Electron Energy Input Research Articles

Concurrent ultraviolet and infrared observations of the north Jovian aurora during Juno's first perijove

The UltraViolet Spectrograph (UVS) and the Jupiter InfraRed Auroral Mapper (JIRAM) observed the north polar aurora before the first perijove of the Juno orbit (PJ1) on 27 August 2016. The UVS bandpass corresponds to the H2 Lyman and Werner bands that are directly excited by collisions of auroral electrons with molecular hydrogen. The spectral window of the JIRAM L-band imager includes some of the brightest H3+ thermal features between 3.3 and 3.6 µm. A series of spatial scans obtained with JIRAM every 30 s is used to build up five quasi-global images, each covering ∼12 min. of observations. JIRAM's best spatial resolution was on the order of 50 km/pixel during this time frame, while UVS has a resolution of about 750 km. Most of the observed large-scale auroral features are similar in the two spectral regions, but important differences are also observed in their morphology and relative intensity. Only a part of the UV-IR differences stems from the higher spatial resolution of JIRAM, as some of them are still present following smoothing of the JIRAM images at the UVS resolution. For example, the JIRAM images show persistent narrow arc structures in the 100°–180° SIII longitude sector at dusk not resolved in the ultraviolet, but consistent with the structure of in situ electron precipitation measured two hours later. The comparison between the H2 intensity and the H3+ radiance measured along two radial cuts from the center of the main emission illustrates the complex relation between the electron energy input, their characteristic energy and the H3+ emission. Low values of the H3+ intensity relative to the H2 brightness are observed in regions of high FUV color ratio corresponding to harder electron precipitation. The rapid loss of H3+ ions reacting with methane near and below the homopause appears to play a significant role in the control of the relative brightness of the two emissions. Cooling of the auroral thermosphere by H3+ radiation is spatially variable relative to the direct particle heating resulting from the precipitated electron flux.

Statistics of “Cold” Early Impulsive Solar Flares in X-Ray and Microwave Domains

Abstract Solar flares often happen after a preflare/preheating phase, which is almost or entirely thermal. In contrast, there are the so-called early impulsive flares that do not show a (significant) preflare heating, but instead often show the Neupert effect—a relationship where the impulsive phase is followed by a gradual, cumulative-like, thermal response. This has been interpreted as a dominance of nonthermal energy release at the impulsive phase, even though a similar phenomenology is expected if the thermal and nonthermal energies are released in comparable amounts at the impulsive phase. Nevertheless, some flares do show a good quantitative correspondence between the nonthermal electron energy input and plasma heating; in such cases, the thermal response was weak, which results in them being called “cold” flares. We undertook a systematic search for such events among early impulsive flares registered by the Konus-Wind instrument in the triggered mode from 11/1994 to 4/2017, and selected 27 cold flares based on relationships between hard X-ray (HXR) (Konus-Wind) and soft X-ray (Geostationary Operational Environmental Satellite) emission. For these events, we put together all available microwave data from different instruments. We obtained temporal and spectral parameters of HXR and microwave emissions of the events and examined correlations between them. We found that, compared to a “mean” flare, the cold flares: (i) are weaker, shorter, and harder in the X-ray domain; (ii) are harder and shorter, but not weaker in the microwaves; (iii) have a significantly higher spectral peak frequencies in the microwaves. We discuss the possible physical reasons for these distinctions and implication of the finding.

