Electron Precipitation Bursts Research Articles

Polar Mesosphere Summer Echoes and Possible Signatures of Pulsating Aurora Detected by the Meteor Radar

AbstractUsing data of the all‐sky interferometric meteor radar (SKiYMET, 36.9 MHz) operating in the Sodankylä Geophysical Observatory (67°22′N, 26°38′E, Finland) we found a specific type of polar mesosphere summer echo (PMSE), the power of which exhibits irregular variations at a frequency of the order of a few Hz. We classified such radar echoes as pulsating PMSE. These echoes were observed in late June‐July in the morning sector (4–12 MLT) during geomagnetic storms. They were received from a narrow range of altitudes near 82 km, which corresponds to the altitude of noctilucent clouds where ice particles of about 50‐nm radii exist. During pulsating PMSE, the SGO ionosonde showed an electron density of the order of 3 × 1011 m−3 around 82 km, and enhanced D‐region ionization was manifested in the cosmic noise absorption. We suggest that the power of PMSE is modulated by bursts of electron precipitation corresponding to the few‐Hz internal modulation of pulsating aurora. During a short precipitation burst of 50–100 keV electrons, additional electrons can attach to the ice particles due to the presence of hyperthermal electrons according to the hypothesis proposed by Rosenberg et al. (2012, https://doi.org/10.1016/j.jastp.2011.10.011). This leads to an increase of the power of PMSE. After the burst is ended, the ice particles are deionized with a characteristic time of about 0.2 s due to attractive interaction with ions.

A New Technique for Estimating the Lifetime of Bursts of Electron Precipitation From Sounding Rocket Measurements

AbstractAt 0735 UT on 13 December 2015, the Rocket Experiment for Neutral Upwelling‐2 experiment launched north toward the auroral cusp region from Andoya, Norway. The instrumented rocket included an electron spectrometer, photometers that measured the auroral redline and greenline, and an instrument that measured ionospheric thermal electron temperature. On the down leg, just south of Svalbard, the rocket entered a region of poleward moving auroral forms that were characterized by narrow structures due to a combination of spatial and temporal variations. A noticeable feature was that the redline to greenline brightness ratio was much smaller than expected. A model is developed that shows that these emissions can be used to estimate the lifetimes of bursty electron precipitation. This model is shown to be consistent with some poleward moving auroral form lifetimes being on the order of 100 ms. The correlation between the precipitation and temperature bursts suggest that some transport occurred.

Remote sensing and modeling of energetic electron precipitation into the lower ionosphere using VLF/LF radio waves and field aligned current data

Abstract. A model for the development of electron density height profiles based on space time distributed ionization sources and reaction rates in the lower ionosphere is described. Special attention is payed to the definition of an auroral oval distribution function for energetic electron energy input into the lower ionosphere based on a Maxwellian energy spectrum. The distribution function is controlled by an activity parameter which is defined proportional to radio signal amplitude disturbances of a VLF/LF transmitter. Adjusting the proportionality constant allows to model precipitation caused VLF/LF signal disturbances using radio wave propagation calculations and to scale the distribution function. Field aligned current (FAC) data from the new Swarm satellite mission are used to constrain the spatial extent of the distribution function. As an example electron precipitation bursts during a moderate substorm on the 12 April 2014 (midnight–dawn) are modeled along the subauroral propagation path from the NFR/TFK transmitter (37.5 kHz, Iceland) to a midlatitude site.

Characteristics of flux-time profiles, temporal evolution, and spatial distribution of radiation-belt electron precipitation bursts in the upper ionosphere before great and giant earthquakes

The analysis of energetic electron observations made by the DEMETER satellite reveals that radiation belt electron precipitation (RBEP) bursts are observed in general several (~1-6 days) before a large (M > 6.5) earthquake (EQ) in the presence of broad band (~1-20 kHz) VLF waves. The EBs show in general a relative peak-to-background flux increase usually < 100, they have a time duration of ~0.5 – 3 min, and their energy spectrum reach up to energies <~500 keV. The RBEP activity is observed as one, two or three EBs throughout a semi-orbit, depended on the magnetic field structure above the EQ epicenter. A statistical analysis has been made for earthquakes in Japan, which reveals a standard temporal variation of the number of EBs, which begins with an incremental rate several days before major earthquakes, and after a maximum, decreases so that the electron precipitation ceases above the epicenter. Some earthquake induced EBs were observed not only in the nightside ionosphere, but also in the dayside ionosphere.

