Pattern of electron and ion precipitation in northern and southern polar regions for northward interplanetary magnetic field conditions

Abstract

Using the data of the DMSP F6 and F7 satellites, we show that there is a systematic difference between ion precipitation in the auroral oval and in the polar cap. The oval ion precipitation is smooth, and the ratio of the total electron number flux to the total ion flux is less than 20. The polar cap ion precipitation is patchy, and the ratio is more than 20. The boundary between these two types of ion precipitation usually can be detected for both northward and southward interplanetary magnetic field by a sharp fall of the ion total number flux below the level 5 x 105-106 ions (cm2s st) -1. Since this determination of the poleward boundary of the auroral oval is efficient for any conditions, we propose to define the polar cap as a region bounded by this boundary. The polar cap determined in this way has roughly the shape of an ellipse aligned along the 1100-2300 MLT meridian with daytime and nighttime boundaries at 80 ø and 72 ø , respectively. Four patterns of electron precipitation in the polar cap are identified for northward IMF conditions: (1) symmetric precipitation in both polarcaps with no distinct regularity. (2) symmetric precipitation with a prevalence of electron fluxes in the morning sector of both polar caps, (3) asymmetric distribution with no correspondence between the patterns in the opposite polar regions, and (4) asymmetric distribution where the increased electron fluxes are observed in opposite sectors (morning or evening) of the opposite polar caps depending on the polarity of the By IMF component. The energy spectra of electrons even in neighboring spikes in the same polar cap may be greatly different, and therefore discrepancy in electron spectra in opposite polar caps cannot be regarded as unique proof for the open character of the magnetic field lines. tions. Following Meng's [1981] suggestion that the polar cap arcs are generated by soft precipitation of the widened auroral oval, Lassen et al. [1988] have proposed defining the polar cap as a near-pole region free of aurora. In this case the polar cap is thought to be teardrop or pear shaped. The horse collar type aurora introduced by Hones et al. [1989] on the basis of satellite data also contains a "slot" bordered by bright arcs with no aurora inside it. Rich and Gussenhoven [1987], using electron measurements, claimed that the polar cap is a small region confined almost entirely to the nightside when IMF is near zero or slightly northward. However, such a concept is evidently not in conformity with great many experimentM data showing the appearance of Sun-aligned arcs along the noon-midnight meridian just in the teardrop-shaped region [Meng and Akasofu, 1976; Ismail et al., 1977; Lassen and Danielsen, 1978; Gussenhoven, 1982; Ismail and Meng, 1982; Gusev and Troshichev, 1986, 1990]. According to the results of Troshichev et al. [1988] the occurrence of Sun-aligned arcs along the noon-midnight 8337 8338 TROSHICHEV AND NISHIDA: PRECIPITATION PATTERNS IN THE POLAR CAP meridian is normal for conditions when Bz >0 and By •0. Another auror...

Structure of the auroral precipitation region in the dawn sector: relationship to convection reversal boundaries and field-aligned currents

“…According to the above suggested classification, it is the boundary b3b between the AO and the SSSL where the auroral electron spectrum sharply softens. In reality, the spectrograms of all the DMSP satellite passes intersecting the dawn sector which have been presented in Troshichev and Nishida (1992) agree with this interpretation. According to this interpretation, the CPIP regime is nothing more than the SSSL regime.…”

“…Yet poleward of this boundary, the DMSP and Viking satellites observed not only intensive ion fluxes, but also intensive fluxes of auroral energy electrons as well. A detailed analysis of DMSP satellite data by Troshichev and Nishida (1992) showed that the TB really separates the highenergy electron region from the softer electron and ion precipitation region located further poleward. According to the above suggested classification, it is the boundary b3b between the AO and the SSSL where the auroral electron spectrum sharply softens.…”

Cross polar cap diameter and voltage as a function of PC index and interplanetary quantities

“…The study of ion and electron characteristics measured by DMSP spacecraft [Troshichev and Nishida, 1992;Gusev and Troshichev, 1992] has shown that the region of regular auroral oval can be identified where the diffuse precipitation of ions E > 1 keV takes place, irrespective of IMF sign and level of magnetic activity. Intensity of ion flux can increase again in connection with spikes of electron precipitation observed inside the polar cap under the influence of the northward IMF, but the ratio of ion number flux to electron number flux in the polar cap turns out to be less than that in the regular oval.…”

Dynamical polar cap: A unifying approach

“…Similarly, polar cap arcs have been reported as following a "teardrop" or "horse-collar" shape (that is, to be excluded from a central high latitude region) [Meng, 1981;Hones et al, 1989]. Others report that polar cap arcs can be found throughout the oval, interspersed with regions of polar rain or void [Hard), et al, 1982;Troshichev and Nishida, 1992]. This latter configuration is closely related to the 0 aurora, which consists of a brief IMF excursions can be highly geoeffective; for example, Greenwald et al [1990] found that 6 min after a change in IMF By reached the ionosphere, the entire radar field of view, 15 ø MLAT by 2.5 hours MLT near noon, had converted to the new convection configuration.…”