The speeds of coronal mass ejection events

The outward speeds of mass ejection events observed with the white light coronagraph experiment on Skylab varied over a range extending from less than 100 km s^–1 to greater than 1200 km s^–1. For all events the average speed within the field of view of the experiment (1.75 to 6 solar radii) was 470 km s^–1. Typically, flare associated events (Importance 1 or greater) traveled faster (775 km s^–1) than events associated with eruptive prominences (330 km s^–1); no flare associated event had a speed less than 360 km s^–1, and only one eruptive prominence associated event had a speed greater than 600 km s^–1. Speeds versus height profiles for a limited number of events indicate that the leading edges of the ejecta move outward with constant or increasing speeds.Metric wavelength type II and IV radio bursts are associated only with events moving faster than about 400 km s^–1; all but two events moving faster than 500 km ^–1 produced either a type II or IV radio burst or both. This suggests that the characteristic speed with which MHD signals propagate in the lower (1.1 to 3 solar radii) corona, where metric wavelength bursts are generated, is about 400 to 500 km s^–1. The fact that the fastest mass ejection events are almost always associated with flares and with metric wavelength type II and IV radio bursts explains why major shock wave disturbances in the solar wind at 1 AU are most often associated with these forms of solar activity rather than with eruptive prominences.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Wide-band average spectra of solar radio bursts

Peak flux spectra of solar radio bursts in a wide frequency band have been statistically determined for different morphological types of bursts, for various ranges of magnetic field of the burst-associated sunspots and also for the bursts occurring in the central and limb region of the solar disk. Important results obtained are: (i) The generalised spectra have two peaks, one near to meter-wave and the other in the centimeter-wave region, the former peak being more pronounced than the latter; (ii) identical spectral shape is observed for the great and impulsive types and also for GRF and PBI types of bursts; (iii) the radio emission intensity is relatively higher in the central part than that in the limb part of the solar disk for frequencies 1–10 GHz, while the reverse is true for frequencies 0.245–1 GHz and 10–35 GHz; (iv) the optical depth of the absorbing layer above the source of a burst is found to be the same for meter to centimeter-wavelength bursts, implying that the radio sources in this wide band have uniform characteristics with respect to optical thickness; (v) in case of simultaneous emission in the dekameter to X-ray band, most of the decimetric bursts are seen to be very prompt and coincident with the associated flare's starting time. The interpretations of the obtained spectra give an insight into the possible generation mechanisms, pointing to the location of the source region in the solar atmosphere.     

19 GHz (1.58 cm) solar radio bursts in the period July 1967 to December 1969

The results of 21/2 yr (July 1967–December 1969) monitoring of solar radio bursts at 19 GHz ( = 1.58 cm) at the Radio and Space Research Station, Slough, are presented. Observations at this frequency are important in helping to define the form of the microwave spectrum of solar bursts since many of the more intense bursts have their spectral peak in the frequency region above 10 GHz. Fifteen bursts with peak flux increases exceeding 1000 × 10^–22 Wm^–2 Hz^–1 were observed during this period.     

A study of energetic solar flare X-rays

A new series of solar flare energetic X-ray events has been detected by an ionization chamber on the OGO-I and OGO-III satellites in free space. These X-rays lie in the range 10–50 keV, and a study has been made of their relationship to 3 and 10 cm radio bursts and with the emission of electrons and protons observed in space. The onset times, times of maximum intensity and total duration are very similar for the radio and X-ray emission. Also, the average decay is similar and usually follows an exponential type behavior. However, this good correlation applies most often to the flash phase of flares, whereas subsequent surges of activity from the same eruption may produce microwave emission or further X-ray bursts not closely correlated. An approximate proportionality is found between the total energy content of the X-rays and of the 3 and 10 cm integrated radio fluxes. These measurements suggest that the X-ray and microwave emission have a common energizing process which determines the time profile of both. The recording of electrons greater than 40 keV by the Interplanetary Monitoring Probe (IMP satellite) has been found to correlate very well with flares producing X-ray and microwave emission provided the propagation path to the sun is favorable. There is evidence that the acceleration of solar protons may not be closely associated with the processes responsible for the production of microwaves, X-rays, and interplanetary electrons.The OGO ionization chamber responds to energies (10–50 keV) intermediate between the soft X-rays giving SID disturbances (1–10 keV) and energetic quanta previously measured with balloons (50–500 keV). Proposed source mechanisms should be capable of covering this range of energies including the most energetic quanta occasionally observed.     

