Region Of Electron Precipitation Research Articles

Spatial extent of the energetic electron precipitation region during substorms

The spatial coverage of the energetic electron precipitation (EEP) region during three substorms has been determined using a combination of ground-based instruments in North America. The primary datasets used to determine the presence or absence of EEP are riometer measurements of cosmic noise absorption (CNA); attenuation of the 10–11 MHz atmospheric noise in the D-region ionosphere determined using the Super Dual Auroral Radar Network (SuperDARN); amplitude variations in subionospheric very low frequency (VLF) propagation observed using the Antarctic-Arctic Radiation-belt (Dynamic) Deposition VLF Atmospheric Research Konsortium (AARDDVARK) VLF receivers, and total electron content measurements derived from global navigation satellite systems. Our results show that substorm EEP during moderate geomagnetic conditions can extend considerably equatorward or poleward of the average latitude limits reported in a previous statistical study of substorm EEP and an EEP model. This evidence is provided by the SuperDARN radars and the radar signature is not always accompanied by cosmic noise absorption measured by the riometers. Further work is required to determine which EEP energies and fluxes the SuperDARN radars are sensitive to, but we can conclude that EEP with sufficient flux to attenuate 10–11 MHz radiowaves by ∼5 dB reaches latitudes well equatorward of the statistical EEP latitude limits previously modelled for >30 keV electrons.

Determining Latitudinal Extent of Energetic Electron Precipitation Using MEPED On‐Board NOAA/POES

AbstractEnergetic Electron Precipitation (EEP) from the plasma sheet and the radiation belts ionizes the polar lower thermosphere and mesosphere. EEP increases the production of NOx and HOx, which will catalytically destroy ozone, an important element of atmospheric dynamics. Therefore, measurement of the latitudinal extent of the precipitation boundaries is important in quantifying the atmospheric effects of the Sun‐Earth interaction. This study uses measurements by the Medium Energy Proton Electron Detector (MEPED) of six NOAA/POES and EUMETSAT/METOP satellites from 2004 to 2014 to determine the latitudinal boundaries of EEP and their variability with geomagnetic activity and solar wind drivers. Variation of the boundaries for different electron energies and Magnetic Local Time (MLT) is studied. Regression analyses are applied to determine the best predictor variable based on solar wind parameters and geomagnetic indices. The highest correlation was found for the pressure‐corrected Dst index when applying a linear regression model. A model of the equatorward EEP boundary is developed separately for three different energy channels, >43, >114, and >292 keV, and for 3 hour MLT sectors. For >43 keV EEP, 80% of the equatorward boundaries predicted by the model are within ±2.2° cgmlat. The model exhibits a solar cycle bias where it systematically exaggerates the equatorward movement of the EEP region during solar minimum. The highest accuracy of the model is found in periods dominated by corotating interaction regions/high speed solar wind streams. The result will be a key element for constructing a model of EEP variability to be applied in atmosphere climate models.

Fluctuation of Lower Ionosphere Associated with Energetic Electron Precipitations during a Substorm

In this paper, using the combined observations of the NOAA 16, LANL-01A, IMAGE satellites, VLF radio wave, and ground-based riometers, we study the fluctuation of lower ionosphere-associated precipitating energetic electrons during a geomagnetic storm on 8 November 2004. Associated with the substorm dispersion injection observed by the LANL-01A satellite, the riometers observed obvious enhancements of ionospheric absorption within the electron isotropic zone, which they attributed to the tail current sheet scattering (TCS) mechanism. Through observations of the NOAA 16 satellite, we found a sharp enhancement of the precipitating electron flux within the anisotropic zone, which entailed an obvious separation of energetic electron precipitation at high latitudes. This energetic electron precipitation within the anisotropic zone leads to the significant enhancement of electron density in the D region, thus resulting in the variations of VLF radio wave amplitudes, which propagate in the middle latitudes. Since the projection of the electron precipitation region within the anisotropic zone is at the inner edge of the plasmapause observed by the IMAGE EUV, the precipitation of energetic electrons should be attributed to the ELF hiss-ring current electron interaction. As a result, the energetic electron precipitations due to the tail current sheet scattering mechanism and wave-particle interaction in the inner magnetosphere were both observed and analyzed as they were associated with a substorm during a geomagnetic storm.

