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| THE ATMOSPHERE |
The atmosphere is an envelope of gas surrounding a planet or satellite (or any astrophysical object). Even the Sun, which is a dense gas itself, has an atmosphere that is referred to as its corona. An atmosphere is bound to its object by the force of gravity. So if an object has an atmosphere, it is because its gravity is strong enough to overcome the natural tendency of the gases in the atmosphere to escape. This tendency exists because the gas particles have random motion related to their temperature. An existing atmosphere is then due to the "tug of war" between the strength of the object's gravity vs. the ability of the gas to expand away from the object due to its hot temperature. So, hot, low gravity objects have little or no atmosphere, and cold, high gravity objects are expected to have a substantial atmosphere, unless some other process took it away. Properties of interest in studying the atmosphere are its density, temperature, gas motion, existence and properties of any clouds, and the type and variety of gas constituents. In the Earth's case the main gas constituents are nitrogen (mostly) and oxygen, each generally in a molecular form, and its clouds are composed of water vapor. "Molecular form" in the Earth's case means, taking nitrogen for example, that two nitrogen atoms are combined to form the more complex particle called a molecule, and this is done by electric and magnetic forces. The Earth's atmosphere is separated into six general divisions, which are called (in order from the lowest layer): the troposphere, the stratosphere, the mesosphere, the thermosphere, the ionosphere (the electrically charged layer where the gas particles are a combination of molecules, atoms and ions), and the exosphere (the outermost region which blends into the magnetosphere ), where the gas molecules are only very weakly bound to the Earth. The thermosphere and ionosphere almost overlap spatially, but the ionosphere can vary temporally. |
Laboratory Branches participating in this area of research:
Codes 691 and 693
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| THE HELIOSPHERE |
The heliosphere is the huge region around the Sun that encloses the Sun's solar wind and its magnetic field, which is "frozen into" and carried out by the wind. It reaches out to the boundary where the solar wind meets the interstellar gas and where the Sun's gas and magnetic field's direct influence ends. This boundary is called the heliopause. The position and shape of the heliopause is not known, but this boundary is believed to lie well beyond the orbit of Pluto (at the planet's farthest point) and maybe several times this distance from the Sun even at its closest point. The heliosphere is not at all spherical in shape. Instead, it is very likely to be elongated along the direction of relative motion of the Sun with respect to the interstellar gas. In fact, the heliosphere is expected to have a tail like shape, similar to the magnetotail of Earth's magnetosphere. Presently there are two very distant active spacecraft, Voyagers 1 and 2, measuring the properties of the solar wind and its magnetic field on their way through the heliosphere toward the heliopause. They have not yet measured any direct indication of the heliopause that we know of. The expectation is that they will encounter it before they cease to function, estimated to be about year 2020 or so. Voyager 1 has traveled about 70 AU from the Earth as of November 1997. It is also believed that a shock wave may exist on each side of the heliopause, because of the relatively fast motion of the gases on both sides. They have not been directly seen either, but remote sensing by radio waves seem to indicate that the inner one may have been detected. It will be exciting to see what the true situation is if and when the two Voyager spacecraft encounter the heliopause and these possible shock waves. We believe that we understand the theory of these "boundaries," but nature has a way of surprising and humbling us. Stay tuned... |
Laboratory Branches participating in this area of research:
Codes 692, 695, and 696
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| THE IONOSPHERE |
| The ionosphere is the part of a planet's atmosphere where its gas is at least partially electrically charged. We refer to this charged gas as being "ionized." In the Earth's case the ionosphere starts at an altitude of about 90 km and extends more than a 1000 km. This electrical charging of an otherwise "neutral" atmospheric gas is the result of the Sun's ultraviolet light bombarding the atmospheric gas, which is mainly oxygen and nitrogen particles (called molecules) in the Earth's case. Specifically this means that the Sun's ultraviolet light knocks electrons off the gas molecules, resulting in a high fraction of electrically charged gas particles, called ions; some neutral molecules remain. As well as Earth, Venus, Mars and Jupiter are examples of planets that are well known to have ionospheres, but the composition of the ionospheric gases are different from Earth's and from each other. The Earth's ionosphere can be studied by instruments on board rockets and balloons, as well as from Earth's surface using radar. |
Laboratory Branches participating in this area of research:
Codes 691, 693 and 696
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| MAGNETIC FIELDS |
Magnetic fields are imagined "lines of force" that arise from the motion of electrically charged particles, either as familiar electrical currents or, on a microscopic scale, as tiny currents working together in magnetized materials. Whenever a charged particle (charged either plus or minus), or an electrical current, moves through a magnetic field there is a magnetic force that is perpendicular to both the magnetic field and the direction of the moving particle (or current). And the strength of the magnetic force depends on the strength of both the magnetic field and the intensity of the moving charged particles, meaning their speed and density. So we see that magnetic forces have a complex character. Magnetic fields are very common in space, because moving electrically charged particles are very common in space, and these act like currents. [In fact, in the part of the universe that is fairly well understood electrically charged gases are the most common form of matter (and in the solar system), although on Earth they are not commonly experienced outside of laboratories; "dark matter" is the part of the universe that is not understood. Under certain conditions these charged gases are called plasmas.] Magnetic fields play a very important role in our everyday experience even if we are not always conscious of their influence. For example, electrical currents driven deep in the Earth generate a global external magnetic field which orients a compass which guides hikers and helps to shield us from very energetic charged particles called galactic cosmic rays and even from energetic charged particles from the Sun. Also the operations of the TV, radio, telephone, electrical motors, electrical generators, computers, parts of car engines, packaging machines, and on and on - depend on magnetic fields. In making such a list it would be almost easier to say that the list should include all "appliances/machines" in a modern society and then list the exceptions! |
