K.-L. Klein

Open magnetic flux tubes in the corona and the transport of solar energetic particles

We investigate how magnetic fields guide energetic particles through the corona into interplanetary space and eventually to a spacecraft near the Earth. A set of seven simple particle events is studied, where energetic electrons (30–500 keV; Wind spacecraft) or protons (5–55 MeV; SoHO) were released together with low-energy electron beams producing metric-to-kilometric type III emission. Imaging of the coronal (metre-wave) part of this emission with the Nancay Radioheliograph is used to identify the open flux tubes that guide these electrons – and by inference all particles detected at 1 AU. Open coronal field lines are also computed using potential magnetic field extrapolations, constrained by a source surface and by SoHO/MDI measurements in the photosphere (code by Schrijver and DeRosa). We find that in all events the type III radio sources lie in open flux tubes in the potential magnetic field extrapolations. The open flux tubes are rooted in small parts of the parent active region, covering a heliocentric angle of a few degrees in the photosphere. But they expand rapidly above the neighbouring closed magnetic structures and cover several tens of degrees in longitude on the source surface. Some of these open field lines are found to connect the parent active region to the footpoint of the nominal Parker spiral on the source surface, within the uncertainty of about ±10 ◦ inherent to the evaluation of its connection longitude. This is so even when the parent active region is as far as 50 ◦ away. In two cases where the coronal flux tubes point to high heliolatitudes, the detection of Langmuir waves at the Wind spacecraft in the ecliptic plane suggests that the interplanetary field lines curve down to the ecliptic before reaching 1 AU. We conclude that non-radial open flux tubes in the corona can transport particles over several tens of degrees in longitude even in simple impulsive particle events. In all events we studied, potential magnetic field models give an adequate description of these structures.

Coronal phenomena at the release of solar energetic electron events

We investigate dynamical processes in the solar corona at the release of electrons (∼30-500 keV) detected by the Wind/3DP experiment, with the aim to clarify the relationship between coronal acceleration and the escape of electrons to interplanetary space. Energetic electrons and plasma in the corona are traced using radio, EUV and X-ray observations. 40 events were identified where the release time of the electrons could be determined within an uncertainty of a few minutes and occurred during the observing hours of the Nancay Radioheliograph. All were accompanied by decametric-to-kilometric type III bursts (Wind/WAVES), and 30 by metric radio emission in the western hemisphere. The main findings from these 30 events are: (i) Electrons detected at Wind are released at the time of distinct episodes of electron acceleration in the corona signalled by radio emission. The release may occur at the start of the radio event or up to an hour later. (ii) The most conspicuous examples of delayed electron release occur in events associated with complex, long lasting (>10 min, up to seveal hours) radio emission. Radio observations suggest that in these cases the earlier accelerated electrons remain confined in the corona or are injected into flux tubes which are not connected to the spacecraft. (iii) Type II bursts revealing shock waves in the corona accompany about a third of the events. But the shock waves occur in general together with type IV radio signatures due to long lasting acceleration not related to the shock. With a few exceptions these type IV emissions have a clearer timing relationship with the electron release to space than the type II bursts. We conclude that the combination of time-extended acceleration at heights ≤0.5 R ○. above the photosphere with the injection of electrons into a variety of closed and open magnetic field structures explains the broad variety of timing shown by the radio observations and the in situ measurements.

First detection of correlated electron beams and plasma jets in radio and soft X-ray data

From a common analysis of solar radio spectral and imaging data of a fast drift burst of type U(N) together with Yohkoh soft X-ray images it is shown that the radio emission is compatible with electron beams injected and reflected in extended loops. The electron beam production coincides with the injection of hot matter, visible as a jetlike soft X-ray feature in the underlying loop system.

Magnetic Activity Associated With Radio Noise Storms

As it crossed the solar disk in May and June 1998, AR 8227 was tracked by TRACE, Yohkoh, SOHO, and many ground-based observatories. We have studied how the evolution of the magnetic field resulted in changes in activity in the corona. In particular, we examine how the evolving field may have led to the acceleration of electrons which emit noise storms observed by the Nangay Radio Heliograph between 30 May and 1 June 1998, in the absence of any flare. The magnetic changes were related to moving magnetic features (MMFs) in the vicinity of the leading spot and are related to the decay of this spot. Within the limits of the instrumental capabilities, the location in time and space of the radio emissions followed the changes observed in the photospheric magnetograms. We have extrapolated the photospheric magnetic field with a linear force-free approximation and find that the active region magnetic field was very close to being potential. These computations show a complex magnetic topology associated to the MMFs. The observed photospheric evolution is expected to drive magnetic reconnection in such complex magnetic topology. We therefore propose that the MMFs are at the origin of the observed metric noise-storms.

Fine structure in a metric type IV burst: Multi-site spectrographic, polarimetric, and heliographic observations

The spectral fine structure of solar radio continua is thought to reveal wave-particle and wave-wave interactions in magnetic traps in the solar corona. We present observations of spectra, polarization, and spatial characteristics of combined emission/extinction features (‘zebra patterns’) during a decimetric/metric type IV event on 5 June, 1990. Very high modulation depths are observed. The size and location of the sources during emission and extinction are determined for the first time. Two remarkable features are found: (1) The sources of emission stripes have finite size, up to nearly 2′; during extinction stripes the brightness is reduced across the whole extent of the unperturbed continuum, which is slightly larger than 2′. (2) During emission stripes the sources drift over distances up to several × 104 km, with apparent velocities up to 105 km s−1. The observed features are briefly discussed with respect to interpretations based on wave-particle interactions and on the scattering of electromagnetic waves.