Astronomy and Astrophysics, Volume 475, Issue 2, November IV 2007, pp.707-715
Context: Investigating the physical properties of solar coronal streamers is important for understanding their role in the global magnetic structure of the extended solar atmosphere, as well as in the generation of the slow solar wind. Aims: We hope to contribute as completely as possible to the ongoing SOHO instruments campaign devoted to the study of the physical characteristics of coronal streamers at various heliocentric distances. Methods: We analyzed ultraviolet H I Ly? and O VI resonance doublet lines observed by UVCS/SOHO in a narrow, mid-latitude streamer structure along different lines of sight during a week in May 2004 and made nearly simultaneous white-light polarized brightness measurements from the LASCO/SOHO C2 coronagraph. Results: Electron densities and temperatures, H I and O VI kinetic temperatures, and outflow velocities were derived from the line intensities and widths, as well as from the O VI line intensity ratio in the 1.6-5 R? range of heights, limited to the central region of the streamer. To our knowledge, the H I outflow velocities obtained in this work are the first ones determined inside a streamer structure. They are significantly lower than those of the O VI ions. This, together with the O VI kinetic temperatures that are much higher than the H I ones, suggest that the absorption of Alfv?n waves at the ion cyclotron frequency might also occur inside streamers. Conclusions: In comparison with other streamers described in the literature, the structure examined in this work generally exhibits lower electron density and neutral hydrogen kinetic temperature. Conversely, the O VI kinetic temperature and outflow velocity radial profiles are consistent with the results for the other examined streamers.
Memorie della Societa Astronomica Italiana, v.78, p.268
The solar jets were first observed by SOHO instruments (EIT, LASCO, UVCS) during the previous solar minimum. They were small,
fast ejections originating from flaring UV bright points within large polar
coronal holes. The obtained data provided us with estimates of the jet plasma
conditions, dynamics, evolution of the electron temperature and heating rate
required to reproduce the observed ionization state. To follow the polar
jets through the solar cycle a special SOHO Joint Observing Program (JOP
155) was designed. It involves a number of SOHO instruments (EIT, CDS, UVCS, LASCO) as well as TRACE. The coordinated observations
have been carried out since April 2002. The data enabled to identify counterparts
of the 1996-1998 solar minimum jets. Their frequency of several events per
day appear comparable to the frequency from the previous solar minimum. The
jets are believed to be triggered by field line reconnection between emerging
magnetic dipole and pre-existing unipolar field. Existing models predict
that the hot jet is formed together with another jet of a cool material.
The particular goal of the coordinated SOHO and TRACE observations was to
look for possible association of the hot and cool plasma ejections. Currently
there is observational evidence that supports these models.
Astrophysical Journal, Volume 659, Issue 2, pp. 1702-1712.
We report on observations acquired in 2003 May during a SOHO-Ulysses quadrature
campaign. The UVCS slit was set normal to the radial of the Sun along the
direction to Ulysses at 1.7 Rsolar, at a northern latitude of
14.5°. From May 25 to May 28, UVCS acquired spectra of several short-lived
ejections that represent the extension at higher altitudes of recursive EIT
jets, imaged in He II λ304. The jets were visible also in LASCO images and
seem to propagate along the radial to Ulysses. UVCS spectra showed an unusually
high emission in cool lines, lasting for about 10-25 minutes, with no evidence
of hot plasma. Analysis of the cool line emission allowed us to infer the
physical parameters (temperature, density, and outward velocity) of jet plasma
and the evolution of these quantities as the jet crossed the UVCS slit. From
these quantities, we estimated the energy needed to produce the jet. We also
looked for any evidence of the events in the in situ data. We conclude by
comparing our results with those of previous works on similar events and
propose a scenario that accounts for the observed magnetic setting of the
source of the jets and allows the jets to be magnetically driven.
Planetary and Space Science, Volume 55, Issue 9, p. 1021-1030.
In the last 10 years more than 1000 sungrazing comets have been discovered
by the LASCO coronagraphs aboard SOHO the spacecraft; from this huge amount
of data it has been possible to study the common origin of these comets and
to explain some of the main peculiarities observed in their lightcurves.
Moreover, the UV Coronagraph Spectrometer (UVCS) aboard SOHO allowed EUV
spectroscopy of sungrazers in the final stage of their trajectory (i.e. between
1.4 and 10 solar radii), but a few sungrazers have been observed with this
instrument. In this paper we review the main results from the UVCS observation
of sungrazers C/1996 Y1, C/2000 C6 and C/2001 C2, discussing also the first
possible detection of two fragments and the determination of the pyroxene
dust grain number density in the latter one. Preliminary results on the UVCS
data interpretation of a sungrazer observed in 2002 (C/2002 S2) are also presented
here.
