Patches in the Polar Ionosphere: UT and Seasonal Dependence

The seasonal and UT dependencies of patches in the polar ionosphere are simulated using the Utah State University time dependent ionospheric model (TDIM). Patch formation 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 IMF. 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 performed 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

Ionospheric Storm Simulations Driven by Magnetospheric MHD and Empirical Models with Data Comparisons

The results of two ionospheric simulations are compared with each other and with ionospheric observations of the southern hemisphere for the magnetic cloud passage event of January 14, 1988. For most of the event one simulation agrees with observations, while the other does not. Electric fields and electron precipitation patterns generated by a magnetospheric MHD model are used as inputs to a physical model of the ionosphere in the successful simulation, while empirical electric fields and electron precipitation are used as the inputs for the second simulation. In spite of ionospheric summer conditions a large and deep polar hole is developed. This is seen in the in situ plasma observations made by the DMSP-F8 satellite. The hole is surprisingly present during both northward and southward IMF conditions. It is deepest for the storm phase of the southward IMF period. A well-defined tongue of ionization is formed during this period. These features have been reproduced by the TDIM-MHD simulation and to a lesser extent by the TDIM-empirical simulation. However, the model simulations have not been able to generate a storm enhanced density where one was observed by DMSP-F8 during the initial phase of the storm. The differences between the two F region ionospheric simulations are attributed to differences in the magnetospheric electric fields and precipitation patterns used as inputs. This study provides a unique first simulation of the ionosphere\u27s response to self-consistent electric field and auroral precipitation patterns over a 24-hour period that leads into a major geomagnetic storm

Modeling Polar Cap \u3ci\u3eF\u3c/i\u3e-Region Patches Using Time Varying Convection

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

Theoretical Study of the Effect of Ionospheric Return Currents on the Electron Temperature

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

Modeled Ionospheric Te Profiles at Mid-Latitudes for Possible IRI Application

The electron temperature (Te) variation in the mid-latitude ionosphere at altitudes between 120 – 800 km has been modelled for various seasonal and solar-cycle conditions. The calculated electron temperatures are consistent with plasma densities and ion temperatures computed from a time-dependent ionospheric model. The Te distribution can be represented by a subset of standard Te profiles. Te above 200 km is controlled by the magnetospheric heat flux into the ionosphere. For realistic values of the magnetospheric heat flux, the maximum electron temperature ranges from 3000 to 10,000 K at 800 km. The effect of increasing the heat flux is to increase the topside temperature but retain the profile shape. Hence, given a topside Te observation and selection of an appropriate profile shape, the entire Te distribution can be computed

Theoretical Study of the Electron Temperature in the High-Latitude Ionosphere for Solar Maximum and Winter Conditions

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 Te 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

Relationship of Theoretical Patch Climatology to Polar Cap Patch Observations

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 B y component, and hence the patch locations are also expected to be B y 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 B y 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 B y 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

Ionospheric Response to an Auroral Substorm

The response of the ionosphere to a representative auroral substorm was simulated. The response was found to be significant at all altitudes in a large spatial region near midnight magnetic local time. In this midnight region, there were Te and Ti hot spots, substantial O+ → NO+ composition changes, non‐Maxwellian velocity distributions, transient ion upwellings, a large‐scale lowering of the F‐layer, ionization peaks that occur in the E‐region, and sharp horizontal gradients. Also, during the expansion phase, the E‐region densities increase due to auroral precipitation, while the plasma densities above 300 km decrease due to the overall lowering of the ionosphere. The net result is that the temporal morphologies of the plasma densities at high and low altitudes are opposite during this part of the substorm. These complex features indicate that care must be exercised when interpreting plasma measurements from both ground‐based and space‐based instruments

Ambiguity in Identification of Polar Cap F-Region Patches: Contrasting Radio and Optical Observation Techniques

The phenomenon referred to as polar cap F-region patches can be observed by many different techniques, including measurements of the radio wave critical frequency, the 630 nm intensity, the in situ electron density, and radio wave coherent scatter from irregularities on the patches. Consequently, the definition of a patch may be technique-dependent or at least ambiguous. In this study we used a physical model of the ionosphere to study the relationship between ground-based 630 nm intensity and simulated critical frequency measurements of patches. The results show that the 630 nm intensity and NmFm are not well correlated without a knowledge of hmF2, the peak altitude of the F-layer. In the polar cap the variation of hmF2 could well be ±100 km, resulting in variations of a factor of four in 630 nm intensity for a constant NmF2 value. Hence, correlating patches observed in 630 nm with NmF2 requires a detailed knowledge of hmF2. Ionospheric model simulations have been parameterized such that the model predictions of the 630 nm intensity — NmF2 — hmF2 dependencies are available as an aid in interpreting patch measurements. These results also indicate that the search for neutral atmospheric gravity waves via their effect on 630 nm emissions is even more difficult than anticipated previously