基于PSP观测的行星际Ⅲ型射电爆发的研究进展

与太阳射电爆发相比,通常认为频率较低的行星际射电爆发产生于远离低日冕的行星际空间.地球电离层的截止导致地基设备无法对其进行观测.美国国家航空航天局(National Aeronautics and Space Administration, NASA)发射的帕克太阳探测器(Parker Solar Probe, PSP)是迄今为止距离太阳最近的空间探测器.其搭载的射电频谱仪能够对10 k Hz–19.17 MHz频段范围内的射电辐射进行观测. PSP能够靠近甚至可能穿越行星际III型射电爆发的辐射源区,因此使用PSP对行星际射电爆发进行观测具有前所未有的优势.简要介绍了目前为止使用PSP的射电观测数据对行星际III型射电爆发的多方面研究,包括爆发的发生率、偏振、散射、截止频率、可能的辐射机制和相关的辐射源区等方面的研究进展,并讨论了其未来的研究前景.     

The Source Regions of Impulsive Solar Electron Events

Low-energy (2–19 keV) impulsive electron events observed in interplanetary space have been traced back to the Sun, using their interplanetary type III radiation and metric/decimetric radio-spectrograms. For the first time we are able to study the highest frequencies and thus the radio signatures closest to the source region. All the selected impulsive solar electron events have been found to be associated with an interplanetary type III burst. This allows to time the particle events at the 2 MHz plasma level and identify the associated coronal radio emissions. Except for 5 out of 27 cases, the electron events were found to be associated with a coronal type III burst in the metric wavelength range. The start frequency yields a lower limit to the density in the acceleration region. We also search for narrow-band spikes at the start of the type III bursts. In about half of the observed cases we find metric spikes or enhancements of type I bursts associated with the start of the electron event. If interpreted as the plasma emission of the acceleration process, the observed average frequency of spikes suggests a source density of the order of 3×10⁸ cm^–3 consistent with the energy cut-off observed.     

Sources of SEP Acceleration during a Flare – CME Event

A high-speed, halo-type coronal mass ejection (CME), associated with a GOES M4.6 soft X-ray flare in NOAA AR 0180 at S12W29 and an EIT wave and dimming, occurred on 9 November 2002. A complex radio event was observed during the same period. It included narrow-band fluctuations and frequency-drifting features in the metric wavelength range, type III burst groups at metric – hectometric wavelengths, and an interplanetary type II radio burst, which was visible in the dynamic radio spectrum below 14 MHz. To study the association of the recorded solar energetic particle (SEP) populations with the propagating CME and flaring, we perform a multi-wavelength analysis using radio spectral and imaging observations combined with white-light, EUV, hard X-ray, and magnetogram data. Velocity dispersion analysis of the particle distributions (SOHO and Wind in situ observations) provides estimates for the release times of electrons and protons. Our analysis indicates that proton acceleration was delayed compared to the electrons. The dynamics of the interplanetary type II burst identify the burst source as a bow shock created by the fast CME. The type III burst groups, with start times close to the estimated electron-release times, trace electron beams travelling along open field lines into the interplanetary space. The type III bursts seem to encounter a steep density gradient as they overtake the type II shock front, resulting in an abrupt change in the frequency drift rate of the type III burst emission. Our study presents evidence in support of a scenario in which electrons are accelerated low in the corona behind the CME shock front, while protons are accelerated later, possibly at the CME bow shock high in the corona.     

Direction-finding measurements of type III radio bursts out of the ecliptic plane

Direction-finding measurements with the plasma wave experiments on the HAWKEYE 1 and IMP 8 satellites are used to find the source locations of type III solar radio bursts in heliocentric latitude and longitude in a frequency range from 31.1 kHz to 500 kHz. IMP 8 has its spin axis perpendicular to the ecliptic plane; hence, by analyzing the spin modulation of the received signals the location of the type III burst projected into the ecliptic plane can be found. HAWKEYE 1 has its spin axis nearly parallel to the ecliptic plane; hence, the location of the source out of the ecliptic plane may also be determined. Using an empirical model for the emission frequency as a function of radial distance from the sun the three-dimensional trajectory of the type III radio source can be determined from direction-finding measurements at different frequencies. Since the electrons which produce these radio emissions follow the magnetic field lines from the Sun these measurements provide information on the three-dimensional structure of the magnetic field in the solar wind. The source locations projected into the ecliptic plane follow an Archimedean spiral. Perpendicular to the ecliptic plane the source locations usually follow a constant heliocentric latitude. When the best fit magnetic field line through the source locations is extrapolated back to the Sun this field line usually originates within a few degrees from the solar flare which produced the radio burst. With direction-finding measurements of this type it is also possible to determine the source size from the modulation factor of the received signals. For a type III event on June 8, 1974, the half angle source size was measured to be 60° at 500 kHz and 40° at 56.2 kHz as viewed from the Sun.Presented at Workshop on Mechanisms for Solar Type III Radio Bursts, Berkeley, California, May 8–9, 1975; see Solar Phys. 46, 433.     

