Relative Velocities and Linewidths in a Coronal Hole and Outside

Relative Doppler velocities and spectral linewidths in a coronal hole and in the quiet Sun region outside have been obtained from Solar and Heliospheric Observatory (SOHO)/Coronal Diagnostic Spectrometer (CDS) observations. Five strong emission lines in the CDS wavelength range (namely, O? iii 599 Å, O?v 630 Å, Ne?vi 562.8 Å, He?ii 304 Å, and Mg?ix 368 Å), whose formation temperatures represent different heights in the solar atmosphere from the lower transition region to the inner corona, have been used in the study. As reported earlier, relative velocities in the coronal hole are generally blueshifted with respect to the quiet Sun, and the magnitude of the blueshifts increases with height. It has been found that the polar coronal hole has larger relative velocities than the equatorial extension in the inner corona. Several localized velocity contours have been found mainly on network brightenings and in the vicinity of the coronal hole boundary. The presence of velocity contours on the network may represent network outflows whereas the latter could be due to localized jets probably arising from magnetic reconnection at the boundary. All spectral lines have larger widths in the coronal hole than in the quiet Sun. In O?v 630 Å an extended low-linewidth region is seen in the coronal hole?–?quiet Sun boundary, which may indicate fresh mass transfer across the boundary. Also polar coronal holes have larger linewidths in comparison with the equatorial extension. Together with larger relative velocities, this suggests that the solar wind emanating from polar hole regions is faster than that from equatorial hole regions.     

Similarities and Differences between Coronal Holes and the Quiet Sun: Are Loop Statistics the Key?

Coronal holes (CH) emit significantly less at coronal temperatures than quiet-Sun regions (QS), but can hardly be distinguished in most chromospheric and lower transition region lines. A key quantity for the understanding of this phenomenon is the magnetic field. We use data from SOHO/MDI to reconstruct the magnetic field in coronal holes and the quiet Sun with the help of a potential magnetic model. Starting from a regular grid on the solar surface we then trace field lines, which provide the overall geometry of the 3D magnetic field structure. We distinguish between open and closed field lines, with the closed field lines being assumed to represent magnetic loops. We then try to compute some properties of coronal loops. The loops in the coronal holes (CH) are found to be on average flatter than in the QS. High and long closed loops are extremely rare, whereas short and low-lying loops are almost as abundant in coronal holes as in the quiet Sun. When interpreted in the light of loop scaling laws this result suggests an explanation for the relatively strong chromospheric and transition region emission (many low-lying, short loops), but the weak coronal emission (few high and long loops) in coronal holes. In spite of this contrast our calculations also suggest that a significant fraction of the cool emission in CHs comes from the open flux regions. Despite these insights provided by the magnetic field line statistics further work is needed to obtain a definite answer to the question if loop statistics explain the differences between coronal holes and the quiet Sun.     

Intensity oscillations at the feet of coronal holes

We investigate the regime of chromospheric oscillations at the bases of coronal holes and compare them with the oscillations in the quiet chromosphere outside coronal holes using time series of spectrograms taken at different times in eight quiet regions on the Sun. As the oscillation parameter being studied, we have chosen the central intensity of the chromospheric Ca II K and H and 849.8-nm lines. The intensity measurements at all spatial points (along the spectrograph slit) have been subjected to a standard Fourier analysis. For the identified areas of the networks, cells, and network boundaries, we have calculated the integrated oscillation powers in several frequency bands. For all frequency bands, the powers of the intensity oscillations at the formation level of the Ca II resonance doublet line cores have been found to be enhanced at the bases of coronal holes approximately by a factor of 1.5. For the “three-minute” band, this enhancement is more pronounced in the network than in the cell, while the opposite is true for the “five-minute” band. The power in the five-minute band is higher than that in the three-minute one both at the bases of coronal holes and outside them, but this ratio in the network for a coronal hole is higher (1.40 ± 0.25 and 1.30 ± 0.10). We interpret this fact and the fact that the power of the three-minute oscillations for nonmagnetic regions changes with height differently at the base of a coronal hole and outside it as an increase in the importance of magnetoacoustic portals at the chromospheric base of the coronal hole.     

