Range Of Heliocentric Distances Research Articles

Evolution of coronal mass ejections with and without sheaths from the inner to the outer heliosphere: Statistical investigation for 1975 to 2022

Aims. This study covers a thorough statistical investigation of the evolution of interplanetary coronal mass ejections (ICMEs) with and without sheaths through a broad heliocentric distance and temporal range. The analysis treats the sheath and magnetic obstacle (MO) separately in order to gain more insight on their physical properties. In detail, we aim to unravel different characteristics of these structures occurring over the inner and outer heliosphere. Methods. The method is based on a large statistical sample of ICMEs probed over different distances in the heliosphere. For this, information about detection times for the sheath and MO from 13 individual ICME catalogs was collected and crosschecked. The time information was then combined into a main catalog that was used as the basis for the statistical investigation. The data analysis based on this catalog covers a large number of spacecraft missions, enabling in situ solar wind measurements from 1975 to 2022. This allowed us to study the differences between solar cycles. Results. All the structures under study (sheath, MO with and without sheath) show the biggest increase in size together with the largest decrease in density at a distance of ∼0.75 AU. At 1 AU, we found different sizes for MOs with and without a sheath, with the former being larger. Up to 1 AU, the upstream solar wind shows the strongest pileup close to the interface with the sheath. For larger distances, the pileup region seems to shift, and it recedes from that interface further into the upstream solar wind. This might refer to a change in the sheath formation mechanism (driven versus non-driven) with heliocentric distance, suggesting the relevance of the CME propagation and the expansion behavior in the outer heliosphere. A comparison to previous studies showed inconsistencies over the solar cycle, which makes more detailed studies necessary in order to fully understand the evolution of ICME structures.

Unraveling the dust activity of naked-eye comet C/2022 E3 (ZTF)

A morphological and photometric analysis of the naked-eye long-period comet C/2022 E3 (ZTF) before perihelion is presented in this study. The observation images taken by the Zwicky Transient Facility survey telescope from July 2022 to October 2022 show a gradually brightening dust coma and a tail with a clear structure. The morphology of the dust coma reveals nonsteady-state emission with an ejection velocity lower than 14 m s−1 for particles larger than 100 µm. According to the syndyne-synchrone analysis, dust particles larger than about 10 µm contribute significantly to the observed tail. The model simulations of the 10 October 2022 image suggest that the radii of large particles lingering near the nucleus range from 0.1 to 1 mm. Assuming that the nucleus of comet E3 is a homogeneous sphere with an albedo of 0.1, the photometry analysis sets the lower and upper limits of the nucleus radius to be 0.81 ± 0.07 km and 2.79 ± 0.01 km, respectively. The dust production rates increased continuously from 241 ± 3 kg s−1 in July to 476 ± 9 kg s−1 in October. The dependence of the ejection velocity v⊥ perpendicular to the orbital plane of comet E3 on the particle size a can be simplified as v⊥ ∝ a−1/2, which indicates that the dust emission is likely driven by gas. The water-production rate is inferred as ~368 ± 72 kg s−1 in October 2022, which is sustained by an equilibrium-sublimating area of 8.2 × 106 m2 at least. The comparative analysis of the characteristics of comet E3 with those of comets belonging to different types shows that the activity profile of long-period comet E3 surprisingly aligns more closely with those of short-period comets within a heliocentric distance range of about [1.7, 3.4] AU, where the images of comet E3 that we used in this study were taken.

Suprathermal H+ Pickup Ion Tails in the Outer Heliosphere

This study provides a detailed analysis of five distant interplanetary shocks observed by the Solar Wind Around Pluto instrument on board New Horizons, which exhibit the signature of a suprathermal H+ pickup ion (PUI) tail in the downstream distribution. These shocks were observed with a PUI data cadence of approximately 24 hr, covering a heliocentric distance range of 23.71–36.75 au. The shock compression ratio varies between approximately 1.4 and 3.2. The H+ PUI density and temperature show a gradual increase across the shock, while the H+ solar wind density shows erratic behavior without a distinct downstream compression. The H+ PUI cooling index variation across the shock displays different characteristics in each shock. This study demonstrates, for the first time, the variation of the number density of downstream H+ PUI tails with the shock compression ratio, revealing an increase in tail density with stronger shocks. Additionally, theoretical estimates of reflected PUI number densities derived from the electrostatic cross-shock potential agree very well with the observed H+ PUI tail densities for stronger shocks.

