Characteristics Of Electron Precipitation Research Articles

Characteristics of Electron Precipitation Directly Driven by Poloidal ULF Waves

AbstractA mechanism recently proposed for magnetospheric electron loss into the atmosphere is the precipitation directly driven by ultralow‐frequency (ULF) waves. In this study, we quantitatively analyze the properties of ULF wave‐induced precipitation by simulating the electron bounce and drift motion in poloidal‐mode waves excited in a dipole magnetic field. Our results reveal that precipitation occurs only when electrons encounter a westward‐directed wave electric field in the magnetosphere, which leads to cross‐field energy enhancements and reduces their mirror heights. The simulations also demonstrate longer duration electron precipitation at the drift‐resonance energy. We calculate the temporal variations of the energy spectrum for precipitating electrons and the total precipitating energy fluxes. These results improve our understanding of ULF wave‐induced electron precipitation as well as provide a point of comparison for observations from balloons or ground‐based instruments.

The atmospheric X-ray imaging spectrometer (AXIS) instrument: Quantifying energetic particle precipitation through bremsstrahlung X-ray imaging.

The Atmospheric X-ray Imaging Spectrometer (AXIS) described in this work is a compact, wide field-of-view, hard x-ray imager. The AXIS instrument will fly onboard the Atmospheric Effects of Precipitation through Energetic X-rays (AEPEX) 6U CubeSat mission and will measure bremsstrahlung x-ray photons in the 50-240keV range with cadmium-zinc-telluride (CZT) detectors using coded aperture optics. AXIS will measure photons generated by energetic particle precipitation for the purpose of determining the spatial scales of precipitation and estimating electron precipitation characteristics. This paper describes the design and testing of the AXIS instrument, including a summary of simulations performed that motivate the shielding, optics, and mechanical design. Testing and characterization is reported that validates the instrument design and shows that the instrument design meets or exceeds the measurement requirements necessary for AEPEX mission success.

Electron precipitation characteristics during isolated, compound, and multi-night substorm events

Abstract. A set of 24 isolated, 46 compound, and 36 multi-night substorm events from the years 2008–2013 have been analysed in this study. Isolated substorm events are defined as single expansion–recovery phase pairs, compound substorms consist of multiple phase pairs, and multi-night substorm events refer to recurring substorm activity on consecutive nights. Approximately 200 nights of substorm activity observed over Fennoscandian Lapland have been analysed for their magnetic disturbance magnitude and the level of cosmic radio noise absorption. Substorm events were automatically detected from the local electrojet index data and visually categorized. We show that isolated substorms have limited lifetimes and spatial extents as compared to the other substorm types. The average intensity (both in absorption and ground-magnetic deflection) of compound and multi-night substorm events is similar. For multi-night substorm events, the first night is rarely associated with the strongest absorption. Instead, the high-energy electron population needed to cause the strongest absorption builds up over 1–2 additional nights of substorm activity. The non-linear relationship between the absorption and the magnetic deflection at high- and low-activity conditions is also discussed. We further collect in situ particle spectra for expansion and recovery phases to construct median precipitation fluxes at energies from 30 eV up to about 800 keV. In the expansion phases the bulk of the spectra show a local maximum flux in the range of a few keV to 10 keV, while in the recovery phases higher fluxes are seen in the range of tens of keV to hundreds of keV. These findings are discussed in the light of earlier observations of substorm precipitation and their atmospheric effects.

Characteristics of Electron Precipitation During 40 Energetic Electron Injections Inferred via Subionospheric VLF Signal Propagation

AbstractEnergetic electron injection events are associated with energetic electron precipitation (EEP) through possible resonant wave‐particle interactions. Previous studies confirm the impacts of injection‐driven precipitation on observed amplitude/phase of subionospheric VLF (very low frequency) signals transmitted from distant artificial transmitters. Currently, there are substantial uncertainties on precipitation characteristics and flux during injection events. In this work we study 40 injection events selected by Van Allen Probes particle data to investigate the changes in amplitude and phase of VLF signals at ground receivers across Canada during particle injection events. We model the ionospheric effect of the EEP flux to find its impact on VLF propagation and characterize the injection events. Typically, we find a clear phase advance of ~40° in the received VLF signal at Fort Smith (Canada, L = 8) transmitted from U.S. Navy communication transmitter NAA at Maine (USA). Comparing to other VLF transmitter‐receiver paths in North America leads us to conclude that effects are only seen on paths with adequately large range ≫200 km) through L > 7. Modeling the VLF phase change indicates that in the majority of events (>90%), less than 10% of the strong scattering limit inferred from particle flux measurements at the Van Allen Probes is required to obtain the observed VLF phase signature. The median precipitating flux during energetic particle injections is less than 4 × 106 el/cm2 s sr (<10% of the strong scattering rate) for electrons above ~40 keV extracted from trapped particles energy spectrum. This implies that strong scattering is not typical for these 40 selected energetic electron injection events.

