Sudden changes in the amplitude of long‐distance subionospheric VLF transmissions have been found at night in association with whistlers. Both increases and decreases in signal strength have been observed, depending on signal frequency and orientation of the receiving antenna. Sample observations at Eights Station in Antarctica of station NSS (Annapolis, Maryland) on 22.3 kHz showed increases in signal strength that averaged 3 db, with rise times of about 2 sec and durations of about 30 sec. Coincident with every rise was a midlatitude (L ≈ 2.5) whistler originating in the northern hemisphere. To explain this association, it is suggested that the whistler dumps energetic (30–300 kev) electrons into the D region. The resulting ionization then alters the properties of the earth‐ionosphere wave guide. The mechanism of precipitation is thought to be pitch angle scattering of trapped electrons that resonate with the magnetic field of the whistler wave near the magnetic equator.

Abstract. Auroral medium frequency (MF) bursts are broadband impulsive radio emissions observed at ground level during the breakup phase of auroral substorms. Measurements made in northern Canada during 1995-1996 show the seasonal and local time dependencies of MF burst emissions, provide case-study evidence for a null in the MF burst spectrum near twice the ionospheric electron gyrofrequency, and establish a correlation between MF burst and impulsive auroral hiss. High time resolution measurements reveal that MF bursts occur in 100-300 /is wave packets which sometimes appear periodic having a period close to the ionsopheric proton gyroperiod. The timescales of the MF burst wave packets are comparable to those of whistler solitary waves recently observed with the FAST satellite.

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...

Abstract. Ground level observations using two vertical loop antennas oriented at 900 to each other reveal the sense of polarization of several types of auroral radio emissions in the frequency range 30-5000 kHz. Auroral hiss is observed to be right elliptically polarized

Abstract. Using data from the PHAZE II sounding rocket, launched from PokerFlat, Alaska, we present high-resolution observations of structure in auroral HF waves at and below the local plasma frequency. These observations were made in the altitude range of 390-945 km where the local plasma frequency is below the electron cyclotron frequency. We observe monochromatic, long-lived, narrowband emissions occuring below the local plasma frequency during times of intense HF wave emission. We have termed these emissions "HF bands" due to their appearance in spectrogram images. These emissions are probably identical to the "spike" emissions identified by previous observers using lower time resolution data from the AUREOL/ARCAD3 satellite which showed a narrow peak spectra below the local plasma frequency. HF bands often occur when the local plasma density is varying and are associated with regions of intense Langmuir wave generation. We investigate the hypothesis that the HF bands are created when a Langmuir wave propagates from a low-density region into a higher density region. The wave moves onto the whistler mode branch and propagates as an HF band. Theoretical calculations of propagation times of whistler mode waves support this hypothesis.