Bursty precipitation driven by chorus waves

The electron precipitation bursts have been shown to be a major sink for the radiation belt relativistic electrons. As an underlying mechanism of such bursts, we propose particle scattering into the loss cone due to nonlinear resonance interaction between electrons and chorus. Stochastic heating due to the coupling leads to diffusion in pitch angle, and the rate of diffusion would be sufficient to account for the emptying of Earth's radiation belt over the time of the main phase of geomagnetic storms. The results obtained in the present paper account for a strong energy dependence in the electron precipitation event and the correlation between the energization and loss processes on macroscopic time scales, which is primarily attributed to the co-operative effects of the coupling. This mechanism of chorus scattering should produce pitch-angle distributions that are energy-dependent and butterfly-shaped. The calculated time scales and the total energy input to the atmosphere from precipitating relativistic electrons are in reasonable agreement with experimental data.

Relativistic electron scattering by electrostatic upper hybrid waves in the radiation belt

[ 1] Observations of particle precipitation into the Earth's atmosphere performed on low-altitude satellites have excited substantial interest in relativistic electron precipitation bursts from the outer zone of the radiation belt. As underlying mechanism of such bursts, we suggest particle scattering into the loss cone due to higher-order cyclotron resonance interaction between relativistic electrons and intense narrow-band upper hybrid waves, which are frequently observed outside the plasmapause. The case of a single wave and the case of a wide wave number spectrum are considered, and approximate expressions for the relativistic particle diffusion coefficients in phase space are calculated for conditions of an inhomogeneous plasma. It is found that relativistic electrons have a preference over lower-energy electrons from the viewpoint of the number of cyclotron resonances that a particle crosses during each bounce period. Owing to the predominantly longitudinal direction of the upper hybrid wave group velocity, the resonant wave-particle interaction can take place over many electron bounce periods, which facilitates the particle scattering into the loss cone. The theory developed in the present paper accounts for two main features of the relativistic electron precipitation bursts, namely, a strong energy dependence in the electron precipitation process and the small-scale burst structure, which is primarily attributed to the localization and strong inhomogeneity of the growth region of the upper hybrid modes responsible for the scattering. The existence of intrinsic temporal structure of the precipitation on timescales comparable to the bounce period is also consistent with the theory due to the high efficiency of the scattering process.

Effective recombination coefficient in the lower ionosphere during bursts of auroral electrons

Sudden ionospheric electron density increases, associated with impulsive electron precipitation bursts and followed by decays, are observed by the EISCAT incoherent scatter radar. The effective recombination coefficient and the ratio of the NO + and O 2 + ion concentrations are estimated from the variations by use of the Sodankylä Ion Chemistry model. Recombination coefficients obtained previously from the same data by a simpler approach are compared with the present results.

Relativistic electron precipitation enhancements near the outer edge of the radiation belt

We examined characteristics of relativistic electron precipitation bursts observed by the Heavy Ion Large Telescope (HILT) experiment onboard the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite. We report on relatively narrow, persistent, latitudinal bands of precipitation with time scales of 10∼30 sec near the outer edge of the radiation belt: these develop and decay with a time scale of a few hours. Acceleration processes more effective than the usual radial diffusion process or scattering process would be needed to explain this strong precipitation band phenomenon. Another prominent signature is microbursts with a time scale down to a few hundred milliseconds. It is suggest that these microbursts are due to wave‐particle interaction involving a relaxation‐oscillator type of mechanism.

Cusp/cleft auroral activity in relation to solar wind dynamic pressure, interplanetary magnetic field Bz and By

