Energetic Electron Precipitation Events Research Articles

Three‐dimensional modeling of subionospheric VLF propagation in the presence of localized D region perturbations associated with lightning

A theoretical model of single‐mode, subionospheric VLF wave propagation in the presence of localized perturbations of the nighttime D region has been developed. Such perturbations could be produced, for example, by lightning‐induced electron precipitation associated with a characteristic type of phase or amplitude perturbation in VLF signals known as “Trimpi” events. Our model assumes that the ionospheric perturbation is slowly varying in the horizontal plane and that mode‐coupling is therefore negligible, and accounts for (1) effects of perturbations with finite extent in the dimension transverse to the great circle (GC) path between transmitter and receiver, and (2) effects of perturbations which lie off the GC path as well as on it. The formulation used for the numerical calculations depends significantly on the mode refractive index of the ambient Earth‐ionosphere waveguide and the mode refractive index in the region of the perturbation. In the calculations, values for the mode refractive index are determined from the electron‐density‐versus‐altitude profiles of both the ambient and perturbed ionospheres. Values for changes in the amplitude and phase of a received signal were obtained from the model and compared with amplitude and phase measurements of a VLF signal received at Palmer Station, Antarctica, from the NPM transmitter (23.4 kHz) in Hawaii during energetic electron precipitation events. The large distance and all‐sea path between the transmitter and receiver make it possible to represent the signal using single waveguide mode theory. The results of varying the location of a perturbation along the GC path as well as off the path in the transverse dimension, varying the horizontal scale of the perturbation, and varying the vertical density profile of the perturbation were all examined. The model shows that positive phase and/or negative amplitude shifts in the received signal are produced by perturbations centered on the GC path, whereas both positive and negative amplitude (or phase) shifts in a single‐mode signal can be produced by perturbations lying in regions off the GC path. Results of the model indicate that the magnitude of the signal scattered by the perturbation towards the receiver continuously decreases with distance away from the GC path, becoming insignificant beyond ∼20 λ. On or near the GC path, it was found that the magnitude of the scattered signal was proportional to the scale of the perturbation parallel to the GC path. Using realistic values for the ground and ionospheric profile parameters, values of the shift in the amplitude and phase of the signal similar to those measured on the NPM signal received at Palmer Station, Antarctica, were obtained using this model. For example, a cylindrically symmetric perturbation of 5 λ in horizontal extent due to a 0.2 second burst of precipitating electrons of ∼2×10−3 ergs cm−2 s−1 flux density can produce amplitude changes of ∼ −0.3 dB and phase changes of ∼2°. Results from the model suggest that the ratio of the shifts in signal phase and amplitude can be used to determine the distance of the perturbation from the GC path.

Bremsstrahlung X ray mappings of an intense, widespread, and pulsating electron precipitation event following a sudden commencement

For the first time, remote bremsstrahlung X ray spatial, temporal, and spectral mappings from a satellite are presented for a strong energetic electron precipitation event following a sudden commencement. After the sudden commencement of July 13, 1982, and at the beginning of a magnetospheric substorm, X rays were observed over a period of more than 4 min from the P78‐1 spacecraft at 600‐km altitude. The X rays above 21 keV were emitted over a magnetic local time (MLT) interval spanning 6 hours. Five precipitating temporal spikes were observed with coherency over wide longitude intervals of 3 hours or more in MLT near midnight. The energy spectra over broad longitude intervals tended to be harder simultaneously at the times of greatest X ray intensity. The power input to the atmosphere from precipitating electrons that produced the X rays emitted within 2200–2230 MLT is estimated to have reached values of 5 × 1016 ergs/s over an area of about 3 × 104 km², leading to an average energy deposition rate of about 160 ergs/cm² s. The precipitation inputs at other magnetic local times displayed nearly simultaneous sharp maxima in time but with lower power inputs.

Eiscat observations of the ionospheric D-region during auroral radio absorption events

Electron densities in the D-region have been observed with EISCAT during energetic electron precipitation events. Sample results are presented which demonstrate the value of the technique in studying variations of electron density with fine temporal and spatial resolution. Different types of absorption event can be characterized in terms of the changes in the incoming electron spectrum inferred from profiles of electron density. We contrast the D-region behaviour of night- and day-time events in terms of precipitating spectrum and absorption profile. A softening of the electron spectrum during the course of a morning event is clearly seen.

