A. Bemporad

Current Sheet Evolution in the Aftermath of a CME Event

We report on SOHO UVCS observations of the coronal restructuring following a coronal mass ejection (CME) on 2002 November 26, at the time of a SOHO-Ulysses quadrature campaign. Starting about 1.5 hr after a CME in the northwest quadrant, UVCS began taking spectra at 1.7 R� , covering emission from both cool and hot plasma. Observations continued, with occasional gaps, for more than 2 days. Emission in the 974.8 8 line of [Fe xviii], indicating temperatures above 6 ; 10 6 K, was observed throughout the campaign in a spatially limited location. Comparison with EITimages shows the [Fe xviii] emission to overlie a growing post-flare loop system formed in the aftermath of the CME. The emission most likely originates in a current sheet overlying the arcade. Analysis of the [Fexviii] emission allows us to infer the evolution of physical parameters in the current sheet over the entire span of ourobservations:inparticular,wegivethetemperatureversustimeinthecurrentsheetandestimateitsdensity.Atthe timeofthequadrature,UlysseswasdirectlyabovethelocationoftheCMEandinterceptedtheejecta.Highionization state Fe was detected by the Ulysses SWICS throughout the magnetic cloud associated with the CME, although its rapid temporal variation suggests bursty, rather than smooth, reconnection in the coronal current sheet. The SOHOUlyssesdatasetprovideduswiththeuniqueopportunityofanalyzingacurrentsheetstructurefromitslowest coronal levels out to its in situ properties. Both the remote and in situ observations are compared with predictions of theoretical CME models.

THE ROLE OF STREAMERS IN THE DEFLECTION OF CORONAL MASS EJECTIONS: COMPARISON BETWEEN STEREO THREE-DIMENSIONAL RECONSTRUCTIONS AND NUMERICAL SIMULATIONS

On 2009 September 21, a filament eruption and the associated coronal mass ejection (CME) were observed by the Solar Terrestrial Relations Observatory (STEREO) spacecraft. The CME originated from the southern hemisphere and showed a deflection of about 15 ◦ toward the heliospheric current sheet (HCS) during the propagation in the COR1 field of view. The CME source region was near the central meridian, but no on-disk CME signatures could be seen from the Earth. The aim of this paper is to provide a physical explanation for the strong deflection of the CME observed on 2009 September 21. The two-sided view of the STEREO spacecraft allows us to reconstruct the three-dimensional travel path of the CME and the evolution of the CME source region. The observations are combined with a magnetohydrodynamic simulation, starting from a magneticfield configuration closely resembling the extrapolated potential field for that date. By applying localized shearing motions, a CME is initiated in the simulation, showing a similar non-radial evolution, structure, and velocity as the observed event. The CME gets deflected toward the current sheet of the larger northern helmet streamer due to an imbalance in the magnetic pressure and tension forces and finally gets into the streamer. This study shows that during solar minima, even CMEs originating from high latitude can be easily deflected toward the HCS, eventually resulting in geoeffective events. How rapidly they undergo this latitudinal migration depends on the strength of both the large-scale coronal magnetic field and the magnetic flux of the erupting filament.

Spectroscopic Detection of Turbulence in Post-CME Current Sheets

Plasma inpost-CMEcurrentsheets(CSs)isexpectedtobehighly turbulent because of the tearing andcoalescence instability and/or local microscopic instabilities. For this reason, in the last decade the inconsistency between the observed (� 10 4 Y10 5 km) and the expected (� 1Y10 m) CS thickness has been tentatively explained in many MHD modelsasaconsequenceof plasmaturbulencethatshouldbeabletosignificantlybroadentheCS.However,fromthe observational point of view, little is known about this subject. A few post-CME CSs have been observed in UVCS spectra as a strong emission in the high-temperature [Fe xviii] line, usually unobservable in the solar corona. In this work, published data on post-CME CSs observed by UVCS are reanalyzed, concentrating for the first time on the evolutionof turbulencederivedfromthenonthermalbroadeningof the[Fe xviii]lineprofiles.Derivedturbulentspeeds are on the order of � 60 km s � 1 a few hours after the CME and slowly decay down to � 30 km s � 1 in the following 2days.Fromthisevolutiontheanomalousdiffusivityduetomicroinstabilitieshasbeenestimated,andthescenarioof multiple small-scale reconnections is tested. Results show that the existence of many (� 10 � 11 to 10 � 17 � CS m � 3 ) microscopic CSs (� CSs) of small sizes (� 10Y10 4 m) could explain not only the high CS temperatures but also the much larger observed thickness of macroscopic CSs, thanks to turbulent broadening. Subject headingg Sun: corona — Sun: coronal mass ejections (CMEs) — Sun: UV radiation — turbulence

