Solar flares in the extreme ultraviolet

Solar-flare observations in the extreme ultraviolet (300–1350 Å) are reported. Some 269 flares observed by the Harvard College Observatory (HCO) experiment on OSO 4 and 211 flares observed by the HCO experiment on OSO 6 have been analyzed. The flares were observed in spectral lines and continua emitted by many ionic species over a temperature range from 10⁴ to 3.5 × 10⁶ K. The EUV data have been correlated with X-ray, H, and radio observations, and a significant number of EUV bursts not associated with reported H, X-ray, or radio bursts have been iden tified and investigated. The results indicate that these latter EUV events are less energetic by about a factor of 2 than EUV bursts associated with — F subflares.     

Solar soft X-rays and solar activity

Flare-associated soft X-ray bursts (8–12 Å) are examined for 283 events observed by OSO-III. These bursts are shown to be predominantly thermal in nature. Their time-profiles are roughly similar to those of the associated H flares, although the X-ray burst begins about two minutes earlier, on the average. The strength of the soft X-ray burst is directly related to the area and brilliance of the flare, the age and flare-richness of the associated plage, and the general level of solar activity at the time of the burst. The peak enhancements in the soft X-ray and H emission rates during flares are of the same order of magnitude, as are the total flare energies radiated at these wavelengths. We estimate that soft X-radiation accounts for up to 10% of a flare's total electromagnetic emission.NRC/NAS Resident Research Associate.     

Gyro-synchrotron emission in a magnetic dipole field for the application to the center-to-limb variation of microwave impulsive bursts

In order to interpret the observed center to limb variations of spectrum and polarization of microwave impulsive bursts, gyro-synchrotron emission from nonthermal electrons trapped in a magnetic dipole field is computed. The theoretical spectrum and polarization are consistent with observed ones if we put an outer boundary of the radio source at a layer of 100-60 G or (7–9) × 10⁴ km in height. Rather small observed center-limb variations in intensity and polarization are attributed to the distribution of , an angle between the magnetic field and the direction of observer, in the radio source emitting the burst, though the intensity and polarization depend strongly on especially at small values of .     

A model for solar radio pulsations at short centimetric band

In this paper, the observed solar radio pulsations during the bursts at 9.375 GHz are considered to be excited by some plasma instability. Under the condition of the conservation of energy in the wave-particle interaction, the saturation time of plasma instabilities is inversely proportional to the initial radiation intensity, which may explain why the repetition rate of the pulsations is directly proportional to the radio burst flux at 9.375 GHz as well as 15 GHz and 22 GHz. It is also predicted that the energy released in an individual pulse increases with increasing the flux of radio bursts, the modularity of the pulsations decreases with increasing the flux of radio bursts, these predictions are consistent with the statistical results at 9.375 GHz in different events. The energy density of the non-thermal particles in these events is estimated from the properties of pulsation. For the typical values of the ambient plasma density (10⁹ cm^–3) and the ratio between the nonthermal and ambient electrons (10^–4), the order of magnitude of the energy density and the average energy of the nonthermal electrons is 10^–4 erg/cm³ and 10 kev, respectively. It is interesting that there are two branches in a statistical relation between the repetition rate and the radio burst flux in a special event on March 11–17, 1989, which just corresponds to two different orders of magnitude for the quasi-quantized energy released in these five bursts. This result may be explained by the different ratios between the thermal and the nonthermal radiations.     

Sunspot motion,flares and type III bursts in McMath 11482

We have studied a series of flares in McMath 11482, 1972 August 19–22, with particular reference to the basis for the flares and comparison with dekameter radio data. We find that the flares were produced by rapid ( 1000 km h^–1) westward motion of a large new p spot. Many flares occur just in front of the spot, and they cease when the motion stops. All flares occurring in front of the spot produce type III bursts, while even strong flares elsewhere in the region produce little or no type III. The time of type III emission agrees perfectly with the start of the H flare. Thus type III bursts are only produced in favorable configurations.Simultaneous K-line movies are compared with H films and show little difference in flare appearance.     

