Large-scale Structure of the Chromospheric Doppler Velocities on the Solar Disk from 2D-spectroscopy within the He I 10830 Å Line (original) (raw)
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Astronomy & Astrophysics, 2018
We investigated the capability of mapping the solar wind outflow velocity of neutral hydrogen atoms by using synergistic visible-light and ultraviolet observations. We used polarised brightness images acquired by the LASCO/SOHO and Mk3/MLSO coronagraphs, and synoptic Lyα line observations of the UVCS/SOHO spectrometer to obtain daily maps of solar wind H I outflow velocity between 1.5 and 4.0 R⊙ on the SOHO plane of the sky during a complete solar rotation (from 1997 June 1 to 1997 June 28). The 28-days data sequence allows us to construct coronal off-limb Carrington maps of the resulting velocities at different heliocentric distances to investigate the space and time evolution of the outflowing solar plasma. In addition, we performed a parameter space exploration in order to study the dependence of the derived outflow velocities on the physical quantities characterising the Lyα emitting process in the corona. Our results are important in anticipation of the future science with the ...
2021
Context. The determination of solar wind H i outflow velocity is fundamental to shedding light on the mechanisms of wind acceleration occurring in the corona. Moreover, it has implications in various astrophysical contexts, such as in the heliosphere and in cometary and planetary atmospheres. Aims. We aim to study the effects of the chromospheric Lyα line profile shape on the determination of the outflow speed of coronal H i atoms via the Doppler dimming technique. This is of particular interest in view of the upcoming measurements of the Metis coronagraph aboard the Solar Orbiter mission. Methods. The Doppler dimming technique exploits the decrease of coronal Lyα radiation in regions where H i atoms flow out in the solar wind. Starting from UV observations of the coronal Lyα line from the UltraViolet Coronagraph Spectrometer (UVCS), aboard the Solar and Heliospheric Observatory (SOHO), and simultaneous measurements of coronal electron densities from pB coronagraphic observations, w...
Solar Wind Outflow and the Chromospheric Magnetic Network
Science, 1999
Observations of outflow velocities in coronal holes (regions of open coronal magnetic field) have recently been obtained with the Solar and Heliospheric Observatory (SOHO) spacecraft. Velocity maps of Ne7+ from its bright resonance line at 770 angstroms, formed at the base of the corona, show a relationship between outflow velocity and chromospheric magnetic network structure, suggesting that the solar wind is rooted at its base t o this structure, emanating from localized regions along boundaries and boundary intersections of magnetic network cells. This apparent relation t o the chromospheric magnetic network and the relatively large outflow velocity signatures will improve understanding of the complex structure and dynamics at the base of the corona and the source region of the solar wind.
Journal of Geophysical Research, 2000
Highly sensitive radio occultation and white light measurements of pathintegrated density have shown that the solar corona comprises three distinct morphological regions, streamer, quiet Sun, and polar coronal hole, which except for the streamer region, extend radially into interplanetary space from 1.15 R s to at least 30 R s. In this paper we build on these results by comparing solar wind flow speeds observed at the same time as path-integrated density. Flow speeds are inferred from the Doppler dimming of O VI lines with the ultraviolet coronagraph spectrometer on the Solar and Heliospheric Observatory, while the simultaneous polarized brightness measurements of path-integrated density are from the High Altitude Observatory Mauna Loa Mk III Kcoronameter. The comparison of global flow speed and density observations in 1997 produces three new results. First, it shows, that the three distinct morphological regions, identified earlier in density measurements, are present in the latitudinal profile of the flow speed in the corona. In particular, the flow speed measurements provide evidence for the quiet Sun as an additional source of fast wind. Second, the comparison shows that flow speed and density are anticorrelated in the solar corona. Third, it demonstrates that the Mk III pB measurements can readily serve as a proxy for velocity distribution in the outer corona. The extensive Mk III data set that spans nearly two solar cycles therefore provides the framework of near-Sun measurements with which connections with solar wind measurements in interplanetary space can be made. Specifically, we show that fast wind regions in the heliosphere, observed directly by Ulysses and Wind plasma measurements and remotely by Nagoya interplanetary scintillation measurements, map radially back to fast wind regions at the Sun identified by the Mk III data.