SPECTROSCOPIC SIGNATURE OF ALFVÉN WAVES DAMPING IN A POLAR CORONAL HOLE UP TO 0.4 SOLAR RADII

Between February 24-25, 2009, the EIS spectrometer onboard the Hinode spacecraft performed special “sit & stare” observations above the South po lar coronal hole continuously over more than 22 hours. Spectra were acquired with the 1” slit pla ced off-limb covering altitudes up to 0.48 R⊙ (3.34× 102 Mm) above the Sun surface, in order to study with EIS the non-t hermal spectral line broadenings. Spectral lines such as Fe xii λ186.88, Fexii λ193.51, Fexii λ195.12 and Fexiii λ202.04 are observed with good statistics up to high altitude s and they have been analyzed in this study. Results show that the FWHM of Fe xii λ195.12 line increases up to ≃ 0.14 R⊙, then decreases higher up. EIS stray light has been estimate d and removed. Derived electron density and non-thermal velocity profiles have bee n used to estimate the total energy flux transported by Alfvén waves o ff-limb in polar coronal hole up to≃ 0.4 R⊙. The computed Alfvén wave energy flux densityfw progressively decays with altitude fromfw ≃ 1.2 · 106 erg cm−2 s−1 at 0.03 R⊙ down to fw ≃ 8.5 · 103 erg cm−2 s−1 at 0.4 R⊙, with an average energy decay rate∆ fw/∆h ≃ −4.5·10−5 erg cm−3 s−1. Hence, this result suggests energy deposition by Alfvén waves in a polar coronal hole, thus providing a signi ficant source for coronal heating. Subject headings: Sun: corona; Sun: oscillations; Sun: UV radiation; line: pr ofiles; waves

Morphology and density structure of post-CME current sheets

Context. Eruption of a coronal mass ejection (CME) drags and “opens” the coronal magnetic field, presumably leading to the formation of a large-scale current sheet and field relaxation by magnetic reconnection. Aims. We analyze the physical characteristics of ray-like coronal features formed in the aftermath of CMEs, to confirm whether interpreting this phenomenon in terms of a reconnecting current sheet is consistent with observations. Methods. The study focuses on measurements of the ray width, density excess, and coronal velocity field as a function of the radial distance. Results. The morphology of the rays implies that they are produced by Petschek-like reconnection in the large-scale current sheet formed in the wake of CME. The hypothesis is supported by the flow pattern, often showing outflows along the ray, and sometimes also inflows into the ray. The inferred inflow velocities range from 3 to 30 km s −1 , and are consistent with the narrow opening-angle of rays, which add up to a few degrees. The density of rays is an order of magnitude higher than in the ambient corona. The densityexcess measurements are compared with the results of the analytical model in which the Petschek-like reconnection geometry is applied to the vertical current sheet, taking into account the decrease in the external coronal density and magnetic field with height. Conclusions. The model results are consistent with the observations, revealing that the main cause of the density excess in rays is a transport of the dense plasma from lower to higher heights by the reconnection outflow.

METIS: a novel coronagraph design for the Solar Orbiter mission

METIS (Multi Element Telescope for Imaging and Spectroscopy) 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 mission profile will bring the Solar Orbiter spacecraft as close to the Sun as 0.3 A.U., and up to 35° out-of-ecliptic providing a unique platform for helio-synchronous observations of the Sun and its polar regions. METIS coronagraph is designed for multi-wavelength imaging and spectroscopy of the solar corona. This presentation gives an overview of the innovative design elements of the METIS coronagraph. These elements include: i) multi-wavelength, reflecting Gregorian-telescope; ii) multilayer coating optimized for the extreme UV (30.4 nm, HeII Lyman-α) with a reflecting cap-layer for the UV (121.6 nm, HI Lyman-α) and visible-light (590-650); iii) inverse external-occulter scheme for reduced thermal load at spacecraft peri-helion; iv) EUV/UV spectrograph using the telescope primary mirror to feed a 1st and 4th-order spherical varied line-spaced (SVLS) grating placed on a section of the secondary mirror; v) liquid crystals electro-optic polarimeter for observations of the visible-light K-corona. The expected performances are also presented.

