Article

SWAN: A study of solar wind anisotropies

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Abstract

The SWAN (solar wind anisotropies) Lyman photometer project is described. It will map the interplanetary hydrogen emission every other day. From these sky maps, the latitude distribution of the solar wind mass flux from equator to pole can be measured, as well as the variation of this distribution. Solar wind mass flux at high latitude is a key boundary condition to the dynamics of the expansion of the solar wind, particularly in the coronal holes, and correlatives studies conducted with other SOHO (solar and heliospheric observatory) coronal instruments. Secondary objectives include the monitoring of known comets, the possible discovery of new comets, and coronal observations.

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... The assumption used and the model itself is described in greater detail in Zoennchen et al. (2015). The parameters of this model are adjusted such that the interplanetary Lyman-α intensities are in good agreement with maps pro- vided by SOHO/SWAN (see also Bertaux et al., 1998). An example is given in Fig. 2. Also the Lyman-α component of stars with a bright UV radiation component (e.g., young O-type stars) was removed from these contaminated LOS observations. ...
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Circumterrestrial Lyman-α column brightness observations from 3–8 Earth radii (Re) have been used to study temporal density variations in the exospheric neutral hydrogen as response to geomagnetic disturbances of different strength, i.e., Dst peak values between −26 and −147 nT. The data used were measured by the two Lyman-α detectors (LAD1/2) onboard both TWINS satellites between the solar minimum of 2008 and near the solar maximum of 2013. The solar Lyman-α flux at 121.6 nm is resonantly scattered near line center by exospheric H atoms and measured by the TWINS LADs. Along a line of sight (LOS), the scattered LOS-column intensity is proportional to the LOS H column density, assuming optically thin conditions above 3 Re. In the case of the eight analyzed geomagnetic storms we found a significant increase in the exospheric Lyman-α flux between 9 and 23 % (equal to the same increase in H column density ΔnH) compared to the undisturbed case short before the storm event. Even weak geomagnetic storms (e.g., Dst peak values ≥ −41 nT) under solar minimum conditions show increases up to 23 % of the exospheric H densities. The strong H density increase in the observed outer exosphere is also a sign of an enhanced H escape flux during storms. For the majority of the storms we found an average time shift of about 11 h between the time when the first significant dynamic solar wind pressure peak (pSW) hits the Earth and the time when the exospheric Lyman-α flux variation reaches its maximum. The results show that the (relative) exospheric density reaction of ΔnH have a tendency to decrease with increasing peak values of Dst index or the Kp index daily sum. Nevertheless, a simple linear correlation between ΔnH and these two geomagnetic indices does not seem to exist. In contrast, when recovering from the peak back to the undisturbed case, the Kp index daily sum and the ΔnH essentially show the same temporal recovery.
... Several instruments capable of mapping the backscattered Lya radiation from interstellar H are currently distributed throughout the heliosphere. SOHO SWAN (Bertaux et al., 1988) has been and is collecting a comprehensive set of Lya full sky maps from L1 over the entire solar cycle with intensity and Doppler shift information (Quemarais et al., 1999). Detectors with a combination of sensitivity to Lya and Lyb are at various distances from the sun on Galileo (Barth et al., 1997), Cassini, the Pioneers (Gangopadhyay et al., 2002) and the Voyagers. ...
