Heliocentric Distance Research Articles

Constraints on the variable nature of the slow solar wind with the Wide-Field Imager on board the Parker Solar Probe

Context. The formation of the slow solar wind remains unclear as we lack a complete understanding of its transient outflows. Aims. In a previous work we analysed the white-light coronal brightness as a function of elongation and time from Wide-Field Imager (WISPR) observations on board the Parker Solar Probe (PSP) mission when PSP reached a minimum heliocentric distance of ≈28 R⊙. We found 4–5 transient outflows per day over a narrow wedge in the PSP orbital plane, which is close to the solar equatorial plane. However, the elongation versus time map (J-map) analysis supplied only lower limits on the number of released density structures due to the small spatial-scales of the transient outflows and line-of-sight integration effects. In this work we place constraints on the properties of slow solar wind transient mass release from the entire solar equatorial plane. Methods. We simulated the release and propagation of transient density structures in the solar equatorial plane for four scenarios: (1) periodic release in time and longitude with random speeds; (2) corotating release in longitude, periodic release in time with random speeds; (3) random release in longitude, periodic release in time and speed; and (4) random release in longitude, time, and speed. Results. The simulations were used in the construction of synthetic J-maps, which are similar to the observed J-map. The scenarios with periodic spatial and temporal releases are consistent with the observations for periods spanning 3°–45°longitude and 1–25 h. The four considered scenarios have similar ranges (35–45 for the minimum values and 96–127 for the maximum values) of released density structures per day from the solar equatorial plane and consequently from the streamer belt, given its proximity to the solar equatorial plane during the WISPR observation. Our results also predict that density structures with sizes in the range 2–8 R⊙ covering 1–20% of the perihelion could have been detectable by PSP in situ observations during that interval. Conclusions. Our estimates of the release rates of density structures from the streamer belt represent a first major step towards assessing their contribution to the slow solar wind mass budget and their potential connection with in situ detections of density structures by PSP.

What long-period comets tell us about the Oort Cloud

Context. The Oort Cloud is located in the farthest outskirts of the Solar System, extending to a heliocentric distance of several tens of thousands of au, and remains the last region of the Solar System where no object has been detected. Thus, all our knowledge of the Oort Cloud has been deduced from the observed long-period comets that are thought to originate from it. Aims. We aimed to retrieve valuable information that might be hidden in the orbital distributions of the observed long-period comets. Such information will allow us to impose constraints not only on the present shape of the Oort Cloud but also on its initial shape 4.5 Gyr ago. This has direct implications for the scenario proposed for its formation. Methods. We used two different databases of long-period comets. First, we calculated the distribution of orbital elements that might carry valuable information about the shape of the Oort Cloud. Then, we compared the distribution with that obtained from two synthetic samples of observable comets. These samples correspond to two considerably different initial configurations: one is a disk model, where we consider a swarm of comets with orbits aligned to the ecliptic plane and with a cometary perihelion close to the giant planets. The other is an isotropic model, where we consider a fully isotropic and thermalized initial distribution of comets. Results. The comparison revealed that the databases contained several features that were in better agreement with the disk model than with the isotropic model. The Oort Cloud contained an initial disk of objects with perihelia close to the planetary region of the Solar System and aphelia extending out to roughly 20 000 au. Some parts of this disk likely remain in the present Solar System. However, the fit to the disk model is poor. The discrepancy between the observational and synthetic results indicates that some dynamical processes in the current Oort Cloud were not included in either model. Conclusions. This initial shape of the Oort Cloud implies that planetary scattering was crucial during its formation. In addition, the fact that some dynamical features are still detec table 4.5 Gyr after the cloud formation imposes constraints on the role of exosolar effects, such as giant molecular clouds, Galactic tides, and the stellar cluster surrounding the Solar System at the time of its formation.

