Abundance Enhancements in Impulsive Solar Energetic-Particle Events with Associated Coronal Mass Ejections

We study the abundances of the elements He through Pb in Fe-rich impulsive solar energetic-particle (SEP) events with measurable abundances of ions with atomic number Z>2 observed on the Wind spacecraft, and their relationship with coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO). On an average the element abundances in these events are similar to coronal abundances at low Z but, for heavier elements, enhancements rise as a power law in the mass-to-charge ratio A/Q of the ions (at coronal temperatures of 2.5?–?3 MK) to a factor of 3 at Ne, 9 at Fe, and 900 for 76≤Z≤82. Energy dependences of abundances are minimal in the 2?–?15 MeV amu^?1 range. The 111 of these Fe-rich impulsive SEP events we found, between November 1994 and August 2013 using the Wind spacecraft, have a 69 % association rate with CMEs. The CMEs are narrow with a median width of 75^°, are characteristically from western longitudes on the Sun, and have a median speed of ≈?600 km?s^?1. Nearly all SEP onsets occur within 1.5?–?5 h of the CME onset. The faster (>?700 km?s^?1), wider CMEs in our sample are related to SEPs with coronal abundances indicating hot coronal plasma with fully ionized He, C, N and O and moderate enhancements of heavier elements, relative to He, but slower (<?700 km?s^?1), narrower CMEs emerge from cooler plasma where higher SEP mass-to-charge ratios, A/Q, yield much greater abundance enhancements, even for C/He and O/He. Apparently, the open magnetic-reconnection region where the impulsive SEPs are accelerated also provides the energy to drive out CME plasma, accounting for a strong, probably universal, impulsive SEP-CME association.     

Coronal Mass Ejections Associated with Slow Long Duration Flares

It is well known that there is a temporal relationship between coronal mass ejections (CMEs) and associated flares. The duration of the acceleration phase is related to the duration of the rise phase of a flare. We investigate CMEs associated with slow long duration events (LDEs), i.e. flares with the long rising phase. We determined the relationships between flares and CMEs and analyzed the CME kinematics in detail. The parameters of the flares (GOES flux, duration of the rising phase) show strong correlations with the CME parameters (velocity, acceleration during main acceleration phase, and duration of the CME acceleration phase). These correlations confirm the strong relation between slow LDEs and CMEs. We also analyzed the relation between the parameters of the CMEs, i.e. a velocity, an acceleration during the main acceleration phase, a duration of the acceleration phase, and a height of a CME at the end of the acceleration phase. The CMEs associated with the slow LDEs are characterized by high velocity during the propagation phase, with the median equal to 1423 km?s^?1. In half of the analyzed cases, the main acceleration was low (a<300 m?s^?2), which suggests that the high velocity is caused by the prolonged acceleration phase (the median for the duration of the acceleration phase is equal 90 minutes). The CMEs were accelerated up to several solar radii (with the median ≈?7 R _⊙), which is much higher than in typical impulsive CMEs. Therefore, slow LDEs may potentially precede extremely strong geomagnetic storms. The analysis of slow LDEs and associated CMEs may give important information for developing more accurate space-weather forecasts, especially for extreme events.     

Latitudinal Distribution of Solar Flares and Their Association with Coronal Mass Ejections

Major solar flare events have been utilised to study the latitudinal frequency distribution of solar flares in northern and southern hemispheres for the period of 1986 to 2003. A statistical analysis has been performed to obtain the correlation between Coronal Mass Ejections (CMEs) and Forbush decrease (Fds) of cosmic ray intensity. Almost the same flares distribution in both hemispheres is found in association with CMEs. In a further analysis, it is noted that a larger number of CME-associated solar flares located in the northern hemisphere are found to be more effective in producing Forbush decreases.     

