arXiv subjects

Jean-Baptiste Dakeyo

Publications and source records attributed to Jean-Baptiste Dakeyo.

10 recordsLinked to original sources

arXiv2026

Direct Measurement of Polar Coronal Hole-like Solar Wind in its Acceleration Phase

The early evolution of fast polar coronal hole (PCH) solar wind remains largely unconstrained by in situ measurements. In March 2025, Parker Solar Probe (Parker) at its closest approach of 9.86 Solar Radii ($R_\odot$) measured outflow from a large equatorial coronal hole (ECH) which was also measured at 1\,au and at intermediate distances by Solar Orbiter (also near its perihelion). At 1\,au the stream properties are consistent with PCH properties established by Ulysses. The stream was measured by Parker substantially below the Alfvén surface, with proton temperatures in excess of 2\,MK and a speed at $\sim$10\,$R_\odot$ which was only $\sim$60\% of its asymptotic value. The Solar Orbiter data indicates that the acceleration is largely complete by 60~$R_{\odot}$. Spherically-polarized fluctuations in the stream are observed to develop from near-transverse and small-angle at Parker to full reversal ``switchbacks'' at Solar Orbiter. Comparison of the implied acceleration profile to historical doppler-dimming measurements suggests that the stream's low coronal acceleration is similar to that of PCH flows. Consistent with previous work, this acceleration requires significantly more energy than can be provided by the observed thermal pressure gradients, with a significant contribution likely from the abundant Alfvénic fluctuation energy observed at Parker. These observations provide unique constraints on models of the radial evolution of the fastest solar wind, and indicate that these wind streams experience gradual, steady acceleration over their first few tens of solar radii of evolution.

astro-ph.SR↗

arXiv2026

Statistical Energy Budget of the Solar Wind with Parker Solar Probe and Solar Orbiter : 1 - Unbalanced Total Energy Radial Evolution

The magneto-hydrodynamic (MHD) description of the solar wind has long been regarded as one of the most successful frameworks for investigating solar wind heating and acceleration. Under a set of simplifying assumptions, it provides a reduced large-scale description that is widely used to study the energetic role of electromagnetic fluctuations in the solar wind energy budget. In practice, applying simplification to MHD can result in the loss of some physics compared to an exact set of equations. However, an observational energy budget can help us to verify whether any energetic information has been lost in the process of simplification. To do so, we directly test the validity of the large-scale non-linear ideal MHD energy budget with solar wind observations from Parker Solar Probe (PSP) and Solar Orbiter (SO). After verifying the reliability of the measurements through comparison with previous studies of similar solar wind properties, we test the conservation of the total energy predicted by the MHD theory. We find that the average total energy increases with radial distance by $56\%$ ($\pm9\%$) between 14 and 203 solar radii. The increase in kinetic energy is not sufficiently compensated by the decrease of the thermal and electromagnetic contributions. We investigate the major sources of observational uncertainty and find that they cannot account for the observed energy increase. These results suggest that the large-scale theoretical description may neglect an energetically significant contribution to solar wind acceleration. Assuming that such a contribution exists, we predict that this extra energy would scale with radial distance as $r^{-0.54\pm0.38}$.

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arXiv2026

On the Radial Evolution of the Solar Wind : The Source Alignment Method Applied to Parker Solar Probe and Solar Orbiter Observations

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 solar radii). 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) between the two probes. This result demonstrates that solar wind acceleration in the inner heliosphere remains significant compared to that occurring below 15 solar radii. Among the different studied plasma parameters, the radial evolution of the electron temperature and plasma density, show the strongest anti-correlation with the increase in bulk velocity.

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arXiv2025

Proton specific entropy as a proxy for the $O^{7+}/O^{6+}$ charge state ratio over heliocentric distance

While the fast solar wind has well-established origins in coronal holes, the source of the slow solar wind remains uncertain. Compositional metrics, such as heavy ion charge state ratios are set in the lower corona, providing insights into solar wind source regions. However, prior to the launch of Solar Orbiter, in situ measurements of heavy ion charge state were limited to distances of 1 AU and beyond. We investigate proton specific entropy as a proxy for the oxygen charge state ratio ($O^{7+}/O^{6+}$),which generally becomes frozen-in below ~1.8 Rsun, leveraging observations from Solar Orbiter's Heavy Ion Sensor and Proton and Alphas Sensor covering 0.28 to 1 AU. Our analysis confirms a strong anti-correlation between specific entropy and the oxygen charge state ratio that persists over a broad range of distances in the inner heliosphere. We categorize observed solar wind into fast solar wind, slow Alfvenic solar wind, and slow solar wind, identifying clear distinctions in specific entropy values and charge state ratios across these types. The work demonstrates the potential to use proton specific entropy as a classifier of solar wind source regions throughout the heliosphere. By establishing the $S_p$-$O^{7+}/O^{6+}$ relationship and quantifying its radial dependence, the specific entropy can be used as a quantity to identify the solar wind source region in the absence of in-situ charge state measurements. This motivates future studies as to the applicability of this proxy to near-Sun observations (such as Parker Solar Probe) and throughout the inner heliosphere.