A theoretical model study of observed correlations between whistler mode waves and energetic electron precipitation events in the magnetosphere

A recently extended test particle computer model of the gyroresonance wave‐particle interaction in the magnetosphere is applied to previously reported cases of observed correlations between whistler mode waves and ionospheric responses to particle precipitation. Three different ionospheric effects, X ray bursts, photoemissions, and D region perturbations, all correlated with VLF waves and believed to be caused by precipitated particles, are considered. The precipitation flux level, the pulse shape, and the associated time delays are computed for the parameters relevant to each case and are compared with values deduced from the data. The results demonstrate that the existing theoretical model can be useful for interpreting experimental results of this kind. Furthermore, the model results and observations, used together, provide a basis for additional diagnostics of the various parameters of the cold and energetic particle distributions in the magnetosphere. For example, when applied to the observed photoemission case (Helliwell et al., 1980) the model results imply that the trapped energetic particle distribution function at the time could be modeled as proportional to E−n/2 with n ≃ 3.5 to 6, where E is the particle energy.

Coordinated measurements of ELF transmission anomalies and the precipitation of energetic particles into the ionosphere

Stimulated by previous findings of a positive qualitative correlation between anomalous ELF signal levels and the occurrence of significant fluxes of precipitating energetic particles, coordinated exercises were conducted between satellite measurements of the precipitating particles and ELF transmissions between the US Navy Wisconsin Test Facility and receiving stations in Maryland, Greenland, Norway, and Italy. Significant fluxes of precipitating electrons were often measured with the Lockheed pay load on the polar‐orbiting satellite 1972–076B. In March‐April 1976 the received signal strengths were found generally to increase during periods when the geomagnetic K indices were higher and when the fluxes of precipitating electrons >160 keV were greater on the average. During other coordinated exercises in March‐April 1975 the general correlations were less evident. Based on the fluxes, energy spectra, and pitch‐angle distributions of the particles, ion‐pair production profiles have been calculated and electron density profiles subsequently obtained with application of known effective electron loss rates. From the coordinated data, correlative studies between signal strengths and electron density profiles have been performed with use of an ELF waveguide mode computer program developed at the Naval Ocean Systems Center. The propagation path to each station of interest is segmented into different regions with an averaging perpendicular to the direction of propagation of the waveguide parameters. With the inclusion of data from several stations, the transmission signal strengths could be studied simultaneously under a variety of ionospheric conditions. The waveguide computer program calculations indicated that energetic electron precipitation events may cause a signal enhancement or degradation, depending upon where the disturbance is occurring along the path. The consistent occurrence during electron precipitation events of signal strength anomalies in either direction is supported by the coordinated satellite/ELF station measurements.

On the morphology of energetic (≥30keV) electron precipitation at the onset of negative magnetic bays

Multiple balloon recordings of bremsstrahlung X-rays supported by recordings of cosmic noise absorption have been used to study in detail energetic (≥30 keV) electron precipitation events occurring near local midnight at the onset of the expansion phase of magnetospheric substorms. This type of precipitation occurs during the first 5–10 min after bay onset and can usually be distinguished from the subsequent bay-associated precipitation by its characteristic time structure, variation in energy spectrum, and higher intensities. During this same interval, the poleward border of the precipitation region moves rapidly towards higher latitudes with speeds of typically 1–2 km/s, whereas the equatorward border seems to move slowly towards lower latitudes. The northward expansion starts just poleward of the lowest latitude reached during the slow equatorward motion of the preceding growth-phase precipitation. The previous narrow precipitation region may thus expand to as much as 10° of invariant latitude within a few minutes. Within the expanding region there are additional intrinsic temporal variations. As the flux of precipitating electrons tends to be most intense and most energetic near the poleward border, recordings made northward of the latitude where the poleward motion started tend to give the appearance of an impulsive precipitation event. The bay-onset precipitation starts abruptly at the onset of Pi 2 magnetic pulsations. Associated with these pulsations there are modulations of the flux of precipitating electrons. An intensified westward electrojet appears to have its center in the equatorward part of the precipitation region. It is suggested that the poleward expansion is associated with the expansion of the plasma sheet earthward of a newly formed X-type neutral line, and is caused by a sudden enhancement of field-line re-connection across the neutral sheet. The intense, more energetic electron precipitation at the poleward border of the precipitation region then takes place along the outer border of the expanding plasma sheet.

