High-latitude Magnetic Field Research Articles

Evolution of the Sun’s activity and the poleward transport of remnant magnetic flux in Cycles 21–24

ABSTRACT Detailed study of the solar magnetic field is crucial to understand its generation, transport, and reversals. The timing of the reversals may have implications on space weather and thus identification of the temporal behaviour of the critical surges that lead to the polar field reversals is important. We analyse the evolution of solar activity and magnetic flux transport in Cycles 21–24. We identify critical surges of remnant flux that reach the Sun’s poles and lead to the polar field reversals. We reexamine the polar field build-up and reversals in their causal relation to the Sun’s low-latitude activity. We further identify the major remnant flux surges and their sources in the time-latitude aspect. We find that special characteristics of individual 11-yr cycles are generally determined by the spatiotemporal organization of emergent magnetic flux and its unusual properties. We find a complicated restructuring of high-latitude magnetic fields in Cycle 21. The global rearrangements of solar magnetic fields were caused by surges of trailing and leading polarities that occurred near the activity maximum. The decay of non-Joy and anti-Hale active regions resulted in the remnant flux surges that disturbed the usual order in magnetic flux transport. We finally show that the leading-polarity surges during cycle minima sometimes link the following cycle and a collective effect of these surges may lead to secular changes in the solar activity. The magnetic field from a Babcock–Leighton dynamo model generally agrees with these observations.

ПРОЯВЛЕНИЯ В ГЕЛИОСФЕРЕ И В ИНТЕНСИВНОСТИ ГКЛ ДВУХ ВЕТВЕЙ СОЛНЕЧНОЙ АКТИВНОСТИ

This paper provides insight into heliospheric processes and galactic cosmic ray (GCR) modulation occurring due to the presence of two branches of solar activity in this solar layer. According to the topology of solar magnetic fields, these branches are called toroidal (active regions, sunspots, flares, coronal mass ejections, etc.) and poloidal (high-latitude magnetic fields, polar coronal holes, zonal unipolar magnetic regions, etc.). The main cause of different manifestations of the two branches on the solar surface and in the heliosphere — the layer at the base of the heliosphere in which the main energetic factor is the magnetic field — is formulated. In this case, the magnetic fields of the poloidal branch, which have a larger scale but a lower intensity, gain an advantage in penetrating into the heliosphere. A connection is shown between the poloidal branch and the heliospheric characteristics (solar wind velocity field, size of the heliosphere, form of the heliospheric current sheet, regular heliospheric magnetic field and its fluctuations) that, according to modern notions, determine GCR propagation in the heliosphere.

MANIFESTATIONS OF TWO BRANCHES OF SOLAR ACTIVITY IN THE HELIOSPHERE AND GCR INTENSITY

This paper provides insight into heliospheric processes and galactic cosmic ray (GCR) modulation occurring due to the presence of two branches of solar activity in this solar layer. According to the topology of solar magnetic fields, these branches are called toroidal (active regions, sunspots, flares, coronal mass ejections, etc.) and poloidal (high-latitude magnetic fields, polar coronal holes, zonal unipolar magnetic regions, etc.). The main cause of different manifestations of the two branches on the solar surface and in the heliosphere — the layer at the base of the heliosphere in which the main energetic factor is the magnetic field — is formulated. In this case, the magnetic fields of the poloidal branch, which have a larger scale but a lower intensity, gain an advantage in penetrating into the heliosphere. A connection is shown between the poloidal branch and the heliospheric characteristics (solar wind velocity field, size of the heliosphere, form of the heliospheric current sheet, regular heliospheric magnetic field and its fluctuations) that, according to modern notions, determine GCR propagation in the heliosphere.

High-Resolution Vector Magnetograms of the Sun’s Poles from Hinode: Flux Distributions and Global Coronal Modeling

The Sun’s polar fields play a leading role in structuring the large-scale solar atmosphere and in determining the interplanetary magnetic field. They are also believed to supply the seed field for the subsequent solar activity cycle. However, present-day synoptic observations do not have sufficient spatial resolution or sensitivity to diagnose accurately the high-latitude magnetic vector field. The high spatial resolution and sensitivity of the full-Stokes observations from the Hinode Solar Optical Telescope Spectro-Polarimeter, observing the poles long-term, allows us to build up a detailed picture of the Cycle 24 polar field reversal, including the changing latitude distribution of the high-latitude flux, and to study the effect on global coronal field models. The Hinode observations provide detailed information on the dominant facular-scale magnetic structure of the polar fields, and their field inclination and flux distribution. Hybrid synoptic magnetograms are constructed from Hinode polar measurements and full-disk magnetograms from the Synoptic Optical Long-term Investigations of the Sun (SOLIS) Vector Spectro-Magnetograph (VSM), and coronal potential field models are calculated. Loss of effective spatial resolution at the highest latitudes presents complications. Possible improvements to synoptic polar data are discussed.