Determination of Electromagnetic Source Direction as an Eigenvalue Problem

Low-frequency solar and interplanetary radio bursts are generated at frequencies below the ionospheric plasma cutoff and must therefore be measured in space, with deployable antenna systems. The problem of measuring both the general direction and polarization of an electromagnetic source is commonly solved by iterative fitting methods such as linear regression that deal simultaneously with both directional and polarization parameters. We have developed a scheme that separates the problem of deriving the source direction from that of determining the polarization, avoiding iteration in a multi-dimensional manifold. The crux of the method is to first determine the source direction independently of concerns as to its polarization. Once the source direction is known, its direct characterization in terms of Stokes vectors, in a single iteration if desired, is relatively simple. This study applies the source-direction determination to radio signatures of flares received by STEREO. We studied two previously analyzed radio type III bursts and found that the results of the eigenvalue decomposition technique are consistent with those obtained previously by Reiner et al. (Solar Phys. 259, 255, 2009). For the type III burst observed on 7 December 2007, the difference in travel times from the derived source location to STEREO A and B is the same as the difference in the onset times of the burst profiles measured by the two spacecraft. This is consistent with emission originating from a single, relatively compact source. For the second event of 29 January 2008, the relative timing does not agree, suggesting emission from two sources separated by 0.1 AU, or perhaps from an elongated region encompassing the apparent source locations.     

Interplanetary baseline observations of type III solar radio bursts

Simultaneous observations of type III radio bursts from spacecraft separated by 0.43 AU have been made using the solar orbiters HELIOS-A and HELIOS-B. The burst beginning at 19:22 UT on March 28, 1976 has been located from the intersection of the source directions measured at each spacecraft, and from burst arrival time differences. The source positions range from 0.03 AU from the Sun at 3000 kHz to 0.08 AU at 585 kHz. The electron density along the burst trajectory, and the exciter velocity (=0.13c) were determined directly, without the need to assume a density model as has been done with single-spacecraft observations. The separation of HELIOS-A and -B has also provided the first measurements of burst directivity at low frequencies. For the March 28 burst the intensity observed from near the source longitude (HELIOS-B) was 3–10 dB greater than that from 60° west of the source (HELIOS-A).     

Fundamental emission for type III bursts in the interplanetary medium: The role of ion-sound turbulence

It is argued (a) that the onset times of type III radio emission and of the streaming electrons implies that type III bursts in the interplanetary medium are generated predominantly at the fundamental, (b) that in view of recent observations of ion-sound waves in the interplanetary medium the theory of the generation of the bursts should be revised to take account of these waves, and (c) the revised theory favours fundamental emission. A detailed discussion of the effect of ion-sound waves on type III bursts is given. The most important results are: (1) Ion-sound waves cause enhanced (over scattering off thermal ions) fundamental emission. (2) Second harmonic emission is also enhanced for T _e> 5 × 10⁵ K, e.g., low in the corona, but is suppressed for T _e< 5 × 10⁵ K, e.g., in the interplanetary medium. (3) The bump-in-the-tail instability for Langmuir waves can be suppressed by the presence of ion-sound waves; it may be replaced by an analogous instability in which fundamental transverse waves are generated directly, with no associated second harmonic, but there are unresolved problems with theory for this process. (4) Very low frequency ion-sound waves can scatter type III radiation. (5) Although the ion-sound waves which have been observed are at too high a frequency to be relevant for these processes, it seems likely that ion-sound waves of the required frequencies are present and do play important roles in the generation of type III bursts.     

Direction finding measurements of type III bursts in both elevation and azimuth

Direction finding measurements with the plasma wave experiments on the HAWKEYE 1 and IMP-8 satellites are used to find the source locations of type III solar radio bursts in elevation (geocentric solar ecliptic latitude) and azimuth (geocentric solar ecliptic longitude) in a frequency range from 31.1 kHz to 500 kHz. IMP-8 has its spin axis perpendicular to the ecliptic plane, hence by analyzing the spin modulation of the signals the location of the type III burst projected into the ecliptic can be found. HAWKEYE 1 has its spin axis nearly parallel to the ecliptic plane, hence the elevation of the source may also be determined. The trajectory of the electrons generating the burst, projected onto the ecliptic plane, follows an Archimedean spiral. Out of the ecliptic plane the trajectory is at a nearly constant heliographic latitude. The electrons originate from a region near a solar flare. With direction measurements of elevation and azimuth along with the modulation factor it is possible to determine the source size. Typical half angle source sizes range from 60° at 500 kHz to 40° at 56.2 kHz as viewed from the sun.     

