1. The output of the sun in all forms, light, solar wind and energetic particles is not constant. It varies with both time and position of the Sun. These changes are called Solar Activity.
2. The condition of the Sun has a major impact on ionospheric radio propagation.
3. As the Sun provides the radiation that governs the state of the ionosphere and hence HF radio propagation, any flares or other disturbances are of great importance.
- Under some circumstances, these can enhance radio communications and the HF radio propagation conditions.
- Under other circumstances, they can disrupt radio communications on the HF bands.
TYPES OF DISTURBANCES:
1. Solar Flares
2. CMEs (Coronal mass ejections)
3. Coronal Holes
1. Solar Flares:
- Solar flares are enormous explosions that occur on the surface of the Sun. They result in the emission of extremely large amount of energy. These occur during solar peak activity.
- Mainly, solar flares result in the increase of the level of solar radiation on Earth.
- This radiation covers the whole electromagnetic spectrum the ultra-violet, X-rays and affect the levels of ionisation in the ionosphere and hence it has an effect on radio communications via the ionosphere.
- Usually, an enhancement in ionospheric propagation is noticed as ionization level of the higher layers of the ionosphere increases.
- However, if the ionization level of the lower layers of the ionosphere starts to rise then this can result in higher levels of attenuation of the radio communications signals.
- It also increases the level of background noise at VHF.
- Flares generally only last for about an hour, after which the surface of the Sun returns to normal.
2. CMEs (Coronal Mass Ejections):
- CMEs are another form of disturbance that can affect radio communications.
- Coronal Mass Ejections (CMEs) are huge bubbles of gas that are threaded with magnetic field lines, and the bubbles are ejected over the space of several hours.
- CMEs can give rise to ionospheric storms. These can provide a short-lived enhancement to ionospheric radio propagation conditions but before long this can result in a blackout to radio communications via the ionosphere.
3. Coronal Holes:
- They are regions where the corona appears dark.
- High-speed solar wind originates from them which has an impact on ionospheric radio propagation conditions and hence on all HF radio communications.