Emission Altitude Research Articles

CO2 non‐LTE limb emissions in Mars' atmosphere as observed by OMEGA/Mars Express

AbstractWe report on daytime limb observations of Mars upper atmosphere acquired by the OMEGA instrument on board the European spacecraft Mars Express. The strong emission observed at 4.3 μm is interpreted as due to CO2 fluorescence of solar radiation and is detected at a tangent altitude in between 60 and 110 km. The main value of OMEGA observations is that they provide simultaneously spectral information and good spatial sampling of the CO2 emission. In this study we analyzed 98 dayside limb observations spanning over more than 3 Martian years, with a very good latitudinal and longitudinal coverage. Thanks to the precise altitude sounding capabilities of OMEGA, we extracted vertical profiles of the non‐local thermodynamic equilibrium (non‐LTE) emission at each wavelength and we studied their dependence on several geophysical parameters, such as the solar illumination and the tangent altitude. The dependence of the non‐LTE emission on solar zenith angle and altitude follows a similar behavior to that predicted by the non‐LTE model. According to our non‐LTE model, the tangent altitude of the peak of the CO2 emission varies with the thermal structure, but the pressure level where the peak of the emission is found remains constant at ∼0.03 ± 0.01 Pa, . This non‐LTE model prediction has been corroborated by comparing SPICAM and OMEGA observations. We have shown that the seasonal variations of the altitude of constant pressure levels in SPICAM stellar occultation retrievals correlate well with the variations of the OMEGA peak emission altitudes, although the exact pressure level cannot be defined with the spectroscopy for the investigation of the characteristics of the atmosphere of Venus (SPICAM) nighttime data. Thus, observed changes in the altitude of the peak emission provide us information on the altitude of the 0.03 Pa pressure level. Since the pressure at a given altitude is dictated by the thermal structure below, the tangent altitude of the peak emission represents then an important piece of information of the atmosphere, of great value for validating general circulation models. We thus compared the altitude of OMEGA peak emission with the altitude of the 0.03 Pa level predicted by the Laboratoire de météorologie dynamique (LMD)‐Mars global circulation model and found that the peak emission altitudes from OMEGA present a much larger variability than the tangent altitude of the 0.03 Pa level predicted by the general circulation model. This variability could be possibly due to unresolved atmospheric waves. Further studies using this strong CO2 limb emission data are proposed.

Space‐based imaging of nighttime medium‐scale traveling ionospheric disturbances using FORMOSAT‐2/ISUAL 630.0 nm airglow observations

AbstractThis paper reports the results of space‐based imaging of nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) in 630.0 nm emission by Imager of Sprites and Upper Atmospheric Lightnings (ISUAL), on board FORMOSAT‐2 satellite. The limb integrated measurements, after removing background, reveal multiple bands of intensity perturbation when projected to a horizontal plane corresponding to the altitude of peak emission, with distinct southwest to northeast orientation in the Southern Hemisphere. Simulations are carried out by artificially introducing MSTID fluctuations in model electron density to confirm if the MSTID could be identified in the ISUAL‐viewing geometry. The ISUAL observations in year 2007 are further used to investigate the MSTID features as well as occurrence characteristics in the Southern Hemisphere, most of which are over the ocean where no ground‐based observations are available. The preliminary statistics shows more MSTID occurrence in solstices with peak in June–July months. Majority of the MSTID perturbations have wavelength in the range 150–300 km, and the wavefronts are aligned at about 30°–50° from the east‐west plane. The statistic results of the orientation of wavefronts indicate that Es layer instability might be important in the MSTID generation.

Comparison of VLT/X-shooter OH and O<sub>2</sub> rotational temperatures with consideration of TIMED/SABER emission and temperature profiles

