North Polar Coronal Hole Research Articles

THE ROLE OF MHD WAVES IN HEATING OF THE SOLAR CORONA

Observations in the Solar North Polar Coronal Hole (NPCH) show that this region has high temperature values (10^6-10^8 K). At this temperature, the coronal plasma loses energy to the transition region below through heat conduction and optically thin emission. It is still a matter of debate how to replace this lost energy in the solar corona and how to maintain the observed temperature values. In this study, we aimed to study the wave theory, which is an important model proposed for the heating problem of the solar corona in NPCH. We assumed a model based on Alfvén/ion cyclotron resonance process with O VI ions by using quasi-linear approximation in NPCH and solve the magnetohydrodynamic (MHD) equations for O VI assuming that the non-thermal contribution to the temperature. Using a Matlab code, we performed 3D MHD numerical solutions of Alfvén waves. Our results show that the damping length scales (0.2-1.8 R) and energy flux densities (10^5-10^7 erg/cm^2 s) of Alfvén waves are similar for both plumes and interplumes in NPCH. As a result of our study, we present the contribution of MHD waves that will cause the acceleration of the solar wind and the heating of the solar corona.

A new model for heating of the Solar North Polar Coronal Hole

This article presents a new model of the North Polar Coronal Hole (NPCH) with the aim of revealing the dissipative/propagative characteristics of magnetohydrodynamic (MHD) waves. We investigate the ...

Diagnostics of a Coronal Hole and the Adjacent Quiet Sun by The Hinode/EUV Imaging Spectrometer (EIS)

A comparison between a Coronal Hole (CH) and the adjacent Quiet-Sun (QS) has been performed using spectroscopic diagnostics of Hinode/ the EUV Imaging Spectrometer (EIS). Coronal funnels play an important role in the formation and propagation of the nascent fast solar wind. Applying Gaussian fitting procedures to the observed line profiles, Doppler velocity, intensity, line width (FWHM) and electron density have been estimated over CH and adjacent QS region of a North Polar Coronal Hole (NPCH). The aim of this study is to identify the coronal funnels based on spectral signatures. Excess width regions (excess FWHM above a threshold level) have been identified in QS and CH. The plasma flow inversion (average red-shifts changing to blue-shifts at a specific height) in CH and excess width regions of QS take place at ~ 5.01$\times$10$^{5}$ K. Furthermore, high density concentration in excess width regions of QS provides an indication that these regions are the footprints of coronal funnels. We have also found that non-thermal velocities of CH are higher in comparison to QS confirming that the CHs are the source regions of fast solar wind. Doppler and non-thermal velocities as recorded by different temperature lines have been also compared with previously published results. As we go from lower to upper solar atmosphere, down-flows are dominated in lower atmosphere while coronal lines are dominated by up-flows with a maximum value of ~ 10-12 km s$^{-1}$ in QS. Non-thermal velocity increases first but after Log T$_{e}$ = 5.47 it decreases further in QS. This trend can be interpreted as a signature of the dissipation of Alfv\'en waves, while increasing trend as reported earlier may attribute to the signature of the growth of Alfv\'en waves at lower heights. Predominance of occurrence of nano-flares around O {\sc vi} formation temperature could also explain non-thermal velocity trend.

CHARGE STATE EVOLUTION IN THE SOLAR WIND. III. MODEL COMPARISON WITH OBSERVATIONS

We test three theoretical models of the fast solar wind with a set of remote sensing observations and in-situ measurements taken during the minimum of solar cycle 23. First, the model electron density and temperature are compared to SOHO/SUMER spectroscopic measurements. Second, the model electron density, temperature, and wind speed are used to predict the charge state evolution of the wind plasma from the source regions to the freeze-in point. Frozen-in charge states are compared with Ulysses/SWICS measurements at 1 AU, while charge states close to the Sun are combined with the CHIANTI spectral code to calculate the intensities of selected spectral lines, to be compared with SOHO/SUMER observations in the north polar coronal hole. We find that none of the theoretical models are able to completely reproduce all observations; namely, all of them underestimate the charge state distribution of the solar wind everywhere, although the levels of disagreement vary from model to model. We discuss possible causes of the disagreement, namely, uncertainties in the calculation of the charge state evolution and of line intensities, in the atomic data, and in the assumptions on the wind plasma conditions. Last, we discuss the scenario where the wind is accelerated from a region located in the solar corona rather than in the chromosphere as assumed in the three theoretical models, and find that a wind originating from the corona is in much closer agreement with observations.

