Thermospheric neutral density response to solar forcing

Recent measurements by the Solar EUV (Extreme Ultra Violet) Experiment (SEE) aboard the Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics satellite (TIMED) provide solar EUV spectral irradiance with adequate spectral and temporal resolution, and thus the opportunity to use solar measurements directly in upper atmospheric general circulation models. Thermospheric neutral density is simulated with the NCAR Thermosphere–Ionosphere–Electrodynamic General Circulation Model (TIEGCM) using TIMED/SEE measurements and using the EUVAC solar proxy model. Neutral density is also calculated using the NRLMSISE-00 empirical model. These modeled densities are then compared to density measurements derived from satellite drag data. It is found that using measured solar irradiance in the general circulation model can improve density calculations compared to using the solar proxy model. It is also found that the general circulation model can improve upon the empirical model in simulating geomagnetic storm effects and the solar cycle variation of neutral density.     

Results of solar observations by the CORONAS-F payload

The CORONAS-F mission experiments and results have been reviewed. The observations with the DIFOS multi-channel photometer in a broad spectral range from 350 to 1500 nm have revealed the dependence of the relative amplitudes of p-modes of the global solar oscillations on the wavelength that agrees perfectly well with the earlier data obtained in a narrower spectral ranges. The SPIRIT EUV observations have enabled the study of various manifestations of solar activity and high-temperature events on the Sun. The data from the X-ray spectrometer RESIK, gamma spectrometer HELICON, flare spectrometer IRIS, amplitude–temporal spectrometer AVS-F, and X-ray spectrometer RPS-1 have been used to analyze the X- and gamma-ray emission from solar flares and for diagnostics of the flaring plasma. The absolute and relative content of various elements (such as potassium, argon, and sulfur) of solar plasma in flares has been determined for the first time with the X-ray spectrometer RESIK. The Solar Cosmic Ray Complex monitored the solar flare effects in the Earth’s environment. The UV emission variations recorded during solar flares in the vicinity of the 120-nm wavelength have been analyzed and the amplitude of relative variations has been determined.     

Comparison of solar soft X-ray irradiance from broadband photometers to a high spectral resolution rocket observation

The solar soft X-ray (XUV; 1–30 nm) radiation is highly variable on all time scales and strongly affects the ionosphere and upper atmosphere of Earth, Mars, as well as the atmospheres and surfaces of other planets and moons in the solar system; consequently, the solar XUV irradiance is important for atmospheric studies and for space weather applications. While there have been several recent measurements of the solar XUV irradiance, detailed understanding of the solar XUV irradiance, especially its variability during flares, has been hampered by the lack of high spectral resolution measurements in this wavelength range. The conversion of the XUV photometer signal into irradiance requires the use of a solar spectral model, but there has not been direct validation of these spectral models for the XUV range. For example, the irradiance algorithm for the XUV Photometer System (XPS) measurements uses multiple CHIANTI spectral models, but validation has been limited to other solar broadband measurements or with comparisons of the atmospheric response to solar variations. A new rocket observation of the solar XUV irradiance with 0.1 nm resolution above 6 nm was obtained on 14 April 2008, and these new results provide a first direct validation of the spectral models used in the XPS data processing. The rocket observation indicates very large differences for the spectral model for many individual emission features, but the differences are significantly smaller at lower resolution, as expected since the spectral models are scaled to match the broadband measurements. While this rocket measurement can help improve a spectral model for quiet Sun conditions, many additional measurements over a wide range of solar activity are needed to fully address the spectral model variations. Such measurements are planned with a similar instrument included on NASA’s Solar Dynamics Observatory (SDO), whose launch is expected in 2009.     

