Weak Brightening Research Articles

Magnetic field fluctuations in the shocked umbral chromosphere

Context.Umbral chromospheric observations show the presence of magnetoacoustic shocks. Several recent studies have reported magnetic field fluctuations associated with these shock waves. The mechanism behind these periodic magnetic field changes is still an unresolved question.Aims.We aim to study the properties and origin of magnetic field fluctuations in the umbral chromosphere.Methods.Temporal series of spectropolarimetric observations were acquired with the GREGOR telescope on 2017 June 18. The chromospheric and photospheric conditions, including the temporal evolution of the magnetic field, were derived from simultaneous inversions of the He I10 830 Å triplet and the Si I10 827 Å line using the HAZEL2 code. The oscillations were interpreted using wavelet analysis and context information from ultraviolet observations acquired with the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (SDO/AIA) and the Interface Region Imaging Spectrograph (IRIS).Results.The chromospheric magnetic field shows strong fluctuations in the sunspot umbra, with peak field strengths of up to 2900 G. This inferred field strength is comparable to the magnetic field strength in the upper photosphere. Magnetic field and velocity umbral oscillations exhibit a strong coherence, with the magnetic field lagging the shock fronts detected in the velocity fluctuations. This points to a common origin of the fluctuations in both parameters, and the analysis of the phase shift between photospheric and chromospheric velocity is consistent with upward wave propagation. These results suggest that the strong inferred magnetic field fluctuations are caused by changes in the response height of the He I10 830 Å line to the magnetic field, which is sensitive to the high photosphere during and right after the shock. The analysis of extreme ultraviolet data shows a weak brightening in a coronal loop rooted in the umbra around the time of the measured magnetic field fluctuations. This coronal activity could possibly have some impact on the inferred fluctuations, but it is not the main driver of the magnetic field oscillations since they are found before the extreme ultraviolet event takes place.Conclusions.Chromospheric magnetic field fluctuations measured with the He I10 830 Å triplet arise due to variations in the opacity of the line. After strong shocks are produced by the propagation of slow magnetoacoustic waves, the response of the line to the magnetic field can be shifted down to the upper photosphere. This is seen as remarkably large fluctuations in the line-of-sight magnetic field strength.

External reconnection and resultant reconfiguration of overlying magnetic fields during sympathetic eruptions of two filaments

Context. Sympathetic eruptions of two solar filaments have been studied for several decades, but the detailed physical process through which one erupting filament triggers another is still under debate. Aims. Here we aim to investigate the physical nature of a sympathetic event involving successive eruptions of two filaments on 2015 November 15-16, which presented abundant sympathetic characteristics. Methods. Combining data from the Solar Dynamics Observatory and other observatories as well as results of nonlinear force-free field (NLFFF) extrapolations, we study the evolution of observational features and magnetic topology during the sympathetic event. Results. The two filaments (north F1 and south F2) were separated by a narrow region of negative polarity, and F1 first erupted, producing a two-ribbon flare. When the outward-spreading ribbon produced by F1 approached stable F2, a weak brightening was observed to the south of F2 and then spread northward, inward approaching F2. Behind this inward-spreading brightening, a dimming region characterized by a plasma density reduction of 30% was extending. NLFFF extrapolations with a time sequence reveal that fields above pre-eruption F1 and F2 constituted a quadrupolar magnetic system with a possible null point. Moreover, the null point kept moving toward F2 and descending within the following hours. We infer that the rising F1 pushed its overlying fields toward the fields above stable F2 and caused successive external reconnection between the overlying fields. From outside to inside (lower and lower in height), the fields above pre-eruption F2 were gradually involved in the reconnection, manifesting as the inward-spreading brightening and extending dimming on the south side of F2. Furthermore, the external reconnection could reconfigure the overlying fields of F2 by transporting magnetic flux from its west part to the east part, which is further verified by the subsequent partial eruption of F2. Conclusions. We propose an integrated evidence chain to demonstrate the critical roles of external magnetic reconnection and the resultant reconfiguration of overlying fields on the sympathetic eruptions of two filaments.

Fine structure events in microwave emission during solar minimumFine structure events in microwave emission during solar minimum

The solar minimum is a period with a relatively smaller number of sunspots and solar eruptions, and has been less studied before. Since the radio signal rapidly responds to the change of solar plasma and magnetic field, we perform a comprehensive analysis of high resolution spectrum data from SBRS and MUSER: 1) a search for solar radio bursts of different kinds in recent solar minima (2007–2009 and 2016–2018); 2) an analysis of several typical radio burst events, negative and positive drifting bursts, for example the November 22, 2015 and August 29, 2016 events; superfine spectral structure events with mini-flares and even without sunspots, for example the March 28, 2008 and July 04, 2017 events. These results show that there were many radio bursts with a fine structure during solar minima. These events occurred not only in powerful flares, but also in faint flares (class C and B by GOES) or even without flares, but in regions related to weak brightenings or ejecta. We assume that the weak solar radio bursts observed by telescopes with high sensitivity and low interference will help us to understand the basic physical characteristics of small-scale solar eruptions.

События с тонкой структурой в микроволновом излучении во время солнечного минимума

The solar minimum is a period with a relatively smaller number of sunspots and solar eruptions, and has been less studied before. Since the radio signal rapidly responds to the change of solar plasma and magnetic field, we perform a comprehensive analysis of high resolution spectrum data from SBRS and MUSER: 1) a search for solar radio bursts of different kinds in recent solar minima (2007–2009 and 2016–2018); 2) an analysis of several typical radio burst events, negative and positive drifting bursts, for example the November 22, 2015 and August 29, 2016 events; superfine spectral structure events with mini-flares and even without sunspots, for example the March 28, 2008 and July 04, 2017 events. These results show that there were many radio bursts with a fine structure during solar minima. These events occurred not only in powerful flares, but also in faint flares (class C and B by GOES) or even without flares, but in regions related to weak brightenings or ejecta. We assume that the weak solar radio bursts observed by telescopes with high sensitivity and low interference will help us to understand the basic physical characteristics of small-scale solar eruptions.

