Solar energetic electron event characteristics as inferred from spacecraft and ground-based observations
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Lataukset204
Pysyvä osoite
Verkkojulkaisu
DOI
Tiivistelmä
Massive eruptions of energy and particles from the surface of the Sun are obvious
signs that the star is active. Forecasting these eruptions, however, is very difficult
but of interest because of the damage that high energy particles can do to spacecraft,
astronauts as well as equipment on the surface of Earth. The Sun is continuously
monitored with, e.g., white-light imaging instruments such as LASCO on-board
SOHO, and ground-based radio frequency instruments like the telescopes in the
RTSN network. This thesis is a statistical study of solar energetic electron events,
observed mainly by Solar Orbiter, combined with white-light observations of coronal
mass ejections (CMEs) and ground-based observations of solar radio bursts. Solar
radio bursts are classified into different types, such as type II and III bursts. They
are bursts of electromagnetic radiation in the radio domain produced by electrons
travelling in the solar atmosphere. The main science question is differentiating the
acceleration mechanisms of solar energetic electrons (SEEs), namely, which electrons
are accelerated in the solar flare and which ones in a CME-driven shock. Solar flares
are quick releases of energy and particles from the surface of the Sun, that can be
seen as bright flashes of light. CMEs are large ejections of particles and magnetic
flux from the Sun. They are often associated with solar flares and sometimes drive
shock waves.
Results of the thesis find confirmation of the so-called big flare syndrome – the
observation that intense flares are associated with bigger events where phenomena
such as fast CMEs and radio bursts appear more frequently. Results of the analysis
point at the importance of shocks in producing the most energetic electron events.
Findings with elemental abundances suggest that they are not reliable indicators
of whether one is observing only flare accelerated particles. Further, this finding
hints in the direction of re-acceleration of particles in shocks, as opposed to events
where a shock is the only source of acceleration. Findings with spectral indices of
the events are in agreement with CME-driven shocks accelerating electrons to flatter
spectra. Results also include a statistically significantly harder spectral index for
those events with an associated type II burst compared to those without one, with
a cut-off value of -4.