The Sun and the Earth: Interactions During Geomagnetic Storms

by Chukwuma Anoruo

The field of solar-terrestrial interactions, the interplay between the Sun and other planetary bodies in the solar system, is a fascinating area of research. My knowledge about this subject as a doctoral student now spans three years. The line of my research delves into the study of major disturbances (geomagnetic storms) of the Earth’s magnetic region (extending from the Earth’s interior out into space) that occur whenever there is a very efficient exchange of energy from the solar wind into the space environment surrounding the Earth. Geomagnetic storms have unique characteristics, and no two storms have the same behavior. Here, I try to study distinctive characteristics of storms’ strength on the electron content of the ionosphere and its trans-ionospheric disturbance of radio signal propagation. This usually has a heavy impact on the Global Positioning System (GPS), causing signal errors and space weather perturbation. This action relative to storm strength causes extra drag on satellites, which modifies the path of radio signals creating errors in positioning information provided by GPS. It also disrupts navigation systems and creates harmful, geomagnetically induced currents in the power grid. Studying this subject is essential for airports to understand periods of safety and for communication with pilots during flights. A general view of the physics of this concept is explained here for a non-expert audience.

Introduction

The solar-terrestrial interaction simply refers to the Earth’s region that is influenced by the physical conditions of the solar interior. Figure 1 shows a diagram of solar-terrestrial interaction. This region is called the Ionosphere and it contains a large amount of electrons that are able to reflect radio signals [1]. The disturbance of this region is called ionospheric disturbance which then causes the Earth’s geomagnetic storms. This disturbance results from streams of energetic particles consisting of electrons and ions (plasma) carried by solar wind into the Earth’s magnetosphere (the magnetic field). The solar wind originates from the Sun’s coronal holes as a result of expansion of the solar atmosphere when the field is open. This wind propagates with a velocity of 400 kilometers per second, hitting the Earth’s atmosphere and causing perturbation of the Earth’s geomagnetic field [2]. The ionospheric disturbance results in instability of the ionosphere, causing the bending of radio signals. This is simply because the ionized electrons affect propagation of radio signals. This level of ionization is dependent upon time of the day, season and latitude [3].

Figure 1. Solar-terrestrial interaction

The parameter widely used to study the degree of ionospheric disturbance during geomagnetic storms is the total electron content (TEC) [4]. TEC is an integrated form of the electron number density that exists between the satellite and the ground receiver. The electron density is the measure of the probability of an electron being present at a specific location. The low, middle and high latitudes of the ionosphere experience different responses during geomagnetic storms. Recent advances in satellite technology have made significant improvements in using TEC to study ionospheric variability [3]. Its measurement has played a significant role in determining the delay that is usually experienced in the radio signal transmitted from a GPS satellite to the ground receiver.

Physics of solar-terrestrial interaction

The disturbance storm time (Dst) measured in nano Tesla (nT) is a major hourly index that is used in defining and quantifying the strength of a storm. It monitors the degree of storm strength caused as the result of geomagnetic field disturbance, which signifies the amount of ions being injected. The collision of these ions with electrons causes electrical excitation of the electron and, consequently, an increase in TEC. This is more frequent during a solar maximum - the active period of the Sun. Since disturbance of Earth’s magnetosphere is associated with geomagnetic storms, they are categorized with a negative intensity of Dst index as seen in Table 1.

Type of storm	Dst range (nT)
Weak	>-50
Moderate	-100 to -50
Intense	-250 to -100
Severe	<-250

Table 1. Classification of geomagnetic storms with different Dst ranges [5]

Figure 2 indicates a schematic diagram of August 1998 storm-like variation in the Dst index featuring a typical geomagnetic storm. The Dst ranges from negative to positive values. During intense storms Dst decreases and increases during calm days.

