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1.
On 10 March 2001 the active region NOAA 9368 produced an unusually impulsive solar flare in close proximity to the solar limb. This flare has previously been studied in great detail, with observations classifying it as a type 1 white-light flare with a very hard spectrum in hard X-rays. The flare was also associated with a type II radio burst and coronal mass ejection. The flare emission characteristics appeared to closely correspond to previous instances of seismic emission from acoustically active flares. Using standard local helioseismic methods, we identified the seismic signatures produced by the flare that, to date, is the least energetic (in soft X-rays) of the flares known to have generated a detectable acoustic transient. Holographic analysis of the flare shows a compact acoustic source strongly correlated with the impulsive hard X-rays, visible continuum, and radio emission. Time?–?distance diagrams of the seismic waves emanating from the flare region also show faint signatures, mainly in the eastern sector of the active region. The strong spatial coincidence between the seismic source and the impulsive visible continuum emission reinforces the theory that a substantial component of the seismic emission seen is a result of sudden heating of the low photosphere associated with the observed visible continuum emission. Furthermore, the low-altitude magnetic loop structure inferred from potential-field extrapolations in the flaring region suggests that there is a significant anti-correlation between the seismicity of a flare and the height of the magnetic loops that conduct the particle beams from the corona. 相似文献
2.
Instances of seismic transients emitted into the solar interior in the impulsive phases of some solar flares offer a promising diagnostic tool, both for understanding the physics of solar flares and for the general development of local helioseismology. Among the prospective contributors to flare acoustic emission that have been considered are: i) chromospheric shocks propelled by pressure transients caused by impulsive thick-target heating of the upper and middle chromosphere by high-energy particles, ii) heating of the photosphere by continuum radiation from the chromosphere or possibly by high-energy protons, and iii) magnetic-force transients caused by magnetic reconnection. Hydrodynamic modeling of chromospheric shocks suggests that radiative losses deplete all but a small fraction of the energy initially deposited into them before they penetrate the photosphere. Comparisons between the spatial distribution of acoustic sources, derived from seismic holography of the surface signatures of flare acoustic emission, and the spatial distributions of sudden changes both in visible-light emission and in magnetic signatures offer a possible means of discriminating between contributions to flare acoustic emission from photospheric heating and magnetic-force transients. In this study we develop and test a means for estimating the seismic intensity and spatial distribution of flare acoustic emission from photospheric heating associated with visible-light emission and compare this with the helioseismic signatures of seismic emission. Similar techniques are applicable to transient magnetic signatures. 相似文献
3.
We study the effect of chromospheric bombardment by an electron beam during solar flares. Using a semi-empirical flare model, we investigate energy balance at temperature minimum level and in the upper photosphere. We show that non-thermal hydrogen ionization (i.e., due to the electrons of the beam) leads to an increase of chromospheric hydrogen continuum emission, H– population, and absorption of photospheric and chromospheric continuum radiation. So, the upper photosphere is radiatively heated by chromospheric continuum radiation produced by the beam. The effect of hydrogen ionization is an enhanced white-light emission both at chromospheric and photospheric level, due to Paschen and H– continua emission, respectively. We then obtain white-light contrasts compatible with observations, obviously showing the link between white-light flares and atmospheric bombardment by electron beams. 相似文献
4.
The Transition Region and Coronal Explorer (TRACE) instrument includes a “white light” imaging capability with novel characteristics.
Many flares with such white-light emission have been detected, and this paper provides an introductory overview of these data.
These observations have 0.5″ pixel size and use the full broad-band response of the CCD sensor; the images are not compromised
by ground-based seeing and have excellent pointing stability as well as high time resolution. The spectral response of the
TRACE white-light passband extends into the UV, so these data capture, for the first time in images, the main radiative energy
of a flare. This initial survey is based on a sample of flares observed at high time resolution for which the Reuven Ramaty
High-Energy Solar Spectroscopic Imager (RHESSI) had complete data coverage, a total of 11 events up to the end of 2004. We
characterize these events in terms of source morphology and contrast against the photosphere. We confirm the strong association
of the TRACE white-light emissions - which include UV as well as visual wavelengths – with hard X-ray sources observed by
RHESSI. The images show fine structure at the TRACE resolution limit, and often show this fine structure to be extended over
large areas rather than just in simple footpoint sources. The white-light emission shows strong intermittency both in space
and in time and commonly contains features unresolved at the TRACE resolution. We detect white-light continuum emission in
flares as weak as GOES C1.6. limited by photon statistics and background solar fluctuations, and support the conclusion of
Neidig (1989) that white-light continuum occurs in essentially all flares. 相似文献
5.
