Auroral Electron Flux Research Articles

Formula omitted] cooling in the jovian aurora: Juno remote sensing observations and modeling

Cooling of Jupiter's auroral thermosphere by H3+ radiation to space is one of the processes controlling the energy balance in the auroral upper atmosphere. The UltraViolet Spectrograph (UVS) and the Jupiter InfraRed Auroral Mapper (JIRAM) on board Juno have observed the Jovian polar aurora from its polar orbit since August 2016. The UVS instrument measures the H2 Lyman and Werner bands whose brightness is a proxy of the precipitated auroral electron flux. The 3.3–3.6 μm spectral window of the JIRAM L-band imager maps the H3+ thermal radiance with unprecedented spatial resolution. Comparison of concurrent observations indicates that the morphological features are similar in the two spectral regions but differences are also observed in the spatial intensity contrast. We compare the total (direct and indirect) particle heating rate and the cooling by H3+ radiation derived from four pairs of simultaneous UVS and JIRAM images. The total auroral cooling power H3+ is in the range 2–4 terawatts in both hemispheres. In all cases, the H3+ cooling in the aurora is found to range between 0.45 and 0.67 time less than the particle collisional heating. In a second step, we use the ultraviolet H2 brightness and FUV color ratio to derive the characteristics of the electron precipitation and model the H3+ radiance for each UVS map pixel. The comparison of the H3+ modeled radiance map with the JIRAM observations shows general good agreement with some local differences. The four spatially integrated H3+ cooling power from the model are in very good agreement with the JIRAM values. These results are important constraints for global magnetosphere-ionosphere-atmosphere models.

Nightside Auroral Electrons at Mars: Upstream Drivers and Ionospheric Impact

AbstractDiscrete aurorae have been observed at Mars by multiple spacecraft, including Mars Express, Mars Atmosphere and Volatile EvolutioN (MAVEN), and most recently the United Arab Emirates Hope spacecraft. Meanwhile, there have been studies on the source particles responsible for producing these detectable aurorae (termed “auroral electrons”). By utilizing empirical criteria to select auroral electrons established by Xu et al. (2022, https://doi.org/10.1029/2022GL097757), we conduct statistical analyses of the impact of upstream drivers on the occurrence rate and fluxes of auroral electrons. We find the occurrence rate increases with upstream dynamic pressure and weakly depends on the interplanetary magnetic field strength. Meanwhile, the integrated auroral electron flux somewhat decreases with increasing upstream drivers. Auroral electron precipitation also occurs more frequently and is more intense over regions of strong crustal fields compared to weak crustal fields. Aside from emissions, auroral electrons are expected to cause significant impact ionization and enhance the plasma density locally. In this study, we also quantify the nightside ionospheric impact of auroral electron precipitation, specifically the thermal ion (O+, , and ) density enhancement, with MAVEN observations. Our results show that the ion density is increased by up to an order of magnitude at low altitudes. The crustal effects on ion density profiles for nominal electron and auroral electron precipitation are also discussed.

THE FEATURES OF PRECIPITATING ELECTRON SPECTRA IN THE RAYED AURORAS

The features of auroral electron fluxes, which form rayed structures in auroras, are investigated. The experimental study was the results of triangulation measurements with equipment recording radiation in a wide wavelength range (380–580 nm). It is shown that the spectra of the precipitating electron flux can be approximated by the sum of two electron fluxes having a power-law energy spectrum and a Maxwellian energy distribution.

Effect of Electron Precipitations on the Effective Recombination Coefficient

The results of an analysis of the possible effect of auroral electron fluxes on the effective recombination coefficient αeff in the ionosphere are presented. It is shown that the αeff value in the E-region of the ionosphere is determined mainly by the physical-chemical properties of the medium. In the F1-layer of the ionosphere, the effective recombination coefficient becomes dependent on both the value of the energy flux and the type of the energy spectrum of the auroral electron flux.

