Polar Field Reversal Research Articles

Time-dependent electroadhesive force degradation

This paper concerns a comprehensive investigation of time-dependent electroadhesion (EA) force degradation. EA shear force tests on different object materials (a PET, glass, ABS, and wood plate) have shown that force degradation was dominated by residual polarization charges trapped in the EA pad dielectric rather than in the substrate dielectric from which the object to be prehended is made. In order to explain this dynamic physical phenomenon, a model of dielectric polarization and depolarization has been proposed. According to the derived relationship between EA force and discharge time, three different methods intended to mitigate this problem has been compared: (1) the natural discharge method, (2) the high voltage resistor discharge method, and (3) the discharge prior to field polarity reversal method. These methods are useful for generating repeatable and stable EA forces, which are required for the characterization of EA pads and their subsequent employment in material handling, mobile robot crawling and climbing tasks.

Chromospheric plasma ejection above a pore

ABSTRACT We present high spatial resolution observations of short-lived transients, ribbons and jet-like events above a pore in Ca ii H images where fine structure, like umbral dots, light bridges and penumbral microfilaments, is present in the underlying photosphere. We found that current layers are formed at the edges of the convective fine structure, due to the shear between their horizontal field and the ambient vertical field. High vertical electric current density patches are observed in the photosphere around these events, which indicates the formation of a current sheet at the reconnection site. In the framework of past studies, low altitude reconnection could be the mechanism that produces such events. The reconnection is caused by an opposite polarity field produced by the bending of field lines by convective downflows at the edge of pore fine structure.

Generation of the Sun’s Radial Magnetic Field by Global Hydrodynamic Flows

A theory on the emergence of global hydrodynamic flows of matter and the generation of global magnetic fields in the solar convective zone is proposed. The unstable profile of differential rotation plays a crucial role in it: all hydrodynamic flows on the Sun (poloidal circulation, torsion oscillations, and spatiotemporal variations of the poloidal flow) are generated due to the loss of stability of differential rotation. None of the models known to us, wherein torsion oscillations and variations of the meridional circulation are typically calculated separately and are thus considered to be independent flows, reproduce this result. The calculations within our model suggest, on the contrary, that the indicated flows are actually toroidal and poloidal components of a single, 3-dimensional global flow. The decisive role of torsion oscillations in the generation of the radial alternating magnetic field is highlighted in the present study. It is demonstrated by numerical simulation that the time-varying radial magnetic field on the surface of the Sun reaches its maximum at the poles, where it changes polarity with a period of ~22 years. This process may be identified with the observed effect of polarity reversal of the polar field within the Hale magnetic cycle. It is demonstrated that the lines of zero values (polarity reversal) of the surface radial magnetic field pass through the maxima of the velocity modulus of zonal flows (torsion oscillations). In addition, the lines of magnetic polarity reversal of the radial field and the maximum velocities of surface zonal flows drift from the poles to the equator. It is noted that the obtained results on the latitudinal evolution of surface zonal flows correlate with the behavior of deep zonal flows determined by processing helioseismological data.

Time‐Latitude Distribution of Prominences for 10 Solar Cycles: A Study Using Kodaikanal, Meudon, and Kanzelhohe Data

AbstractSolar prominences are structures of importance because of their role in polar field reversal. We study the long‐term variation of the time latitude distribution of solar prominences in this article. To accomplish this, we primarily used the digitized disc‐blocked Ca II K spectroheliograms as recorded from Kodaikanal Solar Observatory for the period of 1906–2002. For improving the data statistics we included full disc H images from Meudon and Kanzelhohe Observatory, which are available after 1980. We developed an automated technique to identify the latitudinal locations of prominences in daily images from all three data sets. Derived time‐latitude distribution clearly depicted poleward migration of prominence structures for 10 cycles (15–24). Unlike previous studies, we separated the rate of poleward migration during onset and near pole, using piece‐wise linear fits. In most cases, we found acceleration in poleward migration with the change occurring near 70° latitudes. The derived migration rates for such large number of solar cycles can provide important inputs toward understanding polar field buildup process.