Astronomy and Astrophysics, Volume 472, Issue 1, September II 2007, pp.299-307
Chinese Journal of Astronomy & Astrophysics, Vol. 7, p. 457-476
Large-scale magnetic structures are the main carrier of major eruptions in
the solar atmosphere. These structures are rooted in the photosphere and
are driven by the unceasing motion of the photospheric material through a
series of equilibrium configurations. The motion brings energy into the coronal
magnetic field until the system ceases to be in equilibrium. The catastrophe
theory for solar eruptions indicates that loss of mechanical equilibrium
constitutes the main trigger mechanism of major eruptions, usually shown
up as solar flares, eruptive prominences, and coronal mass ejections (CMEs).
Magnetic reconnection which takes place at the very beginning of the eruption
as a result of plasma instabilities/turbulence inside the current sheet,
converts magnetic energy into heating and kinetic energy that are responsible
for solar flares, and for accelerating both plasma ejecta (flows and CMEs)
and energetic particles. Various manifestations are thus related to one another,
and the physics behind these relationships is catastrophe and magnetic reconnection.
This work reports on recent progress in both theoretical research and observations
on eruptive phenomena showing the above manifestations. We start by displaying
the properties of large-scale structures in the corona and the related magnetic
fields prior to an eruption, and show various morphological features of the
disrupting magnetic fields. Then, in the framework of the catastrophe theory,
we look into the physics behind those features investigated in a succession
of previous works, and discuss the approaches they used.
2007-14: Plasma Outflows in Coronal
Streamers
Noci, Giancarlo; Gavryuseva, Elena
Astrophysical Journal, Volume 658, Issue 1, pp. L63-L66.
In this Letter we show that it is possible to determine slow velocities (~20 km/s) in the extended solar corona by means of the ratio of the resonance doublet of O VI. We apply this technique to a quiescent streamer at solar minimum, observed by the Ultraviolet Coronagraph Spectrometer (UVCS) for 4 consecutive days, and determine the velocity pattern in it. We show that a rapid velocity increase occurs on the lateral sides of the streamer as the distance from the streamer axis increases. We also show that, probably, outflowing plasma exists also in the streamer core. We point out the interest of examining the possible relation of this finding with the reduction of the O VI emission observed by UVCS in the core of some streamers. We also point out the importance of studying the connection of the plasma just outside the v=20 km/s curve with the streamer cusp, to gain insight in the physics of the slow solar wind.
2007-13: Properties of Solar Polar Coronal
Plumes Constrained by Ultraviolet Coronagraph Spectrometer Data
Raouafi, N.-E.; Harvey, J. W.; Solanki, S. K.
Astrophysical Journal, Volume 658, Issue 1, pp. 643-656.
We investigate the plasma dynamics (outflow speed and turbulence) inside polar plumes. We compare line profiles (mainly of O VI) observed by the Ultraviolet Coronagraph Spectrometer (UVCS) instrument on SOHO at the minimum of solar cycle 22-23 with model calculations. We consider Maxwellian velocity distributions with different widths in plume and interplume regions. Electron densities are assumed to be enhanced in plumes and to approach interplume values with increasing height. Different combinations of the outflow and turbulence velocity in the plume regions are considered. We compute line profiles and total intensities of the H I Ly-alpha and the O VI doublets. The observed profile shapes and intensities are reproduced best by a small solar wind speed at low altitudes in plumes that increases with height to reach ambient interplume values above roughly 3-4 Rsolar, combined with a similar variation of the width of the velocity distribution of the scattering atoms/ions. We also find that plumes very close to the pole give narrow profiles at heights above 2.5 Rsolar, which are not observed. This suggests a tendency for plumes to be located away from the pole. We find that the inclusion of plumes in the model computations provides an improved correspondence with the observations and confirms previous results showing that published UVCS observations in polar coronal holes can be roughly reproduced without the need for large temperature anisotropy. The latitude distributions of plumes and magnetic flux distributions are studied by analyzing data from different instruments on SOHO and with SOLIS.
2007-12: Violent Magnetic Activities on the Sun and the Catastrophe Model
Lin J. et al.
ApJ, 658, L123, 2007.
2007-11: The Physical Properties of Coronal Streamers II
Uzzo M., Strachan L., and Vourlidas A.
