Large Particle Events Research Articles

The 26 December 2001 Solar Eruptive Event Responsible for GLE63. II. Multi-Loop Structure of Microwave Sources in a Major Long-Duration Flare

Analysis of the observations of the SOL2001-12-26 event related to ground-level-event GLE63, including microwave spectra and images from NoRH at 17 and 34 GHz, SSRT at 5.7 GHz, and TRACE in 1600 \AA\ has led to the following results. A flare ribbon overlapped with the sunspot umbra, which is typical of large particle events. Atypical were: i) long duration of the flare of more than one hour; ii) moderate intensity of a microwave burst, about $10^4$ sfu; iii) low peak frequency of the gyrosynchrotron spectrum, around 6 GHz; and its insensitivity to the flux increase by more than one order of magnitude. This was accompanied by a nearly constant ratio of the flux emitted by the volume in the high-frequency part of the spectrum to its elevated low-frequency part determined by the area of the source. With the self-similarity of the spectrum, a similarity was observed between the moving microwave sources and the brightest parts of the flare ribbons in 1600 \AA. Comparison of the 17 GHz and 1600 \AA\ images has confirmed that the microwave sources were associated with multiple flare loops, whose footpoints appeared in ultraviolet as intermittent bright kernels. To understand the properties of the event, we simulated its microwave emission using a system of several homogeneous gyrosynchrotron sources above the ribbons. The scatter between the spectra and sizes of the individual sources is determined by the inhomogeneity of the magnetic field within the ribbons. The microwave flux is mainly governed by the magnetic flux passing through the ribbons and the sources. An apparent simplicity of microwave structures is caused by a poorer spatial resolution and dynamic range of the microwave imaging. The results indicate that microwave manifestations of accelerated electrons correspond to the structures observed in thermal emissions, as well-known models predict.

The Properties of Solar Energetic Particle Event-Associated Coronal Mass Ejections Reported in Different CME Catalogs

We compare estimates of the speed and width of coronal mass ejections (CMEs) in several catalogs for the CMEs associated with ~200 solar energetic particle (SEP) events in 2006-2013 that included 25 MeV protons. The catalogs used are: CDAW, CACTUS, SEEDS and CORIMP, all derived from observations by the LASCO coronagraphs on the SOHO spacecraft, the CACTUS catalog derived from the COR2 coronagraphs on the STEREO-A and -B spacecraft, and the DONKI catalog, which uses observations from SOHO and the STEREO spacecraft. We illustrate how, for this set of events, CME parameters can differ considerably in each catalog. The well-known correlation between CME speed and proton event intensity is shown to be similar for most catalogs, but this is largely because it is determined by a few large particle events associated with fast CMEs, and small events associated with slow CMEs. Intermediate particle events "shuffle" in position when speeds from different catalogs are used. Quadrature spacecraft CME speeds do not improve the correlation. CME widths also vary widely between catalogs, and are influenced by plane of the sky projection and how the width is inferred from the coronagraph images. The high degree of association (~50%) between the 25 MeV proton events and "full halo" (360 deg.-width) CMEs as defined in the CDAW catalog is removed when other catalogs are considered. Using CME parameters from the quadrature spacecraft, the SEP intensity is correlated with CME width, which is also correlated with CME speed.

Acceleration of Relativistic Protons During the 20 January 2005 Flare and CME

The origin of relativistic solar protons during large flare/CME events has not been uniquely identified so far. We perform a detailed comparative analysis of the time profiles of relativistic protons detected by the worldwide network of neutron monitors at Earth with electromagnetic signatures of particle acceleration in the solar corona during the large particle event of 20 January 2005. The intensity – time profile of the relativistic protons derived from the neutron monitor data indicates two successive peaks. We show that microwave, hard X-ray, and γ-ray emissions display several episodes of particle acceleration within the impulsive flare phase. The first relativistic protons detected at Earth are accelerated together with relativistic electrons and with protons that produce pion-decay γ rays during the second episode. The second peak in the relativistic proton profile at Earth is accompanied by new signatures of particle acceleration in the corona within ≈1R ⊙ above the photosphere, revealed by hard X-ray and microwave emissions of low intensity and by the renewed radio emission of electron beams and of a coronal shock wave. We discuss the observations in terms of different scenarios of particle acceleration in the corona.

