Carrington Rotation Research Articles

Variations in differential rotation and meridional flow within the Sun’s surface shear layer 1996–2022

We measure differential rotation and meridional flow in the Sun’s surface shear layer by tracking the motions of the magnetic network seen in magnetograms from SOHO/MDI and SDO/HMI over solar cycles 23, 24, and the start of 25 (1996–2022). We examine the axisymmetric flows derived from 15 to 24 daily measurements averaged over individual 27-day Carrington rotations. Variations in the differential rotation include the equatorial torsional oscillation—cyclonic flows centered on the active latitudes with slower flows on the poleward sides of the active latitudes and faster flows equatorward. The fast flow band starts at ∼45° latitude during the declining phase of the previous cycle and drifts equatorward, terminating at the equator at about the time of cycle minimum. Variations in the differential rotation also include a polar oscillation above 45° with faster rotation at cycle maxima and slower rotation at cycle minima. The equatorial variations were stronger in cycle 24 than in cycle 23 but the polar variations were weaker. Variations in the meridional flow include a slowing of the poleward flow in the active latitudes during cycle rise and maximum and a speeding up of the poleward flow during cycle decline and minimum. The slowing in the active latitudes was less pronounced in cycle 24 than in cycle 23. Polar countercells (equatorward flow) extend from the poles down to ∼60° latitude from time to time (1996–2000 and 2016–2022 in the south and 2001–2011 and 2017–2022 in the north). Both axisymmetric flows vary in strength with depth. The rotation rate increases inward while the meridional flow weakens inward.

Direct discontinuous Galerkin method for potential magnetic field solutions

In this paper, we employ the direct discontinuous Galerkin (DDG) method for the first time to extrapolate the coronal potential magnetic field (PF) with the source surface (SS) and call the developed numerical model as the DDG-PFSS solver. In this solver, the Laplace’s equation is solved by means of the time-dependent method, i.e., introducing a pseudo-time term into the Laplace’s equation and changing the boundary value problem into the initial-boundary value problem. The steady-state solution of the initial-boundary value problem is the solution of the Laplace’s equation to be solved. This formulation facilitates the implementation of the DDG discretization. In order to validate the DDG-PFSS solver, we test a problem with the exact solution, which demonstrates the effectiveness and third-order accuracy of the solver. Then we apply it to the extrapolation for the coronal potential magnetic field. We use the integral GONG synoptic magnetogram of Carrington rotation (CR) 2060 as the boundary condition and achieve the global potential magnetic field solution by the DDG-PFSS solver. The numerical results such as the coronal holes and streamer belts derived from the DDG-PFSS solver are in good agreement with those obtained from the spherical harmonic expansion method. Also, based on the numerical magnetic field and Wang-Sheeley-Arge model, the obtained solar wind speed is found to basically capture the structures of the high- and low-speed streams observed at 1 AU. These results suggest that the DDG-PFSS solver can be seen as a contribution to the numerical methods for obtaining the global potential magnetic field solutions of the solar corona.

The Nonpotentiality of Steady-state Coronal Magnetic Field Derived with Time-relaxation Magnetohydrodynamics Simulations Using Helioseismic and Magnetic Imager Three-component Magnetic Field Data

The steady states of the coronal magnetic field obtained with the magnetohydrodynamic (MHD) time-relaxation simulation model are examined. Our electric-field-driven model can introduce the three components of the solar surface magnetic field data maps as the boundary values of an MHD simulation, without violating the divergence-free condition of the magnetic field. The magnetic field in the simulated steady-state solar corona exhibits substantial nonpotentiality in the closed-field streamers. A few choices are allowed in our model, such as the criteria for determining whether or not the horizontal components at the weak-field region are included. The initial magnetic field configuration can be arbitrarily determined. In this work, we examined the differences between the steady states obtained with the information on the horizontal components and with several choices of the simulation setting, and compared the new steady states with those obtained without using the horizontal magnetic field components. We found that nonpotential magnetic structures in the derived steady states well correspond to the observed solar filament structures during a selected period of Carrington Rotation 2106. The difference in the steady state with different boundary treatments is found to be large. The difference caused by the initial magnetic configuration is found to be small.

