Astronomy and Astrophysics, Volume 481, Issue 3, 2008, pp.835-844, (preprint: arXiv:0801.2705)
We provide a guideline to interpret the UVCS emission lines (in particular O VI and Si XII) during shock wave propagation in the outer solar corona. We use a numerical MHD model performing a set of simulations of shock waves generated in the corona and from the result we compute the plasma emission for the O VI and Si XII including the effects of NEI. We analyze the radiative and spectral properties of our model with the support of a detailed radiation model including Doppler dimming and an analytical model for shocks, and, finally, we synthesize the expected O VI 1032A line profile. We explain several spectral features of the observations like the absence of discontinuities in the O VI emission during the shock passage, the brightening of Si XII emission and the width of the lines. We use our model also to give very simple and general predictions for the strength of the line wings due to the ions shock heating and on the line shape for Limb CMEs or Halo CMEs. The emission coming from post-shock region in the solar corona roughly agrees with the emission from a simple planar and adiabatic shock, but the effect of thermal conduction and the magnetic field may be important depending on the event parameters. Doppler dimming significantly influences the O VI emission while Si XII line brightens mainly because of the shock compression. Significant shock heating is responsible for the wide and faint component of the O VI line usually observed which may be taken as a shock signature in the solar corona.
Journal of Astrophysics and Astronomy, vol. 29, issue 1-2, pp. 321-327
Astronomy and Astrophysics, Volume 486, Issue 3, 2008, pp.1009-1013.
We have compared the density distributions of solar corona obtained by SOHO Ultraviolet Coronagraph Spectrometer (UVCS) and Mauna Loa Solar Observatory (MLSO) MK4 coronameter. This is the first attempt to compare the coronal densities estimated by the two instruments. In the spectral data of UVCS, we have selected two emission lines (O VI 1032 Åand 1037.6 Å), which have both radiative and collisional components. The coronal number density is determined from the ratio of these two components. The MK4 coronameter has a field of view ranging from 1.08 to 2.85 solar radii. The coronal density can be determined by inverting MLSO MK4 polarization maps. We find that the mean electron number density in a helmet streamer observed by MK4 on 2003 April 28 is fairly consistent with that observed by UVCS. For a coronal hole and an active region observed on 1999 October 19 and 24, the MK4 coronal densities are close to those from the UVCS within a factor of two; the former values are twice the latter at 1.7 solar radii and closer to the latter at higher altitudes. Our results demonstrate that MK4 polarization data can provide us with a coronal density distribution in a large field of view with a time cadence of about three minutes. We suggest that the MK4 data can be used to derive 2-D density distributions of coronal structures and further to estimate the heights of CME-associated type II shocks.
12th European Solar Physics Meeting, Freiburg, Germany, held September, 8-12, 2008. Online at http://espm.kis.uni-freiburg.de/, p.2.94.
The SOHO polar coronal jets were first observed by SOHO instruments (EIT, LASCO, UVCS) during the previous solar minimum in 1996. They were small, fast coronal eruptions originating from the flaring UV bright points within large polar coronal holes. Data analysis and modeling provided estimates for jet plasma conditions, dynamics, evolution of the electron temperature and heating rate. In the current minimum of solar activity large polar holes are again dominating structures in the solar corona. Hinode observed there a number of polar X-ray ejections occurring with high frequency - about 60 events per day. We will present observations and compare physical properties of the SOHO and Hinode polar coronal jets. We will also discuss possible relation of these two types of solar ejections.
Annales Geophysicae, Volume 26, Issue 10, 2008, pp.3007-3016.