A white-light flare (WLF) on 10 March 2001 was well observed in the Hα line and the Ca ii λ8542 line using the imaging spectrograph installed on the Solar Tower Telescope of Nanjing University. Three small sunspots
appeared in the infrared continuum image. In one sunspot, the infrared continuum is enhanced by 4–6% compared to the preflare
value, making the sunspot almost disappear in the continuum image for about 3 min. A hard X-ray (HXR) source appeared near
the sunspot, the flux of which showed a good time correlation with the profile of the continuum emission. In the sunspot region,
both positive and negative magnetic flux suffered a substantial change. We propose that electron precipitation followed by
radiative back-warming may play the chief role in heating the sunspot. The temperature rise in the lower atmosphere and the
corresponding energy requirement are estimated. The results show that the energy released in a typical WLF is sufficient to
power the sunspot heating. 相似文献
6.
A solar flare with both H and Fe i 5324 emissions was observed in AR 7529 (S13, E65) on 24 June, 1993 at the Bejing Astronomical Observatory. Our calculations show that the Fe i 5324 emission region of the flare was located in the low photosphere at a height of about 180 km above 5000 = 1, which is lower than many previous studies of white-light flares. To study a Fe i 5324 flare, which represents a kind of extreme case in solar flares, would be useful for clarifying some arguments in the researches of white-light flares as well as for understanding the mechanism of solar flares.The synthetic analyses from vairous features of the flare lead to the following possible exciting mechanism of the Fe i 5324 flare: owing to the flow of energetic electrons from the corona and probably also the thermal conduction downward into the lower atmosphere, a condensation with a temperature higher than that below it was formed near the transition region. Then the low photosphere was heated through backwarming. The Fe i 5324 flare occurred as an indicator of the excitation in the low photosphere. 相似文献
7.
H. S. Hudson 《Solar physics》1972,24(2):414-428
Observations indicate that fast electrons in solar flares, which cause the hard X-ray burst and the impulsive microwave burst, lose energy predominantly by collisional processes. This requires a thick-target theory of the emission, for which the electron spectrum inferred from the X-ray spectrum becomes 1.5 powers steeper than in the usual thin-target theory.The low-energy end of this spectrum contains enough energy above about 5 keV to supply the white-light continuum emission occasionally observed in major flares. The penetration of the nonthermal electrons creates long-lived excess ionization which enhances the free-free and free-bound continuum in the heated medium. The emission will occur high above the photosphere at small optical depth in the visible continuum. Thus its spectrum will extend into the infrared and ultraviolet. 相似文献
8.
The underlying physics that generates the excitations in the global low-frequency (<?5.3?mHz) solar acoustic power spectrum is a well-known process that is attributed to solar convection; however, a definitive explanation as to what causes excitations in the high-frequency regime (>?5.3?mHz) has yet to be found. Karoff and Kjeldsen (Astrophys. J. 678, 73??C?76, 2008) concluded that there is a correlation between solar flares and the global high-frequency solar acoustic waves. We have used Global Oscillation Network Group (GONG) helioseismic data in an attempt to verify the Karoff and Kjeldsen (2008) results as well as compare the post-flare acoustic power spectrum to the pre-flare acoustic power spectrum for 31 solar flares. Among the 31 flares analyzed, we observe that a decrease in acoustic power after the solar flare is just as likely as an increase. Furthermore, while we do observe variations in acoustic power that are most likely associated with the usual p-modes associated with solar convection, these variations do not show any significant temporal association with flares. We find no evidence that consistently supports flare-driven high-frequency waves. 相似文献
9.
Recent gamma-ray observations of solar flares have provided a better means for estimating the heating of the solar atmosphere by energetic protons. Such heating has been suggested as the explanation of the continuum emission of the white-light flare. We have analyzed the effects on the photosphere of high-energy particles capable of producing the intense gamma-ray emission observed in the 1978 July 11 flare. Using a simple energy-balance argument and taking into account hydrogen ionization, we have obtained the following conclusions:
- Heating near τ5000 = 1 in the input HSRA model atmosphere is negligible, even for very high fluxes of energetic particles.
- Energy deposition increases with height for the inferred proton spectra, and does not depend strongly upon the assumed angle of incidence. The computed energy inputs fall in the range 10–100 ergs (cm3 s)?1 at the top of the photosphere.
