Solar Eruptive Events Research Articles

A major solar eruptive event in July 2012: Defining extreme space weather scenarios

AbstractA key goal for space weather studies is to define severe and extreme conditions that might plausibly afflict human technology. On 23 July 2012, solar active region 1520 (~141°W heliographic longitude) gave rise to a powerful coronal mass ejection (CME) with an initial speed that was determined to be 2500 ± 500 km/s. The eruption was directed away from Earth toward 125°W longitude. STEREO‐A sensors detected the CME arrival only about 19 h later and made in situ measurements of the solar wind and interplanetary magnetic field. In this paper, we address the question of what would have happened if this powerful interplanetary event had been Earthward directed. Using a well‐proven geomagnetic storm forecast model, we find that the 23–24 July event would certainly have produced a geomagnetic storm that was comparable to the largest events of the twentieth century (Dst ~ −500 nT). Using plausible assumptions about seasonal and time‐of‐day orientation of the Earth's magnetic dipole, the most extreme modeled value of storm‐time disturbance would have been Dst = −1182 nT. This is considerably larger than estimates for the famous Carrington storm of 1859. This finding has far reaching implications because it demonstrates that extreme space weather conditions such as those during March of 1989 or September of 1859 can happen even during a modest solar activity cycle such as the one presently underway. We argue that this extreme event should immediately be employed by the space weather community to model severe space weather effects on technological systems such as the electric power grid.

Improved methods for determining the kinematics of coronal mass ejections and coronal waves

The study of solar eruptive events and associated phenomena is of great importance in the context of solar and heliophysics. Coronal mass ejections (CMEs) and coronal waves are energetic manifestations of the restructuring of the solar magnetic field and mass motion of the plasma. Characterising this motion is vital for deriving the dynamics of these events and thus understanding the physics driving their initiation and propagation. The development and use of appropriate methods for measuring event kinematics is therefore imperative. Traditional approaches to the study of CME and coronal wave kinematics do not return wholly accurate nor robust estimates of the true event kinematics and associated uncertainties. We highlight the drawbacks of these approaches, and demonstrate improved methods for accurate and reliable determination of the kinematics. The Savitzky-Golay filter is demonstrated as a more appropriate fitting technique for CME and coronal wave studies, and a residual resampling bootstrap technique is demonstrated as a statistically rigorous method for the determination of kinematic error estimates and goodness-of-fit tests. It is shown that the scatter on distance-time measurements of small sample size can significantly limit the ability to derive accurate and reliable kinematics. This may be overcome by (i) increasing measurement precision and sampling cadence, and (ii) applying robust methods for deriving the kinematics and reliably determining their associated uncertainties. If a priori knowledge exists and a pre-determined model form for the kinematics is available (or indeed any justified fitting-form to be tested against the data), then its precision can be examined using a bootstrapping technique to determine the confidence interval associated with the model/fitting parameters.

First observational application of a connectivity-based helicity flux density

Measuring the magnetic helicity distribution in the solar corona can help in understanding the trigger of solar eruptive events because magnetic helicity is believed to play a key role in solar activity due to its conservation property. A new method for computing the photospheric distribution of the helicity flux was recently developed. This method takes into account the magnetic field connectivity whereas previous methods were based on photospheric signatures only. This novel method maps the true injection of magnetic helicity in active regions. We applied this method for the first time to an observed active region, NOAA 11158, which was the source of intense flaring activity. We used high-resolution vector magnetograms from the SDO/HMI instrument to compute the photospheric flux transport velocities and to perform a nonlinear force-free magnetic field extrapolation. We determined and compared the magnetic helicity flux distribution using a purely photospheric as well as a connectivity-based method. While the new connectivity-based method confirms the mixed pattern of the helicity flux in NOAA 11158, it also reveals a different, and more correct, distribution of the helicity injection. This distribution can be important for explaining the likelihood of an eruption from the active region. The connectivity-based approach is a robust method for computing the magnetic helicity flux, which can be used to study the link between magnetic helicity and eruptivity of observed active regions.

