M. Pick

On-the-Disk Development of the Halo Coronal Mass Ejection on 1998 May 2

A halo coronal mass ejection (CME) was observed at 15:03 UT on 1998 May 2 by the Solar and Heliospheric Observatory Large-Angle Spectrometric Coronagraph. The observation of the CME was preceded by a major soft X-ray flare in NOAA Active Region 8210, characterized by a delta spot magnetic configuration and some activity in region 8214. A large transequatorial interconnecting loop (TIL) seen in the soft X-rays connected AR 8210 to a faint magnetic field region in the periphery of region 8214. Smaller loop systems were also connecting AR 8210 to other fainter bipolar magnetic structures, the interconnecting loop (IL) east of AR 8210 being one of the most visible. We present here a multiwavelength analysis of the large- and small-scale coronal structures associated with the development of the flare and of the CME, with emphasis placed on radio-imaging data. In the early phases of the flare, the radio emission sources traced the propagation paths of electrons along the TIL and the IL, which are accelerated in the vicinity of AR 8210. Furthermore, jetlike flows were observed in soft X-rays and in Hα in these directions. Significantly, the TIL and IL loop systems disappeared at least partially after the CME. An EUV Imaging Telescope (EIT) dimming region of similar size and shape to the soft X-ray TIL, but noticeably offset from it, was also observed. During the flash phase of the flare, new radio sources appeared, presenting signatures of destabilization and reconnection at discrete locations of the connecting loops. We interpret these as possible signatures of the CME liftoff on the disk. An Hα Moreton wave (blast wave) and an EIT wave were also observed, originating from the flaring AR 8210. The signatures in radio, after the wave propagated high into the corona, include type II-like emissions in the spectra. The radio images link these emissions to fast-moving sources, presumably formed at locations where the blast wave encounters magnetic structures. The opening of the CME magnetic field is revealed by the radio observations, which show large and expanding moving sources overlying the later-seen EIT dimming region.

Evidence for large-scale solar magnetic reconnection from radio and X-ray measurements

Utilizing Yohkoh Soft X-ray Telescope and Nancay radioheliograph data, we present, for the first time, observations of expanding twisted X-ray loops and a series of nonthermal radio bursts that follow the loop expansion in time and space up to ∼12′ distance. The loops were produced during a long-duration C4.7 flare close to disk center on 1994 October 25 at 1049 UT. The series of radio bursts were observed on the southern hemisphere above a weak positive-polarity region. The Kitt Peak magnetogram shows the existence of a weak negative-polarity region on the northern hemisphere at the same heliolongitude. Simultaneously with the nonthermal radio bursts, we observed the appearance of two remote X-ray brightenings and subsequent formation of two coronal holes above these weak (quiet) magnetic regions of opposite polarity, which strongly suggest the involvement of these remote regions in the event. During the 6 hr-long gradual phase of the flare, new X-ray loop connections developed among the active region and the remote quiet regions. A nonthermal radio continuum emission originating from the active region was also observed. We propose that the series of radio bursts, two remote X-ray brightenings, and new coronal loop connections were all signatures of a large-scale reconnection process between the expanding twisted flare loops and overlying transequatorial loops connecting quiet-Sun regions. The reconnection was only partial; the external part of the overlying large-scale fields were pushed out in the solar wind by the expanding twisted loops, leading to the formation of the coronal holes. The interaction between the active region and the large-scale fields seemed to be active during the entire gradual phase of the flare. This scenario may also explain the measurement of high-energy electrons in the interplanetary medium from 74° south heliolatitude as observed by Ulysses.

Development of Coronal Mass Ejections: Radio Shock Signatures

We present observational imaging evidence for the existence of metric radio bursts closely associated with the front edge of coronal mass ejections (CMEs). These radio bursts drift in frequency similarly to type II bursts. They are weak and usually go undetected on spectrograph data. We find the same measured projected velocity for the displacement of, respectively, the radio source (when observed at two or more frequencies) and the CME leading edge. The position of the emitting source coincides with the CME leading edge. Among the events analyzed, the fastest of them, with a velocity over 1400 km s-1, was associated with interplanetary type II bursts.

