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NASA NTRS2019

Properties of DH Type II Radio Bursts and Their Space Weather Implications

We report on the properties of type II radio bursts observed by the Radio and Plasma Wave Experiment (WAVES) onboard the Wind spacecraft over the past two solar cycles. We confirm that the associated coronal mass ejections (CMEs) are fast and wide, more than half the CMEs being halos. About half of the type II bursts extend down to 0.5M hertz, corresponding to a heliocentric distance of tens of solar radii. The DH (Decametric-Hectometric) type II bursts are mostly confined to the active region belt and their occurrence rate follows the solar activity cycle. Type II bursts occurring on the western hemisphere of the Sun and extending to lower frequencies are good indicators of a solar energetic particle event.

Gopalswamy, N.↗
NASA NTRS1975

On the detection of magnetospheric radio bursts from Uranus and Neptune

Earth, Jupiter, and Saturn are sources of intense but sporadic bursts of electromagnetic radiation or magnetospheric radio bursts (MRB). The similarity of the differential power flux spectra of the MRB from all three planets is examined. The intensity of the MRB is scaled for the solar wind power input into a planetary magnetosphere. The possibility of detecting MRB from Uranus and Neptune is considered.

Kennel, C. F.↗
NASA NTRS1982

Electron-cyclotron masers as the source of certain solar and stellar radio bursts

The theory of electron-cyclotron masers as they might apply in the astrophysical context is developed, and it is suggested that such masers provide an explanation for the very bright emission known to be associated with certain kinds of radio bursts observed on the Sun and other stars. Some of the observed properties of solar and stellar radio bursts that seem to require amplification of the radiation are summarized, including millisecond solar spikes, RS CVn binaries, and flares on M dwarf stars. Recent developments in the theory of electron-cyclotron masers are summarized and the current theory is applied to electrons with a loss cone anisotropy, estimating the growth rate and saturation levels. In the interpretation of solar microwave spikes and RS CVn binaries, the mechanisms of gyromagnetic absorption, maser at the second harmonic, polarization, and angular distribution are examined in the light of the theory.

Melrose, D. B.↗
NASA NTRS1976

Possibility of detecting magnetospheric radio bursts from Uranus and Neptune

The intensity of magnetospheric radio bursts (MRBs) is scaled to solar-wind input into planetary magnetospheres and the frequency of emission is scaled to polar surface magnetic-field strength in order to estimate the possibility of detecting MRBs from Uranus and Neptune. A scaling law is derived which relates the ratio of power radiated in MRBs to the solar-wind input for earth, Jupiter, and Saturn. Power-flux spectra of MRBs from these three planets are plotted, and it is shown that Jupiter and Saturn may radiate 1% to 5% of the solar-wind energy input into their magnetospheres. The properties of MRBs from Uranus and Neptune are estimated by assuming a conversion efficiency of 1% to 5%, a bandwidth of half the peak frequency, and conformity of Uranus' and Neptune's dipole moments with the magnetic Bode's law. Based on the results, it is suggested that detection of MRBs from these two planets may be a reasonable cruise-mode radio-astronomy objective on future missions to the outer solar system.

Kennel, C. F.↗
NASA NTRS2012

Emission Patterns of Solar Type III Radio Bursts: Stereoscopic Observations

Simultaneous observations of solar type III radio bursts obtained by the STEREO A, B, and WIND spacecraft at low frequencies from different vantage points in the ecliptic plane are used to determine their directivity. The heliolongitudes of the sources of these bursts, estimated at different frequencies by assuming that they are located on the Parker spiral magnetic field lines emerging from the associated active regions into the spherically symmetric solar atmosphere, and the heliolongitudes of the spacecraft are used to estimate the viewing angle, which is the angle between the direction of the magnetic field at the source and the line connecting the source to the spacecraft. The normalized peak intensities at each spacecraft Rj = Ij /[Sigma]Ij (the subscript j corresponds to the spacecraft STEREO A, B, and WIND), which are defined as the directivity factors are determined using the time profiles of the type III bursts. It is shown that the distribution of the viewing angles divides the type III bursts into: (1) bursts emitting into a very narrow cone centered around the tangent to the magnetic field with angular width of approximately 2 deg and (2) bursts emitting into a wider cone with angular width spanning from [approx] -100 deg to approximately 100 deg. The plots of the directivity factors versus the viewing angles of the sources from all three spacecraft indicate that the type III emissions are very intense along the tangent to the spiral magnetic field lines at the source, and steadily fall as the viewing angles increase to higher values. The comparison of these emission patterns with the computed distributions of the ray trajectories indicate that the intense bursts visible in a narrow range of angles around the magnetic field directions probably are emitted in the fundamental mode, whereas the relatively weaker bursts visible to a wide range of angles are probably emitted in the harmonic mode.

Thejappa, G.↗
NASA NTRS1973

Solar radio bursts at kilometer wavelengths

The potential value of traveling solar radio bursts for investigating energetic particle propagation, and for probing the interplanetary medium is discussed. A general survey of the characteristics of type 3 radio phenomena observed at hectometer and kilometer wavelengths is presented along with a brief discussion of the relationships among type 1 meter noise storms, decametric continuum, and type 3 hectometric storms. Type 3 bursts are analyzed to show how these data provide information about the average energy, dispersion, and trajectory of energetic particles, the interplanetary scale, and magnetic field configuration. The recent observations of type 2 shock wave phenomena at kilometer wavelengths are described, and current research and the direction of future observation are outlined.