Solar Electron Beam—Langmuir Wave Interactions and How They Modify Solar Electron Beam Spectra: Solar Orbiter Observations of a Match Made in the Heliosphere

Solar Orbiter's four in situ instruments have recorded numerous energetic electron events at heliocentric distances between 0.5 and 1 au. We analyze energetic electron fluxes, spectra, pitch-angle distributions, associated Langmuir waves, and type III solar radio bursts for three events to understand what causes modifications in the electron flux and identify the origin and characteristics of features observed in the electron spectrum. We investigate what electron beam properties and solar wind conditions are associated with Langmuir wave growth and spectral breaks in the electron peak flux as a function of energy. We observe velocity dispersion and quasilinear relaxation in the electron flux caused by the resonant wave–particle interactions in the deca-keV range, at the energies at which we observe breaks in the electron spectrum, cotemporal with the local generation of Langmuir waves. We show, via the evolution of the electron flux at the time of the event, that these interactions are responsible for the spectral signatures observed around 10 and 50 keV, confirming the results of simulations by Kontar and Reid. These signatures are independent of pitch-angle scattering. Our findings highlight the importance of using overlapping FOVs when working with data from different sensors. In this work, we exploit observations from all in situ instruments to address, for the first time, how the energetic electron flux is modified by the beam–plasma interactions and results in specific feature appearing in the local spectrum. Our results, corroborated with numerical simulations, can be extended to a wider range of heliocentric distances.

Properties Underlying the Variation of the Magnetic Field Spectral Index in the Inner Solar Wind

Using data from orbits 1−11 of the Parker Solar Probe mission, the magnetic field spectral index was measured across a range of heliocentric distances. The previously observed transition between a value of −5/3 far from the Sun and a value of −3/2 close to the Sun was recovered, with the transition occurring at around 50 R ⊙ and the index saturating at −3/2 as the Sun is approached. A statistical analysis was performed to separate the variation of the index on distance from its dependence on other parameters of the solar wind that are plausibly responsible for the transition, including the cross helicity, residual energy, turbulence age, and magnitude of magnetic fluctuations. Of all parameters considered, the cross helicity was found to be by far the strongest candidate for the underlying variable responsible. The velocity spectral index was also measured and found to be consistent with −3/2 over the range of values of cross helicity measured. Possible explanations for the behavior of the indices are discussed, including the theorized different behavior of imbalanced, compared to balanced, turbulence.

Turbulence Properties of Interplanetary Coronal Mass Ejections in the Inner Heliosphere: Dependence on Proton Beta and Flux Rope Structure

Interplanetary coronal mass ejections (ICMEs) have low proton beta across a broad range of heliocentric distances and a magnetic flux rope structure at large scales, making them a unique environment for studying solar wind fluctuations. Power spectra of magnetic field fluctuations in 28 ICMEs observed between 0.25 and 0.95 au by Solar Orbiter and Parker Solar Probe have been examined. At large scales, the spectra were dominated by power contained in the flux ropes. Subtraction of the background flux rope fields increased the mean spectral index from −5/3 to −3/2 at kd i ≤ 10−3. Rope subtraction also revealed shorter correlation lengths in the magnetic field. The spectral index was typically near −5/3 in the inertial range at all radial distances regardless of rope subtraction and steepened to values consistently below −3 with transition to kinetic scales. The high-frequency break point terminating the inertial range evolved approximately linearly with radial distance and was closer in scale to the proton inertial length than the proton gyroscale, as expected for plasma at low proton beta. Magnetic compressibility at inertial scales did not show any significant correlation with radial distance, in contrast to the solar wind generally. In ICMEs, the distinctive spectral properties at injection scales appear mostly determined by the global flux rope structure while transition-kinetic properties are more influenced by the low proton beta; the intervening inertial range appears independent of both ICME features, indicative of a system-independent scaling of the turbulence.

Col-OSSOS: Evidence for a Compositional Gradient Inherited from the Protoplanetary Disk?

