Fast Solar Wind Research Articles

Linear Stability in the Inner Heliosphere: Helios Re-evaluated

Abstract Wave–particle instabilities driven by departures from local thermodynamic equilibrium have been conjectured to play a role in governing solar wind dynamics. We calculate the statistical variation of linear stability over a large subset of Helios I and II observations of the fast solar wind using a numerical evaluation of the Nyquist stability criterion, accounting for multiple sources of free energy associated with protons and helium including temperature anisotropies and relative drifts. We find that 88% of the surveyed intervals are linearly unstable. The median growth rate of the unstable modes is within an order of magnitude of the turbulent transfer rate, fast enough to potentially impact the turbulent scale-to-scale energy transfer. This rate does not significantly change with radial distance, though the nature of the unstable modes, and which ion components are responsible for driving the instabilities, does vary. The effect of ion–ion collisions on stability is found to be significant; collisionally young wind is much more unstable than collisionally old wind, with very different kinds of instabilities present in the two kinds of wind.

Nonthermal Motions in a Polar Coronal Hole Measured with Hinode/EIS during an on-Orbit Partial Solar Eclipse on 2017 August 21

Abstract We have performed a spectroscopic observation over the south polar coronal hole (PCH) with the Hinode Extreme-ultraviolet (EUV) Imaging Spectrometer (EIS) during an on-orbit partial solar eclipse. In this partial eclipse, the Moon passed through the EIS observing area that was set in the south PCH at the height of 0.9–1.4 solar radii. Using the lunar occultation, we have corrected for the scattered light contamination from bright regions of the Sun that is present in the dark PCH emission line profiles. The nonthermal width of the corrected emission line profile in the PCH increases from the limb toward the high-altitude corona. It has also been confirmed that the nonthermal width tends to decrease beyond ∼1.2 solar radii. These results are consistent with the model in which outward-propagating Alfvén waves start being dissipated at ∼1.2 solar radii, as previously reported. The reduced energy within ∼1.4 solar radii contributes to atmospheric heating and the initial acceleration for the solar wind in the low corona. The remaining energy flux at 1.4 solar radii may be dissipated in the distant corona and is sufficient to provide the additional acceleration required to drive the fast solar wind.

The origin of slow Alfvénic solar wind at solar minimum

ABSTRACT Although the origins of slow solar wind are unclear, there is increasing evidence that at least some of it is released in a steady state on overexpanded coronal hole magnetic field lines. This type of slow wind has similar properties to the fast solar wind, including strongly Alfvénic fluctuations. In this study, a combination of proton, alpha particle, and electron measurements are used to investigate the kinetic properties of a single interval of slow Alfvénic wind at 0.35 au. It is shown that this slow Alfvénic interval is characterized by high alpha particle abundances, pronounced alpha–proton differential streaming, strong proton beams, and large alpha-to-proton temperature ratios. These are all features observed consistently in the fast solar wind, adding evidence that at least some Alfvénic slow solar wind also originates in coronal holes. Observed differences between speed, mass flux, and electron temperature between slow Alfvénic and fast winds are explained by differing magnetic field geometry in the lower corona.

Perpendicular Ion Heating by Cyclotron Resonant Dissipation of Turbulently Generated Kinetic Alfvén Waves in the Solar Wind

Abstract Solar wind ions are observed to be heated in the directions perpendicular to the large-scale magnetic field, with preferential heating given to heavy ions. In the solar corona, this heating may be responsible for the generation of the wind itself. It is likely that this heating results from the dissipation of plasma turbulence, but the specific kinetic mechanism that produces these preferential effects is not known. Solar wind turbulence at proton scales is often characterized in terms of highly oblique kinetic Alfvén waves (KAWs), which have been thought to dissipate through the Landau resonance and yield parallel heating. We show that the quasilinear resonant cyclotron interaction between KAWs and solar wind ions can actually produce perpendicular ion heating. We present an illustrative calculation of a steady, critically balanced spectrum of KAWs acting on homogeneous ion distributions with a plasma β = 0.1, representative of turbulent conditions in the fast solar wind near 20 R s . The KAWs are described here by a two-fluid dispersion relation. We find that thermal protons are strongly heated in the perpendicular direction within a typical quasilinear time of several thousand gyroperiods, which corresponds to only a few minutes at 20 R s . Alpha particles in the same fluctuation field are heated to similar perpendicular thermal speeds, equivalent to the greater than mass proportional perpendicular temperatures that are commonly observed. We discuss improvements to this simple model that may determine whether this mechanism can be responsible for the observed coronal and solar wind ion heating.

