Semi-empirical Atmospheric Models Research Articles

The Impact of Angle-dependent Partial Frequency Redistribution on the Scattering Polarization of the Solar Na i D Lines

The long-standing paradox of the linear polarization signal of the Na i D1 line was recently resolved by accounting for the atom’s hyperfine structure and the detailed spectral structure of the incident radiation field. That modeling relied on the simplifying angle-averaged (AA) approximation for partial frequency redistribution (PRD) in scattering, which potentially neglects important angle–frequency couplings. This work aims at evaluating the suitability of a PRD-AA modeling for the D1 and D2 lines through comparisons with general angle-dependent (AD) PRD calculations in both the absence and presence of magnetic fields. We solved the radiative transfer problem for polarized radiation in a 1D semiempirical atmospheric model with microturbulent and isotropic magnetic fields, accounting for PRD effects and comparing PRD-AA and PRD-AD modelings. The D1 and D2 lines are modeled separately as a two-level atomic system with hyperfine structure. The numerical results confirm that a spectrally structured radiation field induces linear polarization in the D1 line. However, the PRD-AA approximation greatly impacts the Q/I shape, producing an antisymmetric pattern instead of the more symmetric PRD-AD one while presenting a similar sensitivity to magnetic fields between 10 and 200 G. Under the PRD-AA approximation, the Q/I profile of the D2 line presents an artificial dip in its core, which is not found for the PRD-AD case. We conclude that accounting for PRD-AD effects is essential to suitably model the scattering polarization of the Na i D lines. These results bring us closer to exploiting the full diagnostic potential of these lines for the elusive chromospheric magnetic fields.

Longitudinal Variation of Thermospheric Density during Low Solar Activity from APOD Observations

The longitudinal distribution of upper atmospheric density has been broadly studied. However, the studies mostly focused on 24 h averaged distribution. This study presents the longitudinal distribution of thermospheric density at dawn and dusk, using observations collected by the atmospheric density detector onboard the Chinese satellite APOD (Atmospheric Density Detection and Precise Orbit Determination) during low solar activity. The APOD observations show a significant relative longitudinal variation of thermospheric density with global maxima (Δρrmax) near the geomagnetic pole, especially in the winter hemisphere. The annual maximum of Δρrmax appears in the Southern Hemisphere around the June solstices and reaches 26.3% and 39.6% at dawn and dusk, respectively. The auroral heating and meridional wind might play a significant role in the longitudinal variation of thermospheric density. We further compare the APOD observations with the semi-empirical atmospheric model MSIS (Mass Spectrometer Incoherent Scatter Radar) 2.0 predictions under low solar activity conditions. The MSIS 2.0 model reproduces similar longitudinal variations to the observations, with hemispheric asymmetry. The longitudinal variation of thermospheric density from APOD should be related to the distribution of the atmospheric average molecular weight from the model. More observational data are needed to verify the results of this study further.

Neutral Atmospheric Density Measurement Using Insight-HXMT Data by the Earth Occultation Technique

The Earth occultation technique has broad applications in both astronomy and atmospheric density measurements. We construct the background model during the occultation of the Crab Nebula observed by the Insight-Hard X-ray Modulation Telescope (Insight-HXMT) at energies between 6 and 100 keV. We propose a Bayesian atmospheric density retrieval method based on the Earth occultation technique, combining Poisson and Gaussian statistics. By modeling the atmospheric attenuation of X-ray photons during the occultation, we simultaneously retrieved the neutral densities of the atmosphere at different altitude ranges. Our method considers the correlation of densities between neighboring atmospheric layers and reduces the potential systematic bias to which previous work may be subject. Previous analyses based on light-curve fitting or spectral fitting also lost some spectral or temporal information of the data. In contrast to previous work, the occultation data observed by the three telescopes on board Insight-HXMT is fully used in our analysis, further reducing the statistical error in density retrieval. We apply our method to cross-check the (semi)empirical atmospheric models, using 115 sets of occultation data of the Crab Nebula observed by Insight-HXMT. We find that the retrieved neutral density is ∼10%, ∼20%, and ∼25% less than the values of the widely used atmospheric model NRLMSISE-00, in the altitude range of 55–80 km, 80–90 km, and 90–100 km, respectively. We also show that the newly released atmospheric model NRLMSIS 2.0 is generally consistent with our density measurements.

