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.     

EUV observation from the Earth-orbiting satellite,EXCEED

An Earth-orbiting small satellite “EXtreme ultraviolet spectrosCope for ExosphEric Dynamics” (EXCEED) which will be launched in 2012 is under development. The mission will carry out spectroscopic and imaging observation of EUV (Extreme Ultraviolet: 60–145 nm) emissions from tenuous plasmas around the planets (Venus, Mars, Mercury, and Jupiter). It is essential for EUV observation to put on an observing site outside the Earth’s atmosphere to avoid the absorption. It is also essential that the detection efficiency must be very high in order to catch the faint signals from those targets. In this mission, we employ cesium iodide coated microchannel plate as a 2 dimensional photon counting devise which shows 1.5–50 times higher quantum detection efficiency comparing with the bared one. We coat the surface of the grating and entrance mirror with silicon carbides by the chemical vapor deposition method in order to archive the high diffraction efficiency and reflectivity. The whole spectrometer is shielded by the 2 mm thick stainless steel to prevent the contamination caused by the high energy electrons from the inner radiation belt. In this paper, we will introduce the mission overview, its instrument, and their performance.     

EUV emission from normal galaxies

Using data from the Wide Field Camera EUV all-sky survey, we have established upper limits to the EUV flux from a sample of 30 bright, nearby, non-active spiral galaxies. These galaxies were chosen to be those most likely to be detected in the EUV on the basis of (i) low interstellar absorption within our own galaxy, (ii) brightness in other wavebands, (iii) high star formation activity, and (iv) proximity. The derived EUV upper limits are restrictive, and establish for the first time that the EUV flux escaping from galaxies does not constitute a major component of their bolometric luminosity, and in particular that it cannot be the sink for the energy injected into the interstellar medium by supernova explosions, as had been suggested following the failure to detect this power in the X-ray band.     

A study of the plasmasphere density distribution using computed tomography methods from the EUV radiation data

In this paper, we introduce a new topic for imaging plasmasphere density with CT techniques. Both parallel-beam and fan-beam reconstruction formulas are derived with a special weighting function. Different from the traditional X-ray CT, the extreme ultraviolet (EUV) radiation may be totally absorbed by the Earth. Only the portion between the Earth and the detector enters the EUV sensor and be measured. The key step in our derivation is to remove the complete absorption effect with a specific weighting function. Computer-simulation studies are present to verify our formulas for the plasmasphere density.     

脉泽分子云动力学结构的VLBI测量

利用VLBⅠMKⅢ记录系统, 在1665和1667MHz跃迁频率上, 观测活动恒星形成区NGC6334F的左右圆偏振羟基(OH)脉泽分子辐射;介绍了VLBIOH分子谱线观测过程, 给出了基本的观测结果, 并对结果作了简要地分析和讨论.      

太阳活动对EUV辐射在大气中吸收过程的影响

本文利用A-E卫星在太阳活动21周峰年间观测到的EUV辐射资料,高层大气成分的吸收截面,以及MSIS-86热层大气模式,研究了EUV辐射在大气中的吸收过程;在透射比为1/e和0.1/100时分别计算了透射高度随波长及太阳活动的变化。在波长范围50—1050内对37个波段分别求出了透射高度随太阳天顶角的变化。结果表明,当太阳活动增强时各波段的透射高度均升高,而且透射比越大则透射高度随太阳活动的变化也越剧烈。当透射比为一定时,太阳天顶角越大则透射高度随太阳活动的变化也越大。除此之外还存在一个相反效应,即太阳活动会使Chapman函数变小,这反过来又促使透射高度降低。这两种效应的综合作用结果可较好地解释某些电离层观测中的日没效应。     

Cometary kilometric radio waves and plasma waves correlated with ion pick-up effect at comet Halley

From the discrete spectra of the emissions from the comet in the frequency range from 30 to 195 kHz named CKR (Cometary Kilometric Radiation), movements of the bow shock at comet Halley are concluded, i.e., the observed CKR emissions can be interpreted as being generated and propagating from the moving shock. The motion of the shocks are possibly associated with time variation of the solar wind and of the cometary outgassings. By in-situ plasma waves observations using PWP (Plasma Wave Probe) onboard the Sakigake spacecraft, the characteristic spectra of the electrostatic electron plasma waves, the electron cyclotron harmonic waves, and the ion sound waves have been detected during the interval of the Halley's comet fly-by. Compared with the results of a Faraday cup observation and a magnetometer, it is concluded that these plasma wave phenomena are the manifestation of the ion pick-up processes. The ion pick-up processes are taking place even in the remote region within a distance range from 7×10⁶ to 10⁷ km from the cometary nucleus.     

