W. Lyatsky

Solar illumination as cause of the equinoctial preference for geomagnetic activity

Geomagnetic and auroral activity vary seasonally with maxima at equinoxes, as has been known for more than a century. The cause remains under debate. The angle made by the Earths dipole axis with the typical direction of the interplanetary magnetic field (IMF) can explain a portion (about 17%) of the effect. To explain the majority of the equinoctial effect, we suggest that geomagnetic activity peaks when the nightside auroral zones of both hemispheres are in darkness, as happens at equinox. Under such conditions, no conducting path exists in the ionosphere to complete the currents required by solar wind-magnetosphere-ionosphere coupling, and geomagnetic disturbances maximize. To test this theory, the Universal Time (UT) variation of geomagnetic activity was explored. As our model predicts, geomagnetic activity in December, measured by the Am index, evinces a deep minimum around 0300–0600 UT when the auroral oval of both hemispheres are in darkness and a maximum around 1500–1600 UT when the southern nightside oval is sunlit. In June, complementary effects are predicted and observed. Previous studies using the AE index have shown more ambiguous results. Here we show that if AE is resolved into the AU and AL components, the discrepancy disappears, with the AL component following the same pattern as does Am. We thus conclude that the intensity of global geomagnetic activity is well ordered by whether the nightside auroral oval is sunlit in one hemisphere or neither.

Field-aligned currents between conjugate hemispheres

Interhemispheric field-aligned currents flowing between dayside conjugate ionospheres are calculated for different seasons. The currents are a result of redistribution of original three-dimensional currents because of different ionospheric conductivity in dark and sunlit conjugate hemispheres. Interhemispheric currents and related patterns for the electric potential distribution are computed for a reasonable model of the ionospheric conductivity. The interhemispheric field-aligned currents are attached to the terminator position, and they close a part of ionospheric currents, flowing in the summer high-latitude ionosphere, through the conjugate ionosphere of the opposite hemisphere. Although the interhemispheric currents are more significant for summer/winter conditions, they can be observed for equinoctial conditions as well because of a diurnal variation in the terminator position with respect to the geomagnetic poles. Interhemispheric currents in the winter ionosphere are generated mainly by sources in the summer hemisphere. An interesting consequence of the existence of interhemispheric currents is that auroral events can be observed not in the hemisphere with incident field-aligned currents from a magnetospheric source but in the opposite hemisphere.

Traveling convection vortices as seen by the SuperDARN HF radars

Two impulsive traveling convection vortex (TCV) events observed simultaneously by ground based magnetometers and the SuperDARN HF radars in the prenoon sector were studied. In both cases, disturbances traveled westward at speeds of 4–6 km/s. Convection patterns derived from magnetometer measurements and radar observations were overall in reasonable agreement; observed differences at some points might be caused by both the nonuniform ionospheric conductivity distribution and difference in the integration time of the radar and magnetometer data. For one event, the convection patterns obtained from magnetometer data and SuperDARN radar measurements were relatively simple; they can be interpreted as a result of the westward motion of a convection vortex system associated with a pair of field-aligned currents separated in azimuthal direction. This TCV event was associated with relatively low Pc5 pulsation activity, contrary to the second TCV event that was accompannied by a train of Pc5 magnetic pulsations of large amplitude. Convection patterns for the second event were complicated. A simple scenario for the interpretation of the generation of TCVs and Pc5 pulsations is suggested. A sudden impulse in the solar wind dynamic pressure produces disturbances on several boundaries of magnetospheric plasma: on the magnetopause, the LLBL inner edge, and the plasma sheet inner edge. These boundaries are elastic so that surface waves can propagate along them. The high-latitude wave is responsible mainly for TCVs, whereas the low-latitude waves may be responsible for excitation of Pc5 field line resonance pulsations. The scenario explains important features of both TCV events and Pc5 pulsations: both phenomena appear simultaneously and show westward (eastward) propagation, but the TCVs are observed at latitudes close to the LLBL inner edge, whereas the Pc5 pulsations occur at lower latitudes, close to the inner boundary of the plasma sheet.

Interplanetary magnetic field By control of dayside auroras

Abstract The effect of the interplanetary magnetic field (IMF) By component on the dayside auroral oval from Viking UV measurements for March–November 1986 is studied. Observations of dayside auroras from Viking UV images for large positive (15 cases) and negative (22 cases) IMF By (∣By∣>4 nT), suggest that: (1) the intensity of dayside auroras tends to increase for negative IMF By and to decrease for positive By, so that negative IMF By conditions seem preferable for observations of dayside auroras; (2) for negative IMF By, the auroral oval tends to be narrow and continuous throughout the noon meridian without any noon gap or any strong undulation in the auroral distribution. For positive IMF By, a sharp decrease and spreading of auroral activity is frequently observed in the post-noon sector, a strong undulation in the poleward boundary of the auroral oval around noon, and the formation of auroral forms poleward of the oval; and (3) the observed features of dayside auroras are in reasonable agreement with the expected distribution of upward field-aligned currents associated with the IMF By in the noon sector.

