It’s not so easy to distinguish various transients in the solar wind by just looking at the magnetic field and solar wind parameters such as speed, temperature and density. Most can pick out the signature from a coronal mass ejection, featuring increase in magnetic field and decrease in temperature, and a sudden increase in density, speed and magnetic field strength if the CME is driving a shock. Things are getting more complicated when it comes to the effects from a Sector Boundary Crossings (SBC) or from the interaction region related to a fast speed solar wind stream coming from a coronal hole (increase in speed and temperature). A coronal hole rotates together with the Sun: the associated solar wind structure is therefore called a Co-rotating Interaction Region (CIR). When the coronal hole survives several solar rotations, it is recurring and it's important for medium-term space weather predictions. But let's start by having a look on what SBC and (C)IR actually are.
Sector Boundary Crossing
The Sun is a giant magnet with magnetic field lines extending away into space (positive, "away") and other field lines returning from space to the Sun (negative, "towards"), with each solar hemisphere having a different polarity, in general. Along the solar magnetic equator, these opposite field lines run parallel to each other creating a current sheet, which is called the "Heliopsheric Current Sheet" (HCS).
Because the Sun’s rotational axis and its magnetic axis are not always aligned, this sheet gets warped. It is often compared to a ballerina skirt. When the Earth traverses such a fold, a change in the orientation of the magnetic field of the solar wind occurs, which is called a Sector Boundary Crossing (SBC). It quite abruptly changes either from "towards" to "away" from the Sun, or from "away" to "towards" the Sun. This orientation is measured by the "Phi angle", which is oriented "away" when values are between 90 and 270 degrees. Though such a crossing usually may be accompanied by a slight change in e.g. solar wind speed or magnetic field strength, this is not a requirement.
SBCs are usually not associated to big disturbances in the geomagnetic field. A nice example of an SBC occurred late July this year, when early on 28 July the magnetic field changed from "away" to "towards" the Sun, and a few days later (31 July around noon) back to "away" (Phi angle, blue). Though some changes can be seen in the speed (yellow), density (orange) and magnetic field strength (white) of the solar wind, the geomagnetic field remained quiet to unsettled. Data were taken from ACE.
Corotating Interaction Regions
Solar particles can leave the Sun along the open magnetic field lines and at different speeds. Hence, the high speed stream from e.g. a coronal hole may interact with the slower moving normal solar wind ahead. Where the two meet, an interface results in a compressed space of increased density and magnetic field strength. This interface separates the slow from the fast solar wind. When the speed or density difference is big enough, the interface becomes a shock, sometimes already at the Sun-Earth distance. Then the speed continues its gradual increase towards the main speed of the coronal hole. As the stream interaction region passes, there's no obvious change in the direction of the magnetic field, which is usually oscillating during that period. As the stream interaction region has passed, the magnetic field assumes the direction of the field embedded in the high speed stream of the coronal hole, which may or may not be the same as that of the slow solar wind.
As it may be clear from this description, (C)IRs can have quite a complex solar wind signature and have the potential to generate somewhat stronger geomagnetic disturbances than SBCs. A nice example just happened last week, when a stream interaction region related to a negative coronal hole passed by the Earth early on 19 September. Even though no shock was observed and the magnetic field remained mostly directed towards the Sun, the oscillating north-south direction of the magnetic field resulted in minor geomagnetic storming conditions on 19 September. Notice also the low density of the coronal hole’s particle stream once the interface has passed. If the coronal hole survives the coming solar rotation, it will sweep again by Earth in little less than one month.
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