共查询到20条相似文献，搜索用时 500 毫秒
1.
G. Trottet J.-P. Raulin G. Giménez de Castro T. Lüthi A. Caspi C. H. Mandrini M. L. Luoni P. Kaufmann 《Solar physics》2011,273(2):339-361
Solar flares observed in the 200 – 400 GHz radio domain may exhibit a slowly varying and time-extended component which follows
a short (few minutes) impulsive phase and can last for a few tens of minutes to more than one hour. The few examples discussed
in the literature indicate that such long-lasting submillimeter emission is most likely thermal bremsstrahlung. We present
a detailed analysis of the time-extended phase of the 27 October 2003 (M6.7) flare, combining 1 – 345 GHz total-flux radio
measurements with X-ray, EUV, and Hα observations. We find that the time-extended radio emission is, as expected, radiated
by thermal bremsstrahlung. Up to 230 GHz, it is entirely produced in the corona by hot and cool materials at 7 – 16 MK and
1 – 3 MK, respectively. At 345 GHz, there is an additional contribution from chromospheric material at a few 104 K. These results, which may also apply to other millimeter–submillimeter radio events, are not consistent with the expectations
from standard semiempirical models of the chromosphere and transition region during flares, which predict observable radio
emission from the chromosphere at all frequencies where the corona is transparent. 相似文献
2.
This paper reviews the contrasting properties of radio and EUV/X-ray observations for the study of the solar atmosphere. The
emphasis is placed on explaining the nature of radio observations to an EUV/X-ray audience. Radio emission is produced by
mechanisms which are well-understood within classical physics. Bremsstrahlung tends to be dominant at low frequencies, while
gyro-resonance emission from strong magnetic fields produces bright sources at higher frequencies. At most radio frequencies
the images of the Sun are dominated almost everywhere by bremsstrahlung opacity, which may be optically thick or thin depending
on circumstances. Where gyro-resonance sources are present they may be used as sensitive probes of the regions above active
regions where magnetic field strengths exceed several hundred gauss, and this unique capability is one of the strengths of
radio observations. Typically a gyro-resonance radio source shows the temperature on an optically thick surface of constant
magnetic field within the corona. Since each radio frequency corresponds to a different magnetic field strength, the coronal
structure can be `peeled away' by using different frequencies. The peculiarities of radio observing techniques are discussed
and contrasted with EUV/X-ray techniques. Radio observations are strong at determining temperatures and coronal magnetic field
strengths while EUV/X-ray observations better sense densities and reveal coronal magnetic field lines: in this way the two
wavelength domains are nicely complementary. 相似文献
3.
Very Large Array (VLA) observations at wavelengths of 20 and 91 cm have been combined with data from the SOHO and RHESSI solar
missions to study the evolution of transequatorial loops connecting active regions on the solar surface. The radio observations
provide information about the acceleration and propagation of energetic electrons in these large-scale coronal magnetic structures
where energy release and transport take place. On one day, a long-lasting Type I noise storm at 91 cm was seen to intensify
and shift position above the northern hemisphere region following an impulsive hard X-ray burst in the southern hemisphere
footpoint region. VLA 20-cm observations as well as SOHO EIT EUV images showed evolving coronal plasma that appeared to move
across the solar equator during this time period. This suggests that the transequatorial loop acted as a conduit for energetic
particles or fields that may have triggered magnetic changes in the corona where the northern noise storm region was seen.
On another day, a hard X-ray burst detected at the limb was accompanied by impulsive 20- and 91-cm burst emission along a
loop connecting to an active region in the same hemisphere but about 5′ away, again suggesting particle propagation and remote
flare triggering across interconnecting loops. 相似文献
4.
