Global Structure Of Solar Wind Research Articles

WawHelioIonMP: A Semiempirical Tool for the Determination of Latitudinal Variation in the Ionization Rate of Interstellar Hydrogen and the Solar Wind

The latitudinal structure of the solar wind varies during the cycle of solar activity. Analysis of this variation is important for understanding the solar activity and interpretation of observations of heliospheric energetic neutral hydrogen atoms and interstellar neutral (ISN) atoms inside the heliosphere, which yield information on the heliosphere and its interaction with the interstellar medium. Existing methods of retrieving this information from indirect remote-sensing measurements of phenomena, including the heliospheric backscatter glow and interplanetary scintillations of remote radio sources, are challenging to apply in real time. Here, we propose a method WawHelioIonMP of approximate retrieval of latitudinal profiles of the ionization rates of ISN H using a machine-learning-based interpretation of the helioglow. Assuming that we know their history during two past solar cycles and have observations of the helioglow for close-to-circumsolar circles with a radius close to 90°, we derive statistically an algebraic relation between the ionization profiles and lightcurves. With the relation reversed, we are then able to derive the ionization rate profiles based on observed light curves, such as those planned for the GLObal solar Wind Structure (GLOWS) experiment on the forthcoming NASA mission Interstellar Mapping and Acceleration Probe (IMAP). The application of this method is straightforward and rapid because complex simulations are no longer needed. We present the method of retrieval of the profiles of the ionization rates, leaving the discussion of details of the decomposition of the retrieved ionization rate profiles into profiles of the solar wind speed and density to a future paper.

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.

Towards Construction of a Solar Wind \u201cReanalysis\u201d Dataset: Application to the First Perihelion Pass of Parker Solar Probe

Accurate reconstruction of global solar-wind structure is essential for connecting remote and in situ observations of solar plasma, and hence understanding formation and release of solar wind. Information can routinely be obtained from photospheric magnetograms, via coronal and solar-wind modelling, and directly from in situ observations, typically at large heliocentric distances (most commonly near 1 AU). Magnetogram-constrained modelling has the benefit of reconstructing global solar-wind structure, but with relatively large spatial and/or temporal errors. In situ observations, on the other hand, make accurate temporal measurements of solar-wind structure, but are highly localised. We here use a data assimilative (DA) approach to combine these two sources of information as a first step towards producing a solar-wind “reanalysis” dataset that optimally combines model and observation. The physics of solar wind stream interaction is used to extrapolate in heliocentric distance, while the assumption of steady-state solar-wind structure enables extrapolation in longitude. The major challenge is extrapolating in latitude. Using solar-wind speed during the interval of the first perihelion pass of Parker Solar Probe (PSP) in November 2018 as a test bed, we investigate two approaches. The first is to assume the solar wind is two-dimensional and thus has no latitudinal structure within the {pm}, 7^{circ } bounded by the heliographic equatorial and ecliptic planes. The second assumes in situ solar-wind observations are representative of some (small) latitudinal range. We show how observations of the inner heliosphere, such as will be provided by PSP, can be exploited to constrain the latitudinal representivity of solar-wind observations to improve future solar-wind reconstruction and space-weather forecasting.

South Korea's renewed focus on space weather

South Korea's renewed focus on space weather

Development of the 3‐D MHD model of the solar corona‐solar wind combining system

In the framework of integrated numerical space weather prediction, we have developed a 3‐D MHD simulation model of the solar surface‐solar wind system. We report the construction method of the model and its first results. By implementing a grid system with angularly unstructured and increasing radial spacing, we realized a spherical grid that has no pole singularity and realized a fine grid size around the inner boundary and a wide‐range grid up to a size of 1 AU simultaneously. The magnetic field at the inner boundary is specified by the observational data. In order to obtain the supersonic solar wind speed, parameterized source functions are introduced into the momentum and energy equations. These source functions decay exponentially in altitude as widely used in previous studies. The absolute values of the source functions are controlled so as to reflect the topology of the coronal magnetic field. They are increased inside the magnetic flux tube with subradial expansion and reduced inside the magnetic flux tube with overradial expansion. This adjustment aims to reproduce the variation of the solar wind speed according to the coronal magnetic structure. The simulation simultaneously reproduces the plasma‐exit structure, the high‐ and low‐temperature regions, the open and closed magnetic field regions in the corona, the fast and slow solar wind, and the sector structure in interplanetary space. It is confirmed from the comparison with observations that the MHD model successfully reproduces many features of both the fine solar coronal structure and the global solar wind structure.

