Geomagnetic Variations in
the Ionosphere

F.N. Okeke

Department of Physics & Astronomy
P.O. Box 3238, University Post Office
University of Nigeria, Nsukka, Nigeria

Abstract. This paper reviews the theoretical basis and research work in relation to the geomagnetic variations in the ionosphere. Controversies as to the cause and nature of the geomagnetic variations in the ionosphere still exist. Observations from a number of stations like EISCAT, IMAGE and AGO are available. It is suggested that African countries should join these projects to enhance our understanding of these complex phenomena.

Sommaire. Cet article passe en revue les bases théoriques et les travaux de recherche sur les variations géomagnétiques dans l'ionosphère. Des controverses sur la cause et la nature de ces variations existent encore. Des observations à partir de plusieurs stations telles EISCAT, IMAGE et AGO sont disponibles. On suggère que les pays Africains rejoignent ces projets pour faire progresser notre compréhension de ces phénomènes complexes.

1 Introduction

In recent times the magnetic field in the ionosphere has assumed great importance, with availability of data from ground-based stations data, spacecraft, Earth-orbiting satellites, etc. The ionosphere has been described as the part of the earth's upper atmosphere where ions and electrons are present in quantities sufficient to affect the propagation of radio waves. This region is between 70 km and 1000 km high above the earth (e.g. Campbell, 1997). The ionosphere is a conducting medium because of its ionization. In this region wind carries the ionized particles with velocity $\omega$ across the lines of earth's magnetic flux B. This constitutes a conductor cutting the magnetic lines. From Faraday's law of electromagnetic induction, there occurs the induction of electromagnetic force (emf) $\omega$ $\times$ B. This emf causes current to flow in the conducting medium in the direction of ($\omega$ $\times$B). Since in a given region the direction of B is fixed, the wind needs to blow in closed curves for the current to complete its circuit, but the wind progresses instead of blowing in closed curves. Therefore the current is divergent and cannot complete its circuit. As a result, the current accumulates positive and negative charges that polarize the ionization. The polarization field is given by
$E = -\nabla\phi$($\phi$ is the scalar potential). There now exist two sources of electric fields whose sum is given by E$^{\prime}$ = E + $\omega$ $\times$ B. The total E$^{\prime}$ current is non-divergent, i.e. $\nabla$J = 0, and J + $\sigma$ E $^{\prime}$, where J is current density and $\sigma$ is tensor conductivity. Thus the polarization electrostatic field E helps the dynamo-induced field $\omega$ $\times$ B to derive a current that completes the circuit. The magnetic field of this dynamo current is that part of the geomagnetic transient variation (Fig. 1) whose origin is in the ionosphere.

Figure 1: Organogram of Geomagnetic Temporal Variation.

The secular variation of the earth's magnetic field results from the effects of magnetic induction in the fluid outer core and from the effects of magnetic diffusion in the core and the mantle (Bloxham, 1992).

On the other hand transient variations can be explained by electric currents in the mantle, the ionosphere and magnetosphere. We will now briefly discuss each of the variations.

The quiet-day variation refers to the magnetic variation on some days that are free from magnetic disturbances. This comprises two parts: the solar quiet daily variation Sq, which depends on solar time and is a regular diurnal variation, and the lunar daily variation L, which depends on lunar time and is a regular semidiurnal variation. The Sq has been found to have diurnal variation, seasonal variation and latitudinal variation (Chapman & Bartels, 1940; Onwumechili, 1992; Okeke et al., 1998). The lunar daily magnetic variation (L) depends on the lunar time and changes in form with each day of the lunar month, measured from one new moon to the next. It is too small in amplitude to be seen by mere inspection on magnetograms, except at stations along the magnetic equator.

It has long been noted that at the dip equator, the midday eastward polarization field generated by global scale dynamo action gives rise to a downward Hall current. A strong vertical polarization field is set up which opposes the downward flow of current due to the presence of non-conducting boundaries. This field in turn gives rise to the intense Hall current which is called the Equatorial electrojet (EEJ).

