Axial dipole moments of solar active regions in cycles 21-24

The axial dipole moments of emerging active regions control the evolution of the axial dipole moment of the whole photospheric magnetic field and the strength of polar fields. Hale's and Joy's laws of polarity and tilt orientation affect the sign of the axial dipole moment of an active region, determining the normal sign for each solar cycle. If both laws are valid (or both violated), the sign of the axial moment is normal. However, for some active regions, only one of the two laws is violated, and the signs of these axial dipole moments are the opposite of normal. The opposite-sign axial dipole moments can potentially have a significant effect on the evolution of the photospheric magnetic field, including the polar fields.We determine the axial dipole moments of active regions identified from magnetographic observations and study how the axial dipole moments of normal and opposite signs are distributed in time and latitude in solar cycles 21-24.We use active regions identified from the synoptic maps of the photospheric magnetic field measured at the National Solar Observatory (NSO) Kitt Peak (KP) observatory, the Synoptic Optical Long term Investigations of the Sun (SOLIS) vector spectromagnetograph (VSM), and the Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO).We find that, typically, some 30% of active regions have opposite-sign axial dipole moments in every cycle, often making more than 20% of the total axial dipole moment. Most opposite-signed moments are small, but occasional large moments, which can affect the evolution of polar fields on their own, are observed. Active regions with such a large opposite-sign moment may include only a moderate amount of total magnetic flux. We find that in cycles 21-23 the northern hemisphere activates first and shows emergence of magnetic flux over a wider latitude range, while the southern hemisphere activates later, and emergence is concentrated to lower latitudes. We also note that cycle 24 differs from cycles 21-23 in many ways. Cycle 24 is the only cycle where the northern butterfly wing includes more active regions than the southern wing, and where axial dipole moment of normal sign emerges on average later than opposite-signed axial dipole moment. The total axial dipole moment and even the average axial moment of active regions is smaller in cycle 24 than in previous cycles.

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Reconstructing solar magnetic fields from historical observations

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936134 ◽

2019 ◽

Vol 632 ◽

pp. A39 ◽

Cited By ~ 2

Author(s):

I. O. I. Virtanen ◽

I. I. Virtanen ◽

A. A. Pevtsov ◽

K. Mursula

Keyword(s):

Magnetic Field ◽

Dipole Moment ◽

Magnetic Flux ◽

Opposite Sign ◽

Dipole Moments ◽

Active Regions ◽

Photospheric Magnetic Field ◽

Axial Dipole ◽

Axial Moment ◽

Cycle 24

Context. The axial dipole moments of emerging active regions control the evolution of the axial dipole moment of the whole photospheric magnetic field and the strength of polar fields. Hale’s and Joy’s laws of polarity and tilt orientation affect the sign of the axial dipole moment of an active region. If both laws are valid (or both violated), the sign of the axial moment is normal. However, for some active regions, only one of the two laws is violated, and the signs of these axial dipole moments are the opposite of normal. Those opposite-sign active regions can have a significant effect, for example, on the development of polar fields. Aims. Our aim is to determine the axial dipole moments of active regions identified from magnetographic observations and study how the axial dipole moments of normal and opposite signs are distributed in time and latitude in solar cycles 21−24. Methods. We identified active regions in the synoptic maps of the photospheric magnetic field measured at the National Solar Observatory (NSO) Kitt Peak (KP) observatory, the Synoptic Optical Long term Investigations of the Sun (SOLIS) vector spectromagnetograph (VSM), and the Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO), and determined their axial dipole moments. Results. We find that, typically, some 30% of active regions have opposite-sign axial moments in every cycle, often making more than 20% of the total axial dipole moment. Most opposite-signed moments are small, but occasional large moments, which can affect the evolution of polar fields on their own, are observed. Active regions with such a large opposite-sign moment may include only a moderate amount of total magnetic flux. We find that in cycles 21−23 the northern hemisphere activates first and shows emergence of magnetic flux over a wider latitude range, while the southern hemisphere activates later, and emergence is concentrated to lower latitudes. Cycle 24 differs from cycles 21−23 in many ways. Cycle 24 is the only cycle where the northern butterfly wing includes more active regions than the southern wing, and where axial dipole moment of normal sign emerges on average later than opposite-signed axial dipole moment. The total axial dipole moment and even the average axial moment of active regions is smaller in cycle 24 than in previous cycles.