First direct measurement of auroral and equatorial jets in the stratosphere of Jupiter

Context. The tropospheric wind pattern in Jupiter consists of alternating prograde and retrograde zonal jets with typical velocities of up to 100 m s−1 around the equator. At much higher altitudes, in the ionosphere, strong auroral jets have been discovered with velocities of 1−2 km s−1. There is no such direct measurement in the stratosphere of the planet. Aims. In this Letter, we bridge the altitude gap between these measurements by directly measuring the wind speeds in Jupiter’s stratosphere. Methods. We use the Atacama Large Millimeter/submillimeter Array’s very high spectral and angular resolution imaging of the stratosphere of Jupiter to retrieve the wind speeds as a function of latitude by fitting the Doppler shifts induced by the winds on the spectral lines. Results. We detect, for the first time, equatorial zonal jets that reside at 1 mbar, that is, above the altitudes where Jupiter’s quasi-quadrennial oscillation occurs. Most noticeably, we find 300−400 m s−1 nonzonal winds at 0.1 mbar over the polar regions underneath the main auroral ovals. They are in counterrotation and lie several hundred kilometers below the ionospheric auroral winds. We suspect them to be the lower tail of the ionospheric auroral winds. Conclusions. We directly detect, for the first time, strong winds in Jupiter’s stratosphere. They are zonal at low-to-mid latitudes and nonzonal at polar latitudes. The wind system found at polar latitudes may help increase the efficiency of chemical complexification by confining the photochemical products in a region of large energetic electron precipitation.

Precipitation of Energetic Electrons from the Earth’s Radiation Belt Stimulated by High-Power HF Radio Waves for Modification of the Midlatitude Ionosphere

Based on the results of the experiments performed in 2005–2010 within the framework of the Sura—DEMETER program, we analyze the features of the precipitations of energetic electrons (with energies E ≈ 100 keV) from the Earth’s radiation belt. The modification of the ionospheric F2 region was conducted by means of high-power HF O-mode radio waves radiated in the CW regime. The precipitations were detected using the equipment onboard DEMETER, a French microsatellite. The conditions of precipitation appearance were determined, and it was found that the electron precipitation region was stretched along the geomagnetic meridian to a distance of 1300 km; the size of the region in the transverse direction is about 400 km. It was shown by ionosonde measurements that such precipitations lead to increased absorption of radio waves in the lower ionosphere. It is assumed that the mechanism for precipitation of electrons from the Earth’s radiation belt is determined by the interaction of energetic electrons with VLF radio waves, which are generated due to the interaction of the amplitude-unmodulated O-mode pump wave with the ionospheric plasma near the wave reflection height.

Does a Local B‐Minimum Appear in the Tail Current Sheet During a Substorm Growth Phase?

AbstractMagnetic configurations with dBz/dr > 0 in the midtail current sheet are potentially unstable to various instabilities associated with the explosive substorm onset. Their existence is hard to confirm with observations of magnetospheric spacecraft. Here we use remote sensing by low‐altitude spacecraft that measured the loss cone filling rate during electron‐rich solar particle event, providing information about magnetic properties of the tail current sheet. We found a latitudinally localized anisotropic 30 keV electron loss cone region embedded inside an extended region of isotropic solar electron precipitation. It was persistently observed for more than 0.5 h during isolated growth phase event by six Polar Operational Environmental Satellites spacecraft, which crossed the premidnight auroral oval. The embedded anisotropic region was observed 1° poleward of the outer radiation belt boundary over 4–5 h wide magnetic local time sector, suggesting a persistent ridge‐type Bz2/j maximum in the equatorial plasma sheet at distances 15–20 RE. We discuss infrequent observations of such events taking into account recent results of global magnetohydrodynamic simulations.

