Polar Magnetic Field Reversal Research Articles

Analysis of BMR Tilt from AutoTAB Catalog: Hinting toward the Thin Flux Tube Model?

One of the intriguing mechanisms of the Sun is the formation of bipolar magnetic regions (BMRs) in the solar convection zone (CZ), which are observed as regions of concentrated magnetic fields of opposite polarity on the photosphere. These BMRs are tilted with respect to the equatorial line, which statistically increases with latitude. The thin flux tube model, employing the rise of magnetically buoyant flux loops and their twist by Coriolis force, is a popular paradigm for explaining the formation of tilted BMRs. In this study, we assess the validity of the thin flux tube model by analyzing the tracked BMR data obtained through the Automatic Tracking Algorithm for BMRs. Our observations reveal that the tracked BMRs exhibit the expected collective behaviors. We find that the polarity separation of BMRs increases over their normalized lifetime, supporting the assumption of a rising flux tube from the CZ. Moreover, we observe an increasing trend of the tilt with the flux of the BMR, suggesting that rising flux tubes associated with lower flux regions are primarily influenced by drag force and Coriolis force, while in higher flux regions, magnetic buoyancy dominates. Furthermore, we observe Joy’s law dependence for emerging BMRs from their first detection, indicating that at least a portion of the tilt observed in BMRs can be attributed to the Coriolis force. Notably, lower flux regions exhibit a higher amount of fluctuations associated with their tilt measurement compared to stronger flux regions, suggesting that lower flux regions are more susceptible to turbulent convection.

Probing the variations in the timing of the Sun’s polar magnetic field reversals through observations and surface flux transport simulations

ABSTRACT The polar field reversal is a crucial process in the cyclic evolution of the large-scale magnetic field of the Sun. Various important characteristics of a solar cycle, such as its duration and strength, and also the cycle predictability, are determined by the polar field reversal time. While the regular measurements of solar magnetic field have been accumulated for more than half a century, there is no consensus in the heliophysics community concerning the interpretation of the Sun’s polar field measurements and especially the determination of polar field reversal time. There exists a severe problem of non-reproducibility in the reported results even from studies of the same observational data set, and this causes an obstacle to make more accurate forecasts of solar cycle. Here, we analyze the solar magnetograms from four instruments for the last four cycles, to provide a more correct interpretation of the polar field observations and to find more accurate time of the reversals. We show the absence of triple (multiple) reversals in Cycles 21–24, significant variations in the time interval between reversals in the hemispheres and in the time interval between a reversal and a cycle beginning. In order to understand the origin of the reversal time variation, we perform Surface Flux Transport (SFT) simulations and find out that the presence of the ‘anomalous’ bipolar magnetic regions (BMRs) in different phases of a cycle can cause cycle-to-cycle variations of the reversal time within the similar range found in observations.

Focus on software, data acquisition, and instrumentation

Focus on software, data acquisition, and instrumentation

Magnetic Reconnection as the Driver of the Solar Wind

We present EUV solar observations showing evidence for omnipresent jetting activity driven by small-scale magnetic reconnection at the base of the solar corona. We argue that the physical mechanism that heats and drives the solar wind at its source is ubiquitous magnetic reconnection in the form of small-scale jetting activity (a.k.a. jetlets). This jetting activity, like the solar wind and the heating of the coronal plasma, is ubiquitous regardless of the solar cycle phase. Each event arises from small-scale reconnection of opposite-polarity magnetic fields producing a short-lived jet of hot plasma and Alfvén waves into the corona. The discrete nature of these jetlet events leads to intermittent outflows from the corona, which homogenize as they propagate away from the Sun and form the solar wind. This discovery establishes the importance of small-scale magnetic reconnection in solar and stellar atmospheres in understanding ubiquitous phenomena such as coronal heating and solar wind acceleration. Based on previous analyses linking the switchbacks to the magnetic network, we also argue that these new observations might provide the link between the magnetic activity at the base of the corona and the switchback solar wind phenomenon. These new observations need to be put in the bigger picture of the role of magnetic reconnection and the diverse form of jetting in the solar atmosphere.

