Non-polar Coronal Holes Research Articles

Statistical Study of the Occurrence of Coronal Holes in the Solar Corona During Solar Cycle 24

In this article we look at: (a) the occurrence of coronal holes as a function of the different phases of solar cycle 24, (b) the variability of the annual number of coronal holes from 2009 to 2019, (c) the distribution of polar and non-polar coronal holes, irrespective of the phase but also as a function of the different phases of the cycle. From the phase minimum to the descending phase of solar cycle 24, the occurrences of coronal holes observed on the solar disc are 38%, 24%, 9% and 29% respectively. The annual number of coronal holes observed on the solar disc does not always follow the expected evolution of the two components of the solar magnetic field and the annual number of sunspots. Regardless of the phase of the solar cycle, the majority of coronal holes, around 65%, are located in the polar regions. During the early and late phases of the solar cycle, polar coronal holes predominate, whereas during the ascending phase and phase maximum, non-polar coronal holes predominate. The number of polar coronal holes is highest during the minimum phase of the solar cycle and lowest during the maximum phase. During the phases of the solar cycle, coronal hole activity appears to be in phase opposition with sunspot activity. There is a fairly good correlation between variations in the number of polar coronal holes and the expected evolution of the poloidal component of the solar magnetic field.

Evolutionary features of polar and non-polar coronal holes during solar cycle 24 and the rising phase of cycle 25

This paper presents the results of the analysis of the evolution of coronal holes (CHs) on the Sun during the period May 13, 2010 – March 20, 2022, covering Solar Cycle 24. Our study uses images in the extreme-ultraviolet iron line (Fe XII 193 Å) obtained with the Atmospheric Imager Assembly of the Solar Dynamics Observatory (AIA/SDO). To localize CHs and determine their areas, we used the Heliophysics Event Knowledgebase (HEK). We separate the CHs into polar and non-polar and study the evolutionary features of each group. During this period, an asymmetry between the Northern (N) and Southern (S) Hemispheres (N-S or hemispheric asymmetry) is detected both in the solar activity (SA) indices and in the localization of the maximum areas of the polar and non-polar CHs. It is shown that the hemispheric asymmetry of the areas of polar and non-polar CHs varies significantly over time and that the nature of these changes is clearly related to the SA cycle. We find that for most of the period, the polar CHs were predominated generated in the S- hemisphere while the non-polar CHs were dominant in the N- hemisphere. It is found that the maximum and minimum of the hemispheric imbalance in the areas of non-polar CHs are close in time to the maximum and minimum of the asymmetry of the SA indices (the number and areas of sunspots). The maximum hemispheric imbalance of the polar CH areas is observed at the maximum of Cycle 24, and the minimum imbalance is found at the cycle minimum. These results confirm our assumption that these two types of CHs are of a different nature and that the non-polar CHs, like sunspots, are elements of the general magnetic activity.

Statistical Approach on Differential Emission Measure of Coronal Holes using the CATCH Catalog

Coronal holes are large-scale structures in the solar atmosphere that feature a reduced temperature and density in comparison to the surrounding quiet Sun and are usually associated with open magnetic fields. We perform a differential emission measure analysis on the 707 non-polar coronal holes in the Collection of Analysis Tools for Coronal Holes (CATCH) catalog to derive and statistically analyze their plasma properties (i.e. temperature, electron density, and emission measure). We use intensity filtergrams of the six coronal EUV filters from the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory, which cover a temperature range from approx10^{5.5} to 10^{7.5}~mbox{K}. Correcting the data for stray and scattered light, we find that all coronal holes have very similar plasma properties with an average temperature of 0.94 pm0.18~mbox{MK}, a mean electron density of (2.4 pm0.7) times10^{8}~mbox{cm}^{-3}, and a mean emission measure of (2.8 pm1.6) times10^{26}~mbox{cm}^{-5}. The temperature distribution within the coronal holes was found to be largely uniform, whereas the electron density shows a 30 to 40% linear decrease from the boundary towards the inside of the coronal hole. At distances greater than 20″ (approx15~mbox{Mm}) from the nearest coronal hole boundary, the density also becomes statistically uniform. The coronal hole temperature may show a weak solar-cycle dependency, but no statistically significant correlation of plasma properties with solar-cycle variations could be determined throughout the observed period between 2010 and 2019.