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Two-dipole model of the asymmetric Sun

Journal of Space Weather and Space Climate ◽

10.1051/swsc/2020041 ◽

2020 ◽

Vol 10 ◽

pp. 40

Author(s):

Bertalan Zieger ◽

Kalevi Mursula

Keyword(s):

Magnetic Field ◽

Large Scale ◽

Dipole Moments ◽

Active Regions ◽

Opposite Polarity ◽

Heliospheric Current Sheet ◽

Dipole Model ◽

Photospheric Magnetic Field ◽

Photospheric Field ◽

Even Harmonics

The large-scale photospheric magnetic field is commonly thought to be mainly dipolar during sunspot minima, when magnetic fields of opposite polarity cover the solar poles. However, recent studies show that the octupole harmonics contribute comparably to the spatial power of the photospheric field at these times. Also, the even harmonics are non-zero, indicating that the Sun is hemispherically asymmetric with systematically stronger fields in the south during solar minima. We present here an analytical model of two eccentric axial dipoles of different strength, which is physically motivated by the dipole moments produced by decaying active regions. With only four parameters, this model closely reproduces the observed large-scale photospheric field and all significant coefficients of its spherical harmonics expansion, including the even harmonics responsible for the solar hemispheric asymmetry. This two-dipole model of the photospheric magnetic field also explains the southward shift of the heliospheric current sheet observed during recent solar minima.

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Reconstructing solar magnetic fields from historical observations

Astronomy and Astrophysics ◽

10.1051/0004-6361/201732323 ◽

2018 ◽

Vol 616 ◽

pp. A134 ◽

Cited By ~ 2

Author(s):

I. O. I. Virtanen ◽

I. I. Virtanen ◽

A. A. Pevtsov ◽

K. Mursula

Keyword(s):

Magnetic Field ◽

Magnetic Flux ◽

Southern Hemisphere ◽

Northern Hemisphere ◽

Active Regions ◽

Surface Flux ◽

Magnetic Activity ◽

Maunder Minimum ◽

Photospheric Magnetic Field ◽

Sunspot Activity

Aims. Sunspot activity is often hemispherically asymmetric, and during the Maunder minimum, activity was almost completely limited to one hemisphere. In this work, we use surface flux simulation to study how magnetic activity limited only to the southern hemisphere affects the long-term evolution of the photospheric magnetic field in both hemispheres. The key question is whether sunspot activity in one hemisphere is enough to reverse the polarity of polar fields in both hemispheres. Methods. We simulated the evolution of the photospheric magnetic field from 1978 to 2016 using the observed active regions of the southern hemisphere as input. We studied the flow of magnetic flux across the equator and its subsequent motion towards the northern pole. We also tested how the simulated magnetic field is changed when the activity of the southern hemisphere is reduced. Results. We find that activity in the southern hemisphere is enough to reverse the polarity of polar fields in both hemispheres by the cross-equatorial transport of magnetic flux. About 1% of the flux emerging in the southern hemisphere is transported across the equator, but only 0.1%–0.2% reaches high latitudes to reverse and regenerate a weak polar field in the northern hemisphere. The polarity reversals in the northern hemisphere are delayed compared to the southern hemisphere, leading to a quadrupole Sun lasting for several years.