In the present-day Kuiper Belt, the number of compositional classes and the orbital distributions of these classes hold important cosmogonic implications for the solar system. The Colours of the Outer Solar System Origins Survey (Col-OSSOS) recently showed that the observed color distribution of small (H ⪆ 6) trans-Neptunian objects (TNOs) can be accounted for by the existence of only two composition classes, named BrightIR and FaintIR, where the range of colors in each class can be modeled as mixtures of two material end-members. Here, we combine the high-precision photometric measurements of Col-OSSOS with those of previous surveys to investigate the orbital distribution of the two color classes and reinterpret the known color–inclination correlation of TNOs in the context of the BrightIR/FaintIR taxonomy. In addition to confirming the previously reported distinct orbital distributions of these classes, we identify a trend of increasing orbital inclinations toward the bluer end of the optical and near-infrared color distribution of BrightIR objects. Using the output of numerical simulations investigating the orbital evolution of TNOs during their scattering phase with Neptune, we show that this trend could reflect a composition gradient in the early protoplanetary disk, in the range of heliocentric distances over which TNOs from the BrightIR class accreted. However, tensions between this interpretation and the existence of blue contaminants among cold classical TNOs, and possible alternative origins for the detected correlation, currently bear uncertainty on our proposed interpretation.

Boundary Treatment for the Subsonic/Alfvénic Inner Boundary at 2.5 R ⊙ in a Time-dependent 3D Magnetohydrodynamics Solar Wind Simulation Model

A new magnetohydrodynamics (MHD) simulation model of the global solar corona and solar wind is presented. The model covers the range of heliocentric distance from 2.5 solar radii, so that coronal mass ejections at the earliest phase near the Sun can be treated in the future. This model is constructed by introducing a characteristics-based boundary treatment to an existing heliosphere 3D MHD model. In tailoring a set of characteristic equations for this new model, we assume that the coronal magnetic field is open to interplanetary space and that the solar coronal plasma is flowing outward everywhere at 2.5 solar radii. The characteristic equations for the subsonic/Alfvénic inner boundary surface are satisfied by altering the plasma density and/or temperature to maintain a polytropic relationship. In this article, the details of the characteristics-based boundary treatment for the middle of the corona (named CharM) are provided. The quasi-steady states of the solar wind derived from simulations with various choices of a parameter in the boundary treatments are compared and examined. Although further improvements are needed, we apply the new boundary treatment to simulations for three Carrington rotation periods from the minimum to maximum phase of the solar activity cycle, and show that an optimal choice yields a reasonable quasi-steady state of the transonic/Alfvénic solar wind matching the specified subsonic/Alfvénic plasma speed at 2.5 R ⊙.

The Polytropic Behavior of Solar Wind Protons as Observed by the Ulysses Spacecraft during Solar Minimum

We analyze proton bulk parameters derived from Ulysses observations and investigate the polytropic behavior of solar wind protons over a wide range of heliocentric distances and latitudes. The large-scale variations of the proton density and temperature over heliocentric distance indicate that plasma protons are governed by subadiabatic processes (polytropic index γ < 5/3), if we assume protons with three effective kinetic degrees of freedom. From the correlation between the small-scale variations of the plasma density and temperature in selected subintervals, we derive a polytropic index γ ∼ 1.4 on average. Further examination shows that the polytropic index does not have an apparent dependence on the solar wind speed. This agrees with the results of previous analyses of solar wind protons at ∼1 au. We find that the polytropic index varies slightly over the range of the heliocentric distances and heliographic latitudes explored by Ulysses. We also show that the homogeneity of the plasma and the accuracy of the polytropic model applied to the data points vary over Ulysses' orbit. We compare our results with the results of previous studies that derive the polytropic index of solar wind ions within the heliosphere using observations from various spacecraft. We finally discuss the implications of our findings in terms of heating mechanisms and the effective degrees of freedom of the plasma protons.

Thermal Energy Budget of Electrons in the Inner Heliosphere: Parker Solar Probe Observations

We present an observational analysis of the electron thermal energy budget using data from Parker Solar Probe. We use the macroscopic moments, obtained from our fits to the measured electron distribution function, to evaluate the thermal energy budget based on the second moment of the Boltzmann equation. We separate contributions to the overall budget from reversible and irreversible processes. We find that an irreversible thermal energy source must be present in the inner heliosphere over the heliocentric distance range from 0.15 to 0.47 au. The divergence of the heat flux is positive at heliocentric distances below 0.33 au, while beyond 0.33 au, there is a measurable degradation of the heat flux. Expansion effects dominate the thermal energy budget below 0.3 au. Under our steady-state assumption, the free streaming of the electrons is not sufficient to explain the observed thermal energy density budget. We conjecture that the most likely driver for the required heating process is turbulence. Our results are consistent with the known nonadiabatic polytropic index of the electrons, which we measure as 1.18 in the explored range of heliocentric distances.

