Magnetospheric Line Radiation Research Articles

Magnetospheric Line Radiation Observed Close to the Source: Properties and Propagation

AbstractMagnetospheric Line Radiation (MLR) is a type of whistler mode electromagnetic wave phenomenon observed in the inner magnetosphere at frequencies of a few kilohertz, that is characterized by a frequency modulation of the wave intensity. Although such events are quite regularly observed by ground‐based stations and low‐altitude spacecraft, their observations in the equatorial region at larger radial distances (i.e., close to tentative source regions) are extremely limited, likely due to the generally low frequency resolution of available measurements. A systematic search for MLR in continuous intervals of high‐resolution multicomponent wave data obtained by the Van Allen Probes spacecraft detects 15 events. They occur primarily on the dayside at frequencies between about 1 and 5 kHz, propagating with oblique wave normals away from the geomagnetic equator. For one event, simultaneous ground‐based observations are available, providing limits on the spatial extent of the event: it does not extend beyond the high‐density plasmasphere region. An electrostatic wave at a frequency corresponding to the modulation frequency of MLR is observed in three events. This is likely linked to the event formation mechanism and has not been observed before. Our results can lead to an understanding of the formation mechanism of MLR.

A Large‐Scale Magnetospheric Line Radiation Event in the Upper Ionosphere Recorded by the China‐Seismo‐Electromagnetic Satellite

AbstractThis paper reports a large‐scale magnetospheric line radiation (MLR) event during a moderate geomagnetic storm on 11 September 2018, which was well recorded by the China‐Seismo‐Electromagnetic Satellite (CSES) in the upper ionosphere. The event shows a symmetrical propagation feature at the conjugated locations between the two hemispheres, exhibiting a large spatial extension roughly from the latitudes 54°N to 53°S. The parallel structures are visible both in the electric and magnetic fields at a frequency band ranging from the local proton cyclotron frequency to ∼1.6 kHz. The wave intensity of parallel spectral lines was primarily enhanced in high latitude regions, gradually weakening at mid‐low latitudes, and then got absorbed in the equatorial region, presenting a distinct V‐shaped structure. The frequency spacings between neighboring spectral lines roughly vary from ∼80 to 110 Hz at the high latitudes and ∼80–130 Hz at the low latitudes, suggesting a slight variation feature with latitude. The parallel spectral structures of MLR drift between ∼0.39 and 0.57 Hz/s at high latitudes and ∼0.18–0.19 Hz/s at low latitudes. The wave vector analysis shows that the MLR waves are right‐hand polarized, obliquely propagating toward the Earth and in the azimuthal direction, where the Poynting flux is primarily oriented perpendicular to the ambient magnetic field. The other large‐scale MLR events all exhibit similar parallel structures and polarization characteristics, suggesting the universality of such a phenomenon. However, the azimuthal angles differ among different events, showing complex features.

On the use of ELF/VLF emissions triggered by HAARP to simulate PLHR and to study associated MLR events

A spectrogram of Power Line Harmonic Radiation (PLHR) consists of a set of lines with frequency spacing corresponding exactly to 50 or 60 Hz. It is distinct from a spectrogram of Magnetospheric Line Radiation (MLR) where the lines are not equidistant and drift in frequency. PLHR and MLR propagate in the ionosphere and the magnetosphere and are recorded by ground experiments and satellites. If the source of PLHR is evident, the origin of the MLR is still under debate and the purpose of this paper is to understand how MLR lines are formed. The ELF waves triggered by High-frequency Active Auroral Research Program (HAARP) in the ionosphere are used to simulate lines (pulses of different lengths and different frequencies). Several receivers are utilized to survey the propagation of these pulses. The resulting waves are simultaneously recorded by ground-based experiments close to HAARP in Alaska, and by the low-altitude satellite DEMETER either above HAARP or its magnetically conjugate point. Six cases are presented which show that 2-hop echoes (pulses going back and forth in the magnetosphere) are very often observed. The pulses emitted by HAARP return in the Northern hemisphere with a time delay. A detailed spectral analysis shows that sidebands can be triggered and create elements with superposed frequency lines which drift in frequency during the propagation. These elements acting like quasi-periodic emissions are subjected to equatorial amplification and can trigger hooks and falling tones. At the end all these known physical processes lead to the formation of the observed MLR by HAARP pulses. It is shown that there is a tendency for the MLR frequencies of occurrence to be around 2 kHz although the exciting waves have been emitted at lower and higher frequencies.Graphical

