Kalle Huttunen-Heikinmaa

Sources of SEP Acceleration during a Flare – CME Event

Abstract 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 Windin 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.

On the Effect of Pre‐event Background in Determining Solar Particle Event Onset

The onset of a Solar Energetic Particle (SEP) event has been traditionally determined by using the velocity dispersion analysis (VDA), where the event onset at the Sun is determined by fitting the solar onset time and the particles’ path length to match the observed event onset at 1 AU for several SEP energy channels. This approach has been studied by using energetic particle simulations, in order to understand the effect of the interplanetary scattering on the arrival time of the first particles to 1 AU. In these studies, the SEP event onset at 1 AU has been defined as the time when the intensities reach certain percentage of the maximum of the event. This is often not feasible for practical work, as a real event may be complex in its structure, and the pre‐event background may mask the SEP onset to differing degrees in different energies. In order to estimate the usability of the VDA method, we study the simulated SEP onset on a pre‐existing particle background by varying the pre‐event background level,...

Correcting for interplanetary scattering in velocity dispersion analysis of solar energetic particles

To understand the origin of Solar Energetic Particles (SEPs), we must study their injection time relative to other solar eruption manifestations. Traditionally the injection time is determined using the Velocity Dispersion Analysis (VDA) where a linear fit of the observed event onset times at 1 AU to the inverse velocities of SEPs is used to derive the injection time and path length of the first-arriving particles. VDA does not, however, take into account that the particles that produce a statistically observable onset at 1 AU have scattered in the interplanetary space. We use Monte Carlo test particle simulations of energetic protons to study the effect of particle scattering on the observable SEP event onset above pre-event background, and consequently on VDA results. We find that the VDA results are sensitive to the properties of the pre-event and event particle spectra as well as SEP injection and scattering parameters. In particular, a VDA-obtained path length that is close to the nominal Parker spiral length does not imply that the VDA injection time is correct. We study the delay to the observed onset caused by scattering of the particles and derive a simple estimate for the delay time by using the rate of intensity increase at the SEP onset as a parameter. We apply the correction to a magnetically well-connected SEP event of June 10 2000, and show it to improve both the path length and injection time estimates, while also increasing the error limits to better reflect the inherent uncertainties of VDA.

A search for two-component energetic proton events observed on board SOHO

[1] We examined solar energetic particle (SEP) events from May 1996 through April 2000 that showed evidence for two peaks of intensity, separated by 3-24 hours, in the energy range ∼1-20 MeV, observed with the Energetic and Relativistic Nuclei and Electron instrument (ERNE) onboard Solar and Heliospheric Observatory (SOHO). Of 88 clear SEP events observed, 18 exhibited such double peaks, with more than factor of two dips between the peaks in the energy range up to at least 8 MeV. Second peaks were examined in terms of velocity dispersion and change of 4 He/p abundance ratio. Three events showed both velocity dispersion and abundance change, four events just an abundance change, and four showed velocity dispersion and no abundance change. We discuss how these different properties may reflect acceleration processes near the Sun and in the interplanetary medium. In the remaining seven events with neither velocity dispersion nor abundance change, we suggest that the second peak was a spatial feature caused by interplanetary magnetic field structures.