The SPIRIT telescope aboard the CORONAS-F satellite (in orbit from 26 July 2001 to 5 December 2005), observed the off-limb solar corona in the 175 Å (Fe IX, X and XI lines) and 304 Å (He II and Si XI lines) bands. In the coronagraphic mode the mirror was tilted to image the corona at the distance of 1.1...5 Rsun from the solar center, the outer occulter blocked the disk radiation and the detector sensitivity was enhanced. This intermediate region between the fields of view of ordinary extreme-ultraviolet (EUV) telescopes and most of the white-light (WL) coronagraphs is responsible for forming the streamer belt, acceleration of ejected matter and emergence of slow and fast solar wind. We present here the results of continuous coronagraphic EUV observations of the solar corona carried out during two weeks in June and December 2002. The images showed a "diffuse" (unresolved) component of the corona seen in both bands, and non-radial, ray-like structures seen only in the 175 Å band, which can be associated with a streamer base. The correlations between latitudinal distributions of the EUV brightness in the corona and at the limb were found to be high in 304 Å at all distances and in 175 Å only below 1.5 Rsun. The temporal correlation of the coronal brightness along the west radial line, with the brightness at the underlying limb region was significant in both bands, independent of the distance. On 2 February 2003 SPIRIT observed an expansion of a transient associated with a prominence eruption seen only in the 304 Å band. The SPIRIT data have been compared with the corresponding data of the SOHO LASCO, EIT and UVCS instruments.
The Astrophysical Journal, Volume 689, Issue 1, pp. 572-584.
Plasma in post-CME current sheets (CSs) is expected to be highly turbulent because of the tearing and coalescence instability and/or local microscopic instabilities. For this reason, in the last decade the inconsistency between the observed (~104-105 km) and the expected (~1-10 m) CS thickness has been tentatively explained in many MHD models as a consequence of plasma turbulence that should be able to significantly broaden the CS. However, from the 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 evolution of turbulence derived from the nonthermal broadening of the [Fe XVIII] line profiles. Derived turbulent speeds 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 2 days. From this evolution the anomalous diffusivity due to microinstabilities has been estimated, and the scenario of 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 (~10-104 m) could explain not only the high CS temperatures but also the much larger observed thickness of macroscopic CSs, thanks to turbulent broadening.
PARTICLE ACCELERATION AND TRANSPORT IN THE HELIOSPHERE AND BEYOND: 7th Annual International Astrophysics Conference. AIP Conference Proceedings, Volume 1039, pp. 11-19 (2008).
A magnificent phenomenon in major solar eruptions is the creation of a long current sheet connecting solar flares to coronal mass ejections (CMEs). The sheet is important as the site where the magnetic energy is converted efficiently into heat and bulk kinetic energy and where particles are accelerated to high energy. With the development made recently in studying the CME/flare current sheet itself and particle accelerations in both theories and observations, our knowledge about the current sheet, its interior structures, and various processes occurring inside the sheet has been improved significantly. We are weaving various manifestations of solar eruptive processes related to the reconnecting current sheets, and looking into physical mechanisms that connect them to one another in the present work. Implications of these manifestations and the corresponding mechanisms to particle accelerations in the sheet are discussed, and some results we obtained recently for the particle acceleration are also displayed.
The Astrophysical Journal, Volume 677, Issue 2, pp. L137-L140.
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 Rsolar. In 2006 June the Ultraviolet Coronagraph Spectrometer (SOHO UVCS) observed over a period of about 9.2 days H Ly\u03b1 intensity fluctuations at 2.1 Rsolar over a polar coronal hole. The Ly\u03b1 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.
The Astrophysical Journal, Volume 688, Issue 1, pp. 656-668.
The observations of the UVCS SOHO instrument from 1996 May to 1997 May have been analyzed to reconstruct intensity time series of the O VI 1032 Å and H I Ly\u03b1 1216 Å spectral lines at different coronal heliolatitudes from 1.5 to 3.0 Rsolar from Sun center. At solar minimum, some features persist for several rotations, thus allowing analysis of the UV emission as time series modulated at the period of the solar rotation. We find evidence of coronal differential rotation, which significantly differs from that of the photospheric plasma. The estimated equatorial synodic rotation period of the corona at 1.5 Rsolar is 27.48+/-0.10 days. The study of the latitudinal variation shows that the UV corona decelerates toward the photospheric rates from the equator up to the poleward boundary of the midlatitude streamers, reaching a peak of 28.16+/-0.20 days around +30° from the equator at 1.5 Rsolar, while a less evident peak is observed in the northern hemisphere. This result suggests a real north-south rotational asymmetry as a consequence of different activity and weak coupling between the magnetic fields of the two hemispheres. The study of the radial rotation profiles shows that the corona is rotating almost rigidly with height, but we find an abrupt increase by about half a day between 2.3 and 2.5 Rsolar. The larger gradients of the rotation rates are localized at the boundaries between open and closed field lines, suggesting that in these regions the differential rotation might be a source of magnetic stress and, consequently, of energy release.
