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The sun and its activity
Biomed Pharmacother 56 (2002) 243s–246s www.elsevier.com/locate/biopha
Editorial
The sun and its activity 1. Introduction: global properties of the sun
2. The active sun
The sun is a typical main-sequence star in the universe. It has a diameter of 1.4E6 km and an average density of 1.4 g/cm3. Solar energy is generated at its inner core, where the temperature is about 15 million degrees. This energy has been transferred by radiation and convection processes from the inner core to the surface over several hundred years. The solar radiation that we see everyday on earth was thus created several hundred years ago at the center of the sun. The temperature of the sun’s surface, called the photosphere, is six thousand degrees, but its upper atmosphere, called the corona, is several million degrees. The mechanism that heats up the corona to such high temperatures is still a mystery in solar physics. The solar-radiation spectrum peaks at a wavelength of 500 nm, which coincides with the sensitivity peak of the human eye. Solar electromagnetic radiation from X-rays to radio-waves, and occasionally, gamma rays, is emitted in solar flares. X-rays, extreme ultraviolet (EUV), and gamma rays are harmful to humans, but the earth’s atmosphere is only transparent for a limited range of radiation, for example, visible light, radiowaves and some infrared wavelengths. Accordingly, the combination of the earth’s atmosphere and solar radiation provide suitable conditions for life on earth. High-temperature coronal plasma is ejected into interplanetary space as so-called solar wind. Average properties of the solar wind when it reaches the earth’s orbit are a density of 1 g/cc, a magnetic field strength of several nanotesla, and a temperature of one hundred thousand degrees. Abrupt changes in the condition of the solar wind plasma produce various kinds of geomagnetic field disturbances on earth. The overall solar-wind and interplanetary magnetic-field structure was firstly observed in three dimensions by the European explorer “Ulysses” in the 1990s and a detailed study of this data has just started. Many fundamental properties and processes concerning the solar wind plasma and the interplanetary magnetic field are still unexplored in the field of solar physics. For example, why is solar coronal plasma accelerated to a bulk speed of several hundred kilometers per second? The answer to such a fundamental question is still unclear.
Sunspot observation by telescope started at the beginning of the seventeenth century, immediately after the invention of the telescope. A German astronomer, Schwabe, found that the number of sunspots follows an 11-year periodicity. At the beginning of the twentieth century, Hale found that sunspots have a strong magnetic field. Namely, the magnetic-field strength of a big sunspot is about 3000 Gauss, which is equivalent to a strong magnet. A typical group of sunspots has a leading part and a following part whose polarities are different. This means that at the intersection of subsurface flux rope at the solar surface, i.e. the photosphere, corresponds a sunspot group. The lifetime of a sunspot is from several days to several months. A large and complex sunspot group —called a complex of activity— sometimes survives for several months. Magnetic field lines anchored to a sunspot extend to the sun’s upper atmosphere, i.e., the solar corona. Such magnetic field lines can be traced as a coronal loop in a soft-X-ray telescope image, like that obtained by Yohkoh. These coronal loops, i.e., magnetized plasma, interact with each other, and sometimes a huge amount of energy is released abruptly in the form of a so-called solar flare and/or a coronal mass ejection (CME). In the case of a solar flare, the solar atmosphere is heated up to several tens of millions of degrees by the accelerated plasma, and a huge amount of energy is radiated (mainly as electromagnetic radiation). CME is an abrupt ejection of coronal plasma into interplanetary space at a velocity of up to 2000 km/s. Some CMEs are associated with solar flares, but their relationship is still unclear. The energy build-up and trigger process of solar flares and CME are still important unsolved problems in solar physics. To understand the flare-build-up process, it is crucial to observe the three-dimensional structures of the active region and the coronal loop. However, current solar observations can only provide two-dimensional data such as image data or a two-dimensional map of the surface magnetic field. Fig. 1 is an example of analysis on a large X-class flare. Observations with different wavelengths provide information on plasma distribution at different temperatures and
© 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 0 7 5 3 - 3 3 2 2 ( 0 2 ) 0 0 2 9 8 - 6
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Editorial / Biomed Pharmacother 56 (2002) 243s–246s
Fig. 1. Solar active region (sunspot group) observed by different wave lengths.