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.

Statistical Analysis and Catalog of Non-polar Coronal Holes Covering the SDO-Era Using CATCH

Coronal holes are usually defined as dark structures seen in the extreme ultraviolet and X-ray spectrum which are generally associated with open magnetic fields. Deriving reliably the coronal hole boundary is of high interest, as its area, underlying magnetic field, and other properties give important hints as regards high speed solar wind acceleration processes and compression regions arriving at Earth. In this study we present a new threshold-based extraction method, which incorporates the intensity gradient along the coronal hole boundary, which is implemented as a user-friendly SSW-IDL GUI. The Collection of Analysis Tools for Coronal Holes (CATCH) enables the user to download data, perform guided coronal hole extraction and analyze the underlying photospheric magnetic field. We use CATCH to analyze non-polar coronal holes during the SDO-era, based on 193 Å filtergrams taken by the Atmospheric Imaging Assembly (AIA) and magnetograms taken by the Heliospheric and Magnetic Imager (HMI), both on board the Solar Dynamics Observatory (SDO). Between 2010 and 2019 we investigate 707 coronal holes that are located close to the central meridian. We find coronal holes distributed across latitudes of about {pm}, 60^{circ}, for which we derive sizes between 1.6 times 10^{9} and 1.8 times 10^{11}mbox{ km}^{2}. The absolute value of the mean signed magnetic field strength tends towards an average of 2.9pm 1.9 G. As far as the abundance and size of coronal holes is concerned, we find no distinct trend towards the northern or southern hemisphere. We find that variations in local and global conditions may significantly change the threshold needed for reliable coronal hole extraction and thus, we can highlight the importance of individually assessing and extracting coronal holes.

Coronal Holes as Tracers of the Sun’s Global Magnetic Field in Cycles 21–23 of Solar Activity

The dependences of the number and parameters of non-polar coronal holes (CHs) on characteristics of the Sun’s global magnetic field (GMF) are investigated in cycles 21–23 of solar activity (1976–2012) based on data from the CH catalog of the Mountain Astronomical Station of the Main (Pulkovo) Astronomical Observatory of the Russian Academy of Sciences. The influence of variations in the GMF structural arrangement on the CH number and parameters is considered. It is shown that the more stable and steadystate the GMF topology is in the cycle, the higher are the GMF values. The majority of CHs form over periods of stable GMF structure. With a growth in the rate of structural GMF changes, the CH number and the values of all CH parameters decrease. The correlation between the area, extension in latitude and longitude, and magnetic flux of CHs and the intensity of both the calculated coronal field and the observed large-scale photospheric magnetic field is higher in cycles with larger magnetic-field values and with a more stable GMF structure. Non-polar CHs are divided into three groups depending on the GMF value and a rate of the structural GMF rearrangement.

Coronal Hole and Solar Global Magnetic Field Evolution in 1976 – 2012

Coronal hole spatial-temporal evolution is studied and comparison made with that of the solar global magnetic field in cycles 21-23 (1976-2012). The latitude-longitude distribution dynamics of coronal holes and the regularities in the global magnetic field associated with the solar polar field reversal are analyzed. Polar and non-polar coronal hole populations are considered. The investigation reveals some temporal and spatial regularities in coronal hole distributions that match well the global magnetic-field cycle evolution. The results show that the non-polar coronal hole longitudinal distribution follows all configuration changes in the global magnetic-field structure. Reorganizations of the global magnetic-field and coronal hole distributions occur simultaneously during a time interval of a few solar rotations. The cycle evolution of the non-polar coronal holes reflects the transition of the solar global magnetic field from the zonal structure to sectorial andvice versa. Two different type waves of non-polar coronal holes are revealed from their latitudinal distribution. The first one is short poleward waves. They trace the poleward motion of the unipolar photospheric magnetic fields from approximately 35 deg to the associated pole in each hemisphere and the redevelopment of a new-polarity polar CH. Although they start the poleward movement before the change of the polar magnetic field in the associated hemisphere, they reach the pole after the polar reversal. The other type of non-polar CH wave forms two sinusoidal branches associated with the positive- and negative-polarity magnetic fields. The complete period of the wave was equal to approximately 268 CRs (22 years). These wave CHs arrive at high latitudes during declining phases when the new polarity polar CHs are already completely formed.