Abstract. Rotational temperatures Trot derived from lines of the same OH band are an important method to study the dynamics and long-term trends in the mesopause region near 87 km. To measure realistic temperatures, the rotational level populations have to be in local thermodynamic equilibrium (LTE). However, this might not be fulfilled, especially at high emission altitudes. In order to quantify possible non-LTE contributions to the OH Trot as a function of the upper vibrational level v′, we studied a sample of 343 echelle spectra taken with the X-shooter spectrograph at the Very Large Telescope at Cerro Paranal in Chile. These data allowed us to analyse 25 OH bands in each spectrum. Moreover, we could measure lines of O2b(0-1), which peaks at about 94 to 95 km, and O2a(0-0) with an emission peak at about 90 km. The latter altitude is reached in the second half of the night after a rise of several km because of the decay of a daytime population of excited O2. Since the radiative lifetimes for the upper levels of the two O2 bands are relatively long, the derived Trot are not significantly affected by non-LTE contributions. These bands are well suited for a comparison with OH if the differences in the emission profiles are corrected. For different sample averages, we made these corrections by using OH emission, O2a(0-0) emission, and CO2-based temperature profile data from the multi-channel radiometer SABER on the TIMED satellite. The procedure relies on differences of profile-weighted SABER temperatures. For an O2a(0-0)-based reference profile at 90 km, we found a good agreement of the O2 with the SABER-related temperatures, whereas the OH temperatures, especially for the high and even v′, showed significant excesses with a maximum of more than 10 K for v′ = 8. The exact value depends on the selected lines and molecular parameters. We could also find a nocturnal trend towards higher non-LTE effects, particularly for high v′. The amplitude of these variations can be about 2 K or less, which tends to be significantly smaller than the total amount of the non-LTE contributions. The variations revealed may be important for dynamical studies based on Trot derived from OH bands with high v′.

Upper tropospheric humidity changes under constant relative humidity

Abstract. Theoretical derivations are given on the change of upper tropospheric humidity (UTH) in a warming climate. The considered view is that the atmosphere, which is getting moister with increasing temperatures, will retain a constant relative humidity. In the present study, we show that the upper tropospheric humidity, a weighted mean over a relative humidity profile, will change in spite of constant relative humidity. The simple reason for this is that the weighting function that defines UTH changes in a moister atmosphere. Through analytical calculations using observations and through radiative transfer calculations, we demonstrate that two quantities that define the weighting function of UTH can change: the water vapour scale height and the peak emission altitude. Applying these changes to real profiles of relative humidity shows that absolute UTH changes typically do not exceed 1 %. If larger changes would be observed they would be an indication of climatological changes of relative humidity. As such, an increase in UTH between 1980 and 2009 in the northern midlatitudes, as shown by earlier studies using the High-resolution Infrared Radiation Sounder (HIRS) data, may be an indication of an increase in relative humidity as well.

Wide-band, low-frequency pulse profiles of 100 radio pulsars with LOFAR

LOFAR offers the unique capability of observing pulsars across the 10-240 MHz frequency range with a fractional bandwidth of roughly 50%. This spectral range is well-suited for studying the frequency evolution of pulse profile morphology caused by both intrinsic and extrinsic effects: such as changing emission altitude in the pulsar magnetosphere or scatter broadening by the interstellar medium, respectively. The magnitude of most of these effects increases rapidly towards low frequencies. LOFAR can thus address a number of open questions about the nature of radio pulsar emission and its propagation through the interstellar medium. We present the average pulse profiles of 100 pulsars observed in the two LOFAR frequency bands: High Band (120-167 MHz, 100 profiles) and Low Band (15-62 MHz, 26 profiles). We compare them with Westerbork Synthesis Radio Telescope (WSRT) and Lovell Telescope observations at higher frequencies (350 and1400 MHz) in order to study the profile evolution. The profiles are aligned in absolute phase by folding with a new set of timing solutions from the Lovell Telescope, which we present along with precise dispersion measures obtained with LOFAR. We find that the profile evolution with decreasing radio frequency does not follow a specific trend but, depending on the geometry of the pulsar, new components can enter into, or be hidden from, view. Nonetheless, in general our observations confirm the widening of pulsar profiles at low frequencies, as expected from radius-to-frequency mapping or birefringence theories. We offer this catalog of low-frequency pulsar profiles in a user friendly way via the EPN Database of Pulsar Profiles (http://www.epta.eu.org/epndb/).