GRAND MINIMA AND EQUATORWARD PROPAGATION IN A CYCLING STELLAR CONVECTIVE DYNAMO

The 3-D magnetohydrodynamic (MHD) Anelastic Spherical Harmonic (ASH) code, using slope-limited diffusion, is employed to capture convective and dynamo processes achieved in a global-scale stellar convection simulation for a model solar-mass star rotating at three times the solar rate. The dynamo generated magnetic fields possesses many time scales, with a prominent polarity cycle occurring roughly every 6.2 years. The magnetic field forms large-scale toroidal wreaths, whose formation is tied to the low Rossby number of the convection in this simulation. The polarity reversals are linked to the weakened differential rotation and a resistive collapse of the large-scale magnetic field. An equatorial migration of the magnetic field is seen, which is due to the strong modulation of the differential rotation rather than a dynamo wave. A poleward migration of magnetic flux from the equator eventually leads to the reversal of the polarity of the high-latitude magnetic field. This simulation also enters an interval with reduced magnetic energy at low latitudes lasting roughly 16 years (about 2.5 polarity cycles), during which the polarity cycles are disrupted and after which the dynamo recovers its regular polarity cycles. An analysis of this grand minimum reveals that it likely arises through the interplay of symmetric and antisymmetric dynamo families. This intermittent dynamo state potentially results from the simulations relatively low magnetic Prandtl number. A mean-field-based analysis of this dynamo simulation demonstrates that it is of the $\alpha$-$\Omega$ type. The time scales that appear to be relevant to the magnetic polarity reversal are also identified.

Direct observation of closed magnetic flux trapped in the high-latitude magnetosphere.

The structure of Earth's magnetosphere is poorly understood when the interplanetary magnetic field is northward. Under this condition, uncharacteristically energetic plasma is observed in the magnetotail lobes, which is not expected in the textbook model of the magnetosphere. Using satellite observations, we show that these lobe plasma signatures occur on high-latitude magnetic field lines that have been closed by the fundamental plasma process of magnetic reconnection. Previously, it has been suggested that closed flux can become trapped in the lobe and that this plasma-trapping process could explain another poorly understood phenomenon: the presence of auroras at extremely high latitudes, called transpolar arcs. Observations of the aurora at the same time as the lobe plasma signatures reveal the presence of a transpolar arc. The excellent correspondence between the transpolar arc and the trapped closed flux at high altitudes provides very strong evidence of the trapping mechanism as the cause of transpolar arcs.

Reversals of the Sun’s Polar Magnetic Fields in Relation to Activity Complexes and Coronal Holes

A spatiotemporal analysis of long-term measurements of the Sun’s magnetic field was carried out to study changes in its zonal structure and reversals of the polar fields in Cycles 21 – 24. A causal relationship between activity complexes, their remnant magnetic fields, and high-latitude magnetic fields has been demonstrated in the current cycle. The appearance of unipolar magnetic regions near the poles is largely determined by the decay of long-lived activity complexes. The nonuniform distribution of sunspot activity and its north–south asymmetry result in the asymmetry of remnant fields that are transported poleward due to meridional circulation. The asymmetry of high-latitude magnetic fields leads to an asynchrony of polar-field reversals in both hemispheres. The interaction of high-latitude unipolar magnetic regions with the polar fields affects the embedded coronal holes. The evolution of large-scale magnetic fields was also studied in a time–latitude aspect. It is shown that regular reversals of the Sun’s polar fields resulted from cyclic changes in high-latitude magnetic fields. A triple polarity reversal of the polar fields in Cycle 21 and short-term polarity alternations at the poles were interpreted taking into account the interaction of the remnant fields with the Sun’s polar fields.

High-latitude magnetic fields and their time derivatives: interhemispheric similarities

There are two simple and objective measures of geomagnetic activity: the daily range of field components, and the daily maximum of the time derivative of the field. We study these using data from the geomagnetic latitude range of 50–85 deg, covering both the northern and southern hemispheres in 2003–2005. These activity indicators reach their maximum around the magnetic latitude of 70 deg. When comparing northern and southern sites of approximately equal geomagnetic latitudes, there are, in general, no striking interhemispheric differences.

Large-Scale Magnetic Field of the Sun and Evolution of Sunspot Activity

The evolution of the large-scale magnetic field of the Sun has been studied using an algorithm of tomographic inversion. By analyzing line-of-sight magnetograms, we mapped the radial and toroidal components of the Sun’s large-scale magnetic field. The evolution of the radial and toroidal magnetic field components in the 11-year solar cycle has been studied in a time–latitude aspect. It is shown that the toroidal magnetic field of the Sun is causally related to sunspot activity; i.e., the sunspot formation zones drift in latitude and follow the toroidal magnetic fields. The results of our analysis support the idea that the high-latitude toroidal magnetic fields can serve as precursors of sunspot activity. The toroidal fields in the current cycle are anomalously weak and also show a barely noticeable equatorward drift. This behavior of the toroidal magnetic field suggests low activity levels in the current cycle and in the foreseeable future.

