First determination of the angular dependence of rise and decay times of solar radio bursts using multi-spacecraft observations
arxiv(2024)
摘要
Radio photons interact with anisotropic density fluctuations in the
heliosphere, which can alter their trajectory and influence properties deduced
from observations. This is particularly evident in solar radio observations,
where anisotropic scattering leads to highly-directional radio emissions.
Consequently, observers at varying locations will measure different properties,
including different source sizes, source positions, and intensities. However,
it is not known if measurements of the decay time of solar radio bursts are
also affected by the observer's position. Decay times are dominated by
scattering effects, and so are frequently used as proxies of the level of
density fluctuations in the heliosphere, making the identification of any
location-related dependence crucial. We combine multi-vantage observations of
interplanetary Type III bursts from four non-collinear, angularly-separated
spacecraft with simulations, to investigate the dependence of both the decay-
and rise-time measurements on the separation of the observer from the source.
We propose a function to characterise the entire time profile of radio signals,
allowing for the simultaneous estimation of the peak flux, decay time, and rise
time, while demonstrating that the rise phase of radio bursts has a
non-constant, non-exponential growth rate. We determine that the decay and rise
times are independent of the observer's position, identifying them as the only
properties to remain unaffected, thus not requiring corrections for the
observer's location. Moreover, we examine the ratio between the rise and decay
times, finding that it does not depend on the frequency. Therefore, we provide
the first evidence that the rise phase is also significantly impacted by
scattering effects, adding to our understanding of the plasma emission process.
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