Discovery of a New Local Group Dwarf Galaxy Candidate in UNIONS: Boötes V

We present the discovery of Boötes V, a new ultra-faint dwarf galaxy (UFD) candidate. This satellite is detected as a resolved overdensity of stars during an ongoing search for new Local Group dwarf galaxy candidates in the UNIONS photometric data set. It has a physical half-light radius of pc, a V-band magnitude of −4.5 ± 0.4 mag, and resides at a heliocentric distance of approximately 100 kpc. We use Gaia DR3 astrometry to identify member stars, characterize the systemic proper motion, and confirm the reality of this faint stellar system. The brightest star in this system was followed up using Gemini GMOS-N long-slit spectroscopy and is measured to have a metallicity of [Fe/H] = −2.85 ± 0.10 dex and a heliocentric radial velocity of v r = 5.1 ± 13.4 km s−1. Boötes V is larger (in terms of scale radius), more distant, and more metal-poor than the vast majority of globular clusters. It is likely that Boötes V is an UFD, though future spectroscopic studies will be necessary to definitively classify this object.

A New Method of Frequency Fluctuation Estimation and IPS Processing Results Based on the Downlink Signal of Tianwen-1

The radio-occultation observations taken by Tianwen-1 are aiming to study the properties of solar wind. A new method of frequency fluctuation (FF) estimation is presented for processing the down-link signals of Tianwen-1 during the occultation period to study the properties of the coronal plasma at the heliocentric distances of 4.48–19 R ⊙. Because of low S/N as well as the phase fluctuation phenomena caused by solar activity, a Kalman based on polynomial prediction methods is proposed to avoid the phase locked loop loss lock. A new detrend method based on multi-level iteration correction is proposed to estimate Doppler shift to get more accurate power density spectra of FF in the low frequency region. The data analyze procedure is used to get the properties of the solar corona during the occultation. The method was finally verified at the point when the solar offset is 5.7 R ⊙, frequency tracking was successfully performed on data with a carrier-to-noise ratio of about 28 dBHz. The density spectra obtained by the improved method are basically the same when the frequency is greater than 2 mHz, the uncertainty in the result of the rms of the FF obtained by removing the trend term with different order polynomials is less than 3.3%. The data without eliminating interference show a large error for different detrending orders, which justifies the need for an improved approach. Finally, the frequency fluctuation results combined with the information on intensity fluctuation obtained by the new method are compared with the results of the integrated Space Weather Analysis system and theoretical formula, which verifies that the processing results in this paper have a certain degree of credibility.

Physical Properties of the Asteroid Belts from Brightness-limited Surveys

We introduce a general debiasing Bayesian formalism to brightness-limited surveys of solar system objects that constrains the statistical distribution of a population’s physical characteristics such as size, albedo, heliocentric distance, or others to account for the survey detections and measured properties such as brightness. We reanalyze the Dark Energy Camera search for main-belt asteroids of Peña et al. and fit a smooth double power-law size to its size distribution. Using only detections with reliable distances and accurate orbital information, we find no evidence for a significant difference in size distribution between the inner, middle, and outer belts for objects between 0.1 and 1 km in diameter, with q 2 = 2.27 ± 0.05 a good fit for the small-end slope. We show that the intrinsic number density of objects is consistent between these populations and is compatible with observational biases. As spectral classes are distributed unevenly throughout the main belt, these results support the ongoing collisional reprocessing of the asteroid belt for objects smaller than 5 km.

The Trans-Heliospheric Survey

Context.Though the solar wind is characterized by spatial and temporal variability across a wide range of scales, long-term averages of in situ measurements have revealed clear radial trends: changes in average values of basic plasma parameters (e.g., density, temperature, and speed) and a magnetic field with a distance from the Sun.Aims.To establish our current understanding of the solar wind's average expansion through the heliosphere, data from multiple spacecraft needed to be combined and standardized into a single dataset.Methods.In this study, data from twelve heliospheric and planetary spacecraft - Parker Solar Probe (PSP), Helios 1 and 2, Mariner 2 and 10, Ulysses, Cassini, Pioneer 10 and 11, New Horizons, and Voyager 1 and 2 - were compiled into a dataset spanning over three orders of magnitude in heliocentric distance. To avoid introducing artifacts into this composite dataset, special attention was given to the solar cycle, spacecraft heliocentric elevation, and instrument calibration.Results.The radial trend in each parameter was found to be generally well described by a power-law fit, though up to two break points were identified in each fit.Conclusions.These radial trends are publicly released here to benefit research groups in the validation of global heliospheric simulations and in the development of new deep-space missions such as Interstellar Probe.

