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He I 10830 A spectroheliograms obtained during 1974-1979 were used to study the evolution of the Sun's polar coronal holes. Synoptic polar plots show that the holes have decreased in size to the point that only vestigual holes remain and even these remnants fluctuate with detailed sunspot activity.
A small blowout jet was observed at the boundary of the south coronal hole on 2011 February 8 at around 21:00 UT. Images from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) revealed an expanding loop rising from one footpoint of a compact, bipolar bright point. Magnetograms from the Helioseismic Magnetic Imager (HMI) on board SDO showed that the jet was triggered by the cancelation of a parasitic positive polarity feature near the negative pole of the bright point. The jet emission was present for 25 mins and it extended 30 Mm from the bright point. Spectra from the EUV Imaging Spectrometer on board Hinode yielded a temperature and density of 1.6 MK and 0.9-1.7 × 10( exp 8) cu cm for the ejected plasma. Line-of-sight velocities reached up to 250 km/s. The density of the bright point was 7.6 × 10(exp 8) cu cm, and the peak of the bright point's emission measure occurred at 1.3 MK, with no plasma above 3 MK.
Utilizing an iterative scheme, a self-consistent axisymmetric MHD model for the solar wind has been developed. We use this model to evaluate the properties of the solar wind issuing from the open polar coronal hole regions of the Sun, during solar minimum. We explore the variation of solar wind parameters across the extent of the hole and we investigate how these variations are affected by the geometry of the hole and the strength of the field at the coronal base.
We present evidence for significant reflection of Alfven waves in an isothermal, hydrostatic model corona and that heating in coronal holes is provided by Alfven waves. For Alfven waves with periods of 5 min, upward propagating waves are reflected if the temperature is less than 10 exp 6 K, but escape into the solar wind if the temperature is greater than 10 exp 6 K. This sensitive temperature dependence may provide the self-limiting mechanism that has been suspected to exist because the reflected waves result in heating which raises the temperature which, in turn, decreases the reflection. The reflection occurs mostly inside of about 6 solar radii, depending on temperature, wave period, and magnetic field strength and geometry. The importance of this process has often been overlooked due to a poor choice of coronal Alfven speed and temperature. SOHO is well-suited to measure whether the required properties for reflection exist. Solar Probe, however, is the only definitive experiment to show if the waves actually exist to the degree necessary.
He I 10830 A synoptic maps, obtained at the Kitt Peak National Observatory during 1974-1979, show that the sun's polar coronal holes have contracted significantly during 1977-1978. Prior to the accelerated increase of sunspot activity in mid-1977, the area of each polar cap was on the order of 8% of the sun's total surface area, whereas toward the end of 1978 these areas fell below 2% of the total surface area. Synoptic polar plots show that the vestigial holes had irregular shapes and were often well removed from the poles themselves. These results are consistent with the changes that one would expect when the polar magnetic fields are weakening just prior to sunspot maximum
Observations with a white-light coronagraph aboard Skylab are used to determine the boundaries of a coronal hole in the northern polar region and the three-dimensional density structure within the hole between heights of 2 and 5 solar radii. The boundary of the hole is found to be essentially axisymmetric about the polar axis, nearly radial from 3 to 6 solar radii, and located near 25 deg latitude at these heights. The radiances arising from the hole are interpreted as resulting from an axisymmetric density distribution whose logarithmic radial gradient is independent of position within the hole and whose magnitude increases with angular distance away from the hole's axis. The velocity distribution within the hole is obtained from the continuity equation by assuming that the particle flux flowing outward in the hole is similar to that measured for high-speed solar-wind streams at 1 AU, and it is shown that the transition from subsonic to supersonic flow occurs between 2.2 and 3 solar radii.
EUV (Fe XV at 284 A) and radio (at 169 and 408 MHz) observations were made of the coronal hole on May 31, 1973. An inhomogeneous model consisting of hot (a temperature of about 2 x 10 to the 6th K) elements covering 10% of the hole surface surrounded by regions of colder gas (a temperature of about 8 x 10 to the 5th K) is able to explain both observations.
We examined both X-ray and Magnetic field data in order to determine if there is a correlation between emerging magnetic flux and the production of Coronal jets. It was proposed that emerging flux can be a trigger to a coronal jet. The jet is thought to be caused when local bipoles reconnect or when a region of magnetic polarity emerges through a uniform field. In total we studied 15 different jets that occurred over a two day period starting 2011-02-27 00:00:00 UTC and ending 2011-02-28 23:59:55 UTC. All of the jets were contained within a coronal hole that was centered on the disk. Of the 15 that we studied 6 were shown to have an increase of magnetic flux within one hour prior to the creation of the jet and 10 were within 3 hours before the event.
Solar limb-brightening curves are discussed for XUV spectral lines formed in the upper chromosphere and transition zone of a quiet region and a polar coronal hole. The spectra were recorded with a slit spectrograph on Skylab. The lines considered are emitted from ions formed within the temperature range from 10,000 to 220,000 K. The limb-brightening curves cover a region from -4 sec within the limb to +20 sec above it. The data from 0 sec to +20 sec are compared with predictions based on both homogeneous and inhomogeneous models of the transition zone. The limb-brightening curve of the O I line at 1355.6 A indicates that O I is formed in spicules. The limb brightening of the He II line at 1640.4 A is consistent with a temperature of formation between about 40,000 and 90,000 K for He II.
Analysis of 650 microphotometric scans across the solar limb reveals that the H(alpha) chromosphere is slightly taller inside coronal holes than in quiet regions outside holes. The change in height occurs as a step at the hole boundaries; this suggests that the increase with latitude in the average height of spicules found by Lippincott and by Athay was the average result of upward steps at the polar hole boundaries rather than a gradual latitude trend. It is estimated that the power consumed by spicules is of the same order as that returning by conduction from the corona, but the bulk of the spicules (which sets the height of the chromosphere) shows almost no response. It is concluded that spicules are not caused by heat conduction from the corona but are driven from below, suggesting that spicules are more closely connected with the heating of the corona than with its cooling.