Dependence of Properties of Magnetospheric Line Radiation and Quasiperiodic Emissions on Solar Wind Parameters and Geomagnetic Activity

AbstractVery low frequency electromagnetic waves in the inner magnetosphere sometimes exhibit either frequency or time modulation. These phenomena are called, respectively, magnetospheric line radiation (MLR) and quasiperiodic (QP) emissions. Data from Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions spacecraft were used to analyze their properties, such as MLR frequency spacing, QP modulation period, and QP intensity as functions of geomagnetic activity and solar wind parameters. Altogether, 1,152 MLR events and 2,172 QP emissions were analyzed. It is shown that the influence of the analyzed parameters on QP emissions is different for QP events with modulation periods shorter/longer than 20 s. While the properties of QP events with long modulation periods are significantly related to the geomagnetic activity and solar wind parameters, no such dependences are observed for events with short modulation periods. This suggests that there might be two types of QP emissions generated by two different mechanisms. It is further shown that there seems to be no relation between the properties of QP and MLR events observed at the same times. Finally, the event properties do not seem to be related to the whistler occurrence rate.

Correlation Between Power Line Harmonic Radiation and Magnetospheric Line Radiation Over China

Power line harmonic radiation (PLHR) and magnetospheric line radiation (MLR) are recognized as electromagnetic pollutants in the near-earth space, and their correlation has been a controversial issue. An identification procedure based on short-time discrete Fourier transform and Welch power spectrum estimation was applied to detect 114 PLHR events and 328 MLR events that occurred as DEMETER satellite flew over China from 2008 to 2010. PLHR events feature parallel horizontal lines with a frequency interval of 50/100 Hz in spectrogram, whereas MLR events feature multi-parallel spectral lines with frequency drift as a baseline for possible triggered emissions as well as triggered emissions with no baseline. We statistically compared the temporal and geographical distribution, Kp and Dst indices, and propagation characteristics between PLHR and MLR events. PLHR events showed more obvious variations annually than MLR events, and the former had a close relationship with the development of Chinese power system. Both PLHR and MLR presented similar diurnal and seasonal differences, owing likely to the ionospheric state; moreover, they appeared to have no significant connection with geomagnetic activity level. The geographical distributions of PLHR and MLR differed significantly. PLHR events occurred in the range of (20° N–45° N), whereas MLR events were concentrated in southwestern low-latitude regions of (18° N–28°N). Furthermore, PLHR waves were right-hand circularly polarized, whereas MLR waves were approximately linearly polarized. In summary, MLR is more likely caused by natural radiation that is not directly related to PLHR, whereas PLHR is closely related to terrestrial power systems.

Magnetospheric line radiation: 6.5 years of observations by the DEMETER spacecraft

AbstractFrequency‐time spectrograms of electromagnetic waves observed in the inner magnetosphere in the frequency range of about 1–8 kHz are sometimes formed by several nearly horizontal and almost equidistant intense lines. Such events are called magnetospheric line radiation (MLR). We use a list of 1230 MLR events identified in all the data measured by the low‐altitude satellite Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) during the duration of the mission (2004–2010). We compare the occurrence of MLR events with solar wind parameters and geomagnetic indices using a superposed epoch analysis. It is found that MLR events occur more often after periods of enhanced geomagnetic activity, being statistically related to specific solar wind parameters. Moreover, the length of the analyzed time interval allows us to investigate the influence of the solar cycle and the season of the year. The events occur more often during the northern winter and spring than during the northern summer. As for the spatial distribution of the events, they occur less frequently at geomagnetic longitudes of the South Atlantic Anomaly. We analyze energy spectra of electrons precipitating in this area at the times of MLR events, and we derive energy‐latitude plots of electron flux variations related to the MLR occurrence. Finally, we perform a detailed wave analysis of two MLR events for which high‐resolution multicomponent data are available. The events are right‐handed and nearly circularly polarized, propagating at oblique wave normal angles from larger radial distances and larger geomagnetic latitudes.

