Large Solar Flares Research Articles

A solar neutron telescope in Tibet and its capability examined by the 1998 November 28th event

A new solar neutron telescope has been constructed at Yangbajing, Tibet (600 g/cm 2) and has been successfully operated for 8 years. During this time several interesting events were observed in coincidence with large solar flares. In this paper the authors discuss a directional count rate enhancement 1998 November 28 as to whether it is related to solar phenomena or not. We use this event to evaluate the sensitivity of the solar neutron telescope and discuss the detection limit for solar neutrons and high energy solar protons of this detector.

Converging motion of conjugate flaring kernels during two large solar flares

In this paper, we analyze the footpoint motion of two large solar flares using observations made by the Transition Region and Coronal Explorer (TRACE) and Reuven Ramaty High Energy Solar Spectroscopic Imager ( RHESSI). The two flares are the M5.7 flare of March 14, 2002 and the X10 flare of October 29, 2003. They are both classical two-ribbon flares as observed in TRACE 1600 or 171 Å images and have long-duration conjugate hard X-ray (HXR) footpoint emission. We use the ‘center-of-mass’ method to locate the centroids of the UV/EUV flare ribbons. The results are: (1) The conjugate UV/EUV ribbons and HXR footpoints of the two flares show a converging (inward) motion during the impulsive phase. For the two flares, the converging motion lasts about 3 and 10 min, respectively. The usual separation (outward) motion for the flare ribbons and footpoints take place only after the converging motion. (2) During the inward and the outward motion, the conjugate ribbons and footpoints of the two events exhibit a strong unshear motion. In obtaining above results, TRACE UV/EUV and RHESSI HXR data show an overall agreement. The two events demonstrate that the magnetic reconnection for the flares occurs in highly sheared magnetic field. Furthermore, the results support the magnetic model constructed by Ji et al. [Ji, H., Huang, G., Wang, H. Astrophys. J. 660, 893–900, 2007], who proposed that the contracting motion of flaring loops is the signature of the relaxation of sheared magnetic fields.

Are Moreton Waves Coronal Phenomena?

We report on permeability characteristics of the upper solar atmosphere due to the progression of a Moreton wave. An exceptional Moreton wave is tracked to cover most of the Sun, following an unusually large solar X-ray flare observed on 2003 October 29. Using Hα intensity and Doppler measurements, the Moreton wave is tracked for as long as 12 minutes. Moving outward, the wave circumnavigates strong-field active regions. The wave sweeps through solar magnetic neutral lines, disrupting material from filament and filament channels, thereby accentuating the visibility of the wave. We establish that the requirement for the visibility of a Moreton wave is the necessary presence of higher density material in the layers of the corona, besides reaffirming that Moreton waves are observed only when the speed of the disturbance exceeds Mach 2. We suggest that the cause can be a removal of significant amount of material from the solar upper atmosphere due to a coronal mass ejection.

Particle Acceleration by the Sun: Electrons, Hard X-rays/Gamma-rays

Observations of hard X-ray (HXR)/γ-ray continuum and γ-ray lines produced by energetic electrons and ions, respectively, colliding with the solar atmosphere, have shown that large solar flares can accelerate ions up to many GeV and electrons up to hundreds of MeV. Solar energetic particles (SEPs) are observed by spacecraft near 1 AU and by ground-based instrumentation to extend up to similar energies, but it appears that a different acceleration process, one associated with fast Coronal Mass Ejections (CMEs) is responsible. Much weaker SEP events are observed that are generally rich in electrons, 3He, and heavy elements. The energetic particles in these events appear to be similar to those accelerated in flares. The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) mission provides high-resolution spectroscopy and imaging of flare HXRs and γ-rays. The observations of the location, energy spectra, and composition of the flare accelerated energetic particles at the Sun strongly imply that the acceleration is closely related to the magnetic reconnection that releases the energy in solar flares. Here preliminary comparisons of the RHESSI observations with observations of both energetic electrons and ions near 1 AU are reviewed, and the implications for the particle acceleration and escape processes are discussed.

