Vikas S. Sonwalkar

Magnetospherically reflected whistlers as a source of plasmaspheric hiss

Ray-tracing simulations and estimates of whistler wave damping show that magnetospherically reflected whistlers can persist for ∼102 s in a low frequency band (ƒ ∼1 kHz). The combined contribution from whistler rays produced by a single lightning flash but entering the magnetosphere at different points form a continuous hiss-like signal, as observed at a fixed point. Estimates indicate that the total whistler wave energy input into the magnetosphere from lightning discharges may maintain experimentally observed levels of magnetospheric hiss.

An explanation of ground observations of auroral hiss : Role of density depletions and meter-scale irregularities

Auroral hiss is one of the most intense whistler mode plasma wave phenomena observed both on the ground at high latitudes and on spacecraft in the auroral zone. Propagation of auroral hiss from its source region to the ground is poorly understood. The standard whistler mode propagation in a smooth magnetosphere predicts that auroral hiss generated at large wave-normal angles along the auroral field lines by Cerenkov resonance cannot penetrate to the ground. We show that the presence of density depletions along the field lines in the auroral zone and meter-scale density irregularities at altitudes 5000 – 20, 000 km) propagates to lower altitudes (< 3000 – 5000 km) in two modes: (1) a ducted mode guided by field-aligned density depletions and (2) a nonducted mode. The hiss with large wave-normal angle arriving at < 5000 km altitude is scattered by meter-scale irregularities, and about 0.1% to 10% of the scattered hiss has small wave-normal angles which can penetrate to the ground. Our mechanism explains the following features of auroral hiss observed on the ground: (1) the characteristic spectra of continuous and impulsive auroral hiss, (2) the upper and lower frequency cutoffs, (3) the dispersion of impulsive auroral hiss, (4) the location of ionospheric exit points of auroral hiss with respect to visible aurora, and (5) the 2–5 order of magnitude intensity decrease of auroral hiss observed on the ground relative to that observed on spacecraft. Based on the model presented here, we provide methods to infer parameters of density depletions and intensity of lower hybrid waves stimulated by auroral hiss from the ground measurements of auroral hiss together with optical and radar measurements.

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 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.

DE‐1 observations of lower hybrid waves excited by VLF whistler mode waves

Recent satellite data show high amplitude lower hybrid (LH) waves excited by electromagnetic (EM) whistler mode waves throughout magnetospheric regions where small scale magnetic-field-aligned plasma density irregularities exist. One important consequence in the auroral acceleration region is the heating of suprathermal ions by the excited LH waves. To evaluate such heating it is necessary to know the wavelength (λ) range of the waves, information not previously available since most past observations were made with long (l ≥ 75; l=antenna length) electric dipole antennas which have very poor response for λ < l. New observations using the short 9 m electric dipole antenna on the DE-1 spacecraft show that LH waves with λ ≤ 10 m are excited by EM input waves from ground-based VLF transmitters. Sequential observations on the short (9 m) and long (200 m) dipole antennas show that the long antenna generally has no measurable response to LH waves with λ ≤ 26 m. Since most previous correlative observations of LH waves and ion conics in the low altitude auroral acceleration region have involved long electric antennas, it is suggested that the intensity of the LH waves in this region may have been seriously underestimated.

Active Wave Experiments in Space Plasmas: The Z Mode

The term Z mode is space physics notation for the low-frequency branch of the extraordinary (X) mode. It is an internal, or trapped, mode of the plasma confined in frequency between the cutoff frequency fz and the upper-hybrid fre- quency fuh which is related to the electron plasma frequency fpe and the electron cyclotron frequency fce by the expression f 2 uh = f 2 pe + f 2 ce; fz is a function of fpe and fce. These characteristic frequencies are directly related to the electron number density Ne and the magnetic field strength |B|, i.e., fpe(kHz) 2 ≈ 80.6Ne(cm −3 )a nd fce(kHz) 2 ≈ 0.028|B|(nT). The Z mode is further classified as slow or fast depending on whether the phase velocity is lower or higher than the speed of light in vacuum. The Z mode provides a link between the short wavelength λ (large wave number k =2 π/λ ) electrostatic (es) domain and the long λ (small k) electromagnetic (em) domain. An understanding of the generation, propagation and reception of Z-mode waves in space plasma leads to fundamental information on wave/particle interac- tions, Ne, and field-aligned Ne irregularities (FAI) in both active and passive wave experiments. Here we review Z-mode observations and their interpretations from both radio sounders on rockets and satellites and from plasma-wave receivers on satellites. The emphasis will be on the scattering and ducting of sounder-generated Z-mode waves by FAI and on the passive reception of Z-mode waves generated by natural processes such as Cherenkov and cyclotron emission. The diagnostic applica- tions of the observations to understanding ionospheric and magnetospheric plasma processes and structures benefit from the complementary nature of passive and ac- tive plasma-wave experiments.

