Andrei Zhukov

Solar and Heliospheric Phenomena in October-November 2003: Causes and Effects

We present new observational data on the phenomena of extremely high activity on the Sun and in the heliosphere that took place in October–November 2003. A large variety of solar and heliospheric parameters give evidence that the interval under consideration is unique over the entire observation time. Based on these data, comparing them with similar situations in the past and using available theoretical concepts, we discuss possible cause-and-effect connections between the processes observed. The paper includes the first results and conclusions derived by the collaboration “Solar Extreme Events-2003” organized in Russia for detailed investigations of these events. As a result of our consideration, it is beyond question that the physical causes of solar and heliospheric phenomena in October–November 2003 are not exclusively local and do not belong only to the active regions and solar atmosphere above them. The energy reservoirs and driving forces of these processes have a more global nature. In general, they are hidden from an observer, since ultimately their sources lie in the subphotospheric layers of the Sun, where changes that are fast and difficult to predict can sometimes take place (and indeed they do). Solar flares can serve as sufficiently good tracers of these sudden changes and reconstructions on the Sun, although one can still find other diagnostic indicators among the parameters of magnetic fields, motions of matter, and emission characteristics.

November 2004 space weather events: Real‐time observations and forecasts

Space weather events with their solar origin and their distribution through the heliosphere affect the whole magnetosphere-ionosphere-Earth system. Their real-time monitoring and forecasting are important for science and technology. Here we discuss one of the largest space weather events of Solar Cycle 23, in November 2004, which was also one of the most difficult periods to forecast. Nine halo coronal mass ejections (CMEs), interacting on their way through the interplanetary medium and forming two geoeffective interplanetary structures, exemplify the complexity of the event. Real-time and quasi-real-time observations of the ground geomagnetic field show rapid and extensive expansion of the auroral oval to 55° in geomagnetic latitude accompanied by great variability of the ionosphere. Geomagnetically induced currents (GICs) seen in ground networks, such as power grids and pipelines, were significant during the event, although no problems were reported. Forecasts of the CME propagation, global and local ground geomagnetic activity, and ionospheric parameters, issued by several regional warning centers, revealed certain deficiencies in predictions of the interplanetary characteristics of the CME, size of the geomagnetic disturbances, and complexity of the ionospheric variations produced by this event. This paper is a collective report based on the materials presented at the splinter session on November 2004 events during the first European Space Weather Week.

Low polarised emission from the core of coronal mass ejections

Aims. In white-light coronagraph images, cool prominence material is sometimes observed as bright patches in the core of coronal mass ejections (CMEs). If, as generally assumed, this emission is caused by Thomson-scattered light from the solar surface, it should be strongly polarised tangentially to the solar limb. However, the observations of a CME made with the SECCHI/STEREO coronagraphs on 31 August 2007 show that the emission from these bright core patches is exceptionally low polarised. Methods. We used the polarisation ratio method of Moran & Davila (2004) to localise the barycentre of the CME cloud. By analysing the data from both STEREO spacecraft we could resolve the plane-of-the-sky ambiguity this method usually suffers from. Stereoscopic triangulation was used to independently localise the low-polarisation patch relative to the cloud. Results. We demonstrated for the first time that the bright core material is located close to the centre of the CME cloud. We show that the major part of the CME core emission, more than 85% in our case, is Hα radiation and only a small fraction is Thomson-scattered light. Recent calculations also imply that the plasma density in the patch is 8 × 10 8 cm −3 or more compared to 2.6 × 10 6 cm −3 for the Thomson-scattering CME environment surrounding the core material.

The EUI instrument on board the Solar Orbiter mission: from breadboard and prototypes to instrument model validation

The Solar Orbiter mission will explore the connection between the Sun and its heliosphere, taking advantage of an orbit approaching the Sun at 0.28 AU. As part of this mission, the Extreme Ultraviolet Imager (EUI) will provide full-sun and high-resolution image sequences of the solar atmosphere at selected spectral emission lines in the extreme and vacuum ultraviolet. To achieve the required scientific performances under the challenging constraints of the Solar Orbiter mission it was required to further develop existing technologies. As part of this development, and of its maturation of technology readiness, a set of breadboard and prototypes of critical subsystems have thus been realized to improve the overall instrument design. The EUI instrument architecture, its major components and sub-systems are described with their driving constraints and the expected performances based on the breadboard and prototype results. The instrument verification and qualification plan will also be discussed. We present the thermal and mechanical model validation, the instrument test campaign with the structural-thermal model (STM), followed by the other instrument models in advance of the flight instrument manufacturing and AIT campaign.

The technical challenges of the Solar-Orbiter EUI instrument

The Extreme Ultraviolet Imager (EUI) onboard Solar Orbiter consists of a suite of two high-resolution imagers (HRI) and one dual-band full Sun imager (FSI) that will provide EUV and Lyman-α images of the solar atmospheric layers above the photosphere. The EUI instrument is based on a set of challenging new technologies allowing to reach the scientific objectives and to cope with the hard space environment of the Solar Orbiter mission. The mechanical concept of the EUI instrument is based on a common structure supporting the HRI and FSI channels, and a separated electronic box. A heat rejection baffle system is used to reduce the Sun heat load and provide a first protection level against the solar disk straylight. The spectral bands are selected by thin filters and multilayer mirror coatings. The detectors are 10μm pitch back illuminated CMOS Active Pixel Sensors (APS), best suited for the EUI science requirements and radiation hardness. This paper presents the EUI instrument concept and its major sub-systems. The current developments of the instrument technologies are also summarized.