Radio evidence for breakout reconnection in solar eruptive events

Context. Magnetic reconnection is understood to be fundamental to energy release in solar eruptive events (SEEs). In these events reconnection produces a magnetic flux rope above an arcade of hot flare loops. Breakout reconnection, a secondary reconnection high in the corona between this flux rope and the overlying magnetic field, has been hypothesized. Direct observational evidence for breakout reconnection has been elusive, however.Aims. The aim of this study is to establish a plausible interpretation of the combined radio and hard X-ray (HXR) emissions observed during the impulsive phase of the near-limb X3.9-class SEE on 2003 November 03.Methods. We study radio spectra (AIP), simultaneous radio images (Nancay Multi-frequency Radio Heliograph, NRH), and single-frequency polarimeter data (OAT). The radio emission is nonthermal plasma radiation with a complex structure in frequency and time. Emphasis is on the time interval when the HXR flare loop height was observed by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) to be at its minimum and an X-ray source was observed above the top of the arcade loops.Results. Two stationary, meter-wavelength sources are observed radially aligned at 0.18 and 0.41 R ⊙ above the active region and HXR sources. The lower source is apparently associated with the upper reconnection jet of the flare current sheet (CS), and the upper source is apparently associated with breakout reconnection. Sources observed at lower radio frequencies surround the upper source at the expected locations of the breakout reconnection jets.Conclusions. We believe the upper radio source is the most compelling evidence to date for the onset of breakout reconnection during a SEE. The height stationarity of the breakout sources and their dynamic radio spectrum discriminate them from propagating disturbances. Timing and location arguments reveal for the first time that both the earlier described above the flare loop top HXR source and the lower radio source are emission from the upper reconnection jet above the vertical flare CS.

A Challenging Solar Eruptive Event of 18 November 2003 and the Causes of the 20 November Geomagnetic Superstorm. I. Unusual History of an Eruptive Filament

This is the first of four companion papers, which comprehensively analyze a complex eruptive event of 18 November 2003 in active region (AR) 10501 and the causes of the largest Solar Cycle 23 geomagnetic storm on 20 November 2003. Analysis of a complete data set, not considered before, reveals a chain of eruptions to which hard X-ray and microwave bursts responded. A filament in AR 10501 was not a passive part of a larger flux rope, as usually considered. The filament erupted and gave origin to a coronal mass ejection (CME). The chain of events was as follows: i) a presumable eruption at 07:29 UT accompanied by a not reported M1.2 class flare probably associated with the onset of a first southeastern CME (CME1), which most likely is not responsible for the superstorm; ii) a confined eruption (without a CME) at 07:41 UT (M3.2 flare) that destabilized the large filament; iii) the filament acceleration around 07:56 UT; iv) the bifurcation of the eruptive filament that transformed into a large “cloud”; v) an M3.9 flare in AR 10501 associated to this transformation. The transformation of the filament could be due to the interaction of the eruptive filament with the magnetic field in the neighborhood of a null point, located at a height of about 100 Mm above the complex formed by ARs 10501, 10503, and their environment. The CORONAS-F/SPIRIT telescope observed the cloud in 304 Å as a large Y-shaped darkening, which moved from the bifurcation region across the solar disk to the limb. The masses and kinematics of the cloud and the filament were similar. Remnants of the filament were not clearly observed in the second southwestern CME (CME2), previously regarded as a source of the 20 November geomagnetic storm. These facts do not support a simple scenario, in which the interplanetary magnetic cloud is considered as a flux rope formed from a structure initially associated with the pre-eruption filament in AR 10501. Observations suggest a possible additional eruption above the bifurcation region close to solar disk center between 08:07 and 08:17 UT, which could be the source of the 20 November superstorm.

