Wakefield stimulated terahertz radiation from a plasma grating

When a short laser pulse propagates in a corrugated plasma, its wakefield interacts with the density and electric field ripples of the plasma. In the present paper, the modulation of the plasma originates from two counter-propagating long laser pulses, i.e. the corrugated plasma can be as- sumed as a so called plasma grating. PIC (particle in cell) simulations show electromagnetic wave radiation at a frequency just above the plasma frequency when the wakefield interacts with the grating. An electromagnetic instability is proposed as the reason for the emission process. The electrostatic driver of the electromagnetic instability is the beat of wake and grating. That beat mode possess large wavenumber (originating from the small grating wavelength) and small fre- quency (of the order of the plasma frequency) when one concentrates on small ratios of the plasma modulation length to the wavelength of the wakefield. The latter situation occurs when the long laser pulses (which generate the grating) as well as the short laser pulse (which drives the wakefield) have similar frequency ω0 ≫ ωpe where ωpe is the plasma frequency. The coherent volume emission process lasts for a while. It is finally superseded by terahertz transition radiation at the boundaries of the original grating.

Spectral Evolution of the X-Ray Remnant of SN 1987A: A High-resolution Chandra HETG Study

Based on observations with the Chandra X-ray Observatory, we present the latest spectral evolution of the X-ray remnant of SN 1987A (SNR 1987A). We present a high-resolution spectroscopic analysis using our new deep (∼312 ks) Chandra HETG observation taken in 2018 March as well as archival Chandra grating spectroscopic data taken in 2004, 2007, and 2011 with similarly deep exposures (∼170–350 ks). We perform detailed spectral model fits to quantify changing plasma conditions over the last 14 yr. Recent changes in electron temperatures and volume-emission measures suggest that the shocks moving through the inner ring have started interacting with less dense circumstellar material, probably beyond the inner ring. We find significant changes in the X-ray line-flux ratios (among H- and He-like Si and Mg ions) in 2018, consistent with changes in the thermal conditions of the X-ray-emitting plasma that we infer based on the broadband spectral analysis. Post-shock electron temperatures suggested by line-flux ratios are in the range ∼0.8–2.5 keV as of 2018. We do not yet observe any evidence of substantial abundance enhancement, suggesting that the X-ray emission component from the reverse-shocked metal-rich ejecta is not yet significant in the observed X-ray spectrum.

The OH (3-1) nightglow volume emission rate retrieved from OSIRIS measurements: 2001 to 2015

The OH airglow has been used to investigate the chemistry and dynamics of the mesosphere and the lower thermosphere (MLT) for a long time. The infrared imager (IRI) aboard the Odin satellite has been recording the night-time 1.53 µm OH (3-1) emission for more than 15 years (2001–2015), and we have recently processed the complete data set. The newly derived data products contain the volume emission rate profiles and the Gaussian-approximated layer height, thickness, peak intensity and zenith intensity, and their corresponding error estimates. In this study, we describe the retrieval steps for these data products. We also provide data screening recommendations. The monthly zonal averages depict the well-known annual oscillation and semi-annual oscillation signatures, which demonstrate the fidelity of the data set (https://doi.org/10.5281/zenodo.4746506, Li et al., 2021). The uniqueness of this Odin IRI OH long-term data set makes it valuable for studying various topics, for instance, the sudden stratospheric warming events in the polar regions and solar cycle influences on the MLT.

The OH (3-1) nightglow volume emission rate retrieved from OSIRIS measurements: 2001 to 2015

The OH airglow has been used to investigate the chemistry and dynamics of the mesosphere and the lower thermo-sphere (MLT) for a long time. The infrared imager (IRI) aboard the Odin satellite has been recording the nighttime 1.53 μm OH (3-1) emission for more than 15 years (2001–2015) and we have recently processed the complete data set. The newly derived data products contain the volume emission rate profiles and the Gaussian approximated layer height, thickness, peak intensity and zenith intensity, and their corresponding error estimates. In this study, we describe the retrieval steps of these data products. We also provide data screening recommendations. The monthly zonal averages depict the well known annual oscillation and semi-annual oscillation signatures, which demonstrate the fidelity of the data set (https://doi.org/10.5281/zenodo.4746506, Li et al. (2021)). The uniqueness of this Odin-IRI OH long-term data set makes it valuable for studying various topics, for instance, the sudden stratospheric warming events in the polar regions and solar cycle influences on the MLT.

Abel transform of exponential functions for planetary and cometary atmospheres with application to observation of 46P/Wirtanen and to the OI 557.7 nm emission at Mars.

