A flare-associated mechanism for solar surges

An ejection mechanism for solar surges is discussed. The mechanism is closely related to an earlier proposed model for solar flares founded on current disruption. Hence, the observed connection between surges and flares may be explained in a simple way. The mechanism can also offer explanations of other questions connected with surges such as why surges tend to grow up from small bright knots emitting moustache spectra, why strong magnetic field concentrations are observed near the base of surges, why surges often exhibit a helical motion, and why homologous surges are produced. Finally it is pointed out that spicules might be generated by a mechanism similar to that discussed for surges.     

Some statistical properties of surges

All surges observed at the Swedish Astrophysical Station in Anacapri from July 1, 1957 until December 31, 1967 have been studied statistically. In 1958 the Southern hemisphere was most active in producing surges, but thereafter the Northern hemisphere has dominated the activity with increasing rate. The average for the whole period shows that the Northern part of the sun has been twice as active as the Southern part. The latitude distribution for surges has varied with the solar cycle with a pattern similar to the butterfly diagram for sunspots. The polar surges were most frequent in the beginning of the new cycle. The importance distribution is also dependent of the solar cycle, as a higher percentage of surges of importance 1 are found during the solar minimum. Surges of higher importance tend to have longer durations, and they are more often associated with flares. The association rate between surges and radio emission is dependent on both the solar cycle and the importance of the surge. During years with high activity, more than 20% of all surges were followed within 5 min from their start by radio emission, compared to 4% during solar minimum. 17% of surges of importance 2 are closer related to radio emission, but only 10% of surges of importance 1. On the whole, 11% of the surges occurred almost simultaneously with radio bursts. Bursts in discrete frequencies were registered in connection with 8% of the surges, while 11% were followed within 5 min from their start by bursts in spectral observations. All these surges were isolated, i.e. no flares have been reported to occur during their lifetimes. This is also the case for the 18 surges covered by X-ray observations, three of which were closer related to X-ray bursts.     

A Study of Surges: II. On the Relationship between Chromospheric Surges and Coronal Mass Ejections

Liu et al. (Astrophys. J. 628, 1056, 2005a) described one surge – coronal mass ejection (CME) event showing a close relationship between solar chromospheric surge ejection and CME that had not been noted before. In this work, large Hα surges (>72 Mm, or 100 arcsec) are studied. Eight of these were associated with CMEs. According to their distinct morphological features, Hα surges can be classified into three types: jetlike, diffuse, and closed loop. It was found that all of the jetlike surges were associated with jetlike CMEs (with angular widths ≤30 degrees); the diffuse surges were all associated with wide-angle CMEs (e.g., halo); the closed-loop surges were not associated with CMEs. The exclusive relation between Hα surges and CMEs indicates difference in magnetic field configurations. The jetlike surges and related narrow CMEs propagate along coronal fields that are originally open. The unusual transverse mass motions in the diffuse surges are suggested to be due to magnetic reconnections in the corona that produce wide-angle CMEs. For the closed-loop surges, their paths are just outlining stable closed loops close to the solar surface. Thus no CMEs are associated with them.     

The relation between the surges and solar radio emission

The 120 limb surges which have been observed by means of Wrocaw Observatory coronagraph from September 1966 to November 1977 are investigated. The evolution of surges was compared with the radio data during the surges. A correlation between radio bursts and the surges was found, particularly with chains of type I radio bursts, which is the first reliable correlation found of these bursts with non-radio events. The type I correlation only applied for surges without accompanying flare, of which 43% are correlated with this type of radio emission. In 23 of 30 associated events the start of a surge coincided within 5 minutes with the start or an enhancement of the type I storm. If flares were present, the association was not significant.We also compared the maximum height reached by a surge with the frequencies of the radio bursts emitted at the same time and the maximum velocity of the rising surge with the frequency drift of type I chains. No such a correlation was however found.We discuss the possibility that surges are the result of a sudden energy input into the chromosphere related to the type I source in the corona.     

EUV observations of subflares and surges

EUV observations of two subflares and associated surges have been analyzed. At maximum brightness the emission measures and radiative outputs of the subflares were approximately 20% of the corresponding values for the active region. Multiple EUV surges were observed during and following each subflare, with surge material being ejected in a variety of directions, including toward a coronal bright point located outside of the active region. The total energy of the surges appears to be comparable to that radiated by the subflares, a few times 10²⁸ erg. As reported in previous studies of surges, we find that there was no significant emission from these features in spectral lines formed at temperatures T>10⁶K. The ejection of surges in several different directions and nearly simultaneous flaring of various areas of the active region suggest that the primary site of the subflares was magnetically connected to a variety of different areas in the active region and the surrounding quiet region.     