Small‐Scale Magnetic Holes in the Solar Wind Observed by Parker Solar Probe

AbstractThe small‐scale magnetic hole (SSMH), characterized by magnetic field depression, is a structure with the size in the order of proton gyro‐radius. SSMHs near the Earth or other planets have been widely observed in recent years. However, SSMHs in the solar wind near the Sun are rarely investigated due to mission constraints. In the present study, SSMHs in the pristine solar wind within a wide heliocentric distance range are analyzed based on the Parker Solar Probe (PSP) Mission measurements. A total of 2,416 SSMHs are successfully identified during the orbits of PSP from 2nd October, 2018, to 31st December, 2020, with an average occurrence rate of ∼5.8 events/day. The occurrence rate of SSMHs decreases from ∼29.5 to ∼0.6 events/day as the heliocentric distance R increases. The spatial scale of these SSMHs obeys a bi‐log‐normal distribution, with the median scale L ∼ 137 km (∼6 ρp, proton gyro‐radius). As interplanetary magnetic field Bave increases or R decreases, the upper limit of the spatial scale L corresponding to each bin extends to a larger value. The L corresponding to the maximum occurrence rate also increases when Bave increases and R decreases. Besides, the SSMHs tend to occur more frequently in the solar wind environment with weak Bave and high thermal pressure Pt. Our results shed light on the characteristics and the origin of SSMHs in the pristine solar wind.

Three‐Dimensional Structure of the Corona During WHPI Campaign Rotations CR‐2219 and CR‐2223

AbstractDifferential emission measure tomography (DEMT) and white light (WL) tomography were applied to study the three‐dimensional (3D) structure of the global solar corona for two Whole Heliosphere and Planetary Interactions campaign periods, Carrington rotations 2219 and 2223. With DEMT, Solar Dynamics Observatory/Atmospheric Imaging Assembly images were used to reconstruct the 3D coronal electron density and temperature in the range of heliocentric distance 1.02–1.25 R⊙. With WL tomography, Solar and Heliospheric Observatory/Large Angle and Spectrometric COronagraph‐C2 images were used to reconstruct the 3D electron density in the range of heliocentric distance 2.5–6.0 R⊙. The two periods were also simulated with the 3D‐magneto‐hydrodynamic Alfvén Wave Solar Model (AWSoM), and its results compared in detail with the reconstructions. The DEMT analysis reveals a 20% less dense and 20% hotter corona than for rotations corresponding to the solar cycle 23/24 deep minimum. The electron density and temperature of the AWSoM model agree with DEMT results within 10% and 20%, respectively, while its electron density overestimates results of WL tomography up to 75%. The slow (fast) component of the terminal wind speed of the model is found to be associated with field lines characterized by larger (smaller) values of the tomographic density and temperature at the coronal base. DEMT reconstructions reveal the coronal plasma to be ubiquitously characterized by temperature variability of up to ≈45% over spatial scales of order ∼104 km. Taking into account this level of fine‐structure by global models may be consequential for their predictions on wave propagation in the corona.

Modern Faraday Rotation Studies to Probe the Solar Wind

For decades, observations of Faraday rotation have provided unique insights into the plasma density and magnetic field structure of the solar wind. Faraday rotation (FR) is the rotation of the plane of polarization when linearly polarized radiation propagates through a magnetized plasma, such as the solar corona, coronal mass ejection (CME), or stream interaction region. FR measurements are very versatile: they provide a deeper understanding of the large-scale coronal magnetic field over a range of heliocentric distances (especially ≈1.5 to 20 R⊙) not typically accessible to in situ spacecraft observations; detection of small-timescale variations in FR can provide information on magnetic field fluctuations and magnetohydrodynamic wave activity; and measurement of differential FR can be used to detect electric currents. FR depends on the integrated product of the plasma density and the magnetic field component along the line of sight to the observer; historically, models have been used to distinguish between their contributions to FR. In the last two decades, though, new methods have been developed to complement FR observations with independent measurements of the plasma density based on the choice of background radio source: calculation of the dispersion measure (pulsars), measurement of Thomson scattering brightness (radio galaxies), and application of radio ranging and apparent-Doppler tracking (spacecraft). New methods and new technology now make it possible for FR observations of solar wind structures to return not only the magnitude of the magnetic field, but also the full vector orientation. In the case of a CME, discerning the internal magnetic flux rope structure is critical for space weather applications.

