====== Space Physics ======

The main subject of space physics or solar-terrestrial physics is the interaction of electric and magnetic fields with high energy charged particles in outer space. Solar wind particles constantly ejected from the Sun create a space plasma throughout the heliosphere. Whether we send or place spacecraft, satellites or space stations anywhere in the solar system, everything has to move inside this plasma. Just as you need to know the science of the atmosphere to build an airplane that can move in the Earth's atmosphere, so to build spacecraft and satellites that can move in the heliosphere of the solar system, you need to understand the science of the heliosphere, which is part of space physics.

Before the beginning of the space age, space physics was mainly done by observing and detecting various processes in the upper part of the Earth's atmosphere. But currently most of the research on this subject is done by collecting data directly from the sites of the most intense interactions with rockets and spacecraft. Instruments used for collection include cameras, photometers, spectrometers, magnetometers. Although the face of this field has changed completely in the last hundred years, its history is very old.

===== - History of Space Physics =====

Space physics began because of human interest in two things on Earth: the aurora and the geomagnetic field. Humans first saw the aurora long ago, but the existence of the geomagnetic field was not understood before the invention of the compass. There are references to Aurora in the scriptures of many nations of the world. Two and a half thousand years ago Xenophenes of Greece described the aurora as a 'moving mass of burning clouds'. Aurora sightings are mentioned in Chinese records dating back more than 4,000 years.

==== - Aurora and Geomagnetism ====

There were many superstitions and fears about Aurora. The first scientific inquiry into it began in the seventeenth century in Europe. **Galileo** proposed that the aurora is formed when sunlight hits the air rising from the Earth's shadow. He also coined the term 'Aurora Borealis' to mean the northern aurora. French philosopher-priest Pierre Gassendi understood the aurora to be a phenomenon far above the Earth's surface, because an aurora looks the same from two distant places. At the same time, Descartes attributed the aurora to the reflection of light from ice in areas near the North Pole. In the latter half of the seventeenth century, both the activity of the Sun and the aurora decreased greatly, which may be one reason why solar-terrestrial physics did not improve during that time.

In the 18th century Edmund **Halley** hypothesized that the aurora had a relationship with the direction of the geomagnetic field. But the French philosopher **de Mairan** disagreed with Halley, suggesting a sunspot connection with the aurora. Since then, aurora research has been closely associated with the geomagnetic field.

The first allusion to the existence of geomagnetism is found in an eleventh-century Chinese treatise on the compass. In the twelfth century, books on geomagnetism and compasses were also written in Europe, in which it is said that sailors used compasses to determine the direction of north on cloudy days. By the fourteenth century, many ships were using a regular compass.

The difference (called **declination**) between the direction of the magnetic pole and the geographic pole varies from place to place on Earth. We do not know for sure when it was first understood. But in a letter written in Europe in the sixteenth century, it is said that the declination of Rome is 6 degrees and the declination of Nuremberg in Germany is 10 degrees. Portuguese navigator Joao de Castro measured the declination of 43 places along the west coast of India and the Red Sea between 1538 and 1541.

The degree to which the geomagnetic field tilts toward the ground is called the inclination. It is measured with a compass mounted on a pivot, which was probably first made in the late sixteenth century.

Exactly in 1600 William **Gilbert's** famous book 'De Magnete' was published. It clearly states that the whole earth is magnetic. Gilbert, however, assumed that the Earth's magnetic field was constant, which was incorrect. In the seventeenth century it was known that the declination angle changes with time. In the last decade of this century, Edmond Halley's royally funded expeditions across the Atlantic Ocean to the north and south ushered in a new wave of geomagnetism research. I have already mentioned Halley and de Mairan's debate about the cause of the aurora. The seventeenth century began with Gilbert's book in 1600 and ended with Halley's Voyages in 1700.

==== - The 18th-19th Centuries ====

Considerable work was done on the terrestrial part of solar-terrestrial physics by the seventeenth century, but progress on the solar part took longer. Galileo observed sunspots with a telescope, but in the latter half of the seventeenth century the number of sunspots had declined greatly, and no further work could be done on them.

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The 11-year solar cycle shown in the figure above was discovered in 1851. This cycle is related to the Sun's magnetic field. For example, the Sun's magnetic field was very weak during the solar minimum that occurred between 2006-2010. One of the greatest discoveries of space physics in the 18th century was that the compass was constantly moving due to a changing magnetic field. Daily changes in the geomagnetic field were understood through thousands of observations made with compasses in Sweden. The reason for this change is the rotation of the earth on its axis.