Night-time reception of solar radio bursts over wide bandwidths

Spectra of night-time solar radio bursts observed over wide bandwiths are described. The bursts covered frequency ranges of 20–30 and 24–33 MHz without displaying any abrupt frequency structures. It seems that a scatter mechanism of some kind is involved in the night-time propagation.     

71 GHz (4.2 mm) solar radio bursts in the period July 1967 to December 1969

The results of 2 1/2 years (July 1967 – December 1969) monitoring of solar radio bursts at 71 GHz ( = 4.2 mm) at the Radio and Space Research Station, Slough are presented. During this period only seven events were positively identified as 71 GHz bursts. One of these events (6 July, 1968) is among the largest solar bursts ever recorded anywhere in the microwave-millimetre wave band (47000 × 10^–22Wm^–2Hz^–1), and the associated magnetic field may possibly have exceeded 7200 G. Another event (27 March, 1969) has demonstrated that bursts at 71 GHz can be both intense (4700 × 10^–22Wm^–2Hz^–1) and complex. On other occasions, the absence of any detectable event at 71 GHz helps to define the high frequency spectrum of the burst, this being an important factor in determining the initial energy distribution of the electrons ejected by the associated flare. On one such occasion (21 March, 1969) the derived energy distribution index is 8, in contrast with the more usual values of 2–4.1969–1970 NCR-OAR Senior Post-Doctoral Research Associate at Air Force Cambridge Research Laboratories, L. G. Hanscom Field, Bedford, Mass., U.S.A.     

The Interplanetary Responses to the Great Solar Activities in Late October 2003

Based on the observations of the Sun and the interplanetary medium, a series of solar activities in late October 2003 and their consequences are studied comprehensively. Thirteen X-ray flares with importance greater than M-class, six frontside halo coronal mass ejections (CMEs) with span angle larger than 100 and three associated eruptions of filament materials are identified by examining lots of solar observations from October 26 to 29. All these flares were associated with type III radio bursts, all the frontside halo CMEs were accompanied by type II or type II-like radio bursts. Particularly, among these activities, two major solar events caused two extraordinary enhancements (exceeding 1000 particles/(cm²s^–1sterMev^–1) of solar energetic particle (SEP) flux intensity near the Earth, two large ejecta with fast shocks preceding, and two great geomagnetic storms with Dst peak value of –363 and –401 nT, respectively. By using a cross correlation technique and a force-free cylindrical flux rope model, the October 29 magnetic cloud associated with the largest CME are analyzed, including its orientation and the sign of its helicity. It is found that the helicity of the cloud is negative, contrary to the regular statistical pattern that negative- and positive-helical interplanetary magnetic clouds would be expected to come from northern and southern solar hemisphere. Moreover, the relationship between the orientation of magnetic cloud and associated filament is discussed. In addition, some discussion concerning multiple-magnetic-cloud structures and SEP events is also given.     

Spectral features of burst radio emission over the solar cycle

We have investigated common burst spectral features for the 20th cycle of solar activity. The maximum daily radio fluxes in 8 frequency ranges are analysed. For every year the classification of these daily spectra is obtained by cluster analysis methods. There are two spectral minima for average spectra of clusters (in frequency ranges 4–3 and 0.5–0.25 GHz). As a rule their positions do not change during the solar cycle.Every annual spectrum of weak bursts has three minima (in frequency ranges 4–3, 2–1, and 0.5–0.25 GHz). The positions of these minima remain invariable during the solar cycle. But anuual spectra of strong bursts depend essentially on the phase of solar activity.The basic features of most burst spectra can be explained by gyrosynchrotron radiation of thermal and nonthermal electrons and plasma radiation at the plasma frequency and its second harmonic.     

Some statistical properties of surges

All surges observed at the Swedish Astrophysical Station in Anacapri from July 1, 1957 until December 31, 1967 have been studied statistically. In 1958 the Southern hemisphere was most active in producing surges, but thereafter the Northern hemisphere has dominated the activity with increasing rate. The average for the whole period shows that the Northern part of the sun has been twice as active as the Southern part. The latitude distribution for surges has varied with the solar cycle with a pattern similar to the butterfly diagram for sunspots. The polar surges were most frequent in the beginning of the new cycle. The importance distribution is also dependent of the solar cycle, as a higher percentage of surges of importance 1 are found during the solar minimum. Surges of higher importance tend to have longer durations, and they are more often associated with flares. The association rate between surges and radio emission is dependent on both the solar cycle and the importance of the surge. During years with high activity, more than 20% of all surges were followed within 5 min from their start by radio emission, compared to 4% during solar minimum. 17% of surges of importance 2 are closer related to radio emission, but only 10% of surges of importance 1. On the whole, 11% of the surges occurred almost simultaneously with radio bursts. Bursts in discrete frequencies were registered in connection with 8% of the surges, while 11% were followed within 5 min from their start by bursts in spectral observations. All these surges were isolated, i.e. no flares have been reported to occur during their lifetimes. This is also the case for the 18 surges covered by X-ray observations, three of which were closer related to X-ray bursts.     