Ion–Cyclotron Resonance Frequency Interval Dependence on the O VI Ion Number Density in the North Polar Coronal Hole 1.5R–3R Region

The frequency intervals in which O VI ions get in resonance with ion–cyclotron waves are calculated using the kinetic model, for the latest six values found in literature on O VI ion number densities in the 1.5R–3R region of the NPCH. It is found that the common resonance interval is 1.5 kHz to 3 kHz. The R-variations of wave numbers necessary for the above calculations are evaluated numerically, solving the cubic dispersion relation with the dielectric response derived from the quasi-linear Vlasov equation for the left-circularly polarized ion-cyclotron waves.

Investigation of diffusivity and viscosity in solar plasma

For diffusive and viscous plasma, the dispersion relation ω2 = k 2 [v 2 − iω(ν + η)] + νηk 4 is applied for the North Polar Coronal Hole, where we have assumed the angular frequency ω to be a real quantity and the wave number k as a complex quantity. For ω = 2π/τ, we have chosen three values 10−2, 10−3 and 10−4 s for τ. For each value of τ, we have considered three situations: (i) where ν = 0 (no viscosity), (ii) where η = 0 (no magnetic diffusivity) and (iii) where both the diffusivity η and viscosity ν are present. For the cases (i) and (ii), we have obtained two solutions, ±(k r + i k i ). But, for the case (iii), we have obtained two pairs of solutions, ±(k r1 + i k i1) and ±(k r2 + i k i2). These two pairs correspond to the fast-mode and slow-mode waves.

Observations of a pulse-driven cool polar jet by SDO/AIA

Context. We observe a solar jet at north polar coronal hole (NPCH) using SDO AIA 304 {\deg}A image data on 3 August 2010. The jet rises obliquely above the solar limb and then retraces its propagation path to fall back. Aims. We numerically model this observed solar jet by implementing a realistic (VAL-C) model of solar temperature. Methods. We solve two-dimensional ideal magnetohydrodynamic equations numerically to simulate the observed solar jet. We consider a localized velocity pulse that is essentially parallel to the background magnetic field lines and initially launched at the top of the solar photosphere. The pulse steepens into a shock at higher altitudes, which triggers plasma perturbations that exhibit the observed features of the jet. The typical direction of the pulse also clearly exhibits the leading front of the moving jet. Results. Our numerical simulations reveal that a large amplitude initial velocity pulse launched at the top of the solar photosphere produces in general the observed properties of the jet, e.g., upward and backward average velocities, height, width, life-time, and ballistic nature. Conclusions. The close matching between the jet observations and numerical simulations provides first strong evidence for the formation of this jet by a single velocity pulse. The strong velocity pulse is most likely generated by the low- atmospheric reconnection in the polar region which results in triggering of the jet. The downflowing material of the jet most likely vanishes in the next upcoming velocity pulses from lower solar atmosphere, and therefore distinctly launched a single jet upward in the solar atmosphere is observed.

The effect of plume/interplume lanes on ion-cyclotron resonance heating

AbstractThe effect of the Plume/Interplume Lane (PIPL) structure of the solar North Polar Coronal Hole (NPCH) on the ion-cyclotron resonance (ICR) process is investigated. The ICR process in the interplume lanes is much more effective than in the plumes, agreeing with the observations which show the source of fast solar wind is interplume lanes.