Confirmation of EMIC wave‐driven relativistic electron precipitation

AbstractElectromagnetic ion cyclotron (EMIC) waves are believed to be an important source of pitch angle scattering driven relativistic electron loss from the radiation belts. To date, investigations of this precipitation have been largely theoretical in nature, limited to calculations of precipitation characteristics based on wave observations and small‐scale studies. Large‐scale investigation of EMIC wave‐driven electron precipitation has been hindered by a lack of combined wave and precipitation measurements. Analysis of electron flux data from the POES (Polar Orbiting Environmental Satellites) spacecraft has been suggested as a means of investigating EMIC wave‐driven electron precipitation characteristics, using a precipitation signature particular to EMIC waves. Until now the lack of supporting wave measurements for these POES‐detected precipitation events has resulted in uncertainty regarding the driver of the precipitation. In this paper we complete a statistical study comparing POES precipitation measurements with wave data from several ground‐based search coil magnetometers; we further present a case study examining the global nature of this precipitation. We show that a significant proportion of the precipitation events correspond with EMIC wave detections on the ground; for precipitation events that occur directly over the magnetometers, this detection rate can be as high as 90%. Our results demonstrate that the precipitation region is often stationary in magnetic local time, narrow in L, and close to the expected plasmapause position. Predominantly, the precipitation is associated with helium band rising tone Pc1 waves on the ground. The success of this study proves the viability of POES precipitation data for investigating EMIC wave‐driven electron precipitation.

A statistical approach to determining energetic outer radiation belt electron precipitation fluxes

AbstractSubionospheric radio wave data from an Antarctic‐Arctic Radiation‐Belt (Dynamic) Deposition VLF Atmospheric Research Konsortia (AARDDVARK) receiver located in Churchill, Canada, is analyzed to determine the characteristics of electron precipitation into the atmosphere over the range 3 < L < 7. The study advances previous work by combining signals from two U.S. transmitters from 20 July to 20 August 2010, allowing error estimates of derived electron precipitation fluxes to be calculated, including the application of time‐varying electron energy spectral gradients. Electron precipitation observations from the NOAA POES satellites and a ground‐based riometer provide intercomparison and context for the AARDDVARK measurements. AARDDVARK radiowave propagation data showed responses suggesting energetic electron precipitation from the outer radiation belt starting 27 July 2010 and lasting ~20 days. The uncertainty in >30 keV precipitation flux determined by the AARDDVARK technique was found to be ±10%. Peak >30 keV precipitation fluxes of AARDDVARK‐derived precipitation flux during the main and recovery phase of the largest geomagnetic storm, which started on 4 August 2010, were >105 el cm−2 s−1 sr−1. The largest fluxes observed by AARDDVARK occurred on the dayside and were delayed by several days from the start of the geomagnetic disturbance. During the main phase of the disturbances, nightside fluxes were dominant. Significant differences in flux estimates between POES, AARDDVARK, and the riometer were found after the main phase of the largest disturbance, with evidence provided to suggest that >700 keV electron precipitation was occurring. Currently the presence of such relativistic electron precipitation introduces some uncertainty in the analysis of AARDDVARK data, given the assumption of a power law electron precipitation spectrum.

Energetic electron precipitation characteristics observed from Antarctica during a flux dropout event

AbstractData from two autonomous VLF radio receiver systems installed in a remote region of the Antarctic in 2012 is used to take advantage of the juxtaposition of the L = 4.6 contour, and the Hawaii‐Halley, Antarctica, great circle path as it passes over thick Antarctic ice shelf. The ice sheet conductivity leads to high sensitivity to changing D region conditions, and the quasi constant L shell highlights outer radiation belt processes. The ground‐based instruments observed several energetic electron precipitation events over a moderately active 24 h period, during which the outer radiation belt electron flux declined at most energies and subsequently recovered. Combining the ground‐based data with low and geosynchronous orbiting satellite observations on 27 February 2012, different driving mechanisms were observed for three precipitation events with clear signatures in phase space density and electron anisotropy. Comparison between flux measurements made by Polar‐orbiting Operational Environmental Satellites (POES) in low Earth orbit and by the Antarctic instrumentation provides evidence of different cases of weak and strong diffusion into the bounce loss cone, helping to understand the physical mechanisms controlling the precipitation of energetic electrons into the atmosphere. Strong diffusion events occurred as the <600 keV fluxes began to recover as a result of adiabatic transport of electrons. One event appeared to have a factor of about 10 to 100 times more flux than was reported by POES, consistent with weak diffusion into the bounce loss cone. Two events had a factor of about 3 to 10 times more >30 keV flux than was reported by POES, more consistent with strong diffusion conditions.