Continuous optical observations of cusp/cleft auroral activities within ≈ 09‐15 MLT and 70‐76° magnetic latitude are studied in relation to changes in solar wind dynamic pressure and interplanetary magnetic field (IMF) variability. The observed latitudinal movements of the cusp/cleft aurora in response to IMF Bz changes may be explained as an effect of a variable magnetic field intensity in the outer dayside magnetosphere associated with the changing intensity of region 1 field‐aligned currents and associated closure currents. Ground magnetic signatures related to such currents were observed in the present case (January 10, 1993). Strong, isolated enhancements in solar wind dynamic pressure (Δp/p ≥ 0.5) gave rise to equatorward shifts of the cusp/cleft aurora, characteristic auroral transients, and distinct ground magnetic signatures of enhanced convection at cleft latitudes. A sequence of auroral events of ≈ 5‐10 min recurrence time, moving eastward along the poleward boundary of the persistent cusp/cleft aurora in the ≈ 10‐14 MLT sector, during negative IMF Bz and By, conditions, were found to be correlated with brief pulses in solar wind dynamic pressure (0.1 &lt; Δp/p &lt; 0.5). Simultaneous photometer observations from Ny Ålesund, Svalbard, and Danmarkshavn, Greenland, show that the events often appeared on the prenoon side (≈ 10‐12 MLT), before moving into the postnoon sector in the case we study here, when IMF By &lt; 0. In other cases, similar auroral event sequences have been observed to move westward in the prenoon sector, during intervals of positive By. Thus a strong prenoon/postnoon asymmetry of event occurence and motion pattern related to the IMF By polarity is observed. We find that this category of auroral event sequence is stimulated bursts of electron precipitation that originate from magnetosheath plasma that has accessed the dayside magnetosphere in the noon or near‐noon sector, possibly at high latitudes, partly governed by the IMF orientation as well as by solar wind dynamic pressure pulses.

Simultaneous measurements of waves and precipitating electrons near the equator in the outer radiation belt

An investigation of wave‐particle interactions is made using several simultaneous electron and wave measurements performed at near‐equatorial positions from the CRRES satellite. Bursts of electron precipitation were observed, most frequently at local times near dawn. Examples of bursts are presented in which the fluxes of the precipitating electrons and the wave intensities are correlated with coefficients as high as 0.7. During bursts the frequencies of the enhanced waves spanned a wide range from 311 Hz to 3.11 kHz, and the energies of the enhanced electrons were in the range 1.7 keV to 288 keV. The changes of the precipitating fluxes were generally less pronounced at the lowest energies. On the basis of electron‐cyclotron resonant calculations using the cold plasma densities and ambient magnetic fields taken from the CRRES measurements it was found that the wave frequencies and precipitating electron energies were generally consistent with those expected from electron resonance with parallel propagating whistler waves. The electron data of principal concern here were acquired in and about the loss cone with narrow angular resolution spectrometers covering the energy range 340 eV to 5 MeV. The wave data included electric field measurements spanning frequencies from 5 Hz to 400 kHz and magnetic field measurements from 5 Hz to 10 kHz.

Relaxation of transient lower ionospheric disturbances caused by lightning‐whistler‐induced electron precipitation bursts

A quantitative model of the relaxation of transient lower ionospheric (D region) disturbances caused by lightning‐induced electron precipitation is developed, taking advantage of known particular features of the lightning‐induced disturbances, such as the fact that they are produced in typically &lt;1 s and decay over 10–100 s. The model represents the nighttime D region as consisting of only four kinds of charged particles (electrons, positive ions, negative ions, and positive cluster ions) and is particularly suited for description of the detailed behavior of the electron density. Application of the model to some previously modeled disturbances indicates that some of the least known chemical reaction rates in the nighttime D region altitudes may be measurable using subionospheric VLF data. In the production of secondary ionization by precipitating electron bursts, the model calculations indicate the presence of a saturation effect such that the number density of the secondary electrons is not simply equal to the ion pair production rate times the burst duration. In some cases involving precipitation of ∼1‐MeV electrons, the model predicts the formation of new layers of ionization at 50–70 km altitude that represent a different attachment‐detachment quasi‐equilibrium value from that of the unperturbed ambient. Such new layers may exist for up to ∼105 s following electron precipitation bursts.

