Bart van der Holst

We present a method for estimating the heating rate of solar wind (SW) and pickup ion (PUI) protons from their respective energy equations. Here, we estimate the heating rate for SW protons along the New Horizons (NH), Voyager 2 (V2), and Pioneer 10 (P10) trajectories using plasma measurements, rather than the magnetic field measurements. We also compute the heating rate for PUI protons for the first time along the NH direction. We find that the SW proton heating rate in the V2 direction is higher than that along the NH direction between ∼26 and ∼52 au. Along the P10 direction, the proton heating rate exceeds that of V2 between 20 and 38 au, but is less beyond 38 au. The heating rate differs along the spacecraft trajectories in the upwind and downwind directions. The PUI proton heating rate is found to be more than 2 orders of magnitude larger than that of SW protons between ∼26 and ∼52 au. The SW/PUI heating rate correlates with SW speed and SW/PUI temperature. We present a comparison between the injection rate of turbulence energy in the energy-containing range and the intensity profile of energetic particles in the ∼50–110 keV range as a function of distance along the NH and V2 directions. It suggests that turbulence may play an important role in the modulation of energetic particles throughout the heliosphere.

Through a coupling of the Space Weather Modeling Framework and the Space Plasma and Energetic Charged particle Transport on Unstructured Meshes, we perform hybrid modeling of Galactic cosmic rays (GCRs), treated as a test-particle population, and their modulation in the quiet inner heliosphere, treated as a magnetized fluid, during the passage of corotating interaction regions (CIRs), modeled with the Alfvén Wave Solar Atmosphere Model. We find strong evidence that this modulation is due, almost entirely, to the role of diffusive transport on either side of the stream interface separating the interacting fast and slow flows, which dominates over advection and drift effects. In particular, our simulations indicate that parallel diffusion has a greater influence on the GCR response to the passage of the CIR than perpendicular diffusion.

Over the past several decades, observations have shown that minor ions have a higher temperature and flow faster than protons in the solar wind. Theories based on turbulence have been developed that can explain many of these observed phenomena. We present our first step in developing a global multi-ion solar wind model with turbulence by including ion temperatures but not yet including differential streaming. The extent of this model is from the lower transition region (50,000 K temperature) to the corona and inner heliosphere. It uses low-frequency, reflection-driven incompressible turbulence to address coronal heating and solar wind acceleration. The energy partitioning of the dissipation of turbulence to the electrons and various ions is based on stochastic heating and linear Landau and transit-time damping. In order to test the validity of our approach we have carried out a three-dimensional simulation of the solar corona and the solar wind using an idealized dipole magnetic field configuration, calculated the oxygen temperature across the entire domain, and compared it to measurements obtained from the UltraViolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO) satellite and with the Solar Wind Ion Composition Spectrometer (SWICS) on board Advanced Composition Explorer (ACE). The comparison shows that even with the simplified magnetic field configuration the multi-ion model predictions reproduce the phenomenon of preferential heating of heavy ions in both remote-sensing and in situ observations.

We introduce an updated, time-dependent treatment to the split-tail (“croissant-like”) heliosphere model with data-driven solar wind conditions at 1 au to study the evolution of the heliosphere with solar-cycle variations in plasma speed, plasma density, and magnetic field intensity. The model produces a sub-Alfvénic and low beta region, not observed by the Voyagers, ∼20 au ahead of the heliopause. The simulated magnetic field and radial flow depart from Voyager observations in this region, indicating that time-dependent effects alone are not sufficient to understand this regime of the heliosheath. We decompose fast and slow magnetosonic wave modes from time-dependent plasma pulse structures in the heliosheath, using a linear Riemann variable analysis, for the first time. Fast mode waves can both reflect at the heliopause and transmit into the interstellar medium, and their speeds are unaffected by the low beta plasma regime in front of the heliopause. The model reproduces the pf2 jump observed in 2020.4 by Voyager 1 in the interstellar medium, and we attribute the source of pressure fronts observed by Voyager 1 in the interstellar medium, and pressure pulses observed by Voyager 2 in the heliosheath, to fast mode waves that are commonly recurring solar-cycle features. The presence of fast mode waves drive a highly variable termination shock, with average radial speeds of 6.05 au yr ⁻¹ ± 5.37 au yr ⁻¹ in the New Horizons direction. We find that the termination shock has a sinusoidal-like oscillatory motion in the rising phase of the solar cycle and broad inward motions during the declining phase.

The outer heliosphere model in the Space Weather Modeling Framework describes the dynamics of the thermal solar wind, pickup ions, electrons, and neutrals. We have extended this model with turbulence transport and a kinetic description of the pickup ions (PUIs) to better capture the interaction of the solar wind with the interstellar medium. The turbulence energy is enhanced via the isotropization of newly born PUIs. This increased turbulence can further heat the thermal protons and electrons. Additionally, the PUIs are nonthermal. We assume that the PUIs are fast pitch-angle scattered into isotropic distribution through wave–particle interaction. Hence, we simulate the PUI velocity distribution function by solving the PUI transport equations under the assumption that the distribution is isotropic and depends only on the velocity magnitude in the solar wind rest frame. In the PUI transport equations, we include velocity-space advection contributions. We also have additional source terms containing charge exchange. This is the first 3D magnetohydrodynamic model to treat the full kinetic distribution of PUIs. The model, extended with turbulence, is validated against Voyager 2 data. The results are within the range of the observations in the supersonic solar wind.