Sun CubE OnE: A multi-wavelength synoptic solar micro satellite

The Sun cubE onE (SEE) is a 12U CubeSat mission proposed for a phase A/B study to the Italian Space Agency that will investigate Gamma and X-ray fluxes and ultraviolet (UV) solar emission to support studies in Sun-Earth interaction and Space Weather from LEO. More in detail, SEE’s primary goals are to measure the flares emission from soft-X to Gamma ray energy range and to monitor the solar activity in the Fraunhofer Mg II doublet at 280 nm, taking advantage of a full disk imager payload. The Gamma and X-ray fluxes will be studied with unprecedented temporal resolution and with a multi-wavelength approach thanks to the combined use of silicon photodiode and silicon photomultiplier (SiPM) -based detectors. The flare spectrum will be explored from the keV to the MeV range of energies by the same payload, and with a cadence up to 10 kHz and with single-photon detection capabilities to unveil the sources of the solar flares. The energy range covers the same bands used by GOES satellites, which are the standard bands for flare magnitude definition. At the same time SiPM detectors combined with scintillators allow to cover the non-thermal bremsstrahlung emission in the gamma energy range. Given its UV imaging capabilities, SEE will be a key space asset to support detailed studies on solar activity, especially in relation to ultraviolet radiation which strongly interacts with the upper layers of the Earth’s atmosphere, and in relation to space safety, included in the field of human space exploration. The main goal for the UV payload is to study the evolution of the solar UV emission in the Mg II band at two different time scales: yearly variations along the solar cycle and transient variations during flare events. The Mg II index is commonly used as a proxy of the solar activity in the Sun-as-a-star paradigm, in which solar irradiance variations in the UV correlate with the variations in stratospheric ozone concentrations and other physical parameters of the Earth high atmosphere. SEE data will be used together with space and ground-based observatories that provide Solar data (e.g. Solar Orbiter, IRIS, GONG, TSST), high energy particle fluxes (e.g. GOES, MAXI, CSES) and geomagnetic data in a multi-instrument/multi-wavelength/multi-messenger approach.     

Solar spectral irradiance variability in the ultraviolet from SORCE and UARS SOLSTICE

The SOLar-STellar Irradiance Comparison Experiment (SOLSTICE) on the SOlar Radiation and Climate Experiment (SORCE) has been measuring the solar spectral irradiance on a daily basis since early 2003. This time period includes near-solar maximum conditions, the Halloween storms of 2003, and solar minimum conditions. These results can be compared to observations from the SOLSTICE I experiment that flew on the Upper Atmosphere Research Satellite (UARS) during the decline of the previous solar cycle as well as with currently operating missions. We will discuss similarities and differences between the two solar cycles in the long-term ultraviolet irradiance record.     

Patterns of carbon monoxide in the middle atmosphere and effects of solar variability

We determine the spatial-time patterns of zonally averaged carbon monoxide (CO) in the middle atmosphere by applying Principle Component Analysis to the CO data obtained from the Microwave Limb Sounder (MLS) measurements on the Aura satellite in 2004–2012. The first two principal components characterize more than 90% of the CO variability. Both principal components are localized in the low thermosphere near the mesopause. The first principal component is asymmetric relative to the poles. It has opposite signs in the Northern and Southern Hemisphere at mid to high latitudes and strongly oscillates with an annual periodicity. The second principal component has the same sign in both hemispheres and oscillates mainly with a semi-annual frequency. Both principal components are modulated by the 11-year solar cycle and display short-term variations. To test possible correlations of these variations with the short term solar ultraviolet (UV) variability we use the simultaneous measurements of the UV solar radiance from the Solar-Stellar Irradiance Comparison Experiment (SOLSTICE) on the Solar Radiation and Climate Experiment (SORCE) satellite to investigate the correlation between CO in the middle atmosphere and solar UV in 2004–2012. Using a wavelet coherence technique a weak, intermittent 27-day signal is detected in high-frequency parts of the CO principal components.     

