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Upper-Atmosphere Electric-Field Sources 14 ARTHUR D. RICHMOND National Center for Atmospheric Research The Earth's space environment is filled with electrons and positive ions, comprising a plasma of very low den- sity. These charged particles collide only infrequently and are strongly influenced by magnetic and electric fields. In turn, the charged particles affect the distribu- tions of the magnetic and electric fields in space. The space plasma environment, therefore, is dominated by electrodynamic processes. The regions of space involved in creating upper atmosphere electric fields are illus- trated in Figure 14.1 and are described below. The solar wind is a plasma with electron and ion number densities of order 5 X 106 m~3 flowing contin- ually outward from the Sun at a speed of 300-1000 km/ sec. Imbedded within it is the interplanetary magnetic field (IMF), which is maintained by electric currents flowing throughout the solar-wind plasma. The IMF strength at the orbit of the Earth is roughly a factor of 10-4 smaller than the strength of the surface geomag- netic field. Most of the time, an interplanetary field line near the Earth can be traced back to the surface of the Sun, where magnetic fields are ubiquitous. As the Sun rotates (once every 27 days) different magnetic regions influence the IMF near the Earth. The combination of solar-rotation and outward solar-wind flow produce a roughly spiral IMF pattern. In addition, the solar-wind velocity can change dramatically and produce both large-scale and small-scale distortions of the IMF so that 195 the field direction and strength vary greatly. These changes have been found to influence the electrical state of the magnetosphere and ionosphere. The magnetosphere is the region of space where the geomagnetic field has a dominant influence on plasma properties. As the charged particles of the solar wind are deflected by the geomagnetic field, an electric current layer is formed at the boundary between the solar wind and the magnetosphere, called the magnetopause. This current layer distorts the geomagnetic field from the di- pole-like configuration that it would otherwise have and helps to create a long magnetized tail trailing the Earth. Although the full extent of this tail has not yet been determined, it is known to be more than 500 Earth radii. The magnetosphere contains the radiation belt, composed of energetic charged particles trapped in the magnetic field. The number density of electron-ion pairs in the magnetosphere is highly variable, ranging in order of magnitude from a low of 106 m ~ 3 in parts of the tail up to 10~2 m - 3 in the densest portions of the dayside ionosphere. The ionosphere is the ionized component of the Earth's upper atmosphere. It is not distinct from the magnetosphere, but rather forms the base of the magne- tosphere in terms of electrodynamic processes. The lower boundary of the ionosphere is not well defined but can be taken as about 90 km altitude for the present pur

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196 FIGURE 14.1 Configuration of the magnetosphere. The magnetic field is shown by continuous lines, and locations of important magne- tospheric features are pointed out. poses, representing the level where the density of elec- tron-ion pairs falls to roughly 10~ m ~ 3 and below which electric currents become relatively small. The ioniza- tion is formed largely by the effect on the upper atmo- sphere of solar extreme-ultraviolet and x-ray radiation at wavelengths shorter than 102.6 nm, but energetic particles impacting the upper atmosphere from the magnetosphere also create important enhancements. The ionospheric plasma has a temperature on the order of 1000 K, which is much cooler than the energetic plasma farther out in the magnetosphere. Collisions be- tween charged particles and neutral atmospheric mole- cules become important below 200 km altitude and strongly affect the electrodynamic characteristics of the ionosphere. At high latitudes, where magnetic-field lines connect the ionosphere with the outer magnetosphere, the iono- spheric features are quite complex. Ionospheric phe- nomena become better organized in a coordinate system based on the geomagnetic field than in geographic co- ordinates, with the difference arising mainly from the 11 tilt of the dipolar field from the Earth's axis. Differ- ent magnetic coordinate systems exist, but for descrip- tive purposes the differences are not crucial, and the simple terms "magnetic latitude" and "magnetic local time" will suffice here. At high magnetic latitudes, aurora are produced as energetic charged particles, mainly electrons, precipi- tate into the upper atmosphere from the outer magneto- sphere, creating both visible emissions and ionization enhancements. The aurora form a belt around the mag- netic pole, called the auroral oval. The oval is in fact ARTHUR D. RICHMOND roughly circular, of variable size, and has a wider latitu- dinal extent on the nightside than on the dayside of the Earth. The entire oval is shifted toward the nightside, so that aurora appear at lower magnetic latitudes at night (roughly 67) than during the day (roughly 78~. The nighttime particle precipitation also tends to be more intense and widespread than on the dayside. Contained within the auroral oval is the polar cap, where auroras are less frequent but where on occasion very energetic protons from solar flares enter and penetrate relatively deep into the upper atmosphere. A more detailed description of the Earth's space envi- ronment can be found in several of the references listed at the end of this paper, especially in the book of Akasofu and Chapman (1972~. ELECTRODYNAMIC PROCESSES IN SPACE The charged particles of a plasma react strongly to electric and magnetic fields. There is a strong tendency for the particles to short out any electric fields, so that it is often a good approximation to treat the electric field in the frame of reference of the plasma as vanishing: Ep~asma= 0. (14.1) This is often called the magnetohydrodynamic (or MHD) approximation (e. g., Roederer, 1979~. The frame-of-reference choice is important if the plasma is moving and if a magnetic field is present, because the electric field observed in a different reference frame is not the same. If we let E be the electric field in an Earth- fixed reference frame, V be the velocity of the plasma with respect to the Earth, and B be the magnetic-field vector, then a (nonrelativistic) Lorentz transformation yields Ep~asma = E + V X B. (14.2) where the vector product V X B results in a vector di- rected perpendicular to both V and B. The electric field is then simply related to the plasma velocity and mag- netic field by the approximate relation E = - V X B. (14.3) Alternatively, the velocity component perpendicular to B can be related to E and B as V = (E X B)/B2. (14.4) Equations (14.3) and (14.4) express the same fact from two different points of view: the electric field and plasma velocity are closely interrelated and help to de- termine each other. In some cases, as in the solar wind where plasma momentum is high, the electric field quickly adjusts toward the value given by Eq. (14.3~. In other cases, as in the upper ionosphere where electric

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UPPER-ATMOSPHERE ELECTRIC-FIELD SOURCES fields tend to be imposed on the plasma as a result of the dynamo processes to be discussed, the plasma quickly is set into motion at the velocity given by Eq. (14.4~. A further important consequence of the MHD ap- proximation, when combined with the Faraday law of magnetic induction, is the following: all plasma parti- cles lying along a common magnetic-field line at one instant of time will forever remain on a common field line. The magnetic field may vary both temporally and spatially, and the plasma may move from one region of space to another, but plasma ions and electrons will con- tinue to share a field line with the same partners. This result affects both the magnetic-field configuration and the plasma velocity. In the solar wind, the magnetic field is distorted to follow the motions of the plasma. Nearer the Earth, where the magnetic field is so strong that it is not easily distorted, the constraint means that all particles on a dipolar field line must move simultane- ously together to another field line. Convection of plasma thus can be mapped between the outer magneto- sphere and the ionosphere along magnetic-field lines. The electric field similarly maps along the magnetic field. The MHD approximation is useful in interrelating plasma motions with electric and magnetic fields, but it breaks down under a number of important circum- stances, especially where electric current densities are large. Furthermore, the approximation does not explain the distribution of currents, which are a central element in the processes giving rise to upper-atmospheric electric fields. Current flow across magnetic-field lines exerts a force on the medium, a force that must either be bal- anced by other forces, like pressure gradients, or else result in acceleration of the medium. Consideration of force and momentum balance thus is an important part of understanding currents in plasmas. One place where the MHD approximation breaks down is in the lower ionosphere, below 150 km, where collisions between ions and the much more numerous air molecules are sufficiently frequent to prevent the ions from maintaining the velocity given by Eq. (14.4~. In this region electric current readily flows across mag- netic-field lines. As we shall see, neutral-air winds in the lower ionosphere lead to generation of electric currents and fields, and for this reason the height range of roughly 90-150 km is called the "dynamo region" of the ionosphere. Unlike the outer magnetosphere, the ionosphere be- haves as an Ohmic medium, with the current density linearly related to the electric field under most circum- stances. The conductivity, however, is highly aniso- tropic owing to the presence of the geomagnetic field. The conductivity in the direction of the magnetic field is very large, so that the electric-field component in this 197 direction is almost entirely shorted out, and magnetic- field lines are nearly electric equipotential lines at all altitudes above 90 km. The conductivity characteristics perpendicular to the magnetic field depend on the rate of ion-neutral collisions, and they change with altitude as the neutral density varies. Figure 14.2 shows typical mid-latitude conductivity profiles for day and night conditions. In the nighttime auroral oval the conductiv- ity is more akin to the "Day" profile in Figure 14.2 than to the "Night" profile because of ionization production associated with the aurora. Although the ionospheric plasma density typically maximizes at around 300 km altitude, the conductivity perpendicular to the mag- netic field maximizes at around 110 km, with a large day-night difference owing to the day-night difference in ionospheric density. The current component perpen- dicular to the magnetic field flows in a direction as much as 88 different from the electric-field direction, as also shown in Figure 14.2. This effect results from the fact that electrons are relatively little influenced by col- lisions above 80 km and move perpendicular to the elec- tric field as given by Eq. (14.4), while positive ions are strongly affected by collisions below 130 km and are un- able to cancel current carried by the drifting electrons. SOLAR WIND/MAGNETOSPHERE DYNAMO In an analogy with a dynamo-electric machine that generates electricity by rotating a conducting armature through a magnetic field, the motion of a plasma through a magnetic field produces an electromotive 300 - y - ~ 200 Cl 00 o ANGLE BETWEEN CONDUCTIVITY MAGNITUDE CURRENT AND PERPENDICULAR TO MAGNETIC FIELD ELECTRIC FIELD 400 1 1 1 1 / Night \Day 10-4 10-3 Go 45O 90O 10-7 10-6 10-5 S/m FIGURE 14.2 Typical profiles of the ionospheric conductivity com- ponent perpendicular to the geomagnetic field for day and night con- ditions (left) and angle between the current and electric-field compo- nents perpendicular to the geomagnetic field (right).

