A glimpse at the GOCE satellite gravity gradient observations

Since 30 September 2009, following the launch and in-orbit testing of the most sophisticated gravity mission ever built, the European Space Agency (ESA) GOCE satellite is in ‘measurement mode’, providing continuous time series of satellite gravity gradient (SGG) observations and GPS satellite-to-satellite tracking (SST) observations. The availability of GPS SST observations allows the precise reconstruction of the GOCE position and thus the precise geolocation of the SGG observations. The SGG observations are based on the differences between observations taken by pairs of accelerometers, which need to be corrected first by applying a so-called calibration matrix and second by subtracting rotational terms (centrifugal and angular accelerations).     

Orbit determination of BDS-3 satellite based on regional ground tracking station and inter-satellite link observations

The BeiDou global navigation satellite system (BDS-3) has established the Ka-band inter-satellite link (ISL) to realize a two-way ranging function between satellites, which provides a new observation technology for the orbit determination of BDS-3 satellites. Therefore, this study presents a BDS satellite orbit determination model based on ground tracking station (GTS) observations and ISL ranging observations firstly to analyze the impact of the ISL ranging observations on the orbit determination of BDS-3 satellites. Subsequently, considering the data fusion processing, the variance component estimation (VCE) algorithm is applied to the parameter estimation process of the satellite orbit determination. Finally, using the measured data from China’s regional GTS observations and BDS-3 ISL ranging observations, the effects of ISL ranging observations on the orbit determination accuracy of BDS-3 satellites are analyzed. Moreover, the impact of the VCE algorithm on the fusion data processing is evaluated from the aspects of orbit determination accuracy, Ka-band hardware delay parameter stability, and ISL ranging observation residuals. The results show that for China’s regional GTSs, the addition of BDS-3 ISL ranging observations can significantly improve the orbit determination accuracy of BDS-3 satellites. The observed orbit determination accuracy of satellite radial component is improved from 48 cm to 4.1 cm. In addition, when the initial weight ratio between GTS observations and ISL ranging observations is not appropriate, the various indicators which include orbit determination accuracy, ISL hardware delay, and ISL observation residuals were observed to have improved after the adjustment of the VCE algorithm. These results validate the effectiveness of the VCE algorithm for the fusion data processing of the GTS observations and ISL ranging observations.     

Analysis of total visual and CCD V-broadband observations of Comet C/1995 O1 (Hale-Bopp): 1995–2001

We developed a general method for determination of water production rates from groundbased visual observations and applied it to Comet Hale–Bopp. Our main objective is to extend the method to include total visual magnitude observations obtained with CCD detector and V filter in the analysis of total visual magnitudes. We compare the CCD V-broadband careful observations of Liller [Liller, W. Pre-perihelion CCD photometry of Comet 1995 O1 (Hale-Bopp). Planet. Space Sci. 45, 1505–1513, 1997; Liller, W. CCD photometry of Comet C/1995 O1 (Hale-Bopp): 1995–2000. Int. Comet Quart. 23(3), 93–97, 2001] with the total visual magnitude observations from experienced international observers found in the International Comet Quarterly (ICQ) archive. A data set of ∼400 CCD observations covering about the same 6 years time span of the ∼12,000 ICQ selected total visual magnitude observations were used in the analysis. A least-square method applied to the water production rates, yields power laws as a function of the heliocentric distances for the pre- and post-perihelion phases. The average dimension of the nucleus as well as its effective active area is determined and compared with values published in the literature.     

Modeling ground and space based cosmic ray observations

After entering our local astrosphere (called the heliosphere), galactic cosmic rays, as charged particles, are affected by the Sun’s turbulent magnetic field. This causes their intensities to decrease towards the inner heliosphere, a process referred to as modulation. Over the years, cosmic ray modulation has been studied extensively at Earth, utilizing both ground and space based observations. Moreover, modelling cosmic ray modulation and comparing results with observations, insight can be gained into the transport of these particles, as well as offering explanations for observed features. We review some of the most prominent cosmic ray observations made near Earth, how these observations can be modelled and what main insights are gained from this modelling approach. Furthermore, a discussion on drifts, as one of the main modulation processes, are given as well as how drift effects manifest in near Earth observations. We conclude by discussing the contemporary challenges, fuelled by observations, which are presently being investigated. A main challenge is explaining observations made during the past unusual solar minimum.     

A review of the present state of gravity modes oscillations of the sun

Short review of ground-based observations of the Sun and of solar gravity modes detection is presented with the emphasize on 160m oscillation as predominant, long-time phase-coherent oscillation. The limitations imposed by the influence of Earth's atmosphere and geoseismic waves on observations of solar oscillations are pointed out together with a necessity of the observations of solar oscillations with the aid of space-probes.     