On the formation of spot and magnetic cycles in the galactic cosmic ray intensity in the minima of solar activity

The problems of the determination and separation of variations in the galactic cosmic ray intensity, caused by the spot-forming activity and polarity of high-latitude solar magnetic fields, are discussed. Based on the solution of the boundary-value problem, some features in the intensity formation at solar cycle minima are formulated.

The rotation of the plasmapause-like boundary at high latitudes in Saturn's magnetosphere and its relation to the eccentric rotation of the northern and southern auroral ovals

[1] Here we present a study of the rotation of the plasmapause-like density boundary discovered by the Cassini spacecraft at high latitudes in the Saturnian magnetosphere, and compare the results with previously published studies of high-latitude magnetic field perturbations and the eccentric rotation of the auroral ovals. Near the planet the density boundary is located at dipole L values ranging from about 8 to 15, and separates a region of very low densities at high latitudes from a region of higher densities at lower latitudes. We show that the density boundary rotates at different rates in the northern and southern hemispheres, and that the periods are the same as the modulation periods of Saturn kilometric radiation in those hemispheres. We also show that the phase of rotation in a given hemisphere is closely correlated with the phase of the high-latitude magnetic field perturbations observed by Cassini in that hemisphere, and also with the phase of the eccentric rotation of the auroral oval observed by the Hubble Space Telescope.

Magnetospheric period oscillations at Saturn: Comparison of equatorial and high-latitude magnetic field periods with north and south Saturn kilometric radiation periods

Magnetospheric period oscillations at Saturn: Comparison of equatorial and high-latitude magnetic field periods with north and south Saturn kilometric radiation periods

POLAR investigation of the Sun—POLARIS

The POLAR Investigation of the Sun (POLARIS) mission uses a combination of a gravity assist and solar sail propulsion to place a spacecraft in a 0.48 AU circular orbit around the Sun with an inclination of 75° with respect to solar equator. This challenging orbit is made possible by the challenging development of solar sail propulsion. This first extended view of the high-latitude regions of the Sun will enable crucial observations not possible from the ecliptic viewpoint or from Solar Orbiter. While Solar Orbiter would give the first glimpse of the high latitude magnetic field and flows to probe the solar dynamo, it does not have sufficient viewing of the polar regions to achieve POLARIS’s primary objective: determining the relation between the magnetism and dynamics of the Sun’s polar regions and the solar cycle.

22-year variations of the solar rotation and solar activity cycles

We have performed a comparative analysis of the results of our study of the 22-year rotation variations obtained from data on large-scale magnetic fields in the Hα line, magnetographic observations, and spectral-corona observations. All these types of data suggest that the rotation rate at low latitudes slows down at an epoch close to the maximum of odd activity cycles. The 22-year waves of rotation-rate deviation from the mean values drift from high latitudes toward the equator in a time comparable to the magnetic-cycle duration. We discuss the possibility of the generation of a solar magnetic cycle by the interaction of 22-year torsional oscillations with the slowly changing or relic magnetic field. We consider the generation mechanisms of the high-latitude magnetic field through a superposition of the magnetic fields produced by the decay and dissipation of bipolar groups and the relic or slowly changing magnetic field and a superposition of the activity wave from the next activity cycle at high latitudes.

Initial studies of high latitude magnetic field data during different magnetospheric conditions

Abstract We investigate the statistical properties of high-latitude magnetometer data for differing geomagnetic activity. This is achieved by characterizing changes in the nonlinear statistics of the geomagnetic field, by means of the Hurst exponent, measured from a single ground-based magnetometer station. The long-range statistical nature of the geomagnetic field at a local observation site can be described as a multifractional Brownian motion, thus suggesting the statistical structure required of mathematical models of magnetospheric activity. We also find that, in general, the average Hurst exponent for quiet magnetospheric intervals is smaller than that for more active intervals.

Mapping of Poynting flux and widths of inertial Alfvén wave structures in high-latitude plasmas

[1] Downward propagating inertial Alfven waves (IAWs), transporting electromagnetic energy along high-latitude Earth's magnetic field lines, are a well-established phenomenon in the magnetosphere. Such waves have been detected now from Freja, FAST, and Polar satellites, showing that at the altitudes of the former two satellites, the latitudinal widths of the IAW structures are a few electron-skin depths. Since IAWs have a nonzero group velocity component Vg⊥ perpendicular to the Earth's magnetic field B(r), the latitudinal fanning of the finite-width wave must impact the scaling of the wave Poynting flux S with the geocentric altitude r. We estimate the effect of the fanning on the scaling of S on B(r), showing that commonly assumed scaling S ∼ B might not be true for the narrow IAW structures. For the ultra-low-frequency (ULF) waves having frequencies in the range of a few mHz, it turns out that the width of such IAWs becomes nearly constant in altitude and S ∼ B1/2 due to the convergence of magnetic field in longitude only. Like IAWs, even the kinetic Alfven waves are expected to show a similar scaling.

