Precipitation Bursts Research Articles

Perturbations of subionospheric LF and MF signals due to whistler‐induced electron precipitation bursts

First evidence of whistler‐induced burst precipitation effects on subionospherically propagating signals in the LF and MF ranges have been observed at Palmer (L ∼ 2.4) and Siple stations (L ∼ 4.3), Antarctica. The occurrence rate on a 37.2‐kHz LF signal originating in California was usually comparable to that on the more disturbed VLF paths. At Palmer, examples at 37.2 kHz were seen on 70% of the nights during a March‐April 1983 observing period. Perturbations on a ∼1800 km‐long 780‐kHz MF path to Palmer occurred on nights of high activity on VLF paths, but fewer than 10 MF events were usually detected as compared to >50 on the active VLF paths. The MF perturbations were of order 50% in amplitude and were not in general followed by a ∼30‐s decay toward a pre‐event level, as is usually the case for the VLF signals. Test particle modeling of the whistler‐particle interaction supports the idea that the MF signals are affected by ionization extending above the nominal ∼85‐km reflection height for VLF signals and also are consistent with the observed time delay between the 780‐kHz signal perturbation and the whistler‐generating lightning stroke. Fast phase advances correlated with the whistlers were regularly observed on a 12.9‐kHz Omega signal. The data provide a means of inferring the existence of multiple precipitation regions and offer new evidence that whistlers originating in the northern hemisphere can cause significant precipitation in the southern hemisphere, at least in the vicinity of the South Atlantic magnetic anomaly.

Diurnal variation of burst precipitation effects on subionospheric VLF/LF signal propagation near L = 2

Burst precipitation effects on subionospheric VLF/LF signal propagation are studied using data recorded at Palmer Station, Antarctica (L ∼ 2.4) between March 3 and April 7, 1983. Relatively abrupt signal amplitude changes, known as “Trimpi” effects, were observed in conjunction with magnetospheric whistlers, which are inferred to induce electron precipitation that in turn causes enhanced ionization at the 80 to 90‐km altitude level. Earlier findings that Trimpi effects occur predominantly under nighttime ionospheric conditions were confirmed by comparing the sunrise‐sunset terminator position with perturbation activity on three VLF/LF signal paths for which the arrival bearings range from magnetic north to west. Events were found to occur on 70% of the observing nights. Signal paths making larger angles with the terminator were used to deduce that most events were caused by ionospheric effects occurring within ∼1000 km of Palmer Station. Some events were observed after sunrise over the observing station, apparently due to precipitation in dark regions as far as ∼1800 km away. Observed differences between two different paths in the occurrence of this postsunrise continuation of Trimpi events suggest that the Trimpi event activity rate decreases below L ∼2. This may be due to a reduction in whistler activity and/or lower particle scattering efficiency. The average hourly occurrence rate of transmitter NSS events on the 11 most active nights studied increased from sunset to near midnight and then remained at a roughly constant level until sunrise. The influence of the whistler rate and of whistler path locations on Trimpi effects is clearly important and requires further study.

Direct detection of the precipitation of ring current electrons and protons stimulated by artificial VLF emission

Bursts of energetic electron and proton precipitation from the magnetosphere and artificially stimulated by a powerful ground‐based Soviet VLF emitter (F = 19.1 kHz, L = 4.0), were detected during the ESSA experiment aboard the AUREOL‐3 satellite (ARCAD‐3 project) at altitudes of 1500 ‐ 2000 km in the MLT morning sector. Stimulated electron and proton precipitations were clearly recorded in 7 overpasses out of 17 during the period 1 ‐ 25 December 1981. The precipitation zone corresponded to the projection on the ionosphere of the ring current region (L = 2.8 to 3.6), and was displaced equatorward with respect to the location of the VLF transmitter. The intensities of the artificial 0.1 ‐ 2 keV electron fluxes ranged from 104 to 105 (cm² sec ster keV)−1 and those of the 60 ‐ 220 keV protons from 100 to 10² (cm² sec ster keV)−1. The 1.8 keV electron bursts were delayed by about 1 sec with respect to the VLF pulses, and the 110 keV proton fluxes were delayed by about 3 ‐ 5 sec. Results are discussed in terms of both Cerenkov and cyclotron resonance effects between VLF nonducted waves and magnetospheric particles.