How did the solar wind structure change around the solar maximum? From interplanetary scintillation observation

Abstract. Observations from the second Ulysses fast latitude scan show that the global structure of solar wind near solar maximum is much more complex than at solar minimum. Soon after solar maximum, Ulysses observed a polar coronal hole (high speed) plasma with magnetic polarity of the new solar cycle in the Northern Hemisphere. We analyze the solar wind structure at and near solar maximum using interplanetary scintillation (IPS) measurements. To do this, we have developed a new tomographic technique, which improves our ability to examine the complex structure of the solar wind at solar maximum. Our IPS results show that in 1999 and 2000 the total area with speed greater than 700 km s-1 is significantly reduced first in the Northern Hemisphere and then in the Southern Hemisphere. For year 2001, we find that the formation of large areas of fast solar wind around the north pole precedes the formation of large polar coronal holes around the southern pole by several months. The IPS observations show a high level agreement to the Ulysses observation, particularly in coronal holes.Key words. Interplanetary physics (solar wind plasma) – Radio science (remote sensing)

A numerical simulation of the 3-D structure of solar corona in May 1998

The three-dimensional (3-D) coronal structure in Carrington rotation 1935 in May 1998 is obtained with a 3-D numerical model of ideal MHD equations. In this model, the MacCormack-II scheme is employed for the discretization of the governing equations in the computational domain and the non-reflecting projected-characteristic condition is adopted at the inner boundary in order to keep the simulation stable and the solution self-consistent. The observed photospheric magnetic field in Carrington rotation 1935 is used as the initial guessed magnetic field. The 3-D structure of solar corona acquired by our computation rather well agrees with reality. The results of our computation are as follows: (1) The magnetic field on source surface is nearly a radial one, and the non-radial component is weaker than 2μ T near the neutral line. (2) The strength of the radial component of the magnetic field on the source surface does not exhibit large variations except in the vicinity of the neutral line, and this agrees with the observation. (3) The strength of the radial magnetic field component at 1 AU, calculated with inverse square law, agrees with observation better than that given by the potential model. (4) The 3-D numerical coronal structure qualitatively coincides with observation. Besides, the magnetic field dominates global solar wind structure. The density is high and velocity is low in closed magnetic fields or near current sheets, while the density is low and velocity is high in open magnetic fields.

Ulysses' Second Orbit: Remarkably Different Solar Wind

By the time of the 34th ESLAB symposium, dedicated to the memory of John Simpson, Ulysses had nearly reached its peak southerly latitude in its second polar orbit. The global solar wind structure observed thus far in Ulysses’ second orbit is remarkably different from that observed over its first orbit. In particular, Ulysses observed highly irregular solar wind with less periodic stream interaction regions, much more frequent coronal mass ejections. and only a single, short interval of fast solar wind. Ulysses also observed the slowest solar wind seen thus far in its ten-year journey (~270 km s−1). The complicated solar wind structure undoubtedly arises from the more complex coronal structure found around solar activity maximum, when the large polar coronal holes have disappeared and coronal streamers, small-scale corona] holes, and frequent CMEs are found at all heliolatitudes.

A summary of solar wind observations at high latitudes: Ulysses

The Ulysses mission has provided the first in-situ observations of the solar wind covering all solar latitudes from the equator to the poles in both hemispheres. The measurements from the first polar passes, made at near-minimum solar activity conditions, have confirmed the basic picture established on the basis of remote sensing techniques: the high-latitude wind is fast, and originates in the polar coronal holes. The detailed in-situ observations have, however, revealed a number of features related to the global solar wind structure that were not expected: the transition between slow and fast wind was relatively abrupt, followed by a slight increase in speed toward the poles; the mass flux is almost independent of latitude, with only a modest increase at the equator; the momentum flux is significantly higher over the poles than near the equator, suggesting a non-circular cross-section for the flanks of the heliosphere.