High‐Energy Protons Associated with Liftoff of a Coronal Mass Ejection

Large solar energetic particle (SEP) events occur in association with fast coronal mass ejections (CMEs) and flares. We have studied in detail the rise phase of the SEP event of 1998 May 2 observed with the particle telescope ERNE aboard the Solar and Heliospheric Observatory (SOHO) spacecraft and ground-based neutron monitors. Using the ERNE data and numerical modeling of the SEP transport, we present improved evaluations of the solar release profile of deka-MeV protons. The SOHO EIT images are used to study the CME liftoff processes and possible sources of deka-MeV and hecto-MeV proton streams. In a first stage of the deka-MeV proton production, which starts not later than 4 minutes after the radio flash and the Moreton wave start, particles get accelerated from a few MeV through 20 MeV in ≈15 minutes. Both ERNE and neutron monitor data are used to study the release of solar protons in the hecto-MeV range. The proton acceleration to above 400 MeV was completed not later than 15-20 minutes after the onset of the eruption. However, injection profiles of deka-MeV protons and hecto-MeV protons were different. Differences in the release scenarios, energy spectra, and composition of deka-MeV protons versus hecto-MeV protons suggest two different acceleration regions involved, perhaps situated on initially open lines and initially closed lines of the coronal magnetic field. The first SEP productions were followed by a prolonged period of proton reacceleration, which continued in the ~10-100 MeV range for many hours and during which a common energy spectrum was formed.

Extremely large solar high-energy particle events: Occurrence probability and characteristics

The probability of occurrence of extremely large energetic particle (SEP) events is determined by both the general properties of the distribution function, and the character of this function in the region of large particle fluxes (fluences and peak fluxes). Choosing the most authentic function for describing the experimental distribution, we can extrapolate this function into the region of fluxes, which have not been recorded yet till now.

Annual Variations of Spring Aerosol Events Observed by Sun Photometers from 1998 to 2002 over Japan

According to records of meteorological stations of the Japan Meteorological Agency (JMA), the occurrence of Kosa dust events increased during 2000-2002. This was verified by the analyses of aerosol optical depth at 500 nm (τ500) and Angstrom exponent (α) obtained on a minutely basis by sun photometers in JMA from 1998 through 2002. The significant aerosol event (τ500 > 0.3) was classified into “a significant large particle event” (α ≤ 1.0) and “a significant small particle event” (α > 1.0). “Significant large particle event days” in Ryori were clearly more frequent from 2000 to 2002 compared with 1998 and 1999, and corresponded with the frequencies of Kosa dust events in China and Japan. In Yonagunijima, Japan, however, “Significant large particle event days” were not corresponded with the frequencies of Kosa dust events in China. Moreover, the occurrence distribution of the Angstrom exponent (α), an index of particle size distribution, in Yonagunijima showed that typical Kosa events were rare, while significant small particle events occurred frequently. This indicates the dominant influence of relatively small particles on the large spring aerosol optical depth in Yonagunijima, rather than large particles such as Kosa dust. Trajectory analysis suggested that these small particles originated from urban pollution in the east coast of the Asian continent and Taiwan and/or smoke from biomass burning in Southeast Asia.

Observation of solar particle event on board a commercial flight from Los Angeles to New York on 29 October 2003

The enhanced radiation associated with solar particle events and in particular the secondary ionization doses created by the neutron flux in the upper atmosphere resulting from both cosmic ray and solar particle interaction with constituents of the atmosphere, pose an additional significant risk to both aircrew and passengers at aircraft altitude. Whilst the elevated x‐ray component from a solar flare or coronal mass ejection (CME) can be significant, the effects of large particle events to the secondary radiation environment at jet aircraft cruising altitudes, can have consequences for both people and micro electronics used in modern generation aircraft. It has been shown that dose rates increase with higher latitudes and altitudes, and due to the geomagnetic field effect that deflects incoming solar and galactic cosmic rays, we get greater shielding closer to the equator. Generally increasing latitude appears to have a greater effect on increasing secondary radiation than altitude alone and this is certainly so during a solar flare or solar particle event.