New calculations for Si vii/Mg vii and Si ix/Mg ix theoretical line ratios as a tool for plasma density diagnostics and for the estimation of relative element abundance in the solar atmosphere are presented, making use of the EUV spectrum of an active region obtained by the Solar EUV Rocket Telescope and Spectrograph (SERTS). Theoretical line ratios, hitherto not observed, are discussed for diagnosing solar plasma from the observations obtainable with the Coronal Diagnostic Spectrometer (CDS) instrument on board the SOHO mission.In the honour of Dr P. K. Raju, Indian Institute of Astrophysics, Bangalore, on the completion of his 60 years. 相似文献
5.
We present a technique for creating a longitude-resolved image of Jupiter's thermal radio emission. The technique has been applied to VLA data taken on 25 January 1996 at a wavelength of 2 cm. A comparison with infrared data shows a good correlation between radio hot spots and the 5 μm hot spots seen on IRTF images. The brightest spot on the radio image is most likely the hot spot through which the Galileo probe entered Jupiter's atmosphere. We derived the ammonia abundance (= volume mixing ratio) in the hot spot, which is ∼3×10−5, about half that seen in longitude-averaged images of the NEB, or less than 1/3 of the longitude-averaged ammonia abundance in the EZ. This low ammonia abundance probably extends down to at least the 4 bar level. 相似文献
6.
Images taken in the band centered at 30.4 nm are routinely used to map the radiance of the He?ii Ly?α line on the solar disk. That line is one of the strongest, if not the strongest, line in the EUV observed in the solar spectrum, and one of the few lines in that wavelength range providing information on the upper chromosphere or lower transition region. However, when observing the off-limb corona, the contribution from the nearby Si?xi 30.3 nm line can become significant. In this work we aim at estimating the relative contribution of those two lines in the solar corona around the minimum of solar activity. We combine measurements from CDS taken in August 2008 with temperature and density profiles from semiempirical models of the corona to compute the radiances of the two lines, and of other representative coronal lines (e.g. Mg?x 62.5 nm, Si?xii 52.1 nm). Considering both diagnosed quantities from line ratios (temperatures and densities) and line radiances in absolute units, we obtain a good overall match between observations and models. We find that the Si?xi line dominates the He?ii line from just above the limb up to ≈?2?R ⊙ in streamers, while its contribution to narrowband imaging in the 30.4 nm band is expected to become smaller, even negligible in the corona beyond ≈?2?–?3?R ⊙, the precise value being strongly dependent on the coronal temperature profile. 相似文献
7.
We present observations of a coronal hole made with the EUV spectroheliometer of the Harvard College aboard Skylab and with high resolution (2–4) radio telescopes at Culgoora and Fleurs Australia and Bonn, West Germany. We attempt to derive the density and temperature distributions in the transition region and inner corona from the combined observations. No one standard model can explain both sets of observations; characteristically, models based on EUV data yield higher radio brightnesses than are observed, while models based on radio data yield lower EUV line intensities than are observed. The discrepancy is essentially that the electron density inferred from the EUV data is about three times that inferred from the radio data.After examining several possible modifications of the standard models we suggest that the discrepancy would disappear if the abundances of the heavier elements were increased by about a factor of 10. Such increases could result from differential diffusion in the large temperature gradient of the transition region. We conclude therefore that models which incorporate thermal diffusion, as well as mass outflow and departures from ionization equilibrium, offer the greatest hope of reconciling the EUV and radio observations of coronal holes. 相似文献
8.
A comprehensive case and statistical study of CME onsets has been conducted on the solar limb using the CDS, LASCO and EIT
instruments aboard the SOHO spacecraft. This is the first dedicated campaign to establish firmly the EUV signatures of CME
onsets and is based on a series of low-corona observing campaigns made in 2002. The event database consisted of 36 multiple
emission line sequences observed with CDS and the study builds, in particular, on studies of EUV coronal dimming which have
been associated with CME onsets. We witness a range of dimming events in EUV coronal emission line data. Shorter events, commonly
of duration < 4 hours, we find are indirectly associated with CME onsets whereas longer-duration dimmings (> 4 hours) appear
to be either due to coronal evolution or rotational effects. However, for some CME onsets, where the CDS pointing was appropriate,
no dimming was observed. Dimming observed in EIT typically occurred immediately after the launch of a loop or prominence,
and in 5 out of 9 events there is evidence of a matter buildup within the loop before launch. A total of 10 events occurred
where CDS was used to directly observe the CME footprint, but no relationship between these events was found. The results
suggest that the response of the corona to a CME launch differs between the low (1.0 R
⊙≤R≤1.2 R
⊙) and middle (1.2 R
⊙<R≤2.0 R
⊙) corona regions, hence implying a difference between dimming observations conducted with different instruments. 相似文献
9.