Inspection of 19 globular cluster candidates in the Galactic bulge with the VVV survey

Context. The census of the globular clusters (GCs) in the Milky Way is still a work in progress. The advent of new deep surveys has made it possible to discover many new star clusters both in the Galactic disk and bulge, but many of these new candidates have not yet been studied in detail, leaving a veil on their true physical nature. Aims. We explore the nature of 19 new GC candidates in the Galactic bulge by analysing their colour–magnitude diagrams (CMDs) in the near-infrared (NIR) using the VISTA Variables in the Via Láctea Survey (VVV) database. We estimate their main astrophysical parameters: reddening and extinction, distance, total luminosity, mean cluster proper motions (PMs), metallicity, and age. Methods. We obtain the cluster catalogues including the likely cluster members by applying a decontamination procedure on the observed CMDs based on the vector PM diagrams from VIRAC2. We adopt NIR reddening maps in order to calculate the reddening and extinction for each cluster, and then estimate the distance moduli and heliocentric distances. Metallicities and ages are evaluated by fitting theoretical stellar isochrones. We also calculate their luminosities in comparison with known Galactic GCs. Results. We estimate a wide reddening range of 0.25 ⩽ E(J − Ks)⩽2.0 mag and extinction 0.11 ⩽ AKs ⩽ 0.86 mag for the sample clusters, as expected in the bulge regions. The range of heliocentric distances is 6.8 ⩽ D ⩽ 11.4 kpc. This allows us to place these clusters between 0.56 and 3.25 kpc from the Galactic centre, assuming R⊙ = 8.2 kpc. Also, their PMs are kinematically similar to the typical motion of the Galactic bulge, apart from VVV-CL160, which shows different PMs. We also derive their metallicities and ages, finding −1.40⩽ [Fe/H] ⩽ 0.0 dex and t ≈ 8 − 13 Gyr respectively. The luminosities are calculated both in Ks- and V-bands, recovering −3.4 ⩽ MV ⩽ −7.5. We also examine the possible RR Lyrae members found in the cluster fields. Conclusions. Based on their positions, kinematics, metallicities, and ages, and comparing our results with the literature, we conclude that nine candidates are real GCs, seven need more observations to be fully confirmed as GCs, and three candidates are discarded as GCs and appear to be younger open clusters.

BepiColombo’s Cruise Phase: Unique Opportunity for Synergistic Observations

The investigation of multi-spacecraft coordinated observations during the cruise phase of BepiColombo (ESA/JAXA) are reported, with a particular emphasis on the recently launched missions, Solar Orbiter (ESA/NASA) and Parker Solar Probe (NASA). Despite some payload constraints, many instruments onboard BepiColombo are operating during its cruise phase simultaneously covering a wide range of heliocentric distances (0.28 AU–0.5 AU). Hence, the various spacecraft configurations and the combined in-situ and remote sensing measurements from the different spacecraft, offer unique opportunities for BepiColombo to be part of these unprecedented multipoint synergistic observations and for potential scientific studies in the inner heliosphere, even before its orbit insertion around Mercury in December 2025. The main goal of this report is to present the coordinated observation opportunities during the cruise phase of BepiColombo (excluding the planetary flybys). We summarize the identified science topics, the operational instruments, the method we have used to identify the windows of opportunity and discuss the planning of joint observations in the future.