The role of plasma ejections in the development of large solar flares of various durations

Recent observations indicate that relatively strong plasma ejections are accompanied by the formation of systems of coronal loops with two glowing ribbons near their footpoints. However, while two-ribbon flares can sometimes last for many hours, for example, soft X rays, they sometimes decay within tens of minutes. We study here factors affecting the durations of flares using four major flares occurring in July 15–18, 2002, as examples. Various ground-based and satellite observations are used to show that short-duration events involved collimated (narrow) plasma ejections directed to the north and the subsequent formation of compact loops in the leading part of the active region. During one event, a powerful eastward ejection in a wide solid angle was followed by the formation of an extended arch system in the trailing part, which determined the long duration of the flare. It is proposed that in events involving collimated jets and corresponding narrow features in coronal mass ejections (CMEs), systems of coronal loops do form, but post-eruptive energy release either does not occur or is expressed very faintly. So the energy does not go downward from this region, and the plasma is emitted free in the coronal loops. In contrast to such rapid flares, wide ejections and bright, large-scale CMEs are accompanied by the formation and prolonged existence of an extended arch system. Thus, powerful nonstationary solar processes involve a large-scale CME and the flare itself, with the pattern of a particular event determined by the reconnection scenario and the evolution of the ejected plasma.

Seismic Emission from A M9.5-Class Solar Flare

Following the discovery of a few significant seismic sources at 6.0 mHz from the large solar flares of October 28 and 29, 2003, we have extended SOHO/MDI helioseismic observations to moderate M-class flares. We report the detection of seismic waves emitted from the β γ δ active region NOAA 9608 on September 9, 2001. A quite impulsive solar flare of type M9.5 occurred from 20:40 to 20:48 UT. We used helioseismic holography to image seismic emission from this flare into the solar interior and computed time series of egression power maps in 2.0 mHz bands centered at 3.0 and 6.0 mHz. The 6.0 mHz images show an acoustic source associated with the flare some 30 Mm across in the East – West direction and 15 Mm in the North – South direction nestled in the southern penumbra of the main sunspot of AR 9608. This coincides closely with three white-light flare kernels that appear in the sunspot penumbra. The close spatial correspondence between white-light and acoustic emission adds considerable weight to the hypothesis that the acoustic emission is driven by heating of the lower photosphere. This is further supported by a rough hydromechanical model of an acoustic transient driven by sudden heating of the low photosphere. Where direct heating of the low photosphere by protons or high-energy electrons is unrealistic, the strong association between the acoustic source and co-spatial continuum emission can be regarded as evidence supporting the back-warming hypothesis, in which the low photosphere is heated by radiation from the overlying chromosphere. This is to say that a seismic source coincident with strong, sudden radiative emission in the visible continuum spectrum indicates a photosphere sufficiently heated so as to contribute significantly to the continuum emission observed.

Rapid fluctuations of stratospheric electric field following a solar energetic particle event

[1] During January, 2005, there were several large X-class solar flares and associated solar energetic particle (SEP) events. Coincidentally, the MINIS balloon campaign had multiple payloads aloft in the stratosphere above Antarctica measuring dc electric fields, conductivity and x-ray flux. One-to-one increases in the electrical conductivity and decreases to near zero of both the vertical and horizontal electric field components were observed in conjunction with an increase in particle flux at SEP onset. Combined with an atmospheric electric field mapping model, these data are consistent with a shorting out of the global electric circuit and point toward substantial ionospheric convection modifications. Additionally, two subsequent, rapid changes were detected in the vertical electric field component several hours after SEP onset. These changes result in similar fluctuations in the calculated vertical current density. We will describe how rigidity cut-off dynamics may be crucial in understanding these sudden jumps in the vertical electric field.

The Automatic Predictability of Super Geomagnetic Storms from halo CMEs associated with Large Solar Flares

We investigate the relationship between magnetic structures of coronal mass ejection (CME) source regions and geomagnetic storms, in particular, the super storms when the D st index decreases below −200 nT. By examining all full halo CMEs that erupted between 1996 and 2004, we selected 73 events associated with M-class and X-class solar flares, which have a clearly identifiable source region. By analyzing daily full-disk MDI magnetograms, we found that the horizontal gradient of the line-of-sight magnetic field is a viable parameter to identify a flaring magnetic neutral line and thus can be used to predict the possible source region of CMEs. The accuracy of this prediction is about 75%, especially for those associated with X-class flares (up to 89%). The mean orientation of the magnetic structures of source regions was derived and characterized by the orientation angle θ, which is defined to be ≤ 90∘ in the case of the southward orientation and ≥ 90∘, when the magnetic structure is northwardly oriented. The orientation angle was calculated as the median orientation angle of extrapolated field lines relative to the flaring neutral line. We report that for about 92% of super storms (12 out of 13 events) the orientation angle was found to be southward. In the case of intense and moderate storms (D st≥ −200 nT), the relationship is less pronounced (70%, 21 out of 30 events). Our findings demonstrate that the approach presented in this paper can be used to perform an automatic prediction of the occurrence of large X-class flares and super geomagnetic storms.