Validation of the Alfvén Wave Solar Atmosphere Model (AWSoM) with Observations from the Low Corona to 1 au

Abstract We perform a validation study of the latest version of the Alfvén Wave Solar atmosphere Model (AWSoM) within the Space Weather Modeling Framework. To do so, we compare the simulation results of the model with a comprehensive suite of observations for Carrington rotations representative of the solar minimum conditions extending from the solar corona to the heliosphere up to the Earth. In the low corona (r < 1.25 ), we compare with EUV images from both Solar-Terrestrial Relations Observatory-A/EUVI and Solar Dynamics Observatory/Atmospheric Imaging Assembly and to three-dimensional (3D) tomographic reconstructions of the electron temperature and density based on these same data. We also compare the model to tomographic reconstructions of the electron density from Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph observations (2.55 < r < 6.0 ). In the heliosphere, we compare model predictions of solar wind speed with velocity reconstructions from InterPlanetary Scintillation observations. For comparison with observations near the Earth, we use OMNI data. Our results show that the improved AWSoM model performs well in quantitative agreement with the observations between the inner corona and 1 au. The model now reproduces the fast solar wind speed in the polar regions. Near the Earth, our model shows good agreement with observations of solar wind velocity, proton temperature, and density. AWSoM offers an extensive application to study the solar corona and larger heliosphere in concert with current and future solar missions as well as being well suited for space weather predictions.

Assessing the Performance of EUHFORIA Modeling the Background Solar Wind

In order to address the growing need for more accurate space-weather predictions, a new model named EUHFORIA (EUropean Heliospheric FORecasting Information Asset) was recently developed. We present the first results of the performance assessment for the solar-wind modeling with EUHFORIA and identify possible limitations of its present setup. Using the basic EUHFORIA 1.0.4 model setup with the default input parameters, we modeled background solar wind (no coronal mass ejections) and compared the obtained results with Advanced Composition Explorer (ACE) in-situ measurements. For the purposes of statistical study we developed a technique of combining daily EUHFORIA runs into continuous time series. The combined time series were derived for the years 2008 (low solar activity) and 2012 (high solar activity), from which in-situ speed and density profiles were extracted. We find for the low-activity phase a better match between model results and observations compared to the high-activity time interval considered. The quality of the modeled solar-wind parameters is found to be rather variable. Therefore, to better understand the results obtained we also qualitatively inspected characteristics of coronal holes, i.e. the sources of the studied fast streams. We discuss how different characteristics of the coronal holes and input parameters to EUHFORIA influence the modeled fast solar wind, and suggest possibilities for the improvement of the model.

Features of Geophysical Manifestations of Solar Activity in August 2018 According to Muon Hodoscope URAGAN Data

Variations of the muon flux of cosmic rays (using data from the URAGAN muon hodoscope) and perturbations of the Earth’s magnetic field (OMNI database) were analyzed for August 2018, when there were almost no active regions on the solar disk and the number of sunspots was close to zero. The Kp and Dst geoefficiency coefficients were found to be elevated when the Earth entered the region of fast solar wind from coronal holes. Analysis of the non-stationarity factor C(t) for the muon count time series Nµ(t) shows that there are always elevated C(t) values that are ahead of the fast solar wind from coronal holes by (1–3) days. A predictor was also found in the sequence of values C(t) which is 1-day ahead of the geo-effective storm (SSC-power G3) of the solar CME eruption on August 20.

Small-Scale Plasma Fluctuations in Fast and Slow Solar Wind Streams

This study gives a comparison of characteristics of small-scale ion flux fluctuations based on measurements of the SPEKTR-R spacecraft in slow and fast solar wind streams. Fast quasi-stationary streams and fast disturbed streams, associated with transient phenomena in the solar corona, are considered separately. The use of high-time-resolution data from the Bright Monitor of Solar Wind (BMSW) instrument makes it possible to analyze plasma fluctuations in a wide range of scales, including small scales, where kinetic effects can make a significant contribution. It is shown in the study that the nature of small-scale fluctuations can significantly differ in fast streams with the same mean solar wind velocity but having a different nature.