Solar wind rotation rate and shear at coronal hole boundaries

Context.In situ measurements by several spacecraft have revealed that the solar wind is frequently perturbed by transient structures that have been interpreted as magnetic folds, jets, waves, and flux ropes that propagate rapidly away from the Sun over a large range of heliocentric distances. Parker Solar Probe (PSP), in particular, has detected very frequent rotations of the magnetic field vector at small heliocentric radial distances, accompanied by surprisingly large solar wind rotation rates. The physical origin of such magnetic field bends and switchbacks, the conditions for their survival across the interplanetary space, and their relation to solar wind rotation are yet to be clearly understood.Aims.We aim to characterise the global properties of the solar wind flows crossed by PSP, to relate those flows to the rotational state of the low solar corona, and to identify regions of the solar surface and corona that are likely to be sources of switchbacks and bends.Methods.We traced measured solar wind flows from the spacecraft position down to the surface of the Sun to identify their potential source regions, and used a global magneto-hydrodynamic model of the corona and solar wind to analyse the dynamical properties of those regions. We identify regions of the solar corona for which solar wind speed and rotational shear are important and long-lived that can be favourable to the development of magnetic deflections and to their propagation across extended heights in the solar wind.Results.We show that coronal rotation is highly structured, and that enhanced flow shear and magnetic field gradients develop near the boundaries between coronal holes and streamers, and around and above pseudo-streamers, even when such boundaries are aligned with the direction of solar rotation. The exact properties and amplitudes of the shears are a combined effect of the forces exerted by the rotation of the corona and of its magnetic topology. A large fraction of the switchbacks identified by PSP map back to these regions, both in terms of instantaneous magnetic field connectivity and of the trajectories of wind streams that reach the spacecraft.Conclusions.We conclude that these regions of strong shears are likely to leave an imprint on the solar wind over large distances and to increase the transverse speed variability in the slow solar wind. The simulations and connectivity analysis suggest they could be a source of the switchbacks and spikes observed by PSP.

No Activity among 13 Centaurs Discovered in the Pan-STARRS1 Detection Database

Abstract Centaurs are small bodies orbiting in the giant planet region that were scattered inward from their source populations beyond Neptune. Some members of the population display comet-like activity during their transition through the solar system, the source of which is not well understood. The range of heliocentric distances where the active Centaurs have been observed and their median lifetime in the region suggest that this activity is driven neither by water-ice sublimation nor entirely by supervolatiles. Here we present an observational and thermodynamical study of 13 Centaurs discovered in the Pan-STARRS1 detection database aimed at identifying and characterizing active objects beyond the orbit of Jupiter. We find no evidence of activity associated with any of our targets at the time of their observations with the Gemini North telescope in 2017 and 2018, or in archival data from 2013 to 2019. Upper limits on the possible volatile and dust production rates from our targets are 1–2 orders of magnitude lower than production rates in some known comets and are in agreement with values measured for other inactive Centaurs. Our numerical integrations show that the orbits of six of our targets evolved interior to r ∼ 15 au over the past 100,000 yr, where several possible processes could trigger sublimation and outgassing, but their apparent inactivity indicates that either their dust production is below our detection limit or the objects are dormant. Only one Centaur in our sample—2014 PQ70—experienced a sudden decrease in semimajor axis and perihelion distance attributed to the onset of activity for some previously known inactive Centaurs, and therefore it is the most likely candidate for any future outburst. This object should be a target of high interest for any further observational monitoring.