Reflection and transmission of GMIR shock at the heliopause and their relation to the 2- and 3-kHz radio emissions

We use Voyager 2 plasma and magnetic field data together with a one-fluid MHD model to study the interactions of the 1991 GMIR shock with the heliopause. The 1991 GMIR is an extraordinarily large global solar wind structure in radial, longitudinal and latitudinal extents. It has a strong shock at the leading edge. After its penetration through the termination shock, the GMIR shock first propagates through the subsonic solar wind, then interacts with the heliopause. The interaction produces a transmitted shock propagating outward in the interstellar medium, and a reflected shock propagating backward in the subsonic solar wind. We identify the reflected shock and the transmitted shock as the possible source of the radio noise detected at Voyagers. The plasma frequency behind the reflected and the transmitted shock can be, respectively, responsible for the 2- and 3-kHz radio emissions. The two bands of radio noise are emitted from sources on both sides of the heliopause starting at about the same time. If the emission is generated by f p-radiation then the heliopause is located at R∼130 AU. If the emission is generated by 2f p-radiation then R∼150 AU. Because the relative speed of the interstellar plasma with respect to the sun appears to be sub-Alfvenic, it is very unlikely there is a fast-bow shock of the heliosphere.

Interaction of global merged interaction region shock with the heliopause and its relation to the 2‐ and 3‐kHz radio emissions

We use Voyager 2 plasma and magnetic field data together with a one‐fluid magnetohydrodynamics model to study the interactions of the 1991 global merged interaction region (GMIR) shock with the termination shock and the heliopause. The 1991 GMIR is an extraordinarily large global solar wind structure in radial, longitudinal, and latitudinal extents. It has a strong shock at the leading edge. After its penetration through the termination shock, the GMIR shock first propagates through the subsonic solar wind, then interacts with the heliopause. The interaction produces a transmitted shock propagating outward in the interstellar medium, and a reflected shock propagating backward in the subsonic solar wind. We identify the reflected shock and the transmitted shock as the possible source of the radio noise detected at Voyagers (Gurnett, et al., 1993). The plasma frequency behind the reflected and the transmitted shock can be responsible for the 2‐ and 3‐kHz radio emissions, respectively. The two bands of radio noise are emitted from sources on both sides of the heliopause starting at about the same time. If the emissions picked up by Voyager are due to fp radiation, then the heliopause is located at R∼130 AU. If the emissions are due to 2fp radiation, then R∼150 AU. Because the relative speed of the interstellar plasma with respect to the sun appears to be sub‐Alfvenic, it is very unlikely there is a fast‐mode bow shock of the heliosphere.

The global structure of the solar wind in June 1991

A numerical simulation of the global solar wind structure for Carrington rotation 1843 (31 May–28 June, 1991) is performed based on a fully three-dimensional, steady-state MHD model of the solar wind (Usmanov, 1993b). A self-consistent solution for 3-D MHD equations is constructed for the spherical shell extending from the solar photosphere up to 10 AU. Solar magnetic field observations are used to prescribe boundary conditions. The computed distribution of the magnetic field is compared with coronal hole observations and with the IMF measurements made by IMP-8 spacecraft at the Earth's orbit.

The nature of the distant solar wind

This review emphasizes the large scale, global aspects of the solar wind flow at large heliocentric distances and how that structure derives from coronal and interplanetary phenomena. Particular attention is paid to the solar cycle variation, and speculation on its three-dimensional manifestations in the outer heliosphere is offered. Solar cycle variations in the global solar wind structure may lead to systematic variations in the dynamical properties of the interplanetary medium which may have a significant effect upon galactic cosmic ray modulation and the three-dimensional shape of the heliospheric termination shock.