SOLAR MAGNETIC ACTIVITY CYCLES, CORONAL POTENTIAL FIELD MODELS AND ERUPTION RATES

We study the evolution of the observed photospheric magnetic field and the modeled global coronal magnetic field during the past 3 1/2 solar activity cycles observed since the mid-1970s. We use synoptic magnetograms and extrapolated potential-field models based on longitudinal full-disk photospheric magnetograms from the NSO's three magnetographs at Kitt Peak, the Synoptic Optical Long-term Investigations of the Sun (SOLIS) vector spectro-magnetograph (VSM), the spectro-magnetograph and the 512-channel magnetograph instruments, and from the U. Stanford's Wilcox Solar Observatory. The associated multipole field components are used to study the dominant length scales and symmetries of the coronal field. Polar field changes are found to be well correlated with active fields over most of the period studied, except between 2003-6 when the active fields did not produce significant polar field changes. Of the axisymmetric multipoles, only the dipole and octupole follow the poles whereas the higher orders follow the activity cycle. All non-axisymmetric multipole strengths are well correlated with the activity cycle. The tilt of the solar dipole is therefore almost entirely due to active-region fields. The axial dipole and octupole are the largest contributors to the global field except while the polar fields are reversing. This influence of the polar fields extends to modulating eruption rates. According to the Computer Aided CME Tracking (CACTus), Solar Eruptive Event Detection System (SEEDS), and Nobeyama radioheliograph prominence eruption catalogs, the rate of solar eruptions is found to be systematically higher for active years between 2003-2012 than for those between 1997-2002. This behavior appears to be connected with the weakness of the late-cycle 23 polar fields as suggested by Luhmann. We see evidence that the process of cycle 24 field reversal is well advanced at both poles.

Origins of Rolling, Twisting, and Non-radial Propagation of Eruptive Solar Events

We demonstrate that major asymmetries in erupting filaments and CMEs, namely major twists and non-radial motions are typically related to the larger-scale ambient environment around eruptive events. Our analysis of prominence eruptions observed by the STEREO, SDO and SOHO spacecraft shows that prominence spines retain, during the initial phases, the thin ribbon-like topology they had prior to the eruption. This topology allows bending, rolling, and twisting during the early phase of the eruption, but not before. The combined ascent and initial bending of the filament ribbon is non-radial in the same general direction as for the enveloping CME. However, the non-radial motion of the filament is greater than that of the CME. In considering the global magnetic environment around CMEs, as approximated by the Potential Field Source Surface (PFSS) model, we find that the non-radial propagation of both erupting filaments and associated CMEs is correlated with the presence of nearby coronal holes, which deflect the erupting plasma and embedded fields. In addition, CME and filament motions respectively are guided towards weaker field regions, namely null points existing at different heights in the overlying configuration. Due to the presence of the coronal hole, the large-scale forces acting on the CME may be asymmetric. We find that the CME propagates usually non-radially in the direction of least resistance, which is always away from the coronal hole. We demonstrate these results using both low and high latitude examples.

Spatially resolved observations of a split-band coronal type II radio burst

Context. The origin of coronal type-II radio bursts and of their band-splitting are still not fully understood. Aims. To make progress in solving this problem on the basis of one extremely well observed solar eruptive event. Methods. The relative dynamics of multi-thermal eruptive plasmas, observed in detail by the SDO/AIA and of the harmonic type-II burst sources, observed by the NRH at ten frequencies from 445 to 151 MHz, is studied for the partially behind the limb event on 3 November 2010. Special attention is given to the band-splitting of the burst. Analysis is supplemented by investigation of coronal hard X-ray (HXR) sources observed by the RHESSI. Results. It is found that the flare impulsive phase was accompanied by the formation of a double coronal HXR source, whose upper part coincided with the hot (T~10 MK) eruptive plasma blob. The leading edge (LE) of the eruptive plasmas (T~1-2 MK) moved upward from the flare region with the speed of v=900-1400 km/s. The type II burst source initially appeared just above the LE apex and moved with the same speed and in the same direction. After about 20 s it started to move about twice faster, but still in the same direction. At any given moment the low frequency component (LFC) source of the splitted type-II burst was situated above the high frequency component (HFC) source, which in turn was situated above the LE. It is also found that at a given frequency the HFC source was located slightly closer to the photosphere than the LFC source. Conclusions. The shock wave, which could be responsible for the observed type-II radio burst, was initially driven by the multi-temperature eruptive plasmas, but later transformed to a freely propagating blast shock wave. The most preferable interpretation of the type-II burst splitting is that its LFC was emitted from the upstream region of the shock, whereas the HFC - from the downstream region.