In mid-eighteenth century Sweden, the relationship between the aurora and the geomagnetic field was discovered. And at the end of this century, James Cook first saw the Aurora Australis (Aurora of the South Pole), and twenty years after that, Henry **Cavendish** of England used trigonometry to determine the height of the Aurora from 80 to 115 kilometers. Cavendish's calculations were much more precise than those of Halley and de Mairan.

Revolutionary changes came in the 19th century. Measuring the geomagnetic field from many places simultaneously begins with magnetometers. A good mathematical analysis of all the collected data was done by Karl **Gauss** of Germany. As a result, it was understood which part of the field comes from below the ground and which part is created high in the atmosphere by the influence of the sun.

In the middle of this century, Heinrich **Schwabe** of Germany discovered that the number of sunspots on the Sun's surface fluctuated in a cycle of approximately ten years, which we now call the solar cycle. Also, when magnetic observatories were installed in different British colonies, an English scientist analyzed the data from different continents and realized that the geomagnetic disturbance of the earth rises and falls with the solar cycle.

One of the most amazing events in recent history happened in 1859. English amateur astronomer Richard Carrington observed a large white-light flare on the Sun's surface, and at the same time a magnetic observatory in London observed a major disturbance in the Earth's magnetic field. Now we know the cause was the solar wind coming from the sun. The supersonic solar wind connecting the Earth to the Sun then stripped many of our ionosphere's electrons from their atoms, increasing electric currents in the electrically-conducting ionosphere, causing geomagnetic storms on Earth.

Almost every year there is a total solar eclipse somewhere on Earth. The outer part of the Sun and the corona are best seen during the eclipse. But the problem is that the total eclipse lasts only a few minutes which is not long enough to understand the activity on the Sun's surface. The Coronal Mass Ejection (CME) that Carrington actually observed was discovered only after the invention of the coronagraph. Corona can be well observed by creating an artificial eclipse by covering the surface of the sun with this device.

Another great discovery of the 19th century was that of Arctic explorer John Franklin. He realized that the aurora is not uniform all the way to the poles. Now we know that the aurora is most common in the auroral zone, an oval band encircling the pole 20-25 degrees from the magnetic pole. This is best illustrated by the animation of Jupiter's aurora in the last subsection.

JJ Thomson discovered the electron in the last decade of this century. Norwegian space physicist Kristian **Birkeland**, using the work of earlier scientists and inspired by Thomson's electron, proposed that auroras are caused by electron-like particles from the Sun moving along Earth's magnetic field lines. Lord Kelvin strongly opposed any such connection of the Sun with the Earth, finding it absurd that an electron could physically travel from the Sun to the Earth itself. Birkeland's work did not gain much attention until the launch of spacecraft during the Cold War.

==== - Ionosphere ====

The electrically conductive region approximately 100 km above the ground is called the ionosphere. Here the conductivity is much higher because there is less particle-to-particle collision. Scottish meteorologist Balfour Stewart's description of the region in an 1882 article in the Encyclopaedia Britannica is very close to the modern atmospheric-dynamo theory. According to this theory, a current of electrons is generated by the solar wind in the upper atmosphere of the Earth, and the interaction of this current with the geomagnetic field creates the aurora.

Electrical engineers began to contribute to this field in the early 20th century. The radio signals that Marconi sent across the Atlantic were interpreted by Kennelly and Heaviside in 1902 through the ionosphere. Two scientists in the United States measured the altitude or height of the ionosphere by sending radio signals straight up from Earth and measuring the signals that were reflected back; This method is still used, it is called radio sounding.

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The electron density in each layer of the ionosphere is different, which can be measured by reflected radio signals. In this way, three regions named D, E, F have been found starting from approximately 60, 90 and 110 km height. A few hundred electrons are available per cc in the D region, a few thousand in the E region, and a few hundred thousand in the F region.

In addition, as spectroscopy improved in the 20th century, attempts to understand the **color of the aurora** continued. If an oxygen molecule near the ground is excited (excited) before giving off any radiation, it collides with another molecule and becomes de-excited (relaxed), unable to emit light. But at a height of 200 km, the number of atoms is so low that the excited atom starts radiating at a certain frequency, in a certain color, before any other atom can relieve its excitation. That is why the aurora has so many colors. In the aurora below 100 km above the ground, the red-blue light of nitrogen is greater, between 100 and 250 km the green of oxygen is greatest, and above 250 km the red line of oxygen is greater. Auroras are mainly caused by electrons, but in 1939 the first proton auroras were also observed.