J. B. Zirker 《Solar physics》1970,11(1):68-81
The relative populations of levels of highly ionized Fe, Ni and Ca ions have been calculated for physical conditions appropriate to the solar corona. The results are presented in the form of tables. Line intensity ratios in the EUV and visible that are sensitive to electron density are discussed and compared with observations. 相似文献
10.
Werner M. Neupert 《Solar physics》1998,177(1-2):181-190
Using extreme ultraviolet (EUV) images of the solar corona, we have carried out a region-by-region study of the association of coronal emission of Feix–Fexvi with Caii K plage areas and intensities reported in Solar-Geophysical Data. We find that emission is dependent on the area and brightness of the plage, with specific correlations varying with the temperature of formation of the emitting coronal ion. If confirmed and extended, this approach may provide a means of estimating coronal EUV levels associated with solar activity and ultimately a proxy method that is more accurate than the sole use of the centimetric radio flux for estimating the coronal component of solar EUV emission. 相似文献
11.
《天文和天体物理学研究(英文版)》2020,(2)
During non-flaring times,the radio flux of the Sun at wavelengths of a few centimeters to several tens of centimeters mostly originates from thermal bremsstrahlung emission,very similar to extremeultraviolet(EUV) radiation.Owing to such a proximity,it is feasible to investigate the relationship between the EUV emission and radio emission in a quantitative way.In this paper,we reconstruct the radio images of the Sun through the differential emission measure obtained from multi-wavelength EUV images of the Atmospheric Imaging Assembly on board Solar Dynamics Observatory(SDO).Through comparing the synthetic radio images at 6 GHz with those observed by the Siberian Radioheliograph,we find that the predicted radio flux is qualitatively consistent with the observed value,confirming thermal origin of the coronal radio emission during non-flaring times.The results further show that the predicted radio flux is closer to the observations in the case that includes the contribution of plasma with temperatures above 3 MK than in the case of only involving low temperature plasma,as was usually done in the pre-SDO era.We also discuss applications of the method and uncertainties of the results. 相似文献
12.
A large filament was observed during a multi-wavelength coordinated campaign on June 19, 1998 in the Hα line with the Swedish Vacuum Solar Telescope (SVST) at La Palma, in the coronal lines Fe ix/x 171 Å and Fe xi 195 Å with the Transition Region and Coronal Explorer (TRACE) and in EUV lines with the SOHO/CDS spectrometer and the hydrogen Lyman series with the SOHO/SUMER spectrometer. Because of its high-latitude location, it is possible to disentangle the physical properties of the Hα filament and the filament channel seen in EUV lines. TRACE images point out a dark region fitting the Hα fine-structure threads and a dark corridor (filament channel), well extended south of the magnetic inversion line. A similar pattern is observed in the CDS EUV-line images. The opacity of the hydrogen and helium resonance continua at 171 Å is almost two orders of magnitude lower than that at the Hi head (912 Å) and thus similar to the opacity of the Hα line. Since we do not see the filament channel in Hα, this would imply that it should also be invisible in TRACE lines. Thus, the diffuse dark corridor is interpreted as due to the coronal ‘volume blocking’ by a cool plasma which extends to large altitudes. Such extensions were also confirmed by computing the heights from the projection geometry and by simulations of the CDS and TRACE line intensities using the spectroscopic model of EUV filaments (Heinzel, Anzer, and Schmieder, 2003). Finally, our NLTE analysis of selected hydrogen Lyman lines observed by SUMER also leads to a conclusion that the dark filament channel is due to a presence of relatively cool plasma having low densities and being distributed at altitudes reaching the Hα filament. 相似文献
13.