On the Evolution of the Anisotropic Scaling of Magnetohydrodynamic Turbulence in the Inner Heliosphere

We analyze a merged Parker Solar Probe (PSP) and Solar Orbiter (SO) data set covering heliocentric distances 13 R ⊙ ≲ R ≲ 220 R ⊙ to investigate the radial evolution of power and spectral index anisotropy in the wavevector space of solar wind turbulence. Our results show that anisotropic signatures of turbulence display a distinct radial evolution when fast, V sw ≥ 400 km s−1, and slow, V sw ≤ 400 km s−1, wind streams are considered. The anisotropic properties of slow wind in Earth orbit are consistent with a “critically balanced” cascade, but both spectral index anisotropy and power anisotropy diminish with decreasing heliographic distance. Fast streams are observed to roughly retain their near-Sun anisotropic properties, with the observed spectral index and power anisotropies being more consistent with a “dynamically aligned” type of cascade, though the lack of extended fast wind intervals makes it difficult to accurately measure the anisotropic scaling. A high-resolution analysis during the first perihelion of PSP confirms the presence of two subranges within the inertial range, which may be associated with the transition from weak to strong turbulence. The transition occurs at κ d i ≈ 6 × 10−2 and signifies a shift from −5/3 to −2 and from −3/2 to −1.57 scaling in parallel and perpendicular spectra, respectively. Our results provide strong observational constraints for anisotropic theories of MHD turbulence in the solar wind.

Evolution of the Radial Size and Expansion of Coronal Mass Ejections Investigated by Combining Remote and In Situ Observations

A fundamental property of coronal mass ejections (CMEs) is their radial expansion, which determines the increase in the CME radial size and the decrease in the CME magnetic field strength as the CME propagates. CME radial expansion can be investigated either by using remote observations or by in situ measurements based on multiple spacecraft in radial conjunction. However, there have been only few case studies combining both remote and in situ observations. It is therefore unknown if the radial expansion in the corona estimated remotely is consistent with that estimated locally in the heliosphere. To address this question, we first select 22 CME events between the years 2010 and 2013, which were well observed by coronagraphs and by two or three spacecraft in radial conjunction. We use the graduated cylindrical shell model to estimate the radial size, radial expansion speed, and a measure of the dimensionless expansion parameter of CMEs in the corona. The same parameters and two additional measures of the radial-size increase and magnetic-field-strength decrease with heliocentric distance of CMEs based on in situ measurements are also calculated. For most of the events, the CME radial size estimated by remote observations is inconsistent with the in situ estimates. We further statistically analyze the correlations of these expansion parameters estimated using remote and in situ observations, and discuss the potential reasons for the inconsistencies and their implications for the CME space weather forecasting.

What Do Halo CMEs Tell Us about Solar Cycle 25?

It is known that the weak state of the heliosphere due to diminished solar activity in cycle 24 backreacted on coronal mass ejections (CMEs) to make them appear wider for a given speed. One of the consequences of the weak state of the heliosphere is that more CMEs appear as halo CMEs (HCMEs), and halos are formed at shorter heliocentric distances. Current predictions for the strength of solar cycle (SC) 25 range from half to twice the strength of SC 24. We compare the HCME occurrence rate and other properties during the rise phase of cycles 23, 24, and 25 to weigh in on the strength of SC 25. We find that HCME and solar wind properties in SC 25 are intermediate between SCs 23 and 24, but closer to SC 24. The HCME occurrence rate, normalized to the sunspot number, is higher in SCs 24 and 25 than in SC 23. The solar wind total pressure in SC 25 is ∼35% smaller than that in SC 23. Furthermore, the occurrence rates of high-energy solar energetic particle events and intense geomagnetic storms are well below the corresponding values in SC 23, but similar to those in SC 24. We conclude that cycle 25 is likely to be similar to or slightly stronger than cycle 24, in agreement with polar-field precursor methods for cycle 25 prediction.

