Small GO Research Articles

Repeated Type III Burst Groups Associated with a B-Class Flare and a Narrow-Width CME

We have analysed a solar event from 27 September 2021, which included a small GOES B-class flare, a compact and narrow-width CME, and radio type III bursts that appeared in groups. The long-duration, repeated metric type III burst emission indicates continuous electron acceleration at high altitudes. The flaring active region was surrounded by strong magnetic fields and large-scale loops, which guided the outflow of the CME plasmoid and hence the narrow, bullet-like appearance of the CME. Radio imaging and EUV observations confirmed the direction of particle propagation and the depletion of matter from the solar source region. We observed V-shaped type III burst emission lanes, which also explain the field configuration and suggest a possible location for repeated reconnection that occurred at a constant altitude.

2019 International Women’s Day event

Context. We present a detailed analysis of an eruptive event that occurred on 2019 March 8 in the active region AR 12734, which we refer as the International Women’s Day event. The event under study is intriguing based on several aspects: (1) low-coronal eruptive signatures come in ‘pairs’, namely, there is a double-peaked flare, two coronal dimmings, and two extreme ultraviolet (EUV) waves; (2) although the event is characterized by a complete chain of eruptive signatures, the corresponding coronagraphic signatures are weak; and (3) although the source region of the eruption is located close to the center of the solar disc and the eruption is thus presumably Earth-directed, heliospheric signatures are very weak with very weak Earth impact. Aims. In order to understand the initiation and evolution of this particular event, we performed a comprehensive analysis using a combined observational-modeling approach. Methods. We analyzed a number of multi-spacecraft and multi-instrument (both remote-sensing and in situ) observations, including soft X-ray, EUV, radio and white-light emission, as well as plasma, magnetic field, and particle measurements. We employed 3D nonlinear force-free modeling to investigate the coronal magnetic field configuration in and around the active region, the graduated cylindrical shell model to make a 3D reconstruction of the CME geometry, and the 3D magnetohydrodynamical numerical model EUropean Heliospheric FORecasting Information Asset to model the background state of the heliosphere. Results. Our results reveal a two-stage C1.3 flare, associated with two EUV waves that occur in close succession and two-stage coronal dimmings that evolve co-temporally with the flare and type II and III radio bursts. Despite its small GOES class, a clear drop in magnetic free energy and helicity is observed during the flare. White light observations do not unambiguously indicate two separate CMEs, but rather a single entity most likely composed of two sheared and twisted structures corresponding to the two eruptions observed in the low corona. The corresponding interplanetary signatures are that of a small flux rope swith indications of strong interactions with the ambient plasma, which result in a negligible geomagnetic impact. Conclusions. Our results indicate two subsequent eruptions of two systems of sheared and twisted magnetic fields, which already begin to merge in the upper corona and start to evolve further out as a single entity. The large-scale magnetic field significantly influences both the early and the interplanetary evolution of the structure. During the first eruption, the stability of the overlying field was disrupted, enabling the second eruption. We find that during the propagation in the interplanetary space the large-scale magnetic field, that is, the location of heliospheric current sheet between the AR and the Earth, is likely to influence propagation, along with the evolution of the erupted structure(s).

Observations of Transient Active Region Heating with Hinode

Abstract We present observations of transient active region heating events observed with the Extreme Ultraviolet Imaging Spectrometer (EIS) and X-ray Telescope (XRT) on Hinode. This initial investigation focuses on NOAA active region 10940 as observed by Hinode on 2007 February 1 between 12 and 19UT. In these observations we find numerous examples of transient heating events within the active region. The high spatial resolution and broad temperature coverage of these instruments allows us to track the evolution of coronal plasma. The evolution of the emission observed with XRT and EIS during these events is generally consistent with loops that have been heated and are cooling. We have analyzed the most energetic heating event observed during this period, a small GOES B-class flare, in some detail and present some of the spectral signatures of the event, such as relative Doppler shifts at one of the loop footpoints and enhanced line widths during the rise phase of the event. While the analysis of these transient events has the potential to yield insights into the coronal heating mechanism, these observations do not rule out the possibility that there is a strong steady heating level in the active region. Detailed statistical analysis will be required to address this question definitively.

Small GOES flares with intense hard X-ray emission

Large solar flares with intense soft X-ray emission (i.e., high GOES class) generally tend to show a strong hard X-ray emission. However, there are examples of low GOES class events with unusually strong hard X-ray emission. In this paper, we analyse the morphology and physical parameters of such small GOES intensity flares with strong hard X-ray emission, using Yohkoh SXT images and photometric data obtained from INTERBALL-TAIL RF15-I X-ray Photometer. We observe a great variety in the soft X-ray morphology of such flares (a large diversity of loop configurations). Some of these flares do not differ greatly in their morphology from large intense flares, but most flares are generally compact. In spite of their low intensities in soft X-rays, the significant hard X-ray emission is observed by INTERBALL up to 30–60 keV. We briefly discuss some of the possible causes of the soft and hard X-ray emission ratio of these events.

An investigation of small GOES flares with intense hard X-ray bursts

Most solar flare observations show that intense hard X-ray bursts come from large flares that have a large GOES classification (large peak 1 – 8 Å flux). This correlation, known as the “Big Flare Syndrome”, suggests that more intense flares tend to have harder spectra. We have observed 7 flares that are exceptions to this. These flares have small GOES classifications ranging from B1.4 to C5.5 and peak hard X-ray count rates similar to those often observed from M class flares. This paper examines the cause of this anomoly using the Yohkoh Soft X-Ray Telescope, Hard X-Ray Telescope, and Bragg Crystal Spectrometer. Two hypotheses are proposed for the exceptions: (1) flares with multiple magnetic loops and common footpoints, producing multiple hard X-ray emission regions and low density thermal plasma distributed over a large volume, and (2) high densities in the magnetic loops restricting the propagation of the non-thermal electrons in the loop after magnetic reconnection has occurred and suppressing chromospheric evaporation. Two of the flares support the first hypothesis. The other flares either have data missing or are too small to be properly analysed by the Yohkoh instruments.