On association of radio emission with solar surges

About 45% non-flare surges are found to be associated with radio bursts inferred from spectral data. Associated surges are mostly accompanied by type I (24%) and type III (29%) bursts.     

Hα Surges Initiated by Newly-emerging Satellite Magnetic Fields

On July 22, 2011 and in the active region NOAA 11259 there ap- peared the event of the ejection of solar atmospheric Hα surges. According to the full-disc Hα observations of the Big Bear Solar Observatory in United States, three consecutive surges at one and the same place in the north of the main spot of the active region were discovered. The trajectories of these three surges exhib- ited the ?gure of straight lines, and their integral con?guration is like an inverted Eiffel Tower. The ?rst two surges are quite similar, and in each of them there appeared two bright points in the northern part of the main spot. After several minutes, the surges appeared in the midst of bright points. When the bright- ness of the bright points attained the maximum value, the surges spouted out from the midst of bright points. And after reaching the maximum altitude, they quickly vanished. Before the ejection of the third surge took place, no bright points appeared. Besides, its maximal altitude is merely one half of that of the ?rst two surges. Via a comparison with the SDO/HMI (Solar Dynamics Obser- vatory/Helioseismic and Magnetic Imager) data of radial magnetic ?elds, it is found that in more than one hour before the appearance of the ?rst surge there emerged bipolar magnetic ?elds in the region of ejection. Besides, in several min- utes before the ejection of each Hα surge the magnetic ?uxes of positive polarity diminished. Via our analysis it is found that there appeared reconnections be- tween the newly emerging satellite magnetic ?elds and the preexisting magnetic ?elds in the spot, and this caused the continuous ejections of Hα surges.     

2003年10月22日太阳活动区AR0484内日浪事件的研究

利用色球Hα、TRACE／WL、SOHO／EITEuV单色像观测资料及SOHO／MDI光球磁场观测资料，对2003年10月22日太阳活动区AR0484内发生的日浪事件进行了研究．发现：(1)在Ha线心观测上，日浪包含有亮、暗2个分量，这2个分量先后出现而且并不共空间．日浪的亮分量与UV和EUV波段上观测到的喷发具有较好的同时性和共空间性．(2)日浪喷发物质沿着EUV环运动。(3)在光球层，日浪足根处的黑子和磁场有明显的变化．这些观测结果支持日浪的磁重联模型。     

Hα surges and associated Soft X-ray loops

A recurrent H surge was observed on 7 October, 1991 on the western solar limb with the Meudon MSDP spectrograph. The GOES satellite recorded X-ray subflares coincident with all three events. During two of the surges high-resolutionYohkoh Soft X-ray Telescope (SXT) images have been taken. Low X-ray loops overlying the active region where the surges occurred were continuously restructuring. A flare loop appeared at the onset of each surge event and somewhat separated from the footpoint of the surge. The loops are interpreted as causally related to the surges. It is suggested that surges are due to magnetic reconnection between a twisted cool loop and open field lines. Cold plasma bubbles or jets squeezed among untwisting magnetic field lines could correspond to the surge material. No detection was made of either X-ray emission along the path of the surges or X-ray jets, possibly because of the finite detection threshold of theYohkoh SXT.     

X-ray bright surges

We present evidence of X-ray emission from surges that are bright in H. These surges have many features common to flaring arches of Martin and vestka (1988); the basic difference between the two is that in flaring arches cold and hot plasma are injected into clearly defined closed magnetic loops, while in the surges the injection goes into large-scale magnetic field structures of which the second footpoint is usually unknown. Because of the steep density gradient in such large-scale structures, the X-ray visibility of bright surges is limited to a few tens of seconds only. A series of repetitive surges, some of them bright and emitting X-rays, occurred on 8 July, 1980 from footpoints of two large-scale coronal structures, which might have been the legs of an enormous arch at least 600 Mm long.     

The possible role of energetic electrons in the production of surges

Energetic electrons, which play a major role in the explosive phases of flares, are proposed as the energy source for the production of surges. Flare data from a two-year interval are analyzed to show that the probability of having surges associated with flares is greater when there are accompanying decimeter type III bursts or impulsive 8800 MHz bursts. The model of chromospheric heating by impulsive electrons proposed by Hudson is examined and shown to provide an adequate explanation for the origin of flare surges. The proposed surge model is consistent with the temporal evolution of the flare-surge event and the required surge energy. Surges not accompanied by flares can also probably be explained by the model.     

