arXiv subjects

Seiji Yashiro

Publications and source records attributed to Seiji Yashiro.

51 records · Page 3Linked to original sources

arXiv2016

On the Directivity of Low-Frequency Type IV Radio Bursts

An intense type IV radio burst was observed by the STEREO Behind (STB) spacecraft located about 144 degres behind Earth. The burst was associated with a large solar eruption that occurred on the backside of the Sun (N05E151) close to the disk center in the STB view. The eruption was also observed by the STEREO Ahead (STA) spacecraft (located at 149 degrees ahead of Earth) as an eruption close to the west limb (N05W60) in that view. The type IV burst was complete in STB observations in that the envelope reached the lowest frequency and then receded to higher frequencies. The burst was partial viewed from STA, revealing only the edge coming down to the lowest frequency. The type IV burst was not observed at all near Earth because the source was 61 degrees behind the east limb. The eruption was associated with a low-frequency type II burst observed in all three views, although it was not very intense. Solar energetic particles were also observed at both STEREOs and at SOHO, suggesting that the shock was much extended, consistent with the very high speed of the CME (about 2048 km/s). These observations suggest that the type IV emission is directed along a narrow cone above the flare site. We confirm this result statistically using the type IV bursts of solar cycle 23.

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arXiv2016

Unusual Polar Conditions in Solar Cycle 24 and their Implications for Cycle 25

We report on the prolonged solar-maximum conditions until late 2015 at the north-polar region of the Sun indicated by the occurrence of high-latitude prominence eruptions and microwave brightness temperature close to the quiet Sun level. These two aspects of solar activity indicate that the polarity reversal was completed by mid-2014 in the south and late 2015 in the north. . The microwave brightness in the south-polar region has increased to a level exceeding the level of cycle 23/24 minimum, but just started to increase in the north. The north-south asymmetry in the polarity reversal has switched from that in cycle 23. These observations lead us to the hypothesis that the onset of cycle 25 in the northern hemisphere is likely to be delayed with respect to that in the southern hemisphere. We find that the unusual condition in the north is a direct consequence of the arrival of poleward surges of opposite polarity from the active region belt. We also find that multiple rush-to-the-pole episodes were indicated by the prominence eruption locations that lined up at the boundary between opposite polarity surges. The high-latitude prominence eruptions occurred in the boundary between the incumbent polar flux and the insurgent flux of opposite polarity.

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arXiv2015

The Mild Space Weather in Solar Cycle 24

The space weather is extremely mild during solar cycle 24: the number of major geomagnetic storms and high-energy solar energetic particle events are at the lowest since the dawn of the space age. Solar wind measurements at 1 AU using Wind and ACE instruments have shown that there is a significant drop in the density, magnetic field, total pressure, and Alfven speed in the inner heliosphere as a result of the low solar activity. The drop in large space weather events is disproportionately high because the number of energetic coronal mass ejections that cause these events has not decreased significantly. For example, the rate of halo CMEs, which is a good indicator of energetic CMEs, is similar to that in cycle 23, even though the sunspot number has declined by about 40%. The mild space weather seems to be a consequence of the anomalous expansion of CMEs due to the low ambient pressure in the heliosphere. The anomalous expansion results in the dilution of the magnetic contents of CMEs, so the geomagnetic storms are generally weak. CME driven shocks propagating through the weak heliospheric field are less efficient in accelerating energetic particles, so the particles do not attain high energies. Finally, we would like to point out that extreme events such as the 2012 July 23 CMEs that occurred on the backside of the Sun and did not affect Earth except for a small proton event.

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arXiv2015

High-energy solar particle events in cycle 24

The Sun is already in the declining phase of cycle 24, but the paucity of high-energy solar energetic particle (SEP) events continues with only two ground level enhancement (GLE) events as of March 31, 2015. In an attempt to understand this, we considered all the large SEP events of cycle 24 that occurred until the end of 2014. We compared the properties of the associated CMEs with those in cycle 23. We found that the CME speeds in the sky plane were similar, but almost all those cycle-24 CMEs were halos. A significant fraction of (16%) of the frontside SEP events were associated with eruptive prominence events. CMEs associated with filament eruption events accelerate slowly and attain peak speeds beyond the typical GLE release heights. When we considered only western hemispheric events that had good connectivity to the CME nose, there were only 8 events that could be considered as GLE candidates. One turned out to be the first GLE event of cycle 24 (2012 May 17). In two events, the CMEs were very fast (>2000 km/s) but they were launched into a tenuous medium (high Alfven speed). In the remaining five events, the speeds were well below the typical GLE CME speed (~2000 km/s). Furthermore, the CMEs attained their peak speeds beyond the typical heights where GLE particles are released. We conclude that several factors contribute to the low rate of high-energy SEP events in cycle 24: (i) reduced efficiency of shock acceleration (weak heliospheric magnetic field), (ii) poor latitudinal and longitudinal connectivity), and (iii) variation in local ambient conditions (e.g., high Alfven speed).