Thesis
Designing and developing space instruments involves a wide variety of evaluation and simulation techniques in order to ensure correct operation under all possible conditions likely to be encountered in space and to allow parallel development of different subsystems of an instrument. This thesis describes three such evaluation and simulation techniques including real-time processing techniques, devised for two major European space missions, the Solar and Heliospheric Observatory (SOHO) and the X-ray Multi-Mirror (XMM) mission. The design of a Science Data Display Adapter is described, which was developed to provide comprehensive performance evaluation of the detectors of the Grazing Incidence Spectrometer (GIS), part of the Coronal Diagnostic Spectrometer on-board the SOHO, in the absence of the Command and Data Handling System and the Experiment Ground Support Equipment. The requirements to handle high data rates and to have significant display flexibility are discussed. This thesis also describes a user-controlled detector simulator developed to carry out full range tests of the GIS processing electronics in the absence of real detectors, including extreme conditions not easily achievable by other means. With its large degree of flexibility, the simulator provides realistic shapes and a wide range of characteristics for the output events of the Spiral Anode (SPAN). Although the simulator was designed specifically to simulate the SPAN, the design is applicable to any three channel detector system and has since been used for the FONEMA instrument for the Russian Mars96 mission. Finally, two alternative algorithms, which are applied to reduce the telemetry requirements for a Charge Coupled Device based, space-borne, X-ray spectrometer by on-board reconstruction of X-ray events split over two or more adjacent pixels, are described. The algorithms have been developed for the Reflection Grating Spectrometer (RGS) on the XMM, and were also used to study the feasibility of having a single processor Data Pre-Processor subsystem as part of the RGS Digital Electronics. Such design has now been adopted for flight.
Article
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Circumterrestrial Lyman-α column brightness observations above 3 Earth radii (Re) have been used to derive separate 3-D neutral hydrogen density models of the Earth's exosphere for solar minimum (2008, 2010) and near-solar-maximum (2012) conditions. The data used were measured by Lyman-α detectors (LAD1/2) onboard each of the TWINS satellites from very different orbital positions with respect to the exosphere. Exospheric H atoms resonantly scatter the near-line-center solar Lyman-α flux at 121.6 nm. Assuming optically thin conditions above 3Re along a line of sight (LOS), the scattered LOS-column intensity is proportional to the LOS H-column density. We found significant differences in the density distribution of the terrestrial exosphere under different solar conditions. Under solar maximum conditions we found higher H densities and a larger spatial extension compared to solar minimum. After a continuous, 2-month decrease in (27 day averaged) solar activity, significantly lower densities were found. Differences in shape and orientation of the exosphere under different solar conditions exist. Above 3 Re, independent of solar activity, increased H densities appear on the Earth's nightside shifted towards dawn. With increasing distance (as measured at 8Re) this feature is shifted westward/duskward by between −4 and −5° with respect to midnight. Thus, at larger geocentric distance the exosphere seems to be aligned with the aberrated Earth–solar-wind line, defined by the solar wind velocity and the orbital velocity of the Earth. The results presented in this paper are valid for geocentric distances between 3 and 8Re.
Article
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The Solar Wind Anisotropies (SWAN) instrument on board the SOHO spacecraft measures Lyman alpha radiation emanating mainly from neutral hydrogen gas in the solar neighborhood. This gas is part of the local interstellar cloud in which the Sun and the heliosphere are immersed. Measurements of Lyman alpha can be used to infer the local cloud characteristics like the velocity and the direction of the flow, gas temperature, and density. The strong interaction between the Sun and the neutral hydrogen gas also makes possible investigations of solar characteristics by Lyman alpha measurements. In this work we will concentrate on deriving the latitudinal distribution of solar-induced ionization from SWAN Lyman alpha maps measured in 1996 at a time of the solar minimum. From the ionization we derive the distribution of the solar wind mass flux. SWAN Lyman alpha data show that the ionization and the mass flux are nearly flat for all solar latitudes except the narrow belt from −20° to 20° around the solar equator. In this region, ionization and the solar wind mass flux show a definite increase, which can be seen as an intensity depression in the Lyman alpha data from directions near the ecliptic. These results confirm earlier in situ measurements by Ulysses during the present minimum and Lyman alpha measurements by Prognoz satellites 20 years ago.