On the solar surge association with microwave and hard X-ray emission bursts

It is found that 20% solar surges are associated with microwave bursts (2800–15000 MHz) and also that solar surges are not associated with hard X-ray bursts (17–40 keV).     

Longitudinal distribution of the cool solar surges

This investigation shows that solar surges are poorly associated with sudden ionospheric disturbances (X-ray) implying that solar surge material is cool and does not heat the corona. The investigation also shows that solar surges are most prolific at longitudes 80°, 110°, 260°, and 290°.     

An analysis of surges triggered by a small flare

On 22 October, 1980, near the solar central meridian in the western vicinity of a large spot group, a subflare of the two-ribbon type was observed. Three surges were associated with this flare. Their starting points were situated close to the principal flare patches on both sides of a short filament that was visible for only a few hours. True flow velocities and decelerations along the arch-shaped surge trajectories have been determined for two of the surges. The highest velocities were found at the onset of the surges.The event was studied from a series of H on- and off-band flltergrams taken with the Debrecen coronagraph of 53 cm aperture.     

Do surges heat the corona?

A comparison of X-ray filtergrams obtained during the Skylab mission 8 hr before and within 4 hr following 54 active region surges on the disk revealed only 6 cases of long-enduring, large-scale (> 10 000 km) coronal enhancements that might have been associated with surge activity. It is concluded that there is no evidence for any substantial increase in the temperature or amount of coronal material during reported surges.     

Magnetic fields in flares and active prominences

We study the longitudinal magnetic field in a number of active limb prominences showing fields in excess of 30 G. The objects fall into three groups: surges, caps and active region prominences. There appears to be an upper limit of 150–200 G for the field strength in prominences.

A model of surges is presented in which a pre-surge axi-symmetric magnetic field is established by a line current in the corona. We observe particle acceleration in surges that indicates v×B≠0 in these objects during periods comparable to the Alfvén transit time.

The strong fields observed in caps seem to run between parts of active regions in accordance with Hale's law of sunspot group polarities.

     

A study of surges and flares within an active region

Active region 2684 was observed by the Solar Maximum Mission and ground-based observatories simultaneously for over 12 hours on September 23, 1980. During these observations, recurrent surges were detected above an area with complex parasitic magnetic polarity located at the periphery of the active region. The time evolution of the H surges, Civ brightenings and X-ray spikes leads to the conclusion that the energy source is in the corona, from magnetic reconnection. The energy is transported by energetic charged particles along the loops, thereby heating the chromosphere as the particles lose their energy. The divergent motion of the spots corresponding to small dipoles at the base of the surge indicates that there is important magnetic reorganisation. According to the magnetic field-line configuration (large loop or open structures), X-rays can (or cannot) be associated with surges.     

The physical relationship between flares and surges observed in the extreme ultraviolet

A search was made for EUV surges among the EUV flares recorded by the Harvard spectroheliometer on ATM. Out of a large set of partial observations of such flares, a subset of 24 complete events was chosen. More than 24 associated surges were found, many of them multiple events. The flare-surge correlation is therefore considerably higher in the EUV than in H, presumably because EUV surges generally appear in emission, and in high contrast compared to H. In over 70% of the cases, the surges were found to grow out of the flare structure. Making reasonable assumptions, it was possible to infer the magnitude of the gas pressure gradient from the flare core into the surge by using the EUV intensity gradient. The inferred pressure gradient appears sufficient to drive the surge, although higher resolution observations will be required to corroborate this, and rule out the importance of magnetic Lorentz force.     

Chromospheric evolution and the flare activity of super-active region NOAA 6555

Super-active region NOAA 6555 was highly flare productive during the period March 21st–27th, 1991 of its disk passage. We have st udied its chromospheric activity using high spatial resolution Hα filtergrams taken at Udaipur along with MSFC vector magnetograms. A possible relationship of flare productivity and the variation in shear has been explored. Flares were generally seen in those subareas of the active region which possessed closed magnetic field configuration, whereas only minor flares and/or surges occurred in subareas showing open magnetic field configuration. Physical mechanisms responsible for the observed surges are also discussed.     

The recurrent surges in McMATH 9760 and associated surge brightenings

The unique surge activity that occurred in McMath 9760 region during November 7 to November 16, 1968 is described and discussed. The surge activity was unusually intense; over 70 surges were observed, some of which were very highly energetic. Some surges were accompanied by surge brightenings; the latter caused a marked sharp increase in the velocity of the ejected matter. The region was studied at H -0.5 Å. In spite of the enhanced activity, the region appeared to be highly stable.