Riometer

A Relative Ionospheric Opacity meter or short riometer is a device for passive observation of the ionospheric absorption ability in the frequency range of the radio waves.

Principle of operation

A riometer measures the reception level of the cosmic background radiation in the range of the radio waves emitted by stars or galaxies resistant and after passing through the ionosphere, the Earth reaches (Radio window). Although the strength varies with the Earth's rotation, it is nevertheless sufficiently constant and therefore predictable, depending on the region of the sky for earthly standards. Is the absorption heights in particular of up to 110 km is measured, since the majority of the absorbent located in there, lower layers of the ionosphere, known as the D layer takes place.

Depending on the degree of ionization, the absorption in the ionosphere is different in size. If a fault occurs, for example, by a solar radiation outburst increases the ionization in the lower ionosphere and thus the attenuation of radio waves. If the ionosphere quiet, the radio waves pass through, however, this to be attenuated without addition to the expected level. This results in a normal course of the reception field strength over time. The normal, daytime -dependent attenuation of the D- layer is here already included in the calculation.

The example shows such a reception diagram of the VHF Riometers the U.S. HAARP facility. The chart covers a 36 - hour period, which are characterized by a normal damping the first 24 hours. This can be well seen in the correspondence of the blue line of the actual received field strength with the green marked, expected reception field strength. From 14 UTC clock, however, the reception field strength drops sharply; hence the derived marked in red damping increases and is still volatile in the following.

To improve the measurement accuracy of both sensitive receivers are used as antennas also come with a strong directivity against Zenit, which also suppresses noise incident side of human origin.

Frequency selection

Normally, the frequency is observed, which is located in the lower portion of the ultra-short waves. Firstly, it must be high enough so that the waves are not reflected at the ionosphere, on the other hand they should also not be too high, thus increasing with increasing frequency attenuation does not increase too much. Usually a frequency 21-40 MHz is used.

Loss events

Causes

One cause of a loss event may be, for example, the penetration of high-energy electrons from the magnetosphere into the atmosphere due to a disturbance of the solar wind. Similarly, for the occurrence of an Aurora event, which can be regularly observed at high latitudes.

Effects

Loss events happen very often, but not always, go hand in hand with a deterioration of the propagation of short waves. A high-energy solar radiation outburst directly affects the ionization and thus the damping of the D- and E - layer of the ionosphere, which can lead to a so-called short wave fade -out. This tends to occur during the active phase of the sunspot cycle.