Milan Maksimovic

The inner heliosphere hosts a dynamically rich population of dust particles – the Zodiacal Dust Cloud – spanning sizes from nanometers to hundreds of micrometers and originating primarily from comets and asteroids. Over the past two decades, a generation of space missions has transformed our observational grasp of this environment. This review consolidates the advances enabled by the Wind, STEREO, Parker Solar Probe, and Solar Orbiter space missions, which together observe from heliocentric distances of 1 au down to less than 0.05 au. Two complementary observational approaches are addressed in detail. The first is in-situ dust detection via radio and plasma wave instruments, which commonly measure characteristic voltage pulses resulting from hypervelocity dust impacts on spacecraft. We describe the underlying physics, signal interpretation, dust particle mass calibration and machine-learning classification methods for this technique, which constitutes the only in-situ dust measurement capability on current inner-heliosphere missions. The second approach is remote sensing of the Zodiacal Light and F-corona through white-light coronagraph and heliospheric imagers. The combination of these approaches has yielded a consistent picture of the radial brightness profile of the cloud, provided the first observational evidence for a dust depletion zone developing sunward of 35 solar radii and a dust-free zone interior to 5 solar radii. The in-situ measurements established flux densities of dust in hyperbolic trajectories (β-meteoroids) across multiple missions, revealed variable fluxes of nanodust, and revealed solar-cycle modulation of the interstellar dust flux. This review presents these results, discusses their implications, and outlines the prospects offered by near-future missions.

The inner heliosphere hosts a dynamically rich population of dust particles – the Zodiacal Dust Cloud – spanning sizes from nanometers to hundreds of micrometers and originating primarily from comets and asteroids. Over the past two decades, a generation of space missions has transformed our observational grasp of this environment. This review consolidates the advances enabled by the Wind, STEREO, Parker Solar Probe, and Solar Orbiter space missions, which together observe from heliocentric distances of 1 au down to less than 0.05 au. Two complementary observational approaches are addressed in detail. The first is in-situ dust detection via radio and plasma wave instruments, which commonly measure characteristic voltage pulses resulting from hypervelocity dust impacts on spacecraft. We describe the underlying physics, signal interpretation, dust particle mass calibration and machine-learning classification methods for this technique, which constitutes the only in-situ dust measurement capability on current inner-heliosphere missions. The second approach is remote sensing of the Zodiacal Light and F-corona through white-light coronagraph and heliospheric imagers. The combination of these approaches has yielded a consistent picture of the radial brightness profile of the cloud, provided the first observational evidence for a dust depletion zone developing sunward of 35 solar radii and a dust-free zone interior to 5 solar radii. The in-situ measurements established flux densities of dust in hyperbolic trajectories (β-meteoroids) across multiple missions, revealed variable fluxes of nanodust, and revealed solar-cycle modulation of the interstellar dust flux. This review presents these results, discusses their implications, and outlines the prospects offered by near-future missions.

The inner heliosphere hosts a dynamically rich population of dust particles – the Zodiacal Dust Cloud – spanning sizes from nanometers to hundreds of micrometers and originating primarily from comets and asteroids. Over the past two decades, a generation of space missions has transformed our observational grasp of this environment. This review consolidates the advances enabled by the Wind, STEREO, Parker Solar Probe, and Solar Orbiter space missions, which together observe from heliocentric distances of 1 au down to less than 0.05 au. Two complementary observational approaches are addressed in detail. The first is in-situ dust detection via radio and plasma wave instruments, which commonly measure characteristic voltage pulses resulting from hypervelocity dust impacts on spacecraft. We describe the underlying physics, signal interpretation, dust particle mass calibration and machine-learning classification methods for this technique, which constitutes the only in-situ dust measurement capability on current inner-heliosphere missions. The second approach is remote sensing of the Zodiacal Light and F-corona through white-light coronagraph and heliospheric imagers. The combination of these approaches has yielded a consistent picture of the radial brightness profile of the cloud, provided the first observational evidence for a dust depletion zone developing sunward of 35 solar radii and a dust-free zone interior to 5 solar radii. The in-situ measurements established flux densities of dust in hyperbolic trajectories (β-meteoroids) across multiple missions, revealed variable fluxes of nanodust, and revealed solar-cycle modulation of the interstellar dust flux. This review presents these results, discusses their implications, and outlines the prospects offered by near-future missions.

The Lunar Surface Electromagnetics Experiment (LuSEE-Night) is a joint NASA-DOE-ESA low-frequency radio telescope that will reach the lunar far side in 2027. The unknown dielectric properties of the subsurface at the LuSEE-Night landing site impose the most significant limitation for precision instrument calibration, as reflections from the lunar subsurface can change the primary beam at the 10%–20% level. Simulations of these effects have provided insight and concern, showing that the lunar subsurface modeled as a lossy dielectric can absorb a large amount of the power of the sky signal. While this absorption may not strongly impact the signal-to-noise ratio in a sky noise-dominated regime, it could complicate the beam pattern and make the signal more difficult to model and interpret. We have simulated the far-field properties of the LuSEE-Night beam for varying dielectric profiles of the lunar subsurface. We find that varying the properties of the lunar subsurface has the most significant impact around the antenna resonance, impacting its amplitude, position, and width. Conversely, changing the properties of the foreground impacts the data across the band. We use a Bayesian inference pipeline to jointly estimate parameters of a Galactic foreground model and dielectric properties of the lunar subsurface around the LuSEE-Night landing site and find that parameters of both the galaxy and subsurface properties can be estimated jointly. While the modeling is somewhat idealized, we believe that the results are largely robust owing to the fact that spectral variations for plausible subsurface and galaxy models have very different spectral signatures.