heights. In the lower right frame, the three-dimensional structure of the coronal magnetic field is derived by extrapolating the observations of the surface magnetic field. Observational and theoretical studies, focusing on threedimensional structures, are keys to answering the questions concerning flare build-up. Trials of the above-mentioned approaches for understanding three-dimensional structures and their changes are key issues in current solar physics. Many recent studies have revealed the preferable conditions for solar flares. The magnetic-field lines in the solar atmosphere are shaken at the foot point by turbulent motion in the photosphere, and deformed. These deformed magnetic field lines from the original configuration with a simple dipole contain excess energy as electric current; this excess energy is considered to be the energy source of solar flares. Tracking the deformation of the magnetic field lines will provide essential clues in the study of the flare mechanism. However, remote sensing of the solar magnetic field requires a highly sensitive spectroscopic polarimetry technique. To obtain information on the magnetic-field
structure in the upper solar atmosphere, calculation using remote-sensing data of the surface magnetic field is now possible. However, the sensitivity available with current observation techniques is still one or two orders lower than the scientific requirement. To overcome this difficulty, various ambitious projects, like Solar-B, are on-going and new observation techniques are under development. Solar activity has a clear periodicity of 11 years on average, which is called the solar-activity cycle. The origin of this cyclic behaviour is considered to be a kind of electromagnetic generation process which is driven by the internal rotation of the sun (i.e., the dynamo model). The sun rotates at a different angular velocity according to latitude, and different with solid rotation, such as with the earth. The rotation period near the solar equator as observed from the earth is about 27 days, but at a higher latitude, it is longer (i.e., the sun rotates more slowly). The rotation period near the solar pole is about 40 days. Rotation speed varies with not only latitude but also with depth. As a result, this differential rotation creates a strong magnetic field in
Editorial / Biomed Pharmacother 56 (2002) 243s–246s
245s
continuously observed by satellite. Although scientific observations of sunspots only have a three-hundred-year history (i.e., from Galileo’s era), efforts to estimate the history of solar activity through proxy data are continuing. Furthermore, a relationship between climate change and solar activity has been pointed out in regard to the ‘little ice age’ and the Maunder solar-minimum period. Research on this topic has just started and will be an important issue concerning future solar physics.
3. The sun, earth and the human race
Fig. 2. The sun at solar activity maximum (1991) and minimum (1996). (Courtesy of ISAS).
the subsurface convection zone, which acts as the origin of sunspots. This mechanism, called dynamo action, is considered to be the key mechanism in the continuous regeneration of solar magnetic flux with an 11-year periodicity. The difficulty in understanding this solar-cycle mechanism has been a lack of knowledge about the inside of the sun. Recently, solar seismology —a new, powerful tool for solar interior study— has been developed. It tackles the problem inversely, by revealing the physical properties of the solar interior from observations of the oscillation of the solar surface plasma. Solar seismology, coupled with longterm continuous observation data, is required to solve the solar-cycle problem. Fig. 2 shows X-ray images of the 11-year change of the sun’s solar flux. Recently, the change in the total solar flux (solar constant), along with the solar cycle, has been
Fig. 3. Chain of solar, interplanetary and terrestrial disturbances.