Lipid Pore Instability in Bipolar Electrically Stressed Membranes

Molecular dynamics simulations of lipid membranes facilitate the understanding of biophysical phenomena that cannot be observed with conventional experimental methods, including stress-induced pore formation in lipid bilayers. Here we report newly discovered nanosecond-scale stabilities and instabilities of the transient pores formed in lipid bilayers by strong transmembrane electric fields, which promote the formation of bilayer-spanning water bridges and conductive lipid pores through which normally impermeant pharmacological and genetic material can enter cells. These pores are transient, lasting less than 100 ns in the absence of an electric field. But in the presence of a sustaining field, which has a magnitude much lower than that required for pore formation, pores can be stabilized so that they last indefinitely. In biomedical and biotechnological applications that use alternating or bipolar pulsed electric fields for membrane permeabilization, it is important to know the effects of the field polarity reversal. For low-frequency alternating fields or long duration pulses, successive positive and negative cycles are additive. But for bipolar pulses in the nanosecond regime, it has been shown experimentally that the effects of a positive pulse can be “cancelled” by a closely following negative pulse. In molecular simulations we see that lipid electropore stability after field reversal is a function both of the magnitude of the applied field and the length of time the field is applied before the field direction is reversed.

Two-dipole model of the asymmetric Sun

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.

Influence of the asymmetry of the solar magnetic field polarity reversal on geomagnetic activity

Influence of the asymmetry of the solar magnetic field polarity reversal on geomagnetic activity

Electromagnetic Burst Generation during Annihilation of Magnetic Field in Relativistic Laser-Plasma Interaction

We present the results of theoretical studies of formation and evolution of the current sheet in a colliosionless plasma during magnetic reconnection in relativistic limit. Relativistic magnetic reconnection is driven by parallel laser pulses interacting with underdense plasma target. Annihilation of laser created magnetic field of opposite polarity generates strong non-stationary electric field formed in between the region with opposite polarity magnetic field accelerating charged particles within the current sheet. This laser-plasma target configuration is discussed in regard with the laboratory modeling of charged particle acceleration and gamma flash generation in astrophysics. We present the results of 3-dimensional kinetic simulations and theoretical studies on the formation and evolution of the current sheet in a collisionless plasma during magnetic field annihilation in the ultra-relativistic limit. Annihilation of oppositively directed magnetic fields driven by two laser pulses interacting with underdense plasma target is accompanied by an electromagnetic burst generation. The induced strong non-stationary longitudinal electric field accelerates charged particles within the current sheet. Properties of the laser-plasma target configuration are discussed in the context of the laboratory modeling for charged particle acceleration and gamma flash generation in astrophysics.

Opposite Polarity Magnetic Fields and Convective Downflows in a Simulated Sunspot Penumbra

Abstract Recent numerical simulations and observations of sunspots show a significant amount of opposite polarity magnetic fields within the sunspot penumbra. Most of the opposite polarity fields are associated with convective downflows. We present an analysis of 3D MHD simulations through forward modeling of synthetic Stokes profiles of the Fe i 6301.5 Å and Fe i 6302.5 Å lines. The synthetic Stokes profiles are spatially and spectrally degraded considering typical instrument properties. Line bisector shifts of the Fe i 6301.5 Å line are used to determine line-of-sight velocities. Far wing magnetograms are constructed from the Stokes V profiles of the Fe i 6302.5 Å line. While we find an overall good agreement between observations and simulations, the fraction of opposite polarity magnetic fields, the downflow filling factor, and the opposite polarity-downflow association are strongly affected by spatial smearing and presence of strong gradients in the line-of-sight magnetic fields and velocity. A significant fraction of opposite polarity magnetic fields and downflows is hidden in the observations due to typical instrumental noise. Comparing simulations that differ by more than a factor of two in grid spacing, we find that these quantities are robust within the simulations.