Anti-neutrino imprint in solar neutrino flare**This paper is dedicated to the memory of the flaring life of Jonathan Evron, fallen at brightest age 20 on 2nd November 2005 for the Jewish State of Israel.

A future neutrino detector at megaton mass might enlarge the neutrino telescope thresholds revealing cosmic supernova background and largest solar flares (SFs) neutrinos. Indeed the solar energetic (Ep>100 MeV) flare particles (protons, α), while scattering among themselves on solar corona atmosphere must produce prompt charged pions, whose chain decays are source of a solar (electron–muon) neutrino ‘flare’ (at tens or hundreds MeV energy). These brief (minutes) neutrino ‘bursts’ at largest flare peak may overcome by three to five orders of magnitude the steady atmospheric neutrino noise on the Earth, possibly leading to their detection above detection thresholds (in a full mixed three flavour state). Moreover the birth of anti-neutrinos at a few tens of MeV very clearly flares above a null thermal ‘hep’ anti-neutrino solar background and also above a tiny supernova relic and atmospheric noise. The largest prompt solar anti-neutrino ‘burst’ may be well detected in future Super Kamikande (gadolinium implemented) anti-neutrino signatures mostly in inverse beta decay () and rarely in higher energy muon, or even rarest tau neutrino leptons reactions. Our estimate for the recent and exceptional October–November 2003 SFs and January 20th 2005 exceptional flare might lead to a few events above or near unity for existing Super-Kamiokande and above unity for megaton detectors. The ν spectra may reflect in a subtle way the neutrino flavour oscillations and mixing in flight. A comparison of the solar neutrino flare (at their birth place on the Sun and after oscillation on arrival on the Earth) with other neutrino foreground is estimated: it offers an independent track to disentangle the neutrino flavour puzzles and its most secret mixing angles. The sharpest noise-free anti-neutrino imprint maybe its first clean voice.

Current state of the problem of solar flares: New observations and new models

A review of current questions related to the problem of large solar flares is given. The basic physical principles applied in numerical simulation of flares are presented and illustrated. The main attention is given to the phenomenon of magnetic reconnection in large-scale current layers at separators of magnetic field in the corona. This phenomenon is demonstrated within the framework of the Rainbow topological model. The model provides the possibility of explaining specific features of large-scale reconnection as a physical process that makes it possible to accumulate large energy in the form of the magnetic energy of current layers before a flare and to quickly transform this energy to the kinetic energy of particles during a flare. The secondary effects in the solar atmosphere caused by energy fluxes from reconnecting current layers are also discussed. These con- sequences of the primary energy release are responsible for the flare pattern observed in X-ray, optical, UV, and other spectral ranges.

Day–night, seismic, and solar flare effect on the propagation of 24 kHz sub-ionospheric VLF transmitter signals

The preliminary results of a collaborative study on the amplitude variation of 24 kHz sub-ionospheric NAA VLF transmitter signals transmitted from Cutler, Maine (Lat. 44.6°N, Long. 67.2°W) and monitored simultaneously at Budapest (Lat. 47.5°N, Long. 19.17°E), Hungary and Agra (Lat. 27.2°N, Long. 78°E), India are presented. The time segments of the propagation paths are so chosen that they lie in the post-midnight hours over Budapest and sunrise hours over Agra. The results show that the amplitude at Budapest decreases after midnight hours by about 3 dB whereas the same at Agra increases by 5 dB during sunrise normally. The anomalous enhancements and reductions in the amplitude variation during the three month period of July–September 2002 along Cutler–Agra great circle path (GCP) are examined in the light of seismic, solar flares, and magnetic storm effects. It is found that the occasional amplitude reductions are caused by earthquakes ( M > 5) which occurred along the GCP, and the enhancements are caused by solar flares. The magnetic storms do not seem to influence the data except in the case when associated with large solar flares.

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.

The timing of relativistic proton acceleration in the 20 January 2005 flare

Understanding the energy budget in large solar flares requires a good knowledge of how and where the energetic charged particles are accelerated. If they are mainly accelerated by a Coronal Mass Ejection (CME)-driven shock, then they do not have to derive their energy from the flare region. Conversely, if the CME does not accelerate the particles, then the energy must be provided from elsewhere. Resolution of this controversial issue may be aided if we can study events where the timing of the energetic charged particle acceleration may be tightly constrained by the data. We report here on high resolution observations of such an event. The intense ground level solar proton event of 20 January, 2005 had a rise to maximum at the South Pole of around 5 min, with a similar decay time to 1/3 maximum. This suggests that the magnetic connection from the Sun to the Earth was good and that the proton injection was impulsive on the timescale of a few minutes or less. Comparison of the proton onset time with the solar electromagnetic emissions which accompany large flares, together with observations of the coronal mass ejection seen around the injection time suggests that the CME was not responsible for the relativistic ion acceleration. The near-relativistic (~250 keV) electron intensity onset was some 8 min later than the proton onset. Implications of this on the relative injection time of the particles are discussed. It is concluded that while the relativistic protons were not accelerated by the CME-driven shock, the CME may have influenced the release of both flare-accelerated protons and electrons into the interplanetary medium.