ApJ, 671, 912, 2007.
In this paper the plasma properties of three streamers observed in 2003 by Ultraviolet Coronagraph Spectrometer (UVCS) are presented for five heights from 1.75 to 5.0 R$_\odot$. The kinetic temperatures for protons (T$_{k,p}$) and the O$^{5+}$ ions (T$_{k,O}$) are derived as a function of height with preferential heating of O$^{5+}$ over protons recorded. By examining how T$_{k,p}$ varies with latitude at each height, an idea of the magnetic field morphology is found. At 1.75 R$_\odot$ the elemental abundances (O, S, Ar and Fe), electron temperature and electron density are derived from the UV spectral data. All three streamers were quiescent with typical abundance values, however no depleted cores were found. The First Ionization Potential (FIP) effect was detected for all three streamers with a bias of $\sim$4. This is consistent with slow solar wind in situ measurements thereby supporting the hypothetical connection between the two. All three streamers had a higher than expected electron temperature. The electron densities above 1.75 R$_\odot$ are derived from the Large Angle Spectroscopic Coronagraph (LASCO) C2 polarized brightness data. Estimates for the O$^{5+}$ outflow velocities are obtained using the O VI 1032 \AA\, over 1037 \AA\, intensity ratios and the estimated electron densities. All three streamers show evidence of significant outflows at 4.0 and 5.0 R$_\odot$.
2007-10: Tracking UVCS/SOHO Responsivity with Observations of Zeta Tau
B. Valcu, P. L. Smith, L. D. Gardner, J. C. Raymond, M. P. Miralles, J. L. Kohl
Solar Physics, Volume 243, Issue 1, pp.93-104.
2007-9: Transition Region Emission and Energy Input to Thermal Plasma During the Impulsive Phase of Solar Flares
Raymond, J. C.; Holman, G.; Ciaravella, A.; Panasyuk, A.; Ko, Y. -K.;
Kohl, J.
The Astrophysical Journal, Volume 659, Issue 1, pp. 750-757.
The energy released in a solar flare is partitioned between thermal and
non-thermal particle energy and lost to thermal conduction and radiation
over a broad range of wavelengths. It is difficult to determine the conductive
losses and the energy radiated at transition region temperatures during
the impulsive phases of flares. We use UVCS measurements of O VI photons
produced by 5 flares and subsequently scattered by O VI ions in the corona
to determine the 5.0 〈 log T 〈 6.0 transition region luminosities. We compare
them with the rates of increase of thermal energy and the conductive losses
deduced from RHESSI and GOES X-ray data using areas from RHESSI images to
estimate the loop volumes, cross-sectional areas and scale lengths. The
transition region luminosities during the impulsive phase exceed the X-ray
luminosities for the first few minutes, but they are smaller than the rates
of increase of thermal energy unless the filling factor of the X-ray emitting
gas is ~ 0.01. The estimated conductive losses from the hot gas are too
large to be balanced by radiative losses or heating of evaporated plasma,
and we conclude that the area of the flare magnetic flux tubes is much smaller
than the effective area measured by RHESSI during this phase of the flares.
For the 2002 July 23 flare, the energy deposited by non-thermal particles
exceeds the X-ray and UV energy losses and the rate of increase of the thermal
energy.
2007-8: Self-consistent Coronal Heating and Solar Wind Acceleration from Anisotropic Magnetohydrodynamic Turbulence
Cranmer, Steven R.; van Ballegooijen, Adriaan A.; Edgar, Richard J.
The Astrophysical Journal Supplement Series, Volume 171, Issue 2, pp. 520-551.
We present a series of models for the plasma properties along open magnetic
flux tubes rooted in solar coronal holes, streamers, and active regions.
These models represent the first self-consistent solutions that combine:
(1) chromospheric heating driven by an empirically guided acoustic wave spectrum,
(2) coronal heating from Alfven waves that have been partially reflected,
then damped by anisotropic turbulent cascade, and (3) solar wind acceleration
from gradients of gas pressure, acoustic wave pressure, and Alfven wave pressure.
