A. Bemporad

FIRST COMPLETE DETERMINATION OF PLASMA PHYSICAL PARAMETERS ACROSS A CORONAL MASS EJECTION-DRIVEN SHOCK

We report on the study of a fast coronal mass ejection (CME)-driven shock associated with the solar eruption of 2002 March 22. This event was observed in the intermediate corona both in white light and the extreme ultraviolet (EUV) by the LASCO and UVCS instruments on board the Solar and Heliospheric Observatory ,a s well as in metric and decametric wavelengths through space- and ground-based radio observatories. Clear signatures of shock transit are (1) strong type II emission lanes observed after the CME initiation, (2) strong Ovi λλ1032, 1037 line profile broadenings (up to ∼2 × 10 7 K) associated with the shock transit across the UVCS slit field of view, and (3) a density enhancement located in LASCO images above the CME front. Since the UVCS slit was centered at 4.1 R� , in correspondence with the flank of the expanding CME, this observation represents the highest UV detection of a shock obtained so far with the UVCS instrument. White-light and EUV data have been combined in order to estimate not only the shock compression ratio and the plasma temperature, but also the strength of the involved coronal magnetic fields, by applying the Rankine–Hugoniot equations for the general case of oblique shocks. Results show that, for a compression ratio X = 2.06 as derived from LASCO data, the coronal plasma is heated across the shock from an initial temperature of 2.3 × 10 5 Ku p to 1.9 × 10 6 K, while at the same time the magnetic field undergoes a compression from a pre-shock value of ∼0.02 G up to a post-shock field of ∼0.04 G. Magnetic and kinetic energy density increases at the shock are comparable (in agreement with the idea of equipartition of energy), and both are more than two times larger than the thermal energy density increase. This is the first time that a complete characterization of pre- and post-shock plasma physical parameters has been derived in the solar corona.

A comprehensive study of the initiation and early evolution of a coronal mass ejection from ultraviolet and white-light data

In this work we analyze simultaneous UV and white-light (WL) observations of a slow CME that occurred on 2000 January 31. Unlike most CMEs studied in the UV so far, this event was not associated with a flare or filament eruption. Based on vector magnetograph data and magnetic field models, we find that field disruption in an active region (AR) was driven by flux emergence and shearing motions, leading to the CME and to post-CME arcades seen in the EUV. WL images, acquired by the Mark IV coronagraph at the Mauna Loa Observatory, allowed us to identify the CME front, bubble, and core shortly (about 1 hr) after the CME ejection. From polarized brightness (pB) Mauna Loa data we estimated the mass and electron densities of the CME. The CME mass increases with time, indicating that about 2/3 of the mass originates above 1.6 R☉. Analysis of the UV spectra, acquired by the Solar and Heliospheric Observatory Ultraviolet Coronagraph Spectrometer (SOHO UVCS) at 1.6 and 1.9 R☉, allowed us to derive the electron temperature distribution across the CME. The temperature maximizes at the CME core and increases between 1.6 and 1.9 R☉. This event was unusual, in that the leading edge and the CME core were hotter than the ambient corona. We discuss magnetic heating and adiabatic compression as explanations for the high temperatures in the core and leading edge, respectively.

Stereoscopic Reconstruction from STEREO/EUV Imagers Data of the Three-dimensional Shape and Expansion of an Erupting Prominence