On Polar Magnetic Field Reversal in Solar Cycles 21, 22, 23, and 24

The Sun's polar magnetic fields change their polarity near the maximum of sunspot activity. We analyzed the polarity reversal epochs in Solar Cycles 21 to 24. There was a triple reversal in the N-hemisphere in Solar Cycle 24 and single reversals in the rest of cases. Epochs of the polarity reversal from measurements of the Wilcox Solar Observatory (WSO) are compared with ones when the reversals were completed in the N- and S-hemispheres. The reversal times were compared with hemispherical sunspot activity and with the Heliospheric Current Sheet (HCS) tilts, too. It was found that reversals occurred at the epoch of the sunspot activity maximum in Cycles 21 and 23, and after the corresponding maxima in Cycles 22 and 24, and one-two years after maximal HCS tilts calculated in WSO. Reversals in Solar Cycles 21, 22, 23, and 24 were completed first in the N-hemisphere and then in the S-hemisphere after 0.6, 1.1, 0.7, and 0.9 years, respectively. The polarity inversion in the near-polar latitude range \pm(55-90)^\circ occurred from 0.5 to 2.0 years earlier that the times when the reversals were completed in corresponding hemisphere. Using the maximal smoothed WSO polar field as precursor we estimated that amplitude of Solar Cycle 25 will reach 116 \pm 12 in values of smoothed monthly sunspot numbers and will be comparable with the current cycle amplitude equaled to 116.4.

Synchronizing volcanic, sedimentary, and ice core records of Earth's last magnetic polarity reversal.

Reversal of Earth's magnetic field polarity every 105 to 106 years is among the most far-reaching, yet enigmatic, geophysical phenomena. The short duration of reversals make precise temporal records of past magnetic field behavior paramount to understanding the processes that produce them. We correlate new 40Ar/39Ar dates from transitionally magnetized lava flows to astronomically dated sediment and ice records to map the evolution of Earth's last reversal. The final 180° polarity reversal at ~773 ka culminates a complex process beginning at ~795 ka with weakening of the field, succeeded by increased field intensity manifested in sediments and ice, and then by an excursion and weakening of intensity at ~784 ka that heralds a >10 ka period wherein sediments record highly variable directions. The 22 ka evolution of this reversal suggested by our findings is mirrored by a numerical geodynamo simulation that may capture much of the naturally observed reversal process.

Plasmoid-mediated reconnection in solar UV bursts

Context. Ultraviolet bursts are transients in the solar atmosphere with an increased impulsive emission in the extreme UV lasting for one to several tens of minutes. They often show spectral profiles indicative of a bi-directional outflow in response to magnetic reconnection. Aims. To understand UV bursts, we study how motions of magnetic elements at the surface can drive the self-consistent formation of a current sheet resulting in plasmoid-mediated reconnection. In particular, we want to study the role of the height of the reconnection in the atmosphere. Methods. We conducted numerical experiments solving the 2D magnetohydrodynamic equations from the solar surface to the upper atmosphere. Motivated by observations, we drove a small magnetic patch embedded in a larger system of magnetic field of opposite polarity. This type of configuration creates an X-type neutral point in the initial potential field. The models are characterized by the (average) plasma-β at the height of this X point. Results. The driving at the surface stretches the X-point into a thin current sheet, where plasmoids appear, accelerating the reconnection, and a bi-directional jet forms. This is consistent with what is expected for UV bursts or explosive events, and we provide a self-consistent model of the formation of the reconnection region in such events. The gravitational stratification gives a natural explanation for why explosive events are restricted to a temperature range around a few 0.1 MK, and the presence of plasmoids in the reconnection process provides an understanding of the observed variability during the transient events on a timescale of minutes. Conclusions. Our numerical experiments provide a comprehensive understanding of UV bursts and explosive events, in particular of how the atmospheric response changes if the reconnection happens at different plasma-β, that is, at different heights in the atmosphere. This analysis also gives new insight into how UV bursts might be related to the photospheric Ellerman bombs.

