The National Geomagnetic Initiative (1993) / Chapter Skim
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A1: The Magnetosphere, Ionosphere, and Atmosphere
A1: The Magnetosphere, Ionosphere, and Atmosphere
Pages 75-99
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From page 75... ...
It is the interaction of the solar wind with the geomagnetic field that creates the Earth's magnetosphere. The magnetosphere, in turn, stores energy from the solar wind and dissipates it sporadically in geomagnetic storms and substorms that accelerate large fluxes of energetic charged particles and drive large electrical currents; these currents, in part, are diverted down into the ionosphere.
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From page 76... ...
In addition to contributing to the geomagnetic field models that are needed to describe the flow of energy and momentum between the magnetosphere and upper atmosphere, these observations provide important measures of the strength and intensity of the horizontal currents flowing overhead in the ionosphere. Careful analysis of ground magnetic variations can tell us a great deal about the nature of solar wind-magnetosphere-ionosphere couplings at high latitudes, and about the nature of the upper-atmosphere winds that drive the ionospheric dynamo at all latitudes.
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From page 77... ...
At the magnetopause the solar wind interacts with the Earth's field by two major processes: viscous interaction and magnetic reconnection. The viscous interaction occurs along the flanks of the magnetosphere transferring solar wind momentum to closed field lines inside the magnetopause.
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From page 78... ...
The rapid drift of protons to the west and electrons to the east creates a torus of westward current around the Earth, known as the ring current. The electric field produced in the magnetosphere by convection is projected onto the ionosphere along magnetic field lines.
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From page 79... ...
Fluctuations in the interplanetary magnetic field carried by the solar wind affect the rate of dayside reconnection, and hence cause corresponding fluctuations in the convection electric field and all of the current systems mentioned above. These changes are highly filtered and delayed because of the large inductance of the field-aligned current systems.
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From page 80... ...
Even when magnetospheric energy inputs to the upper atmosphere and ionosphere are weak, electric fields and currents are generated in the ionosphere by the dynamo action of the ever-present neutral atmosphere winds. Because the ionosphere is electrically conducting, its motion through the geomagnetic field produces an electromotive force (EMF)
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From page 81... ...
The highly anisotropic nature of ionospheric conductivity can result in strong Hall polarization electric fields, especially at the magnetic equator where the horizontal geometry of magnetic field lines inhibits the discharge of these fields. The electric field drives a strong current along the magnetic equator on the dayside of the Earth, the equatorial electrojet, that produces an enhancement in magnetic perturbations on the ground and at satellite altitudes.
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From page 82... ...
field-line resonances, storm-time compressional waves, and electromagnetic ion cyclotron waves. Other, less structured waves are focused along magnetic field lines directly into the polar cusp and cleft regions.
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From page 83... ...
As a result of intensive study of ground-based observations, many of the magnetic pulsations in this category have been found to be associated with traveling ionospheric plasma vortices produced by moving filamentary field-aligned currents that appear to originate near the inner edge of the {ow-latitude boundary layer. The generation of solitary vortices appears to be related to transients generated by changes in the interplanetary magnetic field or solar wind pressure.
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From page 84... ...
How frequently these occur and under what circumstances remain open questions. Irregular Variations Variations of large-scale field-aligned and ionospheric currents can also cause large, less structured variations in magnetic fields near the Earth's surface.
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From page 85... ...
modulations at Pc 3 frequencies, with clear TMF control of all three types of signals, suggest that at least along field lines connecting to cusp/cleft latitudes there are some fundamental unexplored interactions between various kinds of waves and plasma populations. Simultaneous and carefully synchronized information from magnetometer arrays at a variety of longitudes and latitudes will be especially valuable in determining whether cleft and/or cusp sources are involved in the transmission of these pulsations to the ground at lower latitudes, and can help to provide further tests of the proposed wave-entry mechanisms from an upstream wave source.
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From page 86... ...
Improved representations of the sources of external magnetic field are important for further progress of the geomagnetic community. Because the solar wind is changing with time, these models must be parameterized to include solar wind dynamic pressure and substorm phase as well as dipole tilt.
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From page 87... ...
