4 research outputs found
An inner boundary condition for solar wind models based on coronal density
Accurate forecasting of the solar wind has grown in importance as society becomes increasingly dependent on technology susceptible to space weather events. This work describes an inner boundary condition for ambient solar wind models based on tomography maps of the coronal plasma density gained from coronagraph observations, providing a novel alternative to magnetic extrapolations. The tomographical density maps provide a direct constraint of the coronal structure at heliocentric distances of 4–8 R⊙, thus avoiding the need to model the complex non-radial lower corona. An empirical inverse relationship converts densities to solar wind velocities, which are used as an inner boundary condition by the Heliospheric Upwind Extrapolation (HUXt) model to give ambient solar wind velocity at Earth. The dynamic time warping (DTW) algorithm is used to quantify the agreement between tomography/HUXt output and in situ data. An exhaustive search method is then used to adjust the lower boundary velocity range in order to optimise the model. Early results show up to a 32% decrease in mean absolute error between the modelled and observed solar wind velocities compared to the coupled MAS/HUXt model. The use of density maps gained from tomography as an inner boundary constraint is thus a valid alternative to coronal magnetic models and offers a significant advancement in the field, given the availability of routine space-based coronagraph observations
Derived electron densities from linear polarization observations of the visible-light corona during the 14 December 2020 total solar eclipse
A new instrument was designed to take visible-light (VL) polarized brightness
() observations of the solar corona during the 14 December 2020 total solar
eclipse. The instrument, called the Coronal Imaging Polarizer (CIP), consisted
of a 16 MP CMOS detector, a linear polarizer housed within a piezoelectric
rotation mount, and an f-5.6, 200 mm DSLR lens. Observations were successfully
obtained, despite poor weather conditions, for five different exposure times
(0.001 s, 0.01 s, 0.1 s, 1 s, and 3 s) at six different orientation angles of
the linear polarizer (0\de, 30\de, 60\de, 90\de, 120\de, and 150\de). The
images were manually aligned using the drift of background stars in the sky and
images of different exposure times were combined using a simple signal-to-noise
ratio cut. The polarization and brightness of the local sky is also estimated
and the observations were subsequently corrected. The of the K-corona was
determined using least squares fitting and radiometric calibration was done
relative to the Mauna Loa Solar Observatory (MLSO) K-Cor observations from
the day of the eclipse. The data was then inverted to acquire the coronal
electron density, , for an equatorial streamer and a polar coronal hole,
which agreed very well with previous studies. The effect of changing the number
of polarizer angles used to compute the is also discussed and it is found
that the results vary by up to 13\% when using all six polarizer angles
versus only a select three angles
An Empirical Relationship Between Coronal Density and Solar Wind Velocity in the Middle Corona With Applications to Space Weather
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Constraints on solar wind density and velocity based on coronal tomography and Parker Solar Probe measurements
Previous work has established an empirical relationship between densities gained from coronal rotational
tomography near the ecliptic plane with solar wind outflow speeds at heliocentric distance r₀ = 8R⊙. This work
aims to include solar wind acceleration, and thus velocity profiles out to 1 au. Inner boundary velocities are given
as a function of normalized tomographic densities, ρN, as V₀ = (75 * ᵉ⁻[⁵.²⁺ρᴺ] + 108), and typically range from
100 to 180 km s−1. The subsequent acceleration is defined as V(r) = V₀(1+αIP[1-e⁽⁻[ʳ⁻ʳ⁰]/ʳᴴ⁾]), with αIP ranging between 1.75 and 2.7, and rH between 50 and 35 R⊙ dependent on V0. These acceleration profiles
approximate the distribution of in situ measurements by Parker Solar Probe (PSP) and other measurements at 1 au.
Between 2018 November and 2021 September these constraints are applied using the HUXt model and give good
agreement with in situ observations at PSP, with a ∼6% improvement compared with using a simpler constant
acceleration model previously considered. Given the known tomographical densities at 8 R⊙, we extrapolate
density to 1 au using the model velocities and assuming mass flux conservation. Extrapolated densities agree well
with OMNI measurements. Thus coronagraph-based estimates of densities define the ambient solar wind outflow
speed, acceleration, and density from 8 R⊙ to at least 1 au. This sets a constraint on more advanced models, and a
framework for forecasting that provides a valid alternative to the use of velocities derived from magnetic field
extrapolations