Laxman Adhikari

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.

We investigate the radial evolution of turbulent fluctuations in the inner heliosphere using Parker Solar Probe and Solar Orbiter observations over heliocentric distances of ∼32–90 R _⊙ . We decompose the measured fluctuations of density, velocity, and magnetic field into eight fundamental magnetohydrodynamic modes using the linear mode decomposition technique. The mode-resolved energy results show that the fluctuation energy is highly field-dependent: Velocity fluctuations are primarily Alfvénic, with both forward and backward Alfvén modes exhibiting relatively shallow radial decay, whereas magnetic fluctuations are dominated by transverse nonpropagating incompressible / magnetic island–like components. In particular, the magnetic island mode decreases approximately as δB ⁱ ∼ r ^−1.90 , followed closely by the forward and backward Alfvénic magnetic modes, δB ^A+ ∼ r ^−1.96 and δB ^A− ∼ r ^−2.17 . In contrast, density fluctuations are mainly governed by fast magnetosonic modes, while entropy and slow magnetosonic contributions remain comparatively weaker and decay more rapidly with heliocentric distance. We find that the variance anisotropy of the transverse incompressible magnetic island mode ( δB ⁱ ) and the Alfvén modes ( δB ^A± ) increases slightly with heliocentric distance. These results will help identify which magnetohydrodynamic modes are responsible for carrying turbulent energy in the inner heliosphere and how that energy evolves with distance from the Sun.

Solar wind electron velocity distribution functions (VDFs) consist of a Maxwellian core, a nearly isotropic suprathermal halo, and a field-aligned strahl. Observations show that the strahl gradually broadens into the halo with heliocentric distance, and the most promising candidate for this transition is wave–particle interaction with whistler waves. In previous kinetic models that employ this mechanism, the diffusion coefficients were derived under the simplified assumption that the whistler wave energy spectrum follows a power-law form. Our previous theoretical work generalized these diffusion coefficients to retain the full dependence on the turbulent magnetic energy 〈 δB ² 〉. Building on this generalization, we present the first self-consistent numerical model for solar wind suprathermal electrons that couples a nearly incompressible magnetohydrodynamic turbulence transport model with a kinetic electron transport model. The turbulent magnetic energy 〈 δB ² 〉 for whistler waves at the resonant wavenumber k _r is obtained as a function of energy-containing range turbulence quantities. The coupled model simulates suprathermal electrons from 0.11 au outward and yields VDFs that agree significantly better with observations. Quantitatively, at 1 au, the strahl energy spectrum from the coupled model yields a spectral index κ ≈ 15, consistent with the observed range of 8–16. The coupled model bridges the gap between large-scale turbulence in the energy-containing range and small-scale kinetic scattering, providing a physically based framework to investigate suprathermal electron transport in different solar wind backgrounds.

One of the key mechanisms proposed for coronal heating is dissipation of quasi-two-dimensional (2D) turbulence generated above the photosphere by continual mixing and convective shuffling of open and closed magnetic field lines within the so-called magnetic carpet. This class of heating mechanism is modeled within the Nearly Incompressible—Magnetohydrodynamics (NI-MHD) framework and has previously been shown to supply sufficient energy to drive the solar wind to supersonic and super-Alfvénic speeds and heat the corona to temperatures exceeding one million kelvin. In addition to turbulent heating, the overall energy budget of the low corona and the chromospheric-coronal transition region is strongly influenced by loss mechanisms associated with thermal conduction and radiative cooling, which must therefore be included in realistic models. To that end, we present solutions to a coupled system of NI-MHD turbulence transport equations and solar wind evolution equations that self-consistently incorporates the effects of collisional thermal conduction and radiative cooling. Results show that an interplay between turbulence dissipation and heat-loss mechanisms can result in the expected solar-wind driving as well as coronal heating. The predicted density and thermal conduction flux at the inner boundary, as well as the overall wind speed, are broadly consistent with representative ranges of observed coronal and solar wind parameters. The model results are sensitive to the ratio of von Kármán–Taylor constant to correlation length, as well as to the assumed expansion factor of the associated flux tube. This sensitivity is explored through a parametric survey.

Low-frequency turbulence in the solar chromosphere remains poorly understood. We address (1) the sources of low-frequency turbulence that potentially heat the chromosphere, and (2) how turbulence is transported and dissipated throughout the chromosphere and lower corona. We use particle-in-cell simulations to investigate mixed polarity magnetic fields corresponding to emergent magnetic carpet field in coronal holes or quiet Sun regions for strong (imbalanced) and weak (balanced) guide magnetic fields. The initial mixed polarity magnetic field transitions rapidly to a turbulent state dominated by advected small-scale nonlinear structures, with a minority slab turbulence population, and the emergent field is largely annihilated. Turbulence is anisotropic for an imbalanced magnetic field and more isotropic for balanced cases. We develop a transport model for turbulence advected and dissipated throughout the chromosphere by randomly distributed energy-containing scale dynamical flows described by log-normal statistics. We compute the expectations for the total energy per unit volume 〈 y 〉( h ) J m ⁻³ , the Elsässer specific energy 〈 〈 Z ∞ 2 〉 〉 ( h ) m ² s ⁻² , the heating rate 〈 H ̇ 〉 ( h ) J m ⁻³ s ⁻¹ , and the correlation length 〈 λ 〉( h ) in kilometers as functions of height h above the photosphere. Turbulent energy is injected into the low corona by a random “patchwork” of sites across the transition region surface. The expected energy injection rates 〈 S ̇ 〉 J m ⁻² s ⁻¹ for the chromosphere and at the base of the corona exceed the estimated energy requirements needed to heat both the chromosphere and corona. Similarly, we show that spicules can be heated gradually with increasing height by entrained magnetic carpet and photospheric turbulence.

