122 research outputs found
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.Comment: 12 pages, 4 figures, 1 table, Accepted for publication in ApJ Lett.
on May 6, 201
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
Alfv\'en 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 Alfv\'enic 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
Alfv\'en 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.Comment: 25 pages, 6 figures, 2 table
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).Comment: 9 pages, 5 figures, 2 tables,presented at the 14th International
Astrophysics Conference held in Tampa, FL during April 24-29, 2015. Accepted
for publication in Journal of Physics: Conference Series (JPCS). edited by G.
Zan
Three-Dimensional Image Fusion of SPECT and CT Scans for Locating Sentinel Lymph Nodes in Malignant Melanomas
Image fusion software can derive a fusion image from single photon emission computed tomography and computed tomography scans. We applied a three-dimensional fusion image to detect sentinel lymph nodes (SLNs) in 3 patients with malignant melanomas of the lumbar, vulvar and head region, respectively. During each operation, we detected SLNs at the expected site, as indicated by the fusion images. The three-dimensional image fusion could thus be confirmed as a simple and helpful method for precisely localizing SLNs in these patients
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.Comment: 4 pages, 4 figures, contributed paper to b presented at the URSI
Asia-Pacific Radio Science Conference in Seoul, August 21-25, 201
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.Comment: 13 pages, 4 figures, 1 tabl
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