ABSTRACT
Plasma is a high-energy gas that is responsible for the ionization of a significant number of its atoms or molecules, thus forming equal numbers of positive and negative ions ( electrons). Plasmas exhibit some properties of gases, but differ from gases being good conductors of electricity and being affected by magnetic fields. Although there are free charges and ambipolar pairs in plasmas, overall the negative and positive charges compensate each other. Therefore, plasmas are electrically neutral, better known as quasi-neutrality [1]. Within the known universe, more than 99% of all observable matter is plasma, a state often highly dynamic and far from thermal and mechanical equilibrium. In particular, for our own solar-terrestrial system, various plasma active phenomena frequently occur such as solar flares, solar wind, and coronal mass ejections (CMEs) in the solar atmosphere; interplanetary magnetic clouds and anti-collision shock waves in interplanetary space; and geomagnetic storms and Earth’s aurora occurring in Earth’s magnetosphere and ionosphere. Such phenomena are not only the most important events resulting in changes in the space environment around our anthrosphere, but also provide natural laboratories for a detailed study of the basic plasma processes encountered in astrophysics. A large part of the energy flux released in the solar atmosphere travels into interplanetary space and impacts the Earth and is known as bow shock. The bow shock energizes the magnetosphere, resulting in changes in the composition, energy balance, and the ionospheric dynamics of the plasma sphere and plasma pause [2-4]. The activities of the plasma process of the solar atmosphere have a direct impact over the ionosphere down to troposphere, thus affecting the radio communication in the different frequency bands [5]. The layers of the atmosphere all react in different ways from the solar particles and plasma influx. The D layer [6] is the innermost layer and is one of the most active during solar activity. The D Layer is 60 km (37 mi) to 90 km (56 mi) above the surface of the Earth. Ionization here is due to Lyman series-alpha hydrogen radiation at a wavelength of 121.5 nanometer (nm) ionizing nitric oxide (NO). In addition, with high solar activity hard X-rays (wavelength <1 nm) may ionize (N2, O2). During the night, cosmic rays produce a residual amount of ionization. Recombination is high in the D layer, the net ionization effect is low, but loss of wave energy is great due to frequent collisions of the electrons (about 10 collisions every msec). As a result, high-frequency (HF) radio waves are not reflected by the D layer but suffer loss of energy therein. This is the main reason for absorption of HF radio waves, particularly at 10 MHz and below, with progressively smaller absorption as the frequency gets higher. The absorption is small at night and greatest about midday. The layer reduces greatly after sunset; a small part remains due to galactic cosmic rays. A common example of the D layer in action is the disappearance of distant AM broadcast band stations in the
daytime. During solar proton events (SPEs), ionization can reach unusually high levels in the D-region over high and polar latitudes. Such very rare events are known as polar cap absorption (or PCA) events, because the increased ionization significantly enhances the absorption of radio signals passing through the region. In fact, absorption levels can increase by many tens of dB during intense events, which is enough to absorb most (if not all) transpolar HF radio signal transmissions. Such events typically last less than 24-48 h.