On 2007 May 9, a prominence eruption was observed in the Heii λ304 filter by the two EUV Imagers (EUVI) telescopes aboard the STEREO A and B spacecrafts. The high spatial resolution (∼1. �� 5p ixel −1 ) EUVI images have been used to infer via triangulation the three-dimensional (3D) shape and orientation of the prominence � 12 minutes after the beginning (13:40 UT) of the eruption. At this time, the prominence has the shape of a “hook” with the base anchored at the Sun. The “hook” prominence is highly inclined southward with respect to the radial direction, has an average thickness of 0.061 R� , a length of 0.43 R� , and lies in first approximation on a plane inclined by ∼54. 5 with respect to the line of sight. Thanks to the very high temporal cadence (∼37 s) of EUVI observations it has been possible also to infer the 3D early eruption trajectory. In the following ∼20 minutes the prominence rotates westward, undergoing a strong latitudinal acceleration, ∼3 times larger than the radial acceleration. In this time interval, the prominence expands in a direction mainly parallel to the plane of the sky; the total volume occupied by the plasma increases by a factor of ∼8, while the prominence thickness increases only by ∼12%. This is related to the fact that the early prominence expansion is anisotropic and occurs mainly on a plane parallel to the plane of the sky. Even if the smallscale spatial distribution of the erupting material observed in the Heii EUVI images is quite complex, both the approximately planar shape and the successive planar expansion suggest that on larger spatial scales the prominence can be globally approximated as a two-dimensional “ribbon-like” feature, instead of a 3D twisted flux tube.

Plasma Physical Parameters along Coronal-mass-ejection-driven Shocks. I. Ultraviolet and White-light Observations

In this work, UV and white-light (WL) coronagraphic data are combined to derive the full set of plasma physical parameters along the front of a shock driven by a coronal mass ejection. Pre-shock plasma density, shock compression ratio, speed, and inclination angle are estimated from WL data, while pre-shock plasma temperature and outflow velocity are derived from UV data. The Rankine-Hugoniot (RH) equations for the general case of an oblique shock are then applied at three points along the front located between 2.2 and 2.6 R ☉ at the shock nose and at the two flanks. Stronger field deflection (by ~46°), plasma compression (factor ~2.7), and heating (factor ~12) occur at the nose, while heating at the flanks is more moderate (factor 1.5-3.0). Starting from a pre-shock corona where protons and electrons have about the same temperature (Tp ~ Te ~ 1.5 × 106xa0K), temperature increases derived with RH equations could better represent the proton heating (by dissipation across the shock), while the temperature increase implied by adiabatic compression (factor ~2 at the nose, ~1.2-1.5 at the flanks) could be more representative of electron heating: the transit of the shock causes a decoupling between electron and proton temperatures. Derived magnetic field vector rotations imply a draping of field lines around the expanding flux rope. The shock turns out to be super-critical (sub-critical) at the nose (at the flanks), where derived post-shock plasma parameters can be very well approximated with those derived by assuming a parallel (perpendicular) shock.

Multi Element Telescope for Imaging and Spectroscopy (METIS) coronagraph for the Solar Orbiter mission

METIS, the “Multi Element Telescope for Imaging and Spectroscopy”, is a coronagraph selected by the European Space Agency to be part of the payload of the Solar Orbiter mission to be launched in 2017. The unique profile of this mission will allow 1) a close approach to the Sun (up to 0.28 A.U.) thus leading to a significant improvement in spatial resolution; 2) quasi co-rotation with the Sun, resulting in observations that nearly freeze for several days the large-scale outer corona in the plane of the sky and 3) unprecedented out-of-ecliptic view of the solar corona. This paper describes the experiment concept and the observational tools required to achieve the science drivers of METIS. METIS will be capable of obtaining for the first time: • simultaneous imaging of the full corona in polarized visible-light (590-650 nm) and narrow-band ultraviolet HI Lyman α (121.6 nm); • monochromatic imaging of the full corona in the extreme ultraviolet He II Lyman α (30.4 nm); • spectrographic observations of the HI and He II Ly α in corona. These measurements will allow a complete characterization of the three most important plasma components of the corona and the solar wind, that is, electrons, hydrogen, and helium. This presentation gives an overview of the METIS imaging and spectroscopic observational capabilities to carry out such measurements.

