Solar and stellar activity: diagnostics and indices

AbstractWe summarize the fifty-year concerted effort to place the “activity” of the Sun in the context of the stars. As a working definition of solar activity in the context of stars, we adopt thoseglobally–observablevariations on time scales below thermal time scales, of ~105yr for the convection zone. So defined, activity is dominated by magnetic–field evolution, including the 22–year Hale cycle, the typical time it takes for the quasi-periodic reversal in which the global magnetic–field takes place. This is accompanied by sunspot variations with 11 year periods, known since the time of Schwabe, as well as faster variations due to rotation of active regions and flaring. “Diagnostics and indices” are terms given to theindirectsignatures of varying magnetic–fields, including the photometric (broad-band) variations associated with the sunspot cycle, and variations of the accompanying heated plasma in higher layers of stellar atmospheres seen at special optical wavelengths, and UV and X-ray wavelengths. Our attention is also focussed on the theme of the Symposium by examining evidence for deep and extended minima of stars, and placing the 70–year long solar Maunder Minimum into a stellar context.

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Magnetic Field Topology in Solar Active Regions

International Astronomical Union Colloquium ◽

10.1017/s0252921100029560 ◽

1993 ◽

Vol 141 ◽

pp. 435-438

Author(s):

N. Seehafer

Keyword(s):

Magnetic Field ◽

Magnetic Energy ◽

Mean Field ◽

Active Regions ◽

Field Configuration ◽

Relaxation Processes ◽

Magnetic Field Topology ◽

Solar Active Regions ◽

Global Magnetic Field ◽

Slow Evolution

AbstractIn solar active regions, over extended periods of time the plasma-magnetic field configuration evolves quasistatically through a sequence of nearly force-free equilibrium states. This evolution may be understood as the continual distortion of an existing equilibrium by wavelike disturbances propagating upward from the photosphere and subsequent fast relaxation to a new, neighbouring equilibrium. In the present paper the build-up of magnetic energy, which is presumably necessary for flares and other explosive events, during a quasistatic evolution is considered. If during the slow evolution the magnetic energy is increased, then the relaxation processes represent inverse cascades of energy. We study the conditions under which such cascades are possible within the framework of mean-field MHD. In contrast to the convection zone, where the dynamo for the global magnetic field of the Sun works, the solar atmosphere is convectively stable and the first order smoothing approximation justified. It turns out then that current helicity (B.∇ × B) is an important quantity decisive for whether magnetic energy can be built up.

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Mapping the global magnetic field in the solar corona through magnetoseismology

10.5194/egusphere-egu21-642 ◽

2021 ◽

Author(s):

Zihao Yang ◽

Christian Bethge ◽

Hui Tian ◽

Steven Tomczyk ◽

Richard Morton ◽

...

Keyword(s):

Magnetic Field ◽

Field Strength ◽

Solar Corona ◽

Phase Speed ◽

Magnetic Coupling ◽

Active Regions ◽

Coronal Magnetic Field ◽

Spectral Lines ◽

Mhd Waves ◽

Global Magnetic Field

<p>Magnetoseismology, a technique of magnetic field diagnostics based on observations of magnetohydrodynamic (MHD) waves, has been widely used to estimate the field strengths of oscillating structures in the solar corona. However, previously magnetoseismology was mostly applied to occasionally occurring oscillation events, providing an estimate of only the average field strength or one-dimensional distribution of field strength along an oscillating structure. This restriction could be eliminated if we apply magnetoseismology to the pervasive propagating transverse MHD waves discovered with the Coronal Multi-channel Polarimeter (CoMP). Using several CoMP observations of the Fe XIII 1074.7 nm and 1079.8 nm spectral lines, we obtained maps of the plasma density and wave phase speed in the corona, which allow us to map both the strength and direction of the coronal magnetic field in the plane of sky. We also examined distributions of the electron density and magnetic field strength, and compared their variations with height in the quiet Sun and active regions. Such measurements could provide critical information to advance our understanding of the Sun's magnetism and the magnetic coupling of the whole solar atmosphere.</p>

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Numerical Simulations of Large-scale Solar Magnetic Fields

Australian Journal of Physics ◽

10.1071/ph850999 ◽

1985 ◽

Vol 38 (6) ◽

pp. 999 ◽

Cited By ~ 24

Author(s):

CR DeVore ◽

NR Sheeley Jr ◽

JP Boris ◽

TR Young Jr ◽

KL Harvey

Keyword(s):

Magnetic Field ◽

Magnetic Flux ◽

Large Scale ◽

Sunspot Cycle ◽

Active Regions ◽

The Sun ◽

Solar Magnetic Fields ◽

Field Patterns ◽

Large Scale Magnetic Field ◽

The Magnetic Field

We have solved numerically a transport equation which describes the evolution of the large-scale magnetic field of the Sun. Data derived from solar magnetic observations are used to initialize the computations and to account for the emergence of new magnetic flux during the sunspot cycle. Our objective is to assess the ability of the model to reproduce the observed evolution of the field patterns. We discuss recent results from simulations of individual active regions over a few solar rotations and of the magnetic field of the Sun over sunspot cycle 21.

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Global Evolution of Photospheric Magnetic Fields

Symposium - International Astronomical Union ◽

10.1017/s0074180900044247 ◽

1990 ◽

Vol 138 ◽

pp. 281-295

Author(s):

V. I. Makarov ◽

K. R. Sivaraman

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Large Scale ◽

Active Regions ◽

Magnetic Activity ◽

Field Pattern ◽

The Sun ◽

Coronal Emission ◽

Global Magnetic Field ◽

Global Modes

The main features concerning the evolution of the large scale photospheric magnetic fields derived from synoptic maps as well as from H-alpha synoptic charts are reviewed. The significance of a variety of observations that indicate the presence of a high latitude component as a counterpart to the sunspot phenomenon at lower latitudes is reviewed. It is argued that these two components describe the global magnetic field on the sun. It is demonstrated that this scenario is able to link many phenomena observed on the sun (coronal emission, ephemeral active regions, geomagnetic activity, torsional oscillations, polar faculae and global modes in the magnetic field pattern) with the global magnetic activity.

