The seasonal and UT dependencies of patches in the polar ionosphere are simulated using the Utah State University time dependent ionospheric model (TDIM). Patch fonnation is achieved by changing the plasma convection pattern in response to temporal changes in the interplanetary magnetic field (IMF) By component during periods of southward IMP. This mechanism redirects the plasma flow from the dayside high-density region , which is the source of the tongue of ionization (TOI) density feature, through the throat and leads to patches, rather than a continuous TOI. The model predicts that the patches are absent at winter solstice (northern hemisphere) between 0800 and 1200 UT and that they have their largest seasonal intensity at winter solstice between 2000 and 2400 UT. Between winter solstice and equinox, patches are strong and present all day. Patches are present in summer as well, although their intensity is only tens of percent above the background density. These winter-to-equinox findings are also shown to be consistent with observations. The model was also used to predict times at which patch observations could be perfonned to determine the contributions from other patch mechanisms. This observational window is ± 20 days about winter solstice between 0800 and 1200 UT in the northern hemisphere. In this observational window the TOI is either absent or reduced to a very low density. Hence the time dependent electric field mechanism considered in this study does not produce patches, and if they are observed, then they must be due to some other mechanism.
Creation of polar cap F‐region patches are simulated for the first time using two independent physical models of the high latitude ionosphere. The patch formation is achieved by temporally varying the magnetospheric electric field (ionospheric convection) input to the models. The imposed convection variations are comparable to changes in the convection that result from changes in the By IMF component for southward interplanetary magnetic field (IMF). Solar maximum‐winter simulations show that simple changes in the convection pattern lead to significant changes in the polar cap plasma structuring. Specifically, in winter, as enhanced dayside plasma convects into the polar cap to form the classic tongue‐of‐ionization (TOI) the convection changes produce density structures that are indistinguishable from the observed patches.
The electron temperature (Te) variation in the high-latitude ionosphere at altitudes between 120 and 800 km has been modeled for solar maximum, winter solstice, and strong magnetic activity conditions. The calculated electron temperatures are consistent with the plasma densities and ion temperatures computed from a time-dependent ionospheric model. Heating rates for both solar EUV and auroral precipitation were included. In general, the predicted UT variation of the electron temperature that results from the displacement between the magnetic and geographic poles is only a few hundred degrees. However, in sunlit trough regions, Te hot spots develop, and these hot spots show a marked UT variation, by as much as 2500 K. The dominant parameter controlling the T, variation above 200 km is the magnetospheric heat flux into the ionosphere, which is essentially unknown. For realistic values of the magnetospheric heat flux, the maximum electron temperature ranges from 5000 to 10,000 K at 800 km. A magnetospheric heat flux is particularly effective in enhancing trough electron temperatures. In general, the electron heat flux at high altitudes is uniquely related to the electron temperature and gradient, except on auroral field lines where thermoelectric heat flow is important. 1. INTRODUCTION During the last several years, we have developed a comprehensive model of the convecting high-latitude ionosphere in order to determine the extent to which various chemical and transport processes affect the ion temperature, ion composition, and electron density at Fregion altitudes [cf. $chunk and Raitt, 1980; $ojka et al., 1979, 1981a; $chunk and $ojka, 1982a]. Our numerical model produces time-dependent, three-dimensional distributions for the ion temperature and the ion (NO*, NI, O•, N*, O*, He*) and electron densities. The model takes account of field-aligned diffusion, cross-field electrodynamic drifts, thermospheric winds, polar wind escape, energy-dependent chemical reactions, neutral composition changes, ion production due to solar EUV radiation and auroral precipitation, ion thermal conduction, ion diffusion-thermal heat flow, and local heating and cooling processes. Our model also takes account of the offset between the geomagnetic and geographic poles.In this investigation, we have improved our high-latitude ionospheric model by including the electron energy equation so that we can study the electron temperature variations in the high-latitude F region. The adopted energy equation takes account of thermal conduction, thermoelectric transport, Joule heating, heating due to photoelectrons and auroral electrons, collisional coupling to the thermal ions, and both elastic and inelastic cooling to the neutrals.Although a significant effort has been devoted to studying the electron temperature behavior at middle and low latitudes, in comparison much less effort has been directed toward studying the Te behavior at high latitudes [cf. $chunk and Nagy, 1978]. However, during the last decade, certain trends have been clearly establis...
