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Takaaki Yokoyama - One of the best experts on this subject based on the ideXlab platform.

  • Statistical Study of the reconnection rate in solar flares observed with yohkoh sxt
    The Astrophysical Journal, 2006
    Co-Authors: Kaori Nagashima, Takaaki Yokoyama
    Abstract:

    We report a Statistical Study of flares observed with the Soft X-Ray Telescope (SXT) on board Yohkoh in the year 2000. We measure physical parameters of 77 flares, such as the temporal scale, size, and magnetic flux density, and find that the sizes of flares tend to be distributed more broadly as the GOES class becomes weaker and that there is a lower limit of magnetic flux density that depends on the GOES class. We also examine the relationships among these parameters and find weak correlation between the temporal and spatial scales of the flares. We estimate reconnection inflow velocity, coronal Alfven velocity, and reconnection rate using the observed values. The inflow velocities are distributed from a few km s-1 to several tens of km s-1, and the Alfven velocities in the corona are in the range from 103 to 104 km s-1. Hence, the reconnection rate is 10-3 to 10-2. We find that the reconnection rate in a flare tends to decrease as the GOES class of the flare increases. This value is within 1 order of magnitude of the theoretical maximum value predicted by the Petschek model, although the dependence of the reconnection rate on the magnetic Reynolds number tends to be stronger than that in the Petschek model.

  • Statistical Study of the reconnection rate in solar flares observed with yohkoh sxt
    arXiv: Astrophysics, 2006
    Co-Authors: Kaori Nagashima, Takaaki Yokoyama
    Abstract:

    We report a Statistical Study of flares observed with the Soft X-ray Telescope (SXT) onboard Yohkoh in the year of 2000. We measure physical parameters of 77 flares, such as the temporal scale, the size, and the magnetic flux density and find that the sizes of flares tend to be distributed more broadly as the GOES class becomes weaker and that there is a lower limit of magnetic flux density that depends on the GOES class. We also examine the relationship between these parameters and find weak correlation between temporal and spatial scales of flares. We estimate reconnection inflow velocity, coronal Alfven velocity, and reconnection rate using above observed values. The inflow velocities are distributed from a few km/s to several tens km/s and the Alfven velocities in the corona are in the range from 10^3 to 10^4 km/s. Hence the reconnection rate is 10^-3 - 10^-2. We find that the reconnection rate in a flare tends to decrease as the GOES class of the flare increases. This value is within one order of magnitude from the theoretical maximum value predicted by the Petschek model, although the dependence of the reconnection rate on the magnetic Reynolds number tends to be stronger than that in the Petschek model.

Kaori Nagashima - One of the best experts on this subject based on the ideXlab platform.

  • Statistical Study of the reconnection rate in solar flares observed with yohkoh sxt
    The Astrophysical Journal, 2006
    Co-Authors: Kaori Nagashima, Takaaki Yokoyama
    Abstract:

    We report a Statistical Study of flares observed with the Soft X-Ray Telescope (SXT) on board Yohkoh in the year 2000. We measure physical parameters of 77 flares, such as the temporal scale, size, and magnetic flux density, and find that the sizes of flares tend to be distributed more broadly as the GOES class becomes weaker and that there is a lower limit of magnetic flux density that depends on the GOES class. We also examine the relationships among these parameters and find weak correlation between the temporal and spatial scales of the flares. We estimate reconnection inflow velocity, coronal Alfven velocity, and reconnection rate using the observed values. The inflow velocities are distributed from a few km s-1 to several tens of km s-1, and the Alfven velocities in the corona are in the range from 103 to 104 km s-1. Hence, the reconnection rate is 10-3 to 10-2. We find that the reconnection rate in a flare tends to decrease as the GOES class of the flare increases. This value is within 1 order of magnitude of the theoretical maximum value predicted by the Petschek model, although the dependence of the reconnection rate on the magnetic Reynolds number tends to be stronger than that in the Petschek model.

  • Statistical Study of the reconnection rate in solar flares observed with yohkoh sxt
    arXiv: Astrophysics, 2006
    Co-Authors: Kaori Nagashima, Takaaki Yokoyama
    Abstract:

    We report a Statistical Study of flares observed with the Soft X-ray Telescope (SXT) onboard Yohkoh in the year of 2000. We measure physical parameters of 77 flares, such as the temporal scale, the size, and the magnetic flux density and find that the sizes of flares tend to be distributed more broadly as the GOES class becomes weaker and that there is a lower limit of magnetic flux density that depends on the GOES class. We also examine the relationship between these parameters and find weak correlation between temporal and spatial scales of flares. We estimate reconnection inflow velocity, coronal Alfven velocity, and reconnection rate using above observed values. The inflow velocities are distributed from a few km/s to several tens km/s and the Alfven velocities in the corona are in the range from 10^3 to 10^4 km/s. Hence the reconnection rate is 10^-3 - 10^-2. We find that the reconnection rate in a flare tends to decrease as the GOES class of the flare increases. This value is within one order of magnitude from the theoretical maximum value predicted by the Petschek model, although the dependence of the reconnection rate on the magnetic Reynolds number tends to be stronger than that in the Petschek model.