Radio signatures of a fast coronal mass ejection development on November 6, 1997

The Oporto radiospectrograph and the Nancay radioheliograph recorded a radio event on November 6, 1997, closely related in time with a flare on National Oceanic and Atmospheric Administration (NOAA) active region 8100. At the beginning of the event the radio sources are located on a rather small volume in the vicinity of the flare site. In a timescale of only a few minutes the radio emission sites spread over a large volume in the corona, covering a range of 100° in heliolatitude. During the period of the radio event the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) observed an extremely fast coronal mass ejection (CME), with a velocity around 2000 km s -1 . This CME presents the particularity of having a fast lateral expansion, giving it a shape reminiscent of a coat hanger. There is a very good association between the latitudinal extent and time development of the CME seen by LASCO and the radio sources recorded by the radio instruments.

Solar Source Regions for 3He-rich Solar Energetic Particle Events Identified Using Imaging Radio, Optical, and Energetic Particle Observations

We have identified the sources of six impulsive 3He-rich solar energetic particle events using imaging radio, optical, and energetic ion and electron data, together with calculated coronal fields obtained from extrapolating photospheric magnetograms using a potential field source surface (PFSS) model. These events were all studied in 2006 by Wang et al., who identified the particle sources as typically small, flaring active regions lying next to a coronal hole containing Earth-directed open field lines, located between W33° and W65°. By introducing radio imaging data we were able in one case to conclusively identify which of two simultaneous EUV jets was associated with the particle source. In addition, type III radio burst and energetic electron data introduced in this study constrain the injection times much more accurately than possible with low-energy ion data used in Wang et al. These new observations confirm the source identifications of Wang et al. and remove many of the remaining uncertainties. All of these events were associated with narrow, fast coronal mass ejections (CMEs), which are unusual for 3He-rich solar energetic particle (SEP) events. Although the CMEs generally were ejected in directions well off the ecliptic plane, the PFSS calculations show the presence of magnetic field lines that made it possible for the energetic particle to quickly reach Earth. Some of these impulsive events were observed during periods in which 3He was observed continuously over several days.

REVISITING THE ORIGIN OF IMPULSIVE ELECTRON EVENTS: CORONAL MAGNETIC RESTRUCTURING

We revisit the impulsive beamlike particle events detected in situ from 1997 to 2000 by the Electron, Proton, and Alpha Monitor (EPAM) experiment on the Advanced Composition Explorer spacecraft. We study in detail a subset of events for which there are radio coronal observations from the Nancay Radioheliograph. EPAM measures electrons in the energy range from 40 to 300 keV over a wide range of look directions and with better than 1 minute time resolution, while the Nancay radioheliograph provides images of the solar corona at five different frequencies with time cadence of eight images per second and per frequency. The radio images are complemented with spectral information from a series of radiospectrographs over a wide frequency range (from dm to km wavelengths), white-light coronagraphic images from the Large Angle Spectroscopic Coronagraph (LASCO) on the the Solar and Heliospheric Observatory (SOHO) spacecraft, and EUV images from the Extreme Ultraviolet Imaging Telescope (EIT), also on SOHO. We separate the particle events according to their associated radio emissions in the meter to decameter wavelengths, in radio-simple (only type III bursts) and radio-complex (also type II bursts and/or continua). The electron events in the radio-simple category have rather short durations, are very weak, and show essentially no delay between the onset of type III emission and the inferred release time for the energetic electrons. The electron events in the radio-complex category present variable delays between the onset of type III emission and the inferred release time for the energetic electrons. The inferred release time for the particles in the radio-complex category always coincides with the onset or major changes in the complex radio emissions; this good association suggests that the coronal processes involved in the radio emissions are at the origin of the electron acceleration. The timing and spectral characteristics of the radio emissions, when compared with the properties of the particles seen at EPAM, and the white-light information from LASCO, strongly support an acceleration process in the corona, at variable heights and below the leading edge of the associated coronal mass ejection. The coronal restructuring put in evidence by the radio signatures is the simplest explanation for the origin of those energetic particles.