Recurrent solar density transients in the slow wind observed with the Metis coronagraph

Aims We aim to investigate and characterize the morphology and dynamics of small-scale coronal plasma density inhomogeneities detected as brighter, denser features propagating outward through the solar corona in the visible-light images of the Metis coronagraph on board Solar Orbiter on February 22, 2021. Our main focus is on investigating their possible origin and contribution to the slow wind variability and dynamics and their dependence on coronal magnetic field configurations and structure. Methods. The method adopted is based on the computations of autocorrelation and cross-correlation functions applied to temporal and spatial series of total brightness as a function of the heliocentric distance and solar latitudes. Results. We find that the plasma density inhomogeneities studied here are small-scale structures with typical radial and transverse sizes, as projected on the plane of sky, on the order of 500 Mm and 40 Mm, respectively, and that they are up to 24 times brighter than the ambient solar wind. The brighter density structures exhibit longer lifetime and more stable shape and dimensions as they travel toward the outer edge of the field of view. The enhanced density structures are ejected with a most probable cadence of about 80 min at or below the inner edge of the Metis field of view (within 3.1 R⊙–5.7 R⊙ at the time of observations) in a wide latitudinal region corresponding to the site of a complex web of separatrix and quasi-separatrix layers, as resulting from the simulated magnetohydrodynamic configuration of the west limb of the solar corona. Some of the moving density enhancements clearly show morphological characteristics compatible with the switchback phenomenon, supporting the results indicating that the switchbacks occur at the coronal level. The enhanced density structures were ejected into the ambient slow wind with a mean velocity of about 240 ± 40 km s−1, which is significantly higher than that deduced for the ambient solar wind on the basis of previous Metis observations during the solar minimum of cycle 24. The absence of acceleration observed across the coronagraph field of view suggests that the ejected plasmoids are progressively reaching the expansion rate of the ambient wind. Conclusions. The results suggest that the quasi-periodic enhanced-density plasmoids might be the consequence of reconnection phenomena occurring in the complex web of the separatrix and quasi-separatrix layers present in the solar corona. Moreover, the structural characteristics of some of the detected plasmoids are in favor of the presence of switchbacks that originate during interchange reconnection processes occurring at or below 3 R⊙ in the S-web. The speed of the plasma ejected in the reconnection process is higher than that of the ambient slow solar wind and is likely to be related to the energy involved in the process generating the propagating structures.

Slow Solar Wind Connection Science during Solar Orbiter’s First Close Perihelion Passage

The Slow Solar Wind Connection Solar Orbiter Observing Plan (Slow Wind SOOP) was developed to utilize the extensive suite of remote-sensing and in situ instruments on board the ESA/NASA Solar Orbiter mission to answer significant outstanding questions regarding the origin and formation of the slow solar wind. The Slow Wind SOOP was designed to link remote-sensing and in situ measurements of slow wind originating at open–closed magnetic field boundaries. The SOOP ran just prior to Solar Orbiter’s first close perihelion passage during two remote-sensing windows (RSW1 and RSW2) between 2022 March 3–6 and 2022 March 17–22, while Solar Orbiter was at respective heliocentric distances of 0.55–0.51 and 0.38–0.34 au from the Sun. Coordinated observation campaigns were also conducted by Hinode and IRIS. The magnetic connectivity tool was used, along with low-latency in situ data and full-disk remote-sensing observations, to guide the target pointing of Solar Orbiter. Solar Orbiter targeted an active region complex during RSW1, the boundary of a coronal hole, and the periphery of a decayed active region during RSW2. Postobservation analysis using the magnetic connectivity tool, along with in situ measurements from MAG and SWA/PAS, showed that slow solar wind originating from two out of three of the target regions arrived at the spacecraft with velocities between ∼210 and 600 km s−1. The Slow Wind SOOP, despite presenting many challenges, was very successful, providing a blueprint for planning future observation campaigns that rely on the magnetic connectivity of Solar Orbiter.