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.     

Comparative Analysis of Type ii Bursts and of Thermal and non-Thermal Flare Signatures

The relationship between metric type II radio bursts and solar flares is studied. Well-defined correlations between the properties of type II bursts and the characteristics of associated microwave and soft X-ray bursts are established in two entirely independent data sets. It is shown that the correlations are strongly affected by the wide range of coronal Alfvén velocities involved, comprising values from only 150 up to 800 km s^–1, with a typical value of 400 km s^–1. After careful data analysis it was inferred that type II bursts are more closely related to the soft X-ray bursts than they are to microwave bursts. The correlations indicate that type II burst shocks are preferably generated by flares with a relatively strong thermal component, and that the shocks are probably ignited by the plasma expansion associated with the 'evaporation' process in the transition region. Although the results imply that the majority of metric type II bursts are caused by flares, a simple geometrical consideration shows that a fraction of non-flare type II bursts cannot be explained by behind-limb events and that roughly 10% of metric type II bursts should be attributed to non-flare coronal mass ejections.     

On the origin of solar metric type II bursts

The vast majority of solar flares are not associated with metric Type II radio bursts. For example, for the period February 1980–July 1982, corresponding to the first two and one-half years of the Solar Maximum Mission, 95% of the 2500 flares with peak >25 keV count rates >100 c s^–1lacked associated Type II emission. Even the 360 largest flares, i.e., those having >25 keV peak count rates >1000 c s^–1, had a Type II association rate of only 24%. The lack of a close correlation between flare size and Type II occurrence implies the need for a 'special condition' that distinguishes flares that are accompanied by metric Type II radio bursts from those of comparable size that are not. The leading candidates for this special condition are: (1) an unusually low Alfvén speed in the flaring region; and (2) fast material motion. We present evidence based on SMM and GOES X-ray data and Solwind coronagraph data that argues against the first of these hypotheses and supports the second. Type II bursts linked to flares within 30° of the solar limb are well associated (64%; 49/76) with fast (>400 km s^–1) coronal mass ejections (CMEs); for Type II flares within 15° of the limb, the association rate is 79% (30/38). An examination of the characteristics of 'non-CME' flares associated with Type IIs does not support the flare-initiated blast wave picture that has been proposed for these events and suggests instead that CMEs may have escaped detection. While the degree of Type II–CME association increases with flare size, there are notable cases of small Type II flares whose outstanding attribute is a fast CME. Thus we argue that metric Type II bursts (as well as the Moreton waves and kilometric Type II bursts that may accompany them) have their root cause in fast coronal mass ejections.     

Solar microwave bursts as indicators of the occurrence of solar proton emission

A study has been made of the relation of 19 GHz( = 1.58 cm) solar radio bursts to solar proton emission, with particular reference to the usefulness of relatively long duration bursts with intensities exceeding 50% of the quiet Sun flux (or exceeding 350 × 10^–22 W m^–2 Hz^–1) as indicators of the occurrence of proton events during the four years from 1966–69. 76 to 88% of such bursts are directly associated with solar protons and 60 to 85% of the moderate to large proton events in the four year period could have been predicted from these bursts. The complete microwave spectra of the proton events have also been studied, and have been used to extend the results obtained at 19 GHz to other frequencies, particularly in the 5–20 GHz band. The widely used frequency of 2.8 GHz is not the optimum frequency for this purpose since proton events have a minimum of emission in this region. Most of the radio energy of proton events is at frequencies above 10 GHz. The radio spectra of proton events tend to peak at higher frequencies than most non-proton events, the overall range being 5 to 70 GHz, with a median of 10–12 GHz and a mean of 17 GHz.On leave from the Radio and Space Research Station, Slough, England, as 1969–1970 National Research Council-National Academy of Sciences Senior Post-Doctoral Research Associate at AFCRL.     