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arXiv2014

Spatio-temporal scaling of turbulent photospheric line-of-sight magnetic field in active region NOAA 11158

We study structure and dynamics of the turbulent photospheric magnetic field in the active region NOAA 11158 by characterizing the spatial and temporal scaling properties of the line-of-sight (LOS) component. Using high-resolution high-cadence LOS magnetograms from SDO/HMI, we measured power-law exponents $α$ and $β$ describing wavenumber- ($k$) and frequency-domain ($f$) Fourier power spectra, respectively, and investigated their evolution during the passage of the active region through the field of view of HMI. Flaring active region NOAA 11158 produces an average one-dimensional spatial power spectral density that follows approximately a $k^{-2}$ power law -- a spectrum that suggests parallel MHD fluctuations in an anisotropic turbulent medium. In addition, we found that values of $α$ capture systematic changes in the configuration of LOS photospheric magnetic field during flaring activity in the corona. Position-dependent values of the temporal scaling exponent $β$ showed that, on average, the core of the active region scales with $β>$ 3 surrounded by a diffusive region with an approximately $f^{-2}$-type spectrum. Our results indicate that only about 1 - 3 \% of the studied LOS photospheric magnetic flux displays $β\approxα$, implying that Taylor's hypothesis of frozen-in-flow turbulence is typically invalid for this scalar in presence of turbulent photospheric flows. In consequence, both spatial and temporal variations of the plasma and magnetic field must be included in a complete description of the turbulent evolution of active regions.

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arXiv2014

The Relation Between Large-Scale Coronal Propagating Fronts and Type II Radio Bursts

Large-scale, wave-like disturbances in extreme-ultraviolet (EUV) and type II radio bursts are often associated with coronal mass ejections (CMEs). Both phenomena may signify shock waves driven by CMEs. Taking EUV full-disk images at an unprecedented cadence, the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory has observed the so-called EIT waves or large-scale coronal propagating fronts (LCPFs) from their early evolution, which coincides with the period when most metric type II bursts occur. This article discusses the relation of LCPFs as captured by AIA with metric type II bursts. We show examples of type II bursts without a clear LCPF and fast LCPFs without a type II burst. Part of the disconnect between the two phenomena may be due to the difficulty in identifying them objectively. Furthermore, it is possible that the individual LCPFs and type II bursts may reflect different physical processes and external factors. In particular, the type II bursts that start at low frequencies and high altitudes tend to accompany an extended arc- shaped feature, which probably represents the 3D structure of the CME and the shock wave around it, rather than its near-surface track, which has usually been identified with EIT waves. This feature expands and propagates toward and beyond the limb. These events may be characterized by stretching of field lines in the radial direction, and be distinct from other LCPFs, which may be explained in terms of sudden lateral expansion of the coronal volume. Neither LCPFs nor type II bursts by themselves serve as necessary conditions for coronal shock waves, but these phenomena may provide useful information on the early evolution of the shock waves in 3D when both are clearly identified in eruptive events.

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arXiv2014

Anomalous Expansion of Coronal Mass Ejections during Solar Cycle 24 and its Space Weather Implications

The familiar correlation between the speed and angular width of coronal mass ejections (CMEs) is also found in solar cycle 24, but the regression line has a larger slope: for a given CME speed, cycle 24 CMEs are significantly wider than those in cycle 23. The slope change indicates a significant change in the physical state of the heliosphere, due to the weak solar activity. The total pressure in the heliosphere (magnetic + plasma) is reduced by ~40%, which leads to the anomalous expansion of CMEs explaining the increased slope. The excess CME expansion contributes to the diminished effectiveness of CMEs in producing magnetic storms during cycle 24, both because the magnetic content of the CMEs is diluted and also because of the weaker ambient fields. The reduced magnetic field in the heliosphere may contribute to the lack of solar energetic particles accelerated to very high energies during this cycle.

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arXiv2013

Obscuration of Flare Emission by an Eruptive Prominence

We report on the eclipsing of microwave flare emission by an eruptive prominence from a neighboring region as observed by the Nobeyama Radioheliograph at 17 GHz. The obscuration of the flare emission appears as a dimming feature in the microwave flare light curve. We use the dimming feature to derive the temperature of the prominence and the distribution of heating along the length of the filament. We find that the prominence is heated to a temperature above the quiet Sun temperature at 17 GHz. The duration of the dimming is the time taken by the eruptive prominence in passing over the flaring region. We also find evidence for the obscuration in EUV images obtained by the Solar and Heliospheric Observatory (SOHO) mission.