Interplanetary Shocks between 0.3 and 1.0 au: Helios 1 and 2 Observations

The Helios 1 (H1) and Helios 2 (H2) spacecraft measured the solar winds at a distance between ∼0.3 and 1.0 au from the Sun. With increasing heliocentric distance (r h), the plasma speed is found to increase at ∼34–40 km s−1 au−1 and the density exhibits a sharper fall () compared to the magnetic field magnitude () and the temperature (). Using all available solar wind plasma and magnetic field measurements, we identified 68 and 39 fast interplanetary shocks encountered by H1 and H2, respectively. The overwhelming majority (85%) of the shocks are found to be driven by interplanetary coronal mass ejections (ICMEs). While the two spacecraft encountered more than 73 solar wind high-speed streams (HSSs), only ∼22% had shocks at the boundaries of corotating interaction regions (CIRs) formed by the HSSs. All of the ICME shocks were found to be fast forward (FF) shocks; only four of the CIR shocks were fast reverse shocks. Among all ICME FF shocks (CIR FF shocks), 60% (75%) are quasi-perpendicular with shock normal angles (θ Bn) ≥ 45° relative to the upstream ambient magnetic field, and 40% (25%) are quasi-parallel (θ Bn < 45°). No radial dependences were found in FF shock normal angle and speed. The FF shock Mach number (M ms), magnetic field, and plasma compression ratios are found to increase with increasing r h at the rates of 0.72, 0.89, and 0.98 au−1, respectively. On average, ICME FF shocks are found to be considerably faster (∼20%) and stronger (with ∼28% higher M ms) than CIR FF shocks.

Wavefront error of PHI/HRT on Solar Orbiter at various heliocentric distances

Aims.We use wavefront sensing to characterise the image quality of the High Resolution Telescope (HRT) of the Polarimetric and Helioseismic Imager (SO/PHI) data products during the second remote sensing window of the Solar Orbiter (SO) nominal mission phase. Our ultimate aims are to reconstruct the HRT data by deconvolving with the HRT point spread function (PSF) and to correct for the effects of optical aberrations on the data.Methods.We use a pair of focused–defocused images to compute the wavefront error and derive the PSF of HRT by means of a phase diversity (PD) analysis.Results.The wavefront error of HRT depends on the orbital distance of SO to the Sun. At distances &gt; 0.5 au, the wavefront error is small, and stems dominantly from the inherent optical properties of HRT. At distances &lt; 0.5 au, the thermo-optical effect of the Heat Rejection Entrance Window (HREW) becomes noticeable. We develop an interpolation scheme for the wavefront error that depends on the thermal variation of the HREW with the distance of SO to the Sun. We also introduce a new level of image reconstruction, termed ‘aberration correction’, which is designed to reduce the noise caused by image deconvolution while removing the aberrations caused by the HREW.Conclusions.The computed PSF via phase diversity significantly reduces the degradation caused by the HREW in the near-perihelion HRT data. In addition, the aberration correction increases the noise by a factor of only 1.45 compared to the factor of 3 increase that results from the usual PD reconstructions.