Radiative losses in the chromosphere during a C-class flare

Context. Solar flares release an enormous amount of energy (∼1032 erg) into the corona. A substantial fraction of this energy is transported to the lower atmosphere, which results in chromospheric heating. The mechanisms that transport energy to the lower solar atmosphere during a flare are still not fully understood. Aims. We aim to estimate the temporal evolution of the radiative losses in the chromosphere at the footpoints of a C-class flare, in order to set observational constraints on the electron beam parameters of a RADYN flare simulation. Methods. We estimated the radiative losses from hydrogen, and singly ionized Ca and Mg using semiempirical model atmospheres, which were inferred from a multiline inversion of observed Stokes profiles obtained with the CRISP and CHROMIS instruments on the Swedish 1-m Solar Telescope. The radiative losses were computed taking into account the effect of partial redistribution and non-local thermodynamic equilibrium. To estimate the integrated radiative losses in the chromosphere, the net cooling rates were integrated between the temperature minimum and the height where the temperature reaches 10 kK. We also compared our time series of radiative losses with those from the RADYN flare simulations. Results. We obtained a high spatial-resolution map of integrated radiative losses around the flare peak time. The stratification of the net cooling rate suggests that the Ca IR triplet lines are responsible for most of the radiative losses in the flaring atmosphere. During the flare peak time, the contribution from Ca II H and K and Mg II h and k lines are strong and comparable to the Ca IR triplet (∼32 kW m−2). Since our flare is a relatively weak event, the chromosphere is not heated above 11 kK, which in turn yields a subdued Lyα contribution (∼7 kW m−2) in the selected limits of the chromosphere. The temporal evolution of total integrated radiative losses exhibits sharply rising losses (0.4 kW m−2 s−1) and a relatively slow decay (0.23 kW m−2 s−1). The maximum value of total radiative losses is reached around the flare peak time, and can go up to 175 kW m−2 for a single pixel located at footpoint. After a small parameter study, we find the best model-data consistency in terms of the amplitude of radiative losses and the overall atmospheric structure with a RADYN flare simulation in the injected energy flux of 5 × 1010 erg s−1 cm−2.

Modeling the scattering polarization of the solar Ca I 4227 Å line with angle-dependent partial frequency redistribution

Context. The correct modeling of the scattering polarization signals observed in several strong resonance lines requires taking partial frequency redistribution (PRD) phenomena into account. Modeling scattering polarization with PRD effects is very computationally demanding and the simplifying angle-averaged (AA) approximation is therefore commonly applied. Aims. This work aims to assess the impact and the range of validity of the AA approximation with respect to the general angle-dependent (AD) treatment of PRD effects in the modeling of scattering polarization in strong resonance lines, with a focus on the solar Ca I 4227 Å line. Methods. Spectral line polarization was modeled by solving the radiative transfer problem for polarized radiation, under nonlocal thermodynamic equilibrium conditions, taking PRD effects into account in static one-dimensional semi-empirical atmospheric models presenting arbitrary magnetic fields. The problem was solved through a two-step approach. In step 1, the problem was solved for the intensity only, considering a multilevel atom. In step 2, the problem was solved including polarization, considering a two-level atom with an unpolarized and infinitely sharp lower level, and fixing the lower level population calculated at step 1. Results. The results for the Ca I 4227 Å line show a good agreement between the AA and AD calculations for the Q/I and U/I wings’ signals. However, AA calculations reveal an artificial trough in the line-core peak of the linear polarization profiles, whereas AD calculations show a sharper peak in agreement with the observations. Conclusions. An AD treatment of PRD effects is essential to correctly model the line-core peak of the scattering polarization signal of the Ca I 4227 Å line. By contrast, in the considered static case, the AA approximation seems to be suitable to model the wing scattering polarization lobes and their magnetic sensitivity through magneto-optical effects.