Low-Frequency Lyα Power Spectra Observed by UVCS in a Polar Coronal Hole

The occurrence of f−1 noise in interplanetary magnetic fields (in the 1 × 10−5 to 1 × 10−4 Hz band) and other plasma parameters has now been known for about 20 years and has been recently identified also in the photospheric magnetic fields. However, the relationship between interplanetary and solar fluctuation spectra and the identification of their sources at the Sun are problems that still need to be addressed. Moreover, interplanetary density and magnetic field power spectra show a f−2 interval at frequencies smaller that ~6 × 10−4 Hz whose source on the Sun is at present not fully understood. In this work we report on the first study of low-frequency density fluctuations in the solar corona at 2.1 R☉. In 2006 June the Ultraviolet Coronagraph Spectrometer (SOHO UVCS) observed over a period of about 9.2 days H Lyα intensity fluctuations at 2.1 R☉ over a polar coronal hole. The Lyα intensity power spectra S(f) (related mainly to density fluctuations) showed a S(f) ∝ f−2 frequency interval between 2.6 × 10−6 and 3.0 × 10−5 Hz and a S(f) ∝ f−1 frequency interval between 3.0 × 10−5 and 1.3 × 10−4 Hz. The detection of a f−2 interval, in agreement with interplanetary density and magnetic field power spectra, has been also predicted in solar wind models as a consequence of phase-mixing mechanisms of waves propagating in coronal holes. High-latitude power spectra show a f−1 band approximately in the same frequency interval where f−1 noise has been detected in interplanetary densities, and interplanetary and photospheric magnetic fields, providing a connection between photospheric, coronal, and interplanetary f−1 noises.

An improved version of the Shadow Position Sensor readout electronics on-board the ESA PROBA-3 Mission

PROBA-3 [1] [2] is a Mission of the European Space Agency (ESA) composed by two satellites flying in formation and aimed at achieving unprecedented performance in terms of relative positioning. The mission purpose is, in first place, technological: the repeated formation break and acquisition during each orbit (every about twenty hours) will be useful to demonstrate the efficacy of the closed-loop control system in keeping the formation-flying (FF) and attitude (i.e. the alignment with respect to the Sun) of the system. From the scientific side, instead, the two spacecraft will create a giant instrument about 150 m long: an externally occulted coronagraph named ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun) dedicated to the study of the inner part of the visible solar corona. The two satellites composing the mission are: the Coronagraph Spacecraft (CSC), hosting the Coronagraph Instrument (CI), and the disk-shaped (1.4 m diameter) Occulter Spacecraft (OSC). The PROBA-3 GNC (Guidance, Navigation and Control) system will employ several metrological subsystems to keep and retain the desired relative position and the absolute attitude (i.e. with respect to the Sun) of the aligned spacecraft, when in observational mode. The SPS subsystem [5] is one of these metrological instruments. It is composed of eight silicon photomultipliers (SiPMs), sensors operated in photovoltaic mode [6] that will sense the penumbra light around the Instrument’s pupil so to detect any FF displacement from the nominal position. In proximity of the CDR (Critical Design Review) phase, we describe in the present paper the changes occurred to design in the last year in consequence of the tests performed on the SPS Breadboard (Evaluation Board, EB) and the SPS Development Model (DM) and that will finally lead to the realization of the flight version of the SPS system.

In‐flight UV and polarized‐VL radiometric calibrations of the solar orbiter/METIS imaging coronagraph