Fast solar electrons,interplanetary plasma and km-wave type-III radio bursts observed from the IMP-6 spacecraft

IMP-6 spacecraft observations of low frequency radio emission, fast electrons, and solar wind plasma are used to examine the dynamics of the fast electron streams which generate solar type-III radio bursts. Of twenty solar electron events observed between April, 1971 and August, 1972, four were found to be amenable to detailed analysis. Observations of the direction of arrival of the radio emission at different frequencies were combined with the solar wind density and velocity measurements at 1 AU to define an Archimedean spiral trajectory for the radio burst exciter. The propagation characteristics of the exciter and of the fast electrons observed at 1 AU were then conpared. We find that: (1) the fast electrons excite the radio emission at the second harmonic; (2) the total distance travelled by the electrons was between 30 and 70% longer than the length of the smooth spiral defined by the radio observations; (3) this additional distance travelled is the result of scattering of the electrons in the interplanetary medium; (4) the observations are consistent with negligible true energy loss by the fast electrons.     

太阳米波和分米波Ⅱ型、Ⅲ型射电暴及其精细结构观测研究进展

太阳米波和分米波的射电观测是对太阳爆发过程中耀斑和日冕物质抛射现象研究的重要观测手段。米波和分米波的太阳射电暴以相干等离子体辐射为主导,表现出在时域和频域的多样性和复杂性。其中Ⅱ型射电暴是激波在日冕中运动引起电磁波辐射的结果。在Ⅱ型射电暴方面,首先对米波Ⅱ型射电暴的激波起源问题和米波Ⅱ型射电暴与行星际Ⅱ型射电暴的关系问题进行了讨论;其次,结合Lin-Forbes太阳爆发理论模型对Ⅱ型射电暴的开始时间和起始频率进行讨论:最后,对Ⅱ型射电暴信号中包含的两种射电精细结构,Herringbone结构(即鱼骨结构)和与激波相关的Ⅲ型射电暴也分别进行了讨论。Ⅲ型射电暴是高能电子束在日冕中运动产生电磁波辐射的结果。在Ⅲ型射电暴方面,首先介绍了利用Ⅲ型射电暴对日冕磁场位形和等离子体密度进行研究的具体方法;其次,对利用Ⅲ型射电暴测量日冕温度的最新理论进行介绍;最后,对Ⅲ型射电暴和Ⅱ型射电暴的时间关系、Ⅲ型射电暴和粒子加速以及Ⅲ型射电暴信号中包含的射电精细结构(例如斑马纹、纤维爆发及尖峰辐射)等问题进行讨论并介绍有关的最新研究进展。     

Ulysses observations of nonlinear wave-wave interactions in the source regions of type III solar radio bursts

The Ulysses Unified Radio and Plasma Wave Experiment (URAP) has observed Langmuir, ion-acoustic and associated solar type III radio emissions in the interplanetary medium. Bursts of 50&amp;#x2013;300 Hz (in the spacecraft frame) electric field signals, corresponding to long-wavelength ion-acoustic waves are often observed coincident in time with the most intense Langmuir wave spikes, providing evidence for the electrostatic decay instability. Langmuir waves often occur as envelope solitons, suggesting that strong turbulence processes, such as modulational instability and soliton formation, often coexist with weak turbulence processes, such as electrostatic decay, in a few type III burst source regions.     

Characteristics of type III exciters derived from low frequency radio observations

Low frequency radio observations (2.8 MHz-67 kHz) from the RAE-1 and IMP-6 satellites allow the tracking of type III solar burst exciters out to large distances from the Sun (of the order of 1 AU). A study of the interaction processes between the exciter and the interplanetary medium was made using the time-intensity profiles of the radio emission. We have investigated the change in exciter length with distance from the Sun, and the resulting exciter velocity dispersion which can be deduced from this change. From detailed measurements on 35 simple bursts we have found that the exciter length increases at a faster rate than a constant velocity dispersion would give. We have also investigated the damping of the radio emission and have concluded that some current theories of the damping mechanism give results which are not consistent with the low frequency observations.Work performed while a NRC/NAS Postdoctoral Resident Research Associate.     

Radio evidence for electron acceleration in a cusp-like structure

In the spectrum of the type IV/II radio burst of 1994 July 18 we found a rare superposition of fast and slowly drifting features in the frequency range 100–300 MHz about 1 min before the onset of the shock induced meter wave type II burst. We take this as signatures of the passage of an MHD-like disturbance and the simultaneous injection of two contradirected electron beams. These beams are manifested as an ordinary type III burst and a reverse drift burst starting at the same frequency. Remarkably, the reverse drift burst is visible only due to its absorbtion trace in an underlying continuum patch. We argue that the superimposed burst features are emitted from a common source which covers the top of a closed magnetic field structure and the immediately superposed part (cusp) of the helmet streamer configuration. The radio source is situated immediately at the electron acceleration site.     