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arXiv2025

On the Propagation and Damping of Alfvenic Fluctuations in the Outer Solar Corona and Solar Wind

We analyze \textit{Parker Solar Probe} and \textit{Solar Orbiter} observations to investigate the propagation and dissipation of Alfvénic fluctuations from the outer corona to 1~AU. Conservation of wave-action flux provides the theoretical baseline for how fluctuation amplitudes scale with the Alfvén Mach number $M_a$, once solar-wind acceleration is accounted for. Departures from this scaling quantify the net balance between energy injection and dissipation. Fluctuation amplitudes follow wave-action conservation for $M_a < M_a^{b}$ but steepen beyond this break point, which typically lies near the Alfvén surface ($M_a \approx 1$) yet varies systematically with normalized cross helicity $σ_c$ and fluctuation scale. In slow, quasi-balanced streams, the transition occurs at $M_a \lesssim 1$; in fast, imbalanced wind, WKB-like scaling persists to $M_a \gtrsim 1$. Outer-scale fluctuations maintain wave-action conservation to larger $M_a$ than inertial-range modes. The turbulent heating rate $Q$ is largest below $M_a^{b}$, indicating a preferential heating zone shaped by the degree of imbalance. Despite this, the Alfvénic energy flux $F_a$ remains elevated, and the corresponding damping length $Λ_d = F_a/Q$ remains sufficiently large to permit long-range propagation before appreciable damping occurs. Normalized damping lengths $Λ_d/H_A$, where $H_A$ is the inverse Alfvén-speed scale height, are near unity for $M_a \lesssim M_a^{b}$ but decline with increasing $M_a$ and decreasing $U$, implying that incompressible reflection-driven turbulence alone cannot account for the observed dissipation. Additional damping mechanisms -- such as compressible effects -- are likely required to account for the observed heating rates across much of the parameter space.

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arXiv2025

Multi-spacecraft Measurements of the Evolving Geometry of the Solar Alfvén Surface Over Half a Solar Cycle

The geometry of a star's Alfvén surface determines stellar angular momentum loss, separates a causally distinct 'corona' and stellar wind, and potentially affects exoplanetary habitability. The solar Alfvén 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 Alfvén surface from measurements of the solar wind speed and local Alfvén speed. We show that accounting for solar wind acceleration and mass flux is necessary to achieve reasonable agreement between the scaled location of the Alfvén surface and the locations of direct crossings measured by Parker. We produce continuous 2D equatorial cuts of the Alfvén 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-Alfvénic 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

A parametric study of solar wind properties and composition using fluid and kinetic solar wind models

The physical processes in the solar corona that shape the solar wind remain an active research topic. Modeling efforts have shown that energy and plasma exchanges near the transition region plays a crucial role in modulating solar wind properties. Although these regions cannot be measured in situ, plasma parameters can be inferred from coronal spectroscopy and ionization states of heavy ions, which remain unchanged as they escape the corona. We introduce a new solar wind model extending from the chromosphere to the inner heliosphere, capturing thermodynamic coupling across atmospheric layers. By including neutral and charged particle interactions, we model the transport and ionisation processes of the gas through the transition region, the corona and into the solar wind. Instead of explicitly modeling coronal heating, we link its spatial distribution to large-scale magnetic field properties. Our results confirm that energy deposition strongly affects wind properties through key mechanisms involving chromospheric evaporation, thermal expansion, and magnetic flux expansion. For sources near active regions, the model predicts significant solar wind acceleration, with plasma outflows comparable to those inferred from coronal spectroscopy. For winds from large coronal holes, the model reproduces the observed anticorrelation between charge state and wind speed. However, the predicted charge state ratios are overall lower than observed. Inclusion of a population of energetic electrons enhances both heavy ion charge states and solar wind acceleration, improving agreement with observations.

