Shock-associated Events Research Articles

Characterizing Magnetic Connectivity of Solar Flare Electron Sources to STEREO Spacecraft Using ADAPT-WSA Modeling

Abstract Onsets and intensity profiles of six energetic (E > 30 keV) electron events common to STEREO A and B (STA and STB) spacecraft were analyzed by Klassen et al. with the STEREO Solar Electron and Proton Telescopes when the spacecraft were separated by <70° in solar longitude. All six events were characterized by earlier onsets and higher peak intensities for the spacecraft with magnetic footpoints at the solar longitudes of larger source separations. The 2.5 Rs footpoint locations, based on Parker spiral (PS) calculations with spacecraft solar wind (SW) speeds V sw, are compared with 5 Rs footpoint locations calculated by selected realizations of ADAPT-WSA (Air Force Data Assimilative Photospheric flux Transport—Wang–Sheeley–Arge) solar wind (SW) forecast model runs for each spacecraft. ADAPT-WSA footpoint locations support the Klassen et al. results of azimuthally nonuniform injections from two shock-associated events and confirm locations for the flare source event on 2014 July 17. Substantial footpoint differences of the two methods diminish the disparity of the flare event of 2014 May 2 but exacerbate the case of two flare electron events on 2014 August 1. As limited test cases for a comparison of ADAPT-WSA and PS methods at slightly different source surfaces, the Carrington longitude differences range from several to ∼30°. We review the importance and limitations of methods for determining the solar magnetic footpoints for solar energetic particle studies.

Radio‐rich solar eruptive events

We report on the analysis of a large number of solar eruptive events that produced radio emission in the dekameter‐hectometric (DH) radio window (1–14 MHz), newly opened by the Wind/WAVES experiment. The distinguishing characteristics of coronal mass ejections (CMEs) associated with the DH type II radio bursts are larger‐than‐average width and speed. Flares of all sizes (X‐ray importance B to X) occurring at all longitudes were associated with the DH type II bursts and CMEs. We found a global enhancement in EUV over an area much larger than the flaring active region in the beginning many events. A comparison between the ‘Shock Associated‧ events and microwave bursts shows that at least half of the events do not have temporal relation. A majority of the DH type II bursts were associated with IP shocks and kilometric type II bursts. In particular, we found a very close relationship between the kilometric type II bursts and the IP shocks.

Drift acceleration at interplanetary shocks

Scatter-free acceleration of energetic particles by quasi-perpendicular interplanetary shocks is investigated. A brief review is given on the predictions of the gradient drift acceleration model concerning the energy, time, and angular dependence of the particle flux caused by a single shock encounter interaction. The angular distribution of ions in the energy range 35 keV to 1 MeV has been determined by the low-energy ion spectrometer aboard the International Sun Earth Explorer (ISEE) 3 spacecraft at several shock associated events. Reflections of particles from the shock were clearly identificable by the loss cone in the upstream pitch angle distributions. The measurements were compared to the predictions of the gradient drift acceleration model, showing a qualitative agreement in many respects. However, bidirectional distributions observed at nearly perpendicular shocks cannot be explained in the framework of the single shock encounter mechanism. It is suggested that multiple intersections of the field lines with the surface of the shock, forming magnetic traps on the upstream side, are responsible for the observed bidirectional distributions. Results obtained from numerical test particle simulations are discussed and compared to observations. A qualitative agreement between model calculations and measurements confirms that the energetic particles are trapped and accelerated, due to special field line topology, on the upstream side of the shock. It is also argued that the collapse of the trap by the convection of the field lines through the shock is accompanied by a considerable increase of the particle flux, which may be responsible for the shock spikes.

Drift Acceleration at Interplanetary Shocks

AbstractScatter-free acceleration of energetic particles by quasi-perpendicular interplanetary shocks is investigated. A brief review is given on the predictions of the gradient drift acceleration model concerning the energy, time, and angular dependence of the particle flux caused by a single shock encounter interaction. The angular distribution of ions in the energy range 35 keV to 1 MeV has been determined by the low-energy ion spectrometer aboard the ISEE 3 spacecraft at several shock associated events. Reflections of particles from the shock were clearly identifiable by the loss cone in the upstream pitch angle distributions. The measurements were compared to the predictions of the gradient drift acceleration model, showing a qualitative agreement in many respects. However, bidirectional distributions observed at nearly perpendicular shocks cannot be explained in the framework of the single shock encounter mechanism. It is suggested that multiple intersections of the field lines with the surface of the shock, forming magnetic traps on the upstream side, are responsible for the observed bidirectional distributions. Results obtained from numerical test particle simulations are discussed and compared to observations. A qualitative agreement between model calculations and measurements confirms that the energetic particles are trapped and accelerated, due to special field line topology, on the upstream side of the shock. It is also argued that the collapse of the trap by the convection of the field lines through the shock is accompanied by a considerable increase of the particle flux, which may be responsible for the shock spikes.Subject headings: acceleration of particles — interplanetary medium

