Wind Density Research Articles

Global Sensitivity Analysis and Uncertainty Quantification for Background Solar Wind Using the Alfvén Wave Solar Atmosphere Model

AbstractModeling the impact of space weather events such as coronal mass ejections (CMEs) is crucial to protecting critical infrastructure. The Space Weather Modeling Framework is a state‐of‐the‐art framework that offers full Sun‐to‐Earth simulations by computing the background solar wind, CME propagation, and magnetospheric impact. However, reliable long‐term predictions of CME events require uncertainty quantification (UQ) and data assimilation. We take the first steps by performing global sensitivity analysis (GSA) and UQ for background solar wind simulations produced by the Alfvén Wave Solar atmosphere Model (AWSoM) for two Carrington rotations: CR2152 (solar maximum) and CR2208 (solar minimum). We conduct GSA by computing Sobol' indices that quantify contributions from model parameter uncertainty to the variance of solar wind speed and density at 1 au, both crucial quantities for CME propagation and strength. Sobol' indices also allow us to rank and retain only the most important parameters, which aids in the construction of smaller ensembles for the reduced‐dimension parameter space. We present an efficient procedure for computing the Sobol' indices using polynomial chaos expansion surrogates and space‐filling designs. The PCEs further enable inexpensive forward UQ. Overall, we identify three important model parameters: the multiplicative factor applied to the magnetogram, Poynting flux per magnetic field strength constant used at the inner boundary, and the coefficient of the perpendicular correlation length in the turbulent cascade model in AWSoM.

The Solar Wind Interaction with (1) Ceres: The Role of Interior Conductivity

As a potential “ocean world,” (1) Ceres’ interior may possess relatively high electrical conductivities on the order of 10−4–100 S m−1, suggesting that the solar wind interaction with Ceres may differ from other highly resistive objects such as the Moon. Here, we use a hybrid plasma model to quantify the solar wind interaction with Ceres over a range of scenarios for Ceres’ internal conductivity structure and the upstream solar wind and interplanetary magnetic field (IMF) conditions. Internal models for Ceres include one-, two-, and three-layer conductivity structures that variously include a crust, mantle, and/or subsurface ocean, while modeled solar wind conditions include a nominal case, a high IMF case, and an “extreme” space weather case. To first order, Ceres’ interaction with the solar wind is governed by the draping and enhancement of the IMF over its interior, whether from a moderate-conductivity mantle or a high-conductivity ocean. In turn, IMF draping induces compressional wings in the solar wind density and deceleration in the solar wind speed outside of Ceres. Together, all three effects are readily observable by a hypothetical orbital or landed mission with standard plasma and magnetic field instrumentation. Finally, we also consider the possible effects of unipolar induction within Ceres, which has been previously suggested as a mechanism for conducting bodies in the solar wind. Our model results show that the efficacy of unipolar induction is highly suppressed by the slow magnetic field-line diffusion through Ceres’ interior and, thus, is not a significant contributor to Ceres’ overall interaction with the solar wind.

On the General Correlation between 3D Solar Wind Speed and Density Model and Solar Proxies

The solar wind (SW) is a supersonic outflow of plasma from the solar corona, with the latitudinal speed and density profiles varying with the solar activity. The SW protons charge exchange with the inflowing interstellar neutral atoms and create energetic neutral atoms (ENAs), which bring information on the physical state of the plasma within the boundary region of the heliosphere. The speed of the ENAs depends on their energies, and consequently observations at different energies provide information on different epochs backward in time. Therefore, understanding the history of the evolution of the SW is important to understand this information. In this paper, we extend the work by Porowski et al., who provided the WawHelioIon 3DSW model of the time evolution of latitudinal profiles of the SW speed and density based on results of analysis of interplanetary scintillations (IPSs). Based on results of principal component analysis, we search for correlation between selected solar proxies and the structure of the SW obtained from IPSs and show that it is possible to reproduce the evolution of the SW structure during the past three solar cycles based on the proxies. With this, we extend the history of the evolution of the SW structure back to 1976, i.e., to the epoch when observations of the key proxies—the inclination of the SW current sheet and the solar polar magnetic fields—became available. We point out the potential of the use of the proxies for forecasting the structure of the SW into the future.

