Diffuse Electron Precipitation Research Articles

Возмущения ионосферного радиоканала во время магнитных бурь в ноябре–декабре 2023 г.

This paper presents the results of analysis of oblique ionospheric sounding data obtained with continuous chirp signal on the subauroral paths Magadan—Irkutsk and Norilsk—Irkutsk. It specifies the interplanetary sources of magnetic storms in November–December 2023. It was established that signals propagating outside the great-circle arc and additional diffuse reflections can be found in oblique sounding ionograms in intense magnetospheric convection field. Their appearance can be related to refraction of radio waves on the polar wall of the main ionospheric trough and scattering by small-scale inhomogeneities. Connection has been revealed between variations in the maximum observed frequencies of HF radio wave propagation modes with the spatial position of the main ionospheric trough and the equatorial boundary of diffuse electron precipitation zone.

Disturbances of ionospheric radio channel during magnetic storms in November–December 2023

This paper presents the results of analysis of oblique ionospheric sounding data obtained with continuous chirp signal on the subauroral paths Magadan—Irkutsk and Norilsk—Irkutsk. It specifies the interplanetary sources of magnetic storms in November–December 2023. It was established that signals propagating outside the great-circle arc and additional diffuse reflections can be found in oblique sounding ionograms in intense magnetospheric convection field. Their appearance can be related to refraction of radio waves on the polar wall of the main ionospheric trough and scattering by small-scale inhomogeneities. Connection has been revealed between variations in the maximum observed frequencies of HF radio wave propagation modes with the spatial position of the main ionospheric trough and the equatorial boundary of diffuse electron precipitation zone.

Ionospheric Conductances Due To Electron and Ion Precipitations: A Comparison Between EISCAT and DMSP Estimates

AbstractEnergetic particle precipitation is the major source of electron production that controls the ionospheric Pedersen and Hall conductances at high latitudes. Many studies use empirical formulas to estimate conductances. The particle precipitation spectra measured by the Defense Meteorological Satellite Program (DMSP) Special Sensor J are often used as the input to the empirical formulas. In this study, we evaluate the empirical formulas of ionospheric conductances during four different types of auroral precipitation conditions based on 63 conjugate events observed by DMSP and EISCAT. The conductances calculated from the DMSP data with the empirical formulas are compared with those based on EISCAT measurements with the standard equations. The best correlation between these two is found when the empirical Robinson formulas (Robinson et al., 1987, https://doi.org/10.1029/ja092ia03p02565) are used in the presence of diffuse electron precipitation without ions. In the presence of ion precipitation, the correlation coefficients are smaller, but the correlation improves when the Galand formulas (Galand & Richmond, 2001, https://doi.org/10.1029/1999ja002001) are used to estimate the contribution of ion precipitation to the conductances. We also found that pure ion precipitation can cause the increase of conductances up to 2–7 S for Pedersen and 2.5–10 S for Hall conductances, which is positively correlated with the auroral electrojet index. Overall, the empirical formulas applied to the DMSP particle spectra underestimate the ionospheric conductances.

On the Importance of Using Event‐Specific Wave Diffusion Rates in Modeling Diffuse Electron Precipitation

AbstractA few to tens of keV electron precipitation that carries substantial energy source down to the upper atmosphere to create aurora is manifested as an important magnetosphere‐ionosphere coupling process. The precipitation is usually caused by scattering processes associated with plasma waves in the magnetosphere. The scattering process is often quantified by wave diffusion rates that indicate how fast an electron is scattered. Global models commonly use diffusion coefficients that are derived from statistical wave models. However, due to the statistical nature, many localized, transient features could be smeared out. In this study, we investigate electron precipitation using event‐specific diffusion coefficients that are obtained based on simultaneous in‐situ measured/inferred, rather than statistical, chorus wave dynamics. We find that the application of the event‐specific diffusion coefficients associated with a more dynamic and intense chorus wave model leads more electrons, particularly at several to tens of keV in the dawn‐to‐noon sector at L > 3, to precipitate than using statistical coefficients. The new simulation roughly captures both the intensity and variability of the precipitating flux as detected by the NOAA/POES satellites. Ionospheric electron density in the lower E region (100–120 km) observed by the mid‐latitude Millstone Hill radar is also much better reproduced, while the case using statistical diffusion coefficients underestimates the ionization rate. This study implies the importance of using event‐specific diffusion rates in simulating the diffuse electron precipitation and understanding the magnetosphere‐ionosphere coupling.