Sapaki: Galactic O3If* star possibly born in isolation

Context. The study of high-mass stars found to be isolated in the field of the Milky Way may help to probe the feasibility of the core-accretion mechanism in the case of massive star formation. The existence of truly isolated stars may efficiently probe the possibility that individual massive stars can be born in isolation. Aims. We observed WR67a (hereafter Sapaki), an O3If* star that appears to be isolated close to the center of a well-developed giant cavity that is aptly traced by 8.0 μm hot dust emission. Methods. We acquired medium-resolution (R = 4100) and moderate signal-to-noise (S/N = 95 at 4500 Å) spectra for Sapaki in the range of 3800 − 10 500 Å with the Magellan Echellette (MagE) at Las Campanas Observatory. We computed the line-of-sight total extinctions. Additionally, we restricted its heliocentric distance by using a range of different estimators. Moreover, we measured its radial velocity from several lines in its spectrum. Finally, we analyzed its proper motions from Gaia to examine its possible runaway status. Results. The star has been classified as having the spectral type O3If* given its resemblance to standard examples of the class. In addition, we found that Sapaki is highly obscured, reaching a line-of-sight extinction value of AV = 7.87. We estimated the heliocentric distance to be in the range of d = 4 − 7 kpc. We also estimated its radial velocity to be Vr = −34.2 ± 15.6 km s−1. We may also discard its runaway status solely based on its 2D kinematics. Furthermore, by analyzing proper motions and parallaxes provided by Gaia, we found only one other star with compatible measurements. Conclusions. Given its apparent non-runaway status and the absence of clustering, Sapaki appears to be a solid candidate for isolated high-mass star formation in the Milky Way.

Orbital eccentricity and inclination distribution of main-belt asteroids—The Statistical model revisited

The distribution and evolution of the Main-belt Asteroid (MBA) orbital eccentricity and inclination provide clues for a deeper understanding of the dynamic evolution of our solar system. This paper presents an analysis of the orbital eccentricity and inclination of 460,000 known MBAs and fits MBA eccentricities and inclinations to a distribution model and further onto a dynamic model. We find that outer-belt MBAs fully fit a Rayleigh distribution; the orbital inclination grouped by the absolute magnitude generally not fully conformed to a Rayleigh distribution. Three evolutionary models of the MBAs were built; Model 1 follows the assumptions of a Rayleigh distribution and Model 2 follows the assumptions of a 2D Cauchy distribution; for the only orbital inclination, a Gaussian distribution was assumed as Model 3. For eccentricity, the distribution mode parameter fits by about 0.074∼0.086 in Model 1 and about 0.03 in Model 2 for the cases grouped by the absolute magnitude, the excitation parameter decreases as the magnitude increases. For the cases grouped by the heliocentric distance, the Model 1 fits the samples better than Model 2; the outer belt also fits Model 1 well, and its fit goodness is about 0.21. For inclination, when the samples were grouped by the absolute magnitude, the distribution mode and excitation parameters were consistent as well. Model 3 overall fits the data better than Models 1 and 2. In summary, the MBA excitation mechanism appears basically consistent with all models when the MBAs were classified by absolute magnitude for both eccentricity and inclination. The cases grouped by absolute magnitude can be fit well in Model 1 for eccentricities and Model 3 for inclinations. When the samples classified by heliocentric distance, the eccentricity and inclination distribution mode was correlated with heliocentric distance, and the influence of excitation was highly related to that distance as well.

The Evolution of the 1/f Range within a Single Fast-solar-wind Stream between 17.4 and 45.7 Solar Radii

The power spectrum of magnetic field fluctuations in the fast solar wind (V SW > 500 km s−1) at magnetohydrodynamic scales is characterized by two different power laws on either side of a break frequency f b. The low-frequency range at frequencies f smaller than f b is often viewed as the energy reservoir that feeds the turbulent cascade at f > f b. At heliocentric distances r exceeding 60 solar radii (R s), the power spectrum often has a 1/f scaling at f < f b, i.e., the spectral index is close to −1. In this study, measurements from the Parker Solar Probe's Encounter 10 with the Sun are used to investigate the evolution of the magnetic field power spectrum at f < f b at r < 60 R s during a fast radial scan of a single fast-solar-wind stream. We find that the spectral index in the low-frequency part of the spectrum decreases from approximately −0.61 to −0.94 as r increases from 17.4 to 45.7 R s. Our results suggest that the 1/f spectrum that is often seen at large r in the fast solar wind is not produced at the Sun, but instead develops dynamically as the wind expands outward from the corona into the interplanetary medium.

