1,179 research outputs found
The origin of the warped heliospheric current sheet
The warped heliospheric current sheet in early 1976 was calculated from the observed photospheric magnetic field using a potential field method. Comparisons with measurements of the interplanetary magnetic field polarity in early 1976 obtained at several locations in the heliosphere at Helios 1, Helios 2, Pioneer 11 and Earth show a rather detailed agreement between the computed current sheet and the observations. It appears that the large scale structure of the warped heliospheric current sheet is determined by the structure of the photospheric magnetic field, and that "ballerina skirt" effects may add small scale ripples
Structure and evolution of the large scale solar and heliospheric magnetic fields
Structure and evolution of large scale photospheric and coronal magnetic fields in the interval 1976-1983 were studied using observations from the Stanford Solar Observatory and a potential field model. The solar wind in the heliosphere is organized into large regions in which the magnetic field has a componenet either toward or away from the sun. The model predicts the location of the current sheet separating these regions. Near solar minimum, in 1976, the current sheet lay within a few degrees of the solar equator having two extensions north and south of the equator. Soon after minimum the latitudinal extent began to increase. The sheet reached to at least 50 deg from 1978 through 1983. The complex structure near maximum occasionally included multiple current sheets. Large scale structures persist for up to two years during the entire interval. To minimize errors in determining the structure of the heliospheric field particular attention was paid to decreasing the distorting effects of rapid field evolution, finding the optimum source surface radius, determining the correction to the sun's polar field, and handling missing data. The predicted structure agrees with direct interplanetary field measurements taken near the ecliptic and with coronameter and interplanetary scintillation measurements which infer the three dimensional interplanetary magnetic structure. During most of the solar cycle the heliospheric field cannot be adequately described as a dipole
Reversals of the solar magnetic dipole in the light of observational data and simple dynamo models
Observations show that the photospheric solar magnetic dipole usually does
not vanish during the reversal of the solar magnetic field, which occurs in
each solar cycle. In contrast, mean-field solar dynamo models predict that the
dipole field does become zero. In a recent paper Moss et al. (2013) suggested
that this contradiction can be explained as a large-scale manifestation of
small-scale magnetic fluctuations of the surface poloidal field. Here we
compare this interpretation with WSO (Wilcox Solar Observatory) photospheric
magnetic field data in order to determine the amplitude of magnetic
fluctuations required to explain the phenomenon and to compare the results with
predictions from a simple dynamo model which takes these fluctuations into
account. We demonstrate that the WSO data concerning the magnetic dipole
reversals are very similar to the predictions of our very simple solar dynamo
model, which includes both mean magnetic field and fluctuations. The ratio
between the rms value of the magnetic fluctuations and the mean field is
estimated to be about 2, in reasonable agreement with estimates from sunspot
data. The reversal epoch, during which the fluctuating contribution to the
dipole is larger than that from the mean field, is about 4 months. The memory
time of the fluctuations is about 2 months. Observations demonstrate that the
rms of the magnetic fluctuations is strongly modulated by the phase of the
solar cycle. This gives additional support to the concept that the solar
magnetic field is generated by a single dynamo mechanism rather than also by
independent small-scale dynamo action. A suggestion of a weak nonaxsymmetric
magnetic field of a fluctuating nature arises from the analysis, with a
lifetime of about 1 year.Comment: 9 pages, 10 figures, accepted versio
How much more can sunspots tell us about the solar dynamo?
Sunspot observations inspired solar dynamo theory and continue to do so. Simply counting them established the sunspot cycle and its period. Latitudinal distributions introduced the tough constraint that the source of sunspots moves equator-ward as the cycle progresses. Observations of Hale's polarity law mandated hemispheric asymmetry. How much more can sunspots tell us about the solar dynamo? We draw attention to a few outstanding questions raised by inherent sunspot properties. Namely, how to explain sunspot rotation rates, the incoherence of follower spots, the longitudinal spacing of sunspot groups, and brightness trends within a given sunspot cycle. After reviewing the first several topics, we then present new results on the brightness of sunspots in Cycle 24 as observed with the Helioseismic Magnetic Imager (HMI). We compare these results to the sunspot brightness observed in Cycle 23 with the Michelson Doppler Imager (MDI). Next, we compare the minimum intensities of five sunspots simultaneously observed by the Hinode Solar Optical Telescope Spectropolarimeter (SOT-SP) and HMI to verify that the minimum brightness of sunspot umbrae correlates well to the maximum field strength. We then examine 90 and 52 sunspots in the north and south hemisphere, respectively, from 2010 - 2012. Finally, we conclude that the average maximum field strengths of umbra 40 Carrington Rotations into Cycle 24 are 2690 Gauss, virtually indistinguishable from the 2660 Gauss value observed at a similar time in Cycle 23 with MDI
The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: Optimization of the Spectral Line Inversion Code
The Very Fast Inversion of the Stokes Vector (VFISV) is a Milne-Eddington
spectral line inversion code used to determine the magnetic and thermodynamic
parameters of the solar photosphere from observations of the Stokes vector in
the 6173 A Fe I line by the Helioseismic and Magnetic Imager (HMI) onboard the