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198 force and current flow and can also be considered a dy- namo. The solar wind/magnetosphere dynamo results from the flow of the solar wind around and partly into the magnetosphere, setting up plasma motion in the magnetosphere as well an electric field and currents (e.g., Stern, 1977; Hill, 1979; Roederer, 1979; Cowley, 1982~. All details of this process are not yet understood, but a number of features have become clear. Strong evidence exists that the magnetosphere is partly open, that is, that some magnetic-field lines traced from the Earth extend indefinitely into inter- planetary space (Hill and Wolf, 1977; Stern, 1977; Ly- ons and Williams, 1984~. The amount of magnetic flux that connects to the interplanetary magnetic field may be as great as the entire flux passing through the polar caps. Figure 14.3 shows a schematic figure of the mag- netic-field configuration for the simple case where the IMF is directed southward. If the IMF had an east-west component, as it usually does, we would require a three- dimensional representation of the magnetic-field inter- connection. Details of the magnetic-field configuration are not yet resolved, so Figure 14.3 should be treated more as a conceptual tool than as a true representation of the magnetosphere. The essential features are the four classes of magnetic-field lines denoted in Figure 14.3: (1) closed field lines connected to the Earth in both northern and southern hemispheres; (2) interplanetary field lines unconnected to the Earth; (3) open field lines connecting the northern polar cap to interplanetary space; and (4) open field lines connecting the southern polar cap to interplanetary space. The interplanetary electric field, obtained from Eq. (14.3), is directed out of the page in Figure 14.3. To the extent that the MHD approximation is valid and electric fields map along the magnetic field, the polar iono- sphere is also subject to an electric field out of the page, t ~ \ ~ ~ .~ ::= 14 l ~ ~ ~ ~ FIGURE 14.3 Schematic diagram of magnetic field and plasma flow in the solar wind/magnetosphere environment (from Lyons and Williams, 1984). Continuous lines show the magnetic field for the case where the IMF is purely southward. Open arrows show the plasma velocity direction. Numbers 1-4 denote magnetic regions of different topology, as discussed in the text. ARTHUR D. RICHMOND causing ionospheric plasma to convect antisunward. The magnitude of the ionospheric electric field is greater than that of the interplanetary electric field because the bundling of magnetic-field lines at the ionosphere causes electric potential gradients to intensify. On the other hand, the plasma drift velocity in the upper iono- sphere, given by Eq. (14.4), is much less than the solar- wind velocity because of the inverse dependence on magnetic-field strength. The polar-cap electric field is typically 20 mV/m, giving an ionospheric convection velocity of roughly 300 miser. Other directions of the IMF than shown in Figure 14.3 result in a somewhat altered pattern of polar-cap ionospheric convection, but a usual feature is the general antisunward flow. More about IMF influence on the high-latitude electric field is discussed in a later section. The physical processes that determine the amount of magnetic flux that interconnects the geomagnetic field and the IMF are not well understood and are the subject of much study (e.g., Cowley, 1982~. They clearly in- volve a violation of the MHD approximation since the ionospheric and solar-wind plasmas coexisting on an open magnetic-field line at one instant of time could not have lain on a common magnetic-field line throughout their entire histories. The violations of this approxima- tion occur to some extent throughout the magnetosphere but are particularly important in at least two regions: at the sunward magnetopause and somewhere in the mag- netospheric tail. At the sunward magnetopause plasma flows together through unconnected interplanetary and magnetospheric magnetic fields and flows out north- ward and southward on interconnected magnetic-fields lines (Figure 14.3~. In the tail plasma flows together on interconnected field lines and flows out through uncon- nected magnetospheric and interplanetary magnetic fields. In the closed portion of the magnetosphere plasma flows generally toward the Sun, passing around the Earth on the morning and evening sides (out of the plane of Figure 14.3~. However, some of the outermost portions of the closed-field region convect away from the Sun because of momentum transfer from the nearby solar wind (e. g., Hones, 1983) . Magnetospheric plasma convection has a number of important consequences, one of which is the energiza- tion of plasma and particle precipitation into the iono- sphere. As plasma flows from the tail toward the Earth it is compressionally heated because the volume occu- pied by plasma on neighboring magnetic-field lines is reduced as the magnetic-field strength increases and as the length of field lines decreases. Other particle accel- eration processes also help to energize the plasma. Some of the energized particles precipitate into the ionosphere and create ionization enhancements, especially in the auroral oval. The energized plasma also has an impor

