That is, FEE fluxes are enhanced by energetic electrons coming from the inner ERB across the drift shells. Note that the same mechanism was thought to explain the observations in the inner ERB mentioned above, with electrons coming from the outer belt. For the movement of particles across the drift shells, a significant electric field \(E\) is necessary, the presence of which in the region of the lower L-shells is not obvious. Specifically, according to the generally accepted concept of the electrodynamics of magnetospheric processes, the large-scale convection electric field \(E\) should be shielded from the inner magnetosphere at L < 6. That is, the inner magnetosphere shields the dawn-to-dusk convection (quasi-static) \(E\) by setting up polarization charges (i.e., dusk-to-dawn \(E\)) at the inner edge of the ring current [19]. However, numerous observations showed that some conditions for the penetration of the convection field in L < 6 may arise. The dawn-dusk electric field is shielded near Earth for low geomagnetic activity (low \(Kp\) index), while it has a local maximum at L \(\sim\) 4 during moderate activity, and strong \(E\) was observed near L \(\sim\) 3 during high geomagnetic activity [20]. On the other hand, observations have also shown that the electric field in the L = 2-6 region is very variable during moderate and strong magnetic activity occasionally exceeding 2 mV/m [21]. In this connection, a number of principle questions arise, in particular about the origin of enhanced electric field \(E\) at low L-shells: is it convective (quasi-static) or inductive (transient)? If it is an enhanced large-scale convection field, does it penetrate to \(L\) < 1.5? If it is an induction electric field generated by a time-varying magnetic field B (\(\partial B/\partial t\)), what are the time scales and rates of change of \(B\) at lower L-shells? Penetration electric fields have been studied with ground-based magnetometer and radar measurements. Equatorial electric fields at altitude of ~800 km, mapped along the magnetic field line over distances of only a few hundred kilometers, should approximately correspond to the ionospheric electric field at low latitudes. Studies of the dayside equatorial electrojet showed that the interplanetary electric field partially penetrates from high to lower latitudes in the ionosphere (e.g., [22] and references therein). During southward turning of Bz IMF, the solar wind/magnetospheric electric field penetrates due to enhanced convection (undershielding of the inner magnetosphere). It has been empirically found that the dayside penetration electric field (eastward/dawn-dusk direction) is about 10% of the interplanetary electric field \(E\) (e.g., [23]). During northward turning of Bz IMF, it penetrates at night due to ceasing of convection and the inner magnetospheric shielding the electric field (eastward/dusk-dawn direction) [24]. But in this case, the dayside electric field (westward direction) is only a few percent of the interplanetary electric field \(E\)[23]. In addition, a recent study showed that a response of the equatorial ionosphere to overshielding electric fields is not uniform in an altitude range from 200 to 800 km [25]. That is, with regard to the electric field, the bottomside (200 km) response can differ significantly from topside (800 km) ionosphere. Thus, there are some unexplored issues related to the adequate representation of penetration electric field distribution with respect to altitude and latitude. Recent satellite measurements of quasi-static electric fields by THEMIS and Van Allen Probes confirmed that the magnetospheric convection electric field can penetrate earthward of L = 3 and enhance up to 2 mV/m during geomagnetic storms [26; 27; 28]. These studies suggested that observed temporal variations of electric fields were related to the large-scale convection driven by solar wind. Nevertheless, due to shielding, the electric field below L = 2 is on average weak: less than 1 mV/m [20].