Magnetospheric line radiation event observed simultaneously on board Cluster 1, Cluster 2 and DEMETER spacecraft

We present a case study of a magnetospheric line radiation (MLR) event observed simultaneously by Cluster 1 and Cluster 2 during a perigee passage at a radial distance of about 4 REand, at the same time, by the low‐orbiting DEMETER satellite. This unique constellation enables us to analyze spatiotemporal variability of the phenomenon. Although the Cluster spacecraft are separated by as much as 0.7 L‐shells, the observed wave pattern is the same on both. The analysis of B to E ratios indicates a quasiparallel propagation, which suggests that the waves cross the geomagnetic equator over a significant range of L‐shells, at least 3.9–4.6. Simultaneous observations by the DEMETER satellite separated by about 1.8 hours in MLT from the Cluster spacecraft indicate a significant azimuthal extent of the source. The obtained results show that during the MLR event the same wave pattern is observed over a significant portion of the inner magnetosphere.

Detailed properties of magnetospheric line radiation events observed by the DEMETER spacecraft

Magnetospheric line radiation (MLR) events are electromagnetic waves in the frequency range of about 1–8 kHz observed in the inner magnetosphere that, when presented in a form of frequency‐time spectrograms, consist of several nearly parallel and almost equidistant intense lines. Although many observations of these events have been reported using ground‐based instruments and a survey of a large data set based on low‐altitude satellite data has been published recently, their origin remains unclear. We use low‐altitude satellite observations of MLR events to study their detailed properties, namely, the frequency spacing of individual lines and their frequency drift. Since the satellite, unlike ground observatories, is moving, it allows us to analyze the properties of the events as a function of the position, especially L‐shell. We show that neither the frequency spacing of the events nor their frequency drift varies significantly with the L‐shell where the event is observed. Moreover, the frequency drift is generally positive. The individual lines forming the events cannot be explained as harmonics of the base frequency equal to the frequency spacing. We suggest that a possible generation mechanism might be an interaction between a wave of a carrier frequency and an additional wave with the frequency equal to the observed frequency spacing. We cannot exclude that it comes from human activity (power lines), but a magnetospheric origin is more likely. We suggest that the emissions might be guided by the plasmasphere inner boundary before they deviate to lower L‐shells at altitudes of a few thousands of kilometers.

Conjugate observations on board a satellite and on the ground of a remarkable MLR‐like event

Electromagnetic waves observed by low‐altitude satellites and on the ground have sometimes – when represented as a frequency‐time spectrogram – a form of several nearly parallel and almost equidistant clearly defined lines at frequencies between 1 and 8 kHz. These may drift in frequency and they are usually called Magnetospheric Line Radiation (MLR). Although they have been known for a long time, their origin is still unclear. We present an experimental study using both ground‐based and satellite data, showing for the first time a detailed analysis of the simultaneous observations of an MLR‐like event on the ground and in the conjugate region. We demonstrate that the individual lines forming the event result (at least in this particular case) from elements reflected between the northern and southern hemispheres.