Laboratory Branches participating in this area of research:
Codes 691, 692, 693, 695, and 696
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| THE MAGNETOSPHERE |
The magnetosphere (of a magnetized planet) is a region around the planet formed by the motion of the solar wind ramming into the magnetic field of the planet. It can be thought of as a cavity in the solar wind around the planet caused by the presence of the planet's magnetic field. At the boundary of the magnetosphere, called the magnetopause, the inside pressure of the magnetosphere itself and the outside pressure of the solar wind are equal when the boundary is not moving. (See the figure.) In the earth's case the inside pressure is mainly due to the magnetosphere's own magnetic field. (In the case of Jupiter's magnetosphere, the inside pressure is made up of particle pressure as well as magnetic field pressure making its magnetosphere act in a flimsy way as the solar wind rams into it.) A planet's magnetosphere is not at all spherical in shape. In fact, "-sphere" may be considered, instead, to be a "sphere of influence" of magnetic fields. The magnetosphere is very elongated in shape, because of the consequences of the very directed flow of the solar wind ramming into it. The elongated region extending away from the Sun is called the magnetotail. The magnetotail's axis is approximately aligned with the solar wind flow direction. The magnetic field of the magnetosphere is the result of the combination of the intrinsic magnetic field of the planet plus those fields due to the flowing electrical currents in the magnetosphere and on its boundaries. The flowing currents sometimes vary as a result of changes within the solar wind, and hence, the magnetosphere's field varies. In fact, the influence of the solar wind's variations (measured mainly by its density, speed, and magnetic field) on the magnetosphere help to determine the magnetosphere's changing size, shape, and general state, as well as its magnetic field. Most of the planets of the solar system have been observed to have magnetospheres, including Earth. [Venus is an example of a planet that does not have a magnetosphere and Pluto's case is unknown.] And there are apparently an uncountable number of objects similar to solar system magnetospheres throughout the universe, the exotic and rotating magnetic field regions around pulsars, being examples. |
Laboratory Branches participating in this area of research:
Codes 695 and 696
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| THE SOLAR WIND |
 The solar wind is an electrically charged gas that usually blows almost directly outward from the Sun with a density so low that it is nearly a vacuum in comparison to our atmosphere at the Earth's surface. The solar wind is in actuality the extension of the Sun's corona, its atmosphere. Its density at Earth's distance from the Sun is typically about 6 particles per cubic centimeter, although it is highly variable. It is composed of the same matter as the Sun itself being principally protons (having plus one charge) and an almost equal number of electrons (with a minus one charge) for any given volume of space. However, many other positively charged atoms (called ions) heavier than protons but at even much lower densities are sometimes observed by spacecraft instruments. At Earth's orbital distance from the Sun the speed of the solar wind varies from about 200 km/s to 1200 km/s (i.e., from about 0.5 to 3 in millions of miles per hour) and is typically 420 km/s (one million miles/hr). Frozen into this wind are complex magnetic fields. Such fields owe their existence to electrical currents flowing in the wind. The dynamical motions of the solar wind are usually dominated by its internal gas pressure, measured by its temperature and density, but for some very interesting subregions within the solar wind forces from its own magnetic field dominate over the gas pressure, giving regions that appear in some respects like magnetospheres. These regions can be much larger than any known planetary magnetosphere, however. These regions of magnetic field domination in the solar wind are not observed commonly, but they are the subject of much present day research, because of interest in their origin at the Sun and because of their usually dramatic effects at the Earth. |
Laboratory Branches participating in this area of research:
Codes 692, 695 and 696
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| THE SUN |
The Sun is the Earth's nearest star. Similar to most typical stars, it is a large ball of hot electrically charged gas which obtains its energy from nuclear reactions in its core, much like energy expected from a countless number of hydrogen bombs exploding. This nuclear energy heats the ball of gas up causing it to tend to expand. However, the weight of this gas, which is very dense at the core or central region, causes the ball to try to contract. This "tug of war" between these tendencies to expand and contract at the same time results, at the present time, in a steady situation. The gas ball of the Sun maintains a nearly constant nearly spherical shape with a diameter of about 1.6 million km (i.e., about one million miles across). The Sun's inner regions are so dense that a light particle, called a photon, that is formed near the center takes on average more than 10 million years to get to the surface as it bounces around inside from one material particle to another countless times in its net outward path. The Sun rotates once in about 25 days, but is seen at the Earth's location to rotate once about every 27 days, because the Earth is revolving around the Sun in the same direction as the Sun's rotation but much slower. The Sun has a complex magnetic field that has a main part that switches its field-sense once every 22 years. There are complex features on the Sun's surface and in its vast atmosphere, called the corona, as seen by various kinds of "light" rays (x-rays, ultraviolet, radio, etc.). These are almost entirely due to the strong influence of the Sun's complex magnetic fields at, below, and above its surface, the photosphere. In other words, if there were no magnetic fields on the Sun, there would be a much more uniform appearance to the Sun's surface and atmosphere. The solar wind comes from the expansion of the Sun's corona. |
Laboratory Branches participating in this area of research:
Codes 692, 695 and 696