The Lyman-alpha Solar Telescope (LST) for the ASO-S mission – III. data and potential diagnostics

The Lyman-alpha Solar Telescope (LST) is one of the three payloads onboard the Advanced Space-based Solar Observatory (ASO-S) mission. It aims at imaging the Sun from the disk center up to 2.5 R⊙ targeting solar eruptions, particularly coronal mass ejections (CMEs), solar flares, prominences/filaments and related phenomena, as well as the fast and slow solar wind. The most prominent speciality of LST is the simultaneous observation of the solar atmosphere in both Lyα and white light (WL) with high temporospatial resolution both on the solar disk and the inner corona. New observations in the Lyα line together with traditionalWL observations will provide us with many new insights into solar eruptions and solar wind. LST consists of a Solar Corona Imager (SCI) with a field of view (FOV) of 1.1 – 2.5R⊙, a Solar Disk Imager (SDI) and a full-disk White-light Solar Telescope (WST) with an identical FOV up to 1.2R⊙. SCI has a dual waveband in Lyα (121.6 ± 10 nm) and in WL (700 ± 40 nm), while SDI works in the Lyα waveband of 121.6 ± 7.5 nm and WST works in the violet narrow-band continuum of 360 ± 2.0 nm. To produce high quality science data, careful ground and in-flight calibrations are required. We present our methods for different calibrations including dark field correction, flat field correction, radiometry, instrumental polarization and optical geometry. Based on the data calibration, definitions of the data levels and processing procedures for the defined levels from raw data are described. Plasma physical diagnostics offer key ingredients to understand ejecta and plasma flows in the inner corona, as well as different features on the solar disk including flares, filaments, etc. Therefore, we are making efforts to develop various tools to detect the different features observed by LST, and then to derive their physical parameters, for example, the electron density and temperature of CMEs, the outflow velocity of the solar wind, and the hydrogen density and mass flows of prominences. Coordinated observations and data analyses with the coronagraphs onboard Solar Orbiter, PROBA-3, and Aditya are also briefly discussed.

Observation of Suprathermal Tails of He+ Pickup Ions across Solar Wind Compression Regions with STEREO PLASTIC

The presence of suprathermal tails of solar wind and pickup ions in interplanetary space has been widely observed, even during quiet times with no simultaneous observation of solar energetic particles. One of the persistent characteristics of these tails have been power law spectra of the velocity distribution function with a v−5 dependence in the solar wind reference frame and exponential fall-off at higher energies, but variations in the spectra including those of other species have also been observed. Several attempts to explain the formation of suprathermal tails during quiet times have been made, among them continuing acceleration by compressive fluctuations of the solar wind and the stochastic superposition of exponential, Gaussian, and variable power law spectra from diffusive shock, stochastic, and other acceleration processes. We find here that acceleration is effective within compression regions with and without shocks. In the context of a superposed epoch analysis of the evolution of He+ pickup ion distributions across compression regions, we report on a related study of He+ tails, using STEREO PLASTIC data from 2007 through 2014. Quiet times have been selected based on limiting energetic He fluxes above the tail energies and based on the tail fluxes themselves. We find that the suprathermal tail flux is dependent on the compression strength and varies substantially across the compression region. The strongest tails with spectra somewhat steeper than v−5 occur in the compressed fast solar wind, and they decrease rapidly with distance from the preceding and following compression into the rarefaction region, when using an unbiased data sample and applying a quiet time criterion based on higher energy ions. This may be consistent with the compressions being a potential source of the tails. When applying a quiet time criterion based on the observed tail fluxes, the temporal evolution disappears, possibly implicating a selection of the lower end of a Poisson distribution of tail count rates, rendering such a selection unusable for temporal evolution studies.

Two-fluid Numerical Simulations of the Origin of the Fast Solar Wind

Abstract With the use of our JOANNA code, which solves radiative equations for ion + electron and neutral fluids, we perform realistic 2.5D numerical simulations of plasma outflows associated with the solar granulation. These outflows exhibit physical quantities that are consistent, to the order of magnitude, with the observational findings for mass and energy losses in the upper chromosphere, transition region, and inner corona, and they may originate the fast solar wind.

