R. A. Helliwell

Magnetospheric chorus: Occurrence patterns and normalized frequency

Abstract New characteristics of VLF chorus in the outer magnetosphere are reported. The study is based on more than 400 hours of broadband (0.3–12.5 kHz) data collected by the Stanford University/Stanford Research Institute VLF experiment on OGO 3 during 1966–1967. Bandlimited emissions constitute the dominant form of whistler-mode radiation in the region 4⪝ L⪝ 10 . Magnetospheric chorus occurs mainly from 0300 to 1500 LT, at higher L at noon than at dawn, and moves to lower L during geomagnetic disturbance, in accord with ground observations of VLF chorus. Occurrence is moderate near the equator, lower near 15°, and maximum at high latitudes (far down the field lines). The centre frequency ƒ of the chorus band varies as L−3> and at low latitudes is closely related to the electron gyrofrequency on the dipole field line through the satellite. Based on the measured local gyrofrequency ƒ H , the normalized frequency distribution of chorus observed within 10° of the dipole equator shows two peaks, at ƒ ƒ H ≅ 0.53 and ƒ ƒ H ≅ 0.34 . This bimodal distribution is a persistent statistical feature of near equatorial chorus, independent of L, LT and Kp. However there is considerable variability in individual events, with chorus often observed above, below, and between these statistical peaks; in particular, it is not unusual for single emissions to cross ƒ ƒ H = 0.50 . When two bands are simultaneously present individual emission elements only rarely show one-to-one correlation between bands. For low Kp the median bandwidth of the upper band, gap and lower band are all ∼16% of their centre frequencies, independent of L; for higher Kp the bandwidth of the lower band increases. Bandwidth also increases with latitude beyond ~10°. Starting frequencies of narrowband emissions range throughout the band. The majority of the emissions rise in frequency at a rate between 0.2 and 2.0 kHz/sec; this rate increases with Kp and decreases with L. Falling tones are rarely observed at dipole latitudes ƒ ƒ H ≅ 0.5 , at which the curvature of the refractive index surface vanishes at zero wave normal angle. Near this frequency rays with initial wave normal angles between 0° and −20° are focused along the initial field line for thousands of km, enhancing the phase-bunching of incoming gyroresonant electrons. The upper peak in the bimodal normalized frequency distribution is attributed to this enhancement near the critical frequency, at latitudes of ~5°. Slightly below the critical frequency interference between modes with different ray velocities may contribute to the dip in the bimodal distribution. The lower peak may reflect a corresponding peak in the resonant electron distribution, or guiding in field-aligned density irregularities. The observations are consistent with gyroresonant generation of emissions near the equator, followed by spreading of the radiation over a range of L shells farther down the field lines.

Magnetospheric Effects of Power Line Radiation

Radiation from electrical power lines leaks into the magnetosphere and stimulates strong very-low-frequency wave activity out to many earth radii. Observations in Antarctica show that wave activity induced by power lines tends to occur during the daytime when power consumption is high in the source region in eastern Canada. The wave frequency ranges from 1 to 8 kilohertz. This man-made wave activity may have significant effects on energetic electrons trapped in the earths radiation belts.

Ultra‐low frequency magnetic field measurements in southern California during the Northridge Earthquake of 17 January 1994

Measurements of ultra-low frequency (ULF) magnetic field fluctuations by two independent monitoring systems in Southern California were in progress during January 1994 when the moderately-large M6.7 Northridge earthquake occurred on 17 January. Our two measuring systems are located at Table Mountain, on the other side of the San Gabriel mountains and at a distance of 81 km from the epicenter, and at Pinon Flat, south of Palm Springs and at a distance of 206 km from the epicenter. Both systems operated well throughout the month and without interruption due to the earthquake. As a result of the occurrence of a moderate magnetic storm on 11 January, which was followed by a period of enhanced ULF magnetic activity that persisted until after the time of the earthquake, the sensitivity of our measurements throughout California was reduced for roughly a week before the earthquake took place. Nevertheless, no large signals that could be associated with the earthquake were evident at any time, except for the usual co-seismic shaking response of the detectors. Subsequent removal of the upper atmosphere signals from the Table Mountain measurements, using the measurements from the more distant Pinon Flat location as reference, essentially left no significant residual. Thus, assuming that ULF magnetic fields were produced by the earthquake, their amplitudes were too small to produce obvious increases in the ULF background noise at 81 km from the epicenter, which is in agreement with our earlier estimate of a range of about 100 km for the ULF magnetic field fluctuations observed prior to the M7.1 Loma Prieta earthquake. These results imply that a network of conventional magnetic field detectors spaced less than 100 km apart would be required to detect ULF magnetic field fluctuations prior to earthquakes with magnitudes greater than 7. Under the same conditions, superconducting magnetic field gradiometers could offer greater sensitivity and range.