The Role of Diffuse Electron Precipitation in the Formation of Subauroral Polarization Streams

AbstractThe role of diffuse electron precipitation in the formation of subauroral polarization streams (SAPS) is investigated with the Multiscale Atmosphere‐Geospace Environment (MAGE) model. Diffuse precipitation is derived from the distribution of drifting electrons. SAPS manifest themselves as a separate mesoscale flow channel in the duskside ionosphere, which gradually merges with the primary auroral convection toward dayside as the equatorward auroral boundary approaches the poleward Region‐2 field‐aligned currents (FACs) boundary. SAPS expand to lower latitudes and toward the nightside during the main phase of a geomagnetic storm, associated with magnetotail earthward plasma flows building up the ring current and intensifying Region‐2 FACs and electron precipitation. SAPS shrink poleward and sunward as the interplanetary magnetic field turns northward. When diffuse precipitation is turned off in a controlled MAGE simulation, ring current and duskside Region‐2 FACs become weaker, but subauroral zonal ion drifts are still comparable to auroral convection. However, subauroral and auroral convection manifest as a single broad flow channel without showing any mesoscale structure. SAPS overlap with the downward Region‐2 FACs equatorward of diffuse precipitation, where poleward electric fields are strong due to a low conductance in the subauroral ionosphere. The Region‐2 FACs extend to latitudes lower than the diffuse precipitation because the ring current protons penetrate closer to the Earth than the electrons do. This study reproduces the key physics of SAPS formation and their evolution in the coupled magnetosphere‐ionosphere during a geomagnetic storm. Diffuse electron precipitation is demonstrated to play a critical role in determining SAPS location and structure.

A new ionospheric electron precipitation module coupled with RAM‐SCB within the geospace general circulation model

AbstractElectron precipitation down to the atmosphere due to wave‐particle scattering in the magnetosphere contributes significantly to the auroral ionospheric conductivity. In order to obtain the auroral conductivity in global MHD models that are incapable of capturing kinetic physics in the magnetosphere, MHD parameters are often used to estimate electron precipitation flux for the conductivity calculation. Such an MHD approach, however, lacks self‐consistency in representing the magnetosphere‐ionosphere coupling processes. In this study we improve the coupling processes in global models with a more physical method. We calculate the physics‐based electron precipitation from the ring current and map it to the ionospheric altitude for solving the ionospheric electrodynamics. In particular, we use the BATS‐R‐US (Block Adaptive Tree Scheme‐Roe type‐Upstream) MHD model coupled with the kinetic ring current model RAM‐SCB (Ring current‐Atmosphere interaction Model with Self‐Consistent Magnetic field (B)) that solves pitch angle‐dependent electron distribution functions, to study the global circulation dynamics during the 25–26 January 2013 storm event. Since the electron precipitation loss is mostly governed by wave‐particle resonant scattering in the magnetosphere, we further investigate two loss methods of specifying electron precipitation loss associated with wave‐particle interactions: (1) using pitch angle diffusion coefficients Dαα(E,α) determined from the quasi‐linear theory, with wave spectral and plasma density obtained from statistical observations (named as “diffusion coefficient method”) and (2) using electron lifetimes τ(E) independent on pitch angles inferred from the above diffusion coefficients (named as “lifetime method”). We found that both loss methods demonstrate similar temporal evolution of the trapped ring current electrons, indicating that the impact of using different kinds of loss rates is small on the trapped electron population. However, for the precipitated electrons, the lifetime method hardly captures any precipitation in the large L shell (i.e., 4 < L < 6.5) region, while the diffusion coefficient method produces much better agreement with NOAA/POES measurements, including the spatial distribution and temporal evolution of electron precipitation in the region from the premidnight through the dawn to the dayside. Further comparisons of the precipitation energy flux to DMSP observations indicates that the new physics‐based precipitation approach using diffusion coefficients for the ring current electron loss can explain the diffuse electron precipitation in the dawn sector, such as the enhanced precipitation flux at auroral latitudes and flux drop near the subauroral latitudes, but the traditional MHD approach largely overestimates the precipitation flux at lower latitudes.