Spectral Diagnostics of Solar Photospheric Bright Points

Abstract Through the use of the high-resolution spectral data and the broadband imaging obtained with the Goode Solar Telescope at the Big Bear Solar Observatory on 2013 June 6, the spectra of three typical photospheric bright points (PBPs) have been analyzed. Based on the Hα and Ca ii 8542 Å line profiles, as well as the TiO continuum emission, for the first time, the non-LTE semiempirical atmospheric models for the PBPs are computed. The attractive characteristic is the temperature enhancement in the lower photosphere. The temperature enhancement is about 200–500 K at the same column mass density as in the atmospheric model of the quiet-Sun. The total excess radiative energy of a typical PBP is estimated to be 1 × 1027–2 × 1027 erg, which can be regarded as the lower limit energy of the PBPs. The radiation flux in the visible continuum for the PBPs is about 5.5 × 1010 erg cm−2 s−1. Our result also indicates that the temperature in the atmosphere above PBPs is close to that of a plage. It gives clear evidence that PBPs may contribute significantly to the heating of the plage atmosphere. Using our semiempirical atmospheric models, we estimate self-consistently the average magnetic flux density B in the PBPs. It is shown that the maximum value is about 1 kG, and it decreases toward both higher and lower layers, reminding us of the structure of a flux tube between photospheric granules.

On the Formation of Lyman β and the O i 1027 and 1028 Å Spectral Lines

Abstract We study the potential of Lyman β and the O i 1027 and 1028 Å spectral lines to help in understanding the properties of the chromosphere and transition region (TR). The oxygen transitions are located in the wing of Lyman β, which is a candidate spectral line for the solar missions Solar Orbiter/Spectral Imaging of the Coronal Environment and Solar-C (EUVST). We examine the general spectroscopic properties of the three transitions in the quiet Sun by synthesizing them assuming nonlocal thermal equilibrium and taking into account partial redistribution effects. We estimate the heights where the spectral lines are sensitive to the physical parameters, computing the response functions to temperature and velocity using a 1D semiempirical atmospheric model. We also synthesize the intensity spectrum using the 3D enhanced network simulation computed with the Bifrost code. The results indicate that Lyman β is sensitive to the temperature from the middle chromosphere to the TR, while it is mainly sensitive to the line-of-sight (LOS) velocity at the lower atmospheric layers, around 2000 km above the optical surface. The O i lines form lower in the middle chromosphere, being sensitive to the LOS velocities at heights lower than those covered by Lyman β. The spatial distribution of the intensity signals computed with the Bifrost atmosphere, as well as the inferred velocities from the line core Doppler shift, confirms the previous results. Therefore, these results indicate that the spectral window at 1025 Å contains several spectral lines that complement each other to seamlessly trace the thermal structure and gas dynamics from the middle chromosphere to the lower TR.

The MESAS Project: ALMA Observations of the F-type Stars γ Lep, γ Vir A, and γ Vir B

Abstract The spectrum of stars in the submillimeter to centimeter wavelength range remains poorly constrained due to a lack of data for most spectral types. An accurate characterization of stellar emission in this regime is needed to test stellar atmosphere models, and is also essential for revealing emission associated with unresolved circumstellar debris. We present Atacama Large Millimeter/submillimeter Array observations of the three nearby, main-sequence, debris-poor, F-type stars γ Lep, γ Vir A, and γ Vir B at 0.87 and 1.29 mm. We use these data to constrain semiempirical atmospheric models. We discuss the atmospheric structure of these stars, explore potential short-term variability, and the potential impact on debris disk studies. These results are part of an ongoing campaign to obtain long wavelength observations of debris-poor stars, entitled Measuring the Emission of Stellar Atmospheres at Submillimeter/millimeter wavelengths.