Horizontal profile of a moving red line cusp aurora

AbstractAuroral emission at a wavelength of 630.0 nm (red line) has a long radiative time. In this study, we show how this long radiative time controls the horizontal extent of a moving mesoscale aurora, which is typical of the cusp. Using high time resolution (4 s) observations by an all‐sky imager at Longyearbyen, Svalbard, and observations by the European Incoherent Scatter Svalbard Radar (ESR), we examined the auroral emission enhancements obtained in the cusp during an interval of southward interplanetary magnetic field on 27 November 2011. Simultaneous observations from the all‐sky imager and the ESR clearly show the manner in which auroral emission regions passed through the radar's field of view. When the front edge of the moving auroral structure intersected the radar's field of view, the ESR electron temperature was enhanced. A few minutes later, the ESR electron temperature dropped to the background level, indicating that the mesoscale electron precipitation region shifted away from the radar's field of view. At that time, the auroral emission in the radar's field of view decreased but still had adequate intensities. These results provide evidence demonstrating that moving cusp auroral emission occurs behind the electron precipitation region as well as inside that region. We interpreted this feature semiquantitatively by using the equation of continuity of the density of the excited atomic oxygen. Our model indicates that the maximum intensity in the moving auroral structure occurs at a point along the trailing edge of the electron precipitation region and that the velocity of that region is important for determining how the 630.0 nm aurora emissions extend horizontally.

CORRELATION BETWEEN EMISSION INTENSITIES IN DAYSIDE AURORAL ARCS AND PRECIPITATING ELECTRON SPECTRA

AbstractMore than 20000 dayside auroral arcs of the 557.7 and 630.0 nm emission intensities have been statistically studied, and the dependences of the I557.7/I630.0 ratio on the I557.7 emission intensity have been determined. The 557.7 nm emission intensity has two maximum values in the hot spot and warm spot regions, with average values of 2.2 and 2.9 kR, respectively. But there is a maximum near magnetic noon for 630.0 nm emission intensity, with an average value of 1.5 kR. In the I557.7 emission range 0.1∼10 kR, the I557.7/I630.0 ratio tends to increase from 0.2 to 9. The correlation between emission intensity and precipitating electron spectra has been investigated using 17 cases of DMSP passing through 40 auroral arcs above Chinese Arctic Yellow River Station (YRS). We obtain the equations that the average energy of the electrons is proportional to the I557.7/I630.0 ratio. There is a positive correlation between the total energy flux of the electrons and the I557.7 emission intensity. The typical region of electron precipitation, where the auroral arcs were observed, was the boundary plasma sheet (BPS) in the prenoon and postnoon sectors. We also found some low‐energy precipitating electrons from the region of mantle, where the arcs are located poleward of dayside auroral oval. The magnetic source region of the precipitating electrons with low energy was identified as the low latitude boundary layer (LLBL) adjacent to magnetic noon. Arcs are located in the lower latitude in this region.

First multipoint in situ observations of electron microbursts: Initial results from the NSF FIREBIRD II mission

AbstractWe present initial dual spacecraft observations that for the first time both constrain the spatial scale size and provide spectral properties at medium energies of electron microbursts. We explore individual microburst events that occurred on 2 February 2015 using simultaneous observations made by the twin CubeSats which comprise the National Science Foundation (NSF) Focused Investigations of Relativistic Electron Bursts: Intensity, Range, and Dynamics (FIREBIRD II). During these microburst events, the two identically instrumented FIREBIRD II CubeSats were separated by as little as 11 km while traversing electron precipitation regions in low‐Earth orbit. These coincident microburst events map to size scales >120 km at the equator. Given the prevalence of coincident and noncoincident events we conclude that this is of the same order of magnitude as that of the spatial scale size of electron microburst, an unknown property that is critical for quantifying their overall role in radiation belt dynamics. Finally, we present measurements of electron microbursts showing that precipitation often occurs simultaneously across a broad energy range spanning 200 keV to 1 MeV, a new form of empirical evidence that provides additional insights into the physics of microburst generation mechanisms.