The Association of Solar Flares with Coronal Mass Ejections During the Extended Solar Minimum

We study the association of solar flares with coronal mass ejections (CMEs) during the deep, extended solar minimum of 2007?–?2009, using extreme-ultraviolet (EUV) and white-light (coronagraph) images from the Solar Terrestrial Relations Observatory (STEREO). Although all of the fast (v>900 km?s^?1), wide (θ>100^°) CMEs are associated with a flare that is at least identified in GOES soft X-ray light curves, a majority of flares with relatively high X-ray intensity for the deep solar minimum (e.g. ?1×10^?6 W?m^?2 or C1) are not associated with CMEs. Intense flares tend to occur in active regions with a strong and complex photospheric magnetic field, but the active regions that produce CME-associated flares tend to be small, including those that have no sunspots and therefore no NOAA active-region numbers. Other factors on scales similar to and larger than active regions seem to exist that contribute to the association of flares with CMEs. We find the possible low coronal signatures of CMEs, namely eruptions, dimmings, EUV waves, and Type III bursts, in 91 %, 74 %, 57 %, and 74 %, respectively, of the 35 flares that we associate with CMEs. None of these observables can fully replace direct observations of CMEs by coronagraphs.     

Center-to-Limb Variability of Hot Coronal EUV Emissions During Solar Flares

It is generally accepted that densities of quiet-Sun and active region plasma are sufficiently low to justify the optically thin approximation, and this is commonly used in the analysis of line emissions from plasma in the solar corona. However, the densities of solar flare loops are substantially higher, compromising the optically thin approximation. This study begins with a radiative transfer model that uses typical solar flare densities and geometries to show that hot coronal emission lines are not generally optically thin. Furthermore, the model demonstrates that the observed line intensity should exhibit center-to-limb variability (CTLV), with flares observed near the limb being dimmer than those occurring near disk center. The model predictions are validated with an analysis of over 200 flares observed by the EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO), which uses six lines, with peak formation temperatures between 8.9 and 15.8 MK, to show that limb flares are systematically dimmer than disk-center flares. The data are then used to show that the electron column density along the line of sight typically increases by \(1.76 \times 10^{19}~\mbox{cm}^{-2}\) for limb flares over the disk-center flare value. It is shown that the CTLV of hot coronal emissions reduces the amount of ionizing radiation propagating into the solar system, and it changes the relative intensities of lines and bands commonly used for spectral analysis.     

耀斑日冕硬X射线源观测研究

对于足点被日面边缘遮挡住的耀斑的观测研究是诊断日冕硬X射线辐射的一个重要方法.通过统计分析RHESSI (Reuven Ramaty High-Energy Solar Spectroscopic Imager)卫星观测到的71个此类耀斑硬X射线源发现,前人提出的两类源,即日冕X射线辐射中热辐射与非热辐射源区空间分离较小的源和分离较大的源,在能谱、成像、光变曲线以及GOES持续时间等方面都没有显著的区别,其中辐射区的面积、耀斑总热能以及GOES持续时间与分离距离之间有很好的相关性.这些结果支持近年来提出的一些耀斑统一模型.同时也表明Masuda耀斑只是一类非常特殊的事件,不具有日冕硬X射线辐射的一般特征.     

Longitudinal Distribution of Solar Flares and Their Association with Coronal Mass Ejections and Forbush Decreases

Major Hα solar-flare events of high optical importance have been employed to study their heliographic distribution in longitude around the Sun for the period of 2001 to 2006. A statistical analysis was performed to obtain their relationship with halo/partial-halo CMEs and Forbush decreases (Fds) of cosmic-ray intensity. Our analysis indicates that 63% of the solar flares associated with halo CMEs and Fds occur in the western hemisphere and of 37% of such flares occur in the eastern hemisphere. Similarly, we found that nearly 60% of the solar flares associated with partial- halo CMEs and Fds occur in the western hemisphere and the rest (40%) occur in the eastern hemisphere. Finally, we conclude that the flares in association with CMEs and located in the western hemisphere of the solar disk are more effective in producing Fds. The magnitudes of Fds are observed to be higher when in association of halo CMEs. A slight excess in the eastern hemisphere is found for both the halo and partial-halo CMEs.     