Wind Effects on Small‐Scale River and Creek Plumes

AbstractIn contrast to large river plumes, Coriolis effects are weak, and inertia is quickly depleted so that the fate and structure of small‐scale plumes are more sensitive to tide and wind. Advected alongshore by reversing tidal currents in absence of wind forcing, small buoyant plumes are persistently deflected downwind in presence of alongshore winds and exhibit little tidal variability. The effect of different upwelling/downwelling winds on buoyant outflows ∼10 m3 s−1 is explored. With increasing wind, tidal variability decreases, as does asymmetry in plume characteristics—for strong winds upwelling/downwelling plume structure is similar as the plume is retained closer to the shore. Wind forcing is exerted directly by wind stress on the surface of the plume and indirectly by wind‐driven currents that deflect the upwind boundary of the plume. While inertia and buoyancy dominate the inner plume, and wind dominates the outer plume, the mid‐plume responds to an interaction of wind and buoyancy forcing that can be indexed by a Plume Wedderburn Number Wpl (wind stress vs. density gradients): for weaker winds (Wpl < 1) surface stress enhances stratification through straining, lengthening the reach of low‐salinity waters, whereas for stronger winds (Wpl > 1) surface stress mixes the plume vertically, shortening the reach of low‐salinity waters. However, dilute plume waters extend furthest in strong winds, passively advected several kilometers downwind. Shoreline exposure to outflow transitions from a quasi‐symmetrical tide‐averaged zone of impact under zero‐wind to a heavily skewed zone with persistent weak wind and a one‐sided zone for strong wind.

Variability and Parameter Dependence of Hard X-Ray Obscuration Around Supermassive Black Holes in Active Galaxies

We investigate hard X-ray obscuration of Active Galactic Nuclei (AGNs) by considering photoionization processes for a number of simulated accretion-disk winds in our magnetohydrodynamic (MHD) framework. To follow up our earlier work on calculating synthetic AGN obscuration functions, we are motivated in this work to expand our examination of the dependences of obscuration on individual parameters associated with MHD wind and coronal X-ray such as: X-ray luminosity L ion, photon index Γ, density of the wind n o and its radial gradient p where n ∝ n o r −p . We find that AGN appear to undergo a relatively drastic transition from more obscured state (e.g., N H ≳ 1022 cm−2) to unobscured state (i.e., N H ≲ 1022 cm−2) as p approaches to ∼1.1 in these calculations. In the current formalism, luminosity is linearly coupled to wind density (i.e., L X ∝ n o ), and our calculations also indicate AGN become more obscured at higher luminosity. We further discuss the plausibility of the current model in comparison with observations.

What powers the wind from the black hole accretion disc in GRO J1655−40?

ABSTRACT Black hole accretion discs can produce powerful outflowing plasma (disc winds), seen as blue-shifted absorption lines in stellar and supermassive systems. These winds in quasars have an essential role in controlling galaxy formation across cosmic time, but there is no consensus on how these are physically launched. A single unique observation of a stellar-mass black hole GRO J1655−40 was used to argue that magnetic driving was the only viable mechanism and motivated unified models of magnetic winds in both binaries and quasars. The alternative, X-ray heating (thermal-radiative wind), was ruled out for the low observed luminosity by the high wind density estimated from an absorption line of a metastable level of Fe xxii. Here, we reanalyse these data using a photoionization code that includes cascades from radiative excitation as well as collisions in populating the metastable level. The cascade reduces the inferred wind density by more than an order of magnitude. The derived column is also optically thick, so the source is intrinsically more luminous than observed. We show that a thermal-radiative wind model calculated from a radiation hydrodynamic simulation matches well with the data. We revisit the previous magnetic wind solution and show that this is also optically thick, leading to a larger source luminosity. However, unlike the thermal-radiative wind, it struggles to reproduce the overall ion population at the required density. These results remove the requirement for a magnetic wind in these data and remove the basis of the self-similar unified magnetic wind models extrapolated to quasar outflows.