Acceleration and Storage of Energetic Electrons in Magnetic Loops in the Course of Electric Current Oscillations

A mechanism of electron acceleration and storage of energetic particles in solar and stellar coronal magnetic loops, based on oscillations of the electric current, is considered. The magnetic loop is presented as an electric circuit with the electric current generated by convective motions in the photosphere. Eigenoscillations of the electric current in a loop induce an electric field directed along the loop axis. It is shown that the sudden reductions that occur in the course of type IV continuum and pulsating type III observed in various frequency bands (25?–?180 MHz, 110?–?600 MHz, 0.7?–?3.0 GHz) in solar flares provide evidence for acceleration and storage of the energetic electrons in coronal magnetic loops. We estimate the energization rate and the energy of accelerated electrons and present examples of the storage of energetic electrons in loops in the course of flares on the Sun or on ultracool stars. We also discuss the efficiency of the suggested mechanism as compared with the electron acceleration during the five-minute photospheric oscillations and with the acceleration driven by the magnetic Rayleigh–Taylor instability.     

Global Coronal Seismology

Following the observation and analysis of large-scale coronal-wave-like disturbances, we discuss the theoretical progress made in the field of global coronal seismology. Using simple mathematical techniques we determine average values for the magnetic field together with a magnetic map of the quiet Sun. The interaction between global coronal waves and coronal loops allows us to study loop oscillations in a much wider context, i.e. we connect global and local coronal oscillations.     

Evolution of the Fine Structure of Magnetic Fields in the Quiet Sun: Observations from Sunrise/IMaX and Extrapolations

Observations with the balloon-borne Sunrise/Imaging Magnetograph eXperiment (IMaX) provide high spatial resolution (roughly 100 km at disk center) measurements of the magnetic field in the photosphere of the quiet Sun. To investigate the magnetic structure of the chromosphere and corona, we extrapolate these photospheric measurements into the upper solar atmosphere and analyze a 22-minute long time series with a cadence of 33 seconds. Using the extrapolated magnetic-field lines as tracer, we investigate temporal evolution of the magnetic connectivity in the quiet Sun’s atmosphere. The majority of magnetic loops are asymmetric in the sense that the photospheric field strength at the loop foot points is very different. We find that the magnetic connectivity of the loops changes rapidly with a typical connection recycling time of about 3±1 minutes in the upper solar atmosphere and 12±4 minutes in the photosphere. This is considerably shorter than previously found. Nonetheless, our estimate of the energy released by the associated magnetic-reconnection processes is not likely to be the sole source for heating the chromosphere and corona in the quiet Sun.     

Differentiating Coronal Holes from the Quiet Sun by He 1083 nm Imaging Spectroscopy

The locations of coronal holes are usually based on equivalent-width images in the He i 1083 nm line. However, it is difficult to differentiate coronal holes from the centers of quiet chromospheric network without complementary data and the skill of an experienced observer. Analysis of imaging spectroscopy shows that line half-width and central intensity are correlated differently in coronal holes and a quiet Sun. This fact can be used to form linear combinations of these images in which coronal holes are better separated from the quiet Sun. Coronal hole borders agree well with SOHO/EIT data but can show significant differences from National Solar Observatory maps.     

On midrange periodicities in solar radio flux and sunspot areas

Using the Hilbert-Huang transform technique, we investigate the midrange periodicities in solar radio flux at 2800 MHz (F10.7) and sunspot areas (SAs) from February 1, 1947 to September 30, 2016. The following prominent results are found: (1) The quasi-periodic oscillations of both data sets are not identical, such as the rotational cycle, the midrange periodicities, and the Schwabe cycle. In particular, the midrange periodicities ranging from 37.9 days to 297.3 days are related to the magnetic Rossby-type waves; (2) The 1.3-year and 1.7-year fluctuations in solar activity indicators are surface manifestations (from photosphere to corona) of magnetic flux changes generated deep inside the Sun; (3) At the timescale of the Schwabe cycle, the complicated phase relationships in the three intervals (1947–1958, 1959–1988, and 1989–2016) agree with the produced periodicities of the magnetic Rossby-type waves. The findings indicate that the magnetic Rossby-type waves are the possible physical mechanism behind the midrange periodicities of solar activity indicators. Moreover, the significant change in the relationship between photospheric and coronal activity took place after the maximum of solar cycle 22 could be interpreted by the magnetic Rossby-type waves.     