Measurements from dense magnetometer arrays deployed at auroral latitudes will be essential if further progress is to be made in pulsation research. Applications of Geospace Studies The interaction of the solar wind with the Earth's magnetic field produces a variety of phenomena collectively known as geomagnetic activity.
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From page 88... ...
Other spacecraft that use the magnetic field for orientation suffer degraded attitude control because of large changes in field orientation. Spacecraft at higher altitudes often experience "single event upsets" from solar cosmic rays and energetic particles trapped in the Van Allen radiation belts.
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From page 89... ...
Operational Considerations Need for Comprehensive Magnetic Field Measurements Magnetospheric physics is primarily concerned with the study of processes through which the solar wind interacts with the Earth's magnetic field to create and change the various current systems described previously. The present understanding of these currents has been obtained through innumerable studies utilizing data taken at ground stations and by spacecraft orbiting the Earth.
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From page 90... ...
Another magnetometer at geosynchronous orbit is needed 6 to hours in local time from both GOES East and GOES West. Between geosynchronous orbit and the solar wind, there is no requirement for continuous magnetic field measurements, but significant scientific and operational benefits would result if more satellites in this sector of space carried a magnetometer suitable for scientific measurements of the magnetospheric magnetic field.
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From page 91... ...
When new scientific and operational missions in this region of space are developed, they should include a research-grade magnetometer to support modeling of the magnetospheric field as a function of TMF direction, substorm activity, and other effects. Low-Earth orbit measurements of the electrical currents flowing between the magnetosphere and the ionosphere are also essential to the modeling of both regions.
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From page 92... ...
Many of the existing observatories also need to be upgraded. All observatories must be able to record the magnetic field digitally at a sampling rate of at least ~ vector per minute, and the data must be put into a data base accessible in real time through a computer network such as the one being developed for the International Real-Time Geomagnetic Observatory Network (INTERMAGNET)
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From page 93... ...
In the past, such temporary stations have been custombuilt by various scientific groups and later dismantled. With the development of modern fluxgate magnetometers and the advent of lowcost data retrieval, storage, and communications hardware, appropriate agencies should procure reusable geomagnetic observatory stations.
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From page 94... ...
General Recommendations Data Management Requirements A quantitative understanding of the magnetosphere and the thermosphere requires the acquisition of a comprehensive suite of spacecraft and ground-based observations that must be analyzed in association with complex models. It is essential that the outstanding research questions identified earlier be addressed in the context of an efficient and flexible data management environment capable of presenting a wide variety of data to a physically diverse science community in readily usable fashion.
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From page 95... ...
In summary, the data management requirements to support the present initiative are as follows: · extensive collaboration with existing and planned data systems; · real-time and on-line interactive data bases; · data systems that catalog, display, and deliver data in on-line fashion;
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From page 96... ...
Improvement of these indices through the addition of ground stations for improved spatial coverage, increased time resolution, better removal of uninteresting currents, real-time generation, and rapid dissemination to users are matters of great importance to the geomagnetic community. Accordingly, the relevant agencies should work together to meet the following goals: · upgrade the current network of geomagnetic observatories used for the AE index, replacing analog stations with digital and adding real-time satellite links; · expand the AE network with additional stations at crucial points in eastern Canada, eastern Siberia, and possibly the Southern Hemisphere auroral oval; stations located at higher and lower latitudes to record electrojet activity for expanded or contracted auroral ovals should also be established when resources permit; · calculate the Dst index with I-minute resolution and make it available in real time via network access;
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From page 97... ...
At middle or low latitudes, magnetometers are required at about 30° longitudinal increments to investigate variations due to solar wind dynamic pressure changes, substorm current systems, the ring current, and the ionospheric equatorial electroj et. These scientific objectives can be reached if the following actions are taken: deploy dense (100- to 200-km separation)
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From page 98... ...
To accomplish these goals the following requirements are established: · The interplanetary magnetic field and the solar wind in the vicinity of the Earth must be measured continuously. These data must be made available to users on a near-real-time basis.
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From page 99... ...
A goal should be to model the perturbations in the geomagnetic field at low and middle latitudes to within a total error budget of 10 nanoteslas. Conversely, studies of the magnetospheric magnetic field require improved models of the internal magnetic field for analyzing the ground signatures of magnetospheric activity and pulsations.
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