Annales Geophysicae, Volume 26, Issue 10, 2008, pp.3017-3024.
On 10 11 December 2005 a slow CME occurred in the Western Hemisphere in between two coronal streamers. SOHO/MDI magnetograms show a multipolar magnetic configuration at the photosphere: a complex of active regions located at the CME source and two bipoles at the base of the lateral coronal streamers. White light observations reveal that the CME expansion affects both of them and induces the release of plasma within or close to the nearby streamers. These transient phenomena are possibly due to magnetic reconnections induced by the CME expansion and occurring inside the streamer current sheet or between the CME flanks and the streamer. These events have been observed by the SOHO/UVCS with the spectrometer slit centered at 1.8 R\u022f over about a full day. In this work we focus on the interaction between the CME and the streamer: the UVCS spectral interval included UV lines from ions at different temperatures of maximum formation such as O VI, Si XIII and Al Xi. These data gave us the opportunity to infer the evolution of plasma temperature and density at the reconnection site and adjacent regions. These are relevant to characterize secondary reconnection processes occurring during a CME development.
New Astronomy, Volume 13, Issue 7, p. 526-540.
The flare-related, persistent and abrupt changes in the photospheric magnetic field have been reported by many authors during recent years. These bewildering observational results pose a challenge to the current flare theories in which the photospheric magnetic field usually remains unchanged in the eruption. In this paper, changes in the photosphere magnetic field during the solar eruption are investigated based on the catastrophe model. The results indicate that the projection effect is an important source that yields the change in the observed photospheric magnetic field in the line-of-sight. Furthermore one may observe the change in the normal component of magnetic field if the spectrum line used to measure the photospheric magnetic field does not exactly come from the photospheric surface. Our results also show that the significance of selecting the correct spectral lines to study the photospheric field becomes more apparent for the magnetic configurations with complex boundary condition (or background field).
The Astrophysical Journal, Volume 673, Issue 1, pp. L91-L94.
We carry out model calculations to show that observed outflow velocities of order 7-10 km s-1 of C IV and O VI ions, and 15-20 km s-1 of Ne VIII ions, are not only consistent with models of the solar wind from coronal holes but also place unique constraints on the degree of flow tube expansion as well as the location of the expansion in the transition region/lower corona. We include nonequilibrium effects in the calculations and also show that ions and protons couple well in the line-forming region.
The Astrophysical Journal, Volume 682, Issue 1, pp. 644-653.
A theory for the heating of coronal magnetic flux ropes is developed. The dissipated magnetic energy has two distinct contributions: (1) energy injected into the corona as a result of granule-scale, random footpoint motions and (2) energy from the large-scale, nonpotential magnetic field of the flux rope. The second type of dissipation can be described in terms of hyperdiffusion, a type of magnetic diffusion in which the helicity of the mean magnetic field is conserved. The associated heating rate depends on the gradient of the torsion parameter of the mean magnetic field. A simple model of an active region containing a coronal flux rope is constructed. We find that the temperature and density on the axis of the flux rope are lower than in the local surroundings, consistent with observations of coronal cavities. The model requires that the magnetic field in the flux rope be stochastic in nature, with a perpendicular length scale of the magnetic fluctuations of the order of 1000 km.
