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

Edwin C. Jones (AE4TM) Vanderbilt University, Nashville, TN 37212 Oak Ridge National Laboratory, Oak Ridge, TN 37831 (affiliation 1992-1996)

Raymond A. Greenwald Johns Hopkins Applied Physics Laboratory, Laurel, MD 20723

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, and 3. HF backscatter observed at the middle latitudes. In each of these propagation modes, the Kapuskasing and Goose Bay SuperDARN Radar reflections revealed strong echoes off drifting 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.

The Goose Bay radar summary above indicates echoes off travelling ion clouds of 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. ¹

The Goose Bay radar summary above indicates echoes off similar travelling 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.

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.

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 and Goose Bay, Newfoundland 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 Large Scale Travelling Ionospheric Disturbances (LSTID'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 Kapuskasing 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 of mixed negative and positive values. Finally, the color of the horizontal arrows below the top panel indicate the highest frequency of HF backscatter observed at middle latitudes.

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.

In each of the radar summaries above, the TID velocities are determined by measuring the slopes of the radar echoes in the time dependent power returns. These slopes are believed to represent the average velocities of the electrons. In contrast, the Doppler velocity plots indicate lower velocities for these TID's and the reason for this is as follows. From the figure above, as an electron cloud propagates away from a magnetic pole, each individual electron within the cloud is trapped along a respective magnetic flux line so that the electron cloud expands reducing the total ion density proportional to the square of the distance from the magnetic pole. Recalling that the total electron density NZ required to backscatter a pulse of frequency f is given⁴ by NZ~(constant)f², the ability to backscatter a pulse of frequency f_c falls off as f_c~1/d, where d is the distance from the magnetic pole. As a result, radar pulses experience backscatter at the point where this critical electron density NZ is reached rather than off individual electrons propagating equatorward thereby leading to a lower values in Doppler velocity of the Travelling Ionospheric Disturbances as compared to the velocities obtained from the time dependent power returns.

Correlations between 2m Sporadic-E, Trans-Atlantic openings, and HF backscatter made by shortwave radio stations at the middle latitudes are compared to SuperDARN radar data made within the auroral ovals. Each of these propagation modes as recorded at the middle latitudes occurred when simultaneous radar echoes off drifting ion clouds took place within the auroral ovals. Dense ion clouds formed by the convergence of the magnetic flux lines near the magnetic poles are believed propagate equatorward under the influence of Atmospheric Gravity Waves for each of these propagation modes (note: Atmospheric Gravity Waves are large wavelength neutral pressure waves extending into the thermosphere and not Einstein's gravity).^1-3 Once these dense ion clouds are free from the highly convective plasma currents which disrupt most radio signals, these ion clouds allow for the formation of these interesting and useful propagation modes.

The most striking radar echoes occur during periods when HF backscatter is 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.