Multiwavelength Study of a Solar Eruption from AR NOAA 11112: II. Large-Scale Coronal Wave and Loop Oscillation

We analyze multiwavelength observations of an M2.9/1N flare that occurred in AR NOAA 11112 on 16 October 2010. AIA 211 Å EUV images reveal the presence of a faster coronal wave (decelerating from ≈?1390 to ≈?830 km?s^?1) propagating ahead of a slower wave (decelerating from ≈?416 to ≈?166 km?s^?1) towards the western limb. The dynamic radio spectrum from Sagamore Hill radio telescope shows the presence of a metric type II radio burst, which reveals the presence of a coronal shock wave (speed ≈?800 km?s^?1). The speed of the faster coronal wave, derived from AIA 211 Å images, is found to be comparable to the coronal shock speed. AIA 171 Å high-cadence observations showed that a coronal loop, which was located at a distance of ≈?0.32R _⊙ to the west of the flaring region, started to oscillate by the end of the impulsive phase of the flare. The results indicate that the faster coronal wave may be the first driver of the transversal oscillations of coronal loop. As the slower wave passed through the coronal loop, the oscillations became even stronger. There was a plasmoid eruption observed in EUV and a white-light CME was recorded, having velocity of ≈?340?–?350 km?s^?1. STEREO 195 Å images show an EIT wave, propagating in the same direction as the lower-speed coronal wave observed in AIA, but decelerating from ≈?320 to ≈?254 km?s^?1. These observations reveal the co-existence of both waves (i.e. coronal Moreton and EIT waves), and the type II radio burst seems to be associated with the coronal Moreton wave.     

Periodicities of Sunspot Number and Coronal Index Time Series During Solar Cycle 23

We investigate the presence and temporal evolution of short- and intermediate-term periodicities in the daily data of sunspot numbers and coronal index for the time span from May 1996 to December 2008, which covers the entire Solar Cycle 23. The daily sunspot number data have been analyzed for the full disk, and for northern and southern hemispheres of the Sun. Using the wavelet power spectrum technique, we find a number of quasi-periodic oscillations in all the data sets. We also find a prominent period of 22 to 35 days in the high-frequency range, and detect the Rieger period of 150 to 160 days in both data sets during different phases of Cycle 23. We also detect ∼1.3 year oscillation in both sunspot and coronal index time series. In addition, we find a number of other short and mid-term periods. We discuss possible explanations of the observed periodicities in the light of previous results and existing numerical models.     

The Si IV λ 1393 line in a coronal hole compared to the quiet Sun from OSO-8 observations

We report on studies of the 1393 line of Si iv, formed in the transition region at about 80 000 K, made using the Colorado experiment on OSO-8. Results indicate that the line width is somewhat greater in coronal holes compared to the quiet Sun, implying a difference in the broadening mechanism. There is no evidence that the line is Doppler shifted in coronal holes relative to the quiet Sun implying there is no mass flow in holes, at the 80 000 K level, greater than 4.3 km s^–1. Within the uncertainty of our experiment the integrated line intensities are the same in a coronal hole as in the quiet Sun.     

Coronal Holes Versus Normal Quiet Sun Observed with SUMER

We present a preliminary analysis of spectral lines obtained with the SUMER instrument (Solar Ultraviolet Measurements of Emitted Radiation) onboard the Solar and Heliospheric Observatory (SOHO), as observed during three observing campaigns. From the 70 observed spectral lines, we selected 12, representing 9 ions or atoms, in order to analyse line intensities, shifts and widths in polar coronal holes as well as in the normal quiet Sun. We find that coronal lines show a distinct blueshift in coronal holes relative to the quiet Sun at equal heliospheric angle, while there is no evidence for such a shift for lines formed at temperatures below 10⁵K. The widths of lines formed at temperatures above 3 – 10⁴K are slightly increased inside the coronal hole, but unaffected for lower temperatures. Intensity measurements clearly show the center-to-limb variation, as well as an intensity diminution inside the coronal hole for lines formed above approximately 10⁵K. This revised version was published online in July 2006 with corrections to the Cover Date.     

Contrast of Coronal Holes and Parameters of Associated Solar Wind Streams

It is shown that the contrast of coronal holes, just as their size, determines the velocity of the solar wind streams. Fully calibrated EIT images of the Sun have been used. About 450 measurements in 284 Å have been analyzed. The time interval under examination covers about 1500 days in the declining phase of cycle 23. All coronal holes recorded for this interval in the absence of coronal mass ejections (CMEs) have been studied. The comparison with some other parameters (e.g. density, temperature, magnetic field) was carried out. The correlations with the velocity are rather high (0.70?–?0.89), especially during the periods of moderate activity, and could be used for everyday forecast. The contrast of coronal holes is rather small.     

