Solar radio emissions
Solar radio emissions are a phenomenon resulting from various processes within the Sun that produce electromagnetic radiation, particularly in the radio frequency spectrum. These emissions can be attributed to events such as solar flares, which are explosive releases of energy that occur in the Sun's corona, often associated with sunspots. Solar radio emissions come in different types, including Type I, II, III, IV, and V bursts, each characterized by specific frequencies and durations, with origins linked to solar activity.
The Sun's structure plays a crucial role in these emissions, with energy being generated in the core and transported outward through radiative diffusion and convection processes. In the outer layers, particularly the corona, highly ionized particles create nonthermal radio emissions, which become visible as bursts during solar events. Scientific instruments, such as the Solar Orbiter and the Ulysses spacecraft, have been instrumental in studying these emissions, helping scientists understand their relationship to solar activity and their impact on space weather.
These solar phenomena have significant implications for technology on Earth and in space, as they can affect communication systems, satellites, and even astronauts' safety. Thus, ongoing research into solar radio emissions is essential for predicting and mitigating the effects of solar activity on the solar system.
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Solar radio emissions
Specific solar phenomena lead to the production of radio waves that affect Earth. These can be broken down into several types of radio emissions, many of which appear to be more closely tied to certain solar events than others. Detecting and understanding these emissions are important to both human activities and life on Earth.
Overview
In 1942, investigations began to determine what was “jamming” British radar and allowing German battle cruisers to pass freely through the English Channel. J. S. Hey attacked this problem. Investigations led to the recognition that the Sun, and not any German technology, was the source of this kind of radio interference. Hey learned that “jamming” happened during daytime and was worse when British receiving instruments were pointed in the general direction of the Sun. Hey also discovered that the Sun at that time sported a large group of sunspots. Indeed, solar radio waves were creating the interference.
It is necessary to understand the basic structure and dynamics of the Sun before seeking to understand why radio regions are produced. Two main solar processes emit energy. Beginning closest to the core, radiative diffusion is at work. Photons are created in the center of the Sun and move outward as a result of absorption and reemission by atoms and electrons that make up the interior of the Sun. After they move out past the radiative zone, they undergo the process of convection. Convection is caused by temperature differences that make hot and cool fluids circulate. Hotter gas rises, and cooler gas falls. As cooler gas sinks closer to the Sun’s core, it begins to be heated, causing it to rise again. This progression repeats itself, forming convection cells.
The innermost layer of the Sun’s atmosphere is called the photosphere and is also known as the Sun’s visible “surface.” Most of the light emitted by the Sun escapes through this “surface.” Furthermore, sunspots are found in this region. The umbra, or dark region of a sunspot, emits approximately 30 percent less light than an equally sized area without sunspots. Above the photosphere is the chromosphere. The chromosphere is characterized by an emission line spectrum, whereas the photosphere displays an absorption line spectrum. Vertical spikes of rising gas, or spicules, as well as plages, originate in the chromosphere. The outermost layer of the atmosphere is the corona. Atoms located here are highly ionized due to extremely high kinetic temperatures. This is responsible for nonthermal radio emissions.
Variations in the Sun’s magnetic field often cause explosive emissions of energy in the form of both streams of particles and electromagnetic radiation. Visually, these can be seen as solar flares erupting from the lower corona to the photosphere, specifically where sunspots are located. However, there is more complexity to this phenomenon than this simple picture might suggest. Flares create shock waves that excite the local plasma into oscillation. These oscillations produce radio waves of a frequency equal to that of the plasma oscillations that give rise to them. Occasionally, electrons near the location of a flare will get excited and interact with strong magnetic fields. This interaction will also create intense radio emissions, called radio bursts.
Detailed studies of these solar flares were done by the radio heliograph in Culgoora, Australia. It was discovered that the explosions originated from the corona. However, there were limitations to the effectiveness of the radio heliograph. It could observe at only three radio frequencies: 327 megahertz (MHz), 180 MHz, and 80 MHz. To help overcome this problem, the Clark Lake Radio Observatory was designed to use frequencies ranging from 15 MHz to 125 MHz. This new capability unveiled radio microbursts that are believed to be caused by coronal heating. Radio emission frequencies have been discovered to be a function of electron density. Typical ranges for the lower corona are approximately 100 MHz to one gigahertz (GHz). Near the middle of the corona, the frequencies become ten MHz to 100 MHz. The frequencies for the lower coronal areas are larger than higher up in the Sun’s atmosphere because electron density is greater closer to the core.
The Sun also emits radio waves through synchrotron radiation, the same process by which auroras are produced. Electrons become caught spinning around magnetic field lines and have their motion restricted in two directions: one rotating around the field line and the other in the direction along the magnetic field line. This type of radiation is more intense at radio wavelengths. When plasma expands, it can overcome the Sun’s inward gravitational pull. When this occurs, it is possible for electrons to escape, creating the solar wind.