Richard Woo 《Solar physics》2007,241(2):251-261
In the absence of magnetic field measurements of the solar corona, the density structure of white-light images has provided
important insight into the coronal magnetic field. Recent work sparked by highly sensitive radio occultation measurements
of path-integrated density has elucidated the density structure of unprocessed solar eclipse pictures. This paper does the
same for processed images that reveal low-contrast small-scale structures, specifically Koutchmy’s edge-enhanced white-light
image of the 11 August 1999 solar eclipse. This processed image provides visual evidence for two important results deduced
from radio occultation measurements of small-scale density variations. First, in addition to the closed loops readily seen
at the base of the corona in high-resolution EUV and soft X-ray images, open filamentary structures permeate the corona including
active regions generally thought to be magnetically closed. Observed at the image resolution, the filamentary structures are
1° wide in latitude and an order of magnitude smaller than polar plumes. Second, although inhomogeneities that are convected
along with the solar wind are also present, filamentary structures dominate the image because of their steeper density gradients.
The quantitative profile of polarized brightness (pB) at the base of the corona shows that the filamentary structures have transverse density gradients that are proportional
to their density. This explains why edge-enhanced images, limited in sensitivity to density gradients, tend to detect filamentary
structures more readily in high-density regions (e.g., active regions, streamer stalks, and prominences) than in low-density polar coronal holes, and why filamentary structures
seem more prevalent in solar eclipse pictures during solar maximum. The pB profile at the base of the corona also fills the gap in Doppler measurements there, reinforcing that open ultra-fine-scale
filamentary structures observed by the radio measurements are predominantly radial and that they are an integral part of the
radial expansion of the solar wind. 相似文献
14.
J. Sylwester S. Kuzin Yu. D. Kotov F. Farnik F. Reale 《Journal of Astrophysics and Astronomy》2008,29(1-2):339-343
The scientific goals and construction details of a new design, Polish X-ray spectrophotometer are given. It will be incorporated within the Russian TESIS X and EUV complex aboard the forthcoming CORO-NAS solar mission. SphinX (Solar Photometer in X-rays) will use PIN silicon detectors for high time resolution (0.01 s) measurements of the solar spectra of quiet and active corona in the range 0.5–15 keV. A new filter-fluorescence target concept will be employed to allow for a fast photometry of the solar X-ray flux variations in selected, well defined narrow spectral bands including the Fe XXVI and Fe XXV iron line groups. 相似文献
15.
The abundance of Fe relative to H is obtained by using resonantly scattered intensities of λ 284 of Fe XV that were measured with OSO-7 and resonantly scattered intensities of Lα of H I that were obtained by Gabriel (1971). Because of possible differences in electron densities along lines of sight for these non-simultaneous measurements and in relative calibrations, results are rather uncertain but still indicate that the average Fe abundance relative to H in the corona appears to be at least as large as a recent photospheric abundance. Some limitations in using this method for obtaining abundances are examined for future experiments with simultaneous measurements and well calibrated detectors. 相似文献
16.
We investigate the coronal imaging capabilities of the Solar UltraViolet Imager (SUVI) on board the Geostationary Operational Environmental Satellite-R series spacecraft. Nominally Sun-pointed, SUVI provides solar images in six extreme ultraviolet (EUV) wavelengths. On-orbit data indicated that SUVI had sufficient dynamic range and sensitivity to image the corona to the largest heights above the Sun to date while simultaneously imaging the Sun. We undertook a campaign to investigate the existence of the EUV signal well beyond the nominal Sun-centered imaging area of the solar EUV imagers. We off-pointed the SUVI line of sight by almost one imaging area around the Sun. We present the details of the campaign we conducted when the solar cycle was at near the minimum and some results that confirm that EUV emission is present to beyond three solar radii.
相似文献17.