On the Coronal Mass Ejection Detection Rate during Solar Cycles 23 and 24

Abstract The Solar and Heliospheric Observatory (SOHO) mission’s white light coronagraphs have observed more than 25,000 coronal mass ejections (CMEs) from 1996 January to the end of 2015 July. This period of time covers almost two solar cycles (23 and 24). The basic attributes of CMEs, reported in the SOHO/Large Angle and Spectrometric Coronagraph (LASCO) catalog, during these solar cycles were statistically analyzed. The question of the CME detection rate and its connection to the solar cycles was considered in detail. Based on the properties and detection rate, CMEs can be divided into two categories: regular and specific events. The regular events are pronounced and follow the pattern of sunspot number. On the other hand, the special events are poorer and more correlated with the general conditions of heliosphere and corona. Nevertheless, both groups of CMEs are the result of the same physical phenomenon, viz. release of magnetic energy from closed field regions. It was demonstrated that the enhanced CME rate, since the solar cycle 23 polar-field reversal, is due to a significant decrease of total (magnetic and plasma) heliospheric pressure as well as the changed magnetic pattern of solar corona. CMEs expel free magnetic energy and helicity from the Sun; therefore, they are related to complex solar magnetic field structure. It is also worth emphasizing that the CMEs listed in the SOHO/LASCO catalog are real ejections (not false identification). Their detection rate reflects the global evolution of the magnetic field on the Sun, and not only changes in the magnetic structures associated with sunspots.

Two-step Evolution of a Rising Flux Rope Resulting in a Confined Solar Flare

Abstract Combining the Solar Dynamics Observatory and the New Vacuum Solar Telescope observations, we study a confined flare triggered by a rising flux rope within the trailing sunspots of active region 12733. The flux rope lying above the sheared polarity inversion line can be constructed through magnetic extrapolation but could not be detected in multiwavelength images at the pre-flare stage. The conspicuous shearing motions between the opposite-polarity fields in the photosphere are considered to be responsible for the flux rope formation. The maximum twist of the flux rope is as high as −1.76, and then the flux rope rises due to the kink instability. Only when the flare starts can the flux rope be observed in high-temperature wavelengths. The differential emission measure results confirm that this flux rope is a high-temperature structure. Associated with the rising flux rope, there appear many post-flare loops and a pair of flare ribbons. When the rising flux rope meets and reconnects with the large-scale overlying field lines, a set of large-scale twisted loops are formed, and two flare ribbons propagating in opposite directions appear on the outskirts of the former ribbons, indicating that the twist of the flux rope is transferred to a much larger system. These results imply that the external reconnection between the rising flux rope and the large-scale overlying loops plays an important role in the confined flare formation.

Near-annual variation of the magnetic Sun

Over past 51 years (1968–2018) the general magnetic field of the Sun was measured nearly each day by the Crimean Astrophysical Observatory, the Wilcox Solar Observatory and five others. It is shown that the annual variation of this field (longitudinal component), thought to be caused by the inclination of the Sun’s axis to ecliptic, is splitted into two components with periods 0.954(3) years and 1.034(4) years (with mean amplitudes \(\approx 0.044\) G). But while the first one agrees well with the expected period 0.957(4) years (caused by the 11-year polar field reversals and coupled to the yearly projection effect), the second one is shifted relative to its theoretical value, 1.047(4) years, by \(\Delta = 0.013(5)\) years (99% confidence). The beat period of the two observed components, 12.3(7) years, agrees well with the Jovian period 11.9 years, being close also to the length of the 11-year cycle. Since the modern astronomy rejects a planetary influence on the Sun, the origin of both the 1.034-year periodicity and the shift \(\Delta \) remains to be mysterious.

Evolution of the Sun’s Polar Fields and the Poleward Transport of Remnant Magnetic Flux

Synoptic magnetograms and relevant proxy data were analyzed to study the evolution of the Sun's polar magnetic fields. Time-latitude analysis of large-scale magnetic fields demonstrates cyclic changes in their zonal structure and the polar-field buildup. The time-latitude distributions of the emergent and remnant magnetic flux enable us to examine individual features of recent cycles. The poleward transport of predominantly following polarities contributed much of the polar flux and led to polar-field reversals. Multiple reversals of dominant polarities at the Sun's poles were identified in Cycles 20 and 21. Three-fold reversals were caused by remnant flux surges of following and leading polarities. Time-latitude analysis of solar magnetic fields in Cycles 20--24 revealed zones which are characterized by a predominance of negative (non-Joy's) tilts and appearance of active regions which violate Hale's polarity law. The decay of non-Joy's and anti-Hale's active regions result in remnant flux surges which disturb the usual order in magnetic flux transport and sometimes lead to multiple reversals of polar fields. The analysis of local and large-scale magnetic fields in their causal relation improved our understanding of the Sun's polar field weakening.