The properties of the solar wind, as measured in situ throughout the heliosphere, depend both on the characteristics of its coronal source and on the intrinsic processes governing its interplanetary evolution. Recently, radial and Parker spiral alignment techniques have been applied to Parker Solar Probe (PSP) and Solar Orbiter (SO) observations to investigate the radial evolution of the same solar wind parcel. These studies have shown that the solar wind can undergo significant acceleration even beyond its primary acceleration region (i.e., above 15 R_⊙). However, such radial and Parker spiral alignments are rare in practice, which limits the statistical significance and general applicability of the results. We introduce a new source alignment technique designed to overcome these limitations. Using magnetic backmapping, we associate similar solar wind streams observed by the two spacecraft based on the proximity of their photospheric footpoints, combined with additional in situ stream similarity criteria. Applying the source alignment method to PSP and SO observations, we identify a total of 548 alignment intervals, each lasting 30 minutes. By constructing statistics over all alignments, we find that the solar wind speed increases by an average of 45% per radial decade (approximately 147 km s⁻¹) between the two probes. This result demonstrates that solar wind acceleration in the inner heliosphere remains significant compared to that occurring below 15 R_⊙. Among the different studied plasma parameters, the radial evolution of the electron temperature and plasma density shows the strongest anticorrelation with the increase in bulk velocity.

Context. Hard X-ray (HXR) flares and type III radio bursts are produced by electron beams accelerated during solar flares. Most statistical studies on the relation of the two emissions have focused on coronal type III bursts, which occasionally show a close temporal and spatial association with HXR flares, thereby suggesting a common acceleration site. Mild correlations have been identified when comparing the peak radio flux directly to the peak HXR count rates in associated events, partly due to the distinct emission mechanisms and media. In contrast, systematic studies on the relation between HXRs and interplanetary (IP) type III bursts remain scarce. Aims. Leveraging the Solar Orbiter mission’s unique capability for simultaneous HXR and radio observations, this study aims to establish a quantitative diagnostic link between the energy content and spectral properties of the electron beams that precipitate to produce HXRs at the Sun and those that escape to generate type III bursts in space. The Spectrometer/Telescope for Imaging X-rays (STIX) and the Radio and Plasma Waves instrument (RPW) on board Solar Orbiter are well suited for such a study. Methods. We analyzed 38 events from September 2021 to May 2023 with a clear temporal association between HXR bursts and type III bursts. For each event, we performed HXR spectroscopy at the time of the associated HXR peak to derive the spectral index ( δ ) and nonthermal power of the HXR-emitting electrons. From the RPW dynamic spectra, we measured the type III peak radio flux and fit the time profiles to extract the frequency drift of the burst’s front, peak, and back velocities, which were then converted to exciter velocities. Results. The type III radio peak flux exhibits a strong anticorrelation with the HXR spectral index, δ , and a significant positive correlation with the nonthermal electron power. Furthermore, the analysis of radio exciter velocities shows that the front velocity correlates with both the hardness and power of the HXR-emitting electrons. The peak and back velocities only describe weak relationships. The energy of radio-emitting electron beams estimated from radio observations (∼10 ²² –10 ²⁴ erg) is smaller than the energy derived from HXR-emitting electrons (∼10 ²⁵ –10 ²⁹ erg). However, the two are still correlated. Conclusions. Our results support a common electron accelerator for flares with a temporal association between HXRs and radio, as expected. The peak radio flux correlates with the spectral hardness and energy of escaping electron beams; whereas the front velocity shows a weaker, but still significant correlation with high-energy electron power. Although their energies are correlated, radio-emitting electrons carry only a minor fraction (≲10 ⁻³ ) of the energy contained in HXR-emitting electrons. We find that this difference might be linked to electron escape conditions in acceleration regions.