THE DIRECTIVITY OF SOLAR KILOMETRIC TYPE III BURSTS: ULYSSES–ARTEMIS OBSERVATIONS IN AND OUT OF THE ECLIPTIC PLANE

Comparing the records of the radio spectrographs ARTEMIS (100–500 MHz) on the ground and URAP (1–1000 kHz) on the Ulysses spacecraft, we find that most type III bursts extend from the corona to the solar wind. Using the positions of the associated flares, and assuming an average intensity ratio between these two frequency ranges, we derive for the first time the average radiation pattern of interplanetary type III bursts. We find that at 800 kHz it is shifted east of the radial direction by 30° and has a half-width of about 80° at maximum/10; the shift and width increase towards lower frequencies. Ulysses high-latitude observations show that the cross-section perpendicular to the heliospheric equator is about the same. We interpret these properties by refraction effects in local density gradients.     

Solar Energetic Particle Event of 2005 January 20： Release Times and Possible Sources

Based on cosmic ray data obtained by neutron monitors at the Earth's surface, and data on near-relativistic electrons measured by the WIND satellite, as well as on solar X-ray and radio burst data, the solar energetic particle (SEP) event of 2005 January 20 is studied. The results show that this event is a mixed event where the flare is dominant in the acceleration of the SEPs, the interplanetary shock accelerates mainly solar protons with energies below 130 MeV, while the relativistic protons are only accelerated by the solar flare. The interplanetary shock had an obvious acceleration effect on relativistic electrons with energies greater than 2 MeV. It was found that the solar release time for the relativistic protons was about 06:41 UT, while that for the near-relativistic electrons was about 06:39 UT. The latter turned out to be about 2 min later than the onset time of the interplanetary type III burst.     

Multipoint Observations of Solar Type III Radio Bursts from STEREO and Wind

The twin STEREO and the Wind spacecraft make remote multipoint measurements of interplanetary radio sources of solar origin from widely separated vantage points. One year after launch, the angular separation between the STEREO spacecraft reached 45°, which was ideal for locating solar type III radio sources in the heliosphere by three-spacecraft triangulation measurements from STEREO and Wind. These triangulated source locations enable intrinsic properties of the radio source, such as its beaming characteristics, to be deduced. We present the first three-point measurements of the beaming characteristics for two solar type III radio bursts that were simultaneously observed by the three spacecraft in December of 2007 and in January of 2008. These analyses suggest that individual type III bursts exhibit a wide beaming pattern that is approximately beamed along the direction tangent to the Parker spiral magnetic field line at the source location.     

Velocities of type II solar radio events

Radial velocities for 144 simple but representative type II bursts were determined from measured frequency time histories. The velocity distribution is peaked in the region between 500 and 700 km s ^–1 (with the exact value dependent upon the coronal density model assumed) and skewed towards the larger velocities. In 85 % of the cases it was found that the velocities were constant with height. In the remaining 15 % the drift rate decreased drastically at low frequencies. This tended to occur for events having high initial velocities. The measured velocity is dependent upon the properties of the flare event but does not appear to be related to other characteristics of the radio burst. Comparisons show that the group of type II events studied had a velocity distribution which was comparable with that for coronal mass ejection events seen in association with type II bursts. The measured velocities were however statistically smaller than those of interplanetary type II bursts.     

Type III solar radio burst storms observed at low frequencies

Storms of type III solar radio bursts observed from 5.4 to 0.2 MHz, indicate the quasicontinuous production of type III events observable for a half solar rotation but persisting in some cases for well over a complete rotation. The characteristics of these storms, including the dependance of occurrence and apparent drift rates on the disc position of the associated active region are discussed. The drift rate dependance is shown to be a consequence of the propagation time of emission from the source to the observer. The occurrence rate of a burst every 10 sec observed near CMP implies that if this level of activity persists, then about a quarter of a million exciter packets are released into the interplanetary plasma during a complete rotation. Storm bursts are less intense than most isolated type III's and occur over a more limited frequency range. There appears to be a very close relation between these storms and decametric continuum.     

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.     

Type III solar radio bursts and the fundamental-harmonic hypothesis

The observational evidence is reviewed for the occurrence of type III solar radio bursts in pairs with frequency ratio two to one. We show that the observations can be explained under the hypothesis that there is a tendency for a type III burst to be followed by a second burst within approximately one second. This explanation leads to fewer difficulties than the hypothesis that type III bursts occur in pairs, one member being emitted at the fundamental of the local coronal plasma frequency, the other at its second harmonic. We conclude that in general, type III bursts are emitted at the second harmonic of the plasma frequency and that type III theories should account for this and only under very special circumstances (which are rare) for the emission at the fundamental and the second harmonic.