X-ray and radio emissions in the early stages of solar flares

Radio and X-ray observations are presented for three flares which show significant activity for several minutes prior to the main impulsive increase in the hard X-ray flux. The activity in this ‘pre-flash’ phase is investigated using 3.5 to 461 keV X-ray data from the Solar Maximum Mission, 100 to 1000 MHz radio data from Zürich, and 169 MHz radio-heliograph data from Nançay. The major results of this study are as follows:(1)Decimetric pulsations, interpreted as plasma emission at densities of 109–1010 cm−3, and soft X-rays are observed before any Hα or hard X-ray increase.(2)Some of the metric type III radio bursts appear close in time to hard X-ray peaks but delayed between 0.5 and 1.5 s, with the shorter delays for the bursts with the higher starting frequencies.(3)The starting frequencies of these type III bursts appear to correlate with the electron temperatures derived from isothermal fits to the hard X-ray spectra. Such a correlation is expected if the particles are released at a constant altitude with an evolving electron distribution. In addition to this effect we find evidence for a downward motion of the acceleration site at the onset of the flash phase.(4)In some cases the earlier type III bursts occurred at a different location, far from the main position during the flash phase.(5)The flash phase is characterized by higher hard X-ray temperatures, more rapid increase in X-ray flux, and higher starting frequency of the coincident type III bursts.

The Radio-Coronal Mass Ejection Event on 2001 April 15

On 2001 April 15, the Nancay radioheliograph observed fast-moving, expanding loops in images taken in the wavelength range between 164 and 432 MHz. We were able to follow the progression of the radio loops, starting from a few tenths to more than 1 R☉ above the solar limb, with a time cadence of order seconds. The loops seen in radio agree very well with the features of the coronal mass ejection (CME) seen later, more than 2.5 R☉ above the limb, in white-light images by the Large Angle Spectrometric Coronagraph (LASCO) experiment on board the Solar and Heliospheric Observatory (SOHO) spacecraft. The event is well associated with an energetic electron event seen by the Electron, Proton, and Alpha Monitor (EPAM) experiment on board the Advanced Composition Explorer (ACE) spacecraft. A detailed transport model for the electrons shows that, not only the inferred onset at the Sun, but also the duration of the particle release, are similar for the radio loop and the in situ electron event detected near the Earth.

Combining visibilities from the giant meterwave radio telescope and the Nancay radio heliograph - High dynamic range snapshot images of the solar corona at 327 MHz

We report first results from an ongoing program of combining visibilities from the Giant Meterwave Radio Telescope (GMRT) and the Nancay Radio Heliograph (NRH) to produce composite snapshot images of the sun at meter wavelengths. We describe the data processing, including a specific multi-scale CLEAN algorithm. We present results of a) simulations for two models of the sun at 327 MHz, with differing complexity b) observations of a complex noise storm on the sun at 327 MHz on Aug. 27, 2002. Our results illustrate the capacity of this method to produce high dynamic range snapshot images when the solar corona has structures with scales ranging from the image resolution of 49 �� to the size of the whole sun. We emphasize that snapshot images of a complex object such as the sun, obtained by combining data from both instruments, are far better than images from either instrument alone, because their uv-coverages are very complementary.

Observation by Ulysses of hot (∼270 keV) coronal particles at 32° south heliolatitude and 4.6 AU

An unusual event of streaming 60 keV-2 MeV ions (with energy spectrum peaked ∼270 keV) and of 42–315 keV electrons occurred during the passage of a coronal mass ejection (CME) over the Ulysses spacecraft June 9–13, 1993, located at helioradius 4.6 AU and heliolatitude 32° south. The topology of the interplanetary magnetic field (IMF) within the CME has been identified as a helical magnetic flux rope by Gosling et al. [1994]. The ion and electron pitch angle distributions (PADs) had a bidirectional component in the outer (large-pitch) regions of the flux rope, while there were strongly unidirectional (antisunward) beams in the inner (small-pitch) core of the structure, where the electron PADs also displayed a distinctive depletion of electrons moving inward (sunward). Because the core ion beam was narrow, we can associate the observed energy spectrum in the peak direction of the beam (characterized by a Maxwellian with kT = 270 keV) directly with the spectrum injected in the inner heliosphere. The well-defined spatial structure of the event and the absence of any clear signatures of local interplanetary shock acceleration during the period June 9–17 implies that the injection source could have been a long-lived hot coronal ion population. The “weak scattering” electron PAD implies that the other (antisunward) end of the core of the flux rope was magnetically connected, not back to the sun, but rather to the outer heliosphere.