Design status of ASPIICS, an externally occulted coronagraph for PROBA-3

The “sonic region” of the Sun corona remains extremely difficult to observe with spatial resolution and sensitivity sufficient to understand the fine scale phenomena that govern the quiescent solar corona, as well as phenomena that lead to coronal mass ejections (CMEs), which influence space weather. Improvement on this front requires eclipse-like conditions over long observation times. The space-borne coronagraphs flown so far provided a continuous coverage of the external parts of the corona but their over-occulting system did not permit to analyse the part of the white-light corona where the main coronal mass is concentrated. The proposed PROBA-3 Coronagraph System, also known as ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), with its novel design, will be the first space coronagraph to cover the range of radial distances between ~1.08 and 3 solar radii where the magnetic field plays a crucial role in the coronal dynamics, thus providing continuous observational conditions very close to those during a total solar eclipse. PROBA-3 is first a mission devoted to the in-orbit demonstration of precise formation flying techniques and technologies for future European missions, which will fly ASPIICS as primary payload. The instrument is distributed over two satellites flying in formation (approx. 150m apart) to form a giant coronagraph capable of producing a nearly perfect eclipse allowing observing the sun corona closer to the rim than ever before. The coronagraph instrument is developed by a large European consortium including about 20 partners from 7 countries under the auspices of the European Space Agency. This paper is reviewing the recent improvements and design updates of the ASPIICS instrument as it is stepping into the detailed design phase.

Solar Sources of Geoeffective CMEs: a SOHO/EIT View

Observations of the low solar corona, in particular in the EUV, are an eectiv e means of identifying the solar sources of coronal mass ejections (CMEs). SOHO/EIT, with its continuous 24 hours per day coverage, is well suited to perform this task. Source regions and start times of frontside full and partial halo CMEs (that may be geoeectiv e) can thus be determined. The most frequent EUV signatures of CMEs are coronal dimmings. EIT waves, eruptive lamen ts and post-eruption arcades are also reliable signatures. Frontside halo CMEs with source regions close to the solar disc center have the strongest chance to hit the Earth. The inspection of the EIT data together with photospheric magnetograms may give an idea about the ejected interplanetary ux rope magnetic eld and, in particular, about the presence or absence of southward (geoeectiv e) eld. If a source region is situated close to the solar limb, the corresponding CME also may be geoeectiv e, as the CME-driven shocks have large angular extent. In this case the storm can be produced by the sheath plasma behind the shock, provided it contains strong enough southward interplanetary magnetic eld. Some implications for the operational space weather forecast are discussed. EIT and LASCO are capable to identify the solar sources of the most of geomagnetic storms. In some cases, however, the identication is uncertain, so the observations by the future STEREO mission will be needed for the investigation of similar events.

The extreme UV imager of solar orbiter: from detailed design to flight model

The Extreme Ultraviolet Imager (EUI) on-board the Solar Orbiter mission will provide full-sun and high-resolution image sequences of the solar atmosphere at selected spectral emission lines in the extreme and vacuum ultraviolet. After the breadboarding and prototyping activities that focused on key technologies, the EUI project has completed the design phase and has started the final manufacturing of the instrument and its validation. The EUI instrument has successfully passed its Critical Design Review (CDR). The process validated the detailed design of the Optical Bench unit and of its sub-units (entrance baffles, doors, mirrors, camera, and filter wheel mechanisms), and of the Electronic Box unit. In the same timeframe, the Structural and Thermal Model (STM) test campaign of the two units have been achieved, and allowed to correlate the associated mathematical models. The lessons learned from STM and the detailed design served as input to release the manufacturing of the Qualification Model (QM) and of the Flight Model (FM). The QM will serve to qualify the instrument units and sub-units, in advance of the FM acceptance tests and final on-ground calibration.

Solar Wind Disturbances and Their Sources in the EUV Solar Corona

We investigate possible links between the activity manifestations in the solar corona and conditions in the solar wind. For the reduction of this immense task we have selected 206 events in the solar wind in 1997 – 2000 corresponding to geomagnetic events with Ap > 20 (compiled into a database at http://alpha.sinp.msu.ru/apev). Up to now, 24 events during the epoch of low solar activity (January 1997 – January 1998) are investigated. The solar wind conditions monitored by ACE and WIND spacecraft were traced back to the solar corona observed by SOHO/EIT. The search for coronal signatures which are probably associated with the disturbed solar wind conditions was performed. The coronal sources of these 24 events are identified, namely: eruptions in active regions, filament eruptions and coronal holes. It is shown that halo and partial halo CMEs observed within the SOHO/LASCO sensitivity limits are not necessary indicators of Earth‐directed eruptions, and coronal EUV dimmings can be used as a complementary in...

Design and modelisation of ASPIICS optics

In the framework of development of ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), the Centre Spatial de Liege is responsible of the optical design of the coronagraph and the optics will be manufactured by TOPTEC. The particularity of this coronagraph is to have an external occulter located 150 m ahead of the first imaging lens. This external occulter is re-imaged on an internal occulter which function is - as in a classical externally occulted Lyot coronagraph - to block the sun light diffracted by the external occulter and to reduce the straylight on the detector. The selection of this configuration is driven by the requirement to observe the corona as close as possible to the solar limb (i.e. 1 RSun) without imaging the limb itself. A requirement of 1.08 RSun is specified at optical design level to grant 1.2 Rsun at instrument level. The coronograph instrument is designed to have a field of view of 1.6° x 1.6° with a resolution of less than 6 arcsec. Its performances are limited by diffraction in a 530 – 590 nm wavelength range. This paper presents the optical design and demonstrates that by design the requirements are fulfilled within the misalignment, manufacturing and thermo-elastic error contributions.