Auroral Roar Emissions Research Articles

Polarization observations of 4fce auroral roar emissions

AbstractWe report on the first polarization measurement of auroral roar emissions near 4 times the ionospheric electron cyclotron frequency (4fce). A ground‐based passive receiver that uses orthogonal loop antennas, installed in Iceland, revealed the sense of polarization of 11 events of 4fce roar emissions. In 9 of 11 cases, 4fce roar was left‐handed elliptically polarized, namely, O‐mode waves. The O‐mode 4fce roar was observed under both sunlit and dark ionospheric conditions during geomagnetic storms. For O‐mode 4fce roar generation, satisfaction of the matching condition where upper hybrid frequency (fUH) equals 4fce requires a high‐density, F region ionosphere, even during darkness, which might be attributed to auroral precipitation or tongue of ionization. In two cases, right‐handed elliptically polarized 4fce roar was observed during darkness hours and the main phase of a geomagnetic storm. This polarization indicates that nonlinear coupling of two upper hybrid waves may also work to generate X‐mode 4fce roar.

First observations of 5fce auroral roars

Auroral radio emissions reveal physics of beam‐plasma interactions and provide possibilities to remotely sense ionospheric plasma processes.Sato et al. (2012) recently discovered that auroral roar emissions, long known to occur at two and three times the electron gyrofrequency (fce), also occur at 4fce. Using data from wave receivers in the British Antarctic Survey Automatic Geophysical Observatories, we confirm the existence of 4fce‐roars and observe for the first time 5fce‐roars. A search at higher frequencies did not find higher harmonics. Both 4fce‐ and 5fce‐roars only occur in sunlit conditions near summer solstice. Harmonic roar emission frequencies scale with the strength of the geomagnetic field, and combining data from four observatories suggests that the 4fce‐roar source height lies around 245 km, lower than the 275 km estimated for 2fce‐roar. These observations show that the auroral roar generation mechanism acts under a broader set of plasma conditions than previously considered.

First observations of 4fce auroral roar emissions

This is a report on the first observations of auroral roar emissions near 4 times the ionospheric electron cyclotron frequency (fce) with a passive receiver installed in Svalbard, Norway. 4fce roar emissions were detected from 5.27 to 5.70 MHz during moderate geomagnetic disturbances in 22 days between May and September 2011 only from noon to evening, while no event occurred during the winter season. An analysis of a typical event shows that the electron density profile measured from EISCAT Svalbard dynasonde satisfies the condition that the upper frequency of the 4fce roar is nearly equal to both upper hybrid resonance frequency (fUH) and 4fce at 238‐km altitude. These observations support the idea expanded from the most commonly‐accepted generation mechanism of 2fce and 3fce roar: the origin of 4fce roar is upper hybrid waves favorably generated under the condition of fUH ∼ 4fce in the auroral F‐region ionosphere.

Banded frequency structure from linear mode conversion in inhomogeneous plasmas

Linear mode conversion of unmagnetized Langmuir waves into transverse electromagnetic waves in an inhomogeneous plasma is investigated numerically. It is shown that the presence of a tunneling region and density well near the mode conversion region can significantly influence the mode conversion efficiency. The density cavity acts as a resonator, such that the mode conversion efficiency shifts from zero to a local maximum (up to 100% efficiency) with variation of the cavity width on scales of order the Langmuir wavelength. In this way, Langmuir waves with a smooth (structureless) frequency dependence can be selectively mode converted to produce narrow frequency bands of electromagnetic radiation. Applications are discussed to banded frequency fine structure in electromagnetic emissions at the electron plasma frequency in Earth’s foreshock and the solar wind, type III solar radio bursts, and lower ionospheric auroral roar emissions.

Statistical study of auroral roar emissions observed at South Pole Station

Auroral roar is a band‐limited natural auroral radio emission observed at ground level in the auroral zone at frequencies near 3 MHz and 4.5 MHz, close to 2 and 3 times the electron gyrofrequency. Using semiautomated data analysis techniques, we generate a database of frequencies, amplitudes, and universal times of 2fce‐auroral roar emissions observed at South Pole Station (74° invariant latitude) during 1999. These data confirm that auroral roar emissions are observed at ground level only during times when the ionosphere is not illuminated, and magnetic local time plays a significant role in controlling the occurrence rates of auroral roar emissions observed at ground level. Auroral roar occurrence rates and the ranges of auroral roar amplitudes and frequencies increase during times of high geomagnetic activity. The distribution of auroral roar frequencies is bimodal for large‐amplitude events, which we interpret as evidence that a substantial fraction of the auroral roar emissions observed at ground level originate in the topside ionosphere. Finally, superposed epoch analyses of occurrences of auroral roar relative to substorm onsets inferred from magnetometer and VLF data show that auroral roar emissions occur favorably preceding such events rather than after them, consistent with previous case study and anecdotal evidence.

