Introduction

Man has no obvious sensation of the presence or change of the Earth's magnetic field such as he does for the sensation of rain, wind, or earthquakes. He must rely upon the field's interaction in other physical processes to produce measurable effects. In this chapter we will look at some methods of providing such geomagnetic information.

From a physicist's viewpoint the geomagnetic field we wish to measure has some interesting singular characteristics. It is ever-present; we must take deliberate action to create any required field-free environments. Because of the great spatial extent of the field with respect to available sensor dimensions, only single-point measurements are typically obtained. The natural field is constantly changing and cannot be stopped at will by the experimenter. A conglomeration of Earth-core, magnetospheric, ionospheric, and induced currents all contribute to the simple measurement of a geomagnetic field magnitude and direction at each instant of time; occasionally, special frequency-analysis techniques allow us to identify some of these contributing sources.

The Earth's field changes are not easily stoppered in a bottle and brought to the laboratory for testing like a paleomagnetic rock sample. Those who want the measurement usually must move to a sampling spot that they have selected with care in order to minimize unwanted “noise” and to indicate special upper-atmosphere or deep-Earth characteristics. Everywhere at the Earth's surface, the “steady” field (i.e., slowly varying with respect to the spectral components at active times) is quite strong compared to the relatively infinitesimal fields of rapid (micro) pulsations.

This appendix presents a number of mathematical topics that arise in the book. The review is not meant to be comprehensive; it is limited to only items that could be helpful for understanding the flow of ideas in the chapters of this book.

Variables and Functions

Variable is the name we give to a value of something that changes. When we call a variable independent we mean that it can be any size within a prescribed physical domain of realistic values. A dependent variable is the value that we call a function of the independent variable. On a daily magnetogram, the magnitudes of the scaled H (a “dependent” variable) are dependent upon the selection of the “independent” variable of daily hourly time that we can take to be any value (in the domain) from 0 to 24. Maxwell's equations (Chapter 1) allow a unique field value to be determined from a given source-current distribution; however, given the field values, a number of possible currents might be the source. The dependent-variable field is a function of the independent-variable current. The extreme highest and lowest values of the dependent variable that occur over the domain of the independent variable define the range of the dependent variable.

The term function has a very special meaning in mathematics. When we say, for example, “the variable y is a function of the variable x” it is written as y = f(x).

Introduction

It is the nature of geomagnetic fields to not divulge their sources simply. The observatory magnetometers (Earth-field measuring devices described in Chapter 5) respond to all the fields reaching the local environment, add them together, and limit recording only by the frequency response designed into the instruments. A large part of research in geomagnetism concerns the dissection of field variation recordings to isolate the individual contributing sources, discover the physical processes that cause these currents, and thereby understand another feature of our global environment. Occasionally, a newly revealed feature becomes immediately important and useful to society's needs; usually, its utility is discovered only after many years. One of the first field sources to be discovered (Stewart, 1883; Schuster, 1889, 1908) was a current driven by tidal forces and winds in a conducting region above the Earth that was subsequently named the ionosphere. Such currents are indicated by a recurring field pattern on quiet-time daily recordings. The accurate determination of quiet-day field variation now finds utility in improvement of satellite main-field modeling, in profiling the Earth's electric conductivity, and in establishment of baselines from which magnetospheric disturbances are quantified.

The purpose of this chapter is to explain the origin and behavior of the regularly recurring field variations that have periods of a day or less. Because the principal source for these currents lies in a naturally ionized layer above the Earth, we will examine the basic features of this ionosphere.

Introduction

The science of geomagnetism developed slowly. The earliest writings about compass navigation are credited to the Chinese and dated to 250 years B.C. (Figure 1.1). When Gilbert published the first textbook on geomagnetism in 1600, he concluded that the Earth itself behaved as a great magnet (Gilbert, 1958 reprint) (Figure 1.2). In the early nineteenth century, Gauss (1848) introduced improved magnetic field observation techniques and the spherical harmonic method for geomagnetic field analysis. Not until 1940 did the comprehensive textbook of Chapman and Bartels bring us into the modern age of geomagnetism. The bibliography in the Appendix, Section B.7, lists some of the major textbooks about the Earth's geomagnetic field that are currently in use.