Mapping the electron energy in Jupiter's aurora: Hubble spectral observations

AbstractFar ultraviolet spectral observations have been made with the Hubble Space Telescope in the time‐tag mode using the Space Telescope Imaging Spectrograph (STIS) long slit. The telescope was slewed in such a way that the slit projection scanned from above the polar limb down to midlatitudes, allowing us to build up the first spectral maps of the FUV Jovian aurora. The shorter wavelengths are partly absorbed by the methane layer overlying part of the auroral emission layer. The long‐wavelength intensity directly reflects the precipitated energy flux carried by the auroral electrons. Maps of the intensity ratio of the two spectral regions have been obtained by combining spectral emissions in two wavelength ranges. They show that the amount of absorption by methane varies significantly between the different components of the aurora and inside the main emission region. Some of the polar emissions are associated with the hardest precipitation, although the auroral regions of strong electron precipitation do not necessarily coincide with the highest electron energies. Outputs from an electron transport model are used to create maps of the distribution of the characteristic electron energies. Using model atmospheres adapted to auroral conditions, we conclude that electron energies range between a few tens to several hundred keV. Comparisons of derived energies are in general agreement with those calculated from magnetosphere‐ionosphere coupling models, with values locally exceeding the standard model predictions. These results will provide useful input for three‐dimensional modeling of the distribution of particle heat sources into the high‐latitude Jovian upper atmosphere.

Comparative characteristics of ion and electron precipitation in the dawn and dusk sectors

Characteristics of ion and electron precipitations in the dawn and dusk sectors are investigated by DMSP F6 and F7 satellite observations. It is shown that in the dusk sector the positions of electron and ion precipitation boundaries are nearly coincident for all levels of magnetic activity; however the latitudinal dis� tribution of energy fluxes indicates that the positions of electron and ion precipitation maxima are spatially separated. Maximum energy fluxes of ions is observed at the equatorial precipitation boundary, while those of electrons at the poleward one. In the dawn sector, the electron precipitation region is 3°-4° wider than that of ions. The isotropy boundary in the dusk sector is located in the region of diffuse precipitation (DAZ) near its poleward boundary for all levels of magnetic activity, while in the dawn sector it falls in the region of struc� tured precipitations (AOP). Electron precipitations are dominating in the dawn sector. Here in the region of diffuse precipitation (DAZ), the ion energy fluxes Fi make less than 5% as compared to the electron energy flux Fe. In the region of structured precipitations (AOP), the portion of Fi decreases with increasing magnetic activity from ~10-20% for AL ≈ -100 nT to <5% for AL ≈ -1000 nT. As for the dusk sector, in the AOP region, electron precipitations are dominating as well, while in the DAZ region the ion energy fluxes are significant. In the 1500-1800 MLT sector, the ratio Fi/Fe increases from ~0.7 to ~3.0 with AL changing from -100 nT to -1000 nT.

Auroral glow equatorward from the auroral oval

On the basis of observations for the IGY period (visoplots) it is shown, that during magnetic storms diffuse glow is detected at all latitudes between the lowest latitude of the visually observed auroral glow at the zenith and the auroral oval. The diffuse glow region spatially coincides with the region of soft electron precipitation extending equatorward from the boundary of the oval to the latitude of the plasmopause projections along the magnetic force lines to the ionosphere. Using published materials on the diffuse glow dynamics and SAR arcs at the Yakutsk meridian, as well as simultaneous measurements of the DMSP F9 satellite, we discuss the contribution from low-energy electron precipitation transfered via convection toward Earth from the magnetosphere’s plasma sheet to excitation of 630.0 nm emission in low-intensity (<1.0 kR) SAR arcs.

Magnetospheric location of the equatorward prebreakup arc

We address the long‐standing problem of the location and origin of the equatorwardmost Pre‐Breakup auroral Arc (PBA) by combining energetic particle observations from NOAA Polar Operational Environmental Satellites (POES) overpasses of prebreakup arcs with auroral imaging and magnetospheric observations from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. The prebreakup arc was observed within a few minutes of auroral breakup and ∼1–2 hours in MLT from the breakup meridian. For three ideal conjunctions out of 16 PBA crossings, we also construct a dynamically‐adapted magnetospheric model after adding concurrent magnetic observations by the GOES spacecraft. Model‐predicted isotropy boundaries of energetic particles are compared with observations, informing us about model uncertainties. Direct mapping with adapted models as well as particle flux comparisons between the ionosphere and the magnetosphere confirm that the PBA source lies within the region of the steep equatorial magnetic field gradient, where the equatorial field is also small (5–20 nT). That equatorial location, at roughly 8–10 Re, is likely the earthwardmost edge of the thin cross‐tail current sheet. From observations we find that the prebreakup arc nearly coincides in latitude with the energy‐dispersed, 30–300 keV electron isotropy boundary. Here the non‐adiabatic electron precipitation from the high flux region of the outer radiation belt near its outer edge produces a narrow, intense energetic electron precipitation region, called the energetic electron arc (EEA). Two fortuitous conjunctions with DMSP also confirm that energetic (&gt;20 keV) EEA electrons and lower energy, inverted‐V electrons associated with the PBA are collocated. We suggest that EEA formation is an inherent part of the PBA formation process. By creating an enhanced conductance strip, the EEA (a seed arc) produces ionospheric polarization that leads to field‐aligned current generation and associated field‐aligned electron precipitation. We also discuss implications of our findings for the substorm onset mechanism.