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UPPER-ATMOSPHERE ELECTRIC-FIELD SOURCES tent influence on the flow of electric currents and on the distribution of electric fields (e.g., Spiro and Wolf, 1984~. Energetic particles drift in the Earth's magnetic field-electrons toward the east and positive ions to ward the west so that a westward current flows within the hot plasma. This westward current, flowing in the geomagnetic field, essentially exerts an electromagnetic force on the plasma directed away from the Earth and thus tending to oppose the earthward] convection. Charge separation associated with the current tends to DUSK create an eastward electric-field component, opposite to the nightside westward convection electric field, largely canceling the convection electric field in the inner mag netosphere. The overall pattern of magnetospheric convection tends to map along the magnetic field into the iono sphere even though this mapping is imperfect because of net electric fields that tend to develop within the non uniform energetic plasma. In the upper ionosphere, where Eq. (14.4) is valid, the general convection pat tern looks something like that shown in Figure 14.4. There is antisunward flow over the polar cap and sun ward flow in most of the auroral oval. The dayside magnetopause maps perhaps somewhere near the polar cap auroral oval boundary on the dayside, while the most distant closed field lines in the tail map perhaps somewhere near the polar cap boundary on the night side (the mapping of these outer magnetospheric regions into the ionosphere is not yet well determined). The flow lines in Figure 14.4 correspond to lines of constant electrostatic potential in a steady state. There is a poten tial high on the dawn side of the polar cap and a low on the dusk side, with a potential difference of the order of 50,000 V. The electric-field strength in the auroral oval tends to be somewhat larger than the polar-cap electric field. Currents are an integral part of the electrical circuit associated with the solar wind/magnetosphere dynamo (e.g., Banks, 1979; Roederer, 1979; Stern, 1983; Aka sofu, 1984~. Figure 14.5 shows schematically the cur rent flow near the Earth. Currents flowing along the direction of the magnetic field (field-aligned currents) couple the auroral oval with outer portions of the mag netosphere. The upward and downward currents are connected by cross-field currents in the ionospheric dy namo region. The anisotropy of the dynamo-region con ductivity gives rise to strong current components per pendicular to the electric field in the auroral oval, in the form of eastward and westward auroral electrojets. IONOSPHERIC WIND DYNAMO Winds in the dynamo region have the effect of mov ing an electric conductor (the weakly ionized plasma) 199 SUN ~ :_ ~ ~ AURORAL 8 kV - ~ - - OVAL 00 HRS. FIGURE 14.4 Schematic diagram of the magnetic north polar re- gion showing the auroral oval and ionospheric convection (from Burch, 1977~. The convection contours also represent electric potential contours, with a potential difference of order 8 kV between them. through a magnetic field (the geomagnetic field), which results in the production of an electromotive force and the generation of electric currents and fields (e. g., Aka- sofu and Chapman, 1972; Wagner et al., 1980~. The effective electric field driving the current, E ', is related to the electric field in the earth frame, E, the wind ve- locity u, and the geomagnetic field, B. by E' = E + uXB. (14.5) Thus we may consider that two components of current exist, one driven by the "real" (measurable) electric field E and the other driven by the "dynamo electric field" FIELD ALIGNED ELECTROJET CURRENTS COW:\ ~/// CURRENTS DRIVEN-it\ ~ ~ \ BY SOLAR HEATING ~ ~ \/ EQUATORIAL i=:= / AURORAL OVAL ELECTROJET '5 SOLAR RADIATION ~ RELD AU - ED / ~1CURRFNTR FIGURE 14.5 Schematic diagram of electric currents in the iono- sphere and inner magnetosphere.