Statistical relationship between large‐scale upward field‐aligned currents and electron precipitation

AbstractSimultaneous observations of Birkeland currents by the constellation of Iridium satellites and N2 Lyman‐Birge‐Hopfield (LBH) auroral emissions measured by the Global Ultraviolet Imager (GUVI) onboard the Thermosphere, Ionosphere, and Mesosphere Energetics and Dynamics (TIMED) satellite are used to establish relationships between large‐scale upward field‐aligned currents and electron precipitation during stable current configurations. The electron precipitation was inferred from GUVI data using a statistical relationship between LBH intensity and electron energy flux. LBH emissions with >5% contribution from protons, identified by Lyman‐alpha intensity, were excluded from the analysis. The Birkeland currents were derived with a spatial resolution of 3° in latitude and 2 h in local time. For southward interplanetary magnetic field (IMF), the electron precipitation occurred primarily within and near large‐scale upward currents. The correspondence was less evident for northward IMF, presumably because the spatial variability is large compared to the areas of interest so that the number of events identified is smaller and the derived statistical distributions are less reliable. At dusk, the correlation between upward current and precipitation was especially high, where a larger fraction of the electron precipitation is accelerated downward by a field‐aligned potential difference. Unaccelerated electron precipitation dominated in the morning sector, presumably induced by scattering of eastward‐drifting energetic electrons into the loss cone through interaction with whistler‐mode waves (diffuse precipitation) rather than by field‐aligned acceleration. In the upward Region 1 on the dayside, where the electron precipitation is almost exclusively due to field‐aligned acceleration, a quadratic relationship between current density and electron energy flux was observed, implying a linear current‐voltage relationship in this region. Current density and electron energy flux in the regions of the large‐scale upward currents from pre‐midnight through dawn to noon are essentially uncorrelated, consistent with diffuse electron precipitation dominating the incident energy flux.

Magnetosphere‐ionosphere energy interchange in the electron diffuse aurora

The diffuse aurora has recently been shown to be a major contributor of energy flux into the Earth's ionosphere. Therefore, a comprehensive theoretical analysis is required to understand its role in energy redistribution in the coupled ionosphere‐magnetosphere system. In previous theoretical descriptions of precipitated magnetospheric electrons (E∼ 1 keV), the major focus has been the ionization and excitation rates of the neutral atmosphere and the energy deposition rate to thermal ionospheric electrons. However, these precipitating electrons will also produce secondary electrons via impact ionization of the neutral atmosphere. This paper presents the solution of the Boltzman‐Landau kinetic equation that uniformly describes the entire electron distribution function in the diffuse aurora, including the affiliated production of secondary electrons (E< 600 eV) and their ionosphere‐magnetosphere coupling processes. In this article, we discuss for the first time how diffuse electron precipitation into the atmosphere and the associated secondary electron production participate in ionosphere‐magnetosphere energy redistribution.

Auroral electrojets and boundaries of plasma domains in the magnetosphere during magnetically disturbed intervals