Characteristics of solar coronal source regions producing 3He-rich particle events

We use H, X-ray, and kilometric radio data to examine the solar coronal activity associated with energetic (1 MeV/nucl^–1) ³He-rich particle events observed near Earth. The basis of the study is the 12 ³He-rich events observed in association with impulsive 2 to 100 keV electron events reported by Reames et al. (1985). We find that when H and X-ray brightenings can be associated with ³He/electron events, they have onsets coinciding to within 1 min of that of the associated metric type III bursts. In three or four events we found no associated H or X-ray flares, and in two events even the metric type III bursts were weak or absent. The measured low-energy (2 keV) electron spectra for these events show no evidence of a flattening due to Coulomb collisional losses. These results and several other recent findings are consistent with the idea that the ³He/electron events are due to particle acceleration in the corona well above the associated H and X-ray flares.     

Interferometer observations of a radio burst at 8.6 mm associated with a polarized hard X-ray event

Observations of a radio burst at 8.6 mm wavelength on 1970 November 5, are described with the particular interest on the correspondence between radio and polarized X-ray events. The radio observations were carried out using an interferometer with a half power width of 2.9 at the Dept. of Physics, Nagoya University, and indicated that the location of the radio burst coincided with preceding sunspots and the size of the burst source must be very small, less than about 1. Mechanisms of radio and X-ray emissions are discussed briefly.     

Characteristics of soft solar X-ray bursts

The burst component of the solar X-ray flux in the soft wavelength range 2 < < 12 Å observed from Explorer 33 and Explorer 35 from July 1966 to September 1968 was analyzed. In this period 4028 burst peaks were identified.The differential distributions of the temporal and intensity parameters of the bursts revealed no separation into more than one class of bursts. The most frequently observed value for rise time was 4 min and for decay time was 12 min. The distribution of the ratio of rise to decay time can be represented by an exponential with exponent -2.31 from a ratio of 0.3 to 2.7; the maximum in this distribution occurred at a ratio of 0.3. The values of the total observed flux, divided by the background flux at burst maximum, can be represented by a power law with exponent -2.62 for ratios between 1.5 and 32. The distribution of peak burst fluxes can be represented by a power law with exponent - 1.75 over the range 1–100 milli-erg (cm² sec)^–1. The flux time integral values are given by a power law with exponent -1.44 over the range 1–50 erg cm^–2.The distribution of peak burst flux as a function of H importance revealed a general tendency for larger peak X-ray fluxes to occur with both larger H flare areas and with brighter H flares. There is no significant dependence of X-ray burst occurrence on heliographic longitude; the emission thus lacks directivity.The theory of free-free emission by a thermal electron distribution was applied to a composite quantitative discussion of hard X-ray fluxes (data from Arnoldy et al., 1968; Kane and Winckler, 1969; and Hudson et al., 1969) and soft X-ray fluxes during solar X-ray bursts. Using bursts yielding measured X-ray intensities in three different energy intervals, covering a total range of 1–50 keV, temperatures and emission measures were derived. The emission measure was found to vary from event to event. The peak time of hard X-ray events was found to occur an average of 3 min before the peak time of the corresponding soft X-ray bursts. Thus a changing emission measure during the event is also required. A free-free emission process with temperatures of 12–39 × 10⁶K and with an emission measure in the range 3.6 × 10⁴⁷ to 2.1 × 10⁵⁰ cm^–3 which varies both from event to event and within an individual event is required by the data examined.Now at Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey.     

A comparison of positions and sizes of sources of centimeter and X-ray bursts

We have made a parallel study of three cm- radio bursts, observed on 9 August, 1973 with the NRAO three-element interferometer at 3.7 and 11.1 cm and the associated X-ray flares observed with the S-054 telescope aboard Skylab. Within the errors of our measurements (± 5) the radio and X-ray events are cospatial. We find good agreement between the size of the X-ray kernel and that of the core of the cm burst, while there is evidence that much of the impulsive radio flux was produced in a larger area.