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200 u x B. These two components are not independent, however, because it turns out that the electric field E itself depends on the dynamo electric field u X B. To see how ionospheric winds cause electric fields to be set up, let us for the moment ignore the effects of the solar wind/magnetosphere dynamo. The dynamo elec- tric field associated with the wind will drive a current. In general, this current would tend to converge in some regions of space and cause an accumulation of positive charge, while in other regions of space it would diverge and cause negative charge to accumulate. These charges would create an electric field directed from the positive toward the negative regions, which would cause current to flow tending to drain the charges. An equilibrium state would be attained when the electric-field-driven current drained charge at precisely the rate it was being accumulated by the wind-driven current. Actually, the time scale for this equilibrium to be achieved is ex- tremely rapid, so that the electric field is effectively al- ways in balance with the wind. The magnitude of the electric field is of order 1 mV/m. A net current flows in the entire ionosphere owing to combined action of the wind and electric field, especially on the sunlit side of the Earth (e.g., Takeda and Maeda, 1980, 1981~. Figure 14.5 shows a large-scale current vortex at middle- and low-latitudes flowing counterclockwise in the northern hemisphere. A corresponding clockwise vortex flows in the southern hemisphere. These vortices are known tra- ditionally as the Sq current system because of the nature of the ground-level magnetic variations that they pro- duce: S for solar daily variations and q for quiet levels of magnetic activity. At high latitudes the electric fields and currents produced by the ionospheric wind dynamo are relatively weak in comparison with those of the solar wind/magnetospheric dynamo. Winds in the upper atmosphere change strongly through the course of the day. They are driven in one ARTHUR D. RICHMOND form or another by the daily variation in absorption of solar radiation. Atmospheric heating causes expansion and the creation of horizontal pressure gradients, which drive the global-scale upper-atmospheric winds. Solar- ultraviolet radiation absorption at the height of the dy- namo region drives a major portion of the winds. Ab- sorption by ozone lower down (30-60 km altitude) also affects the clynamo-region winds by generating atmo- spheric tides that can propagate upward as global-scale atmospheric waves (e. g., Forbes, 1982a, 1982b). Figure 14.6 gives an example of three days of wind measure- ments at 100-130 km above Puerto Rico, showing the strong effects of propagating tides. Important contribu- tions to the dynamo also come from altitudes above 130 km, where the conductivity is smaller (Figure 14.2) but where winds tend to be stronger and to vary less in height. Figure 14.7 shows the average global electrostatic po- tential generated by the ionospheric wind dynamo, ex- pressed in magnetic coordinates. The zero potential is arbitrarily defined here such that the average iono- spheric potential over the Earth is zero. The total poten- tial difference of the average pattern (4.7 kV) is smaller than what can usually be expected on any given day and is much smaller than that associated with the solar wind/magnetospheric dynamo. This pattern was de- rived from observations of plasma drifts on magneti- cally quiet days at an altitude of about 300 km. The elec- tric-field maps along magnetic-field lines between hemispheres, providing the symmetry about the mag- netic equator even when the wind dynamo action in op- posite hemispheres is asymmetric. Magnetic-field lines peaking below 300 km in the equatorial region are not represented in Figure 14.7. Electric fields in the equato- rial lower ionosphere have a localized strong enhance- ment of the vertical component associated with the strong anisotropy of the conductivity in the dynamo re AUG 10, 1 974 E 130 20 - 110 100 . . ' ' ' ' ' ' '1 AUG 12, 1974 ~ arm// ~ ~ _ - .~ ~ = . . . . . . . . . oG 10 1 2 14 16 08 10 T I ~ E (AST) AUG 13, 1974 ~- 2 14 16 08 10 12 14 16 Contour level: 20 m/s FIGURE 14.6 Observed eastward (unshaded) and westward (shaded) winds above Puerto Rico curing three daytime periods in August 1974 (from Harper, 1977). The contour level is 20 miser.

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UPPER-ATMOSPHERE ELECTRIC-FIELD SOURCES - ~n 60 Cal 30 - 77~) ~ O - -60~ ~1~== Z 0 3 6 9 12 tO 18 21 24 ~ MAGNET IC LOCAL T Il\/IE ( hours ) FIGURE 14.7 Average quiet-day ionospheric electrostatic potential at 300 km altitude as a function of magnetic local time (from Rich- mond et al., 1980~. The contour level is 500 V, and extreme relative to the global average are labeled in kilovolts. "ion. This enhanced electric field drives an eastward daytime current along the magnetic equator called the equatorial electrojet, as seen in Figure 14.5 (e.g., Forbes, 1981~. VARIABILITY The preceding sections discussed the average patterns of ionospheric electric fields. Substantial deviations from these patterns occur, on a global scale as well as on a localized scale, and with a wide range of time scales. Because the solar wind and particularly the IMF un- dergo large variations (e.g., Hundhausen, 1979), it is not surprising that the electric fields associated with so- lar wind/magnetosphere dynamo action similarly show large variations (e. g., Cowley, 1983; Rostoker, 1983~. The interconnection of the interplanetary and magneto- spheric magnetic fields maximizes when the IMF is southward, as in Figure 14.3. As the IMF direction ro- tates out of the plane of Figure 14.3 the amount of inter- connected magnetic flux appears to lessen. For a north- ward-directed IMF the pattern of magnetic-field interconnection must be quite different from that shown in Figure 14.3, and it is possible that interconnec- tion becomes insignificant, so that the magnetosphere is closed. The east-west component of the IMF affects the interconnection morphology and consequently also the pattern of high-latitude magnetosphere convection. The third IMF component, in the plane of Figure 14.3 but directed toward or away from the Sun, seems to have a less important role in the solar wind/magneto- sphere dynamo than the other two components. How- ever, the toward/away component correlates strongly with the west/east IMF component because of the ten 201 dency for the IMF to assume a spiral pattern around the Sun. Some discussion of IMF effects has therefore taken place in reference to the toward/away component rather than the west/east component. Figure 14.8 shows the average patterns of electric po- tential above 60 magnetic latitude deduced from ground magnetic variations for four different directions of the IMF. Bz is the northward component of the IMF; By is the eastward component (toward the east for an observer on the sunward side of the Earth). The electric fields are stronger for a southward IMF (Bz < 0) than for northward IMF (Bz ~ 0), in accord with the concept that magnetic field interconnection is greater and mag- netospheric convection is stronger for a southward IMF. There are also clear differences in the patterns between the westward (By 0) IMF cases, especially on the sunward side of the Earth. Magnetic storms are dramatic disturbances of the en- tire magnetosphere lasting a time on the order of 1 day, usually produced by a strong enhancement of the solar- wind velocity, density, and/or southward IMF compo- nent (e. g., Akasofu and Chapman, 1972~. The enhance- ments often come from explosive eruptions of plasma ELECTRIC POTENTIAL BOO 12 FIGURE 14.8 Average electric potential patterns above 60N mag- netic latitude as a function of magnetic local time deduced from mag- netic variations at the ground, for four directions of the IMF (see text). The contour level is 4 kV (from Friis-Christensen et al., 1985).