Automated Identification of Coronal Holes from Synoptic EUV Maps

Coronal holes (CHs) are regions of open magnetic field lines in the solar corona and the source of the fast solar wind. Understanding the evolution of coronal holes is critical for solar magnetism as well as for accurate space weather forecasts. We study the extreme ultraviolet (EUV) synoptic maps at three wavelengths (195 Å/193 Å, 171 Å and 304 Å) measured by the Solar and Heliospheric Observatory/Extreme Ultraviolet Imaging Telescope (SOHO/EIT) and the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) instruments. The two datasets are first homogenized by scaling the SDO/AIA data to the SOHO/EIT level by means of histogram equalization. We then develop a novel automated method to identify CHs from these homogenized maps by determining the intensity threshold of CH regions separately for each synoptic map. This is done by identifying the best location and size of an image segment, which optimally contains portions of coronal holes and the surrounding quiet Sun allowing us to detect the momentary intensity threshold. Our method is thus able to adjust itself to the changing scale size of coronal holes and to temporally varying intensities. To make full use of the information in the three wavelengths we construct a composite CH distribution, which is more robust than distributions based on one wavelength. Using the composite CH dataset we discuss the temporal evolution of CHs during the Solar Cycles 23 and 24.     

The anomalous intensities of helium lines in a coronal hole

Observations made at the quiet Sun-centre with the Coronal Diagnostic Spectrometer (CDS) and Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instruments on the Solar and Heliospheric Observatory ( SOHO ) have shown that the intensities of the resonance lines of He  i and He  ii are significantly larger than predicted by emission measure distributions found from other transition region lines. The intensities of the helium lines are observed to be lower in coronal holes than in the quiet Sun. Any theory proposed to account for the behaviour of the helium lines must explain the observations of both the quiet Sun and coronal holes. We use observations made with SOHO to find the physical conditions in a polar coronal hole. The electron pressure is found using the C  iii 1175-Å and N  iii 991.5-Å lines, as the C  iii line at 977.0 Å becomes optically thick in some regions at high latitudes. The mean electron pressure is a factor of ≃2 lower than that at the quiet Sun-centre. The mean coronal electron temperature is     . The helium lines are enhanced with respect to other transition region lines but by factors which are ≃ 30 per cent smaller than at the quiet Sun-centre. The mean ratios of the intensities of the He  i 537.0- and 584.3-Å lines and of the He  i and He  ii 303.8-Å lines vary little with the type of region studied. These ratios are compared with those predicted by models of the transition region, taking into account the radiative transfer in the helium lines. No significant variation is found in the relative abundances of carbon and silicon.     

The Whole Heliosphere Interval in the Context of a Long and Structured Solar Minimum: An Overview from Sun to Earth

Throughout months of extremely low solar activity during the recent extended solar-cycle minimum, structural evolution continued to be observed from the Sun through the solar wind and to the Earth. In 2008, the presence of long-lived and large low-latitude coronal holes meant that geospace was periodically impacted by high-speed streams, even though solar irradiance, activity, and interplanetary magnetic fields had reached levels as low as, or lower than, observed in past minima. This time period, which includes the first Whole Heliosphere Interval (WHI 1: Carrington Rotation (CR) 2068), illustrates the effects of fast solar-wind streams on the Earth in an otherwise quiet heliosphere. By the end of 2008, sunspots and solar irradiance had reached their lowest levels for this minimum (e.g., WHI 2: CR 2078), and continued solar magnetic-flux evolution had led to a flattening of the heliospheric current sheet and the decay of the low-latitude coronal holes and associated Earth-intersecting high-speed solar-wind streams. As the new solar cycle slowly began, solar-wind and geospace observables stayed low or continued to decline, reaching very low levels by June??C?July 2009. At this point (e.g., WHI 3: CR 2085) the Sun?CEarth system, taken as a whole, was at its quietest. In this article we present an overview of observations that span the period 2008??C?2009, with highlighted discussion of CRs 2068, 2078, and 2085. We show side-by-side observables from the Sun??s interior through its surface and atmosphere, through the solar wind and heliosphere and to the Earth??s space environment and upper atmosphere, and reference detailed studies of these various regimes within this topical issue and elsewhere.     

Investigation of Quasi-periodic Solar Oscillations in Sunspots Based on SOHO/MDI Magnetograms

In this work we study quasi-periodic solar oscillations in sunspots, based on the variation of the amplitude of the magnetic field strength and the variation of the sunspot area. We investigate long-period oscillations between three minutes and ten hours. The magnetic field synoptic maps were obtained from the SOHO/MDI. Wavelet (Morlet), global wavelet spectrum (GWS) and fast Fourier transform (FFT) methods are used in the periodicity analysis at the 95?% significance level. Additionally, the quiet Sun area (QSA) signal and an instrumental effect are discussed. We find several oscillation periods in the sunspots above the 95?% significance level: 3??C?5, 10??C?23, 220??C?240, 340 and 470 minutes, and we also find common oscillation periods (10??C?23 minutes) between the sunspot area variation and that of the magnetic field strength. We discuss possible mechanisms for the obtained results, based on the existing models for sunspot oscillations.     