Dynamics of the solar chromosphere
Astronomy and Astrophysics, 2001
We search for signatures of high-frequency oscillations in the upper solar photosphere and low chromosphere in the context of acoustic heating of outer stellar atmospheres. We use ultraviolet image sequences of a quiet center-disk area from the Transition Region and Coronal Explorer (TRACE) mission which were taken with strict cadence regularity. The latter permits more reliable high-frequency diagnosis than in earlier work. Spatial Fourier power maps, spatially averaged coherence and phase-difference spectra, and spatio-temporal (k h , f) decompositions all contain high-frequency features that at first sight seem of considerable intrinsic interest but actually are more likely to represent artifacts of different nature. Spatially averaged phase difference measurement provides the most sensitive diagnostic and indicates the presence of acoustic modulation up to f ≈ 20 mHz (periods down to 50 s) in internetwork areas.
Origins of the Slow and the Ubiquitous Fast Solar Wind
The Astrophysical Journal, 1997
We present in this Letter the first coordinated radio occultation measurements and ultraviolet observations of the inner corona below 5.5 R s , obtained during the Galileo solar conjunction in January 1997, to establish the origin of the slow solar wind. Limits on the flow speed are derived from the Doppler dimming of the resonantly scattered component of the oxygen 1032Å and 1037Å lines as measured with the UltraViolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO). White light images of the corona from the Large Angle Spectroscopic Coronagraph (LASCO) on SOHO taken simultaneously are used to place the Doppler radio scintillation and ultraviolet measurements in the context of coronal structures. These combined observations provide the first direct confirmation of the view recently proposed by Woo and Martin (1997) that the slow solar wind is associated with the axes, also known as stalks, of streamers. Furthermore, the ultraviolet observations also show how the fast solar wind is ubiquitous in the inner corona, and that a velocity shear between the fast and slow solar wind develops along the streamer stalks.
A Study of Chromospheric Oscillations Using the SOHO and TRACE Spacecraft
The Astrophysical Journal, 2001
We analyze line and continuum time-series data of the solar atmosphere, with between 10 and 60 s cadence, using the MDI and SUMER instruments on the SOHO spacecraft and the UV bandpasses on the T RACE satellite. The co-aligned data sets sample spectral features formed from photosphere to the middle transition region, spanning Ðve decades in pressure, under quiet-Sun and plage conditions. We discuss power, phase di †erence, and coherence spectra, and examine data in the time domain. The observed photospheric and chromospheric oscillations are strongly coupled for frequencies between 2 and 8 mHz. Phase coherences decrease with increasing height, with only occasional periods and locations of observable coherence up to heights where transition region emission lines are formed. The middle chromosphere (in the SUMER continua) oscillates in several megameter (Mm) diameter coherent patches with power predominantly in the 5È7 mHz range. The T RACE data, formed in the upper photosphere, show smaller patterns superimposed on these large-scale oscillations, resulting (at least in part) from granulation. At the observed spatial scales, all the observed properties point to p-modes, especially the "" pseudomodes ÏÏ just above the acoustic cuto † frequency, as the dominant mode of the chromospheric dynamics. Smaller scale "" acoustic event ÏÏ drivers, associated with granular dynamics, appear to be less important. The predominant internetwork chromospheric oscillations arise from regions much larger horizontally than vertically. If propagating largely vertically, this can naturally explain why the one-dimensional simulations of Carlsson & Stein might be more successful than expected. The chromospheric response to the p-mode driving is, however, intermittent in space and time. Some of the intermittency appears to result from the interaction of the upward-propagating waves with magnetic Ðelds. Evidence for this includes suppressed intensities and oscillations near quiet-Sun network elements (which we dub "" magnetic shadows ÏÏ), absence of oscillations in internetwork regions neighboring plage magnetic Ðelds, and a change in character of the quiet-Sun internetwork oscillations between the 119 and 104 nm continua formed at 1 and 1.2 Mm. The latter might be caused by canopy Ðelds that form between these heights under typical quiet-Sun conditions. A SUMER-only data set reported by et al. has a WikstÔl factor of 3 more oscillatory power in the 104 nm continuum than the data analyzed here, with stronger coherences extending into the solar transition region. Together, these data support the general picture that the chromosphere oscillates primarily in response to forcing by the p-modes, they are therefore large-scale (several Mm across) waves, and they are often strongly inÑuenced by magnetic e †ects (internetwork Ðelds, or the overlying canopy), before the oscillations even reach the transition region.