Characteristics of a Gradual Filament Eruption and Subsequent CME Propagation in Relation to a Strong Geomagnetic Storm

Abstract An unexpected strong geomagnetic storm occurred on 2018 August 26, which was caused by a slow coronal mass ejection (CME) from a gradual eruption of a large quiet-region filament. We investigate the eruption and propagation characteristics of this CME in relation to the strong geomagnetic storm with remote sensing and in situ observations. Coronal magnetic fields around the filament are extrapolated and compared with extreme ultraviolet observations. We determine the propagation direction and tilt angle of the CME flux rope near the Sun using a graduated cylindrical shell (GCS) model and the Sun-to-Earth kinematics of the CME with wide-angle imaging observations from STEREO A. We reconstruct the flux-rope structure using a Grad–Shafranov technique based on the in situ measurements at the Earth and compare it with those from solar observations and the GCS results. Our conclusions are as follows: (1) the eruption of the filament was unusually slow and occurred in the regions with relatively low critical heights of the coronal field decay index; (2) the axis of the CME flux rope rotated in the corona as well as in interplanetary space, which tended to be aligned with the local heliospheric current sheet; (3) the CME was bracketed between slow and fast solar winds, which enhanced the magnetic field inside the CME at 1 au; (4) the geomagnetic storm was caused by the enhanced magnetic field and a southward orientation of the flux rope at 1 au from the rotation of the flux rope.

The associations of geomagnetic storms, fast solar wind, and stream interaction regions with cardiovascular characteristic in patients with acute coronary syndrome

Abstract It is shown the statistical associations between space weather pattern and humans’ cardiovascular system. We investigated the association between space weather events and cardiovascular characteristics of 4076 randomly selected patients with acute coronary syndrome (ACS) who were admitted for inpatient treatment in Kaunas city, Lithuania during 2000–2005. We hypothesized that days of the space weather events, 1–3 days after, and the period between two events, named as intersection days (1–3 days after the event, which coincided with 1–3 days before the event), might be associated with patients’ cardiovascular characteristics. The multivariate logistic regression was applied, and the patients’ risk was evaluated by odds ratio (OR), adjusting for age, sex, smoking status, the day of the week, and seasonality. During the intersection days of geomagnetic storms (GS), the risk of ACS increases in obese patients (OR=1.72, p = 0.008). The risk of ventricular fibrillation during admission was associated with stream interaction region (SIR) with a lag of 0–3 days (OR=1.44, p = 0.049) The risk of ACS in patients with chronic atrial fibrillation was associated with fast solar wind (FSW) (≥600 km/s) (lag 0–3 days, OR=1.39, p = 0.030) and with days of solar proton event (lag 0–3) going in conjunction with SIR (lag 0–3) (OR=2.06, p = 0.021). During days which were not assigned as GS with a lag of (−3 to 3) days, FSW (lag 0–3) was associated with the risk of ACS in patients with renal disease (OR=1.71, p = 0.008) and days of SIR – with the risk in patients with pulmonary disease (OR=1.53, p = 0.021). A SIR event, days between two space weather events, and FSW without GS may be associated with a risk to human health.

Storm-time mesoscale field-aligned currents and interplanetary parameters

Present paper studies Field aligned currents (FACs) estimated by employing Ampere's law to the magnetic field recorded by CHAMP satellite during 24 geomagnetic storms. Low-pass filtered FACs with a cutoff period of 20 s (scale size~150 km) are used to determine FAC range, which is defined as a peak-to-peak amplitude of FAC density. Thus we are considering only the strongest positive and negative FACs emerging either from Region 1, Region 2, Region 0, or substorm current wedge systems. It is known that the FACs significantly depend on the highly variable solar wind (SW) and interplanetary magnetic field (IMF) conditions and also on the processes internal to magnetospheric-ionospheric system such as substorm. The correlation analysis carried out here shows that sometimes the FAC range, correlates well with SymH, AsyH, AsyD, AL, am and Kp indices (>95% significance), but not always. The variation of the FAC range with magnetic local times shows distinctly different patterns during southward and northward IMF conditions, with peaks near dawn-dusk during southward IMF and near local noon-midnight during northward IMF. These results are in agreement with the earlier reports. However, the seasonal dependence reveals that the noon time peak is essentially associated with the summer season. We have determined a new parameter called ‘occurrence rate of FAC range ≥1 μA/m2’ and examined it under various solar wind and IMF conditions. It is found that the probability of FAC range ≥1 μA/m2 have a clear dependence on the clock angle, suggesting more frequent intensifications during southward IMF. Clear linear dependence on the cone angle demonstrates higher occurrence probability of FAC range ≥ 1 μA/m2 when the IMF is perpendicular to the Sun-Earth line (cone angle nearing 90°). All these results based on the newly defined parameters such as FAC range and probability of FAC range ≥1 μA/m2, for the storm time mesoscale FAC are consistent with the previous studies. The FAC ranges are found to have a linear dependence on the values of IMF BY, BYZ, BT and BZ, though saturation is apparent at higher values of the IMF parameters. FAC range shows distinctly different dependence for slow and fast solar wind, suggesting the importance of the composition and properties of SW in controlling the FAC strengths.

