800 research outputs found
Radio-loud CMEs from the disk center lacking shocks at 1 AU
A coronal mass ejection (CME) associated with a type II burst and originating
close to the center of the solar disk typically results in a shock at Earth in
2-3 days and hence can be used to predict shock arrival at Earth. However, a
significant fraction (about 28%) of such CMEs producing type II bursts were not
associated with shocks at Earth. We examined a set of 21 type II bursts
observed by the Wind/WAVES experiment at decameter-hectometric (DH) wavelengths
that had CME sources very close to the disk center (within a central meridian
distance of 30 degrees), but did not have a shock at Earth. We find that the
near-Sun speeds of these CMEs average to ~644 km/s, only slightly higher than
the average speed of CMEs associated with radio-quiet shocks. However, the
fraction of halo CMEs is only ~30%, compared to 54% for the radio-quiet shocks
and 91% for all radio-loud shocks. We conclude that the disk-center radio-loud
CMEs with no shocks at 1 AU are generally of lower energy and they drive shocks
only close to the Sun and dissipate before arriving at Earth. There is also
evidence for other possible processes that lead to the lack of shock at 1 AU:
(i) overtaking CME shocks merge and one observes a single shock at Earth, and
(ii) deflection by nearby coronal holes can push the shocks away from the
Sun-Earth line, such that Earth misses these shocks. The probability of
observing a shock at 1 AU increases rapidly above 60% when the CME speed
exceeds 1000 km/s and when the type II bursts propagate to frequencies below 1
MHz.Comment: 33 pages, 11 figures, 2 table
Interplanetary shocks lacking type II radio bursts
We report on the radio-emission characteristics of 222 interplanetary (IP)
shocks. A surprisingly large fraction of the IP shocks (~34%) is radio quiet
(i.e., the shocks lacked type II radio bursts). The CMEs associated with the RQ
shocks are generally slow (average speed ~535 km/s) and only ~40% of the CMEs
were halos. The corresponding numbers for CMEs associated with radio loud (RL)
shocks are 1237 km/s and 72%, respectively. The RQ shocks are also accompanied
by lower peak soft X-ray flux. CMEs associated with RQ (RL) shocks are
generally accelerating (decelerating). The kinematics of CMEs associated with
the km type II bursts is similar to those of RQ shocks, except that the former
are slightly more energetic. Comparison of the shock The RQ shocks seem to be
mostly subcritical and quasi-perpendicular. The radio-quietness is predominant
in the rise phase and decreases through the maximum and declining phases of
solar cycle 23. The solar sources of the shock-driving CMEs follow the sunspot
butterfly diagram, consistent with the higher-energy requirement for driving
shocks
Space Weather Application Using Projected Velocity Asymmetry of Halo CMEs
Halo coronal mass ejections (HCMEs) originating from regions close to the
center of the Sun are likely to be responsible for severe geomagnetic storms.
It is important to predict geo-effectiveness of HCMEs using observations when
they are still near the Sun. Unfortunately, coronagraphic observations do not
provide true speeds of CMEs due to the projection effects. In the present
paper, we present a new technique allowing estimate the space speed and
approximate source location using projected speeds measured at different
position angles for a given HCME (velocity asymmetry). We apply this technique
to HCMEs observed during 2001-2002 and find that the improved speeds are better
correlated with the travel times of HCMEs to Earth and with the magnitudes
ensuing geomagnetic storms.Comment: accepted for [publication in Solar Physic
Using the Coronal Evolution to Successfully Forward Model CMEs' In Situ Magnetic Profiles
Predicting the effects of a coronal mass ejection (CME) impact requires
knowing if impact will occur, which part of the CME impacts, and its magnetic
properties. We explore the relation between CME deflections and rotations,
which change the position and orientation of a CME, and the resulting magnetic
profiles at 1 AU. For 45 STEREO-era, Earth-impacting CMEs, we determine the
solar source of each CME, reconstruct its coronal position and orientation, and
perform a ForeCAT (Kay et al. 2015a) simulation of the coronal deflection and
rotation. From the reconstructed and modeled CME deflections and rotations we
determine the solar cycle variation and correlations with CME properties. We
assume no evolution between the outer corona and 1 AU and use the ForeCAT
results to drive the FIDO in situ magnetic field model (Kay et al. 2017a),
allowing for comparisons with ACE and Wind observations. We do not attempt to
reproduce the arrival time. On average FIDO reproduces the in situ magnetic
field for each vector component with an error equivalent to 35% of the average
total magnetic field strength when the total modeled magnetic field is scaled
to match the average observed value. Random walk best fits distinguish between
ForeCAT's ability to determine FIDO's input parameters and the limitations of
the simple flux rope model. These best fits reduce the average error to 30%.
