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1.
We discuss Yohkoh SXT observations of stationary giant post-flare arches which occurred on 3–6 May, 1992 and study in detail the last arch, associated with the flare at 19:02 UT on 5 May, which extended above the west limb. The arch was similar to the first giant arch discovered on board the SMM, on 21–22 May, 1980. We demonstrate that the long lifetimes of these structures necessarily imply additional energy input from the underlying active region: otherwise, conduction would cool these arches in less than one hour and even with the unlikely assumption of conduction inhibited, pure radiative cooling would not produce the temperature decrease observed. All arch tops, although varying in brightness, stayed for several days at a fairly constant altitude of 100 000 km, and the arch studied, on 5–6 May, was just a new brightening of the pre-existing decaying structure. The brightening was apparently due to inflow of hot plasma from the flare region. Yohkoh data confirm that these stationary arches are rare phenomena when compared with the rising arches studied in Paper I and with Uchida et al.'s expanding active regions. 相似文献
2.
We revisit the flare that occurred on 13 January 1992, which is now universally termed the “Masuda flare”. The new analysis
is motivated not just by its uniqueness despite the increasing number of coronal observations in hard X-rays, but also by
the improvement of Yohkoh hard X-ray image processing, which was achieved after the intensive investigations on this celebrated event. Using an uncertainty
analysis, we show that the hard X-ray coronal source is located closer to the soft X-ray loop by about 5000 km (or 7 arcsec)
in the re-calibrated Hard X-ray Telescope (HXT) images than in the original ones. Specifically, the centroid of the M1-band
(23 – 33 keV) coronal source is above the maximum brightness of the Soft X-ray Telescope (SXT) loop by 5000±1000 km (9600
km in the original data) and above the apex of the SXT loop represented by the 30% brightness contour by 2000±1000 km (∼ 7000 km
in the original data). The change is obviously significant, because most coronal sources are above the thermal loop by less
than 6 arcsec. We suggest that this change may account for the discrepancy in the literature, i.e., the spectrum of the coronal emission was reported to be extremely hard below ∼ 20 keV in the pre-calibration investigations,
whereas it was reported to be considerably softer in the literature after the re-calibration done by Sato, Kosugi, and Makishima
(Pub. Astron. Soc. Japan
51, 127, 1999). Still, the coronal spectrum is flatter at lower energies than at higher energies, due to the lack of a similar, co-spatial
source in the L-band (14 – 23 keV), for which a convincing explanation is absent. 相似文献
3.
Kenneth H. Schatten 《Solar physics》1970,12(3):484-491
The Faraday rotation of a radio source (Pioneer 6) occulted by the solar corona has been measured by Levy et al. (1969).
During the course of these measurements, three large-scale transient phenomena were observed. These events were preceded by
subflares and class 1 flares. These transient events are interpreted as evidence for a coronal magnetic bottle at 10 R
⊙. The velocity of propagation for the disturbance is set at 200 km/sec; the dimension of the region, 10 R
⊙; field strength at 10 R
⊙, 0.02 G; particle density, 2.0 × 104/cm3; Alfvén speed, 320 km/sec. From the nature of the observations and the lack of related effects from similar flares on the
interplanetary sector pattern observed at 1 AU, it is suggested that such coronal magnetic bottles expand to perhaps 10–30
R
⊙ and then contract to a few solar radii. Such a phenomena is evidence for an expansion of the corona with a sub-Alfvénic velocity.
It is further suggested that such magnetic bottles may be important in the storage and diffusion of solar generated cosmic
ray particles.
NAS-NRC Postdoctoral Resident Research Associate. 相似文献
4.
On arch-filament systems in spotgroups 总被引：1，自引：0，他引：1
A. Bruzek 《Solar physics》1967,2(4):451-461
Systems of arch-shaped filaments (AFS) occurring in the interspot region of young bipolar groups are studied. Their main characteristics are: Average length: 30000km, average width 20000km, width of individual filaments 1000–3000 km, height of arches 4–15000 km. A typical lifetime of the filaments 30 min; appreciable changes of the system occur within several hours; the lifetime of a system is about three days. - The arch-filament systems bridge the neutral line and connect the regions of the innermost spots of opposite polarity. Material moves along the filaments (v 25–50 km/sec) following the direction of the magnetic field, and sometimes arches are observed rising at a rate of 20 km/sec. They are very dark on the inner disk and appear either in emission or in absorption close to the solar limb. - The occurrence of bright points (moustaches) is found to be closely associated with AFS in young spotgroups. - The possible nature of AFS and their relation to other types of filamentary structures is discussed. 相似文献
5.
