D Region Absorption Documentation

D Region Absorption Prediction Documentation

Introduction

Long-range communications using high frequency (HF) radio waves (3 - 30 MHz) depend on reflection of the signals in the ionosphere. Radio waves are typically reflected near the peak of the F2 layer (~300 km altitude), but along the path to the F2 peak the radio wave signal suffers attenuation due to absorption by the intervening ionosphere.

Absorption is the process by which the energy of radio waves are converted into heat and electromagnetic (EM) noise through interactions between the radio wave, ionospheric electrons, and the neutral atmosphere (For a more extensive description of the absorption process see Davies (1990)). Most of the absorption occurs in the ionospheric D region (50 - 90 km altitude) where the product of the electron density and the electron/neutral collision frequency attains a maximum. Within this region the neutral density is relatively constant over time, so variations in the local electron density drive the total amount of absorption. The electron density is a function of many parameters and normally varies with local time, latitude, season, and over the solar cycle. These "natural" changes are predictable, and affect absorption only moderately at the lowest HF frequencies. Much more significant changes to the electron density, and therefore the absorption strength, are seen as a result of solar x-ray flares (the classic short wave fade).

Solar x-ray flares have significant emission in the 0.1-0.8 nm [1-8 Å] wavelength range. This is important because these wavelengths ionize the D region, dramatically increasing local electron density, and hence the total EM absorption. The flares, which can last from a few minutes to several hours, are rated C, M, or X according to the 0.1-0.8 nm flux as measured by instruments on the GOES satellites. To qualify as a C-class flare the flux, F, must fall within the range 10^-6 <= F < 10^-5 W m^-2, for M-class 10^-5 <= F < 10^-4, and X-class 10^-4 <= F. In standard notation the letters act as multipliers, for example C3.2 equates to a flux of 3.2 x 10^-6 W m^-2.

The C, M, and X classification is based on the full-disk x-ray emission from the sun. During periods of high solar activity, such as solar maximum, the background flux may increase to C-class levels for days at a time, even without flare activity. The D region electron density is directly driven by the total x-ray flux regardless of the source, so these periods of high background flux are equally important to radio absorption.

Due to geometric effects, D region ionization is greatest at the sub-solar point, where the sun is directly overhead. The amount of ionization and absorption falls with distance away from the sub-solar point, reaching zero at the day/night terminator. The night-side of the Earth is unaffected.

Product

The D Region Absorption Product addresses the operational impact of x-ray flux on HF radio communication. It can be found at the following address:

http://swpc.noaa.gov/rt_plots/dregion.html

The main product consists of four dynamic components: a global frequency map, an attenuation bar graph, status messages, and an estimated recovery clock. Each of these components is described below. All of the components update continuously, driven by one-minute GOES X-ray flux data. By updating at this cadence we have assumed that the D-Region recovers quickly from changes in solar flux, which is a reasonable approximation.

To complement the global frequency map, zoomed-in views and a text version are also available by clicking on the map or the Tabular Values link respectively. The text version displays numeric values of the frequency map in 5 degree latitude and 15 degree longitude increments.

Global Frequency Map

The global frequency map graphically illustrates the Highest Affected Frequency (HAF) as a function of latitude and longitude. We define the HAF as the frequency which suffers a loss of 1 dB during vertical propagation from the ground, through the ionosphere, and back to ground. Radio frequencies lower than the HAF suffer an even greater loss as described in the Attenuation Bar Graph section.

The sub-solar point, where HAF reaches its largest value and attenuation is greatest, is marked with a yellow or purple diamond.

To create the global frequency map, the HAF is calculated at the sub-solar point based on the current x-ray flux value. This calculation is made using an empirical formula derived from the following relationships between solar 0.1-0.8 nm x-ray flux and degraded frequency:
             M1.0 -> 15 MHz 
             M5.0 -> 20 MHz 
             X1.0 -> 25 MHz 
             X5.0 -> 30 MHz 
(Space Environmental Forecaster Operations Manual, 21 October 1997)

A fit of these empirically derived relationships results in the following equation for the sub-solar highest affected frequency:
             HAF (MHz) = 10*log[flux (W m^-2)] + 65
At other geographic locations the HAF becomes lower, based on the solar zenith angle (chi) dependence. The degraded frequencies taper off from the maximum as cos ^0.75[chi]. For example, an M5.0 flare shows a HAF of 20 MHz at the sub-solar point decreasing to zero at the day/night terminator.