Whistler induced suppression of VLF noise

New evidence has been found connecting whistlers with transient reductions in the amplitude of magnetospheric VLF noise bands. Until now, the only reported examples of such effects were observed during a several hour period at Siple Station, Antarctica, when bursts of particle precipitation were correlated with whistlers that suppressed a background hiss band. Data acquired at South Pole Station during 1981 showed similar suppression events on 20% of all winter days. A review of data from Siple, Byrd, Eights, and Palmer Stations in Antarctica and Roberval in Quebec, Canada, has since identified many more events. The ground signature of an event is characterized by the attenuation of an existing VLF noise band following the arrival of a whistler. The attenuation typically reaches a maximum of 3–5 dB in 5–10 s, and the noise band recovers to the pre‐event amplitude 15–30 s after the whistler. The noise bands are generally mid‐latitude hiss, but suppression associated with polar chorus has been seen. Regularly observed features of suppression events include multi‐hop echoing of the driving whistler; confinement of the whistler echoes to the frequency band occupied by the suppressed noise; suppression during the first pass of the whistler; continuing suppression by the whistler echoes as the suppressed noise amplitude reaches a minimum and then recovers; and recovery of the noise band to the pre‐event level. Event duration was found to increase with the duration of the whistler echo train. Some data suggest that this phenomenon may be a natural analog of the previously reported quiet band effect in which signals from the VLF transmitter at Siple Station were observed to attenuate portions of noise bands.

Rare ground‐based observations of Siple VLF transmitter signals outside the plasmapause

Signals from the Siple, Antarctica (L ∼4.3), VLF transmitter observed at the conjugate ground station Roberval, Canada, have previously been found to propagate either within the outer plasmasphere or within the region of steep plasmapause density gradients, but on paths with equatorial electron density that is within a factor of 2 of nearby plasmaspheric levels. We report here two cases in which propagation occurred just outside the plasmapause and at plasma trough density levels. In one case a series of 1‐s pulses and frequency ramps was observed; many of the pulses triggered risers with slopes of ∼10 kHz/s, much steeper than those usually observed within the plasmasphere. In the other case, no evidence of triggered emissions was seen on the Siple pulses, but instead efficient echoing occurred and noise band precursors to large wave bursts were initiated. The wave bursts were of a type that has been previously identified as driving transient bursts of electron precipitation into the ionosphere. Both cases occurred under magnetic conditions more disturbed than those typical of strong Siple signal propagation in the outer plasmasphere.

A study of whistlers correlated with bursts of electron precipitation near L = 2

The previously reported Trimpi effect involves observation of amplitude perturbations in subionospherically propagating fixed‐frequency VLF signals. The perturbations are coincident in time with magnetospherically propagating whistlers, and it has been inferred that the whistlers drive energetic electron precipitation of E > 100 keV in a manner such that a localized increase in ionization occurs near 80‐km altitude in the nighttime earth‐ionosphere waveguide. In the present work, VLF broadband, analog chart and DF data from Palmer Station, Antarctica (L ∼ 2.3), recorded in 1978 and 1979 are used to investigate additional features of the Trimpi process. Attention was concentrated on perturbations of the signals NSS (21.4 kHz) and NLK (18.6 kHz) that have arrival bearings at Palmer of 352° and 318°, respectively. In September–October 1978, amplitude perturbations on NSS and NLK at Palmer were as frequent as similar events previously reported from a station at L = 4. A large equinoctial peak in activity was found, as in the previous study. The perturbations on NSS and NLK were usually increases in the 10%–30% range; decreases on NSS were observed on one unusually disturbed day when the plasmapause was equatorward of Palmer. Case studies were performed on five days of high activity. Essentially, all of the 93 events involved exhibited a close time correlation between amplitude perturbation and received whistler. In some cases only a fraction of the observed whistlers appeared to correlate with fixed‐frequency signal changes. In one such case, correlated whistlers were found through DF analysis and frequency‐time pattern comparisons to contain a component not present in most other whistlers. Above a field strength of 13 µV/m, the amplitude of this component was found to be linearly related to NSS perturbation amplitude, while for cases of less than 13 µV/m, no NSS change was detected. It is concluded that the southern hemisphere region within which perturbations occurred for the five case studies extends 200 km east and west of Palmer and ∼400 km northward over the L range 2.4 to 2.1 (at 100‐km altitude). Within this area it is estimated that individual ionospheric perturbations were of order 100 km in the east‐west direction and that the perturbed regions were centered within ∼200 km of the affected great circle paths.

