Theory More About the Solar Wind

The idea of the solar wind first originated when that a comet tail was blown by pressure of sunlight was hypothesized by 17th century astronomer Johannes Kepler as the idea was furthered by 1943 like the Sun emitted a steady stream of charged particles. The solar wind was first measured in speed by the Soviet Luna 1 probe by 1959, which had ventured outside of the Earth's magnetic field as that was confirmed by the U.S. Mariner 2 mission by 1962, on its way to Venus. We live in the extended atmosphere of a magnetically active star which, in part, means that we are constantly in the path of the Sun’s outflow of charged particles, called the solar wind. The space environment of our entire solar system, both near Earth and far beyond Pluto, is determined by our Sun’s activity, which cycles and fluctuates through time. The solar wind is originating in the solar corona, this outermost part of the Sun, and a continuous and varying stream of charged particles. It was the finding that comets' ion tails always point away from the Sun which eventually led in 1958 to the discovery of the solar wind. The corona has such a high temperature that the Sun's gravity is letting the solar wind escape. The solar wind is a plasma of electrons and protons as one still does not know which of several theories offers the best theory for it. In 1958, U.S. astronomer Parker developed a theory showing how the Sun’s hot corona is so hot that it overcomes the Sun’s gravity. According to the theory, the material in the corona expands continuously outwards in all directions, forming a solar wind. A year later, the Soviet spacecraft Luna 1 detected solar wind particles in space, and three years after that, the observations were confirmed by NASA’s Mariner 2 spacecraft. The Mariner 2 detected two distinct streams of solar wind: a slow stream travelling at approximately 215 miles per second and a fast stream zipping through space at twice that speed (the joint NASA-ESA Ulysses solar probe in 1990 proved that during solar minimums, the fast solar wind originates at Sun's poles as the slow one from the equatorial regions). Then, in 1973, the origins of the fast solar wind were identified. X-ray images of the corona taken from Skylab revealed that the fast wind spews from coronal holes. As the solar cycle progresses toward its maximum, the structure of the solar wind changes from two-distinct regimes — fast at the poles and slow at the equator — to a mixed, inhomogeneous flow. The debate about the origins of the slow solar wind hinges on a distinction between what’s known as the closed and open corona. The closed corona refers to regions of the Sun where its magnetic field lines are closed — that is, connected to the solar surface at both ends. Bright helmet streamers — large loops that form over magnetically active regions, shaped like a knight’s pointy helmet — are one such example. The plasma, or ionized gas, travelling along the closed loops of a helmet streamer is for the most part confined to the area near the Sun. The open corona, on the other hand, refers to regions where the magnetic field lines anchor to the Sun at only one end, stretching out into space on the other, thus creating a highway for solar material to escape into space. Coronal holes — the cooler regions at the source of the fast solar wind — are the habitat of open field lines. By the time the slow solar wind leaves the solar corona, it is also flowing on open magnetic field lines, as that’s the only way to get so far from the Sun. But theories differ on whether it started off there, or was instead born on closed field lines only to switch to open field lines somewhere along the way. The expansion factor theory, for example, claims the slow solar wind originates on open field lines, just like the fast wind. Its comparatively slow speed results from the expanding path it takes on its way out of the corona, as magnetic field lines skirt the borders of helmet streamers. Just as water coursing through a pipe slows to a trickle as the pipe expands, plasma traveling along these widening magnetic paths slows down, forming the slow wind. Other theories claim the slow solar wind originates on closed field lines and later switches to open field lines. Accordingly, the slow wind forms when the open field lines from coronal holes bump into the closed field lines at the edges of helmet streamers, explosively rewiring themselves in a magnetic reconnection. The plasma formerly on the streamer’s closed field lines suddenly finds itself on an open field line, where it can escape out to space. The idea that slow solar wind plasma was once on closed field lines is supported by evidence that it once faced the kinds of extreme heating we know happens there. Switchbacks are sudden reversals in the magnetic field of the solar wind first observed by 2018, might be related to the way the Sun maintains and moves magnetic field lines that stretch out into the solar system. They seem to arise from a reconfiguration of open and looped magnetic field lines already in the Sun's atmosphere. When an open magnetic field line encounters a closed magnetic loop they can undergo a process called interchange reconnection. This process would create a outward-flowing S-shaped kink in the newly formed open magnetic field line

->Solar Probe Discoveries by 2019!
NASA's Parker Solar Probe, the closest-ever spacecraft to reach the Sun, released by 2019 a first batch of data it gathered. Quick reversals have been observed in the solar wind's magnetic field and sudden, faster-moving jets of material, which make the solar wind more turbulent. Such reversals likely are localized disturbances in the magnetic field traveling away from the Sun, rather than a change in the magnetic field. Flips also occurs lasting anywhere from a few seconds to several minutes as they are due to solar waves' velocity spikes. Those remnants of structures from the Sun being hurled into space are violently changing the organization of the flows and magnetic field. The solar wind, at its beginning near Sun is rotating along with our star but at a certain distance it transitions from rotating along to flowing directly outwards. A dust-free zone at last, which had been first hypothesized since 1929, was observed beginning at some 3.5 million miles (5.6 million kilometers) from the Sun