A Comparison of Global Magnetofrictional Simulations of the 2015 March 20 Solar Eclipse

The solar corona exhibits a wide range of phenomena, from highly non-potential objects such as solar filaments to near-potential structures such as the open magnetic flux. For any global model to be useful in space weather applications, the model must on a single day reproduce all of these phenomena in the same simulation, using the same set of coronal physics and parameters. The purpose of the present paper is to evaluate whether the evolving magnetofrictional model can achieve this goal. Twenty-eight separate simulations are analyzed, where each tries to reproduce both highly non-potential and near-potential phenomena observed in the solar corona on the same day. This day is chosen to be 2015 March 20, the date of the solar eclipse. The study evaluates how the cadence of bipole data, ideal or nonideal coronal physics, and the variety of helicity injection mechanisms affect the accuracy of the simulations. Results show that significantly better agreement arises when using Advective Flux Transport (AFT) synoptic maps to drive the simulations, as compared to 27-day Carrington rotation synoptic maps. Using the nonideal effect of hyperdiffusion leads to the worst agreement with all coronal phenomena. Alternatively, when running either ideal or ohmic diffusion simulations with helicity condensation or bipoles with a self-helicity, a good agreement with both on-disk and limb structures can be found. This suggests that future studies aiming to simulate the corona and reproduce multiple phenomena on a given day should use data products such as AFT and avoid using the nonideal physics of hyperdiffusion.

Comparing the Performance of a Solar Wind Model from the Sun to 1 au Using Real and Synthetic Magnetograms

The input of the solar wind models plays a significant role in accurate solar wind predictions at 1 au. This work introduces a synthetic magnetogram produced from a dynamo model as an input for magnetohydrodynamics simulations. We perform a quantitative study that compares the space weather modeling framework (SWMF) results for the observed and synthetic solar magnetogram input. For each case, we compare the results for extreme ultraviolet images and extract the simulation data along the Earth trajectory to compare with in situ observations. We initialize the SWMF using the real and synthetic magnetograms for a set of Carrington rotations within solar cycles 23 and 24. Our results help quantify the ability of dynamo models to be used as input to solar wind models and thus provide predictions for the solar wind at 1 au.

Solar Cycle Variability in Coronal Holes and Their Effects on Solar Wind Sources

AbstractVarious upstream spacecraft have now observed the solar wind conditions affecting the Earth since the 1970s, covering over four solar activity cycles. These measurements provide a long term picture of the related patterns in large scale incident plasma and magnetic field parameters of interest for both interpreting cycles in geospace effects, and understanding how the Sun controls our space environment. This paper focuses on the latter, in part to provide context at the start of the new solar cycle 25, and toward establishing connections between the 1 AU ecliptic solar wind behavior and the unprecedented near‐Sun measurements of heliospheric features on Parker Solar Probe and Solar Orbiter. Magnetograph data‐based potential field source surface models provide a basic picture of how the solar wind sources, including those that give rise to corotating high speed streams in the ecliptic, have changed since the beginning of the record of regular solar wind measurements. In particular, they suggest the contributions from low to mid latitude coronal holes dominate the observed cycles (21–24), especially the weaker cycles (23 and 24), impacting upstream measurement interpretations, modeling, and forecasting considerations. For example, recurring features are affected by differential rotation of the Sun's surface field, which through its effects on the corona, can produce solar wind streams reappearing at ∼25–30 days intervals instead of at the canonical 27.3 days Carrington rotation rate. In addition, the conditions that lead to the corotating stream structure that can dominate periods of low solar activity are seen to be more complicated than suggested by the simple concepts of early studies. The overall results illustrate where in the cycles well‐defined, long‐lived large scale structures can be expected, and the advantages of synoptic displays of 1 AU solar wind parameters for anticipating timings of recurring features.