GLOBAL ENERGETICS OF THIRTY-EIGHT LARGE SOLAR ERUPTIVE EVENTS

We have evaluated the energetics of 38 solar eruptive events observed by a variety of spacecraft instruments between February 2002 and December 2006, as accurately as the observations allow. The measured energetic components include: (1) the radiated energy in the GOES 1 - 8 A band; (2) the total energy radiated from the soft X-ray (SXR) emitting plasma; (3) the peak energy in the SXR-emitting plasma; (4) the bolometric radiated energy over the full duration of the event; (5) the energy in flare-accelerated electrons above 20 keV and in flare-accelerated ions above 1 MeV; (6) the kinetic and potential energies of the coronal mass ejection (CME); (7) the energy in solar energetic particles (SEPs) observed in interplanetary space; and (8) the amount of free (nonpotential) magnetic energy estimated to be available in the pertinent active region. Major conclusions include: (1) the energy radiated by the SXR-emitting plasma exceeds, by about half an order of magnitude, the peak energy content of the thermal plasma that produces this radiation; (2) the energy content in flare-accelerated electrons and ions is sufficient to supply the bolometric energy radiated across all wavelengths throughout the event; (3) the energy contents of flare-accelerated electrons and ions are comparable; (4) the energy in SEPs is typically a few percent of the CME kinetic energy (measured in the rest frame of the solar wind); and (5) the available magnetic energy is sufficient to power the CME, the flare-accelerated particles, and the hot thermal plasma.

Radio Pulsating Structures with Coronal Loop Contraction

We present a multi-wavelength study of a solar eruption event on 20 July 2004, comprising observations in Hα, EUV, soft X-rays, and in radio waves with a wide frequency range. The analyzed data show both oscillatory patterns and shock wave signatures during the impulsive phase of the flare. At the same time, large-scale EUV loops located above the active region were observed to contract. Quasi-periodic pulsations with ∼ 10 and ∼ 15 s oscillation periods were detected both in microwave – millimeter waves and in decimeter – meter waves. Our calculations show that MHD oscillations in the large EUV loops – but not likely in the largest contracting loops – could have produced the observed periodicity in radio emission, by triggering periodic magnetic reconnection and accelerating particles. As the plasma emission in decimeter – meter waves traces the accelerated particle beams and the microwave emission shows a typical gyrosynchrotron flux spectrum (emission created by trapped electrons within the flare loop), we find that the particles responsible for the two different types of emission could have been accelerated in the same process. Radio imaging of the pulsed decimetric – metric emission and the shock-generated radio type II burst in the same wavelength range suggest a rather complex scenario for the emission processes and locations. The observed locations cannot be explained by the standard model of flare loops with an erupting plasmoid located above them, driving a shock wave at the CME front.

MAXIMIZING MAGNETIC ENERGY STORAGE IN THE SOLAR CORONA

The energy that drives solar eruptive events such as coronal mass ejections (CMEs) almost certainly originates in coronal magnetic fields. Such energy may build up gradually on timescales of days or longer before its sudden release in an eruptive event, and the presence of free magnetic energy capable of rapid release requires nonpotential magnetic fields and associated electric currents. For magnetic energy to power a CME, that energy must be sufficient to open the magnetic field to interplanetary space, to lift the ejecta against solar gravity, and to accelerate the material to speeds of typically several hundred km s{sup -1}. Although CMEs are large-scale structures, many originate from relatively compact active regions on the solar surface-suggesting that magnetic energy storage may be enhanced when it takes place in smaller magnetic structures. This paper builds on our earlier work exploring energy storage in large-scale dipolar and related bipolar magnetic fields. Here we consider two additional cases: quadrupolar fields and concentrated magnetic bipoles intended to simulate active regions. Our models yield stored energies whose excess over that of the corresponding open field state can be greater than 100% of the associated potential field energy; this contrasts with maximum excess energies of onlymore » about 20% for dipolar and symmetric bipolar configurations. As in our previous work, energy storage is enhanced when we surround a nonpotential field with a strong overlying potential field that acts to 'hold down' the nonpotential flux as its magnetic energy increases.« less

Solar eruptive events

It’s long been known that the Sun plays host to the most energetic explosions in the solar system. But key insights into how they work have only recently become available.