Fig. 3 is a schematic drawing of the relation between solar disturbances and the earth’s corresponding response. Energetic solar flares and/or CMEs sometimes produce solar energetic particle (SEP) events. Flaring solar atmosphere and interplanetary shock waves have been proposed as the mechanism that produces SEP. (‘acceleration’ makes little sense here). Recently, a hybrid model that combines seed-particle acceleration in a solar flare and acceleration in an interplanetary shock wave has been proposed. However, a theory to explain the acceleration mechanism to an energy of several hundred mega-electronvolts has yet to be proposed. CMEs launched from the sun propagate across interplanetary space at up to 2000 km/s and arrive at the earth in a few days. Strongly disturbed magnetic fields caused by the shock wave in front of the CMEs interact with the magnetic field of earth’s magnetosphere and produce a geomagnetic storm. Large geomagnetic storms can lead to a variation in earth’s radiation belt and ionosphere. In addition, X-ray
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Editorial / Biomed Pharmacother 56 (2002) 243s–246s
radiation accompanying solar flares brings about anomalous changes in the ionospheric conditions, causing short-wave fade out of telecommunication signals. Coronal holes, i.e. where coronal X-ray emission is weak, are sources of high-speed solar-wind streams. Such high-speed solar wind interacts with earth’s magnetosphere and causes recurrent-type geomagnetic disturbance. The origin of recurrent-type geomagnetic storms had been a mystery until the soft-X-ray telescope onboard ‘Skylab’ confirmed the existence of coronal holes. These space-weather disturbances have strong influences on long-range communications and navigation in shortwave range, satellite systems, and electric-power lines at high latitudes. Recently, SEPs have influenced manned space activities, especially on extravehicular ones (i.e., astronauts working in space suits outside their shuttle or station), and are causing great concern Therefore, research projects on space-weather phenomena are currently underway in many countries. The purpose of such research is to understand the mechanism of solar disturbance and its influence on the earth and various social systems. Prediction and alert systems for short-term variation of the space
environment, such as space-weather forecasting techniques, are being developed. The study of the relationship between the sun and the earth is in its initial phase. It is known that solar influence is exerted on the earth in various ways. Better scientific understanding of this relationship requires not only understanding the individual elementary processes of the sun and earth, but also their combined processes (My best guess). Moreover, when the relationship with humans (in particular, the human body) is taken into account, it becomes much more complex and difficult to model. This means that understanding the relationship between the sun and humans on earth is a challenging topic that will require long-term, continuous effort. M. Akioka Hiraiso Solar Observatory, Communications Research Laboratory, 3601 Isozaki, Hitachinaka, 311-1202 Ibaraki, Japan E-mail address: [email protected]
Editorial
The sun and its activity 1. Introduction: global properties of the sun
2. The active sun
The sun is a typical main-sequence star in the universe. It has a diameter of 1.4E6 km and an average density of 1.4 g/cm3. Solar energy is generated at its inner core, where the temperature is about 15 million degrees. This energy has been transferred by radiation and convection processes from the inner core to the surface over several hundred years. The solar radiation that we see everyday on earth was thus created several hundred years ago at the center of the sun. The temperature of the sun’s surface, called the photosphere, is six thousand degrees, but its upper atmosphere, called the corona, is several million degrees. The mechanism that heats up the corona to such high temperatures is still a mystery in solar physics. The solar-radiation spectrum peaks at a wavelength of 500 nm, which coincides with the sensitivity peak of the human eye. Solar electromagnetic radiation from X-rays to radio-waves, and occasionally, gamma rays, is emitted in solar flares. X-rays, extreme ultraviolet (EUV), and gamma rays are harmful to humans, but the earth’s atmosphere is only transparent for a limited range of radiation, for example, visible light, radiowaves and some infrared wavelengths. Accordingly, the combination of the earth’s atmosphere and solar radiation provide suitable conditions for life on earth. High-temperature coronal plasma is ejected into interplanetary space as so-called solar wind. Average properties of the solar wind when it reaches the earth’s orbit are a density of 1 g/cc, a magnetic field strength of several nanotesla, and a temperature of one hundred thousand degrees. Abrupt changes in the condition of the solar wind plasma produce various kinds of geomagnetic field disturbances on earth. The overall solar-wind and interplanetary magnetic-field structure was firstly observed in three dimensions by the European explorer “Ulysses” in the 1990s and a detailed study of this data has just started. Many fundamental properties and processes concerning the solar wind plasma and the interplanetary magnetic field are still unexplored in the field of solar physics. For example, why is solar coronal plasma accelerated to a bulk speed of several hundred kilometers per second? The answer to such a fundamental question is still unclear.
