15 research outputs found
Interaction of a Moreton/EIT wave and a coronal hole
We report high-cadence H-alpha observations of a distinct Moreton wave
observed at Kanzelhoehe Solar Observatory associated with the 3B/X3.8 flare and
CME event of 2005 January 17. The Moreton wave can be identified in about 40
H-alpha frames over a period of 7 min. The EIT wave is observed in only one
frame but the derived propagation distance is close to that of the
simultaneously measured Moreton wave fronts indicating that they are closely
associated phenomena. The large angular extent of the Moreton wave allows us to
study the wave kinematics in different propagation directions with respect to
the location of a polar coronal hole (CH). In particular we find that the wave
segment whose propagation direction is perpendicular to the CH boundary
(``frontal encounter'') is stopped by the CH which is in accordance with
observations reported from EIT waves (Thompson et al. 1998). However, we also
find that at a tongue-shaped edge of the coronal hole, where the front
orientation is perpendicular to the CH boundary (the wave ``slides along'' the
boundary), the wave signatures can be found up to 100 Mm inside the CH. These
findings are briefly discussed in the frame of recent modeling results.Comment: 14 pages, 6 figures, accepted for publication in the Ap
Geoeffectiveness of Coronal Mass Ejections in the SOHO era
The main objective of the study is to determine the probability distributions
of the geomagnetic Dst index as a function of the coronal mass ejection (CME)
and solar flare parameters for the purpose of establishing a probabilistic
forecast tool for the geomagnetic storm intensity. Several CME and flare
parameters as well as the effect of successive-CME occurrence in changing the
probability for a certain range of Dst index values, were examined. The results
confirm some of already known relationships between remotely-observed
properties of solar eruptive events and geomagnetic storms, namely the
importance of initial CME speed, apparent width, source position, and the
associated solar flare class. In this paper we quantify these relationships in
a form to be used for space weather forecasting in future. The results of the
statistical study are employed to construct an empirical statistical model for
predicting the probability of the geomagnetic storm intensity based on remote
solar observations of CMEs and flares
Genesis and impulsive evolution of the 2017 September 10 coronal mass ejection
The X8.2 event of 10 September 2017 provides unique observations to study the
genesis, magnetic morphology and impulsive dynamics of a very fast CME.
Combining GOES-16/SUVI and SDO/AIA EUV imagery, we identify a hot ( MK) bright rim around a quickly expanding cavity, embedded inside a much
larger CME shell ( MK). The CME shell develops from a dense set
of large AR loops (0.5 ), and seamlessly evolves into the CME
front observed in LASCO C2. The strong lateral overexpansion of the CME shell
acts as a piston initiating the fast EUV wave. The hot cavity rim is
demonstrated to be a manifestation of the dominantly poloidal flux and
frozen-in plasma added to the rising flux rope by magnetic reconnection in the
current sheet beneath. The same structure is later observed as the core of the
white light CME, challenging the traditional interpretation of the CME
three-part morphology. The large amount of added magnetic flux suggested by
these observations explains the extreme accelerations of the radial and lateral
expansion of the CME shell and cavity, all reaching values of km
s. The acceleration peaks occur simultaneously with the first RHESSI
keV hard X-ray burst of the associated flare, further underlining the
importance of the reconnection process for the impulsive CME evolution.
Finally, the much higher radial propagation speed of the flux rope in relation
to the CME shell causes a distinct deformation of the white light CME front and
shock.Comment: Accepted for publication in the Astrophysical Journa
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
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 approximate to 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.Peer reviewe