Ionospheric Perturbations

Solar radiations (II)

The Sun emits a great number of radiations across all the spectrum but they are mainly concentrated in the light, in respect with the Planck's curve. These radiations are generated during the process of nuclear fusion developing in the sun core, a continuous processus transforming the hydrogen into helium and heavier elements (all those that you can see in its spectrum). The intense energy released during this process radiates away to the sun surface, mainly in the form of gamma rays, X-rays and neutrinos. While these latter leave the sun practically without interaction, they are neutral, all other radiations cross slowly the radiative zone and transform gradually in heat, creating convective flows in the last 100000 km of the sun atmosphere. It is in this convective zone located just below the surface that the solar wind is created, generated by the heat that warms the corona, releasing fast particles into space. This is then magnetic waves present in the corona that speed up the solar wind.

Radiations emitted by the sun are of two types and do not reach the Earth at the same time : there are the electromagnetic emissions and the particle emissions. As these radiations hit the Earth ionosphere at various speeds with different energy levels, effects of their impact varies accordingly and not all shortwaves are concerned by these events but mainly sky waves.

The electromagnetic radiations

The electromagnetic emissions are constituted of X-rays, UV and EUV radiations. Travelling at the velocity of light, they reach the Earth 8.3 minutes after their emission.

Crossing the geomagnetosphere as it didn't exist, the EUV hit first the F-layer of the ionosphere increasing its ionization; atoms and molecules warm up and free one or more electrons. The higher is the solar activity, the stronger is the ionization of the F-layer. Its density is estimated at 2 millions electrons/cm³, what is one hundred time denser that the D-layer. Therefore when we observe many sunspots on the sun surface and an intense solar flux (energy per unit area) at 10.7 cm (2800 MHz), which is correlated with the EUV variability, all active radio amateurs rub one's hands expecting good propagation conditions. Indeed, a strong ionization of the F-layer increases its reflecting power and as it is a function of the frequency used, stronger is the ionization the higher is the maximum usable frequency (MUF) to work DX stations, exceeding regularly 50 MHz in such occasions.

At left, the progression of the sunspot number during the solar cycle 24 (current). At right, the progression of the radio flux at 10.7 cm during the same period. Both curves are correlated over 97 %. Documents SWPC/NOAA.

UV and X-rays radiations penetrate deeper in the ionosphere and at low altitudes are stopped in the D-layer which sees it ionization level suddenly drastically increase, and its nominal height of 90 km does down to 50 km or so. Strongly ionized, the D-layer absorbs with a greater efficiency HF signals, mainly on the Earth side face to the sun and affects the lowest part of the HF spectrum, these are SWF, PCA and other geomagnetic storms. We will come back on these perturbations on the page.

Particle emissions and Coronal Mass Ejections

Particle emissions are constituted of high-energy protons and electrons, and alpha particles (helium nuclei) forming the solar cosmic rays. Fast protons and electrons form also the solar wind which is responsible of the ionization of the high atmosphere of the Earth.

On the other side, when the sun releases huge amount of energy in CME, fast particles constituted of protons and heavy nuclei propagate into space, creating a shockwave moving at speeds between 300 and 1000 km/s while the fastest particles can reach close to 2000 km/s  ! While neutrinos and other electromagnetic radiations reach quickly the Earth, high-energy protons are much slower and we have to wait for 10 to 40 hours in order that the slowest protons of low energy reach the Earth. In the meantime we recorded the first magnetic storms on Earth.

At left, a huge prominence released by the sun on October 26, 2003 at 01:19 UTC. It was quickly associated with a Class-C3 CME (right) that disturbed solar winds currents. Click on the CME image to run an animation (GIF of 349 KB). As usual this flow of particles (protons and gas) stroke the Earth ionosphere in a catastrophic way, perturbing all radio communications, inducing deep fadings (SWF), Polar Cap Absorption (PCA) and blackouts. Documents SOHO/LASCO.

The protons flux emitted by the CME and the solar wind emitted by the solar coronal holes arrive on Earth some two daysafter the emission. The pressure created by the particles cloud on the terrestrial magnetopause is huge and all its structure is modified. From that moment shortwaves communications are interrupted by intermittence. Next, fast protons charged between 3 to 100 MeV arrive at polar caps, attracted by the geomagnetic field open at this place. Lines of this magnetic field are very similar to the one of a bipolar magnet butof astronomical size. This phenomenon occurs simultaneously in both hemispheres.

The geomagnetic field

Now that the solar plasma has came closer to us and is partly trapped in the geomagnetic field, it is interesting to linger on the origin of the latter.

The Earth's magnetic field (or geomagnetic field) is generated in the deep core of the Earth by the differential movements of the magma. It displays slight variations during quiet days and abnormal variations when it is perturbated or active. The field strength is expressed in gauss (CGS) or better in tesla (T) in the System International (SI). 1 gamma (g) is equivalent to 1 nT. The intensity of the geomagnetic field at the Earth's surface is approximately 32000 nT at the equator and 62000 nT at the north pole where the compass needle points downward.

Like a magnet displays two poles surrounded with a magnetic field, the geomagnetic field can be approximated by a centered dipole field, with the axis of the dipole inclined to the earth's rotational axis by about 11.5°. The northern part of the geomagnetic dipole is located near geographic coordinates 78.3° N 69° W near Thule, Greenland. Its southern counterpart is located near 79° S 110° E near Vostok Antarctica base. The pole where the magnetic field is vertical to the Earth's surface, is located near 76° N 101° W, and 66° S 141° E.

Note that the origin of geomagnetic longitude is the meridian passing through the geomagnetic poles (true or dipole model) and the geographic south pole (corrected geomagnetic latitude).

When the geomagnetic field is disturbed at global scale, a phenomenon distincts from regular diurnal variations, we speak of geomagnetic storm. It is defined as occurring when the daily Ap index exceeds 29 (minor storm, see below). The next table characterize the geomagentic activity and components of the geomagnetic field.

As you know, solar and geomagnetic activities can deeply affect radio propagation and thus human activities. Tens of different disturbances occur daily, some are practically unnoticed excepting on recordings, others lead to minor perturbations but some can disturb large area in the ionosphere and the most servere can affect human infrastructures as well.

Let's see what are these geomagnetic and ionospheric disturbances, their origins and how they affect sky wave propagation.

Next chapter

Geomagnetic and ionospheric disturbances