70 research outputs found
Photospheric magnetic structure of coronal holes
In this study, we investigate in detail the photospheric magnetic structure
of 98 coronal holes using line-of-sight magnetograms of SDO/HMI, and for a
subset of 42 coronal holes using HINODE/SOT G-band filtergrams. We divided the
magnetic field maps into magnetic elements and quiet coronal hole regions by
applying a threshold at G. We find that the number of magnetic bright
points in magnetic elements is well correlated with the area of the magnetic
elements (cc=). Further, the magnetic flux of the individual
magnetic elements inside coronal holes is related to their area by a power law
with an exponent of (cc=). Relating the
magnetic elements to the overall structure of coronal holes, we find that on
average () % of the overall unbalanced magnetic flux of the coronal
holes arises from long-lived magnetic elements with lifetimes > 40 hours. About
() % of the unbalanced magnetic flux arises from a very weak
background magnetic field in the quiet coronal hole regions with a mean
magnetic field density of about 0.2 to 1.2 G. This background magnetic field is
correlated to the flux of the magnetic elements with lifetimes of > 40 h
(cc=). The remaining flux arises from magnetic elements with
lifetimes < 40 hours. By relating the properties of the magnetic elements to
the overall properties of the coronal holes, we find that the unbalanced
magnetic flux of the coronal holes is completely determined by the total area
that the long-lived magnetic elements cover (cc=)
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
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&
On the short term stability and tilting motion of a well-observed low-latitude solar coronal hole
The understanding of the solar magnetic coronal structure is tightly linked
to the shape of open field regions, specifically coronal holes. A dynamically
evolving coronal hole coincides with the local restructuring of open to closed
magnetic field, which leads to changes in the interplanetary solar wind
structure. By investigating the dynamic evolution of a fast-tilting coronal
hole, we strive to uncover clues about what processes may drive its
morphological changes, which are clearly visible in EUV filtergrams. Using
combined 193A and 195A EUV observations by AIA/SDO and EUVI/STEREO_A, in
conjunction with line-of-sight magnetograms taken by HMI/SDO, we track and
analyze a coronal hole over 12 days to derive changes in morphology, area and
magnetic field. We complement this analysis by potential field source surface
modeling to compute the open field structure of the coronal hole. We find that
the coronal hole exhibits an apparent tilting motion over time that cannot
solely be explained by solar differential rotation. It tilts at a mean rate of
~3.2{\deg}/day that accelerates up to ~5.4{\deg}/day. At the beginning of May,
the area of the coronal hole decreases by more than a factor of three over four
days (from ~13 * 10^9 km^2 to ~4 * 10^9 km^2), but its open flux remains
constant (~2 * 10^20 Mx). Further, the observed evolution is not reproduced by
modeling that assumes the coronal magnetic field to be potential. In this
study, we present a solar coronal hole that tilts at a rate that has yet to be
reported in literature. The rate exceeds the effect of the coronal hole being
advected by either photospheric or coronal differential rotation. Based on the
analysis we find it likely that this is due to morphological changes in the
coronal hole boundary caused by ongoing interchange reconnection and the
interaction with a newly emerging ephemeral region in its vicinity.Comment: Accepted in A&A September 15, 2023; 10 pages, 8 figure
Coronal Hole Detection and Open Magnetic Flux
Many scientists use coronal hole (CH) detections to infer open magnetic flux. Detection techniques differ in the areas that they assign as open, and may obtain different values for the open magnetic flux. We characterize the uncertainties of these methods, by applying six different detection methods to deduce the area and open flux of a near-disk center CH observed on 2010 September 19, and applying a single method to five different EUV filtergrams for this CH. Open flux was calculated using five different magnetic maps. The standard deviation (interpreted as the uncertainty) in the open flux estimate for this CH ≈ 26%. However, including the variability of different magnetic data sources, this uncertainty almost doubles to 45%. We use two of the methods to characterize the area and open flux for all CHs in this time period. We find that the open flux is greatly underestimated compared to values inferred from in situ measurements (by 2.2–4 times). We also test our detection techniques on simulated emission images from a thermodynamic MHD model of the solar corona. We find that the methods overestimate the area and open flux in the simulated CH, but the average error in the flux is only about 7%. The full-Sun detections on the simulated corona underestimate the model open flux, but by factors well below what is needed to account for the missing flux in the observations. Under-detection of open flux in coronal holes likely contributes to the recognized deficit in solar open flux, but is unlikely to resolve it