Orbit improvement for GTO objects using follow-up observations

The Astronomical Institute of the University of Bern was and is conducting several search campaigns for space debris in the geostationary ring (GEO) and the geostationary transfer orbit (GTO). First tests to build up a catalogue of satellites and space debris were performed recently. As the discovery observations of an object usually cover a time interval of a few minutes only, the first orbits determined are assumed to be circular. For GEO objects a circular orbit is a rather good approximation. This is certainly not the case for GTO objects. Nevertheless, a circular orbit seems to be an acceptable approximation for a short time interval around the observations. The first orbits can be used to acquire follow-up observations, which allow the determination of elliptical orbits. The maximum allowed time span between the detection and the follow-up for a successful recovery is found using simulated GTO orbits and observations. Further follow-up observations are simulated in order to study the orbit improvement process and the required force model. The accuracy of orbits needed to build up a catalogue is studied. The simulations are compared to the results achieved with observations acquired by the 1 m telescope on Tenerife.     

Understanding magnetotail current sheet meso-scale structures using MHD simulations

Recent Cluster observations have strongly supported the existence of meso-scale structure in the magnetotail current sheet. In our study, a magnetohydrodynamic simulation event study exhibited current sheet behavior comparable to that seen in the Cluster observations. Geotail and DoubleStar observations also show that the simulation is providing a realistic representation of the magnetosphere during the period of interest; that is, when the current sheet evidently becomes bifurcated. The magnetohydrodynamic simulation allows us to place the local observations into a global contest. It shows that the observations can be explained in terms of localized reconnection tailward of the Cluster location and the formation of a flux rope nearby. The simulation also features wave-like structure across the current sheet.     

Physical properties of dust grains deduced by optical probing techniques

In-situ space observations of dust in the solar system are seldom possible. On the opposite, remote observations of solar light scattered by dust are relatively easy to perform from Earth- or satellite-based observatories; the evolution of the polarization of light scattered by dust particles as a function of the phase angle may provide information on the physical properties of these particles. Unfortunately, since remote observations are integrated along the line-of-sight of the observer, they can hardly be used to determine local physical properties. We have precisely developed Optical Probe techniques to forge the link between the numerous remote observations and the unique in-situ measurements. A short review of the remote observations of light scattered by cometary dust is first presented. Then, the Optical Probe concept is analyzed. Finally, the OPE instrument, which had been designed to optically probe the inner coma of comet Halley is described; its limitations and its achievements during Halley and Grigg-Skjellerup encounters are discussed.     

Interplanetary spacecraft navigation using pulsars

We demonstrate how observations of pulsars can be used to help navigate a spacecraft travelling in the solar system. We make use of archival observations of millisecond pulsars from the Parkes radio telescope in order to demonstrate the effectiveness of the method and highlight issues, such as pulsar spin irregularities, which need to be accounted for. We show that observations of four millisecond pulsars every seven days using a realistic X-ray telescope on the spacecraft throughout a journey from Earth to Mars can lead to position determinations better than ∼20 km and velocity measurements with a precision of ∼0.1 ms⁻¹.     

Cosmic ray modulation in the outer heliosphere: Predictions for cosmic ray intensities up to the heliopause along Voyager 1 and 2 trajectories

Time dependent cosmic ray modulation in the outer heliosphere is calculated and results are compared to Voyager 1 and 2 observations using a two-dimensional time-dependent cosmic ray transport model. We predict possible future 133–242 MeV proton observations along the Voyager 1 and 2 spacecraft trajectories. Recent theoretical advances in cosmic ray transport parameters are introduced in order to provide a time-dependence for the assumed transport parameters used in the model. This leads to results that are in general compatible with the spacecraft observations in the inner and outer heliosphere over multiple solar cycles. However, for the outer heliosphere, we find that the Voyager 1 and 2 spacecraft observations cannot be fitted with an identical set of parameters along both trajectories. This indicates a possible asymmetric heliosphere or a symmetric heliosphere but with different diffusion parameters in the northern and southern hemispheres, respectively. Furthermore, results indicate that Voyager 2 observations are still under the influence of solar cycle related changes because of the large modulation volume between the heliopause and spacecraft location in contrast to Voyager 1 which shows a steady increase in cosmic ray intensities.     

IUE observations of globular clusters and blue horizontal branch stars

This paper presents a review of ultraviolet photometry and spectroscopy of globular clusters and blue horizontal branch stars. Current observations with IUE are placed within the framework of earlier photometric studies carried out by OAO and ANS. In order to provide basic information on the nature of the horizontal branch star, whose light dominates in the ultraviolet, observations of field horizontal branch stars as well as individual stars in globular clusters are discussed. Finally the IUE observations of globular clusters in the Magellanic Clouds are examined, since they provide a sequence spanning the age range from 7 × 10⁶ years to 10¹⁰ years.     

Cometary dust observations by optical in-situ methods

Remote optical observations of comets provide information only along the whole line of sight and require some assumptions to be interpreted. Due to the advent of cometary space missions, a two-step strategy has been defined to derive without any assumption spatial distribution and physical properties of dust by in-situ optical observations. First, an $\text{Optical Probe Experiment}$, suitable for a fast fly-by, should provide passive in-situ measurements in the direction of the approaching (or receding) comet near encounter; by suitably differencing such observations, the brightness and polarization per $\text{unit volume}$ can be recovered along the trajectory of the spacecraft. Secondly, a $\text{Light Scattering Dust Analyzer}$, suitable for a rendez-vous mission, should permit the determination of the scattering properties of $\text{individual particles}$. Both experiments also provide a connecting link between non-optical in-situ measurements (from mass spectrometers or impact detectors) and remote optical observations.     

