Near real-time modeling of global ionospheric vertical total electron content using hourly IGS data

The International GNSS Service (IGS) has been providing reliable Global Ionospheric Maps (GIMs) since 1998. The Ionosphere Associate Analysis Centers (IAACs) model the global ionospheric Total Electron Content (TEC) and generate the daily GIM products within the context of the IGS. However, the rapid and final daily GIM products have a latency of at least one day and one week or so, respectively. This limits the value of GIM products in real-time GNSS applications. We propose and develop an approach for near real-time modeling of global ionospheric TEC by using the hourly IGS data. We perform an experiment in a real operating environment to generate near real-time GIM (named BUHG) products for more than two years. Final daily GIM products, Precise Point Positioning (PPP) based VTEC resources, and JASON-3 Vertical TEC (VTEC) measurements are collected for testing the performance of BUHG. The results show that the performance of BUHG is very close to that of the daily GIM products. Also, there is good agreement between BUHG and PPP-derived VTEC as well as with JASON-3 VTEC. It is possible that BUHG would be further improved with an increase in available hourly GNSS data.     

The Plasma Ion and Electron Instruments for the Genesis Mission

The Genesis Ion Monitor (GIM) and the Genesis Electron Monitor (GEM) provide 3-dimensional plasma measurements of the solar wind for the Genesis mission. These measurements are used onboard to determine the type of plasma that is flowing past the spacecraft and to configure the solar wind sample collection subsystems in real-time. Both GIM and GEM employ spherical-section electrostatic analyzers followed by channel electron multiplier (CEM) arrays for detection and angle and energy/charge analysis of incident ions and electrons. GIM is of a new design specific to Genesis mission requirements whereas the GEM sensor is an almost exact copy of the plasma electron sensors currently flying on the ACE and Ulysses spacecraft, albeit with new electronics and programming. Ions are detected at forty log-spaced energy levels between ∼ 1 eV and 14 keV by eight CEM detectors, while electrons with energies between ∼ 1 eV and 1.4 keV are measured at twenty log-spaced energy levels using seven CEMs. The spin of the spacecraft is used to sweep the fan-shaped fields-of-view of both instruments across all areas of the sky of interest, with ion measurements being taken forty times per spin and samples of the electron population being taken twenty four times per spin. Complete ion and electron energy spectra are measured every ∼ 2.5 min (four spins of the spacecraft) with adequate energy and angular resolution to determine fully 3-dimensional ion and electron distribution functions. The GIM and GEM plasma measurements are principally used to enable the operational solar wind sample collection goals of the Genesis mission but they also provide a potentially very useful data set for studies of solar wind phenomena, especially if combined with other solar wind data sets from ACE, WIND, SOHO and Ulysses for multi-spacecraft investigations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Studying the Lunar Ionosphere with SELENE Radio Science Experiment

Radio occultation observations of the electron density near the lunar surface were conducted during the SELENE (Kaguya) mission using the Vstar and Rstar sub-satellites. Previous radio occultation measurements conducted in the Soviet lunar missions have indicated the existence of an ionosphere with peak densities of several hundreds of electrons per cubic centimeters above the dayside lunar surface. These densities are difficult to explain theoretically when the removal of plasma by the solar wind is considered, and thus the generation mechanism of the lunar ionosphere is a major issue, with even the validity of previous observations still under debate. The most serious error source in the measurement is the fluctuation of the terrestrial ionosphere which also exists along the ray path. To cope with this difficulty, about 400 observations were conducted using Vstar to enable statistical analysis of the weak signal of the lunar ionosphere. Another method is to utilize Vstar and Rstar with the second one being used to measure the terrestrial ionosphere contribution. The observations will establish the morphology of the lunar ionosphere and will reveal its relationship with various conditions to provide possible clues to the mechanism.     

