Statistical investigation on the formation of sunspot light bridges

Light bridges(LBs) are bright lanes that divide one sunspot umbra into two or more parts. Though frequently observed in sunspots, their formation mechanisms have rarely been studied and thus are not well understood. Here we present results from the first statistical investigation on the formation of LBs. Using observations with the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, we identified 144 LBs within 71 active regions(ARs) over the whole year of 2014. The formation processes of these LBs can be categorized into three groups: penumbral intrusion(type-A), sunspot merging(typeB) and umbral-dot emergence(type-C). The numbers of events in these three categories are 74, 57 and 13, respectively. The duration of the LB formation process is mostly less than 40 hours, with an average of ～20 hours. Most LBs have a maximum length of less than 20. For type-A LBs, we found a positive correlation between the LB length and the duration of the LB formation process, suggesting a similar speed of penumbral intrusion in different sunspots.     

Statistics and investigation of white light solar flares

We discuss the properties of white light flares on the basis of the published accounts of these events, together with the associated H flares, radio bursts, X-ray bursts, proton events and ionosperic distrubances. In addition, spectral plates taken at Purple Mountain Observatory since 1962 have been examined. We found that 5% of the spectrograms of solar flares show variable white-light emission. A minority of the white light flares are associated with H flare of small importance classes. We think these may be caused by perturbations originating in the convective zone below, while the majority accompanied by high-energy events are caused by the bombardment of energetic particles from above.     

X-ray heating of a low-temperature region in chromospheric flares

Part of the proper X-ray emission of a flare is absorbed in the chromosphere and heats the region which creates an optical (in particular Hα) flare emission. The heating of chromosphere by X-ray emission may be responsible for the diffuse halo around the flare kernels. The optical emission of flare kernels, whose main sources of heating are energetic particles and/or thermal fluxes, may be also increased. By simple model calculations the present paper discusses the possibility of such effects for the large flare of 1972 August 7.     

Themis,BBSO, MDI and trace observations of a filament eruption

We describe a filament destabilization which occurred on 5 May 2001 in NOAA AR 9445, before a flare event. The analysis is based on Hα data acquired by THEMIS operating in IPM mode, Hα data and magnetograms obtained at the Big Bear Solar Observatory, MDI magnetograms and 171 Å images taken by TRACE. Observations at 171 Å show that ～ 2.5 hours before the flare peak, the western part of the EUV filament channel seems to split into two parts. The bifurcation of the filament in the Hα line is observed to take place ～ 1.5 hours before the flare peak, while one thread of the filament erupts ～10 min before the peak of the flare. Our analysis of longitudinal magnetograms shows the presence of a knot of positive flux inside a region of negative polarity, which coincides with the site of filament bifurcation. We interpret this event as occurring in two steps: the first step, characterized by the appearance of a new magnetic feature and the successive reconnection in the lower atmosphere between its field lines and the field lines of the old arcade sustaining the filament, leads to a new filament channel and to the observed filament bifurcation; the second step, characterized by the eruption of part of the filament lying on the old PIL, leads to a second reconnection, occurring higher in the corona.     

Frequency-energy distributions of flares and active region transient brightenings

In 1988, Uchida and Shibata proposed a model for compact loop flares as due to the collision of two large amplitude torsional Alfvén wave packets coming up along a coronal magnetic loop, leaking out from the subphotospheric convective layers of the solar atmosphere. We investigate the possibility that active region transient brightenings occur when a single torsional Alfvén wave packet transits a coronal loop. Assuming this related origin for flares and transient brightenings, the statistics of the two phenomena must also be closely related. It is shown that the observed power-law frequency-energy distributions of flares and transient brightenings may be accounted for in a natural way if the energy distribution of the underlying torsional Alfvén wave packets is itself a power law.     

Location of the electron acceleration region in solar flares

Observations of impulsive solar flare X-rays 10 keV by the OGO-5 satellite and the measurements of energetic solar electrons made with the Explorer-35 and Explorer-41 (IMP-5) satellites during the period March 1968–September 1969 have been analyzed in order to determine the ion density in the X-ray source region as well as the location of the electron acceleration region in the solar atmosphere. If we assume that the efficiency of escape of the accelerated electrons into interplanetary space is 10^–3, the observations are found to be consistent with the following interpretation: (i) the ion density in the X-ray source region varies from event to event and lies between 10⁹ and 10¹¹ ions cm^–3 for those events in which the impulsive X-ray emission could be detected; (ii) for those events in which no impulsive emission was detected above threshold, the ion density in the X-ray source was < 10⁹ ions cm^–3; (iii) at least in some small solar flares the region where the electrons are accelerated during the flash phase is located in the lower corona.     

Ephemeral region flares and the diffusion of the network

The flare-like events which are frequently seen in H in apparently quiet regions of the solar disk can in all cases be identified with bipolar features (ephemeral regions, ER) on magnetograms. These events represent the H counterpart of X-ray bright point flares.Statistically, this phenomenon is associated with the proximity of the bipolar features to the super-granulation network, in the sense that an ER is likely to flare during its lifetime if the distance to the nearest network element is less than or equal to its own pole separation. This conclusion is supported by direct study of time sequences of magnetograms and H pictures, which manifest the interaction of ER with the supergranulation network. The flare-like brightenings in some examples occurred in the region of interaction between network flux and one pole of the ER.The consequence of this interaction is that small quantities of network flux are transported over distances of the order of the ER pole separations. This may have an important effect on the long-term diffusion of magnetic flux.     