- H? continuum dominates for column densities as small as 1022 cm?3, but at greater heights hydrogen ionizes sufficiently for the higher continua to dominate the energy balance.
- The total energy deposited in the ‘photospheric’ region of H? dominance could be within a factor of 3 of the necessary energy deposition, by comparison with the white-light flare of 1972 August 7, but the emergent spectrum is quite red so that the intensity excess in the visible band is insufficient to explain the observations.
10.
We consider potential sources of infrared (1 to 1 mm) continuum in solar flares. Several mechanisms should produce detectable fluxes: in the 350 window for ground-based observations, impulsive emission will arise in synchrotron radiation from 1–10 MeV electrons, and possibly thermal (free-free) continuum from the source of the white-light flare; the hot flare plasma responsible for soft X-ray emission will also emit detectable fluxes of free-free continuum in the largest flares. At shorter wavelengths the dominant infrared emission will come from the H flare itself. Observations in the infrared wavelengths will help to complete our picture of flare structure in both the impulsive and gradual phases. 相似文献
11.
Homogeneous plane-parallel model atmospheres for solar flares have been constructed to approximately simulate observations of flares. The wings of the Ca II lines have been used to derive flare upper photosphere models, which indicate temperature increases of ～100 K over the temperature distribution in the pre-existing facula at a height of 300 km above τ5000 = 1. In the case of flares covering sunspots the temperature rise seems to occur much higher in the atmosphere. We solve the transfer and statistical equilibrium equations for a three-level hydrogen atom and a five-level calcium atom in order to obtain the chromospheric flare models. The general properties of flares, including n e, N 2, linear thickness, and Lyman continuum intensity are approximately reproduced. We find that with increasing flare importance the height of the upper chromosphere and transition region occur lower in the solar atmosphere, accounting for the factor of 60–600 increase in pressure in these regions relative to the quiet Sun. The Ca II line profiles agree with observations only by assuming a macro-velocity distribution that increases with height. Also the chromospheric parts of flares appear to be highly inhomogeneous. We show that shock and particle heated flare models do not agree with the observations and propose a thermal response model for flares. In particular, it appears that heating in the photosphere is an essential aspect of flares. 相似文献
12.
A. F. Kowalski S. L. Hawley J. A. Holtzman J. P. Wisniewski E. J. Hilton 《Solar physics》2012,277(1):21-29
The white light during M dwarf flares has long been known to exhibit the broadband shape of a T≈10 000 K blackbody, and the white light in solar-flares is thought to arise primarily from hydrogen recombination. Yet, a
current lack of broad-wavelength coverage solar flare spectra in the optical/near-UV region prohibits a direct comparison
of the continuum properties to determine if they are indeed so different. New spectroscopic observations of a secondary flare
during the decay of a megaflare on the dM4.5e star YZ CMi have revealed multiple components in the white-light continuum of
stellar flares, including both a blackbody-like spectrum and a hydrogen-recombination spectrum. One of the most surprising
findings is that these two components are anti-correlated in their temporal evolution. We combine initial phenomenological
modeling of the continuum components with spectra from radiative hydrodynamic models to show that continuum veiling causes
the measured anti-correlation. This modeling allows us to use the components’ inferred properties to predict how a similar
spatially resolved, multiple-component, white-light continuum might appear using analogies to several solar-flare phenomena.
We also compare the properties of the optical stellar flare white light to Ellerman bombs on the Sun. 相似文献
13.
We analyze particle acceleration processes in large solar flares, using observations of the August, 1972, series of large events. The energetic particle populations are estimated from the hard X-ray and γ-ray emission, and from direct interplanetary particle observations. The collisional energy losses of these particles are computed as a function of height, assuming that the particles are accelerated high in the solar atmosphere and then precipitate down into denser layers. We compare the computed energy input with the flare energy output in radiation, heating, and mass ejection, and find for large proton event flares that:
- The ～10–102 keV electrons accelerated during the flash phase constitute the bulk of the total flare energy.
- The flare can be divided into two regions depending on whether the electron energy input goes into radiation or explosive heating. The computed energy input to the radiative quasi-equilibrium region agrees with the observed flare energy output in optical, UV, and EUV radiation.
- The electron energy input to the explosive heating region can produce evaporation of the upper chromosphere needed to form the soft X-ray flare plasma.
- Very intense energetic electron fluxes can provide the energy and mass for interplanetary shock wave by heating the atmospheric gas to energies sufficient to escape the solar gravitational and magnetic fields. The threshold for shock formation appears to be ～1031 ergs total energy in >20 keV electrons, and all of the shock energy can be supplied by electrons if their spectrum extends down to 5–10 keV.