Kilometric shock-associated events and microwave bursts

The peak times of impulsive microwave bursts are compared with those of shock-associated (SA) kilometric radio events. The first peaks in these two frequency regimes are usually well-correlated in time, but the last peaks of the SA events observed at 1 MHz occur an average of 20 min after the last impulsive microwave peaks. In some cases, the SA events overlap in time with the post-burst increases of microwave bursts; sometimes there is general correspondence in their intensity time profiles. These observations suggest that the earlier components of the SA events are usually caused by electrons accelerated in or near the microwave source region. We discuss the possibility that the later components of some SA events could be associated with nonthermal electrons responsible for microwave post-burst increases, although they have traditionally been attributed to electrons accelerated at type II burst producing shocks in the upper corona.

Charged particle acceleration processes in the interplanetary medium

Selected aspects of energetic (>35 keV) ion acceleration processes in three regions of the heliosphere are reviewed: at interplanetary travelling shocks near 1 AU, in the outer heliosphere and in cometary environments. Clear examples of diffusive acceleration events at quasi-parallel shocks as well as of shock drift acceleration events at quasi-perpendicular shocks near 1 AU have been found. Observations of both individual events and statistical analyses of many events are discussed in the framework of current theoretical models. Specific attention is drawn to the question of ‘seed particles’ for shock-associated events and to the significant number of shocks passing through an ambient interplanetary particle population without affecting it. In the second part of the paper selected observations of particle acceleration processes in the outer heliosphere are discussed, specifically the latitudinal gradient in the energetic ion intensity of shock-acceleration events, derived from Voyager data, and signatures for ‘local’ acceleration of MeV protons, discovered by Pioneer 10. Finally the in-situ observations at comet Giacobini-Zinner and Halley in 1985 and 1986 which have provided clear evidence of ion acceleration processes in the turbulent cometary environment, are placed in the context of current model predictions.

Characteristics of shock-associated fast-drift kilometric radio bursts

The existence of a class of fast-drift, shock-associated (SA), kilometric radio bursts which occur at the time of metric type II emission and which are not entirely the kilometric continuation of metric type III bursts has been reported previously (Cane et al., 1981). In this paper, we establish unambiguous SA event criteria for the purpose of statistically comparing SA events with conventional kilometric type III bursts. We apply these criteria to all long-duration, fast-drift bursts observed by the ISEE-3 spacecraft during a 28-month interval and find that more than 70% of the events satisfying the criteria are associated with the radio signatures of coronal shocks. If a given event in our sample is associated with a metric type II or type IV burst, it is 13 times more likely to satisfy the SA criteria than an event associated only with metric type III activity. Compared with conventional kilometric type III bursts, the characteristics of these SA events are longer duration, higher maximum intensity, and a larger number of components. Differences in these characteristics for the two classes of events are not sufficient to distinguish all SA events from conventional type III bursts. The consistent lack of reported metric type III activity during the latter part of the candidate events suggests that some of the electrons are accelerated high in the corona, at or near the altitude of the shock.

The relationship of shock-associated kilometric radio emission with metric type II bursts and energetic particles

Shock-associated (SA) events are a class of kilometric wavelength solar radio bursts first observed with the ISEE-3 Radio Astronomy Experiment. Cane et al. /1/ noted that these fast drift events are typically associated with metric type II bursts and hypothesized that the SA events were due to electrons accelerated by coronal shocks. We compare SA events from 1978 to 1982 with metric type II bursts and solar energetic particle (SEP) events. Most metric type II bursts are not obviously associated with SA events at 1980 kHz. Metric type II bursts associated with magnetically well connected flares and SA emission are well correlated with SEP events; those without SA emission are poorly correlated with SEP events. The largest SEP events from flares at any longitude are well correlated with SAs. These results are consistent with the hypothesis that the escaping electrons giving rise to SA emission are accelerated in coronal shocks.

The time scales of the scattering of energetic protons in interplanetary space

Observations with the directional spectrometer DFH aboard ISEE3 have been used to obtain results on the scattering time scales of energetic protons. Depending on the duration of the scattering process the particle distribution will be subjected to either phase scattering or full scattering. Our analysis of some representative events shows that full scattering is applicable to shock-associated events.