Sara F. Martin 《Solar physics》1989,121(1-2):215-238
Mass motions are a principal means by which components of solar flares can be distinguished. Typical patterns of mass motions in H are described for chromospheric flare ribbons, remote chromospheric flare patches, flare loops, flaring arches, surges, erupting filaments and some expanding coronal features. Interrelationships between these phenomena are discussed and illustrations of each are presented. 相似文献
6.
We describe the radio signatures that led up to and concluded the solar eruptive event of 14 July 2000 (Bastille Day Event).
These radio signatures provide a means of remotely sensing the associated solar activity and transient phenomena. For many
days prior to the Bastille Day Event kilometric Type III radio storm emissions were observed that were presumably associated
with the active region NOAA 9077. These storm emissions continued until the X5.7 flare at ∼ 10 UT on 14 July 2000 that characterized
the Bastille Day Event, then ceased abruptly. The Bastille Day Event itself produced very intense, complex, long-duration
Type III-like radio emissions, which appear to have been associated with electrons generated (accelerated) deep in the solar
corona. The coronal mass ejection (CME) associated with the Bastille Day Event generated decametric to kilometric Type II
radio emissions as the CME propagated through the solar corona and interplanetary medium. The frequency drift of these Type II
radio emissions are related to the dynamics of the propagating CME and indicate that the CME experienced significant deceleration
as it propagated from the high corona into the interplanetary medium. 相似文献
7.
We analyze hard and soft X-ray, microwave and meter wave radio, interplanetary particle, and optical data for the complex energetic solar event of 22 July 1972. The flare responsible for the observed phenomena most likely occurred 20° beyond the NW limb of the Sun, corresponding to an occultation height of 45 000 km. A group of type III radio bursts at meter wavelengths appeared to mark the impulsive phase of the flare, but no impulsive hard X-ray or microwave burst was observed. These impulsive-phase phenomena were apparently occulted by the solar disk as was the soft X-ray source that invariably accompanies an H flare. Nevertheless essentially all of the characteristic phenomena associated with second-stage acceleration in flares - type II radio burst, gradual second stage hard X-ray burst, meter wave flare continuum (FC II), extended microwave continuum, energetic electrons and ions in the interplanetary medium - were observed. The spectrum of the escaping electrons observed near Earth was approximately the same as that of the solar population and extended to well above 1 MeV.Our analysis of the data leads to the following results: (1) All characteristics are consistent with a hard X-ray source density n
i
108 cm–3 and magnetic field strength 10 G. (2) The second-stage acceleration was a physically distinct phenomenon which occurred for tens of minutes following the impulsive phase. (3) The acceleration occurred continuously throughout the event and was spatially widespread. (4) The accelerating agent was very likely the shock wave associated with the type II burst. (5) The emission mechanism for the meter-wave flare continuum source may have been plasma-wave conversion, rather than gyrosynchrotron emission. 相似文献
8.
