arXiv subjects

Mihailo Martinovic

Publications and source records attributed to Mihailo Martinovic.

8 recordsLinked to original sources

arXiv2026

Wave Emission and Absorption in a Near-Sun Proton-Cyclotron Wave Storm

Quantification of energy transport and dissipation in weakly collisional heliospheric plasmas that are far from local thermodynamic equilibrium is an outstanding scientific problem. A central challenge is determining how non-Maxwellian velocity-space structure affects damping and emission of coherent ion-scale waves, especially compared to simplified analytical models for background plasma velocity distributions. In this work, we study the damping and emission of parallel-propagating proton cyclotron waves for two models of proton velocity distributions measured by the SPAN-I instrument on board Parker Solar Probe during an extended storm of waves with left-hand polarization in the solar wind at a heliocentric distance of 30.1 solar radii. Using the measured velocity distribution rather than a two-component bi-Maxwellian model predicts instabilities consistent with the observed coherent waves. For intervals in which both models predict net damping, the observed VDF model yields weaker damping in 90\% of cases, with a reduction in the integrated heating rate of 0.44 relative to the bi-Maxwellian model. These results suggest that simplified analytical velocity distribution models may overestimate cyclotron damping and underestimate wave emission in the near-Sun solar wind.

astro-ph.SR↗

arXiv2025

Multi-spacecraft Measurements of the Evolving Geometry of the Solar Alfv\'en Surface Over Half a Solar Cycle

The geometry of a star's Alfv\'en surface determines stellar angular momentum loss, separates a causally distinct 'corona' and stellar wind, and potentially affects exoplanetary habitability. The solar Alfv\'en surface is the only such structure that is directly measurable and since 2021, has been routinely measured in situ by NASA's Parker Solar Probe (Parker). We use these unique measurements in concert with Solar Orbiter and L1 in situ data spanning the first half of the Solar Cycle 25 in time and from 0.045 - 1 au in heliocentric distance to develop a radial scaling technique to estimate the morphology of the Alfv\'en surface from measurements of the solar wind speed and local Alfv\'en speed. We show that accounting for solar wind acceleration and mass flux is necessary to achieve reasonable agreement between the scaled location of the Alfv\'en surface and the locations of direct crossings measured by Parker. We produce continuous 2D equatorial cuts of the Alfv\'en surface over half a Solar Cycle (ascending phase and maximum). Parker's earliest crossings clipped outward extrusions, many of which are likely transient related, while more recently Parker has unambiguously sampled deep sub-Alfv\'enic flows. We analyze the average altitude, departure from spherical symmetry, and surface roughness, finding that all are positively correlated to solar activity. For the current modest Solar Cycle, the height varies up to 30\% which corresponds to a near-doubling in angular momentum loss per unit mass loss.

astro-ph.SR↗

arXiv2025

Evidence of Interaction between Ion-Scale Waves and Ion Velocity Distributions in the Solar Wind

Recent in situ observations from Parker Solar Probe (PSP) near perihelia reveal ion beams, temperature anisotropies, and kinetic wave activity. These features are likely linked to solar wind heating and acceleration. During PSP Encounter 17 (at $11.4R_s$) on Sep-26-2023, the PSP/FIELDS instrument detected enhanced ion-scale wave activity associated with deviations from local thermodynamic equilibrium in ion velocity distribution functions (VDFs) observed by the PSP/Solar Probe Analyzers-Ion (SPAN-I). Dense beams (secondary populations) were present in the proton VDFs during this wave activity. Using bi-Maxwellian fits to the proton VDFs, we found that the density of the proton beam population increased during the wave activity and, unexpectedly, surpassed the core population at certain intervals. Interestingly, the wave power was reduced during the intervals when the beam population density exceeded the core density. The drift velocity of the beams decreases from 0.9 to 0.7 of the Alfv\'{e}n speed and the proton core shows a higher temperature anisotropy ($T_\perp/T_\parallel>2.5$) during these intervals. We conclude that the observations during these intervals are consistent with a reconnection event during a heliospheric current sheet crossing. During this event, $\alpha$ particle parameters (density, velocity, and temperature anisotropy) remained nearly constant. Using linear analysis, we examined how the proton beam drives instability or wave dissipation. Furthermore, We investigated the nonlinear evolution of ion kinetic instabilities using hybrid kinetic simulations. This study provides direct clues about energy transfer between particles and waves in the young solar wind.

astro-ph.SR↗

arXiv2025

Modeling hot, anisotropic ion beams in the solar wind motivated by the Parker Solar Probe observations near perihelia

Recent observations of the solar wind ions by the SPAN-I instruments on board the Parker Solar Probe (PSP) spacecraft at solar perihelia (Encounters) 4 and closer find ample evidence of complex anisotropic non-Maxwellian velocity distributions that consist of core, beam, and `hammerhead' (i.e., anisotropic beam) populations. The proton core populations are anisotropic, with T_perp/T||>1, and the beams have super-Alfvenic speed relative to the core (we provide an example from Encounter 17). The alpha-particle population show similar features as the protons. These unstable VDFs are associated with enhanced, right-hand (RH) and left-hand (LH) polarized ion-scale kinetic wave activity, detected by the FIELDS instrument. Motivated by PSP observations, we employ nonlinear hybrid models to investigate the evolution of the anisotropic hot-beam VDFs and model the growth and the nonlinear stage of ion kinetic instabilities in several linearly unstable cases. The models are initialized with ion VDFs motivated by the observational parameters. We find rapidly growing (in terms of proton gyroperiods) combined ion-cyclotron (IC) and magnetosonic (MS) instabilities, which produce LH and RH ion-scale wave spectra, respectively. The modeled ion VDFs in the nonlinear stage of the evolution are qualitatively in agreement with PSP observations of the anisotropic core and `hammerhead' velocity distributions, quantifying the effect of the ion kinetic instabilities on wind plasma heating close to the Sun. We conclude that the wave-particle interactions play an important role in the energy transfer between the magnetic energy (waves) and random particle motion leading to anisotropic solar wind plasma heating.