CME-HSS Interaction and Characteristics Tracked from Sun to Earth
In a thorough study, we investigate the origin of a remarkable plasma and magnetic field configuration observed in situ on June 22, 2011, near L1, which appears to be a magnetic ejecta (ME) and a shock signature engulfed by a solar wind high-speed stream (HSS). We identify the signatures as an Earth-directed coronal mass ejection (CME), associated with a C7.7 flare on June 21, 2011, and its interaction with a HSS, which emanates from a coronal hole (CH) close to the launch site of the CME. The results indicate that the major interaction between the CME and the HSS starts at a height of 1.3 R⊙ up to 3 R⊙. Over that distance range, the CME undergoes a strong north-eastward deflection of at least 30∘ due to the open magnetic field configuration of the CH. We perform a comprehensive analysis for the CME–HSS event using multi-viewpoint data (from the Solar TErrestrial RElations Observatories, the Solar and Heliospheric Observatory and the Solar Dynamics Observatory), and combined modeling efforts (nonlinear force-free field modeling, Graduated Cylindrical Shell CME modeling, and the Forecasting a CME’s Altered Trajectory – ForeCAT model). We aim at better understanding its early evolution and interaction process as well as its interplanetary propagation and related in situ signatures, and finally the resulting impact on the Earth’s magnetosphere
How the area of solar coronal holes affects the properties of high-speed solar wind streams near Earth : An analytical model
Since the 1970s it has been empirically known that the area of solar coronal holes affects the properties of high-speed solar wind streams (HSSs) at Earth. We derive a simple analytical model for the propagation of HSSs from the Sun to Earth and thereby show how the area of coronal holes and the size of their boundary regions affect the HSS velocity, temperature, and density near Earth. We assume that velocity, temperature, and density profiles form across the HSS cross section close to the Sun and that these spatial profiles translate into corresponding temporal profiles in a given radial direction due to the solar rotation. These temporal distributions drive the stream interface to the preceding slow solar wind plasma and disperse with distance from the Sun. The HSS properties at 1 AU are then given by all HSS plasma parcels launched from the Sun that did not run into the stream interface at Earth distance. We show that the velocity plateau region of HSSs as seen at 1 AU, if apparent, originates from the center region of the HSS close to the Sun, whereas the velocity tail at 1 AU originates from the trailing boundary region. Small HSSs can be described to entirely consist of boundary region plasma, which intrinsically results in smaller peak velocities. The peak velocity of HSSs at Earth further depends on the longitudinal width of the HSS close to the Sun. The shorter the longitudinal width of an HSS close to the Sun, the more of its "fastest" HSS plasma parcels from the HSS core and trailing boundary region have impinged upon the stream interface with the preceding slow solar wind, and the smaller is the peak velocity of the HSS at Earth. As the longitudinal width is statistically correlated to the area of coronal holes, this also explains the well-known empirical relationship between coronal hole areas and HSS peak velocities. Further, the temperature and density of HSS plasma parcels at Earth depend on their radial expansion from the Sun to Earth. The radial expansion is determined by the velocity gradient across the HSS boundary region close to the Sun and gives the velocity-temperature and density-temperature relationships at Earth their specific shape. When considering a large number of HSSs, the assumed correlation between the HSS velocities and temperatures close to the Sun degrades only slightly up to 1 AU, but the correlation between the velocities and densities is strongly disrupted up to 1 AU due to the radial expansion. Finally, we show how the number of particles of the piled-up slow solar wind in the stream interaction region depends on the velocities and densities of the HSS and preceding slow solar wind plasma.Peer reviewe
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