The Influence of Plasma Density Irregularities on Whistler-Mode Wave Propagation

Whistler mode (W-mode) waves are profoundly affected by Field- Aligned Density Irregularities (FAI) present in the magnetosphere. These irregu- larities, present in all parts of the magnetosphere, occur at scale lengths ranging from a few meters to several hundred kilometers and larger. Given the spatial sizes of FAI and typical wavelength of W-mode waves found in the magnetosphere, it is convenient to classify FAI into three broad categories: large scale FAI, large scale FAI of duct-type, and small scale FAI. We discuss experimental results and their interpretations which provide physical insight into the effects of FAI on whistler (W) mode wave propagation. It appears that FAI, large or small scale, influence the prop- agation of every kind of W-mode waves originating on the ground or in space. There are two ways FAI can influence W-mode propagation. First, they provide W-mode waves accessibility to regions otherwise not reachable. This has made it possible for W-mode waves to probe remote regions of the magnetosphere, rendering them as a powerful remote sensing tool. Second, they modify the wave structure which may have important consequences for radiation belt dynamics via wave-particle interac- tions. We conclude with a discussion of outstanding questions that must be answered in order to determine the importance of FAI in the propagation of W-mode waves and on the overall dynamics of wave-particle interactions in the magnetosphere.

Testing radio bursts observed on the nightside of Venus for evidence of whistler mode propagation from lightning

Radio burst events recorded on the nightside of Venus by the orbiting electric field detector (OEFD) on Pioneer Venus Orbiter (PVO) have been interpreted as originating in subionospheric lightning. This lightning source interpretation has been subject to repeated challenges. During many of the burst observations, activity occurred in the lowest, or 100 Hz, filter band channel only, while in a smaller number of cases, activity occurred at two or more of the four filter band frequencies 100 Hz, 730 Hz, 5.4 kHz, and 30 kHz. Previous work with the data has been primarily statistical in nature. In some studies, only events with activity limited to the 100-Hz channel were considered; 100 Hz had been found to be lower than typical values (∼100–1000 Hz) of the ambient electron gyrofrequency, and such cases appeared to be candidates for whistler mode propagation from lightning sources to the satellite. In general it was recognized that if the higher-frequency signals were of subionospheric origin, their observation from PVO would require an ionospheric penetration mechanism other than the conventional one associated with excitation of the cold plasma whistler mode at the lower ionospheric boundary. In the present work, methods have been developed for testing the hypothesis that particular burst events were the result of whistler mode propagation of signals from subionospheric lightning sources. The tests allow prediction of the resonance cone angle, wave normal direction, refractive index, wave dispersion, and wave polarization and are believed to represent an improved way of categorizing OEFD burst data for purposes of investigating source/propagation mechanisms. The tests, which are capable of refinement, were applied to observations from 11 periods along seven orbits. Most, of these cases had been illustrated in the literature in support of conflicting interpretations of the observations. The key wave normal test was applied to each of the 11 cases, and the dispersion and polarization tests were also applied to the limited extent that the properties of the particular data sets would permit. The results obtained from the limited data sample indicate that there are at least two main categories of burst events, one for which the assumed vertical wave normal angle was within the allowed cone of angles for whistler mode propagation and one for which this was not the case. Lightning is thus considered to be a candidate source for at least some of the OEFD bursts. Its further assessment as a source must await studies of additional events and, in particular, examination of cases to which the more stringent dispersion and polarization tests can be applied. Four of the five burst events that were found to be inconsistent with the hypothesis of whistler mode propagation from lightning involved receptions at multiple OEFD filter band frequencies, while one involved 100 Hz only. A search for the cause of such events should include possible mechanisms of ionospheric wave penetration at frequencies both above and below the gyrofrequency, as well as plasma instability mechanisms local to the spacecraft.