An auroral precipitation model for the rapid X-ray burster

It is suggested that rotating magnetospheres can be separated into two catgories according to their rotation speeds. Fast rotators such as pulsars, and perhaps Jupiter, are dominated by gravitational and centrifugal effects and have flattened, distended magnetopheres. The dominant forces in slowly rotating magnetospheres: of which the Earth is an example: are gravitational, electric, and magnetic. Because the outer regions of slowly rotating magnetospheres are subject mainly to electric and magnetic forces, magnetospheric substorms and other types of unstable behavior are expected to occur frequently. It is proposed that the rapid X-ray burster may be a slow rotator, whose magnetospheric activity should resemble the Earth's. Moreover, if all X-ray bursters were slow rotators there should be few X-ray bursters that are also pulsars: in accord with the observational data. The proposed relaxation oscillator that turns the rapid bursters on-and-off has a terrestrial analog in pulsating aurorae. Repeated bursts of particle precipitation ( e.g., pulsating aurorae) from a magnetospheric trapping region are expected when particle injection in a substorm results in saturation of the trapping region with more matter than it can contain in equilibrium. Saturation leads to rapid growth of plasma waves, which are turned off again when the resonantmore » particles are removed by precipitation. If there is a continual source of ''accreting'' matter of sufficient strength, the wave growth and precipitation episodes repeat semiregular intervals, leading to ''pulsations'' or ''bursts'' of radiation when the precipitating particles are stopped in the atmosphere. The predicted repitition periods for the rapid burster: in the range 8 to 24 s: are within the uncertainties of the calculation. It is argued that a saturation-resonant interaction-wave growth-precipitation model fits the observations better than models based on hydromagnetic instabilities.« less

Observations of inverted-V electron precipitation

The characteristics of inverted-V electron precipitation fluxes deduced predominantly from observations by the Atmosphere Explorer satellites are reviewed. The energy and pitch angle distributions are presented and shown to be generally in agreement with acceleration by a parallel electrostatic potential. Characteristics of secondary electrons are examined, and effects of beam plasma instabilities on these electrons are discussed. The properties of the monoenergetic component are compared with theoretical models of creating parallel DC electric fields, and found to favor the anomalous resistivity model. The article also discusses relations of inverted-V events with other auroral phenomena including auroras, electrostatic shocks, convective electric field reversals, field-aligned currents and wave emissions. The principal conclusions are: (1) plasma sheet electrons are continuously accelerated to form inverted-V structures in the pre-midnight hemisphere independent of substorm phase, (2) the acceleration processes are probably related to large scale electrostatic wave turbulence observed at altitudes of a few thousand kilometers, (3) narrow bursts of intense electron precipitation fluxes are found to be imbedded within some inverted-V's. It is argued that the narrow bursts of intense electron precipitation have the proper characteristics to cause discrete auroral arcs in the atmosphere. We suggest that these narrow bursts are accelerated by an electrostatic shock at higher altitude and capable of producing discrete auroral arcs below the observing satellite.