OBSERVATIONS OF A TWO-STAGE SOLAR ERUPTIVE EVENT (SEE): EVIDENCE FOR SECONDARY HEATING

We present RHESSI, SDO/AIA, SOHO/LASCO, STEREO, and GOES observations of a partially occulted solar eruptive event (SEE) that occurred at the South-West limb on 8 March, 2011. The GOES X-ray light curve shows two peaks separated by almost two hours that we interpret as two stages of a single event associated with the delayed eruption of a CME. A hot flux rope formed during the first stage and continued expanding and rising throughout the event. The speed of the flux rope decreased from approx.120 to 14 km/s during the decay phase of the first stage and increased again during the second stage to become the CME with a speed of approx.516 km/s. RHESSI and GOES data analyses show that the plasma temperature reached over 20 MK in the first stage, then decreased to approx.10 MK and increased to 15 MK in the second stage. This event provides clear evidence for a secondary heating phase. The enhanced EUV and X-ray emission came from the high corona ( approx.60 arcsec above the limb) in the second stage, approx.40 arcsec higher than the site of the initial flare emission. STEREO-A on-disk observations indicate that the post-flare loops during this stage were of larger scale sizes and spatially distinct from those in the first stage.

Recent Developments of NEMO: Detection of EUV Wave Characteristics

The recent developments in space instrumentation for solar observations and telemetry have caused the necessity of advanced pattern recognition tools for the different classes of solar events. The Extreme ultraviolet Imaging Telescope (EIT) of solar corona on-board SOHO spacecraft has uncovered a new class of eruptive events which are often identified as signatures of Coronal Mass Ejection (CME) initiations on solar disk. It is evident that a crucial task is the development of an automatic detection tool of CMEs precursors. The Novel EIT wave Machine Observing (NEMO) (http://sidc.be/nemo) code is an operational tool that detects automatically solar eruptions using EIT image sequences. NEMO applies techniques based on the general statistical properties of the underlying physical mechanisms of eruptive events on the solar disc. In this work, the most recent updates of NEMO code - that have resulted to the increase of the recognition efficiency of solar eruptions linked to CMEs - are presented. These updates provide calculations of the surface of the dimming region, implement novel clustering technique for the dimmings and set new criteria to flag the eruptive dimmings based on their complex characteristics. The efficiency of NEMO has been increased significantly resulting to the extraction of dimmings observed near the solar limb and to the detection of small-scale events as well. As a consequence, the detection efficiency of CMEs precursors and the forecasts of CMEs have been drastically improved. Furthermore, the catalogues of solar eruptive events that can be constructed by NEMO may include larger number of physical parameters associated to the dimming regions.

THE TEMPERATURE-DEPENDENT NATURE OF CORONAL DIMMINGS

The opening-up of the magnetic field during solar eruptive events is often accompanied by a dimming of the local coronal emission. From observations of filament eruptions recorded with the Extreme-Ultraviolet Imager on STEREO during 2008-2009, it is evident that these dimmings are much more pronounced in 19.5 nm than in the lower-temperature line 17.1 nm, as viewed either on the disk or above the limb. We conclude that most of the cooler coronal plasma is not ejected but remains gravitationally bound when the loops open up. This result is consistent with Doppler measurements by Imada and coworkers, who found that the upflow speeds in a transient coronal hole increased dramatically above a temperature of 1 MK; it is also consistent with the quasistatic behavior of polar plumes, as compared with the hotter interplume regions that are the main source of the fast solar wind. When the open flux reconnects and closes down again, the trapped plasma is initially heated to such high temperatures that it is no longer visible at Fe IX 17.1 nm. Correspondingly, 17.1 nm images show a dark ribbon or ``heat wave'' propagating away from the polarity inversion line and coinciding with the brightened Fe XV 28.4 nm and Fe XII 19.5 nm post-eruptive loops and their footpoint areas. Such dark ribbons provide a clear example of dimmings that are not caused by a density depletion. The propagation of the ``heat wave'' is driven by the closing-down, not the opening-up, of flux and can be observed both off-limb and on-disk.