Lately, it has been discovered that solar flares have a very big influence on Sq current, since they increase conductivity in the ionosphere; hence Sq is enhanced. Also, it was found that solar flare effect (SFE) is higher in stations of EEJ than other stations outside the EEJ zone.

In studying the disturbances in the ionosphere it is better to consider the outstanding disturbances called magnetic storms. Campbell (1997) noted that dynamic processes on the sun deliver a plasma of charged particles and associated fields to the earth's environment, causing geomagnetic disturbances at the earth's surface referred to as `` geomagnetic'' storms. Sometimes solar disturbances occur accompanied by enhancement of the solar photon radiation in parts of the spectrum or by an increase in the velocity and concentration of the solar wind to an extent that the modified portion behaves like a cloud of denser plasma, or by ejection of more energetic protons and electrons with such low concentration that they behave not as a plasma, but as independent charged particles (Ratcliffe, 1972). When the particles reach the vicinity of the earth, they produce sudden ionospheric disturbances (SID), ionospheric storms or magnetic storms. When an intense SID occurs, it is found that there is a simultaneous small change in the magnetic field. The region of modified wind travels out from the sun as a supersonic wave and, when it reaches the magnetosphere, it gives rise to several different phenomena which include:

Figure 2: Schematic diagram of a storm-like variation (labelled Dst in Fig. 1)
featuring a typical geomagnetic storm (after Kamide et al., 1998).

Kamide et al. (1998) noted that in early and recent years of research, a general picture of a typical magnetic storm presents the features shown in Fig. 2. It comprehensively summarises the stages during a magnetic storm. This, first, involves a sudden positive increase in the horizontal component of the earth's magnetic field (H), which is referred to as sudden storm commencement (SSC). This SSC is due to a sudden increase in the solar wind arriving to the earth surface in the form of a shock wave, which provokes the field sudden increase. Then follows a period of arbitrary length in which the elevated field does not change significantly: this is referred to as the Initial phase of the storm. Shortly after the enhanced solar stream reaches the magnetosphere; there is an increase in the number of energetic particles in the magnetospheric trapping region. When many more energetic particles move in their trapped region around the earth, with electrons and protons in opposite directions, they give rise to an increased ring current and hence to the development of a depressed earth magnetic-field H-component. This decrease in H is referred to as the Main phase of the magnetic storm, transpiring over a period from one to a few hours. When new particles are no longer injected, the ring current slowly decays for one or two days. During this time, excess particles are lost and the magnetic field gradually returns to its normal value; in other words the storm ends by a slow recovery over hours to tens of hours which is referred to as the Recovery phase of the magnetic storm. Note that the H-component variation depends on latitudes.

Sometimes comparatively short-lived changes occur in the polar magnetic field called polar or auroral substorms. They are most frequent during the main and recovery phases of magnetic storms. They are also observed during magnetically quiet times; Campbell (1997) noted that only the middle and the low-latitude geomagnetic fields describe the gradual storm-time growth and decay and that satellite observations in the magnetosphere show no single mechanism that closely follows the main and recovery storm phases. He also observed that the idea of substorms arose out of the need to tie together the in situ observations of storm period bursts of activity that are linked together on time scales shorter than the main and recovery phases. He also noted that the intermediate substorm processes are complex and still under some debate among upper atmospheric physicists.

2. Previous work

Over the decades, numerous studies have been conducted on the earth's magnetic field and variations in the ionosphere. All of these are too extensive to review, but a few that are relevant to this study will be reviewed.

Studies of daily variabilities of ranges of geomagnetic solar quiet daily variation (Sq) and its associated ionospheric parameters in low and middle latitudes have been studied by many authors, including Chapman & Stagg (1929), Ogbuehi et al. (1967), Mann & Schlapp (1985, 1987, 1988), Phillips & Briggs (1991) and Okeke et al. (1998). Also studies of day-to-day variabilities of the Sq daily ranges and associated ionospheric parameters in the equatorial zone included those of Onwumechili & Ogbuehi (1965, 1967), MacDougall (1969, 1979).