The only input parameters are the photospheric lower boundary conditions
for the waves and the radial dependence of the background magnetic field
along the flux tube. We have not included multifluid or collisionless effects
(e.g., preferential ion heating) which are not yet fully understood. For
a single choice for the photospheric wave properties, our models produce a
realistic range of slow and fast solar wind conditions by varying only the
coronal magnetic field. Specifically, a two-dimensional model of coronal
holes and streamers at solar minimum reproduces the latitudinal bifurcation
of slow and fast streams seen by Ulysses. The radial gradient of the Alfven
speed affects where the waves are reflected and damped, and thus whether
energy is deposited below or above the Parker critical point. As predicted
by earlier studies, a larger coronal ``expansion factor'' gives rise to a
slower and denser wind, higher temperature at the coronal base, less intense
Alfven waves at 1 AU, and correlative trends for commonly measured ratios
of ion charge states and FIP-sensitive abundances that are in general agreement
with observations. These models offer supporting evidence for the idea that
coronal heating and solar wind acceleration (in open magnetic flux tubes)
can occur as a result of wave dissipation and turbulent cascade.
2007-7: ``Bursty'' Reconnection Following
Solar Eruptions: MHD Simulations and Comparison with Observations
Riley, Pete; Lionello, Roberto; Mikić, Zoran; Linker, Jon; Clark, Eric;
Lin, Jun; Ko, Yuan-Kuen
The Astrophysical Journal, Volume 655, Issue 1, pp. 591-597.
Posteruptive arcades are frequently seen in the aftermath of coronal mass
ejections (CMEs). The formation of these loops at successively higher altitudes,
coupled with the classic ``two-ribbon'' flare seen in Hα, are interpreted
as reconnection of the coronal magnetic field that has been dragged outward
by the CME. White-light observations of ``rays,'' which have been interpreted
as being coincident with the current sheet at the reconnection site underneath
the erupting CME, also provide evidence for its occurrence. ``Blobs'' occasionally
seen within these rays suggest an even richer level of structure. In this
report, we present numerical simulations that reproduce both the observed
rays and the formation and evolution of the blobs. We compare their properties
with SOHO/LASCO observations of similar structures, and relate their formation
to standard theories of reconnection.
2007-6: Coronal mass ejections and the
associated activities on the solar disk observed on October 26, 2003
Bao, X.; Zhang, H.; Lin, J.; Stenborg, G. A.
Astronomy and Astrophysics, Volume 463, Issue 1, February III 2007, pp.321-331
Coronal mass ejections (CMEs) are usually considered fast (slow) if their
velocities are greater (less) than 500 km s-1. It is generally
believed that fast CMEs are well associated with activity manifested on
the solar disk, such as solar flares, and that slow CMEs are often associated
with filament eruptions out of active regions and can hardly leave any signature
on the solar disk. However, this may not be totally true for the cases we
are studying in the present work, where we have explored more explicitly
than in previous studies the relationship between different types of CMEs
and the associated on-disk activities.
Methods: .We analyzed four CMEs that happen to take off near the west limb
of the Sun on October 26, 2003. Their maximum speeds varied from 300 to
1800 km s-1, with average accelerations from about 6 m s-2
up to 330 m s-2. They span over almost the full range of speeds
of typical CMEs, from slow to fast. The evolution and kinematical properties
of each CME at their early stages as well as the corresponding processes
on the solar disk are examined.
Results: .Three out of the four events analyzed, including slow and fast
ones, exhibited apparent acceleration during their early development and
a likely association with solar disk activity as manifested by ribbon flares
and post-event loops. The fourth event (the CME that did not show any clear
association with solar disk activity) was not the slowest one: it had a
median speed (≤400 km s-1). This suggests that some existing
conclusions about the CME-flare correlation need to be re-investigated.
In addition, the correlation of the coronal mass ejection angular widths
to their speeds is also discussed.
2007-5: Features and Properties of Coronal
Mass Ejection/Flare Current Sheets
Lin, J.; Li, J.; Forbes, T. G.; Ko, Y.-K.; Raymond, J. C.; Vourlidas, A.
The Astrophysical Journal, Volume 658, Issue 2, pp. L123-L126.
Solar eruptions occur when magnetic energy is suddenly converted into heat and kinetic energy by magnetic reconnection in a current sheet (CS). It is often assumed that CSs are too thin to be observable because the electric resistivity ηe in CSs is taken to be very small. In this work, we show the implications for the CS thickness d estimated from observations of three eruptions by the UVCS and the LASCO experiments on SOHO. We infer the effective ηe causing the rapid reconnection, which predicts much faster reconnection in a thick CS than that caused by the classical and anomalous resistivities. We find that in these events CSs are observable and have extremely large values of d and ηe, implying that large-scale turbulence is operating within CSs. We also discuss the properties of the so-called hyperresistivity caused by the tearing mode and the relation to our results.
