174 research outputs found
A Helioseismic Perspective on the Depth of the Minimum Between Solar Cycles 23 and 24
The solar-activity-cycle minimum observed between Cycles 23 and 24 is
generally regarded as being unusually deep and long. That minimum is being
followed by one of the smallest amplitude cycles in recent history. We perform
an in-depth analysis of this minimum with helioseismology. We use Global
Oscillation Network Group (GONG) data to demonstrate that the frequencies of
helioseismic oscillations are a sensitive probe of the Sun's magnetic field:
The frequencies of the helioseismic oscillations were found to be
systematically lower in the minimum following Cycle 23 than in the minimum
preceding it. This difference is statistically significant and may indicate
that the Sun's global magnetic field was weaker in the minimum following Cycle
23. The size of the shift in oscillation frequencies between the two minima is
dependent on the frequency of the oscillation and takes the same functional
form as the frequency dependence observed when the frequencies at cycle maximum
are compared with the cycle-minimum frequencies. This implies that the same
near-surface magnetic perturbation is responsible. Finally, we determine that
the difference in the mean magnetic field between the minimum preceding Cycle
23 and that following it is approximately 1G.Comment: Accepted for publication in Solar Physics, 16 pages, 4 figures, 4
table
Properties of quasi-periodic pulsations in solar flares from a single active region
We investigate the properties of a set of solar flares originating from a
single active region (AR) that exhibit QPPs, and look for signs of the QPP
periods relating to AR properties. The AR studied, best known as NOAA 12192,
was unusually long-lived and produced 181 flares. Data from the GOES, EVE,
Fermi, Vernov and NoRH observatories were used to determine if QPPs were
present in the flares. For the soft X-ray GOES and EVE data, the time
derivative of the signal was used. Power spectra of the time series data
(without any form of detrending) were inspected, and flares with a peak above
the 95% confidence level in the spectrum were labelled as having candidate
QPPs. The confidence levels were determined taking account of uncertainties and
the possible presence of red noise. AR properties were determined using HMI
line of sight magnetograms. A total of 37 flares (20% of the sample) show good
evidence of having QPPs, and some of the pulsations can be seen in data from
multiple instruments and in different wavebands. The QPP periods show a weak
correlation with the flare amplitude and duration, but this may be due to an
observational bias. A stronger correlation was found between the QPP period and
duration of the QPP signal, which can be partially but not entirely explained
by observational constraints. No correlations were found with the AR area,
bipole separation, or average magnetic field strength. The fact that a
substantial fraction of the flare sample showed evidence of QPPs using a strict
detection method with minimal processing of the data demonstrates that these
QPPs are a real phenomenon, which cannot be explained by the presence of red
noise or the superposition of multiple unrelated flares. The lack of
correlation between the QPP periods and AR properties implies that the
small-scale structure of the AR is important, and/or that different QPP
mechanisms act in different cases.Comment: 23 pages, 57 figures. Accepted for publication by Astronomy &
Astrophysic
Oscillations in stellar superflares
Two different mechanisms may act to induce quasi-periodic pulsations (QPP) in
whole-disk observations of stellar flares. One mechanism may be
magneto-hydromagnetic (MHD) forces and other processes acting on flare loops as
seen in the Sun. The other mechanism may be forced local acoustic oscillations
due to the high-energy particle impulse generated by the flare (known as
`sunquakes' in the Sun). We analyze short-cadence Kepler data of 257 flares in
75 stars to search for QPP in the flare decay branch or post-flare oscillations
which may be attributed to either of these two mechanisms. About 18 percent of
stellar flares show a distinct bump in the flare decay branch of unknown
origin. The bump does not seem to be a highly-damped global oscillation because
the periods of the bumps derived from wavelet analysis do not correlate with
any stellar parameter. We detected damped oscillations covering several cycles
(QPP), in seven flares on five stars. The periods of these oscillations also do
not correlate with any stellar parameter, suggesting that these may be a due to
flare loop oscillations. We searched for forced global oscillations which might
result after a strong flare. To this end, we investigated the behaviour of the
amplitudes of solar-like oscillations in eight stars before and after a flare.