Semi-empirical model atmospheres for the chromosphere of the sunspot penumbra and umbral flashes

Context. The solar chromosphere and the lower transition region are believed to play a crucial role in the heating of the solar corona. Models that describe the chromosphere (and the lower transition region), accounting for its highly dynamic and structured character are, so far, found to be lacking. This is partly due to the breakdown of complete frequency redistribution (CRD) in the chromospheric layers and also because of the difficulty in obtaining complete sets of observations that adequately constrain the solar atmosphere at all relevant heights. Aims. We aim to obtain semi-empirical model atmospheres that reproduce the features of the Mg II h&k line profiles that sample the middle chromosphere with focus on a sunspot. Methods. We used spectropolarimetric observations of the Ca II 8542 Å spectra obtained with the Swedish 1 m Solar Telescope and used NICOLE inversions to obtain semi-empirical model atmospheres for different features in and around a sunspot. These were used to synthesize Mg II h&k spectra using the RH1.5D code, which we compared with observations taken with the Interface Region Imaging Spectrograph (IRIS). Results. Comparison of the synthetic profiles with IRIS observations reveals that there are several areas, especially in the penumbra of the sunspot, where most of the observed Mg II h&k profiles are very well reproduced. In addition, we find that supersonic hot down-flows, present in our collection of models in the umbra, lead to synthetic profiles that agree well with the IRIS Mg II h&k profiles, with the exception of the line core. Conclusions. We put forward and make available four semi-empirical model atmospheres. Two for the penumbra, reflecting the range of temperatures obtained for the chromosphere, one for umbral flashes, and a model representative of the quiet surroundings of a sunspot.

Solar Polar Brightening and Radius at 100 and 230 GHz Observed by ALMA

Abstract Polar brightening of the Sun at radio frequencies has been studied for almost 50 years and yet a disagreement persists between solar atmospheric models and observations. Some observations reported brightening values much smaller than the expected values obtained from the models, with discrepancies being particularly large at millimeter wavelengths. New clues to calibrate the atmospheric models can be obtained with the advent of the Atacama Large Millimeter/submillimeter Array (ALMA) radio interferometer. In this work, we analyzed the lower limit of the polar brightening observed at 100 and 230 GHz by ALMA, during its Science Verification period, 2015 December 16–20. We find that the average polar intensity is higher than the disk intensity at 100 and 230 GHz, with larger brightness intensities at the south pole in eight of the nine maps analyzed. The observational results were compared with calculations of the millimetric limb brightening emission for two semi-empirical atmospheric models, FAL-C and SSC. Both models presented larger limb intensities than the average observed values. The intensities obtained with the SSC model were closer to the observations, with polar brightenings of 10.5% and 17.8% at 100 and 230 GHz, respectively. This discrepancy may be due to the presence of chromospheric features (like spicules) at regions close to the limb.

SEMI-EMPIRICAL MODELING OF THE PHOTOSPHERE, CHROMOPSHERE, TRANSITION REGION, AND CORONA OF THE M-DWARF HOST STAR GJ 832*

ABSTRACT Stellar radiation from X-rays to the visible provides the energy that controls the photochemistry and mass loss from exoplanet atmospheres. The important extreme ultraviolet (EUV) region (10–91.2 nm) is inaccessible and should be computed from a reliable stellar model. It is essential to understand the formation regions and physical processes responsible for the various stellar emission features to predict how the spectral energy distribution varies with age and activity levels. We compute a state-of-the-art semi-empirical atmospheric model and the emergent high-resolution synthetic spectrum of the moderately active M2 V star GJ 832 as the first of a series of models for stars with different activity levels. We construct a one-dimensional simple model for the physical structure of the star’s chromosphere, chromosphere-corona transition region, and corona using non-LTE radiative transfer techniques and many molecular lines. The synthesized spectrum for this model fits the continuum and lines across the UV-to-optical spectrum. Particular emphasis is given to the emission lines at wavelengths that are shorter than 300 nm observed with the Hubble Space Telescope, which have important effects on the photochemistry of the exoplanet atmospheres. The FUV line ratios indicate that the transition region of GJ 832 is more biased to hotter material than that of the quiet Sun. The excellent agreement of our computed EUV luminosity with that obtained by two other techniques indicates that our model predicts reliable EUV emission from GJ 832. We find that the unobserved EUV flux of GJ 832, which heats the outer atmospheres of exoplanets and drives their mass loss, is comparable to the active Sun.