High-speed stereoscopy of aurora

Abstract. We performed 100 fps stereoscopic imaging of aurora for the first time. Two identical sCMOS cameras equipped with narrow field-of-view lenses (15° by 15°) were directed at magnetic zenith with the north–south base distance of 8.1 km. Here we show the best example that a rapidly pulsating diffuse patch and a streaming discrete arc were observed at the same time with different parallaxes, and the emission altitudes were estimated as 85–95 km and > 100 km, respectively. The estimated emission altitudes are consistent with those estimated in previous studies, and it is suggested that high-speed stereoscopy is useful to directly measure the emission altitudes of various types of rapidly varying aurora. It is also found that variation of emission altitude is gradual (e.g., 10 km increase over 5 s) for pulsating patches and is fast (e.g., 10 km increase within 0.5 s) for streaming arcs.

Long-term variation of OH peak emission altitude and volume emission rate over Indian low latitudes

Using 13 (April 2002–December 2014) years of Sounding of the Atmosphere using Broadband Emission Radiometry (SABER/TIMED) 1.6µm OH airglow emission data, we have studied the long-term variation of OH peak emission altitude and volume emission rate (VER) for 0–10°N latitude and 70–90°E longitude grid. We have noted that, during day time the OH peak emission altitude is varying from 80 to 87km with mean value of 83.5km and from 82 to 88km with mean value of 85km during night time. The signature of semi-annual oscillation (SAO), annual oscillation (AO) and quasi-biennial oscillation (QBO) in the OH peak emission altitude as well as the VER is evident. Our analysis reveals that the SAO and QBO signatures but not the AO signature are very strong in the equatorial region during night time. Apart from the SAO, AO and QBO signatures, the presence of oscillation related to the El Niño oscillation (ENSO) is also noted. After the removal of these oscillations, we find the evidence of the influence of solar activity and a long term trend in the OH emission layer. It is also found good correlation between the mesospheric and stratospheric variations (ECMWF data).

SAR arcs we have seen: Evidence for variability in stable auroral red arcs

AbstractSince 1987, an all‐sky airglow imaging system has operated from a site at the Millstone Hill/Haystack Observatory in Westford, MA. During the ~2.5 solar cycles from 1987 to 2014, many studies using all‐sky images, in conjunction with incoherent scatter radar and satellite data, described subauroral, ionospheric disturbances observed during individual geomagnetic storms. The most prominent storm time optical feature from a subauroral site is a stable auroral red (SAR) arc. The standard use of a SAR arc's position is to locate the ionospheric footprint of the narrow plasmapause‐ring current interaction region where heat conduction from the inner magnetosphere excites emission within the F layer trough. When mapped from an emission altitude of 400 km to the geomagnetic equatorial plane, SAR arcs from Millstone Hill give the location of the plasmapause at radial distances between 2 to 4.5 Earth radii. A total of 314 SAR arcs have been observed during the 27 years of imaging at Millstone Hill. We find no single morphology for all SAR arcs, but rather patterns that occasionally depart from stability in space and time. We have classified these into five categories: longevity, multiplicity, zonal structure, latitudinal inhomogeneity, and tilt with respect to geomagnetic coordinates. In each case, the implications for the inner magnetosphere sources that drive SAR arcs are explored. While individual SAR arc variability characteristics have been noted in previous studies, here we describe for the first time all five types from the same site—an aspect not yet addressed in either magnetosphere or ionosphere modeling studies.

Correlated spin-down rates and radio emission in PSR B1859+07

We study the spin-down changes of PSR B1859$+$07 over a period of more than 28 years of radio observation. We identify that the time derivative of the rotational frequency ($\nu$) varies quasi-periodically with a period of $\sim$350 days, switching mainly between two spin-down states. The profile shape of the pulsar is correlated with the $\dot \nu$ variation, producing two slightly different profile shapes corresponding to high- and low-$\dot \nu$ states. In addition to these two normal emission states, we confirm the occasional flare-state of the pulsar, in which the emission appears early in spin phase compared to that of the common normal emission. The profile of the flare-state is significantly different from that of the two normal emission states. The correlation analysis further shows that the flare-state is not directly linked with the $\dot \nu$ changes. With a simple emission beam model, we estimate the emission altitude of the normal emission to be 240~km, and explain the origin of the flare-state as an emission height variation from the leading edge of the beam. We also argue that the emission of these states can be explained with a partially active beam model. In this scenario, the trailing portion of the radio beam is usually active and the normal emission is produced. The flare-state occurs when the leading edge of the beam becomes active while the trailing part is being blocked. This model estimates a fixed emission altitude of 360~km. However, the cause of the flare-state (i.e. the emission height variation, or the time-dependent activity across the radio beam) is not easily explained.