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202 \ near the Sun's surface but sometimes are a long-lasting, relatively localized feature of the solar wind that sweeps past the Earth as the Sun rotates. During a magnetic storm magnetospheric convection varies strongly but is generally enhanced; plasma energization and precipita- tion into the ionosphere are greatly increased; and elec- tric current flow is much stronger, also varying rapidly in time. Figure 14.9 gives an example of ground-level magnetic fluctuations caused by magnetospheric and ionospheric currents during a large storm. Quiet-day variations in the declination (D), vertical component (Z), and horizontal intensity (H) are seen up until 8:30 UT, when a shock wave in the solar wind hit the magne- tosphere to produce a storm sudden commencement. Perturbations up to a few percent of the total geomag- netic-field strength occurred during the subsequent hours. Storms are composed of a succession of impulsive disturbances lasting 1-3 hours, called substorms (e.g., Akasofu, 1977; Nishida, 1978; McPherron, 1979~. Im- pulsive disturbances with similar characteristics also oc- cur on the average a few times a day even when no storm is in progress, and these are also called substorms. The characteristics of substorms can vary quite considerably from one to another but are often associated with what appears to be a large-scale plasma instability in the mag- netospheric tail. The disturbed electric fields extend be- yond the auroral oval and can even be seen at the mag- netic equator (e. g., Fejer, 1985~. Magnetic storms are predominantly a phenomenon of the solar wind/magnetosphere dynamo, but they affect the ionospheric wind dynamo as well. In addition to strong auroral conductivity enhancements, conductivi- ties can also be altered at lower latitudes at night by over D > =, in AS o Z to j H _ . _ ARTHUR D. RICHMOND an order of magnitude (Rowe and Mathews, 1973), though they still remain well below daytime values. The nighttime ionospheric layer above 200 km altitude can be raised or lowered in response to stormtime electric fields and winds, which changes its conductive proper- ties. During major storms the entire wind system in the dynamo region can be altered by the energy input to the upper atmosphere, so that the pattern of electric-field generation is modified (Blanc and Richmond, 1980~. This effect can raise the potential at the equator by sev- eral thousand volts with respect to high latitudes. Regular changes in the electric fields and currents oc- cur over the course of the 11-year solar cycle and with the changing seasons. Ionospheric conductivities change by up to a factor of 2 as the ionizing solar radiation waxes and wanes along with the trend of sunspots. Ionospheric winds also change as the intensity of solar extreme-ultraviolet radiation increases and decreases (e.g., Forbes and Garrett, 1979~. We know that iono- spheric dynamo currents change by a factor of 2 to 3 with the solar cycle (e.g., Matsushita, 1967), but the variation of electric fields is not yet so extensively docu- mented. At Jicamarca, Peru, however, ionospheric elec- tric fields have been measured on an occasional basis for well over a solar cycle, and the average behavior of the east-west equatorial electric-field component at 300 km altitude is shown in Figure 14.10, as represented by the vertical plasma drift that it produces. The presence of a strong upward drift after sunset at solar sunspot maxi- mum (1968-1971) is not usually present at solar mini- mum (1975-1976~. Clear seasonal variations appear at Jicamarca as well as at higher latitudes. Day-to-day changes in middle- and low-latitude electric fields and FRE )ERICKS BERG V, ~ - 22! 4_ ~ - F _ Izl'~ Fang IZ . ~4 T 6 U.T. 2 O ~2 4 FIGURE 14.9 Magnetogram from Fredericksburg, Virginia, on March 22, 1979, showing variations of the magnetic declination (D)' vertical magnetic field (Z), and horizontal magnetic intensity (H) during a magnetic storm. Scale values are not shown.