Aims . We present the first extensive catalogue of interplanetary Type III solar radio bursts compiled through human participation, and provide a statistical analysis of burst occurrence, peak flux, and frequency drift. Methods . The catalogue is based on Radio and Plasma Waves (RPW) observations from Solar Orbiter, collected via the citizen science campaign Solar Radio Burst Tracker on Zooniverse.org with contributions from 1952 volunteers between February 2020 and March 2025. A dedicated post-processing pipeline extracted each burst’s time-frequency extent, peak flux, and quantitative uncertainties, enabling statistical analyses. Results . The catalogue comprises 15 934 bursts. Burst occurrence strongly correlates with sunspot number and shows modulation with Solar Orbiter’s heliocentric distance. Peak flux distributions across 23 frequencies follow power laws with an average slope of –1.56 ± 0.004, steepening at higher frequencies. This is consistent with earlier studies of smaller samples. At still higher frequencies, slopes plateau around –1.7 to –1.8, resembling self-organized criticality behaviour. Burst occurrence peaks at 1-2 MHz, reflecting efficient electron-beam transport and beam-plasma interaction. Frequency drift rates for 13 074 bursts scale as d f /d t = –0.001 f ^1.37 , differing from prior relations over other frequency ranges. Conclusions . This human-validated, large-scale catalogue provides a robust statistical resource for Type III bursts, capturing solar cycle and observational effects, extending analyses to faint and complex events, and constraining frequency-dependent flux and drift rates. It also offers a benchmark for future automated and machine-learning detection methods.

Context . On February 18, 2025, Solar Orbiter (SO) completed its fourth gravity assist maneuver of Venus (VGAM4) and reached an unprecedented proximity coming within 378 km of the planet. This flyby was necessary to steer the spacecraft into an orbit outside the plane of the ecliptic. Near the closest approach, only the Radio and Plasma Wave (RPW) and Magnetometer (MAG) instruments were operational; this enabled high-cadence measurements to be taken to investigate the plasma properties of the Venusian ionosphere. Aims . The main goal of this study is to derive the electron density and temperature in the ionosphere of Venus using electric potential measurements from RPW, and to characterize them. Methods . During approximately five minutes around the closest approach, the High Frequency Receiver of RPW detected radio emissions of a type naturally generated by planetary ionospheres whose frequency can be related to the electron density. Using quasithermal noise spectroscopy, we inferred the electron temperature at discrete altitudes and solar zenith angles. Results . Solar Orbiter measured an average density and electron temperature in the ionosphere of Venus of 12 385 ± 148 cm ⁻³ and 0.43 ± 0.05 eV, respectively. These values are in agreement with in-situ measurements by Pioneer Venus Orbiter (PVO) obtained at the solar maximum. Binned magnetic fields and temperatures are anticorrelated, which suggests that the magnetic flux ropes, observed in the Venus ionosphere, are more likely non-force-free structures. Conclusions . The findings presented in this paper, together with the measurement from the Parker Solar Probe (PSP) during the third gravity assist, support the conclusion that the plasma density in the Venusian ionosphere above 350 km varies with solar activity, whereas the electron temperature shows a much weaker dependence. Notably, the electron temperature remains consistent across the three missions (SO, PSP, and PVO), despite varying levels of solar activity. This suggests that, over the altitude and solar zenith regions probed, the thermal structure of the Venusian ionosphere is not driven by solar extreme ultraviolet (EUV) heating alone, but is also shaped by external heat sources near the ionopause. Processes such as the damping of whistler mode waves, solar wind ion heating, and thermal conduction from the hot ionosheath appear to play a major role.

Aims . Thermal electron measurements in space plasmas typically suffer at low energies from spacecraft emissions of photo- and secondary electrons and from charging of the spacecraft body. We aim to examine these effects by using numerical simulations in the context of electron measurements acquired by the Electron Analyser System (SWA-EAS) on board the Solar Orbiter mission. Methods . We employed the Spacecraft Plasma Interaction Software to model the interaction of the Solar Orbiter spacecraft with solar wind plasma. In the model, we implemented a virtual detector to simulate the measured electron energy spectra as observed in situ by the SWA-EAS experiment. For comparison with the real SWA-EAS data, numerical simulations were set according to the measured plasma conditions at 0.3 AU. From the simulation results, we derived the electron energy spectra as detected by the virtual SWA-EAS experiment for different electron populations and compared these with both the initial plasma conditions and the corresponding real SWA-EAS data samples. Results . We found a qualitative agreement between the simulated and real data observed in situ by the SWA-EAS detector. Contrary to other space missions, the contamination by cold electrons emitted from the spacecraft is seen well above the spacecraft potential energy threshold. A detailed analysis of the simulated electron energy spectra demonstrates that contamination above the threshold is a result of cold electron fluxes emitted from distant spacecraft surfaces. The relative position of the break in the simulated spectrum with respect to the spacecraft potential slightly deviates from that in the real observations. This may indicate that the real potential of the SWA-EAS detector with respect to ambient plasma differs from the spacecraft potential value measured on board. The overall contamination is shown to be composed of emissions from a number of different sources and their relative contribution varies with the ambient plasma conditions.

Solar radio bursts, and astrophysical radio emissions in general, are observed either in space or on the ground by measuring their fluctuating electric field. Here, we report the first measurement of the magnetic component of a solar radio wave, observed simultaneously by the Solar Orbiter and Parker Solar Probe missions. The observations were made during the type III radio burst on 2021 October 28. The analysis of the wave polarization and magnetic and electric field amplitudes allows us to estimate the refractive index and put constraints on the direction of the wave. The wave is found to be consistent with an ordinary-mode wave and with a source near the southeast limb of the Sun. These results pave the way for future observations and analyses of the magnetic field of radio waves, in particular, for solar radio bursts.