For many of us the first exposure to the concept of an electromagnetic field came with our early exploration of the properties of a magnet. Its strong attraction to other magnets and to objects made of iron indicated immediately that something special was happening in the space between the two solid objects. We accepted words such as field, force field, and lines of force as ways to describe the strength and direction of the push or pull that one magnetic object exerted on another magnetic material that came under its influence. So, to start our subject, I would like to recall a few of our experiences that give reality to the words magnetic field and dipole field.

Introduction

In this chapter we will look at some of the ways in which geomagnetism finds utility in today's world. The main subjects are the impact of the geomagnetic field on modern technological systems and the application of geomagnetism to the discovery of the physical nature of our world. I also include interesting observations for which geomagnetic connections imply future application directions.

Each period range of natural geomagnetic field fluctuations can be identified with special utilization topics. For example, consider the following:

1. (a) For the period range from 0.25 seconds to 1 minute the primary subjects of interest are Earth crust exploration, detection of hidden conductivity anomalies, electric power transformer failures, studies of hydromagnetic wave propagation, and discovery of magnetospheric processes.

2. (b) For the range from 1 minute to 24 hours, studies include the structure of magnetospheric deformation and currents, thermospheric heating and winds, ionospheric currents and tides, and conductivity characteristics of the Earth's lower crust, mantle, and continental coastlines. Geomagnetic storms in this time scale affect a multitude of man-made systems such as satellites, communication systems, electric-power grids, and long pipelines (see Heirtzler et al., 2002).

3. (c) From the range 1 day to 1 year we obtain information about the fluid motions within the Earth's core and at the core–mantle boundary, solar activity and solar sector changes, tropospheric weather changes, and magnetospheric deformation. Our main field magnetic navigation charts are obtained from data in this period range.

Below is a description of computer program files, mentioned throughout this textbook, that may be obtained by the readers in two ways: (1) free of charge at the website prepared by Susan McLean of NGDC/NOAA (http://www.ngdc.noaa.gov/seg/potfld/geomag.shtml) or (2) on a high-density (1.4 Mb) floppy disk sold by NGDC/ NOAA.

At the website main page, select the highlighted “useful computer programs.” The executable files are designed for a DOS-compatible, personal computer. The programs were designed to assist the user in understanding the subject of geomagnetic fields; no claim is made regarding the suitability of the software for any other purpose. No restriction has been placed on the sharing of these programs; also, no warranty (expressed or implied), no endorsement, no guarantee of accuracy, and no responsibility for the program's functioning, can be made by the program authors, the author or publisher of this book, or by the National Geophysical Data Center. The files ending in. EXE are the executable programs; all the other files provide necessary input for some of the programs and must also be copied for proper operation of the set. If the files have been downloaded from the NGDC website, then copy all of them to a disk on your computer before running.

Geomagnetic Coordinates 1940–2005

The GMCORD program provides a determination of the geomagnetic coordinates for any selected global location. The program uses a polynomial fit (see the POLYFIT program below) of the dipole Gauss coefficients from the geomagnetic reference field models (see file ALL-IGRF.TAB below).

Looking back

The astrophysical research discipline we now know as Extreme Ultraviolet astronomy is approximately 30 years old. An observational technique once dismissed as impossible has become established as a significant branch of space astronomy and a major contributor to our knowledge of the Universe. In several areas, the science obtained from EUV observations is unique. For example, the presence of the He II Lyman series in this spectral range provides a diagnostic tool for the study of the second most abundant element in the Universe in the atmospheres of hot stars and in interstellar space. The determination of the ionisation fraction of helium in the local ISM could not have been carried out in any other spectral range.