Numerical models of the solar wind and coronal mass ejections (CMEs) utilize photospheric magnetic field observations to prescribe the inner boundary conditions for the plasma solutions. These magnetic field data are available to the community through various observational instruments, prepared via different methodologies and/or flux transport models. The solar wind solution driven by these maps provides the ambient plasma environment into which CMEs travel. Interaction with the surrounding solar wind impacts CME evolution and propagation in the solar corona and inner heliosphere. In this work, we use different input magnetic field maps for the same time period to drive the global Alfvén Wave Solar atmosphere Model. We obtain the ambient solar wind conditions and compare the plasma properties and magnetic morphology in the corona domain to study the influence of the input maps. To understand how the resulting coronal solutions impact CMEs, we launch eruptions described by analytical flux ropes into these data-driven solutions and compare their evolution in the coronal domain (up to 24 R _⊙ radially). The CMEs achieve varying speeds, deceleration rates, propagation directions, mass, and energies while coupling with the background solar wind. We quantify these differences to show that the different input driving maps can significantly impact the simulated CME propagation in the solar wind plasma. This also highlights the importance of understanding the uncertainties associated with data-driven modeling that become increasingly important in operational models and space weather prediction.

Numerical models of the solar wind and coronal mass ejections (CMEs) utilize photospheric magnetic field observations to prescribe the inner boundary conditions for the plasma solutions. These magnetic field data are available to the community through various observational instruments, prepared via different methodologies and/or flux-transport models. The solar wind solution driven by these maps provides the ambient plasma environment into which CMEs travel, coupling, and interacting with the surrounding plasma and governing the CME evolution and propagation in the solar corona and inner heliosphere. In this work, we use different input magnetic field maps for the same time period to drive the global Alfven Wave Solar atmosphere Model (AWSoM). We obtain the ambient solar wind conditions and compare the plasma properties and magnetic morphology in the coronal domain to study the influence of the input maps. To understand how the resulting coronal solutions impact CMEs, we launch eruptions described by analytical flux ropes into these data-driven solutions and compare their evolution in the coronal domain (up to 24 solar radii radially). The CMEs achieve varying speeds, deceleration rates, propagation directions, mass and energies while coupling with the background solar wind. We quantify these differences to show that the different input driving maps can significantly impact the simulated CME propagation in the solar wind plasma. This also highlights the importance of understanding the uncertainties associated with data-driven modeling that become increasingly important in operational models and space weather prediction.

Magnetic reconnection in a flare current sheet is widely believed to be the main energy release process powering solar flares and coronal mass ejections (CMEs). Modeling this process and determining the channels for the energy release, mass motions, and heating has long been a major goal in space science. We present results from a two-fluid magnetohydrodynamic simulation of an eruptive flare/CME using a newly developed version of the Space Weather Modeling Framework that incorporates two major advances in numerical capability. First, we use the STatistical InjecTion of Condensed Helicity formalism for the energy buildup, so that we start with a potential-field minimum-energy state and slowly form a sheared filament channel over a polarity inversion line as is observed on the Sun. Second, we use a new formulation of the plasma energetics that is explicitly energy conserving while calculating separate electron and ion temperatures and separate parallel and perpendicular pressures, as desired. For this first simulation with our new model, we opted for the nonadiabatic heating to go solely into the protons and for an isotropic pressure. We discuss the resulting energetics of the reconnection and, in particular, the plasma heating in the reconnecting current sheets, mass acceleration, and shock formation. We also discuss the implications of our results for flare/CME observations and for understanding solar eruptions in general.

On 2022 September 5, a large solar energetic particle (SEP) event was detected by Parker Solar Probe (PSP) and Solar Orbiter (SolO) at heliocentric distances of 0.07 and 0.71 au, respectively. PSP observed an unusual velocity dispersion signature: particles below ∼1 MeV exhibited a normal velocity dispersion, while higher-energy particles displayed an inverse velocity arrival (IVA) feature, with the most energetic particles arriving later than those at lower energies. The maximum energy increased from about 20–30 MeV upstream to over 60 MeV downstream of the shock. The arrival of SEPs at PSP was significantly delayed relative to the expected onset of the eruption. In contrast, SolO detected a typical large SEP event characterized by a regular velocity dispersion at all energies up to 100 MeV. To understand these features, we simulate particle acceleration and transport from the shock to the observers with our newly developed SEP model—Particle ARizona and MIchigan Solver on Advected Nodes. Our results reveal that the IVA and delayed particle onset detected by PSP originate from the time-dependent diffusive shock acceleration processes. After shock passage, PSP’s magnetic connectivity gradually shifted due to its high velocity near perihelion, detecting high-energy SEPs streaming sunward. Conversely, SolO maintained a stable magnetic connection to the strong shock region where efficient acceleration was achieved. These results underscore the importance of spatial and temporal dependence in SEP acceleration at interplanetary shocks and provide new insights to understand SEP variations in the inner heliosphere.