Observations of auroral roar emissions at polar cap latitudes: Results from the Early Polar Cap Observatory

Auroral roar, a radio emission occurring near the second and third electron cyclotron harmonics, has been monitored during a 51‐day interval in 1998 from three ground‐based high‐latitude observatories: Churchill (69° corrected geomagnetic latitude (cgm)), Baker Lake (74° cgm), and the Early Polar Cap Observatory (EPCO) at Resolute (84° cgm). These observations reveal a latitude dependence of the local time distribution of auroral roar events. In the auroral zone they occur predominantly in the premidnight‐midnight sector as previously reported, but in the polar cap they occur evenly throughout nighttime hours. Sorting the data by magnetic activity level confirms the effect; for example, the distribution of events at Baker Lake during times of extremely low activity (Kp < 1) is concentrated near magnetic midnight, consistent with Baker Lake lying in the auroral zone when the oval is contracted. The latitude dependence of the local time distribution may be explained by propagation effects, with inefficient propagation in the auroral zone causing only relatively locally generated emissions to be detected there, while stations well poleward of the auroral zone detect emissions from sources widely distributed in longitude and latitude. Statistics of the frequencies of auroral roar events detected at Resolute show evidence of an upper frequency cutoff near 2960 kHz. Ray‐tracing calculations suggest that it is plausible for auroral roar to reach Resolute from sources up to 1500 km distant in the auroral zone, but they do not provide an obvious explanation for the frequency cutoff.

Plasma conditions in auroral roar source regions inferred from radio and radar observations

A medium frequency interferometer (MFI) has been established in Sondrestrom, Greenland, to measure the direction of arrival (DOA) of auroral roar emissions at 2.5–3.0 MHz. Combined with electron density data from the Sondrestrom incoherent scatter radar (ISR), these DOA data provide a powerful test of the most‐cited generation mechanism of auroral roar emissions. This mechanism requires that the relationship between the electron plasma frequency (fpe) and the electron gryofrequency (fce) in the source regions isfpe2 = (n2 − 1)fce2, where n = 2, 3 is the harmonic number of the emission. For each of five auroral roar events studied, to within experimental uncertainty this frequency‐matching condition is met throughout the source region of the emissions inferred from the MFI and ISR data. Furthermore, ray‐tracing analysis indicates that signals at the roar frequency are able to propagate from the source region to the location of the MFI receiver.

Latitudinal dynamics of auroral roar emissions

Auroral roar, a narrowband (δƒ/ƒ < 0.1) emission near 2 and 3 times the ionospheric electron gyrofrequency (2ƒce and 3ƒce), is observed with a meridional chain of LF/MF/HF radio receivers located in northern Canada spanning 67° to 79° invariant latitude. Observations of these emissions are compared with the auroral electrojet location inferred from the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) magnetometer array. Variations in the intensity of the observed auroral roar emissions and in the invariant latitude of the most intense emissions are correlated with movements of the poleward boundary of the electrojet. For example, substorm onsets, which appear as rapid poleward expansions of this boundary, result in screening of the emissions from the underlying ground stations because of precipitation‐induced ionization in the lower ionosphere. In four of the five study days the peak emission intensity is located 0°–9° poleward of the poleward electrojet boundary inferred from the magnetometers. In one case the peak emission intensity is up to 10° equatorward of the poleward electrojet boundary. In all cases, there is a tendency for the latitude of the most intense auroral roar emissions to track the movements of the electrojet location inferred from the magnetometer data. For two examples, the footprint of the Fast Auroral Snapshot (FAST) satellite passes within 3° of one or more of the ground stations, and the satellite detects unstable electron populations in the polewardmost auroral arc, reinforcing the scenario that auroral roar emissions are generated by these electrons in the polewardmost arc and propagate into the polar cap where conditions are often favorable for their detection at ground level.

Propagation of medium frequency (1–4 MHz) auroral radio waves to the ground via the Z‐mode radio window

Recent ground‐based observations of auroral radio waves have identified narrowband emissions near 2 and 3 times the lower ionospheric electron cyclotron frequency (fce) known as auroral roars. In this paper the propagation of these waves in the auroral ionosphere is investigated by means of a ray‐tracing technique. We model one particular scenario in which a large‐scale (tens of kilometers) horizontal density structure, based on density structures observed with the Sondrestrom radar at times of auroral roar emissions, plays a crucial role in both guiding the waves to the ground and enabling mode conversion. The location and the mode characteristics of the initial waves are determined on the basis of local stability properties, which suggests that Z‐mode wave excitation is favored near 2fce. However, since Z‐mode cannot propagate to the ground they must first undergo a mode conversion to one of the free‐space modes (X and O). It is found that for a narrow range of frequencies and initial wave phase angles the trapped Z mode can be converted to O mode via the Ellis radio window. This finding is consistent with the fact that auroral roar emissions are nearly 100% O‐mode polarized. However, it is important to note that the evaluation of the damping of the Z‐mode waves along the ray path is not considered within the context of this preliminary study and will be critical for eventually determining the exact physical scenario of the auroral roar generation mechanism.

