The Astrophysical Journal, Volume 706, Issue 1, pp. 238-243
This paper investigates the evolution of the plasma density fluctuations of the fast and slow solar wind from the solar corona into the interplanetary space. The study is performed by comparing the low-frequency spectra and the phase correlation of the proton density oscillations, measured in the inner heliosphere with the Helios 2 in situ instrumentation, with those due to the large-scale density perturbations observed with UVCS/SOHO in the outer corona. We find that the characteristics of density fluctuations of the fast solar wind are maintained in the transition from the outer corona to the inner heliosphere, thus suggesting a coronal imprint for the heliospheric large-scale 1/f 2 noise spectrum. In contrast, a quick dynamical evolution is observed in the slow wind, which, starting from large-scale fluctuations with strong phase correlations in the outer corona, gives rise to a Kolmogorov-like spectrum and an accumulation of density structures at small scales at 0.3 AU. This can be explained in the framework of nearly incompressible turbulence.
Astronomy and Astrophysics, Volume 506, Issue 2, 2009, pp.901-911.
The relation between current sheets (CSs) associated with flares, revealed by characteristic radio signatures, and current sheets associated with coronal mass ejections (CMEs), detected in coronal ultraviolet (UV) and white light data, has not been analyzed, yet. Aims: We aim at establishing the relationship between CSs associated with a limb flare and CSs associated with the CME that apparently develops after the flare. We use a unique data set, acquired on May 17, 2002, which includes radio and extreme ultraviolet (XUV) observations. Methods: Spectral radio diagnostics, UV spectroscopic techniques, white light coronograph imaging, and (partly) radio imaging are used to illustrate the relation between the CSs and to infer the physical parameters of the radially aligned features that develop in the aftermath of the CME. Results: During the flare, several phenomena are interpreted in accordance with earlier work and with reference to the common eruptive flare scenario as evidence of flare CSs in the low corona. These are drifting pulsating structures in dynamic radio spectra, an erupting filament, expanding coronal loops morphologically recalling the later white light CME, and associated with earlier reported hard X-ray source sites. In the aftermath of the CME, UV spectra allowed us to estimate the CS temperature and density, over the 1.5-2.1 R_\u022f interval of heliocentric altitudes. The UV detected CS, however, appears to be only one of many current sheets that exist underneath the erupting flux rope. A type II burst following the CME radio continuum in time at lower frequencies is considered as the radio signature of a coronal shock excited at the flank of the CME. Conclusions: The results show that we can build an overall scenario where the CME is interpreted in terms of an erupting arcade crossing the limb of the Sun and connected to underlying structures via multiple CSs. Eventually, the observed limb flare seems to be a consequence of the ongoing CME.
Universal Heliophysical Processes, Proceedings of the International Astronomical Union,
IAU Symposium, Volume 257, p. 145-149, 2009.
Recent work on coronal polar plumes (Gabriel et al. 2003, 2005) has aimed at determining the outflow velocity in plume and interplume regions, using the Doppler dimming technique on oxygen VI observations by SUMER and UVCS on SOHO. By comparing observations of SOHO/EIT with plume modelling, we show that the major part of plumes is the result of chance alignments along the line-of-sight of small enhancements in intensity. This confirms the so-called curtain model. These plumes can be attributed to reconnection activity along the boundaries of supergranule cells. A second population of plumes has a lower abundance and arises from surface bright points having a particular magnetic configuration. New observations using the Hinode/EIS spectrometer are in progress, with the aim of providing further insight for this model.
Memorie della Societa Astronomica Italiana Supplement, v.13, p.104 (2009)
Astronomy and Astrophysics, Volume 481, Issue 3, 2008, pp.835-844.