The ratio of profile peak separations as a probe of pulsar radio-beam structure

The known population of pulsars contains objects with four- and five-component profiles, for which the peak-to-peak separations between the inner and outer components can be measured. These Q- and M-type profiles can be interpreted as a result of sightline cut through a nested-cone beam, or through a set of azimuthal fan beams. We show that the ratio RW of the components’ separations provides a useful measure of the beam shape, which is mostly independent of parameters that determine the beam scale and complicate interpretation of simpler profiles. In particular, the method does not depend on the emission altitude and the dipole tilt distribution. The different structures of the radio beam imply manifestly different statistical distributions of RW, with the conal model being several orders of magnitude less consistent with data than the fan-beam model. To bring the conal model into consistency with data, strong effects of observational selection need to be called for, with 80 per cent of Q and M profiles assumed to be undetected because of intrinsic blending effects. It is concluded that the statistical properties of Q and M profiles are more consistent with the fan-shaped beams, than with the traditional nested-cone geometry.

SPICAM observations and modeling of Mars aurorae

Martian aurorae have been detected with the SPICAM instrument on board Mars Express both in the nadir and the limb viewing modes. In this study, we focus on three limb observations to determine both the altitudes and the intensities of the auroral emissions. The CO (a3Π–X1Σ) Cameron bands between 190 and 270nm, the CO (A1Π–X1Σ+) Fourth Positive system (CO 4P) between 135 and 170nm, the CO2+ (B2Σu+–X2Πg) doublet at 289nm, the OI at 297.2nm and the 130.4nm OI triplet emissions have been identified in the spectra and in the time variations of the signals. The intensities of these auroral emissions have been quantified and the altitude of the strongest emission of the CO Cameron bands has been estimated to be 137±27km. The locations of these auroral events have also been determined and correspond to the statistical boundary of open-closed magnetic field lines, in cusp-like structures. The observed altitudes of the auroral emissions are reproduced by a Monte-Carlo model of electron transport in the Martian thermosphere for mono-energetic electrons between 40 and 200eV.No correlation between electron fluxes measured in the upper thermosphere and nadir auroral intensity has been found. Here, we simulate auroral emissions observed both at the limb and at the nadir using electron energy spectra simultaneously measured with the ASPERA-3/ELS instrument. The simulated altitudes are in very good agreement with the observations. We find that predicted vertically integrated intensities for the various auroral emissions are overestimated, probably as a consequence of the inclination and curvature of the magnetic field line threading the aurora. However, the relative brightness of the CO and CO2+ emissions is in good agreement with the observations.

Temporal variations of flux and altitude of sulfur dioxide emissions during volcanic eruptions: implications for long-range dispersal of volcanic clouds

Abstract. Sulfur-rich degassing, which is mostly composed of sulfur dioxide (SO2), plays a major role in the overall impact of volcanism on the atmosphere and climate. The accurate assessment of this impact is currently hampered by the poor knowledge of volcanic SO2 emissions. Here, using an inversion procedure, we show how assimilating snapshots of the volcanic SO2 load derived from the Infrared Atmospheric Sounding Interferometer (IASI) allows for reconstructing both the flux and altitude of the SO2 emissions with an hourly resolution. For this purpose, the regional chemistry-transport model CHIMERE is used to describe the dispersion of SO2 when released in the atmosphere. As proof of concept, we study the 10 April 2011 eruption of the Etna volcano (Italy), which represents one of the few volcanoes instrumented on the ground for the continuous monitoring of SO2 degassing. We find that the SO2 flux time-series retrieved from satellite imagery using the inverse scheme is in agreement with ground observations during ash-poor phases of the eruption. However, large discrepancies are observed during the ash-rich paroxysmal phase as a result of enhanced plume opacity affecting ground-based ultraviolet (UV) spectroscopic retrievals. As a consequence, the SO2 emission rate derived from the ground is underestimated by almost one order of magnitude. Altitudes of the SO2 emissions predicted by the inverse scheme are validated against an RGB image of the Moderate Resolution Imaging Spectroradiometer (MODIS) capturing the near-source atmospheric pathways followed by Etna plumes, in combination with forward trajectories from the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model. At a large distance from the source, modelled SO2 altitudes are compared with independent information on the volcanic cloud height. We find that the altitude predicted by the inverse scheme is in agreement with snapshots of the SO2 height retrieved from recent algorithms exploiting the high spectral resolution of IASI. The validity of the modelled SO2 altitude is further confirmed by the detection of a layer of particles at the same altitude by the spaceborne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). Analysis of CALIOP colour and depolarization ratios suggests that these particles consist of sulfate aerosols formed from precursory volcanic SO2. The reconstruction of emission altitude, through inversion procedures which assimilate volcanic SO2 column amounts, requires specific meteorological conditions, especially sufficient wind shear so that gas parcels emitted at different altitudes follow distinct trajectories. We consequently explore the possibility and limits of assimilating in inverse schemes infrared (IR) imagery of the volcanic SO2 cloud altitude which will render the inversion procedure independent of the wind shear prerequisite.