FORMATION OF CORONAL HOLES ON THE ASHES OF ACTIVE REGIONS

We investigate the formation of isolated non-polar coronal holes (CHs) on the remnants of decaying active regions (ARs) at the minimum/early ascending phase of sunspot activity. We follow the evolution of four bipolar ARs and measure several parameters of their magnetic fields including total flux, imbalance, and compactness. As regions decay, their leading and following polarities exhibit different dissipation rates: loose polarity tends to dissipate faster than compact polarity. As a consequence, we see a gradual increase in flux imbalance inside a dissipating bipolar region, and later a formation of a CH in place of more compact magnetic flux. Out of four cases studied in detail, two CHs had formed at the following polarity of the decaying bipolar AR, and two CHs had developed in place of the leading polarity field. All four CHs contain a significant fraction of magnetic field of their corresponding AR. Using potential field extrapolation, we show that the magnetic field lines of these CHs were closed on the polar CH at the North, which at the time of the events was in imbalance with the polar CH at the South. This topology suggests that the observed phenomenon may play an important role in transformation of toroidal magnetic field to poloidal field, which is a key step in transitioning from an old solar cycle to a new one. The timing of this observed transition may indicate the end of solar cycle 23 and the beginning of cycle 24.

Coronal holes and the solar polar field reversal

The relationship between the solar polar magnetic field reversal and coronal hole evolution is investigated for the time interval from 1996 to 2001. The distribution of coronal holes shows some evolutionary changes in relation to the polarity reversals, and these changes appear to be coordinated with changes in the global magnetic field structure, suggesting that the polar reversal originates from global processes. There are periods when the character of the coronal hole distribution on the solar disk changes significantly, indicating that the magnetic field structure changes. These can be interpreted as periods of rearrangement of the multipole magnetic field structure. It is found that the evolution of the geometric structure of the photospheric magnetic field during these periods is not characterized by a continuous transition from one dominant structure to another, but by relatively sudden rearrangement of the dominant geometrical structure of the magnetic field. These coronal holes and photospheric magnetic field rearrangements coincide with the polar photospheric magnetic field strength variations. The minimum phase of the solar cycle is dominated by the dipole component of the global solar magnetic field. The zonal magnetic field structure and zonal non-polar coronal hole distribution existed at that time. The sectorial magnetic field structure appears when the polar field strength reaches 0.7 of its maximum value in the corresponding hemisphere. This structure was established at dierent times in each hemisphere. In the north hemisphere it has existed since November 1998 and in the south hemisphere it has existed since October 1997. The rearrangement from one configuration to another occurs during a short time period, about 1 2 solar rotations. The coronal hole number and area evolution indicates a redistribution of positive-polarity and negative-polarity photospheric magnetic fields inside these longitudinal sectors, which reflects the solar polar field reversal.

Coronal White‐Light Jets near Sunspot Maximum

During the 1996-1997 activity minimum, the Large Angle and Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO) recorded numerous jetlike ejections above the Sun's polar regions. In a previous study, we showed that these white-light ejections were the outward extensions of extreme-ultraviolet (EUV) jets, which in turn originated from flaring bright points inside the polar coronal holes. Here we investigate a number of jetlike events observed with LASCO during the current sunspot maximum. To identify the solar surface counterparts of these events, we again use Fe XII λ195 images obtained by the EUV Imaging Telescope on SOHO. The white-light jets in our sample have angular widths of ~3°-7° and velocities typically of order 600 km s-1; they tend to be brighter and wider than the polar jets observed near sunspot minimum and are distributed over a much greater range of latitudes. Many of the ejections are recurrent in nature and originate from active regions located inside or near the boundaries of nonpolar coronal holes. We deduce that the jet-producing regions consist of systems of closed magnetic loops partially surrounded by open fields; perturbations in the closed fields caused them to reconnect with the overlying open flux, releasing the trapped energy in the form of jetlike ejections. In some events, the core of the active region erupts, producing fast, collimated ejections with widths of up to ~15°.

