENA observations of the heliotail. IBEX observations of the heliotail have revealed high-latitude lobes of enhanced ENA flux at energies >2 keV. Analyses showed that the high-latitude lobes are nearly aligned with the solar meridian, while also exhibiting a rotation with solar cycle. We show, using steady-state solar wind conditions, that the inclination of the lobes reproduced with commonly used values for the angle ( α BV ) between B ISM and the interstellar flow in the hydrogen deflection plane (40° < α BV < 60°) is inconsistent with the IBEX ENA observations. We report that 0° < α BV < 20° best replicates the heliotail lobe inclinations observed by IBEX. Additionally, our model results indicate that the variation of the solar magnetic field magnitude with solar cycle causes the longitudinal rotation of the lobes observed by IBEX by affecting the inclination of the lobes.
Abstract
The heliosphere is the bubble formed by the solar wind as it interacts with the interstellar medium (ISM). The collimation of the heliosheath (HS) flows by the solar magnetic field in the ...heliotail into distinct north and south columns (jets) is seen in recent global simulations of the heliosphere. However, there is disagreement between the models about how far downtail the two-lobe feature persists and whether the ambient ISM penetrates into the region between the two lobes. Magnetohydrodynamic simulations show that these heliospheric jets become unstable as they move down the heliotail and drive large-scale turbulence. However, the mechanism that produces this turbulence had not been identified. Here we show that the driver of the turbulence is the Rayleigh–Taylor (RT) instability produced by the interaction of neutral H atoms streaming from the ISM with the ionized matter in the HS. The drag between the neutral and ionized matter acts as an effective gravity, which causes an RT instability to develop along the axis of the HS magnetic field. A density gradient exists perpendicular to this axis due to the confinement of the solar wind by the solar magnetic field. The characteristic timescale of the instability depends on the neutral H density in the ISM and for typical values the growth rate is ∼3 years. The instability destroys the coherence of the heliospheric jets and magnetic reconnection ensues, allowing ISM material to penetrate the heliospheric tail. Signatures of this instability should be observable in Energetic Neutral Atom maps from future missions such as the Interstellar Mapping and Acceleration Probe (IMAP). The turbulence driven by the instability is macroscopic and potentially has important implications for particle acceleration.
Abstract Interstellar neutral (ISN) hydrogen is the most abundant species in the outer heliosheath and the very local interstellar medium (VLISM). Charge-exchange collisions in the outer heliosheath ...result in filtration, reducing the ISN hydrogen density inside the heliosphere. Additionally, these atoms are intensively ionized close to the Sun, resulting in a substantial reduction of their density within a few astronomical units from the Sun. The products of this ionization—pickup ions (PUIs)—are detected by charged particle detectors. The Solar Wind Around Pluto instrument on New Horizons provides, for the first time, PUI observations from the distant heliosphere. We analyze the observations collected between 22 and 52 au from the Sun to find the ISN hydrogen density profile and compare the results with predictions from global heliosphere models. We conclude that the density profile derived from the observations is inconsistent with steady-state model predictions. This discrepancy is not explained by time variations close to the Sun and thus may be related to the temporal evolution of the outer boundaries or VLISM conditions. Furthermore, we show that the cold and hot models of ISN hydrogen distribution are not a good approximation closer to the termination shock. Therefore, we recommend a new fiduciary point based on the available New Horizons observations at 40 au from the Sun, at ecliptic direction (285.°62, 1.°94), where the ISN hydrogen density is 0.11 cm −3 . The continued operation of New Horizons should give better insight into the source of the discussed discrepancy.
Abstract
Energetic neutral atom (ENA) models typically require post-processing routines to convert the distributions of plasma and H atoms into ENA maps. Here we investigate how two kinetic-MHD ...models of the heliosphere (the BU and Moscow models) manifest in modeled ENA maps using the same prescription and how they compare with Interstellar Boundary Explorer (IBEX) observations. Both MHD models treat the solar wind as a single-ion plasma for protons, which include thermal solar wind ions, pick-up ions (PUIs), and electrons. Our ENA prescription partitions the plasma into three distinct ion populations (thermal solar wind, PUIs transmitted and ones energized at the termination shock) and models the populations with Maxwellian distributions. Both kinetic-MHD heliospheric models produce a heliotail with heliosheath plasma that is organized by the solar magnetic field into two distinct north and south columns that become lobes of high mass flux flowing down the heliotail; however, in the BU model, the ISM flows between the two lobes at distances in the heliotail larger than 300 au. While our prescription produces similar ENA maps for the two different plasma and H atom solutions at the IBEX-Hi energy range (0.5–6 keV), the modeled ENA maps require a scaling factor of ∼2 to be in agreement with the data. This problem is present in other ENA models with the Maxwellian approximation of multiple ion species and indicates that either a higher neutral density or some acceleration of PUIs in the heliosheath is required.
