Alexander Janzhura

PC index as a proxy of the solar wind energy that entered into the magnetosphere: Development of magnetic substorms

The Polar Cap (PC) index has been approved by the International Association of Geomagnetism and Aeronomy (IAGA XXII Assembly, Merida, Меxico, 2013) as a new index of magnetic activity. The PC index can be considered to be a proxy of the solar wind energy that enters the magnetosphere. This distinguishes PC from AL and Dst indices that are more related to the dissipation of energy through auroral currents or storage of energy in the ring current during magnetic substorms or storms. The association of the PC index with the direct coupling of the solar wind energy into the magnetosphere is based upon analysis of the relationship of PC with parameters in the solar wind, on the one hand, and correlation between the time series of PC and the AL index (substorm development), on the other hand. This paper (the first of a series) provides the results of statistical investigations that demonstrate a strong correlation between the behavior of PC and the development of magnetic substorms. Substorms are classified as isolated and expanded. We found that (1) substorms are preceded by growth in the РС index, (2) sudden substorm expansion onsets are related to “leap” or “reverse” signatures in the PC index which are indicative of a sharp increase in the PC growth rate, (3) substorms start to develop when PC exceeds a threshold level 1.5 ± 0.5 mV/m irrespective of the length of the substorm growth phase, and (4) there is a linear relation between the intensity of substorms and PC for all substorm events.

Special features of procedure for on-line PC index derivation

While the concept of the PC index was being developed, a mechanism of the electric field generation in the polar cap ionosphere was believed to be associated with reconnection of the IMF and geomagnetic field lines at the dayside magnetopause (Dungey, 1961). In the framework of Dungey’s concept, the reconnected field lines are moving across the polar cap at the solar wind speed, i.e. that it takes about 20 minutes to reconfigure the polar cap plasma convection from the pattern characteristic of northward IMF to that characteristic of southward IMF if the IMF suddenly changes its polarity. It is this circumstance that resulted in a choice of PC index resolution (15-min).

Solar wind–magnetosphere–ionosphere coupling and the PC index

The problem of solar wind–magnetosphere–ionosphere coupling is fundamental when monitoring space weather and to newscast substorm and storm development.

PC index response to solar wind geoeffective variations

The polar cap magnetic activity response to interplanetary electric field variations has been thoroughly studied (see Chapter 2) and has provided a basis for the concept of a PC index. That is why only selected cases with a sharp rise in EKL are demonstrated here to highlight the delay time problem of the PC index response to changes in EKL. Events with sudden changes, a sudden rise and a sudden decay in E KL were researched by Stauning and Troshichev (2008) who used solar wind data for 1974-1999 taken mainly from IMP 8 spacecraft located near the Earth (23-35 RE), the additional requirement being that the satellite should definitely be in the disturbed solar wind. The PCN index derived in DMI was used for analysis.

Monitoring of the auroral ionosphere

The ionized atmosphere of the Earth is composed of a series of overlapping layers. In each layer, there is an altitude of maximum density, above and below which the ionization density tends to drop off. The altitude of the peak density in D region is normally around 90 km, but this may decrease considerably to ~ 80 km when the solar X-ray flux is enhanced. The E region peak density occurs at a peak altitude of 110 km. The F region is a combination of two somewhat different regions. The F1 region has an altitude peak near 200 km, but is absent at night. The F2 region has a peak near 300 km during the day and at higher altitudes at night.

A method for the PC index determination

The idea behind PC index derivation is to use current polar cap magnetic data with application of the once-derived regression (calibrating) coefficients α and β connecting the coupling function E KL with value of the polar cap magnetic disturbance.

Physical background (historical outline)

The first examinations of the relations between solar wind variations and geomagnetic activity, represented by the Kp index, displayed the dependence of the Kp value on the solar wind speed v and the interplanetary magnetic field (IMF) intensity B (Coleman et al., 1961; Snyder et al., 1963).

Magnetic disturbances developing under conditions of northward IMF

It has long been known that geomagnetic storms are generated under conditions of southward IMF (BZS) and the interplanetary electric field \(E_Y=vB_{ZS}\) was usually considered as a direct driver of magnetic storms and substorms.

PC index as an indicator of magnetic storm development

Geomagnetic storms are the result of a joint action of magnetopause currents (DCF), which are proportional to the square root of the solar wind dynamic pressure, and ring currents (DR) flowing in the inner magnetosphere (Chapman, 1963). The DR current ground effect typically far exceeds the DCF current effect, which is why magnetic storm intensity is evaluated by the Dst index depicting a longitudinally averaged magnetic field depression at low latitudes (Sugiura, 1976).

PC index as indicator of anomalous atmospheric processes in the winter Antarctica

Existing models of atmospheric variability and change do not take into consideration shortterm changes of solar activity. Indeed, the total energy contributed by the solar wind and cosmic rays in the Earth’s atmosphere is extremely insignificant in comparison with total solar irradiance. But, as distinct from total solar irradiance, the energy of the solar wind and cosmic rays can increase a hundredfold and more in periods of high solar activity. Attempts to find the cause–effect relations between the solar activity variations and weather and climate changeability have had a long story (Wilcox, 1975; Herman and Goldberg, 1978).