Shumann resonances and electromagnetic transparence in the atmosphere of Titan

Among the multiple questions that the CASSINI/HUYGENS mission tries to answer is the likelihood of electric discharges in Titan's atmosphere. The instruments “Huygens Atmospheric Structure Instrument” and “Radio and Plasma Wave Science” will probe the electromagnetic emissions during the Huygens descent and Cassini flybys, respectively. Although no lightning was observed during Voyager's encounters with Titan in 1980 and 1981, this does not exclude the existence of lightning phenomena. Recent investigations show that lightning discharges could occur in the lower atmosphere, such as the detection of methane condensation clouds in the troposphere and the theoretical prediction of an electric field that would be sufficient enough to cause lightning. We present a numerical model of Titan's atmosphere with the aim of calculating the resonance frequencies and the atmospheric transparency to electromagnetic waves. The detection and measurement of these resonances, Schumann frequencies, by the Huygens probe, would show the existence of electric activity connected with lightning discharges in the atmosphere. As it happens with the Schumann frequencies of Earth, losses associated with the electric conductivity will make these frequencies to be lower than the theoretically predicted, the fundamental one being located between 11 and 15 Hz. An analytical study shows that the strong losses associated with the high conductivity make it impossible that an electromagnetic wave generated near the surface with a frequency of 10 MHz or lower reaches the outer part of Titan's atmosphere. Therefore the detection of electromagnetic waves coming from Titan's lower atmosphere by the RPWS instrument is very unlikely.     

Possible DC electric field in the ionosphere related to seismicity

This paper presents the method for calculation of DC electric field in the atmosphere and the ionosphere generated by model distribution of external electric current in the lower atmosphere. Appearance of such current is associated with enhancement of seismic activity that is accompanied by emanation of soil gases into the atmosphere. These gases transfer positive and negative charged aerosols. Atmospheric convection of charged aerosols forms external electric current, which works as a source of conductivity current in the atmosphere–ionosphere electric circuit. It is shown that DC electric field generated in the ionosphere by this current reaches up to 10 mV/m, while the long-term vertical electric field disturbances excited near the Earth surface do not exceed 100 V/m. Such limitation of the near-ground field is caused by the formation of potential barrier for charged particles at the Earth surface in a process of their transport from soil to atmosphere.     

Neutron spectra in the atmosphere from interactions of primary cosmic rays

Spectra of neutrons from interactions of primary cosmic rays in the earth's atmosphere are calculated with the Monte Carlo model fluka for various depths down to sea level. We discuss the environmental models describing the primary cosmic ray spectrum and details of the calculations. Neutron energy spectra are presented for different depths in the atmosphere and for different geographical locations. By comparing results of calculations to measurements on neutron spectra it is shown that fluka may serve as an important tool for the estimation of the radiation environment in the atmosphere.     

Model for induced ionization by galactic cosmic rays in the Earth atmosphere and ionosphere

The ionization profiles produced by galactic cosmic rays in the Earth atmosphere and ionosphere are obtained on the basis of Monte Carlo simulations. Cascade processes in the atmosphere are simulated using CORSIKA 6.52 code with FLUKA 2006 and QGSJET II hadronic interaction subroutines. Proton induced showers are considered using a realistic atmospheric model (US Standard Atmosphere). The energy deposit from different components is taken into account, namely electromagnetic, hadron and muon components. The curvature of the atmosphere is considered in the computer code. On the basis of the computed ionization yield function the ion pair production rate in the atmosphere is obtained for different conditions and locations. The model is applicable to the entire atmosphere, from ground level to upper atmosphere. Several applications of the obtained results are discussed. The Monte Carlo simulation model considers nuclear interactions below the altitude of 35 km. It is compared with analytical–numerical electron production rate model. The latter model which takes into account the electromagnetic interactions above altitudes of 35 km has two main regions of application: above 50 km (thin target model) and between 35 and 50 km (intermediate target model). A good agreement between the CORSIKA results and analytical–numerical model results is found above altitude of 35 km.     

Circulation of the atmosphere from the surface to 100 km

A reference model of the atmospheric circulation on Venus based on available observations is presented. The reference atmosphere has the following main features: (i) the entire atmosphere below 85 km moves predominantly from east to west in the planet's reference system (in the same direction as the rotation of the solid planet itself) with the possible exception of the lowest 10 km where velocities are low, (ii) a jet is present near the cloud-top level at 45° latitude in both hemispheres, with a magnitude of approximately 100 ms^?1, (iii) a weak meridional (north-south component) flow directed towards either pole is superimposed on the zonal (east-west) motion at cloud-top level (about 68 km) altitudes, and, (iv) eddies or wave motions are present in the atmosphere, with amplitudes of less than 15 ms^?1 in the upper atmosphere.     