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Re-Evaluation of the Flare–Type II–CME Association

We have re-evaluated the association of type II solar radio bursts with flares and/or coronal mass ejections (CMEs) using the year 2000 solar maximum data. For this, we consider 52 type II events whose associations with flares or CMEs were absent or not clearly identified and reported. These events are classified as follows; group I: 11 type IIs for which there are no reports of GOES X-ray flares and CMEs; group II: 12 type IIs for which there are no reports of GOES X-ray flares; and group III: 29 type IIs for which the flare locations are not reported. By carefully re-examining their association from GOES X-ray and H, Yohkoh SXT and EIT-EUV data, we attempt to answer the following questions: (i) if there really were no X-ray flares associated with the above 23 type IIs of groups I and II; (ii) whether they can be regarded as backside events whose X-ray emission might have been occulted. From this analysis, we have found that two factors, flare background intensity and flare location, play important roles in the complete reports about flare–type II–CME associations. In the above 23 cases, for more than 50% of the cases in total, the X-ray flares were not noticed and reported, because the background intensity of X-ray flux was high. In the remaining cases, the X-ray intensity might be greatly reduced due to occultation. From the H flare data, Yohkoh SXT data and EIT-EUV data, we found that ten cases out of 23 might be frontside events, and the remaining are backside events. While the flare–type II association is found to be nearly 90%, the type II–CME association is roughly around 75%. This analysis might be useful to reduce some ambiguities regarding the association among type IIs, flares and CMEs.     

Remote flare brightenings and type III reverse slope bursts

We present two large flares which were exceptional in that each produced an extensive chain of H emission patches in remote quiet regions more than 10⁵ km away from the main flare site. They were also unusual in that a large group of the rare type III reverse slope bursts accompanied each flare.The observations suggest that this is no coincidence, but that the two phenomena are directly connected. The onset of about half of the remote H emission patches were found to be nearly simultaneous with RS bursts. One of the flares (August 26, 1979) was also observed in hard X-rays; the RS bursts occurred during hard X-ray spikes. For the other flare (June 16, 1973), soft X-ray filtergrams show coronal loops connecting from the main flare site to the remote H brightenings. There were no other flares in progress during either flare; this, along with the X-ray observations, indicates that the RS burst electrons were generated in these flares and not elsewhere on the Sun. The remote H brightenings were apparently not produced by a blast wave from the main flare; no Moreton waves were observed, and the spatially disordered development of the remote H chains is further evidence against a blast wave. From geometry, time and energy considerations we propose: (1) That the remote H brightenings were initiated by direct heating of the chromosphere by RS burst electrons traveling in closed magnetic loops connecting the flare site to the remote patches; and (2) that after onset, the brightenings were heated by thermal conduction by slower thermal electrons (kT1 keV) which immediately follow the RS burst electrons along the same loops.     

Evidence for electron excitation of type III radio burst emission

Type III radio bursts observed at kilometric wavelengths ( 0.35 MHz) by the OGO-5 spacecraft are compared with > 45 keV solar electron events observed near 1 AU by the IMP-5 and Explorer 35 spacecraft for the period March 1968–November 1969.Fifty-six distinct type III bursts extending to 0.35 MHz ( 50 R equivalent height above the photosphere) were observed above the threshold of the OGO-5 detector; all but two were associated with solar flares. Twenty-six of the bursts were followed 40 min later by > 45 keV solar electron events observed at 1 AU. All of these 26 bursts were identified with flares located west of W 09 solar longitude. Of the bursts not associated with electron events only three were identified with flares west of W 09, 18 were located east of W 09 and 7 occurred during times when electron events would be obscured by high background particle fluxes.Thus almost all type III bursts from the western half of the solar disk observed by OGO-5 above a detection flux density threshold of the order of 10^–13 Wm^–2 Hz^–1 at 0.35 MHz are followed by > 45 keV electrons at 1 AU with a maximum flux of 10 cm^–2 s^–1 ster^–1. If particle propagation effects are taken into account it is possible to account for lack of electron events with the type III bursts from flares east of the central meridian. We conclude that streams of 10–100 keV electrons are the exciting agent for type III bursts and that these same electrons escape into the interplanetary medium where they are observed at 1 AU. The total number of > 45 keV electrons emitted in association with a strong kilometer wavelength type III burst is estimated to be 5 × 10³².     