Article
The Solar and Heliospheric Observatory (SOHO) will carry a set of solar physics experiments which permit a thorough investigation of the solar corona. The emphasis of the mission is on the measurement of the physical properties of coronal structures and the processes occurring therein, leading — it is hoped — to an understanding of the mechanism(s) by which the solar corona is heated and the solar wind is accelerated. The observations will be made from a halo orbit around the L1 Lagrange point on the Earth-Sun line and will range from magnetic field measurements in the photosphere through spectroscopic plasma diagnostics of chromospheric, transition-zone and coronal structures with high spatial and spectral resolution, to coronagraphic observations out to 30 solar radii and to mass spectrometry of the solar wind near one astronomical unit.SOHO is part of the Solar Terrestrial Science Programme (STSP), the first ‘Cornerstone’ in ESA's long-term scientific programme ‘Space Science — Horizon 2000’, and will therefore be flown in connection with the other STSP component, the four-spacecraft Cluster mission, which will investigate plasma structures and processes in the magnetosphere in three dimensions.Both STSP missions, i.e. SOHO and Cluster will address the physics of plasma structures and processes, that are accessible to investigation in the solar-terrestrial context, yet are thought to be examples of plasma processes and structures that are ubiquitous in the cosmos. It is hoped that a cross-fertilisation between the scientific communities associated with SOHO and Cluster will take place, as they investigate the physics of the coronal and magnetospheric plasma with complementary methods and techniques — globally by remote observations, and in detail, by multi-point in-situ measurements.
Article
The Solar and Heliospheric Observatory (SOHO) — a space observatory to be placed, in 1995, 1.5 Gm sunward from the Earth in a halo orbit around the L1 Lagrange point — will investigate:– the solar corona, its heating and expansion into the solar wind, by both studying the radiation emerging from the outer solar atmosphere and in-situ solar wind measurements near 1 AU, and – the structure and dynamics of the solar interior by the method of helioseismology. The science policy evolution leading to this comprehensive observatory concept is described. SOHO's link to the space-plasma-physics mission CLUSTER — devoted to the three-dimensional study of small structures in the magnetosphere — within the Solar Terrestrial Science Programme (STSP) and the embedding of STSP in the much larger International Solar Terrestrial Physics (ISTP) Programme are cited as well.The scientific subjects to be addressed by SOHO are introduced, and their current status assessed. Subsequently, the measurements required to advance these subjects are stated quantitatively and the payload, which will actually perform these measurements, is presented. The mission design, comprising spacecraft, orbit, operations and the data and ground systems are described. The special efforts made to obtain a reliable radiometric calibration of the instruments observing the Sun in the extreme-ultraviolet and to achieve a stable sensitivity through extreme cleanliness of spacecraft and instruments are emphasized and substantiated.
Chapter
The capabilities of space instrumentation are in a continuous state of evolution with the improvement of new techniques such as the metallic multilayer coatings in the Extreme UltraViolet. After a brief review of a few of the new techniques that will provide improved detectors, a survey of current instrumentation is given (Yohkoh, HRTS, MSSA, NIXT). Several missions are in preparation (SOHO, CORONAS); a description of the payload capabilities of these is furnished. Finally, we speculate on the outlook for the new, improved instruments on future missions.
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Comet 2P/Encke was observed with the SOHO/LASCO C2 and C3 coronagraphs over a time interval of 11 days, starting 4 days before its September 2000 perihelion passage and through several broadband visible filters. The lightcurve reveals an outburst which started 4.9 days after perihelion, with the brightness of the coma increasing by 1.5 mag in just a few hours and progressively decreasing thereafter, probably going back to its original state in about 9 days. The color information indicates that an approximately solar color continuum was detected, implying that the observed signals were dominated by solar light scattered off submillimetric dust grains. We propose that the rapid migration of the subsolar point over the southern hemisphere during the perihelion passage activates one or several new active regions enriched in submillimetric grains, with the observed outburst corresponding to the initial blow-off of their mantle. This scenario is consistent with other observations and implies that the south polar region of the nucleus of 2P/Encke has very distinct properties.
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