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arXiv2013

Implications of mass and energy loss due to coronal mass ejections on magnetically-active stars

Analysis of a database of solar coronal mass ejections (CMEs) and associated flares over the period 1996-2007 finds well-behaved power law relationships between the 1-8 AA flare X-ray fluence and CME mass and kinetic energy. We extrapolate these relationships to lower and higher flare energies to estimate the mass and energy loss due to CMEs from stellar coronae, assuming that the observed X-ray emission of the latter is dominated by flares with a frequency as a function of energy dn/dE=kE^-alpha. For solar-like stars at saturated levels of X-ray activity, the implied losses depend fairly weakly on the assumed value of alpha and are very large: M_dot ~ 5x10^-10 M_sun/yr and E_dot ~ 0.1L_sun. In order to avoid such large energy requirements, either the relationships between CME mass and speed and flare energy must flatten for X-ray fluence >~ 10^31 erg, or the flare-CME association must drop significantly below 1 for more energetic events. If active coronae are dominated by flares, then the total coronal energy budget is likely to be up to an order of magnitude larger than the canonical 10^-3 L_bol X-ray saturation threshold. This raises the question of what is the maximum energy a magnetic dynamo can extract from a star? For an energy budget of 1% of L_bol, the CME mass loss rate is about 5x10^-11 M_sun/yr.

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arXiv2012

The Relationship Between the Expansion Speed and Radial Speed of CMEs Confirmed Using Quadrature Observations of the 2011 February 15 CME

It is difficult to measure the true speed of Earth-directed CMEs from a coronagraph located along the Sun-Earth line because of the occulting disk. However, the expansion speed (the speed with which the CME appears to spread in the sky plane) can be measured by such a coronagraph. In order to convert the expansion speed to radial speed (which is important for space weather applications) one can use an empirical relationship between the two that assumes an average width for all CMEs. If we have the width information from quadrature observations, we can confirm the relationship between expansion and radial speeds derived by Gopalswamy et al. (2009a). The STEREO spacecraft were in qudrature with SOHO (STEREO-A ahead of Earth by 87o and STEREO-B 94o behind Earth) on 2011 February 15, when a fast Earth-directed CME occurred. The CME was observed as a halo by the Large-Angle and Spectrometric Coronagraph (LASCO) on board SOHO. The sky-plane speed was measured by SOHO/LASCO as the expansion speed, while the radial speed was measured by STEREO-A and STEREO-B. In addition, STEREO-A and STEREO-B images provided the width of the CME, which is unknown from Earth view. From the SOHO and STEREO measurements, we confirm the relationship between the expansion speed (Vexp) and radial speed (Vrad) derived previously from geometrical considerations (Gopalswamy et al. 2009a): Vrad = 1/2 (1 + cot w)Vexp, where w is the half width of the CME. STEREO-B images of the CME, we found that CME had a full width of 76o, so w = 38o. This gives the relation as Vrad = 1.14 Vexp. From LASCO observations, we measured Vexp = 897 km/s, so we get the radial speed as 1023 km/s. Direct measurement of radial speed yields 945 km/s (STEREO-A) and 1058 km/s (STEREO-B). These numbers are different only by 7.6% and 3.4% (for STEREO-A and STEREO-B, respectively) from the computed value.

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arXiv2011

Coronal Magnetic Field Measurement from EUV Images made by the Solar Dynamics Observatory

By measuring the geometrical properties of the coronal mass ejection (CME) flux rope and the leading shock observed on 2010 June 13 by the Solar Dynamics Observatory (SDO) mission's Atmospheric Imaging Assembly (AIA) we determine the Alfvén speed and the magnetic field strength in the inner corona at a heliocentric distance of ~ 1.4 Rs. The basic measurements are the shock standoff distance (deltaR) ahead of the CME flux rope, the radius of curvature of the flux rope (Rc), and the shock speed. We first derive the Alfvénic Mach number (M) using the relationship, deltaR/Rc = 0.81[(gamma-1) M^2 + 2]/[(gamma+1)(M^2-1)], where gamma is the only parameter that needed to be assumed. For gamma =4/3, the Mach number declined from 3.7 to 1.5 indicating shock weakening within the field of view of the imager. The shock formation coincided with the appearance of a type II radio burst at a frequency of ~300 MHz (harmonic component), providing an independent confirmation of the shock. The shock compression ratio derived from the radio dynamic spectrum was found to be consistent with that derived from the theory of fast mode MHD shocks. From the measured shock speed and the derived Mach number, we found the Alfvén speed to increase from ~140 km/s to 460 km/s over the distance range 1.2 to 1.5 Rs. By deriving the upstream plasma density from the emission frequency of the associated type II radio burst, we determined the coronal magnetic field to be in the range 1.3 to 1.5 G. The derived magnetic field values are consistent with other estimates in a similar distance range. This work demonstrates that the EUV imagers, in the presence of radio dynamic spectra, can be used as coronal magnetometers.