Photoelectron flux variations observed from the FAST satellite

This paper examines high resolution (ΔE/E = 0.15) photoelectron energy spectra from 10 eV to 1 keV, created by solar irradiances between 1.2 and 120 nm. The observations were made from the FAST satellite at ∼3000 km, equatorward of the auroral oval for the July–August, 2002 solar rotation. These data are compared with the solar irradiance observed by the Solar EUV Experiment (SEE) on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite and fluxes calculated using the Field Line Interhemispheric Plasma (FLIP) code. The 41 eV photoelectron flux, which corresponds to solar EUV fluxes near 20 nm, shows a clear solar rotation variation in very good agreement with the EUV flux measurements. This offers the possibility that the 41 eV photoelectron flux could be used as a check on measured solar EUV fluxes near 20 nm. Because of unexpected noise, the solar rotation signal is not evident in the integral photoelectron flux between 156 and 1000 eV corresponding to EUV wavelengths between 0.1 and 7 nm measured by the SEE instrument. Examination of daily averaged photoelectron fluxes at energies between 25 and 500 eV show significant changes in the photoelectron spectra in response X and M class flares. The intensity of photoelectrons produced in this energy region is primarily due to two very narrow EUV wavelength regions at 2.3 and 3 nm driving Auger photoionization in O at 500 eV and N₂ at ∼360 eV. Comparison of calculated and daily averaged electron fluxes shows that the HEUVAC model solar spectrum used in the FLIP code does not reproduce the observed variations in photoelectron intensity. In principle, the 21 discrete photoelectron energy channels could be used to improve the reliability of the solar EUV fluxes at 2.3 and 3 nm inferred from broad band observations. In practice, orbital biases in the way the data were accumulated and/or noise signals arising from natural and anthropogenic longitudinally restricted sources of ionization complicate the application of this technique.     

Thin structure of temporal profiles of solar flares January 15, 17 and 20, 2005 by data of AVS-F apparatus onboard CORONAS-F satellite

The count rate temporal profiles and energy spectra of the solar flares January 15, 17, 20 2005 in hard X-ray and gamma energy bands by data of AVS-F apparatus onboard CORONAS-F satellite are discussed. The energy spectra of these solar flares contain positron line and neutron capture line. Solar flares of January 17 and 20 spectra also contain some nuclear lines. Thin structure with characteristic timescales of 33–92 s is presented on flares temporal profiles in energy bands corresponding to the observed spectral features, which are confirmed by periodogram analysis (confidence level is 99%).     

The HESP/R satellite project

The aim of the HESP/R (High Energy Solar Physics/Radiation) satellite project is to obtain data of γ-ray, hard X-ray soft X-ray, EUV, and visible radiation of solar flares at the next solar maximum in order to study physics of flares. The HESP/R will be a spinning satellite of 4–5 rotations per min., and the spin axis is off-set by a small angle (0.5°–1.0°) from the Sun. Total weight will be 400 kg and launched in 1991 with M-3S-II rocket by ISAS.     

Scientific requirements for future spatially resolved white-light and broad-band high-cadence observations of the Sun

Several important issues are open in the field of solar variability and they wait their solution which up to now was attempted using critical ground-based instrumentations. However, accurate photometric data are attainable only from space. New observational material should be collected with high enough spatial and spectral resolution, covering the whole visible range of the electromagnetic spectrum as well infrared and ultraviolet to reconstruct the total solar irradiance: (1) the absolute contributions of different small-scale structural entities of the solar atmosphere from the white light flares and from micro-flares are still poorly known; (2) we do not know the absolute contributions of different structural elements of the solar atmosphere to the long-term and to the cyclic variations of the solar irradiance, including features of the polar regions of the Sun; (3) the variations of the chromospheric magnetic network are still poorly evaluated; (4) only scarce information is available about the spectral variations of different small-scale features in the high photosphere. Variability of the Sun in white light can be studied with higher spectral, spatial and time resolution using space-born telescopes, which are more appropriate for this purpose than ground based observatories because of better seeing conditions, no interference of the terrestrial atmosphere and a more precise calibration procedure. Scientific requirements for such observations and the possible experimental tools proposed for their solution. Suggested solar studies have broader astrophysical importance.     