Particle Acceleration in Reconnecting Current Sheets in Impulsive Electron-Rich Solar Flares

Electron and proton acceleration in reconnecting current sheets in electron-rich solar flares is considered. A significant three-dimensional magnetic field is assumed in the current sheet where the particles are accelerated by the DC electric field. The tearing instability of a pre-flare current sheet leads to the formation of multiple singular lines of magnetic field where the electric and magnetic fields are coaligned. Magnetized electrons are shown to be accelerated to a few tens of MeV before they leave the vicinity of a singular line. The acceleration time is estimated to be less than 10^–3 s. By contrast, much heavier protons are unmagnetized and their energy gain is more modest. The model explains a high electron-to-proton ratio and the unusually intense gamma-ray continuum above 1 MeV observed in the electron-rich flares.     

Statistical Characteristics of CMEs and Flares Associated with Solar Type II Radio Bursts

We present results from a study of sunspots and faculae on continuum and Caii K images taken at the San Fernando Observatory (SFO) during 1989–1992; a total of approximately 800 images in each bandpass were used. About 18000 red sunspots, 147000 red faculae, and 800000 Caii K faculae were identified based on their contrasts. In addition, we computed the contrasts of pixels on the red images cospatial with Caii K faculae. Sunspot contrasts show a strong dependence on size but no dependence on heliocentric angle. There are continuous but systematic differences among facular regions. We find that the contrast of Caii K faculae is relatively insensitive to heliocentric angle, but is a strong function of facular size, in the sense that larger Caii K faculae are always brighter. The contrast of red faculae is a function of both heliocentric angle and size: the contrast functions show that larger regions contain larger flux tubes, contain deeper flux tubes, and have larger filling factors than small facular regions. Comparisons of cospatial pixels on red and Caii K images show a tight correlation between the average contrast of a region in the continuum and its size and heliocentric angle in the Caii K images. The average contrast of all facular regions is positive everywhere on the disk, even though the largest regions contain flux tubes which appear dark at disk center.     

Survey on Solar X-ray Flares and Associated Coherent Radio Emissions

The radio emission during 201 selected X-ray solar flares was surveyed from 100 MHz to 4 GHz with the Phoenix-2 spectrometer of ETH Zürich. The selection includes all RHESSI flares larger than C5.0 jointly observed from launch until June 30, 2003. Detailed association rates of radio emission during X-ray flares are reported. In the decimeter wavelength range, type III bursts and the genuinely decimetric emissions (pulsations, continua, and narrowband spikes) were found equally frequently. Both occur predominantly in the peak phase of hard X-ray (HXR) emission, but are less in tune with HXRs than the high-frequency continuum exceeding 4 GHz, attributed to gyrosynchrotron radiation. In 10% of the HXR flares, an intense radiation of the above genuine decimetric types followed in the decay phase or later. Classic meter-wave type III bursts are associated in 33% of all HXR flares, but only in 4% are they the exclusive radio emission. Noise storms were the only radio emission in 5% of the HXR flares, some of them with extended duration. Despite the spatial association (same active region), the noise storm variations are found to be only loosely correlated in time with the X-ray flux. In a surprising 17% of the HXR flares, no coherent radio emission was found in the extremely broad band surveyed. The association but loose correlation between HXR and coherent radio emission is interpreted by multiple reconnection sites connected by common field lines.     

