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Zagreb, Croatia

Shanmugaraju A.,Arul Anandar College | Vrsnak B.,Hvar Observatory
Solar Physics | Year: 2014

The speed [v(R)] of coronal mass ejections (CMEs) at various distances from the Sun is modeled (as proposed by Vršnak and Gopalswamy in J. Geophys. Res. 107, 2002, doi:10.1029/2001/JA000120) by using the equation of motion adrag=γ(v-w) and its quadratic form adrag=γ(v-w){pipe}v-w{pipe}, where v and w are the speeds of the CME and solar wind, respectively. We assume that the parameter γ can be expressed as γ=αRβ, where R is the heliocentric distance, and α and β are constants. We extend the analysis of Vršnak and Gopalswamy to obtain a more detailed insight into the dependence of the CME Sun-Earth transit time on the CME speed and the ambient solar-wind speed, for different combinations of α and β. In such a parameter-space analysis, the results obtained confirm that the CME transit time depends strongly on the state of the ambient solar wind. Specifically, we found that: i) for a particular set of values of α and β, a difference in the solar-wind speed causes larger transit-time differences at low CME speeds [v0], than at high v0; ii) the difference between transit times of slow and fast CMEs is larger at low solar-wind speed [w0] than at high w0; iii) transit times of fast CMEs are only slightly influenced by the solar-wind speed. The last item is especially important for space-weather forecasting, since it reduces the number of key parameters that determine the arrival time of fast CMEs, which tend to be more geo-effective than the slow ones. Finally, we compared the drag-based model results with the observational data for two CME samples, consisting of non-interacting and interacting CMEs (Manoharan et al. in J. Geophys. Res.109, 2004). The comparison reveals that the model results are in better agreement with the observations for non-interacting events than for the interacting events. It was also found that for slow CMEs (v0<500 km s-1), there is a deviation between the observations and the model if slow-wind speeds (≈ 300 - 400 km s-1) are taken for the model input. On the other hand, the model values and the observed data agree for both the slow and the fast CMEs if higher solar-wind speeds are assumed. It is also found that the quadratic form of the drag equation reproduces the observed transit times of fast CMEs better than the linear drag model. © 2013 Springer Science+Business Media Dordrecht.

Zhukov A.N.,Moscow State University | Vrnak B.,Hvar Observatory | Veronig A.,University of Graz
Astrophysical Journal | Year: 2012

We present a study of the solar coronal shock wave on 2005 November 14 associated with the GOES M3.9 flare that occurred close to the east limb (S06° E60°). The shock signature, a type II radio burst, had an unusually high starting frequency of about 800MHz, indicating that the shock was formed at a rather low height. The position of the radio source, the direction of the shock wave propagation, and the coronal electron density were estimated using Nançay Radioheliograph observations and the dynamic spectrum of the Green Bank Solar Radio Burst Spectrometer. The soft X-ray, Hα, and Reuven Ramaty High Energy Solar Spectroscopic Imager observations show that the flare was compact, very impulsive, and of a rather high density and temperature, indicating a strong and impulsive increase of pressure in a small flare loop. The close association of the shock wave initiation with the impulsive energy release suggests that the impulsive increase of the pressure in the flare was the source of the shock wave. This is supported by the fact that, contrary to the majority of events studied previously, no coronal mass ejection was detected in association with the shock wave, although the corresponding flare occurred close to the limb. © 2012 The American Astronomical Society. All rights reserved.