Tell-tale Spectral Signatures of MHD-driven Ultrafast Outflows in AGNs

We aim to explore spectral signatures of the predicted multi-ion ultrafast outflows (UFOs) in the broadband X-ray spectra of active galactic nuclei by exploiting an accretion disk-wind model in the context of a simple magnetohydrodynamic (MHD) framework. We are focused primarily on examining the spectral dependences on a number of key properties: (1) ionizing luminosity ratio λ ion, (2) line-of-sight wind density slope p, (3) optical/UV-to-X-ray strength α OX, (4) inclination θ, (5) X-ray photon index Γ, and (6) wind density factor f D . With an emphasis on radio-quiet Seyferts in the sub-Eddington regime, multi-ion UFO spectra are systematically calculated as a function of these parameters to show that MHD-driven UFOs imprint a unique asymmetric absorption line profile with a pronounced blue tail structure on average. Such a characteristic line signature is generic to the simplified MHD disk-wind models presented in this work due to their specific kinematics and density structure. The properties of these absorption line profiles could be utilized as a diagnostics to distinguish between different wind-driving mechanisms or even the specific values of given MHD wind parameters. We also present high-fidelity microcalorimeter simulations in anticipation of the upcoming XRISM/Resolve and Athena/X-IFU instruments to demonstrate that such a “tell-tale” sign may be immune to a spectral contamination by the presence of additional warm absorbers and partially covering gas.

Enhancement of cooling performance in traction motor of electric vehicle using direct slot cooling method

• DSC(Direct Slot Cooling) has been proposed for high-power and high-torque traction motors. • To evaluate the performance of DSC, we compared it with existing motor-cooling technologies, such as water-jacket, end-tip and channel coolings. • DSC enables a 66% and 94% increase in the applied current density compared to that of water-jacket cooling and end-tip cooling, respectively. • By adopting the DSC model, the winding maximum temperature did not exceed 90 °C in the highway driving mode, even when a high current density of 70 A/mm2 was momentarily applied. The torque of a traction motor for electric vehicles is directly proportional to the electric current applied to the motor winding. Since a power increment is directly linked with an increase in torque, traction motors with high electric current are needed to enhance the dynamic performance of electric vehicles. However, heat loss in the winding, which is proportional to the square of the applied electric current, may cause insulation breakdown in the winding or demagnetisation of permanent magnets in the rotor. In this study, we employed direct slot cooling (DSC) to directly cool motor windings, which are the main heat source in a motor, with cooling fluids. To evaluate the performance of DSC, we compared it with existing motor-cooling technologies, such as water-jacket, end-tip and channel cooling. The performances of the cooling methods were compared based on the change in the pumping power of the cooling fluid and the winding current density. Under two real road driving modes (urban dynamometer and highway driving modes), we compared the change in temperature with time in the traction motor of a compact car with the various cooling methods. The proposed DSC model showed good cooling performance; the winding maximum temperature did not exceed 90 °C in the highway driving mode, even when a high current density of 70 A/mm2 was momentarily applied.

Energetic Electron Flux Predictions in the Near‐Earth Plasma Sheet From Solar Wind Driving

AbstractSuprathermal electrons in the near‐Earth plasma sheet are important for inner magnetosphere considerations. They are the source population for outer radiation belt electrons and they pose risks to geosynchronous satellites through their contribution to surface charging. We use empirical modeling to address relationships between solar driving parameters and plasma sheet electron flux. Using Time History of Events and Macroscale Interactions during Substorms, OMNI, and Flare Irradiance Spectral Model Version 2 data, we develop a neural network model to predict differential electron flux from 0.08 to 93 keV in the plasma sheet, at distances from 6 to 12 RE. Driving parameters include solar wind (SW) density and speed, interplanetary magnetic field (IMF) BZ and BY, solar extreme ultraviolet flux, IMF BZ ultra‐low frequency (ULF) wave power, SW‐magnetosphere coupling functions Pα1 and NXCF, and the 4‐hr time history of these parameters. Our model predicts overall plasma sheet electron flux variations with correlation coefficients of between 0.59 and 0.77, and median symmetric accuracy in the 41%–140% range (depending on energy). We find that short time‐scale electron flux variations are not reproduced using short time‐scale inputs. Using a recently published technique to extract information from neural networks, we determine the most important drivers impacting model prediction are VSW, VBS, and IMF BZ. SW‐magnetosphere coupling functions that include IMF clock angle, IMF BZ ULF wave power, and IMF BY have little impact in our model of electron flux in the near‐Earth plasma sheet. The new model, built directly on differential flux, outperforms an existing model that derives fluxes from plasma moments, with the performance improvement increasing with increasing energy.