Temporal Evolution of a Streamer Complex: Coronal and in Situ Plasma Parameters

We report on observations acquired by the Ultraviolet Coronagraph Spectrometer (UVCS) aboard the Solar and Heliospheric Observatory (SOHO), from 2000 June 10 to June 17 at the time of a SOHO-Sun-Ulysses quadrature. UVCS took data at 1.6 and 1.9 R☉ with a slit normal to the solar radius and centered along the radial to Ulysses. A streamer complex was sampled by UVCS throughout the quadrature campaign, giving us the opportunity to derive plasma parameters in different streamers and to compare them with plasma properties measured in situ. Large Angle Spectroscopic Coronagraph images above 2 R☉ helped us understand the temporal evolution of the streamer complex. We derive densities, temperatures, and elemental abundances in two streamers, which have different temperatures and element abundances. In spite of these differences, both structures have the same first ionization potential (FIP) bias. The Fe/O ratio, which may be considered a proxy for the FIP effect, was measured in situ by the Solar Wind Ion Composition Spectrometer aboard the Ulysses spacecraft. Values of Fe/O measured in the corona at the sites where in situ plasma originated agree with in situ Fe/O values.

Rotation of an erupting filament observed by the STEREO EUVI and COR1 instruments

On August 31, 2007, a prominence eruption was observed by the Solar TErrestrial RElations Observatory (STEREO) in the ExtremeUltraViolet Imager (EUVI) 304 images and later on, as the core of a three-part coronal mass ejection (CME) in images acquired by the inner STEREO coronagraph (COR1). Because they were covered by both STEREO spacecraft from right vantage points, these observations provide an excellent opportunity to perform a three-dimensional (3D) prominence reconstruction and study its evolution. We employed the tie-pointing technique to reconstruct the 3D shape and trajectory of the prominence, which has been followed from an heliocentric distance of ∼1.3 up to ∼2.4 R during the first 1.3 h of eruption. Data show evidence for a progressive clockwise prominence rotation by ∼90◦ occurring not only in the early phase of the eruption sampled by EUVI, but also at larger heliocentric distances as seen by COR1. Interestingly, a counter-clockwise rotation of the filament was observed in Hα images in the week before the eruption; the filament does not show a twisted shape. In the same period, the potential field extrapolated at different times shows a clockwise rotation of closed lines overlying the filament. This suggests that a magnetic helicity storage occurred not in the filament itself, but in the global magnetic field configuration of the surrounding corona. Moreover, close inspection to the high-resolution EUVI images revealed a small scale helical feature along the erupting prominence. The sense of rotation of this helix agrees with the observed prominence rotation, providing evidence for the conversion of twist into writhe. The observed rotation of an erupting prominence, if representative of the flux rope rotation, may have a strong impact on the definition of geo-effectiveness of CMEs for space weather forecasting purposes.

Density and Magnetic Field Signatures of Interplanetary 1/f Noise

We investigate the occurrence of 1/f noise in the interplanetary density and the magnetic field at varying heliocentric latitudes. The characteristic spectral amplitudes can be found in Ulysses density and magnetic data in the expected frequency ranges at all available latitudes, ranging from the ecliptic plane to more than 80°. Average spectra indicate a latitudinal variation, with a 1/f density signal becoming more pronounced in higher latitude bands. Azimuthal spectral analysis of solar magnetogram data using the SOHO Michelson Doppler Interferometer also shows 1/f noise in the photospheric magnetic field, most clearly at high latitude. Accordingly, we discuss possibilities that the 1/f signal arises at varying altitudes, possibly surviving coronal dynamics. This raises questions that may be addressed in future studies using spectroscopic, white light, and radio scintillation data.

IDENTIFICATION OF SUPER- AND SUBCRITICAL REGIONS IN SHOCKS DRIVEN BY CORONAL MASS EJECTIONS

In this work, we focus on the analysis of a coronal mass ejection (CME) driven shock observed by the Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph Experiment. We show that white-light coronagraphic images can be employed to estimate the compression ratio X = {rho}{sub d}/{rho}{sub u} all along the front of CME-driven shocks. X increases from the shock flanks (where X {approx_equal} 1.2) to the shock center (where X {approx_equal} 3.0 is maximum). From the estimated X values, we infer the Alfven Mach number for the general case of an oblique shock. It turns out that only a small region around the shock center is supercritical at earlier times, while higher up in the corona the whole shock becomes subcritical. This suggests that CME-driven shocks could be efficient particle accelerators at the initiation phases of the event, while at later times they progressively loose energy, also losing their capability to accelerate high-energy particles. This result has important implications on the localization of particle acceleration sites and in the context of predictive space weather studies.