Figure 2. Schematic diagram of storm strength

At solar maximum, high energetic particles composed of charged plasma from the Earth’s magnetosphere are forced into the Earth’s atmosphere by the solar wind. The solar atmosphere is made up of three layers: a) the photosphere, b) the chromosphere, and c) the corona. This is shown in Figure 3. The photosphere represents the top of the convection zone, where most sunlight is emitted. In this layer there is evidence of dark spots, called sunspots, which are surrounded by brighter areas known as the active regions. The chromosphere and corona lie above the photosphere. In the corona, there are areas that are colder, hence darker, and have low density plasma because of their lower energy and gas levels. Such areas are known as coronal holes. In the coronal holes, the magnetic field is open, and through it, the solar wind propagates outward. On some occasions, huge erupting bubbles known as coronal mass ejections (CMEs) are observed as a result of an eruption. At this time solar wind, originating from the coronal holes where the magnetic field is open, propagates energetic particles with a speed of 400 km/s into the Earth’s atmosphere.

Figure 3. The solar atmosphere

The Interplanetary Magnetic Field (IMF) is a weak field, varying in strength near the Earth. It is the mean plane of the apparent path in the Earth’s region that the Sun follows over the course of one year. Furthermore, it is part of the Sun’s magnetic field carried by the solar wind. The IMF is the main mechanism of solar-terrestrial interaction and regulates the interstellar energetic particle penetration rate. This is the rate at which energetic particles penetrate the Earth’s magnetic field. The kinetic pressure of solar wind modifies the outer region of the Earth’s magnetic field by compressing it — this compression occurs mainly at day time, but can sometimes persist until the night. The orientation of the IMF is a major contributing factor to the entrance of energetic particles from the Sun. The IMF is a vector quantity with three axes, of which the vertical axis (Bz) is important for auroral activity. The main phase of geomagnetic storms occurs when the IMF drifts southward along the Bz axis, while a northward shift coincides with the end of the storm. 

Summary and conclusion

The Earth’s magnetic field protects it from dangerous radiation and high energy plasma from the Sun. However, there is always some part of the plasma and radiation that is able to penetrate through the magnetic field barrier, and this often leads to magnetic disturbances. The magnetic field of the Earth becomes disturbed when hit by energetic particles emitted from the Sun resulting from CME. The actual amount of this depression does not only depend on storm severity but also duration. These magnetic disturbances affect communication and navigation systems and, when severe, may lead to a total blackout. Geomagnetic storms are more frequent during the active period of the Sun. At the minimum solar activity period, the field is at its quietest. The deviation of TEC during disturbed and quiet times gives the actual TEC value which is more valid for determining storm strength. A reliable and accurate representation of the ionosphere is essential in order to understand the impact of storms and how they affect GPS applications. The need to understand the behavior of the Earth’s magnetic field, its origin and variation, is becoming ever more important.

Chukwuma Anoruo
Department of Physics and Astronomy, University of Nigeria, Nsukka, Nigeria


If you have questions or comments concerning Anoruo's post, please leave a comment below, or send him an email. You can also connect with him on ResearchGate.

                                                                                                                                                                   
References


[1] Garner, T. W., Gaussiran, T. L., Tolman, B. W., Harris, R. B., Calfas, R. S., Gallagher, H. (2008). Total electron content measurements in ionospheric physics. Adv. Space Res 42: 720–726.
[2] Lockwood M., Chambodut A., Barnard L.A., Owens M.J., Clarke E., Mendel V., (2018), A homogeneous aa index, secular variation, Journal of Space Weather Space Climate, 8, 27-53.
[3] Kenpankho, P., Supnithi, P., Tsugawa, T & Maruyama, T. (2011). Variations of ionospheric slab thickness observations of Chumphon equatorial magnetic location. Earth PlanetsSpace 63:359-364.
[4] Oron, S., D'ujanga, F. M. &Senyonga, T. (2013).Ionospheric TEC variations during the ascending solar activity phase at an equatorial station. Uganda. Indian Journal of Radio & Space Physics 42:7-17.
[5] Gonzalez,W.D., Joselyn, J.A., Kamide, Y. H., Kroehl, W., Rostoker, G., Tsurutani, B.T., Vasyliunas, V.M. (1994). What is a geomagnetic storm? Journal of Geophysical Research 99:5771–5792.
[6] Buonsanto, M. J. (1999). Ionospheric storms – A review. Space Science Review 88, 563–601.