Constraining planetary mass-loss rates by simulating Parker wind profiles with Cloudy

Models of exoplanet atmospheres based on Parker wind density and velocity profiles are a common choice in fitting spectroscopic observations tracing planetary atmospheric escape. Inferring atmospheric properties using these models often results in a degeneracy between the temperature and the mass-loss rate, and thus provides weak constraints on either parameter. We present a framework that can partially resolve this degeneracy by placing more stringent constraints on the expected thermospheric temperature. We use the photoionization code Cloudy within an iterative scheme to compute the temperature structure of a grid of 1D Parker wind models, including the effects of radiative heating/cooling, as well as the hydrodynamic effects (expansion cooling and heat advection). We constrain the parameter space by identifying models that are not self-consistent through a comparison of the simulated temperature in the He 10 830 Å line-forming region to the temperature assumed in creating the models. We demonstrate this procedure on models based on HD 209458 b. By investigating the Parker wind models with an assumed temperature between 4000 and 12 000 K, and a mass-loss rate between 108 and 1011 g s−1, we are able to rule out a large portion of this parameter space. Furthermore, we fit the models to previous observational data and combine both constraints to find a preferred thermospheric temperature of T = 8200 −1100+1200 K and a mass-loss rate of Ṁ = 10 9.84 −0.27+0.24 g s−1 assuming a fixed atmospheric composition and no gas pressure confinement by the stellar wind. Using the same procedure, we constrain the temperatures and mass-loss rates of WASP-69 b, WASP-52 b, HAT-P-11 b, HAT-P-18 b and WASP-107 b.

Seismological footprint of an anomalous atmospheric activity registered in March 2021, in Baja California, Mexico

On 11 March 2021, a quite short strong signal was recorded by seismic stations of the CICESE Seismic Network, which cannot be associated with any regional or global earthquake. At the CICESE Campus and all along the city of Ensenada, in Baja California, Mexico, people reported vibration of the windows and even a short strong rumbling. Fortunately, houses and buildings did not report any damage. Due to the interaction between the atmosphere and the shallow earth surface, this anomalous atmospheric activity produced a special seismological footprint, with frequencies between 1 and 10 Hz. In this manuscript, we report on the observations of a multiparameter dataset, including seismic data along with wind velocity, wind density, temperature, humidity, atmospheric pressure, and THSW index. The atmospheric perturbation wave was strong enough to be clearly recorded by seismic stations within an area of almost 80 km and to produce some changes in the recorded meteorological parameters. The results from an FK analysis show that the atmospheric activity occurred to the south of Ensenada City and travelled to the north, as shown in the seismic records. We discuss the characteristics of the seismic signals in the frequency domain and the relation to the changes in the atmospheric parameters that could be related to this anomalous atmospheric activity.

Validation of the DSCOVR Spacecraft Mission Space Weather Solar Wind Products

AbstractIn this paper, we present a statistical validation of the Deep Space Climate Observatory (DSCOVR) solar wind data in the operational space weather archive. The DSCOVR observations of the interplanetary magnetic field (IMF), solar wind velocity, density, and temperature were hourly averaged and compared to measurements from NASA's Advanced Composition Explorer (ACE) and Wind spacecraft. Hourly averages, in general, show good correlations between the satellites for the IMF, solar wind velocity Geocentric Solar Ecliptic (GSE) vx‐component, and density. During the period covered by this study (spanning from late July 2016, when DSCOVR went operational, to the end of 2020), the DSCOVR products show no clear evidence of permanent degradation. However, for plasma parameters, there were periods of disagreement with ACE and Wind. The correlation coefficients (Pearson's r) calculated over the entire study period were similar or the same between DSCOVR versus Wind and DSCOVR versus ACE. For comparisons between DSCOVR and Wind, the IMF Bx and By GSE r were 0.94 and 0.96, respectively, while r for the IMF GSE Bz‐component was 0.88. For solar wind velocity, r was found to be 0.96 for the GSE vx‐component, compared with 0.30 for vy and 0.33 for vz. For density, r was found to be 0.84. DSCOVR density observations tend to overestimate compared to Wind values when the solar wind densities are low (below ∼5/cc), while the agreement between the two spacecraft on IMF measurements tends to increase with decreasing spatial separation.

