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The solar wind particles can directly access the high-altitude cusp. They could create large diamagnetic cavities with a size of 6RE (Earth radii) in the latitudinal direction and > 10 R E in the longitudinal direction at an altitude of 7–8RE, and up to 10.5RE in the depth; they could depress the field from more than 260 nT to near 0 nT. Energetic charged particles with energies from 20 keV up to 15 MeV have been detected there, and their intensities are observed to increase by as much as four orders of magnitude during the diamagnetic cavity crossings. Strong electromagnetic fluctuations are measured there as well. The power spectral density of the magnetic fluctuations inside the cavities shows increases by up to five orders of magnitude at O+ gyro-frequency in comparison to the outer Van Allen radiation belt. The cavity electric fields have an amplitude of up to 350 mV/m, and its perpendicular component varies from −300 to 350 mV/m. The phase space densities of both the He++ and O+ ion species in the diamagnetic cavities could be more than two orders of magnitude higher than that in the outer radiation belt at a given magnetic moment. These in situ observations reveal a broad and dynamic region of radiation centered at the Earth's magnetospheric cusp, which is different from the Van Allen radiation belt centered at the Earth's magnetic equator with relatively stable magnetic field. The region of radiation is observed on both open and closed geomagnetic field lines. It can extend to low latitude to reach L-shell values of 7–12.5, providing a direct particle source for the outer radiation belt/ring current.

Observations made by a number of satellites have shown that plasma of solar wind/magnetosheath origin enters the high altitude polar cusp creating diamagnetic cavities of large dimensions. The Earth's dipole field can be excluded from this region in a turbulent manner with the magnitude of the field strength reaching close to 0 nT at times. In these regions of strongly varying magnetic fields, energetic particles are produced with intensities, which exceed those measured in the trapping regions of the magnetosphere for L>6 at a given magnetic moment. These particles can then flow back out of the cusp along field lines that form the magnetopause. A fraction of these particles enter the magnetosphere through the magnetopause on the dusk and dawn flanks. Due to existing gradients in the geomagnetic field, cusp accelerated ions can enter the magnetosphere along the dawn flank and electrons along the dusk flank. For those particles entering near the geomagnetic equatorial plane with pitch angles close to 90°, they will drift along contours of constant magnetic field strength reaching deep into the nightside inner magnetosphere. From observations made by the Polar, Cluster, and ISEE satellites it is shown that this cusp source appears to be capable of providing energetic ions and electrons to the magnetosphere, which form the source population of the subsequent radial diffusion and formation of the radiation belts.

A case study is presented analyzing the properties and spatial extent of the exterior cusp region. The dynamic nature of the exterior cusp makes the large scale properties difficult to measure using a single spacecraft. We use simultaneous cusp observations of the four Cluster spacecraft as well as the Polar spacecraft to monitor the region. On 10–11 March 2006 all five spacecraft observed the exterior cusp in the southern hemisphere mapping out an extended region in latitude spanning 4.5 RE (and possibly up to 9 RE) along the magnetopause. The depth, or dimension of the diamagnetic exterior cusp perpendicular to the magnetopause is measured to be 1.3 RE with simultaneous measurements from the Cluster spacecraft. The multipoint observations provide a unique view of the diamagnetic exterior cusp showing a spatially extended region.

The observation that high speed solar wind streams are correlated with outer radiation belt electrons requires a transducer to convert this mechanical energy to hot electrons. We hypothesize that the high latitude cusp is the ideal location for this acceleration region. We support this hypothesis with two arguments: a forward model to show that the cusp can theoretically accelerate electrons to MeV energies which then are transported to the radiation belts; and, a backward model that deduces a cusp source based on empirical properties of the radiation belt MeV electrons. Accordingly, in the first half we apply the trapping properties of the static equinoctal cusp to deduce the dynamical response of interplanetary transients; in the second half we analyze several peculiar statistics of MeV electron correlations with solar wind as the response of a non-linear, multi-parameter dependence on the solar wind driver. Our model would permit the formulation of more physically accurate MeV electron predictors, which we demonstrate by connecting physical explanations to several empirical predictors recently published.

A possible cusp acceleration mechanism has been investigated during a major geomagnetic storm on May 4, 1998 when the magnetosphere was compressed and eroded. At 5:00–12:05 UT, the WIND spacecraft was about 213 R E upstream from the Earth, the GEOTAIL was in the post dusk magnetosheath, and the POLAR traveled in its outbound orbit from the equatorial radiation belt to the high-altitude dayside cusp and crossed the magnetopause into the magnetosheath. Many instances of in situ ion energization (to >MeV) were observed by POLAR during this period. Simultaneous observations indicated that no comparable flux was observed by WIND. Ion fluxes measured by POLAR were higher than that measured by GEOTAIL, indicating ion source regions near or within the magnetosphere. The measured ions show some interesting features: (1) In the radiation belt, in the magnetopause boundary layer, and in the dayside cusp, most 1– 200 keV / e fluxes were around pitch angles of 90°; while in the magnetosheath, the fluxes came from a broad hemispherical direction between ∼sunward and earthward. (2) At around 6:00 UT, the MeV ion flux was lower in the magnetosheath than in the adjacent magnetosphere. (3) There were two CEP (cusp energetic particle) events with 2–3 orders of magnitudes enhancements of MeV ion fluxes. (4) The MeV ions in one CEP event had a peak flux higher than that of the intense outer radiation belt in the equatorial plane. (5) The 100 keV / e O ions which originated from the ionosphere were observed in both the outer cusp and the magnetopause boundary layer. (6) There were no large magnetic field fluctuations upstream before the bow shock. These observations suggest a likely source of substantial particle energization exists in the cusp region.