Showing papers by "Peter A. Delamere published in 2022"

Dayside Transient Phenomena and Their Impact on the Magnetosphere and Ionosphere

Abstract: Dayside transients, such as hot flow anomalies, foreshock bubbles, magnetosheath jets, flux transfer events, and surface waves, are frequently observed upstream from the bow shock, in the magnetosheath, and at the magnetopause. They play a significant role in the solar wind-magnetosphere-ionosphere coupling. Foreshock transient phenomena, associated with variations in the solar wind dynamic pressure, deform the magnetopause, and in turn generates field-aligned currents (FACs) connected to the auroral ionosphere. Solar wind dynamic pressure variations and transient phenomena at the dayside magnetopause drive magnetospheric ultra low frequency (ULF) waves, which can play an important role in the dynamics of Earth's radiation belts. These transient phenomena and their geoeffects have been investigated using coordinated in-situ spacecraft observations, spacecraft-borne imagers, ground-based observations, and numerical simulations. Cluster, THEMIS, Geotail, and MMS multi-mission observations allow us to track the motion and time evolution of transient phenomena at different spatial and temporal scales in detail, whereas ground-based experiments can observe the ionospheric projections of transient magnetopause phenomena such as waves on the magnetopause driven by hot flow anomalies or flux transfer events produced by bursty reconnection across their full longitudinal and latitudinal extent. Magnetohydrodynamics (MHD), hybrid, and particle-in-cell (PIC) simulations are powerful tools to simulate the dayside transient phenomena. This paper provides a comprehensive review of the present understanding of dayside transient phenomena at Earth and other planets, their geoeffects, and outstanding questions.

Water‐Group Pickup Ions From Europa‐Genic Neutrals Orbiting Jupiter

Abstract: Water‐group gas continuously escapes from Jupiter's icy moons to form co‐orbiting populations of particles or neutral toroidal clouds. These clouds provide insights into their source moons as they reveal loss processes and compositions of their parent bodies, alter local plasma composition, and act as sources and sinks for magnetospheric particles. We report the first observations of H2+ pickup ions in Jupiter's magnetosphere from 13 to 18 Jovian radii and find a density ratio of H2+/H+ = 8 ± 4%, confirming the presence of a neutral H2 toroidal cloud. Pickup ion densities monotonically decrease radially beyond 13 RJ consistent with an advecting Europa‐genic toroidal cloud source. From these observations, we derive a total H2 neutral loss rate from Europa of 1.2 ± 0.7 kg s−1. This provides the most direct estimate of Europa's H2 neutral loss rate to date and underscores the importance of both ion composition and neutral toroidal clouds in understanding satellite‐magnetosphere interactions.

Hybrid Simulations of a Tangential Discontinuity‐Driven Foreshock Bubble Formation in Comparison With a Hot Flow Anomaly Formation

Abstract: Hot flow anomalies (HFAs) and foreshock bubbles (FBs) are significant foreshock transients that can accelerate particles and disturb the magnetosphere‐ionosphere system. Yet, their early formation mechanisms are still not fully understood. To investigate the formation of tangential discontinuity (TD)‐driven FBs and HFAs, we use 2‐D local hybrid simulations where a reflected or an injected warm foreshock ion beam can interact with a TD whose half‐thickness is comparable to the ion inertial scale. We show that the foreshock ions perform a partial gyration within, or across, the TD. Bulk motion differences between partially‐gyrating foreshock ions and fluid‐electrons lead to the generation of currents. As the trigger, these foreshock‐driven currents change the magnetic field topology around the TD and force the frozen‐in solar wind plasma to redistribute along with the field lines, shaping the foreshock transient. This confirms a recently proposed kinetic formation model. The extent of the magnetic field direction change across the TD within the foreshock ion gyromotion determines the current profile and thus the type of foreshock transient that forms. For a thin TD, the foreshock ions generate a current that is much stronger on the upstream side than the downstream side, forming an FB with one upstream compressional boundary. For the same foreshock ion gyroradius and magnetic shear, a thick TD yields comparable foreshock‐driven currents on the upstream and downstream sides, forming an HFA with two compressional boundaries. Our study suggests that the TD thickness is one of the factors that determine the formation of FBs and HFAs.

Broadband Energization of Superthermal Electrons in Jupiter’s Inner Magnetosphere

Abstract: The Juno spacecraft in a polar orbit around Jupiter has observed broadband electron energization signatures at high latitudes (Mauk et al., 2017, https://doi.org/10.1038/nature23648). We investigate the origin of this energization by simulating the propagation of dispersive Alfvén waves from the Io plasma torus to high latitudes. These waves may be triggered by mechanisms such as the moon‐magnetosphere interaction or inflows from radial transport. We build on the initial hybrid gyrofluid‐kinetic electron simulation of Damiano et al. (2019, https://doi.org/10.1029/2018gl081219) to further quantify electron energization by Alfvénic waves, and investigate this process as a local source mechanism for observed broadband superthermal electron populations. We find that the magnitude of energization increases with the wave amplitude, while decreasing the radial wavelength reduces the high‐latitude wave and particle energy flux. Over the examined range of initial conditions we successfully generate broadband electron populations consistent with Juno observations (Mauk et al., 2017, http://doi.org/10.1038/nature23648; Szalay et al., 2018, https://doi.org/10.1029/2018je005752), that contribute to both precipitating populations and those that form trans‐hemispheric beams. Our energized electron distributions are consistent with observed superthermal populations critical for the torus energy budget (Bagenal & Delamere, 2011, https://doi.org/10.1029/2010ja016294) and Io‐related auroral emission.