SWASTi-SW: Space Weather Adaptive Simulation Framework for Solar Wind and Its Relevance to the Aditya-L1 Mission

Solar wind streams, acting as a background, govern the propagation of space weather drivers in the heliosphere, which induce geomagnetic storm activities. Therefore, predictions of the solar wind parameters are the core of space weather forecasts. This work presents an indigenous three-dimensional (3D) solar wind model (SWASTi-SW). This numerical framework for forecasting the ambient solar wind is based on a well-established scheme that uses a semiempirical coronal model and a physics-based inner heliospheric model. This study demonstrates a more generalized version of the Wang–Sheeley–Arge relation, which provides a speed profile input to the heliospheric domain. Line-of-sight observations of GONG and Helioseismic and Magnetic Imager magnetograms are used as inputs for the coronal model, which in turn provides the solar wind plasma properties at 0.1 au. These results are then used as an initial boundary condition for the magnetohydrodynamics model of the inner heliosphere to compute the solar wind properties up to 2.1 au. Along with the validation run for multiple Carrington rotations, the effect of variation of specific heat ratio and study of the stream interaction region (SIR) are also presented. This work showcases the multidirectional features of SIRs and provides synthetic measurements for potential observations from the Solar Wind Ion Spectrometer subsystem of the Aditya Solar wind Particle Experiment payload on board ISRO’s upcoming solar mission Aditya-L1.

A Parametric Study of Performance of Two Solar Wind Velocity Forecasting Models During 2006–2011

AbstractThere is an increasing need for the development of a robust space weather forecasting framework. State‐of‐the‐art MHD space weather forecasting frameworks are based upon the Potential Field Source Surface (PFSS) and Schatten Current Sheet (SCS) extrapolation models for the magnetic field using synoptic magnetograms. These models create a solar wind (SW) background for the simulations using empirical relations of Wang, Sheeley and Arge (WSA), at the inner boundary of heliosphere and have been used to simulate coronal mass ejections for specific cases in previous studies. Besides these MHD frameworks, the Heliospheric Upwind eXtrapolation (HUX) technique can extrapolate SW from inner heliospheric boundaries to L1 and can give a reliable estimate of the SW velocity at L1 comparable to MHD models but in a short computational time. We carried out an extensive parametric study of the performance of the Model1 (PFSS+WSA+HUX) and Model2 (PFSS+SCS+WSA+HUX) for SW velocity prediction at L1. We implemented this framework on 60 Carrington Rotations from CR2047 to CR2107 during 2006–2011, covering the descending and deep minimum phase of solar cycle (SC) 23, and the ascending phase of SC 24. Our results show an unexpected decrease in the performance of the framework during the deep minimum phase of cycle 23, which is attributed to the decrease in the observed coronal hole area. As SC 24 began, this decreasing trend vanished due to an increase in the coronal hole (CH) area at the low and mid‐latitudes, suggesting a good correlation between the performance of the framework and the variation in the CH area.

Implicit Solar Coronal Magnetohydrodynamic (MHD) Modeling with a Low-dissipation Hybridized AUSM-HLL Riemann Solver

In this paper, we develop a 3D implicit single-fluid magnetohydrodynamic (MHD) model to simulate the steady-state solar corona with a wide range of Mach numbers and low plasma β. We employ a low-dissipation advection upstream splitting method (AUSM) to calculate the convective flux in the regions of low Mach numbers for a high resolution, and hybridize the AUSM with Harten-Lax-van Leer Riemann solver in the regions of high Mach numbers to improve the solver’s robustness. The inner boundary condition of no backflow is implemented by numerical flux. A reconstruction method based on the divergence-free radial basis function is adopted to enhance the divergence-free constraint of magnetic field. Also, an anisotropic thermal conduction term is considered; the positivity-preserving reconstruction method is used to prevent the presence of negative thermal pressure and plasma density, and the implicit lower-upper symmetric Gauss Seidel method is implemented for a better convergence rate. After establishing the implicit solar wind MHD model, we employ it to simulate steady-state solar coronal structures in Carrington rotations 2177 and 2212. The simulations demonstrate that the MHD model’s computational efficiency is desirable, and the modeled results are basically in agreement with the solar coronal observations and the mapped in situ measurements from the OMNI archive. Consequently, this implicit MHD model is promising to simulate a complex plasma environment with high-intensity magnetic field and wide-ranging Mach numbers.

