Variations in the ionospheric dynamo current
The ionospheric dynamo is produced by movement of charged particles of the ionosphere across Earth’s main field. This motion is driven by the tidal effects of the Sun and the Moon and by solar heating. The ionospheric dynamo is thus controlled by two parameters: the distribution of winds and the distribution of electrical conductivity in the ionosphere. These parameters are influenced by several factors, including the orbital parameters of Earth, Moon, and Sun; the solar cycle; solar flares; and solar eclipses. Changes in the position of the Sun and the Moon relative to Earth as a result of orbital motions cause variations in distance. This alters the strength of the tides and of solar heating, thereby changing ionospheric wind patterns. These changes are apparent as a seasonal modulation of the winds and hence of the strength of the current.
The second parameter that controls the dynamo current is the electrical conductivity of the ionosphere. Any process that alters ionospheric conductivity changes the current. On the dayside of Earth the dominant source of ionization is sunlight. The amount of ionization depends on the angle at which sunlight enters the atmosphere. Vertical incidence produces more ionization per unit volume than slant entry. For a given hemisphere, normal incidence occurs in summer. Thus, this effect also causes a strong seasonal modulation of the dynamo current.
The degree of atmospheric ionization also depends on the phase of the solar cycle. This 11-year cycle of sunspot activity produces variations in the amount of ultraviolet radiation emitted by the Sun. More sunspots lead to more ultraviolet radiation and increased ionospheric conductivity and hence stronger currents. On a shorter timescale solar flares emit X-rays that penetrate deeper in the atmosphere, temporarily ionizing the D region, the lowest layer of the ionosphere. Dynamo currents are then produced in this layer by whatever winds are present there.
A solar eclipse produces the opposite effect on ionospheric conductivity. The shadow of the Moon as it crosses the ionosphere decreases ionization. Recombination of ionospheric electrons and ions in the absence of light quickly reduces the conductivity. Because the effect is localized and of short duration, its effect on the overall dynamo current is slight.
Magnetic storms—growth of the ring current
The ring current is produced by the drift around Earth of charged particles of the outer Van Allen radiation belt. During quiet conditions the effect of this current at Earth’s surface is negligible (about 20 nanoteslas). Once or twice a month there occurs a phenomenon known as a magnetic storm, during which the intensity of the ring current increases and produces disturbances that are typically on the order of 100 nanoteslas but can be as large as 500 nanoteslas. A variety of phenomena that affect humans occur during magnetic storms. A few of these include increased radiation doses for occupants of transpolar flights, distortion of compass readings in polar regions, disruption of shortwave radio communications, increased corrosion in long pipelines, failure of electrical transmission lines, anomalies in the operations of communications satellites, and potentially lethal doses of radiation for astronauts in interplanetary spacecraft. Efforts have been undertaken to mitigate such serious problems. In the United States, for example, the federal government operates a Space Disturbance Forecast Center in Boulder, Colorado, which monitors the state of the Sun and solar wind and attempts to predict the occurrence of such “space weather.”
Cause of magnetic storms
It is known that magnetic storms are produced by a change in the properties of the solar wind. Magnetically quiet times occur when the solar wind contains a magnetic field called the interplanetary magnetic field (IMF) that has the same direction as Earth’s field on the dayside. Magnetic disturbances occur when this field rotates toward an antiparallel orientation. Normally, the IMF lies in the ecliptic plane, which on the average is roughly parallel to Earth’s magnetic equator. Small departures from this average orientation are caused by rotation of the tilted dipole magnetic field once per day and by revolution of Earth around the Sun once per year. Large departures are caused by changes in the direction of the IMF relative to the ecliptic. Such changes are produced by several phenomena that originate on the Sun.
