ABIERTA O TEMÁTICA LIBRE SIN FINANCIACIÓN

5.3.1

Solar Activity

Most of the energy on Earth’s surface originates from our nearest star, the Sun. It is a 7 × 105_-km

radius star that produces its energy from thermonuclear fusion of hydrogen nuclei into helium. The hot plasma of compressed gases that it forms is associated with a dipolar solar magnetic field that extends outwards into space. The Sun emits EM radiation, and through short-lived, intense and

Figure 5.4: Overview of 30-day averaged sunspot number plotted for a 24-year duration from 1991 [11]. The current solar cycle (2009 − 2020) is the 24th_.

localised outbursts known as solar flares, expels streams of matter known as the solar wind (SW) to space. The solar wind and its magnetic field interact constantly with the Earth and couple with the terrestrial magnetic field and ionised gases in the planet’s vicinity. [22].

Solar sunspots (dark spots on the Sun’s surface, observable in visible light ) are the most active regions of the solar atmosphere, so that their number of appearance is proportional to the intensity of solar activity. Through their protruding strong magnetic flux lines, they eject solar wind (SW) and X-ray flares into space. Therefore, depending on the number of those hotspots on the surface of the Sun, two extremes of solar activity are deduced: A maximum sunspot number corresponding to an intense solar activity (more mass ejections, high-speed solar wind and more intense ionising radiation), and a minimal number of sunspots indicating very weak solar activity. Therefore, solar activity varies through peaks and troughs, and each cycle with a period of ∼ 11 years. The current cycle is the 24th_{since the year 1775.}

The activity of the Sun being the driver of the Earth geomagnetic system, there is more probability of storms/intense activity in the atmosphere in peak solar activity and vice versa. Fig. 5.4 shows the solar cycle of the past 24 years, with the latest quietest period around the year 2009 and a peak of activity in 2014.

5.3.2

Near-Earth Environment

The near-Earth environment is made of concentric regions of plasma whose dynamics are controlled by the magnetic field. It extends from a distance of about 60 km above the ground, up to 10 RE in

the day side of the Earth and may extend to 100 RE in the night side [22]. It is usually referred to

as the magnetosphere.

The magnetosphere owes its name to the fact that the magnetic field intensity is so strong there that it forms a layer of plasma whose interactions are driven by the magnetic field frozen-in to it. The

Figure 5.5: Sketch showing the structure of the magnetosphe with its electric current system. Note that the dayside is towards the left and the nightside is towards the right of the image [12].

ionised particles in the magnetosphere originate mainly from the wind of solar particles that enters in the polar regions and from ionisation by solar radiation. The motion of charged particles in the magnetosphere creates huge currents, such as the westward ring current along the dayside equatorial region and a tail current to the night side. Magnetospheric dynamos drive the wave-interaction and the particle distribution in the whole near-Earth cavity [22]. The different magnetospheric currents are depicted in Fig. 5.5.

Magnetic weather is always used to describe the steadiness of the magnetospheric system. On the ground, it is measured with two indices: The intensity of the currents in the magnetosphere (Dst) and magnetic disturbance (Kp). The Dst measures the intensity of the equatorial ring current in the

magnetosphere from equatorial range Intermagnet observatories [98]. Dst decreases H component in the datasets. The storm intensity can be classified as follows

• intense storm, Dst < −100 nT, • moderate ,DST < −50 nT and • minor,Dst > −30 nT.

The disturbance in the magnetic field is measured from 13 high latitude stations. It is estimated from the mean value of the disturbance levels in the two horizontal components and scaled logarithmically

from 0 to 9 as a function of the severity of the disturbance, where Kp = 0 is for very calm weather

and Kp= 9 indicates extremely disturbed weather.

5.3.3

The Ionosphere and Solid Earth Waveguide

Below 50 km, the atmosphere aggregates 99% of the total mass of gas it contains. Above 60 km, where atoms are rarer, ultra-violet (UV) and X-ray radiation from the Sun heat and ionise the gas. Ionised particles do not recombine easily because of the low ion density. At those heights, there is a permanent density of free electrons and of ions that form a layer called the ionosphere. The ionosphere is highly conducting and can support strong electric currents. It affects radio waves, can generate EM waves and is subject to long-range interactions and instabilities characteristic of a plasma [22]. Plasma media interact strongly with the ambient geomagnetic field which affects the motion of ionised particles, modifies ionospheric electric currents and bulk movement of the plasma. The solid Earth is a spheroidal shaped planet of average radius RE = 6370 km, made of an inner

molten metal core, a molten rock mantle in the outer core, all enclosed in a solid rock layer, called the lithosphere. The latter’s surface is covered at 75% by oceans and continents occupy the remain- ing 25%. The conductivity at the Earth’s surface varies from seawaters’ (5 Ω−1m−1) to granite’s (10−3Ω−1m−1) [99]. In the ELF range, the solid earth can be considered as a PEC because the conductive current exceeds the displacement current resulting in the wave reflection over Earth’s surface. The Earth-ionosphere (E-I) waveguide formed of conductive layers of the upper ionosphere and Earth’s lithosphere form a natural ELF resonator in which EM radiation around the globe excite specific harmonic modes inside the cavity, which are characterised by eigenfrequencies due only to the spherical shape of the cavity, losses and dispersion in the ionosphere. These eigenmodes are called Schumann resonances.

5.3.4

Sources of Excitation

The main source of EM excitation in the E-I cavity is attributed to lightning activity [19, 100]. Lightning can be physically described as a transient current discharge that occurs in the atmosphere due to Coulomb potential that builds up between separated charges in thunderstorm clouds and the ground [29]. They occur randomly in time and in location but are mainly distributed on land areas. They form the background ELF noise detected on ground magnetograms [19]. Amongst these signals, single super-powerful lightning strokes occur once in a minute of time on average worldwide. These Q-bursts as they are called can be tens of time more intense than the average background noise. They are reflected back and forth several times in the Earth-ionosphere cavity between the source and its antipode before being attenuated to undetectable levels, creating an impulse response of the whole cavity [19]. These signals can be detected on the ground above the background signals.