A Three-Order, Divergence-Free Scheme for the Simulation of Solar Wind

In this paper, we present a three-order, divergence-free finite volume scheme to simulate the steady state solar wind ambient. The divergence-free condition of the magnetic field is preserved by the constrained transport (CT) method. The CT method can keep the magnetic fields divergence free if the magnetic fields is divergence free initially. Thus, a least-squares reconstruction of magnetic field with the divergence free constraints is used to make the magnetic fields global solenoidality initially. High order spatial accuracy is obtained through a non-oscillatory hierarchical reconstruction, while a high order time discretization is achieved using a three-order Runge–Kutta scheme. This new model of three order in space and time is validated by numerical results for Carrington rotation 2207. The numerical results show its capability for producing stable reliable results for structured solar wind. The high-order, divergence-free properties of this method make it an ideal tool for the simulations of coronal mass ejection in future.

Rate of Change of Large-Scale Solar-Wind Structure

Quantifying the rate at which the large-scale solar-wind structure evolves is important for both understanding the physical processes occurring in the corona and for space-weather forecast improvement. Models of the global corona and heliosphere typically assume that the ambient solar-wind structure is steady and corotates with the Sun, which is generally expected to be more valid at solar minimum than solar maximum, but this has not been well tested. Similarly, assimilation of solar-wind observations into models requires quantitative knowledge of how the reliability of the observations changes with age. In this study we examine 25 years of near-Earth in situ solar-wind observations and 45 years of observation-constrained solar-wind simulations to determine how much the 1-AU solar-wind speed, V, and radial magnetic-field component, B_{R}, vary between consecutive Carrington rotations (CRs). For the in situ spacecraft observations, we find the rate of change of V and B_{R} is similar during solar maximum and minimum, particularly when transient interplanetary coronal mass ejections are removed from the data. This is somewhat counter to expectations. Conversely, the rate of change in V and B_{R} obtained from global heliospheric simulations is strongly correlated with the solar cycle, with the corona and heliosphere being more variable at solar maximum, as expected. Limiting the analysis of the simulations to the solar equatorial region, however, strongly reduces the difference between solar maximum and minimum, bringing the result into close agreement with the in situ observations. This latitudinal sensitivity is explained in terms of the global solar-wind structure over the solar cycle. For the purposes of assimilating in-ecliptic solar-wind observations, we suggest the uncertainty in V should increase by around 3 km s−1 per day since the observation was made and 0.1 nT per day for B_{R}. For observations made at higher latitude, the effect of observation age will be solar-cycle dependent.

On the Relation Between Coronal Green Line Brightness and Magnetic Fields Intensity

Two-dimensional (2D) solar coronal magnetogram is difficult to be measured directly until now. From the previous knowledge, a general relation has been noticed that the brighter green-line brightness for corona, the higher coronal magnetic field intensity may correspond to. To try to further reveal the relationship between coronal green line brightness and magnetic field intensity, we use the 2D coronal images observed by Yunnan Observatories Green-line Imaging System (YOGIS) of the 10 cm Lijiang coronagraph and the coronal magnetic field maps calculated from the current-free extrapolations with the photospheric magnetograms taken by Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) spacecraft. In our analysis, we identified the coronal loop structures and construct two-dimensional maps of the corresponding magnetic field intensity in the plane of the sky (POS) above the limb. We derive the correlation coefficients between the coronal brightness and the magnetic field intensity for different heights of coronal layers. We further use a linear combination of a Gaussian and a quadratic profile to fit the correlation coefficients distribution, finding a largest correlation coefficient of 0.82 near 1.1 R ⊙ (solar radii) where is almost the top of the closed loop system. For the small closed loop system identified, the correlation coefficient distributions crossing and covering the loop are calculated. We also investigate the correlation with extended heliocentric latitude zones and long period of one whole Carrington Rotation, finding again that the maximum correlation coefficient occurs at the same height. It is the first time for us to find that the correlation coefficients are high (all are larger than 0.8) at the loop-tops and showing poor correlation coefficients with some fluctuations near the feet of the coronal loops. Our findings indicate that, for the heating of the low-latitude closed loops, both DC (dissipation of currents) and AC (dissipation of Alfvén and magnetosonic waves) mechanisms should act simultaneously on the whole closed loop system while the DC mechanisms dominate in the loop-top regions. Therefore, in the distributions of the correlation coefficients with different heights of coronal layers, for both large- and small-scale latitude ranges, the coefficients can reach their maximum values at the same coronal height of 1.1 R ⊙, which may indicate the particular importance of the height of closed loops for studying the coupling of the local emission mechanism and the coronal magnetic fields, which maybe helpful for studying the origin of the low-speed 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.