In the 1880s, a type of audio wave known as a 'whistler' was found on telephone lines in Austria. Whistler is sudden radio noise whose pitch gradually fades away. During the Thirty-One Years' War, which began in 1914, many heard whistler noises when one country intercepted another country's telephone lines. But in the fifties of the twentieth century, it was first understood that its source was actually lightning. The electromagnetic energy of the thunder gets trapped in the magnetic field lines in the upper part of the ionosphere and reverberates back and forth, thus creating the whistler. Whistler's research has shown that the electron density is very high in the uppermost part of the ionosphere, this part is now called the plasmasphere, but some also call it the inner magnetosphere.

Just as there is astronomy for the study of the entire universe outside the earth's atmosphere, there is a field called aeronomy for the study of the upper part of the atmosphere. Many spacecraft have been sent for Aeronomy. The purpose of Canada's first satellite into space after the Soviet Union and the United States was to study aeronomy, specifically the upper part of the ionosphere by radio sounding.

==== - Magnetosphere ====

Extending far beyond the upper part of the ionosphere is our magnetosphere, which is made up of Earth's magnetic field. In this sphere there is a lot of plasma coming from the Sun. Sidney Chapman, one of the founders of this research, first thought that only one charged particle, positive or negative, came from the Sun. But in that case the particles of the same charge would have gone astray before reaching the earth due to mutual repulsion. After criticism from others, Chapman's group later realized that the solar wind coming from the Sun is actually plasma, a neutral stream of roughly equal positive and negative charge. Today we can even propel spacecraft using this plasma.

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A pair of positive-negative charges from the Sun is seen as a magnetic dipole in the Earth's magnetosphere. This dipole causes the sun-facing part of our field to contract, creating a tail on the opposite side. The Sun's plasma surrounds Earth's magnetosphere, and Earth's magnetic field creates a cavity in the solar plasma that travels throughout the Solar System.

The boundary between the Earth's magnetosphere and the solar wind from the Sun, called the magnetopause, was first directly observed by the battery-powered Explorer 1 satellite in 1961. But the character of this boundary has been better known since the solar-powered Explorer 12, a satellite that took data from the magnetosheath beyond the magnetopause. A subsequent combination of more satellite data showed that the solar wind undergoes a major shock before reaching the magnetopause. Its current name is Bow Shock. Such shock waves are seen in front of supersonic aircraft in Earth's atmosphere.

Thomas Gold, who coined the name 'magnetosphere', proposed a concept called 'collisionless shock', whereby shock fronts and waves can be generated without collisions. The above bow shock was proposed seven years after his proposal. Although direct collisions do not act on these shocks, the electric and magnetic fields affect the particles in the same way as collisions. The shock causes the supersonic solar wind to slow down, heat up, and flow around a planet like a sheet. It is called supersonic because the velocity of the solar wind is greater than the velocity of pressure waves in the interplanetary medium, the ratio of these two velocities is the Mach number.

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The bow shock changes the nature of the solar wind. Earth's dipole magnetic field traps many particles in the solar wind inside its fieldline bottle. Field lines go from one pole to another pole. As trapped particles in the solar wind move poleward along the lines, the line density increases, and the magnetic field then pushes them back toward the equator. During this echo-like journey, the particle also drifts, meaning that a particle not only travels north-south, but also moves slightly east or west on each journey. Electrons move eastward in the direction of Earth's rotation, and protons move westward. Another name for the radiation belt created by these bottled particles is the Van Allen Belt.

==== - Solar Wind ====

The importance of the Sun's magnetic field was first well understood in 1908, when George Hale developed the solar magnetograph. The Sun's magnetic field causes particles in its wind to accelerate, and this acceleration causes our auroras. In Chapman's model of geomagnetic storms, the solar wind was seen as a temporary phenomenon, causing disturbances to our planet when the wind arrived. But the problem with this model was discovered through observations of comets.

An astronomer observed in 1943 that a comet's tail does not extend along its radius, falling about 5 degrees behind the radial direction. Eight years later, the solar wind was cited as the cause of this lag. It was thought then that this trailing of the tail could be explained if there was a wind blowing at an average speed of 450 km/s all the time from the Sun. Six years later, Sweden's Hannes Alven said that the plasma in the solar wind is actually magnetized, with the magnetic field enveloping the comet like a mantle, creating a tail of field lines on the opposite side of the Sun.