Solar wind magnetosonic mach number as a control variable for energy dissipation during magnetic storms

During the main phase of many magnetic storms the solar wind Mach number is low and IMF magnitude is large. Under these conditions, the ionospheric potential saturates, and it becomes relatively insensitive to further increases in the IMF magnitude. On the other hand, the dayside merging rate and the potential become sensitive to the solar wind density. This should result in a correlation between the intensity of the auroral electrojets and the solar wind density. In this study we provide a sample of 314 moderate to strong storms and investigate the correlation between Dst index and the energy dissipated in the ionosphere. We show that for lower Mach numbers, this correlation decreases. We also show that the ionospheric indices of the storms with the lower Mach number are less correlated to the geoeffectiveness of the solar wind during these storms.

A global view of Pc3 wave activity in near-Earth space: Results from hybrid-Vlasov simulations

Ultra-low frequency (ULF) waves in the Pc3 range, with periods between 10–45 s, are routinely observed in Earth’s dayside magnetosphere. They are thought to originate in the foreshock, which extends upstream of the quasi-parallel bow shock and is populated with shock-reflected particles. The foreshock is permeated with ULF waves generated by ion beam instabilities, most notably the “30-s” waves whose periods match those of the Pc3 waves and which are carried earthward by the solar wind flow. However, the global picture of Pc3 wave activity from the foreshock to the magnetosphere and its response to changing solar wind conditions is still poorly understood. In this study, we investigate the global distribution and properties of Pc3 waves across near-Earth space using global simulations performed with the hybrid-Vlasov model Vlasiator. The simulations enable us to study the waves in their global context, and compare their properties in the foreshock, magnetosheath and dayside magnetosphere, for different sets of upstream solar wind conditions. We find that in all three regions the Pc3 wave power peaks at higher frequencies when the interplanetary magnetic field (IMF) strength is larger, consistent with previous studies. The Pc3 wave power is significantly enhanced in all three regions for higher solar wind Alfvén Mach number. As this parameter is known to affect the shock properties but has little impact inside the magnetosphere, this brings further support to the magnetospheric waves originating in the foreshock. Other parameters that are found to influence the foreshock wave power are the solar wind density and the IMF cone angle. Inside the magnetosphere, the wave power distribution depends strongly on the IMF orientation, which controls the foreshock position upstream of the bow shock. The wave power is largest when the angle between the IMF and the Sun-Earth line is smallest, suggesting that wave generation and transmission are most efficient in these conditions.

Dynamical Unification of Tidal Disruption Events

The ∼100 tidal disruption events (TDEs) observed so far exhibit a wide range of emission properties both at peak and over their lifetimes. Some TDEs radiate predominantly at X-ray energies, while others radiate chiefly at UV and optical wavelengths. While the peak luminosities across TDEs show distinct properties, the evolutionary behavior can also vary between TDEs with similar peak emission properties. In particular, for optical TDEs, while their UV and optical emissions decline somewhat following the fallback pattern, some events can greatly rebrighten in X-rays at late time. In this Letter, we conduct three-dimensional general relativistic radiation magnetohydrodynamics simulations of TDE accretion disks at varying accretion rates in the regime of super-Eddington accretion. We make use of Monte Carlo radiative transfer simulations to calculate the reprocessed spectra at various inclinations and at different evolutionary stages. We confirm the unified model proposed by Dai et al., which predicts that the observed emission largely depends on the viewing angle of the observer with respect to the disk orientation. Furthermore, we find that disks with higher accretion rates have elevated wind and disk densities, which increases the reprocessing of the high-energy radiation and thus generally augments the optical-to-X-ray flux ratio along a particular viewing angle. This implies that at later times, as the accretion level declines, we expect that more X-rays will leak out along intermediate viewing angles. Such dynamical model for TDEs can provide a natural explanation for the diversity in the emission properties observed in TDEs at peak and along their temporal evolution.

Modeling of Joint Parker Solar Probe–Metis/Solar Orbiter Observations

We present the first theoretical modeling of joint Parker Solar Probe (PSP)–Metis/Solar Orbiter (SolO) quadrature observations. The combined observations describe the evolution of a slow solar wind plasma parcel from the extended solar corona (3.5–6.3 R ⊙) to the very inner heliosphere (23.2 R ⊙). The Metis/SolO instrument remotely measures the solar wind speed finding a range from 96 to 201 km s−1, and PSP measures the solar wind plasma in situ, observing a radial speed of 219.34 km s−1. We find theoretically and observationally that the solar wind speed accelerates rapidly within 3.3–4 R ⊙ and then increases more gradually with distance. Similarly, we find that the theoretical solar wind density is consistent with the remotely and in-situ observed solar wind density. The normalized cross helicity and normalized residual energy observed by PSP are 0.96 and −0.07, respectively, indicating that the slow solar wind is very Alfvénic. The theoretical NI/slab results are very similar to PSP measurements, which is a consequence of the highly magnetic field-aligned radial flow ensuring that PSP can measure slab fluctuations and not 2D ones. Finally, we calculate the theoretical 2D and slab turbulence pressure, finding that the theoretical slab pressure is very similar to that observed by PSP.