However, no clear amplitude change could be detected. We also analyzed the
amplitudes of the self-excited pulsations in two delta Scuti stars and one
gamma Doradus star before and after a flare. Again, no clear amplitude changes
were found. Our conclusions are that a new process needs to be found to explain
the high incidence of bumps in stellar flare light curves, that flare loop
oscillations may have been detected in a few stars and that no conclusive
evidence exists as yet for flare induced global acoustic oscillations
(starquakes).Comment: 13 pages, 14 figures, 3 table
A new efficient method for determining weighted power spectra: detection of low-frequency solar p-modes by analysis of BiSON data
We present a new and highly efficient algorithm for computing a power
spectrum made from evenly spaced data which combines the noise-reducing
advantages of the weighted fit with the computational advantages of the Fast
Fourier Transform (FFT). We apply this method to a 10-year data set of the
solar p-mode oscillations obtained by the Birmingham Solar Oscillations Network
(BiSON) and thereby uncover three new low-frequency modes. These are the l=2,
n=5 and n=7 modes and the l=3, n=7 mode. In the case of the l=2, n=5 modes,
this is believed to be the first such identification of this mode in the
literature. The statistical weights needed for the method are derived from a
combination of the real data and a sophisticated simulation of the instrument
performance. Variations in the weights are due mainly to the differences in the
noise characteristics of the various BiSON instruments, the change in those
characteristics over time and the changing line-of-sight velocity between the
stations and the Sun. It should be noted that a weighted data set will have a
more time-dependent signal than an unweighted set and that, consequently, its
frequency spectrum will be more susceptible to aliasing.Comment: 11 pages, 7 Figures, accepted for publication in MNRAS, Figure 6 had
to be reduced in size to upload and so may be difficult to view on screen in
.ps versio
A Multi-Instrument Investigation of the Frequency Stability of Oscillations Above the Acoustic Cut-Off Frequency with Solar Activity
Below the acoustic cut-off frequency, oscillations are trapped within the
solar interior and become resonant. However, signatures of oscillations persist
above the acoustic cut-off frequency, and these travelling waves are known as
pseudomodes. Acoustic oscillation frequencies are known to be correlated with
the solar cycle, but the pseudomode frequencies are predicted to vary in
anti-phase. We have studied the variation in pseudomode frequencies with time
systematically through the solar cycle. We analyzed Sun-as-a-star data from
Variability of Solar Irradiance and Gravity Oscillations (VIRGO), and Global
Oscillations at Low Frequencies (GOLF), as well as the decomposed data from
Global Oscillation Network (GONG) for harmonic degrees . The
data cover over two solar cycles (1996--2021, depending on instrument). We
split them into overlapping 100-day long segments and focused on two frequency
ranges, namely -- and --. The
frequency shifts between segments were then obtained by fitting the
cross-correlation function between the segments' periodograms. For VIRGO and
GOLF, we found no significant variation of pseudomode frequencies with solar
activity. However, in agreement with previous studies, we found that the
pseudomode frequency variations are in anti-phase with the solar cycle for GONG
data. Furthermore, the pseudomode frequency shifts showed a double-peak feature
at their maximum, which corresponds to solar activity minimum, and is not seen
in solar activity proxies. An, as yet unexplained, pseudo-periodicity in the
amplitude of the variation with harmonic degree is also observed in the
GONG data
Changes in the sensitivity of solar p-mode frequency shifts to activity over three solar cycles
Low-degree solar p-mode observations from the long-lived Birmingham Solar
Oscillations Network (BiSON) stretch back further than any other single
helioseismic data set. Results from BiSON have suggested that the response of
the mode frequency to solar activity levels may be different in different
cycles. In order to check whether such changes can also be seen at higher
degrees, we compare the response of medium-degree solar p-modes to activity
levels across three solar cycles using data from Big Bear Solar Observatory
(BBSO), Global Oscillation Network Group (GONG), Michelson Doppler Imager (MDI)
and Helioseismic and Magnetic Imager (HMI), by examining the shifts in the mode
frequencies and their sensitivity to solar activity levels. We compare these
shifts and sensitivities with those from radial modes from BiSON. We find that
the medium-degree data show small but significant systematic differences
between the cycles, with solar cycle 24 showing a frequency shift about 10 per
cent larger than cycle 23 for the same change in activity as determined by the
10.7 cm radio flux. This may support the idea that there have been changes in
the magnetic properties of the shallow subsurface layers of the Sun that have
the strongest influence on the frequency shifts.Comment: 6 pages, 3 figures, accepted by MNRAS 3rd July 201
Suppression of Dynamically Induced Stochastic Magnetic Behaviour through Materials Engineering
tochastic behavior fundamentally limits the performance and reliability of nanomagnetic devices. Typically, stochastic behavior is assumed to be the result of simple thermal activation, but it may also be “dynamically induced,” i.e., a direct result of the spatial and temporal complexity of magnetization dynamics. Here, we show how materials engineering can be used to comprehensively suppress dynamically induced stochasticity. Using the dynamics of magnetic domain walls in Ni80Fe20 nanowires as a case study, we show how manipulation of the Gilbert damping constant via doping with the rare-earth-element terbium dramatically simplifies domain-wall dynamics. This allows us to obtain quasi-deterministic behaviors from systems that nominally exhibit exceptionally high levels of stochasticity
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