<p>Line-of-sight integration of emissions from planetary and cometary atmospheres is the Abel transform of the emission rate, under the spherical symmetry assumption. Indefinite integrals constructed from the Abel transform integral are useful for implementing remote sensing data analysis methods, such as the numerical inverse Abel transform giving the volume emission rate compatible with the observation. We obtain analytical expressions based on a suitable, non-alternating, series development to compute those indefinite integrals. We establish expressions allowing absolute accuracy control of the convergence of these series depending on the number of terms involved. We compare the analytical method with numerical computation techniques, which are found to be sufficiently accurate as well. Inverse Abel transform fitting is then tested in order to establish that the expected emission rate profiles can be retrieved from the observation of both planetary and cometary atmospheres. We show that the method is robust, especially when Tikhonov regularization is included, although it must be carefully tuned when the observation varies across many orders of magnitude. A first application is conducted over observation of comet 46P/Wirtanen, showing some variability possibly attributable to an evolution of the contamination by dust and icy grains. A second application is considered to deduce the 557.7 nm volume emission rate profile of the metastable oxygen atom in the upper atmosphere of planet Mars.</p>

The O2 aurora observed by the TIMED/SABER satellite

<p>We studied O<sub>2</sub> aurora based on the observations of O<sub>2</sub> emission at 1.27 μm from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument during the nighttime over 18 years. The horizontal structure and vertical profile of O<sub>2</sub> auroral volume emission rate is obtained after removing O<sub>2</sub> nightglow contamination. The O<sub>2</sub> auroral intensity varies between 0.14 and 5.97 kR, and the peak volume emission rate varies between 0.97 × 10<sup>2</sup> and 41.01 × 10<sup>2</sup> photons cm<sup>−3</sup> s<sup>−1</sup>. The O<sub>2 </sub>auroral intensity and peak volume emission rate exponentially increases with increasing Kp index, whereas the peak height decreases with increasing Kp index. The O<sub>2</sub> auroral intensity and peak volume emission rate under solar minimum condition are larger than those under solar maximum condition. The peak height under solar minimum condition is lower than that under solar maximum condition.</p>

THE SIMULATION OF VIBRATIONAL POPULATIONS OF ELECTRONICALLY EXCITED N2 IN TITAN’S UPPER ATMOSPHERE DURING PRECIPITATIONS OF HIGH-ENERGETIC PARTICLES

We study the electronic kinetics of singlet molecular nitrogen in Titan’s upper atmosphere during precipitations of high-energetic particles. Both radiative processes and processes of electron excitation energy transfer during inelastic collisions with N2 and CH4 molecules were considered in the calculation of vibrational populations of electronically excited singlet states a'1Σu–, a1Πg, w1Δu of molecular nitrogen in the upper atmosphere of Titan. It is shown that the calculated volume emission intensities of the Lyman-Birge-Hopfield bands correlate with the profiles of the ion production rate in the atmosphere of Titan during the considered cases of electron precipitation for considered interval of the energies 30-1000 eV of magnetospheric electrons. This fact is explained by the negligible contribution of collisional processes to the vibrational populations a1Πg(v'=0-6) in the considered range of heights above 900 km.

Note on consistency between Kalogerakis–Sharma Mechanism (KSM) and two-step mechanism of atmospheric band emission (762 nm)

For more than 30 years, a two-step mechanism was used to explain observed Atmospheric band emission (762 nm) in mesopause region. A new mechanism, which leads to the formation of electronically excited molecular oxygen that gives this emission, was proposed recently. We show, based on an analytical solution, that the fit-functions for Atmospheric band volume emission in the case of the two-step mechanism and the new Kalogerakis–Sharma Mechanism (KSM) have analogous expression. This derivation solves the problem of consistency between the well-known two-step mechanism and the newly proposed KSM.

Retrieval of daytime mesospheric ozone using OSIRIS observations of O₂ (<i>a</i>¹Δ_g) emission

Improving knowledge of the ozone global distributions in the mesosphere–lower thermosphere (MLT) is a crucial step in understanding the behaviour of the middle atmosphere. However, the concentration of ozone under sunlit conditions in the MLT is often so low that its measurement requires instruments with very high sensitivity. Fortunately, the bright oxygen airglow can serve as a proxy to retrieve the daytime ozone density indirectly, due to the strong connection to ozone photolysis in the Hartley band. The OSIRIS IR imager (hereafter, IRI), one of the instruments on the Odin satellite, routinely measures the oxygen infrared atmospheric band (IRA band) at 1.27 µm. In this paper, we will primarily focus on the detailed description of the steps done for retrieving the calibrated IRA band limb radiance (with <10 % random error), the volume emission rate of O2 (a1Δg) (with <25 % random error) and finally the ozone number density (with <20 % random error). This retrieval technique is applied to a 1-year sample from the IRI dataset. The resulting product is a new ozone dataset with very tight along-track sampling distance (<20 km). The feasibility of the retrieval technique is demonstrated by a comparison of coincident ozone measurements from other instruments aboard the same spacecraft, as well as zonal mean and monthly average comparisons between Odin-OSIRIS (both spectrograph and IRI), Odin-SMR and Envisat-MIPAS. We find that IRI appears to have a positive bias of up to 25 % below 75 km, and up to 50 % in some regions above. We attribute these differences to uncertainty in the IRI calibration as well as uncertainties in the photochemical constants. However, the IRI ozone dataset is consistent with the compared dataset in terms of the overall atmospheric distribution of ozone between 50 and 100 km. If the origin of the bias can be identified before processing the entire dataset, this will be corrected and noted in the dataset description. The retrieval technique described in this paper can be further applied to all the measurements made throughout the 19 year mission, leading to a new, long-term high-resolution ozone dataset in the middle atmosphere.