Since magnetic storms, produced by ring currents in magnetosphere (Ring Disturbances – DR) and magnetic substorms, produced by currents in polar ionosphere (Polar Disturbances – DP) are mainly related to southward IMF polarity, the conclusion was made that magnetic disturbances are caused by the solar plasma fluxes carrying the southward BZS IMF component. Apart from magnetic substorms, developed in the auroral zone, the regular weak magnetic disturbances were revealed also in polar cap. These polar cap variations, named as DP2 disturbances, are observed permanently, irrespective of season and level of magnetic activity, the intensity of disturbances being also dependent on southward BZS IMF component. In addition, specific DP3 and DP4 disturbances, related to BZN and BY IMF influence, were found in polar caps as well. The ionospheric current system DP1 responsible for magnetic substorms is presented in, current systems of the polar cap magnetic disturbances DP2 (b), DP3 (c), DP4 (d) are shown in.
Author(s) Details:
A. Troshichev
Arctic and Antarctic Research Institute, Russia.
Recent Global Research Developments in Southward IMF
1. Importance of Southward IMF:
- The southward pointing IMF (B_s) plays a crucial role in the solar-terrestrial relationship.
- The basic assumption is that magnetic reconnection occurs predominantly at the magnetopause between the southward IMF and Earth’s predominantly northward magnetic field.
- This interaction is ordered by solar magnetospheric coordinates, which vary as the Earth rotates.
- It’s essential to calculate the southward magnetic field in terrestrial magnetic coordinates, accounting for daily and yearly variations.
- Even if the IMF is northward in solar ecliptic coordinates, it can be southward in solar magnetospheric coordinates.
- The Earth’s magnetosphere stores energy gained from this interaction when the IMF is southward in the Earth’s preferred frame, even if it appears northward in the solar wind’s preferred frame [1].
2. Kelvin-Helmholtz Waves Under Southward IMF:
- Kelvin-Helmholtz (KH) waves, driven by velocity shear, are often observed during northward IMF conditions.
- However, they are less frequently observed during southward IMF.
- A recent 3D MHD simulation investigated KH instability under pure southward IMF conditions.
- In the linear phase of KH instability, there are minimal jet flows along both northward and southward directions.
- In the nonlinear growth phase, KH vortices form with a width of approximately 12λ and a duration of about 20 Alfvén times (t_A).
- Jet flows with peak V_z values of around 0.5V_A are distributed in the spine region and the edge of the KH vortex [2].
3. Rectified IMF Components:
- Researchers have investigated the rectified IMF components in two coordinate systems: Geocentric Solar Ecliptic (GSEQ) and Geocentric Solar Magnetospheric (GSM).
- The focus was on the southward pointing component (B_s) in GSM from 1999 to 2017.
- Understanding the temporal variations of B_s is crucial for geomagnetic activity studies [3].
References
1. Russell, C.T. Comment on: “Origin and Characteristics of the Southward Component of the Interplanetary Magnetic Field” by G. Verbanac and M. Bandić. Sol Phys 297, 119 (2022). https://doi.org/10.1007/s11207-022-02043-z
2. Wang, JQ., Yang, Y., Khan, S. et al. Jets with Kelvin-Helmholtz waves at the Earth’s magnetopause under pure southward IMF conditions. Astrophys Space Sci 368, 12 (2023). https://doi.org/10.1007/s10509-023-04168-4
3. Verbanac, G., Bandić, M. Origin and Characteristics of the Southward Component of the Interplanetary Magnetic Field. Sol Phys 296, 183 (2021). https://doi.org/10.1007/s11207-021-01930-1