Auroral Propagation Research Articles

Impacts of Auroral Precipitation on HF Propagation: A Hypothetical Over‐the‐Horizon Radar Case Study

AbstractOver‐the‐horizon radar (OTHR) systems operating in the high‐frequency (HF) band (3–30 MHz) are unique in their ability to detect targets at extreme ranges, offering cost‐effective large‐area surveillance. Due to their reliance on the reflective nature of the ionosphere in this band, OTHR systems are extremely sensitive to ionospheric conditions and can expect significant variations in operational performance. At high latitudes, the presence of auroral enhancements in the E‐Region electron density can substantially modify the coverage area and frequency management of OTHR systems. In this study, HF raytracing is utilized to investigate these impacts for a hypothetical radar under different auroral conditions simulated using the Empirical Canadian High Arctic Ionospheric Model. Aurora were seen to increase maximum usable frequency from 8.5 to 26 MHz whilst also reducing median available range from 2,541 to 1,226 km and changing coverage area by −50.4% to 58.6%, for the greatest differences. Target interception showed large variations in path coverage of between 33%–115% and 0%–107% for two flight paths tested with precipitation toggled. Two distinct auroral propagation modes were observed, noted as the F‐E ducted and Auroral E‐modes. Long‐range coverage provided by the auroral F‐E ducted mode was of limited capacity with low solar activity due to reduced NmF2. F‐mode propagation transitioned to the dominating Auroral E‐mode between Auroral Electrojet index values of 50‐ and 200‐nT. The significant variations in both frequency and coverage observed within this study highlight some aspects of the importance of considering aurora in OTHR modeling and design.

Rapid intensification and propagation of the dayside aurora: Large scale interplanetary pressure pulses (fast shocks)

We present two cases of abrupt dayside auroral brightenings and very fast auroral propagation using the POLAR UV imaging data. The brightenings occur first at noon and then propagate along the auroral oval towards dawn and dusk. Ionospheric speeds of 6 to 11 km/s are determined. The auroral brightenings and motion are associated with the arrival and propagation of interplanetary shocks/pressure waves. The brightening at noon occurs within minutes of the shock compression of the noon‐time magnetopause. The speed of the auroral propagation in the ionosphere towards dawn and dusk corresponds extremely well to the solar wind downstream flow. Our model assumes that shocks/pressure waves compress the outer dayside magnetosphere, and plasma contained therein. This plasma compression leads to the loss cone instability, wave‐particle interactions, and concomitant particle loss into the ionosphere.

The NBS Meteor Burst Communication System

This project was undertaken in 1955 to investigate the properties of the intermittent reception of VHF signals over long distances by meteoric propagation and their communication usefulness. To accomplish this, a complete duplex teletype system operating at about 50 mc was constructed, and results are reported from tests made over a 1277-km east-west path. Comparisons are made of burst transmissions of 10, 20, 40, and 80 times normal teletype speed and of varaitions in a number of control system parameters. For the system under test, the optimum speedup was 40X, which produced a daily average channel capacity of about 40 wpm with a character error rate of about 0.0035 (with the best control system settings). Higher speedup ratios are advocated for future systems. The most serious causes of outages in this type of system are atmospheric noise and sustained multipath distortion from competing modes such as E and auroral propagation.

The geometry of auroral communications

As early as 1939, radio amateurs found auroral ionization useful for communication purposes. Such ionization makes hf and vhf propagation possible over paths as great as several hundred kilometers when other more normal ionospheric propagation modes do not exist. The geometry of reflection is investigated for a variety of transmitter locations based upon the assumption of specular reflection from columnar ionization aligned with the earth's magnetic field lines. The results of the investigations outline the region of useful auroral ionization and the regions on the earth within which the auroral propagation is possible. The probability has been determined of obtaining propagation from a particular transmitter location to any receiver location within the region of propagation. These geometrical studies allow the communicator to predict the most useful transmitter and receiver locations in utilizing auroral ionization for communication purposes. The studies also may suggest methods of minimizing the effects of auroral propagation when it is considered a detrimental propagating mode, for example, when it results in undesirable multipath effects.