Accepting the solar magnetic field also solves a problem of the terrestrial magnetic field. A few days after an eruption on the Sun, the Earth's magnetic field suddenly increases within minutes, called a sudden impulse. Thomas Gold attributed these sudden impulses to collisionless shocks.

In the 1950s, after analyzing the effect of the solar wind on Earth, it was said that this wind spread across the solar system should not contain more than 30 electrons per cc. In the 1960s, the Soviet Luna mission measured the density and magnetic field of this wind for the first time. And the US Mariner 2 mission launched the first interplanetary space probe en route to Venus.

Large amounts of energy are occasionally released from the magnetized plasma of the Sun and Earth; In the case of the Sun, they are called solar flares, in the case of Earth, substorms or geomagnetic storms. This released energy is first stored by the magnetic field. Exactly how was not known until the 1960s. In the 1940s, a solar scientist found some neutral points at the site of solar flares, created by reconnection. It was one of his postdocs who fully discovered the process of energy release through reconnection in 1961.

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As shown in the video, the solar wind can drag some of the magnetized plasma in our magnetosphere from the dayside (where the Sun is) to the nightside, and on the nightside the plasma and field lines can join the two previously separated lines together, called reconnection. During this time, a lot of energy is released through the conversion of magnetic energy into kinetic and other energies. If they are small, they are called substorms, and if they are large, they are called geomagnetic storms.

==== - Interplanetary Voyage ====

The solar wind continues throughout the solar system until the heliopause, where the interstellar wind meets the solar wind. In between, the solar wind faces many obstacles. Each planet has a different magnetosphere.

In the 1960s, humans set foot on the moon. Analysis of rocks brought back from the Moon by the Apollo missions between 1969 and 1973 revealed that the lunar crust is magnetic, while the core is conductive. But the moon has no atmosphere. The first probes sent into deep space, Mariner 2, 4 and 5, went to Mars and Venus, which have atmospheres, but do not have Earth's magnetic moment. Most spacecraft have been sent to these two planets.

Ultraviolet radiation from the Sun heats the outer atmospheres of Venus and Mars to form a neutral layer, called the exosphere, which mixes with the solar wind. The pressure of the planet's ionosphere works against the pressure of the solar wind to create a balance. Although Venus and Mars do not have magnetospheres of their own, the solar wind envelops them like a mantle, which can be compared to the effect of the solar wind on comets.

In the 1970s, Mariner 10 visited Mercury and found a mini-magnetosphere, although it appears to have no atmosphere. The first spacecraft were sent to the outer parts of the solar system in this decade. Pioneer 10 and 11 reached Jupiter in 1973, and Pioneer 11 flew to Saturn the following year. Voyager 1 and 2 were launched in 1977, Voyager 2 reached Uranus in 1986, and Neptune in 1989. Both these spacecraft have now passed the heliopause.

In 1990, the Ulysses mission went to Jupiter not to use the planet as a sling to go further, but to climb up under Jupiter's gravity to observe the Sun's poles. Solar physics is currently a very large field. Most attempts at solar observation have been through remote sensing, but some missions, such as Helios, have attempted to approach the Sun. Day by day we are getting closer to the sun.

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Not much is known about a planet on a flyby mission. Orbiters have to be placed around the planet to map it. An orbiter was sent to Jupiter in 1995, named Galileo. Currently we can also see the planet's aurora, an animation of which is shown above. The Cassini orbiter was sent to Saturn in 2004.

Through various orbiters, we have learned that all four gas giant planets have advanced magnetospheres, bow shocks, magnetopauses and magnetotails. Jupiter's magnetosphere has become nearly disk-like due to rapid rotation and the impact of moons like Io. Such magnetodiscs have also been found around Saturn. Uranus' magnetosphere is too strange to say anything about for now.

The field that began as solar-terrestrial physics about five hundred years ago is what we now call space physics. Because we study not only the effect of the sun on the earth, but also the effect of the sun on many other planets, and many asteroids and comets in this field. And we also study the heliosphere of the Sun itself. Many of our spacecraft are now reaching the far reaches of the solar system. Just as we have destroyed the Earth in the last five hundred years by turning it into a capital building machine, we will probably destroy the entire solar system in the next five hundred years, which is sad.