The Solar Wind Parker Spiral Angle Distributions and Variations at 1 au

Using the data from the Advanced Composition Explorer (ACE) and Wind spacecraft, we statistically studied the Parker spiral angle (PSA) of the solar wind magnetic field from 1998 to 2019 at 1 au. The PSA occurrences over both a Carrington rotation (CR) and a year can be well fitted by a Gaussian distribution. However, large-scale magnetic structures, such as interplanetary coronal mass ejections (ICMEs), can significantly deviate the PSA distribution of a CR from the Gaussian distribution. The PSA distributions of each CR and each year are affected by the solar activity: They are more concentrated at a relatively higher average PSA at solar maximum. There is also a weak anticorrelation between the yearly solar wind speed (v sw) and the average PSA. MESSENGER, Venus Express, and ACE observations at different heliocentric distances within 1 au show that the dominating polarities of the heliospheric magnetic field change greatly from year to year even when the solar activity is on the same level. Our results suggest that the PSA distribution in addition to the sunspot number can provide some new information on the magnetic field variation of the Sun.

Solar Hαexcess during Solar Cycle 24 from full-disk filtergrams of the Chromospheric Telescope

Context.The chromospheric Hαspectral line is a strong line in the spectrum of the Sun and other stars. In the stellar regime, this spectral line is already used as a powerful tracer of stellar activity. For the Sun, other tracers, such as Ca IIK, are typically used to monitor solar activity. Nonetheless, the Sun is observed constantly in Hαwith globally distributed ground-based full-disk imagers.Aims.The aim of this study is to introduce the imaging Hαexcess and deficit as tracers of solar activity and compare them to other established indicators. Furthermore, we investigate whether the active region coverage fraction or the changing Hαexcess in the active regions dominates temporal variability in solar Hαobservations.Methods.We used observations of full-disk Hαfiltergrams of the Chromospheric Telescope and morphological image processing techniques to extract the imaging Hαexcess and deficit, which were derived from the intensities above or below 10% of the median intensity in the filtergrams, respectively. These thresholds allowed us to filter for bright features (plage regions) and dark absorption features (filaments and sunspots). In addition, the thresholds were used to calculate the mean intensityImeanE/Dfor Hαexcess and deficit regions. We describe the evolution of the Hαexcess and deficit during Solar Cycle 24 and compare it to the mean intensity and other well established tracers: the relative sunspot number, the F10.7 cm radio flux, and the Mg IIindex. In particular, we tried to determine how constant the Hαexcess and number density of Hαexcess regions are between solar maximum and minimum. The number of pixels above or below the intensity thresholds were used to calculate the area coverage fraction of Hαexcess and deficit regions on the Sun, which was compared to the imaging Hαexcess and deficit and the respective mean intensities averaged for the length of one Carrington rotation. In addition, we present the Hαexcess and mean intensity variation of selected active regions during their disk passage in comparison to the number of pixels of Hαexcess regions.Results.The Hαexcess and deficit follow the behavior of the solar activity over the course of the cycle. They both peak around solar maximum, whereby the peak of the Hαdeficit is shortly after the solar maximum. Nonetheless, the correlation of the monthly averages of the Hαexcess and deficit is high with a Spearman correlation ofρ = 0.91. The Hαexcess is closely correlated to the chromospheric Mg IIindex with a correlation of 0.95. The highest correlation of the Hαdeficit is found with the F10.7 cm radio flux, with a correlation of 0.89, due to their peaks after the solar activity maximum. Furthermore, the Hαdeficit reflects the cyclic behavior of polar crown filaments and their disappearance shortly before the solar maximum. We investigated the mean intensity distribution for Hαexcess regions for solar minimum and maximum. The shape of the distributions for solar minimum and maximum is very similar, but with different amplitudes. Furthermore, we found that the area coverage fraction of Hαexcess regions and the Hαexcess are strongly correlated with an overall Spearman correlation of 0.92. The correlation between the Hαexcess and the mean intensity of Hαexcess regions is 0.75. The correlation of the area coverage fraction and the mean intensity of Hαexcess regions is in general relatively low (ρ = 0.45) and only for few active regions is this correlation above 0.7. The weak correlation between the area coverage fraction and mean intensity leaves us pessimistic that the degeneracy between these two quantities can be broken for the modeling of unresolved stellar surfaces.