Changes in the Martian atmosphere induced by auroral electron precipitation

Typical auroral events in the Martian atmosphere, such as discrete and diffuse auroral emissions detected by UV spectrometers onboard ESA Mars Express and NASA MAVEN, are investigated. Auroral electron kinetic energy distribution functions and energy spectra of the upward and downward electron fluxes are obtained by electron transport calculations using the kinetic Monte Carlo model. These characteristics of auroral electron fluxes make it possible to calculate both the precipitation-induced changes in the atmosphere and the observed manifestations of auroral events on Mars. In particular, intensities of discrete and diffuse auroral emissions in the UV and visible wavelength ranges (Soret et al., 2016; Bisikalo et al., 2017; Gerard et al., 2017). For these conditions of auroral events, the analysis is carried out, and the contribution of the fluxes of precipitating electrons to the heating and ionization of the Martian atmosphere is estimated. Numerical calculations show that in the case of discrete auroral events the effect of the residual crustal magnetic field leads to a significant increase in the upward fluxes of electrons, which causes a decrease in the rates of heating and ionization of the atmospheric gas in comparison with the calculations without taking into account the residual magnetic field. It is shown that all the above-mentioned impact factors of auroral electron precipitation processes should be taken into account both in the photochemical models of the Martian atmosphere and in the interpretation of observations of the chemical composition and its variations using the ACS instrument onboard ExoMars.

The effect of ring current electron scattering rates on magnetosphere‐ionosphere coupling

AbstractThis simulation study investigated the electrodynamic impact of varying descriptions of the diffuse aurora on the magnetosphere‐ionosphere (M‐I) system. Pitch angle diffusion caused by waves in the inner magnetosphere is the primary source term for the diffuse aurora, especially during storm time. The magnetic local time (MLT) and storm‐dependent electrodynamic impacts of the diffuse aurora were analyzed using a comparison between a new self‐consistent version of the Hot Electron Ion Drift Integrator with varying electron scattering rates and real geomagnetic storm events. The results were compared with Dst and hemispheric power indices, as well as auroral electron flux and cross‐track plasma velocity observations. It was found that changing the maximum lifetime of electrons in the ring current by 2–6 h can alter electric fields in the nightside ionosphere by up to 26%. The lifetime also strongly influenced the location of the aurora, but the model generally produced aurora equatorward of observations.

Variation of Jupiter's aurora observed by Hisaki/EXCEED: 2. Estimations of auroral parameters and magnetospheric dynamics

AbstractJupiter's auroral parameters are estimated from observations by a spectrometer EXCEED (Extreme Ultraviolet Spectroscope for Exospheric Dynamics) on board Japanese Aerospace Exploration Agency's Earth‐orbiting planetary space telescope Hisaki. EXCEED provides continuous auroral spectra covering the wavelength range over 80–148 nm from the whole northern polar region. The auroral electron energy is estimated using a hydrocarbon color ratio adopted for the wavelength range of EXCEED, and the emission power in the long wavelength range 138.5–144.8 nm is used as an indicator of total emitted power before hydrocarbon absorption and auroral electron energy flux. The quasi‐continuous observations by Hisaki provide the auroral electron parameters and their relation under different auroral activity levels. Short‐ (within < one planetary rotation) and long‐term (> one planetary rotation) enhancements of auroral power accompany increases of the electron number flux rather than the electron energy variations. The relationships between the auroral electron energy (~70–400 keV) and flux (1026–1027/s, 0.08–0.9 μA/m2) estimated from the observations over a 40 day interval are in agreement with field‐aligned acceleration theory when incorporating probable magnetospheric parameters. Applying the electron acceleration theory to each observation point, we explore the magnetospheric source plasma variation during these power‐enhanced events. Possible scenarios to explain the derived variations are (i) an adiabatic variation of the magnetospheric plasma under a magnetospheric compression and/or plasma injection, and (ii) a change of the dominant auroral component from the main emission (main aurora) to the emission at the open‐closed boundary.