Investigating datasets with high IRIS burst prevalence

Context. Approximately 0.01% of all Si IV 1394 Å spectra sampled in 2013 and 2014 by the Interface Region Imaging Spectrograph (IRIS) have IRIS burst profiles. However, these events are not evenly distributed across datasets with 19.31% of these spectra being identified in only six (of more than 3500) rasters. Aims. Here, we investigate five of these six datasets, with the aim of understanding why they contain so many IRIS burst profiles. This research will help guide future targeted analyses of IRIS bursts. Methods. We analyse five datasets sampled by the IRIS satellite, studying both Si IV 1394 Å spectra and 1400 Å filter slit-jaw imager (SJI) data. IRIS burst profiles are identified through the use of an automated algorithm. Additionally, we study co-spatial line-of-sight photospheric magnetic field maps sampled by the Solar Dynamics Observatory’s Helioseismic and Magnetic Imager (SDO/HMI) instrument. Results. The majority of identified IRIS burst profiles (12 401 out of 13 904) found in the five datasets analysed here were localised to seven small regions in the time-distance domain (temporal durations of < 4 h and spatial lengths of < 12″ along the slit). The SJI data co-spatial to these regions contained long-lived or repetitive compact brightenings, matching the defined properties of UV bursts, which remained close to the IRIS slit throughout their evolutions. The IRIS burst profiles were not limited to the brightest pixels in the fields of view (FOVs) nor did they comprise the majority of bright (> 500 DN/s) pixels. These IRIS burst profiles occurred co-spatial to evolving (e.g., cancelling) opposite polarity magnetic fields where magnetic reconnection is thought to be possible. Conclusions. More than 10% of the IRIS burst profiles identified during the entirety of 2013 and 2014 are contained in just seven small regions in the time-distance domain where long-lived (lifetimes > 10 min) or repetitive UV bursts occurred along the axis of the IRIS slit.

Numerical simulations of the laser-driven Petschek-type magnetic reconnection

This paper describes a numerical study of the magnetic reconnection between two magnetic fields of opposite polarity. The magnetic fields are created by an electric current in a coil connected to two metal disks. One of the disks is irradiated by a strong laser beam, whereby large amounts of free electrons flow toward the other disk, constituting a closed circuit for the electric current flowing through the coil. Two parallel coils are arranged to connect the two disks, and irradiation of the laser beam on one disk results in parallel electric currents in the two coils, inducing two magnetic fields of opposite polarity in the region between them. The magnetic reconnection that occurs in this region is three-dimensional. This three-dimensional magnetic reconnection is investigated via magnetohydrodynamic numerical simulations. The characteristics of the Petschek-type magnetic reconnection are observed for the first time in such numerical simulations of magnetic reconnection. Changes in the shape of the magnetic field lines form the boundary of the dissipation region and the outflow region. Moreover, the thermal plasma generated by reconnection is strongly confined to the region where the reconnecting current sheet and the slow-mode shock are located, and no leaks of thermal plasma are observed. Comparisons with existing laboratory experiment results confirm that our numerical simulations reproduce the experimental outcomes and provide reasonable explanations for the results observed in laboratories.

Variations in the Correlations of Acceleration and Force of Slow and Fast CMEs with Solar Activity during Solar Cycles 23 – 24

Studying the behavior of coronal mass ejections (CMEs) is important for both solar physics and space weather. The correlations of smoothed monthly mean daily integrated CME acceleration [ $a$ ], mass [ $M$ ], and the force [ $Ma$ ] to drive CMEs with sunspot activity [ $R_{\mathrm{I}}$ ] are analyzed for both slow and fast CMEs and for both Solar Cycles 23 and 24 separately. It is found that $a$ is inversely related to both $R_{\mathrm{I}}$ and $M$ . The correlation between $Ma$ and $R_{\mathrm{I}}$ for both slow and fast CMEs is negative at the rising phase of Solar Cycle 23 and positive otherwise. There is a sharp peak in $\gamma =Ma/R_{\mathrm{I}}$ near the solar minimum (December 2008) for both slow and fast CMEs. However, for fast CMEs, there is a sharp negative peak near the previous solar minimum (August 1996) and another positive peak near the current solar minimum (2019). The positive (negative) peak tends to be related to the solar minimum from a stronger (weaker) to a weaker (stronger) solar cycle. These results suggest that the CME acceleration depends more on the strength of solar activity than on the CME’s speed. Stronger magnetic activity may slow down the CMEs that are too massive or too fast and weaker activity may speed up the CMEs that are less massive or too slow. During a few years’ period of magnetic-field polarity reversal around the solar minimum, the force provided by large-scale magnetic-field structures may not be strong enough to constrain CME motions, leading to the “escape” of CMEs with large $|\gamma |$ .