Converging motion of conjugate flaring kernels during two large solar flares

In this paper, we analyze the footpoint motion of two large solar flares using observations made by the Transition Region and Coronal Explorer (TRACE) and Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The two flares are the M5.7 flare of March 14, 2002 and the X10 flare of October 29, 2003. They are both classical two-ribbon flares as observed in TRACE 1600 or 171 Å images and have long-duration conjugate hard X-ray (HXR) footpoint emission. We use the ‘center-of-mass’ method to locate the centroids of the UV/EUV flare ribbons. The results are: (1) The conjugate UV/EUV ribbons and HXR footpoints of the two flares show a converging (inward) motion during the impulsive phase. For the two flares, the converging motion lasts about 3 and 10 min, respectively. The usual separation (outward) motion for the flare ribbons and footpoints take place only after the converging motion. (2) During the inward and the outward motion, the conjugate ribbons and footpoints of the two events exhibit a strong unshear motion. In obtaining above results, TRACE UV/EUV and RHESSI HXR data show an overall agreement. The two events demonstrate that the magnetic reconnection for the flares occurs in highly sheared magnetic field. Furthermore, the results support the magnetic model constructed by Ji et al. [Ji, H., Huang, G., Wang, H. Astrophys. J. 660, 893–900, 2007], who proposed that the contracting motion of flaring loops is the signature of the relaxation of sheared magnetic fields.     

Which solar EUV indices are best for reconstructing the solar EUV irradiance?

The solar EUV irradiance is of key importance for space weather. Most of the time, however, surrogate quantities such as EUV indices have to be used by lack of continuous and spectrally resolved measurements of the irradiance. The ability of such proxies to reproduce the irradiance from different solar atmospheric layers is usually investigated by comparing patterns of temporal correlations. We consider instead a statistical approach. The TIMED/SEE experiment, which has been continuously operating since February 2002, allows for the first time to compare in a statistical manner the EUV spectral irradiance to five EUV proxies: the sunspot number, the f10.7, Ca K, and Mg II indices, and the He I equivalent width.     

New flare model using recent measurements of the solar ultraviolet irradiance

The solar photon output from the Sun, which was once thought to be constant, varies considerably over time scales from seconds during solar flares to years due to the solar cycle. This is especially true in the wavelengths shorter than 190 nm. These variations cause significant deviations in the Earth and space environment on similar time scales, which then affects many things including satellite drag, radio communications, atmospheric densities and composition of particular atoms, molecules, and ions of Earth and other planets, as well as the accuracy in the Global Positioning System (GPS). The Flare Irradiance Spectral Model (FISM) is an empirical model that estimates the solar irradiance at wavelengths from 0.1 to 190 nm at 1 nm resolution with a time cadence of 60 s. This is a high enough temporal resolution to model variations due to solar flares, for which few accurate measurements at these wavelengths exist. This model also captures variations on the longer time scales of solar rotation (days) and solar cycle (years). Daily average proxies used are the 0–4 nm irradiance, the Mg II c/w, F10.7, as well as the 1 nm bins centered at 30.5 nm, 121.5 (Lyman Alpha), and 36.5 nm. The GOES 0.1–0.8 nm irradiance is used as the flare proxy. The FISM algorithms are given, and results and comparisons are shown that demonstrate the FISM estimations agree within the stated uncertainties to the various measurements of the solar Vacuum Ultraviolet (VUV) irradiance.     

Nitric oxide density enhancements due to solar flares

A differential emission measure technique is used to determine flare spectra using solar observations from the soft X-ray instruments aboard the Thermosphere Ionosphere Mesosphere Energetics Dynamics and Solar Radiation and Climate Experiment satellites. We examine the effect of the solar flare soft X-ray energy input on the nitric oxide (NO) density in the lower thermosphere. The retrieved spectrum of the 28 October 2003 X18 flare is input to a photochemical thermospheric NO model to calculate the predicted flare NO enhancements. Model results are compared to Student Nitric Oxide Explorer Ultraviolet Spectrometer observations of this flare. We present results of this comparison and show that the model and data are in agreement. In addition, the NO density enhancements due to several flares are studied. We present results that show large solar flares can deposit the same amount of 0.1–2 and 0.1–7 nm energy to the thermosphere during a relatively short time as the Sun normally deposits in one day. The NO column density nearly doubles when the daily integrated energy above 5 J m⁻² is doubled.     