Abstract. We investigate variations in the location and intensity of the auroral electrojets during magnetic storms and substorms using a numerical method for estimating the equivalent ionospheric currents based on data from meridian chains of magnetic observatories. Special attention was paid to the complex structure of the electrojets and their interrelationship with diffuse and discrete particle precipitation and field-aligned currents in the dusk sector. During magnetospheric substorms the eastward electrojet (EE) location in the evening sector changes with local time from cusp latitudes (Φ~77°) during early afternoon to latitudes of diffuse auroral precipitation (Φ~65°) equatorward of the auroral oval before midnight. During the main phase of an intense magnetic storm the eastward currents in the noon-early evening sector adjoin to the cusp at Φ~65° and in the pre-midnight sector are located at subauroral latitude Φ~57°. The westward electrojet (WE) is located along the auroral oval from evening through night to the morning sector and adjoins to the polar electrojet (PE) located at cusp latitudes in the dayside sector. The integrated values of the eastward (westward) equivalent ionospheric current during the intense substorm are ~0.5 MA (~1.5 MA), whereas they are 0.7 MA (3.0 MA) during the storm main phase maximum. The latitudes of auroral particle precipitation in the dusk sector are identical with those of both electrojets. The EE in the evening sector is accompanied by particle precipitation mainly from the Alfvén layer but also from the near-Earth part of the central plasma sheet. In the lower-latitude part of the EE the field-aligned currents (FACs) flow into the ionosphere (Region 2 FAC), and at its higher-latitude part the FACs flow out of the ionosphere (Region 1 FAC). During intense disturbances, in addition to the Region 2 FAC and the Region 1 FAC, a Region 3 FAC with the downward current was identified. This FAC is accompanied by diffuse electron precipitation from the plasma sheet boundary layer. Actually, the triple system of FAC is observed in the evening sector and, as a consequence, the WE and the EE overlap. The WE in the evening sector comprises only the high-latitude periphery of the plasma precipitation region and corresponds to the Hall current between the Region 1 FAC and Region 3 FAC. During the September 1998 magnetic storm, two velocity bursts (~2–4 km/s) in the magnetospheric convection were observed at the latitudes of particle precipitation from the central plasma sheet and at subauroral latitudes near the ionospheric trough. These kind of bursts are known as subauroral polarization streams (SAPS). In the evening sector the Alfvén layer equatorial boundary for precipitating ions is located more equatorward than that for electrons. This may favour northward electric field generation between these boundaries and may cause high speed westward ions drift visualized as SAPS. Meanwhile, high speed ion drifts cover a wider range of latitudes than the distance between the equatorward boundaries of ions and electrons precipitation. To summarize the results obtained a new scheme of 3-D currents in the magnetosphere-ionosphere system and a clarified view of interrelated 3-D currents and magnetospheric plasma domains are proposed.

Evidence of near‐Earth breakup location

We report a detailed study of an isolated substorm onset with the FAST spacecraft crossing the equatorward = most auroral arc just when/where it starts to break up. Comprehensive ground (optical, riometer and magnetometer) data combined with the space borne field and particle high resolution observations allowed to infer the following properties of the breakup onset region in this well‐documented event: (1) The arc flux tube stays in the region of considerable plasma pressure gradient wherethe pressure values are close to ∼1–2 nPa. The arc was located (2) just 0.4° poleward of the proton isotropic (b2i) boundary (which roughly gives ∼40 nT estimate for the equatorial magnetic field) and (3) close to the peak of the diffuse electron precipitation. (4) Tsyganenko 96 model for these particular solar wind conditions (which correspond well to both the observed b2i boundary and estimated plasma pressure) maps the arc to the equatorial distance of ∼8RE.All these facts in mutual agreement evidence that the auroral breakup in this particular case was launched in the near‐Earth domain of the magnetotail at r ∼8RE.

Low‐altitude observations of the evolution of substorm injection boundaries

We have analyzed the properties of the evolution of the ion and electron precipitation and the magnetic field perturbations in the auroral oval during substorms using the low‐altitude polar‐orbiting satellites DMSP F6 and F7. We show two examples where the magnetosphere‐ionosphere coupling, based on the character of the particle precipitation and field‐aligned current regions, exhibits different substorm responses. The main difference in the coupling response is reflected in the intensification of the ion keV plasma sheet precipitation flux. In one example (November 29, 1984) the intensification is confined to a narrow latitudinal region at the poleward edge of the region 2 current system. In the other example (November 1, 1984) the intensification encompasses a much wider latitudinal span over which typical estimated ion thermal energies are 15 keV and where there is a proliferation of discrete electron precipitation with peak energies of 500 eV. During the recovery phase, that region develops a very uniform diffuse electron precipitation that can last for long periods (>1 hour) subjected to electrostatic potential drops of ≈450 eV. There are, however, characteristics common to both examples. For instance, the energy distribution in the equatorward edge of the midnight auroral oval ion precipitation is transformed a few minutes into the expansion phase from that expected qualitatively from a steady state earthward convection into an energy distribution where the flux of keV ions is dramatically reduced and only ions and electrons with energies below 1 keV are detected. This precipitation region is colocated with a system of region 2 polarity field‐aligned currents and has a well‐defined poleward boundary that defines the location where the average energy of the ion and electron precipitation starts to decrease monotonically with decreasing latitude. Also, during the expansion phase the intense diffuse electron precipitation that occurs poleward of the region 2 system coincides with strong region 1 field‐aligned currents. Inside this region, ion and electron flux depletion (≈2° wide) develops around the magnetic latitude where the enhancement of the westward electrojet is first detected. During expansion and recovery additional ion precipitation appears in the midnight and dusk passes equatorward of the electron equatorward precipitation boundary in the two cases presented here. The similarity between the ion spectra of this ion precipitation and the spectra of the plasma sheet suggests that portions of plasma sheet plasma become detached from the bulk of the plasma sheet population. The equatorward detachments are separated from the rest of the plasma sheet by a dispersionless boundary. The ion energy cutoff of the dusk detachments exhibits the dispersion in latitude characteristic of steady state earthward transport produced by an enhanced convection electric field. This suggests that the particle transport in the magnetosphere is disrupted at onset in the inner magnetosphere where region 2 currents are generated but continues unimpeded in the earthward region.

