Space Physics and Aeronomy, Ionosphere Dynamics and Applications. Группа авторов

Space Physics and Aeronomy, Ionosphere Dynamics and Applications - Группа авторов


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cap (ECPC) model of the Dungey cycle. If the dayside and nightside reconnection rates are unequal, the proportion of the magnetic flux associated with the Earth's dipole that is open, the open magnetic flux content of the magnetosphere, also known as the “polar cap flux,” FPC, will change with time. Global auroral imagery, used to estimate the size of the polar cap, shows that typically 0.5 GWb of the 8 GWb associated with the Earth's dipole is open, though this can vary between 0.2 and 1.2 GWb (e.g., Milan et al., 2007; Huang et al., 2009). Assuming a polar magnetic field strength of 50,000 nT, with 0.5 GWb of open flux the polar cap is approximately 1,800 km in radius.

      The reconnection rate at the magnetopause is measured by a voltage, the amount of flux that is opened by reconnection in unit time, or equally the rate at which magnetic flux is transported into a region where its topology changes from closed to open. If the dayside and nightside reconnection rates, ΦD and ΦN, are equal, flux is closed as rapidly as it is opened, and FPC is constant. In such a situation, the rate of open and closing flux is also the rate at which flux is transported through the magnetosphere by the Dungey cycle, so ΦD = ΦN = ΦPC. On the other hand, if the rates are unequal, the rate of change of FPC is

Schematic illustrations of (a) Deformation of the magnetopause and resulting plasma flows following a burst of dayside reconnection. (b) (i)–(iv) The flows in the ionosphere excited by a burst of dayside reconnection. (c) Ionospheric flows excited by nightside reconnection. (d) Ionospheric flows excited due to dayside and nightside reconnection, in the limit that the redistribution of flux maintains a circular polar cap at all times.

      (from Cowley & Lockwood, 1992).

Schematic illustrations of the ionospheric convection excited in response to combined dayside and nightside reconnection.

      (from Milan et al., 2012; Reproduced with permission of John Wiley and Sons).

      In the limit that the polar cap remains circular, the cross polar cap potential along the dawn‐dusk meridian is the average of the dayside and nightside reconnection rates (Lockwood, 1991),

      The cross polar cap potential is sometimes defined as the difference between the maximum and minimum potentials in the convection pattern, ∆Φ = Φmax − Φmin, in which case, ∆Φ ≥ ΦPC.

      Observational evidence for the evolution of the flows from day to night across the polar cap following the onset of dayside reconnection, as depicted in the top two panels of Figure 2.10, has been mixed. As a result, significant debate has ensued as to the validity and applicability of the Cowley and Lockwood (1992) paradigm of ionospheric flow excitation.


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