An upper limit of the number and energy of electrons accelerated at an extended coronal shock wave

Type II radio emission is the most direct signature of extended shock waves in the solar corona, and a prime example of electron acceleration at a shock. Yet, the energetics of these electrons is poorly known. On 1996 August 19 and 1997 October 07 type II bursts were observed in association with flares behind the solar limb. While Earth-orbiting X-ray detectors aboard GOES and the Compton Gamma-Ray Observatory did not register emission above background, theUlysses spacecraft, which had the candidate active region in its field of view, saw a major burst during the 1996 event. We use this peculiar configuration to infer the energetics of electrons accelerated in the low corona from the Ulysses observations, and to derive upper limits of the number and energy content of shock-accelerated electrons from the thresholds of the Earth-orbiting detectors. The inferred upper limit is similar to electron numbers estimated for large particle events at 1 AU, but the fluxes and energy fluxes of shock-accelerated electrons are much smaller than those required to produce a conspicuous hard X-ray burst.

A model study of the impact of magnetic field structure on atmospheric composition during solar proton events

During a polarity transition of the Earth's magnetic field, the structure and strength of the field change significantly from their present values. This will alter the global pattern of charged particle precipitation into the atmosphere. Thus, particle precipitation is possible into regions that are at the moment effectively shielded by the Earth's magnetic field. A two‐dimensional global chemistry, photolysis and transport model of the atmosphere has been used to investigate how the increased particle precipitation affects the chemical composition of the middle and lower atmosphere. Ozone losses resulting from large energetic particle events are found to increase significantly, with resultant losses similar to those observed in the Antarctic ozone hole of the 1990s. This results in significant increases in surface UV‐B radiation as well as changes in stratospheric temperature and circulation over a period of several months after large particle events.

Delay in solar energetic particle onsets at high heliographic latitudes

Abstract. Ulysses observations have shown that solar energetic particles (SEPs) can easily reach high heliographic latitudes. To obtain information on the release and propagation of SEPs prior to their arrival at Ulysses, we analyse the onsets of nine large high-latitude particle events. We measure the onset times in several energy channels, and plot them versus inverse particle speed. This allows us to derive an experimental path length and time of release from the solar atmosphere. We repeat the procedure for near-Earth observations by Wind and SOHO. We find that the derived path lengths at Ulysses are 1.06 to 2.45 times the length of a Parker spiral magnetic field line connecting the spacecraft to the Sun. The time of particle release from the Sun is between 100 and 350 min later than the release time derived from in-ecliptic measurements. We find no evidence of correlation between the delay in release and the inverse of the speed of the CME associated with the event, or the inverse of the speed of the corresponding interplanetary shock. The main parameter determining the magnitude of the delay appears to be the difference in latitude between the flare and the footpoint of the spacecraft.Key words. Interplanetary physics (energetic particles) – Solar physics, astrophysics and astronomy (energetic particles, flares and mass ejections)

Cause of Proton Depletion in Solar Impulsive Particle Events

It is found from our reanalysis of the relevant data that the high electron-to-proton ratio (E/P) observed in large gamma-ray line (GRL) flares can be alternatively explained by proton depletion rather than electron richness as formerly suggested. The term impulsive in this paper is exclusively used as the large events associated with some energetic flare phenomena like GRL emissions and meter-wave type II emissions. Consequently, other weaker events that occur much more frequently, and in association with 3He-rich particle events, are outside the scope of this study. We propose that the proton depletion can be explained by either intrinsic E/Ps of escaping particles from the flare itself or a selection effect in the acceleration process at the lower coronal shock. In the latter case, we suggest that shock waves in the lower corona accelerate only a very few protons because there are no such seed particles as are usually supplied by non-Maxwellian suprathermal high-energy tails in the case of upper collisionless atmosphere. Such suprathermal seed particles are actually observed even in the steady solar wind ubiquitously. As for the former case of the explanation, the flare origin hypothesis for large particle events seems unlikely, because of the difference in the emission cone angles between the large and small events. For gradual events, on the other hand, shocks are thought to be driven by coronal mass ejections and can survive even in the upper atmosphere, accompanied by seed particles, resulting in acceleration of plenty of protons.