Spectra and solar energetic protons over 20 GeV in Bastille Day event

Solar energetic particles (SEPs) from large solar flares give important information about the physical process in the solar corona and the heliosphere. Several observations have indicated that solar protons could sometimes be accelerated to at least tens of GeV, even hundreds of GeV, in intense solar energetic process. We studied the solar proton differential energy spectra with energy range of 1–500 MeV at several time intervals during Bastille Day event. It was shown that the spectra could be fitted by a power law function before flare and after flare the power law spectra still existed above 30 MeV although spectra became softer with time. There was a spectral “knee” occurring at ∼30 MeV. We constructed a solar proton differential spectrum from 30 MeV to 3 GeV at peak flux time 10:30 UT and fitted it in the same manner. On the basis of a supposition of having the same power law spectrum in higher energy, we calculated the solar proton integrated fluxes in energy range of from 500 MeV to 20 GeV and compared them with other results obtained from experimental, modelling and theoretical calculations in other big historic SEP events. A Monte Carlo simulation was carried out for a primary proton beam at the top of the atmosphere producing secondary muons on the ground. Based on the simulation, possibility of registering the solar energetic proton beams with energies over 20 GeV was discussed.

X‐Ray Observations of Solar Long‐Duration Flares

Yohkoh X-ray observations during the several-hour decay of six large solar flares show that the main emitting region is in the form of a bright loop-top source whose spatial extent well after the impulsive stage explains most of the broadening of S XV and Ca XIX X-ray lines, which would otherwise imply nonthermal velocities of 100 km s-1 or more. During the decay of each event, the electron temperature and emission measure decline slowly. For the nearly disk center event of 1992 February 27, the volume increases and the deduced lower limit to the electron density Ne decreases from ~1011 cm-3 to just below 1010 cm-3. The nonuniform emission along the loops indicates that Spitzer heat conduction does not operate. If radiation is the only or main energy-loss mechanism, the lower limits to Ne imply upper limits to radiation cooling times of up to 10 hr, less than the flare decay time, so necessitating a continuing energy input. Although magnetic reconnection of previous broken open loops (the Kopp-Pneuman model) has been widely regarded as the source of energy in such flares, the small nonthermal velocities—only 20-50 km s-1 as indicated by SUMER observations—and the lack of spectroscopic indicators of large inflows or outflows argue against it. Other flare models must also explain the relatively small nonthermal velocities. An alternative picture of loop oscillations recently seen in TRACE and Yohkoh data is proposed for the production of observed nonthermal velocities.

Extended decimeter radio emission after large solar flares

The large solar flares of October and November 2003 were accompanied by extremely intense radio emissions at decimeter wavelengths. The radio emission continued long after the main phase of the flares and reached an unprecedented peak flux density at 410 MHz of 6.3(±0.7) × 10 5 solar flux units on 2003, October 28. The unusual number of large flares from the same active regions yields a homologous set ideal for statistical analysis. We have compared the coherent radio emissions (as observed by the Zurich and Trieste instruments) with the X-rays measured by RHESSI and GOES. As major results, we find that the total duration, the peak flux, and radiated energy of the radio emissions correlate with the flare energy released (measured in soft X-rays). Enhanced hard X-rays (>12 keV) are always observed during the time of enhanced radio emission. In 27% of the radio subpeaks covered by RHESSI, we find X-ray subpeaks. The most intense radio emissions are not due to electron beams, but are post-flare emissions apparently not directly related to the primary energy release and acceleration process. These radio emissions, generally classified as Type IV and DCIM bursts, have previously been interpreted by loss-cone emission of trapped electrons. However, radio and hard X-ray subpeaks do not show a detailed correlation or Neupert effect, and long-term trapping can be excluded. Possible acceleration mechanisms after the main flare phase are discussed and compared with the observations.

