70 research outputs found
Validation of heliospheric modeling algorithms through pulsar observations II: simulations with EUHFORIA
In space weather studies and forecasting we employ magnetohydrodynamic (MHD)
simulations which can provide rather accurate reconstruction of the solar wind
dynamics and its evolution. However, all MHD simulations are restricted by the
input data and the modelled solar wind characteristics need to be validated
with different types of observations. That is very difficult, in particular for
the solar wind characteristics close to the Sun, since the majority of in-situ
observations are taken in the vicinity of the Earth. This is why all
alternative methods for estimation of solar wind plasma characteristics are
very important. In this study we utilise low radio frequency observations of
pulsars to probe the total electron content along the line of sight. For the
first time, we compare density estimates from pulsars with predictions from the
3D MHD modelling code; the EUropean Heliospheric FORecasting Information Asset
(EUHFORIA). We find a very good correlation for the solar wind density along a
given line of sight obtained by EUHFORIA and pulsar observations. We also
demonstrate that the pulsar observations can be very useful not only for the
model validation but also for understanding its limitations.Comment: Published in Journal of Advances in Space Researc
The impact of coronal hole characteristics and solar cycle activity in reconstructing coronal holes with EUHFORIA
Modelling with high accuracy the open magnetic field and the fast solar wind in the heliosphere is essential for space weather forecasting purposes. Primary sources of open magnetic field flux are Coronal Holes (CH), uni-polar regions that appear as dark patches in the solar corona when observed in X-ray and extreme-ultraviolet (EUV) images due to having significantly lower density and temperature to their surroundings. Therefore, when assessing how well the open magnetic field and the fast solar wind are modelled one can look at how well the model performs on one of its fundamental functions, that of reconstructing coronal hole areas. In this study we investigate how the CH morphology (i.e. latitudinal position of the centre of mass, area, intensity, elongation) and the solar variability, from high to low activity periods, can affect the results. We also investigated the possibility that the model is reconstructing CHs that are systematically shifted with respect to their observed position. The study is applied on 15 CHs exhibiting different latitudinal position and geometry. We compare the modelled CH areas with boundaries obtained by remote sensing EUV observations using the CATCH tool (Collection of Analysis Tools for Coronal Holes). We found no apparent effect of the CH characteristics on the modelling capabilities. In addition, solar cycle activity seems not to have any effect either. However, we emphasize that our sample is small and this outcome highlights the need for an extended research.Peer reviewe
Reconstructing Coronal Hole Areas With EUHFORIA and Adapted WSA Model : Optimizing the Model Parameters
The adopted Wang-Sheeley-Arge (WSA) model embedded in EUHFORIA (EUropean Heliospheric FORecasting Information Asset) is compared to EUV observations. According to the standard paradigm, coronal holes are sources of open flux; thus, we use remote sensing EUV observations and CATCH (Collection of Analysis Tools for Coronal Holes) to extract CH areas and compare them to the open flux areas modeled by EUHFORIA. From the adopted WSA model we employ only the Potential Field Source Surface (PFSS) model for the inner corona and the Schatten Current Sheet (SCS) model for the outer (PFSS+SCS). The height, R-ss, of the outer boundary of the PFSS, known as the source surface, and the height, R-i, of the inner boundary of the SCS are important parameters affecting the modeled CH areas. We investigate the impact the two model parameters can have in the modeled results. We vary R-ss within the interval [1.4, 3.2]R-circle dot with a step of 0.1R(circle dot), and R-i within the interval [1.3, 2.8]R-circle dot with the same step, and the condition that R-iPeer reviewe
Influence of coronal hole morphology on the solar wind speed at Earth
It has long been known that the high-speed stream (HSS) peak velocity at
Earth directly depends on the area of the coronal hole (CH) on the Sun.