The most spectacular event that may cause a magnetic storm is a solar flare, which is an explosion in the corona of the Sun that releases an enormous amount of energy in the form of outward-streaming particles. The bulk of these particles takes approximately two days to arrive at Earth, where it begins to influence the magnetic field. During transit the solar flare particles catch up with slower particles emitted earlier. The subsequent interaction of the high- and low-speed solar wind components causes a high-pressure region to develop, and this region tilts the IMF out of the plane of the ecliptic. If the IMF is tilted antiparallel to Earth’s field, a magnetic storm results.
Another phenomenon responsible for magnetic storms is the existence of coronal holes around the Sun. X-ray images of the Sun made during the 1970s by the U.S. Skylab astronauts revealed that the corona of the Sun is not homogeneous but often exhibits “holes”—regions within the solar atmosphere in which the density of gas is lower than in adjacent regions and from which charged particles escape with relative ease. Particles from such holes reach higher velocities in their outward expansion than do normal solar wind particles and produce high-speed streams. These streams interact with the slower-speed solar wind emitted from regions without holes and produce the same tilting of the IMF described above. Coronal holes persist for many 27-day solar (equatorial) rotations and, as a consequence, produce recurrent magnetic storms. Coronal holes are the hypothetical “M regions” on the Sun proposed many decades ago to explain recurrent storms that could not be associated with particular solar flares.
Magnetic reconnection
The observed dependence of geomagnetic activity on the orientation of the IMF is explained by most researchers as a consequence of magnetic reconnection. In reconnection, two oppositely directed magnetic fields are brought together by flowing plasmas at an x-type neutral line. Far from the neutral line the magnetic field is frozen in the plasma; however, near the neutral line it becomes unfrozen and diffuses through the plasma, establishing a new configuration of magnetic field lines. On passing through the neutral line, field lines from opposite sides connect and flow rapidly away from the neutral line at right angles to their direction of inflow. In the process, energy originally stored in a strong magnetic field is converted to the kinetic energy of flowing plasma. In addition, the topology of magnetic field lines is changed. At the dayside magnetopause (see the summarizing the configuration of Earth’s outer magnetic field), field lines of the IMF become connected to geomagnetic field lines. Because the IMF is frozen into the solar wind, the portion of the reconnected field line external to the magnetosphere is dragged away from the Sun above and below the polar caps. The portions of the field line inside must follow the external portions; hence, their “feet” appear to drift across the polar caps. This process cannot go on indefinitely, as geomagnetic field lines will be continuously eroded from the dayside unless they are replaced by an internal flow. Such a flow develops after a short lag and follows the same pattern as the return of field lines drawn away from the Sun by viscous interaction. When the flow is fully developed, the flux of magnetic field lines toward the Sun within the magnetosphere balances the flux away from the Sun above and below the polar caps.
For field lines to return from the nightside, they must first disconnect from the solar wind. This occurs at a second x-type neutral line located behind Earth (see the summarizing the configuration of Earth’s magnetic field). There, as on the dayside, oppositely directed field lines are brought together by plasma flows. Reconnection occurs, and the IMF and geomagnetic field lines again become separate entities.
The topology of magnetic field lines produced by the reconnection process accounts for the existence of auroral ovals. Field lines of the polar caps are “open” to the solar wind, whereas those at lower latitudes are “closed” to it, as is evident from the above figure. On the nightside the field lines connecting to the neutral line form a natural boundary for trapping charged particles. The region interior to the “last-closed field lines” is filled with trapped particles and is called the plasma sheet. The projection of the last-closed field lines on the polar atmosphere forms the poleward boundary of the nightside auroral oval. As previously noted, a second boundary forms on the nightside of Earth as particles drift earthward under the influence of magnetospheric convection (driven by both viscous interaction and reconnection) and then enter the region of strong azimuthal drift. This boundary is called the inner edge of the plasma sheet, and it projects as the equatorward edge of the nightside auroral oval.