METIS is an innovative inverted occulted solar coronagraph capable of obtaining for the first time simultaneous imaging of the full corona in linearly polarized visible-light (580-640 nm) and narrow-band (± 10 nm) ultraviolet H I Ly-α (121.6 nm). It has been selected to fly aboard the Solar Orbiter1 spacecraft, whose launch is foreseen in July 2017. Thanks to its own capabilities and exploiting the peculiar opportunities offered by the Solar Orbiter planned orbit, METIS will address some of the still open issues in understanding the physical processes in the corona and inner heliosphere. The Solar Orbiter Nominal Mission Phase (NMP) will be characterized by three scientific observing windows per orbit and METIS will perform at least one in-flight calibration per observing window. The two imaging channels of METIS will be calibrated on ground and periodically checked, verified and re-calibrated in-flight. In particular, radiometric calibration images will be needed to determine the absolute brightness of the solar corona. For UV radiometric calibration a set of targets is represented by continuum-emitting early type bright stars (e.g. A and B spectral types) whose photospheres produce a bright far-ultraviolet continuum spectrum stable over long timescales. These stars represent an important reference standard not only for METIS in-flight calibrations but also for other Solar Orbiter instruments and they will be crucial for instruments cross-calibrations as well. For VL radiometric calibration, a set of linearly polarized stars will be used. These targets shall have a minimum degree of linear polarization (DoLP > 5%) and a detectable magnitude, compatible with the instrument integration times constrained by the desired S/N ratio and the characteristics of the spacecraft orbit dynamics.

In-flight UV and polarized-VL radiometric calibrations of the solar orbiter/METIS imaging coronagraph

METIS is an innovative inverted occulted solar coronagraph capable of obtaining for the first time simultaneous imaging of the full corona in linearly polarized visible-light (580-640 nm) and narrow-band (± 10 nm) ultraviolet H I Ly-α (121.6 nm). It has been selected to fly aboard the Solar Orbiter1 spacecraft, whose launch is foreseen in July 2017. Thanks to its own capabilities and exploiting the peculiar opportunities offered by the Solar Orbiter planned orbit, METIS will address some of the still open issues in understanding the physical processes in the corona and inner heliosphere. The Solar Orbiter Nominal Mission Phase (NMP) will be characterized by three scientific observing windows per orbit and METIS will perform at least one in-flight calibration per observing window. The two imaging channels of METIS will be calibrated on ground and periodically checked, verified and re-calibrated in-flight. In particular, radiometric calibration images will be needed to determine the absolute brightness of the solar corona. For UV radiometric calibration a set of targets is represented by continuum-emitting early type bright stars (e.g. A and B spectral types) whose photospheres produce a bright far-ultraviolet continuum spectrum stable over long timescales. These stars represent an important reference standard not only for METIS in-flight calibrations but also for other Solar Orbiter instruments and they will be crucial for instruments cross-calibrations as well. For VL radiometric calibration, a set of linearly polarized stars will be used. These targets shall have a minimum degree of linear polarization (DoLP > 5%) and a detectable magnitude, compatible with the instrument integration times constrained by the desired S/N ratio and the characteristics of the spacecraft orbit dynamics.

On board processing procedures for the Solar Orbiter METIS coronagraph

Solar Orbiter is an ESA space mission devoted to improve the knowledge of those effects nowadays still not fully understood on the physical mechanisms underlying the behaviour of our star. The mission has a peculiar trajectory that will bring the S/C close to the Sun up to 0.28 AU, exploiting the opportunity to follow up our star as close as never before. METIS, one of the instruments selected to be part of the Solar Orbiter payload, is a coronagraph that will investigate the inner part of the heliosphere performing imaging in the visible band and in the hydrogen Lyman α line @ 121.6 nm. METIS will be able to simultaneously operate the two detectors: an Intensified APS for the UV channel and an APS for the visible light, and a Liquid Crystal Variable Retarder (LCVR) plate, for broadband visible polarimetry. They will be operated by means of the centralised management unit of the instrument, the METIS Processing and Power Unit. This payload subsystem hosts a microprocessor that implements, thanks to the application software, all the needed functionalities to fully control the instrument subsystems and its own processing capabilities. Both sensors will be readout at high rate and the acquired data shall undergo through a preliminary on-board processing to maximize the scientific return and to provide the necessary information to validate the results on ground. Being Solar Orbiter a deepspace mission, some METIS procedures have been designed to provide to the instrument an efficient autonomous behaviour in case of an immediate reaction is required as for arising transient events or occurrence of safety hazards condition. METIS will implement an on-board algorithm for the automatic detection of this kind of events in order to promptly react and autonomously adapt the observing procedure.