EUV astronomy has passed through the development phases that might be deemed typical of a discipline depending on access to space. Beginning with the sounding rocket borne experiments of the early 1970s, the longer duration Apollo–Soyuz Test Project highlighted the potential of the field, with the first reported source detections in 1975. However, it was a further 15 years before the next major advance with the first EUV all-sky survey of the ROSAT WFC (in 1990) followed by the wider spectral coverage of the EUVE survey in 1992. The underlying reasons for this hiatus had more to do with national and international politics, together with the economics of funding opportunities and even the launch delays following the Challenger disaster, than technological limitations.

Compared to the large numbers of coronal sources and white dwarf stars found in the EUV all-sky survey catalogues, the number of cataclysmic variable detections is small, numbering only ≈25 objects. Nevertheless, many of these systems were bright enough to have been suitable targets for spectroscopic observations with EUVE, but these studies were complicated by the fact that the sources are highly variable and the exposure times needed to produce good signal-to-noise were at least 50 ks for the brightest sources, much longer than the binary and rotation periods. Hence, with EUVE it was only possible to obtain observations of the time averaged spectrum, except in periods of outburst, which lasted for several days.

Cataclysmic variables can be conveniently divided into two groups: (i) those where the white dwarf does not have a significant magnetic field (<0.1–1 MG) and accretion onto its surface occurs via an accretion disc (e.g. figure 9.1); (ii) the polars–systems in which the white dwarf does have a strong magnetic field (≈10–100 MG) which prevents the formation of an accretion disc and channels accrete material onto the magnetic poles of the white dwarf along the field lines (figure 9.2). A small number of magnetic systems, the intermediate polars, have weaker fields (B ≈ 1–10 MG) allowing a partial disc to accumulate which is disrupted by the field in its inner regions. As in the polars, accretion onto the white dwarf follows the field lines onto the magnetic poles (figure 9.2).

Astrophysical significance of the EUV

The Extreme Ultraviolet (EUV) nominally spans the wavelength range from 100 to 1000 Å, although for practical purposes the edges are often somewhat indistinct as instrument band-passes extend shortward into the soft X-ray or longward into the far ultraviolet (far UV). Like X-ray emission, the production of EUV photons is primarily associated with the existence of hot gas in the Universe. Indeed, X-ray astronomy has long been established as a primary tool for studying a diverse range of astronomical objects from stars through to clusters of galaxies. An important question is what information can EUV observations provide that cannot be obtained from other wavebands? In broad terms, studying photons with energies between ultraviolet (UV) and X-ray ranges means examining gas with intermediate temperature. However, the situation is really more complex. For example, EUV studies of hot thin plasma in stars deal mainly with temperatures between a few times 105 and a few times 106 K, while hot blackbody-like objects such as white dwarfs are bright EUV sources at temperatures a factor of 10 below these. Perhaps the most significant contribution EUV observations can make to astrophysics in general is by providing access to the most important spectroscopic features of helium – the He I and He II ground state continua together with the He I and He II resonance lines.

This book is the first comprehensive description of the development of the discipline of astronomy in the Extreme Ultraviolet (EUV) wavelength range (≈ 100–1000 Å), from its beginnings in the late 1960s through to the results of the latest satellite missions flown during the 1990s. It is particularly timely to publish this work now as the Extreme Ultraviolet Explorer, the last operational cosmic EUV observatory, was shut down in 2001 and re-entered the Earth's atmosphere in early 2002. Although new EUV telescopes are being designed, it will be several years before a new orbital observatory can come to fruition. Hence, for a while, progress beyond that reported in this book will be slow.

We intended this book to be for astrophysicists and space scientists wanting a general introduction to both the observational techniques and the scientific results from EUV astronomy. Consequently, our goal has been to collect together in a single volume material on the early history, the instrumentation and the detailed study of particular groups of astronomical objects. EUV observations of the Sun are not within the scope of this current work, since the Sun can be observed in far more detail than most sources of EUV emission, providing material for a book on its own. We have found it useful to deal with the subject in its historical context. Therefore, we do not have specific chapters on instrumentation but integrate such material into the development of the scientific results on a mission-by-mission basis.

