The HAARP 30 MHz Riometer
What is a riometer?
A riometer is a passive scientific instrument used to observe ionospheric absorption, particularly absorption
at altitudes less than 110 km caused by electron precipitation. The word riometer stands for Relative
Ionospheric Opacity Meter
How does a Riometer Work?
Riometers measure the strength of radio noise originating from stars or galaxies and arriving at the earth after
passing through the ionosphere. The sky is filled with stars and galaxies that emit a broad spectrum of radio
noise and the noise is strong enough to be picked up using sensitive receiving equipment. Because some regions
of the sky are noiser than others, this noise varies on a predictable basis as the Earth rotates. Although noise
due to stars or galaxies may change over very long time frames, it is constant enough to be considered a repeatable
function of Local Sidereal Time.
Depending on the amount of ionization present, radio signals passing through the ionosphere may suffer losses (or
become weaker) in a process called absorption. Imagine the ionosphere as a set of louvers. If it is disturbed, the
louvers close and signals arriving from outside of the earth's vicinity do not pass through very well. If the
ionosphere is "quiet," the louvers are open fully and signals pass through easily.
If there were no sources of absorption in the earth's atmosphere, the cosmic noise measured by the riometer would
be exactly the same at corresponding times during each successive Sidereal day. The "Quiet Day Curve" is this
expected, or "no-absorption" diurnal noise level. (In this context,"quiet" means that the ionosphere is undisturbed
by solar events.) Any difference between the actual measurement and the Quiet Day Curve is attributed to ionospheric
absorption.
The riometer uses a sensitive receiver which is typically tuned to a frequency near the lower end of the Very High
Frequency (VHF) region. The frequency is chosen to be high enough that radio waver are not reflected by the ionosphere
but pass through it. At the same time, ionospheric absorption gets less as the frequency is increased, so the frequency
should not be too high if good measurement resolution is desired. Traditionally, frequencies in the 21 to 40 MHz range
have been used. A large number of riometers world wide including the one at HAARP use a common frequency, 30 MHz.
The riometer is intended to measure the ionospheric absorption directly above its location. Medium to high gain antennas
pointed at the zenith are used. Such antennas also suppress interfering, man made radio signals that may propagate into the
location at low angles.
In operation, the riometer listens to the background cosmic radio noise throughout the day. If that noise is the
same as the expected (or quiet day curve) noise, we know that it is not being affected by the ionosphere before it
reaches the earth's surface. If the received noise is less than the quiet day curve, we know the ionosphere has absorbed
some of the noise signal. The riometer uses a conversion algorithm to calculate an estimate of the amount of absorption
thus observed. A simple relation can be used to determine the amount of absorption that would be caused at other
frequencies.
The HAARP VHF Riometer
The HAARP VHF Riometer operates at a frequency near 30 MHz. The receiver measures the total power in a bandwidth of
30 kHz and compares the result with quiet day data developed over a long period of riometer operation at the HAARP
location. The difference between the current measured noise power and the expected power is converted to ionospheric
absorption and plotted on a continually updating
chart.
The HAARP VHF Riometer uses a 2 X 2 array of five element yagi antennas to. The antenna array has a reception
pattern that encompasses a fairly large portion of the overhead sky. As a result, the HAARP VHF riometer does not
give information on where (in the sky) a particular absorption event may be occurring but, rather, provides an
overall or integrated measure of ionospheric absorption above the site.
The chart below is an example of data obtained using the HAARP VHF Riometer. The chart covers the 36 hour time period
from 1200 UTC on 28 January 2007 until 0000 UTC on 30 January 2007. There are three curves on this chart: (1) The
green curve is the Quiet Day Curve, (2) The blue curve plots the actual measured data and (3) The red curve is the
calculated ionospheric absorption. In the early part of the chart (to the left) the green and blue curves overlap,
indicating that the cosmic noise observed by the riometer is very close to the expected value. Beginning at about
1400 UTC on 29 January, however, the measured noise (blue curve) drops below the quiet day curve and the ionospheric
absorption (red curve), as calculated by the riometer, begins to rise. After a short time, the measured noise returns
to the quiet level (at 1600 UTC) but then drops again in response to another absorption event. (Compare this period
of disturbed conditions with the quiet period 24 hours earlier in the chart at 1400 UTC on 28 January.)
How is the riometer used scientifically?
Riometers are most sensitive to ionospheric absorption occurring at altitudes between 50 and 110 km. Absorption at
these altitudes can be caused in several ways. During daylight hours, for example, the sun causes ionization in the
"D layer" at altitudes near 80 km. This ionization occurs each day throughout the year and is a predictable function
of the sun's zenith angle. This regular and periodic absorption is accounted for in the "quiet day curve."
Another type of absorption event is caused by high energy electrons precipitating into the earth's atmosphere from
the magnetosphere as a result of a disturbance in the solar wind, for example. The altitude to which these particles
penetrate depends on their initial energy. Auroral precipitation, commonly observed at high latitudes, produces
absorption at altitudes of 90 - 100 km. Riometers are capable of observing auroral precipitation events that would
not necessarily be visible optically.
Absorption events shown by riometers are very frequently (but not always) associated with poor HF sky-wave propagation
conditions. When the sun is above the horizon, an energetic solar flare will cause nearly instantaneous increases in
the ionization of the D and E layers, producing an abrupt short wave fade-out. Riometers will clearly indicate these
transient events that are common during the active portion of the solar cycle.
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