Stone, R. G.↗
NASA NTRS2004

Solar Flares, Type III Radio Bursts, CMEs, and Energetic Particles

Despite the fact that it has been well known since the earliest observations that solar energetic particle events are well associated with solar flares it is often considered that the association is not physically significant. Instead, in large events, the particles are considered to be only accelerated at a shock driven by the coronal mass ejection (CME) that is also always present. If particles are accelerated in the associated flare, it is claimed that such particles do not find access to open field lines and therefore do not escape from the low corona. However recent work has established that long lasting type III radio bursts extending to low frequencies are associated with all prompt solar particle events. Such bursts establish the presence of open field lines. Furthermore, tracing the radio bursts to the lowest frequencies, generated near the observer, shows that the radio producing electrons gain access to a region of large angular extent. It is likely that the electrons undergo cross field transport and it seems reasonable that ions do also. Such observations indicate that particle propagation in the inner heliosphere is not yet fully understood. They also imply that the contribution of flare particles in major particle events needs to be properly addressed.

Cane, H. V.↗
NASA NTRS1972

Energetic electrons from solar flares and associated type 3 radio bursts from metric to hectometric wave frequencies

Distinct Kev electron events as observed by satellites near the earth are, in general, associated with solar flares which are accompained by the emission of both metric and hectometric type 3 radio bursts. The positions of these flares are mainly on the western hemisphere of the sun. These results show that Kev electrons propagate under the control of the magnetic field in the interplanetary space and that, while propagating through this space, these electrons excite type 3 radio bursts from metric to hectometric wave frequencies. Emission characteristics of hectometric type 3 bursts are briefly considered in relation to the positions of associated flares.

Sakurai, K.↗
NASA NTRS1989

Impulsiveness and energetics in solar flares with and without type II radio bursts - A comparison of hard X-ray characteristics for over 2500 solar flares

The hard X-ray characteristics of more than 2500 solar flares are used to study the relative size, impulsiveness, and energetics of flares with and without type II radio bursts. A quantitative definition of the hard X-ray impulsiveness is introduced, which may be applied to a large number of events unambiguously. It is found that the flares with type II bursts are generally not significantly larger, more impulsive, or more energetic than those without type II bursts. Also, no evidence is found to suggest a simple classification of the flares as either 'impulsive' or 'gradual'. Because type II bursts are present even in small flares with relatively unimpulsive energy releases, it is concluded that changes in the ambient conditions of the solar atmosphere causing an unusually low Alfven speed may be important in the generation of the shock wave that produces type II radio bursts.

Pearson, Douglas H.↗
NASA NTRS1987

The speeds of electrons that excite solar radio bursts of type III

Evidence is presented that solar type III radio bursts at kilometric wavelengths are excited by electrons with average speeds of 0.14 c; i.e., in good agreement with in situ measurements by Lin et al. (1981; 1986), but considerably lower than the generally accepted values of 0.3 to 0.5 c. A set of 28 bursts for which electrons and/or plasma waves were observed at ISEE-3 is examined, and it is found that the initial parts of all bursts were due to plasma radiation at the fundamental, and that the fastest electrons that produce radio emission range from 0.25 c down to 0.07 c (average 0.14 c). The slower electrons, those that produce fundamental radiation at approximately the time of burst peak, have an average speed of 0.06 c and a range from about 0.10 c down to 0.03 c.There is no evidence in the data for a systematic increase or decrease of exciting electron speed with distance from the sun.

Dulk, G. A.↗
NASA NTRS1972

Locations of solar radio bursts

Measurements of the positions of the sources of solar radio bursts at decameter wavelengths, 20- to 60-MHz range, provide information about physical conditions in a region of the corona extending from about one-half to several solar radii from the surface of the sun. Position measurements in the 20- to 60-MHz range were made with the Clark Lake sweep-frequency grating interferometer. Although the resolution of the instrument is sufficient to measure source position accurately, the highly variable effect of refraction in the earth's ionosphere severely limits the observations, distorting position determinations to such an extent as to make much of the position data useless unless ionospheric refraction can be taken into account. A method was developed for the removal of the refractive effect from the data to permit determination of true source position. With this technique it is considered possible to measure spatial extent of radio sources that will provide information about emission processes and physical parameters in the corona.

Fitzenreiter, R.↗
NASA NTRS1985

Evidence of scattering effects on the sizes of interplanetary Type III radio bursts

An analysis is conducted of 162 interplanetary Type III radio bursts; some of these bursts have been observed in association with fast electrons and Langmuir wave events at 1 AU and, in addition, have been subjected to in situ plasma parameter measurements. It is noted that the sizes of burst sources are anomalously large, compared to what one would anticipate on the basis of the interplanetary plasma density distribution, and that the variation of source size with frequency, when compared with the plasma frequency variation measured in situ, implies that the source sizes expand with decreasing frequency to fill a cone whose apex is at the sun. It is also found that some local phenomenon near the earth controls the apparent size of low frequency Type III sources.

Steinberg, J. L.↗
NASA NTRS1984

Type III radio bursts in the interplanetary medium - The role of propagation

Interplanetary type III radio burst observations are analyzed in order to ascertain the role played by propagation effects between the true source and the observer. Large source altitudes are noted, together with an increasing angular size of sources with increasing angular distance from the sun's center. These and other observations furnish strong evidence for the theory that propagation effects, group delays, ducting and/or scattering significantly affect the observed heights, sizes, and brightness temperatures of interplanetary type III bursts. This would be true irrespective of whether the bursts are due to plasma radiation at the fundamental or at the harmonic, and the effects would extend to the arrival times of the radiation to a greater or lesser extent, depending on the path from the source to the observer.

Steinberg, J. L.↗