Medium Frequency Burst Research Articles

A Statistical Study of Auroral Medium Frequency Bursts and Anomalous Incoherent Scatter Radar Echoes

AbstractDifferent types of incoherent scatter radar (ISR) echoes are observed associated with aurora, including some which have been interpreted as signatures of cavitating Langmuir turbulence (CLT). Akbari et al. (2013) https://doi.org/10.1002/jgra.50314 discussed two instances of correlation between CLT and naturally occurring radio emissions called medium frequency burst (MFB) which occur at substorm onsets. Based on that observation, radio detections of MFB from Toolik Lake Observatory have been applied to investigate occurrence of CLT in ISR data from Poker Flat Incoherent Scatter Radar and their possible correlation with MFB. Of 131 MFB events, 25 occurred within 15 min of an ISR echo detection, compared to 6 of 116 intervals of a control set with similar local time and seasonal distribution. The difference is significant at the 10−4 level, suggesting that ISR echoes are more probable during substorm onset times identified using MFB as a proxy. However, only four observed ISR echoes coincident with one MFB event showed both specific characteristics consistent with CLT. Furthermore, investigation of the angle of arrival of MFB suggests that the electromagnetic emissions do not originate from the plasma volume where the ISR detects the echoes. The small number of coincident ISR echoes and MFB is expected due to the different volumes in which the emissions and the echoes are detected. 50% of the MFB events occurred within 20 min of a substorm onset independently identified versus 8% of the control set intervals, confirming the correlation of MFB with substorm onsets.

Estimating Polar Cap Density and Medium‐Frequency Burst Source Heights Using 2fce Roar Radio Emissions

AbstractCompeting theories exist for the generation mechanism of auroral medium‐frequency burst (MFB). In an effort to constrain MFB source heights, this study analyzes 33 events in which MFB and auroral 2fce roar co‐occurred at Sondrestrom, Greenland. Using measurements from an array of receiving antennas, direction‐of‐arrival calculations indicate that in a given co‐occurrence, the elevation angle of MFB typically is higher than that of roar. Ray tracing is used to determine source heights of the MFB signals. Density profiles are obtained from the International Reference Ionosphere (IRI) and shifted in magnitude until each event's roar signals originate at heights where the frequency‐matching condition for 2fce roar generation is satisfied. This shifting method is validated using density measurements from the Sondrestrom incoherent scatter radar (ISR) facility for the two events with available ISR data. After shifting, ray tracing demonstrates that in 25 of the 33 events, burst originates at a height of about 200 km, lower than the typical altitude of peak electron density. However, ISR measurements show that the density profile is enhanced at low altitudes while MFB is observed, peaking in the E region rather than the F region. This finding implies that the MFB sources at 200 km are on the topside of the density peak, in a region of downward pointing density gradient, in qualitative agreement with the mechanism of MFB generation by Langmuir waves in the topside ionosphere. These results also suggest a new method of estimating density in the polar cap using roar signals to calibrate IRI profiles.

Polarization measurements of unusual cases of medium frequency burst emissions extending below 1.5\xa0MHz

Auroral medium frequency burst (MFB) is a radio emission of natural auroral origin associated with substorm expansion phase and observable at ground level. The emission usually occurs at frequencies above 1500 kHz, but occasionally it extends to a sharp lower cutoff frequency at 1300–1500 kHz depending on the observing site, with a frequency gap below the cutoff and sporadic emission below the gap extending to frequencies as low as 1000 kHz. These low-frequency MFB components lie below the electron gyrofrequency and hence could represent either whistler or LO-modes. Recently, using crossed antennas and a two-channel receiver at Toolik Lake, Alaska, polarization of these low-frequency MFB components was measured for the first time and found to be left-hand. This observation eliminates whistler mode as a possibility and requires the low-frequency components be LO-mode in the ionosphere, which constrains their source location since it requires that the frequency exceeds the L-cutoff frequency. In these occasional events marked by a cutoff and low-frequency MFB components, the latter probably originate at high altitudes (>,800 km) and reach the ground through extraordinary low-density polar cap ionosphere.