The interpretation of the XUV solar spectrum

This paper reviews (a) the earth's ionosphere, and (b) the solar atmosphere, in relation to the recent observations of solar XUV. The expected ionospheric characteristics are derived as directly as possible from the XUV observations and then compared with the well-known D-, E-, and F-layer formations. The comparison leads to (1) a high ionospheric recombination coefficient decreasing rapidly with height, (2) contributions to the E-layer from both UV and X-rays, and (3) very little difference in the solar cycle variations from the D-, E-, and F-layers although intensity variations are greater from high than low ions. The flux measurements of the identified XUV solar emission lines give information on the numbers of ions in the solar atmosphere. This makes it possible to derive (1) the amount of solar material in each temperature range, (2) the chemical abundances, and (3) the physical differences between quiet solar atmosphere, centres of activity, and flares. When the new dielectronic recombination coefficients are well investigated it should be possible to redetermine the distribution of solar material with temperature. The fitting of such results to chromospheric and coronal models provides many problems.     

不同GNSS星载原子钟周期特性分析

高精度的卫星时钟修正是全球卫星导航系统实时精密单点定位和授时服务的重要基础。为了提高GNSS钟差预报精度,需要对GNSS星载原子钟的周期特性进行分析。基于2016年全年的GNSS精密卫星钟差数据,利用中位数方法进行了数据预处理,使用多项式拟合模型分析了卫星钟的拟合残差,利用频谱分析法分析了BDS、GPS卫星钟差的周期特性,全面分析了BDS、GPS星载原子钟的周期特性。分析结果表明：除Cs钟外,其他卫星钟差都表现出较好的周期特性,BDS、GPS的主周期项基本在12h、24h、6h附近;同时不同的轨道、原子钟,其钟差周期项不同,而相同的轨道类型,其钟差周期项也存在一定差异;卫星的钟差主周期分别近似为其卫星轨道周期的1/2倍、1倍、2倍。     

Observations of Vertical Reflections from the Topside Martian Ionosphere

The Martian ionosphere has for the first time been probed by a low frequency topside radio wave sounder experiment (MARSIS) (Gurnett et al., 2005). The density profiles in the Martian ionosphere have for the first time been observed for solar zenith angles less than 48 degrees. The sounder spectrograms typically have a single trace of echoes, which are controlled by reflections from the ionosphere in the direction of nadir. With the local density at the spacecraft derived from the sounder measurements and using the lamination technique the spectrograms are inverted to electron density profiles. The measurements yield electron density profiles from the sub-solar region to past the terminator. The maximum density varies in time with the solar rotation period, indicating control of the densities by solar ionizing radiation. Electron density increases associated with solar flares were observed. The maximum electron density varies with solar zenith angle as predicted by theory. The altitude profile of electron densities between the maximum density and about 170m altitude is well approximated by a classic Chapman layer. The neutral scale height is close to 10 to 13 km. At altitudes above 180 km the densities deviate from and are larger than inferred by the Chapman layer. At altitudes above the exobase the density decrease was approximated by an exponential function with scale heights between 24 and 65 km. The densities in the top side ionosphere above the exobase tends to be larger than the densities extrapolated from the Chapman layer fitted to the measurements at lower altitudes, implying more efficient upward diffusion above the collision dominated photo equilibrium region.     

Two-fluid 2.5D MHD Simulations of the Fast Solar Wind in Coronal Holes and the Relation to UVCS Observations

Recent SOHO/UVCS observations indicate that the perpendicular proton and ion temperatures are much larger than electron temperatures. In the present study we simulate numerically the solar wind flow in a coronal hole with the two-fluid approach. We investigate the effects of electron and proton temperatures on the solar wind acceleration by nonlinear waves. In the model the nonlinear waves are generated by Alfvén waves with frequencies in the 10^-3 Hz range, driven at the base of the coronal hole. The resulting electron and proton flow profile exhibits density and velocity fluctuations. The fluctuations may steepen into shocks as they propagate away from the sun. We calculate the effective proton temperature by combining the thermal and wave velocity of the protons, and find qualitative agreement with the proton kinetic temperature increase with height deduced from the UVCS Ly-α observations by Kohl et al. (1998). This revised version was published online in June 2006 with corrections to the Cover Date.     