The quiet sun brightness temperature at 408 MHz

The flux of the radio quiet Sun and the brightness temperature at 408 MHz (73 cm) are derived from measurements with the E-W Nançay interferometer and the E-W arm of the Medicina North Cross. It is shown that the lowest envelopes, which defined the radio quiet Sun, correspond to transits of extended coronal holes across the disk of the Sun.     

Statistical Analysis of the Relationships among Coronal Holes,Corotating Interaction Regions,and Geomagnetic Storms

We have examined the relationships among coronal holes (CHs), corotating interaction regions (CIRs), and geomagnetic storms in the period 1996?–?2003. We have identified 123 CIRs with forward and reverse shock or wave features in ACE and Wind data and have linked them to coronal holes shown in National Solar Observatory/Kitt Peak (NSO/KP) daily He i 10?830 Å maps considering the Sun?–?Earth transit time of the solar wind with the observed wind speed. A sample of 107 CH?–?CIR pairs is thus identified. We have examined the magnetic polarity, location, and area of the CHs as well as their association with geomagnetic storms (Dst≤?50 nT). For all pairs, the magnetic polarity of the CHs is found to be consistent with the sunward (or earthward) direction of the interplanetary magnetic fields (IMFs), which confirms the linkage between the CHs and the CIRs in the sample. Our statistical analysis shows that (1) the mean longitude of the center of CHs is about 8°E, (2) 74% of the CHs are located between 30°S and 30°N (i.e., mostly in the equatorial regions), (3) 46% of the CIRs are associated with geomagnetic storms, (4) the area of geoeffective coronal holes is found to be larger than 0.12% of the solar hemisphere area, and (5) the maximum convective electric field E _y in the solar wind is much more highly correlated with the Dst index than any other solar or interplanetary parameter. In addition, we found that there is also a semiannual variation of CIR-associated geomagnetic storms and discovered new tendencies as follows: For negative-polarity coronal holes, the percentage (59%; 16 out of 27 events) of CIRs associated with geomagnetic storms in the first half of the year is much larger than that (25%; 6 out of 24 events) in the second half of the year and the occurrence percentage (63%; 15 out of 24 events) of CIR-associated storms in the southern hemisphere is significantly larger than that (26%; 7 out of 27 events) in the northern hemisphere. Positive-polarity coronal holes exhibit an opposite tendency.     

A Phenomenological Study of the Cosmic Ray Variations over the Past 9400 Years, and Their Implications Regarding Solar Activity and the Solar Dynamo

Two 9400-year long ¹⁰Be data records from the Arctic and Antarctic and a ¹⁴C record of equal length were used to investigate the periodicities in the cosmic radiation incident on Earth throughout the past 9400 years. Fifteen significant periodicities between 40 and 2320 years are observed in the ¹⁰Be and ¹⁴C records, there being close agreement between the periodicities in each record. We found that the periodic variations in the galactic cosmic radiation are the primary cause for periods <?250 years, with minor contributions of terrestrial origin possible >?250 years. The spectral line for the Gleissberg (87-year) periodicity is narrow, indicating a stability of ≈?0.5 %. The 9400-year record contains 26 Grand Minima (GM) similar to the Maunder Minimum, most of which occurred as sequences of 2?–?7 GM with intervals of 800?–?1200 years in between, in which there were no GM. The intervals between the GM sequences are characterised by high values of the modulation function. Periodicities <?150 years are observed in both the GM intervals and the intervals in between. The longer-period variations such as the de Vries (208-year) cycle have high amplitudes during the GM sequences and are undetectable in between. There are three harmonically related pairs of periodicities (65 and 130 years), (75 and 150 years), and (104 and 208 years). The long periodicities at 350, 510, and 708 years closely approximate 4, 6, and 8 times the Gleissberg period (87 years). The well-established properties of cosmic-ray modulation theory and the known dependence of the heliospheric magnetic field on the solar magnetic fields lead us to speculate that the periodicities evident in the paleo-cosmic-ray record are also present in the solar magnetic fields and in the solar dynamo. The stable, narrow natures of the Gleissberg and other periodicities suggest that there is a strong “frequency control” in the solar dynamo, in strong contrast to the variable nature (8?–?15 years) of the Schwabe (11-year) solar cycle.