The solar wind is made of a sea of electrically-charged electrons and ions which have separated at extremely high temperatures and also carries the interplanetary magnetic field along for the ride, forging a magnetic connection between the Sun and Earth and the other planets in the solar system. Its particles flow at 250 miles per second (400km/s), and they are so dispersed that interplanetary space at Earth’s orbit has about a thousand times fewer particles in one cubic inch of space than the best laboratory vacuum on Earth. The solar wind temperature is reaching some million degrees Fahrenheit (million degrees Celsius) warm, as it can move as fast as 466 miles per second (750 km/s) or a million mile per hour. The farther the wind gets from its source, the Sun, the faster and hotter it gets. The solar wind never journeys slower than 161 miles per second. The faster the solar wind, the more helium is present in it. The solar wind plasma is hotter than expected as the cause of that heating might be hidden in its turbulences as current becomes very intense, creating high magnetic stress regions and sometimes a phenomenon known as magnetic reconnection. There is no consensus on what powers the solar wind’s acceleration, its extreme variability, or its remarkably high temperatures. The vacuum of interplanetary space, on a other hand, is filled with turbulence and swirling vortices which can account for. The solar wind at Earth is about 70 times hotter than one might expect from the temperature of the solar corona and how much it expands as it crosses from the Sun to us. In terms of temperatures, it is the energy released by turbulences which heat the solar wind -turbulent motion cascades down into motion on smaller and smaller scales until it hits the level of the fundamental gyrations of the particles about the magnetic field, where it becomes heat- Turbulences also mix up the wind, leading to the swift variability. The solar wind is mostly made of hydrogen. There are two types of solar wind, the fast, and slow one. Fast solar wind emanates from the interior of coronal holes as the slow solar wind, on the other hand, is associated with hotter regions around the equator. The most definitive difference between fast and slow solar wind is their composition. The amount of heavy elements and their charge state, or number of electrons, differ between the two types of wind. Density and charge state composition of the slow solar wind rises and falls every 90 minutes. That 'periodic density structures' were discovered about 2000 as they display the telltale fingerprints of magnetic reconnection

More About the Interface Between the Solar Corona and The Solar Wind
In deep space, the solar wind is turbulent and gusty. It is the Sun itself which causes the turbulence in the solar wind, from fine-grained structures which exist in the outer corona, the source of the solar wind. Corona is a dynamic, persistently moving and changing structure. As the inner corona is turbulent, scientists considered the outer corona to be smooth and homogenous which revealed itself to be not the case. Where does the corona end and the solar wind begin? One definition points to the Alfvén surface, a theoretical boundary where the solar wind starts moving faster than waves can travel backward through it but there isn’t a clean Alfvén surface. The Alfvén zone is a no-man's land, where the solar wind gradually disconnects from the Sun, rather than a single clear boundary. The observations reveal a patchy framework where, at a given distance from the Sun, some plasma is moving fast enough to stop backward communication, and nearby streams are not. The streams are close enough, and fine enough, to jumble the natural boundary of the Alfvén surface to create a wide, partially-disconnected region between the corona and the solar wind. How tangled we think the magnetic field gets in the corona at 10 solar radii means that some interesting physics is happening around there. Coronal streamers themselves — also known as helmet streamers, because they resemble a knight’s pointy helmet — are bright structures that develop over regions of the Sun with enhanced magnetic activity are composed of myriad fine strands that together average to produce a brighter feature. Blobs oozing from the Sun every 90 minutes could also shed light on the solar wind's beginnings

The solar wind is a plasma of electrons and protons. In the solar wind, electrons are over a thousand times lighter than the ions. Electrons are very small, negatively charged particles. Protons, are larger positively charged particles like ionized hydrogen and helium. Most of the particles in the solar wind are hydrogen and helium atoms as the wind also carries 'heavy' atoms (that is elements heavier than hydrogen and helium, such as carbon and oxygen) with most of their electrons stripped away. Such particles, for example, collide with neutral atoms in a comet's atmosphere. The solar wind shows constant turbulence at every size scale: long streaming jets, smaller whirling eddies, and even microscopic movements as charged particles circle in miniature orbits. Through it all, great magnetic waves and electric currents move through, stirring up the particles even more. The plasma possess electric and magnetic fields generated by the motions of proton and electrons, which in turn is heating those particles. Length and effects of the magnetic waves are known as these start long as long wavelength fluctuations, but lose energy – while getting shorter – over time. Loss of energy in the waves transfer energy to the solar wind particles, heating them. The magnetic waves transfer heat to the particles at different rates depending on their wavelength. The largest waves lose energy at a continuous rate until they make it down to about 100-kilometer wavelength. They then lose energy even more quickly before they hit around 2-kilometer wavelength and return to more or less the previous rate. In a plasma each particle is electrically charged so movement is governed as much by the laws of electromagnetics as it is by the fundamental laws of gravity and motion