From the Sun to the Earth: The 13 May 2005 Coronal Mass Ejection

We report the results of a multi-instrument, multi-technique, coordinated study of the solar eruptive event of 13 May 2005. We discuss the resultant Earth-directed (halo) coronal mass ejection (CME), and the effects on the terrestrial space environment and upper Earth atmosphere. The interplanetary CME (ICME) impacted the Earth’s magnetosphere and caused the most-intense geomagnetic storm of 2005 with a Disturbed Storm Time (Dst) index reaching −263 nT at its peak. The terrestrial environment responded to the storm on a global scale. We have combined observations and measurements from coronal and interplanetary remote-sensing instruments, interplanetary and near-Earth in-situ measurements, remote-sensing observations and in-situ measurements of the terrestrial magnetosphere and ionosphere, along with coronal and heliospheric modelling. These analyses are used to trace the origin, development, propagation, terrestrial impact, and subsequent consequences of this event to obtain the most comprehensive view of a geo-effective solar eruption to date. This particular event is also part of a NASA-sponsored Living With a Star (LWS) study and an on-going US NSF-sponsored Solar, Heliospheric, and INterplanetary Environment (SHINE) community investigation.

RE-FLARING OF A POST-FLARE LOOP SYSTEM DRIVEN BY FLUX ROPE EMERGENCE AND TWISTING

In this letter, we study in detail the evolution of the post-flare loops on 2005 January 15 that occurred between two consecutive solar eruption events, both of which generated a fast halo CME and a major flare. The post-flare loop system, formed after the first CME/flare eruption, evolved rapidly, as manifested by the unusual accelerating rise motion of the loops. Through nonlinear force-free field (NLFFF) models, we obtain the magnetic structure over the active region. It clearly shows that the flux rope below the loops also kept rising accompanied with increasing twist and length. Finally, the post-flare magnetic configuration evolved to a state that resulted in the second CME/flare eruption. This is an event in which the post-flare loops can re-flare in a short period of $\sim$16 hr following the first CME/flare eruption. The observed re-flaring at the same location is likely driven by the rapid evolution of the flux rope caused by the magnetic flux emergence and the rotation of the sunspot. This observation provides valuable information on CME/flare models and their prediction.

Geoefficiency of solar eruptive events

The sources of geomagnetic disturbances during 1999–2003 are discussed. The relation between geomagnetic activity and the rate of coronal mass ejections (CMEs), their parameters, and the dynamics of solar photospheric magnetic fields is considered. It is shown that during the reorganization of unipolar regions of the photospheric magnetic field, the number of CMEs increases and their parameters change. The geomagnetic disturbance level also increases in these periods.

Coronal mass ejections: structure and dynamics

We discuss in this paper some issues related to the relationship, currently thought to be relatively strong, between large scale solar eruptive events and twisted magnetic flux ropes (TFRs). We first consider the possibility to prove the presence of a TFR in a pre-eruptive configuration by using a model along with observational information provided by a vector magnetograph. Thus we examine, in the framework of a generic model in which the coronal field is driven into an evolution by changes imposed at the photospheric level, several mechanisms which may lead to the formation and the disruption of a TFR. We consider in particular the development of a MHD instability, and we address the issues of the energy and helicity contents of an erupting configuration.

A study of eruptive solar events with negative radio bursts

Solar events of June 15/16, 2000, June 1/2, 2002, February 6, 2002, and February 7, 2002, have been studied. These events probably belong to a poorly studied class of explosive eruptions. In such events disintegration of the magnetic structure of an eruptive filament and dispersing of its fragments as a cloud over a considerable part of the solar surface are possible. The analysis of SOHO/EIT extreme ultraviolet images obtained in the 195 A and 304 A channels has revealed the appearance of dimmings of various shapes and propagation of a coronal wave for June 1/2, 2002. In all the events the Nobeyama, Learmonth, and Ussuriysk observatories recorded negative radio bursts at several frequencies in the 1–10 GHz range. Most likely, these bursts were due to absorption of solar radio emission in clouds produced by fragments of filaments. Absorption of the solar background radiation can be observed as a depression of the emission in the 304 A channel. A model has been developed, which permits one to estimate parameters of absorbing plasma such as temperature, optical thickness, area of the absorbing cloud, and its height above the chromosphere from the radio absorption observed at several frequencies. The obtained values of the temperature, 8000–9000 K, demonstrate that the absorber was the material of an erupted cool filament. The model estimate of the masses of the ejecta in the considered events were ∼1015 g, which is comparable to masses of typical filaments and coronal mass ejections.