Modeled and observed equatorial thermospheric winds and temperatures

AbstractThermospheric winds and temperatures must be correctly specified to understand the impacts of lower atmosphere processes on the upper atmosphere and to measure the global effects of high‐latitude magnetospheric processes. Fabry‐Perot interferometers can estimate these parameters by measuring the characteristic 630.0 nm emission that is produced at around 250 km altitude. These sophisticated instruments exist at only a few locations globally, so models are often employed to provide wind and temperature estimates elsewhere. This study is composed of two parts. First, observing system simulation experiments estimate the accuracy of Fabry‐Perot interferometer observations using the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM) and the Whole Atmosphere Community Climate Model eXtended (WACCM‐X). Atmospheric observational error sources are found to be very small across two test periods (September 2000 and September 2010) and using two different “truth” models. The largest magnitude wind observation error is found to be 16.9 m/s, root‐mean‐square errors are 2.3 m/s, and the bias is 0.9 m/s. The largest‐magnitude temperature observation error is found to be 63.7 K, root‐mean‐square errors over the test period are 6.7 K, and the bias is 2.8 K. Modeled redline emission altitudes vary by over 100 km, far more than was expected. Second, several models (TIEGCM, WACCM‐X, the Horizontal Wind Model, and the Mass Spectrometer Incoherent Scatter model) are assessed using interferometer winds and temperatures from Cariri and Cajazeiras, Brazil, as ground truth. In the best cases, the models reproduce wind variability without systematic biases but show no ability to predict instantaneous values, although temperatures are modeled more accurately.

Lunar semidiurnal tide in the terrestrial airglow

We report on lunar semidiurnal tidal signatures in several parameters of the terrestrial airglow, including OI green line emission rates, OH(3-1) emission rates, as well as OH emission altitude, atomic oxygen, and OH(3-1) rotational temperature in the mesosphere/lower thermosphere region. The parameters were retrieved from spaceborne measurements of nightglow emissions at low latitudes with the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography instrument on the Envisat satellite. The identified lunar tidal signatures in airglow emissions have amplitudes of a few percent and are highly significant statistically. Moreover, the signatures observed in the different parameters analyzed show a coherent behavior consistent with the view that they are caused by vertical motions associated with vertical transport of atomic oxygen and adiabatic cooling/heating. The observed lunar semidiurnal tidal signature in temperature is in good agreement with model simulations with the global scale wave model.

OH populations and temperatures from simultaneous spectroscopic observations of 25 bands