Effect of the solar activity variation on the Global Ionosphere Thermosphere Model (GITM)

Abstract. The accuracy of global atmospheric models used to predict the middle/lower thermosphere characteristics is still an open topic. Uncertainties in the prediction of the gas properties in the thermosphere lead to inaccurate computations of the drag force on space objects (i.e. satellites or debris). Currently the lifetime of space objects and therefore the population of debris in low Earth orbit (LEO) cannot be quantified with a satisfactory degree of accuracy. In this paper, the Global Ionosphere Thermosphere Model (GITM) developed at the University of Michigan has been validated in order to provide detailed simulations of the thermosphere. First, a sensitivity analysis has been performed to investigate the effect of the boundary conditions on the final simulations results. Then, results of simulations have been compared with flight measurements from the CHallenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) satellites and with existing semi-empirical atmospheric models (IRI and MSIS). The comparison shows a linear dependency of the neutral density values with respect to the solar activity. In particular, GITM shows an over-predicting or under-predicting behaviour under high or low solar activity respectively. The reasons for such behaviour can be attributed to a wrong implementation of the chemical processes or the gas transport properties in the model.

CHROMOSPHERIC HEATING BY ACOUSTIC WAVES COMPARED TO RADIATIVE COOLING

ABSTRACT Acoustic and magnetoacoustic waves are among the possible candidate mechanisms that heat the upper layers of the solar atmosphere. A weak chromospheric plage near the large solar pore NOAA 11005 was observed on 2008 October 15, in the Fe i 617.3 nm and Ca ii 853.2 nm lines of the Interferometric Bidimemsional Spectrometer attached to the Dunn Solar Telescope. In analyzing the Ca ii observations (with spatial and temporal resolutions of 0.″4 and 52 s) the energy deposited by acoustic waves is compared to that released by radiative losses. The deposited acoustic flux is estimated from the power spectra of Doppler oscillations measured in the Ca ii line core. The radiative losses are calculated using a grid of seven one-dimensional hydrostatic semi-empirical model atmospheres. The comparison shows that the spatial correlation of the maps of radiative losses and acoustic flux is 72%. In a quiet chromosphere, the contribution of acoustic energy flux to radiative losses is small, only about 15%. In active areas with a photospheric magnetic-field strength between 300 and 1300 G and an inclination of 20°–60°, the contribution increases from 23% (chromospheric network) to 54% (a plage). However, these values have to be considered as lower limits and it might be possible that the acoustic energy flux is the main contributor to the heating of bright chromospheric network and plages.

Spectropolarimetric capabilities of Ca ii 8542 Å line

The next generation of space and ground-based solar missions aim to study the magnetic properties of the solar chromosphere using the infrared Ca II lines and the He I 10830 {\AA} line. The former seem to be the best candidates to study the stratification of magnetic fields in the solar chromosphere and their relation to the other thermodynamical properties underlying the chromospheric plasma. The purpose of this work is to provide a detailed analysis of the diagnostic capabilities of the Ca II 8542 {\AA} line, anticipating forthcoming observational facilities. We study the sensitivity of the Ca II 8542 {\AA} line to perturbations applied to the physical parameters of reference semi-empirical 1D model atmospheres using response functions and we make use of 3D MHD simulations to examine the expected polarization signals for moderate magnetic field strengths. Our results indicate that the Ca II 8542 {\AA} line is mostly sensitive to the layers enclosed between $\log$ $\tau=[0,-5.5]$, under the physical conditions that are present in our model atmospheres. In addition, the simulated magnetic flux tube generates strong longitudinal signals in its centre and moderate transversal signals, due to the vertical expansion of magnetic field lines, in its edge. Thus, observing the Ca II 8542 {\AA} line we will be able to infer the 3D geometry of moderate magnetic field regions.