Evidence of Alfvénic Activity in Jupiter's Mid‐To‐High Latitude Magnetosphere

Abstract: Using a combination of Juno magnetometer and plasma data, we show evidence of Alfvénic turbulence within the mid‐to‐high latitude magnetosphere with sufficient conditions to trigger auroral particle acceleration. We analyze 12 events that, in agreement with theoretical results, are found to be dissipative at the electron inertial scale. Furthermore, these events contain significant Poynting flux in the range ∼0.8–20 mW/m2 at ionospheric altitudes. This is sufficient to generate auroral emissions. We confirm that such events are incompressible, confirming their Alfvénicity, occur at dissipative scales, have intermittent features present and are multifractal in nature. These results illustrate the importance of turbulence in the mid‐to‐high latitudes of Jupiter's magnetosphere as a driver of particle acceleration.

Jupiter's Sheared Flow Unstable Magnetopause Boundary Observed by Juno

Abstract: The interaction between the solar wind and giant magnetospheres (i.e., Jupiter's and Saturn's magnetospheres) is fundamentally important for magnetospheric physics, in which the viscous interaction (presumably driven by the Kelvin‐Helmholtz (KH) instability) is often expected to play an important role. Previous simulation and theory studies suggested that Jupiter's low‐latitude dawn side flank region is KH unstable due to the large sheared flow between the fast tailward solar wind and magnetospheric sunward corotational flow. The onset of the KH instability can strongly modify the magnetopause boundary layer, which is consistent with the identification of 194 boundary crossings during Juno's first 14 orbits. We applied four different boundary normal analysis methods to illustrate that there is always a wide range of local normal directions, suggesting Jupiter's dawn‐side flank region is often highly perturbed. The distribution of local normal directions is insensitive to the upstream solar wind dynamic pressure, Juno's inward/outward boundary crossing direction, and the location along the z direction of the boundary crossing. We also used a 2‐D magnetohydrodynamic simulation to demonstrate that such a wide distribution can be formed by the KH instability even at the beginning of the nonlinear stage.

Dynamic Jovian Magnetosphere Responses to Enhanced Solar Wind Ram Pressure: Implications for Auroral Activities

Abstract: The main emission (ME) of the Jovian aurora is thought to be related to the current system associated with the breakdown of plasma corotation in the middle magnetosphere. According to the mainstream corotation breakdown model, the intensity of the Jovian ME is expected to decrease when the solar wind (SW) ram pressure increases, which is not fully consistent with auroral observations. In addition to the field‐aligned current (FAC), Alfvénic power (AP) play an important role in regulating planetary auroral emissions. We use three‐dimensional global simulations to investigate how these proxies of auroral emission respond to enhanced SW ram pressure. We found that during SW compression, both FAC and AP experience up‐down‐up trends, which is not revealed by any previous simulations, while could potentially explain many observations. The results suggest that different Jovian auroral activities including brightening or dimming can be observed during SW compression period.

The Roles of Flux Tube Entropy and Effective Gravity in the Inward Plasma Transport at Saturn

Abstract: The inward plasma transport at the Saturnian magnetosphere is examined using the flux tube interchange stability formalism developed by Southwood & Kivelson. Seven events are selected. Three cases are considered: (1) the injected flux tube and ambient plasmas are nonisotropic, (2) the injected flux tube and ambient plasmas are isotropic, and (3) the injected flux tube plasma is isotropic, but the ambient plasma is nonisotropic. Case 1 may be relevant for fresh injections, while case 3 may be relevant for old injections. For cases 1 and 2, all but one event have negative stability conditions, suggesting that the flux tube should be moving inward. For case 3, the injections located at L > 11 have negative stability conditions, while four out of five of the injections at L < 9 have positive stability conditions. The positive stability condition for small L suggests that the injection may be near its equilibrium position and possibly oscillating thereabouts—hence the outward transport if the flux tube overshot the equilibrium position. The flux tube entropy plays an important role in braking the plasma inward transport. When the stability condition is positive, it is because the entropy term, which is positive, counters and dominates the effective gravity term, which is negative for all the events. The ambient plasma and drift-out from adjacent injections can affect the stability and the inward motion of the injected flux tube. The results have implications for inward plasma transport in the Jovian magnetosphere, as well as other fast-rotating planetary magnetospheres.