An Electric-field-driven Global Coronal Magnetohydrodynamics Simulation Model Using Helioseismic and Magnetic Imager Vector-magnetic-field Synoptic Map Data

We present the simulation methodology and results of our new data-driven global coronal magnetohydrodynamics (MHD) simulation model. In this model, the solar-surface electric field is first calculated such that the curl will satisfy both the induction equation and the given temporal variations of the solar-surface magnetic field. We use the synoptic maps of the Helioseismic and Magnetic Imager three-component vector-magnetic-field data to specify the solar-surface magnetic-field vector for a period from Carrington Rotations (CRs) 2106 to 2110. A set of whole-Sun three-component electric-field maps are obtained for each CR transition interval of about 27.3 days. Using the inverted electric field as the driving variable, our new global coronal MHD model, with the angular resolution of π/64, can trace the evolution of the three-dimensional coronal magnetic field that matches the specified time-dependent solar-surface magnetic-field maps and simultaneously satisfies the divergence-free condition. A set of additional boundary treatments are introduced to control the contribution of the horizontal components of the magnetic field at the weak-field regions. The strength of the solar-surface magnetic field is limited to 20 Gauss for the sake of computational stability in this study. With these numerical treatments, the nonpotential coronal features, such as twisted loop structures, and their eruptive outward motions are obtained. This present model, capable of introducing three-component solar-surface magnetic-field observation data to coronal MHD simulations, is our first step toward a better model framework for the solar corona and hence solar wind.

Tracking Movement of Long-lived Equatorial Coronal Holes from Analysis of Long-term McIntosh Archive Data

Features at the Sun’s surface and atmosphere are constantly changing due to its magnetic field. The McIntosh Archive provides a long-term (45 yr) record of these features, digitized from hand-drawn synoptic maps by Patrick McIntosh. Utilizing this data, we create stack plots for coronal holes, i.e., Hovmöller-type plots of latitude bands, for all longitudes, stacked in time, allowing tracking of coronal hole movement. Using a newly developed two-step method of centroid calculation, which includes a Fourier descriptor to represent a coronal hole’s boundary and calculate the centroid by the use of Green’s theorem, we calculate the centroids of 31 unique, long-lived equatorial coronal holes for successive Carrington rotations during the entire solar cycle 23, and estimate their slopes (time versus longitude) as the coronal holes evolve. We compute coronal hole centroid drift speeds from these slopes, and find an eastward (prograde) pattern that is actually retrograde with respect to the local differential rotation. By discussing the plausible physical mechanisms which could cause these long-lived equatorial coronal holes to drift retrograde, we identify either classical or magnetically modified westward-propagating solar Rossby waves, with a speed of a few tens to a few hundreds of meters per second, to be the best candidate for governing the drift of deep-rooted, long-lived equatorial coronal holes. To explore plausible physics of why long-lived equatorial coronal holes appear few in number during solar minimum/early rising phase more statistics are required, which will be studied in future.