Abstract. OH rotational temperatures are widely used to derive mesopause temperatures and their variations. Since most data sets are only based on a fixed set of lines of a single band, it is important to know possible systematic uncertainties related to the choice of lines. Therefore, a comprehensive study of as many OH bands as possible is desirable. For this purpose, astronomical echelle spectrographs at large telescopes are the most suitable instruments. They offer a wide wavelength coverage, relatively high spectral resolution, and high sensitivity. Moreover, since each ground-based astronomical observation has an imprint of the Earth's atmosphere, the data archives of large astronomical facilities are a treasure for atmospheric studies. For our project, we used archival data of the medium-resolution X-shooter echelle spectrograph operated by the European Southern Observatory at Cerro Paranal in Chile. The instrument can simultaneously observe all OH bands that are accessible from ground. We reduced and analysed a set of 343 high-quality spectra taken between 2009 and 2013 to measure OH line intensities and to derive rotational and vibrational temperatures of 25 bands between 0.58 and 2.24 μm. We studied the influence of the selected line set, OH band, upper vibrational level v′, and the molecular data on the derived level populations and temperatures. The rotational temperature results indicate differences by several degrees depending on the selection. The temperatures for bands of even and odd v′ show deviations which increase with v′. A study of the temporal variations revealed that the nocturnal variability pattern changes for v′ from 2 to 9. In particular, the spread of temperatures tends to increase during the night, and the time of the minimum temperature depends on v′. The vibrational temperatures depend on the range of v′ used for their determination, since the higher vibrational levels from 7 to 9 seem to be overpopulated compared to the lower levels. The vibrational temperature tends to increase during the night, while the intensity decreases. Our results support the assumption that the OH emission altitude depends on v′. Moreover, the emission layer appears to rise in the course of the night, which makes the OH thermalisation less efficient. The derived rotational temperatures and their change with v′ seem to be significantly affected by non-equilibrium populations.

Unusual depletion of OI 630.0 nm dayglow and simultaneous mesopause heating during the penetration of interplanetary electric field over dip equator

AbstractEvidence for simultaneous changes in the thermosphere and mesopause associated with penetration electric fields using the OI 630.0 nm and OH (8, 3) dayglow measurements are presented. An unusual decrease in the thermospheric OI 630.0 nm dayglow and an intense heating (~30 K) at OH emission altitudes over a magnetic dip equatorial station, Trivandrum, India, were observed on 9 April 2006, coincident with the penetration of a noontime westward interplanetary electric field. The Sounding of the Atmosphere using Broadband Emission Radiometry observations on board TIMED satellite also revealed heating at the mesopause heights during this period. In addition, ionosonde and magnetometer observations indicated the presence of strong penetration electric fields. We propose that strong heating at the mesopause resulted in the intrusion of additional neutrals, like N2, into the emission altitudes and quenched the O(1D) atoms therein. The reduced Cowling conductivity at the E region inferred using magnetometer observations further corroborates this proposed mechanism.

Variability of OH(3–1) emission altitude from 2003 to 2011: Long-term stability and universality of the emission rate–altitude relationship

Vertical volume emission rate profiles of the OH(3–1) Meinel emission near the mesopause are retrieved from nighttime limb-emission observations with the SCIAMACHY (Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY) instrument on the Envisat satellite. Mean emission altitudes are determined by weighting altitude with the vertical OH(3–1) volume emission rate profile. Analysis of the SCIAMACHY data set from January 2003 to December 2011 shows a clear semi-annual variation of the mean emission altitudes at low latitudes – where SCIAMACHY performs measurements throughout the year – with an amplitude of 0.5–1.0km. Confirming earlier studies, we find a near constant, or universal, scaling of mean OH emission altitude and vertically integrated emission rate, which can be employed by ground-based observers to infer indirect information on OH emission altitude and its variability, if measurements of OH emission rates are performed. For this purpose we provide climatological fit parameters for the altitude/emission rate relationship. No obvious long-term trends or 11-year solar cycle signatures are present in the OH emission altitude time series, partly contradicting earlier studies. The long-term stability in OH emission altitude at the local time of the SCIAMACHY nighttime observations (10 p.m.) strengthens the use of ground-based OH rotational temperature measurements to study middle atmospheric climate change.

Fire evolution in the radioactive forests of Ukraine and Belarus: future risks for the population and the environment