Occurrence rate of SAR arcs during the 23rd solar activity cycle

The occurrence rate of SAR arcs during 1997–2007 has been analyzed based on the photometric observations at the Yakutsk meridian (Maimaga station, corrected geomagnetic coordinates: 57°N, 200°E). SAR arcs appeared in 114 cases (∼500 h) during ∼370 nights of observations (∼3170 h). The occurrence frequency of SAR arcs increases to 27% during the growth phase of solar activity and has a clearly defined maximum at a decline of cycle 23. The SAR arc registration probability corresponds to the variations in geomagnetic activity in this solar cycle. The dates, intervals of UT, and geomagnetic latitudes of SAR arc observations at the Yakutsk meridian are presented.     

Physical characteristics of the upper layers of Saturn's atmosphere

Analysis of polarimetric observations of Saturn was carried out. In the long wave-length spectral range (λ > 0.5μm) polarimetric observations do not contradict the model of spherical or irregular randomly oriented particles. In the short wave-length spectral interval (λ < 0.5μm) it is necessary to take into account the scattering by oriented particles.     

Kuafu and the studies of CME initiation

We first briefly review the current trend in the studies of coronal mass ejections (CMEs), then summarize some recent efforts in understanding the CME initiation. Emphasis has been put on the studies of Earth-directed CMEs whose associated surface activity and large scale magnetic source have been well identified. The data analysis by combining the MDI full disc magnetograms, vector magnetograms of active regions, EUV waves and dimmings, non-thermal radio sources, and the SOHO LASCO observations has shed new light in understanding the CME magnetism. However, the current studies seem to invoke new observations in a few aspects: (1) The observations which enable us to trace CMEs from the earliest associated surface activity to its initial acceleration and key development in the low corona in the height of 1–3 R; (2) The imaging spectroscopic observations which can be used to diagnose the early plasma outflow and the line-of-sight velocity in understanding the kinematics of CMEs; (3) The accurate timing from primary magnetic energy release, manifested by chromospheric activity, non-thermal radio bursts, and EUV, X-ray and γ-ray emissions, to the CME initiation, early acceleration and propagation, and the consequences in the interplanetary space and magnetosphere. The Kuafu Mission will meet the basic requirement for the new observations in CME initiation studies and serve as a monitor of space weather of the Sun–Earth system.     

Evolution of galaxy cluster scaling from XMM-Newton observation of the galaxy clusters at z ⩾ 0.4

We present here new XMM-Newton observations of 3 relatively cool clusters at z ≈ 0.4, complemented by archival observations of 3 other clusters at similar redshift. We derived the M–T and R–T relations from the hydrostatic equation using an isothermal temperature distribution.     

Interpretation of hard X-ray images during the impulsive phase of a limb flare

We show that the observations of a limb flare, in which a hard X-ray (16–30 keV) source is seen at the boundary between two interacting magnetic structures, indicate the presence of hot (T ? 6 × 10⁷ K) plasma within the region. Non thermal bremsstrahlung processes do not agree with these observations. We discuss the possible causes of the heating.     

Large-scale waves in the solar corona: The continuing debate

Ten years after the first observation of large-scale wave-like coronal disturbances with the EIT instrument aboard SOHO, the most crucial questions concerning these “EIT waves” are still being debated controversially – what is their actual physical nature, and how are they launched? Possible explanations include MHD waves or shocks, launched by flares or driven by coronal mass ejections (CMEs), as well as models where coronal waves are not actually waves at all, but generated by successive “activation” of magnetic fieldlines in the framework of a CME. Here, we discuss recent observations that might help to discriminate between the different models. We focus on strong coronal wave events that do show chromospheric Moreton wave signatures. It is stressed that multiwavelength observations with high time cadence are particularly important, ideally when limb events with CME observations in the low corona are available. Such observations allow for a detailed comparison of the kinematics of the wave, the CME and the associated type II radio burst. For Moreton-associated coronal waves, we find strong evidence for the wave/shock scenario. Furthermore, we argue that EIT waves are actually generated by more than one physical process, which might explain some of the issues which have made the interpretation of these phenomena so controversial.     

GMRT monitoring of Cyg X-1 and Cyg X-3

We present the results of low frequency radio observations of the X-ray binaries, Cygnus X-1 and Cygnus X-3, during different X-ray states. The low frequency observations were made for the first time for these sources at 0.61 and 1.28 GHz using the Giant Meter-wave Radio Telescope (GMRT) between 2003 and 2004. Both Cyg X-1 and Cyg X-3 are highly variable at low radio frequencies. We also compare our data with the observations at 15 GHz conducted by the Ryle telescope. Spectral turnover is seen for both the sources below 2 GHz. The data suggest that the change in the radio flux density in both the sources is correlated to the X-ray hardness ratio and follows a similar behavior pattern.