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

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

Statistical comparison of WINDII auroral green line emission rate with DMSP/SSJ4 electron energy input for high and low solar flux years

[1] The atmospheric response to precipitating auroral electrons under varying conditions of solar illumination is investigated using a statistical approach with respect to magnetic local time (MLT), solar zenith angle (χ), and geomagnetic latitude (MLAT) in the northern high-latitude region. Vertical profiles of daytime auroral O(1S) emission rate are obtained from UARS/WINDII measurements by removing the airglow components. These are compared with precipitating electron and proton input observed with DMSP-F10 and -F12 SSJ4 from which total energy and average energy are obtained. In order to investigate the solar flux effect, 2 a of data are examined, 1993 (high solar flux) and 1996 (low solar flux). Total electron energy flux increases with increasing solar zenith angle from 40° to 160° so that the energy flux during nighttime is much greater than in the daytime. During the daytime (χ ≤ 90°) the auroral atmospheric response correlates well with electron energy input yielding auroral production rates of 1.2 and 1.6 kR erg−1 cm2 s, but this relation breaks down beyond χ = 90°. It is suspected that nighttime thin sheet auroral profiles are not correctly inverted for this limb-viewing imager. A strong anticorrelation is found between the altitude of peak auroral volume emission rate and the average precipitating electron energy with slopes of 12.1 km keV−1 and 5.7 km keV−1 for 1993 and 1996, respectively. The dependence of auroral emission rate on solar illumination conditions is consistent with earlier work by Newell et al. (1996b) and others, but the clear dependence on solar zenith angle has not been demonstrated before.

Forecasting electron precipitation based on predicted geomagnetic activity

We have developed a statistical model of precipitating electrons based on 10 years worth of NOAA-12 Space Environment Monitor (SEM) data. For each hour of UT, the electron spectral shape is defined for solar activity (using F 10.7), geomagnetic activity (using Dst, Kp, and PC), invariant latitude (40 to 90°N and −40° to −90°S), and every hour of magnetic local time to form a climatology. The current version of our climatology includes a description of the precipitating differential number flux, unfolded from the integral high-energy telescopes of SEM and its lower-energy differential measurements. In total, the energy range covered by SEM is from 300 eV to 1 MeV. We use real-time Dst predictions from several sites on the Internet which forecast Dst up to 1 h ahead of time to predict the corresponding precipitating electron flux. Our electron precipitation forecast contributes to space weather applications by predicting a spectral description of atmospheric electron energy input. These electron energy spectra can be used to generate atmospheric energy input, ionization rates and height-integrated Hall and Pedersen conductivities, which are important parameters to models such as AMIE and TIME-GCM. Our data could also improve the space environment description used in spacecraft charging models.

Hydrophobic Coatings from Plasma Polymerized Vinyltrimethylsilane

Thin-films have been deposited onto silicon and metal substrates by plasma polymerization of vinyltrimethylsilane (VTMS), to provide hydrophobic coatings for dropwise condensation of steam. The deposition conditions have been sampled by a set of statistically designed experiments, and the films' thicknesses, water droplet contact angles, elemental compositions (by XPS), and –CHx contents have been measured. There is a strong correlation between these film properties and their deposition conditions. The films that continue to provide successful long-term dropwise condensation had been deposited at a low rate onto heated substrates, such that they have high water contact angle and high –CHx content. This required a plasma that provided a low electron energy input per precursor species collision.

Hydrophobic Coatings from Plasma Polymerized Vinyltrimethylsilane

Thin-films have been deposited onto silicon and metal substrates by plasma polymerization of vinyltrimethylsilane (VTMS), to provide hydrophobic coatings for dropwise condensation of steam. The deposition conditions have been sampled by a set of statistically designed experiments, and the films' thicknesses, water droplet contact angles, elemental compositions (by XPS), and –CHx contents have been measured. There is a strong correlation between these film properties and their deposition conditions. The films that continue to provide successful long-term dropwise condensation had been deposited at a low rate onto heated substrates, such that they have high water contact angle and high –CHx content. This required a plasma that provided a low electron energy input per precursor species collision.