Sunspot observation by telescope started at the beginning of the seventeenth century, immediately after the invention of the telescope. A German astronomer, Schwabe, found that the number of sunspots follows an 11-year periodicity. At the beginning of the twentieth century, Hale found that sunspots have a strong magnetic field. Namely, the magnetic-field strength of a big sunspot is about 3000 Gauss, which is equivalent to a strong magnet. A typical group of sunspots has a leading part and a following part whose polarities are different. This means that at the intersection of subsurface flux rope at the solar surface, i.e. the photosphere, corresponds a sunspot group. The lifetime of a sunspot is from several days to several months. A large and complex sunspot group —called a complex of activity— sometimes survives for several months. Magnetic field lines anchored to a sunspot extend to the sun’s upper atmosphere, i.e., the solar corona. Such magnetic field lines can be traced as a coronal loop in a soft-X-ray telescope image, like that obtained by Yohkoh. These coronal loops, i.e., magnetized plasma, interact with each other, and sometimes a huge amount of energy is released abruptly in the form of a so-called solar flare and/or a coronal mass ejection (CME). In the case of a solar flare, the solar atmosphere is heated up to several tens of millions of degrees by the accelerated plasma, and a huge amount of energy is radiated (mainly as electromagnetic radiation). CME is an abrupt ejection of coronal plasma into interplanetary space at a velocity of up to 2000 km/s. Some CMEs are associated with solar flares, but their relationship is still unclear. The energy build-up and trigger process of solar flares and CME are still important unsolved problems in solar physics. To understand the flare-build-up process, it is crucial to observe the three-dimensional structures of the active region and the coronal loop. However, current solar observations can only provide two-dimensional data such as image data or a two-dimensional map of the surface magnetic field. Fig. 1 is an example of analysis on a large X-class flare. Observations with different wavelengths provide information on plasma distribution at different temperatures and
© 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 0 7 5 3 - 3 3 2 2 ( 0 2 ) 0 0 2 9 8 - 6
244s
Editorial / Biomed Pharmacother 56 (2002) 243s–246s
Fig. 1. Solar active region (sunspot group) observed by different wave lengths.
heights. In the lower right frame, the three-dimensional structure of the coronal magnetic field is derived by extrapolating the observations of the surface magnetic field. Observational and theoretical studies, focusing on threedimensional structures, are keys to answering the questions concerning flare build-up. Trials of the above-mentioned approaches for understanding three-dimensional structures and their changes are key issues in current solar physics. Many recent studies have revealed the preferable conditions for solar flares. The magnetic-field lines in the solar atmosphere are shaken at the foot point by turbulent motion in the photosphere, and deformed. These deformed magnetic field lines from the original configuration with a simple dipole contain excess energy as electric current; this excess energy is considered to be the energy source of solar flares. Tracking the deformation of the magnetic field lines will provide essential clues in the study of the flare mechanism. However, remote sensing of the solar magnetic field requires a highly sensitive spectroscopic polarimetry technique. To obtain information on the magnetic-field
structure in the upper solar atmosphere, calculation using remote-sensing data of the surface magnetic field is now possible. However, the sensitivity available with current observation techniques is still one or two orders lower than the scientific requirement. To overcome this difficulty, various ambitious projects, like Solar-B, are on-going and new observation techniques are under development. Solar activity has a clear periodicity of 11 years on average, which is called the solar-activity cycle. The origin of this cyclic behaviour is considered to be a kind of electromagnetic generation process which is driven by the internal rotation of the sun (i.e., the dynamo model). The sun rotates at a different angular velocity according to latitude, and different with solid rotation, such as with the earth. The rotation period near the solar equator as observed from the earth is about 27 days, but at a higher latitude, it is longer (i.e., the sun rotates more slowly). The rotation period near the solar pole is about 40 days. Rotation speed varies with not only latitude but also with depth. As a result, this differential rotation creates a strong magnetic field in
Editorial / Biomed Pharmacother 56 (2002) 243s–246s
245s
continuously observed by satellite. Although scientific observations of sunspots only have a three-hundred-year history (i.e., from Galileo’s era), efforts to estimate the history of solar activity through proxy data are continuing. Furthermore, a relationship between climate change and solar activity has been pointed out in regard to the ‘little ice age’ and the Maunder solar-minimum period. Research on this topic has just started and will be an important issue concerning future solar physics.