电离层顶部电子含量对GNSS电离层层析的影响分析

卫星信号射线上的总电子含量(slant total electron content, STEC)是像素基全球卫星导航系统电离层层析(computerized ionospheric tomography, CIT)建模的必要数据来源，但电离层层析通常忽略1 000 km以上的顶部电子含量，为弄清这部分电子含量对层析结果的影响，利用NeQuick2模型计算站星视线上的STEC与其在电离层区域内的STEC比值来改正原始数据，并分别利用改正前后的STEC进行电离层层析。结果显示，电离层顶部电子含量占比约为10%，白天占比略大于黑夜，与测高仪站的数据相比，改正后的均方根值比改正前提高了20%以上；与Swarm卫星提供的电子剖面数据对比，改正后的层析结果精度较改正前提升了19.6%左右，且该方法受地磁扰动影响较为明显。总的来说，利用CIT进行小尺度电离层探测，可较直观地看出，顶部电子含量对层析结果的影响较大，需要采取相应手段予以剔除。     

Theory of formation of the ionosphere

Current knowledge about the solar radiation and absorption and ionization cross sections of atmospheric gases is reviewed. Next the main observed features of ionospheric layers are summarized. Using CIRA 1965 model atmospheres the heights of the peak of the ionization rate are calculated for a number of solar emission lines and it is made clear which of these lines are responsible for the formation of E and F1 layers. The mechanism of electron removal in the F and upper E regions as well as in the lower regions is considered, and the mechanism of formation and some behaviours of each ionospheric layer is discussed. In particular, the equatorial F2 layer is briefly considered. Discrepancies are pointed out between the values of the recombination coefficient and the rate constant for ion-atom interchange reaction obtained from ionospheric observations and from laboratory experiments. Inconsistency of the values of the intensity of solar radiation measured by rocket techniques and inferred from ionospheric considerations is also noted. Some evidence is presented suggesting that corpuscular radiation may be responsible for part of the ionization in the ionosphere even in temperate latitudes.     

Magnetic Fields in the Inner Heliosphere

The structure of Heliospheric Magnetic Field (HMF) is a function of both the coronal conditions from which it originates and dynamic processes which take place in the solar wind. The division between the inner and outer regions of the heliosphere is the result of dynamic processes which form large scale structures with increasing heliocentric distance. The structure of the HMF is normally described in the reference frame based on Parker's geometric model, but is better understood as an extension of potential field models of the corona. The Heliospheric Current Sheet (HCS) separates the two dominant polarities in the heliosphere; its large scale geometry near solar minimum is well understood but its topology near solar maximum remains to be investigated by Ulysses. At solar minimum, Corotating Interaction Regions (CIRs) dominate the near-equatorial heliosphere and extend their influence to mid-latitudes; the polar regions of the heliosphere are dominated by uniform fast solar wind streams and large amplitude, long wavelength, mostly transverse magnetic fluctuations. Coronal Mass Ejections (CMEs) introduce transient variability into the large scale heliospheric structure and may dominate the inner heliosphere near solar maximum at all latitudes.     

The Middle Atmospheric Ozone Response to the 11-Year Solar Cycle

Because of its chemical and radiative properties, atmospheric ozone constitutes a key element of the Earth’s climate system. Absorption of sunlight by ozone in the ultraviolet wavelength range is responsible for stratospheric heating, and determines the temperature structure of the middle atmosphere. Changes in middle atmospheric ozone concentrations result in an altered radiative input to the troposphere and to the Earth’s surface, with implications on the energy balance and the chemical composition of the lower atmosphere. Although a wide range of ground- and satellite-based measurements of its integrated content and of its vertical distribution have been performed since several decades, a number of uncertainties still remain as to the response of middle atmospheric ozone to changes in solar irradiance over decadal time scales. This paper presents an overview of achieved findings, including a discussion of commonly applied data analysis methods and of their implication for the obtained results. We suggest that because it does not imply least-squares fitting of prescribed periodic or proxy data functions into the considered times series, time-domain analysis provides a more reliable method than multiple regression analysis for extracting decadal-scale signals from observational ozone datasets. Applied to decadal ground-based observations, time-domain analysis indicates an average middle atmospheric ozone increase of the order of 2% from solar minimum to solar maximum, which is in reasonable agreement with model results.     

