Space physics, or solar-terrestrial physics, deals primarily with the interaction of electric and magnetic fields with high-energy charged particles in outer space. Charged particles from the solar wind constantly ejecting 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 make airplanes that can move in the Earth’s atmosphere, so to make spacecraft and satellites that can move in the heliosphere of the solar system, you need to understand the science of the heliosphere, and this science is called 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, since people were fascinated by the aurora, surprised by the attraction of magnets, since the beginning of space physics.

Space physics began because of human interest in two things on Earth: auroras and geomagnetism. Evidence of seeing the aurora existed long ago, but the existence of geomagnetism 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 defined the aurora as a ‘moving mass of burning clouds’. Chinese records date back more than four thousand years to aurora sightings.

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 realized that the aurora is a phenomenon far above the Earth’s surface, because the 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.

In the 18th century Edmund Halley hypothesized a geomagnetic relationship to the aurora. But the French philosopher de Meran disagreed with Halley, suggesting a sunspot connection with the aurora. Since then, aurora research has been closely associated with geomagnetism.

The first allusion to the existence of geomagnets is found in an eleventh-century Chinese treatise on the compass. In the twelfth century, books were also written about geomagnets and compasses in Europe, in which sailors used compasses to fix 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 inclination of the geomagnetic field towards 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 the declension was known to change over 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 Meeran’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.

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 through binoculars, but in the latter half of the 17th century the sunspots became much smaller (the ‘Mondar Minimum’ from 1645 to 1700) and no further work was possible.

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 always moving. Daily changes in the geomagnetic field can be 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 the mid-eighteenth century, the geomagnetic connection with the aurora was discovered in Sweden. 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 Meeran.

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 is generated much higher in the atmosphere.

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 data from different continents and realized that the Earth’s geomagnetic disturbance rises 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, 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 that connects the Earth to the Sun then stripped many of the electrons in our ionosphere from the atoms, causing an increase in electric current in the conductive 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 therefore first discovered 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.

JJ Thomson discovered the electron in the last decade of this century. Norwegian space physicist Kristian Birklund, 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. Birklund’s work did not gain much attention until the launch of spacecraft during the Cold War.

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.

The electron density in each layer of the ionosphere is different, which is detected in the reflected radio signal. Thus, three layers called DEF are found starting from approximately 60, 90 and 110 km height. Several hundreds of electrons are available per cc in the D layer, several thousand in the E layer, and several hundred thousand in the RF layer.

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, it collides with another molecule before emitting any radiation and becomes de-excited, giving off 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 25 km the red line of more oxygen. 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. Whistler also heard a lot during the Thirty-One Years’ War, which began in 1914, when one country intercepted the telephone lines of another country. 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 extremely high in the upper part of the ionosphere, this part is now called the plasmasphere, but some also call it the inner magnetosphere.

Astronomy is the study of the upper part of the atmosphere, just as astronomy is for the study of the entire universe outside the Earth’s 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 the upper part of the ionosphere by means of aeronomy, specifically radio sounding (echoes).

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 run spacecraft using this plasma.

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 Gould, 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, 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.

The bow shock allows the nature of the solar wind to sail. Many particles in this wind trap the Earth’s dipole magnetic field in its fieldline bottleneck. 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 a journey like an echo, the particle also drifts, meaning that a particle not only moves 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.

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. Chapman’s model of geomagnetic storms saw the solar wind as a temporary phenomenon, when the wind disturbs our planet. But the problem with this model was discovered through observations of comets.

An astronomer noted in 1943 that the comet’s tail does not stay along the radius of its orbit, falling about 5 degrees behind the radial direction. Eight years later, the solar wind was cited as the cause of this lag. It was then thought that if there was a wind blowing at an average of 450 km/s all the time from the Sun, then this trailing of the tail could be explained. Six years ago, Sweden’s Hannes Alven said that the plasma in the solar wind is actually magnetized, 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 solves one 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 Gould also attributed collisionless shocks to sudden impulses.

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. The density and magnetic field were first measured by the Soviet Luna mission in the 1960s. And the US Mariner 2 mission launched the first interplanetary space probe en route to Venus.

A large amount of energy is suddenly released from the magnetized plasma surrounding the Sun and Earth; Solar flares on the Sun, substorms on Earth. The magnetic field acts to store this energy for release. How it was not known before the sixties. 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.

As shown in the video, the solar wind can drag some of our magnetosphere’s magnetized plasma from the dayside to the nightside, and on the nightside, the effects of all this plasma and field lines can cause the two previously distant lines to join together, a process known as 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.

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.