Extreme energetic particle decreases near geostationary orbit: A manifestation of current diversion within the inner plasma sheet

A qualitative model of magnetic field reconfiguration as might result from neutral line formation in the central plasma sheet late in a substorm growth phase is considered. It is suggested that magnetic reconnection probably begins before the substorm expansion phase and that cross‐tail current is enhanced across the plasma sheet both earthward and tailward of a limited region near the neutral line. Such an enhanced cross‐tail current earthward of the original X line region may contribute to thinning the plasma sheet substantially, and this would in turn affect the drift currents in that location, thus enhancing the current even closer toward the Earth. In this way a redistribution and progressive diversion of normal cross‐tail current throughout much of the inner portion of the plasma sheet could occur. The resulting intensified current, localized at the inner edge of the plasma sheet, would lead to a very thin plasma confinement region. This would explain the very taillike field and extreme particle dropouts often seen late in substorm growth phases. When the currents cannot be further enhanced within the inner plasma sheet (and when diffuse electron precipitation has sufficiently increased ionospheric conductivities), an intense (≳106 A), very rapid increase of current flow through the auroral ionosphere would be expected. This latter phase, in combination with magnetic reconnection reaching field lines of the plasma sheet boundary layer and the tail lobes, would constitute the substorm expansion phase in this scenario.

Extreme energetic particle decreases near geostationary orbit: A manifestation of current diversion within the inner plasma sheet

We consider a qualitative model of cross-tail current flow and suggest that when magnetic reconnection begins, the current effectively flows across the plasma sheet both earthward and tailward of the disruption region near the neutral line. We show that an enhanced cross-tail current earthward of this region would thin the plasma sheet substantially due to the magnetic pinch effect often studied in laboratory plasmas. This thinning would in turn affect the drift currents even closer toward the earth. In this way, a disruption and effective diversion of the cross-tail current throughout much of the inner portion of the plasma sheet could result and intense cross-tail current localized at the inner edge of the plasma sheet would lead to a very thin current-carrying region. This would explain the very taillike field and extreme particle dropouts often seen late in substorm growth phases. When the currents cannot be further enhanced within the inner plasma sheet (and when diffuse electron precipitation has sufficiently increased ionospheric conductivities) we expect an intense (≥10 6 A), very rapid increase of current flow through the auroral ionosphere. This latter phase would basically constitute the substorm expansion phase.

The absence of region 1/Region 2 field‐aligned currents during prolonged quiet times

Using magnetic field and precipitating particle data from the DMSP/F7 satellite, we have found that, during extended periods of geomagnetic quiet, the large‐scale field‐aligned current (FAC) system known as Region 1/Region 2 either does not exist or is too small to be observable. A clear dayside cleft FAC system is observed on the preferred side of local noon for the indicated by the IMF By component and the cleft electron precipitation extends far into the region normally considered to be the polar cap. These quiet periods represent times when energy input from the solar wind is low; the IMF is maintained at near zero or slightly northward levels and the solar wind speed is low. For such times, the Kp index is ≤ 1 for several consecutive 3 hour periods. In these cases, the auroral zone is marked by diffuse electron precipitation, and only small‐scale or fine‐scale FAC's. In the polar cap, electron precipitation is characterized by polar rain and there are no FAC systems (including the NBZ system). Some previous reports imply that one should always find this large‐scale Region 1/Region 2 system during quiet times. We suggest that the Region 1/Region 2 system vanishes 10's of mins to hrs after the IMF has turned northward.