Recentest measurements have shown that the cascade of turbulence occurs through the action of a special kind of traveling waves – named Alfvén waves after Nobel laureate Hannes Alfvén, who discovered them in 1941. Alfvén waves are generated when magnetic field lines, such as those coming from the Sun or a disk around a black hole, interact with charged particles, or ions, and become twisted or coiled into a helical shape. Alfvén waves point perpendicular to the magnetic field. This is in contrast to previous studies, which in the 1970’s examined magnetic waves closer to the Sun and magnetic waves running parallel to the magnetic field, which can send particles moving in tight circular orbits – a process known as cyclotron resonance -- thus giving them a kick in both energy and temperature. The perpendicular waves found here, on the other hand, create electric fields that efficiently transfer energy to particles by, essentially, pushing them to move faster. The process – known as Landau damping – helped heat electrons. But, since much of the change in temperature with distance from the Sun is due to changes in the proton temperature, it was crucial to understand how they obtained their energy. Since hot electrons do not heat protons very well at all, this couldn’t be the mechanism. Landau damping is what adds energy to both protons and electrons – at least near Earth – also helps explain the odd rate change in wave fluctuations as well. When the wavelengths are about 100 kilometers or a bit shorter, the electric fields of these perpendicular waves heat protons very efficiently. So, at these lengths, the waves transfer energy quickly to the surrounding protons -- offering an explanation why the magnetic waves suddenly begin to lose energy at a faster rate. Waves that are about two kilometers, however, do not interact efficiently with protons because the electric fields oscillate too fast to push them. Instead these shorter waves begin to push and heat electrons efficiently and quickly deplete all the energy in the waves. Not all the energy being dissipated by the protons step is what explains why the remaining energy in the wave continues its journey toward smaller scales, wavelengths where it can pass to the electrons step. Kinetic Alfvén waves, on a other hand, are energy transporters in plasmas: when they move through, electrons traveling at the right speed get trapped in the weak spots of the wave’s magnetic field. Because the field is stronger on either side of such spots, the electrons bounce back and forth as if bordered by two walls, in what is known as a magnetic mirror in the wave. As a result, the electrons aren't distributed evenly throughout. Some scientists think that such a process with kinetic Alfvén waves are key to how the solar wind is heated to extreme temperatures

A phenomenon called 'charge exchange,' discovered in the mid-1990's like a unexpected X-rays source at the head of comet Hyakutake, occurs when the solar wind collides with Earth's exosphere and neutral gas in interplanetary space. When highly charged heavy ions in the solar wind collide with neutral atoms found in space, heavy ions 'steal' a electron from the neutrals, a exchange that puts the heavy ions in a short-lived excited state. As they relax, they emit soft X-rays ('soft,' for X-rays close to the ultraviolet range of electromagnectic spectrum). Charge exchange since has been found in numerous places, like comets, interplanetary wind, possibly supernova remnants, and galactic halos. Soft X-ray emissions also exist in the atmospheres of Venus and Mars. Soft X-ray emissions also have been observed in Earth's exosphere. The solar wind which carries electric and magnetic fields interacts with pockets of neutral gas, where the electrons and ions are still tightly bound together, and picks up electrons from those

The range of interaction with the solar wind is quite diverse in the solar system. The average of magnetic field lines on their way in space from the Sun to Earth forms a steady path following a distinct spiral because of the Sun’s rotation. A new model is showing solar energetic particles spiraling out much wider and farther than previously theorized as they even find their way to even the far side of the Sun, no matter what type of event first propelled them, a result of turbulence in solar material. During its journey to Pluton by 2010, the New Horizons probe, by about 2016, collected three years' worth of measurements of the solar wind helping to improve models of the space environment throughout the solar sysem. During their move to the outskirts of the solar system, solar particles pick up a acceleration boost that kicks them up just past their original speed and maybe the seeds of extremely energetic particles called anomalous cosmic rays, which are super-fast, energetic rays coming back inwards the solar system. Further away, at lower energies, rays are thought to play a role at shaping the boundary where the solar wind hits interstellar space, a region now explored by the Voyager 2 mission (when that mission travelled the same path than New Horizons, solar activity was much more important). Coronal mass ejections also affect the solar wind and the way it interacts with planets. The farther away the Sun, the less CMEs and collisions of two different-speed solar wind streams are discernible. Smaller structures tend to be worn down or clump together as they travel outwards, creating fewer but bigger features. Pattern acquired at the Sun from the region the solar wind originates from are still seen far away however. Characteristics of the solar wind –including speed, density, and temperature– are shaped by the region of the Sun it flows from function of the rotation of varied wind-producing regions, yielding miscellaneous patterns