Determination of Coronal Mass Ejection Physical Parameters from a Combination of Polarized Visible Light and UV Lyα Observations

Visible-light observations of Coronal Mass Ejections (CMEs) performed with coronagraphs and heliospheric imagers (in primis on board the Solar and Heliospheric Observatory and STEREO missions) have offered the best way to study the kinematics and geometrical structure of these fundamental events so far. Nevertheless, it has been widely demonstrated that only combination of multi-wavelength data (including X-ray spectra, EUV images, EUV-UV spectra, and radio dynamic spectra) can provide complete information on the plasma temperature and density distributions, non-thermal motions, magnetic fields, and other physical parameters, for both CMEs and CME-related phenomena. In this work, we analyze three CMEs by combining simultaneous data acquired in the polarized visible light by the LASCO-C2 coronagraph and in the UV H I Lyα line (1216 Å) by the UVCS spectrometer, in order to estimate the CME plasma electron density (using the polarization-ratio technique to infer the 3D structure of the CME) and temperature (from the comparison between the expected and measured Lyα intensities) along the UVCS field of view. This analysis is primarily aimed at testing the diagnostic methods that will be applied to coronagraphic observations of CMEs delivered by the Metis instrument on board the next ESA-Solar Orbiter mission. We find that CME cores are usually associated with cooler plasma (T∼ {10}6 K), and that a significant increase of the electron temperatures is observed from the core to the front of the CME (where T\gt {10}6.3 K), which seems to be correlated, in all cases, with the morphological structure of the CME as derived from visible-light images

Physical Conditions of Coronal Plasma at the Transit of a Shock Driven by a Coronal Mass Ejection

We report here on the determination of plasma physical parameters across a shock driven by a coronal mass ejection using white light (WL) coronagraphic images and radio dynamic spectra (RDS). The event analyzed here is the spectacular eruption that occurred on 2011 June 7, a fast CME followed by the ejection of columns of chromospheric plasma, part of them falling back to the solar surface, associated with a M2.5 flare and a type-II radio burst. Images acquired by the Solar and Heliospheric Observatory/LASCO coronagraphs (C2 and C3) were employed to track the CME-driven shock in the corona between 2-12 R☉ in an angular interval of about 110°. In this interval we derived two-dimensional (2D) maps of electron density, shock velocity, and shock compression ratio, and we measured the shock inclination angle with respect to the radial direction. Under plausible assumptions, these quantities were used to infer 2D maps of shock Mach number MA and strength of coronal magnetic fields at the shock's heights. We found that in the early phases (2-4 R☉) the whole shock surface is super-Alfvénic, while later on (i.e., higher up) it becomes super-Alfvénic only at the nose. This is in agreement with the location for the source of the observed type-II burst, as inferred from RDS combined with the shock kinematic and coronal densities derived from WL. For the first time, a coronal shock is used to derive a 2D map of the coronal magnetic field strength over intervals of 10 R☉ altitude and ̃110° latitude

A database of synthetic images in WL and UV filters to test diagnostic and modeling techniques to be applied on the future Metis data

In this report we describe how Metis synthetic images have been created to develop, test and optimize diagnostic tools for the inversion of combined WL and UV future images, and the determination of 2D maps of electron density and solar wind. We used FORWARD package and a coronal 3D model in order to create a baseline of WL and UV coronagraphic images representative of future Metis data acquired at different s/c distances and periods of solar activity cycle

On the Possibilities of Detecting Helium D3_3 Line Polarization with Metis

Space coronagraph Metis on board of the Solar Orbiter offers us new capabilities for studying eruptive prominences and coronal mass ejections (CME). Its two spectral channels, hydrogen L$\alpha$ and visible-light (VL) will provide, for the first time, co-aligned and co-temporal images to study dynamics and plasma properties of CMEs. Moreover, with the VL channel (580 - 640 nm) we find an exciting possibility to detect the helium D$_3$ line (587.73 nm) and its linear polarization. The aim of this study is to predict the diagnostics potential of this line regarding the CME thermal and magnetic structure. For a grid of models we first compute the intensity of the D$_3$ line together with VL continuum intensity due to Thomson scattering on core electrons. We show that the Metis VL channel will detect a mixture of both, with predominance of the helium emission at intermediate temperatures between 30 - 50,000 K. Then we use the code HAZEL to compute the degree of linear polarization detectable in the VL channel. This is a mixture of D$_3$ scattering polarization and continuum polarization. The former one is lowered in the presence of a magnetic field and the polarization axis is rotated (Hanle effect). Metis has the capability of measuring $Q/I$ and $U/I$ polarization degrees and we show their dependence on temperature and magnetic field. At $T$=30,000 K we find a significant lowering of $Q/I$ which is due to strongly enhanced D$_3$ line emission, while depolarization at 10 G amounts roughly to 10 \%.Comment: 11 pages, 6 figures, accepted for publication in Ap

Measuring coronal magnetic fields with remote sensing observations of shock waves

