SuperDARN Radar for Predicting Rare Propagations

Identifying Unique and Specific Propagation Modes in Over-the-Horizon SuperDARN Radar

Edwin C. Jones (AE4TM) and Raymond A. Greenwald (Johns Hopkins Applied Physics Laboratory)

Abstract

The following document is a brief summary of some correlations made between several HF and VHF propagation modes observed at the middle latitudes by shortwave radio operators and simultaneous over-the-horizon SuperDARN radar reflections observed within the auroral ovals. A good understanding of the various backscatter features in over-the-horizon HF radar reflections is paramount to our full understanding of the dynamics of the ionosphere. The specific propagation modes discussed here include 1. Sporadic-E propagation observed on 2 meters, 2. Trans-Atlantic openings after dark between the US and Europe during the solar minimum of 1996, 3. HF backscatter observed at the middle latitudes, and 4. Trans-auroral HF propagation. In each of these propagation modes, the Kapuskasing and Goose Bay SuperDARN Radar reflections revealed strong echoes off slow moving (<50 m/s) ion clouds. Interestingly, each propagation mode showed features unique and specific to that mode. Finally, the OTH research radar data are shown to be a useful tool for propagation predictions in conjunction with the propagation forecasts for the SFI and K-Index levels.


Sporadic-E Propagation

The Goose Bay radar summary above indicates echoes off slow moving ion clouds having negative Doppler velocities and average reflection powers of 10dB. These reflections were observed on May 18-19, 1996 and October 6, 1996 when wide area 2 meter communications occurred between WV, VA, MO, AR, OH, KY, IL, and TN. During these unusual VHF propagation conditions, the Sunspot Numbers (SSN) were zero and the Solar Flux Index (SFI) was no more than 70. No geomagnetic disturbances were occurring as indicated by low Planetary A-indices of 3-5. Furthermore, middle latitude HF communications by shortwave radio operators were disrupted suggesting that these 2 meter openings were due to sporadic-E propagation. One plausible and widely accepted model for the formation of sporadic-E clouds is the ionospheric wind shear model involving atmospheric gravity waves with vertical ionic compression.(Ref 1)


TA: Trans-Atlantic Propagation

The Goose Bay radar summary above indicates echoes off similar slow moving ion clouds but of positive Doppler velocities and average reflection powers of 15dB. These radar echoes occurred on April 26-27, 1996 and also on September 7, 1996 when the SSN were 0/15 (respectively), the SFI were 68/69 (respectively), and the Planetary-A index were 5/10 (respectively). On April 26, 1996 the Barcelona, Spain stations EA3GM and EA3OT worked very low power mobile HF stations in Knoxville, Tennessee (AE4TM) and Atlanta, Georgia and well after dark on 14.201 MHz. On September 7, 1996, portable to portable shortwave contacts were made between the US and Europe on 20 meters. These contacts included but were not limited to DL0LI, G3TBK, G3WAS, and OZ9EDR contacting a mobile Nashville, Tennessee (AE4TM) shortwave station. Such contacts are rare at these low solar flux levels and are commonly attributed to Trans-Atlantic Openings.

Role of TID's in Trans-Atlantic Openings

The radar summary for 26-April-1996 and 7-Sept-1996 from the previous figure was image enhanced to reveal fine details within the radar echoes. This was accomplished by adjusting the power scale from 0-8dB, the Doppler velocity scale from -25m/s to +25m/s, and the spectral width from 0-50m/s. Interestingly, this enhancement reveals weak radar echoes off moving ionospheric structures (marked with the sloping black lines) that are moving toward the radar at an average speed of 390km/hr. Such signatures are typically characteristic of Travelling Ionospheric Disturbances (TID's) suggesting that TID's may play a role in the formation of these Trans-Atlantic propagation mediums. Unfortunately, radar data during the 1996 solar minimum are limited because several of the SuperDARN radars were not operational during these events.


Backscatter Propagation

The most striking radar echoes take place during periods of middle-latitude HF backscatter allowing shortwave communications into the normal radio blind zones. In the Kapuskasing, Ontario SuperDARN radar data shown above simultaneously observed HF backscatter were taking place at the middle USA latitudes. Detailed analyses of these HF backscatter events strongly suggest that Travelling Ionospheric Disturbances (TID's) due to Atmospheric Gravity Waves (AGW's) are responsible for these phenomenon and these analyses are published in detail at http://ecjones.org/backscatter.html . Note here that these reflections cannot be ascribed to Bragg scattering off ocean waves during the specific dates of these events because the Hudson Bay which is directly north of this SuperDARN radar site is solid ice during each of these observations. Note here that the radar echoes are all very intense and although not obvious from the 0-30dB scaling, an average of 45dB returns are recorded in the raw data archives on these dates. In contrast to the Trans-Atlantic openings, the Doppler velocities are negative. Finally, the color of the horizontal arrows below the top panel indicate the highest frequency of HF backscatter observed at middle latitudes.