Type II radio bursts and particle acceleration

328 particle events recorded during 30 months from January 1, 1966 to June 30, 1968 (taken from the new Catalog of Solar Particle Events, 1955–1969) are compared with the occurrence of 166 type II radio bursts during the same period. The results of this comparison give a convincing evidence that proton acceleration to higher energies in flares (the second acceleration step) is closely connected with the type II burst occurrence. The shock wave appears to originate near the time when the impulsive burst occurs, and the second acceleration step follows immediately the first one; in some cases the second step sets in while the first step is still in progress.A detailed analysis indicates that we may need even three different acceleration mechanisms in flares: The first one gives rise to electrons which produce the microwave and hard X-ray bursts (and it probably also accelerates protons to low energies); the second, which sometimes coincides (but mostly does not coincide) with the first one, produces beams of electrons which give rise to type III bursts; and the third one, characterized by the type II burst-producing shock wave, accelerates (on some, rather rare occasions) the particles, preaccelerated by the first mechanism, to higher energies.Mitteilungen aus dem Fraunhofer Institut Nr. 125.Grant of Stifterverband für die Deutsche Wissenschaft.     

Coronal electric currents produced by photospheric motions

Hard X-ray bursts have been observed from two 1B flares located in the same sunspot region and separated in time by about 70 min on 8 December 1970. The bursts are composed by many flashes of 2 to 20 seconds duration. Power spectrum analysis reveals no strong periodicities although a significant peak appears at 7.5 s. The characteristic times of 2 to 20 s do not seem likely to be electron transit times since the flare size is so small. These times are evidently connected with the acceleration of the electrons. Fitting Takakura's model of the radio noise region to the observations we find this region to be small but the density of electrons above 100 keV is high, on the order of 10⁷ cm^-3.     

Electron beams and associated rapid fluctuations in solar flares

Recent observations show that the rapid fluctuations in radio, hard X-ray, and H emissions are closely associated with type III and microwave (or decimetric) bursts during the impulsive and/or preimpulsive phases of solar flares.In order to clarify the physical processes of these observed phenomena, this paper proposes a tentative model of two acceleration regions A (source of type III bursts) and B (source of microwave or decimetric bursts) formed in the neutral sheet and at the top of a flaring loop, respectively; and also suggests that the electron beams streaming from region A and/or region B downward to the chromosphere are responsible for the rapid fluctuations in the different emissions mentioned above during the impulsive and/or pre-impulsive phases of solar flares.     

Second-stage acceleration in a limb-occulted flare

We analyze hard and soft X-ray, microwave and meter wave radio, interplanetary particle, and optical data for the complex energetic solar event of 22 July 1972. The flare responsible for the observed phenomena most likely occurred 20° beyond the NW limb of the Sun, corresponding to an occultation height of 45 000 km. A group of type III radio bursts at meter wavelengths appeared to mark the impulsive phase of the flare, but no impulsive hard X-ray or microwave burst was observed. These impulsive-phase phenomena were apparently occulted by the solar disk as was the soft X-ray source that invariably accompanies an H flare. Nevertheless essentially all of the characteristic phenomena associated with second-stage acceleration in flares - type II radio burst, gradual second stage hard X-ray burst, meter wave flare continuum (FC II), extended microwave continuum, energetic electrons and ions in the interplanetary medium - were observed. The spectrum of the escaping electrons observed near Earth was approximately the same as that of the solar population and extended to well above 1 MeV.Our analysis of the data leads to the following results: (1) All characteristics are consistent with a hard X-ray source density n _i 10⁸ cm^–3 and magnetic field strength 10 G. (2) The second-stage acceleration was a physically distinct phenomenon which occurred for tens of minutes following the impulsive phase. (3) The acceleration occurred continuously throughout the event and was spatially widespread. (4) The accelerating agent was very likely the shock wave associated with the type II burst. (5) The emission mechanism for the meter-wave flare continuum source may have been plasma-wave conversion, rather than gyrosynchrotron emission.     