2007-4: Coronal transients and metric
type II radio bursts. II. Accelerations at low coronal heights
Mancuso, S.
Astronomy and Astrophysics, Volume 463, Issue 3, March I 2007, pp.1137-1141
In the companion Paper I, Mancuso & Raymond (2004) investigated the
relationship between type ii and coronal mass ejection (CME) activity for
a sample of twenty-nine CME/shock events that occurred between March and
December 1999. Most of the events appeared to lead the type ii emission
locations by several minutes and the two sets of speeds were not well-correlated,
in apparent disagreement with a CME-driven origin interpretation of the
coronal shocks. The above discrepancies were attributed to an artifact effect
due to geometry, favoring emission at the flanks of the CME leading edges
in correspondence with denser low-Alfvén-speed coronal structures,
where shock strengths are enhanced. An important caveat in the analysis
carried out in Paper I is that the conclusions that supported the CME-driven
shock front-flank scenario were based on sunward-extrapolated CME trajectories
deduced from the analysis of Large Angle and Spectrometric Coronagraph (SOHO/LASCO)
observations in the outer corona that might not be accurate at low coronal
heights where significant accelerations should be present before the CMEs
acquire the speeds inferred by coronagraphic images. In the present paper,
we re-examine the above relationship for a subset of ten events by integrating
the LASCO measurements with ancillary CME observations taken by other instruments
(MLSO/Mk4, SOHO/EIT, and UVCS) at coronal heights comparable to the typical
type ii radio emissions (1.2-2.5 Rȯ). We investigate the bias
introduced in the sunward extrapolation of LASCO data and discuss whether
the presence of previously undetected CME accelerations at low coronal heights
might have affected the conclusions put forth in the CME-driven shock front-flank
scenario proposed in Paper I. We show that the new set of observations neither
solve the problem of the timing between CMEs and shocks nor improve the
correlation between CME and shock speeds, although acceleration effects
are found to be important at the typical metric type ii heights and must
be taken into account for a proper analysis of the CME/shock relationship.
Astronomy and Astrophysics, Volume 464, Issue 2, March III 2007, pp.753-760
Context: .Detailed hydrodynamic modeling explained several features of
a fragment of the core of a Coronal Mass Ejection observed with SoHO/UVCS
at 1.7 Rȯ on 12 December 1997, but some questions remained unsolved.
Aims: .We investigate the role of the magnetic fields in the thermal insulation
and the expansion of an ejected fragment (cloud) traveling upwards in the
outer corona.
Methods: .We perform MHD simulations including the effects of thermal conduction
and radiative losses of a dense spherical or cylindrical cloud launched
upwards in the outer corona, with various assumptions on the strength and
topology of the ambient magnetic field; we also consider the case of a cylindrical
cloud with an internal magnetic field component along its axis.
Results: .We find that a weak ambient magnetic field (β � 20) with open
topology provides both significant thermal insulation and large expansion.
The cylindrical cloud expands more than the spherical one.
2007-2: Wind in the Solar Corona: Dynamics
and Composition
Antonucci, Ester
Space Science Reviews, Volume 124, Issue 1-4, pp. 35-50
The dynamics of the solar corona as observed during solar minimum with the Ultraviolet Coronagraph Spectrometer, UVCS, on SOHO is discussed. The large quiescent coronal streamers existing during this phase of the solar cycle are very likely composed by sub-streamers, formed by closed loops and separated by open field lines that are channeling a slow plasma that flows close to the heliospheric current sheet. The polar coronal holes, with magnetic topology significantly varying from their core to their edges, emit fast wind in their central region and slow wind close to the streamer boundary. The transition from fast to slow wind then appears to be gradual in the corona, in contrast with the sharp transition between the two wind regimes observed in the heliosphere. It is suggested that speed, abundance and kinetic energy of the wind are modulated by the topology of the coronal magnetic field. Energy deposition occurs both in the slow and fast wind but its effect on the kinetic temperature and expansion rate is different for the slow and fast wind.
2007-1: A Comprehensive Study of the Initiation and Early Evolution of a Coronal Mass Ejection from Ultraviolet and White-Light Data
Bemporad, A.; Raymond, J.; Poletto, G.; Romoli, M.
The Astrophysical Journal, Volume 655, Issue 1, pp. 576-590.
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 Rsolar. Analysis of the UV spectra, acquired by the Solar and Heliospheric Observatory Ultraviolet Coronagraph Spectrometer (SOHO UVCS) at 1.6 and 1.9 Rsolar, 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 Rsolar. 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