Ko Y.-K.,U.S. Navy | Raymond J.C.,Harvard - Smithsonian Center for Astrophysics | Vrsnak B.,Hvar Observatory
Astrophysical Journal | Year: 2010

A post-coronal mass ejection (CME) current sheet (CS) is a common feature developed behind an erupting flux rope in CME models. Observationally, white light observations have recorded many occurrences of a thin ray appearing behind a CME eruption that closely resembles a post-CME CS in its spatial correspondence and morphology. UV and X-ray observations further strengthen this interpretation by the observations of high-temperature emission at locations consistent with model predictions. The next question then becomes whether the properties inside a post-CME CS predicted by a model agree with observed properties. In this work, we assume that the post-CME CS is a consequence of Petschek-like reconnection and that the observed ray-like structure is bounded by a pair of slow mode shocks developed from the reconnection site. We perform time-dependent ionization calculations and model the UV line emission. We find that such a model is consistent with SOHO/UVCS observations of the post-CME CS. The change of Fe xviii emission in one event implies an inflow speed of ∼10 km s-1 and a corresponding reconnection rate of MA ∼ 0.01. We calculate the expected X-ray emission for comparison with X-ray observations by Hinode/XRT, as well as the ionic charge states as would be measured in situ at 1 AU. We find that the predicted count rate for Hinode/XRT agrees with what was observed in a post-CME CS on 2008 April 9, and the predicted ionic charge states are consistent with high ionization states commonly measured in the interplanetary CMEs. The model results depend strongly on the physical parameters in the ambient corona, namely the coronal magnetic field, the electron density, and temperature during the CME event. It is crucial to obtain these ambient coronal parameters and as many facets of the CS properties as possible by observational means so that the post-CME CS models can be scrutinized more effectively. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

Veronig A.M.,University of Graz | Muhr N.,University of Graz | Kienreich I.W.,University of Graz | Temmer M.,University of Graz | And 2 more authors.
Astrophysical Journal Letters | Year: 2010

We present first observations of a dome-shaped large-scale extreme-ultraviolet coronal wave, recorded by the Extreme Ultraviolet Imager instrument on board STEREO-B on 2010 January 17. The main arguments that the observed structure is the wave dome (and not the coronal mass ejection, CME) are (1) the spherical form and sharpness of the dome's outer edge and the erupting CME loops observed inside the dome; (2) the low-coronal wave signatures above the limb perfectly connecting to the on-disk signatures of the wave; (3) the lateral extent of the expanding dome which is much larger than that of the coronal dimming; and (4) the associated high-frequency type II burst indicating shock formation low in the corona. The velocity of the upward expansion of the wave dome (ν ∼ 650 km s-1) is larger than that of the lateral expansion of the wave (ν ∼ 280 km s-1), indicating that the upward dome expansion is driven all the time, and thus depends on the CME speed, whereas in the lateral direction it is freely propagating after the CME lateral expansion stops. We also examine the evolution of the perturbation characteristics: first the perturbation profile steepens and the amplitude increases. Thereafter, the amplitude decreases with r -2.5 ± 0.3, the width broadens, and the integral below the perturbation remains constant. Our findings are consistent with the spherical expansion and decay of a weakly shocked fast-mode MHD wave. © 2010. The American Astronomical Society. All rights reserved.

Rotter T.,University of Graz | Veronig A.M.,University of Graz | Temmer M.,University of Graz | Vrsnak B.,Hvar Observatory
Solar Physics | Year: 2012

We analyze the relationship between the coronal hole (CH) characteristics on the Sun (area, position, and intensity levels) and the corresponding solar wind parameters (solar wind speed v, proton temperature T, proton density n, and magnetic field strength B) measured in situ at 1 AU with a 6-h time resolution. We developed a histogram-based intensity thresholding method to obtain fractional CH areas from SOHO/EIT 195 Å images. The algorithm was applied to 6-h cadence EIT 195 Å images for the year 2005, which were characterized by a low solar activity. In calculating well-defined peaks of the solar wind parameters corresponding to the peaks in CH area, we found that the solar wind speed v shows a high correlation with correlation coefficient cc=0. 78, medium correlation for T and B with cc=0. 41 and cc=0. 41. No significant correlation was found with the proton density n. Applying an intensity-weighted CH area did not improve the relations, since the size and the mean intensity of the CH areas are not independent parameters but strongly correlated (cc=- 0. 72). Comparison of the fractional CH areas derived from GOES/SXI and SOHO/EIT and the related solar wind predictions shows no systematic differences (cc=0. 79). © 2012 Springer Science+Business Media B.V.

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