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UPPER-ATMOSPHERE ELECTRIC-FIELD SOURCES mars 40 20 ~ I ~ ~ ~ JlCAMARCa VERTICAL DRIFTS WINTER SOLSTICE 8 May-7Aug . . ~ EQUINOX -20 - 4 hb-7May 8 Aug-9 Nov MA R I\ _ ,_ ~` _ j,~ BE _` _ my' 20 o SUMMER SOLSTICE 10 Nov-3 Feb .~ ~ ~ At . , , ~ ~ ., 1, , ~ . ~ I ~ ~ ~ ~ 16 20 00 04 08 LOCAL TIME (75 W) 08 12 FIGURE 14.10 Average quiet-day vertical component of the plasma drift velocity caused by east-west electric fields over Jicamarca, Peru, as a function of local time (from Fejer et al., 1979~. 1968-1971 (a) are sunspot cycle maximum years, while 1975-1976 (b) are sunspot cycle minimum years. currents also occur, produced by corresponding changes in the ionospheric winds. Rapid variations with time scales ranging from sec- onds to hours are a common feature of ionospheric elec- tric fields and currents. The magnitude of the fluctua- tions is often as large as that of the regular daily variations, generally increasing with magnetic latitude up to the auroral oval. Besides storm and substorm phe- nomena, global-scale disturbances in the electric fields and currents also occur with the arrival of solar-wind shocks, with fluctuations of the IMP, and with rapid ionosphere conductivity changes during solar flares. Lo- calized ionospheric electric-field fluctuations may be as- sociated with wind or conductivity irregularities or with small-scale magnetospheric processes. For example, lo- calized quasi-periodic oscillations in the electric fields and currents with periods ranging from a fraction of a second to minutes, called pulsations, are often observed at high latitudes (e. g., Nishida, 1978~. EFFECTS OF UPPER-ATMOSPHERIC ELECTRIC FIELDS AND CURRENTS Electric fields and currents interact strongly with the upper atmosphere and help determine its behavior 203 (e.g., Banks, 1979~. The drifting ions, as they interact collisionally with neutral molecules, exert a force on the air and tend to bring it toward the ion motion. Above 200 km altitude this effect can be important: at high latitudes winds are common that approach the rapid ve- locity of the convecting plasma (e.g., Meriwether, 1983), while at low latitudes, where plasma drifts are much smaller, the collisional interaction tends to retard the winds driven by pressure gradients. Of even greater importance is the heating of the upper atmosphere caused by currents in the auroral region. The heating can make a significant contribution to the upper-atmo- spheric energy budget and can even be the dominant heat source above 120 km during magnetic storms. As the temperature increases the upper atmosphere ex- pands, and the drag on near-Earth satellites is in- creased, changing their orbits (e. g., Joselyn, 1982) . The ionosphere is affected in many ways by electric fields. Above 200 km, where the chemical lifetimes of ions range from several minutes to hours, rapid convec- tion of ionization at high latitudes can bring dense day- sicle plasma to the nightside of the Earth in some places, cause stagnation and prolonged nighttime decay of ion- ization at other places, and generally produce highly complex patterns of ionization density (e. g., Sojka et al., 1983~. Plasma temperatures and chemical reaction rates are also affected by the rapid ion convection through the air. Even at middle and low latitudes plasma drifts have an important influence on the upper ionosphere, pri- marily by raising or lowering the layer into regions of lower or higher neutral density, so that chemical decay is retarded or accelerated. During magnetic storms the plasma-drift effects on the ionosphere are not only in- tensified but are also supplemented by indirect effects through modification of the neutral atmosphere (e.g., Prolss, 1980~. Auroral heating induces atmospheric con- vection that alters the molecular composition of the up- per atmosphere and leads to more rapid chemical loss of the ionization, even at middle latitudes. Winds gener- ated by the magnetic storm impart motion to the ioniza- tion along the direction of the magnetic field, causing redistribution of the plasma as well as further modifica- tion of the loss rate. All these ionospheric phenomena affect radio-wave transmissions that reflect off the iono- sphere. Radio waves at frequencies greater than about 30 MHz do not normally reflect off the ionosphere and are much less influenced by large-scale plasma density vari- ations than are lower frequencies. However, these waves are affected by small-scale plasma irregularities that cause radio signals to scintillate, undergoing sub- stantial amplitude and phase modulations (e. g., Aarons, 1982~. The scintillations are bothersome for transmissions between satellites and the ground. Elec . . .