Different degrees of association between the two parameters have been shown by
many authors. In this study, we revisit this association in greater detail for
a sample of 45 nonpolar CHs during the minimum phase of solar cycle 24. The aim
is to understand how CHs of different properties influence the HSS peak speeds
observed at Earth and draw from this to improve solar wind modeling. The
characteristics of the CHs of our sample were extracted based on the Collection
of Analysis Tools for Coronal Holes (CATCH) which employs an intensity
threshold technique applied to extreme-ultraviolet (EUV) filtergrams. We first
examined all the correlations between the geometric characteristics of the CHs
and the HSS peak speed and duration at Earth, for the entire sample. The CHs
were then categorized in different groups based on morphological criteria, such
as the aspect ratio, the orientation angle and the geometric complexity, a
parameter which is often neglected when the formation of the fast solar wind at
Earth is studied. Our results, confirmed also by the bootstrapping technique,
show that all three aforementioned morphological criteria play a major role in
determining the HSS peak speed at 1 AU. Therefore, they need to be taken into
consideration for empirical models that aim to forecast the fast solar wind at
Earth based on the observed CH solar sources.Comment: Accepted by the Astronomy & Astrophysics journa
Solar flares with and without SOHO/LASCO coronal mass ejections and type II shocks
We analyse of a set of radio rich (accompanied by type IV or II bursts) solar
flares and their association with SOHO/LASCO Coronal Mass Ejections in the
period 1998 2000. The intensity, impulsiveness and energetics of these events
are investigated. We find that, on the average, flares associated both with
type IIs and CMEs are more impulsive and more energetic than flares associated
with type IIs only (without CME reported), as well as flares accompanied by
type IV continua but not type II shocks. From the last two classes, flares with
type II bursts (without CMEs reported) are the shortest in duration and the
most impulsive.Comment: Advances in Space Research, Volume 38, Issue 5, p. 1007-101
Statistical survey of coronal mass ejections and interplanetary type II bursts
Coronal mass ejections (CMEs) are responsible for most severe space weather events, such as solar energetic particle events and geomagnetic storms at Earth. Type II radio bursts are slow drifting emissions produced by beams of suprathermal electrons accelerated at CME-driven shock waves propagating through the corona and interplanetary medium. Here, we report a statistical study of 153 interplanetary type II radio bursts observed by the two STEREO spacecraft between 2008 March and 2014 August. The shock associated radio emission was compared with CME parameters included in the Heliospheric Cataloguing, Analysis and Techniques Service catalog. We found that faster CMEs are statistically more likely to be associated with the interplanetary type II radio bursts. We correlate frequency drifts of interplanetary type II bursts with white-light observations to localize radio sources with respect to CMEs. Our results suggest that interplanetary type II bursts are more likely to have a source region situated closer to CME flanks than CME leading edge regions
LOFAR observations of the quiet solar corona
The quiet solar corona emits meter-wave thermal bremsstrahlung. Coronal radio
emission can only propagate above that radius, , where the local
plasma frequency eqals the observing frequency. The radio interferometer LOw
Frequency ARray (LOFAR) observes in its low band (10 -- 90 MHz) solar radio
emission originating from the middle and upper corona. We present the first
solar aperture synthesis imaging observations in the low band of LOFAR in 12
frequencies each separated by 5 MHz. From each of these radio maps we infer
, and a scale height temperature, . These results can be combined
into coronal density and temperature profiles. We derived radial intensity
profiles from the radio images. We focus on polar directions with simpler,
radial magnetic field structure. Intensity profiles were modeled by ray-tracing
simulations, following wave paths through the refractive solar corona, and
including free-free emission and absorption. We fitted model profiles to
observations with and as fitting parameters. In the low corona,
solar radii, we find high scale height temperatures up to
2.2e6 K, much more than the brightness temperatures usually found there. But if
all values are combined into a density profile, this profile can be
fitted by a hydrostatic model with the same temperature, thereby confirming
this with two independent methods. The density profile deviates from the
hydrostatic model above 1.5 solar radii, indicating the transition into the
solar wind. These results demonstrate what information can be gleaned from
solar low-frequency radio images. The scale height temperatures we find are not
only higher than brightness temperatures, but also than temperatures derived
from coronograph or EUV data. Future observations will provide continuous
frequency coverage, eliminating the need for local hydrostatic density models
Exploring the Circular Polarisation of Low-Frequency Solar Radio Bursts with LOFAR
The Sun is an active star that often produces numerous bursts of electromagnetic radiation at radio wavelengths. Low frequency radio bursts have recently been brought back to light with the advancement of novel radio interferometers. However, their polarisation properties have not yet been explored in detail, especially with the Low Frequency Array (LOFAR), due to difficulties in calibrating the data and accounting for instrumental leakage. Here, using a unique method to correct the polarisation observations, we explore the circular polarisation of different sub-types of solar type III radio bursts and a type I noise storm observed with LOFAR, which occurred during March-April 2019. We analysed six individual radio bursts from two different dates. We present the first Stokes V low frequency images of the Sun with LOFAR in tied-array mode observations. We find that the degree of circular polarisation for each of the selected bursts increases with frequency for fundamental emission, while this trend is either not clear or absent for harmonic emission. The type III bursts studied, that are part of a long-lasting type III storm, can have different senses of circular polarisation, occur at different locations and have different propagation directions. This indicates that the type III bursts forming a classical type III storm do not necessarily have a common origin, but instead they indicate the existence of multiple, possibly unrelated acceleration processes originating from solar minimum active regions.Peer reviewe
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