Advances in Space Research, 2000
In October 1997, we made a coordinated observation of the solar photosphere and chromosphere at Hlda observatory (Kyoto umverslty, Japan) and at Telde observatory (Tenenfe) over a 10 day We obtamed imaging data senes contmuously durmg 6 hr 45 mm m G-band (4308 A) observed with the Domeless Solar Telescope (DST) at Hlda on 24th October (effective FOV, 96"x99") Addltlonally, m this observation, we simultaneously observed Image senes of the chromosphere durmg the latter 4 hr 10 mm m H (Y lme center and H a! &O 6 A From these data set, we could detect that emerging flux tubes crossed the photosphere to the chromosphere and that 'convective collapse' phenomena appeared at the stage of the spot formation Moreover, we confirmed that the hfetlme of mesogranulatlon was about 4000 set (70 mm) from the temporal evolution of velocity patterns We show here only a summary of these observations 0
The Influence of Magnetic Field on Oscillations in the Solar Chromosphere
The Astrophysical Journal, 2006
Two sequences of solar images obtained by the Transition Region and Coronal Explorer in three UV passbands are studied using wavelet and Fourier analysis and compared to the photospheric magnetic flux measured by the Michelson Doppler Interferometer on the Solar Heliospheric Observatory to study wave behaviour in differing magnetic environments. Wavelet periods show deviations from the theoretical cutoff value and are interpreted in terms of inclined fields. The variation of wave speeds indicates that a transition from dominant fast-magnetoacoustic waves to slow modes is observed when moving from network into plage and umbrae. This implies preferential transmission of slow modes into the upper atmosphere, where they may lead to heating or be detected in coronal loops and plumes.
The solar wind as seen by SOHO/SWAN since 1996: comparison with SOHO/LASCO C2 coronal densities
We update the SOHO/SWAN H Lyman-α brightness analysis to cover the 1996-2008 time interval. A forward model applied to the intensity maps provides the latitude and time dependence of the interstellar Hydrogen ionisation rate over more than a full solar cycle. The hydrogen ionisation, being almost entirely due to charge-exchange with solar wind ions, reflects closely the solar wind flux. Our results show that the solar wind latitudinal structure during the present solar minimum is strikingly different from the previous minimum, with a much wider slow solar wind equatorial belt which persists until at least the end of 2008. We compute absolute values of the in-ecliptic H ionisation rates using OMNI solar wind data and use them to calibrate our ionisation rates at all heliographic latitudes. We then compare the resulting fluxes with the synoptic LASCO/C2 electron densities at 6 solar radii. The two time-latitude patterns are strikingly similar over all the cycle. This comparison shows that 6R s densities can be used to infer the solar wind type close to its source, with high (resp. low) densities tracing the slow (resp. fast) solar wind, simply because the density reflects at which altitude occurs the acceleration. The comparison between the two minima suggests that the fast polar wind acceleration occurs at larger distance during the current minimum compared to the previous one. This difference, potentially linked to the magnetic field decrease or(and) the coronal temperature decrease should be reproduced by solar wind expansion models .