Models for quasi‐periodic electric fields and associated electron precipitation in the auroral zone

On four rocket flights, quasi‐periodic electric fields were observed in the nighttime auroral oval and in the polar cleft. These fields had periods of between 0.5 and 3 s and amplitudes from 2 to 30 mV/m and exhibited left‐hand and right‐hand elliptical polarization. Electric field intensifications often coincided with bursts of energetic electron precipitation, on occasion modulated with nearly the same period as the fields. The events in the auroral oval were associated with substorms and visual auroral activity. The spectral and polarization properties of the observed fields suggest that they represented the electric components of Pc 1 or Pi 1 micropulsations. To model these waves and the concurrent electron flux variations, different mechanisms for wave excitation, electron acceleration, and wave‐particle interaction are considered. One likely interpretation attributes the micropulsations to Birkeland current chopping by an unstable double layer located at ≃1‐RE altitude. The double layer is also assumed to accelerate the observed electrons, the electron flux variations being due either to the inherent variations of the double layer or to its interaction with the micropulsations. Because of the sparcity of parameters measured so far the models are provisional. Follow‐up experiments are proposed to investigate the problem further.

Whistlers and VLF noises propagating just outside the plasmapause

Ground‐based recordings of broad‐band whistler mode signals propagating along the outer ‘surface’ of the plasmasphere, just beyond the region of steep plasmapause density gradients, exhibit a number of features that have not been observed in other field line regions. The features include extension of signal frequencies into the range ≃0.5–0.8 fHeq, where fHeq is the equatorial electron gyrofrequency of the path, and echoing or repeated propagation over the path at frequencies above 0.5 fHeq. The effects are frequently exhibited by tne ‘knee trace,’ the whistler component propagating just beyond the plasma‐pause in the low‐density or plasma trough region.The unusual features include VLF noise bands and bursts that are frequently triggered or ‘activated’ by knee whistler traces or echoes of knee traces. The noise bands and bursts tend to occur within the frequency range 0.4–0.8 fHeq and near the frequency of an amplitude peak in knee whistler trace activity. The wave activity is documented in VLF data from Eights, Antarctica (L≃4), for 1963 and 1965 and from Siple, Antarctica (L≃4.2), for 1973 through 1975. Certain features of the whistler and noise activity and of equatorial electron densities deduced from whistlers support earlier findings that the plasmapause equatorial radius may on occasion be irregular, varying by up to 1 RE within the longitudinal viewing range (≃30°) of a whistler receiver. Propagation effects such as guidance along the outer plasmapause surface and coupling of wave energy into and out of the ionosphere are not yet well understood. The VLF noise effects are of a kind recently found to be associated with detectable bursts of electron precipitation into the nighttime lower ionosphere. Because of the special propagation and warm plasma effects associated with the plasmapause, that region appears to offer advantages for experiments on magnetospheric wave injection both from the ground and from satellites.

Morphology and fine time structure of an early-morning electron precipitation event

Simultaneous balloon recordings of auroral-zone X-rays from precipitating electrons, covering a range of L-values from ≈5 to ≈7.5, are presented. The precipitation event was observed in the early morning sector (from about 0200 to 0500 local magnetic time), and was associated with a negative magnetic bay. Before the bay, precipitation associated with the growth phase of the substorm was observed at high L-values. After bay onset, precipitation was observed over the whole range of L-values covered, but with a delayed onset in the southern part of the precipitation region as compared with the onset of cosmic noise absorption in the local midnight sector. At high L-values the X-ray flux was completely unstructured and drizzle-like, both before and after bay onset. At low L-values, where precipitation occurred only after bay onset, the event was splash-like with X-ray bursts of typically 4–6 sec duration apparently rising out of the cosmic-ray background. The precipitation bursts had spatial extensions of 300–400 km. They were accompanied by weak magnetic impulses which were, both temporally and spatially, closely related to the X-ray bursts. The unstructured precipitation at high L-values was apparently associated with and extending along the auroral electrojet, presumably representing freshly accelerated particles. The highly structured and burst-like precipitation to the south seems to have come from a cloud of electrons drifting out from the acceleration region, from which wave-particle instabilities or some other mechanism caused electrons to be precipitated.