Characteristics of short‐duration electron precipitation bursts and their relationship with VLF wave activity

Energetic (&gt;6 keV) electron data from the SEEP payload on the low altitude (∼200 km) polar orbiting S81‐1 satellite indicate a high rate of occurrence of short duration (&lt;0.6 s) electron precipitation bursts. Characteristics of events observed at night (2230 MLT) versus daytime (1030 MLT) and at midlatitudes (2&lt;L&lt;3) versus higher latitudes (L&gt;3) were distinctly different in several ways. For 2&lt;L&lt;3 the daytime bursts occurred approximately uniformly in longitude and were equally distributed between the northern and southern hemispheres. The nighttime bursts in the same L shell range occurred approximately twice as often on a worldwide basis and were observed predominantly in the northern hemisphere and at longitudes of 260°E to 320°E. In a significant number of the nighttime events at 2&lt;L&lt;3 the median electron energy increased with time during the burst, but most of the other spectra showed no well‐defined trend. During some of the nighttime bursts broad peaks were observed in the energy spectra, but these peaks were not so evident in the daytime bursts. On higher L shells, L&gt;3, narrow electron precipitation bursts (&lt;0.3 s duration) were frequently observed poleward of the plasmapause, more often near noon than near midnight and much more frequently than at 2&lt;L&lt;3. Comparison of the electron data with simultaneous VLF wave data from Palmer (L≃2.4) and Siple (L≃4.3) stations in Antarctica indicated a varying degree of association of electron bursts with whistlers and chorus emissions. Several of the electron bursts observed at nighttime and at 2&lt;L&lt;3 were correlated with lightning‐generated whistlers observed at Palmer Station. When daytime bursts at higher latitudes (L&gt;3) were observed on satellite passes within ±50° of the Siple meridian, chorus was invariably detected at Siple, but correlation of electron bursts with individual chorus spectral elements was not evident. The lack of such correlation may be due to the limited spatial extent of flux tubes excited by individual chorus elements which are possibly generated without mutual coherence in multiple high altitude magnetospheric locations. Within one hour of all four of the satellite passes within 400 km of Siple with the highest rate (≥10 individual bursts per pass) of burst occurrence, overhead electron precipitation was clearly detected by ground based sensors at Siple Station. Quasi‐periodic bursts of several seconds duration were observed on one or more of the photometer, riometer, and magnetic pulsation sensors and many of these bursts were correlated with clustered VLF chorus bursts. Hence, there is at least association of chorus with electron precipitation.

Electron precipitation bursts in the nighttime slot region measured simultaneously from two satellites

Based on data acquired in 1982 with the Stimulated Emission of Energetic Particles payload on the low‐altitude (170‐280 km) S81‐1 spacecraft and the Space Environment Monitor instrumentation on the NOAA 6 satellite (800–830 km), a study has been made of short‐duration nighttime electron precipitation bursts at L = 2.0–3.5. From 54 passes of each satellite across the slot region simultaneously in time, 21 bursts were observed on the NOAA 6 spacecraft, and 76 on the S81‐1 satellite. Five events, probably associated with lightning, were observed simultaneously from the two spacecraft within 1.2 s, providing a measure of the spatial extent of the bursts. This limited sample indicates that the intensity of precipitation events falls off with width in longitude and L shell but individual events extend as much as 5° in invariant latitude and 43° in longitude. The number of events above a given flux observed in each satellite was found to be approximately inversely proportional to the flux. The time average energy input to the atmosphere over the longitude range 180°E to 360°E at a local time of 2230 directly from short‐duration bursts spanning a wide range of intensity enhancements was estimated to be about 6 × 10−6 ergs/cm² s in the northern hemisphere and about 1.5 × 10−5 ergs/cm² s in the southern hemisphere. In the south, this energy precipitation rate is lower than that from electrons in the drift loss cone by about 2 orders of magnitude. However, on the basis of these data alone we cannot discount weak bursts from being a major contributor to populating the drift loss cone with electrons which ultimately precipitate into the atmosphere.

Direct observation of magnetospheric electron precipitation stimulated by lightning

Plasmaspheric electron precipitation bursts stimulated by observed lightning flashes have been studied using in situ rocket techniques under night-time conditions at Wallops Island, Virginia, on 23 August 1984. In one case, the rocket-observed ligntning flash was located by a ground-based network to be off the coast of Virginia at 74.8°W, 35.9°N, which was approximately 200km southeast of the rocket. Electron precipitation (> 40 keV) caused by simultaneous 21.4 kHz coded transmissions from a VLF radio transmitter at Annapolis, Maryland, was found to be negligible compared with the observed fluxes and energies of precipitating electrons stimulated by lightning. The results confirm that lightning (which can occur up to 100 times per second globally) can activate ionizing radiation sources within the magnetosphere, and that these may affect local middle atmospheric electrical structure in a measurable way.

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 &gt;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.

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 &gt; 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.

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.