Enthalpy-based modeling of tomographically reconstructed quiet-Sun coronal loops

The structure of the solar corona is made of magnetic flux tubes or loops. Due to the lack of contrast with their environment, observing and studying coronal loops in the quiet Sun is extremely difficult. In this work we use a differential emission measure tomographic (DEMT) technique to reconstruct, from a series of EUV images covering an entire solar rotation, the average 3D distribution of the thermal properties of the coronal plasma. By combining the DEMT products with extrapolations of the global coronal magnetic field, we reconstruct coronal loops and obtain the energy input required to keep them at the typical million-degree temperatures of the corona. We statistically study a large number of reconstructed loops for Carrington rotation (CR) 2082 obtaining a series of typical average loops of different lengths. We look for relations between the thermal properties and the lengths of the constructed typical loops and find similar results to those found in a previous work (Mac Cormack et al., 2020).. We also analyze the typical loop properties by comparing them with the zero-dimensional (0D) hydrodynamic model Enthalpy-Based Thermal Evolution of Loops (EBTEL, Klimchuk et al., 2008). We explore two heating scenarios. In the first one, we apply a constant heating rate assuming that typical loops are in quasi-static equilibrium. In the second scenario we heat the plasma in the loops using short impulsive events. We find that the reconstructed typical loops are overdense with respect to quasi-static equilibrium solutions of the hydrodynamic model. Impulsive heating, on the other hand, reproduces better the observed densities and temperatures for the shorter and approximately semicircular loops. The thermal properties of longer loops cannot be correctly reproduced with the EBTEL model. We suggest that to properly assess the physical characteristics of the analyzed loops in future works, it would be necessary to use a more sophisticated 1D model, with which to study the loop temperature and density profiles and test localized heating at different locations along the loops.

Testing the Alfvén-wave Model of the Solar Wind with Interplanetary Scintillation

Abstract Understanding the mechanism(s) of solar wind acceleration is important in astrophysics and geophysics. A promising model of solar wind acceleration is known as the wave/turbulence-driven (WTD) model, in which Alfvén waves feed energy to the solar wind. In this study, we tested the WTD model with global measurements of wind speed from interplanetary scintillation (IPS) observations. For Carrington rotations in minimal and maximal activity phases, we selected field lines calculated by the potential-field source-surface method in high and midlatitudes and compared the simulated and observed wind velocities. The simulation was performed in a self-consistent manner by solving the magnetohydrodynamic equations from the photosphere to the solar wind. In high-latitude regions, the simulated solar wind velocity agrees better with the IPS observation than with the classical Wang–Sheeley empirical estimation, both in maximal and minimal activity phases. In midlatitude regions, the agreement worsens, possibly because of the inaccuracy of the WTD model and/or the magnetic-field extrapolation. Our results indicate that the high-latitude solar wind is likely to be driven by waves and turbulence and that the physics-based prediction of the solar wind velocity is highly feasible with an improved magnetic-field extrapolation.

Solar Wind Velocities at Comets C/2011 L4 Pan‐STARRS and C/2013 R1 Lovejoy Derived Using a New Image Analysis Technique

AbstractThe ion tails of bright comets have long been considered as a natural tracers of the solar wind (SW) near these objects. Studies of comets and their ion tails allow inexpensive monitoring of key SW structures in the inner heliosphere, much of which is otherwise only accessible by in situ SW spacecraft measurements. Here, we present a novel technique to mine the rich archive of amateur, professional and spacecraft observations of cometary ion tails. To demonstrate this, we focus on Near‐Sun comet C/2011 L4 (Pan‐STARRS) during Carrington Rotations (CR) 2134 and 2135 and comet C/2013 R1 (Lovejoy) during CR 2118. We outline the technique’s shortcomings, including its geometric limitations, and present a catalog of radial SW velocities derived in the near‐comet environment and information on the heliospheric conditions inferred from the measured SW. Complementary measurements, derived from folding ion rays and a velocity profile map built from consecutive images, are provided as an alternative means of quantifying the SW ‐cometary ionosphere interaction. We find that comets are generally good indicators of SW structure, but the quality of the results is strongly dependent on the observing geometry.