The SOHO CELIAS/SEM EUV database from SC23 minimum to the present

The SOHO Solar EUV Monitor has been in operation since December 1995 onboard the SOHO spacecraft. This instrument is a highly stable transmission grating solar extreme ultraviolet spectrometer. It has made nearly continuous full disk solar irradiance measurements both within an 8 nm bandpass centered at 30.4 nm and throughout the 0.1 to 50 nm solar flux region since launch. The 30.4 nm flux, the 0.1 to 50 nm flux and the extracted soft X-ray (0.1 to 5 nm) flux are presented and compared with the behavior of solar proxies.     

双通道辐射量计算太阳极紫外辐射E10.7指数方法初探

从太阳极紫外辐射研究的重要性出发, 介绍了太阳极紫外辐射E_10.7指 数及其作用, 详细阐述了利用两个能道的太阳辐射观测值计算极紫外辐射E_10.7指数的计算方法. 利用该方法对实测太阳辐射数据进行处理, 计算获得了2000-2005年的每日E_10.7指数, 并将计算结果 与Solar2000模式的输出结果进行对比分析, 验证了该计算方法的可行性, 对比结果表明, 最大相对误差在20%以内, 平均相对误差均在10%以内.      

Active region properties and irradiance variations

Total Solar Irradiance (TSI) has been measured for more than three decades. These observations demonstrate that total irradiance changes on time scales ranging from minutes to years and decades. Considerable efforts have been made to understand the physical origin of irradiance variations and to model the observed changes using measures of sunspots and faculae. In this paper, we study the short-term variations in TSI during the declining portion and minimum of solar cycle 22 and the rising portion of cycle 23 (1993–1998). This time interval of low solar activity allows us to study the effect of individual sunspot groups on TSI in detail. In this paper, we indicate that the effect of sunspot groups on total irradiance may depend on their type in the Zürich classification system and/or their evolution, and on their magnetic configuration. Some uncertainties in the data and other effects are also discussed.     

Distinct EUV minimum of the solar irradiance (16–40 nm) observed by SolACES spectrometers onboard the International Space Station (ISS) in August/September 2009

In the field of terrestrial climatology the continuous monitoring of the solar irradiance with highest possible accuracy is an important goal. SolACES as a part of the ESA mission SOLAR on the ISS is measuring the short-wavelength solar EUV irradiance from 16–150 nm. This data will be made available to the scientific community to investigate the impact of the solar irradiance variability on the Earth’s climate as well as the thermospheric/ionospheric interactions that are pursued in the TIGER program. Since the successful launch with the shuttle mission STS-122 on February 7th, 2008, SolACES initially recorded the low EUV irradiance during the extended solar activity minimum. Thereafter it has been observing the EUV irradiance during the increasing solar activity with enhanced intensity and changing spectral composition. SolACES consists of three grazing incidence planar grating spectrometers. In addition there are two three-signal ionisation chambers, each with exchangeable band-pass filters to determine the absolute EUV fluxes repeatedly during the mission. One important problem of space-borne instrumentation recording the solar EUV irradiance is the degradation of the spectrometer sensitivity. The two double ionisation chambers of SolACES, which could be re-filled with three different gases for each recording, allow the recalibration of the efficiencies of the three SolACES spectrometers from time to time.     

Solar EUV and decimetric indices and thermospheric models

Data bases and limits of applicability of existing empirical thermospheric models are reviewed by using these models together with solar EUV irradiance data in studying the solar activity effect on composition, density and temperature. For two rather short aeronomy missions of the AEROS A and B satellites solar EUV indices as proposed by Schmidtke are used in comparison with the 10.7 cm solar flux F in determining the solar activity effect in in-situ composition measurements sampled by the same satellites at 250, 310 and 380 km altitude. No advantage of solar EUV indices over F could be determined.