482 research outputs found
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
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
Prediction Space Weather Using an Asymmetric Cone Model for Halo CMEs
Halo coronal mass ejections (HCMEs) are responsible of the most severe
geomagnetic storms. A prediction of their geoeffectiveness and travel time to
Earth's vicinity is crucial to forecast space weather.
Unfortunately coronagraphic observations are subjected to projection effects
and do not provide true characteristics of CMEs. Recently, Michalek (2006, {\it
Solar Phys.}, {\bf237}, 101) developed an asymmetric cone model to obtain the
space speed, width and source location of HCMEs. We applied this technique to
obtain the parameters of all front-sided HCMEs observed by the SOHO/LASCO
experiment during a period from the beginning of 2001 until the end of 2002
(solar cycle 23). These parameters were applied for the space weather forecast.
Our study determined that the space speeds are strongly correlated with the
travel times of HCMEs within Earth's vicinity and with the magnitudes related
to geomagnetic disturbances
The Peculiar Behavior of Halo Coronal Mass Ejections in Solar Cycle 24
We report on a remarkable finding that the halo coronal mass ejections (CMEs)
in cycle 24 are more abundant than in cycle 23, although the sunspot number in
cycle 24 has dropped by about 40%. We also find that the distribution of
halo-CME source locations is different in cycle 24: the longitude distribution
of halos is much flatter with the number of halos originating at central
meridian distance >/=60 degrees twice as large as that in cycle 23. On the
other hand, the average speed and the associated soft X-ray flare size are the
same in the two cycles, suggesting that the ambient medium into which the CMEs
are ejected is significantly different. We suggest that both the higher
abundance and larger central meridian longitudes of halo CMEs can be explained
as a consequence of the diminished total pressure in the heliosphere in cycle
24 (Gopalswamy et al. 2014). The reduced total pressure allows CMEs expand more
than usual making them appear as halos.Comment: 12 pages, 5 figures, accepted for publication in the Astrophysical
Journal Letters, April 7, 201
The First Ground Level Enhancement Event of Solar Cycle 24: Direct Observation of Shock Formation and Particle Release Heights
We report on the 2012 May 17 Ground Level Enhancement (GLE) event, which is
the first of its kind in Solar Cycle 24. This is the first GLE event to be
fully observed close to the surface by the Solar Terrestrial Relations
Observatory (STEREO) mission. We determine the coronal mass ejection (CME)
height at the start of the associated metric type II radio burst (i.e., shock
formation height) as 1.38 Rs (from the Sun center). The CME height at the time
of GLE particle release was directly measured from a STEREO image as 2.32 Rs,
which agrees well with the estimation from CME kinematics. These heights are
consistent with those obtained for cycle-23 GLEs using back-extrapolation. By
contrasting the 2012 May 17 GLE with six other non-GLE eruptions from
well-connected regions with similar or larger flare size and CME speed, we find
that the latitudinal distance from the ecliptic is rather large for the non-GLE
events due to a combination of non-radial CME motion and unfavorable solar B0
angle, making the connectivity to Earth poorer. We also find that the coronal
environment may play a role in deciding the shock strength.Comment: 16 pages, 4 figures, 1 tabl
Statistical Analysis of Periodic Oscillations in LASCO Coronal Mass Ejection Speeds
A large set of coronal mass ejections (CMEs, 3463) has been selected to study their periodic oscillations in speed in the Solar and Heliospheric Observatory (SOHO) missions Large Angle and Spectrometric Coronagraph (LASCO) field of view. These events, reported in the SOHOLASCO catalog in the period of time 19962004, were selected based on having at least 11 height-time measurements. This selection criterion allows us to construct at least ten-point speed distance profiles and evaluate kinematic properties of CMEs with a reasonable accuracy. To identify quasi-periodic oscillations in the speed of the CMEs a sinusoidal function was fitted to speed distance profiles and the speed time profiles. Of the considered events 22 revealed periodic velocity fluctuations. These speed oscillations have on average amplitude equal to 87 kms(exp -1) and period 7.8R /241 min (in distance-time). The study shows that speed oscillations are a common phenomenon associated with CME propagation implying that all the CMEs have a similar magnetic flux-rope structure. The nature of oscillations can be explained in terms of magnetohydrodynamic (MHD) waves excited during the eruption process. More accurate detection of these modes could, in the future, enable us to characterize magnetic structures in space (space seismology)
Spatial Relationship between Solar Flares and Coronal Mass Ejections
We report on the spatial relationship between solar flares and coronal mass
ejections (CMEs) observed during 1996-2005 inclusive. We identified 496
flare-CME pairs considering limb flares (distance from central meridian > 45
deg) with soft X-ray flare size > C3 level. The CMEs were detected by the Large
Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric
Observatory (SOHO). We investigated the flare positions with respect to the CME
span for the events with X-class, M-class, and C-class flares separately. It is
found that the most frequent flare site is at the center of the CME span for
all the three classes, but that frequency is different for the different
classes. Many X-class flares often lie at the center of the associated CME,
while C-class flares widely spread to the outside of the CME span. The former
is different from previous studies, which concluded that no preferred flare
site exists. We compared our result with the previous studies and conclude that
the long-term LASCO observation enabled us to obtain the detailed spatial
relation between flares and CMEs. Our finding calls for a closer flare-CME
relationship and supports eruption models typified by the CSHKP magnetic
reconnection model.Comment: 7 pages; 4 figures; Accepted by the Astrophysical Journa
Commission 10: Solar Activity
Commission 10 aims at the study of various forms of solar activity, including networks, plages, pores, spots, fibrils, surges, jets, filaments/prominences, coronal loops, flares, coronal mass ejections (CMEs), solar cycle, microflares, nanoflares, coronal heating etc., which are all manifestation of the interplay of magnetic fields and solar plasma. Increasingly important is the study of solar activities as sources of various disturbances in the interplanetary space and near-Earth “space weather”.
Over the past three years a major component of research on the active Sun has involved data from the RHESSI spacecraft. This review starts with an update on current and planned solar observations from spacecraft. The discussion of solar flares gives emphasis to new results from RHESSI, along with updates on other aspects of flares. Recent progress on two theoretical concepts, magnetic reconnection and magnetic helicity is then summarized, followed by discussions of coronal loops and heating, the magnetic carpet and filaments. The final topic discussed is coronal mass ejections and space weather.
The discussions on each topic is relatively brief, and intended as an outline to put the extensive list of references in context.
The review was prepared jointly by the members of the Organizing Committee, and the names of the primary contributors to the various sections are indicated in parentheses
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