An investigation comparing ground‐based techniques that quantify auroral electron flux and conductance

AbstractWe present three case studies that examine optical and radar methods for specifying precipitating auroral flux parameters and conductances. Three events were chosen corresponding to moderate nonsubstorm auroral activity with 557.7 nm intensities greater than 1kR. A technique that directly fits the electron number density from a forward electron transport model to alternating code incoherent scatter radar data is presented. A method for determining characteristic energy using neutral temperature observations is compared against estimates from the incoherent scatter radar. These techniques are focused on line‐of‐sight observations that are aligned with the local geomagnetic field. Good agreement is found between the optical and incoherent scatter radar methods for estimates of the average energy, energy flux, and conductances. The Pedersen conductance predicted by Robinson et al. (1987) is in very good agreement with estimates calculated from the incoherent scatter radar observations. However, we present an updated form of the relation by Robinson et al. (1987), ΣH/ΣP=0.57〈E〉0.53, which was found to be more consistent with the incoherent scatter radar observations. These results are limited to similar auroral configurations as in these case studies. Case studies are presented that quantify auroral electron flux parameters and conductance estimates which can be used to specify the magnitude of energy dissipated within the ionosphere resulting from magnetospheric driving.

Characteristics of monoenergetic and broadband auroral electron precipitation as observed by the Science and Technology Satellite‐I

AbstractPreviously, the pitch angle distribution of monoenergetic and broadband electron precipitation has been investigated mainly by case studies. The main focus of this study is quantitative comparison of pitch angle distributions between monoenergetic and broadband electron precipitations using long‐term observations on board one platform. From December 2003 to October 2004, Science and Technology Satellite‐I (altitude∼680 km) regularly observed auroral electron flux and cold ambient plasma parameters during quiet and moderately disturbed conditions. Monoenergetic electron precipitation has notable perpendicular anisotropy, while broadband electron precipitation is much more field aligned. As for other features of monoenergetic and broadband electron precipitation, the characteristic energy of precipitating electrons is slightly higher for monoenergetic (around 1 keV) than for broadband electron precipitation (from several hundred eV to 1 keV). For both monoenergetic and broadband types, the characteristic energy and energy flux do not show clear correlation with cold ambient plasma density/temperature.

Auroral electron precipitation and flux tube erosion in Titan’s upper atmosphere

Cassini dasta shows that Titan’s atmosphere strongly depletes the electron content in Saturn’s flux tubes, producing features known as electron bite-outs, which indicate that the flux of auroral electrons decreases over time. To understand this process we have developed a time-dependent two-stream model, which uses field line geometries and drift paths calculated by a three-dimensional multi-fluid model of Titan’s plasma interaction. The boundary conditions of the model account for the time-dependent reduction or increase in electron flux along Saturn’s magnetic field lines because of the loss or production of electrons in Titan’s atmosphere. The modification of the auroral electron flux depends on the electron bounce period in Saturn’s outer magnetosphere; therefore, we also calculate electron bounce periods along several Kronian field lines accounting for both the magnetic mirroring force and the field-aligned electric potential in Saturn’s plasma sheet. We use the time-dependent two-stream model to calculate how the reduction in the auroral electron flux affects electron impact ionization and energy deposition rates in Titan’s upper atmosphere. We find that the flux of higher energy (>50eV) electrons entering Titan’s atmosphere is strongly reduced over time, resulting in smaller ionization and energy deposition rates below ∼1300km altitude. Finally, we show that sample spectrograms produced from our calculations are consistent with CAPS-ELS data.

On the feasibility of characterizing jovian auroral electrons via [formula omitted] infrared line-emission analysis

Here we investigate the feasibility of characterizing the jovian auroral electron energy and flux via H3+ infrared (IR) emission line analysis. Ground based telescopes can monitor jovian infrared auroral activities continuously for an extended time interval compared to the more restricted temporal coverage of ultraviolet (UV) observations. Since the departure from local thermodynamic equilibrium (LTE) varies with vibrational levels and altitude, measurements of the relative emission line intensities reveal the altitude of emission and hence the electron energy. The combination of three H3+ line-intensity ratios is required to determine the electron energy and the background temperature. The feasibility issue is evaluated by studying how the observational error propagates into the error of the estimated electron energy. We have found several best sets of H3+ lines from which the intensity-ratios can be utilized for the present purpose. Using these lines in the observed 2- and 4- micron wavelength ranges, we can estimate the electron energy and the background temperature within errors of a factor of ∼3.5% and 3%, respectively, if the observation error is 1%. Since saturnian H3+ emissions vary far more substantially according to temperature variations, the method described here is not applicable to observations of Saturn.