Observations of Solar Type II bursts at frequencies 10–30 MHz

We present the results of radio telescope UTR-2 observations of solar Type II radio bursts in the 10–30 MHz frequency range. These events possess a fine structure consisting of fast drift sub-bursts similar to Type III bursts. The frequency drift rate of the Type II bursts at decameter wavelengths is smaller than 0.1 MHz s^–1. One of these bursts with herringbone structure has a wave-like backbone that almost does not drift. The features of the observed bursts are discussed.     

A New Solar Broadband Radio Spectrometer (SBRS) in China

A new radio spectrometer, Solar Broadband Radio Spectrometer (SBRS) with characteristics of high time resolution, high-frequency resolution, high sensitivity, and wide frequency coverage in the microwave region is described. Its function is to monitor solar radio bursts in the frequency range of 0.7–7.6 GHz with time resolution of 1–10 ms. SBRS consists of five `component spectrometers' which work in five different wave bands (0.7–1.5 GHz, 1.0–2.0 GHz, 2.6–3.8 GHz, 4.5–7.5 GHz, and 5.2–7.6 GHz, respectively). A combination of multi-channel and scanning techniques is adopted. The component spectrometers are attached to different antennas which are separately located at Beijing, Kunming, and Nanjing. Close attention was paid to solve the problems of sensitivity, dynamic range, interference-resistance, data acquisition, and handling a large amount of data. The SBRS was put into operation in the 23th solar maximum activity period, and has proved itself to be a valuable instrument for the study of solar bursts in microwaves.     

Note on solar hard X-ray bursts

Observationally solar X bursts fall into three different categories : soft X bursts (E < 10 keV), deka-keV bursts (10–150 keV), and very hard X bursts or deci-MeV bursts (200–1000 keV). The first kind is quasi-thermal, the last kind is non-thermal. The real existence of the third kind of burst looks probable but has not yet been proved by direct observations. The difference between deci-MeV and deka-keV bursts may mainly be a matter of geometry of the emitting plasma.     

Effects of turbulence development in solar surges

Several authors have claimed for correlations between surges (dark features) and various kinds of solar emissions (radio, microwave, X-ray). In this paper we propose a model to explain such correlations, in particular presenting the properties of the instabilities resulting from the coupling between material flow, connected to the appearance of a surge, and magnetic field topology. As a consequence of such instability a turbulent energy cascade to small characteristic lengths grows up. Depending of the relevant parameters of the surge (dark feature), different regimes can be found, producing different levels of electrons acceleration and mass motion deceleration. We try then to correlate the different developments of the instability with the behavior observed in type I and type III radio bursts related to surges.Proceedings of the Second CERSA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.     

Flare-time sudden enhancements of low frequency field strength and associated meter wave solar radio bursts

A number of meter wavelength solar radio bursts of spectral Type-III have been observed by means of a solar radio spectroscope (40–240 MHz) simultaneously with sudden enhancements of low frequency (164 KHz) field strength (SES's) of Radio Tashkent which are known to take place due to the enhancements of D-layer ionization caused by flare-time solar X-rays.The association between the solar X-ray flares as detected by the SES's and the Type-III meter-wave solar bursts is discussed. It is found that the association of SES's and meter wave solar bursts, which implies the ejection of flare-time electrons towards the photosphere as well as corona, is about 72%.     

Longitudinal distribution of the cool solar surges

This investigation shows that solar surges are poorly associated with sudden ionospheric disturbances (X-ray) implying that solar surge material is cool and does not heat the corona. The investigation also shows that solar surges are most prolific at longitudes 80°, 110°, 260°, and 290°.     

On the relationships between sfe (crochet) and solar X-ray and microwave bursts

Geomagnetic crochets (sfe) observed at Kodaikanal over the period 1966–71 have been studied in relation to solar X-ray bursts observed by NRL satellite (SOLRAD-9) in the 0.5–3 Å, 1–8 Å and 8–20 Å bands and radio bursts observed in the frequency range 1000–17000 MHz. The amplitude of sfe is linearly correlated with the peak intensities of X-ray bursts in the 1–8 Å and 8–20 Å bands. The single frequency correlation of sfe with radio bursts is a flat maximum in the frequency range 2000–3750 MHz. Following the spectral classification of AFCRL for microwave bursts, it is noticed that sfe are mostly associated with the A type burst spectra and are very poorly correlated with bursts with the G, C and M type spectra. These features differ from those of other SID's reported earlier.