MHD waves in the solar north polar coronal hole

AbstractThe effects, hitherto not treated, of the temperature and the number density gradients, both in the parallel and the perpendicular direction to the magnetic field, of O VI ions, on the MHD wave propagation characteristics in the solar North Polar Coronal Hole are investigated. We investigate the magnetosonic wave propagation in a resistive MHD regime where only the thermal conduction is taken into account. Heat conduction across the magnetic field is treated in a non‐classical approach wherein the heat is assumed to be conducted by the plasma waves emitted by ions and absorbed at a distance from the source by other ions. Anisotropic temperature and the number density distributions of O VI ions revealed the chaotic nature of MHD standing wave, especially near the plume/interplume lane borders. Attenuation length scales of the fast mode is shown not to be smoothly varying function of the radial distance from the Sun (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A Study of Polar Jet Parameters Based on Hinode XRT Observations

Abstract Hinode/SOHO campaign 7197 is the most extensive study of polar jet formation and evolution from within both the north and south polar coronal holes so far. For the first time, this study showed that the appearance of X-ray jets in the solar coronal holes occurs at very high frequency–about 60 jets d$^{-1}$ on average. Using observations collected by the X-Ray Telescope on Hinode, a number of physical parameters from a large sample of jets were statistically studied. We measured the apparent outward velocity, the height, the width and the lifetime of the jets. In our sample, all of these parameters show peaked distributions with maxima at 160 km s$^{-1}$ for the outward velocity, 5$\times$10$^4$km for the height, 8$\times$10$^3$km for the width, and about 10 min for the lifetime of the jets. We also present the first statistical study of jet transverse motions, which obtained transverse velocities of 0–35 km s$^{-1}$. These values were obtained on the basis of a larger (in terms of frequency) and better sampled set of events than what was previously statistically studied (Shimojo et al. 1996, PASJ, 48, 123). The results were made possible by the unique characteristics of XRT. We describe the methods used to determine the characteristics and set some future goals. We also show that despite some possible selection effects, jets preferably occur inside the polar coronal holes.

Polar Coronal Holes During Solar Cycles 22 and 23

Data from the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses and synoptic maps from Kitt Peak are used to analyze the polar coronal holes of solar activity cycles 22 and 23 (from 1990 to end of 2003). In the beginning of the declining phase of solar cycles 22 and 23, the north polar coronal holes (PCHs) appear about one year earlier than the ones in the south polar region. The solar wind velocity and the solar wind ionic charge composition exhibit a characteristic dependence on the solar wind source position within a PCH. From the center toward the boundary of a young PCH, the solar wind velocity decreases, coinciding with a shift of the ionic charge composition toward higher charge states. However, for an old PCH, the ionic charge composition does not show any obvious change, although the latitude evolution of the velocity is similar to that of a young PCH.

Chromospheric Internetwork Oscillations at Various Locations of the Quiet Sun

We analyze oscillation behaviours in chromospheric internetwork regions using spectral observations of the CII 1334 angstrom line obtained with the Solar Ultraviolet Measurements of Emitted Radiation spectrograph (SUMER) aboard Solar and Heliospheric Observatory (SOHO). Three areas, 26 x 120 arcsec(2) each, at the various latitudes from the disk center to the north polar coronal hole, were rastered with a cadence of about 40-60 s in the solar minimum year. We obtained the time evolution of two-dimensional (2D) line intensity, continuum and line core shift. The continuum and the line shift show similar to 3 min chromospheric oscillations in the internetwork regions underlying the coronal hole as well as at the disk center: We find that the CII 1334 angstrom line shift oscillates with an average speed of similar to 1.7 km s(-1), independent of the latitude, while its coherent scale decreases with latitude. On the other hand, the oscillation amplitude of the continuum around the 1334 angstrom and the phase delay between the Doppler shift and continuum slightly increase with latitude.

The Nascent Solar Wind: Origin and Acceleration

High-speed solar wind is known to originate in polar coronal holes, which, however, are made up of two components: bright, high-density regions known as and dark, weakly emitting low-density regions known as interplumes. Recent space observations have shown that the width of UV lines is larger in interplume regions, while observations of the ratio of the O VI doublet lines at 1032 and 1037 A, at 1.7 solar radii, suggest higher outflows in interplume regions than in plumes at that altitude. These results favor interplume regions as sources of the fast solar wind. The present work aims at investigating the outflow speed versus altitude properties of the O VI and H I ions, at heights below 2 solar radii, in both plumes and interplume regions. To this end, we examined Solar Ultraviolet Measurement of Emitted Radiation (SUMER) and Ultraviolet Coronagraph Spectrometer (UVCS) observations of a north polar coronal hole taken on 1996 June 3, over the altitude range between 1 and 2 solar radii, and through a Doppler dimming analysis of our data, we show that interplume areas may be really identified as sources of fast wind streams. The behavior of plumes, on the contrary, can be interpreted in terms of static structures embedded in the interplume ambient. We conclude by comparing our results with the predictions of theoretical models of the solar wind and giving an empirical estimate of the heating rate, per particle, for H I and O VI ions in interplume regions at 1.75 and 2.0 solar radii.