A P Dimmock - One of the best experts on this subject based on the ideXlab platform.

  • a Statistical Study of magnetic field fluctuations in the dayside magnetosheath and their dependence on upstream solar wind conditions
    Journal of Geophysical Research, 2014
    Co-Authors: A P Dimmock, K Nykyri, T I Pulkkinen
    Abstract:

    The magnetosheath functions as a natural interface connecting the interplanetary and magnetospheric plasma. Since the magnetosheath houses the shocked solar wind, it is populated with abundant magnetic field turbulence which are generated both locally and externally. Although the steady state magnetosheath is to date relatively well understood, the same cannot be said of transient magnetic perturbations due to their kinetic nature and often complex and numerous generation mechanisms. The current manuscript presents a Statistical Study of magnetic field fluctuations in the dayside magnetosheath as a function of upstream solar wind conditions. We concentrate on the ambient higher-frequency fluctuations in the range of 0.1 Hz → 2 Hz. We show evidence that the dawn (quasi-parallel) flank is visibly prone to higher-amplitude magnetic perturbations compared to the dusk (quasi-perpendicular) region. Our Statistical data also suggest that the magnitude of turbulence can be visibly enhanced close to the magnetopause during periods of southward interplanetary magnetic field orientations. Faster (> 400 km s−1) solar wind velocities also appear to drive higher-amplitude perturbations compared to slower speeds. The spatial distribution also suggests some dependence on the magnetic pileup region at the subsolar magnetopause.

  • a Statistical Study of the cross shock electric potential at low mach number quasi perpendicular bow shock crossings using cluster data
    Journal of Geophysical Research, 2012
    Co-Authors: A P Dimmock, S D Bale, M A Balikhin, V Krasnoselskikh, S N Walker, Y Hobara
    Abstract:

    [1] The cross-shock electrostatic potential at the front of collision-less shocks plays a key role in the distribution of energy at the shock front. Multipoint measurements such as those provided by the Cluster II mission provide an ideal framework for the Study of the cross-shock potential because of their ability to distinguish between temporal and spacial variations at the shock front. We present a Statistical Study of the cross-shock potential calculated for around 50 crossings of the terrestrial bow shock. The Statistical dependency of the normalized (with resect to upstream ion kinetic energy) cross-shock potential (ΦK) on the upstream Alfven Mach number is in good agreement with analytical results that predict decrease of Φk with increasing Mach number.

I V Zimovets - One of the best experts on this subject based on the ideXlab platform.

  • relationships between photospheric vertical electric currents and hard x ray sources in solar flares Statistical Study
    arXiv: Solar and Stellar Astrophysics, 2020
    Co-Authors: I V Zimovets, I N Sharykin
    Abstract:

    There are still debates whether particle acceleration in solar flares may occur due to interruption of electric currents flowing along magnetic loops. To contribute to this problem, we performed the first Statistical Study of relationships between flare hard X-ray (HXR; $50-100$ keV) sources observed by the \textit{Ramaty High-Energy Solar Spectroscopic Imager} (RHESSI) and photospheric vertical electric currents (PVECs, $j_{r}$) calculated using vector magnetograms obtained with the Helioseismic and Magnetic Imager (HMI) on-board the \textit{Solar Dynamics Observatory} (SDO). A sample of 48 flares, from C3.0 to X3.1 class, observed in central part of the solar disk by both instruments in 2010--2015 was analyzed. We found that $\approx 70$\% of all HXR sources overlapped with islands or ribbons of enhanced ($\left| j_{r} \right| \gtrsim 10^{4}$ statampere~cm$^{-2}$) PVECs. However, less than $\approx 40$\% of the HXR sources overlapped with PVEC maxima, with an accuracy of $\pm 3^{\prime\prime}$. More than in half of the flares there were HXR sources outside regions of enhanced PVECs. We found no correlation between intensity of the HXR sources and PVEC density or total PVEC under them. No systematic dissipation of PVECs under the HXR sources was found during the flares. Collectively, the results do not support the current-interruption flare models. However, the results indicate the importance of the presence of longitudinal currents in flare regions. Understanding of their specific role in the processes of energy release, plasma heating, and acceleration of particles requires further investigation.

