Space Weather

Space Weather Woman



The following information is from an article "Understanding Solar Indices", by Ian Poole, G3YWX in QST, September 2002

Conditions are continually varying on the HF bands as a result of the varying levels of ionization in the ionosphere. The radiation coming chiefly from the Sun hits the upper ionosphere, causing molecules to ionize, creating positive ions and free electrons. A state of “dynamic equilibrium” exists. The free electrons that affect radio waves recombine with the positive ions to reform molecules. When levels of ionization are higher when there are more free electrons) the ionosphere is more capable of bending back radio signals to Earth. Also, high levels of ionization mean high maximum usable frequencies and better HF conditions. The level of ionization at any given point above the Earth is dependent upon a number of factors including the time of day, the season and most important of all the sunspot cycle. It is found that the level of radiation from the Sun increases as the number of sunspots increases. Accordingly, the level of radiation received from the Sun peaks around the top of the sunspot cycle. In fact, it is the bright area just around the sunspot called the plage that emits most of the extra radiation. At the sunspot peak the level of geomagnetic activity also rises. This happens as the Sun emits vast quantities of particles. There is normally a steady flow of these particles, but at times such as when there are solar flares the level of these emissions greatly increases. When they hit the Earth’s magnetic field it becomes disturbed, creating a magnetic storm that can be detected at points around the globe. Another effect is that the ionosphere itself can be disturbed, giving rise to an ionospheric storm. This will degrade HF communications and when particularly bad it can lead to a total HF blackout.

Solar Flux
A measure known as the solar flux is used as the basic indicator of solar activity, and to determine the level or radiation being received from the Sun. The solar flux is measured in solar flux units (SFU) and is the amount of radio noise or flux that is emitted at a frequency of 2800 MHz (10.7 cm). The Penticton Radio Observatory in British Columbia, Canada reports this measure daily. The solar flux is closely related to the amount of ionization and hence the electron concentration in the F2 region. As a result it gives a very good indication of conditions for long-distance communication. The figure for the solar flux can vary from as low as 50 or so to as high as 300. Low values indicate that the maximum useable frequency will be low and overall conditions will not be very good, particularly on the higher HF bands. Conversely, high values generally indicate there is sufficient ionization to support long-distance communication at higher than-normal frequencies. However, remember that it takes a few days of high values for conditions to improve. Typically values in excess of 200 will be measured during the peak of a sunspot cycle with high values of up to 300 being experienced for shorter periods.

Geomagnetic Activity
There are two indices that are used to determine the level of geomagnetic activity: the A index and the K index. These give indications of the severity of the magnetic fluctuations and hence the disturbance to the ionosphere. The first of the two indices used to measure geomagnetic activity is the K index. Each magnetic observatory calibrates its magnetometer so that its K index describes the same level of magnetic disturbance, no matter whether the observatory is located in the auroral regions or at the Earth’s equator. At three hourly intervals starting at 0000 UTC each day, the maximum deviations from the quiet day curve at a particular observatory are determined and the largest value is selected. This value is then manipulated mathematically and the K index is calculated for that location. The K index is a “quasi logarithmic” number and as such cannot be averaged to give a longer-term view of the state of the Earth’s magnetic field. Thus was born the A index, a daily average.