Particle precipitation during NEIAL events: simultaneous ground based nighttime observations at Svalbard

Abstract. In this paper we present Naturally Enhanced Ion Acoustic Lines (NEIALs) observed with the EISCAT Svalbard Radar (ESR) together with auroral emissions observed with the Meridian Scanning Photometer (MSP). This is the first report of NEIALs observed during nighttime at Svalbard. Previously, NEIALs have been associated with a strong red line intensity (>10 kR), which exceeds the green line intensities. The high intensity in the red line emission is a sign of abundant low energy electron precipitation. In our observations, one of the NEIAL events was accompanied by the red line emissions far below the previously reported intensities. This happened when the green line intensity exceeds the red line intensity. In this work we discuss the behaviour of electron precipitation characteristics and optical emissions during NEIAL events on the nightside, and we suggest that intensity enhancements in the 844.6 nm emission line could be a better candidate than the 630.0 nm emission as an optical signature for NEIALs.

Reconstruction of electron precipitation characteristics from a set of multiwavelength digital all‐sky auroral images

We present a method for reconstructing the precipitating electron flux from a set of multiwavelength digital all‐sky auroral images. The method involves solving a large linear inversion problem, and it works for any number of all‐sky imager stations and wavelengths. The idea in our method is to invert the energy‐dependent precipitating electron flux directly and not the three‐dimensional volume emission rate as an intermediate step. In this way, the inversion is automatically restricted to physical emission rate profiles. We show the effectiveness of the method in the all‐sky camera case by simulated examples. The method is also applied to real data. The method is capable of correctly reconstructing not only auroral arc positions but also the precipitating electron energy and number fluxes, although not necessarily the full differential flux. A natural limitation is that arcs that are significantly off‐zenith and are viewed sideways by only one all‐sky camera station are smeared out. However, even for these arcs the average arc position and energy characteristics come out with reasonable accuracy.

A case study of electron precipitation in the late substorm growth phase on and nearby a preonset arc

Abstract. We follow the electron precipitation characteristics on and nearby a preonset arc using the high resolution Freja TESP instrument. Our data coverage extends from about 10 min before onset up to 1 min before onset. The arc is the most equatorward one (around MLAT 62°) of a system of growth phase arcs, and it was close to the radiation belt precipitation. Within the preonset arc, inverted-V type precipitation dominates. Poleward of the arc we also find some precipitation regions, and here there is systematically a cold electron population superposed with a warm population. Using single and double Maxwellian fits to the measured electron spectra we find the ionosphere-magnetosphere coupling parameters (field-aligned conductance K and the parallel potential drop V) as well as the effective source plasma properties (density and temperature) during the event. Compared to typical expansion phase features, the preonset parallel potential drop is smaller by a factor of ten, the electron temperature is smaller by a factor of at least five, and the field-aligned conductance is about the same or larger. The fact that there are two isotropic superposed electron populations on the poleward side of the preonset arc suggests that the distance between warm trapped electrons on dipolar field lines and colder electrons on open field lines has become so small near the onset that mixing e.g. due to finite electron Larmor radius effects can take place.Key words. Ionosphere · (ionosophere-magnetosphere interactions) · Magnetospheric physics (auroral phenomena; storms and substorms).

A case study of electron precipitation in the late substorm growth phase on and nearby a preonset arc

<strong class="journal-contentHeaderColor">Abstract.</strong> We follow the electron precipitation characteristics on and nearby a preonset arc using the high resolution Freja TESP instrument. Our data coverage extends from about 10 min before onset up to 1 min before onset. The arc is the most equatorward one (around MLAT 62^°) of a system of growth phase arcs, and it was close to the radiation belt precipitation. Within the preonset arc, inverted-V type precipitation dominates. Poleward of the arc we also find some precipitation regions, and here there is systematically a cold electron population superposed with a warm population. Using single and double Maxwellian fits to the measured electron spectra we find the ionosphere-magnetosphere coupling parameters (field-aligned conductance <i>K</i> and the parallel potential drop <i>V</i>) as well as the effective source plasma properties (density and temperature) during the event. Compared to typical expansion phase features, the preonset parallel potential drop is smaller by a factor of ten, the electron temperature is smaller by a factor of at least five, and the field-aligned conductance is about the same or larger. The fact that there are two isotropic superposed electron populations on the poleward side of the preonset arc suggests that the distance between warm trapped electrons on dipolar field lines and colder electrons on open field lines has become so small near the onset that mixing e.g. due to finite electron Larmor radius effects can take place.<br><br><b>Key words.</b> Ionosphere · (ionosophere-magnetosphere interactions) · Magnetospheric physics (auroral phenomena; storms and substorms).