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

A model is proposed for recent ground‐based observations of auroral roar emissions, detected at 2Ωe and 3Ωe, where Ωe is the local electron cyclotron frequency in the source region, between 200 and 500 km above the Earth's surface. Electron cyclotron maser emission is a likely mechanism to account for these emissions because it naturally produces coherent radiation at harmonics of Ωe. A theory for auroral roar emissions has already been proposed, whereby maser‐generated second (X2) and third (X3) harmonic x mode radiation is amplified in the source region by multiple reflections off the walls of the density cavity in which they are produced. After many reflections the X2 and X3 waves propagate along the density cavity to a ground‐based observer. However, it is demonstrated here with ray‐tracing calculations that it is highly probable that maser‐generated X2 and X3 radiation is reabsorbed at lower altitudes and thus cannot be detected at the ground. An indirect maser mechanism is proposed instead, where maser‐generated z mode waves at Ωe grow to high levels in the source region and then undergo repeated nonlinear wave‐wave coalescence to produce second‐ and third‐harmonic waves that propagate directly to the ground. The z mode waves must satisfy the necessary kinematic constraints to produce observable second‐ and third‐harmonic radiation. The dependence of the z mode maser on the temperature and functional form of the unstable electron distribution is discussed, along with the conditions required for the coalescence processes to proceed and produce the observed levels of radiation.

Further investigation of auroral roar fine structure

In April 1996, a downconverting receiver was operated in Churchill, Manitoba, Canada, to increase the statistics about the recently discovered fine structure of auroral roar emissions. Auroral roar is found to be both structured and unstructured. A wide variety of previously unknown tonal features drifting in a complicated manner were recorded. These structured features can be classified according to their duration, frequency drift, and grouping with like features. Typically, 95% of the structured features last less than 1 s. The slope of drifting features is more commonly negative than positive with a magnitude typically less than a few kHz s−1 and a maximum of ∼ 800 kHz s−1. The minimum bandwidth of features is 6 Hz or less, and typical separation between similar features is ∼ 400 Hz. These measurements form a basis for reviewing proposed generation mechanisms of auroral roar including a localized source model and laser cavity mechanism.

Fine structure of auroral roar emissions

In September 1994, a narrowband downconverting receiver was installed at Circle Hot Springs, Alaska, to complement ongoing stepped frequency receiver observations of auroral roar, a relatively narrowband emission at frequencies roughly 2 and 3 times the ionospheric electron gyrofrequency (ƒce). By good fortune, auroral roar events were captured at 0957 UT (0057 LT) on September 6, 1994, and at 1015 UT (0115 LT) on October 23, 1994. The narrowband recording with a center frequency of 2.853 MHz and bandwidth of 10 kHz revealed that these auroral roar events were composed of multiple narrowband features which drift in frequency in a complicated pattern. Both rising tones and falling tones were observed, and the slopes of the discrete features varied from nearly flat to 100 kHz/s. When the downconverted signals are played through a loudspeaker, they resembled structured VLF emissions such as chorus. These data show that auroral roar, like auroral kilometric radiation, is at least sometimes characterized by fine frequency structures previously not resolved by stepped frequency analyzers. The existence of such fine structures has significance for theories of the generation mechanism of auroral roar, which remains unexplained.

A new type of auroral radio emission observed at medium frequencies (∼1350–3700 kHz) using ground‐based receivers

A new form of broadband emissions (“MF bursts”) appearing between ∼ 1350–3700 kHz has been identified in 8 months of data (October 1993–May 1994) from ground‐based LF/MF/HF receiving systems at Circle Hot Springs (65.5°N, 144.7°W) and Two Rivers (64.9°N, 146.9°W), Alaska. These emissions are often observed simultaneously with 2ƒce and 3ƒce auroral roar emissions, auroral hiss, and absorption of AM broadcast band transmitters. MF bursts display significant amplitude variations on timescales as short as 10 ms, but the variations do not correlate with the fine structures seen in simultaneous auroral hiss and auroral roar. The generation mechanism of these emissions is unknown.