Electron beam measurements for a laboratory simulation of auroral kilometric radiation**This paper was presented as an invited talk at the 28th International Conference on Phenomena in Ionized Gases (ICPIG XXVIII) held in Prague, Czech Republic on 15–20 July 2007. See http://stacks.iop.org/0963-0252/17/2

Efficient (∼1%) electron cyclotron radio emissions are produced in the X-mode from regions of locally depleted plasma in the Earth's polar magnetosphere. These emissions are commonly referred to as auroral kilometric radiation. Two populations of electrons exist with rotational kinetic energy to contribute to this effect, the downward propagating auroral electron flux which acquires transverse momentum due to conservation of the magnetic moment as it experiences an increasing magnetic field and the mirrored component of this flux. This paper demonstrates the production of an electron beam having a controlled velocity spread for use in an experiment to investigate the available free energy in the earthbound electron flux. The experiment was scaled to microwave frequencies and used an electron gun to inject an electron beam into a controlled region of increasing magnetic field produced by a set of solenoids reproducing the magnetospheric situation. Results are presented of the measurements of diode voltage, beam current as a function of magnetic mirror ratio and estimates of the line density versus electron pitch angle consistent with the formation of a horseshoe velocity distribution and demonstrating control of the electron distribution in velocity space.

Pc5 pulsations on the ground, in the magnetosphere, and in the electron precipitation: Event of 19 January 2005

In the first part of our paper, we consider the pattern of geomagnetic pulsations in the Pc5 range in the North European area, based on an event of 19 January 2005. Intense pulsations, observed in Lovozero at Kola Peninsula, were accompanied by auroras north of the station, recorded by an all‐sky TV camera. In the second part, we consider the global pattern of amplitudes and phases of geomagnetic pulsations for this event, which includes data from nearly conjugate stations and the magnetic data of GOES 10 and 12, confirming the quasi‐stationary model for magnetic field distribution on the ground. In the event, the auroral luminosity oscillated with the same 5 min period as the magnetic field. Magnetic data from Scandinavia and data from North Europe riometers show analogous pulsations. The maximal intensity of geomagnetic pulsations in this region occurred near the pulsating aurora, but there was a reversal of pulsation polarity in the X‐component. In the area of phase change the value of the Z‐component is maximal. We suggest that geomagnetic pulsating variations, observed on the surface, are determined by Biot‐Savart's law for a three‐dimensional current system, the extra‐ionospheric part of which is spatially coincident with auroral electron flux. The electric field in the ionosphere is found from the current continuity condition and the value of Pedersen conductivity. The directions to the pulsating current, calculated by using the magnetic data from Lovozero, are close to the directions of the auroral area. We also claim that this approach is applicable to all short periodic oscillations observed on the surface. Satellite data indicates the same periods, but with oscillations that appear to be poloidal on GOES10 and toroidal on GOES12, suggesting the traveling character of the wave.

Further study of flickering auroral roar emission: 2. Theory and numerical calculations