On the propagation and mode conversion of auroral medium frequency bursts

AbstractAuroral medium frequency (MF) bursts are broadband, impulsive radio emissions associated with local substorm onsets. MF bursts consist of a characteristic fine structure whereby the higher frequencies arrive 10–100 ms before the lower frequencies. LaBelle (2011a) proposed that MF bursts originate as Langmuir/Z mode waves on the topside of the ionosphere that mode‐convert to LO mode waves and propagate to ground level, with the fine structure resulting by propagation delays due to the topside ionospheric density profile. We investigate three aspects of this mechanism. First, full‐wave calculations are used to simulate the MF burst fine structure using a realistic ionospheric density profile. The delay between the highest and lowest frequencies is 21 ms. This value is smaller than the experimentally determined delays of ∼100 ms presented in Bunch and LaBelle (2009), but differences between the topside electron number density profile used in the simulations and the number density profile during disturbed conditions make comparisons only approximate. Second, the Landau damping of Langmuir/Z mode waves on the topside ionosphere is calculated, assuming the electron distribution function consists of a cold background population (ne0) and a warm secondary population (nse). The Landau damping is small when nse/ne0 = 0.04% (consistent with Maggs and Lotko (1981)) but is significant when nse/ne0 > 0.4%. Finally, full‐wave calculations are used to determine the mode conversion efficiency from Langmuir/Z mode waves to LO mode waves. These imply that waves would suffer an attenuation of wave energy density of approximately 5–10% if they are generated with their wave vectors in a narrow cone centered around the local magnetic field. Taken together, these calculations suggest that for small values of nse/ne0 <0.4%, the mechanism proposed by LaBelle (2011a) is a plausible explanation for the origin of MF bursts.

DEMETER observations of bursty MF emissions and their relation to ground‐level auroral MF burst

AbstractA survey of medium frequency (MF) electric field data from selected orbits of the Detection of Electro‐Magnetic Emissions Transmitted from Earthquakes (DEMETER) spacecraft reveals 68 examples of a new type of bursty MF emissions occurring at high latitudes associated with auroral phenomena. These resemble auroral MF burst, a natural radio emission observed at ground level near local substorm onsets. Similar to MF burst, the bursty MF waves observed by DEMETER have broadband, impulsive frequency structure covering 1.5–3.0 MHz, amplitudes of 50–100 μV/m, an overall occurrence rate of ∼0.76% with higher occurrence during active times, and strong correlation with auroral hiss. The magnetic local time distribution of the MF waves observed by DEMETER shows peak occurrence rate near 18 MLT, somewhat earlier than the equivalent peak in the occurrence rate of ground level MF burst, though propagation effects and differences in the latitudes sampled by the two techniques may explain this discrepancy. Analysis of solar wind and SuperMAG data suggests that while the bursty MF waves observed by DEMETER are associated with enhanced auroral activity, their coincidence with substorm onset may not be as exact as that of ground level MF burst. One conjunction occurs in which MF burst is observed at Churchill, Manitoba, within 8 min of MF emissions detected by DEMETER on field lines approximately 1000 km southeast of Churchill. These observations may plausibly be associated with the same auroral event detected by ground level magnetometers at several Canadian observatories. Although it is uncertain, the balance of the evidence suggests that the bursty MF waves observed with DEMETER are the same phenomenon as the ground level MF burst. Hence, theories of MF burst generation in the ionosphere, such as beam‐generated Langmuir waves excited over a range of altitudes or strong Langmuir turbulence generating a range of frequencies within a narrow altitude range, need to be revisited to see whether they predict in situ detection of MF burst.

