Large Polar Coronal Holes Research Articles

Heliospheric 3D structure and CME propagation as seen from SOHO: Recent lessons for space weather predictions

Observations from Skylab, Helios, Ulysses, and SOHO have demonstrated how closely the 3D heliosphere is related to the underlying coronal structure. Around solar activity minimum, the large polar coronal holes dominate the major part of the heliosphere, through the high-speed solar wind streams emanating from them. A distinctively different type of solar wind is restricted to a narrow near-equatorial belt (about 30 0 in latitude). Its magnetic topology is dominated by strong multipole components and multiple current sheets. Owing to the new telescopes on SOHO, various effects of CME disturbances propagating through the heliosphere can now be observed in much greater detail: optically from the photosphere out to 32 R s, and later on by in situ spacecraft. It appears that the prediction reliability of space weather at Earth's orbit can be raised substantially in the near future.

Coupling between high and low latitudes as observed with LASCO in the solar corona and in interplanetary space

Abstract From Skylab, Helios and Ulysses observations we had learnt already how closely the 3D heliosphere is related to the underlying coronal structure. Around solar activity minimum, large polar coronal holes dominate the major part of the heliosphere, through the high-speed solar wind streams emanating from them. A different type of solar wind is restricted to a narrow near-equatorial belt (about 30 degrees in latitude). The magnetic topology is dominated by strong multipole components and multiple current sheets in the upper corona, and by a large-scale dipole field further outside, respectively. New observations by LASCO on SOHO cover both regions and may reveal clues to the processes responsible for the generation of the different types of solar wind.

Latitudinal solar wind velocity variations from polar coronal holes: A self‐consistent MHD model

Coronal holes are regions of the solar corona magnetically open to the interplanetary medium where the plasma flows outward to form the solar wind. An axisymmetric magnetohydrodynamic model has been developed that describes both the magnetic structure of the solar corona and the outward flow of coronal plasma in a self‐consistent manner, considering a dipolar field at the solar surface. Equilibrium solutions were found for a field at the solar pole of 10 G and a variety of plasma boundary conditions at 1 AU in order to explore the latitudinal dependence of the solar wind velocity from polar coronal holes. As expected, two large polar coronal holes were found, extending down to 32° latitude, separated by a belt of closed coronal helmets. The coronal helmet belt extended to 1.7 Rs, and above this height all flux lines were open and connected to one of the polar coronal holes, from which all solar wind flowed. Three cases of latitudinal velocity variation at 1 AU were considered: (1) a sharp transition between a slow wind at low latitudes and a fast wind at high latitude (in accordance with Ulysses' observations), (2) a linear latitudinal increase of velocity from slow wind at the equator to fast wind over the pole, and (3) a constant velocity irrespective of latitude. In all cases a fast wind flowed from the poles, but there always existed a low‐latitude band of slow solar wind close to the Sun. This slow wind flowed out from the boundary of both coronal holes and formed a band of slow wind on either side of the heliospheric current sheet. The transition between slow and fast wind close to the Sun was sharply defined in all cases.

Magnetic observations during the recent declining phase of solar activity

Changes in the heliospheric magnetic field during the recent declining phase in solar activity are reviewed and compared with observations during past sunspot cycles. The study is based principally on data obtained by IMP-8 and Ulysses. The field magnitude is found to have increased during the declining phase until it reached a maximum value of 11.5nT in ≈ 1991.5, approximately two years after sunspot maximum. The field of the sun's south pole became negative after a reversal in early 1990. The sector structure disappeared at Ulysses in April 1993 when the latitude of the spacecraft was −30 revealing a low inclination of the heliospheric current sheet. A large outburst of solar activity in March 1991 caused four CME's and numerous shocks at the location of Ulysses. Following a delay of more than a year, a series of recurrent high speed streams and Corotating Interaction Regions commenced in July 1992 which were observed by IMP-8, Ulysses and Voyager 2. In all these respects, the behavior of the magnetic field mimics that seen in the two earlier sunspot cycles. The comprehensive data set suggests a correlation between |B| and sunspot number. The major solar cycle variations in the radial component (and magnitude) of the field have been successfully reproduced by a recent model consisting of a tilted solar dipole, whose strength and tilt undergo characteristic changes over the sunspot cycle, and the heliospheric current sheet. The large outbursts of activity in mid-1972, mid-1982 and the first quarter of 1991 may represent a characteristic last “gasp” of solar activity before the sun evolves to a different state. The recurrent high speed streams in 1973, 1984 and 1992 accompany the development of large asymetrical polar coronal holes and the growth in intensity of the polar cap fields. After they endure for about one year, the polar coronal holes recede and the high speed streams are replaced by weaker streams more characteristic of solar minimum.