Solar cosmic rays in the near Earth space and the atmosphere

Solar energetic particles (SEPs) constitute a distinct population of energetic charged particles, which can be often observed in the near Earth space. SEP penetration into the Earth’s magnetosphere is a complicated process depending on particle magnetic rigidity and geomagnetic field structure. Particles in the several MeV energy range can only access to periphery of the magnetosphere and the polar cap regions, while the GeV particles can arrive at equatorial latitudes. Solar protons with energies higher than 100 MeV may be observed in the atmosphere above ∼30 km, and those with energies more than 1 GeV may be recorded even at the sea level. There are some observational evidences of SEP influence on atmospheric processes. Intruding into the atmosphere, SEPs affect middle atmosphere odd-nitrogen and ozone chemistry. Since spatial and temporal variations of SEP fluxes in the near Earth space are controlled by solar activity, SEPs may present an important link between solar activity and climate. The paper outlines dynamics of SEP fluxes in the near Earth space during the last decades. This can be useful for tracing relationship between SEPs and atmospheric processes.     

Possible contribution of different energy sources to the production of organics in Titan's atmosphere.

A quantitative comparison of the products arising from the irradiation of a Titan's simulated atmosphere is presented. The energy sources used represent some of the main events that occur in the satellite's atmosphere. All of the compounds identified are classified in the hydrocarbon and nitrile chemical families. Almost all of the detected compounds in Titan's atmosphere are produced by one or more energy sources. The compounds with the highest energy yields include the C2 hydrocarbons, methanonitrile and ethanonitrile. The possibility of using some of the produced organics as tracer compounds during the Huygens descend to identify the leading energy form in the different atmospheric levels remains open.     

Planetary and gravity waves in the mesosphere and lower thermosphere

Planetary and gravity waves contribute significantly to the variability of atmospheric parameters in the middle atmosphere. In the mesosphere and lower thermosphere the wave fluctuations are sufficiently large to often mask the prevailing or mean state of the atmosphere. This review summarizes current knowledge about the motion, temperature and density fields associated with both large and small scale waves and stresses improved understanding that has come from recent ground based, rocket and satellite investigations.     

Modeling the (upper) solar atmosphere including the magnetic field

The atmosphere of the Sun is highly structured and dynamic in nature. From the photosphere and chromosphere into the transition region and the corona plasma-β changes from above to below one, i.e., while in the lower atmosphere the energy density of the plasma dominates, in the upper atmosphere the magnetic field plays the governing role – one might speak of a “magnetic transition”. Therefore the dynamics of the overshooting convection in the photosphere, the granulation, is shuffling the magnetic field around in the photosphere. This leads not only to a (re-)structuring of the magnetic field in the upper atmosphere, but induces also the dynamic reaction of the coronal plasma, e.g., due to reconnection events. Therefore the (complex) structure and the interaction of various magnetic patches is crucial to understand the structure, dynamics and heating of coronal plasma as well as its acceleration into the solar wind.

The present article will emphasize the need for three-dimensional modeling accounting for the complexity of the solar atmosphere to understand these processes. Some advances on 3D modeling of the upper solar atmosphere in magnetically closed as well as open regions will be presented together with diagnostic tools to compare these models to observations. This highlights the recent success of these models which in many respects closely match the observations.     

New insights into Titan's organic chemistry in the gas and aerosol phases.

Titan, the largest satellite of Saturn, with a dense atmosphere very rich in organics, and many couplings in the various parts of its "geofluid", is a reference for studying prebiotic chemistry on a planetary scale. New data have been obtained from experiments simulating this organic chemistry (gas and aerosol phases), within the right ranges of temperature and a careful avoiding of any chemical contamination. They show a very good agreement with the observational data, demonstrating for the first time the formation of all the organic species already detected in Titan atmosphere including, at last, C4N2, together with many other species not yet detected in Titan. This strongly suggests the presence of more complex organics in Titan's atmosphere and surface, including high molecular weight polyynes and cyanopolyynes. The NASA-ESA Cassini-Huygens mission has been successfully launched in October 1997. The Cassini spacecraft will reach the Saturn system in 2004 and become an orbiter around Saturn, while the Huygens probe will penetrate into Titan's atmosphere. In situ measurements, in particular from Huygens GC-MS and ACP instruments, will provide a detailed analysis of the organics present in the air, aerosols, and surface. This very ambitious mission should yield much information of crucial importance for our knowledge of the complexity of Titan's chemistry, and, more generally for the field of exobiology.     

Advances in Researches on the Middle and Upper Atmosphere in 2010-2012

This article summarizes the researches on the middle and upper atmosphere by Chinese scientists in 2010?2012. The focuses are placed on the advances in construction of ground-based remote sensing facilities, the mean state and long-term changes in the middle atmosphere circulation, the prevailing dynamical processes, and the coupling of the middle atmospheric layers.      