Equatorial and Low Latitude Ionospheric Effects During Sudden Stratospheric Warming Events

There are several external sources of ionospheric forcing, including these are solar wind-magnetospheric processes and lower atmospheric winds and waves. In this work we review the observed ion-neutral coupling effects at equatorial and low latitudes during large meteorological events called sudden stratospheric warming (SSW). Research in this direction has been accelerated in recent years mainly due to: (1) extensive observing campaigns, and (2) solar minimum conditions. The former has been instrumental to capture the events before, during, and after the peak SSW temperatures and wind perturbations. The latter has permitted a reduced forcing contribution from solar wind-magnetospheric processes. The main ionospheric effects are clearly observed in the zonal electric fields (or vertical E×B drifts), total electron content, and electron and neutral densities. We include results from different ground- and satellite-based observations, covering different longitudes and years. We also present and discuss the modeling efforts that support most of the observations. Given that SSW can be forecasted with a few days in advance, there is potential for using the connection with the ionosphere for forecasting the occurrence and evolution of electrodynamic perturbations at low latitudes, and sometimes also mid latitudes, during arctic winter warmings.     

Soho Observations of Density Fluctuations in Coronal Holes

In recent UVCS/SOHO White Light Channel (WLC) observations we found quasi-periodic variations in the polarized brightness (pB) in the polar coronal holes at heliocentric distances of 1.9 to 2.45 solar radii. The motivation for the observation is the 2.5D MHD model of solar wind acceleration by nonlinear waves, that predicts compressive fluctuations in coronal holes. In February 1998 we performed new observations using the UVCS/WLC in the coronal hole and obtained additional data. The new data corroborate our earlier findings with higher statistical significance. The new longer observations show that the power spectrum peaks in the 10–12 minute range. These timescales agree with EIT observations of brightness fluctuations in polar plumes. We performed preliminary LASCO/C2 observations in an effort to further establish the coronal origin of the fluctuations. This revised version was published online in June 2006 with corrections to the Cover Date.     

Short Term,Direct Indices of Solar Variability

Indices of solar activity relevant for understanding and modelling solar irradiance variability are identified, and their temporal characteristics compared. Reproducing observed solar irradiance variability requires a minimum of two different types of indices — an index for irradiance depletion by sunspots and an index for global irradiance enhancement by faculae and network. When combined with appropriate wavelength-dependent parameterizations of sunspot and facular contrasts and center-to-limb functions, these indices permit the construction of empirical models of daily, monthly and annual solar total and spectral irradiances. The models are compared with observations at selected wavelengths and for the total irradiance. While the models replicate much of the rotational and 11-year cycle variance in contemporary irradiance databases, differences exist because of either the presence of variability mechanisms additional to solar magnetism, or of unresolved instrumental effects in the databases. The reconstruction of solar irradiance in the past requires speculation about the extent of intercycle fluctuations in the global facular index, or in other, as yet unspecified, variability mechanisms.     

Coronal Heating and Solar Wind Acceleration: Future Work on Observations

The space-based observatories SOHO and TRACE have shown some very interesting results on the structure and dynamics of the Sun and its atmosphere, e.g., the extremely high ion temperatures or the enormous variability in the corona. But one question is still open to debate: how to use these data to distinguish between different types of physical heating processes, as, e.g., absorption of high-frequency Alfvén-waves or reconnection events? This paper will discuss some possibilities on how to tackle this type of question. These include observations of ion temperature anisotropies and electron temperatures in the corona as well as measurements of coronal magnetic fields. Emphasis will be put on simultaneous observations of the whole solar atmosphere from the photosphere into the solar wind and on solar-stellar connections. In the light of these ideas new proposed space missions as well as ground based efforts will be discussed.     

Relativistic electron events in interplanetary space

This review is concerned with relativistic electron events observed in interplanetary space. The different types of event are identified and illustrated. The relationships between solar X-ray and radio emissions and relativistic electrons are examined, and the relevance of the observations to solar flare acceleration models is discussed. A statistical analysis of electron spectra, the electron/proton ratio and propagation from the flare site to the Earth is presented. A model is outlined which can account for the release of electrons from the Sun in a manner consistent with observations of energetic solar particles and electromagnetic solar radiation.The literature survey for this review was concluded in May 1973.     