A search for ELF/VLF emissions induced by earthquakes as observed in the ionosphere by the DE 2 satellite

Satellite observations of ELF/VLF wave activity by groups from both the Soviet Union and France have indicated the possibility of ELF/VLF radio emissions generated by earthquakes. However, an examination of ELF/VLF wave data from the low-altitude (apogee ∼ 1300 km, perigee ∼300 km, inclination ∼90°) Dynamics Explorer 2 (DE 2) satellite showed no clearly distinguishable ELF/VLF signatures associated with earthquakes. After an initial survey of approximately 5000 DE 2 orbits, ELF and VLF wave data were selected from 63 satellite orbits, called earthquake orbits, in which the ionospheric footprint of the DE 2 crossed the geographic latitude while passing within ±20° geographic longitude of the epicenters of imminent or recent earthquakes of magnitude ≥5.0. ELF/VLF noise measured near the epicenters was analyzed for occurrence rates and average spectra, as well as for peak and mean electric field intensities in three spectrometers covering a frequency range of 4 Hz - 512 kHz in 20 channels. The same analysis was then repeated for 61 carefully matched control orbits when there were no imminent or recent earthquakes within ±20° geographic longitude or within ±10° geographic latitude of the satellite footprint. These control orbits resembled the earthquake orbits with respect to latitude, longitude, local time, and geomagnetic index Kp. Sixty-three percent of the earthquake orbits showed an ELF or VLF emission above 10 µV/m in at least one of the 20 channels when the satellite passed near an epicenter. The same analysis performed on control orbit data yielded a 62% chance of observing similar emissions. Moreover, these results did not change when geomagnetic latitudes, instead of geographic latitudes, were considered. Further analyses failed to indicate any significant differences between the ELF/VLF noise measured on earthquake orbits and control orbits with regard to the general nature of the spectra, the frequency of occurrence of emissions, and peak and mean values of the electric field of the emissions.

Simultaneous triggered VLF emissions and energetic electron distributions observed on POLAR with PWI and HYDRA

We report simultaneous observations of energetic 1-20 keV electrons and VLF emissions triggered within the plasmasphere by pulses from ground based VLF transmitters, using the PWI and HYDRA instruments on the POLAR spacecraft. The 1-20 keV electrons have the correct energy to interact with the input pulses through gyoresonance. Emissions are generated by the pulses only when the particle flux is enhanced well above background and the particle pitch angle distribution is very highly anisotropic, with the average equatorial pitch angle exceeding ∼ 75°. Because of these high pitch angles, the particles are trapped typically within 7° of the magnetic equator. Only pulses which propagate within whistler mode ducts are observed to trigger emissions. The observed pitch angle anisoptropies are much larger than those assumed in present models of the VLF emission triggering phenomenon, and thus our observations provide a new starting point for understanding the emission process.

Intensity of Discrete VLF Emissions

Very low frequency whistler mode noise from the magnetosphere frequently appears in the form of narrow band tones of variable frequency called discrete VLF emissions. These tones may appear spontaneously or they may be triggered by transmissions from ground based VLF stations (Helliwell, 1965). They can be observed from within the plasmasphere out to the magnetopause and at frequencies from 300 to 30000 Hz. A phenomenological theory of discrete emissions has been advanced, based on cyclotron resonance between energetic electrons and narrow band whistler mode waves traveling along the static magnetic field (Helliwell, 1967). An important feature of this theory is the maintenance of oscillations through feedback between the waves and the electrons over a path that is long compared with the wavelength.

Power line radiation in the magnetosphere

Abstract VLF radiation from electrical power transmission lines stimulates nonlinear wave-particle and wave-wave interactions in the magnetosphere, resulting in wave growth, triggering of emissions, and entrainment of other natural or manmade VLF waves. Examples of these effects will be reviewed using both ground-based and satellite data. In many instances, the interpretation of data is aided by Siple transmitter results that show similar spectral characteristics.