Generation of a magnetospheric electric field
An important consequence of reconnection is that it produces a magnetospheric electric field, as does viscous interaction. This comes about as a result of the connection between the interplanetary and geomagnetic fields. This process can be understood as follows. In the solar wind the Lorentz force separates positive and negative charges, just as it does in the magnetospheric boundary layer. These charges accumulate at boundaries within the solar wind where either the velocity or the orientation of the IMF changes. There is an electric field between these boundaries. Because magnetic field lines have nearly infinite conductivity, the electric field originating in the solar wind is projected by magnetic field lines into the magnetosphere and onto the polar caps. The effect of this field depends on its strength and the length of the dayside x line. The voltage, or potential, drop caused by any electric field depends on the distance over which the field is applied. In dayside reconnection not all interplanetary magnetic field lines connect to Earth. Most slip around the magnetosphere. Consequently, the voltage applied to the polar cap is that which exists in the solar wind between the field lines that are reconnected at the ends of the x line. Usually this is 10–20 percent of the total voltage across a distance equal to the diameter of the magnetosphere. Even so, it can be as large as 200,000 volts.
A magnetic storm can be explained relatively simply in terms of the concept of magnetic reconnection described above. A solar flare or high-speed solar wind stream creates a high-pressure region in the solar wind. The leading edge of this region reaches Earth and presses the magnetopause earthward. The sudden earthward motion and accompanying increase in strength of the magnetopause current cause an abrupt increase in the magnetic field at Earth’s surface known as the storm sudden commencement. In most cases the pressure remains high for a number of hours and causes a larger-than-normal surface field. This interval is called the initial phase of a magnetic storm. Eventually, the IMF turns toward the south, antiparallel to Earth’s field, and magnetic reconnection begins. Closed magnetic field lines are eroded from the dayside and added to the polar caps, increasing their diameter. The aurora, which occurs in two ovals immediately equatorward of the polar caps, moves to lower latitudes. Within about an hour the nightside neutral line begins to return a sufficient amount of flux to the dayside, and convection approaches equilibrium.
Magnetic reconnection drives magnetospheric convection much more efficiently than does viscous interaction. Consequently, all phenomena associated with convection are much enhanced over quiet times. Convecting particles approach closer to Earth before they are deflected by drift in the main field. Field-aligned currents and the ionospheric electrojets driven by the convection electric field are much stronger. In addition, particles drifting across the main field gain more energy. This process of energization occurs at all times but is much enhanced during strong convection. It is caused by the dawn-to-dusk electric field across the magnetosphere. Any positive charge that drifts in the direction of an electric field gains energy from the field. Since positive charges on the nightside drift toward dusk, they gain energy. Similarly, electrons gain energy drifting toward dawn opposite to the electric field. On the dayside the drifts are reversed and particles lose energy. The combination of effects from more particles drifting faster closer to Earth enhances the nightside ring current and reduces the magnetic field on Earth’s surface.
If the magnetospheric electric field remained steady, the particles drifting around Earth would lose their energy on the dayside and convect to the magnetopause, where they would be lost to the solar wind. If the electric field across the magnetosphere is suddenly reduced by a northward turning of the IMF, however, many particles that would have been returned to the solar wind by convection are trapped on drift paths closed around Earth. These particles rapidly separate into a doughnut-shaped ring that forms a symmetrical ring of current around the planet. Subsequent cycles of increase and decrease in the magnetospheric electric field trap additional particles and increase the energy of those already trapped. By this and another process described below, the ring current grows and produces the main phase of a magnetic storm.
A second and more spectacular phenomenon also contributes to the development of the storm main phase. This phenomenon is known as a magnetospheric substorm. The term substorm is used because such an event is observed during the development of the main phase of a storm. Since events of this kind occur more frequently at times when there is no significant growth of the ring current, they are treated below as a separate topic. As will be shown, the main effect of a substorm is energization and injection of particles into the inner magnetosphere in a localized region near midnight. Although the particles do not appear to have an immediate effect on the strength of the ring current, they are usually trapped on closed drift paths and are available for subsequent energization by fluctuations in the magnetospheric electric field. Many of the dramatic and often detrimental effects attributed to magnetic storms are actually caused by particularly intense substorms that accompany them. Both phenomena are linked by the same fundamental processes.