P. Janardhan 《Journal of Astrophysics and Astronomy》2006,27(2-3):201-207
At the Sun-Earth distance of one astronomical unit (1 AU), the solar wind is known to be strongly supersonic and super Alfvenic
with Mach and Alfven numbers being on average 12 and 9 respectively. Also, solar wind densities (average ∼10cm-3) and velocities (average ∼450kms-1) at 1AU, are known to be inversely correlated with low velocities having higher than average densities andvice versa. However, on May 11 and 12 1999 the Earth was engulfed by an unusually low density (< 0.1cm-3) and low velocity (< 350km s-1) solar wind with an Alfven Mach number significantly less than 1. This was a unique low-velocity, low-density, sub-Alfvénic
solar wind flow which spacecraft observations have shown lasted more than 24 hours. One consequence of this extremely tenuous
solar wind was a spectacular expansion of the Earth’s magnetosphere and bow shock. The expanding bow shock was observed by
several spacecraft and reached record upstream distances of nearly 60 Earth radii, the lunar orbit. The event was so dramatic
that it has come to be known asthe solar wind disappearance event. Though extensive studies of this event were made by many authors in the past, it has only been recently shown that the unusual
solar wind flows characterizing this event originated from a small coronal hole in the vicinity of a large active region on
the Sun. These recent results have put to rest speculation that such events are associated with global phenomenon like the
periodic solar polar field reversal that occurs at the maximum of each solar cycle. In this paper we revisit the 11 May 1999
event, look at other disappearance events that have ocurred in the past, examine the reasons why speculations about the association
of such events with global phenomena like solar polar field reversals were made and also examine the role of transient coronal
holes as a possible solar source for such events. 相似文献
9.
A detailed analysis of characteristics of coronal mass ejections and flares associated with deca-hectometer wavelength type-II radio bursts (DH-CMEs and DH-flares) observed in the period 1997–2008 is presented. A sample of 62 limb events is divided into two populations known as after-flare CMEs (AF-CMEs) and before-flare CMEs (BF-CMEs) based on the relative timing of the flare and CME onsets. On average, AF-CMEs (1589 km s−1) have more speed than the BF-CMEs (1226 km s−1) and the difference between mean values are highly significant (P∼2%). The average CME nose height at the time of type-II start is at larger distance for AF-CMEs than the BF-CMEs (4.89 and 3.84 R o, respectively). We found a good anti-correlation for accelerating (R a=−0.89) and decelerating (R d=−0.78) AF-CMEs. In the case of decelerating BF-CMEs, the correlation seems to be similar to that for decelerating AF-CMEs (R d=−0.83). The number of decelerating AF-CMEs is 51% only; where as, the number of decelerating BF-CMEs is 83%. The flares associated with BF-CMEs have shorter rise and decay times than flares related to AF-CMEs. We found statistically significant differences between the two sets of associated DH-type-II bursts characteristics: starting frequency (P∼4%), drift rate (P∼1%), and ending frequency (P∼6%). The delay time analysis of DH-type-II start and flare onset times shows that the time lags are longer in AF-CME events than in BF-CME events (P≪1%). From the above results, the AF-CMEs which are associated with DH-type-II bursts are found to be more energetic, associated with long duration flares and DH-type-IIs of lower ending frequencies. 相似文献
10.
Recent observations with EUV imaging instruments such as SOHO/EIT and TRACE have shown evidence for flare-like processes at
the bottom end of the energy scale, in the range of E
th≈1024–1027 erg. Here we compare these EUV nanoflares with soft X-ray microflares and hard X-ray flares across the entire energy range.
From the observations we establish empirical scaling laws for the flare loop length, L(T)∼T, the electron density, n
e(T)∼T
2, from which we derive scaling laws for the loop pressure, p(T)∼T
3, and the thermal energy, E
th∼T
6. Extrapolating these scaling laws into the picoflare regime we find that the pressure conditions in the chromosphere constrain a height level for flare loop footpoints, which scales
with h
eq(T)∼T
−0.5. Based on this chromospheric pressure limit we predict a lower cutoff of flare loop sizes at L
∖min≲5 Mm and flare energies E
∖min≲1024 erg. We show evidence for such a rollover in the flare energy size distribution from recent TRACE EUV data. Based on this
energy cutoff imposed by the chromospheric boundary condition we find that the energy content of the heated plasma observed
in EUV, SXR, and HXR flares is insufficient (by 2–3 orders of magnitude) to account for coronal heating. 相似文献
11.