The FIDO results are sensitive to changes of order a degree in the CME
latitude, longitude, and tilt, suggesting that accurate space weather
predictions require accurate measurements of a CME's position and orientation.Comment: accepted in JGR: Space Physic
Prominence eruption initiated by helical kink-instability of an embedded flux rope
We study the triggering mechanism of a limb-prominence eruption and the
associated coronal mass ejection near AR 12342 using SDO and LASCO/SOHO
observations. The prominence is seen with an embedded flux thread (FT) at one
end and bifurcates from the middle to a different footpoint location. The
morphological evolution of the FT is similar to an unstable flux rope (FR),
which we regard as prominence embedded FR. The FR twist exceeds the critical
value. In addition, the morphology of the prominence plasma in 304\AA~images
marks the helical nature of the magnetic skeleton with a total of 2.96 turns
along arc length. The potential field extrapolation model indicates that the
critical height of the background magnetic field gradient falls within the
inner corona (105Mm) consistent with the extent of coronal plasma loops. These
results suggest that the helical kink instability in the embedded FR caused the
slow rise of the prominence to a height of the torus instability domain.
Moreover, the differential emission measure analysis unveils heating of the
prominence plasma to coronal temperatures during eruption, suggesting a
reconnection-related heating underneath the upward rising embedded FR. The
prominence starts with a slow rise motion of 10km/s, followed by fast and slow
acceleration phases having an average acceleration of ,
in C2, C3 field of view respectively. As predicted by previous numerical
simulations, the observed synchronous kinematic profiles of the CME leading
edge and the core supports the involved FR instability in the prominence
initiation.Comment: Accepted in ApJ, 13 pages, 9 figure
Two dimensional imaging observations of meter-decameter bursts associated with the February 1986 flare activity
An analysis is presented of the two dimensional imaging observations of a flare observed on 3 Feb. l986 using the Clark Lake Multifrequency Radioheliograph. The flare produced almost all types of Meter-decimeter radio emission: enhanced storm radiation, type III/V bursts, II and IV and flare continuum. The flare continuum had early (FCE) and late (FC II) components and the type II occurred during the period between these two components. Comparing the source positions of type III/V and FCE it was found that these bursts must have occurred along adjacent open and closed field lines, respectively. The positional analysis of type II and FC II implies that the nonthermal electrons responsible for FC II need not be accelerated by type II shock and this conclusion is further supported by the close association of FC II with a microwave peak. Using the positional and temporal analysis of all these bursts and the associated hard X-ray and microwave emissions, a schematic model is developed for the magnetic field configuration in the flaring region in which the nonthermal particles responsible for these bursts are confined or along which they propagate
Width of Radio-Loud and Radio-Quiet CMEs
In the present paper we report on the difference in angular sizes between
radio-loud and radio-quiet CMEs. For this purpose we compiled these two samples
of events using Wind/WAVES and SOHO/LASCO observations obtained during
1996-2005. It is shown that the radio-loud CMEs are almost two times wider than
the radio-quiet CMEs (considering expanding parts of CMEs). Furthermore we show
that the radio-quiet CMEs have a narrow expanding bright part with a large
extended diffusive structure. These results were obtained by measuring the CME
widths in three different ways.Comment: Solar Physic, in pres
Magnetic Field Strength in the Upper Solar Corona Using White-light Shock Structures Surrounding Coronal Mass Ejections
To measure the magnetic field strength in the solar corona, we examined 10
fast (> 1000 km/s) limb CMEs which show clear shock structures in SOHO/LASCO
images. By applying piston-shock relationship to the observed CME's standoff
distance and electron density compression ratio, we estimated the Mach number,
Alfven speed, and magnetic field strength in the height range 3 to 15 solar
radii (Rs). Main results from this study are: (1) the standoff distance
observed in solar corona is consistent with those from a magnetohydrodynamic
(MHD) model and near-Earth observations; (2) the Mach number as a shock
strength is in the range 1.49 to 3.43 from the standoff distance ratio, but
when we use the density compression ratio, the Mach number is in the range 1.47
to 1.90, implying that the measured density compression ratio is likely to be
underestimated due to observational limits; (3) the Alfven speed ranges from
259 to 982 km/s and the magnetic field strength is in the range 6 to 105 mG
when the standoff distance is used; (4) if we multiply the density compression
ratio by a factor of 2, the Alfven speeds and the magnetic field strengths are
consistent in both methods; (5) the magnetic field strengths derived from the
shock parameters are similar to those of empirical models and previous
estimates.Comment: Accepted for publication in ApJ, 11 Figures, 1 Tabl
Solar Cycle Variation of CMEs and CIRs
Coronal mass ejections (CMEs) and high-speed solar wind streams (HSS) are two solar phenomena that produce large-scale structures in the interplanetary (IP) medium. CMEs evolve into interplanetary CMEs (ICMEs) and the HSS result in corotating interaction regions (CIRs) when they interact with preceding slow solar wind. CMEs and CIRs originate from closed (active region and filament region) and open (corona) hole) magnetic field regions on the Sun, respectively. These two types of mass emissions from the Sun are responsible for the largest effects on the heliosphere, particularly on Earth's space environment. This paper discussed how these structures and their solar sources vary with the solar cycle and the consequent changes in the geospace impact
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