Spatial–temporal evolution of photospheric weak-field shifts in solar cycles 21–24

Context. Weak magnetic field elements make a dominant contribution to the total magnetic field on the solar surface. Even so, little is known of their long-term occurrence. Aims. We study the long-term spatial–temporal evolution of the weak-field shift and skewness of the distribution of photospheric magnetic field values during solar cycles 21−24 in order to clarify the role and relation of the weak field values to the overall magnetic field evolution. Methods. We used Wilcox Solar Observatory (WSO) and the Synoptic Optical Long-term Investigations of the Sun Vector SpectroMagnetograph synoptic maps to calculate weak-field shifts for each latitude bin of each synoptic map, and thereby constructed a time–latitude butterfly diagram for shifts. We also calculated butterfly diagrams for skewness for all field values and for weak field values only. Results. The weak-field shifts and (full-field) skewness depict a similar spatial–temporal solar cycle evolution to that of the large-scale surface magnetic field. The field distribution has a systematic non-zero weak-field shift and a large skewness already at (and after) the emergence of the active region, even at the highest resolution. We find evidence for coalescence of opposite-polarity fields during the surge evolution. This is clearly more effective at the supergranulation scale. However, a similar dependence of magnetic field coalescence on spatial resolution was not found in the unipolar regions around the poles. Conclusions. Our results give evidence for the preference of even the weakest field elements toward the prevailing magnetic polarity since the emergence of an active region, and for a systematic coalescence of stronger magnetic fields of opposite polarities to produce weak fields during surge evolution and at the poles. We also find that the supergranulation process is reduced or turned off in the unipolar regions around the poles. These observations improve the understanding not only of the development of the weakest magnetic field elements, but also of the dynamics of magnetic fields at large, and even of processes below the solar surface.

Precision measurements of cosmic ray electron and positron spectra above 50 MeV with the PAMELA magnetic spectrometer

The PAMELA experiment with magnetic spectrometer operated almost ten years on board of the Resurs DK1 satellite. The satellite was launched on 15 June 2006 on polar orbit with an inclination of 70° and an altitude of 350–610 km. The spectrometer continuously measured charged cosmic ray particles in wide energy range from about 50 MeV up to several TeV. In this work the spectra of electrons and positrons averaged over one year were obtained from July of 2006 until January 2016, i.e. from the end of 23th and at the beginning of 24th solar cycle including the period of interplanetary magnetic field polarity reversal. These precise long duration time-dependent measurements of the electron and positron spectra are important to estimate possible contributions of exotic cosmic ray sources such as dark matter annihilation or decay.

Local interstellar spectra and solar modulation of cosmic ray electrons and positrons

Low energy cosmic rays are modulated by the solar activity when they propagation in the heliosphere, leading to ambiguities in understanding their acceleration at sources and propagation in the Milky Way. By means of the precise measurements of the e−,e+,e−+e+, and e+/(e−+e+) spectra by AMS-02 near the Earth, as well as the very low energy measurements of the e−+e+ fluxes by Voyager-1 far away from the Sun, we derive the local interstellar spectra (LIS) of e− and e+ components individually. Our method is based on a non-parametric description of the LIS of e− and e+ and a force-field solar modulation model. We then obtain the evolution of the solar modulation parameters based on the derived LIS and the monthly fluxes of cosmic ray e− and e+ measured by AMS-02. To better fit the monthly data, additional renormalization factors for e− and e+ have been multiplied to the modulated fluxes. We find that the inferred solar modulation parameters of positrons are in good agreement with that of cosmic ray nuclei, and the time evolutions of the solar modulation parameters of electrons and positrons differ after the reversal of the heliosphere magnetic field polarity, which shows clearly the charge-sign dependent modulation effect.