Susan M. P. McKenna-Lawlor 《Solar physics》2011,272(2):257-299
During the International Geophysical Year (IGY, 1957/1958) Dunsink Observatory near Dublin in Ireland was a World Data Centre
for Solar Activity. In this circumstance, Hα Lyot Heliograph records secured on a daily basis between 07:00 – 14:00 UT at
the Cape of Good Hope (then an integral link in a network of similar instruments contributing during the IGY to global monitoring
of solar chromospheric activity) were routinely sent to Dunsink for analysis and dissemination. The investigations carried
out at Dunsink on these data resulted, inter alia, in the discovery of the Flare Nimbus phenomenon. The nimbus comprises a dark expanding halo seen in the plage regions around
major flares at, or within a few minutes of, the time of flare maximum intensity in Hα light. It reaches its greatest extent
about 30 minutes after flare maximum. Its maximum dimensions (estimated visually) lie in the range 2 – 4×105 km and its duration ranges from ∼ 1 – 2 hours. Within the nimbus the striation pattern is either completely destroyed or
loses its pre-flare configuration. An account of this phenomenon and its interpretation appeared primarily, although not exclusively,
in the locally produced Dunsink Observatory Publications which are not now easily accessible to the world community of solar researchers. Also, at around the time when the nimbus
was first identified and recorded in Lyot Heliograph data at several observatories, techniques in solar physics shifted towards
high resolution narrow field observations. Under these conditions no further examples of the nimbus were recorded and the
subject has remained dormant over several decades. The present paper again places the scientific results obtained with regard
to the nimbus in the public domain, together with an account of the evolution within the scientific community of an explanation
of this phenomenon. It is suggested here for the first time, in the light of present day data concerning coronal mass ejections
(CMEs) and coronal dimming, that the nimbus provides a signature of CME-related reorganization of the magnetic field in the
chromosphere (such that the transverse magnetic field component decreases and transforms into the line of sight component
as the vector field stretches out). Coronal dimming provides a complementary signature of CME-related mass depletion in the
corona. 相似文献
12.
Energetic Particle Fluxes during the Bastille Day Solar Eruption 总被引：2，自引：0，他引：2
We report on our observations of solar energetic particle fluxes of p, He, C, O, Ne, Mg, Si, and Fe ions measured by the Energetic and Relativistic Nucleon and Electron (ERNE) experiment associated
with the Bastille Day solar flare and coronal mass ejection (CME) on 14 July 2000. We observed two clear maxima of the Fe/O
ratio at the energies 8.5–15 MeV nucl−1. The first Fe/O maximum occurred ∼ 3 hours after the beginning of the particle event, and the second maximum ∼ 22 hours after
the first one at the arrival of the shock associated with the Bastille Day eruption. We also observed a change in the energy
spectrum of oxygen concurrent with a change in the direction of the interplanetary magnetic field at the start of the second
enhancement of the Fe/O ratio. We propose an interpretation of the particle event where observed interplanetary particle fluxes
are associated with two different particle sources near the Sun and in interplanetary space. We suggest that heavy ions observed
during the first period of the Fe/O enhancement were released when a coronal shock reached a magnetic foot point connected
to 1 AU. The second maximum of Fe/O occurred when spacecraft encountered Fe-rich material stored in magnetic field flux tubes
early in the event and was possibly reaccelerated by the interplanetary shock. 相似文献
13.
The `ribbons' of two-ribbon flares show complicated patterns reflecting the linkages of coronal magnetic field lines through
the lower solar atmosphere. We describe the morphology of the EUV ribbons of the July 14, 2000 flare, as seen in SOHO, TRACE,
and Yohkoh data, from this point of view. A successful co-alignment of the TRACE, SOHO/MDI and Yohkoh/HXT data has allowed us to locate the EUV ribbon positions on the underlying field to within ∼ 2′′, and thus to investigate
the relationship between the ribbons and the field, and also the sites of electron precipitation. We have also made a determination
of the longitudinal magnetic flux involved in the flare reconnection event, an important parameter in flare energetic considerations.
There are several respects in which the observations differ from what would be expected in the commonly-adopted models for
flares. Firstly, the flare ribbons differ in fine structure from the (line-of-sight) magnetic field patterns underlying them,
apparently propagating through regions of very weak and probably mixed polarity. Secondly, the ribbons split or bifurcate.