Spectroscopic observations and models of umbral light bridges

Profiles of the Nai D lines in two moderately bright umbral light bridges are analyzed. A simple method of elimination of stray light is presented. The corrected continuum intensities ( 588 nm) of the light bridges are 0.51 and 0.43. For each light bridge, using optimization technique, a semi-empirical model is derived. The models exhibit a steep temperature rise towards the subphotospheric layers. Generally, the temperature of the light bridges under investigation is lower than the temperature of the penumbra as well as the temperature of bright umbral dots.     

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.     

Magnetic fields and flares in the region CMP 20 September 1963

The position of bright knots of 30 flares at their very beginning relative to the high-resolution isogauss maps of the longitudinal component (H ) and maps of the transverse component (H ) of magnetic field are considered for seven days during the passage of the active and large spot group in Sept. 1963 (see Table I and maps on Figures 1–8).The flare bright knots occur simultaneously in regions of opposite magnetic polarity, and the majority of these knots are adjacent to neutral line H = 0, although not coinciding precisely with this line (Figure 9). Lenticular form of flare knots and the motions of bright material of flares is restrained by transversal field H . Also flares are closely associated (83%) with so-called bifurcated regions, where specific crossing of transverse components takes place (Figures 4–5). There is well-expressed (80%) coincidence of flare knots with the strongest (positive or negative) electric currents as determined from the relation j = c/4 rot H. The relation of results obtained to some existing theories of flares is briefly discussed.U.S. Nat. Acad. of Science - U.S.S.R. Acad. Nauk. Exchange Scientist Program; now at CSIRO Division of Physics, Australia.     

Plasma jet and shock formation during current loop coalescence in solar flares

We report on the results of plasma jet and shock formation during the current loop coalescence in solar flares. It is shown by a theoretical model based on the ideal MHD equation that the spiral, two-sided plasma jet can be explosively driven by the plasma rotational motion induced during the two current loop coalescence process. The maximum velocity of the jet can exceed the Alfvén velocity, depending on the plasma (= c _s ² v _A ² ) ratio. The acceleration time getting to the maximum jet velocity is quite short and le than 1 s. The rebound following the plasma collapse driven by magnetic pinch effect can strongly induce super-Alfvénic flow. We present the condition of the shock formation. We briefly discuss the high-energy particle acceleration during the plasma collapse as well as by the shocks.     

Plasma radiation diagnostics of the primary energy release region in solar flares

The possibility is investigated that the plasma turbulence used in many recent models of the primary energy release and acceleration in solar flares should be detectable by radiation near the fundamental and second harmonic of the plasma frequency. Formulae are derived for fundamental emission due to the combination of ion-acoustic and Langmuir plasma turbulence and for second harmonic emission due to the combination of two Langmuir waves. These results are applied to recent primary energy release and acceleration models which shows that either such radiation should be detectable and possibly distinguishable with suitable microwave interferometers or that its absence places fairly stringent constraints on the possible level of Langmuir or Langmuir and ion-acoustic waves in these models.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The production of lithium in the solar chromosphere and photosphere during white light flares

Recently new values of the lithium formation rate in low energy flares have been reported in the literature. These values are applied to the white light flare phenomenon on the Sun. It is found that the formation rate in the chromosphere is much larger than in the upper photosphere and that the ratio between the time integrated flare created abundance and the initial photospheric abundance is modest in the chromosphere and small in the upper photosphere. The yield of Li⁶ in the upper photosphere is, however, comparable to the upper limit of Li⁶ there.     

A comparison of the oscillations in sunspot umbrae,penumbrae, and the surrounding photosphere

Time series of the nonsplit Fei 7090 Å line have been observed in several sunspots with a 100 x 100 diode array corresponding to 48 arc sec times 1.39 Å. The spatial behaviour of Doppler motions along one fixed slit position has been studied as a function of time. Former results are confirmed, that the power in the five minute range decreases from the photosphere to the umbra, where, however, values still well above the noise level are measured. Regarding the penumbra, the power tends to exhibit a maximum at locations where the line-of-sight component of a radial horizontal field should be maximal. This indicates that the direction of the oscillatory velocities might be influenced by the magnetic field or the Evershed flow. No significant power is found in the 3 min range. An exception might be seen in a small patch at the limb of the umbra of one spot.     

The heating of the temperature minimum region in solar flares — A reassessment

As a sequel to the work by Machado et al. (1978), we discuss and evaluate the suggestions made by these authors on how to possibly reconcile the observed temperature enhancements at temperature-minimum levels in solar flares with some form of theoretical heating mechanism. After establishing the H^– LTE assumption used by Machado et al., we then consider EUV irradiation, and joule heating by steady currents, as heating mechanisms. We find that, unless there are strong inhomogeneities associated with either mechanism, neither can reasonably be reconciled with observations. It is concluded that detailed, high resolution (both spatial and temporal) measurements are necessary to further our understanding of the flare process at temperature-minimum levels.On leave from: Department of Astronomy, The University, Glasgow G12 8QQ, Scotland, U. K.     

Cyclical variability of prominences, CMEs and flares

Solar flares, prominences and CMEs are well known manifestations of solar activity. For many years, qualitative studies were made about the cyclical behaviour of such phenomena. Nowadays, more quantitative studies have been undertaken with the aim to understand the solar cycle dependence of such phenomena as well as peculiar behaviour, such as asymmetries and periodicities, occurring within the solar cycle. Here, we plan to review the more recent research concerning all these topics.