Ben-Zion Kozlovsky 《Solar physics》1968,5(3):410-416
In this paper we attempted to relate the relative abundance measurements of the solar wind at the earth's orbit to conditions in the solar corona. We followed the distribution of ionization stages of oxygen and helium by integrating the coupled rate equations outward from the corona to the earth's orbit. We concluded that the material observed in the solar wind at the earth's orbit must be a superposition of contributions from hotter and cooler regions of the corona.Supported in part by the National Science Foundation [GP-7976, formerly GP-5391] and the Office of Naval Research [Nonr-220(47)]. 相似文献
18.
Very-Large-Array (VLA) observations of the Sun at 20, 91 and 400 cm have been combined with data from the SOHO, TRACE and Wind solar missions to study the properties of long-lasting Type I noise storms and impulsive metric and decimetric bursts during solar flares and associated coronal mass ejections. These radio observations provide information about the acceleration and propagation of energetic electrons in the low and middle corona as well as their interactions with large-scale magnetic structures where energy release and transport takes place. For one flare and its associated CME, the VLA detected impulsive 20 and 91 cm bursts that were followed about ten minutes later by 400 cm burst emission that appeared to move outward into the corona. This event was also detected by the Waves experiment on Wind which showed intense, fast-drifting interplanetary Type III bursts following the metric and decimetric bursts detected by the VLA. For another event, impulsive 91 cm emission was detected about a few minutes prior to impulsive bursts at 20.7 cm, suggesting an inwardly propagating beam of electrons that excited burst emission at lower levels and shorter wavelengths. We also find evidence for significant changes in the intensity of Type I noise storms in the same or nearby active region during impulsive decimetric bursts and CMEs. These changes might be attributed to flare-initiated heating of the Type I radio source plasma by outwardly-propagating flare ejecta or to the disruption of ambient magnetic fields by the passage of a CME. 相似文献
19.
Type-I bursts (i.e. noise storms) are the earliest-known type of solar radio emission at the meter wavelength. They are believed to be excited by non-thermal energetic electrons accelerated in the corona. The underlying dynamic process and exact emission mechanism still remain unresolved. Here, with a combined analysis of extreme ultraviolet (EUV), radio and photospheric magnetic field data of unprecedented quality recorded during a type-I storm on 30 July 2011, we identify a good correlation between the radio bursts and the co-spatial EUV and magnetic activities. The EUV activities manifest themselves as three major brightening stripes above a region adjacent to a compact sunspot, while the magnetic field there presents multiple moving magnetic features (MMFs) with persistent coalescence or cancelation and a morphologically similar three-part distribution. We find that the type-I intensities are correlated with those of the EUV emissions at various wavelengths with a correlation coefficient of 0.7?–?0.8. In addition, in the region between the brightening EUV stripes and the radio sources there appear consistent dynamic motions with a series of bi-directional flows, suggesting ongoing small-scale reconnection there. Mainly based on the induced connection between the magnetic motion at the photosphere and the EUV and radio activities in the corona, we suggest that the observed type-I noise storms and the EUV brightening activities are the consequence of small-scale magnetic reconnection driven by MMFs. This is in support of the original proposal made by Bentley et al. (Solar Phys. 193, 227, 2000). 相似文献
20.
Richard F. Donnelly 《Solar physics》1971,109(1):37-58
Several progressions in the temporal characteristics of full-disk solar UV and EUV fluxes have been identified that raise many questions about the solar physics involved. The collective effect of numerous enhancements smaller than scaled plages contribute significantly to the solar cycle variations, especially for emissions from the cooler portions of the corona and the chromosphere. Active-region remnants are suggested to have a strong role even in solar-rotation induced variations late in an episode of major activity. Although cool coronal EUV emissions are long lasting, the persistence of the solar-rotation induced variations is even greater at photospheric UV wavelengths. Gyroresonance and possibly nonthermal radio emission at centimeter wavelengths are suggested to be particularly important during the first solar rotation of an episode of major activity. 相似文献