Plasma draining and replenishing near a solar active region inferred from cross-correlation between radiation intensity and Doppler shift

Using observations from the EUV Imaging Spectrometer (EIS) onboard Hinode, we exam the plasma dynamics around the edge of the active region 10977, possibly associated with the source of nascent slow/intermediate solar wind. The correlation between the temporal profiles of the radiation intensity and Doppler shift for each emission line are analyzed. And three small regions with positive correlations for all the five emission lines are selected for a detailed analysis. In this work, Doppler blue (red) shift is defined as negative (positive). We find that in Region 1, the radiation intensity (Doppler velocity) decreases by about 15% (about 3 km s−1), and logarithmical differential emission measures (lg(DEMs)) reduces by about 0.06–0.10% at all temperatures, called “weak dimming”, during a 30-min interval. In Region 2 and Region 3, however, the radiation intensity (Doppler velocity) increases by about 15% (about 3 km s−1), and lg(DEMs) increases by about 0.06%–0.10% at all temperatures, called “weak brightening”. Such weak dimming (weak brightening) could reflect a slow draining (replenishing) of plasma in the solar wind flux tubes, possibly due to a larger (smaller) outflow flux at high altitude than at low altitude. These suggest that the plasma supply could be intermittent with an alternation of draining and replenishing, for which the underlying physical process is yet unknown, at the source region of slow/intermediate solar wind.

A blowout surge from the eruption of a miniature filament confined by large coronal loops

By means of Hα, EUV, soft X-ray, hard X-ray, and photospheric magnetic field observations, we report the surge-like eruption of a small-scale filament, called “blowout surge” according to recent observations, occurring on a plage region around AR 10876 on 1 May 2006. Along magnetic polarity reversal boundaries with obvious magnetic cancelations, the filament was located underneath a compact coronal arcade and close to one end of large coronal loops around the AR’s periphery. The filament started to erupt about 8min before the main impulsive phase of a small two-ribbon flare, which had two Hα blue-wing kernels connected by hard X-ray loop-top sources on the both sides of the filament. After the flare end, the filament further underwent a distant eruption following a path nearly along the preexisting large loops, and thus looked like an Hα surge and an EUV jet. During the eruption, a small coronal dimming was formed near the flare, while weak brightenings appeared around the remote end of the large loops. We interpret these joint observations as the filament eruption being confined and guided by the large loops. The filament eruption, initially embedded in one footpoint region of the large loops, can break away from the magnetic restraint of the overlying compact arcade, but might be still limited inside the large loops. As a result, the eruption took a surge form that can only expand laterally along the large loops rather than erupt radially.

Magnetic Interaction: A Transequatorial Jet and Interconnecting Loops

We present, to our knowledge for the first time, a rare observation of direct magnetic interaction between a transequatorial jet and interconnecting loops (IL) in the southern hemisphere. The jet originated from a flare and appeared to move outward along open field lines, but it passed so close to the IL that its edge met with one of the IL ends. As a result, the IL began to erupt, weak brightenings appeared at the meeting site, and a nearby dark feature was disturbed. After the eruption, in addition to a looplike dimming due to the disappearance of the IL, a dimming region was formed around its another end, which was very probably caused by the expansion or opening of its field lines and represented its evacuated feet. Two coronal mass ejections (CMEs) were observed within 2 hr in association with the event. One was related to the flare and the jet, while the other was due to the IL eruption. These observations suggest that a sole flare can not only trigger a CME but also simultaneously trigger an IL eruption by means of its interaction with a jet, so can lead to two interdependent CMEs, i. e., a sympathetic CME pair physically connected by the jet/IL interaction.

Alternating bursts of low energy ions and electrons near the substorm onset

Abstract. The substorm associated behavior of the low energy particles (30 eV–28.5 keV) near the earthward edge of the plasma sheet is examined using data from CRRES during the late growth and early expansion phases of a substorm on 12 March 1991 and their significance for the substorm onset mechanism is discussed. In this substorm, the CRRES was located on L ~6.3 and ~20° westward of the substorm onset and observed the sequence of the alternating bursts of the low energy ions and electrons. The bursts of the 0.633–9.6 keV ions occurred 1–2 min before the (7.31–21.7 keV) electron bursts. The first ion burst happened 2min before the substorm onset, at the moment of weak brightening of the most equatorial pre-breakup arc near the latitude ~62°. The alternation of the ion and electron bursts may be a signature of a drift-Alfvén ballooning instability on the inner edge of the plasma sheet near substorm onset.

Plasmoid ejection and auroral brightenings

Geotail plasma and magnetic field observations of 24 plasmoids between 21 and 29 RE have been compared with Polar ultraviolet observations of auroral brightenings. Both single and multiple plasmoids almost always corresponded to brightenings, but the brightenings were sometimes weak and spatially limited and did not always grow to a global substorm. Even a case where a plasmoid event occurred with fast postplasmoid flow corresponded to a weak brightening but no substorm. Some brightenings did not correspond to plasmoids, but these brightenings were observed away from the longitude of Geotail, indicating that plasmoids have a small longitudinal extent in the near tail. The plasmoids were occasionally observed before the brightenings but more frequently were observed 0–2 min after the brightenings, with the delays probably due to the transit time to the observation point. It seems likely that formation of a near‐Earth neutral line causes each brightening in the polar ionosphere, but these formations do not always lead to a full‐fledged substorm. What additional circumstance causes development of a full, large‐scale substorm remains an open question.