Comparison of thermal and nonthermal hard X-ray emission in electron-heated solar flares

Using the results of numerical simulations of the solar atmospheric response to heating by nonthermal electron beams during solar flares, we have calculated the spatial and temporal evolution of both (i) the direct (beam-target) nonthermal bremsstrahlung and (ii) the thermal bremsstrahlung arising from the hot plasma energized by the electron beam. Typically, we find that below a certain ‘cross-over’ energy E *, the emission is dominated by the thermal component, while at higher energies the direct bremsstrahlung component becomes more important. This ‘cross-over’ energy is dependent on the position within the loop, generally increasing with height. We have also investigated the dependence of the ‘cross-over’ energy E * on the parameters of the electron energy input. At the time of peak electron flux injection the ‘cross-over’ energy E * can, for plausible parameters, be as high as 52 keV at the top 1″ pixel, and as low as 16 keV at the bottom 1″ pixel. We conclude that a possible reassessment of SMM HXIS data as an indicator of the ‘thermal’ or ‘nonthermal’ character of the primary energy release (based primarily on the geometric properties of the hard X-ray source) is required. Our results also point to the minimum photon energy that future instruments should observe (where practical, giving due consideration to detector sensitivity) in order to be sure that, in the context of the thick-target interpretation, the nonthermal component is not swamped by the self-consistent thermal counterpart created by the beam heating.

Electrodynamics of the polar cusp ionosphere; A case study

By using continuous ground‐based observations of polar cusp auroras and electromagnetic parameters obtained from satellites in polar orbit, electrodynamic processes in the polar cusp have been investigated. HILAT satellite measurements of magnetic field, precipitating electrons and ion drift components above auroral structures observed from the ground permit detailed investigations of the cusp ionosphere. Information on Birkeland currents, Poynting flux, height integrated Pedersen conductivity and Joule heat dissipation rate has been derived. Similar latitudinal structures were observed from HILAT (electron precipitation/ion drift) and in the simultaneous photometer profiles obtained from the ground ∼200 km to the east of the satellite pass. Intermediate scale (tens of kilometers) structures of enhanced electron precipitation (peak energy flux ∼10 ergs/cm²s (0.01 W/m²) and average energy ∼0.1 keV), strong northward electric fields (peak values ∼200 mV/m), and Birkeland current densities ∼10 µA/m² were observed. Rather complex patterns of Birkeland currents were connected to these structures. From correlated latitudinal variations in electric and magnetic field components the downward directed Poynting flux and the ionospheric Joule heat dissipation rate within one of the auroral forms could be estimated. The dissipation rate in the center of this structure was found to be a factor of 2 higher than the electron energy input rate. The satellite pass occurred within an interval (interplanetary magnetic field (IMF) Bz < 0) characterized by a series of poleward moving auroral structures (east‐west motion not resolved by the present technique), each appearing at the cusp equatorward boundary. The auroral forms were dominated by red oxygen emission (I630.0 nm/I557.7 nm ≃5 and I630.0 nm ≃5 kR), which is typical in the cusp region.

Electron fluxes and correlations with quiet time auroral arcs

Primary electron fluxes, measured by two satellites as they crossed quiet time (Kp = 1, Σ Kp = 11+) auroral arcs, are presented for times near 1420 UT on November 12, 1972. The DMSP 30 satellite measured the optical intensity at 0.6–1.0 µm full width at half maximum (FWHM) and precipitated electrons with 0.22 ≤ E ≤ 19.7 keV near 2100 MLT. The 72-1 satellite crossed the auroral arc system near 2200 MLT and measured 0.36- to 16.5-keV electrons at all pitch angles. A detailed description of the electron morphology that includes comparisons of electron fluxes with optical intensities is presented. We find that the optical intensities extending from 2100 to 2200 MLT correlate best with electrons with E > 1 keV. The strongest emission bands at these near-infrared wavelengths are N2(IP) (first-positive band of N2) and N2+(M) (Meinel band of N2+). We estimate that between 4 and 10% of the primary electron energy input is converted to an optical intensity of 140 kR between 0.6 and 1.0 µm FWHM and is equivalent to a weak IBC 2 in the 5577-A green line of atomic oxygen.