In this paper, we analyze the current and future status of forests in Ukraine and Belarus that were contaminated after the nuclear disaster in 1986. Using several models, together with remote‐sensing data and observations, we studied how climate change in these forests may affect fire regimes. We investigated the possibility of 137Cs displacement over Europe by studying previous fire events, and examined three fire scenarios that depended on different emission altitudes of 137Cs, assuming that 10% of the forests were affected by fires. Field measurements and modeling simulations confirmed that numerous radioactive contaminants are still present at these sites in extremely large quantities.Forests in Eastern Europe are characterized by large, highly fire‐prone patches that are conducive to the development of extreme crown fires. Since 1986, there has been a positive correlation between extreme fire events and drought in the two contaminated regions. Litter carbon storage in the area has doubled since 1986 due to increased tree mortality and decreased decomposition rates; dead trees and accumulating litter in turn can provide fuel for wildfires that pose a high risk of redistributing radioactivity in future years. Intense fires in 2002, 2008, and 2010 resulted in the displacement of 137Cs to the south; the cumulative amount of 137Cs re‐deposited over Europe was equivalent to 8% of that deposited following the initial Chernobyl disaster. However, a large amount of 137Cs still remains in these forests, which could be remobilized along with a large number of other dangerous, long‐lived, refractory radionuclides. We predict that an expanding flammable area associated with climate change will lead to a high risk of radioactive contamination with characteristic fire peaks in the future. Current fire‐fighting infrastructure in the region is inadequate due to understaffing and lack of funding. Our data yield the first cogent predictions for future fire incidents and provide scientific insights that could inform and spur evidence‐based policy decisions concerning highly contaminated regions around the world, such as those of Chernobyl.

HF Accelerated Electron Fluxes, Spectra, and Ionization

Wave particle interactions, an essential aspect of laboratory, terrestrial, and astrophysical plasmas, have been studied for decades by transmitting high power HF radio waves into Earth’s weakly ionized space plasma, to use it as a laboratory without walls. Application to HF electron acceleration remains an active area of research (Gurevich in Usp Fizicheskikh Nauk 177(11):1145–1177, 2007) today. HF electron acceleration studies began when plasma line observations proved (Carlson et al. in J Atmos Terr Phys 44:1089–1100, 1982) that high power HF radio wave-excited processes accelerated electrons not to ~eV, but instead to −100 times thermal energy (10 s of eV), as a consequence of inelastic collision effects on electron transport. Gurevich et al (J Atmos Terr Phys 47:1057–1070, 1985) quantified the theory of this transport effect. Merging experiment with theory in plasma physics and aeronomy, enabled prediction (Carlson in Adv Space Res 13:1015–1024, 1993) of creating artificial ionospheres once ~GW HF effective radiated power could be achieved. Eventual confirmation of this prediction (Pedersen et al. in Geophys Res Lett 36:L18107, 2009; Pedersen et al. in Geophys Res Lett 37:L02106, 2010; Blagoveshchenskaya et al. in Ann Geophys 27:131–145, 2009) sparked renewed interest in optical inversion to estimate electron spectra in terrestrial (Hysell et al. in J Geophys Res Space Phys 119:2038–2045, 2014) and planetary (Simon et al. in Ann Geophys 29:187–195, 2011) atmospheres. Here we present our unpublished optical data, which combined with our modeling, lead to conclusions that should meaningfully improve future estimates of the spectrum of HF accelerated electron fluxes. Photometric imaging data can significantly improve detection of emissions near ionization threshold, and confirm depth of penetration of accelerated electrons many km below the excitation altitude. Comparing observed to modeled emission altitude shows future experiments need electron density profiles to derive more accurate HF electron flux spectra.

Vertical emissivity profiles of Jupiter's northern H3+ and H2 infrared auroras observed by Subaru/IRCS

AbstractWe resolved the vertical emissivity profiles of H3+ overtone, H3+ hot overtone, and H2 emission lines of the Jovian northern auroras in K band obtained in December 2011 observed by the IR Camera and Spectrograph of the Subaru 8.2 m telescope with the adaptive optics system (AO188). The spatial resolution achieved was ~0.2 arcsec, corresponding to ~600 km at Jupiter. We derived the vertical emissivity profiles at three polar regions close to the Jovian limb. The H3+ overtone and H3+ hot overtone lines had similar peak altitudes of 700–900 km and 680–950 km above the 1 bar level, which were 100–300 km and 150–420 km lower, respectively, than the model values. On the contrary, the H2 peak emission altitude was high, 590–720 km above the 1 bar level. It was consistent with the value expected for precipitation of ~1 keV electron, which favors a higher‐altitude emissivity profile. We concluded that the lower peak altitudes of H3+ overtone and hot overtone lines were caused by the nonlocal thermodynamic equilibrium effect stronger than the model assumption. We could reproduce the observational emissivity profiles from the model by including this effect. It has been proposed that neutral H2 and ionized H3+ emissions can have different source altitudes because of their different morphologies and velocities; however, our observed results with a general circulation model show that the peak emission altitudes of H3+ and H2 can be similar even with different velocities.