Thirdly, the amount of line-of-sight flux passed over by the ribbons in the negative and positive fields is not equal. Fourthly,
the strongest hard X-ray sources are observed to originate in stronger field regions. Based on a comparison between HXT and
EUV time-profiles we suggest that emission in the EUV ribbons is caused by electron bombardment of the lower atmosphere, supporting
the hypothesis that flare ribbons map out the chromospheric footpoints of magnetic field lines newly linked by reconnection.
We describe the interpretation of our observations within the standard model, and the implications for the distribution of
magnetic fields in this active region. 相似文献
14.
The flare plasma temperature calculated from GOES-11 (1.5–12.4 and 3.1–24.8 keV) data is compared with the solar nonthermal
fluxes in various energy ranges in the December 6, 2006 event. Particle acceleration and plasma heating episodes took place
in the pre-flare and impulsive phases; a hard (ACS SPI > 150 keV) X-ray emission was observed 5 min before the onset of the GOES X-ray flare and was not accompanied by a temperature
rise. A close correlation has been found between the flare plasma temperature and the hard X-ray intensity. The temperature
delayed by 0.4 min turned out to be directly proportional to the logarithm of the ACS SPI count rate within the first 3 min
of the impulsive phase. This shows that the accelerated electrons responsible for the X-ray emission were the main plasma
heating source in the pre-flare and impulsive phases. The correlation between the temperature and the hard X-ray intensity
disappears after the observation of a resonance peak at a frequency of 245 MHz. Significant electron fluxes may no longer
be able to effectively heat the expanding plasma when its density in the interaction region reaches ∼109 cm−3. The observations of the July 23, 2002 and December 5, 2006 events confirm the trends found. 相似文献
15.
D. J. Mullan 《Solar physics》1977,54(1):183-206
Short-lived increases in the brightness of many red dwarfs have been observed for the last 30 yr, and a variety of more or less exotic models have been proposed to account for such flares. Information about flares in the Sun has progressed greatly in recent years as a result of spacecraft experiments, and properties of coronal flare plasma are becoming increasingly better known. In this paper, after briefly reviewing optical, radio and X-ray observations of stellar flares, we show how a simplified model which describes conductive plus radiative cooling of the coronal flare plasma in solar flares has been modified to apply to optical and X-ray stellar flare phenomena. This model reproduces many characteristic features of stellar flares, including the mean UBV colors of flare light, the direction of flare decay in the two-color diagram, precursors, Stillstands, secondary maxima, lack of sensitivity of flare color to flare amplitude, low flux of flare X-rays, distinction between so-called spike flares and slow flares, Balmer jumps of as much as 6–8, and emission line redshifts up to 3000 km s–1. In all probability, therefore, stellar flares involve physical processes which are no more exotic (and no less!) than those in solar flares. Advantages of observing stellar flares include the possibilities of (i) applying optical diagnostics to coronal flare plasma, whereas this is almost impossible in the Sun, and (ii) testing solar flare models in environments which are not generally accessible in the solar atmosphere. 相似文献
16.
The spatial and temporal evolution of the high temperature plasma in the flare of 1973 June 15 has been studied using the flare images photographed by the NRL XUV spectroheliograph on Skylab.The overall event involves the successive activations of a number of different loops and arches bridging the magnetic neutral line. The spatial shifts and brightenings observed in the Fe xxiii–xxiv lines are interpreted as the activation of new structures. These continued for four or five minutes after the end of the microwave burst phase, implying additional energy-release unrelated to the nonthermal phase of the flare. A shear component observed in the coronal magnetic field may be a factor in the storage and release of the flare energy.The observed Fe xxiii–xxiv intensities define a post-burst heating phase during which the temperature remained approximately constant at 13 × 106 K while the Fe xxiv intensity and 0–3 Å flux rose to peak values. This phase coincided with the activation of the densest structure (N
e
= 2 × 1011 cm–3). Heating of higher loops continued into the decay phase, even as the overall temperature and flux declined with the fading of the lower Fe xxiv arches.The observed morphology of individual flaring arches is consistent with the idea of energy release at altitude in the arch (coincident with a bright, energetic core in the Fe xxiv image) and energy flow downward into the ribbons. The Doppler velocity of the Fe xxi 1354 Å line is less than 5 km s–1, indicating that the hot plasma region is stationary.The relation of this flare to the larger class of flares associated with filament eruptions and emerging magnetic flux is discussed. 相似文献
17.