On the geomagnetic disturbances, McPherron (1972), and Burch ( 1972, 1973) showed from the results of their work that geomagnetic disturbances tend to occur when the Interplanetary Magnetic Field (IMF) is directed southward, while intervals of relative calm result when the IMF is directed northward, indicating that the transfer of energy from the solar wind to the earth's magnetosphere is more efficient when the IMF Bz component is negative. Extended intervals of Bz can sometimes lead to a build-up of energy in the magnetosphere dissipated in the form of substorms. It has long been established that the onset of a substorm can be related to disturbances in the solar wind and/or IMF. In most recent studies by Hortwitz (1985), McPherron et al. (1986) and Henderson et al. (1996), it was shown that substorm onset could also occur in the absence of major directional changes in the IMF and with no apparent wind. Fairfield & Cahill (1966) carried out the first observations to show a positive correlation between geomagnetic activity and the direction of the planetary magnetic field, using data from the Explorer 12 spacecraft together with data from several ground-based magnetometer stations.

More work has been carried out on geomagnetic activities. Lui et al. (1990) attributed the initiation of substorms to a kinetic instability due to particle drift across the magnetic field. Other work by Hones et al. (1973; 1984), Nishida et al. (1981) and Cattell et al. (1986) suggested that the initiation could be due to the formation of a magnetic neutral line within the cross-tail current sheet at a radial distance of about 15-20 earth radii. Lyon (1995) proposed that the expansion phase of substorms results from a reduction in the large-scale electric field imparted to the magnetosphere from the solar wind, following a growth phase of 30 minutes or more due to an electrical enhancement. Lyon (1996) pointed out the need to formulate a clearer global description of substorms that is consistent within the ionosphere and the magnetosphere, and which distinguishes them from other types of geomagnetic activity.

Work by Henderson et al. (1996) showed that magnetospheric substorms can indeed occur in the absence of an identifiable trigger in either the IMF or the solar wind dynamic pressure.

Fairfield & Cahill (1966) carried out an observation to show a positive correlation between geomagnetic activity and the direction of the planetary magnetic field, using data from the Explorer 12 spacecraft together with data from several ground-based magnetometer stations.

3. Conclusion

A review is given of many studies of the geomagnetic field in the ionosphere. In the review it is clearly seen that ground observations from a number of stations are available; for example, magnetometer data from the auroral stations and the stations of the European Incoherent Scatter (EISCAT) magnetometer cross, all-sky camera data, International Monitor for Auroral Geomagnetic Effects (IMAGE), also observations from satellites, coherent scatter radar, etc., are also available. The recent magnetometer measurements are being made under the beams of the Dual Aurora Network (DARN) and super-DARN radars. However, for the very high geomagnetic latitudes (Artic Ocean and Antarctic plateau) spaced ground-based facilities are used in Antarctica, i.e. the Automatic Geophysical Observatories (AGOs).

It has been noted that satellite observations in magnetospheres show no single mechanism that closely follows the main and recovery storm phases. It was also noted that the intermediate substorm processes are complex and still under some debate. Therefore, it is necessary that more complex observations be carried out using more modern instruments and a new location. If the station is located in Africa, this will give room for comparison of data obtained from other European and Asian countries. Lately, it has been found that solar flares have a big influence on the Sq current and hence on the EEJ current, and that it is higher in stations of EEJ than at other stations. Nigeria, being near the equator, experiences EEJ current so it becomes very necessary to participate in research of activities in the ionosphere of which EEJ is part.

Of all the works reviewed, there has not been a universally accepted cause of the complex activities like the substorm, EEJ counter electrojet (CEJ), and none of the operating stations are in Africa. It becomes necessary, therefore, that since South Africa has already started with AGOs, other African countries should support the project. It is extremely important that we have an African-based monitoring network to be able to exchange our observations and data with other continents. I mentioned Nigeria particularly, because there are so many activities yet to be studied, particularly along the dip equator. This will give better understanding and clarification of some of these complex phenomena.

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WGSSA
2000-02-28