Two recent papers report ground‐based observations of ∼3–30 Hz pulsations in the amplitude of 2fce auroral roar radio emissions. These pulsations occur in groups which last for ∼1 s with adjacent pulsation groups spaced from one to several seconds. We put forth that these pulsations reflect periodic modulation of precipitating auroral electron fluxes, which could include periodic downward field‐aligned bursts (FABs) and periodically modulated inverted‐V electron precipitation, similar to those that could account for flickering aurora. We calculate time and frequency variations in the growth rate of Z‐mode waves in the ionosphere for two different models of time‐varying electron distribution functions, one corresponding to acceleration by electromagnetic ion cyclotron waves (EMIC) and the other corresponding to a simple time‐varying accelerating potential (TVP). On the basis of these numerical calculations, the reported characteristics of ∼10 Hz flickering roar appear consistent with modulations induced in the electron distribution function via interaction with EMIC waves, since the modulation associated with EMIC waves results in flickering of only the lower portion of the auroral roar frequency band, as in most of the observations. The few observed cases of higher‐frequency flickering roar, at ∼20–30 Hz, fall above the O+ cyclotron frequency for the altitude range on interest, and may represent a different source of electron flux modulation such as a varying electrostatic potential. The radio observations provide an additional means of probing flickering phenomenon and in particular may put constraints on the form of modulations of the electron distribution function at the auroral roar source height of a few hundred kilometers.

Electrostatic Noise Spectrum at the Electron Cyclotron Frequency and Electromagnetic Emission in the Inhomogeneous Plasma of the Topside Ionosphere

Measurements of the wave emission of the topside ionosphere made onboard the APEX satellite using the electric component of the wave field in the 0.1–10 MHz frequency band are presented. At middle latitudes a wave intensity decrease was observed in the broad-band spectrum of the electrostatic noise at the electron cyclotron frequency. It is shown that a break in the spectrum of electrostatic modes at the electron cyclotron frequency (the absence of the plasma eigen-frequencies) may be a cause of the observed effect. The increase of the intensity at the electron cyclotron frequency in the ionospheric trough and at latitudes above the trough region as compared to middle latitudes may be explained by the capture by plasma irregularities of the electromagnetic emission of the auroral electron fluxes.

Morphology and seasonal variations of global auroral proton precipitation observed by IMAGE‐FUV

Observations with the FUV imagers on board the IMAGE satellite have been used to map the auroral electron and proton energy fluxes during the summer and winter solstices of 2000, in order to construct a statistical view of the global auroral proton precipitation. The distribution for electrons compare well both in morphology and in magnitude with those obtained previously with the Polar‐UVI instruments and with an empirical auroral precipitation model based on DMSP data. The proton morphology also closely resembles the statistical ion oval derived from DMSP data, showing a “C‐shaped” morphology with a minimum located in the morning sector. The precipitation proton auroral power is on the order of 2.2 GW for an average Kp value of 2, also in close agreement with the values of the DMSP empirical model. The FUV data also reveal the presence of seasonal effects in the proton precipitation. Specifically, the latitudinal width of the proton oval is larger in summer than in winter so that the globally precipitated proton power is 1.5 times higher in summer than in winter. The occurrence probability of intense proton auroras (with energy flux >0.5 mW m−2) is also shown to be nearly three times higher in summer than in winter. This seasonal effect in the proton precipitation contrasts with those observed for electrons, where intense electron events occur more often in winter than in summer. We discuss a mechanism that may account for these results based on the presence of field‐aligned potential drops which accelerate auroral electrons downward in regions of upward directed field‐aligned current, while suppressing the precipitating magnetospheric proton flux. The presence of such field‐aligned potentials is dependent on the differing solar illumination in winter and summer.

Modulation of auroral field-aligned electron fluxes under two inverted-V structures at different altitudes

[1] This paper presents observations of field-aligned electron flux bursts from the auroral sounding rocket flight, Alaska 2000, launched February 25, 2000 from Poker Flat, Alaska. In one particular energetic electron burst, the field-aligned electron flux is observed to be modulated at 22–30 Hz at energies up to 30 keV. By assuming that the observed energy dispersion in the bursts is caused by time-of-flight effects, we obtain characteristic source altitudes of 87 dispersive electron bursts, and find that the average characteristic source altitude of the bursts is 4015 ± 684 km in the first inverted-V and 5411 ± 972 km in the second inverted-V. This result implies that the electron-wave or other interaction depends more strongly on which arc it occurs in, rather than another factor such as energy of the electrons, since dispersive bursts at a wide range of energies and durations were used to arrive at the result.