Coma environment of comet C/2017 K2 around the water ice sublimation boundary observed with VLT/MUSE

We report a new imaging spectroscopic observation of Oort cloud comet C/2017 K2 (hereafter K2) with the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope on its way to perihelion at 2.53 au, around a heliocentric distance where H2O ice begins to play a key role in comet activation. Normalized reflectances over 6500–8500 Å for its inner (cometocentric distance ρ ≈ 103 km) and outer (ρ ≈ 2 × 104 km) comae are 9.7 ± 0.5 and 7.2 ± 0.3 % (103 Å)−1, respectively, the latter being consistent with the slope observed when the comet was beyond the orbit of Saturn. The dust coma of K2 at the time of observation appears to contain three distinct populations: millimeter-sized chunks prevailing at ρ ≲ 103 km; a 105 km steady-state dust envelope; and fresh anti-sunward jet particles. The dust chunks dominate the continuum signal and are distributed over a similar radial distance scale as the coma region with redder dust than nearby. They also appear to be co-spatial with OI1D, suggesting that the chunks may accommodate H2O ice with a fraction (≳1%) of refractory materials. The jet particles do not colocate with any gas species detected. The outer coma spectrum contains three significant emissions from C2(0,0) Swan band, OI1D, and CN(1,0) red band, with an overall deficiency in NH2. Assuming that all OI1D flux results from H2O dissociation, we compute an upper limit on the water production rate QH2O of ~7 × 1028 molec s−1 (with an uncertainty of a factor of two). The production ratio log[QC2/QCN] of K2 suggests that the comet has a typical carbon chain composition, with the value potentially changing with distance from the Sun. Our observations suggest that dust chunks (&gt;0.1 mm) containing water ice and near K2’s nucleus emitted beyond 4 au may be responsible for its very low gas rotational temperature and the discrepancy between its optical and infrared lights reported at similar heliocentric distances.

Langmuir waves associated with magnetic holes in the solar wind

Context.Langmuir waves (electrostatic waves near the electron plasma frequency) are often observed in the solar wind and may play a role in the energy dissipation of electrons. The largest amplitude Langmuir waves are typically associated with type II and III solar radio bursts and planetary foreshocks. In addition, Langmuir waves not related to radio bursts occur in the solar wind, but their source is not well understood. Langmuir waves have been observed inside isolated magnetic holes, suggesting that magnetic holes play an important role in the generation of Langmuir waves.Aims.We provide the statistical distribution of Langmuir waves in the solar wind at different heliocentric distances. In particular, we investigate the relationship between magnetic holes and Langmuir waves. We identify possible source regions of Langmuir waves in the solar wind, other than radio bursts, by analyzing the local plasma conditions.Methods.We analyzed data from Solar Orbiter’s Radio and Plasma Waves (RPW) and Magnetometer (MAG) instruments. We used the triggered electric field snapshots and onboard statistical data (STAT) of the Time Domain Sampler (TDS) of RPW to identify Langmuir waves and investigate their properties. The plasma densities were derived from the spacecraft potential estimated by RPW. The MAG data were used to monitor the background magnetic field and detect magnetic holes, which are defined as regions with an isolated decrease in |B| of 50% or more compared to the background level. The statistical analysis was performed on data from 2020 to 2021, comprising heliocentric distances between 0.5 AU and 1 AU.Results.We show that 78% of the Langmuir waves in the solar wind not connected to radio bursts occur in regions of local magnetic field depletions, including the regions classified as isolated magnetic holes. We also show that the Langmuir waves occur more frequently inside magnetic holes than in any other region in the solar wind, which indicates that magnetic holes are important source regions of solar wind Langmuir waves. We find that Langmuir waves associated with magnetic holes in the solar wind typically have lower amplitudes than those associated with radio bursts.