Spatial distribution of nutrient loads and thresholds in large shallow lakes: The case of Chaohu Lake, China

Persistent eutrophication frequently causes toxic algal blooms, which is a serious threat to drinking water safety, food security, and public health. Nutrient thresholds, the maximum nutrient load that an aquatic ecosystem can absorb while meeting management objective, are key to avoiding and reducing blooms. The determination of thresholds relies on nutrient load-response curves. The spatial heterogeneity of large shallow lakes in terms of lake characteristics results in different curves shape among areas of the lake, which leads to spatial differences in thresholds. However, the spatial heterogeneity of thresholds is typically neglected; there are few methods to analyse the relationship between river loads and lake-specific area thresholds. Here, we proposed the Area Threshold Analysis Framework to analyse the spatial patterns of nutrient loads and thresholds in lakes. We first quantified the flow and load of the rivers entering the lake. The lake was then zoned and modelled to analyse area thresholds. Finally, an intuitive link between the nutrient loads of specific rivers and the thresholds of specific areas of the lake was established. The results showed a nonlinear without hysteresis response in all areas of Chaohu Lake; the nutrient loads and thresholds were highly variable in space, showing a trend of increasing and then decreasing from west to east. Flow density and wind might be important in influencing the spatial distribution of thresholds. The different effects of the wind and flow density on total phosphorus (TP) and chlorophyll a (Chl-a) lead to large differences in the thresholds with TP and Chl-a as management objectives, respectively. The large gap between nutrient loads and thresholds made it important for management to consider appropriate management goals to deal with unrealistic nutrient reductions. Achieving year-round Chl-a ≤ 30 µg/l in Chaohu Lake was easier than TP ≤ 0.05 mg/l, since for the latter, the Shiwuli, Paihe, and Nanfei river loads needed reductions of >80 %. In addition, for Area 1 and 2, it was more practical to first start meeting TP ≤ 0.05 mg/l for 2 or 3 seasons of the year than for 4 seasons. Overall, we developed a new framework for spatial threshold analysis and established an intuitive link between nutrient loads and thresholds in large shallow lakes. These results are valuable for understanding the threshold properties of spatially heterogeneous ecosystems in general and provide a reference for watershed nutrient management and ecological restoration of lakes.

Energy dependent response of the dayside Martian ionospheric electrons to solar forcing

We study the response of Auger electrons, He-II photoelectrons, and thermal electrons in the dayside Martian ionosphere to the solar soft X-rays (SXR; 0.1–7 nm), 30.5 nm, and the extreme ultraviolet (EUV; 20–90 nm) irradiances, respectively. For this purpose, the suprathermal electron fluxes measured by the Solar Wind Electron Analyzer (SWEA) and densities of the thermal electrons measured by Langmuir Probe and Waves (LPW) instruments, both on the Mars Atmosphere and Volatile Evolution (MAVEN) mission, have been used. The data used in the present study span from January 2015 (Martian Year (MY) 32, solar longitude, Ls = 263°) to December 2019 (MY 35, Ls = 110°), which falls in the declining phase of solar cycle 24. The solar irradiances are taken from the extreme ultraviolet monitor instrument on MAVEN and also from the Flare Irradiance Spectral Model for Mars. The results of the present study show that the fluxes of the suprathermal electrons and the densities of the thermal electrons are drastically reduced during solar minimum. The Auger electrons show a significant correlation and have an almost linear relationship with the SXR irradiance; both of which are independent of altitude. The response of the He-II photoelectrons to 30.5 nm solar irradiance deviates slightly from the linear relationship, particularly in the altitude range of 225–350 km and the solar zenith angle (SZA) range of 45°-65°. At these altitude and SZA ranges, the thermal electrons show a power law dependence on the EUV irradiance. The responses of the He-II photoelectrons and thermal electrons to their respective solar irradiances decrease on either side of the altitude and SZA range of maximum response (225–350 km and 45°-65°, respectively). The energy dependent response of the electrons to the solar irradiances and their altitude and SZA variation are explained by considering their dependence on electron temperature, and ionization and the neutral heating efficiencies. The responses of the He-II photoelectrons and thermal electrons to their respective solar irradiances decrease near the terminator.