MLR events and associated triggered emissions observed by DEMETER

This paper gives an overview of different sets of new Magnetospheric Line Radiation (MLR) observed by the satellite DEMETER. Different types of emissions have been observed: emissions called Power Line Harmonic Radiation (PLHR) with frequency lines exactly separated by 50/100 or 60/120 Hz, emissions with frequency lines not exactly separated by 50/100 or 60/120 Hz and drifting in frequency (MLR). By comparison with past observations one can say that some MLR events are due to man-made PLHR which may suffer a non-linear gyro-resonant interaction at the magnetic equator. It is also shown that periodic emissions are very often associated with the MLR. In this case the origin of these waves is natural. The lines are produced by the periodicity and the frequency band limits of the individual elements which causes the appearance of lines on the spectrograms. Finally the paper shows that MLR can trigger emissions.

Survey of magnetospheric line radiation events observed by the DEMETER spacecraft

Magnetospheric line radiation (MLR) events are electromagnetic waves in the frequency range between about 1 and 8 kHz that, when presented as a frequency‐time spectrogram, take the form of nearly parallel and clearly defined lines, which sometimes drift slightly in frequency. They have been observed both by satellites and ground‐based instruments, but their origin is still unclear. We present a survey of these MLR waves observed by the DEMETER spacecraft (at an altitude of about 700 km). Three years of VLF Survey mode data were manually searched for MLR events, creating the largest event satellite database of about 650 events, which was then used to investigate the wave properties and geographical occurrence. Finally, the most favorable geomagnetic conditions (Kp and Dst indices) for the occurrence of MLR events have been found. It is shown that MLR events occur mostly at L > 2 (upper limit is given by a limitation of the spacecraft), they occur primarily inside the plasmasphere, and there is a lower number of events occurring over the Atlantic Ocean than elsewhere on the globe. The MLR events occur more often during the day and usually during, or after, periods of higher magnetic activity. Their frequencies usually lay between about 2 and 6 kHz, with the total frequency bandwidth of an observation being below 2 kHz in the majority of cases. Moreover, it is shown that the longitudinal dimensions of the MLR events can be as large as 100° and they can last for up to a few hours. Finally, we discuss a possibility that MLR events may be triggered by power line harmonic radiation (PLHR) and we report an event supporting this hypothesis.

Simultaneous observation on board a satellite and on the ground of large‐scale magnetospheric line radiation

Very Low Frequency (VLF) spectrograms show sometimes sets of lines called MLR (Magnetospheric Line Radiation) with frequency spacing close to 50 or 60 Hz. It is very tempting to attribute these MLR to Power Line Harmonic Radiation (PLHR). PLHR are the ELF and VLF waves radiated by electric power systems at the harmonic frequencies of 50 or 60 Hz. Here we show for the first time large scale MLR observed simultaneously on ground and on board a low altitude satellite which is flying over the same zone. This two hours event is observed over a large area in the Northern hemisphere (∼7,400,000 km2) and in the conjugate region. It is hypothesized that these MLR are due to PLHR propagating in the ionosphere and the magnetosphere. When they cross the equator, the PLHR undergo a nonlinear interaction with particles and may play a role in the dynamics of the radiation belts.

Comparison of magnetospheric line radiation and power line harmonic radiation: A systematic survey using the DEMETER spacecraft

Results of a systematic search for magnetospheric line radiation (MLR) observed by the DEMETER spacecraft since the beginning of the mission are presented. DEMETER is a French microsatellite (altitude of orbit about 700 km, inclination 98°) designed to study electromagnetic phenomena connected with seismic or man‐made activity that has been launched in June 2004. An automatic identification procedure of possible MLR events has been used in order to analyze a large amount of measured data. It is shown that there are two principally different classes of events: (1) events with frequency spacing of 50/100 or 60/120 Hz (power line harmonic radiation, PLHR) and (2) events with a different frequency spacing. The first class of events is generated by power systems on the Earth's surface, with frequency spacing well corresponding to the fundamental frequency of the radiating power system. On the other hand, the second class is most probably generated in a completely natural way. All the detected events are thoroughly analyzed, and different properties of the two classes are statistically demonstrated. We have found that PLHR events occur both during low and high geomagnetic activity, with none of them significantly preferred. However, MLR events occur more frequently under disturbed conditions. Most of the PLHR events are observed at frequencies of 2 to 3 kHz. On the other hand, MLR events most frequently occur at frequencies below 2 kHz and seem to be more intense than PLHR. Additionally, PLHR events are more intense during the night than during the day, and there is about the same number of PLHR events observed during the day and during the night. On the contrary, no dependence of MLR peak intensities on magnetic local time was found, and more MLR events were observed during the day than during the night, although this difference is not statistically very significant. Finally, there is a group of MLR events with characteristics corresponding to the previous spacecraft observations of equatorial noise.