Variations in Abundance Enhancements in Impulsive Solar Energetic-Particle Events and Related CMEs and Flares

We study event-to-event variations in the abundance enhancements of the elements He through Pb for Fe-rich impulsive solar energetic-particle (SEP) events, and their relationship with properties of associated coronal mass ejections (CMEs) and solar flares. Using a least-squares procedure we fit the power-law enhancement of element abundances as a function of their mass-to-charge ratio A/Q to determine both the power and the coronal temperature (which determines Q) in each of 111 impulsive SEP events identified previously. Individual SEP events with the steepest element enhancements, e.g. ～?(A/Q)⁶, tend to be smaller, lower-fluence events with steeper energy spectra that are associated with B- and C-class X-ray flares, with cooler (～?2.5 MK) coronal plasma, and with narrow (°), slower (?1) CMEs. On the other hand, higher-fluence SEP events have flatter energy spectra, less-dramatic heavy-element enhancements, e.g. ～?(A/Q)³, and come from somewhat hotter coronal plasma (～?3.2 MK) associated with C-, M-, and even X-class X-ray flares and with wider CMEs. Enhancements in ³He/⁴He are uncorrelated with those in heavy elements. However, events with ³He/⁴He≥0.1 are even more strongly associated with narrow, slow CMEs, with cooler coronal plasma, and with B- and C-class X-ray flares than are other Fe-rich impulsive SEP events with smaller enhancements of ³He.     

Sunspot Motions Associated with Flares

The evolution of five bipolar sunspot groups during their disk passage leading to flares are analysed and studied using Kodaikanal Observatory photoheliogram and spectroheliogram data. The changes in the orientation angle observed in the spot groups show that sunspot proper motion plays an important role in introducing non-potential character to the field lines. This in turn develops shear and once the shear reaches a critical value, the flare eruption is triggered. The rotational motions in the sunspots are measured from the change in their orientation angle and are given as a measure of shear. The sunspots considered for analyses in the present study are not associated with any filament activity.     

Solar Wind Speed and Expansion Rate of the Coronal Magnetic Field in Solar Maximum and Minimum Phases

Relationships between solar wind speed and expansion rate of the coronal magnetic field have been studied mainly by in-ecliptic observations of artificial satellites and some off-ecliptic data by Ulysses. In this paper, we use the solar wind speed estimated by interplanetary scintillation (IPS) observations in the whole heliosphere. Two synoptic maps of SWS estimated by IPS observations are constructed for two Carrington rotations CR 1830 and 1901; CR 1830 starting on the 11th of June, 1990 is in the maximum phase of solar activity cycle and CR 1901 starting on the 29th of September, 1995 is in the minimum phase. Each of the maps consist of 64800 (360×180) data points. Similar synoptic maps of expansion rate of the coronal magnetic field (RBR) calculated by the so-called potential model are also constructed under a radial field assumption for CR 1830 and CR1901. Highly significant correlation (r=–0.66) is found between the SWS and the RBR during CR1901 in the solar minimum phase; that is, high-speed winds emanate from photospheric areas corresponding to low expansion rate of the coronal magnetic field and low speed winds emanate from photospheric areas of high expansion rate. A similar result is found during CR 1830 in solar maximum phase, though the correlation is relatively low (r=–0.29). The correlation is improved when both the data during CR 1830 and CR 1901 are used together; the correlation coefficient becomes –0.67 in this case. These results suggest that the correlation analysis between the SWS and the RBR can be applied to estimate the solar wind speed from the expansion rate of the coronal magnetic field, though the correlation between them may depend on the solar activity cycle. We need further study of correlation analysis for the entire solar cycle to get an accurate empirical equation for the estimation of solar wind speed. If the solar wind speed is estimated successfully by an empirical equation, it can be used as an initial condition of a solar wind model for space weather forecasts.     

Investigation of X-Ray Flares with Long Rising Phases

We have used Yohkoh and GOES X-ray observations to investigate flares with a long rising phase. We have found that a characteristic feature of such flares is a long time interval, Δ t ≥ 20 min, between the temperature maximum and the maximum of the emission measure. We have carried out detailed analysis for 10 limb flares of this type. Time variation of the heating function, E_H(t), has been determined for their loop-top X-ray kernels. The time variation of E_H(t), together with the temperature–density diagnostic diagrams, have been used to explain the large value of the time interval, Δ t. The main point is that for these flares the heating function E_H(t) decreases so slowly after the temperature maximum, that for the long time, Δ t, the energy flux reaching flare foot points is sufficient to maintain significant chromospheric evaporation. Investigation of the flare evolution in the temperature–density diagnostic diagrams allowed us to work out a new method of determination of the density for flare kernels. This method can be applied to all the kernels for which their altitudes can be estimated. The advantage of this method is that for the density determination it is not necessary to assume what is the extension of the emitting plasma along the line of sight.     