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Emergence of Twisted Magnetic Flux Related Sigmoidal Brightening

International Astronomical Union Colloquium ◽

10.1017/s0252921100064630 ◽

2000 ◽

Vol 179 ◽

pp. 263-264

Author(s):

K. Sundara Raman ◽

K. B. Ramesh ◽

R. Selvendran ◽

P. S. M. Aleem ◽

K. M. Hiremath

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Magnetic Flux ◽

Ultra Violet ◽

Active Regions ◽

Morphological Properties ◽

Energy Storage And Conversion ◽

The Sun ◽

X Rays ◽

The Magnetic Field

Extended AbstractWe have examined the morphological properties of a sigmoid associated with an SXR (soft X-ray) flare. The sigmoid is cospatial with the EUV (extreme ultra violet) images and in the optical part lies along an S-shaped Hαfilament. The photoheliogram shows flux emergence within an existingδtype sunspot which has caused the rotation of the umbrae giving rise to the sigmoidal brightening.It is now widely accepted that flares derive their energy from the magnetic fields of the active regions and coronal levels are considered to be the flare sites. But still a satisfactory understanding of the flare processes has not been achieved because of the difficulties encountered to predict and estimate the probability of flare eruptions. The convection flows and vortices below the photosphere transport and concentrate magnetic field, which subsequently appear as active regions in the photosphere (Rust & Kumar 1994 and the references therein). Successive emergence of magnetic flux, twist the field, creating flare productive magnetic shear and has been studied by many authors (Sundara Ramanet al.1998 and the references therein). Hence, it is considered that the flare is powered by the energy stored in the twisted magnetic flux tubes (Kurokawa 1996 and the references therein). Rust & Kumar (1996) named the S-shaped bright coronal loops that appear in soft X-rays as ‘Sigmoids’ and concluded that this S-shaped distortion is due to the twist developed in the magnetic field lines. These transient sigmoidal features tell a great deal about unstable coronal magnetic fields, as these regions are more likely to be eruptive (Canfieldet al.1999). As the magnetic fields of the active regions are deep rooted in the Sun, the twist developed in the subphotospheric flux tube penetrates the photosphere and extends in to the corona. Thus, it is essentially favourable for the subphotospheric twist to unwind the twist and transmit it through the photosphere to the corona. Therefore, it becomes essential to make complete observational descriptions of a flare from the magnetic field changes that are taking place in different atmospheric levels of the Sun, to pin down the energy storage and conversion process that trigger the flare phenomena.

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Abrupt Changes of the Photospheric Magnetic Field in Active Regions and the Impulsive Phase of Solar Flares (Preprint)

10.21236/ada566133 ◽

2012 ◽

Author(s):

E. W. Cliver ◽

G. J. Petrie ◽

A. G. Ling

Keyword(s):

Magnetic Field ◽

Solar Flares ◽

Active Regions ◽

Impulsive Phase ◽

Photospheric Magnetic Field ◽

Abrupt Changes

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Source-region characteristics of anemone active regions in the ascending phase of solar cycle 24

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038927 ◽

2020 ◽

Vol 642 ◽

pp. A233

Author(s):

R. Sharma ◽

C. Cid

Keyword(s):

Magnetic Field ◽

Solar Cycle ◽

Minimum Distance ◽

Source Region ◽

Coronal Holes ◽

Active Regions ◽

Yearly Average ◽

Solar Cycle 24 ◽

Cycle 24 ◽

Rise Phase

Context. Active regions in close proximity to coronal holes, also known as anemone regions, are the best candidates for studying the interaction between closed and open magnetic field topologies at the Sun. Statistical investigation of their source-region characteristics can provide vital clues regarding their possible association with energetic events, relevant from space weather perspectives. Aims. The main goal of our study is to understand the distinct properties of flaring and non-flaring anemone active regions and their host coronal holes, by examining spatial and magnetic field distributions during the rise phase of the solar cycle, in the years 2011–2014. Methods. Anemone regions were identified from the minimum-distance threshold, estimated using the data available in the online catalogs for on-disk active regions and coronal holes. Along with the source-region area and magnetic field characteristics, associated filament and flare cases were also located. Regions with and without flare events were further selected for a detailed statistical examination to understand the major properties of the energetic events, both eruptive and confined, at the anemone-type active regions. Results. Identified anemone regions showed weak asymmetry in their spatial distribution over the solar disk, with yearly average independent from mean sunspot number trend, during the rise phase of solar cycle 24. With the progression in solar cycle, the area and minimum-distance parameters indicated a decreasing trend in their magnitudes, while the magnetic field characteristics indicated an increase in their estimated magnitudes. More than half of the regions in our database had an association with a filament structure, and nearly a third were linked with a magnetic reconnection (flare) event. Anemone regions with and without flares had clear distinctions in their source-region characteristics evident from the distribution of their properties and density analysis. The key differences included larger area and magnetic field magnitudes for flaring anemone regions, along with smaller distances between the centers of the active region and its host coronal hole.