Representations of solar EUV fluxes for aeronomical applications

The development of significantly improved representations of solar EUV inputs for computer-aided investigations of the terrestrial thermosphere and ionosphere has become attractive particularly for the present solar cycle which has been covered by reasonably complete and continuous EUV observations from the AE-E Satellite. These representations try to satisfy some rather incongruous $\text{requirements of spectral detail}$, regarding (a) the strong wavelength-dependence in the $\text{terrestrial}$ atmospheric cross sections of the various types of EUV photon interactions, (b) the great differences in the relative amplitudes of the various types of variations in the full-disk fluxes of $\text{solar}$ emissions at different wavelengths, and (c) the persisting desire to use only a $\text{small number of daily indices}$ as actual input variables for computational models supposed to cover the entire EUV wavelength range (remembering the great success of empirical thermospheric models using only two indices). These general physical and specific aeronomical demands indeed outline a very difficult task. The present study, based mainly on AE-E satellite observations during 1976–1979, represents an exploratory step, only clarifying some important developmental aspects, without recommending any specific formulations for immediately practicable adoption in aeronomical modelling at this time.     

Numerical Simulation for the 16 August 1999 EUV Brightenings

The 16 August 1999 EUV brightenings are numerically simulated by a third-order upwind compact scheme, basing on the TRACE observation. The present simulation can give a possible explanation to its formation and evolution. The numerical results show that the initial reconnection jets at around X-point are responsible for the occurrence of EUV brightening. The strong and superposed ejections caused by the first and second coalescence of magnetic islands are possibly related to the lifted material which initially appeared as absorption features and Later EUV-emitting structures respectively. The bi-directional reconnection jets may correspond to the lifted material that either continued to move upward along the apparently open field lines or fell down to the surface.      

The detection of energetic cometary and solar particles by the EPONA instrument on the Giotto mission

EPONA is an energetic particle detector system incorporating totally depleted silicon surface barrier layer detectors. Active and passive background shielding will be employed and, by applying various techniques, particles of different species, including electrons, protons, alpha particles and pick-up ions of cometary origin may be detected over a wide spectrum of energies extending from the tens of KeV into the MeV range.

The instrument can operate in two modes namely (a) in a cruise phase or storage mode and (b) in a real time mode. During the real time mode, observations at high spatial (octosectoring) and temporal (0.5s) resolution in the cometary environment permit studies to be made of accelerated particles at the bow shock and/or in the tail of the comet. In conjunction with magnetic field measurements on board Giotto, observations of energetic electrons and their anisotropies can determine whether the magnetic field lines in the cometary tail are open or closed. Further, the absorption of low energy solar particles in the cometary atmosphere can be measured and such data would provide an integral value of the pertaining gas and dust distribution. Solar particle background measurements during encounter may also be used to correct the measurements of other spacecraft borne instruments potentially vulnerable to such radiation.

Solar particle flux measurements, obtained during the cruise phase will, when combined with simultaneous observations made by other spacecraft at different heliographic longitudes, provide information concerning solar particle propagation in the corona and in interplanetary space.     

Electromagnetic field dissipation and corpuscular bombardment and their implications in atmospheric modelling

The dissipation of energy of electric fields and currents in the polar auroral atmosphere is a major source of energy for the thermosphere ranging locally up to 100 ergs cm^?2 sec^?1 and perhaps more during the most intense disturbance. Globally the input of energy to the thermosphere can often exceed that due to solar EUV radiation. This energy source is always significant in polar regions and its variable strength with respect to that of the solar EUV radiation determines the behaviour of the middle and low latitude thermosphere. It is extremely difficult to model because of its variability in space and time. Nevertheless understanding the dynamics and composition of the global thermosphere is dependent upon incorporation of this source realistically into models. A further important aspect of this energy source is the consequences of its action in changing the density and composition of the thermosphere globally leading to subsequent changes in the absorption of solar EUV radiation. The ring current may also, at times, be a significant source of energy to the low latitude thermosphere.     

EUV imaging of near-Venus space

Japan’s Venus Climate Orbiter (the Planet-C spacecraft) will be launched in 2008 and will reach an orbit in the ecliptic plane around Venus in 2009. We propose two eXtreme UltraViolet (XUV) imagers to take global two-dimensional snapshots of near-Venus space, including the Venus ionosphere and the interaction region between the solar wind plasma and the Venus ionospheric plasma. The imagers detect the resonantly scattering emissions of oxygen ions (O II 83.4 nm) and atoms (O I 130.3 nm), neutral helium (He I 58.4 nm), and hydrogen (H Ly-α 121.6 nm). Scientific goals are to investigate mechanisms of momentum and mass transfer across the ionopause, of convection in the upper atmosphere and ionosphere, and of atmospheric escape. Especially, we emphasize that sequential images of the O II 83.4-nm emission will enable us to understand temporal evolution of the vortex produced by the Kelvin–Helmholtz (K–H) instability. Though the wave structure due to the K–H instability is generated also at the terrestrial magnetopause, oxygen ions are too tenuous to detect the emission. On the other hand, at the Venus ionopause oxygen ions have enough density to image the resonance emission, i.e., the Venus ionosphere plays a role as a space laboratory for plasma physics.     