Contrast of Coronal Holes and Parameters of Associated Solar Wind Streams

It is shown that the contrast of coronal holes, just as their size, determines the velocity of the solar wind streams. Fully calibrated EIT images of the Sun have been used. About 450 measurements in 284 Å have been analyzed. The time interval under examination covers about 1500 days in the declining phase of cycle 23. All coronal holes recorded for this interval in the absence of coronal mass ejections (CMEs) have been studied. The comparison with some other parameters (e.g. density, temperature, magnetic field) was carried out. The correlations with the velocity are rather high (0.70?–?0.89), especially during the periods of moderate activity, and could be used for everyday forecast. The contrast of coronal holes is rather small.     

Solar Wind Effects Associated with a Recurrent,High-Latitude Coronal Brightness Enhancement

During the descent of Ulysses following the 2001 solar north pole passage, the SOHO LASCO C2 telescope recorded a particularly strong sequence of recurrent polarization brightness (pB) features at latitudes of around 55°. As Ulysses passed overhead, solar rotation swept the interplanetary extensions of these persistent coronal structures over the spacecraft. Comparison of solar remote sensing and Ulysses in situ observations through 2002 reveals the solar wind effects of very bright and recurrent K-coronal structures at high solar latitudes and of a steeply inclined heliospheric neutral sheet (HNS). Despite the high level of solar activity, the HNS at high latitude still organizes solar wind stream structure much as it did near the previous solar minimum. The recurrent coronal streamers originate slow solar wind and mark the northern extremity of a very tilted HNS whose passage at Ulysses is accompanied by slow, dense solar wind, enhanced temperature, depressed α abundance, enhanced magnetic fields, and magnetic field directional changes that evolve with spacecraft latitude.     

Radio Emission of Flares and Coronal Mass Ejections

We review recent progress on our understanding of radio emission from solar flares and coronal mass ejections (CMEs) with emphasis on those aspects of the subject that help us address questions about energy release and its properties, the configuration of flare?–?CME source regions, coronal shocks, particle acceleration and transport, and the origin of solar energetic particle (SEP) events. Radio emission from electron beams can provide information about the electron acceleration process, the location of injection of electrons in the corona, and the properties of the ambient coronal structures. Mildly relativistic electrons gyrating in the magnetic fields of flaring loops produce radio emission via the gyrosynchrotron mechanism, which provides constraints on the magnetic field and the properties of energetic electrons. CME detection at radio wavelengths tracks the eruption from its early phase and reveals the participation of a multitude of loops of widely differing scale. Both flares and CMEs can ignite shock waves and radio observations offer the most robust tool to study them. The incorporation of radio data into the study of SEP events reveals that a clear-cut distinction between flare-related and CME-related SEP events is difficult to establish.     

Heavy Ion Abundances in Impulsive Solar Flares: Influence of Pre-Acceleration in a Current Sheet

Competition between stochastic energy gains and collisional energy losses is known to lead to preferential acceleration of heavy ions in flare loops. Ion acceleration in a reconnecting current sheet is shown to mitigate the influence of collisional energy losses on stochastic particle acceleration in impulsive solar flares. This effect decreases the sensitivity of the resulting abundance ratios on initial ion charge states. The resulting abundances are determined by the fact that the energy loss rate falls off rapidly with increasing energy. As an example, the expected Fe/O enhancement ratios are computed and shown to be comparable with those observed with ACE SEPICA in several impulsive flares in 1998. One consequence of the model is that the preferential acceleration of heavy ions can occur only when the plasma gas pressure is large enough, m _e/m _p, which may explain the observed correlation between the heavy ion enrichment and selective ³He acceleration in impulsive flares.