GianLuca Israel 《Astrophysics and Space Science》2007,308(1-4):25-31
On 27th December 2004 SGR 1806–20, one of the most active Soft γ-ray Repeaters (SGRs), displayed an extremely rare event, also known as giant flare, during which up to 1047 ergs were released in the ∼1–1000 keV range in less than 1 s. Before and after the giant flare we carried out IR observations
by using adaptive optics (NAOS-CONICA) mounted on VLT which provided images of unprecedented quality (FWHM better than 0.1″).
We discovered the likely IR counterpart to SGR 1806–20 based on positional coincidence with the VLA uncertainty region and
flux variability of a factor of about 2 correlated with that at higher energies.
Moreover, by analysing the Rossi-XTE/PCA data we have discovered rapid Quasi-Periodic Oscillations (QPOs) in the pulsating
tail of the 27th December 2004 giant flare of SGR 1806–20. QPOs at ∼92.5 Hz are detected in a 50 s interval starting 170 s
after the onset of the giant flare. These QPOs appear to be associated with increased emission by a relatively hard unpulsed
component and are seen only over phases of the 7.56 s spin period pulsations away from the main peak. QPOs at ∼18 and ∼30 Hz
are also detected ∼200–300 s after the onset of the giant flare. This is the first time that QPOs are unambiguously detected
in the flux of a Soft Gamma-ray Repeater, or any other isolated neutron star. We interpret the highest QPOs in terms of the
coupling of toroidal seismic modes with Alfvén waves propagating along magnetospheric field lines. The lowest frequency QPO
might instead provide indirect evidence on the strength of the internal magnetic field of the neutron star.
相似文献
18.
Large-scale active coronal phenomena in Yohkoh SXT images 总被引：1，自引：0，他引：1
Zdenk vestka Frantiek Fárník Hugh S. Hudson Yutaka Uchida Paul Hick James R. Lemen 《Solar physics》1995,161(2):331-363
We have found several occurrences of slowly rising giant arches inYohkoh images. These are similar to the giant post-flare arches previously discovered by SMM instruments in the 80s. However, we see them now with 3–5 times better spatial resolution and can recognize well their loop-like structure. As a rule, these arches followeruptive flares with gradual soft X-ray bursts, and rise with speeds of 1.1–2.4 km s–1 which keep constant for >5 to 24 hours, reaching altitudes up to 250 000 km above the solar limb. These arches differ from post-flare loop systems by their (much higher) altitudes, (much longer) lifetimes, and (constant) speed of growth. One event appears to be a rise of a transequatorial interconnecting loop.In the event of 21–22 February 1992 one can see both the loop system, rising with a gradually decreasing speed to an altitude of 120 000 km, and the arch, emerging from behind the loops and continuing to rise with a constant speed for many more hours up to 240 000 km above the solar limb. In the event of 2–3 November 1991 three subsequent rising large-scale coronal systems can be recognized: first a fast one with speed increasing with altitude and ceasing to be visible at about 300 000 km. This most probably shows the X-ray signature of a coronal mass ejection (CME). A second one, with gradually decreasing speed, might represent very high rising flare loops. A third one continues to rise slowly with a constant speed up to 230 000 km (and up to 285 000 km after the speed begins to decay), and this is the giant arch. This event, including an arch revival on November 4–5, is very similar to rising giant arches observed by the SMM on 6–7 November 1980. Other events of this kind were observed on 27–28 April 1992, 15 March 1993, and 4–6 November 1993, all seen above the solar limb, where it is much easier to identify them.The temperature in the brightest part of the arch of 2–3 November 1991 was increasing with its altitude, from 2 to 4 × 106 K, which seems to be an effect of slower cooling at lower densities. Under an assumption of line-of-sight thickness of 50 000 km, the emission measure indicates densities from 1.1 × 1010 cm–3 at an altitude of 150 000 km to 1.0 × 109 cm–3 at 245 000 km 11.5 hours later. It appears that the arch is composed of plasma of widely different temperatures, and that hot plasma rises faster than the cool component. Thus the whole arch expands upward, and its density gradient increases with time, which explains whyYohkoh images show only the lowest and coolest parts of the expanding structure. The whole arch may represent an energy in excess of 1031 erg, and more if conduction contributes to the arch cooling.We suggest that the rise of the arch is initiated by a CME which removes the magnetic field and plasma in the upper corona, and the coronal structures remaining below this cavity begin to expand into the vacuum left behind the CME. However, we are unable to explain why the speed of rise stays constant for so many hours. 相似文献
19.