Some practical aspects of auroral propagation

Assoclated with visible auroral activity there is some kind of reflection or scattering mechanism that can result in radio communication in the VHF band over distances up to 1000 km with low power transmitters and probably up to 2000 km with high power transmitters. The transmission-loss experienced over such a circuit is somewhat less than is involved in regular VHF scatter-communication but is much greater than for sporadic E openings at VHF. For auroral propagation the direction of arrival is deviated from the great circle plane towards the direction of auroral activity, whereas for regular VHF scatter-transmission, and for sporadic E transmission, propagation is approximately geodesic on most occasions. For auroral propagation the bandwidth available for communication is almost always severeley limited, frequently to the extent that amplitude-modulated (AM) radio-telephony is impossib1e. Moreover the ratio of the fading rate to the carrier frequency is rather more than a power of ten greater than is customary for ionospheric transmiseion under quiet ionospheric conditions. Auroral propagation occurs most frequently at night and is hardly ever experienced between sunrise and noon. It occurs most frequently at the equinoxes and more often in years of high sunspot activity.

Auroral echoes observed north of the auroral zone on 51.9 mc/sec

During November 1954, a simple radar system, designed for observation of auroral echoes at 51.9 Mc/sec, was operated at Point Barrow, Alaska. Because this location is north of the accepted maximum of the auroral zone, most of the visible aurora is seen to the south of the observing station. The radar used a continuously-rotating antenna to see with equal sensitivity in all directions, but more than 90 per cent of the echoes were obtained from directions north of east and west. Echoes were obtained only from 500 to 1100 km. These effects are explained by the theory of Moore as enlarged by Booker, Gartlein, and Nichols, requiring near-perpendicularity of radio ray paths to the lines of the earth's magnetic field. During visible aurora, propagation at 51.7 Mc/sec was investigated over an 800-km path from College to Barrow, across the auroral zone. Bursts of signal due to meteor ionization were readily observed. Propagation associated with aurora was almost non-existent, even with visible aurora at the mid-path. If the theory of auroral echoes of Harang and Landmark were true, auroral propagation should have been readily detected.

VHF auroral and sporadic-Epropagation from Cedar Rapids, Iowa, to Ithaca, New York

A 50-mc transmitter operating continuously at Cedar Rapids, to the west of Ithaca, has been monitored for more than two years, using a northward-pointing antenna intended to reduce signals from the west due to E -scatter and bursts due to meteors. Associated with visible auroral activity, enhanced signals are heard, having a very rapid fading rate, characteristic of auroral propagation previously described. Continuous recordings are made on an Esterline-Angus chart moving 6 inches per hour. Much stronger sporadic- E signals are observed occasionally entering the sidelobes of the receiving antenna, but can usually be distinguished from auroral propagation by the appearance of the trace or clarity of the beat frequency. This method of identification and separation is thought to be effective because each mode of propagation has its own diurnal and seasonal variations agreeing with characteristics found previously by others.

Polarization of Radio Echoes from Auroræ

NOTABLE variations in the polarization of radio echoes from aurorae have been observed at Saskatoon (52.1° N., 106.6° W.), using frequencies of 56 and 106.5 Mc./s. Theories of very high-frequency radio propagation predict the maintenance of the transmitted state of polarization under normal ionospheric conditions. An examination by Moore1 of amateur radio transmissions by ‘auroral propagation’ indicated that this was also the case when aurora was present. Observations by Bowles2 confirmed this conclusion, although exceptions occurred when aurora was overhead. More recently, Harang and Landmark3, in reporting on back-scatter echoes during aurora, mentioned that 35 Mc./s. echoes were sometimes completely depolarized and sometimes nearly plane polarized, and that 74 Mc./s. echoes were plane polarized in the same sense as the transmitted signals. The Saskatoon observations are apparently the only ones that have been made so far where it has been possible to verify the auroral origin of the echoes4.