Developing the Cambrian and Ordovician Magnetic Polarity Time Scale: Current Data and Attempt of Synthesis

Abstract—The time scale of the geomagnetic polarity is the quintessence of our knowledge about the changes in the polarity of the Earth’s magnetic field in the geological past. The pattern of the Earth’s magnetic field polarity reversals is relatively well known for the Cenozoic, Mesozoic, and Late Paleozoic. However, our understanding of the behavior of geomagnetic polarity in the more ancient epochs is non-systematic and fragmentary. Obtaining the reliable high-quality information for the Lower and Middle Paleozoic should become the next important step.This paper compiles the main data yielded by recent studies on magnetic stratigraphy of the Lower Paleozoic (Cambrian and Ordovician) of the Siberian Platform and other regions conducted by our team and our colleagues. The synthesis of these data allows us to propose a working model of the Early Paleozoic magnetic polarity time scale which embodies the current knowledge about the character of changes in the geomagnetic field polarity on this interval of the geological time. The process of geomagnetic field polarity reversals in the Early Paleozoic is described by the following most important characteristics: (a) the existence of a superchron of reversed polarity during a significant part of the Early and Middle Ordovician (the third Phanerozoic superchron Moyero); (b) a high (probably peaking in the Phanerozoic) frequency of the geomagnetic reversals in the Middle Cambrian; (c) a decrease in the frequency of geomagnetic reversals in Late Cambrian and Tremadocian with the approach of the superchron; (d) the presence of anomalous period in the behavior of the geomagnetic field close to the Phanerozoic/Precambrian boundary; and (e) the absence of a double superchron in the Ordovician (Algeo, 1996).

Nonaxisymmetric Component of Solar Activity: the Vector of the Longitudinal Asymmetry

The vector representation of sunspots is used to study the nonaxisymmetric features of the solar activity distribution (sunspot data from Greenwich–USAF/NOAA, 1874–2016). Each sunspot is represented by a polar vector with modulus equal to the sunspot area and the phase equal to the sunspot heliolongitude. The vector sum of these individual vectors defines both the magnitude of the longitudinal asymmetry and the dominating longitude of the sunspot distribution. These characteristics are to a large extent free from the influence of a stochastic component and emphasize the nonaxisymmetric component of the solar activity. The longitudinal asymmetry follows the 11-year solar cycle with the amplitude three times lower than the sunspot area maximum. Longitudinal asymmetry is mostly determined by large sunspots: 82% of the longitudinal asymmetry is given by sunspots with area from 100 to 2000 MSH. Longitudinal asymmetries of the northern and southern hemispheres are connected only weakly (correlation coefficient $R = 0.29$ ). The longitudinal distribution displays a maximum at the longitude $\sim 180^{\circ }$ during ascent-maximum and at $\sim 0^{\circ }/360^{\circ }$ during descent-minimum. The active longitude changes with the reversals of polarity of the local (minimum of solar activity) and global magnetic fields (reversal of polar magnetic field).

Magnetic Flux Emergence in a Coronal Hole

A joint campaign of various space-borne and ground-based observatories, comprising the Japanese Hinode mission (HOP~338, 20\,--\,30~September 2017), the GREGOR solar telescope, and the \textit{Vacuum Tower Telescope} (VTT), investigated numerous targets such as pores, sunspots, and coronal holes. In this study, we focus on the coronal hole region target. On 24~September 2017, a very extended non-polar coronal hole developed patches of flux emergence, which contributed to the decrease of the overall area of the coronal hole. These flux emergence patches erode the coronal hole and transform the area into a more quiet-Sun-like area, whereby bipolar magnetic structures play an important role. Conversely, flux cancellation leads to the reduction of opposite-polarity magnetic fields and to an increase in the area of the coronal hole. Other global coronal hole characteristics, including the evolution of the associated magnetic flux and the aforementioned area evolution in the EUV, are studied using data of the \textit{Helioseismic and Magnetic Imager} (HMI) and \textit{Atmospheric Imaging Assembly} (AIA) onboard the \textit{Solar Dynamics Observatory} (SDO). The interplanetary medium parameters of the solar wind display parameters compatible with the presence of the coronal hole. Furthermore, a particular transient is found in those parameters.

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.