  • excitation of kink oscillations of coronal loops Statistical Study
    Astronomy and Astrophysics, 2015
    Co-Authors: V M Nakariakov, I V Zimovets
    Abstract:

    Context. Solar flares are often accompanied by kink (transverse) oscillations of coronal loops. Despite intensive Study of these oscillations in recent years, the mechanisms that excite them are still not known. Aims. We aim to clarify the excitation mechanisms for these kink oscillations of coronal loops. Methods. Weanalysed 58 kink-oscillation events observed by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) during its first four years (2010‐2014) with the use of the JHelioviewer. Association of these oscillation events with flares, lower coronal (r 1.4R� ) eruptions and plasma ejections, coronal mass ejections (CMEs), and coronal Type-II radio bursts is studied. Results. We find that 44 of these 58 oscillation events (76%) were associated with CMEs observed in the white light emission. Moreover, 57 events (98%) were accompanied by lower coronal eruptions/ejections (LCEs) observed in the extreme-ultraviolet band in the parental active regions. In the remaining event an LCE was not clearly seen, but it was definitely associated with a CME too. The main observational finding is that the kink oscillations were excited by the deviation of loops from their equilibria by a nearby LCE in 55 events (95%). In three remaining events, it was difficult to reliably determine the cause of the oscillations because of limitations in the observational data. We also found that 53 events (91%) were associated with flares. In five remaining events, the parental active regions were behind the limb and we could not directly see flare sites. It indicates that there is a close relationship between these two kinds of solar activity. However, the estimated speeds of a hypothetical driver of kink oscillations by flares were found to be lower than 500 kms −1 in 80% of the cases. Such low speeds do not favour the association of the oscillation excitation with a shock wave, as usually assumed. That only 23 (40%) of the oscillation events were found to be associated with coronal Type-II radio bursts also goes against the shock wave mechanism for the excitation of kink oscillations. Conclusions. The Statistical analysis shows that the most probable mechanism for exciting the kink oscillations of coronal loops is the deviation of loops from their equilibrium by nearby eruptions or plasma ejections rather than a blast shock wave ignited by a flare.

B Semelin - One of the best experts on this subject based on the ideXlab platform.

  • star formation efficiency in galaxy interactions and mergers a Statistical Study
    Astronomy and Astrophysics, 2007
    Co-Authors: P Di Matteo, F Combes, A L Melchior, B Semelin
    Abstract:

    We investigate the enhancement of star formation efficiency in galaxy interactions and mergers by numerical simulations of several hundred galaxy collisions. All morphological types along the Hubble sequence are considered in the initial conditions of the two colliding galaxies, with varying bulge-to-disk ratios and gas mass fractions. Different types of orbits are simulated, direct and retrograde, according to the initial relative energy and impact parameter, and the resulting star formation history is compared to that occuring in the two galaxies when they are isolated. Our principal results are (1) retrograde encounters have greater star formation efficiency (SFE) than direct encounters, (2) the amount of gas available in the galaxy is not the main parameter governing the SFE in the burst phase, (3) there is a negative correlation between the amplitude of the star forming burst and the tidal forces exerted per unit of time, which is due to the large amount of gas dragged outside the galaxy by tidal tails in strong interactions, (4) globally, the Kennicutt-Schmidt law is seen to apply Statistically for isolated galaxies, interacting pairs and mergers, (5) enhanced star formation occurs essentially in nuclear starbursts, triggered by inward gas flows driven by non-axisymmetries in the galaxy disks. Direct encounters develop more pronounced asymmetries than retrograde ones. Based on these Statistical results we derive general laws for the enhancement of star formation in galaxy interactions and mergers, as a function of the main parameters of the encounter.

  • star formation efficiency in galaxy interactions and mergers a Statistical Study
    arXiv: Astrophysics, 2007
    Co-Authors: P Di Matteo, F Combes, A L Melchior, B Semelin
    Abstract:

    We investigate the enhancement of star formation efficiency in galaxy interactions and mergers, by numerical simulations of several hundred galaxy collisions. All morphological types along the Hubble sequence are considered in the initial conditions of the two colliding galaxies, with varying bulge-to-disk ratios and gas mass fractions. Different types of orbits are simulated, direct and retrograde, according to the initial relative energy and impact parameter, and the resulting star formation history is compared to that occuring in the two galaxies when they are isolated. Our principal results are: (1) retrograde encounters have a larger star formation efficiency (SFE) than direct encounters; (2) the amount of gas available in the galaxy is not the main parameter governing the SFE in the burst phase; (3) there is an anticorrelation between the amplitude of the star forming burst and the tidal forces exerted per unit of time, which is due to the large amount of gas dragged outside the galaxy by tidal tails in strong interactions; (4) globally, the Kennicutt-Schmidt law is retrieved Statistically for isolated galaxies, interacting pairs and mergers; (5) the enhanced star formation is essentially occurring in nuclear starbursts, triggered by inward gas flows driven by non-axisymmetries in the galaxy disks. Direct encounters develop more pronounced asymmetries than retrograde ones. Based on these Statistical results, we derive general laws for the enhancement of star formation in galaxy interactions and mergers, as a function of the main parameters of the encounter.