Longitudinal differences in electron precipitation near L = 4

The differences in the electron precipitation characteristics as seen above Siple Station, Antarctica, and the Kerguelen Islands have been studied. These two sites are both in the southern hemisphere at nearly the same magnetic latitude (L=4). The two stations are at longitudes that place them roughly equal distances east and west of the center of the South Atlantic magnetic anomaly at this L value. With respect to the direction of electron drift, Siple is upstream of the anomaly, and Kerguelen is downstream. The primary data used in the study were counting rates from rocket‐borne, parachute‐deployed, sodium iodide scintillation counters at altitudes of 80 to 30 km and VLF wave data from both ground‐based and rocket‐borne receivers. Nine rocket flights are involved, five from Kerguelen and four from Siple. The flights were made under a range of geomagnetic and VLF activity conditions. The data differ in two major respects. First, the precipitation background at Kerguelen is very low, with high levels of wave activity being required to produce any detectable precipitation, an observation which indicates that there is a range of longitude and/or magnetic local time in which the electron pitch angle diffusion rate is 2 orders of magnitude lower than the long‐term global average rate. Second, X ray microbursts were found to be essentially absent at Kerguelen and very common at Siple. This observation appears to support models of the microburst generation process which predict maximum pitch angle scatterings of only a few tenths of a degree.

D-region signatures of substorm growth phase and onset observed by EISCAT

High resolution electron density measurements by EISCAT during the pre-onset phase and onset of an auroral absorption substorm are used to investigate the characteristics of electron precipitation during these substorm phases. The development of the pre-onset phase is the result of a uniform increase of electron fluxes with energies of a few tens of keV, with no particularly hard component. The absorption spike observed at substorm onset contains fine structure when investigated at 10 s resolution, indicating a rapid hardening of the precipitating spectrum at the onset.

Energetic particle precipitation into the high‐latitude ionosphere and the auroral electrojets: 3. Characteristics of electron precipitation into the morning sector auroral oval

Data from the University of Alberta meridian line of magnetometers are utilized to define the poleward and equatorward borders of the westward electrojet in the morning sector. Soft particle spectrometer data from the Isis 2 polar orbiting satellite are organized in the framework of the westward electrojet for cases where the satellite orbital path took it close to the meridian line of magnetometers. It is found that, in the morning sector, the energetic electron data are organized into four distinct regions as a function of latitude. The southernmost region (D) features trapped electrons across a broad energy range. Region C, just poleward of Region D, contains fluxes of hot (several keV) electrons with a Maxwellian spectrum which are precipitating into the southern portion of the westward electrojet. These Region B fluxes are colocated with the poleward portion of the westward electrojet and are less capable of creating ionospheric Hall conductivity than are their more energetic Region C counterparts. Finally, Region A, poleward of the northern border of the westward electrojet, contains irregular latitudinally localized regions of precipitating electrons whose spectrum contains drifting Maxwellians and which are evidently being accelerated along the field lines on which they are being observed. The energies of electrons in Region A are of the order of a few hundred electron volts and rarely exceed ∼1–2 keV. The phenomenology of Region A is remarkably similar to the acceleration region in the poleward portion of the auroral oval in the evening sector although the peak energies of the electrons are smaller in the morning sector compared to the evening sector. Our results suggest that the Harang discontinuity does not signify any significant asymmetry in convection drift paths in the magnetosphere and that asymmetries noted in ionospheric current flow and auroral luminosity are due solely to asymmetries in the energies to which electrons may be accelerated in the morning compared to the evening sector. We present a model of plasma convective flow in the magnetosphere which is consistent with the observations reported in this paper.

Self‐consistent particle and parallel electrostatic field distributions in the magnetospheric‐ionospheric auroral region

The variation of the self‐consistent electrostatic potential along the magnetic field is calculated by application of the principle of quasi‐neutrality to the plasma components distributed along an auroral field line. The equilibrium plasma consists of hot anisotropic magnetospheric plasma, ionospheric plasma evaporated or extracted upward by the parallel electrostatic field, and backscattered electrons. It is shown that the above charged particle populations can support a potential difference of up to several Kilovolts between the equator and the ionosphere along an auroral field line. Moreover, the corresponding parallel electric field has the proper signature to account for electron precipitation characteristics. Comparisons between theoretical and observed electron precipitation fluxes lead to estimates for the various physical parameters in the model.