Cusp confinement zones on quiet and disturbed dayside magnetosphere

The possibility of the charged particle confinement in the dayside regions of magnetosphere cusps is investigated. The analysis, on the basis of Tsyganenko geomagnetic field model, consisted of a numerical simulation of the single charged particle trajectories passing through the regions of the high latitude magnetic field minima. It is shown that the magnetic field topology with off-equatorial field minima itself does not guarantee a particle trap in cusps. In determined conditions relatively stable particle traps exist at different levels of magnetic disturbance, and the topology strongly depends on the seasonal influence. The particles captured in the zones form a kind of cusp radiation belt where particles drift with periods of several minutes conserving the 1st and 2nd adiabatic invariants. This capture and other features of the cusp trapped radiation are discussed.

What Is Missing from Our Understanding of Long‐Term Solar and Heliospheric Activity?

The heliospheric magnetic field is associated with changes in space weather, cosmic-ray flux, and likely climate. This field is determined by the largest scale patterns of magnetism at the solar surface, dominated by the lower latitude active regions during cycle maximum and by the circumpolar fields during cycle minimum. Whereas the magnetic field in the activity belt is readily studied, the high-latitude field is much less accessible, and its study requires a combination of modeling and observation. Current models hold that the high-latitude magnetic field on the Sun is determined solely by the accumulation of field transported poleward from lower latitude active regions. We test this hypothesis by simulating the evolution of the magnetic field at the solar surface and in the heliosphere during the last 340 yr using a state-of-the-art model that incorporates all processes that are known to contribute significantly to the evolution of the large-scale patterns in the solar field. We find that if only the emergence frequency of magnetic bipoles is varied in accordance with observed sunspot records, the polar-cap field reservoir does not match measurements during past years. Based on comparisons of our simulations with observed polar fluxes over the last few decades and with the proxy for the heliospheric flux formed by 340 yr of 10Be ice-core data, we suggest that the high-latitude field may be subject to decay on a timescale of 5-10 yr. We discuss the consequences of this finding for our understanding of the Sun-Earth connection and explore inferences for the coupling of the Sun's internal magnetic field to the heliospheric field.

Noon ionospheric signatures of a sudden commencement following a solar wind pressure pulse

Abstract. Using experimental data from the Cutlass Super-DARN HF radars and from a subset of ground magnetometers of the IMAGE Scandinavian chain, the response of the ionosphere in the noon sector to a solar wind pressure increase is studied. The emphasis is on the signature of the convection vortices and of the Hall currents that are associated with the pair of opposite parallel currents flowing along the morning and afternoon high-latitude magnetic field lines. We show that the sudden commencement is characterised by an equatorward convection, immediately followed (within less than 3 min) by a strong poleward plasma motion. These results are shown to agree qualitatively with the global model of sudden commencement of Araki (1994).Key words. Ionosphere (plasma convection; electric fields and currents) – Magnetospheric physics (solar wind-magnetosphere interactions)

Spatially Resolved Far Ultraviolet Spectroscopy of the Jovian Aurora

Spatially resolved spectra in four 50-Å FUV spectral windows were obtained across the jovian aurora with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope. Nearly simultaneous ultraviolet imaging makes it possible to correlate the intensity variations along the STIS slit with those observed in the images and to characterize the global auroral context prevailing at the time of the observations. Spectra at ∼1-Å resolution taken in pairs included an unabsorbed window and a spectral region affected by hydrocarbon absorption. Both sets of spectra correspond to an aurora with a main oval brightness of about 130 kilorayleighs of H 2 emission. The far ultraviolet color ratios I(1550–1620 Å)/ I(1230–1300 Å) are 2.3 and 5.9 for the noon and morning sectors of the main oval, respectively. We use an interactive model coupling the energy degradation of incoming energetic electrons, auroral temperature and composition, and synthetic H 2 spectra to fit the intensity distribution of the H 2 lines. It is found that the model best fitting globally the spectra has a soft energy component in addition to a 10 erg cm −2 s −1 flux of 80 keV electrons. It provides an effective H 2 temperature of 540 K. The relative intensity of temperature-sensitive H 2 lines indicates differences between the auroral main oval and polar cap emissions. The amount of methane absorption across the polar region is shown to vary in a way consistent with temperature. For the second spectral pair, the polar cap shows a higher attenuation by CH 4, indicating a harder precipitation along high-latitude magnetic field lines.