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.

Small-amplitude fluctuations in the magnetized solar wind are typically measured by a single spacecraft. In the magnetohydrodynamic description, fluctuations are expressed in terms of the fundamental modes admitted by the system. An important question is how to resolve observed sets of fluctuations, typically plasma moments such as density, velocity, pressure, and magnetic field, into their constituent fundamental modal components. In particular, identifying these components for the same plasma parcel and their properties observed at two radially aligned locations has not yet been investigated in detail. We apply the linear mode decomposition technique recently developed by G. P. Zank et al. to a plasma parcel measured by Solar Orbiter at 0.88 au and Wind at 0.99 au during their radial alignment on 2021 November 9, identifying eight modes and examining how their properties evolve with radial distance in the inner heliosphere. Our study shows that the contribution of transverse magnetic energy to the total transverse magnetic energy corresponding to Alfvén modes decreases by ∼17%, while that of the magnetic island and fast modes increases by ∼15% and ∼2%, respectively. We find that the transverse magnetic energy decreases by 86% in forward Alfvén and 69% in backward Alfvén and increases by 9% in magnetic island modes. Similarly, the forward Alfvén transverse kinetic energy decreases by 61%, while the backward Alfvén kinetic energy decreases by 13%. This behavior reflects a possible resonant interaction between counterpropagating Alfvén modes that generate zero-frequency magnetic islands, thereby reducing energy imbalance.

Although the drift effect is well known to play an important role in the transport process of energetic charged particles in the heliosphere, the widely adopted standard drift coefficient only reflects the uniformity of the large-scale magnetic field. The drift arising from the turbulent electromagnetic field was investigated by Stawicki with quasilinear theory. However, some higher-order terms in the Fokker–Planck coefficient were ignored in his model in order to simplify a laborious calculation. We revisit Stawicki’s model by incorporating higher-order terms and discuss extensively the properties of the general form of the drift coefficient. We propose a new parametric form of the suppression factor ( f _s ), defined as the ratio of the net and standard drift coefficients, which is very distinct from some previous models. According to the calculated f _s , we find that suppression occurs at a similar level during the solar minimum (2008/1–2010/12) and solar maximum (2013/1–2015/12).

The characterization of solar wind fluctuations provides insight into magnetofluid turbulence. The slow solar wind with high Alfvénicity, the Alfvénic slow solar wind (ASSW), possesses a similar bulk speed as the slow wind, but with fluctuations largely similar to those found in the fast wind. ASSWs have attracted less attention from the turbulence community compared to fast wind flows. G. P. Zank et al. developed a linear mode decomposition (LMD) technique that identifies eight modes in the magnetized solar wind plasma, namely entropy, fast and slow (+/−) magnetosonic (MS), Alfvén (+/−), and magnetic island modes. Using the LMD technique, we present a comparative study of the characteristic modes found in the ASSW, classical slow solar wind (CSSW), and fast solar wind (FSW) events identified by the WIND spacecraft. We investigate the properties of the eight magnetohydrodynamic modes and their spectral characteristic differences in the three solar wind plasma environments at 1 au. We show that in the CSSW, ASSW, and FSW intervals (i) fast MS mode density fluctuations are dominant compared to entropy and slow MS modes, (ii) magnetic island modes clearly dominate Alfvén modes for the transverse fluctuating magnetic field, and (iii) slow MS modes show no significant power spectral density contribution compared to the Alfvén and fast modes in the frequency spectra for velocity fluctuations. The ASSW exhibits characteristics intermediate to the CSSW and FSW in entropy modes, compressibility, the magnetic island-to-Alfvén mode magnetic energy ratio, and kinetic variance anisotropy and magnetic energy variance anisotropy corresponding to Alfvén modes.

We analyze ∼40–200 keV energetic particle observations from the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) on board the New Horizons (NH) spacecraft from 2007 to 2024 and compare them with similar measurements from the Low Energy Charged Particle experiments on the Voyager 1 and Voyager 2 spacecraft when they were at comparable heliocentric distances (5–60 au). In both Voyager data sets, particle intensities generally decrease with increasing radial distance from the Sun, reaching a minimum in the outer heliosphere before rising again prior to their respective encounters with the heliospheric termination shock (TS). This radial behavior in the intensity-time profiles is described as a heliospheric valley. The NH/PEPSSI time series from 5 au (2007) to 60 au (2024) exhibits a similar decrease in particle fluxes with distance. Analysis of the radial dependence of energetic particle intensities from all three spacecraft normalized by observations at 1 au to account for the solar cycle effects reveals an approximate piece-wise composite power-law relationship, with a slope break (steeper decline) beyond ∼33 au. This break may reflect differences in the dominant transport and acceleration mechanisms operating in the two regions demarcated by this radial break. In addition, a radial scaling method is applied to Voyager observations to best match the NH data. This comparison provides an estimate for the NH TS crossing between 2027 (68 au) and 2034 (83 au).