Variation of coronal line widths on and off the disk

We present observations of a Mg x 625 Å coronal line, obtained with the cds instrument on SoHO, extending from the disk part of the coronal hole to ~90 000 km above the limb in the north polar coronal hole. Observations were performed in polar plumes and inter-plume lanes. To obtain a sufficiently high signal-to-noise ratio the observations were made over long periods of time and subsequent time frames were summed up. For the off-limb observations we notice a turnover point, around 65 000 km above the limb, where the line widths seem to suddenly decrease or level-off. The initial linear increase of line width with altitude supports previous observations and is consistent with an interpretation of linear undamped Alfvèn waves propagating outwards in open field regions. The turnover point seems to indicate the location where a change of physics takes place. We find that this turnover occurs at approximately the same line width value for each of the datasets examined, suggesting that the turnover occurs whenever the non-thermal velocity reaches a certain key velocity. For the on-disk data we find larger widths in the deep coronal hole as compared to the adjacent quiet Sun regions, suggesting the presence of additional waves and/or turbulence in the coronal hole.

Alfven waves in the inner polar coronal hole

We investigate the Alfven wave propagation characteristics in the inner zone (r < 1.35R� ) of the North Polar Coronal Hole. The linear, incompressible magnetohydrodynamic (MHD) approximation reveals two wave modes with different phase velocities. Alfven wave with the slower phase velocity is quickly damped; in other words it cannot propagate in the coronal plasma. On the other hand, the higher phase velocity Alfven waves can propagate along the magnetic field lines. Mechanical energy flux density of these waves is found to be high enough to replace the coronal energy which is lost via optically thin emission and through heat conduction to the chromosphere below. We show that Alfven waves propagating along the magnetic flux tubes go through refraction and get damped via viscous dissipation and resistivity. The energy flux density of the waves is of the order of 10 6 erg cm −2 s −1 . The radial profile of the energy flux density in the region 1.05-1.35R shows a rise until about 1.15R where it peaks and further on declines less steeply than rising arm. This result is used to interpret Si VIII data taken by the Solar Ultraviolet Measurements of Emitted Radiation instrument on board Solar and Heliospheric Observatory. Si VIII lines like many other ionic lines show non- thermal broadening. The observed radial profile of the non-thermal line-of-sight velocity of Si VIII shows a plateau which coincides with the peak of the energy flux density we derived. We claim that this coincidence is not accidental but indicates the fact that the decrease in the energy flux density of damped Alfven waves causes the plateau formation in Si VIII non-thermal line-of-sight velocity profile.

The Sun at solar minimum: North ‐ south asymmetry of the polar coronal holes

Data from the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses and synoptic charts derived from Kitt Peak magnetograms are used to compare the south and north polar coronal holes which existed during the declining/minimum phase of the solar activity cycle from 1992 to 1997. The kinetic properties of the solar wind emanating from the two polar coronal holes, as represented by solar wind speed, do not differ significantly. However, the electron temperature in the two coronal holes inferred from ionic charge composition data, namely the O7+/O6+ ratio, show consistent differences, with the south polar hole being 10 to 15% hotter. The ground‐based magnetograms show that the north polar coronal hole covers a larger part of the solar surface than the southern one. The total magnetic flux and, specifically, the flux density of the north polar coronal hole is considerably lower for the whole interval of time between 1992 and 1997. This strongly indicates that the difference in coronal hole temperature between the southern and northern coronal hole is intrinsic and is not due to the fact that the Ulysses observations in the south and north coronal hole streams were made at different phases of the solar cycle. Thus the differences found represent a real north‐south asymmetry during this time period.