Measurement of the open magnetic flux in the inner heliosphere down to 0.13 AU

Context.Robustly interpreting sets of in situ spacecraft data of the heliospheric magnetic field (HMF) for the purpose of probing the total unsigned magnetic flux in the heliosphere is critical for constraining global coronal models as well as understanding the large scale structure of the heliosphere itself. The heliospheric flux (ΦH) is expected to be a spatially conserved quantity with a possible secular dependence on the solar cycle and equal to the measured radial component of the HMF weighted by the square of the measurement’s heliographic distance (BRR2). It is also expected to constitute a direct measurement of the total unsigned magnetic flux escaping the corona (Φopen). Previous work indicates that measurements of ΦHexceed the value predicted by standard coronal models (the “open flux problem”). However, the value of the open flux derived from in situ measurements remains uncertain because it depends on the method employed to derive it. Past derivations also pointed towards an increase in ΦHwith heliocentric distance, although this may also be related to its method of computation.Aims.In this work, we attempt to determine a more robust estimate of the heliospheric magnetic flux (ΦH) using data from the FIELDS instrument on board Parker Solar Probe (PSP), to analyse how susceptible it is to overestimation and a dependence on time and space, as well as considering how it compares to simple estimates of Φopenfrom potential field source surface (PFSS) models.Methods.We compared computations of the heliospheric magnetic flux using different methods of data processing on magnetic field data from PSP, STEREO A, and Wind. Measured radial trends in fluctuations and background magnetic structure were used to generate synthetic data to analyse their effect on the estimate ofBRR2. The resulting best estimates were computed as a function of time and space and then compared to estimates from PFSS models.Results.Radially varying fluctuations of the HMF vector as well as large-scale variations in the inclination of the Parker spiral angle are shown to have a non-trivial effect on the 1D distributions ofBRR2. This causes the standard statistical metrics of the mean and mode (the most probable values) to evolve with radius, independently of the central value about which the vector fluctuates. In particular, the mean systematically underestimates ΦHforR&lt; 0.8 AU and increases close to 1 AU. We attempt to mitigate for this by using the “Parker spiral method” of projecting the vector onto the background Parker spiral direction (which requires vector fluctuations to be evenly distributed about a central value). Even with this method, we find evidence of a small enhancement in flux close to 1 AU. The fraction of field which is locally inverted in a given time interval grows with radial distance from the Sun which remains a possible physical reason for this excess but is essentially negligible at PSP’s perihelia distances where the impact of fluctuations in general is also much reduced. The Parker spiral method (PSM) and most probable values converge close to the Sun. Our derived best estimate for the time interval studied is ~2.5−0.6+0.3nT AU2. To the extent probed by PSP, no strong dependence on latitude or longitude is apparent, although at 1 AU, the spread of measured values appears to grow at the highest latitudes. The best estimate of the heliospheric flux is significantly larger than estimates from PFSS models studied here, which predict values from 1.2–1.8 nT AU2, depending on the choice of magnetogram or source surface height.Conclusions.Of the methods for computing the heliospheric flux over a wide range of heliocentric distances using only magnetic field data considered in this work, the most robust choice is to use the PSM. The decay of fluctuations and weakening importance of local flux inversions at smaller heliocentric distances indicate that the measurement is most accurate close to the sun and that it is justified for us to consider that ΦH~ Φopenfor these measurements. The determined value is too high to be explained via PFSS models. Contemporary magnetohydrodynamic models with the same photospheric input are unlikely to close this gap. Therefore, the most likely solutions remain in improvements of coronal models, for example, through improved boundary conditions via the direct measurement of the photospheric field in the solar polar regions or through the inclusion of missing physical processes such as time-dependent or non-potential effects, which can produce a contribution to the open flux that is not rooted in obvious coronal holes.

A living catalog of stream interaction regions in the Parker Solar Probe era

Stream interaction regions (SIRs) and corotating interaction regions (CIRs) are important phenomena in heliospheric physics. These large-scale structures vary temporally and spatially, both in latitude and with radial distance. The additions of Parker Solar Probe (PSP) and Solar Orbiter have allowed for investigations into the radial evolution of these structures over a wide range of heliocentric distances for the first time since the Helios era. To better enable investigations of SIRs and CIRs within the inner heliosphere, we have developed a living catalog of SIR and CIR observations by Parker Solar Probe with corresponding observations by STEREO-A as well as ACE and Wind at 1 au. The methodology used for the identification of events and the generation of this catalog, the initial catalog of PSP observations spanning orbits one through five along with corresponding 1 au observations, and information on accessing the living catalog for future studies is described. This list of SIR and CIR events from PSP and corresponding observations from other heliophysics missions will enable case studies utilizing unique orbital arrangements, as well as aid in future statistical studies to further understand the properties and evolution of these structures.

Turbulent Proton Heating Rate in the Solar Wind from 5–45 R ⊙

Abstract Various remote sensing observations have been used so far to probe the turbulent properties of the solar wind. Using the recently reported density modulation indices that are derived using angular broadening observations of Crab Nebula during 1952–2013, we measured the solar wind proton heating using the kinetic Alfvén wave dispersion equation. The estimated heating rates vary from ≈1.58 × 10−14 to 1.01 × 10−8 erg cm−3 s−1 in the heliocentric distance range of 5–45 R ⊙. Further, we found that heating rates vary with the solar cycle in correlation with density modulation indices. The models derived using in situ measurements (for example, electron/proton density, temperature, and magnetic field) that the recently launched Parker Solar Probe observations (planned closest perihelia 9.86 R ⊙ from the center of the Sun) are useful in the estimation of the turbulent heating rate precisely. Further, we compared our heating rate estimates with the one derived using previously reported remote sensing and in situ observations.