The Coronal Mass Ejections are plasma clouds expelled from the Sun into the interplanetary medium. We study the propagation of shock waves in the solar corona generated during Coronal Mass Ejections by means of a numerical multi-dimensional MHD model. The model describes the MHD evolution of a compressible plasma in an ambient magnetic field including tensor thermal conduction, radiative losses as main physical effects. We use the MHD version of the FLASH parallel hydrodynamic code with adaptive mesh refinement, originally developed at the University of Chicago USA). The code is highly modular and made efficiently parallel with the Message Passing Interface library. We analyze the diagnostic signatures of shock fronts generated by supersonic CME fragments detectable with the UltraViolet Coronagraphic Spectrometer on board the SoHO mission. To this aim we perform 3D MHD simulations of the shock propagation for the time it takes to cross the UVCS slit positioned at a distance of a few solar radii from the solar surface. In the presence of highly effective thermal conduction the simulation takes 200000 time steps to cover 1000 s of evolution. Considering a 3-D domain of 256x256x512 grid cells this kind of simulations requires thousands of hours of computer time and therefore high performance computing (HPC) systems. The simulations were run on the CINECA IBM/SP5 HPC cluster within the INAF/CINECA agreement. We show simulation results and some implications for UVCS observations.
Earth, Moon, and Planets, Volume 104, Issue 1-4, pp. 105-108, (2009).
We report on the analysis of a fast (>2,000 km/s) CME-driven shock event observed with the UVCS telescope operating aboard SoHO on 23 July 2002. The same shock was also detected in the metric band by several ground-based radiospectrographs. The peculiarity of this event is the presence in the radio spectra of two intense metric type II bursts features drifting at different rates, together with clear shock/related broadenings of the O VI doublet lines observed by UVCS that were found to be temporally associated with the above radio features. The nature of these multiple radio lanes in the metric band is still under debate. One possible explanation is that they are produced by multiple shock waves generated by different ejections or, alternatively, by the flare and the associated CME. Also, emission from the upstream and downstream shock regions can produce split bands. By adopting a plausible CME model, together with a detailed analysis of the white-light, UV, and radio data associated with this event, we are able to conclude that both the radio and the UV shock signatures were produced by a single shock wave surface generated by the expanding CME.
Living Reviews in Solar Physics, vol. 6, no. 3
, (2009).
Coronal holes are the darkest and least active regions of the Sun, as observed both on the solar disk and above the solar limb. Coronal holes are associated with rapidly expanding open magnetic fields and the acceleration of the high-speed solar wind. This paper reviews measurements of the plasma properties in coronal holes and how these measurements are used to reveal details about the physical processes that heat the solar corona and accelerate the solar wind. It is still unknown to what extent the solar wind is fed by flux tubes that remain open (and are energized by footpoint-driven wave-like fluctuations), and to what extent much of the mass and energy is input intermittently from closed loops into the open-field regions. Evidence for both paradigms is summarized in this paper. Special emphasis is also given to spectroscopic and coronagraphic measurements that allow the highly dynamic non-equilibrium evolution of the plasma to be followed as the asymptotic conditions in interplanetary space are established in the extended corona. For example, the importance of kinetic plasma physics and turbulence in coronal holes has been affirmed by surprising measurements from the UVCS instrument on SOHO that heavy ions are heated to hundreds of times the temperatures of protons and electrons. These observations point to specific kinds of collisionless Alfvén wave damping (i.e., ion cyclotron resonance), but complete theoretical models do not yet exist. Despite our incomplete knowledge of the complex multi-scale plasma physics, however, much progress has been made toward the goal of understanding the mechanisms ultimately responsible for producing the observed properties of coronal holes.
Acta Astronomica Sinica, vol. 50, no. 3, p. 271-288, (2009).
The kinematic characteristic of electrons after acceleration by a super-Dreicer electric field Ez is investigated in the reconnecting current sheet (RCS) with a guiding magnetic field Bz by simulations of testing particles. Firstly, the influence of the different directions of the electric filed on the acceleration of particles, changing the trajectories of electrons, is discussed when assuming the magnetic field as a constant. If Bz parallels to Ez, the pitch-angle of accelerated electrons is close to 180 degree; If Bz anti-parallels to Ez, the pitch-angle is close to 0 degree. The different directions of the guiding magnetic field lead to different regions where electrons are accelerated by the electric field but will not change the final energy distribution of electrons-in form of a power-law distribution E-gamma;. ? approximates to 2.9 in typical coronal conditions. Further research indicates that ? depends on the strength of both the guiding field and electric field and the scale of the current sheet. Secondly, the kinematic characteristic of accelerated particles in the current sheet including multiple X- and O-points is further investigated. Particles in the RCS with X- and O- points are trapped and get the maximal acceleration in the accelerating region. The final energy spectrum shows a multiple power-law distribution.