Global electrodynamics observed during the initial and main phases of the July 1991 magnetic storm

Data acquired by seven fortuitously positioned satellites in the interplanetary medium, the magnetosphere, and the topside ionosphere, as well as from numerous ground magnetometers have allowed us to follow the evolution of global currents and electric fields during the geomagnetic storm on July 8–9, 1991. An interplanetary disturbance collided with the magnetosphere at ∼1636 UT on July 8th, compressing the magnetopause inside of geostationary orbit for about 3 hours, as observed by magnetometers on two GOES satellites. The resulting storm developed in four segments with a 7 hour lull separating the initial and main phases. The initial phase was characterized by (1) an extensive, DP 2 current system that produced an AE perturbation of ∼3500 nT, with reporting stations on the dayside, (2) a twenty‐fold increase in auroral electron fluxes, and (3) the immediate appearance of electric fields in the inner magnetosphere. The main phase intensification and earthward movement of the ring current are associated with southward turnings of the interplanetary magnetic field (IMF) and a polar cap potential increase above 200 kV. A significant fraction of the ring current growth was observed prior to the first of several substorms that occurred during the main and recovery phases. The beginning of recovery was characterized by a diminution of the southward IMF component, the cross polar cap potential and magnetospheric electric fields. It was also marked by a substorm onset. Considering the timing of Dst modulations relative to substorm occurrence, we conclude that during this magnetic storm, the electric fields responsible for ring current transport/energization were mostly associated with the DP 2 current system. Finally, in the evening‐sector penetration electric fields of the initial and main phases led to the formation of upward moving equatorial plasma bubbles that were detected by two DMSP satellites in the topside ionosphere.

Optical effects in the aurora caused by ionospheric HF heating

Abstract Ionospheric heating experiments were done by the EISCAT Heater in Tromso on 15–19 November, 1993. A low-light TV camera was installed at the VLF receiving station at Porojarvi about 100 km to the south-east of Tromso. The spectral analysis of the auroral luminosity variations showed that the brightness of the aurora varied at the modulation frequency of the heating wave. The results of this analysis and the numerical simulations of the auroral luminosity variations caused by the HF heating are shown. The variations of the optical emission intensity at the heating frequency occur during the auroral ionosphere modification. The observed intensity variation of the auroral green line during the interval of enhanced electron temperature is explained by a decreasing rate of the O2+ ion dissociative recombination when the electron temperature increases. The brightness variation depends on the characteristic energy and the intensity of the auroral electron flux and the heating wave parameters. The artificial luminosity pulsations caused by HF heating are estimated.

Initial comparison of POLAR UVI and Sondrestrom IS radar estimates for auroral electron energy flux

Calibrated images from the POLAR satellite ultraviolet imager (UVI) in the 165.5 to 174.5 nm portion of the N2 Lyman‐Birge‐Hopfield band (LBH‐long) can be used to estimate the energy flux (FE) of auroral electrons precipitating into the high‐latitude ionosphere. Similarly, electron density profiles, as measured by ground‐based incoherent‐scatter radar, can be used to estimate FE and mean energy (Eo) by solving a system of linear equations relating the E‐region ionization rate profile to a family of monoenergetic ion production profiles. A coordinated POLAR/Sondrestrom radar experiment, designed as an initial comparison of POLAR UVI and ground‐based estimates of FE for a stable auroral arc, was executed during a POLAR apogee on May 20, 1996 at the Sondrestrom radar facility (lat. 66.99°N, long. 50.95°W). Reconstructed energy distributions, based on radar‐measured Ne profiles, indicate an approximately 2 keV Maxwellian source with an energy flux of from 6.4 to 14 mW m−2. LBH‐long images, binned over 0.5° of latitude and 1.0° of longitude, were used to derive energy flux as well. The UVI‐derived FE time history agrees favorably with radar estimates both in absolute magnitude and in the trend for this period. This experiment suggests that reliable estimates for the precipitating electron source energy and its ionospheric response can be derived from either ground‐based radar or POLAR UVI images during summertime conditions.