216 research outputs found
3-Phase Evolution of a Coronal Hole, Part I: 360{\deg} remote sensing and in-situ observations
We investigate the evolution of a well-observed, long-lived, low-latitude
coronal hole (CH) over 10 solar rotations in the year 2012. By combining EUV
imagery from STEREO-A/B and SDO we are able to track and study the entire
evolution of the CH having a continuous 360 coverage of the Sun. The
remote sensing data are investigated together with in-situ solar wind plasma
and magnetic field measurements from STEREO-A/B, ACE and WIND. From this we
obtain how different evolutionary states of the CH as observed in the solar
atmosphere (changes in EUV intensity and area) affect the properties of the
associated high-speed stream measured at AU. Most distinctly pronounced for
the CH area, three development phases are derived: a) growing, b) maximum, and
c) decaying phase. During these phases the CH area a) increases over a duration
of around three months from about to , b) keeps a rather constant area for about one month
of , and c) finally decreases in the
following three months below until the CH
cannot be identified anymore. The three phases manifest themselves also in the
EUV intensity and in in-situ measured solar wind proton bulk velocity.
Interestingly, the three phases are related to a different range in solar wind
speed variations and we find for the growing phase a range of
~km~s, for the maximum phase ~km~s, and for the
decaying phase a more irregular behavior connected to slow and fast solar wind
speed of ~km~s.Comment: Accepted for publication in Ap
Initiation of coronal mass ejections by sunspot rotation
We study a filament eruption, two-ribbon flare, and coronal mass ejection (CME) that occurred in NOAA Active Region 10898 on 6 July 2006. The filament was located South of a strong sunspot that dominated the region. In the evolution leading up to the eruption, and for some time after it, a counter-clockwise rotation of the sunspot of about 30 degrees was observed. We suggest that the rotation triggered the eruption by progressively expanding the magnetic field above the filament. To test this scenario, we study the effect of twisting the initially potential field overlying a pre-existing flux-rope, using three-dimensional zero-β MHD simulations. We first consider a relatively simple and symmetric system, and then study a more complex and asymmetric magnetic configuration, whose photospheric-flux distribution and coronal structure are guided by the observations and a potential field extrapolation. In both cases, we find that the twisting leads to the expansion of the overlying field. As a consequence of the progressively reduced magnetic tension, the flux-rope quasi-statically adapts to the changed environmental field, rising slowly. Once the tension is sufficiently reduced, a distinct second phase of evolution occurs where the flux-rope enters an unstable regime characterised by a strong acceleration. Our simulations thus suggest a new mechanism for the triggering of eruptions in the vicinity of rotating sunspots
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
The role of initial density profiles in simulations of coronal wave - coronal hole interaction
Interactions between global coronal waves (CWs) and coronal holes (CHs)
reveal many interesting features of reflected waves and coronal hole boundaries
(CHB) but have fairly been studied so far. Magnetohydrodynamic (MHD)
simulations can help us to better understand what is happening during these
interaction events, and therefore, to achieve a broader understanding of the
parameters involved. In this study, we perform for the first time 2D MHD
simulations of a CW-CH interaction including a realistic initial wave density
profile that consists of an enhanced as well as a depleted wave part. We vary
several initial parameters, such as the initial density amplitudes of the
incoming wave, the CH density, and the CHB width, which are all based on actual
measurements. We analyse the effects of different incident angles on the
interaction features and we use the corresponding time-distance plots to detect
specific features of the incoming and the reflected wave. We found that a
particular combination of a small CH density, a realistic initial density
profile and a sufficiently small incident angle leads to remarkable interaction
features, such as a large density amplitude of the reflected wave with respect
to the incoming one. The parameter studies in this paper provide a tool to
compare time-distance plots based on observational measurements to those
created from simulations and therefore enable us to derive interaction
parameters from observed CW-CH interaction events that usually cannot be
obtained directly. The simulation results in this study are augmented by
analytical expressions for the reflection coefficient of the CW-CH interaction
which allows us to verify the simulations results in an additional way. This
work is the first of a series of studies aiming to finally reconstruct actual
observed CW-CH interaction events by means of MHD-simulations
A statistical study of long-term evolution of coronal hole properties as observed by SDO
The study of the evolution of coronal holes (CHs) is especially important in
the context of high-speed solar wind streams (HSS) emanating from them. Stream
interaction regions may deliver large amount of energy into the Earths system,
cause geomagnetic storms, and shape interplanetary space. By statistically
analysing 16 long-living CHs observed by the SDO, we focus on coronal,
morphological and underlying photospheric magnetic field characteristics as
well as investigate the evolution of the associated HSSs. We use CATCH to
extract and analyse CHs using observations taken by AIA and HMI. We derive
changes in the CH properties and correlate them to the CH evolution. Further we
analyse the properties of the HSS signatures near 1au from OMNI data by
manually extracting the peak bulk velocity of the solar wind plasma. We find
that the area evolution of CHs mostly shows a rough trend of growing to a
maximum followed by a decay. No correlation of the area evolution to the
evolution of the signed magnetic flux and signed magnetic flux density enclosed
in the projected CH area was found. From this we conclude that the magnetic
flux within the extracted CH boundaries is not the main cause for its area
evolution. We derive CH area change rates (growth and decay) of 14.2 +/- 15.0 *
10^8 km^2/day showing a reasonable anti-correlation (cc =-0.48) to the solar
activity, approximated by the sunspot number. The change rates of the signed
mean magnetic flux density (27.3 +/- 32.2 mG/day) and the signed magnetic flux
(30.3 +/- 31.5 * 10^18 Mx/day) were also found to be dependent on solar
activity (cc =0.50 and cc =0.69 respectively) rather than on the individual CH
evolutions. Further we find that the CH area-to-HSS peak velocity relation is
valid for each CH over its evolution but revealing significant variations in
the slopes of the regression lines.Comment: Accepted at A&
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