Attenuation Bar Graph

A bar graph on the right-hand side of the graphic displays the expected attenuation in decibels as a function of frequency for vertical radio wave propagation. This graph is only valid at the sub-solar point, although users can re-create it for any location using the tabular HAF data. The displayed values can also be scaled to approximately account for oblique radio wave propagation using the 1/sin(α) dependence, where α is the elevation angle of the propagation path.

To create the attenuation graph, we use the fact that the HAF suffers absorption of approximately 1 dB for vertical radio wave propagation at the sub-solar point. Attenuation at other frequencies (f) is calculated by scaling the strength of absorption using the relationship:

absorption[f] (dB) = (HAF)²/f²

(Davies, 1990; and Stonehocker, 1970)

Status Messages

Text messages appear at the bottom of the frequency map based on the following criteria:
Condition Message
               flux <= C2.0  or
               flux <= 2*Background      Normal Background Conditions
   
        C2.0 < flux <  M1.0   or
2*Background < flux <  M1.0              Elevated x-ray flux
 
       M1.0 <= flux <  X1.0              Moderate x-ray flux
          
               flux >= X1.0              Extreme x-ray flux
          
               flux data missing         Unknown Conditions
  
Where Background refers to the previous day's background x-ray flux. It is estimated by using the lower of two values, either (a) the average of 1-minute data between 0800 UT and 1600 UT, or (b) the average of the 0000 UT to 0800 UT and the 1600 UT to 2400 UT data.

In addition to the text messages, a status window in the lower right corner displays the current x-ray flux trend, either increasing or decreasing. This window is only visible when the flux is above normal background conditions, as defined above.

Estimated Recovery Clock

After an x-ray event (defined as flux greater than M1 levels) peaks and the flux begins to decrease, a clock near the lower right corner displays the Estimated Recovery Time to normal background conditions. The estimate is based on the following empirically derived values relating the magnitude of a flare to the statistical average of the flare duration:
             M1.0 ->  25 minutes 
             M5.0 ->  40 minutes 
             X1.0 ->  60 minutes 
             X5.0 -> 120 minutes
(Space Environmental Forecaster Operations Manual, 21 October 1997)

The above values are fit using the following set of equations, where k = log[2 * background flux]. In order to determine which equation to use, the algorithm sequentially tests the current x-ray flux against the listed criteria.

Criteria Time Remaining (min)
        log[flux] <= -5.7  or
                  <= k                        0
      
 -5.7 < log[flux] <  -5.0  or    
    k < log[flux] <  -5.0          25*(k - log[flux])/(k + 5)
     
-5.0 <= log[flux] <= -3.3          32.19*(log[flux])^2 +
                                   323.45*(log[flux]) + 837.2
             
        log[flux] > -3.3           100*(log[flux]) + 450
  
The program uses the current x-ray flux to re-calculate the estimated recovery time every minute. This is because the duration of a specific x-ray flare may deviate significantly from the statistical average. It is therefore possible that the clock will not countdown sequentially as it updates. For example, if the duration of the flare is extremely short, the clock might read 25 min, 20 min, 15 min, 5 min after each update. Conversely, if the duration is especially long, the clock might read 25 min, 25 min, 25 min, 24 min, ... as it updates.

When the flux is increasing or no event is in progress, the dialog box displays "No Estimate".

References

Davies, K., Ionospheric Radio. Peregrinus Ltd., London, UK. 1990.

Stonehocker, G.H., Advanced Telecommunication Forecasting Technique, in Ionospheric Forecasting, AGARD CONF. Proc. No. 49, Advisory Group for Aerospace Research and Development, NATO; Agy, V. (Ed), p27-1, 1970.

Space Environmental Forecaster Operations Manual, page 4.3.1, 55^th Space Weather Squadron, Falcon AFB, USAF, 21 October 1997.

Please send questions and comments to Tim.Fuller-Rowell@noaa.gov.