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arXiv2011

The Strength and Radial Profile of Coronal Magnetic Field from the Standoff Distance of a CME-driven Shock

We determine the coronal magnetic field strength in the heliocentric distance range 6 to 23 solar radii (Rs) by measuring the shock standoff distance and the radius of curvature of the flux rope during the 2008 March 25 coronal mass ejection (CME) imaged by white-light coronagraphs. Assuming the adiabatic index, we determine the Alfven Mach number, and hence the Alfven speed in the ambient medium using the measured shock speed. By measuring the upstream plasma density using polarization brightness images, we finally get the magnetic field strength upstream of the shock. The estimated magnetic field decreases from ~48 mG around 6 Rs to 8 mG at 23 Rs. The radial profile of the magnetic field can be described by a power law in agreement with other estimates at similar heliocentric distances.

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arXiv2010

Comprehensive Analysis of Coronal Mass Ejection Mass and Energy Properties Over a Full Solar Cycle

The LASCO coronagraphs, in continuous operation since 1995, have observed the evolution of the solar corona and coronal mass ejections (CMEs) over a full solar cycle with high quality images and regular cadence. This is the first time that such a dataset becomes available and constitutes a unique resource for the study of CMEs. In this paper, we present a comprehensive investigation of the solar cycle dependence on the CME mass and energy over a full solar cycle (1996-2009) including the first in-depth discussion of the mass and energy analysis methods and their associated errors. Our analysis provides several results worthy of further studies. It demonstrates the possible existence of two event classes; 'normal' CMEs reaching constant mass for $>10$ R$_{\sun}$ and 'pseudo' CMEs which disappear in the C3 FOV. It shows that the mass and energy properties of CME reach constant levels, and therefore should be measured, only above $\sim 10 R_\sun$. The mass density ($g/R_\sun^2$) of CMEs varies relatively little ($<$ order of magnitude) suggesting that the majority of the mass originates from a small range in coronal heights. We find a sudden reduction in the CME mass in mid-2003 which may be related to a change in the electron content of the large scale corona and we uncover the presence of a six-month periodicity in the ejected mass from 2003 onwards.

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arXiv2009

Large Geomagnetic Storms Associated with Limb Halo Coronal Mass Ejections

Solar cycle 23 witnessed the observation of hundreds of halo coronal mass ejections (CMEs), thanks to the high dynamic range and extended field of view of the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) mission. More than two thirds of halo CMEs originating on the front side of the Sun have been found to be geoeffective (Dst =< -50 nT). The delay time between the onset of halo CMEs and the peak of ensuing geomagnetic storms has been found to depend on the solar source location (Gopalswamy et al., 2007). In particular, limb halo CMEs (source longitude > 45deg) have a 20% shorter delay time on the average. It was suggested that the geomagnetic storms due to limb halos must be due to the sheath portion of the interplanetary CMEs (ICMEs) so that the shorter delay time can be accounted for. We confirm this suggestion by examining the sheath and ejecta portions of ICMEs from Wind and ACE data that correspond to the limb halos. Detailed examination showed that three pairs of limb halos were interacting events. Geomagnetic storms following five limb halos were actually produced by other disk halos. The storms followed by four isolated limb halos and the ones associated with interacting limb halos, were all due to the sheath portions of ICMEs.

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arXiv2004

The Log-normal Distributions of Physical Quantities of Flare associated Coronal Mass Ejections (CMEs), and Flare/CME Model of Gamma-ray Bursts

We investigated the statistical distributions of physical quantities of solar flares and associated coronal mass ejections (CMEs). We found that the distributions of the X-ray peak fluxes of CME-related flares, their time intervals, and speeds of associated CMEs are in good agreement with log-normal distributions. One possible interpretation of this is that only large energetic mass ejections can escape from the solar corona, which become CMEs in the interplanetary space. This ``filtering effect'' may be the reason why statistical distributions of some physical quantities are similar to log-normal distribution. It is known that the distributions of several physical quantities of gamma-ray bursts (GRBs) are also log-normal distributions, and that the time variations of gamma-ray intensity in GRBs are very similar to those of hard X-rays of solar flares. On the basis of these similarities and physical consideration of magnetohydrodynamic properties of an accretion disk around a black hole, which is supposed to be situated in the central engine of a GRB, we propose a new model of the central engine of GRBs, the {\it flare/CME model}, in which GRBs are formed by nonsteady jets consisting of intermittent ejections of mass (like solar CMEs) associated with reconnection events (like solar flares) in the accretion disk corona. Such nonsteady MHD jets would produce many shocks as well as local reconnection events far from the central engine. In this model, only large energetic mass ejections can escape from the accretion disk corona, so that statistical distributions of some physical quantities are similar to log-normal distributions.

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