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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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Solar Cycle, Maunder Minimum and Pandemic Influenza

Journal of Infectious Diseases & Case Reports ◽

10.47363/jidscr/2021(1)128 ◽

2020 ◽

pp. 1-4

Author(s):

N Chandra Wickramasinghe ◽

◽

Robert Temple ◽

Keyword(s):

Magnetic Field ◽

Influenza Virus ◽

Sunspot Cycle ◽

Maunder Minimum ◽

Moon System ◽

Influenza Pandemics ◽

Dust Clouds ◽

The Earth ◽

Cometary Panspermia ◽

Cosmic Origin

We explore the idea that influenza pandemics may arise from the transference of new virions (new sub-types of the influenza virus) of cosmic origin in general accord with the theory of cometary panspermia. Such a transfer process will be modulated by the sunspot cycle and through its role in affecting the interplanetary magnetic field configurations in the Earth’s vicinity. Transfers of virus could take place directly from comets or indirectly from a transient repository represented for instance by the Kordylewski if dust clouds at the L4 and L5 Lagrange libration points of the Earth-Moon system. In either case an active sun appears to be a perequisite for effective transfers. The long remission of influenza pandemics throughout the period 1645-1715, during the Maunder sunspot minimum, might be understood on the basis of our model.

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Joint Discussion 8 Solar and stellar activity cycles

Proceedings of the International Astronomical Union ◽

10.1017/s174392130701054x ◽

2006 ◽

Vol 2 (14) ◽

pp. 271-272

Author(s):

Alexander G. Kosovichev ◽

Klaus G. Strassmeier

Keyword(s):

Magnetic Field ◽

Time Scales ◽

Tree Ring ◽

Solar Magnetic Field ◽

Sunspot Cycle ◽

Magnetic Cycle ◽

Stellar Activity ◽

Tree Ring Data ◽

Activity Cycles

The solar magnetic field and its associated atmospheric activity exhibits periodic variations on a number of time scales. The 11-year sunspot cycle and its underlying 22-year magnetic cycle are, besides the 5-minute oscillation, the most widely known. Amplitudes and periods range from a few parts per million (ppm) and 2–3 minutes for p-modes in sunspots, a few 10 ppm and 10 minutes for the granulation turn around, a few 100 ppm and weeks for the lifetime of plages and faculae, 1000 ppm and 27 days for the rotational signal from spots, to the long-term cycles of 90 yr (Gleissberg cycle), 200 - 300 yr (Wolf, Spörer, Maunder minima), 2,400 yr from 14C tree-ring data, and possibly in excess of 100,000 yr.

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Accounting for small bipolar magnetic regions in solar irradiance reconstructions

10.5194/egusphere-egu21-8323 ◽

2021 ◽

Author(s):

Bernhard Hofer ◽

Natalie A. Krivova ◽

Sami K. Solanki ◽

Robert Cameron ◽

Chi-Ju Wu ◽

...

Keyword(s):

Time Scales ◽

Solar Irradiance ◽

Climate Models ◽

Transport Model ◽

Active Regions ◽

Surface Flux ◽

Realistic Model ◽

Maunder Minimum ◽

Flux Transport ◽

Reasonable Description

<p>Historical solar irradiance is a critical input to climate models. As no direct measurements are available before 1978, reconstructions of past irradiance changes are employed instead. Such reconstructions are based on the knowledge that solar irradiance on time scales of interest to climate studies is modulated by the evolution of the solar surface magnetic structures, such as sunspots and faculae. This calls for historical records or proxies of such features. The longest direct, and thus mostly used, record is the sunspot number. It allows a reasonable description of the emergence and evolution of active regions, which are larger magnetic regions containing sunspots. At the same time, a significant amount of the magnetic flux on the Sun emerges in the form of the so-called ephemeral magnetic regions, which are weaker short-lived bipolar regions that do not contain sunspots. Due to their high frequency, ephemeral regions are an important source of the irradiance variability, especially on time scales longer than the solar cycle. Difficulties in their proper accounting are a main reason for the high uncertainty in the secular irradiance variability. Existing models either do not account for their evolution at all or link them linearly to active regions. We use a new, more realistic model of the ephemeral region emergence, relying on recent independent solar observations, as input to a surface flux transport model (SFTM) to simulate the evolution of the magnetic field in such regions. The latter can then be used to reconstruct the solar irradiance since the Maunder minimum.</p>

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Observations of Magnetic Field Changes in Active Regions

Symposium - International Astronomical Union ◽

10.1017/s0074180900022877 ◽

1971 ◽

Vol 43 ◽

pp. 422-427 ◽

Cited By ~ 7

Author(s):

K. L. Harvey ◽

W. C. Livingston ◽

J. W. Harvey ◽

C. D. Slaughter

Keyword(s):

Magnetic Field ◽

Time Scales ◽

Active Regions ◽

Time Sequence

A time sequence of longitudinal magnetograms of two active regions, McMath Regions 9281 and 9760, have indicated magnetic field changes occurring in localized areas with time scales of the order of hours. We believe the observed field changes are evolutionary in nature, rather than related to the occurrence of small flares. Three examples of evolutionary magnetic changes are discussed.

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