Properties of Solar Polar Coronal Hole Plasmas Observed above the Limb

We determine the line-of-sight emission measure distribution and nonthermal motions as a function of height above the limb in the north and south polar coronal holes. These quantities are derived from extreme-ultraviolet (EUV) spectra obtained from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO) spacecraft. The SUMER slit was oriented along the north-south direction for all the observations, and the spatial resolution is about 1''. The spectra were obtained from a number of different types of observations in 1996. We select a group of emission lines for analysis for which, under the usual assumption of ionization equilibrium, the maximum emissivities span the temperature range from about 3 × 105 K up to about 1.1 × 106 K. We compare our results with recently published similar observations of a west limb quiet-Sun streamer region, with other coronal hole results based on SUMER spectra, and with earlier observations of the quiet Sun and coronal holes obtained from Skylab and rocket spectra. We find that the electron temperature in the polar holes increases with height above the limb, that the emission measure distribution of plasma located at line-of-sight heights less than about 60'' peaks at a temperature of about 9 × 105 K, and that nonthermal motions sometimes, but not always, increase slightly with height above the limb. When observed, these increases level off above the limb at about 120''. We speculate that the increases with height above the limb may be a manifestation of the fast solar wind. They may also be due to the reduction in transition region structures with increasing limb height. We also discuss wave heating as a cause of the line width increases.

The Relationship of Solar Abundance Measurements to the Electron Temperature in a Polar Coronal Hole

We discuss the behavior of the intensity of the Mg VI λ1191.64 spectral line relative to the intensity of the Ne VI λ1005.78 spectral line as a function of height above the limb in the solar north polar coronal hole. The intensities of Mg VI lines relative to Ne VI lines have been shown to be excellent indicators of element abundance variations due to the first ionization potential (FIP) effect. We find that the Mg VI/Ne VI intensity ratio increases with height above the limb by factors ranging from 1.7 to 4 over a height range extending from about 6'' above the limb to 28'' above the limb. We conclude that this intensity ratio increase is primarily due to an increase of electron temperature with height, rather than the result of an FIP effect, and therefore caution must be exercised in using any Mg VI/Ne VI line ratio as an abundance diagnostic above the limb in the polar holes. At 6'' above the limb, the Mg VI/Ne VI line ratio indicates that the solar Mg/Ne abundance ratio is probably within a factor of 2 of the photospheric abundance ratio. The spectra we use were recorded by the Solar Ultraviolet Measurements of Emitted Radiation spectrometer on the Solar and Heliospheric Observatory spacecraft.

Polar Plumes and Inter-plume regions as observed by SUMER on SOHO

We present observations of O vi 1032 A line profiles obtained with the SUMER instrument on SOHO extending from the solar disk to 1.5 R⊙ above the limb in the north polar coronal hole. Variations of the intensity and linewidth in the polar plume and inter-plume regions are investigated. We find an anti-correlation between the intensity and the linewidth in the plume and inter-plume regions with detailed plume structures been seen out to 1.5 R⊙. Possible implications regarding the magnetic topologies of these two regions and related heating mechanisms are discussed. The O vi linewidth measurements are combined with UVCS output to provide an overview of its variations with height extending up to 3.5 R⊙. We find a linear increase of the linewidth from 1 to 1.2 R⊙, then a plateau followed by a sharp increase around 1.5 R⊙.

Long-Period Oscillations in Polar Plumes as Observed by cds on Soho

We examine spectral time series of the transition region line O v 629A, observed with the Coronal Diagnostic Spectrometer (CDS) on the SOHO spacecraft in July 1997. Both Fourier and wavelet transforms have been applied independently to the analysis of plume oscillations in order to find the most reliable periods. The wavelet analysis allows us to derive the duration as well as the periods of the oscillations. Our observations indicate the presence of compressional waves with periods of 10–25 min. We have also detected a 11±1 min periodicity in the network regions of the north polar coronal hole. The waves are produced in short bursts with coherence times of about 30 min. We interpret these oscillations as outward propagating slow magneto-acoustic waves, which may contribute significantly to the heating of the lower corona by compressive dissipation and which may also provide enough energy flux for the acceleration of the fast solar wind. The data support the idea that the same driver is responsible for the network and plume oscillations with the network providing the magnetic channel through which the waves propagate upwards from the lower atmosphere to the plumes.