OBSERVED VARIABILITY OF THE SOLAR Mg II h SPECTRAL LINE

The Mg II h&k doublet are two of the primary spectral lines observed by the Sun-pointing Interface Region Imaging Spectrograph (IRIS). These lines are tracers of the magnetic and thermal environment that spans from the photosphere to the upper chromosphere. We use a double gaussian model to fit the Mg II h profile for a full-Sun mosaic dataset taken 24-Aug-2014. We use the ensemble of high-quality profile fits to conduct a statistical study on the variability of the line profile as it relates the magnetic structure, dynamics, and center-to-limb viewing angle. The average internetwork profile contains a deeply reversed core and is weakly asymmetric at h2. In the internetwork, we find a strong correlation between h3 wavelength and profile asymmetry as well h1 width and h2 width. The average reversal depth of the h3 core is inversely related to the magnetic field. Plage and sunspots exhibit many profiles which do not contain a reversal. These profiles also occur infrequently in the internetwork. We see indications of magnetically aligned structures in plage and network in statistics associated with the line core, but these structures are not clear or extended in the internetwork. The center-to-limb variations are compared with predictions of semi-empirical model atmospheres. We measure a pronounced limb darkening in the line core which is not predicted by the model. The aim of this work is to provide a comprehensive measurement baseline and preliminary analysis on the observed structure and formation of the Mg II profiles observed by IRIS.

Diagnostics of Ellerman bombs with high-resolution spectral data

Ellerman bombs (EBs) are tiny brightenings often observed near sunspots. The most impressive characteristic of EB spectra is the two emission bumps in both wings of the Hα and Ca II 8542Å lines. High-resolution spectral data of three small EBs were obtained on 2013 June 6 with the largest solar telescope, the 1.6 m New Solar Telescope at the Big Bear Solar Observatory. The characteristics of these EBs are analyzed. The sizes of the EBs are in the range of 0.3′ – 0.8′ and their durations are only 3–5 min. Our semi-empirical atmospheric models indicate that the heating occurs around the temperature minimum region with a temperature increase of 2700–3000 K, which is surprisingly higher than previously thought. The radiative and kinetic energies are estimated to be as high as 5 × 1025 − 3.0 × 1026 erg despite the small size of these EBs. Observations of the magnetic field show that the EBs just appeared in a parasitic region with mixed polarities and were accompanied by mass motions. Nonlinear force-free field extrapolation reveals that the three EBs are connected with a series of magnetic field lines associated with bald patches, which strongly implies that these EBs should be produced by magnetic reconnection in the solar lower atmosphere. According to the lightcurves and the estimated magnetic reconnection rate, we propose that there is a three phase process in EBs: pre-heating, flaring and cooling phases.

Semiempirical photospheric models of a solar flare on May 28, 2012

The variation of the photosphere physical state during the decay phase of SF/B6.8-class solar flare on May 28, 2012 in active region NOAA 11490 is studied.We used the data of the spectropolarimetric observations with the French–Italian solar telescope THEMIS (Tenerife, Spain). Semi-empirical model atmospheres are derived from the inversion with SIR (Stokes Inversion based on Response functions) code. The inversion was based on Stokes profiles of six photospheric lines. Each model atmosphere has a two-component structure: a magnetic flux tube and non-magnetic surroundings. The Harvard Smithsonian Reference Atmosphere (HSRA) has been adopted for the surroundings. The macroturbulent velocity and the filling factor were assumed to be constant with the depth. The optical depth dependences of the temperature, magnetic field strength, and line-of-sight velocity are obtained from inversion.According to the received model atmospheres, the parameters of the magnetic field and the thermodynamical parameters changed during the decay phase of the flare. The model atmospheres showed that the photosphere remained in a disturbed state during observations after the maximum of the flare. There are temporal changes in the temperature and the magnetic field strength optical depth dependences. The temperature enhancement in the upper photospheric layers is found in the flaring atmospheres relative to the quiet-Sun model. The downflows are found in the low and upper photosphere at the decay phase of the flare.

