Understanding Midlatitude Riometers

 Here we will attempt to answer the following questions:

• What does the riometer measure?
• What does an ideal riometer output look like?
• What does riometer data tell us?
• What effects does the sun have on the riometer?
• What effects do astronomical sources other than the sun have on the riometer?
• What effects does weather have on the riometer?
• What effects do human radio transmissions have on the riometer?
• How can different types of interference be identified in riometer data?

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What does the riometer measure?

A riometer receives radio frequency energy from an area of the sky directly overhead.  The riometer antenna typically has rather broad coverage, so much of the sky is seen by the riometer.  The BLO riometer measures radio energy between 25.2-25.8 MHz.

The riometer is intended to receive radio energy from natural astronomical sources, including the sun, Jupiter, the radio star Cassiopeia A, the center of the Milky Way, and numerous lesser sources.  The riometer produces a voltage in response to the received radio energy; for our unit, zero volts means no energy being received, and a full-scale reading of about 7.2 V indicates maximum received energy.  A calibration curve may be used to express the riometer output voltage in terms of noise power, usually degrees Kelvin (K) or watts per hertz (W/Hz.)

Changes in the riometer output may be caused by:

1. movement of radio sources (noted above) across the sky;
2. changes in radio output by the sources (the sun and Jupiter are the major variable astronomical sources);
3. absorption of the radio energy by the ionosphere;
4. variable radio energy from sources on earth.

We are primarily interested in (3), ionospheric absorption effects.  The other causes of riometer changes are complications we must deal with.

What does an ideal riometer output look like?

The ideal riometer output for one day is called the Quiet Day Curve (QDC).  A day is “quiet” if there are no outbursts from the sun, no ionospheric disturbances, and no interference from terrestrial sources (weather or human.)

A QDC for the Bear Lake Observatory riometer is shown below.  The time scale is Universal Time (UT), allowing other scientific observations to be compared easily.  The left side of the plot, 0000 UT, is 6 pm local time and the sun is setting.  Sunrise is around 1200 UT.  The regularly-spaced downward spikes are calibration pulses.

We can see that riometer output is high and fairly constant at night (0200-1200 UT).  Ripples at night are due to astronomical sources moving across the sky.

As soon as the sun rises, the ionospheric D region is created by solar radiation ionizing the atmosphere between altitudes of about 60-90 km.  The D region then absorbs radio energy passing through it; absorption is inversely proportional to the square of the frequency, with strongest absorption effects seen below about 5 MHz.  Maximum absorption occurs around local noon, about 2000 UT, then decreases through the afternoon until the sun sets.  The absorption curve from dawn to dusk on a quiet day is related to the cosine of the solar zenith angle.

What does riometer data tell us?

Riometers are used to study the ionospheric D region by measuring radio wave absorption.  The smooth drop in radio energy from dawn to noon and the increase from noon to dusk is well-understood in terms of the sun’s ionizing effect on the D region and the radio wave absorption that results.  However, the depth of daytime absorption varies considerably, especially in the winter.  This variation is believed to be caused by changes in the chemical composition of the D region, which are in turn due to winds blowing between high and middle latitudes.  This process is not fully understood, so observing absorption and comparing it to other measurements may yield clues to what is happening in this part of the atmosphere.

Riometers also show the impact of solar flares and charged particles on the D region.  These effects are related to solar and geomagnetic disturbances, and are of interest in the study of space weather.  Space weather effects at midlatitudes are rather poorly understood.

Observations of normal D region absorption and solar flares are limited to daytime hours.  Absorption due to particle precipitation can occur at night in conjunction with auroral disturbances, but these events are most common at high latitudes.  At midlatitudes, auroral absorption may only occur a few times each year, depending on the latitude and the current progress of the 11-year solar sunspot cycle.

What effects does the sun have on the riometer?

The sun is responsible for producing D-region ionization and thus the radio wave absorption shown in the quiet day curve.  This effect is observed every day.  The sun also produces other strong effects when sunspots are active.

Solar flares are intense bursts of radiation associated with sunspots.  Flares may occur at various wavelengths, from visible light to ultraviolet and x-rays.  Ultraviolet and x-ray flares have the greatest effect on D-region ionization, and often cause communications blackouts on shortwave radio frequencies that last from a few minutes to a few hours.  X-ray flares are monitored by GOES satellites and measurements are available in real time from the Internet.  Ultraviolet flares are not routinely measured at present, and their occurrence is deduced from ionospheric effects such as the riometer measures.