Muscle force production with low and medium frequency burst modulated biphasic pulsed currents

Objective: Russian current, a medium frequency burst modulated alternating current (BMAC), is widely used for NMES, but has not been shown to elicit forces near voluntary maximum. In contrast, low frequency BMAC has been shown to produce greater force production than Russian and most recently, medium frequency burst modulated biphasic pulsed current (BMBPC) elicited greater force than Russian. Whether low frequency BMBPC yields greater force than medium frequency BMBPC is unknown. This study examined elicited forces using BMBPC with low and medium frequency carrier currents.Design: A cross-over design where percent maximal isometric knee extensor forces (%MVIF) elicited using BMBPC with low or medium kilohertz carrier frequencies were compared in 23 subjects. Perceived discomfort was also assessed. Data were compared using paired samples t-tests.Results: 98.4% of the MVIF was elicited with the low frequency BMBPC which was significantly greater (p < 0.001) than the 40.4% yielded by the medium frequency current. Cohen’s d effect size of 2.146 indicated a “huge effect”. Perceived discomfort of the low frequency current was 5.7/10 and was significantly greater (p < 0.001) than the medium frequency current (3.6/10).Conclusion: BMBPC with low frequency carrier current elicits forces approximating maximal volitional force. These findings offer new evidence with strong clinical implications when using NMES.

Experimental tests of a topside generation mechanism for auroral medium frequency radio emissions

Medium frequency (MF) burst is a ground‐level, impulsive auroral radio emission at 1.3–4.5 MHz associated with substorm onset. Using continuous wave measurements, Bunch and LaBelle (2009) discovered that MF burst consists of a structured feature that appears as a “backwards seven” on a frequency‐time spectrogram. LaBelle (2011) suggested that this fine structure results from Langmuir waves on the topside of the ionosphere that mode‐convert into L‐mode electromagnetic waves. This theory predicts that the lower frequency boundary of a “backwards‐seven” fine structure must exceed the maximum L‐mode cutoff (fL) along the wave propagation path. To test this prediction, we analyzed 186 fine structures from two sites over multiple seasons. To account for the possibility of wave refraction into regions of lower density, the values for fL were calculated assuming that the maximum plasma frequency ranged from 0–5% below the upper frequency boundary of the observed fine structure; for these conditions 96–100% of the fine structures had a lower frequency boundary above fL. The theory further predicts that the MF burst upper frequency boundary must lie below the maximum ionospheric plasma frequency along the source magnetic field line. We analyzed fifteen MF bursts that occurred while the Sondrestrom Incoherent Scatter Radar (ISR) measured the local ionospheric density. In all cases, the upper frequency boundary was below the maximum ionospheric plasma frequency measured by the radar. These two studies are consistent with the idea that MF bursts originate as Langmuir waves on the topside of the ionosphere that subsequently mode‐convert to L‐mode waves.

An explanation for the fine structure of MF burst emissions

[1] Medium Frequency Burst (MFB) is a broadband auroral radio emission observed at ground-level at the onset of auroral substorms. Recent high-resolution measurements show it to be composed of fine structures with “backwards seven”-shaped leading edges when displayed as a frequency-time spectrogram. With a simple one-dimensional model, the group delay of Langmuir waves originating on the F-region topside and converting linearly to L-mode can be made to match the shape of the leading edge by adjusting the topside density profile, but for plausible density profiles the fit implies relatively low energy electron beams (few hundred keV). If this mechanism explains the emissions, it would be consistent with their excitation by relatively low energy Alfvénically accelerated electron beams in the leading edges of poleward-expanding auroral arcs during substorm onset.

Theoretical constraints on the generation mechanism of auroral medium frequency burst radio emissions

[1] Auroral medium frequency (MF) burst is an impulsive auroral radio emission observed at ground level between ∼1.3 and 4.5 MHz for a few minutes at substorm onsets. Despite multiple suggested theories, the exact generation mechanism for MF burst remains unknown. The foremost theory suggests that MF burst originates from Langmuir or upper hybrid waves excited over a range of altitudes in the bottomside or topside F region, which linearly mode convert to the electromagnetic LO mode, gaining access to ground level. Analysis of plasmas with two electron components implies the existence of additional normal modes, commonly referred to as the electron acoustic (EA) and electron cyclotron sound (ECS) modes, which have been suggested to potentially play a role in the generation of MF burst. Analytical calculations using Waves in Homogeneous, Anisotropic Multicomponent Plasmas, or WHAMP, and other dispersion-solving calculations are applied to applicable ionospheric normal modes (RX, LO, Langmuir or upper hybrid, EA, and ECS), compared to previous results, and applied to MF burst and the auroral plasma environment. Computation of resonant parallel energies suggests that, in the presence of an auroral electron distribution, instability of EA, ECS, LX, and Langmuir or upper hybrid modes is possible. The same analysis suggests that LO and RX modes are unlikely to be directly excited. Excitation of the EA or ECS mode in the frequency range applicable to MF burst is found to be more favorable for higher ionospheric plasma densities (fpe/fce > 2.36) and higher secondary electron temperatures (Th ≈ 100s of eV). Growth rate calculations suggest that growth of the EA or ECS mode is excitable by the auroral electron beam with higher growth rates for the ECS mode.