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204 trio fields are involved in producing the ionospheric ir- regularities, often by plasma instabilities resulting from drifts or from steep density gradients created by nonuni- form convection (e.g., Fejer and Kelley, 1980~. The ir- regularities tend to be strongest at high latitudes, where electric fields and their consequences are greatest, but the equatorial region is also subject to strong irregulari- ties, both in the equatorial electrojet and in the night- time upper ionosphere. Rapid magnetic fluctuations caused by changing ionospheric currents disturb geophysical surveys of magnetic anomalies in the Earth's crust. In addition, the fluctuations induce electric currents in the Earth. Analysis of the Earth currents can be useful for geophys- ical studies, but when they enter large man-made struc- tures like electric transmission lines and pipelines they can cause disruptive electrical signals and corrosion. Lanzerotti and Gregori discuss these problems in more detail in Chapter 16, this volume. Finally, it should be noted that the ionospheric elec- tric field extends down into lower atmospheric regions. In fact, the presence of the field at lower altitudes has been used for many years to measure ionospheric elec- tric fields from stratospheric balloons (e.g., Mozer and Lucht, 1974) . Roble and Tzur (Chapter 15, this volume) discuss the relation of the ionospheric electric field to the global atmospheric electric circuit. SOME OUTSTANDING PROBLEMS The entire range of subjects discussed in this chapter is undergoing scientific investigation, but there are cer- tain topics that currently present questions of funda- mental importance to the understanding of upper-at- mospheric electric-field sources. Since the beginning of the artificial satellite era mea- surements of space plasmas, energetic particles, and electromagnetic fields have provided a broad picture of the interaction of the solar wind with the magneto- sphere. Yet theory still is unable to explain quantita- tively, without the use of ad hoc parameterizations, how much magnetic flux interconnects the Earth with the solar wind or what the details of the magnetic- and electric-field configurations are in the vicinity of the magnetopause and in the distant magnetospheric tail. Measurements alone cannot answer these questions be- cause of the impossibility of measuring simultaneously the entire spatial structure of the continuously changing magnetosphere. However, some future observational programs may aid in the solution of the problem by combining continuous monitoring of electrodynamic features of the solar wind and of the entire auroral oval ARTHUR D. RICHMOND ionosphere with spot measurements in the poorly ex- plored high-latitude magnetopause and distant tail re- gions. Because solar wind/magnetosphere dynamo processes intimately involve magnetospheric plasma energiza- tion, ionospheric conductivity alterations, and coupling of magnetospheric and ionospheric electric fields and currents, the dynamo cannot be fully understood with- out taking account of the magnetosphere-ionosphere in- teract~ons. Theoretical models incorporating the mu- tual interactions have made great strides of progress in recent years (e. g., Spiro and Wolf, 1984), but much re- mains to be learned. Observational programs that can measure a large number of the important phenomena involved in the interactions are a necessary adjunct to theoretical studies. The distribution of ionospheric conductivity is impor- tant in determining the distributions of both electric fields and currents in the ionosphere and magneto- sphere, yet our understanding of nightside conductivi- ties is still rudimentary. We know that the conductivi- ties vary greatly but they are difficult to measure when the ionization density is low, and we do not yet fully understand the nature of nighttime ionization sources. In the auroral oval and polar cap the nightside conduc- tivities can be highly structured, having features correl- ated with structured features of the electric fields and currents. Because the conductivity features are so vari- able, useful models are difficult to construct, although a number of encouraging developments in modeling au- roral-oval conductivities have been made in the past few years (Reiff, 1984~. Concerning the ionospheric wind dynamo, theoreti- cal models and observations are in general agreement, except that the morphology of nighttime electric fields is not yet well explained, a problem related to the conduc- tivity uncertainties. The nature of day-do-day variabil- ity in the electric fields and currents, as well as smaller- scale variations, are only poorly understood at present. Winds in the dynamo region, especially those of a tidal nature, exhibit day-to-day variability that has not been fully explained and is currently unpredictable. It ap- pears that day-to-day variability in the distribution of atmospheric heating as well as changes in atmospheric tidal propagation characteristics and interactions with other wave motions are largely to blame, but these con- ditions are not now monitored on a global scale. Ad- vances in this area may be forthcoming in the wake of intensified study of the middle atmosphere (10-120 km altitude). In addition, further clarification of upper-at- mospheric circulation changes during magnetic storms is needed in order to understand how storms affect the ionospheric wind dynamo.

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UPPER-ATMOSPHERE ELECTRIC-FIELD SOURCES ACKNOWLEDGMENT Most of this chapter was prepared while the author was employed at the NOAA Space Environment Labo ratory. REFERENCES Aarons, J. (1982). Global morphology of ionospheric scintillations, Proc. ISEE 70, 360-378. Akasofu, S.-I. (1977~. Physics of Magnetospheric Substorms, D. Reidel, Dordrecht, Holland. Akasofu, S.-I. (1984~. The Magnetospheric currents: An introduction, in Magnetospheric Currents, T. A. Potemra, ea., American Geo- physical Union, Washington, D.C., pp. 29-48. Akasofu, S.-I., and S. Chapman (1972~. Solar-Terrestrial Physics, Clarendon Press, Oxford. Banks, P. M. (1979~. Magnetosphere, ionosphere and atmosphere in- teractions, in Solar System Plasma Physics, Vol. II, C. F. Kennel, L. J. Lanzerotti, and E. N. Parker, eds., North-Holland Publ. Co., Amsterdam, pp. 57-103. Blanc, M., and A. D. Richmond (1980~. The ionospheric disturbance dynamo, J. Geophys. Res. 85, 1669-1686. Burch, J. L. (1977~. 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