4 research outputs found
Evolution of active and polar photospheric magnetic fields during the rise of Cycle 24 compared to previous cycles
The evolution of the photospheric magnetic field during the declining phase
and minimum of Cycle 23 and the recent rise of Cycle 24 are compared with the
behavior during previous cycles. We used longitudinal full-disk magnetograms
from the NSO's three magnetographs at Kitt Peak, the Synoptic Optical Long-term
Investigations of the Sun (SOLIS) Vector Spectro-Magnetograph (VSM), the
Spectromagnetograph and the 512-Channel Magnetograph instruments, and
longitudinal full-disk magnetograms from the Mt. Wilson 150-foot tower. We
analyzed 37 years of observations from these two observatories that have been
observing daily, weather permitting, since 1974, offering an opportunity to
study the evolving relationship between the active region and polar fields in
some detail over several solar cycles. It is found that the annual averages of
a proxy for the active region poloidal magnetic field strength, the magnetic
field strength of the high-latitude poleward streams, and the time derivative
of the polar field strength are all well correlated in each hemisphere. These
results are based on statistically significant cyclical patterns in the active
region fields and are consistent with the Babcock-Leighton phenomenological
model for the solar activity cycle. There was more hemispheric asymmetry in the
activity level, as measured by total and maximum active region flux, during
late Cycle 23 (after around 2004), when the southern hemisphere was more
active, and Cycle 24 up to the present, when the northern hemisphere has been
more active, than at any other time since 1974. The active region net proxy
poloidal fields effectively disappeared in both hemispheres around 2004, and
the polar fields did not become significantly stronger after this time. We see
evidence that the process of Cycle 24 field reversal has begun at both poles.Comment: Accepted for publication in Solar Physic
Lifetimes of High-Degree p Modes in the Quiet and Active Sun
We study variations of the lifetimes of high-degree solar p-modes in the
quiet and active Sun with the solar activity cycle. The lifetimes in the degree
range 300 - 600 and frequency 2.5 - 4.5 mHz were computed from SOHO/MDI data in
an area including active regions and quiet Sun using the time-distance
technique. We applied our analysis to the data in four different phases of
solar activity: in 1996 (at minimum), 1998 (rising phase), 2000 (at maximum)
and 2003 (declining phase). The results from the area with active regions show
that the lifetime decreases as activity increases. The maximal lifetime
variations are between solar minimum in 1996 and maximum in 2000; the relative
variation averaged over all mode degree values and frequencies is a decrease of
about 13%. The lifetime reductions relative to 1996 are about 7% in 1998 and
about 10% in 2003. The lifetime computed in the quiet region still decreases
with solar activity although the decrease is smaller. On average, relative to
1996, the lifetime decrease is about 4% in 1998, 10% in 2000 and 8% in 2003.
Thus, measured lifetime increases when regions of high magnetic activity are
avoided. Moreover, the lifetime computed in quiet regions also shows variations
with activity cycle.Comment: 13 pages, 5 figures; Accepted for publication in Solar Physic