Experimental tests of the generation mechanism of auroral medium frequency burst radio emissions

[1] Medium frequency (MF) burst is an impulsive auroral radio emission at 1.3–4.5 MHz commonly detected by ground-based instruments for a few minutes at substorm onsets. It is thought to arise from mode conversion radiation. The Dartmouth College MF radio interferometer at Toolik Field Station, Alaska (68.51 invariant latitude), measured spectra, amplitudes, and directions of arrival (DOA) of 47 MF burst events during 2006– 2007 and 49 events during 2007–2008. Statistical analysis of these events shows that they come predominantly from the south and east of Toolik, as expected because propagation conditions are more favorable poleward and westward of the active auroral arcs than equatorward or eastward during premidnight (westward moving) substorm onset activity. Case studies of a selected MF burst event on 20 November 2007 show that motions of the radio emissions qualitatively track the motions of auroral arcs simultaneously observed with all-sky camera. Case studies of DOA data of selected MF burst events on 31 January and 20 November 2007 show that higher-frequency components of MF burst arrive at higher elevation angles than lower-frequency components. Statistical studies confirm this trend. Ray-tracing analysis shows that this trend implies that sources of the higherfrequency components of the MF burst are at higher altitudes than those of the lowerfrequency components. The analysis also shows that the MF burst comes from the bottomside F region ionosphere. These observations are consistent with a mechanism of MF burst emission whereby the emissions originate from mode conversion of Langmuir or upper hybrid waves excited over a range of altitudes in the bottomside F region.

Fully resolved observations of auroral medium frequency burst radio emissions

[1] Auroral MF burst is a naturally occurring spontaneous emission characterizing substorm onsets. Between March 21 and April 19, 2008, a full-waveform receiver at Toolik Lake, Alaska, detected 23 examples of MF burst. The high-resolution continuous waveform data show that MF burst consists of a superposition of structured emissions, defined as those with sharp boundaries on frequency-time diagrams, and unstructured emissions which are more diffuse. Structured emissions account for 30–40% of the wave power, although this should be considered a lower bound since unstructured emission could represent weak or overlapping structured emissions. Structured emissions have a repeatable frequency-time pattern in which their leading edge decreases from a well-defined upper bound, at first slowly and then more rapidly over about 100 ms. This timescale is too long to be explained by wave propagation delay alone. Assuming emission at the local electron plasma frequency in the bottomside ionosphere, the source must move downward at 400–1000 km/s in order to explain the observed time delay.

Auroral medium frequency burst radio emission associated with the 23 March 2007 THEMIS study substorm

Auroral medium frequency (MF) burst is an impulsive auroral radio emission associated with substorm onset detected by ground‐based instruments between 1.3 and 4.5 MHz. On 23 March 2007 an MF burst emission was detected by the Dartmouth radio interferometer located near Toolik Lake, Alaska. This emission temporally coincides with the onset of the 23 March 2007 Time History of Events and Macroscale Interactions during Substorms (THEMIS) study substorm. Directions of arrival computed using the Dartmouth radio interferometer for this event also coincide spatially with the location of the expanding auroral arcs to the south observed by the all‐sky imager at Fort Yukon, Alaska. This observation represents the first example of a direction of arrival measurement for MF burst. It strongly supports the association of MF burst with intense auroral arcs accompanying substorm onset. The direction of arrival of the MF burst is consistent with the direction to the eastern edge of the substorm onset location determined by multiple data sets during this substorm and suggests that location of MF burst radio emissions may be an effective method of locating substorm onsets, much as radio atmospherics are used to locate lightning.

Observations of auroral medium frequency bursts

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 μs 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.