Journal of Geophysical Research, Volume 113, Issue A7, CiteID A00B02
We examine phenomena associated with eruptions in the two different regimes of the solar corona and the terrestrial magnetosphere. We find striking similarities between the speeds of shrinking magnetic field lines in the corona and dipolarization fronts traversing the magnetosphere. We also examine the similarities between supra-arcade downflows observed during solar flares and bursty bulk flows seen in the magnetotail and find that these phenomena have remarkably similar speeds, velocity profiles, and size scales. Thus we show manifest similarities in the magnetic reconfiguration in response to the ejection of coronal mass ejections in the corona and the ejection of plasmoids in the magnetotail. The subsequent return of loops to a quasi-potential state in the corona and field dipolarization in the magnetotail are physical analogs and trigger similar phenomena such as downflows, which provides key insights into the underlying drivers of the plasma dynamics.
The Astrophysical Journal, Volume 683, Issue 2, pp. 1168-1179.
We construct subsynoptic maps of the ultraviolet-line fluxes, electron density, and elemental abundances for an east limb corona at 1.63 solar radii from 2000 September 20 to October 1. The data, covering position angles of 85°-126°, were taken from the Ultraviolet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory (SOHO), and the maps are based on the variation of these parameters along the field of view of the UVCS slit. Combining these maps with the limb synoptic maps made from SOHO Extreme Ultraviolet Imaging Telescope, SOHO Large Angle and Spectrometric Coronagraph, and Yohkoh Soft X-ray Telescope observations provides a large-scale, distinct view of the contrast between different coronal structures in different physical properties and their relation to the underlying disk and magnetic field structures. During this time period, the east limb corona mainly consisted of three streamers and two dark areas that exhibited very different plasma properties. We construct a three-dimensional MHD coronal model that incorporates energy transport processes, and compare the large-scale coronal properties of the model with those of the observation. The comparison investigates areas of different abundances and densities, and their possible association with open and closed magnetic field structures. We find a good indication that the open field regions, which we believe to be the slow-wind source regions in this case, have lower coronal density and higher abundance values than the closed field regions. This is true for absolute abundance, and probably also for the FIP bias. Therefore, such synoptic maps can be very useful for identifying solar wind source regions.
The Astrophysical Journal, Volume 686, Issue 2, pp. 1372-1382.
In the wake of the 2003 November 4 coronal mass ejection associated with the largest solar flare of the last sunspot cycle, a current sheet (CS) was observed by the Ultraviolet Coronagraph Spectrometer (UVCS) as a narrow bright feature in the [Fe XVIII] (106.8 K) line. This is the first UV observation in which the CS evolution is followed from its onset. UV spectra provide diagnostics of electron temperature, emission measure, Doppler shift, line width, and size of the CS as function of time. Since the UVCS slit was inside the Mark IV K-coronameter (MK4) field of view, the combination of UV spectra and MK4 white light data provides estimates of the electron density and depth along the line of sight of the CS. The thickness of the CS in the [Fe XVIII] line is far larger than classical or anomalous resistivity would predict, and it might indicate an effective resistivity much larger than anomalous resistivity, such as that due to hyperdiffusion. The broad [Fe XVIII] line profiles in the CS cannot be explained as thermal widths. They result from a combination of bulk motions and turbulence. The Petschek reconnection mechanism and turbulent reconnection may be consistent with the observations.
Journal of Astrophysics and Astronomy, vol. 29, issue 1-2, pp. 187-193.
JGR, Volume 113, Issue A11, CiteID A11107
Magnetic reconnection plays a crucial role in violent energy conversion occurring in the environments of high electrical conductivity, such as the solar atmosphere, magnetosphere, and fusion devices. We focus on the morphological features of the process in two different environments, the solar atmosphere and the geomagnetic tail. In addition to indirect evidence that indicates reconnection in progress or having just taken place, such as auroral manifestations in the magnetosphere and the flare loop system in the solar atmosphere, more direct evidence of reconnection in the solar and terrestrial environments is being collected. Such evidence includes the reconnection inflow near the reconnecting current sheet, and the outflow along the sheet characterized by a sequence of plasmoids. Both turbulent and unsteady Petschek-type reconnection processes could account for the observations. We also discuss other relevant observational consequences of both mechanisms in these two settings. While on face value, these are two completely different physical environments, there emerge many commonalities, for example, an Alfven speed of the same order of magnitude, a key parameter determining the reconnection rate. This comparative study is meant as a contribution to current efforts aimed at isolating similarities in processes occurring in very different contexts in the heliosphere, and even in the universe.