Position of proton precipitation during auroral intensifications

The unique spectrographic observations of auroras on the Kola Peninsula, simultaneously performed in 1970 at Loparskaya and Kem stations using C-180-S cameras, have been analyzed by up-to-date digital data processing. The position and dynamics of proton precipitation relative to other manifestations of auroral and substorm activity (auroral arcs and electrojets) under moderately and weakly disturbed conditions have been analyzed. Several previously known regularities in the morphology of proton auroras have been confirmed. It has been indicated that the direction of motion of the proton band equatorward boundary in the evening sector changes at a sign reversal of the IMF Z component. Weak breakups affect the poleward boundary of the proton band but do not influence the position of the equatorward boundary of this band, which results in the expansion of the proton emission region. When a disturbance is stronger, the proton emission disappears near an active electron arc and subsequently appears poleward of its position before intensification. Short-term proton precipitation is also observed in the region of active electron precipitation during an intense breakup in the form of N–S structures.

What can we learn from HF signal scattered from a discrete arc?

Abstract. We present observations of a discrete southward propagating arc which appeared in the mid-night sector at latitudes equatorward of main substorm activity. The arc observations were made simultaneously by the ALFA (Auroral Light Fine Analysis) optical camera, the SuperDARN-CUTLASS HF radar and the Demeter satellite during a coordinated multi-instrumental campaign conducted at the KEOPS/ESRANGE site in December 2006. The SuperDARN HF signal which is often lost in the regions of strong electron precipitation yields in our case clear backscatter from an isolated arc of weak intensity. Consequently we are able to study arc dynamics, the formation of meso-scale irregularities of the electron density along the arc, compare the arc motion with the convection of surrounding plasma and discuss the contribution of ionospheric ions in the arc erosion and its propagation.

Simultaneous DMSP, all-sky camera, and IMAGE FUV observations of the brightening arc at a substorm pseudo-breakup

Abstract Auroral particles, field-aligned currents, and plasma convections in the vicinity of the brightening arc at substorm onset are still not well understood, since it is very rare to have conjugate satellite measurements above the brightening arc. In this paper, we investigate the characteristics of auroral particles and fields associated with the brightening arc at a pseudo-onset of substorm on October 31, 2000, using ground all-sky TV images, IMAGE FUV auroral images, and particle, magnetic field, and plasma flow data obtained by the DMSP F12 satellite. The arc brightening at Tixie (66.0°MLAT), Russia, occurred at 1004 UT (18.75 MLT) coincident with a coherent Pi 2 pulsation at midlatitudes and with the DMSP crossing above the arc. The brightening arc did not develop on a global scale, indicating that this event is a pseudo auroral breakup, which occurred ≈16 min before the major substorm expansion onset. IMAGE auroral images indicate that the longitude of the brightening center was ≈2.5 h nightside of Tixie. The DMSP data show that the precipitating particles associated with the brightening arc correspond to an electron inverted-V structure at the equatorward edge of the electron precipitation region. The arc was located in the energetic (&gt;1 keV) ion precipitation region, near the equatorward boundary of the upward region 1 field-aligned current, and at the peak of the sunward convection velocity. These facts indicate that the brightening arc at duskside of the onset local time was located in the inner plasma sheet at the inner edge of the region 1 current source in the sunward convection region.