Thermal structure of the Martian atmosphere retrieved from the IR spectrometry in the 15 μm CO2 band: input to MIRA

This paper describes one of the sources of the data concerning the thermal structure of the Martian atmosphere, based on the thermal IR spectrometry method. It allows to investigate the Martian atmosphere below 55 km by retrieving the temperature profiles from the 15 μm CO₂ band. This approach enables to reach the vertical resolution of several kilometers and the temperature accuracy of several Kelvins. An aerosol abundance, which influences the temperature profile, is obtained from the continuum of the same spectrum parallel with the temperature profile and is taken into account in the temperature retrieval procedure in a self consistent way. Although this method has the limited vertical resolution, it possesses a significant advantage: the thermal IR spectrometry allows to monitor the temperature profiles with a good coverage both in space and local time. The Planetary Fourier spectrometer on board of Mars Express has the spectral range from 250 to 8000 cm⁻¹ and a high spectral resolution of about 2 cm⁻¹. Vertical temperature profiles retrieval is one of the main scientific goals of the experiment. The important data are expected to be obtained on the vertical thermal structure of the atmosphere, and its dependence on latitude, longitude, season, local time, clouds and dust loadings. These results should give a significant input in the future MIRA, being included in the Chapter “Structure of the atmosphere from the surface to 100 km”.     

Reactive-diffusion waves in the mesospheric photochemical system

The influence of horizontal eddy diffusion on photochemical oscillations of minor gas constituents in the mesopause region of the Earth’s atmosphere is investigated. It is shown that, when the spectrum of oscillations contains a pronounced second subharmonic (nonlinear response of the photochemical system to diurnal variations of solar radiation), reaction-diffusion waves in the form of travelling fronts and pulses of the phase of two-day oscillations may be excited in this region of the atmosphere.     

Trapped antiprotons produced by cosmic rays in the Earth's magnetosphere.

The existence of significant fluxes of antiparticles in the Earth magnetosphere has been predicted on theoretical considerations in this article. These antiparticles (positrons or antiprotons) at several hundred kilometers of altitudes, we believe are not of direct extraterrestrial origin, but are the natural products of nuclear reactions of the high energy primary cosmic rays (CR) and trapped protons (TP) confined in the terrestrial radiation belt, with the constituents of terrestrial atmosphere. Extraterrestrial positrons and antiprotons born in nuclear reactions of the same CR particles passing through only 5-7 g/cm2 of interstellar matter, exhibit lower fluxes compared to the antiprotons born at hundreds of g/cm2 in the atmosphere, which when confined in the magnetic field of the Earth (in any other planet), get accumulated. We present the results of the computations of the antiproton fluxes at 10 MeV to several GeV energies due to CR particle interactions with the matter in the interstellar space, and also with the residual atmosphere at altitudes of approximately 1000 km over the Earth's surface. The estimates show that the magnetospheric antiproton fluxes are greater by two orders of magnitude compared to the extraterrestrial fluxes measured at energies <1-2 GeV.     

Advances in the Researches of the Middle and Upper Atmosphere in China

In this paper we summarize the research results by Chinese scientists in 2016-2018. The focuses are placed on the researches of the middle and upper atmosphere, specifically the researches associated with groundbased observation capability development, dynamical processes, and properties of circulation and chemistryclimate coupling of the middle atmospheric layers.      

X-ray emission from the planetary nebula NGC 1360

The EXOSAT observatory has detected the nucleus of NGC 1360 in four photometric energy bands. The data rules out that the emission is from blackbody origin. Initial fits made with LTE model atmosphere spectra require the presence of highly ionized Oxygen and Neon in the stellar atmosphere.     

Progress in trend studies: Highlights of the TREND2004 Workshop

TREND2004, the third IAGA/ICMA Workshop Long Term Changes and Trends in the Atmosphere, presented a lot of new information and in some way summarized progress over the last few years in the field of long-term trends in the atmosphere–ionosphere system with emphasis on the middle atmosphere and ionosphere. The paper briefly summarizes the results of the Workshop with focus on the mesosphere, ionosphere and thermosphere and presents some new findings. Progress in recent years was substantial in trends in thermospheric density, mesospheric and mesopause region temperature, and in the lower ionosphere (ionized component). On the other hand, principal contradictions remain in trends in the F2 region of the ionosphere, particularly in interpretation of the observed trends.     

Impact of the solar wind dynamics on the ozone density variations

The analysis of the baloon and rocket measurement data reveals the changes of temperature in the polar and subpolar middle atmosphere with approaching of the magnetopause of the magnetosphere to the Earth. The data of the most observation points show the notable warming of the middle atmosphere in this situation, but there were some stations (“Vostok” in Antarctica and “Frobisher Bay” in Arctic) where the reverse dependence between these parameters was registered. The experimental data show that the total ozone content (TOC) decreases with approaching of the magnetopause to the Earth under both atmosphere warming or cooling at high altitudes. The drop in the TOC values is closely connected to increase of atmospheric electric field magnitude. The obtained results possibly could be explained in framework of the global electric circuit model.     

Advances in the Researches of the Middle and Upper Atmosphere in China in 2014—2016

In this report the research results by Chinese scientists in 2014-2016 are summarized. The focuses are placed on the researches of the middle and upper atmosphere, specifically the researches associated with ground-based observation capability development, dynamical processes, and properties of circulation and chemistry-climate coupling of the middle atmospheric layers.