astro-ph.SR↗

arXiv2025

Differentiating the acceleration mechanisms in the slow and Alfvénic slow solar wind

In the corona, plasma is accelerated to hundreds of kilometers per second, and heated to temperatures hundreds of times hotter than the Sun's surface, before it escapes to form the solar wind. Decades of space-based experiments have shown that the energization process does not stop after it escapes. Instead, the solar wind continues to accelerate and it cools far more slowly than a freely-expanding adiabatic gas. Recent work suggests that fast solar wind requires additional momentum beyond what can be provided by the observed thermal pressure gradients alone whereas it is sufficient for the slowest wind. The additional acceleration for fast wind can be provided through an Alfvén wave pressure gradient. Beyond this fast-slow categorization, however, a subset of slow solar wind exhibits high Alfvénicity that suggest Alfvén waves could play a larger role in its acceleration compared to conventional slow wind outflows. Through a well-timed conjunction between Solar Orbiter and Parker Solar Probe, we trace the energetics of slow wind to compare with a neighboring Alfvénic slow solar wind stream. An analysis that integrates remote and heliospheric properties and modeling of the two distinct solar wind streams finds Alfvénic slow solar wind behaves like fast wind, where a wave pressure gradient is required to reconcile its full acceleration, while non-Alfvénic slow wind can be driven by its non-adiabatic electron and proton thermal pressure gradients. Derived coronal conditions of the source region indicate good model compatibility but extended coronal observations are required to effectively trace solar wind energetics below Parker's orbit.

astro-ph.SR↗

arXiv2023

Proton and electron temperatures in the solar wind and their correlations with the solar wind speed

The heating and acceleration of the solar wind remains one of the fundamental unsolved problems in heliophysics. It is usually observed that the proton temperature $T_i$ is highly correlated with the solar wind speed $V_{SW}$, while the electron temperature $T_e$ shows anti-correlation or no clear correlation with the solar wind speed. Here we inspect both Parker Solar Probe (PSP) and WIND data and compare the observations with simulation results. PSP observations below 30 solar radii clearly show a positive correlation between proton temperature and wind speed and a negative correlation between electron temperature and wind speed. One year (2019) of WIND data confirm that proton temperature is positively correlated with solar wind speed, but the electron temperature increases with the solar wind speed for slow wind while it decreases with the solar wind speed for fast wind. Using a one-dimensional Alfvén-wave-driven solar wind model with different proton and electron temperatures, we for the first time find that if most of the dissipated Alfvén wave energy heats the ions instead of electrons, a positive $T_i-V_{SW}$ correlation and a negative $T_e-V_{SW}$ correlation arise naturally. If the electrons gain a small but finite portion of the dissipated wave energy, the $T_e-V_{SW}$ correlation evolves with radial distance to the Sun such that the negative correlation gradually turns positive. The model results show that Alfvén waves are one of the possible explanations of the observed evolution of proton and electron temperatures in the solar wind.

astro-ph.SR↗

arXiv2022

Acceleration of polytropic solar wind: Parker Solar Probe observation and one-dimensional model

The acceleration of the solar coronal plasma to supersonic speeds is one of the most fundamental yet unresolved problem in heliophysics. Despite the success of Parker's pioneering theory on an isothermal solar corona, the realistic solar wind is observed to be non-isothermal, and the decay of its temperature with radial distance usually can be fitted to a polytropic model. In this work, we use Parker Solar Probe data from the first nine encounters to estimate the polytropic index of solar wind protons. The estimated polytropic index varies roughly between 1.25 and 1.5 and depends strongly on solar wind speed, faster solar wind on average displaying a smaller polytropic index. We comprehensively analyze the 1D spherically symmetric solar wind model with polytropic index $γ\in [1,5/3]$. We derive a closed algebraic equation set for transonic stellar flows, i.e. flows that pass the sound point smoothly. We show that an accelerating wind solution only exists in the parameter space bounded by $C_0/C_g < 1$ and $(C_0/C_g)^2 > 2(γ-1)$ where $C_0$ and $C_g$ are the surface sound speed and one half of the escape velocity of the star, and no stellar wind exists for $γ> 3/2$. With realistic solar coronal temperatures, the observed solar wind with $γ\gtrsim 1.25$ cannot be explained by the simple polytropic model. We show that mechanisms such as strong heating in the lower corona that leads to a thick isothermal layer around the Sun and large-amplitude Alfvén wave pressure are necessary to remove the constraint in $γ$ and accelerate the solar wind to high speeds.

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