Observational data relating to the evolution of the polar magnetic field around sunspot maximum is examined. Particular emphasis is given to coronal hole observations performed during the last two solar maxima. Long-term averages of the latitudinal dependence of the photospheric magnetic field and the evolutionary pattern of the polar crown filaments are used to trace the poleward motion of the reversal of the large-scale field and are compared to the redevelopment of polar holes. Within the context of phenomenological models of the solar cycle, it is concluded that: (1) the process of polarity reversal and redevelopment of polar holes is discontinuous, with surges of flux of old-cycle polarity interrupting the poleward migration of new-cycle flux; (2) contrary to the Babcock (1961) hypothesis, the polar crown disappears months after the magnetic pole reversal; and (3) the observations support suggestions of a poleward meridional flow around solar maximum that cannot be accounted for by Leighton-type (1964) diffusion.
The intensities and timescales of gradual solar energetic particle (SEP) events at 1 AU may depend not only on the characteristics of shocks driven by coronal mass ejections (CMEs), but also on large-scale coronal and interplanetary structures. It has long been suspected that the presence of coronal holes (CHs) near the CMEs or near the 1-AU magnetic footpoints may be an important factor in SEP events. We used a group of 41 E (is) approx. 20 MeV SEP events with origins near the solar central meridian to search for such effects. First we investigated whether the presence of a CH directly between the sources of the CME and of the magnetic connection at 1 AU is an important factor. Then we searched for variations of the SEP events among different solar wind (SW) stream types: slow, fast, and transient. Finally, we considered the separations between CME sources and CH footpoint connections from 1 AU determined from four-day forecast maps based on Mount Wilson Observatory and the National Solar Observatory synoptic magnetic-field maps and the Wang-Sheeley-Arge model of SW propagation. The observed in-situ magnetic-field polarities and SW speeds at SEP event onsets tested the forecast accuracies employed to select the best SEP/CH connection events for that analysis. Within our limited sample and the three analytical treatments, we found no statistical evidence for an effect of CHs on SEP event peak intensities, onset times, or rise times. The only exception is a possible enhancement of SEP peak intensities in magnetic clouds.
Skylab and ground-based observations in the extreme UV range of the spectrum are used to analyze characteristics of coronal holes, and to correlate their development with variations in the solar cycle. The mergence of bipolar magnetic regions (BMR) during the declining phase of the cycle is held reponsible for the formation of large equatorial holes (M-regions, which cause recurring geomagnetic storms). The BMR mergence model can also be applied to polar cap holes.
White-light measurements made by the Mk III Mauna Loa K-coronameter and the SOHO LASCO C2 and C3 coronagraphs, extending from 1.15 to 30 Ro, have been combined to show that the boundaries of polar coronal holes, as determined by measurements of path-integrated density, extend approximately radially into interplanetery space.
A description is presented of a sequence of one-dimensional fluid flow models of the transition zone and the inner corona. A hydrostatic model atmosphere in reasonable agreement with observations of closed, large-scale coronal structures found in the quiet sun is considered and various physical effects are introduced, one at a time, observing the response of the model. As a result of the investigations, a model is developed of the plasma flow in a coronal hole. It is shown that the data severely circumscribe the allowable range of possible models.
Recent results concerning streams and magnetic fields in the inner solar system are reviewed. Observations have shown that MHD streams are bounded by thin shear layers within 1 AU, probably because they originate in coronal holes which have sharp boundaries. The properties of Alfvenic fluctuations in streams cannot be fully explained on the basis of the hypothesis that they are plane, transverse Alfven waves. A more complete and accurate description might be that they represent nonplanar general Alfven waves weakly coupled to a compressive mode and moving through a medium containing tangential discontinuities and other convected inhomogeneities.
This letter presents measurements of small Doppler shifts in the line center position of 629A OV obtained with a new high-resolution EUV spectrometer flown aboard a sounding rocket. A major result is the detection of an apparent systematic outflow (relative to the quiet sun) of approximately 3 km/s average and 5 km/s maximum in the solar transition region underlying a well-defined low-latitude coronal hole. This is reminiscent of a similar apparent outflow observed by Cushman and Rense in the coronal line 303A Si XI. The hypothesis that this is evidence for acceleration of the high speed solar wind deep in the transition region and inner corona is explored briefly.
Jets (transient/collimated plasma ejections) occur frequently throughout the solar corona and contribute mass/energy to the corona and solar wind. By combining numerical simulations and high-resolution observations, we have made substantial progress recently on determining the energy buildup and release processes in these jets. Here we describe a study of 27 equatorial coronal-hole jets using Solar Dynamics Observatory/Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager observations on 2013 June 27–28 and 2014 January 8–10. Out of 27 jets, 18 (67%) are associated with mini-filament ejections; the other nine (33%) do not show mini-filament eruptions but do exhibit mini-flare arcades and other eruptive signatures. This indicates that every jet in our sample involved a filament-channel eruption. From the complete set of events, six jets (22%) are apparently associated with tiny flux-cancellation events at the polarity inversion line, and two jets (7%) are associated with sympathetic eruptions of filaments from neighboring bright points. Potential-field extrapolations of the source-region photospheric magnetic fields reveal that all jets originated in the fan-spine topology of an embedded bipole associated with an extreme ultraviolet coronal bright point. Hence, all our jets are in agreement with the breakout model of solar eruptions. We present selected examples and discuss the implications for the jet energy buildup and initiation mechanisms.