Characteristics and evolution of sheath and leading edge structures of interplanetary coronal mass ejections in the inner heliosphere based on Helios and Parker Solar Probe observations

Context. We investigated the plasma and magnetic field characteristics of the upstream regions of interplanetary coronal mass ejections (ICMEs) and their evolution as function of distance to the Sun in the inner heliosphere. Results are related both to the development of interplanetary shocks, sheath regions, and compressed solar wind plasma ahead of the magnetic ejecta (ME). Aims. From a sample of 45 ICMEs observed by Helios 1/2 and the Parker Solar Probe, we aim to identify four main density structures; namely shock, sheath, leading edge, and ME itself. We compared characteristic parameters (proton particle density, plasma-beta, temperature, magnetic field strength, proton bulk speed, and duration) to the upstream solar wind in order to investigate the interrelation between the different density structures. Methods. For the statistical investigation, we used plasma and magnetic field measurements from 40 well-observed Helios 1/2 events from 1974–1981. Helios data cover the distance range from 0.3–1 au. For comparison, we added a sample of five ICMEs observed with the Parker Solar Probe from 2019–2021 over the distance range of 0.32–0.75 au. Results. It is found that the sheath structure consists of compressed plasma as a consequence of the turbulent solar wind material following the shock and lies ahead of a region of compressed ambient solar wind. The region of compressed solar wind plasma is typically found directly in front of the magnetic driver and seems to match the bright leading edge commonly observed in remote sensing observations of CMEs. From the statistically derived density evolution over distance, we find the CME sheath becomes denser than the ambient solar wind at about 0.06 au. From 0.09–0.28 au, the sheath structure density starts to dominate over the density within the ME. The ME density seems to fall below the ambient solar wind density over 0.45–1.18 au. Besides the well-known expansion of the ME, the sheath size shows a weak positive correlation with distance, while the leading edge seems not to expand with distance from the Sun. We further find a moderate anti-correlation between sheath density and local solar wind plasma speed upstream of the ICME shock. An empirical relation is derived connecting the ambient solar wind speed with sheath and leading edge density. We provide constraints to these results in this paper. Conclusions. The average starting distance for actual sheath formation could be as close as 0.06 au. The early strong ME expansion quickly ceases with distance from the Sun and might lead to a dominance in the sheath density between 0.09 and 0.28 au. The leading edge can be understood as a separate structure of compressed ambient solar wind directly ahead of the ME and is likely the bright leading edge of CMEs often seen in coronagraph images. The results allow for better interpretation of ICME evolution and possibly the observed mass increase due to enlargement of the sheath material. The empirical relation between sheath and leading edge density and ambient solar wind speed can be used for more detailed modeling of ICME evolution in the inner heliosphere.

WR 63: a multiple system (O+O) + WR?

ABSTRACT The spectrum of the Wolf–Rayet (WR) star WR 63 contains spectral lines of two different O stars that show regular radial velocity (RV) variations with amplitudes of ∼160 and ∼225 km s−1 on a ∼4.03 d period. The light curve shows two narrow eclipses that are 0.2 mag deep on the same period as the RV changes. On the other hand, our data show no significant RV variations for the WR spectral lines. Those findings are compatible with WR 63 being a triple system composed of two non-interacting late-O stars orbiting a WR star on a period longer than 1000 d. The amplitude of the WR spectral line-profile variability reaches 7–8 per cent of the line intensity and seems related to a 0.04 mag periodic photometric variation. Large wind density structures are a possible origin for this variability, but our data are not sufficient to verify this. Our analysis shows that, should the three stars be bound, they would be coeval with an age of about 5.9 ± 1.4 Myr. The distance to the O stars is estimated to be $3.4\, \pm \, 0.5$ kpc. Their dynamical masses are 14.3 ± 0.1 and 10.3 ± 0.1 M⊙. Using rotating single-star evolutionary tracks, we estimate their initial masses to be 18 ± 2 and 16 ± 2 M⊙ for the primary and the secondary, respectively. Regular spectral monitoring is required in the future to detect RV variations of the WR star that would prove that it is gravitationally bound to the close O+OB system and to determine its mass.