Northern hemisphere atmospheric effects due to the July 2000 Solar Proton Event

The third largest solar proton event in the past thirty years took place during July 14–16, 2000, and had a significant impact on the earth's atmosphere. These energetic protons produced both HOx (H, OH, HO2) and NOx (N, NO, NO2) constituents in the mesosphere and upper stratosphere at polar latitudes (> 60° geomagnetic) of both hemispheres. The temporal evolution of increases in NO and NO2 during the event at northern polar latitudes were measured by the UARS HALOE instrument. Increases in mesospheric NOx of over 50 ppbv were found in the HALOE measurements. Measurements from the UARS HALOE and NOAA 14 SBUV/2 instruments indicate short‐term (∼day) middle mesospheric ozone decreases of over 70% caused by short‐lived HOx during the event with a longer‐term (several days) upper stratospheric ozone depletion of up to 9% caused by longer‐lived NOx. We believe this is the first time that the three constituents NO, NO2, and ozone were all measured simultaneously during a proton event. The observations constitute a dramatic confirmation of the impact of a large particle event in the control of ozone in the polar middle atmosphere and offer the opportunity to test theories of constituent changes driven by particle precipitation.

The Origin of Solar Energetic Particle Events: Coronal Acceleration versus Shock Wave Acceleration

We review evidence that led to the view that acceleration at shock waves driven by coronal mass ejections (CMEs) is responsible for large particle events detected at 1 AU. It appears that even if the CME bow shock acceleration is a possible model for the origin of rather low energy ions, it faces difficulties on account of the production of ions far above 1 MeV: (i) although shock waves have been demonstrated to accelerate ions to energies of some MeV nucl−1 in the interplanetary medium, their ability to achieve relativistic energies in the solar environment is unproven; (ii) SEP events producing particle enhancements at energies ≥100 MeV are also accompanied by flares; those accompanied only by fast CMEs have no proton signatures above 50 MeV. We emphasize detailed studies of individual high energy particle events which provide strong evidence that time-extended particle acceleration which occurs in the corona after the impulsive flare contributes to particle fluxes in space. It appears thus that the CME bow shock scenario has been overvalued and that long lasting coronal energy release processes have to be taken into account when searching for the origin of high energy SEP events.

3He Enhancements in Large Solar Energetic Particle Events.

We have measured the 3He abundance from approximately 0.5 to 2 MeV nucleon-1 in 12 large solar energetic particle (SEP) events during the period 1997 November-1999 June. In five of the events, the 3He time-intensity profile is similar to the 4He time-intensity profile, indicating a common acceleration and transport origin for the two species. The average 3He/4He ratio during these events is &parl0;1.9+/-0.2&parr0;x10-3, a factor of approximately 5 enhancement over the solar wind value. During this same survey period, we have also measured the low-energy ion intensities during quieter periods in between the large-particle events. We find 3He and Fe remnants from impulsive events present on a majority of the days, implying that they fill a substantial volume (>50%) of the in-ecliptic interplanetary medium during our survey. We suggest that these suprathermal ions may therefore be a source population that is available for further acceleration by interplanetary shocks that accompany large SEP events, thereby leading to the 3He enhancements in a significant fraction of large SEP events. This impulsive SEP event material might also account for recent observations of large solar particle events with energetic particle ionization states that have a wide range of ionization states that encompass values expected for both gradual and impulsive solar SEP events.

IMP-8 observations of the spectra, composition, and variability of solar heavy ions at high energies relevant to manned space missions

In more than 25 years of almost continuous observations, the University of Chicago’s Cosmic Ray Telescope (CRT) on IMP-8 has amassed a unique database on high-energy solar heavy ions of potential relevance to manned spaceflight. In the very largest particle events, IMP-8/CRT has even observed solar Fe ions above the Galactic cosmic ray background up to ∼800 MeV/nucleon, an energy sufficiently high to penetrate nearly 25 g/cm 2 of shielding. IMP-8/CRT observations show that high-energy heavy-ion spectra are often surprisingly hard power laws, without the exponential roll-offs suggested by stochastic acceleration fits to lower energy measurements alone. Also, in many solar particle events the Fe/O ratio grows with increasing energy, contrary to the notion that ions with higher mass-to-charge ratios should be less abundant at higher energies. Previous studies of radiation hazards for manned spaceflight have often assumed heavy-ion composition and steeply-falling energy spectra inconsistent with these observations. Conclusions based on such studies should therefore be re-assessed. The significant event-to-event variability observed in the high-energy solar heavy ions also has important implications for strategies in building probabilistic models of solar particle radiation hazards.