Outflows at the Edges of an Active Region in a Coronal Hole: A Signature of Active Region Expansion?
Outflows of plasma at the edges of active regions surrounded by quiet Sun are now a common observation with the Hinode satellite. While there is observational evidence to suggest that the outflows are originating in the magnetic field surrounding
the active regions, there is no conclusive evidence that reveals how they are driven. Motivated by observations of outflows
at the periphery of a mature active region embedded in a coronal hole, we have used a three-dimensional simulation to emulate
the active region’s development in order to investigate the origin and driver of these outflows. We find that outflows are
accelerated from a site in the coronal hole magnetic field immediately surrounding the active region and are channelled along
the coronal hole field as they rise through the atmosphere. The plasma is accelerated simply as a result of the active region
expanding horizontally as it develops. Many of the characteristics of the outflows generated in the simulation are consistent
with those of observed outflows: velocities up to 45 km s−1, properties akin to the coronal hole, proximity to the active region’s draining loops, expansion with height, and projection
over monopolar photospheric magnetic concentrations. Although the horizontal expansion occurs as a consequence of the active
region’s development in the simulation, expansion is also a general feature of established active regions. Hence, it is entirely
possible and plausible that the expansion acceleration mechanism displayed in the simulation is occurring in active regions
on the Sun and, in addition to reconnection, is driving the outflows observed at their edges. 相似文献
20.
We studied the evolution of a small eruptive flare (GOES class C1) from its onset phase using multi-wavelength observations
that sample the flare atmosphere from the chromosphere to the corona. The main instruments involved were the Coronal Diagnostic
Spectrometer (CDS) aboard SOHO and facilities at the Dunn Solar Tower of the National Solar Observatory/Sacramento Peak. Transition
Region and Coronal Explorer (TRACE) together with Ramaty High-Energy Spectroscopic Imager (RHESSI) also provided images and
spectra for this flare. Hα and TRACE images display two loop systems that outline the pre-reconnection and post-reconnection magnetic field lines and
their topological changes revealing that we are dealing with an eruptive confined flare. RHESSI data do not record any detectable
emission at energies ≥25 keV, and the observed count spectrum can be well fitted with a thermal plus a non-thermal model of
the photon spectrum. A non-thermal electron flux F ≈ 5 × 1010 erg cm−2 s−1 is determined. The reconstructed images show a very compact source whose peak emission moves along the photospheric magnetic
inversion line during the flare. This is probably related to the motion of the reconnection site, hinting at an arcade of
small loops that brightens successively. The analysis of the chromospheric spectra (Ca II K, He I D3 and Hγ, acquired with a four-second temporal cadence) shows the presence of a downward velocity (between 10 and 20 km s−1) in a small region intersected by the spectrograph slit. The region is included in an area that, at the time of the maximum
X-ray emission, shows upward motions at transition region (TR) and coronal levels. For the He I 58.4 and O v 62.97 lines, we determine a velocity of ≈−40 km s−1 while for the Fe XIX 59.22 line a velocity of ≈−80 km s−1 is determined with a two-component fitting. The observations are discussed in the framework of available hydrodynamic simulations
and they are consistent with the scenario outlined by Fisher (1989). No explosive evaporation is expected for a non-thermal
electron beam of the observed characteristics, and no gentle evaporation is allowed without upward chromospheric motion. It
is suggested that the energy of non-thermal electrons can be dissipated to heat the high-density plasma, where possibly the
reconnection occurs. The consequent conductive flux drives the evaporation process in a regime that we can call sub-explosive. 相似文献