Solar radio emissions come in several types. Type I is mainly narrow-band bursts that occur frequently and can last on the order of hours to days. Type II radio bursts have a wavelength of approximately one meter. These bursts are usually caused by solar flares. Along with solar flares, Type II bursts can be created from highly energetic particles. They are also slow “drift bursts” that begin at high frequencies and drift to lower frequencies. Type III bursts are fast drift radio bursts because their emission frequency decreases over time. They can occur from energetic electrons escaping along magnetic field lines. Typically, these bursts have wavelengths in the meter range but have also been discovered in the decimeter range as well. The energies of the particles that create these bursts range from one to 100 kilo-electron volts (keV). Bursts that have broadband qualities are Type IV and Type V. Type IV are also related to solar flares. Type V most commonly last for a few minutes but can last for longer periods of time if the frequency is decreased; this type has also been observed simultaneously with Type III bursts.
Knowledge Gained
It was once believed that Type II radio bursts correlated with energetic solar flares in that these bursts occur only from high-energy flares. Investigation of radio bursts, however, led to the determination that Type II bursts do, in fact, occur with less intense flares as well as high-intensity flares.
The robotic Cassini orbiter was outfitted with a radio and plasma wave science (RPWS) instrument designed to measure electric fields, magnetic fields, and electron density while that spacecraft orbits the planet Saturn. Even though Cassini’s main mission was to explore Saturn and its large satellite Titan, in 2003, with the spacecraft still en route to the Saturn system (Saturn orbit insertion was in mid-2004), Cassini unexpectedly picked up two Type III radio bursts coming from the Sun, one on October 28 and the other on November 4. These bursts were caused by intense solar flares. The orbiter was approximately eight astronomical units (AU) away from the Sun at the time when it detected these flares. That these solar flares were so intense was evidenced by the detection of those radio bursts at such a great distance from the Sun. Solar flares have a huge effect on the “space weather” of the entire solar system.
Ulysses is a joint National Aeronautics and Space Administration (NASA) and European Space Agency (ESA) spacecraft devoted to studies of the Sun. Launched in October 1990 from the space shuttleDiscovery on mission STS-41, Ulysses was sent on a trajectory rising up above the ecliptic plane in order to fly around the poles of the Sun and thereby study the Sun at all latitudes. Its mission was to understand what is happening during solar maximum, the period in the solar cycle when a copious number of sunspots are present.
Ulysses recorded coronal mass ejections and discovered an irregular solar wind that was not very periodic. This mission also made it possible to look closely at Type III radio bursts from the Sun because Ulysses was equipped with a unified radio and plasma wave instrument (URAP). One of the functions of URAP is to reveal the direction, angular size, and polarization of radio sources located in the heliosphere, the area of space that encompasses the Sun’s magnetic field and the solar wind. Investigating radio bursts is important because it can lead to mapping out the pattern of the Sun’s magnetic field by the detection of energetic particles making up these long-wavelength electromagnetic waves. Scientists have questioned how beams of particles of Type III emissions can stay together long enough to be observed. Data from Ulysses addressed this by making observations of the solar plasma’s electric field. It discovered that disturbed plasma can stabilize the beams and “hold” the particles and beams together. These bunches have been termed envelope solitons. Ulysses also recorded that more shock waves occur during periods when sunspot activity increases.
Context
NASA’s Solar Dynamics Observatory (SDO) is another orbiter designed to explore the Sun. In 2008, it was under construction at the Goddard Space Flight Center, and it was launched in 2010. SDO’s purpose is to focus on coronal mass ejections, sunspots, and solar flares. This idea of investigating so-called space weather is important because violent weather can endanger astronauts, satellites, probes, and airplanes flying near the poles of the Earth. The Daniel K. Inouye Solar Telescope, funded by the National Science Foundation, and located on Hawaii, has also provided scientists with images that show the magnetic fields of solar features that affect "space weather," including sunspots and flares.
Launched in 2020 as another joint effort between NASA and the ESA, the Solar Orbiter flies near the Sun, taking measurements of radio emissions and plasma waves that can later be compared to visual measurements to make correlations. One of the main tasks of Solar Orbiter is to further investigate interrelationships between solar flares, coronal mass ejections, and radio bursts. The Solar Orbiter has a radio spectrometer (RAS) to measure solar radio emissions of frequencies in the range of 100 kilohertz (kHz) to one GHz. Because of the large range of frequencies that it can observe, RAS has three component spectrometers: one ranging from 100 kHz to sixteen MHz, a second from sixteen to 200 MHz, and a third from 200 MHz to one GHz. It can observe plasma variations originating at the low coronal level of the Sun’s atmosphere. The Solar Orbiter has taken the closest ever images to the Sun.
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