- High energy protons are accelerated later than the 10–102 keV electrons and most of them escape to the interplanetary medium. The energetic protons are not a significant contributor to the energization of flare phenomena. The observations are consistent with shock-wave acceleration of the protons and other nuclei, and also of electrons to relativistic energies.
- The flare white-light continuum emission is consistent with a model of free-bound transitions in a plasma with strong non-thermal ionization produced in the lower solar chromosphere by energetic electrons. The white-light continuum is inconsistent with models of photospheric heating by the energetic particles. A threshold energy of ～5×1030 ergs in >20 keV electrons is required for detectable white-light emission.
14.
Donald C. Norquist 《Solar physics》2011,269(1):111-127
Designing a statistical solar flare forecasting technique can benefit greatly from knowledge of the flare frequency of occurrence
with respect to sunspot groups. This study analyzed sunspot groups and Hα and X-ray flares reported for the period 1997 – 2007.
Annual catalogs were constructed, listing the days that numbered sunspot groups were observed (designated sunspot group-days,
SSG-Ds) and for each day a record for each associated Hα flare of importance category one or greater and normal or bright
brightness and for each X-ray flare of intensity C 5 or higher. The catalogs were then analyzed to produce frequency distributions
of SSG-Ds by year, sunspot group class, likelihood of producing at least one flare overall and by sunspot group class, and
frequency of occurrence of numbers of flares per day and flare intensity category. Only 3% of SSG-Ds produced a substantial
Hα flare and 7% had a significant X-ray flare. We found that mature, complex sunspot groups were more likely than simple sunspot
groups to produce a flare, but the latter were more prevalent than the former. More than half of the SSG-Ds with flares had
a maximum intensity flare greater than the lowest category (C-class of intensity five and higher). The fact that certain sunspot
group classes had flaring probabilities significantly higher than the combined probabilities of the intensity categories when
all SSG-Ds were considered suggest that it might be best to first predict the flaring probability. For sunspot groups found
likely to flare, a separate diagnosis of maximum flare intensity category appears feasible. 相似文献
15.
We examine empirical atmospheric structures that are consistent with enhanced white-light continuum emission in solar flares. This continuum can be produced either by hydrogen bound-free emission in an enhanced region in the upper chromosphere, or by H- emission in an enhanced region around the temperature minimum. In the former case, weak Paschen jumps in the spectrum will be present, with the spectrum being dominated by a strong Balmer continuum, while in the latter case the spectrum exhibits a weaker, flat enhancement over the entire visible spectrum.We find that when proper account is taken of radiative backwarming processes, the two enhanced atmospheric regions above are not independent, in that irradiation by Balmer continuum photons from the upper chromosphere creates sufficient heating around the temperature minimum to account for the temperature enhancements there. Thus the problem of main phase white-light flare production reduces to one of creating temperature enhancements of order 104 K in the upper chromosphere; radiative backwarming then naturally accounts for the enhancements of order 100 K around the temperature minimum.Heating by electron and proton bombardment, and by XUV irradiation from above, are then considered as candidates for creating the necessary enhancements in the upper chromosphere. We find that electron bombardment can be ruled out, whereas bombardment by protons in the few-MeV energy range is a viable candidate, but one without strong observational support. The XUV irradiation hypothesis is examined by incorporating it self-consistently into the PANDORA radiative transfer algorithm used to construct the empirical model atmospheres; we find that the introduction of XUV radiation, with flux and spectrum appropriate to white-light flare events, does indeed produce sufficient radiative heating in the upper chromosphere to balance the radiative losses associated with the required temperature enhancements.In summary, we find that the radiative coupling of (i) the upper chromosphere and temperature minimum regions (through Balmer continuum photons) and (ii) the transition region and upper chromosphere (through XUV photons) can account for white-light emission in solar flares.Presidential Young Investigator. 相似文献
16.