DE 1 VLF observations during activity wave injection experiments

We report on coordinated high-altitude satellite observations in support of one of the first space-based very low frequency (VLF) wave injection experiments, namely the USSR Aktivny mission. The Aktivny satellite (A) was designed to carry a VLF transmitter (nominal frequency approximately 10 kHz, transmitter power approximately 10 kW) coupled to a 20-m-diameter loop antenna in a nearly polar orbit (83 deg inclination, apogee approximately 2500 km, perigee approximately 500 km). We focus our attention on conjunction experiments between the Aktivny and DE 1 satellites. Because of problems in the deployment of the loop antenna, the radiated power capability of the antenna was significantly reduced. Although this substantially reduced the expectation of receiving detectable signal levels on the satellite, the DE 1/Aktivny conjunction experiments were nevertheless carried out as a means of possibly placing an upper limit on the radiated power. During the period November 1989 through April 1990, a total of 10 DE 1/Aktivny wave injection sessions were conducted. During each session the Aktivny transmitter operated at 10.537 kHz with 1 s On - 1 s Off format, for a period of 6 min centered around the conjunction time. During three conjunction periods (December 12, 26, and 27, 1989) both DE 1 and Aktivny were in the southern hemisphere, and DE 1 was at relatively low altitudes (ranging from 6211 to 14,810 km), thus providing the best conjunction possibilities according to the ray tracing criteria developed above. On most days, Omega transmitter signals as well as commonly occuring natural wave phenomena such as whistlers (0(+)) and hiss were clearly seen well above the background level, but there was no evidence of the Aktivny 1 s On/ 1 s Off pattern. Though no Aktivny signals were detected by the LWR on the DE 1 satellite, the experimental constraints allow us to place an upper limit on the total power radiated by the Aktivny transmitter in the whistler-mode. Using experimental parameters, and the minimum detectable signal level of 0.05 muV/m for LWR, we find the upper limit on the total power radiated by the Aktivny satellite in the whistler-mode to be approximately 10 mW. Several recommendations for future space-based wave injection experiments are presented.

Properties of the magnetospheric hot plasma distribution deduced from whistler mode wave injection at 2400 Hz: Ground-based detection of azimuthal structure in magnetospheric hot plasmas

Siple station VLF wave injection experiments aimed at finding the properties of the magnetospheric hot plasma were conducted for a 9-hour period between 1705 and 0210 UT on January 23–24, 1988. A special frequency versus time format, lasting l min and transmitted every 5 min, consisted of a sequence of pulses, frequency ramps, and parabolas, all in a 1-kHz range centered on 2400 Hz. The transmitted signals, after propagating along a geomagnetic field-aligned duct, were recorded at Lake Mistissini, Canada. At various times during the 9-hour interval the Siple signals showed features characteristic of wave-particle interactions, including wave growth, sidebands, and triggered emissions. Our observations, primarily at 2400 Hz, show that (1) there were no correlations between the initial levels, the growth rates, and the saturation levels of constant-frequency pulses; (2) in general, the values of growth rate and saturation level of two pulses injected within 30 s were nearly the same; (3) the initial level, growth rate, and saturation level showed temporal variations over 5–15 min and 1–2 hour timescales; (4) the leading edges of constant-frequency signals underwent spatial amplification; and (5) under conditions of saturation the received signal bandwidth (∼ 20 Hz) remained constant over a 1-hour period, although the saturation level and growth rate varied during the same period. On the assumption that gyroresonant interactions were responsible for the observed wave growth and saturation, the timescales over which those phenomena varied provide constraints on the possible energetic electron population within the duct. In the reference frame of the duct (L ∼ 5.1, Ne ∼ 280 cm−3) the particle fluxes showed no variation over a 30-s timescale but varied over 5–15 min and 1–2 hour timescales. The 5–15 min timescale variations indicate longitudinal structures ranging from ∼ 0.2° or ∼100 km (in the equatorial plane) for electrons with energy E = 0.6 keV and pitch angle α = 40°, to ∼ 5° or ∼2800 km for electrons with energy E = 11 keV and pitch angle α = 80°. The hour-long time variations indicate longitudinal structures ranging from ∼ 2° or ∼1100 km (in the equatorial plane) for electrons with energy E = 0.6 keV and pitch angle α = 40°, to ∼ 45° or ∼25,000 km for electrons with energy E = 11 keV and pitch angle α = 80°. We conclude that ground-based active and passive wave experiments have substantial potential for investigating properties of the hot plasma of the magnetosphere.