We report sample results on Saturn magnetospheric energetic ion spectral shapes using measurements obtained from the Magnetospheric Imaging Instrument (MIMI) suite onboard Cassini. The ion ...intensities are measured by the Charge Energy Mass Spectrometer (CHEMS) that covers the energy range of 3 to 236 keV/e, the Low Energy Magnetospheric Measurements System (LEMMS) covering the energy range of 0.024 < E < 18 MeV, and the Ion Neutral Camera (INCA) that provides ion measurements in the ion mode at the energy range ∼5.5 to >220 keV for protons. The data used cover several passes from the period 1 July 2004 to 10 April 2007, at various latitudes over the dipole L range 5 < L < 20 RS. The spectra generally show a power law in energy form at larger L values but display a flattening/relative peak at lower (L < 10) values centered at ∼50 to ∼100 keV and can be fit by a κ distribution function with characteristic kT ranging from ∼10 to ∼100 keV. The results are consistent with the assumption that energetic protons are heated adiabatically as they move inward to stronger magnetic fields, in contrast to the singly ionized oxygen that seems to be heated locally at each L shell. The lack of any trend of the O+ temperature versus L shell implies that nonadiabatic energization mechanisms and charge exchange with Saturn's neutral gas cloud play an important role for ion energetics.
Abstract
Cassini data are consistent with a global electric field in Saturn's magnetosphere that points approximately antisunward. The inner radial extent of this field was initially established ...using Saturn orbit insertion data but measurements of ultrarelativistic electrons from that pass cast some doubt on whether the electric field reaches all the way to the A ring. It was not until the so-called ring-grazing and proximal orbits near the end of the mission in 2017 that relevant data were again obtained on magnetic field lines that connect to the region just outward of the main rings. Here we report on the energetic charged particle data during those orbits, showing that electron observations at a wide range of energies are consistent with an electric field that influences charged particle drift paths near the outer edge of the A ring. We include a very detailed analysis of Cassini's ultrarelativistic electron measurements (channel E7 in the text) and argue they provide no information about the electric field. This result further strengthens the case of several studies that have used the presence of the electric field to explain signatures of acceleration in the data.
Abstract
Global models of the heliosphere are critical tools used in the interpretation of heliospheric observations. There are several three-dimensional magnetohydrodynamic (MHD) heliospheric models ...that rely on different strategies and assumptions. Until now only one paper has compared global heliosphere models, but without magnetic field effects. We compare the results of two different MHD models, the BU and Moscow models. Both models use identical boundary conditions to compare how different numerical approaches and physical assumptions contribute to the heliospheric solution. Based on the different numerical treatments of discontinuities, the BU model allows for the presence of magnetic reconnection, while the Moscow model does not. Both models predict collimation of the solar outflow in the heliosheath by the solar magnetic field and produce a split tail where the solar magnetic field confines the charged solar particles into distinct north and south columns that become lobes. In the BU model, the interstellar medium (ISM) flows between the two lobes at large distances due to MHD instabilities and reconnection. Reconnection in the BU model at the port flank affects the draping of the interstellar magnetic field in the immediate vicinity of the heliopause. Different draping in the models cause different ISM pressures, yielding different heliosheath thicknesses and boundary locations, with the largest effects at high latitudes. The BU model heliosheath is 15% thinner and the heliopause is 7% more inwards at the north pole relative to the Moscow model. These differences in the two plasma solutions may manifest themselves in energetic neutral atom measurements of the heliosphere.
An extensive analysis of Cassini Ion and Neutral Camera (INCA) and Charge Energy Mass Spectrometer (CHEMS) measurements of ~6–231 keV ion anisotropies acquired during selected spin and stare periods ...for nearly all mission orbits has been completed. Based on this analysis, we find that the computed azimuthal speed of Saturn's magnetodisk plasma increases within the inner and middle magnetosphere. Beyond the orbit of Titan, in the magnetotail, the calculated rotation speed remains roughly constant with increasing distance from Saturn. The component of convection parallel to Saturn's spin axis is smaller and on average nearly zero. The radial speed of plasma shows distinct local time dependence and increases outward with increasing distance down the magnetotail. The magnetodisk flow remains primarily azimuthal to large distances, indicating the plasma is still under the influence of Saturn as it transits across the nightside. Tailward flows have been observed in the region near the dawn and dusk magnetotail flanks. The plasma flow in the predawn quadrant is much more disorganized than that in the premidnight quadrant. The hydrogen and oxygen hot ion temperatures increase with decreasing distance to Saturn. Some plasma evidently escapes into a dusk boundary layer at the dusk magnetotail flank, while the remaining plasma primarily moves across the magnetotail and is entrained into the flow of a boundary layer at the dawn flank of the magnetotail. When scaled to the magnetopause standoff distance and corotation fraction, the convection speed of plasma in the magnetospheres of Jupiter and Saturn is similarly organized.
Key Points
Saturn's magnetospheric convection pattern has been extracted from Cassini data
The escape of plasma from the system occurs primarily at the magnetotail flanks
The magnetotail convection pattern contains a strong dawn‐dusk asymmetry
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