Chromosphere of K giant stars

We aim to constrain the geometrical extent of the chromosphere of non-binary K giant stars and detect any spatial structures in the chromosphere. We performed observations with the CHARA interferometer and the VEGA beam combiner at optical wavelengths. We observed seven non-binary K giant stars. We measured the ratio of the radii of the photosphere to the chromosphere using the interferometric measurements in the Halpha and the Ca II infrared triplet line cores. For beta Ceti, spectro-interferometric observations are compared to an non-local thermal equilibrium (NLTE) semi-empirical model atmosphere including a chromosphere. The NLTE computations provide line intensities and contribution functions that indicate the relative locations where the line cores are formed and can constrain the size of the limb-darkened disk of the stars with chromospheres. We measured the angular diameter of seven K giant stars and deduced their fundamental parameters: effective temperatures, radii, luminosities, and masses. We determined the geometrical extent of the chromosphere for four giant stars. The chromosphere extents obtained range between 16% to 47% of the stellar radius. The NLTE computations confirm that the Ca II/849 nm line core is deeper in the chromosphere of ? Cet than either of the Ca II/854 nm and Ca II/866 nm line cores. We present a modified version of a semi-empirical model atmosphere derived by fitting the Ca II triplet line cores of this star. In four of our targets, we also detect the signature of a differential signal showing the presence of asymmetries in the chromospheres. Conclusions. It is the first time that geometrical extents and structure in the chromospheres of non-binary K giant stars are determined by interferometry. These observations provide strong constrains on stellar atmosphere models.

Estimation of secular density variations in the upper atmosphere from 1964–2007 satellite drag data

Orbital parameters of several artificial satellites of the Earth were analyzed for 1964–2007 and secular variations of the atmospheric density were estimated for the last 30–40 years. The analysis was based on the information about orbital parameters of 17 satellites and high-precision numerical integrations of the equations of motion with allowance for basic perturbing factors and spatiotemporal density variations, calculated from measured solar activity indices using the NRLMSISE-00 atmosphere model. The results demonstrate the presence of long-term variations in the atmospheric density not presented in modern atmosphere models. During solar-activity cycle 21, the atmospheric density became 0.4 to 19% higher (depending on height) than in cycle 20. It decreased by 1.0 to 11% (depending on height) in cycle 22 as compared to cycle 21. Both decreases and increases were observed in the atmospheric density during cycle 23, but with much smaller gradients. The results cannot be explained only by the growing concentration of greenhouse gases. Possible causes of the density variations and possible ways to take them into account in modern empirical and semiempirical atmospheric models are discussed.

Synthetic analysis of a two-ribbon microflare

High-resolution Stokes spectral data of Hα, Ca II 8542 Å, and Fe 16302.5 Å lines for a two-ribbon microflare (TRMF) were simultaneously obtained by the THEMIS telescope on 2002 September 5. We derive the intensity, velocity, and longitudinal magnetic field maps. The hard X-ray emission observed by RHESSI provides evidence of nonthermal particle acceleration in the TRMF. Using Hα and Ca II 8542 Å line profiles and a non-LTE calculation, we obtain semi-empirical atmospheric models for the two brightest kernels of the TRMF. Our result indicates that the temperature enhancement in the chromosphere is more than 2500 K. The kinetic and radiative energies at the kernels are also estimated, resulting in an estimate of the total energy of the TRMF of about 2.4 ×1029 erg. Observations indicate that the TRMF results from the low coronal magnetic reconnection following the eruption of a small filament. However, the local temperature “bump" in the chromosphere presents a puzzle for such a standard flare model. A possible solution to this is discussed.