Advances in Space Research, Volume 44, Issue 4, p. 451-456, (2009).
We report on the analysis of two fast CME-driven shocks observed with the UltraViolet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory (SOHO). The first event, detected on 2002 March 22 at 4.1 R\u022f with the UVCS slit placed in correspondence with the flank of the expanding CME surface, represents the highest UV detection of a shock obtained so far with the UVCS instrument in the corona. The second one, detected on 2002 July 23 at 1.6 R\u022f with the UVCS slit placed in correspondence with the front of the expanding CME surface, shows an anomalous deficiency of ion heating with respect to what observed in previous CME/shocks observed by UVCS, possibly reflecting the effect of different coronal plasma conditions over the solar cycle. From the two different sets of observations we derived an estimate for the shock compression ratio X, which turns out to be X = 2.4 ± 0.2 and X = 2.2 ± 0.1, respectively, for the first and second event. Comparison between the two events provides complementary perspectives on the dynamical evolution of CME-driven shocks.
The Astrophysical Journal, Volume 700, Issue 2, pp. 1716-1731 (2009).
The origin of the Moreton wave observed in the chromosphere and the EIT wave observed in the corona during the eruption remains an active research subject. We investigate numerically in this work the evolutionary features of the magnetic configuration that includes a current-carrying flux rope, which is used to model the filament, after the loss of equilibrium in the system takes place in a catastrophic fashion. Rapid motions of the flux rope following the catastrophe invoke the velocity vortices behind the rope, and may also invoke slow- and fast-mode shocks in front of the rope. The velocity vortices at each side of the flux rope propagate roughly horizontally away from the area where they are produced, and both shocks expand toward the flank of the flux rope. The fast shock may eventually reach the bottom boundary and produce two echoes moving back into the corona, but the slow one and the vortices totally decay somewhere in the lower corona before arriving of the bottom boundary. The interaction of the fast shock with the boundary leads to disturbance that accounts for the Moreton wave observed in H\u03b1, and the disturbance in the corona caused by the slow shock and the velocity vortices should account for the EIT wave whose speed is about 40% that of the Moreton wave. The implication of these results to the observed correlation of the type II radio burst to the fast- and the slow-mode shocks and that of EIT waves to coronal mass ejections and flares has also been discussed.
The Astrophysical Journal, Volume 702, Issue 2, pp. 1604-1614 (2009).
We analyze measured proton and electron temperatures in the high-speed solar wind in order to calculate the separate rates of heat deposition for protons and electrons. When comparing with other regions of the heliosphere, the fast solar wind has the lowest density and the least frequent Coulomb collisions. This makes the fast wind an optimal testing ground for studies of collisionless kinetic processes associated with the dissipation of plasma turbulence. Data from the Helios and Ulysses plasma instruments were collected to determine mean radial trends in the temperatures and the electron heat conduction flux between 0.29 and 5.4 AU. The derived heating rates apply specifically for these mean plasma properties and not for the full range of measured values around the mean. We found that the protons receive about 60% of the total plasma heating in the inner heliosphere, and that this fraction increases to approximately 80% by the orbit of Jupiter. A major factor affecting the uncertainty in this fraction is the uncertainty in the measured radial gradient of the electron heat conduction flux. The empirically derived partitioning of heat between protons and electrons is in rough agreement with theoretical predictions from a model of linear Vlasov wave damping. For a modeled power spectrum consisting only of Alfvénic fluctuations, the best agreement was found for a distribution of wavenumber vectors that evolves toward isotropy as distance increases.
Journal of Geophysical Research, Volume 114, Issue A9, CiteID A09103.
Previous formulations of heating and transport associated with strong magnetohydrodynamic (MHD) turbulence are generalized to incorporate separate internal energy equations for electrons and protons. Electron heat conduction is included. Energy is supplied by turbulent heating that affects both electrons and protons and is exchanged between them via collisions. Comparison to available Ulysses data shows that a reasonable accounting for the data is provided when (1) the energy exchange timescale is very long and (2) the deposition of heat due to turbulence is divided, with 60% going to proton heating and 40% into electron heating. Heat conduction, determined here by an empirical fit, plays a major role in describing the electron data.