The Astrophysical Journal, Volume 674, Issue 2, pp. 1167-1178.
A systematic, comprehensive survey of H I Ly? and O VI doublet profiles in polar coronal holes is performed using spectroscopic observation by the SOHO Ultraviolet Coronagraph Spectrometer (UVCS) at solar minimum in 1996-1997. By decomposing coronal hole profiles into component Gaussians, we determine detailed height distributions of profile parameters for respective components. In particular, we show that for each of the O VI ??103.2, 103.8 doublet lines, the narrow and broad components have competing intensities within heliocentric height 1.3-2.1 Rsolar, and that the considerable profile change occurring in this height range is due to changes not only of the widths of individual components but also of the mutual proportion between the narrow- and the broad-component intensities. We also show that the ratios between the O VI doublet intensities, I?103.2/I?103.8, for the narrow-narrow and the broad-broad component pairs have contrasting values and exhibit opposing radial dependences; the O VI ratio between the former pair increases with height from 2 at height 1 Rsolar to 4 at 1.7 Rsolar, while that between the latter decreases with height from 2-3 at <1.8 Rsolar to less than 1 at 2.5 Rsolar. We discuss implications of our results for the interpretation of the observed broad widths of the O VI doublet.
AdSpR 41, 138-143. 2008
It is common to use imaging instruments such as EUV and X-ray imagers and coronagraphs to study large-scale phenomena such as coronal mass ejections and coronal waves. Although high resolution spectroscopy is generally limited to a small field of view, its importance in understanding global phenomena should not be under-estimated. I will review current spectroscopic observations of large-scale dynamic phenomena such as global coronal waves and coronal mass ejections. The aim is to determine plasma parameters such as flows, temperatures and densities to obtain a physical understanding of these phenomena.
Astrophysical Journal, Volume 678, Issue 2, pp. 1480-1497.
We present a detailed analysis of oxygen ion velocity distributions in the extended solar corona, based on observations made with the Ultraviolet Coronagraph Spectrometer (UVCS) on the SOHO spacecraft. Polar coronal holes at solar minimum are known to exhibit broad line widths and unusual intensity ratios of the O VI ?? 1032, 1037 emission line doublet. The traditional interpretation of these features has been that the oxygen ions have a strong temperature anisotropy, with the temperature perpendicular to the magnetic field being much larger than the temperature parallel to the field. However, recent work by Raouafi and Solanki suggested that it may be possible to model the observations using an isotropic velocity distribution. In this paper we analyze an expanded data set to show that the original interpretation of an anisotropic distribution is the only one that is fully consistent with the observations. It is necessary to search the full range of ion plasma parameters to determine the values with the highest probability of agreement with the UVCS data. The derived ion outflow speeds and perpendicular kinetic temperatures are consistent with earlier results, and there continues to be strong evidence for preferential ion heating and acceleration with respect to hydrogen. At heliocentric heights above 2.1 solar radii, every UVCS data point is more consistent with an anisotropic distribution than with an isotropic distribution. At heights above 3 solar radii, the exact probability of isotropy depends on the electron density chosen to simulate the line-of-sight distribution of O VI emissivity. The most realistic electron densities (which decrease steeply from 3 to 6 solar radii) produce the lowest probabilities of isotropy and most-probable temperature anisotropy ratios that exceed 10. We also use UVCS O VI absolute intensities to compute the frozen-in O5+ ion concentration in the extended corona; the resulting range of values is roughly consistent with recent downward revisions in the oxygen abundance.