Solar wind charge exchange in cometary atmospheres

Context. Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet, mass-loading the solar wind through an effective conversion of fast light solar wind ions into slow heavy cometary ions. Aims. To understand these processes and place them in the context of a solar wind plasma interacting with a neutral atmosphere, numerical or analytical models are necessary. Inputs of these models, such as collision cross sections and chemistry, are crucial. Methods. Book-keeping and fitting of experimentally measured charge-changing and ionization cross sections of hydrogen and helium particles in a water gas are discussed, with emphasis on the low-energy/low-velocity range that is characteristic of solar wind bulk speeds (<20 keV u−1/2000 km s−1). Results. We provide polynomial fits for cross sections of charge-changing and ionization reactions, and list the experimental needs for future studies. To take into account the energy distribution of the solar wind, we calculated Maxwellian-averaged cross sections and fitted them with bivariate polynomials for solar wind temperatures ranging from 105 to 106 K (12–130 eV). Conclusions. Single- and double-electron captures by He2+ dominate at typical solar wind speeds. Correspondingly, single-electron capture by H+ and single-electron loss by H− dominate at these speeds, resulting in the production of energetic neutral atoms (ENAs). Ionization cross sections all peak at energies above 20 keV and are expected to play a moderate role in the total ion production. However, the effect of solar wind Maxwellian temperatures is found to be maximum for cross sections peaking at higher energies, suggesting that local heating at shock structures in cometary and planetary environments may favor processes previously thought to be negligible. This study is the first part in a series of three on charge exchange and ionization processes at comets, with a specific application to comet 67P/Churyumov-Gerasimenko and the Rosetta mission.

Dependence of 3D Self-correlation Level Contours on the Scales in the Inertial Range of Solar Wind Turbulence

Abstract The self-correlation level contours at the 1010 cm scale reveal a 3D isotropic feature in the slow solar wind and a quasi-anisotropic feature in the fast solar wind. However, the 1010 cm scale is approximately near the low-frequency break (outer scale of turbulence cascade), especially in the fast wind. How the self-correlation level contours behave with dependence on the scales in the inertial range of solar wind turbulence remains unknown. Here we present the 3D self-correlation function level contours and their dependence on the scales in the inertial range for the first time. We use data at 1 au from instruments on the Wind spacecraft in the period 2005–2018. We show the 3D isotropic self-correlation level contours of the magnetic field in the inertial range of both slow and fast solar wind turbulence. We also find that the self-correlation level contours of the velocity in the inertial range present 2D anisotropy with an elongation in the perpendicular direction and 2D isotropy in the plane perpendicular to the mean magnetic field. These results indicate differences between the magnetic field and the velocity, providing new clues to interpret the solar wind turbulence on the inertial scale.