A new inversion method of plasma density distribution of plasmasphere in the geomagnetic equatorial plane from IMAGE data

The plasma density distribution of plasmasphere in the geomagnetic equatorial plane can help us study the magnetosphere like plasmasphere, ionosphere and their kinetics. In this paper, we introduce a new inversion method, GE-ART, to calculate the plasma density distribution in the geomagnetic equatorial plane from the Extreme Ultraviolet (EUV) data of IMAGE satellite under the assumption that the plasma density is constant along each geomagnetic field line. The new GE-ART algorithm was derived from the traditional Algebraic Reconstruction Techniques (ART) in Computed Tomography (CT) which was different from the several existing methods. In this new method, each value of the EUV image data was back-projected evenly to the geomagnetic field lines intersected by this EUV sight. A 3-D inversion matrix was produced by the contributions of all the voxels contained in the plasmasphere covered by the EUV sensor. That is, we considered that each value of the EUV image data was relative to the plasma densities of all the voxels passed through by the corresponding EUV radiation, which is the biggest difference to all the existing inversion methods. Finally, the GE-ART algorithm was evaluated by the real EUV data from the IMAGE satellite.     

多谱段全日面极紫外成像仪

在极紫外波段对太阳进行成像观测是研究太阳活动、日冕中等离子体物理特性的重要手段.传统极紫外成像仪或光谱仪无法同时实现高光谱分辨率和大视场的太阳成像.本文设计了一种新型太阳极紫外多谱段成像系统,采用无狭缝光栅分光方式实现了高光谱分辨率和空间分辨率的全日面成像,成像视场可达47',光谱分辨率每像素2×10-3 nm,空间分辨率每像素1.4',全日面时间分辨率优于60s.通过分析谱线的全日面成像图和系统响应,表明成像仪能大范围的观测太阳活动形态演化,为太阳物理研究和空间天气预报提供更完整的观测数据.      

Solar flare electron acceleration: Comparing theories and observations

A popular scenario for electron acceleration in solar flares is transit-time damping of low-frequency MHD waves excited by reconnection and its outflows. The scenario requires several processes in sequence to yield energetic electrons of the observed large number. Until now there was very little evidence for this scenario, as it is even not clear where the flare energy is released. RHESSI measurements of bremsstrahlung by non-thermal flare electrons yield energy estimates as well as the position where the energy is deposited. Thus quantitative measurements can be put into the frame of the global magnetic field configuration as seen in coronal EUV line observations. We present RHESSI observations combined with TRACE data that suggest primary energy inputs mostly into electron acceleration and to a minor fraction into coronal heating and primary motion. The more sensitive and lower energy X-ray observations by RHESSI have found also small events (C class) at the time of the acceleration of electron beams exciting meter wave Type III bursts. However, not all RHESSI flares involve Type III radio emissions. The association of other decimeter radio emissions, such as narrowband spikes and pulsations, with X-rays is summarized in view of electron acceleration.     

Collisional profiles of ionized calcium perturbed by helium

The spectra of many cool DZ white dwarfs show extremely broad resonance lines of ionized calcium and magnesium in the optical and ultraviolet region. When the line center is strongly saturated, these wings may become a significant source of additional opacity in cool white dwarf atmospheres. The omission of this opacity results in systematic errors in the predicted flux distribution and hence in the effective temperatures and element abundances. In a continuation of Homeier et al. (2007) and Allard et al. (2012), where we presented absorption spectra of Na–He in cool white dwarfs, we extend this work to the Ca⁺–He system.     

Solar EUV flux variation and flare brightenings

On 2010 February 8, the Extreme ultraviolet (EUV) flux variation in 195 Å and flare brightening has been examined in different sizes of active regions by using SOHO/EIT, MDI and Hα$\alpha$ observational data. These three active regions represent a large active region with a sunspot group, a moderate active region without a sunspot and a small region with weak plage in Hα$\alpha$ band respectively. Our study shows that the main full disk EUV flux comes from active regions, especially from large active regions. The sudden increases of EUV flux are corresponding to the EUV flare brightenings. For the large active region, the local EUV 195 Å flux peaks are well correlated to that of the GOES X-ray flux. The EUV 195 Å flux peaking time of M-class flares delay GOES X-ray flux a few minutes. For the moderate active region, the local EUV 195 Å flux is not well correlated to GOES X-ray flux. The EUV 195 Å flare brightenings in the moderate active region appeared in the duration of sudden increase of its own local EUV flux. For the small active region, the local EUV 195  Å flux varied almost independently of the GOES X-ray flux. Our study suggests that for an active region its local EUV 195 Å flux is more closely correlated to the EUV flare brightening than the full disk GOES X-ray flux.     

Cometary magnetospheres: a tutorial

The nucleus of an active comet, such as comet Halley near its perihelion, produces large quantities of gas and dust. The resulting cometary atmosphere, or coma, extends more than a million kilometers into space, where it interacts with the solar wind. An “induced” cometary magnetosphere is a consequence of this interaction. Cometary ion pick-up and mass loading of the solar wind starts to take place at very large cometocentric distances. Eventually this mass loading leads to the formation of a weak cometary bow shock. Even closer to the nucleus, collisional processes, such as ion-neutral chemistry, become important. Other features of the magnetosphere of an active comet include a magnetic barrier, a magnetotail, and a diamagnetic cavity near the nucleus. X-ray emission from comets is produced by the interaction of the solar wind with cometary neutrals and this topic is also discussed. A broad review of the cometary magnetosphere will be given in this paper.