Are equatorial electron cyclotron waves responsible for diffuse auroral electron precipitation?

On the basis of theoretical calculations of electron diffusion coefficients and of OGO 5 data, Lyons [1974] suggested that electrostatic electron cyclotron harmonic waves had amplitudes large enough to cause the strong pitch angle diffusion of plasma sheet keV electrons and to be responsible for diffuse auroral precipitation. However, recent measurements of the wave location and amplitude performed aboard the GEOS spacecraft have brought new pieces of information challenging these conclusions. Our calculations are based on the theoretical tool developed by Lyons [1974] but take into account the recently observed wave confinement within a few degrees from the magnetic equator. Under these conditions, we avoid the numerical averaging of the pitch angle diffusion coefficient over the whole line of force, and we can derive an analytical expression of the minimum wave amplitude required to cause strong pitch angle diffusion for plasma sheet electrons. This expression has the advantage to be easily tractable in further calculations, and permits us to evaluate the dependence of the results on parameters that are not reported by observations, such as the wave number spectrum. Our results are found to differ from Lyons's [1974] predictions by a factor of about 2.5, and typically, a wave amplitude of more than 2 mV m−1 is required to put 1‐keV electrons on strong diffusion. On the other hand, on the basis of a statistical analysis of electron cyclotron wave amplitudes measured in the nightside plasmasheet by the GEOS 2 spacecraft, we show that most of the time (>91 %), this typical value of 2 mV m−1 is not reached. This result appears inconsistent with the hypothesis that diffuse auroras, which are a permanent feature of the auroral zones, are due solely to electrostatic electron cyclotron waves. This leads us to the conclusion that these waves are not the only cause of diffuse electron precipitation. It is suggested that other mechanisms involving for instance the dynamics of the ions (such as field‐aligned currents) could play an important role.

A statistical study of the ‘instantaneous’ nightside auroral oval: The equatorward boundary of electron precipitation as observed by the Isis 1 and 2 satellites

Electron spectrograms from 351 passes of the Isis 1 and 2 satellites were utilized to study statistically the effects of the interplanetary magnetic field (IMF), substorm activity, and the earth's dipole tilt angle on the latitude of the equatorward boundary of the nightside (2000–0400 magnetic local time) ‘instantaneous’ auroral oval. The boundary location (in invariant latitude) of the instantaneous oval at hourly local time intervals was identified in terms of the equatorward boundary of the diffuse >100-eV electron precipitation. The following characteristics were noted: (1) The north-south component (Bz) of the IMF plays the dominant role in controlling the motion of this boundary. The invariant latitude of the boundary is shown to shift by approximately ±4° depending on the direction of the IMF (northward and southward, respectively) relative to its position corresponding to Bz = 0. This indicates an inward motion of the associated boundary in the magnetotail by about 5 earth radii when the IMF changes its direction from northward to southward with large magnitude. There is a significant difference in the amount of the shift between the evening and morning sectors; i.e., for the same decrease in Bz value the boundary moves more equatorward in the morning sector than in the evening sector. When the obtained oval particle boundary was projected onto the equatorial plane of the magnetotail along magnetic field lines, good agreement was found between the projected boundary and the drift boundary (the Alfven layer) of low-energy electrons in the presence of the dawn-dusk electric field. Thus this agreement gives new evidence showing that the diffuse electron precipitation that produces the diffuse aurora originates near and at the inner boundary of the plasma sheet. (2) Substorm activity seems to have a separate role in determining the latitude of the equatorward boundary of the nightside auroral precipitation region. The boundary during substorm periods is statistically found to be 2°–3° lower in invariant latitude than that during quiet times. Even a simple classification into quiet and disturbed conditions improves the accuracy with which the auroral oval location can be inferred. By combining the IMF effect and the substorm effect it is indicated that the boundary is located at the lowest latitudes when a substorm takes place during a southward IMF with large magnitude, whereas the boundary is located in the highest latitudes when the IMF has a northward component during quiet times. (3) The equatorward boundary of the nightside auroral oval is located at higher latitudes in the winter hemisphere than in the summer hemisphere, although this effect of the earth's dipole tilt is usually smaller than the effects of the IMF and substorm activity.