Observations of the solar wind from coronal holes

The solar wind emanating from coronal holes (CH) constitutes a quasi-stationary flow whose properties change only slowly with the evolution of the hole itself. Some of the properties of the wind from coronal holes depend on whether the source is a large polar coronal hole or a small near-equatorial hole. The speed of polar CH flows is usually between 700 and 800 km/s, whereas the speed from the small equatorial CH flows is generally lower and can be <400 km/s. At 1 AU, the average particle and energy fluxes from polar CH are 2.5xl08cm-2 sec-1 and 2.0 erg cm-2 s-1. This particle flux is significantly less than the 4xl08 cm-2 sec-1 observed in the slow, interstream wind, but the energy fluxes are approximately the same. Both the particle and energy fluxes from small equatorial holes are somewhat smaller than the fluxes from the large polar coronal holes.

Ulysses at 50° south: constant immersion in the high‐speed solar wind

We present speed observations from the Ulysses solar wind plasma experiment through 50° south latitude. The pronounced speed modulation arising from solar rotation and the tilt of the heliomagnetic current sheet has nearly disappeared. Ulysses is now observing wind speeds in the 700 to 800 km s−1 range, with a magnetic polarity indicating an origin in the large south polar coronal hole. The strong compressions, rarefactions, and shock waves previously seen have weakened or disappeared. Occasional coronal mass ejections characterized by low plasma density caused by radial expansion have been observed. The coronal configuration was simple and stable in 1993, indicating that the observed solar wind changes were caused by increasing spacecraft latitude. Trends in prevailing speed with increasing latitude support previous findings. A decrease in peak speed southward of 40° latitude may indicate that the fastest solar wind comes from the equatorial extensions of the polar coronal holes.

Solar wind composition and expectations for high solar latitudes

Instruments on board the Ulysses spacecraft are to investigate, for the first time, the solar wind in the three principal domains of solar latitude. (1) During Ulysses' flight in the ecliptic plane the solar wind near the current sheet was continuously measured, and its composition was determined with unprecedented sensitivity and mass resolution by SWICS, the Solar Wind Ion Composition Spectrometer. (2) Recently, it has been argued that EUV and flare particle observations indicate strong abundance anomalies in erupting prominences and impulsive flares. When Ulysses will cross the latitudes where such events mostly occur, we expect to provide complementary solar wind composition data, aiming to identify the processes that cause these anomalies. (3) When reaching the high solar latitudes, Ulysses will probe the solar wind emanating from the large polar coronal holes. There, we expect to use composition measurements to study the differences between high and low solar latitudes as regards the chromospheric structure and the coronal and interplanetary expansion.

UV observations of the extended corona

Abstract Measurements of the intensities and profiles of the resonantly scattered hydrogen Lyman-alpha line at 1216 A have provided important insights on the physical conditions in the extended corona where the solar wind originates. Comparisons of measured kinetic temperatures with predictions of models suggest that there is proton heating and/or a nonthermal contribution to the motions of coronal protons between 1.5 and 4 solar radii. Comparisons of measured Lyman-alpha intensities with predictions of models provide evidence for subsonic flow for r ⊙ in a quiet region of the corona and two polar coronal holes observed near solar maximum. There is evidence for supersonic flow ( ∼ 200 km s −1 ) at r ≈ 2 R ⊙ in a large polar coronal hole observed in 1982. Future measurements of HI Lyman alpha and other coronal lines will greatly expand the quality and quantity of UV spectroscopic measurements of the extended corona.