3. The sun, earth and the human race
Fig. 2. The sun at solar activity maximum (1991) and minimum (1996). (Courtesy of ISAS).
the subsurface convection zone, which acts as the origin of sunspots. This mechanism, called dynamo action, is considered to be the key mechanism in the continuous regeneration of solar magnetic flux with an 11-year periodicity. The difficulty in understanding this solar-cycle mechanism has been a lack of knowledge about the inside of the sun. Recently, solar seismology —a new, powerful tool for solar interior study— has been developed. It tackles the problem inversely, by revealing the physical properties of the solar interior from observations of the oscillation of the solar surface plasma. Solar seismology, coupled with longterm continuous observation data, is required to solve the solar-cycle problem. Fig. 2 shows X-ray images of the 11-year change of the sun’s solar flux. Recently, the change in the total solar flux (solar constant), along with the solar cycle, has been
Fig. 3. Chain of solar, interplanetary and terrestrial disturbances.
Fig. 3 is a schematic drawing of the relation between solar disturbances and the earth’s corresponding response. Energetic solar flares and/or CMEs sometimes produce solar energetic particle (SEP) events. Flaring solar atmosphere and interplanetary shock waves have been proposed as the mechanism that produces SEP. (‘acceleration’ makes little sense here). Recently, a hybrid model that combines seed-particle acceleration in a solar flare and acceleration in an interplanetary shock wave has been proposed. However, a theory to explain the acceleration mechanism to an energy of several hundred mega-electronvolts has yet to be proposed. CMEs launched from the sun propagate across interplanetary space at up to 2000 km/s and arrive at the earth in a few days. Strongly disturbed magnetic fields caused by the shock wave in front of the CMEs interact with the magnetic field of earth’s magnetosphere and produce a geomagnetic storm. Large geomagnetic storms can lead to a variation in earth’s radiation belt and ionosphere. In addition, X-ray
246s
Editorial / Biomed Pharmacother 56 (2002) 243s–246s
radiation accompanying solar flares brings about anomalous changes in the ionospheric conditions, causing short-wave fade out of telecommunication signals. Coronal holes, i.e. where coronal X-ray emission is weak, are sources of high-speed solar-wind streams. Such high-speed solar wind interacts with earth’s magnetosphere and causes recurrent-type geomagnetic disturbance. The origin of recurrent-type geomagnetic storms had been a mystery until the soft-X-ray telescope onboard ‘Skylab’ confirmed the existence of coronal holes. These space-weather disturbances have strong influences on long-range communications and navigation in shortwave range, satellite systems, and electric-power lines at high latitudes. Recently, SEPs have influenced manned space activities, especially on extravehicular ones (i.e., astronauts working in space suits outside their shuttle or station), and are causing great concern Therefore, research projects on space-weather phenomena are currently underway in many countries. The purpose of such research is to understand the mechanism of solar disturbance and its influence on the earth and various social systems. Prediction and alert systems for short-term variation of the space
environment, such as space-weather forecasting techniques, are being developed. The study of the relationship between the sun and the earth is in its initial phase. It is known that solar influence is exerted on the earth in various ways. Better scientific understanding of this relationship requires not only understanding the individual elementary processes of the sun and earth, but also their combined processes (My best guess). Moreover, when the relationship with humans (in particular, the human body) is taken into account, it becomes much more complex and difficult to model. This means that understanding the relationship between the sun and humans on earth is a challenging topic that will require long-term, continuous effort. M. Akioka Hiraiso Solar Observatory, Communications Research Laboratory, 3601 Isozaki, Hitachinaka, 311-1202 Ibaraki, Japan E-mail address: [email protected]