Non-relativistic solar electrons

This review summarizes both the direct spacecraft observations of non-relativistic solar electrons, and observations of the X-ray and radio emission generated by these particles at the Sun and in the interplanetary medium. These observations bear on three physical processes basic to energetic particle phenomena: (1) the acceleration of particles in tenuous plasmas; (2) the propagation of energetic charged particles in a disordered magnetic field, and (3) the interaction of energetic charged particles with tenuous plasmas to produce electromagnetic radiation. Because these electrons are frequently accelerated and emitted by the Sun, mostly in small and relatively simple flares, it is possible to define a detailed physical picture of these processes.In many small solar flares non-relativistic electrons accelerated during flash phase constitute the bulk of the total flare energy. Thus the basic flare mechanism in these flares essentially converts the available flare energy into fast electrons. Non-relativistic electrons exhibit a wide variety of propagation modes in the interplanetary medium, ranging from diffusive to essentially scatter-free. This variability in the propagation may be explained in terms of the distribution of interplanetary magnetic field fluctuations. Type III solar radio burst emission is generated by these electrons as they travel out to 1 AU and beyond. Recent in situ observations of these electrons at 1 AU, accompanied by simultaneous observations of the low frequency radio emission generated by them at 1 AU provide quantitative information on the plasma processes involved in the generation of type III bursts.     

Doppler scintillation measurements of the heliospheric current sheet and coronal streamers close to the Sun

Prominent enhancements in Doppler scintillation lasting a fraction of a day (solar source several degrees wide) and overlying the neutral line represent the signature of the heliospheric current sheet and the apparent interplanetary manifestation of coronal streamers near the Sun. This first detection of coronal streamers in radio scintillation measurements provides the link betweenin situ measurements of the spatial wavenumber spectrum of electron density fluctuations beyond 0.3 AU and earlier measurements deduced from radio scintillation and scattering observations inside 0.3 AU. Significant differences between the density spectra of fast streams and slow solar wind associated with the heliospheric current sheet near the Sun reinforce the emerging picture that high- and low-speed flows are organized by the large-scale solar magnetic field, and that while the contrast between solar wind properties of the two flows is highest near the Sun, it undergoes substantial erosion in the ecliptic plane as the solar wind expands.     

Solar Variability and the Search for Corresponding Climate Signals

Instrumental and paleodata from the last centuries are investigated to get circumstantial evidence for external influences on the Earth's climate machine. Such influences could be of extraterrestrial and/or anthropogenic origin. Anthropogenic influences are separated from solar on superdecadal time scales and on a hemispheric level using a non-linear regression model. The function to be explained is the northern hemispheric temperature. The model contains two forcing components explicitly: A parameterized anthropogenic component, which describes the aggregated effect of greenhouse gases, aerosols and other anthropogenic climate impacts. A solar component, which describes the solar variability history. The solution of the regression model allows, under certain assumptions, a functional separation of the variability components and provides an estimation of their relative contributions to global warming during the last 140 years.     

OpenGGCM Simulations for the THEMIS Mission

The THEMIS mission provides unprecedented multi-point observations of the magnetosphere in conjunction with an equally unprecedented dense network of ground measurements. However, coverage of the magnetosphere is still sparse. In order to tie together the THEMIS observations and to understand the data better, we will use the Open Geospace General Circulation Model (OpenGGCM), a global model of the magnetosphere-ionosphere system. OpenGGCM solves the magnetohydrodynamic (MHD) equations in the outer magnetosphere and couples via field aligned current (FAC), electric potential, and electron precipitation to a ionosphere potential solver and the Coupled Thermosphere Ionosphere Model (CTIM). The OpenGGCM thus provides a global comprehensive view of the magnetosphere-ionosphere system. An OpenGGCM simulation of one of the first substorms observed by THEMIS on 23 March 2007 shows that the OpenGGCM reproduces the observed substorm signatures very well, thus laying the groundwork for future use of the OpenGGCM to aid in understanding THEMIS data and ultimately contributing to a comprehensive model of the substorm process.