Astronomy and Astrophysics, Volume 499, Issue 3, 2009, pp.905-916.
Context: Eruption of a coronal mass ejection (CME) drags and \u201copens\u201d 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 density-excess 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.
Monthly Notices of the Royal Astronomical Society, Volume 395, Issue 4, pp. 2183-2188.
Episodic ejection of plasma blobs has been observed in many black hole systems. While steady, continuous jets are believed to be associated with large-scale open magnetic fields, what causes the episodic ejection of blobs remains unclear. Here by analogy with the coronal mass ejection on the Sun, we propose a magnetohydrodynamical model for episodic ejections from black holes associated with the closed magnetic fields in an accretion flow. Shear and turbulence of the accretion flow deform the field and result in the formation of a flux rope in the disc corona. Energy and helicity are accumulated and stored until a threshold is reached. The system then loses its equilibrium and the flux rope is thrust outward by the magnetic compression force in a catastrophic way. Our calculations show that for parameters appropriate for the black hole in our Galactic centre, the plasmoid can attain relativistic speeds in about 35 min.
The Astrophysical Journal, Volume 693, Issue 1, pp. 1022-1028 (2009).
Density fluctuations of the low and midlatitude solar corona plasma are analyzed during the recent solar minimum period. Long time series of the intensity of the neutral hydrogen Ly\u03b1, 1216 Å, line have been observed with the UltraViolet Coronagraph Spectrometer/Solar and Heliospheric Observatory at 1.7 R sun, in low-latitude streamers and in regions where the slow solar wind is accelerated. Their frequency composition is investigated by using three different techniques, namely the Fourier, the Hurst, and the phase coherence analyses. The Fourier analysis reveals the existence of low-frequency f \u2013\u03b1 power spectra in the range from ~3 × 10\u20136 Hz to ~10\u20134 Hz, corresponding to periods from a few hours to a few days. The coronal density fluctuations are dominated by discontinuities separating structures with a minimum characteristic timescale of about 3 hr and a corresponding spatial scale of about 3 × 104 km. The nonlinear analysis technique based on the structure functions shows that for large timescales the coronal density fluctuations are statistically self-affine and give rise to an average Hurst exponent langHrang = 0.654 ± 0.008. This indicates that the process underlying the variability of the corona and the slow wind at coronal level is a persistent mechanism, generating correlations among the plasma density fluctuations. Finally, the analysis based on the phase coherence index shows a high degree of phase synchronization of the coronal density variations for large timescales, which shows that the solar corona is dominated by phase coherent structures. The results of the analysis suggest a coupling of the variability of the solar corona and the photospheric dynamics induced by the convection at supergranular scale.
The Astrophysical Journal Letters, Volume 697, Issue 1, pp. L72-L76 (2009).
In this paper, we analyze transient UV brightenings in spectra acquired by SOHO/UltraViolet Coronagraph Spectrometer (UVCS) on 2003 June 2 in association with a coronal mass ejection (CME) that occurred at the West limb of the Sun at 08:54 UT. Brightenings have been observed in lines from cool (C III, O VI), intermediate (Si VIII, Si XII), and high ([Fe XVIII]) temperature ions over about 7 hr from the CME. Brightenings in cool lines are interpreted in terms of mini-ejections that appear at the time of, and after, the passage of the CME front through the UVCS slit. We give here their temperature and density and we point out that, assuming a spherical shape, a few of these mini-CMEs can provide a mass comparable to that quoted for typical CMEs. Hot lines, like the [Fe XVIII] line at 974.9 Å which shows up in the CME associated current sheet (CS), undergo transient brightness as well, but hot lines brightenings are more difficult to interpret. We propose here a scenario where they are signatures of the passage through the UVCS slit of plasmoids similar to those observed in the filamentary CS of the magnetotail that form as a consequence of the tearing-mode instability or of a time-dependent Petschek-type reconnection.
The Astrophysical Journal, Volume 695, Issue 1, pp. 221-237 (2009).