A Secondary Fan-spine Magnetic Structure in Active Region 11897

Abstract Fan-spine is a special topology in solar atmosphere and is closely related to magnetic null point, as well as circular-ribbon flares, which can provide important information for understanding the intrinsic 3D nature of solar flares. However, the fine structure within the fan has rarely been investigated. In present paper, we investigate a secondary fan-spine (SFS) structure within the fan of a larger fan-spine topology. On 2013 November 18, this large fan-spine structure was traced out owing to the partial eruption of a filament, which caused a circular-ribbon flare in NOAA Active Region 11897. The extrapolated 3D magnetic fields and squashing factor Q maps depict distinctly this fan-spine topology, its surrounding quasi-separatrix layer (QSL) halo, and a smaller quasi-circular ribbon with high Q located in the center, which implies the existence of fine structure within the fan. The imaging observations, extrapolated 3D fields, and Q maps on November 17 show that there indeed exists an SFS surrounded by a QSL, which is enveloped by another QSL halo corresponding to the overlying larger dome-shaped fan. Moreover, the material flows caused by the null-point reconnection are also detected along this SFS. After checking the evolution of the underneath magnetic fields, we suggest that the continuous emergence of magnetic flux within the central parasitic region encompassed by the opposite-polarity fields results in the formation of the SFS under the large fan.

Precision Measurement of the monthly cosmic Ray fluxes e−, e+, p, He) with the Alpha Magnetic Spectrometer on the ISS

The precision measurements of the monthly cosmic ray fluxes with Alpha Magnetic Spectrometer on the International Space Station are presented. Individual electron, positron, proton and helium spectra have been measured for each Bartel’s rotation period (27 days) in the time range from May 2011 to May 2017. This period covers the ascending phase of solar cycle #24 together with the reversal of the Sun’s magnetic field polarity through the minimum. The fluxes reveal a characteristic time dependence below 20 GeV. The data show a strong charge-sign dependent effects corresponding to the the polarity reversal of the solar magnetic field.

Flows and magnetic field structures in reconnection regions of simulations of the solar atmosphere: Do flux pile-up models work?

Aims. We study the process of magnetic field annihilation and reconnection in simulations of magnetised solar photosphere and chromosphere with magnetic fields of opposite polarities and constant numerical resistivity. Methods. Exact analytical solutions for reconnective annihilations were used to interpret the features of magnetic reconnection in simulations of flux cancellation in the solar atmosphere. We used MURaM high-resolution photospheric radiative magneto-convection simulations to demonstrate the presence of magnetic field reconnection consistent with the magnetic flux pile-up models. Also, a simulated data-driven chromospheric magneto-hydrodynamic simulation is used to demonstrate magnetic field and flow structures, which are similar to the theoretically predicted ones. Results. Both simulations demonstrate flow and magnetic field structures roughly consistent with accelerated reconnection with magnetic flux pile-up. The presence of standard Sweet–Parker type reconnection is also demonstrated in stronger photospheric magnetic fields.

Positive and Negative Photospheric Fields in Solar Cycles 21–24

Distribution of the positive and negative photospheric fields is studied on the base of synoptic maps of the photospheric magnetic field produced by the National Solar Observatory Kitt Peak (NSO Kitt Peak) for 1976-2016. In the analysis only the sign of the field irrespective of its strength is taken into account, emphasizing the role of the fields with average and weak strengths. Time changes in the positive and negative magnetic fields for two hemispheres, for the high latitudes and for the sunspot zone as well as their imbalances are considered. Distributions of fields of opposite polarities, which are defined mostly by the fields of the high latitudes, change with a 22-year cycle. The polarity imbalance in each hemisphere is closely connected with the dipole moment $g_{10}$. The imbalance of high latitudes for two hemispheres changes with the 22-year period and coincides with the sign of both the polar field in the southern hemisphere, and the quadrupole moment $g_{20}$. For the magnetic fields of the sunspot zone during ~75% of time a connection of the magnetic field imbalance with the quadrupole moment taken with the reversed sign $- g_{20}$ and with the sign of the polar field in the northern hemisphere is observed. The received results testify to cyclic changes in the polarity imbalance.