Solar noise bursts are produced by flares and other solar disturbances.  They occur on a wide range of radio frequencies, from HF up to microwaves.  Solar noise bursts appear as increases in the riometer output during the day, often pushing the output to its maximum level.  Bursts may last anywhere from a few minutes to several hours.

The plot below shows the effects of an extremely large x-ray flare on the BLO riometer.  The flare occurred before 1800 UT (mid-morning local time), and the riometer output dropped to nearly zero indicating strong absorption.  A short time later, a strong solar noise burst pushed the riometer output to its 7 V maximum level.  The noise burst gave way to absorption at about 1800 UT, then noise again raised the riometer output to its maximum level.  Thus solar noise bursts can hide the absorption effects of solar x-ray flares in riometer data, complicating analysis.

A common measure of solar energy output is the F10.7 index, based on solar microwave radiation with a 10.7 cm wavelength.  The F10.7 index is related to the ionization levels found in the E and F regions of the ionosphere.  High F10.7 values tend to occur when flares and radio noise bursts are observed, so the index may be used to identify quiet and active periods for further study.

What effects do astronomical sources other than the sun have on the riometer?

Astronomical sources, excluding the sun, produce small amounts of radio energy.  Listening on a shortwave radio, the sources sound like static varying from faint to moderate levels (depending on the sensitivity of the radio and the antenna used.)

For radio stars and galaxies, changes in riometer output are due to the sources moving in and out of the antenna beam.  Jupiter, however, can produce radio bursts in the riometer frequency range.  These bursts are somewhat predictable and may show up in the riometer data depending on time of day and ionospheric conditions.

Two such effects are shown in the riometer data for 24 August 2005 shown earlier.  At sunset (about 0100 UT) Jupiter is setting as daytime absorption ends, and it is an active time for Jupiter, so the small bump at 0100 UT is likely the result of a Jovian noise burst.  Before dawn, around 1000 UT, Cassiopeia A is at its highest point in the sky, and the riometer level rises as the radio star passes through the riometer antenna beam.

What effects does weather have on the riometer?

Lightning is the weather phenomenon that has the greatest effect on HF radio and riometers.  A lightning discharge is a powerful electrical impulse that creates a burst of radio noise over a wide range of frequencies.  A single lightning discharge in the vicinity of the riometer will produce a spike in the riometer output.  A nearby thunderstorm can create a noise burst signature on the riometer similar to solar noise bursts.  More distant thunderstorms will create weaker noise bursts, even if the thunderstorm is somewhat over the horizon from the riometer.  It is possible for lightning noise to propagate over thousands of kilometers due to ionospheric refraction, though this effect is generally relatively weak and mostly observed at night.

An example is shown below.  Lightning noise is seen in the HF receiver scan (left) as a vertical spike around 1600-1700 UT and smaller spikes at about 1300 and 1900 UT.  This thunderstorm noise may explain some of the interference seen in the riometer data for the day (right), where the sinusoidal quiet day curve is nearly unrecognizable.  Solar activity also contributes to the disruption of the quiet day curve on this day.

What effects do human radio transmissions have on the riometer?

Human radio transmissions in the riometer frequency range may have the same appearance as lightning and solar noise bursts, with the amplitude depending on the power and proximity of the transmission.  It is therefore very important to choose a riometer frequency range away from popular communications bands.  A few bands have been reserved for radio astronomy observations, and riometers usually make use of these bands.  The most commonly-used radio astronomy band is at 38 MHz.  The BLO riometer is set up in the 25.5 MHz radio astronomy band.  The traditional riometer frequency of 30 MHz is not in a protected band, and often has interference from amateur radio and citizen’s band (CB) transmissions.

Unfortunately, the riometer bandwidth is wider than the radio astronomy band, and not everyone observes international band designations, so interference from human radio transmissions is practically unavoidable.  Human interference is most common in the winter when propagation conditions allow signals to travel thousands of kilometers.

How can different types of interference be identified in riometer data?

Generally solar noise bursts, lightning noise, and human interference will all appear as increases in the riometer output level, often taking it to the maximum.  Identification of the source of noise bursts and interference in riometer data usually relies on circumstantial evidence.

• A list of solar noise bursts is available from the NOAA Space Environment Center.  This list is not necessarily comprehensive, and may not show weak noise bursts that still have a significant effect on the riometer.
• Weather observations for the riometer area can help to identify periods when lightning interference is likely.
• Human interference can sometimes be observed directly by tuning a shortwave radio across the riometer operating frequency band when the riometer is showing signs of noise or interference.

Identifying solar and Jovian noise bursts is scientifically interesting.  Other noise sources are nuisances.  Since there is rarely much that can be done about them, identifying those sources is not a priority during normal operation.