Sequential Measurements of Cosmic-Ray Neutron Spectrum and Dose Rate at Sea Level in Sendai, Japan

The cosmic-ray neutron energy spectrum and dose rate were measured sequentially for two years from April 2001 up to March 2003 by using three neutron detectors, a 3He-loaded multi-moderator detector (Bonner ball), 12.7 cm diameter by 12.7 cm long NE213 organic liquid scintillator, and high-sensitivity rem (dose equivalent) counter at the Kawauchi campus of Tohoku University in Sendai, Japan of geomagnetic latitude, 29°N, and cutoff rigidity, 10.43 GV. The neutron spectrum has three major peaks, thermal energy peak, evaporation peak around 1 MeV and cascade peak around 100 MeV. The ambient neutron dose equivalent rates measured by the rem counter, and the Bonner ball keep almost constant values of 4.0 and 6.5 (nSv/h), respectively, throughout this time period, after atmospheric pressure correction, and it often decreased about 30% after a large Solar Flare, that is called as the Forbush decrease. The total neutron flux was also obtained by the Bonner ball measurements to be 7.5×10-3 (n cm−2.s−1) in average. The altitude variation of neutron flux and dose was also investigated by comparing the measured results with other results measured at Mt. Fuji area and aboard an airplane, where the cutoff rigidities are similar.

Seismic Emission from the Solar Flares of 2003 October 28 and 29

We report the detection of seismic waves emitted from powerful solar flares that occurred in NOAA Active Region 10486 on 2003 October 28 and 29. We used helioseismic holography to image the seismic sources of the waves. This technique was previously used to image the source of seismic emission from the large solar flare of 1996 July 9. Egression power maps at 6 mHz with a 2 mHz bandwidth reveal multiple compact acoustic sources strongly associated with the footpoints of a coronal loop that hosted the flares. The total acoustic energy in the flare signatures is a very small fraction of the total energy radiated by the flares. The acoustic signatures are co-aligned with hard X-ray signatures, suggesting a direct link between energetic particles accelerated during the flare and the acoustic waves as a hydrodynamic response of the chromosphere, or possibly the underlying photosphere, to these particles at the footpoints of the loop. There is also evidence of high-energy protons impinging onto the chromosphere in the neighborhoods of the acoustic sources. Observations of emission in the D1 line of neutral sodium at the onset of the October 29 flare show evidence of a downward-propagating shock/condensation at the onset of the flare. Concurrent Global Oscillation Network Group (GONG) intensity observations show significant radiative emission with a sudden onset in the compact region encompassing the acoustic signature. Most flares appear to be acoustically inactive. Photospheric heating by high-energy protons is likely to be a major factor in seismic emission from acoustically active flares.

X‐ray magnitude of the 4 November 2003 solar flare inferred from the ionospheric attenuation of the galactic radio background

We use observations from a pair of 20.1 MHz riometers to constrain the magnitude and timing of the largest solar X‐ray flare on record, a flare that saturated the detectors on the GOES‐12 satellite. Using recordings of the sudden ionospheric disturbance associated with the flare, we show that by correcting the riometer absorption measurements for the solar zenith angle, applying a model for the effects of solar X‐rays on the ionosphere, and, finally, calibrating the resulting data against the solar 0.1–0.8 nm flux from the GOES‐12 satellite, we are able to closely reproduce the temporal X‐ray profiles of large solar flares. We demonstrate this using two reference flares to show that we can reproduce the flare peak magnitude to approximately ±10% and the time of the peak to better than 3 min, even when we simulate saturations of the GOES data at 50–60% of the peak flare magnitude. Analysis of the 4 November 2003 flare is complicated by the presence of radio interference near the flare peak; however, a polynomial interpolation can be used to infer the flare characteristics at the peak. By applying this analysis and considering the likely error bounds, we constrain the peak flare magnitude to lie in the range 3.4–4.8 mW/m2 (X34–X48) peaking at 1944–1948 UT. The best fit gives a peak flux of 4.0 mW/m2 (X40), peaking at 1946 UT. Taking the three independent published estimates for the peak X‐ray magnitude that have associated error bounds, the weighted mean X‐ray peak is found to be approximately 4.0 mW/m2 (X40), peaking close to 1946 UT.

Solar neutron events in association with large solar flares in November 2003

The Sun was intensely active from late October to the beginning of November 2003. A series of 11 X class solar flares occurred in NOAA regions 484, 486 and 488. Unique among this series of flares were those occurring on November 2 and 4 in which solar neutrons were observed by the ground based neutron monitors located at Mt. Chacaltaya, Bolivia and Haleakala, Hawaii, respectively. In these flares, intense emission of hard X-rays and γ-rays have been observed by the satellites. It seems that production of solar neutrons coincided with the production of the hard electromagnetic radiations of these two flares.