astro-ph.SR↗

arXiv2024

Estimated Heating Rates Due to Cyclotron Damping of Ion-scale Waves Observed by Parker Solar Probe

Circularly polarized waves consistent with parallel-propagating ion cyclotron waves (ICWs) and fast magnetosonic waves (FMWs) are often observed by Parker Solar Probe (PSP) at ion kinetic scales. Such waves damp energy via the cyclotron resonance, with such damping expected to play a significant role in the enhanced, anisotropic heating of the solar wind observed in the inner heliosphere. We employ a linear plasma dispersion solver, PLUME, to evaluate frequencies of ICWs and FMWs in the plasma rest frame and Doppler-shift them to the spacecraft frame, calculating their damping rates at frequencies where persistently high values of circular polarization are observed. We find such ion-scale waves are observed during $20.37\%$ of PSP Encounters 1 and 2 observations and their plasma frame frequencies are consistent with them being transient ICWs. We estimate significant ICW dissipation onto protons, consistent with previous empirical estimates for the total turbulent damping rates, indicating that ICW dissipation could account for the observed enhancements in the proton temperature and its anisotropy with respect to the mean magnetic field.

astro-ph.SR↗

arXiv2020

Turbulence characteristics of switchbacks and non-switchbacks intervals observed by \emph{Parker Solar Probe}

We use \emph{Parker Solar Probe} (\emph{PSP}) in-situ measurements to analyze the characteristics of solar wind turbulence during the first solar encounter covering radial distances between $35.7R_\odot$ and $41.7R_\odot$. In our analysis we isolate so-called switchback (SB) intervals (folded magnetic field lines) from non-switchback (NSB) intervals, which mainly follow the Parker spiral field. Using a technique based on conditioned correlation functions, we estimate the power spectra of Elsasser, magnetic and bulk velocity fields separately in the SB and NSB intervals. In comparing the turbulent energy spectra of the two types of intervals, we find the following characteristics: 1) The decorrelation length of the backward-propagating Elsasser field $z^-$ is larger in the NSB intervals than the one in the SB intervals; 2) the magnetic power spectrum in SB intervals is steeper, with spectral index close to -5/3, than in NSB intervals, which have a spectral index close to -3/2; 3) both SB and NSB turbulence are imbalanced with NSB having the largest cross-helicity, 4) the residual energy is larger in the SB intervals than in NSB, and 5) the analyzed fluctuations are dominated by Alfvénic fluctuations that are propagating in the \emph{sunward} (\emph{anti-sunward}) direction for the SB (NSB) turbulence. These observed features provide further evidence that the switchbacks observed by \emph{PSP} are associated with folded magnetic field lines giving insight into their turbulence nature.

physics.space-ph↗

arXiv2020

Proton Temperature Anisotropy Variations in Inner Heliosphere Estimated with First Parker Solar Probe Observations

We report proton temperature anisotropy variations in the inner heliosphere with Parker Solar Probe (PSP) observations. Using a linear fitting method, we derive proton temperature anisotropy with temperatures measured by the Solar Probe Cup (SPC) from the SWEAP instrument suite and magnetic field observations from the FIELDS instrument suite. The observed radial dependence of temperature variations in the fast solar wind implies stronger perpendicular heating and parallel cooling than previous results from Helios measurements made at larger radial distances. The anti-correlation between proton temperature anisotropy and parallel plasma beta is retained in fast solar wind. However, the temperature anisotropies of the slow solar wind seem to be well constrained by the mirror and parallel firehose instabilities. The perpendicular heating of the slow solar wind inside 0.24 AU may contribute to its same trend up against mirror instability thresholds as fast solar wind. These results suggest that we may see stronger anisotropy heating than expected in inner heliosphere.

physics.space-ph↗

arXiv2019

Linear Stability in the Inner Heliosphere: Helios Reëvaluated

Wave-particle instabilities driven by departures from local thermodynamic equilibrium have been conjectured to play a role in governing solar wind dynamics. We calculate the statistical variation of linear stability over a large subset of Helios I and II fast solar wind observations using a numerical evaluation of the Nyquist stability criterion, accounting for multiple sources of free energy associated with protons and helium including temperature anisotropies and relative drifts. We find that 88\% of the surveyed intervals are linearly unstable. The median growth rate of the unstable modes is within an order of magnitude of the turbulent transfer rate, fast enough to potentially impact the turbulent scale-to-scale energy transfer. This rate does not significantly change with radial distance, though the nature of the unstable modes, and which ion components are responsible for driving the instabilities, does vary. The effect of ion-ion collisions on stability is found to be significant; collisionally young wind is much more unstable than collsionally old wind, with very different kinds of instabilities present in the two kinds of wind.

astro-ph.SR↗