A coordinated two-satellite study of energetic electron precipitation events

A new technique for studying the spatial/temporal variations of energetic electron precipitation events is investigated. Data are presented in which precipitating electrons were measured simultaneously on two coordinated polar-orbiting satellites and the bremsstrahlung produced by the electrons precipitating into the atmosphere was observed from one of the satellites. Two electron spectrometers measuring the intensities and energy spectra of electrons of > 130 keV were located on the oriented satellite 1971-089A (altitude, ≃800 km), whereas a single similar spectrometer measuring electrons of > 160 keV was located on the spinning low-altitude (∼750 km) satellite 1972-076B. The X rays of >50 keV were measured with a 50-cm³ germanium spectrometer placed on the 1972-076B satellite. With the coordinated data a study is made of events in which large fluctuations were observed in the precipitating energetic electron intensities. In the examples presented the satellite X ray data alone demonstrate that the spatially integrated electron influx was constant in time, and when the X ray data are combined with the direct electron measurements from the two satellites, the resulting data suggest that the major features in the flux profiles were primarily spatial in nature. The combination of X ray and electron measurements from two satellites is shown to provide an important method for studying and attempting to resolve spatial and temporal effects.

Satellite observations of Bremsstrahlung from widespread energetic electron precipitation events

This is a preliminary report of the first satellite observations that have been made of the intensities and energy spectra of the bremsstrahlung >50 keV produced in the atmosphere by intense widespread precipitations of energetic electrons from the outer radiation belt. The measurements afford the opportunity to investigate nearly simultaneously the energetic electron behavior on a worldwide scale during magnetic disturbances. With such a technique, one may study different features of the precipitation processes than those covered by satellite-borne particle detectors, which can provide more detailed particle information in a local region but without simultaneous measurements from many satellites can provide worldwide particle precipitation information only from long-term statistical studies. The observations are made with a 50-cm³ Ge(Li) gamma ray spectrometer that was launched aboard the spin-stabilized satellite 1972–076B into a nearly circular polar orbit at 750-km altitude on October 2, 1972. Correlative measurements of precipitating electrons are also made with an energetic electron spectrometer on board the same satellite. The longitude and hence local time extents of the precipitations are derived by least square fitting techniques from the gamma ray spectrometer responses at various spin angle orientations and satellite locations. The observations indicate the occurrence of nearly simultaneous precipitations over a wide range of local times. Significant variations occur in the intensity versus local time, but in the limited number of cases studied to date the intensities near noon are generally greater than those in the early morning hours.

Simultaneous forward-scatter, riometer, and bremsstrahlung observations of a daytime electron precipitation event in the auroral zone

Ionospheric scatter propagation and absorption during an energetic electron precipitation event during daylight in the auroral zone are examined, using bremsstrahlung observations at balloon altitude to compute the precipitating electron flux. The electron spectrum responsible for the event is described as the sum of two exponential spectra, one soft (characteristic energy E 0 = 9 keV) and the other hard ( E 0 = 40 keV). Analysis of the random and systematic errors of the observations places the effective scattering height at 75 ± 2 km.

Spatial characteristics of auroral-zone X-ray microbursts

An array of four narrowly collimated X-ray detectors, each viewing a separate part of the sky, was used to study spatial and temporal characteristics of bremsstrahlung X rays produced by energetic electron precipitation in the auroral zone. Each telescope viewed a circle of about 20-km diameter at the stopping heights of electrons (90–100 km). The telescope arrays were carried to small atmospheric depths of 2–3 g/cm² using large-volume balloons. The data revealed that the impulsive-type energetic electron precipitation events, microbursts, are not distributed uniformly in the X-ray production plane. The data further showed that each microburst precipitation region is on the average 40±14-km radius. An energetic electron precipitation event that occurred on September 16, 1965, was shaped similarly to bands aligned in the north-south direction drifting eastward with a velocity of about 250–300 m/sec. Microbursts occurred during this event, indicating that the propagating electron source region and microbursts were related.