Coronal Inflows and the Sun’s Nonaxisymmetric Open Flux

Wang et al. recently described white-light coronagraph observations of faint coronal features moving inward toward the Sun at heliocentric distances of 2-6 R☉. In a study of these inflows during 1996-2000, we have found that they occur along bends of the coronal streamer belt and are especially common when the magnetic field has a four-sector structure. The measured inflow rate is dominated by episodic bursts that are correlated with the occurrence of nonpolar coronal holes and other indicators of the Sun's nonaxisymmetric open flux. However, the inflow rate has only a broad long-term correlation with conventional indicators of solar activity like the sunspot number and coronal mass ejection rate. We conclude that most inflows indicate collapsing field lines that occur as nonpolar coronal holes are subjected to photospheric motions and the eruptions of new flux.

Cosmic ray transport in a heliospheric magnetic field with non-polar coronal holes

The simple tilted dipole picture of Corotating Interaction Regions which prevailed during the first polar pass of Ulysses no longer applies since the Sun entered a more active phase. Recent observations show that CIRs still persist, though the large polar coronal holes of solar minimum shrink to smaller areas and move to lower latitudes. We present 3-D simulations for the cosmic-ray intensity variations in a model with non-polar high speed streams. Latitudinal and recurrent time-variations are discussed, but more detailed and realistic simulations are required before quantitative comparisons with observations can be made.

Coronal Hole Evolution During 1996-1999

Spatial and temporal distributions of coronal holes for the rising phase of the solar cycle during 1996–1999 are considered. Connections between the number of non-polar coronal holes on the solar disk and the Wolf number, the mean solar photospheric magnetic field, and the solar flux density at 2800 mHz are analyzed. Peculiarities of the photospheric magnetic field structure of the regions corresponding to coronal hole locations and comparison with `clear' ones are discussed.

Characteristics of interplanetary magnetic clouds in relation to their solar association

Interplanetary magnetic clouds (MCs) are always associated with a coronal mass ejection. In some particular cases, they also have been related to the occurrence of near‐surface solar events such as filament eruptions and flares. In this paper we use a sample of 67 MCs observed between 1967 and 1982 at 1 AU to obtain better statistics of their near‐surface solar association, considering Hα flares, filament eruptions, X‐ray events, and coronal holes. We also study possible differences in the characteristics of the clouds depending on their solar association. We find that 57% of the MCs observed at 1 AU were associated with flares (F), 21% were associated with filament eruptions (P), 7% were associated with both (FP), and 15% were not associated with any reported near‐surface solar event. Most of the associated flares occurred at the west of the solar central meridian, and most of the associated filament eruptions occurred at the east. In 95% of the cases for which the coronal hole information was available, a nonpolar coronal hole was near the region of the solar event. In the cases where the cloud was not associated with a flare or filament eruption, a coronal hole was always near the solar central meridian. This suggests that coronal holes may be playing a role in the formation of interplanetary magnetic clouds. We consider the spatial profiles of magnetic field and plasma parameters inside MCs, observed as temporal variations as clouds pass detectors at 1 AU. The average magnitudes of these parameters inside the cloud were also considered. The profiles observed were the following: flat (A‐type), with a maximum (B‐type), descending (C‐type), ascending (D‐type), with a minimum (E‐type), fluctuating (F‐type), and with two minima (G‐type). We find that the most common profiles and the average values of the different parameters vary for different solar associations. We also find some correlations between the cloud parameters, which may appear different in clouds associated with F or P. However, as there is a fraction of MCs not associated with near‐surface solar events, it is possible that this kind of activity is not essential for the formation of an interplanetary magnetic cloud, but only adds particular characteristics to the clouds.