Introduction

The first true space observatories incorporating imaging telescopes and providing access to the soft X-ray band, but providing some overlapping response into the EUV were flown in the late 1970s and early 1980s. With the Einstein and European X-ray Astronomy Satellite (EXOSAT) satellites, launched in 1978 and 1983 respectively, long exposure times coupled with high point source sensitivity became available to high-energy astronomers for the first time. This progress depended mainly on developments in optics and detector technology but also coincided with a more sophisticated understanding of the physical processes involved in soft X-ray and EUV emission and a better appreciation of the potential significance of observations in these wavebands. This chapter describes the Einstein and EXOSAT missions detailing both the telescope and detector technology. Developments in the understanding of the physical emission processes are outlined to set the context for discussion of the most significant results from these observatories, relevant to the broad field of EUV astronomy.

Parallel developments in far-UV astronomy, arising mainly from the International Ultraviolet Explorer (IUE) observatory, provide an important complement to the work of Einstein and EXOSAT. However, since the technology of IUE is quite different to that used at shorter wavelengths we concentrate solely on the appropriate scientific results. Finally, the Voyager ultraviolet spectrometer (UVS), while mainly designed for planetary studies, was used during cruise phases of the mission, providing the first stellar EUV spectra in the region just below the Lyman limit at 912 Å.

Emission from B stars

Prior to the EUV sky surveys, O and B stars exhibiting strong mass-loss were expected to be a minor, but nevertheless important, group of EUV sources, the emission arising from hot, shocked gas in the stellar winds. Little thought was given to the likelihood of detecting photospheric EUV flux since photospheric helium was expected to restrict emission to the longest EUV wavelengths, most affected by interstellar attenuation. Nevertheless, the existence of the so-called β CMa tunnel of low column density, extending over distances of 200–300 pc (e.g. Welsh 1991) promoted the hope that a few such objects might be detected in this direction at wavelengths longward of 504 Å. The subsequent detection of the B2 II star ∊ CMa (Adhara, d = 188 pc) in the 500–740 Å (tin) filter during the EUVE sky search was not, therefore, particularly remarkable. However, the intensity of the flux recorded outshone all other non-solar sources of EUV radiation, including the well-known hot white dwarf HZ 43, previously believed to be the brightest EUV source, although this star remains the brightest object at the shortest EUV wavelengths (Vallerga et al. 1993).

The magnitude of the detected EUVE tin count rate (98 ± 10 counts s−1) was a strong indication that the line-of-sight column density was even lower than the upper limit of 3 × 1018 cm−2 estimated indirectly by Welsh (1991) from NaI absorption line studies.

Introduction

Even before the launches of the Einstein and EXOSAT observatories, it was clear that the next major step forward in EUV astronomy should be a survey of the entire sky, along the lines of the X-ray sky surveys of the 1970s. Such a survey was necessary to map out the positions of all sources of EUV radiation and determine the best directions in which to observe. Indeed, the groups at University of California, Berkeley had been selected by NASA to fly such an experiment on the Orbiting Solar Observatory (OSO) J satellite but, unfortunately, the OSO series was cancelled after the flight of OSO-I. Following the success of the Apollo–Soyuz mission in 1975, scientific interest was revived in the survey concept and the Extreme Ultraviolet Explorer (EUVE) mission was subsequently approved in 1976. Interest in the EUV waveband was also growing in Europe. Having successfully flown a series of imaging X-ray astronomy experiments between 1976 and 1978, the Massachussetts Institute of Technology (MIT)/University of Leicester collaboration sought a new direction of research, with the imminent launch of Einstein, and began development of a new imaging telescope operating in the EUV. This was seen as a direct extension of the mirror technology already refined in the soft X-ray and was combined with the MCP detector expertise acquired from work on the Einstein HRI.