Hai-Min Wang 《中国天文和天体物理学报》2009,9(2)
White-light flares are considered to be the most energetic flaring events that are observable in the optical broad-band continuum of the solar spectrum. They have not been commonly observed. Observations of white-light flares with sub-arcsecond resolution have been very rare. The continuous high resolution observations of Hinode provide a unique opportunity to systematically study the white-light flares with a spatial resolution around 0.2 arcsec. We surveyed all the flares above GOES magnitude C5.0 since the launch of Hinode in 2006 October. 13 of these kinds of flares were covered by the Hinode G-band observations. We analyzed the peak contrasts and equivalent areas (calculated via integrated excess emission contrast) of these flares as a function of the GOES X-ray flux, and found that the cut-off visibility is likely around M1 flares under the observing limit of Hinode. Many other observational and physical factors should affect the visibility of white-light flares; as the observing conditions are improved, smaller flares are likely to have detectable white-light emissions. We are cautious that this limiting visibility is an overestimate, because G-band observations contain emissions from the upper atmosphere.Among the 13 events analyzed, only the M8.7 flare of 2007 June 4 had near-simultaneous observations in both the G-band and the blue continuum. The blue continuum had a peak contrast of 94% vs. 175% in G-band for this event. The equivalent area in the blue continuum is an order of magnitude lower than that in the G-band. Very recently, Jess et al.studied a C2.0 flare with a peak contrast of 300% in the blue continuum. Compared to the events presented in this letter, that event is probably an unusual white-light flare: a very small kernel with a large contrast that can be detected in high resolution observations. 相似文献
17.
We derive an occurrence frequency for white-light flares (WLF) of 15.5 ± 4.5 yr?1 during a 2.6 year period following the maximum of solar cycle 21. This compares with a frequency 5–6 yr?1 derived by McIntosh and Donnelly (1972) during solar cycle 20. We find that the higher frequency of the more recently observed WLFs is due to the availability of patrol data at shorter wavelengths (λ ? 4000 Å), where the contrast of the flare emission is increased; the improved contrast has allowed less energetic (and hence more frequently occurring) events to be classified as WLFs. We find that sufficient conditions for the occurrence of a WLF are: active region magnetic class = delta; sunspot penumbra class = K, with spot group area ≥ 500 millionths of the solar hemisphere; 1–8 Å X-ray burst class ≥ X2. 相似文献
18.
Rapidly moving transient features have been detected in magnetic and Doppler images of super-active region NOAA 10486 during the X17/4B flare of 28 October 2003 and the X10/2B flare of 29 October 2003. Both these flares were extremely energetic white-light events. The transient features appeared during impulsive phases of the flares and moved with speeds ranging from 30 to 50 km?s?1. These features were located near the previously reported compact acoustic (Donea and Lindsey, Astrophys. J. 630, 1168, 2005) and seismic sources (Zharkova and Zharkov, Astrophys. J. 664, 573, 2007). We examine the origin of these features and their relationship with various aspects of the flares, viz., hard X-ray emission sources and flare kernels observed at different layers: i) photosphere (white-light continuum), ii) chromosphere (Hα 6563 Å), iii) temperature minimum region (UV 1600 Å), and iv) transition region (UV 284 Å). 相似文献
19.
Observational properties of two white-light flares (WLFs), on June 15, 1991, and June 26, 1999, are presented and compared. This is of particular interest, because the former was one of the most intense flares of X-ray class X12, while the latter was a compact flare of class M2.3. Significant differences between some flare parameters (GOES class, Hα classification, the number of WLF kernels and their location in the sunspot group, the size and duration of the WLF emission, and the peak flux density of the microwave emission) have been found. However, both these events had approximately the same powers of the emission per unit area in continuum near 658.0 nm: E = 1.5 × 107 and 1.1. × 107 erg cm?2 s?1 nm?1. There is generally a good temporal coincidence between the microwave and hard X-ray emissions and the WLF emission during the impulsive phase, but the light curve of the WLF emission on June 26, 1999, shows a stronger correlation with the X-ray emission in the energy range 14–23 keV. Both flares can be classified by their spectral characteristics as type I white-light flares. 相似文献
20.
Horizontal motion has been studied of the matter along the active region at different heights of the photosphere (115–580
km) in the initial phase of the two-ribbon solar flare on September 4, 1990, near the solar limb, accompanied by the ejection.
Photospheric velocities varied in the range −3.5 ... 2.5 km/s. The direction of motion in the photosphere and the chromosphere
was mainly toward the observer. Kinematic elements have been discovered in the structure of the horizontal velocity field.
Their size reduced as they approached the maximum of the flare from 7–12 to 4–5 Mm, and the velocity amplitude decreased.
Throughout the whole investigated active region, vortex motions were observed in the photosphere and chromosphere. Temporal
changes in the horizontal velocity field in node areas and in their vicinity were oscillatory in nature and occurred almost
simultaneously along the entire height of the photosphere. 相似文献