Is magnetospheric line radiation man‐made?

Magnetospheric line radiation (MLR) events are relatively narrowband VLF signals (∼30 Hz) that sometimes drift in frequency and that have been observed in both ground‐based and satellite data sets. We present the result of a survey undertaken on the basis of measurements made of MLR events observed at Halley station, Antarctica (75°35'S, 26°33'W, L≈4.3) during June, July, September, and December 1995, specifically to examine whether there is a link between MLR and power line harmonic radiation (PLHR). We find that (1) the diurnal variation of MLR occurrence at Halley does not resemble the expected load pattern in the industrialized conjugate hemisphere; (2) MLR does not show the pronounced east‐west asymmetry in the distribution of arrival directions which would be the case if it was linked to North American electrical load; (3) MLR does not show an immediate association with geomagnetic activity, as would be expected from increases in PLHR levels produced by geomagnetically‐induced currents saturating transformers; and (4) there is no evidence of a Sunday, weekend, or other 7‐day cycle in the occurrence of MLR. Taken together these results strongly suggest that MLR is a natural VLF emission and is not primarily caused by PLHR. In addition, Halley data have been examined to determine whether the intensity of all types of VLF emissions are influenced by PLHR. We find that (1) there is no significant difference between weekdays and weekends over the frequency range 0.5–9.3 kHz and (2) there is no consistent change in wave intensity that is observed around any of the major North American holiday periods. It is concluded that PLHR is not a significant influence on geospace as viewed from Halley.

Temporal properties of magnetospheric line radiation

Magnetospheric line radiation (MLR) events are relatively narrowband VLF signals that sometimes drift in frequency and have been observed in both ground‐based and satellite data sets. We present the results of a survey undertaken on the basis of measurements made of MLR events observed at Halley, Antarctica (75°30′S, 26°54′W, L ≈ 4.3), in June, July, September, and December 1995, specifically looking at the temporal properties of Halley MLR events. We find that (1) single MLR lines described in previous papers tend to be comprised of up to 3 lines with widths of 5–10 Hz. (2) The multiple lines show highly variable spacings (e.g., < 10 to ∼100 Hz). (3) The rate at which MLR lines appears from the noise is ∼0.1 to 0.2 dB s−1, considerably smaller than previously reported for other VLF emissions. MLR exhibits slow growth, relative frequency stability, and long lifetimes in comparison with other coherent whistler mode emissions and thus may be generated by a separate mechanism. (4) The diurnal occurrence of MLR observed at Halley is twin‐peaked, indicating an association with a combination of chorus and midlatitude hiss. (5) MLR occurrence rates can vary greatly within a given month, but the proportion of MLR present as part of the overall Halley wave activity is roughly constant (10–13%) throughout the year. (6) The occurrence of MLR activity at Halley is weakly linked to geomagnetic activity but only 24–48 hours after very large storms (Kp > 6), which affects only 8% of the total MLR events in this study. For smaller storms, there is little effect, although MLR events tend not to occur when the geomagnetic activity has been quiet in the previous 48 hours. (7) There is no dependence of MLR occurrence rates upon the instantaneous levels of geomagnetic activity. (8) The average duration of a typical MLR event at Halley is ∼ 30 min, quite similar to previous reports.