Location of HF Backscatter Relative to the Auroral Convections

To illustrate where the general location of the HF backscatter observed by shortwave radio operators is taking place with respect to the auroral ovals, two examples of backscatter are shown against two auroral ionic convection maps constructed with data from the SuperDARN radar network. The figure above illustrates a date where 12m backscatter was observed (left) and 10m backscatter was observed (right). In the 3 1/2 years HF backscatter was extensively studied at the middle-latitudes by radio station AE4TM, this backscatter was never observed to have occurred within the high velocity convection zone of the auroral ovals.

Trans-Auroral Shortwave Propagation

Finally, the last figure above illustrates Trans-Auroral HF propagation. Auroras are well known for their irregular shapes and constant movements and these lead to heavy signal fading (QSB) of the transmitted signals. This QSB can also result from multiple reflections within these auroral layers causing rapid phase shifting. At 30MHz, an auroral signal is easily recognized by a "under-water-like" modulation. Because of these extreme and sudden phase shifts, narrow band modes such as CW and digital are the most reliable modes for these Trans-Auroral contacts.

The above figure specifically illustrates the movements of high velocity auroral plasma currents during a Pactor-II digital link between AE4TM in Nashville, Tennessee and UK8AWY in Tashkent, Uzbekistan on 23-Feb-2002. These currents were calculated by the collective Doppler data of the northern hemisphere SuperDARN radar network. At one point in this data link, the flow of data ceased and according to the SuperDARN data this corresponded to a sudden drop in ion velocity along the line connecting Tashkent and Nashville just east of Greenland.

Summary

Correlations between 2m Sporadic-E, Trans-Atlantic openings, HF backscatter, and Trans-Auroral made by shortwave radio stations at the middle latitudes are compared to SuperDARN radar data made within the auroral ovals. In all of these propagation types, radar echoes off slow (<50 m/s) ion drifts in the auroral ovals were observed with these radars. These ion clouds are believed to originate within the auroral ovals under the influence of atmospheric gravity waves and propagate towards the opposite pole.(Refs 1-3) With the exception of the Trans-Atlantic openings, the Doppler velocities observed by the radar are negative. The reason for these findings are unknown at present but may be related to the fact that the Trans-Atlantic openings occur in the night sky whereas the other propagation modes occur during the daytime. The most striking radar echoes occur during periods of HF backscatter as observed at the middle-latitudes with echoes averaging 45dB above background noise levels followed by 15dB echoes during Trans-Atlantic openings and 10dB echoes during 2m Sporadic-E openings. Finally, it is possible that the relative echo reflection powers are simply related to the relative size of the ion clouds reflecting these rf signals. These findings have never been published prior to this work and these findings illustrate how HF over-the-horizon radar could be used to predict the occurrence of these various HF/VHF propagation modes for shortwave radio operators. Prior to over-the-horizon research radars (OHR), the solar elements were the only source of information available for the prediction of these various interesting propagation modes.


References

1. J.D. Mathews, Journal of Atmospheric and Terrestrial Physics 60, pp. 413-435 (1998).

2.  R.L. Balthazor and R.J. Moffett, "A study of atmospheric gravity waves and travelling ionospheric disturbances at equatorial latitudes", Ann. Geophysicae 15, pp. 1048-1056 (1997).

3.  J.W. MacDougall, D.A. Andre, G.J. Sofko, C.-S. Huang, and A.V. Koustov, "Travelling ionospheric disturbance properties deduced from Super Dual Auroral Radar measurements", Ann. Geophysicae 18, pp. 1550-1559 (2001).

 

Really, really cool stuff...I love theoretical Physics as a brain teaser. Too bad we can't manufacture "slow moving ion clouds but of positive Doppler velocities and average reflection powers of 15dB". Too much opposition from Enviornmentalist wackos!

Since we can't make them....where can the average Ham get access to a detector capable of assessing when "slow moving ion clouds but of positive Doppler velocities and average reflection powers of 15dB" are between himself and those rare DX stations? More importantly, how can we convince those rare DX stations to get on the air during such favorable conditions?

 

Actually anyone with Internet access can access real time data from the Superdarn network, Very interesting it is too
http://superdarn.jhuapl.edu/
there is a java applet that allows you to view and manipulate live data from the various Auroral radar stations

 

Cool information. I'm pretty interested in finding out just when and where these openings are going to be. I don't quite understand all these solar levels and such. So a web site for getting a brief text description of when and where would be great for me.

73's