Substorm development as observed by Interball UV imager and 2-D magnetic array

Abstract Results of the study of two substorms from Interball auroral UV measurements and two-dimensional patterns of equivalent ionospheric currents derived from the MACCS/CANOPUS and Greenland magnetometer arrays are presented. Substorm development in 2-D equivalent ionospheric current patterns may be described in terms of the formation of two vortices in the equivalent currents: a morning vortex related to downward field-aligned current and an evening vortex related to upward field-aligned current. Poleward propagation of the magnetic disturbances during substorm expansive phase was found to be associated mainly with a poleward displacement of the morning vortex, whereas the evening vortex remained approximately at the same position. As a result, the initial quasi-azimuthal separation of the vortices was replaced by their quasi-meridional separation at substorm maximum. Interball UV images during this period showed the formation of a bright auroral border at the poleward edge of substorm auroral bulge. The auroral UV images showed also that the auroral distribution in the region between the polar border and the main auroral oval tends to have a form of bubbles or petals growing from a bright protuberant region on the equatorward boundary of the auroral oval. However, the resolution of the UV imager was not sufficient for the reliable separation of such the structures, therefore, this result should be considered as preliminary. Overlapping of the auroral UV images onto equivalent current patterns shows that the bright substorm surge was well collocated with the evening vortex whereas the poleward auroral border did not coincide with any evident feature in equivalent ionospheric currents and was located several degrees equatorward of the morning current vortex center related to downward field-aligned current. The ground-based magnetic array allowing us to obtain instantaneous patterns of equivalent ionospheric currents gives a possibility to propose a new index for substorm activity such as the magnitude of the total current between the centers of the morning and evening vortices. Such integral index would not depend on where the substorm is located and be unaffected by the migration of substorm activity poleward or equatorward.

Field‐aligned currents in the polar cap at small IMF Bz and By inferred from SuperDARN radar observations

Routine SuperDARN observations of the ionospheric plasma convection and field-aligned currents (FACs) in the high-latitude ionosphere are used to study current systems established at small interplanetary magnetic field (IMF) B z and B y , By statistical averaging of available data sets we show that under this IMF condition the ionospheric convection pattern consists of two (evening and morning) convection cells that are similar in shape. The flow intensity inside the central polar cap is noticeably depressed so that plasma entering the polar cap flows around its border, predominantly along the lines of equal magnetic latitude, so that the convection cells are of a crescent-like shape. This global pattern of plasma flow is associated with the effect of the region 0 field-aligned currents coexisting with the region 1 and region 2 field-aligned currents. SuperDARN observations of FACs for individual events support this conclusion. FACs were derived by analyzing the vorticity of the SuperDARN convection maps. We show that region 0 currents for small IMF B z and B y can exist in time sectors way off the magnetic noon. Thus radar observations support earlier findings from satellite magnetometer measurements of the region 0 current system at high latitudes during both the prenoon and afternoon at small IMF intensities. Because the region 0 FACs occur during small IMF intensities, it is suggested that quasi-viscous processes play a role in their generation.

Ionospheric convection and equivalent ionospheric currents in the dayside high‐latitude winter ionosphere

Ionospheric convection inferred from Super Dual Auroral Radar Network (SuperDARN) HF radar measurements is compared with an equivalent ionospheric convection derived from ground magnetometer data in the dayside winter high-latitude ionosphere. Although there was general agreement between observed convection patterns produced by radars and magnetometers, there were significant differences in details. The orientation of equivalent convection vectors inferred from magnetic data was often opposite to the convection vectors determined by the SuperDARN radars in the poleward part of the convection vortex structure, though the agreement was reasonable in its equatorward part. The magnitudes of convection vectors determined from radar data and those inferred from magnetometer data were often different. The observed differences are attributed to strong horizontal inhomogeneity in the ionospheric conductivity distribution for winter conditions. It is possible that magnetic disturbances in the dark high-latitude ionosphere are strongly affected by field-aligned currents at the terminator that separates regions of the sunlit highly conducting ionosphere and dark poorly conducting ionosphere.

Possible Role of ion Demagnetization in the Plasma Sheet in Auroral arc and Substorm Generation

Ion demagnetization in the plasma sheet causes the formation of field-aligned current that can trigger a magnetosphere-ionosphere coupling feedback instability, which may play an important role in substorm and auroral arc generation. Since field-aligned currents close ionospheric currents, their magnitude is controlled by ionospheric conductivity. The cause of instability is the impact of increasing upward field-aligned currents on ionospheric conductivity, which in turn stimulates an increase in the field-aligned currents. When the magnitude of these currents becomes sufficiently large for the acceleration of precipitating electrons, a feedback mechanism becomes possible. Upward field-aligned currents increase the ionospheric conductivity that stimulates an explosion-like increase in field-aligned currents. It is believed that this instability may be related to substorm generation. Demagnetization of hot ions in the plasma sheet leads to the motion of magnetospheric electrons through a spatial gradient of ion population. Field-aligned currents, because of their effect on particle acceleration and the magnitude of ionospheric conductivity, can also lead to another type of instability associated with the breaking of the earthward convection flow into convection streams. The growth rate of this instability is maximum for structures with sizes less than the ion Larmor radius in the equatorial plane. This may lead to the formation of auroral arcs with widths of the order of 10 km. This instability is able to explain many features of auroral arcs, including their conjugacy in opposite hemispheres. However, it cannot explain very narrow (less than 1 km) arcs.

Eastward convection jet at the poleward boundary of the nightside auroral oval

The statistical study of the azimuthal convection flow in the midnight sector, as measured by the Saskatoon and the Stokkseyri SuperDARN radars, reveals the existence of an enhanced eastward convection stream around the poleward boundary of the auroral oval. The stream occupies two to three degrees and is located at geomagnetic latitudes 72–75°, which is indeed significantly poleward of the position of the center of the auroral oval. Poleward of the eastward convection stream, a westward convection stream is detected by the Saskatoon radar though not evident in the Stokkseyri radar measurements. The existence of the eastward convection stream at the poleward edge of the nightside auroral oval is very consistent with earlier results from the Akebono spacecraft. Such plasma flows are the source of possible plasma instabilities in the ionospheric E and F-regions.