In the present work, we use SOHO/SUMER, SOHO/UVCS, SOHO/EIT, SOHO/LASCO, STEREO/EUVI, and Hinode/EIS coordinated observations of an active region (AR 10989) at the west limb taken on 2008 April 8 to study the cooling of coronal loops. The cooling plasma is identified using the intensities of SUMER spectral lines emitted at temperatures in the 4.15 <= log T <= 5.45 range. EIS and SUMER spectral observations are used to measure the physical properties of the loops. We found that before cooling took place these loops were filled with coronal hole-like plasma, with temperatures in the 5.6 <= log T <= 5.9 range. SUMER spectra also allowed us to determine the plasma temperature, density, emission measure, element abundances, and dynamic status during the cooling process. The ability of EUVI to observe the emitting region from a different direction allowed us to measure the volume of the emitting region and estimate its emission measure. Comparison with values measured from line intensities provided us with an estimate of the filling factor. UVCS observations of the coronal emission above the active region showed no streamer structure associated with AR 10989 at position angles between 242°and 253fdg EIT, LASCO, and EUVI-A narrowband images and UVCS spectral observations were used to discriminate between different scenarios and monitor the behavior of the active region in time. The present study provides the first detailed measurements of the physical properties of cooling loops, a very important benchmark for theoretical models of loop cooling and condensation.
The Astrophysical Journal, Volume 693, Issue 2, pp. 1666-1677 (2009).
A discussion of the thickness of current sheets in solar eruptions, d, led Lin et al. in 2007 to estimate very large values for the effective resistivity, \u03b7 e . Here, we address some questions raised by that paper. We apply the limb synoptic map technique and find d between 5.0 × 104 and 4.6 × 105 km, increasing with both time and altitude. The possibility that large apparent d and \u03b7 e result from projection effects is examined and rejected. We derive theoretical scaling laws relating d to other observables that corroborate this conclusion and thus help confine both d and \u03b7 e to a reasonable range. The possible impact of our results on the existing models of particle acceleration in reconnecting current sheets is also briefly discussed.
The Astrophysical Journal, Volume 691, Issue 1, pp. 794-805 (2009).
In the present work we use a deep-exposure spectrum taken by the SUMER spectrometer in a polar coronal hole in 1996 to measure the ion temperatures of a large number of ions at many different heights above the limb between 0.03 and 0.17 solar radii. We find that the measured ion temperatures are almost always larger than the electron temperatures and exhibit a nonmonotonic dependence on the charge-to-mass ratio. We use these measurements to provide empirical constraints to a theoretical model of ion heating and acceleration based on gradually replenished ion-cyclotron waves. We compare the wave power required to heat the ions to the observed levels to a prediction based on a model of anisotropic magnetohydrodynamic turbulence. We find that the empirical heating model and the turbulent cascade model agree with one another, and explain the measured ion temperatures, for charge-to-mass ratios smaller than about 0.25. However, ions with charge-to-mass ratios exceeding 0.25 disagree with the model; the wave power that they require to be heated to the measured ion temperatures shows an increase with charge-to-mass ratio (i.e., with increasing frequency) that cannot be explained by a traditional cascade model. We discuss possible additional processes that might be responsible for the inferred surplus of wave power.
The Astrophysical Journal, Volume 692, Issue 2, pp. 1271-1286 (2009).
The Ultraviolet Coronagraph Spectrometer (UVCS) observed Doppler-shifted material of a partial halo coronal mass ejection (CME) on 2001 December 13. The observed ratio of [O V]/O V] is a reliable density diagnostic important for assessing the state of the plasma. Earlier UVCS observations of CMEs found evidence that the ejected plasma is heated long after the eruption. We have investigated the heating rates, which represent a significant fraction of the CME energy budget. The parameterized heating and radiative and adiabatic cooling have been used to evaluate the temperature evolution of the CME material with a time-dependent ionization state model. The functional form of a flux-rope model for interplanetary magnetic clouds was also used to parameterize the heating. We find that continuous heating is required to match the UVCS observations. To match the O VI bright knots, a higher heating rate is required such that the heating energy is greater than the kinetic energy. The temperatures for the bright knots in Ly\u03b1 and C III emission indicate that smaller heating rates are required for those regions. In the context of the flux-rope model, about 75% of the magnetic energy must go into heat in order to match the O VI observations. We derive tighter constraints on the heating than earlier analyses, and we show that thermal conduction with the Spitzer conductivity is not sufficient to account for the heating at large heights.