Solar Dynamics Observatory (SDO). We report on the modifications made to the
original VFISV inversion code in order to optimize its operation within the HMI
data pipeline and provide the smoothest solution in active regions. The changes
either sped up the computation or reduced the frequency with which the
algorithm failed to converge to a satisfactory solution. Additionally, coding
bugs which were detected and fixed in the original VFISV release, are reported
here.Comment: Accepted for publication in Solar Physic
Evidence for a Latitudinal Gradient of the Cosmic Ray Intensity Associated with a Change in the Tilt of the Heliospheric Current Sheet
Since mid-1985, the average flux of >70 MeV/nucleon cosmic rays at Voyager 2 (r_2 ∼ 17 AU, Θ_2 ∼ 0°) has been ∼3-5% greater than that at Voyager 1 (r_1 ∼ 24 AU, Θ_1 ∼ 26°N). This is the first direct observation over such a large radial range in which the galactic cosmic ray flux closer to the sun is higher than the flux farther from the sun for an extended period of time. This observation is consistent with the presence of a negative latitudinal gradient G_Θ=−0.36 ± 0.05 (or −0.60 ± 0.08) %/deg, assuming a coexistent radial gradient G_r of 1 (or 2) %/AU. We suggest that the appearance of this persistent negative latitudinal gradient may be due to the abrupt, large decrease of the heliospheric current sheet tilt to ∼20° in early 1985
The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: SHARPs -- Space-weather HMI Active Region Patches
A new data product from the Helioseismic and Magnetic Imager (HMI) onboard
the Solar Dynamics Observatory (SDO) called Space-weather HMI Active Region
Patches (SHARPs) is now available. SDO/HMI is the first space-based instrument
to map the full-disk photospheric vector magnetic field with high cadence and
continuity. The SHARP data series provide maps in patches that encompass
automatically tracked magnetic concentrations for their entire lifetime; map
quantities include the photospheric vector magnetic field and its uncertainty,
along with Doppler velocity, continuum intensity, and line-of-sight magnetic
field. Furthermore, keywords in the SHARP data series provide several
parameters that concisely characterize the magnetic-field distribution and its
deviation from a potential-field configuration. These indices may be useful for
active-region event forecasting and for identifying regions of interest. The
indices are calculated per patch and are available on a twelve-minute cadence.
Quick-look data are available within approximately three hours of observation;
definitive science products are produced approximately five weeks later. SHARP
data are available at http://jsoc.stanford.edu and maps are available in either
of two different coordinate systems. This article describes the SHARP data
products and presents examples of SHARP data and parameters.Comment: 27 pages, 7 figures. Accepted to Solar Physic
The Interaction of New and Old Magnetic Fluxes at the Beginning of Solar Cycle 23
The 11-year cycle of solar activity follows Hale's law by reversing the
magnetic polarity of leading and following sunspots in bipolar regions during
the minima of activity. In the 1996-97 solar minimum, most solar activity
emerged in narrow longitudinal zones - `active longitudes' but over a range in
latitude. Investigating the distribution of solar magnetic flux, we have found
that the Hale sunspot polarity reversal first occurred in these active zones.
We have estimated the rotation rates of the magnetic flux in the active zones
before and after the polarity reversal. Comparing these rotation rates with the
internal rotation inferred by helioseismology, we suggest that both `old' and
`new' magnetic fluxes were probably generated in a low-latitude zone near the
base of the solar convection zone. The reversal of active region polarity
observed in certain longitudes at the beginning of a new solar cycle suggests
that the phenomenon of active longitudes may give fundamental information about
the mechanism of the solar cycle. The non-random distribution of old-cycle and
new-cycle fluxes presents a challenge for dynamo theories, most of which assume
a uniform longitudinal distribution of solar magnetic fields.Comment: 4 pages, 5 figures; accepted for publication in ApJ Letter
Solar mean magnetic field variability: A wavelet approach to Wilcox Solar Observatory and SOHO/Michelson Doppler Imager observations
Solar mean magnetic field (SMMF) measurements from the Wilcox Solar Observatory and with the SOHO/MDI instrument are described and analyzed. Even though two completely different methods of observation are used, the two data sets obtained show a strong similarity. Using continuous wavelet transforms, SMMF variability is found at a number of temporal scales. Detected SMMF signals with a 1–2 year period are considered to be linked to variations in the internal rotation of the Sun. Intermediate SMMF oscillations with a period of 80–200 days are probably connected to the evolution of large active regions. We also find evidence for 90 min variations with coronal mass ejections as a suggested origin
Solar Magnetic Field Reversals and the Role of Dynamo Families
The variable magnetic field of the solar photosphere exhibits periodic
reversals as a result of dynamo activity occurring within the solar interior.
We decompose the surface field as observed by both the Wilcox Solar Observatory
and the Michelson Doppler Imager into its harmonic constituents, and present
the time evolution of the mode coefficients for the past three sunspot cycles.
The interplay between the various modes is then interpreted from the
perspective of general dynamo theory, where the coupling between the primary
and secondary families of modes is found to correlate with large-scale polarity
reversals for many examples of cyclic dynamos. Mean-field dynamos based on the
solar parameter regime are then used to explore how such couplings may result
in the various long-term trends in the surface magnetic field observed to occur
in the solar case.Comment: Accepted to ApJ; comments/corrections to this article are welcome via
e-mail, even after publicatio
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