Auroral particles associated with a substorm brightening arc

We investigated the characteristics of auroral precipitating particles associated with a brightened arc at a pseudo‐substorm, using auroral images of a high‐time resolution (0.25 s) all‐sky TV camera, particle data from the DMSP F10 spacecraft, and high‐latitude magnetograms. The arc brightening occurred at 0407 UT (∼20 MLT) on December 30, 1994, at Fort Smith (67 MLAT), Canada, coincident with high‐latitude Pi 2 magnetic pulsations. The brightening appeared without expansion (pseudo‐breakup), and Fort Smith was located duskside of the main onset region. From the simultaneous DMSP particle data, the brightened arc is found to correspond to an electron inverted‐V structure near the equatorward edge of the electron precipitation region, suggesting the pseudo‐breakup of the auroral substorm occurred in the inner part of the plasma sheet.

FAST and IMAGE‐FUV observations of a substorm onset

On 6 February 2002 the NASA FAST satellite transited a substorm break up arc approximately 1 min after substorm onset as identified by the NASA IMAGE satellite far ultraviolet (FUV) instrument complement. These IMAGE data show that both the intense electron and proton precipitation features seen by FAST were not present prior to substorm onset. The most intense onset poleward surge was produced by superthermal electrons, and their energy spectrum and field‐aligned angular distribution were consistent with wave accelerated electrons. The low energy ion fluxes in the E × B direction and magnetometer measurements confirmed the presence of waves in this feature. Thus the leading edge of the auroral surge was not produced by quasi‐static field‐aligned currents and related “inverted V” electric fields. The onset arc was also the poleward boundary of intense energetic protons. The FUV proton auroral images indicate that substantial part of the ion energy was carried by protons of energy &gt;25 keV. Equatorward of the surge, there was a broader region of electron precipitation with embedded quasi‐static “inverted V” electric fields. The FAST particle and magnetometer measurements are consistent with a field‐aligned current structure in which the bulk of the upward current was carried by the “inverted V” precipitation region. The region of the superthermal electrons carried very little net current. The high‐density upward field‐aligned current carried by the superthermal electrons was presumably balanced by oppositely directed downward currents carried by cold electrons. The ground onset location was separated from the closed/open field line boundary by an extended region of closed field lines. In this region there was some weak plasma sheet electron and proton precipitation, but there was no clear signature of any distant, prebreakup substorm onset activity.

Influence of magnetospheric processes on winter HF radar spectra characteristics

Abstract. This study investigates further the relationship between regions of the magnetosphere and the characteristics of HF radar Doppler spectra recorded in the ionospheric projection of those regions. It builds on earlier work, which has reported a relationship between the Doppler spectral width and the ionospheric projection of the magnetospheric cusp region, by introducing novel techniques for classifying the Doppler spectra recorded by the SuperDARN radars. We first review the geophysical factors that can condition the characteristics of the autocorrelation function (ACF) data produced by the radars. This leads to a classification scheme of the ACF data which is then applied to a large database compiled from winter data taken by the Northern Hemisphere Super-DARN radars. This statistical study shows that the ACF characteristics are not randomly distributed in space, but rather are spatially organized in the ionosphere. This paper suggests that these regions are ordered primarily by the low energy (&lt;approx&gt; 1 keV) electron precipitation region and the presence of intense ULF wave activity.Key words. Ionosphere (auroral ionosphere; ionosphere-magnetosphere interactions; plasma convection)

Coordinated Akebono and EISCAT observations of suprathermal ion outflows in the nightside inverted-V region

We have investigated ion outflows observed by the Akebono satellite and the EISCAT radar in the nightside auroral region on February 16, 1993. The Akebono satellite at about 7000 km altitude observed the region of suprathermal ion outflows and inverted-V type electron precipitation alternately with a horizontal separation of 70–150 km at the ionospheric level. These two regions corresponded to the upward and downward field-aligned current region, respectively, and intense ELF waves were observed in the ion outflow region. From the EISCAT VHF radar observation (Common Program 7 mode), it has been suggested that the ion outflow region and the enhanced electron temperature region were aligned along geomagnetic field lines with vertical and horizontal separations of 200–400 and 70–80 km, respectively and these two regions convected equatorward across the EISCAT radar at Tromsø site. Based on these results, we propose a model for this ion outflow as follows. In the nightside auroral region, downward FAC regions exist near the edge of the inverted-V type electron precipitation regions. ELF waves are excited probably by a plasma instability due to the upward thermal electron beam carrying the downward FACs, and these ELF waves cause transverse ion heating at the top of the ionosphere. The produced ion conics contribute significantly to ion outflow.