Lexicon: HSS, CIR (fast solar wind)
High Speed Stream (HSS)
☞ Fast solar wind from coronal holes
Fast solar wind, which originates from so-called coronal holes (not real holes, CH for short), forms a high-speed stream. The limit to slow solar wind is 400 km/s. Everything above this is considered fast, everything below is slow. Slow solar wind originates from ‘streamers’ that form between positively and negatively polarised coronal holes or from ‘pseudo-streamers’ above active sunspot regions. A characteristic difference is the plasma density: high in slow solar wind, low in fast solar wind.
At the maximum of the approximate 11-year solar cycle, a high-speed stream is rarely faster than 550 km/s. In the declining phase of the solar cycle, it can sometimes exceed 800 km/s when the coronal holes at the solar equator increase significantly in size and number. In the solar minimum there is almost exclusively slow solar wind as there are only huge coronal holes at the poles of the sun whose fast solar wind does not reach us.
LEFT: View from north to south on the planetary plane (ecliptic), in the centre the Sun, the yellow dot is the Earth
CENTRE: Side view/cross-section of the Sun-Earth axis
RIGHT: Cross-section of the outer circle in 2.2 earth radii
Yellow and orange fast solar wind from coronal holes
The interplay of fast and slow solar wind leads to the formation of a spiral in the heliosphere – the Parker spiral
High-speed streams and in particular the embedded SIRs (Stream Interaction Regions) or CIRs (Corotating Interaction Regions) are responsible for about 10% of all major geomagnetic storms – especially in the declining phase of the solar cycle.
They can then predictably trigger powerful geomagnetic storms every 27 days or so for several months, which can last for days once they have rotated around the sun and arrived back in the geoeffective zone. Aurora fans are happy because they can even plan their Tromsø holiday weeks or months in advance. Satellite operators, on the other hand, are sweating because they have to deal with the ongoing effects of a strongly disturbed magnetic field for days on end.
In interplanetary space, a high-speed stream can accelerate slow ICMEs, decelerate fast ICMEs, deform them, push them away and interact with them in other complex ways. ICMEs can be embedded in them and difficult to detect when they emerge unnoticed and without a conspicuous flare (X-ray or radio flares/ bursts) in the vicinity of coronal holes and thus cause completely surprising strong geomagnetic storms (stealth CMEs).
Fast solar wind near ICMEs is always a challenge for predicting space weather.
Stream Interaction Regions (SIR) & Corotating Interaction Regions (CIR)
A stream interaction region with condensed plasma and shock waves produced by it:
Source: Jian et al (2006)
The terms CIR and SIR are often used synonymously, although by definition only a Stream Interaction Region that orbits the sun at least a second time is called a Corotating Interaction Region. If two or more Interaction Regions meet and merge, we have a Merged Interaction Region, MIR, and the zoo is complete. MIRs can be found in the outer reaches of the heliosphere (from Jupiter’s orbit onwards). To make matters worse, a distinction is rarely made between CIR, SIR, HSS and“fast solar wind” in everyday space weather operations, but this is not particularly bad in practice.
Source: Kataoka and Miyoshi (2006)
High-speed streams of coronal holes lead to auroras relatively reliably, especially at high latitudes, but rather rarely at mid-latitudes, e.g. in Central Europe – here especially around the equinoxes and when the magnetic polarity is right (negatively polarised in February, March, April, May – positively polarised in August, September, October, November).
Note: We want SPAM! September plus – April minus