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Reconstructing solar magnetic fields from historical observations

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935606 ◽

2019 ◽

Vol 627 ◽

pp. A11

Author(s):

I. O. I. Virtanen ◽

I. I. Virtanen ◽

A. A. Pevtsov ◽

L. Bertello ◽

A. Yeates ◽

...

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Large Scale ◽

Active Regions ◽

Long Term Evolution ◽

Photospheric Magnetic Field ◽

Solar Magnetic Fields ◽

Term Evolution ◽

Large Scale Field

Aims. The evolution of the photospheric magnetic field has only been regularly observed since the 1970s. The absence of earlier observations severely limits our ability to understand the long-term evolution of solar magnetic fields, especially the polar fields that are important drivers of space weather. Here, we test the possibility to reconstruct the large-scale solar magnetic fields from Ca II K line observations and sunspot magnetic field observations, and to create synoptic maps of the photospheric magnetic field for times before modern-time magnetographic observations. Methods. We reconstructed active regions from Ca II K line synoptic maps and assigned them magnetic polarities using sunspot magnetic field observations. We used the reconstructed active regions as input in a surface flux transport simulation to produce synoptic maps of the photospheric magnetic field. We compared the simulated field with the observed field in 1975−1985 in order to test and validate our method. Results. The reconstruction very accurately reproduces the long-term evolution of the large-scale field, including the poleward flux surges and the strength of polar fields. The reconstruction has slightly less emerging flux because a few weak active regions are missing, but it includes the large active regions that are the most important for the large-scale evolution of the field. Although our reconstruction method is very robust, individual reconstructed active regions may be slightly inaccurate in terms of area, total flux, or polarity, which leads to some uncertainty in the simulation. However, due to the randomness of these inaccuracies and the lack of long-term memory in the simulation, these problems do not significantly affect the long-term evolution of the large-scale field.

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Predicting cycle 24 using various dynamo-based tools

Annales Geophysicae ◽

10.5194/angeo-26-259-2008 ◽

2008 ◽

Vol 26 (2) ◽

pp. 259-267 ◽

Cited By ~ 6

Author(s):

M. Dikpati

Keyword(s):

Solar Cycle ◽

Magnetic Flux ◽

Active Regions ◽

Flux Transport ◽

Delayed Onset ◽

Drift Speed ◽

Sensitivity Tests ◽

The Mean ◽

Cycle 24

Abstract. Various dynamo-based techniques have been used to predict the mean solar cycle features, namely the amplitude and the timings of onset and peak. All methods use information from previous cycles, including particularly polar fields, drift-speed of the sunspot zone to the equator, and remnant magnetic flux from the decay of active regions. Polar fields predict a low cycle 24, while spot zone migration and remnant flux both lead to predictions of a high cycle 24. These methods both predict delayed onset for cycle 24. We will describe how each of these methods relates to dynamo processes. We will present the latest results from our flux-transport dynamo, including some sensitivity tests and how our model relates to polar fields and spot zone drift methods.

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Steady states of non-axial dipolar rods driven by rotating fields

Soft Matter ◽

10.1039/c9sm01671f ◽

2020 ◽

Vol 16 (5) ◽

pp. 1201-1210

Author(s):

Jorge L. C. Domingos ◽

Everton A. de Freitas ◽

W. P. Ferreira

Keyword(s):

Magnetic Field ◽

Dipole Moment ◽

External Magnetic Field ◽

Steady States ◽

Dimensional System ◽

Two Dimensional ◽

Axial Dipole ◽

Two Dimensional System ◽

Magnetic Colloids

We investigate a two-dimensional system of magnetic colloids with anisotropic geometry (rods) and non-axial dipole moment subjected to an oscillating external magnetic field.

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