The Stanford University ELF/VLF Radiometer Project: Measurement of the Global Distribution of ELF/VLF Electromagnetic Noise

S tanford University is currently conducting a global survey of electromagnetic noise in the 10 32,000 Hz (ELF/V LF) frequency band using a network of eight computer-controlled receiving systems, or ‘radiom eters.’ One goal of this m easurem ent program is to improve communication in the E L F/V L F band by providing more upto-date and complete inform ation about the properties of E L F/V L F noise (both na tu ra l and man-made) than is currently available—the last extensive survey of noise in the same frequency band was made over two decades ago. In this p resentation we describe the Stanford E L F /V L F noise m easurem ent project, including the instrum enta tion comprising each of the radiometers, the form of the ir analog and digital m easurem ents (which are made under the control of a minicomputer), and the d a ta processing techniques th a t will be used. T he results of previous noise surveys are briefly reviewed and the significance of the overall decline of noise power with increasing frequency revealed by these surveys and other studies is discussed in the context of the scientific applications of the noise d a ta obtained by the radiom eter network.

Simultaneous observations of VLF ground transmitter signals on the DE 1 and COSMOS 1809 satellites : detection of a magnetospheric caustic and a duct

Khabarovsk transmitter signals (15.0 kHz, 48°N, 135°E) were observed on the high-altitude (∼15000 km) Dynamic Explorer 1 (DE 1) and the low-altitude (∼960) km COSMOS 1809 satellites during a 9-day period in August 1989. On 7 out of 9 days the linear wave receiver (LWR) on the DE 1 satellite detected signals from the Khabarovsk transmitter. In addition, the DE 1 satellite also detected signals from the Alpha transmitter (11.9-15.6 kHz) in Russia and an Omega transmitter (10.2-13.6 kHz) in Australia, as well as natural VLF emissions such as hiss, chorus, whistlers, and wideband impulsive signals. On two days, August 23 and 27, 1989, observations of the Khabarovsk transmitter signals were simultaneously carried out at high altitude on the DE 1 satellite and at low altitude on the COSMOS 1809 satellite. Analysis of data from these 2 days has led to several new results on the propagation of whistler mode signals in the Earth’s magnetosphere. New evidence was found of previously reported propagation phenomena, such as (1) confinement of transmitter signals in the conjugate hemisphere at ionospheric heights (∼1000 km), (2) observation of direct multipath propagation on both DE 1 and COSMOS 1809, (3) detection of ionospheric irregularities of ≤ 100 km scale size with a few percent enhancement in electron density, believed to be responsible for the observed multipath propagation. We report the first detection of an exterior caustic surface near L ∼ 3.5 for VLF ground transmitter signals injected into the magnetosphere; the location of the caustic surface depended on the signal frequency, and the electric and magnetic fields decreased by several hundred decibels per L shell in the dark (shadow) side of the caustic. We also report the first direct detection of a magnetospheric duct at L = 2.94 which was believed to be responsible for the ducted propagation of Khabarovsk signals observed on the COSMOS 1809 satellite; the measured duct parameters were: ΔL ∼ 0.06 and ΔNe, ∼ 10 - 13%. The duct width at the equator was ∼367 km. Our study also indicates that duct end points can extend down to at least ∼1000 km. The peak electric and magnetic fields of ducted Khabarovsk transmitter signals at ∼1000 km were 520 µV/m and 36 pT respectively. Estimated field strengths of these signals inside the duct at the geomagnetic equator were 57 µV/m and 12 pT for electric and magnetic field respectively. The results of two-dimensional ray tracing simulations were consistent with the observations of the nonducted whistler-mode propagation of Khabarovsk (15 kHz) and Alpha (11.9 kHz) signals from the transmitter location to the DE 1 and COSMOS 1809 satellites. Our results have direct implications for the question of accessibility of waves injected from the ground to various regions of the ionosphere and the magnetosphere. In situ measurements of electric and magnetic fields of Khabarovsk transmitter signals inside a duct may well prove to be the critical measurements needed to differentiate between the small signal and large signal theories of wave particle interactions.

Global Measurements of Low-Frequency Radio Noise

We report illustrative results obtained by Stanford University’s global survey of ELF/VLF radio noise (frequencies in the range 10 Hz – 32 kHz). Particular comparison is made between the noise measurements made at high (polar) latitudes with those at lower latitudes. Although most of the natural ELF/VLF noise observed everywhere in the world is lightning-generated, the high-latitude noise often contains additional components that are of magnetospheric origin. In the data we have examined, this noise consists predominantly of polar chorus, which is concentrated in the range 300 Hz–2 kHz. It produces a characteristic signature in the noise statistics. Less frequent occurrences of broad-band (auroral) hiss can occasionally mask most or all of the lightning-generated noise in the ELF/VLF range.