Abstract. During a southward orientation of the interplanetary magnetic field (IMF), patches are often observed moving antisunward across the polar cap. In saying "patches" we refer to structures in which the F region electron densities are enhanced relative to lower background levels; we do not in this paper consider patches which are observed optically (see J. J. Sojka et al., Ambiguity in identificiation of polar cap F region patches, submitted to the Journal of Atmospheric and Terrestrial Physics, 1995). The patches can be modeled by a process which involves the "chopping up" of the tongue of ionization (TOI) [Sojka et al., 1993a]. Various mechanisms for chopping the TOI have been suggested; our preferred method is to introduce temporal changes in the convection electric field pattern. In any case the present study is quite independent of any particular mechanism, so long as the TOI is considered to be the source of the patches. In this study we have used the Utah State University Time-Dependent Ionospheric Model (TDIM) to model the TOI for various IMF By orientations. In our simulations the location of the TOI in the polar cap is mainly determined by the IMF By component, and hence the patch locations are also expected to be By dependent. This suggests that a polar ground-based instrument may not see patches even when they are present in the polar ionosphere. This is because the typical field of view of a ground-based instrument, such as an all-sky camera, covers less than 10% of the polar region. The TDIM simulation results were used to predict the By dependence of patches that different ground-based sites would observe. Eureka (Canada) at the magnetic pole is predicted not to observe patches for southward IMF conditions if the By component is strongly negative. Sondrestrom (Greenland) and NyAlesund (Svalbard), although at similar cusp latitudes, are expected to see quite different diurnal responses to patches. At Sondrestrom, patches are seen at noon in winter; both sites should see them in the premidnight sector. These model predictions are the "groundwork" for detailed patch observation-model comparisons at all three sites. IntroductionBuchau et al. [1983] have shown that polar F region density structures come in two main forms, that is, patches and sun-aligned arcs. The patches were found to correlate with times of strong geomagnetic activity. Today we also know that the patches form under southward interplanetary magnetic field (IMF) conditions. Weber et al. [1984] showed that the patches were transported into the polar cap rather than formed locally by precipitation. These observations show that the patches convect across the polar cap at the prevailing convection speed. This convection is predominantly antisunward for southward IMF conditions. The convection can be traced backward, through the cusp, into a dayside region equatorward of the auroral ionosphere. Hence the origin of the patches could be equatorward of the cusp region, in the cusp region, or a combination of both. Weber et al. [1984, ...
An electron heat flow can occur in a partially ionized plasma in response to either an electron temperature gradient (thermal conduction) or an electron current (thermoelectric heat flow). The former process has been extensively studied, while the latter process has received relatively little attention. Therefore a time‐dependent three‐dimensional model of the high‐latitude ionosphere was used to study the effect of field‐aligned ionospheric return currents on auroral electron temperatures for different seasonal and solar cycle conditions as well as for different upper boundary heat fluxes. The results of this study lead to the following conclusions: (1) The average, large‐scale, return current densities, which are a few microamps per square meter, are too small to affect auroral electron temperatures. (2) Current densities greater than about 10−5 A m−2 are needed for thermoelectric heat flow to be important. (3) The thermoelectric effect displays a marked solar cycle and seasonal dependence. (4) Thermoelectric heat transport corresponds to an upward flow of electron energy. (5) This energy flow can be either a source or sink of electron energy, depending on the altitude and geophysical conditions. (6) Thermoelectric heat transport is typically a sink above 300 km and acts to lower ambient electron temperatures by as much as 2000 K for field‐aligned return current densities of the order of 5 × 10 −5 A m−2. For this case, the electron temperature decreases with altitude above 300 km with a gradient that can exceed 1 K km−1. Also, the electron temperature can drop below both the ion and neutral temperatures in the upper F region owing to thermoelectric cooling. (7) A downward magnetospheric heat flux in combinations with an upward thermoelectric heat flux can produce steep positive electron temperature gradients in the topside ionosphere.
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