Magnetospheric line radiation observations at Halley, Antarctica

Magnetospheric line Radiation (MLR) events are relatively narrowband VLF signals that sometimes drift in frequency, and have been observed in both ground based and satellite data sets. Line radiation has been attributed by some authors to be power line harmonic radiation (PLHR), generated from harmonics of the power transmission frequency (50 or 60 Hz) and radiated into the ionosphere and magnetosphere by long power lines. We present the result of a survey undertaken on the basis of measurements made of MLR events observed at Halley station, Antarctica (75°30′S, 26°54′W, L ≈ 4.3) in part of June 1995. Particular attention is given to the frequency spacing, drift rates, and amplitude of the MLR lines. MLR is present in 7.0% of the minute‐long VLF recordings made at Halley. The MLR lines rise in frequency as often as they fall. However, these lines do not necessarily rise or fall monotonically and can oscillate while drifting. The Halley MLR has a wide range of line spacings and does not preferentially show spacings near harmonics of electrical transmission frequencies, either 50 Hz or 60 Hz. There is no correlation between the frequency drifts of the local 50 Hz Halley electrical supply and those of the observed MLR lines. The distribution of MLR line spacings observed in the Halley data has a roughly exponential form, suggesting a different mechanism for MLR than for PLHR.

Discrete VLF emissions (7–9 kHz) displaying unusual banded and periodic structure

Rising frequency VLF emissions having unusually high frequency and exhibiting banded structure were recorded between 14.55 and 15.30 U.T. on 28 June 1972 by the VLF goniometer receiver at Halley, Antarctica. The risers were split into two frequency groups, one with frequencies in the range 6.0–7.7 kHz and the other with frequencies between 7.8 and 9.4 kHz, the former being more numerous. The gap between the lower and upper frequency risers is superficially similar to, though at a higher frequency than, the missing bands in emissions observed by satellites. However, it is found to be unlikely that the risers received at Halley can be explained by any of the mechanisms advanced to explain the banded satellite-observed emissions. Several other explanations are considered and it is shown that the most likely is partial suppression by magnetospheric line radiation propagating in the same duct. The risers are interpreted as being generated via cyclotron resonance with counterstreaming electrons. A computer program based on Helliwell's (1967) phenomenological theory is used to determine the generation region and electron energies involved. Both frequency groups of risers display a 4 min periodicity in occurrence. It is shown that this time period is consistent with that required to replenish the flux of resonant electrons, by eastwards drift into the duct, after the emissions have been quenched due to the reduction of the flux by pitch-angle diffusion into the loss cone.

ISEE 1 observations of VLF line radiation in the Earth's magnetosphere

A search has been carried out for magnetospheric line radiation in the VLF data acquired during the period October 1977–August 1979 by the Stanford University VLF receiver on the ISEE 1 satellite, while the satellite position varied between L = 2 and L = 8 and the satellite longitude lay in the range 50°W to 110°W. This magnetopsheric region encompassed the magnetic field lines linking three stations, Eights, Siple and Roberval, from which most ground‐based data on VLF line radiation has been obtained and also included one of the main regions in which the occurrence of VLF chorus has been actively linked to power line radiation. Line radiation was detected on only 5 of 90 orbits, and all examples occurred at frequencies below 4 kHz. The most clearly defined example of line radiation was acquired near L = 5 under conditions of very low magnetic activity during which the plasma‐pause was located at L ∼ 7. In this particular case, the strongest lines in the spectrum exhibited a frequency change of approximately 22 Hz/min over a 9‐min period, while weaker lines changed frequency as slowly as 6 Hz/min. The line radiation was detected during a period when whistler mode echoing was quite pronounced on lower L shells and may actually have been a scattered component of line radiation echoing between hemispheres on a magnetic shell of lower L value. In all instances the S/N ratio or the absolute intensity of the lines was quite low. Thus, in agreement with earlier work, we conclude that very little of the background VLF wave energy in the outer magnetosphere is contained directly in VLF line radiation. However, the catalytic role of line radiation in controlling VLF wave‐particle interactions remains to be assessed.