Recent works demonstrated that remote sensing observations of shock waves propagating into the corona and associated with major solar eruptions can be used to derive the strength of coronal magnetic fields met by the shock over a very large interval of heliocentric distances and latitudes. This opinion article will summarize most recent results obtained on this topic and will discuss the weaknesses and strengths of these techniques to open a constructive discussion with the scientific community

Plasma Physical Parameters along CME-driven Shocks. II. Observation-Simulation Comparison

In this work, we compare the spatial distribution of the plasma parameters along the 1999 June 11 coronal mass ejection (CME)-driven shock front with the results obtained from a CME-like event simulated with the FLIPMHD3D code, based on the FLIP-MHD particle-in-cell method. The observational data are retrieved from the combination of white-light coronagraphic data (for the upstream values) and the application of the Rankine-Hugoniot equations (for the downstream values). The comparison shows a higher compression ratio X and Alfvénic Mach number MA at the shock nose, and a stronger magnetic field deflection d toward the flanks, in agreement with observations. Then, we compare the spatial distribution of MA with the profiles obtained from the solutions of the shock adiabatic equation relating MA, X, and {θ }{Bn} (the angle between the upstream magnetic field and the shock front normal) for the special cases of parallel and perpendicular shock, and with a semi-empirical expression for a generically oblique shock. The semi-empirical curve approximates the actual values of MA very well, if the effects of a non-negligible shock thickness {δ }{sh} and plasma-to magnetic pressure ratio {β }u are taken into account throughout the computation. Moreover, the simulated shock turns out to be supercritical at the nose and sub-critical at the flanks. Finally, we develop a new one-dimensional Lagrangian ideal MHD method based on the GrAALE code, to simulate the ion-electron temperature decoupling due to the shock transit. Two models are used, a simple solar wind model and a variable-γ model. Both produce results in agreement with observations, the second one being capable of introducing the physics responsible for the additional electron heating due to secondary effects (collisions, Alfvén waves, etc.)

Detection of Coronal Mass Ejections at L1 and Forecast of Their Geoeffectiveness

A novel tool aimed to detect solar coronal mass ejections (CMEs) at the Lagrangian point L1 and to forecast their geoeffectiveness is presented in this paper. This approach is based on the analysis of in situ magnetic field and plasma measurements to compute some important magnetohydrodynamic quantities of the solar wind (the total pressure, the magnetic helicity, and the magnetic and kinetic energy), which are used to identify the CME events, that is their arrival and transit times, and to assess their likelihood for impacting the Earths magnetosphere. The method is essentially based on the comparison of the topological properties of the CME magnetic field configuration and of the CME energetic budget with those of the quasi-steady ambient solar wind. The algorithm performances are estimated by testing the tool on solar wind data collected in situ by the Wind spacecraft from 2005 to 2016. In the scanned 12 yr time interval, it results that (i) the procedure efficiency is of 86% for the weakest magnetospheric disturbances, increasing with the level of the geomagnetic storming, up to 100% for the most intense geomagnetic events, (ii) zero false positive predictions are produced by the algorithm, and (iii) the mean delay between the potentially geoeffective CME detection and the geomagnetic storm onset if of 4 hr, with a 98% 2-8 hr confidence interval. Hence, this new technique appears to be very promising in forecasting space weather phenomena associated to CMEs

Acceleration of Solar Energetic Particles through CME-driven Shock and Streamer Interaction

On 2013 June 21, a solar prominence eruption was observed, accompanied by an M2.9 class flare, a fast coronal mass ejection, and a type II radio burst. The concomitant emission of solar energetic particles (SEPs) produced a significant proton flux increase, in the energy range 4-100 MeV, measured by the Low and High Energy Telescopes on board the Solar TErrestrial RElations Observatory (STEREO)-B spacecraft. Only small enhancements, at lower energies, were observed at the STEREO-A and Geostationary Operational Environmental Satellite (GOES) spacecraft. This work investigates the relationship between the expanding front, coronal streamers, and the SEP fluxes observed at different locations. Extreme-ultraviolet data, acquired by the Atmospheric Imaging Assembly (AIA) instrument on board the Solar Dynamics Observatory (SDO), were used to study the expanding front and its interaction with streamer structures in the low corona. The 3D shape of the expanding front was reconstructed and extrapolated at different times by using SDO/AIA, STEREO/Sun Earth Connection Coronal and Heliospheric Investigation, and Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph observations with a spheroidal model. By adopting a potential field source surface approximation and estimating the magnetic connection of the Parker spiral, below and above 2.5 R ⊙, we found that during the early expansion of the eruption, the front had a strong magnetic connection with STEREO-B (between the nose and flank of the eruption front) while having a weak connection with STEREO-A and GOES. The obtained results provide evidence, for the first time, that the interaction between an expanding front and streamer structures can be responsible for the acceleration of high-energy SEPs up to at least 100 MeV, as it favors particle trapping and hence increases the shock acceleration efficiency