Kappa-Maxwellian Electrons and Bi-Maxwellian Protons in a Two-fluid Model for Fast Solar Wind

Abstract Modeling fast solar wind based on kinetic theory is an important task for scientists. In this paper, we present a two-fluid model for fast solar wind with anisotropic Kappa-Maxwellian electrons and Bi-Maxwellian protons. In the simulation, the energy exchange between the plasma particles and low-frequency Alfvén waves is considered. A set of eleven coupled equations is derived by applying the zeroth- to fourth-order moments of the Vlasov equation and the modified electromagnetic Maxwell equations. A characteristic of the Kappa distribution (indicated by κ index) is explicit in the equation for the parallel component of the electron heat flux (parallel to the ambient magnetic field line) and differs from the equation derived for the proton heat flux due to the different nature of the distributions. Within the large κ index, the equations for the two-fluid model tend to the equations obtained by the Maxwellian distribution. Using an iterated Crank–Nicolson method, the coupled equations are numerically solved for the fast solar wind conditions. We show that at (0.3–1) au from the Sun, the electron density, components of temperature, and components of heat flux follow the power-law behavior. We also showed that near the Earth, the flow speed (electron or proton) increases with decreasing κ. We concluded that through applying the small κ index (the non-Maxwellian distribution), the extraordinary nature of the solar atmosphere, with its temperature of several million kelvin for electrons, has been captured.

Nonequilibrium Ionization Effects on Coronal Plasma Diagnostics and Elemental Abundance Measurements

Abstract Plasma diagnostics and elemental abundance measurements are crucial to help us understand the formation and dynamics of the solar wind. Here we use a theoretical solar wind model to study the effect of nonequilibrium ionization (NEI) on plasma diagnostic techniques applied to line intensities emitted by the fast solar wind. We find that NEI almost always changes the spectral line intensities with up to 120% difference for the lighter elements and for higher charge states of Fe even below 1.5 solar radii (R s ). The measured plasma density, temperature, and differential emission measure are only slightly affected by NEI. However, NEI significantly affects the first-ionization potential (FIP) bias and abundance ratio measurements, producing an error of up to a factor 4 at 1.5 R s for the Mg-to-Ne, Fe-to-S, and Ar-to-Fe ratios when EI is assumed. We conclude that it is very important to consider the NEI effect when spectral line intensities are synthesized and the FIP bias and elemental abundance are measured.

3D Feature of Self-correlation Level Contours at 1010 cm Scale in Solar Wind Turbulence

Abstract The self-correlation level contours at 1010 cm scale reveal a 2D isotropic feature in both the slow solar wind fluctuations and the fast solar wind fluctuations. However, this 2D isotropic feature is obtained based on the assumption of axisymmetry with respect to the mean magnetic field. Whether the self-correlation level contours are still 3D isotropic remains unknown. Here we perform for the first time a 3D self-correlation level contours analysis on the solar wind turbulence. We construct a 3D coordinate system based on the mean magnetic field direction and the maximum fluctuation direction identified by the minimum-variance analysis method. We use data with 1 hr intervals observed by WIND spacecraft from 2005 to 2018. We find, on one hand, in the slow solar wind, the self-correlation level contour surfaces for both the magnetic field and the velocity field are almost spherical, which indicates a 3D isotropic feature. On the other hand, there is a weak elongation in one of the perpendicular directions in the fast solar wind fluctuations. The 3D feature of the self-correlation level contours surfaces cannot be explained by the existing theory.

On the Fast Solar Wind Heating and Acceleration Processes: A Statistical Study Based on the UVCS Survey Data

Abstract The UltraViolet Coronagraph Spectrometer (UVCS) on board the SOlar and Heliospheric Observatory has almost continuously observed, throughout the whole solar cycle 23, the UV solar corona. This work addresses the first-ever statistical analysis of the daily UVCS observations, performed in the O vi channel, of the northern polar coronal hole, between 1.5 and 3 R ⊙, during the period of low solar activity from 1996 April to 1997 December. The study is based on the investigation, at different heights, of the correlation between the variance of the O vi 1031.92 Å spectral line and the O vi 1031.92, 1037.61 Å doublet intensity ratio, which are proxies of the kinetic temperature of the O5+ ions and of the speed of the oxygen component of the fast solar wind, respectively. This analysis allows the clear identification of the sonic point in polar coronal holes at the distance of 1.9 R ⊙. The results show that heat addition below the sonic point does not lead to an increase of the outflow speed. As a matter of fact, the coronal plasma is heated more efficiently in the subsonic region, while its acceleration occurs more effectively in the region of supersonic flow. So, within the panorama of the Parker Solar Probe and Solar Orbiter missions, the statistical analysis of the historical UVCS data appears to be very promising in providing unique clues to some still unsolved problems, as the coronal heating, in the solar corona.