Statistical and case studies of radio emissions observed near 2ƒ_ce and 3ƒ_ce in the auroral zone

Abstract

Receivers located at Two Rivers, Alaska (64.9øN, 146.9øW), and Circle Hot Springs, Alaska (65.5øN, 144.7øW), have been used to monitor the spectrum between 0.05-4.8 MHz for extended periods. Seasonal and diurnal effects of auroral roar, a weak narrow band radio emission near 2 and 3 times the ionospheric electron cyclotron frequency (f•), have been determined. Many individual auroral roar events correlate with magnetic activity, and superposed epoch analysis using planetary K indices shows a correlation between magnetic activity and auroral roar commencement. For a 3f• roar event on March 23, 1992, riometer, magnetometer, photometer, and all-sky camera data are available and show that individual bursts of 3f• auroral roar are associated with intensifications of the aurora, as was known previously for 2f• roar. Finally, two possible generation mechanisms are investigated in detail: nonlinear interaction between lower hybrid and electrostatic upper hybrid waves producing electromagnetic waves, and the feasibility of a lower ionospheric decameter maser for direct generation of X mode waves at harmonics of f•. Introduction Although reports of natural radio emissions from aurora have appeared occasionally during the past four (s• •vi•ws by Ellyett [1969], LaBelk [1989], and LaBelk and Weatherwax [1992]), only in the past few years has a systematic study been done covering the 0.05-to 4.8-MHz frequency range for extended time periods. Surprisingly, this rather ordinary frequency range, which includes the familiar AM broadcast band, exhibits phenomena of auroral origin which are neither well described observationally nor explained theoretically. The study reported here uses passive groundbased receivers at remote high-latitude regions in the northern hemisphere where interesting and still unexplained radio events have been reported in previous papers. These phenomena include broadband noise enhancements [Weatherwax et al., 1994a], bursty broadband emissions at 1.4-3.5 MHz [Weatherwax et al., 1994b], and narrow-band emissions near 2 [Kellogg and Monson, 1979; 1984] and 3 times [Weatherwax et al., 1993] the ionospheric electron cyclotron frequency (fc,). Emissions near 2fc• were first observed at Churchill, Manitoba (latitude 58.8øN, longitude 94.0øW, magnetic latitude 69.7øN), by Kellogg and Monson [1979], who labeled them auroral "roar." These emissions occurred at frequencies between 2.9 and 3.1 MHz and lasted up to tens of minutes. Kellogg and Monson [1984] observed 69 events during several campaigns over three winters at Churchill. The most commonly observed frequency of 3.03 MHz corresponds to 2f• at an altitude of 275 km above Churchill. The events were usually associated with auroral breakup or with westward traveling surges. Attempts to observe similar events from Cleary, Alaska (latitude 65.1øN, longitude 147.8øW, magnetic latitude 64.8øN), during a 2-week campaign in 1982, were unsuccessful. Recent observations of 2f• auroral roar near Fairbanks, Alaska, were reported by Weatherwax et al. [1993]. These emission...

A z mode electron‐cyclotron maser model for bottomside ionospheric harmonic radio emissions

“…One question that remains to be answered is why the third-harmonic source is statistically at higher altitudes than the second-harmonic source over many auroral roar events [Weatherwax et al, 1995] …”

Auroral radio emission and absorption of medium frequency radio waves observed in Iceland

“…To clarify the frequency spectra and occurrence character of the auroral radio emissions, it is necessary to take into account the nature of radio wave absorption in the ionosphere, as pointed out by Weatherwax et al (1995). This is because occurrence of the auroral radio emissions observed on the ground is certainly controlled by the collisional damping effect in the D and E regions of the ionosphere.…”

The polarization of auroral radio emissions

“…For example, several researchers have suggested that auroral roar may be generated in the X-mode by the cyclotron maser mechanism operating at F-region altitudes [Weatherwax et al, 1995;Yoon et al, 1996]. This mechanism predicts that auroral roar should be right elliptically polarized.…”

“…This mechanism predicts that auroral roar should be right elliptically polarized. Another possible mechanism of auroral roar involves the conversion of upper hybrid waves to electromagnetic waves [Gough and Urban, 1983;Weatherwax et al, 1995]. This mechanism predicts either left-elliptical polarization (LP) or right-elliptical polarization (RP), depending on the mode conversion process.…”