Geomagnetic Effects of Solar Wind

Our Sun is not merely a source of electromagnetic radiation. An incessant slow flow of low-energy (1.5 to 10 keV) particles¹ —protons, electrons and ions— coasts away along the Sun’s magnetic field (nominally 5 nT)² into interplanetary space. This combination of the interplanetary magnetic field carrying solar particles is called the slow solar wind.

Coronal holes & fast solar wind

Holes in the Sun’s corona are a source of additional high-speed particle streams, constituting the fast solar wind. If a coronal hole (CH) is Earth-directed, these high-speed particles reach Earth in approximately five days. Coronal holes typically last between six and eight Sun rotations. This implies that, in little over 27 days time, the same coronal hole may be again pointing towards Earth.

Earth-orbiting satellites provide real-time proton flux readings. Solar protons with energies between 1 to 200 MeV are responsible for most of the ionisation of the D-layer³ and, consequently, for the ionospheric absorption of HF radio waves. Nonetheless, Earth’s geomagnetic field shields off most solar particles. Only at high latitudes around the geomagnetic poles, can solar particles easily enter Earth’s ionosphere. Earth’s nominal geomagnetic field strength has local variations. The maximum nominal field strength is 66 µT at the magnetic poles, and the local minimum is 24 µT.⁴

CMEs & polar cap absorption

Occasionally, any type of active solar region, including flares, may produce a coronal mass ejection (CME). The energetic particles of an Earth directed-CME will reach Earth in between 19 hours and five days, depending on the speed of these particles. If the Earth-directed CME is particularly dense, the proton flux above the geomagnetic poles will increase dramatically, resulting in polar cap absorption (PCA). Its severity is expressed from S1 to S5 on NOAA’s (poorly named) solar radiation storm scale. HF and VHF (3–300 MHz) signals will be subject to absorption above the poles, whereas VLF and LF (3–300 kHz) radio waves will be reflected at lower altitudes than normal. Following the end of a PCA event, trans-polar radio paths may be disturbed for days, up to weeks.

Geomagnetic disturbances

The magnetic field of the fast solar wind can disturb Earth’s geomagnetic field. This is especially the case when the magnetic field of the solar wind opposes Earth’s magnetic field; i.e. the B_z component of the solar wind (perpendicular to the ecliptic plane) is negative (pointing upwards). —Remember: Despite a looming pole reversal, Earth’s geomagnetic north still corresponds to a south pole in Earth’s bar magnet dipole model!— A disturbed geomagnetic field results in auroral ovals that are larger, more intense and lower in magnetic latitude than under normal, quiet conditions. This is indicated by larger planetary K_p values. Severe geomagnetic storms occur when K_p > 4. Their severity is expressed from G1 to G5 on NOAA’s geomagnetic storm scale.

Aurora generates crackling noise on lower HF

Satellite planning

References

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