Ignition of MHD shocks associated with solar flares

We have selected single frequency recordings of 28 high-frequency type II bursts characterized by a starting frequency greater than 237 MHz to estimate as accurately as possible the launch-time of the flare-associated MHD shocks. We established the time associations between metric type II burst onsets and the time characteristics of the microwave and X-ray fluxes of the associated flares. The associated flares were impulsive events with rise times most often about 1 min in the hard X-ray range and 1–2 min in the microwave wavelength range. The majority of the type II bursts from our sample started about 1 min after the maximum of the microwave burst. Launch times of MHD shocks producing type II bursts were obtained using the 10 × Saito coronal model and shock velocities estimated from burst characteristics at different frequencies. Back-extrapolations of type II recordings indicate that MHD shocks are launched in the time interval prior to the maximum of the first peak in the associated microwave burst, most probably at the beginning of the rapid increase of the microwave burst.     

Shock waves and type II radiobursts in the interplanetary medium

The planetary radio astronomy experiment on the Voyager spacecraft observed several type II solar radiobursts at frequencies below 1.3 MHz; these correspond to shock waves at distances between 20R and 1 AU from the Sun. We study the characteristics of these bursts and discuss the information that they give on shock waves in the interplanetary medium and on the origin of the high energy electrons which give rise to the radioemission. The relatively frequent occurence of type II bursts at large distances from the Sun favors the hypothesis of the emission by a longitudinal shockwave. The observed spectral characteristics reveal that the source of emission is restricted to only a small portion of the shock. From the relation between type II bursts, type III bursts and optical flares, we suggest that some of the type II bursts could be excited by type III burst fast electrons which catch up the shock and are then trapped.     

Hα and hard X-ray development in two-ribbon flares

Morphological features of two-ribbon flares have been studied, using simultaneous ISEE-3 hard X-ray records and high-resolution Big Bear H movies for more than 20 events. Long-lasting and complex hard X-ray bursts are almost invariably found associated with flares of the two-ribbon type. We find at least three events, namely March 31, 1979, April 10, 1980, and July 1, 1980, where the occurrence of individual spikes in hard X-ray radiation coincides with suddenly enhanced H emission covering the sunspot penumbra. There definitely exist important ( 1B) two-ribbon H flares without significant hard X-ray emission.     

Sharp Decreases of Solar Metric Radio Storm Emission

We discuss a little-known variety of sharp decreases of long-duration meter-wavelength noise storms and type IV bursts. A survey of the IZMIRAN and AIP radio observations shows that a decrease or nearly complete disappearance of the continuum and bursts developing over tens of minutes without a subsequent recovery of the radio flux occasionally occurs. The decrease is usually preceded by a short-duration (several tens of minutes) enhancement of the radio emission. In these events, the onset of the flux decrease drifts from high to low frequencies with a rate of –(0.05–0.35) MHz s^–1, comparable to the drift rates of noise-storm onsets and of chains of type I bursts. White-light coronagraph observations, as well as the characteristics of the accompanying microwave and soft X-ray emissions, provide evidence that such radio decreases appear to be associated with coronal mass ejections (CMEs) and post-CME phenomena. Yohkoh/SXT images show radio flux decrease events which are accompanied by significant rearrangements of coronal structures. We suggest that the radio flux variations are caused by CME interactions with pre-existing coronal arcade structures which are sources of noise storms and energetic electron acceleration. The fact that the noise-storm decreases develop with delays of several tens of minutes relative to the associated microwave burst peak, when the corresponding CME front is located at heights of several R , however, is not explained.