The influence of solar proton events on the ozone layer

Our two-dimensional chemistry and transport atmospheric model was used to simulate the influence of solar proton events (SPEs) over the 1965 to 1995 time period. Extremely large particle events with huge fluxes of very high energy protons occurred in August 1972 and October 1989 and caused large increases in long-lived NOY constituents, which affected ozone for several months to years past the events. Our simulations show that the SPE-produced NOY constituents in the upper stratosphere caused direct ozone losses. However, interference of the NOY constituents with the halogen loss cycles for ozone destruction actually led to some total ozone production. We predict a seasonal dependence for the impact on polar ozone, with larger (smaller) decreases predicted for SPEs occurring in the fall or winter (spring or summer).

Ulysses and WIND particle observations of the November 1997 solar events

The episode of intense solar activity on 4 and 6 November 1997 resulted in large and extended particle events observed by the Ulysses and WIND spacecraft. Ulysses was at a heliocentric distance of 5.34 AU, very close to the solar equator and 100° west in heliolongitude from earth. WIND was near the earth at 0.99 AU from the Sun and in the ecliptic plane. Both spacecraft detected particle events and magnetic field structures related to this intense activity. A likely scenario for these particle events is proposed. The differences in the particle intensity profiles observed by both spacecraft are explained in terms of their location with respect to the solar activity site, the existence of interplanetary structures traveling towards the spacecraft, and the nature of particle transport from the source up to the two spacecraft.

Comment on the Solar Flare Debate

The controversy surrounding the “Solar Flare Myth” [Miller et al., 1995] is understandable, given the fairly extreme positions taken by the proponents and opponents of the “myth” [Gosling, 1993]. Having spent some time in the field of solar energetic particle events and their terrestrial effects, extending back even to the time when such events were unrecognized, I felt compelled to comment on one aspect of the controversy that I believe is clearly wrong as stated by the “myth” supporters. It is the statement that large energetic particle events have little, if any, connection with flares. This is directly contradictory to the observations made back in the late 1950s by ground‐based riometers, as described in several papers published in the late 1950s and early 1960s [e.g., Reid and Leinbach, 1959, 1961].

New findings challenge beliefs about solar‐terrestrial physics

Solar flares are intense, short‐lived brightenings that occur near sunspots on the Sun's surface. Large geomagnetic storms, intense auroral displays, large energetic particle events in interplanetary space, and major shock wave disturbances in the solar wind often occur in close association with large solar flares. Over the years, the common association of these events in near‐Earth space with solar flares led to a paradigm of cause and effect in which large solar flares came to be understood as the fundamental cause of these disturbances.Certain aspects of this paradigm were developed in the early 1930s [e.g., Hale, 1931], and by the early 1960s it had become part of the underlying dogma central to the discipline of solar‐terrestrial physics. This paradigm still dominates the popular perception of the relationship between solar activity and interplanetary and geomagnetic events and continues to provide much of the pragmatic rationale for the study of the solar flare phenomenon.

Coronal Abundances of Neon and Magnesium Isotopes from Solar Energetic Particles

The relative abundances of 22 to 70 Me V nucleon^(-l) Ne and Mg isotopes from two large flare-associated solar energetic particle (SEP) events in 1992 October and November, are determined from data taken by the Mass Spectrometer Telescope on the polar-orbiting SAMPEX satellite. After converting the measured abundances to coronal values, the average ^(22)Ne/^(20)Ne ratio is 0.076 ± 0.008, consistent with the solar wind value of 0.073 but significantly less than that of previous SEP measurements, in which the abundance of ^(22)Ne was substantially enhanced. The average ^(26)Mg/^(24)Mg ratio is 0.152 ± 0.012, consistent with both previous SEP and other solar system measurements.