Solar activity relations in energetic electron events measured by the MESSENGER mission

Aims. We perform a statistical study of the relations between the properties of solar energetic electron (SEE) events measured by the MESSENGER mission from 2010 to 2015 and the parameters of the respective parent solar activity phenomena in order to identify the potential correlations between them. During the time of analysis, the MESSENGER heliocentric distance varied between 0.31 and 0.47 au. Methods. We used a published list of 61 SEE events measured by MESSENGER, which includes information on the near-relativistic electron peak intensities, the peak-intensity energy spectral indices, and the measured X-ray peak intensity of the flares related to the SEE events. Taking advantage of multi-viewpoint remote-sensing observations, we reconstructed, whenever possible, the associated coronal mass ejections (CMEs) and shock waves; and we determined the three-dimensional (3D) properties (location, speed, and width) of the CMEs and the maximum speed of the 3D CME-driven shocks in the corona. We used different methods (Spearman, Pearson, and a Bayesian approach, namely the Kelly method to linear regression) to estimate the correlation coefficients between the flare intensity, maximum speed at the apex of the CME-driven shock, CME speed at the apex, and CME width with the electron peak intensities and with the energy spectral indices. In this statistical study, we considered and addressed the limitations of the particle instrument on board MESSENGER (elevated background intensity level, anti-Sun pointing). Results. There is an asymmetry to the east in the range of connection angles (CAs) for which the SEE events present the highest peak intensities, where the CA is the longitudinal separation between the footpoint of the magnetic field connecting to the spacecraft and the flare location. Based on this asymmetry, we define a subsample of well-connected events as when −65° ≤ CA ≤ +33°. For the well-connected sample, we find moderate to strong correlations between the near-relativistic electron peak intensity and the 3D CME-driven shock maximum speed at the apex (Spearman: cc = 0.53 ± 0.05; Pearson: cc = 0.65 ± 0.04; Kelly: cc = 0.87 ± 0.20), the flare peak intensity (Spearman: cc = 0.63 ± 0.03; Pearson: cc = 0.59 ± 0.03; Kelly: cc = 0.74 ± 0.30), and the 3D CME speed at the apex (Spearman: cc = 0.50 ± 0.04; Pearson: cc = 0.46 ± 0.03; Kelly: cc = 0.60 ± 0.39). When including poorly connected events (full sample), the relations between the peak intensities and the solar-activity phenomena are blurred, showing lower correlation coefficients. Conclusions. Based on the comparison of the correlation coefficients presented in this study using near 0.4 au data, (1) both flare and shock-related processes may contribute to the acceleration of near relativistic electrons in large SEE events, in agreement with previous studies based on near 1 au data; and (2) the maximum speed of the CME-driven shock is a better parameter to investigate particle-acceleration-related mechanisms than the average CME speed, as suggested by the stronger correlation with the SEE peak intensities.

Particle-in-cell modelling of comet 67P/Churyumov-Gerasimenko

Context.Sufficiently far from the Sun, all comets go through a phase of low activity. Rosetta observations at large heliocentric distances of approximately 3 au showed that the plasma at a low-activity comet is affected by both steady state electric fields and low-frequency waves.Aims.Our goal is to provide a model for the electric fields in the inner coma at a low-activity comet and to simulate waves and field structures farther away from the nucleus.Methods.We compare analytical models for the convective, ambipolar, and polarisation electric fields to the results of an electrostatic particle-in-cell simulation of a scaled-down low-activity comet.Results.We find good agreement between the steady state field model and the simulation results close to the nucleus. At larger cometocentric distances, waves dominate the electric field. These waves are interpreted as the scaled-down electrostatic limit of the previously observed singing comet waves. The comet ion density is not spherically symmetric.Conclusions.Low-activity comets can be modelled using electrostatic particle-in-cell simulations of a scaled-down system. Outside the innermost part of the coma (r≳ 40 km), the plasma is not spherically symmetric and the electric field is dominated by waves.